Congestion Avoidance in Computer Networks With a Connectionless Network Layer Ra j Jain, K. K. Ramakrishnan, Dah-Ming Chiu Digital Equipment Corp oration 550 King St. LKG1-2/A19 Littleton, MA 01460 DEC-TR-506 c Copyright 1988, Digital Equipment Corp oration. All rights reserved. Version:June 1, 1997 Abstract Widespread use of computer networks and the use of varied technology for the interconnection of computers has made congestion a signi cant problem. In this rep ort, we summarize our research on congestion avoidance. We compare the concept of congestion avoidance with that of congestion control. Brie y, congestion control is a recovery mechanism, while conges- tion avoidance is a prevention mechanism. A congestion control scheme helps the network to recover from the congestion state while a congestion avoidance scheme allows a network to op erate in the region of low delay and high throughput with minimal queuing, thereby preventing it from entering the congested state in which packets are lost due to bu er shortage. A number of p ossible alternatives for congestion avoidance were identi ed. From these alternatives we selected one called the binary feedback scheme in which the network uses a single bit in the network layer header to feed back the congestion information to its users, which then increase or decrease their load to make optimal use of the resources. The concept of global optimality in a distributed system is de ned in terms of eciency and fairness such that they can b e indep endently quanti ed and apply to anynumber of resources and users. The prop osed scheme has b een simulated and shown to be globally ecient, fair, resp onsive, convergent, robust, distributed, and con guration-indep endent. queuing and congestion. 1 INTRODUCTION We are concerned here with congestion avoidance rather than congestion control. Brie y, a conges- Congestion in computer networks is b ecoming a sig- tion avoidance scheme allows a network to op erate in ni cant problem due to increasing use of the net- the region of low delay and high throughput. These works, as well as due to increasing mismatch in link schemes prevent a network from entering the con- sp eeds caused byintermixing of old and new technol- gested state in which the packets are lost. We will ogy. Recent technological advances such as lo cal area elab orate on this p oint in the next section where the networks LANs and b er optic LANs have resulted terms ow control, congestion control, and conges- in a signi cant increase in the bandwidths of com- tion avoidance will b e de ned and their relationship puter network links. However, these new technolo- to each other discussed. gies must co exist with the old low bandwidth media We studied a number of alternative schemes for such as the twisted pair. This heterogeneity has re- congestion avoidance. Based on a numb er of require- sulted in mismatch of arrival and service rates in the ments describ ed later in this rep ort, we selected an intermediate no des in the network, causing increased 1 p erformance. A ow control scheme protects the alternative called the binary feedback scheme for de- destination from b eing o o ded by the source. tailed study. This scheme uses only a single bit in Some of the alternatives that have b een describ ed the network layer header to feed back the congestion in the literature are window ow-control, Xon/Xo information from the network to users, which then in- [7], rate ow-control [5], etc. In the window ow- crease or decrease their load on the network to make control scheme, the destination sp eci es a limit on ecient and fair use of the resources. We present the numb er of packets that the source may send with- precise de nitions of eciency and fairness that can out further p ermission from the destination. b e used for other distributed systems as well. Let us now extend the con guration to include a This rep ort is a summary of our work in the area of communication subnet see Figure 1b consisting of congestion avoidance in connectionless networks. We routers and links that have limited memory, band- have tried to make this summary as self-contained width, and pro cessing sp eeds. Now the source must and brief as p ossible. For further information, the not only ob ey the directives from the destination, but reader is encouraged to read detailed rep orts in [16, also from all the routers and links in the network. 22,4,23]. Without this additional control the source may send packets at a rate to o fast for the network, leading to queuing, bu er over ow, packet losses, retrans- 2 CONCEPTS missions, and p erformance degradation. A conges- tion control scheme protects the network from b eing In this section we de ne the basic concepts of ow o o ded by its users transp ort entities at source and control, congestion control, and congestion avoid- destination no des. ance. In connection-oriented networks the congestion problem is generally solved by reserving the resources at all routers during connection setup. In connec- tionless networks it can b e done by explicit messages choke packets from the network to the sources [19], or by implicit means such as timeout on a packet loss. In [15, 13, 21], a number of alternatives have b een discussed and a timeout-based scheme has b een analyzed in detail. Traditional congestion control schemes help im- prove the p erformance after congestion has o ccurred. Figure 2 shows general patterns of resp onse time and throughput of a network as the network load increases. If the load is small, throughput gener- ally keeps up with the load. As the load increases, throughput increases. After the load reaches the net- work capacity, throughput stops increasing. If the Figure 1: load is increased any further, the queues start build- ing, p otentially resulting in packets b eing dropp ed. Throughput may suddenly drop when the load in- Consider the simple con guration shown in Figure creases b eyond this p oint and the network is said to 1a, in which two no des are directly connected via be congested. The resp onse-time curve follows a simi- a link. Without any control, the source may send lar pattern. At rst the resp onse time increases little packets at a rate to o fast for the destination. This with load. When the queues start building up, the may cause bu er over ow at the destination, lead- resp onse time increases linearly until nally, as the ing to packet losses, retransmissions, and degraded 2 Ascheme that allows the network to op erate at the knee is called a congestion avoidance scheme, as distinguished from a congestion control scheme that tries to keep the network op erating in the zone to the left of the cli . A prop erly designed congestion avoidance scheme will ensure that the users are en- couraged to increase their trac load as long as this do es not signi cantly a ect the resp onse time and are required to decrease them if that happ ens. Thus, the network load oscillates around the knee. Congestion control schemes are still required, however, to protect the network should it reach the cli due to transient changes in the network. The distinction between congestion control and congestion avoidance is similar to that b etween dead- lo ck recovery and deadlo ck avoidance. Congestion control pro cedures are curative and the avoidance pro cedures are preventive in nature. The p oint at which a congestion control scheme is called up on dep ends up on the amount of memory available in the routers, whereas the p oint at which a conges- tion avoidance scheme is invoked is indep endent of the memory size. We elab orate further on these concepts in [16]. 3 ALTERNATIVES Figure 2: Congestion control and congestion avoidance are dy- namic system control issues. Like all other control queues start over owing, the resp onse time increases schemes they consist of two parts: a feedback mecha- drastically. nism and a control mechanism. The feedback mecha- The p oint at which throughput approaches zero is nism allows the system network to inform its users called the p oint of congestion col lapse. This is also sources or destinations of the current state of the the p oint at which the resp onse time approaches in- system, and the control mechanism allows the users nity. The purp ose of a congestion control scheme to adjust their loads on the system. such as [15, 3] is to detect the fact that the network The problem of congestion control has b een dis- has reached the p oint of congestion collapse resulting cussed extensively in the literature. A number of in packet losses, and to reduce the load so that the feedback mechanisms have b een prop osed. If we network returns to an uncongested state. extend those mechanisms to op erate the network We call the p oint of congestion collapse a cli due around the knee rather than the cli , we obtain the fact that the throughput falls o rapidly after this congestion avoidance mechanisms. For the feedback p oint. We use the term knee to describ e the p oint mechanisms wehave the following alternatives: after which the increase in the throughput is small, but after which a signi cant increase in the resp onse 1. Congestion feedback via packets sent from time results.