draft-ietf-tsvwg-byte-pkt-congest-12.txt   rfc7141.txt 
Transport Area Working Group B. Briscoe Internet Engineering Task Force (IETF) B. Briscoe
Internet-Draft BT Request for Comments: 7141 BT
Updates: 2309 (if approved) J. Manner BCP: 41 J. Manner
Intended status: BCP Aalto University Updates: 2309, 2914 Aalto University
Expires: May 11, 2014 November 07, 2013 Category: Best Current Practice February 2014
ISSN: 2070-1721
Byte and Packet Congestion Notification Byte and Packet Congestion Notification
draft-ietf-tsvwg-byte-pkt-congest-12
Abstract Abstract
This document provides recommendations of best current practice for This document provides recommendations of best current practice for
dropping or marking packets using any active queue management (AQM) dropping or marking packets using any active queue management (AQM)
algorithm, including random early detection (RED), BLUE, pre- algorithm, including Random Early Detection (RED), BLUE, Pre-
congestion notification (PCN) and newer schemes such as CoDel Congestion Notification (PCN), and newer schemes such as CoDel
(Controlled Delay) and PIE (Proportional Integral controller (Controlled Delay) and PIE (Proportional Integral controller
Enhanced). We give three strong recommendations: (1) packet size Enhanced). We give three strong recommendations: (1) packet size
should be taken into account when transports detect and respond to should be taken into account when transports detect and respond to
congestion indications, (2) packet size should not be taken into congestion indications, (2) packet size should not be taken into
account when network equipment creates congestion signals (marking, account when network equipment creates congestion signals (marking,
dropping), and therefore (3) in the specific case of RED, the byte- dropping), and therefore (3) in the specific case of RED, the byte-
mode packet drop variant that drops fewer small packets should not be mode packet drop variant that drops fewer small packets should not be
used. This memo updates RFC 2309 to deprecate deliberate used. This memo updates RFC 2309 to deprecate deliberate
preferential treatment of small packets in AQM algorithms. preferential treatment of small packets in AQM algorithms.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This memo documents an Internet Best Current Practice.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 5741.
This Internet-Draft will expire on May 11, 2014. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7141.
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.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology and Scoping . . . . . . . . . . . . . . . . . 6 1.1. Terminology and Scoping . . . . . . . . . . . . . . . . . 6
1.2. Example Comparing Packet-Mode Drop and Byte-Mode Drop . . 7 1.2. Example Comparing Packet-Mode Drop and Byte-Mode Drop . . 7
2. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 9 2. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 9
2.1. Recommendation on Queue Measurement . . . . . . . . . . . 9 2.1. Recommendation on Queue Measurement . . . . . . . . . . . 9
2.2. Recommendation on Encoding Congestion Notification . . . . 10 2.2. Recommendation on Encoding Congestion Notification . . . 10
2.3. Recommendation on Responding to Congestion . . . . . . . . 11 2.3. Recommendation on Responding to Congestion . . . . . . . 11
2.4. Recommendation on Handling Congestion Indications when 2.4. Recommendation on Handling Congestion Indications When
Splitting or Merging Packets . . . . . . . . . . . . . . . 12 Splitting or Merging Packets . . . . . . . . . . . . . . 12
3. Motivating Arguments . . . . . . . . . . . . . . . . . . . . . 12 3. Motivating Arguments . . . . . . . . . . . . . . . . . . . . 13
3.1. Avoiding Perverse Incentives to (Ab)use Smaller Packets . 12 3.1. Avoiding Perverse Incentives to (Ab)use Smaller Packets . 13
3.2. Small != Control . . . . . . . . . . . . . . . . . . . . . 14 3.2. Small != Control . . . . . . . . . . . . . . . . . . . . 14
3.3. Transport-Independent Network . . . . . . . . . . . . . . 14 3.3. Transport-Independent Network . . . . . . . . . . . . . . 14
3.4. Partial Deployment of AQM . . . . . . . . . . . . . . . . 15 3.4. Partial Deployment of AQM . . . . . . . . . . . . . . . . 16
3.5. Implementation Efficiency . . . . . . . . . . . . . . . . 17 3.5. Implementation Efficiency . . . . . . . . . . . . . . . . 17
4. A Survey and Critique of Past Advice . . . . . . . . . . . . . 17 4. A Survey and Critique of Past Advice . . . . . . . . . . . . 17
4.1. Congestion Measurement Advice . . . . . . . . . . . . . . 18 4.1. Congestion Measurement Advice . . . . . . . . . . . . . . 18
4.1.1. Fixed Size Packet Buffers . . . . . . . . . . . . . . 18 4.1.1. Fixed-Size Packet Buffers . . . . . . . . . . . . . . 18
4.1.2. Congestion Measurement without a Queue . . . . . . . . 19 4.1.2. Congestion Measurement without a Queue . . . . . . . 19
4.2. Congestion Notification Advice . . . . . . . . . . . . . . 20 4.2. Congestion Notification Advice . . . . . . . . . . . . . 20
4.2.1. Network Bias when Encoding . . . . . . . . . . . . . . 20 4.2.1. Network Bias When Encoding . . . . . . . . . . . . . 20
4.2.2. Transport Bias when Decoding . . . . . . . . . . . . . 22 4.2.2. Transport Bias When Decoding . . . . . . . . . . . . 22
4.2.3. Making Transports Robust against Control Packet 4.2.3. Making Transports Robust against Control Packet
Losses . . . . . . . . . . . . . . . . . . . . . . . . 23 Losses . . . . . . . . . . . . . . . . . . . . . . . 23
4.2.4. Congestion Notification: Summary of Conflicting 4.2.4. Congestion Notification: Summary of Conflicting
Advice . . . . . . . . . . . . . . . . . . . . . . . . 24 Advice . . . . . . . . . . . . . . . . . . . . . . . 24
5. Outstanding Issues and Next Steps . . . . . . . . . . . . . . 25 5. Outstanding Issues and Next Steps . . . . . . . . . . . . . . 25
5.1. Bit-congestible Network . . . . . . . . . . . . . . . . . 25 5.1. Bit-congestible Network . . . . . . . . . . . . . . . . . 25
5.2. Bit- & Packet-congestible Network . . . . . . . . . . . . 25 5.2. Bit- and Packet-Congestible Network . . . . . . . . . . . 26
6. Security Considerations . . . . . . . . . . . . . . . . . . . 26 6. Security Considerations . . . . . . . . . . . . . . . . . . . 26
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 27
8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 26 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
10. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 28 9.1. Normative References . . . . . . . . . . . . . . . . . . 28
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 9.2. Informative References . . . . . . . . . . . . . . . . . 29
11.1. Normative References . . . . . . . . . . . . . . . . . . . 28 Appendix A. Survey of RED Implementation Status . . . . . . . . 33
11.2. Informative References . . . . . . . . . . . . . . . . . . 28 Appendix B. Sufficiency of Packet-Mode Drop . . . . . . . . . . 34
Appendix A. Survey of RED Implementation Status . . . . . . . . . 32 B.1. Packet-Size (In)Dependence in Transports . . . . . . . . 35
Appendix B. Sufficiency of Packet-Mode Drop . . . . . . . . . . . 34 B.2. Bit-Congestible and Packet-Congestible Indications . . . 38
B.1. Packet-Size (In)Dependence in Transports . . . . . . . . . 35 Appendix C. Byte-Mode Drop Complicates Policing Congestion
B.2. Bit-Congestible and Packet-Congestible Indications . . . . 38 Response . . . . . . . . . . . . . . . . . . . . . . 39
Appendix C. Byte-mode Drop Complicates Policing Congestion
Response . . . . . . . . . . . . . . . . . . . . . . 39
Appendix D. Changes from Previous Versions . . . . . . . . . . . 40
1. Introduction 1. Introduction
This document provides recommendations of best current practice for This document provides recommendations of best current practice for
how we should correctly scale congestion control functions with how we should correctly scale congestion control functions with
respect to packet size for the long term. It also recognises that respect to packet size for the long term. It also recognises that
expediency may be necessary to deal with existing widely deployed expediency may be necessary to deal with existing widely deployed
protocols that don't live up to the long term goal. protocols that don't live up to the long-term goal.
When signalling congestion, the problem of how (and whether) to take When signalling congestion, the problem of how (and whether) to take
packet sizes into account has exercised the minds of researchers and packet sizes into account has exercised the minds of researchers and
practitioners for as long as active queue management (AQM) has been practitioners for as long as active queue management (AQM) has been
discussed. Indeed, one reason AQM was originally introduced was to discussed. Indeed, one reason AQM was originally introduced was to
reduce the lock-out effects that small packets can have on large reduce the lock-out effects that small packets can have on large
packets in drop-tail queues. This memo aims to state the principles packets in tail-drop queues. This memo aims to state the principles
we should be using and to outline how these principles will affect we should be using and to outline how these principles will affect
future protocol design, taking into account the existing deployments future protocol design, taking into account pre-existing deployments.
we have already.
The question of whether to take into account packet size arises at The question of whether to take into account packet size arises at
three stages in the congestion notification process: three stages in the congestion notification process:
Measuring congestion: When a congested resource measures locally how Measuring congestion: When a congested resource measures locally how
congested it is, should it measure its queue length in time, bytes congested it is, should it measure its queue length in time,
or packets? bytes, or packets?
Encoding congestion notification into the wire protocol: When a Encoding congestion notification into the wire protocol: When a
congested network resource signals its level of congestion, should congested network resource signals its level of congestion, should
it drop / mark each packet dependent on the size of the particular the probability that it drops/marks each packet depend on the size
packet in question? of the particular packet in question?
Decoding congestion notification from the wire protocol: When a Decoding congestion notification from the wire protocol: When a
transport interprets the notification in order to decide how much transport interprets the notification in order to decide how much
to respond to congestion, should it take into account the size of to respond to congestion, should it take into account the size of
each missing or marked packet? each missing or marked packet?
Consensus has emerged over the years concerning the first stage, Consensus has emerged over the years concerning the first stage,
which Section 2.1 records in the RFC Series. In summary: If possible which Section 2.1 records in the RFC Series. In summary: If
it is best to measure congestion by time in the queue, but otherwise possible, it is best to measure congestion by time in the queue;
the choice between bytes and packets solely depends on whether the otherwise, the choice between bytes and packets solely depends on
resource is congested by bytes or packets. whether the resource is congested by bytes or packets.
The controversy is mainly around the last two stages: whether to The controversy is mainly around the last two stages: whether to
allow for the size of the specific packet notifying congestion i) allow for the size of the specific packet notifying congestion i)
when the network encodes or ii) when the transport decodes the when the network encodes or ii) when the transport decodes the
congestion notification. congestion notification.
Currently, the RFC series is silent on this matter other than a paper Currently, the RFC series is silent on this matter other than a paper
trail of advice referenced from [RFC2309], which conditionally trail of advice referenced from [RFC2309], which conditionally
recommends byte-mode (packet-size dependent) drop [pktByteEmail]. recommends byte-mode (packet-size dependent) drop [pktByteEmail].
Reducing drop of small packets certainly has some tempting
advantages: i) it drops less control packets, which tend to be small Reducing the number of small packets dropped certainly has some
and ii) it makes TCP's bit-rate less dependent on packet size. tempting advantages: i) it drops fewer control packets, which tend to
However, there are ways of addressing these issues at the transport be small and ii) it makes TCP's bit rate less dependent on packet
layer, rather than reverse engineering network forwarding to fix the size. However, there are ways of addressing these issues at the
problems. transport layer, rather than reverse engineering network forwarding
to fix the problems.
This memo updates [RFC2309] to deprecate deliberate preferential This memo updates [RFC2309] to deprecate deliberate preferential
treatment of packets in AQM algorithms solely because of their size. treatment of packets in AQM algorithms solely because of their size.
It recommends that (1) packet size should be taken into account when It recommends that (1) packet size should be taken into account when
transports detect and respond to congestion indications, (2) not when transports detect and respond to congestion indications, (2) not when
network equipment creates them. This memo also adds to the network equipment creates them. This memo also adds to the
congestion control principles enumerated in BCP 41 [RFC2914]. congestion control principles enumerated in BCP 41 [RFC2914].
In the particular case of Random early Detection (RED), this means In the particular case of Random Early Detection (RED), this means
that the byte-mode packet drop variant should not be used to drop that the byte-mode packet drop variant should not be used to drop
fewer small packets, because that creates a perverse incentive for fewer small packets, because that creates a perverse incentive for
transports to use tiny segments, consequently also opening up a DoS transports to use tiny segments, consequently also opening up a DoS
vulnerability. Fortunately all the RED implementers who responded to vulnerability. Fortunately, all the RED implementers who responded
our admittedly limited survey (Section 4.2.4) have not followed the to our admittedly limited survey (Section 4.2.4) have not followed
earlier advice to use byte-mode drop, so the position this memo the earlier advice to use byte-mode drop, so the position this memo
argues for seems to already exist in implementations. argues for seems to already exist in implementations.
However, at the transport layer, TCP congestion control is a widely However, at the transport layer, TCP congestion control is a widely
deployed protocol that doesn't scale with packet size (i.e. its deployed protocol that doesn't scale with packet size (i.e., its
reduction in rate does not take into account the size of a lost reduction in rate does not take into account the size of a lost
packet). To date this hasn't been a significant problem because most packet). To date, this hasn't been a significant problem because
TCP implementations have been used with similar packet sizes. But, most TCP implementations have been used with similar packet sizes.
as we design new congestion control mechanisms, this memo recommends But, as we design new congestion control mechanisms, this memo
that we should build in scaling with packet size rather than assuming recommends that we build in scaling with packet size rather than
we should follow TCP's example. assuming that we should follow TCP's example.
This memo continues as follows. First it discusses terminology and This memo continues as follows. First, it discusses terminology and
scoping. Section 2 gives the concrete formal recommendations, scoping. Section 2 gives concrete formal recommendations, followed
followed by motivating arguments in Section 3. We then critically by motivating arguments in Section 3. We then critically survey the
survey the advice given previously in the RFC series and the research advice given previously in the RFC Series and the research literature
literature (Section 4), referring to an assessment of whether or not (Section 4), referring to an assessment of whether or not this advice
this advice has been followed in production networks (Appendix A). has been followed in production networks (Appendix A). To wrap up,
To wrap up, outstanding issues are discussed that will need outstanding issues are discussed that will need resolution both to
resolution both to inform future protocol designs and to handle inform future protocol designs and to handle legacy AQM deployments
legacy (Section 5). Then security issues are collected together in (Section 5). Then security issues are collected together in
Section 6 before conclusions are drawn in Section 8. The interested Section 6 before conclusions are drawn in Section 7. The interested
reader can find discussion of more detailed issues on the theme of reader can find discussion of more detailed issues on the theme of
byte vs. packet in the appendices. byte vs. packet in the appendices.
This memo intentionally includes a non-negligible amount of material This memo intentionally includes a non-negligible amount of material
on the subject. For the busy reader Section 2 summarises the on the subject. For the busy reader, Section 2 summarises the
recommendations for the Internet community. recommendations for the Internet community.
1.1. Terminology and Scoping 1.1. Terminology and Scoping
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
This memo applies to the design of all AQM algorithms, for example, This memo applies to the design of all AQM algorithms, for example,
Random Early Detection (RED) [RFC2309], BLUE [BLUE02], Pre-Congestion Random Early Detection (RED) [RFC2309], BLUE [BLUE02], Pre-Congestion
Notification (PCN) [RFC5670], Controlled Delay (CoDel) Notification (PCN) [RFC5670], Controlled Delay (CoDel) [CoDel], and
[I-D.nichols-tsvwg-codel] and the Proportional Integral controller the Proportional Integral controller Enhanced (PIE) [PIE].
Enhanced (PIE) [I-D.pan-tsvwg-pie]. Throughout, RED is used as a Throughout, RED is used as a concrete example because it is a widely
concrete example because it is a widely known and deployed AQM known and deployed AQM algorithm. There is no intention to imply
algorithm. There is no intention to imply that the advice is any that the advice is any less applicable to the other algorithms, nor
less applicable to the other algorithms, nor that RED is preferred. that RED is preferred.
Congestion Notification: Congestion notification is a changing Congestion Notification: Congestion notification is a changing
signal that aims to communicate the probability that the network signal that aims to communicate the probability that the network
resource(s) will not be able to forward the level of traffic load resource(s) will not be able to forward the level of traffic load
offered (or that there is an impending risk that they will not be offered (or that there is an impending risk that they will not be
able to). able to).
The `impending risk' qualifier is added, because AQM systems set a The 'impending risk' qualifier is added, because AQM systems set a
virtual limit smaller than the actual limit to the resource, then virtual limit smaller than the actual limit to the resource, then
notify when this virtual limit is exceeded in order to avoid notify the transport when this virtual limit is exceeded in order
uncontrolled congestion of the actual capacity. to avoid uncontrolled congestion of the actual capacity.
Congestion notification communicates a real number bounded by the Congestion notification communicates a real number bounded by the
range [ 0 , 1 ]. This ties in with the most well-understood range [ 0 , 1 ]. This ties in with the most well-understood
measure of congestion notification: drop probability. measure of congestion notification: drop probability.
Explicit and Implicit Notification: The byte vs. packet dilemma Explicit and Implicit Notification: The byte vs. packet dilemma
concerns congestion notification irrespective of whether it is concerns congestion notification irrespective of whether it is
signalled implicitly by drop or using Explicit Congestion signalled implicitly by drop or explicitly using ECN [RFC3168] or
Notification (ECN [RFC3168] or PCN [RFC5670]). Throughout this PCN [RFC5670]. Throughout this document, unless clear from the
document, unless clear from the context, the term marking will be context, the term 'marking' will be used to mean notifying
used to mean notifying congestion explicitly, while congestion congestion explicitly, while 'congestion notification' will be
notification will be used to mean notifying congestion either used to mean notifying congestion either implicitly by drop or
implicitly by drop or explicitly by marking. explicitly by marking.
Bit-congestible vs. Packet-congestible: If the load on a resource Bit-congestible vs. Packet-congestible: If the load on a resource
depends on the rate at which packets arrive, it is called packet- depends on the rate at which packets arrive, it is called 'packet-
congestible. If the load depends on the rate at which bits arrive congestible'. If the load depends on the rate at which bits
it is called bit-congestible. arrive, it is called 'bit-congestible'.
Examples of packet-congestible resources are route look-up engines Examples of packet-congestible resources are route look-up engines
and firewalls, because load depends on how many packet headers and firewalls, because load depends on how many packet headers
they have to process. Examples of bit-congestible resources are they have to process. Examples of bit-congestible resources are
transmission links, radio power and most buffer memory, because transmission links, radio power, and most buffer memory, because
the load depends on how many bits they have to transmit or store. the load depends on how many bits they have to transmit or store.
Some machine architectures use fixed size packet buffers, so Some machine architectures use fixed-size packet buffers, so
buffer memory in these cases is packet-congestible (see buffer memory in these cases is packet-congestible (see
Section 4.1.1). Section 4.1.1).
The path through a machine will typically encounter both packet- The path through a machine will typically encounter both packet-
congestible and bit-congestible resources. However, currently, a congestible and bit-congestible resources. However, currently, a
design goal of network processing equipment such as routers and design goal of network processing equipment such as routers and
firewalls is to size the packet-processing engine(s) relative to firewalls is to size the packet-processing engine(s) relative to
the lines in order to keep packet processing uncongested even the lines in order to keep packet processing uncongested, even
under worst case packet rates with runs of minimum size packets. under worst-case packet rates with runs of minimum-size packets.
