transport: ACK Frequency extension — ACK_FREQUENCY (0xaf) + IMMEDIATE_ACK (0x1f)#242
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) Implements draft-ietf-quic-ack-frequency so peers (quinn / quiche / msquic all ship it) can tune our ACK cadence: - src/frames/ack_frequency.zig (new): ACK_FREQUENCY (0xaf) parse/ serialize — sequence number, ack-eliciting threshold, requested max ack delay (µs), reordering threshold — and IMMEDIATE_ACK (0x1f). 0xaf > 0x3f so the type is a 2-byte varint on the wire; both frame loops already varint-decode the type. - quic_tls.zig: advertise + parse the min_ack_delay transport parameter (0xff04de1b, MICROSECONDS). Advertised by default at 1000 µs (1 ms — our drive-tick ACK granularity; well under the 25 ms max_ack_delay we advertise). Set TransportParamsOpts.min_ack_delay_us = 0 to disable. - io.zig frame loops (server + client): 0x1f sets ack_immediate; 0xaf is sequence-gated (stale frames ignored per draft §4), malformed bodies → FRAME_ENCODING_ERROR, requested delay below our advertised min_ack_delay → PROTOCOL_VIOLATION. - ACK cadence: with no ACK_FREQUENCY received the flush path is UNCHANGED (every drive tick). Once armed, flushAllConnAppAcks holds a conn's ACK until the eliciting-packet threshold is crossed, the requested max ack delay expires, a reorder event beyond the reordering threshold occurs, or IMMEDIATE_ACK arrives. Ack-eliciting counting piggybacks on noteFrameReceived (anything but PADDING/ACK/ CONNECTION_CLOSE). The interop client keeps its eager per-batch flush — acking more often than requested is explicitly permitted by the draft. - PTO probes (both sides) ride an IMMEDIATE_ACK alongside the PING when the peer advertised min_ack_delay, so probes are answered without waiting out the peer's (possibly relaxed) delayed-ack timer. - Peer TP validation: min_ack_delay > max_ack_delay is treated defensively as not-advertised (extension disabled for that peer) rather than tearing down the handshake from the void apply path. Tests: frame codec round-trip + truncation, TP round-trip + omit-when- zero, ConnState apply/stale/violation gating, threshold + delay-timer + IMMEDIATE_ACK flush gating, reorder trigger incl. threshold=0 disable. 296/297 pass (1 pre-existing skip). Local interop smoke: handshake + transfer green with both endpoints advertising the new TP.
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Closes #187.
Summary
Implements draft-ietf-quic-ack-frequency: the
min_ack_delaytransport parameter (0xff04de1b), the ACK_FREQUENCY frame (0xaf), and the IMMEDIATE_ACK frame (0x1f). quinn / quiche / msquic all ship this; until now those peers saw zquic as extension-less and fell back to the RFC 9000 baseline.What's in
src/frames/ack_frequency.zig(new): frame codec + unit tests. Note 0xaf > 0x3f → 2-byte varint type on the wire (0x40 0xaf); both 1-RTT frame loops already varint-decode types so dispatch is clean.max_ack_delaywe advertise).TransportParamsOpts.min_ack_delay_us = 0disables. Peer's value is parsed and stored; a peer advertisingmin_ack_delay > max_ack_delayis defensively treated as not-advertised rather than tearing down the handshake from inside the void TP-apply path (draft says TRANSPORT_PARAMETER_ERROR — noted inline).IMMEDIATE_ACK→ flush the pending app-ACK now.ACK_FREQUENCY→ sequence-gated apply (stale/duplicate ignored per draft §4); malformed body → FRAME_ENCODING_ERROR close; requested max ack delay below our advertisedmin_ack_delay→ PROTOCOL_VIOLATION close.flushAllConnAppAcksholds a conn's ACK until the ack-eliciting threshold is crossed, the requested delay expires, a reorder beyond the reordering threshold occurs, or IMMEDIATE_ACK arrives. The interop client keeps its eager per-batch flush (acking more often than requested is explicitly permitted by the draft).Not in (deliberate)
Sending ACK_FREQUENCY to tune the peer's cadence (an embedder-facing API). Receive-side honor is the interop-relevant half; the send half can follow when an embedder wants it.
Tests
handshake+transfergreen with both endpoints advertising the new TP.