SSCA SIP Protocols & Call Flow Fundamentals 4 — Questions and Answers
Question 1: What SIP response code indicates that the server understood the request but is refusing to fulfill it, without the client being able to change this by re-sending?
- 403 Forbidden (Correct answer)
- 401 Unauthorized
- 407 Proxy Authentication Required
- 480 Temporarily Unavailable
Correct answer: 403 Forbidden
403 Forbidden means the server understood the request but will not authorize it, regardless of credentials.
Question 2: In SIP, the To header tag is added by whom and when?
- The UAS adds it in the response to establish a dialog (Correct answer)
- The UAC adds it in the initial INVITE
- The proxy adds it when forking
- The registrar adds it during registration
Correct answer: The UAS adds it in the response to establish a dialog
The UAS (called party) adds the 'tag' parameter to the To header in its response, which together with the From tag creates the dialog.
Question 3: Which SIP method is used to end a subscription established with SUBSCRIBE?
- SUBSCRIBE with Expires: 0 (Correct answer)
- UNSUBSCRIBE
- CANCEL
- BYE
Correct answer: SUBSCRIBE with Expires: 0
A subscription is terminated by sending a SUBSCRIBE request with an Expires header value of 0.
Question 4: What is the role of the P-Asserted-Identity header in SIP?
- To convey the verified identity of the caller within a trusted network (Correct answer)
- To assert the called party's identity
- To encrypt the From header
- To specify the billing identity
Correct answer: To convey the verified identity of the caller within a trusted network
P-Asserted-Identity carries the authenticated identity of the originating user as determined by a trusted network element.
Question 5: In SIP, what happens when a proxy receives a request with Max-Forwards: 0?
- It returns a 483 Too Many Hops response (Correct answer)
- It forwards the request anyway with Max-Forwards: 0
- It drops the request silently
- It returns a 408 Request Timeout
Correct answer: It returns a 483 Too Many Hops response
When a proxy receives Max-Forwards: 0 and cannot process the request itself, it must return 483 Too Many Hops.
Question 6: Which component of a SIP URI identifies the user part versus the host part?
- The '@' symbol separates user from host (user@host) (Correct answer)
- The ':' colon separates them
- The ';' semicolon separates them
- The '?' question mark separates them
Correct answer: The '@' symbol separates user from host (user@host)
A SIP URI is structured as sip:user@host:port;parameters, where '@' separates the user portion from the domain/host.
Question 7: What is the purpose of the Supported header in SIP?
- To list the SIP extensions the UAC or UAS supports (Correct answer)
- To indicate which codecs are supported
- To specify supported transport protocols
- To list supported authentication methods
Correct answer: To list the SIP extensions the UAC or UAS supports
The Supported header lists option tags for SIP extensions that the sender supports, allowing negotiation of capabilities.
SSCA SIP Protocols & Call Flow Fundamentals 4
What SIP response code indicates that the server understood the request but is refusing to fulfill it, without the client being able to change this by re-sending?