Therefore, packet-congestion is currently rare [RFC6077; S.3.3], Therefore, packet congestion is currently rare (see Section 3.3 of
but there is no guarantee that it will not become more common in [RFC6077]), but there is no guarantee that it will not become more
future. common in the future.
Note that information is generally processed or transmitted with a Note that information is generally processed or transmitted with a
minimum granularity greater than a bit (e.g. octets). The minimum granularity greater than a bit (e.g., octets). The
appropriate granularity for the resource in question should be appropriate granularity for the resource in question should be
used, but for the sake of brevity we will talk in terms of bytes used, but for the sake of brevity we will talk in terms of bytes
in this memo. in this memo.
Coarser Granularity: Resources may be congestible at higher levels Coarser Granularity: Resources may be congestible at higher levels
of granularity than bits or packets, for instance stateful of granularity than bits or packets, for instance stateful
firewalls are flow-congestible and call-servers are session- firewalls are flow-congestible and call-servers are session-
congestible. This memo focuses on congestion of connectionless congestible. This memo focuses on congestion of connectionless
resources, but the same principles may be applicable for resources, but the same principles may be applicable for
congestion notification protocols controlling per-flow and per- congestion notification protocols controlling per-flow and per-
session processing or state. session processing or state.
RED Terminology: In RED whether to use packets or bytes when RED Terminology: In RED, whether to use packets or bytes when
measuring queues is called respectively "packet-mode queue measuring queues is called, respectively, 'packet-mode queue
measurement" or "byte-mode queue measurement". And whether the measurement' or 'byte-mode queue measurement'. And whether the
probability of dropping a particular packet is independent or probability of dropping a particular packet is independent or
dependent on its size is called respectively "packet-mode drop" or dependent on its size is called, respectively, 'packet-mode drop'
"byte-mode drop". The terms byte-mode and packet-mode should not or 'byte-mode drop'. The terms 'byte-mode' and 'packet-mode'
be used without specifying whether they apply to queue measurement should not be used without specifying whether they apply to queue
or to drop. measurement or to drop.
1.2. Example Comparing Packet-Mode Drop and Byte-Mode Drop 1.2. Example Comparing Packet-Mode Drop and Byte-Mode Drop
Taking RED as a well-known example algorithm, a central question Taking RED as a well-known example algorithm, a central question
addressed by this document is whether to recommend RED's packet-mode addressed by this document is whether to recommend RED's packet-mode
drop variant and to deprecate byte-mode drop. Table 1 compares how drop variant and to deprecate byte-mode drop. Table 1 compares how
packet-mode and byte-mode drop affect two flows of different size packet-mode and byte-mode drop affect two flows of different size
packets. For each it gives the expected number of packets and of packets. For each it gives the expected number of packets and of
bits dropped in one second. Each example flow runs at the same bit- bits dropped in one second. Each example flow runs at the same bit
rate of 48Mb/s, but one is broken up into small 60 byte packets and rate of 48 Mbps, but one is broken up into small 60 byte packets and
the other into large 1500 byte packets. the other into large 1,500 byte packets.
To keep up the same bit-rate, in one second there are about 25 times To keep up the same bit rate, in one second there are about 25 times
more small packets because they are 25 times smaller. As can be seen more small packets because they are 25 times smaller. As can be seen
from the table, the packet rate is 100,000 small packets versus 4,000 from the table, the packet rate is 100,000 small packets versus 4,000
large packets per second (pps). large packets per second (pps).
Parameter Formula Small packets Large packets Parameter Formula Small packets Large packets
-------------------- -------------- ------------- ------------- -------------------- --------------- ------------- -------------
Packet size s/8 60B 1,500B Packet size s/8 60 B 1,500 B
Packet size s 480b 12,000b Packet size s 480 b 12,000 b
Bit-rate x 48Mbps 48Mbps Bit rate x 48 Mbps 48 Mbps
Packet-rate u = x/s 100kpps 4kpps Packet rate u = x/s 100 kpps 4 kpps
Packet-mode Drop Packet-mode Drop
Pkt loss probability p 0.1% 0.1% Pkt-loss probability p 0.1% 0.1%
Pkt loss-rate p*u 100pps 4pps Pkt-loss rate p*u 100 pps 4 pps
Bit loss-rate p*u*s 48kbps 48kbps Bit-loss rate p*u*s 48 kbps 48 kbps
Byte-mode Drop MTU, M=12,000b Byte-mode Drop MTU, M=12,000 b
Pkt loss probability b = p*s/M 0.004% 0.1% Pkt-loss probability b = p*s/M 0.004% 0.1%
Pkt loss-rate b*u 4pps 4pps Pkt-loss rate b*u 4 pps 4 pps
Bit loss-rate b*u*s 1.92kbps 48kbps Bit-loss rate b*u*s 1.92 kbps 48 kbps
Table 1: Example Comparing Packet-mode and Byte-mode Drop Table 1: Example Comparing Packet-Mode and Byte-Mode Drop
For packet-mode drop, we illustrate the effect of a drop probability For packet-mode drop, we illustrate the effect of a drop probability
of 0.1%, which the algorithm applies to all packets irrespective of of 0.1%, which the algorithm applies to all packets irrespective of
size. Because there are 25 times more small packets in one second, size. Because there are 25 times more small packets in one second,
it naturally drops 25 times more small packets, that is 100 small it naturally drops 25 times more small packets, that is, 100 small
packets but only 4 large packets. But if we count how many bits it packets but only 4 large packets. But if we count how many bits it
drops, there are 48,000 bits in 100 small packets and 48,000 bits in drops, there are 48,000 bits in 100 small packets and 48,000 bits in
4 large packets--the same number of bits of small packets as large. 4 large packets -- the same number of bits of small packets as large.
The packet-mode drop algorithm drops any bit with the same The packet-mode drop algorithm drops any bit with the same
probability whether the bit is in a small or a large packet. probability whether the bit is in a small or a large packet.
For byte-mode drop, again we use an example drop probability of 0.1%, For byte-mode drop, again we use an example drop probability of 0.1%,
but only for maximum size packets (assuming the link maximum but only for maximum size packets (assuming the link maximum
transmission unit (MTU) is 1,500B or 12,000b). The byte-mode transmission unit (MTU) is 1,500 B or 12,000 b). The byte-mode
algorithm reduces the drop probability of smaller packets algorithm reduces the drop probability of smaller packets
proportional to their size, making the probability that it drops a proportional to their size, making the probability that it drops a
small packet 25 times smaller at 0.004%. But there are 25 times more small packet 25 times smaller at 0.004%. But there are 25 times more
small packets, so dropping them with 25 times lower probability small packets, so dropping them with 25 times lower probability
results in dropping the same number of packets: 4 drops in both results in dropping the same number of packets: 4 drops in both
cases. The 4 small dropped packets contain 25 times less bits than cases. The 4 small dropped packets contain 25 times less bits than
the 4 large dropped packets: 1,920 compared to 48,000. the 4 large dropped packets: 1,920 compared to 48,000.
The byte-mode drop algorithm drops any bit with a probability The byte-mode drop algorithm drops any bit with a probability
proportionate to the size of the packet it is in. proportionate to the size of the packet it is in.
2. Recommendations 2. Recommendations
This section gives recommendations related to network equipment in This section gives recommendations related to network equipment in
Sections 2.1 and 2.2, and in Sections 2.3 and 2.4 we discuss the Sections 2.1 and 2.2, and we discuss the implications on transport
implications on the transport protocols. protocols in Sections 2.3 and 2.4.
2.1. Recommendation on Queue Measurement 2.1. Recommendation on Queue Measurement
Ideally, an AQM would measure the service time of the queue to Ideally, an AQM would measure the service time of the queue to
measure congestion of a resource. However service time can only be measure congestion of a resource. However service time can only be
measured as packets leave the queue, where it is not always expedient measured as packets leave the queue, where it is not always expedient
to implement a full AQM algorithm. To predict the service time as to implement a full AQM algorithm. To predict the service time as
packets join the queue, an AQM algorithm needs to measure the length packets join the queue, an AQM algorithm needs to measure the length
of the queue. of the queue.
In this case, if the resource is bit-congestible, the AQM In this case, if the resource is bit-congestible, the AQM
implementation SHOULD measure the length of the queue in bytes and, implementation SHOULD measure the length of the queue in bytes and,
if the resource is packet-congestible, the implementation SHOULD if the resource is packet-congestible, the implementation SHOULD
measure the length of the queue in packets. Subject to the measure the length of the queue in packets. Subject to the
exceptions below, no other choice makes sense, because the number of exceptions below, no other choice makes sense, because the number of
packets waiting in the queue isn't relevant if the resource gets packets waiting in the queue isn't relevant if the resource gets
congested by bytes and vice versa. For example, the length of the congested by bytes and vice versa. For example, the length of the
queue into a transmission line would be measured in bytes, while the queue into a transmission line would be measured in bytes, while the
length of the queue into a firewall would be measured in packets. length of the queue into a firewall would be measured in packets.
To avoid the pathological effects of drop tail, the AQM can then To avoid the pathological effects of tail drop, the AQM can then
transform this service time or queue length into the probability of transform this service time or queue length into the probability of
dropping or marking a packet (e.g. RED's piecewise linear function dropping or marking a packet (e.g., RED's piecewise linear function
between thresholds). between thresholds).
What this advice means for RED as a specific example: What this advice means for RED as a specific example:
1. A RED implementation SHOULD use byte mode queue measurement for 1. A RED implementation SHOULD use byte-mode queue measurement for
measuring the congestion of bit-congestible resources and packet measuring the congestion of bit-congestible resources and packet-
mode queue measurement for packet-congestible resources. mode queue measurement for packet-congestible resources.
2. An implementation SHOULD NOT make it possible to configure the 2. An implementation SHOULD NOT make it possible to configure the
way a queue measures itself, because whether a queue is bit- way a queue measures itself, because whether a queue is bit-
congestible or packet-congestible is an inherent property of the congestible or packet-congestible is an inherent property of the
queue. queue.
Exceptions to these recommendations might be necessary, for instance Exceptions to these recommendations might be necessary, for instance
where a packet-congestible resource has to be configured as a proxy where a packet-congestible resource has to be configured as a proxy
bottleneck for a bit-congestible resource in an adjacent box that bottleneck for a bit-congestible resource in an adjacent box that
does not support AQM. does not support AQM.
The recommended approach in less straightforward scenarios, such as The recommended approach in less straightforward scenarios, such as
fixed size packet buffers, resources without a queue and buffers fixed-size packet buffers, resources without a queue, and buffers
comprising a mix of packet and bit-congestible resources, is comprising a mix of packet and bit-congestible resources, is
discussed in Section 4.1. For instance, Section 4.1.1 explains that discussed in Section 4.1. For instance, Section 4.1.1 explains that
the queue into a line should be measured in bytes even if the queue the queue into a line should be measured in bytes even if the queue
consists of fixed-size packet-buffers, because the root-cause of any consists of fixed-size packet buffers, because the root cause of any
congestion is bytes arriving too fast for the line--packets filling congestion is bytes arriving too fast for the line -- packets filling
buffers are merely a symptom of the underlying congestion of the buffers are merely a symptom of the underlying congestion of the
line. line.
2.2. Recommendation on Encoding Congestion Notification 2.2. Recommendation on Encoding Congestion Notification
When encoding congestion notification (e.g. by drop, ECN or PCN), the When encoding congestion notification (e.g., by drop, ECN, or PCN),
probability that network equipment drops or marks a particular packet the probability that network equipment drops or marks a particular
to notify congestion SHOULD NOT depend on the size of the packet in packet to notify congestion SHOULD NOT depend on the size of the
question. As the example in Section 1.2 illustrates, to drop any bit packet in question. As the example in Section 1.2 illustrates, to
with probability 0.1% it is only necessary to drop every packet with drop any bit with probability 0.1%, it is only necessary to drop
probability 0.1% without regard to the size of each packet. every packet with probability 0.1% without regard to the size of each
packet.
This approach ensures the network layer offers sufficient congestion This approach ensures the network layer offers sufficient congestion
information for all known and future transport protocols and also information for all known and future transport protocols and also
ensures no perverse incentives are created that would encourage ensures no perverse incentives are created that would encourage
transports to use inappropriately small packet sizes. transports to use inappropriately small packet sizes.
What this advice means for RED as a specific example: What this advice means for RED as a specific example:
1. The RED AQM algorithm SHOULD NOT use byte-mode drop, i.e. it 1. The RED AQM algorithm SHOULD NOT use byte-mode drop, i.e., it
ought to use packet-mode drop. Byte-mode drop is more complex, ought to use packet-mode drop. Byte-mode drop is more complex,
it creates the perverse incentive to fragment segments into tiny it creates the perverse incentive to fragment segments into tiny
pieces and it is vulnerable to floods of small packets. pieces and it is vulnerable to floods of small packets.
2. If a vendor has implemented byte-mode drop, and an operator has 2. If a vendor has implemented byte-mode drop, and an operator has
turned it on, it is RECOMMENDED to switch it to packet-mode drop, turned it on, it is RECOMMENDED that the operator use packet-mode
after establishing if there are any implications on the relative drop instead, after establishing if there are any implications on
performance of applications using different packet sizes. The the relative performance of applications using different packet
unlikely possibility of some application-specific legacy use of sizes. The unlikely possibility of some application-specific
byte-mode drop is the only reason that all the above legacy use of byte-mode drop is the only reason that all the
recommendations on encoding congestion notification are not above recommendations on encoding congestion notification are not
phrased more strongly. phrased more strongly.
RED as a whole SHOULD NOT be switched off. Without RED, a drop RED as a whole SHOULD NOT be switched off. Without RED, a tail-
tail queue biases against large packets and is vulnerable to drop queue biases against large packets and is vulnerable to
floods of small packets. floods of small packets.
Note well that RED's byte-mode queue drop is completely orthogonal to Note well that RED's byte-mode queue drop is completely orthogonal to
byte-mode queue measurement and should not be confused with it. If a byte-mode queue measurement and should not be confused with it. If a
RED implementation has a byte-mode but does not specify what sort of RED implementation has a byte-mode but does not specify what sort of
byte-mode, it is most probably byte-mode queue measurement, which is byte-mode, it is most probably byte-mode queue measurement, which is
fine. However, if in doubt, the vendor should be consulted. fine. However, if in doubt, the vendor should be consulted.
A survey (Appendix A) showed that there appears to be little, if any, A survey (Appendix A) showed that there appears to be little, if any,
installed base of the byte-mode drop variant of RED. This suggests installed base of the byte-mode drop variant of RED. This suggests
skipping to change at page 11, line 19 skipping to change at page 11, line 28
deployment impact. deployment impact.
2.3. Recommendation on Responding to Congestion 2.3. Recommendation on Responding to Congestion
When a transport detects that a packet has been lost or congestion When a transport detects that a packet has been lost or congestion
marked, it SHOULD consider the strength of the congestion indication marked, it SHOULD consider the strength of the congestion indication
as proportionate to the size in octets (bytes) of the missing or as proportionate to the size in octets (bytes) of the missing or
marked packet. marked packet.
In other words, when a packet indicates congestion (by being lost or In other words, when a packet indicates congestion (by being lost or
marked) it can be considered conceptually as if there is a congestion marked), it can be considered conceptually as if there is a
indication on every octet of the packet, not just one indication per congestion indication on every octet of the packet, not just one
packet. indication per packet.
To be clear, the above recommendation solely describes how a To be clear, the above recommendation solely describes how a
transport should interpret the meaning of a congestion indication, as transport should interpret the meaning of a congestion indication, as
a long term goal. It makes no recommendation on whether a transport a long term goal. It makes no recommendation on whether a transport
should act differently based on this interpretation. It merely aids should act differently based on this interpretation. It merely aids
interoperablity between transports, if they choose to make their interoperability between transports, if they choose to make their
actions depend on the strength of congestion indications. actions depend on the strength of congestion indications.
This definition will be useful as the IETF transport area continues This definition will be useful as the IETF transport area continues
its programme of; its programme of:
o updating host-based congestion control protocols to take account o updating host-based congestion control protocols to take packet
of packet size size into account, and
o making transports less sensitive to losing control packets like o making transports less sensitive to losing control packets like
SYNs and pure ACKs. SYNs and pure ACKs.
What this advice means for the case of TCP: What this advice means for the case of TCP:
1. If two TCP flows with different packet sizes are required to run 1. If two TCP flows with different packet sizes are required to run
at equal bit rates under the same path conditions, this SHOULD be at equal bit rates under the same path conditions, this SHOULD be
done by altering TCP (Section 4.2.2), not network equipment (the done by altering TCP (Section 4.2.2), not network equipment (the
latter affects other transports besides TCP). latter affects other transports besides TCP).
2. If it is desired to improve TCP performance by reducing the 2. If it is desired to improve TCP performance by reducing the
chance that a SYN or a pure ACK will be dropped, this SHOULD be chance that a SYN or a pure ACK will be dropped, this SHOULD be
done by modifying TCP (Section 4.2.3), not network equipment. done by modifying TCP (Section 4.2.3), not network equipment.
To be clear, we are not recommending at all that TCPs under To be clear, we are not recommending at all that TCPs under
equivalent conditions should aim for equal bit-rates. We are merely equivalent conditions should aim for equal bit rates. We are merely
saying that anyone trying to do such a thing should modify their TCP saying that anyone trying to do such a thing should modify their TCP
algorithm, not the network. algorithm, not the network.
These recommendations are phrased as 'SHOULD' rather than 'MUST', These recommendations are phrased as 'SHOULD' rather than 'MUST',
because there may be cases where expediency dictates that because there may be cases where expediency dictates that
compatibility with pre-existing versions of a transport protocol make compatibility with pre-existing versions of a transport protocol make
the recommendations impractical. the recommendations impractical.
2.4. Recommendation on Handling Congestion Indications when Splitting 2.4. Recommendation on Handling Congestion Indications When Splitting
or Merging Packets or Merging Packets
Packets carrying congestion indications may be split or merged in Packets carrying congestion indications may be split or merged in
some circumstances (e.g. at a RTP/RTCP transcoder or during IP some circumstances (e.g., at an RTP / RTP Control Protocol (RTCP)
fragment reassembly). Splitting and merging only make sense in the transcoder or during IP fragment reassembly). Splitting and merging
context of ECN, not loss. only make sense in the context of ECN, not loss.
The general rule to follow is that the number of octets in packets The general rule to follow is that the number of octets in packets
with congestion indications SHOULD be equivalent before and after with congestion indications SHOULD be equivalent before and after
merging or splitting. This is based on the principle used above; merging or splitting. This is based on the principle used above;
that an indication of congestion on a packet can be considered as an that an indication of congestion on a packet can be considered as an
indication of congestion on each octet of the packet. indication of congestion on each octet of the packet.
