TCP/IP

Hour 1. What Is TCP/IP?
What You'll Learn in This Hour:

Networks and network protocols

History of TCP/IP

Important features of TCP/IP

TCP/IP is a protocol system—a collection of protocols that support network communications. The answer to the question What is a protocol? must begin with the question What is a network?

This hour describes what a network is and shows why networks need protocols. You'll also learn what TCP/IP is, what it does, and where it began.

At the completion of this hour, you'll be able to

Define the term network

Explain what a network protocol suite is

Explain what TCP/IP is

Discuss the history of TCP/IP

List some important features of TCP/IP

Identify the organizations that oversee TCP/IP and the Internet

Explain what RFCs are and where to find them

Networks and Protocols
A network is a collection of computers or computer-like devices that can communicate across a common transmission medium, as shown in Figure 1.1.

Figure 1.1. A typical local network.


In a network, requests and data from one computer pass across the transmission medium (which might be a network cable or a phone line) to another computer. In Figure 1.1, computer A must be able to send a message or request to computer B. Computer B must be able to understand computer A's message and respond to it by sending a message back to computer A.

A computer interacts with the world through one or more applications that perform specific tasks and manage input and output. If that computer is part of a network, some of those applications must be capable of communicating with applications on other network computers. A network protocol is a system of common rules that helps define the complex process of transferring data. The data travels from an application on one computer, through the computer's network hardware, across the transmission medium to the correct destination, and up through the destination computer's network hardware to a receiving application (see Figure 1.2).

Figure 1.2. The role of a network protocol suite.


The protocols of TCP/IP define the network communication process and, more importantly, define how a unit of data should look and what information it should contain so that a receiving computer can interpret the message correctly. TCP/IP and its related protocols form a complete system defining how data should be processed, transmitted, and received on a TCP/IP network. A system of related protocols, such as the TCP/IP protocols, is called a protocol suite.

The actual act of formatting and processing TCP/IP transmissions is performed by a software component known as the vendor's implementation of TCP/IP. For instance, Microsoft TCP/IP is a software component that enables Windows computers to process TCP/IP-formatted data and thus to participate in a TCP/IP network. As you read this book, be aware of the following distinction:

A TCP/IP standard is a system of rules defining communication on TCP/IP networks.

A TCP/IP implementation is a software component that performs the functions that enable a computer to participate in a TCP/IP network.

The purpose of the TCP/IP standards is to ensure the compatibility of all TCP/IP implementations regardless of version or vendor.

By the Way

The important distinction between the TCP/IP standards and a TCP/IP implementation is often blurred in popular discussions of TCP/IP, and this is sometimes confusing for readers. For instance, authors often talk about the layers of the TCP/IP model providing services for other layers. In fact, it is not the TCP/IP model that provides services. The TCP/IP model defines the services that should be provided. The vendor software implementations of TCP/IP actually provide these services.
The Development of TCP/IP
Present-day TCP/IP networking represents the synthesis of two developments that began in the 1970s and have subsequently revolutionized the world of computing:

The Internet

The local area network

The Internet
TCP/IP's design is a result of its historical role as the protocol system for what was to become the Internet. The Internet, like so many other high-tech developments, grew from research originally performed by the United States Department of Defense. In the late 1960s, Defense Department officials began to notice that the military was accumulating a large and diverse collection of computers. Some of those computers weren't networked, and others were grouped in small, closed networks with incompatible proprietary protocols.

Proprietary, in this case, means that the technology is controlled by a private entity (such as a corporation). That entity might not have any interest in divulging enough information about the protocol so that users can use it to connect to other (rival) network protocols.

Defense officials began to wonder if it would be possible for these disparate computers to share information. Accustomed as they were to considerations of security, the Defense Department reasoned that, if such a network were possible, it would likely become a target for military attack. One of the primary requirements of this new network, therefore, was that it must be decentralized. Critical services must not be concentrated in a few vulnerable failure points. Because every failure point is vulnerable in the age of the missile, they wanted a network with no failure points at all—where a bomb could land on any part of the infrastructure without bringing down the whole network. These visionary soldiers created a network that became known as ARPAnet, named for the Defense Department's Advanced Research Projects Agency (ARPA). The protocol system that supported this interconnectable, decentralized network was the beginning of what we now know as TCP/IP.

A few years later, when the National Science Foundation wanted to build a network to connect research institutions, it adopted ARPAnet's protocol system and began to build what we know as the Internet. As you'll learn later in this book, the original decentralized vision of ARPAnet survives to this day in the design of the TCP/IP protocol system and is a big part of the success of TCP/IP and the Internet.

Two important features of TCP/IP that provide for this decentralized environment are as follows:

End node verification— The two computers that are actually communicating—called the end nodes because they are at each end of the chain passing the message—are responsible for acknowledging and verifying the transmission. All computers basically operate as equals, and there is no central scheme for overseeing communications.

Dynamic routing— Nodes are connected through multiple paths, and the routers choose a path for the data based on present conditions. You'll learn more about routing and router paths in later hours.

