Split stream writes into packet chunks

src/bin/dosh-client.rs

@@ -7966,31 +7966,34 @@ async fn queue_or_send_stream_data(
     stream_next_send_offset: &mut HashMap<u64, u64>,
     stream_sent_data: &mut HashMap<u64, BTreeMap<u64, PendingStreamChunk>>,
 ) -> Result<()> {
-    if !opened_streams.contains(&stream_id)
-        || stream_send_credit.get(&stream_id).copied().unwrap_or(0) < bytes.len()
-        || stream_pending_data
-            .get(&stream_id)
-            .is_some_and(|pending| !pending.is_empty())
-    {
-        stream_pending_data
-            .entry(stream_id)
-            .or_default()
-            .push_back(bytes);
-        return Ok(());
-    }
-    *stream_send_credit.entry(stream_id).or_default() -= bytes.len();
-    let offset = *stream_next_send_offset.entry(stream_id).or_default();
-    stream_next_send_offset.insert(stream_id, offset.saturating_add(bytes.len() as u64));
-    stream_sent_data.entry(stream_id).or_default().insert(
-        offset,
-        PendingStreamChunk {
+    for chunk in dosh::transport::split_stream_data_bytes(bytes, STREAM_INITIAL_WINDOW) {
+        if !opened_streams.contains(&stream_id)
+            || stream_send_credit.get(&stream_id).copied().unwrap_or(0) < chunk.len()
+            || stream_pending_data
+                .get(&stream_id)
+                .is_some_and(|pending| !pending.is_empty())
+        {
+            stream_pending_data
+                .entry(stream_id)
+                .or_default()
+                .push_back(chunk);
+            continue;
+        }
+        *stream_send_credit.entry(stream_id).or_default() -= chunk.len();
+        let offset = *stream_next_send_offset.entry(stream_id).or_default();
+        stream_next_send_offset.insert(stream_id, offset.saturating_add(chunk.len() as u64));
+        stream_sent_data.entry(stream_id).or_default().insert(
             offset,
-            bytes: bytes.clone(),
-            last_sent: Instant::now(),
-            attempts: 1,
-        },
-    );
-    send_stream_data(socket, addr, cred, send_seq, stream_id, offset, bytes).await
+            PendingStreamChunk {
+                offset,
+                bytes: chunk.clone(),
+                last_sent: Instant::now(),
+                attempts: 1,
+            },
+        );
+        send_stream_data(socket, addr, cred, send_seq, stream_id, offset, chunk).await?;
+    }
+    Ok(())
 }
 
