Internet DRAFT - draft-briscoe-conex-re-ecn-motiv
draft-briscoe-conex-re-ecn-motiv
Transport Area Working Group B. Briscoe, Ed.
Internet-Draft A. Jacquet
Intended status: Historic BT
Expires: September 12, 2014 T. Moncaster
Moncaster.com
A. Smith
BT
March 11, 2014
Re-ECN: A Framework for adding Congestion Accountability to TCP/IP
draft-briscoe-conex-re-ecn-motiv-03
Abstract
This document describes a framework for using a new protocol called
re-ECN (re-inserted explicit congestion notification), which can be
deployed incrementally around unmodified routers. Re-ECN allows
accurate congestion monitoring throughout the network thus enabling
the upstream party at any trust boundary in the internetwork to be
held responsible for the congestion they cause, or allow to be
caused. So, networks can introduce straightforward accountability
for congestion and policing mechanisms for incoming traffic from end-
customers or from neighbouring network domains. As well as giving
the motivation for re-ECN this document also gives examples of
mechanisms that can use the protocol to ensure data sources respond
correctly to congestion. And it describes example mechanisms that
ensure the dominant selfish strategy of both network domains and end-
points will be to use the protocol honestly.
Note concerning Intended Status: If this draft were ever published as
an RFC it would probably have historic status. There is limited
space in the IP header, so re-ECN had to compromise by requiring the
receiver to be ECN-enabled otherwise the sender could not use re-ECN.
Re-ECN was a precursor to chartering of the IETF's Congestion
Exposure (ConEx) working group, but during chartering there were
still too few ECN receivers enabled, therefore it was decided to
pursue other compromises in order to fit a similar capability into
the IP header.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 12, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Re-ECN Protocol in Brief . . . . . . . . . . . . . . . . . 5
1.3. The Re-ECN Framework . . . . . . . . . . . . . . . . . . . 6
1.4. Solving Hard Problems . . . . . . . . . . . . . . . . . . 7
1.5. The Rest of this Document . . . . . . . . . . . . . . . . 8
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 8
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Policing Congestion Response . . . . . . . . . . . . . . . 9
3.1.1. The Policing Problem . . . . . . . . . . . . . . . . . 9
3.1.2. The Case Against Bottleneck Policing . . . . . . . . . 10
4. Re-ECN Incentive Framework . . . . . . . . . . . . . . . . . . 11
4.1. Revealing Congestion Along the Path . . . . . . . . . . . 11
4.1.1. Positive and Negative Flows . . . . . . . . . . . . . 13
4.2. Incentive Framework Overview . . . . . . . . . . . . . . . 13
4.3. Egress Dropper . . . . . . . . . . . . . . . . . . . . . . 17
4.4. Ingress Policing . . . . . . . . . . . . . . . . . . . . . 19
4.5. Inter-domain Policing . . . . . . . . . . . . . . . . . . 21
4.6. Inter-domain Fail-safes . . . . . . . . . . . . . . . . . 24
4.7. The Case against Classic Feedback . . . . . . . . . . . . 25
4.8. Simulations . . . . . . . . . . . . . . . . . . . . . . . 26
5. Other Applications of Re-ECN . . . . . . . . . . . . . . . . . 26
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5.1. DDoS Mitigation . . . . . . . . . . . . . . . . . . . . . 26
5.2. End-to-end QoS . . . . . . . . . . . . . . . . . . . . . . 28
5.3. Traffic Engineering . . . . . . . . . . . . . . . . . . . 28
5.4. Inter-Provider Service Monitoring . . . . . . . . . . . . 28
6. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 28
7. Incremental Deployment . . . . . . . . . . . . . . . . . . . . 29
7.1. Incremental Deployment Features . . . . . . . . . . . . . 29
7.2. Incremental Deployment Incentives . . . . . . . . . . . . 30
8. Architectural Rationale . . . . . . . . . . . . . . . . . . . 35
9. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 38
9.1. Policing Rate Response to Congestion . . . . . . . . . . . 38
9.2. Congestion Notification Integrity . . . . . . . . . . . . 38
9.3. Identifying Upstream and Downstream Congestion . . . . . . 39
10. Security Considerations . . . . . . . . . . . . . . . . . . . 40
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40
12. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 40
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 40
14. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 40
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
15.1. Normative References . . . . . . . . . . . . . . . . . . . 40
15.2. Informative References . . . . . . . . . . . . . . . . . . 41
Appendix A. Example Egress Dropper Algorithm . . . . . . . . . . 43
Appendix B. Policer Designs to ensure Congestion
Responsiveness . . . . . . . . . . . . . . . . . . . 44
B.1. Per-user Policing . . . . . . . . . . . . . . . . . . . . 44
B.2. Per-flow Rate Policing . . . . . . . . . . . . . . . . . . 45
Appendix C. Downstream Congestion Metering Algorithms . . . . . . 47
C.1. Bulk Downstream Congestion Metering Algorithm . . . . . . 48
C.2. Inflation Factor for Persistently Negative Flows . . . . . 48
Appendix D. Re-TTL . . . . . . . . . . . . . . . . . . . . . . . 49
Appendix E. Argument for holding back the ECN nonce . . . . . . . 50
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Authors' Statement: Status (to be removed by the RFC Editor)
Although the re-ECN protocol is intended to make a simple but far-
reaching change to the Internet architecture, the most immediate
priority for the authors is to delay any move of the ECN nonce to
Proposed Standard status. The argument for this position is
developed in Appendix E.
1. Introduction
This document aims to:
o Describe the motivation for wanting to introduce re-ECN;
o Provide a very brief description of the protocol;
o The framework within which the protocol sits;
o To show how a number of hard problems become much easier to solve
once re-ECN is available in IP.
This introduction starts with a run through of these 4 points.
1.1. Motivation
Re-ECN is proposed as a means of allowing accurate monitoring of
congestion throughout the Internet. The current Internet relies on
the vast majority of end-systems running TCP and reacting to detected
congestion by reducing their sending rates. Thus congestion control
is conducted by the collaboration of the majority of end-systems.
In this situation it is possible for applications that are
unresponsive to congestion to take whatever share of bottleneck
resources they want from responsive flows, the responsive flows
reduce their sending rate in face of congestion and effectively get
out of the way of unresponsive flows. An increasing proportion of
such applications could lead to congestion collapse being more common
[RFC3714]. Each network has no visibility of whole path congestion
and can only respond to congestion on a local basis.
Using re-ECN will allow any point along a path to calculate
congestion both upstream and downstream of that point. As a
consequence of this policing of congestion /could/ be carried out in
the network if end-systems fail to do so. Re-ECN enables flows and
users to be policed and for policing to happen at network ingress and
at network borders.
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1.2. Re-ECN Protocol in Brief
In re-ECN each sender makes a prediction of the congestion that each
flow will cause and signals that prediction within the IP headers of
that flow. The prediction is based on, but not limited to, feedback
received from the receiver. Sending a prediction of the congestion
gives network equipment a view of the congestion downstream and
upstream.
In order to explain this mechanism we introduce the notion of IP
packets carrying different, notional values dependent on the state of
their header flags:
o Negative - are those marked by queues when incipient congestion is
detected. This is exactly the same as ECN [RFC3168];
o Positive - are sent by the sender in proportion to the number of
bytes in packets that have been marked negative according to
feedback received from the receiver;
o Cautious - are sent whenever the sender cannot be sure of the
correct amount of positive bytes to inject into the network for
example, at the start of a flow to indicate that feedback has not
been established;
o Cancelled - packets sent by the sender as positive that get marked
as negative by queues in the network due to incipient congestion;
o Neutral - normal IP packets but show queues that they can be
marked negative.
A flow starts to transmit packets. No feedback has been established
so a number of cautious packets are sent (see the protocol definition
[Re-TCP] for an analysis of how many cautious packets should be sent
at flow start). The rest are sent as neutral.
The packets traverse a congested queue. A fraction are marked
negative as an indication of incipient congestion.
The packets are received by the receiver. The receiver feeds back to
the sender a count of the number of packets that have been marked
negative. This feedback can be provided either by the transport
(e.g. TCP) or by higher-layer control messages.
The sender receives the feedback and then sends a number of positive
packets in proportion to the bytes represented by packets that have
been marked negative. It is important to note that congestion is
revealed by the fraction of marked packets rather than a field in the
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IP header. This is due to the limited code points available and
includes use of the last unallocated bit (sometimes called the evil
bit [RFC3514]). Full details of the code points used is given in
[Re-TCP]. This lack of codepoints is, however, the case with IPv4.
ECN is similarly restricted.
The number of bytes inside the negative packets and positive packets
should therefore be approximately equal at the termination point of
the flow. To put it another way, the balance of negative and
positive should be zero.
1.3. The Re-ECN Framework
The introducion of the protocol enables 3 things:
o Gives a view of whole path congestion;
o Enables policing of flows;
o It allows networks to monitor the flow of congestion across their
borders.
At any point in the network a device can calculate the upstream
congestion by calculating the fraction of bytes in negative packets
to total packets. This it could do using ECN by calculating the
fraction of packets marked Congestion Experienced.
Using re-ECN a device in the network can calculate downstream
congestion by subtracting the fraction of negative packets from the
fraction of positive packets.
A user can be restricted to only causing a certain amount of
congestion. A Policer could be introduced at the ingress of a
network that counts the number of positive packets being sent and
limits the sender if that sender ties to transmit more positive
packets than their allowance.
A user could deliberately ignore some or all of the feedback and
transmit packets with a zero or much lower proportion of positive
packets than negative packets. To solve this a Dropper is proposed.
This would be placed at the egress of a network. If the number of
negative packets exceeds the number of positive packets then the flow
could be dropped or some other sanction enacted.
Policers and droppers could be used between networks in order to
police bulk traffic. A whole network harbouring users causing
congestion in downstream networks can be held responsible or policed
by its downstream neighbour.
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1.4. Solving Hard Problems
We have already shown that by making flows declare the level of
congestion they are causing that they can be policed, more
specifically these are the kind of problems that can be solved:
o mitigating distributed denial of service (DDoS);
o simplifying differentiation of quality of service (QoS);
o policing compliance to congestion control;
o inter-provider service monitoring;
o etc.
Uniquely, re-ECN manages to enable solutions to these problems
without unduly stifling innovative new ways to use the Internet.
This was a hard balance to strike, given it could be argued that DDoS
is an innovative way to use the Internet. The most valuable insight
was to allow each network to choose the level of constraint it wishes
to impose. Also re-ECN has been carefully designed so that networks
that choose to use it conservatively can protect themselves against
the congestion caused in their network by users on other networks
with more liberal policies.
For instance, some network owners want to block applications like
voice and video unless their network is compensated for the extra
share of bottleneck bandwidth taken. These real-time applications
tend to be unresponsive when congestion arises. Whereas elastic TCP-
based applications back away quickly, ending up taking a much smaller
share of congested capacity for themselves. Other network owners
want to invest in large amounts of capacity and make their gains from
simplicity of operation and economies of scale.
While we have designed re-ECN so that networks can choose to deploy
stringent policing, this does not imply we advocate that every
network should introduce tight controls on those that cause
congestion. Re-ECN has been specifically designed to allow different
networks to choose how conservative or liberal they wish to be with
respect to policing congestion. But those that choose to be
conservative can protect themselves from the excesses that liberal
networks allow their users.
Re-ECN allows the more conservative networks to police out flows that
have not asked to be unresponsive to congestion---not because they
are voice or video---just because they don't respond to congestion.
But it also allows other networks to choose not to police.
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Crucially, when flows from liberal networks cross into a conservative
network, re-ECN enables the conservative network to apply penalties
to its neighbouring networks for the congestion they allow to be
caused. And these penalties can be applied to bulk data, without
regard to flows.
Then, if unresponsive applications become so dominant that some of
the more liberal networks experience congestion collapse [RFC3714],
they can change their minds and use re-ECN to apply tighter controls
in order to bring congestion back under control.
