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Transport Area Working Group B. Briscoe
Internet-Draft BT & UCL
Intended status: Standards Track A. Jacquet
Expires: January 10, 2008 A. Salvatori
M. Koyabe
T. Moncaster
BT
July 09, 2007
Re-ECN: Adding Accountability for Causing Congestion to TCP/IP
draft-briscoe-tsvwg-re-ecn-tcp-04
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document introduces a new protocol for explicit congestion
notification (ECN), termed re-ECN, which can be deployed
incrementally around unmodified routers. The protocol arranges an
extended ECN field in each packet so that, as it crosses any
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interface in an internetwork, it will carry a truthful prediction of
congestion on the remainder of its path. Then the upstream party at
any trust boundary in the internetwork can be held responsible for
the congestion they cause, or allow to be caused. So, networks can
introduce straightforward accountability and policing mechanisms for
incoming traffic from end-customers or from neighbouring network
domains. The purpose of this document is to specify the re-ECN
protocol at the IP layer and to give guidelines on any consequent
changes required to transport protocols. It includes the changes
required to TCP both as an example and as a specification. It also
gives examples of mechanisms that can use the protocol to ensure data
sources respond correctly to congestion. And it describes example
mechanisms that ensure the dominant selfish strategy of both network
domains and end-points will be to set the extended ECN field
honestly.
Authors' Statement: Status (to be removed by the RFC Editor)
Although the re-ECN protocol is intended to make a simple but far-
reaching change to the Internet architecture, the most immediate
priority for the authors is to delay any move of the ECN nonce to
Proposed Standard status. The argument for this position is
developed in Appendix I.
Changes from previous drafts (to be removed by the RFC Editor)
Full diffs created using the rfcdiff tool are available at
<http://www.cs.ucl.ac.uk/staff/B.Briscoe/pubs.html#retcp>
From -03 to -04 (current version):
Clarified reasons for holding back ECN nonce (Section 3.2 &
Appendix I).
Clarified Figure 1.
Added Section 4.1.1.1 on equivalence of drops and ECN marks.
Improved precision of Section 5.6 on IP in IP tunnels.
Explained the RTT fairness is possible to enforce, but unlikely to
be required (Section 6.1.3 & Appendix F).
Explained that bulk per-user policing should be adequate but per-
flow policing is also possible if desired, though it is not likely
to be necessary (Section 6.1.5 & Appendix G).
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Reinforced need for passive policing at inter-domain borders to
enable all-optical networking (Section 6.1.6).
Minor editorial changes throughout.
From -02 to -03:
Started guidelines for re-ECN support in DCCP and SCTP.
Added annex on limitations of nonce mechanism.
Minor editorial changes throughout.
From -01 to -02:
Explanation on informal terminology in Section 3.4 clarified.
IPv6 wire protocol encoding added (Section 5.2).
Text on (non-)issues with tunnels, encryption and link layer
congestion notification added (Section 5.6 & Section 5.7).
Section added giving evolvability arguments against encouraging
bottleneck policing (Section 6.1.2). And text on re-ECN's
evolvability by design added to Section 6.1.3
Text on inter-domain policing (Section 6.1.6) and inter-domain
fail-safes (Section 6.1.7) added.
From -00 to -01:
Encoding of re-ECN wire protocol changed for reasons given in
Appendix B and consequently draft substantially re-written.
Substantial text added in sections on applications, incremental
deployment, architectural rationale and security considerations.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 7
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Background and Applicability . . . . . . . . . . . . . . . 8
3.2. Re-ECN Abstracted Network Layer Wire Protocol (IPv4 or
v6) . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3. Re-ECN Protocol Operation . . . . . . . . . . . . . . . . 11
3.4. Informal Terminology . . . . . . . . . . . . . . . . . . . 13
4. Transport Layers . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1.1. RECN mode: Full re-ECN capable transport . . . . . . . 16
4.1.2. RECN-Co mode: Re-ECT Sender with a Vanilla or
Nonce ECT Receiver . . . . . . . . . . . . . . . . . . 20
4.1.3. Capability Negotiation . . . . . . . . . . . . . . . . 21
4.1.4. Extended ECN (EECN) Field Settings during Flow
Start or after Idle Periods . . . . . . . . . . . . . 23
4.1.5. Pure ACKS, Retransmissions, Window Probes and
Partial ACKs . . . . . . . . . . . . . . . . . . . . . 26
4.2. Other Transports . . . . . . . . . . . . . . . . . . . . . 27
4.2.1. General Guidelines for Adding Re-ECN to Other
Transports . . . . . . . . . . . . . . . . . . . . . . 27
4.2.2. Guidelines for adding Re-ECN to RSVP or NSIS . . . . . 28
4.2.3. Guidelines for adding Re-ECN to DCCP . . . . . . . . . 28
4.2.4. Guidelines for adding Re-ECN to SCTP . . . . . . . . . 28
5. Network Layer . . . . . . . . . . . . . . . . . . . . . . . . 28
5.1. Re-ECN IPv4 Wire Protocol . . . . . . . . . . . . . . . . 28
5.2. Re-ECN IPv6 Wire Protocol . . . . . . . . . . . . . . . . 30
5.3. Router Forwarding Behaviour . . . . . . . . . . . . . . . 31
5.4. Justification for Setting the First SYN to FNE . . . . . . 32
5.5. Control and Management . . . . . . . . . . . . . . . . . . 33
5.5.1. Negative Balance Warning . . . . . . . . . . . . . . . 33
5.5.2. Rate Response Control . . . . . . . . . . . . . . . . 34
5.6. IP in IP Tunnels . . . . . . . . . . . . . . . . . . . . . 34
5.7. Non-Issues . . . . . . . . . . . . . . . . . . . . . . . . 35
6. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 36
6.1. Policing Congestion Response . . . . . . . . . . . . . . . 36
6.1.1. The Policing Problem . . . . . . . . . . . . . . . . . 36
6.1.2. The Case Against Bottleneck Policing . . . . . . . . . 37
6.1.3. Re-ECN Incentive Framework . . . . . . . . . . . . . . 38
6.1.4. Egress Dropper . . . . . . . . . . . . . . . . . . . . 45
6.1.5. Policing . . . . . . . . . . . . . . . . . . . . . . . 47
6.1.6. Inter-domain Policing . . . . . . . . . . . . . . . . 48
6.1.7. Inter-domain Fail-safes . . . . . . . . . . . . . . . 52
6.1.8. Simulations . . . . . . . . . . . . . . . . . . . . . 53
6.2. Other Applications . . . . . . . . . . . . . . . . . . . . 53
6.2.1. DDoS Mitigation . . . . . . . . . . . . . . . . . . . 53
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6.2.2. End-to-end QoS . . . . . . . . . . . . . . . . . . . . 54
6.2.3. Traffic Engineering . . . . . . . . . . . . . . . . . 54
6.2.4. Inter-Provider Service Monitoring . . . . . . . . . . 54
6.3. Limitations . . . . . . . . . . . . . . . . . . . . . . . 54
7. Incremental Deployment . . . . . . . . . . . . . . . . . . . . 55
7.1. Incremental Deployment Features . . . . . . . . . . . . . 55
7.2. Incremental Deployment Incentives . . . . . . . . . . . . 57
8. Architectural Rationale . . . . . . . . . . . . . . . . . . . 61
9. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 64
9.1. Policing Rate Response to Congestion . . . . . . . . . . . 64
9.2. Congestion Notification Integrity . . . . . . . . . . . . 65
9.3. Identifying Upstream and Downstream Congestion . . . . . . 66
10. Security Considerations . . . . . . . . . . . . . . . . . . . 66
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 68
12. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 68
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 68
14. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 69
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 69
15.1. Normative References . . . . . . . . . . . . . . . . . . . 69
15.2. Informative References . . . . . . . . . . . . . . . . . . 70
Appendix A. Precise Re-ECN Protocol Operation . . . . . . . . . . 73
Appendix B. Justification for Two Codepoints Signifying Zero
Worth Packets . . . . . . . . . . . . . . . . . . . . 74
Appendix C. ECN Compatibility . . . . . . . . . . . . . . . . . . 76
Appendix D. Packet Marking During Flow Start . . . . . . . . . . 77
Appendix E. Example Egress Dropper Algorithm . . . . . . . . . . 77
Appendix F. Re-TTL . . . . . . . . . . . . . . . . . . . . . . . 77
Appendix G. Policer Designs to ensure Congestion
Responsiveness . . . . . . . . . . . . . . . . . . . 78
G.1. Per-user Policing . . . . . . . . . . . . . . . . . . . . 78
G.2. Per-flow Rate Policing . . . . . . . . . . . . . . . . . . 79
Appendix H. Downstream Congestion Metering Algorithms . . . . . . 82
H.1. Bulk Downstream Congestion Metering Algorithm . . . . . . 82
H.2. Inflation Factor for Persistently Negative Flows . . . . . 83
Appendix I. Argument for holding back the ECN nonce . . . . . . . 84
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 85
Intellectual Property and Copyright Statements . . . . . . . . . . 88
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1. Introduction
This document aims:
o To provide a complete specification of the addition of the re-ECN
protocol to IP and guidelines on how to add it to transport layer
protocols, including a complete specification of re-ECN in TCP as
an example;
o To show how a number of hard problems become much easier to solve
once re-ECN is available in IP.
A general statement of the problem solved by re-ECN is to provide
sufficient information in each IP datagram to be able to hold senders
and whole networks accountable for the congestion they cause
downstream, before they cause it. But the every-day problems that
re-ECN can solve are much more recognisable than this rather generic
statement: mitigating distributed denial of service (DDoS);
simplifying differentiation of quality of service (QoS); policing
compliance to congestion control; and so on.
Uniquely, re-ECN manages to enable solutions to these problems
without unduly stifling innovative new ways to use the Internet.
This was a hard balance to strike, given it could be argued that DDoS
is an innovative way to use the Internet. The most valuable insight
was to allow each network to choose the level of constraint it wishes
to impose. Also re-ECN has been carefully designed so that networks
that choose to use it conservatively can protect themselves against
the congestion caused in their network by users on other networks
with more liberal policies.
For instance, some network owners want to block applications like
voice and video unless their network is compensated for the extra
share of bottleneck bandwidth taken. These real-time applications
tend to be unresponsive when congestion arises. Whereas elastic TCP-
based applications back away quickly, ending up taking a much smaller
share of congested capacity for themselves. Other network owners
want to invest in large amounts of capacity and make their gains from
simplicity of operation and economies of scale.
Re-ECN allows the more conservative networks to police out flows that
have not asked to be unresponsive to congestion---not because they
are voice or video---just because they don't respond to congestion.
But it also allows other networks to choose not to police.
Crucially, when flows from liberal networks cross into a conservative
network, re-ECN enables the conservative network to apply penalties
to its neighbouring networks for the congestion they allow to be
caused. And these penalties can be applied to bulk data, without
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regard to flows.
Then, if unresponsive applications become so dominant that some of
the more liberal networks experience congestion collapse [RFC3714],
they can change their minds and use re-ECN to apply tighter controls
in order to bring congestion back under control.
Re-ECN works by arranging that each packet arrives at each network
element carrying a view of expected congestion on its own downstream
path, albeit averaged over multiple packets. Most usefully,
congestion on the remainder of the path becomes visible in the IP
header at the first ingress. Many of the applications of re-ECN
involve a policer at this ingress using the view of downstream
congestion arriving in packets to police or control the packet rate.
Importantly, the scheme is recursive: a whole network harbouring
users causing congestion in downstream networks can be held
responsible or policed by its downstream neighbour.
This document is structured as follows. First an overview of the re-
ECN protocol is given (Section 3), outlining its attributes and
explaining conceptually how it works as a whole. The two main parts
of the document follow, as described above. That is, the protocol
specification divided into transport (Section 4) and network
(Section 5) layers, then the applications it can be put to, such as
policing DDoS, QoS and congestion control (Section 6). Although
these applications do not require standardisation themselves, they
are described in a fair degree of detail in order to explain how re-
ECN can be used. Given re-ECN proposes to use the last undefined bit
in the IPv4 header, we felt it necessary to outline the potential
that re-ECN could release in return for being given that bit.
Deployment issues discussed throughout the document are brought
together in Section 7, which is followed by a brief section
explaining the somewhat subtle rationale for the design from an
architectural perspective (Section 8). We end by describing related
work (Section 9), listing security considerations (Section 10) and
finally drawing conclusions (Section 12).
2. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document first specifies a protocol, then describes a framework
that creates the right incentives to ensure compliance to the
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protocol. This could cause confusion because the second part of the
document considers many cases where malicious nodes may not comply
with the protocol. When such contingencies are described, if any of
the above keywords are not capitalised, that is deliberate. So, for
instance, the following two apparently contradictory sentences would
be perfectly consistent: i) x MUST do this; ii) x may not do this.
3. Protocol Overview
3.1. Background and Applicability
First we briefly recap the essentials of the ECN protocol [RFC3168].
Two bits in the IP protocol (v4 or v6) are assigned to the ECN field.
The sender clears the field to "00" (Not-ECT) if either end-point
transport is not ECN-capable. Otherwise it indicates an ECN-capable
transport (ECT) using either of the two code-points "10" or "01"
(ECT(0) and ECT(1) resp.).
ECN-capable routers probabilistically set "11" if congestion is
experienced (CE), the marking probability increasing with the length
of the queue at its egress link (typically using the RED
algorithm [RFC2309]). However, they still drop rather than mark Not-
ECT packets. With multiple ECN-capable routers on a path, a flow of
packets accumulates the fraction of CE marking that each router adds.
The combined effect of the packet marking of all the routers along
the path signals congestion of the whole path to the receiver. So,
for example, if one router early in a path is marking 1% of packets
and another later in a path is marking 2%, flows that pass through
both routers will experience approximately 3% marking (see Appendix A
for a precise treatment).
The choice of two ECT code-points in the ECN field [RFC3168]
permitted future flexibility, optionally allowing the sender to
encode the experimental ECN nonce [RFC3540] in the packet stream.
The nonce is designed to allow a sender to check the integrity of
congestion feedback. But Section 9.2 explains that it still gives no
control over how fast the sender transmits as a result of the
feedback. On the other hand, re-ECN is designed both to ensure that
congestion is declared honestly and that the sender's rate responds
appropriately.
Re-ECN is based on a feedback arrangement called `re-
feedback' [Re-fb]. The word is short for either receiver-aligned,
re-inserted or re-echoed feedback. But it actually works even when
no feedback is available. In fact it has been carefully designed to
work for single datagram flows. It also encourages aggregation of
single packet flows by congestion control proxies. Then, even if the
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traffic mix of the Internet were to become dominated by short
messages, it would still be possible to control congestion
effectively and efficiently.
Changing the Internet's feedback architecture seems to imply
considerable upheaval. But re-ECN can be deployed incrementally at
the transport layer around unmodified routers using existing fields
in IP (v4 or v6). However it does also require the last undefined
bit in the IPv4 header, which it uses in combination with the 2-bit
ECN field to create four new codepoints. Nonetheless, changes to IP
routers are RECOMMENDED in order to improve resilience against DoS
attacks. Similarly, re-ECN works best if both the sender and
receiver transports are re-ECN-capable, but it can work with just
sender support. Section 7.1 summarises the incremental deployment
strategy.
The re-ECN protocol makes no changes and has no effect on the TCP
congestion control algorithm or on other rate responses to
congestion. Re-ECN is only concerned with enabling the ingress
network to police that a source is complying with a congestion
control algorithm, which is orthogonal to congestion control itself.
Before re-ECN can be considered worthy of using up the last bit in
the IP header, we must be sure that all our claims are robust. We
have gradually been reducing the list of outstanding issues, but the
few that still remain are listed in Section 6.3. We expect new
attacks may still be found, but we offer the re-ECN protocol on the
basis that it is built on fairly solid theoretical foundations and,
so far, it has proved possible to keep it relatively robust.
3.2. Re-ECN Abstracted Network Layer Wire Protocol (IPv4 or v6)
The re-ECN wire protocol uses the two bit ECN field broadly as in
RFC3168 [RFC3168] as described above, but with five differences of
detail (brought together in a list in Section 7.1). This
specification defines a new re-ECN extension (RE) flag. We will
defer the definition of the actual position of the RE flag in the
IPv4 & v6 headers until Section 5. Until then it will suffice to use
an abstraction of the IPv4 and v6 wire protocols by just calling it
the RE flag.
Unlike the ECN field, the RE flag is intended to be set by the sender
and remain unchanged along the path, although it can be read by
network elements that understand the re-ECN protocol. It is feasible
that a network element MAY change the setting of the RE flag, perhaps
acting as a proxy for an end-point, but such a protocol would have to
be defined in another specification (e.g. [Re-PCN]).
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Although the RE flag is a separate, single bit field, it can be read
as an extension to the two-bit ECN field; the three concatenated bits
in what we will call the extended ECN field (EECN) making eight
codepoints. We will use the RFC3168 names of the ECN codepoints to
describe settings of the ECN field when the RE flag setting is "don't
care", but we also define the following six extended ECN codepoint
names for when we need to be more specific.
RFC3168 ECN defines uses for all four codepoints of the two-bit ECN
field. This memo widens the codepoint space to eight, and uses six
codepoints. One of re-ECN's codepoints is an alternative use of the
codepoint set aside in RFC3168 for the ECN nonce (ECT(1)).
Transports not using re-ECN can still use the ECN nonce, while those
using re-ECN do not need to as long as the sender is also checking
for transport protocol compliance [I-D.moncaster-tcpm-rcv-cheat].
The case for doing this is given in Appendix I. Two re-ECN
codepoints are given compatible uses to those defined in RFC3168
(Not-ECT and CE). The other codepoint used by RFC3168 (ECT(0)) isn't
used for re-ECN. Altogether this leave one codepoint of the eight
unused and available for future use.
+-------+------------+------+--------------+------------------------+
| ECN | RFC3168 | RE | Extended ECN | Re-ECN meaning |
| field | codepoint | flag | codepoint | |
+-------+------------+------+--------------+------------------------+
| 00 | Not-ECT | 0 | Not-RECT | Not re-ECN-capable |
| | | | | transport |
| 00 | Not-ECT | 1 | FNE | Feedback not |
| | | | | established |
| 01 | ECT(1) | 0 | Re-Echo | Re-echoed congestion |
| | | | | and RECT |
| 01 | ECT(1) | 1 | RECT | Re-ECN capable |
| | | | | transport |
| 10 | ECT(0) | 0 | --- | Legacy ECN use only |
| | | | | |
| 10 | ECT(0) | 1 | --CU-- | Currently unused |
| | | | | |
| 11 | CE | 0 | CE(0) | Re-Echo canceled by |
| | | | | congestion experienced |
| 11 | CE | 1 | CE(-1) | Congestion experienced |
+-------+------------+------+--------------+------------------------+
Table 1: Extended ECN Codepoints
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3.3. Re-ECN Protocol Operation
In this section we will give an overview of the operation of the re-
ECN protocol for TCP/IP, leaving a detailed specification to the
following sections. Other transports will be discussed later.
In summary, the protocol adds a third `re-echo' stage to the existing
TCP/IP ECN protocol. Whenever the network adds CE congestion
signalling to the IP header on the forward data path, the receiver
feeds it back to the ingress using TCP, then the sender re-echoes it
into the forward data path using the RE flag in the next packet.
Prior to receiving any feedback a sender will not know which setting
of the RE flag to use, so it sets the feedback not established (FNE)
codepoint. The network reads the FNE codepoint conservatively as
equivalent to re-echoed congestion.
Specifically, once a flow is established, a re-ECN sender always
initialises the ECN field to ECT(1). And it usually sets the RE flag
to "1". Whenever a router re-marks a packet to CE, the receiver
feeds back this event to the sender. On receiving this feedback, the
re-ECN sender will clear the RE flag to "0" in the next packet it
sends.
We chose to set and clear the RE flag this way round to ease
incremental deployment (see Section 7.1). To avoid confusion we will
use the term `blanking' (rather than marking) when the RE flag is
cleared to "0". So, over a stream of packets, we will talk of the
`RE blanking fraction' as the fraction of octets in packets with the
RE flag cleared to "0".
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_ _ _ _
/ \ / \ / \ / \
| S |--| 0 | - - - - - - - - | i |--| D |
\ _ / \ _ / \ _ / \ _ /
. . . .
