draft-ietf-tsvwg-byte-pkt-congest-03.txt   draft-ietf-tsvwg-byte-pkt-congest-04.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: April 27, 2011 October 24, 2010 Expires: September 15, 2011 March 14, 2011
Byte and Packet Congestion Notification Byte and Packet Congestion Notification
draft-ietf-tsvwg-byte-pkt-congest-03 draft-ietf-tsvwg-byte-pkt-congest-04
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 three strong recommendations: congestion notification (PCN). We give three strong recommendations:
(1) packet size should be taken into account when transports read (1) packet size should be taken into account when transports read
congestion indications, (2) packet size should not be taken into congestion indications, (2) packet size should not be taken into
account when network equipment creates congestion signals (marking, account when network equipment creates congestion signals (marking,
dropping), and therefore (3) the byte-mode packet drop variant of the dropping), and therefore (3) the byte-mode packet drop variant of the
skipping to change at page 1, line 38 skipping to change at page 1, line 38
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 27, 2011. This Internet-Draft will expire on September 15, 2011.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Terminology and Scoping . . . . . . . . . . . . . . . . . 7 1.1. Terminology and Scoping . . . . . . . . . . . . . . . . . 7
1.2. Why now? . . . . . . . . . . . . . . . . . . . . . . . . . 8 2. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 8
2. Motivating Arguments . . . . . . . . . . . . . . . . . . . . . 10 2.1. Recommendation on Queue Measurement . . . . . . . . . . . 8
2.1. Scaling Congestion Control with Packet Size . . . . . . . 10 2.2. Recommendation on Notifying Congestion . . . . . . . . . . 9
2.2. Transport-Independent Network . . . . . . . . . . . . . . 10 2.3. Recommendation on Responding to Congestion . . . . . . . . 10
2.3. Avoiding Perverse Incentives to (Ab)use Smaller Packets . 11 2.4. Recommendation on Handling Congestion Indications when
2.4. Small != Control . . . . . . . . . . . . . . . . . . . . . 12 Splitting or Merging Packets . . . . . . . . . . . . . . . 11
2.5. Implementation Efficiency . . . . . . . . . . . . . . . . 13 3. Motivating Arguments . . . . . . . . . . . . . . . . . . . . . 11
3. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 13 3.1. Scaling Congestion Control with Packet Size . . . . . . . 11
3.1. Recommendation on Queue Measurement . . . . . . . . . . . 13 3.2. Transport-Independent Network . . . . . . . . . . . . . . 12
3.2. Recommendation on Notifying Congestion . . . . . . . . . . 13 3.3. Avoiding Perverse Incentives to (Ab)use Smaller Packets . 13
3.3. Recommendation on Responding to Congestion . . . . . . . . 14 3.4. Small != Control . . . . . . . . . . . . . . . . . . . . . 14
3.4. Recommended Future Research . . . . . . . . . . . . . . . 15 3.5. Implementation Efficiency . . . . . . . . . . . . . . . . 14
3.6. Why now? . . . . . . . . . . . . . . . . . . . . . . . . . 14
4. A Survey and Critique of Past Advice . . . . . . . . . . . . . 15 4. A Survey and Critique of Past Advice . . . . . . . . . . . . . 15
4.1. Congestion Measurement Advice . . . . . . . . . . . . . . 16 4.1. Congestion Measurement Advice . . . . . . . . . . . . . . 16
4.1.1. Fixed Size Packet Buffers . . . . . . . . . . . . . . 16 4.1.1. Fixed Size Packet Buffers . . . . . . . . . . . . . . 17
4.1.2. Congestion Measurement without a Queue . . . . . . . . 17 4.1.2. Congestion Measurement without a Queue . . . . . . . . 18
4.2. Congestion Notification Advice . . . . . . . . . . . . . . 18 4.2. Congestion Notification Advice . . . . . . . . . . . . . . 18
4.2.1. Network Bias when Encoding . . . . . . . . . . . . . . 18 4.2.1. Network Bias when Encoding . . . . . . . . . . . . . . 18
4.2.2. Transport Bias when Decoding . . . . . . . . . . . . . 20 4.2.2. Transport Bias when Decoding . . . . . . . . . . . . . 20
4.2.3. Making Transports Robust against Control Packet 4.2.3. Making Transports Robust against Control Packet
Losses . . . . . . . . . . . . . . . . . . . . . . . . 21 Losses . . . . . . . . . . . . . . . . . . . . . . . . 22
4.2.4. Congestion Notification: Summary of Conflicting 4.2.4. Congestion Notification: Summary of Conflicting
Advice . . . . . . . . . . . . . . . . . . . . . . . . 22 Advice . . . . . . . . . . . . . . . . . . . . . . . . 22
4.2.5. RED Implementation Status . . . . . . . . . . . . . . 23 4.2.5. RED Implementation Status . . . . . . . . . . . . . . 23
5. Outstanding Issues and Next Steps . . . . . . . . . . . . . . 24 5. Outstanding Issues and Next Steps . . . . . . . . . . . . . . 24
5.1. Bit-congestible World . . . . . . . . . . . . . . . . . . 24 5.1. Bit-congestible World . . . . . . . . . . . . . . . . . . 24
5.2. Bit- & Packet-congestible World . . . . . . . . . . . . . 25 5.2. Bit- & Packet-congestible World . . . . . . . . . . . . . 25
6. Security Considerations . . . . . . . . . . . . . . . . . . . 26 6. Security Considerations . . . . . . . . . . . . . . . . . . . 26
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 27 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 27
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
9. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 28 9. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 28
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
10.1. Normative References . . . . . . . . . . . . . . . . . . . 28 10.1. Normative References . . . . . . . . . . . . . . . . . . . 28
10.2. Informative References . . . . . . . . . . . . . . . . . . 29 10.2. Informative References . . . . . . . . . . . . . . . . . . 29
Appendix A. Idealised Wire Protocol . . . . . . . . . . . . . . . 32 Appendix A. Idealised Wire Protocol . . . . . . . . . . . . . . . 33
A.1. Protocol Coding . . . . . . . . . . . . . . . . . . . . . 32 A.1. Protocol Coding . . . . . . . . . . . . . . . . . . . . . 33
A.2. Example Scenarios . . . . . . . . . . . . . . . . . . . . 34 A.2. Example Scenarios . . . . . . . . . . . . . . . . . . . . 34
A.2.1. Notation . . . . . . . . . . . . . . . . . . . . . . . 34 A.2.1. Notation . . . . . . . . . . . . . . . . . . . . . . . 34
A.2.2. Bit-congestible resource, equal bit rates (Ai) . . . . 34 A.2.2. Bit-congestible resource, equal bit rates (Ai) . . . . 35
A.2.3. Bit-congestible resource, equal packet rates (Bi) . . 35 A.2.3. Bit-congestible resource, equal packet rates (Bi) . . 36
A.2.4. Pkt-congestible resource, equal bit rates (Aii) . . . 36 A.2.4. Pkt-congestible resource, equal bit rates (Aii) . . . 37
