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.