draft-ietf-aqm-recommendation-05.txt   draft-ietf-aqm-recommendation-06.txt 
Network Working Group F. Baker, Ed. Network Working Group F. Baker, Ed.
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Obsoletes: 2309 (if approved) G. Fairhurst, Ed. Obsoletes: 2309 (if approved) G. Fairhurst, Ed.
Intended status: Best Current Practice University of Aberdeen Intended status: Best Current Practice University of Aberdeen
Expires: December 24, 2014 June 24, 2014 Expires: January 2, 2015 July 1, 2014
IETF Recommendations Regarding Active Queue Management IETF Recommendations Regarding Active Queue Management
draft-ietf-aqm-recommendation-05 draft-ietf-aqm-recommendation-06
Abstract Abstract
This memo presents recommendations to the Internet community This memo presents recommendations to the Internet community
concerning measures to improve and preserve Internet performance. It concerning measures to improve and preserve Internet performance. It
presents a strong recommendation for testing, standardization, and presents a strong recommendation for testing, standardization, and
widespread deployment of active queue management (AQM) in network widespread deployment of active queue management (AQM) in network
devices, to improve the performance of today's Internet. It also devices, to improve the performance of today's Internet. It also
urges a concerted effort of research, measurement, and ultimate urges a concerted effort of research, measurement, and ultimate
deployment of AQM mechanisms to protect the Internet from flows that deployment of AQM mechanisms to protect the Internet from flows that
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 18, 2014. This Internet-Draft will expire on January 2, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 1.1. Congestion Collapse . . . . . . . . . . . . . . . . . . . 3
2. The Need For Active Queue Management . . . . . . . . . . . . 4 1.2. Active Queue Management to Manage Latency . . . . . . . . 3
2.1. AQM and Multiple Queues . . . . . . . . . . . . . . . . . 8 1.3. Document Overview . . . . . . . . . . . . . . . . . . . . 4
2.2. AQM and Explicit Congestion Marking (ECN) . . . . . . . . 8 1.4. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. The Need For Active Queue Management . . . . . . . . . . . . 5
2.1. AQM and Multiple Queues . . . . . . . . . . . . . . . . . 9
2.2. AQM and Explicit Congestion Marking (ECN) . . . . . . . . 9
2.3. AQM and Buffer Size . . . . . . . . . . . . . . . . . . . 9 2.3. AQM and Buffer Size . . . . . . . . . . . . . . . . . . . 9
3. Managing Aggressive Flows . . . . . . . . . . . . . . . . . . 9 3. Managing Aggressive Flows . . . . . . . . . . . . . . . . . . 10
4. Conclusions and Recommendations . . . . . . . . . . . . . . . 12 4. Conclusions and Recommendations . . . . . . . . . . . . . . . 13
4.1. Operational deployments SHOULD use AQM procedures . . . . 13 4.1. Operational deployments SHOULD use AQM procedures . . . . 14
4.2. Signaling to the transport endpoints . . . . . . . . . . 13 4.2. Signaling to the transport endpoints . . . . . . . . . . 14
4.2.1. AQM and ECN . . . . . . . . . . . . . . . . . . . . . 14 4.2.1. AQM and ECN . . . . . . . . . . . . . . . . . . . . . 15
4.3. AQM algorithms deployed SHOULD NOT require operational 4.3. AQM algorithms deployed SHOULD NOT require operational
tuning . . . . . . . . . . . . . . . . . . . . . . . . . 15 tuning . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.4. AQM algorithms SHOULD respond to measured congestion, not 4.4. AQM algorithms SHOULD respond to measured congestion, not
application profiles. . . . . . . . . . . . . . . . . . . 17 application profiles. . . . . . . . . . . . . . . . . . . 18
4.5. AQM algorithms SHOULD NOT be dependent on specific 4.5. AQM algorithms SHOULD NOT be dependent on specific
transport protocol behaviours . . . . . . . . . . . . . . 17 transport protocol behaviours . . . . . . . . . . . . . . 19
4.6. Interactions with congestion control algorithms . . . . . 18 4.6. Interactions with congestion control algorithms . . . . . 19
4.7. The need for further research . . . . . . . . . . . . . . 19 4.7. The need for further research . . . . . . . . . . . . . . 20
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
6. Security Considerations . . . . . . . . . . . . . . . . . . . 20 6. Security Considerations . . . . . . . . . . . . . . . . . . . 21
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 21
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Normative References . . . . . . . . . . . . . . . . . . 21 9.1. Normative References . . . . . . . . . . . . . . . . . . 22
9.2. Informative References . . . . . . . . . . . . . . . . . 22 9.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 25 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction 1. Introduction
The Internet protocol architecture is based on a connectionless end- The Internet protocol architecture is based on a connectionless end-
to-end packet service using the Internet Protocol, whether IPv4 to-end packet service using the Internet Protocol, whether IPv4
[RFC0791] or IPv6 [RFC2460]. The advantages of its connectionless [RFC0791] or IPv6 [RFC2460]. The advantages of its connectionless
design: flexibility and robustness, have been amply demonstrated. design: flexibility and robustness, have been amply demonstrated.
