This message is to describe an attack on the problem of grouping
transferred calls in regard to CDR. This proposal is to use a
"References" header in the INVITE that establishes the latter,
transferred dialog to show that it was derived from the first, original
dialog.
Here are some more details:
We are working on popularizing the following mechanism: When one
dialog is related to another dialog in some way, the INVITE that
establishes the new dialog contains a "References" header that gives
the Call-Id of the old dialog. Thus message logs will record the
connection between the two dialogs.
In the case of a blind transfer, the new INVITE will look something
like this:
INVITE sips:[email protected] SIP/2.0
>>> References: [email protected];rel=refer
Via: SIP/2.0/TLS client.biloxi.example.com:5061
;branch=z9hG4bKnashds1
Max-Forwards: 70
From: Bob <sips:[email protected]>;tag=8675309
To: Carol <sips:[email protected]>
Call-ID: [email protected]
CSeq: 1 INVITE
Contact: <sips:[email protected]>
Referred-By: <[email protected]>
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces
Content-Type: application/sdp
The marked References header shows that this INVITE was generated due
to a REFER that was received within the dialog with Call-Id
"[email protected]".
Because SIP elements ignore headers whose names they do not know, this
is upward-compatible. We have implemented References in a few places
in the sipXecs stack and have had no interoperation problems with any
other SIP elements.
The Internet-Draft that defines References is available at
http://tools.ietf.org/html/draft-worley-references-03. The example
involving REFER is in section 3.2.
In a perfect world, the UA that receives the REFER would add the
References header automatically. But of course, it doesn't. But we
can instruct it to do so by modifying the Refer-To URI in the REFER:
REFER sips:[email protected] SIP/2.0
Via: SIP/2.0/TLS client.biloxi.example.com:5061
;branch=z9hG4bKnashds8
Max-Forwards: 70
From: Alice <sips:[email protected]>;tag=1234567
To: Bob <sips:[email protected]>;tag=314159
Call-ID: [email protected]
CSeq: 101 REFER
>>> Refer-To:
>>> <sips:[email protected]?references=12345601%40atlanta.example.com%3brel%3drefer>
Referred-By: <[email protected]>
Contact: <sips:[email protected]>
We already modify the Refer-To headers of REFERs that pass through the
proxy, so we shouldn't have too much difficulty adding this machinery.
If there already is a References header-value, we can add our value to
the current value, because the References header allows multiple
values, separated by commas.
Dale
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