SSCA Session Management 2 — Questions and Answers
Question 1: Which SIP method is used to re-negotiate the parameters of an existing session without terminating it?
- re-INVITE (Correct answer)
- UPDATE
- NOTIFY
- REFER
Correct answer: re-INVITE
A re-INVITE is sent within an established dialog to modify session parameters such as codecs, media addresses, or to place a call on hold.
Question 2: What is the purpose of the UPDATE method in SIP session management?
- To terminate a session gracefully
- To modify session parameters before or during a session without changing the dialog state (Correct answer)
- To subscribe to event notifications
- To redirect a call to another URI
Correct answer: To modify session parameters before or during a session without changing the dialog state
The UPDATE method (RFC 3311) allows a UAC to modify session parameters mid-call without affecting the dialog state, unlike re-INVITE.
Question 3: When a SIP session is placed on hold using RFC 3264, what value is set in the SDP 'c=' line or 'a=' attribute?
- a=inactive
- a=sendonly
- c=0.0.0.0 or a=sendonly (Correct answer)
- a=recvonly
Correct answer: c=0.0.0.0 or a=sendonly
RFC 3264 specifies that hold is indicated by setting the connection address to 0.0.0.0 or by using the 'a=sendonly' attribute in the SDP offer.
Question 4: Which SIP header field uniquely identifies a dialog and is used to match subsequent requests to an established session?
- Call-ID alone
- From tag alone
- The combination of Call-ID, From tag, and To tag (Correct answer)
- The Contact header
Correct answer: The combination of Call-ID, From tag, and To tag
A SIP dialog is uniquely identified by the triple: Call-ID, the From tag, and the To tag, which together distinguish it from all other dialogs.
Question 5: What happens to a SIP session if a re-INVITE is rejected with a 488 Not Acceptable Here response?
- The session is immediately terminated
- The original session parameters remain in effect (Correct answer)
- The UA must send a BYE to re-establish the call
- The session is placed on hold automatically
Correct answer: The original session parameters remain in effect
If a re-INVITE is rejected, the original session parameters negotiated during the initial INVITE remain in effect and the session continues unchanged.
Question 6: What is the role of the CSeq header in SIP session management?
- It identifies the transport protocol used
- It orders transactions within a dialog and prevents duplicate request processing (Correct answer)
- It specifies the maximum number of hops a request can traverse
- It carries the session description payload type
Correct answer: It orders transactions within a dialog and prevents duplicate request processing
The CSeq header contains a sequence number and method name that orders requests within a dialog and allows UAs to detect and discard retransmitted or out-of-order requests.
Question 7: Which timer in RFC 4028 (Session Timers) triggers a re-INVITE or UPDATE to refresh a SIP session?
- Timer A
- Timer B
- Session-Expires header value (Correct answer)
- Timer T1
Correct answer: Session-Expires header value
RFC 4028 defines the Session-Expires header value as the session interval; when it expires without a refresh, the session is terminated.
SSCA Session Management 2
Which SIP method is used to re-negotiate the parameters of an existing session without terminating it?