Internet Engineering Task Force (IETF) M. Mathis

Internet Engineering Task Force (IETF) M. Mathis
Request for Comments: 7713 Google, Inc.
Category: Informational B. Briscoe
ISSN: 2070-1721 BT
December 2015

Congestion Exposure (ConEx) Concepts, Abstract Mechanism,
and Requirements

Abstract

This document describes an abstract mechanism by which senders inform
the network about the congestion recently encountered by packets in
the same flow. Today, network elements at any layer may signal
congestion to the receiver by dropping packets or by Explicit
Congestion Notification (ECN) markings, and the receiver passes this
information back to the sender in transport-layer feedback. The
mechanism described here enables the sender to also relay this
congestion information back into the network in-band at the IP layer,
such that the total amount of congestion from all elements on the
path is revealed to all IP elements along the path, where it could,
for example, be used to provide input to traffic management. This
mechanism is called Congestion Exposure, or ConEx. The companion
document, "Congestion Exposure (ConEx) Concepts and Use Cases"
(RFC 6789), provides the entry point to the set of ConEx
documentation.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7713.

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Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
3. Requirements for the ConEx Abstract Mechanism . . . . . . . . 7
3.1. Requirements for ConEx Signals . . . . . . . . . . . . . 7
3.2. Constraints on the Audit Function . . . . . . . . . . . . 8
3.3. Requirements for Non-abstract ConEx Specifications . . . 9
4. Encoding Congestion Exposure . . . . . . . . . . . . . . . . 12
4.1. Naive Encoding . . . . . . . . . . . . . . . . . . . . . 12
4.2. Null Encoding . . . . . . . . . . . . . . . . . . . . . . 13
4.3. ECN-Based Encoding . . . . . . . . . . . . . . . . . . . 13
4.4. Independent Bits . . . . . . . . . . . . . . . . . . . . 14
4.5. Codepoint Encoding . . . . . . . . . . . . . . . . . . . 14
4.6. Units Implied by an Encoding . . . . . . . . . . . . . . 15
5. Congestion Exposure Components . . . . . . . . . . . . . . . 16
5.1. Network Devices (Not Modified) . . . . . . . . . . . . . 16
5.2. Modified Senders . . . . . . . . . . . . . . . . . . . . 16
5.3. Receivers (Optionally Modified) . . . . . . . . . . . . . 17
5.4. Policy Devices . . . . . . . . . . . . . . . . . . . . . 17
5.4.1. Congestion Monitoring Devices . . . . . . . . . . . . 18
5.4.2. Rest-of-Path Congestion Monitoring . . . . . . . . . 18
5.4.3. Congestion Policers . . . . . . . . . . . . . . . . . 18
5.5. Audit . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6. Support for Incremental Deployment . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.1. Normative References . . . . . . . . . . . . . . . . . . 27
8.2. Informative References . . . . . . . . . . . . . . . . . 27
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30

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1. Introduction

This document describes an abstract mechanism by which, to a first
approximation, senders inform the network about the congestion
encountered by packets earlier in the same flow. It is not a
complete protocol specification because it is known that designing an
encoding (e.g., packet formats, codepoint allocations, etc.) is
likely to entail compromises that preclude some uses of the protocol.
The goal of this document is to provide a framework for developing
and testing algorithms to evaluate the benefits of the ConEx protocol
and to evaluate the consequences of the compromises in various
different encoding designs. This document lays out requirements for
concrete protocol specifications.

A companion document [RFC6789] provides the entry point to the set of
ConEx documentation. It outlines concepts that are prerequisites to
understanding why ConEx is useful, and it outlines various ways that
ConEx might be used.

2. Overview

As typical end-to-end transport protocols continually seek out more
network capacity, network elements signal whenever congestion
results, and the transports are responsible for controlling this
network congestion [RFC5681]. The more a transport tries to use
capacity that others want to use, the more congestion signals will be
attributable to that transport. Likewise, the more transport
sessions sustained by a user and the longer the user sustains them,
the more congestion signals will be attributable to that user. The
goal of ConEx is to ensure that the resulting congestion signals are
sufficiently visible and robust, because they are an ideal metric for
networks to use as the basis of traffic management or other related
functions.

Networks indicate congestion by three possible signals: packet loss,
ECN marking, or queueing delay. ECN marking and some packet loss may
be the outcome of Active Queue Management (AQM), which the network
uses to warn senders to reduce their rates. Packet loss is also the
natural consequence of complete exhaustion of a buffer or other
network resource. Some experimental transport protocols and TCP
variants infer impending congestion from increasing queuing delay.
However, delay is too amorphous to use as a congestion metric. In
this and other ConEx documents, the term 'congestion signals' is
generally used solely for ECN markings and packet losses because they
are unambiguous signals of congestion.

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In both cases, the congestion signals follow the route indicated in
Figure 1. A congested network device sends a signal in the data
stream on the forward path to the transport receiver, the receiver
passes it back to the sender through transport-level feedback, and
the sender makes some congestion control adjustment.

This document extends the capabilities of the Internet protocol suite
with the addition of a new Congestion Exposure signal. To a first
approximation, this signal (also shown in Figure 1) relays the
congestion information from the transport sender back through the
internetwork layer where it is visible to any interested
internetwork-layer devices along the forward path. This document
frames the engineering problem of designing the ConEx Signal. The
requirements are described in Section 3 and some example encodings
are presented in Section 4. Section 5 describes all of the protocol
components.

This new signal is expressly designed to support a variety of new
policy mechanisms that might be used to instrument, monitor, or
manage traffic. The policy devices are not shown in Figure 1 but
might be placed anywhere along the forward data path (see
Section 5.4).

,---------. ,---------.
|Transport| |Transport|
| Sender | . |Receiver |
| | /|___________________________________________| |
| ,-<---------------Congestion-Feedback-Signals--<--------. |
| | |/ | | |
| | |\ Transport Layer Feedback Flow | | |
| | | \ ___________________________________________| | |
| | | \| | | |
| | | ' ,-----------. . | | |
| | |_____________| |_______________|\ | | |
| | | IP Layer | | Data Flow \ | | |
| | | |(Congested)| \ | | |
| | | | Network |--Congestion-Signals--->-' |
| | | | Device | \| |
| | | | | /| |
| `----------->--(new)-IP-Layer-ConEx-Signals-------->| |
| | | | / | |
| |_____________| |_______________ / | |
| | | | |/ | |
`---------' `-----------' ' `---------'

Figure 1: The Flow of Congestion and ConEx Signals

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Since the policy devices can affect how traffic is treated, it is
assumed that there is an intrinsic motivation for users,
applications, or operating systems to understate the congestion that
they are causing. Therefore, it is important to be able to audit
ConEx Signals and to be able to apply sufficient sanction to
discourage cheating of congestion policies. The general approach to
auditing is to count signals on the forward path to confirm that
there are never fewer ConEx Signals than congestion signals. Many
ConEx design constraints come from the need to assure that the audit
function is sufficiently robust. The audit function is described in
Section 5.5; however, significant portions of this document (and
prior research [Refb-dis]) are motivated by issues relating to the
audit function and making it robust.

