The FTP client and the other the FTP server

If we have two host on a local area network (LAN) such as an Ethernet, Both running FTP, Figure 1.2 show the protocol involved


We have labeled one application box the FTP client and the Other FTP server. Most network applications are designed so that one end is the client and the other side the server. The server provides some type of service to clients, in this case access to files on the server host. In the remote login application, telnet, the service provide to the client is the ability to login to the server's host.

Each layer has one or more protocols for the communicating with its peer at the same layer. One protocol, for example, allows the two TCP layers to communicate, and another protocol lest the two IP layers communicate.

On the right side of Figure 1.2 we have noted that normally the application layer is a user process while the lower three layers are usually implemented in the kernel (the operating system). Although this isn't a requirement, it's typical and this is the way it's done under Unix.

There is another critical difference between the top layer in Figure 1.2 and the lower three layers. The application layer is concerned with the details of the application and not with the movement of data across the network. The lower three layers know nothing about the application but handle all the communication details.

We show four protocols in Figure 1.2, each at a different layer. FTP is an application layer protocol, TCP is a transport layer protocol, IP is a network layer protocol, and the Ethernet protocols operate at the link layer. The TCP/IP protocol suite is a combination of many protocols. Although the commonly used name for the entire protocol suite is TCP/IP, TCP and IP are only two of the protocols. (An alternative name is the Internet Protocol Suite.)

The purpose of the network interface layer and the application layer are obvious-the former handles the details of the communication media (Ethernet, token ring, etc.) while the latter handles one specific user application (FTP, Telnet, etc.). But on first glance the difference between the network layer and the transport layer is somewhat hazy. Why is there a distinction between the two? To understand the reason, we have to expand our perspective from a single network to a collection of networks.

One of the reasons for the phenomenal growth in networking during the 1980s was the realization that an island consisting of a stand-alone computer made little sense. A few stand-alone systems were collected together into a network. While this was progress, during the 1990s we have come to realize that this new, bigger island consisting of a single network doesn't make sense either. People are combining multiple networks together into an internetwork, or an internet. An internet is a collection of networks that all use the same protocol suite.

The easiest way to build an internet is to connect two or more networks with a router. This is often a special-purpose hardware box for connecting networks. The nice thing about routers is that they provide connections to many different types of physical networks: Ethernet, token ring, point-to-point links, FDDI (Fiber Distributed Data Interface), and so on.

These boxes are also called IP routers, but we'll use the term router.

Historically these boxes were called gateways, and this term is used throughout much of the TCP/IP literature. Today the term gateway is used for an application gateway: a process that connects two different protocol suites (say, TCP/IP and IBM's SNA) for one particular application (often electronic mail or file transfer).