Internet DRAFT - draft-briscoe-conex-re-ecn-tcp
draft-briscoe-conex-re-ecn-tcp
Transport Area Working Group B. Briscoe, Ed.
Internet-Draft A. Jacquet
Intended status: Historic BT
Expires: January 5, 2015 T. Moncaster
Moncaster.com
A. Smith
BT
July 04, 2014
Re-ECN: Adding Accountability for Causing Congestion to TCP/IP
draft-briscoe-conex-re-ecn-tcp-04
Abstract
This document introduces re-ECN (re-inserted explicit congestion
notification), which is intended to make a simple but far-reaching
change to the Internet architecture. The sender uses the IP header
to reveal the congestion that it expects on the end-to-end path. The
protocol works by arranging an extended ECN field in each packet so
that, as it crosses any interface in an internetwork, it will carry a
truthful prediction of congestion on the remainder of its path. It
can be deployed incrementally around unmodified routers. 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 briefly gives examples of
mechanisms that can use the protocol to ensure data sources respond
sufficiently to congestion, but these are described more fully in a
companion document.
Note concerning Intended Status: If this draft were ever published as
an RFC it would probably have historic status. There is limited
space in the IP header, so re-ECN had to compromise by requiring the
receiver to be ECN-enabled otherwise the sender could not use re-ECN.
Re-ECN was a precursor to chartering of the IETF's Congestion
Exposure (ConEx) working group, but during chartering there were
still too few ECN receivers enabled, therefore it was decided to
pursue other compromises in order to fit a similar capability into
the IP header.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 5, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Simplified Re-ECN Protocol . . . . . . . . . . . . . . . 6
4.1.1. Congestion Control and Policing the Protocol . . . . 6
4.1.2. Background and Applicability . . . . . . . . . . . . 7
4.2. Re-ECN Abstracted Network Layer Wire Protocol (IPv4 or
v6) . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Re-ECN Protocol Operation . . . . . . . . . . . . . . . . 9
4.4. Positive and Negative Flows . . . . . . . . . . . . . . . 11
5. Network Layer . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Re-ECN IPv4 Wire Protocol . . . . . . . . . . . . . . . . 12
5.2. Re-ECN IPv6 Wire Protocol . . . . . . . . . . . . . . . . 14
5.3. Router Forwarding Behaviour . . . . . . . . . . . . . . . 15
5.4. Justification for Setting the First SYN to FNE . . . . . 16
5.5. Control and Management . . . . . . . . . . . . . . . . . 17
5.5.1. Negative Balance Warning . . . . . . . . . . . . . . 17
5.5.2. Rate Response Control . . . . . . . . . . . . . . . . 18
5.6. IP in IP Tunnels . . . . . . . . . . . . . . . . . . . . 18
5.7. Non-Issues . . . . . . . . . . . . . . . . . . . . . . . 19
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6. Transport Layers . . . . . . . . . . . . . . . . . . . . . . 20
6.1. TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.1.1. RECN mode: Full Re-ECN capable transport . . . . . . 21
6.1.2. Drops and Marks . . . . . . . . . . . . . . . . . . . 23
6.1.3. Safety against Long Pure ACK Loss Sequences . . . . . 24
6.1.4. RECN-Co mode: Re-ECT Sender with a RFC3168 compliant
ECN Receiver . . . . . . . . . . . . . . . . . . . . 25
6.1.5. Capability Negotiation . . . . . . . . . . . . . . . 26
6.1.6. Extended ECN (EECN) Field Settings during Flow Start
or after Idle Periods . . . . . . . . . . . . . . . . 28
6.1.7. Pure ACKS, Retransmissions, Window Probes and Partial
ACKs . . . . . . . . . . . . . . . . . . . . . . . . 31
6.2. Other Transports . . . . . . . . . . . . . . . . . . . . 32
6.2.1. General Guidelines for Adding Re-ECN to Other
Transports . . . . . . . . . . . . . . . . . . . . . 32
6.2.2. Guidelines for adding Re-ECN to RSVP or NSIS . . . . 33
6.2.3. Guidelines for adding Re-ECN to DCCP . . . . . . . . 33
6.2.4. Guidelines for adding Re-ECN to SCTP . . . . . . . . 33
7. Incremental Deployment . . . . . . . . . . . . . . . . . . . 33
8. Related Work . . . . . . . . . . . . . . . . . . . . . . . . 35
8.1. Congestion Notification Integrity . . . . . . . . . . . . 35
9. Security Considerations . . . . . . . . . . . . . . . . . . . 36
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
11. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 38
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 38
13. Comments Solicited . . . . . . . . . . . . . . . . . . . . . 38
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 39
14.1. Normative References . . . . . . . . . . . . . . . . . . 39
14.2. Informative References . . . . . . . . . . . . . . . . . 40
Appendix A. Precise Re-ECN Protocol Operation . . . . . . . . . 42
Appendix B. Justification for Two Codepoints Signifying Zero
