Re: [rtcweb] SDP and ssrc-group,

 From a formal perspective, I don't feel it is correct to describe the 
semantical meaning of an ssrc by referring to a payload.

Best regards
Sergio
On 29/10/2014 8:48, Roni Even wrote:
> Hi,
> We discussed this mapping issue in a previous version of the application token draft http://tools.ietf.org/html/draft-even-mmusic-application-token-02 example in section 1.
> We thought of having a mapping from appId to "pt" (section 3.3.1)
> Roni
>
>> -----Original Message-----
>> From: rtcweb [mailto:rtcweb-bounces@ietf.org] On Behalf Of I?aki Baz Castillo
>> Sent: 21 October, 2014 7:26 PM
>> To: rtcweb@ietf.org
>> Subject: [rtcweb] SDP and ssrc-group,
>>
>> 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
> _______________________________________________
> rtcweb mailing list
> rtcweb@ietf.org
> https://www.ietf.org/mailman/listinfo/rtcweb