TCPIP Protocol Architecture Computer Networks

A brief description of the ISO OSI model from a network data communication standpoint follows: Application Layer This layer is the level of the protocol hierarchy where the user−accessed network processes reside. This includes all of the processes that users interact with directly, as well as other processes at this level of which users are not necessarily aware. This is actually the layer that is continuously upgraded; each new network application specifies a new application−specific protocol to be managed by this layer. Presentation Layer This layer provides standard data presentation routines. In the exchange of data, the cooperating applications must agree on how data is represented. Session Layer This layer manages the sessions connections between cooperating applications. Transport Layer This layer guarantees that the receiving site gets the data exactly as it was sent, i.e., it ensures the completeness of the data transportation. Network Layer This layer manages connections across the network and isolates the upper layer protocols from the details of the underlying network. It handles the addressing and delivery of data. Data Link Layer This layer handles a reliable delivery of data across the underling network. The layer is basically divided into two sublayers: Logical Link Control LLC, the hardware independent sublayer • Media Access Control MAC, the hardware dependent sublayer • Physical Layer This layer defines the characteristics of the network interface hardware needed to carry the data transmission signal. Details such as voltage levels, number and location of interface pins, signal modulation, and so on, are specified here. Ethernet standard IEEE 802.3 is an example. It is not mandatory for each layer to exist in any practical implementation. Unnecessary layers can be skipped, as in the case of the router in Figure 14.4. It has the same effect as when several layers are joined together into one layer.

14.3.3 TCPIP Protocol Architecture

The ISO OSI Reference Model presents the most common way to describe protocol architecture. The International Standard Organization established the model, and it assists in the implementation of any inter−computer communication technology. In that light, the OSI model could be used also to fully describe TCPIP protocols. However, sometimes it is easier and more convenient to use other, simpler protocol models to describe or rather, to understand specific protocols; noncrucial layers for specific protocols could be ignored. For example, for an easier understanding of TCPIP protocols, another architectural model that more closely matches the structure of TCPIP is more appropriate. This model is known as the Four Layer TCPIP Architectural Model and is presented in Figure 14.5. APPLICATION LAYER 329 End−to−end data delivery INTERNET LAYER Datagrams and data routing NETWORK ACCESS LAYER Access to physical network PHYSICAL LAYER Network media not included in the TCPIP model Figure 14.5: The four−layer TCPIP protocol architecture. The model provides a pictorial representation of the layers in the TCPIP protocol hierarchy. Data is processed in the same way as in the ISO OSI model, except that fewer layers take part in its processing. Data is passed down or up the protocol stack when it is being sent to or received from a network. The flow of data is represented in Figure 14.6. Figure 14.6: Data flow through the TCPIP protocol stack. Each layer in the stack adds control information to ensure proper delivery. This control information is called a header, because it is placed in front of data to be transmitted. Each layer treats all of the information it receives from the layer above as data and places its own header in front of that information. The addition of delivery information at every layer is called encapsulation. When data is received the opposite occurs. Each layer strips off its header and checks it before passing the data on to the layer above. Therefore each layer treats the information it receives from the lower layer as both a header and data. The TCPIP protocol stack does not include the physical layer, i.e., the stack does not treat physical network itself. However, the lowest layer depends on the implemented physical network. Each layer has its own independent data structures, compatible with data structures in the neighboring layers. Conceptually this is not mandatory, but a strictly defined interface between layers implies such compatibility; otherwise, it would be more difficult to build the interface. Still, each layer has its own terminology to describe data structures. Figure 14.7 shows the terms used 330 Figure 14.7: Data structures in the TCPIP protocol stack.

14.4 TCPIP Layers and Protocols