diff --git a/bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseComponent.java b/bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseComponent.java
index 2771302a2..53ed7416a 100644
--- a/bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseComponent.java
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseComponent.java
@@ -190,6 +190,18 @@ public interface DatabaseComponent extends TransactionManager {
 	Collection<Message> generateBatch(Transaction txn, ContactId c,
 			int maxLength, int 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, int maxLatency) throws DbException;
+
 	/**
 	 * Returns an offer for the given contact for transmission over a
 	 * transport with the given maximum latency, or null if there are no
@@ -446,6 +458,17 @@ public interface DatabaseComponent extends TransactionManager {
 	MessageStatus getMessageStatus(Transaction txn, ContactId c, MessageId m)
 			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
+	 * messages that have already been sent and are not yet due for
+	 * retransmission.
+	 * <p/>
+	 * Read-only.
+	 */
+	Map<MessageId, Integer> getUnackedMessagesToSend(Transaction txn,
+			ContactId c) throws DbException;
+
 	/**
 	 * Returns the next time (in milliseconds since the Unix epoch) when a
 	 * message is due to be deleted, or {@link #NO_CLEANUP_DEADLINE}
diff --git a/bramble-core/src/main/java/org/briarproject/bramble/db/Database.java b/bramble-core/src/main/java/org/briarproject/bramble/db/Database.java
index 705913ee7..cb386ab5c 100644
--- a/bramble-core/src/main/java/org/briarproject/bramble/db/Database.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/db/Database.java
@@ -496,11 +496,28 @@ 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.
 	 * <p/>
+	 * Unlike {@link #getUnackedMessagesToSend(Object, ContactId)} this method
+	 * does not return messages that have already been sent unless they are
+	 * due for retransmission.
+	 * <p/>
 	 * Read-only.
 	 */
 	Collection<MessageId> getMessagesToSend(T txn, ContactId c, int maxLength,
 			int maxLatency) throws DbException;
 
+	/**
+	 * Returns the IDs of all messages that are eligible to be sent to the
+	 * given contact, together with their raw lengths.
+	 * <p/>
+	 * Unlike {@link #getMessagesToSend(Object, ContactId, int, int)} 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)
+			throws DbException;
+
 	/**
 	 * Returns the IDs of any messages that need to be validated.
 	 * <p/>
diff --git a/bramble-core/src/main/java/org/briarproject/bramble/db/DatabaseComponentImpl.java b/bramble-core/src/main/java/org/briarproject/bramble/db/DatabaseComponentImpl.java
index b2422f9bc..d4c8ef682 100644
--- a/bramble-core/src/main/java/org/briarproject/bramble/db/DatabaseComponentImpl.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/db/DatabaseComponentImpl.java
@@ -436,6 +436,32 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return messages;
 	}
 
+	@Override
+	public Collection<Message> generateBatch(Transaction transaction,
+			ContactId c, Collection<MessageId> ids, int 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.updateExpiryTimeAndEta(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,
@@ -714,6 +740,16 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return status;
 	}
 
+	@Override
+	public Map<MessageId, Integer> getUnackedMessagesToSend(
+			Transaction transaction,
+			ContactId c) throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		return db.getUnackedMessagesToSend(txn, c);
+	}
+
 	@Override
 	public Map<MessageId, MessageState> getMessageDependencies(
 			Transaction transaction, MessageId m) throws DbException {
diff --git a/bramble-core/src/main/java/org/briarproject/bramble/db/JdbcDatabase.java b/bramble-core/src/main/java/org/briarproject/bramble/db/JdbcDatabase.java
index 224b20bf1..32212193b 100644
--- a/bramble-core/src/main/java/org/briarproject/bramble/db/JdbcDatabase.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/db/JdbcDatabase.java
@@ -51,6 +51,7 @@ 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;
@@ -344,6 +345,11 @@ abstract class JdbcDatabase implements Database<Connection> {
 			"CREATE INDEX IF NOT EXISTS statusesByContactIdTimestamp"
 					+ " ON statuses (contactId, timestamp)";
 
+	private static final String
+			INDEX_STATUSES_BY_CONTACT_ID_TX_COUNT_TIMESTAMP =
+			"CREATE INDEX IF NOT EXISTS statusesByContactIdTxCountTimestamp"
+					+ " ON statuses (contactId, txCount, timestamp)";
+
 	private static final String INDEX_MESSAGES_BY_CLEANUP_DEADLINE =
 			"CREATE INDEX IF NOT EXISTS messagesByCleanupDeadline"
 					+ " ON messages (cleanupDeadline)";
@@ -570,6 +576,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 			s.executeUpdate(INDEX_MESSAGE_DEPENDENCIES_BY_DEPENDENCY_ID);
 			s.executeUpdate(INDEX_STATUSES_BY_CONTACT_ID_GROUP_ID);
 			s.executeUpdate(INDEX_STATUSES_BY_CONTACT_ID_TIMESTAMP);
+			s.executeUpdate(INDEX_STATUSES_BY_CONTACT_ID_TX_COUNT_TIMESTAMP);
 			s.executeUpdate(INDEX_MESSAGES_BY_CLEANUP_DEADLINE);
 			s.close();
 		} catch (SQLException e) {
@@ -2259,6 +2266,37 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	@Override
+	public Map<MessageId, Integer> getUnackedMessagesToSend(Connection txn,
+			ContactId c) throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			String sql = "SELECT length, messageId FROM statuses"
+					+ " WHERE contactId = ? AND state = ?"
+					+ " AND groupShared = TRUE AND messageShared = TRUE"
+					+ " AND deleted = FALSE AND seen = FALSE"
+					+ " ORDER BY txCount, timestamp";
+			ps = txn.prepareStatement(sql);
+			ps.setInt(1, c.getInt());
+			ps.setInt(2, DELIVERED.getValue());
+			rs = ps.executeQuery();
+			Map<MessageId, Integer> results = new LinkedHashMap<>();
+			while (rs.next()) {
+				int length = rs.getInt(1);
+				MessageId id = new MessageId(rs.getBytes(2));
+				results.put(id, length);
+			}
+			rs.close();
+			ps.close();
+			return results;
+		} catch (SQLException e) {
+			tryToClose(rs, LOG, WARNING);
+			tryToClose(ps, LOG, WARNING);
+			throw new DbException(e);
+		}
+	}
+
 	@Override
 	public Collection<MessageId> getMessagesToValidate(Connection txn)
 			throws DbException {