However, these advantages are not without cost: careful design is However, these advantages are not without cost: careful design is
required to provide good service under heavy load. In fact, lack of required to provide good service under heavy load. In fact, lack of
attention to the dynamics of packet forwarding can result in severe attention to the dynamics of packet forwarding can result in severe
service degradation or "Internet meltdown". This phenomenon was service degradation or "Internet meltdown". This phenomenon was
first observed during the early growth phase of the Internet in the first observed during the early growth phase of the Internet in the
mid 1980s [RFC0896][RFC0970], and is technically called "congestive mid 1980s [RFC0896][RFC0970], and is technically called "congestive
collapse". collapse" and was a key focus of RFC2309.
Since 1998, when RFC2309 was written, the Internet has become used
for a variety traffic of traffic. In the current Internet and 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 experienced by
applications. This document replaces RFC2309, and while there is
still a need to avoid congestion collapse, there is now also a focus
on reducing network latency using the same technology.
1.1. Congestion Collapse
The original fix for Internet meltdown was provided by Van Jacobsen. The original fix for Internet meltdown was provided by Van Jacobsen.
Beginning in 1986, Jacobsen developed the congestion avoidance Beginning in 1986, Jacobsen developed the congestion avoidance
mechanisms [Jacobson88] that are now required for implementations of mechanisms [Jacobson88] that are now required for implementations of
the Transport Control Protocol (TCP) [RFC0768] [RFC1122]. These the Transport Control Protocol (TCP) [RFC0768] [RFC1122]. These
mechanisms operate in Internet hosts to cause TCP connections to mechanisms operate in Internet hosts to cause TCP connections to
"back off" during congestion. We say that TCP flows are "responsive" "back off" during congestion. We say that TCP flows are "responsive"
to congestion signals (i.e., marked or dropped packets) from the to congestion signals (i.e., marked or dropped packets) from the
network. It is primarily these TCP congestion avoidance algorithms network. It is primarily these TCP congestion avoidance algorithms
that prevent the congestive collapse of today's Internet. Similar that prevent the congestive collapse of today's Internet. Similar
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been done on Internet dynamics since 1988, and the Internet has been done on Internet dynamics since 1988, and the Internet has
grown. It has become clear that the congestion avoidance mechanisms grown. It has become clear that the congestion avoidance mechanisms
[RFC5681], while necessary and powerful, are not sufficient to [RFC5681], while necessary and powerful, are not sufficient to
provide good service in all circumstances. Basically, there is a provide good service in all circumstances. Basically, there is a
limit to how much control can be accomplished from the edges of the limit to how much control can be accomplished from the edges of the
network. Some mechanisms are needed in the network devices to network. Some mechanisms are needed in the network devices to
complement the endpoint congestion avoidance mechanisms. These complement the endpoint congestion avoidance mechanisms. These
mechanisms may be implemented in network devices that include mechanisms may be implemented in network devices that include
routers, switches, and other network middleboxes. routers, switches, and other network middleboxes.
