draft-ietf-aqm-recommendation-00.txt   draft-ietf-aqm-recommendation-01.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: BCP University of Aberdeen Intended status: Best Current Practice University of Aberdeen
Expires: April 17, 2014 October 17, 2013 Expires: August 3, 2014 January 30, 2014
IETF Recommendations Regarding Active Queue Management IETF Recommendations Regarding Active Queue Management
draft-ietf-aqm-recommendation-00 draft-ietf-aqm-recommendation-01
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
are not sufficiently responsive to congestion notification. are not sufficiently responsive to congestion notification.
The note largely repeats the recommendations of RFC 2309, updated The note largely repeats the recommendations of RFC 2309, updated
after fifteen years of experience and new research. after fifteen years of experience and new research.
Status of this Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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 April 13, 2014. This Internet-Draft will expire on August 3, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2013 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
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include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 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
3. Managing Aggressive Flows . . . . . . . . . . . . . . . . . . 8 3. Managing Aggressive Flows . . . . . . . . . . . . . . . . . . 8
4. Conclusions and Recommendations . . . . . . . . . . . . . . . 10 4. Conclusions and Recommendations . . . . . . . . . . . . . . . 10
4.1. Operational deployments SHOULD use AQM procedures . . . . 11 4.1. Operational deployments SHOULD use AQM procedures . . . 11
4.2. Signaling to the transport endpoints . . . . . . . . . . . 11 4.2. Signaling to the transport endpoints . . . . . . . . . . 11
4.2.1. AQM and ECN . . . . . . . . . . . . . . . . . . . . . 12 4.2.1. AQM and ECN . . . . . . . . . . . . . . . . . . . . . 12
4.3. AQM algorithms deployed SHOULD NOT require operational 4.3. AQM algorithms deployed SHOULD NOT require operational
tuning . . . . . . . . . . . . . . . . . . . . . . . . . . 13 tuning . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.4. AQM algorithms SHOULD respond to measured congestion, 4.4. AQM algorithms SHOULD respond to measured congestion, not
not application profiles. . . . . . . . . . . . . . . . . 14 application profiles. . . . . . . . . . . . . . . . . . . 14
4.5. AQM algorithms SHOULD NOT be dependent on specific 4.5. AQM algorithms SHOULD NOT be dependent on specific
transport protocol behaviours . . . . . . . . . . . . . . 14 transport protocol behaviours . . . . . . . . . . . . . . 15
4.6. Interactions with congestion control algorithms . . . . . 15 4.6. Interactions with congestion control algorithms . . . . . 15
4.7. The need for further research . . . . . . . . . . . . . . 16 4.7. The need for further research . . . . . . . . . . . . . . 16
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17 6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 17 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 18
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . . 18 9.1. Normative References . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . . 18 9.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 21 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
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
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2. The Need For Active Queue Management 2. The Need For Active Queue Management
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
queue, accept packets for the queue until the maximum length is queue, accept packets for the queue until the maximum length is
reached, then reject (drop) subsequent incoming packets until the reached, then reject (drop) subsequent incoming packets until the
queue decreases because a packet from the queue has been transmitted. queue decreases because a packet from the queue has been transmitted.
This technique is known as "tail drop", since the packet that arrived This technique is known as "tail drop", since the packet that arrived
most recently (i.e., the one on the tail of the queue) is dropped most recently (i.e., the one on the tail of the queue) is dropped
when the queue is full. This method has served the Internet well for when the queue is full. This method has served the Internet well for
years, but it has two important drawbacks. years, but it has two important drawbacks:
1. Lock-Out 1. 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 queue space, preventing other connections flows to monopolize queue space, preventing other connections
from getting room in the queue. This "lock-out" phenomenon is from getting room in the queue. This "lock-out" phenomenon is
often the result of synchronization or other timing effects. often the result of synchronization or other timing effects.
2. Full Queues 2. Full Queues
The tail drop discipline allows queues to maintain a full (or, The tail drop discipline allows queues to maintain a full (or,
almost full) status for long periods of time, since tail drop almost full) status for long periods of time, since tail drop
signals congestion (via a packet drop) only when the queue has signals congestion (via a packet drop) only when the queue has
become full. It is important to reduce the steady-state queue become full. It is important to reduce the steady-state queue
size, and this is perhaps the most important goal for queue size, and this is perhaps the most important goal for queue
management. management.
