[SIPForum-discussion] [Sip-implementors] ptime/Maxptime supportin SDP

Fri Jun 1 05:12:25 UTC 2007

As per RFC 3264, the offerer should place codecs in the order of
preference in "m" line.

v=0

o=carol 28908764872 28908764872 IN IP4 100.3.6.6

s=-

t=0 0

c=IN IP4 192.0.2.4

m=audio 0 RTP/AVP 0 1 3

a=rtpmap:0 PCMU/8000

a=rtpmap:1 1016/8000

a=rtpmap:3 GSM/8000

m=video 0 RTP/AVP 31 34

a=rtpmap:31 H261/90000

a=rtpmap:34 H263/90000

In the above media attribute line codec 0 has high priority then follows
1 and 3.

It is answerer to negotiate based on this preference.

Cheers

   S

________________________________

From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of Manpreet Singh
Sent: Friday, June 01, 2007 1:21 AM
To: balasubramanian_neelakantan at quintum.com;
sip-implementors at cs.columbia.edu; discussion at sipforum.org
Subject: Re: [SIPForum-discussion] [Sip-implementors] ptime/Maxptime
supportin SDP

I was reading some threads on this last year and there is says this way
there is no way to find out the priority of the codecs to be used. So if
one does implement this how would the priority be decided? So in the
example below, which has higher preference, payload 18 or 4?

Is this defined somewhere formally?

thanks

________________________________

From: Bala Neelakantan [mailto:neel at quintum.com] 
Sent: Wednesday, May 16, 2007 10:34 PM
To: 'Manpreet Singh'; sip-implementors at cs.columbia.edu
Subject: RE: [Sip-implementors] ptime/Maxptime support in SDP

For that, you have to have multiple m lines, with each having just one
codec, and the ptime can be specified for that. 

In your example, you can try separate m= line for each codec in the
Offer.

m=audio 22836 RTP/AVP 4
c=IN IP4 216.168.161.230 
a=rtpmap:18 G729/8000 
a=fmtp:18 annexb=yes;

a=ptime:20 
a=maxptime:40 

m=audio 22838 RTP/AVP 18

a=rtpmap:4 G723/8000 
a=fmtp:4 bitrate=6.3;annexa=yes;

a=ptime:30 
a=maxpime:60 

Thanks,

Neel

-----Original Message-----
From: Manpreet Singh [mailto:msingh at ibasis.net] 
Sent: Wednesday, May 16, 2007 3:28 PM
To: balasubramanian_neelakantan at quintum.com;
sip-implementors at cs.columbia.edu
Subject: RE: [Sip-implementors] ptime/Maxptime support in SDP

Well each codec is defined as the media attribute so for each media
attribute, one can define packetization to be used. Different codec
support different packetizations so they have to be defined explicitly
and we cant use a session level ptime for all media attribiutes.

So how does one specify packetization in the SDP for each codec used is
really my question. Lets assume a UA wants to use G723 with 30 ms
packetization and G729 with 40 ms packetization.

-----Original Message----- 
From: Bala Neelakantan [mailto:neel at quintum.com] 
Sent: Wednesday, May 16, 2007 4:15 PM 
To: 'Manpreet Singh'; sip-implementors at cs.columbia.edu 
Subject: RE: [Sip-implementors] ptime/Maxptime support in SDP 

Ptime is media attribute.  It can't be specified per codec. 

>From 2327. 

   a=ptime:<packet time> 
       This gives the length of time in milliseconds represented by the 
       media in a packet. This is probably only meaningful for audio 
       data.  It should not be necessary to know ptime to decode RTP or 
       vat audio, and it is intended as a recommendation for the 
       encoding/packetisation of audio.  It is a media attribute, and is

       not dependent on charset. 

You can see some notes on usage of ptime in rfc3264. 

>From RFC 4040. 

   Optional parameters: ptime, maxptime 

             "ptime" gives the length of time in milliseconds 
             represented by the media in a packet, as described in RFC 
             2327 [6]. 

             "maxptime" represents the maximum amount of media, which 
             can be encapsulated in each packet, expressed as time in 
             milliseconds, as described in RFC 3267 [9]. 

Thanks, 
Neel 

> -----Original Message----- 
> From: sip-implementors-bounces at cs.columbia.edu 
> [mailto:sip-implementors- bounces at cs.columbia.edu] On Behalf Of 
> Manpreet Singh 
> Sent: Wednesday, May 16, 2007 2:22 PM 
> To: sip-implementors at cs.columbia.edu 
> Subject: [Sip-implementors] ptime/Maxptime support in SDP 
> 
> Whats the right implementation for ptime/maxptime in the SDP. I read 
> few recommendations but what is the correct way of doing it? 
> 
> m=audio 22836 RTP/AVP 4 18 
> c=IN IP4 216.168.161.230 
> a=rtpmap:18 G729/8000 
> a=fmtp:18 annexb=yes;ptime=20 
> a=maxptime:40 
> a=rtpmap:4 G723/8000 
> a=fmtp:4 bitrate=6.3;annexa=yes;ptime=30 a=maxpime:60 
> 
> OR 
> 
> m=audio 22836 RTP/AVP 4 18 
> c=IN IP4 216.168.161.230 
> a=rtpmap:18 G729/8000 
> a=fmtp:18 annexb=yes; 
> a=ptime:20 
> a=maxptime:40 
> a=rtpmap:4 G723/8000 
> a=fmtp:4 bitrate=6.3;annexa=yes 
> a=ptime:30 
> a=maxpime:60 
> 
> is this how we can define ptime and maxptime per media attribute? Any 
> place this has been defined? 
> 
> Thanks 
> _______________________________________________ 
> Sip-implementors mailing list 
> Sip-implementors at cs.columbia.edu 
> https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors 

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