draft-ietf-tsvwg-byte-pkt-congest-00.txt   draft-ietf-tsvwg-byte-pkt-congest-01.txt 
Transport Area Working Group B. Briscoe Transport Area Working Group B. Briscoe
Internet-Draft BT & UCL Internet-Draft BT
Intended status: Informational August 07, 2008 Updates: 2309 (if approved) October 23, 2009
Expires: February 8, 2009 Intended status: Informational
Expires: April 26, 2010
Byte and Packet Congestion Notification Byte and Packet Congestion Notification
draft-ietf-tsvwg-byte-pkt-congest-00 draft-ietf-tsvwg-byte-pkt-congest-01
Status of this Memo Status of this Memo
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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). The primary conclusion is that packet congestion notification (PCN). The primary conclusion is that packet
size should be taken into account when transports read congestion size should be taken into account when transports read congestion
indications, not when network equipment writes them. Reducing drop indications, not when network equipment writes them. Reducing drop
of small packets has some tempting advantages: i) it drops less of small packets has some tempting advantages: i) it drops less
control packets, which tend to be small and ii) it makes TCP's bit- 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 rate less dependent on packet size. However, there are ways of
addressing these issues at the transport layer, rather than reverse addressing these issues at the transport layer, rather than reverse
engineering network forwarding to fix specific transport problems. engineering network forwarding to fix specific transport problems.
Network layer algorithms like the byte-mode packet drop variant of Network layer algorithms like 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. consequently also opening up a DoS vulnerability.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Motivating Arguments . . . . . . . . . . . . . . . . . . . . . 8 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 9
2.1. Scaling Congestion Control with Packet Size . . . . . . . 8 2. Motivating Arguments . . . . . . . . . . . . . . . . . . . . . 9
2.1. Scaling Congestion Control with Packet Size . . . . . . . 9
2.2. Avoiding Perverse Incentives to (ab)use Smaller Packets . 10 2.2. Avoiding Perverse Incentives to (ab)use Smaller Packets . 10
2.3. Small != Control . . . . . . . . . . . . . . . . . . . . . 11 2.3. Small != Control . . . . . . . . . . . . . . . . . . . . . 12
3. Working Definition of Congestion Notification . . . . . . . . 11 2.4. Implementation Efficiency . . . . . . . . . . . . . . . . 12
4. Congestion Measurement . . . . . . . . . . . . . . . . . . . . 12 3. Working Definition of Congestion Notification . . . . . . . . 12
4.1. Congestion Measurement by Queue Length . . . . . . . . . . 12 4. Congestion Measurement . . . . . . . . . . . . . . . . . . . . 13
4.1.1. Fixed Size Packet Buffers . . . . . . . . . . . . . . 12 4.1. Congestion Measurement by Queue Length . . . . . . . . . . 13
4.2. Congestion Measurement without a Queue . . . . . . . . . . 13 4.1.1. Fixed Size Packet Buffers . . . . . . . . . . . . . . 13
5. Idealised Wire Protocol Coding . . . . . . . . . . . . . . . . 14 4.2. Congestion Measurement without a Queue . . . . . . . . . . 14
6. The State of the Art . . . . . . . . . . . . . . . . . . . . . 15 5. Idealised Wire Protocol Coding . . . . . . . . . . . . . . . . 15
6.1. Congestion Measurement: Status . . . . . . . . . . . . . . 16 6. The State of the Art . . . . . . . . . . . . . . . . . . . . . 17
6.2. Congestion Coding: Status . . . . . . . . . . . . . . . . 17 6.1. Congestion Measurement: Status . . . . . . . . . . . . . . 17
6.2.1. Network Bias when Encoding . . . . . . . . . . . . . . 17 6.2. Congestion Coding: Status . . . . . . . . . . . . . . . . 18
6.2.2. Transport Bias when Decoding . . . . . . . . . . . . . 19 6.2.1. Network Bias when Encoding . . . . . . . . . . . . . . 18
6.2.2. Transport Bias when Decoding . . . . . . . . . . . . . 20
6.2.3. Making Transports Robust against Control Packet 6.2.3. Making Transports Robust against Control Packet
Losses . . . . . . . . . . . . . . . . . . . . . . . . 20 Losses . . . . . . . . . . . . . . . . . . . . . . . . 21
6.2.4. Congestion Coding: Summary of Status . . . . . . . . . 21 6.2.4. Congestion Coding: Summary of Status . . . . . . . . . 22
7. Outstanding Issues and Next Steps . . . . . . . . . . . . . . 23 7. Outstanding Issues and Next Steps . . . . . . . . . . . . . . 24
7.1. Bit-congestible World . . . . . . . . . . . . . . . . . . 23 7.1. Bit-congestible World . . . . . . . . . . . . . . . . . . 24
7.2. Bit- & Packet-congestible World . . . . . . . . . . . . . 23 7.2. Bit- & Packet-congestible World . . . . . . . . . . . . . 24
8. Security Considerations . . . . . . . . . . . . . . . . . . . 24 8. Security Considerations . . . . . . . . . . . . . . . . . . . 25
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 25 9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 26
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 27 11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 28
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
12.1. Normative References . . . . . . . . . . . . . . . . . . . 27 12.1. Normative References . . . . . . . . . . . . . . . . . . . 28
12.2. Informative References . . . . . . . . . . . . . . . . . . 27 12.2. Informative References . . . . . . . . . . . . . . . . . . 29
Editorial Comments . . . . . . . . . . . . . . . . . . . . . . . . Editorial Comments . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A. Example Scenarios . . . . . . . . . . . . . . . . . . 31 Appendix A. Example Scenarios . . . . . . . . . . . . . . . . . . 32
A.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 31 A.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 32
A.2. Bit-congestible resource, equal bit rates (Ai) . . . . . . 31 A.2. Bit-congestible resource, equal bit rates (Ai) . . . . . . 32
A.3. Bit-congestible resource, equal packet rates (Bi) . . . . 32 A.3. Bit-congestible resource, equal packet rates (Bi) . . . . 33
A.4. Pkt-congestible resource, equal bit rates (Aii) . . . . . 33 A.4. Pkt-congestible resource, equal bit rates (Aii) . . . . . 34
A.5. Pkt-congestible resource, equal packet rates (Bii) . . . . 34 A.5. Pkt-congestible resource, equal packet rates (Bii) . . . . 35
Appendix B. Congestion Notification Definition: Further Appendix B. Congestion Notification Definition: Further
Justification . . . . . . . . . . . . . . . . . . . . 34 Justification . . . . . . . . . . . . . . . . . . . . 35
Appendix C. Byte-mode Drop Complicates Policing Congestion Appendix C. Byte-mode Drop Complicates Policing Congestion
Response . . . . . . . . . . . . . . . . . . . . . . 35 Response . . . . . . . . . . . . . . . . . . . . . . 36
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 36 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 37
Intellectual Property and Copyright Statements . . . . . . . . . . 37
Relationship to existing RFCs
To be removed by the RFC Editor on publication (with appropriate
changes to the 'Updates:' header and the RFC Index as appropriate).