 #[allow(clippy::too_many_arguments)]
@@ -8008,9 +8011,22 @@ async fn flush_stream_pending_data(
     let Some(pending) = stream_pending_data.get_mut(&stream_id) else {
         return Ok(());
     };
-    while let Some(bytes) = pending.front() {
+    let chunk_limit = dosh::transport::stream_data_chunk_limit(STREAM_INITIAL_WINDOW);
+    while let Some(front_len) = pending.front().map(Vec::len) {
+        if front_len > chunk_limit {
+            let Some(bytes) = pending.pop_front() else {
+                break;
+            };
+            for chunk in dosh::transport::split_stream_data_bytes(bytes, STREAM_INITIAL_WINDOW)
+                .into_iter()
+                .rev()
+            {
+                pending.push_front(chunk);
+            }
+            continue;
+        }
         let credit = stream_send_credit.get(&stream_id).copied().unwrap_or(0);
-        if credit < bytes.len() {
+        if credit < front_len {
             break;
         }
         let Some(bytes) = pending.pop_front() else {
@@ -9605,9 +9621,9 @@ mod tests {
         LOCAL_SLEEP_REPAINT_RETRY_WINDOW, LocalForward, MAX_PENDING_USER_INPUT_BYTES,
         NativeIdentityContext, POST_RECONNECT_STALE_INPUT_GRACE, POST_SUBMIT_ALL_INPUT_HOLD,
         PendingStreamOpen, PredictMode, Predictor, RESTART_STATUS_SCRIPT, RemoteForward,
-        STALE_TERMINAL_INPUT_AFTER, STARTUP_INPUT_HOLD, SshConfig, SshPathTokenContext,
-        StartupGateMode, StatusAction, UpdateOptions, UpdateRole, auth_allows, cache_key,
-        cache_server_prefix, cleanup_stream_state, clear_cached_credentials,
+        STALE_TERMINAL_INPUT_AFTER, STARTUP_INPUT_HOLD, STREAM_INITIAL_WINDOW, SshConfig,
+        SshPathTokenContext, StartupGateMode, StatusAction, UpdateOptions, UpdateRole, auth_allows,
+        cache_key, cache_server_prefix, cleanup_stream_state, clear_cached_credentials,
         effective_update_artifact_tag, ensure_tui_safe_status_overlay, expand_ssh_path_tokens,
         input_contains_focus_in, input_matches_escape, is_local_status_target,
         is_resume_response_for_client, latest_release_download_url,
@@ -9615,21 +9631,22 @@ mod tests {
         newest_client_trace_path_from, parse_dynamic_forward, parse_escape_key,
         parse_local_forward, parse_remote_forward, parse_single_remote_path, parse_ssh_config,
         parse_trace_line, parse_trace_options, parse_trace_report_options, parse_trace_summary,
-        parse_update_options, post_submit_hold_duration, queue_pending_user_input,
-        queue_stale_pending_user_input, raw_contains_host_table, recv_response_until,
-        refresh_live_addr, release_tag_download_url, release_tag_from_effective_url,
-        release_version_from_tag, render_status_clear, render_status_overlay, requested_env,
-        resolved_startup_command, retire_stream_state, retransmit_stream_opens,
-        rewrite_forward_command, sanitize_trace_name, selected_predict_mode, selected_udp_host,
-        server_version_mismatch, should_flush_terminal_input_after_contact,
-        should_health_log_client_start, should_hold_during_startup_gate,
-        should_hold_post_submit_input, should_reconnect_before_input_for_local_sleep,
-        should_repaint_idle_terminal, should_strip_unowned_terminal_reports,
-        split_after_command_submit, split_trace_tokens, ssh_command_target, ssh_config_uses_proxy,
-        ssh_destination_host, ssh_username, ssh_with_user, startup_command, status_ssh_target,
-        strip_stale_mouse_reports, strip_terminal_focus_reports, strip_unowned_terminal_reports,
-        summarize_trace_file, summarize_trace_file_with_mode, terminal_private_mode_transition,
-        toml_bare_key_or_quoted, top_trace_events, trace_report_warnings, unix_update_script,
+        parse_update_options, post_submit_hold_duration, queue_or_send_stream_data,
+        queue_pending_user_input, queue_stale_pending_user_input, raw_contains_host_table,
+        recv_response_until, refresh_live_addr, release_tag_download_url,
+        release_tag_from_effective_url, release_version_from_tag, render_status_clear,
+        render_status_overlay, requested_env, resolved_startup_command, retire_stream_state,
+        retransmit_stream_opens, rewrite_forward_command, sanitize_trace_name,
+        selected_predict_mode, selected_udp_host, server_version_mismatch,
+        should_flush_terminal_input_after_contact, should_health_log_client_start,
+        should_hold_during_startup_gate, should_hold_post_submit_input,
+        should_reconnect_before_input_for_local_sleep, should_repaint_idle_terminal,
+        should_strip_unowned_terminal_reports, split_after_command_submit, split_trace_tokens,
+        ssh_command_target, ssh_config_uses_proxy, ssh_destination_host, ssh_username,
+        ssh_with_user, startup_command, status_ssh_target, strip_stale_mouse_reports,
+        strip_terminal_focus_reports, strip_unowned_terminal_reports, summarize_trace_file,
+        summarize_trace_file_with_mode, terminal_private_mode_transition, toml_bare_key_or_quoted,
+        top_trace_events, trace_report_warnings, unix_update_script,
         update_binary_version_for_installer, update_installer_url, update_version_status,
         upsert_managed_block, valid_forward_host, vscode_safe_alias, wake_repaint_retry_deadline,
         windows_update_script,
@@ -10860,6 +10877,85 @@ mod tests {
         assert_eq!(pending[&44].attempts, 2);
     }
 
