If the HSS returns `DIAMETER_ERROR_USER_UNKNOWN` in UAA, what SIP response does the I-CSCF send to the UE?

Q. If the HSS returns DIAMETER_ERROR_USER_UNKNOWN in UAA, what SIP response does the I-CSCF send to the UE?

a) 480 Temporarily Unavailable
b) 403 Forbidden
c) 404 Not Found
d) 200 OK


Check 🔎

:arrow_right: Correct answer: Letter c.

When the HSS replies to the I-CSCF with DIAMETER_ERROR_USER_UNKNOWN in response to a User-Authorization-Request (UAR), it indicates that the public identity (IMPU) is not known to the IMS system.

In this case, the I-CSCF responds to the UE with a SIP 404 Not Found.

  • This maps directly from the Diameter error to SIP semantics.
  • It tells the UE that the requested user identity simply does not exist in the IMS domain.

Q. If the HSS returns DIAMETER_ERROR_USER_UNKNOWN in UAA, what SIP response does the I-CSCF send to the UE?

a) :x: 480 Temporarily Unavailable
→ Incorrect. This implies a temporary network failure, not a non-existent user.

b) :x: 403 Forbidden
→ Incorrect. This indicates unauthorized access, not that the user doesn’t exist.

c) :white_check_mark: 404 Not Found
→ Correct. 404 is returned when the IMPU is unknown in the HSS, as per DIAMETER_ERROR_USER_UNKNOWN. Unknown user leads to SIP 404 to the UE.

d) :x: 200 OK
→ Incorrect. This is a successful response, which is not valid when the user is unknown.