Re: [rtcweb] SDP and ssrc-group,

Hi Peter,

This is the rtcweb list, so while I appreciate the info about how Chrome 
implements it, in regards to this discussion it is irrelevant. As far as 
I know there is no spec that allow us to know just by looking at the SDP 
which ssrc belongs to each stream. Please, correct me if I am wrong.

Best regards
Sergio
On 21/10/2014 19:57, Peter Thatcher wrote:
> I'm not sure where it's specified, but here's where it's implemented:
>
> https://code.google.com/p/webrtc/source/browse/trunk/talk/media/webrtc/webrtcvideoengine.cc#4108
>
> It only supports 2 SSRCs for a FID group.  It would ignore any more 
> than that.
>
> On Tue, Oct 21, 2014 at 10:40 AM, Iñaki Baz Castillo <ibc@aliax.net 
> <mailto:ibc@aliax.net>> wrote:
>
>     How is that? Where is that specified? What about if I include 3
>     ssrc values in the ssrc-group? What is each one for?
>
>     On 21 Oct 2014 19:31, "Peter Thatcher" <pthatcher@google.com
>     <mailto:pthatcher@google.com>> wrote:
>
>         345259865 is "real"
>         2693756249 <tel:2693756249> is rtx
>
>         On Tue, Oct 21, 2014 at 9:25 AM, Iñaki Baz Castillo
>         <ibc@aliax.net <mailto:ibc@aliax.net>> wrote:
>
>             Hi,
>
>             May I know which SSRC (345259865 or 2693756249
>             <tel: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 <tel: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 <tel:2693756249> cname:erS7E/KHLYKTejNs
>             a=ssrc:2693756249 <tel:2693756249>
>             msid:DWpWct9bWKzTMNYZn5bKVgwZ8Mfy2EtfqBY5
>             c0134f05-e7c2-4afd-a979-4e224de5eb91
>             a=ssrc:2693756249 <tel:2693756249>
>             mslabel:DWpWct9bWKzTMNYZn5bKVgwZ8Mfy2EtfqBY5
>             a=ssrc:2693756249 <tel: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
>             <ibc@aliax.net <mailto:ibc@aliax.net>>
>
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>
>
>
>
>
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