Plan for Progress  

The Open System Interconnection Reference Model or OSI Reference Model or OSI Model is basically a conceptual description for layered communications and computer network protocol design. It was created as part of the Open Systems Interconnection or OSI initiative. In its most simplistic form, it separates network architecture into seven distinct layers: the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. Because of this it is frequently referred to as the OSI Seven Layer Model. A layer is a collection of ideally similar functions that enable services to the layer directly above it and gets service from the layer beneath it. On each single layer an instance provides services to the instances at the layer above and requests service from the layer beneath. An example of this relates to a layer that provides error-free communications across a network which then provides the path required by applications above it, while it calls the next lower layer to send and receive packets that comprise the contents of the path.

Work on a layered model of network architecture was initialized in the year 1977. Simultaneously the International Organization for Standardization or ISO started to develop its OSI framework architecture. OSI has two major constituents: an abstract model of networking, called the Basic Reference Model or seven-layer model and a set of specific protocols. It is important to note that the standard documents that describe the OSI model can be freely downloaded from the ITU-T as the X.200-series of recommendations. A number of the protocol specifications can also be accessed as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model are available from ISO, however not all of these are available at no cost.

Every area of the OSI design evolved from experiences with the CYCLADES network, which also had a great impact on the design of the internet. The latest design has been carefully documented in ISO 7498 and several of its addenda. In this latest model, a networking system is segmented into layers. Within each layer, one or more entities establish its functionality. Each entity will interact directly only with the layer just underneath it, and enables facilities for use by the layer above it. Protocols will allow an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions ideally describe the functionality provided to an (N)-layer by an (N-1) layer, where N is one of the seven layers of protocols operating in the local host.

Neither the OSI Reference Model nor OSI protocols dictate any programming interfaces, other than a purposely abstract service specifications. Protocol specifications precisely define the interfaces between multiple computers, but the software interfaces within computers are implementation-specific.

Take for example, Microsoft Windows’ Winsock, and Unix’s Berkeley sockets and System V Transport Layer Interface, are interfaces between applications (Layer 5 and above) and the transport (Layer 4). NDIS and ODI are interfaces between the media in Layer 2 and the network protocol in Layer 3. Interface standards, with the exception of the Physical Layer to media, are estimated implementations of OSI Service Specifications.

ARP is a very significant constituent of IP networking. ARP is essentially used to link OSI Layer 3, Network, to OSI Layer 2, Data-Link. In more simplistic terms, that means that ARP is used to connect our IP address to our Ethernet address or MAC Addressing. To permit communication with any device on your network, there must be an Ethernet MAC address for that device. If the device is not on a LAN, then a default gateway may be utilized or in other words a router. Where the router is used it becomes the destination MAC address that your PC will use to communicate .

There are two forms of ARP entries; these are static and dynamic. For the most part, dynamic ARP entries will be utilized. This simply means that the ARP entry, the Ethernet MAC to IP address link, is kept on a device for a relatively lengthy duration of time, once it is being used. The static ARP entry contrasts with the dynamic ARP entry. When using a static ARP entry, the user manually entersthe link between the Ethernet MAC address and the IP address. Because of the painstaking effort of using this method and the overall associated positives with using dynamic ARP entries, dynamic ARP entries are subsequently used more often than not.

To develop an ARP entry an ARP protocol is issued. For example if a PC wants to communicate with host www.thisisanexample.com. Before it can appropriately carry out this task, it has to first resolve the hostname with the DNS server. If DNS resolves it successfully with an address of 111.112.111.112. before the PC can make actual contact with that IP address, it must then first resolve the IP address to the MAC address. To accomplish this, it must perform an ARP request. This is a broadcast that goes to the local LAN that will try to assess who has IP address 111.112.111.112 and what is the Ethernet MAC address. If server responds and says I have IP address 111.112.111.112 and my MAC address is 1234.4567.890B the PC will place that entry into its local ARP cache and it will remain there until the entry has not been used and the ARP cache timeout has expired.

Additionally, there is also the Inverse ARP protocol. This protocol executes the opposing task that the ARP protocol performs. Inverse ARP will ask all devices if they have the MAC address xxxx.xxxx.xxxx and, if so, what the specific device’s IP address is.

Regardless of the IP device being used, it will have an ARP cache. This ARP cache can be used for troubleshooting of network connectivity. If the ARP is working as it should and at optimum performance, a user will have a dynamic ARP entry that is complete with both MAC and IP values being available. If a user does not have a complete entry, he or she can clear the ARP cache and try to communicate once more by pinging the device with which the user wishes to communicate.