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35c05b654a
Plug a KCP session in front of the VP8 wire so the upper layer (mux/curl tunnels) gets reliable, ordered, message-oriented delivery on top of an otherwise unreliable carrier. Why this fixes the random 0 B/s stalls -------------------------------------- The previous design had no retransmits and no ACKs. A single dropped VP8 frame (from drainOutbound on micro-reconnect, the inbound 'default:' overflow drop, or RTP loss invalidating a partial assembly) created a hole in mux.Stream.nextSeq that would never be filled: the receiver parked all subsequent frames in outOfOrder forever and curl read 0 B/s until a brand new mux stream id was opened. KCP layer details ----------------- * Stream mode + 4-byte big-endian length prefix. Message mode would be ideal but UDPSession.Write fragments anything > MSS *outside* of kcp.Send, leaving every fragment with frg=0 and breaking PeekSize-based reassembly. Length-prefix framing under stream mode is the canonical workaround. * nodelay(1, 10ms, fast-resend=2, no congestion control) — KCP turbo preset, identical to kcptun/shadowsocks tuning. Recovers from burst losses in tens of ms instead of seconds. * SndWnd/RcvWnd 4096 segments to absorb the multi-segment burst that a single VP8 sample can carry. * MTU 1400 (kcp-go hardcodes mtuLimit=1500). Wire compatibility with VP8 keepalives -------------------------------------- KCP packets always start with the LE conv id (0x01 0xEE 0xFF 0xC0). VP8 keepalive frames start with 0x30. The receive path filters by the magic byte before handing the buffer to KCP, so keepalives never poison the KCP state machine. Features() now advertises Reliable+Ordered. Drop-on-overflow paths in the carrier are kept — KCP detects the loss via SACK and retransmits. Refs: transport/vp8-kcp