+    #[tokio::test]
+    async fn client_stream_send_splits_large_writes() {
+        let receiver = tokio::net::UdpSocket::bind("127.0.0.1:0").await.unwrap();
+        let sender = tokio::net::UdpSocket::bind("127.0.0.1:0").await.unwrap();
+        let addr = receiver.local_addr().unwrap();
+        let cred = CachedCredential {
+            server: "host".to_string(),
+            session: "term".to_string(),
+            mode: "forward-only".to_string(),
+            udp_host: "127.0.0.1".to_string(),
+            udp_port: addr.port(),
+            client_id: [1; 16],
+            session_key: [2; 32],
+            session_key_id: protocol::session_key_id(&[2; 32]),
+            attach_ticket: Vec::new(),
+            attach_ticket_psk: [3; 32],
+            last_rendered_seq: 0,
+        };
+        let mut send_seq = 10;
+        let stream_id = 44;
+        let opened_streams = HashSet::from([stream_id]);
+        let mut stream_send_credit = HashMap::from([(stream_id, STREAM_INITIAL_WINDOW)]);
+        let mut stream_pending_data = HashMap::new();
+        let mut stream_next_send_offset = HashMap::new();
+        let mut stream_sent_data = HashMap::new();
+        let bytes = vec![9; dosh::transport::MAX_STREAM_DATA_BYTES * 2 + 11];
+
+        queue_or_send_stream_data(
+            &sender,
+            addr,
+            &cred,
+            &mut send_seq,
+            stream_id,
+            bytes,
+            &opened_streams,
+            &mut stream_send_credit,
+            &mut stream_pending_data,
+            &mut stream_next_send_offset,
+            &mut stream_sent_data,
+        )
+        .await
+        .unwrap();
+
+        assert!(stream_pending_data.is_empty());
+        assert_eq!(stream_sent_data[&stream_id].len(), 3);
+        assert_eq!(
+            stream_next_send_offset[&stream_id],
+            (dosh::transport::MAX_STREAM_DATA_BYTES * 2 + 11) as u64
+        );
+        assert_eq!(send_seq, 13);
+
+        let mut seen = Vec::new();
+        let mut buf = vec![0u8; u16::MAX as usize];
+        for _ in 0..3 {
+            let (n, _) = tokio::time::timeout(Duration::from_secs(1), receiver.recv_from(&mut buf))
+                .await
+                .unwrap()
+                .unwrap();
+            let packet = protocol::decode(&buf[..n]).unwrap();
+            assert_eq!(packet.header.kind, PacketKind::StreamData);
+            let plain =
+                protocol::decrypt_body(&packet, &cred.session_key, protocol::CLIENT_TO_SERVER)
+                    .unwrap();
+            let data: protocol::StreamData = protocol::from_body(&plain).unwrap();
+            seen.push((data.offset, data.bytes.len()));
+        }
+        assert_eq!(
+            seen,
+            vec![
+                (0, dosh::transport::MAX_STREAM_DATA_BYTES),
+                (
+                    dosh::transport::MAX_STREAM_DATA_BYTES as u64,
+                    dosh::transport::MAX_STREAM_DATA_BYTES
+                ),
+                ((dosh::transport::MAX_STREAM_DATA_BYTES * 2) as u64, 11)
+            ]
+        );
+    }
+
     #[tokio::test]
     async fn client_stream_retransmit_uses_observed_rtt() {
         let receiver = tokio::net::UdpSocket::bind("127.0.0.1:0").await.unwrap();

