网络与网际互联.pptVIP

The delivery of packets to their destinations in a network such as the one shown in Figure 3.7 is the collective responsibility of the routers located at connection points. Unless the source and destination hosts are on the same LAN, the packet has to be transmitted in a series of hops, passing through router nodes. The determination of routes for the transmission of packets to their destinations is the responsibility of a routing algorithm implemented by a program in the network layer at each node. A router exchanges information about the network with its neighbouring nodes by sending a summary of its routing table using a router information protocol (RIP). The RIP actions performed at a router are described informally as follows: 1. Periodically, and whenever the local routing table changes, send the table (in a summary form) to all accessible neighbours. That is, send an RIP packet containing a copy of the table on each non-faulty outgoing link. 2. When a table is received from a neighbouring router, if the received table shows a route to a new destination, or a better (lower-cost) route to an existing destination, update the local table with the new route. If the table was received on link n and it gives a different cost than the local table for a route that begins with link n, replace the cost in the local table with the new cost. This is done because the new table was received from a router that is closer to the relevant destination and is therefore always more authoritative for routes that pass through it. The capacity of a network is limited by the performance of its communication links and switching nodes. When the load at any particular link or node approaches its capacity, queues will build up at hosts trying to send packets and at intermediate nodes holding packets whose onward transmission is blocked by other traffic. If the load continues at the same high level, the queues will continue to grow until they reach the limit of available buffer space.