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
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algorithms, such as CBQ. This is because per-flow scheduling algorithms, such as CBQ. This is because per-flow scheduling
algorithms by themselves do not control the overall queue size or algorithms by themselves do not control the overall queue size or
the size of individual queues. AQM is needed to control the the size of individual queues. AQM is needed to control the
overall average queue sizes, so that arriving bursts can be overall average queue sizes, so that arriving bursts can be
accommodated without dropping packets. In addition, AQM should accommodated without dropping packets. In addition, AQM should
be used to control the queue size for each individual flow or be used to control the queue size for each individual flow or
class, so that they do not experience unnecessarily high delay. class, so that they do not experience unnecessarily high delay.
Therefore, AQM should be applied across the classes or flows as Therefore, AQM should be applied across the classes or flows as
well as within each class or flow. well as within each class or flow.
In short, scheduling algorithms and queue management should be In short, scheduling algorithms and queue management should be seen
seen as complementary, not as replacements for each other. as complementary, not as replacements for each other.
It is also important to differentiate the choice of buffer size for a
queue in a switch/router or other network device, and the
threshold(s) and other parameters that determine how and when an AQM
algorithm operates. One the one hand, the optimum buffer size is a
function of operational requirements and should generally be sized to
be sufficient to buffer the largest normal traffic burst that is
expected. This size depends on the number and burstiness of traffic
arriving at the queue and the rate at which traffic leaves the queue.
Different types of traffic and deployment scenarios will lead to
different requirements. On the other hand, the choice of AQM
algorithm and associated parameters is a function of the way in which
congestion is experienced and the required reaction to achieve
acceptable performance. This latter topic is the primary topic of
the following sections.
3. Managing Aggressive Flows 3. Managing Aggressive Flows
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
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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 presently unresponsive applications continue to be
important.Network devices need to be able to protect themselves important.Network devices need to be able to protect themselves
against unresponsive flows, and mechanisms to accomplish this against unresponsive flows, and mechanisms to accomplish this
must be developed and deployed. Deployment of such mechanisms must be developed and deployed. Deployment of such mechanisms
would provide an incentive for all applications to become would provide an incentive for all applications to become
responsive by either using a congestion-controlled transport responsive by either using a congestion-controlled transport
(e.g. TCP, SCTP, DCCP) or by incorporating their own congestion (e.g. TCP, SCTP, DCCP) or by incorporating their own congestion
control in the application. [RFC5405]. control in the application. [RFC5405].
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.
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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 granularity
of a "flow" when considering a queue management method. There are a of a "flow" when considering a queue management method. There are a
few "natural" answers: 1) a transport (e.g. TCP or UDP) flow (source few "natural" answers: 1) a transport (e.g. TCP or UDP) flow (source
address/port, destination address/port, DSCP); 2) a source/ address/port, destination address/port, DSCP); 2) a source/
destination host pair (IP addresses, DSCP); 3) a given source host or destination host pair (IP addresses, DSCP); 3) a given source host or
a given destination host. We suggest that the source/destination a given destination host. We suggest that the source/destination
host pair gives the most appropriate granularity in many host pair gives the most appropriate granularity in many
circumstances. However, it is possible that different vendors/ circumstances. However, it is possible that different vendors/
providers could set different granularities for defining a flow (as a providers could set different granularities for defining a flow (as a
way of "distinguishing" themselves from one another), or that way of "distinguishing" themselves from one another), or that
different granularities could be chosen for different places in the different granularities could be chosen for different places in the
network. It may be the case that the granularity is less important network. It may be the case that the granularity is less important
than the fact that a network device needs to be able to deal with than the fact that a network device needs to be able to deal with
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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. However, 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 on. To the
contrary, available research [Papagiannaki] says that even high speed contrary, available research [Papagiannaki] says that even high speed
links in network cores that are normally very stable in depth and links in network cores that are normally very stable in depth and
behavior experience occasional issues that need moderation. behavior experience occasional issues that need moderation.
4.1. Operational deployments SHOULD use AQM procedures 4.1. Operational deployments SHOULD use AQM procedures
AQM procedures are designed to minimize delay induced in the network AQM procedures are designed to minimize delay induced in the network
by queues that have filled as a result of host behavior. Marking and by queues that have filled as a result of host behavior. Marking and
loss behaviors provide a signal that buffers within network devices loss behaviors provide a signal that buffers within network devices
are becoming unnecessarily full, and that the sender would do well to are becoming unnecessarily full, and that the sender would do well to
moderate its behavior. moderate its behavior.
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 data segments in flight, such as in a queue. o Delaying transport segments (packets) in flight, such as in a
queue.
o Dropping data segments in transit. o Dropping transport segments (packets) in transit.
o Marking data segments, such as using Explicit Congestion o Marking transport segments (packets), such as using Explicit
Control[RFC3168] [RFC4301] [RFC4774] [RFC6040] [RFC6679]. Congestion Control[RFC3168] [RFC4301] [RFC4774] [RFC6040]
[RFC6679].
