Subject: RE: [Sipping] b2bua and cc-transfer

From: "Peter Musgrave" <[email-masked]>
Date: Mon, 30 Apr 2007 08:12:58 -0600

Hi all, 

In addition to mapping the Call Id and the contact header, the B2B will
need to map the to-tag and from-tag information in the Replaces header. 

According to RFC3891 Section 3:

The User Agent Server

(UAS) matches the to-tag and from-tag parameters as if they were tags
present in an incoming request.  In other words, the to-tag parameter is
compared to the local tag, and the from-tag parameter is compared to the
remote tag.

This introduces an additional wrinkle. 

+-----+             +------+           +------+            +-----+

|UA -A|             | UA-B |           |b2bua |            |UB-C |

+-----+             +------+           +------+            +-----+

                                     maps: call-id

                                           Contact

Assume C calls B and establishes a dialog. B then calls A and sends A a
REFER to C with a Replaces telling it to replace the dialog C has with
B. So the B2B must adjust the call id, and ensure that the to-tag in the
resulting INVITE with replaces matches the local tag provided by C in
it's initial call to B (i.e. the tag in the From header). 

The other scenario is as above but when the initial call is caused by B
calling C. In this case C's local tag will be in the To: header. When
the transfer occurs and an INVITE with replaces goes through to C, the
B2B now needs to ensure the to-tag value comes from the To: header in
the initial dialog. 

As long as the B2B keeps track of "local" and "remote" tags for each
dialog, this mapping should be fairly straight-forward. 

Peter

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