Congestion Exposure (ConEx) Concepts and Abstract Mechanism
draft-ietf-conex-abstract-mech-04
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Congestion Exposure (ConEx) Working M. Mathis
Group Google, Inc
Internet-Draft B. Briscoe
Intended status: Informational BT
Expires: September 13, 2012 March 12, 2012
Congestion Exposure (ConEx) Concepts and Abstract Mechanism
draft-ietf-conex-abstract-mech-04
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 September 13, 2012.
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
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7
2. Requirements for the ConEx Abstract Mechanism . . . . . . . . 7
2.1. Requirements for ConEx Signals . . . . . . . . . . . . . . 7
2.2. Requirements for the Audit Function . . . . . . . . . . . 8
2.3. Requirements for non-abstract ConEx specifications . . . . 9
3. Encoding Congestion Exposure . . . . . . . . . . . . . . . . . 10
3.1. Naive Encoding . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Null Encoding . . . . . . . . . . . . . . . . . . . . . . 11
3.3. ECN Based Encoding . . . . . . . . . . . . . . . . . . . . 11
3.4. Independent Bits . . . . . . . . . . . . . . . . . . . . . 12
3.5. Codepoint Encoding . . . . . . . . . . . . . . . . . . . . 12
3.6. Units Implied by an Encoding . . . . . . . . . . . . . . . 13
4. Congestion Exposure Components . . . . . . . . . . . . . . . . 15
4.1. Network Devices (Not modified) . . . . . . . . . . . . . . 15
4.2. Modified Senders . . . . . . . . . . . . . . . . . . . . . 15
4.3. Receivers (Optionally Modified) . . . . . . . . . . . . . 15
4.4. Policy Devices . . . . . . . . . . . . . . . . . . . . . . 16
4.4.1. Congestion Monitoring Devices . . . . . . . . . . . . 16
4.4.2. Rest-of-Path Congestion Monitoring . . . . . . . . . . 16
4.4.3. Congestion Policers . . . . . . . . . . . . . . . . . 17
4.5. Audit . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.5.1. Using Credit to Simplify Audit . . . . . . . . . . . . 20
5. Support for Incremental Deployment . . . . . . . . . . . . . . 20
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
7. Security Considerations . . . . . . . . . . . . . . . . . . . 23
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
9. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
10.2. Informative References . . . . . . . . . . . . . . . . . . 23
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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.
As transport protocols continually seek out more network capacity,
network elements signal whenever congestion results, and the
transports are responsible for controlling this network
congestion.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. ConEx ensures 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.
Therefore 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
the sender makes some congestion control adjustment.
This document extends the capabilities of the Internet protocol suite
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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 all internetwork layer
devices along the forward path. This document frames the engineering
problem of designing the ConEx signal. The requirements are
described in Section 2 and some example encoding are presented in
Section 3.
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 4.4
,---------. ,---------.
|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 4.4 and Section 4.5,
respectively.
Figure 1
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
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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
and compare congestion signals and ConEx signals on the forward path.
Many ConEx design constraints come from the need to assure that the
audit function is sufficiently robust. The audit function is
described in Section 4.5, 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 a
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.
[g]Flow-state for auditing is discussed further in Section 4.5. 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
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 typically gauge
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resource consumption and congestion in terms of packets. This 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 5.
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
be 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. Therefore most 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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1.1. 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 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 4.5.1)
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.
2. 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.
2.1. Requirements for ConEx Signals
Ideally, all the following requirements would be met by a Congestion
Exposure Signal. However it is already known that some compromises
will be necessary, and therefore all the requirements 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 a requirement:
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
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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 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 4.5). The simplest mechanism to compensate for the round
trip delay between the signals is to include a "credit" signal to
cover the yet to be observed congestion that might occur during
this delay. (see Section 4.5.1 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 or vantage points.
2.2. Requirements for the Audit Function
The role and constraints on the audit function are described in
Section 4.5. 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 [RFC3448]. 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) so that senders cannot gain from
understating congestion. Audit sanctions SHOULD remove any gain
from playing off losses at the audit function against higher
allowed throughput at a congestion policer;
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
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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];
2.3. 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 any conflated signals or any other effects
that are known to compromise signal integrity;
C. A specification for signal units (bytes vs packets, etc), any
approximations allowed and algorithms to do any implied
conversions or accounting;
D. If the units are bytes a definition of which headers are
included in the size of the packet;
E. How tunnels should propagate the ConEx encoding;
F. Whether the encoding fields are mutable or not, to ensure that
header authentication, checksum calculation, etc. process them
correctly.
G. Definition of any extensibility;
H. Backward and forward compatibility and potential migration
strategies;
I. Any (hopefully optional) modification to data-plane forwarding
dependent on the encoding (e.g. preferential discard,
interaction with Diffserv, ECN etc.);
J. Any warning or error messages relevant to the encoding.
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. @@@More TBA, incl ops & management@@@
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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.
