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[WIP] Optimize TCP read implementation for backpressure handling #75

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[WIP] Optimize TCP read implementation for backpressure handling #75

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Copilot Oct 15, 2025
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Copilot Oct 15, 2025
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105 changes: 54 additions & 51 deletions src/stream/tcb.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -43,6 +43,7 @@ pub(super) enum PacketType {
/// - `unordered_packets` is the bytes stream received from the lower device,
/// which can be acknowledged and extracted by `consume_unordered_packets` method
/// then can be read by upstream application via `Tcp::poll_read` method.
/// - `ordered_packets` is the list of contiguous packets ready to be read by the application.
#[derive(Debug, Clone)]
pub(crate) struct Tcb {
seq: SeqNum,
Expand All @@ -53,6 +54,7 @@ pub(crate) struct Tcb {
state: TcpState,
inflight_packets: BTreeMap<SeqNum, InflightPacket>,
unordered_packets: BTreeMap<SeqNum, Vec<u8>>,
ordered_packets: Vec<Vec<u8>>,
duplicate_ack_count: usize,
duplicate_ack_count_helper: SeqNum,
}
Expand All @@ -72,6 +74,7 @@ impl Tcb {
state: TcpState::Listen,
inflight_packets: BTreeMap::new(),
unordered_packets: BTreeMap::new(),
ordered_packets: Vec::new(),
duplicate_ack_count: 0,
duplicate_ack_count_helper: seq.into(),
}
Expand Down Expand Up @@ -106,46 +109,40 @@ impl Tcb {
self.unordered_packets.insert(seq, buf);
}
pub(super) fn get_available_read_buffer_size(&self) -> usize {
READ_BUFFER_SIZE.saturating_sub(self.get_unordered_packets_total_len())
let total_buffered = self.get_unordered_packets_total_len() + self.get_ordered_packets_total_len();
READ_BUFFER_SIZE.saturating_sub(total_buffered)
}
#[inline]
pub(crate) fn get_unordered_packets_total_len(&self) -> usize {
self.unordered_packets.values().map(|p| p.len()).sum()
}
#[inline]
pub(crate) fn get_ordered_packets_total_len(&self) -> usize {
self.ordered_packets.iter().map(|p| p.len()).sum()
}

pub(super) fn consume_unordered_packets(&mut self, max_bytes: usize) -> Option<Vec<u8>> {
let mut data = Vec::new();
let mut remaining_bytes = max_bytes;

while remaining_bytes > 0 {
if let Some(seq) = self.unordered_packets.keys().next().copied() {
if seq != self.ack {
break; // sequence number is not continuous, stop extracting
}

// remove and get the first packet
let mut payload = self.unordered_packets.remove(&seq).unwrap();
let payload_len = payload.len();

if payload_len <= remaining_bytes {
// current packet can be fully extracted
data.extend(payload);
self.ack += payload_len as u32;
remaining_bytes -= payload_len;
} else {
// current packet can only be partially extracted
let remaining_payload = payload.split_off(remaining_bytes);
data.extend_from_slice(&payload);
self.ack += remaining_bytes as u32;
self.unordered_packets.insert(self.ack, remaining_payload);
break;
}
} else {
break; // no more packets to extract
pub(super) fn consume_unordered_packets(&mut self) {
while let Some(seq) = self.unordered_packets.keys().next().copied() {
if seq != self.ack {
break; // sequence number is not continuous, stop extracting
}

// remove and get the first packet
let payload = self.unordered_packets.remove(&seq).unwrap();
let payload_len = payload.len();

// Move the packet to ordered_packets
self.ordered_packets.push(payload);
self.ack += payload_len as u32;
}
}

if data.is_empty() { None } else { Some(data) }
pub(super) fn pop_ordered_packet(&mut self) -> Option<Vec<u8>> {
if self.ordered_packets.is_empty() {
None
} else {
Some(self.ordered_packets.remove(0))
}
}

pub(super) fn increase_seq(&mut self) {
Expand Down Expand Up @@ -359,26 +356,32 @@ mod tests {
tcb.add_unordered_packet(SeqNum(1500), vec![2; 500]); // seq=1500, len=500
tcb.add_unordered_packet(SeqNum(2000), vec![3; 500]); // seq=2000, len=500

// test 1: extract up to 700 bytes
let data = tcb.consume_unordered_packets(700).unwrap();
assert_eq!(data.len(), 700); // extract 500 + 200
assert_eq!(data[..500], vec![1; 500]); // the first packet
assert_eq!(data[500..700], vec![2; 200]); // the first 200 bytes of the second packet
assert_eq!(tcb.ack, SeqNum(1700)); // ack increased by 700
assert_eq!(tcb.unordered_packets.len(), 2); // remaining two packets
assert_eq!(tcb.unordered_packets.get(&SeqNum(1700)).unwrap().len(), 300); // the second packet remaining 300 bytes
assert_eq!(tcb.unordered_packets.get(&SeqNum(2000)).unwrap().len(), 500); // the third packet unchanged

// test 2: extract up to 800 bytes
let data = tcb.consume_unordered_packets(800).unwrap();
assert_eq!(data.len(), 800); // extract 300 bytes of the second packet and the third packet
assert_eq!(data[..300], vec![2; 300]); // the remaining 300 bytes of the second packet
assert_eq!(data[300..800], vec![3; 500]); // the third packet
assert_eq!(tcb.ack, SeqNum(2500)); // ack increased by 800
assert_eq!(tcb.unordered_packets.len(), 0); // no remaining packets