Re-ECN reduces the need for complex network equipment to perform
these functions.
1.5. The Rest of this Document
This document is structured as follows. First the motivation for the
new protocol is given (Section 3) followed by the incentive framework
that is possible with the protocol Section 4. Section 5 then
describes other important applications re-ECN, such as policing DDoS,
QoS and congestion control. Although these applications do not
require standardisation themselves, they are described in a fair
degree of detail in order to explain how re-ECN can be used. Given
re-ECN proposes to use the last undefined bit in the IPv4 header, we
felt it necessary to outline the potential that re-ECN could release
in return for being given that bit.
Deployment issues discussed throughout the document are brought
together in Section 7, which is followed by a brief section
explaining the somewhat subtle rationale for the design from an
architectural perspective (Section 8). We end by describing related
work (Section 9), listing security considerations (Section 10) and
finally drawing conclusions (Section 12).
2. 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].
This document first specifies a protocol, then describes a framework
that creates the right incentives to ensure compliance to the
protocol. This could cause confusion because the second part of the
document considers many cases where malicious nodes may not comply
with the protocol. When such contingencies are described, if any of
the above keywords are not capitalised, that is deliberate. So, for
instance, the following two apparently contradictory sentences would
be perfectly consistent: i) x MUST do this; ii) x may not do this.
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3. Motivation
3.1. Policing Congestion Response
3.1.1. The Policing Problem
The current Internet architecture trusts hosts to respond voluntarily
to congestion. Limited evidence shows that the large majority of
end-points on the Internet comply with a TCP-friendly response to
congestion. But telephony (and increasingly video) services over the
best effort Internet are attracting the interest of major commercial
operations. Most of these applications do not respond to congestion
at all. Those that can switch to lower rate codecs.
Of course, the Internet is intended to support many different
application behaviours. But the problem is that this freedom can be
exercised irresponsibly. The greater problem is that we will never
be able to agree on where the boundary is between responsible and
irresponsible. Therefore re-ECN is designed to allow different
networks to set their own view of the limit to irresponsibility, and
to allow networks that choose a more conservative limit to push back
against congestion caused in more liberal networks.
As an example of the impossibility of setting a standard for
fairness, mandating TCP-friendliness would set the bar too high for
unresponsive streaming media, but still some would say the bar was
too low [relax-fairness]. Even though all known peer-to-peer
filesharing applications are TCP-compatible, they can cause a
disproportionate amount of congestion, simply by using multiple flows
and by transferring data continuously relative to other short-lived
sessions. On the other hand, if we swung the other way and set the
bar low enough to allow streaming media to be unresponsive, we would
also allow denial of service attacks, which are typically
unresponsive to congestion and consist of multiple continuous flows.
Applications that need (or choose) to be unresponsive to congestion
can effectively take (some would say steal) whatever share of
bottleneck resources they want from responsive flows. Whether or not
such free-riding is common, inability to prevent it increases the
risk of poor returns for investors in network infrastructure, leading
to under-investment. An increasing proportion of unresponsive or
free-riding demand coupled with persistent under-supply is a broken
economic cycle. Therefore, if the current, largely co-operative
consensus continues to erode, congestion collapse could become more
common in more areas of the Internet [RFC3714].
While we have designed re-ECN so that networks can choose to deploy
stringent policing, this does not imply we advocate that every
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network should introduce tight controls on those that cause
congestion. Re-ECN has been specifically designed to allow different
networks to choose how conservative or liberal they wish to be with
respect to policing congestion. But those that choose to be
conservative can protect themselves from the excesses that liberal
networks allow their users.
3.1.2. The Case Against Bottleneck Policing
The state of the art in rate policing is the bottleneck policer,
which is intended to be deployed at any forwarding resource that may
become congested. Its aim is to detect flows that cause
significantly more local congestion than others. Although operators
might solve their immediate problems by deploying bottleneck
policers, we are concerned that widespread deployment would make it
extremely hard to evolve new application behaviours. We believe the
IETF should offer re-ECN as the preferred protocol on which to base
solutions to the policing problems of operators, because it would not
harm evolvability and, frankly, it would be far more effective (see
later for why).
Approaches like [XCHOKe] & [pBox] are nice approaches for rate
policing traffic without the benefit of whole path information (such
as could be provided by re-ECN). But they must be deployed at
bottlenecks in order to work. Unfortunately, a large proportion of
traffic traverses at least two bottlenecks (in two access networks),
particularly with the current traffic mix where peer-to-peer file-
sharing is prevalent. If ECN were deployed, we believe it would be
likely that these bottleneck policers would be adapted to combine ECN
congestion marking from the upstream path with local congestion
knowledge. But then the only useful placement for such policers
would be close to the egress of the internetwork.
But then, if these bottleneck policers were widely deployed (which
would require them to be more effective than they are now), the
Internet would find itself with one universal rate adaptation policy
(probably TCP-friendliness) embedded throughout the network. Given
TCP's congestion control algorithm is already known to be hitting its
scalability limits and new algorithms are being developed for high-
speed congestion control, embedding TCP policing into the Internet
would make evolution to new algorithms extremely painful. If a
source wanted to use a different algorithm, it would have to first
discover then negotiate with all the policers on its path,
particularly those in the far access network. The IETF has already
traveled that path with the Intserv architecture and found it
constrains scalability [RFC2208].
Anyway, if bottleneck policers were ever widely deployed, they would
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be likely to be bypassed by determined attackers. They inherently
have to police fairness per flow or per source-destination pair.
Therefore they can easily be circumvented either by opening multiple
flows (by varying the end-point port number); or by spoofing the
source address but arranging with the receiver to hide the true
return address at a higher layer.
4. Re-ECN Incentive Framework
The aim is to create an incentive environment that ensures optimal
sharing of capacity despite everyone acting selfishly (including
lying and cheating). Of course, the mechanisms put in place for this
can lie dormant wherever co-operation is the norm.
4.1. Revealing Congestion Along the Path
Throughout this document we focus on path congestion. But some forms
of fairness, particularly TCP's, also depend on round trip time. If
TCP-fairness is required, we also propose to measure downstream path
delay using re-feedback. We give a simple outline of how this could
work in Appendix D. However, we do not expect this to be necessary,
as researchers tend to agree that only congestion control dynamics
need to depend on RTT, not the rate that the algorithm would converge
on after a period of stability.
Recall that re-ECN can be used to measure path congestion at any
point on the path. End-systems know the whole path congestion. The
receiver knows this by the ratio of negative packets to all other
packets it observes. The sender knows this same information via the
feedback.
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+---+ +----+ +----+ +---+
| S |--| Q1 |----------------| Q2 |--| R |
+---+ +----+ +----+ +---+
. . . .
^ . . . .
| . . . .
| . positive fraction . .
3% |-------------------------------+=======
| . . | .
2% | . . | .
| . . negative fraction | .
1% | . +----------------------+ .
| . | . .
0% +--------------------------------------->
^ ^ ^
L M N Observation points
Figure 1: A 2-Queue Example (Imprecise)
Figure 1 uses a simple network to illustrate how re-ECN allows queues
to measure downstream congestion. The receiver counts negative
packets as being 3% of all received packets. This fraction is fed
back to the sender. The sender sets 3% of its packets to be positive
to match this. This fraction of positive packets can be observed
along the path. This is shown by the horizontal line at 3% in the
figure. The negative fraction is shown by the stepped line which
rises to meet the positive fraction line with steps at at each queue
where packets are marked negative. Two queues are shown (Q1 and Q2)
that are currently congested. Each time packets pass through a
fraction are marked red; 1% at Q1 and 2% at Q2). The approximate
downstream congestion can be measured at the observation points shown
along the path by subtracting the negative fraction from the positive
fraction, as shown in the table below. [Re-TCP] [ref other document]
derives these approximations from a precise analysis).
+-------------------+------------------------------+
| Observation point | Approx downstream congestion |
+-------------------+------------------------------+
| L | 3% - 0% = 3% |
| M | 3% - 1% = 2% |
| N | 3% - 3% = 0% |
+-------------------+------------------------------+
Table 1: Downstream Congestion Measured at Example Observation Points
All along the path, whole-path congestion remains unchanged so it can
be used as a reference against which to compare upstream congestion.
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The difference predicts downstream congestion for the rest of the
path. Therefore, measuring the fractions of negative and positive
packets at any point in the Internet will reveal upstream, downstream
and whole path congestion.
Note: to be absolutely clear these fractions are averages that would
result from the behaviour of the protocol handler mechanically
sending positive packets in direct response to incoming feedback---we
are not saying any protocol handler has to work with these average
fractions directly.
4.1.1. Positive and Negative Flows
In section Section 1.2 we introduced the notion of IP packets having
different values (negative, positive, cautious, cancelled and
neutral). So positive and cautious packets have a value of +1,
negative -1, and cancelled and neutral have zero value.
In the rest of this document we will loosely talk of positive or
negative flows. A negative flow is one where more negative bytes
than positive bytes arrive at the reciever. Likewise positive flows
are where more positive bytes arrive than negative bytes. Both of
these indicate that the wrong amount of positive bytes have been
sent.
4.2. Incentive Framework Overview
Figure 2 sketches the incentive framework that we will describe piece
by piece throughout this section. We will do a first pass in
overview, then return to each piece in detail. We re-use the earlier
example of how downstream congestion is derived by subtracting
upstream congestion from path congestion (Figure 1) but depict
multiple trust boundaries to turn it into an internetwork. For
clarity, only downstream congestion is shown (the difference between
the two earlier plots). The graph displays downstream path
congestion seen in a typical flow as it traverses an example path
from sender S to receiver R, across networks N1, N2 & N3. Everyone
is shown using re-ECN correctly, but we intend to show why everyone
would /choose/ to use it correctly, and honestly.
Three main types of self-interest can be identified:
o Users want to transmit data across the network as fast as
possible, paying as little as possible for the privilege. In this
respect, there is no distinction between senders and receivers,
but we must be wary of potential malice by one on the other;
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o Network operators want to maximise revenues from the resources
they invest in. They compete amongst themselves for the custom of
users.
o Attackers (whether users or networks) want to use any opportunity
to subvert the new re-ECN system for their own gain or to damage
the service of their victims, whether targeted or random.
policer dropper
| |
| |
S <-----N1----> <---N2---> <---N3--> R domain
| |
| |
Border Gateways
Figure 2: Incentive Framework
Source congestion control: We want to ensure that the sender will
throttle its rate as downstream congestion increases. Whatever
the agreed congestion response (whether TCP-compatible or some
enhanced QoS), to some extent it will always be against the
sender's interest to comply.
Ingress policing: But it is in all the network operators' interests
to encourage fair congestion response, so that their investments
are employed to satisfy the most valuable demand. The re-ECN
protocol ensures packets carry the necessary information about
their own expected downstream congestion so that N1 can deploy a
policer at its ingress to check that S1 is complying with whatever
congestion control it should be using (Section 4.4). If N1 is
extremely conservative it could police each flow, but it is likely
to just police the bulk amount of congestion each customer causes
without regard to flows, or if it is extremely liberal it need not
police congestion control at all. Whatever, it is always
preferable to police traffic at the very first ingress into an
internetwork, before non-compliant traffic can cause any damage.
Edge egress dropper: If the policer ensures the source has less
right to a high rate the higher it declares downstream congestion,
the source has a clear incentive to understate downstream
congestion. But, if flows of packets are understated when they
enter the internetwork, they will have become negative by the time
they leave. So, we introduce a dropper at the last network
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egress, which drops packets in flows that persistently declare
negative downstream congestion (see Section 4.3 for details).
Inter-domain traffic policing: But next we must ask, if congestion
arises downstream (say in N3), what is the ingress network's
(N1's) incentive to police its customers' response? If N1 turns a
blind eye, its own customers benefit while other networks suffer.
This is why all inter-domain QoS architectures (e.g. Intserv,
Diffserv) police traffic each time it crosses a trust boundary.