A.2.5. Pkt-congestible resource, equal packet rates (Bii) . . 37 A.2.5. Pkt-congestible resource, equal packet rates (Bii) . . 37
Appendix B. Byte-mode Drop Complicates Policing Congestion Appendix B. Byte-mode Drop Complicates Policing Congestion
Response . . . . . . . . . . . . . . . . . . . . . . 37 Response . . . . . . . . . . . . . . . . . . . . . . 38
Appendix C. Changes from Previous Versions . . . . . . . . . . . 39
Appendix C. Changes from Previous Versions . . . . . . . . . . . 38
1. Introduction 1. Introduction
This memo is initially concerned with how we should correctly scale This memo is initially concerned with how we should correctly scale
congestion control functions with packet size for the long term. But congestion control functions with packet size for the long term. But
it also recognises that expediency may be necessary to deal with it also recognises that expediency may be necessary to deal with
existing widely deployed protocols that don't live up to the long existing widely deployed protocols that don't live up to the long
term goal. term goal.
When notifying congestion, the problem of how (and whether) to take When notifying congestion, the problem of how (and whether) to take
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congestion notification. congestion notification.
Currently, the RFC series is silent on this matter other than a paper Currently, the RFC series is silent on this matter other than a paper
trail of advice referenced from [RFC2309], which conditionally trail of advice referenced from [RFC2309], which conditionally
recommends byte-mode (packet-size dependent) drop [pktByteEmail]. recommends byte-mode (packet-size dependent) drop [pktByteEmail].
Reducing drop of small packets certainly has some tempting Reducing drop of small packets certainly has some tempting
advantages: i) it drops less control packets, which tend to be small 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. and ii) it makes TCP's bit-rate less dependent on packet size.
However, there are ways of addressing these issues at the transport However, there are ways of addressing these issues at the transport
layer, rather than reverse engineering network forwarding to fix the layer, rather than reverse engineering network forwarding to fix the
problems of one specific transport. problems of one specific transport, as byte-mode variant of RED was
designed to do.
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 deliberate preferential treatment for small packets in AQM against deliberate preferential treatment for small packets in AQM
algorithms and to record this advice within the RFC series. It algorithms and to record this advice within the RFC series. It
recommends that (1) packet size should be taken into account when recommends that (1) packet size should be taken into account when
transports read congestion indications, (2) not when network transports read congestion indications, (2) not when network
equipment writes them. equipment writes them.
In particular this means that the byte-mode packet drop variant of In particular this means that the byte-mode packet drop variant of
RED should not be used to drop fewer small packets, because that RED should not be used to drop fewer small packets, because that
creates a perverse incentive for transports to use tiny segments, creates a perverse incentive for transports to use tiny segments,
consequently also opening up a DoS vulnerability. Fortunately all consequently also opening up a DoS vulnerability. Fortunately all
the RED implementers who responded to our survey (Section 4.2.4) have 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 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.
However, at the transport layer, TCP congestion control is a widely However, at the transport layer, TCP congestion control is a widely
deployed protocol that we argue doesn't scale correctly with packet deployed protocol that doesn't scale correctly with packet size. To
size. To date this hasn't been a significant problem because most date this hasn't been a significant problem because most TCPs have
TCPs have been used with similar packet sizes. But, as we design new been used with similar packet sizes. But, as we design new
congestion controls, we should build in scaling with packet size congestion controls, we should build in scaling with packet size
rather than assuming we should follow TCP's example. rather than assuming we should follow TCP's example.
This memo continues as follows. First it discusses terminology and This memo continues as follows. First it discusses terminology and
scoping and why it is relevant to publish this memo now. Section 2 scoping. Section 2 gives the concrete formal recommendations,
gives motivating arguments for the recommendations that are formally followed by motivating arguments in Section 3. We then critically
stated in Section 3, which follows. We then critically survey the survey the advice given previously in the RFC series and the research
advice given previously in the RFC series and the research literature literature (Section 4), followed by an assessment of whether or not
(Section 4), followed by an assessment of whether or not this advice this advice has been followed in production networks (Section 4.2.5).
has been followed in production networks (Section 4.2.5). To wrap To wrap up, outstanding issues are discussed that will need
up, outstanding issues are discussed that will need resolution both resolution both to inform future protocols designs and to handle
to inform future protocols designs and to handle legacy (Section 5). legacy (Section 5). Then security issues are collected together in
Then security issues are collected together in Section 6 before Section 6 before conclusions are drawn in Section 7. The interested
conclusions are drawn in Section 7. The interested reader can find reader can find discussion of more detailed issues on the theme of
discussion of more detailed issues on the theme of byte vs. packet in byte vs. packet in the appendices.
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 3 to read on the subject. A busy reader can jump right into Section 2 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
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they have to process. Examples of bit-congestible resources are they have to process. Examples of bit-congestible resources are
transmission links, radio power and most buffer memory, because transmission links, radio power and most buffer memory, because
the load depends on how many bits they have to transmit or store. the load depends on how many bits they have to transmit or store.
