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Network Working Group                                          A. MankinRequest for Comments: 1254                                         MITRE                                                         K. Ramakrishnan                                           Digital Equipment Corporation                                                                 Editors                                                             August 1991Gateway Congestion Control SurveyStatus of this Memo   This memo provides information for the Internet community.  It is a   survey of some of the major directions and issues.  It does not   specify an Internet standard.  Distribution of this memo is   unlimited.Abstract   The growth of network intensive Internet applications has made   gateway congestion control a high priority.  The IETF Performance and   Congestion Control Working Group surveyed and reviewed gateway   congestion control and avoidance approaches.  The purpose of this   paper is to present our review of the congestion control approaches,   as a way of encouraging new discussion and experimentation.  Included   in the survey are Source Quench, Random Drop, Congestion Indication   (DEC Bit), and Fair Queueing.  The task remains for Internet   implementors to determine and agree on the most effective mechanisms   for controlling gateway congestion.1.  Introduction   Internet users regularly encounter congestion, often in mild forms.   However, severe congestion episodes have been reported also; and   gateway congestion remains an obstacle for Internet applications such   as scientific supercomputing data transfer.  The need for Internet   congestion control originally became apparent during several periods   of 1986 and 1987, when the Internet experienced the "congestion   collapse" condition predicted by Nagle [Nag84].  A large number of   widely dispersed Internet sites experienced simultaneous slowdown or   cessation of networking services for prolonged periods.  BBN, the   firm responsible for maintaining the then backbone of the Internet,   the ARPANET, responded to the collapse by adding link capacity   [Gar87].   Much of the Internet now uses as a transmission backbone the National   Science Foundation Network (NSFNET). Extensive monitoring and   capacity planning are being done for the NSFNET backbone; still, asPerformance and Congestion Control Working Group                [Page 1]

RFC 1254           Gateway Congestion Control Survey         August 1991   the demand for this capacity grows, and as resource-intensive   applications such as wide-area file system management [Sp89]   increasingly use the backbone, effective congestion control policies   will be a critical requirement.   Only a few mechanisms currently exist in Internet hosts and gateways   to avoid or control congestion.  The mechanisms for handling   congestion set forth in the specifications for the DoD Internet   protocols are limited to:      Window flow control in TCP [Pos81b], intended primarily for      controlling the demand on the receiver's capacity, both in terms      of processing and buffers.      Source quench in ICMP, the message sent by IP to request that a      sender throttle back [Pos81a].   One approach to enhancing Internet congestion control has been to   overlay the simple existing mechanisms in TCP and ICMP with more   powerful ones.  Since 1987, the TCP congestion control policy, Slow-   start, a collection of several algorithms developed by Van Jacobson   and Mike Karels [Jac88], has been widely adopted. Successful Internet   experiences with Slow-start led to the Host Requirements RFC [HREQ89]   classifying the algorithms as mandatory for TCP.  Slow-start modifies   the user's demand when congestion reaches such a point that packets   are dropped at the gateway.  By the time such overflows occur, the   gateway is congested.  Jacobson writes that the Slow-start policy is   intended to function best with a complementary gateway policy   [Jac88].1.1  Definitions   The characteristics of the Internet that we are interested in include   that it is, in general, an arbitrary mesh-connected network.  The   internetwork protocol is connectionless.  The number of users that   place demands on the network is not limited by any explicit   mechanism; no reservation of resources occurs and transport layer   set-ups are not disallowed due to lack of resources.  A path from a   source to destination host may have multiple hops, through several   gateways and links.  Paths through the Internet may be heterogeneous   (though homogeneous paths also exist and experience congestion).   That is, links may be of different speeds.  Also, the gateways and   hosts may be of different speeds or may be providing only a part of   their processing power to communication-related activity.  The   buffers for storing information flowing through Internet gateways are   finite.  The nature of the internet protocol is to drop packets when   these buffers overflow.Performance and Congestion Control Working Group                [Page 2]

RFC 1254           Gateway Congestion Control Survey         August 1991   Gateway congestion arises when the demand for one or more of the   resources of the gateway exceeds the capacity of that resource.  The   resources include transmission links, processing, and space used for   buffering.  Operationally, uncongested gateways operate with little   queueing on average, where the queue is the waiting line for a   particular resource of the gateway.  One commonly used quantitative   definition [Kle79] for when a resource is congested is when the   operating point is greater than the point at which resource power is   maximum, where resource power is defined as the ratio of throughput   to delay (SeeSection 2.2).  At this operating point, the average   queue size is close to one, including the packet in service.  Note   that this is a long-term average queue size.  Several definitions   exist for the timescale of averaging for congestion detection and   control, such as dominant round-trip time and queue regeneration   cycle (seeSection 2.1).   Mechanisms for handling congestion may be divided into two   categories, congestion recovery and congestion avoidance.  Congestion   recovery tries to restore an operating state, when demand has already   exceeded capacity.  Congestion avoidance is preventive in nature.  It   tries to keep the demand on the network at or near the point of   maximum power, so that congestion never occurs.  Without congestion   recovery, the network may cease to operate entirely (zero   throughput), whereas the Internet has been operating without   congestion avoidance for a long time.  Overall performance may   improve with an effective congestion avoidance mechanism.  Even if   effective congestion avoidance was in place, congestion recovery   schemes would still be required, to retain throughput in the face of   sudden changes (increase of demand, loss of resources) that can lead   to congestion.   In this paper, the term "user" refers to each individual transport   (TCP or another transport protocol) entity.  For example, a TCP   connection is a "user" in this terminology.  The terms "flow" and   "stream" are used by some authors in the same sense.  Some of the   congestion control policies discussed in this paper, such as   Selective Feedback Congestion Indication and Fair Queueing aggregate   multiple TCP connections from a single host (or between a source   host-destination host pair) as a virtual user.   The term "cooperating transport entities" will be defined as a set of   TCP connections (for example) which follow an effective method of   adjusting their demand on the Internet in response to congestion.   The most restrictive interpretation of this term is that the   transport entities follow identical algorithms for congestion control   and avoidance.  However, there may be some variation in these   algorithms.  The extent to which heterogeneous end-system congestion   control and avoidance may be accommodated by gateway policies shouldPerformance and Congestion Control Working Group                [Page 3]

