draft-ietf-tsvwg-byte-pkt-congest-02.txt   draft-ietf-tsvwg-byte-pkt-congest-03.txt 
Transport Area Working Group B. Briscoe Transport Area Working Group B. Briscoe
Internet-Draft BT Internet-Draft BT
Updates: 2309 (if approved) J. Manner Updates: 2309 (if approved) J. Manner
Intended status: Informational Aalto University Intended status: Informational Aalto University
Expires: January 13, 2011 July 12, 2010 Expires: April 27, 2011 October 24, 2010
Byte and Packet Congestion Notification Byte and Packet Congestion Notification
draft-ietf-tsvwg-byte-pkt-congest-02 draft-ietf-tsvwg-byte-pkt-congest-03
Abstract Abstract
This memo concerns dropping or marking packets using active queue This memo concerns dropping or marking packets using active queue
management (AQM) such as random early detection (RED) or pre- management (AQM) such as random early detection (RED) or pre-
congestion notification (PCN). We give two strong recommendations: congestion notification (PCN). We give three strong recommendations:
(1) packet size should not be taken into account when transports read (1) packet size should be taken into account when transports read
congestion indications, not when network equipment writes them, and congestion indications, (2) packet size should not be taken into
(2) byte-mode packet drop variant of AQM algorithms, such as RED, account when network equipment creates congestion signals (marking,
should not be used to drop fewer small packets. dropping), and therefore (3) the byte-mode packet drop variant of the
RED AQM algorithm that drops fewer small packets should not be used.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 13, 2011. This Internet-Draft will expire on April 27, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Terminology and Scoping . . . . . . . . . . . . . . . . . 6 1.1. Terminology and Scoping . . . . . . . . . . . . . . . . . 7
1.2. Why now? . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.2. Why now? . . . . . . . . . . . . . . . . . . . . . . . . . 8
2. Motivating Arguments . . . . . . . . . . . . . . . . . . . . . 8 2. Motivating Arguments . . . . . . . . . . . . . . . . . . . . . 10
2.1. Scaling Congestion Control with Packet Size . . . . . . . 8 2.1. Scaling Congestion Control with Packet Size . . . . . . . 10
2.2. Avoiding Perverse Incentives to (ab)use Smaller Packets . 10 2.2. Transport-Independent Network . . . . . . . . . . . . . . 10
2.3. Small != Control . . . . . . . . . . . . . . . . . . . . . 11 2.3. Avoiding Perverse Incentives to (Ab)use Smaller Packets . 11
2.4. Implementation Efficiency . . . . . . . . . . . . . . . . 11 2.4. Small != Control . . . . . . . . . . . . . . . . . . . . . 12
3. The State of the Art . . . . . . . . . . . . . . . . . . . . . 11 2.5. Implementation Efficiency . . . . . . . . . . . . . . . . 13
3.1. Congestion Measurement: Status . . . . . . . . . . . . . . 12 3. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 13
3.1.1. Fixed Size Packet Buffers . . . . . . . . . . . . . . 13 3.1. Recommendation on Queue Measurement . . . . . . . . . . . 13
3.1.2. Congestion Measurement without a Queue . . . . . . . . 14 3.2. Recommendation on Notifying Congestion . . . . . . . . . . 13
3.2. Congestion Coding: Status . . . . . . . . . . . . . . . . 14 3.3. Recommendation on Responding to Congestion . . . . . . . . 14
3.2.1. Network Bias when Encoding . . . . . . . . . . . . . . 14 3.4. Recommended Future Research . . . . . . . . . . . . . . . 15
3.2.2. Transport Bias when Decoding . . . . . . . . . . . . . 16 4. A Survey and Critique of Past Advice . . . . . . . . . . . . . 15
3.2.3. Making Transports Robust against Control Packet 4.1. Congestion Measurement Advice . . . . . . . . . . . . . . 16
Losses . . . . . . . . . . . . . . . . . . . . . . . . 17 4.1.1. Fixed Size Packet Buffers . . . . . . . . . . . . . . 16
3.2.4. Congestion Coding: Summary of Status . . . . . . . . . 18 4.1.2. Congestion Measurement without a Queue . . . . . . . . 17
4. Outstanding Issues and Next Steps . . . . . . . . . . . . . . 20 4.2. Congestion Notification Advice . . . . . . . . . . . . . . 18
4.1. Bit-congestible World . . . . . . . . . . . . . . . . . . 20 4.2.1. Network Bias when Encoding . . . . . . . . . . . . . . 18
4.2. Bit- & Packet-congestible World . . . . . . . . . . . . . 21 4.2.2. Transport Bias when Decoding . . . . . . . . . . . . . 20
5. Recommendation and Conclusions . . . . . . . . . . . . . . . . 22 4.2.3. Making Transports Robust against Control Packet
5.1. Recommendation on Queue Measurement . . . . . . . . . . . 22 Losses . . . . . . . . . . . . . . . . . . . . . . . . 21
5.2. Recommendation on Notifying Congestion . . . . . . . . . . 23 4.2.4. Congestion Notification: Summary of Conflicting
5.3. Recommendation on Responding to Congestion . . . . . . . . 24 Advice . . . . . . . . . . . . . . . . . . . . . . . . 22
5.4. Recommended Future Research . . . . . . . . . . . . . . . 24 4.2.5. RED Implementation Status . . . . . . . . . . . . . . 23
6. Security Considerations . . . . . . . . . . . . . . . . . . . 24 5. Outstanding Issues and Next Steps . . . . . . . . . . . . . . 24
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25 5.1. Bit-congestible World . . . . . . . . . . . . . . . . . . 24
8. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 25 5.2. Bit- & Packet-congestible World . . . . . . . . . . . . . 25
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25 6. Security Considerations . . . . . . . . . . . . . . . . . . . 26
9.1. Normative References . . . . . . . . . . . . . . . . . . . 25 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 27
9.2. Informative References . . . . . . . . . . . . . . . . . . 26 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
Appendix A. Congestion Notification Definition: Further 9. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 28
Justification . . . . . . . . . . . . . . . . . . . . 30 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Appendix B. Idealised Wire Protocol . . . . . . . . . . . . . . . 30 10.1. Normative References . . . . . . . . . . . . . . . . . . . 28
B.1. Protocol Coding . . . . . . . . . . . . . . . . . . . . . 30 10.2. Informative References . . . . . . . . . . . . . . . . . . 29
B.2. Example Scenarios . . . . . . . . . . . . . . . . . . . . 32 Appendix A. Idealised Wire Protocol . . . . . . . . . . . . . . . 32
B.2.1. Notation . . . . . . . . . . . . . . . . . . . . . . . 32 A.1. Protocol Coding . . . . . . . . . . . . . . . . . . . . . 32
B.2.2. Bit-congestible resource, equal bit rates (Ai) . . . . 32 A.2. Example Scenarios . . . . . . . . . . . . . . . . . . . . 34
B.2.3. Bit-congestible resource, equal packet rates (Bi) . . 33 A.2.1. Notation . . . . . . . . . . . . . . . . . . . . . . . 34
B.2.4. Pkt-congestible resource, equal bit rates (Aii) . . . 34 A.2.2. Bit-congestible resource, equal bit rates (Ai) . . . . 34
B.2.5. Pkt-congestible resource, equal packet rates (Bii) . . 35 A.2.3. Bit-congestible resource, equal packet rates (Bi) . . 35
A.2.4. Pkt-congestible resource, equal bit rates (Aii) . . . 36
A.2.5. Pkt-congestible resource, equal packet rates (Bii) . . 37
Appendix B. Byte-mode Drop Complicates Policing Congestion
Response . . . . . . . . . . . . . . . . . . . . . . 37
Appendix C. Byte-mode Drop Complicates Policing Congestion Appendix C. Changes from Previous Versions . . . . . . . . . . . 38
Response . . . . . . . . . . . . . . . . . . . . . . 35
Appendix D. Changes from Previous Versions . . . . . . . . . . . 36
1. Introduction 1. Introduction
This memo is initially concerned with how we should correctly scale
congestion control functions with packet size for the long term. But
it also recognises that expediency may be necessary to deal with
existing widely deployed protocols that don't live up to the long
term goal.
When notifying congestion, the problem of how (and whether) to take When notifying 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 drop-tail queues. This memo aims to state the principles
we should be using and to come to conclusions on what these we should be using and to come to conclusions on what these
principles will mean for future protocol design, taking into account principles will mean for future protocol design, taking into account
the deployments we have already. the deployments we have already.
The byte vs. packet dilemma arises at three stages in the congestion The byte vs. packet dilemma arises at three stages in the congestion
notification process: notification process:
Measuring congestion: When the congested resource decides locally to Measuring congestion: When the congested resource decides locally to
measure how congested it is. (Should the queue measure its length measure how congested it is, should the queue measure its length
in bytes or packets?); in bytes or packets?
Coding congestion notification into the wire protocol: When the Encoding congestion notification into the wire protocol: When the
congested resource decides whether to notify the level of congested network resource decides whether to notify the level of
congestion on each particular packet. (When a queue considers congestion by dropping or marking a particular packet, should its
whether to notify congestion by dropping or marking a particular decision depend on the byte-size of the particular packet being
packet, should its decision depend on the byte-size of the dropped or marked?
particular packet being dropped or marked?);
Decoding congestion notification from the wire protocol: When the Decoding congestion notification from the wire protocol: When the
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 the transport take into account to respond to congestion, should it take into account the byte-
the byte-size of each missing or marked packet?). size of 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:
whether queues are measured in bytes or packets, termed byte-mode whether queues are measured in bytes or packets, termed byte-mode
queue measurement or packet-mode queue measurement. This memo queue measurement or packet-mode queue measurement. This memo
records this consensus in the RFC Series. In summary the choice records this consensus in the RFC Series. In summary the choice
solely depends on whether the resource is congested by bytes or solely depends on whether the resource is congested by bytes or
packets. packets.
The controversy is mainly around the last two stages to do with The controversy is mainly around the last two stages: whether to
encoding congestion notification into packets: whether to allow for allow for the size of the specific packet notifying congestion i)
the size of the specific packet notifying congestion i) when the when the network encodes or ii) when the transport decodes the
network encodes or ii) when the transport decodes the congestion congestion notification.
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
and ii) it makes TCP's bit-rate less dependent on packet size.
However, there are ways of addressing these issues at the transport
layer, rather than reverse engineering network forwarding to fix the
problems of one specific transport.
The primary purpose of this memo is to build a definitive consensus The primary purpose of this memo is to build a definitive consensus
against such deliberate preferential treatment for small packets in against deliberate preferential treatment for small packets in AQM
AQM algorithms and to record this advice within the RFC series. algorithms and to record this advice within the RFC series. It
Fortunately all the implementers who responded to our survey recommends that (1) packet size should be taken into account when
(Section 3.2.4) have not followed the earlier advice, so the transports read congestion indications, (2) not when network
equipment writes them.
In particular this means that the byte-mode packet drop variant of
RED should not be used to drop fewer small packets, because that
creates a perverse incentive for transports to use tiny segments,
consequently also opening up a DoS vulnerability. Fortunately all
the RED implementers who responded to our survey (Section 4.2.4) have
not followed the earlier advice to use byte-mode drop, so the
consensus this memo argues for seems to already exist in consensus this memo argues for seems to already exist in
implementations. implementations.
The primary conclusion of this memo is that packet size should be However, at the transport layer, TCP congestion control is a widely
taken into account when transports read congestion indications, not deployed protocol that we argue doesn't scale correctly with packet
when network equipment writes them. Reducing drop of small packets size. To date this hasn't been a significant problem because most
has some tempting advantages: i) it drops less control packets, which TCPs have been used with similar packet sizes. But, as we design new
tend to be small and ii) it makes TCP's bit-rate less dependent on congestion controls, we should build in scaling with packet size
packet size. However, there are ways of addressing these issues at rather than assuming we should follow TCP's example.
the transport layer, rather than reverse engineering network
forwarding to fix specific transport problems.
The second conclusion is that network layer algorithms like the byte-
mode packet drop variant of RED should not be used to drop fewer
small packets, because that creates a perverse incentive for
transports to use tiny segments, consequently also opening up a DoS
vulnerability.
This memo is initially concerned with how we should correctly scale
congestion control functions with packet size for the long term. But
it also recognises that expediency may be necessary to deal with
existing widely deployed protocols that don't live up to the long
term goal. It turns out that the 'correct' variant of RED to deploy
seems to be the one everyone has deployed, and no-one who responded
to our survey has implemented the other variant. However, at the
transport layer, TCP congestion control is a widely deployed protocol
that we argue doesn't scale correctly with packet size. To date this
hasn't been a significant problem because most TCPs have been used
with similar packet sizes. But, as we design new congestion
controls, we should build in scaling with packet size rather than
assuming we should follow TCP's example.