^ . . . .
| . . . .
| . RE blanking fraction . .
3% |-------------------------------+=======
| . . | .
2% | . . | .
| . . CE marking fraction | .
1% | . +----------------------+ .
| . | . .
0% +--------------------------------------->
^ 0 ^ i ^ resource index
0 ^ 1 ^ 2 observation points
| |
1.00% 2.00% marking fraction
Figure 1: A 2-Router Example (Imprecise)
Figure 1 uses a simple network to illustrate how re-ECN allows
routers to measure downstream congestion. The horizontal axis
represents the index of each congestible resource (typically queues)
along a path through the Internet. There may be many routers on the
path, but we assume only two are currently congested (those with
resource index 0 and i). The two superimposed plots show the
fraction of each extended ECN codepoint in a flow observed along this
path. Given about 3% of packets reaching the destination are marked
CE, in response to feedback the sender will blank the RE flag in
about 3% of packets it sends. Then approximate downstream congestion
can be measured at the observation points shown along the path by
subtracting the CE marking fraction from the RE blanking fraction, as
shown in the table below (Appendix A derives these approximations
from a precise analysis).
+-------------------+------------------------------+
| Observation point | Approx downstream congestion |
+-------------------+------------------------------+
| 0 | 3% - 0% = 3% |
| 1 | 3% - 1% = 2% |
| 2 | 3% - 3% = 0% |
+-------------------+------------------------------+
Table 2: Downstream Congestion Measured at Example Observation Points
All along the path, whole-path congestion remains unchanged so it can
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be used as a reference against which to compare upstream congestion.
The difference predicts downstream congestion for the rest of the
path. Therefore, measuring the fractions of each codepoint at any
point in the Internet will reveal upstream, downstream and whole path
congestion.
Note that we have introduced discussion of marking and blanking
fractions solely for illustration. To be absolutely clear, these
fractions are averages that would result from the behaviour of a TCP
protocol handler mechanically blanking outgoing packets in direct
response to incoming feedback---we are not saying any protocol
handler works with these average fractions directly.
3.4. Informal Terminology
In the rest of this memo we will loosely talk of positive or negative
flows, meaning flows where the moving average of the downstream
congestion metric is persistently positive or negative. The notion
of a negative metric arises because it is derived by subtracting one
metric from another. Of course actual downstream congestion cannot
be negative, only the metric can (whether due to time lags or
deliberate malice).
Just as we will loosely talk of positive and negative flows, we will
also talk of positive or negative packets, meaning packets that
contribute positively or negatively to the downstream congestion
metric.
Therefore we will talk of packets having `worth' of +1, 0 or -1,
which, when multiplied by their size, indicates their contribution to
the downstream congestion metric.
Figure 2 shows the main state transitions of the system once a flow
is established, showing the worth of packets in each state. When the
network congestion marks a packet it decrements its worth (moving
from the left of the main square to the right). When the sender
blanks the RE flag in order to re-echo congestion it increments the
worth of a packet (moving from the bottom of the main square to the
top).
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Sender state Sent Worth Received Worth
packet packet
+----------------------------------------------------+
| ^
V |
Congestion echoed -->Re-Echo +1 --+---> CE(0) 0 --+
(positive) | (canceled) |
V network |
| congestion |
| |
Flow established --> RECT 0 ----+-> CE(-1) -1 --+
^ (neutral) | | (negative)
| | |
| no V V
| congestion | |
+-----------<--------------+-+
Figure 2: Re-ECN System State Diagram (bootstrap not shown)
The idea is that every time the network decrements the worth of a
packet, the sender increments the worth of a later packet. Then,
over time, as many positive octets should arrive at the receiver as
negative. Note we have said octets not packets, so if packets are of
different sizes, the worth should be incremented on enough octets to
balance the octets in negative packets arriving at the receiver. It
is this balance that will allow the network to hold the sender
accountable for the congestion it causes, as we shall see. The
informal outline below uses TCP as an example transport, but the idea
would be broadly similar for any transport that adapts its rate to
congestion.
We will start with the sender in `flow established' state. Normally,
as acknowledgements of earlier packets arrive that don't feedback any
congestion, the congestion window can be opened, so the sender goes
round the smaller sub-loop, sending RECT packets (worth 0) and
returning to the flow established state to send another one. If a
router congestion marks one of the packets, it decrements the
packet's worth. The sender will have been continuing to traverse
round the smaller feedback loop every time acknowledgements arrive.
But when congestion feedback returns from this packet that was marked
with -1 worth (the largest loop in the figure) the sender jumps to
the congestion echoed state in order to re-echo the congestion,
incrementing the worth of the next packet to +1 by blanking its RE
flag. The sender then returns to the flow established state and
continues round the smaller loop, sending packets worth 0. Note that
the size of the loops is just an artefact of the figure; it is not
meant to imply that one loop is slower than the other - they are both
the same end to end feedback loop.
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If a packet carrying re-echoed congestion happens to also be
congestion marked, the +1 worth added by the sender will be cancelled
out by the -1 network congestion marking. Although the two worth
values correctly cancel out, neither the congestion marking nor the
re-echoed congestion are lost, because the RE bit and the ECN field
are orthogonal. So, whenever this happens, the receiver will
correctly detect and re-echo the new congestion event as well (the
top sub-loop). When we need to distinguish, we will sometimes call a
packet marked RECT 'neutral' (0 worth), while we will call the CE(0)
marking 'canceled' (also 0 worth). If a re-echoed packet isn't
unlucky enough to be further congestion marked, the sender will
return to the flow established state and continue to send RECT
packets (worth 0).
The table below specifies unambiguously the worth of each extended
ECN codepoint. Note the order is different from the previous table
to better show how the worth increments and decrements. The FNE
codepoint is an exception. It is used in the flow bootstrap process
(explained later) and has the same positive (+1) worth as a packet
with the Re-Echo codepoint.
+--------+------+----------------+-------+--------------------------+
| ECN | RE | Extended ECN | Worth | Re-ECN meaning |
| field | bit | codepoint | | |
+--------+------+----------------+-------+--------------------------+
| 00 | 0 | Not-RECT | ... | Not re-ECN-capable |
| | | | | transport |
| 01 | 0 | Re-Echo | +1 | Re-echoed congestion and |
| | | | | RECT |
| 10 | 0 | --- | ... | Legacy ECN use only |
| 11 | 0 | CE(0) | 0 | Re-Echo canceled by |
| | | | | congestion experienced |
| 00 | 1 | FNE | +1 | Feedback not established |
| 01 | 1 | RECT | 0 | Re-ECN capable transport |
| 10 | 1 | --CU-- | ... | Currently unused |
| | | | | |
| 11 | 1 | CE(-1) | -1 | Congestion experienced |
+--------+------+----------------+-------+--------------------------+
Table 3: 'Worth' of Extended ECN Codepoints
4. Transport Layers
4.1. TCP
Re-ECN capability at the sender is essential. At the receiver it is
optional, as long as the receiver has a basic (`vanilla flavour')
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RFC3168-compliant ECN-capable transport (ECT) [RFC3168]. Given re-
ECN is not the first attempt to define the semantics of the ECN
field, we give a table below summarising what happens for various
combinations of capabilities of the sender S and receiver R, as
indicated in the first four columns below. The last column gives the
mode a half-connection should be in after the first two of the three
TCP handshakes.
+--------+--------------+------------+---------+--------------------+
| Re-ECT | ECT-Nonce | ECT | Not-ECT | S-R |
| | (RFC3540) | (RFC3168) | | Half-connection |
| | | | | Mode |
+--------+--------------+------------+---------+--------------------+
| SR | | | | RECN |
| S | R | | | RECN-Co |
| S | | R | | RECN-Co |
| S | | | R | Not-ECT |
+--------+--------------+------------+---------+--------------------+
Table 4: Modes of TCP Half-connection for Combinations of ECN
Capabilities of Sender S and Receiver R
We will describe what happens in each mode, then describe how they
are negotiated. The abbreviations for the modes in the above table
mean:
RECN: Full re-ECN capable transport
RECN-Co: Re-ECN sender in compatibility mode with a
vanilla [RFC3168] ECN receiver or an [RFC3540] ECN nonce-capable
receiver. Implementation of this mode is OPTIONAL.
Not-ECT: Not ECN-capable transport, as defined in [RFC3168] for when
at least one of the transports does not understand even basic ECN
marking.
Note that we use the term Re-ECT for a host transport that is re-ECN-
capable but RECN for the modes of the half connections between hosts
when they are both Re-ECT. If a host transport is Re-ECT, this fact
alone does NOT imply either of its half connections will necessarily
be in RECN mode, at least not until it has confirmed that the other
host is Re-ECT.
4.1.1. RECN mode: Full re-ECN capable transport
In full RECN mode, for each half connection, both the sender and the
receiver each maintain an unsigned integer counter we will call ECC
(echo congestion counter). The receiver maintains a count, modulo 8,
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of how many times a CE marked packet has arrived during the half-
connection. Once a RECN connection is established, the three TCP
option flags (ECE, CWR & NS) used for ECN-related functions in other
versions of ECN are used as a 3-bit field for the receiver to
repeatedly tell the sender the current value of ECC whenever it sends
a TCP ACK. We will call this the echo congestion increment (ECI)
field. This overloaded use of these 3 option flags as one 3-bit ECI
field is shown in Figure 4. The actual definition of the TCP header,
including the addition of support for the ECN nonce, is shown for
comparison in Figure 3. This specification does not redefine the
names of these three TCP option flags, it merely overloads them with
another definition once a flow is established.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| | | N | C | E | U | A | P | R | S | F |
| Header Length | Reserved | S | W | C | R | C | S | S | Y | I |
| | | | R | E | G | K | H | T | N | N |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 3: The (post-ECN Nonce) definition of bytes 13 and 14 of the
TCP Header
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| | | | U | A | P | R | S | F |
| Header Length | Reserved | ECI | R | C | S | S | Y | I |
| | | | G | K | H | T | N | N |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 4: Definition of the ECI field within bytes 13 and 14 of the
TCP Header, overloading the current definitions above for established
RECN flows.
Receiver Action in RECN Mode
Every time a CE marked packet arrives at a receiver in RECN mode,
the receiver transport increments its local value of ECC modulo 8
and MUST echo its value to the sender in the ECI field of the next
ACK. It MUST repeat the same value of ECI in every subsequent ACK
until the next CE event, when it increments ECI again.
The increment of the local ECC values is modulo 8 so the field
value simply wraps round back to zero when it overflows. The
least significant bit is to the right (labelled bit 9).
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A receiver in RECN mode MAY delay the echo of a CE to the next
delayed-ACK, which would be necessary if ACK-withholding were
implemented.
Sender Action in RECN Mode
On the arrival of every ACK, the sender compares the ECI field
with its own ECC value, then replaces its local value with that
from the ACK. The difference D is assumed to be the number of CE
marked packets that arrived at the receiver since it sent the
previously received ACK (but see below for the sender's safety
strategy). Whenever the ECI field increments by D (and/or d drops
are detected), the sender MUST clear the RE flag to "0" in the IP
header of the next D' data packets it sends (where D' = D + d),
effectively re-echoing each single increment of ECI. Otherwise
the data sender MUST send all data packets with RE set to "1".
As a general rule, once a flow is established, as well as setting
or clearing the RE flag as above, a data sender in RECN mode MUST
always set the ECN field to ECT(1). However, the settings of the
extended ECN field during flow start are defined in Section 4.1.4.
As we have already emphasised, the re-ECN protocol makes no
changes and has no effect on the TCP congestion control algorithm.
So, each increment of ECI (or detection of a drop) also triggers
the standard TCP congestion response, but with no more than one
congestion response per round trip, as usual.
A TCP sender also acts as the receiver for the other half-
connection. The host will maintain two ECC values S.ECC and R.ECC
as sender and receiver respectively. Every TCP header sent by a
host in RECN mode will also repeat the prevailing value of R.ECC
in its ECI field. If a sender in RECN mode has to retransmit a
packet due to a suspected loss, the re-transmitted packet MUST
carry the latest prevailing value of R.ECC when it is re-
transmitted, which will not necessarily be the one it carried
originally.
4.1.1.1. Drops and Marks
Re-ECN is based on the ECN protocol [RFC3168] which in turn is
typically based on the RED algorithm [RFC2309]. This algorithm marks
packets as CE with a probability that increases as the size of the
router queue increases. Howeverif the queue becomes too full then it
will revert to dropping packets. Because of this it is important
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that re-ECN treats each packet drop it detects as if it were actually
a CE mark. This ensures that it can continue to correctly echo
congestion even through a highly congested path.
In order to ensure that drops are correctly echoed the sender needs
to add the number of drops detected per RTT to the difference in ECI
value waiting to be echoed. A drop is defined as set out in
[RFC2581] -- if the connection is in slow start then a single
duplicate aknowledgement will be treated as an indication of a drop.
When the system is in the congestion avoidance stage then 3 duplicate
acknowledgements will be treated as a sign of a drop. In all cases,
if a re-transmission time-out occurs then that will be treatd as a
drop.
4.1.1.2. Safety against Long Pure ACK Loss Sequences
The ECI method was chosen for echoing congestion marking because a
re-ECN sender needs to know about every CE mark arriving at the
receiver, not just whether at least one arrives within a round trip
time (which is all the ECE/CWR mechanism supported). And, as pure
ACKs are not protected by TCP reliable delivery, we repeat the same
ECI value in every ACK until it changes. Even if many ACKs in a row
are lost, as soon as one gets through, the ECI field it repeats from
previous ACKs that didn't get through will update the sender on how
many CE marks arrived since the last ACK got through.
The sender will only lose a record of the arrival of a CE mark if all
the ACKS are lost (and all of them were pure ACKs) for a stream of
data long enough to contain 8 or more CE marks. So, if the marking
fraction was p, at least 8/p pure ACKs would have to be lost. For
example, if p was 5%, a sequence of 160 pure ACKs would all have to
be lost. To protect against such extremely unlikely events, if a re-
ECN sender detects a sequence of pure ACKs has been lost it SHOULD
assume the ECI field wrapped as many times as possible within the
sequence.
Specifically, if a re-ECN sender receives an ACK with an
acknowledgement number that acknowledges L segments since the
previous ACK but with a sequence number unchanged from the previously
received ACK, it SHOULD conservatively assume that the ECI field
incremented by D' = L - ((L-D) mod 8), where D is the apparent
increase in the ECI field. For example if the ACK arriving after 9
pure ACK losses apparently increased ECI by 2, the assumed increment
of ECI would still be 2. But if ECI apparently increased by 2 after
11 pure ACK losses, ECI should be assumed to have increased by 10.
A re-ECN sender MAY implement a heuristic algorithm to predict beyond
reasonable doubt that the ECI field probably did not wrap within a
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sequence of lost pure ACKs. But such an algorithm is NOT REQUIRED.
Such an algorithm MUST NOT be used unless it is proven to work even
in the presence of correlation between high ACK loss rate on the back
channel and high CE marking rate on the forward channel.
Whatever assumption a re-ECN sender makes about potentially lost CE
marks, both its congestion control and its re-echoing behaviour
SHOULD be consistent with the assumption it makes.
4.1.2. RECN-Co mode: Re-ECT Sender with a Vanilla or Nonce ECT Receiver
If the half-connection is in RECN-Co mode, ECN feedback proceeds no
differently to that of vanilla ECN. In other words, the receiver
sets the ECE flag repeatedly in the TCP header and the sender
responds by setting the CWR flag. Although RECN-Co mode is used when
the receiver has not implemented the re-ECN protocol, the sender can
infer enough from its vanilla ECN feedback to set or clear the RE
flag reasonably well. Specifically, every time the receiver toggles
the ECE field from "0" to "1" (or a loss is detected), as well as
setting CWR in the TCP flags, the re-ECN sender MUST blank the RE
flag of the next packet to "0" as it would do in full RECN mode.
Otherwise, the data sender SHOULD send all other packets with RE set
to "1". Once a flow is established, a re-ECN data sender in RECN-Co
mode MUST always set the ECN field to ECT(1).
If a CE marked packet arrives at the receiver within a round trip
time of a previous mark, the receiver will still be echoing ECE for
the last CE mark. Therefore, such a mark will be missed by the
sender. Of course, this isn't of concern for congestion control, but
it does mean that very occasionally the RE blanking fraction will be
understated. Therefore flows in RECN-Co mode may occasionally be
mistaken for very lightly cheating flows and consequently might
suffer a small number of packet drops through an egress dropper
(Section 6.1.4). We expect re-ECN would be deployed for some time
before policers and droppers start to enforce it. So, given there is
not much ECN deployment yet anyway, this minor problem may affect
only a very small proportion of flows, reducing to nothing over the
years as vanilla ECN hosts upgrade. The use of RECN-Co mode would
need to be reviewed in the light of experience at the time of re-ECN
deployment.
RECN-Co mode is OPTIONAL. Re-ECN implementers who want to keep their
code simple, MAY choose not to implement this mode. If they do not,
a re-ECN sender SHOULD fall back to vanilla ECT mode in the presence
of an ECN-capable receiver. It MAY choose to fall back to the ECT-
Nonce mode, but if re-ECN implementers don't want to be bothered with
RECN-Co mode, they probably won't want to add an ECT-Nonce mode
either.
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4.1.2.1. Re-ECN support for the ECN Nonce
A TCP half-connection in RECN-Co mode MUST NOT support the ECN
Nonce [RFC3540]. This means that the sending code of a re-ECN
implementation will never need to include ECN Nonce support. Re-ECN
is intended to provide wider protection than the ECN nonce against
congestion control misbehaviour, and re-ECN only requires support
from the sender, therefore it is preferable to specifically rule out
the need for dual sender implementations. As a consequence, a re-ECN
capable sender will never set ECT(0), so it will be easier for
network elements to discriminate re-ECN traffic flows from other ECN
traffic, which will always contain some ECT(0) packets.
However, a re-ECN implementation MAY OPTIONALLY include receiving
code that complies with the ECN Nonce protocol when interacting with
a sender that supports the ECN nonce (rather than re-ECN), but this
support is NOT REQUIRED.
RFC3540 allows an ECN nonce sender to choose whether to sanction a
receiver that does not ever set the nonce sum. Given re-ECN is
intended to provide wider protection than the ECN nonce against
congestion control misbehaviour, implementers of re-ECN receivers MAY
choose not to implement backwards compatibility with the ECN nonce
capability. This may be because they deem that the risk of sanctions
is low, perhaps because significant deployment of the ECN nonce seems
unlikely at implementation time.
4.1.3. Capability Negotiation
During the TCP hand-shake at the start of a connection, an originator
of the connection (host A) with a re-ECN-capable transport MUST
indicate it is Re-ECT by setting the TCP options NS=1, CWR=1 and
ECE=1 in the initial SYN.
A responding Re-ECT host (host B) MUST return a SYN ACK with flags
CWR=1 and ECE=0. The responding host MUST NOT set this combination
of flags unless the preceding SYN has already indicated Re-ECT
support as above. A Re-ECT server (B) can use either setting of the
NS flag combined with this type of SYN ACK in response to a SYN from
a Re-ECT client (A). Normally a Re-ECT server will reply to a Re-ECT
client with NS=0, but in the special circumstance below it can return
a SYN ACK with NS=1.
If the initial SYN from Re-ECT client A is marked CE(-1), a Re-ECT
server B MUST increment its local value of ECC. But B cannot reflect
the value of ECC in the SYN ACK, because it is still using the 3 bits
to negotiate connection capabilities. So, server B MUST set the
alternative TCP header flags in its SYN ACK: NS=1, CWR=1 and ECE=0.
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These handshakes are summarised in Table 5 below, with X meaning
`don't care'. The handshakes used for the other flavours of ECN are
also shown for comparison. To compress the width of the table, the
headings of the first four columns have been severely abbreviated, as
follows:
R: *R*e-ECT
N: ECT-*N*once (RFC3540)
E: *E*CT (RFC3168)
I: Not-ECT (*I*mplicit congestion notification).
These correspond with the same headings used in Table 4. Indeed, the
resulting modes in the last two columns of the table below are a more
comprehensive way of saying the same thing as Table 4.