src/bin/dosh-server.rs

@@ -3363,67 +3363,69 @@ async fn queue_or_send_stream_data_to_client(
     stream_id: u64,
     bytes: Vec<u8>,
 ) -> Result<()> {
-    let send = {
+    let sends = {
         let mut locked = state.lock().expect("server state poisoned");
-        let mut send = None;
+        let mut sends = Vec::new();
         if let Some(client) = locked.client_mut(&client_id) {
-            if !client.opened_streams.contains(&stream_id)
-                || client
-                    .stream_send_credit
-                    .get(&stream_id)
-                    .copied()
-                    .unwrap_or(0)
-                    < bytes.len()
-                || client
-                    .stream_pending_data
-                    .get(&stream_id)
-                    .is_some_and(|pending| !pending.is_empty())
-            {
+            for chunk in dosh::transport::split_stream_data_bytes(bytes, STREAM_INITIAL_WINDOW) {
+                if !client.opened_streams.contains(&stream_id)
+                    || client
+                        .stream_send_credit
+                        .get(&stream_id)
+                        .copied()
+                        .unwrap_or(0)
+                        < chunk.len()
+                    || client
+                        .stream_pending_data
+                        .get(&stream_id)
+                        .is_some_and(|pending| !pending.is_empty())
+                {
+                    client
+                        .stream_pending_data
+                        .entry(stream_id)
+                        .or_default()
+                        .push_back(chunk);
+                    continue;
+                }
+                *client.stream_send_credit.entry(stream_id).or_default() -= chunk.len();
+                let offset = *client.stream_next_send_offset.entry(stream_id).or_default();
                 client
-                    .stream_pending_data
+                    .stream_next_send_offset
+                    .insert(stream_id, offset.saturating_add(chunk.len() as u64));
+                client.send_seq += 1;
+                let body = protocol::to_body(&StreamData {
+                    stream_id,
+                    offset,
+                    bytes: chunk.clone(),
+                })?;
+                let packet = protocol::encode_encrypted(
+                    PacketKind::StreamData,
+                    client_id,
+                    client.send_seq,
+                    client.last_acked,
+                    &client.session_key,
+                    SERVER_TO_CLIENT,
+                    &body,
+                )?;
+                client
+                    .stream_sent_data
                     .entry(stream_id)
                     .or_default()
-                    .push_back(bytes);
-                return Ok(());
-            }
-            *client.stream_send_credit.entry(stream_id).or_default() -= bytes.len();
-            let offset = *client.stream_next_send_offset.entry(stream_id).or_default();
-            client
-                .stream_next_send_offset
-                .insert(stream_id, offset.saturating_add(bytes.len() as u64));
-            client.send_seq += 1;
-            let body = protocol::to_body(&StreamData {
-                stream_id,
-                offset,
-                bytes: bytes.clone(),
-            })?;
-            let packet = protocol::encode_encrypted(
-                PacketKind::StreamData,
-                client_id,
-                client.send_seq,
-                client.last_acked,
-                &client.session_key,
-                SERVER_TO_CLIENT,
-                &body,
-            )?;
-            client
-                .stream_sent_data
-                .entry(stream_id)
-                .or_default()
-                .insert(
-                    offset,
-                    PendingStreamChunk {
+                    .insert(
                         offset,
-                        bytes,
-                        last_sent: Instant::now(),
-                        attempts: 1,
-                    },
-                );
-            send = Some((client.endpoint, packet));
+                        PendingStreamChunk {
+                            offset,
+                            bytes: chunk,
+                            last_sent: Instant::now(),
+                            attempts: 1,
+                        },
+                    );
+                sends.push((client.endpoint, packet));
+            }
         }
-        send
+        sends
     };
-    if let Some((endpoint, packet)) = send {
+    for (endpoint, packet) in sends {
         let _ = send_udp(socket, &packet, endpoint).await?;
     }
     Ok(())
@@ -3443,16 +3445,30 @@ async fn flush_stream_pending_data_to_client(
                 let Some(pending) = client.stream_pending_data.get_mut(&stream_id) else {
                     return Ok(());
                 };
-                let Some(bytes) = pending.front() else {
+                let Some(front_len) = pending.front().map(Vec::len) else {
                     client.stream_pending_data.remove(&stream_id);
                     return Ok(());
                 };
+                if front_len > dosh::transport::stream_data_chunk_limit(STREAM_INITIAL_WINDOW) {
+                    let Some(bytes) = pending.pop_front() else {
+                        client.stream_pending_data.remove(&stream_id);
+                        return Ok(());
+                    };
+                    for chunk in
+                        dosh::transport::split_stream_data_bytes(bytes, STREAM_INITIAL_WINDOW)
+                            .into_iter()
+                            .rev()
+                    {
+                        pending.push_front(chunk);
+                    }
+                    continue;
+                }
                 let credit = client
                     .stream_send_credit
                     .get(&stream_id)
                     .copied()
                     .unwrap_or(0);
-                if credit < bytes.len() {
+                if credit < front_len {
                     return Ok(());
                 }
                 let Some(bytes) = pending.pop_front() else {
@@ -5259,6 +5275,108 @@ mod tests {
         assert_eq!(data.bytes, b"hello");
     }
 
