SSCA Session Management 4 — Questions and Answers
Question 1: What is the significance of the 'tag' parameter in the From and To headers of a SIP session?
- It indicates the priority of the request
- It serves as part of the dialog identifier and differentiates multiple calls from the same UA (Correct answer)
- It carries the display name of the user
- It specifies the transport layer used for the session
Correct answer: It serves as part of the dialog identifier and differentiates multiple calls from the same UA
Tags are random strings added to From and To headers that, combined with the Call-ID, uniquely identify a dialog and distinguish parallel calls from the same address.
Question 2: Which SIP method is used to query the capabilities of a remote UA without establishing a session?
- REGISTER
- INFO
- OPTIONS (Correct answer)
- SUBSCRIBE
Correct answer: OPTIONS
The OPTIONS method allows a UA or proxy to query the capabilities (supported methods, codecs, extensions) of a remote UA without initiating a call.
Question 3: In SIP session management, what does the Record-Route header accomplish?
- It records the route taken by a REGISTER request
- It ensures that subsequent mid-dialog requests traverse the same proxy path (Correct answer)
- It lists codecs in order of preference for the session
- It identifies the user's location for emergency routing
Correct answer: It ensures that subsequent mid-dialog requests traverse the same proxy path
Proxies insert themselves into the Record-Route header so that subsequent in-dialog requests (like re-INVITEs and BYEs) continue to be routed through them.
Question 4: What is the function of the Route header in a SIP request sent within an established dialog?
- It specifies the codec route from sender to receiver
- It contains the list of proxies the request must traverse, built from the Record-Route header (Correct answer)
- It identifies the user's home domain for authentication
- It sets the path for media streams between endpoints
Correct answer: It contains the list of proxies the request must traverse, built from the Record-Route header
The Route header is populated from the stored route set (derived from Record-Route headers) and directs mid-dialog requests through the previously established proxy path.
Question 5: A SIP UA sends a BYE with no prior INVITE having been answered. What is the likely outcome?
- The proxy creates a new session using the BYE parameters
- The remote UA responds with 481 Call/Transaction Does Not Exist (Correct answer)
- The BYE is treated as a CANCEL and processed accordingly
- The session is put on hold until a new INVITE arrives
Correct answer: The remote UA responds with 481 Call/Transaction Does Not Exist
A BYE sent outside of an established dialog will receive a 481 Call/Transaction Does Not Exist response because no matching dialog is found.
Question 6: Which SIP header indicates the maximum number of forwards (proxy hops) a request is allowed to traverse?
- Max-Breadth
- Via count
- Max-Forwards (Correct answer)
- Path-Limit
Correct answer: Max-Forwards
The Max-Forwards header is decremented by each proxy and server; if it reaches zero before reaching the destination, a 483 Too Many Hops response is returned.
Question 7: What does the SIP 'Contact' header convey within a session dialog?
- The billing address for the session
- The direct URI where the UA can be reached for subsequent in-dialog requests (Correct answer)
- The codec preferences in priority order
- The proxy server address for future registrations
Correct answer: The direct URI where the UA can be reached for subsequent in-dialog requests
The Contact header provides the direct SIP URI of the UA, allowing subsequent in-dialog requests to be sent directly to the UA rather than through the registrar.
SSCA Session Management 4
What is the significance of the 'tag' parameter in the From and To headers of a SIP session?