Use eager retransmission if the transport is lossy and cheap.

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

@@ -22,6 +22,11 @@ public interface TransportConnectionWriter {
 	 */
 	int getMaxIdleTime();
 
+	/**
+	 * Returns true if the transport is lossy and cheap.
+	 */
+	boolean isLossyAndCheap();
+
 	/**
 	 * Returns an output stream for writing to the transport connection.
 	 */

bramble-api/src/main/java/org/briarproject/bramble/api/plugin/duplex/AbstractDuplexTransportConnection.java

@@ -79,6 +79,11 @@ public abstract class AbstractDuplexTransportConnection
 			return plugin.getMaxIdleTime();
 		}
 
+		@Override
+		public boolean isLossyAndCheap() {
+			return false;
+		}
+
 		@Override
 		public OutputStream getOutputStream() throws IOException {
 			return AbstractDuplexTransportConnection.this.getOutputStream();

bramble-api/src/main/java/org/briarproject/bramble/api/plugin/simplex/SimplexPlugin.java

@@ -15,6 +15,12 @@ import javax.annotation.Nullable;
 @NotNullByDefault
 public interface SimplexPlugin extends Plugin {
 
+	/**
+	 * Returns true if the transport is likely to lose streams and the cost of
+	 * transmitting redundant copies of data is cheap.
+	 */
+	boolean isLossyAndCheap();
+
 	/**
 	 * Attempts to create and return a reader for the given transport
 	 * properties. Returns null if a reader cannot be created.

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

@@ -16,7 +16,7 @@ public interface SyncSessionFactory {
 			PriorityHandler handler);
 
 	SyncSession createSimplexOutgoingSession(ContactId c, TransportId t,
-			int maxLatency, StreamWriter streamWriter);
+			int maxLatency, boolean eager, StreamWriter streamWriter);
 
 	SyncSession createDuplexOutgoingSession(ContactId c, TransportId t,
 			int maxLatency, int maxIdleTime, StreamWriter streamWriter,

bramble-core/src/main/java/org/briarproject/bramble/connection/OutgoingSimplexSyncConnection.java

@@ -71,8 +71,10 @@ class OutgoingSimplexSyncConnection extends SyncConnection implements Runnable {
 		StreamWriter streamWriter = streamWriterFactory.createStreamWriter(
 				w.getOutputStream(), ctx);
 		ContactId c = requireNonNull(ctx.getContactId());
+		// Use eager retransmission if the transport is lossy and cheap
 		return syncSessionFactory.createSimplexOutgoingSession(c,
-				ctx.getTransportId(), w.getMaxLatency(), streamWriter);
+				ctx.getTransportId(), w.getMaxLatency(), w.isLossyAndCheap(),
+				streamWriter);
 	}
 }
 

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/AbstractRemovableDrivePlugin.java

@@ -92,6 +92,11 @@ abstract class AbstractRemovableDrivePlugin implements SimplexPlugin {
 		throw new UnsupportedOperationException();
 	}
 
+	@Override
+	public boolean isLossyAndCheap() {
+		return true;
+	}
+
 	@Override
 	public TransportConnectionReader createReader(TransportProperties p) {
 		try {

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/FileTransportWriter.java

@@ -36,6 +36,11 @@ class FileTransportWriter implements TransportConnectionWriter {
 		return plugin.getMaxIdleTime();
 	}
 
+	@Override
+	public boolean isLossyAndCheap() {
+		return plugin.isLossyAndCheap();
+	}
+
 	@Override
 	public OutputStream getOutputStream() {
 		return out;

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/RemovableDriveWriterTask.java

@@ -106,6 +106,11 @@ class RemovableDriveWriterTask extends RemovableDriveTaskImpl
 			return delegate.getMaxIdleTime();
 		}
 
+		@Override
+		public boolean isLossyAndCheap() {
+			return delegate.isLossyAndCheap();
+		}
+
 		@Override
 		public OutputStream getOutputStream() throws IOException {
 			return delegate.getOutputStream();

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/TransportOutputStreamWriter.java

@@ -1,8 +1,8 @@
 package org.briarproject.bramble.plugin.file;
 
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.plugin.Plugin;
 import org.briarproject.bramble.api.plugin.TransportConnectionWriter;
+import org.briarproject.bramble.api.plugin.simplex.SimplexPlugin;
 
 import java.io.OutputStream;
 import java.util.logging.Logger;
@@ -17,10 +17,10 @@ class TransportOutputStreamWriter implements TransportConnectionWriter {
 	private static final Logger LOG =
 			getLogger(TransportOutputStreamWriter.class.getName());
 
-	private final Plugin plugin;
+	private final SimplexPlugin plugin;
 	private final OutputStream out;
 
-	TransportOutputStreamWriter(Plugin plugin, OutputStream out) {
+	TransportOutputStreamWriter(SimplexPlugin plugin, OutputStream out) {
 		this.plugin = plugin;
 		this.out = out;
 	}
@@ -35,6 +35,11 @@ class TransportOutputStreamWriter implements TransportConnectionWriter {
 		return plugin.getMaxIdleTime();
 	}
 
+	@Override
+	public boolean isLossyAndCheap() {
+		return plugin.isLossyAndCheap();
+	}
+
 	@Override
 	public OutputStream getOutputStream() {
 		return out;

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

@@ -60,12 +60,12 @@ class SyncSessionFactoryImpl implements SyncSessionFactory {
 
 	@Override
 	public SyncSession createSimplexOutgoingSession(ContactId c, TransportId t,
-			int maxLatency, StreamWriter streamWriter) {
+			int maxLatency, boolean eager, StreamWriter streamWriter) {
 		OutputStream out = streamWriter.getOutputStream();
 		SyncRecordWriter recordWriter =
 				recordWriterFactory.createRecordWriter(out);
 		return new SimplexOutgoingSession(db, dbExecutor, eventBus, c, t,
-				maxLatency, false, streamWriter, recordWriter);
+				maxLatency, eager, streamWriter, recordWriter);
 	}
 
 	@Override

bramble-core/src/test/java/org/briarproject/bramble/test/TestTransportConnectionWriter.java

@@ -35,6 +35,11 @@ public class TestTransportConnectionWriter
 		return 60_000;
 	}
 
+	@Override
+	public boolean isLossyAndCheap() {
+		return false;
+	}
+
 	@Override
 	public OutputStream getOutputStream() {
 		return out;