This memo intends to update RFC2309, which stated an interim view but
requested that further research was needed on this topic.
Changes from Previous Versions 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 briscoe-byte-pkt-mark-02 to ietf-byte-pkt-congest-00 (this From -00 to -01 (this version):
version):
Added note on relationship to existing RFCs * Minor clarifications throughout and updated references
Posed the question of whether packet-congestion could become From briscoe-byte-pkt-mark-02 to ietf-byte-pkt-congest-00:
* Added note on relationship to existing RFCs
* Posed the question of whether packet-congestion could become
common and deferred it to the IRTF ICCRG. Added ref to the 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:
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 2.
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 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 2.3).
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 6.2.1) and on congestion carving (Section 4.1.1 & Section 6.2.1) and on congestion
measurement in wireless link technologies without queues measurement in wireless link technologies without queues
(Section 4.2). (Section 4.2).
Added section on 'Making Transports Robust against Control * Added section on 'Making Transports Robust against Control
Packet Losses' (Section 6.2.3) with existing & new material Packet Losses' (Section 6.2.3) 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.
Emphasised that drop-tail suffers from similar problems to * Emphasised that drop-tail suffers from similar problems to
byte-mode drop, so only byte-mode drop should be turned off, byte-mode drop, so only byte-mode drop should be turned off,
not RED itself. not RED itself.
Clarified the original apparent motivations for recommending * Clarified the original apparent motivations for recommending
byte-mode drop included protecting SYNs and pure ACKs more than byte-mode drop included protecting SYNs and pure ACKs more than
equalising the bit rates of TCPs with different segment sizes. equalising the bit rates of TCPs with different segment sizes.
Removed some conjectured motivations. Removed some conjectured motivations.
Added support for updates to TCP in progress (ackcc & ecn-syn- * Added support for updates to TCP in progress (ackcc & ecn-syn-
ack). ack).
Updated survey results with newly arrived data. * Updated survey results with newly arrived data.
Pulled all recommendations together into the conclusions. * Pulled all recommendations together into the conclusions.
Moved some detailed points into two additional appendices and a * Moved some detailed points into two additional appendices and a
note. note.
Considerable clarifications throughout. * Considerable clarifications throughout.
Updated references
Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", * Updated references
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1. Introduction 1. Introduction
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.
Note that the byte vs. packet dilemma concerns congestion Note that the byte vs. packet dilemma concerns congestion
notification irrespective of whether it is signalled implicitly by notification irrespective of whether it is signalled implicitly by
drop or using explicit congestion notification (ECN [RFC3168] or PCN drop or using explicit congestion notification (ECN [RFC3168] or PCN
[I-D.eardley-pcn-marking-behaviour]). Throughout this document, [I-D.ietf-pcn-marking-behaviour]). Throughout this document, unless
unless clear from the context, the term marking will be used to mean clear from the context, the term marking will be used to mean
notifying congestion explicitly, while congestion notification will notifying congestion explicitly, while congestion notification will
be used to mean notifying congestion either implicitly by drop or be used to mean notifying congestion either implicitly by drop or
explicitly by marking. explicitly by marking.
If the load on a resource depends on the rate at which packets If the load on a resource depends on the rate at which packets
arrive, it is called packet-congestible. If the load depends on the arrive, it is called packet-congestible. If the load depends on the
rate at which bits arrive it is called bit-congestible. rate at which bits arrive it is called bit-congestible.
Examples of packet-congestible resources are route look-up engines Examples of packet-congestible resources are route look-up engines
and firewalls, because load depends on how many packet headers they and firewalls, because load depends on how many packet headers they
have to process. Examples of bit-congestible resources are have to process. Examples of bit-congestible resources are
transmission links, and most buffer memory, because the load depends transmission links, radio power and most buffer memory, because the
on how many bits they have to transmit or store. Some machine load depends on how many bits they have to transmit or store. Some
architectures use fixed size packet buffers, so buffer memory in machine architectures use fixed size packet buffers, so buffer memory
these cases is packet-congestible (see Section 4.1.1). in these cases is packet-congestible (see Section 4.1.1).
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 used, appropriate granularity for the resource in question SHOULD be used,
but for the sake of brevity we will talk in terms of bytes in this but for the sake of brevity we will talk in terms of bytes in this
memo. memo.
Resources may be congestible at higher levels of granularity than Resources may be congestible at higher levels of granularity than
packets, for instance stateful firewalls are flow-congestible and packets, for instance stateful firewalls are flow-congestible and
call-servers are session-congestible. This memo focuses on call-servers are session-congestible. This memo focuses on
congestion of connectionless resources, but the same principles may congestion of connectionless resources, but the same principles may
be applied for congestion notification protocols controlling per-flow be applicable for congestion notification protocols controlling per-
and per-session processing or state. flow and per-session processing or state.
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 how Measuring congestion When the congested resource decides locally how
to measure how congested it is. (Should the queue be measured in to measure how congested it is. (Should the queue be measured in
bytes or packets?); bytes or packets?);
Coding congestion notification into the wire protocol: When the Coding congestion notification into the wire protocol: When the
congested resource decides how to notify the level of congestion. congested resource decides how to notify the level of congestion.
skipping to change at page 7, line 8 skipping to change at page 7, line 40
decodes congestion notification. In RED, the variant that reduces decodes congestion notification. In RED, the variant that reduces
drop probability for packets based on their size in bytes is called drop probability for packets based on their size in bytes is called
byte-mode drop, while the variant that doesn't is called packet mode byte-mode drop, while the variant that doesn't is called packet mode
drop. Whether queues are measured in bytes or packets is an drop. Whether queues are measured in bytes or packets is an
orthogonal choice, termed byte-mode queue measurement or packet-mode orthogonal choice, termed byte-mode queue measurement or packet-mode
queue measurement. queue measurement.