It is useful to distinguish between two classes of algorithms related 1.2. Active Queue Management to Manage Latency
to congestion control: "queue management" versus "scheduling"
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 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.
Active Queue Management (AQM) is a technology that manages the size
of the queues that build in network buffers. 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 algorithms. To a rough approximation, queue management algorithms
manage the length of packet queues by marking or dropping packets manage the length of packet queues by marking or dropping packets
when necessary or appropriate, while scheduling algorithms determine when necessary or appropriate, while scheduling algorithms determine
which packet to send next and are used primarily to manage the which packet to send next and are used primarily to manage the
allocation of bandwidth among flows. While these two mechanisms are allocation of bandwidth among flows. While these two mechanisms are
closely related, they address different performance issues and closely related, they address different performance issues and
operate on different timescales. Both may be used in combination. operate on different timescales. Both may be used in combination.
1.3. Document Overview
This memo highlights two performance issues: This memo highlights two performance issues:
The first issue is the need for an advanced form of queue management The first issue is the need for an advanced form of queue management
that we call "Active Queue Management", AQM. Section 2 summarizes that we call "Active Queue Management", AQM. Section 2 summarizes
the benefits that active queue management can bring. A number of AQM the benefits that active queue management can bring. A number of AQM
procedures are described in the literature, with different procedures are described in the literature, with different
characteristics. This document does not recommend any of them in characteristics. This document does not recommend any of them in
particular, but does make recommendations that ideally would affect particular, but does make recommendations that ideally would affect
the choice of procedure used in a given implementation. the choice of procedure used in a given implementation.
The second issue, discussed in Section 3 of this memo, is the The second issue, discussed in Section 3 of this memo, is the
potential for future congestive collapse of the Internet due to flows potential for future congestive collapse of the Internet due to flows
that are unresponsive, or not sufficiently responsive, to congestion that are unresponsive, or not sufficiently responsive, to congestion
indications. Unfortunately, while scheduling can mitigate some of indications. Unfortunately, while scheduling can mitigate some of
the side-effects of sharing a network queue with an unresponsive the side-effects of sharing a network queue with an unresponsive
flow, there is currently no consensus solution to controlling the flow, there is currently no consensus solution to controlling the
congestion caused by such aggressive flows. Methods such as congestion caused by such aggressive flows. Methods such as
congestion exposure (ConEx) [RFC6789] offer a framework [CONEX] that congestion exposure (ConEx) [RFC6789] offer a framework [CONEX] that
can update network devices to alleviate these effcects. Significant can update network devices to alleviate these effects. Significant
research and engineering will be required before any solution will be research and engineering will be required before any solution will be
available. It is imperative that work to mitigate the impact of available. It is imperative that work to mitigate the impact of
unresponsive flows is energetically pursued, to ensure the future unresponsive flows is energetically pursued, to ensure the future
stability of the Internet. stability of the Internet.
Section 4 concludes the memo with a set of recommendations to the Section 4 concludes the memo with a set of recommendations to the
Internet community concerning these topics. Internet community concerning these topics.
The discussion in this memo applies to "best-effort" traffic, which The discussion in this memo applies to "best-effort" traffic, which
is to say, traffic generated by applications that accept the is to say, traffic generated by applications that accept the
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effective when the adaption occurs on time scales of a single Round effective when the adaption occurs on time scales of a single Round
Trip Time (RTT) or a small number of RTTs, for elastic traffic Trip Time (RTT) or a small number of RTTs, for elastic traffic
[RFC1633]. [RFC1633].
[RFC2309] resulted from past discussions of end-to-end performance, [RFC2309] resulted from past discussions of end-to-end performance,
Internet congestion, and Random Early Discard (RED) in the End-to-End Internet congestion, and Random Early Discard (RED) in the End-to-End
Research Group of the Internet Research Task Force (IRTF). This Research Group of the Internet Research Task Force (IRTF). This
update results from experience with this and other algorithms, and update results from experience with this and other algorithms, and
the AQM discussion within the IETF[AQM-WG]. the AQM discussion within the IETF[AQM-WG].
