[rtcweb] SDP and ssrc-group,

[Iñaki Baz Castillo <[e-mail]>]

Hi,

May I know which SSRC (345259865 or 2693756249) will be used for the
real media stream (plus RED and FEC) and which SSRC will be used for
RTX?



--------------------------
m=video 62164 RTP/SAVPF 100 116 117 96
a=mid:video
a=rtpmap:100 VP8/90000
a=rtpmap:116 red/90000
a=rtpmap:117 ulpfec/90000
a=rtpmap:96 rtx/90000
a=fmtp:96 apt=100
a=ssrc-group:FID 345259865 2693756249
a=ssrc:345259865 cname:erS7E/KHLYKTejNs
a=ssrc:345259865 msid:DWpWct9bWKzTMNYZn5bKVgwZ8Mfy2EtfqBY5
c0134f05-e7c2-4afd-a979-4e224de5eb91
a=ssrc:345259865 mslabel:DWpWct9bWKzTMNYZn5bKVgwZ8Mfy2EtfqBY5
a=ssrc:345259865 label:c0134f05-e7c2-4afd-a979-4e224de5eb91
a=ssrc:2693756249 cname:erS7E/KHLYKTejNs
a=ssrc:2693756249 msid:DWpWct9bWKzTMNYZn5bKVgwZ8Mfy2EtfqBY5
c0134f05-e7c2-4afd-a979-4e224de5eb91
a=ssrc:2693756249 mslabel:DWpWct9bWKzTMNYZn5bKVgwZ8Mfy2EtfqBY5
a=ssrc:2693756249 label:c0134f05-e7c2-4afd-a979-4e224de5eb91
-------------------------------




RFC 5576 does not clarify it at all:

http://tools.ietf.org/html/rfc5576#section-4.2

--------------------------------------------------
4.2.  The "ssrc-group" Media Attribute

   a=ssrc-group:<semantics> <ssrc-id> ...

   [..]

   The <semantics> parameter is taken from the specification of the
   "group" attribute [RFC3388].  The initial semantic values defined for
   the "ssrc-group" attribute are FID (Flow Identification) [RFC3388]
   and FEC (Forward Error Correction) [RFC4756].  In each case, the
   relationship among the grouped sources is the same as the
   relationship among corresponding sources in media streams grouped
   using the SDP "group" attribute.
--------------------------------------------------



The referenced RFC 3388 neither clarifies it:

---------------------------------------------------
7.4 FID Semantics

   Several "m" lines grouped together using FID semantics form a media
   flow.  A media agent handling a media flow that comprises several "m"
   lines MUST send a copy of the media to every "m" line part of the
   flow as long as the codecs and the direction attribute present in a
   particular "m" line allow it.

   It is assumed that the application uses only one codec at a time to
   encode the media produced.  This codec MAY change dynamically during
   the session, but at any particular moment only one codec is in use.

   The application encodes the media using the current codec and checks
   one by one all the "m" lines that are part of the flow.  If a
   particular "m" line contains the codec being used and the direction
   attribute is "sendonly" or "sendrecv", a copy of the encoded media is
   sent to the address/port specified in that particular media stream.
   If either the "m" line does not contain the codec being used or the
   direction attribute is neither "sendonly" nor "sendrecv", nothing is
   sent over this media stream.
----------------------------------------------------




So, how is the usage of ssrc-group? Where is it really defined?

Can I put more than 2 ssrc together in the same ssrc-group line?

How should the receiver interpret it?

Does a ssrc-group always mean RTX usage? Where is that specified in
the above SDP?

Does not the above SDP look a complete mixture of hacks and workarounds?




-- 
Iñaki Baz Castillo
<[e-mail]>