Some machine architectures use fixed size packet buffers, so Some machine architectures use fixed size packet buffers, so
buffer memory in these cases is packet-congestible (see buffer memory in these cases is packet-congestible (see
Section 4.1.1). Section 4.1.1).
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 Therefore, packet-congestion is currently rare [RFC6077; S.3.3],
[I-D.irtf-iccrg-welzl; S.3.3], but there is no guarantee that it but there is no guarantee that it will not become common with
will not become common with future technology trends. 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 bits or packets, for instance stateful of granularity than bits or packets, for instance stateful
firewalls are flow-congestible and call-servers are session- firewalls are flow-congestible and call-servers are session-
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RED Terminology: In RED, whether to use packets or bytes when RED Terminology: In RED, whether to use packets or bytes when
measuring queues is called respectively packet-mode queue measuring queues is called respectively packet-mode queue
measurement or byte-mode queue measurement. And whether the measurement or byte-mode queue measurement. And whether the
probability of dropping a packet is independent or dependent on probability of dropping a packet is independent or dependent on
its byte-size is called respectively packet-mode drop or byte-mode its byte-size is called respectively packet-mode drop or byte-mode
drop. The terms byte-mode and packet-mode should not be used drop. The terms byte-mode and packet-mode should not be used
without specifying whether they apply to queue measurement or to without specifying whether they apply to queue measurement or to
drop. drop.
1.2. Why now? 2. Recommendations
Now is a good time to discuss whether fairness between different 2.1. Recommendation on Queue Measurement
sized packets would best be implemented in network equipment, or at
the transport, for a number of reasons:
1. The IETF pre-congestion notification (PCN) working group is Queue length is usually the most correct and simplest way to measure
standardising the external behaviour of a PCN congestion congestion of a resource. To avoid the pathological effects of drop
notification (AQM) algorithm [RFC5670]; 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).
2. [RFC2309] says RED may either take account of packet size or not If the resource is bit-congestible, the implementation SHOULD measure
when dropping, but gives no recommendation between the two, the length of the queue in bytes. If the resource is packet-
referring instead to advice on the performance implications in an congestible, the implementation SHOULD measure the length of the
email [pktByteEmail], which recommends byte-mode drop. Further, queue in packets. No other choice makes sense, because the number of
just before RFC2309 was issued, an addendum was added to the packets waiting in the queue isn't relevant if the resource gets
archived email that revisited the issue of packet vs. byte-mode congested by bytes and vice versa.
drop in its last paragraph, making the recommendation less clear-
cut;
3. Without the present memo, the only advice in the RFC series on Corollaries:
packet size bias in AQM algorithms would be a reference to an
archived email in [RFC2309] (including an addendum at the end of
the email to correct the original).
4. The IRTF Internet Congestion Control Research Group (ICCRG) 1. A RED implementation SHOULD use byte mode queue measurement for
recently took on the challenge of building consensus on what measuring the congestion of bit-congestible resources and packet
common congestion control support should be required from network mode queue measurement for packet-congestible resources.
forwarding functions in future [I-D.irtf-iccrg-welzl]. The wider
Internet community needs to discuss whether the complexity of
adjusting for packet size should be in the network or in
transports;
5. Given there are many good reasons why larger path max 2. "An Admin SHOULD NOT be able to configure the way a queue
transmission units (PMTUs) would help solve a number of scaling measures itself, because wether a queue is bit-congestible or
issues, we don't want to create any bias against large packets packet-congestible is a property of the resource."
that is greater than their true cost;
6. The IETF audio/video transport (AVT) working group is The recommended approach in less straightforward scenarios, such as
standardising how the real-time protocol (RTP) should feedback fixed size buffers, and resources without a queue, is discussed in
and respond to explicit congestion notification (ECN) Section 4.1.
[I-D.ietf-avt-ecn-for-rtp].
7. The IETF has started to consider the question of fairness between 2.2. Recommendation on Notifying Congestion
flows that use different packet sizes (e.g. in the small-packet
variant of TCP-friendly rate control, TFRC-SP [RFC4828]). Given
transports with different packet sizes, if we don't decide
whether the network or the transport should allow for packet
size, it will be hard if not impossible to design any transport
protocol so that its bit-rate relative to other transports meets
design guidelines [RFC5033] (Note however that, if the concern
were fairness between users, rather than between flows
[Rate_fair_Dis], relative rates between flows would have to come
under run-time control rather than being embedded in protocol
designs).
2. Motivating Arguments When notifying congestion, a network device SHOULD treat all packets
equally, regardless of their size. Therefore, the probability that
network equipment drops or marks a packet to notify congestion SHOULD
NOT depend on the size of the packet. For instance, to drop any bit
with probability 0.1% it is only necessary to drop every packet with
probability 0.1% without regard to the size of each packet.
This means that 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 2.3). 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.
2.3. Recommendation on Responding to Congestion
When a transport detects that a packet has been lost or congestion
marked, it SHOULD consider the strength of the congestion indication
as proportionate to the size in octets of the missing or marked
packet.
In other words, when a packet indicates congestion (by being lost or
marked) it can be considered conceptually as if there is a congestion
indication on every octet of the packet, not just one indication per
packet.
Therefore, 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.
2.4. Recommendation on Handling Congestion Indications when Splitting
or Merging Packets
Packets carrying congestion indications may be split or merged (e.g.
at a transcoder or during fragment reassembly). Splitting and
merging only make sense in the context of ECN, not loss.
The general rule to follow is that the number of octets in packets
with congestion indications should be roughly the same before and
after merging or splitting. This is based on the principle used
above; that an indication of congestion on a packet can be considered
as an indication of congestion on each octet of the packet.
One can think of a splitting or merging process as if all the
incoming congestion-marked octets increment a counter and all the
outgoing marked octets decrement the same counter. In order to
ensure that congestion indications remain timely, even the smallest
positive remainder in the conceptual counter should trigger the next
outgoing packet to be marked (causing the counter to go negative).
