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 sipforum.org
[mailto:discussion-bouncessipforum.org] On Behalf Of
Manpreet Singh
Sent: Friday, June 01, 2007 1:21 AM
To: balasubramanian_neelakantanquintum.com;
sip-implementorscs.columbia.edu; discussionsipforum.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:neelquintum.com]
Sent: Wednesday, May 16, 2007 10:34 PM
To: 'Manpreet Singh'; sip-implementorscs.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:msinghibasis.net]
Sent: Wednesday, May 16, 2007 3:28 PM
To: balasubramanian_neelakantanquintum.com;
sip-implementorscs.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:neelquintum.com]
Sent: Wednesday, May 16, 2007 4:15 PM
To: 'Manpreet Singh'; sip-implementorscs.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-bouncescs.columbia.edu
> [mailto:sip-implementors- bouncescs.columbia.edu] On Behalf
Of
> Manpreet Singh
> Sent: Wednesday, May 16, 2007 2:22 PM
> To: sip-implementorscs.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-implementorscs.columbia.edu
> https://lists.cs.columbia.edu/cucslists/listinf
o/sip-implementors
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