RFC 1254           Gateway Congestion Control Survey         August 1991   be a subject of future research. The role played in Internet   performance of non-cooperating transport entities is discussed inSection 5.1.2  Goals and Scope of This Paper   The task remains for Internet implementors to determine effective   mechanisms for controlling gateway congestion.  There has been   minimal common practice on which to base recommendations for Internet   gateway congestion control.  In this survey, we describe the   characteristics of one experimental gateway congestion management   policy, Random Drop, and several that are better-known:  Source   Quench, Congestion Indication, Selective Feedback Congestion   Indication, and Fair Queueing, both Bit-Round and Stochastic.  A   motivation for documenting Random Drop is that it has as primary   goals low overhead and suitability for scaling up for Internets with   higher speed links.  Both of these are important goals for future   gateway implementations that will have fast links, fast processors,   and will have to serve large numbers of interconnected hosts.   The structure of this paper is as follows.  First, we discuss   performance goals, including timescale and fairness considerations.   Second, we discuss the gateway congestion control policies.  Random   Drop is sketched out, with a recommendation for using it for   congestion recovery and a separate section on its use as congestion   avoidance.  Third, since gateway congestion control in itself does   not change the end-systems' demand, we briefly present the effective   responses to these policies by two end-system congestion control   schemes, Slow-start and End-System Congestion Indication.  Among our   conclusions, we address the issues of transport entities that do not   cooperate with gateway congestion control.  As an appendix, because   of the potential interactions with gateway congestion policies, we   report on a scheme to help in controlling the performance of Internet   gateways to connection-oriented subnets (in particular, X.25).   Resources in the current Internet are not charged to users of them.   Congestion avoidance techniques cannot be expected to help when users   increase beyond the capacity of the underlying facilities.  There are   two possible solutions for this, increase the facilities and   available bandwidth, or forcibly reduce the demand.  When congestion   is persistent despite implemented congestion control mechanisms,   administrative responses are needed.  These are naturally not within   the scope of this paper.  Also outside the scope of this paper are   routing techniques that may be used to relocate demand away from   congested individual resources (e.g., path-splitting and load-   balancing).Performance and Congestion Control Working Group                [Page 4]

RFC 1254           Gateway Congestion Control Survey         August 19912.  Performance Goals   To be able to discuss design and use of various mechanisms for   improving Internetwork performance, we need to have clear performance   goals for the operation of gateways and sets of end-systems.   Internet experience shows that congestion control should be based on   adaptive principles; this requires efficient computation of metrics   by algorithms for congestion control.  The first issue is that of the   interval over which these metrics are estimated and/or measured.2.1  Interval for Measurement/Estimation of Performance Metrics   Network performance metrics may be distorted if they are computed   over intervals that are too short or too long relative to the dynamic   characteristics of the network.  For instance, within a small   interval, two FTP users with equal paths may appear to have sharply   different demands, as an effect of brief, transient fluctuations in   their respective processing.  An overly long averaging interval   results in distortions because of the changing number of users   sharing the resource measured during the time.  It is similarly   important for congestion control mechanisms exerted at end systems to   find an appropriate interval for control.   The first approach to the monitoring, or averaging, interval for   congestion control is one based on round-trip times.  The rationale   for it is as follows:  control mechanisms must adapt to changes in   Internet congestion as quickly as possible.  Even on an uncongested   path, changed conditions will not be detected by the sender faster   than a round-trip time.  The effect of a sending end-system's control   will also not be seen in less than a round-trip time in the entire   path as well as at the end systems.  For the control mechanism to be   adaptive, new information on the path is needed before making a   modification to the control exerted.  The statistics and metrics used   in congestion control must be able to provide information to the   control mechanism so that it can make an informed decision.   Transient information which may be obsolete before a change is made   by the end-system should be avoided.  This implies the   monitoring/estimating interval is one lasting one or more round   trips.  The requirements described here give bounds on:      How short an interval:  not small enough that obsolete information      is used for control;      How long:  not more than the period at which the end-system makes      changes.   But, from the point of view of the gateway congestion control policy,   what is a round-trip time?  If all the users of a given gateway havePerformance and Congestion Control Working Group                [Page 5]

RFC 1254           Gateway Congestion Control Survey         August 1991   the same path through the Internet, they also have the same round-   trip time through the gateway.  But this is rarely the case.   A meaningful interval must be found for users with both short and   long paths. Two approaches have been suggested for estimating this   dynamically, queue regeneration cycle and frequency analysis.   Use of the queue regeneration cycle has been described as part of the   Congestion Indication policy.  The time period used for averaging   here begins when a resource goes from the idle to busy state.  The   basic interval for averaging is a "regeneration cycle" which is in   the form of busy and idle intervals.  Because an average based on a   single previous regeneration may become old information, the   recommendation in [JRC87] is to average over the sum of two   intervals, that is, the previous (busy and idle) period, and the time   since the beginning of the current busy period.   If the gateway users are window-based transport entities, it is   possible to see how the regeneration interval responds to their   round-trip times.  If a user with a long round-trip time has the   dominant traffic, the queue length may be zero only when that user is   awaiting acknowledgements.  Then the users with short paths will   receive gateway congestion information that is averaged over several   of their round-trip times.  If the short path traffic dominates the   activity in the gateway, i.e., the connections with shorter round-   trip times are the dominant users of the gateway resources, then the   regeneration interval is shorter and the information communicated to   them can be more timely. In this case, users with longer paths   receive, in one of their round-trip times, multiple samples of the   dominant traffic; the end system averaging is based on individual   user's intervals, so that these multiple samples are integrated   appropriately for these connections with longer paths.   The use of frequency analysis has been described by [Jac89]. In this   approach, the gateway congestion control is done at intervals based   on spectral analysis of the traffic arrivals.  It is possible for   users to have round-trip times close to each other, but be out of   phase from each other. A spectral analysis algorithm detects this.   Otherwise, if multiple round-trip times are significant, multiple   intervals will be identified.  Either one of these will be   predominant, or several will be comparable. An as yet difficult   problem for the design of algorithms accomplishing this technique is   the likelihood of "locking" to the frequency of periodic traffic of   low intensity, such as routing updates.Performance and Congestion Control Working Group                [Page 6]

