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3fc1810a46
First step toward RFC 4588 RTX retransmission with SSRC multiplexing (#4671). This PR adds the config, the server-side groundwork, the tests and the test tools. RTX negotiation, the uplink unwrap and the downlink RTX send path come in a follow-up PR. ## Config New per-vhost `nack_prefer_rtx` directive, off by default. It can be overridden with `SRS_VHOST_RTC_NACK_PREFER_RTX`. ``` vhost __defaultVhost__ { rtc { nack_prefer_rtx on; } } ``` `rtc.rtx.conf` and `rtc.plain.conf` are sample configs for the two modes. `regression-test.conf` turns RTX on. ## Server changes - **NACK simulator fix:** simulated drops (and `drop_for_pt`) now happen before SRTP unprotect. Before, a dropped packet had already been unprotected, so its retransmission was rejected as a replay. - SDP writes `a=ssrc-group` lines. `SrsRtxPayloadDes` uses the offered clock rate and reads `apt` from the fmtp. - A play NACK that names an RTX SSRC is ignored. - RTX counters show up as cumulative totals in the `rtx=(s,r,u,p)` stats group. - Detailed NACK logs are compiled in only for `--simulator=on` or debug-drop builds. --------- Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>