Subject: ISUP inside SIP body

From: Gonzalo Camarillo <[email-masked]>
Date: Mon, 19 Jul 1999 15:05:24 +0300

Hi,

Last week in Oslo there were some talks about ISUP messages in SIP
bodies.
Jonathan made a good comment. What happens if we receive for example a
REL ISUP message in a INVITE request?

This situation will probably never happen, but other similar scenarios
could represent a problem.

The behaviour of an MGC would have to be different, depending on the
callee (a SIP client or another MGC).

For example, if we receive in the MGC an ACM message from the ISUP side,
our SIP-ISUP state machine will probably say that we have to send a "180
Ringing" response (or 183) to the SIP side.
But the ACM might contain a parameter that indicates that the B-party
(the ISUP side) is not being alerted ('early ACM').

So, if the A-party was a SIP phone, we should't send anything, because
for SIP nothing happened.

But if the A-party was another MGC, this ACM has to be sent (SIP INFO
method or 180 reponse?) in order to provide ISUP transparency.
SIP INFO sounds good, but "180 Ringing" would have contradictory
information in the ACM...

The point is, if we are just using SIP for transporting ISUP messages,
should we care about the information that SIP provides, or we have just
to get the ISUP message as it is, and put it on the PSTN network?

In SIGTRAN they do not want to use TCP because (among other reasons)
before sending the first IAM the TCP handshake takes too long...
If SIP is used, we have this timing problem also...

What I am saying is that providing ISUP-SIP-ISUP services is more
complicated that many people think, and there are lots of things that
have to be taken into account (besides puting some messages inside
anothers)...

All these issues will be addressed in a draft coming up soon.

Comments are welcome,

Gonzalo
-- 
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