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

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

Hi,

On Sun, 8 Jun 2025 at 01:33, Michael Niedermayer <michael@niedermayer.cc> wrote:
>
> 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

Thank you

I found also another place where this rollover could be problematic

diff --git a/libavformat/rtpdec.c b/libavformat/rtpdec.c
index 7664ab58bd..aa1ea46469 100644
--- a/libavformat/rtpdec.c
+++ b/libavformat/rtpdec.c
@@ -670,10 +670,10 @@ static void finalize_packet(RTPDemuxContext *s,
AVPacket *pkt, uint32_t timestam

     if (s->last_rtcp_ntp_time != AV_NOPTS_VALUE && s->ic->nb_streams > 1) {
         int64_t addend;
-        int delta_timestamp;
+        int32_t delta_timestamp;

         /* compute pts from timestamp with received ntp_time */
-        delta_timestamp = timestamp - s->last_rtcp_timestamp;
+        delta_timestamp = (int32_t)(timestamp - s->last_rtcp_timestamp);
         /* convert to the PTS timebase */
         addend = av_rescale(s->last_rtcp_ntp_time - s->first_rtcp_ntp_time,
                             s->st->time_base.den,

But to be honest I don't have time to properly test it, but I could
send a patch with an RFT tag if you want.

Regards,


>
> thx
>
> [...]
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> Asymptotically faster algorithms should always be preferred if you have
> asymptotical amounts of data
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