Bump version numbers for 1.4.8 release.

Update tor bridges

See merge request briar/briar!1653

bramble-android/build.gradle

@@ -15,8 +15,8 @@ android {
 	defaultConfig {
 		minSdkVersion 16
 		targetSdkVersion 30
-		versionCode 10406
+		versionCode 10408
-		versionName "1.4.6"
+		versionName "1.4.8"
 		consumerProguardFiles 'proguard-rules.txt'
 
 		testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"

bramble-android/src/main/java/org/briarproject/bramble/network/AndroidNetworkManager.java

@@ -11,7 +11,10 @@ import android.net.LinkAddress;
 import android.net.LinkProperties;
 import android.net.Network;
 import android.net.NetworkInfo;
+import android.net.wifi.WifiInfo;
+import android.net.wifi.WifiManager;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.event.EventBus;
 import org.briarproject.bramble.api.event.EventExecutor;
 import org.briarproject.bramble.api.lifecycle.Service;
@@ -21,7 +24,6 @@ import org.briarproject.bramble.api.network.event.NetworkStatusEvent;
 import org.briarproject.bramble.api.nullsafety.MethodsNotNullByDefault;
 import org.briarproject.bramble.api.nullsafety.ParametersNotNullByDefault;
 import org.briarproject.bramble.api.system.TaskScheduler;
-import org.briarproject.bramble.api.system.TaskScheduler.Cancellable;
 
 import java.net.Inet4Address;
 import java.net.InetAddress;
@@ -38,6 +40,7 @@ import javax.annotation.Nullable;
 import javax.inject.Inject;
 
 import static android.content.Context.CONNECTIVITY_SERVICE;
+import static android.content.Context.WIFI_SERVICE;
 import static android.content.Intent.ACTION_SCREEN_OFF;
 import static android.content.Intent.ACTION_SCREEN_ON;
 import static android.net.ConnectivityManager.CONNECTIVITY_ACTION;
@@ -111,15 +114,37 @@ class AndroidNetworkManager implements NetworkManager, Service {
 
 	@Override
 	public NetworkStatus getNetworkStatus() {
-		NetworkInfo net = connectivityManager.getActiveNetworkInfo();
+		// https://issuetracker.google.com/issues/175055271
-		boolean connected = net != null && net.isConnected();
+		try {
-		boolean wifi = false, ipv6Only = false;
+			NetworkInfo net = connectivityManager.getActiveNetworkInfo();
-		if (connected) {
+			boolean connected = net != null && net.isConnected();
-			wifi = net.getType() == TYPE_WIFI;
+			boolean wifi = false, ipv6Only = false;
-			if (SDK_INT >= 23) ipv6Only = isActiveNetworkIpv6Only();
+			if (connected) {
-			else ipv6Only = areAllAvailableNetworksIpv6Only();
+				wifi = net.getType() == TYPE_WIFI;
+				if (SDK_INT >= 23) ipv6Only = isActiveNetworkIpv6Only();
+				else ipv6Only = areAllAvailableNetworksIpv6Only();
+			}
+			return new NetworkStatus(connected, wifi, ipv6Only);
+		} catch (SecurityException e) {
+			logException(LOG, WARNING, e);
+			// Without the ConnectivityManager we can't detect whether we have
+			// internet access. Assume we do, which is probably less harmful
+			// than assuming we don't. Likewise, assume the connection is
+			// IPv6-only. Fall back to the WifiManager to detect whether we
+			// have a wifi connection.
+			LOG.info("ConnectivityManager is broken, guessing connectivity");
+			boolean connected = true, wifi = false, ipv6Only = true;
+			WifiManager wm = (WifiManager) app.getSystemService(WIFI_SERVICE);
+			if (wm != null) {
+				WifiInfo info = wm.getConnectionInfo();
+				if (info != null && info.getIpAddress() != 0) {
+					LOG.info("Connected to wifi");
+					wifi = true;
+					ipv6Only = false;
+				}
+			}
+			return new NetworkStatus(connected, wifi, ipv6Only);
 		}
-		return new NetworkStatus(connected, wifi, ipv6Only);
 	}
 
 	/**
@@ -130,23 +155,29 @@ class AndroidNetworkManager implements NetworkManager, Service {
 	 */
 	@TargetApi(23)
 	private boolean isActiveNetworkIpv6Only() {
-		Network net = connectivityManager.getActiveNetwork();
+		// https://issuetracker.google.com/issues/175055271
-		if (net == null) {
+		try {
-			LOG.info("No active network");
+			Network net = connectivityManager.getActiveNetwork();
+			if (net == null) {
+				LOG.info("No active network");
+				return false;
+			}
+			LinkProperties props = connectivityManager.getLinkProperties(net);
+			if (props == null) {
+				LOG.info("No link properties for active network");
+				return false;
+			}
+			boolean hasIpv6Unicast = false;
+			for (LinkAddress linkAddress : props.getLinkAddresses()) {
+				InetAddress addr = linkAddress.getAddress();
+				if (addr instanceof Inet4Address) return false;
+				if (!addr.isMulticastAddress()) hasIpv6Unicast = true;
+			}
+			return hasIpv6Unicast;
+		} catch (SecurityException e) {
+			logException(LOG, WARNING, e);
 			return false;
 		}
-		LinkProperties props = connectivityManager.getLinkProperties(net);
-		if (props == null) {
-			LOG.info("No link properties for active network");
-			return false;
-		}
-		boolean hasIpv6Unicast = false;
-		for (LinkAddress linkAddress : props.getLinkAddresses()) {
-			InetAddress addr = linkAddress.getAddress();
-			if (addr instanceof Inet4Address) return false;
-			if (!addr.isMulticastAddress()) hasIpv6Unicast = true;
-		}
-		return hasIpv6Unicast;
 	}
 
 	/**

bramble-android/src/main/java/org/briarproject/bramble/plugin/file/AndroidRemovableDrivePlugin.java

@@ -32,13 +32,22 @@ class AndroidRemovableDrivePlugin extends RemovableDrivePlugin {
 	InputStream openInputStream(TransportProperties p) throws IOException {
 		String uri = p.get(PROP_URI);
 		if (isNullOrEmpty(uri)) throw new IllegalArgumentException();
-		return app.getContentResolver().openInputStream(Uri.parse(uri));
+		try {
+			return app.getContentResolver().openInputStream(Uri.parse(uri));
+		} catch (SecurityException e) {
+			throw new IOException(e);
+		}
 	}
 
 	@Override
 	OutputStream openOutputStream(TransportProperties p) throws IOException {
 		String uri = p.get(PROP_URI);
 		if (isNullOrEmpty(uri)) throw new IllegalArgumentException();
-		return app.getContentResolver().openOutputStream(Uri.parse(uri), "wt");
+		try {
+			return app.getContentResolver()
+					.openOutputStream(Uri.parse(uri), "wt");
+		} catch (SecurityException e) {
+			throw new IOException(e);
+		}
 	}
 }

bramble-android/src/main/java/org/briarproject/bramble/plugin/tcp/AndroidLanTcpPlugin.java

@@ -175,16 +175,24 @@ class AndroidLanTcpPlugin extends LanTcpPlugin {
 	@TargetApi(21)
 	@Nullable
 	private InetAddress getWifiClientIpv6Address() {
-		for (Network net : connectivityManager.getAllNetworks()) {
+		// https://issuetracker.google.com/issues/175055271
-			NetworkCapabilities caps =
+		try {
-					connectivityManager.getNetworkCapabilities(net);
+			for (Network net : connectivityManager.getAllNetworks()) {
-			if (caps == null || !caps.hasTransport(TRANSPORT_WIFI)) continue;
+				NetworkCapabilities caps =
-			LinkProperties props = connectivityManager.getLinkProperties(net);
+						connectivityManager.getNetworkCapabilities(net);
-			if (props == null) continue;
+				if (caps == null || !caps.hasTransport(TRANSPORT_WIFI)) {
-			for (LinkAddress linkAddress : props.getLinkAddresses()) {
+					continue;
-				InetAddress addr = linkAddress.getAddress();
+				}
-				if (isIpv6LinkLocalAddress(addr)) return addr;
+				LinkProperties props =
+						connectivityManager.getLinkProperties(net);
+				if (props == null) continue;
+				for (LinkAddress linkAddress : props.getLinkAddresses()) {
+					InetAddress addr = linkAddress.getAddress();
+					if (isIpv6LinkLocalAddress(addr)) return addr;
+				}
 			}
+		} catch (SecurityException e) {
+			logException(LOG, WARNING, e);
 		}
 		return null;
 	}
@@ -227,12 +235,17 @@ class AndroidLanTcpPlugin extends LanTcpPlugin {
 	// network's socket factory may try to connect via another network
 	private SocketFactory getSocketFactory() {
 		if (SDK_INT < 21) return SocketFactory.getDefault();
-		for (Network net : connectivityManager.getAllNetworks()) {
+		// https://issuetracker.google.com/issues/175055271
-			NetworkCapabilities caps =
+		try {
-					connectivityManager.getNetworkCapabilities(net);
+			for (Network net : connectivityManager.getAllNetworks()) {
-			if (caps != null && caps.hasTransport(TRANSPORT_WIFI)) {
+				NetworkCapabilities caps =
-				return net.getSocketFactory();
+						connectivityManager.getNetworkCapabilities(net);
+				if (caps != null && caps.hasTransport(TRANSPORT_WIFI)) {
+					return net.getSocketFactory();
+				}
 			}
+		} catch (SecurityException e) {
+			logException(LOG, WARNING, e);
 		}
 		LOG.warning("Could not find suitable socket factory");
 		return SocketFactory.getDefault();

bramble-android/src/main/java/org/briarproject/bramble/plugin/tor/AndroidTorPluginFactory.java

@@ -11,62 +11,35 @@ import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.Backoff;
 import org.briarproject.bramble.api.plugin.BackoffFactory;
 import org.briarproject.bramble.api.plugin.PluginCallback;
-import org.briarproject.bramble.api.plugin.TorConstants;
 import org.briarproject.bramble.api.plugin.TorControlPort;
 import org.briarproject.bramble.api.plugin.TorDirectory;
 import org.briarproject.bramble.api.plugin.TorSocksPort;
-import org.briarproject.bramble.api.plugin.TransportId;
-import org.briarproject.bramble.api.plugin.duplex.DuplexPlugin;
-import org.briarproject.bramble.api.plugin.duplex.DuplexPluginFactory;
 import org.briarproject.bramble.api.system.AndroidWakeLockManager;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.system.LocationUtils;
 import org.briarproject.bramble.api.system.ResourceProvider;
 import org.briarproject.bramble.api.system.WakefulIoExecutor;
-import org.briarproject.bramble.util.AndroidUtils;
 
 import java.io.File;
 import java.util.concurrent.Executor;
-import java.util.logging.Logger;
 
+import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 import javax.net.SocketFactory;
 
+import static org.briarproject.bramble.util.AndroidUtils.getSupportedArchitectures;
+
 @Immutable
 @NotNullByDefault
-public class AndroidTorPluginFactory implements DuplexPluginFactory {
+public class AndroidTorPluginFactory extends TorPluginFactory {
 
-	private static final Logger LOG =
-			Logger.getLogger(AndroidTorPluginFactory.class.getName());
-
-	private static final int MAX_LATENCY = 30 * 1000; // 30 seconds
-	private static final int MAX_IDLE_TIME = 30 * 1000; // 30 seconds
-	private static final int MIN_POLLING_INTERVAL = 60 * 1000; // 1 minute
-	private static final int MAX_POLLING_INTERVAL = 10 * 60 * 1000; // 10 mins
-	private static final double BACKOFF_BASE = 1.2;
-
-	private final Executor ioExecutor, wakefulIoExecutor;
 	private final Application app;
-	private final NetworkManager networkManager;
-	private final LocationUtils locationUtils;
-	private final EventBus eventBus;
-	private final SocketFactory torSocketFactory;
-	private final BackoffFactory backoffFactory;
-	private final ResourceProvider resourceProvider;
-	private final CircumventionProvider circumventionProvider;
-	private final BatteryManager batteryManager;
 	private final AndroidWakeLockManager wakeLockManager;
-	private final Clock clock;
-	private final File torDirectory;
-	private int torSocksPort;
-	private int torControlPort;
-	private final CryptoComponent crypto;
 
 	@Inject
 	AndroidTorPluginFactory(@IoExecutor Executor ioExecutor,
 			@WakefulIoExecutor Executor wakefulIoExecutor,
-			Application app,
 			NetworkManager networkManager,
 			LocationUtils locationUtils,
 			EventBus eventBus,
@@ -75,80 +48,43 @@ public class AndroidTorPluginFactory implements DuplexPluginFactory {
 			ResourceProvider resourceProvider,
 			CircumventionProvider circumventionProvider,
 			BatteryManager batteryManager,
-			AndroidWakeLockManager wakeLockManager,
 			Clock clock,
+			CryptoComponent crypto,
 			@TorDirectory File torDirectory,
 			@TorSocksPort int torSocksPort,
 			@TorControlPort int torControlPort,
-			CryptoComponent crypto) {
+			Application app,
-		this.ioExecutor = ioExecutor;
+			AndroidWakeLockManager wakeLockManager) {
-		this.wakefulIoExecutor = wakefulIoExecutor;
+		super(ioExecutor, wakefulIoExecutor, networkManager, locationUtils,
+				eventBus, torSocketFactory, backoffFactory, resourceProvider,
+				circumventionProvider, batteryManager, clock, crypto,
+				torDirectory, torSocksPort, torControlPort);
 		this.app = app;
-		this.networkManager = networkManager;
-		this.locationUtils = locationUtils;
-		this.eventBus = eventBus;
-		this.torSocketFactory = torSocketFactory;
-		this.backoffFactory = backoffFactory;
-		this.resourceProvider = resourceProvider;
-		this.circumventionProvider = circumventionProvider;
-		this.batteryManager = batteryManager;
 		this.wakeLockManager = wakeLockManager;
-		this.clock = clock;
-		this.torDirectory = torDirectory;
-		this.torSocksPort = torSocksPort;
-		this.torControlPort = torControlPort;
-		this.crypto = crypto;
 	}
 
+	@Nullable
 	@Override
-	public TransportId getId() {
+	String getArchitectureForTorBinary() {
-		return TorConstants.ID;
+		for (String abi : getSupportedArchitectures()) {
-	}
+			if (abi.startsWith("x86_64")) return "x86_64_pie";
-
+			else if (abi.startsWith("x86")) return "x86_pie";
-	@Override
+			else if (abi.startsWith("arm64")) return "arm64_pie";
-	public long getMaxLatency() {
+			else if (abi.startsWith("armeabi")) return "arm_pie";
-		return MAX_LATENCY;
-	}
-
-	@Override
-	public DuplexPlugin createPlugin(PluginCallback callback) {
-
-		// Check that we have a Tor binary for this architecture
-		String architecture = null;
-		for (String abi : AndroidUtils.getSupportedArchitectures()) {
-			if (abi.startsWith("x86_64")) {
-				architecture = "x86_64";
-				break;
-			} else if (abi.startsWith("x86")) {
-				architecture = "x86";
-				break;
-			} else if (abi.startsWith("arm64")) {
-				architecture = "arm64";
-				break;
-			} else if (abi.startsWith("armeabi")) {
-				architecture = "arm";
-				break;
-			}
 		}
-		if (architecture == null) {
+		return null;
-			LOG.info("Tor is not supported on this architecture");
+	}
-			return null;
-		}
-		// Use position-independent executable
-		architecture += "_pie";
 
-		Backoff backoff = backoffFactory.createBackoff(MIN_POLLING_INTERVAL,
+	@Override
-				MAX_POLLING_INTERVAL, BACKOFF_BASE);
+	TorPlugin createPluginInstance(Backoff backoff,
-		TorRendezvousCrypto torRendezvousCrypto =
+			TorRendezvousCrypto torRendezvousCrypto, PluginCallback callback,
-				new TorRendezvousCryptoImpl(crypto);
+			String architecture) {
-		AndroidTorPlugin plugin = new AndroidTorPlugin(ioExecutor,
+		return new AndroidTorPlugin(ioExecutor,
 				wakefulIoExecutor, app, networkManager, locationUtils,
 				torSocketFactory, clock, resourceProvider,
 				circumventionProvider, batteryManager, wakeLockManager,
 				backoff, torRendezvousCrypto, callback, architecture,
 				MAX_LATENCY, MAX_IDLE_TIME, torDirectory, torSocksPort,
 				torControlPort);
-		eventBus.addListener(plugin);
-		return plugin;
 	}
 }

bramble-android/src/main/java/org/briarproject/bramble/system/AndroidTaskScheduler.java

@@ -8,6 +8,7 @@ import android.content.Intent;
 import android.os.Process;
 import android.os.SystemClock;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.lifecycle.Service;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.system.AlarmListener;
@@ -116,10 +117,12 @@ class AndroidTaskScheduler implements TaskScheduler, Service, AlarmListener {
 		long dueMillis = now + MILLISECONDS.convert(delay, unit);
 		Runnable wakeful = () ->
 				wakeLockManager.executeWakefully(task, executor, "TaskHandoff");
-		Future<?> check = scheduleCheckForDueTasks(delay, unit);
+		// Acquire the lock before scheduling the check to ensure the check
-		ScheduledTask s = new ScheduledTask(wakeful, dueMillis, check,
+		// doesn't access the task queue before the task has been added
-				cancelled);
+		ScheduledTask s;
 		synchronized (lock) {
+			Future<?> check = scheduleCheckForDueTasks(delay, unit);
+			s = new ScheduledTask(wakeful, dueMillis, check, cancelled);
 			tasks.add(s);
 		}
 		return s;
@@ -136,6 +139,7 @@ class AndroidTaskScheduler implements TaskScheduler, Service, AlarmListener {
 		return schedule(wrapped, executor, delay, unit, cancelled);
 	}
 
+	@GuardedBy("lock")
 	private Future<?> scheduleCheckForDueTasks(long delay, TimeUnit unit) {
 		Runnable wakeful = () -> wakeLockManager.runWakefully(
 				this::runDueTasks, "TaskScheduler");
@@ -206,7 +210,7 @@ class AndroidTaskScheduler implements TaskScheduler, Service, AlarmListener {
 		private final Future<?> check;
 		private final AtomicBoolean cancelled;
 
-		public ScheduledTask(Runnable task, long dueMillis,
+		private ScheduledTask(Runnable task, long dueMillis,
 				Future<?> check, AtomicBoolean cancelled) {
 			this.task = task;
 			this.dueMillis = dueMillis;

bramble-android/src/main/java/org/briarproject/bramble/util/AndroidUtils.java

@@ -4,6 +4,7 @@ import android.annotation.SuppressLint;
 import android.bluetooth.BluetoothAdapter;
 import android.content.Context;
 import android.os.Build;
+import android.os.Looper;
 import android.provider.Settings;
 
 import org.briarproject.bramble.api.Pair;
@@ -134,4 +135,8 @@ public class AndroidUtils {
 			return null;
 		}
 	}
+
+	public static boolean isUiThread() {
+		return Looper.myLooper() == Looper.getMainLooper();
+	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/Cancellable.java

@@ -0,0 +1,6 @@
+package org.briarproject.bramble.api;
+
+public interface Cancellable {
+
+	void cancel();
+}

bramble-api/src/main/java/org/briarproject/bramble/api/StringMap.java

@@ -1,8 +1,14 @@
 package org.briarproject.bramble.api;
 
+import org.briarproject.bramble.util.StringUtils;
+
+import java.util.ArrayList;
 import java.util.Hashtable;
+import java.util.List;
 import java.util.Map;
 
+import javax.annotation.Nullable;
+
 public abstract class StringMap extends Hashtable<String, String> {
 
 	protected StringMap(Map<String, String> m) {
@@ -52,4 +58,31 @@ public abstract class StringMap extends Hashtable<String, String> {
 	public void putLong(String key, long value) {
 		put(key, String.valueOf(value));
 	}
+
+	@Nullable
+	public int[] getIntArray(String key) {
+		String s = get(key);
+		if (s == null) return null;
+		// Handle empty string because "".split(",") returns {""}
+		if (s.length() == 0) return new int[0];
+		String[] intStrings = s.split(",");
+		int[] ints = new int[intStrings.length];
+		try {
+			for (int i = 0; i < ints.length; i++) {
+				ints[i] = Integer.parseInt(intStrings[i]);
+			}
+		} catch (NumberFormatException e) {
+			return null;
+		}
+		return ints;
+	}
+
+	public void putIntArray(String key, int[] value) {
+		List<String> intStrings = new ArrayList<>();
+		for (int integer : value) {
+			intStrings.add(String.valueOf(integer));
+		}
+		// Puts empty string if input array value is empty
+		put(key, StringUtils.join(intStrings, ","));
+	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/client/ClientHelper.java

@@ -9,7 +9,8 @@ import org.briarproject.bramble.api.data.BdfList;
 import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.db.Transaction;
 import org.briarproject.bramble.api.identity.Author;
-import org.briarproject.bramble.api.mailbox.MailboxPropertiesUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.properties.TransportProperties;
@@ -19,10 +20,9 @@ import org.briarproject.bramble.api.sync.MessageId;
 
 import java.security.GeneralSecurityException;
 import java.util.Collection;
+import java.util.List;
 import java.util.Map;
 
-import javax.annotation.Nullable;
-
 @NotNullByDefault
 public interface ClientHelper {
 
@@ -127,16 +127,17 @@ public interface ClientHelper {
 			BdfDictionary properties) throws FormatException;
 
 	/**
-	 * Parse and validate the property dictionary of a Mailbox property update
+	 * Parse and validate the elements of a Mailbox update message.
-	 * message.
 	 *
-	 * @return the properties for using the Mailbox, or null if there is no
+	 * @return the parsed update message
-	 * Mailbox available
+	 * @throws FormatException if the message elements are invalid
-	 * @throws FormatException if the properties are not valid
 	 */
-	@Nullable
+	MailboxUpdate parseAndValidateMailboxUpdate(BdfList clientSupports,
-	MailboxPropertiesUpdate parseAndValidateMailboxPropertiesUpdate(
+			BdfList serverSupports, BdfDictionary properties)
-			BdfDictionary properties) throws FormatException;
+			throws FormatException;
+
+	List<MailboxVersion> parseMailboxVersionList(BdfList bdfList)
+			throws FormatException;
 
 	/**
 	 * Retrieves the contact ID from the group metadata of the given contact

bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseComponent.java

@@ -33,11 +33,18 @@ import java.util.List;
 import java.util.Map;
 
 import javax.annotation.Nullable;
+import javax.annotation.concurrent.ThreadSafe;
 
 /**
  * Encapsulates the database implementation and exposes high-level operations
  * to other components.
+ * <p>
+ * With the exception of the {@link #open(SecretKey, MigrationListener)} and
+ * {@link #close()} methods, which must not be called concurrently, the
+ * database can be accessed from any thread. See {@link TransactionManager}
+ * for locking behaviour.
  */
+@ThreadSafe
 @NotNullByDefault
 public interface DatabaseComponent extends TransactionManager {
 
@@ -193,26 +200,15 @@ public interface DatabaseComponent extends TransactionManager {
 			throws DbException;
 
 	/**
-	 * Returns a batch of messages for the given contact, with a total length
+	 * Returns a batch of messages for the given contact, for transmission over
-	 * less than or equal to the given length, for transmission over a
+	 * a transport with the given maximum latency. The total length of the
-	 * transport with the given maximum latency. Returns null if there are no
+	 * messages, including record headers, will be no more than the given
-	 * sendable messages that fit in the given length.
+	 * capacity. Returns null if there are no sendable messages that would fit
+	 * in the given capacity.
 	 */
 	@Nullable
 	Collection<Message> generateBatch(Transaction txn, ContactId c,
-			int maxLength, long maxLatency) throws DbException;
+			long capacity, long maxLatency) throws DbException;
-
-	/**
-	 * Returns a batch of messages for the given contact containing the
-	 * messages with the given IDs, for transmission over a transport with
-	 * the given maximum latency.
-	 * <p/>
-	 * If any of the given messages are not in the database or are not visible
-	 * to the contact, they are omitted from the batch without throwing an
-	 * exception.
-	 */
-	Collection<Message> generateBatch(Transaction txn, ContactId c,
-			Collection<MessageId> ids, long maxLatency) throws DbException;
 
 	/**
 	 * Returns an offer for the given contact for transmission over a
@@ -232,15 +228,16 @@ public interface DatabaseComponent extends TransactionManager {
 			throws DbException;
 
 	/**
-	 * Returns a batch of messages for the given contact, with a total length
+	 * Returns a batch of messages for the given contact, for transmission over
-	 * less than or equal to the given length, for transmission over a
+	 * a transport with the given maximum latency. Only messages that have been
-	 * transport with the given maximum latency. Only messages that have been
+	 * requested by the contact are returned. The total length of the messages,
-	 * requested by the contact are returned. Returns null if there are no
+	 * including record headers, will be no more than the given capacity.
-	 * sendable messages that fit in the given length.
+	 * Returns null if there are no sendable messages that have been requested
+	 * by the contact and would fit in the given capacity.
 	 */
 	@Nullable
 	Collection<Message> generateRequestedBatch(Transaction txn, ContactId c,
-			int maxLength, long maxLatency) throws DbException;
+			long capacity, long maxLatency) throws DbException;
 
 	/**
 	 * Returns the contact with the given ID.
@@ -344,6 +341,30 @@ public interface DatabaseComponent extends TransactionManager {
 	Collection<MessageId> getMessageIds(Transaction txn, GroupId g,
 			Metadata query) throws DbException;
 
+	/**
+	 * Returns the IDs of some messages received from the given contact that
+	 * need to be acknowledged, up to the given number of messages.
+	 * <p/>
+	 * Read-only.
+	 */
+	Collection<MessageId> getMessagesToAck(Transaction txn, ContactId c,
+			int maxMessages) throws DbException;
+
+	/**
+	 * Returns the IDs of some messages that are eligible to be sent to the
+	 * given contact over a transport with the given maximum latency. The total
+	 * length of the messages including record headers will be no more than the
+	 * given capacity.
+	 * <p/>
+	 * Unlike {@link #getUnackedMessagesToSend(Transaction, ContactId)} this
+	 * method does not return messages that have already been sent unless they
+	 * are due for retransmission.
+	 * <p/>
+	 * Read-only.
+	 */
+	Collection<MessageId> getMessagesToSend(Transaction txn, ContactId c,
+			long capacity, long maxLatency) throws DbException;
+
 	/**
 	 * Returns the IDs of any messages that need to be validated.
 	 * <p/>
@@ -460,15 +481,30 @@ public interface DatabaseComponent extends TransactionManager {
 	MessageStatus getMessageStatus(Transaction txn, ContactId c, MessageId m)
 			throws DbException;
 
+	/**
+	 * Returns the message with the given ID for transmission to the given
+	 * contact over a transport with the given maximum latency. Returns null
+	 * if the message is no longer visible to the contact.
+	 *
+	 * @param markAsSent True if the message should be marked as sent.
+	 * If false it can be marked as sent by calling
+	 * {@link #setMessagesSent(Transaction, ContactId, Collection, long)}.
+	 */
+	@Nullable
+	Message getMessageToSend(Transaction txn, ContactId c, MessageId m,
+			long maxLatency, boolean markAsSent) throws DbException;
+
 	/**
 	 * Returns the IDs of all messages that are eligible to be sent to the
-	 * given contact, together with their raw lengths. This may include
+	 * given contact.
-	 * messages that have already been sent and are not yet due for
+	 * <p>
-	 * retransmission.
+	 * Unlike {@link #getMessagesToSend(Transaction, ContactId, long, long)}
+	 * this method may return messages that have already been sent and are
+	 * not yet due for retransmission.
 	 * <p/>
 	 * Read-only.
 	 */
-	Map<MessageId, Integer> getUnackedMessagesToSend(Transaction txn,
+	Collection<MessageId> getUnackedMessagesToSend(Transaction txn,
 			ContactId c) throws DbException;
 
 	/**
@@ -648,6 +684,13 @@ public interface DatabaseComponent extends TransactionManager {
 	void removeTransportKeys(Transaction txn, TransportId t, KeySetId k)
 			throws DbException;
 
+	/**
+	 * Records an ack for the given messages as having been sent to the given
+	 * contact.
+	 */
+	void setAckSent(Transaction txn, ContactId c, Collection<MessageId> acked)
+			throws DbException;
+
 	/**
 	 * Sets the cleanup timer duration for the given message. This does not
 	 * start the message's cleanup timer.
@@ -694,6 +737,13 @@ public interface DatabaseComponent extends TransactionManager {
 	void setMessageState(Transaction txn, MessageId m, MessageState state)
 			throws DbException;
 
+	/**
+	 * Records the given messages as having been sent to the given contact
+	 * over a transport with the given maximum latency.
+	 */
+	void setMessagesSent(Transaction txn, ContactId c,
+			Collection<MessageId> sent, long maxLatency) throws DbException;
+
 	/**
 	 * Adds dependencies for a message
 	 */

bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseExecutor.java

@@ -18,6 +18,10 @@ import static java.lang.annotation.RetentionPolicy.RUNTIME;
  * submitted, tasks are not run concurrently, and submitting a task will never
  * block. Tasks must not run indefinitely. Tasks submitted during shutdown are
  * discarded.
+ * <p>
+ * It is not mandatory to use this executor for database tasks. The database
+ * can be accessed from any thread, but this executor's guarantee that tasks
+ * are run in the order they're submitted may be useful in some cases.
  */
 @Qualifier
 @Target({FIELD, METHOD, PARAMETER})

bramble-api/src/main/java/org/briarproject/bramble/api/db/Transaction.java

@@ -45,6 +45,9 @@ public class Transaction {
 	/**
 	 * Attaches an event to be broadcast when the transaction has been
 	 * committed. The event will be broadcast on the {@link EventExecutor}.
+	 * Events and {@link #attach(Runnable) tasks} are submitted to the
+	 * {@link EventExecutor} in the order they were attached to the
+	 * transaction.
 	 */
 	public void attach(Event e) {
 		if (actions == null) actions = new ArrayList<>();
@@ -54,6 +57,9 @@ public class Transaction {
 	/**
 	 * Attaches a task to be executed when the transaction has been
 	 * committed. The task will be run on the {@link EventExecutor}.
+	 * {@link #attach(Event) Events} and tasks are submitted to the
+	 * {@link EventExecutor} in the order they were attached to the
+	 * transaction.
 	 */
 	public void attach(Runnable r) {
 		if (actions == null) actions = new ArrayList<>();

bramble-api/src/main/java/org/briarproject/bramble/api/db/TransactionManager.java

@@ -1,51 +1,95 @@
 package org.briarproject.bramble.api.db;
 
+import org.briarproject.bramble.api.event.EventExecutor;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import javax.annotation.Nullable;
+import javax.annotation.concurrent.ThreadSafe;
 
+/**
+ * An interface for managing database transactions.
+ * <p>
+ * Read-only transactions may access the database concurrently. Read-write
+ * transactions access the database exclusively, so starting a read-only or
+ * read-write transaction will block until there are no read-write
+ * transactions in progress.
+ * <p>
+ * Failing to {@link #endTransaction(Transaction) end} a transaction will
+ * prevent other callers from accessing the database, so it is recommended to
+ * use the {@link #transaction(boolean, DbRunnable)},
+ * {@link #transactionWithResult(boolean, DbCallable)} and
+ * {@link #transactionWithNullableResult(boolean, NullableDbCallable)} methods
+ * where possible, which handle committing or aborting the transaction on the
+ * caller's behalf.
+ * <p>
+ * Transactions are not reentrant, i.e. it is not permitted to start a
+ * transaction on a thread that already has a transaction in progress.
+ */
+@ThreadSafe
 @NotNullByDefault
 public interface TransactionManager {
 
 	/**
-	 * Starts a new transaction and returns an object representing it.
+	 * Starts a new transaction and returns an object representing it. This
-	 * <p/>
+	 * method acquires the database lock, which is held until
-	 * This method acquires locks, so it must not be called while holding a
+	 * {@link #endTransaction(Transaction)} is called.
-	 * lock.
 	 *
-	 * @param readOnly true if the transaction will only be used for reading.
+	 * @param readOnly True if the transaction will only be used for reading,
+	 * in which case the database lock can be shared with other read-only
+	 * transactions.
 	 */
 	Transaction startTransaction(boolean readOnly) throws DbException;
 
 	/**
 	 * Commits a transaction to the database.
+	 * {@link #endTransaction(Transaction)} must be called to release the
+	 * database lock.
 	 */
 	void commitTransaction(Transaction txn) throws DbException;
 
 	/**
-	 * Ends a transaction. If the transaction has not been committed,
+	 * Ends a transaction. If the transaction has not been committed by
-	 * it will be aborted. If the transaction has been committed,
+	 * calling {@link #commitTransaction(Transaction)}, it is aborted and the
-	 * any events attached to the transaction are broadcast.
+	 * database lock is released.
-	 * The database lock will be released in either case.
+	 * <p>
+	 * If the transaction has been committed, any
+	 * {@link Transaction#attach events} attached to the transaction are
+	 * broadcast and any {@link Transaction#attach(Runnable) tasks} attached
+	 * to the transaction are submitted to the {@link EventExecutor}. The
+	 * database lock is then released.
 	 */
 	void endTransaction(Transaction txn);
 
 	/**
-	 * Runs the given task within a transaction.
+	 * Runs the given task within a transaction. The database lock is held
+	 * while running the task.
+	 *
+	 * @param readOnly True if the transaction will only be used for reading,
+	 * in which case the database lock can be shared with other read-only
+	 * transactions.
 	 */
 	<E extends Exception> void transaction(boolean readOnly,
 			DbRunnable<E> task) throws DbException, E;
 
 	/**
 	 * Runs the given task within a transaction and returns the result of the
-	 * task.
+	 * task. The database lock is held while running the task.
+	 *
+	 * @param readOnly True if the transaction will only be used for reading,
+	 * in which case the database lock can be shared with other read-only
+	 * transactions.
 	 */
 	<R, E extends Exception> R transactionWithResult(boolean readOnly,
 			DbCallable<R, E> task) throws DbException, E;
 
 	/**
 	 * Runs the given task within a transaction and returns the result of the
-	 * task, which may be null.
+	 * task, which may be null. The database lock is held while running the
+	 * task.
+	 *
+	 * @param readOnly True if the transaction will only be used for reading,
+	 * in which case the database lock can be shared with other read-only
+	 * transactions.
 	 */
 	@Nullable
 	<R, E extends Exception> R transactionWithNullableResult(boolean readOnly,

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxConstants.java

@@ -0,0 +1,37 @@
+package org.briarproject.bramble.api.mailbox;
+
+import static java.util.concurrent.TimeUnit.HOURS;
+import static org.briarproject.bramble.api.transport.TransportConstants.MAX_FRAME_LENGTH;
+import static org.briarproject.bramble.api.transport.TransportConstants.MAX_PAYLOAD_LENGTH;
+import static org.briarproject.bramble.api.transport.TransportConstants.STREAM_HEADER_LENGTH;
+import static org.briarproject.bramble.api.transport.TransportConstants.TAG_LENGTH;
+
+public interface MailboxConstants {
+
+	/**
+	 * The maximum length of a file that can be uploaded to or downloaded from
+	 * a mailbox.
+	 */
+	int MAX_FILE_BYTES = 1024 * 1024;
+
+	/**
+	 * The maximum length of the plaintext payload of a file, such that the
+	 * ciphertext is no more than {@link #MAX_FILE_BYTES}.
+	 */
+	int MAX_FILE_PAYLOAD_BYTES =
+			(MAX_FILE_BYTES - TAG_LENGTH - STREAM_HEADER_LENGTH)
+					/ MAX_FRAME_LENGTH * MAX_PAYLOAD_LENGTH;
+
+	/**
+	 * The number of connection failures
+	 * that indicate a problem with the mailbox.
+	 */
+	int PROBLEM_NUM_CONNECTION_FAILURES = 5;
+
+	/**
+	 * The time in milliseconds since the last connection success
+	 * that need to pass to indicates a problem with the mailbox.
+	 */
+	long PROBLEM_MS_SINCE_LAST_SUCCESS = HOURS.toMillis(1);
+
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxManager.java

@@ -2,6 +2,8 @@ package org.briarproject.bramble.api.mailbox;
 
 import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.mailbox.event.OwnMailboxConnectionStatusEvent;
 
 import javax.annotation.Nullable;
 
@@ -41,4 +43,14 @@ public interface MailboxManager {
 	 */
 	boolean checkConnection();
 
+	/**
+	 * Unpairs the owner's mailbox and tries to wipe it.
+	 * As this makes a network call, it should be run on the {@link IoExecutor}.
+	 *
+	 * @return true if we could wipe the mailbox, false if we couldn't.
+	 * It is advised to inform the user to wipe the mailbox themselves,
+	 * if we failed to wipe it.
+	 */
+	@IoExecutor
+	boolean unPair() throws DbException;
 }

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxProperties.java

@@ -2,6 +2,9 @@ package org.briarproject.bramble.api.mailbox;
 
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
+import java.util.List;
+
+import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 
 @Immutable
@@ -11,12 +14,37 @@ public class MailboxProperties {
 	private final String baseUrl;
 	private final MailboxAuthToken authToken;
 	private final boolean owner;
+	private final List<MailboxVersion> serverSupports;
+	@Nullable
+	private final MailboxFolderId inboxId; // Null for own mailbox
+	@Nullable
+	private final MailboxFolderId outboxId; // Null for own mailbox
 
+	/**
+	 * Constructor for properties used by the mailbox's owner.
+	 */
 	public MailboxProperties(String baseUrl, MailboxAuthToken authToken,
-			boolean owner) {
+			List<MailboxVersion> serverSupports) {
 		this.baseUrl = baseUrl;
 		this.authToken = authToken;
-		this.owner = owner;
+		this.owner = true;
+		this.serverSupports = serverSupports;
+		this.inboxId = null;
+		this.outboxId = null;
+	}
+
+	/**
+	 * Constructor for properties used by a contact of the mailbox's owner.
+	 */
+	public MailboxProperties(String baseUrl, MailboxAuthToken authToken,
+			List<MailboxVersion> serverSupports, MailboxFolderId inboxId,
+			MailboxFolderId outboxId) {
+		this.baseUrl = baseUrl;
+		this.authToken = authToken;
+		this.owner = false;
+		this.serverSupports = serverSupports;
+		this.inboxId = inboxId;
+		this.outboxId = outboxId;
 	}
 
 	public String getBaseUrl() {
@@ -35,4 +63,18 @@ public class MailboxProperties {
 	public boolean isOwner() {
 		return owner;
 	}
+
+	public List<MailboxVersion> getServerSupports() {
+		return serverSupports;
+	}
+
+	@Nullable
+	public MailboxFolderId getInboxId() {
+		return inboxId;
+	}
+
+	@Nullable
+	public MailboxFolderId getOutboxId() {
+		return outboxId;
+	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxPropertiesUpdate.java

@@ -1,41 +0,0 @@
-package org.briarproject.bramble.api.mailbox;
-
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-
-import javax.annotation.concurrent.Immutable;
-
-@Immutable
-@NotNullByDefault
-public class MailboxPropertiesUpdate {
-
-	private final String onion;
-	private final MailboxAuthToken authToken;
-	private final MailboxFolderId inboxId;
-	private final MailboxFolderId outboxId;
-
-	public MailboxPropertiesUpdate(String onion,
-			MailboxAuthToken authToken, MailboxFolderId inboxId,
-			MailboxFolderId outboxId) {
-		this.onion = onion;
-		this.authToken = authToken;
-		this.inboxId = inboxId;
-		this.outboxId = outboxId;
-	}
-
-	public String getOnion() {
-		return onion;
-	}
-
-	public MailboxAuthToken getAuthToken() {
-		return authToken;
-	}
-
-	public MailboxFolderId getInboxId() {
-		return inboxId;
-	}
-
-	public MailboxFolderId getOutboxId() {
-		return outboxId;
-	}
-
-}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxSettingsManager.java

@@ -7,6 +7,8 @@ import org.briarproject.bramble.api.db.Transaction;
 import org.briarproject.bramble.api.lifecycle.LifecycleManager;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
+import java.util.List;
+
 import javax.annotation.Nullable;
 
 @NotNullByDefault
@@ -26,6 +28,8 @@ public interface MailboxSettingsManager {
 	void setOwnMailboxProperties(Transaction txn, MailboxProperties p)
 			throws DbException;
 
+	void removeOwnMailboxProperties(Transaction txn) throws DbException;
+
 	MailboxStatus getOwnMailboxStatus(Transaction txn) throws DbException;
 
 	void recordSuccessfulConnection(Transaction txn, long now)
@@ -47,7 +51,8 @@ public interface MailboxSettingsManager {
 		 * @param txn A read-write transaction
 		 * @param ownOnion Our new mailbox's onion (56 base32 chars)
 		 */
-		void mailboxPaired(Transaction txn, String ownOnion)
+		void mailboxPaired(Transaction txn, String ownOnion,
+				List<MailboxVersion> serverSupports)
 				throws DbException;
 
 		/**

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxStatus.java

@@ -4,6 +4,9 @@ import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import javax.annotation.concurrent.Immutable;
 
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.PROBLEM_MS_SINCE_LAST_SUCCESS;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.PROBLEM_NUM_CONNECTION_FAILURES;
+
 @Immutable
 @NotNullByDefault
 public class MailboxStatus {
@@ -56,4 +59,12 @@ public class MailboxStatus {
 	public int getAttemptsSinceSuccess() {
 		return attemptsSinceSuccess;
 	}
+
+	/**
+	 * @return true if this status indicates a problem with the mailbox.
+	 */
+	public boolean hasProblem(long now) {
+		return attemptsSinceSuccess >= PROBLEM_NUM_CONNECTION_FAILURES &&
+				(now - lastSuccess) >= PROBLEM_MS_SINCE_LAST_SUCCESS;
+	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxUpdate.java

@@ -0,0 +1,31 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.List;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public class MailboxUpdate {
+	private final boolean hasMailbox;
+	private final List<MailboxVersion> clientSupports;
+
+	public MailboxUpdate(List<MailboxVersion> clientSupports) {
+		this(clientSupports, false);
+	}
+
+	MailboxUpdate(List<MailboxVersion> clientSupports, boolean hasMailbox) {
+		this.clientSupports = clientSupports;
+		this.hasMailbox = hasMailbox;
+	}
+
+	public List<MailboxVersion> getClientSupports() {
+		return clientSupports;
+	}
+
+	public boolean hasMailbox() {
+		return hasMailbox;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxUpdateManager.java

@@ -9,31 +9,31 @@ import org.briarproject.bramble.api.sync.ClientId;
 import javax.annotation.Nullable;
 
 @NotNullByDefault
-public interface MailboxPropertyManager {
+public interface MailboxUpdateManager {
 
 	/**
-	 * The unique ID of the mailbox property client.
+	 * The unique ID of the mailbox update (properties) client.
 	 */
 	ClientId CLIENT_ID =
 			new ClientId("org.briarproject.bramble.mailbox.properties");
 
 	/**
-	 * The current major version of the mailbox property client.
+	 * The current major version of the mailbox update (properties) client.
 	 */
-	int MAJOR_VERSION = 0;
+	int MAJOR_VERSION = 2;
 