// test 3: no data to extract
let data = tcb.consume_unordered_packets(1000);
// test 1: consume contiguous packets
tcb.consume_unordered_packets();
assert_eq!(tcb.ack, SeqNum(2500)); // ack increased by 1500
assert_eq!(tcb.unordered_packets.len(), 0); // all packets consumed
assert_eq!(tcb.ordered_packets.len(), 3); // three packets in ordered list

// test 2: pop first ordered packet
let data = tcb.pop_ordered_packet().unwrap();
assert_eq!(data.len(), 500);
assert_eq!(data, vec![1; 500]);
assert_eq!(tcb.ordered_packets.len(), 2);

// test 3: pop second ordered packet
let data = tcb.pop_ordered_packet().unwrap();
assert_eq!(data.len(), 500);
assert_eq!(data, vec![2; 500]);
assert_eq!(tcb.ordered_packets.len(), 1);

// test 4: pop third ordered packet
let data = tcb.pop_ordered_packet().unwrap();
assert_eq!(data.len(), 500);
assert_eq!(data, vec![3; 500]);
assert_eq!(tcb.ordered_packets.len(), 0);

// test 5: no more data to extract
let data = tcb.pop_ordered_packet();
assert!(data.is_none());
}

Expand Down
20 changes: 11 additions & 9 deletions src/stream/tcp.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -623,7 +623,7 @@ async fn tcp_main_logic_loop(
if flags & ACK == ACK {
if len > 0 {
tcb.add_unordered_packet(incoming_seq, payload);
extract_data_n_write_upstream(&up_packet_sender, &mut tcb, network_tuple, &data_tx, &read_notify)?;
extract_data_n_write_upstream(&up_packet_sender, &mut tcb, network_tuple, &read_notify)?;
}
tcb.change_state(TcpState::Established);
}
Expand All @@ -650,7 +650,7 @@ async fn tcp_main_logic_loop(
PacketType::NewPacket => {
tcb.add_unordered_packet(incoming_seq, payload);
let nt = network_tuple;
extract_data_n_write_upstream(&up_packet_sender, &mut tcb, nt, &data_tx, &read_notify)?;
extract_data_n_write_upstream(&up_packet_sender, &mut tcb, nt, &read_notify)?;
write_notify.lock().unwrap().take().map(|w| w.wake_by_ref()).unwrap_or(());
}
PacketType::Ack => {
Expand Down Expand Up @@ -711,7 +711,7 @@ async fn tcp_main_logic_loop(
} else if flags == (ACK | PSH) && pkt_type == PacketType::NewPacket {
if !payload.is_empty() && tcb.get_ack() == incoming_seq {
tcb.add_unordered_packet(incoming_seq, payload);
extract_data_n_write_upstream(&up_packet_sender, &mut tcb, network_tuple, &data_tx, &read_notify)?;
extract_data_n_write_upstream(&up_packet_sender, &mut tcb, network_tuple, &read_notify)?;
}
} else {
// unnormal case, we do nothing here
Expand Down Expand Up @@ -769,7 +769,7 @@ async fn tcp_main_logic_loop(
if len > 0 {
// if the other side is still sending data, we need to deal with it like PacketStatus::NewPacket
tcb.add_unordered_packet(incoming_seq, payload);
extract_data_n_write_upstream(&up_packet_sender, &mut tcb, network_tuple, &data_tx, &read_notify)?;
extract_data_n_write_upstream(&up_packet_sender, &mut tcb, network_tuple, &read_notify)?;
write_notify.lock().unwrap().take().map(|w| w.wake_by_ref()).unwrap_or(());
}
let new_state = tcb.get_state();
Expand Down Expand Up @@ -799,7 +799,7 @@ async fn tcp_main_logic_loop(
} else {
// if the other side is still sending data, we need to deal with it like PacketStatus::NewPacket
tcb.add_unordered_packet(incoming_seq, payload);
extract_data_n_write_upstream(&up_packet_sender, &mut tcb, network_tuple, &data_tx, &read_notify)?;
extract_data_n_write_upstream(&up_packet_sender, &mut tcb, network_tuple, &read_notify)?;
write_notify.lock().unwrap().take().map(|w| w.wake_by_ref()).unwrap_or(());
}
if flags & FIN == FIN {
Expand Down Expand Up @@ -833,7 +833,6 @@ fn extract_data_n_write_upstream(
up_packet_sender: &PacketSender,
tcb: &mut Tcb,
network_tuple: NetworkTuple,
data_tx: &tokio::sync::mpsc::UnboundedSender<Vec<u8>>,
read_notify: &std::sync::Arc<std::sync::Mutex<Option<Waker>>>,
) -> std::io::Result<()> {
let (state, seq, ack) = (tcb.get_state(), tcb.get_seq(), tcb.get_ack());
Expand All @@ -843,10 +842,13 @@ fn extract_data_n_write_upstream(
return Ok(());
}

if let Some(data) = tcb.consume_unordered_packets(8192) {
let before_len = tcb.get_ordered_packets_total_len();
tcb.consume_unordered_packets();
let after_len = tcb.get_ordered_packets_total_len();

if after_len > before_len {
let hint = if state == TcpState::Established { "normally" } else { "still" };
log::trace!("{network_tuple} {state:?}: {l_info} {hint} receiving data, len = {}", data.len());
data_tx.send(data).map_err(|e| std::io::Error::new(BrokenPipe, e))?;
log::trace!("{network_tuple} {state:?}: {l_info} {hint} receiving data, new ordered bytes = {}", after_len - before_len);
read_notify.lock().unwrap().take().map(|w| w.wake_by_ref()).unwrap_or(());
write_packet_to_device(up_packet_sender, network_tuple, tcb, None, ACK, None, None)?;
}
Expand Down
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