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fix(proto): move PTO cap after max_ack_delay addition

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The cap was applied before max_ack_delay * backoff, so the final PTO
could exceed MAX_PTO_INTERVAL with high backoff. Also trim comments.
This commit is contained in:
dignifiedquire
2026-03-21 13:51:49 +01:00
parent f10c1f2774
commit 65099697e6
1 changed files with 3 additions and 16 deletions
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noq-proto/src/connection/mod.rs
+3 -16
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@@ -3387,16 +3387,6 @@ impl Connection {
let backoff = 2u32.pow(pto_count.min(MAX_BACKOFF_EXPONENT));
let mut duration = path.rtt.pto_base() * backoff;
// Cap PTO backoff post-handshake to prevent excessively long retransmission
// gaps after connectivity loss. Without this cap, a path that loses connectivity
// for a few seconds can have PTO backed off to 30s+, making recovery impossibly
// slow even after connectivity returns. Picoquic uses a similar cap of 2s
// (PICOQUIC_LARGE_RETRANSMIT_TIMER). Only applied post-handshake: during
// handshake, natural backoff is needed to avoid flooding on unreachable peers.
if !self.is_handshaking() {
duration = duration.min(MAX_PTO_INTERVAL);
}
if path_id == PathId::ZERO
&& path.in_flight.ack_eliciting == 0
&& !self.peer_completed_address_validation(PathId::ZERO)
@@ -3430,6 +3420,8 @@ impl Connection {
}
// Include max_ack_delay and backoff for ApplicationData.
duration += self.ack_frequency.max_ack_delay_for_pto() * backoff;
// Cap PTO post-handshake to prevent long retransmit gaps after connectivity loss.
duration = duration.min(MAX_PTO_INTERVAL);
}
let Some(last_ack_eliciting) = pns.time_of_last_ack_eliciting_packet else {
continue;
@@ -3547,7 +3539,6 @@ impl Connection {
self.reset_keep_alive(path_id, now);
self.reset_idle_timeout(now, space_id, path_id);
self.permit_idle_reset = true;
self.receiving_ecn |= ecn.is_some();
if let Some(x) = ecn {
let space = &mut self.spaces[space_id];
@@ -7279,11 +7270,7 @@ fn get_max_ack_delay(params: &TransportParameters) -> Duration {
// Prevents overflow and improves behavior in extreme circumstances
const MAX_BACKOFF_EXPONENT: u32 = 16;
/// Maximum PTO interval after backoff. Prevents retransmission gaps from growing
/// unbounded during connectivity loss. After a silent gap, the PTO may have backed
/// off exponentially (e.g. 30ms * 2^10 = 30s). Capping at 2s ensures prompt recovery
/// once connectivity returns — the next PTO probe fires within 2s, gets ACKed, and
/// normal operation resumes. Matches picoquic's PICOQUIC_LARGE_RETRANSMIT_TIMER.
// Cap on PTO after backoff, matching picoquic's PICOQUIC_LARGE_RETRANSMIT_TIMER.
const MAX_PTO_INTERVAL: Duration = Duration::from_secs(2);
/// Minimal remaining size to allow packet coalescing, excluding cryptographic tag