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Global Call IP Technology Guide — November 2007 89
Dialogic Corporation
IP Call Scenarios
activated such as forking or sequential searching which may result in the triggered INVITE
reaching the wrong User Agent. To prevent an incorrect UA answering the INVITE, a Require:
replaces header field is included in the Refer-To. This ensures that only the UA which matches the
Replaces dialog will answer the INVITE, since any incorrect UA which supports Replaces will
reply with a 481 and a UA which does not support Replaces will reply with a 420.
Note that there is still no guarantee that the correct endpoint will be reached, and the result of this
second REFER may also be a failure. In that case, the Transferor could fall back to unattended
transfer or give up on the transfer entirely. Since two REFERs are sent within the dialog, creating
two distinct subscriptions, the Transferee uses the ‘id’ parameter in the Event header field to
distinguish notifications for the two subscriptions.
Figure 32. SIP Attended Call Transfer, Recovery from URI Not Routable
A
(Transferring,
Transferor)
App
A
(Transferring,
Transferor)
IP CCLib
B
(Transferred,
Transferee)
3rd Party
Screening Proxy
C
(Transferred To,
Transfer Target)
3rd Party
GCEV_INVOKE_
XFER_FAIL
(CRNp, 403)
gc_InvokeXfer
(CRNp, CRNs,
TransferredToContact)
Transferred call between B and C is connected.
Primary and secondary calls are dropped and released.
Post condition:
GCEV_INVOKE_
XFER(CRNp)
GCEV_INVOKE_
XFER_ACCEPTED
(CRNP)
INVITE
200 OK
Primary call between A and B is connected (not shown).
Secondary (consultation) call between A and C is connected (not shown).
Pre condition:
gc_InvokeXfer
(CRNp, CRNs,
TransferredToAOR)
REFER (Refer-To:sip:
TransferredToContact?Replaces=secondaryCall)
202 Accepted
GCEV_INVOKE_
XFER_ACCEPTED
(CRNP)
NOTIFY (403 Forbidden)
Subscription-State = terminated
200 OK
REFER (Refer-To:sip:
TransferredToAOR?Replaces=secondaryCall)
202 Accepted
NOTIFY (200 OK)
Subscription-State = terminated
200 OK
INVITE
INVITE
403 Forbidden
200 OK
Normal primary and secondary
call cleanup not shown.