SSCA Proxy Servers 3

Question 1

In the SIP trapezoid model, how many proxies are typically involved in a call between two different domains?

Accepted Answer

Two

Answer

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'?

Accepted Answer

It tries each destination in order, waiting for a failure response before trying the next

Answer

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?

Accepted Answer

NAPTR record

Answer

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?

Accepted Answer

The proxy adds its own Via header on top of the existing ones

Answer

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?

Accepted Answer

A B2BUA terminates and re-originates SIP dialogs, maintaining full control, unlike a proxy which only forwards

Answer

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.

(SSCA) Sip School Certified Associate Practice Test

(SSCA) Sip School Certified Associate Practice Test

SSCA Proxy Servers 3