The above rule is not phrased with the word "MUST" to allow the The above rule is not phrased with the word 'MUST' to allow the
following exception. There are cases where pre-existing protocols following exception. There are cases in which pre-existing protocols
were not designed to conserve congestion marked octets (e.g. IP were not designed to conserve congestion-marked octets (e.g., IP
fragment reassembly [RFC3168] or loss statistics in RTCP receiver fragment reassembly [RFC3168] or loss statistics in RTCP receiver
reports [RFC3550] before ECN was added [RFC6679]). When any such reports [RFC3550] before ECN was added [RFC6679]). When any such
protocol is updated, it SHOULD comply with the above rule to conserve protocol is updated, it SHOULD comply with the above rule to conserve
marked octets. However, the rule may be relaxed if it would marked octets. However, the rule may be relaxed if it would
otherwise become too complex to interoperate with pre-existing otherwise become too complex to interoperate with pre-existing
implementations of the protocol. implementations of the protocol.
One can think of a splitting or merging process as if all the One can think of a splitting or merging process as if all the
incoming congestion-marked octets increment a counter and all the incoming congestion-marked octets increment a counter and all the
outgoing marked octets decrement the same counter. In order to outgoing marked octets decrement the same counter. In order to
ensure that congestion indications remain timely, even the smallest ensure that congestion indications remain timely, even the smallest
positive remainder in the conceptual counter should trigger the next positive remainder in the conceptual counter should trigger the next
outgoing packet to be marked (causing the counter to go negative). outgoing packet to be marked (causing the counter to go negative).
3. Motivating Arguments 3. Motivating Arguments
This section is informative. It justifies the recommendations given This section is informative. It justifies the recommendations made
in the previous section. in the previous section.
3.1. Avoiding Perverse Incentives to (Ab)use Smaller Packets 3.1. Avoiding Perverse Incentives to (Ab)use Smaller Packets
Increasingly, it is being recognised that a protocol design must take Increasingly, it is being recognised that a protocol design must take
care not to cause unintended consequences by giving the parties in care not to cause unintended consequences by giving the parties in
the protocol exchange perverse incentives [Evol_cc][RFC3426]. Given the protocol exchange perverse incentives [Evol_cc] [RFC3426]. Given
there are many good reasons why larger path maximum transmission there are many good reasons why larger path maximum transmission
units (PMTUs) would help solve a number of scaling issues, we do not units (PMTUs) would help solve a number of scaling issues, we do not
want to create any bias against large packets that is greater than want to create any bias against large packets that is greater than
their true cost. their true cost.
Imagine a scenario where the same bit rate of packets will contribute Imagine a scenario where the same bit rate of packets will contribute
the same to bit-congestion of a link irrespective of whether it is the same to bit congestion of a link irrespective of whether it is
sent as fewer larger packets or more smaller packets. A protocol sent as fewer larger packets or more smaller packets. A protocol
design that caused larger packets to be more likely to be dropped design that caused larger packets to be more likely to be dropped
than smaller ones would be dangerous in both the following cases: than smaller ones would be dangerous in both of the following cases:
Malicious transports: A queue that gives an advantage to small Malicious transports: A queue that gives an advantage to small
packets can be used to amplify the force of a flooding attack. By packets can be used to amplify the force of a flooding attack. By
sending a flood of small packets, the attacker can get the queue sending a flood of small packets, the attacker can get the queue
to discard more traffic in large packets, allowing more attack to discard more large-packet traffic, allowing more attack traffic
traffic to get through to cause further damage. Such a queue to get through to cause further damage. Such a queue allows
allows attack traffic to have a disproportionately large effect on attack traffic to have a disproportionately large effect on
regular traffic without the attacker having to do much work. regular traffic without the attacker having to do much work.
Non-malicious transports: Even if an application designer is not Non-malicious transports: Even if an application designer is not
actually malicious, if over time it is noticed that small packets actually malicious, if over time it is noticed that small packets
tend to go faster, designers will act in their own interest and tend to go faster, designers will act in their own interest and
use smaller packets. Queues that give advantage to small packets use smaller packets. Queues that give advantage to small packets
create an evolutionary pressure for applications or transports to create an evolutionary pressure for applications or transports to
send at the same bit-rate but break their data stream down into send at the same bit rate but break their data stream down into
tiny segments to reduce their drop rate. Encouraging a high tiny segments to reduce their drop rate. Encouraging a high
volume of tiny packets might in turn unnecessarily overload a volume of tiny packets might in turn unnecessarily overload a
completely unrelated part of the system, perhaps more limited by completely unrelated part of the system, perhaps more limited by
header-processing than bandwidth. header processing than bandwidth.
Imagine two unresponsive flows arrive at a bit-congestible Imagine that two unresponsive flows arrive at a bit-congestible
transmission link each with the same bit rate, say 1Mbps, but one transmission link each with the same bit rate, say 1 Mbps, but one
consists of 1500B and the other 60B packets, which are 25x smaller. consists of 1,500 B and the other 60 B packets, which are 25x
Consider a scenario where gentle RED [gentle_RED] is used, along with smaller. Consider a scenario where gentle RED [gentle_RED] is used,
the variant of RED we advise against, i.e. where the RED algorithm is along with the variant of RED we advise against, i.e., where the RED
configured to adjust the drop probability of packets in proportion to algorithm is configured to adjust the drop probability of packets in
each packet's size (byte mode packet drop). In this case, RED aims proportion to each packet's size (byte-mode packet drop). In this
to drop 25x more of the larger packets than the smaller ones. Thus, case, RED aims to drop 25x more of the larger packets than the
for example if RED drops 25% of the larger packets, it will aim to smaller ones. Thus, for example, if RED drops 25% of the larger
drop 1% of the smaller packets (but in practice it may drop more as packets, it will aim to drop 1% of the smaller packets (but, in
congestion increases [RFC4828; Appx B.4]). Even though both flows practice, it may drop more as congestion increases; see Appendix B.4
arrive with the same bit rate, the bit rate the RED queue aims to of [RFC4828]). Even though both flows arrive with the same bit rate,
pass to the line will be 750kbps for the flow of larger packets but the bit rate the RED queue aims to pass to the line will be 750 kbps
990kbps for the smaller packets (because of rate variations it will for the flow of larger packets but 990 kbps for the smaller packets
actually be a little less than this target). (because of rate variations, it will actually be a little less than
this target).
Note that, although the byte-mode drop variant of RED amplifies small Note that, although the byte-mode drop variant of RED amplifies
packet attacks, drop-tail queues amplify small packet attacks even small-packet attacks, tail-drop queues amplify small-packet attacks
more (see Security Considerations in Section 6). Wherever possible even more (see Security Considerations in Section 6). Wherever
neither should be used. possible, neither should be used.
3.2. Small != Control 3.2. Small != Control
Dropping fewer control packets considerably improves performance. It Dropping fewer control packets considerably improves performance. It
is tempting to drop small packets with lower probability in order to is tempting to drop small packets with lower probability in order to
improve performance, because many control packets tend to be smaller improve performance, because many control packets tend to be smaller
(TCP SYNs & ACKs, DNS queries & responses, SIP messages, HTTP GETs, (TCP SYNs and ACKs, DNS queries and responses, SIP messages, HTTP
etc). However, we must not give control packets preference purely by GETs, etc). However, we must not give control packets preference
virtue of their smallness, otherwise it is too easy for any data purely by virtue of their smallness, otherwise it is too easy for any
source to get the same preferential treatment simply by sending data data source to get the same preferential treatment simply by sending
in smaller packets. Again we should not create perverse incentives data in smaller packets. Again, we should not create perverse
to favour small packets rather than to favour control packets, which incentives to favour small packets rather than to favour control
is what we intend. packets, which is what we intend.
Just because many control packets are small does not mean all small Just because many control packets are small does not mean all small
packets are control packets. packets are control packets.
So, rather than fix these problems in the network, we argue that the So, rather than fix these problems in the network, we argue that the
transport should be made more robust against losses of control transport should be made more robust against losses of control
packets (see 'Making Transports Robust against Control Packet Losses' packets (see Section 4.2.3).
in Section 4.2.3).
3.3. Transport-Independent Network 3.3. Transport-Independent Network
TCP congestion control ensures that flows competing for the same TCP congestion control ensures that flows competing for the same
resource each maintain the same number of segments in flight, resource each maintain the same number of segments in flight,
irrespective of segment size. So under similar conditions, flows irrespective of segment size. So under similar conditions, flows
with different segment sizes will get different bit-rates. with different segment sizes will get different bit rates.
To counter this effect it seems tempting not to follow our To counter this effect, it seems tempting not to follow our
recommendation, and instead for the network to bias congestion recommendation, and instead for the network to bias congestion
notification by packet size in order to equalise the bit-rates of notification by packet size in order to equalise the bit rates of
flows with different packet sizes. However, in order to do this, the flows with different packet sizes. However, in order to do this, the
queuing algorithm has to make assumptions about the transport, which queuing algorithm has to make assumptions about the transport, which
become embedded in the network. Specifically: become embedded in the network. Specifically:
o The queuing algorithm has to assume how aggressively the transport o The queuing algorithm has to assume how aggressively the transport
will respond to congestion (see Section 4.2.4). If the network will respond to congestion (see Section 4.2.4). If the network
assumes the transport responds as aggressively as TCP NewReno, it assumes the transport responds as aggressively as TCP NewReno, it
will be wrong for Compound TCP and differently wrong for Cubic will be wrong for Compound TCP and differently wrong for Cubic
TCP, etc. To achieve equal bit-rates, each transport then has to TCP, etc. To achieve equal bit rates, each transport then has to
guess what assumption the network made, and work out how to guess what assumption the network made, and work out how to
replace this assumed aggressiveness with its own aggressiveness. replace this assumed aggressiveness with its own aggressiveness.
o Also, if the network biases congestion notification by packet size o Also, if the network biases congestion notification by packet
it has to assume a baseline packet size--all proposed algorithms size, it has to assume a baseline packet size -- all proposed
use the local MTU (for example see the byte-mode loss probability algorithms use the local MTU (for example, see the byte-mode loss
formula in Table 1). Then if the non-Reno transports mentioned probability formula in Table 1). Then if the non-Reno transports
above are trying to reverse engineer what the network assumed, mentioned above are trying to reverse engineer what the network
they also have to guess the MTU of the congested link. assumed, they also have to guess the MTU of the congested link.
Even though reducing the drop probability of small packets (e.g. Even though reducing the drop probability of small packets (e.g.,
RED's byte-mode drop) helps ensure TCP flows with different packet RED's byte-mode drop) helps ensure TCP flows with different packet
sizes will achieve similar bit rates, we argue this correction should sizes will achieve similar bit rates, we argue that this correction
be made to any future transport protocols based on TCP, not to the should be made to any future transport protocols based on TCP, not to
network in order to fix one transport, no matter how predominant it the network in order to fix one transport, no matter how predominant
is. Effectively, favouring small packets is reverse engineering of it is. Effectively, favouring small packets is reverse engineering
network equipment around one particular transport protocol (TCP), of network equipment around one particular transport protocol (TCP),
contrary to the excellent advice in [RFC3426], which asks designers contrary to the excellent advice in [RFC3426], which asks designers
to question "Why are you proposing a solution at this layer of the to question "Why are you proposing a solution at this layer of the
protocol stack, rather than at another layer?" protocol stack, rather than at another layer?"
In contrast, if the network never takes account of packet size, the In contrast, if the network never takes packet size into account, the
transport can be certain it will never need to guess any assumptions transport can be certain it will never need to guess any assumptions
the network has made. And the network passes two pieces of that the network has made. And the network passes two pieces of
information to the transport that are sufficient in all cases: i) information to the transport that are sufficient in all cases: i)
congestion notification on the packet and ii) the size of the packet. congestion notification on the packet and ii) the size of the packet.
Both are available for the transport to combine (by taking account of Both are available for the transport to combine (by taking packet
packet size when responding to congestion) or not. Appendix B checks size into account when responding to congestion) or not. Appendix B
that these two pieces of information are sufficient for all relevant checks that these two pieces of information are sufficient for all
scenarios. relevant scenarios.
When the network does not take account of packet size, it allows When the network does not take packet size into account, it allows
transport protocols to choose whether to take account of packet size transport protocols to choose whether or not to take packet size into
or not. However, if the network were to bias congestion notification account. However, if the network were to bias congestion
by packet size, transport protocols would have no choice; those that notification by packet size, transport protocols would have no
did not take account of packet size themselves would unwittingly choice; those that did not take into account packet size themselves
become dependent on packet size, and those that already took account would unwittingly become dependent on packet size, and those that
of packet size would end up taking account of it twice. already took packet size into account would end up taking it into
account twice.
3.4. Partial Deployment of AQM 3.4. Partial Deployment of AQM
In overview, the argument in this section runs as follows: In overview, the argument in this section runs as follows:
o Because the network does not and cannot always drop packets in o Because the network does not and cannot always drop packets in
proportion to their size, it shouldn't be given the task of making proportion to their size, it shouldn't be given the task of making
drop signals depend on packet size at all. drop signals depend on packet size at all.
o Transports on the other hand don't always want to make their rate o Transports on the other hand don't always want to make their rate
response proportional to the size of dropped packets, but if they response proportional to the size of dropped packets, but if they
want to, they always can. want to, they always can.
The argument is similar to the end-to-end argument that says "Don't The argument is similar to the end-to-end argument that says "Don't
do X in the network if end-systems can do X by themselves, and they do X in the network if end systems can do X by themselves, and they
want to be able to choose whether to do X anyway." Actually the want to be able to choose whether to do X anyway". Actually the
following argument is stronger; in addition it says "Don't give the following argument is stronger; in addition it says "Don't give the
network task X that could be done by the end-systems, if X is not network task X that could be done by the end systems, if X is not
deployed on all network nodes, and end-systems won't be able to tell deployed on all network nodes, and end systems won't be able to tell
whether their network is doing X, or whether they need to do X whether their network is doing X, or whether they need to do X
themselves." In this case, the X in question is "making the response themselves." In this case, the X in question is "making the response
to congestion depend on packet size". to congestion depend on packet size".
We will now re-run this argument taking each step in more depth. The We will now re-run this argument reviewing each step in more depth.
argument applies solely to drop, not to ECN marking. The argument applies solely to drop, not to ECN marking.
A queue drops packets for either of two reasons: a) to signal to host A queue drops packets for either of two reasons: a) to signal to host
congestion controls that they should reduce the load and b) because congestion controls that they should reduce the load and b) because
there is no buffer left to store the packets. Active queue there is no buffer left to store the packets. Active queue
management tries to use drops as a signal for hosts to slow down management tries to use drops as a signal for hosts to slow down
(case a) so that drop due to buffer exhaustion (case b) should not be (case a) so that drops due to buffer exhaustion (case b) should not
necessary. be necessary.
AQM is not universally deployed in every queue in the Internet; many AQM is not universally deployed in every queue in the Internet; many
cheap Ethernet bridges, software firewalls, NATs on consumer devices, cheap Ethernet bridges, software firewalls, NATs on consumer devices,
etc implement simple tail-drop buffers. Even if AQM were universal, etc implement simple tail-drop buffers. Even if AQM were universal,
it has to be able to cope with buffer exhaustion (by switching to a it has to be able to cope with buffer exhaustion (by switching to a
behaviour like tail-drop), in order to cope with unresponsive or behaviour like tail drop), in order to cope with unresponsive or
excessive transports. For these reasons networks will sometimes be excessive transports. For these reasons networks will sometimes be
dropping packets as a last resort (case b) rather than under AQM dropping packets as a last resort (case b) rather than under AQM
control (case a). control (case a).
When buffers are exhausted (case b), they don't naturally drop When buffers are exhausted (case b), they don't naturally drop
packets in proportion to their size. The network can only reduce the packets in proportion to their size. The network can only reduce the
probability of dropping smaller packets if it has enough space to probability of dropping smaller packets if it has enough space to
store them somewhere while it waits for a larger packet that it can store them somewhere while it waits for a larger packet that it can
drop. If the buffer is exhausted, it does not have this choice. drop. If the buffer is exhausted, it does not have this choice.
Admittedly tail-drop does naturally drop somewhat fewer small Admittedly tail drop does naturally drop somewhat fewer small
packets, but exactly how few depends more on the mix of sizes than packets, but exactly how few depends more on the mix of sizes than
the size of the packet in question. Nonetheless, in general, if we the size of the packet in question. Nonetheless, in general, if we
wanted networks to do size-dependent drop, we would need universal wanted networks to do size-dependent drop, we would need universal
deployment of (packet-size dependent) AQM code, which is currently deployment of (packet-size dependent) AQM code, which is currently
unrealistic. unrealistic.
A host transport cannot know whether any particular drop was a A host transport cannot know whether any particular drop was a
deliberate signal from an AQM or a sign of a queue shedding packets deliberate signal from an AQM or a sign of a queue shedding packets
due to buffer exhaustion. Therefore, because the network cannot due to buffer exhaustion. Therefore, because the network cannot
universally do size-dependent drop, it should not do it all. universally do size-dependent drop, it should not do it all.
Whereas universality is desirable in the network, diversity is Whereas universality is desirable in the network, diversity is
desirable between different transport layer protocols - some, like desirable between different transport-layer protocols -- some, like
NewReno TCP [RFC5681], may not choose to make their rate response standards track TCP congestion control [RFC5681], may not choose to
proportionate to the size of each dropped packet, while others will make their rate response proportionate to the size of each dropped
(e.g. TFRC-SP [RFC4828]). packet, while others will (e.g., TCP-Friendly Rate Control for Small
Packets (TFRC-SP) [RFC4828]).
3.5. Implementation Efficiency 3.5. Implementation Efficiency
Biasing against large packets typically requires an extra multiply Biasing against large packets typically requires an extra multiply
and divide in the network (see the example byte-mode drop formula in and divide in the network (see the example byte-mode drop formula in
Table 1). Allowing for packet size at the transport rather than in Table 1). Taking packet size into account at the transport rather
the network ensures that neither the network nor the transport needs than in the network ensures that neither the network nor the
to do a multiply operation--multiplication by packet size is transport needs to do a multiply operation -- multiplication by
effectively achieved as a repeated add when the transport adds to its packet size is effectively achieved as a repeated add when the
count of marked bytes as each congestion event is fed to it. Also transport adds to its count of marked bytes as each congestion event
the work to do the biasing is spread over many hosts, rather than is fed to it. Also, the work to do the biasing is spread over many
concentrated in just the congested network element. These aren't hosts, rather than concentrated in just the congested network
principled reasons in themselves, but they are a happy consequence of element. These aren't principled reasons in themselves, but they are
the other principled reasons. a happy consequence of the other principled reasons.
4. A Survey and Critique of Past Advice 4. A Survey and Critique of Past Advice
This section is informative, not normative. This section is informative, not normative.
The original 1993 paper on RED [RED93] proposed two options for the The original 1993 paper on RED [RED93] proposed two options for the
RED active queue management algorithm: packet mode and byte mode. RED active queue management algorithm: packet mode and byte mode.
Packet mode measured the queue length in packets and dropped (or Packet mode measured the queue length in packets and dropped (or
marked) individual packets with a probability independent of their marked) individual packets with a probability independent of their
size. Byte mode measured the queue length in bytes and marked an size. Byte mode measured the queue length in bytes and marked an
skipping to change at page 17, line 43 skipping to change at page 18, line 11
further work, it was stated that no recommendation had been made on further work, it was stated that no recommendation had been made on
whether the queue size should be measured in bytes or packets, but whether the queue size should be measured in bytes or packets, but
noted that the difference could be significant. noted that the difference could be significant.
