This Service Definition defines ACSE services for application-association control in an open systems interconnection environment. ACSE supports two modes of communication service: connection-oriented and connectionless. The ACSE connection-oriented service is provided by the use of the connection-oriented ACSE protocol (see ITU-T Rec. X.227 | ISO/IEC 8650-1) in conjunction with the connection-oriented presentation-service (see ITU-T Rec. X.216 | ISO/IEC 8822). The ACSE connection-oriented service assumes as a minimum the use of the presentation-service connection-oriented Kernel functional unit. The ACSE connectionless service (A-UNIT-DATA) is provided by the use of the connectionless ACSE protocol (see ITU-T Rec. X.237 | ISO/IEC 10035- 1) in conjunction with the connectionless presentation-service (P-UNIT-DATA). Three functional units are defined in the ACSE. The mandatory Kernel functional unit is used to establish and release application-associations. The ACSE includes two optional functional units. The optional Authentication functional unit supports the exchange of information in support of authentication during association establishment. It provides additional facilities for exchanging information in support of authentication during association establishment without adding services. The ACSE authentication facilities may be used to support a limited class of authentication methods. The second optional functional unit supports the negotiation of application context during association establishment. The ACSE services apply to a wide range of application-process communications. This Service Definition does not specify individual implementations or products, nor does it constrain theimplementation of entities and interfaces within a computer system. No requirement is made for conformance to this Service Definition.