fix: repair lost video packets with retransmissions - #98
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October 4, 2026 21:06
A lost video packet turned into a keyframe recovery and a visible freeze, while the official client repairs the same losses unseen: in a session on the same route it requested 133 packets and received all of them. OpenNOW requested packets only once their gap had already been given up, and the video replay window was RFC 3711's 64 packets, 7.5 ms at ~8,500 packets/s, less than one round trip, so a resent packet was rejected as a replay anyway. - Missing packets are requested while their gap is still open, on the official client's logged schedule: 1 ms after the loss, then up to 3 retries 4 ms apart (NvstNackTracker). - A gap whose first packet was requested waits up to the official 52 ms for it instead of becoming loss once the reorder window passes it. - The video replay window holds 2,048 packets, the official NACK queue length. Audio keeps 64. - The counters line adds replayed, late and duplicate drops and the packets a retransmission repaired.
The seat ignored the RTCP NACKs: 173 requests for 564 packets repaired 4. The official client announces rtpNackVersion 2 and, for that version, sends control command 0x317 (NvscClientPipeline:: createAndSendNackRequest -> ServerControl::sendRtpNackRequest) rather than an RTCP NACK. NvstRtpNackRequest builds its payload as RtpSourceQueueExtV2::createNackRequest does: version, stream index and entry count, then per entry a little-endian u16 sequence number and a u64 mask of the following 64 packets. Requests go out on the partially reliable control stream through the video pipeline's bundle; the RTCP NACK stays as the fallback before the bundle is up. Receiver loss tests run on a fixed clock, so a slow test machine no longer turns their gaps into retransmission waits.
A retry waited 4 ms against a ~15 ms round trip, so a lost packet was requested up to four times before the first answer could arrive; in a two-second loss burst ~720 resent copies were dropped as duplicates. With useRtdForRtpNackToggle the official client waits the round trip plus 4 ms, and the receiver now does the same, from the control connection's measured round trip. The counters line adds nackRetries.
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Included review availability: This review used your included allowance. Your plan provides up to 1 included review per hour; 0 remain after this review. 📝 WalkthroughWalkthroughThe receiver tracks missing video packets and requests retransmissions. Requests can use the control channel or the existing SRTCP NACK path. The SRTP replay window is configurable and is set to 2,048 packets for video reception. ChangesNVST Packet Retransmission
Priority: ➖ Normal Estimated code review effort: 4 (Complex) | ~45 minutes Change: Bug fix Sequence Diagram(s)sequenceDiagram
participant NvstVideoReceiver
participant NvstMjolnirReceiver
participant NvstVideoPipeline
participant PartiallyReliableControlChannel
NvstVideoReceiver->>NvstMjolnirReceiver: Emit retransmission request event
NvstMjolnirReceiver->>NvstVideoPipeline: Forward requested sequence numbers
NvstVideoPipeline->>PartiallyReliableControlChannel: Send RTP NACK command
Suggested reviewers: Merge Risk: ⚪ Minimal · up to No actionable regression was established in the retransmission path. The PR is mergeable after normal checks. Security Architecture ReviewSecurity architecture risk: 🔵 Low · up to The recovery design preserves packet authentication, duplicate rejection, stream isolation, and bounded retries and waits. No material security regression was established in the inspected paths. Some uncertainty remains around remote-server handling and delivery guarantees for the new retransmission command. Retained concerns Security review detailsSecurity Blast Radius
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Review comments at @GFN/NVST/BifrostFree/NvstVideoReceiver.swift:
- Around line 714-725: Update requestMissingPackets so nackTracker.due only
receives indices the transport will actually send, using the supported request
limit; alternatively, update NvstMjolnirReceiver.handle to send all due indices
in chunks before they are recorded as requested. Apply the behavior to both the
control-channel and SRTCP fallback paths.
