SIP Provisional Response Acknowledgment

A.

SIP Provisional Response Acknowledgment (PRACK) is used to acknowledge the receipt of provisional responses (1xx) to INVITE requests. When a SIP client receives a provisional response, it sends a PRACK message to acknowledge the receipt of the response. This allows the client to confirm that it received the response, and also informs the sender that the client is ready to receive the final response (2xx, 3xx, 4xx, 5xx, or 6xx).

However, not all provisional responses require a PRACK message. The correct answer to this question is (C) 199 Early Dialog Terminated, which is not a typical 1xx response and is not acknowledged with a PRACK message.

The other 1xx responses, on the other hand, are typically acknowledged with a PRACK message. Let's take a look at each of them:

A. 100 Trying: This is a standard response that indicates that the server has received the request but has not yet completed processing it. A PRACK message is typically sent to acknowledge this response.

B. 180 Ringing: This response indicates that the destination user agent has received the INVITE request and is alerting the user. A PRACK message is typically sent to acknowledge this response.

D. 183 Session in Progress: This response indicates that the destination user agent has received the INVITE request and is providing information about the progress of the call setup. A PRACK message is typically sent to acknowledge this response.

E. 182 Queued: This response indicates that the destination user agent has received the INVITE request but cannot complete it immediately. A PRACK message is typically sent to acknowledge this response.

In summary, a PRACK message is typically used to acknowledge the receipt of provisional responses (1xx) to INVITE requests, but not all 1xx responses require a PRACK message. The 199 Early Dialog Terminated response does not require a PRACK message.