1.1. Requirements Language 1.4. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
2. The Need For Active Queue Management 2. The Need For Active Queue Management
Active Queue Management (AQM) is a method that allows network devices 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 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 a queue. Although AQM can be applied across a range of deployment
enviroments, the recommendations in this document are directed to use environments, the recommendations in this document are directed to
in the general Internet. It is expected that the principles and use in the general Internet. It is expected that the principles and
guidance are also applicable to a wide range of environments, but may guidance are also applicable to a wide range of environments, but may
require tuning for specific types of link/network (e.g. to require tuning for specific types of link/network (e.g. to
accommodate the traffic patterns found in data centres, the accommodate the traffic patterns found in data centres, the
challenges of wireless infrastructure, or the higher delay challenges of wireless infrastructure, or the higher delay
encountered on satellite Internet links). The remainder of this encountered on satellite Internet links). The remainder of this
section identifies the need for AQM and the advantages of deploying section identifies the need for AQM and the advantages of deploying
the method. the method.
The traditional technique for managing the queue length in a network The traditional technique for managing the queue length in a network
device is to set a maximum length (in terms of packets) for each device is to set a maximum length (in terms of packets) for each
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The naive assumption might be that there is a simple tradeoff The naive assumption might be that there is a simple tradeoff
between delay and throughput, and that the recommendation that between delay and throughput, and that the recommendation that
queues be maintained in a "non-full" state essentially translates queues be maintained in a "non-full" state essentially translates
to a recommendation that low end-to-end delay is more important to a recommendation that low end-to-end delay is more important
than high throughput. However, this does not take into account than high throughput. However, this does not take into account
the critical role that packet bursts play in Internet the critical role that packet bursts play in Internet
performance. For example, even though TCP constrains the performance. For example, even though TCP constrains the
congestion window of a flow, packets often arrive at network congestion window of a flow, packets often arrive at network
devices in bursts [Leland94]. If the queue is full or almost devices in bursts [Leland94]. If the queue is full or almost
full, an arriving burst will cause multiple packets to be full, an arriving burst will cause multiple packets to be dropped
dropped. This can result in a global synchronization of flows from the same flow. Bursts of loss can result in a global
throttling back, followed by a sustained period of lowered link synchronization of flows throttling back, followed by a sustained
utilization, reducing overall throughput. period of lowered link utilization, reducing overall throughput
[Flo94], [Zha90]
The point of buffering in the network is to absorb data bursts The goal of buffering in the network is to absorb data bursts and
and to transmit them during the (hopefully) ensuing bursts of to transmit them during the (hopefully) ensuing bursts of
silence. This is essential to permit transmission of bursts of silence. This is essential to permit transmission of bursts of
data. Normally small queues are preferred in network devices, data. Normally small queues are preferred in network devices,
with sufficient queue capacity to absorb the bursts. The with sufficient queue capacity to absorb the bursts. The
counter-intuitive result is that maintaining normally-small counter-intuitive result is that maintaining normally-small
queues can result in higher throughput as well as lower end-to- queues can result in higher throughput as well as lower end-to-
end delay. In summary, queue limits should not reflect the end delay. In summary, queue limits should not reflect the
steady state queues we want to be maintained in the network; steady state queues we want to be maintained in the network;
instead, they should reflect the size of bursts that a network instead, they should reflect the size of bursts that a network
device needs to absorb. device needs to absorb.
2. Lock-Out 2. Lock-Out
In some situations tail drop allows a single connection or a few In some situations tail drop allows a single connection or a few
flows to monopolize the queue space starving other connection flows to monopolize the queue space starving other connection
preventing them from getting room in the queue. preventing them from getting room in the queue [Flo92].