+----+---+---+---+------------+-------------+-----------+-----------+
| R | N | E | I | SYN A-B | SYN ACK B-A | A-B Mode | B-A Mode |
+----+---+---+---+------------+-------------+-----------+-----------+
| | | | | NS CWR ECE | NS CWR ECE | | |
| AB | | | | 1 1 1 | X 1 0 | RECN | RECN |
| A | B | | | 1 1 1 | 1 0 1 | RECN-Co | ECT-Nonce |
| A | | B | | 1 1 1 | 0 0 1 | RECN-Co | ECT |
| A | | | B | 1 1 1 | 0 0 0 | Not-ECT | Not-ECT |
| B | A | | | 0 1 1 | 0 0 1 | ECT-Nonce | RECN-Co |
| B | | A | | 0 1 1 | 0 0 1 | ECT | RECN-Co |
| B | | | A | 0 0 0 | 0 0 0 | Not-ECT | Not-ECT |
+----+---+---+---+------------+-------------+-----------+-----------+
Table 5: TCP Capability Negotiation between Originator (A) and
Responder (B)
As soon as a re-ECN capable TCP server receives a SYN, it MUST set
its two half-connections into the modes given in Table 5. As soon as
a re-ECN capable TCP client receives a SYN ACK, it MUST set its two
half-connections into the modes given in Table 5. The half-
connections will remain in these modes for the rest of the
connection, including for the third segment of TCP's three-way hand-
shake (the ACK).
{ToDo: Consider SYNs within a connection.}
Recall that, if the SYN ACK reflects the same flag settings as the
preceding SYN (because there is a broken legacy implementation that
behaves this way), RFC3168 specifies that the whole connection MUST
revert to Not-ECT.
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Also note that, whenever the SYN flag of a TCP segment is set
(including when the ACK flag is also set), the NS, CWR and ECE flags
MUST NOT be interpreted as the 3-bit ECI value, which is only set as
a copy of the local ECC value in non-SYN packets.
4.1.4. Extended ECN (EECN) Field Settings during Flow Start or after
Idle Periods
If the originator (A) of a TCP connection supports re-ECN it MUST set
the extended ECN (EECN) field in the IP header of the initial SYN
packet to the feedback not established (FNE) codepoint.
FNE is a new extended ECN codepoint defined by this specification
(Section 3.2). The feedback not established (FNE) codepoint is used
when the transport does not have the benefit of ECN feedback so it
cannot decide whether to set or clear the RE flag.
If after receiving a SYN the server B has set its sending half-
connection into RECN mode or RECN-Co mode, it MUST set the extended
ECN field in the IP header of its SYN ACK to the feedback not
established (FNE) codepoint. Note the careful wording here, which
means that Re-ECT server B MUST set FNE on a SYN ACK whether it is
responding to a SYN from a Re-ECT client or from a client that is
merely ECN-capable.
The original ECN specification [RFC3168] required SYNs and SYN ACKs
to use the Not-ECT codepoint of the ECN field. The aim was to
prevent well-known DoS attacks such as SYN flooding being able to
gain from the advantage that ECN capability afforded over drop at
ECN-capable routers.
For a SYN ACK, Kuzmanovic [I-D.ietf-tcpm-ecnsyn] has shown that this
caution was unnecessary, and proposes to allow a SYN ACK to be ECN-
capable to improve performance. We have gone further by proposing to
make the initial SYN ECN-capable too. By stipulating the FNE
codepoint for the initial SYN, we comply with RFC3168 in word but not
in spirit, because we have indeed set the ECN field to Not-ECT, but
we have extended the ECN field with another bit. And it will be seen
(Section 5.3) that we have defined one setting of that bit to mean an
ECN-capable transport. Therefore, by proposing that the FNE
codepoint MUST be used on the initial SYN of a connection, we have
(deliberately) made the initial SYN ECN-capable. Section 5.4
justifies deciding to make the initial SYN ECN-capable.
Once a TCP half connection is in RECN mode or RECN-Co mode, FNE will
have already been set on the initial SYN and possibly the SYN ACK as
above. But each re-ECN sender will have to set FNE cautiously on a
few data packets as well, given a number of packets will usually have
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to be sent before sufficient congestion feedback is received. The
behaviour will be different depending on the mode of the half-
connection:
RECN mode: Given the constraints on TCP's initial window [RFC3390]
and its exponential window increase during slow start
phase [RFC2581], it turns out that the sender SHOULD set FNE on
the first and third data packets in its flow, assuming equal sized
data packets once a flow is established. Appendix D presents the
calculation that led to this conclusion. Below, after running
through the start of an example TCP session, we give the intuition
learned from that calculation.
RECN-Co mode: A re-ECT sender that switches into re-ECN
compatibility mode or into Not-ECT mode (because it has detected
the corresponding host is not re-ECN capable) MUST limit its
initial window to 1 segment. The reasoning behind this constraint
is given in Section 5.4. Having set this initial window, a re-ECN
sender in RECN-Co mode SHOULD set FNE on the first and third data
packets in a flow, as for RECN mode.
+----+------+----------------+-------+-------+---------------+------+
| | Data | TCP A(Re-ECT) | IP A | IP B | TCP B(Re-ECT) | Data |
+----+------+----------------+-------+-------+---------------+------+
| | Byte | SEQ ACK CTL | EECN | EECN | SEQ ACK CTL | Byte |
| -- | ---- | ------------- | ----- | ----- | ------------- | ---- |
| 1 | | 0100 SYN | FNE | --> | R.ECC=0 | |
| | | CWR,ECE,NS | | | | |
| 2 | | R.ECC=0 | <-- | FNE | 0300 0101 | |
| | | | | | SYN,ACK,CWR | |
| 3 | | 0101 0301 ACK | RECT | --> | R.ECC=0 | |
| 4 | 1000 | 0101 0301 ACK | FNE | --> | R.ECC=0 | |
| 5 | | R.ECC=0 | <-- | FNE | 0301 1102 ACK | 1460 |
| 6 | | R.ECC=0 | <-- | RECT | 1762 1102 ACK | 1460 |
| 7 | | R.ECC=0 | <-- | FNE | 3222 1102 ACK | 1460 |
| 8 | | 1102 1762 ACK | RECT | --> | R.ECC=0 | |
| 9 | | R.ECC=0 | <-- | RECT | 4682 1102 ACK | 1460 |
| 10 | | R.ECC=0 | <-- | RECT | 6142 1102 ACK | 1460 |
| 11 | | 1102 3222 ACK | RECT | --> | R.ECC=0 | |
| 12 | | R.ECC=0 | <-- | RECT | 7602 1102 ACK | 1460 |
| 13 | | R.ECC=1 | <*- | RECT | 9062 1102 ACK | 1460 |
| | | ... | | | | |
+----+------+----------------+-------+-------+---------------+------+
Table 6: TCP Session Example #1
Table 6 shows an example TCP session, where the server B sets FNE on
its first and third data packets (lines 5 & 7) as well as on the
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initial SYN ACK as previously described. The left hand half of the
table shows the relevant settings of headers sent by client A in
three layers: the TCP payload size; TCP settings; then IP settings.
The right hand half gives equivalent columns for server B. The only
TCP settings shown are the sequence number (SEQ), acknowledgement
number (ACK) and the relevant control (CTL) flags that A sets in the
TCP header. The IP columns show the setting of the extended ECN
(EECN) field.
Also shown on the receiving side of the table is the value of the
receiver's echo congestion counter (R.ECC) after processing the
incoming EECN header. Note that, once a host sets a half-connection
into RECN mode, it MUST initialise its local value of ECC to zero.
The intuition that Appendix D gives for why a sender should set FNE
on the first and third data packets is as follows. At line 13, a
packet sent by B is shown with an '*', which means it has been
congestion marked by an intermediate router from RECT to CE(-1). On
receiving this CE marked packet, client A increments its ECC counter
to 1 as shown. This was the 7th data packet B sent, but before
feedback about this event returns to B, it might well have sent many
more packets. Indeed, during exponential slow start, about as many
packets will be in flight (unacknowledged) as have been acknowledged.
So, when the feedback from the congestion event on B's 7th segment
returns, B will have sent about 7 further packets that will still be
in flight. At that stage, B's best estimate of the network's packet
marking fraction will be 1/7. So, as B will have sent about 14
packets, it should have already marked 2 of them as FNE in order to
have marked 1/7; hence the need to have set the first and third data
packets to FNE.
Client A's behaviour in Table 6 also shows FNE being set on the first
SYN and the first data packet (lines 1 & 4), but in this case it
sends no more data packets, so of course, it cannot, and does not
need to, set FNE again. Note that in the A-B direction there is no
need to set FNE on the third part of the three-way hand-shake (line
3---the ACK).
Note that in this section we have used the word SHOULD rather than
MUST when specifying how to set FNE on data segments before positive
congestion feedback arrives (but note that the word MUST was used for
FNE on the SYN and SYN ACK). FNE is only RECOMMENDED for the first
and third data segments to entertain the possibility that the TCP
transport has the benefit of other knowledge of the path, which it
re-uses from one flow for the benefit of a newly starting flow. For
instance, one flow can re-use knowledge of other flows between the
same hosts if using a Congestion Manager [RFC3124] or when a proxy
host aggregates congestion information for large numbers of flows.
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After an idle period of more than 1 second, a re-ECN sender transport
MUST set the EECN field of the packet that resumes the connection to
FNE. Note that this next packet may be sent a very long time later,
a packet does NOT have to be sent after 1 second of idling. In order
that the design of network policers can be deterministic, this
specification deliberately puts an absolute lower limit on how long a
connection can be idle before the packet that resumes the connection
must be set to FNE, rather than relating it to the connection round
trip time. We use the lower bound of the retransmission timeout
(RTO) [RFC2988], which is commonly used as the idle period before TCP
must reduce to the restart window [RFC2581]. Note our specification
of re-ECN's idle period is NOT intended to change the idle period for
TCP's restart, nor indeed for any other purposes.
{ToDo: Describe how the sender falls back to legacy modes if packets
don't appear to be getting through (to work round firewalls
discarding packets they consider unusual).}
4.1.5. Pure ACKS, Retransmissions, Window Probes and Partial ACKs
A re-ECN sender MUST clear the RE flag to "0" and set the ECN field
to Not-ECT in pure ACKs, retransmissions and window probes, as
specified in [RFC3168]. Our eventual goal is for all packets to be
sent with re-ECN enabled, and we believe the semantics of the ECI
field go a long way towards being able to achieve this. However, we
have not completed a full security analysis for these cases,
therefore, currently we merely re-state current practice.
We must also reconcile the facts that congestion marking is applied
to packets but acknowledgements cover octet ranges and acknowledged
octet boundaries need not match the transmitted boundaries. The
general principle we work to is to remain compatible with TCP's
congestion control which is driven by congestion events at packet
granularity while at the same time aiming to blank the RE flag on at
least as many octets in a flow as have been marked CE.
Therefore, a re-ECN TCP receiver MUST increment its ECC value as many
times as CE marked packets have been received. And that value MUST
be echoed to the sender in the first available ACK using the ECI
field. This ensures the TCP sender's congestion control receives
timely feedback on congestion events at the same packet granularity
that they were generated on congested routers.
Then, a re-ECN sender stores the difference D between its own ECC
value and the incoming ECI field by incrementing a counter R. Then, R
is decremented by 1 each subsequent packet that is sent with the RE
flag blanked, until R is no longer positive. Using this technique,
whenever a re-ECN transport sends a not re-ECN capable (NRECN) packet
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(e.g. a retransmission), the remaining packets required to have the
RE flag blanked will be automatically carried over to subsequent
packets, through the variable R.
This does not ensure precisely the same number of octets have RE
blanked as were CE marked. But we believe positive errors will
cancel negative over a long enough period. {ToDo: However, more
research is needed to prove whether this is so. If it is not, it may
be necessary to increment and decrement R in octets rather than
packets, by incrementing R as the product of D and the size in octets
of packets being sent (typically the MSS).}
4.2. Other Transports
4.2.1. General Guidelines for Adding Re-ECN to Other Transports
Re-ECT sender transports that have established the receiver transport
is at least ECN-capable (not necessarily re-ECN capable) MUST blank
the RE codepoint in packets carrying at least as many octets as
arrive at receiver with the CE codepoint set. Re-ECN-capable sender
transports should always initialise the ECN field to the ECT(1)
codepoint once a flow is established.
If the sender transport does not have sufficient feedback to even
estimate the path's CE rate, it SHOULD set FNE continuously. If the
sender transport has some, perhaps stale, feedback to estimate that
the path's CE rate is nearly definitely less than E%, the transport
MAY blank RE in packets for E% of sent octets, and set the RECT
codepoint for the remainder.
The following sections give guidelines on how re-ECN support could be
added to RSVP or NSIS, to DCCP, and to SCTP - although separate
Internet drafts will be necessary to document the exact mechanics of
re-ECN in each of these protocols.
{ToDo: Give a brief outline of what would be expected for each of the
following:
o UDP fire and forget (e.g. DNS)
o UDP streaming with no feedback
o UDP streaming with feedback
}
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4.2.2. Guidelines for adding Re-ECN to RSVP or NSIS
A separate I-D has been submitted [Re-PCN] describing how re-ECN can
be used in an edge-to-edge rather than end-to-end scenario. It can
then be used by downstream networks to police whether upstream
networks are blocking new flow reservations when downstream
congestion is too high, even though the congestion is in other
operators' downstream networks. This relates to current IETF work on
Admission Control over Diffserv using Pre-Congestion Notification
(PCN) [PCN-arch].
4.2.3. Guidelines for adding Re-ECN to DCCP
Beside adjusting the initial features negotiation sequence, operating
re-ECN in DCCP [RFC4340] could be achieved by defining a new option
to be added to acknowledgments, that would include a multibit field
where the destination could copy its ECC.
4.2.4. Guidelines for adding Re-ECN to SCTP
Annex 1 in [RFC2960] gives the specifications for SCTP to support
ECN. Similar steps should be taken to support re-ECN. Beside
adjusting the initial features negotiation sequence, operating re-ECN
in SCTP could be achieved by defining a new control chunk, that would
include a multibit field where the destination could copy its ECC
5. Network Layer
5.1. Re-ECN IPv4 Wire Protocol
The wire protocol of the ECN field in the IP header remains largely
unchanged from [RFC3168]. However, an extension to the ECN field we
call the RE (re-ECN extension) flag (Section 3.2) is defined in this
document. It doubles the extended ECN codepoint space, giving 8
potential codepoints. The semantics of the extra codepoints are
backward compatible with the semantics of the 4 original codepoints
[RFC3168] (Section 7.1 collects together and summarises all the
changes defined in this document).
For IPv4, this document proposes that the new RE control flag will be
positioned where the `reserved' control flag was at bit 48 of the
IPv4 header (counting from 0). Alternatively, some would call this
bit 0 (counting from 0) of byte 7 (counting from 1) of the IPv4
header (Figure 5).
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0 1 2
+---+---+---+
| R | D | M |
| E | F | F |
+---+---+---+
Figure 5: New Definition of the Re-ECN Extension (RE) Control Flag at
the Start of Byte 7 of the IPv4 Header
The semantics of the RE flag are described in outline in Section 3
and specified fully in Section 4. The RE flag is always considered
in conjunction with the 2-bit ECN field, as if they were concatenated
together to form a 3-bit extended ECN field. If the ECN field is set
to either the ECT(1) or CE codepoint, when the RE flag is blanked
(cleared to "0") it represents a re-echo of congestion experienced by
an early packet. If the ECN field is set to the Not-ECT codepoint,
when the RE flag is set to "1" it represents the feedback not
established (FNE) codepoint, which signals that the packet was sent
without the benefit of congestion feedback.
It is believed that the FNE codepoint can simultaneously serve other
purposes, particularly where the start of a flow needs distinguishing
from packets later in the flow. For instance it would have been
useful to identify new flows for tag switching and might enable
similar developments in the future if it were adopted. It is similar
to the state set-up bit idea designed to protect against memory
exhaustion attacks. This idea was proposed informally by David Clark
and documented by Handley and Greenhalgh [Steps_DoS]. The FNE
codepoint can be thought of as a `soft-state set-up flag', because it
is idempotent (i.e. one occurrence of the flag is sufficient but
further occurrences achieve the same effect if previous ones were
lost).
We are sure there will probably be other claims pending on the use of
bit 48. We know of at least two [ARI05], [RFC3514] but neither have
been pursued in the IETF, so far, although the present proposal would
meet the needs of the former.
The security flag proposal (commonly known as the evil bit) was
published on 1 April 2003 as Informational RFC 3514, but it was not
adopted due to confusion over whether evil-doers might set it
inappropriately. The present proposal is backward compatible with
RFC3514 because if re-ECN compliant senders were benign they would
correctly clear the evil bit to honestly declare that they had just
received congestion feedback. Whereas evil-doers would hide
congestion feedback by setting the evil bit continuously, or at least
more often than they should. So, evil senders can be identified,
because they declare that they are good less often than they should.
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5.2. Re-ECN IPv6 Wire Protocol
For IPv6, this document proposes that the new RE control flag will be
positioned as the first bit of the option field of a new Congestion
hop by hop option header (Figure 6).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr ext Len | Option Type | Opt Length =4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Reserved for future use |
|E| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Definition of a New IPv6 Congestion Hop by Hop Option
Header containing the Re-ECN Extension (RE) Control Flag
0 1 2 3 4 5 6 7 8
+-+-+-+-+-+-+-+-+-
|AIU|C|Option ID|
+-+-+-+-+-+-+-+-+-
Figure 7: Congestion Hop by Hop Option Type Encoding
The Hop-by-Hop Options header enables packets to carry information to
be examined and processed by routers or nodes along the packet's
delivery path, including the source and destination nodes. For re-
ECN, the two bits of the Action If Unrecognized (AIU) flag of the
Congestion extension header MUST be set to "00" meaning if
unrecognized `skip over option and continue processing the header'.
Then, any routers or a receiver not upgraded with the optional re-ECN
features described in this memo will simply ignore this header. But
routers with these optional re-ECN features or a re-ECN policing
function, will process this Congestion extension header.
The `C' flag MUST be set to "1" to specify that the Option Data
(currently only the RE control flag) can change en-route to the
packet's final destination. This ensures that, when an
Authentication header (AH [RFC2402]) is present in the packet, for
any option whose data may change en-route, its entire Option Data
field will be treated as zero-valued octets when computing or
verifying the packet's authenticating value.
Although the RE control flag should not be changed along the path, we
expect that the rest of this option field that is currently `Reserved
for future use' could be used for a multi-bit congestion notification
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field which we would expect to change en route. As the RE flag does
not need end-to-end authentication, we set the C flag to '1'.
{ToDo: A Congestion Hop by Hop Option ID will need to be registered
with IANA.}
5.3. Router Forwarding Behaviour
Re-ECN works well without modifying the forwarding behaviour of any
routers. However, below, two OPTIONAL changes to forwarding
behaviour are defined which respectively enhance performance and
improve a router's discrimination against flooding attacks. They are
both OPTIONAL additions that we propose MAY apply by default to all
Diffserv per-hop scheduling behaviours (PHBs) [RFC2475] and ECN
marking behaviours [RFC3168]. Specifications for PHBs MAY define
different forwarding behaviours from this default, but this is NOT
REQUIRED. [Re-PCN] is one example.
FNE indicates ECT:
The FNE codepoint tells a router to assume that the packet was
sent by an ECN-capable transport (see Section 5.4). Therefore an
FNE packet MAY be marked rather than dropped. Note that the FNE
codepoint has been intentionally chosen so that, to legacy routers
(which do not inspect the RE flag) an FNE packet appears to be
Not-ECT so it will be dropped by legacy AQM algorithms.
A network operator MUST NOT configure a router to ECN mark rather
than drop FNE packets unless it can guarantee that FNE packets
will be rate limited, either locally or upstream. The ingress
policers discussed in Section 6.1.5 would count as rate limiters
for this purpose.
Preferential Drop: If a re-ECN capable router experiences very high
load so that it has to drop arriving packets (e.g. a DoS attack),
it MAY preferentially drop packets within the same Diffserv PHB
using the preference order for extended ECN codepoints given in
Table 7. Preferential dropping can be difficult to implement on
some hardware, but if feasible it would discriminate against
attack traffic if done as part of the overall policing framework
of Section 6.1.3. If nowhere else, routers at the egress of a
network SHOULD implement preferential drop (stronger than the MAY
above). For simplicity, preferences 4 & 5 MAY be merged into one
preference level.