We have already shown that re-ECN gives a trustworthy measure of
the expected downstream congestion that a flow will cause by
subtracting negative volume from positive at any intermediate
point on a path. N3 (say) can use this measure to police all the
responses to congestion of all the sources beyond its upstream
neighbour (N2), but in bulk with one very simple passive
mechanism, rather than per flow, as we will now explain.
Emulating policing with inter-domain congestion penalties: Between
high-speed networks, we would rather avoid per-flow policing, and
we would rather avoid holding back traffic while it is policed.
Instead, once re-ECN has arranged headers to carry downstream
congestion honestly, N2 can contract to pay N3 penalties in
proportion to a single bulk count of the congestion metrics
crossing their mutual trust boundary (Section 4.5). In this way,
N3 puts pressure on N2 to suppress downstream congestion, for
every flow passing through the border interface, even though they
will all start and end in different places, and even though they
may all be allowed different responses to congestion. The figure
depicts this downward pressure on N2 by the solid downward arrow
at the egress of N2. Then N2 has an incentive either to police
the congestion response of its own ingress traffic (from N1) or to
emulate policing by applying penalties to N1 in turn on the basis
of congestion counted at their mutual boundary. In this recursive
way, the incentives for each flow to respond correctly to
congestion trace back with each flow precisely to each source,
despite the mechanism not recognising flows (see Section 5.2).
Inter-domain congestion charging diversity: Any two networks are
free to agree any of a range of penalty regimes between themselves
but they would only provide the right incentives if they were
within the following reasonable constraints. N2 should expect to
have to pay penalties to N3 where penalties monotonically increase
with the volume of congestion and negative penalties are not
allowed. For instance, they may agree an SLA with tiered
congestion thresholds, where higher penalties apply the higher the
threshold that is broken. But the most obvious (and useful) form
of penalty is where N3 levies a charge on N2 proportional to the
volume of downstream congestion N2 dumps into N3. In the
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explanation that follows, we assume this specific variant of
volume charging between networks - charging proportionate to the
volume of congestion.
We must make clear that we are not advocating that everyone should
use this form of contract. We are well aware that the IETF tries
to avoid standardising technology that depends on a particular
business model. And we strongly share this desire to encourage
diversity. But our aim is merely to show that border policing can
at least work with this one model, then we can assume that
operators might experiment with the metric in other models (see
Section 4.5 for examples). Of course, operators are free to
complement this usage element of their charges with traditional
capacity charging, and we expect they will as predicted by
economics.
No congestion charging to users: Bulk congestion penalties at trust
boundaries are passive and extremely simple, and lose none of
their per-packet precision from one boundary to the next (unlike
Diffserv all-address traffic conditioning agreements, which
dissipate their effectiveness across long topologies). But at any
trust boundary, there is no imperative to use congestion charging.
Traditional traffic policing can be used, if the complexity and
cost is preferred. In particular, at the boundary with end
customers (e.g. between S and N1), traffic policing will most
likely be more appropriate. Policer complexity is less of a
concern at the edge of the network. And end-customers are known
to be highly averse to the unpredictability of congestion
charging.
NOTE WELL: This document neither advocates nor requires congestion
charging for end customers and advocates but does not require
inter-domain congestion charging.
Competitive discipline of inter-domain traffic engineering: With
inter-domain congestion charging, a domain seems to have a
perverse incentive to fake congestion; N2's profit depends on the
difference between congestion at its ingress (its revenue) and at
its egress (its cost). So, overstating internal congestion seems
to increase profit. However, smart border routing [Smart_rtg] by
N1 will bias its routing towards the least cost routes. So, N2
risks losing all its revenue to competitive routes if it
overstates congestion (see Section 5.3). In other words, if N2 is
the least congested route, its ability to raise excess profits is
limited by the congestion on the next least congested route.
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Closing the loop: All the above elements conspire to trap everyone
between two opposing pressures, ensuring the downstream congestion
metric arrives at the destination neither above nor below zero.
So, we have arrived back where we started in our argument. The
ingress edge network can rely on downstream congestion declared in
the packet headers presented by the sender. So it can police the
sender's congestion response accordingly.
Evolvability of congestion control: We have seen that re-ECN enables
policing at the very first ingress. We have also seen that, as
flows continue on their path through further networks downstream,
re-ECN removes the need for further per-domain ingress policing of
all the different congestion responses allowed to each different
flow. This is why the evolvability of re-ECN policing is so
superior to bottleneck policing or to any policing of different
QoS for different flows. Even if all access networks choose to
conservatively police congestion per flow, each will want to
compete with the others to allow new responses to congestion for
new types of application. With re-ECN, each can introduce new
controls independently, without coordinating with other networks
and without having to standardise anything. But, as we have just
seen, by making inter-domain penalties proportionate to bulk
downtream congestion, downstream networks can be agnostic to the
specific congestion response for each flow, but they can still
apply more penalty the more liberal the ingress access network has
been in the response to congestion it allowed for each flow.
We now take a second pass over the incentive framework, filling in
the detail.
4.3. Egress Dropper
As traffic leaves the last network before the receiver (domain N3 in
Figure 2), the fraction of positive octets in a flow should match the
fraction of negative octets introduced by congestion marking (red
packets), leaving a balance of zero. If it is less (a negative
flow), it implies that the source is understating path congestion
(which will reduce the penalties that N2 owes N3).
If flows are positive, N3 need take no action---this simply means its
upstream neighbour is paying more penalties than it needs to, and the
source is going slower than it needs to. But, to protect itself
against persistently negative flows, N3 will need to install a
dropper at its egress. Appendix A gives a suggested algorithm for
this dropper. There is no intention that the dropper algorithm needs
to be standardised, it is merely provided to show that an efficient,
robust algorithm is possible. But whatever algorithm is used must
meet the criteria below:
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o It SHOULD introduce minimal false positives for honest flows;
o It SHOULD quickly detect and sanction dishonest flows (minimal
false negatives);
o It SHOULD be invulnerable to state exhaustion attacks from
malicious sources. For instance, if the dropper uses flow-state,
it should not be possible for a source to send numerous packets,
each with a different flow ID, to force the dropper to exhaust its
memory capacity (rationale for SHOULD: Continuously sending keep-
alive packets might be perfectly reasonable behaviour, so we can't
distinguish a deliberate attack from reasonable levels of such
behaviour. Therefore it is strictly impossible to be invulnerable
to such an attack);
o It MUST introduce sufficient loss in goodput so that malicious
sources cannot play off losses in the egress dropper against
higher allowed throughput. Salvatori [CLoop_pol] describes this
attack, which involves the source understating path congestion
then inserting forward error correction (FEC) packets to
compensate expected losses;
o It MUST NOT be vulnerable to `identity whitewashing', where a
transport can label a flow with a new ID more cheaply than paying
the cost of continuing to use its current ID.
Note that the dropper operates on flows but we would like it not to
require per-flow state. This is why we have been careful to ensure
that all flows MUST start with a cautious packet. If a flow does not
start with a cautious packet, a dropper is likely to treat it
unfavourably. This risk makes it worth sending a cautious packet at
the start of a flow, even though there is a cost to the sender of
doing so (positive `worth'). Indeed, with cautious packets, the rate
at which a sender can generate new flows can be limited (Appendix B).
In this respect, cautious packets work like Handley's state set-up
bit [Steps_DoS].
Appendix A also gives an example dropper implementation that
aggregates flow state. Dropper algorithms will often maintain a
moving average across flows of the fraction of positive packets.
When maintaining an average across flows, a dropper SHOULD only allow
flows into the average if they start with a cautious packet, but it
SHOULD NOT include cautious packets in the positive packet average.
A sender sends cautious packets when it does not have the benefit of
feedback from the receiver. So, counting cautious packets would be
likely to make the average unnecessarily positive, providing headroom
(or should we say footroom?) for dishonest (negative) traffic.
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If the dropper detects a persistently negative flow, it SHOULD drop
sufficient negative and neutral packets to force the flow to not be
negative. Drops SHOULD be focused on just sufficient packets in
misbehaving flows to remove the negative bias while doing minimal
extra harm.
4.4. Ingress Policing
Access operators who wish to limit the congeston that a sender is
able to cause can deploy policers at the very first ingress to the
internetwork. Re-ECN has been designed to avoid the need for
bottleneck policing so that we can avoid a future where a single rate
adaptation policy is embedded throughout the network. Instead, re-
ECN allows the particular rate adaptation policy to be solely agreed
bilaterally between the sender and its ingress access provider ([ref
other document] discusses possible ways to signal between them),
which allows congestion control to be policed, but maintains its
evolvability, requiring only a single, local box to be updated.
Appendix B gives examples of per-user policing algorithms. But there
is no implication that these algorithms are to be standardised, or
that they are ideal. The ingress rate policer is the part of the re-
ECN incentive framework that is intended to be the most flexible.
Once endpoint protocol handlers for re-ECN and egress droppers are in
place, operators can choose exactly which congestion response they
want to police, and whether they want to do it per user, per flow or
not at all.
The re-ECN protocol allows these ingress policers to easily perform
bulk per-user policing (Appendix B.1). This is likely to provide
sufficient incentive to the user to correctly respond to congestion
without needing the policing function to be overly complex. If an
access operator chose they could use per-flow policing according to
the widely adopted TCP rate adaptation ( Appendix B.2) or other
alternatives, however this would introduce extra complexity to the
system.
If a per-flow rate policer is used, it should use path (not
downstream) congestion as the relevant metric, which is represented
by the fraction of octets in packets with positive (positive and
cautious packets) and cancelled packets. Of course, re-ECN provides
all the information a policer needs directly in the packets being
policed. So, even policing TCP's AIMD algorithm is relatively
straightforward (Appendix B.2).
Note that we have included cancelled packets in the measure of path
congestion. cancelled packets arise when the sender sends a positive
packet in response to feedback, but then this positive packet just
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happens to be congestion marked itself. One would not normally
expect many cancelled packets at the first ingress because one would
not normally expect much congestion marking to have been necessary
that soon in the path. However, a home network or campus network may
well sit between the sending endpoint and the ingress policer, so
some congestion may occur upstream of the policer. And if congestion
does occur upstream, some cancelled packets should be visible, and
should be taken into account in the measure of path congestion.
But a much more important reason for including cancelled packets in
the measure of path congestion at an ingress policer is that a sender
might otherwise subvert the protocol by sending cancelled packets
instead of neutral packets. Like neutral, cancelled packets are
worth zero, so the sender knows they won't be counted against any
quota it might have been allowed. But unlike neutral packets,
cancelled packets are immune to congestion marking, because they have
already been congestion marked. So, it is both correct and useful
that cancelled packets should be included in a policer's measure of
path congestion, as this removes the incentive the sender would
otherwise have to mark more packets as cancelled than it should.
An ingress policer should also ensure that flows are not already
negative when they enter the access network. As with cancelled
packets, the presence of negative packets will typically be unusual.
Therefore it will be easy to detect negative flows at the ingress by
just detecting negative packets then monitoring the flow they belong
to.
Of course, even if the sender does operate its own network, it may
arrange not to congestion mark traffic. Whether the sender does this
or not is of no concern to anyone else except the sender. Such a
sender will not be policed against its own network's contribution to
congestion, but the only resulting problem would be overload in the
sender's own network.
Finally, we must not forget that an easy way to circumvent re-ECN's
defences is for the source to turn off re-ECN support, by setting the
Not-RECT codepoint, implying RFC3168 compliant traffic. Therefore an
ingress policer should put a general rate-limit on Not-RECT traffic,
which SHOULD be lax during early, patchy deployment, but will have to
become stricter as deployment widens. Similarly, flows starting
without a cautious packet can be confined by a strict rate-limit used
for the remainder of flows that haven't proved they are well-behaved
by starting correctly (therefore they need not consume any flow
state---they are just confined to the `misbehaving' bin if they carry
an unrecognised flow ID).
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4.5. Inter-domain Policing
One of the main design goals of re-ECN is for border security
mechanisms to be as simple as possible, otherwise they will become
the pinch-points that limit scalability of the whole internetwork.
