Chap5 - 5 3 5 units 5 units All links have a capacity of 10...

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5 3 5 units 5 units All links have a capacity of 10 units Routing in Data Networks 5.1 INTRODUCTION We have frequently referred to the routing algorithm as the network layer protocol that guides packets through the communication subnet to their correct destination. The times at which routing decisions are made depend on whether the network uses datagrams or virtual circuits. In a datagram network, two successive packets of the same user pair may travel along different routes, and a routing decision is necessary for each individual packet (see Fig. 5.1). In a virtual circuit network, a routing decision is made when each virtual circuit is set up. The routing algorithm is used to choose the communication path for the virtual circuit. All packets of the virtual circuit subsequently use this path up to the time that the virtual circuit is either terminated or rerouted for some reason (see Fig. 5.2). Routing in a network typically involves a rather complex collection of algorithms that work more or less independently and yet support each other by exchanging services or information. The complexity is due to a number of reasons. First, routing requires coordination between all the nodes of the subnet rather than just a pair of modules as, for example, in data link and transport layer protocols. Second, the routing system must 363
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364 Routing in Data Networks Chap. 5 cope with link and node failures, requiring redirection of traffic and an update of the databases maintained by the system. Third, to achieve high performance, the routing algorithm may need to modify its routes when some areas within the network become congested. The main emphasis will be on two aspects of the routing problem. The first has to do with selecting routes to achieve high performance. In Sections 5.2.3 to 5.2.5, we discuss algorithms based on shortest paths that are commonly used in practice. In Sections 5.5, 5.6, and 5.7 we describe sophisticated routing algorithms that try to achieve near optimal performance. The second aspect of routing that we will emphasize is broadcasting routing-related information (including link and node failures and repairs) to all network nodes. This issue and the subtleties associated with it are examined in Section 5.3. The introductory sections set the stage for the main development. The remainder of this section explains in nonmathematical terms the main objectives in the routing problem and provides an overview of current routing practice. Sections 5.2.1 to 5.2.3 present some of the main notions and results of graph theory, principally in connection with shortest paths and minimum weight spanning trees. Section 5.4 uses the material on graph theory to describe methods for topological design of networks. Finally, Section 5.8 reviews the routing system of the Codex network and its relation to the optimal routing algorithm of Section 5.7.
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Chap5 - 5 3 5 units 5 units All links have a capacity of 10...

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