[SIPForum-discussion] RTP ssrc change during a call
Gunnar Hellström
gunnar.hellstrom at omnitor.se
Tue Aug 16 05:32:42 UTC 2011
Hi Raghul,
RFC 3550 RTP says that there are two cases when the SSRC can change:
1. If an SSRC conflict is detected ( someone else using the same SSRC )
2. If the source changes its transport address.
" If a source changes its source transport address, it must also choose
a new SSRC identifier to avoid being interpreted as a looped source (see
Section 8.2)."
Another place mentions:
"Since the SSRC identifier may change if a conflict is discovered or a
program is restarted"
Section A.1 contains a description of checks to do to decide if an RTP
packet is valid. Decisions on new appearing SSRC is included in that
description.
Summary: There are cases when the behaviour you have seen is valid. It
should also be indicated in RTCP.
Regards
Gunnar
-----------------------
Raghul Prasanna skrev 2011-08-12 10:36:
> Hello All,
>
> I have seen a call where the far end initially starts with a SSRC and
> then the next rtp from the same source has different ssrc field, I am
> looking for the wireshark I had so that I can share it with you guys,
> will post it if I find that.
>
> Is this acceptable? I remember the user saying one way audio or
> something like that.
>
> This is not a conference call, because I was wondering if the far end
> was a mixer and the rtp was actually coming from different source, but
> then realised that it will have CSRC if it was from a mixer to
> identify the actual source.
>
> Anyways is it OK to change SSRC by a source during a call, if it can
> what will be effect on the call?
>
> Thanks,
> Raghul
>
>
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