SSCA SIP Troubleshooting & Diagnostic Tools 4

Question 1

Which SIP method is used specifically to query a SIP server's capabilities without initiating a call?

Accepted Answer

OPTIONS

Answer

The OPTIONS method is used to query a SIP UA or proxy for its supported methods, codecs, and extensions.

Question 2

In a PCAP trace, you notice SIP INVITEs being retransmitted every 500ms. What does this indicate?

Accepted Answer

No response has been received and UDP retransmission timer T1 is firing

Answer

UDP SIP transactions use timer T1 (default 500ms) for retransmissions when no provisional or final response is received.

Question 3

What does the 'Via' header in a SIP response reveal during troubleshooting?

Accepted Answer

The path the request traveled through proxies

Answer

Each proxy adds a Via header entry on the request, and responses are routed back following the Via stack, making it a hop-by-hop path record.

Question 4

A call to a PSTN number fails with '502 Bad Gateway'. From the SIP proxy's perspective, what occurred?

Accepted Answer

The downstream gateway or server returned an invalid response

Answer

502 Bad Gateway means the proxy received an invalid or unexpected response from the upstream server or gateway it contacted.

Question 5

When troubleshooting NAT traversal for SIP, which header field in the INVITE is most critical to inspect for correct IP addressing?

Accepted Answer

Contact header and SDP 'c=' (connection) line

Answer

The Contact header must contain a routable IP, and the SDP 'c=' line must have the correct media IP; private IPs here cause NAT traversal failures.

(SSCA) Sip School Certified Associate Practice Test