The TCP/IP Reference Model

Let us now turn from the OSI reference model to the reference model used in the grandparent of all wide area computer networks, the ARPANET, and its successor, the worldwide Internet.

The ARPANET was a research network sponsored by the DoD (US Department of Defense).  It eventually connected hundreds of universities and government installations, using leased telephone lines. When satellite and radio networks were added later, the existing protocols had trouble interworking with them, so a new reference architecture was needed. Thus, the ability to connect multiple networks in a seamless way was one of the major design goals from he very beginning. This architecture later become known as the TCP/IP Reference Model, after its two primary protocols. It was first defined in (Cerf and Kahn, 1974).

The design philosophy behind the model is discussed in (Clark, 1988).

Given the DoD’s worry that some of its precious hosts, routers, and internetwork gateways might get blown o pieces at a moment’s notice, another major goal was that the network be able to survive loss of subnet hardware, with existing conversations not being broken off. In other words, DoD wanted connections to remain intact as long as the source and destination machines were functioning, even if some of he machines or transmission lines in between were suddenly put out of operation. Furthermore, flexible architecture was needed since applications with divergent requirements were envisioned, ranging from transferring files to real-time speech transmission.

The Internet Layer:

All these requirements led to the choice of a packet-switching network based on a connectionless internetwork layer. This layer, called the internet layer, is the linchpin that holds the whole architecture together. Its job is to permit hosts to inject packets into any network and have them travel independently to the destination. They may even arrive in a different order than they were sent, in which case it is the job of higher layers to rearrange them.

The internet layer defines an official packet format and protocol called IP (Internet Protocol). The job of the internet layer is to deliver IP packets where they are supposed to go.

Packet routing is clearly the major issue here, as is avoiding congestion. For these reasons, it is reasonable to say that the TCP/IP internet layer is similar in functionality to the OSI network layer.

The Transport Layer:

The layer above the internet layer in the TCP/IP model is now usually called the transport layer. It is designed to allow peer entities on the source and destination hosts to carry on a conversation, just as in the OSI transport layer. Two end-to-end transport protocols have been defined here.

The first one, TCP (Transmission Control Protocol), is a reliable connection-oriented protocol that allows a byte stream originating on one machine to be delivered without error on any other machine in the internet. It fragments the incoming bytes stream into discreate message and passes each one on to the internet layer.

The second protocol in this layer, UDP (User Datagram Protocol), is an unreliable, connectionless protocol for applications that do not want TCP’s sequencing or flow control and wish to provide their own. It is also widely used for one-shot, client-server-type request queries and applications in which prompt delivery is more important that accurate delivery, such as transmitting speech or video.

The Application Layer:

The TCP/IP model does not have session or presentation layers. No need for them was perceived, so they were not included. Experience with the OSI model has proven this view correct: they are of little use to most applications.

On top of the transport layer is the application layer. It contains all the higher-level protocols. The early ones included virtual terminal (TELNET), file transfer (FTP), and electronic mail (SMTP).

The virtual terminal protocol allows a user on one machine o log onto a distant machine and work there. The file transfer protocol provides a way to move data efficiently from one machine to another.

The Host-to-Network Layer:

Below the internet layer is a great void. The TCP/IP reference model does not really say much about what happens here, except to point out that the host has to connect to the network using some protocols so it can send IP packets to it. This protocol is not defined and varies from host to host and network to network.

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