Re: [rtcweb] SDP and ssrc-group,

Looking at the SDP, the Payload Type seems like the clearest way to differentiate.  As it is, SSRC does not appear linked to the potential payloads.

> On 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
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