draft-ietf-aqm-recommendation-03.txt   draft-ietf-aqm-recommendation-04.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: August 19, 2014 February 15, 2014 Expires: November 15, 2014 May 14, 2014
IETF Recommendations Regarding Active Queue Management IETF Recommendations Regarding Active Queue Management
draft-ietf-aqm-recommendation-03 draft-ietf-aqm-recommendation-04
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 August 19, 2014. This Internet-Draft will expire on November 15, 2014.
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.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
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. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. The Need For Active Queue Management . . . . . . . . . . . . 4 2. The Need For Active Queue Management . . . . . . . . . . . . 4
2.1. AQM and Multiple Queues . . . . . . . . . . . . . . . . . 7
2.2. AQM and Explicit Congestion Marking (ECN) . . . . . . . . 8
2.3. AQM and Buffer Size . . . . . . . . . . . . . . . . . . . 8
3. Managing Aggressive Flows . . . . . . . . . . . . . . . . . . 8 3. Managing Aggressive Flows . . . . . . . . . . . . . . . . . . 8
4. Conclusions and Recommendations . . . . . . . . . . . . . . . 10 4. Conclusions and Recommendations . . . . . . . . . . . . . . . 11
4.1. Operational deployments SHOULD use AQM procedures . . . 11 4.1. Operational deployments SHOULD use AQM procedures . . . . 12
4.2. Signaling to the transport endpoints . . . . . . . . . . 12 4.2. Signaling to the transport endpoints . . . . . . . . . . 12
4.2.1. AQM and ECN . . . . . . . . . . . . . . . . . . . . . 13 4.2.1. AQM and ECN . . . . . . . . . . . . . . . . . . . . . 13
4.3. AQM algorithms deployed SHOULD NOT require operational 4.3. AQM algorithms deployed SHOULD NOT require operational
tuning . . . . . . . . . . . . . . . . . . . . . . . . . 13 tuning . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.4. AQM algorithms SHOULD respond to measured congestion, not 4.4. AQM algorithms SHOULD respond to measured congestion, not
application profiles. . . . . . . . . . . . . . . . . . . 15 application profiles. . . . . . . . . . . . . . . . . . . 16
4.5. AQM algorithms SHOULD NOT be dependent on specific 4.5. AQM algorithms SHOULD NOT be dependent on specific
transport protocol behaviours . . . . . . . . . . . . . . 15 transport protocol behaviours . . . . . . . . . . . . . . 17
4.6. Interactions with congestion control algorithms . . . . . 16 4.6. Interactions with congestion control algorithms . . . . . 17
4.7. The need for further research . . . . . . . . . . . . . . 17 4.7. The need for further research . . . . . . . . . . . . . . 18
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18 6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 18 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 19
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
9.1. Normative References . . . . . . . . . . . . . . . . . . 19 9.1. Normative References . . . . . . . . . . . . . . . . . . 20
9.2. Informative References . . . . . . . . . . . . . . . . . 19 9.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 22 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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".
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 that are now required in TCP implementations [Jacobson88] mechanisms [Jacobson88] that are now required for implementations of
[RFC1122]. These mechanisms operate in Internet hosts to cause TCP the Transport Control Protocol (TCP) [RFC0768] [RFC1122]. These
connections to "back off" during congestion. We say that TCP flows mechanisms operate in Internet hosts to cause TCP connections to
are "responsive" to congestion signals (i.e., marked or dropped "back off" during congestion. We say that TCP flows are "responsive"
packets) from the network. It is primarily these TCP congestion to congestion signals (i.e., marked or dropped packets) from the
avoidance algorithms that prevent the congestive collapse of today's network. It is primarily these TCP congestion avoidance algorithms
Internet. Similar algorithms are specified for other non-TCP that prevent the congestive collapse of today's Internet. Similar
transports. algorithms are specified for other non-TCP transports.
However, that is not the end of the story. Considerable research has However, that is not the end of the story. Considerable research has
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 TCP congestion avoidance grown. It has become clear that the TCP congestion avoidance
mechanisms [RFC5681], while necessary and powerful, are not mechanisms [RFC5681], while necessary and powerful, are not
sufficient to provide good service in all circumstances. Basically, sufficient to provide good service in all circumstances. Basically,
there is a limit to how much control can be accomplished from the there is a limit to how much control can be accomplished from the
edges of the network. Some mechanisms are needed in the network edges of the network. Some mechanisms are needed in the network
devices to complement the endpoint congestion avoidance mechanisms. devices to complement the endpoint congestion avoidance mechanisms.
These mechanisms may be implemented in network devices that include These 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 It is useful to distinguish between two classes of algorithms related
to congestion control: "queue management" versus "scheduling" to congestion control: "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 AQM mechanisms allocation of bandwidth among flows. While these two mechanisms are
are closely related, they address different performance issues. closely related, they address different performance issues and
operate on different timescales. Both may be used in combination.
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, there is currently no consensus solution indications. Unfortunately, while scheduling can mitigate some of
to controlling congestion caused by such aggressive flows; the side-effects of sharing a network queue with an unresponsive
significant research and engineering will be required before any flow, there is currently no consensus solution to controlling the
solution will be available. It is imperative that this work be congestion caused by such aggressive flows; significant research and
energetically pursued, to ensure the future stability of the engineering will be required before any solution will be available.
