[Bug 759770] New: problem with sequence number in RTP-Info-header when rtponviftimestamp plugin present

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=759770

            Bug ID: 759770
           Summary: problem with sequence number in RTP-Info-header when
                    rtponviftimestamp plugin present
    Classification: Platform
           Product: GStreamer
           Version: unspecified
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-rtsp-server
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: branko.subasic at axis.com
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

When the RTSP server creates the RTP-Info header it asks the payloaders for the
sequence number and RTP time stamps. However, when the rtponviftimestamp
element is present, after the actual payloader, these values will not be taken
from the first packet pushed by the payloader. This is because the payloaders
always store the info for the last payloaded packet, butrtponviftimestamp the
rtponviftimestamp element caches one buffer (it needs to do this to know
whether to set the e-bit in the extension header or not). This means that the
payloader will push (at least) two buffers while pre-rolling.

One way to fix this would be to make the payloader store the sequence number
and RTP time stamp for the "pre-roll packet", e.g. the first packet payloaded
when going to paused state. However, that would mean that the payloader would
have to behave differently in paused and playing states. In paused state it
would have to provide the sequence number and rtp timestamp for the "preroll
packet", but in playing state it would have to return the values for the last
payloaded packet.
Or the RTSP server would have to ask for them in different ways.

Another way would be to use an upstream custom query to ask for the sequence
number and RTP time stamp. When the rtponviftimestamp element is present it
would answer the query, when it's not the payloader would answer it.

I propese the latter approach. It requires changes in rtpbasepayload,
rtponviftimestamp, and gst-rtsp-server. But it is a generic solution which does
not require the payloaders to behave differently in different states.

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