Worth Packets . . . . . . . . . . . . . . . . . . . 43
Appendix C. ECN Compatibility . . . . . . . . . . . . . . . . . 44
Appendix D. Packet Marking with FNE During Flow Start . . . . . 46
Appendix E. Argument for holding back the ECN nonce . . . . . . 48
Appendix F. Alternative Terminology Used in Other Documents . . 50
Appendix G. Changes from previous drafts (to be removed by the
RFC Editor) . . . . . . . . . . . . . . . . . . . . 50
1. Introduction
AUTHORS' STATEMENT (to be removed by the RFC Editor): The most
immediate priority for the authors is to delay any move of the ECN
nonce to Proposed Standard status, in order to leave options open for
the future. The argument for this position is developed in
Appendix E.
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This document provides a complete specification for 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. The motivation behind this proposal is
given in [I-D.re-ecn-motiv], but we include a brief summary here.
Re-ECN is intended to allow senders to inform the network of the
level of congestion they expect their flows to see. This information
is currently only visible at the transport layer. ECN [RFC3168]
reveals the upstream congestion state of any path by monitoring the
rate of CE marks. The receiver then informs the sender when they
have seen a marked packet. Re-ECN builds on ECN by providing new
codepoints that allow the sender to declare the level of congestion
they expect on the forward path. It is closely related to ECN and
indeed we define a compatibility mode to allow a re-ECN sender to
communicate with an ECN receiver.
If a sender understates expected congestion compared to actual
congestion then the network could discard packets or enact some other
sanction. A policer can also be introduced at the ingress of
networks that can limit the level of congestion being caused.
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.
It is important to add a few key points.
o In any standard network it always takes one round trip before any
feedback is received. For this reason a sender must make a
conservative prediction by transmitting IP packets with a special
Cautious marking when it is unsure of the state of the network.
o It should be noted that the prediction is carried in-band in
normal data packets and for many transports feedback can be
carried in the normal acknowledgements or control packets.
o The re-ECN protocol is independent of the transport. In TCP,
acknowledgments are used to convey the feedback from receiver to
sender. This memo concentrates on TCP as an example transport
protocol, however the re-ECN protocol is compatible with any
transport where feedback can be sent from receiver to sender.
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This document is structured as follows. First an overview of the re-
ECN protocol is given (Section 4), outlining its attributes and
explaining conceptually how it works as a whole. The two main parts
of the document follow. That is, the protocol specification divided
into network (Section 5) and transport (Section 6) layers.
Deployment issues discussed throughout the document are brought
together in Section 7. Related work is discussed in (Section 8).
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].
3. Terminology
{ToDo: No attempt has been made to bring terminology into line with
that agreed within the ConEx working group. For instance the term
dropper remains unchanged, even though the ConEx w-g has decided to
call it an audit function (which is actually a much better term).}
The following terminology is used throughout this memo. Some of this
terminology has changed as this draft has been revised. Therefore,
to help avoid confusion, Appendix F sets out all the alternative
terminology that has been used in other re-ECN related documents.
o Neutral packet - a packet that is able to be congestion marked by
an ECN or re-ECN queue.
o Negative packet - a Neutral packet that has been congestion marked
by an ECN or re-ECN queue.
o Positive packet - a packet that has been marked by the sender to
indicate the expected level of congestion along its path. In
general Positive packets should only be sent in response to
feedback received from the receiver.*
o Cancelled packet - a Positive Packet that has been congestion
marked by an ECN or re-ECN queue.
o Cautious packet - a packet that has been marked by the sender to
indicate the expected level of congestion along its path. In
general Cautious packets should be used when there is insufficient
feedback to be confident about the congestion state of the
network.*
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* the difference between positive and cautious packets is
explained in detail later in the document along with guidelines on
the use of Cautious packets.