 	/**
-	 * The current minor version of the mailbox property client.
+	 * The current minor version of the mailbox update (properties) client.
 	 */
 	int MINOR_VERSION = 0;
 
 	/**
-	 * The number of properties required for a (non-empty) update message.
+	 * The number of properties required for an update message with a mailbox.
 	 */
 	int PROP_COUNT = 4;
 
 	/**
-	 * The required properties of a non-empty update message.
+	 * The required properties of an update message with a mailbox.
 	 */
 	String PROP_KEY_ONION = "onion";
 	String PROP_KEY_AUTHTOKEN = "authToken";
@@ -57,11 +57,16 @@ public interface MailboxPropertyManager {
 	 */
 	String MSG_KEY_LOCAL = "local";
 
-	@Nullable
+	/**
-	MailboxPropertiesUpdate getLocalProperties(Transaction txn, ContactId c)
+	 * Key in the client's local group for storing the clientSupports list that
+	 * was last sent out.
+	 */
+	String GROUP_KEY_SENT_CLIENT_SUPPORTS = "sentClientSupports";
+
+	MailboxUpdate getLocalUpdate(Transaction txn, ContactId c)
 			throws DbException;
 
 	@Nullable
-	MailboxPropertiesUpdate getRemoteProperties(Transaction txn, ContactId c)
+	MailboxUpdate getRemoteUpdate(Transaction txn, ContactId c)
 			throws DbException;
 }

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxUpdateWithMailbox.java

@@ -0,0 +1,30 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.List;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public class MailboxUpdateWithMailbox extends MailboxUpdate {
+
+	private final MailboxProperties properties;
+
+	public MailboxUpdateWithMailbox(List<MailboxVersion> clientSupports,
+			MailboxProperties properties) {
+		super(clientSupports, true);
+		if (properties.isOwner()) throw new IllegalArgumentException();
+		this.properties = properties;
+	}
+
+	public MailboxUpdateWithMailbox(MailboxUpdateWithMailbox o,
+			List<MailboxVersion> newClientSupports) {
+		this(newClientSupports, o.getMailboxProperties());
+	}
+
+	public MailboxProperties getMailboxProperties() {
+		return properties;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxVersion.java

@@ -0,0 +1,44 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public class MailboxVersion implements Comparable<MailboxVersion> {
+
+	private final int major;
+	private final int minor;
+
+	public MailboxVersion(int major, int minor) {
+		this.major = major;
+		this.minor = minor;
+	}
+
+	public int getMajor() {
+		return major;
+	}
+
+	public int getMinor() {
+		return minor;
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		if (o instanceof MailboxVersion) {
+			MailboxVersion v = (MailboxVersion) o;
+			return major == v.major && minor == v.minor;
+		}
+		return false;
+	}
+
+	@Override
+	public int compareTo(MailboxVersion v) {
+		int c = major - v.major;
+		if (c != 0) {
+			return c;
+		}
+		return minor - v.minor;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/RemoteMailboxPropertiesUpdateEvent.java

@@ -1,36 +0,0 @@
-package org.briarproject.bramble.api.mailbox;
-
-import org.briarproject.bramble.api.contact.ContactId;
-import org.briarproject.bramble.api.event.Event;
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-
-import javax.annotation.Nullable;
-import javax.annotation.concurrent.Immutable;
-
-/**
- * An event that is broadcast when {@link MailboxPropertiesUpdate} are received
- * from a contact.
- */
-@Immutable
-@NotNullByDefault
-public class RemoteMailboxPropertiesUpdateEvent extends Event {
-
-	private final ContactId contactId;
-	@Nullable
-	private final MailboxPropertiesUpdate mailboxPropertiesUpdate;
-
-	public RemoteMailboxPropertiesUpdateEvent(ContactId contactId,
-			@Nullable MailboxPropertiesUpdate mailboxPropertiesUpdate) {
-		this.contactId = contactId;
-		this.mailboxPropertiesUpdate = mailboxPropertiesUpdate;
-	}
-
-	public ContactId getContact() {
-		return contactId;
-	}
-
-	@Nullable
-	public MailboxPropertiesUpdate getMailboxPropertiesUpdate() {
-		return mailboxPropertiesUpdate;
-	}
-}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/event/MailboxProblemEvent.java

@@ -0,0 +1,19 @@
+package org.briarproject.bramble.api.mailbox.event;
+
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * An event that is broadcast by {@link MailboxSettingsManager} when
+ * recording a connection failure for own Mailbox
+ * that has persistent for long enough for the mailbox owner to become active
+ * and fix the problem with the mailbox.
+ */
+@Immutable
+@NotNullByDefault
+public class MailboxProblemEvent extends Event {
+
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/event/OwnMailboxConnectionStatusEvent.java

@@ -1,6 +1,8 @@
-package org.briarproject.bramble.api.mailbox;
+package org.briarproject.bramble.api.mailbox.event;
 
 import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.mailbox.MailboxStatus;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import javax.annotation.concurrent.Immutable;

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/event/RemoteMailboxUpdateEvent.java

@@ -0,0 +1,34 @@
+package org.briarproject.bramble.api.mailbox.event;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * An event that is broadcast when {@link MailboxUpdate} are received
+ * from a contact.
+ */
+@Immutable
+@NotNullByDefault
+public class RemoteMailboxUpdateEvent extends Event {
+
+	private final ContactId contactId;
+	private final MailboxUpdate mailboxUpdate;
+
+	public RemoteMailboxUpdateEvent(ContactId contactId,
+			MailboxUpdate mailboxUpdate) {
+		this.contactId = contactId;
+		this.mailboxUpdate = mailboxUpdate;
+	}
+
+	public ContactId getContact() {
+		return contactId;
+	}
+
+	public MailboxUpdate getMailboxUpdate() {
+		return mailboxUpdate;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/plugin/TorConstants.java

@@ -1,5 +1,7 @@
 package org.briarproject.bramble.api.plugin;
 
+import static java.util.concurrent.TimeUnit.SECONDS;
+
 public interface TorConstants {
 
 	TransportId ID = new TransportId("org.briarproject.bramble.tor");
@@ -10,8 +12,9 @@ public interface TorConstants {
 	int DEFAULT_SOCKS_PORT = 59050;
 	int DEFAULT_CONTROL_PORT = 59051;
 
-	int CONNECT_TO_PROXY_TIMEOUT = 5000; // Milliseconds
+	int CONNECT_TO_PROXY_TIMEOUT = (int) SECONDS.toMillis(5);
-	int EXTRA_SOCKET_TIMEOUT = 30000; // Milliseconds
+	int EXTRA_CONNECT_TIMEOUT = (int) SECONDS.toMillis(120);
+	int EXTRA_SOCKET_TIMEOUT = (int) SECONDS.toMillis(30);
 
 	// Local settings (not shared with contacts)
 	String PREF_TOR_NETWORK = "network2";

bramble-api/src/main/java/org/briarproject/bramble/api/record/RecordWriter.java

@@ -12,4 +12,6 @@ public interface RecordWriter {
 	void flush() throws IOException;
 
 	void close() throws IOException;
+
+	long getBytesWritten();
 }

bramble-api/src/main/java/org/briarproject/bramble/api/sync/DeferredSendHandler.java

@@ -0,0 +1,15 @@
+package org.briarproject.bramble.api.sync;
+
+import java.util.Collection;
+
+/**
+ * An interface for holding the IDs of messages sent and acked during an
+ * outgoing {@link SyncSession} so they can be recorded in the DB as sent
+ * or acked at some later time.
+ */
+public interface DeferredSendHandler {
+
+	void onAckSent(Collection<MessageId> acked);
+
+	void onMessageSent(MessageId sent);
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/SyncRecordWriter.java

@@ -20,4 +20,6 @@ public interface SyncRecordWriter {
 	void writePriority(Priority p) throws IOException;
 
 	void flush() throws IOException;
+
+	long getBytesWritten();
 }

bramble-api/src/main/java/org/briarproject/bramble/api/system/TaskScheduler.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.api.system;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.util.concurrent.Executor;
@@ -16,6 +17,8 @@ public interface TaskScheduler {
 	 * <p>
 	 * If the platform supports wake locks, a wake lock will be held while
 	 * submitting and running the task.
+	 *
+	 * @return A {@link Cancellable} for cancelling the task.
 	 */
 	Cancellable schedule(Runnable task, Executor executor, long delay,
 			TimeUnit unit);
@@ -27,17 +30,11 @@ public interface TaskScheduler {
 	 * <p>
 	 * If the platform supports wake locks, a wake lock will be held while
 	 * submitting and running the task.
+	 *
+	 * @return A {@link Cancellable} for cancelling all future executions of
+	 * the task.
 	 */
 	Cancellable scheduleWithFixedDelay(Runnable task, Executor executor,
 			long delay, long interval, TimeUnit unit);
 
-	interface Cancellable {
-
-		/**
-		 * Cancels the task if it has not already started running. If the task
-		 * is {@link #scheduleWithFixedDelay(Runnable, Executor, long, long, TimeUnit) periodic},
-		 * all future executions of the task are cancelled.
-		 */
-		void cancel();
-	}
 }

bramble-api/src/test/java/org/briarproject/bramble/test/TestUtils.java

@@ -20,7 +20,12 @@ import org.briarproject.bramble.api.identity.Author;
 import org.briarproject.bramble.api.identity.AuthorId;
 import org.briarproject.bramble.api.identity.Identity;
 import org.briarproject.bramble.api.identity.LocalAuthor;
-import org.briarproject.bramble.api.mailbox.MailboxPropertiesUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.properties.TransportProperties;
 import org.briarproject.bramble.api.sync.ClientId;
@@ -221,6 +226,19 @@ public class TestUtils {
 				getAgreementPublicKey(), verified);
 	}
 
+	public static MailboxProperties getMailboxProperties(boolean owner,
+			List<MailboxVersion> serverSupports) {
+		String baseUrl = "http://" + getRandomString(56) + ".onion"; // TODO
+		MailboxAuthToken authToken = new MailboxAuthToken(getRandomId());
+		if (owner) {
+			return new MailboxProperties(baseUrl, authToken, serverSupports);
+		}
+		MailboxFolderId inboxId = new MailboxFolderId(getRandomId());
+		MailboxFolderId outboxId = new MailboxFolderId(getRandomId());
+		return new MailboxProperties(baseUrl, authToken, serverSupports,
+				inboxId, outboxId);
+	}
+
 	public static void writeBytes(File file, byte[] bytes)
 			throws IOException {
 		FileOutputStream outputStream = new FileOutputStream(file);
@@ -273,22 +291,38 @@ public class TestUtils {
 		return Math.sqrt(getVariance(samples));
 	}
 
-	public static boolean isOptionalTestEnabled(Class testClass) {
+	public static boolean isOptionalTestEnabled(Class<?> testClass) {
 		String optionalTests = System.getenv("OPTIONAL_TESTS");
 		return optionalTests != null &&
 				asList(optionalTests.split(",")).contains(testClass.getName());
 	}
 
-	public static boolean mailboxPropertiesUpdateEqual(
+	public static boolean mailboxUpdateEqual(@Nullable MailboxUpdate a,
-			@Nullable MailboxPropertiesUpdate a,
+			@Nullable MailboxUpdate b) {
-			@Nullable MailboxPropertiesUpdate b) {
+		if (a == null || b == null) {
+			return a == b;
+		}
+		if (!a.hasMailbox() && !b.hasMailbox()) {
+			return a.getClientSupports().equals(b.getClientSupports());
+		} else if (a.hasMailbox() && b.hasMailbox()) {
+			MailboxUpdateWithMailbox am = (MailboxUpdateWithMailbox) a;
+			MailboxUpdateWithMailbox bm = (MailboxUpdateWithMailbox) b;
+			return am.getClientSupports().equals(bm.getClientSupports()) &&
+					mailboxPropertiesEqual(am.getMailboxProperties(),
+							bm.getMailboxProperties());
+		}
+		return false;
+	}
+
+	public static boolean mailboxPropertiesEqual(@Nullable MailboxProperties a,
+			@Nullable MailboxProperties b) {
 		if (a == null || b == null) {
 			return a == b;
 		}
 		return a.getOnion().equals(b.getOnion()) &&
 				a.getAuthToken().equals(b.getAuthToken()) &&
-				a.getInboxId().equals(b.getInboxId()) &&
+				a.isOwner() == b.isOwner() &&
-				a.getOutboxId().equals(b.getOutboxId());
+				a.getServerSupports().equals(b.getServerSupports());
 	}
 
 	public static boolean hasEvent(Transaction txn,

bramble-core/build.gradle

@@ -16,7 +16,7 @@ dependencies {
 	implementation 'org.bitlet:weupnp:0.1.4'
 	implementation 'net.i2p.crypto:eddsa:0.2.0'
 	implementation 'org.whispersystems:curve25519-java:0.5.0'
-	implementation 'org.briarproject:jtorctl:0.3'
+	implementation 'org.briarproject:jtorctl:0.4'
 
 	//noinspection GradleDependency
 	implementation "com.squareup.okhttp3:okhttp:$okhttp_version"

bramble-core/src/main/java/org/briarproject/bramble/client/ClientHelperImpl.java

@@ -25,7 +25,10 @@ import org.briarproject.bramble.api.identity.Author;
 import org.briarproject.bramble.api.identity.AuthorFactory;
 import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
 import org.briarproject.bramble.api.mailbox.MailboxFolderId;
-import org.briarproject.bramble.api.mailbox.MailboxPropertiesUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.properties.TransportProperties;
@@ -39,12 +42,13 @@ import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.security.GeneralSecurityException;
+import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
 
-import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 
@@ -52,12 +56,12 @@ import static org.briarproject.bramble.api.client.ContactGroupConstants.GROUP_KE
 import static org.briarproject.bramble.api.identity.Author.FORMAT_VERSION;
 import static org.briarproject.bramble.api.identity.AuthorConstants.MAX_AUTHOR_NAME_LENGTH;
 import static org.briarproject.bramble.api.identity.AuthorConstants.MAX_PUBLIC_KEY_LENGTH;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_COUNT;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_COUNT;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_AUTHTOKEN;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_AUTHTOKEN;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_INBOXID;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_INBOXID;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_ONION;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_ONION;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_OUTBOXID;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_OUTBOXID;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_ONION_LENGTH;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_ONION_LENGTH;
 import static org.briarproject.bramble.api.properties.TransportPropertyConstants.MAX_PROPERTIES_PER_TRANSPORT;
 import static org.briarproject.bramble.api.properties.TransportPropertyConstants.MAX_PROPERTY_LENGTH;
 import static org.briarproject.bramble.util.ValidationUtils.checkLength;
@@ -412,11 +416,28 @@ class ClientHelperImpl implements ClientHelper {
 	}
 
 	@Override
-	@Nullable
+	public MailboxUpdate parseAndValidateMailboxUpdate(BdfList clientSupports,
-	public MailboxPropertiesUpdate parseAndValidateMailboxPropertiesUpdate(
+			BdfList serverSupports, BdfDictionary properties)
-			BdfDictionary properties) throws FormatException {
+			throws FormatException {
+		List<MailboxVersion> clientSupportsList =
+				parseMailboxVersionList(clientSupports);
+		List<MailboxVersion> serverSupportsList =
+				parseMailboxVersionList(serverSupports);
+
+		// We must always learn what Mailbox API version(s) the client supports
+		if (clientSupports.isEmpty()) {
+			throw new FormatException();
+		}
 		if (properties.isEmpty()) {
-			return null;
+			// No mailbox -- cannot claim to support any API versions!
+			if (!serverSupports.isEmpty()) {
+				throw new FormatException();
+			}
+			return new MailboxUpdate(clientSupportsList);
+		}
+		// Mailbox must be accompanied by the Mailbox API version(s) it supports
+		if (serverSupports.isEmpty()) {
+			throw new FormatException();
 		}
 		// Accepting more props than we need, for forward compatibility
 		if (properties.size() < PROP_COUNT) {
@@ -435,9 +456,26 @@ class ClientHelperImpl implements ClientHelper {
 		checkLength(inboxId, UniqueId.LENGTH);
 		byte[] outboxId = properties.getRaw(PROP_KEY_OUTBOXID);
 		checkLength(outboxId, UniqueId.LENGTH);
-		return new MailboxPropertiesUpdate(onion,
+		String baseUrl = "http://" + onion + ".onion"; // TODO
-				new MailboxAuthToken(authToken), new MailboxFolderId(inboxId),
+		MailboxProperties props = new MailboxProperties(baseUrl,
-				new MailboxFolderId(outboxId));
+				new MailboxAuthToken(authToken), serverSupportsList,
+				new MailboxFolderId(inboxId), new MailboxFolderId(outboxId));
+		return new MailboxUpdateWithMailbox(clientSupportsList, props);
+	}
+
+	@Override
+	public List<MailboxVersion> parseMailboxVersionList(BdfList bdfList)
+			throws FormatException {
+		List<MailboxVersion> list = new ArrayList<>();
+		for (int i = 0; i < bdfList.size(); i++) {
+			BdfList element = bdfList.getList(i);
+			if (element.size() != 2) {
+				throw new FormatException();
+			}
+			list.add(new MailboxVersion(element.getLong(0).intValue(),
+					element.getLong(1).intValue()));
+		}
+		return list;
 	}
 
 	@Override

bramble-core/src/main/java/org/briarproject/bramble/db/Database.java

@@ -406,6 +406,12 @@ interface Database<T> {
 	Collection<MessageId> getMessageIds(T txn, GroupId g, Metadata query)
 			throws DbException;
 
+	/**
+	 * Returns the length of the given message in bytes, including the
+	 * message header.
+	 */
+	int getMessageLength(T txn, MessageId m) throws DbException;
+
 	/**
 	 * Returns the metadata for all delivered messages in the given group.
 	 * <p/>
@@ -496,7 +502,8 @@ interface Database<T> {
 
 	/**
 	 * Returns the IDs of some messages that are eligible to be sent to the
-	 * given contact, up to the given total length.
+	 * given contact. The total length of the messages including record headers
+	 * will be no more than the given capacity.
 	 * <p/>
 	 * Unlike {@link #getUnackedMessagesToSend(Object, ContactId)} this method
 	 * does not return messages that have already been sent unless they are
@@ -504,20 +511,20 @@ interface Database<T> {
 	 * <p/>
 	 * Read-only.
 	 */
-	Collection<MessageId> getMessagesToSend(T txn, ContactId c, int maxLength,
+	Collection<MessageId> getMessagesToSend(T txn, ContactId c, long capacity,
 			long maxLatency) throws DbException;
 
 	/**
 	 * Returns the IDs of all messages that are eligible to be sent to the
-	 * given contact, together with their raw lengths.
+	 * given contact.
 	 * <p/>
-	 * Unlike {@link #getMessagesToSend(Object, ContactId, int, long)} this
+	 * Unlike {@link #getMessagesToSend(Object, ContactId, long, long)} this
 	 * method may return messages that have already been sent and are not yet
 	 * due for retransmission.
 	 * <p/>
 	 * Read-only.
 	 */
-	Map<MessageId, Integer> getUnackedMessagesToSend(T txn, ContactId c)
+	Collection<MessageId> getUnackedMessagesToSend(T txn, ContactId c)
 			throws DbException;
 
 	/**
@@ -598,13 +605,14 @@ interface Database<T> {
 
 	/**
 	 * Returns the IDs of some messages that are eligible to be sent to the
-	 * given contact and have been requested by the contact, up to the given
+	 * given contact and have been requested by the contact. The total length
-	 * total length.
+	 * of the messages including record headers will be no more than the given
+	 * capacity.
 	 * <p/>
 	 * Read-only.
 	 */
 	Collection<MessageId> getRequestedMessagesToSend(T txn, ContactId c,
-			int maxLength, long maxLatency) throws DbException;
+			long capacity, long maxLatency) throws DbException;
 
 	/**
 	 * Returns all settings in the given namespace.

bramble-core/src/main/java/org/briarproject/bramble/db/DatabaseComponentImpl.java

@@ -75,7 +75,6 @@ import org.briarproject.bramble.api.transport.TransportKeys;
 
 import java.util.ArrayList;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.Executor;
@@ -87,6 +86,7 @@ import javax.annotation.Nullable;
 import javax.annotation.concurrent.ThreadSafe;
 import javax.inject.Inject;
 
+import static java.util.Collections.singletonList;
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.api.sync.Group.Visibility.INVISIBLE;
@@ -424,13 +424,14 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	@Nullable
 	@Override
 	public Collection<Message> generateBatch(Transaction transaction,
-			ContactId c, int maxLength, long maxLatency) throws DbException {
+			ContactId c, long capacity, long maxLatency) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
 		Collection<MessageId> ids =
-				db.getMessagesToSend(txn, c, maxLength, maxLatency);
+				db.getMessagesToSend(txn, c, capacity, maxLatency);
+		if (ids.isEmpty()) return null;
 		long totalLength = 0;
 		List<Message> messages = new ArrayList<>(ids.size());
 		for (MessageId m : ids) {
@@ -439,38 +440,11 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 			messages.add(message);
 			db.updateRetransmissionData(txn, c, m, maxLatency);
 		}
-		if (ids.isEmpty()) return null;
 		db.lowerRequestedFlag(txn, c, ids);
 		transaction.attach(new MessagesSentEvent(c, ids, totalLength));
 		return messages;
 	}
 
-	@Override
-	public Collection<Message> generateBatch(Transaction transaction,
-			ContactId c, Collection<MessageId> ids, long maxLatency)
-			throws DbException {
-		if (transaction.isReadOnly()) throw new IllegalArgumentException();
-		T txn = unbox(transaction);
-		if (!db.containsContact(txn, c))
-			throw new NoSuchContactException();
-		long totalLength = 0;
-		List<Message> messages = new ArrayList<>(ids.size());
-		List<MessageId> sentIds = new ArrayList<>(ids.size());
-		for (MessageId m : ids) {
-			if (db.containsVisibleMessage(txn, c, m)) {
-				Message message = db.getMessage(txn, m);
-				totalLength += message.getRawLength();
-				messages.add(message);
-				sentIds.add(m);
-				db.updateRetransmissionData(txn, c, m, maxLatency);
-			}
-		}
-		if (messages.isEmpty()) return messages;
-		db.lowerRequestedFlag(txn, c, sentIds);
-		transaction.attach(new MessagesSentEvent(c, sentIds, totalLength));
-		return messages;
-	}
-
 	@Nullable
 	@Override
 	public Offer generateOffer(Transaction transaction, ContactId c,
@@ -505,13 +479,14 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	@Nullable
 	@Override
 	public Collection<Message> generateRequestedBatch(Transaction transaction,
-			ContactId c, int maxLength, long maxLatency) throws DbException {
+			ContactId c, long capacity, long maxLatency) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
 		Collection<MessageId> ids =
-				db.getRequestedMessagesToSend(txn, c, maxLength, maxLatency);
+				db.getRequestedMessagesToSend(txn, c, capacity, maxLatency);
+		if (ids.isEmpty()) return null;
 		long totalLength = 0;
 		List<Message> messages = new ArrayList<>(ids.size());
 		for (MessageId m : ids) {
@@ -520,7 +495,6 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 			messages.add(message);
 			db.updateRetransmissionData(txn, c, m, maxLatency);
 		}
-		if (ids.isEmpty()) return null;
 		db.lowerRequestedFlag(txn, c, ids);
 		transaction.attach(new MessagesSentEvent(c, ids, totalLength));
 		return messages;
@@ -635,6 +609,24 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getMessageIds(txn, g, query);
 	}
 
+	@Override
+	public Collection<MessageId> getMessagesToAck(Transaction transaction,
+			ContactId c, int maxMessages) throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		return db.getMessagesToAck(txn, c, maxMessages);
+	}
+
+	@Override
+	public Collection<MessageId> getMessagesToSend(Transaction transaction,
+			ContactId c, long capacity, long maxLatency) throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		return db.getMessagesToSend(txn, c, capacity, maxLatency);
+	}
+
 	@Override
 	public Collection<MessageId> getMessagesToValidate(Transaction transaction)
 			throws DbException {
@@ -740,10 +732,29 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return status;
 	}
 
+	@Nullable
 	@Override
-	public Map<MessageId, Integer> getUnackedMessagesToSend(
+	public Message getMessageToSend(Transaction transaction, ContactId c,
-			Transaction transaction,
+			MessageId m, long maxLatency, boolean markAsSent)
-			ContactId c) throws DbException {
+			throws DbException {
+		if (transaction.isReadOnly()) throw new IllegalArgumentException();
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		if (!db.containsVisibleMessage(txn, c, m)) return null;
+		Message message = db.getMessage(txn, m);
+		if (markAsSent) {
+			db.updateRetransmissionData(txn, c, m, maxLatency);
+			db.lowerRequestedFlag(txn, c, singletonList(m));
+			transaction.attach(new MessagesSentEvent(c, singletonList(m),
+					message.getRawLength()));
+		}
+		return message;
+	}
+
+	@Override
+	public Collection<MessageId> getUnackedMessagesToSend(
+			Transaction transaction, ContactId c) throws DbException {
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
@@ -1069,6 +1080,20 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		db.removeTransportKeys(txn, t, k);
 	}
 
+	@Override
+	public void setAckSent(Transaction transaction, ContactId c,
+			Collection<MessageId> acked) throws DbException {
+		if (transaction.isReadOnly()) throw new IllegalArgumentException();
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		List<MessageId> visible = new ArrayList<>(acked.size());
+		for (MessageId m : acked) {
+			if (db.containsVisibleMessage(txn, c, m)) visible.add(m);
+		}
+		db.lowerAckFlag(txn, c, visible);
+	}
+
 	@Override
 	public void setCleanupTimerDuration(Transaction transaction, MessageId m,
 			long duration) throws DbException {
@@ -1115,7 +1140,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		if (old == INVISIBLE) db.addGroupVisibility(txn, c, g, v == SHARED);
 		else if (v == INVISIBLE) db.removeGroupVisibility(txn, c, g);
 		else db.setGroupVisibility(txn, c, g, v == SHARED);
-		List<ContactId> affected = Collections.singletonList(c);
+		List<ContactId> affected = singletonList(c);
 		transaction.attach(new GroupVisibilityUpdatedEvent(affected));
 	}
 
@@ -1163,6 +1188,28 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		transaction.attach(new MessageStateChangedEvent(m, false, state));
 	}
 
+	@Override
+	public void setMessagesSent(Transaction transaction, ContactId c,
+			Collection<MessageId> sent, long maxLatency) throws DbException {
+		if (transaction.isReadOnly()) throw new IllegalArgumentException();
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		long totalLength = 0;
+		List<MessageId> visible = new ArrayList<>(sent.size());
+		for (MessageId m : sent) {
+			if (db.containsVisibleMessage(txn, c, m)) {
+				visible.add(m);
+				totalLength += db.getMessageLength(txn, m);
+				db.updateRetransmissionData(txn, c, m, maxLatency);
+			}
+		}
+		db.lowerRequestedFlag(txn, c, visible);
+		if (!visible.isEmpty()) {
+			transaction.attach(new MessagesSentEvent(c, visible, totalLength));
+		}
+	}
+
 	@Override
 	public void addMessageDependencies(Transaction transaction,
 			Message dependent, Collection<MessageId> dependencies)

bramble-core/src/main/java/org/briarproject/bramble/db/JdbcDatabase.java

@@ -51,7 +51,6 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
-import java.util.LinkedHashMap;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
@@ -70,12 +69,14 @@ import static java.sql.Types.BOOLEAN;
 import static java.sql.Types.INTEGER;
 import static java.sql.Types.VARCHAR;
 import static java.util.Arrays.asList;
+import static java.util.logging.Level.FINE;
 import static java.util.logging.Level.INFO;
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.api.db.DatabaseComponent.NO_CLEANUP_DEADLINE;
 import static org.briarproject.bramble.api.db.DatabaseComponent.TIMER_NOT_STARTED;
 import static org.briarproject.bramble.api.db.Metadata.REMOVE;
+import static org.briarproject.bramble.api.record.Record.RECORD_HEADER_BYTES;
 import static org.briarproject.bramble.api.sync.Group.Visibility.INVISIBLE;
 import static org.briarproject.bramble.api.sync.Group.Visibility.SHARED;
 import static org.briarproject.bramble.api.sync.Group.Visibility.VISIBLE;
@@ -102,6 +103,11 @@ abstract class JdbcDatabase implements Database<Connection> {
 	// Package access for testing
 	static final int CODE_SCHEMA_VERSION = 50;
 
+	/**
+	 * The maximum number of idle connections to keep open.
+	 */
+	private static final int MAX_CONNECTION_POOL_SIZE = 1;
+
 	// Time period offsets for incoming transport keys
 	private static final int OFFSET_PREV = -1;
 	private static final int OFFSET_CURR = 0;
@@ -363,7 +369,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 	private final Condition connectionsChanged = connectionsLock.newCondition();
 
 	@GuardedBy("connectionsLock")
-	private final LinkedList<Connection> connections = new LinkedList<>();
+	private final LinkedList<Connection> connectionPool = new LinkedList<>();
 
 	@GuardedBy("connectionsLock")
 	private int openConnections = 0;
@@ -571,7 +577,8 @@ abstract class JdbcDatabase implements Database<Connection> {
 		connectionsLock.lock();
 		try {
 			if (closed) throw new DbClosedException();
-			txn = connections.poll();
+			txn = connectionPool.poll();
+			logConnectionCounts();
 		} finally {
 			connectionsLock.unlock();
 		}
@@ -582,7 +589,14 @@ abstract class JdbcDatabase implements Database<Connection> {
 				txn.setAutoCommit(false);
 				connectionsLock.lock();
 				try {
+					// The DB may have been closed since the check above
+					if (closed) {
+						tryToClose(txn, LOG, WARNING);
+						throw new DbClosedException();
+					}
 					openConnections++;
+					logConnectionCounts();
+					connectionsChanged.signalAll();
 				} finally {
 					connectionsLock.unlock();
 				}
@@ -593,67 +607,91 @@ abstract class JdbcDatabase implements Database<Connection> {
 		return txn;
 	}
 
-	@Override
+	@GuardedBy("connectionsLock")
-	public void abortTransaction(Connection txn) {
+	private void logConnectionCounts() {
-		try {
+		if (LOG.isLoggable(FINE)) {
-			txn.rollback();
+			LOG.fine(openConnections + " connections open, "
-			connectionsLock.lock();
+					+ connectionPool.size() + " in pool");
-			try {
-				connections.add(txn);
-				connectionsChanged.signalAll();
-			} finally {
-				connectionsLock.unlock();
-			}
-		} catch (SQLException e) {
-			// Try to close the connection
-			logException(LOG, WARNING, e);
-			tryToClose(txn, LOG, WARNING);
-			// Whatever happens, allow the database to close
-			connectionsLock.lock();
-			try {
-				openConnections--;
-				connectionsChanged.signalAll();
-			} finally {
-				connectionsLock.unlock();
-			}
 		}
 	}
 
 	@Override
-	public void commitTransaction(Connection txn) throws DbException {
+	public void abortTransaction(Connection txn) {
+		// The transaction may have been aborted due to an earlier exception,
+		// so close the connection rather than returning it to the pool
 		try {
-			txn.commit();
+			txn.rollback();
 		} catch (SQLException e) {
-			throw new DbException(e);
+			logException(LOG, WARNING, e);
 		}
+		closeConnection(txn);
+	}
+
+	private void closeConnection(Connection txn) {
+		tryToClose(txn, LOG, WARNING);
 		connectionsLock.lock();
 		try {
-			connections.add(txn);
+			openConnections--;
+			logConnectionCounts();
 			connectionsChanged.signalAll();
 		} finally {
 			connectionsLock.unlock();
 		}
 	}
 
-	void closeAllConnections() throws SQLException {
+	@Override
+	public void commitTransaction(Connection txn) throws DbException {
+		// If the transaction commits successfully then return the connection
+		// to the pool, otherwise close it
+		try {
+			txn.commit();
+			returnConnectionToPool(txn);
+		} catch (SQLException e) {
+			logException(LOG, WARNING, e);
+			closeConnection(txn);
+			throw new DbException(e);
+		}
+	}
+
+	private void returnConnectionToPool(Connection txn) {
+		boolean shouldClose;
+		connectionsLock.lock();
+		try {
+			shouldClose = connectionPool.size() >= MAX_CONNECTION_POOL_SIZE;
+			if (shouldClose) openConnections--;
+			else connectionPool.add(txn);
+			logConnectionCounts();
+			connectionsChanged.signalAll();
+		} finally {
+			connectionsLock.unlock();
+		}
+		if (shouldClose) tryToClose(txn, LOG, WARNING);
+	}
+
+	void closeAllConnections() {
 		boolean interrupted = false;
 		connectionsLock.lock();
 		try {
 			closed = true;
-			for (Connection c : connections) c.close();
+			for (Connection c : connectionPool) tryToClose(c, LOG, WARNING);
-			openConnections -= connections.size();
+			openConnections -= connectionPool.size();
-			connections.clear();
+			connectionPool.clear();
 			while (openConnections > 0) {
+				if (LOG.isLoggable(INFO)) {
+					LOG.info("Waiting for " + openConnections
+							+ " connections to be closed");
+				}
 				try {
 					connectionsChanged.await();
 				} catch (InterruptedException e) {
 					LOG.warning("Interrupted while closing connections");
 					interrupted = true;
 				}
-				for (Connection c : connections) c.close();
+				for (Connection c : connectionPool) tryToClose(c, LOG, WARNING);
-				openConnections -= connections.size();
+				openConnections -= connectionPool.size();
-				connections.clear();
+				connectionPool.clear();
 			}
+			LOG.info("All connections closed");
 		} finally {
 			connectionsLock.unlock();
 		}
@@ -1879,6 +1917,31 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	@Override
+	public int getMessageLength(Connection txn, MessageId m)
+			throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			String sql = "SELECT length from messages"
+					+ " WHERE messageId = ? AND state = ?";
+			ps = txn.prepareStatement(sql);
+			ps.setBytes(1, m.getBytes());
+			ps.setInt(2, DELIVERED.getValue());
+			rs = ps.executeQuery();
+			if (!rs.next()) throw new DbStateException();
+			int length = rs.getInt(1);
+			if (rs.next()) throw new DbStateException();
+			rs.close();
+			ps.close();
+			return length;
+		} catch (SQLException e) {
+			tryToClose(rs, LOG, WARNING);
+			tryToClose(ps, LOG, WARNING);
+			throw new DbException(e);
+		}
+	}
+
 	@Override
 	public Map<MessageId, Metadata> getMessageMetadata(Connection txn,
 			GroupId g) throws DbException {
@@ -2227,8 +2290,8 @@ abstract class JdbcDatabase implements Database<Connection> {
 	}
 
 	@Override
-	public Collection<MessageId> getMessagesToSend(Connection txn, ContactId c,
+	public Collection<MessageId> getMessagesToSend(Connection txn,
-			int maxLength, long maxLatency) throws DbException {
+			ContactId c, long capacity, long maxLatency) throws DbException {
 		long now = clock.currentTimeMillis();
 		PreparedStatement ps = null;
 		ResultSet rs = null;
@@ -2248,12 +2311,11 @@ abstract class JdbcDatabase implements Database<Connection> {
 			ps.setLong(4, maxLatency);
 			rs = ps.executeQuery();
 			List<MessageId> ids = new ArrayList<>();
-			int total = 0;
 			while (rs.next()) {
 				int length = rs.getInt(1);
-				if (total + length > maxLength) break;
+				if (capacity < RECORD_HEADER_BYTES + length) break;
 				ids.add(new MessageId(rs.getBytes(2)));
-				total += length;
+				capacity -= RECORD_HEADER_BYTES + length;
 			}
 			rs.close();
 			ps.close();
@@ -2266,12 +2328,12 @@ abstract class JdbcDatabase implements Database<Connection> {
 	}
 
 	@Override
-	public Map<MessageId, Integer> getUnackedMessagesToSend(Connection txn,
+	public Collection<MessageId> getUnackedMessagesToSend(Connection txn,
 			ContactId c) throws DbException {
 		PreparedStatement ps = null;
 		ResultSet rs = null;
 		try {
-			String sql = "SELECT length, messageId FROM statuses"
+			String sql = "SELECT messageId FROM statuses"
 					+ " WHERE contactId = ? AND state = ?"
 					+ " AND groupShared = TRUE AND messageShared = TRUE"
 					+ " AND deleted = FALSE AND seen = FALSE"
@@ -2280,15 +2342,11 @@ abstract class JdbcDatabase implements Database<Connection> {
 			ps.setInt(1, c.getInt());
 			ps.setInt(2, DELIVERED.getValue());
 			rs = ps.executeQuery();
-			Map<MessageId, Integer> results = new LinkedHashMap<>();
+			List<MessageId> ids = new ArrayList<>();
-			while (rs.next()) {
+			while (rs.next()) ids.add(new MessageId(rs.getBytes(1)));
-				int length = rs.getInt(1);
-				MessageId id = new MessageId(rs.getBytes(2));
-				results.put(id, length);
-			}
 			rs.close();
 			ps.close();
-			return results;
+			return ids;
 		} catch (SQLException e) {
 			tryToClose(rs, LOG, WARNING);
 			tryToClose(ps, LOG, WARNING);
@@ -2401,6 +2459,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 				MessageId m = new MessageId(rs.getBytes(1));
 				GroupId g = new GroupId(rs.getBytes(2));
 				Collection<MessageId> messageIds = ids.get(g);
+				//noinspection Java8MapApi
 				if (messageIds == null) {
 					messageIds = new ArrayList<>();
 					ids.put(g, messageIds);
@@ -2527,7 +2586,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 
 	@Override
 	public Collection<MessageId> getRequestedMessagesToSend(Connection txn,
-			ContactId c, int maxLength, long maxLatency) throws DbException {
+			ContactId c, long capacity, long maxLatency) throws DbException {
 		long now = clock.currentTimeMillis();
 		PreparedStatement ps = null;
 		ResultSet rs = null;
@@ -2547,12 +2606,11 @@ abstract class JdbcDatabase implements Database<Connection> {
 			ps.setLong(4, maxLatency);
 			rs = ps.executeQuery();
 			List<MessageId> ids = new ArrayList<>();
-			int total = 0;
 			while (rs.next()) {
 				int length = rs.getInt(1);
-				if (total + length > maxLength) break;
+				if (capacity < RECORD_HEADER_BYTES + length) break;
 				ids.add(new MessageId(rs.getBytes(2)));
-				total += length;
+				capacity -= RECORD_HEADER_BYTES + length;
 			}
 			rs.close();
 			ps.close();
@@ -2706,6 +2764,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 				ContactId c = new ContactId(rs.getInt(1));
 				TransportId t = new TransportId(rs.getString(2));
 				Collection<TransportId> transportIds = ids.get(c);
+				//noinspection Java8MapApi
 				if (transportIds == null) {
 					transportIds = new ArrayList<>();
 					ids.put(c, transportIds);

bramble-core/src/main/java/org/briarproject/bramble/io/TimeoutMonitorImpl.java

@@ -1,10 +1,10 @@
 package org.briarproject.bramble.io;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.io.TimeoutMonitor;
 import org.briarproject.bramble.api.lifecycle.IoExecutor;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.system.TaskScheduler;
-import org.briarproject.bramble.api.system.TaskScheduler.Cancellable;
 import org.briarproject.bramble.api.system.Wakeful;
 
 import java.io.IOException;

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ApiCall.java

@@ -0,0 +1,21 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.mailbox.MailboxApi.TolerableFailureException;
+
+/**
+ * An interface for calling an API endpoint with the option to retry the call.
+ */
+interface ApiCall {
+
+	/**
+	 * This method makes a synchronous call to an API endpoint and returns
+	 * true if the call should be retried, in which case the method may be
+	 * called again on the same {@link ApiCall} instance after a delay.
+	 *
+	 * @return True if the API call needs to be retried, or false if the API
+	 * call succeeded or {@link TolerableFailureException failed tolerably}.
+	 */
+	@IoExecutor
+	boolean callApi();
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ConnectivityChecker.java

@@ -0,0 +1,32 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * An interface for checking whether a mailbox is reachable.
+ */
+@ThreadSafe
+@NotNullByDefault
+interface ConnectivityChecker {
+
+	/**
+	 * Destroys the checker. Any current connectivity check is cancelled.
+	 */
+	void destroy();
+
+	/**
+	 * Starts a connectivity check if needed and calls the given observer when
+	 * the check succeeds. If a check is already running then the observer is
+	 * called when the check succeeds. If a connectivity check has recently
+	 * succeeded then the observer is called immediately.
+	 */
+	void checkConnectivity(MailboxProperties properties,
+			ConnectivityObserver o);
+
+	interface ConnectivityObserver {
+		void onConnectivityCheckSucceeded();
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ConnectivityCheckerImpl.java

@@ -0,0 +1,111 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.system.Clock;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+abstract class ConnectivityCheckerImpl implements ConnectivityChecker {
+
+	/**
+	 * If no more than this much time has elapsed since the last connectivity
+	 * check succeeded, consider the result to be fresh and don't check again.
+	 * <p>
+	 * Package access for testing.
+	 */
+	static final long CONNECTIVITY_CHECK_FRESHNESS_MS = 10_000;
+
+	private final Object lock = new Object();
+
+	protected final Clock clock;
+	private final MailboxApiCaller mailboxApiCaller;
+
+	@GuardedBy("lock")
+	private boolean destroyed = false;
+
+	@GuardedBy("lock")
+	@Nullable
+	private Cancellable connectivityCheck = null;
+
+	@GuardedBy("lock")
+	private long lastConnectivityCheckSucceeded = 0;
+
+	@GuardedBy("lock")
+	private final List<ConnectivityObserver> connectivityObservers =
+			new ArrayList<>();
+
+	/**
+	 * Creates an {@link ApiCall} for checking whether the mailbox is
+	 * reachable. The {@link ApiCall} should call
+	 * {@link #onConnectivityCheckSucceeded(long)} if the check succeeds.
+	 */
+	abstract ApiCall createConnectivityCheckTask(MailboxProperties properties);
+
+	ConnectivityCheckerImpl(Clock clock, MailboxApiCaller mailboxApiCaller) {
+		this.clock = clock;
+		this.mailboxApiCaller = mailboxApiCaller;
+	}
+
+	@Override
+	public void destroy() {
+		synchronized (lock) {
+			destroyed = true;
+			connectivityObservers.clear();
+			if (connectivityCheck != null) {
+				connectivityCheck.cancel();
+				connectivityCheck = null;
+			}
+		}
+	}
+
+	@Override
+	public void checkConnectivity(MailboxProperties properties,
+			ConnectivityObserver o) {
+		boolean callNow = false;
+		synchronized (lock) {
+			if (destroyed) return;
+			if (connectivityCheck == null) {
+				// No connectivity check is running
+				long now = clock.currentTimeMillis();
+				if (now - lastConnectivityCheckSucceeded
+						> CONNECTIVITY_CHECK_FRESHNESS_MS) {
+					// The last connectivity check is stale, start a new one
+					connectivityObservers.add(o);
+					ApiCall task =
+							createConnectivityCheckTask(properties);
+					connectivityCheck = mailboxApiCaller.retryWithBackoff(task);
+				} else {
+					// The last connectivity check is fresh
+					callNow = true;
+				}
+			} else {
+				// A connectivity check is running, wait for it to succeed
+				connectivityObservers.add(o);
+			}
+		}
+		if (callNow) o.onConnectivityCheckSucceeded();
+	}
+
+	protected void onConnectivityCheckSucceeded(long now) {
+		List<ConnectivityObserver> observers;
+		synchronized (lock) {
+			if (destroyed) return;
+			connectivityCheck = null;
+			lastConnectivityCheckSucceeded = now;
+			observers = new ArrayList<>(connectivityObservers);
+			connectivityObservers.clear();
+		}
+		for (ConnectivityObserver o : observers) {
+			o.onConnectivityCheckSucceeded();
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ContactMailboxConnectivityChecker.java

