queue length is above the minimum threshold, RED starts dropping packets. The rate of packet drop increases linearly as the average queue size increases, until the average queue size reaches the maximum threshold.
  • Maximum threshold: When the average queue size is above the maximum threshold, all packets are dropped.
  • Mark probability denominator: This number is the fraction of packets that are dropped when the average queue depth is at the maximum threshold. For example, if the denominator is 512, one out of every 512 packets is dropped when the average queue is at the maximum threshold. The linear increase of packet drops from the minimum threshold (0 drops) to the maximum threshold is based on this parameter and the queue size between the minimum and maximum thresholds.
    • The minimum threshold value should be set high enough to maximize link utilization. If the minimum threshold is too low, packets may be dropped unnecessarily, and the transmission link will not be fully used. The difference between the maximum threshold and the minimum threshold should be large enough to avoid global synchronization. If the difference is too small, many packets may be dropped at once, resulting in global synchronization. RED Modes
    Figure shows the three RED dropping modes used in TCP that are based on the average queue size: TCP Traffic Before and After RED
    Figure shows TCP throughput behavior compared to link bandwidth in a congested network scenario where the tail-drop and RED mechanisms are in use on the link. Before RED, when all sessions slow down, congestion on the router interface is removed and all TCP sessions restart their transmission at about the same time. Again, the router interface quickly becomes congested, causing tail drop. As a result, all TCP sessions back off again. This behavior cycles constantly, resulting in a link that is generally underutilized. After RED is applied, RED randomly drops packets, influencing a small number of sessions at a time, before the interface reaches congestion. Overall throughput of sessions is increased, as is average link utilization. Global synchronization is very unlikely to occur because of selective, but random, dropping of adaptive traffic.
    Content 4.6 Congestion Avoidance 4.6.4 Weighted Random Early Detection Interestingly, Cisco does not support RED. Instead, Cisco supports weighted random early detection (WRED) which combines RED with IP precedence or DSCP and performs packet dropping based on IP precedence or DSCP markings. For example, a packet with an IP precedence value of 0 might have a minimum threshold of 20 packets, whereas a packet with an IP precedence of 1 might have a minimum threshold of 25 packets. In this example, packets with an IP precedence of 0 would start to be discarded before packets with an IP precedence of 1.As with RED, WRED monitors the average queue length in the router and determines when to begin discarding packets based on the length of the interface queue. When the average queue length exceeds the user-specified minimum threshold, WRED begins to randomly drop packets with a certain probability. If the average length of the queue continues to increase so that it becomes larger than the user-specified maximum threshold, WRED reverts to a tail-drop packet-discard strategy, in which all incoming packets are dropped. The idea behind using WRED is to maintain the queue length at a level somewhere below the maximum threshold and to implement different drop policies for different classes of traffic. WRED can selectively discard lower-priority traffic when the interface becomes congested and can provide differentiated performance characteristics for different classes of service. WRED can also be configured to produce nonweighted RED behavior. For interfaces configured to use Resource Reservation Protocol (RSVP), WRED chooses packets from other flows to drop rather than the RSVP flows. Also, IP precedence or DSCP helps determine which packets are dropped, because traffic at a lower priority has a higher drop rate than traffic at a higher priority (and, therefore, lower-priority traffic is more likely to be throttled back). In addition, WRED statistically drops more packets from large users than from small users. The traffic sources that generate the most traffic are more likely to be slowed than traffic sources that generate little traffic. WRED reduces the chances of tail drop by selectively dropping packets when the output interface begins to show signs of congestion. By dropping some packets early rather than waiting until the queue is full, WRED avoids dropping large numbers of packets at once and minimizes the chances of global synchronization. As a result, WRED helps maximize the utilization of transmission lines. WRED treats non-IP traffic as precedence 0, the lowest precedence. Therefore, non-IP traffic, in general, is more likely to be dropped than IP traffic. WRED should be used wherever there is a potential congested link (bottleneck), which could very well be an access or edge link; however, WRED is normally used in the core routers of a network rather than at the network edge. Edge routers assign IP precedence or DSCP to packets as they enter the network. WRED uses these assigned values to determine how to treat different types of traffic. Note that Cisco does not recommend WRED for any voice queue, although you may enable WRED on an interface carrying voice traffic. WRED will not throttle back voice traffic because voice traffic is User Datagram Protocol (UDP)-based. The network itself should be designed not to lose voice packets because lost voice packets result in reduced voice quality. WRED controls congestion by affecting prioritized traffic other than voice, and avoiding congestion helps to ensure voice quality. Figure summarizes these points. WRED Building Blocks
    Figure shows how to implement WRED and the parameters used by WRED to influence packet-drop decisions. The router constantly updates the WRED algorithm with the calculated average queue length, which is based on the recent history of queue lengths. Configured in the traffic profile are the parameters that define the drop characteristics used by WRED (minimum threshold, maximum threshold, and mark probability denominator). These parameters define the WRED probability slopes. When a packet arrives at the output queue, the IP precedence or DSCP value is used to select the correct WRED profile for the packet. The packet is then passed to WRED for processing. Based on the selected traffic profile and the average queue length, WRED calculates the probability for dropping the current packet and either drops the packet or passes it to the output queue. If the queue is already full, the packet is dropped. Otherwise, the packet is eventually transmitted out to the interface. If the average queue length is greater than the minimum threshold but less than the maximum threshold, based on the drop probability, WRED either queues the packet or performs a random drop. Class-Based WRED
    Traditionally, Cisco IOS software used stand-alone RED and WRED mechanisms to avoid congestion on an interface. Those mechanisms can perform a differentiated drop based on the IP precedence or DSCP value. The class-based weighted fair queuing (CBWFQ) system supports the use of WRED