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

[Suhas Nandakumar <[e-mail]>]

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 represent the 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