Subject: Re: [Sip] INFO method used for ISUP to SIP IW (2nd round)

From: [email-masked]
Date: Wed, 20 Jul 2005 22:34:30 EDT

To all,

Recently, there was a discussion of the question of whether or not the INFO 
can be sent before the dialog has been established. There seemed to be various 
opinions on this, but no clear answer. This is a very important issue for 
SIP-T.

As I noted before, INFO is defined in RFC 2976 where it states that it 
follows the rules for BYE as contained in RFC 2543. RFC 2543 is now obsolete. In 
fact, the main question about the use of INFO relates to its use during early 
dialog, a concept which was defined after INFO was defined.

RFC 3261 states that the BYE is used:
in the caller to callee direction, either in confirmed or early dialog state
in the callee to called direction, on confirmed dialog only

RFC 3398 indicates that the INFO is used to carry encapsulated ISUP:
in the caller to called direction, even if INVITE transaction is not finished 
(i.e., in both early and confirmed dialog state)
in the callee to caller direction,  even before answer (i.e., in both early 
and confirmed dialog states)

So, the use of INFO in RFC 3398 does not always follow the rules for BYE as 
stated in RFC 2976.

Q.1912.5, on the other hand, describes how "transparent" messages are 
encapsulated, and it says they are carried in the following:
a) 183 Session Progress provisional response if this is sent by the I-IWU in 
the backward direction before a confirmed dialog is established.
b) INFO message in all other cases. 

So the ITU-T Recommendation does have the INFO following the rules for BYE.

Is this a correct interpretation?

Mike Pierce
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