Network+ Study Guide

Introducing TCP/IP

Because TCP/IP is so central to working with the Internet and with intranets, you should understand it in detail. You’ll start with some background on TCP/IP and how it came about and then move on to the descriptions of the technical goals defined by the original designers. Then you’ll get a look at how TCP/IP compares to a theoretical model, the Open Systems Interconnect (OSI) model.

A Brief History of TCP/IP

The TCP/IP protocol was first proposed in 1973, but it was not until 1983 that a standardized version was developed and adopted for wide area use. In that same year, TCP/IP became the official transport mechanism for all connections to ARPAnet, a forerunner of the Internet.

Much of the original work on TCP/IP was done at the University of California at Berkeley, where computer scientists were also working on the Berkeley version of Unix (which eventually grew into the Berkeley Software Distribution [BSD] series of Unix releases). TCP/IP was added to the BSD releases, which in turn was made available to universities and other institutions for the cost of a distribution tape. Thus, TCP/IP began to spread in the academic world, laying the foundation for today’s explosive growth of the Internet and of intranets as well.

During this time, the TCP/IP family continued to evolve and add new members. One of the most important aspects of this growth was the continuing development of the certification and testing program carried out by the U.S. government to ensure that the published standards, which were free, were met. Publication ensured that the developers did not change anything or add any features specific to their own needs. This open approach has continued to the present day; use of the TCP/IP family of protocols virtually guarantees a trouble-free connection between many hardware and software platforms.

TCP/IP Design Goals

When the U.S. Department of Defense began to define the TCP/IP network protocols, their design goals included the following:

• TCP/IP had to be independent of all hardware and software manufacturers. Even today, this is fundamentally why TCP/IP makes such good sense in the corporate world: It is not tied to IBM, Novell, Microsoft, DEC, or any other specific company.

• It had to have good built-in failure recovery. Because TCP/IP was originally a military proposal, the protocol had to be able to continue operating even if large parts of the network suddenly disappeared from view, say, after an enemy attack.

• It had to handle high error rates and still provide completely reliable end-to-end service.

• It had to be efficient and have a low data overhead. The majority of data packets using the IP protocol have a simple, 20-byte header, which means better performance in comparison with other networks. A simple protocol translates directly into faster transmissions, giving more efficient service.

• It had to allow the addition of new networks without any service disruptions.

As a result, TCP/IP was developed with each component performing unique and vital functions that allowed all the problems involved in moving data between machines over networks to be solved in an elegant and efficient way. Before looking at both TCP and IP individually, you should understand where TCP/IP fits into the broader world of network protocols and, particularly, how it compares to the theoretical reference model published by the International Organization for Standardization (ISO) as the OSI model.

There are several benefits to using the TCP/IP networking protocol:

• TCP/IP is a widely published open standard and is completely independent of any hardware or software manufacturer.

• TCP/IP can send data between different computer systems running completely different operating systems, from small PCs all the way to mainframes and everything in between.

• TCP/IP is separated from the underlying hardware and will run over Ethernet, Token Ring, or X.25 networks and even over dial-up telephone lines.

• TCP/IP is a routable protocol, which means it can send datagrams over a specific route, thus reducing traffic on other parts of the network.

• TCP/IP has reliable and efficient data-delivery mechanisms.

• TCP/IP uses a common addressing scheme. Therefore, any system can address any other system, even in a network as large as the Internet. (We will look at this addressing scheme in the “Understanding IP Addressing” section later in this chapter.)

The popularity that the TCP/IP family of protocols enjoys today did not arise just because the protocols were there, or even because the U.S. government mandated their use. They are popular because they are robust, solid protocols that solve many of the most difficult networking problems, and do so in an elegant and efficient way.

TCP/IP and the OSI Model

As you learned in Chapter 2, “The OSI Model,” the OSI model divides computer-to-computer communications into seven connected layers; TCP/IP uses the Department of Defense (DoD) model, which describes communications in only four layers, as Figure 3.1 shows. Each successively higher layer builds on the functions provided by the layers below.

Figure 3.1: A comparison of the seven-layer OSI model, the four-layer DoD model, and how TCP/IP maps to each model

As you may remember from the OSI model, these layers are as follows:

Application Layer  The highest layer; defines the manner in which applications interact with the network—including databases, e-mail, and terminal-emulation programs.

Presentation Layer  Defines the way in which data is formatted, presented, converted, and encoded.

Session Layer  Coordinates communications and maintains the session for as long as it is needed—performing security, logging, and administrative functions.

Transport Layer  Defines protocols for structuring messages and supervises the validity of the transmission by doing some error checking.

Network Layer  Defines data-routing protocols to ensure that the information arrives at the correct destination node.

Data Link Layer  Validates the integrity of the flow of the data from one node to another by synchronizing blocks of data and controlling the flow.

Physical Layer  Defines the mechanism for communicating with the transmission medium and the interface hardware.

In the DoD model, the four layers are as follows:

Process/Application Layer  The highest layer; applications such as FTP, Telnet, and others interact through this layer.

Host-to-Host Layer  TCP and other protocols add transport data to the data packet.

Internet Layer  Adds IP information to the packet.

Network Access Layer  Defines the mechanism for communicating with the transmission medium and the interface hardware.

Each layer adds its own header and trailer data to the basic data packet and encapsulates the data from the layer above. On the receiving end, this header information is stripped, one layer at a time, until the data arrives at its final destination.

Now let’s look at how TCP and IP work together.