+    #[tokio::test]
+    async fn server_stream_send_splits_large_writes() {
+        let (pty_tx, _pty_rx) = mpsc::unbounded_channel();
+        let mut state = ServerState::new(ServerConfig::default(), [0u8; 32], pty_tx);
+        let client_id = [11u8; 16];
+        let session_key = [12u8; 32];
+        let receiver = UdpSocket::bind("127.0.0.1:0").await.unwrap();
+        let sender = Arc::new(UdpSocket::bind("127.0.0.1:0").await.unwrap());
+        let endpoint = receiver.local_addr().unwrap();
+        state.sessions.insert(
+            "test".to_string(),
+            Session {
+                pty: None,
+                parser: vt100::Parser::new(24, 80, 100),
+                restored_screen: None,
+                clients: HashMap::from([(
+                    client_id,
+                    ClientState {
+                        endpoint,
+                        mode: "forward-only".to_string(),
+                        session_key,
+                        last_acked: 0,
+                        replay: ReplayWindow::default(),
+                        send_seq: 1,
+                        cols: 80,
+                        rows: 24,
+                        last_seen: Instant::now(),
+                        pending: VecDeque::new(),
+                        allowed_forwardings: Vec::new(),
+                        stream_writers: HashMap::new(),
+                        opened_streams: HashSet::from([42]),
+                        stream_send_credit: HashMap::from([(42, STREAM_INITIAL_WINDOW)]),
+                        stream_pending_data: HashMap::new(),
+                        stream_pending_opens: HashMap::new(),
+                        stream_next_send_offset: HashMap::new(),
+                        stream_sent_data: HashMap::new(),
+                        stream_next_recv_offset: HashMap::new(),
+                        stream_recv_pending: HashMap::new(),
+                        stream_retired: HashSet::new(),
+                        stream_retired_order: VecDeque::new(),
+                        stream_retransmit_srtt: None,
+                        epoch: 0,
+                        epoch_started: Instant::now(),
+                        epoch_packets: 0,
+                        previous_session_key: None,
+                    },
+                )]),
+                output_seq: 0,
+                recent: VecDeque::new(),
+                empty_since: None,
+                holder_control: None,
+                persistent: false,
+                bytes_since_persist: 0,
+                last_persisted_seq: 0,
+                last_screen_persist_at: Instant::now(),
+            },
+        );
+        state.client_index.insert(client_id, "test".to_string());
+        let state = Arc::new(Mutex::new(state));
+        let bytes = vec![3; dosh::transport::MAX_STREAM_DATA_BYTES * 2 + 17];
+
+        queue_or_send_stream_data_to_client(&state, &sender, client_id, 42, bytes)
+            .await
+            .unwrap();
+
+        {
+            let locked = state.lock().unwrap();
+            let client = locked.sessions["test"].clients.get(&client_id).unwrap();
+            assert!(client.stream_pending_data.is_empty());
+            assert_eq!(client.stream_sent_data[&42].len(), 3);
+            assert_eq!(
+                client.stream_next_send_offset[&42],
+                (dosh::transport::MAX_STREAM_DATA_BYTES * 2 + 17) as u64
+            );
+        }
+
+        let mut seen = Vec::new();
+        let mut buf = vec![0u8; u16::MAX as usize];
+        for _ in 0..3 {
+            let (n, _) = tokio::time::timeout(Duration::from_secs(1), receiver.recv_from(&mut buf))
+                .await
+                .unwrap()
+                .unwrap();
+            let packet = protocol::decode(&buf[..n]).unwrap();
+            assert_eq!(packet.header.kind, PacketKind::StreamData);
+            let plain = protocol::decrypt_body(&packet, &session_key, SERVER_TO_CLIENT).unwrap();
+            let data: StreamData = protocol::from_body(&plain).unwrap();
+            seen.push((data.offset, data.bytes.len()));
+        }
+        assert_eq!(
+            seen,
+            vec![
+                (0, dosh::transport::MAX_STREAM_DATA_BYTES),
+                (
+                    dosh::transport::MAX_STREAM_DATA_BYTES as u64,
+                    dosh::transport::MAX_STREAM_DATA_BYTES
+                ),
+                ((dosh::transport::MAX_STREAM_DATA_BYTES * 2) as u64, 17)
+            ]
+        );
+    }
+
     #[tokio::test]
     async fn blocked_stream_data_does_not_block_terminal_frames() {
         let (pty_tx, _pty_rx) = mpsc::unbounded_channel();

