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

[kaushik at ncoretech.com]

I guess it is "answerer's" prerogative to select the preferred codec.

But I agree that there should be a way for offerer also to
indicate preferences. Not sure how this can be done.

Also, it is not clear what should happen if answerer responds with
both these codecs.

Regards,
Kaushik

Quoting Manpreet Singh <msingh at ibasis.net>:

> 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 <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- <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
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