INTRODUCTION

We are interested in visualization over networks. In this lecture, we provide some basic background to networking. Our main reference is Stevens [1].

Computer networking is a diverse topic without some abstraction. The commonly accepted abstract model for networks is the OSI open systems interconnection Model (which we simply call the OSI model). OSI stands for International Standards Organization. This model defines seven layers, which in increasing levels of abstraction are

1. physical
2. data link
3. network
4. transport
5. session
6. presentation
7. application

A simplified version of the OSI model with only 4 layers is seen in the next table:

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LAYER	DATA	PROTOCOLS	BYTE SIZE
process	messages	TFTP	N
transport	messages	UDP	N+4
network	packets	IP	N+4+8
data link	frames	Ethernet	N+4+8+20

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• In Column 1, we merge the first two (calling it `data link') and the last three (calling it `process') layers of the original OSI model. We also invert the order of the layers.
• Column 2 of this table gives the names of the unit of data sent in the specified layer.
• Column 3 gives a typical application, TFTP ('Trivial File Transfer Protocol') and the names of the protocols used in each layer.
• Column 4 shows how N bytes of data at the process level is augmented with header (and trailer) bytes at each level.