@@ -0,0 +1,39 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+
+import java.io.IOException;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+class ContactMailboxConnectivityChecker extends ConnectivityCheckerImpl {
+
+	private final MailboxApi mailboxApi;
+
+	ContactMailboxConnectivityChecker(Clock clock,
+			MailboxApiCaller mailboxApiCaller, MailboxApi mailboxApi) {
+		super(clock, mailboxApiCaller);
+		this.mailboxApi = mailboxApi;
+	}
+
+	@Override
+	ApiCall createConnectivityCheckTask(MailboxProperties properties) {
+		if (properties.isOwner()) throw new IllegalArgumentException();
+		return new SimpleApiCall() {
+			@Override
+			void tryToCallApi() throws IOException, ApiException {
+				if (!mailboxApi.checkStatus(properties)) {
+					throw new ApiException();
+				}
+				// Call the observers and cache the result
+				onConnectivityCheckSucceeded(clock.currentTimeMillis());
+			}
+		};
+	}
+
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxApi.java

@@ -7,6 +7,9 @@ import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
 import org.briarproject.bramble.api.mailbox.MailboxFileId;
 import org.briarproject.bramble.api.mailbox.MailboxFolderId;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.io.File;
 import java.io.IOException;
@@ -16,8 +19,21 @@ import java.util.List;
 import javax.annotation.Nonnull;
 import javax.annotation.concurrent.Immutable;
 
+import static java.util.Collections.singletonList;
+
+@NotNullByDefault
 interface MailboxApi {
 
+	/**
+	 * Mailbox API versions that we support as a client. This is reported to our
+	 * contacts by {@link MailboxUpdateManager}.
+	 */
+	List<MailboxVersion> CLIENT_SUPPORTS = singletonList(
+			new MailboxVersion(1, 0));
+
+	List<MailboxVersion> getServerSupports(MailboxProperties properties)
+			throws IOException, ApiException;
+
 	/**
 	 * Sets up the mailbox with the setup token.
 	 *
@@ -25,7 +41,7 @@ interface MailboxApi {
 	 * @return the owner token
 	 * @throws ApiException for 401 response.
 	 */
-	MailboxAuthToken setup(MailboxProperties properties)
+	MailboxProperties setup(MailboxProperties properties)
 			throws IOException, ApiException;
 
 	/**

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxApiCaller.java

@@ -0,0 +1,32 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import static java.util.concurrent.TimeUnit.DAYS;
+import static java.util.concurrent.TimeUnit.MINUTES;
+
+@NotNullByDefault
+interface MailboxApiCaller {
+
+	/**
+	 * The minimum interval between retries in milliseconds.
+	 */
+	long MIN_RETRY_INTERVAL_MS = MINUTES.toMillis(1);
+
+	/**
+	 * The maximum interval between retries in milliseconds.
+	 */
+	long MAX_RETRY_INTERVAL_MS = DAYS.toMillis(1);
+
+	/**
+	 * Asynchronously calls the given API call on the {@link IoExecutor},
+	 * automatically retrying at increasing intervals until the API call
+	 * returns false or retries are cancelled.
+	 *
+	 * @return A {@link Cancellable} that can be used to cancel any future
+	 * retries.
+	 */
+	Cancellable retryWithBackoff(ApiCall apiCall);
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxApiCallerImpl.java

@@ -0,0 +1,98 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.system.TaskScheduler;
+
+import java.util.concurrent.Executor;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static java.lang.Math.min;
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+
+@Immutable
+@NotNullByDefault
+class MailboxApiCallerImpl implements MailboxApiCaller {
+
+	private final TaskScheduler taskScheduler;
+	private final Executor ioExecutor;
+
+	@Inject
+	MailboxApiCallerImpl(TaskScheduler taskScheduler,
+			@IoExecutor Executor ioExecutor) {
+		this.taskScheduler = taskScheduler;
+		this.ioExecutor = ioExecutor;
+	}
+
+	@Override
+	public Cancellable retryWithBackoff(ApiCall apiCall) {
+		Task task = new Task(apiCall);
+		task.start();
+		return task;
+	}
+
+	private class Task implements Cancellable {
+
+		private final ApiCall apiCall;
+		private final Object lock = new Object();
+
+		@GuardedBy("lock")
+		@Nullable
+		private Cancellable scheduledTask = null;
+
+		@GuardedBy("lock")
+		private boolean cancelled = false;
+
+		@GuardedBy("lock")
+		private long retryIntervalMs = MIN_RETRY_INTERVAL_MS;
+
+		private Task(ApiCall apiCall) {
+			this.apiCall = apiCall;
+		}
+
+		private void start() {
+			synchronized (lock) {
+				if (cancelled) throw new AssertionError();
+				ioExecutor.execute(this::callApi);
+			}
+		}
+
+		@IoExecutor
+		private void callApi() {
+			synchronized (lock) {
+				if (cancelled) return;
+			}
+			// The call returns true if we should retry
+			if (apiCall.callApi()) {
+				synchronized (lock) {
+					if (cancelled) return;
+					scheduledTask = taskScheduler.schedule(this::callApi,
+							ioExecutor, retryIntervalMs, MILLISECONDS);
+					// Increase the retry interval each time we retry
+					retryIntervalMs =
+							min(MAX_RETRY_INTERVAL_MS, retryIntervalMs * 2);
+				}
+			} else {
+				synchronized (lock) {
+					scheduledTask = null;
+				}
+			}
+		}
+
+		@Override
+		public void cancel() {
+			Cancellable scheduledTask;
+			synchronized (lock) {
+				cancelled = true;
+				scheduledTask = this.scheduledTask;
+				this.scheduledTask = null;
+			}
+			if (scheduledTask != null) scheduledTask.cancel();
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxApiImpl.java

@@ -14,6 +14,7 @@ import org.briarproject.bramble.api.mailbox.MailboxFileId;
 import org.briarproject.bramble.api.mailbox.MailboxFolderId;
 import org.briarproject.bramble.api.mailbox.MailboxId;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.io.File;
@@ -56,7 +57,24 @@ class MailboxApiImpl implements MailboxApi {
 	}
 
 	@Override
-	public MailboxAuthToken setup(MailboxProperties properties)
+	public List<MailboxVersion> getServerSupports(MailboxProperties properties)
+			throws IOException, ApiException {
+		if (!properties.isOwner()) throw new IllegalArgumentException();
+		Response response = sendGetRequest(properties, "/versions");
+		if (response.code() != 200) throw new ApiException();
+
+		ResponseBody body = response.body();
+		if (body == null) throw new ApiException();
+		try {
+			JsonNode node = mapper.readTree(body.string());
+			return parseServerSupports(node);
+		} catch (JacksonException e) {
+			throw new ApiException();
+		}
+	}
+
+	@Override
+	public MailboxProperties setup(MailboxProperties properties)
 			throws IOException, ApiException {
 		if (!properties.isOwner()) throw new IllegalArgumentException();
 		Request request = getRequestBuilder(properties.getAuthToken())
@@ -75,17 +93,40 @@ class MailboxApiImpl implements MailboxApi {
 			if (tokenNode == null) {
 				throw new ApiException();
 			}
-			String ownerToken = tokenNode.textValue();
+			return new MailboxProperties(properties.getBaseUrl(),
-			return MailboxAuthToken.fromString(ownerToken);
+					MailboxAuthToken.fromString(tokenNode.textValue()),
+					parseServerSupports(node));
 		} catch (JacksonException | InvalidMailboxIdException e) {
 			throw new ApiException();
 		}
 	}
 
+	private List<MailboxVersion> parseServerSupports(JsonNode node)
+			throws ApiException {
+		List<MailboxVersion> serverSupports = new ArrayList<>();
+		ArrayNode serverSupportsNode = getArray(node, "serverSupports");
+		for (JsonNode versionNode : serverSupportsNode) {
+			if (!versionNode.isObject()) throw new ApiException();
+			ObjectNode objectNode = (ObjectNode) versionNode;
+			JsonNode majorNode = objectNode.get("major");
+			JsonNode minorNode = objectNode.get("minor");
+			if (majorNode == null || !majorNode.isNumber()) {
+				throw new ApiException();
+			}
+			if (minorNode == null || !minorNode.isNumber()) {
+				throw new ApiException();
+			}
+			int major = majorNode.asInt();
+			int minor = minorNode.asInt();
+			if (major < 0 || minor < 0) throw new ApiException();
+			serverSupports.add(new MailboxVersion(major, minor));
+		}
+		return serverSupports;
+	}
+
 	@Override
 	public boolean checkStatus(MailboxProperties properties)
 			throws IOException, ApiException {
-		if (!properties.isOwner()) throw new IllegalArgumentException();
 		Response response = sendGetRequest(properties, "/status");
 		if (response.code() == 401) throw new ApiException();
 		return response.isSuccessful();

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxManagerImpl.java

@@ -102,23 +102,55 @@ class MailboxManagerImpl implements MailboxManager {
 		try {
 			MailboxProperties props = db.transactionWithNullableResult(true,
 					mailboxSettingsManager::getOwnMailboxProperties);
+			if (props == null) throw new DbException();
 			success = api.checkStatus(props);
-		} catch (DbException | IOException | MailboxApi.ApiException e) {
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+			// we don't treat this is a failure to record
+			return false;
+		} catch (IOException | MailboxApi.ApiException e) {
+			// we record this as a failure
 			success = false;
 			logException(LOG, WARNING, e);
 		}
-		if (success) {
+		try {
-			try {
+			recordCheckResult(success);
-				// we are only recording successful connections here
+		} catch (DbException e) {
-				// as those update the UI and failures might be false negatives
+			logException(LOG, WARNING, e);
-				db.transaction(false, txn ->
-						mailboxSettingsManager.recordSuccessfulConnection(txn,
-								clock.currentTimeMillis()));
-			} catch (DbException e) {
-				logException(LOG, WARNING, e);
-			}
 		}
 		return success;
 	}
 
+	private void recordCheckResult(boolean success) throws DbException {
+		long now = clock.currentTimeMillis();
+		db.transaction(false, txn -> {
+			if (success) {
+				mailboxSettingsManager.recordSuccessfulConnection(txn, now);
+			} else {
+				mailboxSettingsManager.recordFailedConnectionAttempt(txn, now);
+			}
+		});
+	}
+
+	@Override
+	public boolean unPair() throws DbException {
+		MailboxProperties properties = db.transactionWithNullableResult(true,
+				mailboxSettingsManager::getOwnMailboxProperties);
+		if (properties == null) {
+			// no more mailbox, that's strange but possible if called in quick
+			// succession, so let's return true this time
+			return true;
+		}
+		boolean wasWiped;
+		try {
+			api.wipeMailbox(properties);
+			wasWiped = true;
+		} catch (IOException | MailboxApi.ApiException e) {
+			logException(LOG, WARNING, e);
+			wasWiped = false;
+		}
+		db.transaction(false,
+				mailboxSettingsManager::removeOwnMailboxProperties);
+		return wasWiped;
+	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxModule.java

@@ -1,34 +1,39 @@
 package org.briarproject.bramble.mailbox;
 
+import org.briarproject.bramble.api.FeatureFlags;
 import org.briarproject.bramble.api.client.ClientHelper;
 import org.briarproject.bramble.api.contact.ContactManager;
 import org.briarproject.bramble.api.data.MetadataEncoder;
 import org.briarproject.bramble.api.lifecycle.LifecycleManager;
 import org.briarproject.bramble.api.mailbox.MailboxManager;
-import org.briarproject.bramble.api.mailbox.MailboxPropertyManager;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.sync.validation.ValidationManager;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.versioning.ClientVersioningManager;
 
+import java.util.List;
+
 import javax.inject.Inject;
 import javax.inject.Singleton;
 
 import dagger.Module;
 import dagger.Provides;
 
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.CLIENT_ID;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.CLIENT_ID;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.MAJOR_VERSION;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.MAJOR_VERSION;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.MINOR_VERSION;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.MINOR_VERSION;
+import static org.briarproject.bramble.mailbox.MailboxApi.CLIENT_SUPPORTS;
 
 @Module
 public class MailboxModule {
 
 	public static class EagerSingletons {
 		@Inject
-		MailboxPropertyValidator mailboxPropertyValidator;
+		MailboxUpdateValidator mailboxUpdateValidator;
 		@Inject
-		MailboxPropertyManager mailboxPropertyManager;
+		MailboxUpdateManager mailboxUpdateManager;
 	}
 
 	@Provides
@@ -56,31 +61,44 @@ public class MailboxModule {
 
 	@Provides
 	@Singleton
-	MailboxPropertyValidator provideMailboxPropertyValidator(
+	MailboxUpdateValidator provideMailboxUpdateValidator(
-			ValidationManager validationManager, ClientHelper clientHelper,
+			ValidationManager validationManager,
-			MetadataEncoder metadataEncoder, Clock clock) {
+			ClientHelper clientHelper,
-		MailboxPropertyValidator validator = new MailboxPropertyValidator(
+			MetadataEncoder metadataEncoder,
+			Clock clock,
+			FeatureFlags featureFlags) {
+		MailboxUpdateValidator validator = new MailboxUpdateValidator(
 				clientHelper, metadataEncoder, clock);
-		validationManager.registerMessageValidator(CLIENT_ID, MAJOR_VERSION,
+		if (featureFlags.shouldEnableMailbox()) {
-				validator);
+			validationManager.registerMessageValidator(CLIENT_ID,
+					MAJOR_VERSION, validator);
+		}
 		return validator;
 	}
 
+	@Provides
+	List<MailboxVersion> provideClientSupports() {
+		return CLIENT_SUPPORTS;
+	}
+
 	@Provides
 	@Singleton
-	MailboxPropertyManager provideMailboxPropertyManager(
+	MailboxUpdateManager provideMailboxUpdateManager(
+			FeatureFlags featureFlags,
 			LifecycleManager lifecycleManager,
 			ValidationManager validationManager, ContactManager contactManager,
 			ClientVersioningManager clientVersioningManager,
 			MailboxSettingsManager mailboxSettingsManager,
-			MailboxPropertyManagerImpl mailboxPropertyManager) {
+			MailboxUpdateManagerImpl mailboxUpdateManager) {
-		lifecycleManager.registerOpenDatabaseHook(mailboxPropertyManager);
+		if (featureFlags.shouldEnableMailbox()) {
-		validationManager.registerIncomingMessageHook(CLIENT_ID, MAJOR_VERSION,
+			lifecycleManager.registerOpenDatabaseHook(mailboxUpdateManager);
-				mailboxPropertyManager);
+			validationManager.registerIncomingMessageHook(CLIENT_ID,
-		contactManager.registerContactHook(mailboxPropertyManager);
+					MAJOR_VERSION, mailboxUpdateManager);
-		clientVersioningManager.registerClient(CLIENT_ID, MAJOR_VERSION,
+			contactManager.registerContactHook(mailboxUpdateManager);
-				MINOR_VERSION, mailboxPropertyManager);
+			clientVersioningManager.registerClient(CLIENT_ID, MAJOR_VERSION,
-		mailboxSettingsManager.registerMailboxHook(mailboxPropertyManager);
+					MINOR_VERSION, mailboxUpdateManager);
-		return mailboxPropertyManager;
+			mailboxSettingsManager.registerMailboxHook(mailboxUpdateManager);
+		}
+		return mailboxUpdateManager;
 	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxPairingTaskFactoryImpl.java

@@ -4,8 +4,8 @@ import org.briarproject.bramble.api.crypto.CryptoComponent;
 import org.briarproject.bramble.api.db.DatabaseComponent;
 import org.briarproject.bramble.api.event.EventExecutor;
 import org.briarproject.bramble.api.mailbox.MailboxPairingTask;
-import org.briarproject.bramble.api.mailbox.MailboxPropertyManager;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.system.Clock;
 
@@ -24,7 +24,7 @@ class MailboxPairingTaskFactoryImpl implements MailboxPairingTaskFactory {
 	private final Clock clock;
 	private final MailboxApi api;
 	private final MailboxSettingsManager mailboxSettingsManager;
-	private final MailboxPropertyManager mailboxPropertyManager;
+	private final MailboxUpdateManager mailboxUpdateManager;
 
 	@Inject
 	MailboxPairingTaskFactoryImpl(
@@ -34,20 +34,20 @@ class MailboxPairingTaskFactoryImpl implements MailboxPairingTaskFactory {
 			Clock clock,
 			MailboxApi api,
 			MailboxSettingsManager mailboxSettingsManager,
-			MailboxPropertyManager mailboxPropertyManager) {
+			MailboxUpdateManager mailboxUpdateManager) {
 		this.eventExecutor = eventExecutor;
 		this.db = db;
 		this.crypto = crypto;
 		this.clock = clock;
 		this.api = api;
 		this.mailboxSettingsManager = mailboxSettingsManager;
-		this.mailboxPropertyManager = mailboxPropertyManager;
+		this.mailboxUpdateManager = mailboxUpdateManager;
 	}
 
 	@Override
 	public MailboxPairingTask createPairingTask(String qrCodePayload) {
 		return new MailboxPairingTaskImpl(qrCodePayload, eventExecutor, db,
 				crypto, clock, api, mailboxSettingsManager,
-				mailboxPropertyManager);
+				mailboxUpdateManager);
 	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxPairingTaskImpl.java

@@ -11,9 +11,9 @@ import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
 import org.briarproject.bramble.api.mailbox.MailboxPairingState;
 import org.briarproject.bramble.api.mailbox.MailboxPairingTask;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
-import org.briarproject.bramble.api.mailbox.MailboxPropertiesUpdate;
-import org.briarproject.bramble.api.mailbox.MailboxPropertyManager;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
@@ -51,7 +51,7 @@ class MailboxPairingTaskImpl implements MailboxPairingTask {
 	private final Clock clock;
 	private final MailboxApi api;
 	private final MailboxSettingsManager mailboxSettingsManager;
-	private final MailboxPropertyManager mailboxPropertyManager;
+	private final MailboxUpdateManager mailboxUpdateManager;
 
 	private final Object lock = new Object();
 	@GuardedBy("lock")
@@ -68,7 +68,7 @@ class MailboxPairingTaskImpl implements MailboxPairingTask {
 			Clock clock,
 			MailboxApi api,
 			MailboxSettingsManager mailboxSettingsManager,
-			MailboxPropertyManager mailboxPropertyManager) {
+			MailboxUpdateManager mailboxUpdateManager) {
 		this.payload = payload;
 		this.eventExecutor = eventExecutor;
 		this.db = db;
@@ -76,7 +76,7 @@ class MailboxPairingTaskImpl implements MailboxPairingTask {
 		this.clock = clock;
 		this.api = api;
 		this.mailboxSettingsManager = mailboxSettingsManager;
-		this.mailboxPropertyManager = mailboxPropertyManager;
+		this.mailboxUpdateManager = mailboxUpdateManager;
 		state = new MailboxPairingState.QrCodeReceived();
 	}
 
@@ -115,9 +115,7 @@ class MailboxPairingTaskImpl implements MailboxPairingTask {
 	private void pairMailbox() throws IOException, ApiException, DbException {
 		MailboxProperties mailboxProperties = decodeQrCodePayload(payload);
 		setState(new MailboxPairingState.Pairing());
-		MailboxAuthToken ownerToken = api.setup(mailboxProperties);
+		MailboxProperties ownerProperties = api.setup(mailboxProperties);
-		MailboxProperties ownerProperties = new MailboxProperties(
-				mailboxProperties.getBaseUrl(), ownerToken, true);
 		long time = clock.currentTimeMillis();
 		db.transaction(false, txn -> {
 			mailboxSettingsManager
@@ -127,9 +125,9 @@ class MailboxPairingTaskImpl implements MailboxPairingTask {
 			// timers for contacts who doesn't have their own mailbox. This way,
 			// data stranded on our old mailbox will be re-uploaded to our new.
 			for (Contact c : db.getContacts(txn)) {
-				MailboxPropertiesUpdate remoteProps = mailboxPropertyManager
+				MailboxUpdate update = mailboxUpdateManager.getRemoteUpdate(
-						.getRemoteProperties(txn, c.getId());
+						txn, c.getId());
-				if (remoteProps == null) {
+				if (update == null || !update.hasMailbox()) {
 					db.resetUnackedMessagesToSend(txn, c.getId());
 				}
 			}
@@ -179,10 +177,10 @@ class MailboxPairingTaskImpl implements MailboxPairingTask {
 		LOG.info("QR code is valid");
 		byte[] onionPubKey = Arrays.copyOfRange(bytes, 1, 33);
 		String onion = crypto.encodeOnion(onionPubKey);
-		String baseUrl = "http://" + onion + ".onion";
+		String baseUrl = "http://" + onion + ".onion"; // TODO
 		byte[] tokenBytes = Arrays.copyOfRange(bytes, 33, 65);
 		MailboxAuthToken setupToken = new MailboxAuthToken(tokenBytes);
-		return new MailboxProperties(baseUrl, setupToken, true);
+		return new MailboxProperties(baseUrl, setupToken, new ArrayList<>());
 	}
 
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxSettingsManagerImpl.java

@@ -8,11 +8,14 @@ import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
 import org.briarproject.bramble.api.mailbox.MailboxStatus;
-import org.briarproject.bramble.api.mailbox.OwnMailboxConnectionStatusEvent;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
+import org.briarproject.bramble.api.mailbox.event.MailboxProblemEvent;
+import org.briarproject.bramble.api.mailbox.event.OwnMailboxConnectionStatusEvent;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.settings.Settings;
 import org.briarproject.bramble.api.settings.SettingsManager;
 
+import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.CopyOnWriteArrayList;
 
@@ -28,8 +31,10 @@ class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 
 	// Package access for testing
 	static final String SETTINGS_NAMESPACE = "mailbox";
+	// TODO: This currently stores the base URL, not the 56-char onion address
 	static final String SETTINGS_KEY_ONION = "onion";
 	static final String SETTINGS_KEY_TOKEN = "token";
+	static final String SETTINGS_KEY_SERVER_SUPPORTS = "serverSupports";
 	static final String SETTINGS_KEY_LAST_ATTEMPT = "lastAttempt";
 	static final String SETTINGS_KEY_LAST_SUCCESS = "lastSuccess";
 	static final String SETTINGS_KEY_ATTEMPTS = "attempts";
@@ -55,9 +60,18 @@ class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 		String onion = s.get(SETTINGS_KEY_ONION);
 		String token = s.get(SETTINGS_KEY_TOKEN);
 		if (isNullOrEmpty(onion) || isNullOrEmpty(token)) return null;
+		int[] ints = s.getIntArray(SETTINGS_KEY_SERVER_SUPPORTS);
+		// We know we were paired, so we must have proper serverSupports
+		if (ints == null || ints.length == 0 || ints.length % 2 != 0) {
+			throw new DbException();
+		}
+		List<MailboxVersion> serverSupports = new ArrayList<>();
+		for (int i = 0; i < ints.length - 1; i += 2) {
+			serverSupports.add(new MailboxVersion(ints[i], ints[i + 1]));
+		}
 		try {
 			MailboxAuthToken tokenId = MailboxAuthToken.fromString(token);
-			return new MailboxProperties(onion, tokenId, true);
+			return new MailboxProperties(onion, tokenId, serverSupports);
 		} catch (InvalidMailboxIdException e) {
 			throw new DbException(e);
 		}
@@ -69,9 +83,28 @@ class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 		Settings s = new Settings();
 		s.put(SETTINGS_KEY_ONION, p.getBaseUrl());
 		s.put(SETTINGS_KEY_TOKEN, p.getAuthToken().toString());
+		List<MailboxVersion> serverSupports = p.getServerSupports();
+		int[] ints = new int[serverSupports.size() * 2];
+		int i = 0;
+		for (MailboxVersion v : serverSupports) {
+			ints[i++] = v.getMajor();
+			ints[i++] = v.getMinor();
+		}
+		s.putIntArray(SETTINGS_KEY_SERVER_SUPPORTS, ints);
 		settingsManager.mergeSettings(txn, s, SETTINGS_NAMESPACE);
 		for (MailboxHook hook : hooks) {
-			hook.mailboxPaired(txn, p.getOnion());
+			hook.mailboxPaired(txn, p.getOnion(), p.getServerSupports());
+		}
+	}
+
+	@Override
+	public void removeOwnMailboxProperties(Transaction txn) throws DbException {
+		Settings s = new Settings();
+		s.put(SETTINGS_KEY_ONION, "");
+		s.put(SETTINGS_KEY_TOKEN, "");
+		settingsManager.mergeSettings(txn, s, SETTINGS_NAMESPACE);
+		for (MailboxHook hook : hooks) {
+			hook.mailboxUnpaired(txn);
 		}
 	}
 
@@ -110,6 +143,7 @@ class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 		settingsManager.mergeSettings(txn, newSettings, SETTINGS_NAMESPACE);
 		MailboxStatus status = new MailboxStatus(now, lastSuccess, newAttempts);
 		txn.attach(new OwnMailboxConnectionStatusEvent(status));
+		if (status.hasProblem(now)) txn.attach(new MailboxProblemEvent());
 	}
 
 	@Override

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxUpdateManagerImpl.java

@@ -8,6 +8,7 @@ import org.briarproject.bramble.api.contact.ContactId;
 import org.briarproject.bramble.api.contact.ContactManager.ContactHook;
 import org.briarproject.bramble.api.crypto.CryptoComponent;
 import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfEntry;
 import org.briarproject.bramble.api.data.BdfList;
 import org.briarproject.bramble.api.data.MetadataParser;
 import org.briarproject.bramble.api.db.DatabaseComponent;
@@ -18,11 +19,13 @@ import org.briarproject.bramble.api.lifecycle.LifecycleManager.OpenDatabaseHook;
 import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
 import org.briarproject.bramble.api.mailbox.MailboxFolderId;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
-import org.briarproject.bramble.api.mailbox.MailboxPropertiesUpdate;
-import org.briarproject.bramble.api.mailbox.MailboxPropertyManager;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager.MailboxHook;
-import org.briarproject.bramble.api.mailbox.RemoteMailboxPropertiesUpdateEvent;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
+import org.briarproject.bramble.api.mailbox.event.RemoteMailboxUpdateEvent;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.sync.Group;
 import org.briarproject.bramble.api.sync.Group.Visibility;
@@ -35,6 +38,7 @@ import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.versioning.ClientVersioningManager;
 import org.briarproject.bramble.api.versioning.ClientVersioningManager.ClientVersioningHook;
 
+import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
 
@@ -44,10 +48,11 @@ import javax.inject.Inject;
 import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_DO_NOT_SHARE;
 
 @NotNullByDefault
-class MailboxPropertyManagerImpl implements MailboxPropertyManager,
+class MailboxUpdateManagerImpl implements MailboxUpdateManager,
 		OpenDatabaseHook, ContactHook, ClientVersioningHook,
 		IncomingMessageHook, MailboxHook {
 
+	private final List<MailboxVersion> clientSupports;
 	private final DatabaseComponent db;
 	private final ClientHelper clientHelper;
 	private final ClientVersioningManager clientVersioningManager;
@@ -59,12 +64,14 @@ class MailboxPropertyManagerImpl implements MailboxPropertyManager,
 	private final Group localGroup;
 
 	@Inject
-	MailboxPropertyManagerImpl(DatabaseComponent db, ClientHelper clientHelper,
+	MailboxUpdateManagerImpl(List<MailboxVersion> clientSupports,
+			DatabaseComponent db, ClientHelper clientHelper,
 			ClientVersioningManager clientVersioningManager,
 			MetadataParser metadataParser,
 			ContactGroupFactory contactGroupFactory, Clock clock,
 			MailboxSettingsManager mailboxSettingsManager,
 			CryptoComponent crypto) {
+		this.clientSupports = clientSupports;
 		this.db = db;
 		this.clientHelper = clientHelper;
 		this.clientVersioningManager = clientVersioningManager;
@@ -80,12 +87,46 @@ class MailboxPropertyManagerImpl implements MailboxPropertyManager,
 	@Override
 	public void onDatabaseOpened(Transaction txn) throws DbException {
 		if (db.containsGroup(txn, localGroup.getId())) {
-			return;
+			try {
+				BdfDictionary meta = clientHelper.getGroupMetadataAsDictionary(
+						txn, localGroup.getId());
+				BdfList sent = meta.getList(GROUP_KEY_SENT_CLIENT_SUPPORTS);
+				if (clientHelper.parseMailboxVersionList(sent)
+						.equals(clientSupports)) {
+					return;
+				}
+			} catch (FormatException e) {
+				throw new DbException();
+			}
+			// Our current clientSupports list has changed compared to what we
+			// last sent out.
+			for (Contact c : db.getContacts(txn)) {
+				MailboxUpdate latest = getLocalUpdate(txn, c.getId());
+				MailboxUpdate updated;
+				if (latest.hasMailbox()) {
+					updated = new MailboxUpdateWithMailbox(
+							(MailboxUpdateWithMailbox) latest, clientSupports);
+				} else {
+					updated = new MailboxUpdate(clientSupports);
+				}
+				Group g = getContactGroup(c);
+				storeMessageReplaceLatest(txn, g.getId(), updated);
+			}
+		} else {
+			db.addGroup(txn, localGroup);
+			// Set things up for any pre-existing contacts
+			for (Contact c : db.getContacts(txn)) {
+				addingContact(txn, c);
+			}
 		}
-		db.addGroup(txn, localGroup);
+
-		// Set things up for any pre-existing contacts
+		try {
-		for (Contact c : db.getContacts(txn)) {
+			BdfDictionary meta = BdfDictionary.of(new BdfEntry(
-			addingContact(txn, c);
+					GROUP_KEY_SENT_CLIENT_SUPPORTS,
+					encodeSupportsList(clientSupports)));
+			clientHelper.mergeGroupMetadata(txn, localGroup.getId(), meta);
+		} catch (FormatException e) {
+			throw new DbException();
 		}
 	}
 
@@ -100,11 +141,15 @@ class MailboxPropertyManagerImpl implements MailboxPropertyManager,
 		db.setGroupVisibility(txn, c.getId(), g.getId(), client);
 		// Attach the contact ID to the group
 		clientHelper.setContactId(txn, g.getId(), c.getId());
-		// If we are paired, create and send props to the newly added contact
 		MailboxProperties ownProps =
 				mailboxSettingsManager.getOwnMailboxProperties(txn);
 		if (ownProps != null) {
-			createAndSendProperties(txn, c, ownProps.getOnion());
+			// We are paired, create and send props to the newly added contact
+			createAndSendUpdateWithMailbox(txn, c, ownProps.getServerSupports(),
+					ownProps.getOnion());
+		} else {
+			// Not paired, but we still want to get our clientSupports sent
+			sendUpdateNoMailbox(txn, c);
 		}
 	}
 
@@ -114,17 +159,17 @@ class MailboxPropertyManagerImpl implements MailboxPropertyManager,
 	}
 
 	@Override
-	public void mailboxPaired(Transaction txn, String ownOnion)
+	public void mailboxPaired(Transaction txn, String ownOnion,
-			throws DbException {
+			List<MailboxVersion> serverSupports) throws DbException {
 		for (Contact c : db.getContacts(txn)) {
-			createAndSendProperties(txn, c, ownOnion);
+			createAndSendUpdateWithMailbox(txn, c, serverSupports, ownOnion);
 		}
 	}
 
 	@Override
 	public void mailboxUnpaired(Transaction txn) throws DbException {
 		for (Contact c : db.getContacts(txn)) {
-			sendEmptyProperties(txn, c);
+			sendUpdateNoMailbox(txn, c);
 		}
 	}
 
@@ -156,8 +201,8 @@ class MailboxPropertyManagerImpl implements MailboxPropertyManager,
 			}
 			ContactId c = clientHelper.getContactId(txn, m.getGroupId());
 			BdfList body = clientHelper.getMessageAsList(txn, m.getId());
-			MailboxPropertiesUpdate p = parseProperties(body);
+			MailboxUpdate u = parseUpdate(body);
-			txn.attach(new RemoteMailboxPropertiesUpdateEvent(c, p));
+			txn.attach(new RemoteMailboxUpdateEvent(c, u));
 			// Reset message retransmission timers for the contact. Avoiding
 			// messages getting stranded:
 			// - on our mailbox, if they now have a mailbox but didn't before
@@ -171,70 +216,82 @@ class MailboxPropertyManagerImpl implements MailboxPropertyManager,
 	}
 
 	@Override
-	@Nullable
+	public MailboxUpdate getLocalUpdate(Transaction txn, ContactId c)
-	public MailboxPropertiesUpdate getLocalProperties(Transaction txn,
+			throws DbException {
-			ContactId c) throws DbException {
+		MailboxUpdate local = getUpdate(txn, db.getContact(txn, c), true);
-		return getProperties(txn, db.getContact(txn, c), true);
+		// An update (with or without mailbox) is created when contact is added
+		if (local == null) {
+			throw new DbException();
+		}
+		return local;
 	}
 
 	@Override
 	@Nullable
-	public MailboxPropertiesUpdate getRemoteProperties(Transaction txn,
+	public MailboxUpdate getRemoteUpdate(Transaction txn, ContactId c)
-			ContactId c) throws DbException {
+			throws DbException {
-		return getProperties(txn, db.getContact(txn, c), false);
+		return getUpdate(txn, db.getContact(txn, c), false);
 	}
 
 	/**
 	 * Creates and sends an update message to the given contact. The message
-	 * holds our own mailbox's onion, and generated unique properties. All of
+	 * holds our own mailbox's onion, generated unique properties, and lists of
-	 * which the contact needs to communicate with our Mailbox.
+	 * supported Mailbox API version(s). All of which the contact needs to
+	 * communicate with our Mailbox.
 	 */
-	private void createAndSendProperties(Transaction txn,
+	private void createAndSendUpdateWithMailbox(Transaction txn, Contact c,
-			Contact c, String ownOnion) throws DbException {
+			List<MailboxVersion> serverSupports, String ownOnion)
-		MailboxPropertiesUpdate p = new MailboxPropertiesUpdate(ownOnion,
+			throws DbException {
+		String baseUrl = "http://" + ownOnion + ".onion"; // TODO
+		MailboxProperties properties = new MailboxProperties(baseUrl,
 				new MailboxAuthToken(crypto.generateUniqueId().getBytes()),
+				serverSupports,
 				new MailboxFolderId(crypto.generateUniqueId().getBytes()),
 				new MailboxFolderId(crypto.generateUniqueId().getBytes()));
+		MailboxUpdate u =
+				new MailboxUpdateWithMailbox(clientSupports, properties);
 		Group g = getContactGroup(c);
-		storeMessageReplaceLatest(txn, g.getId(), p);
+		storeMessageReplaceLatest(txn, g.getId(), u);
 	}
 
 	/**
-	 * Sends an empty update message to the given contact. The empty update
+	 * Sends an update message with empty properties to the given contact. The
-	 * indicates for the receiving contact that we no longer have a Mailbox that
+	 * empty update indicates for the receiving contact that we don't have any
-	 * they can use.
+	 * Mailbox that they can use. It still includes the list of Mailbox API
+	 * version(s) that we support as a client.
 	 */
-	private void sendEmptyProperties(Transaction txn, Contact c)
+	private void sendUpdateNoMailbox(Transaction txn, Contact c)
 			throws DbException {
 		Group g = getContactGroup(c);
-		storeMessageReplaceLatest(txn, g.getId(), null);
+		MailboxUpdate u = new MailboxUpdate(clientSupports);
+		storeMessageReplaceLatest(txn, g.getId(), u);
 	}
 
 	@Nullable
-	private MailboxPropertiesUpdate getProperties(Transaction txn,
+	private MailboxUpdate getUpdate(Transaction txn, Contact c, boolean local)
-			Contact c, boolean local) throws DbException {
+			throws DbException {
-		MailboxPropertiesUpdate p = null;
+		MailboxUpdate u = null;
 		Group g = getContactGroup(c);
 		try {
 			LatestUpdate latest = findLatest(txn, g.getId(), local);
 			if (latest != null) {
 				BdfList body =
 						clientHelper.getMessageAsList(txn, latest.messageId);
-				p = parseProperties(body);
+				u = parseUpdate(body);
 			}
 		} catch (FormatException e) {
 			throw new DbException(e);
 		}
-		return p;
+		return u;
 	}
 
 	private void storeMessageReplaceLatest(Transaction txn, GroupId g,
-			@Nullable MailboxPropertiesUpdate p) throws DbException {
+			MailboxUpdate u) throws DbException {
 		try {
 			LatestUpdate latest = findLatest(txn, g, true);
 			long version = latest == null ? 1 : latest.version + 1;
 			Message m = clientHelper.createMessage(g, clock.currentTimeMillis(),
-					encodeProperties(version, p));
+					encodeProperties(version, u));
 			BdfDictionary meta = new BdfDictionary();
 			meta.put(MSG_KEY_VERSION, version);
 			meta.put(MSG_KEY_LOCAL, true);
@@ -266,23 +323,36 @@ class MailboxPropertyManagerImpl implements MailboxPropertyManager,
 		return null;
 	}
 
-	private BdfList encodeProperties(long version,
+	private BdfList encodeProperties(long version, MailboxUpdate u) {
-			@Nullable MailboxPropertiesUpdate p) {
 		BdfDictionary dict = new BdfDictionary();
-		if (p != null) {
+		BdfList serverSupports = new BdfList();
-			dict.put(PROP_KEY_ONION, p.getOnion());
+		if (u.hasMailbox()) {
-			dict.put(PROP_KEY_AUTHTOKEN, p.getAuthToken().getBytes());
+			MailboxUpdateWithMailbox um = (MailboxUpdateWithMailbox) u;
-			dict.put(PROP_KEY_INBOXID, p.getInboxId().getBytes());
+			MailboxProperties properties = um.getMailboxProperties();
-			dict.put(PROP_KEY_OUTBOXID, p.getOutboxId().getBytes());
+			serverSupports = encodeSupportsList(properties.getServerSupports());
+			dict.put(PROP_KEY_ONION, properties.getOnion());
+			dict.put(PROP_KEY_AUTHTOKEN, properties.getAuthToken());
+			dict.put(PROP_KEY_INBOXID, properties.getInboxId());
+			dict.put(PROP_KEY_OUTBOXID, properties.getOutboxId());
 		}
-		return BdfList.of(version, dict);
+		return BdfList.of(version, encodeSupportsList(u.getClientSupports()),
+				serverSupports, dict);
 	}
 
-	@Nullable
+	private BdfList encodeSupportsList(List<MailboxVersion> supportsList) {
-	private MailboxPropertiesUpdate parseProperties(BdfList body)
+		BdfList supports = new BdfList();
-			throws FormatException {
+		for (MailboxVersion version : supportsList) {
-		BdfDictionary dict = body.getDictionary(1);
+			supports.add(BdfList.of(version.getMajor(), version.getMinor()));
-		return clientHelper.parseAndValidateMailboxPropertiesUpdate(dict);
+		}
+		return supports;
+	}
+
+	private MailboxUpdate parseUpdate(BdfList body) throws FormatException {
+		BdfList clientSupports = body.getList(1);
+		BdfList serverSupports = body.getList(2);
+		BdfDictionary dict = body.getDictionary(3);
+		return clientHelper.parseAndValidateMailboxUpdate(clientSupports,
+				serverSupports, dict);
 	}
 
 	private Group getContactGroup(Contact c) {

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxUpdateValidator.java

@@ -15,15 +15,15 @@ import org.briarproject.bramble.api.system.Clock;
 
 import javax.annotation.concurrent.Immutable;
 
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.MSG_KEY_LOCAL;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.MSG_KEY_LOCAL;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.MSG_KEY_VERSION;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.MSG_KEY_VERSION;
 import static org.briarproject.bramble.util.ValidationUtils.checkSize;
 
 @Immutable
 @NotNullByDefault
-class MailboxPropertyValidator extends BdfMessageValidator {
+class MailboxUpdateValidator extends BdfMessageValidator {
 
-	MailboxPropertyValidator(ClientHelper clientHelper,
+	MailboxUpdateValidator(ClientHelper clientHelper,
 			MetadataEncoder metadataEncoder, Clock clock) {
 		super(clientHelper, metadataEncoder, clock);
 	}
@@ -31,14 +31,19 @@ class MailboxPropertyValidator extends BdfMessageValidator {
 	@Override
 	protected BdfMessageContext validateMessage(Message m, Group g,
 			BdfList body) throws InvalidMessageException, FormatException {
-		// Version, properties
+		// Version, Properties, clientSupports, serverSupports
-		checkSize(body, 2);
+		checkSize(body, 4);
 		// Version
 		long version = body.getLong(0);
 		if (version < 0) throw new FormatException();
+		// clientSupports
+		BdfList clientSupports = body.getList(1);
+		// serverSupports
+		BdfList serverSupports = body.getList(2);
 		// Properties
-		BdfDictionary dictionary = body.getDictionary(1);
+		BdfDictionary dictionary = body.getDictionary(3);
-		clientHelper.parseAndValidateMailboxPropertiesUpdate(dictionary);
+		clientHelper.parseAndValidateMailboxUpdate(clientSupports,
+				serverSupports, dictionary);
 		// Return the metadata
 		BdfDictionary meta = new BdfDictionary();
 		meta.put(MSG_KEY_VERSION, version);

bramble-core/src/main/java/org/briarproject/bramble/mailbox/OwnMailboxConnectivityChecker.java

@@ -0,0 +1,75 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.TransactionManager;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+
+import java.io.IOException;
+import java.util.logging.Logger;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+@ThreadSafe
+@NotNullByDefault
+class OwnMailboxConnectivityChecker extends ConnectivityCheckerImpl {
+
+	private static final Logger LOG =
+			getLogger(OwnMailboxConnectivityChecker.class.getName());
+
+	private final MailboxApi mailboxApi;
+	private final TransactionManager db;
+	private final MailboxSettingsManager mailboxSettingsManager;
+
+	OwnMailboxConnectivityChecker(Clock clock,
+			MailboxApiCaller mailboxApiCaller,
+			MailboxApi mailboxApi,
+			TransactionManager db,
+			MailboxSettingsManager mailboxSettingsManager) {
+		super(clock, mailboxApiCaller);
+		this.mailboxApi = mailboxApi;
+		this.db = db;
+		this.mailboxSettingsManager = mailboxSettingsManager;
+	}
+
+	@Override
+	ApiCall createConnectivityCheckTask(MailboxProperties properties) {
+		if (!properties.isOwner()) throw new IllegalArgumentException();
+		return () -> {
+			try {
+				return checkConnectivityAndStoreResult(properties);
+			} catch (DbException e) {
+				logException(LOG, WARNING, e);
+				return true; // Retry
+			}
+		};
+	}
+
+	private boolean checkConnectivityAndStoreResult(
+			MailboxProperties properties) throws DbException {
+		try {
+			if (!mailboxApi.checkStatus(properties)) throw new ApiException();
+			LOG.info("Own mailbox is reachable");
+			long now = clock.currentTimeMillis();
+			db.transaction(false, txn -> mailboxSettingsManager
+					.recordSuccessfulConnection(txn, now));
+			// Call the observers and cache the result
+			onConnectivityCheckSucceeded(now);
+			return false; // Don't retry
+		} catch (IOException | ApiException e) {
+			LOG.warning("Own mailbox is unreachable");
+			logException(LOG, WARNING, e);
+			long now = clock.currentTimeMillis();
+			db.transaction(false, txn -> mailboxSettingsManager
+					.recordFailedConnectionAttempt(txn, now));
+		}
+		return true; // Retry
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/SimpleApiCall.java

