DataChannel: add bufferedAmount back-pressure surface (W3C §6.2)#45
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DataChannel had no flow-control surface: send()/sendBinary() pushed to SCTP with no visibility into how much was still in flight, so callers couldn't back-pressure large transfers. - SCTP: tally acked application bytes per stream when a SACK clears the retransmit queue (excluding DCEP control + the empty-message padding byte) and report them via a new onBytesAcked callback. - DataChannel: bufferedAmount (un-acked app bytes; += on send, -= on ack), bufferedAmountLowThreshold, and onBufferedAmountLow (fires on the downward threshold crossing). PeerConnection routes onBytesAcked to the owning channel. This gives app-level back-pressure: a caller that pauses while bufferedAmount is high and resumes on onBufferedAmountLow is paced by the peer's SACK rate. (Internal SCTP send flow control — holding chunks when the remote a_rwnd is exhausted — and binaryType are left as BACKLOG follow-ups; binaryType has no Dart analog since messages are always Uint8List.) Tested: SCTP onBytesAcked accounting (per-stream, empty=0) via two state machines; and the full bufferedAmount lifecycle against real Chrome in Scenario 1 (rose to 66583, drained to 0, onBufferedAmountLow fired). The stats loopback harness was extracted to test/peer_connection/loopback.dart for reuse. A separate finding — webdartc↔webdartc data channels never finish the DCEP open — is recorded in BACKLOG (it blocked a pure-loopback DC test). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Addresses the DataChannel flow-control gap from the RFC/W3C divergence audit.
Problem
DataChannelhad no flow-control surface:send()/sendBinary()pushed straight to SCTP with no visibility into how much was still in flight, so callers couldn't back-pressure large transfers.Change
onBytesAcked(streamId, bytes)callback — same streamId fan-out idiom asonData/onStreamReset.bufferedAmount(un-acked app bytes;+=on send,-=on ack),bufferedAmountLowThreshold, andonBufferedAmountLow(fires on the downward threshold crossing).PeerConnectionroutesonBytesAckedto the owning channel.This gives app-level back-pressure: a caller that pauses while
bufferedAmountis high and resumes ononBufferedAmountLowis paced by the peer's SACK rate.Scope / follow-ups (BACKLOG)
bufferedAmountis un-acked (sent-but-not-acked) rather than the W3C "queued-but-not-yet-sent", becausesendDatatransmits immediately — documented on the API and in BACKLOG.a_rwndis exhausted) andbinaryType(no Dart analog — messages are alwaysUint8List) are left as BACKLOG items.onOpennever fires in pure loopback; DC e2e tests all use a browser). Recorded in BACKLOG — it's why the integration test runs against Chrome rather than loopback.Tests
onBytesAcked(buffered_amount_test.dart): per-stream tally, empty-message = 0, two state machines.bufferedAmount rose to 66583, drained to 0, onBufferedAmountLow fired.dart analyzeclean; 664 unit + 22 e2e pass. Reviewed with/simplify.🤖 Generated with Claude Code