3. 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
[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.
3.1. 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
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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.
3.2. Null Encoding
In limited contexts 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.
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).
3.3. ECN Based Encoding
The re-ECN specification [I-D.briscoe-tsvwg-re-ecn-tcp] presents an
IPv4 implementation of ConEx 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 bit and/or code points, these
signals had to partially share code points.
The central theme of the re-ECN work was an audit mechanism that
provides sufficient disincentives against misrepresenting congestion
[I-D.briscoe-tsvwg-re-ecn-motiv]. It is analyzed extensively in
Briscoe's PhD dissertation [Refb-dis].
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Re-ECN is an example of one chosen set of compromises attempting to
meet the requirements of Section 2. 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 overcome this
problem by being able to represent both loss and ECN based
congestion, and assuming that both signals must be supported
indefinitely.
For a tutorial background on re-ECN motivation and techniques, see
[Re-fb, FairerFaster].
3.4. 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 (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 4.5 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. 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.
3.5. 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.
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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.
3.6. Units Implied by an Encoding
The following comments apply generally to all the other encodings.
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.
We could require that a ConEx encoding specifies whether ConEx
markings are in units of bytes or packets. But the problem is deeper
than that: we do not even know whether congestion signals themselves
(loss & ECN) are in units of bytes or packets.
Therefore a ConEx encoding SHOULD specify whether it assumes units of
bytes or packets for both ConEx markings and for congestion
indications.
[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.
For example, to implement ConEx in bytes, the sender maintains a
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counter of outstanding bytes to be ConEx-marked. When the SACK
options report the size of a loss, this is added to the counter, and
whenever the counter is positive the next data packet is ConEx-marked
and its size subtracted from the counter. Then, if one 1500B packet
is lost, even if subsequent packets to be sent are all 600B, the
sender will compensate by Conex-marking enough small packets. In
this case, the sender will ConEx-mark the next three 600B packets
before the counter goes negative (1500 - 3*600 = -300), which
indicates that it has sent sufficient ConEx marked small packets to
compensate for the lost large packet. It will hold over the negative
remainder towards the next loss. As long as the remainder is kept
negative, the ConEx markings will be on the safe side for audit
purposes.
With TCP-ECN the sender knows the size in bytes of packets going out,
but ECN feedback is in units of packets not bytes. In some TCP
implementations, ECN markings are easy to convert to marked bytes,
while in others it requires significant work. Therefore even if a
ConEx encoding specifies that markings should be interpreted in
bytes, it SHOULD allow implementers some leeway to approximate.
Experiments with these approximations will determine whether they are
sufficient for different patterns of packet size variations.
If an encoding is specified in units of bytes, the encoding SHOULD
also specify which headers to include in the size of a packet. Bit-
congestion is caused by all the bits transmitted with packets,
including lower layer frame headers, trailers etc. However, a
transport endpoint cannot know the size of the frame header on a
packet when it caused congestion at some other link in the Internet,
or what size frame header will be used at the audit function.
Therefore, it will be practical to define the size of a packet as
including the layer 3 header that encapsulates the transport header
associated with the ConEx transport sender, but not any more lower
layer headers, nor any tunnel headers (which a transport is unlikely
to be aware of anyway, because they will already have been stripped
before the transport sees the segment).
It is appropriate to defer the definition of units to the (non-
abstract) encoding specification, because this choice will need to be
made in normative language, and the present document is only
informative. It may seem that this could lead to interoperability
problems if more than one encoding is specified. However, one
encoding is unlikely to have to interact with another: the
interactions between ConEx implementations in senders, policy devices
and audit devices can only happen in the context of one encoding on
the wire.
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4. Congestion Exposure Components
The components shown in Figure 1 are described in more detail.
4.1. 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.
4.2. Modified Senders
The sending transport needs to be modified to send Congestion
Exposure Signals in response to congestion feedback signals (For
example see [I-D.conex-tcp-mods]). 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,
because it is believed that ConEx without ECN is harder to audit, and
thus potentially exposed to fraud. 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.
4.3. 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.conex-accurate-ecn]). This was the approach taken when adding
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re-ECN to TCP [I-D.briscoe-tsvwg-re-ecn-tcp].
4.4. 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, the auditing devices referred to in the previous section
primarily enforce compliance with the ConEx protocol and do not need
to be configured with any client-specific policy.
4.4.1. 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 4.4.3), 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 between
networks 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.
4.4.2. Rest-of-Path Congestion Monitoring
ConEx signals indicate the level of congestion along a whole path
from source to destination. In contrast when ECN signals are
monitored in the middle of a network, they indicate the level of
congestion experienced so far on the path.
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.
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4.4.3. 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
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 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.