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+-------+-----+------------+-------+------------+-------------------+
| ECN | RE | Extended | Worth | Drop Pref | Re-ECN meaning |
| field | bit | ECN | | (1 = drop | |
| | | codepoint | | 1st) | |
+-------+-----+------------+-------+------------+-------------------+
| 01 | 0 | Re-Echo | +1 | 5/4 | Re-echoed |
| | | | | | congestion and |
| | | | | | RECT |
| 00 | 1 | FNE | +1 | 4 | Feedback not |
| | | | | | established |
| 11 | 0 | CE(0) | 0 | 3 | Re-Echo canceled |
| | | | | | by congestion |
| | | | | | experienced |
| 01 | 1 | RECT | 0 | 3 | Re-ECN capable |
| | | | | | transport |
| 11 | 1 | CE(-1) | -1 | 3 | Congestion |
| | | | | | experienced |
| 10 | 1 | --CU-- | n/a | 2 | Currently Unused |
| 10 | 0 | --- | n/a | 2 | Legacy ECN use |
| | | | | | only |
| 00 | 0 | Not-RECT | n/a | 1 | Not |
| | | | | | re-ECN-capable |
| | | | | | transport |
+-------+-----+------------+-------+------------+-------------------+
Table 7: Drop Preference of EECN Codepoints (Sorted by `Worth')
The above drop preferences are arranged to preserve packets with
more positive worth (Section 3.4), given senders of positive
packets must have honestly declared downstream congestion. This
is explained fully in Section 6 on applications, particularly when
the application of re-ECN to protect against DDoS attacks is
described.
5.4. Justification for Setting the First SYN to FNE
Congested routers may mark an FNE packet to CE(-1) (Section 5.3), and
the initial SYN MUST be set to FNE by Re-ECT client A
(Section 4.1.4). So an initial SYN may be marked CE(-1) rather than
dropped. This seems dangerous, because the sender has not yet
established whether the receiver is a legacy one that does not
understand congestion marking. It also seems to allow malicious
senders to take advantage of ECN marking to avoid so much drop when
launching SYN flooding attacks. Below we explain the features of the
protocol design that remove both these dangers.
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ECN-capable initial SYN with a Not-ECT server: If the TCP server B
is re-ECN capable, provision is made for it to feedback a possible
congestion marked SYN in the SYN ACK (Section 4.1.4). But if the
TCP client A finds out from the SYN ACK that the server was not
ECN-capable, the TCP client MUST consider the first SYN as
congestion marked before setting itself into Not-ECT mode.
Section 4.1.4 mandates that such a TCP client MUST also set its
initial window to 1 segment. In this way we remove the need to
cautiously avoid setting the first SYN to Not-RECT. This will
give worse performance while deployment is patchy, but better
performance once deployment is widespread.
SYN flooding attacks can't exploit ECN-capability: Malicious hosts
may think they can use the advantage that ECN-marking gives over
drop in launching classic SYN-flood attacks. But Section 5.3
mandates that a router MUST only be configured to treat packets
with the FNE codepoint as ECN-capable if FNE packets are rate
limited. Introduction of the FNE codepoint was a deliberate move
to enable transport-neutral handling of flow-start and flow state
set-up in the IP layer where it belongs. It then becomes possible
to protect against flooding attacks of all forms (not just SYN
flooding) without transport-specific inspection for things like
the SYN flag in TCP headers. Then, for instance, SYN flooding
attacks using IPSec ESP encryption can also be rate limited at the
IP layer.
It might seem pedantic going to all this trouble to enable ECN on the
initial packet of a flow, but it is motivated by a much wider concern
to ensure safe congestion control will still be possible even if the
application mix evolves to the point where the majority of flows
consist of a single window or even a single packet. It also allows
denial of service attacks to be more easily isolated and prevented.
5.5. Control and Management
5.5.1. Negative Balance Warning
A new ICMP message type is being considered so that a dropper can
warn the apparent sender of a flow that it has started to sanction
the flow. The message would have similar semantics to the `Time
exceeded' ICMP message type. To ensure the sender has to invest some
work before the network will generate such a message, a dropper
SHOULD only send such a message for flows that have demonstrated that
they have started correctly by establishing a positive record, but
have later gone negative. The threshold is up to the implementation.
The purpose of the message is to deconfuse the cause of drops from
other causes, such as congestion or transmission losses. The dropper
would send the message to the sender of the flow, not the receiver.
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If we did define this message type, it would be REQUIRED for all re-
ECT senders to parse and understand it. Note that a sender MUST only
use this message to explain why losses are occurring. A sender MUST
NOT take this message to mean that losses have occurred that it was
not aware of. Otherwise, spoof messages could be sent by malicious
sources to slow down a sender (c.f. ICMP source quench).
However, the need for this message type is not yet confirmed, as we
are considering how to prevent it being used by malicious senders to
scan for droppers and to test their threshold settings. {ToDo:
Complete this section.}
5.5.2. Rate Response Control
As discussed in Section 6.1.5 the sender's access operator will be
expected to use bulk per-user policing, but they might choose to
introduce a per-flow policer. In cases where operators do introduce
per-flow policing, there may be a need for a sender to send a request
to the ingress policer asking for permission to apply a non-default
response to congestion (where TCP-friendly is assumed to be the
default). This would require the sender to know what message
format(s) to use and to be able to discover how to address the
policer. The required control protocol(s) are outside the scope of
this document, but will require definition elsewhere.
The policer is likely to be local to the sender and inline, probably
at the ingress interface to the internetwork. So, discovery should
not be hard. A variety of control protocols already exist for some
widely used rate-responses to congestion. For instance DCCP
congestion control identifiers (CCIDs [RFC4340]) fulfil this role and
so does QoS signalling (e.g. and RSVP request for controlled load
service is equivalent to a request for no rate response to
congestion, but with admission control).
5.6. IP in IP Tunnels
For re-ECN to work correctly through IP in IP tunnels, it needs
slightly different tunnel handling to regular ECN [RFC3168].
Currently there is some incosistency between how the handling of IP
in IP tunnels is defined in [RFC3168] and how it is defined in
[RFC4301], but re-ECN would work fine with the IPsec behaviour. This
inconsistency is addressed in a new Internet Draft [ECN-tunnel] that
proposes to update RFC3168 tunnel behaviour to bring it into line
with IPsec. Ideally, for re-ECN to work through a tunnel, the tunnel
entry should copy both the RE flag and the ECN field from the inner
to the outer IP header. Then at the tunnel exit, any congestion
marking of the outer ECN field should overwrite the inner ECN field
(unless the inner field is Not-ECT in which case an alarm should be
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raised). The RE flag shouldn't change along a path, so the outer RE
flag should be the same as the inner. If it isn't a management alarm
should be raised. This behaviour is the same as the full-
functionality variant of [RFC3168] at tunnel exit, but different at
tunnel entry.
If tunnels are left as they are specified in [RFC3168], whether the
limited or full-functionality variants are used, a problem arises
with re-ECN if a tunnel crosses an inter-domain boundary, because the
difference between positive and negative markings will not be
correctly accounted for. In a limited functionality ECN tunnel, the
flow will appear to be legacy traffic, and therefore may be wrongly
rate limited. In a full-functionality ECN tunnel, the result will
depend whether the tunnel entry copies the inner RE flag to the outer
header or the RE flag in the outer header is always cleared. If the
former, the flow will tend to be too positive when accounted for at
borders. If the latter, it will be too negative. If the rules set
out in [ECN-tunnel] are followed then this will not be an issue.
5.7. Non-Issues
The following issues might seem to cause unfavourable interactions
with re-ECN, but we will explain why they don't:
o Various link layers support explicit congestion notification, such
as Frame Relay and ATM. Explicit congestion notification is
proposed to be added to other link layers, such as Ethernet
(802.3ar Ethernet congestion management) and MPLS [ECN-MPLS];
o Encryption and IPSec.
In the case of congestion notification at the link layer, each
particular link layer scheme either manages congestion on the link
with its own link-level feedback (the usual arrangement in the cases
of ATM and Frame Relay), or congestion notification from the link
layer is merged into congestion notification at the IP level when the
frame headers are decapsulated at the end of the link (the
recommended arrangement in the Ethernet and MPLS cases). Given the
RE flag is not intended to change along the path, this means that
downstream congestion will still be measureable at any point where IP
is processed on the path by subtracting positive from negative
markings.
In the case of encryption, as long as the tunnel issues described in
Section 5.6 are dealt with, payload encryption itself will not be a
problem. The design goal of re-ECN is to include downstream
congestion in the IP header so that it is not necessary to bury into
inner headers. Obfuscation of flow identifiers is not a problem for
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re-ECN policing elements. Re-ECN doesn't ever require flow
identifiers to be valid, it only requires them to be unique. So if
an IPSec encapsulating security payload (ESP [RFC2406]) or an
authentication header (AH [RFC2402]) is used, the security parameters
index (SPI) will be a sufficient flow identifier, as it is intended
to be unique to a flow without revealing actual port numbers.
In general, even if endpoints use some locally agreed scheme to hide
port numbers, re-ECN policing elements can just consider the pair of
source and destination IP addresses as the flow identifier. Re-ECN
encourages endpoints to at least tell the network layer that a
sequence of packets are all part of the same flow, if indeed they
are. The alternative would be for the sender to make each packet
appear to be a new flow, which would require them all to be marked
FNE in order to avoid being treated with the bulk of malicious flows
at the egress dropper. Given the FNE marking is worth +1 and
networks are likely to rate limit FNE packets, endpoints are given an
incentive not to set FNE on each packet. But if the sender really
does want to hide the flow relationship between packets it can choose
to pay the cost of multiple FNE packets, which in the long run will
compensate for the extra memory required on network policing elements
to process each flow.
6. Applications
6.1. Policing Congestion Response
6.1.1. The Policing Problem
The current Internet architecture trusts hosts to respond voluntarily
to congestion. Limited evidence shows that the large majority of
end-points on the Internet comply with a TCP-friendly response to
congestion. But telephony (and increasingly video) services over the
best effort Internet are attracting the interest of major commercial
operations. Most of these applications do not respond to congestion
at all. Those that can switch to lower rate codecs, still have a
lower bound below which they must become unresponsive to congestion.
Of course, the Internet is intended to support many different
application behaviours. But the problem is that this freedom can be
exercised irresponsibly. The greater problem is that we will never
be able to agree on where the boundary is between responsible and
irresponsible. Therefore re-ECN is designed to allow different
networks to set their own view of the limit to irresponsibility, and
to allow networks that choose a more conservative limit to push back
against congestion caused in more liberal networks.
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As an example of the impossibility of setting a standard for
fairness, mandating TCP-friendliness would set the bar too high for
unresponsive streaming media, but still some would say the bar was
too low. Even though all known peer-to-peer filesharing applications
are TCP-compatible, they can cause a disproportionate amount of
congestion, simply by using multiple flows and by transferring data
continuously relative to other short-lived sessions. On the other
hand, if we swung the other way and set the bar low enough to allow
streaming media to be unresponsive, we would also allow denial of
service attacks, which are typically unresponsive to congestion and
consist of multiple continuous flows.
Applications that need (or choose) to be unresponsive to congestion
can effectively take (some would say steal) whatever share of
bottleneck resources they want from responsive flows. Whether or not
such free-riding is common, inability to prevent it increases the
risk of poor returns for investors in network infrastructure, leading
to under-investment. An increasing proportion of unresponsive or
free-riding demand coupled with persistent under-supply is a broken
economic cycle. Therefore, if the current, largely co-operative
consensus continues to erode, congestion collapse could become more
common in more areas of the Internet [RFC3714].
While we have designed re-ECN so that networks can choose to deploy
stringent policing, this does not imply we advocate that every
network should introduce tight controls on those that cause
congestion. Re-ECN has been specifically designed to allow different
networks to choose how conservative or liberal they wish to be with
respect to policing congestion. But those that choose to be
conservative can protect themselves from the excesses that liberal
networks allow their users.
6.1.2. The Case Against Bottleneck Policing
The state of the art in rate policing is the bottleneck policer,
which is intended to be deployed at any forwarding resource that may
become congested. Its aim is to detect flows that cause
significantly more local congestion than others. Although operators
might solve their immediate problems by deploying bottleneck
policers, we are concerned that widespread deployment would make it
extremely hard to evolve new application behaviours. We believe the
IETF should offer re-ECN as the preferred protocol on which to base
solutions to the policing problems of operators, because it would not
harm evolvability and, frankly, it would be far more effective (see
later for why).
Approaches like [XCHOKe] & [pBox] are nice approaches for rate
policing traffic without the benefit of whole path information (such
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as could be provided by re-ECN). But they must be deployed at
bottlenecks in order to work. Unfortunately, a large proportion of
traffic traverses at least two bottlenecks (in two access networks),
particularly with the current traffic mix where peer-to-peer file-
sharing is prevalent. If ECN were deployed, we believe it would be
likely that these bottleneck policers would be adapted to combine ECN
congestion marking from the upstream path with local congestion
knowledge. But then the only useful placement for such policers
would be close to the egress of the internetwork.
But then, if these bottleneck policers were widely deployed (which
would require them to be more effective than they are now), the
Internet would find itself with one universal rate adaptation policy
(probably TCP-friendliness) embedded throughout the network. Given
TCP's congestion control algorithm is already known to be hitting its
scalability limits and new algorithms are being developed for high-
speed congestion control, embedding TCP policing into the Internet
would make evolution to new algorithms extremely painful. If a
source wanted to use a different algorithm, it would have to first
discover then negotiate with all the policers on its path,
particularly those in the far access network. The IETF has already
traveled that path with the Intserv architecture and found it
constrains scalability [RFC2208].
Anyway, if bottleneck policers were ever widely deployed, they would
be likely to be bypassed by determined attackers. They inherently
have to police fairness per flow or per source-destination pair.
Therefore they can easily be circumvented either by opening multiple
flows (by varying the end-point port number); or by spoofing the
source address but arranging with the receiver to hide the true
return address at a higher layer.
6.1.3. Re-ECN Incentive Framework
The aim is to create an incentive environment that ensures optimal
sharing of capacity despite everyone acting selfishly (including
lying and cheating). Of course, the mechanisms put in place for this
can lie dormant wherever co-operation is the norm.
Throughout this document we focus on path congestion. But some forms
of fairness, particularly TCP's, also depend on round trip time. If
TCP-fairness is required, we also propose to measure downstream path
delay using re-feedback. We give a simple outline of how this could
work in Appendix F. However, we do not expect this to be necessary,
as researchers tend to agree that only congestion control dynamics
need to depend on RTT, not the rate that the algorithm would converge
on after a period of stability.
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Figure 8 sketches the incentive framework that we will describe piece
by piece throughout this section. We will do a first pass in
overview, then return to each piece in detail. We re-use the earlier
example of how downstream congestion is derived by subtracting
upstream congestion from path congestion (Figure 1) but depict
multiple trust boundaries to turn it into an internetwork. For
clarity, only downstream congestion is shown (the difference between
the two earlier plots). The graph displays downstream path
congestion seen in a typical flow as it traverses an example path
from sender S to receiver R, across networks N1, N2 & N4. Everyone
is shown using re-ECN correctly, but we intend to show why everyone
would /choose/ to use it correctly, and honestly.
Three main types of self-interest can be identified:
o Users want to transmit data across the network as fast as
possible, paying as little as possible for the privilege. In this
respect, there is no distinction between senders and receivers,
but we must be wary of potential malice by one on the other;
o Network operators want to maximise revenues from the resources
they invest in. They compete amongst themselves for the custom of
users.
o Attackers (whether users or networks) want to use any opportunity
to subvert the new re-ECN system for their own gain or to damage
the service of their victims, whether targeted or random.
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policer
|
|
S <-----N1----> <---N2---> <---N4--> R domain
| : :
A\|/: :
| V : :
3% |---------+ :
| : | :
2% | : +-----------------------+ :
| : downstream congestion | :
1% | : | :
| : | :
0% +---------------------------------+=====-->
0 i ^ resource index
| | /|\
1.00% 2.00% | marking fraction
|
dropper
Figure 8: Incentive Framework, showing creation of opposing pressures
to under-declare and over-declare downstream congestion, using a
policer and a dropper
Source congestion control: We want to ensure that the sender will
throttle its rate as downstream congestion increases. Whatever
the agreed congestion response (whether TCP-compatible or some
enhanced QoS), to some extent it will always be against the
sender's interest to comply.
Ingress policing: But it is in all the network operators' interests
to encourage fair congestion response, so that their investments
are employed to satisfy the most valuable demand. The re-ECN
protocol ensures packets carry the necessary information about
their own expected downstream congestion so that N1 can deploy a
policer at its ingress to check that S1 is complying with whatever
congestion control it should be using (Section 6.1.5). If N1 is
extremely conservative it could police each flow, but it is likely
to just police the bulk amount of congestion each customer causes
without regard to flows, or if it is extremely liberal it need not
police congestion control at all. Whatever, it is always
preferable to police traffic at the very first ingress into an
internetwork, before non-compliant traffic can cause any damage.
Edge egress dropper: If the policer ensures the source has less
right to a high rate the higher it declares downstream congestion,
the source has a clear incentive to understate downstream
congestion. But, if flows of packets are understated when they
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enter the internetwork, they will have become negative by the time
they leave. So, we introduce a dropper at the last network
egress, which drops packets in flows that persistently declare
negative downstream congestion (see Section 6.1.4 for details).
..competitive routing
.' : '.
.' p e n a l:t i e s '.
: | : \ :
A : | : | :
|S <-----N1----> <---N2---> <---N4--> R domain
| : | : | :
| V | : | :
3% |--------+ | : | :
| | V V V V
2% | +-----------------------+
| downstream congestion |
1% | : |
| : |
0% +--------------------------------+=====-->
0 ^ i resource index
| /|\ |
1.00% | 2.00% marking fraction
|
sanctions
Figure 9: Incentives at Inter-domain Borders
Inter-domain traffic policing: But next we must ask, if congestion
arises downstream (say in N4), what is the ingress network's
(N1's) incentive to police its customers' response? If N1 turns a
blind eye, its own customers benefit while other networks suffer.
This is why all inter-domain QoS architectures (e.g. Intserv,
Diffserv) police traffic each time it crosses a trust boundary.
We have already shown that re-ECN gives a trustworthy measure of
the expected downstream congestion that a flow will cause by
subtracting negative volume from positive at any intermediate
point on a path. N4 (say) can use this measure to police all the
responses to congestion of all the sources beyond its upstream
neighbour (N2), but in bulk with one very simple passive
mechanism, rather than per flow, as we will now explain using
Figure 9.
Emulating policing with inter-domain congestion penalties: Between
high-speed networks, we would rather avoid per-flow policing, and
we would rather avoid holding back traffic while it is policed.
Instead, once re-ECN has arranged headers to carry downstream
congestion honestly, N2 can contract to pay N4 penalties in
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proportion to a single bulk count of the congestion metrics
crossing their mutual trust boundary (Section 6.1.6). In this
way, N4 puts pressure on N2 to suppress downstream congestion, for
every flow passing through the border interface, even though they
will all start and end in different places, and even though they
may all be allowed different responses to congestion. The figure
depicts this downward pressure on N2 by the solid downward arrow
at the egress of N2. Then N2 has an incentive either to police
the congestion response of its own ingress traffic (from N1) or to
emulate policing by applying penalties to N1 in turn on the basis
of congestion counted at their mutual boundary. In this recursive
way, the incentives for each flow to respond correctly to
congestion trace back with each flow precisely to each source,
despite the mechanism not recognising flows (see Section 6.2.2).
Inter-domain congestion charging diversity: Any two networks are
free to agree any of a range of penalty regimes between themselves
but they would only provide the right incentives if they were
within the following reasonable constraints. N2 should expect to
have to pay penalties to N4 where penalties monotonically increase
with the volume of congestion and negative penalties are not
allowed. For instance, they may agree an SLA with tiered
congestion thresholds, where higher penalties apply the higher the
threshold that is broken. But the most obvious (and useful) form
of penalty is where N4 levies a charge on N2 proportional to the
volume of downstream congestion N2 dumps into N4. In the
explanation that follows, we assume this specific variant of
volume charging between networks - charging proportionate to the
volume of congestion.