All the above terms have related IP codepoints as defined in
(Section 5).
4. Protocol Overview
4.1. Simplified Re-ECN Protocol
We describe here the simplified re-ECN protocol. To simplify the
description we assume packets and segments are synonymous.
Packets are sent from a sender to a receiver. In Figure 1 the queues
(Q1 and Q2) are ECN enabled as per RFC 3168 [RFC3168]. If congestion
occurs then packets are marked with the congestion experienced (CE)
flag exactly as in the ECN protocol [RFC3168]; the routers do not
need to be modified and do not need to know the re-ECN protocol. The
receiver constantly informs the sender of the current count of
Negative packets it has seen. The sender uses this information
determine how many Positive packets it must send into the network.
The receiver's aim is to balance the number of bytes that have been
congestion marked with the number of Positive bytes it has sent.
+--------- Feedback----------+
| |
v |
+---+ +----+ +----+ +---+
| | | | | | | |
| S |--->| Q1 |--->| Q2 |--->| R |
| | | | | | | |
+---+ +----+ +----+ +---+
Figure 1: Simple Re-ECN
4.1.1. Congestion Control and Policing the Protocol
The arrangement of the protocol ensures that packets carry a
declaration of the amount of congestion that will be experienced on
the path. The re-ECN protocol is orthogonal to any congestion
control algorithms, but can be used to ensure that congestion control
is being applied by the sender.
In general we assume that there will be a policer at the network
ingress which can rate limit traffic based on the amount of
congestion declared.
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At the network egress there is a dropper which can impose sanctions
on flows that incorrectly declare congestion.
Policers and droppers are explained in more detail in
[I-D.re-ecn-motiv].
4.1.2. Background and Applicability
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 not a new congestion control protocol, rather
it is orthogonal to congestion control itself. Re-ECN is concerned
with revealing information about congestion so that users and
networks can be held accountable for the congestion they cause, or
allow to be caused.
Re-ECN builds on ECN so 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 queues probabilistically set this field to "11" if
congestion is experienced (CE). In general this marking probability
will increase 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
queues on a path, a flow of packets accumulates the fraction of CE
marking that each queue adds. The combined effect of the packet
marking of all the queues along the path signals congestion of the
whole path to the receiver. So, for example, if one queue early in a
path is marking 1% of packets and another later in a path is marking
2%, flows that pass through both queues 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 8.1 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,
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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
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 queues 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, we RECOMMEND
adding optional preferential drop to IP queues based on the re-ECN
fields 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 6.1.4).
4.2. 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). 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. When we don't need to choose between IPv4
and v6 wire protocols it will suffice call it the RE flag.
Unlike the ECN field, the RE flag is intended to be set by the sender
and SHOULD 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. [I-D.re-pcn-border-cheat]).
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) giving 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.
One of re-ECN's codepoints is an alternative use of the codepoint set
aside in RFC3168 for the ECN nonce (ECT(1)). Transports using re-ECN
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do not need to use the ECN nonce as long as the sender is also
checking for transport protocol compliance [tcp-rcv-cheat]. The case
for doing this is given in Appendix E. 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 by ECN or re-
ECN and available for future use.
+--------+-------------+-------+-----------+------------------------+
| ECN | RFC3168 | RE | EECN | re-ECN meaning |
| field | codepoint | flag | codepoint | |
+--------+-------------+-------+-----------+------------------------+
| 00 | Not-ECT | 0 | Not-ECT | Not re-ECN-capable |
| | | | | transport (Legacy) |
| 00 | --- | 1 | FNE | Feedback not |
| | | | | established (Cautious) |
| 01 | ECT(1) | 0 | Re-Echo | Re-echoed congestion |
| | | | | and RECT (Positive) |
| 01 | --- | 1 | RECT | Re-ECN capable |
| | | | | transport (Neutral) |
| 10 | ECT(0) | 0 | ECT(0) | RFC3168 ECN use only |
| 10 | --- | 1 | --CU-- | Currently unused |
| 11 | CE | 0 | CE(0) | Re-Echo cancelled by |
| | | | | CE (Cancelled) |
| 11 | --- | 1 | CE(-1) | Congestion Experienced |
| | | | | (Negative) |
+--------+-------------+-------+-----------+------------------------+
Table 1: Extended ECN Codepoints
4.3. 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.
