Inside the 200 SIP response, which SIP header includes the S-CSCF address stored inside it?

Interview
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Q. Inside the 200 SIP response, which SIP header includes the S-CSCF address stored inside it?

a) Path Header
b) Service Route Header
c) Contact Header
d) Via Header

Check 🔎

:arrow_right: Correct answer: Letter b.

The Service-Route header is inserted by the S-CSCF in the 200 OK response to a REGISTER request. It contains the S-CSCF’s own address, allowing the UE to route subsequent SIP requests (e.g., INVITE, MESSAGE) via the same S-CSCF that handled the registration.

Q. Inside the 200 SIP response, which SIP header includes the S-CSCF address stored inside it?

a) :x: Path Header
→ Incorrect. The Path header is included in the REGISTER request, not the response, and it contains the P-CSCF’s address, used for routing toward the UE.

b) :white_check_mark: Service Route Header
→ Correct. This header appears in the 200 OK of the REGISTER response and stores the S-CSCF’s URI, ensuring proper routing of future SIP requests from the UE.

c) :x: Contact Header
→ Incorrect. The Contact header holds the UE’s SIP URI or IP address, not the S-CSCF’s.

d) :x: Via Header
→ Incorrect. The Via header tracks SIP message traversal, showing which proxies or UAs the message has passed through, not specifically storing the S-CSCF address.