The Local Area Network (LAN)
As the Internet began to emerge around universities and research institutions, another network concept, the local area network (LAN) was also taking form. LANs developed along with the computer industry and were a response to the need for offices to share computer resources.

Early LAN protocols did not provide Internet access and were designed around proprietary protocol systems. Many did not support routing of any kind. Eventually, some companies began to want a protocol that would connect their incompatible, noncontiguous LANs, and they looked to TCP/IP. As the Internet became more popular, LAN users began to clamor for Internet access, and a variety of solutions began to emerge for getting LAN users connected. Specialized gateways provided the protocol translation necessary for these local networks to reach the Internet. Gradually, LAN software vendors began to provide more complete support for TCP/IP. Recent versions of NetWare, Mac OS, and Windows have continued to expand the role of TCP/IP on local networks. TCP/IP grew up around Unix, and all Unix variants are fluent in TCP/IP. The recent popularity of Unix-based systems such as Linux, BSD, Solaris, and Apple OS X has increased the dominance of TCP/IP in the networking world.

By the Way

The term gateway is used inconsistently in discussions of TCP/IP. A gateway is sometimes just an ordinary router that connects a LAN to a larger network (see the discussion of routers later in this hour), and sometimes the term is used to refer to a routing device that performs some form of protocol translation.



As you'll learn in Hour 3, "The Network Access Layer," the need to accommodate local area networks has caused considerable innovation in the implementation of the hardware-conscious protocols that underlie TCP/IP.

TCP/IP Features
TCP/IP includes many important features that you'll learn about in this book. In particular, pay close attention to the way the TCP/IP protocol suite addresses the following problems:

Logical addressing

Routing

Name service

Error control and flow control

Application support

These issues are at the heart of TCP/IP. The following sections introduce these important features. You'll learn more about these features later in this book.

Logical Addressing
A network adapter has a unique and permanent physical address. The physical address is a number that was given to the card at the factory. On a local area network, low-lying hardware-conscious protocols deliver data across the physical network using the adapter's physical address. There are many network types, and each has a different way of delivering data. On a basic ethernet network, for example, a computer sends messages directly onto the transmission medium. The network adapter of each computer listens to every transmission on the local network to determine whether a message is addressed to its own physical address.

By the Way

As you'll learn in Hour 9, "Network Hardware," today's ethernet networks are a bit more complicated than the idealized scenario of a computer sending messages directly onto the transmission line. Ethernet networks sometimes contain hardware devices such as switches and hubs to manage the signal.



On large networks, of course, every network adapter can't listen to every message. (Imagine your computer listening to every piece of data sent over the Internet.) As the transmission medium becomes more populated with computers, a physical addressing scheme cannot function efficiently. Network administrators often segment networks using devices such as routers to reduce network traffic. On routed networks, administrators need a way to subdivide the network into smaller subnetworks (called subnets) and impose a hierarchical design so that a message can travel efficiently to its destination. TCP/IP provides this subnetting capability through logical addressing. A logical address is an address configured through the network software. In TCP/IP, a computer's logical address is called an IP address. As you'll learn in Hour 4, "The Internet Layer," and Hour 5, "Subnetting," an IP address can include

A network ID number identifying a network

A subnet ID number identifying a subnet on the network

A host ID number identifying the computer on the subnet

The IP addressing system also lets the network administrator impose a sensible numbering scheme on the network so that the progression of addresses reflects the internal organization of the network.

By the Way

If your network is isolated from the Internet, you are free to use any IP addresses you want (as long as your network follows the basic rules for IP addressing). If your network will be part of the Internet, however, Internet Corporation for Assigned Names and Numbers (ICANN), which was formed in 1998, will assign a network ID to your network, and that network ID will form the first part of the IP address.

TCP/IP Features
TCP/IP includes many important features that you'll learn about in this book. In particular, pay close attention to the way the TCP/IP protocol suite addresses the following problems:

Logical addressing

Routing

Name service

Error control and flow control

Application support

These issues are at the heart of TCP/IP. The following sections introduce these important features. You'll learn more about these features later in this book.

Logical Addressing
A network adapter has a unique and permanent physical address. The physical address is a number that was given to the card at the factory. On a local area network, low-lying hardware-conscious protocols deliver data across the physical network using the adapter's physical address. There are many network types, and each has a different way of delivering data. On a basic ethernet network, for example, a computer sends messages directly onto the transmission medium. The network adapter of each computer listens to every transmission on the local network to determine whether a message is addressed to its own physical address.

By the Way

As you'll learn in Hour 9, "Network Hardware," today's ethernet networks are a bit more complicated than the idealized scenario of a computer sending messages directly onto the transmission line. Ethernet networks sometimes contain hardware devices such as switches and hubs to manage the signal.



