Abstract

Abstract-This paper presents Random Early Detection (RED) gateways for congestion avoidance in packet-switched networks. The gateway detects incipient congestion by computing the average queue size. The gateway could notify connections of congestion either by dropping packets arriving at the gateway or by setting a bit in packet headers. When the average queue size exceeds a preset threshold, the gateway drops or marks each arriving packet with a certain probability, where the exact probability is a function of the average queue size. RED gateways keep the average queue size low while allowing occasional bursts of packets in the queue. During congestion, the probability that the gateway notifies a particular connection to reduce its window is roughly proportional to that connection's share of the bandwidth through the gateway. RED gateways are designed to accompany a transport-layer congestion control protocol such as TCP. The RED gateway has no bias against bursty traffic and avoids the global synchronization of many connections decreasing their window at the same time. Simulations of a TCP/IP network are used to illustrate the performance of RED gateways. lNTRODucTION I N high-speed ne!works with connections with large delay bandwidth products. gateways are likely to be designed with correspondingly large maximum queues to acccomodate transient congestion. In the current Internet, the TCP transport protocol detects congestion only after a packet has been dropped al the gateway. However, it would clearly be undesirable to have large queues (possibly on the order of a delay bandwidth product) that were full much of the time; this would significantly increase the average delay in the network. Therefore, with increasingly high-speed networks, it is increasingly important to have mechanisms that keep throughput high but avemge queue sizes low. In the absence of explicit feedback from the gateway, there are a number of mechanisms that have been proposed for transport-layer protocols to maintain high throughput and low delay in the network. Some of these proposed mechanisms are designed to work with current gateways [15] could infer congestion from the estimated bottleneck service time or from changes in throughput or end-to-end delay, as well as from packet drops or other methods. Nevertheless, the view of an individual connection is limited by the time scales of the connection, the traffic pattern of the connection, the lack of knowledge of the number of congested gateways, the possibilities of routing changes, or other difficulties in distinguishing propagation delay from persistent queueing delay. The most effective detection of congestion can occur in the gateway itself. The gateway can reliably distinguish between propagation delay and persistent queueing delay. Only the gateway has a unified view of the queueing behavior over time; the perspective of individual connections is limited by the packet arrival patterns for those connections. In addition, a gateway is shared by many active connections with a wide range of round-trip times, tolerances of delay, throughput requirements, etc. Decisions about the duration and magnitude of transient congestion to be allowed at the gateway are best made by the gateway itself. The method of monitoring the average queue size at the gateway, and of notifying connections of incipient congestion, is based on the assumption that it will continue to be useful to have queues at the gateway where traffic from a number of connections is multiplexed together with FIFO scheduling. Not only is FIFO scheduling useful for sharing delay among connections and reducing delay for a particular connection during its periods of burstiness [4], but it scales well and is easy to implement efficiently. In an alternate approach, some congestion control mechanisms that use variants of Fair Queueing [20] or hop-by-hop flow control schemes [22] propose that the gateway scheduling algorithm make use of per-connection state for every active connection. We would suggest instead that per-connection gateway mechanisms be used only in those circumstances where gateway scheduling mechanisms without per-connection mechanisms are clearly inadequate. The DECbit congestion avoidance scheme [ 18], described later in this paper, is an early example of congestion detection at the gateway; DECbit gateways give explicit feedback when the average queue size exceeds a certain threshold. This paper proposes a different congestion avoidance mechanism at the gateway, RED (Random Early Detection) gateways. with somewhat different methods for detecting congestion and choosing which connections to notify of this congestion. While the principles behind RED gateways are fairly general and RED gateways can be useful in controlling the average queue size even in a network where the transport protocol 1063-6692/93$03.00" O 199.7 IEEE