SIP Session Management 3 — Questions and Answers
Question 1: What happens if a SIP UAS receives a re-INVITE with an SDP offer it cannot support?
- It responds with 488 Not Acceptable Here (Correct answer)
- It responds with 404 Not Found
- It responds with 300 Multiple Choices
- It sends a BYE to terminate the session
Correct answer: It responds with 488 Not Acceptable Here
488 Not Acceptable Here indicates the SDP offer parameters (e.g., codecs) in the request are not acceptable to the UAS.
Question 2: Which SIP mechanism allows a UA to detect if a session is still active when no media is flowing?
- Session timers (RFC 4028) (Correct answer)
- Registration refresh
- PRACK retransmission
- 180 Ringing keepalive
Correct answer: Session timers (RFC 4028)
RFC 4028 session timers use periodic re-INVITEs or UPDATE requests to verify both sides are still alive.
Question 3: In SIP session timers, what does the 'refresher' parameter in the Session-Expires header indicate?
- Which UA is responsible for sending the refresh request (Correct answer)
- The maximum session duration in seconds
- The codec to use during refresh
- The proxy that handles the refresh
Correct answer: Which UA is responsible for sending the refresh request
The refresher=uac or refresher=uas parameter specifies which endpoint must send the periodic re-INVITE or UPDATE to keep the session alive.
Question 4: What SIP response is sent by a UAS when it receives a BYE request that references a dialog it cannot find?
- 481 Call/Transaction Does Not Exist (Correct answer)
- 404 Not Found
- 486 Busy Here
- 403 Forbidden
Correct answer: 481 Call/Transaction Does Not Exist
481 indicates the BYE references a call-ID or dialog that the UAS has no record of, often due to a crash or restart.
Question 5: During a SIP call transfer using REFER, which header carries the transfer target's URI?
- Refer-To (Correct answer)
- Contact
- P-Asserted-Identity
- Referred-By
Correct answer: Refer-To
The Refer-To header contains the URI of the transfer target that the referee should establish a new session with.
Question 6: What is the role of NOTIFY in a REFER-based call transfer?
- Report the status of the REFER subscription (e.g., 100 Trying, 200 OK) (Correct answer)
- Tear down the original session
- Negotiate new codecs for the transferred call
- Authenticate the transfer target
Correct answer: Report the status of the REFER subscription (e.g., 100 Trying, 200 OK)
After a REFER, NOTIFY messages carry sipfrag bodies reporting the progress of the new INVITE to the transfer target.
Question 7: Which condition causes SIP early dialogs to be created?
- Receipt of a provisional response (1xx) with a To tag (Correct answer)
- Receipt of a 200 OK with a From tag
- Sending a REGISTER request
- Receipt of a 4xx response
Correct answer: Receipt of a provisional response (1xx) with a To tag
When a provisional response like 180 Ringing includes a To tag, an early dialog is established before the session is confirmed.
SIP Session Management 3
What happens if a SIP UAS receives a re-INVITE with an SDP offer it cannot support?