The congestion and ConEx Signals shown in Figure 1 represent a series
of discrete events: ECN marks or lost packets, carried by the forward
data stream and fed back into the internetwork layer. The policy and
audit functions are most likely to act on the accumulated values of
these signals, for which we use the term "volume". For example,
"traffic volume" is the total number of bytes delivered optionally
over a specified time interval and over some aggregate of traffic
(e.g., all traffic from a site), while "loss volume" is the total
amount of bytes discarded from some aggregate over an interval. The
term "congestion-volume" is defined precisely in [RFC6789]. Note
that volume per unit time is average rate.

A design goal of the ConEx protocol is that the important policy
mechanisms can be implemented per logical link without per-flow state
(see Section 5.4). However, the trade-off is that per-flow state
could be needed to audit ConEx Signals (Section 5.5). This is
justified in that i) auditing at the edges, with a limited number of
flows, enables policy elsewhere, including in the core, without any
per-flow state; ii) auditing can use soft flow state, which does not
require route pinning.

There is a long standing argument over units of congestion: bytes vs
packets (see [RFC7141] and its references). Section 4.6 explains why
this problem must be addressed carefully. However, this document
does not take a strong position on this issue. Nonetheless, it does
require that the units of congestion must be an explicitly stated
property of any proposed encoding, and the consequences of that
design decision must be evaluated along with other aspects of the
design.

To be successful, the ConEx protocol needs to have the property that
the relevant stakeholders each have the incentive to unilaterally
start on each stage of partial deployment, which in turn creates

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incentives for further deployment. Furthermore, legacy systems that
will never be upgraded do not become a barrier to deploying ConEx.
Issues relating to partial deployment are described in Section 6.

Note that ConEx Signals are not intended to be used for fine-grained
congestion control. They are anticipated to be most useful at longer
time scales and/or at coarser granularity than single microflows.
For example, the total congestion caused by a user might serve as an
input to higher-level policy or accountability functions designed to
create incentives for improving user behavior, such as choosing to
send large quantities of data at off-peak times, at lower data rates,
or with less aggressive protocols such as Low Extra Delay Background
Transport (LEDBAT) [RFC6817]; see [RFC6789].

Ultimately, ConEx Signals have the potential to provide a mechanism
to regulate global Internet congestion. From the earliest days of
research on congestion control, there has been a concern that there
is no mechanism to prevent transport designers from incrementally
making protocols more aggressive without bound and spiraling to a
"tragedy of the commons" Internet congestion collapse. The "TCP
friendly" paradigm was created in part to forestall this failure.
However, it no longer commands any authority because it has little to
say about the Internet of today, which has moved beyond the scaling
range of standard TCP. As a consequence, many transports and
applications are opening arbitrarily large numbers of connections or
using arbitrary levels of aggressiveness. ConEx represents a
recognition that the IETF cannot regulate this space directly because
it concerns the behaviour of users and applications, not individual
transport protocols. Instead, the IETF can give network operators
the protocol tools to arbitrate the space themselves with better bulk
traffic management. This, in turn, should create incentives for
users and designers of applications and of transport protocols to be
more mindful about contributing to congestion.

2.1. Terminology

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 RFC 2119 [RFC2119].

ConEx Signals in IP packet headers from the sender to the network:

Not-ConEx: The transport (or at least this packet) is not using
ConEx.

ConEx-Capable: The transport is using ConEx. This is the opposite
of Not-ConEx.

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ConEx Signal: A signal in a packet sent by a ConEx-capable
transport. It carries at least one of the following signals:

Re-Echo-Loss: The transport has experienced a loss.

Re-Echo-ECN: The transport has detected an ECN Congestion
Experienced (CE) mark.

Credit: The transport is building up credit to signal advance
notice of the risk of packets contributing to congestion, in
contrast to signalling only after inherently delayed feedback
of actual congestion.

ConEx-Not-Marked: The transport is ConEx-capable but is not
signaling Re-Echo-Loss, Re-Echo-ECN, or Credit.

ConEx-Marked: At least one of Re-Echo-Loss, Re-Echo-ECN, or Credit.

ConEx-Re-Echo: At least one of Re-Echo-Loss or Re-Echo-ECN.

3. Requirements for the ConEx Abstract Mechanism

First-time readers may wish to skim this section, since it is more
understandable having read the entire document.

3.1. Requirements for ConEx Signals

Ideally, all the following requirements would be met by a Congestion
Exposure Signal:

a. The ConEx Signal SHOULD be visible to internetwork-layer devices
along the entire path from the transport sender to the transport
receiver. Equivalently, it SHOULD be present in the IPv4 or IPv6
header and in the outermost IP header if using IP-in-IP
tunneling. It MAY need to be visible if other encapsulating
headers are used to interconnect networks. The ConEx Signal
SHOULD be immutable once set by the transport sender. A
corollary of these requirements is that the chosen ConEx encoding
SHOULD pass silently without modification through preexisting
networking gear.

b. The ConEx Signal SHOULD be useful under only partial deployment.
A minimal deployment SHOULD only require changes to transport
senders. Furthermore, partial deployment SHOULD create
incentives for additional deployment, both in terms of enabling
ConEx on more devices and adding richer features to existing
devices. Nonetheless, ConEx deployment need never be universal,

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and it is anticipated that some hosts and some transports may
never support the ConEx protocol and some networks may never use
the ConEx Signals.

c. The ConEx Signal SHOULD be timely. There will be a minimum delay
of one RTT and often longer if the transport protocol sends
infrequent feedback (consider Real-time Transport Control
Protocol (RTCP) [RFC3550] [RFC6679], for example).

d. The ConEx Signal SHOULD be accurate and auditable. The general
approach for auditing is to observe the volume of congestion
signals and ConEx Signals on the forward data path and verify
that the ConEx Signals do not underrepresent the congestion
signals (see Section 5.5).

e. The ConEx Signals for packet loss and ECN marking SHOULD have
distinct encodings because they are likely to require different
auditing techniques.

f. Additionally, there SHOULD be an auditable ConEx Credit signal.
A sender can use Credit to indicate potential future congestion,
for example, as is often seen during startup. ConEx Credit is
intended to overestimate congestion actually experienced across
the network.

It is already known that implementing ConEx Signals is likely to
entail some compromises, and therefore, all the requirements above
are expressed with the keyword "SHOULD" rather than "MUST". The only
mandatory requirement is that a concrete protocol description MUST
give sound reasoning if it chooses not to meet some requirement.