When RED was recommended for general deployment in 1998 [RFC2309], When RED was recommended for general deployment in 1998 [RFC2309],
the two modes were mentioned implying the choice between them was a the two modes were mentioned implying the choice between them was a
question of performance, referring to a 1997 email [pktByteEmail] for question of performance, referring to a 1997 email [pktByteEmail] for
advice on tuning. A later addendum to this email introduced the advice on tuning. A later addendum to this email introduced the
insight that there are in fact two orthogonal choices: insight that there are in fact two orthogonal choices:
o whether to measure queue length in bytes or packets (Section 4.1) o whether to measure queue length in bytes or packets (Section 4.1),
and
o whether the drop probability of an individual packet should depend o whether the drop probability of an individual packet should depend
on its own size (Section 4.2). on its own size (Section 4.2).
The rest of this section is structured accordingly. The rest of this section is structured accordingly.
4.1. Congestion Measurement Advice 4.1. Congestion Measurement Advice
The choice of which metric to use to measure queue length was left The choice of which metric to use to measure queue length was left
open in RFC2309. It is now well understood that queues for bit- open in RFC 2309. It is now well understood that queues for bit-
congestible resources should be measured in bytes, and queues for congestible resources should be measured in bytes, and queues for
packet-congestible resources should be measured in packets packet-congestible resources should be measured in packets
[pktByteEmail]. [pktByteEmail].
Congestion in some legacy bit-congestible buffers is only measured in Congestion in some legacy bit-congestible buffers is only measured in
packets not bytes. In such cases, the operator has to set the packets not bytes. In such cases, the operator has to take into
thresholds mindful of a typical mix of packets sizes. Any AQM account a typical mix of packet sizes when setting the thresholds.
algorithm on such a buffer will be oversensitive to high proportions Any AQM algorithm on such a buffer will be oversensitive to high
of small packets, e.g. a DoS attack, and under-sensitive to high proportions of small packets, e.g., a DoS attack, and under-sensitive
proportions of large packets. However, there is no need to make to high proportions of large packets. However, there is no need to
allowances for the possibility of such legacy in future protocol make allowances for the possibility of such a legacy in future
design. This is safe because any under-sensitivity during unusual protocol design. This is safe because any under-sensitivity during
traffic mixes cannot lead to congestion collapse given the buffer unusual traffic mixes cannot lead to congestion collapse given that
will eventually revert to tail drop, discarding proportionately more the buffer will eventually revert to tail drop, which discards
large packets. proportionately more large packets.
4.1.1. Fixed Size Packet Buffers 4.1.1. Fixed-Size Packet Buffers
The question of whether to measure queues in bytes or packets seems The question of whether to measure queues in bytes or packets seems
to be well understood. However, measuring congestion is confusing to be well understood. However, measuring congestion is confusing
when the resource is bit congestible but the queue into the resource when the resource is bit-congestible but the queue into the resource
is packet congestible. This section outlines the approach to take. is packet-congestible. This section outlines the approach to take.
Some, mostly older, queuing hardware allocates fixed sized buffers in Some, mostly older, queuing hardware allocates fixed-size buffers in
which to store each packet in the queue. This hardware forwards to which to store each packet in the queue. This hardware forwards
the line in one of two ways: packets to the line in one of two ways:
o With some hardware, any fixed sized buffers not completely filled o With some hardware, any fixed-size buffers not completely filled
by a packet are padded when transmitted to the wire. This case, by a packet are padded when transmitted to the wire. This case
should clearly be treated as packet-congestible, because both should clearly be treated as packet-congestible, because both
queuing and transmission are in fixed MTU-sized units. Therefore queuing and transmission are in fixed MTU-size units. Therefore,
the queue length in packets is a good model of congestion of the the queue length in packets is a good model of congestion of the
link. link.
o More commonly, hardware with fixed size packet buffers transmits o More commonly, hardware with fixed-size packet buffers transmits
packets to line without padding. This implies a hybrid forwarding packets to the line without padding. This implies a hybrid
system with transmission congestion dependent on the size of forwarding system with transmission congestion dependent on the
packets but queue congestion dependent on the number of packets, size of packets but queue congestion dependent on the number of
irrespective of their size. packets, irrespective of their size.
Nonetheless, there would be no queue at all unless the line had Nonetheless, there would be no queue at all unless the line had
become congested--the root-cause of any congestion is too many become congested -- the root cause of any congestion is too many
bytes arriving for the line. Therefore, the AQM should measure bytes arriving for the line. Therefore, the AQM should measure
the queue length as the sum of all the packet sizes in bytes that the queue length as the sum of all the packet sizes in bytes that
are queued up waiting to be serviced by the line, irrespective of are queued up waiting to be serviced by the line, irrespective of
whether each packet is held in a fixed size buffer. whether each packet is held in a fixed-size buffer.
In the (unlikely) first case where use of padding means the queue In the (unlikely) first case where use of padding means the queue
should be measured in packets, further confusion is likely because should be measured in packets, further confusion is likely because
the fixed buffers are rarely all one size. Typically pools of the fixed buffers are rarely all one size. Typically, pools of
different sized buffers are provided (Cisco uses the term 'buffer different-sized buffers are provided (Cisco uses the term 'buffer
carving' for the process of dividing up memory into these pools carving' for the process of dividing up memory into these pools
[IOSArch]). Usually, if the pool of small buffers is exhausted, [IOSArch]). Usually, if the pool of small buffers is exhausted,
arriving small packets can borrow space in the pool of large buffers, arriving small packets can borrow space in the pool of large buffers,
but not vice versa. However, there is no need to consider all this but not vice versa. However, there is no need to consider all this
complexity, because the root-cause of any congestion is still line complexity, because the root cause of any congestion is still line
overload--buffer consumption is only the symptom. Therefore, the overload -- buffer consumption is only the symptom. Therefore, the
length of the queue should be measured as the sum of the bytes in the length of the queue should be measured as the sum of the bytes in the
queue that will be transmitted to line, including any padding. In queue that will be transmitted to the line, including any padding.
the (unusual) case of transmission with padding this means the sum of In the (unusual) case of transmission with padding, this means the
the sizes of the small buffers queued plus the sum of the sizes of sum of the sizes of the small buffers queued plus the sum of the
the large buffers queued. sizes of the large buffers queued.
We will return to borrowing of fixed sized buffers when we discuss We will return to borrowing of fixed-size buffers when we discuss
biasing the drop/marking probability of a specific packet because of biasing the drop/marking probability of a specific packet because of
its size in Section 4.2.1. But here we can repeat the simple rule its size in Section 4.2.1. But here, we can repeat the simple rule
for how to measure the length of queues of fixed buffers: no matter for how to measure the length of queues of fixed buffers: no matter
how complicated the buffering scheme is, ultimately a transmission how complicated the buffering scheme is, ultimately a transmission
line is nearly always bit-congestible so the number of bytes queued line is nearly always bit-congestible so the number of bytes queued
up waiting for the line measures how congested the line is, and it is up waiting for the line measures how congested the line is, and it is
rarely important to measure how congested the buffering system is. rarely important to measure how congested the buffering system is.
4.1.2. Congestion Measurement without a Queue 4.1.2. Congestion Measurement without a Queue
AQM algorithms are nearly always described assuming there is a queue AQM algorithms are nearly always described assuming there is a queue
for a congested resource and the algorithm can use the queue length for a congested resource and the algorithm can use the queue length
to determine the probability that it will drop or mark each packet. to determine the probability that it will drop or mark each packet.
But not all congested resources lead to queues. For instance, power But not all congested resources lead to queues. For instance, power-
limited resources are usually bit-congestible if energy is primarily limited resources are usually bit-congestible if energy is primarily
required for transmission rather than header processing, but it is required for transmission rather than header processing, but it is
rare for a link protocol to build a queue as it approaches maximum rare for a link protocol to build a queue as it approaches maximum
power. power.
Nonetheless, AQM algorithms do not require a queue in order to work. Nonetheless, AQM algorithms do not require a queue in order to work.
For instance spectrum congestion can be modelled by signal quality For instance, spectrum congestion can be modelled by signal quality
using target bit-energy-to-noise-density ratio. And, to model radio using the target bit-energy-to-noise-density ratio. And, to model
power exhaustion, transmission power levels can be measured and radio power exhaustion, transmission-power levels can be measured and
compared to the maximum power available. [ECNFixedWireless] proposes compared to the maximum power available. [ECNFixedWireless] proposes
a practical and theoretically sound way to combine congestion a practical and theoretically sound way to combine congestion
notification for different bit-congestible resources at different notification for different bit-congestible resources at different
layers along an end to end path, whether wireless or wired, and layers along an end-to-end path, whether wireless or wired, and
whether with or without queues. whether with or without queues.
In wireless protocols that use request to send / clear to send (RTS / In wireless protocols that use request to send / clear to send
CTS) control, such as some variants of IEEE802.11, it is reasonable (RTS / CTS) control, such as some variants of IEEE802.11, it is
to base an AQM on the time spent waiting for transmission reasonable to base an AQM on the time spent waiting for transmission
opportunities (TXOPs) even though wireless spectrum is usually opportunities (TXOPs) even though the wireless spectrum is usually
regarded as congested by bits (for a given coding scheme). This is regarded as congested by bits (for a given coding scheme). This is
because requests for TXOPs queue up as the spectrum gets congested by because requests for TXOPs queue up as the spectrum gets congested by
all the bits being transferred. So the time that TXOPs are queued all the bits being transferred. So the time that TXOPs are queued
directly reflects bit congestion of the spectrum. directly reflects bit congestion of the spectrum.
4.2. Congestion Notification Advice 4.2. Congestion Notification Advice
4.2.1. Network Bias when Encoding 4.2.1. Network Bias When Encoding
4.2.1.1. Advice on Packet Size Bias in RED 4.2.1.1. Advice on Packet-Size Bias in RED
The previously mentioned email [pktByteEmail] referred to by The previously mentioned email [pktByteEmail] referred to by
[RFC2309] advised that most scarce resources in the Internet were [RFC2309] advised that most scarce resources in the Internet were
bit-congestible, which is still believed to be true (Section 1.1). bit-congestible, which is still believed to be true (Section 1.1).
But it went on to offer advice that is updated by this memo. It said But it went on to offer advice that is updated by this memo. It said
that drop probability should depend on the size of the packet being that drop probability should depend on the size of the packet being
considered for drop if the resource is bit-congestible, but not if it considered for drop if the resource is bit-congestible, but not if it
is packet-congestible. The argument continued that if packet drops is packet-congestible. The argument continued that if packet drops
were inflated by packet size (byte-mode dropping), "a flow's fraction were inflated by packet size (byte-mode dropping), "a flow's fraction
of the packet drops is then a good indication of that flow's fraction of the packet drops is then a good indication of that flow's fraction
of the link bandwidth in bits per second". This was consistent with of the link bandwidth in bits per second". This was consistent with
a referenced policing mechanism being worked on at the time for a referenced policing mechanism being worked on at the time for
detecting unusually high bandwidth flows, eventually published in detecting unusually high bandwidth flows, eventually published in
1999 [pBox]. However, the problem could and should have been solved 1999 [pBox]. However, the problem could and should have been solved
by making the policing mechanism count the volume of bytes randomly by making the policing mechanism count the volume of bytes randomly
dropped, not the number of packets. dropped, not the number of packets.
A few months before RFC2309 was published, an addendum was added to A few months before RFC 2309 was published, an addendum was added to
the above archived email referenced from the RFC, in which the final the above archived email referenced from the RFC, in which the final
paragraph seemed to partially retract what had previously been said. paragraph seemed to partially retract what had previously been said.
It clarified that the question of whether the probability of It clarified that the question of whether the probability of
dropping/marking a packet should depend on its size was not related dropping/marking a packet should depend on its size was not related
to whether the resource itself was bit congestible, but a completely to whether the resource itself was bit-congestible, but a completely
orthogonal question. However the only example given had the queue orthogonal question. However, the only example given had the queue
measured in packets but packet drop depended on the size of the measured in packets but packet drop depended on the size of the
packet in question. No example was given the other way round. packet in question. No example was given the other way round.
In 2000, Cnodder et al [REDbyte] pointed out that there was an error In 2000, Cnodder et al. [REDbyte] pointed out that there was an error
in the part of the original 1993 RED algorithm that aimed to in the part of the original 1993 RED algorithm that aimed to
distribute drops uniformly, because it didn't correctly take into distribute drops uniformly, because it didn't correctly take into
account the adjustment for packet size. They recommended an account the adjustment for packet size. They recommended an
algorithm called RED_4 to fix this. But they also recommended a algorithm called RED_4 to fix this. But they also recommended a
further change, RED_5, to adjust drop rate dependent on the square of further change, RED_5, to adjust the drop rate dependent on the
relative packet size. This was indeed consistent with one implied square of the relative packet size. This was indeed consistent with
motivation behind RED's byte mode drop--that we should reverse one implied motivation behind RED's byte-mode drop -- that we should
engineer the network to improve the performance of dominant end-to- reverse engineer the network to improve the performance of dominant
end congestion control mechanisms. This memo makes a different end-to-end congestion control mechanisms. This memo makes a
recommendations in Section 2. different recommendations in Section 2.
By 2003, a further change had been made to the adjustment for packet By 2003, a further change had been made to the adjustment for packet
size, this time in the RED algorithm of the ns2 simulator. Instead size, this time in the RED algorithm of the ns2 simulator. Instead
of taking each packet's size relative to a `maximum packet size' it of taking each packet's size relative to a 'maximum packet size', it
was taken relative to a `mean packet size', intended to be a static was taken relative to a 'mean packet size', intended to be a static
value representative of the `typical' packet size on the link. We value representative of the 'typical' packet size on the link. We
have not been able to find a justification in the literature for this have not been able to find a justification in the literature for this
change, however Eddy and Allman conducted experiments [REDbias] that change; however, Eddy and Allman conducted experiments [REDbias] that
assessed how sensitive RED was to this parameter, amongst other assessed how sensitive RED was to this parameter, amongst other
things. However, this changed algorithm can often lead to drop things. This changed algorithm can often lead to drop probabilities
probabilities of greater than 1 (which gives a hint that there is of greater than 1 (which gives a hint that there is probably a
probably a mistake in the theory somewhere). mistake in the theory somewhere).
On 10-Nov-2004, this variant of byte-mode packet drop was made the On 10-Nov-2004, this variant of byte-mode packet drop was made the
default in the ns2 simulator. It seems unlikely that byte-mode drop default in the ns2 simulator. It seems unlikely that byte-mode drop
has ever been implemented in production networks (Appendix A), has ever been implemented in production networks (Appendix A);
therefore any conclusions based on ns2 simulations that use RED therefore, any conclusions based on ns2 simulations that use RED
without disabling byte-mode drop are likely to behave very without disabling byte-mode drop are likely to behave very
differently from RED in production networks. differently from RED in production networks.
4.2.1.2. Packet Size Bias Regardless of AQM 4.2.1.2. Packet-Size Bias Regardless of AQM
The byte-mode drop variant of RED (or a similar variant of other AQM The byte-mode drop variant of RED (or a similar variant of other AQM
algorithms) is not the only possible bias towards small packets in algorithms) is not the only possible bias towards small packets in
queueing systems. We have already mentioned that tail-drop queues queuing systems. We have already mentioned that tail-drop queues
naturally tend to lock-out large packets once they are full. naturally tend to lock out large packets once they are full.
But also queues with fixed sized buffers reduce the probability that But also, queues with fixed-size buffers reduce the probability that
small packets will be dropped if (and only if) they allow small small packets will be dropped if (and only if) they allow small
packets to borrow buffers from the pools for larger packets (see packets to borrow buffers from the pools for larger packets (see
Section 4.1.1). Borrowing effectively makes the maximum queue size Section 4.1.1). Borrowing effectively makes the maximum queue size
for small packets greater than that for large packets, because more for small packets greater than that for large packets, because more
buffers can be used by small packets while less will fit large buffers can be used by small packets while less will fit large
packets. Incidentally, the bias towards small packets from buffer packets. Incidentally, the bias towards small packets from buffer
borrowing is nothing like as large as that of RED's byte-mode drop. borrowing is nothing like as large as that of RED's byte-mode drop.
Nonetheless, fixed-buffer memory with tail drop is still prone to Nonetheless, fixed-buffer memory with tail drop is still prone to
lock-out large packets, purely because of the tail-drop aspect. So, lock out large packets, purely because of the tail-drop aspect. So,
fixed size packet-buffers should be augmented with a good AQM fixed-size packet buffers should be augmented with a good AQM
algorithm and packet-mode drop. If an AQM is too complicated to algorithm and packet-mode drop. If an AQM is too complicated to
implement with multiple fixed buffer pools, the minimum necessary to implement with multiple fixed buffer pools, the minimum necessary to
prevent large packet lock-out is to ensure smaller packets never use prevent large-packet lockout is to ensure that smaller packets never
the last available buffer in any of the pools for larger packets. use the last available buffer in any of the pools for larger packets.
4.2.2. Transport Bias when Decoding 4.2.2. Transport Bias When Decoding
The above proposals to alter the network equipment to bias towards The above proposals to alter the network equipment to bias towards
smaller packets have largely carried on outside the IETF process. smaller packets have largely carried on outside the IETF process.
Whereas, within the IETF, there are many different proposals to alter Whereas, within the IETF, there are many different proposals to alter
transport protocols to achieve the same goals, i.e. either to make transport protocols to achieve the same goals, i.e., either to make
the flow bit-rate take account of packet size, or to protect control the flow bit rate take into account packet size, or to protect
packets from loss. This memo argues that altering transport control packets from loss. This memo argues that altering transport
protocols is the more principled approach. protocols is the more principled approach.
A recently approved experimental RFC adapts its transport layer A recently approved experimental RFC adapts its transport-layer
protocol to take account of packet sizes relative to typical TCP protocol to take into account packet sizes relative to typical TCP
packet sizes. This proposes a new small-packet variant of TCP- packet sizes. This proposes a new small-packet variant of TCP-
friendly rate control [RFC5348] called TFRC-SP [RFC4828]. friendly rate control (TFRC [RFC5348]), which is called TFRC-SP
Essentially, it proposes a rate equation that inflates the flow rate [RFC4828]. Essentially, it proposes a rate equation that inflates
by the ratio of a typical TCP segment size (1500B including TCP the flow rate by the ratio of a typical TCP segment size (1,500 B
header) over the actual segment size [PktSizeEquCC]. (There are also including TCP header) over the actual segment size [PktSizeEquCC].
other important differences of detail relative to TFRC, such as using (There are also other important differences of detail relative to
virtual packets [CCvarPktSize] to avoid responding to multiple losses TFRC, such as using virtual packets [CCvarPktSize] to avoid
per round trip and using a minimum inter-packet interval.) responding to multiple losses per round trip and using a minimum
inter-packet interval.)
