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Send immediate ACKs after RMSS bytes of data #1002

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Oct 21, 2024
Merged

Send immediate ACKs after RMSS bytes of data #1002

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79 changes: 55 additions & 24 deletions src/socket/tcp.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2057,20 +2057,20 @@ impl<'a> Socket<'a> {

// Handle delayed acks
if let Some(ack_delay) = self.ack_delay {
if self.ack_to_transmit() || self.window_to_update() {
if self.ack_to_transmit() {
self.ack_delay_timer = match self.ack_delay_timer {
AckDelayTimer::Idle => {
tcp_trace!("starting delayed ack timer");

AckDelayTimer::Waiting(cx.now() + ack_delay)
}
// RFC1122 says "in a stream of full-sized segments there SHOULD be an ACK
// for at least every second segment".
// For now, we send an ACK every second received packet, full-sized or not.
AckDelayTimer::Waiting(_) => {
AckDelayTimer::Waiting(_) if self.immediate_ack_to_transmit() => {
tcp_trace!("delayed ack timer already started, forcing expiry");
AckDelayTimer::Immediate
}
timer @ AckDelayTimer::Waiting(_) => {
tcp_trace!("waiting until delayed ack timer expires");
timer
}
AckDelayTimer::Immediate => {
tcp_trace!("delayed ack timer already force-expired");
AckDelayTimer::Immediate
Expand Down Expand Up @@ -2183,6 +2183,24 @@ impl<'a> Socket<'a> {
}
}

/// Return whether to send ACK immediately due to the amount of unacknowledged data.
///
/// RFC 9293 states "An ACK SHOULD be generated for at least every second full-sized segment or
/// 2*RMSS bytes of new data (where RMSS is the MSS specified by the TCP endpoint receiving the
/// segments to be acknowledged, or the default value if not specified) (SHLD-19)."
///
/// Note that the RFC above only says "at least 2*RMSS bytes", which is not a hard requirement.
/// In practice, we follow the Linux kernel's empirical value of sending an ACK for every RMSS
/// byte of new data. For details, see
/// <https://elixir.bootlin.com/linux/v6.11.4/source/net/ipv4/tcp_input.c#L5747>.
fn immediate_ack_to_transmit(&self) -> bool {
if let Some(remote_last_ack) = self.remote_last_ack {
remote_last_ack + self.remote_mss < self.remote_seq_no + self.rx_buffer.len()
} else {
false
}
}

/// Return whether we should send ACK immediately due to significant window updates.
///
/// ACKs with significant window updates should be sent immediately to let the sender know that
Expand Down Expand Up @@ -7361,15 +7379,15 @@ mod test {
}

#[test]
fn test_delayed_ack_every_second_packet() {
let mut s = socket_established();
fn test_delayed_ack_every_rmss() {
let mut s = socket_established_with_buffer_sizes(DEFAULT_MSS * 2, DEFAULT_MSS * 2);
s.set_ack_delay(Some(ACK_DELAY_DEFAULT));
send!(
s,
TcpRepr {
seq_number: REMOTE_SEQ + 1,
ack_number: Some(LOCAL_SEQ + 1),
payload: &b"abc"[..],
payload: &[0; DEFAULT_MSS - 1],
..SEND_TEMPL
}
);
Expand All @@ -7380,35 +7398,48 @@ mod test {
send!(
s,
TcpRepr {
seq_number: REMOTE_SEQ + 1 + 3,
seq_number: REMOTE_SEQ + 1 + (DEFAULT_MSS - 1),
ack_number: Some(LOCAL_SEQ + 1),
payload: &b"def"[..],
payload: &b"a"[..],
..SEND_TEMPL
}
);

// Every 2nd packet, ACK is sent without delay.
// No ACK is immediately sent.
recv_nothing!(s);

send!(
s,
TcpRepr {
seq_number: REMOTE_SEQ + 1 + DEFAULT_MSS,
ack_number: Some(LOCAL_SEQ + 1),
payload: &b"a"[..],
..SEND_TEMPL
}
);

// RMSS+1 bytes of data has been received, so ACK is sent without delay.
recv!(
s,
Ok(TcpRepr {
seq_number: LOCAL_SEQ + 1,
ack_number: Some(REMOTE_SEQ + 1 + 6),
window_len: 58,
ack_number: Some(REMOTE_SEQ + 1 + (DEFAULT_MSS + 1)),
window_len: (DEFAULT_MSS - 1) as u16,
..RECV_TEMPL
})
);
}

#[test]
fn test_delayed_ack_three_packets() {
let mut s = socket_established();
fn test_delayed_ack_every_rmss_or_more() {
let mut s = socket_established_with_buffer_sizes(DEFAULT_MSS * 2, DEFAULT_MSS * 2);
s.set_ack_delay(Some(ACK_DELAY_DEFAULT));
send!(
s,
TcpRepr {
seq_number: REMOTE_SEQ + 1,
ack_number: Some(LOCAL_SEQ + 1),
payload: &b"abc"[..],
payload: &[0; DEFAULT_MSS],
..SEND_TEMPL
}
);
Expand All @@ -7419,30 +7450,30 @@ mod test {
send!(
s,
TcpRepr {
seq_number: REMOTE_SEQ + 1 + 3,
seq_number: REMOTE_SEQ + 1 + DEFAULT_MSS,
ack_number: Some(LOCAL_SEQ + 1),
payload: &b"def"[..],
payload: &b"a"[..],
..SEND_TEMPL
}
);

send!(
s,
TcpRepr {
seq_number: REMOTE_SEQ + 1 + 6,
seq_number: REMOTE_SEQ + 1 + (DEFAULT_MSS + 1),
ack_number: Some(LOCAL_SEQ + 1),
payload: &b"ghi"[..],
payload: &b"b"[..],
..SEND_TEMPL
}
);

// Every 2nd (or more) packet, ACK is sent without delay.
// RMSS+2 bytes of data has been received, so ACK is sent without delay.
recv!(
s,
Ok(TcpRepr {
seq_number: LOCAL_SEQ + 1,
ack_number: Some(REMOTE_SEQ + 1 + 9),
window_len: 55,
ack_number: Some(REMOTE_SEQ + 1 + (DEFAULT_MSS + 2)),
window_len: (DEFAULT_MSS - 2) as u16,
..RECV_TEMPL
})
);
Expand Down
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