We must make clear that we are not advocating that everyone should
use this form of contract. We are well aware that the IETF tries
to avoid standardising technology that depends on a particular
business model. And we strongly share this desire to encourage
diversity. But our aim is merely to show that border policing can
at least work with this one model, then we can assume that
operators might experiment with the metric in other models (see
Section 6.1.6 for examples). Of course, operators are free to
complement this usage element of their charges with traditional
capacity charging, and we expect they will as predicted by
economics.
No congestion charging to users: Bulk congestion penalties at trust
boundaries are passive and extremely simple, and lose none of
their per-packet precision from one boundary to the next (unlike
Diffserv all-address traffic conditioning agreements, which
dissipate their effectiveness across long topologies). But at any
trust boundary, there is no imperative to use congestion charging.
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Traditional traffic policing can be used, if the complexity and
cost is preferred. In particular, at the boundary with end
customers (e.g. between S and N1), traffic policing will most
likely be more appropriate. Policer complexity is less of a
concern at the edge of the network. And end-customers are known
to be highly averse to the unpredictability of congestion
charging.
NOTE WELL: This document neither advocates nor requires congestion
charging for end customers and advocates but does not require
inter-domain congestion charging.
Competitive discipline of inter-domain traffic engineering: With
inter-domain congestion charging, a domain seems to have a
perverse incentive to fake congestion; N2's profit depends on the
difference between congestion at its ingress (its revenue) and at
its egress (its cost). So, overstating internal congestion seems
to increase profit. However, smart border routing [Smart_rtg] by
N1 will bias its routing towards the least cost routes. So, N2
risks losing all its revenue to competitive routes if it
overstates congestion (see Section 6.2.3). In other words, if N2
is the least congested route, its ability to raise excess profits
is limited by the congestion on the next least congested route.
This pressure on N2 to remain competitive is represented by the
dotted downward arrow at the ingress to N2 in Figure 9.
Closing the loop: All the above elements conspire to trap everyone
between two opposing pressures (the downward and upward arrows in
Figure 8 & Figure 9), ensuring the downstream congestion metric
arrives at the destination neither above nor below zero. So, we
have arrived back where we started in our argument. The ingress
edge network can rely on downstream congestion declared in the
packet headers presented by the sender. So it can police the
sender's congestion response accordingly.
Evolvability of congestion control: We have seen that re-ECN enables
policing at the very first ingress. We have also seen that, as
flows continue on their path through further networks downstream,
re-ECN removes the need for further per-domain ingress policing of
all the different congestion responses allowed to each different
flow. This is why the evolvability of re-ECN policing is so
superior to bottleneck policing or to any policing of different
QoS for different flows. Even if all access networks choose to
conservatively police congestion per flow, each will want to
compete with the others to allow new responses to congestion for
new types of application. With re-ECN, each can introduce new
controls independently, without coordinating with other networks
and without having to standardise anything. But, as we have just
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seen, by making inter-domain penalties proportionate to bulk
downtream congestion, downstream networks can be agnostic to the
specific congestion response for each flow, but they can still
apply more penalty the more liberal the ingress access network has
been in the response to congestion it allowed for each flow.
6.1.3.1. The Case against Classic Feedback
A system that produces an optimal outcome as a result of everyone's
selfish actions is extremely powerful. Especially one that enables
evolvability of congestion control. But why do we have to change to
re-ECN to achieve it? Can't classic congestion feedback (as used
already by standard ECN) be arranged to provide similar incentives
and similar evolvability? Superficially it can. Kelly's seminal
work showed how we can allow everyone the freedom to evolve whatever
congestion control behaviour is in their application's best interest
but still optimise the whole system of networks and users by placing
a price on congestion to ensure responsible use of this
freedom [Evol_cc]). Kelly used ECN with its classic congestion
feedback model as the mechanism to convey congestion price
information. The mechanism could be thought of as volume charging;
except only the volume of packets marked with congestion experienced
(CE) was counted.
However, below we explain why relying on classic feedback /required/
congestion charging to be used, while re-ECN achieves the same
powerful outcome (given it is built on Kelly's foundations), but does
not /require/ congestion charging. In brief, the problem with
classic feedback is that the incentives have to trace the indirect
path back to the sender---the long way round the feedback loop. For
example, if classic feedback were used in Figure 8, N2 would have had
to influence N1 via all of N4, R & S rather than directly.
Inability to agree what is happening downstream: In order to police
its upstream neighbour's congestion response, the neighbours
should be able to agree on the congestion to be responded to.
Whatever the feedback regime, as packets change hands at each
trust boundary, any path metrics they carry are verifiable by both
neighbours. But, with a classic path metric, they can only agree
on the /upstream/ path congestion.
Inaccessible back-channel: The network needs a whole-path congestion
metric if it wants to control the source. Classically, whole path
congestion emerges at the destination, to be fed back from
receiver to sender in a back-channel. But, in any data network,
back-channels need not be visible to relays, as they are
essentially communications between the end-points. They may be
encrypted, asymmetrically routed or simply omitted, so no network
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element can reliably intercept them. The congestion charging
literature solves this problem by charging the receiver and
assuming this will cause the receiver to refer the charges to the
sender. But, of course, this creates unintended side-effects...
`Receiver pays' unacceptable: In connectionless datagram networks,
receivers and receiving networks cannot prevent reception from
malicious senders, so `receiver pays' opens them to `denial of
funds' attacks.
End-user congestion charging unacceptable: Even if 'denial of funds'
were not a problem, we know that end-users are highly averse to
the unpredictability of congestion charging and anyway, we want to
avoid restricting network operators to just one retail tariff.
But with classic feedback only an upstream metric is available, so
we cannot avoid having to wrap the `receiver pays' money flow
around the feedback loop, necessarily forcing end-users to be
subjected to congestion charging.
To summarise so far, with classic feedback, policing congestion
response without losing evolvability /requires/ congestion charging
of end-users and a `receiver pays' model, whereas, with re-ECN, it is
still possible to influence incentives using congestion charging but
using the safer `sender pays' model. However, congestion charging is
only likely to be appropriate between domains. So, without losing
evolvability, re-ECN enables technical policing mechanisms that are
more appropriate for end users than congestion pricing.
We now take a second pass over the incentive framework, filling in
the detail.
6.1.4. Egress Dropper
As traffic leaves the last network before the receiver (domain N4 in
Figure 8), the fraction of positive octets in a flow should match the
fraction of negative octets introduced by congestion marking, leaving
a balance of zero. If it is less (a negative flow), it implies that
the source is understating path congestion (which will reduce the
penalties that N2 owes N4).
If flows are positive, N4 need take no action---this simply means its
upstream neighbour is paying more penalties than it needs to, and the
source is going slower than it needs to. But, to protect itself
against persistently negative flows, N4 will need to install a
dropper at its egress. Appendix E gives a suggested algorithm for
this dropper. There is no intention that the dropper algorithm needs
to be standardised, it is merely provided to show that an efficient,
robust algorithm is possible. But whatever algorithm is used must
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meet the criteria below:
o It SHOULD introduce minimal false positives for honest flows;
o It SHOULD quickly detect and sanction dishonest flows (minimal
false negatives);
o It MUST be invulnerable to state exhaustion attacks from malicious
sources. For instance, if the dropper uses flow-state, it should
not be possible for a source to send numerous packets, each with a
different flow ID, to force the dropper to exhaust its memory
capacity;
o It MUST introduce sufficient loss in goodput so that malicious
sources cannot play off losses in the egress dropper against
higher allowed throughput. Salvatori [CLoop_pol] describes this
attack, which involves the source understating path congestion
then inserting forward error correction (FEC) packets to
compensate expected losses.
Note that the dropper operates on flows but we would like it not to
require per-flow state. This is why we have been careful to ensure
that all flows MUST start with a packet marked with the FNE
codepoint. If a flow does not start with the FNE codepoint, a
dropper is likely to treat it unfavourably. This risk makes it worth
setting the FNE codepoint at the start of a flow, even though there
is a cost to the sender of setting FNE (positive `worth'). Indeed,
with the FNE codepoint, the rate at which a sender can generate new
flows can be limited (Appendix G). In this respect, the FNE
codepoint works like Handley's state set-up bit [Steps_DoS].
Appendix E also gives an example dropper implementation that
aggregates flow state. Dropper algorithms will often maintain a
moving average across flows of the fraction of RE blanked packets.
When maintaining an average across flows, a dropper SHOULD only allow
flows into the average if they start with FNE, but it SHOULD NOT
include packets with the FNE codepoint set in the average. A sender
sets the FNE codepoint when it does not have the benefit of feedback
from the receiver. So, counting packets with FNE cleared would be
likely to make the average unnecessarily positive, providing headroom
(or should we say footroom?) for dishonest (negative) traffic.
If the dropper detects a persistently negative flow, it SHOULD drop
sufficient negative and neutral packets to force the flow to not be
negative. Drops SHOULD be focused on just sufficient packets in
misbehaving flows to remove the negative bias while doing minimal
extra harm.
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6.1.5. Policing
Access operators who wish to limit the congeston that a sender is
able to cause can deploy policers at the very first ingress to the
internetwork. Re-ECN has been designed to avoid the need for
bottleneck policing so that we can avoid a future where a single rate
adaptation policy is embedded throughout the network. Instead, re-
ECN allows the particular rate adaptation policy to be solely agreed
bilaterally between the sender and its ingress access provider
(Section 5.5.2 discusses possible ways to signal between them), which
allows congestion control to be policed, but maintains its
evolvability, requiring only a single, local box to be updated.
Appendix G gives examples of per-user policing algorithms. But there
is no implication that these algorithms are to be standardised, or
that they are ideal. The ingress rate policer is the part of the re-
ECN incentive framework that is intended to be the most flexible.
Once endpoint protocol handlers for re-ECN and egress droppers are in
place, operators can choose exactly which congestion response they
want to police, and whether they want to do it per user, per flow or
not at all.
The re-ECN protocol allows these ingress policers to easily perform
bulk per-user policing (Appendix G.1). This is likely to provide
sufficient incentive to the user to correctly respond to congestion
without needing the policing function to be overly complex. If an
access operator chose they could use per-flow policing according to
the widely adopted TCP rate adaptation ( Appendix G.2) or other
alternatives, however this would introduce extra complexity to the
system.
If a per-flow rate policer is used, it should use path (not
downstream) congestion as the relevant metric, which is represented
by the fraction of octets in packets with positive (Re-Echo and FNE)
and canceled (CE(0)) markings. Of course, re-ECN provides all the
information a policer needs directly in the packets being policed.
So, even policing TCP's AIMD algorithm is relatively straightforward
(Appendix G.2).
Note that we have included canceled packets in the measure of path
congestion. Canceled packets arise when the sender re-echoes earlier
congestion, but then this Re-Echo packet just happens to be
congestion marked itself. One would not normally expect many
canceled packets at the first ingress because one would not normally
expect much congestion marking to have been necessary that soon in
the path. However, a home network or campus network may well sit
between the sending endpoint and the ingress policer, so some
congestion may occur upstream of the policer. And if congestion does
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occur upstream, some canceled packets should be visible, and should
be taken into account in the measure of path congestion.
But a much more important reason for including canceled packets in
the measure of path congestion at an ingress policer is that a sender
might otherwise subvert the protocol by sending canceled packets
instead of neutral (RECT) packets. Like neutral, canceled packets
are worth zero, so the sender knows they won't be counted against any
quota it might have been allowed. But unlike neutral packets,
canceled packets are immune to congestion marking, because they have
already been congestion marked. So, it is both correct and useful
that canceled packets should be included in a policer's measure of
path congestion, as this removes the incentive the sender would
otherwise have to mark more packets as canceled than it should.
An ingress policer should also ensure that flows are not already
negative when they enter the access network. As with canceled
packets, the presence of negative packets will typically be unusual.
Therefore it will be easy to detect negative flows at the ingress by
just detecting negative packets then monitoring the flow they belong
to.
Of course, even if the sender does operate its own network, it may
arrange not to congestion mark traffic. Whether the sender does this
or not is of no concern to anyone else except the sender. Such a
sender will not be policed against its own network's contribution to
congestion, but the only resulting problem would be overload in the
sender's own network.
Finally, we must not forget that an easy way to circumvent re-ECN's
defences is for the source to turn off re-ECN support, by setting the
Not-RECT codepoint, implying legacy traffic. Therefore an ingress
policer should put a general rate-limit on Not-RECT traffic, which
SHOULD be lax during early, patchy deployment, but will have to
become stricter as deployment widens. Similarly, flows starting
without an FNE packet can be confined by a strict rate-limit used for
the remainder of flows that haven't proved they are well-behaved by
starting correctly (therefore they need not consume any flow state---
they are just confined to the `misbehaving' bin if they carry an
unrecognised flow ID).
6.1.6. Inter-domain Policing
One of the main design goals of re-ECN is for border security
mechanisms to be as simple as possible, otherwise they will become
the pinch-points that limit scalability of the whole internetwork.
We want to avoid per-flow processing at borders and to keep to
passive mechanisms that can monitor traffic in parallel to
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forwarding, rather than having to filter traffic inline---in series
with forwarding. Such passive, off-line mechanisms are essential for
future high-speed all-optical border interconnection where packets
cannot be buffered while they are checked for policy compliance.
So far, we have been able to keep the border mechanisms simple,
despite having had to harden them against some subtle attacks on the
re-ECN design. The mechanisms are still passive and avoid per-flow
processing.
The basic accounting mechanism at each border interface simply
involves accumulating the volume of packets with positive worth (Re-
Echo and FNE), and subtracting the volume of those with negative
worth: CE(-1). Even though this mechanism takes no regard of flows,
over an accounting period (say a month) this subtraction will account
for the downstream congestion caused by all the flows traversing the
interface, wherever they come from, and wherever they go to. The two
networks can agree to use this metric however they wish to determine
some congestion-related penalty against the upstream network.
Although the algorithm could hardly be simpler, it is spelled out
using pseudo-code in Appendix H.1.
Various attempts to subvert the re-ECN design have been made. In all
cases their root cause is persistently negative flows. But, after
describing these attacks we will show that we don't actually have to
get rid of all persistently negative flows in order to thwart the
attacks.
In honest flows, downstream congestion is measured as positive minus
negative volume. So if all flows are honest (i.e. not persistently
negative), adding all positive volume and all negative volume without
regard to flows will give an aggregate measure of downstream
congestion. But such simple aggregation is only possible if no flows
are persistently negative. Unless persistently negative flows are
completely removed, they will reduce the aggregate measure of
congestion. The aggregate may still be positive overall, but not as
positive as it would have been had the negative flows been removed.
In Section 6.1.4 we discussed how to sanction traffic to remove, or
at least to identify, persistently negative flows. But, even if the
sanction for negative traffic is to discard it, unless it is
discarded at the exact point it goes negative, it will wrongly
subtract from aggregate downstream congestion, at least at any
borders it crosses after it has gone negative but before it is
discarded.
We rely on sanctions to deter dishonest understatement of congestion.
But even the ultimate sanction of discard can only be effective if
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the sender is bothered about the data getting through to its
destination. A number of attacks have been identified where a sender
gains from sending dummy traffic or it can attack someone or
something using dummy traffic even though it isn't communicating any
information to anyone:
o A host can send traffic with no positive markings towards its
intended destination, aiming to transmit as much traffic as any
dropper will allow [Bauer06]. It may add forward error correction
(FEC) to repair as much drop as it experiences.
o A host can send dummy traffic into the network with no positive
markings and with no intention of communicating with anyone, but
merely to cause higher levels of congestion for others who do want
to communicate (DoS). So, to ride over the extra congestion,
everyone else has to spend more of whatever rights to cause
congestion they have been allowed.
o A network can simply create its own dummy traffic to congest
another network, perhaps causing it to lose business at no cost to
the attacking network. This is a form of denial of service
perpetrated by one network on another. The preferential drop
measures in Section 5.3 provide crude protection against such
attacks, but we are not overly worried about more accurate
prevention measures, because it is already possible for networks
to DoS other networks on the general Internet, but they generally
don't because of the grave consequences of being found out. We
are only concerned if re-ECN increases the motivation for such an
attack, as in the next example.
o A network can just generate negative traffic and send it over its
border with a neighbour to reduce the overall penalties that it
should pay to that neighbour. It could even initialise the TTL so
it expired shortly after entering the neighbouring network,
reducing the chance of detection further downstream. This attack
need not be motivated by a desire to deny service and indeed need
not cause denial of service. A network's main motivator would
most likely be to reduce the penalties it pays to a neighbour.
But, the prospect of financial gain might tempt the network into
mounting a DoS attack on the other network as well, given the gain
would offset some of the risk of being detected.
The first step towards a solution to all these problems with negative
flows is to be able to estimate the contribution they make to
downstream congestion at a border and to correct the measure
accordingly. Although ideally we want to remove negative flows
themselves, perhaps surprisingly, the most effective first step is to
cancel out the polluting effect negative flows have on the measure of
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downstream congestion at a border. It is more important to get an
unbiased estimate of their effect, than to try to remove them all. A
suggested algorithm to give an unbiased estimate of the contribution
from negative flows to the downstream congestion measure is given in
Appendix H.2.
Although making an accurate assessment of the contribution from
negative flows may not be easy, just the single step of neutralising
their polluting effect on congestion metrics removes all the gains
networks could otherwise make from mounting dummy traffic attacks on
each other. This puts all networks on the same side (only with
respect to negative flows of course), rather than being pitched
against each other. The network where this flow goes negative as
well as all the networks downstream lose out from not being
reimbursed for any congestion this flow causes. So they all have an
interest in getting rid of these negative flows. Networks forwarding
a flow before it goes negative aren't strictly on the same side, but
they are disinterested bystanders---they don't care that the flow
goes negative downstream, but at least they can't actively gain from
making it go negative. The problem becomes localised so that once a
flow goes negative, all the networks from where it happens and beyond
downstream each have a small problem, each can detect it has a
problem and each can get rid of the problem if it chooses to. But
negative flows can no longer be used for any new attacks.
Once an unbiased estimate of the effect of negative flows can be
made, the problem reduces to detecting and preferably removing flows
that have gone negative as soon as possible. But importantly,
complete eradication of negative flows is no longer critical---best
endeavours will be sufficient.
For instance, let us consider the case where a source sends traffic
with no positive markings at all, hoping to at least get as much
traffic delivered as network-based droppers will allow. The flow is
likely to go at least slightly negative in the first network on the
path (N1 if we use the example network layout in Figure 9). If all
networks use the algorithm in Appendix H.2 to inflate penalties at
their border with an upstream network, they will remove the effect of
negative flows. So, for instance, N2 will not be paying a penalty to
N1 for this flow. Further, because the flow contributes no positive
markings at all, a dropper at the egress will completely remove it.
The remaining problem is that every network is carrying a flow that
is causing congestion to others but not being held to account for the
congestion it is causing. Whenever the fail-safe border algorithm
(Section 6.1.7) or the border algorithm to compensate for negative
flows (Appendix H.2) detects a negative flow, it can instantiate a
focused dropper for that flow locally. It may be some time before
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the flow is detected, but the more strongly negative the flow is, the
more quickly it will be detected by the fail-safe algorithm. But, in
the meantime, it will not be distorting border incentives. Until it
is detected, if it contributes to drop anywhere, its packets will
tend to be dropped before others if routers use the preferential drop
rules in Section 5.3, which discriminate against non-positive
packets. All networks below the point where a flow goes negative
(N1, N2 and N4 in this case) have an incentive to remove this flow,
but the router where it first goes negative (in N1) can of course
remove the problem for everyone downstream.
In the case of DDoS attacks, Section 6.2.1 describes how re-ECN
mitigates their force.
6.1.7. Inter-domain Fail-safes
The mechanisms described so far create incentives for rational
network operators to behave. That is, one operator aims to make
another behave responsibly by applying penalties and expects a
rational response (i.e. one that trades off costs against benefits).
It is usually reasonable to assume that other network operators will
behave rationally (policy routing can avoid those that might not).
But this approach does not protect against the misconfigurations and
accidents of other operators.
Therefore, we propose the following two mechanisms at a network's
borders to provide "defence in depth". Both are similar:
Highly positive flows: A small sample of positive packets should be
picked randomly as they cross a border interface. Then subsequent
packets matching the same source and destination address and DSCP
should be monitored. If the fraction of positive marking is well
above a threshold (to be determined by operational practice), a
management alarm SHOULD be raised, and the flow MAY be
automatically subject to focused drop.
