SIP Protocol Fundamentals 3 — Questions and Answers
Question 1: What SIP response code indicates that the callee's phone is ringing?
- 100 Trying
- 180 Ringing (Correct answer)
- 181 Call Is Being Forwarded
- 183 Session Progress
Correct answer: 180 Ringing
180 Ringing is sent by the UAS to inform the UAC that the called party's device is alerting the user.
Question 2: Which SIP method is used to query the capabilities of a SIP server or UA?
- INFO
- OPTIONS (Correct answer)
- PING
- QUERY
Correct answer: OPTIONS
OPTIONS allows a UAC to query a server or UA about its supported methods, extensions, codecs, and other capabilities.
Question 3: What is the function of the SIP 'CSeq' header?
- Identifies the call session globally
- Orders requests within a dialog and names the method (Correct answer)
- Specifies codec sequence preference
- Controls the hop count of messages
Correct answer: Orders requests within a dialog and names the method
CSeq contains a sequence number and method name, used to order transactions and detect retransmissions within a dialog.
Question 4: Which transport protocol is preferred for SIP in environments requiring reliability and large messages?
- UDP
- TCP (Correct answer)
- SCTP
- ICMP
Correct answer: TCP
TCP is preferred when reliability is needed or when SIP messages exceed the MTU size, though UDP is more common for basic deployments.
Question 5: What does a SIP 302 Moved Temporarily response instruct the UAC to do?
- Retry the request after a delay
- Resend the request to an alternate URI provided in the Contact header (Correct answer)
- Authenticate with the proxy
- Terminate the session immediately
Correct answer: Resend the request to an alternate URI provided in the Contact header
302 Moved Temporarily provides an alternate Contact URI and instructs the UAC to retry the request at that new address.
Question 6: In SIP, what is an 'address-of-record' (AOR)?
- The IP address of the UA's NIC
- A SIP URI that serves as a public address for a user (Correct answer)
- The MAC address bound to a SIP phone
- The FQDN of the SIP proxy
Correct answer: A SIP URI that serves as a public address for a user
An AOR is a SIP URI (e.g., sip:alice@example.com) that represents a user's persistent public identity, independent of their device location.
Question 7: Which SIP header limits the number of proxies that can forward a request?
- Max-Hops
- Max-Forwards (Correct answer)
- TTL
- Route-Limit
Correct answer: Max-Forwards
The Max-Forwards header starts at a value (typically 70) and is decremented by each proxy; reaching 0 causes a 483 Too Many Hops response.
SIP Protocol Fundamentals 3
What SIP response code indicates that the callee's phone is ringing?