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00 01 02 03 04 05 06 07 08 09 10 11
12 13 RFC 7713
Congestion Exposure (ConEx) Working M. Mathis
Group Google, Inc
Internet-Draft B. Briscoe
Intended status: Informational BT
Expires: April 25, 2013 October 22, 2012
Congestion Exposure (ConEx) Concepts and Abstract Mechanism
draft-ietf-conex-abstract-mech-06
Abstract
This document describes an abstract mechanism by which senders inform
the network about the congestion encountered by packets earlier in
the same flow. Today, network elements at any layer may signal
congestion to the receiver by dropping packets or by 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 "ConEx Concepts
and Use Cases" provides the entry-point to the set of ConEx
documentation.
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-
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 April 25, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . . . 7
3. Requirements for the ConEx Abstract Mechanism . . . . . . . . 7
3.1. Requirements for ConEx Signals . . . . . . . . . . . . . . 7
3.2. Requirements for the Audit Function . . . . . . . . . . . 8
3.3. Requirements for non-abstract ConEx specifications . . . . 9
4. Encoding Congestion Exposure . . . . . . . . . . . . . . . . . 10
4.1. Naive Encoding . . . . . . . . . . . . . . . . . . . . . . 11
4.2. Null Encoding . . . . . . . . . . . . . . . . . . . . . . 11
4.3. ECN Based Encoding . . . . . . . . . . . . . . . . . . . . 12
4.4. Independent Bits . . . . . . . . . . . . . . . . . . . . . 12
4.5. Codepoint Encoding . . . . . . . . . . . . . . . . . . . . 13
4.6. Units Implied by an Encoding . . . . . . . . . . . . . . . 13
5. Congestion Exposure Components . . . . . . . . . . . . . . . . 14
5.1. Network Devices (Not modified) . . . . . . . . . . . . . . 14
5.2. Modified Senders . . . . . . . . . . . . . . . . . . . . . 14
5.3. Receivers (Optionally Modified) . . . . . . . . . . . . . 15
5.4. Policy Devices . . . . . . . . . . . . . . . . . . . . . . 15
5.4.1. Congestion Monitoring Devices . . . . . . . . . . . . 15
5.4.2. Rest-of-Path Congestion Monitoring . . . . . . . . . . 16
5.4.3. Congestion Policers . . . . . . . . . . . . . . . . . 16
5.5. Audit . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.5.1. Using Credit to Simplify Audit . . . . . . . . . . . . 19
6. Support for Incremental Deployment . . . . . . . . . . . . . . 20
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
10. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 23
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
11.1. Normative References . . . . . . . . . . . . . . . . . . . 24
11.2. Informative References . . . . . . . . . . . . . . . . . . 24
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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.
A companion document [I-D.ietf-conex-concepts-uses] provides the
entry point to the set of ConEx documentation. It outlines concepts
that are pre-requisites 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. ConEx
is only concerned with ECN markings and packet losses, because they
are unambiguous signals of congestion.
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
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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 encoding 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. They are described in
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-------->| |
| | | | / | |
| |_____________| |_______________ / | |
| | | | |/ | |
`---------' `-----------' ' `---------'
Not shown are policy devices that use the ConEx Signal to monitor or
manage traffic and audit devices to monitor the accuracy of ConEx
signals. These devices might be anywhere along the forward path.
The are discussed in detail in Section 5.4 and Section 5.5,
respectively.
Figure 1
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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. It is important to be able to audit ConEx signals, and to
be able 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])
is 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
[I-D.ietf-conex-concepts-uses]. Note that volume per unit time is
(average) rate.
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 be implemented for heavily aggregated traffic in the
core of the Internet with complexity akin to accumulating marking
volumes per logical link. Ideally it would also be possible to audit
ConEx signals without per flow state, however this is not always
possible. Since auditing can be done near the edges of the network
where traffic is less aggregated, per flow state is more easily
tolerated. Also, the flow-state required for 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.
