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libavformat/rtpdec: Fix RTP timestamp wraparound in Producer Reference Time

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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>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
This commit is contained in:
Clément Péron
2025-06-07 10:06:19 +02:00
committed by Michael Niedermayer
parent 869e288b3a
commit 8a9cbf99a5
1 changed files with 3 additions and 2 deletions
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5
libavformat/rtpdec.c
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@ -587,7 +587,8 @@ void ff_rtp_parse_set_crypto(RTPDemuxContext *s, const char *suite,
} }
static int rtp_set_prft(RTPDemuxContext *s, AVPacket *pkt, uint32_t timestamp) { 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 *prft =
(AVProducerReferenceTime *) av_packet_new_side_data( (AVProducerReferenceTime *) av_packet_new_side_data(
@ -596,7 +597,7 @@ static int rtp_set_prft(RTPDemuxContext *s, AVPacket *pkt, uint32_t timestamp) {
return AVERROR(ENOMEM); return AVERROR(ENOMEM);
rtcp_time = ff_parse_ntp_time(s->last_rtcp_ntp_time) - NTP_OFFSET_US; 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); delta_time = av_rescale_q(delta_timestamp, s->st->time_base, AV_TIME_BASE_Q);
prft->wallclock = rtcp_time + delta_time; prft->wallclock = rtcp_time + delta_time;