3.2. Constraints on the Audit Function

The role of the audit function and constraints on it are described in
Section 5.5. There is no intention to standardise the audit
function. However, it is necessary to lay down the following
normative constraints on audit behaviour so that transport designers
will know what to design against and implementers of audit devices
will know what pitfalls to avoid:

Minimal False Hits: Audit SHOULD introduce minimal false hits for
honest flows.

Minimal False Misses: Audit SHOULD quickly detect and sanction
dishonest flows, ideally on the first dishonest packet.

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Transport Oblivious: Audit SHOULD NOT be designed around one
particular rate response, such as any particular TCP congestion
control algorithm or one particular resource-sharing regime such
as TCP friendliness [RFC5348]. An important goal is to give
ingress networks the freedom to unilaterally allow different rate
responses to congestion and different resource sharing regimes
[Evol_cc] without having to coordinate with other networks over
details of individual flow behaviour.

Sufficient Sanction: Audit SHOULD introduce sufficient sanction
(e.g., loss in goodput) such that senders cannot gain from
understating congestion.

Proportionate Sanction: To the extent that the audit might be
subject to false hits, the sanction SHOULD be proportionate to the
degree to which congestion is understated. If the audit over-
punishes, attackers will find ways to harness it into amplifying
attacks on others. Ideally the audit should, in the long run,
cause the user to get no better performance than they would get by
being accurate.

Manage Memory Exhaustion: Audit SHOULD be able to counter state-
exhaustion attacks. For instance, if the audit function uses flow
state, it should not be possible for senders to exhaust its memory
capacity by gratuitously sending numerous packets, each with a
different flow ID.

Identifier Accountability: Audit SHOULD 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 [CheapPseud].

3.3. Requirements for Non-abstract ConEx Specifications

An experimental ConEx specification SHOULD describe the following
protocol details:

Network Layer:

A. the specific ConEx Signal encodings with packet formats, bit
fields, and/or codepoints;

B. an inventory of invalid combinations of flags or invalid
codepoints in the encoding, as well as whether security
gateways should normalise, discard, or ignore such invalid
encodings, and what values they should be considered
equivalent to by ConEx-aware elements;

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C. an inventory of any conflated signals or any other effects
that are known to compromise signal integrity;

D. whether the source is responsible for allowing for the round-
trip delay in ConEx Signals (e.g., using a Credit marking),
and if so, whether Credit is maintained for the duration of a
flow or degrades over time, and what defines the end of the
duration of a flow;

E. a specification for signal units (bytes vs. packets, etc.),
any approximations allowed, and the algorithms to do any
implied conversions or accounting;

F. if the units are bytes, a definition of which headers are
included in the size of the packet;

G. how tunnels should propagate the ConEx encoding;

H. whether the encoding fields are mutable or not, to ensure that
header authentication, checksum calculation, etc., process
them correctly; a ConEx encoding field SHOULD be immutable
end-to-end, then endpoints can detect if it has been tampered
with in transit;

I. if a specific encoding allows mutability (e.g., at proxies),
then an inventory of invalid transitions between codepoints;
in all encodings, transitions from any ConEx marking to Not-
ConEx MUST be invalid;

J. a statement that the ConEx encoding is only applicable to
unicast and anycast and that forwarding elements should
silently ignore any ConEx signalling on multicast packets
(they should be forwarded unchanged);

K. the definition of any extensibility;

L. backward and forward compatibility and potential migration
strategies; in all cases, a ConEx encoding MUST be arranged so
that legacy transport senders implicitly send Not-ConEx;

M. any (optional) modification to data-plane forwarding dependent
on the encoding (e.g., preferential discard, interaction with
Diffserv, ECN, etc.); and

N. any warning or error messages relevant to the encoding.

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Note regarding item J on multicast: A multicast tree may involve
different levels of congestion on each leg. Any traffic
management can only monitor or control multicast congestion at or
near each receiver. It would make no sense for the sender to try
to expose "whole-path congestion" in sent packets because it
cannot hope to describe all the differing congestion levels on
every leg of the tree.

Transport Layer:

A. a specification of any required changes to congestion feedback
in particular transport protocols;

B. a specification (or, minimally, a recommendation) for how a
transport should estimate credits at the beginning of a
connection and while it is in progress;

C. a specification of whether any other protocol options should
(or must) be enabled along with an implementation of ConEx
(e.g., at least attempting to negotiate ECN and Selective
Acknowledgement (SACK) capability);

D. a specification of any configuration that a ConEx stack may
require (or, preferably, confirmation that it requires no
configuration); and

E. a specification of the statistics that a protocol stack should
log for each type of marking on a per-flow or aggregate basis.

Security:

A. an example of a strong audit algorithm suitable for detecting
if a single flow is misstating congestion; this algorithm
should present minimal false results but need not have optimal
scaling properties (e.g., may need per-flow state).

B. an example of an audit algorithm suitable for detecting
misstated congestion in a large aggregate (e.g., no per-flow
state).

C. a definition of the level of ConEx-Re-Echo and ConEx-Credit
signals that will be sufficient to pass audit (see
Section 5.5).

The possibility exists that these specifications overconstrain the
ConEx design and can not be fully satisfied. An important part of
the evaluation of any particular design will be a thorough inventory
of all ways in which it might fail to satisfy these specifications.

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4. Encoding Congestion Exposure

Most protocol specifications start with a description of packet
formats and codepoints with their associated meanings. This document
does not: It is already known that choosing the encoding for ConEx is
likely to entail some engineering compromises that have the potential
to reduce the protocol's usefulness in some settings. For instance,
the experimental ConEx encoding chosen for IPv6 [CONEX-DESTOPT] had
to make compromises on tunnelling. Rather than making these
engineering choices prematurely, this document sidesteps the encoding
problem by making it abstract. It describes several different
representations of ConEx Signals, none of which are specified to the
level of specific bits or codepoints.

The goal of this approach is to be as complete as possible for
discovering the potential usage and capabilities of the ConEx
protocol, so we have some hope of making optimal design decisions
when choosing the encoding. Even if experiments reveal particular
problems due to the encoding, then this document will still serve as
a reference model.

4.1. Naive Encoding

For tutorial purposes, it is helpful to describe a naive encoding of
the ConEx protocol for TCP and similar protocols: set a bit (not
specified here) in the IP header on each retransmission and on each
ECN-signalled window reduction. Network devices along the forward
path can see this bit and act on it. For example, any device along
the path might limit the rate of all traffic if the rate of marked
(congested) packets exceeds a threshold.

This simple encoding is sufficient to illustrate many of the benefits
envisioned for ConEx. At first glance, it looks like it might
motivate people to deploy and use it. It is a one-line code change
that a small number of OS developers and content providers could
unilaterally deploy across a significant fraction of all Internet
traffic. However, this encoding does not support auditing so it
would also motivate users and/or applications to misrepresent the
congestion that they are causing [RFC3514]. As a consequence, the
naive encoding is not likely to be trusted and thus creates its own
disincentives for deployment.

