[Live-devel] live555 media server and multiple RTSP joins

[Brody, Ben]

Hi,

We are experiencing an issue with a product on the project that I'm working on. It is an analogue video encoder that uses the live555 media server. When we make multiple RTSP joins to the device, each new RTSP join causes the video sessions already in progress to glitch.

I noticed one of the updates to live555 from 2016 that we think could be related:

2016.01.20:
- When a server calls "startStream()" to start a RTSP stream for a client, we now no longer
  make a slight adjustment to the RTP timestamp sequence (using the "presetNextTimestamp()" call)
  if there is already another ongoing stream using the same "RTPSink".  The effect of this is
  only minor, but it ensures that the addition of an addition 'destination' to an ongoing RTSP/RTP
  stream does not cause any change to the contents of the RTP/RTCP packets.
  (Thanks to Erik Montnemery for noting this issue.)

I believe that this update was implemented by making sure that the timestamp is only adjusted if "hasMultipleDestinations" is false.

>From RTPSink.cpp:

  if (!groupsockBeingUsed().hasMultipleDestinations()) {
    // Don't adjust the timestamp stream if we already have another destination ongoing
    fTimestampBase = tsNow;
    fNextTimestampHasBeenPreset = True;
  }

>From Groupsock.hh:

Boolean hasMultipleDestinations() const { return fDests != NULL && fDests->fNext != NULL; }

What I'm trying to figure out is whether hasMultipleDestinations will be true after the first RTSP join to the server. Clearly at that point fDests will not be NULL. However, I'm suspecting that fDests->fNext might be NULL until a second RTSP client joins the stream. Looking at the Groupsock OutputSocket constructor that seems to initialise fDests with a new destRecord that has the next parameter set to NULL. I think that fDests->fNext is only non-NULL after a new destination is added using addDestination.

Does the first RTSP join both construct a destination and add a destination?

If not, why does hasMultipleDestinations include "&& fDests->fNext != NULL"?

Thank you in advance and sorry for dredging up such an old change.

Regards, Ben.




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Ben Brody MEng CEng MIET MINCOSE
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