Flow-state for auditing is discussed further in Section 5.5. In
summary: i) flow state for auditing does not require route pinning;
ii) auditing at the edges, with limited per flow state, enables
policy in the core, without any per flow state.
There is a long standing argument over units of congestion: bytes vs
packets (see [I-D.ietf-tsvwg-byte-pkt-congest] and its references).
This document does not take a strong position on this issue.
However, we make the following observations: the most expensive links
in the Internet, in terms of cost per bit, are all at lower data
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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, which may
prove to be problematic for application protocols that have irregular
packet sizes, such as BGP, SPDY and some variable rate video encoding
schemes. 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 must have the property that the
relevant stakeholders each have the incentive to unilaterally start
on each stage of partial deployment, which in turn creates 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, 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 LEDBAT
[I-D.ietf-ledbat-congestion] (see [I-D.ietf-conex-concepts-uses]).
Ultimately ConEx signals have the potential to provide a mechanism to
regulate global Internet congestion. From the earliest days of
congestion control research 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 application and of transport protocols to be more
mindful about contributing to congesting.
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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 ConEx-
capable.
ConEx-Capable: The transport is ConEx-Capable. This is the opposite
of Not-ConEx.
ConEx Signal: 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 experienced an ECN mark.
Credit: The transport is building up credit to allow for any
future delay in expected ConEx signals (see Section 5.5.1)
ConEx-Not-Marked: The transport is ConEx-capable but is signaling
none of Re-Echo-Loss, Re-Echo-ECN or Credit.
ConEx-Marked: At least one of Re-Echo-Loss, Re-Echo-ECN or Credit.
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. 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 pre-existing 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,
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.
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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 RTCP [RFC3550] for example). This
delay complicates auditing, and SHOULD be minimized.
d. The ConEx signal SHOULD be accurate and auditable. The general
approach is to observe the volume of congestion signals and ConEx
signals on the forward data path and verify that the ConEx
signals do not under-represent the congestion signals (see
Section 5.5). The simplest mechanism to compensate for the round
trip delay between the signals is for the sender to include a
"credit" signal to cover the yet to be observed congestion that
might occur during this delay. (see Section 5.5.1 for details).
Furthermore, the ConEx signals for packet loss and ECN marking
SHOULD have distinct encodings because they are likely to require
different auditing techniques.
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. Requirements for the Audit Function
The role and constraints on the audit function 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;
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;
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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 audit over-
punishes, attackers will find ways to harness it into amplifying
attacks on others. Ideally 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 code points;
B. An inventory of invalid combinations of flags or invalid
codepoints in the encoding. Whether security gateways should
normalise, discard or ignore such invalid encodings, and what
values they should be considered equivalent to by ConEx-aware
elements;
C. An inventory of any conflated signals or any other effects
that are known to compromise signal integrity;
D. A specification for signal units (bytes vs packets, etc), any
approximations allowed and algorithms to do any implied
conversions or accounting;
E. If the units are bytes a definition of which headers are
included in the size of the packet;
F. How tunnels should propagate the ConEx encoding;
G. Whether the encoding fields are mutable or not, to ensure that
header authentication, checksum calculation, etc. process them
correctly.
H. 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)
I. Definition of any extensibility;
J. Backward and forward compatibility and potential migration
strategies;
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K. Any (optional) modification to data-plane forwarding dependent
on the encoding (e.g. preferential discard, interaction with
Diffserv, ECN etc.);
L. Any warning or error messages relevant to the encoding.
Note regarding item H 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 new
connection.
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 SACK
capability);
D. A specification of any configuration that a ConEx stack may
require (or preferably confirmation that it requires no
configuration);
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).
The possibility exists that these specifications over constrain 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.
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
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[I-D.ietf-conex-destopt] had to make compromises on tunnelling.
Rather than making these engineering choices prematurely, this
document side steps 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 code points.
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 signaled 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.
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 matching each new header against the history
to count duplicate data.
