Subject: [MMUSIC] Re: [Sipping] Media descriptor problem in SDP

From: Gonzalo Camarillo <[email-masked]>
Date: Thu, 06 Mar 2003 15:12:12 +0200

Louis,

I have removed the SIPPING mailing list from the cc and added MMUSIC,
since this is an MMUSIC related question.

If you want to provide different parameters for a particular codec, you
should define different dynamic payload types (e.g., 96 and 97).

Gonzalo

Louis Guindon wrote:
> 
> Hi,
> 
> We are receiving the following SDP with an INVITE...
>   m=audio 5004 RTP/AVP 8 8
>   a=fmtp:8 ptime:20
>   a=fmtp:8 ptime:30
>   a=sendrecv
>   a=ptime:20
> 
> After looking in the RFC2327 ABNF for those fields, I cannot find any problem with the grammar. However, it's doesn't make sense to me that we can put the media format twice on the media announcements line. Moreover, it doesn't make more sense to put two attribut for the same media format with different parameters.
> 
> Any idea where I can find about the validity/invadility of SDP packet?
> 
> Regards,
> 
> Louis Guindon
> Software Design Engineer
> Mediatrix Telecom, Inc.
> [email-masked]
> Tel :  819-829-8749 ext. 242
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---

From: Magnus Westerlund <[email-masked]>
Date: Thu, 06 Mar 2003 14:33:46 +0100

Hi Louis,

There are actually some errors in this SDP you present. It is however 
not a clear syntax error.

If you look in draft-ietf-rtp-mime-06.txt you will see that chapter 3 
tells how to map the ptime parameter to the SDP. And it shall not be 
part of the a=fmtp line but go into its own line. Unfortunately you 
can't specify these parameters on individual payload type basis. They 
apply in general to all payload formats that allow them in that media 
block.

Also there is an error using the same PT twice on the m= line. So if you 
of some reasons want to have two PTs for PCMA then your SDP should be 
written like this utilizing dynamic PTs as Gonzalo pointed out:

m=audio 5004 RTP/AVP 8 96
a=rtpmap:96 PCMA/8000
a=sendrecv
a=ptime:20

I hope this clarifies things. If the only thing you would like to 
accomplish is to have different frame lengths a single PT is enough. If 
you in the SDP likes to clarify that you will use 20 or 30 but never 
more than 30 ms then you could write this as:

m=audio 5004 RTP/AVP 8 96
a=sendrecv
a=ptime:20
a=maxptime:30

Regards

Magnus

PS. Please don't send confidentiality message like your initial message 
contained as the mailing list you sent to is an open list having a 
public archive.

-- 

Magnus Westerlund 

Multimedia Technologies, Ericsson Research ERA/TVA/A
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