Interoperability Between Machine-to-Machine Communication System and IP Multimedia Subsystem - Interoperability and Open-Source Solutions for the Internet of Things - page 111

Fig. 6. M2M to IMS video service
10 The S-CSCF requests an Authentication Vector (AV) from the HSS to be
used to challenge the user as part of the authentication procedure. The
Cx MULTIMEDIA AUTHENTICATION REQUEST (MAR) contains
the public and private user address, as well as the selected S-CSCF
address.
11 The HSS stores the S-CSCF address and returns an AV (including nonce
value) in the Cx MULTIMEDIA AUTHENTICATION ANSWER (MAA).
12-14 The S-CSCF invokes an Authentication Challenge by returning 401
UNAUHORISED towards the UE-A. The WWW-Authenticate header
of the message includes the nonce.
15 Based on the received information, the UE computes the response and
returns this value as well as the nonce in a new REGISTER request sent
to the P-CSCF.
16 The P-CSCF proxies the REGISTER to the selected I-CSCF.
17 The I-CSCF sends a Cx USER AUTHORISATION REQUEST (UAR)
(Diameter) containing the user's private and public address to HSS.
18 The HSS returns (UAA) the S-CSCF URI that was previously received
from the S-CSCF (6).
19 The REGISTER request is forwarded from the I-CSCF to the S-CSCF.
20 The S-CSCF Sends the Server Assignment Request to HSS to notify that
it is serving the user and to request user data.
21 The HSS sends the Server Assignment Answer to S-CSCF. HSS stores
the name of the S-CSCF and provides the user data to the S-CSCF.