Nonetheless, this Naive encoding does present a clear mental model of
how the ConEx protocol might function under various uses. It is
useful for thought experiments where it can be stipulated that all
participants are honest and it does illustrate some of the incentives
that might be introduced by ConEx.

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4.2. Null Encoding

In limited contexts, it is possible to implement ConEx-like functions
without any signals at all by measuring rest-of-path congestion
directly from TCP headers. The algorithm is to keep at least one RTT
of past TCP headers and match each new header against the history to
count duplicate data.

This could implement many ConEx policies, without any explicit
protocol. It is fairly easy to implement, at least at low rate
(e.g., in a software-based edge router). However, it would only be
useful in cases where the network operator can see the TCP headers.
At the time of writing (2014), those cases are the majority of
traffic because UDP, IPsec, and VPN tunnels are used far less than
Secure Socket Layer (SSL) or Transport Layer Security (TLS) over
TCP/IP, which do not hide TCP sequence numbers from network devices.
However, anyone specifically intending to avoid the attention of a
congestion policy device would only have to hide their TCP headers
from the network operator (e.g., by using a VPN tunnel).

4.3. ECN-Based Encoding

The re-ECN specification [RE-ECN-TCP] presents an encoding of ConEx
in IPv4 and IPv6 that was tightly integrated with ECN encoding in
order to fit into the IPv4 header. Any individual packet may need to
represent any ECN codepoint and any ConEx Signal value independently.
So, ideally, their encoding should be entirely independent. However,
given the limited number of header bits and/or codepoints, re-ECN
chooses to partially share codepoints and to re-echo both losses and
ECN with just one codepoint.

The central theme of the re-ECN work is an audit mechanism that
provides sufficient disincentives against misrepresenting congestion
[RE-ECN-MOTIVATION]. It is analyzed extensively in Briscoe's PhD
dissertation [Refb-dis]. For a tutorial background on re-ECN
motivation and techniques, see [Re-fb] and [FairerFaster].

Re-ECN is an example of one chosen set of compromises attempting to
meet the requirements of Section 3. The present document takes a
step back, aiming to state the ideal requirements in order to allow
the Internet community to assess whether different compromises might
be better.

The problem with re-ECN is that it requires that receivers be ECN
enabled in addition to sender changes. Newer encodings
[CONEX-DESTOPT] overcome this problem by being able to represent loss
and ECN-based congestion separately.

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4.4. Independent Bits

This encoding involves flag bits, each of which the sender can set
independently to indicate to the network one of the following four
signals:

ConEx (Not-ConEx): The transport is (or is not) using ConEx with
this packet (network-layer encoding requirement L in Section 3.3
says the protocol must be arranged so that legacy transport
senders implicitly send Not-ConEx).

Re-Echo-Loss (Not-Re-Echo-Loss): The transport has (or has not)
experienced a loss.

Re-Echo-ECN (Not-Re-Echo-ECN): The transport has (or has not)
experienced ECN-signalled congestion.

Credit (Not-Credit): The transport is (or is not) building up
congestion credit (see Section 5.5 on the audit function).

A packet with ConEx set, combined with all the three other flags
cleared, implies ConEx-Not-Marked.

This encoding does not imply any exclusion property among the
signals. Multiple types of congestion (ECN, loss) can be signalled
on the same ACK. So, ideally, a ConEx sender would be able to
reflect these in the next packet. However, there will be many
invalid combinations of flags (e.g., Not-ConEx combined with any of
the ConEx-Marked flags), which a malicious sender could use to
advantage against naive policy devices that only check each flag
separately.

As long as the packets in a flow have uniform sizes, it does not
matter whether the units of congestion are packets or bytes.
However, if an application sends very irregular packet sizes, it may
be necessary for the sender to mark multiple packets to avoid being
in technical violation of an audit function measuring in bytes (see
Section 4.6).

4.5. Codepoint Encoding

This encoding involves signaling one of the following five
codepoints:

ENUM {Not-ConEx, ConEx-Not-Marked, Re-Echo-Loss, Re-Echo-ECN, Credit}

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Each named codepoint has the same meaning as in the encoding using
independent bits in the previous section. The use of any one
codepoint implies the negative of all the others.

Inherently, the semantics of most of the enumerated codepoints are
mutually exclusive. 'Credit' is the only one that might need to be
used in combination with either Re-Echo-Loss or Re-Echo-ECN, but even
that requirement is questionable. It must not be forgotten that the
enumerated encoding loses the flexibility to signal these two
combinations, whereas the encoding with four independent bits is not
so limited. Alternatively, two extra codepoints could be assigned to
these two combinations of semantics. The comment in the previous
section about units also applies.

4.6. Units Implied by an Encoding

The following comments apply generally to all the other encodings.

Congestion can be due to exhaustion of bit-carrying capacity or
exhaustion of packet-processing power. When a packet is discarded or
marked to indicate congestion, there is no easy way to know whether
the lost or marked packet signifies bit congestion or packet
congestion. The above ConEx encodings that rely on marking packets
suffer from the same ambiguity.

This problem is most acute when audit needs to check that one count
of markings matches another. For example, if there are ConEx
markings on three large (1500 B) packets, is that sufficient to match
the loss of five small (60 B) packets? If a packet marking is
defined to mean all the bytes in the packet are marked, then we have
4500 B of ConEx-Marked data against 300 B of lost data, which is
easily sufficient. If instead we are counting packets, then we have
three ConEx packets against five lost packets, which is not
sufficient. This problem will not arise when all the packets in a
flow are the same size, but a choice needs to be made for flows in
which packet sizes vary, such as BGP, SPDY, and some variable-rate
video encoding schemes.

Whether to use bytes or packets is not obvious. For instance, the
most expensive links in the Internet, in terms of cost per bit, are
all at lower data rates, where transmission times are large and
packet sizes are important. In order for a policy to consider wire
time, it needs to know the number of congested bytes. However, high
speed networking equipment and the transport protocols themselves
sometimes gauge resource consumption and congestion in terms of
packets.

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[RFC7141] advises that congestion indications should be interpreted
in units of bytes when responding to congestion, at least on today's
Internet. [RFC6789] takes the same view in its definition of
congestion-volume, again, for today's Internet.

In any TCP implementation, this is simple to achieve for varying size
packets given that TCP SACK tracks losses in bytes. If an encoding
is specified in units of bytes, the encoding should also specify
which headers to include in the size of a packet (see network-layer
requirement F in Section 3.3).

RFC 7141 constructs an argument for why equipment tends to be built
so that the bottleneck will be the bit-carrying capacity of its
interfaces, not its packet-processing capacity. However, RFC 7141
acknowledges that the position may change in future and notes that
new techniques will need to be developed to distinguish packet and
bit congestion.

