SIP Request Method for Reliability of SIP 1xx Response Types

B.

In order to achieve reliability for provisional responses, we do nearly the same thing.

Reliable provisional responses are retransmitted by the TU with an exponentialbackoff.

Those retransmissions cease when a PRACK message is received.

The PRACK request plays the same role as ACK, but for provisional responses.

There is an important difference, however.

PRACK is a normal SIP message, like BYE.

As such, its own.

The SIP (Session Initiation Protocol) is an application-layer protocol used for establishing, modifying, and terminating real-time sessions that involve voice, video, messaging, and other communications applications and services. SIP uses a request/response model to initiate and manage sessions between two or more endpoints.

SIP responses are used to convey the status or result of a request. Responses are grouped into classes based on the first digit of the three-digit response code, with the following classes:

• 1xx: Informational responses
• 2xx: Success responses
• 3xx: Redirection responses
• 4xx: Client error responses
• 5xx: Server error responses
• 6xx: Global failure responses

When a SIP client sends a request, it expects a response from the server indicating the success or failure of the request. In some cases, the response may be delayed or lost, which can cause the client to retransmit the request or take other actions that may lead to duplicate requests or incorrect session state.

To address this issue, SIP defines a reliability mechanism for certain types of responses, known as provisional responses or 1xx responses. Provisional responses indicate that the server has received the request and is processing it, but the final status is not yet known. Provisional responses include 100 Trying, 180 Ringing, and 183 Session Progress.

The SIP reliability mechanism for 1xx responses is based on the PRACK (Provisional ACK) method, which enables the client to acknowledge the receipt of the 1xx response and to request retransmission of the response if it is lost. PRACK messages contain a sequence number that enables the server to detect and discard duplicate requests.

Therefore, the correct answer to the question is B. PRACK, as it is the SIP request method that enables reliability of 1xx response types. The other SIP request methods listed in the answers have different purposes:

• ACK (Acknowledge) is used to confirm the successful receipt of a final response to an INVITE request.
• OPTIONS is used to query the capabilities of a SIP server or endpoint.
• CANCEL is used to terminate a pending request.
• REGISTER is used by a SIP client to register its address-of-record with a SIP registrar server.