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 The ACSE supports two modes of communication: connection-mode and connectionless-mode. The ACSE service definition (see ITU-T Rec. X.217 bis | ISO/IEC 15953) includes both modes of communication. This Recommendation | International Standard only includes the connection mode of communication. This Recommendation | International Standard for the connectionless mode of communication is contained in ITU-T Rec. X.237 bis | ISO/IEC 15955. This Recommendation | International Standard defines procedures that are applicable to instances of communication between systems which wish to interconnect in an open systems interconnection environment in a connection mode. This Recommendation | International Standard includes the Kernel functional unit that is used to establish and release ASO-associations. The Authentication functional unit provides additional facilities for exchanging information in support of authentication during association establishment without adding new services. The ACSE authentication facilities can be used to support a limited class of authentication methods. The ASO-context negotiation functional unit provides the additional facility for the recipient to select the ASO-context from a list offered by the initiator 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 specifies: procedures for the transfer of information for ASO-association control and the authentication of ASOs and application-entities; and the abstract syntax for the representation of the ACSE APDUs. The ACSE procedures are defined in terms of: the interactions among peer ACSE protocol machines through the use of Presentation services or supporting ACSE services; and the interaction between an ACSE protocol machine and its service-user. This Recommendation | International Standard also specifies conformance requirements for systems implementing these procedures. It does not contain tests which can be used to demonstrate conformance.