rtsp: Parse SSRC attributes in the SDP

When feeding input RTP packets to the depacketizer via custom IO,
it needs to pick the right stream using the payload type for
RTP packets, and using the SSRC for RTCP packets. If the first
packet is an RTCP packet, we don't (currently) know the SSRC
yet and thus can't pick the right RTP depacketizer to handle it.

By parsing the SSRC attribute in the SDP, we can map initial
RTCP packets to the right stream.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtsp.c

@@ -565,6 +565,10 @@ static void sdp_parse_line(AVFormatContext *s, SDPParseState *s1,
                 s1->seen_fmtp = 1;
                 av_strlcpy(s1->delayed_fmtp, buf, sizeof(s1->delayed_fmtp));
             }
+        } else if (av_strstart(p, "ssrc:", &p) && s->nb_streams > 0) {
+            rtsp_st = rt->rtsp_streams[rt->nb_rtsp_streams - 1];
+            get_word(buf1, sizeof(buf1), &p);
+            rtsp_st->ssrc = strtoll(buf1, NULL, 10);
         } else if (av_strstart(p, "range:", &p)) {
             int64_t start, end;
 
@@ -824,6 +828,8 @@ int ff_rtsp_open_transport_ctx(AVFormatContext *s, RTSPStream *rtsp_st)
     if (!rtsp_st->transport_priv) {
          return AVERROR(ENOMEM);
     } else if (CONFIG_RTPDEC && rt->transport == RTSP_TRANSPORT_RTP) {
+        RTPDemuxContext *rtpctx = rtsp_st->transport_priv;
+        rtpctx->ssrc = rtsp_st->ssrc;
         if (rtsp_st->dynamic_handler) {
             ff_rtp_parse_set_dynamic_protocol(rtsp_st->transport_priv,
                                               rtsp_st->dynamic_protocol_context,

libavformat/rtsp.h

@@ -457,6 +457,9 @@ typedef struct RTSPStream {
     /** Enable sending RTCP feedback messages according to RFC 4585 */
     int feedback;
 
+    /** SSRC for this stream, to allow identifying RTCP packets before the first RTP packet */
+    uint32_t ssrc;
+
     char crypto_suite[40];
     char crypto_params[100];
 } RTSPStream;