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].
However, all the implementers who responded to our survey have not However, all the implementers who responded to our survey
followed this advice. The primary purpose of this memo is to build a (Section 6.2.4) have not followed this advice. The primary purpose
definitive consensus against deliberate preferential treatment for of this memo is to build a definitive consensus against deliberate
small packets in AQM algorithms and to record this advice within the preferential treatment for small packets in AQM algorithms and to
RFC series. record this advice within the RFC series.
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 the network layer, 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 packet vs. byte issue requires speedy resolution because the
IETF pre-congestion notification (PCN) working group is about to IETF pre-congestion notification (PCN) working group is about to
standardise the external behaviour of a PCN congestion standardise the external behaviour of a PCN congestion
notification (AQM) algorithm [I-D.eardley-pcn-marking-behaviour]; notification (AQM) algorithm [I-D.ietf-pcn-marking-behaviour];
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 para, making the recommendation less clear-cut; drop in its last para, making the recommendation less clear-cut;
3. Without the present memo, the only advice in the RFC series on 3. Without the present memo, the only advice in the RFC series on
skipping to change at page 8, line 40 skipping to change at page 9, line 25
congestion notification in Section 3 then determining the correct way congestion notification in Section 3 then determining the correct way
to measure congestion (Section 4) and to design an idealised to measure congestion (Section 4) and to design an idealised
congestion notification protocol (Section 5). It then surveys the congestion notification protocol (Section 5). It then surveys the
advice given previously in the RFC series, the research literature advice given previously in the RFC series, the research literature
and the deployed legacy (Section 6) before listing outstanding issues and the deployed legacy (Section 6) before listing outstanding issues
(Section 7) that will need resolution both to achieve the ideal (Section 7) that will need resolution both to achieve the ideal
protocol and to handle legacy. After discussing security protocol and to handle legacy. After discussing security
considerations (Section 8) strong recommendations for the way forward considerations (Section 8) strong recommendations for the way forward
are given in the conclusions (Section 9). are given in the conclusions (Section 9).
1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Motivating Arguments 2. Motivating Arguments
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. Here we try to design a test to see which
approach scales with packet size. approach scales with packet size.
Imagine a bit-congestible link shared by many flows, so that each Given bit-congestible is the more common case, consider a bit-
busy period tends to cause packets to be lost from different flows. congestible link shared by many flows, so that each busy period tends
to cause packets to be lost from different flows. The test compares
The test compares two identical scenarios with the same applications, two identical scenarios with the same applications, the same numbers
the same numbers of sources and the same load. But the sources break of sources and the same load. But the sources break the load into
the load into large packets in one scenario and small packets in the large packets in one scenario and small packets in the other. Of
other. Of course, because the load is the same, there will be course, because the load is the same, there will be proportionately
proportionately more packets in the small packet case. more packets in the small packet case.
The test of whether a congestion control scales with packet size is The test of whether a congestion control scales with packet size is
that it should respond in the same way to the same congestion that it should respond in the same way to the same congestion
excursion, irrespective of the size of the packets that the bytes excursion, irrespective of the size of the packets that the bytes
causing congestion happen to be broken down into. causing congestion happen to be 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 or many small packets. So for the same congestion excursion, the
same amount of bytes have to be shed to get the load back to its 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 more packets
will have to be discarded to shed the same bytes. 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, they will respond irrespective of the size of the packet dropped, those with smaller
more to the same congestion excursion, failing our test. On the packets will respond more to the same congestion excursion, failing
other hand, if they respond proportionately less when smaller packets our test. On the other hand, if they respond proportionately less
are dropped/marked, overall they will be able to respond the same to when smaller packets are dropped/marked, overall they will be able to
the same congestion excursion. respond the 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), not just to dropped or marked packets irrespective
of packet size (as TCP does). of packet size (as TCP does).
The email [pktByteEmail] referred to by RFC2309 says the question of The email [pktByteEmail] referred to by RFC2309 says the question of
whether a packet's own size should affect its drop probability 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 the network layer should not be optimised for whatever But we argue the network layer should not be optimised for whatever
skipping to change at page 10, line 23 skipping to change at page 11, line 15
as fewer larger packets or more smaller packets. A protocol design as fewer larger packets or more smaller packets. A protocol design
that caused larger packets to be more likely to be dropped than that caused larger packets to be more likely to be dropped than
smaller ones would be dangerous in this case: smaller ones would be dangerous in this case:
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. The regular traffic without the attacker having to do much work.
byte-mode drop variant of RED amplifies small packet attacks.
Drop-tail queues amplify small packet attacks even more than RED Note that, although the byte-mode drop variant of RED amplifies
byte-mode drop (see the Security Considerations section small packet attacks, drop-tail queues amplify small packet
Section 8). Wherever possible neither should be used. attacks even more (see Security Considerations in Section 8).
Wherever possible neither should be used.
Normal transports: Even if a transport is not malicious, if it finds Normal transports: Even if a transport is not malicious, if it finds
small packets go faster, it will tend to act in its own interest small packets go faster, it will tend to act in its own interest
and use them. Queues that give advantage to small packets create and use them. Queues that give advantage to small packets create
an evolutionary pressure for transports to send at the same bit- an evolutionary pressure for transports to send at the same bit-
rate but break their data stream down into tiny segments to reduce rate but break their data stream down into tiny segments to reduce
their drop rate. Encouraging a high volume of tiny packets might their drop rate. Encouraging a high volume of tiny packets might
in turn unnecessarily overload a completely unrelated part of the in turn unnecessarily overload a completely unrelated part of the
system, perhaps more limited by header-processing than bandwidth. system, perhaps more limited by header-processing than bandwidth.
