e: sec-agree Proxy-Require: sec-agree 2) The P-CSCF entity transfers the REGISTER message to the I-CSCF (Interrogating-CSCF) entity, including its identity in the header Path. The P-CSCF entity can find the IP address of the I-CSCF entity by doing a domain name system (DNS) resolution on the basis of the domain name of Alice's UA entity. 78 VoLTE and ViLTE The P-CSCF entity provides the type of mobile network and the identity of the cell in the header P-Access-Network-Info. This information is provided by the PCRF entity. The P-CSCF inserts the header Require containing the value path to ensure that the S-CSCF (Serving-CSCF) entity will take account of the header Path. If the S-CSCF entity does not support this header, it sends back a response 420 Bad extension. The P-CSCF entity declares in the parameter integrity-protected of the header Authorization that security has not been established with the UA entity. The P-CSCF entity removes the headers Require and Proxy - Require which contained the sec-agree because IPSec security will be implemented by the P-CSCF entity. REGISTER sip:ims.mnc01.mcc208.3gppnetwork.org SIP/2.0 Path: <sip:pcscf@homeA.net;lr> Require: path Authorization: Digest sername="alice_private@homeA.net" realm=" ims.mnc01.mcc208.3gppnetwork.org",nonce=" , uri="sip:i ims.mnc01.mcc208.3gppnetwork.org ", response="", integrity-protected="no" 3) I-CSCF entity transmits to the HSS entity the DIAMETER UAR (User-Authorization-Request) message to retrieve the list of S-CSCF entities that can be assigned to the UA entity. 4) I-CSCF entity performs the selection of an S-CSCF entity to which it forwards the REGISTER request from the list of S-CSCF entities received in the DIAMETER User-Authorization-Answer (UAA) message. 5) The I-CSCF entity replaces the initial URI (sip:ims mnc01.mcc208.3gppnetwork.org) with that of the S-CSCF (sip:scscf.homeA.net)\: and sends the REGISTER request to the S- CSCF entity selected. REGISTER sip:scscf.homeA.net SIP/2.0 Basic Procedures 6) The S-CSCF entity transmits to the HSS 