Subject: [Sip] m= lines with multiple ports

From: Christer Holmberg <[email-masked]>
Date: Fri, 12 Oct 2001 10:57:29 +0300

Hi,

More on the SIP media handling discussion.

At the moment, I think most people agree that the number of m= lines in
the request and response MUST match.

Now, SDP allows the including of multiple ports (so called port pairs)
in a single m= line, so the UAC may send:

m=audio 2000/2 RTP/AVP 31

Now, there are a few scenarios:

1. The UAS only supports the stream on one port. In that case, it could
indicate that by sending:

m=audio 3000 RTP/AVP 31
m=audio 0 RTP/AVP 31

However, this breaks the matching rule...

2. The UAS supports both streams, but can not use a port pair for them,
so again it must use separate m= lines, by sending:

m=audio 3000 RTP/AVP 31
m=audio 4000 RTP/AVP 31

However, this breaks the matching rule...

3. The UAS doesn't accept this at all, by sending:

m=audio 0 RTP/AVP 31

...or, should it send:

m=audio 0/0 RTP/AVP 31

This does match the rule.

I don't know how often this would happend in SIP, but since SDP allows
it it MAY happend. The question is: shall we forbid that kind of SDP use
in SIP, or how shall we handle it?

SDP also allows us to have a c= line indicating multiple IP addresses,
which makes this even more complicated. However, in that case we are
talking about multicasting, so let's forget about that for now.

Regards,

Christer Holmberg
Ercisson Finland

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