Re: [rtcweb] SDP and ssrc-group,

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>
>
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