[Sip] Re: [MMUSIC] RE: [Sip-implementors] supporting just one codec at a time

[Flemming Andreasen <[e-mail]>]

Jerry Yin wrote:

> Hi there,
>
> After I read the simcap
> (http://www.ietf.org/internet-drafts/draft-andreasen-mmusic-sdp-simcap-04.tx
> t), I found that it is not backwards compatible with the SDP (RFC 2327).
> Here's my comments.
>
> 1. the rtpmap's format in RFC 2327 is
>
>     a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding
>      parameters>].
> While in simcap, it is :
>     a=rtpmap:<payload type> telephone-event

No. There's simply an example which has made an editorial mistake in omitting
the <clock rate>.

>
>
>     Where is the "telephone-event" defined as an encoding name?

RFC 2833.

> If the UA is
> implemented according to RFC 2327, how can it talk to the UAs implemented
> according to the simcap?
>

Per RFC 2327, unknown attributes are simply ignored and simcap just defines a
couple of extra attributes that can be ignored at will.


>
> 2. The "m=" header and "a=rtpmap" in RFC 2327 is good enough to describe the
> media attributes already. I don't know why it is necessary to define a new
> attributes of "a=cdsc" and "a=cpar". There are some redundant information
> here, I think.
>

SDP has well-known shortcomings when it comes to capability description and
negotiation. See e.g.
http://www.ietf.org/internet-drafts/draft-ietf-mmusic-sdpng-req-01.txt for an
analysis of these.



>
> 3. If we want to describe a subset of capabilities among all capabilities
> the SDP claimed, we can simply introduce one new attribute to describe the
> subset.

> For example,
>    a=subset subset_codec_number : total_codec_number.
> This way, even some UAs don't understand a = subset, they still can talk
> with the one supports this attribute by at list one media streaming.
>

They can, but not with the semantics you intended.

>
> Here's an example:
> v=0
> o=- 25678 753849 IN IP4 128.96.41.1
> s=-
> c=IN IP4 128.96.41.1
> t=0 0
> m=audio 3456 RTP/AVP 96 97 98
> a=rtpmap:96 L8/8000
> a=rtpmap:97 L16/8000
> a=rtpmap:98 L16/11025/2
> a=subset 2:3
> m=video 3458 RTP/AVP 26 31
> a=rtpmap:26 JPEG/90000
> a=rtpmap:31 H261/90000
> a=subset 1:2
>
> This packet means the sender can support two audio streams among three
> different codecs {L8, L16, and L16-stereo} and one video stream of JPEG or
> H261.

This doesn't work if the receiver doesn't understand the "subset" attribute,
hence it's not backwards compatible.

-- Flemming

--
Flemming Andreasen
Cisco Systems



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