Persistently negative flows: A small sample of congestion marked
(negative) packets should be picked randomly as they cross a
border interface. Then subsequent packets matching the same
source and destination address and DSCP should be monitored. If
the balance of positive minus negative markings is persistently
negative, a management alarm SHOULD be raised, and the flow MAY be
automatically subject to focused drop.
Both these mechanisms rely on the fact that highly positive (or
negative) flows will appear more quickly in the sample by selecting
randomly solely from positive (or negative) packets.
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6.1.8. Simulations
Simulations of policer and dropper performance done for the multi-bit
version of re-feedback have been included in section 5 "Dropper
Performance" of [Re-fb]. Simulations of policer and dropper for the
re-ECN version described in this document are work in progress.
6.2. Other Applications
6.2.1. DDoS Mitigation
A flooding attack is inherently about congestion of a resource.
Because re-ECN ensures the sources causing network congestion
experience the cost of their own actions, it acts as a first line of
defence against DDoS. As load focuses on a victim, upstream queues
grow, requiring honest sources to pre-load packets with a higher
fraction of positive packets. Once downstream routers are so
congested that they are dropping traffic, they will be CE marking the
traffic they do forward 100%. Honest sources will therefore be
sending Re-Echo 100% (and therefore being severely rate-limited at
the ingress).
Senders under malicious control can either do the same as honest
sources, and be rate-limited at ingress, or they can understate
congestion by sending more neutral RECT packets than they should. If
sources understate congestion (i.e. do not re-echo sufficient
positive packets) and the preferential drop ranking is implemented on
routers (Section 5.3), these routers will preserve positive traffic
until last. So, the neutral traffic from malicious sources will all
be automatically dropped first. Either way, the malicious sources
cannot send more than honest sources.
Further, hosts under malicious control will tend to be re-used for
many different attacks. They will therefore build up a long term
history of causing congestion. Therefore, as long as the population
of potentially compromisable hosts around the Internet is limited,
the per-user policing algorithms in Appendix G.1 will gradually
throttle down zombies and other launchpads for attacks. Therefore,
widespread deployment of re-ECN could considerably dampen the force
of DDoS. Certainly, zombie armies could hold their fire for long
enough to be able to build up enough credit in the per-user policers
to launch an attack. But they would then still be limited to no more
throughput than other, honest users.
Inter-domain traffic policing (see Section 6.1.6)ensures that any
network that harbours compromised `zombie' hosts will have to bear
the cost of the congestion caused by traffic from zombies in
downstream networks. Such networks will be incentivised to deploy
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per-user policers that rate-limit hosts that are unresponsive to
congestion so they can only send very slowly into congested paths.
As well as protecting other networks, the extremely poor performance
at any sign of congestion will incentivise the zombie's owner to
clean it up. However, the host should behave normally when using
uncongested paths.
Uniquely, re-ECN handles DDoS traffic without relying on the validity
of identifiers in packets. Certainly the egress dropper relies on
uniqueness of flow identifiers, but not their validity. So if a
source spoofs another address, re-ECN works just as well, as long as
the attacker cannot imitate all the flow identifiers of another
active flow passing through the same dropper (see Section 6.3).
Similarly, the ingress policer relies on uniqueness of flow IDs, not
their validity. Because a new flow will only be allowed any rate at
all if it starts with FNE, and the more FNE packets there are
starting new flows, the more they will be limited. Essentially a re-
ECN policer limits the bulk of all congestion entering the network
through a physical interface; limiting the congestion caused by each
flow is merely an optional extra.
6.2.2. End-to-end QoS
{ToDo: (Section 3.3.2 of [Re-fb] entitled `Edge QoS' gives an outline
of the text that will be added here).}
6.2.3. Traffic Engineering
{ToDo: }
6.2.4. Inter-Provider Service Monitoring
{ToDo: }
6.3. Limitations
The known limitations of the re-ECN approach are:
o We still cannot defend against the attack described in Section 10
where a malicious source sends negative traffic through the same
egress dropper as another flow and imitates its flow identifiers,
allowing a malicious source to cause an innocent flow to
experience heavy drop.
o Re-feedback for TTL (re-TTL) would also be desirable at the same
time as re-ECN. Unfortunately this requires a further standards
action for the mechanisms briefly described in Appendix F
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o Traffic must be ECN-capable for re-ECN to be effective. The only
defence against malicious users who turn off ECN capbility is that
networks are expected to rate limit Not-ECT traffic and to apply
higher drop preference to it during congestion. Although these
are blunt instruments, they at least represent a feasible scenario
for the future Internet where Not-ECT traffic co-exists with re-
ECN traffic, but as a severely hobbled under-class. We recommend
(Section 7.1) that while accommodating a smooth initial transition
to re-ECN, policing policies should gradually be tightened to rate
limit Not-ECT traffic more strictly in the longer term.
o When checking whether a flow is balancing positive markings with
congestion marking, re-ECN can only account for congestion
marking, not drops. So, whenever a sender experiences drop, it
does not have to re-echo the congestion event. Nonetheless, it is
hardly any advantage to be able to send faster than other flows
only if your traffic is dropped and the other traffic isn't.
o We are considering the issue of whether it would be useful to
truncate rather than drop packets that appear to be malicious, so
that the feedback loop is not broken but useful data can be
removed.
7. Incremental Deployment
7.1. Incremental Deployment Features
The design of the re-ECN protocol started from the fact that the
current ECN marking behaviour of routers was sufficient and that re-
feedback could be introduced around these routers by changing the
sender behaviour but not the routers. Otherwise, if we had required
routers to be changed, the chance of encountering a path that had
every router upgraded would be vanishly small during early
deployment, giving no incentive to start deployment. Also, as there
is no new forwarding behaviour, routers and hosts do not have to
signal or negotiate anything.
However, networks that choose to protect themselves using re-ECN do
have to add new security functions at their trust boundaries with
others. They distinguish legacy traffic by its ECN field. Traffic
from Not-ECT transports is distinguishable by its Not-RECT marking.
Traffic from legacy ECN transports is distinguished from re-ECN by
which of ECT(0) or ECT(1) is used. We chose to use ECT(1) for re-ECN
traffic deliberately. Existing ECN sources set ECT(0) on either 50%
(the nonce) or 100% (the default) of packets, whereas re-ECN does not
use ECT(0) at all. We can use this distinguishing feature of legacy
ECN traffic to separate it out for different treatment at the various
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border security functions: egress dropping, ingress policing and
border policing.
The general principle we adopt is that an egress dropper will not
drop any legacy traffic, but ingress and border policers will limit
the bulk rate of legacy traffic that can enter each network. Then,
during early re-ECN deployment, operators can set very permissive (or
non-existent) rate-limits on legacy traffic, but once re-ECN
implementations are generally available, legacy traffic can be rate-
limited increasingly harshly. Ultimately, an operator might choose
to block all legacy traffic entering its network, or at least only
allow through a trickle.
Then, as the limits are set more strictly, the more legacy ECN
sources will gain by upgrading to re-ECN. Thus, towards the end of
the voluntary incremental deployment period, legacy transports can be
given progressively stronger encouragement to upgrade.
The following list of minor changes, brings together all the points
where Re-ECN semantics for use of the two-bit ECN field are different
compared to RFC3168:
o A re-ECN sender sets ECT(1) by default, whereas an RFC3168 sender
sets ECT(0) by default (Section 3.3);
o No provision is necessary for a re-ECN capable source transport to
use the ECN nonce (Section 4.1.2.1);
o Routers MAY preferentially drop different extended ECN codepoints
(Section 5.3);
o Packets carrying the feedback not established (FNE) codepoint MAY
optionally be marked rather than dropped by routers, even though
their ECN field is Not-ECT (with the important caveat in
Section 5.3);
o Packets may be dropped by policing nodes because of apparent
misbehaviour, not just because of congestion (Section 6);
o Tunnel entry behaviour is still to be defined, but may have to be
different from RFC3168 (Section 5.6).
None of these changes REQUIRE any modifications to routers. Also
none of these changes affect anything about end to end congestion
control; they are all to do with allowing networks to police that end
to end congestion control is well-behaved.
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7.2. Incremental Deployment Incentives
It would only be worth standardising the re-ECN protocol if there
existed a coherent story for how it might be incrementally deployed.
In order for it to have a chance of deployment, everyone who needs to
act must have a strong incentive to act, and the incentives must
arise in the order that deployment would have to happen. Re-ECN
works around unmodified ECN routers, but we can't just discuss why
and how re-ECN deployment might build on ECN deployment, because
there is precious little to build on in the first place. Instead, we
aim to show that re-ECN deployment could carry ECN with it. We focus
on commercial deployment incentives, although some of the arguments
apply equally to academic or government sectors.
ECN deployment:
ECN is largely implemented in commercial routers, but generally
not as a supported feature, and it has largely not been deployed
by commercial network operators. It has been released in many
Unix-based operating systems, but not in proprietary OSs like
Windows or those in many mobile devices. For detailed deployment
status, see [ECN-Deploy]. We believe the reason ECN deployment
has not happened is twofold:
* ECN requires changes to both routers and hosts. If someone
wanted to sell the improvement that ECN offers, they would have
to co-ordinate deployment of their product with others. An ECN
server only gives any improvement on an ECN network. An ECN
network only gives any improvement if used by ECN devices.
Deployment that requires co-ordination adds cost and delay and
tends to dilute any competitive advantage that might be gained.
* ECN `only' gives a performance improvement. Making a product a
bit faster (whether the product is a device or a network),
isn't usually a sufficient selling point to be worth the cost
of co-ordinating across the industry to deploy it. Network
operators tend to avoid re-configuring a working network unless
launching a new product.
ECN and re-ECN for Edge-to-edge Assured QoS:
We believe the proposal to provide assured QoS sessions using a
form of ECN called pre-congestion notification (PCN) [PCN-arch] is
most likely to break the deadlock in ECN deployment first. It
only requires edge-to-edge deployment so it does not require
endpoint support. It can be deployed in a single network, then
grow incrementally to interconnected networks. And it provides a
different `product' (internetworked assured QoS), rather than
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merely making an existing product a bit faster.
Not only could this assured QoS application kick-start ECN
deployment, it could also carry re-ECN deployment with it; because
re-ECN can enable the assured QoS region to expand to a large
internetwork where neighbouring networks do not trust each other.
[Re-PCN] argues that re-ECN security should be built in to the QoS
system from the start, explaining why and how.
If ECN and re-ECN were deployed edge-to-edge for assured QoS,
operators would gain valuable experience. They would also clear
away many technical obstacles such as firewall configurations that
block all but the legacy settings of the ECN field and the RE
flag.
ECN in Access Networks:
The next obstacle to ECN deployment would be extension to access
and backhaul networks, where considerable link layer differences
makes implementation non-trivial, particularly on congested
wireless links. ECN and re-ECN work fine during partial
deployment, but they will not be very useful if the most congested
elements in networks are the last to support them. Access network
support is one of the weakest parts of this deployment story. All
we can hope is that, once the benefits of ECN are better
understood by operators, they will push for the necessary link
layer implementations as deployment proceeds.
Policing Unresponsive Flows:
Re-ECN allows a network to offer differentiated quality of service
as explained in Section 6.2.2. But we do not believe this will
motivate initial deployment of re-ECN, because the industry is
already set on alternative ways of doing QoS. Despite being much
more complicated and expensive, the alternative approaches are
here and now.
But re-ECN is critical to QoS deployment in another respect. It
can be used to prevent applications from taking whatever bandwidth
they choose without asking.
Currently, applications that remain resolute in their lack of
response to congestion are rewarded by other TCP applications. In
other words, TCP is naively friendly, in that it reduces its rate
in response to congestion whether it is competing with friends
(other TCPs) or with enemies (unresponsive applications).
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Therefore, those network owners that want to sell QoS will be keen
to ensure that their users can't help themselves to QoS for free.
Given the very large revenues at stake, we believe effective
policing of congestion response will become highly sought after by
network owners.
But this does not necessarily argue for re-ECN deployment.
Network owners might choose to deploy bottleneck policers rather
than re-ECN-based policing. However, under Related Work
(Section 9) we argue that bottleneck policers are inherently
vulnerable to circumvention.
Therefore we believe there will be a strong demand from network
owners for re-ECN deployment so they can police flows that do not
ask to be unresponsive to congestion, in order to protect their
revenues from flows that do ask (QoS). In particular, we suspect
that the operators of cellular networks will want to prevent VoIP
and video applications being used freely on their networks as a
more open market develops in GPRS and 3G devices.
Initial deployments are likely to be isolated to single cellular
networks. Cellular operators would first place requirements on
device manufacturers to include re-ECN in the standards for mobile
devices. In parallel, they would put out tenders for ingress and
egress policers. Then, after a while they would start to tighten
rate limits on Not-ECT traffic from non-standard devices and they
would start policing whatever non-accredited applications people
might install on mobile devices with re-ECN support in the
operating system. This would force even independent mobile device
manufacturers to provide re-ECN support. Early standardisation
across the cellular operators is likely, including interconnection
agreements with penalties for excess downstream congestion.
We suspect some fixed broadband networks (whether cable or DSL)
would follow a similar path. However, we also believe that larger
parts of the fixed Internet would not choose to police on a per-
flow basis. Some might choose to police congestion on a per-user
basis in order to manage heavy peer-to-peer file-sharing, but it
seems likely that a sizeable majority would not deploy any form of
policing.
This hybrid situation begs the question, "How does re-ECN work for
networks that choose to using policing if they connect with others
that don't?" Traffic from non-ECN capable sources will arrive
from other networks and cause congestion within the policed, ECN-
capable networks. So networks that chose to police congestion
would rate-limit Not-ECT traffic throughout their network,
particularly at their borders. They would probably also set
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higher usage prices in their interconnection contracts for
incoming Not-ECT and Not-RECT traffic. We assume that
interconnection contracts between networks in the same tier will
include congestion penalties before contracts with provider
backbones do.
A hybrid situation could remain for all time. As was explained in
the introduction, we believe in healthy competition between
policing and not policing, with no imperative to convert the whole
world to the religion of policing. Networks that chose not to
deploy egress droppers would leave themselves open to being
congested by senders in other networks. But that would be their
choice.
The important aspect of the egress dropper though is that it most
protects the network that deploys it. If a network does not
deploy an egress dropper, sources sending into it from other
networks will be able to understate the congestion they are
causing. Whereas, if a network deploys an egress dropper, it can
know how much congestion other networks are dumping into it, and
apply penalties or charges accordingly. So, whether or not a
network polices its own sources at ingress, it is in its interests
to deploy an egress dropper.
Host support:
In the above deployment scenario, host operating system support
for re-ECN came about through the cellular operators demanding it
in device standards (i.e. 3GPP). Of course, increasingly, mobile
devices are being built to support multiple wireless technologies.
So, if re-ECN were stipulated for cellular devices, it would
automatically appear in those devices connected to the wireless
fringes of fixed networks if they coupled cellular with WiFi or
Bluetooth technology, for instance. Also, once implemented in the
operating system of one mobile device, it would tend to be found
in other devices using the same family of operating system.
Therefore, whether or not a fixed network deployed ECN, or
deployed re-ECN policers and droppers, many of its hosts might
well be using re-ECN over it. Indeed, they would be at an
advantage when communicating with hosts across Re-ECN policed
networks that rate limited Not-RECT traffic.
Other possible scenarios:
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The above is thankfully not the only plausible scenario we can
think of. One of the many clubs of operators that meet regularly
around the world might decide to act together to persuade a major
operating system manufacturer to implement re-ECN. And they may
agree between them on an interconnection model that includes
congestion penalties.
Re-ECN provides an interesting opportunity for device
manufacturers as well as network operators. Policers can be
configured loosely when first deployed. Then as re-ECN take-up
increases, they can be tightened up, so that a network with re-ECN
deployed can gradually squeeze down the service provided to legacy
devices that have not upgraded to re-ECN. Many device vendors
rely on replacement sales. And operating system companies rely
heavily on new release sales. Also support services would like to
be able to force stragglers to upgrade. So, the ability to
throttle service to legacy operating systems is quite valuable.
Also, policing unresponsive sources may not be the only or even
the first application that drives deployment. It may be policing
causes of heavy congestion (e.g. peer-to-peer file-sharing). Or
it may be mitigation of denial of service. Or we may be wrong in
thinking simpler QoS will not be the initial motivation for re-ECN
deployment. Indeed, the combined pressure for all these may be
the motivator, but it seems optimistic to expect such a level of
joined-up thinking from today's communications industry. We
believe a single application alone must be a sufficient motivator.
In short, everyone gains from adding accountability to TCP/IP,
except the selfish or malicious. So, deployment incentives tend
to be strong.
8. Architectural Rationale
In the Internet's technical community, the danger of not responding
to congestion is well-understood, as well as its attendant risk of
congestion collapse [RFC3714]. However, one side of the Internet's
commercial community considers that the very essence of IP is to
provide open access to the internetwork for all applications. They
see congestion as a symptom of over-conservative investment, and rely
on revising application designs to find novel ways to keep
applications working despite congestion. They argue that the
Internet was never intended to be solely for TCP-friendly
applications. Meanwhile, another side of the Internet's commercial
community believes that it is worthwhile providing a network for
novel applications only if it has sufficient capacity, which can
happen only if a greater share of application revenues can be
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/assured/ for the infrastructure provider. Otherwise the major
investments required would carry too much risk and wouldn't happen.
The lesson articulated in [Tussle] is that we shouldn't embed our
view on these arguments into the Internet at design time. Instead we
should design the Internet so that the outcome of these arguments can
get decided at run-time. Re-ECN is designed in that spirit. Once
the protocol is available, different network operators can choose how
liberal they want to be in holding people accountable for the
congestion they cause. Some might boldly invest in capacity and not
police its use at all, hoping that novel applications will result.
Others might use re-ECN for fine-grained flow policing, expecting to
make money selling vertically integrated services. Yet others might
sit somewhere half-way, perhaps doing coarse, per-user policing. All
might change their minds later. But re-ECN always allows them to
interconnect so that the careful ones can protect themselves from the
liberal ones.
The incentive-based approach used for re-ECN is based on Gibbens and
Kelly's arguments [Evol_cc] on allowing endpoints the freedom to
evolve new congestion control algorithms for new applications. They
ensured responsible behaviour despite everyone's self-interest by
applying pricing to ECN marking, and Kelly had proved stability and
optimality in an earlier paper.
Re-ECN keeps all the underlying economic incentives, but rearranges
the feedback. The idea is to allow a network operator (if it
chooses) to deploy engineering mechanisms like policers at the front
of the network which can be designed to behave /as if/ they are
responding to congestion prices. Rather than having to subject users
to congestion pricing, networks can then use more traditional
charging regimes (or novel ones). But the engineering can constrain
the overall amount of congestion a user can cause. This provides a
buffer against completely outrageous congestion control, but still
makes it easy for novel applications to evolve if they need different
congestion control to the norms. It also allows novel charging
regimes to evolve.
Despite being achieved with a relatively minor protocol change, re-
ECN is an architectural change. Previously, Internet congestion
could only be controlled by the data sender, because it was the only
one both in a position to control the load and in a position to see
information on congestion. Re-ECN levels the playing field. It
recognises that the network also has a role to play in moderating
(policing) congestion control. But policing is only truly effective
at the first ingress into an internetwork, whereas path congestion
was previously only visible at the last egress. So, re-ECN
democratises congestion information. Then the choice over who
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actually controls congestion can be made at run-time, not design
time---a bit like an aircraft with dual controls. And different
operators can make different choices. We believe non-architectural
approaches to this problem are unlikely to offer more than partial
solutions (see Section 9).
Importantly, re-ECN does NOT REQUIRE assumptions about specific
congestion responses to be embedded in any network elements, except
at the first ingress to the internetwork if that level of control is
desired by the ingress operator. But such tight policing will be a
matter of agreement between the source and its access network
operator. The ingress operator need not police congestion response
at flow granularity; it can simply hold a source responsible for the
aggregate congestion it causes, perhaps keeping it within a monthly
congestion quota. Or if the ingress network trusts the source, it
can do nothing.
Therefore, the aim of the re-ECN protocol is NOT solely to police
TCP-friendliness. Re-ECN preserves IP as a generic network layer for
all sorts of responses to congestion, for all sorts of transports.
Re-ECN merely ensures truthful downstream congestion information is
available in the network layer for all sorts of accountability
applications.
