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IETF Recommendations Regarding Active Queue Management

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7567.
Authors Fred Baker , Gorry Fairhurst
Last updated 2020-01-21 (Latest revision 2015-02-25)
Replaces draft-baker-aqm-recommendation
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Best Current Practice
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Richard Scheffenegger
Shepherd write-up Show Last changed 2014-12-09
IESG IESG state Became RFC 7567 (Best Current Practice)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Martin Stiemerling
Send notices to (None)
IANA IANA review state Version Changed - Review Needed
IANA action state No IANA Actions
Network Working Group                                      F. Baker, Ed.
Internet-Draft                                             Cisco Systems
Obsoletes: 2309 (if approved)                          G. Fairhurst, Ed.
Intended status: Best Current Practice            University of Aberdeen
Expires: August 29, 2015                               February 25, 2015

         IETF Recommendations Regarding Active Queue Management


   This memo presents recommendations to the Internet community
   concerning measures to improve and preserve Internet performance.  It
   presents a strong recommendation for testing, standardization, and
   widespread deployment of active queue management (AQM) in network
   devices, to improve the performance of today's Internet.  It also
   urges a concerted effort of research, measurement, and ultimate
   deployment of AQM mechanisms to protect the Internet from flows that
   are not sufficiently responsive to congestion notification.

   The note replaces the recommendations of RFC 2309 based on fifteen
   years of experience and new research.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 29, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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   ( in effect on the date of
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   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Congestion Collapse . . . . . . . . . . . . . . . . . . .   3
     1.2.  Active Queue Management to Manage Latency . . . . . . . .   4
     1.3.  Document Overview . . . . . . . . . . . . . . . . . . . .   5
     1.4.  Changes to the recommendations of RFC2309 . . . . . . . .   6
     1.5.  Requirements Language . . . . . . . . . . . . . . . . . .   6
   2.  The Need For Active Queue Management  . . . . . . . . . . . .   6
     2.1.  AQM and Multiple Queues . . . . . . . . . . . . . . . . .  10
     2.2.  AQM and Explicit Congestion Marking (ECN) . . . . . . . .  10
     2.3.  AQM and Buffer Size . . . . . . . . . . . . . . . . . . .  11
   3.  Managing Aggressive Flows . . . . . . . . . . . . . . . . . .  11
   4.  Conclusions and Recommendations . . . . . . . . . . . . . . .  14
     4.1.  Operational deployments SHOULD use AQM procedures . . . .  15
     4.2.  Signaling to the transport endpoints  . . . . . . . . . .  16
       4.2.1.  AQM and ECN . . . . . . . . . . . . . . . . . . . . .  17
     4.3.  AQM algorithm deployment SHOULD NOT require operational
           tuning  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     4.4.  AQM algorithms SHOULD respond to measured congestion, not
           application profiles. . . . . . . . . . . . . . . . . . .  19
     4.5.  AQM algorithms SHOULD NOT be dependent on specific
           transport protocol behaviours . . . . . . . . . . . . . .  20
     4.6.  Interactions with congestion control algorithms . . . . .  21
     4.7.  The need for further research . . . . . . . . . . . . . .  22
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  23

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   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  24
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  24
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  25
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   The Internet protocol architecture is based on a connectionless end-
   to-end packet service using the Internet Protocol, whether IPv4
   [RFC0791] or IPv6 [RFC2460].  The advantages of its connectionless
   design: flexibility and robustness, have been amply demonstrated.
   However, these advantages are not without cost: careful design is
   required to provide good service under heavy load.  In fact, lack of
   attention to the dynamics of packet forwarding can result in severe
   service degradation or "Internet meltdown".  This phenomenon was
   first observed during the early growth phase of the Internet in the
   mid 1980s [RFC0896][RFC0970], and is technically called "congestion
   collapse" and was a key focus of RFC2309.

   Although wide-scale congestion collapse is not common in the
   Internet, the presence of localised congestion collapse is by no
   means rare.  It is therefore important to continue to avoid
   congestion collapse.

   Since 1998, when RFC2309 was written, the Internet has become used
   for a variety of traffic.  In the current Internet, low latency is
   extremely important for many interactive and transaction-based
   applications.  The same type of technology that RFC2309 advocated for
   combating congestion collapse is also effective at limiting delays to
   reduce the interaction delay (latency) experienced by applications
   [Bri15].  High or unpredictable latency can impact the performance of
   the control loops used by ene-to-end protocols (including congestion
   control algorithms using TCP).  There is now also a focus on reducing
   network latency using the same technology.

   The mechanisms decsribed in this document may be implemented in
   network devices on the path between end-points that include routers,
   switches, and other network middleboxes.  The methods may also be
   implemented in the networking stacks within endpoint devices that
   connect to the network.

1.1.  Congestion Collapse

   The original fix for Internet meltdown was provided by Van Jacobsen.
   Beginning in 1986, Jacobsen developed the congestion avoidance
   mechanisms [Jacobson88] that are now required for implementations of

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   the Transport Control Protocol (TCP) [RFC0793] [RFC1122].  ([RFC7414]
   provides a roadmap to help identify TCP-related documents.)  These
   mechanisms operate in Internet hosts to cause TCP connections to
   "back off" during congestion.  We say that TCP flows are "responsive"
   to congestion signals (i.e., packets that are dropped or marked with
   explicit congestion notification [RFC3168]).  It is primarily these
   TCP congestion avoidance algorithms that prevent the congestion
   collapse of today's Internet.  Similar algorithms are specified for
   other non-TCP transports.

   However, that is not the end of the story.  Considerable research has
   been done on Internet dynamics since 1988, and the Internet has
   grown.  It has become clear that the congestion avoidance mechanisms
   [RFC5681], while necessary and powerful, are not sufficient to
   provide good service in all circumstances.  Basically, there is a
   limit to how much control can be accomplished from the edges of the
   network.  Some mechanisms are needed in network devices to complement
   the endpoint congestion avoidance mechanisms.  These mechanisms may
   be implemented in network devices.

1.2.  Active Queue Management to Manage Latency

   Internet latency has become a focus of attention to increase the
   responsiveness of Internet applications and protocols.  One major
   source of delay is the build-up of queues in network devices.
   Queueing occurs whenever the arrival rate of data at the ingress to a
   device exceeds the current egress rate.  Such queueing is normal in a
   packet-switched network and is often necessary to absorb bursts in
   transmission and perform statistical multiplexing of traffic, but
   excessive queueing can lead to unwanted delay, reducing the
   performance of some Internet applications.

   RFC 2309 introduced the concept of "Active Queue Management" (AQM), a
   class of technologies that, by signaling to common congestion-
   controlled transports such as TCP, manages the size of queues that
   build in network buffers.  RFC 2309 also describes a specific AQM
   algorithm, Random Early Detection (RED), and recommends that this be
   widely implemented and used by default in routers.

