There seems to be a problem in draft-ietf-mmusic-sdp-comedia-00.
The problem is essentially the same as the one that section 3
(Source-Port Considerations) attempts to address, but I believe the
problem is actually a larger one that the draft doesn't address.
Consider an endpoint like a gateway that might be establishing many
connections simultaneously, and assume it wants to act in
direction:passive or direction:both. To do so, it hands out SDP
specifying the port number it will listen on for connections. If it uses
the same port number for multiple concurrent connections, then it will
have difficulty in telling which incoming connection belongs to each
pending call.
The <source-port> is supposed to make it possible to sort this out. But
that can only distinguish between incoming connections from the same
address - requests from different addresses could be using the same
port.
Perhaps the draft assumes that the connection will be from the c= in the
sdp from the other endpoint. But that is a pretty shaky assumption.
The only certain solution I can see without changing the draft is to
dedicate a listening port for the duration of a single invitation. But
this is a costly solution. Alternately, the syntax could be extended to:
a=direction:<role> <source-port> <source-address>
I believe this would solve the problem if <source-port> and
<source-address> are provided. Ensuring this would require making them
mandatory rather than optional as currently specified.
Paul Kyzivat
Cisco Systems
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