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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. This is
currently (2012) the vast majority of traffic because UDP, IPSEC and
VPN tunnels are used far less than SSL or 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 [I-D.briscoe-conex-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. ConEx and ECN are
orthogonal signals in the sense that any individual packet may need
to represent any one of the 4 possible combinations of signal values.
Ideally their encoding should be entirely independent. However,
given the limited number of header bits and/or code points, re-ECN
chooses to partially share code points 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
[I-D.briscoe-conex-re-ecn-motiv]. It is analyzed extensively in
Briscoe's PhD dissertation [Refb-dis]. For a tutorial background on
re-ECN motivation and techniques, see [Re-fb, 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
[I-D.ietf-conex-destopt] overcome this problem by being able to
represent loss and ECN based congestion separately.
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:
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ConEx (Not-ConEx) The transport is (or is not) using ConEx with this
packet (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-signaled congestion
Credit (Not-Credit) The transport is (or is not) building up
congestion credit (see Section 5.5 on the audit function)
This encoding does not imply any exclusion property among the
signals. Multiple types of congestion (ECN, loss) can be signalled
on the same ACK. However, there will be many invalid combinations of
flags (e.g. Not-ConEx combined with any of the ConEx-marked flags),
which could be used 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 the audit function.
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}
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.
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Congestion can be due to exhaustion of bit-carry 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 (1500B) packets, is that sufficient to match the loss
of 5 small (60B) packets? If a packet-marking is defined to mean all
the bytes in the packet are marked, then we have 4500B of Conex
marked data against 300B of lost data, which is easily sufficient.
If instead we are counting packets, then we have 3 ConEx packets
against 5 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.
Therefore a ConEx encoding SHOULD explicity specify whether it
assumes units of bytes or packets for both congestion indications and
ConEx markings.
[I-D.ietf-tsvwg-byte-pkt-congest] advises that congestion indications
SHOULD be interpreted in units of bytes when responding to
congestion, at least on today's Internet. In any TCP implementation
this is simple to achieve for varying size packets, given 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.
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 [I-D.ietf-conex-tcp-modifications]).
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
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least attempt to negotiate ECN (a ConEx transport protocol spec 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
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 is 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.
If the transport receiver does not support ECN, then it's native loss
signaling mechanism (required for compliance with existing congestion
control standards) will be sufficient for the Sender to generate
ConEx signals.
A traditional ECN implementation (RFC 3168 for TCP) 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 an optional
modification to the receiver could be recommended for precision (see
[I-D.kuehlewind-tcpm-accurate-ecn]). This was the approach taken
when adding re-ECN to TCP [I-D.briscoe-conex-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.
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
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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 traffic bit-rate, on the assumption that high
bit-rate is more likely to cause congestion.
A congestion policer monitors all ConEx traffic entering a network,
or some identifiable subset. Using ConEx 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.
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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 [CongPol] for details.
Note that the policing action is to introduce a throttle (delay
through traffic) immediately upstream of the congestion policer.
This throttle could include 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 there will be an intrinsic
motivation for users to understate the congestion that they are
causing. Without strong audit functions the ConEx signal is likely
to become inaccurate to the point of being useless. The most
important feature of an encoding design is likely to be the
robustness of the auditing it supports.
The general approach is to compare the volume of ConEx signals to
direct measures of actual congestion volume. The credit approach
described in Section 5.5.1 can be used to guarantee that this is a
strict bound: if the actual congestion exceeds the ConEx signal, then
some congestion was understated and some sanction should be applied
to the traffic. Although sanctions are beyond the scope of this
document, an example sanction might be to throttle the traffic
immediately upstream of the 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:
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.
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TCP-specific loss auditing: For non-encrypted standard TCP traffic
on a single path, an auditor could 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.
Predominant bottleneck loss auditing: For networks designed so that
losses predominantly occur due to Active Queue Management under
the control of one IP-aware 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
not know which losses it could cheat and which ones it couldn't
without getting caught.
Generic loss auditing: For congestion signaled 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 necessarily identify retransmissions
or missing packets. Furthermore the missing packet might simply
be taking a different route.
