Hi Bob,
Pardon me as I am going into a slightly different,
but related topic. I had a doubt regarding what you
wrote in your mail. Some time ago, even I thought
that breaking a line of the form
m=audio 3456 RTP/AVP 2 9 15
into 3 lines like:
m=audio 3456 RTP/AVP 2
m=audio 3458 RTP/AVP 9
m=audio 3460 RTP/AVP 15
would help the originating UA get an exact match for
*all* of the codecs that it supports and that it
can then decide (on getting a reply) as to which one
to choose.
However, the recent draft on offer-answer
(draft-rosenberg-mmusic-sdp-offer-answer-00.txt)
adds another dimension of difference to the 2 offers
shown above. In particular, the draft states that:
"If multiple media streams of the same type are present,
it means that the offerer wishes to send (and/or receive),
multiple streams at the same time."
^^^^^^^^^^^^^^^^^
This would mean that in the second offer shown above,
the UA receiving the offer would try to encode & send us
as many streams that are acceptable to it *simultaneously*.
Question: Is this solution then okay, since the UA
that proposed this SDP (in this case, us)
did not actually intend to cause this?
We wanted to choose *one* codec!
In the first offer, however, the other end will send
only one stream and it has the freedom to change the
codecs any time in the middle of the call, adding some
(little/more) complexity on the proposer's (our) end to
look at RTP packets and decide the codec that is in use.
The idea of using OPTIONS seems like the only reasonable
choice to me. However, please correct me if the above
understanding is false!
Thanks in advance,
Siddharth.
=================================
Siddharth Toshniwal
Software Engineer
Hughes Software Systems, Bangalore
http://www.hssworld.com
"Bob Penfield" <[EMAIL PROTECTED]> on 11/27/2001 09:19:11 PM
To: "Vikram Varma" <[EMAIL PROTECTED]>, "'Shen, Eran '"
<[EMAIL PROTECTED]>, "'Sip-Implementors \(E-mail\) '"
<[EMAIL PROTECTED]>
cc: (bcc: Siddharth J Toshniwal/HSSBLR)
Subject: Re: [Sip-implementors] Problem with media channels in SIP.
By offering:
m=audio 3456 RTP/AVP 2 9 15
you are saying you can receive any of those codecs on port 3456.
Maybe you could use a different port for each codec and offer:
m=audio 3456 RTP/AVP 2
m=audio 3458 RTP/AVP 9
m=audio 3460 RTP/AVP 15
You should then get an answer:
m=audio 5004 RTP/AVP 2
m=audio 5006 RTP/AVP 9
m=audio 0 RTP/AVP 15
Note "15" was disabled by setting the port number to 0.
You cannot send SDP in the ACK because you sent it in the INVITE.
If you need to change, you must re-INVITE.
Another alternative would be send an OPTIONS request to get back the
capabilities of the other end. Then you could narrow down the codecs you
offer. But not all UAs support OPTIONS.
cheers,
(-:bob
Robert F. Penfield
Chief Software Architect
Acme Packet, Inc.
130 New Boston Street
Woburn, MA 01801
[EMAIL PROTECTED]
_______________________________________________
Sip-implementors mailing list
[EMAIL PROTECTED]
http://lists.cs.columbia.edu/mailman/listinfo/sip-implementors
