[SIP] SCTP as transport for SIP

["Fairlie-Cuninghame, Robert" <[e-mail]>]

Hi there,

I have a query with your "SCTP as transport for SIP" draft
(http://www.cs.columbia.edu/~hgs/sip/drafts/draft-rosenberg-sip-sctp-00.txt)
,

For instance:

             If the transport is SCTP, the entity SHOULD look for
             exising open SCTP connections to the given IP address and
             port. The number of streams within the connection, and the
             mapping of SIP requests to streams, are at the discretion
             of the entity. However, it is RECOMMENDED that some kind of
             stateless hash be used so that requests for the same call
             all be mapped into the same stream. If no SCTP connection
             is established, one is opened and the request sent over it.

The draft says that the mapping of the SIP requests to streams is up to the
entity. How can this be? A SIP server would normally be only listening to
particular streams to receive SIP requests, the other streams may be unused
or used for some other purpose. The "lookup method" (SRV or whatever) must
surely specify an IP address, SCTP port and stream number to locate the SIP
service via SCTP?

What is more, due to the unidirectional nature of the SCTP streams, surely
the reverse stream must be specified to the endpoint in the Via (or
elsewhere). [I don't think a reverse DNS SRV lookup could be guaranteed -
the UAC may not even have a hostname.]

Does this not mean that we need to add an SCTP stream id URL parameter ?

For instance, some lookup yields a SIP URL (ie Request-URI) of 
sip:host.company.com;transport=sctp;sctp-stream=13 (or something else -
maybe "transport=sctp-13")

and then the Via: would similarly need to specify the reverse stream (which
may very easily be to a different stream or even to a different SCTP port).

I think there are quite a few consideration that SCTP raises above and
beyond other transport protocols (even when you ignore the issues related to
SCTP association establishment). I'll be interested to hear your opinions.

Regards, 

Robert.