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 Introduction to TCP/IP The Information Factory is providing you the entire contents of Section 1 of our video training course "Introduction to TCP/IP" so that you may get a feel for the content and technical level of the course. SECTION 1 INTRODUCTION TO TCP/IP The use of networks has changed the face of Information Technologies by allowing hundreds of thousands of computers around the world to intercommunicate. Originally, computer networks were pictured as a single large machine with all data storage and functionality centrally located. All of the terminals connected in a star pattern to this single central computer. Networks have evolved to where we now see powerful computers on desktops tied together in small networks that are linked to larger networks. These larger networks also intercommunicate so that a computer on a desktop can exchange data with a mainframe, minicomputer or another desktop computer somewhere else across the globe. This concept of communicating between computers across multiple networks is called "Internetworking."
As shown in Figure 1.1, many companies use Local Area Networks to tie together the computers in a work group or department. The Local Area Networks are interconnected using devices like Bridges or Routers so that a building may contain an interconnected group of Local Area Networks (LANs). A business can then use communications links to tie their Local Area Networks together across a Wide Area Network (WAN). When a company internetworks across LANs and WANs, the computers must have a method of recognizing each other across the intervening networks. As an example how does a program running on the computer called "Y" send data to a program on the computer called "X". When the data is transmitted from "Y," it must find a path across the intervening networks, and then be moved through computer "X" to the correct receiving program. In addition if the computers are different brands or use different operating systems, the data must be changed into the appropriate codes and file formats. In the example shown in Figure 1.1, the data had to traverse a Local Area Network, a Wide Area Network, and finally another Local Area Network to get to the correct computer. This brings us to our questions about internetworking and computer communications which are:
In this course we are going to answer these questions by showing how companies can use a suite of protocols called TCP/IP to provide these functions. The acronym TCP/IP translates into Transport Control Protocol and Internet Protocol. TCP/IP is designed to solve modern computer communication problems by allowing different computers to communicate across multiple networks. IP or the "Internet Protocol" will provide the thread that allows data to find its way between two computers across multiple networks. TCP or the "Transport Control Protocol" will provide the "reliable transport" or assurance that data is sent and received between the two computers and their respective application programs. We will also look at the "Services" that TCP/IP can provide to assist communications between programs and computers by providing end to end network level functions like file transfer, remote terminal log-ins and electronic mail. All of these various protocols and services are combined into what is known as the TCP/IP "Suite" of protocols.
The Layers of the TCP/IP Suite The TCP/IP "suite" of protocols is built up in the computer in "layers" of software each designed to perform a specific function toward making communications across networks as transparent as possible to the computers and programs. Referring to Figure 1.2 (above) the layers in TCP/IP can be seen as fitting between the user's application program and the actual communications network. Each of the layers can be described as performing the following functions: Application Layer This layer is the actual application or program being run by the computer or end user which is going to use the communications functions in TCP/IP. Although the application is not a part of the TCP/IP suite, it will invoke TCP or one of the services in the next lower layer. Services Layer If an application is not written to talk to TCP/IP directly, it can use one of the functions in the Service Layer. The services in this layer are actually standard programs that provide services such as file transfer, remote host logon, electronic mail transfer and a host of other services. Once one of the services is invoked, the application then passes the data to the service for transmission. Transport Layer The service layer will then invoke the transport function where the data will be assembled into blocks called Message Transfer Units. The Transport layer will count and verify transmission of each MTU across the network. The Transport Layer provides a virtual connection between the two end computer involved in the data transfer. Network Layer The Transport layer sends the data down to the Network Layer for actual movement and routing of the data between the network nodes or intervening computers. The network layer acts as the entrance or gateway to the internetworking function moving the data between the two computers that are in communication. The Network layer provides the internetworking function by routing data across the intervening networks. Link Layer The Link layer is the actual interface to the communications circuit, Local Area Network or modem that performs the transmission. Link Layers are functions like Ethernet 802.3, Modem interfaces, packet networks, etc. Why a Layered Protocol Suite? By layering the protocols, it is possible to separate the various functions involved in internetworking. Although the layering may appear to be more complex, in reality it is easier to maintain layered protocols. Since each function is in a separate layer it simplifies the updating and testing of changes to TCP/IP. This concept of a group or "suite" of protocols in a stack handling all of the functions in communications, makes it difficult to understand TCP/IP when viewed against the traditional concept of communications protocols. Originally communications protocols simply took care of handling errors on a local communications link. The older protocols were usually linked into the specific application program or designed around a closed set of proprietary standards or communications hardware. With TCP/IP we have to think of a series of services stacked on top of each other performing different parts of the overall communications and internetworking function. The Internet It is also important at this time to understand that the term Internet can have two meanings. First, when we use the term "internet" and show it in lower case letters, we are referring to a method of communicating across multiple networks, or a collection of networks that intercommunicate. The networks can be a collection within a single corporation or business, or it can be a collection of networks among several companies. Second, when we refer to "The Internet" or "Internet" in capital letters, we are talking about the networks that form the original global Internet that developed and maintains TCP/IP standards. The Internet grew out the Department of Defense ARPANet which connected multiple universities and research institutions throughout the United States, Alaska and Hawaii. The network allowed these