Internet. It is imperative that this work be energetically pursued, to ensure
the future 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
occasional loss, duplication, or reordering of traffic in flight. It occasional loss, duplication, or reordering of traffic in flight. It
also applies to other traffic, such as real-time traffic that can 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 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 effective when the adaption occurs on time scales of a single Round
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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. This can result in a global synchronization of flows dropped. This can result in a global synchronization of flows
throttling back, followed by a sustained period of lowered link throttling back, followed by a sustained period of lowered link
utilization, reducing overall throughput. utilization, reducing overall throughput.
The point of buffering in the network is to absorb data bursts The point of buffering in the network is to absorb data bursts
and to transmit them during the (hopefully) ensuing bursts of and to transmit them during the (hopefully) ensuing bursts of
silence. This is essential to permit the transmission of bursty 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.
Besides tail drop, two alternative queue disciplines that can be Besides tail drop, two alternative queue disciplines that can be
applied when a queue becomes full are "random drop on full" or "drop applied when a queue becomes full are "random drop on full" or "head
front on full". Under the random drop on full discipline, a network drop on full". When a new packet arrives at a full queue using the
device drops a randomly selected packet from the queue (which can be random drop on full discipline, the network device drops a randomly
an expensive operation, since it naively requires an O(N) walk selected packet from the queue (which can be an expensive operation,
through the packet queue) when the queue is full and a new packet since it naively requires an O(N) walk through the packet queue).
arrives. Under the "drop front on full" discipline [Lakshman96], the When a new packet arrives at a full queue using the head drop on full
network device drops the packet at the front of the queue when the discipline, the network device drops the packet at the front of the
queue is full and a new packet arrives. Both of these solve the queue [Lakshman96]. Both of these solve the lock-out problem, but
lock-out problem, but neither solves the full-queues problem neither solves the full-queues problem described above.
described above.
We know in general how to solve the full-queues problem for We know in general how to solve the full-queues problem for
"responsive" flows, i.e., those flows that throttle back in response "responsive" flows, i.e., those flows that throttle back in response
to congestion notification. In the current Internet, dropped packets to congestion notification. In the current Internet, dropped packets
provide a critical mechanism indicating congestion notification to provide a critical mechanism indicating congestion notification to
hosts. The solution to the full-queues problem is for network hosts. The solution to the full-queues problem is for network
devices to drop packets before a queue becomes full, so that hosts devices to drop packets before a queue becomes full, so that hosts
can respond to congestion before buffers overflow. We call such a can respond to congestion before buffers overflow. We call such a
proactive approach AQM. By dropping packets before buffers overflow, proactive approach AQM. By dropping packets before buffers overflow,
AQM allows network devices to control when and how many packets to AQM allows network devices to control when and how many packets to
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While AQM can manage queue lengths and reduce end-to-end latency 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 even in the absence of end-to-end congestion control, it will be
able to reduce packet drops only in an environment that continues able to reduce packet drops only in an environment that continues
to be dominated by end-to-end congestion control. to be dominated by end-to-end congestion control.
2. Provide a lower-delay interactive service 2. Provide a lower-delay interactive service
By keeping a small average queue size, AQM will reduce the delays By keeping a small average queue size, AQM will reduce the delays
experienced by flows. This is particularly important for experienced by flows. This is particularly important for
interactive applications such as short Web transfers, POP/IMAP, interactive applications such as short web transfers, POP/IMAP,
Telnet traffic, or interactive audio-video sessions, whose DNS, terminal traffic (telnet, ssh, mosh, RDP, etc), gaming or
subjective (and objective) performance is better when the end-to- interactive audio-video sessions, whose subjective (and
end delay is low. objective) performance is better when the end-to-end delay is
low.
3. Avoid lock-out behavior 3. Avoid lock-out behavior
AQM can prevent lock-out behavior by ensuring that there will AQM can prevent lock-out behavior by ensuring that there will
almost always be a buffer available for an incoming packet. For almost always be a buffer available for an incoming packet. For
the same reason, AQM can prevent a bias against low capacity, but the same reason, AQM can prevent a bias against low capacity, but
highly bursty, flows. highly bursty, flows.
Lock-out is undesirable because it constitutes a gross unfairness Lock-out is undesirable because it constitutes a gross unfairness
among groups of flows. However, we stop short of calling this among groups of flows. However, we stop short of calling this
benefit "increased fairness", because general fairness among benefit "increased fairness", because general fairness among
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. For control may receive more capacity than a flow that does. AQM can
example, a router may maintain per-flow state to achieve general therefore be combined with a scheduling mechanism that divides
fairness by a per-flow scheduling algorithm such as Fair Queueing network traffic between multiple queues (section 2.1).
(FQ) [Demers90], or a Class-Based Queue scheduling algorithm such
as CBQ [Floyd95].