Section 4.5.1 of this TFRC-SP spec discusses the implications of Section 4.5.1 of the TFRC-SP specification discusses the implications
operating in an environment where queues have been configured to drop of operating in an environment where queues have been configured to
smaller packets with proportionately lower probability than larger drop smaller packets with proportionately lower probability than
ones. But it only discusses TCP operating in such an environment, larger ones. But it only discusses TCP operating in such an
only mentioning TFRC-SP briefly when discussing how to define environment, only mentioning TFRC-SP briefly when discussing how to
fairness with TCP. And it only discusses the byte-mode dropping define fairness with TCP. And it only discusses the byte-mode
version of RED as it was before Cnodder et al pointed out it didn't dropping version of RED as it was before Cnodder et al. pointed out
sufficiently bias towards small packets to make TCP independent of that it didn't sufficiently bias towards small packets to make TCP
packet size. independent of packet size.
So the TFRC-SP spec doesn't address the issue of which of the network So the TFRC-SP specification doesn't address the issue of whether the
or the transport _should_ handle fairness between different packet network or the transport _should_ handle fairness between different
sizes. In its Appendix B.4 it discusses the possibility of both packet sizes. In Appendix B.4 of RFC 4828, it discusses the
TFRC-SP and some network buffers duplicating each other's attempts to possibility of both TFRC-SP and some network buffers duplicating each
deliberately bias towards small packets. But the discussion is not other's attempts to deliberately bias towards small packets. But the
conclusive, instead reporting simulations of many of the discussion is not conclusive, instead reporting simulations of many
possibilities in order to assess performance but not recommending any of the possibilities in order to assess performance but not
particular course of action. recommending any particular course of action.
The paper originally proposing TFRC with virtual packets (VP-TFRC) The paper originally proposing TFRC with virtual packets (VP-TFRC)
[CCvarPktSize] proposed that there should perhaps be two variants to [CCvarPktSize] proposed that there should perhaps be two variants to
cater for the different variants of RED. However, as the TFRC-SP cater for the different variants of RED. However, as the TFRC-SP
authors point out, there is no way for a transport to know whether authors point out, there is no way for a transport to know whether
some queues on its path have deployed RED with byte-mode packet drop some queues on its path have deployed RED with byte-mode packet drop
(except if an exhaustive survey found that no-one has deployed it!-- (except if an exhaustive survey found that no one has deployed it! --
see Appendix A). Incidentally, VP-TFRC also proposed that byte-mode see Appendix A). Incidentally, VP-TFRC also proposed that byte-mode
RED dropping should really square the packet-size compensation-factor RED dropping should really square the packet-size compensation factor
(like that of Cnodder's RED_5, but apparently unaware of it). (like that of Cnodder's RED_5, but apparently unaware of it).
Pre-congestion notification [RFC5670] is an IETF technology to use a Pre-congestion notification [RFC5670] is an IETF technology to use a
virtual queue for AQM marking for packets within one Diffserv class virtual queue for AQM marking for packets within one Diffserv class
in order to give early warning prior to any real queuing. The PCN in order to give early warning prior to any real queuing. The PCN-
marking algorithms have been designed not to take account of packet marking algorithms have been designed not to take into account packet
size when forwarding through queues. Instead the general principle size when forwarding through queues. Instead, the general principle
has been to take account of the sizes of marked packets when has been to take the sizes of marked packets into account when
monitoring the fraction of marking at the edge of the network, as monitoring the fraction of marking at the edge of the network, as
recommended here. recommended here.
4.2.3. Making Transports Robust against Control Packet Losses 4.2.3. Making Transports Robust against Control Packet Losses
Recently, two RFCs have defined changes to TCP that make it more Recently, two RFCs have defined changes to TCP that make it more
robust against losing small control packets [RFC5562] [RFC5690]. In robust against losing small control packets [RFC5562] [RFC5690]. In
both cases they note that the case for these two TCP changes would be both cases, they note that the case for these two TCP changes would
weaker if RED were biased against dropping small packets. We argue be weaker if RED were biased against dropping small packets. We
here that these two proposals are a safer and more principled way to argue here that these two proposals are a safer and more principled
achieve TCP performance improvements than reverse engineering RED to way to achieve TCP performance improvements than reverse engineering
benefit TCP. RED to benefit TCP.
Although there are no known proposals, it would also be possible and Although there are no known proposals, it would also be possible and
perfectly valid to make control packets robust against drop by perfectly valid to make control packets robust against drop by
requesting a scheduling class with lower drop probability, by re- requesting a scheduling class with lower drop probability, which
marking to a Diffserv code point [RFC2474] within the same behaviour would be achieved by re-marking to a Diffserv code point [RFC2474]
aggregate. within the same behaviour aggregate.
Although not brought to the IETF, a simple proposal from Wischik Although not brought to the IETF, a simple proposal from Wischik
[DupTCP] suggests that the first three packets of every TCP flow [DupTCP] suggests that the first three packets of every TCP flow
should be routinely duplicated after a short delay. It shows that should be routinely duplicated after a short delay. It shows that
this would greatly improve the chances of short flows completing this would greatly improve the chances of short flows completing
quickly, but it would hardly increase traffic levels on the Internet, quickly, but it would hardly increase traffic levels on the Internet,
because Internet bytes have always been concentrated in the large because Internet bytes have always been concentrated in the large
flows. It further shows that the performance of many typical flows. It further shows that the performance of many typical
applications depends on completion of long serial chains of short applications depends on completion of long serial chains of short
messages. It argues that, given most of the value people get from messages. It argues that, given most of the value people get from
the Internet is concentrated within short flows, this simple the Internet is concentrated within short flows, this simple
expedient would greatly increase the value of the best efforts expedient would greatly increase the value of the best-effort
Internet at minimal cost. A similar but more extensive approach has Internet at minimal cost. A similar but more extensive approach has
been evaluated on Google servers [GentleAggro]. been evaluated on Google servers [GentleAggro].
The proposals discussed in this sub-section are experimental The proposals discussed in this sub-section are experimental
approaches that are not yet in wide operational use, but they are approaches that are not yet in wide operational use, but they are
existence proofs that transports can make themselves robust against existence proofs that transports can make themselves robust against
loss of control packets. The examples are all TCP-based, but loss of control packets. The examples are all TCP-based, but
applications over non-TCP transports could mitigate loss of control applications over non-TCP transports could mitigate loss of control
packets by making similar use of Diffserv, data duplication, FEC etc. packets by making similar use of Diffserv, data duplication, FEC,
etc.
4.2.4. Congestion Notification: Summary of Conflicting Advice 4.2.4. Congestion Notification: Summary of Conflicting Advice
+-----------+----------------+-----------------+--------------------+ +-----------+-----------------+-----------------+-------------------+
| transport | RED_1 (packet | RED_4 (linear | RED_5 (square byte | | transport | RED_1 (packet- | RED_4 (linear | RED_5 (square |
| cc | mode drop) | byte mode drop) | mode drop) | | cc | mode drop) | byte-mode drop) | byte-mode drop) |
+-----------+----------------+-----------------+--------------------+ +-----------+-----------------+-----------------+-------------------+
| TCP or | s/sqrt(p) | sqrt(s/p) | 1/sqrt(p) | | TCP or | s/sqrt(p) | sqrt(s/p) | 1/sqrt(p) |
| TFRC | | | | | TFRC | | | |
| TFRC-SP | 1/sqrt(p) | 1/sqrt(sp) | 1/(s.sqrt(p)) | | TFRC-SP | 1/sqrt(p) | 1/sqrt(s*p) | 1/(s*sqrt(p)) |
+-----------+----------------+-----------------+--------------------+ +-----------+-----------------+-----------------+-------------------+
Table 2: Dependence of flow bit-rate per RTT on packet size, s, and Table 2: Dependence of flow bit rate per RTT on packet size, s, and
drop probability, p, when network and/or transport bias towards small drop probability, p, when there is network and/or transport bias
packets to varying degrees towards small packets to varying degrees
Table 2 aims to summarise the potential effects of all the advice Table 2 aims to summarise the potential effects of all the advice
from different sources. Each column shows a different possible AQM from different sources. Each column shows a different possible AQM
behaviour in different queues in the network, using the terminology behaviour in different queues in the network, using the terminology
of Cnodder et al outlined earlier (RED_1 is basic RED with packet- of Cnodder et al. outlined earlier (RED_1 is basic RED with packet-
mode drop). Each row shows a different transport behaviour: TCP mode drop). Each row shows a different transport behaviour: TCP
[RFC5681] and TFRC [RFC5348] on the top row with TFRC-SP [RFC4828] [RFC5681] and TFRC [RFC5348] on the top row with TFRC-SP [RFC4828]
below. Each cell shows how the bits per round trip of a flow depends below. Each cell shows how the bits per round trip of a flow depends
on packet size, s, and drop probability, p. In order to declutter on packet size, s, and drop probability, p. In order to declutter
the formulae to focus on packet-size dependence they are all given the formulae to focus on packet-size dependence, they are all given
per round trip, which removes any RTT term. per round trip, which removes any RTT term.
Let us assume that the goal is for the bit-rate of a flow to be Let us assume that the goal is for the bit rate of a flow to be
independent of packet size. Suppressing all inessential details, the independent of packet size. Suppressing all inessential details, the
table shows that this should either be achievable by not altering the table shows that this should either be achievable by not altering the
TCP transport in a RED_5 network, or using the small packet TFRC-SP TCP transport in a RED_5 network, or using the small packet TFRC-SP
transport (or similar) in a network without any byte-mode dropping transport (or similar) in a network without any byte-mode dropping
RED (top right and bottom left). Top left is the `do nothing' RED (top right and bottom left). Top left is the 'do nothing'
scenario, while bottom right is the `do-both' scenario in which bit- scenario, while bottom right is the 'do both' scenario in which the
rate would become far too biased towards small packets. Of course, bit rate would become far too biased towards small packets. Of
if any form of byte-mode dropping RED has been deployed on a subset course, if any form of byte-mode dropping RED has been deployed on a
of queues that congest, each path through the network will present a subset of queues that congest, each path through the network will
different hybrid scenario to its transport. present a different hybrid scenario to its transport.
Whatever, we can see that the linear byte-mode drop column in the Whatever the case, we can see that the linear byte-mode drop column
middle would considerably complicate the Internet. It's a half-way in the middle would considerably complicate the Internet. Even if
house that doesn't bias enough towards small packets even if one one believes the network should be doing the biasing, linear byte-
believes the network should be doing the biasing. Section 2 mode drop is a half-way house that doesn't bias enough towards small
recommends that _all_ bias in network equipment towards small packets packets. Section 2 recommends that _all_ bias in network equipment
should be turned off--if indeed any equipment vendors have towards small packets should be turned off -- if indeed any equipment
implemented it--leaving packet-size bias solely as the preserve of vendors have implemented it -- leaving packet-size bias solely as the
the transport layer (solely the leftmost, packet-mode drop column). preserve of the transport layer (solely the leftmost, packet-mode
drop column).
In practice it seems that no deliberate bias towards small packets In practice, it seems that no deliberate bias towards small packets
has been implemented for production networks. Of the 19% of vendors has been implemented for production networks. Of the 19% of vendors
who responded to a survey of 84 equipment vendors, none had who responded to a survey of 84 equipment vendors, none had
implemented byte-mode drop in RED (see Appendix A for details). implemented byte-mode drop in RED (see Appendix A for details).
5. Outstanding Issues and Next Steps 5. Outstanding Issues and Next Steps
5.1. Bit-congestible Network 5.1. Bit-congestible Network
For a connectionless network with nearly all resources being bit- For a connectionless network with nearly all resources being bit-
congestible the recommended position is clear--that the network congestible, the recommended position is clear -- the network should
should not make allowance for packet sizes and the transport should. not make allowance for packet sizes and the transport should. This
This leaves two outstanding issues: leaves two outstanding issues:
o How to handle any legacy of AQM with byte-mode drop already o The question of how to handle any legacy AQM deployments using
deployed; byte-mode drop;
o The need to start a programme to update transport congestion o The need to start a programme to update transport congestion
control protocol standards to take account of packet size. control protocol standards to take packet size into account.
A survey of equipment vendors (Section 4.2.4) found no evidence that A survey of equipment vendors (Section 4.2.4) found no evidence that
byte-mode packet drop had been implemented, so deployment will be byte-mode packet drop had been implemented, so deployment will be
sparse at best. A migration strategy is not really needed to remove sparse at best. A migration strategy is not really needed to remove
an algorithm that may not even be deployed. an algorithm that may not even be deployed.
A programme of experimental updates to take account of packet size in A programme of experimental updates to take packet size into account
transport congestion control protocols has already started with in transport congestion control protocols has already started with
TFRC-SP [RFC4828]. TFRC-SP [RFC4828].
5.2. Bit- & Packet-congestible Network 5.2. Bit- and Packet-Congestible Network
The position is much less clear-cut if the Internet becomes populated The position is much less clear-cut if the Internet becomes populated
by a more even mix of both packet-congestible and bit-congestible by a more even mix of both packet-congestible and bit-congestible
resources (see Appendix B.2). This problem is not pressing, because resources (see Appendix B.2). This problem is not pressing, because
most Internet resources are designed to be bit-congestible before most Internet resources are designed to be bit-congestible before
packet processing starts to congest (see Section 1.1). packet processing starts to congest (see Section 1.1).
The IRTF Internet congestion control research group (ICCRG) has set The IRTF's Internet Congestion Control Research Group (ICCRG) has set
itself the task of reaching consensus on generic forwarding itself the task of reaching consensus on generic forwarding
mechanisms that are necessary and sufficient to support the mechanisms that are necessary and sufficient to support the
Internet's future congestion control requirements (the first Internet's future congestion control requirements (the first
challenge in [RFC6077]). The research question of whether packet challenge in [RFC6077]). The research question of whether packet
congestion might become common and what to do if it does may in the congestion might become common and what to do if it does may in the
future be explored in the IRTF (the "Challenge 3: Packet Size" in future be explored in the IRTF (the "Challenge 3: Packet Size" in
[RFC6077]). [RFC6077]).
Note that sometimes it seems that resources might be congested by Note that sometimes it seems that resources might be congested by
neither bits nor packets, e.g. where the queue for access to a neither bits nor packets, e.g., where the queue for access to a
wireless medium is in units of transmission opportunities. However, wireless medium is in units of transmission opportunities. However,
the root cause of congestion of the underlying spectrum is overload the root cause of congestion of the underlying spectrum is overload
of bits (see Section 4.1.2). of bits (see Section 4.1.2).
6. Security Considerations 6. Security Considerations
This memo recommends that queues do not bias drop probability due to This memo recommends that queues do not bias drop probability due to
packets size. For instance dropping small packets less often than packets size. For instance, dropping small packets less often than
large creates a perverse incentive for transports to break down their large ones creates a perverse incentive for transports to break down
flows into tiny segments. One of the benefits of implementing AQM their flows into tiny segments. One of the benefits of implementing
was meant to be to remove this perverse incentive that drop-tail AQM was meant to be to remove this perverse incentive that tail-drop
queues gave to small packets. queues gave to small packets.
In practice, transports cannot all be trusted to respond to In practice, transports cannot all be trusted to respond to
congestion. So another reason for recommending that queues do not congestion. So another reason for recommending that queues not bias
bias drop probability towards small packets is to avoid the drop probability towards small packets is to avoid the vulnerability
vulnerability to small packet DDoS attacks that would otherwise to small-packet DDoS attacks that would otherwise result. One of the
result. One of the benefits of implementing AQM was meant to be to benefits of implementing AQM was meant to be to remove tail drop's
remove drop-tail's DoS vulnerability to small packets, so we DoS vulnerability to small packets, so we shouldn't add it back
shouldn't add it back again. again.
If most queues implemented AQM with byte-mode drop, the resulting If most queues implemented AQM with byte-mode drop, the resulting
network would amplify the potency of a small packet DDoS attack. At network would amplify the potency of a small-packet DDoS attack. At
the first queue the stream of packets would push aside a greater the first queue, the stream of packets would push aside a greater
proportion of large packets, so more of the small packets would proportion of large packets, so more of the small packets would
survive to attack the next queue. Thus a flood of small packets survive to attack the next queue. Thus a flood of small packets
would continue on towards the destination, pushing regular traffic would continue on towards the destination, pushing regular traffic
with large packets out of the way in one queue after the next, but with large packets out of the way in one queue after the next, but
suffering much less drop itself. suffering much less drop itself.
Appendix C explains why the ability of networks to police the Appendix C explains why the ability of networks to police the
response of _any_ transport to congestion depends on bit-congestible response of _any_ transport to congestion depends on bit-congestible
network resources only doing packet-mode not byte-mode drop. In network resources only doing packet-mode drop, not byte-mode drop.
summary, it says that making drop probability depend on the size of In summary, it says that making drop probability depend on the size
the packets that bits happen to be divided into simply encourages the of the packets that bits happen to be divided into simply encourages
bits to be divided into smaller packets. Byte-mode drop would the bits to be divided into smaller packets. Byte-mode drop would
therefore irreversibly complicate any attempt to fix the Internet's therefore irreversibly complicate any attempt to fix the Internet's
incentive structures. incentive structures.
7. IANA Considerations 7. Conclusions
This document has no actions for IANA.
8. Conclusions
This memo identifies the three distinct stages of the congestion This memo identifies the three distinct stages of the congestion
notification process where implementations need to decide whether to notification process where implementations need to decide whether to
take packet size into account. The recommendations provided in take packet size into account. The recommendations provided in
Section 2 of this memo are different in each case: Section 2 of this memo are different in each case:
o When network equipment measures the length of a queue, if it is o When network equipment measures the length of a queue, if it is
not feasible to use time it is recommended to count in bytes if not feasible to use time; it is recommended to count in bytes if
the network resource is congested by bytes, or to count in packets the network resource is congested by bytes, or to count in packets
if is congested by packets. if is congested by packets.
o When network equipment decides whether to drop (or mark) a packet, o When network equipment decides whether to drop (or mark) a packet,
it is recommended that the size of the particular packet should it is recommended that the size of the particular packet should
not be taken into account not be taken into account.
o However, when a transport algorithm responds to a dropped or o However, when a transport algorithm responds to a dropped or
marked packet, the size of the rate reduction should be marked packet, the size of the rate reduction should be
proportionate to the size of the packet. proportionate to the size of the packet.
In summary, the answers are 'it depends', 'no' and 'yes' respectively In summary, the answers are 'it depends', 'no', and 'yes',
respectively.
For the specific case of RED, this means that byte-mode queue For the specific case of RED, this means that byte-mode queue
measurement will often be appropriate but the use of byte-mode drop measurement will often be appropriate, but the use of byte-mode drop
is very strongly discouraged. is very strongly discouraged.
At the transport layer the IETF should continue updating congestion At the transport layer, the IETF should continue updating congestion
control protocols to take account of the size of each packet that control protocols to take into account the size of each packet that
indicates congestion. Also the IETF should continue to make indicates congestion. Also, the IETF should continue to make
protocols less sensitive to losing control packets like SYNs, pure protocols less sensitive to losing control packets like SYNs, pure
ACKs and DNS exchanges. Although many control packets happen to be ACKs, and DNS exchanges. Although many control packets happen to be
small, the alternative of network equipment favouring all small small, the alternative of network equipment favouring all small
packets would be dangerous. That would create perverse incentives to packets would be dangerous. That would create perverse incentives to
split data transfers into smaller packets. split data transfers into smaller packets.
The memo develops these recommendations from principled arguments The memo develops these recommendations from principled arguments
concerning scaling, layering, incentives, inherent efficiency, concerning scaling, layering, incentives, inherent efficiency,
security and policeability. But it also addresses practical issues security, and 'policeability'. It also addresses practical issues
such as specific buffer architectures and incremental deployment. such as specific buffer architectures and incremental deployment.