   With an appropriate set of parameters, RED is an effective algorithm.
   However, dynamically predicting this set of parameters was found to
   be difficult.  As a result, RED has not been enabled by default, and
   its present use in the Internet is limited.  Other AQM algorithms
   have been developed since RC2309 was published, some of which are
   self-tuning within a range of applicability.  Hence, while this memo
   continues to recommend the deployment of AQM, it no longer recommends
   that RED or any other specific algorithm is used as a default;
   instead it provides recommendations on how to select appropriate

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   algorithms and that a recommended algorithm is able to automate any
   required tuning for common deployment scenarios.

   Deploying AQM in the network can significantly reduce the latency
   across an Internet path and since writing RFC2309, this has become a
   key motivation for using AQM in the Internet.  In the context of AQM,
   it is useful to distinguish between two related classes of
   algorithms: "queue management" versus "scheduling" algorithms.  To a
   rough approximation, queue management algorithms manage the length of
   packet queues by marking or dropping packets when necessary or
   appropriate, while scheduling algorithms determine which packet to
   send next and are used primarily to manage the allocation of
   bandwidth among flows.  While these two mechanisms are closely
   related, they address different performance issues and operate on
   different timescales.  Both may be used in combination.

1.3.  Document Overview

   The discussion in this memo applies to "best-effort" traffic, which
   is to say, traffic generated by applications that accept the
   occasional loss, duplication, or reordering of traffic in flight.  It
   also applies to other traffic, such as real-time traffic that can
   adapt its sending rate to reduce loss and/or delay.  It is most
   effective when the adaption occurs on time scales of a single Round
   Trip Time (RTT) or a small number of RTTs, for elastic traffic

   Two performance issues are highlighted:

   The first issue is the need for an advanced form of queue management
   that we call "Active Queue Management", AQM.  Section 2 summarizes
   the benefits that active queue management can bring.  A number of AQM
   procedures are described in the literature, with different
   characteristics.  This document does not recommend any of them in
   particular, but does make recommendations that ideally would affect
   the choice of procedure used in a given implementation.

   The second issue, discussed in Section 4 of this memo, is the
   potential for future congestion collapse of the Internet due to flows
   that are unresponsive, or not sufficiently responsive, to congestion
   indications.  Unfortunately, while scheduling can mitigate some of
   the side-effects of sharing a network queue with an unresponsive
   flow, there is currently no consensus solution to controlling the
   congestion caused by such aggressive flows.  Methods such as
   congestion exposure (ConEx) [RFC6789] offer a framework [CONEX] that
   can update network devices to alleviate these effects.  Significant
   research and engineering will be required before any solution will be
   available.  It is imperative that work to mitigate the impact of

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   unresponsive flows is energetically pursued, to ensure acceptable
   performance and the future stability of the Internet.

   Section 4 concludes the memo with a set of recommendations to the
   Internet community on the use of AQM and recommendations for defining
   AQM algorithms.

1.4.  Changes to the recommendations of RFC2309

   This memo replaces the recommendations in [RFC2309], which resulted
   from past discussions of end-to-end performance, Internet congestion,
   and RED in the End-to-End Research Group of the Internet Research
   Task Force (IRTF).  It follows experience with this and other
   algorithms, and the AQM discussion within the IETF [AQM-WG].

   While RFC2309 described AQM in terms of the length of a queue.  This
   memo changes this, to use AQM to refer to any method that allows
   network devices to control either the queue length and/or the mean
   time that a packet spends in a queue.

   This memo also explicitly obsoletes the recommendation that Random
   Early Detection (RED) was to be used as the default AQM mechanism for
   the Internet.  This is replaced by a detailed set of recommendations
   for selecting an appropriate AQM algorithm.  As in RFC2309, this memo
   also motivates the need for continued research, but clarifies the
   research with examples appropriate at the time that this memo is

1.5.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  The Need For Active Queue Management

   Active Queue Management (AQM) is a method that allows network devices
   to control the queue length or the mean time that a packet spends in
   a queue.  Although AQM can be applied across a range of deployment
   environments, the recommendations in this document are directed to
   use in the general Internet.  It is expected that the principles and
   guidance are also applicable to a wide range of environments, but may
   require tuning for specific types of link/network (e.g. to
   accommodate the traffic patterns found in data centres, the
   challenges of wireless infrastructure, or the higher delay
   encountered on satellite Internet links).  The remainder of this
   section identifies the need for AQM and the advantages of deploying
   AQM methods.

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   The traditional technique for managing the queue length in a network
   device is to set a maximum length (in terms of packets) for each
   queue, accept packets for the queue until the maximum length is
   reached, then reject (drop) subsequent incoming packets until the
   queue decreases because a packet from the queue has been transmitted.
   This technique is known as "tail drop", since the packet that arrived
   most recently (i.e., the one on the tail of the queue) is dropped
   when the queue is full.  This method has served the Internet well for
   years, but it has four important drawbacks:

   1.  Full Queues

       The tail drop discipline allows queues to maintain a full (or,
       almost full) status for long periods of time, since tail drop
       signals congestion (via a packet drop) only when the queue has
       become full.  It is important to reduce the steady-state queue
       size, and this is perhaps the most important goal for queue

       The naive assumption might be that there is a simple tradeoff
       between delay and throughput, and that the recommendation that
       queues be maintained in a "non-full" state essentially translates
       to a recommendation that low end-to-end delay is more important
       than high throughput.  However, this does not take into account
       the critical role that packet bursts play in Internet
       performance.  For example, even though TCP constrains the
       congestion window of a flow, packets often arrive at network
       devices in bursts [Leland94].  If the queue is full or almost
       full, an arriving burst will cause multiple packets to be dropped
       from the same flow.  Bursts of loss can result in a global
       synchronization of flows throttling back, followed by a sustained
       period of lowered link utilization, reducing overall throughput
       [Flo94], [Zha90]

       The goal of buffering in the network is to absorb data bursts and
       to transmit them during the (hopefully) ensuing bursts of
       silence.  This is essential to permit transmission of bursts of
       data.  Normally small queues are preferred in network devices,
       with sufficient queue capacity to absorb the bursts.  The
       counter-intuitive result is that maintaining normally-small
       queues can result in higher throughput as well as lower end-to-
       end delay.  In summary, queue limits should not reflect the
       steady state queues we want to be maintained in the network;
       instead, they should reflect the size of bursts that a network
       device needs to absorb.

   2.  Lock-Out

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       In some situations tail drop allows a single connection or a few
       flows to monopolize the queue space starving other connections,
       preventing them from getting room in the queue [Flo92].

   3.  Mitigating the Impact of Packet Bursts

       Large burst of packets can delay other packets, disrupting the
       control loop (e.g. the pacing of flows by the TCP ACK-Clock), and
       reducing the performance of flows that share a common bottleneck.

