[AVT] Re: [Sip] RE: [Sip-implementors] media description in SDP is for sender or receiver?

[Magnus Westerlund <magnus.westerlund at ericsson.com>]

Hi,

(cc also AVT as this is an AVT WG matter)

It took AVT a long time until actually offer/answer sections started 
appearing in the RTP payload formats. It is basically only the last year 
or two that they have been included.

In general parameters applies for the stream the offerer/answer wants to 
receive. However config is a bit problematic. It provides information 
that concerns the bit-stream that the encoder will send out. Thus config 
applies from sender to receiver.

This is at least spelled out in RFC 3016:

"config: This parameter SHALL be used to indicate the configuration
      of the corresponding MPEG-4 Visual bitstream.  It SHALL NOT be
      used to indicate the codec capability in the capability exchange
      procedure."

So the interpretation I make is the following:
profile-level-id in an offer for a sendrecv stream tells the answerer 
the highest level and profile it may use to send media to the offerer.

The config string tells the answerer about the bit-stream it will 
receive from the offerer.

The issue with all parameters that relate to stream properties rather 
than capabilities or reception configurations that the receiver can 
determine for the sender is that they are based on assumptions about the 
other end-points capabilities. The offerer assumes that the answerer is 
at least as capable as the offerer and can handle the config string.

I think it is currently unspecified if the answerer may downgrade the 
received profile-level-id to a lower capability in its answer. I think 
that is reasonable, but it do generates some issues in regards to the 
config string. Because suddenly the config that the offerer provided may 
no longer be valid. The offerer would be forced to re-invite with a new 
offer that matches the answerer reception capabilities.

If one is interested in how terribly complicated this becomes should 
take a look at the H.264/MPEG AVC RTP payload format in RFC 3984.

RFC 3016 would benefit from an update to include the necessary 
offer/answer section that describe this. So volunteers are very welcome 
to start this work. Simply contact the authors and the AVT WG chairs.

As the above is my interpretation, if the interpretation made in 
implementations differ, then that fact should be commented on.

Cheers

Magnus Westerlund
AVT WG Chair

Ling, Fan wrote:
> Since the "config=" field in my example is specific configuration for
> MPEG4 simple profile video bitstream, its interpretation should "depends
> on the parameters", however, I can't find such interpretation in MPEG4
> RTP RFC3016.


--

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
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S-164 80 Stockholm, Sweden | mailto: magnus.westerlund at ericsson.com

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