R request. Alice's UA entity defines the format of the notifications of registration events in the header Accept (application/reginfo+xml) 26) The P-CSCF entity removes the headers Route containing its own identity and replaces the header P-Preferred-Identity with the header -Asserted-Identity containing Alice's asserted URI identity. Basic Procedures The P-CSCF adds the headers Via and Record Route containing its own identity. The header Record Route enables us to construct the route taken by subsequent requests. The S-CSCF authorizes the subscription with the response 200 OK because it trusts the content of the header P-Asserted-Identity inserted by the P-CSCF and because Alice has subscribed to the registration event service. 27), 28) The response 200 OK is routed through the addresses contained in the headers Via. 29) Receipt of the message 200 OK triggers a subscription on the part of the P-CSCF entity to discover the state of registration of Alice's UA entity. 30) The S-CSCF entity responds with 200 OK. 31) The 200 OK response to the REGISTER message triggers the SUBSCRIBE request from the TAS entity to have knowledge of the registration status of Alice's UA entity (or Bob's). 32) The S-CSCF entity responds with 200 OK. 33), 34) The S-CSCF entity notifies Alice's UA entity of the registration of its identities by sending a NOTIFY request. The S-CSCF reports the state of the subscription in the header Subscription-State (value active) and its duration in the parameter expires. The notification of registration elements is produced in an XML (eXtensible markup language) document of type reginfo attached to the SIP (session initiation protocol) message. 35), 36) The 200 OK response of the mobile is routed from addresses contained in the Via headers. 37) The S-CSCF entity notifies to the P-CSCF entity the registration status of Alice's UA entity (or Bob's) by sending a NOTIFY request. 38) The P-CSCF entity responds with 200 OK. VoLTE and ViLTE 39) The S-CSCF entity notifies to the TAS entity the registration sta