In contrast, AQM is needed even for network devices that use per- 2.1. AQM and Multiple Queues
flow scheduling algorithms such as FQ or class-based scheduling
algorithms, such as CBQ. This is because per-flow scheduling A network device may use per-flow or per-class queuing with a
algorithms by themselves do not control the overall queue size or scheduling algorithm to either prioritise certain applications or
the size of individual queues. AQM is needed to control the classes of traffic, or to provide isolation between different traffic
overall average queue sizes, so that arriving bursts can be flows within a common class. For example, a router may maintain per-
accommodated without dropping packets. In addition, AQM should flow state to achieve general fairness by a per-flow scheduling
be used to control the queue size for each individual flow or algorithm such as various forms of Fair Queueing (FQ) [Dem90],
class, so that they do not experience unnecessarily high delay. including Weighted Fair Queuing (WFQ), Stochastic Fairness Queueing
Therefore, AQM should be applied across the classes or flows as (SFQ) [McK90] Deficit Round Robin (DRR) [Shr96] and/or a Class-Based
well as within each class or flow. 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 realise 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 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 In short, scheduling algorithms and queue management should be seen
as complementary, not as replacements for each other. 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) An AQM method may use Explicit Congestion Notification (ECN)
[RFC3168] instead of dropping to mark packets under mild or moderate [RFC3168] instead of dropping to mark packets under mild or moderate
congestion (see Section 4.2.1). 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 4.2.1provides
recommendations on the use of ECN with AQM.
It is also important to differentiate the choice of buffer size for a 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 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.
Different types of traffic and deployment scenarios will lead to Different types of traffic and deployment scenarios will lead to
different requirements. AQM frees a designer from having to the different requirements. AQM frees a designer from having to the
limit buffer space to achieve acceptable performance, allowing limit buffer space to achieve acceptable performance, allowing
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One of the keys to the success of the Internet has been the One of the keys to the success of the Internet has been the
congestion avoidance mechanisms of TCP. Because TCP "backs off" congestion avoidance mechanisms of TCP. Because TCP "backs off"
during congestion, a large number of TCP connections can share a during congestion, a large number of TCP connections can share a
single, congested link in such a way that link bandwidth is shared single, congested link in such a way that link bandwidth is shared
reasonably equitably among similarly situated flows. The equitable reasonably equitably among similarly situated flows. The equitable
sharing of bandwidth among flows depends on all flows running sharing of bandwidth among flows depends on all flows running
compatible congestion avoidance algorithms, i.e., methods conformant compatible congestion avoidance algorithms, i.e., methods conformant
with the current TCP specification [RFC5681]. with the current TCP specification [RFC5681].
We call a flow "TCP-friendly" when it has a congestion response that In this document a flow is known as "TCP-friendly" when it has a
approximates the average response expected of a TCP flow. One congestion response that approximates the average response expected
example method of a TCP-friendly scheme is the TCP-Friendly Rate of a TCP flow. One example method of a TCP-friendly scheme is the
Control algorithm [RFC5348]. In this document, the term is used more TCP-Friendly Rate Control algorithm [RFC5348]. In this document, the
generally to describe this and other algorithms that meet these term is used more generally to describe this and other algorithms
goals. that meet these goals.
It is convenient to divide flows into three classes: (1) TCP Friendly It is convenient to divide flows into three classes: (1) TCP Friendly
flows, (2) unresponsive flows, i.e., flows that do not slow down when flows, (2) unresponsive flows, i.e., flows that do not slow down when
congestion occurs, and (3) flows that are responsive but are not TCP- congestion occurs, and (3) flows that are responsive but are not TCP-
friendly. The last two classes contain more aggressive flows that friendly. The last two classes contain more aggressive flows that
pose significant threats to Internet performance, which we will now pose significant threats to Internet performance, which we will now
discuss. discuss.
1. TCP-Friendly flows 1. TCP-Friendly flows
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document) and does not itself provide mechanisms to prevent document) and does not itself provide mechanisms to prevent
congestion collapse and establish a degree of fairness [RFC5405]. congestion collapse and establish a degree of fairness [RFC5405].
There is a growing set of UDP-based applications whose congestion There is a growing set of UDP-based applications whose congestion
avoidance algorithms are inadequate or nonexistent (i.e, a flow avoidance algorithms are inadequate or nonexistent (i.e, a flow
that does not throttle its sending rate when it experiences that does not throttle its sending rate when it experiences
congestion). Examples include some UDP streaming applications congestion). Examples include some UDP streaming applications
for packet voice and video, and some multicast bulk data for packet voice and video, and some multicast bulk data
transport. If no action is taken, such unresponsive flows could transport. If no action is taken, such unresponsive flows could
lead to a new congestive collapse [RFC2309]. lead to a new congestive collapse [RFC2309].
In general, UDP-based applications need to incorporate effective In general, UDP-based applications need to incorporate effective
congestion avoidance mechanisms [RFC5405]. Further research and congestion avoidance mechanisms [RFC5405]. Further research and
development of ways to accomplish congestion avoidance for development of ways to accomplish congestion avoidance for
presently unresponsive applications continue to be important. presently unresponsive applications continue to be important.
Network devices need to be able to protect themselves against Network devices need to be able to protect themselves against
unresponsive flows, and mechanisms to accomplish this must be unresponsive flows, and mechanisms to accomplish this must be
developed and deployed. Deployment of such mechanisms would developed and deployed. Deployment of such mechanisms would
provide an incentive for all applications to become responsive by provide an incentive for all applications to become responsive by
either using a congestion-controlled transport (e.g. TCP, SCTP, either using a congestion-controlled transport (e.g. TCP, SCTP
DCCP) or by incorporating their own congestion control in the [RFC4960] and DCCP [RFC4340].) or by incorporating their own
application [RFC5405]. congestion control in the application [RFC5405].
Lastly, some applications (e.g. current web browsers) open a
large numbers of 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.
