SSCA SIP Troubleshooting & Diagnostic Tools 5 — Questions and Answers
Question 1: What does a '487 Request Terminated' response indicate in a SIP transaction?
- The call completed normally
- The INVITE was terminated by a CANCEL before it was answered (Correct answer)
- The server ran out of resources
- The codec negotiation failed
Correct answer: The INVITE was terminated by a CANCEL before it was answered
487 Request Terminated is the final response to an INVITE that was cancelled by the UAC using a CANCEL method.
Question 2: During SIP troubleshooting, the 'Record-Route' header is used to:
- Log all call durations to a billing system
- Force subsequent in-dialog requests through specific proxies (Correct answer)
- Specify the preferred audio codec order
- Authenticate the calling party
Correct answer: Force subsequent in-dialog requests through specific proxies
Record-Route causes proxies to insert themselves into the dialog path so that all in-dialog requests (like re-INVITEs and BYE) pass through them.
Question 3: Which tool can be used to simulate SIP load by generating thousands of simultaneous SIP calls for stress testing?
- sipsak
- SIPp (Correct answer)
- sngrep
- Wireshark
Correct answer: SIPp
SIPp is a performance testing tool that can simulate thousands of simultaneous SIP UAC/UAS scenarios with detailed statistics.
Question 4: A SIP re-INVITE is sent mid-call. Which scenario would NOT require a re-INVITE?
- Placing a call on hold
- Changing the media codec
- Requesting a new call-ID for the session (Correct answer)
- Adding a video stream to an audio call
Correct answer: Requesting a new call-ID for the session
The Call-ID identifies a dialog and cannot be changed mid-call; hold, codec changes, and stream additions all legitimately trigger re-INVITEs.
Question 5: When a SIP proxy returns '301 Moved Permanently', what action should the UAC take?
- Retry the request to the same address after a delay
- Send the request to the new address in the Contact header and update stored location (Correct answer)
- Authenticate using the WWW-Authenticate header
- Send a CANCEL to abort the transaction
Correct answer: Send the request to the new address in the Contact header and update stored location
301 Moved Permanently tells the UAC to redirect the request to the URI in the Contact header and permanently store the new address.
Question 6: What information does the 'Reason' header in a SIP BYE message provide during troubleshooting?
- The codec used during the call
- The cause code explaining why the call was terminated (Correct answer)
- The total call duration in seconds
- The media server IP address
Correct answer: The cause code explaining why the call was terminated
The Reason header carries a protocol and cause code (e.g., Q.850 cause 16) that explains why the call was released, aiding disconnect analysis.
Question 7: In SIP over TLS (SIPS), on which default port does signaling occur?
- 5060
- 5061 (Correct answer)
- 443
- 8443
Correct answer: 5061
SIPS (SIP over TLS) uses port 5061 by default, while plain SIP uses 5060.
SSCA SIP Troubleshooting & Diagnostic Tools 5
What does a '487 Request Terminated' response indicate in a SIP transaction?