Which SIP header carries the user's IMSI during IMS registration?

Interview
1

Q. Which SIP header carries the user’s IMSI during IMS registration?

a) Via
b) Contact
c) Route
d) Authorization

Check 🔎

:arrow_right: Correct answer: Letter d.

During IMS registration, the user’s IMSI (typically formatted as the private identity — IMSI@mncXXX.mccYYY.3gppnetwork.org) is embedded in the Authorization header as part of the SIP REGISTER request. This header is used for digest authentication in the IMS AKA (Authentication and Key Agreement) procedure.

Though the IMSI itself is not explicitly shown in clear text, it is represented in the private identity and carried in the Authorization header when responding to a 401 Unauthorized challenge from the IMS core.

Option Analysis:

a) :x: Via
→ Incorrect. It is used for routing SIP messages back through the network and does not include user-specific identity information.

b) :x: Contact
→ Incorrect. It contains the URI where the user agent can be reached, not authentication credentials or the IMSI.

c) :x: Route
→ Incorrect. This header is used for routing SIP messages to specific proxy servers; it does not contain user identity.

d) :white_check_mark: Authorization
→ Correct. This header carries the user’s private identity (IMSI-based) credentials used for authentication during IMS registration.