This memo continues as follows. Terminology and scoping are This memo continues as follows. First it discusses terminology and
discussed next, and the reasons to make the recommendations presented scoping and why it is relevant to publish this memo now. Section 2
in this memo now are given in Section 1.2. Motivating arguments for gives motivating arguments for the recommendations that are formally
our advice are given in Section 2. We then survey the advice given stated in Section 3, which follows. We then critically survey the
previously in the RFC series, the research literature and the advice given previously in the RFC series and the research literature
deployed legacy (Section 3) before listing outstanding issues (Section 4), followed by an assessment of whether or not this advice
(Section 4) that will need resolution both to inform future protocols has been followed in production networks (Section 4.2.5). To wrap
designs and to handle legacy. We then give concrete recommendations up, outstanding issues are discussed that will need resolution both
for the way forward in (Section 5). We finally give security to inform future protocols designs and to handle legacy (Section 5).
considerations in Section 6. The interested reader can also find Then security issues are collected together in Section 6 before
further discussions about the theme of byte vs. packet in the conclusions are drawn in Section 7. The interested reader can find
appendices. discussion of more detailed issues on the theme of 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. A busy reader can jump right into Section 5 to read on the subject. A busy reader can jump right into Section 3 to read
a summary of the recommendations for the Internet community. a summary of the 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].
Congestion Notification: Rather than aim to achieve what many have Congestion Notification: Rather than aim to achieve what many have
tried and failed, this memo will not try to define congestion. It tried and failed, this memo will not try to define congestion. It
skipping to change at page 6, line 32 skipping to change at page 7, line 30
serve. L is the instantaneous offered load. serve. L is the instantaneous offered load.
The phrase `unwilling to serve' is added, because AQM systems The phrase `unwilling to serve' is added, because AQM systems
(e.g. RED, PCN [RFC5670]) set a virtual limit smaller than the (e.g. RED, PCN [RFC5670]) set a virtual limit smaller than the
actual limit to the resource, then notify when this virtual limit actual limit to the resource, then notify when this virtual limit
is exceeded in order to avoid congestion of the actual capacity. is exceeded in order to avoid congestion of the actual capacity.
Note that the denominator is offered load, not capacity. Note that the denominator is offered load, not capacity.
Therefore congestion notification is a real number bounded by the Therefore congestion notification is a real number bounded by the
range [0,1]. This ties in with the most well-understood measure range [0,1]. This ties in with the most well-understood measure
of congestion notification: drop fraction (often loosely called of congestion notification: drop probability (often loosely called
loss rate). It also means that congestion has a natural loss rate). It also means that congestion has a natural
interpretation as a probability; the probability of offered interpretation as a probability; the probability of offered
traffic not being served (or being marked as at risk of not being traffic not being served (or being marked as at risk of not being
served). Appendix A describes a further incidental benefit that served).
arises from using load as the denominator of congestion
notification.
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 using explicit congestion
notification (ECN [RFC3168] or PCN [RFC5670]). Throughout this notification (ECN [RFC3168] or PCN [RFC5670]). Throughout this
document, unless clear from the context, the term marking will be document, unless clear from the context, the term marking will be
used to mean notifying congestion explicitly, while congestion used to mean notifying congestion explicitly, while congestion
notification will be used to mean notifying congestion either notification will be used to mean notifying congestion either
implicitly by drop or explicitly by marking. implicitly by drop or explicitly by marking.
skipping to change at page 7, line 12 skipping to change at page 8, line 9
congestible. If the load depends on the rate at which bits arrive congestible. If the load depends on the rate at which bits arrive
it is called bit-congestible. 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 3.1.1). Section 4.1.1).
Currently a design goal of network processing equipment such as Currently a design goal of network processing equipment such as
routers and firewalls is to keep packet processing uncongested routers and firewalls is to keep packet processing uncongested
even under worst case bit rates with minimum packet sizes. even under worst case bit rates with minimum packet sizes.
Therefore, packet-congestion is currently rare, but there is no Therefore, packet-congestion is currently rare
guarantee that it will not become common with future technology [I-D.irtf-iccrg-welzl; S.3.3], but there is no guarantee that it
trends. will not become common with future technology trends.
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 packets, for instance stateful firewalls are of granularity than bits or packets, for instance stateful
flow-congestible and call-servers are session-congestible. This firewalls are flow-congestible and call-servers are session-
memo focuses on congestion of connectionless resources, but the congestible. This memo focuses on congestion of connectionless
same principles may be applicable for congestion notification resources, but the same principles may be applicable for
protocols controlling per-flow and per-session processing or congestion notification protocols controlling per-flow and per-
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 respectively called packet-mode or byte-mode measuring queues is called respectively packet-mode queue
queue measurement. And if the probability of dropping a packet measurement or byte-mode queue measurement. And whether the
depends on its byte-size it is called byte-mode drop, whereas if probability of dropping a packet is independent or dependent on
the drop probability is independent of a packet's byte-size it is its byte-size is called respectively packet-mode drop or byte-mode
called packet-mode drop. drop. The terms byte-mode and packet-mode should not be used
without specifying whether they apply to queue measurement or to
drop.
1.2. Why now? 1.2. Why now?
Now is a good time to discuss whether fairness between different Now is a good time to discuss whether fairness between different
sized packets would best be implemented in the network layer, or at sized packets would best be implemented in network equipment, or at
the transport, for a number of reasons: the transport, for a number of reasons:
1. The packet vs. byte issue requires speedy resolution because the 1. The IETF pre-congestion notification (PCN) working group is
IETF pre-congestion notification (PCN) working group is
standardising the external behaviour of a PCN congestion standardising the external behaviour of a PCN congestion
notification (AQM) algorithm [RFC5670]; notification (AQM) algorithm [RFC5670];
2. [RFC2309] says RED may either take account of packet size or not 2. [RFC2309] says RED may either take account of packet size or not
when dropping, but gives no recommendation between the two, when dropping, but gives no recommendation between the two,
referring instead to advice on the performance implications in an referring instead to advice on the performance implications in an
email [pktByteEmail], which recommends byte-mode drop. Further, email [pktByteEmail], which recommends byte-mode drop. Further,
just before RFC2309 was issued, an addendum was added to the just before RFC2309 was issued, an addendum was added to the
archived email that revisited the issue of packet vs. byte-mode archived email that revisited the issue of packet vs. byte-mode
drop in its last paragraph, making the recommendation less clear- drop in its last paragraph, making the recommendation less clear-
skipping to change at page 8, line 32 skipping to change at page 9, line 32
forwarding functions in future [I-D.irtf-iccrg-welzl]. The wider forwarding functions in future [I-D.irtf-iccrg-welzl]. The wider
Internet community needs to discuss whether the complexity of Internet community needs to discuss whether the complexity of
adjusting for packet size should be in the network or in adjusting for packet size should be in the network or in
transports; transports;
5. Given there are many good reasons why larger path max 5. Given there are many good reasons why larger path max
transmission units (PMTUs) would help solve a number of scaling transmission units (PMTUs) would help solve a number of scaling
issues, we don't want to create any bias against large packets issues, we don't want to create any bias against large packets
that is greater than their true cost; that is greater than their true cost;
6. The IETF has started to consider the question of fairness between 6. The IETF audio/video transport (AVT) working group is
standardising how the real-time protocol (RTP) should feedback
and respond to explicit congestion notification (ECN)
[I-D.ietf-avt-ecn-for-rtp].
7. The IETF has started to consider the question of fairness between
flows that use different packet sizes (e.g. in the small-packet flows that use different packet sizes (e.g. in the small-packet
variant of TCP-friendly rate control, TFRC-SP [RFC4828]). Given variant of TCP-friendly rate control, TFRC-SP [RFC4828]). Given
transports with different packet sizes, if we don't decide transports with different packet sizes, if we don't decide
whether the network or the transport should allow for packet whether the network or the transport should allow for packet
size, it will be hard if not impossible to design any transport size, it will be hard if not impossible to design any transport
protocol so that its bit-rate relative to other transports meets protocol so that its bit-rate relative to other transports meets
design guidelines [RFC5033] (Note however that, if the concern design guidelines [RFC5033] (Note however that, if the concern
were fairness between users, rather than between flows were fairness between users, rather than between flows
[Rate_fair_Dis], relative rates between flows would have to come [Rate_fair_Dis], relative rates between flows would have to come
under run-time control rather than being embedded in protocol under run-time control rather than being embedded in protocol
designs). designs).
2. Motivating Arguments 2. Motivating Arguments
In this section, we evaluate the topic of packet vs. byte based
congestion notifications and motivate the recommendations given in
this document.
2.1. Scaling Congestion Control with Packet Size 2.1. Scaling Congestion Control with Packet Size
There are two ways of interpreting a dropped or marked packet. It There are two ways of interpreting a dropped or marked packet. It
can either be considered as a single loss event or as loss/marking of can either be considered as a single loss event or as loss/marking of
the bytes in the packet. Here we try to design a test to see which the bytes in the packet.
approach scales with packet size.
Given bit-congestible is the more common case (see Section 1.1), Consider a bit-congestible link shared by many flows (bit-congestible
consider a bit-congestible link shared by many flows, so that each is the more common case, see Section 1.1), so that each busy period
busy period tends to cause packets to be lost from different flows. tends to cause packets to be lost from different flows. Consider
The test compares two identical scenarios with the same applications, further two sources that have the same data rate but break the load
the same numbers of sources and the same load. But the sources break into large packets in one application (A) and small packets in the
the load into large packets in one scenario and small packets in the other (B). Of course, because the load is the same, there will be
other. Of course, because the load is the same, there will be proportionately more packets in the small packet flow (B).
proportionately more packets in the small packet case.
The test of whether a congestion control scales with packet size is If a congestion control scales with packet size it should respond in
that it should respond in the same way to the same congestion the same way to the same congestion excursion, irrespective of the
excursion, irrespective of the size of the packets that the bytes size of the packets that the bytes causing congestion happen to be
causing congestion happen to be broken down into. broken down into.
A bit-congestible queue suffering a congestion excursion has to drop A bit-congestible queue suffering a congestion excursion has to drop
or mark the same excess bytes whether they are in a few large packets or mark the same excess bytes whether they are in a few large packets
or many small packets. So for the same congestion excursion, the (A) or many small packets (B). So for the same congestion excursion,
same amount of bytes have to be shed to get the load back to its the same amount of bytes have to be shed to get the load back to its
operating point. But, of course, for smaller packets more packets operating point. But, of course, for smaller packets (B) more
will have to be discarded to shed the same bytes. packets will have to be discarded to shed the same bytes.
If all the transports interpret each drop/mark as a single loss event If all the transports interpret each drop/mark as a single loss event
irrespective of the size of the packet dropped, those with smaller irrespective of the size of the packet dropped, those with smaller
packets will respond more to the same congestion excursion, failing packets (B) will respond more to the same congestion excursion. On
our test. On the other hand, if they respond proportionately less the other hand, if they respond proportionately less when smaller
when smaller packets are dropped/marked, overall they will be able to packets are dropped/marked, overall they will be able to respond the
respond the same to the same congestion excursion. same to the same congestion excursion.
Therefore, for a congestion control to scale with packet size it Therefore, for a congestion control to scale with packet size it
should respond to dropped or marked bytes (as TFRC-SP [RFC4828] should respond to dropped or marked bytes (as TFRC-SP [RFC4828]
effectively does), not just to dropped or marked packets irrespective effectively does), instead of dropped or marked packets (as TCP
of packet size (as TCP does). does).
The email [pktByteEmail] referred to by RFC2309 says the question of 2.2. Transport-Independent Network
whether a packet's own size should affect its drop probability
"depends on the dominant end-to-end congestion control mechanisms".
But we argue the network layer should not be optimised for whatever
transport is predominant.
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. But even with different segment sizes will get different bit rates.
though reducing the drop probability of small packets helps ensure
TCPs with different packet sizes will achieve similar bit rates, we Even though reducing the drop probability of small packets (e.g.
argue this correction should be made to TCP itself, not to the RED's byte-mode drop) helps ensure TCPs with different packet sizes
will achieve similar bit rates, we argue this correction should be
made to any future transport protocols based on TCP, not to the
network in order to fix one transport, no matter how prominent it is. network in order to fix one transport, no matter how prominent it is.
Effectively, favouring small packets is reverse engineering of
network equipment around one particular transport protocol (TCP),
contrary to the excellent advice in [RFC3426], which asks designers
to question "Why are you proposing a solution at this layer of the
protocol stack, rather than at another layer?"
Effectively, favouring small packets is reverse engineering of the RFC2309 refers to an email [pktByteEmail] for advice on how RED
network layer around TCP, contrary to the excellent advice in should allow for different packet sizes. The email says the question
[RFC3426], which asks designers to question "Why are you proposing a of whether a packet's own size should affect its drop probability
solution at this layer of the protocol stack, rather than at another "depends on the dominant end-to-end congestion control mechanisms".
layer?" But we argue network equipment should not be specialised for whatever
transport is predominant. No matter how convenient it is, we SHOULD
NOT hack the network solely to allow for omissions from the design of
one transport protocol, even if it is as predominant as TCP.
2.2. Avoiding Perverse Incentives to (ab)use Smaller Packets 2.3. 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]. Again, the protocol exchange perverse incentives [Evol_cc][RFC3426]. Again,
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 this case: than smaller ones would be dangerous in this case:
Normal transports: Even if a transport is not actually malicious, if
it finds small packets go faster, over time it will tend to act in
its own interest and use them. Queues that give advantage to
small packets create an evolutionary pressure for transports to
send at the same bit-rate but break their data stream down into
tiny segments to reduce their drop rate. Encouraging a high
volume of tiny packets might in turn unnecessarily overload a
completely unrelated part of the system, perhaps more limited by
header-processing than bandwidth.