We want to avoid per-flow processing at borders and to keep to
passive mechanisms that can monitor traffic in parallel to
forwarding, rather than having to filter traffic inline---in series
with forwarding. Such passive, off-line mechanisms are essential for
future high-speed all-optical border interconnection where packets
cannot be buffered while they are checked for policy compliance.
So far, we have been able to keep the border mechanisms simple,
despite having had to harden them against some subtle attacks on the
re-ECN design. The mechanisms are still passive and avoid per-flow
processing.
The basic accounting mechanism at each border interface simply
involves accumulating the volume of packets with positive worth
(positive and cautious packets), and subtracting the volume of those
with negative worth (red packets). Even though this mechanism takes
no regard of flows, over an accounting period (say a month) this
subtraction will account for the downstream congestion caused by all
the flows traversing the interface, wherever they come from, and
wherever they go to. The two networks can agree to use this metric
however they wish to determine some congestion-related penalty
against the upstream network. Although the algorithm could hardly be
simpler, it is spelled out using pseudo-code in Appendix C.1.
Various attempts to subvert the re-ECN design have been made. In all
cases their root cause is persistently negative flows. But, after
describing these attacks we will show that we don't actually have to
get rid of all persistently negative flows in order to thwart the
attacks.
In honest flows, downstream congestion is measured as positive minus
negative volume. So if all flows are honest (i.e. not persistently
negative), adding all positive volume and all negative volume without
regard to flows will give an aggregate measure of downstream
congestion. But such simple aggregation is only possible if no flows
are persistently negative. Unless persistently negative flows are
completely removed, they will reduce the aggregate measure of
congestion. The aggregate may still be positive overall, but not as
positive as it would have been had the negative flows been removed.
In Section 4.3 we discussed how to sanction traffic to remove, or at
least to identify, persistently negative flows. But, even if the
sanction for negative traffic is to discard it, unless it is
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discarded at the exact point it goes negative, it will wrongly
subtract from aggregate downstream congestion, at least at any
borders it crosses after it has gone negative but before it is
discarded.
We rely on sanctions to deter dishonest understatement of congestion.
But even the ultimate sanction of discard can only be effective if
the sender is bothered about the data getting through to its
destination. A number of attacks have been identified where a sender
gains from sending dummy traffic or it can attack someone or
something using dummy traffic even though it isn't communicating any
information to anyone:
o A host can send traffic with no positive packets towards its
intended destination, aiming to transmit as much traffic as any
dropper will allow [Bauer06]. It may add forward error correction
(FEC) to repair as much drop as it experiences.
o A host can send dummy traffic into the network with no positive
packets and with no intention of communicating with anyone, but
merely to cause higher levels of congestion for others who do want
to communicate (DoS). So, to ride over the extra congestion,
everyone else has to spend more of whatever rights to cause
congestion they have been allowed.
o A network can simply create its own dummy traffic to congest
another network, perhaps causing it to lose business at no cost to
the attacking network. This is a form of denial of service
perpetrated by one network on another. The preferential drop
measures in [ref other document] provide crude protection against
such attacks, but we are not overly worried about more accurate
prevention measures, because it is already possible for networks
to DoS other networks on the general Internet, but they generally
don't because of the grave consequences of being found out. We
are only concerned if re-ECN increases the motivation for such an
attack, as in the next example.
o A network can just generate negative traffic and send it over its
border with a neighbour to reduce the overall penalties that it
should pay to that neighbour. It could even initialise the TTL so
it expired shortly after entering the neighbouring network,
reducing the chance of detection further downstream. This attack
need not be motivated by a desire to deny service and indeed need
not cause denial of service. A network's main motivator would
most likely be to reduce the penalties it pays to a neighbour.
But, the prospect of financial gain might tempt the network into
mounting a DoS attack on the other network as well, given the gain
would offset some of the risk of being detected.
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The first step towards a solution to all these problems with negative
flows is to be able to estimate the contribution they make to
downstream congestion at a border and to correct the measure
accordingly. Although ideally we want to remove negative flows
themselves, perhaps surprisingly, the most effective first step is to
cancel out the polluting effect negative flows have on the measure of
downstream congestion at a border. It is more important to get an
unbiased estimate of their effect, than to try to remove them all. A
suggested algorithm to give an unbiased estimate of the contribution
from negative flows to the downstream congestion measure is given in
Appendix C.2.
Although making an accurate assessment of the contribution from
negative flows may not be easy, just the single step of neutralising
their polluting effect on congestion metrics removes all the gains
networks could otherwise make from mounting dummy traffic attacks on
each other. This puts all networks on the same side (only with
respect to negative flows of course), rather than being pitched
against each other. The network where this flow goes negative as
well as all the networks downstream lose out from not being
reimbursed for any congestion this flow causes. So they all have an
interest in getting rid of these negative flows. Networks forwarding
a flow before it goes negative aren't strictly on the same side, but
they are disinterested bystanders---they don't care that the flow
goes negative downstream, but at least they can't actively gain from
making it go negative. The problem becomes localised so that once a
flow goes negative, all the networks from where it happens and beyond
downstream each have a small problem, each can detect it has a
problem and each can get rid of the problem if it chooses to. But
negative flows can no longer be used for any new attacks.
Once an unbiased estimate of the effect of negative flows can be
made, the problem reduces to detecting and preferably removing flows
that have gone negative as soon as possible. But importantly,
complete eradication of negative flows is no longer critical---best
endeavours will be sufficient.
For instance, let us consider the case where a source sends traffic
with no positive packets at all, hoping to at least get as much
traffic delivered as network-based droppers will allow. The flow is
likely to go at least slightly negative in the first network on the
path (N1 if we use the example network layout in Figure 2). If all
networks use the algorithm in Appendix C.2 to inflate penalties at
their border with an upstream network, they will remove the effect of
negative flows. So, for instance, N2 will not be paying a penalty to
N1 for this flow. Further, because the flow contributes no positive
packets at all, a dropper at the egress will completely remove it.
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The remaining problem is that every network is carrying a flow that
is causing congestion to others but not being held to account for the
congestion it is causing. Whenever the fail-safe border algorithm
(Section 4.6) or the border algorithm to compensate for negative
flows (Appendix C.2) detects a negative flow, it can instantiate a
focused dropper for that flow locally. It may be some time before
the flow is detected, but the more strongly negative the flow is, the
more quickly it will be detected by the fail-safe algorithm. But, in
the meantime, it will not be distorting border incentives. Until it
is detected, if it contributes to drop anywhere, its packets will
tend to be dropped before others if queues use the preferential drop
rules in [ref other document], which discriminate against non-
positive packets. All networks below the point where a flow goes
negative (N1, N2 and N3 in this case) have an incentive to remove
this flow, but the queue where it first goes negative (in N1) can of
course remove the problem for everyone downstream.
In the case of DDoS attacks, Section 5.1 describes how re-ECN
mitigates their force.
4.6. Inter-domain Fail-safes
The mechanisms described so far create incentives for rational
network operators to behave. That is, one operator aims to make
another behave responsibly by applying penalties and expects a
rational response (i.e. one that trades off costs against benefits).
It is usually reasonable to assume that other network operators will
behave rationally (policy routing can avoid those that might not).
But this approach does not protect against the misconfigurations and
accidents of other operators.
Therefore, we propose the following two mechanisms at a network's
borders to provide "defence in depth". Both are similar:
Highly positive flows: A small sample of positive packets should be
picked randomly as they cross a border interface. Then subsequent
packets matching the same source and destination address and DSCP
should be monitored. If the fraction of positive packets is well
above a threshold (to be determined by operational practice), a
management alarm SHOULD be raised, and the flow MAY be
automatically subject to focused drop.
Persistently negative flows: A small sample of congestion marked
(red) packets should be picked randomly as they cross a border
interface. Then subsequent packets matching the same source and
destination address and DSCP should be monitored. If the balance
of positive packets minus negative packets (measured in bytes) is
persistently negative, a management alarm SHOULD be raised, and
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the flow MAY be automatically subject to focused drop.
Both these mechanisms rely on the fact that highly positive (or
negative) flows will appear more quickly in the sample by selecting
randomly solely from positive (or negative) packets.
4.7. The Case against Classic Feedback
A system that produces an optimal outcome as a result of everyone's
selfish actions is extremely powerful. Especially one that enables
evolvability of congestion control. But why do we have to change to
re-ECN to achieve it? Can't classic congestion feedback (as used
already by standard ECN) be arranged to provide similar incentives
and similar evolvability? Superficially it can. Kelly's seminal
work showed how we can allow everyone the freedom to evolve whatever
congestion control behaviour is in their application's best interest
but still optimise the whole system of networks and users by placing
a price on congestion to ensure responsible use of this
freedom [Evol_cc]). Kelly used ECN with its classic congestion
feedback model as the mechanism to convey congestion price
information. The mechanism could be thought of as volume charging;
except only the volume of packets marked with congestion experienced
(CE) was counted.
However, below we explain why relying on classic feedback /required/
congestion charging to be used, while re-ECN achieves the same
powerful outcome (given it is built on Kelly's foundations), but does
not /require/ congestion charging. In brief, the problem with
classic feedback is that the incentives have to trace the indirect
path back to the sender---the long way round the feedback loop. For
example, if classic feedback were used in Figure 2, N2 would have had
to influence N1 via all of N3, R & S rather than directly.
Inability to agree what is happening downstream: In order to police
its upstream neighbour's congestion response, the neighbours
should be able to agree on the congestion to be responded to.
Whatever the feedback regime, as packets change hands at each
trust boundary, any path metrics they carry are verifiable by both
neighbours. But, with a classic path metric, they can only agree
on the /upstream/ path congestion.
Inaccessible back-channel: The network needs a whole-path congestion
metric if it wants to control the source. Classically, whole path
congestion emerges at the destination, to be fed back from
receiver to sender in a back-channel. But, in any data network,
back-channels need not be visible to relays, as they are
essentially communications between the end-points. They may be
encrypted, asymmetrically routed or simply omitted, so no network
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element can reliably intercept them. The congestion charging
literature solves this problem by charging the receiver and
assuming this will cause the receiver to refer the charges to the
sender. But, of course, this creates unintended side-effects...
`Receiver pays' unacceptable: In connectionless datagram networks,
receivers and receiving networks cannot prevent reception from
malicious senders, so `receiver pays' opens them to `denial of
funds' attacks.
End-user congestion charging unacceptable in many societies: Even if
'denial of funds' were not a problem, we know that end-users are
highly averse to the unpredictability of congestion charging and
anyway, we want to avoid restricting network operators to just one
retail tariff. But with classic feedback only an upstream metric
is available, so we cannot avoid having to wrap the `receiver
pays' money flow around the feedback loop, necessarily forcing
end-users to be subjected to congestion charging.
To summarise so far, with classic feedback, policing congestion
response without losing evolvability /requires/ congestion charging
of end-users and a `receiver pays' model, whereas, with re-ECN, it is
still possible to influence incentives using congestion charging but
using the safer `sender pays' model. However, congestion charging is
only likely to be appropriate between domains. So, without losing
evolvability, re-ECN enables technical policing mechanisms that are
more appropriate for end users than congestion pricing.
4.8. Simulations
Simulations of policer and dropper performance done for the multi-bit
version of re-feedback have been included in section 5 "Dropper
Performance" of [Re-fb]. Simulations of policer and dropper for the
re-ECN version described in this document are work in progress.
5. Other Applications of Re-ECN
5.1. DDoS Mitigation
A flooding attack is inherently about congestion of a resource.
Because re-ECN ensures the sources causing network congestion
experience the cost of their own actions, it acts as a first line of
defence against DDoS. As load focuses on a victim, upstream queues
grow, requiring honest sources to pre-load packets with a higher
fraction of positive packets. Once downstream queues are so
congested that they are dropping traffic, they will be marking to
negative the traffic they do forward 100%. Honest sources will
therefore be sending positive packets 100% (and therefore being
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severely rate-limited at the ingress).