3. Motivating Arguments
In this section, we evaluate the topic of packet vs. byte based In this section, we evaluate the topic of packet vs. byte based
congestion notifications and motivate the recommendations given in congestion notifications and motivate the recommendations given in
this document. this document.
2.1. Scaling Congestion Control with Packet Size 3.1. 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. the bytes in the packet.
Consider a bit-congestible link shared by many flows (bit-congestible Consider a bit-congestible link shared by many flows (bit-congestible
is the more common case, see Section 1.1), so that each busy period is the more common case, see Section 1.1), so that each busy period
tends to cause packets to be lost from different flows. Consider tends to cause packets to be lost from different flows. Consider
further two sources that have the same data rate but break the load further two sources that have the same data rate but break the load
into large packets in one application (A) and small packets in the into large packets in one application (A) and small packets in the
skipping to change at page 10, line 49 skipping to change at page 12, line 29
packets (B) will respond more to the same congestion excursion. On packets (B) will respond more to the same congestion excursion. On
the other hand, if they respond proportionately less when smaller the other hand, if they respond proportionately less when smaller
packets are dropped/marked, overall they will be able to respond the packets are dropped/marked, overall they will be able to 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), instead of dropped or marked packets (as TCP effectively does), instead of dropped or marked packets (as TCP
does). does).
2.2. Transport-Independent Network 3.2. Transport-Independent Network
TCP congestion control ensures that flows competing for the same TCP congestion control ensures that flows competing for the same
resource each maintain the same number of segments in flight, resource each maintain the same number of segments in flight,
irrespective of segment size. So under similar conditions, flows irrespective of segment size. So under similar conditions, flows
with different segment sizes will get different bit rates. with different segment sizes will get different bit rates.
Even though reducing the drop probability of small packets (e.g. Even though reducing the drop probability of small packets (e.g.
RED's byte-mode drop) helps ensure TCPs with different packet sizes RED's byte-mode drop) helps ensure TCPs with different packet sizes
will achieve similar bit rates, we argue this correction should be will achieve similar bit rates, we argue this correction should be
made to any future transport protocols based on TCP, not to the made to any future transport protocols based on TCP, not to the
skipping to change at page 11, line 27 skipping to change at page 13, line 7
RFC2309 refers to an email [pktByteEmail] for advice on how RED RFC2309 refers to an email [pktByteEmail] for advice on how RED
should allow for different packet sizes. The email says the question should allow for different packet sizes. The email says the question
of whether a packet's own size should affect its drop probability of whether a packet's own size should affect its drop probability
"depends on the dominant end-to-end congestion control mechanisms". "depends on the dominant end-to-end congestion control mechanisms".
But we argue network equipment should not be specialised for whatever But we argue network equipment should not be specialised for whatever
transport is predominant. No matter how convenient it is, we SHOULD transport is predominant. No matter how convenient it is, we SHOULD
NOT hack the network solely to allow for omissions from the design of NOT hack the network solely to allow for omissions from the design of
one transport protocol, even if it is as predominant as TCP. one transport protocol, even if it is as predominant as TCP.
2.3. Avoiding Perverse Incentives to (Ab)use Smaller Packets 3.3. 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:
skipping to change at page 12, line 15 skipping to change at page 13, line 42
Encouraging a high volume of tiny packets might in turn Encouraging a high volume of tiny packets might in turn
unnecessarily overload a completely unrelated part of the system, unnecessarily overload a completely unrelated part of the system,
perhaps more limited by header-processing than bandwidth. 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, RED aims
drops 25% of the larger packets, it will aim to drop 1% of the to drop 25x more of the larger packets than the smaller ones. Thus,
smaller packets (but in practice it may drop more as congestion for example if RED drops 25% of the larger packets, it will aim to
increases [RFC4828; S.B.4]). Even though both flows arrive with the drop 1% of the smaller packets (but in practice it may drop more as
same bit rate, the bit rate the RED queue aims to pass to the line congestion increases [RFC4828; S.B.4]). Even though both flows
will be 750k for the flow of larger packet but 990k for the smaller arrive with the same bit rate, the bit rate the RED queue aims to
packets (but because of rate variation it will be less than this pass to the line will be 750Kbit for the flow of larger packet but
target). 990Kbit for the smaller packets (but because of rate variation it
will be less than this target).
Note that, although the byte-mode drop variant of RED amplifies small Note that, although the byte-mode drop variant of RED amplifies small
packet attacks, drop-tail queues amplify small packet attacks even packet attacks, drop-tail queues amplify small packet attacks even
more (see Security Considerations in Section 6). Wherever possible more (see Security Considerations in Section 6). Wherever possible
neither should be used. neither should be used.
2.4. Small != Control 3.4. 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, we argue So again, rather than fix these problems in the network, we 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 4.2.3). Losses' in Section 4.2.3).
2.5. Implementation Efficiency 3.5. 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. Recommendations 3.6. Why now?
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 Now is a good time to discuss whether fairness between different
of packet size sized packets would best be implemented in network equipment, or at
the transport, for a number of reasons:
o making transports less sensitive to losing control packets like 1. The IETF pre-congestion notification (PCN) working group is
SYNs and pure ACKs. standardising the external behaviour of a PCN congestion
notification (AQM) algorithm [RFC5670];
Corollaries: 2. [RFC2309] says RED may either take account of packet size or not
when dropping, but gives no recommendation between the two,
referring instead to advice on the performance implications in an
email [pktByteEmail], which recommends byte-mode drop. Further,
just before RFC2309 was issued, an addendum was added to the
archived email that revisited the issue of packet vs. byte-mode
drop in its last paragraph, making the recommendation less clear-
cut. RFC2309 is currently the only advice in the RFC series on
packet size bias in AQM algorithms;
1. If two TCPs with different packet sizes are required to run at 3. The IRTF Internet Congestion Control Research Group (ICCRG)
equal bit rates under the same path conditions, this SHOULD be recently took on the challenge of building consensus on what
done by altering TCP (Section 4.2.2), not network equipment, common congestion control support should be required from network
which would otherwise affect other transports besides TCP. forwarding functions in future [RFC6077]. The wider Internet
community needs to discuss whether the complexity of adjusting
for packet size should be in the network or in transports;
2. If it is desired to improve TCP performance by reducing the 4. Given there are many good reasons why larger path max
chance that a SYN or a pure ACK will be dropped, this should be transmission units (PMTUs) would help solve a number of scaling
done by modifying TCP (Section 4.2.3), not network equipment. issues, we don't want to create any bias against large packets
that is greater than their true cost;