3. Non-TCP-friendly Transport Protocols 3. Non-TCP-friendly Transport Protocols
A second threat is posed by transport protocol implementations A second threat is posed by transport protocol implementations
that are responsive to congestion, but, either deliberately or that are responsive to congestion, but, either deliberately or
through faulty implementation, are not TCP-friendly. Such through faulty implementation, are not TCP-friendly. Such
applications may gain an unfair share of the available network applications may gain an unfair share of the available network
capacity. capacity.
For example, the popularity of the Internet has caused a For example, the popularity of the Internet has caused a
skipping to change at page 10, line 27 skipping to change at page 11, line 7
more aggressive flows in classes 2 and 3 clearly poses a threat to more aggressive flows in classes 2 and 3 clearly poses a threat to
future Internet stability. There is an urgent need for measurements future Internet stability. There is an urgent need for measurements
of current conditions and for further research into the ways of of current conditions and for further research into the ways of
managing such flows. This raises many difficult issues in managing such flows. This raises many difficult issues in
identifying and isolating unresponsive or non-TCP-friendly flows at identifying and isolating unresponsive or non-TCP-friendly flows at
an acceptable overhead cost. Finally, there is as yet little an acceptable overhead cost. Finally, there is as yet little
measurement or simulation evidence available about the rate at which measurement or simulation evidence available about the rate at which
these threats are likely to be realized, or about the expected these threats are likely to be realized, or about the expected
benefit of algorithms for managing such flows. benefit of algorithms for managing such flows.
Another topic requiring consideration is the appropriate granularity Another topic requiring consideration is the appropriate
of a "flow" when considering a queue management method. There are a granugranularity of a "flow" when considering a queue management
few "natural" answers: 1) a transport (e.g. TCP or UDP) flow (source method. There are a few "natural" answers: 1) a transport (e.g. TCP
address/port, destination address/port, Differentiated Services Code or UDP) flow (source address/port, destination address/port,
Point - DSCP); 2) a source/destination host pair (IP addresses, protocol); 2) Differentiated Services Code Point, DSCP; 3) a source/
DSCP); 3) a given source host or a given destination host. We destination host pair (IP address); 4) a given source host or a given
suggest that the source/destination host pair gives the most destination host, or various combinations of the above.
appropriate granularity in many circumstances. However, it is
possible that different vendors/providers could set different The source/destination host pair gives an appropriate granularity in
granularities for defining a flow (as a way of "distinguishing" many circumstances, However, different vendors/providers use
themselves from one another), or that different granularities could different granularities for defining a flow (as a way of
be chosen for different places in the network. It may be the case "distinguishing" themselves from one another), and different
that the granularity is less important than the fact that a network granularities may be chosen for different places in the network. It
device needs to be able to deal with more unresponsive flows at may be the case that the granularity is less important than the fact
*some* granularity. The granularity of flows for congestion 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 management is, at least in part, a question of policy that needs to
be addressed in the wider IETF community. be addressed in the wider IETF community.
4. Conclusions and Recommendations 4. Conclusions and Recommendations
The IRTF, in publishing [RFC2309], and the IETF in subsequent The IRTF, in publishing [RFC2309], and the IETF in subsequent
discussion, has developed a set of specific recommendations regarding discussion, has developed a set of specific recommendations regarding
the implementation and operational use of AQM procedures. This the implementation and operational use of AQM procedures. The
document updates these to include: updated recommendations provided by this document are summarised as:
1. Network devices SHOULD implement some AQM mechanism to manage 1. Network devices SHOULD implement some AQM mechanism to manage
queue lengths, reduce end-to-end latency, and avoid lock-out queue lengths, reduce end-to-end latency, and avoid lock-out
phenomena within the Internet. phenomena within the Internet.
2. Deployed AQM algorithms SHOULD support Explicit Congestion 2. Deployed AQM algorithms SHOULD support Explicit Congestion
Notification (ECN) as well as loss to signal congestion to Notification (ECN) as well as loss to signal congestion to
endpoints. endpoints.
3. The algorithms that the IETF recommends SHOULD NOT require 3. The algorithms that the IETF recommends SHOULD NOT require
skipping to change at page 11, line 34 skipping to change at page 12, line 17
without incurring undue loss or undue round trip delay. without incurring undue loss or undue round trip delay.
7. Research, engineering, and measurement efforts are needed 7. Research, engineering, and measurement efforts are needed
regarding the design of mechanisms to deal with flows that are regarding the design of mechanisms to deal with flows that are
unresponsive to congestion notification or are responsive, but unresponsive to congestion notification or are responsive, but
are more aggressive than present TCP. are more aggressive than present TCP.
These recommendations are expressed using the word "SHOULD". This is These recommendations are expressed using the word "SHOULD". This is
in recognition that there may be use cases that have not been in recognition that there may be use cases that have not been
envisaged in this document in which the recommendation does not envisaged in this document in which the recommendation does not
apply. However, care should be taken in concluding that one's use apply. Therefore, care should be taken in concluding that one's use
case falls in that category; during the life of the Internet, such case falls in that category; during the life of the Internet, such
use cases have been rarely if ever observed and reported on. To the use cases have been rarely if ever observed and reported. To the
contrary, available research [Papagiannaki] says that even high speed contrary, available research [Choi04] says that even high speed links
links in network cores that are normally very stable in depth and in network cores that are normally very stable in depth and behavior
behavior experience occasional issues that need moderation. experience occasional issues that need moderation. The
recommendations are detailed in the following sections.
