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

[Michael Niedermayer <michael@niedermayer.cc>]

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Hi

On Sat, Jun 07, 2025 at 10:06:19AM +0200, Clément Péron wrote:
> 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;

will apply

thx

[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Asymptotically faster algorithms should always be preferred if you have
asymptotical amounts of data

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