RFC 1254           Gateway Congestion Control Survey         August 19912.2  Power and its Relationship to the Operating Point   Performance goals for a congestion control/avoidance strategy embody   a conflict in that they call for as high a throughput as possible,   with as little delay as possible.  A measure that is often used to   reflect the tradeoff between these goals is power, the ratio of   throughput to delay.  We would like to maximize the value of power   for a given resource.  In the standard expression for power,     Power = (Throughput^alpha)/Delay   the exponent alpha is chosen for throughput, based on the relative   emphasis placed on throughput versus delay: if throughput is more   important, then a value of A alpha greater than one is chosen.  If   throughput and delay are equally important (e.g., both bulk transfer   traffic and interactive traffic are equally important), then alpha   equal to one is chosen. The operating point where power is maximized   is the "knee" in the throughput and delay curves.  It is desirable   that the operating point of the resource be driven towards the knee,   where power is maximized.  A useful property of power is that it is   decreasing whether the resource is under- or over-utilized relative   to the knee.   In an internetwork comprising nodes and links of diverse speeds and   utilization, bottlenecks or concentrations of demand may form.  Any   particular user can see a single bottleneck, which is the slowest or   busiest link or gateway in the path (or possibly identical "balanced"   bottlenecks).  The throughput that the path can sustain is limited by   the bottleneck.  The delay for packets through a particular path is   determined by the service times and queueing at each individual hop.   The queueing delay is dominated by the queueing at the bottleneck   resource(s).  The contribution to the delay over other hops is   primarily the service time, although the propagation delay over   certain hops, such as a satellite link, can be significant.  We would   like to operate all shared resources at their knee and maximize the   power of every user's bottleneck.   The above goal underscores the significance of gateway congestion   control.  If techniques can be found to operate gateways at their   resource knee, it can improve Internet performance broadly.2.3  Fairness   We would like gateways to allocate resources fairly to users.  A   concept of fairness is only relevant when multiple users share a   gateway and their total demand is greater than its capacity.  If   demands were equal, a fair allocation of the resource would be to   provide an equal share to each user.  But even over short intervals,Performance and Congestion Control Working Group                [Page 7]

RFC 1254           Gateway Congestion Control Survey         August 1991   demands are not equal.  Identifying the fair share of the resource   for the user becomes hard.  Having identified it, it is desirable to   allocate at least this fair share to each user.  However, not all   users may take advantage of this allocation.  The unused capacity can   be given to other users.  The resulting final allocation is termed a   maximally fair allocation.  [RJC87] gives a quantitative method for   comparing the allocation by a given policy to the maximally fair   allocation.   It is known that the Internet environment has heterogeneous transport   entities, which do not follow the same congestion control policies   (our definition of cooperating transports). Then, the controls given   by a gateway may not affect all users and the congestion control   policy may have unequal effects.  Is "fairness" obtainable in such a   heterogeneous community?  In Fair Queueing, transport entities with   differing congestion control policies can be insulated from each   other and each given a set share of gateway bandwidth.   It is important to realize that since Internet gateways cannot refuse   new users, fairness in gateway congestion control can lead to all   users receiving small (sub-divided) amounts of the gateway resources   inadequate to meet their performance requirements.  None of the   policies described in this paper currently addresses this.  Then,   there may be policy reasons for unequal allocation of the gateway   resources.  This has been addressed by Bit-Round Fair Queueing.2.4  Network Management   Network performance goals may be assessed by measurements in either   the end-system or gateway frame of reference.  Performance goals are   often resource-centered and the measurement of the global performance   of "the network," is not only difficult to measure but is also   difficult to define.  Resource-centered metrics are more easily   obtained, and do not require synchronization.  That resource-centered   metrics are appropriate ones for use in optimization of power is   shown by [Jaf81].   It would be valuable for the goal of developing effective gateway   congestion handling if Management Information Base (MIB) objects   useful for evaluating gateway congestion were developed.  The   reflections on the control interval described above should be applied   when network management applications are designed for this purpose.   In particular, obtaining an instantaneous queue length from the   managed gateway is not meaningful for the purposes of congestion   management.Performance and Congestion Control Working Group                [Page 8]

RFC 1254           Gateway Congestion Control Survey         August 19913.  Gateway Congestion Control Policies   There have been proposed a handful of approaches to dealing with   congestion in the gateway. Some of these are Source Quench, Random   Drop, Congestion Indication, Selective Feedback Congestion   Indication, Fair Queueing, and Bit-Round Fair Queueing.  They differ   in whether they use a control message, and indeed, whether they view   control of the end-systems as necessary, but none of them in itself   lowers the demand of users and consequent load on the network.  End-   system policies that reduce demand in conjunction with gateway   congestion control are described inSection 4.3.1  Source Quench   The method of gateway congestion control currently used in the   Internet is the Source Quench message of theRFC-792 [Pos81a]   Internet Control Message Protocol (ICMP). When a gateway responds to   congestion by dropping datagrams, it may send an ICMP Source Quench   message to the source of the dropped datagram.  This is a congestion   recovery policy.   The Gateway Requirements RFC,RFC-1009 [GREQ87], specifies that   gateways should only send Source Quench messages with a limited   frequency, to conserve CPU resources during the time of heavy load.   We note that operating the gateway for long periods under such loaded   conditions should be averted by a gateway congestion control policy.   A revised Gateway Requirements RFC is being prepared by the IETF.   Another significant drawback of the Source Quench policy is that its   details are discretionary, or, alternatively, that the policy is   really a family of varied policies.  Major Internet gateway   manufacturers have implemented a variety of source quench   frequencies.  It is impossible for the end-system user on receiving a   Source Quench to be certain of the circumstances in which it was   issued.  This makes the needed end-system response problematic:  is   the Source Quench an indication of heavy congestion, approaching   congestion, a burst causing massive overload, or a burst slightly   exceeding reasonable load?   To the extent that gateways drop the last arrived datagram on   overload, Source Quench messages may be distributed unfairly.  This   is because the position at the end of the queue may be unfairly often   occupied by the packets of low demand, intermittent users, since   these do not send regular bursts of packets that can preempt most of   the queue space.   [Fin89] developed algorithms for when to issue Source Quench and for   responding to it with a rate-reduction in the IP layer on the sendingPerformance and Congestion Control Working Group                [Page 9]

RFC 1254           Gateway Congestion Control Survey         August 1991   host.  The system controls end-to-end performance of connections in a   manner similar to the congestion avoidance portion of Slow-start TCP   [Jac88].3.2  Random Drop   Random Drop is a gateway congestion control policy intended to give   feedback to users whose traffic congests the gateway by dropping   packets on a statistical basis.  The key to this policy is the   hypothesis that a packet randomly selected from all incoming traffic   will belong to a particular user with a probability proportional to   the average rate of transmission of that user.  Dropping a randomly   selected  packet results in users which generate much traffic having   a greater number of packets dropped compared with those generating   little traffic.  The selection of packets to be dropped is completely   uniform.  Therefore, a user who generates traffic of an amount below   the "fair share" (as defined inSection 2.3) may also experience a   small amount of packet loss at a congested gateway. This character of   uniformity is in fact a primary goal that Random Drop attempts to   achieve.   The other primary goal that Random Drop attempts to achieve is a   theoretical overhead which is scaled to the number of shared   resources in the gateway rather than the number of its users.  If a   gateway congestion algorithm has more computation the more users   there are, this can lead to processing demands that are higher as   congestion increases.  Also the low-overhead goal of Random Drop   addresses concerns about the scale of gateway processing that will be   required in the mid-term Internet as gateways with fast processors   and links are shared by very large active sets of users.3.2.1  For Congestion Recovery   Random Drop has been proposed as an improvement to packet dropping at   the operating point where the gateway's packet buffers overflow.   This is using Random Drop strictly as a congestion recovery   mechanism.   In Random Drop congestion recovery, instead of dropping the last   packet to arrive at the queue, a packet is selected randomly from the   queue.  Measurements of an implementation of Random Drop Congestion   Recovery [Man90] showed that a user with a low demand, due to a   longer round-trip time path than other users of the gateway, had a   higher drop rate with RDCR than without.  The throughput accorded to   users with the same round-trip time paths was nearly equal with RDCR   as compared to without it.  These results suggest that RDCR should be   avoided unless it is used within a scheme that groups traffic more or   less by round-trip time.Performance and Congestion Control Working Group               [Page 10]