   4.  Control loop synchronization

       Congestion control, like other end-to-end mechanisms, introduces
       a control loop between hosts.  Sessions that share a common
       network bottleneck can therefore become synchronised, introducing
       periodic disruption (e.g.  jitter/loss). "lock-out" is often also
       the result of synchronization or other timing effects

   Besides tail drop, two alternative queue management disciplines that
   can be applied when a queue becomes full are "random drop on full" or
   "head drop on full".  When a new packet arrives at a full queue using
   the random drop on full discipline, the network device drops a
   randomly selected packet from the queue (which can be an expensive
   operation, since it naively requires an O(N) walk through the packet
   queue).  When a new packet arrives at a full queue using the head
   drop on full discipline, the network device drops the packet at the
   front of the queue [Lakshman96].  Both of these solve the lock-out
   problem, but neither solves the full-queues problem described above.

   We know in general how to solve the full-queues problem for
   "responsive" flows, i.e., those flows that throttle back in response
   to congestion notification.  In the current Internet, dropped packets
   provide a critical mechanism indicating congestion notification to
   hosts.  The solution to the full-queues problem is for network
   devices to drop or ECN-mark packets before a queue becomes full, so
   that hosts can respond to congestion before buffers overflow.  We
   call such a proactive approach AQM.  By dropping or ECN-marking
   packets before buffers overflow, AQM allows network devices to
   control when and how many packets to drop.

   In summary, an active queue management mechanism can provide the
   following advantages for responsive flows.

   1.  Reduce number of packets dropped in network devices

       Packet bursts are an unavoidable aspect of packet networks
       [Willinger95].  If all the queue space in a network device is
       already committed to "steady state" traffic or if the buffer

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       space is inadequate, then the network device will have no ability
       to buffer bursts.  By keeping the average queue size small, AQM
       will provide greater capacity to absorb naturally-occurring
       bursts without dropping packets.

       Furthermore, without AQM, more packets will be dropped when a
       queue does overflow.  This is undesirable for several reasons.
       First, with a shared queue and the tail drop discipline, this can
       result in unnecessary global synchronization of flows, resulting
       in lowered average link utilization, and hence lowered network
       throughput.  Second, unnecessary packet drops represent a waste
       of network capacity on the path before the drop point.

       While AQM can manage queue lengths and reduce end-to-end latency
       even in the absence of end-to-end congestion control, it will be
       able to reduce packet drops only in an environment that continues
       to be dominated by end-to-end congestion control.

   2.  Provide a lower-delay interactive service

       By keeping a small average queue size, AQM will reduce the delays
       experienced by flows.  This is particularly important for
       interactive applications such as short web transfers, POP/IMAP,
       DNS, terminal traffic (telnet, ssh, mosh, RDP, etc), gaming or
       interactive audio-video sessions, whose subjective (and
       objective) performance is better when the end-to-end delay is

   3.  Avoid lock-out behavior

       AQM can prevent lock-out behavior by ensuring that there will
       almost always be a buffer available for an incoming packet.  For
       the same reason, AQM can prevent a bias against low capacity, but
       highly bursty, flows.

       Lock-out is undesirable because it constitutes a gross unfairness
       among groups of flows.  However, we stop short of calling this
       benefit "increased fairness", because general fairness among
       flows requires per-flow state, which is not provided by queue
       management.  For example, in a network device using AQM with only
       FIFO scheduling, two TCP flows may receive very different share
       of the network capacity simply because they have different round-
       trip times [Floyd91], and a flow that does not use congestion
       control may receive more capacity than a flow that does.  AQM can
       therefore be combined with a scheduling mechanism that divides
       network traffic between multiple queues (section 2.1).

   4.  Reduce the probability of control loop synchronization

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       The probability of network control loop synchronization can be
       reduced if network devices introduce randomness in the AQM
       functions that trigger congestion avoidance at the sending host.

2.1.  AQM and Multiple Queues

   A network device may use per-flow or per-class queuing with a
   scheduling algorithm to either prioritize certain applications or
   classes of traffic, limit the rate of transmission, or to provide
   isolation between different traffic flows within a common class.  For
   example, a router may maintain per-flow state to achieve general
   fairness by a per-flow scheduling algorithm such as various forms of
   Fair Queueing (FQ) [Dem90] [Sut99], including Weighted Fair Queuing
   (WFQ), Stochastic Fairness Queueing (SFQ) [McK90] Deficit Round Robin
   (DRR) [Shr96], [Nic12], and/or a Class-Based Queue scheduling
   algorithm such as CBQ [Floyd95].  Hierarchical queues may also be
   used e.g., as a part of a Hierarchical Token Bucket (HTB), or
   Hierarchical Fair Service Curve (HFSC) [Sto97].  These methods are
   also used to realize a range of Quality of Service (QoS) behaviours
   designed to meet the need of traffic classes (e.g. using the
   integrated or differentiated service models).

   AQM is needed even for network devices that use per-flow or per-class
   queuing, because scheduling algorithms by themselves do not control
   the overall queue size or the size of individual queues.  AQM
   mechanisms might need to control the overall queue sizes, to ensure
   that arriving bursts can be accommodated without dropping packets.
   AQM should also be used to control the queue size for each individual
   flow or class, so that they do not experience unnecessarily high
   delay.  Using a combination of AQM and scheduling between multiple
   queues has been shown to offer good results in experimental and some
   types of operational use.

   In short, scheduling algorithms and queue management should be seen
   as complementary, not as replacements for each other.

2.2.  AQM and Explicit Congestion Marking (ECN)

   An AQM method may use Explicit Congestion Notification (ECN)
   [RFC3168] instead of dropping to mark packets under mild or moderate
   congestion.  ECN-marking can allow a network device to signal
   congestion at a point before a transport experiences congestion loss
   or additional queuing delay [ECN-Benefit].  Section 4.2.1 describes
   some of the benefits of using ECN with AQM.

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2.3.  AQM and Buffer Size

   It is important to differentiate the choice of buffer size for a
   queue in a switch/router or other network device, and the
   threshold(s) and other parameters that determine how and when an AQM
   algorithm operates.  The optimum buffer size is a function of
   operational requirements and should generally be sized to be
   sufficient to buffer the largest normal traffic burst that is
   expected.  This size depends on the number and burstiness of traffic
   arriving at the queue and the rate at which traffic leaves the queue.

   One objective of AQM is to minimize the effect of lock-out, where one
   flow prevents other flows from effectively gaining capacity.  This
   need can be illustrated by a simple example of drop-tail queuing when
   a new TCP flow injects packets into a queue that happens to be almost
   full.  A TCP flow's congestion control algorithm [RFC5681] increases
   the flow rate to maximize its effective window.  This builds a queue
   in the network, inducing latency to the flow and other flows that
   share this queue.  Once a drop-tail queue fills, there will also be
   loss.  A new flow, sending its initial burst, has an enhanced
   probability of filling the remaining queue and dropping packets.  As
   a result, the new flow can be effectively prevented from effectively
   sharing the queue for a period of many RTTs.  In contrast, AQM can
   minimize the mean queue depth and therefore reducing the probability
   that competing sessions can materially prevent each other from
   performing well.

   AQM frees a designer from having to limit the buffer space assigned
   to a queue to achieve acceptable performance, allowing allocation of
   sufficient buffering to satisfy the needs of the particular traffic
   pattern.  Different types of traffic and deployment scenarios will
   lead to different requirements.  The choice of AQM algorithm and
   associated parameters is therefore a function of the way in which
   congestion is experienced and the required reaction to achieve
   acceptable performance.  This latter is the primary topic of the
   following sections.

