SSCA SIP Protocols & Call Flow Fundamentals 3 — Questions and Answers
Question 1: What is the function of the Record-Route header in SIP?
- To keep a proxy in the signaling path for subsequent requests (Correct answer)
- To record the audio route for media
- To log the call for billing purposes
- To specify the return path for responses
Correct answer: To keep a proxy in the signaling path for subsequent requests
Record-Route is inserted by proxies that want to remain in the signaling path for all subsequent in-dialog requests.
Question 2: In SIP digest authentication, what is the purpose of the 'nonce' value?
- To prevent replay attacks by providing a unique challenge each time (Correct answer)
- To encrypt the password
- To identify the authentication realm
- To specify the hashing algorithm
Correct answer: To prevent replay attacks by providing a unique challenge each time
The nonce is a server-generated unique value included in the WWW-Authenticate challenge to prevent replay attacks.
Question 3: Which SIP header field is used to specify the maximum number of times a request can be forwarded by proxies?
- Max-Forwards (Correct answer)
- Via
- Route
- Hop-Count
Correct answer: Max-Forwards
Max-Forwards limits the number of proxy hops; each proxy decrements it by one and rejects the request if it reaches zero.
Question 4: In SIP call flow, which message confirms that ringing has been answered and the session is established?
- 200 OK to INVITE (Correct answer)
- ACK
- 183 Session Progress
- 180 Ringing
Correct answer: 200 OK to INVITE
200 OK to INVITE signals that the called party has accepted the call, establishing the dialog.
Question 5: What does the SIP REFER method primarily accomplish?
- Instructs the recipient to contact a third party, enabling call transfer (Correct answer)
- Refers to a call record in a database
- Registers a SIP endpoint with a proxy
- Refreshes an active registration
Correct answer: Instructs the recipient to contact a third party, enabling call transfer
REFER is used to implement call transfer by telling the recipient to send a request (usually INVITE) to the URI in the Refer-To header.
Question 6: Which SIP dialog identifier is formed by the combination of Call-ID, local tag, and remote tag?
- Dialog ID (Correct answer)
- Transaction ID
- Session ID
- Branch ID
Correct answer: Dialog ID
A SIP dialog is uniquely identified by the Call-ID, the local tag (in From), and the remote tag (in To).
Question 7: In SIP, what is 'forking' as performed by a proxy server?
- Forwarding an INVITE to multiple endpoints simultaneously or sequentially (Correct answer)
- Splitting the media stream to multiple destinations
- Branching the SDP offer to multiple codecs
- Dividing billing between two parties
Correct answer: Forwarding an INVITE to multiple endpoints simultaneously or sequentially
Forking allows a proxy to send an incoming INVITE to multiple registered contact addresses for the same AOR.
SSCA SIP Protocols & Call Flow Fundamentals 3
What is the function of the Record-Route header in SIP?