@@ -0,0 +1,39 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+import org.briarproject.bramble.mailbox.MailboxApi.TolerableFailureException;
+
+import java.io.IOException;
+import java.util.logging.Logger;
+
+import static java.util.logging.Level.INFO;
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+/**
+ * Convenience class for making simple API calls that don't return values.
+ */
+@NotNullByDefault
+public abstract class SimpleApiCall implements ApiCall {
+
+	private static final Logger LOG = getLogger(SimpleApiCall.class.getName());
+
+	abstract void tryToCallApi()
+			throws IOException, ApiException, TolerableFailureException;
+
+	@Override
+	public boolean callApi() {
+		try {
+			tryToCallApi();
+			return false; // Succeeded, don't retry
+		} catch (IOException | ApiException e) {
+			logException(LOG, WARNING, e);
+			return true; // Failed, retry with backoff
+		} catch (TolerableFailureException e) {
+			logException(LOG, INFO, e);
+			return false; // Failed tolerably, don't retry
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/plugin/PollerImpl.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.plugin;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.Pair;
 import org.briarproject.bramble.api.connection.ConnectionManager;
 import org.briarproject.bramble.api.connection.ConnectionRegistry;
@@ -27,7 +28,6 @@ import org.briarproject.bramble.api.properties.TransportProperties;
 import org.briarproject.bramble.api.properties.TransportPropertyManager;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.system.TaskScheduler;
-import org.briarproject.bramble.api.system.TaskScheduler.Cancellable;
 import org.briarproject.bramble.api.system.Wakeful;
 import org.briarproject.bramble.api.system.WakefulIoExecutor;
 

bramble-core/src/main/java/org/briarproject/bramble/plugin/tor/TorPlugin.java

@@ -2,6 +2,7 @@ package org.briarproject.bramble.plugin.tor;
 
 import net.freehaven.tor.control.EventHandler;
 import net.freehaven.tor.control.TorControlConnection;
+import net.freehaven.tor.control.TorNotRunningException;
 
 import org.briarproject.bramble.PoliteExecutor;
 import org.briarproject.bramble.api.Pair;
@@ -321,6 +322,8 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 				LOG.info("Tor has already built a circuit");
 				state.getAndSetCircuitBuilt(true);
 			}
+		} catch (TorNotRunningException e) {
+			throw new RuntimeException(e);
 		} catch (IOException e) {
 			throw new PluginException(e);
 		}
@@ -492,6 +495,8 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 			} else {
 				response = controlConnection.addOnion(privKey, portLines);
 			}
+		} catch (TorNotRunningException e) {
+			throw new RuntimeException(e);
 		} catch (IOException e) {
 			logException(LOG, WARNING, e);
 			return;
@@ -540,30 +545,38 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 
 	protected void enableNetwork(boolean enable) throws IOException {
 		state.enableNetwork(enable);
-		controlConnection.setConf("DisableNetwork", enable ? "0" : "1");
+		try {
+			controlConnection.setConf("DisableNetwork", enable ? "0" : "1");
+		} catch (TorNotRunningException e) {
+			throw new RuntimeException(e);
+		}
 	}
 
 	private void enableBridges(boolean enable, List<BridgeType> bridgeTypes)
 			throws IOException {
-		if (enable) {
+		try {
-			Collection<String> conf = new ArrayList<>();
+			if (enable) {
-			conf.add("UseBridges 1");
+				Collection<String> conf = new ArrayList<>();
-			File obfs4File = getObfs4ExecutableFile();
+				conf.add("UseBridges 1");
-			if (bridgeTypes.contains(MEEK)) {
+				File obfs4File = getObfs4ExecutableFile();
-				conf.add("ClientTransportPlugin meek_lite exec " +
+				if (bridgeTypes.contains(MEEK)) {
-						obfs4File.getAbsolutePath());
+					conf.add("ClientTransportPlugin meek_lite exec " +
+							obfs4File.getAbsolutePath());
+				}
+				if (bridgeTypes.contains(DEFAULT_OBFS4) ||
+						bridgeTypes.contains(NON_DEFAULT_OBFS4)) {
+					conf.add("ClientTransportPlugin obfs4 exec " +
+							obfs4File.getAbsolutePath());
+				}
+				for (BridgeType bridgeType : bridgeTypes) {
+					conf.addAll(circumventionProvider.getBridges(bridgeType));
+				}
+				controlConnection.setConf(conf);
+			} else {
+				controlConnection.setConf("UseBridges", "0");
 			}
-			if (bridgeTypes.contains(DEFAULT_OBFS4) ||
+		} catch (TorNotRunningException e) {
-					bridgeTypes.contains(NON_DEFAULT_OBFS4)) {
+			throw new RuntimeException(e);
-				conf.add("ClientTransportPlugin obfs4 exec " +
-						obfs4File.getAbsolutePath());
-			}
-			for (BridgeType bridgeType : bridgeTypes) {
-				conf.addAll(circumventionProvider.getBridges(bridgeType));
-			}
-			controlConnection.setConf(conf);
-		} else {
-			controlConnection.setConf("UseBridges", "0");
 		}
 	}
 
@@ -577,6 +590,8 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 				controlConnection.setConf("DisableNetwork", "1");
 				controlConnection.shutdownTor("TERM");
 				controlSocket.close();
+			} catch (TorNotRunningException e) {
+				throw new RuntimeException(e);
 			} catch (IOException e) {
 				logException(LOG, WARNING, e);
 			}
@@ -708,7 +723,11 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 			});
 			Map<Integer, String> portLines =
 					singletonMap(80, "127.0.0.1:" + port);
-			controlConnection.addOnion(blob, portLines);
+			try {
+				controlConnection.addOnion(blob, portLines);
+			} catch (TorNotRunningException e) {
+				throw new RuntimeException(e);
+			}
 			return new RendezvousEndpoint() {
 
 				@Override
@@ -718,7 +737,11 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 
 				@Override
 				public void close() throws IOException {
-					controlConnection.delOnion(localOnion);
+					try {
+						controlConnection.delOnion(localOnion);
+					} catch (TorNotRunningException e) {
+						throw new RuntimeException(e);
+					}
 					tryToClose(ss, LOG, WARNING);
 				}
 			};
@@ -746,18 +769,8 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 	public void orConnStatus(String status, String orName) {
 		if (LOG.isLoggable(INFO)) LOG.info("OR connection " + status);
 
-		//noinspection IfCanBeSwitch
+		if (status.equals("CONNECTED")) state.onOrConnectionConnected();
-		if (status.equals("LAUNCHED")) state.onOrConnectionLaunched();
-		else if (status.equals("FAILED")) state.onOrConnectionFailed();
-		else if (status.equals("CONNECTED")) state.onOrConnectionConnected();
 		else if (status.equals("CLOSED")) state.onOrConnectionClosed();
-
-		if ((status.equals("FAILED") || status.equals("CLOSED")) &&
-				state.getNumOrConnections() == 0) {
-			// Check whether we've lost connectivity
-			updateConnectionStatus(networkManager.getNetworkStatus(),
-					batteryManager.isCharging());
-		}
 	}
 
 	@Override
@@ -771,9 +784,6 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 	@Override
 	public void message(String severity, String msg) {
 		if (LOG.isLoggable(INFO)) LOG.info(severity + " " + msg);
-		if (msg.startsWith("Switching to guard context")) {
-			state.onSwitchingGuardContext();
-		}
 	}
 
 	@Override
@@ -855,11 +865,6 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 			if (s.getNamespace().equals(ID.getString())) {
 				LOG.info("Tor settings updated");
 				settings = s.getSettings();
-				// Works around a bug introduced in Tor 0.3.4.8.
-				// https://trac.torproject.org/projects/tor/ticket/28027
-				// Could be replaced with callback.transportDisabled()
-				// when fixed.
-				disableNetwork();
 				updateConnectionStatus(networkManager.getNetworkStatus(),
 						batteryManager.isCharging());
 			}
@@ -872,16 +877,6 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 		}
 	}
 
-	private void disableNetwork() {
-		connectionStatusExecutor.execute(() -> {
-			try {
-				if (state.isTorRunning()) enableNetwork(false);
-			} catch (IOException ex) {
-				logException(LOG, WARNING, ex);
-			}
-		});
-	}
-
 	private void updateConnectionStatus(NetworkStatus status,
 			boolean charging) {
 		connectionStatusExecutor.execute(() -> {
@@ -978,12 +973,20 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 	}
 
 	private void enableConnectionPadding(boolean enable) throws IOException {
-		controlConnection.setConf("ConnectionPadding", enable ? "1" : "0");
+		try {
+			controlConnection.setConf("ConnectionPadding", enable ? "1" : "0");
+		} catch (TorNotRunningException e) {
+			throw new RuntimeException(e);
+		}
 	}
 
 	private void useIpv6(boolean ipv6Only) throws IOException {
-		controlConnection.setConf("ClientUseIPv4", ipv6Only ? "0" : "1");
+		try {
-		controlConnection.setConf("ClientUseIPv6", ipv6Only ? "1" : "0");
+			controlConnection.setConf("ClientUseIPv4", ipv6Only ? "0" : "1");
+			controlConnection.setConf("ClientUseIPv6", ipv6Only ? "1" : "0");
+		} catch (TorNotRunningException e) {
+			throw new RuntimeException(e);
+		}
 	}
 
 	@ThreadSafe
@@ -1007,13 +1010,14 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 		private ServerSocket serverSocket = null;
 
 		@GuardedBy("this")
-		private int orConnectionsPending = 0, orConnectionsConnected = 0;
+		private int orConnectionsConnected = 0;
 
 		private synchronized void setStarted() {
 			started = true;
 			callback.pluginStateChanged(getState());
 		}
 
+		@SuppressWarnings("BooleanMethodIsAlwaysInverted")
 		private synchronized boolean isTorRunning() {
 			return started && !stopped;
 		}
@@ -1071,63 +1075,38 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 			if (reasonsDisabled != 0) return DISABLED;
 			if (!networkInitialised) return ENABLING;
 			if (!networkEnabled) return INACTIVE;
-			return bootstrapped && circuitBuilt ? ACTIVE : ENABLING;
+			return bootstrapped && circuitBuilt && orConnectionsConnected > 0
+					? ACTIVE : ENABLING;
 		}
 
 		private synchronized int getReasonsDisabled() {
 			return getState() == DISABLED ? reasonsDisabled : 0;
 		}
 
-		private synchronized void onOrConnectionLaunched() {
-			orConnectionsPending++;
-			logOrConnections();
-		}
-
-		private synchronized void onOrConnectionFailed() {
-			orConnectionsPending--;
-			if (orConnectionsPending < 0) {
-				LOG.warning("Count was zero before connection failed");
-				orConnectionsPending = 0;
-			}
-			logOrConnections();
-		}
-
 		private synchronized void onOrConnectionConnected() {
-			orConnectionsPending--;
+			int oldConnected = orConnectionsConnected;
-			if (orConnectionsPending < 0) {
-				LOG.warning("Count was zero before connection connected");
-				orConnectionsPending = 0;
-			}
 			orConnectionsConnected++;
 			logOrConnections();
+			if (oldConnected == 0) callback.pluginStateChanged(getState());
 		}
 
 		private synchronized void onOrConnectionClosed() {
+			int oldConnected = orConnectionsConnected;
 			orConnectionsConnected--;
 			if (orConnectionsConnected < 0) {
 				LOG.warning("Count was zero before connection closed");
 				orConnectionsConnected = 0;
 			}
 			logOrConnections();
-		}
+			if (orConnectionsConnected == 0 && oldConnected != 0) {
-
+				callback.pluginStateChanged(getState());
-		private synchronized void onSwitchingGuardContext() {
+			}
-			// Tor doesn't seem to report events for connections belonging to
-			// the old guard context, so we have to reset the counters
-			orConnectionsPending = 0;
-			orConnectionsConnected = 0;
-			logOrConnections();
-		}
-
-		private synchronized int getNumOrConnections() {
-			return orConnectionsPending + orConnectionsConnected;
 		}
 
 		@GuardedBy("this")
 		private void logOrConnections() {
 			if (LOG.isLoggable(INFO)) {
-				LOG.info("OR connections: " + orConnectionsPending
+				LOG.info(orConnectionsConnected + " OR connections connected");
-						+ " pending, " + orConnectionsConnected + " connected");
 			}
 		}
 	}

bramble-core/src/main/java/org/briarproject/bramble/plugin/tor/TorPluginFactory.java

@@ -0,0 +1,134 @@
+package org.briarproject.bramble.plugin.tor;
+
+import org.briarproject.bramble.api.battery.BatteryManager;
+import org.briarproject.bramble.api.crypto.CryptoComponent;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.network.NetworkManager;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.Backoff;
+import org.briarproject.bramble.api.plugin.BackoffFactory;
+import org.briarproject.bramble.api.plugin.PluginCallback;
+import org.briarproject.bramble.api.plugin.TorConstants;
+import org.briarproject.bramble.api.plugin.TorControlPort;
+import org.briarproject.bramble.api.plugin.TorDirectory;
+import org.briarproject.bramble.api.plugin.TorSocksPort;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.plugin.duplex.DuplexPlugin;
+import org.briarproject.bramble.api.plugin.duplex.DuplexPluginFactory;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.api.system.LocationUtils;
+import org.briarproject.bramble.api.system.ResourceProvider;
+import org.briarproject.bramble.api.system.WakefulIoExecutor;
+
+import java.io.File;
+import java.util.concurrent.Executor;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+import javax.net.SocketFactory;
+
+import static java.util.logging.Level.INFO;
+import static java.util.logging.Logger.getLogger;
+
+@Immutable
+@NotNullByDefault
+abstract class TorPluginFactory implements DuplexPluginFactory {
+
+	protected static final Logger LOG =
+			getLogger(TorPluginFactory.class.getName());
+
+	protected static final int MAX_LATENCY = 30 * 1000; // 30 seconds
+	protected static final int MAX_IDLE_TIME = 30 * 1000; // 30 seconds
+	private static final int MIN_POLLING_INTERVAL = 60 * 1000; // 1 minute
+	private static final int MAX_POLLING_INTERVAL = 10 * 60 * 1000; // 10 mins
+	private static final double BACKOFF_BASE = 1.2;
+
+	protected final Executor ioExecutor, wakefulIoExecutor;
+	protected final NetworkManager networkManager;
+	protected final LocationUtils locationUtils;
+	protected final EventBus eventBus;
+	protected final SocketFactory torSocketFactory;
+	protected final BackoffFactory backoffFactory;
+	protected final ResourceProvider resourceProvider;
+	protected final CircumventionProvider circumventionProvider;
+	protected final BatteryManager batteryManager;
+	protected final Clock clock;
+	protected final CryptoComponent crypto;
+	protected final File torDirectory;
+	protected final int torSocksPort;
+	protected final int torControlPort;
+
+	TorPluginFactory(@IoExecutor Executor ioExecutor,
+			@WakefulIoExecutor Executor wakefulIoExecutor,
+			NetworkManager networkManager,
+			LocationUtils locationUtils,
+			EventBus eventBus,
+			SocketFactory torSocketFactory,
+			BackoffFactory backoffFactory,
+			ResourceProvider resourceProvider,
+			CircumventionProvider circumventionProvider,
+			BatteryManager batteryManager,
+			Clock clock,
+			CryptoComponent crypto,
+			@TorDirectory File torDirectory,
+			@TorSocksPort int torSocksPort,
+			@TorControlPort int torControlPort) {
+		this.ioExecutor = ioExecutor;
+		this.wakefulIoExecutor = wakefulIoExecutor;
+		this.networkManager = networkManager;
+		this.locationUtils = locationUtils;
+		this.eventBus = eventBus;
+		this.torSocketFactory = torSocketFactory;
+		this.backoffFactory = backoffFactory;
+		this.resourceProvider = resourceProvider;
+		this.circumventionProvider = circumventionProvider;
+		this.batteryManager = batteryManager;
+		this.clock = clock;
+		this.crypto = crypto;
+		this.torDirectory = torDirectory;
+		this.torSocksPort = torSocksPort;
+		this.torControlPort = torControlPort;
+	}
+
+	@Nullable
+	abstract String getArchitectureForTorBinary();
+
+	abstract TorPlugin createPluginInstance(Backoff backoff,
+			TorRendezvousCrypto torRendezvousCrypto, PluginCallback callback,
+			String architecture);
+
+	@Override
+	public TransportId getId() {
+		return TorConstants.ID;
+	}
+
+	@Override
+	public long getMaxLatency() {
+		return MAX_LATENCY;
+	}
+
+	@Override
+	public DuplexPlugin createPlugin(PluginCallback callback) {
+		// Check that we have a Tor binary for this architecture
+		String architecture = getArchitectureForTorBinary();
+		if (architecture == null) {
+			LOG.warning("Tor is not supported on this architecture");
+			return null;
+		}
+
+		if (LOG.isLoggable(INFO)) {
+			LOG.info("The selected architecture for Tor is " + architecture);
+		}
+
+		Backoff backoff = backoffFactory.createBackoff(MIN_POLLING_INTERVAL,
+				MAX_POLLING_INTERVAL, BACKOFF_BASE);
+		TorRendezvousCrypto torRendezvousCrypto =
+				new TorRendezvousCryptoImpl(crypto);
+		TorPlugin plugin = createPluginInstance(backoff, torRendezvousCrypto,
+				callback, architecture);
+		eventBus.addListener(plugin);
+		return plugin;
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/record/RecordWriterImpl.java

@@ -19,6 +19,8 @@ class RecordWriterImpl implements RecordWriter {
 	private final OutputStream out;
 	private final byte[] header = new byte[RECORD_HEADER_BYTES];
 
+	private long bytesWritten = 0;
+
 	RecordWriterImpl(OutputStream out) {
 		this.out = out;
 	}
@@ -31,6 +33,7 @@ class RecordWriterImpl implements RecordWriter {
 		ByteUtils.writeUint16(payload.length, header, 2);
 		out.write(header);
 		out.write(payload);
+		bytesWritten += RECORD_HEADER_BYTES + payload.length;
 	}
 
 	@Override
@@ -42,4 +45,9 @@ class RecordWriterImpl implements RecordWriter {
 	public void close() throws IOException {
 		out.close();
 	}
+
+	@Override
+	public long getBytesWritten() {
+		return bytesWritten;
+	}
 }

bramble-core/src/main/java/org/briarproject/bramble/rendezvous/RendezvousPollerImpl.java

@@ -1,6 +1,7 @@
 package org.briarproject.bramble.rendezvous;
 
 import org.briarproject.bramble.PoliteExecutor;
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.Pair;
 import org.briarproject.bramble.api.connection.ConnectionManager;
 import org.briarproject.bramble.api.contact.PendingContact;
@@ -42,7 +43,6 @@ import org.briarproject.bramble.api.rendezvous.event.RendezvousConnectionOpenedE
 import org.briarproject.bramble.api.rendezvous.event.RendezvousPollEvent;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.system.TaskScheduler;
-import org.briarproject.bramble.api.system.TaskScheduler.Cancellable;
 import org.briarproject.bramble.api.system.Wakeful;
 
 import java.security.GeneralSecurityException;

bramble-core/src/main/java/org/briarproject/bramble/socks/SocksModule.java

@@ -10,6 +10,7 @@ import dagger.Module;
 import dagger.Provides;
 
 import static org.briarproject.bramble.api.plugin.TorConstants.CONNECT_TO_PROXY_TIMEOUT;
+import static org.briarproject.bramble.api.plugin.TorConstants.EXTRA_CONNECT_TIMEOUT;
 import static org.briarproject.bramble.api.plugin.TorConstants.EXTRA_SOCKET_TIMEOUT;
 
 @Module
@@ -20,6 +21,6 @@ public class SocksModule {
 		InetSocketAddress proxy = new InetSocketAddress("127.0.0.1",
 				torSocksPort);
 		return new SocksSocketFactory(proxy, CONNECT_TO_PROXY_TIMEOUT,
-				EXTRA_SOCKET_TIMEOUT);
+				EXTRA_CONNECT_TIMEOUT, EXTRA_SOCKET_TIMEOUT);
 	}
 }

bramble-core/src/main/java/org/briarproject/bramble/socks/SocksSocket.java

@@ -26,15 +26,18 @@ class SocksSocket extends Socket {
 			"Address type not supported"
 	};
 
+	@SuppressWarnings("MismatchedReadAndWriteOfArray")
 	private static final byte[] UNSPECIFIED_ADDRESS = new byte[4];
 
 	private final SocketAddress proxy;
-	private final int connectToProxyTimeout, extraSocketTimeout;
+	private final int connectToProxyTimeout;
+	private final int extraConnectTimeout, extraSocketTimeout;
 
 	SocksSocket(SocketAddress proxy, int connectToProxyTimeout,
-			int extraSocketTimeout) {
+			int extraConnectTimeout, int extraSocketTimeout) {
 		this.proxy = proxy;
 		this.connectToProxyTimeout = connectToProxyTimeout;
+		this.extraConnectTimeout = extraConnectTimeout;
 		this.extraSocketTimeout = extraSocketTimeout;
 	}
 
@@ -66,7 +69,7 @@ class SocksSocket extends Socket {
 
 		// Use the supplied timeout temporarily, plus any configured extra
 		int oldTimeout = getSoTimeout();
-		setSoTimeout(timeout + extraSocketTimeout);
+		setSoTimeout(timeout + extraConnectTimeout);
 
 		// Connect to the endpoint via the proxy
 		sendConnectRequest(out, host, port);

bramble-core/src/main/java/org/briarproject/bramble/socks/SocksSocketFactory.java

@@ -11,18 +11,21 @@ import javax.net.SocketFactory;
 class SocksSocketFactory extends SocketFactory {
 
 	private final SocketAddress proxy;
-	private final int connectToProxyTimeout, extraSocketTimeout;
+	private final int connectToProxyTimeout;
+	private final int extraConnectTimeout, extraSocketTimeout;
 
 	SocksSocketFactory(SocketAddress proxy, int connectToProxyTimeout,
-			int extraSocketTimeout) {
+			int extraConnectTimeout, int extraSocketTimeout) {
 		this.proxy = proxy;
 		this.connectToProxyTimeout = connectToProxyTimeout;
+		this.extraConnectTimeout = extraConnectTimeout;
 		this.extraSocketTimeout = extraSocketTimeout;
 	}
 
 	@Override
 	public Socket createSocket() {
-		return new SocksSocket(proxy, connectToProxyTimeout, extraSocketTimeout);
+		return new SocksSocket(proxy, connectToProxyTimeout,
+				extraConnectTimeout, extraSocketTimeout);
 	}
 
 	@Override

bramble-core/src/main/java/org/briarproject/bramble/sync/DuplexOutgoingSession.java

@@ -13,11 +13,13 @@ import org.briarproject.bramble.api.lifecycle.event.LifecycleEvent;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.plugin.event.TransportInactiveEvent;
+import org.briarproject.bramble.api.record.Record;
 import org.briarproject.bramble.api.sync.Ack;
 import org.briarproject.bramble.api.sync.Message;
 import org.briarproject.bramble.api.sync.Offer;
 import org.briarproject.bramble.api.sync.Priority;
 import org.briarproject.bramble.api.sync.Request;
+import org.briarproject.bramble.api.sync.SyncConstants;
 import org.briarproject.bramble.api.sync.SyncRecordWriter;
 import org.briarproject.bramble.api.sync.SyncSession;
 import org.briarproject.bramble.api.sync.Versions;
@@ -47,8 +49,9 @@ import static java.util.logging.Level.INFO;
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.STOPPING;
-import static org.briarproject.bramble.api.record.Record.MAX_RECORD_PAYLOAD_BYTES;
+import static org.briarproject.bramble.api.record.Record.RECORD_HEADER_BYTES;
 import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_IDS;
+import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_LENGTH;
 import static org.briarproject.bramble.api.sync.SyncConstants.SUPPORTED_VERSIONS;
 import static org.briarproject.bramble.util.LogUtils.logException;
 
@@ -71,6 +74,16 @@ class DuplexOutgoingSession implements SyncSession, EventListener {
 			NEXT_SEND_TIME_DECREASED = () -> {
 	};
 
+	/**
+	 * The batch capacity must be at least {@link Record#RECORD_HEADER_BYTES}
+	 * + {@link SyncConstants#MAX_MESSAGE_LENGTH} to ensure that maximum-size
+	 * messages can be selected for transmission. Larger batches will mean
+	 * fewer round-trips between the DB and the output stream, but each
+	 * round-trip will block the DB for longer.
+	 */
+	private static final int BATCH_CAPACITY =
+			(RECORD_HEADER_BYTES + MAX_MESSAGE_LENGTH) * 2;
+
 	private final DatabaseComponent db;
 	private final Executor dbExecutor;
 	private final EventBus eventBus;
@@ -296,8 +309,7 @@ class DuplexOutgoingSession implements SyncSession, EventListener {
 						db.transactionWithNullableResult(false, txn -> {
 							Collection<Message> batch =
 									db.generateRequestedBatch(txn, contactId,
-											MAX_RECORD_PAYLOAD_BYTES,
+											BATCH_CAPACITY, maxLatency);
-											maxLatency);
 							setNextSendTime(db.getNextSendTime(txn, contactId));
 							return batch;
 						});

bramble-core/src/main/java/org/briarproject/bramble/sync/EagerSimplexOutgoingSession.java

@@ -0,0 +1,66 @@
+package org.briarproject.bramble.sync;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.sync.SyncRecordWriter;
+import org.briarproject.bramble.api.transport.StreamWriter;
+
+import java.io.IOException;
+import java.util.Collection;
+import java.util.logging.Logger;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.logging.Level.INFO;
+import static java.util.logging.Logger.getLogger;
+
+/**
+ * A {@link SimplexOutgoingSession} that sends messages eagerly, ie
+ * regardless of whether they're due for retransmission.
+ */
+@ThreadSafe
+@NotNullByDefault
+class EagerSimplexOutgoingSession extends SimplexOutgoingSession {
+
+	private static final Logger LOG =
+			getLogger(EagerSimplexOutgoingSession.class.getName());
+
+	EagerSimplexOutgoingSession(DatabaseComponent db,
+			EventBus eventBus,
+			ContactId contactId,
+			TransportId transportId,
+			long maxLatency,
+			StreamWriter streamWriter,
+			SyncRecordWriter recordWriter) {
+		super(db, eventBus, contactId, transportId, maxLatency, streamWriter,
+				recordWriter);
+	}
+
+	@Override
+	void sendMessages() throws DbException, IOException {
+		for (MessageId m : loadUnackedMessageIdsToSend()) {
+			if (isInterrupted()) break;
+			Message message = db.transactionWithNullableResult(false, txn ->
+					db.getMessageToSend(txn, contactId, m, maxLatency, true));
+			if (message == null) continue; // No longer shared
+			recordWriter.writeMessage(message);
+			LOG.info("Sent message");
+		}
+	}
+
+	private Collection<MessageId> loadUnackedMessageIdsToSend()
+			throws DbException {
+		Collection<MessageId> ids = db.transactionWithResult(true, txn ->
+				db.getUnackedMessagesToSend(txn, contactId));
+		if (LOG.isLoggable(INFO)) {
+			LOG.info(ids.size() + " unacked messages to send");
+		}
+		return ids;
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/sync/MailboxOutgoingSession.java

@@ -0,0 +1,117 @@
+package org.briarproject.bramble.sync;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.sync.Ack;
+import org.briarproject.bramble.api.sync.DeferredSendHandler;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.sync.SyncRecordWriter;
+import org.briarproject.bramble.api.transport.StreamWriter;
+
+import java.io.IOException;
+import java.util.Collection;
+import java.util.logging.Logger;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.lang.Math.min;
+import static java.util.Collections.emptyList;
+import static java.util.logging.Level.INFO;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.api.record.Record.RECORD_HEADER_BYTES;
+import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_IDS;
+import static org.briarproject.bramble.api.sync.SyncConstants.MESSAGE_HEADER_LENGTH;
+
+/**
+ * A {@link SimplexOutgoingSession} for sending and acking messages via a
+ * mailbox. The session uses a {@link DeferredSendHandler} to record the IDs
+ * of the messages sent and acked during the session so that they can be
+ * recorded in the DB as sent or acked after the file has been successfully
+ * uploaded to the mailbox.
+ */
+@ThreadSafe
+@NotNullByDefault
+class MailboxOutgoingSession extends SimplexOutgoingSession {
+
+	private static final Logger LOG =
+			getLogger(MailboxOutgoingSession.class.getName());
+
+	private final DeferredSendHandler deferredSendHandler;
+	private final long initialCapacity;
+
+	MailboxOutgoingSession(DatabaseComponent db,
+			EventBus eventBus,
+			ContactId contactId,
+			TransportId transportId,
+			long maxLatency,
+			StreamWriter streamWriter,
+			SyncRecordWriter recordWriter,
+			DeferredSendHandler deferredSendHandler,
+			long capacity) {
+		super(db, eventBus, contactId, transportId, maxLatency, streamWriter,
+				recordWriter);
+		this.deferredSendHandler = deferredSendHandler;
+		this.initialCapacity = capacity;
+	}
+
+	@Override
+	void sendAcks() throws DbException, IOException {
+		while (!isInterrupted()) {
+			Collection<MessageId> idsToAck = loadMessageIdsToAck();
+			if (idsToAck.isEmpty()) break;
+			recordWriter.writeAck(new Ack(idsToAck));
+			deferredSendHandler.onAckSent(idsToAck);
+			LOG.info("Sent ack");
+		}
+	}
+
+	private Collection<MessageId> loadMessageIdsToAck() throws DbException {
+		long idCapacity = (getRemainingCapacity() - RECORD_HEADER_BYTES)
+				/ MessageId.LENGTH;
+		if (idCapacity <= 0) return emptyList(); // Out of capacity
+		int maxMessageIds = (int) min(idCapacity, MAX_MESSAGE_IDS);
+		Collection<MessageId> ids = db.transactionWithResult(true, txn ->
+				db.getMessagesToAck(txn, contactId, maxMessageIds));
+		if (LOG.isLoggable(INFO)) {
+			LOG.info(ids.size() + " messages to ack");
+		}
+		return ids;
+	}
+
+	private long getRemainingCapacity() {
+		return initialCapacity - recordWriter.getBytesWritten();
+	}
+
+	@Override
+	void sendMessages() throws DbException, IOException {
+		for (MessageId m : loadMessageIdsToSend()) {
+			if (isInterrupted()) break;
+			// Defer marking the message as sent
+			Message message = db.transactionWithNullableResult(true, txn ->
+					db.getMessageToSend(txn, contactId, m, maxLatency, false));
+			if (message == null) continue; // No longer shared
+			recordWriter.writeMessage(message);
+			deferredSendHandler.onMessageSent(m);
+			LOG.info("Sent message");
+		}
+	}
+
+	private Collection<MessageId> loadMessageIdsToSend() throws DbException {
+		long capacity = getRemainingCapacity();
+		if (capacity < RECORD_HEADER_BYTES + MESSAGE_HEADER_LENGTH) {
+			return emptyList(); // Out of capacity
+		}
+		Collection<MessageId> ids = db.transactionWithResult(true, txn ->
+				db.getMessagesToSend(txn, contactId, capacity, maxLatency));
+		if (LOG.isLoggable(INFO)) {
+			LOG.info(ids.size() + " messages to send");
+		}
+		return ids;
+	}
+
+}

bramble-core/src/main/java/org/briarproject/bramble/sync/SimplexOutgoingSession.java

@@ -3,7 +3,6 @@ package org.briarproject.bramble.sync;
 import org.briarproject.bramble.api.contact.ContactId;
 import org.briarproject.bramble.api.contact.event.ContactRemovedEvent;
 import org.briarproject.bramble.api.db.DatabaseComponent;
-import org.briarproject.bramble.api.db.DatabaseExecutor;
 import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.event.Event;
 import org.briarproject.bramble.api.event.EventBus;
@@ -13,9 +12,10 @@ import org.briarproject.bramble.api.lifecycle.event.LifecycleEvent;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.plugin.event.TransportInactiveEvent;
+import org.briarproject.bramble.api.record.Record;
 import org.briarproject.bramble.api.sync.Ack;
 import org.briarproject.bramble.api.sync.Message;
-import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.sync.SyncConstants;
 import org.briarproject.bramble.api.sync.SyncRecordWriter;
 import org.briarproject.bramble.api.sync.SyncSession;
 import org.briarproject.bramble.api.sync.Versions;
@@ -23,15 +23,7 @@ import org.briarproject.bramble.api.sync.event.CloseSyncConnectionsEvent;
 import org.briarproject.bramble.api.transport.StreamWriter;
 
 import java.io.IOException;
-import java.util.ArrayList;
 import java.util.Collection;
-import java.util.Iterator;
-import java.util.Map;
-import java.util.Map.Entry;
-import java.util.concurrent.BlockingQueue;
-import java.util.concurrent.Executor;
-import java.util.concurrent.LinkedBlockingQueue;
-import java.util.concurrent.atomic.AtomicInteger;
 import java.util.logging.Logger;
 
 import javax.annotation.concurrent.ThreadSafe;
@@ -40,8 +32,9 @@ import static java.util.logging.Level.INFO;
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.STOPPING;
-import static org.briarproject.bramble.api.record.Record.MAX_RECORD_PAYLOAD_BYTES;
+import static org.briarproject.bramble.api.record.Record.RECORD_HEADER_BYTES;
 import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_IDS;
+import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_LENGTH;
 import static org.briarproject.bramble.api.sync.SyncConstants.SUPPORTED_VERSIONS;
 import static org.briarproject.bramble.util.LogUtils.logException;
 
@@ -57,38 +50,40 @@ class SimplexOutgoingSession implements SyncSession, EventListener {
 	private static final Logger LOG =
 			getLogger(SimplexOutgoingSession.class.getName());
 
-	private static final ThrowingRunnable<IOException> CLOSE = () -> {
+	/**
-	};
+	 * The batch capacity must be at least {@link Record#RECORD_HEADER_BYTES}
+	 * + {@link SyncConstants#MAX_MESSAGE_LENGTH} to ensure that maximum-size
+	 * messages can be selected for transmission. Larger batches will mean
+	 * fewer round-trips between the DB and the output stream, but each
+	 * round-trip will block the DB for longer.
+	 */
+	static final int BATCH_CAPACITY =
+			(RECORD_HEADER_BYTES + MAX_MESSAGE_LENGTH) * 2;
 
-	private final DatabaseComponent db;
+	protected final DatabaseComponent db;
-	private final Executor dbExecutor;
+	protected final EventBus eventBus;
-	private final EventBus eventBus;
+	protected final ContactId contactId;
-	private final ContactId contactId;
+	protected final TransportId transportId;
-	private final TransportId transportId;
+	protected final long maxLatency;
-	private final long maxLatency;
+	protected final StreamWriter streamWriter;
-	private final boolean eager;
+	protected final SyncRecordWriter recordWriter;
-	private final StreamWriter streamWriter;
-	private final SyncRecordWriter recordWriter;
-	private final AtomicInteger outstandingQueries;
-	private final BlockingQueue<ThrowingRunnable<IOException>> writerTasks;
 
 	private volatile boolean interrupted = false;
 
-	SimplexOutgoingSession(DatabaseComponent db, Executor dbExecutor,
+	SimplexOutgoingSession(DatabaseComponent db,
-			EventBus eventBus, ContactId contactId, TransportId transportId,
+			EventBus eventBus,
-			long maxLatency, boolean eager, StreamWriter streamWriter,
+			ContactId contactId,
+			TransportId transportId,
+			long maxLatency,
+			StreamWriter streamWriter,
 			SyncRecordWriter recordWriter) {
 		this.db = db;
-		this.dbExecutor = dbExecutor;
 		this.eventBus = eventBus;
 		this.contactId = contactId;
 		this.transportId = transportId;
 		this.maxLatency = maxLatency;
-		this.eager = eager;
 		this.streamWriter = streamWriter;
 		this.recordWriter = recordWriter;
-		outstandingQueries = new AtomicInteger(2); // One per type of record
-		writerTasks = new LinkedBlockingQueue<>();
 	}
 
 	@IoExecutor
@@ -98,22 +93,13 @@ class SimplexOutgoingSession implements SyncSession, EventListener {
 		try {
 			// Send our supported protocol versions
 			recordWriter.writeVersions(new Versions(SUPPORTED_VERSIONS));
-			// Start a query for each type of record
-			dbExecutor.execute(this::generateAck);
-			if (eager) dbExecutor.execute(this::loadUnackedMessageIds);
-			else dbExecutor.execute(this::generateBatch);
-			// Write records until interrupted or no more records to write
 			try {
-				while (!interrupted) {
+				sendAcks();
-					ThrowingRunnable<IOException> task = writerTasks.take();
+				sendMessages();
-					if (task == CLOSE) break;
+			} catch (DbException e) {
-					task.run();
+				logException(LOG, WARNING, e);
-				}
-				streamWriter.sendEndOfStream();
-			} catch (InterruptedException e) {
-				LOG.info("Interrupted while waiting for a record to write");
-				Thread.currentThread().interrupt();
 			}
+			streamWriter.sendEndOfStream();
 		} finally {
 			eventBus.removeListener(this);
 		}
@@ -122,11 +108,10 @@ class SimplexOutgoingSession implements SyncSession, EventListener {
 	@Override
 	public void interrupt() {
 		interrupted = true;
-		writerTasks.add(CLOSE);
 	}
 
-	private void decrementOutstandingQueries() {
+	boolean isInterrupted() {
-		if (outstandingQueries.decrementAndGet() == 0) writerTasks.add(CLOSE);
+		return interrupted;
 	}
 
 	@Override
@@ -146,110 +131,33 @@ class SimplexOutgoingSession implements SyncSession, EventListener {
 		}
 	}
 
-	@DatabaseExecutor
+	void sendAcks() throws DbException, IOException {
-	private void loadUnackedMessageIds() {
+		while (!isInterrupted()) if (!generateAndSendAck()) break;
-		if (interrupted) return;
-		try {
-			Map<MessageId, Integer> ids = db.transactionWithResult(true, txn ->
-					db.getUnackedMessagesToSend(txn, contactId));
-			if (LOG.isLoggable(INFO)) {
-				LOG.info(ids.size() + " unacked messages to send");
-			}
-			if (ids.isEmpty()) decrementOutstandingQueries();
-			else dbExecutor.execute(() -> generateEagerBatch(ids));
-		} catch (DbException e) {
-			logException(LOG, WARNING, e);
-			interrupt();
-		}
 	}
 
-	@DatabaseExecutor
+	private boolean generateAndSendAck() throws DbException, IOException {
-	private void generateEagerBatch(Map<MessageId, Integer> ids) {
+		Ack a = db.transactionWithNullableResult(false, txn ->
-		if (interrupted) return;
+				db.generateAck(txn, contactId, MAX_MESSAGE_IDS));
-		// Take some message IDs from `ids` to form a batch
+		if (LOG.isLoggable(INFO))
-		Collection<MessageId> batchIds = new ArrayList<>();
+			LOG.info("Generated ack: " + (a != null));
-		long totalLength = 0;
+		if (a == null) return false; // No more acks to send
-		Iterator<Entry<MessageId, Integer>> it = ids.entrySet().iterator();
+		recordWriter.writeAck(a);
-		while (it.hasNext()) {
-			// Check whether the next message will fit in the batch
-			Entry<MessageId, Integer> e = it.next();
-			int length = e.getValue();
-			if (totalLength + length > MAX_RECORD_PAYLOAD_BYTES) break;
-			// Add the message to the batch
-			it.remove();
-			batchIds.add(e.getKey());
-			totalLength += length;
-		}
-		if (batchIds.isEmpty()) throw new AssertionError();
-		try {
-			Collection<Message> batch =
-					db.transactionWithResult(false, txn ->
-							db.generateBatch(txn, contactId, batchIds,
-									maxLatency));
-			writerTasks.add(() -> writeEagerBatch(batch, ids));
-		} catch (DbException e) {
-			logException(LOG, WARNING, e);
-			interrupt();
-		}
-	}
-
-	@IoExecutor
-	private void writeEagerBatch(Collection<Message> batch,
-			Map<MessageId, Integer> ids) throws IOException {
-		if (interrupted) return;
-		for (Message m : batch) recordWriter.writeMessage(m);
-		LOG.info("Sent eager batch");
-		if (ids.isEmpty()) decrementOutstandingQueries();
-		else dbExecutor.execute(() -> generateEagerBatch(ids));
-	}
-
-	@DatabaseExecutor
-	private void generateAck() {
-		if (interrupted) return;
-		try {
-			Ack a = db.transactionWithNullableResult(false, txn ->
-					db.generateAck(txn, contactId, MAX_MESSAGE_IDS));
-			if (LOG.isLoggable(INFO))
-				LOG.info("Generated ack: " + (a != null));
-			if (a == null) decrementOutstandingQueries();
-			else writerTasks.add(() -> writeAck(a));
-		} catch (DbException e) {
-			logException(LOG, WARNING, e);
-			interrupt();
-		}
-	}
-
-	@IoExecutor
-	private void writeAck(Ack ack) throws IOException {
-		if (interrupted) return;
-		recordWriter.writeAck(ack);
 		LOG.info("Sent ack");
-		dbExecutor.execute(this::generateAck);
+		return true;
 	}
 
-	@DatabaseExecutor
+	void sendMessages() throws DbException, IOException {
-	private void generateBatch() {
+		while (!isInterrupted()) if (!generateAndSendBatch()) break;
-		if (interrupted) return;
-		try {
-			Collection<Message> b =
-					db.transactionWithNullableResult(false, txn ->
-							db.generateBatch(txn, contactId,
-									MAX_RECORD_PAYLOAD_BYTES, maxLatency));
-			if (LOG.isLoggable(INFO))
-				LOG.info("Generated batch: " + (b != null));
-			if (b == null) decrementOutstandingQueries();
-			else writerTasks.add(() -> writeBatch(b));
-		} catch (DbException e) {
-			logException(LOG, WARNING, e);
-			interrupt();
-		}
 	}
 
-	@IoExecutor
+	private boolean generateAndSendBatch() throws DbException, IOException {
-	private void writeBatch(Collection<Message> batch) throws IOException {
+		Collection<Message> b = db.transactionWithNullableResult(false, txn ->
-		if (interrupted) return;
+				db.generateBatch(txn, contactId, BATCH_CAPACITY, maxLatency));
-		for (Message m : batch) recordWriter.writeMessage(m);
+		if (LOG.isLoggable(INFO))
+			LOG.info("Generated batch: " + (b != null));
+		if (b == null) return false; // No more messages to send
+		for (Message m : b) recordWriter.writeMessage(m);
 		LOG.info("Sent batch");
-		dbExecutor.execute(this::generateBatch);
+		return true;
 	}
 }

bramble-core/src/main/java/org/briarproject/bramble/sync/SyncRecordWriterImpl.java

