Re: [MMUSIC] ISSUE1: SDP Attributes Multiplexing - Payload Type Analysis

[Harald Alvestrand <[e-mail]>]

I agree with Bo's analysis concluding that IDENTICAL-PER-SSRC is likely 
not needed, and that situations where one has one SSRC in multiple media 
descriptions are probably a Bad Thing (I can't think of a case where it 
would represent anything but a bug).

IDENTICAL-PER-PT seems like a sensible thing to add.

On 06/23/2014 05:36 AM, Suhas Nandakumar wrote:
> Hello All
>
>    Continuing the discussions from London on analyzing multiplexing 
> behavior for SDP attributes that are defined per  Payload Type, I was 
> thinking on making following changes to the 
> draft-ietf-mmusic-sdp-mux-attributes-01.
>
> 1. Add new category called IDENTICAL-PER-PT to the SDP Attribute 
> analysis framework in Section 4 which is defined as
>
> IDENTICAL-PER-PT: Attributes that define the RTP payload configuration 
> on per Payload Type basis and MUST have identical values across all 
> the media descriptions for a given RTP Payload Type when repeated.
>
> 2. Similarly add another category IDENTICAL-PER-SSRC to accommodate 
> source level SDP attributes.
>
> IDENTICAL-PER-SSRC: Attributes that correspond to the individual RTP 
> Packet Streams in a given RTP Session and MUST have identical values 
> when repeated across multiple media description for a given SSRC.
>
> 3. For all the attributes that directly/indirectly impact codec 
> configuration and packetization of the media, I have attempted to 
> assign a SDP MUX category and provide some insights on how such a 
> decision was made (wherever applicable)
>
> *Attribute: a=ptime, a=maxptime*
> Mux Category: IDENTICAL-PER-PT
> Notes: For a given codec the audio packetization time must be same if 
> the PT is repeated. This will ensure that the RTP receiver 
> can unambiguously decode the incoming audio packets.
>
>
> *Attribute: a=framerate*
> Mux Category: IDENTICAL-PER-PT
>
> *Attribute: a=rtpmap*
> Mux Category: IDENTICAL-PER-PT
>
>
> *Attribute: a=fmtp*
> Mux Category: IDENTICAL-PER-PT
> Notes:
>
> *Attribute: a=rtcp-fb*
> Mux Category: NORMAL
> Notes: Since the feedback messages are reported per SSRC, the category 
> of NORMAL suffices.
>
> *Attribute: a=depend (RFC5583)*
> Mux Category: IDENTICAL-PER-PT
> Notes:
> Since each dependency identifies a unique RTP packetization  and thus 
> MUST be identified via unique Payload Type.
> Similarly if the Payload Type is repeated across media descriptions, 
> they MUST represent the Identical codec configuration.
>
> *Attribute: a=ssrc:xyz  fmtp:<fmt> ... *
> Mux Category: IDENTICAL-PER-PT
> Notes:
> RFC5576 says this :
> "
> Within a media stream, "ssrc" attributes with the same value of
> <ssrc-id> describe different attributes of the same media sources.
> Across media streams, <ssrc-id> values are not correlated (unless
> correlation is indicated by media-stream grouping or some other
> mechanism) and MAY be repeated.
> "
> Thus if the RTP Payload Type used to specify fmtp parameters  is repeated, it MUST representthe same codec configuration.
> example:
> m=video ...
> a=rtpmap:98 VP8/90000
> a=ssrc:12345 fmtp:98max-fr=15; max-fs-1200
> m=video ...
> a=rtpmap:98 VP8/90000
>
> a=ssrc:56789 fmtp:98 max-fr=15; max-fs-1200
> *Attribute: a=ssrc:xyz  cname: ....*
> Mux Category: IDENTICAL-PER-SSRC
> Notes: RFC5576 defines the identical requirement as it is.
>
> *
> *
> *Attribute: a=imageattr*
> Mux Category: IDENTICAL-PER-PT
>
>
> *Attribute: a=framesize*
> Mux Category: IDENTICAL-PER-PT
>
>
> *Attributes: a=fec-source-flow, a=fec-repair-flow, a=repair-window*
> Mux Category: NORMAL
>
>
> I believe these cover all PT scoped SDP attributes. Please let me know 
> your thoughts and comments.
>
> Cheers
> Suhas Nandakumar
>
>
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