SSCA Registration Process 4 — Questions and Answers
Question 1: How does a UA de-register all of its active bindings at once from a registrar?
- Send REGISTER with Expires: 0 and a specific Contact URI
- Send REGISTER with Contact: * and Expires: 0 (Correct answer)
- Send a BYE request to the registrar
- Send REGISTER with no Contact header
Correct answer: Send REGISTER with Contact: * and Expires: 0
Using 'Contact: *' with 'Expires: 0' is the wildcard de-registration method defined in RFC 3261 that removes all bindings for the AOR.
Question 2: What is a SIP 'binding' as managed by a registrar?
- A TLS certificate linking the UA to the domain
- The association between an AOR and one or more Contact URIs with an expiration time (Correct answer)
- A persistent TCP connection between the UA and the registrar
- The authentication token for a registered UA
Correct answer: The association between an AOR and one or more Contact URIs with an expiration time
A binding maps an Address-of-Record to a device-specific Contact URI and has a lifetime controlled by the Expires value.
Question 3: Can a single SIP Address-of-Record (AOR) have multiple simultaneous bindings?
- No, only one Contact URI can be active at a time
- Yes, but only if they use different transports
- Yes, multiple Contact URIs with different q-values can be registered simultaneously (Correct answer)
- Yes, but only if all share the same Call-ID
Correct answer: Yes, multiple Contact URIs with different q-values can be registered simultaneously
RFC 3261 allows multiple bindings per AOR; the q-value parameter on each Contact indicates relative preference for routing.
Question 4: What does the 'q' parameter in a Contact header indicate during SIP registration?
- The quality of service level for that contact
- The queue position for routing
- The relative preference (priority) of that contact URI when multiple are registered (Correct answer)
- The number of queued registration attempts
Correct answer: The relative preference (priority) of that contact URI when multiple are registered
The q-value (0.0–1.0) indicates priority; higher q-values are tried first when the registrar or proxy forks requests to multiple bindings.
Question 5: What happens to a specific binding when a UA sends a REGISTER with Expires: 0 for that single Contact URI?
- All bindings for the AOR are removed
- Only that specific Contact URI binding is removed (Correct answer)
- The binding is suspended but not deleted
- The registrar ignores the request and keeps the binding active
Correct answer: Only that specific Contact URI binding is removed
Setting Expires: 0 for a specific Contact URI removes only that binding, leaving other bindings for the same AOR intact.
Question 6: What is the consequence if a UA fails to re-register before its binding expires?
- The registrar sends a 480 Temporarily Unavailable to the UA
- The binding is automatically renewed for another full period
- The binding is silently removed and the UA becomes unreachable via that AOR (Correct answer)
- The registrar sends a NOTIFY to the UA warning of expiry
Correct answer: The binding is silently removed and the UA becomes unreachable via that AOR
SIP registration uses soft state; if the UA does not refresh before expiry, the registrar discards the binding without notification.
Question 7: When a UA re-registers with an updated Contact URI for an existing AOR, what does the registrar do?
- It rejects the REGISTER with a 409 Conflict
- It replaces the old binding with the new Contact URI and updated expiry (Correct answer)
- It adds the new Contact URI as an additional binding alongside the old one
- It requires the UA to de-register first before accepting a new Contact
Correct answer: It replaces the old binding with the new Contact URI and updated expiry
If the same Call-ID and a higher CSeq are used with an existing Contact, the registrar updates (replaces) that binding with the new information.
SSCA Registration Process 4
How does a UA de-register all of its active bindings at once from a registrar?