The end to end design principle does not say that all functions
should be moved out of the lower layers---only those functions that
are not generic to all higher layers. Re-ECN adds a function to the
network layer that is generic, but was omitted: accountability for
causing congestion. Accountability is not something that an end-user
can provide to themselves. We believe re-ECN adds no more than is
sufficient to hold each flow accountable, even if it consists of a
single datagram.
"Accountability" implies being able to identify who is responsible
for causing congestion. However, at the network layer it would NOT
be useful to identify the cause of congestion by adding individual or
organisational identity information, NOR by using source IP
addresses. Rather than bringing identity information to the point of
congestion, we bring downstream congestion information to the point
where the cause can be most easily identified and dealt with. That
is, at any trust boundary congestion can be associated with the
physically connected upstream neighbour that is directly responsible
for causing it (whether intentionally or not). A trust boundary
interface is exactly the place to police or throttle in order to
directly mitigate congestion, rather than having to trace the
(ir)responsible party in order to shut them down.
Some considered that ECN itself was a layering violation. The
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reasoning went that the interface to a layer should provide a service
to the higher layer and hide how the lower layer does it. However,
ECN reveals the state of the network layer and below to the transport
layer. A more positive way to describe ECN is that it is like the
return value of a function call to the network layer. It explicitly
returns the status of the request to deliver a packet, by returning a
value representing the current risk that a packet will not be served.
Re-ECN has similar semantics, except the transport layer must try to
guess the return value, then it can use the actual return value from
the network layer to modify the next guess.
The guiding principle behind all the discussion in Section 6.1.6 on
Policing is that any gain from subverting the protocol should be
precisely neutralised, rather than punished. If a gain is punished
to a greater extent than is sufficient to neutralise it, it will most
likely open up a new vulnerability, where the amplifying effect of
the punishment mechanism can be turned on others.
For instance, if possible, flows should be removed as soon as they go
negative, but we do NOT RECOMMEND any attempts to discard such flows
further upstream while they are still positive. Such over-zealous
push-back is unnecessary and potentially dangerous. These flows have
paid their `fare' up to the point they go negative, so there is no
harm in delivering them that far. If someone downstream asks for a
flow to be dropped as near to the source as possible, because they
say it is going to become negative later, an upstream node cannot
test the truth of this assertion. Rather than have to authenticate
such messages, re-ECN has been designed so that flows can be dropped
solely based on locally measurable evidence. A message hinting that
a flow should be watched closely to test for negativity is fine. But
not a message that claims that a positive flow will go negative
later, so it should be dropped. .
9. Related Work
{Due to lack of time, this section is incomplete. The reader is
referred to the Related Work section of [Re-fb] for a brief selection
of related ideas.}
9.1. Policing Rate Response to Congestion
ATM network elements send congestion back-pressure
messages [ITU-T.I.371] along each connection, duplicating any end to
end feedback because they don't trust it. On the other hand, re-ECN
ensures information in forwarded packets can be used for congestion
management without requiring a connection-oriented architecture and
re-using the overhead of fields that are already set aside for end to
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end congestion control (and routing loop detection in the case of re-
TTL in Appendix F).
We borrowed ideas from policers in the literature [pBox],[XCHOKe],
AFD etc. for our rate equation policer. However, without the benefit
of re-ECN they don't police the correct rate for the condition of
their path. They detect unusually high /absolute/ rates, but only
while the policer itself is congested, because they work by detecting
prevalent flows in the discards from the local RED queue. These
policers must sit at every potential bottleneck, whereas our policer
need only be located at each ingress to the internetwork. As Floyd &
Fall explain [pBox], the limitation of their approach is that a high
sending rate might be perfectly legitimate, if the rest of the path
is uncongested or the round trip time is short. Commercially
available rate policers cap the rate of any one flow. Or they
enforce monthly volume caps in an attempt to control high volume
file-sharing. They limit the value a customer derives. They might
also limit the congestion customers can cause, but only as an
accidental side-effect. They actually punish traffic that fills
troughs as much as traffic that causes peaks in utilisation. In
practice network operators need to be able to allocate service by
cost during congestion, and by value at other times.
9.2. Congestion Notification Integrity
The choice of two ECT code-points in the ECN field [RFC3168]
permitted future flexibility, optionally allowing the sender to
encode the experimental ECN nonce [RFC3540] in the packet stream.
This mechanism has since been included in the specifications of DCCP
[RFC4340].
The ECN nonce is an elegant scheme that allows the sender to detect
if someone in the feedback loop - the receiver especially - tries to
claim no congestion was experienced when in fact congestion led to
packet drops or ECN marks. For each packet it sends, the sender
chooses between the two ECT codepoints in a pseudo-random sequence.
Then, whenever the network marks a packet with CE, if the receiver
wants to deny congestion happened, she has to guess which ECT
codepoint was overwritten. She has only a 50:50 chance of being
correct each time she denies a congestion mark or a drop, which
ultimately will give her away.
The purpose of a network-layer nonce should primarily be protection
of the network, while a transport-layer nonce would be better used to
protect the sender from cheating receivers. Now, the assumption
behind the ECN nonce is that a sender will want to detect whether a
receiver is suppressing congestion feedback. This is only true if
the sender's interests are aligned with the network's, or with the
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community of users as a whole. This may be true for certain large
senders, who are under close scrutiny and have a reputation to
maintain. But we have to deal with a more hostile world, where
traffic may be dominated by peer-to-peer transfers, rather than
downloads from a few popular sites. Often the `natural' self-
interest of a sender is not aligned with the interests of other
users. It often wishes to transfer data quickly to the receiver as
much as the receiver wants the data quickly.
In contrast, the re-ECN protocol enables policing of an agreed rate-
response to congestion (e.g. TCP-friendliness) at the sender's
interface with the internetwork. It also ensures downstream networks
can police their upstream neighbours, to encourage them to police
their users in turn. But most importantly, it requires the sender to
declare path congestion to the network and it can remove traffic at
the egress if this declaration is dishonest. So it can police
correctly, irrespective of whether the receiver tries to suppress
congestion feedback or whether the sender ignores genuine congestion
feedback. Therefore the re-ECN protocol addresses a much wider range
of cheating problems, which includes the one addressed by the ECN
nonce.
9.3. Identifying Upstream and Downstream Congestion
Purple [Purple] proposes that routers should use the CWR flag in the
TCP header of ECN-capable flows to work out path congestion and
therefore downstream congestion in a similar way to re-ECN. However,
because CWR is in the transport layer, it is not always visible to
network layer routers and policers. Purple's motivation was to
improve AQM, not policing. But, of course, nodes trying to avoid a
policer would not be expected to allow CWR to be visible.
10. Security Considerations
This whole memo concerns the deployment of a secure congestion
control framework. However, below we list some specific security
issues that we are still working on:
o Malicious users have ability to launch dynamically changing
attacks, exploiting the time it takes to detect an attack, given
ECN marking is binary. We are concentrating on subtle
interactions between the ingress policer and the egress dropper in
an effort to make it impossible to game the system.
o There is an inherent need for at least some flow state at the
egress dropper given the binary marking environment, which leads
to an apparent vulnerability to state exhaustion attacks. An
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egress dropper design with bounded flow state is in write-up.
o A malicious source can spoof another user's address and send
negative traffic to the same destination in order to fool the
dropper into sanctioning the other user's flow. To prevent or
mitigate these two different kinds of DoS attack, against the
dropper and against given flows, we are considering various
protection mechanisms. Section 5.5.1 discusses one of these.
o A malicious client can send requests using a spoofed source
address to a server (such as a DNS server) that tends to respond
with single packet responses. This server will then be tricked
into having to set FNE on the first (and only) packet of all these
wasted responses. Given packets marked FNE are worth +1, this
will cause such servers to consume more of their allowance to
cause congestion than they would wish to. In general, re-ECN is
deliberately designed so that single packet flows have to bear the
cost of not discovering the congestion state of their path. One
of the reasons for introducing re-ECN is to encourage short flows
to make use of previous path knowledge by moving the cost of this
lack of knowledge to sources that create short flows. Therefore,
we in the long run we might expect services like DNS to aggregate
single packet flows into connections where it brings benefits.
However, this attack where DNS requests are made from spoofed
addresses genuinely forces the server to waste its resources. The
only mitigating feature is that the attacker has to set FNE on
each of its requests if they are to get through an egress dropper
to a DNS server. The attacker therefore has to consume as many
resources as the victim, which at least implies re-ECN does not
unwittingly amplify this attack.
Having highlighted outstanding security issues, we now explain the
design decisions that were taken based on a security-related
rationale. It may seem that the six codepoints of the eight made
available by extending the ECN field with the RE flag have been used
rather wastefully to encode just five states. In effect the RE flag
has been used as an orthogonal single bit, using up four codepoints
to encode the three states of positive, neutral and negative worth.
The mapping of the codepoints in an earlier version of this proposal
used the codepoint space more efficiently, but the scheme became
vulnerable to network operators bypassing congestion penalties by
focusing congestion marking on positive packets. Appendix B explains
why fixing that problem while allowing for incremental deployment,
would have used another codepoint anyway. So it was better to use
this orthogonal encoding scheme, which greatly simplified the whole
protocol and brought with it some subtle security benefits (see the
last paragraph of Appendix B).
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With the scheme as now proposed, once the RE flag is set or cleared
by the sender or its proxy, it should not be written by the network,
only read. So the endpoints can detect if any network maliciously
alters the RE flag. IPSec AH integrity checking does not cover the
IPv4 option flags (they were considered mutable---even the one we
propose using for the RE flag that was `currently unused' when IPSec
was defined). But it would be sufficient for a pair of endpoints to
make random checks on whether the RE flag was the same when it
reached the egress as when it left the ingress. Indeed, if IPSec AH
had covered the RE flag, any network intending to alter sufficient RE
flags to make a gain would have focused its alterations on packets
without authenticating headers (AHs).
The security of re-ECN has been deliberately designed to not rely on
cryptography.
11. IANA Considerations
This memo includes no request to IANA (yet).
If this memo was to progress to standards track, it would list:
o The new RE flag in IPv4 (Section 5.1) and its extension with the
ECN field to create a new set of extended ECN (EECN) codepoints;
o The definition of the EECN codepoints for default Diffserv PHBs
(Section 3.2)
o The new extension header for IPv6 (Section 5.2);
o The new combinations of flags in the TCP header for capability
negotiation (Section 4.1.3);
o The new ICMP message type (Section 5.5.1).
12. Conclusions
{ToDo:}
13. Acknowledgements
Sebastien Cazalet and Andrea Soppera contributed to the idea of re-
feedback. All the following have given helpful comments: Andrea
Soppera, David Songhurst, Peter Hovell, Louise Burness, Phil Eardley,
Steve Rudkin, Marc Wennink, Fabrice Saffre, Cefn Hoile, Steve Wright,
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John Davey, Martin Koyabe, Carla Di Cairano-Gilfedder, Alexandru
Murgu, Nigel Geffen, Pete Willis, John Adams (BT), Sally Floyd
(ICIR), Joe Babiarz, Kwok Ho-Chan (Nortel), Stephen Hailes, Mark
Handley (who developed the attack with canceled packets), Adam
Greenhalgh (who developed the attack on DNS) (UCL), Jon Crowcroft
(Uni Cam), David Clark, Bill Lehr, Sharon Gillett, Steve Bauer (who
complemented our own dummy traffic attacks with others), Liz Maida
(MIT), and comments from participants in the CRN/CFP Broadband and
DoS-resistant Internet working groups.
14. Comments Solicited
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF Transport Area working group's mailing list
<tsvwg@ietf.org>, and/or to the authors.
15. References
15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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 Avoidance in the
Internet", RFC 2309, April 1998.
[RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control Transmission
Protocol", RFC 2960, October 2000.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC3390] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
Initial Window", RFC 3390, October 2002.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
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Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion Control ID 2: TCP-like
Congestion Control", RFC 4341, March 2006.
[RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
Datagram Congestion Control Protocol (DCCP) Congestion
Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
March 2006.
15.2. Informative References
[ARI05] Adams, J., Roberts, L., and A. IJsselmuiden, "Changing the
Internet to Support Real-Time Content Supply from a Large
Fraction of Broadband Residential Users", BT Technology
Journal (BTTJ) 23(2), April 2005.
[Bauer06] Bauer, S., Faratin, P., and R. Beverly, "Assessing the
assumptions underlying mechanism design for the Internet",
Proc. Workshop on the Economics of Networked Systems
(NetEcon06) , June 2006, <http://www.cs.duke.edu/nicl/
netecon06/papers/ne06-assessing.pdf>.
[CLoop_pol]
Salvatori, A., "Closed Loop Traffic Policing", Politecnico
Torino and Institut Eurecom Masters Thesis ,
September 2005.
[ECN-Deploy]
Floyd, S., "ECN (Explicit Congestion Notification) in
TCP/IP; Implementation and Deployment of ECN", Web-page ,
May 2004,
<http://www.icir.org/floyd/ecn.html#implementations>.
[ECN-MPLS]
Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", draft-ietf-tsvwg-ecn-mpls-01 (work in
progress), June 2007.
[ECN-tunnel]
Briscoe, B., "Layered Encapsulation of Congestion
Notification", draft-briscoe-tsvwg-ecn-tunnel-00 (work in
progress), July 2007.
[Evol_cc] Gibbens, R. and F. Kelly, "Resource pricing and the
evolution of congestion control", Automatica 35(12)1969--
1985, December 1999,
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<http://www.statslab.cam.ac.uk/~frank/evol.html>.
[I-D.ietf-tcpm-ecnsyn]
Kuzmanovic, A., "Adding Explicit Congestion Notification
(ECN) Capability to TCP's SYN/ACK Packets",
draft-ietf-tcpm-ecnsyn-01 (work in progress),
October 2006.
[I-D.moncaster-tcpm-rcv-cheat]
Moncaster, T., "A TCP Test to Allow Senders to Identify
Receiver Non-Compliance",
draft-moncaster-tcpm-rcv-cheat-01 (work in progress),
June 2007.
[ITU-T.I.371]
ITU-T, "Traffic Control and Congestion Control in
{B-ISDN}", ITU-T Rec. I.371 (03/04), March 2004.
[Jiang02] Jiang, H. and D. Dovrolis, "The Macroscopic Behavior of
the TCP Congestion Avoidance Algorithm", ACM SIGCOMM
CCR 32(3)75-88, July 2002,
<http://doi.acm.org/10.1145/571697.571725>.
[Mathis97]
Mathis, M., Semke, J., Mahdavi, J., and T. Ott, "The
Macroscopic Behavior of the TCP Congestion Avoidance
Algorithm", ACM SIGCOMM CCR 27(3)67--82, July 1997,
<http://doi.acm.org/10.1145/263932.264023>.
[PCN-arch]
Eardley, P., Babiarz, J., Chan, K., Charny, A., Geib, R.,
Karagiannis, G., Menth, M., and T. Tsou, "Pre-Congestion
Notification Architecture",
draft-eardley-pcn-architecture-00 (work in progress),
June 2007.
[Purple] Pletka, R., Waldvogel, M., and S. Mannal, "PURPLE:
Predictive Active Queue Management Utilizing Congestion
Information", Proc. Local Computer Networks (LCN 2003) ,
October 2003.
[RFC2208] Mankin, A., Baker, F., Braden, B., Bradner, S., O'Dell,
M., Romanow, A., Weinrib, A., and L. Zhang, "Resource
ReSerVation Protocol (RSVP) Version 1 Applicability
Statement Some Guidelines on Deployment", RFC 2208,
September 1997.
[RFC2402] Kent, S. and R. Atkinson, "IP Authentication Header",
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RFC 2402, November 1998.
[RFC2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion Manager",
RFC 3124, June 2001.
[RFC3514] Bellovin, S., "The Security Flag in the IPv4 Header",
RFC 3514, April 2003.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
RFC 3540, June 2003.
[RFC3714] Floyd, S. and J. Kempf, "IAB Concerns Regarding Congestion
Control for Voice Traffic in the Internet", RFC 3714,
March 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[Re-PCN] Briscoe, B., "Emulating Border Flow Policing using Re-ECN
on Bulk Data", draft-briscoe-tsvwg-re-ecn-border-cheat-01
(work in progress), March 2006.
[Re-fb] Briscoe, B., Jacquet, A., Di Cairano-Gilfedder, C.,
Salvatori, A., Soppera, A., and M. Koyabe, "Policing
Congestion Response in an Internetwork Using Re-Feedback",
ACM SIGCOMM CCR 35(4)277--288, August 2005, <http://
www.acm.org/sigs/sigcomm/sigcomm2005/
techprog.html#session8>.
[Savage99]
Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
"TCP congestion control with a misbehaving receiver", ACM
SIGCOMM CCR 29(5), October 1999,
<http://citeseer.ist.psu.edu/savage99tcp.html>.
[Smart_rtg]
Goldenberg, D., Qiu, L., Xie, H., Yang, Y., and Y. Zhang,
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"Optimizing Cost and Performance for Multihoming", ACM
SIGCOMM CCR 34(4)79--92, October 2004,
<http://citeseer.ist.psu.edu/698472.html>.
[Steps_DoS]
Handley, M. and A. Greenhalgh, "Steps towards a DoS-
resistant Internet Architecture", Proc. ACM SIGCOMM
workshop on Future directions in network architecture
(FDNA'04) pp 49--56, August 2004.
[Tussle] Clark, D., Sollins, K., Wroclawski, J., and R. Braden,
"Tussle in Cyberspace: Defining Tomorrow's Internet", ACM
SIGCOMM CCR 32(4)347--356, October 2002,
<http://www.acm.org/sigcomm/sigcomm2002/papers/
tussle.pdf>.
[XCHOKe] Chhabra, P., Chuig, S., Goel, A., John, A., Kumar, A.,
Saran, H., and R. Shorey, "XCHOKe: Malicious Source
Control for Congestion Avoidance at Internet Gateways",
Proceedings of IEEE International Conference on Network
Protocols (ICNP-02) , November 2002,
<http://www.cc.gatech.edu/~akumar/xchoke.pdf>.
[pBox] Floyd, S. and K. Fall, "Promoting the Use of End-to-End
Congestion Control in the Internet", IEEE/ACM Transactions
on Networking 7(4) 458--472, August 1999,
<http://www.aciri.org/floyd/end2end-paper.html>.
Appendix A. Precise Re-ECN Protocol Operation
{ToDo: fix this}
The protocol operation described in Section 3.3 was an approximation.
In fact, standard ECN router marking combines 1% and 2% marking into
slightly less than 3% whole-path marking, because routers
deliberately mark CE whether or not it has already been marked by
another router upstream. So the combined marking fraction would
actually be 100% - (100% - 1%)(100% - 2%) = 2.98%.
To generalise this we will need some notation.
o j represents the index of each resource (typically queues) along a
path, ranging from 0 at the first router to n-1 at the last.
o m_j represents the fraction of octets *m*arked CE by a particular
router (whether or not they are already marked) because of
congestion of resource j.
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o u_j represents congestion *u*pstream of resource j, being the
fraction of CE marking in arriving packet headers (before
marking).
o p_j represents *p*ath congestion, being the fraction of packets
arriving at resource j with the RE flag blanked (excluding Not-
RECT packets).
o v_j denotes expected congestion downstream of resource j, which
can be thought of as a *v*irtual marking fraction, being derived
from two other marking fractions.
Observed fractions of each particular codepoint (u, p and v) and
router marking rate m are dimensionless fractions, being the ratio of
two data volumes (marked and total) over a monitoring period. All
measurements are in terms of octets, not packets, assuming that line
resources are more congestible than packet processing.
The path congestion (RE blanking fraction) set by the sender should
reflect the upstream congestion (CE marking fraction) fed back from
the destination. Therefore in the steady state
p_0 = u_n
= 1 - (1 - m_1)(1 - m_2)...
Similarly, at some point j in the middle of the network, if p = 1 -
(1 - u_j)(1 - v_j), then
v_j = 1 - (1 - p)/(1 - u_j)
~= p - u_j; if u_j << 100%
So, between the two routers in the example in Section 3.3, congestion
downstream is
v_1 = 100.00% - (100% - 2.98%) / (100% - 1.00%)
= 2.00%,
or a useful approximation of downstream congestion is
v_1 ~= 2.98% - 1.00%
~= 1.98%.
