[SIPForum-discussion] Early dialog can be replaced if Transfer Target is the reciepient of dialog (early) during Attendant Call Transfer

Vivek Batra Vivek.Batra at matrixtelesol.com
Thu May 7 04:29:05 UTC 2009

Hi Experts,


I have one straight question may be I am not able to read between the words.

As per RFC 3891 Section 3:


If the Replaces header field matches an early dialog that was not
   initiated by this UA, it returns a 481 (Call/Transaction Does Not
   Exist) response to the new INVITE, and leaves the matched dialog
If we correlate the above RFC statement with the Attendant Call Transfer,
does it mean that if Transfer Target receives the INVITE (replaces) header
and it matches with the early dialog which is not initiated by the transfer
target, transfer target should not replace the early dialog and return 481
Call Leg Doesn't Exist.
Please consider the following:
Transferor                               Transferee
Transfer Target
------------One way Speech---------------> (1)
-----------------------> (2)
Trying------------------------------------------- (3)
<--------------------------------------------180 Ringing (with TO Tag)
--------------------- (4)
------------REFER (Replaces) ------------> (5)
<--------------202 Accepted------------------ (6)
(Replaces) -----------> (7)
In the above case of Attendant Call Transfer, when Transfer Target receives
the INVITE (Replaces) in message 7 and dialog information receives in
replaces header of INVITE matches with early dialog (between Transfer Target
and Transferor), can Transfer Target replaces the early dialog by sending
any suitable 4XX response to Transferor and send 180 Ringing to Transferee.
If Yes, how we correlate it with the RFC Section 3 which states to not
replace the early dialog if not initiated by the Transfer Target. Transfer
Target in this case is the recipient of the dialog (early).
Best Regards,
Vivek Batra


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