src/transport.rs

@@ -34,6 +34,7 @@ pub const ADAPTIVE_RETRANSMIT_MIN: Duration = Duration::from_millis(10);
 pub const DEFAULT_KEEPALIVE_AFTER: Duration = Duration::from_secs(2);
 pub const DEFAULT_RETIRED_STREAM_TOMBSTONES: usize = 16 * 1024;
 pub const SERVICE_TARGET_PREFIX: &str = "@dosh-";
+pub const MAX_STREAM_DATA_BYTES: usize = 60 * 1024;
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct TransportConfig {
@@ -362,29 +363,35 @@ impl StreamMux {
         if bytes.is_empty() {
             return Ok(Vec::new());
         }
-        if !self.opened_streams.contains(&stream_id) {
-            if self.pending_opens.contains_key(&stream_id) {
+        let chunks = split_stream_data_bytes(bytes, self.config.initial_window);
+        let mut out = Vec::new();
+        if !self.opened_streams.contains(&stream_id) && !self.pending_opens.contains_key(&stream_id)
+        {
+            bail!("stream {stream_id} is not open");
+        }
+        for chunk in chunks {
+            if !self.opened_streams.contains(&stream_id) {
                 self.pending_data
                     .entry(stream_id)
                     .or_default()
-                    .push_back(bytes);
-                return Ok(Vec::new());
+                    .push_back(chunk);
+                continue;
             }
-            bail!("stream {stream_id} is not open");
+            if self.send_credit.get(&stream_id).copied().unwrap_or(0) < chunk.len()
+                || self
+                    .pending_data
+                    .get(&stream_id)
+                    .is_some_and(|pending| !pending.is_empty())
+            {
+                self.pending_data
+                    .entry(stream_id)
+                    .or_default()
+                    .push_back(chunk);
+                continue;
+            }
+            out.push(self.send_data_now(stream_id, chunk)?);
         }
-        if self.send_credit.get(&stream_id).copied().unwrap_or(0) < bytes.len()
-            || self
-                .pending_data
-                .get(&stream_id)
-                .is_some_and(|pending| !pending.is_empty())
-        {
-            self.pending_data
-                .entry(stream_id)
-                .or_default()
-                .push_back(bytes);
-            return Ok(Vec::new());
-        }
-        Ok(vec![self.send_data_now(stream_id, bytes)?])
+        Ok(out)
     }
 
     pub fn handle_data(&mut self, data: StreamData) -> Result<Option<IncomingStreamData>> {
@@ -600,11 +607,30 @@ impl StreamMux {
 