Imagine two flows arrive at a bit-congestible transmission link each Imagine two unresponsive flows arrive at a bit-congestible
with the same bit rate, say 1Mbps, but one consists of 1500B and the transmission link each with the same bit rate, say 1Mbps, but one
other 60B packets, which are 25x smaller. Consider a scenario where consists of 1500B and the other 60B packets, which are 25x smaller.
gentle RED [gentle_RED] is used, along with the variant of RED we Consider a scenario where gentle RED [gentle_RED] is used, along with
advise against, i.e. where the RED algorithm is configured to adjust the variant of RED we advise against, i.e. where the RED algorithm is
the drop probability of packets in proportion to each packet's size configured to adjust the drop probability of packets in proportion to
(byte mode packet drop). In this case, if RED drops 25% of the each packet's size (byte mode packet drop). In this case, if RED
larger packets, it will aim to drop 1% of the smaller packets (but in drops 25% of the larger packets, it will aim to drop 1% of the
practice it may drop more as congestion increases smaller packets (but in practice it may drop more as congestion
[RFC4828](S.B.4)[Note_Variation]). Even though both flows arrive increases [RFC4828](S.B.4)[Note_Variation]). Even though both flows
with the same bit rate, the bit rate the RED queue aims to pass to arrive with the same bit rate, the bit rate the RED queue aims to
the line will be 750k for the flow of larger packet but 990k for the pass to the line will be 750k for the flow of larger packet but 990k
smaller packets (but because of rate variation it will be less than for the smaller packets (but because of rate variation it will be
this target). It can be seen that this behaviour reopens the same less than this target).
denial of service vulnerability that drop tail queues offer to floods
of small packet, though not necessarily as strongly (see Section 8). It can be seen that this behaviour reopens the same denial of service
vulnerability that drop tail queues offer to floods of small packet,
though not necessarily as strongly (see Section 8).
2.3. Small != Control 2.3. 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
skipping to change at page 11, line 28 skipping to change at page 12, line 26
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 layer, we
argue that the transport should be made more robust against losses of argue 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 6.2.3). Losses' in Section 6.2.3).
2.4. Implementation Efficiency
Allowing for packet size at the transport rather than in the network
ensures that neither the network nor the transport needs to do a
multiply operation--multiplication by packet size is effectively
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
principled reason in itself, but it is a happy consequence of the
other principled reasons.
3. Working Definition of Congestion Notification 3. Working Definition of Congestion Notification
Rather than aim to achieve what many have tried and failed, this memo Rather than aim to achieve what many have tried and failed, this memo
will not try to define congestion. It will give a working definition will not try to define congestion. It will give a working definition
of what congestion notification should be taken to mean for this of what congestion notification should be taken to mean for this
document. Congestion notification is a changing signal that aims to document. Congestion notification is a changing signal that aims to
communicate the ratio E/L, where E is the instantaneous excess load communicate the ratio E/L, where E is the instantaneous excess load
offered to a resource that it cannot (or would not) serve and L is offered to a resource that it cannot (or would not) serve and L is
the instantaneous offered load. the instantaneous offered load.
The phrase `would not serve' is added, because AQM systems (e.g. The phrase `would not serve' is added, because AQM systems (e.g.
RED, PCN [I-D.eardley-pcn-marking-behaviour]) use a virtual capacity RED, PCN [I-D.ietf-pcn-marking-behaviour]) use a virtual capacity
smaller than actual capacity, then notify congestion of this virtual smaller than actual capacity, then notify congestion of this virtual
capacity in order to avoid congestion of the actual capacity. capacity in order to avoid congestion of the actual capacity.
Note that the denominator is offered load, not capacity. Therefore Note that the denominator is offered load, not capacity. Therefore
congestion notification is a real number bounded by the range [0,1]. congestion notification is a real number bounded by the range [0,1].
This ties in with the most well-understood form of congestion This ties in with the most well-understood measure of congestion
notification: drop rate. It also means that congestion has a natural notification: drop fraction (often loosely called loss rate). It
interpretation as a probability; the probability of offered traffic also means that congestion has a natural interpretation as a
not being served (or being marked as at risk of not being served). probability; the probability of offered traffic not being served (or
Appendix B describes a further incidental benefit that arises from being marked as at risk of not being served). Appendix B describes a
using load as the denominator of congestion notification. further incidental benefit that arises from using load as the
denominator of congestion notification.
4. Congestion Measurement 4. Congestion Measurement
4.1. Congestion Measurement by Queue Length 4.1. Congestion Measurement by Queue Length
Queue length is usually the most correct and simplest way to measure Queue length is usually the most correct and simplest way to measure
congestion of a resource. To avoid the pathological effects of drop congestion of a resource. To avoid the pathological effects of drop
tail, an AQM function can then be used to transform queue length into 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 the probability of dropping or marking a packet (e.g. RED's
piecewise linear function between thresholds). If the resource is piecewise linear function between thresholds). If the resource is
skipping to change at page 14, line 42 skipping to change at page 15, line 50
case bit rates with minimum packet sizes. Therefore, packet- case bit rates with minimum packet sizes. Therefore, packet-
congestion is currently rare, but there is no guarantee that it will congestion is currently rare, but there is no guarantee that it will
not become common with future technology trends. 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). Incidentally, it also ensures neither the network nor Section 2.2):
the transport needs to do a multiply operation--multiplication by
packet size is effectively achieved as a repeated add when the
transport adds to its count of marked bytes as each congestion event
is fed to it:
o 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 congestion' into a packet stream should mark the idealised `packet
field in each packet with probability p_p irrespective of the congestion' field in each packet with probability p_p
packet's size. The transport should then take a packet with the irrespective of the packet's size. The transport should then
packet congestion field marked to mean just one mark, irrespective take a packet with the packet congestion field marked to mean
of the packet size. just one mark, irrespective of the packet size.
o 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 A show that transports can extract The worked examples in Appendix A 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:
Ratio-based: Established transport congestion controls like TCP's Ratio-based: Established transport congestion controls like TCP's
[RFC2581] aim to achieve equal segment rates per RTT through the [RFC5681] aim to achieve equal segment rates per RTT through the
same bottleneck--TCP friendliness [RFC3448]. They work with the same bottleneck--TCP friendliness [RFC3448]. They work with the
ratio of dropped to delivered segments (or marked to unmarked ratio of dropped to delivered segments (or marked to unmarked
segments in the case of ECN). The example scenarios show that segments in the case of ECN). The example scenarios show that
these ratio-based transports are effectively the same whether these ratio-based transports are effectively the same whether
counting in bytes or packets, because the units cancel out. counting in bytes or packets, because the units cancel out.
