Re: [rtcweb] SDP and ssrc-group,

I hope I can rule my SDP logic based on standards rather than on how Chrome
implements some features.

My question was generic: if I receive the above SDP, how do I know which
payloads each ssrc is supposed to transport?
On 21 Oct 2014 19:57, "Peter Thatcher" <pthatcher@google.com> 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>
> 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
>>>>
>>>
>>>
>