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TCP/IP protocol architecture The TCP/IP communications suite was designed with modularity in mind. This means that instead of developing a solution which integrates all aspects of communications in one single piece of code, the designers wisely chose to break the puzzle into its constituent components and deal with them individually while recognizing the interdependence tying the pieces together. Thus, TCP/IP evolved into a suite of protocols specifying interdependent solutions to the different pieces of the communications puzzle. This approach to problem solving is normally referred to as the layering approach. Consequently, hereafter, reference will be made to the TCP/IP suite as a layered suite of communications. The four-layer model of the TCP/ IP communications architecture is based on an understanding of data communications that involves four sets of interdependent processes: application representative processes (application layer), host representative processes (host-tohost layer), network representative processes (Internet layer), and media access and delivery representative process (network access layer). Each set of processes takes care of the needs of entities it represents whenever an application engages in the exchange of data with its counterpart on the network. These process sets are grouped into the following four layers: application layer, host-to-host (also known as transport) layer, internet layer, and network access layer. Each of these layers may be implemented in separate, yet interdependent, pieces of software code.

Application Layer Application representative processes take care of reconciling differences in the data syntax between the platforms on which the communicating applications are running. Communicating with an IBM mainframe, for example, might involve character translation between the EBCDIC and ASCII character sets. While performing the translation task the application layer (for instance, application representative process) need not have (and shouldn't care to have) any understanding of how the underlying protocols (for instance, at the host-to-host layer) handles the transmission of translated characters between hosts. Examples of protocols supported at the application layer include FTP, TELNET, NFS, and DNS.

Host-to-Host Transport Layer Host representative processes (for example, the host-to-host, or transport, layer) take care of communicating data reliably between applications running on hosts across the network. It is the responsibility of the host representative process to guarantee the reliability and integrity of the data being exchanged, without confusing the identities of the communication applications. For this reason the host-to-host layer is provided with the mechanism necessary to allow it to make the distinction between the applications on whose behalf it is making data deliveries. In other words, assume that two hosts, tenor and alto, are connected to the same network. Furthermore, assume that a user on host alto is logged in to FTP on host tenor. Also, while using FTP to transfer files, the user is utilizing TELNET to login in to host tenor to edit a document. In this scenario, data exchanged between both hosts could be due to TELNET, FTP, or both. It is the responsibility of the host-to-host layer, hereafter called the transport layer, to make sure that data is sent and delivered to its intended party. What originates from FTP at either end of the connection should be delivered to FTP at the other end. Likewise, TELNET-generated traffic should be delivered to TELNET at the other end, not to FTP. To achieve this, as will be discussed later, the transport layer at both ends of the connection must cooperate in clearly marking data packets so that the nature of the communicating applications is easily identifiable. Protocols operating at the transport layer include both UDP (User Datagram Prot