{ToDo: This section to be updated to explain that the sender re-
echoes losses in the same way as ECN markings.}
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 sends Cautious packets by setting the
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FNE codepoint. The network reads the FNE codepoint conservatively as
equivalent to re-echoed congestion.
Specifically, once feedback from an ECN or re-ECN capable flow is
established, a re-ECN sender always initialises the ECN field to
ECT(1). And it usually sets the RE flag to "1" indicating a Neutral
packet. Whenever a queue 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
(indicating a Positive packet).
We chose to set and clear the RE flag this way round to ease
incremental deployment (see Section 7). 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".
+---+ +----+ +----+ +---+
| S |--| Q1 |----------------| Q2 |--| R |
+---+ +----+ +----+ +---+
. . . .
^ . . . .
| . . . .
| . RE blanking fraction . .
3% |-------------------------------+=======
| . . | .
2% | . . | .
| . . CE marking fraction | .
1% | . +----------------------+ .
| . | . .
0% +--------------------------------------->
^ ^ ^
L M N Observation points
Figure 2: A 2-Queue Example (Imprecise)
Figure 2 uses a simple network to illustrate how re-ECN allows queues
to measure downstream congestion. The receiver views a CE marking
fraction of 3% which is fed back to the sender. The sender sets an
RE blanking fraction of 3% to match this. This RE blanking fraction
can be observed along the path as the RE flag is not changed by
network nodes once set by the sender. This is shown by the
horizontal line at 3% in the figure. The CE marked fraction is shown
by the stepped line which rises to meet the RE blanking fraction line
with steps at each queue where packets are marked. Two queues are
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shown (Q1 and Q2) that are currently congested. Each time packets
pass through a fraction are marked; 1% at Q1 and 2% at Q2). The
approximate downstream congestion can be measured at the observation
points shown along the path by subtracting the CE marking fraction
from the RE blanking fraction, as shown in the table below
(Appendix A derives these approximations from a precise analysis).
NB due to the unary nature of ECN marking and the equivalent unary
nature of re-ECN blanking, the precise fraction of marked bytes must
be calculated by maintaining a moving average of the number of
packets that have been marked as a proportion of the total number of
packets.
Along the path the fraction of packets that had their RE field
cleared remains unchanged so it can 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. We are not saying any protocol
handler will work with these average fractions directly. In fact the
protocol actually requires the number of marked and blanked bytes to
balance by the time the packet reaches the receiver.
4.4. Positive and Negative Flows
In Section 3 we introduced the terms Positive, Neutral, Negative,
Cautious and Cancelled. This terminology is based on the requirement
to balance the proportion of bytes marked as CE with the proportion
of bytes that are re-echo marked. 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. A negative flow is one where more CE marked
packets than re-ECN blanked packets arrive. Likewise in positive
flows more re-ECN blanked packets arrive than CE marked packets. 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).
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. The worth of each type of packet
is given below in Table 2. The idea is that most flows start with
zero worth. 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.
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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.
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 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.
+---------+-------+---------------+-------+-------------------------+
| ECN | RE | Extended ECN | Worth | Re-ECN Term |
| field | bit | codepoint | | |
+---------+-------+---------------+-------+-------------------------+
| 00 | 0 | Not-RECT | ... | --- |
| 00 | 1 | FNE | +1 | Cautious |
| 01 | 0 | Re-Echo | +1 | Positive |
| 10 | 0 | Legacy | ... | RFC3168 ECN use only |
| 11 | 0 | CE(0) | 0 | Cancelled |
| 01 | 1 | RECT | 0 | Neutral |
| 10 | 1 | --CU-- | ... | Currently unused |
| 11 | 1 | CE(-1) | -1 | Negative |
+---------+-------+---------------+-------+-------------------------+
Table 2: 'Worth' of Extended ECN Codepoints
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 4.2) 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 collects together and summarises all the changes
defined in this document).