4.1. Operational deployments SHOULD use AQM procedures 4.1. Operational deployments SHOULD use AQM procedures
AQM procedures are designed to minimize the delay induced in the AQM procedures are designed to minimize the delay and buffer
network by queues that have filled as a result of host behavior. exhaustion induced in the network by queues that have filled as a
Marking and loss behaviors provide a signal that buffers within result of host behavior. Marking and loss behaviors provide a signal
network devices are becoming unnecessarily full, and that the sender that buffers within network devices are becoming unnecessarily full,
would do well to moderate its behavior. 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
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 should be used to control the queue size for each individual
flow or class, so that they do not experience unnecessarily high
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
in rate. The signalling methods include: in rate. The signalling methods include:
o Delaying transport segments (packets) in flight, such as in a o Delaying transport segments (packets) in flight, such as in a
queue. queue.
o Dropping transport segments (packets) in transit. o Dropping transport segments (packets) in transit.
o Marking transport segments (packets), such as using Explicit o Marking transport segments (packets), such as using Explicit
Congestion Control[RFC3168] [RFC4301] [RFC4774] [RFC6040] Congestion Control[RFC3168] [RFC4301] [RFC4774] [RFC6040]
[RFC6679]. [RFC6679].
The use of scheduling mechanisms, such as priority queuing, classful Increased network latency is used as an implicit signal of
queuing, and fair queuing, is often effective in networks to help a
network serve the needs of a range of applications. Network
operators can use these methods to manage traffic passing a choke
point. This is discussed in [RFC2474] and [RFC2475].
Increased network latency can be used as an implicit signal of
congestion. E.g., in TCP additional delay can affect ACK Clocking congestion. E.g., in TCP additional delay can affect ACK Clocking
and has the result of reducing the rate of transmission of new data. and has the result of reducing the rate of transmission of new data.
In RTP, network latency impacts the RTCP-reported RTT and increased In the Real Time Protocol (RTP), network latency impacts the RTCP-
latency can trigger a sender to adjust its rate. Methods such as reported RTT and increased latency can trigger a sender to adjust its
LEDBAT [RFC6817] assume increased latency as a primary signal of rate. Methods such as Low Extra Delay Background Transport (LEDBAT)
congestion. [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], It is essential that all Internet hosts respond to loss [RFC5681],
[RFC5405][RFC4960][RFC4340]. Packet dropping by network devices that [RFC5405][RFC4960][RFC4340]. Packet dropping by network devices that
are under load has two effects: It protects the network, which is the are under load has two effects: It protects the network, which is the
primary reason that network devices drop packets. The detection of primary reason that network devices drop packets. The detection of
loss also provides a signal to a reliable transport (e.g. TCP, SCTP) loss also provides a signal to a reliable transport (e.g. TCP, SCTP)
that there is potential congestion using a pragmatic heuristic; "when that there is potential congestion using a pragmatic heuristic; "when
the network discards a message in flight, it may imply the presence the network discards a message in flight, it may imply the presence
of faulty equipment or media in a path, and it may imply the presence 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 of congestion. To be conservative, a transport must assume it may be
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the dropped traffic can affect other flows. Hence, congestion the dropped traffic can affect other flows. Hence, congestion
signalling by loss is not entirely positive; it is a necessary evil. signalling by loss is not entirely positive; it is a necessary evil.
4.2.1. AQM and ECN 4.2.1. AQM and ECN
Explicit Congestion Notification (ECN) [RFC4301] [RFC4774] [RFC6040] Explicit Congestion Notification (ECN) [RFC4301] [RFC4774] [RFC6040]
[RFC6679] is a network-layer function that allows a transport to [RFC6679] is a network-layer function that allows a transport to
receive network congestion information from a network device without receive network congestion information from a network device without
incurring the unintended consequences of loss. ECN includes both incurring the unintended consequences of loss. ECN includes both
transport mechanisms and functions implemented in network devices, transport mechanisms and functions implemented in network devices,
the latter rely upon using AQM to decider whether to ECN-mark. the latter rely upon using AQM to decider when and whether to ECN-
mark.
Congestion for ECN-capable transports is signalled by a network Congestion for ECN-capable transports is signalled by a network
device setting the "Congestion Experienced (CE)" codepoint in the IP device setting the "Congestion Experienced (CE)" codepoint in the IP
header. This codepoint is noted by the remote receiving end point header. This codepoint is noted by the remote receiving end point
and signalled back to the sender using a transport protocol and signalled back to the sender using a transport protocol
mechanism, allowing the sender to trigger timely congestion control. mechanism, allowing the sender to trigger timely congestion control.
The decision to set the CE codepoint requires an AQM algorithm The decision to set the CE codepoint requires an AQM algorithm
configured with a threshold. Non-ECN capable flows (the default) are configured with a threshold. Non-ECN capable flows (the default) are
dropped under congestion. dropped under congestion.
Network devices SHOULD use an AQM algorithm that marks ECN-capable Network devices SHOULD use an AQM algorithm that marks ECN-capable
traffic when making decisions about the response to congestion. traffic when making decisions about the response to congestion.
Network devices need to implement this method by marking ECN-capable Network devices need to implement this method by marking ECN-capable
traffic or by dropping non-ECN-capable traffic. traffic or by dropping non-ECN-capable traffic.
