rtpenc: Start the sequence numbers from a random offset

Expose the current sequence number via an AVOption - this can
be used both for setting the initial sequence number, or for
querying the current number.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpenc.c

@@ -35,6 +35,7 @@ static const AVOption options[] = {
     { "payload_type", "Specify RTP payload type", offsetof(RTPMuxContext, payload_type), AV_OPT_TYPE_INT, {.i64 = -1 }, -1, 127, AV_OPT_FLAG_ENCODING_PARAM },
     { "ssrc", "Stream identifier", offsetof(RTPMuxContext, ssrc), AV_OPT_TYPE_INT, { .i64 = 0 }, INT_MIN, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM },
     { "cname", "CNAME to include in RTCP SR packets", offsetof(RTPMuxContext, cname), AV_OPT_TYPE_STRING, { .str = NULL }, 0, 0, AV_OPT_FLAG_ENCODING_PARAM },
+    { "seq", "Starting sequence number", offsetof(RTPMuxContext, seq), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 65535, AV_OPT_FLAG_ENCODING_PARAM },
     { NULL },
 };
 
@@ -124,6 +125,13 @@ static int rtp_write_header(AVFormatContext *s1)
         /* Round the NTP time to whole milliseconds. */
         s->first_rtcp_ntp_time = (s1->start_time_realtime / 1000) * 1000 +
                                  NTP_OFFSET_US;
+    // Pick a random sequence start number, but in the lower end of the
+    // available range, so that any wraparound doesn't happen immediately.
+    // (Immediate wraparound would be an issue for SRTP.)
+    if (s->seq < 0)
+        s->seq = av_get_random_seed() & 0x0fff;
+    else
+        s->seq &= 0xffff; // Use the given parameter, wrapped to the right interval
 
     if (s1->packet_size) {
         if (s1->pb->max_packet_size)
@@ -309,7 +317,7 @@ void ff_rtp_send_data(AVFormatContext *s1, const uint8_t *buf1, int len, int m)
     avio_write(s1->pb, buf1, len);
     avio_flush(s1->pb);
 
-    s->seq++;
+    s->seq = (s->seq + 1) & 0xffff;
     s->octet_count += len;
     s->packet_count++;
 }

libavformat/rtpenc.h

@@ -31,7 +31,7 @@ struct RTPMuxContext {
     int payload_type;
     uint32_t ssrc;
     const char *cname;
-    uint16_t seq;
+    int seq;
     uint32_t timestamp;
     uint32_t base_timestamp;
     uint32_t cur_timestamp;

libavformat/version.h

@@ -31,7 +31,7 @@
 
 #define LIBAVFORMAT_VERSION_MAJOR 54
 #define LIBAVFORMAT_VERSION_MINOR 21
-#define LIBAVFORMAT_VERSION_MICRO  1
+#define LIBAVFORMAT_VERSION_MICRO  2
 
 #define LIBAVFORMAT_VERSION_INT AV_VERSION_INT(LIBAVFORMAT_VERSION_MAJOR, \
                                                LIBAVFORMAT_VERSION_MINOR, \