The present document defines a transport protocol for use in the IP multimedia (IM) Core Network (CN) subsystem based on Diameter. The present document is applicable to: The Cx interface between the I-CSCF/S-CSCF and the HSS. Whenever it is possible this document specifies the requirements for this protocol by reference to specifications produced by the IETF within the scope of Diameter. Where this is not possible@ extensions to Diameter are defined within this document.