In a packet switched network each packet sent by a

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Unformatted text preview: , then that computer will have multiple addresses, one for each active interface). In a packet-switched network, each packet sent by a sender contains the address of the destination. It also usually contains the address of the sender, which allows applications and other protocols running at the destination to send packets back. All this information is in the packet’s header, which also may include some other useful fields. When a switch gets a packet, it consults a table keyed by the destination address to determine which link to send the packet on in order to reach the destination. This process is also known as a table lookup, and the table in question is termed the routing table.2 The selected link is called the outgoing link. The combination of the destination address and outgoing link is called 2 In practice, in high-speed networks, the routing table is distinct from the forwarding table. The former contains both the route to use for any destination and other properties of the route, such as the cost. The latter is a table that contains only the route, and is usually placed in faster memory because it has to be consulted on every packet. CHAPTER 18. NETWORK ROUTING - I 4 WITHOUT ANY FAILURES Figure 18-2: A simple network topology showing the routing table at node B. The route for a destination is marked with an oval. The three links at node B are L0, L1, and L2; these names aren’t visible at the other nodes but are internal to node B. the route used by the switch for the destination. Note that the route is different from the path between source and destination in the topology; the sequence of routes at individual switches produces a sequence of links, which in turn leads to a path (assuming that the routing and forwarding procedures are working correctly). Figure 18-2 shows a routing table and routes at a node in a simple network. Because data may be corrupted when sent over a link (uncorrected bit errors) or because of bugs in switch implementations, it is customary to incl...
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This document was uploaded on 02/26/2014 for the course CS 6.02 at MIT.

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