Indeed a limited survey of RED implementations is discussed, which Indeed, a limited survey of RED implementations is discussed, which
shows there appears to be little, if any, installed base of RED's shows there appears to be little, if any, installed base of RED's
byte-mode drop. Therefore it can be deprecated with little, if any, byte-mode drop. Therefore, it can be deprecated with little, if any,
incremental deployment complications. incremental deployment complications.
The recommendations have been developed on the well-founded basis The recommendations have been developed on the well-founded basis
that most Internet resources are bit-congestible not packet- that most Internet resources are bit-congestible, not packet-
congestible. We need to know the likelihood that this assumption congestible. We need to know the likelihood that this assumption
will prevail longer term and, if it might not, what protocol changes will prevail in the longer term and, if it might not, what protocol
will be needed to cater for a mix of the two. The IRTF Internet changes will be needed to cater for a mix of the two. The IRTF
Congestion Control Research Group (ICCRG) is currently working on Internet Congestion Control Research Group (ICCRG) is currently
these problems [RFC6077]. working on these problems [RFC6077].
9. Acknowledgements 8. Acknowledgements
Thank you to Sally Floyd, who gave extensive and useful review Thank you to Sally Floyd, who gave extensive and useful review
comments. Also thanks for the reviews from Philip Eardley, David comments. Also thanks for the reviews from Philip Eardley, David
Black, Fred Baker, David Taht, Toby Moncaster, Arnaud Jacquet and Black, Fred Baker, David Taht, Toby Moncaster, Arnaud Jacquet, and
Mirja Kuehlewind as well as helpful explanations of different Mirja Kuehlewind, as well as helpful explanations of different
hardware approaches from Larry Dunn and Fred Baker. We are grateful hardware approaches from Larry Dunn and Fred Baker. We are grateful
to Bruce Davie and his colleagues for providing a timely and to Bruce Davie and his colleagues for providing a timely and
efficient survey of RED implementation in Cisco's product range. efficient survey of RED implementation in Cisco's product range.
Also grateful thanks to Toby Moncaster, Will Dormann, John Regnault, Also, grateful thanks to Toby Moncaster, Will Dormann, John Regnault,
Simon Carter and Stefaan De Cnodder who further helped survey the Simon Carter, and Stefaan De Cnodder who further helped survey the
current status of RED implementation and deployment and, finally, current status of RED implementation and deployment, and, finally,
thanks to the anonymous individuals who responded. thanks to the anonymous individuals who responded.
Bob Briscoe and Jukka Manner were partly funded by Trilogy, a Bob Briscoe and Jukka Manner were partly funded by Trilogy and
research project (ICT- 216372) supported by the European Community Trilogy 2, research projects (ICT-216372, ICT-317756) supported by
under its Seventh Framework Programme. The views expressed here are the European Community under its Seventh Framework Programme. The
those of the authors only. views expressed here are those of the authors only.
10. Comments Solicited
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF Transport Area working group mailing list
<tsvwg@ietf.org>, and/or to the authors.
11. References 9. References
11.1. Normative References 9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Indicate Requirement Levels", BCP 14, Requirement Levels", BCP 14, RFC 2119, March 1997.
RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, [RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
"The Addition of Explicit Congestion S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Notification (ECN) to IP", RFC 3168, Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
September 2001. S., Wroclawski, J., and L. Zhang, "Recommendations on
Queue Management and Congestion Avoidance in the
Internet", RFC 2309, April 1998.
11.2. Informative References [RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC
2914, September 2000.
[BLUE02] Feng, W-c., Shin, K., Kandlur, D., and D. [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
Saha, "The BLUE active queue management of Explicit Congestion Notification (ECN) to IP", RFC
algorithms", IEEE/ACM Transactions on 3168, September 2001.
Networking 10(4) 513--528, August 2002, <h
ttp://dx.doi.org/10.1109/
TNET.2002.801399>.
[CCvarPktSize] Widmer, J., Boutremans, C., and J-Y. Le 9.2. Informative References
Boudec, "Congestion Control for Flows with
Variable Packet Size", ACM CCR 34(2) 137--
151, 2004,
<http://doi.acm.org/10.1145/
997150.997162>.
[CHOKe_Var_Pkt] Psounis, K., Pan, R., and B. Prabhaker, [BLUE02] Feng, W-c., Shin, K., Kandlur, D., and D. Saha, "The BLUE
"Approximate Fair Dropping for Variable active queue management algorithms", IEEE/ACM Transactions
Length Packets", IEEE Micro 21(1):48--56, on Networking 10(4) 513-528, August 2002,
January-February 2001, <http:// <http://dx.doi.org/10.1109/TNET.2002.801399>.
www.stanford.edu/~balaji/papers/
01approximatefair.pdf}>.
[DRQ] Shin, M., Chong, S., and I. Rhee, "Dual- [CCvarPktSize]
Resource TCP/AQM for Processing- Widmer, J., Boutremans, C., and J-Y. Le Boudec, "End-to-
Constrained Networks", IEEE/ACM end congestion control for TCP-friendly flows with
Transactions on Networking Vol 16, issue variable packet size", ACM CCR 34(2) 137-151, April 2004,
2, April 2008, <http://dx.doi.org/10.1109/ <http://doi.acm.org/10.1145/997150.997162>.
TNET.2007.900415>.
[DupTCP] Wischik, D., "Short messages", [CHOKe_Var_Pkt]
Philosphical Transactions of the Royal Psounis, K., Pan, R., and B. Prabhaker, "Approximate Fair
Society A 366(1872):1941-1953, June 2008, Dropping for Variable-Length Packets", IEEE Micro
<http://rsta.royalsocietypublishing.org/ 21(1):48-56, January-February 2001,
content/366/1872/1941.full.pdf+html>. <http://ieeexplore.ieee.org/xpl/
articleDetails.jsp?arnumber=903061>.
[ECNFixedWireless] Siris, V., "Resource Control for Elastic [CoDel] Nichols, K. and V. Jacobson, "Controlled Delay Active
Traffic in CDMA Networks", Proc. ACM Queue Management", Work in Progress, February 2013.
MOBICOM'02 , September 2002, <http://
www.ics.forth.gr/netlab/publications/
resource_control_elastic_cdma.html>.
[Evol_cc] Gibbens, R. and F. Kelly, "Resource [DRQ] Shin, M., Chong, S., and I. Rhee, "Dual-Resource TCP/AQM
pricing and the evolution of congestion for Processing-Constrained Networks", IEEE/ACM
control", Automatica 35(12)1969--1985, Transactions on Networking Vol 16, issue 2, April 2008,
December 1999, <http:// <http://dx.doi.org/10.1109/TNET.2007.900415>.
www.statslab.cam.ac.uk/~frank/evol.html>.
[GentleAggro] Flach, T., Dukkipati, N., Terzis, A., [DupTCP] Wischik, D., "Short messages", Philosophical Transactions
Raghavan, B., Cardwell, N., Cheng, Y., of the Royal Society A 366(1872):1941-1953, June 2008,
Jain, A., Hao, S., Katz-Bassett, E., and <http://rsta.royalsocietypublishing.org/content/366/1872/
R. Govindan, "Reducing Web Latency: the 1941.full.pdf+html>.
Virtue of Gentle Aggression", ACM SIGCOMM
CCR 43(4)159--170, August 2013, <http://
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[I-D.nichols-tsvwg-codel] Nichols, K. and V. Jacobson, "Controlled [ECNFixedWireless]
Delay Active Queue Management", Siris, V., "Resource Control for Elastic Traffic in CDMA
draft-nichols-tsvwg-codel-01 (work in Networks", Proc. ACM MOBICOM'02 , September 2002,
progress), February 2013. <http://www.ics.forth.gr/netlab/publications/
resource_control_elastic_cdma.html>.
[I-D.pan-tsvwg-pie] Pan, R., Natarajan, P., Piglione, C., and [Evol_cc] Gibbens, R. and F. Kelly, "Resource pricing and the
M. Prabhu, "PIE: A Lightweight Control evolution of congestion control", Automatica
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progress), December 2012. S0005109899001351>.
[IOSArch] Bollapragada, V., White, R., and C. [GentleAggro]
Murphy, "Inside Cisco IOS Software Flach, T., Dukkipati, N., Terzis, A., Raghavan, B.,
Architecture", Cisco Press: CCIE Cardwell, N., Cheng, Y., Jain, A., Hao, S., Katz-Bassett,
Professional Development ISBN13: 978-1- E., and R. Govindan, "Reducing web latency: the virtue of
57870-181-0, July 2000. gentle aggression", ACM SIGCOMM CCR 43(4)159-170, August
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[PktSizeEquCC] Vasallo, P., "Variable Packet Size [IOSArch] Bollapragada, V., White, R., and C. Murphy, "Inside Cisco
Equation-Based Congestion Control", ICSI IOS Software Architecture", Cisco Press: CCIE Professional
Technical Report tr-00-008, 2000, <http:// Development ISBN13: 978-1-57870-181-0, July 2000.
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[RED93] Floyd, S. and V. Jacobson, "Random Early [PIE] Pan, R., Natarajan, P., Piglione, C., Prabhu, M.,
Detection (RED) gateways for Congestion Subramanian, V., Baker, F., and B. Steeg, "PIE: A
Avoidance", IEEE/ACM Transactions on Lightweight Control Scheme To Address the Bufferbloat
Networking 1(4) 397--413, August 1993, <ht Problem", Work in Progress, February 2014.
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red.html>.
[REDbias] Eddy, W. and M. Allman, "A Comparison of [PktSizeEquCC]
RED's Byte and Packet Modes", Computer Vasallo, P., "Variable Packet Size Equation-Based
Networks 42(3) 261--280, June 2003, <http: Congestion Control", ICSI Technical Report tr-00-008,
//www.ir.bbn.com/documents/articles/ 2000, <http://http.icsi.berkeley.edu/ftp/global/pub/
redbias.ps>. techreports/2000/tr-00-008.pdf>.
[REDbyte] De Cnodder, S., Elloumi, O., and K. [RED93] Floyd, S. and V. Jacobson, "Random Early Detection (RED)
Pauwels, "RED behavior with different gateways for Congestion Avoidance", IEEE/ACM Transactions
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799, July 2000, <http://www.icir.org/ abs_all.jsp?arnumber=251892>.
floyd/red/Elloumi99.pdf>.
[RFC2309] Braden, B., Clark, D., Crowcroft, J., [REDbias] Eddy, W. and M. Allman, "A Comparison of RED's Byte and
Davie, B., Deering, S., Estrin, D., Floyd, Packet Modes", Computer Networks 42(3) 261--280, June
S., Jacobson, V., Minshall, G., Partridge, 2003,
C., Peterson, L., Ramakrishnan, K., <http://www.ir.bbn.com/documents/articles/redbias.ps>.
Shenker, S., Wroclawski, J., and L. Zhang,
"Recommendations on Queue Management and
Congestion Avoidance in the Internet",
RFC 2309, April 1998.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. [REDbyte] De Cnodder, S., Elloumi, O., and K. Pauwels, "Effect of
Black, "Definition of the Differentiated different packet sizes on RED performance", Proc. 5th IEEE
Services Field (DS Field) in the IPv4 and Symposium on Computers and Communications (ISCC) 793-799,
IPv6 Headers", RFC 2474, December 1998. July 2000, <http://ieeexplore.ieee.org/xpls/
abs_all.jsp?arnumber=860741>.
[RFC2914] Floyd, S., "Congestion Control [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
Principles", BCP 41, RFC 2914, "Definition of the Differentiated Services Field (DS
September 2000. Field) in the IPv4 and IPv6 Headers", RFC 2474, December
1998.
[RFC3426] Floyd, S., "General Architectural and [RFC3426] Floyd, S., "General Architectural and Policy
Policy Considerations", RFC 3426, Considerations", RFC 3426, November 2002.
November 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
R., and V. Jacobson, "RTP: A Transport Jacobson, "RTP: A Transport Protocol for Real-Time
Protocol for Real-Time Applications", Applications", STD 64, RFC 3550, July 2003.
STD 64, RFC 3550, July 2003.
[RFC3714] Floyd, S. and J. Kempf, "IAB Concerns [RFC3714] Floyd, S. and J. Kempf, "IAB Concerns Regarding Congestion
Regarding Congestion Control for Voice Control for Voice Traffic in the Internet", RFC 3714,
Traffic in the Internet", RFC 3714, March 2004.
March 2004.
[RFC4828] Floyd, S. and E. Kohler, "TCP Friendly [RFC4828] Floyd, S. and E. Kohler, "TCP Friendly Rate Control
Rate Control (TFRC): The Small-Packet (SP) (TFRC): The Small-Packet (SP) Variant", RFC 4828, April
Variant", RFC 4828, April 2007. 2007.
[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. [RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Widmer, "TCP Friendly Rate Control (TFRC): Friendly Rate Control (TFRC): Protocol Specification", RFC
Protocol Specification", RFC 5348, 5348, September 2008.
September 2008.
[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S., and [RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S., and K.
K. Ramakrishnan, "Adding Explicit Ramakrishnan, "Adding Explicit Congestion Notification
Congestion Notification (ECN) Capability (ECN) Capability to TCP's SYN/ACK Packets", RFC 5562, June
to TCP's SYN/ACK Packets", RFC 5562, 2009.
June 2009.
[RFC5670] Eardley, P., "Metering and Marking [RFC5670] Eardley, P., "Metering and Marking Behaviour of PCN-
Behaviour of PCN-Nodes", RFC 5670, Nodes", RFC 5670, November 2009.
November 2009.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
"TCP Congestion Control", RFC 5681, Control", RFC 5681, September 2009.
September 2009.
[RFC5690] Floyd, S., Arcia, A., Ros, D., and J. [RFC5690] Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
Iyengar, "Adding Acknowledgement Acknowledgement Congestion Control to TCP", RFC 5690,
Congestion Control to TCP", RFC 5690, February 2010.
February 2010.
[RFC6077] Papadimitriou, D., Welzl, M., Scharf, M., [RFC6077] Papadimitriou, D., Welzl, M., Scharf, M., and B. Briscoe,
and B. Briscoe, "Open Research Issues in "Open Research Issues in Internet Congestion Control", RFC
Internet Congestion Control", RFC 6077, 6077, February 2011.
February 2011.
[RFC6679] Westerlund, M., Johansson, I., Perkins, [RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
C., O'Hanlon, P., and K. Carlberg, and K. Carlberg, "Explicit Congestion Notification (ECN)
"Explicit Congestion Notification (ECN) for RTP over UDP", RFC 6679, August 2012.
for RTP over UDP", RFC 6679, August 2012.
[RFC6789] Briscoe, B., Woundy, R., and A. Cooper, [RFC6789] Briscoe, B., Woundy, R., and A. Cooper, "Congestion
"Congestion Exposure (ConEx) Concepts and Exposure (ConEx) Concepts and Use Cases", RFC 6789,
Use Cases", RFC 6789, December 2012. December 2012.
[Rate_fair_Dis] Briscoe, B., "Flow Rate Fairness: [Rate_fair_Dis]
Dismantling a Religion", ACM Briscoe, B., "Flow Rate Fairness: Dismantling a Religion",
CCR 37(2)63--74, April 2007, <http:// ACM CCR 37(2)63-74, April 2007,
portal.acm.org/citation.cfm?id=1232926>. <http://portal.acm.org/citation.cfm?id=1232926>.
[gentle_RED] Floyd, S., "Recommendation on using the [gentle_RED]
"gentle_" variant of RED", Web page , Floyd, S., "Recommendation on using the "gentle_" variant
March 2000, <http://www.icir.org/floyd/ of RED", Web page , March 2000,
red/gentle.html>. <http://www.icir.org/floyd/red/gentle.html>.
[pBox] Floyd, S. and K. Fall, "Promoting the Use [pBox] Floyd, S. and K. Fall, "Promoting the Use of End-to-End
of End-to-End Congestion Control in the Congestion Control", IEEE/ACM Transactions on Networking
Internet", IEEE/ACM Transactions on 7(4) 458--472, August 1999, <http://ieeexplore.ieee.org/
Networking 7(4) 458--472, August 1999, <ht xpls/abs_all.jsp?arnumber=793002>.
tp://www.aciri.org/floyd/
end2end-paper.html>.
[pktByteEmail] Floyd, S., "RED: Discussions of Byte and [pktByteEmail]
Packet Modes", email , March 1997, <http:/ Floyd, S., "RED: Discussions of Byte and Packet Modes",
/www-nrg.ee.lbl.gov/floyd/ email, March 1997,
REDaveraging.txt>. <http://ee.lbl.gov/floyd/REDaveraging.txt>.
Appendix A. Survey of RED Implementation Status Appendix A. Survey of RED Implementation Status
This Appendix is informative, not normative. This Appendix is informative, not normative.
In May 2007 a survey was conducted of 84 vendors to assess how widely In May 2007 a survey was conducted of 84 vendors to assess how widely
drop probability based on packet size has been implemented in RED drop probability based on packet size has been implemented in RED
Table 3. About 19% of those surveyed replied, giving a sample size Table 3. About 19% of those surveyed replied, giving a sample size
of 16. Although in most cases we do not have permission to identify of 16. Although in most cases we do not have permission to identify
the respondents, we can say that those that have responded include the respondents, we can say that those that have responded include
skipping to change at page 33, line 18 skipping to change at page 33, line 26
and Alcatel-Lucent. The others range across the large network and Alcatel-Lucent. The others range across the large network
equipment vendors at L3 & L2, firewall vendors, wireless equipment equipment vendors at L3 & L2, firewall vendors, wireless equipment
vendors, as well as large software businesses with a small selection vendors, as well as large software businesses with a small selection
of networking products. All those who responded confirmed that they of networking products. All those who responded confirmed that they
have not implemented the variant of RED with drop dependent on packet have not implemented the variant of RED with drop dependent on packet
size (2 were fairly sure they had not but needed to check more size (2 were fairly sure they had not but needed to check more
thoroughly). At the time the survey was conducted, Linux did not thoroughly). At the time the survey was conducted, Linux did not
implement RED with packet-size bias of drop, although we have not implement RED with packet-size bias of drop, although we have not
investigated a wider range of open source code. investigated a wider range of open source code.
+-------------------------------+----------------+-----------------+ +-------------------------------+----------------+--------------+
| Response | No. of vendors | %age of vendors | | Response | No. of vendors | % of vendors |
+-------------------------------+----------------+-----------------+ +-------------------------------+----------------+--------------+
| Not implemented | 14 | 17% | | Not implemented | 14 | 17% |
| Not implemented (probably) | 2 | 2% | | Not implemented (probably) | 2 | 2% |
| Implemented | 0 | 0% | | Implemented | 0 | 0% |
| No response | 68 | 81% | | No response | 68 | 81% |
| Total companies/orgs surveyed | 84 | 100% | | Total companies/orgs surveyed | 84 | 100% |
+-------------------------------+----------------+-----------------+ +-------------------------------+----------------+--------------+
Table 3: Vendor Survey on byte-mode drop variant of RED (lower drop Table 3: Vendor Survey on byte-mode drop variant of RED (lower drop
probability for small packets) probability for small packets)
Where reasons have been given, the extra complexity of packet bias Where reasons were given for why the byte-mode drop variant had not
code has been most prevalent, though one vendor had a more principled been implemented, the extra complexity of packet-bias code was most
reason for avoiding it--similar to the argument of this document. prevalent, though one vendor had a more principled reason for
avoiding it -- similar to the argument of this document.