3. Control loop synchronisation 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 synchronisation
Congestion control, like other end-to-end mechanisms, introduces Congestion control, like other end-to-end mechanisms, introduces
a control loop between hosts. Sessions that share a common network a control loop between hosts. Sessions that share a common
bottleneck can therefore become synchronised, introducing network bottleneck can therefore become synchronised, introducing
periodic disruption (e.g. jitter/loss). "lock-out" is often also periodic disruption (e.g. jitter/loss). "lock-out" is often also
the result of synchronization or other timing effects. the result of synchronization or other timing effects
Besides tail drop, two alternative queue management disciplines that Besides tail drop, two alternative queue management disciplines that
can be applied when a queue becomes full are "random drop on full" or 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 "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 the random drop on full discipline, the network device drops a
randomly selected packet from the queue (which can be an expensive randomly selected packet from the queue (which can be an expensive
operation, since it naively requires an O(N) walk through the packet 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 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 drop on full discipline, the network device drops the packet at the
front of the queue [Lakshman96]. Both of these solve the lock-out front of the queue [Lakshman96]. Both of these solve the lock-out
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flows requires per-flow state, which is not provided by queue flows requires per-flow state, which is not provided by queue
management. For example, in a network device using AQM with only management. For example, in a network device using AQM with only
FIFO scheduling, two TCP flows may receive very different share FIFO scheduling, two TCP flows may receive very different share
of the network capacity simply because they have different round- of the network capacity simply because they have different round-
trip times [Floyd91], and a flow that does not use congestion trip times [Floyd91], and a flow that does not use congestion
control may receive more capacity than a flow that does. AQM can control may receive more capacity than a flow that does. AQM can
therefore be combined with a scheduling mechanism that divides therefore be combined with a scheduling mechanism that divides
network traffic between multiple queues (section 2.1). network traffic between multiple queues (section 2.1).
4. Reduce the probability of control loop synchronisation 4. Reduce the probability of control loop synchronisation
The probability of network control loop synchronisation can be The probability of network control loop synchronisation can be
reduced by introducing randomness in the AQM functions used by reduced by introducing randomness in the AQM functions used by
network devices that trigger congestion avoidance at the sending network devices that trigger congestion avoidance at the sending
host. host.
2.1. AQM and Multiple Queues 2.1. AQM and Multiple Queues
A network device may use per-flow or per-class queuing with a A network device may use per-flow or per-class queuing with a
scheduling algorithm to either prioritise certain applications or scheduling algorithm to either prioritise certain applications or
classes of traffic, or to provide isolation between different traffic classes of traffic, limit the rate of transmission, or to provide
flows within a common class. For example, a router may maintain per- isolation between different traffic flows within a common class. For
flow state to achieve general fairness by a per-flow scheduling example, a router may maintain per-flow state to achieve general
algorithm such as various forms of Fair Queueing (FQ) [Dem90], fairness by a per-flow scheduling algorithm such as various forms of
including Weighted Fair Queuing (WFQ), Stochastic Fairness Queueing Fair Queueing (FQ) [Dem90], including Weighted Fair Queuing (WFQ),
(SFQ) [McK90] Deficit Round Robin (DRR) [Shr96] and/or a Class-Based Stochastic Fairness Queueing (SFQ) [McK90] Deficit Round Robin (DRR)
Queue scheduling algorithm such as CBQ [Floyd95]. Hierarchical [Shr96], and/or a Class-Based Queue scheduling algorithm such as CBQ
queues may also be used e.g., as a part of a Hierarchical Token [Floyd95]. Hierarchical queues may also be used e.g., as a part of a
Bucket (HTB), or Hierarchical Fair Service Curve (HFSC) [Sto97] . Hierarchical Token Bucket (HTB), or Hierarchical Fair Service Curve
These methods are also used to realise a range of Quality of Service (HFSC) [Sto97] . These methods are also used to realise a range of
(QoS) behaviours designed to + meet the need of traffic classes (e.g. Quality of Service (QoS) behaviours designed to meet the need of
using the integrated or differentiated service models). 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 AQM is needed even for network devices that use per-flow or per-class
queuing, because scheduling algorithms by themselves do not control queuing, because scheduling algorithms by themselves do not control
the overall queue size or the size of individual queues. AQM the overall queue size or the size of individual queues. AQM
