Preface Standards development within the Information Technology sector is harmonized with international standards development. Through the CSA Technical Committee on Information Technology (TCIT), Canadians serve as the Canadian Advisory Committee (CAC) on ISO/IEC Joint Technical Committee 1 on Information Technology (ISO/IEC JTC1) for the Standards Council of Canada (SCC), the ISO member body for Canada and sponsor of the Canadian National Committee of the IEC. Also, as a member of the International Telecommunication Union (ITU), Canada participates in the International Telegraph and Telephone Consultative Committee (ITU-T). Scope This Recommendation | International Standard defines ACSE services for ASO-association control in an open systems interconnection environment. ACSE supports two modes of communication service: connection-mode and connectionless-mode. The ACSE connection-mode service is provided by the use of the connection-mode ACSE protocol (see ITU-T Rec. X.227 bis | ISO/IEC 15954). The ACSE connectionless-mode service (A-UNIT-DATA) is provided by the use of the connectionless-mode ACSE protocol (see ITU-T Rec. X.237 bis | ISO/IEC 15955). Five functional units are defined in the ACSE. The mandatory Kernel functional unit is used to establish and release ASO-associations. The optional Authentication functional unit 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 optional ASO-context negotiation functional unit allows multiple ASO-contexts to be offered during association establishment. The optional higher level association functional unit provides for the facility to identify ASO-associations and transparently pass data to child ASOs and allows the ASO-context or the presentation context on an ASO-association to be modified during the lifetime of the association. This Recommendation | International Standard does not specify individual implementations or products, nor does it constrain the implementation of entities and interfaces within a computer system. No requirement is made for conformance to this Service definition.