RFC 1254           Gateway Congestion Control Survey         August 19913.2.2  For Congestion Avoidance   Random Drop is also proposed as a congestion avoidance policy   [Jac89].  The intent is to initiate dropping packets when the gateway   is anticipated to become congested and remain so unless there is some   control exercised.  This  implies  selection  of  incoming packets to   be randomly dropped at a rate derived from identifying the level of   congestion at the gateway.  The rate is the number of arrivals   allowed between drops. It depends on the current operating point and   the prediction of congestion.   A part of the policy is to determine that congestion will soon occur   and that the gateway is beginning to operate beyond the knee of the   power curve.  With a suitably chosen interval (Section 2.1), the   number of packets from each individual user in a sample over that   interval is proportional to each user's demand on the gateway.  Then,   dropping one or more random packets indicates to some user(s) the   need to reduce the level of demand that is driving the gateway beyond   the desired operating point.  This is the goal that a policy of   Random Drop for congestion avoidance attempts to achieve.   There are several parameters to be determined for a Random Drop   congestion avoidance policy. The first is an interval, in terms of   number of packet arrivals, over which packets are dropped with   uniform probability.  For instance, in a sample implementation, if   this interval spanned 2000 packet arrivals, and a suitable   probability of drop was 0.001, then two random variables would be   drawn in a uniform distribution in the range of 1 to 2,000.  The   values drawn would be used by counting to select the packets dropped   at arrival.  The second parameter is the value for the probability of   drop.  This parameter would be a function of an estimate of the   number of users, their appropriate control intervals, and possibly   the length of time that congestion has persisted.  [Jac89] has   suggested successively increasing the probability of drop when   congestion persists over multiple control intervals.  The motivation   for increasing the packet drop probability is that the implicit   estimate of the number of users and random selection of their packets   to drop does not guarantee that all users have received enough   signals to decrease demand.  Increasing the probability of drop   increases the probability that enough feedback is provided.   Congestion detection is also needed in Random Drop congestion   avoidance, and could be implemented in a variety of ways.  The   simplest is a static threshold, but dynamically averaged measures of   demand or utilization are suggested.   The packets dropped in Random Drop congestion avoidance would not be   from the initial inputs to the gateway.  We suggest that they would   be selected only from packets destined for the resource which isPerformance and Congestion Control Working Group               [Page 11]

RFC 1254           Gateway Congestion Control Survey         August 1991   predicted to be approaching congestion.  For example, in the case of   a gateway with multiple outbound links, access to each individual   link is treated as a separate resource, the Random Drop policy is   applied at each link independently.  Random Drop congestion avoidance   would provide uniform treatment of all cooperating transport users,   even over individual patterns of traffic multiplexed within one   user's stream.  There is no aggregation of users.   Simulation studies [Zha89], [Has90] have presented evidence that   Random Drop is not fair across cooperating and non-cooperating   transport users.  A transport user whose sending policies included   Go-Back-N retransmissions and did not include Slow-start received an   excessive share of bandwidth from a simple implementation of Random   Drop.  The simultaneously active Slow-start users received unfairly   low shares.  Considering this, it can be seen that when users do not   respond to control, over a prolonged period, the Random Drop   congestion avoidance mechanism would have an increased probability of   penalizing users with lower demand.  Their packets dropped, these   users exert the controls leading to their giving up bandwidth.   Another problem can be seen to arise in Random Drop [She89] across   users whose communication paths are of different lengths.  If the   path spans congested resources at multiple gateways, then the user's   probability of receiving an unfair drop and subsequent control (if   cooperating) is exponentially increased.  This is a significant   scaling problem.   Unequal paths cause problems for other congestion avoidance policies   as well.  Selective Feedback Congestion Indication was devised to   enhance Congestion Indication specifically because of the problem of   unequal paths.  In Fair Queueing by source-destination pairs, each   path gets its own queue in all the gateways.3.3  Congestion Indication   The Congestion Indication policy is often referred to as the DEC Bit   policy. It was developed at DEC [JRC87], originally for the Digital   Network Architecture (DNA).  It has also been specified for the   congestion avoidance of the ISO protocols TP4 and CLNP [NIST88].   Like Source Quench, it uses explicit communications from the   congested gateway to the user.  However, to use the lowest possible   network resources for indicating congestion, the information is   communicated in a single bit, the Congestion Experienced Bit, set in   the network header of the packets already being forwarded by a   gateway.  Based on the condition of this bit, the end-system user   makes an adjustment to the sending window. In the NSP transport   protocol of DECNET, the source makes an adjustment to its window; in   the ISO transport protocol, TP4, the destination makes thisPerformance and Congestion Control Working Group               [Page 12]

