Subject: [AVTCORE] SSRC in FIR CCM message

From: Olivier Crête <[email-masked]>
Date: Tue, 30 Aug 2011 17:00:20 -0400

Hi,

Can the SSRC in the FIR message defined in RFC 5104 be different from
the Media SSRC in the rtcp fb header that's just before it? I'm curious
as to why it's duplicated in there? Or did I miss something ?

-- 
Olivier Crête
[email-masked]

---

From: "Rachel Huang(yihong)" <[email-masked]>
Date: Wed, 31 Aug 2011 06:19:34 +0000

Hi, Olivier,

I guess RFC 5104 has described the difference and usage of the two SSRCs. Check it in 4.3.1.2 of RFC5104, the first paragraph.

Best Regards!
Rachel

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of Olivier Crête
Sent: Wednesday, August 31, 2011 5:00 AM
To: [email-masked]
Subject: [AVTCORE] SSRC in FIR CCM message

Hi,

Can the SSRC in the FIR message defined in RFC 5104 be different from the Media SSRC in the rtcp fb header that's just before it? I'm curious as to why it's duplicated in there? Or did I miss something ?

---

From: Magnus Westerlund <[email-masked]>
Date: Wed, 31 Aug 2011 14:04:36 +0200

> I guess RFC 5104 has described the difference and usage of the two SSRCs. Check it in 4.3.1.2 of RFC5104, the first paragraph.

Rachel is correct, and to quote the first paragraph of that section:

4.3.1.2.  Semantics

   Within the common packet header for feedback messages (as defined in
   section 6.1 of [RFC4585]), the "SSRC of packet sender" field
   indicates the source of the request, and the "SSRC of media source"
   is not used and SHALL be set to 0.  The SSRCs of the media senders to
   which the FIR command applies are in the corresponding FCI entries.
   A FIR message MAY contain requests to multiple media senders, using
   one FCI entry per target media sender.

The last sentence reveals why we selected this design. This was to
enable to include multiple SSRCs in one FIR request message. Otherwise
one would need to duplicate the feedback header for each SSRC to request
on.

Cheers

Magnus Westerlund

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