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: a) procedures for the transfer of information for ASO-association control and the authentication of ASOs and application-entities; and b) the abstract syntax for the representation of the ACSE APDUs. The ACSE procedures are defined in terms of: a) the interactions among peer ACSE protocol machines through the use of Presentation services or supporting ACSE services; and b) 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.