     fn flush_pending_data(&mut self, stream_id: u64) -> Result<Vec<OutgoingStreamPacket>> {
         let mut out = Vec::new();
+        let chunk_limit = stream_data_chunk_limit(self.config.initial_window);
         while let Some(front_len) = self
             .pending_data
             .get(&stream_id)
             .and_then(|pending| pending.front().map(Vec::len))
         {
+            if front_len > chunk_limit {
+                let Some(bytes) = self
+                    .pending_data
+                    .get_mut(&stream_id)
+                    .and_then(VecDeque::pop_front)
+                else {
+                    break;
+                };
+                if let Some(pending) = self.pending_data.get_mut(&stream_id) {
+                    for chunk in split_stream_data_bytes(bytes, self.config.initial_window)
+                        .into_iter()
+                        .rev()
+                    {
+                        pending.push_front(chunk);
+                    }
+                }
+                continue;
+            }
             if self.send_credit.get(&stream_id).copied().unwrap_or(0) < front_len {
                 break;
             }
@@ -796,6 +822,21 @@ pub fn stream_data_within_receive_window(
     end <= expected_offset.saturating_add(window)
 }
 
+pub fn stream_data_chunk_limit(receive_window: usize) -> usize {
+    MAX_STREAM_DATA_BYTES.min(receive_window.max(1))
+}
+
+pub fn split_stream_data_bytes(bytes: Vec<u8>, receive_window: usize) -> Vec<Vec<u8>> {
+    if bytes.is_empty() {
+        return Vec::new();
+    }
+    let limit = stream_data_chunk_limit(receive_window);
+    if bytes.len() <= limit {
+        return vec![bytes];
+    }
+    bytes.chunks(limit).map(Vec::from).collect()
+}
+
 pub struct DoshTransport {
     socket: Arc<UdpSocket>,
     role: SessionRole,
@@ -1328,6 +1369,63 @@ mod tests {
         assert_eq!(data.bytes, b"!");
     }
 
+    #[test]
+    fn send_data_splits_large_writes_into_packet_sized_chunks() {
+        let mut mux = StreamMux::new(TransportConfig::default());
+        mux.open_stream(1, "@dosh-test", 0).unwrap();
+        mux.handle_open_ok(StreamOpenOk { stream_id: 1 })
+            .unwrap()
+            .unwrap();
+
+        let bytes = vec![7; MAX_STREAM_DATA_BYTES * 2 + 13];
+        let sent = mux.send_data(1, bytes).unwrap();
+
+        assert_eq!(sent.len(), 3);
+        let first: StreamData = decode(&sent[0]);
+        let second: StreamData = decode(&sent[1]);
+        let third: StreamData = decode(&sent[2]);
+        assert_eq!(first.offset, 0);
+        assert_eq!(first.bytes.len(), MAX_STREAM_DATA_BYTES);
+        assert_eq!(second.offset, MAX_STREAM_DATA_BYTES as u64);
+        assert_eq!(second.bytes.len(), MAX_STREAM_DATA_BYTES);
+        assert_eq!(third.offset, (MAX_STREAM_DATA_BYTES * 2) as u64);
+        assert_eq!(third.bytes.len(), 13);
+    }
+
+    #[test]
+    fn queued_large_write_flushes_in_window_sized_chunks_after_open() {
+        let mut mux = StreamMux::new(TransportConfig {
+            initial_window: 5,
+            ..TransportConfig::default()
+        });
+        mux.open_stream(1, "@dosh-test", 0).unwrap();
+        assert!(mux.send_data(1, b"hello!".to_vec()).unwrap().is_empty());
+
+        let flushed = mux
+            .handle_open_ok(StreamOpenOk { stream_id: 1 })
+            .unwrap()
+            .unwrap();
+
+        assert_eq!(flushed.len(), 1);
+        let first: StreamData = decode(&flushed[0]);
+        assert_eq!(first.offset, 0);
+        assert_eq!(first.bytes, b"hello");
+        assert_eq!(mux.pending_bytes(1), 1);
+
+        let flushed = mux
+            .handle_window_adjust(StreamWindowAdjust {
+                stream_id: 1,
+                received_offset: 5,
+                bytes: 5,
+            })
+            .unwrap()
+            .unwrap();
+        assert_eq!(flushed.len(), 1);
+        let second: StreamData = decode(&flushed[0]);
+        assert_eq!(second.offset, 5);
+        assert_eq!(second.bytes, b"!");
+    }
+
     #[test]
     fn stream_open_ack_lowers_retransmit_timeout_from_observed_rtt() {
         let mut mux = StreamMux::new(TransportConfig {