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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