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 traffic in large packets, allowing more attack
traffic to get through to cause further damage. Such a queue traffic to get through to cause further damage. Such a queue
allows attack traffic to have a disproportionately large effect on allows 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.
Note that, although the byte-mode drop variant of RED amplifies Non-malicious transports: Even if a transport is not actually
small packet attacks, drop-tail queues amplify small packet malicious, if it finds small packets go faster, over time it will
attacks even more (see Security Considerations in Section 6). tend to act in its own interest and use them. Queues that give
Wherever possible neither should be used. advantage to small packets create an evolutionary pressure for
transports to send at the same bit-rate but break their data
stream down into tiny segments to reduce their drop rate.
Encouraging a high volume of tiny packets might in turn
unnecessarily overload a completely unrelated part of the system,
perhaps more limited by header-processing than bandwidth.
Imagine two unresponsive flows arrive at a bit-congestible Imagine 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 1Mbps, but one
consists of 1500B and the other 60B packets, which are 25x smaller. consists of 1500B and the other 60B packets, which are 25x smaller.
Consider a scenario where gentle RED [gentle_RED] is used, along with Consider a scenario where gentle RED [gentle_RED] is used, along with
the variant of RED we advise against, i.e. where the RED algorithm is the variant of RED we advise against, i.e. where the RED algorithm is
configured to adjust the drop probability of packets in proportion to configured to adjust the drop probability of packets in proportion to
each packet's size (byte mode packet drop). In this case, if RED each packet's size (byte mode packet drop). In this case, if RED
drops 25% of the larger packets, it will aim to drop 1% of the drops 25% of the larger packets, it will aim to drop 1% of the
smaller packets (but in practice it may drop more as congestion smaller packets (but in practice it may drop more as congestion
increases [RFC4828](S.B.4)). Even though both flows arrive with the increases [RFC4828; S.B.4]). Even though both flows arrive with the
same bit rate, the bit rate the RED queue aims to pass to the line same bit rate, the bit rate the RED queue aims to pass to the line
will be 750k for the flow of larger packet but 990k for the smaller will be 750k for the flow of larger packet but 990k for the smaller
packets (but because of rate variation it will be less than this packets (but because of rate variation it will be less than this
target). target).
It can be seen that this behaviour reopens the same denial of service Note that, although the byte-mode drop variant of RED amplifies small
vulnerability that drop tail queues offer to floods of small packet, packet attacks, drop-tail queues amplify small packet attacks even
though not necessarily as strongly (see Section 6). more (see Security Considerations in Section 6). Wherever possible
neither should be used.
2.3. Small != Control 2.4. Small != Control
It is tempting to drop small packets with lower probability to It is tempting to drop small packets with lower probability to
improve performance, because many control packets are small (TCP SYNs improve performance, because many control packets are small (TCP SYNs
& ACKs, DNS queries & responses, SIP messages, HTTP GETs, etc) and & ACKs, DNS queries & responses, SIP messages, HTTP GETs, etc) and
dropping fewer control packets considerably improves performance. dropping fewer control packets considerably improves performance.
However, we must not give control packets preference purely by virtue However, we must not give control packets preference purely by virtue
of their smallness, otherwise it is too easy for any data source to of their smallness, otherwise it is too easy for any data source to
get the same preferential treatment simply by sending data in smaller get the same preferential treatment simply by sending data in smaller
packets. Again we should not create perverse incentives to favour packets. Again we should not create perverse incentives to favour
small packets rather than to favour control packets, which is what we small packets rather than to favour control packets, which is what we
intend. 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 again, rather than fix these problems in the network layer, we So again, rather than fix these problems in the network, we argue
argue that the transport should be made more robust against losses of that the transport should be made more robust against losses of
control packets (see 'Making Transports Robust against Control Packet control packets (see 'Making Transports Robust against Control Packet
Losses' in Section 3.2.3). Losses' in Section 4.2.3).
2.4. Implementation Efficiency 2.5. Implementation Efficiency
Allowing for packet size at the transport rather than in the network Allowing for packet size at the transport rather than in the network
ensures that neither the network nor the transport needs to do a ensures that neither the network nor the transport needs to do a
multiply operation--multiplication by packet size is effectively multiply operation--multiplication by packet size is effectively
achieved as a repeated add when the transport adds to its count of achieved as a repeated add when the transport adds to its count of
marked bytes as each congestion event is fed to it. This isn't a marked bytes as each congestion event is fed to it. This isn't a
principled reason in itself, but it is a happy consequence of the principled reason in itself, but it is a happy consequence of the
other principled reasons. other principled reasons.
3. The State of the Art 3. Recommendations
3.1. Recommendation on Queue Measurement
Queue length is usually the most correct and simplest way to measure
congestion of a resource. To avoid the pathological effects of drop
tail, an AQM function can then be used to transform queue length into
the probability of dropping or marking a packet (e.g. RED's
piecewise linear function between thresholds).
If the resource is bit-congestible, the implementation SHOULD measure
the length of the queue in bytes. If the resource is packet-
congestible, the implementation SHOULD measure the length of the
queue in packets. No other choice makes sense, because the number of
packets waiting in the queue isn't relevant if the resource gets
congested by bytes and vice versa.
Corollaries:
1. Whether a resource is bit-congestible or packet-congestible is a
property of the resource, so an admin should not ever need to, or
be able to, configure the way a queue measures itself.
2. If RED is used, the implementation SHOULD use byte mode queue
measurement for measuring the congestion of bit-congestible
resources and packet mode queue measurement for packet-
congestible resources.
The recommended approach in less straightforward scenarios, such as
fixed size buffers, and resources without a queue, is discussed in
Section 4.1.
3.2. Recommendation on Notifying Congestion
The Internet's congestion notification protocols (drop, ECN & PCN)
SHOULD NOT take account of packet size when congestion is notified by
network equipment. Allowance for packet size is only appropriate
when the transport responds to congestion (See Recommendation 3.3).
This approach offers sufficient and correct congestion information
for all known and future transport protocols and also ensures no
perverse incentives are created that would encourage transports to
use inappropriately small packet sizes.
Corollaries:
1. AQM algorithms such as RED SHOULD NOT use byte-mode drop, which
deflates RED's drop probability for smaller packet sizes. RED's
byte-mode drop has no enduring advantages. It is more complex,
it creates the perverse incentive to fragment segments into tiny
pieces and it reopens the vulnerability to floods of small-
packets that drop-tail queues suffered from and AQM was designed
to remove.
2. If a vendor has implemented byte-mode drop, and an operator has
turned it on, it is strongly RECOMMENDED that it SHOULD be turned
off. Note that RED as a whole SHOULD NOT be turned off, as
without it, a drop tail queue also biases against large packets.
But note also that turning off byte-mode drop may alter the
relative performance of applications using different packet
sizes, so it would be advisable to establish the implications
before turning it off.
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 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 fine. However, if in
doubt, the vendor should be consulted.
The byte mode packet drop variant of RED was recommended in the past
(see Section 4.2.1 for how thinking evolved). However, our survey of
84 vendors across the industry (Section 4.2.5) has found that none of
the 19% who responded have implemented byte mode drop in RED. Given
there appears to be little, if any, installed base it seems we can
deprecate byte-mode drop in RED with little, if any, incremental
deployment impact.
3.3. Recommendation on Responding to Congestion
Instead of network equipment biasing its congestion notification in
favour of small packets, the IETF transport area should continue its
programme of;
o updating host-based congestion control protocols to take account
of packet size
o making transports less sensitive to losing control packets like
SYNs and pure ACKs.
Corollaries:
1. If two TCPs with different packet sizes are required to run at
equal bit rates under the same path conditions, this SHOULD be
done by altering TCP (Section 4.2.2), not network equipment,
which would otherwise affect other transports besides TCP.
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
done by modifying TCP (Section 4.2.3), not network equipment.
3.4. Recommended Future Research
The above conclusions cater for the Internet as it is today with most
resources being primarily bit-congestible. A secondary conclusion of
this memo is that research is needed to determine whether there might
be more packet-congestible resources in the future. Then further
research would be needed to extend the Internet's congestion
notification (drop or ECN) so that it would be able to handle a more
even mix of bit-congestible and packet-congestible resources.
4. A Survey and Critique of Past Advice
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
individual packet with probability in proportion to its size individual packet with probability in proportion to its size
(relative to the maximum packet size). In the paper's outline of (relative to the maximum packet size). In the paper's outline of
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. This email clarified that there were in fact two advice on tuning. A later addendum to this email introduced the
orthogonal choices: whether to measure queue length in bytes or insight that there are in fact two orthogonal choices:
packets (Section 3.1 below) and whether the drop probability of an
individual packet should depend on its own size (Section 3.2 below).
3.1. Congestion Measurement: Status o whether to measure queue length in bytes or packets (Section 4.1)
o whether the drop probability of an individual packet should depend
on its own size (Section 4.2).
The rest of this section is structured accordingly.
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 RFC2309. 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.
Where buffers are not configured or legacy buffers cannot be
configured to the above guideline, we do not have to make allowances
for such legacy in future protocol design. If a bit-congestible
buffer is measured in packets, the operator will have set the
thresholds mindful of a typical mix of packets sizes. Any AQM
algorithm on such a buffer will be oversensitive to high proportions
of small packets, e.g. a DoS attack, and undersensitive to high
proportions of large packets. But an operator can safely keep such a
legacy buffer because any undersensitivity during unusual traffic
mixes cannot lead to congestion collapse given the buffer will
eventually revert to tail drop, discarding proportionately more large
packets.
Some modern queue implementations give a choice for setting RED's Some modern queue implementations give a choice for setting RED's
thresholds in byte-mode or packet-mode. This may merely be an thresholds in byte-mode or packet-mode. This may merely be an
administrator-interface preference, not altering how the queue itself administrator-interface preference, not altering how the queue itself
is measured but on some hardware it does actually change the way it is measured but on some hardware it does actually change the way it
measures its queue. Whether a resource is bit-congestible or packet- measures its queue. Whether a resource is bit-congestible or packet-
congestible is a property of the resource, so an admin should not congestible is a property of the resource, so an admin should not
ever need to, or be able to, configure the way a queue measures ever need to, or be able to, configure the way a queue measures
itself. itself.
We believe the question of whether to measure queues in bytes or NOTE: Congestion in some legacy bit-congestible buffers is only
packets is fairly well understood these days. The only outstanding measured in packets not bytes. In such cases, the operator has to
issues concern how to measure congestion when the queue is bit set the thresholds mindful of a typical mix of packets sizes. Any
congestible but the resource is packet congestible or vice versa. AQM algorithm on such a buffer will be oversensitive to high
proportions of small packets, e.g. a DoS attack, and undersensitive
to high proportions of large packets. However, there is no need to
make allowances for the possibility of such legacy in future protocol
design. This is safe because any undersensitivity during unusual
traffic mixes cannot lead to congestion collapse given the buffer
will eventually revert to tail drop, discarding proportionately more
large packets.
There is no controversy over what should be done. It's just you have 4.1.1. Fixed Size Packet Buffers
to be an expert in probability to work out what should be done
(summarised in the following section) and, even if you have, it's not
always easy to find a practical algorithm to implement it.
3.1.1. Fixed Size Packet Buffers Although the question of whether to measure queues in bytes or
packets is fairly well understood these days, measuring congestion is
not straightforward when the resource is bit congestible but the
queue is packet congestible or vice versa. This section outlines the
approach to take. There is no controversy over what should be done,
you just need to be expert in probability to work it out. And, even
if you know what should be done, it's not always easy to find a
practical algorithm to implement it.
Some, mostly older, queuing hardware sets aside fixed sized buffers Some, mostly older, queuing hardware sets aside fixed sized buffers
in which to store each packet in the queue. Also, with some in which to store each packet in the queue. Also, with some
hardware, any fixed sized buffers not completely filled by a packet hardware, any fixed sized buffers not completely filled by a packet
are padded when transmitted to the wire. If we imagine a theoretical are padded when transmitted to the wire. If we imagine a theoretical
forwarding system with both queuing and transmission in fixed, MTU- forwarding system with both queuing and transmission in fixed, MTU-
sized units, it should clearly be treated as packet-congestible, sized units, it should clearly be treated as packet-congestible,
because the queue length in packets would be a good model of because the queue length in packets would be a good model of
congestion of the lower layer link. congestion of the lower layer link.
skipping to change at page 14, line 8 skipping to change at page 17, line 42
We now return to the issue we temporarily set aside: small packets We now return to the issue we temporarily set aside: small packets
borrowing space in larger buffers. In this case, the only difference borrowing space in larger buffers. In this case, the only difference
is that the pools for smaller packets have a maximum queue size that is that the pools for smaller packets have a maximum queue size that
includes all the pools for larger packets. And every time a packet includes all the pools for larger packets. And every time a packet
takes a larger buffer, the current queue size has to be incremented takes a larger buffer, the current queue size has to be incremented
for all queues in the pools of buffers less than or equal to the for all queues in the pools of buffers less than or equal to the
buffer size used. buffer size used.
We will return to borrowing of fixed sized buffers when we discuss We will return to borrowing of fixed sized 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 3.2.1. But here we can give a simple summary of its size in Section 4.2.1. But here we can give a at least one
the present discussion on how to measure the length of queues of simple rule for how to measure the length of queues of fixed buffers:
fixed buffers: no matter how complicated the scheme is, ultimately no matter how complicated the scheme is, ultimately any fixed buffer
any fixed buffer system will need to measure its queue length in system will need to measure its queue length in packets not bytes.
packets not bytes.