Safe deployment of ECN requires that network devices drop excessive Safe deployment of ECN requires that network devices drop excessive
traffic, even when marked as originating from an ECN-capable traffic, even when marked as originating from an ECN-capable
transport. This is a necessary safety precaution because (1) A non- transport. This is a necessary safety precaution because:
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 similarly "hide" an ECN mark. In normal operation
such cases should be very uncommon, however overload protection is
desirable to protect traffic from misconfigured or malicous use of
ECN.
Network devices SHOULD use an algorithm to drop excessive traffic, 1. A non-conformant, broken or malicious receiver could conceal an
even when marked as originating from an ECN-capable transport. 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-
marking).
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 algorithms deployed SHOULD NOT require operational tuning 4.3. AQM algorithms deployed SHOULD NOT require operational tuning
A number of AQM algorithms have been proposed. Many require some A number of AQM algorithms have been proposed. Many require some
form of tuning or setting of parameters for initial network form of tuning or setting of parameters for initial network
conditions. This can make these algorithms difficult to use in conditions. This can make these algorithms difficult to use in
operational networks. operational networks.
AQM algorithms need to consider both "initial conditions" and AQM algorithms need to consider both "initial conditions" and
"operational conditions". The former includes values that exist "operational conditions". The former includes values that exist
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to a reasonable performance for typical network operational to a reasonable performance for typical network operational
conditions. This is expected to ease deployment and operation. conditions. This is expected to ease deployment and operation.
Initial conditions, such as the interface rate and MTU size or Initial conditions, such as the interface rate and MTU size or
other values derived from these, MAY be required by an AQM other values derived from these, MAY be required by an AQM
algorithm. algorithm.
o MAY support further manual tuning that could improve performance o MAY support further manual tuning that could improve performance
in a specific deployed network. Algorithms that lack such in a specific deployed network. Algorithms that lack such
variables are acceptable, but if such variables exist, they SHOULD variables are acceptable, but if such variables exist, they SHOULD
be externalized (made visible to the operator). Guidance needs to be externalized (made visible to the operator). Guidance needs to
be provided on the cases where autotuning is unlikely to achieve be provided on the cases where auto-tuning is unlikely to achieve
satisfactory performance and to identify the set of parameters satisfactory performance and to identify the set of parameters
that can be tuned. This is expected to enable the algorithm to be that can be tuned. For example, the expected response of an
deployed in networks that have specific characteristics (variable/ algorithm may need to be configured to accommodate the largest
larger delay; networks where capacity is impacted by interactions expected Path RTT, since this value can not be known at
with lower layer mechanisms, etc). initialisation. 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 o MAY provide logging and alarm signals to assist in identifying if
an algorithm using manual or auto-tuning is functioning as an algorithm using manual or auto-tuning is functioning as
expected. (e.g., this could be based on an internal consistency expected. (e.g., this could be based on an internal consistency
check between input, output, and mark/drop rates over time). This check between input, output, and mark/drop rates over time). This
is expected to encourage deployment by default and allow operators is expected to encourage deployment by default and allow operators
to identify potential interactions with other network functions. to identify potential interactions with other network functions.
Hence, self-tuning algorithms are to be preferred. Algorithms Hence, self-tuning algorithms are to be preferred. Algorithms
recommended for general Internet deployment by the IETF need to be recommended for general Internet deployment by the IETF need to be
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are using and can hence make their judgments about whether to use 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 small or large packets based on the data they wish to send and the
expected impact on the delay or throughput, or other performance expected impact on the delay or throughput, or other performance
parameter. When a transport or application responds to a dropped or parameter. When a transport or application responds to a dropped or
marked packet, the size of the rate reduction should be proportionate marked packet, the size of the rate reduction should be proportionate
to the size of the packet that was sent [Byte-pkt]. to the size of the packet that was sent [Byte-pkt].
AQM-enabled system MAY instantiate different instances of an AQM AQM-enabled system MAY instantiate different instances of an AQM
algorithm to be applied within the same traffic class. Traffic algorithm to be applied within the same traffic class. Traffic
classes may be differentiated based on an Access Control List (ACL), classes may be differentiated based on an Access Control List (ACL),
the packet DiffServ Code Point (DSCP) [RFC5559], setting of the ECN the packet Differentiated Services Code Point (DSCP) [RFC5559],
field[RFC3168] [RFC4774], a multi-field (MF) classifier that combines 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, the values of a set of protocol fields (e.g. IP address, transport,
ports) or an equivalent codepoint at a lower layer. This ports) or an equivalent codepoint at a lower layer. This
recommendation goes beyond what is defined in RFC 3168, by allowing recommendation goes beyond what is defined in RFC 3168, by allowing
that an implementation MAY use more than one instance of an AQM that an implementation MAY use more than one instance of an AQM
algorithm to handle both ECN-capable and non-ECN-capable packets. algorithm to handle both ECN-capable and non-ECN-capable packets.
4.5. AQM algorithms SHOULD NOT be dependent on specific transport 4.5. AQM algorithms SHOULD NOT be dependent on specific transport
protocol behaviours protocol behaviours
In deploying AQM, network devices need to support a range of Internet In deploying AQM, network devices need to support a range of Internet
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Transport protocols and applications need timely signals of Transport protocols and applications need timely signals of
congestion. The time taken to detect and respond to congestion is congestion. The time taken to detect and respond to congestion is
increased when network devices queue packets in buffers. It can be increased when network devices queue packets in buffers. It can be
difficult to detect tail losses at a higher layer and this may difficult to detect tail losses at a higher layer and this may
sometimes require transport timers or probe packets to detect and sometimes require transport timers or probe packets to detect and
respond to such loss. Loss patterns may also impact timely respond to such loss. Loss patterns may also impact timely
detection, e.g. the time may be reduced when network devices do not detection, e.g. the time may be reduced when network devices do not
drop long runs of packets from the same flow. drop long runs of packets from the same flow.