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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 3).
0 1 2
+---+---+---+
| R | D | M |
| E | F | F |
+---+---+---+
Figure 3: 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 4
and specified fully in Section 6. 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 latter.
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
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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.
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 4).
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 4: 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 5: 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 [RFC4302]) is present in the packet, for
any option whose data may change en-route, its entire Option Data
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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
field which we would expect to change en route. Therefore, as
changes to the RE flag could be detected end-to-end without
authentication (see Section 9), we set the C flag to '1'.
5.3. Router Forwarding Behaviour
{ToDo: Consider a section on how whole protocol interworks with drop.
Perhaps in Protocol Overview.}
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. [I-D.re-pcn-border-cheat] 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 RFC3168
compliant 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 queue 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 [I-D.re-ecn-motiv] would count as rate
limiters for this purpose.
Preferential Drop: If a re-ECN capable router queue 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 3. 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
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policing framework of [I-D.re-ecn-motiv]. 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.
The tabulated drop preferences are arranged to preserve packets
with more positive worth (Section 4.4), given senders of positive
packets must have honestly declared downstream congestion. A full
treatment of this is provided in the companion document describing
the motivation and architecture for re-ECN [I-D.re-ecn-motiv]
particularly when the application of re-ECN to protect against
DDoS attacks is described.
+-------+-----+------------+-------+------------+-------------------+
| 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 | RFC3168 ECN use |
| | | | | | only |
| 00 | 0 | Not-RECT | n/a | 1 | Not Re-ECN- |
| | | | | | capable transport |
+-------+-----+------------+-------+------------+-------------------+
Table 3: Drop Preference of EECN Codepoints (Sorted by `Worth')
5.4. Justification for Setting the First SYN to FNE
the initial SYN MUST be set to FNE by Re-ECT client A (Section 6.1.6)
and (Section 5.3) says a queue MAY optionally treat an FNE packet as
ECN capable, 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 RFC3168 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
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launching SYN flooding attacks. Below we explain the features of the
protocol design that remove both these dangers.
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 6.1.6). But if the
TCP client A finds out from the SYN ACK that the server was not
ECN-capable, the TCP client MUST conservatively consider the first
SYN as congestion marked before setting itself into Not-ECT mode.
Section 6.1.6 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 somewhere. 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.
{ToDo: Give alternative where initial packet is Not-RECT and last ACK
of three-way handshake is FNE. Explain this will give better
performance while deployment is patchy, but worse performance once
deployment is high.}
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
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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.
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 [I-D.re-ecn-motiv] 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
Ideally, for re-ECN to work through IP in IP tunnels, 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 CE marking of
the outer ECN field should overwrite the inner ECN field (unless the
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inner field is Not-ECT in which case an alarm should be 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 requirement is satisfied by the latest specification for
handling ECN through IP tunnels [RFC6040] as well as by IPsec
[RFC4301]. However, it is not satisfied by the ingress behaviour
specified in [RFC3168] although at least the full-functionality
variant of the egress behaviour is fine. RFC6040 updates RFC3168,
but it is likely that many legacy non-IPsec IP-in-IP tunnels will
exist.
If legacy tunnels are left as specified in [RFC3168], whether the
limited or full-functionality variants is 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 RFC3168 compliant 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 [RFC6040] 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 [RFC5129];
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 measurable at any point where IP
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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
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 [RFC4835]) or an
authentication header (AH [RFC4302]) 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.
{ToDo: Add a note about it being useful that the AH header does not
cover the RE flag, referring to Section 9.}
6. Transport Layers
6.1. TCP
Re-ECN capability at the sender is essential. At the receiver it is
optional, as long as the receiver has a basic 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.
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+--------+--------------+------------+---------+--------------------+
| Re-ECT | ECT-Nonce | ECT | Not-ECT | S-R Half- |
| | (RFC3540) | (RFC3168) | | 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 RFC3168
compliant [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.