The use of scheduling mechanisms, such as priority queuing, classful The use of scheduling mechanisms, such as priority queuing, classful
queuing, and fair queuing, is often effective in networks to help a queuing, and fair queuing, is often effective in networks to help a
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]. point. This is discussed in [RFC2474] and [RFC2475].
Increased network latency can be used as an implicit signal of 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 RTP, network latency impacts the RTCP-reported RTT and increased
latency can trigger a sender to adjust its rate. Methods such as latency can trigger a sender to adjust its rate. Methods such as
LEDBAT [RFC6817] assume increased latency as a primary signal of LEDBAT [RFC6817] assume increased latency as a primary signal of
congestion. congestion.
It is essential that all Internet hosts respond to loss [RFC5681], It is essential that all Internet hosts respond to loss [RFC5681],
[RFC5405][RFC2960][RFC4340]. Packet dropping by network devices that [RFC5405][RFC2960][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 transport must the latter." of congestion. To be conservative transport must the latter."
Unreliable transports (e.g. using UDP) need to similarly react to Unreliable transports (e.g. using UDP) need to similarly react to
loss [RFC5405] loss [RFC5405]
Network devices SHOULD use use an AQM algorithm to determine the Network devices SHOULD use use an AQM algorithm to determine the
packets that are effected by congestion. packets that are marked or discarded due to congestion.
Loss also has an effect on the efficiency of a flow and can Loss also has an effect on the efficiency of a flow and can
significantly impact some classes of application. In reliable significantly impact some classes of application. In reliable
transports the dropped data must be subsequently retransmitted. transports the dropped data must be subsequently retransmitted.
While other applications/transports may adapt to the absence of lost While other applications/transports may adapt to the absence of lost
data, this still implies inefficient use of available capacity and data, this still implies inefficient use of available capacity and
the dropped traffic can affect other flows. Hence, loss is not the dropped traffic can affect other flows. Hence, loss is not
entirely positive; it is a necessary evil. entirely positive; it is a necessary evil.
4.2.1. AQM and ECN 4.2.1. AQM and ECN
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Network devices SHOULD use an algorithm to drop excessive traffic, Network devices SHOULD use an algorithm to drop excessive traffic,
even when marked as originating from an ECN-capable transport. even when 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
"operational conditions". The former includes values that exist
before any experience is gathered about the use of the algorithm,
such as the configured speed of interface, support for full duplex
communication, interface MTU and other properties of the link. The
latter includes information observed from monitoring the size of the
queue, experienced queueing delay, rate of packet discards, etc.
This document therefore recommends that AQM algorithm proposed for This document therefore recommends that AQM algorithm proposed for
deployment in the Internet: deployment in the Internet:
o SHOULD NOT require tuning of initial or configuration parameters. o SHOULD NOT require tuning of initial or configuration parameters.
An algorithm needs to provide a default behaviour that auto-tunes An algorithm needs to provide a default behaviour that auto-tunes
to a reasonable performance for typical network conditions. This to a reasonable performance for typical network conditions. This
is expected to ease deployment and operation. is expected to ease deployment and operation.
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. Guidance needs to be provided on the cases where be externalized (made visible to the operator). Guidance needs to
autotuning is unlikely to achieve satisfactory performance and to be provided on the cases where autotuning is unlikely to achieve
identify the set of parameters that can be tuned. This is satisfactory performance and to identify the set of parameters
expected to enable the algorithm to be deployed in networks that that can be tuned. This is expected to enable the algorithm to be
have specific characteristics (variable/larger delay; networks deployed in networks that have specific characteristics (variable/
were capacity is impacted by interactions with lower layer larger delay; networks were capacity is impacted by interactions
mechanisms, etc) 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
designed so that they do not require operational (especially manual) designed so that they do not require operational (especially manual)
configuration or tuning. configuration or tuning.
4.4. AQM algorithms SHOULD respond to measured congestion, not 4.4. AQM algorithms SHOULD respond to measured congestion, not
application profiles. application profiles.
Not all applications transmit packets of the same size. Although Not all applications transmit packets of the same size. Although
applications may be characterised by particular profiles of packet applications may be characterised by particular profiles of packet
size this should not be used as the basis for AQM (see next section). size this should not be used as the basis for AQM (see next section).