3.  Managing Aggressive Flows

   One of the keys to the success of the Internet has been the
   congestion avoidance mechanisms of TCP.  Because TCP "backs off"
   during congestion, a large number of TCP connections can share a
   single, congested link in such a way that link bandwidth is shared
   reasonably equitably among similarly situated flows.  The equitable
   sharing of bandwidth among flows depends on all flows running
   compatible congestion avoidance algorithms, i.e., methods conformant
   with the current TCP specification [RFC5681].

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   In this document a flow is known as "TCP-friendly" when it has a
   congestion response that approximates the average response expected
   of a TCP flow.  One example method of a TCP-friendly scheme is the
   TCP-Friendly Rate Control algorithm [RFC5348].  In this document, the
   term is used more generally to describe this and other algorithms
   that meet these goals.

   There are a variety of types of network flow.  Some convenient
   classes that describe flows are: (1) TCP Friendly flows, (2)
   unresponsive flows, i.e., flows that do not slow down when congestion
   occurs, and (3) flows that are responsive but are less responsive to
   congestion than TCP.  The last two classes contain more aggressive
   flows that can pose significant threats to Internet performance.

   1.  TCP-Friendly flows

       A TCP-friendly flow responds to congestion notification within a
       small number of path Round Trip Times (RTT), and in steady-state
       it uses no more capacity than a conformant TCP running under
       comparable conditions (drop rate, RTT, packet size, etc.).  This
       is described in the remainder of the document.

   2.  Non-Responsive Flows

       A flow that does not adjust its rate in response to congestion
       notification within a small number of path RTTs, can also use
       more capacity than a conformant TCP running under comparable
       conditions.  There is a growing set of applications whose
       congestion avoidance algorithms are inadequate or nonexistent
       (i.e., a flow that does not throttle its sending rate when it
       experiences congestion).

       The User Datagram Protocol (UDP) [RFC0768] provides a minimal,
       best-effort transport to applications and upper-layer protocols
       (both simply called "applications" in the remainder of this
       document) and does not itself provide mechanisms to prevent
       congestion collapse and establish a degree of fairness [RFC5405].
       Examples that use UDP include some streaming applications for
       packet voice and video, and some multicast bulk data transport.
       Other traffic, when aggregated may also become unresponsive to
       congestion notification.  If no action is taken, such
       unresponsive flows could lead to a new congestion collapse
       [RFC2914].  Some applications can even increase their traffic
       volume in response to congestion (e.g. by adding forward error
       correction when loss is experienced), with the possibility that
       they contribute to congestion collapse.

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       In general, applications need to incorporate effective congestion
       avoidance mechanisms [RFC5405].  Research continues to be needed
       to identify and develop ways to accomplish congestion avoidance
       for presently unresponsive applications.  Network devices need to
       be able to protect themselves against unresponsive flows, and
       mechanisms to accomplish this must be developed and deployed.
       Deployment of such mechanisms would provide an incentive for all
       applications to become responsive by either using a congestion-
       controlled transport (e.g.  TCP, SCTP [RFC4960] and DCCP
       [RFC4340].) or by incorporating their own congestion control in
       the application [RFC5405], [RFC6679].

   3.  Transport Flows that are less responsive than TCP

       A second threat is posed by transport protocol implementations
       that are responsive to congestion, but, either deliberately or
       through faulty implementation, reduce less than a TCP flow would
       have done in response to congestion.  This covers a spectrum of
       behaviours between (1) and (2).  If applications are not
       sufficiently responsive to congestion signals, they may gain an
       unfair share of the available network capacity.

       For example, the popularity of the Internet has caused a
       proliferation in the number of TCP implementations.  Some of
       these may fail to implement the TCP congestion avoidance
       mechanisms correctly because of poor implementation.  Others may
       deliberately be implemented with congestion avoidance algorithms
       that are more aggressive in their use of capacity than other TCP
       implementations; this would allow a vendor to claim to have a
       "faster TCP".  The logical consequence of such implementations
       would be a spiral of increasingly aggressive TCP implementations,
       leading back to the point where there is effectively no
       congestion avoidance and the Internet is chronically congested.

       Another example could be an RTP/UDP video flow that uses an
       adaptive codec, but responds incompletely to indications of
       congestion or responds over an excessively long time period.
       Such flows are unlikely to be responsive to congestion signals in
       a timeframe comparable to a small number of end-to-end
       transmission delays.  However, over a longer timescale, perhaps
       seconds in duration, they could moderate their speed, or increase
       their speed if they determine capacity to be available.

       Tunneled traffic aggregates carrying multiple (short) TCP flows
       can be more aggressive than standard bulk TCP.  Applications
       (e.g., web browsers primarily supporting HTTP 1.1 and peer-to-
       peer file-sharing) have exploited this by opening multiple
       connections to the same endpoint.

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       Lastly, some applications (e.g., web browsers primarily
       supporting HTTP 1.1) open a large numbers of succesive short TCP
       flows for a single session.  This can lead to each individual
       flow spending the majority of time in the exponential TCP slow
       start phase, rather than in TCP congestion avoidance.  The
       resulting traffic aggregate can therefore be much less responsive
       than a single standard TCP flow.

   The projected increase in the fraction of total Internet traffic for
   more aggressive flows in classes 2 and 3 could pose a threat to the
   performance of the future Internet.  There is therefore an urgent
   need for measurements of current conditions and for further research
   into the ways of managing such flows.  This raises many difficult
   issues in finding methods with an acceptable overhead cost that can
   identify and isolate unresponsive flows or flows that are less
   responsive than TCP.  Finally, there is as yet little measurement or
   simulation evidence available about the rate at which these threats
   are likely to be realized, or about the expected benefit of
   algorithms for managing such flows.

   Another topic requiring consideration is the appropriate granularity
   of a "flow" when considering a queue management method.  There are a
   few "natural" answers: 1) a transport (e.g.,TCP or UDP) flow (source
   address/port, destination address/port, protocol); 2) Differentiated
   Services Code Point, DSCP; 3) a source/destination host pair (IP
   address); 4) a given source host or a given destination host, or
   various combinations of the above; 5) a subscriber or site receiving
   the Internet service (enterprise or residential).

   The source/destination host pair gives an appropriate granularity in
   many circumstances, However, different vendors/providers use
   different granularities for defining a flow (as a way of
   "distinguishing" themselves from one another), and different
   granularities may be chosen for different places in the network.  It
   may be the case that the granularity is less important than the fact
   that a network device needs to be able to deal with more unresponsive
   flows at *some* granularity.  The granularity of flows for congestion
   management is, at least in part, a question of policy that needs to
   be addressed in the wider IETF community.

