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:

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

• 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.