SSCA Registration Process 2 — Questions and Answers
Question 1: Which SIP header in a REGISTER request identifies the Address-of-Record (AOR) being registered?
- Contact
- From
- To (Correct answer)
- P-Asserted-Identity
Correct answer: To
The To header identifies the AOR — the public address the UA wants to make reachable.
Question 2: What is the purpose of the Contact header in a SIP REGISTER request?
- To specify the registrar's address
- To list all active bindings on the server
- To provide the UA's actual device URI for routing incoming calls (Correct answer)
- To authenticate the user with the domain
Correct answer: To provide the UA's actual device URI for routing incoming calls
The Contact header carries the device-specific URI (e.g., sip:alice@192.168.1.5:5060) that the registrar will bind to the AOR.
Question 3: Which response code does a SIP registrar return upon successful registration?
- 202 Accepted
- 200 OK (Correct answer)
- 204 No Content
- 201 Created
Correct answer: 200 OK
A 200 OK response confirms the registration was accepted and includes the current binding list in Contact headers.
Question 4: What does the CSeq header contain in a SIP REGISTER request?
- A session identifier and the method name REGISTER (Correct answer)
- The expiration time for the binding
- The registrar's domain name
- A timestamp used for replay protection
Correct answer: A session identifier and the method name REGISTER
CSeq contains a monotonically increasing sequence number followed by the method name, e.g., 'CSeq: 1 REGISTER'.
Question 5: Which header uniquely identifies a SIP registration dialog across all REGISTER transactions from a UA?
- Via
- Call-ID (Correct answer)
- From-tag
- Contact
Correct answer: Call-ID
Call-ID combined with the From tag uniquely identifies a registration; the registrar uses Call-ID to detect retransmissions.
Question 6: In a REGISTER response, how does the registrar communicate the actual accepted binding duration to the UA?
- Via the Expires header in the response
- Via the Contact header's 'expires' parameter or the Expires header (Correct answer)
- Via a 183 Session Progress message
- Via the Warning header
Correct answer: Via the Contact header's 'expires' parameter or the Expires header
The registrar communicates accepted duration through the 'expires' parameter on each Contact header or the Expires header in the 200 OK.
Question 7: What is the significance of the Via header in a SIP REGISTER request?
- It specifies the AOR being registered
- It carries authentication credentials
- It indicates the transport and address for the registrar to send the response (Correct answer)
- It lists all known registrars in order of preference
Correct answer: It indicates the transport and address for the registrar to send the response
The Via header tells the registrar which transport (UDP/TCP/TLS) and address to use when sending the response back to the UA.
SSCA Registration Process 2
Which SIP header in a REGISTER request identifies the Address-of-Record (AOR) being registered?