3.1.2. 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, But not all congested resources lead to queues. For instance,
wireless spectrum is bit-congestible (for a given coding scheme), wireless spectrum is bit-congestible (for a given coding scheme),
because interference increases with the rate at which bits are because interference increases with the rate at which bits are
transmitted. But wireless link protocols do not always maintain a transmitted. But wireless link protocols do not always maintain a
queue that depends on spectrum interference. Similarly, power queue that depends on spectrum interference. Similarly, power
limited resources are also usually bit-congestible if energy is limited resources are also usually bit-congestible if energy is
primarily required for transmission rather than header processing, primarily required for transmission rather than header processing,
but it is rare for a link protocol to build a queue as it approaches but it is rare for a link protocol to build a queue as it approaches
maximum power. maximum power.
skipping to change at page 14, line 39 skipping to change at page 18, line 25
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 target bit-energy-to-noise-density ratio. And, to model radio
power exhaustion, transmission power levels can be measured and 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.
3.2. Congestion Coding: Status 4.2. Congestion Notification Advice
3.2.1. Network Bias when Encoding 4.2.1. Network Bias when Encoding
The previously mentioned email [pktByteEmail] referred to by The previously mentioned email [pktByteEmail] referred to by
[RFC2309] gave advice we now disagree with. It said that drop [RFC2309] advised that most scarce resources in the Internet were
probability should depend on the size of the packet being considered bit-congestible, which is still believed to be true (Section 1.1).
for drop if the resource is bit-congestible, but not if it is packet- But it went on to give advice we now disagree with. It said that
congestible, but advised that most scarce resources in the Internet drop probability should depend on the size of the packet being
were currently bit-congestible. The argument continued that if considered for drop if the resource is bit-congestible, but not if it
packet drops were inflated by packet size (byte-mode dropping), "a is packet-congestible. The argument continued that if packet drops
flow's fraction of the packet drops is then a good indication of that were inflated by packet size (byte-mode dropping), "a flow's fraction
flow's fraction of the link bandwidth in bits per second". This was of the packet drops is then a good indication of that flow's fraction
consistent with a referenced policing mechanism being worked on at of the link bandwidth in bits per second". This was consistent with
the time for detecting unusually high bandwidth flows, eventually a referenced policing mechanism being worked on at the time for
published in 1999 [pBox]. [The problem could and should have been detecting unusually high bandwidth flows, eventually published in
solved by making the policing mechanism count the volume of bytes 1999 [pBox]. However, the problem could and should have been solved
randomly dropped, not the number of packets.] by making the policing mechanism count the volume of bytes randomly
dropped, not the number of packets.
A few months before RFC2309 was published, an addendum was added to A few months before RFC2309 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 byte-size of the measured in packets but packet drop depended on the byte-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 drop rate dependent on the square of
relative packet size. This was indeed consistent with one implied relative packet size. This was indeed consistent with one implied
motivation behind RED's byte mode drop--that we should reverse motivation behind RED's byte mode drop--that we should reverse
engineer the network to improve the performance of dominant end-to- engineer the network to improve the performance of dominant end-to-
end congestion control mechanisms. end congestion control mechanisms. But it is not consistent with the
present recommendations of Section 3.
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 for this change in the have not been able to find a justification in the literature for this
literature, however Eddy and Allman conducted experiments [REDbias] change, however Eddy and Allman conducted experiments [REDbias] that
that assessed how sensitive RED was to this parameter, amongst other assessed how sensitive RED was to this parameter, amongst other
things. No-one seems to have pointed out that this changed algorithm things. No-one seems to have pointed out that this changed algorithm
can often lead to drop probabilities of greater than 1 [which should can often lead to drop probabilities of greater than 1 (which should
ring alarm bells hinting that there's a mistake in the theory ring alarm bells hinting that there's a mistake in the theory
somewhere]. On 10-Nov-2004, this variant of byte-mode packet drop somewhere).
was made the default in the ns2 simulator.
On 10-Nov-2004, this variant of byte-mode packet drop was made the
default in the ns2 simulator. None of the responses to our
admittedly limited survey of implementers (Section 4.2.5) found any
variant of byte-mode drop had been implemented. Therefore any
conclusions based on ns2 simulations that use RED without disabling
byte-mode drop are likely to be highly questionable.
The byte-mode drop variant of RED is, of course, not the only The byte-mode drop variant of RED is, of course, not the only
possible bias towards small packets in queueing algorithms. We have possible bias towards small packets in queueing systems. We have
already mentioned that tail-drop queues naturally tend to lock-out already mentioned that tail-drop queues naturally tend to lock-out
large packets once they are full. But also queues with fixed sized large packets once they are full. But also queues with fixed sized
buffers reduce the probability that small packets will be dropped if buffers reduce the probability that small packets will be dropped if
(and only if) they allow small packets to borrow buffers from the (and only if) they allow small packets to borrow buffers from the
pools for larger packets. As was explained in Section 3.1.1 on fixed pools for larger packets. As was explained in Section 4.1.1 on fixed
size buffer carving, borrowing effectively makes the maximum queue size buffer carving, borrowing effectively makes the maximum queue
size for small packets greater than that for large packets, because size for small packets greater than that for large packets, because
more buffers can be used by small packets while less will fit large more buffers can be used by small packets while less will fit large
packets. packets.
In itself, the bias towards small packets caused by buffer borrowing In itself, the bias towards small packets caused by buffer borrowing
is perfectly correct. Lower drop probability for small packets is is perfectly correct. Lower drop probability for small packets is
legitimate in buffer borrowing schemes, because small packets legitimate in buffer borrowing schemes, because small packets
genuinely congest the machine's buffer memory less than large genuinely congest the machine's buffer memory less than large
packets, given they can fit in more spaces. The bias towards small packets, given they can fit in more spaces. The bias towards small
skipping to change at page 16, line 28 skipping to change at page 20, line 21
mode drop. 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 a lock-out large packets, purely because of the tail-drop aspect. So a
good AQM algorithm like RED with packet-mode drop should be used with good AQM algorithm like RED with packet-mode drop should be used with
fixed buffer memories where possible. If RED is too complicated to fixed buffer memories where possible. If RED 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 lock-out is to ensure smaller packets never use
the last available buffer in any of the pools for larger packets. the last available buffer in any of the pools for larger packets.
3.2.2. 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
(unless one counts a reference in an informational RFC to an archived (unless one counts a reference in an informational RFC to an archived
email!). Whereas, within the IETF, there are many different email!). Whereas, within the IETF, there are many different
proposals to alter transport protocols to achieve the same goals, proposals to alter transport protocols to achieve the same goals,
i.e. either to make the flow bit-rate take account of packet size, or i.e. either to make the flow bit-rate take account of packet size, or
to protect control packets from loss. This memo argues that altering to protect control packets from loss. This memo argues that altering
transport protocols is the more principled approach. transport protocols is the more principled approach.
skipping to change at page 17, line 27 skipping to change at page 21, line 20
conclusive, instead reporting simulations of many of the conclusive, instead reporting simulations of many of the
possibilities in order to assess performance but not recommending any possibilities in order to assess performance but not recommending any
particular course of action. 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 Section 3.2.4). Incidentally, VP-TFRC also proposed that byte- see Section 4.2.4). Incidentally, VP-TFRC also proposed that byte-
mode RED dropping should really square the packet size compensation mode RED dropping should really square the packet size compensation
factor (like that of RED_5, but apparently unaware of it). factor (like that of Cnodder's RED_5, but apparently unaware of it).
Pre-congestion notification [I-D.ietf-pcn] is a proposal to use a Pre-congestion notification [RFC5670] is a proposal to use a virtual
virtual queue for AQM marking for packets within one Diffserv class queue for AQM marking for packets within one Diffserv class in order
in order to give early warning prior to any real queuing. The to give early warning prior to any real queuing. The proposed PCN
proposed PCN marking algorithms have been designed not to take marking algorithms have been designed not to take account of packet
account of packet size when forwarding through queues. Instead the size when forwarding through queues. Instead the general principle
general principle has been to take account of the sizes of marked has been to take account of the sizes of marked packets when
packets when monitoring the fraction of marking at the edge of the monitoring the fraction of marking at the edge of the network, as
network. recommended here.
3.2.3. 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 TCP changes would be both cases they note that the case for these two TCP changes would be
weaker if RED were biased against dropping small packets. We argue weaker if RED were biased against dropping small packets. We argue
here that these two proposals are a safer and more principled way to here that these two proposals are a safer and more principled way to
achieve TCP performance improvements than reverse engineering RED to achieve TCP performance improvements than reverse engineering RED to
benefit TCP. benefit TCP.
Although no proposals exist as far as we know, it would also be Although no proposals exist as far as we know, it would also be
possible and perfectly valid to make control packets robust against possible and perfectly valid to make control packets robust against
drop by explicitly requesting a lower drop probability using their drop by explicitly requesting a lower drop probability using their
Diffserv code point [RFC2474] to request a scheduling class with Diffserv code point [RFC2474] to request a scheduling class with
lower drop. lower drop.
The re-ECN protocol proposal [I-D.briscoe-tsvwg-re-ecn-tcp] is
designed so that transports can be made more robust against losing
control packets. It gives queues an incentive to optionally give
preference against drop to packets with the 'feedback not
established' codepoint in the proposed 'extended ECN' field. Senders
have incentives to use this codepoint sparingly, but they can use it
on control packets to reduce their chance of being dropped. For
instance, the proposed modification to TCP for re-ECN uses this
codepoint on the SYN and SYN-ACK.
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 efforts
Internet at minimal cost. Internet at minimal cost.
3.2.4. Congestion Coding: Summary of Status 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 byte |
| cc | mode drop) | byte mode drop) | mode drop) | | cc | mode drop) | byte mode drop) | 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(sp) | 1/(s.sqrt(p)) |
+-----------+----------------+-----------------+--------------------+ +-----------+----------------+-----------------+--------------------+
Table 1: Dependence of flow bit-rate per RTT on packet size s and Table 1: Dependence of flow bit-rate per RTT on packet size s and
drop rate p when network and/or transport bias towards small packets drop rate p when network and/or transport bias towards small packets
to varying degrees to varying degrees
Table 1 aims to summarise the positions we may now be in. Each Table 1 aims to summarise the potential effects of all the advice
column shows a different possible AQM behaviour in different queues from different sources. Each column shows a different possible AQM
in the network, using the terminology of Cnodder et al outlined behaviour in different queues in the network, using the terminology
earlier (RED_1 is basic RED with packet-mode drop). Each row shows a of Cnodder et al outlined earlier (RED_1 is basic RED with packet-
different transport behaviour: TCP [RFC5681] and TFRC [RFC3448] on mode drop). Each row shows a different transport behaviour: TCP
the top row with TFRC-SP [RFC4828] below. Suppressing all [RFC5681] and TFRC [RFC3448] on the top row with TFRC-SP [RFC4828]
inessential details the table shows that independence from packet below.
size should either be achievable by not altering the TCP transport in
a RED_5 network, or using the small packet TFRC-SP transport in a Let us assume that the goal is for the bit-rate of a flow to be
network without any byte-mode dropping RED (top right and bottom independent of packet size. Suppressing all inessential details, the
left). Top left is the `do nothing' scenario, while bottom right is table shows that this should either be achievable by not altering the
the `do-both' scenario in which bit-rate would become far too biased TCP transport in a RED_5 network, or using the small packet TFRC-SP
towards small packets. Of course, if any form of byte-mode dropping transport (or similar) in a network without any byte-mode dropping
RED has been deployed on a selection of congested queues, each path RED (top right and bottom left). Top left is the `do nothing'
will present a different hybrid scenario to its transport. scenario, while bottom right is the `do-both' scenario in which bit-
rate would become far too biased towards small packets. Of course,
if any form of byte-mode dropping RED has been deployed on a subset
of queues that congest, each path through the network will present a
different hybrid scenario to its transport.
Whatever, we can see that the linear byte-mode drop column in the Whatever, we can see that the linear byte-mode drop column in the
middle considerably complicates the Internet. It's a half-way house middle considerably complicates the Internet. It's a half-way house
that doesn't bias enough towards small packets even if one believes that doesn't bias enough towards small packets even if one believes
the network should be doing the biasing. We argue below that _all_ the network should be doing the biasing. Section 3 recommends that
network layer bias towards small packets should be turned off--if _all_ bias in network equipment towards small packets should be
indeed any equipment vendors have implemented it--leaving packet size turned off--if indeed any equipment vendors have implemented it--
bias solely as the preserve of the transport layer (solely the leaving packet size bias solely as the preserve of the transport
leftmost, packet-mode drop column). layer (solely the leftmost, packet-mode drop column).