RFC 1254           Gateway Congestion Control Survey         August 1991   adjustment in the window offered to the sender.   This policy attempts to avoid congestion by setting the bit whenever   the average queue length over the previous queue regeneration cycle   plus part of the current cycle is one or more.  The feedback is   determined independently at each resource.3.4  Selective Feedback Congestion Indication   The simple Congestion Indication policy works based upon the total   demand on the gateway.  The total number of users or the fact that   only a few of the users might be causing congestion is not   considered.  For fairness, only those users who are sending more than   their fair share should be asked to reduce their load, while others   could attempt to increase where possible.  In Selective Feedback   Congestion Indication, the Congestion Experienced Bit is used to   carry out this goal.   Selective Feedback works by keeping a count of the number of packets   sent by different users since the beginning of the queue averaging   interval.  This is equivalent to monitoring their throughputs. Based   on the total throughput, a fair share for each user is determined and   the congestion bit is set, when congestion approaches, for the users   whose demand is higher than their fair share.  If the gateway is   operating below the throughput-delay knee, congestion indications are   not set.   A min-max algorithm used to determine the fair share of capacity and   other details of this policy are described in [RJC87].  One parameter   to be computed is the capacity of each resource to be divided among   the users.  This metric depends on the distribution of inter-arrival   times and packet sizes of the users.  Attempting to determine these   in real time in the gateway is unacceptable.  The capacity is instead   estimated from on the throughput seen when the gateway is operating   in congestion, as indicated by the average queue length.  In   congestion (above the knee), the service rate of the gateway limits   its throughput.  Multiplying the throughput obtained at this   operating point by a capacity factor (between 0.5 and 0.9) to adjust   for the distributions yields an acceptable capacity estimate in   simulations.   Selective Feedback Congestion Indication takes as input a vector of   the number of packets sent by each source-destination pair of end-   systems.  Other alternatives include 1) destination address, 2)   input/output link, and 3) transport connection (source/destination   addresses and ports).   These alternatives give different granularities for fairness.  In thePerformance and Congestion Control Working Group               [Page 13]

RFC 1254           Gateway Congestion Control Survey         August 1991   case where paths are the same or round-trip times of users are close   together, throughputs are equalized similarly by basing the selective   feedback on source or destination address.  In fact, if the RTTs are   close enough, the simple congestion indication policy would result in   a fair allocation.  Counts based on source/destination pairs ensure   that paths with different lengths and network conditions get a fair   throughput at the individual gateways.  Counting packets based on   link pairs has a low overhead, but may result in unfairness to users   whose demand is below the fair share and are using a long path.   Counts based on transport layer connection identifiers, if this   information was available to Internet gateways, would make good   distinctions, since the differences of demand of different   applications and instances of applications would be separately   monitored.   Problems with Selective Feedback Congestion Indication include that   the gateway has significant processing to do.  With the feasible   choice of aggregation at the gateway, unfairness across users within   the group is likely.  For example, an interactive connection   aggregated with one or more bulk transfer connections will receive   congestion indications though its own use of the gateway resources is   very low.3.5  Fair Queueing   Fair Queueing is the policy of maintaining separate gateway output   queues for individual end-systems by source-destination pair.  In the   policy as proposed by [Nag85], the gateway's processing and link   resources are distributed to the end-systems on a round-robin basis.   On congestion, packets are dropped from the longest queue.  This   policy leads to equal allocations of resources to each source-   destination pair.  A source-destination pair that demands more than a   fair share simply increases its own queueing delay and congestion   drops.3.5.1  Bit-Round Fair Queueing   An enhancement of Nagle Fair Queueing, the Bit-Round Fair Queuing   algorithm described and simulated by [DKS89] addresses several   shortcomings of Nagle's scheme. It computes the order of service to   packets using their lengths, with a technique that emulates a bit-   by-bit round-robin discipline, so that long packets do not get an   advantage over short ones.  Otherwise the round-robin would be   unfair, for example, giving more bandwidth to hosts whose traffic is   mainly long packets than to hosts sourcing short packets.   The aggregation of users of a source-destination pair by Fair   Queueing has the property of grouping the users whose round-trips arePerformance and Congestion Control Working Group               [Page 14]

RFC 1254           Gateway Congestion Control Survey         August 1991   similar. This may be one reason that the combination of Bit-Round   Fair Queueing with Congestion Indication had particularly good   simulated performance in [DKS89].   The aggregation of users has the expected drawbacks, as well.  A   TELNET user in a queue with an FTP user does not get delay benefits;   and host pairs with high volume of connections get treated the same   as a host pair with small number of connections and as a result gets   unfair services.   A problem can be mentioned about Fair Queueing, though it is related   to implementation, and perhaps not properly part of a policy   discussion.  This is a concern that the resources (processing) used   for determining where to queue incoming packets would themselves be   subject to congestion, but not to the benefits of the Fair Queuing   discipline.  In a situation where the gateway processor was not   adequate to the demands on it, the gateway would need an alternative   policy for congestion control of the queue awaiting processing.   Clever implementation can probably find an efficient way to route   packets to the queues they belong in before other input processing is   done, so that processing resources can be controlled, too.  There is   in addition, the concern that bit-by-bit round FQ requires non-FCFS   queueing even within the same source destination pairs to allow for   fairness to different connections between these end systems.   Another potential concern about Fair Queueing is whether it can scale   up to gateways with very large source-destination populations.  For   example, the state in a Fair Queueing implementation scales with the   number of active end-to-end paths, which will be high in backbone   gateways.3.5.2  Stochastic Fairness Queuing   Stochastic Fairness Queueing (SFQ) has been suggested as a technique   [McK90] to address the implementation issues relating to Fair   Queueing.  The first overhead that is reduced is that of looking up   the source-destination address pair in an incoming packet and   determining which queue that packet will have to be placed in.  SFQ   does not require as many memory accesses as Fair Queueing to place   the packet in the appropriate queue.  SFQ is thus claimed to be more   amenable to implementation for high-speed networks [McK90].   SFQ uses a simple hash function to map from the source-destination   address pair to a fixed set of queues.  Since the assignment of an   address pair to a queue is probabilistic, there is the likelihood of   multiple address pairs colliding and mapping to the same queue.  This   would potentially degrade the additional fairness that is gained with   Fairness Queueing.  If two or more address pairs collide, they wouldPerformance and Congestion Control Working Group               [Page 15]

RFC 1254           Gateway Congestion Control Survey         August 1991   continue to do so.  To deal with the situation when such a collision   occurs, SFQ periodically perturbs the hash function so that these   address pairs will be unlikely to collide subsequently.   The price paid for achieving this implementation efficiency is that   SFQ requires a potentially large number of queues (we must note   however, that these queues may be organized orthogonally from the   buffers in which packets are stored. The buffers themselves may be   drawn from a common pool, and buffers in each queue organized as a   linked list pointed to from each queue header).  For example, [McK90]   indicates that to get good, consistent performance, we may need to   have up to 5 to 10 times the number of active source-destination   pairs. In a typical gateway, this may require around 1000 to 2000   queues.   [McK90] reports simulation results with SFQ. The particular hash   function that is studied is using the HDLC's cyclic redundancy check   (CRC).  The hash function is perturbed by multiplying each byte by a   sequence number in the range 1 to 255 before applying the CRC.  The   metric considered is the standard deviation of the number of packets   output per source-destination pair.  A perfectly fair policy would   have a standard deviation of zero and an unfair policy would have a   large standard deviation.  In the example studied (which has up to 20   source-destination (s-d) pairs going through a single overloaded   gateway), SFQ with 1280 queues (i.e., 64 times the number of source-   destination pairs), approaches about 3 times the standard deviation   of Fairness Queueing.  This must be compared to a FCFS queueing   discipline having a standard deviation which is almost 175 times the   standard deviation of Fairness Queueing.   It is conjectured in [McK90] that a good value for the interval in   between perturbations of the hash function appears to be in the area   between twice the queue flush time of the stochastic fairness queue   and one-tenth the average conversation time between a source-   destination pair.   SFQ also may alleviate the anticipated scaling problems that may be   an issue with Fair Queueing by not strictly requiring the number of   queues equal to the possible source-destination pairs that may be   presenting a load on a particular gateway. However, SFQ achieves this   property by trading off some of the fairness for implementation   efficiency.   [McK90] examines alternative strategies for implementation of Fair   Queueing and SFQ and estimates their complexity on common hardware   platforms (e.g., a Motorola 68020).  It is suggested that mapping an   IP address pair may require around 24 instructions per packet for   Fair Queueing in the best case; in contrast SFQ requires 10Performance and Congestion Control Working Group               [Page 16]