@@ -85,4 +85,9 @@ class SyncRecordWriterImpl implements SyncRecordWriter {
 	public void flush() throws IOException {
 		writer.flush();
 	}
+
+	@Override
+	public long getBytesWritten() {
+		return writer.getBytesWritten();
+	}
 }

bramble-core/src/main/java/org/briarproject/bramble/sync/SyncSessionFactoryImpl.java

@@ -64,8 +64,13 @@ class SyncSessionFactoryImpl implements SyncSessionFactory {
 		OutputStream out = streamWriter.getOutputStream();
 		SyncRecordWriter recordWriter =
 				recordWriterFactory.createRecordWriter(out);
-		return new SimplexOutgoingSession(db, dbExecutor, eventBus, c, t,
+		if (eager) {
-				maxLatency, eager, streamWriter, recordWriter);
+			return new EagerSimplexOutgoingSession(db, eventBus, c, t,
+					maxLatency, streamWriter, recordWriter);
+		} else {
+			return new SimplexOutgoingSession(db, eventBus, c, t,
+					maxLatency, streamWriter, recordWriter);
+		}
 	}
 
 	@Override

bramble-core/src/main/java/org/briarproject/bramble/sync/validation/ValidationManagerImpl.java

@@ -233,6 +233,9 @@ class ValidationManagerImpl implements ValidationManager, Service,
 		if (v == null) {
 			if (LOG.isLoggable(WARNING)) LOG.warning("No validator for " + cv);
 		} else {
+			if (LOG.isLoggable(INFO)) {
+				LOG.info("Validating message for " + cv.getClientId());
+			}
 			try {
 				MessageContext context = v.validateMessage(m, g);
 				storeMessageContextAsync(m, g.getClientId(),
@@ -323,6 +326,9 @@ class ValidationManagerImpl implements ValidationManager, Service,
 		ClientMajorVersion cv = new ClientMajorVersion(c, majorVersion);
 		IncomingMessageHook hook = hooks.get(cv);
 		if (hook == null) return ACCEPT_DO_NOT_SHARE;
+		if (LOG.isLoggable(INFO)) {
+			LOG.info("Delivering message for " + c);
+		}
 		try {
 			return hook.incomingMessage(txn, m, meta);
 		} catch (DbException e) {

bramble-core/src/main/java/org/briarproject/bramble/system/TaskSchedulerImpl.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.system;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.system.TaskScheduler;
 

bramble-core/src/main/resources/bridges

@@ -8,19 +8,23 @@ d Bridge obfs4 193.11.166.194:27015 2D82C2E354D531A68469ADF7F878FA6060C6BACA cer
 d Bridge obfs4 193.11.166.194:27020 86AC7B8D430DAC4117E9F42C9EAED18133863AAF cert=0LDeJH4JzMDtkJJrFphJCiPqKx7loozKN7VNfuukMGfHO0Z8OGdzHVkhVAOfo1mUdv9cMg iat-mode=0
 d Bridge obfs4 193.11.166.194:27025 1AE2C08904527FEA90C4C4F8C1083EA59FBC6FAF cert=ItvYZzW5tn6v3G4UnQa6Qz04Npro6e81AP70YujmK/KXwDFPTs3aHXcHp4n8Vt6w/bv8cA iat-mode=0
 d Bridge obfs4 209.148.46.65:443 74FAD13168806246602538555B5521A0383A1875 cert=ssH+9rP8dG2NLDN2XuFw63hIO/9MNNinLmxQDpVa+7kTOa9/m+tGWT1SmSYpQ9uTBGa6Hw iat-mode=0
+d Bridge obfs4 146.57.248.225:22 10A6CD36A537FCE513A322361547444B393989F0 cert=K1gDtDAIcUfeLqbstggjIw2rtgIKqdIhUlHp82XRqNSq/mtAjp1BIC9vHKJ2FAEpGssTPw iat-mode=0
+d Bridge obfs4 45.145.95.6:27015 C5B7CD6946FF10C5B3E89691A7D3F2C122D2117C cert=TD7PbUO0/0k6xYHMPW3vJxICfkMZNdkRrb63Zhl5j9dW3iRGiCx0A7mPhe5T2EDzQ35+Zw iat-mode=0
 d Bridge obfs4 51.222.13.177:80 5EDAC3B810E12B01F6FD8050D2FD3E277B289A08 cert=2uplIpLQ0q9+0qMFrK5pkaYRDOe460LL9WHBvatgkuRr/SL31wBOEupaMMJ6koRE6Ld0ew iat-mode=0
 n Bridge obfs4 46.226.107.197:10300 A38FD6BDFD902882F5F5B9B7CCC95602A20B0BC4 cert=t8tA9q2AeGlmp/dO6oW9bkY5RqqmvqjArCEM9wjJoDnk6XtnaejkF0JTA7VamdyOzcvuBg iat-mode=0
 n Bridge obfs4 185.181.11.86:443 A961609729E7FDF520B4E81F1F1B8FA1045285C3 cert=e5faG9Zk4Ni+e7z2YgGfevyKPQlMvkVGi4ublSsHYjaBovKeNXpOhbeFxzbZZoAzxAoGUQ iat-mode=0
 n Bridge obfs4 85.242.211.221:8042 A36A938DD7FDB8BACC846BA326EE0BA0D89A9252 cert=1AN6Pt1eFca3Y/WYD2TGAU3Al9cO4eouXE9SX63s66Z/ks3tVmgQ5GeXi1B5DOvx6Il7Zw iat-mode=0
-n Bridge obfs4 172.105.22.69:80 CBD17B33192A879433AB37C9E142541BD3459ABD cert=rk5YmpKypLsjlS4tjkYaZNBweYMa5tWQRhZ8Q2WRleNOgrhSceKo59BA8kp6kVfaMPXnSw iat-mode=0
-n Bridge obfs4 46.128.93.192:7346 5D28B8E1D117B8720D56A8513CF32509DCA1D84F cert=ED6tZP50eF0vno09F5gFvoWTMdcWFEX2FtwXOUYRevjzKg30/y701f61Vycnh6HO9gkaMw iat-mode=0
 n Bridge obfs4 74.104.165.202:9002 EF432018A6AA5D970B2F84E39CD30A147030141C cert=PhppfUusY85dHGvWtGTybZ1fED4DtbHmALkNMIOIYrAz1B4xN7/2a5gyiZe1epju1BOHVg iat-mode=0
 n Bridge obfs4 70.34.242.31:443 7F026956402CDFF4BCBA8E11EE9C50E3FE0A2B72 cert=hP/KU7JATSfWH3HwS5Er/YLT0J+bRO3+s2fWx2yirrgf37EyrWvm/BQshoNje8WfUm6CBw iat-mode=0
-n Bridge obfs4 192.3.163.88:57145 DEB62DE9643E5956CA4FA78035B48C9BBABE7F29 cert=RMz2z9uVVrioUhx0A8xUmiftRe2RpcXiqIuDfisdIomcHDf82nzfn83X/ixGUiA4rLCAdw iat-mode=0
 n Bridge obfs4 93.95.226.151:41185 460B0CFFC0CF1D965F3DE064E08BA1915E7C916A cert=inluPzp5Jp5OzZar1eQb4dcQ/YlAj/v0kHAUCoCr3rmLt03+pVuVTjoH4mRy4+acXpn+Gw iat-mode=0
 n Bridge obfs4 120.29.217.52:5223 40FE3DB9800272F9CDC76422F8ED7883280EE96D cert=/71PS4l8c/XJ4DIItlH9xMqNvPFg2RUTrHvPlQWh48u5et8h/yyyjCcYphUadDsfBWpaGQ iat-mode=0
-v Bridge 5.45.96.40:9001 8723B591712AAA03FB92000370BD356AB4997FA7
+n Bridge obfs4 83.97.179.29:1199 D83068BFAA28E71DB024B786E1E803BE14257127 cert=IduGtt05tM59Xmvo0oVNWgIRgY4OGPJjFP+y2oa6RMDHQBL/GRyFOOgX70iiazNAIJNkPw iat-mode=0
+n Bridge obfs4 207.181.244.13:40132 37FE8D782F5DD2BAEEDAAB8257B701344676B6DD cert=f5Hbfn3ToMzH170cV8DfLly3vRynveidfOfDcbradIDtbLDX15V2yQ8eEH2CPKQJmQR2Hg iat-mode=0
+n Bridge obfs4 76.255.201.112:8888 96CF36C2ECCFB7376AB6BE905BECD2C2AE8AEFCD cert=+q0pjaiM0JMqHL/BKqCRD+pjflaw/S406eUDF7CnFgamvQW3l2HVLJhQ6uX9P8zff0PLGg iat-mode=0
+n Bridge obfs4 65.108.159.114:14174 E1AD374BA9F34BD98862D128AC54D40C7DC138AE cert=YMkxMSBN2OOd99AkJpFaEUyKkdqZgJt9oVJEgO1QnT37n/Vc2yR4nhx4k4VkPLfEP1f4eg iat-mode=0
+n Bridge obfs4 185.177.207.138:8443 53716FE26F23C8C6A71A2BC5D9D8DC64747278C7 cert=6jcYVilMEzxdsWghSrQFpYYJlkkir/GPIXw/EnddUK3S8ToVpMG8u1SwMOEdEs735RrMHw iat-mode=0
+n Bridge obfs4 176.123.2.253:1933 B855D141CE6C4DE0F7EA4AAED83EBA8373FA8191 cert=1rOoSaRagc6PPR//paIl+ukv1N+xWKCdBXMFxK0k/moEwH0lk5bURBrUDzIX35fVzaiicQ iat-mode=0
 v Bridge 135.181.113.164:54444 74AF4CCA614C454B7D3E81FF8BACD78CEBC7D7DE
-v Bridge 152.44.197.85:10507 FF07DF6B4720DA4C50F1A025662D50916D6223F6
+v Bridge 92.243.15.235:9001 477EAD3C04036B48235F1F27FC91420A286A4B7F
-v Bridge 209.216.78.21:443 C870D381E7264CDB83BAEEBF074804808CCCDB8D
+v Bridge 77.96.91.103:443 ED000A75B79A58F1D83A4D1675C2A9395B71BE8E
 m Bridge meek_lite 192.0.2.2:2 97700DFE9F483596DDA6264C4D7DF7641E1E39CE url=https://meek.azureedge.net/ front=ajax.aspnetcdn.com

bramble-core/src/test/java/org/briarproject/bramble/client/ClientHelperImplTest.java

@@ -21,9 +21,10 @@ import org.briarproject.bramble.api.db.Metadata;
 import org.briarproject.bramble.api.db.Transaction;
 import org.briarproject.bramble.api.identity.Author;
 import org.briarproject.bramble.api.identity.AuthorFactory;
-import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
-import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
-import org.briarproject.bramble.api.mailbox.MailboxPropertiesUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.sync.GroupId;
 import org.briarproject.bramble.api.sync.Message;
 import org.briarproject.bramble.api.sync.MessageFactory;
@@ -41,24 +42,26 @@ import java.util.HashMap;
 import java.util.Map;
 import java.util.Random;
 
+import static java.util.Collections.singletonList;
 import static org.briarproject.bramble.api.identity.AuthorConstants.MAX_AUTHOR_NAME_LENGTH;
 import static org.briarproject.bramble.api.identity.AuthorConstants.MAX_PUBLIC_KEY_LENGTH;
 import static org.briarproject.bramble.api.identity.AuthorConstants.MAX_SIGNATURE_LENGTH;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_AUTHTOKEN;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_AUTHTOKEN;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_INBOXID;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_INBOXID;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_ONION;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_ONION;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_OUTBOXID;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_OUTBOXID;
 import static org.briarproject.bramble.test.TestUtils.getAuthor;
+import static org.briarproject.bramble.test.TestUtils.getMailboxProperties;
 import static org.briarproject.bramble.test.TestUtils.getMessage;
 import static org.briarproject.bramble.test.TestUtils.getRandomBytes;
 import static org.briarproject.bramble.test.TestUtils.getRandomId;
 import static org.briarproject.bramble.test.TestUtils.getSignaturePrivateKey;
 import static org.briarproject.bramble.test.TestUtils.getSignaturePublicKey;
-import static org.briarproject.bramble.test.TestUtils.mailboxPropertiesUpdateEqual;
+import static org.briarproject.bramble.test.TestUtils.mailboxUpdateEqual;
 import static org.briarproject.bramble.util.StringUtils.getRandomString;
 import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
@@ -95,25 +98,31 @@ public class ClientHelperImplTest extends BrambleMockTestCase {
 			messageFactory, bdfReaderFactory, bdfWriterFactory, metadataParser,
 			metadataEncoder, cryptoComponent, authorFactory);
 
-	private final MailboxPropertiesUpdate validMailboxPropsUpdate;
+	private final MailboxUpdateWithMailbox validMailboxUpdateWithMailbox;
+	private final BdfList emptyClientSupports;
+	private final BdfList someClientSupports;
+	private final BdfList emptyServerSupports;
+	private final BdfList someServerSupports;
 
 	public ClientHelperImplTest() {
-		validMailboxPropsUpdate = new MailboxPropertiesUpdate(
+		emptyClientSupports = new BdfList();
-				"pg6mmjiyjmcrsslvykfwnntlaru7p5svn6y2ymmju6nubxndf4pscryd",
+		someClientSupports = BdfList.of(BdfList.of(1, 0));
-				new MailboxAuthToken(getRandomId()),
+		emptyServerSupports = new BdfList();
-				new MailboxFolderId(getRandomId()),
+		someServerSupports = BdfList.of(BdfList.of(1, 0));
-				new MailboxFolderId(getRandomId()));
+		validMailboxUpdateWithMailbox = new MailboxUpdateWithMailbox(
+				singletonList(new MailboxVersion(1, 0)),
+				getMailboxProperties(false,
+						singletonList(new MailboxVersion(1, 0))));
 	}
 
-	private BdfDictionary getValidMailboxPropsUpdateDict() {
+	private BdfDictionary getValidMailboxUpdateWithMailboxDict() {
 		BdfDictionary dict = new BdfDictionary();
-		dict.put(PROP_KEY_ONION, validMailboxPropsUpdate.getOnion());
+		MailboxProperties properties =
-		dict.put(PROP_KEY_AUTHTOKEN, validMailboxPropsUpdate.getAuthToken()
+				validMailboxUpdateWithMailbox.getMailboxProperties();
-				.getBytes());
+		dict.put(PROP_KEY_ONION, properties.getOnion());
-		dict.put(PROP_KEY_INBOXID, validMailboxPropsUpdate.getInboxId()
+		dict.put(PROP_KEY_AUTHTOKEN, properties.getAuthToken());
-				.getBytes());
+		dict.put(PROP_KEY_INBOXID, properties.getInboxId());
-		dict.put(PROP_KEY_OUTBOXID, validMailboxPropsUpdate.getOutboxId()
+		dict.put(PROP_KEY_OUTBOXID, properties.getOutboxId());
-				.getBytes());
 		return dict;
 	}
 
@@ -546,94 +555,151 @@ public class ClientHelperImplTest extends BrambleMockTestCase {
 		}});
 	}
 
-	@Test
+	@Test(expected = FormatException.class)
-	public void testParseEmptyMailboxPropsUpdate() throws Exception {
+	public void testRejectsMailboxUpdateWithEmptyClientSupports()
+			throws Exception {
 		BdfDictionary emptyPropsDict = new BdfDictionary();
-		MailboxPropertiesUpdate parsedProps = clientHelper
+		clientHelper.parseAndValidateMailboxUpdate(emptyClientSupports,
-				.parseAndValidateMailboxPropertiesUpdate(emptyPropsDict);
+				emptyServerSupports, emptyPropsDict
-		assertNull(parsedProps);
+		);
 	}
 
 	@Test
-	public void testParseValidMailboxPropsUpdate() throws Exception {
+	public void testParseMailboxUpdateNoMailbox() throws Exception {
-		MailboxPropertiesUpdate parsedProps = clientHelper
+		BdfDictionary emptyPropsDict = new BdfDictionary();
-				.parseAndValidateMailboxPropertiesUpdate(
+		MailboxUpdate parsedUpdate = clientHelper.parseAndValidateMailboxUpdate(
-						getValidMailboxPropsUpdateDict());
+				someClientSupports, emptyServerSupports, emptyPropsDict);
-		assertTrue(mailboxPropertiesUpdateEqual(validMailboxPropsUpdate,
+		assertFalse(parsedUpdate.hasMailbox());
-				parsedProps));
 	}
 
 	@Test(expected = FormatException.class)
-	public void testRejectsMailboxPropsUpdateOnionNotDecodable()
+	public void testRejectsMailboxUpdateNoMailboxWithSomeServerSupports()
 			throws Exception {
-		BdfDictionary propsDict = getValidMailboxPropsUpdateDict();
+		BdfDictionary emptyPropsDict = new BdfDictionary();
+		clientHelper.parseAndValidateMailboxUpdate(someClientSupports,
+				someServerSupports, emptyPropsDict);
+	}
+
+	@Test(expected = FormatException.class)
+	public void testRejectsMailboxUpdateShortSupports() throws Exception {
+		clientHelper.parseAndValidateMailboxUpdate(BdfList.of(BdfList.of(1)),
+				emptyServerSupports, new BdfDictionary());
+	}
+
+	@Test(expected = FormatException.class)
+	public void testRejectsMailboxUpdateLongSupports() throws Exception {
+		clientHelper.parseAndValidateMailboxUpdate(
+				BdfList.of(BdfList.of(1, 0, 0)), emptyServerSupports,
+				new BdfDictionary());
+	}
+
+	@Test(expected = FormatException.class)
+	public void testRejectsMailboxUpdateNonIntSupports() throws Exception {
+		clientHelper.parseAndValidateMailboxUpdate(
+				BdfList.of(BdfList.of(1, "0")), emptyServerSupports,
+				new BdfDictionary()
+		);
+	}
+
+	@Test(expected = FormatException.class)
+	public void testRejectsMailboxUpdateNonListSupports() throws Exception {
+		clientHelper.parseAndValidateMailboxUpdate(
+				BdfList.of("non-list"), emptyServerSupports,
+				new BdfDictionary());
+	}
+
+	@Test
+	public void testParseValidMailboxUpdateWithMailbox() throws Exception {
+		MailboxUpdate parsedUpdate = clientHelper.parseAndValidateMailboxUpdate(
+				someClientSupports, someServerSupports,
+				getValidMailboxUpdateWithMailboxDict());
+		assertTrue(
+				mailboxUpdateEqual(validMailboxUpdateWithMailbox,
+						parsedUpdate));
+	}
+
+	@Test(expected = FormatException.class)
+	public void rejectsMailboxUpdateWithEmptyServerSupports() throws Exception {
+		clientHelper.parseAndValidateMailboxUpdate(someClientSupports,
+				emptyServerSupports, getValidMailboxUpdateWithMailboxDict());
+	}
+
+	@Test(expected = FormatException.class)
+	public void testRejectsMailboxUpdateOnionNotDecodable() throws Exception {
+		BdfDictionary propsDict = getValidMailboxUpdateWithMailboxDict();
 		String badOnion = "!" + propsDict.getString(PROP_KEY_ONION)
 				.substring(1);
 		propsDict.put(PROP_KEY_ONION, badOnion);
-		clientHelper.parseAndValidateMailboxPropertiesUpdate(propsDict);
+		clientHelper.parseAndValidateMailboxUpdate(someClientSupports,
+				emptyServerSupports, propsDict);
 	}
 
 	@Test(expected = FormatException.class)
-	public void testRejectsMailboxPropsUpdateOnionWrongLength()
+	public void testRejectsMailboxUpdateOnionWrongLength() throws Exception {
-			throws Exception {
+		BdfDictionary propsDict = getValidMailboxUpdateWithMailboxDict();
-		BdfDictionary propsDict = getValidMailboxPropsUpdateDict();
 		String tooLongOnion = propsDict.getString(PROP_KEY_ONION) + "!";
 		propsDict.put(PROP_KEY_ONION, tooLongOnion);
-		clientHelper.parseAndValidateMailboxPropertiesUpdate(propsDict);
+		clientHelper.parseAndValidateMailboxUpdate(someClientSupports,
+				emptyServerSupports, propsDict
+		);
 	}
 
 	@Test(expected = FormatException.class)
-	public void testRejectsMailboxPropsUpdateInboxIdWrongLength()
+	public void testRejectsMailboxUpdateInboxIdWrongLength() throws Exception {
-			throws Exception {
+		BdfDictionary propsDict = getValidMailboxUpdateWithMailboxDict();
-		BdfDictionary propsDict = getValidMailboxPropsUpdateDict();
 		propsDict.put(PROP_KEY_INBOXID, getRandomBytes(UniqueId.LENGTH + 1));
-		clientHelper.parseAndValidateMailboxPropertiesUpdate(propsDict);
+		clientHelper.parseAndValidateMailboxUpdate(someClientSupports,
+				someServerSupports, propsDict);
 	}
 
 	@Test(expected = FormatException.class)
-	public void testRejectsMailboxPropsUpdateOutboxIdWrongLength()
+	public void testRejectsMailboxUpdateOutboxIdWrongLength() throws Exception {
-			throws Exception {
+		BdfDictionary propsDict = getValidMailboxUpdateWithMailboxDict();
-		BdfDictionary propsDict = getValidMailboxPropsUpdateDict();
 		propsDict.put(PROP_KEY_OUTBOXID, getRandomBytes(UniqueId.LENGTH + 1));
-		clientHelper.parseAndValidateMailboxPropertiesUpdate(propsDict);
+		clientHelper.parseAndValidateMailboxUpdate(someClientSupports,
+				someServerSupports, propsDict);
 	}
 
 	@Test(expected = FormatException.class)
-	public void testRejectsMailboxPropsUpdateAuthTokenWrongLength()
+	public void testRejectsMailboxUpdateAuthTokenWrongLength()
 			throws Exception {
-		BdfDictionary propsDict = getValidMailboxPropsUpdateDict();
+		BdfDictionary propsDict = getValidMailboxUpdateWithMailboxDict();
 		propsDict.put(PROP_KEY_AUTHTOKEN, getRandomBytes(UniqueId.LENGTH + 1));
-		clientHelper.parseAndValidateMailboxPropertiesUpdate(propsDict);
+		clientHelper.parseAndValidateMailboxUpdate(someClientSupports,
+				someServerSupports, propsDict);
 	}
 
 	@Test(expected = FormatException.class)
-	public void testRejectsMailboxPropsUpdateMissingOnion() throws Exception {
+	public void testRejectsMailboxUpdateMissingOnion() throws Exception {
-		BdfDictionary propsDict = getValidMailboxPropsUpdateDict();
+		BdfDictionary propsDict = getValidMailboxUpdateWithMailboxDict();
 		propsDict.remove(PROP_KEY_ONION);
-		clientHelper.parseAndValidateMailboxPropertiesUpdate(propsDict);
+		clientHelper.parseAndValidateMailboxUpdate(someClientSupports,
+				someServerSupports, propsDict
+		);
 	}
 
 	@Test(expected = FormatException.class)
-	public void testRejectsMailboxPropsUpdateMissingAuthToken()
+	public void testRejectsMailboxUpdateMissingAuthToken() throws Exception {
-			throws Exception {
+		BdfDictionary propsDict = getValidMailboxUpdateWithMailboxDict();
-		BdfDictionary propsDict = getValidMailboxPropsUpdateDict();
 		propsDict.remove(PROP_KEY_AUTHTOKEN);
-		clientHelper.parseAndValidateMailboxPropertiesUpdate(propsDict);
+		clientHelper.parseAndValidateMailboxUpdate(someClientSupports,
+				someServerSupports, propsDict);
 	}
 
 	@Test(expected = FormatException.class)
-	public void testRejectsMailboxPropsUpdateMissingInboxId() throws Exception {
+	public void testRejectsMailboxUpdateMissingInboxId() throws Exception {
-		BdfDictionary propsDict = getValidMailboxPropsUpdateDict();
+		BdfDictionary propsDict = getValidMailboxUpdateWithMailboxDict();
 		propsDict.remove(PROP_KEY_INBOXID);
-		clientHelper.parseAndValidateMailboxPropertiesUpdate(propsDict);
+		clientHelper.parseAndValidateMailboxUpdate(someClientSupports,
+				someServerSupports, propsDict);
 	}
 
 	@Test(expected = FormatException.class)
-	public void testRejectsMailboxPropsUpdateMissingOutboxId()
+	public void testRejectsMailboxUpdateMissingOutboxId() throws Exception {
-			throws Exception {
+		BdfDictionary propsDict = getValidMailboxUpdateWithMailboxDict();
-		BdfDictionary propsDict = getValidMailboxPropsUpdateDict();
 		propsDict.remove(PROP_KEY_OUTBOXID);
-		clientHelper.parseAndValidateMailboxPropertiesUpdate(propsDict);
+		clientHelper.parseAndValidateMailboxUpdate(someClientSupports,
+				someServerSupports, propsDict);
 	}
 
 }

bramble-core/src/test/java/org/briarproject/bramble/db/DatabaseComponentImplTest.java

@@ -72,6 +72,7 @@ import static java.util.Collections.emptyMap;
 import static java.util.Collections.singletonList;
 import static java.util.concurrent.TimeUnit.HOURS;
 import static org.briarproject.bramble.api.db.DatabaseComponent.TIMER_NOT_STARTED;
+import static org.briarproject.bramble.api.record.Record.RECORD_HEADER_BYTES;
 import static org.briarproject.bramble.api.sync.Group.Visibility.INVISIBLE;
 import static org.briarproject.bramble.api.sync.Group.Visibility.SHARED;
 import static org.briarproject.bramble.api.sync.Group.Visibility.VISIBLE;
@@ -94,11 +95,15 @@ import static org.briarproject.bramble.test.TestUtils.getTransportId;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
 public class DatabaseComponentImplTest extends BrambleMockTestCase {
 
+	private static final int BATCH_CAPACITY =
+			(RECORD_HEADER_BYTES + MAX_MESSAGE_LENGTH) * 2;
+
 	@SuppressWarnings("unchecked")
 	private final Database<Object> database = context.mock(Database.class);
 	private final ShutdownManager shutdownManager =
@@ -298,11 +303,11 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 			throws Exception {
 		context.checking(new Expectations() {{
 			// Check whether the contact is in the DB (which it's not)
-			exactly(19).of(database).startTransaction();
+			exactly(25).of(database).startTransaction();
 			will(returnValue(txn));
-			exactly(19).of(database).containsContact(txn, contactId);
+			exactly(25).of(database).containsContact(txn, contactId);
 			will(returnValue(false));
-			exactly(19).of(database).abortTransaction(txn);
+			exactly(25).of(database).abortTransaction(txn);
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, eventBus,
 				eventExecutor, shutdownManager);
@@ -356,6 +361,39 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 			// Expected
 		}
 
+		try {
+			db.transaction(false, transaction ->
+					db.getMessageToSend(transaction, contactId, messageId, 123,
+							true));
+			fail();
+		} catch (NoSuchContactException expected) {
+			// Expected
+		}
+
+		try {
+			db.transaction(true, transaction ->
+					db.getMessagesToAck(transaction, contactId, 123));
+			fail();
+		} catch (NoSuchContactException expected) {
+			// Expected
+		}
+
+		try {
+			db.transaction(true, transaction ->
+					db.getMessagesToSend(transaction, contactId, 123, 456));
+			fail();
+		} catch (NoSuchContactException expected) {
+			// Expected
+		}
+
+		try {
+			db.transaction(true, transaction ->
+					db.getUnackedMessagesToSend(transaction, contactId));
+			fail();
+		} catch (NoSuchContactException expected) {
+			// Expected
+		}
+
 		try {
 			db.transaction(true, transaction ->
 					db.getUnackedMessageBytesToSend(transaction, contactId));
@@ -439,6 +477,15 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 			// Expected
 		}
 
+		try {
+			db.transaction(false, transaction ->
+					db.setAckSent(transaction, contactId,
+							singletonList(messageId)));
+			fail();
+		} catch (NoSuchContactException expected) {
+			// Expected
+		}
+
 		try {
 			db.transaction(false, transaction ->
 					db.setContactAlias(transaction, contactId, alias));
@@ -456,6 +503,15 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 			// Expected
 		}
 
+		try {
+			db.transaction(false, transaction ->
+					db.setMessagesSent(transaction, contactId,
+							singletonList(messageId), 123));
+			fail();
+		} catch (NoSuchContactException expected) {
+			// Expected
+		}
+
 		try {
 			db.transaction(false, transaction ->
 					db.setSyncVersions(transaction, contactId, emptyList()));
@@ -918,12 +974,14 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 			oneOf(database).containsContact(txn, contactId);
 			will(returnValue(true));
 			oneOf(database).getMessagesToSend(txn, contactId,
-					MAX_MESSAGE_LENGTH * 2, maxLatency);
+					BATCH_CAPACITY, maxLatency);
 			will(returnValue(ids));
+			// First message
 			oneOf(database).getMessage(txn, messageId);
 			will(returnValue(message));
 			oneOf(database).updateRetransmissionData(txn, contactId, messageId,
 					maxLatency);
+			// Second message
 			oneOf(database).getMessage(txn, messageId1);
 			will(returnValue(message1));
 			oneOf(database).updateRetransmissionData(txn, contactId, messageId1,
@@ -937,7 +995,7 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 
 		db.transaction(false, transaction ->
 				assertEquals(messages, db.generateBatch(transaction, contactId,
-						MAX_MESSAGE_LENGTH * 2, maxLatency)));
+						BATCH_CAPACITY, maxLatency)));
 	}
 
 	@Test
@@ -1001,12 +1059,14 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 			oneOf(database).containsContact(txn, contactId);
 			will(returnValue(true));
 			oneOf(database).getRequestedMessagesToSend(txn, contactId,
-					MAX_MESSAGE_LENGTH * 2, maxLatency);
+					BATCH_CAPACITY, maxLatency);
 			will(returnValue(ids));
+			// First message
 			oneOf(database).getMessage(txn, messageId);
 			will(returnValue(message));
 			oneOf(database).updateRetransmissionData(txn, contactId,
 					messageId, maxLatency);
+			// Second message
 			oneOf(database).getMessage(txn, messageId1);
 			will(returnValue(message1));
 			oneOf(database).updateRetransmissionData(txn, contactId,
@@ -1020,7 +1080,73 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 
 		db.transaction(false, transaction ->
 				assertEquals(messages, db.generateRequestedBatch(transaction,
-						contactId, MAX_MESSAGE_LENGTH * 2, maxLatency)));
+						contactId, BATCH_CAPACITY, maxLatency)));
+	}
+
+	@Test
+	public void testGetMessageToSendMessageNotVisible() throws Exception {
+		context.checking(new Expectations() {{
+			oneOf(database).startTransaction();
+			will(returnValue(txn));
+			oneOf(database).containsContact(txn, contactId);
+			will(returnValue(true));
+			oneOf(database).containsVisibleMessage(txn, contactId, messageId);
+			will(returnValue(false));
+			oneOf(database).commitTransaction(txn);
+		}});
+		DatabaseComponent db = createDatabaseComponent(database, eventBus,
+				eventExecutor, shutdownManager);
+
+		db.transaction(false, transaction ->
+				assertNull(db.getMessageToSend(transaction, contactId,
+						messageId, maxLatency, false)));
+	}
+
+	@Test
+	public void testGetMessageToSendMessageNotMarkedAsSent() throws Exception {
+		context.checking(new Expectations() {{
+			oneOf(database).startTransaction();
+			will(returnValue(txn));
+			oneOf(database).containsContact(txn, contactId);
+			will(returnValue(true));
+			oneOf(database).containsVisibleMessage(txn, contactId, messageId);
+			will(returnValue(true));
+			oneOf(database).getMessage(txn, messageId);
+			will(returnValue(message));
+			oneOf(database).commitTransaction(txn);
+		}});
+		DatabaseComponent db = createDatabaseComponent(database, eventBus,
+				eventExecutor, shutdownManager);
+
+		db.transaction(false, transaction ->
+				assertEquals(message, db.getMessageToSend(transaction,
+						contactId, messageId, maxLatency, false)));
+	}
+
+	@Test
+	public void testGetMessageToSendMessageMarkedAsSent() throws Exception {
+		context.checking(new Expectations() {{
+			oneOf(database).startTransaction();
+			will(returnValue(txn));
+			oneOf(database).containsContact(txn, contactId);
+			will(returnValue(true));
+			oneOf(database).containsVisibleMessage(txn, contactId, messageId);
+			will(returnValue(true));
+			oneOf(database).getMessage(txn, messageId);
+			will(returnValue(message));
+			oneOf(database).updateRetransmissionData(txn, contactId, messageId,
+					maxLatency);
+			oneOf(database).lowerRequestedFlag(txn, contactId,
+					singletonList(messageId));
+			oneOf(database).commitTransaction(txn);
+			oneOf(eventBus).broadcast(with(any(MessagesSentEvent.class)));
+		}});
+		DatabaseComponent db = createDatabaseComponent(database, eventBus,
+				eventExecutor, shutdownManager);
+
+		db.transaction(false, transaction ->
+				assertEquals(message, db.getMessageToSend(transaction,
+						contactId, messageId, maxLatency, true)));
 	}
 
 	@Test
@@ -1245,6 +1371,62 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 				db.receiveRequest(transaction, contactId, r));
 	}
 
+	@Test
+	public void testSetAckSent() throws Exception {
+		Collection<MessageId> acked = asList(messageId, messageId1);
+		context.checking(new Expectations() {{
+			oneOf(database).startTransaction();
+			will(returnValue(txn));
+			oneOf(database).containsContact(txn, contactId);
+			will(returnValue(true));
+			// First message is still visible to the contact - flag lowered
+			oneOf(database).containsVisibleMessage(txn, contactId, messageId);
+			will(returnValue(true));
+			// Second message is no longer visible - flag not lowered
+			oneOf(database).containsVisibleMessage(txn, contactId, messageId1);
+			will(returnValue(false));
+			oneOf(database)
+					.lowerAckFlag(txn, contactId, singletonList(messageId));
+			oneOf(database).commitTransaction(txn);
+		}});
+		DatabaseComponent db = createDatabaseComponent(database, eventBus,
+				eventExecutor, shutdownManager);
+
+		db.transaction(false, transaction ->
+				db.setAckSent(transaction, contactId, acked));
+	}
+
+	@Test
+	public void testSetMessagesSent() throws Exception {
+		long maxLatency = 123456;
+		Collection<MessageId> sent = asList(messageId, messageId1);
+		context.checking(new Expectations() {{
+			oneOf(database).startTransaction();
+			will(returnValue(txn));
+			oneOf(database).containsContact(txn, contactId);
+			will(returnValue(true));
+			// First message is still visible to the contact - mark as sent
+			oneOf(database).containsVisibleMessage(txn, contactId, messageId);
+			will(returnValue(true));
+			oneOf(database).getMessageLength(txn, messageId);
+			will(returnValue(message.getRawLength()));
+			oneOf(database).updateRetransmissionData(txn, contactId, messageId,
+					maxLatency);
+			// Second message is no longer visible - don't mark as sent
+			oneOf(database).containsVisibleMessage(txn, contactId, messageId1);
+			will(returnValue(false));
+			oneOf(database).lowerRequestedFlag(txn, contactId,
+					singletonList(messageId));
+			oneOf(database).commitTransaction(txn);
+			oneOf(eventBus).broadcast(with(any(MessagesSentEvent.class)));
+		}});
+		DatabaseComponent db = createDatabaseComponent(database, eventBus,
+				eventExecutor, shutdownManager);
+
+		db.transaction(false, transaction ->
+				db.setMessagesSent(transaction, contactId, sent, maxLatency));
+	}
+
 	@Test
 	public void testChangingVisibilityFromInvisibleToVisibleCallsListeners()
 			throws Exception {

bramble-core/src/test/java/org/briarproject/bramble/db/DatabasePerformanceTest.java

@@ -33,7 +33,9 @@ import java.util.Random;
 import java.util.logging.Logger;
 
 import static java.util.logging.Level.OFF;
+import static org.briarproject.bramble.api.record.Record.RECORD_HEADER_BYTES;
 import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_IDS;
+import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_LENGTH;
 import static org.briarproject.bramble.api.sync.validation.MessageState.DELIVERED;
 import static org.briarproject.bramble.test.TestUtils.deleteTestDirectory;
 import static org.briarproject.bramble.test.TestUtils.getAuthor;
@@ -97,6 +99,9 @@ public abstract class DatabasePerformanceTest extends BrambleTestCase {
 	// All our transports use a maximum latency of 30 seconds
 	private static final int MAX_LATENCY = 30 * 1000;
 
+	private static final int BATCH_CAPACITY =
+			(RECORD_HEADER_BYTES + MAX_MESSAGE_LENGTH) * 2;
+
 	protected final File testDir = getTestDirectory();
 	private final File resultsFile = new File(getTestName() + ".tsv");
 	protected final Random random = new Random();
@@ -471,7 +476,7 @@ public abstract class DatabasePerformanceTest extends BrambleTestCase {
 		benchmark(name, db -> {
 			Connection txn = db.startTransaction();
 			db.getMessagesToSend(txn, pickRandom(contacts).getId(),
-					MAX_MESSAGE_IDS, MAX_LATENCY);
+					BATCH_CAPACITY, MAX_LATENCY);
 			db.commitTransaction(txn);
 		});
 	}
@@ -522,7 +527,7 @@ public abstract class DatabasePerformanceTest extends BrambleTestCase {
 		benchmark(name, db -> {
 			Connection txn = db.startTransaction();
 			db.getRequestedMessagesToSend(txn, pickRandom(contacts).getId(),
-					MAX_MESSAGE_IDS, MAX_LATENCY);
+					BATCH_CAPACITY, MAX_LATENCY);
 			db.commitTransaction(txn);
 		});
 	}

bramble-core/src/test/java/org/briarproject/bramble/db/JdbcDatabaseTest.java

@@ -57,7 +57,6 @@ import java.util.concurrent.atomic.AtomicLong;
 
 import static java.util.Arrays.asList;
 import static java.util.Collections.emptyList;
-import static java.util.Collections.singleton;
 import static java.util.Collections.singletonList;
 import static java.util.Collections.singletonMap;
 import static java.util.concurrent.TimeUnit.SECONDS;
@@ -65,6 +64,7 @@ import static org.briarproject.bramble.api.db.DatabaseComponent.NO_CLEANUP_DEADL
 import static org.briarproject.bramble.api.db.DatabaseComponent.TIMER_NOT_STARTED;
 import static org.briarproject.bramble.api.db.Metadata.REMOVE;
 import static org.briarproject.bramble.api.identity.AuthorConstants.MAX_AUTHOR_NAME_LENGTH;
+import static org.briarproject.bramble.api.record.Record.RECORD_HEADER_BYTES;
 import static org.briarproject.bramble.api.sync.Group.Visibility.INVISIBLE;
 import static org.briarproject.bramble.api.sync.Group.Visibility.SHARED;
 import static org.briarproject.bramble.api.sync.Group.Visibility.VISIBLE;
@@ -350,14 +350,14 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		// The message is sendable, but too large to send
 		assertOneMessageToSendLazily(db, txn);
 		assertOneMessageToSendEagerly(db, txn);
+		long capacity = RECORD_HEADER_BYTES + message.getRawLength() - 1;
 		Collection<MessageId> ids =
-				db.getMessagesToSend(txn, contactId, message.getRawLength() - 1,
+				db.getMessagesToSend(txn, contactId, capacity, MAX_LATENCY);
-						MAX_LATENCY);
 		assertTrue(ids.isEmpty());
 
 		// The message is just the right size to send
-		ids = db.getMessagesToSend(txn, contactId, message.getRawLength(),
+		capacity = RECORD_HEADER_BYTES + message.getRawLength();
-				MAX_LATENCY);
+		ids = db.getMessagesToSend(txn, contactId, capacity, MAX_LATENCY);
 		assertEquals(singletonList(messageId), ids);
 
 		db.commitTransaction(txn);
@@ -396,16 +396,15 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		Collection<MessageId> ids = db.getMessagesToAck(txn, contactId, 1234);
 		assertEquals(asList(messageId, messageId1), ids);
 
-		// Remove both message IDs
+		// Lower the ack flag
 		db.lowerAckFlag(txn, contactId, asList(messageId, messageId1));
 
-		// Both message IDs should have been removed
+		// No message IDs should be returned
 		assertFalse(
 				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, false));
 		assertFalse(
 				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, true));
-		assertEquals(emptyList(), db.getMessagesToAck(txn,
+		assertEquals(emptyList(), db.getMessagesToAck(txn, contactId, 1234));
-				contactId, 1234));
 
 		// Raise the ack flag again
 		db.raiseAckFlag(txn, contactId, messageId);
@@ -2603,7 +2602,7 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 			Connection txn) throws Exception {
 		assertFalse(
 				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, true));
-		Map<MessageId, Integer> unacked =
+		Collection<MessageId> unacked =
 				db.getUnackedMessagesToSend(txn, contactId);
 		assertTrue(unacked.isEmpty());
 		assertEquals(0, db.getUnackedMessageBytesToSend(txn, contactId));
@@ -2613,10 +2612,9 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 			Connection txn) throws Exception {
 		assertTrue(
 				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, true));
-		Map<MessageId, Integer> unacked =
+		Collection<MessageId> unacked =
 				db.getUnackedMessagesToSend(txn, contactId);
-		assertEquals(singleton(messageId), unacked.keySet());
+		assertEquals(singletonList(messageId), unacked);
-		assertEquals(message.getRawLength(), unacked.get(messageId).intValue());
 		assertEquals(message.getRawLength(),
 				db.getUnackedMessageBytesToSend(txn, contactId));
 	}

bramble-core/src/test/java/org/briarproject/bramble/mailbox/ConnectivityCheckerImplTest.java