6.1.1. 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 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, modulo 8, 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 7. The actual definition of the TCP header,
including the addition of support for the ECN nonce, is shown for
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comparison in Figure 6. 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 6: 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 7: 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 and MUST
echo its value, modulo 8, 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).
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 (D = (ECI + 8 - ECC mod 8) mod 8)
is assumed to be the number of CE marked packets that arrived at
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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 6.1.6.
As we have already emphasised, the re-ECN protocol makes no
changes and has no effect on the TCP congestion control algorithm.
So, the first increment of ECI (or detection of a drop) in a RTT
triggers the standard TCP congestion response, no more than one
congestion response per round trip, as usual. However, the sender
re-echoes every increment of ECI irrespective of RTTs.
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.
6.1.2. Drops and Marks
Re-ECN is based on the ECN protocol [RFC3168] . In turn the
congestion markings ECN uses are 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.
However, if the queue becomes too full then it will revert to
dropping packets. Because of this it is important that a re-ECN
sender 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. Drop detection is defined as set out in
[RFC2581] -- if the connection is in slow start then a single
duplicate acknowledgement will be treated as an indication of a drop.
When the system is in the congestion avoidance stage then 3 duplicate
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acknowledgements will be treated as a sign of a drop. In all cases,
if a re-transmission time-out occurs then that will be treated as a
drop.
6.1.3. 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
sequence of lost pure ACKs. But such an algorithm is OPTIONAL. 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.
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6.1.4. RECN-Co mode: Re-ECT Sender with a RFC3168 compliant ECN
Receiver
If the half-connection is in RECN-Co mode, ECN feedback proceeds no
differently to that of RFC3168 compliant 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 RFC3168 compliant 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. 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
RFC3168 compliant 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 RFC3168 compliant 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.
6.1.4.1. 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
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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.
6.1.5. 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 flags 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. Normally a Re-ECT server (B) will reply to a Re-
ECT client with NS=0, but 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.
These handshakes are summarised in Table 5 below, with X indicating
NS can be either 1 or 0 depending respectively on whether congestion
had been experienced or not. 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)
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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 delaying mode changes if using SYN cookies (will also
affect next section).}
{ToDo: consider RSTs within a connection.}
Recall that, if the SYN ACK reflects the same flag settings as the
preceding SYN (because there is a broken RFC3168 compliant
implementation that behaves this way), RFC3168 specifies that the
whole connection MUST revert to Not-ECT.
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
( i.e the ECI field of the SYN-ACK) 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.
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6.1.6. 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 4.2). 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. This is because FNE indicates the transport is
ECN capable as well as re-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 [RFC5562] has shown that this caution was
unnecessary, and allows a SYN ACK to be ECN-capable to improve
performance. 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 gone further by
proposing to make the initial SYN ECN-capable too. 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
to be sent before sufficient congestion feedback is received. The
behaviour will be different depending on the mode of the half-
connection:
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RECN mode: Given the constraints on TCP's initial window [RFC3390]
and its exponential window increase during slow start
phase [RFC5681], it turns out that the sender SHOULD set FNE on
the first and third data packets in its flow after the initial
3-way handshake, 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.
{ToDo: unfortunately the calculation was based on erroneous
assumptions; see [I-D.conex-tcp-mods] for a better approach.}
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
initial SYN ACK as previously described. The left hand half of the
table shows the relevant settings of headers sent by client A in
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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 the relevant
sending host 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 queue 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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{ToDo: There is probably scope for re-writing the above in a
different way so that it says MUST unless some other knowledge of the
path is available. See earlier note pointing out FNE on 1st & 3rd is
too few.}
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) [RFC6298], which is commonly used as the idle period before TCP
must reduce to the restart window [RFC5681]. 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 RFC3168 modes if packets
don't appear to be getting through (to work round firewalls
discarding packets they consider unusual).}
{ToDo: Possible future capabilities for changing Slow Start}
6.1.7. 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
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timely feedback on congestion events at the same packet granularity
that they were generated on congested queues.
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 packet (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).}
6.2. Other Transports
6.2.1. General Guidelines for Adding Re-ECN to Other Transports
As a general rule, 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 for 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)
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o UDP streaming with no feedback
o UDP streaming with feedback
}
6.2.2. Guidelines for adding Re-ECN to RSVP or NSIS
A separate I-D has been submitted [I-D.re-pcn-border-cheat]
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) [RFC5559].
