Subject: [Sip] Multiple 'm=' lines

From: "Chris Boulton" <[email-masked]>
Date: Fri, 24 Aug 2007 12:01:29 +0100

All,

Just looking for a clarification which seems to be escaping me.  I
apologise if it's a trivial question.  I have a scenario where a UAS
receives an INVITE which has multiple media lines (m=).  I realise that
if the UAS maybe does not wish/or support one of the media lines then it
can simply set the port to 0 and respond in the positive.  My question
is: What SIP error response code could the UAS use in this situation if
the call can not proceed?  I am currently working on a draft in the
MediaCtrl work group where it would be useful to signal to the UAC that
the lack of support for a media line was the reason for failure.  I
found the following from Paul K:

If you are unwilling to do the audio without the video, and are also
unwilling to accept a session with no media at all, then the error you
want to use is probably 488. The 415 error means you don't understand
the media type of a required mime body part in the message.

Sorry if I am missing something. Just wondering if there is something
missing here.  I suppose the 488 could contain a message body as per RFC
3261.  I'm not sure a 'Warning' header does the trick.

Thanks.

Chris.

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From: [email-masked]
Date: Sat, 25 Aug 2007 23:22:11 -0400

From: "Chris Boulton" <[email-masked]>

   receives an INVITE which has multiple media lines (m=3D).  I realise that

You haven't explained what your problem is.

In regard to Paul's answer, he is correct.  The 415 response means "I
did not understand the MIME media type of the body (carried in the
Content-Type header)".  The 488 response means "I understood the media
description (the body), but it was unacceptable to me for establishing
a dialog."

In regard to multiple m- lines, you aren't thinking about the question
correctly.  The correct approach is to examine all the m- lines, and
from that set, ignore any that describe media types that the UA is
unable or unwilling to process.  Then examine those that remain, and
see if among them are enough media that the UA can establish
communication.  If the answer is yes, accept the call.  If the answer
is no (probably because no m- lines remain), send a 488 response.

In general, you don't reject a call because one m- line is *not*
something you can deal with, you reject a call because there is *no*
m- line that *is* something you can deal with.

Dale