Networking Concept

Introduction

Installing the Linux operating system is only the first step toward creating a fully functional departmental server or Web site. Almost all computers are now networked in some way to other devices therefore a basic understanding of networking and issues related to the topic will be essential to feeling comfortable with Linux servers.

This introductory chapter forms the foundation on which the following network configuration and troubleshooting chapters will be built. These chapters will then introduce the remaining chapters that cover Linux troubleshooting, general software installation and the configuration of many of the most popular Linux applications used in corporate departments and Small Office/Home Office (SOHO) environments.

Familiarity with the concepts explained in the following sections will help answer many of the daily questions often posed by coworkers, friends, and even yourself. It will help make the road to Linux mastery less perilous, a road that begins with an understanding of the OSI networking model and TCP/IP.

The OSI Networking Model

The Open System Interconnection (OSI) model, developed by the International Organization for Standardization, defines how the various hardware and software components involved in data communication should interact with each other.

A good analogy would be a traveler who prepares herself to return home through many dangerous kingdoms by obtaining permits to enter each country at the very beginning of the trip. At each frontier our friend has to hand over a permit to enter the country. Once inside, she asks the border guards for directions to reach the next frontier and displays the permit for that new kingdom as proof that she has a legitimate reason for wanting to go there.

In the OSI model each component along the data communications path is assigned a layer of responsibility, in other words, a kingdom over which it rules. Each layer extracts the permit, or header information, it needs from the data and uses this information to correctly forward what's left to the next layer. This layer also strips away its permit and forwards the data to the next layer, and so the cycle continues for seven layers.

The very first layer of the OSI model describes the transmission attributes of the cabling or wireless frequencies used at each "link" or step along the way. Layer 2 describes the error correction methodologies to be used on the link; layer 3 ensures that the data can hop from link to link on the way to the final destination described in its header. When the data finally arrives, the layer 4 header is used to determine which locally installed software application should receive it. The application uses the guidelines of layer 5 to keep track of the various communications sessions it has with remote computers and uses layer 6 to verify that the communication or file format is correct. Finally, layer 7 defines what the end user will see in the form of an interface, be it graphical on a screen or otherwise. A description of the functions of each layer in the model can be seen in Table 2-1.

Table 2-1: The Seven OSI Layers

Layer	Name	Description	Application
7	Application	The user interface to the application	telnet FTP sendmail
6	Presentation	Converts data from one presentation format to another. For example, e-mail text entered into Outlook Express being converted into SMTP mail formatted data.
5	Session	Manages continuing requests and responses between the applications at both ends over the various established connections.
4	Transport	Manages the establishment and tearing down of a connection. Ensures that unacknowledged data is retransmitted. Correctly re-sequences data packets that arrive in the wrong order. After the packet's overhead bytes have been stripped away, the resulting data is said to be a segment.	TCP UDP
3	Network	Handles the routing of data between links that are not physically connected together. After the link's overhead bytes have been stripped away, the resulting data is said to be a packet.	IP ARP
2	Link	Error control and timing of bits speeding down the wire between two directly connected devices. Data sent on a link is said to be structured in frames.	Ethernet ARP
1	Physical	Defines the electrical and physical characteristics of the network cabling and interfacing hardware	Ethernet

source: LHN