ITU X.225 (ISO 8327)

ITU X.225 (also known as ISO 8327) is a connection-oriented session layer protocol in the Open Systems Interconnection (OSI) model.  The ITU X.225 protocol specification is a recommendation of the International Telecommunication Union (ITU).

The session layer is layer five in the seven-layer telecommunications model. Session layer protocols provide services for coordinating communication between local and remote applications, establishing, managing and terminating connections. ITU X.225 and other session layer protocols use suspend/resume or checkpoint/rollback capabilities to ensure synchronization of things such as audio and video. Session layer protocols also manage connection security. 

Other protocols that may be used in this layer include AppleTalk RPC Session Control Protocol (SCP) and Zone Information Protocol (ZIP).