But unlike a bit-rate policer, it removes a token only when it
forwards a packet that is 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 is likely to include a queue with its own AQM, which
potentially increases the whole path congestion, to reduce the rest
of path congestion.
4.5. 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 being useless. The most important
feature of an encoding design is likely to be 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 technique described
in Section 4.5.1 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,
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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, ECN marking or dropping
traffic.
This document does not preclude "statistical auditing", where the
audit function indicates some sort of probability that a particular
flow is under reporting congestion, however this design choice
greatly complicates designing an appropriate sanction, because of the
possibility of a false hit.
To facilitate ConEx deployment, not-ConEx traffic might be treated as
a special case of understating congestion, but with a different
sanction. For example an ISP might apply a data rate cap to not-
ConEx traffic, while applying a congestion volume cap to ConEx marked
traffic. With suitable parameters this is likely to give ConEx
marked traffic a much larger share of the network during off peak
hours. (Note that in this example the ConEx auditor is also acting
as a ConEx policy device.) Another option to facilitate deployment
is for the auditor to act as a ConEx proxy, and insert ConEx signals
in packets in behalf of the sender. Such a device is outside of the
scope of this document, but nonetheless potentially useful for
supporting ConEx for legacy systems.
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.
The auditing function should be able to trigger sufficient sanction
to discourage understating congestion[Salvatori05]. This potentially
requires designing the sanction in consort with the policy functions,
even though they might be implemented in different parts of the
network. Note that in the future it might prove to be desirable to
provide advise on uniformly implementing sanctions, because
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. Sampling
techniques can also be used to bound the total auditing memory
footprint, although the implementer must be wary of "identifier white
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washing" attacks to hide cheating connection among chaff.
At some point in the future, when ConEx is built into all transport
protocol implementations, it may not be necessary to audit all
traffic all the time. Auditing might be needed only to identify
rogue actors and prevent them from gaining any long term advantage by
cheating.
A ConEx auditor might use one of the following techniques:
ECN Auditing: 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.
Note that this technique requires no per flow state.
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 down stream 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. This is a good model for most
consumer access networks where audit accuracy could well be
sufficient even if losses occasionally occur at other nodes in the
network, such as border gateways.
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.
Note that this technique requires no per flow state.
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.
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4.5.1. 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 transport.
The simplest mechanism to compensate for the round trip delay between
the signals is to 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
responsible for both minimizing feedback delay and for the risk that
packets in flight will cause congestion to others before the source
can react.
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 ever the balance does go negative, the
audit function can immediately start punishing a flow, without any
grace period.
Note that although per flow state might be required to count losses,
this balance requirement applies both to individual flows and to flow
aggregates. For example with the "predominant bottleneck" approach
in the previous section (which does not require per flow state), an
auditor can detect understated congestion merely by comparing the
total volume of ConEx signals (Re-Echo-Loss, Re-Echo-ECN and Credit)
to the sum of the total volumes of AQM drops and ECN marks.
A specific encoding SHOULD describe the tradeoffs of three
interrelated design decisions: whether the audit is strict or
statistical; how to recommend estimating the initial credit per flow;
and to what extent the sanction needs to avoid over penalizing flows
which a false audit failures.
5. 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
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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 3 on Encoding
Congestion Exposure).
senders: ConEx requires a modification to the source in order to
send ConEx packet markings (see Section 4.2). 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
packets.
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
congestion-causing traffic, and otherwise allow complete freedom.
This greater freedom acts as an inducement for the source to
volunteer ConEx information.
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 Re-Echo 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, it will be best if ConEx uses it (see
Section 4.3).
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.
Queues: No modification to queues is needed for ConEx.
However, once ConEx is deployed, it is possible that a queue
implementation could take advantage of the ConEx information in
packets. For instance, it has been suggested
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[I-D.briscoe-tsvwg-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 2). Also, monitoring rest-of-path congestion is not
accurate if there are congested non-ECN queues upstream of the
monitoring point (Section 4.4.2).
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 packets may safely traverse a network that ignores them.
Networks MUST NOT change ConEx packets to Not-ConEx. If
necessary, endpoints SHOULD be able to detect if a network is
removing ConEx signals.
An operator can deploy policy devices (Section 4.4) 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 other
don't.
An operator can deploy audit devices Section 4.5 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 4.4).
6. IANA Considerations
This memo includes no request to IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
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7. Security Considerations
Significant parts of this whole document are about auditability of
ConEx Signals, in particular Section 4.5.
8. Acknowledgements
This document was improved by review comments from Toby Moncaster,
Nandita Dukkipati, Mirja Kuehlewind, Caitlin Bestler and John Leslie.
9. 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.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in
RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119,
March 1997.
10.2. 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, <http
://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, <ht
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tp://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/
refb/#fairfastip>.
[I-D.briscoe-tsvwg-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-tsvwg-re-
ecn-tcp-motivation-02 (work in
progress), October 2010.
