SSCA SIP Headers 4 — Questions and Answers
Question 1: What is the role of the Referred-By header in SIP REFER?
- It identifies the party that initiated the transfer (Correct answer)
- It specifies the target URI of the transfer
- It carries authentication credentials
- It indicates the codec to use after transfer
Correct answer: It identifies the party that initiated the transfer
The Referred-By header identifies the SIP URI of the user who sent the REFER request, providing accountability in call transfer scenarios.
Question 2: Which header in a SIP response provides the address where the client should send future requests for the same user?
- Record-Route
- Via
- Contact (Correct answer)
- Service-Route
Correct answer: Contact
The Contact header in a response provides the direct URI of the UAS, used by the UAC to address subsequent in-dialog requests.
Question 3: What does the SIP History-Info header capture?
- A record of URI changes that occurred due to retargeting or forwarding (Correct answer)
- Call duration history for billing
- Previous authentication attempts
- Codec negotiation history
Correct answer: A record of URI changes that occurred due to retargeting or forwarding
History-Info records the sequence of Request-URIs used as a request was retargeted, enabling call forwarding history to be conveyed.
Question 4: Which SIP header is used to indicate that a response is final and terminates the transaction?
- Contact
- Via
- No header — the response code indicates finality (Correct answer)
- Expires
Correct answer: No header — the response code indicates finality
In SIP, a 2xx–6xx response code itself signals finality; no special header is needed to indicate the response is final.
Question 5: What does the Min-Expires header in a 423 response indicate?
- The minimum registration duration the registrar will accept (Correct answer)
- The minimum time before a retry is allowed
- The shortest call duration supported
- The minimum subscription interval
Correct answer: The minimum registration duration the registrar will accept
A registrar returns 423 Interval Too Brief with a Min-Expires header when the client's requested expiration is shorter than the registrar's minimum.
Question 6: Which header allows a SIP proxy to challenge a request for authentication on behalf of itself (not the UAS)?
- Authorization
- WWW-Authenticate
- Proxy-Authenticate (Correct answer)
- Proxy-Authorization
Correct answer: Proxy-Authenticate
Proxy-Authenticate is sent in a 407 Proxy Authentication Required response, allowing a proxy to challenge the client for credentials.
Question 7: What is the significance of the 'tag' parameter in the From and To headers?
- It uniquely identifies a dialog endpoint and differentiates multiple dialogs with the same Call-ID (Correct answer)
- It specifies the codec preference of that endpoint
- It carries the cryptographic hash for integrity
- It indicates the SIP version being used
Correct answer: It uniquely identifies a dialog endpoint and differentiates multiple dialogs with the same Call-ID
The tag parameter combined with Call-ID and the other endpoint's tag forms the unique dialog identifier in SIP.
SSCA SIP Headers 4
What is the role of the Referred-By header in SIP REFER?