Our survey was of vendor implementations, so we cannot be certain Our survey was of vendor implementations, so we cannot be certain
about operator deployment. But we believe many queues in the about operator deployment. But we believe many queues in the
Internet are still tail-drop. The company of one of the co-authors Internet are still tail drop. The company of one of the co-authors
(BT) has widely deployed RED, but many tail-drop queues are bound to (BT) has widely deployed RED; however, many tail-drop queues are
still exist, particularly in access network equipment and on bound to still exist, particularly in access network equipment and on
middleboxes like firewalls, where RED is not always available. middleboxes like firewalls, where RED is not always available.
Routers using a memory architecture based on fixed size buffers with Routers using a memory architecture based on fixed-size buffers with
borrowing may also still be prevalent in the Internet. As explained borrowing may also still be prevalent in the Internet. As explained
in Section 4.2.1, these also provide a marginal (but legitimate) bias in Section 4.2.1, these also provide a marginal (but legitimate) bias
towards small packets. So even though RED byte-mode drop is not towards small packets. So even though RED byte-mode drop is not
prevalent, it is likely there is still some bias towards small prevalent, it is likely there is still some bias towards small
packets in the Internet due to tail drop and fixed buffer borrowing. packets in the Internet due to tail-drop and fixed-buffer borrowing.
Appendix B. Sufficiency of Packet-Mode Drop Appendix B. Sufficiency of Packet-Mode Drop
This Appendix is informative, not normative. This Appendix is informative, not normative.
Here we check that packet-mode drop (or marking) in the network gives Here we check that packet-mode drop (or marking) in the network gives
sufficiently generic information for the transport layer to use. We sufficiently generic information for the transport layer to use. We
check against a 2x2 matrix of four scenarios that may occur now or in check against a 2x2 matrix of four scenarios that may occur now or in
the future (Table 4). The horizontal and vertical dimensions have the future (Table 4). Checking the two scenarios in each of the
been chosen because each tests extremes of sensitivity to packet size horizontal and vertical dimensions tests the extremes of sensitivity
in the transport and in the network respectively. to packet size in the transport and in the network respectively.
Note that this section does not consider byte-mode drop at all. Note that this section does not consider byte-mode drop at all.
Having deprecated byte-mode drop, the goal here is to check that Having deprecated byte-mode drop, the goal here is to check that
packet-mode drop will be sufficient in all cases. packet-mode drop will be sufficient in all cases.
+-------------------------------+-----------------+-----------------+ +-------------------------------+-----------------+-----------------+
| Transport | a) Independent | b) Dependent on | | Transport -> | a) Independent | b) Dependent on |
| | of packet size | packet size of | | ----------------------------- | of packet size | packet size of |
| Network | of congestion | congestion | | Network | of congestion | congestion |
| | notifications | notifications | | | notifications | notifications |
+-------------------------------+-----------------+-----------------+ +-------------------------------+-----------------+-----------------+
| 1) Predominantly | Scenario a1) | Scenario b1) | | 1) Predominantly bit- | Scenario a1) | Scenario b1) |
| bit-congestible network | | | | congestible network | | |
| 2) Mix of bit-congestible and | Scenario a2) | Scenario b2) | | 2) Mix of bit-congestible and | Scenario a2) | Scenario b2) |
| pkt-congestible network | | | | pkt-congestible network | | |
+-------------------------------+-----------------+-----------------+ +-------------------------------+-----------------+-----------------+
Table 4: Four Possible Congestion Scenarios Table 4: Four Possible Congestion Scenarios
Appendix B.1 focuses on the horizontal dimension of Table 4 checking Appendix B.1 focuses on the horizontal dimension of Table 4 checking
that packet-mode drop (or marking) gives sufficient information, that packet-mode drop (or marking) gives sufficient information,
whether or not the transport uses it--scenarios b) and a) whether or not the transport uses it -- scenarios b) and a)
respectively. respectively.
Appendix B.2 focuses on the vertical dimension of Table 4, checking Appendix B.2 focuses on the vertical dimension of Table 4, checking
that packet-mode drop gives sufficient information to the transport that packet-mode drop gives sufficient information to the transport
whether resources in the network are bit-congestible or packet- whether resources in the network are bit-congestible or packet-
congestible (these terms are defined in Section 1.1). congestible (these terms are defined in Section 1.1).
Notation: To be concrete, we will compare two flows with different Notation: To be concrete, we will compare two flows with different
packet sizes, s_1 and s_2. As an example, we will take s_1 = 60B packet sizes, s_1 and s_2. As an example, we will take
= 480b and s_2 = 1500B = 12,000b. s_1 = 60 B = 480 b and s_2 = 1,500 B = 12,000 b.
A flow's bit rate, x [bps], is related to its packet rate, u A flow's bit rate, x [bps], is related to its packet rate, u
[pps], by [pps], by
x(t) = s.u(t). x(t) = s*u(t).
In the bit-congestible case, path congestion will be denoted by In the bit-congestible case, path congestion will be denoted by
p_b, and in the packet-congestible case by p_p. When either case p_b, and in the packet-congestible case by p_p. When either case
is implied, the letter p alone will denote path congestion. is implied, the letter p alone will denote path congestion.
B.1. Packet-Size (In)Dependence in Transports B.1. Packet-Size (In)Dependence in Transports
In all cases we consider a packet-mode drop queue that indicates In all cases, we consider a packet-mode drop queue that indicates
congestion by dropping (or marking) packets with probability p congestion by dropping (or marking) packets with probability p
irrespective of packet size. We use an example value of loss irrespective of packet size. We use an example value of loss
(marking) probability, p=0.1%. (marking) probability, p=0.1%.
A transport like RFC5681 TCP treats a congestion notification on any A transport like TCP as specified in RFC 5681 treats a congestion
packet whatever its size as one event. However, a network with just notification on any packet whatever its size as one event. However,
the packet-mode drop algorithm does give more information if the a network with just the packet-mode drop algorithm gives more
transport chooses to use it. We will use Table 5 to illustrate this. information if the transport chooses to use it. We will use Table 5
to illustrate this.
We will set aside the last column until later. The columns labelled We will set aside the last column until later. The columns labelled
"Flow 1" and "Flow 2" compare two flows consisting of 60B and 1500B 'Flow 1' and 'Flow 2' compare two flows consisting of 60 B and
packets respectively. The body of the table considers two separate 1,500 B packets respectively. The body of the table considers two
cases, one where the flows have equal bit-rate and the other with separate cases, one where the flows have an equal bit rate and the
equal packet-rates. In both cases, the two flows fill a 96Mbps link. other with equal packet rates. In both cases, the two flows fill a
Therefore, in the equal bit-rate case they each have half the bit- 96 Mbps link. Therefore, in the equal bit rate case, they each have
rate (48Mbps). Whereas, with equal packet-rates, flow 1 uses 25 half the bit rate (48Mbps). Whereas, with equal packet rates, Flow 1
times smaller packets so it gets 25 times less bit-rate--it only gets uses 25 times smaller packets so it gets 25 times less bit rate -- it
1/(1+25) of the link capacity (96Mbps/26 = 4Mbps after rounding). In only gets 1/(1+25) of the link capacity (96 Mbps / 26 = 4 Mbps after
contrast flow 2 gets 25 times more bit-rate (92Mbps) in the equal rounding). In contrast Flow 2 gets 25 times more bit rate (92 Mbps)
packet rate case because its packets are 25 times larger. The packet in the equal packet rate case because its packets are 25 times
rate shown for each flow could easily be derived once the bit-rate larger. The packet rate shown for each flow could easily be derived
was known by dividing bit-rate by packet size, as shown in the column once the bit rate was known by dividing the bit rate by packet size,
labelled "Formula". as shown in the column labelled 'Formula'.
Parameter Formula Flow 1 Flow 2 Combined Parameter Formula Flow 1 Flow 2 Combined
----------------------- ----------- ------- ------- -------- ----------------------- ----------- -------- -------- --------
Packet size s/8 60B 1,500B (Mix) Packet size s/8 60 B 1,500 B (Mix)
Packet size s 480b 12,000b (Mix) Packet size s 480 b 12,000 b (Mix)
Pkt loss probability p 0.1% 0.1% 0.1% Pkt loss probability p 0.1% 0.1% 0.1%
EQUAL BIT-RATE CASE EQUAL BIT RATE CASE
Bit-rate x 48Mbps 48Mbps 96Mbps Bit rate x 48 Mbps 48 Mbps 96 Mbps
Packet-rate u = x/s 100kpps 4kpps 104kpps Packet rate u = x/s 100 kpps 4 kpps 104 kpps
Absolute pkt-loss-rate p*u 100pps 4pps 104pps Absolute pkt-loss rate p*u 100 pps 4 pps 104 pps
Absolute bit-loss-rate p*u*s 48kbps 48kbps 96kbps Absolute bit-loss rate p*u*s 48 kbps 48 kbps 96 kbps
Ratio of lost/sent pkts p*u/u 0.1% 0.1% 0.1% Ratio of lost/sent pkts p*u/u 0.1% 0.1% 0.1%
Ratio of lost/sent bits p*u*s/(u*s) 0.1% 0.1% 0.1% Ratio of lost/sent bits p*u*s/(u*s) 0.1% 0.1% 0.1%
EQUAL PACKET-RATE CASE EQUAL PACKET RATE CASE
Bit-rate x 4Mbps 92Mbps 96Mbps Bit rate x 4 Mbps 92 Mbps 96 Mbps
Packet-rate u = x/s 8kpps 8kpps 15kpps Packet rate u = x/s 8 kpps 8 kpps 15 kpps
Absolute pkt-loss-rate p*u 8pps 8pps 15pps Absolute pkt-loss rate p*u 8 pps 8 pps 15 pps
Absolute bit-loss-rate p*u*s 4kbps 92kbps 96kbps Absolute bit-loss rate p*u*s 4 kbps 92 kbps 96 kbps
Ratio of lost/sent pkts p*u/u 0.1% 0.1% 0.1% Ratio of lost/sent pkts p*u/u 0.1% 0.1% 0.1%
Ratio of lost/sent bits p*u*s/(u*s) 0.1% 0.1% 0.1% Ratio of lost/sent bits p*u*s/(u*s) 0.1% 0.1% 0.1%
Table 5: Absolute Loss Rates and Loss Ratios for Flows of Small and Table 5: Absolute Loss Rates and Loss Ratios for Flows of Small and
Large Packets and Both Combined Large Packets and Both Combined
So far we have merely set up the scenarios. We now consider So far, we have merely set up the scenarios. We now consider
congestion notification in the scenario. Two TCP flows with the same congestion notification in the scenario. Two TCP flows with the same
round trip time aim to equalise their packet-loss-rates over time. round-trip time aim to equalise their packet-loss rates over time;
That is the number of packets lost in a second, which is the packets that is, the number of packets lost in a second, which is the packets
per second (u) multiplied by the probability that each one is dropped per second (u) multiplied by the probability that each one is dropped
(p). Thus TCP converges on the "Equal packet-rate" case, where both (p). Thus, TCP converges on the case labelled 'Equal packet rate' in
flows aim for the same "Absolute packet-loss-rate" (both 8pps in the the table, where both flows aim for the same absolute packet-loss
table). rate (both 8 pps in the table).
Packet-mode drop actually gives flows sufficient information to Packet-mode drop actually gives flows sufficient information to
measure their loss-rate in bits per second, if they choose, not just measure their loss rate in bits per second, if they choose, not just
packets per second. Each flow can count the size of a lost or marked packets per second. Each flow can count the size of a lost or marked
packet and scale its rate-response in proportion (as TFRC-SP does). packet and scale its rate response in proportion (as TFRC-SP does).
The result is shown in the row entitled "Absolute bit-loss-rate", The result is shown in the row entitled 'Absolute bit-loss rate',
where the bits lost in a second is the packets per second (u) where the bits lost in a second is the packets per second (u)
multiplied by the probability of losing a packet (p) multiplied by multiplied by the probability of losing a packet (p) multiplied by
the packet size (s). Such an algorithm would try to remove any the packet size (s). Such an algorithm would try to remove any
imbalance in bit-loss-rate such as the wide disparity in the "Equal imbalance in the bit-loss rate such as the wide disparity in the case
packet-rate" case (4kbps vs. 92kbps). Instead, a packet-size- labelled 'Equal packet rate' (4k bps vs. 92 kbps). Instead, a
dependent algorithm would aim for equal bit-loss-rates, which would packet-size-dependent algorithm would aim for equal bit-loss rates,
drive both flows towards the "Equal bit-rate" case, by driving them which would drive both flows towards the case labelled 'Equal bit
to equal bit-loss-rates (both 48kbps in this example). rate', by driving them to equal bit-loss rates (both 48 kbps in this
example).
The explanation so far has assumed that each flow consists of packets The explanation so far has assumed that each flow consists of packets
of only one constant size. Nonetheless, it extends naturally to of only one constant size. Nonetheless, it extends naturally to
flows with mixed packet sizes. In the right-most column of Table 5 a flows with mixed packet sizes. In the right-most column of Table 5,
flow of mixed size packets is created simply by considering flow 1 a flow of mixed-size packets is created simply by considering Flow 1
and flow 2 as a single aggregated flow. There is no need for a flow and Flow 2 as a single aggregated flow. There is no need for a flow
to maintain an average packet size. It is only necessary for the to maintain an average packet size. It is only necessary for the
transport to scale its response to each congestion indication by the transport to scale its response to each congestion indication by the
size of each individual lost (or marked) packet. Taking for example size of each individual lost (or marked) packet. Taking, for
the "Equal packet-rate" case, in one second about 8 small packets and example, the case labelled 'Equal packet rate', in one second about 8
8 large packets are lost (making closer to 15 than 16 losses per small packets and 8 large packets are lost (making closer to 15 than
second due to rounding). If the transport multiplies each loss by 16 losses per second due to rounding). If the transport multiplies
its size, in one second it responds to 8*480b and 8*12,000b lost each loss by its size, in one second it responds to 8*480 and
bits, adding up to 96,000 lost bits in a second. This double checks 8*12,000 lost bits, adding up to 96,000 lost bits in a second. This
correctly, being the same as 0.1% of the total bit-rate of 96Mbps. double checks correctly, being the same as 0.1% of the total bit rate
For completeness, the formula for absolute bit-loss-rate is p(u1*s1+ of 96 Mbps. For completeness, the formula for absolute bit-loss rate
u2*s2). is p(u1*s1+u2*s2).
Incidentally, a transport will always measure the loss probability Incidentally, a transport will always measure the loss probability
the same irrespective of whether it measures in packets or in bytes. the same, irrespective of whether it measures in packets or in bytes.
In other words, the ratio of lost to sent packets will be the same as In other words, the ratio of lost packets to sent packets will be the
the ratio of lost to sent bytes. (This is why TCP's bit rate is same as the ratio of lost bytes to sent bytes. (This is why TCP's
still proportional to packet size even when byte-counting is used, as bit rate is still proportional to packet size, even when byte
recommended for TCP in [RFC5681], mainly for orthogonal security counting is used, as recommended for TCP in [RFC5681], mainly for
reasons.) This is intuitively obvious by comparing two example orthogonal security reasons.) This is intuitively obvious by
flows; one with 60B packets, the other with 1500B packets. If both comparing two example flows; one with 60 B packets, the other with
flows pass through a queue with drop probability 0.1%, each flow will 1,500 B packets. If both flows pass through a queue with drop
lose 1 in 1,000 packets. In the stream of 60B packets the ratio of probability 0.1%, each flow will lose 1 in 1,000 packets. In the
bytes lost to sent will be 60B in every 60,000B; and in the stream of stream of 60 B packets, the ratio of lost bytes to sent bytes will be
1500B packets, the loss ratio will be 1,500B out of 1,500,000B. When 60 B in every 60,000 B; and in the stream of 1,500 B packets, the
the transport responds to the ratio of lost to sent packets, it will loss ratio will be 1,500 B out of 1,500,000 B. When the transport
measure the same ratio whether it measures in packets or bytes: 0.1% responds to the ratio of lost to sent packets, it will measure the
in both cases. The fact that this ratio is the same whether measured same ratio whether it measures in packets or bytes: 0.1% in both
in packets or bytes can be seen in Table 5, where the ratio of lost cases. The fact that this ratio is the same whether measured in
to sent packets and the ratio of lost to sent bytes is always 0.1% in packets or bytes can be seen in Table 5, where the ratio of lost
all cases (recall that the scenario was set up with p=0.1%). packets to sent packets and the ratio of lost bytes to sent bytes is
always 0.1% in all cases (recall that the scenario was set up with
p=0.1%).
This discussion of how the ratio can be measured in packets or bytes This discussion of how the ratio can be measured in packets or bytes
is only raised here to highlight that it is irrelevant to this memo! is only raised here to highlight that it is irrelevant to this memo!
Whether a transport depends on packet size or not depends on how this Whether or not a transport depends on packet size depends on how this
ratio is used within the congestion control algorithm. ratio is used within the congestion control algorithm.
So far we have shown that packet-mode drop passes sufficient So far, we have shown that packet-mode drop passes sufficient
information to the transport layer so that the transport can take information to the transport layer so that the transport can take bit
account of bit-congestion, by using the sizes of the packets that congestion into account, by using the sizes of the packets that
indicate congestion. We have also shown that the transport can indicate congestion. We have also shown that the transport can
choose not to take packet size into account if it wishes. We will choose not to take packet size into account if it wishes. We will
now consider whether the transport can know which to do. now consider whether the transport can know which to do.
B.2. Bit-Congestible and Packet-Congestible Indications B.2. Bit-Congestible and Packet-Congestible Indications
As a thought-experiment, imagine an idealised congestion notification As a thought-experiment, imagine an idealised congestion notification
protocol that supports both bit-congestible and packet-congestible protocol that supports both bit-congestible and packet-congestible
resources. It would require at least two ECN flags, one for each of resources. It would require at least two ECN flags, one for each of
bit-congestible and packet-congestible resources. the bit-congestible and packet-congestible resources.
1. A packet-congestible resource trying to code congestion level p_p 1. A packet-congestible resource trying to code congestion level p_p
into a packet stream should mark the idealised `packet into a packet stream should mark the idealised 'packet
congestion' field in each packet with probability p_p congestion' field in each packet with probability p_p
irrespective of the packet's size. The transport should then irrespective of the packet's size. The transport should then
take a packet with the packet congestion field marked to mean take a packet with the packet congestion field marked to mean
just one mark, irrespective of the packet size. just one mark, irrespective of the packet size.
2. A bit-congestible resource trying to code time-varying byte- 2. A bit-congestible resource trying to code time-varying byte-
congestion level p_b into a packet stream should mark the `byte congestion level p_b into a packet stream should mark the 'byte
congestion' field in each packet with probability p_b, again congestion' field in each packet with probability p_b, again
irrespective of the packet's size. Unlike before, the transport irrespective of the packet's size. Unlike before, the transport
should take a packet with the byte congestion field marked to should take a packet with the byte congestion field marked to
count as a mark on each byte in the packet. count as a mark on each byte in the packet.