Senders under malicious control can either do the same as honest
sources, and be rate-limited at ingress, or they can understate
congestion by sending more neutral RECT packets than they should. If
sources understate congestion (i.e. do not re-echo sufficient
positive packets) and the preferential drop ranking is implemented on
queues ([ref othe document]), these queues will preserve positive
traffic until last. So, the neutral traffic from malicious sources
will all be automatically dropped first. Either way, the malicious
sources cannot send more than honest sources.
Further, hosts under malicious control will tend to be re-used for
many different attacks. They will therefore build up a long term
history of causing congestion. Therefore, as long as the population
of potentially compromisable hosts around the Internet is limited,
the per-user policing algorithms in Appendix B.1 will gradually
throttle down zombies and other launchpads for attacks. Therefore,
widespread deployment of re-ECN could considerably dampen the force
of DDoS. Certainly, zombie armies could hold their fire for long
enough to be able to build up enough credit in the per-user policers
to launch an attack. But they would then still be limited to no more
throughput than other, honest users.
Inter-domain traffic policing (see Section 4.5)ensures that any
network that harbours compromised `zombie' hosts will have to bear
the cost of the congestion caused by traffic from zombies in
downstream networks. Such networks will be incentivised to deploy
per-user policers that rate-limit hosts that are unresponsive to
congestion so they can only send very slowly into congested paths.
As well as protecting other networks, the extremely poor performance
at any sign of congestion will incentivise the zombie's owner to
clean it up. However, the host should behave normally when using
uncongested paths.
Uniquely, re-ECN handles DDoS traffic without relying on the validity
of identifiers in packets. Certainly the egress dropper relies on
uniqueness of flow identifiers, but not their validity. So if a
source spoofs another address, re-ECN works just as well, as long as
the attacker cannot imitate all the flow identifiers of another
active flow passing through the same dropper (see Section 6).
Similarly, the ingress policer relies on uniqueness of flow IDs, not
their validity. Because a new flow will only be allowed any rate at
all if it starts with a cautious packet, and the more cautious
packets there are starting new flows, the more they will be limited.
Essentially a re-ECN policer limits the bulk of all congestion
entering the network through a physical interface; limiting the
congestion caused by each flow is merely an optional extra.
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5.2. End-to-end QoS
{ToDo: (Section 3.3.2 of [Re-fb] entitled `Edge QoS' gives an outline
of the text that will be added here).}
5.3. Traffic Engineering
Classic feedback makes congestion-based traffic engineering
inefficient too. Network N3 can see which of its two alternative
upstream networks N2 and N3 are less congested. But it is N1 that
makes the routing decision. This is why current traffic engineering
requires a continuous message stream from congestion monitors to the
routing controller. And even then the monitors can only be trusted
for /intra-/domain traffic engineering. The trustworthiness of re-
ECN enables /inter-/domain traffic engineering without messaging
overhead. {ToDo: Elaborate}
5.4. Inter-Provider Service Monitoring
{ToDo: }
6. Limitations
{ToDo:See also: slide of limitations}
The known limitations of the re-ECN approach are:
o We still cannot defend against the attack described in Section 10
where a malicious source sends negative traffic through the same
egress dropper as another flow and imitates its flow identifiers,
allowing a malicious source to cause an innocent flow to
experience heavy drop.
o Re-feedback for TTL (re-TTL) would also be desirable at the same
time as re-ECN. Unfortunately this requires a further standards
action for the mechanisms briefly described in Appendix D
o Traffic must be ECN-capable for re-ECN to be effective. The only
defence against malicious users who turn off ECN capbility is that
networks are expected to rate limit Not-ECT traffic and to apply
higher drop preference to it during congestion. Although these
are blunt instruments, they at least represent a feasible scenario
for the future Internet where Not-ECT traffic co-exists with re-
ECN traffic, but as a severely hobbled under-class. We recommend
(Section 7.1) that while accommodating a smooth initial transition
to re-ECN, policing policies should gradually be tightened to rate
limit Not-ECT traffic more strictly in the longer term.
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o When checking whether a flow is balancing positive packets with
negative packets (measured in bytes), re-ECN can only account for
congestion marking, not drops. So, whenever a sender experiences
drop, it does not have to re-echo the congestion event by sending
positive packet(s). Nonetheless, it is hardly any advantage to be
able to send faster than other flows only if your traffic is
dropped and the other traffic isn't.
o We are considering the issue of whether it would be useful to
truncate rather than drop packets that appear to be malicious, so
that the feedback loop is not broken but useful data can be
removed.
{ToDo: Monopolies over Routes}
7. Incremental Deployment
7.1. Incremental Deployment Features
The design of the re-ECN protocol started from the fact that the
current ECN marking behaviour of queues was sufficient and that re-
feedback could be introduced around these queues by changing the
sender behaviour but not the routers. Otherwise, if we had required
routers to be changed, the chance of encountering a path that had
every router upgraded would be vanishly small during early
deployment, giving no incentive to start deployment. Also, as there
is no new forwarding behaviour, routers and hosts do not have to
signal or negotiate anything.
However, networks that choose to protect themselves using re-ECN do
have to add new security functions at their trust boundaries with
others. They distinguish legacy traffic by its ECN field. Traffic
from Not-ECT transports is distinguishable by its Not-ECT marking .
Traffic from RFC3168 compliant ECN transports is distinguished from
re-ECN by which of ECT(0) or ECT(1) is used. We chose to use ECT(1)
for re-ECN traffic deliberately. Existing ECN sources set ECT(0) on
either 50% (the nonce) or 100% (the default) of packets, whereas re-
ECN does not use ECT(0) at all. We can use this distinguishing
feature of RFC3168 compliant ECN traffic to separate it out for
different treatment at the various border security functions: egress
dropping, ingress policing and border policing.
The general principle we adopt is that an egress dropper will not
drop any legacy traffic, but ingress and border policers will limit
the bulk rate of legacy traffic (Not-ECT, ECT(0) and those amrked
with the unused codepoint as defined in [Re-TCP]) that can enter each
network. Then, during early re-ECN deployment, operators can set
very permissive (or non-existent) rate-limits on legacy traffic, but
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once re-ECN implementations are generally available, legacy traffic
can be rate-limited increasingly harshly. Ultimately, an operator
might choose to block all legacy traffic entering its network, or at
least only allow through a trickle.
Then, as the limits are set more strictly, the more RFC3168 ECN
sources will gain by upgrading to re-ECN. Thus, towards the end of
the voluntary incremental deployment period, RFC3168 compliant
transports can be given progressively stronger encouragement to
upgrade.
7.2. Incremental Deployment Incentives
It would only be worth standardising the re-ECN protocol if there
existed a coherent story for how it might be incrementally deployed.
In order for it to have a chance of deployment, everyone who needs to
act must have a strong incentive to act, and the incentives must
arise in the order that deployment would have to happen. Re-ECN
works around unmodified ECN routers, but we can't just discuss why
and how re-ECN deployment might build on ECN deployment, because
there is precious little to build on in the first place. Instead, we
aim to show that re-ECN deployment could carry ECN with it. We focus
on commercial deployment incentives, although some of the arguments
apply equally to academic or government sectors.
ECN deployment:
ECN is largely implemented in commercial routers, but generally
not as a supported feature, and it has largely not been deployed
by commercial network operators. ECN has been implemented in most
Unix-based operating systems for some time. Microsoft first
implemented ECN in Windows Vista, but it is only on by default for
the server end of a TCP connection. Unfortunately the client end
had to be turned off by default, because a non-zero ECN field
triggers a bug in a legacy home gateway which makes it crash. For
detailed deployment status, see [ECN-Deploy]. We believe the
reason ECN deployment has not happened is twofold:
* ECN requires changes to both routers and hosts. If someone
wanted to sell the improvement that ECN offers, they would have
to co-ordinate deployment of their product with others. An ECN
server only gives any improvement on an ECN network. An ECN
network only gives any improvement if used by ECN devices.
Deployment that requires co-ordination adds cost and delay and
tends to dilute any competitive advantage that might be gained.
* ECN `only' gives a performance improvement. Making a product a
bit faster (whether the product is a device or a network),
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isn't usually a sufficient selling point to be worth the cost
of co-ordinating across the industry to deploy it. Network
operators tend to avoid re-configuring a working network unless
launching a new product.
ECN and Re-ECN for Edge-to-edge Assured QoS:
We believe the proposal to provide assured QoS sessions using a
form of ECN called pre-congestion notification (PCN) [RFC5559] is
most likely to break the deadlock in ECN deployment first. It
only requires edge-to-edge deployment so it does not require
endpoint support. It can be deployed in a single network, then
grow incrementally to interconnected networks. And it provides a
different `product' (internetworked assured QoS), rather than
merely making an existing product a bit faster.
Not only could this assured QoS application kick-start ECN
deployment, it could also carry re-ECN deployment with it; because
re-ECN can enable the assured QoS region to expand to a large
internetwork where neighbouring networks do not trust each other.
[Re-PCN] argues that re-ECN security should be built in to the QoS
system from the start, explaining why and how.
If ECN and re-ECN were deployed edge-to-edge for assured QoS,
operators would gain valuable experience. They would also clear
away many technical obstacles such as firewall configurations that
block all but the RFC3168 settings of the ECN field and the RE
flag.
ECN in Access Networks:
The next obstacle to ECN deployment would be extension to access
and backhaul networks, where considerable link layer differences
makes implementation non-trivial, particularly on congested
wireless links. ECN and re-ECN work fine during partial
deployment, but they will not be very useful if the most congested
elements in networks are the last to support them. Access network
support is one of the weakest parts of this deployment story. All
we can hope is that, once the benefits of ECN are better
understood by operators, they will push for the necessary link
layer implementations as deployment proceeds.
Policing Unresponsive Flows:
Re-ECN allows a network to offer differentiated quality of service
as explained in Section 5.2. But we do not believe this will
motivate initial deployment of re-ECN, because the industry is
already set on alternative ways of doing QoS. Despite being much
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more complicated and expensive, the alternative approaches are
here and now.
But re-ECN is critical to QoS deployment in another respect. It
can be used to prevent applications from taking whatever bandwidth
they choose without asking.
Currently, applications that remain resolute in their lack of
response to congestion are rewarded by other TCP applications. In
other words, TCP is naively friendly, in that it reduces its rate
in response to congestion whether it is competing with friends
(other TCPs) or with enemies (unresponsive applications).
Therefore, those network owners that want to sell QoS will be keen
to ensure that their users can't help themselves to QoS for free.
Given the very large revenues at stake, we believe effective
policing of congestion response will become highly sought after by
network owners.
But this does not necessarily argue for re-ECN deployment.
Network owners might choose to deploy bottleneck policers rather
than re-ECN-based policing. However, under Related Work
(Section 9) we argue that bottleneck policers are inherently
vulnerable to circumvention.
Therefore we believe there will be a strong demand from network
owners for re-ECN deployment so they can police flows that do not
ask to be unresponsive to congestion, in order to protect their
revenues from flows that do ask (QoS). In particular, we suspect
that the operators of cellular networks will want to prevent VoIP
and video applications being used freely on their networks as a
more open market develops in GPRS and 3G devices.
Initial deployments are likely to be isolated to single cellular
networks. Cellular operators would first place requirements on
device manufacturers to include re-ECN in the standards for mobile
devices. In parallel, they would put out tenders for ingress and
egress policers. Then, after a while they would start to tighten
rate limits on Not-ECT traffic from non-standard devices and they
would start policing whatever non-accredited applications people
might install on mobile devices with re-ECN support in the
operating system. This would force even independent mobile device
manufacturers to provide re-ECN support. Early standardisation
across the cellular operators is likely, including interconnection
agreements with penalties for excess downstream congestion.
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We suspect some fixed broadband networks (whether cable or DSL)
would follow a similar path. However, we also believe that larger
parts of the fixed Internet would not choose to police on a per-
flow basis. Some might choose to police congestion on a per-user
basis in order to manage heavy peer-to-peer file-sharing, but it
seems likely that a sizeable majority would not deploy any form of
policing.