Given this document describes an abstract ConEx mechanism, it is
intended to be timeless. Therefore, it does not take a strong
position on this issue. However, a ConEx encoding will need to
explicitly specify whether it assumes units of bytes or packets
consistently for both congestion indications and ConEx markings (see
network-layer requirement E in Section 3.3). It may help to refer to
the guidance in [RFC7141].

5. Congestion Exposure Components

The components shown in Figure 1 as well as policy and audit are
described in more detail.

5.1. Network Devices (Not Modified)

Congestion signals originate from network devices as they do today.
A congested router, switch, or other network device can discard or
ECN-mark packets when it is congested.

5.2. Modified Senders

The sending transport needs to be modified to send Congestion
Exposure signals in response to congestion feedback signals (e.g.,
for the case of a TCP transport, see [TCP-MODIFICATION]). We want to
permit ConEx without ECN (e.g., if the receiver does not support
ECN). However, we want to encourage a ConEx sender to at least
attempt to negotiate ECN (a ConEx transport protocol specification
may require this) because it is believed that ConEx without ECN is
harder to audit and thus potentially exposed to cheating. Since
honest users have the potential to benefit from stronger mechanisms

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to manage traffic, they have an incentive to deploy ConEx and ECN
together. This incentive is not sufficient to prevent a dishonest
user from constructing (or configuring) a sender that enables ConEx
after choosing not to negotiate ECN, but it should be sufficient to
prevent this from being the sustained default case for any
significant pool of users.

Permitting ConEx without ECN is necessary to facilitate bootstrapping
other parts of ConEx deployment.

5.3. Receivers (Optionally Modified)

Any receiving transport may already feedback sufficiently useful
signals to the sender so that it does not need to be altered.

The native loss or ECN signaling mechanism required for compliance
with existing congestion control standards (e.g., RTCP, Stream
Control Transmission Protocol (SCTP)) will typically be sufficient
for the Sender to generate ConEx Signals.

TCP's loss feedback is sufficient for ConEx if SACK is used
[RFC2018]. However, the original specification for ECN in TCP
[RFC3168] signals congestion no more than once per round trip. The
sender may require more precise feedback from the receiver otherwise
it is at risk of appearing to be understating its ConEx Signals.

Ideally, ConEx should be added to a transport like TCP without
mandatory modifications to the receiver. But in the TCP-ECN case, an
optional modification to the receiver could be recommended for
precision (see [RFC7560], which is based on the approach originally
taken when adding re-ECN to TCP [RE-ECN-TCP]).

5.4. Policy Devices

Policy devices are characterised by a need to be configured with a
policy related to the users or neighboring networks being served. In
contrast, auditing devices solely enforce compliance with the ConEx
protocol and do not need to be configured with any client-specific
policy.

One of the design goals of the ConEx protocol is that none of the
important policy mechanisms requires per-flow state and that policy
mechanisms can even be implemented for heavily aggregated traffic in
the core of the Internet with complexity akin to accumulating marking
volumes per logical link. Of course, policy mechanisms may sometimes
choose to focus down on individual flows, but ConEx aims to make
aggregate policy devices feasible.

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5.4.1. Congestion Monitoring Devices

Policy devices can typically be decomposed into two functions:
i) monitoring the ConEx Signal to compare it with a policy; then ii)
acting in some way on the result. Various actions might be invoked
against 'out of contract' traffic, such as policing (see
Section 5.4.3), re-routing, or downgrading the class of service.

Alternatively, a policy device might not act directly on the traffic,
but instead report to management systems that are designed to control
congestion indirectly. For instance, the reports might trigger
capacity upgrades, penalty clauses in contracts, levy charges based
on congestion, or merely send warnings to clients who are causing
excessive congestion.

Nonetheless, whatever action is invoked, the congestion monitoring
function will always be a necessary part of any policy device.

5.4.2. Rest-of-Path Congestion Monitoring

ConEx Signals indicate the level of congestion along a whole path
from source to destination. In contrast, ECN signals monitored in
the middle of a network indicate the level of congestion experienced
so far on the path (of course, only in ECN-capable traffic).

If a monitor in the middle of a network (e.g., at a network border)
measures both of these signals, it can subtract the level of ECN
(path so far) from the level of ConEx (whole path) to derive a
measure of the congestion that packets are likely to experience
between the monitoring point and their destination (rest-of-path
congestion).

It will often be preferable for policy devices to monitor rest-of-
path congestion if they can, because it is a measure of the
downstream congestion that the policy device can directly influence
by controlling the traffic passing through it.

5.4.3. Congestion Policers

A congestion policer can be implemented in a very similar way to a
bit-rate policer, but its effect can be focused solely on traffic of
users causing congestion downstream, which ConEx Signals make
visible. Without ConEx Signals, the only way to mitigate congestion
is to blindly limit the traffic bit-rate on the assumption that high
bit-rate is more likely to cause congestion.

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A congestion policer monitors all ConEx traffic entering a network or
some identifiable subset. Using ConEx Signals and/or Credit signals
(and preferably subtracting ECN signals to yield rest-of-path
congestion), it measures the amount of congestion that this traffic
is contributing somewhere downstream. If this persistently exceeds a
policy-configured 'congestion-bit-rate', the congestion policer can
limit all the monitored ConEx traffic.

A congestion policer can be implemented by a simple token bucket
applied to an aggregate. But unlike a bit-rate policer, it removes
tokens only when it forwards packets that are ConEx-Marked,
effectively treating Not-ConEx-Marked packets as invisible.
Consequently, because tokens give the right to send congested bits,
the fill rate of the token bucket will represent the allowed
congestion-bit-rate. This should provide sufficient traffic
management without having to additionally constrain the straight bit-
rate at all. See [ISOLATION-POLICING] for details.

Note that the policing action could be to introduce a throttle
(discard some traffic) immediately upstream of the congestion
monitor. Alternatively, this throttle could introduce delay using a
queue with its own AQM, which potentially increases the whole path
congestion. In effect, the congestion policer has moved the
congestion earlier in the path and focused it on one user to protect
downstream resources by reducing the congestion in the rest of the
path.

5.5. Audit

The most critical aspect of ConEx is the capability to support robust
auditing. It can be assumed that sanctions based on ConEx Signals
will create an intrinsic motivation for users to understate the
congestion that they are causing. So, without strong audit
functions, the ConEx Signal would become understated to the point of
being useless. Therefore, the most important feature of an encoding
design is likely to be the robustness of the auditing it supports.

The general goal of an auditor is to make sure that any ConEx-enabled
traffic is sent with sufficient ConEx-Re-Echo and ConEx-Credit
signals. A concrete definition of the ConEx protocol MUST define
what sufficient means.