6.2.3. 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.
6.2.4. Guidelines for adding Re-ECN to SCTP
Appendix A in [RFC4960] 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
7. Incremental Deployment
The design of the re-ECN protocol started from the fact that the
current ECN marking behaviour of queues was sufficient and that re-
feedback could be introduced around these queues by changing the
sender behaviour but not the routers. Otherwise, if we had required
routers to be changed, the chance of encountering a path that had
every router upgraded would be vanishingly small during early
deployment, giving no incentive to start deployment. Also, as there
is no new forwarding behaviour, routers and hosts do not have to
signal or negotiate anything.
However, networks that choose to protect themselves using re-ECN do
have to add new security functions at their trust boundaries with
others. They distinguish legacy traffic by its ECN field. Traffic
from Not-ECT transports is distinguishable by its Not-ECT marking.
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Traffic from RFC3168 compliant ECN transports is distinguished from
re-ECN by which of ECT(0) or ECT(1) is used. We chose to use ECT(1)
for re-ECN traffic deliberately. Existing ECN sources set ECT(0) on
either 50% (the nonce) or 100% (the default) of packets, whereas re-
ECN does not use ECT(0) at all. We can use this distinguishing
feature of RFC3168 compliant ECN traffic to separate it out for
different treatment at the various border security functions: egress
dropping, ingress policing and border policing.
The general principle we adopt is that an egress dropper will not
drop any legacy traffic, but ingress and border policers will limit
the bulk rate of legacy traffic (Not-ECT, ECT(0) and those marked
with the unused codepoint) 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 RFC3168 ECN
sources will gain by upgrading to re-ECN. Thus, towards the end of
the voluntary incremental deployment period, RFC3168 compliant
transports can be given progressively stronger encouragement to
upgrade.
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 4.3);
o No provision is necessary for a re-ECN capable source transport to
use the ECN nonce (Section 6.1.4.1);
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 ;
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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.
8. Related Work
8.1. 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].
{ToDo: DCCP provides nonce support - how does this affect the RFC?}
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
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
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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.
{ToDo: Ensure we address the early ACK problem.}
9. Security Considerations
{ToDo: Describe attacks by networks on flows and by spoofing
sources.} {ToDo: Re-ECN & DNS servers}
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
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.
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
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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).
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.
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10. 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 4.2)
o The Hop-by-Hop option ID for the new extension header for IPv6
(Section 5.2);
o The new combinations of flags in the TCP header for capability
negotiation (Section 6.1.5);
11. Conclusions
{ToDo:}
12. Acknowledgements
Sebastien Cazalet and Andrea Soppera contributed to the idea of re-
feedback. All the following have given helpful comments: Andrea
Soppera, David Songhurst, Peter Hovell, Louise Burness, Phil Eardley,
Steve Rudkin, Marc Wennink, Fabrice Saffre, Cefn Hoile, Steve Wright,
John Davey, Martin Koyabe, Carla Di Cairano-Gilfedder, Alexandru
Murgu, Nigel Geffen, Pete Willis, John Adams (BT), Sally Floyd
(ICIR), Joe Babiarz, Kwok Ho-Chan (Nortel), Stephen Hailes, Mark
Handley (who developed the attack with 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), Meral Shirazipour (Ericsson) and comments from participants in
the CRN/CFP Broadband and DoS-resistant Internet working groups.A
special thank you to Alessandro Salvatori for coming up with fiendish
attacks on re-ECN.
13. Comments Solicited
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF Congestion Exposure (ConEx) working group's
mailing list <conex@ietf.org>, and/or to the authors.
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14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[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.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December
2005.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion Control ID 2: TCP-like
Congestion Control", RFC 4341, March 2006.
[RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
Datagram Congestion Control Protocol (DCCP) Congestion
Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
March 2006.
[RFC4835] Manral, V., "Cryptographic Algorithm Implementation
Requirements for Encapsulating Security Payload (ESP) and
Authentication Header (AH)", RFC 4835, April 2007.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC
4960, September 2007.
