Provides a general description of security services and related mechanisms, which may be provided by the Reference Model, and defines the positions within where the services and mechanisms may be provided. Basic security services and mechanisms and their appropriate placement have been identified for all layers of the Basic Reference Model. Additional security measures may be needed in end-systems, installations and organizations. OSI security functions are concerned only with those visible aspects of a communications path which permit and systems to achieve the secure transfer of information between them. It is not an implementation specification, nor is it a basis for appraising the conformance of actual implementations.