It is for this reason that it is desirable to motivate the
deploying of ECN, even though ECN is not strictly required for
ConEx.
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
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signals. Note that in the future it might prove to be desirable to
provide advice on uniformly implementing sanctions, because otherwise
insufficient sanctions impairs 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.
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.
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 must be wary of
"identifier white washing when caught" tactics where a source cheats
until caught by sampling, then simply discards that flow ID and
starts cheating with a new one.
5.5.1. Using Credit to Simplify Audit
At the audit function, there will be an inherent delay of at least
one round trip between a congestion signal and the subsequent ConEx
signal it triggers, as shown in Figure 1. However, the audit
function cannot be expected to wait for a round trip to check that
one signal balances the other, because that requires excessive state
and the auditor can't easily determine the RTT of each flow.
The simplest mechanism to compensate for the round trip delay between
the signals is to have the sender include a "credit" signal to cover
the yet to be observed congestion that might occur during this delay.
The transport signals sufficient credit in advance to cover
congestion expected during its feedback delay. Then, the audit
function does not need to make allowance for round trip delays that
it cannot quantify. This design choice correctly makes the transport
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responsible for both minimizing feedback delay and for the risk that
packets in flight will cause congestion to others before the source
can react.
Making the source responsible for allowing for the round trip delay
in ConEx signals is a design choice that needs to be consistently
applied. Any such requirement SHOULD be specified in a particular
ConEx encoding specification.
For example, imagine the audit function keeps a running account of
the balance between actual congestion signals (loss or ECN), which it
counts as negative, and ConEx signals, which it counts as positive.
Having made the transport responsible for round trip delays, it will
be expected to have pre-loaded the audit function with some credit at
the start. Therefore, if the balance ever goes negative, the audit
function can immediately start punishing a flow, without any grace
period.
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 of ConEx that
support incremental deployment, 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
marked.
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. 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
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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.
Receivers: A ConEx source should be able to work without a modified
receiver. However, without sufficiently precise congestion
feedback from the receiver, the source may have to conservatively
send extra ConEx markings in order to avoid understating
congestion. The need for more precise receiver feedback is not
exclusive to ConEx, for instance Data Centre TCP (DCTCP [DCTCP])
uses precise feedback to good effect. Nonetheless, if a receiver
offers precise feedback, [I-D.kuehlewind-tcpm-accurate-ecn] it
will be best if ConEx uses it (see Section 5.3).
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 thoretically 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 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
[I-D.briscoe-conex-re-ecn-tcp] 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, 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. Networks MUST NOT change ConEx marked packets to Not-ConEx.
If necessary, endpoints SHOULD be able to detect if a network is
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removing ConEx signals.
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. See [I-D.ietf-conex-concepts-uses] for further
discussion of deployment incentives for networks and 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 policy
devices (Section 5.4).
7. IANA Considerations
This memo includes no request to IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
8. Security Considerations
The only known risk associated with ConEx is that users and
applications are very likely to be motivated to under-represent 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 under-representation. 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
pseudocode for both deterministic and statistical audit functions
designed to thwart these attacks and analyses the effectiveness of an
implementation.
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However, [Refb-dis] is specific to the re-ECN protocol, which
signalled ECN & loss together, whereas ConEx 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.
It is planned to document all known attacks and their defences
(including all the above) in the RFC series. In the interim,
[Refb-dis] and its references should be referred to for details and
ways to address these attacks.
9. Acknowledgements
This document was improved by review comments from Toby Moncaster,
Nandita Dukkipati, Mirja Kuehlewind, Caitlin Bestler and John Leslie.
10. Comments Solicited
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF Congestion Exposure (ConEx) working group
mailing list <conex@ietf.org>, and/or to the authors.
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11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use
in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119,
March 1997.
11.2. Informative References
[CheapPseud] Friedman, E. and P. Resnick, "The
Social Cost of Cheap Pseudonyms",
Journal of Economics and
Management Strategy 10(2)173--
199, 1998.
[CongPol] Jacquet, A., Briscoe, B., and T.