Other methods exist, e.g. Differentiated Services queueing, Pre- Other methods exist, e.g. Differentiated Services queueing, Pre-
Congestion Notification (PCN) [RFC5559], that can be used to Congestion Notification (PCN) [RFC5559], that can be used to
differentiate and police classes of application. Network devices may differentiate and police classes of application. Network devices may
combine AQM with these traffic classification mechanisms and perform combine AQM with these traffic classification mechanisms and perform
AQM only on specific queues within a network device. AQM only on specific queues within a network device.
An AQM algorithm should not deliberately try to prejudice the size of An AQM algorithm should not deliberately try to prejudice the size of
packet that performs best (i.e. preferentially drop/mark based only packet that performs best (i.e. preferentially drop/mark based only
on packet size). Procedures for selecting packets to mark/drop on packet size). Procedures for selecting packets to mark/drop
SHOULD observe actual or projected time a packet is in a queue (bytes SHOULD observe actual or projected time a packet is in a queue (bytes
at a rate being an analog to time). When an AQM algorithm decides at a rate being an analog to time). When an AQM algorithm decides
whether to drop (or mark) a packet, it is RECOMMENDED that the size whether to drop (or mark) a packet, it is RECOMMENDED that the size
of the particular packet should not be taken into account [Byte-pkt]. of the particular packet should not be taken into account [Byte-pkt].
Applications (or transports) generally know the packet size that they Applications (or transports) generally know the packet size that they
are using and can hence make their judgements 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
algorithm to be applied within the same traffic class. Traffic
classes may be differentiated based on an Access Control List (ACL),
the packet DiffServ Code Point (DSCP) [RFC5559], setting of the ECN
field[RFC3168] [RFC4774] or an equivalent codepoint at a lower layer.
This recommendation goes beyond what is defined in RFC 3168, by
allowing more than one instance of an AQM 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
traffic and SHOULD NOT make implicit assumptions about the traffic and SHOULD NOT make implicit assumptions about the
characteristics desired by the set transports/applications the characteristics desired by the set transports/applications the
network supports. That is, AQM methods should be opaque to the network supports. That is, AQM methods should be opaque to the
choice of transport and application. choice of transport and application.
AQM algorithms are often evaluated by considering TCP [RFC0793] with AQM algorithms are often evaluated by considering TCP [RFC0793] with
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4.6. Interactions with congestion control algorithms 4.6. Interactions with congestion control algorithms
Applications and transports need to react to received implicit or Applications and transports need to react to received implicit or
explicit signals that indicate the presence of congestion. This explicit signals that indicate the presence of congestion. This
section identifies issues that can impact the design of transport section identifies issues that can impact the design of transport
protocols when using paths that use AQM. protocols when using paths that use AQM.
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 increased when network devices queue packets in buffers. It can be
difficult to detect tail losses at a higher layer and may sometimes difficult to detect tail losses at a higher layer and may sometimes
require transport timers or probe packets to detect and respond to require transport timers or probe packets to detect and respond to
such loss. Loss patterns may also impact timely detection, e.g. the such loss. Loss patterns may also impact timely detection, e.g. the
time may be reduced when network devices do not drop long runs of time may be reduced when network devices do not drop long runs of
packets from the same flow. packets from the same flow.
A common objective is to deliver data from its source end point to A common objective is to deliver data from its source end point to
its destination in the least possible time. When speaking of TCP its destination in the least possible time. When speaking of TCP
performance, the terms "knee" and "cliff" area defined by [Jain94]. performance, the terms "knee" and "cliff" area defined by [Jain94].
They respectively refer to the minimum congestion window that They respectively refer to the minimum congestion window that
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available network capacity. As a result, the behavior of any elastic available network capacity. As a result, the behavior of any elastic
transport congestion control algorithm designed to minimise delivery transport congestion control algorithm designed to minimise delivery
time should seek to use an effective window at or above the knee and time should seek to use an effective window at or above the knee and
well below the cliff. Choice of an appropriate rate can well below the cliff. Choice of an appropriate rate can
significantly impact the loss and delay experienced not only by a significantly impact the loss and delay experienced not only by a
flow, but by other flows that share the same 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 maximise throughput, but may desire objectives. They may not wish to maximise 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
upon specific transport responses to congestion signals. upon specific transport responses to congestion signals.
4.7. The need for further research 4.7. The need for further research
The second recommendation of [RFC2309] called for further research The second recommendation of [RFC2309] called for further research
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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". Where "Lemmings" are flash
of "mice" that the network inadvertently tries to signal to as if crowds of "mice" that the network inadvertently tries to signal to as
they were elephant flows, resulting in head of line blocking in data if they were elephant flows, resulting in head of line blocking in
center applications. data 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 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
transport methods on applications.
Hence, this document therefore reiterates the call of RFC 2309: we Hence, this document therefore reiterates the call of RFC 2309: we
need continuing research as applications develop. need continuing research as applications develop.