3.4. Recommended Future Research 5. 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].
The above conclusions cater for the Internet as it is today with most 6. The IETF has started to consider the question of fairness between
resources being primarily bit-congestible. A secondary conclusion of flows that use different packet sizes (e.g. in the small-packet
this memo is that research is needed to determine whether there might variant of TCP-friendly rate control, TFRC-SP [RFC4828]). Given
be more packet-congestible resources in the future. Then further transports with different packet sizes, if we don't decide
research would be needed to extend the Internet's congestion whether the network or the transport should allow for packet
notification (drop or ECN) so that it would be able to handle a more size, it will be hard if not impossible to design any transport
even mix of bit-congestible and packet-congestible resources. protocol so that its bit-rate relative to other transports meets
design guidelines [RFC5033] (Note however that, if the concern
were fairness between users, rather than between flows
[Rate_fair_Dis], relative rates between flows would have to come
under run-time control rather than being embedded in protocol
designs).
4. A Survey and Critique of Past Advice 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
skipping to change at page 16, line 12 skipping to change at page 16, line 25
o whether the drop probability of an individual packet should depend o whether the drop probability of an individual packet should depend
on its own size (Section 4.2). on its own size (Section 4.2).
The rest of this section is structured accordingly. The rest of this section is structured accordingly.
4.1. Congestion Measurement Advice 4.1. Congestion Measurement Advice
The choice of which metric to use to measure queue length was left The choice of which metric to use to measure queue length was left
open in RFC2309. It is now well understood that queues for bit- open in 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
[pktByteEmail].
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.
skipping to change at page 19, line 16 skipping to change at page 19, line 34
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. But it is not consistent with the end congestion control mechanisms. But it is not consistent with the
present recommendations of Section 3. present recommendations of Section 2.
By 2003, a further change had been made to the adjustment for packet By 2003, a further change had been made to the adjustment for packet
size, this time in the RED algorithm of the ns2 simulator. Instead size, this time in the RED algorithm of the ns2 simulator. Instead
of taking each packet's size relative to a `maximum packet size' it of taking each packet's size relative to a `maximum packet size' it
was taken relative to a `mean packet size', intended to be a static was taken relative to a `mean packet size', intended to be a static
value representative of the `typical' packet size on the link. We value representative of the `typical' packet size on the link. We
have not been able to find a justification in the literature for this have not been able to find a justification in the literature for this
change, however Eddy and Allman conducted experiments [REDbias] that change, however Eddy and Allman conducted experiments [REDbias] that
assessed how sensitive RED was to this parameter, amongst other assessed how sensitive RED was to this parameter, amongst other
things. No-one seems to have pointed out that this changed algorithm things. No-one seems to have pointed out that this changed algorithm
skipping to change at page 22, line 51 skipping to change at page 23, line 23
RED (top right and bottom left). Top left is the `do nothing' RED (top right and bottom left). Top left is the `do nothing'
scenario, while bottom right is the `do-both' scenario in which bit- scenario, while bottom right is the `do-both' scenario in which bit-
rate would become far too biased towards small packets. Of course, 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 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 of queues that congest, each path through the network will present a
different hybrid scenario to its transport. 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. Section 3 recommends that the network should be doing the biasing. Section 2 recommends that
_all_ bias in network equipment towards small packets should be _all_ bias in network equipment towards small packets should be
turned off--if indeed any equipment vendors have implemented it-- turned off--if indeed any equipment vendors have implemented it--
leaving packet size bias solely as the preserve of the transport leaving packet size bias solely as the preserve of the transport
layer (solely the leftmost, packet-mode drop column). layer (solely the leftmost, packet-mode drop column).
4.2.5. RED Implementation Status 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
skipping to change at page 26, line 8 skipping to change at page 26, line 26
folds both bit congestion and packet congestion into one signal folds both bit congestion and packet congestion into one signal
(either loss or ECN). (either loss or ECN).
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 [RFC6077]). Therefore, rather than not giving
not giving this problem any thought at all, just because it is hard this problem any thought at all, just because it is hard and
and currently hypothetical, we defer the question of whether packet 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 [RFC6077]).
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
skipping to change at page 27, line 50 skipping to change at page 28, line 19
that most Internet resources are bit-congestible not packet- that most Internet resources are bit-congestible not packet-
congestible. We need to know the likelihood that this assumption congestible. We need to know the likelihood that this assumption
will prevail longer term and, if it might not, what protocol changes will prevail longer term and, if it might not, what protocol changes
will be needed to cater for a mix of the two. These questions have will be needed to cater for a mix of the two. These questions have
been delegated to the IRTF. been delegated to the IRTF.