A common objective is to deliver data from its source end point to A common objective of an elastic transport congestion control
its destination in the least possible time. When speaking of TCP protocol is to allow an application to deliver the maximum rate of
performance, the terms "knee" and "cliff" area defined by [Jain94]. data without inducing excessive delays when packets are queued in a
They respectively refer to the minimum congestion window that buffers within the network. To achieve this, a transport should try
maximizes throughput and the maximum congestion window that avoids to operate at rate below the inflexion point of the load/delay curve
loss. An application that transmits at the rate determined by this (the bend of what is sometimes called a "hockey-stick" curve). When
window has the effect of maximizing the rate or throughput. For the the congestion window allows the load to approach this bend, the end-
sender, exceeding the cliff is ineffective, as it (by definition) to-end delay starts to rise - a result of congestion, as packets
induces loss; operating at a point close to the cliff has a negative probabilistically arrive at non-overlapping times. On the one hand,
impact on other traffic and applications, triggering operator a transport that operates above this point can experience congestion
activities, such as those discussed in [RFC6057]. Operating below loss and could also trigger operator activities, such as those
the knee reduces the throughput, since the sender fails to use discussed in [RFC6057]. On the other hand, a flow may achieve both
available network capacity. As a result, the behavior of any elastic near-maximum throughput and low latency when it operates close to
transport congestion control algorithm designed to minimize delivery this knee point, with minimal contribution to router congestion.
time should seek to use an effective window at or above the knee and Choice of an appropriate rate/congestion window can therefore
well below the cliff. Choice of an appropriate rate can significantly impact the loss and delay experienced by a flow and
significantly impact the loss and delay experienced not only by a will impact other flows that share a common network queue.
flow, but by other flows that share the same queue.
Some applications may send less than permitted by the congestion Some applications may send less than permitted by the congestion
control window (or rate). Examples include multimedia codecs that control window (or rate). Examples include multimedia codecs that
stream at some natural rate (or set of rates) or an application that stream at some natural rate (or set of rates) or an application that
is naturally interactive (e.g., some web applications, gaming, is naturally interactive (e.g., some web applications, gaming,
transaction-based protocols). Such applications may have different transaction-based protocols). Such applications may have different
objectives. They may not wish to maximize throughput, but may desire objectives. They may not wish to maximize throughput, but may desire
a lower loss rate or bounded delay. a lower loss rate or bounded delay.
The correct operation of an AQM-enabled network device MUST NOT rely The correct operation of an AQM-enabled network device MUST NOT rely
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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
AQM.
o Research into the use of and deployment of ECN alongside AQM. o Research into the use of and deployment of ECN alongside AQM.
o Tools for enabling AQM (and ECN) deployment and measuring the o Tools for enabling AQM (and ECN) deployment and measuring the
performance. performance.
o Methods for mitigating the impact of non-conformant and malicious o Methods for mitigating the impact of non-conformant and malicious
flows. flows.
o Research to understand the implications of using new network and o Research to understand the implications of using new network and
transport methods on applications. transport methods on applications.
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6. Security Considerations 6. Security Considerations
While security is a very important issue, it is largely orthogonal to While security is a very important issue, it is largely orthogonal to
the performance issues discussed in this memo. the performance issues discussed in this memo.
Many deployed network devices use queueing methods that allow Many deployed network devices use queueing methods that allow
unresponsive traffic to capture network capacity, denying access to unresponsive traffic to capture network capacity, denying access to
other traffic flows. This could potentially be used as a denial-of- other traffic flows. This could potentially be used as a denial-of-
service attack. This threat could be reduced in network devices service attack. This threat could be reduced in network devices
deploy AQM or some form of scheduling. We note, however, that a deploy AQM or some form of scheduling. We note, however, that a
denial-of-service attack may create unresponsive traffic flows that denial-of-service attack that results in unresponsive traffic flows
may be indistinguishable from other traffic flows (e.g. tunnels may be indistinguishable from other traffic flows (e.g. tunnels
carrying aggregates of short flows, high-rate isochronous carrying aggregates of short flows, high-rate isochronous
applications). New methods therefore may remain vulnerable, and this applications). New methods therefore may remain vulnerable, and this
document recommends that ongoing research should consider ways to document recommends that ongoing research should consider ways to
mitigate such attacks. mitigate such attacks.
7. Privacy Considerations 7. Privacy Considerations
This document, by itself, presents no new privacy issues. This document, by itself, presents no new privacy issues.
skipping to change at page 18, line 48 skipping to change at page 20, line 10
The original recommendation in [RFC2309] was written by the End-to- The original recommendation in [RFC2309] was written by the End-to-
End Research Group, which is to say Bob Braden, Dave Clark, Jon End Research Group, which is to say Bob Braden, Dave Clark, Jon
Crowcroft, Bruce Davie, Steve Deering, Deborah Estrin, Sally Floyd, Crowcroft, Bruce Davie, Steve Deering, Deborah Estrin, Sally Floyd,
Van Jacobson, Greg Minshall, Craig Partridge, Larry Peterson, KK Van Jacobson, Greg Minshall, Craig Partridge, Larry Peterson, KK
Ramakrishnan, Scott Shenker, John Wroclawski, and Lixia Zhang. This Ramakrishnan, Scott Shenker, John Wroclawski, and Lixia Zhang. This
is an edited version of that document, with much of its text and is an edited version of that document, with much of its text and
arguments unchanged. arguments unchanged.
