Subject: INVITE ACK Accord

From: [email-masked] (Lawrence Conroy)
Date: Sun, 29 Nov 1998 23:33:13 +0000

A couple of "lurkies" that might bite the unwary - SIP draft 10 of 12th
November 1998 says (on page 23, in section 4.2.2) that "The ACK request MAY
contain a
message body with the final session description to be used by the callee. If
the ACK message body is empty, the callee uses the session description in
the INVITE request".

*  If there is NOT a message body in the ACK, then this implies that the SIP
Gateway must store that held in the original INVITE. Obvious, but worth
remembering, as it can be quite a lot of state variable space for a busy
server, given the possible time-out values for INVITE/ACK.

*  Also, one ASSUMES that the PINT service request (as held in the original
INVITE) is the same as that held in the ACK (if there is a "full" request,
with message body). This is a HIDDEN assumption within SIP (or, if not, I
can't find it on glancing through).

ACK is not, itself, acknowledged with a response, so a "badly-behaved"
client wouldn't know what's going on - a server might be able to handle the
request as passed in the INVITE message, but NOT be able to handle a
different request passed in the message body of an ACK.

It isn't mentioned in the current PINT draft, but as it also isn't mentioned
in the current SIP draft either, I suggest that the PINT draft should include
some comment like:

"If the PINT Client generates an ACK with message bodies, then
these message bodies MUST have the same content as the associated INVITE
message"

All the best, Lawrence
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| Lawrence Conroy,    | "These Opinions must be mine, 'cos if they    |
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