 the SDP offer (a=curr:qos local none, a=curr:qos remote none) that the resource reservation has not been established for the local and remote UA entities. Alice's UA entity indicates in the SDP offer (a=des:qos mandatory local sendrecv) that the precondition of resource reservation is mandatory for the local UA entity. Alice's UA entity indicates in the SDP offer (a=des:qos none remote sendrecv) that the precondition of resource reservation is unknown for the remote UA entity. INVITE sip:bob@homeB.net SIP/2.0 Require: precondition, sec-agree Proxy-Require: sec-agree Supported: 100rel Allow: INVITE, ACK, CANCEL, BYE, PRACK, UPDATE, REFER, MESSAGE Content-Type: application/sdp Content-Length : (...) a=curr:qos local none a=curr:qos remote none a=des:qos mandatory local sendrecv a=des:qos none remote sendrecv 2) The P-CSCF entity responds to the mobile with 100 Trying message that allows for blocking the retransmission timer of the INVITE request. 3) The P-CSCF entity removes the header Route containing its own identity. VoLTE and ViLTE The P-CSCF entity replaces the header P-Preferred-Identity - with the header -Asserted-Identity - containing the asserted URI of Alice. The P-CSCF entity adds the headers Via and Record Route containing its own identity. The header Record Route enables us to construct the route to be taken by subsequent requests. The P-CSCF entity forwards the request to the S-CSCF entity whose identity is contained in the header Route. 4) The S-CSCF entity responds to P-CSCF entity with 100 Trying message that allows for blocking the retransmission timer of the INVITE request. 5) The S-CSCF entity removes the header Route containing its own identity. The S-CSCF entity adds the headers Via and Record Route containing its own identity. The HSS entity has provided, during registration, the S-CSCF entity with the user profile, containing the types of media authorized by the service offered. The S-CSCF entity examines the information contained in the SDP message carried by the INVITE request. If the S-C