4.  Conclusions and Recommendations

   The IRTF, in publishing [RFC2309], and the IETF in subsequent
   discussion, has developed a set of specific recommendations regarding
   the implementation and operational use of AQM procedures.  The
   recommendations provided by this document are summarised as:

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   1.  Network devices SHOULD implement some AQM mechanism to manage
       queue lengths, reduce end-to-end latency, and avoid lock-out
       phenomena within the Internet.

   2.  Deployed AQM algorithms SHOULD support Explicit Congestion
       Notification (ECN) as well as loss to signal congestion to

   3.  AQM algorithms SHOULD NOT require tuning of initial or
       configuration parameters in common use cases.

   4.  AQM algorithms SHOULD respond to measured congestion, not
       application profiles.

   5.  AQM algorithms SHOULD NOT interpret specific transport protocol

   6.  Transport protocol congestion control algorithms SHOULD maximize
       their use of available capacity (when there is data to send)
       without incurring undue loss or undue round trip delay.

   7.  Research, engineering, and measurement efforts are needed
       regarding the design of mechanisms to deal with flows that are
       unresponsive to congestion notification or are responsive, but
       are more aggressive than present TCP.

   These recommendations are expressed using the word "SHOULD".  This is
   in recognition that there may be use cases that have not been
   envisaged in this document in which the recommendation does not
   apply.  Therefore, care should be taken in concluding that one's use
   case falls in that category; during the life of the Internet, such
   use cases have been rarely if ever observed and reported.  To the
   contrary, available research [Choi04] says that even high speed links
   in network cores that are normally very stable in depth and behavior
   experience occasional issues that need moderation.  The
   recommendations are detailed in the following sections.

4.1.  Operational deployments SHOULD use AQM procedures

   AQM procedures are designed to minimize the delay and buffer
   exhaustion induced in the network by queues that have filled as a
   result of host behavior.  Marking and loss behaviors provide a signal
   that buffers within network devices are becoming unnecessarily full,
   and that the sender would do well to moderate its behavior.

   The use of scheduling mechanisms, such as priority queuing, classful
   queuing, and fair queuing, is often effective in networks to help a
   network serve the needs of a range of applications.  Network

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   operators can use these methods to manage traffic passing a choke
   point.  This is discussed in [RFC2474] and [RFC2475].  When
   scheduling is used AQM should be applied across the classes or flows
   as well as within each class or flow:

   o  AQM mechanisms need to control the overall queue sizes, to ensure
      that arriving bursts can be accommodated without dropping packets.

   o  AQM mechanisms need to allow combination with other mechanisms,
      such as scheduling, to allow implementation of policies for
      providing fairness between different flows.

   o  AQM should be used to control the queue size for each individual
      flow or class, so that they do not experience unnecessarily high

4.2.  Signaling to the transport endpoints

   There are a number of ways a network device may signal to the end
   point that the network is becoming congested and trigger a reduction
   in rate.  The signalling methods include:

   o  Delaying transport segments (packets) in flight, such as in a

   o  Dropping transport segments (packets) in transit.

   o  Marking transport segments (packets), such as using Explicit
      Congestion Control[RFC3168] [RFC4301] [RFC4774] [RFC6040]

   Increased network latency is used as an implicit signal of
   congestion.  E.g., in TCP additional delay can affect ACK Clocking
   and has the result of reducing the rate of transmission of new data.
   In the Real Time Protocol (RTP), network latency impacts the RTCP-
   reported RTT and increased latency can trigger a sender to adjust its
   rate.  Methods such as Low Extra Delay Background Transport (LEDBAT)
   [RFC6817] assume increased latency as a primary signal of congestion.
   Appropriate use of delay-based methods and the implications of AQM
   presently remains an area for further research.

   It is essential that all Internet hosts respond to loss [RFC5681],
   [RFC5405][RFC4960][RFC4340].  Packet dropping by network devices that
   are under load has two effects: It protects the network, which is the
   primary reason that network devices drop packets.  The detection of
   loss also provides a signal to a reliable transport (e.g., TCP, SCTP)
   that there is potential congestion using a pragmatic heuristic; "when
   the network discards a message in flight, it may imply the presence

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   of faulty equipment or media in a path, and it may imply the presence
   of congestion.  To be conservative, a transport must assume it may be
   the latter."  Applications using unreliable transports (e.g.,using
   UDP) need to similarly react to loss [RFC5405]

   Network devices SHOULD use an AQM algorithm to measure local
   congestion and to determine the packets to mark or drop so that the
   congestion is managed.

   In general, dropping multiple packets from the same sessions in the
   same RTT is ineffective, and can reduce throughput.  Also, dropping
   or marking packets from multiple sessions simultaneously can have the
   effect of synchronizing them, resulting in increasing peaks and
   troughs in the subsequent traffic load.  Hence, AQM algorithms SHOULD
   randomize dropping in time, to reduce the probability that congestion
   indications are only experienced by a small proportion of the active

   Loss due to dropping also has an effect on the efficiency of a flow
   and can significantly impact some classes of application.  In
   reliable transports the dropped data must be subsequently
   retransmitted.  While other applications/transports may adapt to the
   absence of lost data, this still implies inefficient use of available
   capacity and the dropped traffic can affect other flows.  Hence,
   congestion signalling by loss is not entirely positive; it is a
   necessary evil.

4.2.1.  AQM and ECN

   Explicit Congestion Notification (ECN) [RFC4301] [RFC4774] [RFC6040]
   [RFC6679] is a network-layer function that allows a transport to
   receive network congestion information from a network device without
   incurring the unintended consequences of loss.  ECN includes both
   transport mechanisms and functions implemented in network devices,
   the latter rely upon using AQM to decider when and whether to ECN-

   Congestion for ECN-capable transports is signalled by a network
   device setting the "Congestion Experienced (CE)" codepoint in the IP
   header.  This codepoint is noted by the remote receiving end point
   and signalled back to the sender using a transport protocol
   mechanism, allowing the sender to trigger timely congestion control.
   The decision to set the CE codepoint requires an AQM algorithm
   configured with a threshold.  Non-ECN capable flows (the default) are
   dropped under congestion.

   Network devices SHOULD use an AQM algorithm that marks ECN-capable
   traffic when making decisions about the response to congestion.

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   Network devices need to implement this method by marking ECN-capable
   traffic or by dropping non-ECN-capable traffic.

   Safe deployment of ECN requires that network devices drop excessive
   traffic, even when marked as originating from an ECN-capable
   transport.  This is a necessary safety precaution because:

   1.  A non-conformant, broken or malicious receiver could conceal an
       ECN mark, and not report this to the sender;

   2.  A non-conformant, broken or malicious sender could ignore a
       reported ECN mark, as it could ignore a loss without using ECN;

   3.  A malfunctioning or non-conforming network device may "hide" an
       ECN mark (or fail to correctly set the ECN codepoint at an egress
       of a network tunnel).

   In normal operation, such cases should be very uncommon, however
   overload protection is desirable to protect traffic from
   misconfigured or malicious use of ECN (e.g., a denial-of-service
   attack that generates ECN-capable traffic that is unresponsive to CE-

   An AQM algorithm that supports ECN needs to define the threshold and
   algorithm for ECN-marking.  This threshold MAY differ from that used
   for dropping packets that are not marked as ECN-capable, and SHOULD
   be configurable.