RFC 1254           Gateway Congestion Control Survey         August 1991   instructions in the worst case.  The primary source of this gain is   that SFQ avoids a comparison of the s-d address pair on the packet to   the identity of the queue header.  The relative benefit of SFQ over   Fair Queueing is anticipated to be greater when the addresses are   longer.   SFQ offers promising implemenatation benefits.  However, the price to   be paid in terms of having a significantly larger number of queues   (and the consequent data structures and their management) than the   number of s-d pairs placing a load on the gateway is a concern.  SFQ   is also attractive in that it may be used in concert with the DEC-bit   scheme for Selective Feedback Congestion Indication to provide   fairness as well as congestion avoidance.4.  END-SYSTEM CONGESTION CONTROL POLICIES   Ideally in gateway congestion control, the end-system transport   entities adjust (decrease) their demand in response to control   exerted by the gateway.  Schemes have been put in practice for   transport entities to adjust their demand dynamically in response to   congestion feedback.  To accomplish this, in general, they call for   the user to gradually increase demand until information is received   that the load on the gateway is too high.  In response to this   information, the user decreases load, then begins an exploratory   increases again.  This cycle is repeated continuously, with the goal   that the total demand will oscillate around the optimal level.   We have already noted that a Slow-start connection may give up   considerable bandwidth when sharing a gateway with aggressive   transport entities.  There is currently no way to enforce that end-   systems use a congestion avoidance policy, though the Host   Requirements RFC [HR89] has defined Slow-start as mandatory for TCP.   This adverse effect on Slow-start connections is mitigated by the   Fair Queueing policy.  Our conclusions discuss further the   coexistence of different end-system strategies.   This section briefly presents two fielded transport congestion   control and avoidance schemes, Slow-start and End-System Congestion   Indication, and the responses by means of which they cooperate with   gateway policies.  They both use the control paradigm described   above.  Slow-start, as mentioned inSection 1, was developed by   [Jac88], and widely fielded in the BSD TCP implementation.  End-   system Congestion Indication was developed by [JRC87].  It is fielded   in DEC's Digital Network Architecture, and has been specified as well   for ISO TP4 [NIST88].   Both Slow-start and End-system Congestion Indication view the   relationship between users and gateways as a control system. TheyPerformance and Congestion Control Working Group               [Page 17]

RFC 1254           Gateway Congestion Control Survey         August 1991   have feedback and control components, the latter further broken down   into a procedure bringing a new connection to equilibrium, and a   procedure to maintain demand at the proper level.  They make use of   policies for increasing and decreasing the transport sender's window   size.  These require the sender to follow a set of self-restraining   rules which dynamically adjust the send window whenever congestion is   sensed.   A predecessor of these, CUTE, developed by [Jai86], introduced the   use of retransmission timeouts as congestion feedback.  The Slow-   start scheme was also designed to use timeouts in the same way.  The   End-System policies for Congestion Indication use the Congestion   Experienced Bit delivered in the network header as the primary   feedback, but retransmission timeouts also provoke an end-system   congestion response.   In reliable transport protocols like TCP and TP4, the retransmission   timer must do its best to satisfy two conflicting goals. On one hand,   the timer must rapidly detect lost packets. And, on the other hand,   the timer must minimize false alarms.  Since the retransmit timer is   used as a congestion signal in these end-system policies, it is all   the more important that timeouts reliably correspond to packet drops.   One important rule for retransmission is to avoid bad sampling in the   measurements that will be used in estimating the round-trip delay.   [KP87] describes techniques to ensure accurate sampling.  The Host   Requirements RFC [HR89] makes these techniques mandatory for TCP.   The utilization of a resource can be defined as the ratio of its   average arrival rate to its mean service rate. This metric varies   between 0 and 1.0. In a state of congestion, one or more resources   (link, gateway buffer, gateway CPU) has a utilization approaching   1.0.  The average delay (round trip time) and its variance approach   infinity. Therefore, as the utilization of a network increases, it   becomes increasingly important to take into account the variance of   the round trip time in estimating it for the retransmission timeout.   The TCP retransmission timer is based on an estimate of the round   trip time.  [Jac88] calls the round trip time estimator the single   most important feature of any protocol implementation that expects to   survive a heavy load. The retransmit timeout procedure inRFC-793   [Pos81b] includes a fixed parameter, beta, to account for the   variance in the delay. An estimate of round trip time using the   suggested values for beta is valid for a network which is at most 30%   utilized.  Thus, theRFC-793 retransmission timeout estimator will   fail under heavy congestion, causing unnecessary retransmissions that   add to the load, and making congestive loss detection impossible.   Slow-start TCP uses the mean deviation as an estimate of the variancePerformance and Congestion Control Working Group               [Page 18]

