SIP Provisional Response Acknowledgment Message Characteristics

AB.

The SIP Provisional Response Acknowledgment message, also known as PRACK, is used to acknowledge a provisional response to an INVITE request. Here are the descriptions of the characteristics of PRACK:

A. It must be acknowledged with a "200 OK" message.

• This statement is incorrect. The PRACK message is acknowledged with a "PRACK" message.

B. It is sent by the UAS after receiving the "Supported: PRACK" header in a previous message from the UA.

• This statement is incorrect. The PRACK message is sent by the UAC, not the UAS.

C. It is sent by the UAC after receiving the "Require: 100rel" header in a previous message from the UAS.

• This statement is correct. The UAC sends the PRACK message to acknowledge the provisional response from the UAS after receiving the "Require: 100rel" header in a previous message from the UAS.

D. It must be acknowledged with an ACK message.

• This statement is incorrect. The PRACK message is not acknowledged with an ACK message. Instead, it is acknowledged with a "PRACK" message.

E. It is sent by the UAC after receiving the "Require: PRACK" header in a previous message from the UAS.

• This statement is incorrect. The UAC sends the PRACK message to acknowledge the provisional response from the UAS after receiving the "Require: 100rel" header in a previous message from the UAS.

F. It is sent by the UAS after receiving the "Supported: 100rel" header in a previous message from the UA.

• This statement is incorrect. The PRACK message is sent by the UAC, not the UAS.

Therefore, the correct answers are C and D.