4.2.5. RED Implementation Status
A survey has been conducted of 84 vendors to assess how widely drop A survey has been conducted of 84 vendors to assess how widely drop
probability based on packet size has been implemented in RED. Prior probability based on packet size has been implemented in RED. Prior
to the survey, an individual approach to Cisco received confirmation to the survey, an individual approach to Cisco received confirmation
that, having checked the code-base for each of the product ranges, that, having checked the code-base for each of the product ranges,
Cisco has not implemented any discrimination based on packet size in Cisco has not implemented any discrimination based on packet size in
any AQM algorithm in any of its products. Also an individual any AQM algorithm in any of its products. Also an individual
approach to Alcatel-Lucent drew a confirmation that it was very approach to Alcatel-Lucent drew a confirmation that it was very
likely that none of their products contained RED code that likely that none of their products contained RED code that
implemented any packet-size bias. implemented any packet-size bias.
skipping to change at page 19, line 43 skipping to change at page 23, line 31
Turning to our more formal survey (Table 2), about 19% of those Turning to our more formal survey (Table 2), about 19% of those
surveyed have replied so far, giving a sample size of 16. Although surveyed have replied so far, giving a sample size of 16. Although
we do not have permission to identify the respondents, we can say we do not have permission to identify the respondents, we can say
that those that have responded include most of the larger vendors, that those that have responded include most of the larger vendors,
covering a large fraction of the market. They range across the large covering a large fraction of the market. They range across the large
network equipment vendors at L3 & L2, firewall vendors, wireless network equipment vendors at L3 & L2, firewall vendors, wireless
equipment vendors, as well as large software businesses with a small equipment vendors, as well as large software businesses with a small
selection of networking products. So far, all those who have selection of networking products. So far, all those who have
responded have confirmed that they have not implemented the variant responded have confirmed that they have not implemented the variant
of RED with drop dependent on packet size (2 were fairly sure they of RED with drop dependent on packet size (2 were fairly sure they
had not but needed to check more thoroughly). had not but needed to check more thoroughly). We have established
that Linux does not implement RED with packet size drop bias,
although we have not investigated a wider range of open source code.
+-------------------------------+----------------+-----------------+ +-------------------------------+----------------+-----------------+
| Response | No. of vendors | %age of vendors | | Response | No. of vendors | %age 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 2: Vendor Survey on byte-mode drop variant of RED (lower drop Table 2: 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 have been given, the extra complexity of packet bias
code has been most prevalent, though one vendor had a more principled code has been most prevalent, though one vendor had a more principled
reason for avoiding it--similar to the argument of this document. We reason for avoiding it--similar to the argument of this document.
have established that Linux does not implement RED with packet size
drop bias, although we have not investigated a wider range of open
source code.
Finally, we repeat that RED's byte mode drop is not the only way to Finally, we repeat that RED's byte mode drop SHOULD be disabled, but
bias towards small packets--tail-drop tends to lock-out large packets active queue management such as RED SHOULD be enabled wherever
very effectively. Our survey was of vendor implementations, so we possible if we are to eradicate bias towards small packets--without
cannot be certain about operator deployment. But we believe many any AQM at all, tail-drop tends to lock-out large packets very
queues in the Internet are still tail-drop. The company of one of effectively.
the co-authors (BT) has widely deployed RED, but there are bound to
be many tail-drop queues, particularly in access network equipment Our survey was of vendor implementations, so we cannot be certain
and on middleboxes like firewalls, where RED is not always available. about operator deployment. But we believe many queues in the
Internet are still tail-drop. The company of one of the co-authors
(BT) has widely deployed RED, but many tail-drop queues are there are
bound to still exist, particularly in access network equipment and on
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 3.2.1, 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.
4. Outstanding Issues and Next Steps 5. Outstanding Issues and Next Steps
4.1. Bit-congestible World 5.1. Bit-congestible World
For a connectionless network with nearly all resources being bit- For a connectionless network with nearly all resources being bit-
congestible we believe the recommended position is now unarguably congestible we believe the recommended position is now unarguably
clear--that the network should not make allowance for packet sizes clear--that the network should not make allowance for packet sizes
and the transport should. This leaves two outstanding issues: and the transport should. This leaves two outstanding issues:
o How to handle any legacy of AQM with byte-mode drop already o How to handle any legacy of AQM with byte-mode drop already
deployed; deployed;
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 account of packet size.
The sample of returns from our vendor survey Section 3.2.4 suggest The sample of returns from our vendor survey Section 4.2.4 suggest
that byte-mode packet drop seems not to be implemented at all let that byte-mode packet drop seems not to be implemented at all let
alone deployed, or if it is, it is likely to be very sparse. alone deployed, or if it is, it is likely to be very sparse.
Therefore, we do not really need a migration strategy from all but Therefore, we do not really need a migration strategy from all but
nothing to nothing. nothing to nothing.
A programme of standards updates to take account of packet size in A programme of standards updates to take account of packet size in
transport congestion control protocols has started with TFRC-SP transport congestion control protocols has started with TFRC-SP
[RFC4828], while weighted TCPs implemented in the research community [RFC4828], while weighted TCPs implemented in the research community
[WindowPropFair] could form the basis of a future change to TCP [WindowPropFair] could form the basis of a future change to TCP
congestion control [RFC5681] itself. congestion control [RFC5681] itself.
4.2. Bit- & Packet-congestible World 5.2. Bit- & Packet-congestible World
Nonetheless, a connectionless network with both bit-congestible and Nonetheless, the position is much less clear-cut if the Internet
packet-congestible resources is a different matter. If we believe we becomes populated by a more even mix of both packet-congestible and
should allow for this possibility in the future, this space contains bit-congestible resources. If we believe we should allow for this
a truly open research issue. possibility in the future, this space contains a truly open research
issue.
We develop the concept of an idealised congestion notification We develop the concept of an idealised congestion notification
protocol that supports both bit-congestible and packet-congestible protocol that supports both bit-congestible and packet-congestible
resources in Appendix B. The congestion notification requires at resources in Appendix A. This congestion notification requires at
least two flags for congestion of bit-congestible and packet- least two flags for congestion of bit-congestible and packet-
congestible resources. This hides a fundamental problem--much more congestible resources. This hides a fundamental problem--much more
fundamental than whether we can magically create header space for yet fundamental than whether we can magically create header space for yet
another ECN flag in IPv4, or whether it would work while being another ECN flag in IPv4, or whether it would work while being
deployed incrementally. A congestion notification protocol must deployed incrementally. Distinguishing drop from delivery naturally
survive a transition from low levels of congestion to high. Marking provides just one congestion flag--it is hard to drop a packet in two
two states is feasible with explicit marking, but much harder if ways that are distinguishable remotely. This is a similar problem to
packets are dropped. Also, it will not always be cost-effective to that of distinguishing wireless transmission losses from congestive
implement AQM at every low level resource, so drop will often have to losses.
suffice. Distinguishing drop from delivery naturally provides just
one congestion flag--it is hard to drop a packet in two ways that are This problem would not be solved even if ECN were universally
distinguishable remotely. This is a similar problem to that of deployed. A congestion notification protocol must survive a
distinguishing wireless transmission losses from congestive losses. transition from low levels of congestion to high. Marking two states
is feasible with explicit marking, but much harder if packets are
dropped. Also, it will not always be cost-effective to implement AQM
at every low level resource, so drop will often have to suffice.
We should also note that, strictly, packet-congestible resources are We should also note that, strictly, packet-congestible resources are
actually cycle-congestible because load also depends on the actually cycle-congestible because load also depends on the
complexity of each look-up and whether the pattern of arrivals is complexity of each look-up and whether the pattern of arrivals is
amenable to caching or not. Further, this reminds us that any amenable to caching or not. Further, this reminds us that any
solution must not require a forwarding engine to use excessive solution must not require a forwarding engine to use excessive
processor cycles in order to decide how to say it has no spare processor cycles in order to decide how to say it has no spare
processor cycles. processor cycles.
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 layer is (irrespective of whether more function in the network is desirable).
desirable). Consequently the premise is that CPU congestion will Consequently the premise is that CPU congestion will become more
become more common. DRQ is a proposed modification to the RED common. DRQ is a proposed modification to the RED algorithm that
algorithm that folds both bit congestion and packet congestion into folds both bit congestion and packet congestion into one signal
one signal (either loss or ECN). (either loss or ECN).
The problem of signalling packet processing congestion is not The problem of signalling packet processing congestion is not
pressing, as most Internet resources are designed to be bit- pressing, as most Internet resources are designed to be bit-
congestible before packet processing starts to congest (see congestible before packet processing starts to congest (see
Section 1.1). However, the IRTF Internet congestion control research Section 1.1). However, the IRTF Internet congestion control research
group (ICCRG) has set itself the task of reaching consensus on group (ICCRG) has set itself the task of reaching consensus on
generic forwarding mechanisms that are necessary and sufficient to generic forwarding mechanisms that are necessary and sufficient to
support the Internet's future congestion control requirements (the support the Internet's future congestion control requirements (the
first challenge in [I-D.irtf-iccrg-welzl]). Therefore, rather than first challenge in [I-D.irtf-iccrg-welzl]). Therefore, rather than
not giving this problem any thought at all, just because it is hard not giving this problem any thought at all, just because it is hard
and currently hypothetical, we defer the question of whether packet and currently hypothetical, we defer the question of whether packet
congestion might become common and what to do if it does to the IRTF congestion might become common and what to do if it does to the IRTF
(the 'Small Packets' challenge in [I-D.irtf-iccrg-welzl]). (the 'Small Packets' challenge in [I-D.irtf-iccrg-welzl]).
5. Recommendation and Conclusions
5.1. Recommendation on Queue Measurement
Queue length is usually the most correct and simplest way to measure
congestion of a resource. To avoid the pathological effects of drop
tail, an AQM function can then be used to transform queue length into
the probability of dropping or marking a packet (e.g. RED's
piecewise linear function between thresholds).
If the resource is bit-congestible, the length of the queue SHOULD be
measured in bytes. If the resource is packet-congestible, the length
of the queue SHOULD be measured in packets. No other choice makes
sense, because the number of packets waiting in the queue isn't
relevant if the resource gets congested by bytes and vice versa. We
discuss the implications on RED's byte mode and packet mode for
measuring queue length in Section 3.
NOTE WELL that RED's byte-mode queue measurement is fine, being
completely orthogonal to byte-mode drop. If a 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 fine. However, if in
doubt, the vendor should be consulted.
5.2. Recommendation on Notifying Congestion
The strong recommendation is that AQM algorithms such as RED SHOULD
NOT use byte-mode drop. More generally, the Internet's congestion
notification protocols (drop, ECN & PCN) SHOULD take account of
packet size when the notification is read by the transport layer, NOT
when it is written by the network layer. This approach offers
sufficient and correct congestion information for all known and
future transport protocols and also ensures no perverse incentives
are created that would encourage transports to use inappropriately
small packet sizes.
The alternative of deflating RED's drop probability for smaller
packet sizes (byte-mode drop) has no enduring advantages. It is more
complex, it creates the perverse incentive to fragment segments into
tiny pieces and it reopens the vulnerability to floods of small-
packets that drop-tail queues suffered from and AQM was designed to
remove.
Byte-mode drop is a change to the network layer that makes allowance
for an omission from the design of TCP, effectively reverse
engineering the network layer to contrive to make two TCPs with
different packet sizes run at equal bit rates (rather than packet
rates) under the same path conditions.
It also improves TCP performance by reducing the chance that a SYN or
a pure ACK will be dropped, because they are small. But we SHOULD
NOT hack the network layer to improve or fix certain transport
protocols. No matter how predominant a transport protocol is (even
if it's TCP), trying to correct for its failings by biasing towards
small packets in the network layer creates a perverse incentive to
break down all flows from all transports into tiny segments.
So far, our survey of 84 vendors across the industry has drawn
responses from about 19%, none of whom have implemented the byte mode
packet drop variant of RED. Given there appears to be little, if
any, installed base it seems we can recommend removal of byte-mode
drop from RED with little, if any, incremental deployment impact.
If a vendor has implemented byte-mode drop, and an operator has
turned it on, it is strongly RECOMMENDED that it SHOULD be turned
off. Note that RED as a whole SHOULD NOT be turned off, as without
it, a drop tail queue also biases against large packets. But note
also that turning off byte-mode may alter the relative performance of
applications using different packet sizes, so it would be advisable
to establish the implications before turning it off.
5.3. Recommendation on Responding to Congestion
Instead of network equipment biasing its congestion notification for
small packets, the IETF transport area should continue its programme
of updating congestion control protocols to take account of packet
size and to make transports less sensitive to losing control packets
like SYNs and pure ACKS.
5.4. Recommended Future Research
The above conclusions cater for the Internet as it is today with
most, if not all, resources being primarily bit-congestible. A
secondary conclusion of this memo is that we may see more packet-
congestible resources in the future, so research may be needed to
extend the Internet's congestion notification (drop or ECN) so that
it can handle a mix of bit-congestible and packet-congestible
resources.
6. Security Considerations 6. Security Considerations
This draft recommends that queues do not bias drop probability This draft recommends that queues do not bias drop probability
towards small packets as this creates a perverse incentive for towards small packets as this creates a perverse incentive for
transports to break down their flows into tiny segments. One of the transports to break down their flows into tiny segments. One of the
benefits of implementing AQM was meant to be to remove this perverse benefits of implementing AQM was meant to be to remove this perverse
incentive that drop-tail queues gave to small packets. Of course, if incentive that drop-tail queues gave to small packets. Of course, if
transports really want to make the greatest gains, they don't have to transports really want to make the greatest gains, they don't have to
respond to congestion anyway. But we don't want applications that respond to congestion anyway. But we don't want applications that
are trying to behave to discover that they can go faster by using are trying to behave to discover that they can go faster by using
skipping to change at page 24, line 52 skipping to change at page 26, line 43
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 B 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 not byte-mode drop. In
summary, it says that making drop probability depend on the size of summary, it says that making drop probability depend on the size of
the packets that bits happen to be divided into simply encourages the the packets that bits happen to be divided into simply encourages the
bits to be divided into smaller packets. Byte-mode drop would 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. Acknowledgements 7. Conclusions
This memo strongly recommends that the size of an individual packet
that is dropped or marked should only be taken into account when a
transport reads this as a congestion indication, not when network
equipment writes it. The memo therefore strongly deprecates using
RED's byte-mode of packet drop in network equipment.