   Network devices SHOULD use an algorithm to drop excessive traffic
   (e.g., at some level above the threshold for CE-marking), even when
   the packets are marked as originating from an ECN-capable transport.

4.3.  AQM algorithm deployment SHOULD NOT require operational tuning

   A number of AQM algorithms have been proposed.  Many require some
   form of tuning or setting of parameters for initial network
   conditions.  This can make these algorithms difficult to use in
   operational networks.

   AQM algorithms need to consider both "initial conditions" and
   "operational conditions".  The former includes values that exist
   before any experience is gathered about the use of the algorithm,
   such as the configured speed of interface, support for full duplex
   communication, interface MTU and other properties of the link.  The
   latter includes information observed from monitoring the size of the
   queue, experienced queueing delay, rate of packet discard, etc.

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   This document therefore specifies that AQM algorithms that are
   proposed for deployment in the Internet have the following

   o  AQM algorithm deployment SHOULD NOT require tuning.  An algorithm
      MUST provide a default behaviour that auto-tunes to a reasonable
      performance for typical network operational conditions.  This is
      expected to ease deployment and operation.  Initial conditions,
      such as the interface rate and MTU size or other values derived
      from these, MAY be required by an AQM algorithm.

   o  MAY support further manual tuning that could improve performance
      in a specific deployed network.  Algorithms that lack such
      variables are acceptable, but if such variables exist, they SHOULD
      be externalized (made visible to the operator).  Guidance needs to
      be provided on the cases where auto-tuning is unlikely to achieve
      acceptable performance and to identify the set of parameters that
      can be tuned.  For example, the expected response of an algorithm
      may need to be configured to accommodate the largest expected Path
      RTT, since this value can not be known at initialization.  This
      guidance is expected to enable the algorithm to be deployed in
      networks that have specific characteristics (paths with variable/
      larger delay; networks where capacity is impacted by interactions
      with lower layer mechanisms, etc).

   o  MAY provide logging and alarm signals to assist in identifying if
      an algorithm using manual or auto-tuning is functioning as
      expected. (e.g., this could be based on an internal consistency
      check between input, output, and mark/drop rates over time).  This
      is expected to encourage deployment by default and allow operators
      to identify potential interactions with other network functions.

   Hence, self-tuning algorithms are to be preferred.  Algorithms
   recommended for general Internet deployment by the IETF need to be
   designed so that they do not require operational (especially manual)
   configuration or tuning.

4.4.  AQM algorithms SHOULD respond to measured congestion, not
      application profiles.

   Not all applications transmit packets of the same size.  Although
   applications may be characterized by particular profiles of packet
   size this should not be used as the basis for AQM (see next section).
   Other methods exist, e.g., Differentiated Services queueing, Pre-
   Congestion Notification (PCN) [RFC5559], that can be used to
   differentiate and police classes of application.  Network devices may
   combine AQM with these traffic classification mechanisms and perform
   AQM only on specific queues within a network device.

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   An AQM algorithm should not deliberately try to prejudice the size of
   packet that performs best (i.e., Preferentially drop/mark based only
   on packet size).  Procedures for selecting packets to mark/drop
   SHOULD observe the actual or projected time that a packet is in a
   queue (bytes at a rate being an analog to time).  When an AQM
   algorithm decides whether to drop (or mark) a packet, it is
   RECOMMENDED that the size of the particular packet should not be
   taken into account [RFC7141].

   Applications (or transports) generally know the packet size that they
   are using and can hence make their judgments about whether to use
   small or large packets based on the data they wish to send and the
   expected impact on the delay or throughput, or other performance
   parameter.  When a transport or application responds to a dropped or
   marked packet, the size of the rate reduction should be proportionate
   to the size of the packet that was sent [RFC7141].

   AQM-enabled system MAY instantiate different instances of an AQM
   algorithm to be applied within the same traffic class.  Traffic
   classes may be differentiated based on an Access Control List (ACL),
   the packet Differentiated Services Code Point (DSCP) [RFC5559],
   enabling use of the ECN field (i.e., any of ECT(0), ECT(1) or
   CE)[RFC3168] [RFC4774], a multi-field (MF) classifier that combines
   the values of a set of protocol fields (e.g., IP address, transport,
   ports) or an equivalent codepoint at a lower layer.  This
   recommendation goes beyond what is defined in RFC 3168, by allowing
   that an implementation MAY use more than one instance of an AQM
   algorithm to handle both ECN-capable and non-ECN-capable packets.

4.5.  AQM algorithms SHOULD NOT be dependent on specific transport
      protocol behaviours

   In deploying AQM, network devices need to support a range of Internet
   traffic and SHOULD NOT make implicit assumptions about the
   characteristics desired by the set transports/applications the
   network supports.  That is, AQM methods should be opaque to the
   choice of transport and application.

   AQM algorithms are often evaluated by considering TCP [RFC0793] with
   a limited number of applications.  Although TCP is the predominant
   transport in the Internet today, this no longer represents a
   sufficient selection of traffic for verification.  There is
   significant use of UDP [RFC0768] in voice and video services, and
   some applications find utility in SCTP [RFC4960] and DCCP [RFC4340].
   Hence, AQM algorithms should also demonstrate operation with
   transports other than TCP and need to consider a variety of
   applications.  Selection of AQM algorithms also needs to consider use
   of tunnel encapsulations that may carry traffic aggregates.

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   AQM algorithms SHOULD NOT target or derive implicit assumptions about
   the characteristics desired by specific transports/applications.
   Transports and applications need to respond to the congestion signals
   provided by AQM (i.e., dropping or ECN-marking) in a timely manner
   (within a few RTT at the latest).

4.6.  Interactions with congestion control algorithms

   Applications and transports need to react to received implicit or
   explicit signals that indicate the presence of congestion.  This
   section identifies issues that can impact the design of transport
   protocols when using paths that use AQM.

   Transport protocols and applications need timely signals of
   congestion.  The time taken to detect and respond to congestion is
   increased when network devices queue packets in buffers.  It can be
   difficult to detect tail losses at a higher layer and this may
   sometimes require transport timers or probe packets to detect and
   respond to such loss.  Loss patterns may also impact timely
   detection, e.g., the time may be reduced when network devices do not
   drop long runs of packets from the same flow.

   A common objective of an elastic transport congestion control
   protocol is to allow an application to deliver the maximum rate of
   data without inducing excessive delays when packets are queued in a
   buffers within the network.  To achieve this, a transport should try
   to operate at rate below the inflexion point of the load/delay curve
   (the bend of what is sometimes called a "hockey-stick" curve)
   [Jain94].  When the congestion window allows the load to approach
   this bend, the end-to-end delay starts to rise - a result of
   congestion, as packets probabilistically arrive at non-overlapping
   times.  On the one hand, a transport that operates above this point
   can experience congestion loss and could also trigger operator
   activities, such as those discussed in [RFC6057].  On the other hand,
   a flow may achieve both near-maximum throughput and low latency when
   it operates close to this knee point, with minimal contribution to
   router congestion.  Choice of an appropriate rate/congestion window
   can therefore significantly impact the loss and delay experienced by
   a flow and will impact other flows that share a common network queue.

