parent transfer of data context refers to the other operational elements that are needed to between peer session layer entities and ensures that the appropriate support the specific nature of the communication. These are defined quality of service, which was requested by the originating session 12/10 Open systems interconnection reference model entity, is maintained. In this regard, the transport layer is required to 12.7 Conclusions select the network layer service that most closely matches the requested requirement. In addition, it may also have to invoke The OSI architecture establishes a generic foundation for im- additional functionality to ensure the proper quality of service. Five plementation and evolution of distributed information systems to classes of connection-mode protocol have been defined in ISO 8073 support a diversity of operational requirements. Whilst the richness to accommodate the variety of operational conditions. The transport of functionality that is available can lead to significant complexity, service is defined in ISO 8072. There is also a connectionless- mode not all the capabilities are needed for every application. It is import- transport protocol specified in ISO 8602. ant to select only those functions that are needed for particular operational configurations, and no more. Therefore, the complexity 12.5.5 Network layer will vary as needed to fulfill specific requirements. OSI is a living architecture that will facilitate evolution to new The network layer provides the functions for the telecommunication technologies and operational requirements, but the basic structure resources that provide the paths for transfer of data between sys- must be preserved to ensure an orderly transition on a common items. Real telecommunication services, such as may be provided by basis. New functions may be added to layers in the future, whilst a public data network, are defined by OSI as subnetworks. Paths some of the existing functions will become disused. Even some between systems may involve swi