RFC 1254           Gateway Congestion Control Survey         August 1991   to improve the estimate. As a rough view of what happens with the   Slow-start retransmission calculation, delays can change by   approximately two standard deviations without the timer going off in   a false alarm.  The same method of estimation may be applicable to   TP4.  The procedure is:           Error     = Measured - Estimated           Estimated = Estimated + Gain_1 * Error           Deviation = Deviation + Gain_2 * (|Error| - Deviation)           Timeout   = Estimated + 2 * Deviation           Where:  Gain_1, Gain_2 are gain factors.4.1  Response to No Policy in Gateway   Since packets must be dropped during congestion because of the finite   buffer space, feedback of congestion to the users exists even when   there is no gateway congestion policy.  Dropping the packets is an   attempt to recover from congestion, though it needs to be noted that   congestion collapse is not prevented by packet drops if end-systems   accelerate their sending rate in these conditions.  The accurate   detection of congestive loss by all retransmission timers in the   end-systems is extremely important for gateway congestion recovery.4.2  Response to Source Quench   Given that a Source Quench message has ambiguous meaning, but usually   is issued for congestion recovery, the response of Slow-start to a   Source Quench is to return to the beginning of the cycle of increase.   This is an early response, since the Source Quench on overflow will   also lead to a retransmission timeout later.4.3 Response to Random Drop   The end-system's view of Random Drop is the same as its view of loss   caused by overflow at the gateway. This is a drawback of the use of   packet drops as congestion feedback for congestion avoidance: the   decrease policy on congestion feedback cannot be made more drastic   for overflows than for the drops the gateway does for congestion   avoidance.  Slow-start responds rapidly to gateway feedback.  In a   case where the users are cooperating (all Slow-start), a desired   outcome would be that this responsiveness would lead quickly to a   decreased probability of drop.  There would be, as an ideal, a self-   adjusting overall amount of control.Performance and Congestion Control Working Group               [Page 19]

RFC 1254           Gateway Congestion Control Survey         August 19914.4  Response to Congestion Indication   Since the Congestion Indication mechanism attempts to keep gateways   uncongested, cooperating end-system congestion control policies need   not reduce demand as much as with other gateway policies.  The   difference between the Slow-start response to packet drops and the   End-System Congestion Indication response to Congestion Experienced   Bits is primarily in the decrease policy.  Slow-start decreases the   window to one packet on a retransmission timeout.  End-System   Congestion Indication decreases the window by a fraction (for   instance, to 7/8 of the original value), when the Congestion   Experienced Bit is set in half of the packets in a sample spanning   two round-trip times (two windows full).4.5  Response to Fair Queuing   A Fair Queueing policy may issue control indications, as in the   simulated Bit-Round Fair Queueing with DEC Bit, or it may depend only   on the passive effects of the queueing.  When the passive control is   the main effect (perhaps because most users are not responsive to   control indications), the behavior of retransmission timers will be   very important.  If retransmitting users send more packets in   response to increases in their delay and drops, Fair Queueing will be   prone to degraded performance, though collapse (zero throughput for   all users) may be prevented for a longer period of time.5.  Conclusions   The impact of users with excessive demand is a driving force as   proposed gateway policies come closer to implementation.  Random Drop   and Congestion Indication can be fair only if the transport entities   sharing the gateway are all cooperative and reduce demand as needed.   Within some portions of the Internet, good behavior of end-systems   eventually may be enforced through administration.  But for various   reasons, we can expect non-cooperating transports to be a persistent   population in the Internet.  Congestion avoidance mechanisms will not   be deployed all at once, even if they are adopted as standards.   Without enforcement, or even with penalties for excessive demand,   some end-systems will never implement congestion avoidance   mechanisms.   Since it is outside the context of any of the gateway policies, we   will mention here a suggestion for a relatively small-scale response   to users which implement especially aggressive policies. This has   been made experimentally by [Jac89].  It would implement a low   priority queue, to which the majority of traffic is not routed.  The   candidate traffic to be queued there would be identified by a cache   of recent recipients of whatever control indications the gatewayPerformance and Congestion Control Working Group               [Page 20]

RFC 1254           Gateway Congestion Control Survey         August 1991   policy makes.  Remaining in the cache over multiple control intervals   is the criterion for being routed to the low priority queue.  In   approaching or established congestion, the bandwidth given to the   low-service queue is decreased.   The goal of end-system cooperation itself has been questioned.  As   [She89] points out, it is difficult to define.  A TCP implementation   that retransmits before it determines that has been loss indicated   and in a Go-Back-N manner is clearly non-cooperating.  On the other   hand, a transport entity with selective acknowledgement may demand   more from the gateways than TCP, even while responding to congestion   in a cooperative way.   Fair Queueing maintains its control of allocations regardless of the   end-system congestion avoidance policies.  [Nag85] and [DKS89] argue   that the extra delays and congestion drops that result from   misbehavior could work to enforce good end-system policies.  Are the   rewards and penalties less sharply defined when one or more   misbehaving systems cause the whole gateway's performance to be less?   While the tax on all users imposed by the "over-users" is much less   than in gateways with other policies, how can it be made sufficiently   low?   In the sections on Selective Feedback Congestion Indication and Bit-   Round Fair Queueing we have pointed out that more needs to be done on   two particular fronts:      How can increased computational overhead be avoided?      The allocation of resources to source-destination pairs is, in      many scenarios, unfair to individual users. Bit-Round Fair      Queueing offers a potential administrative remedy, but even if it      is applied, how should the unequal allocations be propagated      through multiple gateways?   The first question has been taken up by [McK90].   Since Selective Feedback Congestion Indication (or Congestion   Indication used with Fair Queueing) uses a network bit, its use in   the Internet requires protocol support; the transition of some   portions of the Internet to OSI protocols may make such a change   attractive in the future.  The interactions between heterogeneous   congestion control policies in the Internet will need to be explored.   The goals of Random Drop Congestion Avoidance are presented in this   survey, but without any claim that the problems of this policy can be   solved.  These goals themselves, of uniform, dynamic treatment of   users (streams/flows), of low overhead, and of good scalingPerformance and Congestion Control Working Group               [Page 21]

RFC 1254           Gateway Congestion Control Survey         August 1991   characteristics in large and loaded networks, are significant.Appendix:  Congestion and Connection-oriented Subnets   This section presents a recommendation for gateway implementation in   an areas that unavoidably interacts with gateway congestion control,   the impact of connection-oriented subnets, such as those based on   X.25.   The need to manage a connection oriented service (X.25) in order to   transport datagram traffic (IP) can cause problems for gateway   congestion control.  Being a pure datagram protocol, IP provides no   information delimiting when a pair of IP entities need to establish a   session between themselves.  The solution involves compromise among   delay, cost, and resources.  Delay is introduced by call   establishment when a new X.25 SVC (Switched Virtual Circuit) needs to   be established, and also by queueing delays for the physical line.   Cost includes any charges by the X.25 network service provider.   Besides the resources most gateways have (CPU, memory, links), a   maximum supported or permitted number of virtual circuits may be in   contest.   SVCs are established on demand when an IP packet needs to be sent and   there is no SVC established or pending establishment to the   destination IP entity.  Optionally, when cost considerations, and   sufficient numbers of unused virtual circuits allow, redundant SVCs   may be established between the same pair of IP entities.  Redundant   SVCs can have the effect of improving performance when coping with   large end-to-end delay, small maximum packet sizes and small maximum   packet windows.  It is generally preferred though, to negotiate large   packet sizes and packet windows on a single SVC.  Redundant SVCs must   especially be discouraged when virtual circuit resources are small   compared with the number of destination IP entities among the active   users of the gateway.   SVCs are closed after some period of inactivity indicates that   communication may have been suspended between the IP entities.  This   timeout is significant in the operation of the interface.  Setting   the value too low can result in closing of the SVC even though the   traffic has not stopped.  This results in potentially large delays   for the packets which reopen the SVC and may also incur charges   associated with SVC calls.  Also, clearing of SVCs is, by definition,   nongraceful.  If an SVC is closed before the last packets are   acknowledged, there is no guarantee of delivery.  Packet losses are   introduced by this destructive close independent of gateway traffic   and congestion.   When a new circuit is needed and all available circuits are currentlyPerformance and Congestion Control Working Group               [Page 22]