(Incidentally, this is why TCP's bit rate is still proportional to (Incidentally, this is why TCP's bit rate is still proportional to
packet size even when byte-counting is used, as recommended for packet size even when byte-counting is used, as recommended for
TCP in [I-D.ietf-tcpm-rfc2581bis], mainly for orthogonal security TCP in [RFC5681], mainly for orthogonal security reasons.)
reasons.)
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.
skipping to change at page 20, line 7 skipping to change at page 21, line 11
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 6.2.4). Incidentally, VP-TFRC also proposed that byte- see Section 6.2.4). 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 RED_5, but apparently unaware of it).
Pre-congestion notification [I-D.eardley-pcn-marking-behaviour] is a Pre-congestion notification [I-D.ietf-pcn-marking-behaviour] is a
proposal to use a virtual queue for AQM marking for packets within proposal to use a virtual queue for AQM marking for packets within
one Diffserv class in order to give early warning prior to any real one Diffserv class in order to give early warning prior to any real
queuing. The proposed PCN marking algorithms have been designed not queuing. The proposed PCN marking algorithms have been designed not
to take account of packet size when forwarding through queues. to take account of packet size when forwarding through queues.
Instead the general principle has been to take account of the sizes Instead the general principle has been to take account of the sizes
of marked packets when monitoring the fraction of marking at the edge of marked packets when monitoring the fraction of marking at the edge
of the network. of the network.
6.2.3. Making Transports Robust against Control Packet Losses 6.2.3. Making Transports Robust against Control Packet Losses
skipping to change at page 20, line 32 skipping to change at page 21, line 36
small packets. We argue here that these two proposals are a safer small packets. We argue here that these two proposals are a safer
and more principled way to achieve TCP performance improvements than and more principled way to achieve TCP performance improvements than
reverse engineering RED to benefit TCP. reverse engineering RED to 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 [Re-TCP] is designed so that transports The re-ECN protocol proposal [I-D.briscoe-tsvwg-re-ecn-tcp] is
can be made more robust against losing control packets. It gives designed so that transports can be made more robust against losing
queues an incentive to optionally give preference against drop to control packets. It gives queues an incentive to optionally give
packets with the 'feedback not established' codepoint in the proposed preference against drop to packets with the 'feedback not
'extended ECN' field. Senders have incentives to use this codepoint established' codepoint in the proposed 'extended ECN' field. Senders
sparingly, but they can use it on control packets to reduce their have incentives to use this codepoint sparingly, but they can use it
chance of being dropped. For instance, the proposed modification to on control packets to reduce their chance of being dropped. For
TCP for re-ECN uses this codepoint on the SYN and SYN-ACK. 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
skipping to change at page 21, line 25 skipping to change at page 22, line 30
+-----------+----------------+-----------------+--------------------+ +-----------+----------------+-----------------+--------------------+
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 positions we may now be in. Each
column shows a different possible AQM behaviour in different queues column shows a different possible AQM behaviour in different queues
in the network, using the terminology of Cnodder et al outlined in the network, using the terminology of Cnodder et al outlined
earlier (RED_1 is basic RED with packet-mode drop). Each row shows a earlier (RED_1 is basic RED with packet-mode drop). Each row shows a
different transport behaviour: TCP [RFC2581] and TFRC [RFC3448] on different transport behaviour: TCP [RFC5681] and TFRC [RFC3448] on
the top row with TFRC-SP [RFC4828] below. Suppressing all the top row with TFRC-SP [RFC4828] below. Suppressing all
inessential details the table shows that independence from packet inessential details the table shows that independence from packet
size should either be achievable by not altering the TCP transport in 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 a RED_5 network, or using the small packet TFRC-SP transport in a
network without any byte-mode dropping RED (top right and bottom network without any byte-mode dropping RED (top right and bottom
left). Top left is the `do nothing' scenario, while bottom right is left). Top left is the `do nothing' scenario, while bottom right is
the `do-both' scenario in which bit-rate would become far too biased 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 towards small packets. Of course, if any form of byte-mode dropping
RED has been deployed on a selection of congested queues, each path RED has been deployed on a selection of congested queues, each path
will present a different hybrid scenario to its transport. will present a different hybrid scenario to its transport.
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The sample of returns from our vendor survey Section 6.2.4 suggest The sample of returns from our vendor survey Section 6.2.4 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 [RFC2581] itself. congestion control [RFC5681] itself.
7.2. Bit- & Packet-congestible World 7.2. Bit- & Packet-congestible World
Nonetheless, a connectionless network with both bit-congestible and Nonetheless, a connectionless network with both bit-congestible and
packet-congestible resources is a different matter. If we believe we packet-congestible resources is a different matter. If we believe we
should allow for this possibility in the future, this space contains should allow for this possibility in the future, this space contains
a truly open research issue. a truly open research issue.
The idealised wire protocol coding described in Section 5 requires at The idealised wire protocol coding described in Section 5 requires at
least two flags for congestion of bit-congestible and packet- least two flags for congestion of bit-congestible and packet-
skipping to change at page 26, line 18 skipping to change at page 27, line 23
dropped, because they are small. But we SHOULD NOT hack the network dropped, because they are small. But we SHOULD NOT hack the network
layer to improve or fix certain transport protocols. No matter how layer to improve or fix certain transport protocols. No matter how
predominant a transport protocol is (even if it's TCP), trying to predominant a transport protocol is (even if it's TCP), trying to
correct for its failings by biasing towards small packets in the correct for its failings by biasing towards small packets in the
network layer creates a perverse incentive to break down all flows network layer creates a perverse incentive to break down all flows
from all transports into tiny segments. from all transports into tiny segments.