Whether network equipment should measure the length of a queue by
counting bytes or counting packets is a different question to whether
it should take into account the size of each packet being dropped or
marked. The answer depends on whether the network resource is
congested respectively by bytes or by packets. This means that RED's
byte-mode queue measurement will often be appropriate even though
byte-mode drop is strongly deprecated.
At the transport layer the IETF should continue updating congestion
control protocols to take account of the size of each packet that
indicates congestion. Also the IETF should continue to make
transports less sensitive to losing control packets like SYNs, pure
ACKs and DNS exchanges. Although many control packets happen to be
small, the alternative of network equipment favouring all small
packets would be dangerous. That would create perverse incentives to
split data transfers into smaller packets.
The memo develops these recommendations from principled arguments
concerning scaling, layering, incentives, inherent efficiency,
security and policability. But it also addresses practical issues
such as specific buffer architectures and incremental deployment.
Indeed a limited survey of RED implementations is included, which
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,
incremental deployment complications.
The recommendations have been developed on the well-founded basis
that most Internet resources are bit-congestible not packet-
congestible. We need to know the likelihood that this assumption
will prevail longer term and, if it might not, what protocol changes
will be needed to cater for a mix of the two. These questions have
been delegated to the IRTF.
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, Toby comments. Also thanks for the reviews from Philip Eardley, Toby
Moncaster and Arnaud Jacquet as well as helpful explanations of Moncaster and Arnaud Jacquet as well as helpful explanations of
different hardware approaches from Larry Dunn and Fred Baker. I am different hardware approaches from Larry Dunn and Fred Baker. I am
grateful to Bruce Davie and his colleagues for providing a timely and grateful 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 are partly funded by Trilogy, a research Bob Briscoe and Jukka Manner are partly funded by Trilogy, a research
project (ICT- 216372) supported by the European Community under its project (ICT- 216372) supported by the European Community under its
Seventh Framework Programme. The views expressed here are those of Seventh Framework Programme. The views expressed here are those of
the authors only. the authors only.
8. Comments Solicited 9. Comments Solicited
Comments and questions are encouraged and very welcome. They can be Comments and questions are encouraged and very welcome. They can be
addressed to the IETF Transport Area working group mailing list addressed to the IETF Transport Area working group mailing list
<tsvwg@ietf.org>, and/or to the authors. <tsvwg@ietf.org>, and/or to the authors.
9. References 10. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in
RFCs to Indicate Requirement Levels",
BCP 14, RFC 2119, March 1997.
[RFC2309] Braden, B., Clark, D., Crowcroft, J.,
Davie, B., Deering, S., Estrin, D.,
Floyd, S., Jacobson, V., Minshall,
G., Partridge, C., Peterson, L.,
Ramakrishnan, K., Shenker, S.,
Wroclawski, J., and L. Zhang,
"Recommendations on Queue Management
and Congestion Avoidance in the
Internet", RFC 2309, April 1998.
[RFC3168] Ramakrishnan, K., Floyd, S., and D.
Black, "The Addition of Explicit
Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC3426] Floyd, S., "General Architectural and
Policy Considerations", RFC 3426,
November 2002.
[RFC5033] Floyd, S. and M. Allman, "Specifying 10.1. Normative References
New Congestion Control Algorithms",
BCP 133, RFC 5033, August 2007.
9.2. Informative References [RFC2119] Bradner, S., "Key words for use in RFCs
to Indicate Requirement Levels", BCP 14,
RFC 2119, March 1997.
[CCvarPktSize] Widmer, J., Boutremans, C., and J-Y. [RFC2309] Braden, B., Clark, D., Crowcroft, J.,
Le Boudec, "Congestion Control for Davie, B., Deering, S., Estrin, D.,
Flows with Variable Packet Size", ACM Floyd, S., Jacobson, V., Minshall, G.,
CCR 34(2) 137--151, 2004, <http:// Partridge, C., Peterson, L.,
doi.acm.org/10.1145/997150.997162>. Ramakrishnan, K., Shenker, S.,
Wroclawski, J., and L. Zhang,
"Recommendations on Queue Management and
Congestion Avoidance in the Internet",
RFC 2309, April 1998.
[DRQ] Shin, M., Chong, S., and I. Rhee, [RFC3168] Ramakrishnan, K., Floyd, S., and D.
"Dual-Resource TCP/AQM for Black, "The Addition of Explicit
Processing-Constrained Networks", Congestion Notification (ECN) to IP",
IEEE/ACM Transactions on RFC 3168, September 2001.
Networking Vol 16, issue 2,
April 2008, <http://dx.doi.org/
10.1109/TNET.2007.900415>.
[DupTCP] Wischik, D., "Short messages", Royal [RFC3426] Floyd, S., "General Architectural and
Society workshop on networks: Policy Considerations", RFC 3426,
modelling and control , November 2002.
September 2007, <http://
www.cs.ucl.ac.uk/staff/ucacdjw/
Research/shortmsg.html>.
[ECNFixedWireless] Siris, V., "Resource Control for [RFC5033] Floyd, S. and M. Allman, "Specifying New
Elastic Traffic in CDMA Networks", Congestion Control Algorithms", BCP 133,
Proc. ACM MOBICOM'02 , RFC 5033, August 2007.
September 2002, <http://
www.ics.forth.gr/netlab/publications/
resource_control_elastic_cdma.html>.
[Evol_cc] Gibbens, R. and F. Kelly, "Resource 10.2. Informative References
pricing and the evolution of
congestion control",
Automatica 35(12)1969--1985,
December 1999, <http://
www.statslab.cam.ac.uk/~frank/
evol.html>.
[I-D.briscoe-tsvwg-re-ecn-tcp] Briscoe, B., Jacquet, A., Moncaster, [CCvarPktSize] Widmer, J., Boutremans, C., and J-Y. Le
T., and A. Smith, "Re-ECN: Adding Boudec, "Congestion Control for Flows
Accountability for Causing Congestion with Variable Packet Size", ACM CCR 34(2)
to TCP/IP", 137--151, 2004, <http://doi.acm.org/
draft-briscoe-tsvwg-re-ecn-tcp-08 10.1145/997150.997162>.
(work in progress), September 2009.
[I-D.ietf-pcn] Eardley, P., "Metering and marking [DRQ] Shin, M., Chong, S., and I. Rhee, "Dual-
behaviour of PCN-nodes", Resource TCP/AQM for Processing-
draft-ietf-pcn-marking-behaviour-05 Constrained Networks", IEEE/ACM
(work in progress), August 2009. Transactions on Networking Vol 16, issue
2, April 2008, <http://dx.doi.org/
10.1109/TNET.2007.900415>.
[I-D.irtf-iccrg-welzl] Welzl, M., Scharf, M., Briscoe, B., [DupTCP] Wischik, D., "Short messages", Royal
and D. Papadimitriou, "Open Research Society workshop on networks: modelling
Issues in Internet Congestion and control , September 2007, <http://
Control", draft-irtf-iccrg-welzl- www.cs.ucl.ac.uk/staff/ucacdjw/Research/
congestion-control-open-research-07 shortmsg.html>.
(work in progress), June 2010.
[IOSArch] Bollapragada, V., White, R., and C. [ECNFixedWireless] Siris, V., "Resource Control for Elastic
Murphy, "Inside Cisco IOS Software Traffic in CDMA Networks", Proc. ACM
Architecture", Cisco Press: CCIE MOBICOM'02 , September 2002, <http://
Professional Development ISBN13: 978- www.ics.forth.gr/netlab/publications/
1-57870-181-0, July 2000. resource_control_elastic_cdma.html>.
[MulTCP] Crowcroft, J. and Ph. Oechslin, [Evol_cc] Gibbens, R. and F. Kelly, "Resource
"Differentiated End to End Internet pricing and the evolution of congestion
Services using a Weighted control", Automatica 35(12)1969--1985,
Proportional Fair Sharing TCP", December 1999, <http://
CCR 28(3) 53--69, July 1998, <http:// www.statslab.cam.ac.uk/~frank/evol.html>.
www.cs.ucl.ac.uk/staff/J.Crowcroft/
hipparch/pricing.html>.
[PktSizeEquCC] Vasallo, P., "Variable Packet Size [I-D.conex-concepts-uses] Briscoe, B., Woundy, R., Moncaster, T.,
Equation-Based Congestion Control", and J. Leslie, "ConEx Concepts and Use
ICSI Technical Report tr-00-008, Cases",
2000, <http://http.icsi.berkeley.edu/ draft-moncaster-conex-concepts-uses-01
ftp/global/pub/techreports/2000/ (work in progress), July 2010.
tr-00-008.pdf>.
[RED93] Floyd, S. and V. Jacobson, "Random [I-D.ietf-avt-ecn-for-rtp] Westerlund, M., Johansson, I., Perkins,
Early Detection (RED) gateways for C., and K. Carlberg, "Explicit Congestion
Congestion Avoidance", IEEE/ACM Notification (ECN) for RTP over UDP",
Transactions on Networking 1(4) 397-- draft-ietf-avt-ecn-for-rtp-02 (work in
413, August 1993, <http:// progress), July 2010.
www.icir.org/floyd/papers/red/
red.html>.
[REDbias] Eddy, W. and M. Allman, "A Comparison [I-D.irtf-iccrg-welzl] Welzl, M., Scharf, M., Briscoe, B., and
of RED's Byte and Packet Modes", D. Papadimitriou, "Open Research Issues
Computer Networks 42(3) 261--280, in Internet Congestion Control", draft-
June 2003, <http://www.ir.bbn.com/ irtf-iccrg-welzl-congestion-control-open-
documents/articles/redbias.ps>. research-08 (work in progress),
September 2010.
[REDbyte] De Cnodder, S., Elloumi, O., and K. [IOSArch] Bollapragada, V., White, R., and C.
Pauwels, "RED behavior with different Murphy, "Inside Cisco IOS Software
packet sizes", Proc. 5th IEEE Architecture", Cisco Press: CCIE
Symposium on Computers and Professional Development ISBN13: 978-1-
Communications (ISCC) 793--799, 57870-181-0, July 2000.
July 2000, <http://www.icir.org/
floyd/red/Elloumi99.pdf>.
[RFC2474] Nichols, K., Blake, S., Baker, F., [MulTCP] Crowcroft, J. and Ph. Oechslin,
and D. Black, "Definition of the "Differentiated End to End Internet
Differentiated Services Field (DS Services using a Weighted Proportional
Field) in the IPv4 and IPv6 Headers", Fair Sharing TCP", CCR 28(3) 53--69,
RFC 2474, December 1998. July 1998, <http://www.cs.ucl.ac.uk/
staff/J.Crowcroft/hipparch/pricing.html>.
[RFC3448] Handley, M., Floyd, S., Padhye, J., [PktSizeEquCC] Vasallo, P., "Variable Packet Size
and J. Widmer, "TCP Friendly Rate Equation-Based Congestion Control", ICSI
Control (TFRC): Protocol Technical Report tr-00-008, 2000, <http:/
Specification", RFC 3448, /http.icsi.berkeley.edu/ftp/global/pub/
January 2003. techreports/2000/tr-00-008.pdf>.
[RFC3714] Floyd, S. and J. Kempf, "IAB Concerns [RED93] Floyd, S. and V. Jacobson, "Random Early
Regarding Congestion Control for Detection (RED) gateways for Congestion
Voice Traffic in the Internet", Avoidance", IEEE/ACM Transactions on
RFC 3714, March 2004. Networking 1(4) 397--413, August 1993, <h
ttp://www.icir.org/floyd/papers/red/
red.html>.
[RFC4782] Floyd, S., Allman, M., Jain, A., and [REDbias] Eddy, W. and M. Allman, "A Comparison of
P. Sarolahti, "Quick-Start for TCP RED's Byte and Packet Modes", Computer
and IP", RFC 4782, January 2007. Networks 42(3) 261--280, June 2003, <http
://www.ir.bbn.com/documents/articles/
redbias.ps>.
[RFC4828] Floyd, S. and E. Kohler, "TCP [REDbyte] De Cnodder, S., Elloumi, O., and K.
Friendly Rate Control (TFRC): The Pauwels, "RED behavior with different
Small-Packet (SP) Variant", RFC 4828, packet sizes", Proc. 5th IEEE Symposium
April 2007. on Computers and Communications
(ISCC) 793--799, July 2000, <http://
www.icir.org/floyd/red/Elloumi99.pdf>.
[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, [RFC2474] Nichols, K., Blake, S., Baker, F., and D.
S., and K. Ramakrishnan, "Adding Black, "Definition of the Differentiated
Explicit Congestion Notification Services Field (DS Field) in the IPv4 and
(ECN) Capability to TCP's SYN/ACK IPv6 Headers", RFC 2474, December 1998.
