SSCA Registration Process 5 — Questions and Answers
Question 1: What is third-party registration in SIP?
- A UA registering through a third-party STUN server
- One UA submitting a REGISTER request on behalf of another UA's AOR (Correct answer)
- Registration that requires three-way handshake authentication
- A SIP registrar delegating registration to a backup server
Correct answer: One UA submitting a REGISTER request on behalf of another UA's AOR
Third-party registration occurs when a UA (e.g., a PBX) sends a REGISTER for a different user's AOR using the From header to identify the requestor.
Question 2: What is the primary role of a SIP registrar in the registration process?
- To forward REGISTER requests to the destination UA
- To authenticate users and route calls end-to-end
- To accept REGISTER requests and maintain AOR-to-Contact bindings in a location service (Correct answer)
- To assign IP addresses to SIP user agents
Correct answer: To accept REGISTER requests and maintain AOR-to-Contact bindings in a location service
The registrar stores AOR-to-Contact bindings in a location service database, enabling proxy servers to route incoming calls to the correct device.
Question 3: What is the Path header used for during SIP registration?
- To specify the file path of the UA's configuration
- To list DNS SRV records for the registrar domain
- To record intermediate proxy routes so that inbound requests traverse the same path (Correct answer)
- To indicate the network path quality between the UA and registrar
Correct answer: To record intermediate proxy routes so that inbound requests traverse the same path
The Path header (RFC 3327) allows edge proxies to insert themselves into the routing path for subsequent inbound requests to the registered UA.
Question 4: Which RFC is the primary specification defining the SIP REGISTER method and registration process?
- RFC 2543
- RFC 3261 (Correct answer)
- RFC 3327
- RFC 5626
Correct answer: RFC 3261
RFC 3261 is the core SIP specification that defines the REGISTER method, registrar behavior, binding management, and authentication flow.
Question 5: What does 'soft state' mean in the context of SIP registration?
- Registration data is stored in RAM and lost on reboot
- The registration binding expires automatically and must be periodically refreshed by the UA (Correct answer)
- The registrar can modify bindings without UA consent
- Authentication credentials are cached softly without re-verification
Correct answer: The registration binding expires automatically and must be periodically refreshed by the UA
Soft state means the binding has a finite lifetime (Expires) and disappears unless the UA actively refreshes it before expiry.
Question 6: What is a GRUU (Globally Routable UA URI) and how does it relate to SIP registration?
- A special registrar URI used for load balancing across multiple servers
- A globally unique, routable URI assigned to a specific UA instance, obtained during registration (Correct answer)
- A URI format that replaces the AOR in all SIP requests
- A group routing URI that forks calls to all registered contacts simultaneously
Correct answer: A globally unique, routable URI assigned to a specific UA instance, obtained during registration
Defined in RFC 5627, a GRUU is a URI that routes to a specific UA device instance, obtained from the registrar during registration via the '+sip.instance' feature tag.
Question 7: What is the recommended timing for a UA to send a registration refresh relative to its binding expiry?
- Exactly when the Expires timer reaches zero
- Immediately after receiving the 200 OK for the initial registration
- Before the Expires timer runs out — typically around half the expiry interval (Correct answer)
- Only after the registrar sends a re-REGISTER request to the UA
Correct answer: Before the Expires timer runs out — typically around half the expiry interval
RFC 3261 recommends re-registering well before expiry (commonly at half the expiry interval) to account for network delays and retransmissions.
SSCA Registration Process 5
What is third-party registration in SIP?