Subject: [Sip] SRV + NAPTR

From: Henning Schulzrinne <[email-masked]>
Date: Thu, 27 Dec 2001 08:41:01 -0700

I tried to think of other options for indicating the protocol including:

- Use in the +rs field, as in SIP+UDP. The 'rs' field appears to be
protocol-specific in any event, so this is harmless.

- Use of URI rewriting, to something like URL;transport=tcp. Ugly.

Both of these have the disadvantage that the protocol is specified
twice, creating another error case where the two entries contradict each
other.

Also, the algorithm specified in Section 4 of 2915 doesn't quite work -
the very first (in terms of order) entry always matches and would have
to be chosen, regardless of client capabilities. I doubt that this is a
problem, but a strict API that just implements matching according to the
RFC would likely fail.

Another possible kludge: construct a pseudo-URI such as 

sip:[email-masked];transports=TCP,UDP

and a regexp such as

!sip:.*;transports=.*udp!  -- some UDP reference
!sip:.*;transports=.*tcp!

Needless to say, this is rather ugly. However, somebody should talk to
the DNS experts and find out whether they consider the matching to be
according to the rules.

Since NAPTR, like SRV, also has preferences (in addition to order), you
could get multiple selection mechanisms. Some advice to the implementor
might be helpful here.

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