5. IANA Considerations 5. IANA Considerations
This memo asks the IANA for no new parameters. This memo asks the IANA for no new parameters.
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
skipping to change at page 18, line 10 skipping to change at page 18, line 33
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]
Internet Engineering Task Force, Work in Progress, "Byte and Internet Engineering Task Force, Work in Progress,
and Packet Congestion Notification "Byte and Packet Congestion Notification (draft-ietf-
(draft-ietf-tsvwg-byte-pkt-congest)", July 2013. tsvwg-byte-pkt-congest)", July 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", of Explicit Congestion Notification (ECN) to IP", RFC
RFC 3168, September 2001. 3168, September 2001.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005. Internet Protocol", RFC 4301, December 2005.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the [RFC4774] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124, Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, November 2006. RFC 4774, November 2006.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines [RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405, for Application Designers", BCP 145, RFC 5405, November
November 2008. 2008.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009. Control", RFC 5681, September 2009.
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion [RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
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] [Demers90]
Demers, A., Keshav, S., and S. Shenker, "Analysis and 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.
[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.
skipping to change at page 19, line 28 skipping to change at page 20, line 8
Patent Office 5377327, December 1994. Patent Office 5377327, December 1994.
[Lakshman96] [Lakshman96]
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 , Version)", IEEE/ACM Transactions on Networking , February
February 1994. 1994.
[Papagiannaki] [Papagiannaki]
Sprint ATL, KAIST, University of Minnesota, Sprint ATL, Sprint ATL, KAIST, University of Minnesota, Sprint ATL,
and Intel ResearchIETF, "Analysis of Point-To-Point Packet and Intel ResearchIETF, "Analysis of Point-To-Point Packet
Delay In an Operational Network", IEEE Infocom 2004, Delay In an Operational Network", IEEE Infocom 2004, March
March 2004, 2004, <http://www.ieee-infocom.org/2004/Papers/37_4.PDF>.
<http://www.ieee-infocom.org/2004/Papers/37_4.PDF>.
[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, [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
September 1981. 1981.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
RFC 793, September 1981. 793, September 1981.
[RFC0896] Nagle, J., "Congestion control in IP/TCP internetworks", [RFC0896] Nagle, J., "Congestion control in IP/TCP internetworks",
RFC 896, January 1984. RFC 896, January 1984.
[RFC0970] Nagle, J., "On packet switches with infinite storage", [RFC0970] Nagle, J., "On packet switches with infinite storage", RFC
RFC 970, December 1985. 970, December 1985.
[RFC1122] Braden, R., "Requirements for Internet Hosts - [RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989. Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1633] Braden, B., Clark, D., and S. Shenker, "Integrated [RFC1633] Braden, B., Clark, D., and S. Shenker, "Integrated
Services in the Internet Architecture: an Overview", Services in the Internet Architecture: an Overview", RFC
RFC 1633, June 1994. 1633, June 1994.
[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering, [RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G., S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker, Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
S., Wroclawski, J., and L. Zhang, "Recommendations on S., Wroclawski, J., and L. Zhang, "Recommendations on
Queue Management and Congestion Avoidance in the Queue Management and Congestion Avoidance in the
Internet", RFC 2309, April 1998. Internet", RFC 2309, April 1998.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998. (IPv6) Specification", RFC 2460, December 1998.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS "Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474, Field) in the IPv4 and IPv6 Headers", RFC 2474, December
December 1998. 1998.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998. Services", RFC 2475, December 1998.
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C., [RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control Transmission Zhang, L., and V. Paxson, "Stream Control Transmission
Protocol", RFC 2960, October 2000. Protocol", RFC 2960, October 2000.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006. Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", [RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC
RFC 4960, September 2007. 4960, September 2007.
[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP [RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification", Friendly Rate Control (TFRC): Protocol Specification", RFC
RFC 5348, September 2008. 5348, September 2008.
[RFC5559] Eardley, P., "Pre-Congestion Notification (PCN) [RFC5559] Eardley, P., "Pre-Congestion Notification (PCN)
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,
skipping to change at page 21, line 22 skipping to change at page 21, line 48
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
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
to be further updated. July 2013 to be further updated.
July 2013
-00 WG Draft - Updated transport recommendations; revised deployment -00 WG Draft - Updated transport recommendations; revised deployment
configuration section; numerous minor edits. Oct 2013 configuration section; numerous minor edits.
Oct 2013
-01 WG Draft - Updated transport recommendations; revised deployment
configuration section; numerous minor edits.
Jan 2014 - Feedback from WG.
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
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