Which SIP Response does the P-CSCF use to know the S-CSCF address the user registered on?

Interview
1

Q. Which SIP Response does the P-CSCF use to know the S-CSCF address the user registered on?

a) 183 Session Progress
b) 401 Unauthorized
c) 200 OK
d) 380 Alternative Services

Check 🔎

:arrow_right: Correct answer: Letter c.

During the IMS registration process, once the HSS selects and returns the S-CSCF to the I-CSCF, the REGISTER request is routed to the S-CSCF. After successful authentication and registration, the S-CSCF responds with a 200 OK, which is passed back to the P-CSCF.

This 200 OK response contains critical headers such as:

  • Path header: used by the P-CSCF to maintain the SIP signaling path.
  • Contact and Service-Route headers: these include the S-CSCF URI, informing the P-CSCF of the serving call session control function.

Q. Which SIP Response does the P-CSCF use to know the S-CSCF address the user registered on?

a) :x: 183 Session Progress
→ Incorrect. This is used for early media during session setup — not for registration.

b) :x: 401 Unauthorized
→ Incorrect. Used for SIP authentication challenges, not for learning S-CSCF addresses.

c) :white_check_mark: 200 OK
→ Correct. After successful SIP REGISTER, the S-CSCF returns a 200 OK, containing the Service-Route header that tells the P-CSCF which S-CSCF is serving the user.

d) :x: 380 Alternative Services
→ Incorrect. This response is used to indicate alternative services or routes, but not used in this context.