[FFmpeg-devel] [PATCH] avformat/whip: add RTX initial support (PR #20544)

[Jack Lau via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

PR #20544 opened by Jack Lau (JackLau)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20544
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20544.patch

Refer to RFC 4588.

Add and set the basic param of RTX like
ssrc, payload_type, srtp.

Modify the SDP to add RTX info so that
the peer be able to parse the RTX packet.

There are more pateches to make RTX really
work.

Signed-off-by: Jack Lau <jacklau1222@qq.com>


>From 3c0be07aa6eb29d135401b14b58c1d1baee33b4f Mon Sep 17 00:00:00 2001
From: Jack Lau <jacklau1222@qq.com>
Date: Thu, 18 Sep 2025 09:23:31 +0800
Subject: [PATCH] avformat/whip: add RTX initial support

Refer to RFC 4588.

Add and set the basic param of RTX like
ssrc, payload_type, srtp.

Modify the SDP to add RTX info so that
the peer be able to parse the RTX packet.

There are more pateches to make RTX really
work.

Signed-off-by: Jack Lau <jacklau1222@qq.com>
---
 libavformat/whip.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/libavformat/whip.c b/libavformat/whip.c
index cfdc861c5b..472c6dbf16 100644
--- a/libavformat/whip.c
+++ b/libavformat/whip.c
@@ -122,6 +122,7 @@
 /* Referring to Chrome's definition of RTP payload types. */
 #define WHIP_RTP_PAYLOAD_TYPE_H264 106
 #define WHIP_RTP_PAYLOAD_TYPE_OPUS 111
+#define WHIP_RTP_PAYLOAD_TYPE_VIDEO_RTX 105
 
 /**
  * The STUN message header, which is 20 bytes long, comprises the
@@ -229,12 +230,14 @@ typedef struct WHIPContext {
     /* The SSRC of the audio and video stream, generated by the muxer. */
     uint32_t audio_ssrc;
     uint32_t video_ssrc;
+    uint32_t video_rtx_ssrc;
 
     uint16_t audio_first_seq;
     uint16_t video_first_seq;
     /* The PT(Payload Type) of stream, generated by the muxer. */
     uint8_t audio_payload_type;
     uint8_t video_payload_type;
+    uint8_t video_rtx_payload_type;
     /**
      * This is the SDP offer generated by the muxer based on the codec parameters,
      * DTLS, and ICE information.
@@ -290,6 +293,7 @@ typedef struct WHIPContext {
     /* The SRTP send context, to encrypt outgoing packets. */
     SRTPContext srtp_audio_send;
     SRTPContext srtp_video_send;
+    SRTPContext srtp_video_rtx_send;
     SRTPContext srtp_rtcp_send;
     /* The SRTP receive context, to decrypt incoming packets. */
     SRTPContext srtp_recv;
@@ -602,9 +606,11 @@ static int generate_sdp_offer(AVFormatContext *s)
 
     whip->audio_ssrc = av_lfg_get(&whip->rnd);
     whip->video_ssrc = whip->audio_ssrc + 1;
+    whip->video_rtx_ssrc = whip->video_ssrc + 1;
 
     whip->audio_payload_type = WHIP_RTP_PAYLOAD_TYPE_OPUS;
     whip->video_payload_type = WHIP_RTP_PAYLOAD_TYPE_H264;
+    whip->video_rtx_payload_type = WHIP_RTP_PAYLOAD_TYPE_VIDEO_RTX;
 
     av_bprintf(&bp, ""
         "v=0\r\n"
@@ -657,7 +663,7 @@ static int generate_sdp_offer(AVFormatContext *s)
         }
 
         av_bprintf(&bp, ""
-            "m=video 9 UDP/TLS/RTP/SAVPF %u\r\n"
+            "m=video 9 UDP/TLS/RTP/SAVPF %u %u\r\n"
             "c=IN IP4 0.0.0.0\r\n"
             "a=ice-ufrag:%s\r\n"
             "a=ice-pwd:%s\r\n"
@@ -671,9 +677,13 @@ static int generate_sdp_offer(AVFormatContext *s)
             "a=rtpmap:%u %s/90000\r\n"
             "a=fmtp:%u level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=%02x%02x%02x\r\n"
             "a=rtcp-fb%u nack\r\n"
+            "a=rtpmap:%u rtx/90000\r\n"
+            "a=fmtp:%u apt=%u\r\n"
+            "a=ssrc-group:FID %u %u\r\n"
             "a=ssrc:%u cname:FFmpeg\r\n"
             "a=ssrc:%u msid:FFmpeg video\r\n",
             whip->video_payload_type,
+            whip->video_rtx_payload_type,
             whip->ice_ufrag_local,
             whip->ice_pwd_local,
             whip->dtls_fingerprint,
@@ -684,6 +694,11 @@ static int generate_sdp_offer(AVFormatContext *s)
             profile_iop,
             level,
             whip->video_payload_type,
+            whip->video_rtx_payload_type,
+            whip->video_rtx_payload_type,
+            whip->video_payload_type,
+            whip->video_ssrc,
+            whip->video_rtx_ssrc,
             whip->video_ssrc,
             whip->video_ssrc);
     }
@@ -1408,6 +1423,12 @@ static int setup_srtp(AVFormatContext *s)
         goto end;
     }
 
+    ret = ff_srtp_set_crypto(&whip->srtp_video_rtx_send, suite, buf);
+    if (ret < 0) {
+        av_log(whip, AV_LOG_ERROR, "Failed to set crypto for video rtx send\n");
+        goto end;
+    }
+
     ret = ff_srtp_set_crypto(&whip->srtp_rtcp_send, suite, buf);
     if (ret < 0) {
         av_log(whip, AV_LOG_ERROR, "Failed to set crypto for rtcp send\n");
@@ -1924,6 +1945,7 @@ static av_cold void whip_deinit(AVFormatContext *s)
     av_freep(&whip->key_file);
     ff_srtp_free(&whip->srtp_audio_send);
     ff_srtp_free(&whip->srtp_video_send);
+    ff_srtp_free(&whip->srtp_video_rtx_send);
     ff_srtp_free(&whip->srtp_rtcp_send);
     ff_srtp_free(&whip->srtp_recv);
     ffurl_close(whip->dtls_uc);
-- 
2.49.1

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