 the Authorization header is also derived from the IMSI private identity. The Contact header contains the parameter + g.3gpp.ics indicating that the MSC server entity implements the ICS service. REGISTER sip:ics.mnc01.mcc208.3gppnetwork.org SIP/2.0 From: <sip:20810999999999@ics.mnc01.mcc208.3gppnetwork.org>; tag=4fa3 <sip:20810999999999@ics.mnc01.mcc208.3gppnetwork.org: Contact: <sip:[5555::aaa:bbb:ccc:ddd]>;expires=600000; +g.3gpp.ics="server" Authorization: Digest username=" 20810999999999@ics.mnc01.mcc208.3gppnetwork.org ", realm=" ics.mnc01.mcc208.3gppnetwork.org ", nonce="", integrity-protected="auth-done", uri="sip: ics.mnc01.mcc208.3gppnetwork.org ", response="" 2) The I-CSCF entity sends to the HSS entity the message DIAMETER user-authorization-request (UAR) to retrieve the list of the S-CSCF entities that can be assigned to the UA entity. 3) The I-CSCF entity performs the selection of an S-CSCF entity to which it forwards the REGISTER request, from the list of the S-CSCF entities received in the message DIAMETER user-authorization-answer (UAA). 4) The I-CSCF entity replaces the initial URI identity (sip: ics.mnc01.mcc208.3gppnetwork.org) by that of the S-CSCF entity (sip:scscf.HomeA.net or sip:scscf.HomeB.net) and transmits the SIP REGISTER request to the selected S-CSCF entity. VoLTE and ViLTE I-CSCF S-CSCF Server Attachment procedure SIP REGISTER DIAMETER UAR DIAMETER UAA SIP REGISTER DIAMETER SAR DIAMETER SAA SIP 200 OK SIP 200 OK SIP REGISTER SIP 200 OK Subscription Subscription Figure 9.4. Mobile registration to IMS network with ICS function 5) As the mobile has been authenticated by the MSC server entity, the S-CSCF entity sends to the HSS entity the message DIAMETER server- assignment-request (SAR) to retrieve the mobile profile. 6) The HSS entity transmits the profile of the mobile in the message DIAMETER server-assignment-answer (SAA). 7), 8) The SIP 200 OK response follows the reverse route taken by the REGISTER request. 9) The S-CSCF entity registers the mobile to the SCC AS entity. 10) The SC