Appendix B. Justification for Two Codepoints Signifying Zero Worth
Packets
It may seem a waste of a codepoint to set aside two codepoints of the
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Extended ECN field to signify zero worth (RECT and CE(0) are both
worth zero). The justification is subtle, but worth recording.
The original version of re-ECN ([Re-fb] and draft-00 of this memo)
used three codepoints for neutral (ECT(1)), positive (ECT(0)) and
negative (CE) packets. The sender set packets to neutral unless re-
echoing congestion, when it set them positive, in much the same way
that it blanks the RE flag in the current protocol. However, routers
were meant to mark congestion by setting packets negative (CE)
irrespective of whether they had previously been neutral or positive.
However, we did not arrange for senders to remember which packet had
been sent with which codepoint, or for feedback to say exactly which
packets arrived with which codepoints. The transport was meant to
inflate the number of positive packets it sent to allow for a few
being wiped out by congestion marking. We (wrongly) assumed that
routers would congestion mark packets indiscriminately, so the
transport could infer how many positive packets had been marked and
compensate accordingly by re-echoing. But this created a perverse
incentive for routers to preferentially congestion mark positive
packets rather than neutral ones.
We could have removed this perverse incentive by requiring re-ECN
senders to remember which packets they had sent with which codepoint.
And for feedback from the receiver to identify which packets arrived
as which. Then, if a positive packet was congestion marked to
negative, the sender could have re-echoed twice to maintain the
balance between positive and negative at the receiver.
Instead, we chose to make re-echoing congestion (blanking RE)
orthogonal to congestion notification (marking CE), which required a
second neutral codepoint (the orthogonal scheme forms the main square
of four codepoints in Figure 2). Then the receiver would be able to
detect and echo a congestion event even if it arrived on a packet
that had originally been positive.
If we had added extra complexity to the sender and receiver
transports to track changes to individual packets, we could have made
it work, but then routers would have had an incentive to mark
positive packets with half the probability of neutral packets. That
in turn would have led router algorithms to become more complex.
Then senders wouldn't know whether a mark had been introduced by a
simple or a complex router algorithm. That in turn would have
required another codepoint to distinguish between legacy ECN and new
re-ECN router marking.
Once the cost of IP header codepoint real-estate was the same for
both schemes, there was no doubt that the simpler option for
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endpoints and for routers should be chosen. The resulting protocol
also no longer needed the tricky inflation/deflation complexity of
the original (broken) scheme. It was also much simpler to understand
conceptually.
A further advantage of the new orthogonal four-codepoint scheme was
that senders owned sole rights to change the RE flag and routers
owned sole rights to change the ECN field. Although we still arrange
the incentives so neither party strays outside their dominion, these
clear lines of authority simplify the matter.
Finally, a little redundancy can be very powerful in a scheme such as
this. In one flow, the proportion of packets changed to CE should be
the same as the proportion of RECT packets changed to CE(-1) and the
proportion of Re-Echo packets changed to CE(0). Double checking
using such redundant relationships can improve the security of a
scheme (cf. double-entry book-keeping or the ECN Nonce).
Alternatively, it might be necessary to exploit the redundancy in the
future to encode an extra information channel.
Appendix C. ECN Compatibility
The rationale for choosing the particular combinations of SYN and SYN
ACK flags in Section 4.1.3 is as follows.
Choice of SYN flags: A re-ECN sender can work with vanilla ECN
receivers so we wanted to use the same flags as would be used in
an ECN-setup SYN [RFC3168] (CWR=1, ECE=1). But at the same time,
we wanted a server (host B) that is Re-ECT to be able to recognise
that the client (A) is also Re-ECT. We believe also setting NS=1
in the initial SYN achieves both these objectives, as it should be
ignored by vanilla ECT receivers and by ECT-Nonce receivers. But
senders that are not Re-ECT should not set NS=1. At the time ECN
was defined, the NS flag was not defined, so setting NS=1 should
be ignored by existing ECT receivers (but testing against
implementations may yet prove otherwise). The ECN Nonce
RFC [RFC3540] is silent on what the NS field might be set to in
the TCP SYN, but we believe the intent was for a nonce client to
set NS=0 in the initial SYN (again only testing will tell).
Therefore we define a Re-ECN-setup SYN as one with NS=1, CWR=1 &
ECE=1
Choice of SYN ACK flags: Choice of SYN ACK: The client (A) needs to
be able to determine whether the server (B) is Re-ECT. The
original ECN specification required an ECT server to respond to an
ECN-setup SYN with an ECN-setup SYN ACK of CWR=0 and ECE=1. There
is no room to modify this by setting the NS flag, as that is
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already set in the SYN ACK of an ECT-Nonce server. So we used the
only combination of CWR and ECE that would not be used by existing
TCP receivers: CWR=1 and ECE=0. The original ECN specification
defines this combination as a non-ECN-setup SYN ACK, which remains
true for vanilla and Nonce ECTs. But for re-ECN we define it as a
Re-ECN-setup SYN ACK. We didn't use a SYN ACK with both CWR and
ECE cleared to 0 because that would be the likely response from
most Not-ECT receivers. And we didn't use a SYN ACK with both CWR
and ECE set to 1 either, as at least one broken receiver
implementation echoes whatever flags were in the SYN into its SYN
ACK. Therefore we define a Re-ECN-setup SYN ACK as one with CWR=1
& ECE=0.
Choice of two alternative SYN ACKs: the NS flag may take either
value in a Re-ECN-setup SYN ACK. Section 5.4 REQUIRES that a Re-
ECT server MUST set the NS flag to 1 in a Re-ECN-setup SYN ACK to
echo congestion experienced (CE) on the initial SYN. Otherwise a
Re-ECN-setup SYN ACK MUST be returned with NS=0. The only current
known use of the NS flag in a SYN ACK is to indicate support for
the ECN nonce, which will be negotiated by setting CWR=0 & ECE=1.
Given the ECN nonce MUST NOT be used for a RECN mode connection, a
Re-ECN-setup SYN ACK can use either setting of the NS flag without
any risk of confusion, because the CWR & ECE flags will be
reversed relative to those used by an ECN nonce SYN ACK.
Appendix D. Packet Marking During Flow Start
{ToDo: Write up proof that sender should mark FNE on first and third
data packets, even with the largest allowed initial window.}
Appendix E. Example Egress Dropper Algorithm
{ToDo: Write up the basic algorithm with flow state, then the
aggregated one.}
Appendix F. Re-TTL
This Appendix gives an overview of a proposal to be able to overload
the TTL field in the IP header to monitor downstream propagation
delay. This is included to show that it would be possible to take
account of RTT if it was deemed desirable.
Delay re-feedback can be achieved by overloading the TTL field,
without changing IP or router TTL processing. A target value for TTL
at the destination would need standardising, say 16. If the path hop
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count increased by more than 16 during a routing change, it would
temporarily be mistaken for a routing loop, so this target would need
to be chosen to exceed typical hop count increases. The TCP wire
protocol and handlers would need modifying to feed back the
destination TTL and initialise it. It would be necessary to
standardise the unit of TTL in terms of real time (as was the
original intent in the early days of the Internet).
In the longer term, precision could be improved if routers
decremented TTL to represent exact propagation delay to the next
router. That is, for a router to decrement TTL by, say, 1.8 time
units it would alternate the decrement of every packet between 1 & 2
at a ratio of 1:4. Although this might sometimes require a seemingly
dangerous null decrement, a packet in a loop would still decrement to
zero after 255 time units on average. As more routers were upgraded
to this more accurate TTL decrement, path delay estimates would
become increasingly accurate despite the presence of some legacy
routers that continued to always decrement the TTL by 1.
Appendix G. Policer Designs to ensure Congestion Responsiveness
G.1. Per-user Policing
User policing requires a policer on the ingress interface of the
access router associated with the user. At that point, the traffic
of the user hasn't diverged on different routes yet; nor has it mixed
with traffic from other sources.
In order to ensure that a user doesn't generate more congestion in
the network than her due share, a modified bulk token-bucket is
maintained with the following parameter:
o b_0 the initial token level
o r the filling rate
o b_max the bucket depth
The same token bucket algorithm is used as in many areas of
networking, but how it is used is very different:
o all traffic from a user over the lifetime of their subscription is
policed in the same token bucket.
o only positive and canceled packets (Re-Echo, FNE and CE(0))
consume tokens
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Such a policer will allow network operators to throttle the
contribution of their users to network congestion. This will require
the appropriate contractual terms to be in place between operators
and users. For instance: a condition for a user to subscribe to a
given network service may be that she should not cause more than a
volume C_user of congestion over a reference period T_user, although
she may carry forward up to N_user times her allowance at the end of
each period. These terms directly set the parameter of the user
policer:
o b_0 = C_user
o r = C_user/T_user
o b_max = b_0 * (N_user +1)
Besides the congestion budget policer above, another user policer may
be necessary to further rate-limit FNE packets, if they are to be
marked rather than dropped (see discussion in Section 5.3.). Rate-
limiting FNE packets will prevent high bursts of new flow arrivals,
which is a very useful feature in DoS prevention. A condition to
subscribe to a given network service would have to be that a user
should not generate more than C_FNE FNE packets, over a reference
period T_FNE, with no option to carry forward any of the allowance at
the end of each period. These terms directly set the parameters of
the FNE policer:
o b_0 = C_FNE
o r = C_FNE/T_FNE
o b_max = b_0
T_FNE should be a much shorter period than T_user: for instance T_FNE
could be in the order of minutes while T_user could be in order of
weeks.
G.2. Per-flow Rate Policing
Whilst we believe that simple per-user policing would be sufficient
to ensure senders comply with congestion control, some operators may
wish to police the rate response of each flow to congestion as well.
Although we do not believe this will be neceesary, we include this
section to show how one could perform per-flow policing using
enforcement of TCP-fairness as an example. Per-flow policing aims to
enforce congestion responsiveness on the shortest information
timescale on a network path: packet roundtrips.
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This again requires that the appropriate terms be agreed between a
network operator and its users, where a congestion responsiveness
policy might be required for the use of a given network service
(perhaps unless the user specifically requests otherwise).
As an example, we describe below how a rate adaptation policer can be
designed when the applicable rate adaptation policy is TCP-
compliance. In that context, the average throughput of a flow will
be expected to be bounded by the value of the TCP throughput during
congestion avoidance, given in Mathis' formula [Mathis97]
x_TCP = k * s / ( T * sqrt(m) )
where:
o x_TCP is the throughput of the TCP flow in packets per second,
o k is a constant upper-bounded by sqrt(3/2),
o s is the average packet size of the flow,
o T is the roundtrip time of the flow,
o m is the congestion level experienced by the flow.
We define the marking period N=1/m which represents the average
number of packets between two positive or canceled packets. Mathis'
formula can be re-written as:
x_TCP = k*s*sqrt(N)/T
We can then get the average inter-mark time in a compliant TCP flow,
dt_TCP, by solving (x_TCP/s)*dt_TCP = N which gives
dt_TCP = sqrt(N)*T/k
We rely on this equation for the design of a rate-adaptation policer
as a variation of a token bucket. In that case a policer has to be
set up for each policed flow. This may be triggered by FNE packets,
with the remainder of flows being all rate limited together if they
do not start with an FNE packet.
Where maintaining per flow state is not a problem, for instance on
some access routers, systematic per-flow policing may be considered.
Should per-flow state be more constrained, rate adaptation policing
could be limited to a random sample of flows exhibiting positive or
canceled packets.
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As in the case of user policing, only positive or canceled packets
will consume tokens, however the amount of tokens consumed will
depend on the congestion signal.
When a new rate adaptation policer is set up for flow j, the
following state is created:
o a token bucket b_j of depth b_max starting at level b_0
o a timestamp t_j = timenow()
o a counter N_j = 0
o a roundtrip estimate T_j
o a filling rate r
When the policing node forwards a packet of flow j with no Re-Echo:
o . the counter is incremented: N_j += 1
When the policing node forwards a packet of flow j carrying a
congestion mark (CE):
o the counter is incremented: N_j += 1
o the token level is adjusted: b_j += r*(timenow()-t_j) - sqrt(N_j)*
T_j/k
o the counter is reset: N_j = 0
o the timer is reset: t_j = timenow()
An implementation example will be given in a later draft that avoids
having to extract the square root.
Analysis: For a TCP flow, for r= 1 token/sec, on average,
r*(timenow()-t_j)-sqrt(N_j)* T_j/k = dt_TCP - sqrt(N)*T/k = 0
This means that the token level will fluctuate around its initial
level. The depth b_max of the bucket sets the timescale on which the
rate adaptation policy is performed while the filling rate r sets the
trade-off between responsiveness and robustness:
o the higher b_max, the longer it will take to catch greedy flows
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o the higher r, the fewer false positives (greedy verdict on
compliant flows) but the more false negatives (compliant verdict
on greedy flows)
This rate adaptation policer requires the availability of a roundtrip
estimate which may be obtained for instance from the application of
re-feedback to the downstream delay Appendix F or passive estimation
[Jiang02].
When the bucket of a policer located at the access router (whether it
is a per-user policer or a per-flow policer) becomes empty, the
access router SHOULD drop at least all packets causing the token
level to become negative. The network operator MAY take further
sanctions if the token level of the per-flow policers associated with
a user becomes negative.
Appendix H. Downstream Congestion Metering Algorithms
H.1. Bulk Downstream Congestion Metering Algorithm
To meter the bulk amount of downstream congestion in traffic crossing
an inter-domain border an algorithm is needed that accumulates the
size of positive packets and subtracts the size of negative packets.
We maintain two counters:
V_b: accumulated congestion volume
B: total data volume (in case it is needed)
A suitable pseudo-code algorithm for a border router is as follows:
====================================================================
V_b = 0
B = 0
for each re-ECN-capable packet {
b = readLength(packet) /* set b to packet size */
B += b /* accumulate total volume */
if readEECN(packet) == (Re-Echo || FNE) {
V_b += b /* increment... */
} elseif readEECN(packet) == CE(-1) {
V_b -= b /* ...or decrement V_b... */
} /*...depending on EECN field */
}
====================================================================
At the end of an accounting period this counter V_b represents the
congestion volume that penalties could be applied to, as described in
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Section 6.1.6.
For instance, accumulated volume of congestion through a border
interface over a month might be V_b = 5PB (petabyte = 10^15 byte).
This might have resulted from an average downstream congestion level
of 1% on an accumulated total data volume of B = 500PB.
H.2. Inflation Factor for Persistently Negative Flows
The following process is suggested to complement the simple algorithm
above in order to protect against the various attacks from
persistently negative flows described in Section 6.1.6. As explained
in that section, the most important and first step is to estimate the
contribution of persistently negative flows to the bulk volume of
downstream pre-congestion and to inflate this bulk volume as if these
flows weren't there. The process below has been designed to give an
unbiased estimate, but it may be possible to define other processes
that achieve similar ends.
While the above simple metering algorithm is counting the bulk of
traffic over an accounting period, the meter should also select a
subset of the whole flow ID space that is small enough to be able to
realistically measure but large enough to give a realistic sample.
Many different samples of different subsets of the ID space should be
taken at different times during the accounting period, preferably
covering the whole ID space. During each sample, the meter should
count the volume of positive packets and subtract the volume of
negative, maintaining a separate account for each flow in the sample.
It should run a lot longer than the large majority of flows, to avoid
a bias from missing the starts and ends of flows, which tend to be
positive and negative respectively.
Once the accounting period finishes, the meter should calculate the
total of the accounts V_{bI} for the subset of flows I in the sample,
and the total of the accounts V_{fI} excluding flows with a negative
account from the subset I. Then the weighted mean of all these
samples should be taken a_S = sum_{forall I} V_{fI} / sum_{forall I}
V_{bI}.
If V_b is the result of the bulk accounting algorithm over the
accounting period (Appendix H.1) it can be inflated by this factor
a_S to get a good unbiased estimate of the volume of downstream
congestion over the accounting period a_S.V_b, without being polluted
by the effect of persistently negative flows.
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Appendix I. Argument for holding back the ECN nonce
The ECN nonce is a mechanism that allows a /sending/ transport to
detect if drop or ECN marking at a congested router has been
suppressed by a node somewhere in the feedback loop---another router
or the receiver.
Space for the ECN nonce was set aside in [RFC3168] (currently
proposed standard) while the full nonce mechanism is specified in
[RFC3540] (currently experimental). The specifications for [RFC4340]
(currently proposed standard) requires that "Each DCCP sender SHOULD
set ECN Nonces on its packets...". It also mandates as a requirement
for all CCID profiles that "Any newly defined acknowledgement
mechanism MUST include a way to transmit ECN Nonce Echoes back to the
sender.", therefore:
o The CCID profile for TCP-like Congestion Control [RFC4341]
(currently proposed standard) says "The sender will use the ECN
Nonce for data packets, and the receiver will echo those nonces in
its Ack Vectors."
o The CCID profile for TCP-Friendly Rate Control (TFRC) [RFC4342]
recommends that "The sender [use] Loss Intervals options' ECN
Nonce Echoes (and possibly any Ack Vectors' ECN Nonce Echoes) to
probabilistically verify that the receiver is correctly reporting
all dropped or marked packets."
The primary function of the ECN nonce is to protect the integrity of
the information about congestion: ECN marks and packet drops.
However, when the nonce is used to protect the integrity of
information about packet drops, rather than ECN marks, a transport
layer nonce will always be sufficient (because a drop loses the
transport header as well as the ECN field in the network header),
which would avoid using scarce IP header codepoint space. Similarly,
a transport layer nonce would protect against a receiver sending
early acknowledgements [Savage99].
If the ECN nonce reveals integrity problems with the information
about congestion, the sending transport can use that knowledge for
two functions:
o to protect its own resources, by allocating them in proportion to
the rates that each network path can sustain, based on congestion
control,
o and to protect congested routers in the network, by slowing down
drastically its connection to the destination with corrupt
congestion information.
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If the sending transport chooses to act in the interests of congested
routers, it can reduce its rate if it detects some malicious party in
the feedback loop may be suppressing ECN feedback. But it would only
be useful to congested routers when /all/ senders using them are
trusted to act in interest of the congested routers.
In the end, the only essential use of a network layer nonce is when
sending transports (e.g. large servers) want to allocate their /own/
resources in proportion to the rates that each network path can
sustain, based on congestion control. In that case, the nonce allows
senders to be assured that they aren't being duped into giving more
of their own resources to a particular flow. And if congestion
suppression is detected, the sending transport can rate limit the
offending connection to protect its own resources. Certainly, this
is a useful function, but the IETF should carefully decide whether
such a single, very specific case warrants IP header space.
In contrast, re-ECN allows all routers to fully protect themselves
from such attacks, without having to trust anyone - senders,
receivers, neighbouring networks. Re-ECN is therefore proposed in
preference to the ECN nonce on the basis that it addresses the
generic problem of accountability for congestion of a network's
resources at the IP layer.
Delaying the ECN nonce is justified because the applicability of the
ECN nonce seems too limited for it to consume a two-bit codepoint in
the IP header. It therefore seems prudent to give time for an
alternative way to be found to do the one function the nonce is
essential for.
Moreover, while we have re-designed the re-ECN codepoints so that
they do not prevent the ECN nonce progressing, the same is not true
the other way round. If the ECN nonce started to see some deployment
(perhaps because it was blessed with proposed standard status),
incremental deployment of re-ECN would effectively be impossible,
because re-ECN marking fractions at inter-domain borders would be
polluted by unknown levels of nonce traffic.
The authors are aware that re-ECN must prove it has the potential it
claims if it is to displace the nonce. Therefore, every effort has
been made to complete a comprehensive specification of re-ECN so that
its potential can be assessed. We therefore seek the opinion of the
Internet community on whether the re-ECN protocol is sufficiently
useful to warrant standards action.
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Authors' Addresses
Bob Briscoe
BT & UCL
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 645196
Email: bob.briscoe@bt.com
URI: http://www.cs.ucl.ac.uk/staff/B.Briscoe/
Arnaud Jacquet
BT
B54/70, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 647284
Email: arnaud.jacquet@bt.com
URI:
Alessandro Salvatori
BT
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Email: alessandro.salvatori@gmail.com
Martin Koyabe
BT
PP2a Rigel House, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 646923
Email: martin.koyabe@bt.com
URI:
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Toby Moncaster
BT
B54/70, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 648734
Email: toby.moncaster@bt.com
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