@@ -0,0 +1,201 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.mailbox.ConnectivityChecker.ConnectivityObserver;
+import org.briarproject.bramble.test.BrambleMockTestCase;
+import org.jmock.Expectations;
+import org.junit.Test;
+
+import javax.annotation.Nonnull;
+
+import static org.briarproject.bramble.mailbox.ConnectivityCheckerImpl.CONNECTIVITY_CHECK_FRESHNESS_MS;
+import static org.briarproject.bramble.mailbox.MailboxApi.CLIENT_SUPPORTS;
+import static org.briarproject.bramble.test.TestUtils.getMailboxProperties;
+
+public class ConnectivityCheckerImplTest extends BrambleMockTestCase {
+
+	private final Clock clock = context.mock(Clock.class);
+	private final MailboxApiCaller mailboxApiCaller =
+			context.mock(MailboxApiCaller.class);
+	private final ApiCall apiCall = context.mock(ApiCall.class);
+	private final Cancellable task = context.mock(Cancellable.class);
+	private final ConnectivityObserver observer1 =
+			context.mock(ConnectivityObserver.class, "1");
+	private final ConnectivityObserver observer2 =
+			context.mock(ConnectivityObserver.class, "2");
+
+	private final MailboxProperties properties =
+			getMailboxProperties(true, CLIENT_SUPPORTS);
+	private final long now = System.currentTimeMillis();
+
+	@Test
+	public void testFirstObserverStartsCheck() {
+		ConnectivityCheckerImpl checker = createChecker();
+
+		// When checkConnectivity() is called a check should be started
+		context.checking(new Expectations() {{
+			oneOf(clock).currentTimeMillis();
+			will(returnValue(now));
+			oneOf(mailboxApiCaller).retryWithBackoff(apiCall);
+			will(returnValue(task));
+		}});
+
+		checker.checkConnectivity(properties, observer1);
+
+		// When the check succeeds the observer should be called
+		context.checking(new Expectations() {{
+			oneOf(observer1).onConnectivityCheckSucceeded();
+		}});
+
+		checker.onConnectivityCheckSucceeded(now);
+
+		// The observer should not be called again when subsequent checks
+		// succeed
+		checker.onConnectivityCheckSucceeded(now);
+	}
+
+	@Test
+	public void testObserverIsAddedToExistingCheck() {
+		ConnectivityCheckerImpl checker = createChecker();
+
+		// When checkConnectivity() is called a check should be started
+		context.checking(new Expectations() {{
+			oneOf(clock).currentTimeMillis();
+			will(returnValue(now));
+			oneOf(mailboxApiCaller).retryWithBackoff(apiCall);
+			will(returnValue(task));
+		}});
+
+		checker.checkConnectivity(properties, observer1);
+
+		// When checkConnectivity() is called again before the first check
+		// succeeds, the observer should be added to the existing check
+		checker.checkConnectivity(properties, observer2);
+
+		// When the check succeeds both observers should be called
+		context.checking(new Expectations() {{
+			oneOf(observer1).onConnectivityCheckSucceeded();
+			oneOf(observer2).onConnectivityCheckSucceeded();
+		}});
+
+		checker.onConnectivityCheckSucceeded(now);
+
+		// The observers should not be called again when subsequent checks
+		// succeed
+		checker.onConnectivityCheckSucceeded(now);
+	}
+
+	@Test
+	public void testFreshResultIsReused() {
+		ConnectivityCheckerImpl checker = createChecker();
+
+		// When checkConnectivity() is called a check should be started
+		context.checking(new Expectations() {{
+			oneOf(clock).currentTimeMillis();
+			will(returnValue(now));
+			oneOf(mailboxApiCaller).retryWithBackoff(apiCall);
+			will(returnValue(task));
+		}});
+
+		checker.checkConnectivity(properties, observer1);
+
+		// When the check succeeds the observer should be called
+		context.checking(new Expectations() {{
+			oneOf(observer1).onConnectivityCheckSucceeded();
+		}});
+
+		checker.onConnectivityCheckSucceeded(now);
+
+		// When checkConnectivity() is called again within
+		// CONNECTIVITY_CHECK_FRESHNESS_MS the observer should be called with
+		// the result of the recent check
+		context.checking(new Expectations() {{
+			oneOf(clock).currentTimeMillis();
+			will(returnValue(now + CONNECTIVITY_CHECK_FRESHNESS_MS));
+			oneOf(observer2).onConnectivityCheckSucceeded();
+		}});
+
+		checker.checkConnectivity(properties, observer2);
+	}
+
+	@Test
+	public void testStaleResultIsNotReused() {
+		ConnectivityCheckerImpl checker = createChecker();
+
+		// When checkConnectivity() is called a check should be started
+		context.checking(new Expectations() {{
+			oneOf(clock).currentTimeMillis();
+			will(returnValue(now));
+			oneOf(mailboxApiCaller).retryWithBackoff(apiCall);
+			will(returnValue(task));
+		}});
+
+		checker.checkConnectivity(properties, observer1);
+
+		// When the check succeeds the observer should be called
+		context.checking(new Expectations() {{
+			oneOf(observer1).onConnectivityCheckSucceeded();
+		}});
+
+		checker.onConnectivityCheckSucceeded(now);
+
+		// When checkConnectivity() is called again after more than
+		// CONNECTIVITY_CHECK_FRESHNESS_MS another check should be started
+		context.checking(new Expectations() {{
+			oneOf(clock).currentTimeMillis();
+			will(returnValue(now + CONNECTIVITY_CHECK_FRESHNESS_MS + 1));
+			oneOf(mailboxApiCaller).retryWithBackoff(apiCall);
+			will(returnValue(task));
+		}});
+
+		checker.checkConnectivity(properties, observer2);
+
+		// When the check succeeds the observer should be called
+		context.checking(new Expectations() {{
+			oneOf(observer2).onConnectivityCheckSucceeded();
+		}});
+
+		checker.onConnectivityCheckSucceeded(
+				now + CONNECTIVITY_CHECK_FRESHNESS_MS + 1);
+	}
+
+	@Test
+	public void testCheckIsCancelledWhenCheckerIsDestroyed() {
+		ConnectivityCheckerImpl checker = createChecker();
+
+		// When checkConnectivity() is called a check should be started
+		context.checking(new Expectations() {{
+			oneOf(clock).currentTimeMillis();
+			will(returnValue(now));
+			oneOf(mailboxApiCaller).retryWithBackoff(apiCall);
+			will(returnValue(task));
+		}});
+
+		checker.checkConnectivity(properties, observer1);
+
+		// When the checker is destroyed the check should be cancelled
+		context.checking(new Expectations() {{
+			oneOf(task).cancel();
+		}});
+
+		checker.destroy();
+
+		// If the check runs anyway (cancellation came too late) the observer
+		// should not be called
+		checker.onConnectivityCheckSucceeded(now);
+	}
+
+	private ConnectivityCheckerImpl createChecker() {
+
+		return new ConnectivityCheckerImpl(clock, mailboxApiCaller) {
+			@Override
+			@Nonnull
+			protected ApiCall createConnectivityCheckTask(
+					@Nonnull MailboxProperties properties) {
+				return apiCall;
+			}
+		};
+	}
+}

bramble-core/src/test/java/org/briarproject/bramble/mailbox/ContactMailboxConnectivityCheckerTest.java

@@ -0,0 +1,98 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.mailbox.ConnectivityChecker.ConnectivityObserver;
+import org.briarproject.bramble.test.BrambleMockTestCase;
+import org.briarproject.bramble.test.CaptureArgumentAction;
+import org.jmock.Expectations;
+import org.jmock.lib.action.DoAllAction;
+import org.junit.Test;
+
+import java.io.IOException;
+import java.util.concurrent.atomic.AtomicReference;
+
+import static org.briarproject.bramble.mailbox.MailboxApi.CLIENT_SUPPORTS;
+import static org.briarproject.bramble.test.TestUtils.getMailboxProperties;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class ContactMailboxConnectivityCheckerTest extends BrambleMockTestCase {
+
+	private final Clock clock = context.mock(Clock.class);
+	private final MailboxApiCaller mailboxApiCaller =
+			context.mock(MailboxApiCaller.class);
+	private final MailboxApi mailboxApi = context.mock(MailboxApi.class);
+	private final Cancellable task = context.mock(Cancellable.class);
+	private final ConnectivityObserver observer =
+			context.mock(ConnectivityObserver.class);
+
+	private final MailboxProperties properties =
+			getMailboxProperties(false, CLIENT_SUPPORTS);
+	private final long now = System.currentTimeMillis();
+
+	@Test
+	public void testObserverIsCalledWhenCheckSucceeds() throws Exception {
+		ContactMailboxConnectivityChecker checker = createChecker();
+		AtomicReference<ApiCall> apiCall = new AtomicReference<>(null);
+
+		// When checkConnectivity() is called a check should be started
+		context.checking(new Expectations() {{
+			oneOf(clock).currentTimeMillis();
+			will(returnValue(now));
+			oneOf(mailboxApiCaller).retryWithBackoff(with(any(ApiCall.class)));
+			will(new DoAllAction(
+					new CaptureArgumentAction<>(apiCall, ApiCall.class, 0),
+					returnValue(task)
+			));
+		}});
+
+		checker.checkConnectivity(properties, observer);
+
+		// When the check succeeds the observer should be called
+		context.checking(new Expectations() {{
+			oneOf(mailboxApi).checkStatus(properties);
+			will(returnValue(true));
+			oneOf(clock).currentTimeMillis();
+			will(returnValue(now));
+			oneOf(observer).onConnectivityCheckSucceeded();
+		}});
+
+		// The call should not be retried
+		assertFalse(apiCall.get().callApi());
+	}
+
+	@Test
+	public void testObserverIsNotCalledWhenCheckFails() throws Exception {
+		ContactMailboxConnectivityChecker checker = createChecker();
+		AtomicReference<ApiCall> apiCall = new AtomicReference<>(null);
+
+		// When checkConnectivity() is called a check should be started
+		context.checking(new Expectations() {{
+			oneOf(clock).currentTimeMillis();
+			will(returnValue(now));
+			oneOf(mailboxApiCaller).retryWithBackoff(with(any(ApiCall.class)));
+			will(new DoAllAction(
+					new CaptureArgumentAction<>(apiCall, ApiCall.class, 0),
+					returnValue(task)
+			));
+		}});
+
+		checker.checkConnectivity(properties, observer);
+
+		// When the check fails, the observer should not be called
+		context.checking(new Expectations() {{
+			oneOf(mailboxApi).checkStatus(properties);
+			will(throwException(new IOException()));
+		}});
+
+		// The call should be retried
+		assertTrue(apiCall.get().callApi());
+	}
+
+	private ContactMailboxConnectivityChecker createChecker() {
+		return new ContactMailboxConnectivityChecker(clock, mailboxApiCaller,
+				mailboxApi);
+	}
+}

bramble-core/src/test/java/org/briarproject/bramble/mailbox/MailboxApiCallerImplTest.java

@@ -0,0 +1,135 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.system.TaskScheduler;
+import org.briarproject.bramble.test.BrambleMockTestCase;
+import org.briarproject.bramble.test.CaptureArgumentAction;
+import org.jmock.Expectations;
+import org.jmock.lib.action.DoAllAction;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.Executor;
+import java.util.concurrent.atomic.AtomicReference;
+
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+import static org.briarproject.bramble.mailbox.MailboxApiCaller.MAX_RETRY_INTERVAL_MS;
+import static org.briarproject.bramble.mailbox.MailboxApiCaller.MIN_RETRY_INTERVAL_MS;
+
+public class MailboxApiCallerImplTest extends BrambleMockTestCase {
+
+	private final TaskScheduler taskScheduler =
+			context.mock(TaskScheduler.class);
+	private final Executor ioExecutor = context.mock(Executor.class);
+	private final ApiCall apiCall = context.mock(ApiCall.class);
+	private final Cancellable scheduledTask = context.mock(Cancellable.class);
+
+	private final MailboxApiCallerImpl caller =
+			new MailboxApiCallerImpl(taskScheduler, ioExecutor);
+
+	@Test
+	public void testSubmitsTaskImmediately() {
+		// Calling retryWithBackoff() should schedule the first try immediately
+		AtomicReference<Runnable> runnable = new AtomicReference<>(null);
+		context.checking(new Expectations() {{
+			oneOf(ioExecutor).execute(with(any(Runnable.class)));
+			will(new CaptureArgumentAction<>(runnable, Runnable.class, 0));
+		}});
+
+		caller.retryWithBackoff(apiCall);
+
+		// When the task runs, the API call should be called. The call
+		// returns false, so no retries should be scheduled
+		context.checking(new Expectations() {{
+			oneOf(apiCall).callApi();
+			will(returnValue(false));
+		}});
+
+		runnable.get().run();
+	}
+
+	@Test
+	public void testDoesNotRetryTaskIfCancelled() {
+		// Calling retryWithBackoff() should schedule the first try immediately
+		AtomicReference<Runnable> runnable = new AtomicReference<>(null);
+		context.checking(new Expectations() {{
+			oneOf(ioExecutor).execute(with(any(Runnable.class)));
+			will(new CaptureArgumentAction<>(runnable, Runnable.class, 0));
+		}});
+
+		Cancellable returned = caller.retryWithBackoff(apiCall);
+
+		// When the task runs, the API call should be called. The call
+		// returns true, so a retry should be scheduled
+		context.checking(new Expectations() {{
+			oneOf(apiCall).callApi();
+			will(returnValue(true));
+			oneOf(taskScheduler).schedule(with(any(Runnable.class)),
+					with(ioExecutor), with(MIN_RETRY_INTERVAL_MS),
+					with(MILLISECONDS));
+			will(new DoAllAction(
+					new CaptureArgumentAction<>(runnable, Runnable.class, 0),
+					returnValue(scheduledTask)
+			));
+		}});
+
+		runnable.get().run();
+
+		// When the Cancellable returned by retryWithBackoff() is cancelled,
+		// the scheduled task should be cancelled
+		context.checking(new Expectations() {{
+			oneOf(scheduledTask).cancel();
+		}});
+
+		returned.cancel();
+
+		// Cancelling again should have no effect
+		returned.cancel();
+
+		// If the scheduled task runs anyway (cancellation came too late),
+		// the API call should not be called and no further tries should be
+		// scheduled
+		runnable.get().run();
+	}
+
+	@Test
+	public void testDoublesRetryIntervalUntilMaximumIsReached() {
+		// The expected retry intervals increase from the min to the max
+		List<Long> expectedIntervals = new ArrayList<>();
+		for (long interval = MIN_RETRY_INTERVAL_MS;
+				interval <= MAX_RETRY_INTERVAL_MS; interval *= 2) {
+			expectedIntervals.add(interval);
+		}
+		// Once the interval reaches the maximum it should be capped
+		expectedIntervals.add(MAX_RETRY_INTERVAL_MS);
+		expectedIntervals.add(MAX_RETRY_INTERVAL_MS);
+
+		// Calling retryWithBackoff() should schedule the first try immediately
+		AtomicReference<Runnable> runnable = new AtomicReference<>(null);
+		context.checking(new Expectations() {{
+			oneOf(ioExecutor).execute(with(any(Runnable.class)));
+			will(new CaptureArgumentAction<>(runnable, Runnable.class, 0));
+		}});
+
+		caller.retryWithBackoff(apiCall);
+
+		// Each time the task runs, the API call returns true, so a retry
+		// should be scheduled with a longer interval
+		for (long interval : expectedIntervals) {
+			context.checking(new Expectations() {{
+				oneOf(apiCall).callApi();
+				will(returnValue(true));
+				oneOf(taskScheduler).schedule(with(any(Runnable.class)),
+						with(ioExecutor), with(interval), with(MILLISECONDS));
+				will(new DoAllAction(
+						new CaptureArgumentAction<>(
+								runnable, Runnable.class, 0),
+						returnValue(scheduledTask)
+				));
+			}});
+
+			runnable.get().run();
+		}
+	}
+}

bramble-core/src/test/java/org/briarproject/bramble/mailbox/MailboxApiTest.java

@@ -9,6 +9,7 @@ import org.briarproject.bramble.api.mailbox.MailboxFileId;
 import org.briarproject.bramble.api.mailbox.MailboxFolderId;
 import org.briarproject.bramble.api.mailbox.MailboxId;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
 import org.briarproject.bramble.mailbox.MailboxApi.MailboxContact;
 import org.briarproject.bramble.mailbox.MailboxApi.MailboxFile;
@@ -34,9 +35,12 @@ import okio.Buffer;
 
 import static java.util.Collections.singletonList;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
+import static org.briarproject.bramble.mailbox.MailboxApi.CLIENT_SUPPORTS;
 import static org.briarproject.bramble.test.TestUtils.getContactId;
+import static org.briarproject.bramble.test.TestUtils.getMailboxProperties;
 import static org.briarproject.bramble.test.TestUtils.getRandomBytes;
 import static org.briarproject.bramble.test.TestUtils.getRandomId;
+import static org.briarproject.bramble.test.TestUtils.mailboxPropertiesEqual;
 import static org.briarproject.bramble.test.TestUtils.readBytes;
 import static org.briarproject.bramble.test.TestUtils.writeBytes;
 import static org.briarproject.bramble.util.StringUtils.getRandomString;
@@ -79,12 +83,109 @@ public class MailboxApiTest extends BrambleTestCase {
 			contactId, contactToken, contactInboxId, contactOutboxId);
 
 	@Test
-	public void testSetup() throws Exception {
+	public void testGetServerSupports() throws Exception {
-		String validResponse = "{\"token\":\"" + token2 + "\"}";
+		String validVersions = "[ {\"major\":1,\"minor\":0} ]";
+		String validResponse = makeVersionsResponse(validVersions);
 		String invalidResponse = "{\"foo\":\"bar\"}";
-		String invalidTokenResponse = "{\"token\":{\"foo\":\"bar\"}}";
+		String invalidVersionsResponse = makeVersionsResponse("42");
-		String invalidTokenResponse2 =
+		String invalidVersionsResponse2 = makeVersionsResponse("[ 1,0 ]");
-				"{\"token\":\"" + getRandomString(64) + "\"}";
+		String invalidVersionsResponse3 =
+				makeVersionsResponse("[ {\"major\":1, \"minor\":-1} ]");
+
+		MockWebServer server = new MockWebServer();
+		server.enqueue(new MockResponse().setBody(validResponse));
+		server.enqueue(new MockResponse().setBody(invalidResponse));
+		server.enqueue(new MockResponse().setResponseCode(401));
+		server.enqueue(new MockResponse().setResponseCode(500));
+		server.enqueue(new MockResponse().setBody(invalidVersionsResponse));
+		server.enqueue(new MockResponse().setBody(invalidVersionsResponse2));
+		server.enqueue(new MockResponse().setBody(invalidVersionsResponse3));
+		server.start();
+		String baseUrl = getBaseUrl(server);
+		List<MailboxVersion> versions = singletonList(new MailboxVersion(1, 0));
+		MailboxProperties properties =
+				new MailboxProperties(baseUrl, token, new ArrayList<>());
+		MailboxProperties properties2 =
+				new MailboxProperties(baseUrl, token2, new ArrayList<>());
+
+		RecordedRequest request;
+
+		// valid response with valid token
+		assertEquals(versions, api.getServerSupports(properties));
+		request = server.takeRequest();
+		assertEquals("/versions", request.getPath());
+		assertEquals("GET", request.getMethod());
+		assertToken(request, token);
+
+		// invalid response
+		assertThrows(ApiException.class,
+				() -> api.getServerSupports(properties));
+		request = server.takeRequest();
+		assertEquals("/versions", request.getPath());
+		assertEquals("GET", request.getMethod());
+		assertToken(request, token);
+
+		// 401 response
+		assertThrows(ApiException.class,
+				() -> api.getServerSupports(properties2));
+		request = server.takeRequest();
+		assertEquals("/versions", request.getPath());
+		assertEquals("GET", request.getMethod());
+		assertToken(request, token2);
+
+		// 500 response
+		assertThrows(ApiException.class,
+				() -> api.getServerSupports(properties));
+		request = server.takeRequest();
+		assertEquals("/versions", request.getPath());
+		assertEquals("GET", request.getMethod());
+		assertToken(request, token);
+
+		// invalid non-array serverSupports response
+		assertThrows(ApiException.class,
+				() -> api.getServerSupports(properties));
+		request = server.takeRequest();
+		assertEquals("/versions", request.getPath());
+		assertEquals("GET", request.getMethod());
+		assertToken(request, token);
+
+		// invalid non-object in serverSupports array response
+		assertThrows(ApiException.class,
+				() -> api.getServerSupports(properties));
+		request = server.takeRequest();
+		assertEquals("/versions", request.getPath());
+		assertEquals("GET", request.getMethod());
+		assertToken(request, token);
+
+		// invalid negative minor version in serverSupports response
+		assertThrows(ApiException.class,
+				() -> api.getServerSupports(properties));
+		request = server.takeRequest();
+		assertEquals("/versions", request.getPath());
+		assertEquals("GET", request.getMethod());
+		assertToken(request, token);
+	}
+
+	private String makeVersionsResponse(String versions) {
+		return "{\"serverSupports\":" + versions + "}";
+	}
+
+	@Test
+	public void testSetup() throws Exception {
+		String validVersions = "[ {\"major\":1,\"minor\":0} ]";
+		String validResponse = makeSetupResponse(
+				"\"" + token2 + "\"", validVersions);
+		String invalidResponse = "{\"foo\":\"bar\"}";
+		String invalidTokenResponse = makeSetupResponse(
+				"{\"foo\":\"bar\"}", validVersions);
+		String invalidTokenResponse2 = makeSetupResponse(
+				"\"" + getRandomString(64) + "\"", validVersions);
+		String invalidVersionsResponse = makeSetupResponse(
+				"\"" + token2 + "\"", "42");
+		String invalidVersionsResponse2 = makeSetupResponse(
+				"\"" + token2 + "\"", "[ 1,0 ]");
+		String invalidVersionsResponse3 = makeSetupResponse(
+				"\"" + token2 + "\"", "[ {\"major\":1, \"minor\":-1} ]");
 
 		MockWebServer server = new MockWebServer();
 		server.enqueue(new MockResponse().setBody(validResponse));
@@ -94,15 +195,18 @@ public class MailboxApiTest extends BrambleTestCase {
 		server.enqueue(new MockResponse().setResponseCode(500));
 		server.enqueue(new MockResponse().setBody(invalidTokenResponse));
 		server.enqueue(new MockResponse().setBody(invalidTokenResponse2));
+		server.enqueue(new MockResponse().setBody(invalidVersionsResponse));
+		server.enqueue(new MockResponse().setBody(invalidVersionsResponse2));
+		server.enqueue(new MockResponse().setBody(invalidVersionsResponse3));
 		server.start();
 		String baseUrl = getBaseUrl(server);
 		MailboxProperties properties =
-				new MailboxProperties(baseUrl, token, true);
+				new MailboxProperties(baseUrl, token, new ArrayList<>());
 		MailboxProperties properties2 =
-				new MailboxProperties(baseUrl, token2, true);
+				new MailboxProperties(baseUrl, token2, new ArrayList<>());
 
 		// valid response with valid token
-		assertEquals(token2, api.setup(properties));
+		mailboxPropertiesEqual(properties2, api.setup(properties));
 		RecordedRequest request1 = server.takeRequest();
 		assertEquals("/setup", request1.getPath());
 		assertEquals("PUT", request1.getMethod());
@@ -149,12 +253,38 @@ public class MailboxApiTest extends BrambleTestCase {
 		assertEquals("/setup", request7.getPath());
 		assertEquals("PUT", request7.getMethod());
 		assertToken(request7, token);
+
+		// invalid non-array serverSupports response
+		assertThrows(ApiException.class, () -> api.setup(properties));
+		RecordedRequest request8 = server.takeRequest();
+		assertEquals("/setup", request8.getPath());
+		assertEquals("PUT", request8.getMethod());
+		assertToken(request8, token);
+
+		// invalid non-object in serverSupports array response
+		assertThrows(ApiException.class, () -> api.setup(properties));
+		RecordedRequest request9 = server.takeRequest();
+		assertEquals("/setup", request9.getPath());
+		assertEquals("PUT", request9.getMethod());
+		assertToken(request9, token);
+
+		// invalid negative minor version in serverSupports response
+		assertThrows(ApiException.class, () -> api.setup(properties));
+		RecordedRequest request10 = server.takeRequest();
+		assertEquals("/setup", request10.getPath());
+		assertEquals("PUT", request10.getMethod());
+		assertToken(request10, token);
+	}
+
+	private String makeSetupResponse(String token, String versions) {
+		return "{\"token\":" + token + "," +
+				"\"serverSupports\":" + versions + "}";
 	}
 
 	@Test
 	public void testSetupOnlyForOwner() {
 		MailboxProperties properties =
-				new MailboxProperties("", token, false);
+				getMailboxProperties(false, CLIENT_SUPPORTS);
 		assertThrows(
 				IllegalArgumentException.class,
 				() -> api.setup(properties)
@@ -170,9 +300,9 @@ public class MailboxApiTest extends BrambleTestCase {
 		server.start();
 		String baseUrl = getBaseUrl(server);
 		MailboxProperties properties =
-				new MailboxProperties(baseUrl, token, true);
+				new MailboxProperties(baseUrl, token, new ArrayList<>());
 		MailboxProperties properties2 =
-				new MailboxProperties(baseUrl, token2, true);
+				new MailboxProperties(baseUrl, token2, new ArrayList<>());
 
 		assertTrue(api.checkStatus(properties));
 		RecordedRequest request1 = server.takeRequest();
@@ -190,16 +320,6 @@ public class MailboxApiTest extends BrambleTestCase {
 		assertToken(request3, token);
 	}
 
-	@Test
-	public void testStatusOnlyForOwner() {
-		MailboxProperties properties =
-				new MailboxProperties("", token, false);
-		assertThrows(
-				IllegalArgumentException.class,
-				() -> api.checkStatus(properties)
-		);
-	}
-
 	@Test
 	public void testWipe() throws Exception {
 		MockWebServer server = new MockWebServer();
@@ -210,9 +330,9 @@ public class MailboxApiTest extends BrambleTestCase {
 		server.start();
 		String baseUrl = getBaseUrl(server);
 		MailboxProperties properties =
-				new MailboxProperties(baseUrl, token, true);
+				new MailboxProperties(baseUrl, token, new ArrayList<>());
 		MailboxProperties properties2 =
-				new MailboxProperties(baseUrl, token2, true);
+				new MailboxProperties(baseUrl, token2, new ArrayList<>());
 
 		api.wipeMailbox(properties);
 		RecordedRequest request1 = server.takeRequest();
@@ -242,7 +362,7 @@ public class MailboxApiTest extends BrambleTestCase {
 	@Test
 	public void testWipeOnlyForOwner() {
 		MailboxProperties properties =
-				new MailboxProperties("", token, false);
+				getMailboxProperties(false, CLIENT_SUPPORTS);
 		assertThrows(IllegalArgumentException.class, () ->
 				api.wipeMailbox(properties));
 	}
@@ -256,7 +376,7 @@ public class MailboxApiTest extends BrambleTestCase {
 		server.start();
 		String baseUrl = getBaseUrl(server);
 		MailboxProperties properties =
-				new MailboxProperties(baseUrl, token, true);
+				new MailboxProperties(baseUrl, token, new ArrayList<>());
 
 		// contact gets added as expected
 		api.addContact(properties, mailboxContact);
@@ -288,7 +408,7 @@ public class MailboxApiTest extends BrambleTestCase {
 	@Test
 	public void testAddContactOnlyForOwner() {
 		MailboxProperties properties =
-				new MailboxProperties("", token, false);
+				getMailboxProperties(false, CLIENT_SUPPORTS);
 		assertThrows(IllegalArgumentException.class, () ->
 				api.addContact(properties, mailboxContact));
 	}
@@ -303,7 +423,7 @@ public class MailboxApiTest extends BrambleTestCase {
 		server.start();
 		String baseUrl = getBaseUrl(server);
 		MailboxProperties properties =
-				new MailboxProperties(baseUrl, token, true);
+				new MailboxProperties(baseUrl, token, new ArrayList<>());
 
 		// contact gets deleted as expected
 		api.deleteContact(properties, contactId);
@@ -340,7 +460,7 @@ public class MailboxApiTest extends BrambleTestCase {
 	@Test
 	public void testDeleteContactOnlyForOwner() {
 		MailboxProperties properties =
-				new MailboxProperties("", token, false);
+				getMailboxProperties(false, CLIENT_SUPPORTS);
 		assertThrows(IllegalArgumentException.class, () ->
 				api.deleteContact(properties, contactId));
 	}
@@ -367,7 +487,7 @@ public class MailboxApiTest extends BrambleTestCase {
 		server.start();
 		String baseUrl = getBaseUrl(server);
 		MailboxProperties properties =
-				new MailboxProperties(baseUrl, token, true);
+				new MailboxProperties(baseUrl, token, new ArrayList<>());
 
 		// valid response with two contacts
 		assertEquals(singletonList(contactId), api.getContacts(properties));
@@ -432,7 +552,7 @@ public class MailboxApiTest extends BrambleTestCase {
 	@Test
 	public void testGetContactsOnlyForOwner() {
 		MailboxProperties properties =
-				new MailboxProperties("", token, false);
+				getMailboxProperties(false, CLIENT_SUPPORTS);
 		assertThrows(
 				IllegalArgumentException.class,
 				() -> api.getContacts(properties)
@@ -452,7 +572,7 @@ public class MailboxApiTest extends BrambleTestCase {
 		server.start();
 		String baseUrl = getBaseUrl(server);
 		MailboxProperties properties =
-				new MailboxProperties(baseUrl, token, true);
+				new MailboxProperties(baseUrl, token, new ArrayList<>());
 
 		// file gets uploaded as expected
 		api.addFile(properties, contactInboxId, file);
@@ -511,7 +631,7 @@ public class MailboxApiTest extends BrambleTestCase {
 		server.start();
 		String baseUrl = getBaseUrl(server);
 		MailboxProperties properties =
-				new MailboxProperties(baseUrl, token, true);
+				new MailboxProperties(baseUrl, token, new ArrayList<>());
 
 		// valid response with one file
 		List<MailboxFile> received1 = api.getFiles(properties, contactInboxId);
@@ -607,7 +727,7 @@ public class MailboxApiTest extends BrambleTestCase {
 		server.start();
 		String baseUrl = getBaseUrl(server);
 		MailboxProperties properties =
-				new MailboxProperties(baseUrl, token, true);
+				new MailboxProperties(baseUrl, token, new ArrayList<>());
 
 		// file gets downloaded as expected
 		api.getFile(properties, contactOutboxId, name, file1);
@@ -651,7 +771,7 @@ public class MailboxApiTest extends BrambleTestCase {
 		server.start();
 		String baseUrl = getBaseUrl(server);
 		MailboxProperties properties =
-				new MailboxProperties(baseUrl, token, true);
+				new MailboxProperties(baseUrl, token, new ArrayList<>());
 
 		// file gets deleted as expected
 		api.deleteFile(properties, contactInboxId, name);
@@ -715,7 +835,7 @@ public class MailboxApiTest extends BrambleTestCase {
 		server.start();
 		String baseUrl = getBaseUrl(server);
 		MailboxProperties properties =
-				new MailboxProperties(baseUrl, token, true);
+				new MailboxProperties(baseUrl, token, new ArrayList<>());
 
 		// valid response with one folders
 		assertEquals(singletonList(id1), api.getFolders(properties));
@@ -784,7 +904,7 @@ public class MailboxApiTest extends BrambleTestCase {
 	@Test
 	public void testGetFoldersOnlyForOwner() {
 		MailboxProperties properties =
-				new MailboxProperties("", token, false);
+				getMailboxProperties(false, CLIENT_SUPPORTS);
 		assertThrows(IllegalArgumentException.class, () ->
 				api.getFolders(properties));
 	}

bramble-core/src/test/java/org/briarproject/bramble/mailbox/MailboxIntegrationTest.java

@@ -21,6 +21,7 @@ import org.junit.rules.TemporaryFolder;
 
 import java.io.File;
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 
@@ -89,17 +90,16 @@ public class MailboxIntegrationTest extends BrambleTestCase {
 		assumeTrue(isOptionalTestEnabled(MailboxIntegrationTest.class));
 
 		if (ownerProperties != null) return;
-		MailboxProperties setupProperties =
+		MailboxProperties setupProperties = new MailboxProperties(
-				new MailboxProperties(URL_BASE, SETUP_TOKEN, true);
+				URL_BASE, SETUP_TOKEN, new ArrayList<>());
-		MailboxAuthToken ownerToken = api.setup(setupProperties);
+		ownerProperties = api.setup(setupProperties);
-		ownerProperties = new MailboxProperties(URL_BASE, ownerToken, true);
 	}
 
 	@AfterClass
 	// we can't test wiping as a regular test as it stops the mailbox
 	public static void wipe() throws IOException, ApiException {
 		if (!isOptionalTestEnabled(MailboxIntegrationTest.class)) return;
-		
+
 		api.wipeMailbox(ownerProperties);
 
 		// check doesn't work anymore
@@ -107,14 +107,30 @@ public class MailboxIntegrationTest extends BrambleTestCase {
 				api.checkStatus(ownerProperties));
 
 		// new setup doesn't work as mailbox is stopping
-		MailboxProperties setupProperties =
+		MailboxProperties setupProperties = new MailboxProperties(
-				new MailboxProperties(URL_BASE, SETUP_TOKEN, true);
+				URL_BASE, SETUP_TOKEN, new ArrayList<>());
 		assertThrows(ApiException.class, () -> api.setup(setupProperties));
 	}
 
 	@Test
 	public void testStatus() throws Exception {
+		// Owner calls status endpoint
 		assertTrue(api.checkStatus(ownerProperties));
+
+		// Owner adds contact
+		ContactId contactId = new ContactId(1);
+		MailboxContact contact = getMailboxContact(contactId);
+		MailboxProperties contactProperties = new MailboxProperties(
+				ownerProperties.getBaseUrl(), contact.token,
+				new ArrayList<>(), contact.inboxId, contact.outboxId);
+		api.addContact(ownerProperties, contact);
+
+		// Contact calls status endpoint
+		assertTrue(api.checkStatus(contactProperties));
+
+		// Owner deletes contact again to leave clean state for other tests
+		api.deleteContact(ownerProperties, contactId);
+		assertEquals(emptyList(), api.getContacts(ownerProperties));
 	}
 
 	@Test
@@ -151,7 +167,8 @@ public class MailboxIntegrationTest extends BrambleTestCase {
 		ContactId contactId = new ContactId(1);
 		MailboxContact contact = getMailboxContact(contactId);
 		MailboxProperties contactProperties = new MailboxProperties(
-				ownerProperties.getBaseUrl(), contact.token, false);
+				ownerProperties.getBaseUrl(), contact.token,
+				new ArrayList<>(), contact.inboxId, contact.outboxId);
 		api.addContact(ownerProperties, contact);
 
 		// upload a file for our contact

bramble-core/src/test/java/org/briarproject/bramble/mailbox/MailboxPairingTaskImplTest.java

@@ -9,9 +9,11 @@ import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
 import org.briarproject.bramble.api.mailbox.MailboxPairingState;
 import org.briarproject.bramble.api.mailbox.MailboxPairingTask;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
-import org.briarproject.bramble.api.mailbox.MailboxPropertyManager;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
-import org.briarproject.bramble.api.mailbox.OwnMailboxConnectionStatusEvent;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
+import org.briarproject.bramble.api.mailbox.event.OwnMailboxConnectionStatusEvent;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.test.BrambleMockTestCase;
 import org.briarproject.bramble.test.DbExpectations;
@@ -23,10 +25,11 @@ import org.junit.Test;
 import java.io.IOException;
 import java.nio.ByteBuffer;
 import java.nio.charset.Charset;
-import java.util.Collections;
+import java.util.ArrayList;
 import java.util.concurrent.Executor;
 import java.util.concurrent.atomic.AtomicInteger;
 
+import static java.util.Collections.singletonList;
 import static org.briarproject.bramble.test.TestUtils.getContact;
 import static org.briarproject.bramble.test.TestUtils.getRandomBytes;
 import static org.briarproject.bramble.test.TestUtils.getRandomId;
@@ -46,25 +49,25 @@ public class MailboxPairingTaskImplTest extends BrambleMockTestCase {
 	private final MailboxApi api = context.mock(MailboxApi.class);
 	private final MailboxSettingsManager mailboxSettingsManager =
 			context.mock(MailboxSettingsManager.class);
-	private final MailboxPropertyManager mailboxPropertyManager =
+	private final MailboxUpdateManager mailboxUpdateManager =
-			context.mock(MailboxPropertyManager.class);
+			context.mock(MailboxUpdateManager.class);
 	private final MailboxPairingTaskFactory factory =
 			new MailboxPairingTaskFactoryImpl(executor, db, crypto, clock, api,
-					mailboxSettingsManager, mailboxPropertyManager);
+					mailboxSettingsManager, mailboxUpdateManager);
 
 	private final String onion = getRandomString(56);
 	private final byte[] onionBytes = getRandomBytes(32);
-	private final String onionAddress = "http://" + onion + ".onion";
+	private final String baseUrl = "http://" + onion + ".onion"; // TODO
 	private final MailboxAuthToken setupToken =
 			new MailboxAuthToken(getRandomId());
 	private final MailboxAuthToken ownerToken =
 			new MailboxAuthToken(getRandomId());
 	private final String validPayload = getValidPayload();
 	private final long time = System.currentTimeMillis();
-	private final MailboxProperties setupProperties =
+	private final MailboxProperties setupProperties = new MailboxProperties(
-			new MailboxProperties(onionAddress, setupToken, true);
+			baseUrl, setupToken, new ArrayList<>());
-	private final MailboxProperties ownerProperties =
+	private final MailboxProperties ownerProperties = new MailboxProperties(
-			new MailboxProperties(onionAddress, ownerToken, true);
+			baseUrl, ownerToken, new ArrayList<>());
 
 	@Test
 	public void testInitialQrCodeReceivedState() {
@@ -98,22 +101,24 @@ public class MailboxPairingTaskImplTest extends BrambleMockTestCase {
 			oneOf(crypto).encodeOnion(onionBytes);
 			will(returnValue(onion));
 			oneOf(api).setup(with(matches(setupProperties)));
-			will(returnValue(ownerToken));
+			will(returnValue(ownerProperties));
 			oneOf(clock).currentTimeMillis();
 			will(returnValue(time));
 		}});
 		Contact contact1 = getContact();
 		Transaction txn = new Transaction(null, false);
+		MailboxUpdate updateNoMailbox = new MailboxUpdate(
+				singletonList(new MailboxVersion(47, 11)));
 		context.checking(new DbExpectations() {{
 			oneOf(db).transaction(with(false), withDbRunnable(txn));
 			oneOf(mailboxSettingsManager).setOwnMailboxProperties(
 					with(txn), with(matches(ownerProperties)));
 			oneOf(mailboxSettingsManager).recordSuccessfulConnection(txn, time);
 			oneOf(db).getContacts(txn);
-			will(returnValue(Collections.singletonList(contact1)));
+			will(returnValue(singletonList(contact1)));
-			oneOf(mailboxPropertyManager).getRemoteProperties(txn,
+			oneOf(mailboxUpdateManager).getRemoteUpdate(txn,
 					contact1.getId());
-			will(returnValue(null));
+			will(returnValue(updateNoMailbox));
 			oneOf(db).resetUnackedMessagesToSend(txn, contact1.getId());
 		}});
 
@@ -174,7 +179,7 @@ public class MailboxPairingTaskImplTest extends BrambleMockTestCase {
 			oneOf(crypto).encodeOnion(onionBytes);
 			will(returnValue(onion));
 			oneOf(api).setup(with(matches(setupProperties)));
-			will(returnValue(ownerToken));
+			will(returnValue(ownerProperties));
 			oneOf(clock).currentTimeMillis();
 			will(returnValue(time));
 		}});
@@ -206,7 +211,8 @@ public class MailboxPairingTaskImplTest extends BrambleMockTestCase {
 		return new PredicateMatcher<>(MailboxProperties.class, p1 ->
 				p1.getAuthToken().equals(p2.getAuthToken()) &&
 						p1.getBaseUrl().equals(p2.getBaseUrl()) &&
-						p1.isOwner() == p2.isOwner());
+						p1.isOwner() == p2.isOwner() &&
+						p1.getServerSupports().equals(p2.getServerSupports()));
 	}
 
 }

bramble-core/src/test/java/org/briarproject/bramble/mailbox/MailboxPropertyValidatorTest.java

@@ -1,99 +0,0 @@
-package org.briarproject.bramble.mailbox;
-
-import org.briarproject.bramble.api.FormatException;
-import org.briarproject.bramble.api.client.ClientHelper;
-import org.briarproject.bramble.api.data.BdfDictionary;
-import org.briarproject.bramble.api.data.BdfEntry;
-import org.briarproject.bramble.api.data.BdfList;
-import org.briarproject.bramble.api.data.MetadataEncoder;
-import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
-import org.briarproject.bramble.api.mailbox.MailboxFolderId;
-import org.briarproject.bramble.api.mailbox.MailboxPropertiesUpdate;
-import org.briarproject.bramble.api.mailbox.MailboxPropertyManager;
-import org.briarproject.bramble.api.sync.Group;
-import org.briarproject.bramble.api.sync.Message;
-import org.briarproject.bramble.api.system.Clock;
-import org.briarproject.bramble.test.BrambleMockTestCase;
-import org.jmock.Expectations;
-import org.junit.Test;
-
-import java.io.IOException;
-
-import static org.briarproject.bramble.test.TestUtils.getGroup;
-import static org.briarproject.bramble.test.TestUtils.getMessage;
-import static org.briarproject.bramble.test.TestUtils.getRandomId;
-import static org.junit.Assert.assertEquals;
-
-public class MailboxPropertyValidatorTest extends BrambleMockTestCase {
-
-	private final ClientHelper clientHelper = context.mock(ClientHelper.class);
-
-	private final BdfDictionary bdfDict;
-	private final MailboxPropertiesUpdate mailboxProps;
-	private final Group group;
-	private final Message message;
-	private final MailboxPropertyValidator mpv;
-
-	public MailboxPropertyValidatorTest() {
-		// Just dummies, clientHelper is mocked so our test is a bit shallow;
-		//  not testing
-		//  {@link ClientHelper#parseAndValidateMailboxPropertiesUpdate(BdfDictionary)}
-		bdfDict = BdfDictionary.of(new BdfEntry("foo", "bar"));
-		mailboxProps = new MailboxPropertiesUpdate("baz",
-				new MailboxAuthToken(getRandomId()),
-				new MailboxFolderId(getRandomId()),
-				new MailboxFolderId(getRandomId()));
-
-		group = getGroup(MailboxPropertyManager.CLIENT_ID,
-				MailboxPropertyManager.MAJOR_VERSION);
-		message = getMessage(group.getId());
-
-		MetadataEncoder metadataEncoder = context.mock(MetadataEncoder.class);
-		Clock clock = context.mock(Clock.class);
-		mpv = new MailboxPropertyValidator(clientHelper, metadataEncoder,
-				clock);
-	}
-
-	@Test
-	public void testValidateMessageBody() throws IOException {
-		BdfList body = BdfList.of(4, bdfDict);
-
-		context.checking(new Expectations() {{
-			oneOf(clientHelper).parseAndValidateMailboxPropertiesUpdate(
-					bdfDict);
-			will(returnValue(mailboxProps));
-		}});
-
-		BdfDictionary result =
-				mpv.validateMessage(message, group, body).getDictionary();
-		assertEquals(4, result.getLong("version").longValue());
-	}
-
-	@Test(expected = FormatException.class)
-	public void testValidateWrongVersionValue() throws IOException {
-		BdfList body = BdfList.of(-1, bdfDict);
-		mpv.validateMessage(message, group, body);
-	}
-
-	@Test(expected = FormatException.class)
-	public void testValidateWrongVersionType() throws IOException {
-		BdfList body = BdfList.of(bdfDict, bdfDict);
-		mpv.validateMessage(message, group, body);
-	}
-
-	@Test
-	public void testEmptyPropertiesReturnsNull() throws IOException {
-		BdfDictionary emptyBdfDict = new BdfDictionary();
-		BdfList body = BdfList.of(42, emptyBdfDict);
-
-		context.checking(new Expectations() {{
-			oneOf(clientHelper).parseAndValidateMailboxPropertiesUpdate(
-					emptyBdfDict);
-			will(returnValue(null));
-		}});
-
-		BdfDictionary result =
-				mpv.validateMessage(message, group, body).getDictionary();
-		assertEquals(42, result.getLong("version").longValue());
-	}
-}

bramble-core/src/test/java/org/briarproject/bramble/mailbox/MailboxSettingsManagerImplTest.java