8. Acknowledgements 8. Acknowledgements
Thank you to Sally Floyd, who gave extensive and useful review Thank you to Sally Floyd, who gave extensive and useful review
comments. Also thanks for the reviews from Philip Eardley, Toby comments. Also thanks for the reviews from Philip Eardley, Toby
Moncaster and Arnaud Jacquet as well as helpful explanations of Moncaster, Arnaud Jacquet and Mirja Kuehlewind as well as helpful
different hardware approaches from Larry Dunn and Fred Baker. I am explanations of different hardware approaches from Larry Dunn and
grateful to Bruce Davie and his colleagues for providing a timely and Fred Baker. We are grateful to Bruce Davie and his colleagues for
efficient survey of RED implementation in Cisco's product range. providing a timely and efficient survey of RED implementation in
Also grateful thanks to Toby Moncaster, Will Dormann, John Regnault, Cisco's product range. Also grateful thanks to Toby Moncaster, Will
Simon Carter and Stefaan De Cnodder who further helped survey the Dormann, John Regnault, Simon Carter and Stefaan De Cnodder who
current status of RED implementation and deployment and, finally, further helped survey the current status of RED implementation and
thanks to the anonymous individuals who responded. deployment and, finally, 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.
9. 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.
10. References 10. References
10.1. Normative References 10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs [RFC2119] Bradner, S., "Key words for use in
to Indicate Requirement Levels", BCP 14, RFCs to Indicate Requirement Levels",
RFC 2119, March 1997. BCP 14, RFC 2119, March 1997.
[RFC2309] Braden, B., Clark, D., Crowcroft, J., [RFC2309] Braden, B., Clark, D., Crowcroft, J.,
Davie, B., Deering, S., Estrin, D., Davie, B., Deering, S., Estrin, D.,
Floyd, S., Jacobson, V., Minshall, G., Floyd, S., Jacobson, V., Minshall,
Partridge, C., Peterson, L., G., Partridge, C., Peterson, L.,
Ramakrishnan, K., Shenker, S., Ramakrishnan, K., Shenker, S.,
Wroclawski, J., and L. Zhang, Wroclawski, J., and L. Zhang,
"Recommendations on Queue Management and "Recommendations on Queue Management
Congestion Avoidance in the Internet", and Congestion Avoidance in the
RFC 2309, April 1998. Internet", RFC 2309, April 1998.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. [RFC3168] Ramakrishnan, K., Floyd, S., and D.
Black, "The Addition of Explicit Black, "The Addition of Explicit
Congestion Notification (ECN) to IP", Congestion Notification (ECN) to IP",
RFC 3168, September 2001. RFC 3168, September 2001.
[RFC3426] Floyd, S., "General Architectural and [RFC3426] Floyd, S., "General Architectural and
Policy Considerations", RFC 3426, Policy Considerations", RFC 3426,
November 2002. November 2002.
[RFC5033] Floyd, S. and M. Allman, "Specifying New [RFC5033] Floyd, S. and M. Allman, "Specifying
Congestion Control Algorithms", BCP 133, New Congestion Control Algorithms",
RFC 5033, August 2007. BCP 133, RFC 5033, August 2007.
10.2. Informative References 10.2. Informative References
[CCvarPktSize] Widmer, J., Boutremans, C., and J-Y. Le [CCvarPktSize] Widmer, J., Boutremans, C., and J-Y.
Boudec, "Congestion Control for Flows Le Boudec, "Congestion Control for
with Variable Packet Size", ACM CCR 34(2) Flows with Variable Packet Size", ACM
137--151, 2004, <http://doi.acm.org/ CCR 34(2) 137--151, 2004, <http://
10.1145/997150.997162>. doi.acm.org/10.1145/997150.997162>.
[DRQ] Shin, M., Chong, S., and I. Rhee, "Dual- [DRQ] Shin, M., Chong, S., and I. Rhee,
Resource TCP/AQM for Processing- "Dual-Resource TCP/AQM for
Constrained Networks", IEEE/ACM Processing-Constrained Networks",
Transactions on Networking Vol 16, issue IEEE/ACM Transactions on
2, April 2008, <http://dx.doi.org/ Networking Vol 16, issue 2,
10.1109/TNET.2007.900415>. April 2008, <http://dx.doi.org/
10.1109/TNET.2007.900415>.
[DupTCP] Wischik, D., "Short messages", Royal [DupTCP] Wischik, D., "Short messages", Royal
Society workshop on networks: modelling Society workshop on networks:
and control , September 2007, <http:// modelling and control ,
www.cs.ucl.ac.uk/staff/ucacdjw/Research/ September 2007, <http://
shortmsg.html>. www.cs.ucl.ac.uk/staff/ucacdjw/
Research/shortmsg.html>.
[ECNFixedWireless] Siris, V., "Resource Control for Elastic [ECNFixedWireless] Siris, V., "Resource Control for
Traffic in CDMA Networks", Proc. ACM Elastic Traffic in CDMA Networks",
MOBICOM'02 , September 2002, <http:// Proc. ACM MOBICOM'02 ,
www.ics.forth.gr/netlab/publications/ September 2002, <http://
resource_control_elastic_cdma.html>. www.ics.forth.gr/netlab/publications/
resource_control_elastic_cdma.html>.
[Evol_cc] Gibbens, R. and F. Kelly, "Resource [Evol_cc] Gibbens, R. and F. Kelly, "Resource
pricing and the evolution of congestion pricing and the evolution of
control", Automatica 35(12)1969--1985, congestion control",
December 1999, <http:// Automatica 35(12)1969--1985,
www.statslab.cam.ac.uk/~frank/evol.html>. December 1999, <http://
www.statslab.cam.ac.uk/~frank/
evol.html>.
[I-D.conex-concepts-uses] Briscoe, B., Woundy, R., Moncaster, T., [I-D.ietf-avt-ecn-for-rtp] Westerlund, M., Johansson, I.,
and J. Leslie, "ConEx Concepts and Use Perkins, C., and K. Carlberg,
Cases", "Explicit Congestion Notification
draft-moncaster-conex-concepts-uses-01 (ECN) for RTP over UDP",
(work in progress), July 2010. draft-ietf-avt-ecn-for-rtp-03 (work
in progress), October 2010.
[I-D.ietf-avt-ecn-for-rtp] Westerlund, M., Johansson, I., Perkins, [I-D.ietf-conex-concepts-uses] Briscoe, B., Woundy, R., Moncaster,
C., and K. Carlberg, "Explicit Congestion T., and J. Leslie, "ConEx Concepts
Notification (ECN) for RTP over UDP", and Use Cases",
draft-ietf-avt-ecn-for-rtp-02 (work in draft-ietf-conex-concepts-uses-00
progress), July 2010. (work in progress), November 2010.