Packets", RFC 5562, June 2009.
[RFC5670] Eardley, P., "Metering and Marking [RFC3448] Handley, M., Floyd, S., Padhye, J., and
Behaviour of PCN-Nodes", RFC 5670, J. Widmer, "TCP Friendly Rate Control
November 2009. (TFRC): Protocol Specification",
RFC 3448, January 2003.
[RFC5681] Allman, M., Paxson, V., and E. [RFC3714] Floyd, S. and J. Kempf, "IAB Concerns
Blanton, "TCP Congestion Control", Regarding Congestion Control for Voice
RFC 5681, September 2009. Traffic in the Internet", RFC 3714,
March 2004.
[RFC5690] Floyd, S., Arcia, A., Ros, D., and J. [RFC4828] Floyd, S. and E. Kohler, "TCP Friendly
Iyengar, "Adding Acknowledgement Rate Control (TFRC): The Small-Packet
Congestion Control to TCP", RFC 5690, (SP) Variant", RFC 4828, April 2007.
February 2010.
[Rate_fair_Dis] Briscoe, B., "Flow Rate Fairness: [RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S.,
Dismantling a Religion", ACM and K. Ramakrishnan, "Adding Explicit
CCR 37(2)63--74, April 2007, <http:// Congestion Notification (ECN) Capability
portal.acm.org/ to TCP's SYN/ACK Packets", RFC 5562,
citation.cfm?id=1232926>. June 2009.
[WindowPropFair] Siris, V., "Service Differentiation [RFC5670] Eardley, P., "Metering and Marking
and Performance of Weighted Window- Behaviour of PCN-Nodes", RFC 5670,
Based Congestion Control and Packet November 2009.
Marking Algorithms in ECN Networks",
Computer Communications 26(4) 314--
326, 2002, <http://www.ics.forth.gr/
netgroup/publications/
weighted_window_control.html>.
[gentle_RED] Floyd, S., "Recommendation on using [RFC5681] Allman, M., Paxson, V., and E. Blanton,
the "gentle_" variant of RED", Web "TCP Congestion Control", RFC 5681,
page , March 2000, <http:// September 2009.
www.icir.org/floyd/red/gentle.html>.
[pBox] Floyd, S. and K. Fall, "Promoting the [RFC5690] Floyd, S., Arcia, A., Ros, D., and J.
Use of End-to-End Congestion Control Iyengar, "Adding Acknowledgement
in the Internet", IEEE/ACM Congestion Control to TCP", RFC 5690,
Transactions on Networking 7(4) 458-- February 2010.
472, August 1999, <http://
www.aciri.org/floyd/
end2end-paper.html>.
[pktByteEmail] Yes and J. Doe, "Missing for now", [Rate_fair_Dis] Briscoe, B., "Flow Rate Fairness:
RFC 0000, May 2006. Dismantling a Religion", ACM
CCR 37(2)63--74, April 2007, <http://
portal.acm.org/citation.cfm?id=1232926>.
[xcp-spec] Falk, A., "Specification for the [WindowPropFair] Siris, V., "Service Differentiation and
Explicit Control Protocol (XCP)", Performance of Weighted Window-Based
draft-falk-xcp-spec-03 (work in Congestion Control and Packet Marking
progress), July 2007. Algorithms in ECN Networks", Computer
Communications 26(4) 314--326, 2002, <htt
p://www.ics.forth.gr/netgroup/
publications/
weighted_window_control.html>.
Appendix A. Congestion Notification Definition: Further Justification [gentle_RED] Floyd, S., "Recommendation on using the
"gentle_" variant of RED", Web page ,
March 2000, <http://www.icir.org/floyd/
red/gentle.html>.
In Section 1.1 on the definition of congestion notification, load not [pBox] Floyd, S. and K. Fall, "Promoting the Use
capacity was used as the denominator. This also has a subtle of End-to-End Congestion Control in the
significance in the related debate over the design of new transport Internet", IEEE/ACM Transactions on
protocols--typical new protocol designs (e.g. in XCP [xcp-spec] & Networking 7(4) 458--472, August 1999, <h
Quickstart [RFC4782]) expect the sending transport to communicate its ttp://www.aciri.org/floyd/
desired flow rate to the network and network elements to end2end-paper.html>.
progressively subtract from this so that the achievable flow rate
emerges at the receiving transport.
Congestion notification with total load in the denominator can serve [pktByteEmail] Floyd, S., "RED: Discussions of Byte and
a similar purpose (though in retrospect not in advance like XCP & Packet Modes", email , March 1997, <http:
QuickStart). Congestion notification is a dimensionless fraction but //www-nrg.ee.lbl.gov/floyd/
each source can extract necessary rate information from it because it REDaveraging.txt>.
already knows what its own rate is. Even though congestion
notification doesn't communicate a rate explicitly, from each
source's point of view congestion notification represents the
fraction of the rate it was sending a round trip ago that couldn't
(or wouldn't) be served by available resources.
Appendix B. Idealised Wire Protocol Appendix A. Idealised Wire Protocol
We will start by inventing an idealised congestion notification We will start by inventing an idealised congestion notification
protocol before discussing how to make it practical. The idealised protocol before discussing how to make it practical. The idealised
protocol is shown to be correct using examples later in this protocol is shown to be correct using examples later in this
appendix. appendix.
B.1. Protocol Coding A.1. Protocol Coding
Congestion notification involves the congested resource coding a Congestion notification involves the congested resource coding a
congestion notification signal into the packet stream and the congestion notification signal into the packet stream and the
transports decoding it. The idealised protocol uses two different transports decoding it. The idealised protocol uses two different
(imaginary) fields in each datagram to signal congestion: one for (imaginary) fields in each datagram to signal congestion: one for
byte congestion and one for packet congestion. byte congestion and one for packet congestion.
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
skipping to change at page 31, line 18 skipping to change at page 33, line 9
distinguish between bit and packet congestion [RFC3714]. Currently, distinguish between bit and packet congestion [RFC3714]. Currently,
packet-congestion is not the common case, but there is no guarantee packet-congestion is not the common case, but there is no guarantee
that it will not become common with future technology trends. that it will not become common with future technology trends.
The idealised wire protocol is given below. It accounts for packet The idealised wire protocol is given below. It accounts for packet
sizes at the transport layer, not in the network, and then only in sizes at the transport layer, not in the network, and then only in
the case of bit-congestible resources. This avoids the perverse the case of bit-congestible resources. This avoids the perverse
incentive to send smaller packets and the DoS vulnerability that incentive to send smaller packets and the DoS vulnerability that
would otherwise result if the network were to bias towards them (see would otherwise result if the network were to bias towards them (see
the motivating argument about avoiding perverse incentives in the motivating argument about avoiding perverse incentives in
Section 2.2): Section 2.3):
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.
The worked examples in Appendix B.2 show that transports can extract The worked examples in Appendix A.2 show that transports can extract
sufficient and correct congestion notification from these protocols sufficient and correct congestion notification from these protocols
for cases when two flows with different packet sizes have matching for cases when two flows with different packet sizes have matching
bit rates or matching packet rates. Examples are also given that mix bit rates or matching packet rates. Examples are also given that mix
these two flows into one to show that a flow with mixed packet sizes these two flows into one to show that a flow with mixed packet sizes
would still be able to extract sufficient and correct information. would still be able to extract sufficient and correct information.
Sufficient and correct congestion information means that there is Sufficient and correct congestion information means that there is
sufficient information for the two different types of transport sufficient information for the two different types of transport
requirements: requirements:
skipping to change at page 32, line 16 skipping to change at page 34, line 8
Absolute-target-based: Other congestion controls proposed in the Absolute-target-based: Other congestion controls proposed in the
research community aim to limit the volume of congestion caused to research community aim to limit the volume of congestion caused to
a constant weight parameter. [MulTCP][WindowPropFair] are a constant weight parameter. [MulTCP][WindowPropFair] are
examples of weighted proportionally fair transports designed for examples of weighted proportionally fair transports designed for
cost-fair environments [Rate_fair_Dis]. In this case, the cost-fair environments [Rate_fair_Dis]. In this case, the
transport requires a count (not a ratio) of dropped/marked bytes transport requires a count (not a ratio) of dropped/marked bytes
in the bit-congestible case and of dropped/marked packets in the in the bit-congestible case and of dropped/marked packets in the
packet congestible case. packet congestible case.
B.2. Example Scenarios A.2. Example Scenarios
B.2.1. Notation A.2.1. Notation
To prove our idealised wire protocol (Appendix B.1) is correct, we To prove our idealised wire protocol (Appendix A.1) is correct, we
will compare two flows with different packet sizes, s_1 and s_2 [bit/ will compare two flows with different packet sizes, s_1 and s_2 [bit/
pkt], to make sure their transports each see the correct congestion pkt], to make sure their transports each see the correct congestion
notification. Initially, within each flow we will take all packets notification. Initially, within each flow we will take all packets
as having equal sizes, but later we will generalise to flows within as having equal sizes, but later we will generalise to flows within
which packet sizes vary. A flow's bit rate, x [bit/s], is related to which packet sizes vary. A flow's bit rate, x [bit/s], is related to
its packet rate, u [pkt/s], by its packet rate, u [pkt/s], by
x(t) = s.u(t). x(t) = s.u(t).
We will consider a 2x2 matrix of four scenarios: We will consider a 2x2 matrix of four scenarios:
skipping to change at page 32, line 42 skipping to change at page 34, line 34
+-----------------------------+------------------+------------------+ +-----------------------------+------------------+------------------+
| resource type and | A) Equal bit | B) Equal pkt | | resource type and | A) Equal bit | B) Equal pkt |
| congestion level | rates | rates | | congestion level | rates | rates |
+-----------------------------+------------------+------------------+ +-----------------------------+------------------+------------------+
| i) bit-congestible, p_b | (Ai) | (Bi) | | i) bit-congestible, p_b | (Ai) | (Bi) |
| ii) pkt-congestible, p_p | (Aii) | (Bii) | | ii) pkt-congestible, p_p | (Aii) | (Bii) |
+-----------------------------+------------------+------------------+ +-----------------------------+------------------+------------------+
Table 3 Table 3
B.2.2. Bit-congestible resource, equal bit rates (Ai) A.2.2. Bit-congestible resource, equal bit rates (Ai)
Starting with the bit-congestible scenario, for two flows to maintain Starting with the bit-congestible scenario, for two flows to maintain
equal bit rates (Ai) the ratio of the packet rates must be the equal bit rates (Ai) the ratio of the packet rates must be the
inverse of the ratio of packet sizes: u_2/u_1 = s_1/s_2. So, for inverse of the ratio of packet sizes: u_2/u_1 = s_1/s_2. So, for
instance, a flow of 60B packets would have to send 25x more packets instance, a flow of 60B packets would have to send 25x more packets
to achieve the same bit rate as a flow of 1500B packets. If a to achieve the same bit rate as a flow of 1500B packets. If a
congested resource marks proportion p_b of packets irrespective of congested resource marks proportion p_b of packets irrespective of
size, the ratio of marked packets received by each transport will size, the ratio of marked packets received by each transport will
still be the same as the ratio of their packet rates, p_b.u_2/p_b.u_1 still be the same as the ratio of their packet rates, p_b.u_2/p_b.u_1
= s_1/s_2. So of the 25x more 60B packets sent, 25x more will be = s_1/s_2. So of the 25x more 60B packets sent, 25x more will be
marked than in the 1500B packet flow, but 25x more won't be marked marked than in the 1500B packet flow, but 25x more won't be marked
too. too.
In this scenario, the resource is bit-congestible, so it always uses In this scenario, the resource is bit-congestible, so it always uses
our idealised bit-congestion field when it marks packets. Therefore our idealised bit-congestion field when it marks packets. Therefore
the transport should count marked bytes not packets. But it doesn't the transport should count marked bytes not packets. But it doesn't
actually matter for ratio-based transports like TCP (Appendix B.1). actually matter for ratio-based transports like TCP (Appendix A.1).
The ratio of marked to unmarked bytes seen by each flow will be p_b, The ratio of marked to unmarked bytes seen by each flow will be p_b,
as will the ratio of marked to unmarked packets. Because they are as will the ratio of marked to unmarked packets. Because they are
ratios, the units cancel out. ratios, the units cancel out.
If a flow sent an inconsistent mixture of packet sizes, we have said If a flow sent an inconsistent mixture of packet sizes, we have said
it should count the ratio of marked and unmarked bytes not packets in it should count the ratio of marked and unmarked bytes not packets in
order to correctly decode the level of congestion. But actually, if order to correctly decode the level of congestion. But actually, if
all it is trying to do is decode p_b, it still doesn't matter. For all it is trying to do is decode p_b, it still doesn't matter. For
instance, imagine the two equal bit rate flows were actually one flow instance, imagine the two equal bit rate flows were actually one flow
at twice the bit rate sending a mixture of one 1500B packet for every at twice the bit rate sending a mixture of one 1500B packet for every
thirty 60B packets. 25x more small packets will be marked and 25x thirty 60B packets. 25x more small packets will be marked and 25x
more will be unmarked. The transport can still calculate p_b whether more will be unmarked. The transport can still calculate p_b whether
it uses bytes or packets for the ratio. In general, for any it uses bytes or packets for the ratio. In general, for any
algorithm which works on a ratio of marks to non-marks, either bytes algorithm which works on a ratio of marks to non-marks, either bytes
or packets can be counted interchangeably, because the choice cancels or packets can be counted interchangeably, because the choice cancels
out in the ratio calculation. out in the ratio calculation.
