Systems Programming by Richard John Anthony

Author:Richard John Anthony
Language: eng
Format: epub
ISBN: 9780128008171
Publisher: Elsevier Inc.
Published: 2015-02-24T16:00:00+00:00

Figure 4.28 A core network of seven routers and the routing table for Router B.

The router's use of the routing table is illustrated with a couple of examples: (1) A packet arrives from Router A, with a destination network address of 12.13.0.0. The router looks up the destination address 12.13.0.0 and finds that the current best known route is via link 2. This means that the packet will be passed to Router C. 2. A packet arrives from Router D, with a destination network address of 12.1.0.0. The router looks up the destination address and finds that the current best known route is via link 1, so the packet is passed to Router A.

In distance vector routing (which is the basis of the example shown in Figure 4.28), each router only has partial knowledge of the entire network topology and does not have to know the full configuration of all routers and links. In this case, Router B does not need to know the actual connectivity beyond its immediate neighbors, but through the exchange of routing protocol messages with its neighbors, it will learn the best routes (in terms of the outgoing links to use) to the destinations.

The actual way in which the router chooses the output link is determined by the actual routing protocol in use and the actual metrics used by that protocol, which define what is meant by “best” route (e.g., the least number of hops, least loaded, and most reliable). In some cases, there will only be one output link that can move the packet on its way in the right direction, so regardless of the routing protocol, this link will be chosen. Things are more complex when there are many possible routes to choose from and where the network traffic conditions are highly dynamic, leading to fluctuations in link utilization and queue lengths in the routers themselves. Under such circumstances, it can be difficult to predict which link the router will select for any specific packet and two consecutive packets sent by the same source node and heading to the same destination node may actually be passed along different links by a given router; see Figure 4.29.

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