So far, our survey of 84 vendors across the industry has drawn So far, our survey of 84 vendors across the industry has drawn
responses from about 19%, none of whom have implemented the byte mode responses from about 19%, none of whom have implemented the byte mode
packet drop variant of RED. Given there appears to be little, if packet drop variant of RED. Given there appears to be little, if
any, installed base recommending removal of byte-mode drop from RED any, installed base it seems we can recommend removal of byte-mode
is possibly only a paper exercise with few, if any, incremental drop from RED with little, if any, incremental deployment impact.
deployment issues.
If a vendor has implemented byte-mode drop, and an operator has If a vendor has implemented byte-mode drop, and an operator has
turned it on, it is strongly RECOMMENDED that it SHOULD be turned 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 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 it, a drop tail queue also biases against large packets. But note
also that turning off byte-mode may alter the relative performance of also that turning off byte-mode may alter the relative performance of
applications using different packet sizes, so it would be advisable applications using different packet sizes, so it would be advisable
to establish the implications before turning it off. to establish the implications before turning it off.
Instead, the IETF transport area should continue its programme of Instead, the IETF transport area should continue its programme of
skipping to change at page 27, line 8 skipping to change at page 28, line 8
most, if not all, resources being primarily bit-congestible. A most, if not all, resources being primarily bit-congestible. A
secondary conclusion of this memo is that we may see more packet- secondary conclusion of this memo is that we may see more packet-
congestible resources in the future, so research may be needed to congestible resources in the future, so research may be needed to
extend the Internet's congestion notification (drop or ECN) so that extend the Internet's congestion notification (drop or ECN) so that
it can handle a mix of bit-congestible and packet-congestible it can handle a mix of bit-congestible and packet-congestible
resources. resources.
10. Acknowledgements 10. 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 Toby Moncaster and Arnaud comments. Also thanks for the reviews from Philip Eardley, Toby
Jacquet. I am grateful to Bruce Davie and his colleagues for Moncaster and Arnaud Jacquet as well as helpful explanations of
providing a timely and efficient survey of RED implementation in different hardware approaches from Larry Dunn and Fred Baker. I am
Cisco's product range. Also grateful thanks to Toby Moncaster, Will grateful to Bruce Davie and his colleagues for providing a timely and
Dormann, John Regnault, Simon Carter and Stefaan De Cnodder who efficient survey of RED implementation in Cisco's product range.
further helped survey the current status of RED implementation and Also grateful thanks to Toby Moncaster, Will Dormann, John Regnault,
deployment and, finally, thanks to the anonymous individuals who Simon Carter and Stefaan De Cnodder who further helped survey the
responded. current status of RED implementation and deployment and, finally,
thanks to the anonymous individuals who responded.
Bob Briscoe is partly funded by Trilogy, a research project (ICT-
216372) supported by the European Community under its Seventh
Framework Programme. The views expressed here are those of the
author only.
11. Comments Solicited 11. 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.
12. References 12. References
12.1. Normative References 12.1. Normative References
skipping to change at page 28, line 28 skipping to change at page 29, line 34
Siris, V., "Resource Control for Elastic Traffic in CDMA Siris, V., "Resource Control for Elastic Traffic in CDMA
Networks", Proc. ACM MOBICOM'02 , September 2002, <http:// Networks", Proc. ACM MOBICOM'02 , September 2002, <http://
www.ics.forth.gr/netlab/publications/ www.ics.forth.gr/netlab/publications/
resource_control_elastic_cdma.html>. resource_control_elastic_cdma.html>.
[Evol_cc] Gibbens, R. and F. Kelly, "Resource pricing and the [Evol_cc] Gibbens, R. and F. Kelly, "Resource pricing and the
evolution of congestion control", Automatica 35(12)1969-- evolution of congestion control", Automatica 35(12)1969--
1985, December 1999, 1985, December 1999,
<http://www.statslab.cam.ac.uk/~frank/evol.html>. <http://www.statslab.cam.ac.uk/~frank/evol.html>.
[I-D.eardley-pcn-marking-behaviour] [I-D.briscoe-tsvwg-re-ecn-tcp]
Eardley, P., "Marking behaviour of PCN-nodes", Briscoe, B., Jacquet, A., Moncaster, T., and A. Smith,
draft-eardley-pcn-marking-behaviour-01 (work in progress), "Re-ECN: Adding Accountability for Causing Congestion to
June 2008. TCP/IP", draft-briscoe-tsvwg-re-ecn-tcp-07 (work in
progress), March 2009.
[I-D.falk-xcp-spec]
Falk, A., "Specification for the Explicit Control Protocol
(XCP)", draft-falk-xcp-spec-03 (work in progress),
July 2007.
[I-D.floyd-tcpm-ackcc] [I-D.floyd-tcpm-ackcc]
Floyd, S. and I. Property, "Adding Acknowledgement Floyd, S., "Adding Acknowledgement Congestion Control to
Congestion Control to TCP", draft-floyd-tcpm-ackcc-02 TCP", draft-floyd-tcpm-ackcc-06 (work in progress),
(work in progress), November 2007. July 2009.
[I-D.ietf-pcn-marking-behaviour]
Eardley, P., "Metering and marking behaviour of PCN-
nodes", draft-ietf-pcn-marking-behaviour-05 (work in
progress), August 2009.
[I-D.ietf-tcpm-ecnsyn] [I-D.ietf-tcpm-ecnsyn]
Floyd, S., "Adding Explicit Congestion Notification (ECN) Floyd, S., "Adding Explicit Congestion Notification (ECN)
Capability to TCP's SYN/ACK Packets", Capability to TCP's SYN/ACK Packets",
draft-ietf-tcpm-ecnsyn-05 (work in progress), draft-ietf-tcpm-ecnsyn-10 (work in progress), May 2009.
February 2008.
[I-D.ietf-tcpm-rfc2581bis]
Allman, M., "TCP Congestion Control",
draft-ietf-tcpm-rfc2581bis-03 (work in progress),
September 2007.
[I-D.irtf-iccrg-welzl-congestion-control-open-research] [I-D.irtf-iccrg-welzl-congestion-control-open-research]
Papadimitriou, D., "Open Research Issues in Internet Welzl, M., Scharf, M., Briscoe, B., and D. Papadimitriou,
Congestion Control", "Open Research Issues in Internet Congestion Control",
draft-irtf-iccrg-welzl-congestion-control-open-research-00 draft-irtf-iccrg-welzl-congestion-control-open-research-05
(work in progress), July 2007. (work in progress), September 2009.
