[FFmpeg-devel] [PATCH v2] rtpdec: Fix RTP timestamp wraparound in Producer Reference Time

["Clément Péron" <peron.clem-at-gmail.com@ffmpeg.org>]

The rtp_set_prft() function incorrectly calculates the timestamp delta
when RTP timestamps wrap around the 32-bit boundary. The current code:

    delta_timestamp = (int64_t)timestamp - (int64_t)s->last_rtcp_timestamp;

treats both timestamps as large positive values, causing wraparound to
produce a large negative delta instead of the correct small positive delta.

For example, with a 90kHz video clock:
- last_rtcp_timestamp = 0xFFFFFF00 (near wraparound)
- timestamp = 0x00000100 (after wraparound)
- Current result: delta ≈ -4.3 billion ticks ≈ -47,721 seconds
- Expected result: delta ≈ +512 ticks ≈ +0.006 seconds

This causes prft->wallclock to jump backward by approximately:
- 90kHz video: ~47,721 seconds (~13.25 hours)
- 48kHz audio: ~89,478 seconds (~24.9 hours)
- 8kHz audio: ~536,871 seconds (~6.2 days)

Fix by casting the subtraction result to int32_t, which correctly
handles wraparound through modular arithmetic:

    delta_timestamp = (int32_t)(timestamp - s->last_rtcp_timestamp);

This ensures the delta is always in the range [-2^31, 2^31-1], making
wraparound produce the correct small positive values.

Fixes timing jumps in applications that rely on Producer Reference Time
for media synchronization.

Signed-off-by: Clément Péron <peron.clem@gmail.com>
---
 libavformat/rtpdec.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/libavformat/rtpdec.c b/libavformat/rtpdec.c
index 729bf83685..7664ab58bd 100644
--- a/libavformat/rtpdec.c
+++ b/libavformat/rtpdec.c
@@ -633,7 +633,8 @@ void ff_rtp_parse_set_crypto(RTPDemuxContext *s, const char *suite,
 }
 
 static int rtp_set_prft(RTPDemuxContext *s, AVPacket *pkt, uint32_t timestamp) {
-    int64_t rtcp_time, delta_timestamp, delta_time;
+    int64_t rtcp_time, delta_time;
+    int32_t delta_timestamp;
 
     AVProducerReferenceTime *prft =
         (AVProducerReferenceTime *) av_packet_new_side_data(
@@ -642,7 +643,7 @@ static int rtp_set_prft(RTPDemuxContext *s, AVPacket *pkt, uint32_t timestamp) {
         return AVERROR(ENOMEM);
 
     rtcp_time = ff_parse_ntp_time(s->last_rtcp_ntp_time) - NTP_OFFSET_US;
-    delta_timestamp = (int64_t)timestamp - (int64_t)s->last_rtcp_timestamp;
+    delta_timestamp = (int32_t)(timestamp - s->last_rtcp_timestamp);
     delta_time = av_rescale_q(delta_timestamp, s->st->time_base, AV_TIME_BASE_Q);
 
     prft->wallclock = rtcp_time + delta_time;
-- 
2.48.1

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