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GFN/NVST/BifrostFree/NvstMjolnirReceiver.swiftGFN/NVST/BifrostFree/NvstNackTracker.swiftGFN/NVST/BifrostFree/NvstRtpNackRequest.swiftGFN/NVST/BifrostFree/NvstVideoReceiver.swiftGFN/NVST/BifrostFree/SrtpCryptography.swiftOPN/Stream/NvstBifrostFreeTransport.swiftOPN/Stream/NvstBifrostFreeVideo.swiftOPN/Stream/NvstVideoPipeline.swiftTests/GFN/NVST/NvstMjolnirFeedbackTests.swiftTests/GFN/NVST/NvstMjolnirReceiverTests.swiftTests/GFN/NVST/NvstNackTrackerTests.swiftTests/GFN/NVST/NvstRtpNackRequestTests.swiftTests/GFN/NVST/SrtpCryptographyTests.swift
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A scan could collect up to 136 missing packets and the tracker counted all of them as requested, but a 0x317 request names at most 64. The rest were never asked for, and their gap still waited up to 52 ms for them. The scan now stops at 64. A request the control channel could not take, before the bundle is up, goes out as the RTCP NACK instead of being counted as sent.
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What this fixes
When a few video packets are lost on the way, OpenNOW freezes for a moment and asks for a new keyframe. The official client loses the same packets on the same route but asks the server to resend them, gets them back in time, and nothing shows. With this change OpenNOW repairs lost packets the same way:
0x317(rtpNackVersion2), not an RTCP NACK.Why
OpenNOW already sent an RTCP NACK, but only at the moment it gave up on a gap and asked for a keyframe, so a resend could never arrive in time. Three more things blocked it:
rtpNackVersion: 2, and for that version it sends control command0x317instead (NvscClientPipeline::createAndSendNackRequest→ServerControl::sendRtpNackRequest).How we got here
RTP NACK Stats: numNackedPacketsRequested=133, numNackedPacketsReceived=133, numNackedPacketsUtilized=124. It lost packets too, and repaired all of them. Its NACK settings: queue 2048, initial delay 1 ms, backoff 4 ms, 3 retries, 52 ms wait,useRtdForRtpNackToggle: 1, "extra wait time before sending NACK retry: 4 ms".libBifrost2:RtpSourceQueueExtV2::createNackRequestbuilds version 2 requests as a u8 version, a u8 stream index and a u8 entry count, then per entry a little-endian u16 sequence number and a little-endian u64 mask of the following 64 packets, at most 64 sequence numbers per request.ServerControl::sendRtpNackRequestsends that as command0x317. With it, the server answered.useRtdForRtpNackToggle, removed them.Results (live, Release build, same route)
With the round-trip retry, the last three runs sent 0 retries and received 0 duplicate packets. Decode time, decoded-frame-to-screen time (~5–7 ms) and the round trip (~15–21 ms) are unchanged; the only added delay is the wait for a resend while a packet is missing, about one round trip, instead of a keyframe recovery.
Test setup
What changed, for reviewers
NvstNackTracker(new): which missing packets are due a request, and when.NvstVideoReceiver: while a gap is open it emits.retransmissionWanted(scanned at most once per millisecond, and at once when a new gap opens); a gap whose first packet was requested waits up to 52 ms instead of ending on the reorder window; the replay window isSrtpReplayWindow(size: 2048). New counters: replayed, repaired and retries.SrtpReplayWindow: configurable size, stored as a ring bitmap. The default stays 64, and audio keeps it.NvstRtpNackRequest(new): the0x317payload. It is sent throughNvstVideoPipeline.requestRetransmissionon the partially reliable control stream; the RTCP NACK stays as the fallback before the bundle is up.replayed=,late=,dup=,nackRepaired=andnackRetries=.Testing
NvstNackTrackerTests(schedule, retries, round-trip retry),NvstRtpNackRequestTests(byte layout, sequence wrap, 64-packet limit), three receiver tests (a resend arriving 77 packets later repairs the gap with no recovery; a resend that never comes ends the wait; a 100-packet gap requests only the first 64), and the wide replay window. The receiver loss tests now run on a fixed clock, so a slow machine cannot turn their gaps into resend waits.swift test --scratch-path .build/sharedon the stream, NVST and settings suites (786 tests; 647 NVST tests after the review fix) and strict SwiftLint with the baseline: clean. Xcode Release build ofmainwith this PR and the companion prefilter PR combined: 0 warnings.Notes
0x20a) still report zeros. Reporting the real counts is a separate change.Summary by CodeRabbit