INTRODUCTION

International Standards Organization (ISO) specification of OSI model and protocols are based on a number of abstract descriptions which specify essential requirements but make no mention of how these requirements should be met by an implementation.

Typically each OSI layer has two sets of specifications: One for the services provided by the protocol and one for the protocol. Service specifications for each layer are normally described in a high level of abstraction. The interfaces for the service definition is left at the level of primitive. Furthermore, the ISO specifications do not deal with any details concerning the mechanism used to exchange primitives across a layer interface.

Lack of a standard interface or a standard mechanism for exchange of primitives across a layer interface can cause severe problems in implementation of machine independent OSI software.

OSI Reference Model offers a number of features that are well aligned with today's software implementation methodology. To name a few:

• OSI model subdivides the communications functions into logically separate modules.
• Exclusive peer-to-peer interaction.
• A well defined service definition expected from each layer.
• Independence of protocol specification from the services expected from the protocol.

A well defined model, architecture and interface provides for integration of independent implementations of OSI protocols. It is the aim of this book to refine the abstract descriptions of the OSI model and create a software implementation environment (A model, an architecture and a platform) that facilitates the implementation of OSI.

Given today's state of technology a number of observations are made.

• The specification of the bottom five layers of OSI have been adopted as ISO standards and are well understood.
• C language has been recognized as a standard for portable and efficient implementation of data communication software.
• A correct, efficient and environment independent implementation of OSI protocols is very desirable.

The author therefore feels that the time is right for creation of a software environment that can facilitate the realization of OSI. Such a platform should be:

• Operating Environment (Operating System/ CPU/ Hardware) independent.
• Efficient.
• Consistent.
• Expandable.

This has been the motivation behind the creation of "Open C Environment" (OCE) for implementation of OSI. OCE is a set of architectural guide lines, conventions and a platform for portable implementation of OSI software. Independent implementations of OSI protocols that adhere to OCE can easily be integrated to realize real open systems.

A platform of facilities that can ease the implementation of OSI protocols is provided. One of the primary goals of this book is to formally define the interface to this platform of facilities.