[I-D.briscoe-tsvwg-re-ecn-tcp] Briscoe, B., Jacquet, A.,
Moncaster, T., and A. Smith, "Re-
ECN: Adding Accountability for
Causing Congestion to TCP/IP",
draft-briscoe-tsvwg-re-ecn-tcp-09
(work in progress), October 2010.
[I-D.conex-accurate-ecn] Kuehlewind, M. and R.
Scheffenegger, "Accurate ECN
Feedback in TCP", draft-
kuehlewind-conex-accurate-ecn-01
(work in progress), October 2011.
[I-D.conex-tcp-mods] Kuehlewind, M. and R.
Scheffenegger, "TCP modifications
for Congestion Exposure", draft-
kuehlewind-conex-tcp-
modifications-00 (work in
progress), July 2011.
[I-D.ietf-conex-concepts-uses] Briscoe, B., Woundy, R., and A.
Cooper, "ConEx Concepts and Use
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Cases",
draft-ietf-conex-concepts-uses-03
(work in progress), October 2011.
[I-D.ietf-conex-destopt] Krishnan, S., Kuehlewind, M., and
C. Ucendo, "IPv6 Destination
Option for Conex",
draft-ietf-conex-destopt-01 (work
in progress), October 2011.
[I-D.ietf-ledbat-congestion] Shalunov, S., Hazel, G., and J.
Iyengar, "Low Extra Delay
Background Transport (LEDBAT)",
draft-ietf-ledbat-congestion-03
(work in progress), October 2010.
[I-D.ietf-tsvwg-byte-pkt-congest] Briscoe, B. and J. Manner, "Byte
and Packet Congestion
Notification", draft-ietf-tsvwg-
byte-pkt-congest-03 (work in
progress), October 2010.
[I-D.sridharan-tcpm-ctcp] Sridharan, M., Tan, K., Bansal,
D., and D. Thaler, "Compound TCP:
A New TCP Congestion Control for
High-Speed and Long Distance
Networks",
draft-sridharan-tcpm-ctcp-02 (work
in progress), November 2008.
[IntDesPrinciples] Clark, D., "The Design Philosophy
of the DARPA Internet Protocols",
ACM SIGCOMM CCR 18(4)106--114,
August 1988, <http://www.acm.org/
sigcomm/ccr/archive/1995/jan95/
ccr-9501-clark.pdf>.
[RFC0791] Postel, J., "Internet Protocol",
STD 5, RFC 791, September 1981.
[RFC2309] Braden, B., Clark, D., Crowcroft,
J., Davie, B., Deering, S.,
Estrin, D., Floyd, S., Jacobson,
V., Minshall, G., Partridge, C.,
Peterson, L., Ramakrishnan, K.,
Shenker, S., Wroclawski, J., and
L. Zhang, "Recommendations on
Queue Management and Congestion
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Avoidance in the Internet",
RFC 2309, April 1998.
[RFC3168] Ramakrishnan, K., Floyd, S., and
D. Black, "The Addition of
Explicit Congestion Notification
(ECN) to IP", RFC 3168,
September 2001.
[RFC3448] Handley, M., Floyd, S., Padhye,
J., and J. Widmer, "TCP Friendly
Rate Control (TFRC): Protocol
Specification", RFC 3448,
January 2003.
[RFC3514] Bellovin, S., "The Security Flag
in the IPv4 Header", RFC 3514,
April 1 2003.
[RFC3540] Spring, N., Wetherall, D., and D.
Ely, "Robust Explicit Congestion
Notification (ECN) Signaling with
Nonces", RFC 3540, June 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.
[RFC5670] Eardley, P., "Metering and Marking
Behaviour of PCN-Nodes", RFC 5670,
November 2009.
[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, <h
ttp://www.acm.org/sigs/sigcomm/
sigcomm2005/
techprog.html#session8>.
Mathis & Briscoe Expires September 13, 2012 [Page 26]
Internet-Draft ConEx Concepts and Abstract Mechanism March 2012
[Refb-dis] Briscoe, B., "Re-feedback: Freedom
with Accountability for Causing
Congestion in a Connectionless
Internetwork", UCL PhD
Dissertation , 2009, <http://
bobbriscoe.net/projects/
refb/#refb-dis>.
[Salvatori05] Salvatori, A., "Closed Loop
Traffic Policing", Politecnico
Torino and Institut Eurecom
Masters Thesis , September 2005.
[Vegas] Brakmo, L. and L. Peterson, "TCP
Vegas: End-to-End Congestion
Avoidance on a Global Internet",
IEEE Journal on Selected Areas in
Communications 13(8)1465--80,
October 1995, <http://
ieeexplore.ieee.org/iel1/49/9740/
00464716.pdf?arnumber=464716>.
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/
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