Internet Architecture

In this article i written about the internet architecture.

The Internet architecture, which is also sometimes called the TCP/IP architecture after

its two main protocols. The Internet architecture evolved out of experiences with an earlier packet-switched network called the ARPANET. Both the Internet and the ARPANET were funded by the Advanced Research Projects Agency (ARPA), one of the R&D funding agencies of the U.S. Department of Defense. The Internet and ARPANET were around before the OSI architecture, and the experience gained from building them was a major influence on the OSI reference model. While the seven-layer OSI model can, with some imagination, be applied to the Internet, a four-layer model is often used instead. At the lowest level are a wide variety of network protocols, denoted NET1, NET2, and so on. In practice, these protocols are implemented by a combination of hardware (e.g., a network adaptor) and software (e.g., a network device driver). For example, you might find Ethernet or Fiber Distributed Data Interface (FDDI) protocols at this layer. (These protocols in turn may actually involve several sublayers, but the Internet architecture does not presume anything about them.) The second layer consists of a single protocol—the Internet Protocol (IP). This is the protocol that supports the interconnection of multiple networking technologies into a single, logical internetwork. The third layer contains two main protocols—the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). TCP and UDP provide alternative logical channels to application programs: TCP provides a reliable byte-stream channel, and UDP provides an unreliable datagram delivery channel (datagram may be thought of as a synonym for message). In the language of the Internet, TCP and UDP are sometimes called end-to-end protocols, although it is equally correct to refer to them as transport protocols.
Running above the transport layer are a range of application protocols, such as FTP, TFTP (Trivial File Transport Protocol), Telnet (remote login), and SMTP (Simple Mail Transfer Protocol, or electronic mail), that enable the interoperation of popular applications. To understand the difference between an application layer protocol and an application, think of all the different World Wide Web browsers that are available (e.g., Mosaic, Netscape, Internet Explorer, Lynx, etc.). There are a similarly large number of different implementations of Web servers. The reason that you can use any one of these application programs to access a particular site on the Web is because they all conform to the same application layer protocol: HTTP (HyperText Transport Protocol). Confusingly, the same word sometimes applies to both an application and the application layer protocol that it uses (e.g., FTP). The Internet architecture has three features that are worth highlighting.

First, as the Internet architecture does not imply strict layering. The application is free to bypass the defined transport layers and to directly use IP or one of the underlying networks. In fact, programmers are free to define new channel abstractions or applications that run on top of any of the existing protocols.

Second, if you look closely at the protocol graph you will notice an hourglass shape—wide at the top, narrow in the middle, and wide at the bottom. This shape actually reflects the central philosophy of the architecture. That is, IP serves as the focal point for the architecture—it defines a common method for exchanging packets among a wide collection of networks. Above IP can be arbitrarily many transport protocols, each offering a different channel abstraction to application programs.

Thus, the issue of delivering messages from host to host is completely separated from the issue of providing a useful process-to-process communication service. Below IP, the architecture allows for arbitrarily many different network technologies, ranging from Ethernet to FDDI to ATM to single point-to-point links.

A final attribute of the Internet architecture (or more accurately, of the IETF culture) is that in order for someone to propose a new protocol to be included in the architecture, they must produce both a protocol specification and at least one (and preferably two) representative implementations of the specification. The existence of working implementations is required for standards to be adopted by the IETF. This cultural assumption of the design community helps to ensure that the architecture’s protocols can be efficiently implemented.