SSCA SIP Troubleshooting & Diagnostic Tools 4 — Questions and Answers
Question 1: Which SIP method is used specifically to query a SIP server's capabilities without initiating a call?
- INFO
- NOTIFY
- OPTIONS (Correct answer)
- SUBSCRIBE
Correct answer: OPTIONS
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?
- The call was successfully forked to multiple endpoints
- The UAS is returning 180 Ringing responses
- No response has been received and UDP retransmission timer T1 is firing (Correct answer)
- The proxy is performing load balancing
Correct answer: No response has been received and UDP retransmission timer T1 is firing
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?
- The codec negotiated during the call
- The path the request traveled through proxies (Correct answer)
- The registration expiry time
- The authentication realm
Correct answer: The path the request traveled through proxies
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?
- The UAC sent a malformed INVITE
- The downstream gateway or server returned an invalid response (Correct answer)
- The proxy's registration database is offline
- The TLS certificate is expired
Correct answer: The downstream gateway or server returned an invalid response
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?
- To header
- Contact header and SDP 'c=' (connection) line (Correct answer)
- Call-ID header
- CSeq header
Correct answer: Contact header and SDP 'c=' (connection) line
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.
Question 6: Which SIP response class (1xx–6xx) indicates a definitive failure that will not be retried automatically?
- 1xx Provisional
- 2xx Success
- 3xx Redirection
- 6xx Global Failure (Correct answer)
Correct answer: 6xx Global Failure
6xx Global Failure responses indicate that the request has definitively failed for all possible destinations and should not be retried.
Question 7: In Asterisk SIP debugging, which CLI command enables verbose SIP message logging to the console?
- sip set debug on (Correct answer)
- sip enable logging
- debug sip verbose
- set verbose sip
Correct answer: sip set debug on
The Asterisk CLI command 'sip set debug on' enables full SIP packet logging to the Asterisk console for troubleshooting.
SSCA SIP Troubleshooting & Diagnostic Tools 4
Which SIP method is used specifically to query a SIP server's capabilities without initiating a call?