The need for an updated document was agreed to in the tsvarea meeting The need for an updated document was agreed to in the tsvarea meeting
at IETF 86. This document was reviewed on the aqm@ietf.org list. at IETF 86. This document was reviewed on the aqm@ietf.org list.
Comments came from Colin Perkins, Richard Scheffenegger, Dave Taht, Comments were received from Colin Perkins, Richard Scheffenegger,
and many others. Dave Taht, John Leslie, David Collier-Brown and many others.
Gorry Fairhurst was in part supported by the European Community under Gorry Fairhurst was in part supported by the European Community under
its Seventh Framework Programme through the Reducing Internet its Seventh Framework Programme through the Reducing Internet
Transport Latency (RITE) project (ICT-317700). Transport Latency (RITE) project (ICT-317700).
9. References 9. References
9.1. Normative References 9.1. Normative References
[Byte-pkt] [Byte-pkt]
skipping to change at page 20, line 5 skipping to change at page 21, line 13
Notification", RFC 6040, November 2010. Notification", RFC 6040, November 2010.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P., [RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN) and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, August 2012. for RTP over UDP", RFC 6679, August 2012.
9.2. Informative References 9.2. Informative References
[AQM-WG] "IETF AQM WG", . [AQM-WG] "IETF AQM WG", .
[Demers90] [Choi04] Sprint ATL, Burlingame, CA, , , , and , "Analysis of
Demers, A., Keshav, S., and S. Shenker, "Analysis and 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: 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]
Welzl, M. and G. Fairhurst, "The Benefits to Applications
of using Explicit Congestion Notification (ECN)", IETF
(Work-in-Progress) , February 2014.
[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 20, line 39 skipping to change at page 22, line 11
Lakshman, TV., Neidhardt, A., and T. Ott, "The Drop From Lakshman, TV., Neidhardt, A., and T. Ott, "The Drop From
Front Strategy in TCP Over ATM and Its Interworking with Front Strategy in TCP Over ATM and Its Interworking with
Other Control Features", IEEE Infocomm , 1996. Other Control Features", IEEE Infocomm , 1996.
[Leland94] [Leland94]
Leland, W., Taqqu, M., Willinger, W., and D. Wilson, "On Leland, W., Taqqu, M., Willinger, W., and D. Wilson, "On
the Self-Similar Nature of Ethernet Traffic (Extended the Self-Similar Nature of Ethernet Traffic (Extended
Version)", IEEE/ACM Transactions on Networking , February Version)", IEEE/ACM Transactions on Networking , February
1994. 1994.
[Papagiannaki] [McK90] McKenney, PE. and G. Varghese, "Stochastic Fairness
Sprint ATL, KAIST, University of Minnesota, Sprint ATL, Queuing", http://www2.rdrop.com/~paulmck/scalability/paper
and Intel ResearchIETF, "Analysis of Point-To-Point Packet /sfq.2002.06.04.pdf , 1990.
Delay In an Operational Network", IEEE Infocom 2004, March
2004, <http://www.ieee-infocom.org/2004/Papers/37_4.PDF>. [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, [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980. August 1980.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981. 1981.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981. 793, September 1981.
skipping to change at page 22, line 9 skipping to change at page 23, line 35
Architecture", RFC 5559, June 2009. Architecture", RFC 5559, June 2009.
[RFC6057] Bastian, C., Klieber, T., Livingood, J., Mills, J., and R. [RFC6057] Bastian, C., Klieber, T., Livingood, J., Mills, J., and R.
Woundy, "Comcast's Protocol-Agnostic Congestion Management Woundy, "Comcast's Protocol-Agnostic Congestion Management
System", RFC 6057, December 2010. System", RFC 6057, December 2010.
[RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind, [RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
"Low Extra Delay Background Transport (LEDBAT)", RFC 6817, "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
December 2012. December 2012.
[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.
[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.
Appendix A. Change Log Appendix A. Change Log
Initial Version: March 2013 Initial Version: March 2013
skipping to change at page 22, line 42 skipping to change at page 24, line 38
-01 WG Draft - Updated transport recommendations; revised deployment -01 WG Draft - Updated transport recommendations; revised deployment
configuration section; numerous minor edits. configuration section; numerous minor edits.
Jan 2014 - Feedback from WG. Jan 2014 - Feedback from WG.
-02 WG Draft - Minor edits Feb 2014 - Mainly language fixes. -02 WG Draft - Minor edits Feb 2014 - Mainly language fixes.
-03 WG Draft - Minor edits Feb 2013 - Comments from David Collier- -03 WG Draft - Minor edits Feb 2013 - Comments from David Collier-
Brown and David Taht. Brown and David Taht.
-04 WG Draft - Minor edits May 3013 - 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.
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)
University of Aberdeen University of Aberdeen
 End of changes. 44 change blocks. 
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