On large networks, of course, every network adapter can't listen to every message. (Imagine your computer listening to every piece of data sent over the Internet.) As the transmission medium becomes more populated with computers, a physical addressing scheme cannot function efficiently. Network administrators often segment networks using devices such as routers to reduce network traffic. On routed networks, administrators need a way to subdivide the network into smaller subnetworks (called subnets) and impose a hierarchical design so that a message can travel efficiently to its destination. TCP/IP provides this subnetting capability through logical addressing. A logical address is an address configured through the network software. In TCP/IP, a computer's logical address is called an IP address. As you'll learn in Hour 4, "The Internet Layer," and Hour 5, "Subnetting," an IP address can include

A network ID number identifying a network

A subnet ID number identifying a subnet on the network

A host ID number identifying the computer on the subnet

The IP addressing system also lets the network administrator impose a sensible numbering scheme on the network so that the progression of addresses reflects the internal organization of the network.

By the Way

If your network is isolated from the Internet, you are free to use any IP addresses you want (as long as your network follows the basic rules for IP addressing). If your network will be part of the Internet, however, Internet Corporation for Assigned Names and Numbers (ICANN), which was formed in 1998, will assign a network ID to your network, and that network ID will form the first part of the IP address. (See Hours 4 and 5.)



In TCP/IP, a logical address is resolved to and from the corresponding hardware-specific physical address using the ARP and RARP protocols, which are discussed in Hour 4.

Routing
A router is a special device that can read logical addressing information and direct data across the network to its destination. At the simplest level, a router divides a local subnet from the larger network (see Figure 1.3).

Figure 1.3. A router connecting a LAN to a large network.


Data addressed to another computer or device on the local subnet does not cross the router and therefore doesn't clutter up the transmission lines of the greater network. If data is addressed to a computer outside the subnet, the router forwards the data accordingly. As has already been mentioned this hour, very large networks such as the Internet include many routers and provide multiple paths from the source to the destination (see Figure 1.4).

Figure 1.4. A routed network.


TCP/IP includes protocols that define how the routers will find a path through the network. You'll learn more about TCP/IP routing and routing protocols in Hour 10, "Routing."

By the Way

As you'll also learn in Hour 9, network devices such as bridges, switches, and smart hubs also can filter traffic and reduce network traffic. Because these devices work with physical addresses rather than logical addresses, they cannot perform the complex routing functions shown in Figure 1.4.



Name Resolution
Although the numeric IP address is probably more user friendly than the network adapter's prefabricated physical address, the IP address is still designed for the convenience of the computer rather than the convenience of the user. People might have trouble remembering whether a computer's address is 111.121.131.146 or 111.121.131.156. TCP/IP, therefore, provides for a parallel structure of user-oriented alphanumeric names, called domain names or DNS names. This mapping of domain names to an IP address is called name resolution. Special computers called name servers store tables showing how to translate these domain names to and from IP addresses.

The computer addresses commonly associated with email or the World Wide Web are expressed as DNS names (for example, www.microsoft.com, falcon.ukans.edu, and idir.net). TCP/IP's name service system provides for a hierarchy of name servers that supply domain name/IP address mappings for DNS-registered computers on the network. This means that the everyday user rarely has to enter or decipher an actual IP address.

DNS is the name resolution system for the Internet and is the most common name resolution method. However, some TCP/IP networks also support other methods for resolving alphanumeric names to IP addresses. Another common name resolution scheme is the Windows Internet Name Services (WINS) for resolving Microsoft Windows NetBIOS names to IP addresses.

You'll learn more about TCP/IP name resolution in Hour 11, "Name Resolution."

Error Control and Flow Control
The TCP/IP protocol suite provides features that ensure the reliable delivery of data across the network. These features include checking data for transmission errors (to ensure that the data that arrives is exactly what was sent) and acknowledging successful receipt of a network message. TCP/IP's Transport layer (see Hour 6, "The Transport Layer") defines many of these error-checking, flow-control, and acknowledgment functions through the TCP protocol. Lower-level protocols at TCP/IP's Network Access layer (see Hour 3) also play a part in the overall system of error control.

Application Support
Several network applications might be running on the same computer. The protocol software must provide some means for determining which incoming packet belongs with each application. In TCP/IP, this interface from the network to the applications is accomplished through a system of logical channels called ports. Each port has a number that is used to identify the port. You can think of these ports as logical pipelines within the computer through which data can flow from the application to (and from) the protocol software (see Figure 1.5).

Figure 1.5. Applications access the network through port addresses.


Hour 6 describes TCP and UDP ports at TCP/IP's Transport layer. You'll learn more about application support and TCP/IP's Application layer in Hour 7, "The Application Layer."

The TCP/IP suite also includes a number of ready-made applications designed to assist with various network tasks. Some typical TCP/IP utilities are shown in Table 1.1. You'll learn more about these TCP/IP utilities in Part IV, "TCP/IP Utilities."

Table 1.1. Typical TCP/IP Utilities Utility
Purpose

ftp
File Transfer

lpr
Printing

ping
Configuration/Troubleshooting

route
Configuration/Troubleshooting

telnet
Remote Terminal Access

traceroute
Configuration/Troubleshooting



By the Way

TCP/IP is actually entering into a new phase at the time of this writing. New technologies such as wireless networks, virtual private networks, and network address translation are adding new complexities that the creators of TCP/IP wouldn't have imagined. You'll learn more about these technologies in later chapters.