[I-D.irtf-iccrg-welzl] Welzl, M., Scharf, M., Briscoe, B., and [IOSArch] Bollapragada, V., White, R., and C.
D. Papadimitriou, "Open Research Issues Murphy, "Inside Cisco IOS Software
in Internet Congestion Control", draft- Architecture", Cisco Press: CCIE
irtf-iccrg-welzl-congestion-control-open- Professional Development ISBN13: 978-
research-08 (work in progress), 1-57870-181-0, July 2000.
September 2010.
[IOSArch] Bollapragada, V., White, R., and C. [MulTCP] Crowcroft, J. and Ph. Oechslin,
Murphy, "Inside Cisco IOS Software "Differentiated End to End Internet
Architecture", Cisco Press: CCIE Services using a Weighted
Professional Development ISBN13: 978-1- Proportional Fair Sharing TCP",
57870-181-0, July 2000. CCR 28(3) 53--69, July 1998, <http://
www.cs.ucl.ac.uk/staff/J.Crowcroft/
hipparch/pricing.html>.
[MulTCP] Crowcroft, J. and Ph. Oechslin, [PktSizeEquCC] Vasallo, P., "Variable Packet Size
"Differentiated End to End Internet Equation-Based Congestion Control",
Services using a Weighted Proportional ICSI Technical Report tr-00-008,
Fair Sharing TCP", CCR 28(3) 53--69, 2000, <http://http.icsi.berkeley.edu/
July 1998, <http://www.cs.ucl.ac.uk/ ftp/global/pub/techreports/2000/
staff/J.Crowcroft/hipparch/pricing.html>. tr-00-008.pdf>.
[PktSizeEquCC] Vasallo, P., "Variable Packet Size [RED93] Floyd, S. and V. Jacobson, "Random
Equation-Based Congestion Control", ICSI Early Detection (RED) gateways for
Technical Report tr-00-008, 2000, <http:/ Congestion Avoidance", IEEE/ACM
/http.icsi.berkeley.edu/ftp/global/pub/ Transactions on Networking 1(4) 397--
techreports/2000/tr-00-008.pdf>. 413, August 1993, <http://
www.icir.org/floyd/papers/red/
red.html>.
[RED93] Floyd, S. and V. Jacobson, "Random Early [REDbias] Eddy, W. and M. Allman, "A Comparison
Detection (RED) gateways for Congestion of RED's Byte and Packet Modes",
Avoidance", IEEE/ACM Transactions on Computer Networks 42(3) 261--280,
Networking 1(4) 397--413, August 1993, <h June 2003, <http://www.ir.bbn.com/
ttp://www.icir.org/floyd/papers/red/ documents/articles/redbias.ps>.
red.html>.
[REDbias] Eddy, W. and M. Allman, "A Comparison of [REDbyte] De Cnodder, S., Elloumi, O., and K.
RED's Byte and Packet Modes", Computer Pauwels, "RED behavior with different
Networks 42(3) 261--280, June 2003, <http packet sizes", Proc. 5th IEEE
://www.ir.bbn.com/documents/articles/ Symposium on Computers and
redbias.ps>. Communications (ISCC) 793--799,
July 2000, <http://www.icir.org/
floyd/red/Elloumi99.pdf>.
[REDbyte] De Cnodder, S., Elloumi, O., and K. [RFC2474] Nichols, K., Blake, S., Baker, F.,
Pauwels, "RED behavior with different and D. Black, "Definition of the
packet sizes", Proc. 5th IEEE Symposium Differentiated Services Field (DS
on Computers and Communications Field) in the IPv4 and IPv6 Headers",
(ISCC) 793--799, July 2000, <http:// RFC 2474, December 1998.
www.icir.org/floyd/red/Elloumi99.pdf>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. [RFC3448] Handley, M., Floyd, S., Padhye, J.,
Black, "Definition of the Differentiated and J. Widmer, "TCP Friendly Rate
Services Field (DS Field) in the IPv4 and Control (TFRC): Protocol
IPv6 Headers", RFC 2474, December 1998. Specification", RFC 3448,
January 2003.
[RFC3448] Handley, M., Floyd, S., Padhye, J., and [RFC3714] Floyd, S. and J. Kempf, "IAB Concerns
J. Widmer, "TCP Friendly Rate Control Regarding Congestion Control for
(TFRC): Protocol Specification", Voice Traffic in the Internet",
RFC 3448, January 2003. RFC 3714, March 2004.
[RFC3714] Floyd, S. and J. Kempf, "IAB Concerns [RFC4828] Floyd, S. and E. Kohler, "TCP
Regarding Congestion Control for Voice Friendly Rate Control (TFRC): The
Traffic in the Internet", RFC 3714, Small-Packet (SP) Variant", RFC 4828,
March 2004. April 2007.
[RFC4828] Floyd, S. and E. Kohler, "TCP Friendly [RFC5562] Kuzmanovic, A., Mondal, A., Floyd,
Rate Control (TFRC): The Small-Packet S., and K. Ramakrishnan, "Adding
(SP) Variant", RFC 4828, April 2007. Explicit Congestion Notification
(ECN) Capability to TCP's SYN/ACK
Packets", RFC 5562, June 2009.
[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S., [RFC5670] Eardley, P., "Metering and Marking
and K. Ramakrishnan, "Adding Explicit Behaviour of PCN-Nodes", RFC 5670,
Congestion Notification (ECN) Capability November 2009.
to TCP's SYN/ACK Packets", RFC 5562,
June 2009.
[RFC5670] Eardley, P., "Metering and Marking [RFC5681] Allman, M., Paxson, V., and E.
Behaviour of PCN-Nodes", RFC 5670, Blanton, "TCP Congestion Control",
November 2009. RFC 5681, September 2009.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, [RFC5690] Floyd, S., Arcia, A., Ros, D., and J.
