Introduction

 

An Introduction to Computer Networks

AN OVERVIEW OF NETWORKS

Somewhere there might be a field of interest in which the order of presentation of topics is well agreed upon.

Computer networking is not it.

There are many interconnections in the field of networking, as in most technical fields, and it is difficult

to find an order of presentation that does not involve endless “forward references” to future chapters; this

is true even if – as is done here – a largely bottom-up ordering is followed. I have therefore taken here a

different approach: this first chapter is a summary of the essentials – LANs, IP and TCP – across the board,

and later chapters expand on the material here.

Local Area Networks, or LANs, are the “physical” networks that provide the connection between machines

within, say, a home, school or corporation. LANs are, as the name says, “local”; it is the IP, or Internet

Protocol, layer that provides an abstraction for connecting multiple LANs into, well, the Internet. Finally,

TCP deals with transport and connections and actually sending user data.

This chapter also contains some important other material. The section on datagram forwarding, central

to packet-based switching and routing, is essential. This chapter also discusses packets generally, congestion, and sliding windows, but those topics are revisited in later chapters. Firewalls and network address

translation are also covered here and not elsewhere.

1.1 Layers

These three topics – LANs, IP and TCP – are often called layers; they constitute the Link layer, the Internetwork layer, and the Transport layer respectively. Together with the Application layer (the software you use),

these form the “four-layer model” for networks. A layer, in this context, corresponds strongly to the idea

of a programming interface or library, with the understanding that a given layer communicates directly only

with the two layers immediately above and below it. An application hands off a chunk of data to the TCP

library, which in turn makes calls to the IP library, which in turn calls the LAN layer for actual delivery. An

application does not interact directly with the IP and LAN layers at all.

The LAN layer is in charge of actual delivery of packets, using LAN-layer-supplied addresses. It is often

conceptually subdivided into the “physical layer” dealing with, eg, the analog electrical, optical or radio

signaling mechanisms involved, and above that an abstracted “logical” LAN layer that describes all the

digital – that is, non-analog – operations on packets; see 2.1.4 The LAN Layer. The physical layer is

generally of direct concern only to those designing LAN hardware; the kernel software interface to the LAN

corresponds to the logical LAN layer.

Application

Transport

IP

Logical LAN

Physical LAN

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