   Some applications may send less than permitted by the congestion
   control window (or rate).  Examples include multimedia codecs that
   stream at some natural rate (or set of rates) or an application that
   is naturally interactive (e.g., some web applications, interactive
   server-based gaming, transaction-based protocols).  Such applications
   may have different objectives.  They may not wish to maximize
   throughput, but may desire a lower loss rate or bounded delay.

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   The correct operation of an AQM-enabled network device MUST NOT rely
   upon specific transport responses to congestion signals.

4.7.  The need for further research

   The second recommendation of [RFC2309] called for further research
   into the interaction between network queues and host applications,
   and the means of signaling between them.  This research has occurred,
   and we as a community have learned a lot.  However, we are not done.

   We have learned that the problems of congestion, latency and buffer-
   sizing have not gone away, and are becoming more important to many
   users.  A number of self-tuning AQM algorithms have been found that
   offer significant advantages for deployed networks.  There is also
   renewed interest in deploying AQM and the potential of ECN.

   Traffic patterns can depend on the network deployment scenario, and
   Internet research therefore needs to consider the implications of a
   diverse range of application interactions.  This includes ensuring
   that combinations of mechanisms, as well as combinations of traffic
   patterns, do not interact and result in either significantly reduced
   flow throughput or significantly increased latency.

   At the time of writing (in 2015), an obvious example of further
   research is the need to consider the many-to-one communication
   patterns found in data centers, known as incast [Ren12], (e.g.,
   produced by Map/Reduce applications).  Such anlaysis needs to study
   not only each application traffic type, but should also include
   combinations of types of traffic.

   Research also needs to consider the need to extend our taxonomy of
   transport sessions to include not only "mice" and "elephants", but
   "lemmings"?  Where "Lemmings" are flash crowds of "mice" that the
   network inadvertently tries to signal to as if they were elephant
   flows, resulting in head of line blocking in a data center deployment

   Examples of other required research include:

   o  Research into new AQM and scheduling algorithms.

   o  Appropriate use of delay-based methods and the implications of

   o  Research into suitable algorithms for marking ECN-capable packets
      that do not require operational configuration or tuning for common

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   o  Experience in the deployment of ECN alongside AQM.

   o  Tools for enabling AQM (and ECN) deployment and measuring the

   o  Methods for mitigating the impact of non-conformant and malicious

   o  Research to understand the implications of using new network and
      transport methods on applications.

   Hence, this document therefore reiterates the call of RFC 2309: we
   need continuing research as applications develop.

5.  IANA Considerations

   This memo asks the IANA for no new parameters.

6.  Security Considerations

   While security is a very important issue, it is largely orthogonal to
   the performance issues discussed in this memo.

   This recommendation requires algorithms to be independent of specific
   transport or application behaviors.  Therefore a network device does
   not require visibility or access to upper layer protocol information
   to implement an AQM algorithm.  This ability to operate in an
   application-agnostic fashion is therefore an example of a privacy-
   enhancing feature.

   Many deployed network devices use queueing methods that allow
   unresponsive traffic to capture network capacity, denying access to
   other traffic flows.  This could potentially be used as a denial-of-
   service attack.  This threat could be reduced in network devices that
   deploy AQM or some form of scheduling.  We note, however, that a
   denial-of-service attack that results in unresponsive traffic flows
   may be indistinguishable from other traffic flows (e.g., tunnels
   carrying aggregates of short flows, high-rate isochronous
   applications).  New methods therefore may remain vulnerable, and this
   document recommends that ongoing research should consider ways to
   mitigate such attacks.

7.  Privacy Considerations

   This document, by itself, presents no new privacy issues.

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8.  Acknowledgements

   The original version of this document describing best current
   practice was based on the informational text of [RFC2309].  This was
   written by the End-to-End Research Group, which is to say Bob Braden,
   Dave Clark, Jon Crowcroft, Bruce Davie, Steve Deering, Deborah
   Estrin, Sally Floyd, Van Jacobson, Greg Minshall, Craig Partridge,
   Larry Peterson, KK Ramakrishnan, Scott Shenker, John Wroclawski, and
   Lixia Zhang.  Although there are important differences, many of the
   key arguments in the present document remain unchanged from those in
   RFC 2309.

   The need for an updated document was agreed to in the tsvarea meeting
   at IETF 86.  This document was reviewed on the list.
   Comments were received from Colin Perkins, Richard Scheffenegger,
   Dave Taht, John Leslie, David Collier-Brown and many others.

   Gorry Fairhurst was in part supported by the European Community under
   its Seventh Framework Programme through the Reducing Internet
   Transport Latency (RITE) project (ICT-317700).

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP", RFC
              3168, September 2001.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4774]  Floyd, S., "Specifying Alternate Semantics for the
              Explicit Congestion Notification (ECN) Field", BCP 124,
              RFC 4774, November 2006.

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405, November

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, September 2009.

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, November 2010.

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   [RFC6679]  Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
              and K. Carlberg, "Explicit Congestion Notification (ECN)
              for RTP over UDP", RFC 6679, August 2012.

   [RFC7141]  Briscoe, B. and J. Manner, "Byte and Packet Congestion
              Notification", BCP 41, RFC 7141, February 2014.

9.2.  Informative References

   [AQM-WG]   "IETF AQM WG", .

   [Bri15]    Briscoe, Bob., Brunstrom, Anna., Petlund, Andreas., Hayes,
              David., Ros, David., Tsang, Ing-Jyh., Gjessing, Stein.,
              Fairhurst, Gorry., Griwodz, Carsten., and Michael. Welzl,
              "Reducing Internet Latency: A Survey of Techniques and
              their Merit, IEEE Communications Surveys & Tutorials",

   [CONEX]    Mathis, M. and B. Briscoe, "The Benefits to Applications
              of using Explicit Congestion Notification (ECN)", IETF
              (Work-in-Progress) draft-ietf-conex-abstract-mech, March

   [Choi04]   Choi, Baek-Young., Moon, Sue., Zhang, Zhi-Li.,
              Papagiannaki, K., and C. Diot, "Analysis of Point-To-Point
              Packet Delay In an Operational Network", March 2004.

   [Dem90]    Demers, A., Keshav, S., and S. Shenker, "Analysis and
              Simulation of a Fair Queueing Algorithm, Internetworking:
              Research and Experience", SIGCOMM Symposium proceedings on
              Communications architectures and protocols , 1990.

              Welzl, M. and G. Fairhurst, "The Benefits to Applications
              of using Explicit Congestion Notification (ECN)", IETF
              (Work-in-Progress) , February 2014.

   [Flo92]    Floyd, S. and V. Jacobsen, "On Traffic Phase Effects in
              Packet-Switched Gateways", 1992.

   [Flo94]    Floyd, S. and V. Jacobsen, "The Synchronization of
              Periodic Routing Messages,
    ", 1994.

