In a SIP REFER-based call transfer, which SIP message is being used by the recipient to notify the originator that the final recipient was successfully contacted?
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A. B. C. D. E.B.
The Refer method always begins within the context of an existing call and starts with the originator.
The originator sends a Refer request to the recipient (user agent receiving the Refer request) to initiate a triggered Invite request.
The triggered Invite request uses the SIP URL contained in the Refer-To header as the destination of the Invite request.
The recipient then contacts the resource in the Refer-To header (final-recipient), and returns a SIP 202 (Accepted) response to the originator.
The recipient also must notify the originator of the outcome of the Refer transaction--whether the final-recipient was successfully or unsuccessfully contacted.
The notification is accomplished using the Notify Method, SIPs event notification mechanism.
A Notify message with a message body of SIP 200
If the call was successful, a call OK indicates a successful transfer, while a body of SIP 503 Service Unavailable indicates an unsuccessful transfer between the recipient and the final-recipient results.
In a SIP REFER-based call transfer, the REFER method is used to initiate a transfer request. The REFER message contains information about the call being transferred and the target recipient who will be taking over the call. The recipient of the REFER message then sends a response to the originator to indicate the status of the transfer.
The correct answer to this question is B. NOTIFY with a message body of 200 OK.
When the recipient successfully answers the call, they send a SIP NOTIFY message with a message body of 200 OK to the originator. The message body contains information about the status of the transfer and indicates that the final recipient has been successfully contacted.
The NOTIFY message is used in SIP to provide asynchronous notifications about events that occur during the course of a SIP session. In this case, the NOTIFY message is being used to provide status updates on the transfer process.
The other answer options are incorrect for the following reasons:
A. 200 OK: This is a standard SIP response code that indicates a successful request. However, it is not used specifically in the context of a SIP REFER-based call transfer to indicate that the final recipient has been successfully contacted.
C. 202 Accepted: This response code is used to indicate that the request has been accepted for processing but is not yet complete. It is not used specifically in the context of a SIP REFER-based call transfer to indicate that the final recipient has been successfully contacted.
D. 100 Trying: This response code is used to indicate that the server has received the request and is processing it, but a final response is not yet available. It is not used specifically in the context of a SIP REFER-based call transfer to indicate that the final recipient has been successfully contacted.
E. 200 BYE: This response code is used in the context of a SIP BYE message to indicate that the call has been terminated. It is not used specifically in the context of a SIP REFER-based call transfer to indicate that the final recipient has been successfully contacted.