ven at the website. TRAFFIC MANAGERS TRAFFIC MANAGERS One of the most sophisticated, but rarely used, techniques (similar to the approach taken by frame relay in the early 1990s) is the Explicit Congestion Notification (ECN) technique. In ECN, the pack- ets that suffer due to congestion are marked appropriately and forwarded to their destination. Upon reception, the receiver notifies the sender of these packets, which slows down its transmission pace. A similar method used in connection-oriented ATM networks is the Early Forward Congestion Indication (EFCI). The most common algorithms used to resolve traffic congestion are RED, WRED, and RED with In and Out (RIO) in IP networks (which may or may not be connectionless if MPLS is involved), and EPD and PPD in (connection-oriented) ATM and cell-based MPLS networks. RED is a probabilistic method allowing the random discarding of packets as soon as the corre- sponding queue is filled beyond a prespecified threshold. As shown in Figure 15.1, this probability increases when packets fill the queue beyond the threshold level. The curve is composed of multiple points that depend on the actual application realm and network requirements. With RED, the same curve will be applicable if all the traffic has the same priority. If this is not the case and traffic has dif- ferent priorities, then several RED curves will be applicable. WRED curves do not have to be of the same geometric profile. For example, in Figure 15.2, we show a completely different profile of more aggressive action in a WRED environment that could be used between the profiles shown in Figures 15.1 and 15.2. If we follow a three-color marking exam- ple like the one we described earlier in the chapter, green packets can be discarded based on the pro- file shown in Figure 15.1 and yellow packets can be discarded based on the profile shown in Fig- ure 15.2. Red packets can be simply dropped if no bandwidth is available. The ability to adapt thresh- olds and drop rates to the various traffic classes makes WRED more