Subject: RE: [MMUSIC] comments on draft-xu-mmusic-sdp-codec-param

From: "Christer Holmberg (JO/LMF)" <[email-masked]>
Date: Thu, 24 Nov 2005 08:09:23 +0100

Hi,

A solution pretty much identical to the proposed x-codecparam attribute,
where the payload type is present (similar to the fmtp attribute), has
been discussed before - and the discussion seems to come up over and
over again. So, I agree that we need to find a solution for this. The
same discussion has also taken place in other standardization bodies,
related e.g. to H.248 usage of SDP.

The draft proposes that one can list ptime values in the attribute. I
also think it should be possible to list the min and max value, as has
been propsed by others.

Something like: a=x-codecparam: <PT> "max"=<time> "min"=<time>

Personally I don't think using fmtp would be the best choise for this,
since the syntax of the attribute would be generic, and not codec
specific.

Regards,

Christer
Ericsson

> -----Original Message-----
> From: [email-masked] 
> [mailto:[email-masked]] On Behalf Of Randell Jesup
> Sent: 12. marraskuuta 2005 7:42
> To: Elwell, John
> Cc: IETF MMUSIC WG
> Subject: Re: [MMUSIC] comments on draft-xu-mmusic-sdp-codec-param
> 
> "Elwell, John" <[email-masked]> writes:
> >I think the ptime area is worth addressing. It appears that some 
> >implementations have a minimum packet time for receipt, below which 
> >they are likely to encounter performance problems. Likewise some may 
> >have a maximum packet time for receipt, governed by buffer sizes. It 
> >would be good to have a way of expressing these upper and 
> lower bounds 
> >to what a device is able to receive. The existing ptime parameter is 
> >unable to express this. Of course, these bounds are likely to differ 
> >according to codec, e.g., for G.711 a device might accept 
> between 5 ms 
> >and 20ms, but for G.723 it might accept between 30ms and 60ms.
> 
>         And then there's iLBC, which uses 20 or 30ms (only), 
> with specific offer/answer rules (20ms default, use 30ms if 
> either side requests it).  
> This is done via fmtp (a=fmtp XX mode=NN), though.
> 
> --
> Randell Jesup, Worldgate (developers of the Ojo videophone), 
> ex-Amiga OS team [email-masked]
> 
> 
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> https://www1.ietf.org/mailman/listinfo/mmusic
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---

From: Magnus Westerlund <[email-masked]>
Date: Thu, 24 Nov 2005 11:17:37 +0100

Hi,

I also support work on a more generic per payload type specified 
packetization indication and negotiation parameter. I agree with 
Jonathan that a new attribute shall be used. Changing ptime at this 
point in time would be a nightmare.

I would also like to point out: STOP USING X-!!!

Sorry for shouting, but this bad idea seems impossible to kill, 
independently of if it is SDP, SIP or RTSP.

I also have grave concerns with an mechanism specifying alternative 
addresses within a single media block. As each alternative address for 
this usage would be its own RTP session using it in this way has several 
severe issues. They all originates from the assumption that one media 
block is its own RTP session (or part of a session) and indicates 
individual streams. Putting multiple streams with different semantics in 
the same media block do break:

- SRTP
- ULP (now that we have fixed it)
- pre-conditions

I would also like to point out that some work has been done for allowing 
the indication using the grouping of media lines framework to indicate 
that different streams are alternatives to each other where only one of 
them should be used at any point in time. See FID semantics in RFC 3388 
which would allow to indicate different media lines and c= for different 
codecs part of the same media flow, thus allowing redirection to 
transcoder for specific codecs.

Cheers

Magnus

-- 

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
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