Subject: [AVT] Questions on RFC 3984

From: "Wu, Duanpei" <[email-masked]>
Date: Fri, 8 Apr 2005 16:34:30 -0700

Hi,

We tried to follow the Rfc 3984 protocol but got problems with the
symmetric parameters in capability negotiation using SIP/SDP. It would
be appreciated if anyone can provide the helps. 

As described in Section 8.2.2 (page 52) (Usage with the SDP Offer/Answer
Model), 

--- Quoted from 8.2.2 in Rfc 3984 ---

When H.264 is offered over RTP using SDP in an Offer/Answer model [7]
   for negotiation for unicast usage, the following limitations and
   rules apply:

   o  The parameters identifying a media format configuration for H.264
      are "profile-level-id", "packetization-mode", and, if required by
      "packetization-mode", "sprop-deint-buf-req".  These three
      parameters MUST be used symmetrically; i.e., the answerer MUST
      either maintain all configuration parameters or remove the media
      format (payload type) completely, if one or more of the parameter
      values are not supported

---- end of Quote ---

If one endpoint has the capability supporting the baseline profile and
level 3.0 (42801E) and offers with 

m=video 49170 RTP/AVP 97
a=rtpmap:97 H264/90000
a=fmtp:97 profile-level-id=42801E;

It does not means or imply that the endpoint will support level 2.1, 1.0
etc. To support the different level, one has to include it in the
offers, such as 4280I5 for level 2.1. If my understanding is correct,
the description seems making it different from the H.264 recommendation
on purpose as quoted here,

---------- Quoted from A.2.1 in H.264 ---------
Decoders conforming to the Baseline profile at a specific level shall be
capable of decoding all bitstreams in which profile_idc is equal to 66
or constraint_set0_flag is equal to 1 and in which level_idc represents
a level less than or equal to the specified level.

----------------- End of Quote
------------------------------------------

I have also gotten confused about the phrases in Section 8.2.2 (page 56)
with "....

 * Parameters declaring a configuration point are not downgradable,
   with the exception of the level part of the "profile-level-id"
   parameter.  This expresses values a receiver expects to be used
   and must be used verbatim on the sender side.

..."

It likely means that the level does not need exact matching, that
contradicts with the rules as I have quoted above from page 52 in
RFC3984.

Or I did not understand them correctly in some points.

Thank you for your time.

Duanpei Wu
Technical Leader
Cisco Systems, Inc
USA

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