Cisco - IOS Technologies - Quality - Random Early Detection

Random Early Detection (RED)

Random Early Detection (RED) is a congestion avoidance mechanism designed for packet switched networks that aims to control the average queue size by indicating to the end hosts when they should temporarily stop sending packets. RED takes advantage of TCP’s congestion control mechanism. By randomly dropping packets prior to periods of high congestion, RED tells the packet source to decrease its transmission rate. Assuming the packet source is using TCP, it will decrease its transmission rate until all the packets reach their destination, indicating that the congestion is cleared. You can use RED as a way to cause TCP to back off traffic. TCP not only pauses, but it also restarts quickly and adapts its transmission rate to the rate that the network can support. RED distributes losses in time and maintains normally low queue depth while absorbing spikes. When enabled on an interface, RED begins dropping packets when congestion occurs at a rate you select during configuration.

RED is recommended only for TCP/IP networks. RED is not recommended for protocols, such as AppleTalk or Novell Netware, that respond to dropped packets by retransmitting the packets at the same rate.

Weighted Random Early Detection

Cisco’s implementation of RED, called Weighted Random Early Detection (WRED), combines the capabilities of the RED algorithm with IP Precedence. This combination provides for preferential traffic handling for higher priority packets. It can selectively discard lower priority traffic when the interface begins to get congested, and provide differentiated performance characteristics for different classes of service. WRED differs from other congestion management techniques such as queuing strategies because it attempts to anticipate and avoid congestion rather than controlling congestion once it occurs. Figure 1 illustrates how WRED works.

By randomly dropping packets prior to periods of high congestion, WRED tells the packet source to decrease its transmission rate. Assuming the packet source is using TCP, it will decrease its transmission rate until all the packets reach their destination, indicating that the congestion is cleared. WRED generally drops packets selectively based on IP Precedence. Packets with a higher IP Precedence are less likely to be dropped than packets with a lower precedence. Thus, higher priority traffic is delivered with a higher probability than lower priority traffic. However, you can also configure WRED to ignore IP precedence when making drop decisions so that non weighted RED behavior is achieved. WRED is also RSVP-aware, and can provide integrated services controlled-load QoS service.

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 buffer is full, WRED avoids dropping large numbers of packets at once and minimizes the chances of global synchronization. Thus, WRED allows the transmission line to be used fully at all times. In addition, WRED statistically drops more packets from large users than small. Therefore, traffic sources that generate the most traffic are more likely to be slowed down than traffic sources that generate little traffic.

Why Use Weighted Random Early Detection?

WRED is useful on any output interface where you expect to have congestion. However, WRED is usually used in the core routers of a network, rather than the nework’s edge. Edge routers assign IP precedences to packets as they enter the network. WRED uses these precedences to determine how it treats different types of traffic. WRED provides separate thresholds and weights for different IP precedences, allowing you to provide different qualities of service for different traffic. Standard traffic may be dropped more frequently than premium traffic during periods of congestion.