sdp: Add an option for sending RTCP packets to the source of the last packets

An SDP description normally only contains the target IP address
and port for the packets. This means that we don't really have
any clue where to send the RTCP RR packets - previously they're
sent to the destination IP written in the SDP (at the same port),
which rarely is the actual peer. And if the source for the packets
is on a different port than the destination, it's never correct.

With a new option, we can choose to send the packets to the
address that the latest packet on each socket arrived from.
---
Some may even argue that this should be the default - perhaps,
but I'd rather keep it optional at first. Additionally, I'm not
sure if sending RTCP RR directly back to the source is
desireable for e.g. multicast.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtsp.c

@@ -96,6 +96,7 @@ const AVOption ff_rtsp_options[] = {
 static const AVOption sdp_options[] = {
     RTSP_FLAG_OPTS("sdp_flags", "SDP flags"),
     { "custom_io", "Use custom IO", 0, AV_OPT_TYPE_CONST, {.i64 = RTSP_FLAG_CUSTOM_IO}, 0, 0, DEC, "rtsp_flags" },
+    { "rtcp_to_source", "Send RTCP packets to the source address of received packets", 0, AV_OPT_TYPE_CONST, {.i64 = RTSP_FLAG_RTCP_TO_SOURCE}, 0, 0, DEC, "rtsp_flags" },
     RTSP_MEDIATYPE_OPTS("allowed_media_types", "Media types to accept from the server"),
     RTSP_REORDERING_OPTS(),
     { NULL },
@@ -2172,9 +2173,10 @@ static int sdp_read_header(AVFormatContext *s)
                         namebuf, sizeof(namebuf), NULL, 0, NI_NUMERICHOST);
             ff_url_join(url, sizeof(url), "rtp", NULL,
                         namebuf, rtsp_st->sdp_port,
-                        "?localport=%d&ttl=%d&connect=%d", rtsp_st->sdp_port,
-                        rtsp_st->sdp_ttl,
-                        rt->rtsp_flags & RTSP_FLAG_FILTER_SRC ? 1 : 0);
+                        "?localport=%d&ttl=%d&connect=%d&write_to_source=%d",
+                        rtsp_st->sdp_port, rtsp_st->sdp_ttl,
+                        rt->rtsp_flags & RTSP_FLAG_FILTER_SRC ? 1 : 0,
+                        rt->rtsp_flags & RTSP_FLAG_RTCP_TO_SOURCE ? 1 : 0);
 
             append_source_addrs(url, sizeof(url), "sources",
                                 rtsp_st->nb_include_source_addrs,

libavformat/rtsp.h

@@ -401,6 +401,8 @@ typedef struct RTSPState {
                                           source address and port. */
 #define RTSP_FLAG_LISTEN      0x2    /**< Wait for incoming connections. */
 #define RTSP_FLAG_CUSTOM_IO   0x4    /**< Do all IO via the AVIOContext. */
+#define RTSP_FLAG_RTCP_TO_SOURCE 0x8 /**< Send RTCP packets to the source
+                                          address of received packets. */
 
 typedef struct RTSPSource {
     char addr[128]; /**< Source-specific multicast include source IP address (from SDP content) */