If a ConEx-enabled transport does not carry sufficient ConEx Signals,
then an auditor is likely to apply some sanction to that traffic.
Although sanctions are beyond the scope of this document, an example
sanction might be to throttle the traffic immediately upstream of the

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auditor to prevent the user from getting any advantage by
understating congestion. Such a throttle would likely include some
combination of delaying or dropping traffic.

A ConEx auditor might use one of the following techniques:

Generic loss auditing: For congestion signalled by loss, totally
accurate auditing is not believed to be possible in the general
case because it involves a network node detecting the absence of
some packets when it cannot always necessarily identify
retransmissions or missing packets. The missing packet might
simply be taking a different route, or the IP payload may be
encrypted.

It is for this reason that it is desirable to motivate the
deploying of ECN, even though ECN is not strictly required for
ConEx.

ECN auditing: Directly observe and compare the volume of ECN and
ConEx marks. Since the volume of ECN marks rises monotonically
along a path, ECN auditing is most accurate when located near the
transport receiver. For this reason, ECN should be monitored
downstream of the predominant bottleneck.

TCP-specific loss auditing: For non-encrypted standard TCP traffic
on a single path, a tactical audit approach could be to measure
losses by detecting retransmissions, which appear as duplicate
sequence numbers upstream of the loss and out of order data
downstream of the loss. Since some reordering is present in the
Internet, such a loss estimator would be most accurate near the
sender. Such an audit device should treat non-ECN-capable packets
with encrypted IP payload as Not-ConEx, even if they claim to be
ConEx-capable, unless the operator is also using one of the other
two techniques below that can audit such packets against losses.

Predominant bottleneck loss auditing: For networks designed so that
losses predominantly occur under the control of one IP-aware
bottleneck node on the path, the auditor could be located at this
bottleneck. It could simply compare ConEx Signals with actual
local packet discards (and ECN marks). This is a good model for
most consumer access networks where audit accuracy could well be
sufficient even if losses occasionally occur elsewhere in the
network.

Although the auditor at the predominant bottleneck would not be
able to count losses at other nodes, transports would not know
where losses were occurring either. Therefore, a transport would

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not know which losses it could cheat and which ones it couldn't
without getting caught.

ECN tunnel loss auditing: A network operator can arrange IP-in-IP
tunnels (or IP-in-MPLS, etc.) so that any losses within the
tunnels are deferred until the tunnel egress. Then, the audit
function can be deployed at the egress and be aware of all losses.
This is possible by enabling ECN marking on switches and routers
within a tunnel, irrespective of whether end systems support ECN,
by exploiting a side effect of the way tunnels handle the ECN
field. After encapsulation at the tunnel ingress, the network
should arrange for any non-ECN packets (with '00' in the ECN field
of the outer) to be set to the ECN-capable transport (ECT(0))
codepoint. Then, if they experience congestion at one of the ECN-
capable switches or routers within the tunnel, some will be ECN-
marked rather than immediately dropped. However, when the tunnel
decapsulator strips the outer from such an ECN-marked packet, if
it finds the inner header has '00' in the ECN field (meaning that
the endpoints do not support ECN), it will automatically drop the
packet, assuming it complies with [RFC6040]. Thus, an audit
function at the decapsulator can know which packets would have
been dropped within the tunnel (and even which are genuinely ECN-
marked for the end-to-end protocol). Non-ECN end systems outside
the tunnel see no sign of the use of ECN internally.

In addition, other audit techniques may be identified in the future.

[Refb-dis] gives a comprehensive inventory of attacks against audit
proposed by various people. It includes pseudocode for both
deterministic and statistical audit functions designed to thwart
these attacks and analyses the effectiveness of an implementation.
Although this work is specific to the re-ECN protocol, most of the
material is useful for designing and assessing audit of other
specific ConEx encodings, against both ECN and loss.

The auditing function should be able to trigger sufficient sanction
to discourage understating congestion [Salvatori05]. This seems to
require designing the sanction in concert with the policy functions,
even though they might be implemented in different parts of the
network. However, [Refb-dis] proves audit and policy functions can
be independent as long as audit drops sufficient traffic to
'normalise' actual congestion signals to be no greater than ConEx
Signals.

Similarly, the job of incentivising the sending of ConEx-enabled
packets is proper solely to policy devices independent of the audit
function. The audit function's job is policy neutral, so it should
be solely confined to checking for correctness within those packets

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that have been marked as ConEx-capable. Even if there are Not-ConEx
packets mixed with ConEx packets within a flow, audit will not need
to monitor any Not-ConEx packets.

Note that in the future it might prove to be desirable to provide
advice on uniformly implementing sanctions, because otherwise
insufficient sanctions could impair the ability to implement policy
elsewhere in the network.

Some of the audit algorithms require per-flow state. This cost is
expected to be tolerable because these techniques are most apropos
near the edges of the network where traffic is generally much less
aggregated so the state need not overwhelm any one device. The flow
state required for the audit creates itself as it detects new flows.
Therefore, a flow will not fail if it is re-routed away from the
audit box currently holding its flow state, so auditing does not
require route pinning and works fine with multipath flows.

Holding flow state seems to create a vulnerability to attacks that
exhaust the auditor's memory by opening numerous new short flows.
The audit function can protect itself from this attack by not
allocating new flow state unless a ConEx-Marked packet arrives (e.g.,
credit at the start of a flow). Because policy devices rate limit
ConEx-Marked packets, this sets a natural limit to the rate at which
a source can create flow state in audit devices. The auditor would
treat all the remaining flows without any ConEx-Marked packets as a
single misbehaving aggregate.

Auditing can be distributed and redundant. One flow may be audited
in multiple places, using multiple techniques. Some audit techniques
do not require any per-flow state and can be applied to aggregate
traffic. These might be able to detect the presence of understated
congestion at large scale and support recursively hunting for
individual flows that are understating their congestion. Even at
large scales, flows can be randomly selected for individual auditing.

Sampling techniques can also be used to bound the total auditing
memory footprint, although the implementer needs to counter the
tactic where a source cheats until caught by sampling, then simply
discards that flow ID and starts cheating with a new one (termed
'identifier whitewashing when caught').

For the concrete ConEx protocol encoding defined in [CONEX-DESTOPT],
ConEx Credit and ConEx-Re-Echo signals are intended to be audited
separately. The Credit signal can be audited directly against actual
congestion (loss and ECN). However, there will be an inherent delay
of at least one round trip between a congestion signal and the
subsequent ConEx-Re-Echo signal it triggers, as shown in Figure 1.

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Therefore, ConEx-Re-Echo signals will need to be audited with some
allowance for this delay. Further discussion of design and
implementation choices for functions intended to audit this concrete
ConEx encoding can be found in [CONEX-AUDIT].