However, where an absolute target rather than relative volume of However, where an absolute target rather than relative volume of
congestion caused is important (Appendix B.1), as it is for congestion caused is important (Appendix A.1), as it is for
congestion accountability [Rate_fair_Dis], the transport must count congestion accountability [Rate_fair_Dis], the transport must count
marked bytes not packets, in this bit-congestible case. Aside from marked bytes not packets, in this bit-congestible case. Aside from
the goal of congestion accountability, this is how the bit rate of a the goal of congestion accountability, this is how the bit rate of a
transport can be made independent of packet size; by ensuring the transport can be made independent of packet size; by ensuring the
rate of congestion caused is kept to a constant weight rate of congestion caused is kept to a constant weight
[WindowPropFair], rather than merely responding to the ratio of [WindowPropFair], rather than merely responding to the ratio of
marked and unmarked bytes. marked and unmarked bytes.
Note the unit of byte-congestion-volume is the byte. Note the unit of byte-congestion-volume is the byte.
B.2.3. Bit-congestible resource, equal packet rates (Bi) A.2.3. Bit-congestible resource, equal packet rates (Bi)
If two flows send different packet sizes but at the same packet rate, If two flows send different packet sizes but at the same packet rate,
their bit rates will be in the same ratio as their packet sizes, x_2/ their bit rates will be in the same ratio as their packet sizes, x_2/
x_1 = s_2/s_1. For instance, a flow sending 1500B packets at the x_1 = s_2/s_1. For instance, a flow sending 1500B packets at the
same packet rate as another sending 60B packets will be sending at same packet rate as another sending 60B packets will be sending at
25x greater bit rate. In this case, if a congested resource marks 25x greater bit rate. In this case, if a congested resource marks
proportion p_b of packets irrespective of size, the ratio of packets proportion p_b of packets irrespective of size, the ratio of packets
received with the byte-congestion field marked by each transport will received with the byte-congestion field marked by each transport will
be the same, p_b.u_2/p_b.u_1 = 1. be the same, p_b.u_2/p_b.u_1 = 1.
skipping to change at page 34, line 29 skipping to change at page 36, line 20
If the two flows are mixed into one, of bit rate x1+x2, with equal If the two flows are mixed into one, of bit rate x1+x2, with equal
packet rates of each size packet, the ratio p_b will still be packet rates of each size packet, the ratio p_b will still be
measurable by counting the ratio of marked to unmarked bytes (or measurable by counting the ratio of marked to unmarked bytes (or
packets because the ratio cancels out the units). However, if the packets because the ratio cancels out the units). However, if the
absolute volume of congestion is required, the transport must count absolute volume of congestion is required, the transport must count
the sum of congestion marked bytes, which indeed gives a correct the sum of congestion marked bytes, which indeed gives a correct
measure of the rate of byte-congestion p_b(x_1 + x_2) caused by the measure of the rate of byte-congestion p_b(x_1 + x_2) caused by the
combined bit rate. combined bit rate.
B.2.4. Pkt-congestible resource, equal bit rates (Aii) A.2.4. Pkt-congestible resource, equal bit rates (Aii)
Moving to the case of packet-congestible resources, we now take two Moving to the case of packet-congestible resources, we now take two
flows that send different packet sizes at the same bit rate, but this flows that send different packet sizes at the same bit rate, but this
time the pkt-congestion field is marked by the resource with time the pkt-congestion field is marked by the resource with
probability p_p. As in scenario Ai with the same bit rates but a probability p_p. As in scenario Ai with the same bit rates but a
bit-congestible resource, the flow with smaller packets will have a bit-congestible resource, the flow with smaller packets will have a
higher packet rate, so more packets will be both marked and unmarked, higher packet rate, so more packets will be both marked and unmarked,
but in the same proportion. but in the same proportion.
This time, the transport should only count marks without taking into This time, the transport should only count marks without taking into
skipping to change at page 35, line 10 skipping to change at page 37, line 5
flow of our example, as required. flow of our example, as required.
But if the transport is interested in the absolute number of packet But if the transport is interested in the absolute number of packet
congestion, it should just count how many marked packets arrive. For congestion, it should just count how many marked packets arrive. For
instance, a flow sending 60B packets will see 25x more marked packets instance, a flow sending 60B packets will see 25x more marked packets
than one sending 1500B packets at the same bit rate, because it is than one sending 1500B packets at the same bit rate, because it is
sending more packets through a packet-congestible resource. sending more packets through a packet-congestible resource.
Note the unit of packet congestion is a packet. Note the unit of packet congestion is a packet.
B.2.5. Pkt-congestible resource, equal packet rates (Bii) A.2.5. Pkt-congestible resource, equal packet rates (Bii)
Finally, if two flows with the same packet rate, pass through a Finally, if two flows with the same packet rate, pass through a
packet-congestible resource, they will both suffer the same packet-congestible resource, they will both suffer the same
proportion of marking, p_p, irrespective of their packet sizes. On proportion of marking, p_p, irrespective of their packet sizes. On
detecting that the pkt-congestion field is marked, the transport detecting that the pkt-congestion field is marked, the transport
should count packets, and it will be able to extract the ratio p_p of should count packets, and it will be able to extract the ratio p_p of
marked to unmarked packets from both flows, irrespective of packet marked to unmarked packets from both flows, irrespective of packet
sizes. sizes.
Even if the transport is monitoring the absolute amount of packets Even if the transport is monitoring the absolute amount of packets
congestion over a period, still it will see the same amount of packet congestion over a period, still it will see the same amount of packet
congestion from either flow. congestion from either flow.
And if the two equal packet rates of different size packets are mixed And if the two equal packet rates of different size packets are mixed
together in one flow, the packet rate will double, so the absolute together in one flow, the packet rate will double, so the absolute
volume of packet-congestion will accumulate at twice the rate of volume of packet-congestion will accumulate at twice the rate of
either flow, 2p_p.u_1 = p_p(u_1+u_2). either flow, 2p_p.u_1 = p_p(u_1+u_2).
Appendix C. Byte-mode Drop Complicates Policing Congestion Response Appendix B. Byte-mode Drop Complicates Policing Congestion Response
This appendix explains why the ability of networks to police the This appendix 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. network resources only doing packet-mode not byte-mode drop.
To be able to police a transport's response to congestion when To be able to police a transport's response to congestion when
fairness can only be judged over time and over all an individual's fairness can only be judged over time and over all an individual's
flows, the policer has to have an integrated view of all the flows, the policer has to have an integrated view of all the
congestion an individual (not just one flow) has caused due to all congestion an individual (not just one flow) has caused due to all
traffic entering the Internet from that individual. This is termed traffic entering the Internet from that individual. This is termed
congestion accountability. congestion accountability.
But a byte-mode drop algorithm has to depend on the local MTU of the But a byte-mode drop algorithm has to depend on the local MTU of the
line - an algorithm needs to use some concept of a 'normal' packet line - an algorithm needs to use some concept of a 'normal' packet
size. Therefore, one dropped or marked packet is not necessarily size. Therefore, one dropped or marked packet is not necessarily
equivalent to another unless you know the MTU at the queue where it equivalent to another unless you know the MTU at the queue where it
was dropped/marked. To have an integrated view of a user, we believe was dropped/marked. To have an integrated view of a user, we believe
congestion policing has to be located at an individual's attachment congestion policing has to be located at an individual's attachment
point to the Internet [I-D.briscoe-tsvwg-re-ecn-tcp]. But from there point to the Internet [I-D.conex-concepts-uses]. But from there it
it cannot know the MTU of each remote queue that caused each drop/ cannot know the MTU of each remote queue that caused each drop/mark.
mark. Therefore it cannot take an integrated approach to policing Therefore it cannot take an integrated approach to policing all the
all the responses to congestion of all the transports of one responses to congestion of all the transports of one individual.
individual. Therefore it cannot police anything. Therefore it cannot police anything.
The security/incentive argument _for_ packet-mode drop is similar. The security/incentive argument _for_ packet-mode drop is similar.
Firstly, confining RED to packet-mode drop would not preclude Firstly, confining RED to packet-mode drop would not preclude
bottleneck policing approaches such as [pBox] as it seems likely they bottleneck policing approaches such as [pBox] as it seems likely they
could work just as well by monitoring the volume of dropped bytes could work just as well by monitoring the volume of dropped bytes
rather than packets. Secondly packet-mode dropping/marking naturally rather than packets. Secondly packet-mode dropping/marking naturally
allows the congestion notification of packets to be globally allows the congestion notification of packets to be globally
meaningful without relying on MTU information held elsewhere. meaningful without relying on MTU information held elsewhere.
Because we recommend that a dropped/marked packet should be taken to Because we recommend that a dropped/marked packet should be taken to
skipping to change at page 36, line 27 skipping to change at page 38, line 21
packets or across different size flows [Rate_fair_Dis]. Therefore packets or across different size flows [Rate_fair_Dis]. Therefore
policing would work naturally with just simple packet-mode drop in policing would work naturally with just simple packet-mode drop in
RED. RED.
In summary, making drop probability depend on the size of the packets In summary, making drop probability depend on the size of the packets
that bits happen to be divided into simply encourages the bits to be that bits happen to be divided into simply encourages the bits to be
divided into smaller packets. Byte-mode drop would therefore divided into smaller packets. Byte-mode drop would therefore
irreversibly complicate any attempt to fix the Internet's incentive irreversibly complicate any attempt to fix the Internet's incentive
structures. structures.
Appendix D. Changes from Previous Versions Appendix C. Changes from Previous Versions
To be removed by the RFC Editor on publication. To be removed by the RFC Editor on publication.
Full incremental diffs between each version are available at Full incremental diffs between each version are available at
<http://www.cs.ucl.ac.uk/staff/B.Briscoe/pubs.html#byte-pkt-congest> <http://www.cs.ucl.ac.uk/staff/B.Briscoe/pubs.html#byte-pkt-congest>
or or
<http://tools.ietf.org/wg/tsvwg/draft-ietf-tsvwg-byte-pkt-congest/> <http://tools.ietf.org/wg/tsvwg/draft-ietf-tsvwg-byte-pkt-congest/>
(courtesy of the rfcdiff tool): (courtesy of the rfcdiff tool):
From -01 to -02 (this version): 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 * Restructured the whole document for (hopefully) easier reading
and clarity. The concrete recommendation, in RFC2119 language, and clarity. The concrete recommendation, in RFC2119 language,
is now in Section 5. is now in Section 7.
From -00 to -01: From -00 to -01:
* Minor clarifications throughout and updated references * Minor clarifications throughout and updated references
From briscoe-byte-pkt-mark-02 to ietf-byte-pkt-congest-00: From briscoe-byte-pkt-mark-02 to ietf-byte-pkt-congest-00:
* Added note on relationship to existing RFCs * Added note on relationship to existing RFCs
* Posed the question of whether packet-congestion could become * Posed the question of whether packet-congestion could become
common and deferred it to the IRTF ICCRG. Added ref to the common and deferred it to the IRTF ICCRG. Added ref to the
dual-resource queue (DRQ) proposal. dual-resource queue (DRQ) proposal.
* Changed PCN references from the PCN charter & architecture to * Changed PCN references from the PCN charter & architecture to
the PCN marking behaviour draft most likely to imminently the PCN marking behaviour draft most likely to imminently
become the standards track WG item. become the standards track WG item.
From -01 to -02: From -01 to -02:
skipping to change at page 37, line 41 skipping to change at page 40, line 26
new transports unless we decide whether the network or the new transports unless we decide whether the network or the
transport is allowing for packet size. transport is allowing for packet size.
* Added statement explaining the horizon of the memo is long * Added statement explaining the horizon of the memo is long
term, but with short term expediency in mind. term, but with short term expediency in mind.
* Added material on scaling congestion control with packet size * Added material on scaling congestion control with packet size
(Section 2.1). (Section 2.1).
* Separated out issue of normalising TCP's bit rate from issue of * Separated out issue of normalising TCP's bit rate from issue of
preference to control packets (Section 2.3). preference to control packets (Section 2.4).
* Divided up Congestion Measurement section for clarity, * Divided up Congestion Measurement section for clarity,
including new material on fixed size packet buffers and buffer including new material on fixed size packet buffers and buffer
carving (Section 3.1.1 & Section 3.2.1) and on congestion carving (Section 4.1.1 & Section 4.2.1) and on congestion
measurement in wireless link technologies without queues measurement in wireless link technologies without queues
(Section 3.1.2). (Section 4.1.2).
* Added section on 'Making Transports Robust against Control * Added section on 'Making Transports Robust against Control
Packet Losses' (Section 3.2.3) with existing & new material Packet Losses' (Section 4.2.3) with existing & new material
included. included.
* Added tabulated results of vendor survey on byte-mode drop * Added tabulated results of vendor survey on byte-mode drop
variant of RED (Table 2). variant of RED (Table 2).
From -00 to -01: From -00 to -01:
* Clarified applicability to drop as well as ECN. * Clarified applicability to drop as well as ECN.
* Highlighted DoS vulnerability. * Highlighted DoS vulnerability.
 End of changes. 148 change blocks. 
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