@@ -6,19 +6,23 @@ import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
 import org.briarproject.bramble.api.mailbox.MailboxStatus;
-import org.briarproject.bramble.api.mailbox.OwnMailboxConnectionStatusEvent;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
+import org.briarproject.bramble.api.mailbox.event.OwnMailboxConnectionStatusEvent;
 import org.briarproject.bramble.api.settings.Settings;
 import org.briarproject.bramble.api.settings.SettingsManager;
 import org.briarproject.bramble.test.BrambleMockTestCase;
 import org.jmock.Expectations;
 import org.junit.Test;
 
+import java.util.List;
 import java.util.Random;
 
+import static java.util.Arrays.asList;
 import static org.briarproject.bramble.mailbox.MailboxSettingsManagerImpl.SETTINGS_KEY_ATTEMPTS;
 import static org.briarproject.bramble.mailbox.MailboxSettingsManagerImpl.SETTINGS_KEY_LAST_ATTEMPT;
 import static org.briarproject.bramble.mailbox.MailboxSettingsManagerImpl.SETTINGS_KEY_LAST_SUCCESS;
 import static org.briarproject.bramble.mailbox.MailboxSettingsManagerImpl.SETTINGS_KEY_ONION;
+import static org.briarproject.bramble.mailbox.MailboxSettingsManagerImpl.SETTINGS_KEY_SERVER_SUPPORTS;
 import static org.briarproject.bramble.mailbox.MailboxSettingsManagerImpl.SETTINGS_KEY_TOKEN;
 import static org.briarproject.bramble.mailbox.MailboxSettingsManagerImpl.SETTINGS_NAMESPACE;
 import static org.briarproject.bramble.mailbox.MailboxSettingsManagerImpl.SETTINGS_UPLOADS_NAMESPACE;
@@ -40,6 +44,9 @@ public class MailboxSettingsManagerImplTest extends BrambleMockTestCase {
 	private final Random random = new Random();
 	private final String onion = getRandomString(64);
 	private final MailboxAuthToken token = new MailboxAuthToken(getRandomId());
+	private final List<MailboxVersion> serverSupports =
+			asList(new MailboxVersion(1, 0), new MailboxVersion(1, 1));
+	private final int[] serverSupportsInts = {1, 0, 1, 1};
 	private final ContactId contactId1 = new ContactId(random.nextInt());
 	private final ContactId contactId2 = new ContactId(random.nextInt());
 	private final ContactId contactId3 = new ContactId(random.nextInt());
@@ -67,6 +74,7 @@ public class MailboxSettingsManagerImplTest extends BrambleMockTestCase {
 		Settings settings = new Settings();
 		settings.put(SETTINGS_KEY_ONION, onion);
 		settings.put(SETTINGS_KEY_TOKEN, token.toString());
+		settings.putIntArray(SETTINGS_KEY_SERVER_SUPPORTS, serverSupportsInts);
 
 		context.checking(new Expectations() {{
 			oneOf(settingsManager).getSettings(txn, SETTINGS_NAMESPACE);
@@ -77,6 +85,7 @@ public class MailboxSettingsManagerImplTest extends BrambleMockTestCase {
 		assertNotNull(properties);
 		assertEquals(onion, properties.getBaseUrl());
 		assertEquals(token, properties.getAuthToken());
+		assertEquals(serverSupports, properties.getServerSupports());
 		assertTrue(properties.isOwner());
 	}
 
@@ -86,8 +95,10 @@ public class MailboxSettingsManagerImplTest extends BrambleMockTestCase {
 		Settings expectedSettings = new Settings();
 		expectedSettings.put(SETTINGS_KEY_ONION, onion);
 		expectedSettings.put(SETTINGS_KEY_TOKEN, token.toString());
-		MailboxProperties properties =
+		expectedSettings.putIntArray(SETTINGS_KEY_SERVER_SUPPORTS,
-				new MailboxProperties(onion, token, true);
+				serverSupportsInts);
+		MailboxProperties properties = new MailboxProperties(onion, token,
+				serverSupports);
 
 		context.checking(new Expectations() {{
 			oneOf(settingsManager).mergeSettings(txn, expectedSettings,

bramble-core/src/test/java/org/briarproject/bramble/mailbox/MailboxUpdateManagerImplTest.java

@@ -11,12 +11,12 @@ import org.briarproject.bramble.api.data.MetadataParser;
 import org.briarproject.bramble.api.db.DatabaseComponent;
 import org.briarproject.bramble.api.db.Metadata;
 import org.briarproject.bramble.api.db.Transaction;
-import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
-import org.briarproject.bramble.api.mailbox.MailboxFolderId;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
-import org.briarproject.bramble.api.mailbox.MailboxPropertiesUpdate;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
-import org.briarproject.bramble.api.mailbox.RemoteMailboxPropertiesUpdateEvent;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
+import org.briarproject.bramble.api.mailbox.event.RemoteMailboxUpdateEvent;
 import org.briarproject.bramble.api.sync.Group;
 import org.briarproject.bramble.api.sync.GroupId;
 import org.briarproject.bramble.api.sync.Message;
@@ -30,30 +30,33 @@ import org.junit.Test;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Random;
 
 import static java.util.Collections.singletonList;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.CLIENT_ID;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.CLIENT_ID;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.MAJOR_VERSION;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.GROUP_KEY_SENT_CLIENT_SUPPORTS;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.MSG_KEY_LOCAL;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.MAJOR_VERSION;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.MSG_KEY_VERSION;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.MSG_KEY_LOCAL;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_AUTHTOKEN;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.MSG_KEY_VERSION;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_INBOXID;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_AUTHTOKEN;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_ONION;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_INBOXID;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_OUTBOXID;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_ONION;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_OUTBOXID;
 import static org.briarproject.bramble.api.sync.Group.Visibility.SHARED;
 import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_DO_NOT_SHARE;
 import static org.briarproject.bramble.test.TestUtils.getContact;
 import static org.briarproject.bramble.test.TestUtils.getGroup;
+import static org.briarproject.bramble.test.TestUtils.getMailboxProperties;
 import static org.briarproject.bramble.test.TestUtils.getMessage;
 import static org.briarproject.bramble.test.TestUtils.getRandomId;
 import static org.briarproject.bramble.test.TestUtils.hasEvent;
-import static org.briarproject.bramble.test.TestUtils.mailboxPropertiesUpdateEqual;
+import static org.briarproject.bramble.test.TestUtils.mailboxUpdateEqual;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 
-public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
+public class MailboxUpdateManagerImplTest extends BrambleMockTestCase {
 
 	private final DatabaseComponent db = context.mock(DatabaseComponent.class);
 	private final ClientHelper clientHelper = context.mock(ClientHelper.class);
@@ -71,32 +74,62 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 	private final Group localGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
 	private final BdfDictionary propsDict;
 	private final BdfDictionary emptyPropsDict = new BdfDictionary();
-	private final MailboxPropertiesUpdate props;
+	private final List<MailboxVersion> someClientSupportsList;
+	private final BdfList someClientSupports;
+	private final List<MailboxVersion> newerClientSupportsList;
+	private final BdfList newerClientSupports;
+	private final List<MailboxVersion> someServerSupportsList;
+	private final BdfList someServerSupports;
+	private final BdfList emptyServerSupports = new BdfList();
+	private final MailboxProperties updateProps;
+	private final MailboxUpdateWithMailbox updateWithMailbox;
+	private final MailboxUpdate updateNoMailbox;
 	private final MailboxProperties ownProps;
 
-	public MailboxPropertyManagerImplTest() {
+	public MailboxUpdateManagerImplTest() {
-		ownProps = new MailboxProperties("http://bar.onion",
+		Random rnd = new Random();
-				new MailboxAuthToken(getRandomId()), true);
+		someClientSupportsList = singletonList(new MailboxVersion(
-		props = new MailboxPropertiesUpdate(ownProps.getOnion(),
+				rnd.nextInt(), rnd.nextInt()));
-				new MailboxAuthToken(getRandomId()),
+		someClientSupports = BdfList.of(BdfList.of(
-				new MailboxFolderId(getRandomId()),
+				someClientSupportsList.get(0).getMajor(),
-				new MailboxFolderId(getRandomId()));
+				someClientSupportsList.get(0).getMinor()));
+		newerClientSupportsList = singletonList(new MailboxVersion(
+				someClientSupportsList.get(0).getMajor(),
+				someClientSupportsList.get(0).getMinor() + 1));
+		newerClientSupports = BdfList.of(BdfList.of(
+				newerClientSupportsList.get(0).getMajor(),
+				newerClientSupportsList.get(0).getMinor()));
+
+		someServerSupportsList = singletonList(new MailboxVersion(
+				rnd.nextInt(), rnd.nextInt()));
+		someServerSupports = BdfList.of(BdfList.of(
+				someServerSupportsList.get(0).getMajor(),
+				someServerSupportsList.get(0).getMinor()));
+
+		updateNoMailbox = new MailboxUpdate(someClientSupportsList);
+
+		updateProps = getMailboxProperties(false, someServerSupportsList);
+		ownProps = new MailboxProperties(updateProps.getBaseUrl(),
+				updateProps.getAuthToken(), someServerSupportsList);
+		updateWithMailbox = new MailboxUpdateWithMailbox(someClientSupportsList,
+				updateProps);
 		propsDict = new BdfDictionary();
-		propsDict.put(PROP_KEY_ONION, props.getOnion());
+		propsDict.put(PROP_KEY_ONION, updateProps.getOnion());
-		propsDict.put(PROP_KEY_AUTHTOKEN, props.getAuthToken().getBytes());
+		propsDict.put(PROP_KEY_AUTHTOKEN, updateProps.getAuthToken());
-		propsDict.put(PROP_KEY_INBOXID, props.getInboxId().getBytes());
+		propsDict.put(PROP_KEY_INBOXID, updateProps.getInboxId());
-		propsDict.put(PROP_KEY_OUTBOXID, props.getOutboxId().getBytes());
+		propsDict.put(PROP_KEY_OUTBOXID, updateProps.getOutboxId());
 	}
 
-	private MailboxPropertyManagerImpl createInstance() {
+	private MailboxUpdateManagerImpl createInstance(
+			List<MailboxVersion> clientSupports) {
 		context.checking(new Expectations() {{
 			oneOf(contactGroupFactory).createLocalGroup(CLIENT_ID,
 					MAJOR_VERSION);
 			will(returnValue(localGroup));
 		}});
-		return new MailboxPropertyManagerImpl(db, clientHelper,
+		return new MailboxUpdateManagerImpl(clientSupports, db,
-				clientVersioningManager, metadataParser, contactGroupFactory,
+				clientHelper, clientVersioningManager, metadataParser,
-				clock, mailboxSettingsManager, crypto);
+				contactGroupFactory, clock, mailboxSettingsManager, crypto);
 	}
 
 	@Test
@@ -104,6 +137,10 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 		Transaction txn = new Transaction(null, false);
 		Contact contact = getContact();
 		Group contactGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
+		Map<MessageId, BdfDictionary> messageMetadata = new LinkedHashMap<>();
+		BdfDictionary sentDict = BdfDictionary.of(new BdfEntry(
+				GROUP_KEY_SENT_CLIENT_SUPPORTS,
+				someClientSupports));
 
 		context.checking(new Expectations() {{
 			oneOf(db).containsGroup(txn, localGroup.getId());
@@ -111,6 +148,8 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 			oneOf(db).addGroup(txn, localGroup);
 			oneOf(db).getContacts(txn);
 			will(returnValue(singletonList(contact)));
+
+			// addingContact()
 			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
 					MAJOR_VERSION, contact);
 			will(returnValue(contactGroup));
@@ -124,9 +163,20 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 					contact.getId());
 			oneOf(mailboxSettingsManager).getOwnMailboxProperties(txn);
 			will(returnValue(null));
+			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
+					MAJOR_VERSION, contact);
+			will(returnValue(contactGroup));
+			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
+					contactGroup.getId());
+			will(returnValue(messageMetadata));
+			expectStoreMessage(txn, contactGroup.getId(), 1, someClientSupports,
+					emptyServerSupports, emptyPropsDict, true);
+
+			oneOf(clientHelper).mergeGroupMetadata(txn, localGroup.getId(),
+					sentDict);
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
 		t.onDatabaseOpened(txn);
 	}
 
@@ -137,6 +187,9 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 		Contact contact = getContact();
 		Group contactGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
 		Map<MessageId, BdfDictionary> messageMetadata = new LinkedHashMap<>();
+		BdfDictionary sentDict = BdfDictionary.of(new BdfEntry(
+				GROUP_KEY_SENT_CLIENT_SUPPORTS,
+				someClientSupports));
 
 		context.checking(new Expectations() {{
 			oneOf(db).containsGroup(txn, localGroup.getId());
@@ -144,6 +197,8 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 			oneOf(db).addGroup(txn, localGroup);
 			oneOf(db).getContacts(txn);
 			will(returnValue(singletonList(contact)));
+
+			// addingContact()
 			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
 					MAJOR_VERSION, contact);
 			will(returnValue(contactGroup));
@@ -158,35 +213,201 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 			oneOf(mailboxSettingsManager).getOwnMailboxProperties(txn);
 			will(returnValue(ownProps));
 			oneOf(crypto).generateUniqueId();
-			will(returnValue(props.getAuthToken()));
+			will(returnValue(updateProps.getAuthToken()));
 			oneOf(crypto).generateUniqueId();
-			will(returnValue(props.getInboxId()));
+			will(returnValue(updateProps.getInboxId()));
 			oneOf(crypto).generateUniqueId();
-			will(returnValue(props.getOutboxId()));
+			will(returnValue(updateProps.getOutboxId()));
 			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
 					MAJOR_VERSION, contact);
 			will(returnValue(contactGroup));
 			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
 					contactGroup.getId());
 			will(returnValue(messageMetadata));
-			expectStoreMessage(txn, contactGroup.getId(), propsDict, 1, true);
+			expectStoreMessage(txn, contactGroup.getId(), 1, someClientSupports,
+					someServerSupports, propsDict, true);
+
+			oneOf(clientHelper).mergeGroupMetadata(txn, localGroup.getId(),
+					sentDict);
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
 		t.onDatabaseOpened(txn);
 	}
 
 	@Test
-	public void testDoesNotCreateGroupsAtStartupIfAlreadyCreated()
+	public void testUnchangedClientSupportsOnSecondStartup() throws Exception {
-			throws Exception {
 		Transaction txn = new Transaction(null, false);
 
+		Contact contact = getContact();
+		Group contactGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
+
+		Map<MessageId, BdfDictionary> emptyMessageMetadata =
+				new LinkedHashMap<>();
+		BdfDictionary sentDict = BdfDictionary.of(new BdfEntry(
+				GROUP_KEY_SENT_CLIENT_SUPPORTS,
+				someClientSupports));
+
+		context.checking(new Expectations() {{
+			oneOf(db).containsGroup(txn, localGroup.getId());
+			will(returnValue(false));
+			oneOf(db).addGroup(txn, localGroup);
+			oneOf(db).getContacts(txn);
+			will(returnValue(singletonList(contact)));
+
+			// addingContact()
+			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
+					MAJOR_VERSION, contact);
+			will(returnValue(contactGroup));
+			oneOf(db).addGroup(txn, contactGroup);
+			oneOf(clientVersioningManager).getClientVisibility(txn,
+					contact.getId(), CLIENT_ID, MAJOR_VERSION);
+			will(returnValue(SHARED));
+			oneOf(db).setGroupVisibility(txn, contact.getId(),
+					contactGroup.getId(), SHARED);
+			oneOf(clientHelper).setContactId(txn, contactGroup.getId(),
+					contact.getId());
+			oneOf(mailboxSettingsManager).getOwnMailboxProperties(txn);
+			will(returnValue(null));
+			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
+					MAJOR_VERSION, contact);
+			will(returnValue(contactGroup));
+			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
+					contactGroup.getId());
+			will(returnValue(emptyMessageMetadata));
+			expectStoreMessage(txn, contactGroup.getId(), 1, someClientSupports,
+					emptyServerSupports, emptyPropsDict, true);
+
+			oneOf(clientHelper).mergeGroupMetadata(txn, localGroup.getId(),
+					sentDict);
+		}});
+
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
+		t.onDatabaseOpened(txn);
+
 		context.checking(new Expectations() {{
 			oneOf(db).containsGroup(txn, localGroup.getId());
 			will(returnValue(true));
+			oneOf(clientHelper).getGroupMetadataAsDictionary(txn,
+					localGroup.getId());
+			will(returnValue(sentDict));
+			oneOf(clientHelper).parseMailboxVersionList(someClientSupports);
+			will(returnValue(someClientSupportsList));
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		t = createInstance(someClientSupportsList);
+		t.onDatabaseOpened(txn);
+	}
+
+	@Test
+	public void testSendsUpdateWhenClientSupportsChangedOnSecondStartup()
+			throws Exception {
+		Transaction txn = new Transaction(null, false);
+
+		Contact contact = getContact();
+		Group contactGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
+
+		Map<MessageId, BdfDictionary> emptyMessageMetadata =
+				new LinkedHashMap<>();
+		BdfDictionary sentDict = BdfDictionary.of(new BdfEntry(
+				GROUP_KEY_SENT_CLIENT_SUPPORTS,
+				someClientSupports));
+
+		context.checking(new Expectations() {{
+			oneOf(db).containsGroup(txn, localGroup.getId());
+			will(returnValue(false));
+			oneOf(db).addGroup(txn, localGroup);
+			oneOf(db).getContacts(txn);
+			will(returnValue(singletonList(contact)));
+
+			// addingContact()
+			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
+					MAJOR_VERSION, contact);
+			will(returnValue(contactGroup));
+			oneOf(db).addGroup(txn, contactGroup);
+			oneOf(clientVersioningManager).getClientVisibility(txn,
+					contact.getId(), CLIENT_ID, MAJOR_VERSION);
+			will(returnValue(SHARED));
+			oneOf(db).setGroupVisibility(txn, contact.getId(),
+					contactGroup.getId(), SHARED);
+			oneOf(clientHelper).setContactId(txn, contactGroup.getId(),
+					contact.getId());
+			oneOf(mailboxSettingsManager).getOwnMailboxProperties(txn);
+			will(returnValue(null));
+			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
+					MAJOR_VERSION, contact);
+			will(returnValue(contactGroup));
+			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
+					contactGroup.getId());
+			will(returnValue(emptyMessageMetadata));
+			expectStoreMessage(txn, contactGroup.getId(), 1, someClientSupports,
+					emptyServerSupports, emptyPropsDict, true);
+
+			oneOf(clientHelper).mergeGroupMetadata(txn, localGroup.getId(),
+					sentDict);
+		}});
+
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
+		t.onDatabaseOpened(txn);
+
+		BdfDictionary metaDictionary = BdfDictionary.of(
+				new BdfEntry(MSG_KEY_VERSION, 1),
+				new BdfEntry(MSG_KEY_LOCAL, true)
+		);
+		Map<MessageId, BdfDictionary> messageMetadata = new LinkedHashMap<>();
+		MessageId messageId = new MessageId(getRandomId());
+		messageMetadata.put(messageId, metaDictionary);
+		BdfList body = BdfList.of(1, someClientSupports, someServerSupports,
+				propsDict);
+		BdfDictionary newerSentDict = BdfDictionary.of(new BdfEntry(
+				GROUP_KEY_SENT_CLIENT_SUPPORTS,
+				newerClientSupports));
+
+		context.checking(new Expectations() {{
+			oneOf(db).containsGroup(txn, localGroup.getId());
+			will(returnValue(true));
+
+			// Find out that we are now on newerClientSupportsList
+			oneOf(clientHelper).getGroupMetadataAsDictionary(txn,
+					localGroup.getId());
+			will(returnValue(sentDict));
+			oneOf(clientHelper).parseMailboxVersionList(someClientSupports);
+			will(returnValue(someClientSupportsList));
+
+			oneOf(db).getContacts(txn);
+			will(returnValue(singletonList(contact)));
+			oneOf(db).getContact(txn, contact.getId());
+			will(returnValue(contact));
+
+			// getLocalUpdate()
+			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
+					MAJOR_VERSION, contact);
+			will(returnValue(contactGroup));
+			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
+					contactGroup.getId());
+			will(returnValue(messageMetadata));
+			oneOf(clientHelper).getMessageAsList(txn, messageId);
+			will(returnValue(body));
+			oneOf(clientHelper).parseAndValidateMailboxUpdate(
+					someClientSupports, someServerSupports, propsDict);
+			will(returnValue(updateWithMailbox));
+			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
+					MAJOR_VERSION, contact);
+			will(returnValue(contactGroup));
+
+			// storeMessageReplaceLatest()
+			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
+					contactGroup.getId());
+			will(returnValue(messageMetadata));
+			expectStoreMessage(txn, contactGroup.getId(), 2,
+					newerClientSupports, someServerSupports, propsDict, true);
+			oneOf(db).removeMessage(txn, messageId);
+
+			oneOf(clientHelper).mergeGroupMetadata(txn, localGroup.getId(),
+					newerSentDict);
+		}});
+
+		t = createInstance(newerClientSupportsList);
 		t.onDatabaseOpened(txn);
 	}
 
@@ -196,6 +417,7 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 		Transaction txn = new Transaction(null, false);
 		Contact contact = getContact();
 		Group contactGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
+		Map<MessageId, BdfDictionary> messageMetadata = new LinkedHashMap<>();
 
 		context.checking(new Expectations() {{
 			// Create the group and share it with the contact
@@ -212,9 +434,17 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 					contact.getId());
 			oneOf(mailboxSettingsManager).getOwnMailboxProperties(txn);
 			will(returnValue(null));
+			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
+					MAJOR_VERSION, contact);
+			will(returnValue(contactGroup));
+			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
+					contactGroup.getId());
+			will(returnValue(messageMetadata));
+			expectStoreMessage(txn, contactGroup.getId(), 1, someClientSupports,
+					emptyServerSupports, emptyPropsDict, true);
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
 		t.addingContact(txn, contact);
 	}
 
@@ -242,21 +472,22 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 			oneOf(mailboxSettingsManager).getOwnMailboxProperties(txn);
 			will(returnValue(ownProps));
 			oneOf(crypto).generateUniqueId();
-			will(returnValue(props.getAuthToken()));
+			will(returnValue(updateProps.getAuthToken()));
 			oneOf(crypto).generateUniqueId();
-			will(returnValue(props.getInboxId()));
+			will(returnValue(updateProps.getInboxId()));
 			oneOf(crypto).generateUniqueId();
-			will(returnValue(props.getOutboxId()));
+			will(returnValue(updateProps.getOutboxId()));
 			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
 					MAJOR_VERSION, contact);
 			will(returnValue(contactGroup));
 			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
 					contactGroup.getId());
 			will(returnValue(messageMetadata));
-			expectStoreMessage(txn, contactGroup.getId(), propsDict, 1, true);
+			expectStoreMessage(txn, contactGroup.getId(), 1, someClientSupports,
+					someServerSupports, propsDict, true);
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
 		t.addingContact(txn, contact);
 	}
 
@@ -273,7 +504,7 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 			oneOf(db).removeGroup(txn, contactGroup);
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
 		t.removingContact(txn, contact);
 	}
 
@@ -284,7 +515,8 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 		Contact contact = getContact();
 		GroupId contactGroupId = new GroupId(getRandomId());
 		Message message = getMessage(contactGroupId);
-		BdfList body = BdfList.of(1, propsDict);
+		BdfList body = BdfList.of(1, someClientSupports, someServerSupports,
+				propsDict);
 		Metadata meta = new Metadata();
 		BdfDictionary metaDictionary = BdfDictionary.of(
 				new BdfEntry(MSG_KEY_VERSION, 1),
@@ -308,16 +540,16 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 			will(returnValue(contact.getId()));
 			oneOf(clientHelper).getMessageAsList(txn, message.getId());
 			will(returnValue(body));
-			oneOf(clientHelper).parseAndValidateMailboxPropertiesUpdate(
+			oneOf(clientHelper).parseAndValidateMailboxUpdate(
-					propsDict);
+					someClientSupports, someServerSupports, propsDict);
-			will(returnValue(props));
+			will(returnValue(updateWithMailbox));
 			oneOf(db).resetUnackedMessagesToSend(txn, contact.getId());
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
 		assertEquals(ACCEPT_DO_NOT_SHARE,
 				t.incomingMessage(txn, message, meta));
-		assertTrue(hasEvent(txn, RemoteMailboxPropertiesUpdateEvent.class));
+		assertTrue(hasEvent(txn, RemoteMailboxUpdateEvent.class));
 	}
 
 	@Test
@@ -327,7 +559,8 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 		Contact contact = getContact();
 		GroupId contactGroupId = new GroupId(getRandomId());
 		Message message = getMessage(contactGroupId);
-		BdfList body = BdfList.of(1, propsDict);
+		BdfList body = BdfList.of(1, someClientSupports, someServerSupports,
+				propsDict);
 		Metadata meta = new Metadata();
 		BdfDictionary metaDictionary = BdfDictionary.of(
 				new BdfEntry(MSG_KEY_VERSION, 2),
@@ -359,16 +592,16 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 			will(returnValue(contact.getId()));
 			oneOf(clientHelper).getMessageAsList(txn, message.getId());
 			will(returnValue(body));
-			oneOf(clientHelper).parseAndValidateMailboxPropertiesUpdate(
+			oneOf(clientHelper).parseAndValidateMailboxUpdate(
-					propsDict);
+					someClientSupports, someServerSupports, propsDict);
-			will(returnValue(props));
+			will(returnValue(updateWithMailbox));
 			oneOf(db).resetUnackedMessagesToSend(txn, contact.getId());
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
 		assertEquals(ACCEPT_DO_NOT_SHARE,
 				t.incomingMessage(txn, message, meta));
-		assertTrue(hasEvent(txn, RemoteMailboxPropertiesUpdateEvent.class));
+		assertTrue(hasEvent(txn, RemoteMailboxUpdateEvent.class));
 	}
 
 	@Test
@@ -399,14 +632,14 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 			oneOf(db).deleteMessageMetadata(txn, message.getId());
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
 		assertEquals(ACCEPT_DO_NOT_SHARE,
 				t.incomingMessage(txn, message, meta));
-		assertFalse(hasEvent(txn, RemoteMailboxPropertiesUpdateEvent.class));
+		assertFalse(hasEvent(txn, RemoteMailboxUpdateEvent.class));
 	}
 
 	@Test
-	public void testCreatesAndStoresLocalPropertiesWithNewVersionOnPairing()
+	public void testCreatesAndStoresLocalUpdateWithNewVersionOnPairing()
 			throws Exception {
 		Contact contact = getContact();
 		List<Contact> contacts = singletonList(contact);
@@ -429,27 +662,28 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 			oneOf(db).getContacts(txn);
 			will(returnValue(contacts));
 			oneOf(crypto).generateUniqueId();
-			will(returnValue(props.getAuthToken()));
+			will(returnValue(updateProps.getAuthToken()));
 			oneOf(crypto).generateUniqueId();
-			will(returnValue(props.getInboxId()));
+			will(returnValue(updateProps.getInboxId()));
 			oneOf(crypto).generateUniqueId();
-			will(returnValue(props.getOutboxId()));
+			will(returnValue(updateProps.getOutboxId()));
 			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
 					MAJOR_VERSION, contact);
 			will(returnValue(contactGroup));
 			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
 					contactGroup.getId());
 			will(returnValue(messageMetadata));
-			expectStoreMessage(txn, contactGroup.getId(), propsDict, 2, true);
+			expectStoreMessage(txn, contactGroup.getId(), 2, someClientSupports,
+					someServerSupports, propsDict, true);
 			oneOf(db).removeMessage(txn, latestId);
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
-		t.mailboxPaired(txn, ownProps.getOnion());
+		t.mailboxPaired(txn, ownProps.getOnion(), someServerSupportsList);
 	}
 
 	@Test
-	public void testStoresEmptyLocalPropertiesWithNewVersionOnUnpairing()
+	public void testStoresLocalUpdateNoMailboxWithNewVersionOnUnpairing()
 			throws Exception {
 		Contact contact = getContact();
 		List<Contact> contacts = singletonList(contact);
@@ -477,18 +711,17 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
 					contactGroup.getId());
 			will(returnValue(messageMetadata));
-			expectStoreMessage(txn, contactGroup.getId(), emptyPropsDict,
+			expectStoreMessage(txn, contactGroup.getId(), 2, someClientSupports,
-					2, true);
+					emptyServerSupports, emptyPropsDict, true);
 			oneOf(db).removeMessage(txn, latestId);
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
 		t.mailboxUnpaired(txn);
 	}
 
 	@Test
-	public void testGetRemoteProperties()
+	public void testGetRemoteUpdate() throws Exception {
-			throws Exception {
 		Transaction txn = new Transaction(null, false);
 		Contact contact = getContact();
 		Group contactGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
@@ -497,34 +730,34 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 				new BdfEntry(MSG_KEY_LOCAL, false)
 		);
 		Map<MessageId, BdfDictionary> messageMetadata = new LinkedHashMap<>();
-		MessageId fooUpdateId = new MessageId(getRandomId());
+		MessageId messageId = new MessageId(getRandomId());
-		messageMetadata.put(fooUpdateId, metaDictionary);
+		messageMetadata.put(messageId, metaDictionary);
-		BdfList fooUpdate = BdfList.of(1, propsDict);
+		BdfList body = BdfList.of(1, someClientSupports, someServerSupports,
+				propsDict);
 
 		context.checking(new Expectations() {{
 			oneOf(db).getContact(txn, contact.getId());
 			will(returnValue(contact));
-			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
+			oneOf(contactGroupFactory)
-					MAJOR_VERSION, contact);
+					.createContactGroup(CLIENT_ID, MAJOR_VERSION, contact);
 			will(returnValue(contactGroup));
 			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
 					contactGroup.getId());
 			will(returnValue(messageMetadata));
-			oneOf(clientHelper).getMessageAsList(txn, fooUpdateId);
+			oneOf(clientHelper).getMessageAsList(txn, messageId);
-			will(returnValue(fooUpdate));
+			will(returnValue(body));
-			oneOf(clientHelper).parseAndValidateMailboxPropertiesUpdate(
+			oneOf(clientHelper).parseAndValidateMailboxUpdate(
-					propsDict);
+					someClientSupports, someServerSupports, propsDict);
-			will(returnValue(props));
+			will(returnValue(updateWithMailbox));
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
-		MailboxPropertiesUpdate remote =
+		MailboxUpdate remote = t.getRemoteUpdate(txn, contact.getId());
-				t.getRemoteProperties(txn, contact.getId());
+		assertTrue(mailboxUpdateEqual(remote, updateWithMailbox));
-		assertTrue(mailboxPropertiesUpdateEqual(remote, props));
 	}
 
 	@Test
-	public void testGetRemotePropertiesReturnsNullBecauseNoUpdate()
+	public void testGetRemoteUpdateReturnsNullBecauseNoUpdate()
 			throws Exception {
 		Transaction txn = new Transaction(null, false);
 		Contact contact = getContact();
@@ -543,13 +776,12 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 			will(returnValue(emptyMessageMetadata));
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
-		assertNull(t.getRemoteProperties(txn, contact.getId()));
+		assertNull(t.getRemoteUpdate(txn, contact.getId()));
 	}
 
 	@Test
-	public void testGetRemotePropertiesReturnsNullBecauseEmptyUpdate()
+	public void testGetRemoteUpdateNoMailbox() throws Exception {
-			throws Exception {
 		Transaction txn = new Transaction(null, false);
 		Contact contact = getContact();
 		Group contactGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
@@ -558,9 +790,10 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 				new BdfEntry(MSG_KEY_LOCAL, false)
 		);
 		Map<MessageId, BdfDictionary> messageMetadata = new LinkedHashMap<>();
-		MessageId fooUpdateId = new MessageId(getRandomId());
+		MessageId messageId = new MessageId(getRandomId());
-		messageMetadata.put(fooUpdateId, metaDictionary);
+		messageMetadata.put(messageId, metaDictionary);
-		BdfList fooUpdate = BdfList.of(1, emptyPropsDict);
+		BdfList body = BdfList.of(1, someClientSupports, emptyServerSupports,
+				emptyPropsDict);
 
 		context.checking(new Expectations() {{
 			oneOf(db).getContact(txn, contact.getId());
@@ -571,20 +804,20 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
 					contactGroup.getId());
 			will(returnValue(messageMetadata));
-			oneOf(clientHelper).getMessageAsList(txn, fooUpdateId);
+			oneOf(clientHelper).getMessageAsList(txn, messageId);
-			will(returnValue(fooUpdate));
+			will(returnValue(body));
-			oneOf(clientHelper).parseAndValidateMailboxPropertiesUpdate(
+			oneOf(clientHelper).parseAndValidateMailboxUpdate(
-					emptyPropsDict);
+					someClientSupports, emptyServerSupports, emptyPropsDict);
-			will(returnValue(null));
+			will(returnValue(updateNoMailbox));
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
-		assertNull(t.getRemoteProperties(txn, contact.getId()));
+		MailboxUpdate remote = t.getRemoteUpdate(txn, contact.getId());
+		assertTrue(mailboxUpdateEqual(remote, updateNoMailbox));
 	}
 
 	@Test
-	public void testGetLocalProperties()
+	public void testGetLocalUpdate() throws Exception {
-			throws Exception {
 		Transaction txn = new Transaction(null, false);
 		Contact contact = getContact();
 		Group contactGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
@@ -593,9 +826,10 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 				new BdfEntry(MSG_KEY_LOCAL, true)
 		);
 		Map<MessageId, BdfDictionary> messageMetadata = new LinkedHashMap<>();
-		MessageId fooUpdateId = new MessageId(getRandomId());
+		MessageId messageId = new MessageId(getRandomId());
-		messageMetadata.put(fooUpdateId, metaDictionary);
+		messageMetadata.put(messageId, metaDictionary);
-		BdfList fooUpdate = BdfList.of(1, propsDict);
+		BdfList body = BdfList.of(1, someClientSupports, someServerSupports,
+				propsDict);
 
 		context.checking(new Expectations() {{
 			oneOf(db).getContact(txn, contact.getId());
@@ -606,46 +840,20 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
 					contactGroup.getId());
 			will(returnValue(messageMetadata));
-			oneOf(clientHelper).getMessageAsList(txn, fooUpdateId);
+			oneOf(clientHelper).getMessageAsList(txn, messageId);
-			will(returnValue(fooUpdate));
+			will(returnValue(body));
-			oneOf(clientHelper).parseAndValidateMailboxPropertiesUpdate(
+			oneOf(clientHelper).parseAndValidateMailboxUpdate(
-					propsDict);
+					someClientSupports, someServerSupports, propsDict);
-			will(returnValue(props));
+			will(returnValue(updateWithMailbox));
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
-		MailboxPropertiesUpdate local =
+		MailboxUpdate local = t.getLocalUpdate(txn, contact.getId());
-				t.getLocalProperties(txn, contact.getId());
+		assertTrue(mailboxUpdateEqual(local, updateWithMailbox));
-		assertTrue(mailboxPropertiesUpdateEqual(local, props));
 	}
 
 	@Test
-	public void testGetLocalPropertiesReturnsNullBecauseNoUpdate()
+	public void testGetLocalUpdateNoMailbox() throws Exception {
-			throws Exception {
-		Transaction txn = new Transaction(null, false);
-		Contact contact = getContact();
-		Group contactGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
-		Map<MessageId, BdfDictionary> emptyMessageMetadata =
-				new LinkedHashMap<>();
-
-		context.checking(new Expectations() {{
-			oneOf(db).getContact(txn, contact.getId());
-			will(returnValue(contact));
-			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
-					MAJOR_VERSION, contact);
-			will(returnValue(contactGroup));
-			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
-					contactGroup.getId());
-			will(returnValue(emptyMessageMetadata));
-		}});
-
-		MailboxPropertyManagerImpl t = createInstance();
-		assertNull(t.getLocalProperties(txn, contact.getId()));
-	}
-
-	@Test
-	public void testGetLocalPropertiesReturnsNullBecauseEmptyUpdate()
-			throws Exception {
 		Transaction txn = new Transaction(null, false);
 		Contact contact = getContact();
 		Group contactGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
@@ -654,9 +862,10 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 				new BdfEntry(MSG_KEY_LOCAL, true)
 		);
 		Map<MessageId, BdfDictionary> messageMetadata = new LinkedHashMap<>();
-		MessageId fooUpdateId = new MessageId(getRandomId());
+		MessageId messageId = new MessageId(getRandomId());
-		messageMetadata.put(fooUpdateId, metaDictionary);
+		messageMetadata.put(messageId, metaDictionary);
-		BdfList fooUpdate = BdfList.of(1, emptyPropsDict);
+		BdfList body = BdfList.of(1, someClientSupports, emptyServerSupports,
+				emptyPropsDict);
 
 		context.checking(new Expectations() {{
 			oneOf(db).getContact(txn, contact.getId());
@@ -667,21 +876,24 @@ public class MailboxPropertyManagerImplTest extends BrambleMockTestCase {
 			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
 					contactGroup.getId());
 			will(returnValue(messageMetadata));
-			oneOf(clientHelper).getMessageAsList(txn, fooUpdateId);
+			oneOf(clientHelper).getMessageAsList(txn, messageId);
-			will(returnValue(fooUpdate));
+			will(returnValue(body));
-			oneOf(clientHelper).parseAndValidateMailboxPropertiesUpdate(
+			oneOf(clientHelper).parseAndValidateMailboxUpdate(
-					emptyPropsDict);
+					someClientSupports, emptyServerSupports, emptyPropsDict);
-			will(returnValue(null));
+			will(returnValue(updateNoMailbox));
 		}});
 
-		MailboxPropertyManagerImpl t = createInstance();
+		MailboxUpdateManagerImpl t = createInstance(someClientSupportsList);
-		assertNull(t.getLocalProperties(txn, contact.getId()));
+		MailboxUpdate local = t.getLocalUpdate(txn, contact.getId());
+		assertTrue(mailboxUpdateEqual(local, updateNoMailbox));
 	}
 
 	private void expectStoreMessage(Transaction txn, GroupId g,
-			BdfDictionary properties, long version, boolean local)
+			long version, BdfList clientSupports, BdfList serverSupports,
+			BdfDictionary properties, boolean local)
 			throws Exception {
-		BdfList body = BdfList.of(version, properties);
+		BdfList body = BdfList.of(version, clientSupports, serverSupports,
+				properties);
 		Message message = getMessage(g);
 		long timestamp = message.getTimestamp();
 		BdfDictionary meta = BdfDictionary.of(

bramble-core/src/test/java/org/briarproject/bramble/mailbox/MailboxUpdateValidatorTest.java

@@ -0,0 +1,116 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfEntry;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.data.MetadataEncoder;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
+import org.briarproject.bramble.api.sync.Group;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.test.BrambleMockTestCase;
+import org.jmock.Expectations;
+import org.junit.Test;
+
+import java.io.IOException;
+import java.util.List;
+
+import static java.util.Collections.singletonList;
+import static org.briarproject.bramble.test.TestUtils.getGroup;
+import static org.briarproject.bramble.test.TestUtils.getMailboxProperties;
+import static org.briarproject.bramble.test.TestUtils.getMessage;
+import static org.junit.Assert.assertEquals;
+
+public class MailboxUpdateValidatorTest extends BrambleMockTestCase {
+
+	private final ClientHelper clientHelper = context.mock(ClientHelper.class);
+
+	private final BdfDictionary bdfDict;
+	private final BdfList emptyServerSupports;
+	private final BdfList someClientSupports;
+	private final BdfList someServerSupports;
+	private final MailboxUpdateWithMailbox updateMailbox;
+	private final MailboxUpdate updateNoMailbox;
+	private final Group group;
+	private final Message message;
+	private final MailboxUpdateValidator muv;
+
+	public MailboxUpdateValidatorTest() {
+		// Just dummies, clientHelper is mocked so our test is a bit shallow;
+		//  not testing
+		//  {@link ClientHelper#parseAndValidateMailboxUpdate(BdfList, BdfList, BdfDictionary)}
+		emptyServerSupports = new BdfList();
+		someClientSupports = BdfList.of(BdfList.of(1, 0));
+		List<MailboxVersion> someClientSupportsList =
+				singletonList(new MailboxVersion(1, 0));
+		someServerSupports = BdfList.of(BdfList.of(1, 0));
+		bdfDict = BdfDictionary.of(new BdfEntry("foo", "bar"));
+
+		MailboxProperties props = getMailboxProperties(false,
+				singletonList(new MailboxVersion(1, 0)));
+		updateMailbox = new MailboxUpdateWithMailbox(
+				singletonList(new MailboxVersion(1, 0)), props);
+		updateNoMailbox = new MailboxUpdate(someClientSupportsList);
+
+
+		group = getGroup(MailboxUpdateManager.CLIENT_ID,
+				MailboxUpdateManager.MAJOR_VERSION);
+		message = getMessage(group.getId());
+
+		MetadataEncoder metadataEncoder = context.mock(MetadataEncoder.class);
+		Clock clock = context.mock(Clock.class);
+		muv = new MailboxUpdateValidator(clientHelper, metadataEncoder,
+				clock);
+	}
+
+	@Test
+	public void testValidateMessageBody() throws IOException {
+		BdfList body =
+				BdfList.of(4, someClientSupports, someServerSupports, bdfDict);
+
+		context.checking(new Expectations() {{
+			oneOf(clientHelper).parseAndValidateMailboxUpdate(
+					someClientSupports, someServerSupports, bdfDict);
+			will(returnValue(updateMailbox));
+		}});
+
+		BdfDictionary result =
+				muv.validateMessage(message, group, body).getDictionary();
+		assertEquals(4, result.getLong("version").longValue());
+	}
+
+	@Test(expected = FormatException.class)
+	public void testValidateWrongVersionValue() throws IOException {
+		BdfList body = BdfList.of(-1, bdfDict);
+		muv.validateMessage(message, group, body);
+	}
+
+	@Test(expected = FormatException.class)
+	public void testValidateWrongVersionType() throws IOException {
+		BdfList body = BdfList.of(bdfDict, bdfDict);
+		muv.validateMessage(message, group, body);
+	}
+
+	@Test
+	public void testEmptyProperties() throws IOException {
+		BdfDictionary emptyBdfDict = new BdfDictionary();
+		BdfList body = BdfList.of(42, someClientSupports, emptyServerSupports,
+				emptyBdfDict);
+
+		context.checking(new Expectations() {{
+			oneOf(clientHelper).parseAndValidateMailboxUpdate(
+					someClientSupports, emptyServerSupports, emptyBdfDict);
+			will(returnValue(updateNoMailbox));
+		}});
+
+		BdfDictionary result =
+				muv.validateMessage(message, group, body).getDictionary();
+		assertEquals(42, result.getLong("version").longValue());
+	}
+}

bramble-core/src/test/java/org/briarproject/bramble/mailbox/OwnMailboxConnectivityCheckerTest.java