[IOSArch] Bollapragada, V., White, R., and C. Murphy, "Inside Cisco [IOSArch] Bollapragada, V., White, R., and C. Murphy, "Inside Cisco
IOS Software Architecture", Cisco Press: CCIE Professional IOS Software Architecture", Cisco Press: CCIE Professional
Development ISBN13: 978-1-57870-181-0, July 2000. Development ISBN13: 978-1-57870-181-0, July 2000.
[MulTCP] Crowcroft, J. and Ph. Oechslin, "Differentiated End to End [MulTCP] Crowcroft, J. and Ph. Oechslin, "Differentiated End to End
Internet Services using a Weighted Proportional Fair Internet Services using a Weighted Proportional Fair
Sharing TCP", CCR 28(3) 53--69, July 1998, <http:// Sharing TCP", CCR 28(3) 53--69, July 1998, <http://
www.cs.ucl.ac.uk/staff/J.Crowcroft/hipparch/pricing.html>. www.cs.ucl.ac.uk/staff/J.Crowcroft/hipparch/pricing.html>.
skipping to change at page 29, line 47 skipping to change at page 30, line 48
[REDbyte] De Cnodder, S., Elloumi, O., and K. Pauwels, "RED behavior [REDbyte] De Cnodder, S., Elloumi, O., and K. Pauwels, "RED behavior
with different packet sizes", Proc. 5th IEEE Symposium on with different packet sizes", Proc. 5th IEEE Symposium on
Computers and Communications (ISCC) 793--799, July 2000, Computers and Communications (ISCC) 793--799, July 2000,
<http://www.icir.org/floyd/red/Elloumi99.pdf>. <http://www.icir.org/floyd/red/Elloumi99.pdf>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS "Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474, Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998. December 1998.
[RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[RFC3448] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP [RFC3448] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification", Friendly Rate Control (TFRC): Protocol Specification",
RFC 3448, January 2003. RFC 3448, January 2003.
[RFC3714] Floyd, S. and J. Kempf, "IAB Concerns Regarding Congestion [RFC3714] Floyd, S. and J. Kempf, "IAB Concerns Regarding Congestion
Control for Voice Traffic in the Internet", RFC 3714, Control for Voice Traffic in the Internet", RFC 3714,
March 2004. March 2004.
[RFC4782] Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick- [RFC4782] Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick-
Start for TCP and IP", RFC 4782, January 2007. Start for TCP and IP", RFC 4782, January 2007.
[RFC4828] Floyd, S. and E. Kohler, "TCP Friendly Rate Control [RFC4828] Floyd, S. and E. Kohler, "TCP Friendly Rate Control
(TFRC): The Small-Packet (SP) Variant", RFC 4828, (TFRC): The Small-Packet (SP) Variant", RFC 4828,
April 2007. April 2007.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
[Rate_fair_Dis] [Rate_fair_Dis]
Briscoe, B., "Flow Rate Fairness: Dismantling a Religion", Briscoe, B., "Flow Rate Fairness: Dismantling a Religion",
ACM CCR 37(2)63--74, April 2007, ACM CCR 37(2)63--74, April 2007,
<http://portal.acm.org/citation.cfm?id=1232926>. <http://portal.acm.org/citation.cfm?id=1232926>.
[Re-TCP] Briscoe, B., Jacquet, A., Moncaster, T., and A. Smith,
"Re-ECN: Adding Accountability for Causing Congestion to
TCP/IP", draft-briscoe-tsvwg-re-ecn-tcp-05 (work in
progress), January 2008.
[WindowPropFair] [WindowPropFair]
Siris, V., "Service Differentiation and Performance of Siris, V., "Service Differentiation and Performance of
Weighted Window-Based Congestion Control and Packet Weighted Window-Based Congestion Control and Packet
Marking Algorithms in ECN Networks", Computer Marking Algorithms in ECN Networks", Computer
Communications 26(4) 314--326, 2002, <http:// Communications 26(4) 314--326, 2002, <http://
www.ics.forth.gr/netgroup/publications/ www.ics.forth.gr/netgroup/publications/
weighted_window_control.html>. weighted_window_control.html>.
[gentle_RED] [gentle_RED]
Floyd, S., "Recommendation on using the "gentle_" variant Floyd, S., "Recommendation on using the "gentle_" variant
skipping to change at page 30, line 50 skipping to change at page 31, line 47
[pBox] Floyd, S. and K. Fall, "Promoting the Use of End-to-End [pBox] Floyd, S. and K. Fall, "Promoting the Use of End-to-End
Congestion Control in the Internet", IEEE/ACM Transactions Congestion Control in the Internet", IEEE/ACM Transactions
on Networking 7(4) 458--472, August 1999, on Networking 7(4) 458--472, August 1999,
<http://www.aciri.org/floyd/end2end-paper.html>. <http://www.aciri.org/floyd/end2end-paper.html>.
[pktByteEmail] [pktByteEmail]
Floyd, S., "RED: Discussions of Byte and Packet Modes", Floyd, S., "RED: Discussions of Byte and Packet Modes",
email , March 1997, email , March 1997,
<http://www-nrg.ee.lbl.gov/floyd/REDaveraging.txt>. <http://www-nrg.ee.lbl.gov/floyd/REDaveraging.txt>.
[xcp-spec]
Falk, A., "Specification for the Explicit Control Protocol
(XCP)", draft-falk-xcp-spec-03 (work in progress),
July 2007.
(Expired)
Editorial Comments Editorial Comments
[Note_Variation] The algorithm of the byte-mode drop variant of RED [Note_Variation] The algorithm of the byte-mode drop variant of RED
switches off any bias towards small packets switches off any bias towards small packets
whenever the smoothed queue length dictates that whenever the smoothed queue length dictates that
the drop probability of large packets should be the drop probability of large packets should be
100%. In the example in the Introduction, as the 100%. In the example in the Introduction, as the
large packet drop probability varies around 25% the large packet drop probability varies around 25% the
small packet drop probability will vary around 1%, small packet drop probability will vary around 1%,
but with occasional jumps to 100% whenever the but with occasional jumps to 100% whenever the
instantaneous queue (after drop) manages to sustain instantaneous queue (after drop) manages to sustain
a length above the 100% drop point for longer than a length above the 100% drop point for longer than
the queue averaging period. the queue averaging period.
