- Route: Contains the P-CSCF address learnt during P-CSCF discovery, plus the elements from the Service-Route header from registration. The P-CSCF URI contains the port number learnt during the security agreement negotiation.
- Event: This field is populated with the value "reg" to specify the use of the registration state package.
- Accept: This field is populated with the value "application/reginfo+xml".
- Security-Verify: Contains the security agreement as represented by the previously received Security-Server header.