   [Floyd91]  Floyd, S., "Connections with Multiple Congested Gateways
              in Packet-Switched Networks Part 1: One-way Traffic.",
              Computer Communications Review , October 1991.

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   [Floyd95]  Floyd, S. and V. Jacobson, "Link-sharing and Resource
              Management Models for Packet Networks", IEEE/ACM
              Transactions on Networking , August 1995.

              Jacobson, V., "Congestion Avoidance and Control", SIGCOMM
              Symposium proceedings on Communications architectures and
              protocols , August 1988.

   [Jain94]   Jain, Raj., Ramakrishnan, KK., and Chiu. Dah-Ming,
              "Congestion avoidance scheme for computer networks", US
              Patent Office 5377327, December 1994.

              Lakshman, TV., Neidhardt, A., and T. Ott, "The Drop From
              Front Strategy in TCP Over ATM and Its Interworking with
              Other Control Features", IEEE Infocomm , 1996.

              Leland, W., Taqqu, M., Willinger, W., and D. Wilson, "On
              the Self-Similar Nature of Ethernet Traffic (Extended
              Version)", IEEE/ACM Transactions on Networking , February

   [McK90]    McKenney, PE. and G. Varghese, "Stochastic Fairness
              sfq.2002.06.04.pdf , 1990.

   [Nic12]    Nichols, K., "Controlling Queue Delay", Communications of
              the ACM Vol. 55 No. 11, July, 2012, pp.42-50. , July 2002.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

   [RFC0896]  Nagle, J., "Congestion control in IP/TCP internetworks",
              RFC 896, January 1984.

   [RFC0970]  Nagle, J., "On packet switches with infinite storage", RFC
              970, December 1985.

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   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1633]  Braden, B., Clark, D., and S. Shenker, "Integrated
              Services in the Internet Architecture: an Overview", RFC
              1633, June 1994.

   [RFC2309]  Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
              S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
              Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
              S., Wroclawski, J., and L. Zhang, "Recommendations on
              Queue Management and Congestion Avoidance in the
              Internet", RFC 2309, April 1998.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474, December

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, December 1998.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41, RFC
              2914, September 2000.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol", RFC
              4960, September 2007.

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification", RFC
              5348, September 2008.

   [RFC5559]  Eardley, P., "Pre-Congestion Notification (PCN)
              Architecture", RFC 5559, June 2009.

   [RFC6057]  Bastian, C., Klieber, T., Livingood, J., Mills, J., and R.
              Woundy, "Comcast's Protocol-Agnostic Congestion Management
              System", RFC 6057, December 2010.

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   [RFC6789]  Briscoe, B., Woundy, R., and A. Cooper, "Congestion
              Exposure (ConEx) Concepts and Use Cases", RFC 6789,
              December 2012.

   [RFC6817]  Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
              December 2012.

   [RFC7414]  Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
              Zimmermann, "A Roadmap for Transmission Control Protocol
              (TCP) Specification Documents", RFC 7414, February 2015.

   [Ren12]    Ren, Y., Zhao, Y., and P. Liu, "A survey on TCP Incast in
              data center networks, International Journal of
              Communication Systems, Volume 27, Issue 8, pages
              1160-117", 1990.

   [Shr96]    Shreedhar, M. and G. Varghese, "Efficient Fair Queueing
              Using Deficit Round Robin", IEEE/ACM Transactions on
              Networking Vol 4, No. 3 , July 1996.

   [Sto97]    Stoica, I. and H. Zhang, "A Hierarchical Fair Service
              Curve algorithm for Link sharing, real-time and priority
              services", ACM SIGCOMM , 1997.

   [Sut99]    Suter, B., "Buffer Management Schemes for Supporting TCP
              in Gigabit Routers with Per-flow Queueing", IEEE Journal
              on Selected Areas in Communications Vol. 17 Issue 6, June,
              1999, pp. 1159-1169. , 1999.

              Willinger, W., Taqqu, M., Sherman, R., Wilson, D., and V.
              Jacobson, "Self-Similarity Through High-Variability:
              Statistical Analysis of Ethernet LAN Traffic at the Source
              Level", SIGCOMM Symposium proceedings on Communications
              architectures and protocols , August 1995.

   [Zha90]    Zhang, L. and D. Clark, "Oscillating Behavior of Network
              Traffic: A Case Study Simulation,
              Oscillating-Behavior-of-Network-Traffic-1990.pdf", 1990.

Appendix A.  Change Log

   RFC-Editor please remove this appendix before publication.

   Initial Version:  March 2013

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   Minor update of the algorithms that the IETF recommends SHOULD NOT
   require operational (especially manual) configuration or tuning
      April 2013

   Major surgery.  This draft is for discussion at IETF-87 and expected
   to be further updated.
      July 2013

   -00 WG Draft - Updated transport recommendations; revised deployment
   configuration section; numerous minor edits.
      Oct 2013

   -01 WG Draft - Updated transport recommendations; revised deployment
   configuration section; numerous minor edits.
      Jan 2014 - Feedback from WG.

   -02 WG Draft - Minor edits  Feb 2014 - Mainly language fixes.

   -03 WG Draft - Minor edits  Feb 2013 - Comments from David Collier-
      Brown and David Taht.

   -04 WG Draft - Minor edits  May 2014 - Comments during WGLC: Provided
      some introductory subsections to help people (with subsections and
      better text). - Written more on the role scheduling.  - Clarified
      that ECN mark threshold needs to be configurable. - Reworked your
      "knee" para.  Various updates in response to feedback.

   -05 WG Draft - Minor edits  June 2014 - New text added to address
      further comments, and improve introduction - adding context,
      reference to Conex, linking between sections, added text on

   -06 WG Draft - Minor edits  July 2014 - Reorganised the introduction
      following WG feedback to better explain how this relates to the
      original goals of RFC2309.  Added item on packet bursts.  Various
      minor corrections incorporated - no change to main

   -07 WG Draft - Minor edits  July 2014 - Replaced ID REF by RFC 7141.
      Changes made to introduction following inputs from Wes Eddy and
      John Leslie.  Corrections and additions proposed by Bob Briscoe.

   -08 WG Draft - Minor edits  August 2014 - Review comments from John
      Leslie and Bob Briscoe.  Text corrections including; updated
      Acknowledgments (RFC2309 ref) s/congestive/congestion/g; changed
      the more bold language from RFC2309 to reflect a more considered
      perceived threat to Internet Performance; modified the category
      that is not-TCP-like to be "less responsive to congestion than

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      TCP" and more clearkly noted that represents a range of

   -09 WG Draft - Minor edits  Jan 2015 - Edits following LC comments.

   -10 WG Draft - Minor edits  Feb 2015 - Update following IESG Review

   -11 WG Draft - Minor edits  Feb 2015 - Resolution of last issues.

Authors' Addresses

   Fred Baker (editor)
   Cisco Systems
   Santa Barbara, California  93117


   Godred Fairhurst (editor)
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen, Scotland  AB24 3UE


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