The OSI Reference Model, in its simplest terms is a seven layered design designed to make software development, product evolution, modular engineering and multi vendor interoperability easier

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Introduction

Introduction Developed by the International Standards Organisation (ISO) in the 1980s and often described simply as The Stack, The OSI Reference Model, in its simplest terms is a seven layered design designed to make software development, product evolution, modular engineering and multi vendor interoperability easier. This enables the solutions offered by one layer to be updated without adversely affecting the other layers thus making it easier for both application and hardware developers alike. By the time the OSI Reference Model had been fully developed The Defence Advanced Research Project Agency had continued to develop what has now become the de facto standard for the internet, the four layered TCP/IP Protocol Stack. TCP/IP attempts to create a heterogeneous network with open protocols that are independent of operating system and architectural difference. TCP/IP protocols are available to everyone, and are developed and changed by consensus. Everyone is free to develop products to meet these open protocol specifications. The Department of Defences development of The TCP/IP Protocol Stack halted further development of the OSI Reference Model and indeed other vendors such as Netware, AppleTalk, Xerox and 3com developed there own standards. Although based on the OSI Reference Model, the concepts of protocol hierarchy are somewhat different. This report will focus on the different protocol layers and their functions within the OSI Reference Model and the TCP/IP Protocol Stack, some comparisons with other vendor specific stacks will also be made.

Middle

If they are not, Layer 6 does not attempt to fix things by itself; it simply fails. Session Maintains end to end communication sessions of the logical link between users. Such function involves a session, which may in turn support one or more presentation spaces above it and one or more applications above that. The session layer is also responsible for reporting upper layer errors. Transport The workhorse of the stack, and the part that most people think of first when they consider networking operations. Maintains the flow of data from source to destination. Provides error checking and recovery of data between the devices, The transport layer can also sequence and acknowledge data requests. In the TCP/IP Model, The Transport layer (also known as the Host-to-Host Transport layer) is responsible for providing the Application layer with session and datagram communication services. The core protocols of the Transport layer are Transmission Control Protocol (TCP) a guaranteed connection orientated protocol and the User Datagram Protocol (UDP) a best effort non guaranteed protocol. This layer is also important for routing as some sophisticated networking products use packet-processors, which can assist with efficient handling of data across layers, typically up to the tcp/udp level. For example, a packet processor may know about tcp header formats and can provide efficient checksum procedures based on that knowledge. For devices such as routers this is important, because as the packet traverses the system (received from one machine, and sent to another), fields in the header (such as the time to live (TTL)

Conclusion

XNS is used as a protocol today only in networks by vendors that adopt some of the standards that XNS provide. These numbers are shrinking still and very few new networks are being based on XNS. Internetworking Technologies handbook Conclusion "The quarter-century-old OSI model describes a layered network architecture that spans from the Physical Layer of networking (connectors, wires, voltages, etc.) up to the Application Layer which delivers reconstituted data to the applications. There are several important properties of the model. The duty of each layer is well defined and documented. Each layer's job is to facilitate the transfer of information between adjacent layers. It is possible to implement something new at any layer (or even create sub-layers) provided interlayer semantics are respected. Another useful feature of this schema is to focus attention on the appropriate layer when a problem arises. A trivial example might be packet collision on Ethernet. Although it may affect higher layers, it is not within the scope, for example, of a Layer 7 protocol to deal with this. Two additional principles in the specification of the model are that separate layers perform functions that differ in their technology, and, that similar functions are placed in the same layer. This model has proven to be quite successful as a design and teaching tool, as evidenced by its standardization and its longevity". Andrew S. Patrick The OSI model plays an important role in standardizing communications between disparate systems. Were it not for the standards and specifications originally outlined within in the OSI model, linking Apple, Novell and other systems would certainly prove much more challenging.

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