Chapter 2: Networking: 2.1 OSI Networking Model

UNIX Hints & Hacks

Chapter 2: Networking

Sections in this Chapter:
2.1 OSI Networking Model	2.4 Remote Network Configurations	2.7 Unmounting and Busy Devices
2.2 Finding the Failure	2.5 Shutdown, Halt, or Reboot over the Network	2.8 Static Routing or routed
2.3 Hiding Files with NFS	2.6 Talking NFS3 to NFS2	2.9 Get the Ethernet Address with arp

Chapter 2
Networking

As it applies to information technology, a network is a series of points or nodes that are interconnected by communication paths. It is these nodes that you are responsible to have functioning and talking on the network.

Networking has an important role in UNIX administration. A UNIX workstation or server is extremely versatile in how it can be configured on a network. This chapter discusses unique networking issues that can come up during the course of a day.

UNIX system administrators deal with anything from NFS mounts to network configurations and mounting and unmounting network devices. In many cases, changes to the network affect not only one machine but multiple machines. The administrator must be careful to understand what is affected by the networking issues that are being handled at that time.

Working on a network is much more dangerous than some believe. The modification on one system can affect an entire network. This is true no matter where you are or what system you are on. If you, like me, usually have four or more shells to different systems on your console at any given time, you must pay close attention the location of each window. If you don't pay attention to where each window is remotely logged in to, you might find yourself modifying the wrong system.

2.1 OSI Networking Model

2.1.1 Description

You must familiarize yourself with networking concepts and the Open Systems Interconnection (OSI) network model.

Reason

The OSI model helps you understand how data is moved across a network and which network layers manipulate the data in certain ways between nodes. There are seven layers to the model. Each of the seven layers provides a set of functions to the layer above and also relies on the layer below.

Layer 7 The application layer--At this layer, a user and a computer interface to a network. It refers to the user programs themselves, such as file transfers and terminal emulations.

Layer 6 The presentation layer--This layer is usually part of an operating system that converts incoming and outgoing data from one presentation format to another. For example, it converts a text stream into a pop-up window with the newly arrived text.

Layer 5 The session layer--This layer manages the establishment of a continuing series of requests and responses between the applications at each end. It provides a cleaner interface to the transport layer and synchronization to recover from transport layer failures.

Layer 4 The transport layer--This layer makes sure data gets delivered to a specific process on a specific machine. It manages the end-to-end control and deals with error-correction.

Layer 3 The network layer--This layer handles the routing of data between two hosts and any congestion that might develop.

Layer 2 The link (or data-link) layer--This layer deals with communication between two machines sharing a common physical channel. It also handles lost, damaged, and duplicated frames for error control and timeouts.

Layer 1 The physical layer--It is concerned with insuring that when one side sends a 1 bit, the other side receives a 1 bit.

Real World Experience

In large IS or IT organizations, it is sometimes difficult to receive help at a moment's notice from other groups within the organization. Using basic troubleshooting techniques, you can narrow down the layer at which the problem might exist. Depending on the problem, you can provide insight to others that you need to enlist help from. If the problem is not at your end, you can more accurately explain and justify where (or in what layer) the problem exists. This can aid the others in providing a quicker response time for solving your problem.

Many UNIX environments work closely with the networking groups within an organization. You might be asked in a job interview whether you can name all the layers. This is expected knowledge, but isn't required by most interviewers for a senior level administrator position. They usually are impressed if you can name all seven layers (and in the correct order). Providing the definitions and functions adds even more to your credibility.

Other Resources

World Wide Web:

A TCP/IP primer-- http://www.sunworld.com/swol-11-1995/swol-11-sysadmin.html

Internet protocols-- http://oac3.hsc.uth.tmc.edu/staff/snewton/tcp-tutorial/