Subject: [SIP] Receiving unrecognized To tags

From: [email-masked]
Date: Wed, 6 Sep 2000 18:04:37 -0500

Section 7.4.18 of rfc-bis says a 481 is returned if "the server received an
INVITE with a To tag that does not match the local tag value."
 
Section 10.1.1 says, "An incoming request is accepted unless the Call-ID
value matches an existing call and the request has a tag value of To header
that does not match the user agent server's tag value."
 
Section 11.5 says, "It is possible that the To header in an INVITE request
has a tag, but the UAS believes this to be a new call. [...] The UAS MAY
either accept or reject the request."
 
What if a UA receives an INVITE with an unknown Call ID and an unknown To
tag? Sections 7 and 11 would seem to imply that the INVITE could be rejected
with a 481, and section 10 would seem to imply that a new call leg should be
created. Which is correct? Or am I misreading something?
 
If a UA always uses the same tag value, shouldn't it automatically reject
any request that has a different To tag? On the other hand, if a UA for some
reason generates a new tag for every call, I suppose it wouldn't necessarily
be able to tell whether an unrecognized To tag was one that it had in fact
previously issued.
 
Also, I'm a little confused about the forking issue. Suppose an INVITE gets
forked to two UA's, one of which ends up as the UAS in a lengthy call. Later
on, a re-INVITE from the UAC gets forked to the same two UA's. To the one
that is not in the call, couldn't the wording of section 11.5 apply, so that
the UA that is not involved in the call could reply to the re-INVITE with a
200 OK? I would have thought that the existence of the tag in the INVITE
would indicate to this UA that it should not accept the INVITE.

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