Re: [rtcweb] SDP and ssrc-group,

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> 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> wrote:
>
>> 345259865 is "real"
>> 2693756249 is rtx
>>
>> On Tue, Oct 21, 2014 at 9:25 AM, Iñaki Baz Castillo <ibc@aliax.net>
>> wrote:
>>
>>> 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
>>> <ibc@aliax.net>
>>>
>>> _______________________________________________
>>> rtcweb mailing list
>>> rtcweb@ietf.org
>>> https://www.ietf.org/mailman/listinfo/rtcweb
>>>
>>
>>