 interesting for today's QoS needs in advanced IP networks. Probability of dropping a packet randomly Queue-Fill Percentage FIGURE 15.1 The principle of a RED curve tying the probability of randomly discarding packets based on how full a specific queue is Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. TRAFFIC MANAGERS 324 NETWORK PROCESSORS Probability of dropping a packet randomly Queue-Fill Percentage FIGURE 15.2 In a WRED environment, multiple RED curves are created for traffic of different priorities. This aggressive curve could be combined with the RED curve shown in Figure 15.1 for packets that are marked differently. If the two algorithms are compared, it is apparent that RED is designed to prevent the tail-drop, which is caused by sudden bursts of traffic. Typical backbone routers can buffer up to around 100 ms worth of traffic per port. If a more significant burst of traffic occurs, buffers will overfill and oncom- ing packets will start to get dropped. For example, the phenomenon of tail-drop has affected TCP flows by causing them to decrease their rates only for them to increase later. In many occasions, unde- sirable oscillations can occur, which lead to instability in the application's performance and the net- work's traffic utilization. RIO is a variation of the RED technique, as shown in Figure 15.3. In this example, two traffic classes are assumed. One is prioritized (also called In) and the other is nonprioritized (also called Out). The packet-discarding probability for nonprioritized traffic lies between the buffered queue sizes Min and Max, while the packet-discarding probability for prioritized traffic lies between the buffered queue sizes Max and Total. If the queue size of Max is exceeded, all arriving nonprioritized traffic packets will be dropped. The two shaded regions are called the Graduated Dropping Regions for t