TCP/IP Network Architecture & Reference Model

The TCP/IP network architecture is a set of protocols that allow communication across multiple diverse networks. The architecture evolved out of research that had the original objective of transferring packets across three different packet networks: the ARPANET packet-switching network, a packet radio network, and a packet satellite network. The military orientation of the research placed a premium on robustness with regard to failures in the network and on flexibility in operating over diverse networks. This environment led to a set of protocols that are highly effective in enabling communications among the many different types of computer systems and networks. Today, the Internet has become the primary fabric for interconnecting the world’s computers. In this section we introduce the TCP/IP network architecture and TCP/IP is the main protocol for carrying information.

Following Figure shows the TCP/IP network architecture, which consists of four layers. The application layer provides services that can be used by other applications. For example, protocols have been developed for remote login, for e-mail, for file transfer, and for network management.

TCP/IP network architecture

The application layer programs are intended to run directly over the transport layer. Two basic types of services are offered in the transport layer. The first service consists of reliable connection-oriented transfer of a byte stream, which is provided by the Transmission Control Protocol (TCP). The second service consists of best effort connectionless transfer of individual messages, which is provided by the User Datagram Protocol (UDP). This service provides no mechanisms for error recovery or flow control. UDP is used for applications that require quick but reliable delivery is not guaranteed.

The TCP/IP model does not require strict layering. In other words, the application layer has the option of bypassing intermediate layers. For example, an application layer may run directly over the Internet layer.

The Internet layer handles the transfer of information across multiple networks through the use of gateways or routers. The Internet layer corresponds to the part of the OSI network layer that is concerned with the transfer of packets between machines that are connected to different networks. It must therefore deal with the routing of packets across these networks as well as with the control of congestion. A key aspect of the Internet layer is the definition of globally unique addresses for machines that are attached to the Internet. The Internet layer provides a single service, namely, best-effort connectionless packet transfer. IP packets are exchanged between routers without a connection setup; the packets are routed independently, and so they may traverse different paths. For this reason, IP packets are also called datagrams. The connectionless approach makes the system robust; that is, if failures occur in the network, the packets are routed around the points of failure; there is no need to set up the connections. The gateways that interconnect the intermediate networks may discard packets when congestion occurs. The responsibility for recovery from these losses is passed on to the transport layer.

Finally, the network interface layer is concerned with the network-specific aspects of the transfer of packets. As such, it must deal with parts equivalent to OSI network layer and data link layer. Various interfaces are available for connecting end computer systems to specific networks such as X.25, ATM, frame relay, Ethernet, and token ring.

The network interface layer is particularly concerned with the protocols that access the intermediate networks. At each gateway the network access protocol encapsulates the IP packet into a packet of the underlying network or link. The IP packet is recovered at the exit gateway of the give n network. This gateway must then encapsulate the IP packet into new packet of the type of the next network or link. This approach provides a clear separation of the internet layer from the technology dependent network interface layer. This approach also allows the internet layer to provide a data transfer service that is transparent in the sense of not depending on the details of the underlying networks.