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PART III Playing with Routers > CHAPTER 14 Scheduling Packets - Pg. 339

C H A P T E R 14 Scheduling Packets A schedule defends from chaos and whim -- Annie Dillard From arranging vacations to making appointments, we are constantly scheduling activ- ities. A busy router is no exception. Routers must schedule the handling of routing updates, management queries, and, of course, data packets. Data packets must be scheduled in real time by the forwarding processors within each line card. This chapter concentrates on the efficient scheduling of data packets while allowing certain classes of data packets to receive different service from other classes. Returning to our picture of a router (Figure 14.1), recall that packets enter on input links and are looked up using the address lookup component. Address lookup provides an output link number, and the packet is switched to the output link by the switching system. Once the packet arrives at the output port, the packet could be placed in a FIFO (first in, first out) queue. If congestion occurs and the output link buffers fill up, packets arriving at the tail of the queue are dropped. Many routers use such a default output-link scheduling mechanism, often