). This is the usual method.
The other option is for the device to alert the user with some form of message and allow
that user to accept or deny the session in accordance with the media type required.
For example, if the session requires a text message to be received by the subscriber,
the operator may want to give the subscriber the option of approving the session prior
to establishing it. This last option is beyond the scope of the IMS specifications but is
being reviewed by the industry as a possible option.
If the device is able to support the session, the device will send the response 200 OK
using the same path that was used to route the request. At this point the destination
subscriber device has calculated the dialog ID for the session and has saved this dialog
ID for subsequent requests and responses for the same session.
Once the requestor has received the response from the called subscriber, an ACK can
be returned and the session can begin. The ACK contains the same BRANCH value as
found in the VIA header in the request. A sample of these messages follows.
INVITE sip:travis.russell@tcg.com sip/2.0
VIA: SIP/2.0/UDP pchome101@aol.com:5060;branch= z9hG4bK74gh5
FROM: Deby Russell
; tag=9hz34567sl
TO: Travis Russell travis.russell@tcg.com
MAX FORWARDS: 70
CALL-ID: 82167534@126.18.27.0
CSeq: 1 INVITE
134 Chapter 5
CONTACT: Deby Russell
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