webrtc: take the fork's loss recovery for sending without a congestion window

rustdesk-org/webrtc 825a0a48: without a congestion window a chunk is lost
once three chunks sent after its latest transmission are acked, counted in
send order so retransmitted chunks are covered too, and the fast retransmit
sends every lost chunk at once, as KCP nc=1 does; before, a lost
retransmission waited for T3-rtx. Also fixes the delayed SACK timer never
re-arming, the switch applying to established associations, T3-rtx
resending one chunk when the peer's window is full, and bounds new data to
1 MiB / 1024 chunks in flight like KCP's snd_wnd.

Simulated 35ms one-way, random loss both ways, 30 fps, frames later than
200ms out of 1200: 12 KB at 5% loss 996 -> 55 (KCP 61); 40 KB at 2% loss
1183 -> 20 (KCP 39).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019aokqJuhjvB3kijXtAg5Ns

Cargo.lock

@@ -9706,7 +9706,7 @@ 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=825a0a4862818f74406d8e1cc25be72259228235#825a0a4862818f74406d8e1cc25be72259228235"
 dependencies = [
  "arc-swap",
  "async-trait",
@@ -9746,7 +9746,7 @@ 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=825a0a4862818f74406d8e1cc25be72259228235#825a0a4862818f74406d8e1cc25be72259228235"
 dependencies = [
  "async-trait",
  "bitflags 1.3.2",

Cargo.toml

@@ -220,10 +220,11 @@ libxdo-sys = { path = "libs/libxdo-sys-stub" }
 # one comes out byte-swapped, fails to bind, and ICE gathers no IPv6 host candidate on Windows.
 # webrtc-sctp: RFC 4960's 1s RTO floor makes a single loss cost 1-3s on a link whose RTT is 24-64ms,
 # and fast retransmit cannot cover a request/response exchange; INITIAL_MTU 1228 also fragments on
-# IPv6. The fork commit carries the arithmetic.
+# IPv6; and its AIMD pins a lossy long-haul link to MSS/(RTT*sqrt(p)), so a switch sends without
+# a congestion window, as KCP does - on by default, `allow-webrtc-congestion-control` opts back in.
 # Pinned by rev, not branch: a fork branch can be rewritten out from under the lockfile.
-webrtc-util = { git = "https://github.com/rustdesk-org/webrtc", rev = "0a84ba37fbca940b82f230fb469d1b3a3172abf6" }
-webrtc-sctp = { git = "https://github.com/rustdesk-org/webrtc", rev = "0a84ba37fbca940b82f230fb469d1b3a3172abf6" }
+webrtc-util = { git = "https://github.com/rustdesk-org/webrtc", rev = "825a0a4862818f74406d8e1cc25be72259228235" }
+webrtc-sctp = { git = "https://github.com/rustdesk-org/webrtc", rev = "825a0a4862818f74406d8e1cc25be72259228235" }
 
 [package.metadata.winres]
 LegalCopyright = "Copyright © 2026 Purslane Tech Pte. Ltd. All rights reserved."