mechanisms need to control the overall queue sizes, to ensure that mechanisms need to control the overall queue sizes, to ensure that
arriving bursts can be accommodated without dropping packets. AQM arriving bursts can be accommodated without dropping packets. AQM
should also be used to control the queue size for each individual should also be used to control the queue size for each individual
flow or class, so that they do not experience unnecessarily high flow or class, so that they do not experience unnecessarily high
delay. Using a combination of AQM and scheduling between multiple delay. Using a combination of AQM and scheduling between multiple
queues has been shown to offer good results in experimental and some queues has been shown to offer good results in experimental and some
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2.3. AQM and Buffer Size 2.3. AQM and Buffer Size
It is important to differentiate the choice of buffer size for a It is important to differentiate the choice of buffer size for a
queue in a switch/router or other network device, and the queue in a switch/router or other network device, and the
threshold(s) and other parameters that determine how and when an AQM threshold(s) and other parameters that determine how and when an AQM
algorithm operates. One the one hand, the optimum buffer size is a algorithm operates. One the one hand, the optimum buffer size is a
function of operational requirements and should generally be sized to function of operational requirements and should generally be sized to
be sufficient to buffer the largest normal traffic burst that is be sufficient to buffer the largest normal traffic burst that is
expected. This size depends on the number and burstiness of traffic expected. This size depends on the number and burstiness of traffic
arriving at the queue and the rate at which traffic leaves the queue. arriving at the queue and the rate at which traffic leaves the queue.
The simplest mechanism starts with a new or building session
attacking a queue that is full. One or more sessions, following
algorithms similar to those of [RFC5681], maximizes its effective
window, maximizing its impact on a queue somewhere in the network and
the effect of that queue on both its own latency and that of
competing sessions. It also maximizes the probability of loss from
that queue. A new session, sending its initial burst, has an
enhanced probability of filling the remaining queue and dropping
packets. As a result, the new session can be effectively prevented
from sharing the queue effectively for a period of many RTTs. One
objective of AQM is to minimize the effect of lock-out by minimizing
mean queue depth and therefore the probability that competing
sessions can materially prevent each other from performing well.
Different types of traffic and deployment scenarios will lead to Different types of traffic and deployment scenarios will lead to
different requirements. different requirements.
AQM frees a designer from having to the limit buffer space to achieve AQM frees a designer from having to the limit buffer space to achieve
acceptable performance, allowing allocation of sufficient buffering acceptable performance, allowing allocation of sufficient buffering
to satisfy the needs of the particular traffic pattern. On the other to satisfy the needs of the particular traffic pattern. On the other
hand, the choice of AQM algorithm and associated parameters is a hand, the choice of AQM algorithm and associated parameters is a
function of the way in which congestion is experienced and the function of the way in which congestion is experienced and the
required reaction to achieve acceptable performance. This latter required reaction to achieve acceptable performance. This latter
topic is the primary topic of the following sections. topic is the primary topic of the following sections.
skipping to change at page 13, line 28 skipping to change at page 14, line 36
network serve the needs of a range of applications. Network network serve the needs of a range of applications. Network
operators can use these methods to manage traffic passing a choke operators can use these methods to manage traffic passing a choke
point. This is discussed in [RFC2474] and [RFC2475]. When point. This is discussed in [RFC2474] and [RFC2475]. When
scheduling is used AQM should be applied across the classes or flows scheduling is used AQM should be applied across the classes or flows
as well as within each class or flow: as well as within each class or flow:
o AQM mechanisms need to control the overall queue sizes, to ensure o AQM mechanisms need to control the overall queue sizes, to ensure
that arriving bursts can be accommodated without dropping packets. that arriving bursts can be accommodated without dropping packets.
o AQM mechanisms need to allow combination with other mechanisms, o AQM mechanisms need to allow combination with other mechanisms,
such as scheduling, to allow implementation of polices for such as scheduling, to allow implementation of policies for
providing fairness between different flows. providing fairness between different flows.
o AQM should be used to control the queue size for each individual o AQM should be used to control the queue size for each individual
flow or class, so that they do not experience unnecessarily high flow or class, so that they do not experience unnecessarily high
delay. delay.
