SSCA SIP Protocols & Call Flow Fundamentals 5 — Questions and Answers
Question 1: In SIP, which message flow completes the three-way handshake for an INVITE transaction?
- INVITE → 200 OK → ACK (Correct answer)
- INVITE → 180 → 200 OK
- INVITE → ACK → 200 OK
- INVITE → TRYING → ACK
Correct answer: INVITE → 200 OK → ACK
The SIP INVITE transaction uses a three-way handshake: INVITE from caller, 200 OK from callee, and ACK from caller.
Question 2: What SIP header is used by a UAC to indicate the address where it wants to receive future in-dialog requests?
- Contact (Correct answer)
- To
- From
- Route
Correct answer: Contact
The Contact header provides a direct SIP URI for future requests within the dialog, bypassing the registrar.
Question 3: Which SIP response class indicates server-side errors?
- 5xx (Correct answer)
- 4xx
- 6xx
- 3xx
Correct answer: 5xx
5xx responses indicate that the server failed to fulfill an apparently valid request due to a server-side error.
Question 4: In SIP presence, which method does a watcher use to receive real-time state updates from a presentity?
- SUBSCRIBE/NOTIFY (Correct answer)
- REGISTER/NOTIFY
- OPTIONS/INFO
- MESSAGE/INFO
Correct answer: SUBSCRIBE/NOTIFY
A watcher sends SUBSCRIBE to the presence agent, which then sends NOTIFY messages whenever the presentity's state changes.
Question 5: What does the SIP 'timer B' control in the INVITE transaction?
- The time a client waits for a final response before considering the transaction failed (Correct answer)
- The interval between retransmissions of the INVITE
- The time to wait for an ACK after sending 200 OK
- The registration expiry timer
Correct answer: The time a client waits for a final response before considering the transaction failed
Timer B is the INVITE transaction timeout — if no final response arrives before Timer B fires, the client considers the transaction failed.
Question 6: Which SIP header field allows a UAC to specify a preferred language for the response?
- Accept-Language (Correct answer)
- Content-Language
- Language
- Preferred-Language
Correct answer: Accept-Language
Accept-Language is used by the UAC to indicate its preferred language for reason phrases and warning text in responses.
Question 7: In SIP, what distinguishes a stateful proxy from a stateless proxy?
- A stateful proxy maintains transaction state and can perform forking and retransmission handling (Correct answer)
- A stateful proxy stores call recordings
- A stateless proxy requires authentication for every request
- A stateful proxy only forwards requests without modifying them
Correct answer: A stateful proxy maintains transaction state and can perform forking and retransmission handling
Stateful proxies maintain transaction state, enabling features like forking, aggregating responses, and handling retransmissions intelligently.
SSCA SIP Protocols & Call Flow Fundamentals 5
In SIP, which message flow completes the three-way handshake for an INVITE transaction?