This hides a fundamental problem--much more fundamental than whether This hides a fundamental problem -- much more fundamental than
we can magically create header space for yet another ECN flag, or whether we can magically create header space for yet another ECN
whether it would work while being deployed incrementally. flag, or whether it would work while being deployed incrementally.
Distinguishing drop from delivery naturally provides just one Distinguishing drop from delivery naturally provides just one
implicit bit of congestion indication information--the packet is implicit bit of congestion indication information -- the packet is
either dropped or not. It is hard to drop a packet in two ways that either dropped or not. It is hard to drop a packet in two ways that
are distinguishable remotely. This is a similar problem to that of are distinguishable remotely. This is a similar problem to that of
distinguishing wireless transmission losses from congestive losses. distinguishing wireless transmission losses from congestive losses.
This problem would not be solved even if ECN were universally This problem would not be solved, even if ECN were universally
deployed. A congestion notification protocol must survive a deployed. A congestion notification protocol must survive a
transition from low levels of congestion to high. Marking two states transition from low levels of congestion to high. Marking two states
is feasible with explicit marking, but much harder if packets are is feasible with explicit marking, but it is much harder if packets
dropped. Also, it will not always be cost-effective to implement AQM are dropped. Also, it will not always be cost-effective to implement
at every low level resource, so drop will often have to suffice. AQM at every low-level resource, so drop will often have to suffice.
We are not saying two ECN fields will be needed (and we are not We are not saying two ECN fields will be needed (and we are not
saying that somehow a resource should be able to drop a packet in one saying that somehow a resource should be able to drop a packet in one
of two different ways so that the transport can distinguish which of two different ways so that the transport can distinguish which
sort of drop it was!). These two congestion notification channels sort of drop it was!). These two congestion notification channels
are a conceptual device to illustrate a dilemma we could face in the are a conceptual device to illustrate a dilemma we could face in the
future. Section 3 gives four good reasons why it would be a bad idea future. Section 3 gives four good reasons why it would be a bad idea
to allow for packet size by biasing drop probability in favour of to allow for packet size by biasing drop probability in favour of
small packets within the network. The impracticality of our thought small packets within the network. The impracticality of our thought
experiment shows that it will be hard to give transports a practical experiment shows that it will be hard to give transports a practical
way to know whether to take account of the size of congestion way to know whether or not to take into account the size of
indication packets or not. congestion indication packets.
Fortunately, this dilemma is not pressing because by design most Fortunately, this dilemma is not pressing because by design most
equipment becomes bit-congested before its packet-processing becomes equipment becomes bit-congested before its packet processing becomes
congested (as already outlined in Section 1.1). Therefore transports congested (as already outlined in Section 1.1). Therefore,
can be designed on the relatively sound assumption that a congestion transports can be designed on the relatively sound assumption that a
indication will usually imply bit-congestion. congestion indication will usually imply bit congestion.
Nonetheless, although the above idealised protocol isn't intended for Nonetheless, although the above idealised protocol isn't intended for
implementation, we do want to emphasise that research is needed to implementation, we do want to emphasise that research is needed to
predict whether there are good reasons to believe that packet predict whether there are good reasons to believe that packet
congestion might become more common, and if so, to find a way to congestion might become more common, and if so, to find a way to
somehow distinguish between bit and packet congestion [RFC3714]. somehow distinguish between bit and packet congestion [RFC3714].
Recently, the dual resource queue (DRQ) proposal [DRQ] has been made Recently, the dual resource queue (DRQ) proposal [DRQ] has been made
on the premise that, as network processors become more cost on the premise that, as network processors become more cost-
effective, per packet operations will become more complex effective, per-packet operations will become more complex
(irrespective of whether more function in the network is desirable). (irrespective of whether more function in the network is desirable).
Consequently the premise is that CPU congestion will become more Consequently the premise is that CPU congestion will become more
common. DRQ is a proposed modification to the RED algorithm that common. DRQ is a proposed modification to the RED algorithm that
folds both bit congestion and packet congestion into one signal folds both bit congestion and packet congestion into one signal
(either loss or ECN). (either loss or ECN).
Finally, we note one further complication. Strictly, packet- Finally, we note one further complication. Strictly, packet-
congestible resources are often cycle-congestible. For instance, for congestible resources are often cycle-congestible. For instance, for
routing look-ups load depends on the complexity of each look-up and routing lookups, load depends on the complexity of each lookup and
whether the pattern of arrivals is amenable to caching or not. This whether or not the pattern of arrivals is amenable to caching. This
also reminds us that any solution must not require a forwarding also reminds us that any solution must not require a forwarding
engine to use excessive processor cycles in order to decide how to engine to use excessive processor cycles in order to decide how to
say it has no spare processor cycles. say it has no spare processor cycles.
Appendix C. Byte-mode Drop Complicates Policing Congestion Response Appendix C. Byte-Mode Drop Complicates Policing Congestion Response
This section is informative, not normative. This section is informative, not normative.
There are two main classes of approach to policing congestion There are two main classes of approach to policing congestion
response: i) policing at each bottleneck link or ii) policing at the response: (i) policing at each bottleneck link or (ii) policing at
edges of networks. Packet-mode drop in RED is compatible with the edges of networks. Packet-mode drop in RED is compatible with
either, while byte-mode drop precludes edge policing. either, while byte-mode drop precludes edge policing.
The simplicity of an edge policer relies on one dropped or marked The simplicity of an edge policer relies on one dropped or marked
packet being equivalent to another of the same size without having to packet being equivalent to another of the same size without having to
know which link the drop or mark occurred at. However, the byte-mode know which link the drop or mark occurred at. However, the byte-mode
drop algorithm has to depend on the local MTU of the line--it needs drop algorithm has to depend on the local MTU of the line -- it needs
to use some concept of a 'normal' packet size. Therefore, one to use some concept of a 'normal' packet size. Therefore, one
dropped or marked packet from a byte-mode drop algorithm is not dropped or marked packet from a byte-mode drop algorithm is not
necessarily equivalent to another from a different link. A policing necessarily equivalent to another from a different link. A policing
function local to the link can know the local MTU where the function local to the link can know the local MTU where the
congestion occurred. However, a policer at the edge of the network congestion occurred. However, a policer at the edge of the network
cannot, at least not without a lot of complexity. cannot, at least not without a lot of complexity.
The early research proposals for type (i) policing at a bottleneck The early research proposals for type (i) policing at a bottleneck
link [pBox] used byte-mode drop, then detected flows that contributed link [pBox] used byte-mode drop, then detected flows that contributed
disproportionately to the number of packets dropped. However, with disproportionately to the number of packets dropped. However, with
no extra complexity, later proposals used packet mode drop and looked no extra complexity, later proposals used packet-mode drop and looked
for flows that contributed a disproportionate amount of dropped bytes for flows that contributed a disproportionate amount of dropped bytes
[CHOKe_Var_Pkt]. [CHOKe_Var_Pkt].
Work is progressing on the congestion exposure protocol (ConEx Work is progressing on the Congestion Exposure (ConEx) protocol
[RFC6789]), which enables a type (ii) edge policer located at a [RFC6789], which enables a type (ii) edge policer located at a user's
user's attachment point. The idea is to be able to take an attachment point. The idea is to be able to take an integrated view
integrated view of the effect of all a user's traffic on any link in of the effect of all a user's traffic on any link in the
the internetwork. However, byte-mode drop would effectively preclude internetwork. However, byte-mode drop would effectively preclude
such edge policing because of the MTU issue above. such edge policing because of the MTU issue above.
Indeed, making drop probability depend on the size of the packets Indeed, making drop probability depend on the size of the packets
that bits happen to be divided into would simply encourage the bits that bits happen to be divided into would simply encourage the bits
to be divided into smaller packets in order to confuse policing. In to be divided into smaller packets in order to confuse policing. In
contrast, as long as a dropped/marked packet is taken to mean that contrast, as long as a dropped/marked packet is taken to mean that
all the bytes in the packet are dropped/marked, a policer can remain all the bytes in the packet are dropped/marked, a policer can remain
robust against bits being re-divided into different size packets or robust against sequences of bits being re-divided into different size
across different size flows [Rate_fair_Dis]. packets or across different size flows [Rate_fair_Dis].
Appendix D. Changes from Previous Versions
To be removed by the RFC Editor on publication.
Full incremental diffs between each version are available at
<http://tools.ietf.org/wg/tsvwg/draft-ietf-tsvwg-byte-pkt-congest/>
(courtesy of the rfcdiff tool):
From -11 to -12: Following the second pass through the IESG:
* Section 2.1 [Barry Leiba]:
+ s/No other choice makes sense,/Subject to the exceptions
below, no other choice makes sense,/
+ s/Exceptions to these recommendations MAY be necessary
/Exceptions to these recommendations may be necessary /
* Sections 3.2 and 4.2.3 [Joel Jaeggli]:
+ Added comment to section 4.2.3 that the examples given are
not in widespread production use, but they give evidence
that it is possible to follow the advice given.
+ Section 4.2.3:
- OLD: Although there are no known proposals, it would also
be possible and perfectly valid to make control packets
robust against drop by explicitly requesting a lower drop
probability using their Diffserv code point [RFC2474] to
request a scheduling class with lower drop.
NEW: Although there are no known proposals, it would also
be possible and perfectly valid to make control packets
robust against drop by requesting a scheduling class with
lower drop probability, by re-marking to a Diffserv code
point [RFC2474] within the same behaviour aggregate.
- appended "Similarly applications, over non-TCP transports
could make any packets that are effectively control
packets more robust by using Diffserv, data duplication,
FEC etc."
+ Updated Wischik ref and added "Reducing Web Latency: the
Virtue of Gentle Aggression" ref.
* Expanded more abbreviations (CoDel, PIE, MTU).
* Section 1. Intro [Stephen Farrell]:
+ In the places where the doc desribes the dichotomy between
'long-term goal' and 'expediency' the words long term goal
and expedient have been introduced, to more explicitly refer
back to this introductory para (S.2.1 & S.2.3).
+ Added explanation of what scaling with packet size means.
* Conclusions [Benoit Claise]:
+ OLD: For the specific case of RED, this means that byte-mode
queue measurement will often be appropriate although byte-
mode drop is strongly deprecated.
NEW: For the specific case of RED, this means that byte-mode
queue measurement will often be appropriate but the use of
byte-mode drop is very strongly discouraged.
From -10 to -11: Following a further WGLC:
* Abstract: clarified that advice applies to all AQMs including
newer ones
* Abstract & Intro: changed 'read' to 'detect', because you don't
read losses, you detect them.
* S.1. Introduction: Disambiguated summary of advice on queue
measurement.
* Clarified that the doc deprecates any preference based solely
on packet size, it's not only against preferring smaller
packets.
* S.4.1.2. Congestion Measurement without a Queue: Explained
that a queue of TXOPs represents a queue into spectrum
congested by too many bits.
* S.5.2: Bit- & Packet-congestible Network: Referred to
explanation in S.4.1.2 to make the point that TXOPs are not a
primary unit of workload like bits and packets are, even though
you get queues of TXOPs.
* 6. Security: Disambiguated 'bias towards'.
* 8. Conclusions: Made consistent with recommendation to use
time if possible for queue measurement.
From -09 to -10: Following IESG review:
* Updates 2309: Left header unchanged reflecting eventual IESG
consensus [Sean Turner, Pete Resnick].
* S.1 Intro: This memo adds to the congestion control principles
enumerated in BCP 41 [Pete Resnick]
* Abstract, S.1, S.1.1, s.1.2 Intro, Scoping and Example: Made
applicability to all AQMs clearer listing some more example
AQMs and explained that we always use RED for examples, but
this doesn't mean it's not applicable to other AQMs. [A number
of reviewers have described the draft as "about RED"]
* S.1 & S.2.1 Queue measurement: Explained that the choice
between measuring the queue in packets or bytes is only
relevant if measuring it in time units is infeasible [So as not
to imply that we haven't noticed the advances made by PDPC &
CoDel]
* S.1.1. Terminology: Better explained why hybrid systems
congested by both packets and bytes are often designed to be
treated as bit-congestible [Richard Barnes].
* S.2.1. Queue measurement advice: Added examples. Added a
counter-example to justify SHOULDs rather than MUSTs. Pointed
to S.4.1 for a list of more complicated scenarios. [Benson
Schliesser, OpsDir]
* S2.2. Recommendation on Encoding Congestion Notification:
Removed SHOULD treat packets equally, leaving only SHOULD NOT
drop dependent on packet size, to avoid it sounding like we're
saying QoS is not allowed. Pointed to possible app-specific
legacy use of byte-mode as a counter-example that prevents us
saying MUST NOT. [Pete Resnick]
* S.2.3. Recommendation on Responding to Congestion: capitalised
the two SHOULDs in recommendations for TCP, and gave possible
counter-examples. [noticed while dealing with Pete Resnick's
point]
* S2.4. Splitting & Merging: RTCP -> RTP/RTCP [Pete McCann, Gen-
ART]
* S.3.2 Small != Control: many control packets are small ->
...tend to be small [Stephen Farrell]
* S.3.1 Perverse incentives: Changed transport designers to app
developers [Stephen Farrell]
* S.4.1.1. Fixed Size Packet Buffers: Nearly completely re-
written to simplify and to reverse the advice when the
underlying resource is bit-congestible, irrespective of whether
the buffer consists of fixed-size packet buffers. [Richard
Barnes & Benson Schliesser]
* S.4.2.1.2. Packet Size Bias Regardless of AQM: Largely re-
written to reflect the earlier change in advice about fixed-
size packet buffers, and to primarily focus on getting rid of
tail-drop, not various nuances of tail-drop. [Richard Barnes &
Benson Schliesser]
* Editorial corrections [Tim Bray, AppsDir, Pete McCann, Gen-ART
and others]
* Updated refs (two I-Ds have become RFCs). [Pete McCann]
From -08 to -09: Following WG last call:
* S.2.1: Made RED-related queue measurement recommendations
clearer
* S.2.3: Added to "Recommendation on Responding to Congestion" to
make it clear that we are definitely not saying transports have
to equalise bit-rates, just how to do it and not do it, if you
want to.
* S.3: Clarified motivation sections S.3.3 "Transport-Independent
Network" and S.3.5 "Implementation Efficiency"
* S.3.4: Completely changed motivating argument from "Scaling
Congestion Control with Packet Size" to "Partial Deployment of
AQM".
From -07 to -08:
* Altered abstract to say it provides best current practice and
highlight that it updates RFC2309
* Added null IANA section
* Updated refs
From -06 to -07:
* A mix-up with the corollaries and their naming in 2.1 to 2.3
fixed.
From -05 to -06:
* Primarily editorial fixes.
From -04 to -05:
* Changed from Informational to BCP and highlighted non-normative
sections and appendices
* Removed language about consensus
* Added "Example Comparing Packet-Mode Drop and Byte-Mode Drop"
* Arranged "Motivating Arguments" into a more logical order and
completely rewrote "Transport-Independent Network" & "Scaling
Congestion Control with Packet Size" arguments. Removed "Why
Now?"
* Clarified applicability of certain recommendations
* Shifted vendor survey to an Appendix
* Cut down "Outstanding Issues and Next Steps"
* Re-drafted the start of the conclusions to highlight the three
distinct areas of concern
* Completely re-wrote appendices
* Editorial corrections throughout.
From -03 to -04:
* Reordered Sections 2 and 3, and some clarifications here and
there based on feedback from Colin Perkins and Mirja
Kuehlewind.
From -02 to -03 (this version)
* Structural changes:
+ Split off text at end of "Scaling Congestion Control with
Packet Size" into new section "Transport-Independent
Network"
+ Shifted "Recommendations" straight after "Motivating
Arguments" and added "Conclusions" at end to reinforce
Recommendations
+ Added more internal structure to Recommendations, so that
recommendations specific to RED or to TCP are just
corollaries of a more general recommendation, rather than
being listed as a separate recommendation.
+ Renamed "State of the Art" as "Critical Survey of Existing
Advice" and retitled a number of subsections with more
descriptive titles.
+ Split end of "Congestion Coding: Summary of Status" into a
new subsection called "RED Implementation Status".
+ Removed text that had been in the Appendix "Congestion
Notification Definition: Further Justification".
* Reordered the intro text a little.
* Made it clearer when advice being reported is deprecated and
when it is not.
* Described AQM as in network equipment, rather than saying "at
the network layer" (to side-step controversy over whether
functions like AQM are in the transport layer but in network
equipment).
* Minor improvements to clarity throughout
From -01 to -02:
* Restructured the whole document for (hopefully) easier reading
and clarity. The concrete recommendation, in RFC2119 language,
is now in Section 8.
From -00 to -01:
* Minor clarifications throughout and updated references
From briscoe-byte-pkt-mark-02 to ietf-byte-pkt-congest-00:
* Added note on relationship to existing RFCs
* Posed the question of whether packet-congestion could become
common and deferred it to the IRTF ICCRG. Added ref to the
dual-resource queue (DRQ) proposal.
* Changed PCN references from the PCN charter & architecture to
the PCN marking behaviour draft most likely to imminently
become the standards track WG item.
From -01 to -02:
* Abstract reorganised to align with clearer separation of issue
in the memo.
* Introduction reorganised with motivating arguments removed to
new Section 3.
* Clarified avoiding lock-out of large packets is not the main or
only motivation for RED.
* Mentioned choice of drop or marking explicitly throughout,
rather than trying to coin a word to mean either.
* Generalised the discussion throughout to any packet forwarding
function on any network equipment, not just routers.
* Clarified the last point about why this is a good time to sort
out this issue: because it will be hard / impossible to design
new transports unless we decide whether the network or the
transport is allowing for packet size.
* Added statement explaining the horizon of the memo is long
term, but with short term expediency in mind.
* Added material on scaling congestion control with packet size
(Section 3.4).
* Separated out issue of normalising TCP's bit rate from issue of
preference to control packets (Section 3.2).
* Divided up Congestion Measurement section for clarity,
including new material on fixed size packet buffers and buffer
carving (Section 4.1.1 & Section 4.2.1) and on congestion
measurement in wireless link technologies without queues
(Section 4.1.2).
* Added section on 'Making Transports Robust against Control
Packet Losses' (Section 4.2.3) with existing & new material
included.
* Added tabulated results of vendor survey on byte-mode drop
variant of RED (Table 3).
From -00 to -01:
* Clarified applicability to drop as well as ECN.
* Highlighted DoS vulnerability.
* Emphasised that drop-tail suffers from similar problems to
byte-mode drop, so only byte-mode drop should be turned off,
not RED itself.
* Clarified the original apparent motivations for recommending
byte-mode drop included protecting SYNs and pure ACKs more than
equalising the bit rates of TCPs with different segment sizes.
Removed some conjectured motivations.
* Added support for updates to TCP in progress (ackcc & ecn-syn-
ack).
* Updated survey results with newly arrived data.
* Pulled all recommendations together into the conclusions.
* Moved some detailed points into two additional appendices and a
note.
* Considerable clarifications throughout.
* Updated references
Authors' Addresses Authors' Addresses
Bob Briscoe Bob Briscoe
BT BT
B54/77, Adastral Park B54/77, Adastral Park
Martlesham Heath Martlesham Heath
Ipswich IP5 3RE Ipswich IP5 3RE
UK UK
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