Managing TCP/IP Networks P2

Now that we have an appreciation for the evolution of the Internet and the TCP/IP protocol suite, let us turn our attention to the structure of the protocol suite. However, since the TCP/IP protocol suite has a layered structure, we will ®rst examine the ISO Reference Model and the subdivision of its second layer by the Institute of Electrical and Electronic Engineers (IEEE) to provide a standardized frame of reference.2.3 THE ISO REFERENCE MODELThe International Organization for Standardization is an agency of the United Nations headquartered in Geneva, Switzerland. The ISO is tasked with the development of worldwide standards to...
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Managing TCP/IP Networks P218 THE TCP/IP PROTOCOL SUITE Now that we have an appreciation for the evolution of the Internet and theTCP/IP protocol suite, let us turn our attention to the structure of theprotocol suite. However, since the TCP/IP protocol suite has a layeredstructure, we will ®rst examine the ISO Reference Model and the subdivisionof its second layer by the Institute of Electrical and Electronic Engineers(IEEE) to provide a standardized frame of reference.2.3 THE ISO REFERENCE MODELThe International Organization for Standardization is an agency of the UnitedNations headquartered in Geneva, Switzerland. The ISO is tasked with thedevelopment of worldwide standards to facilitate the international exchangeof goods and services. The membership of the ISO consists of the nationalstandards organization of most countries, with over 100 countries participat-ing in its work. One of the most notable achievements of the ISO in the ®eld ofdata communications was its development of the seven-layer Open SystemsInterconnection (OSI) Reference Model. This model de®nes the communica-tions process as a set of seven layers, with speci®c functions isolated andassociated with each layer. Figure 2.2 illustrates the seven layers of the ISO Reference Model. Eachlayer covers lower layer processes, effectively isolating them from higher layerfunctions. In this way, each layer performs a set of functions necessary toprovide a set of services to the layer above it. Layer isolation permits thecharacteristics of a given layer to change without impacting the remainder ofthe model, provided that the supporting services remain the same. Thislayering was developed as a mechanism to enable users to mix and matchOSI-conforming communications products to tailor their communicationssystems to satisfy a particular networking requirement. Although OSI-conforming communications products never gained a signi®cant degree ofacceptance, the OSI Reference Model provides a framework for comparingFigure 2.2 The International Organization for Standardization (ISO) Open SystemInterconnection (OSI) Reference Model2.3 THE ISO REFERENCE MODEL 19and contrasting the features and structure of other protocol suites. Inaddition, by understanding the structure of the model and the subdivision ofits second layer by the IEEE, we can also obtain an appreciation of thecapabilities and limitations of other protocol suites as well as the manner bywhich those suites support data ¯ow from source to destination.2.3.1 Layers of the OSI Reference ModelWith the exception of layers 1 and 7, each layer in the ISO Reference Model isbounded by the layers above and below it. Layer 1, the physical layer, whichis responsible for moving bits in electrical or optical form, can be consideredto be bound below by the interconnecting medium over which transmission¯ows. In comparison, layer 7 is the upper layer and has no upper boundary.Within each layer is a group of functions that can be viewed as providing a setof de®ned services to the layer that bounds it from above, resulting in layer nusing the services of layer n-1. To obtain an appreciation of the manner inwhich the ISOs Reference Model operates, let us turn our attention to each ofthe layers in the model.Layer 1: the physical layerAt the lowest or most basic layer, the physical layer represents a set of rulesthat speci®es the electrical, optical, and physical connection between devicesand the transmission medium. Typically, the physical layer can include thecoding method by which data is placed onto the medium as well as thecabling interface to include the operation of different pins on the cablingconnection.Layer 2: the data link layerThe data link layer de®nes how a device gains access to the medium speci®edby the physical layer as well as the data formats to include framing, errorcontrol procedures, and other link control activities. The data formatspeci®cation includes procedures employed to correct transmission errors,thus, layer 2 becomes responsible for the reliable delivery of information. At the data link layer information is grouped into entities referred to asframes. As a minimum, each frame contains control information that enablesthe receiver to synchronize itself to an incoming frame, addressinginformation that identi®es the source and destination, a ®eld containingthe actual information being transmitted from source to destination, and a®eld used for verifying the integrity of the data. One important characteristic of data link protocols is the fact that they donot have network addresses and as such are non-routable. As we will notelater in this chapter, Ethernet, Token-Ring, and FDDI represent examples ofdata link protocols.20 THE TCP/ ...