@@ -0,0 +1,117 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.db.TransactionManager;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.mailbox.ConnectivityChecker.ConnectivityObserver;
+import org.briarproject.bramble.test.BrambleMockTestCase;
+import org.briarproject.bramble.test.CaptureArgumentAction;
+import org.briarproject.bramble.test.DbExpectations;
+import org.jmock.Expectations;
+import org.jmock.lib.action.DoAllAction;
+import org.junit.Test;
+
+import java.io.IOException;
+import java.util.concurrent.atomic.AtomicReference;
+
+import static org.briarproject.bramble.mailbox.MailboxApi.CLIENT_SUPPORTS;
+import static org.briarproject.bramble.test.TestUtils.getMailboxProperties;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class OwnMailboxConnectivityCheckerTest extends BrambleMockTestCase {
+
+	private final Clock clock = context.mock(Clock.class);
+	private final MailboxApiCaller mailboxApiCaller =
+			context.mock(MailboxApiCaller.class);
+	private final MailboxApi mailboxApi = context.mock(MailboxApi.class);
+	private final TransactionManager db =
+			context.mock(TransactionManager.class);
+	private final MailboxSettingsManager mailboxSettingsManager =
+			context.mock(MailboxSettingsManager.class);
+	private final Cancellable task = context.mock(Cancellable.class);
+	private final ConnectivityObserver observer =
+			context.mock(ConnectivityObserver.class);
+
+	private final MailboxProperties properties =
+			getMailboxProperties(true, CLIENT_SUPPORTS);
+	private final long now = System.currentTimeMillis();
+
+	@Test
+	public void testObserverIsCalledWhenCheckSucceeds() throws Exception {
+		OwnMailboxConnectivityChecker checker = createChecker();
+		AtomicReference<ApiCall> apiCall = new AtomicReference<>(null);
+		Transaction txn = new Transaction(null, false);
+
+		// When checkConnectivity() is called a check should be started
+		context.checking(new Expectations() {{
+			oneOf(clock).currentTimeMillis();
+			will(returnValue(now));
+			oneOf(mailboxApiCaller).retryWithBackoff(with(any(ApiCall.class)));
+			will(new DoAllAction(
+					new CaptureArgumentAction<>(apiCall, ApiCall.class, 0),
+					returnValue(task)
+			));
+		}});
+
+		checker.checkConnectivity(properties, observer);
+
+		// When the check succeeds, the success should be recorded in the DB
+		// and the observer should be called
+		context.checking(new DbExpectations() {{
+			oneOf(mailboxApi).checkStatus(properties);
+			will(returnValue(true));
+			oneOf(clock).currentTimeMillis();
+			will(returnValue(now));
+			oneOf(db).transaction(with(false), withDbRunnable(txn));
+			oneOf(mailboxSettingsManager).recordSuccessfulConnection(txn, now);
+			oneOf(observer).onConnectivityCheckSucceeded();
+		}});
+
+		// The call should not be retried
+		assertFalse(apiCall.get().callApi());
+	}
+
+	@Test
+	public void testObserverIsNotCalledWhenCheckFails() throws Exception {
+		OwnMailboxConnectivityChecker checker = createChecker();
+		AtomicReference<ApiCall> apiCall = new AtomicReference<>(null);
+		Transaction txn = new Transaction(null, false);
+
+		// When checkConnectivity() is called a check should be started
+		context.checking(new Expectations() {{
+			oneOf(clock).currentTimeMillis();
+			will(returnValue(now));
+			oneOf(mailboxApiCaller).retryWithBackoff(with(any(ApiCall.class)));
+			will(new DoAllAction(
+					new CaptureArgumentAction<>(apiCall, ApiCall.class, 0),
+					returnValue(task)
+			));
+		}});
+
+		checker.checkConnectivity(properties, observer);
+
+		// When the check fails, the failure should be recorded in the DB and
+		// the observer should not be called
+		context.checking(new DbExpectations() {{
+			oneOf(mailboxApi).checkStatus(properties);
+			will(throwException(new IOException()));
+			oneOf(clock).currentTimeMillis();
+			will(returnValue(now));
+			oneOf(db).transaction(with(false), withDbRunnable(txn));
+			oneOf(mailboxSettingsManager)
+					.recordFailedConnectionAttempt(txn, now);
+		}});
+
+		// The call should be retried
+		assertTrue(apiCall.get().callApi());
+	}
+
+	private OwnMailboxConnectivityChecker createChecker() {
+		return new OwnMailboxConnectivityChecker(clock, mailboxApiCaller,
+				mailboxApi, db, mailboxSettingsManager);
+	}
+}

bramble-core/src/test/java/org/briarproject/bramble/plugin/PollerImplTest.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.plugin;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.connection.ConnectionManager;
 import org.briarproject.bramble.api.connection.ConnectionRegistry;
 import org.briarproject.bramble.api.contact.ContactId;
@@ -20,7 +21,6 @@ import org.briarproject.bramble.api.properties.TransportProperties;
 import org.briarproject.bramble.api.properties.TransportPropertyManager;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.system.TaskScheduler;
-import org.briarproject.bramble.api.system.TaskScheduler.Cancellable;
 import org.briarproject.bramble.test.BrambleMockTestCase;
 import org.briarproject.bramble.test.ImmediateExecutor;
 import org.briarproject.bramble.test.RunAction;

bramble-core/src/test/java/org/briarproject/bramble/rendezvous/RendezvousPollerImplTest.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.rendezvous;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.connection.ConnectionManager;
 import org.briarproject.bramble.api.contact.PendingContact;
 import org.briarproject.bramble.api.contact.PendingContactState;
@@ -27,7 +28,6 @@ import org.briarproject.bramble.api.rendezvous.event.RendezvousConnectionOpenedE
 import org.briarproject.bramble.api.rendezvous.event.RendezvousPollEvent;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.system.TaskScheduler;
-import org.briarproject.bramble.api.system.TaskScheduler.Cancellable;
 import org.briarproject.bramble.test.BrambleMockTestCase;
 import org.briarproject.bramble.test.CaptureArgumentAction;
 import org.briarproject.bramble.test.DbExpectations;

bramble-core/src/test/java/org/briarproject/bramble/sync/EagerSimplexOutgoingSessionTest.java

@@ -0,0 +1,134 @@
+package org.briarproject.bramble.sync;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.sync.Ack;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.sync.SyncRecordWriter;
+import org.briarproject.bramble.api.sync.Versions;
+import org.briarproject.bramble.api.transport.StreamWriter;
+import org.briarproject.bramble.test.BrambleMockTestCase;
+import org.briarproject.bramble.test.DbExpectations;
+import org.junit.Test;
+
+import static java.util.Arrays.asList;
+import static java.util.Collections.emptyList;
+import static java.util.Collections.singletonList;
+import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_BODY_LENGTH;
+import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_IDS;
+import static org.briarproject.bramble.test.TestUtils.getContactId;
+import static org.briarproject.bramble.test.TestUtils.getMessage;
+import static org.briarproject.bramble.test.TestUtils.getRandomId;
+import static org.briarproject.bramble.test.TestUtils.getTransportId;
+
+public class EagerSimplexOutgoingSessionTest extends BrambleMockTestCase {
+
+	private static final int MAX_LATENCY = Integer.MAX_VALUE;
+
+	private final DatabaseComponent db = context.mock(DatabaseComponent.class);
+	private final EventBus eventBus = context.mock(EventBus.class);
+	private final StreamWriter streamWriter = context.mock(StreamWriter.class);
+	private final SyncRecordWriter recordWriter =
+			context.mock(SyncRecordWriter.class);
+
+	private final ContactId contactId = getContactId();
+	private final TransportId transportId = getTransportId();
+	private final Ack ack =
+			new Ack(singletonList(new MessageId(getRandomId())));
+	private final Message message = getMessage(new GroupId(getRandomId()),
+			MAX_MESSAGE_BODY_LENGTH);
+	private final Message message1 = getMessage(new GroupId(getRandomId()),
+			MAX_MESSAGE_BODY_LENGTH);
+
+	@Test
+	public void testNothingToSendEagerly() throws Exception {
+		EagerSimplexOutgoingSession session =
+				new EagerSimplexOutgoingSession(db, eventBus, contactId,
+						transportId, MAX_LATENCY, streamWriter, recordWriter);
+
+		Transaction noAckTxn = new Transaction(null, false);
+		Transaction noIdsTxn = new Transaction(null, true);
+
+		context.checking(new DbExpectations() {{
+			// Add listener
+			oneOf(eventBus).addListener(session);
+			// Send the protocol versions
+			oneOf(recordWriter).writeVersions(with(any(Versions.class)));
+			// No acks to send
+			oneOf(db).transactionWithNullableResult(with(false),
+					withNullableDbCallable(noAckTxn));
+			oneOf(db).generateAck(noAckTxn, contactId, MAX_MESSAGE_IDS);
+			will(returnValue(null));
+			// No messages to send
+			oneOf(db).transactionWithResult(with(true),
+					withDbCallable(noIdsTxn));
+			oneOf(db).getUnackedMessagesToSend(noIdsTxn, contactId);
+			will(returnValue(emptyList()));
+			// Send the end of stream marker
+			oneOf(streamWriter).sendEndOfStream();
+			// Remove listener
+			oneOf(eventBus).removeListener(session);
+		}});
+
+		session.run();
+	}
+
+	@Test
+	public void testSomethingToSendEagerly() throws Exception {
+		EagerSimplexOutgoingSession session =
+				new EagerSimplexOutgoingSession(db, eventBus, contactId,
+						transportId, MAX_LATENCY, streamWriter, recordWriter);
+
+		Transaction ackTxn = new Transaction(null, false);
+		Transaction noAckTxn = new Transaction(null, false);
+		Transaction idsTxn = new Transaction(null, true);
+		Transaction msgTxn = new Transaction(null, false);
+		Transaction msgTxn1 = new Transaction(null, false);
+
+		context.checking(new DbExpectations() {{
+			// Add listener
+			oneOf(eventBus).addListener(session);
+			// Send the protocol versions
+			oneOf(recordWriter).writeVersions(with(any(Versions.class)));
+			// One ack to send
+			oneOf(db).transactionWithNullableResult(with(false),
+					withNullableDbCallable(ackTxn));
+			oneOf(db).generateAck(ackTxn, contactId, MAX_MESSAGE_IDS);
+			will(returnValue(ack));
+			oneOf(recordWriter).writeAck(ack);
+			// No more acks
+			oneOf(db).transactionWithNullableResult(with(false),
+					withNullableDbCallable(noAckTxn));
+			oneOf(db).generateAck(noAckTxn, contactId, MAX_MESSAGE_IDS);
+			will(returnValue(null));
+			// Two messages to send
+			oneOf(db).transactionWithResult(with(true), withDbCallable(idsTxn));
+			oneOf(db).getUnackedMessagesToSend(idsTxn, contactId);
+			will(returnValue(asList(message.getId(), message1.getId())));
+			// Try to send the first message - it's no longer shared
+			oneOf(db).transactionWithNullableResult(with(false),
+					withNullableDbCallable(msgTxn));
+			oneOf(db).getMessageToSend(msgTxn, contactId, message.getId(),
+					MAX_LATENCY, true);
+			will(returnValue(null));
+			// Send the second message
+			oneOf(db).transactionWithNullableResult(with(false),
+					withNullableDbCallable(msgTxn1));
+			oneOf(db).getMessageToSend(msgTxn1, contactId, message1.getId(),
+					MAX_LATENCY, true);
+			will(returnValue(message1));
+			oneOf(recordWriter).writeMessage(message1);
+			// Send the end of stream marker
+			oneOf(streamWriter).sendEndOfStream();
+			// Remove listener
+			oneOf(eventBus).removeListener(session);
+		}});
+
+		session.run();
+	}
+}

bramble-core/src/test/java/org/briarproject/bramble/sync/MailboxOutgoingSessionTest.java

@@ -0,0 +1,231 @@
+package org.briarproject.bramble.sync;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.sync.Ack;
+import org.briarproject.bramble.api.sync.DeferredSendHandler;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.sync.SyncRecordWriter;
+import org.briarproject.bramble.api.sync.Versions;
+import org.briarproject.bramble.api.transport.StreamWriter;
+import org.briarproject.bramble.test.BrambleMockTestCase;
+import org.briarproject.bramble.test.DbExpectations;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static java.util.Collections.emptyList;
+import static java.util.Collections.singletonList;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.MAX_FILE_PAYLOAD_BYTES;
+import static org.briarproject.bramble.api.record.Record.RECORD_HEADER_BYTES;
+import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_BODY_LENGTH;
+import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_IDS;
+import static org.briarproject.bramble.test.TestUtils.getContactId;
+import static org.briarproject.bramble.test.TestUtils.getMessage;
+import static org.briarproject.bramble.test.TestUtils.getRandomId;
+import static org.briarproject.bramble.test.TestUtils.getTransportId;
+
+public class MailboxOutgoingSessionTest extends BrambleMockTestCase {
+
+	private static final int MAX_LATENCY = Integer.MAX_VALUE;
+
+	private final DatabaseComponent db = context.mock(DatabaseComponent.class);
+	private final EventBus eventBus = context.mock(EventBus.class);
+	private final StreamWriter streamWriter = context.mock(StreamWriter.class);
+	private final SyncRecordWriter recordWriter =
+			context.mock(SyncRecordWriter.class);
+	private final DeferredSendHandler deferredSendHandler =
+			context.mock(DeferredSendHandler.class);
+
+	private final ContactId contactId = getContactId();
+	private final TransportId transportId = getTransportId();
+	private final Message message = getMessage(new GroupId(getRandomId()),
+			MAX_MESSAGE_BODY_LENGTH);
+	private final Message message1 = getMessage(new GroupId(getRandomId()),
+			MAX_MESSAGE_BODY_LENGTH);
+	private final int versionRecordBytes = RECORD_HEADER_BYTES + 1;
+
+	@Test
+	public void testNothingToSend() throws Exception {
+		MailboxOutgoingSession session = new MailboxOutgoingSession(db,
+				eventBus, contactId, transportId, MAX_LATENCY,
+				streamWriter, recordWriter, deferredSendHandler,
+				MAX_FILE_PAYLOAD_BYTES);
+
+		Transaction noAckIdTxn = new Transaction(null, true);
+		Transaction noMsgIdTxn = new Transaction(null, true);
+
+		long capacityForMessages = MAX_FILE_PAYLOAD_BYTES - versionRecordBytes;
+
+		context.checking(new DbExpectations() {{
+			// Add listener
+			oneOf(eventBus).addListener(session);
+			// Send the protocol versions
+			oneOf(recordWriter).writeVersions(with(any(Versions.class)));
+			// Calculate capacity for acks
+			oneOf(recordWriter).getBytesWritten();
+			will(returnValue((long) versionRecordBytes));
+			// No messages to ack
+			oneOf(db).transactionWithResult(with(true),
+					withDbCallable(noAckIdTxn));
+			oneOf(db).getMessagesToAck(noAckIdTxn, contactId, MAX_MESSAGE_IDS);
+			will(returnValue(emptyList()));
+			// Calculate capacity for messages
+			oneOf(recordWriter).getBytesWritten();
+			will(returnValue((long) versionRecordBytes));
+			// No messages to send
+			oneOf(db).transactionWithResult(with(true),
+					withDbCallable(noMsgIdTxn));
+			oneOf(db).getMessagesToSend(noMsgIdTxn, contactId,
+					capacityForMessages, MAX_LATENCY);
+			will(returnValue(emptyList()));
+			// Send the end of stream marker
+			oneOf(streamWriter).sendEndOfStream();
+			// Remove listener
+			oneOf(eventBus).removeListener(session);
+		}});
+
+		session.run();
+	}
+
+	@Test
+	public void testSomethingToSend() throws Exception {
+		MailboxOutgoingSession session = new MailboxOutgoingSession(db,
+				eventBus, contactId, transportId, MAX_LATENCY,
+				streamWriter, recordWriter, deferredSendHandler,
+				MAX_FILE_PAYLOAD_BYTES);
+
+		Transaction ackIdTxn = new Transaction(null, true);
+		Transaction noAckIdTxn = new Transaction(null, true);
+		Transaction msgIdTxn = new Transaction(null, true);
+		Transaction msgTxn = new Transaction(null, true);
+
+		int ackRecordBytes = RECORD_HEADER_BYTES + MessageId.LENGTH;
+		long capacityForMessages =
+				MAX_FILE_PAYLOAD_BYTES - versionRecordBytes - ackRecordBytes;
+
+		context.checking(new DbExpectations() {{
+			// Add listener
+			oneOf(eventBus).addListener(session);
+			// Send the protocol versions
+			oneOf(recordWriter).writeVersions(with(any(Versions.class)));
+			// Calculate capacity for acks
+			oneOf(recordWriter).getBytesWritten();
+			will(returnValue((long) versionRecordBytes));
+			// One message to ack
+			oneOf(db).transactionWithResult(with(true),
+					withDbCallable(ackIdTxn));
+			oneOf(db).getMessagesToAck(ackIdTxn, contactId, MAX_MESSAGE_IDS);
+			will(returnValue(singletonList(message.getId())));
+			// Send the ack
+			oneOf(recordWriter).getBytesWritten();
+			will(returnValue((long) versionRecordBytes));
+			oneOf(recordWriter).writeAck(with(any(Ack.class)));
+			oneOf(deferredSendHandler)
+					.onAckSent(singletonList(message.getId()));
+			// No more messages to ack
+			oneOf(db).transactionWithResult(with(true),
+					withDbCallable(noAckIdTxn));
+			oneOf(db).getMessagesToAck(noAckIdTxn, contactId, MAX_MESSAGE_IDS);
+			will(returnValue(emptyList()));
+			// Calculate capacity for messages
+			oneOf(recordWriter).getBytesWritten();
+			will(returnValue((long) versionRecordBytes + ackRecordBytes));
+			// One message to send
+			oneOf(db).transactionWithResult(with(true),
+					withDbCallable(msgIdTxn));
+			oneOf(db).getMessagesToSend(msgIdTxn, contactId,
+					capacityForMessages, MAX_LATENCY);
+			will(returnValue(singletonList(message1.getId())));
+			// Send the message
+			oneOf(db).transactionWithNullableResult(with(true),
+					withNullableDbCallable(msgTxn));
+			oneOf(db).getMessageToSend(msgTxn, contactId, message1.getId(),
+					MAX_LATENCY, false);
+			will(returnValue(message1));
+			oneOf(recordWriter).writeMessage(message1);
+			oneOf(deferredSendHandler).onMessageSent(message1.getId());
+			// Send the end of stream marker
+			oneOf(streamWriter).sendEndOfStream();
+			// Remove listener
+			oneOf(eventBus).removeListener(session);
+		}});
+
+		session.run();
+	}
+
+	@Test
+	public void testAllCapacityUsedByAcks() throws Exception {
+		// The file has enough capacity for a max-size ack record, another
+		// ack record with one message ID, and a few bytes left over
+		long capacity = RECORD_HEADER_BYTES + MessageId.LENGTH * MAX_MESSAGE_IDS
+				+ RECORD_HEADER_BYTES + MessageId.LENGTH + MessageId.LENGTH - 1;
+
+		MailboxOutgoingSession session = new MailboxOutgoingSession(db,
+				eventBus, contactId, transportId, MAX_LATENCY,
+				streamWriter, recordWriter, deferredSendHandler, capacity);
+
+		Transaction ackIdTxn1 = new Transaction(null, true);
+		Transaction ackIdTxn2 = new Transaction(null, true);
+
+		int firstAckRecordBytes =
+				RECORD_HEADER_BYTES + MessageId.LENGTH * MAX_MESSAGE_IDS;
+		int secondAckRecordBytes = RECORD_HEADER_BYTES + MessageId.LENGTH;
+
+		List<MessageId> idsInFirstAck = new ArrayList<>(MAX_MESSAGE_IDS);
+		for (int i = 0; i < MAX_MESSAGE_IDS; i++) {
+			idsInFirstAck.add(new MessageId(getRandomId()));
+		}
+		List<MessageId> idsInSecondAck =
+				singletonList(new MessageId(getRandomId()));
+
+		context.checking(new DbExpectations() {{
+			// Add listener
+			oneOf(eventBus).addListener(session);
+			// Send the protocol versions
+			oneOf(recordWriter).writeVersions(with(any(Versions.class)));
+			// Calculate capacity for acks
+			oneOf(recordWriter).getBytesWritten();
+			will(returnValue((long) versionRecordBytes));
+			// Load the IDs for the first ack record
+			oneOf(db).transactionWithResult(with(true),
+					withDbCallable(ackIdTxn1));
+			oneOf(db).getMessagesToAck(ackIdTxn1, contactId, MAX_MESSAGE_IDS);
+			will(returnValue(idsInFirstAck));
+			// Send the first ack record
+			oneOf(recordWriter).writeAck(with(any(Ack.class)));
+			oneOf(deferredSendHandler).onAckSent(idsInFirstAck);
+			// Calculate remaining capacity for acks
+			oneOf(recordWriter).getBytesWritten();
+			will(returnValue((long) versionRecordBytes + firstAckRecordBytes));
+			// Load the IDs for the second ack record
+			oneOf(db).transactionWithResult(with(true),
+					withDbCallable(ackIdTxn2));
+			oneOf(db).getMessagesToAck(ackIdTxn2, contactId, 1);
+			will(returnValue(idsInSecondAck));
+			// Send the second ack record
+			oneOf(recordWriter).writeAck(with(any(Ack.class)));
+			oneOf(deferredSendHandler).onAckSent(idsInSecondAck);
+			// Not enough capacity left for another ack
+			oneOf(recordWriter).getBytesWritten();
+			will(returnValue((long) versionRecordBytes + firstAckRecordBytes
+					+ secondAckRecordBytes));
+			// Not enough capacity left for any messages
+			oneOf(recordWriter).getBytesWritten();
+			will(returnValue((long) versionRecordBytes + firstAckRecordBytes
+					+ secondAckRecordBytes));
+			// Send the end of stream marker
+			oneOf(streamWriter).sendEndOfStream();
+			// Remove listener
+			oneOf(eventBus).removeListener(session);
+		}});
+
+		session.run();
+	}
+}

bramble-core/src/test/java/org/briarproject/bramble/sync/SimplexOutgoingSessionTest.java

@@ -14,18 +14,12 @@ import org.briarproject.bramble.api.sync.Versions;
 import org.briarproject.bramble.api.transport.StreamWriter;
 import org.briarproject.bramble.test.BrambleMockTestCase;
 import org.briarproject.bramble.test.DbExpectations;
-import org.briarproject.bramble.test.ImmediateExecutor;
 import org.junit.Test;
 
-import java.util.LinkedHashMap;
-import java.util.Map;
-import java.util.concurrent.Executor;
-
-import static java.util.Collections.emptyMap;
 import static java.util.Collections.singletonList;
-import static org.briarproject.bramble.api.record.Record.MAX_RECORD_PAYLOAD_BYTES;
 import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_BODY_LENGTH;
 import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_IDS;
+import static org.briarproject.bramble.sync.SimplexOutgoingSession.BATCH_CAPACITY;
 import static org.briarproject.bramble.test.TestUtils.getContactId;
 import static org.briarproject.bramble.test.TestUtils.getMessage;
 import static org.briarproject.bramble.test.TestUtils.getRandomId;
@@ -41,21 +35,18 @@ public class SimplexOutgoingSessionTest extends BrambleMockTestCase {
 	private final SyncRecordWriter recordWriter =
 			context.mock(SyncRecordWriter.class);
 
-	private final Executor dbExecutor = new ImmediateExecutor();
 	private final ContactId contactId = getContactId();
 	private final TransportId transportId = getTransportId();
 	private final Ack ack =
 			new Ack(singletonList(new MessageId(getRandomId())));
 	private final Message message = getMessage(new GroupId(getRandomId()),
 			MAX_MESSAGE_BODY_LENGTH);
-	private final Message message1 = getMessage(new GroupId(getRandomId()),
-			MAX_MESSAGE_BODY_LENGTH);
 
 	@Test
 	public void testNothingToSend() throws Exception {
 		SimplexOutgoingSession session = new SimplexOutgoingSession(db,
-				dbExecutor, eventBus, contactId, transportId, MAX_LATENCY,
+				eventBus, contactId, transportId, MAX_LATENCY,
-				false, streamWriter, recordWriter);
+				streamWriter, recordWriter);
 
 		Transaction noAckTxn = new Transaction(null, false);
 		Transaction noMsgTxn = new Transaction(null, false);
@@ -74,7 +65,7 @@ public class SimplexOutgoingSessionTest extends BrambleMockTestCase {
 			oneOf(db).transactionWithNullableResult(with(false),
 					withNullableDbCallable(noMsgTxn));
 			oneOf(db).generateBatch(noMsgTxn, contactId,
-					MAX_RECORD_PAYLOAD_BYTES, MAX_LATENCY);
+					BATCH_CAPACITY, MAX_LATENCY);
 			will(returnValue(null));
 			// Send the end of stream marker
 			oneOf(streamWriter).sendEndOfStream();
@@ -85,44 +76,11 @@ public class SimplexOutgoingSessionTest extends BrambleMockTestCase {
 		session.run();
 	}
 
-	@Test
-	public void testNothingToSendEagerly() throws Exception {
-		SimplexOutgoingSession session = new SimplexOutgoingSession(db,
-				dbExecutor, eventBus, contactId, transportId, MAX_LATENCY,
-				true, streamWriter, recordWriter);
-
-		Transaction noAckTxn = new Transaction(null, false);
-		Transaction noIdsTxn = new Transaction(null, true);
-
-		context.checking(new DbExpectations() {{
-			// Add listener
-			oneOf(eventBus).addListener(session);
-			// Send the protocol versions
-			oneOf(recordWriter).writeVersions(with(any(Versions.class)));
-			// No acks to send
-			oneOf(db).transactionWithNullableResult(with(false),
-					withNullableDbCallable(noAckTxn));
-			oneOf(db).generateAck(noAckTxn, contactId, MAX_MESSAGE_IDS);
-			will(returnValue(null));
-			// No messages to send
-			oneOf(db).transactionWithResult(with(true),
-					withDbCallable(noIdsTxn));
-			oneOf(db).getUnackedMessagesToSend(noIdsTxn, contactId);
-			will(returnValue(emptyMap()));
-			// Send the end of stream marker
-			oneOf(streamWriter).sendEndOfStream();
-			// Remove listener
-			oneOf(eventBus).removeListener(session);
-		}});
-
-		session.run();
-	}
-
 	@Test
 	public void testSomethingToSend() throws Exception {
 		SimplexOutgoingSession session = new SimplexOutgoingSession(db,
-				dbExecutor, eventBus, contactId, transportId, MAX_LATENCY,
+				eventBus, contactId, transportId, MAX_LATENCY,
-				false, streamWriter, recordWriter);
+				streamWriter, recordWriter);
 
 		Transaction ackTxn = new Transaction(null, false);
 		Transaction noAckTxn = new Transaction(null, false);
@@ -140,23 +98,23 @@ public class SimplexOutgoingSessionTest extends BrambleMockTestCase {
 			oneOf(db).generateAck(ackTxn, contactId, MAX_MESSAGE_IDS);
 			will(returnValue(ack));
 			oneOf(recordWriter).writeAck(ack);
+			// No more acks
+			oneOf(db).transactionWithNullableResult(with(false),
+					withNullableDbCallable(noAckTxn));
+			oneOf(db).generateAck(noAckTxn, contactId, MAX_MESSAGE_IDS);
+			will(returnValue(null));
 			// One message to send
 			oneOf(db).transactionWithNullableResult(with(false),
 					withNullableDbCallable(msgTxn));
 			oneOf(db).generateBatch(msgTxn, contactId,
-					MAX_RECORD_PAYLOAD_BYTES, MAX_LATENCY);
+					BATCH_CAPACITY, MAX_LATENCY);
 			will(returnValue(singletonList(message)));
 			oneOf(recordWriter).writeMessage(message);
-			// No more acks
-			oneOf(db).transactionWithNullableResult(with(false),
-					withNullableDbCallable(noAckTxn));
-			oneOf(db).generateAck(noAckTxn, contactId, MAX_MESSAGE_IDS);
-			will(returnValue(null));
 			// No more messages
 			oneOf(db).transactionWithNullableResult(with(false),
 					withNullableDbCallable(noMsgTxn));
 			oneOf(db).generateBatch(noMsgTxn, contactId,
-					MAX_RECORD_PAYLOAD_BYTES, MAX_LATENCY);
+					BATCH_CAPACITY, MAX_LATENCY);
 			will(returnValue(null));
 			// Send the end of stream marker
 			oneOf(streamWriter).sendEndOfStream();
@@ -166,63 +124,4 @@ public class SimplexOutgoingSessionTest extends BrambleMockTestCase {
 
 		session.run();
 	}
-
-	@Test
-	public void testSomethingToSendEagerly() throws Exception {
-		SimplexOutgoingSession session = new SimplexOutgoingSession(db,
-				dbExecutor, eventBus, contactId, transportId, MAX_LATENCY,
-				true, streamWriter, recordWriter);
-
-		Map<MessageId, Integer> unacked = new LinkedHashMap<>();
-		unacked.put(message.getId(), message.getRawLength());
-		unacked.put(message1.getId(), message1.getRawLength());
-
-		Transaction ackTxn = new Transaction(null, false);
-		Transaction noAckTxn = new Transaction(null, false);
-		Transaction idsTxn = new Transaction(null, true);
-		Transaction msgTxn = new Transaction(null, false);
-		Transaction msgTxn1 = new Transaction(null, false);
-
-		context.checking(new DbExpectations() {{
-			// Add listener
-			oneOf(eventBus).addListener(session);
-			// Send the protocol versions
-			oneOf(recordWriter).writeVersions(with(any(Versions.class)));
-			// One ack to send
-			oneOf(db).transactionWithNullableResult(with(false),
-					withNullableDbCallable(ackTxn));
-			oneOf(db).generateAck(ackTxn, contactId, MAX_MESSAGE_IDS);
-			will(returnValue(ack));
-			oneOf(recordWriter).writeAck(ack);
-			// No more acks
-			oneOf(db).transactionWithNullableResult(with(false),
-					withNullableDbCallable(noAckTxn));
-			oneOf(db).generateAck(noAckTxn, contactId, MAX_MESSAGE_IDS);
-			will(returnValue(null));
-			// Two messages to send
-			oneOf(db).transactionWithResult(with(true), withDbCallable(idsTxn));
-			oneOf(db).getUnackedMessagesToSend(idsTxn, contactId);
-			will(returnValue(unacked));
-			// Send the first message
-			oneOf(db).transactionWithResult(with(false),
-					withDbCallable(msgTxn));
-			oneOf(db).generateBatch(msgTxn, contactId,
-					singletonList(message.getId()), MAX_LATENCY);
-			will(returnValue(singletonList(message)));
-			oneOf(recordWriter).writeMessage(message);
-			// Send the second message
-			oneOf(db).transactionWithResult(with(false),
-					withDbCallable(msgTxn1));
-			oneOf(db).generateBatch(msgTxn1, contactId,
-					singletonList(message1.getId()), MAX_LATENCY);
-			will(returnValue(singletonList(message1)));
-			oneOf(recordWriter).writeMessage(message1);
-			// Send the end of stream marker
-			oneOf(streamWriter).sendEndOfStream();
-			// Remove listener
-			oneOf(eventBus).removeListener(session);
-		}});
-
-		session.run();
-	}
 }

bramble-core/src/test/java/org/briarproject/bramble/system/TestTaskScheduler.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.system;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.system.TaskScheduler;

bramble-core/src/test/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementIntegrationTest.java

@@ -345,11 +345,17 @@ public class TransportKeyAgreementIntegrationTest
 					.canSendOutgoingStreams(aliceId, DUPLEX_TRANSPORT_ID));
 		}
 
-		// Sync initial client versioning updates
+		// Sync client versioning update from Alice to Bob
 		syncMessage(alice, bob, bobId, 1, true);
+		// Sync client versioning update and ack from Bob to Alice
 		syncMessage(bob, alice, aliceId, 1, true);
-		syncMessage(alice, bob, bobId, 1, true);
+		// Sync second client versioning update, mailbox properties and ack
-		sendAcks(bob, alice, aliceId, 1);
+		// from Alice to Bob
+		syncMessage(alice, bob, bobId, 2, true);
+		// Sync mailbox properties and ack from Bob to Alice
+		syncMessage(bob, alice, aliceId, 1, true);
+		// Sync final ack from Alice to Bob
+		sendAcks(alice, bob, bobId, 1);
 
 		return new Pair<>(aliceId, bobId);
 	}

bramble-core/witness.gradle

@@ -35,7 +35,7 @@ dependencyVerification {
         'org.apache-extras.beanshell:bsh:2.0b6:bsh-2.0b6.jar:a17955976070c0573235ee662f2794a78082758b61accffce8d3f8aedcd91047',
         'org.bitlet:weupnp:0.1.4:weupnp-0.1.4.jar:88df7e6504929d00bdb832863761385c68ab92af945b04f0770b126270a444fb',
         'org.bouncycastle:bcprov-jdk15to18:1.70:bcprov-jdk15to18-1.70.jar:7df4c54f29ce2dd616dc3b198ca4db3dfcc79e3cb397c084a0aff97b85c0bf38',
-        'org.briarproject:jtorctl:0.3:jtorctl-0.3.jar:f2939238a097898998432effe93b0334d97a787972ab3a91a8973a1d309fc864',
+        'org.briarproject:jtorctl:0.4:jtorctl-0.4.jar:4e61f59dc9f3984438a7151c4df8d7c1f83d5fb3eb8c151acfc794a8fef85a36',
         'org.checkerframework:checker-compat-qual:2.5.3:checker-compat-qual-2.5.3.jar:d76b9afea61c7c082908023f0cbc1427fab9abd2df915c8b8a3e7a509bccbc6d',
         'org.checkerframework:checker-qual:2.5.2:checker-qual-2.5.2.jar:64b02691c8b9d4e7700f8ee2e742dce7ea2c6e81e662b7522c9ee3bf568c040a',
         'org.codehaus.mojo.signature:java16:1.1:java16-1.1.signature:53799223a2c98dba2d0add810bed76315460df285c69e4f397ae6098f87dd619',

bramble-java/src/main/java/org/briarproject/bramble/plugin/tor/UnixTorPluginFactory.java

@@ -9,13 +9,9 @@ import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.Backoff;
 import org.briarproject.bramble.api.plugin.BackoffFactory;
 import org.briarproject.bramble.api.plugin.PluginCallback;
-import org.briarproject.bramble.api.plugin.TorConstants;
 import org.briarproject.bramble.api.plugin.TorControlPort;
 import org.briarproject.bramble.api.plugin.TorDirectory;
 import org.briarproject.bramble.api.plugin.TorSocksPort;
-import org.briarproject.bramble.api.plugin.TransportId;
-import org.briarproject.bramble.api.plugin.duplex.DuplexPlugin;
-import org.briarproject.bramble.api.plugin.duplex.DuplexPluginFactory;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.system.LocationUtils;
 import org.briarproject.bramble.api.system.ResourceProvider;
@@ -23,43 +19,18 @@ import org.briarproject.bramble.api.system.WakefulIoExecutor;
 
 import java.io.File;
 import java.util.concurrent.Executor;
-import java.util.logging.Logger;
 
+import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 import javax.net.SocketFactory;
 
 import static java.util.logging.Level.INFO;
-import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.util.OsUtils.isLinux;
 
 @Immutable
 @NotNullByDefault
-public class UnixTorPluginFactory implements DuplexPluginFactory {
+public class UnixTorPluginFactory extends TorPluginFactory {
-
-	private static final Logger LOG =
-			getLogger(UnixTorPluginFactory.class.getName());
-
-	private static final int MAX_LATENCY = 30 * 1000; // 30 seconds
-	private static final int MAX_IDLE_TIME = 30 * 1000; // 30 seconds
-	private static final int MIN_POLLING_INTERVAL = 60 * 1000; // 1 minute
-	private static final int MAX_POLLING_INTERVAL = 10 * 60 * 1000; // 10 mins
-	private static final double BACKOFF_BASE = 1.2;
-
-	private final Executor ioExecutor, wakefulIoExecutor;
-	private final NetworkManager networkManager;
-	private final LocationUtils locationUtils;
-	private final EventBus eventBus;
-	private final SocketFactory torSocketFactory;
-	private final BackoffFactory backoffFactory;
-	private final ResourceProvider resourceProvider;
-	private final CircumventionProvider circumventionProvider;
-	private final BatteryManager batteryManager;
-	private final Clock clock;
-	private final File torDirectory;
-	private int torSocksPort;
-	private int torControlPort;
-	private final CryptoComponent crypto;
 
 	@Inject
 	UnixTorPluginFactory(@IoExecutor Executor ioExecutor,
@@ -73,74 +44,39 @@ public class UnixTorPluginFactory implements DuplexPluginFactory {
 			CircumventionProvider circumventionProvider,
 			BatteryManager batteryManager,
 			Clock clock,
+			CryptoComponent crypto,
 			@TorDirectory File torDirectory,
 			@TorSocksPort int torSocksPort,
-			@TorControlPort int torControlPort,
+			@TorControlPort int torControlPort) {
-			CryptoComponent crypto) {
+		super(ioExecutor, wakefulIoExecutor, networkManager, locationUtils,
-		this.ioExecutor = ioExecutor;
+				eventBus, torSocketFactory, backoffFactory, resourceProvider,
-		this.wakefulIoExecutor = wakefulIoExecutor;
+				circumventionProvider, batteryManager, clock, crypto,
-		this.networkManager = networkManager;
+				torDirectory, torSocksPort, torControlPort);
-		this.locationUtils = locationUtils;
-		this.eventBus = eventBus;
-		this.torSocketFactory = torSocketFactory;
-		this.backoffFactory = backoffFactory;
-		this.resourceProvider = resourceProvider;
-		this.circumventionProvider = circumventionProvider;
-		this.batteryManager = batteryManager;
-		this.clock = clock;
-		this.torDirectory = torDirectory;
-		this.torSocksPort = torSocksPort;
-		this.torControlPort = torControlPort;
-		this.crypto = crypto;
 	}
 
+	@Nullable
 	@Override
-	public TransportId getId() {
+	String getArchitectureForTorBinary() {
-		return TorConstants.ID;
+		if (!isLinux()) return null;
-	}
+		String arch = System.getProperty("os.arch");
-
-	@Override
-	public long getMaxLatency() {
-		return MAX_LATENCY;
-	}
-
-	@Override
-	public DuplexPlugin createPlugin(PluginCallback callback) {
-		// Check that we have a Tor binary for this architecture
-		String architecture = null;
-		if (isLinux()) {
-			String arch = System.getProperty("os.arch");
-			if (LOG.isLoggable(INFO)) {
-				LOG.info("System's os.arch is " + arch);
-			}
-			if (arch.equals("amd64")) {
-				architecture = "linux-x86_64";
-			} else if (arch.equals("aarch64")) {
-				architecture = "linux-aarch64";
-			} else if (arch.equals("arm")) {
-				architecture = "linux-armhf";
-			}
-		}
-		if (architecture == null) {
-			LOG.info("Tor is not supported on this architecture");
-			return null;
-		}
-
 		if (LOG.isLoggable(INFO)) {
-			LOG.info("The selected architecture for Tor is " + architecture);
+			LOG.info("System's os.arch is " + arch);
 		}
+		if (arch.equals("amd64")) return "linux-x86_64";
+		else if (arch.equals("aarch64")) return "linux-aarch64";
+		else if (arch.equals("arm")) return "linux-armhf";
+		return null;
+	}
 
-		Backoff backoff = backoffFactory.createBackoff(MIN_POLLING_INTERVAL,
+	@Override
-				MAX_POLLING_INTERVAL, BACKOFF_BASE);
+	TorPlugin createPluginInstance(Backoff backoff,
-		TorRendezvousCrypto torRendezvousCrypto =
+			TorRendezvousCrypto torRendezvousCrypto, PluginCallback callback,
-				new TorRendezvousCryptoImpl(crypto);
+			String architecture) {
-		UnixTorPlugin plugin = new UnixTorPlugin(ioExecutor, wakefulIoExecutor,
+		return new UnixTorPlugin(ioExecutor, wakefulIoExecutor,
 				networkManager, locationUtils, torSocketFactory, clock,
 				resourceProvider, circumventionProvider, batteryManager,
 				backoff, torRendezvousCrypto, callback, architecture,
 				MAX_LATENCY, MAX_IDLE_TIME, torDirectory, torSocksPort,
 				torControlPort);
-		eventBus.addListener(plugin);
-		return plugin;
 	}
 }

bramble-java/src/test/java/org/briarproject/bramble/plugin/tor/BridgeTest.java

@@ -89,6 +89,9 @@ public class BridgeTest extends BrambleTestCase {
 	private final static long MEEK_TIMEOUT = MINUTES.toMillis(6);
 	private final static int UNREACHABLE_BRIDGES_ALLOWED = 6;
 	private final static int ATTEMPTS_PER_BRIDGE = 5;
+	// Use different ports from Briar Desktop to avoid conflicts
+	private final static int SOCKS_PORT = DEFAULT_SOCKS_PORT + 10;
+	private final static int CONTROL_PORT = DEFAULT_CONTROL_PORT + 10;
 
 	private final static Logger LOG = getLogger(BridgeTest.class.getName());
 
@@ -164,8 +167,8 @@ public class BridgeTest extends BrambleTestCase {
 		factory = new UnixTorPluginFactory(ioExecutor, wakefulIoExecutor,
 				networkManager, locationUtils, eventBus, torSocketFactory,
 				backoffFactory, resourceProvider, bridgeProvider,
-				batteryManager, clock, torDir, DEFAULT_SOCKS_PORT,
+				batteryManager, clock, crypto, torDir,
-				DEFAULT_CONTROL_PORT, crypto);
+				SOCKS_PORT, CONTROL_PORT);
 	}
 
 	@After

briar-android/build.gradle

@@ -26,8 +26,8 @@ android {
 	defaultConfig {
 		minSdkVersion 16
 		targetSdkVersion 30
-		versionCode 10406
+		versionCode 10408
-		versionName "1.4.6"
+		versionName "1.4.8"
 		applicationId "org.briarproject.briar.android"
 
 		vectorDrawables.useSupportLibrary = true
@@ -100,6 +100,7 @@ dependencies {
 	implementation project(path: ':briar-core', configuration: 'default')
 	implementation project(path: ':bramble-core', configuration: 'default')
 	implementation project(':bramble-android')
+	implementation 'org.briarproject:dont-kill-me-lib:0.2.2'
 
 	implementation 'androidx.fragment:fragment:1.3.4'
 	implementation 'androidx.preference:preference:1.1.1'
@@ -114,7 +115,7 @@ dependencies {
 	implementation 'de.hdodenhof:circleimageview:3.1.0'
 	implementation 'com.google.zxing:core:3.3.3'  // newer version need minSdk 24
 	implementation 'uk.co.samuelwall:material-tap-target-prompt:3.3.0'
-	implementation 'com.vanniktech:emoji-google:0.6.0' // newer versions need minSdk 21
+	implementation 'com.vanniktech:emoji-google:0.7.0' // newer versions need minSdk 21
 	implementation 'com.github.kobakei:MaterialFabSpeedDial:1.2.1'
 	implementation 'com.github.chrisbanes:PhotoView:2.3.0'
 	def glideVersion = '4.11.0'