Moncaster, "Policing Freedom to
Use the Internet Resource Pool",
Proc ACM Workshop on Re-
Architecting the Internet
(ReArch'08) , December 2008, <htt
p://bobbriscoe.net/projects/
refb/#polfree>.
[DCTCP] Alizadeh, M., Greenberg, A.,
Maltz, D., Padhye, J., Patel, P.,
Prabhakar, B., Sengupta, S., and
M. Sridharan, "Data Center TCP
(DCTCP)", ACM SIGCOMM
CCR 40(4)63--74, October 2010, <h
ttp://portal.acm.org/
citation.cfm?id=1851192>.
[Evol_cc] Gibbens, R. and F. Kelly,
"Resource pricing and the
evolution of congestion control",
Automatica 35(12)1969--1985,
December 1999, <http://
www.statslab.cam.ac.uk/~frank/
evol.html>.
[FairerFaster] Briscoe, B., "A Fairer, Faster
Internet Protocol", IEEE
Spectrum Dec 2008:38--43,
December 2008, <http://
bobbriscoe.net/projects/
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refb/#fairfastip>.
[I-D.briscoe-conex-re-ecn-motiv] Briscoe, B., Jacquet, A.,
Moncaster, T., and A. Smith, "Re-
ECN: A Framework for adding
Congestion Accountability to
TCP/IP", draft-briscoe-conex-re-
ecn-motiv-00 (work in progress),
April 2012.
[I-D.briscoe-conex-re-ecn-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-00
(work in progress), April 2012.
[I-D.ietf-conex-concepts-uses] Briscoe, B., Woundy, R., and A.
Cooper, "ConEx Concepts and Use
Cases",
draft-ietf-conex-concepts-uses-05
(work in progress), July 2012.
[I-D.ietf-conex-destopt] Krishnan, S., Kuehlewind, M., and
C. Ucendo, "IPv6 Destination
Option for ConEx",
draft-ietf-conex-destopt-03 (work
in progress), September 2012.
[I-D.ietf-conex-tcp-modifications] Kuehlewind, M. and R.
Scheffenegger, "TCP modifications
for Congestion Exposure", draft-
ietf-conex-tcp-modifications-02
(work in progress), May 2012.
[I-D.ietf-ledbat-congestion] Shalunov, S., Hazel, G., Iyengar,
J., and M. Kuehlewind, "Low Extra
Delay Background Transport
(LEDBAT)",
draft-ietf-ledbat-congestion-10
(work in progress),
September 2012.
[I-D.ietf-tsvwg-byte-pkt-congest] Briscoe, B. and J. Manner, "Byte
and Packet Congestion
Notification", draft-ietf-tsvwg-
byte-pkt-congest-08 (work in
progress), August 2012.
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[I-D.kuehlewind-tcpm-accurate-ecn] Kuehlewind, M. and R.
Scheffenegger, "Accurate ECN
Feedback Option in TCP", draft-
kuehlewind-tcpm-accurate-ecn-
option-01 (work in progress),
July 2012.
[RFC3514] Bellovin, S., "The Security Flag
in the IPv4 Header", RFC 3514,
April 2003.
[RFC3550] Schulzrinne, H., Casner, S.,
Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for
Real-Time Applications", STD 64,
RFC 3550, July 2003.
[RFC5348] Floyd, S., Handley, M., Padhye,
J., and J. Widmer, "TCP Friendly
Rate Control (TFRC): Protocol
Specification", RFC 5348,
September 2008.
[RFC5681] Allman, M., Paxson, V., and E.
Blanton, "TCP Congestion
Control", RFC 5681,
September 2009.
[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>.
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Authors' Addresses
Matt Mathis
Google, Inc
1600 Amphitheater Parkway
Mountain View, California 93117
USA
EMail: mattmathis at google.com
Bob Briscoe
BT
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 645196
EMail: bob.briscoe@bt.com
URI: http://bobbriscoe.net/
Mathis & Briscoe Expires April 25, 2013 [Page 27]
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