Appendix A: OSI Implementation

Appendix A: OSI Implementation

Overview

Since some of my examples in this book use MAILbus 400 and Digital X.500, both of which rely on DECnet/OSI, it is worth making a few comments about DECnet. It is an implementation of the OSI model, and as such, will give you an idea of what the theory can look like in practice.

Keep in mind the following points, however: First, the OSI model is not very detailed. As a result, while there is a core of common protocols, not everyone implements everything that is optional.

Digital also has the responsibility of maintaining backward compatibility with its previous version of DECnet (Phase IV), which was not OSI compliant. So it has added modules that make Phase IV requests continue to work in a Phase V network. You would not find these anywhere else.

The OSI model does not define how the management of the network needs to be implemented. DECnet has something called NCL. This too is Digital-specific, although you should find equivalent functionality in any OSI network.

In Figure A-1, you can see how a typical DECnet OSI node can have multiple stacks running concurrently. On the left you see what is largely a CLNS stack running across an ethernet or FDDI cable. On the right you see a CONS (X.25) stack.

Figure A-1. Some of DECnet?s Protocols

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