6. Support for Incremental Deployment

The ConEx abstract protocol described so far is intended to support
incremental deployment in every possible respect. For convenience,
the following list collects together all the features that support
incremental deployment in the concrete ConEx specifications and
points to further information on each:

Packets: The wire protocol encoding allows each packet to indicate
whether it is using ConEx or not (see Section 4 on
Encoding Congestion Exposure).

Senders: ConEx requires a modification to the source in order to
send ConEx packet markings (see Section 5.2). Although ConEx
support can be indicated on a packet-by-packet basis, it is likely
that all the packets in a flow will either consistently support
ConEx or consistently not. It is also likely that, if the
implementation of a transport protocol supports ConEx, all the
packets sent from that host using that protocol will be ConEx-
Capable.

The implementations of some of the transport protocols on a host
might not support ConEx (e.g., the implementation of DNS over UDP
might not support ConEx, while perhaps RTP over UDP and TCP will).
Any non-upgraded transports and non-upgraded hosts will simply
continue to send regular Not-ConEx packets as always.

A network operator can create incentives for senders to
voluntarily reveal ConEx information (see the item on incremental
deployment by 'Networks' below).

Receivers: A ConEx source should be able to work with the regular
receiver for the transport in question without requiring any
ConEx-specific modifications. This is true for modern transport
protocols (RTCP, SCTP, etc.) and it is even true for TCP, as long
as the receiver supports SACK, which is widely deployed anyway.
However, it is not true for ECN feedback in TCP. The need for
more precise ECN feedback in TCP is not exclusive to ConEx; for
instance, Data Centre TCP [DCTCP] uses precise feedback to good
effect. Therefore, if a receiver offers precise feedback,
[RFC7560] it will be best if ConEx uses it (see Section 5.3).

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Alternatively, without sufficiently precise congestion feedback
from the receiver, the source may have to conservatively send
extra ConEx markings in order to avoid understating congestion.

Proxies: Although it was stated above that ConEx requires a
modification to the source, ConEx Signals could theoretically be
introduced by a proxy for the source as long as it can intercept
feedback from the receiver. Similarly, more precise feedback
could theoretically be provided by a proxy for the receiver rather
than modifying the receiver itself.

Forwarding: No modification to forwarding or queuing is needed for
ConEx.

However, once some ConEx is deployed, it is possible that a queue
implementation could optionally take advantage of the ConEx
information in packets. For instance, it has been suggested
[CONEX-DESTOPT] that a queue would be more robust against flooding
if it preferentially discarded Not-ConEx packets then Not-Marked
ConEx packets.

A ConEx sender re-echoes congestion whether the queues signaling
congestion are ECN enabled or not. Nonetheless, an operator
relying on ConEx Signals is recommended to enable ECN in queues
wherever possible. This is because auditing works best if most
congestion is indicated by ECN rather than loss (see Section 3).
Also, monitoring rest-of-path congestion is not accurate if there
are congested non-ECN queues upstream of the monitoring point
(Section 5.4.2).

Networks: If a subset of traffic sources (or proxies) use ConEx
Signals to reveal congestion in the internetwork layer, a network
operator can choose (or not) to use this information for traffic
management. As long as the end-to-end ConEx Signals are present,
each network can unilaterally choose to use them -- independently
of whether other networks do.

ConEx marked packets may safely traverse a network that ignores
them. ConEx Signals are defined to remain unchanged once set by
the sender, but some encodings may allow changes in transit (e.g.,
by proxies). In no circumstances will a network node change
ConEx-Capable packets to Not-ConEx (network-layer encoding
requirement I in Section 3.3). If necessary, endpoints should be
able to detect if a network is removing ConEx Signals (network-
layer encoding requirement H in Section 3.3).

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An operator can deploy policy devices (Section 5.4) wherever
traffic enters its network in order to monitor the downstream
congestion that incoming traffic contributes to and control it if
necessary. A network operator can create incentives for the
developers of sending applications and transports to voluntarily
reveal ConEx information. Without ConEx information, a network
operator tends to have to limit the bit-rate or volume from a site
more than is necessary, just in case it might congest others.
With ConEx information, the operator can solely limit congestion-
causing traffic and otherwise allow complete freedom. This
greater freedom acts as an inducement for the source to volunteer
ConEx information. An operator may also monitor whether a source
transport has sent ConEx packets and treat the same transport with
greater suspicion (e.g., a more stringent rate limit) whenever it
selectively sends packets without ConEx support. See [RFC6789]
for further discussion of deployment incentives for networks and
references to scenarios where some networks use ConEx-based policy
devices and others don't.

An operator can deploy audit devices (Section 5.5) unilaterally
within its own network to verify that traffic sources are not
understating ConEx information. From the viewpoint of one network
operator (say N_a), it only cares that the level of ConEx
signaling is sufficient to cover congestion in its own network.
If traffic continues into a congested downstream network (say
N_b), it is of no concern to the first network (N_a) if the end-
to-end ConEx signaling is insufficient to cover the congestion in
N_b as well. This is N_b's concern, and N_b can both detect such
anomalous traffic and deal with it using ConEx-based audit devices
itself.

7. Security Considerations

The only known risk associated with ConEx is that users and
applications are very likely to be motivated to underrepresent the
congestion that they are causing. Significant portions of this
document are about mechanisms to audit the ConEx Signals and create
sufficient sanction to inhibit such underrepresentation. In
particular, see Section 5.5.

Security attacks and their defences are best discussed against a
concrete protocol specification, not the abstract mechanism of this
document. A concrete ConEx protocol will need to be accompanied by a
document describing how the protocol and its audit mechanisms defend
against likely attacks. [Refb-dis] will be a useful source for such
a document. It gives a comprehensive inventory of attacks against
audit that have been proposed by various parties. It includes

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pseudocode for both deterministic and statistical audit functions
designed to thwart these attacks and analyses the effectiveness of an
implementation.

However, [Refb-dis] is specific to the re-ECN protocol, which
signalled ECN and loss together, whereas the concrete ConEx protocol
defined in [CONEX-DESTOPT] signals them separately. Therefore,
although likely attacks will be similar, there will be more
combinations of attacks to worry about, and defences and their
analysis are likely to be a little different for ConEx.

The main known attacks that a security document for a concrete ConEx
protocol will need to address are listed below and [Refb-dis] should
be referred to for how re-ECN was designed to defend against similar
attacks:

o Attacks on the audit function (see Section 7.5 of [Refb-dis]):

Flow ID Whitewashing: Designing the audit function so that a
source cannot gain from starting a new flow once audit has
detected cheating in a previous flow.

Dragging Down an Aggregate: Avoiding audit discarding packets
from all flows within an aggregate, which would allow one flow
to pull down the average so that the audit function would
discard packets from all flows, not just the offending flow.