4.2. Signaling to the transport endpoints 4.2. Signaling to the transport endpoints
There are a number of ways a network device may signal to the end 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 point that the network is becoming congested and trigger a reduction
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We have learned that the problems of congestion, latency and buffer- We have learned that the problems of congestion, latency and buffer-
sizing have not gone away, and are becoming more important to many sizing have not gone away, and are becoming more important to many
users. A number of self-tuning AQM algorithms have been found that users. A number of self-tuning AQM algorithms have been found that
offer significant advantages for deployed networks. There is also offer significant advantages for deployed networks. There is also
renewed interest in deploying AQM and the potential of ECN. renewed interest in deploying AQM and the potential of ECN.
In 2013, an obvious example of further research is the need to In 2013, an obvious example of further research is the need to
consider the use of Map/Reduce applications in data centers; do we consider the use of Map/Reduce applications in data centers; do we
need to extend our taxonomy of TCP/SCTP sessions to include not only need to extend our taxonomy of TCP/SCTP sessions to include not only
"mice" and "elephants", but "lemmings". "Lemmings" are flash crowds "mice" and "elephants", but "lemmings"? "Lemmings" are flash crowds
of "mice" that the network inadvertently try to signal to as if they of "mice" that the network inadvertently try to signal to as if they
were elephant flows, resulting in head of line blocking in data were elephant flows, resulting in head of line blocking in data
center applications. center applications.
Examples of other required research include: Examples of other required research include:
o Research into new AQM and scheduling algorithms. o Research into new AQM and scheduling algorithms.
o Appropriate use of delay-based methods and the implications of o Appropriate use of delay-based methods and the implications of
AQM. AQM.
skipping to change at page 22, line 14 skipping to change at page 23, line 18
9.2. Informative References 9.2. Informative References
[AQM-WG] "IETF AQM WG", . [AQM-WG] "IETF AQM WG", .
[CONEX] Mathis, M. and B. Briscoe, "The Benefits to Applications [CONEX] Mathis, M. and B. Briscoe, "The Benefits to Applications
of using Explicit Congestion Notification (ECN)", IETF of using Explicit Congestion Notification (ECN)", IETF
(Work-in-Progress) draft-ietf-conex-abstract-mech, March (Work-in-Progress) draft-ietf-conex-abstract-mech, March
2014. 2014.
[Choi04] Sprint ATL, Burlingame, CA, , , , and , "Analysis of [Choi04] Choi, Baek-Young., Moon, Sue., Zhang, Zhi-Li.,
Point-To-Point Packet Delay In an Operational Network", Papagiannaki, K., and C. Diot, "Analysis of Point-To-Point
March 2004. Packet Delay In an Operational Network", March 2004.
[Dem90] Demers, A., Keshav, S., and S. Shenker, "Analysis and [Dem90] Demers, A., Keshav, S., and S. Shenker, "Analysis and
Simulation of a Fair Queueing Algorithm, Internetworking: Simulation of a Fair Queueing Algorithm, Internetworking:
Research and Experience", SIGCOMM Symposium proceedings on Research and Experience", SIGCOMM Symposium proceedings on
Communications architectures and protocols , 1990. Communications architectures and protocols , 1990.
[ECN-Benefit] [ECN-Benefit]
Welzl, M. and G. Fairhurst, "The Benefits to Applications Welzl, M. and G. Fairhurst, "The Benefits to Applications
of using Explicit Congestion Notification (ECN)", IETF of using Explicit Congestion Notification (ECN)", IETF
(Work-in-Progress) , February 2014. (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,
http://ee.lbl.gov/papers/sync_94.pdf", 1994.