RFC 1254           Gateway Congestion Control Survey         August 1991   in use, there is a temptation to pick one to close (possibly using   some Least Recently Used criterion).  But if connectivity demands are   larger than available circuit resources, this strategy can lead to a   type of thrashing where circuits are constantly being closed and   reopened.  In the worst case, a circuit is opened, a single packet   sent and the circuit closed (without a guarantee of packet delivery).   To counteract this, each circuit should be allowed to remain open a   minimum amount of time.   One possible SVC strategy is to dynamically change the time a circuit   will be allowed to remain open based on the number of circuits in   use.  Three administratively controlled variables are used, a minimum   time, a maximum time and an adaptation factor in seconds per   available circuit.  In this scheme, a circuit is closed after it has   been idle for a time period equal to the minimum plus the adaptation   factor times the number of available circuits, limited by the maximum   time.  By administratively adjusting these variables, one has   considerable flexibility in adjusting the SVC utilization to meet the   needs of a specific gateway.Acknowledgements   This paper is the outcome of discussions in the Performance and   Congestion Control Working Group between April 1988 and July 1989.   Both PCC WG and plenary IETF members gave us helpful reviews of   earlier drafts.  Several of the ideas described here were contributed   by the members of the PCC WG.  The Appendix was written by Art   Berggreen.  We are particularly thankful also to Van Jacobson, Scott   Shenker, Bruce Schofield, Paul McKenney, Matt Mathis, Geof Stone, and   Lixia Zhang for participation and reviews.References   [DKS89] Demers, A., Keshav, S., and S. Shenker, "Analysis and   Simulation of a Fair Queueing Algorithm", Proceedings of SIGCOMM '89.   [Fin89] Finn, G., "A Connectionless Congestion Control Algorithm",   Computer Communications Review, Vol. 19, No. 5, October 1989.   [Gar87] Gardner, M., "BBN Report on the ARPANET", Proceedings of the   McLean IETF, SRI-NIC IETF-87/3P, July 1987.   [GREQ87] Braden R., and J. Postel, "Requirements for Internet   Gateways",RFC 1009, USC/Information Sciences Institute, June 1987.   [HREQ89] Braden R., Editor, "Requirements for Internet Hosts --   Communications Layers",RFC 1122, Internet Engineering Task Force,   October 1989.Performance and Congestion Control Working Group               [Page 23]

RFC 1254           Gateway Congestion Control Survey         August 1991   [Has90] Hashem, E., "Random Drop Congestion Control", M.S. Thesis,   Massachusetts Institute of Technology, Department of Computer   Science, 1990.   [Jac88] Jacobson, V., "Congestion Avoidance and Control", Proceedings   of SIGCOMM '88.   [Jac89] Jacobson, V., "Presentations to the IETF Performance and   Congestion Control Working Group".   [Jaf81] Jaffe, J., "Bottleneck Flow Control", IEEE Transactions on   Communications, COM-29(7), July, 1981.   [Jai86] Jain, R., "A Timeout-based Congestion Control Scheme for   Window Flow-controlled Networks", IEEE Journal on Selected Areas in   Communications, SAC-4(7), October 1986.   [JRC87] Jain, R., Ramakrishnan, K., and D. Chiu, "Congestion   Avoidance in Computer Networks With a Connectionless Network Layer",   Technical Report DEC-TR-506, Digital Equipment Corporation.   [Kle79] Kleinrock, L., "Power and Deterministic Rules of Thumb for   Probabilistic Problems in Computer Communications",  Proceedings of   the ICC '79.   [KP87] Karn, P., and C. Partridge, "Improving Round Trip Estimates in   Reliable Transport Protocols", Proceedings of SIGCOMM '87.   [Man90] Mankin, A., "Random Drop Congestion Control", Proceedings of   SIGCOMM '90.   [McK90] McKenney, P., "Stochastic Fairness Queueing", Proceedings of   INFOCOM '90.   [Nag84] Nagle, J., "Congestion Control in IP/TCP Internetworks",RFC896, FACC Palo Alto, 6 January 1984.   [Nag85] Nagle, J., "On Packet Switches With Infinite Storage",RFC970, FACC Palo Alto, December 1985.   [NIST88] NIST, "Stable Implementation Agreements for OSI Protocols,   Version 2, Edition 1", National Institute of Standards and Technology   Special Publication 500-162, December 1988.   [Pos81a] Postel, J., "Internet Control Message Protocol - DARPA   Internet Program Protocol Specification",RFC-792, USC/Information   Sciences Institute, September 1981.Performance and Congestion Control Working Group               [Page 24]

RFC 1254           Gateway Congestion Control Survey         August 1991   [Pos81b] Postel, J., "Transmission Control Protocol - DARPA Internet   Program Protocol Specification",RFC-793, DARPA, September 1981.   [RJC87] Ramakrishnan, K., Jain, R., and D. Chiu, "A Selective Binary   Feedback Scheme for General Topologies", Technical Report DEC-TR-510,   Digital Equipment Corporation.   [She89] Shenker, S., "Correspondence with the IETF Performance and   Congestion Control Working Group".   [Sp89] Spector, A., and M. Kazar, "Uniting File Systems", Unix   Review, Vol.  7, No. 3, March 1989.   [Zha89] Zhang, L., "A New Architecture for Packet Switching Network   Protocols", Ph.D Thesis, Massachusetts Institute of Technology,   Department of Computer Science, 1989.Security Considerations   Security issues are not discussed in this memo.Authors' Addresses   Allison Mankin   The MITRE Corporation   M/S W425   7525 Colshire Drive   McLean, VA  22102   Email: mankin@gateway.mitre.org   K.K. Ramakrishnan   Digital Equipment Corporation   M/S LKG1-2/A19   550 King Street   Littleton, MA  01754   Email: rama@kalvi.enet.dec.comPerformance and Congestion Control Working Group               [Page 25]