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.