This hybrid situation begs the question, "How does re-ECN work for
networks that choose to using policing if they connect with others
that don't?" Traffic from non-ECN capable sources will arrive
from other networks and cause congestion within the policed, ECN-
capable networks. So networks that chose to police congestion
would rate-limit Not-ECT traffic throughout their network,
particularly at their borders. They would probably also set
higher usage prices in their interconnection contracts for
incoming Not-ECT and Not-RECT traffic. We assume that
interconnection contracts between networks in the same tier will
include congestion penalties before contracts with provider
backbones do.
A hybrid situation could remain for all time. As was explained in
the introduction, we believe in healthy competition between
policing and not policing, with no imperative to convert the whole
world to the religion of policing. Networks that chose not to
deploy egress droppers would leave themselves open to being
congested by senders in other networks. But that would be their
choice.
The important aspect of the egress dropper though is that it most
protects the network that deploys it. If a network does not
deploy an egress dropper, sources sending into it from other
networks will be able to understate the congestion they are
causing. Whereas, if a network deploys an egress dropper, it can
know how much congestion other networks are dumping into it, and
apply penalties or charges accordingly. So, whether or not a
network polices its own sources at ingress, it is in its interests
to deploy an egress dropper.
Host support:
In the above deployment scenario, host operating system support
for re-ECN came about through the cellular operators demanding it
in device standards (i.e. 3GPP). Of course, increasingly, mobile
devices are being built to support multiple wireless technologies.
So, if re-ECN were stipulated for cellular devices, it would
automatically appear in those devices connected to the wireless
fringes of fixed networks if they coupled cellular with WiFi or
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Bluetooth technology, for instance. Also, once implemented in the
operating system of one mobile device, it would tend to be found
in other devices using the same family of operating system.
Therefore, whether or not a fixed network deployed ECN, or
deployed re-ECN policers and droppers, many of its hosts might
well be using re-ECN over it. Indeed, they would be at an
advantage when communicating with hosts across re-ECN policed
networks that rate limited Not-RECT traffic.
Other possible scenarios:
The above is thankfully not the only plausible scenario we can
think of. One of the many clubs of operators that meet regularly
around the world might decide to act together to persuade a major
operating system manufacturer to implement re-ECN. And they may
agree between them on an interconnection model that includes
congestion penalties.
Re-ECN provides an interesting opportunity for device
manufacturers as well as network operators. Policers can be
configured loosely when first deployed. Then as re-ECN take-up
increases, they can be tightened up, so that a network with re-ECN
deployed can gradually squeeze down the service provided to
RFC3168 compliant devices that have not upgraded to re-ECN. Many
device vendors rely on replacement sales. And operating system
companies rely heavily on new release sales. Also support
services would like to be able to force stragglers to upgrade.
So, the ability to throttle service to RFC3168 compliant operating
systems is quite valuable.
Also, policing unresponsive sources may not be the only or even
the first application that drives deployment. It may be policing
causes of heavy congestion (e.g. peer-to-peer file-sharing). Or
it may be mitigation of denial of service. Or we may be wrong in
thinking simpler QoS will not be the initial motivation for re-ECN
deployment. Indeed, the combined pressure for all these may be
the motivator, but it seems optimistic to expect such a level of
joined-up thinking from today's communications industry. We
believe a single application alone must be a sufficient motivator.
In short, everyone gains from adding accountability to TCP/IP,
except the selfish or malicious. So, deployment incentives tend
to be strong.
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8. Architectural Rationale
In the Internet's technical community, the danger of not responding
to congestion is well-understood, as well as its attendant risk of
congestion collapse [RFC3714]. However, one side of the Internet's
commercial community considers that the very essence of IP is to
provide open access to the internetwork for all applications. They
see congestion as a symptom of over-conservative investment, and rely
on revising application designs to find novel ways to keep
applications working despite congestion. They argue that the
Internet was never intended to be solely for TCP-friendly
applications. Meanwhile, another side of the Internet's commercial
community believes that it is worthwhile providing a network for
novel applications only if it has sufficient capacity, which can
happen only if a greater share of application revenues can be
/assured/ for the infrastructure provider. Otherwise the major
investments required would carry too much risk and wouldn't happen.
The lesson articulated in [Tussle] is that we shouldn't embed our
view on these arguments into the Internet at design time. Instead we
should design the Internet so that the outcome of these arguments can
get decided at run-time. Re-ECN is designed in that spirit. Once
the protocol is available, different network operators can choose how
liberal they want to be in holding people accountable for the
congestion they cause. Some might boldly invest in capacity and not
police its use at all, hoping that novel applications will result.
Others might use re-ECN for fine-grained flow policing, expecting to
make money selling vertically integrated services. Yet others might
sit somewhere half-way, perhaps doing coarse, per-user policing. All
might change their minds later. But re-ECN always allows them to
interconnect so that the careful ones can protect themselves from the
liberal ones.
The incentive-based approach used for re-ECN is based on Gibbens and
Kelly's arguments [Evol_cc] on allowing endpoints the freedom to
evolve new congestion control algorithms for new applications. They
ensured responsible behaviour despite everyone's self-interest by
applying pricing to ECN marking, and Kelly had proved stability and
optimality in an earlier paper.
Re-ECN keeps all the underlying economic incentives, but rearranges
the feedback. The idea is to allow a network operator (if it
chooses) to deploy engineering mechanisms like policers at the front
of the network which can be designed to behave /as if/ they are
responding to congestion prices. Rather than having to subject users
to congestion pricing, networks can then use more traditional
charging regimes (or novel ones). But the engineering can constrain
the overall amount of congestion a user can cause. This provides a
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buffer against completely outrageous congestion control, but still
makes it easy for novel applications to evolve if they need different
congestion control to the norms. It also allows novel charging
regimes to evolve.
Despite being achieved with a relatively minor protocol change, re-
ECN is an architectural change. Previously, Internet congestion
could only be controlled by the data sender, because it was the only
one both in a position to control the load and in a position to see
information on congestion. Re-ECN levels the playing field. It
recognises that the network also has a role to play in moderating
(policing) congestion control. But policing is only truly effective
at the first ingress into an internetwork, whereas path congestion
was previously only visible at the last egress. So, re-ECN
democratises congestion information. Then the choice over who
actually controls congestion can be made at run-time, not design
time---a bit like an aircraft with dual controls. And different
operators can make different choices. We believe non-architectural
approaches to this problem are unlikely to offer more than partial
solutions (see Section 9).
Importantly, re-ECN does not require assumptions about specific
congestion responses to be embedded in any network elements, except
at the first ingress to the internetwork if that level of control is
desired by the ingress operator. But such tight policing will be a
matter of agreement between the source and its access network
operator. The ingress operator need not police congestion response
at flow granularity; it can simply hold a source responsible for the
aggregate congestion it causes, perhaps keeping it within a monthly
congestion quota. Or if the ingress network trusts the source, it
can do nothing.
Therefore, the aim of the re-ECN protocol is NOT solely to police
TCP-friendliness. Re-ECN preserves IP as a generic network layer for
all sorts of responses to congestion, for all sorts of transports.
Re-ECN merely ensures truthful downstream congestion information is
available in the network layer for all sorts of accountability
applications.
The end to end design principle does not say that all functions
should be moved out of the lower layers---only those functions that
are not generic to all higher layers. Re-ECN adds a function to the
network layer that is generic, but was omitted: accountability for
causing congestion. Accountability is not something that an end-user
can provide to themselves. We believe re-ECN adds no more than is
sufficient to hold each flow accountable, even if it consists of a
single datagram.
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"Accountability" implies being able to identify who is responsible
for causing congestion. However, at the network layer it would NOT
be useful to identify the cause of congestion by adding individual or
organisational identity information, NOR by using source IP
addresses. Rather than bringing identity information to the point of
congestion, we bring downstream congestion information to the point
where the cause can be most easily identified and dealt with. That
is, at any trust boundary congestion can be associated with the
physically connected upstream neighbour that is directly responsible
for causing it (whether intentionally or not). A trust boundary
interface is exactly the place to police or throttle in order to
directly mitigate congestion, rather than having to trace the
(ir)responsible party in order to shut them down.
Some considered that ECN itself was a layering violation. The
reasoning went that the interface to a layer should provide a service
to the higher layer and hide how the lower layer does it. However,
ECN reveals the state of the network layer and below to the transport
layer. A more positive way to describe ECN is that it is like the
return value of a function call to the network layer. It explicitly
returns the status of the request to deliver a packet, by returning a
value representing the current risk that a packet will not be served.
Re-ECN has similar semantics, except the transport layer must try to
guess the return value, then it can use the actual return value from
the network layer to modify the next guess.
The guiding principle behind all the discussion in Section 4.5 on
Policing is that any gain from subverting the protocol should be
precisely neutralised, rather than punished. If a gain is punished
to a greater extent than is sufficient to neutralise it, it will most
likely open up a new vulnerability, where the amplifying effect of
the punishment mechanism can be turned on others.
For instance, if possible, flows should be removed as soon as they go
negative, but we do NOT RECOMMEND any attempts to discard such flows
further upstream while they are still positive. Such over-zealous
push-back is unnecessary and potentially dangerous. These flows have
paid their `fare' up to the point they go negative, so there is no
harm in delivering them that far. If someone downstream asks for a
flow to be dropped as near to the source as possible, because they
say it is going to become negative later, an upstream node cannot
test the truth of this assertion. Rather than have to authenticate
such messages, re-ECN has been designed so that flows can be dropped
solely based on locally measurable evidence. A message hinting that
a flow should be watched closely to test for negativity is fine. But
not a message that claims that a positive flow will go negative
later, so it should be dropped. .
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9. Related Work
{Due to lack of time, this section is incomplete. The reader is
referred to the Related Work section of [Re-fb] for a brief selection
of related ideas.}
9.1. Policing Rate Response to Congestion
ATM network elements send congestion back-pressure
messages [ITU-T.I.371] along each connection, duplicating any end to
end feedback because they don't trust it. On the other hand, re-ECN
ensures information in forwarded packets can be used for congestion
management without requiring a connection-oriented architecture and
re-using the overhead of fields that are already set aside for end to
end congestion control (and routing loop detection in the case of re-
TTL in Appendix D).
We borrowed ideas from policers in the literature [pBox],[XCHOKe],
AFD etc. for our rate equation policer. However, without the benefit
of re-ECN they don't police the correct rate for the condition of
their path. They detect unusually high /absolute/ rates, but only
while the policer itself is congested, because they work by detecting
prevalent flows in the discards from the local RED queue. These
policers must sit at every potential bottleneck, whereas our policer
need only be located at each ingress to the internetwork. As Floyd &
Fall explain [pBox], the limitation of their approach is that a high
sending rate might be perfectly legitimate, if the rest of the path
is uncongested or the round trip time is short. Commercially
available rate policers cap the rate of any one flow. Or they
enforce monthly volume caps in an attempt to control high volume
file-sharing. They limit the value a customer derives. They might
also limit the congestion customers can cause, but only as an
accidental side-effect. They actually punish traffic that fills
troughs as much as traffic that causes peaks in utilisation. In
practice network operators need to be able to allocate service by
cost during congestion, and by value at other times.
9.2. Congestion Notification Integrity
The choice of two ECT code-points in the ECN field [RFC3168]
permitted future flexibility, optionally allowing the sender to
encode the experimental ECN nonce [RFC3540] in the packet stream.
This mechanism has since been included in the specifications of DCCP
[RFC4340].
The ECN nonce is an elegant scheme that allows the sender to detect
if someone in the feedback loop - the receiver especially - tries to
claim no congestion was experienced when in fact congestion led to
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packet drops or ECN marks. For each packet it sends, the sender
chooses between the two ECT codepoints in a pseudo-random sequence.