Dragging Down a Spoofed Flow ID: An attacker understates ConEx
markings in packets that spoof another flow, which fools the
audit function into dropping the genuine user's packets.

o Attacks by networks on other networks (see Section 8.2 of
[Refb-dis]):

Dummy Traffic: Sending dummy traffic across a border with
understated ConEx markings to bring down the average ConEx
markings in the aggregate of border traffic. This attack can
be combined with a TTL that expires before the packets reach an
audit function.

Signal Poisoning with 'Cancelled' Marking: Sending high volumes
of valid packets that are both ConEx-Marked and ECN-marked,
which seems to represent congestion upstream, but it makes
these packets immune to being further ECN-marked downstream.

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It is planned to document all known attacks and their defences
(including all of the above) in the RFC series against a concrete
ConEx protocol specification. In the interim, [Refb-dis] and its
references should be referred to for details and ways to address
these attacks in the case of re-ECN.

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

8.2. Informative References

[CheapPseud]
Friedman, E. and P. Resnick, "The Social Cost of Cheap
Pseudonyms", Journal of Economics and Management Strategy,
Volume 10, Issue 2, pp. 173-199,
DOI 10.1111/j.1430-9134.2001.00173.x, Summer 2001.

[CONEX-AUDIT]
Wagner, D. and M. Kuehlewind, "Auditing of Congestion
Exposure (ConEx) signals", Work in Progress,
draft-wagner-conex-audit-01, February 2014.

[CONEX-DESTOPT]
Krishnan, S., Kuehlewind, M., and C. Ucendo, "IPv6
Destination Option for Congestion Exposure (ConEx)", Work
in Progress, draft-ietf-conex-destopt-11, October 2015.

[DCTCP] Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel,
P., Prabhakar, B., Sengupta, S., and M. Sridharan, "Data
Center TCP (DCTCP)", ACM SIGCOMM Computer Communication
Review, Volume 40, Issue 4, pages 63-74,
DOI 10.1145/1851182.1851192, October 2010,
<http://portal.acm.org/citation.cfm?id=1851192>.

[Evol_cc] Gibbens, R. and F. Kelly, "Resource pricing and the
evolution of congestion control", Automatica, Volume 35,
Issue 12, pages 1969-1985,
DOI 10.1016/S0005-1098(99)00135-1, December 1999,
<http://www.sciencedirect.com/science/article/pii/
S0005109899001351>.

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RFC 7713 ConEx Concepts and Abstract Mechanism December 2015

[FairerFaster]
Briscoe, B., "A Fairer, Faster Internet Protocol", IEEE
Spectrum, pages 38-43, DOI 10.1109/MSPEC.2008.4687368,
December 2008,
<http://spectrum.ieee.org/telecom/standards/
a-fairer-faster-internet-protocol>.

[ISOLATION-POLICING]
Briscoe, B., "Network Performance Isolation using
Congestion Policing", Work in Progress,
draft-briscoe-conex-policing-01, February 2014.

[RE-ECN-MOTIVATION]
Briscoe, B., Jacquet, A., Moncaster, T., and A. Smith,
"Re-ECN: A Framework for adding Congestion Accountability
to TCP/IP", Work in Progress,
draft-briscoe-conex-re-ecn-motiv-03, March 2014.

[RE-ECN-TCP]
Briscoe, B., Jacquet, A., Moncaster, T., and A. Smith,
"Re-ECN: Adding Accountability for Causing Congestion to
TCP/IP", Work in Progress,
draft-briscoe-conex-re-ecn-tcp-04, July 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 Computer Communication Review, Volume 35,
Issue 4, pages 277--288, DOI 10.1145/1090191.1080124,
August 2005,
<http://portal.acm.org/citation.cfm?id=1080091.1080124>.

[Refb-dis] Briscoe, B., "Re-feedback: Freedom with Accountability for
Causing Congestion in a Connectionless Internetwork", PhD
Dissertation, University College London, May 2009,
<http://discovery.ucl.ac.uk/16274/>.

[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996,
<http://www.rfc-editor.org/info/rfc2018>.

[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<http://www.rfc-editor.org/info/rfc3168>.

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RFC 7713 ConEx Concepts and Abstract Mechanism December 2015

[RFC3514] Bellovin, S., "The Security Flag in the IPv4 Header",
RFC 3514, DOI 10.17487/RFC3514, April 2003,
<http://www.rfc-editor.org/info/rfc3514>.

[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.

[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 5348, DOI 10.17487/RFC5348, September 2008,
<http://www.rfc-editor.org/info/rfc5348>.

[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<http://www.rfc-editor.org/info/rfc5681>.

[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC6040, November
2010, <http://www.rfc-editor.org/info/rfc6040>.

[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
2012, <http://www.rfc-editor.org/info/rfc6679>.

[RFC6789] Briscoe, B., Ed., Woundy, R., Ed., and A. Cooper, Ed.,
"Congestion Exposure (ConEx) Concepts and Use Cases",
RFC 6789, DOI 10.17487/RFC6789, December 2012,
<http://www.rfc-editor.org/info/rfc6789>.

[RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
"Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
DOI 10.17487/RFC6817, December 2012,
<http://www.rfc-editor.org/info/rfc6817>.

[RFC7141] Briscoe, B. and J. Manner, "Byte and Packet Congestion
Notification", BCP 41, RFC 7141, DOI 10.17487/RFC7141,
February 2014, <http://www.rfc-editor.org/info/rfc7141>.

[RFC7560] Kuehlewind, M., Ed., Scheffenegger, R., and B. Briscoe,
"Problem Statement and Requirements for Increased Accuracy
in Explicit Congestion Notification (ECN) Feedback",
RFC 7560, DOI 10.17487/RFC7560, August 2015,
<http://www.rfc-editor.org/info/rfc7560>.

Mathis & Briscoe Informational [Page 29]

RFC 7713 ConEx Concepts and Abstract Mechanism December 2015

[Salvatori05]
Salvatori, A., "Closed Loop Traffic Policing", Politecnico
Torino and Institut Eurecom Masters Thesis, September
2005.

[TCP-MODIFICATION]
Kuehlewind, M. and R. Scheffenegger, "TCP modifications
for Congestion Exposure", Work in Progress, draft-ietf-
conex-tcp-modifications-10, October 2015.

Acknowledgments

This document was improved by review comments from Toby Moncaster,
Nandita Dukkipati, Mirja Kuehlewind, Caitlin Bestler, Marcelo Bagnulo
Braun, John Leslie, Ingemar Johansson, and David Wagner.

Bob Briscoe's work on this specification received part-funding from
the European Union's Seventh Framework Programme FP7/2007-2013 under
the Trilogy 2 project, grant agreement no. 317756. The views
expressed here are solely those of the authors.

Authors' Addresses

Matt Mathis
Google, Inc.
1600 Amphitheater Parkway
Mountain View, California 93117
United States

Email: [email protected]

Bob Briscoe
BT (now at Simula Research Laboratory)

Email: [email protected]
URI: http://bobbriscoe.net/

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