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webrtc: send over SCTP without a congestion window, as KCP does

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The same link that streams over KCP crawls over WebRTC. webrtc-sctp runs
RFC 4960's AIMD: a fast retransmit halves cwnd, a T3 drops it to one MTU, and
slow start only rebuilds it while data is queued behind it. Where the loss is
random rather than congestion - a lossy long-haul link - the rate settles at
the Mathis ceiling MSS/(RTT*sqrt(p)) however idle the link is: about 1.3 Mbps
at 70ms RTT and 1% loss, 0.6 Mbps at 5%, while 1080p wants 2-5 Mbps. KCP's
turbo profile (nc=1) has no congestion window at all.

The fork now carries a switch that bypasses the two places gating sends on
cwnd, and hbb_common turns it on for every peer connection unless
`allow-webrtc-congestion-control` is set - the same opt-in KCP has in
`allow-kcp-congestion-control`, for the reason at `get_kcp_cc_enabled`.
Sender-side only; a browser or an older build on the other end interoperates.

Measured over a simulated link (35ms one-way, random loss both ways, 12 KB
frames at 30fps, 300 frames): at 1% loss the window stretches 9.9s of video to
20.7s with a mean latency of 5.5s; without it the stream stays realtime at a
mean of 113ms. At 3%: 47s and 15s against 10.2s and 290ms.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019aokqJuhjvB3kijXtAg5Ns
This commit is contained in:
rustdesk
2026-09-02 11:26:07 +08:00
parent c2fd869eeb
commit 72c445aeee
3 changed files with 7 additions and 6 deletions
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Cargo.lock generated
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@@ -9707,7 +9707,7 @@ dependencies = [
[[package]]
name = "webrtc-sctp"
version = "0.12.0"
source = "git+https://github.com/rustdesk-org/webrtc?rev=0a84ba37fbca940b82f230fb469d1b3a3172abf6#0a84ba37fbca940b82f230fb469d1b3a3172abf6"
source = "git+https://github.com/rustdesk-org/webrtc?rev=65fbe090326bf3abe8480d159b6bb07b5f509c22#65fbe090326bf3abe8480d159b6bb07b5f509c22"
dependencies = [
"arc-swap",
"async-trait",
@@ -9747,7 +9747,7 @@ dependencies = [
[[package]]
name = "webrtc-util"
version = "0.11.0"
source = "git+https://github.com/rustdesk-org/webrtc?rev=0a84ba37fbca940b82f230fb469d1b3a3172abf6#0a84ba37fbca940b82f230fb469d1b3a3172abf6"
source = "git+https://github.com/rustdesk-org/webrtc?rev=65fbe090326bf3abe8480d159b6bb07b5f509c22#65fbe090326bf3abe8480d159b6bb07b5f509c22"
dependencies = [
"async-trait",
"bitflags 1.3.2",