Then, whenever the network marks a packet with CE, if the receiver
wants to deny congestion happened, she has to guess which ECT
codepoint was overwritten. She has only a 50:50 chance of being
correct each time she denies a congestion mark or a drop, which
ultimately will give her away.
The purpose of a network-layer nonce should primarily be protection
of the network, while a transport-layer nonce would be better used to
protect the sender from cheating receivers. Now, the assumption
behind the ECN nonce is that a sender will want to detect whether a
receiver is suppressing congestion feedback. This is only true if
the sender's interests are aligned with the network's, or with the
community of users as a whole. This may be true for certain large
senders, who are under close scrutiny and have a reputation to
maintain. But we have to deal with a more hostile world, where
traffic may be dominated by peer-to-peer transfers, rather than
downloads from a few popular sites. Often the `natural' self-
interest of a sender is not aligned with the interests of other
users. It often wishes to transfer data quickly to the receiver as
much as the receiver wants the data quickly.
In contrast, the re-ECN protocol enables policing of an agreed rate-
response to congestion (e.g. TCP-friendliness) at the sender's
interface with the internetwork. It also ensures downstream networks
can police their upstream neighbours, to encourage them to police
their users in turn. But most importantly, it requires the sender to
declare path congestion to the network and it can remove traffic at
the egress if this declaration is dishonest. So it can police
correctly, irrespective of whether the receiver tries to suppress
congestion feedback or whether the sender ignores genuine congestion
feedback. Therefore the re-ECN protocol addresses a much wider range
of cheating problems, which includes the one addressed by the ECN
nonce.
9.3. Identifying Upstream and Downstream Congestion
Purple [Purple] proposes that queues should use the CWR flag in the
TCP header of ECN-capable flows to work out path congestion and
therefore downstream congestion in a similar way to re-ECN. However,
because CWR is in the transport layer, it is not always visible to
network layer routers and policers. Purple's motivation was to
improve AQM, not policing. But, of course, nodes trying to avoid a
policer would not be expected to allow CWR to be visible.
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10. Security Considerations
{ToDo: enrich this section}{ToDo: Describe attacks by networks on
flows (and by spoofing sources).} {ToDo: Re-ECN & DNS servers}
Nearly the whole of this document concerns security.
11. IANA Considerations
This memo includes no request to IANA.
12. Conclusions
{ToDo:}
13. Acknowledgements
Sebastien Cazalet and Andrea Soppera contributed to the idea of re-
feedback. All the following have given helpful comments: Andrea
Soppera, David Songhurst, Peter Hovell, Louise Burness, Phil Eardley,
Steve Rudkin, Marc Wennink, Fabrice Saffre, Cefn Hoile, Steve Wright,
John Davey, Martin Koyabe, Carla Di Cairano-Gilfedder, Alexandru
Murgu, Nigel Geffen, Pete Willis, John Adams (BT), Sally Floyd
(ICIR), Joe Babiarz, Kwok Ho-Chan (Nortel), Stephen Hailes, Mark
Handley (who developed the attack with cancelled packets), Adam
Greenhalgh (who developed the attack on DNS) (UCL), Jon Crowcroft
(Uni Cam), David Clark, Bill Lehr, Sharon Gillett, Steve Bauer (who
complemented our own dummy traffic attacks with others), Liz Maida
(MIT), and comments from participants in the CRN/CFP Broadband and
DoS-resistant Internet working groups.A special thank you to
Alessandro Salvatori for coming up with fiendish attacks on re-ECN.
14. Comments Solicited
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF Transport Area working group's mailing list
<tsvwg@ietf.org>, and/or to the authors.
15. References
15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The
Addition of Explicit Congestion Notification (ECN)
to IP", RFC 3168, September 2001.
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15.2. Informative References
[Bauer06] Bauer, S., Faratin, P., and R. Beverly, "Assessing
the assumptions underlying mechanism design for the
Internet", Proc. Workshop on the Economics of
Networked Systems (NetEcon06) , June 2006, <http://
www.cs.duke.edu/nicl/netecon06/papers/
ne06-assessing.pdf>.
[CLoop_pol] Salvatori, A., "Closed Loop Traffic Policing",
Politecnico Torino and Institut Eurecom Masters
Thesis , September 2005.
[ECN-Deploy] Floyd, S., "ECN (Explicit Congestion Notification)
in TCP/IP; Implementation and Deployment of ECN",
Web-page , May 2004, <http://www.icir.org/floyd/
ecn.html#implementations>.
[Evol_cc] Gibbens, R. and F. Kelly, "Resource pricing and the
evolution of congestion control",
Automatica 35(12)1969--1985, December 1999, <http:/
/www.sciencedirect.com/science/article/pii/
S0005109899001351>.
[ITU-T.I.371] ITU-T, "Traffic Control and Congestion Control in
B-ISDN", ITU-T Rec. I.371 (03/04), March 2004.
[Jiang02] Jiang, H. and D. Dovrolis, "The Macroscopic
Behavior of the TCP Congestion Avoidance
Algorithm", ACM SIGCOMM CCR 32(3)75-88, July 2002,
<http://doi.acm.org/10.1145/571697.571725>.
[Mathis97] Mathis, M., Semke, J., Mahdavi, J., and T. Ott,
"The Macroscopic Behavior of the TCP Congestion
Avoidance Algorithm", ACM SIGCOMM CCR 27(3)67--82,
July 1997,
<http://doi.acm.org/10.1145/263932.264023>.
[Purple] Pletka, R., Waldvogel, M., and S. Mannal, "PURPLE:
Predictive Active Queue Management Utilizing
Congestion Information", Proc. Local Computer
Networks (LCN 2003) , October 2003.
[RFC2208] Mankin, A., Baker, F., Braden, B., Bradner, S.,
O'Dell, M., Romanow, A., Weinrib, A., and L. Zhang,
"Resource ReSerVation Protocol (RSVP) Version 1
Applicability Statement Some Guidelines on
Deployment", RFC 2208, September 1997.
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[RFC3514] Bellovin, S., "The Security Flag in the IPv4
Header", RFC 3514, April 2003.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust
Explicit Congestion Notification (ECN) Signaling
with Nonces", RFC 3540, June 2003.
[RFC3714] Floyd, S. and J. Kempf, "IAB Concerns Regarding
Congestion Control for Voice Traffic in the
Internet", RFC 3714, March 2004.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
March 2006.
[RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram
Congestion Control Protocol (DCCP) Congestion
Control ID 2: TCP-like Congestion Control",
RFC 4341, March 2006.
[RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
Datagram Congestion Control Protocol (DCCP)
Congestion Control ID 3: TCP-Friendly Rate Control
(TFRC)", RFC 4342, March 2006.
[RFC5559] Eardley, P., "Pre-Congestion Notification (PCN)
Architecture", RFC 5559, June 2009.
[Re-PCN] Briscoe, B., "Emulating Border Flow Policing using
Re-PCN on Bulk Data",
draft-briscoe-re-pcn-border-cheat-03 (work in
progress), October 2009.
[Re-TCP] Briscoe, B., Jacquet, A., Moncaster, T., and A.
Smith, "Re-ECN: Adding Accountability for Causing
Congestion to TCP/IP",
draft-briscoe-conex-re-ecn-tcp-03 (work in
progress), March 2014.
[Re-fb] Briscoe, B., Jacquet, A., Di Cairano-Gilfedder, C.,
Salvatori, A., Soppera, A., and M. Koyabe,
"Policing Congestion Response in an Internetwork
Using Re-Feedback", ACM SIGCOMM CCR 35(4)277--288,
August 2005, <http://www.acm.org/sigs/sigcomm/
sigcomm2005/techprog.html#session8>.
[Savage99] Savage, S., Cardwell, N., Wetherall, D., and T.
Anderson, "TCP congestion control with a
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misbehaving receiver", ACM SIGCOMM CCR 29(5),
October 1999,
<http://citeseer.ist.psu.edu/savage99tcp.html>.
[Smart_rtg] Goldenberg, D., Qiu, L., Xie, H., Yang, Y., and Y.
Zhang, "Optimizing Cost and Performance for
Multihoming", ACM SIGCOMM CCR 34(4)79--92,
October 2004,
<http://citeseer.ist.psu.edu/698472.html>.
[Steps_DoS] Handley, M. and A. Greenhalgh, "Steps towards a
DoS-resistant Internet Architecture", Proc. ACM
SIGCOMM workshop on Future directions in network
architecture (FDNA'04) pp 49--56, August 2004.
[Tussle] Clark, D., Sollins, K., Wroclawski, J., and R.
Braden, "Tussle in Cyberspace: Defining Tomorrow's
Internet", ACM SIGCOMM CCR 32(4)347--356,
October 2002, <http://www.acm.org/sigcomm/
sigcomm2002/papers/tussle.pdf>.
[XCHOKe] Chhabra, P., Chuig, S., Goel, A., John, A., Kumar,
A., Saran, H., and R. Shorey, "XCHOKe: Malicious
Source Control for Congestion Avoidance at Internet
Gateways", Proceedings of IEEE International
Conference on Network Protocols (ICNP-02) ,
November 2002,
<http://www.cc.gatech.edu/~akumar/xchoke.pdf>.
[pBox] Floyd, S. and K. Fall, "Promoting the Use of End-
to-End Congestion Control in the Internet", IEEE/
ACM Transactions on Networking 7(4) 458--472,
August 1999, <http://ieeexplore.ieee.org/xpls/
abs_all.jsp?arnumber=793002>.
[relax-fairness] Briscoe, B., Moncaster, T., and L. Burness,
"Problem Statement: Transport Protocols Don't Have
To Do Fairness",
draft-briscoe-tsvwg-relax-fairness-01 (work in
progress), July 2008.
Appendix A. Example Egress Dropper Algorithm
{ToDo: Write up the basic algorithm with flow state, then the
aggregated one.}
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Appendix B. Policer Designs to ensure Congestion Responsiveness
B.1. Per-user Policing
User policing requires a policer on the ingress interface of the
access router associated with the user. At that point, the traffic
of the user hasn't diverged on different routes yet; nor has it mixed
with traffic from other sources.
In order to ensure that a user doesn't generate more congestion in
the network than her due share, a modified bulk token-bucket is
maintained with the following parameter:
o b_0 the initial token level
o r the filling rate
o b_max the bucket depth
The same token bucket algorithm is used as in many areas of
networking, but how it is used is very different:
o all traffic from a user over the lifetime of their subscription is
policed in the same token bucket.
o only positive and cancelled packets (positive, cautious and
cancelled) consume tokens
Such a policer will allow network operators to throttle the
contribution of their users to network congestion. This will require
the appropriate contractual terms to be in place between operators
and users. For instance: a condition for a user to subscribe to a
given network service may be that she should not cause more than a
volume C_user of congestion over a reference period T_user, although
she may carry forward up to N_user times her allowance at the end of
each period. These terms directly set the parameter of the user
policer:
o b_0 = C_user
o r = C_user/T_user
o b_max = b_0 * (N_user +1)
Besides the congestion budget policer above, another user policer may
be necessary to further rate-limit cautious packets, if they are to
be marked rather than dropped (see discussion in [ref other
document].). Rate-limiting cautious packets will prevent high bursts
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of new flow arrivals, which is a very useful feature in DoS
prevention. A condition to subscribe to a given network service
would have to be that a user should not generate more than C_cautious
cautious packets, over a reference period T_cautious, with no option
to carry forward any of the allowance at the end of each period.
These terms directly set the parameters of the cautious packet
policer:
o b_0 = C_cautious
o r = C_cautious/T_cautious
o b_max = b_0
T_cautious should be a much shorter period than T_user: for instance
T_cautious could be in the order of minutes while T_user could be in
order of weeks.
B.2. Per-flow Rate Policing
Whilst we believe that simple per-user policing would be sufficient
to ensure senders comply with congestion control, some operators may
wish to police the rate response of each flow to congestion as well.
