SIP: Understanding REGISTER Method and Authentication Process

The SIP REGISTER method plays a vital role in authenticating users and managing their presence on a given VoIP network. In this article, we’ll delve into the intricacies of the SIP REGISTER method and explore the authentication process that underlies it. By gaining a deeper understanding of how SIP registration works, you’ll be better equipped to troubleshoot issues and optimize your VoIP infrastructure for reliable, high-quality communications.

REGISTER Method:

The SIP REGISTER method is used by User Agents (UA) to register its current location with the SIP registrar. The REGISTER message contains various headers that provide important information for successful registration. Let’s look at an example SIP registration sequence and break it down for basic understanding.

Important Headers in a REGISTER Message:

• To: This header contains the address of the user being registered. It typically includes the SIP URI (Uniform Resource Identifier) of the user, such as “sip:username@example.com.”
• Contact: The Contact header specifies the address and other relevant information where the UA can be reached. It includes the UA’s SIP URI and an optional expiration time.
• Expires: This header indicates the duration (in seconds) for which the registration remains valid. The default expiration value is 3600 seconds (1 hour).

• Upon receiving a REGISTER message, the registrar server may respond with a 401 Unauthorized status code. This indicates that the UA needs to authenticate itself before the registration can proceed. The server includes a WWW-Authenticate header in the response, specifying the authentication method and realm.

• After receiving the 401 Unauthorized response, the UA needs to authenticate itself using the Digest Authentication method. It calculates a response using the user’s credentials, the realm provided in the WWW-Authenticate header, and other parameters. The UA then resends the REGISTER message with an additional Authorization header containing the authentication information.

• If the authentication is successful, the registrar server responds with a 200 OK status code. This indicates that the registration was accepted, and the UA is now registered with the specified expiration time. The default expiration value is typically set to 3600 seconds (1 hour). To refresh registration, UAs should send REGISTER at half the interval of current registration expiration. If the user agent does not re-register before the registration expires, its binding will be removed by the registrar.

Conclusion:

Understanding REGISTER message and authentication process is crucial for smooth SIP trace analysis. By including the important headers in a REGISTER message, handling 401 responses, and employing digest realm-based authentication, User Agents can establish and maintain their registration with the registrar server effectively. Additionally, being aware of the default expiration value and the necessity for periodic re-registration ensures continuous communication according to the SIP RFC guidelines.