SSCA Proxy Servers 3 — Questions and Answers
Question 1: In the SIP trapezoid model, how many proxies are typically involved in a call between two different domains?
- One
- Two (Correct answer)
- Three
- Four
Correct answer: Two
The SIP trapezoid involves two proxies — one in the caller's domain (outbound proxy) and one in the callee's domain (inbound proxy).
Question 2: What does a SIP proxy do when it performs 'sequential forking'?
- It tries each destination in order, waiting for a failure response before trying the next (Correct answer)
- It sends the request to all destinations simultaneously
- It modifies the SDP for each branch separately
- It caches the response for future identical requests
Correct answer: It tries each destination in order, waiting for a failure response before trying the next
Sequential forking tries potential destinations one at a time, moving to the next only after receiving a failure or timeout from the current attempt.
Question 3: Which DNS record type does a SIP proxy typically query first to resolve the next-hop server for a SIP URI?
- A record
- MX record
- NAPTR record (Correct answer)
- PTR record
Correct answer: NAPTR record
A SIP proxy queries NAPTR (Naming Authority Pointer) records first to determine the transport protocol and then uses SRV and A records to find the server address.
Question 4: What happens to the Via header when a SIP proxy forwards a request?
- The proxy removes all existing Via headers
- The proxy adds its own Via header on top of the existing ones (Correct answer)
- The proxy replaces the topmost Via with its own
- The proxy moves the Via header to the bottom of the stack
Correct answer: The proxy adds its own Via header on top of the existing ones
Each forwarding proxy pushes its own Via header to the top of the Via header stack so responses can be routed back along the same path.
Question 5: What is a B2BUA (Back-to-Back User Agent) and how does it differ from a standard SIP proxy?
- A B2BUA terminates and re-originates SIP dialogs, maintaining full control, unlike a proxy which only forwards (Correct answer)
- A B2BUA only supports TCP while a proxy supports all transports
- A B2BUA cannot handle REGISTER requests while a proxy can
- A B2BUA uses SRTP exclusively while a proxy uses RTP
Correct answer: A B2BUA terminates and re-originates SIP dialogs, maintaining full control, unlike a proxy which only forwards
A B2BUA acts as both a UAS and UAC, terminating the incoming dialog and creating a new outgoing dialog, giving it full call control unlike a proxy which merely forwards.
Question 6: Which SIP response code does a proxy return when the Max-Forwards header value reaches zero?
- 480 Temporarily Unavailable
- 482 Loop Detected
- 483 Too Many Hops (Correct answer)
- 484 Address Incomplete
Correct answer: 483 Too Many Hops
When Max-Forwards reaches 0, the proxy returns 483 Too Many Hops to prevent infinite routing loops.
Question 7: In SIP, what is the role of an 'outbound proxy' for a SIP UA?
- It registers the UA's AOR in the location database
- It receives all outgoing requests from the UA and routes them toward the destination (Correct answer)
- It terminates media streams on behalf of the UA
- It handles only REGISTER requests from the UA
Correct answer: It receives all outgoing requests from the UA and routes them toward the destination
An outbound proxy receives all outgoing SIP requests from the UA and is responsible for routing them toward the appropriate destination.
SSCA Proxy Servers 3
In the SIP trapezoid model, how many proxies are typically involved in a call between two different domains?