[Floyd91] Floyd, S., "Connections with Multiple Congested Gateways [Floyd91] Floyd, S., "Connections with Multiple Congested Gateways
in Packet-Switched Networks Part 1: One-way Traffic.", in Packet-Switched Networks Part 1: One-way Traffic.",
Computer Communications Review , October 1991. Computer Communications Review , October 1991.
[Floyd95] Floyd, S. and V. Jacobson, "Link-sharing and Resource [Floyd95] Floyd, S. and V. Jacobson, "Link-sharing and Resource
Management Models for Packet Networks", IEEE/ACM Management Models for Packet Networks", IEEE/ACM
Transactions on Networking , August 1995. Transactions on Networking , August 1995.
[Jacobson88] [Jacobson88]
Jacobson, V., "Congestion Avoidance and Control", SIGCOMM Jacobson, V., "Congestion Avoidance and Control", SIGCOMM
skipping to change at page 25, line 12 skipping to change at page 26, line 21
on Selected Areas in Communications Vol. 17 Issue 6, June, on Selected Areas in Communications Vol. 17 Issue 6, June,
1999, pp. 1159-1169. , 1999. 1999, pp. 1159-1169. , 1999.
[Willinger95] [Willinger95]
Willinger, W., Taqqu, M., Sherman, R., Wilson, D., and V. Willinger, W., Taqqu, M., Sherman, R., Wilson, D., and V.
Jacobson, "Self-Similarity Through High-Variability: Jacobson, "Self-Similarity Through High-Variability:
Statistical Analysis of Ethernet LAN Traffic at the Source Statistical Analysis of Ethernet LAN Traffic at the Source
Level", SIGCOMM Symposium proceedings on Communications Level", SIGCOMM Symposium proceedings on Communications
architectures and protocols , August 1995. architectures and protocols , August 1995.
[Zha90] Zhang, L. and D. Clark, "Oscillating Behavior of Network
Traffic: A Case Study Simulation,
http://groups.csail.mit.edu/ana/Publications/Zhang-DDC-
Oscillating-Behavior-of-Network-Traffic-1990.pdf", 1990.
Appendix A. Change Log Appendix A. Change Log
Initial Version: March 2013 Initial Version: March 2013
Minor update of the algorithms that the IETF recommends SHOULD NOT Minor update of the algorithms that the IETF recommends SHOULD NOT
require operational (especially manual) configuration or tuningdate: require operational (especially manual) configuration or tuningdate:
April 2013 April 2013
Major surgery. This draft is for discussion at IETF-87 and expected Major surgery. This draft is for discussion at IETF-87 and expected
skipping to change at page 26, line 5 skipping to change at page 27, line 16
some introductory subsections to help people (with subsections and some introductory subsections to help people (with subsections and
better text). - Written more on the role scheduling. - Clarified better text). - Written more on the role scheduling. - Clarified
that ECN mark threshold needs to be configurable. - Reworked your that ECN mark threshold needs to be configurable. - Reworked your
"knee" para. Various updates in response to feedback. "knee" para. Various updates in response to feedback.
-05 WG Draft - Minor edits June 2014 - New text added to address -05 WG Draft - Minor edits June 2014 - New text added to address
further comments, and improve introduction - adding context, further comments, and improve introduction - adding context,
reference to Conex, linking between sections, added text on reference to Conex, linking between sections, added text on
synchronisation. synchronisation.
-06 WG Draft - Minor edits July 2014 - Reorganised the introduction
following WG feedback to better explain how this relates to the
original goals of RGFC2309. Added item on packet bursts. Various
minor corrections incorporatd - no change to main recommendations.
Authors' Addresses Authors' Addresses
Fred Baker (editor) Fred Baker (editor)
Cisco Systems Cisco Systems
Santa Barbara, California 93117 Santa Barbara, California 93117
USA USA
Email: fred@cisco.com Email: fred@cisco.com
Godred Fairhurst (editor) Godred Fairhurst (editor)
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