Appendix A. Example Scenarios Appendix A. Example Scenarios
A.1. Notation A.1. Notation
To prove the two sets of assertions in the idealised wire protocol To prove our idealised wire protocol (Section 5) is correct, we will
(Section 5) are true, we will compare two flows with different packet compare two flows with different packet sizes, s_1 and s_2 [bit/pkt],
sizes, s_1 and s_2 [bit/pkt], to make sure their transports each see to make sure their transports each see the correct congestion
the correct congestion notification. Initially, within each flow we notification. Initially, within each flow we will take all packets
will take all packets as having equal sizes, but later we will as having equal sizes, but later we will generalise to flows within
generalise to flows within which packet sizes vary. A flow's bit which packet sizes vary. A flow's bit rate, x [bit/s], is related to
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:
+-----------------------------+------------------+------------------+ +-----------------------------+------------------+------------------+
| 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) |
skipping to change at page 32, line 40 skipping to change at page 33, line 42
However, where an absolute target rather than relative volume of However, where an absolute target rather than relative volume of
congestion caused is important (Section 5), as it is for congestion congestion caused is important (Section 5), as it is for congestion
accountability [Rate_fair_Dis], the transport must count marked bytes accountability [Rate_fair_Dis], the transport must count marked bytes
not packets, in this bit-congestible case. Aside from the goal of not packets, in this bit-congestible case. Aside from the goal of
congestion accountability, this is how the bit rate of a transport congestion accountability, this is how the bit rate of a transport
can be made independent of packet size; by ensuring the rate of can be made independent of packet size; by ensuring the rate of
congestion caused is kept to a constant weight [WindowPropFair], congestion caused is kept to a constant weight [WindowPropFair],
rather than merely responding to the ratio of marked and unmarked rather than merely responding to the ratio of marked and unmarked
bytes. bytes.
Note the unit of byte-congestion volume is the byte. Note the unit of byte-congestion-volume is the byte.
A.3. Bit-congestible resource, equal packet rates (Bi) A.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
skipping to change at page 34, line 8 skipping to change at page 35, line 11
combined packet rate times the marking probability, p_p(u_1+u_2), 26x combined packet rate times the marking probability, p_p(u_1+u_2), 26x
faster than packet congestion accumulates in the single 1500B packet faster than packet congestion accumulates in the single 1500B packet
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 packets. Note the unit of packet congestion is a packet.
A.5. Pkt-congestible resource, equal packet rates (Bii) A.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.
skipping to change at page 34, line 34 skipping to change at page 35, line 37
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 B. Congestion Notification Definition: Further Justification Appendix B. Congestion Notification Definition: Further Justification
In Section 3 on the definition of congestion notification, load not In Section 3 on the definition of congestion notification, load not
capacity was used as the denominator. This also has a subtle capacity was used as the denominator. This also has a subtle
significance in the related debate over the design of new transport significance in the related debate over the design of new transport
protocols--typical new protocol designs (e.g. in XCP protocols--typical new protocol designs (e.g. in XCP [xcp-spec] &
[I-D.falk-xcp-spec] & Quickstart [RFC4782]) expect the sending Quickstart [RFC4782]) expect the sending transport to communicate its
transport to communicate its desired flow rate to the network and desired flow rate to the network and network elements to
network elements to progressively subtract from this so that the progressively subtract from this so that the achievable flow rate
achievable flow rate emerges at the receiving transport. emerges at the receiving transport.
Congestion notification with total load in the denominator can serve Congestion notification with total load in the denominator can serve
a similar purpose (though in retrospect not in advance like XCP & a similar purpose (though in retrospect not in advance like XCP &
QuickStart). Congestion notification is a dimensionless fraction but QuickStart). Congestion notification is a dimensionless fraction but
each source can extract necessary rate information from it because it each source can extract necessary rate information from it because it
already knows what its own rate is. Even though congestion already knows what its own rate is. Even though congestion
notification doesn't communicate a rate explicitly, from each notification doesn't communicate a rate explicitly, from each
source's point of view congestion notification represents the source's point of view congestion notification represents the
fraction of the rate it was sending a round trip ago that couldn't fraction of the rate it was sending a round trip ago that couldn't
(or wouldn't) be served by available resources. After they were (or wouldn't) be served by available resources. After they were
skipping to change at page 35, line 22 skipping to change at page 36, line 24
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 with byte-mode drop, one dropped or marked packet is not But a byte-mode drop algorithm has to depend on the local MTU of the
necessarily equivalent to another unless you know the MTU that caused line - an algorithm needs to use some concept of a 'normal' packet
it to be dropped/marked. To have an integrated view of a user, we size. Therefore, one dropped or marked packet is not necessarily
equivalent to another unless you know the MTU at the queue that where
it was dropped/marked. To have an integrated view of a user, we
believe congestion policing has to be located at an individual's believe congestion policing has to be located at an individual's
attachment point to the Internet [Re-TCP]. But from there it cannot attachment point to the Internet [I-D.briscoe-tsvwg-re-ecn-tcp]. But
know the MTU of each remote queue that caused each drop/mark. from there it cannot know the MTU of each remote queue that caused
Therefore it cannot take an integrated approach to policing all the each drop/mark. Therefore it cannot take an integrated approach to
responses to congestion of all the transports of one individual. policing 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 36, line 9 skipping to change at page 37, line 14
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.
Author's Address Author's Address
Bob Briscoe Bob Briscoe
BT & UCL BT
B54/77, Adastral Park B54/77, Adastral Park
Martlesham Heath Martlesham Heath
Ipswich IP5 3RE Ipswich IP5 3RE
UK UK
Phone: +44 1473 645196 Phone: +44 1473 645196
Email: bob.briscoe@bt.com Email: bob.briscoe@bt.com
URI: http://www.cs.ucl.ac.uk/staff/B.Briscoe/ URI: http://bobbriscoe.net/
Full Copyright Statement
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Acknowledgment
This document was produced using xml2rfc v1.33 (of
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