Although we do not believe this will be neceesary, we include this
section to show how one could perform per-flow policing using
enforcement of TCP-fairness as an example. Per-flow policing aims to
enforce congestion responsiveness on the shortest information
timescale on a network path: packet roundtrips.
This again requires that the appropriate terms be agreed between a
network operator and its users, where a congestion responsiveness
policy might be required for the use of a given network service
(perhaps unless the user specifically requests otherwise).
As an example, we describe below how a rate adaptation policer can be
designed when the applicable rate adaptation policy is TCP-
compliance. In that context, the average throughput of a flow will
be expected to be bounded by the value of the TCP throughput during
congestion avoidance, given in Mathis' formula [Mathis97]
x_TCP = k * s / ( T * sqrt(m) )
where:
o x_TCP is the throughput of the TCP flow in packets per second,
o k is a constant upper-bounded by sqrt(3/2),
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o s is the average packet size of the flow,
o T is the roundtrip time of the flow,
o m is the congestion level experienced by the flow.
We define the marking period N=1/m which represents the average
number of packets between two positive or cancelled packets. Mathis'
formula can be re-written as:
x_TCP = k*s*sqrt(N)/T
We can then get the average inter-mark time in a compliant TCP flow,
dt_TCP, by solving (x_TCP/s)*dt_TCP = N which gives
dt_TCP = sqrt(N)*T/k
We rely on this equation for the design of a rate-adaptation policer
as a variation of a token bucket. In that case a policer has to be
set up for each policed flow. This may be triggered by cautious
packets, with the remainder of flows being all rate limited together
if they do not start with a cautious packet.
Where maintaining per flow state is not a problem, for instance on
some access routers, systematic per-flow policing may be considered.
Should per-flow state be more constrained, rate adaptation policing
could be limited to a random sample of flows exhibiting positive or
cancelled packets.
As in the case of user policing, only positive or cancelled packets
will consume tokens, however the amount of tokens consumed will
depend on the congestion signal.
When a new rate adaptation policer is set up for flow j, the
following state is created:
o a token bucket b_j of depth b_max starting at level b_0
o a timestamp t_j = timenow()
o a counter N_j = 0
o a roundtrip estimate T_j
o a filling rate r
When the policing node forwards a packet of flow j with no positive
packets:
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o . the counter is incremented: N_j += 1
When the policing node forwards a packet of flow j carrying a
negative packet:
o the counter is incremented: N_j += 1
o the token level is adjusted: b_j += r*(timenow()-t_j) - sqrt(N_j)*
T_j/k
o the counter is reset: N_j = 0
o the timer is reset: t_j = timenow()
An implementation example will be given in a later draft that avoids
having to extract the square root.
Analysis: For a TCP flow, for r= 1 token/sec, on average,
r*(timenow()-t_j)-sqrt(N_j)* T_j/k = dt_TCP - sqrt(N)*T/k = 0
This means that the token level will fluctuate around its initial
level. The depth b_max of the bucket sets the timescale on which the
rate adaptation policy is performed while the filling rate r sets the
trade-off between responsiveness and robustness:
o the higher b_max, the longer it will take to catch greedy flows
o the higher r, the fewer false positives (greedy verdict on
compliant flows) but the more false negatives (compliant verdict
on greedy flows)
This rate adaptation policer requires the availability of a roundtrip
estimate which may be obtained for instance from the application of
re-feedback to the downstream delay Appendix D or passive estimation
[Jiang02].
When the bucket of a policer located at the access router (whether it
is a per-user policer or a per-flow policer) becomes empty, the
access router SHOULD drop at least all packets causing the token
level to become negative. The network operator MAY take further
sanctions if the token level of the per-flow policers associated with
a user becomes negative.
Appendix C. Downstream Congestion Metering Algorithms
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C.1. Bulk Downstream Congestion Metering Algorithm
To meter the bulk amount of downstream congestion in traffic crossing
an inter-domain border an algorithm is needed that accumulates the
size of positive packets and subtracts the size of negative packets.
We maintain two counters:
V_b: accumulated congestion volume
B: total data volume (in case it is needed)
A suitable pseudo-code algorithm for a border router is as follows:
====================================================================
V_b = 0
B = 0
for each Re-ECN-capable packet {
b = readLength(packet) /* set b to packet size */
B += b /* accumulate total volume */
if readEECN(packet) == (positive || cautious {
V_b += b /* increment... */
} elseif readEECN(packet) == negative {
V_b -= b /* ...or decrement V_b... */
} /*...depending on EECN field */
}
====================================================================
At the end of an accounting period this counter V_b represents the
congestion volume that penalties could be applied to, as described in
Section 4.5.
For instance, accumulated volume of congestion through a border
interface over a month might be V_b = 5PB (petabyte = 10^15 byte).
This might have resulted from an average downstream congestion level
of 1% on an accumulated total data volume of B = 500PB.
{ToDo: Include algorithm for precise downstream congestion.}
C.2. Inflation Factor for Persistently Negative Flows
The following process is suggested to complement the simple algorithm
above in order to protect against the various attacks from
persistently negative flows described in Section 4.5. As explained
in that section, the most important and first step is to estimate the
contribution of persistently negative flows to the bulk volume of
downstream pre-congestion and to inflate this bulk volume as if these
flows weren't there. The process below has been designed to give an
unbiased estimate, but it may be possible to define other processes
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that achieve similar ends.
While the above simple metering algorithm is counting the bulk of
traffic over an accounting period, the meter should also select a
subset of the whole flow ID space that is small enough to be able to
realistically measure but large enough to give a realistic sample.
Many different samples of different subsets of the ID space should be
taken at different times during the accounting period, preferably
covering the whole ID space. During each sample, the meter should
count the volume of positive packets and subtract the volume of
negative, maintaining a separate account for each flow in the sample.
It should run a lot longer than the large majority of flows, to avoid
a bias from missing the starts and ends of flows, which tend to be
positive and negative respectively.
Once the accounting period finishes, the meter should calculate the
total of the accounts V_{bI} for the subset of flows I in the sample,
and the total of the accounts V_{fI} excluding flows with a negative
account from the subset I. Then the weighted mean of all these
samples should be taken a_S = sum_{forall I} V_{fI} / sum_{forall I}
V_{bI}.
If V_b is the result of the bulk accounting algorithm over the
accounting period (Appendix C.1) it can be inflated by this factor
a_S to get a good unbiased estimate of the volume of downstream
congestion over the accounting period a_S.V_b, without being polluted
by the effect of persistently negative flows.
Appendix D. Re-TTL
This Appendix gives an overview of a proposal to be able to overload
the TTL field in the IP header to monitor downstream propagation
delay. This is included to show that it would be possible to take
account of RTT if it was deemed desirable.
Delay re-feedback can be achieved by overloading the TTL field,
without changing IP or router TTL processing. A target value for TTL
at the destination would need standardising, say 16. If the path hop
count increased by more than 16 during a routing change, it would
temporarily be mistaken for a routing loop, so this target would need
to be chosen to exceed typical hop count increases. The TCP wire
protocol and handlers would need modifying to feed back the
destination TTL and initialise it. It would be necessary to
standardise the unit of TTL in terms of real time (as was the
original intent in the early days of the Internet).
In the longer term, precision could be improved if routers
decremented TTL to represent exact propagation delay to the next
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router. That is, for a router to decrement TTL by, say, 1.8 time
units it would alternate the decrement of every packet between 1 & 2
at a ratio of 1:4. Although this might sometimes require a seemingly
dangerous null decrement, a packet in a loop would still decrement to
zero after 255 time units on average. As more routers were upgraded
to this more accurate TTL decrement, path delay estimates would
become increasingly accurate despite the presence of some RFC3168
compliant routers that continued to always decrement the TTL by 1.
Appendix E. Argument for holding back the ECN nonce
The ECN nonce is a mechanism that allows a /sending/ transport to
detect if drop or ECN marking at a congested router has been
suppressed by a node somewhere in the feedback loop---another router
or the receiver.
Space for the ECN nonce was set aside in [RFC3168] (currently
proposed standard) while the full nonce mechanism is specified in
[RFC3540] (currently experimental). The specifications for [RFC4340]
(currently proposed standard) requires that "Each DCCP sender SHOULD
set ECN Nonces on its packets...". It also mandates as a requirement
for all CCID profiles that "Any newly defined acknowledgement
mechanism MUST include a way to transmit ECN Nonce Echoes back to the
sender.", therefore:
o The CCID profile for TCP-like Congestion Control [RFC4341]
(currently proposed standard) says "The sender will use the ECN
Nonce for data packets, and the receiver will echo those nonces in
its Ack Vectors."
o The CCID profile for TCP-Friendly Rate Control (TFRC) [RFC4342]
recommends that "The sender [use] Loss Intervals options' ECN
Nonce Echoes (and possibly any Ack Vectors' ECN Nonce Echoes) to
probabilistically verify that the receiver is correctly reporting
all dropped or marked packets."
The primary function of the ECN nonce is to protect the integrity of
the information about congestion: ECN marks and packet drops.
However, when the nonce is used to protect the integrity of
information about packet drops, rather than ECN marks, a transport
layer nonce will always be sufficient (because a drop loses the
transport header as well as the ECN field in the network header),
which would avoid using scarce IP header codepoint space. Similarly,
a transport layer nonce would protect against a receiver sending
early acknowledgements [Savage99].
If the ECN nonce reveals integrity problems with the information
about congestion, the sending transport can use that knowledge for
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two functions:
o to protect its own resources, by allocating them in proportion to
the rates that each network path can sustain, based on congestion
control,
o and to protect congested routers in the network, by slowing down
drastically its connection to the destination with corrupt
congestion information.
If the sending transport chooses to act in the interests of congested
routers, it can reduce its rate if it detects some malicious party in
the feedback loop may be suppressing ECN feedback. But it would only
be useful to congested routers when /all/ senders using them are
trusted to act in interest of the congested routers.
In the end, the only essential use of a network layer nonce is when
sending transports (e.g. large servers) want to allocate their /own/
resources in proportion to the rates that each network path can
sustain, based on congestion control. In that case, the nonce allows
senders to be assured that they aren't being duped into giving more
of their own resources to a particular flow. And if congestion
suppression is detected, the sending transport can rate limit the
offending connection to protect its own resources. Certainly, this
is a useful function, but the IETF should carefully decide whether
such a single, very specific case warrants IP header space.
In contrast, Re-ECN allows all routers to fully protect themselves
from such attacks, without having to trust anyone - senders,
receivers, neighbouring networks. Re-ECN is therefore proposed in
preference to the ECN nonce on the basis that it addresses the
generic problem of accountability for congestion of a network's
resources at the IP layer.
Delaying the ECN nonce is justified because the applicability of the
ECN nonce seems too limited for it to consume a two-bit codepoint in
the IP header. It therefore seems prudent to give time for an
alternative way to be found to do the one function the nonce is
essential for.
Moreover, while we have re-designed the Re-ECN codepoints so that
they do not prevent the ECN nonce progressing, the same is not true
the other way round. If the ECN nonce started to see some deployment
(perhaps because it was blessed with proposed standard status),
incremental deployment of Re-ECN would effectively be impossible,
because Re-ECN marking fractions at inter-domain borders would be
polluted by unknown levels of nonce traffic.
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The authors are aware that Re-ECN must prove it has the potential it
claims if it is to displace the nonce. Therefore, every effort has
been made to complete a comprehensive specification of Re-ECN so that
its potential can be assessed. We therefore seek the opinion of the
Internet community on whether the Re-ECN protocol is sufficiently
useful to warrant standards action.
Authors' Addresses
Bob Briscoe (editor)
BT
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 645196
EMail: bob.briscoe@bt.com
URI: http://bobbriscoe.net/
Arnaud Jacquet
BT
B54/70, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 647284
EMail: arnaud.jacquet@bt.com
URI:
Toby Moncaster
Moncaster.com
Dukes
Layer Marney
Colchester CO5 9UZ
UK
EMail: toby@moncaster.com
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Alan Smith
BT
B54/76, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 640404
EMail: alan.p.smith@bt.com
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