This part of ISO 7498: a) provides a general description of security services and related mechanisms, which may be provided by the Reference Model; and, b) defines the positions within the Reference Model where the services and mechanisms may be provided. This part of ISO 7498 extends the field of application of ISO 7498, to cover secure communications between open systems. Basic security services and mechanisms and their appropriate placement have been identified for all layers of the Basic Reference Model. In addition, the architectural relationships of the security services and mechanisms to the Basic Reference Model have been identified. Additional security measures may be needed in endsystems, installations and organizations. These measures apply in various application contexts. The definition of security services needed to support such additional security measures is outside the scope of this standard. OSI security functions are concerned only with those visible aspects of a communications path which permit end systems to achieve the secure transfer of information between them. OSI Security is not concerned with security measures needed in end systems, installations, and organizations, except where these have implications on the choice and position of security services visible in OSI. These latter aspects of security may be standardized but not within the scope of OSI standards. This part of ISO 7498 adds to the concepts and principles defined in ISO 7498; it does not modify them. It is not an implementation specification, nor is it a basis for appraising the conformance of actual implementations.