institutions to send and receive data and electronic mail on various scientific projects. This is the same network that in the 1960's and 70's did most of the early research on packet switching techniques. Over the years since its inception, the Internet has grown so that today it represents the largest community of on-line users throughout the world. Internet users today represent major corporations, educational institutions, military research organizations and individuals with home computers. Today's Internet has grown into a core of several networks provided by different institutions or companies that are interconnected to public and private networks around the globe. Where Did TCP/IP Come From? The members of the Internet developed TCP/IP as a method of moving data and electronic mail between the various computers on the Internet. Through an Organization called the Internet Advisory Board, the members of the Internet not only manage their network but, they maintain and continue to develop the TCP/IP standards. The standards are published in a series of documents called Request For Comments or RFC's. Because TCP/IP is controlled and maintained by a group of users and not a recognized standards body like the CCITT, IEEE or ANSI, many people refer to it as a de facto Standard. In reality the control and release of changes to TCP/IP follow well developed peer reviews which are maintained by the Internet Architecture Board (IAB) through the Internet Engineering Task Force (IETF). Further information on The Internet can be obtained from: DDN Network Information Center 14200 Park Meadow Drive Suite 200 Chantilly, VA 22021 SUMMARY The main intent of this course is to provide a basic understanding of how internetworking functions using the TCP/IP protocol. The course will explain the functions of TCP/IP by breaking down each of the layers or functions in the protocol suite and explaining them separately. At the end of the course, the final section will show how the various parts of TCP/IP tie together and provide internetworking. In Section 2 we will deal with the IP or Internet Protocol to show how it provides the addressing function and moves data across multiple networks. In Section 3 we will describe how TCP or the Transport Control Protocol moves the data between computers and insures "reliable delivery" or "transport." In Section 4 we will return to IP and explain how internetworking addresses are constructed along with a lesson in how IP networks "Learn" where all of the other computers are located. In Section 5 we will explain the services that TCP/IP provides to send mail, move files or to provide remote terminal log-in across different hosts. Finally in Section 6 we will review TCP/IP and show how the pieces fit together to make a suite of protocols that provide reliable communications for internetworking. TERMS TO REVIEW SECTION 1 Bridges Bridges are devices that provide connections between Local Area Networks below the IP or Network Layer. The bridge can be directly connected to both LAN's or two bridges can be remotely connected using Wide Area Networks. Internetworking Internetworking is the ability to move information between computers on two or more different networks. When properly implemented, internetworking functions take care of networking transparently to the user's application program. Internet Protocol The Internet Protocol (IP) provides the protocol and addressing that will move data across multiple networks. Link Layer In the ISO terminology, a link layer is where the local link protocol takes care of framing and error handling. In TCP/IP the link layer provides the local communications protocol and interface to the network. Examples of link layers can be Ethernet, PPP, HDLC or SLIP. Local Area Network A Local Area Network (or LAN) is a network that ties together devices in a department, building or campus. Generally the LAN can be a network that is entirely on a user's property and does not have any parts that are provided by outside PTT's or other carriers. Network Layer In communications protocols, the network layer is how computers communicate across multiple networks. The network layer knows the addresses and/or routes to other computers. The network layer in TCP/IP is the Internet Protocol (or IP) layer. Reliable Transport This is the methods and protocols that provide confirmation that data is sent correctly between two computers. Routers Routers are devices that provide internetworking by routing Network Layer traffic across various Link Layer interfaces. Routers not only move the data between different networks but, they change the physical or link level protocols to match each part of the transmission network. Service Layer The Service Layer in TCP/IP provides a number of functions to transfer data across networks and simplify communications for application programs. As an example, the services layer in TCP/IP has built-in file transfer and electronic mail delivery programs which can be invoked by applications. TCP/IP This is the acronym for Transport Control Protocol/Internet Protocol which defines a series of protocols designed to simplify internetwork communications. Transport Layer In TCP/IP, the transport layer is where TCP resides. The transport layer provides the end-to-end reliable communications. Transport Control Protocol The Transport Control Protocol (or TCP) provides end-to-end reliable delivery of data. When two computers send information using TCP/IP, the TCP portion of the protocol suite will insure that all of the data is sent across the network and correctly received at the remote computer. Wide Area Network The Wide Area Network (or WAN) provides a network that connects one or more sites across public boundaries (or beyond a user's property). The examples would be a network that connects offices in different cities and/or different countries. The transmission circuits used in a WAN are usually rented from public service providers such as PTT's or Value Added Network services. In some cases a WAN can be built using microwave, fiber optics or other privately owned facilities. QUESTIONS FOR REVIEW SECTION 1 1. A group of LANs and/or WANs can be tied together into an? _____________.
2. Internetworking is?_________________________________ _______________.
3. The "thread" for data across multiple networks in TCP/IP is?____ ____________________.
4. Reliable transport of data between two computers on an internet using TCP/IP is provided by?___________________________.
5. TCP/IP has more than one protocol it is therefore known as?__ ___________________________________.
ANSWERS FOR REVIEW SECTION 1 1. A group of LANs and/or WANs can be tied together into an Internetwork.
2. Internetworking is communicating between computers across multiple networks.
3. The "thread" for data across multiple networks in TCP/IP is the Internet Protocol or IP.
4. Reliable transport of data between two computers on an internet using TCP/IP is provided by the Transport Control Protocol.
5. TCP/IP has more than one protocol it is therefore known as a "suite" of protocols or a layered protocol.
END OF SECTION 1 This completes Section 1 of Introduction to TCP/IP. If you are having trouble understanding the material, we suggest that you review the videotape for Section 1. If you feel confident with the subject matter, please go on to Section 2.
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