The standard and expected behavior for SIP authentication, as implemented by Avaya Aura® Session Manager, follows the RFC 3261 challenge-response mechanism. When a SIP station (User Agent) attempts to register for the first time, it sends a REGISTER request without authentication credentials. The Session Manager, acting as the registrar, challenges this request by responding with a 401 Unauthorized message. This message contains a WWW-Authenticate header with the necessary information (realm and nonce) for the station to build a proper credentialed request. The station then sends a second REGISTER request, this time including an Authorization header with the correct credentials, which then results in a 200 OK if successful.

B. The first registration request is not expected to be successful. It is part of the challenge-response mechanism and is intentionally rejected to prompt the endpoint for credentials.

C. The traceSM utility on Session Manager is a comprehensive diagnostic tool that captures all SIP traffic processed by the server, not just registrations from specific device types like AST.

D. traceSM is used for troubleshooting and analysis; therefore, it displays all SIP messages, including both successful and failed registration attempts, to provide a complete view of the interactions.

1. Avaya Aura® Session Manager Documentation: The troubleshooting guide explicitly shows this sequence. In a sample trace of a successful registration, the first message from the endpoint is a REGISTER, which is immediately followed by a 401 Unauthorized from the Session Manager. The endpoint then sends a second REGISTER with authentication details.

Source: Troubleshooting Avaya Aura® Session Manager, Release 8.1, November 2020, Page 138, "Example of a successful registration trace".

2. IETF RFC 3261 - SIP: Session Initiation Protocol: This foundational document defines the SIP protocol. It specifies that a registrar can challenge a REGISTER request for which it requires authentication by rejecting it with a 401 response.

Source: IETF RFC 3261, Section 10.2.1 "Creating the Initial REGISTER Request" and Section 22.2 "User Agent Authentication".

3. Avaya Aura® Session Manager Administration Guide: The documentation on traceSM confirms its role in capturing all SIP messages for monitoring and troubleshooting, which contradicts the limitations suggested in options C and D.

Source: Administering Avaya Aura® Session Manager, Release 8.1.3, May 2022, Chapter 11: "SIP entity monitoring", Section "Tracing SIP messages with traceSM".