SSCA Proxy Servers 4

Question 1

When a SIP proxy detects a routing loop, which response code should it return?

Accepted Answer

482 Loop Detected

Answer

A proxy returns 482 Loop Detected when it finds its own address in the Via header stack, indicating the request has looped back.

Question 2

What is the purpose of the 'branch' parameter in the Via header added by a SIP proxy?

Accepted Answer

It identifies the transaction and must start with the magic cookie 'z9hG4bK'

Answer

The branch parameter uniquely identifies a transaction and RFC 3261 requires compliant implementations to begin it with the magic cookie 'z9hG4bK'.

Question 3

How does a SIP proxy using strict routing (RFC 2543 behavior) differ in handling the Request-URI compared to loose routing?

Accepted Answer

Strict routing requires the proxy's address to be in the Request-URI, which the proxy replaces with the next Route header

Answer

In strict routing, the proxy must be the Request-URI target; it pops itself from the route set and places the next route entry as the new Request-URI.

Question 4

Which SIP method is a proxy NOT permitted to fork by default, according to RFC 3261?

Accepted Answer

REGISTER

Answer

REGISTER requests should not be forked by a proxy because they are intended for a specific registrar, not multiple destinations.

Question 5

What information does a SIP proxy use from the location service (registrar database) when routing an incoming request?

Accepted Answer

The registered Contact URI(s) mapped to the Address of Record (AOR)

Answer

The proxy queries the location service using the AOR from the Request-URI to retrieve the registered Contact URI(s) for the actual device(s) to forward the request to.

(SSCA) Sip School Certified Associate Practice Test

(SSCA) Sip School Certified Associate Practice Test

SSCA Proxy Servers 4