"TCP Congestion Control", RFC 5681, Iyengar, "Adding Acknowledgement
September 2009. Congestion Control to TCP", RFC 5690,
February 2010.
[RFC5690] Floyd, S., Arcia, A., Ros, D., and J. [RFC6077] Papadimitriou, D., Welzl, M., Scharf,
Iyengar, "Adding Acknowledgement M., and B. Briscoe, "Open Research
Congestion Control to TCP", RFC 5690, Issues in Internet Congestion
February 2010. Control", RFC 6077, February 2011.
[Rate_fair_Dis] Briscoe, B., "Flow Rate Fairness: [Rate_fair_Dis] Briscoe, B., "Flow Rate Fairness:
Dismantling a Religion", ACM Dismantling a Religion", ACM
CCR 37(2)63--74, April 2007, <http:// CCR 37(2)63--74, April 2007, <http://
portal.acm.org/citation.cfm?id=1232926>. portal.acm.org/
citation.cfm?id=1232926>.
[WindowPropFair] Siris, V., "Service Differentiation and [WindowPropFair] Siris, V., "Service Differentiation
Performance of Weighted Window-Based and Performance of Weighted Window-
Congestion Control and Packet Marking Based Congestion Control and Packet
Algorithms in ECN Networks", Computer Marking Algorithms in ECN Networks",
Communications 26(4) 314--326, 2002, <htt Computer Communications 26(4) 314--
p://www.ics.forth.gr/netgroup/ 326, 2002, <http://www.ics.forth.gr/
publications/ netgroup/publications/
weighted_window_control.html>. weighted_window_control.html>.
[gentle_RED] Floyd, S., "Recommendation on using the [gentle_RED] Floyd, S., "Recommendation on using
"gentle_" variant of RED", Web page , the "gentle_" variant of RED", Web
March 2000, <http://www.icir.org/floyd/ page , March 2000, <http://
red/gentle.html>. www.icir.org/floyd/red/gentle.html>.
[pBox] Floyd, S. and K. Fall, "Promoting the Use [pBox] Floyd, S. and K. Fall, "Promoting the
of End-to-End Congestion Control in the Use of End-to-End Congestion Control
Internet", IEEE/ACM Transactions on in the Internet", IEEE/ACM
Networking 7(4) 458--472, August 1999, <h Transactions on Networking 7(4) 458--
ttp://www.aciri.org/floyd/ 472, August 1999, <http://
end2end-paper.html>. www.aciri.org/floyd/
end2end-paper.html>.
[pktByteEmail] Floyd, S., "RED: Discussions of Byte and [pktByteEmail] Floyd, S., "RED: Discussions of Byte
Packet Modes", email , March 1997, <http: and Packet Modes", Web page Red Queue
//www-nrg.ee.lbl.gov/floyd/ Management, March 1997, <Available
REDaveraging.txt>. at: http://ee.lbl.gov/floyd/
REDaveraging.txt>.
Appendix A. 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.
A.1. Protocol Coding A.1. Protocol Coding
skipping to change at page 33, line 9 skipping to change at page 33, line 42
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 2.3): Section 3.3):
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
skipping to change at page 37, line 43 skipping to change at page 38, line 24
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.conex-concepts-uses]. But from there it point to the Internet [I-D.ietf-conex-concepts-uses]. But from there
cannot know the MTU of each remote queue that caused each drop/mark. it cannot know the MTU of each remote queue that caused each drop/
Therefore it cannot take an integrated approach to policing all the mark. Therefore it cannot take an integrated approach to policing
responses to congestion of all the transports of one individual. all the responses to congestion of all the transports of one
Therefore it cannot police anything. individual. 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 38, line 31 skipping to change at page 39, line 15
Appendix C. 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 -03 to -04:
* Reordered Sections 2 and 3, and some clarifications here and
there based on feedback from Colin Perkins and Mirja
Kuehlewind.
From -02 to -03 (this version) From -02 to -03 (this version)
* Structural changes: * Structural changes:
+ Split off text at end of "Scaling Congestion Control with + Split off text at end of "Scaling Congestion Control with
Packet Size" into new section "Transport-Independent Packet Size" into new section "Transport-Independent
Network" Network"
+ Shifted "Recommendations" straight after "Motivating + Shifted "Recommendations" straight after "Motivating
Arguments" and added "Conclusions" at end to reinforce Arguments" and added "Conclusions" at end to reinforce
skipping to change at page 39, line 51 skipping to change at page 40, line 40
* 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:
* Abstract reorganised to align with clearer separation of issue * Abstract reorganised to align with clearer separation of issue
in the memo. in the memo.
* Introduction reorganised with motivating arguments removed to * Introduction reorganised with motivating arguments removed to
new Section 2. new Section 3.
* Clarified avoiding lock-out of large packets is not the main or * Clarified avoiding lock-out of large packets is not the main or
only motivation for RED. only motivation for RED.
* Mentioned choice of drop or marking explicitly throughout, * Mentioned choice of drop or marking explicitly throughout,
rather than trying to coin a word to mean either. rather than trying to coin a word to mean either.
* Generalised the discussion throughout to any packet forwarding * Generalised the discussion throughout to any packet forwarding
function on any network equipment, not just routers. function on any network equipment, not just routers.
* Clarified the last point about why this is a good time to sort * Clarified the last point about why this is a good time to sort
out this issue: because it will be hard / impossible to design out this issue: because it will be hard / impossible to design
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 3.1).
* 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 2.4). preference to control packets (Section 3.4).
* 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 4.1.1 & Section 4.2.1) 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 4.1.2). (Section 4.1.2).
* Added section on 'Making Transports Robust against Control * Added section on 'Making Transports Robust against Control
Packet Losses' (Section 4.2.3) with existing & new material Packet Losses' (Section 4.2.3) with existing & new material
included. included.
 End of changes. 84 change blocks. 
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