[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S., and K.
Ramakrishnan, "Adding Explicit Congestion Notification
(ECN) Capability to TCP's SYN/ACK Packets", RFC 5562, June
2009.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
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[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, November 2010.
14.2. 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.
[I-D.conex-tcp-mods]
Kuehlewind, M. and R. Scheffenegger, "TCP modifications
for Congestion Exposure", draft-ietf-conex-tcp-
modifications-05 (work in progress), February 2014.
[I-D.re-ecn-motiv]
Briscoe, B., Jacquet, A., Moncaster, T., and A. Smith,
"Re-ECN: A Framework for adding Congestion Accountability
to TCP/IP", draft-briscoe-conex-re-ecn-motiv-03 (work in
progress), March 2014.
[I-D.re-pcn-border-cheat]
Briscoe, B., "Emulating Border Flow Policing using Re-PCN
on Bulk Data", draft-briscoe-re-pcn-border-cheat-03 (work
in progress), October 2009.
[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.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[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 1 2003.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces", RFC
3540, June 2003.
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[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, January 2008.
[RFC5559] Eardley, P., "Pre-Congestion Notification (PCN)
Architecture", RFC 5559, June 2009.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298, June
2011.
[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>.
[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.
[tcp-rcv-cheat]
Moncaster, T., Briscoe, B., and A. Jacquet, "A TCP Test to
Allow Senders to Identify Receiver Non-Compliance", draft-
moncaster-tcpm-rcv-cheat-03 (work in progress), July 2014.
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Appendix A. Precise Re-ECN Protocol Operation
The protocol operation in Section 4.3 was described as an
approximation. In fact, standard ECN marking at a queue combines 1%
and 2% marking into slightly less than 3% whole-path marking, because
queues deliberately mark CE whether or not it has already been marked
by another queue 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 queue to n-1 at the last.
o m_j represents the fraction of octets to be |*m|arked CE by a
particular queue (whether or not they are already marked) because
of congestion of resource j.
o u_j represents congestion signals arriving from |*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
queue 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 upstream congestion (CE marking fraction) from the viewpoint
of the destination, which it feeds back to the sender. 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, given p = 1
- (1 - u_j)(1 - v_j), then
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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 4.3, 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
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.
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Instead, we chose to make re-echoing congestion (blanking RE)
orthogonal to congestion notification (marking CE), which required a
second neutral codepoint. 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 RFC3168 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
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 6.1.5 is as follows.
Choice of SYN flags: A Re-ECN sender can work with RFC3168 compliant
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
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setting NS=1 in the initial SYN achieves both these objectives, as
it should be ignored by RFC3168 compliant 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
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 RFC3168 compliant 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.
{ToDo: include the text below, either here, or in the algorithm
sections} At an egress dropper, well-behaved RFC3168 compliant flows
will appear to consist mostly of ECT(0) packets, with a few CE(0)
packet. And, if the legacy source is setting the ECN nonce, the
majority of packets will be an equal mix of ECT(0) and ECT(1) packets
(the latter appearing to be Re-Echo packets in Re-ECN terms). None
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of these three packet markings is negative, so an egress dropper can
handle all legacy flows in bulk and, as long as they don't send any
packets using Re-ECN markings, it need not drop any legacy packets.
So, as soon as an ECT(0) packet is seen, its flow ID can be added to
the set of known legacy flows (a single Bloom filter would suffice).
But, if any packets in flows classified as RFC3168 compliant are
marked with any other marking than the three expected, the flow can
be removed from the RFC3168 set, to be treated in bulk with mis-
behaving Re-ECN flows---the remainder of flow IDs that require no
flow state to be held.
To an ingress Re-ECN policer, legacy ECN flows will appear as very
highly congested paths. When policers are first deployed they can be
configured permissively, allowing through both `RFC3168' ECN and
misbehaving Re-ECN flows. Then, as the threshold is set more
strictly, the more RFC3168 ECN sources will gain by upgrading to Re-
ECN. Thus, towards the end of the voluntary incremental deployment
period, RFC3168 transports can be given progressively stronger
encouragement to upgrade.
Appendix D. Packet Marking with FNE During Flow Start
FNE (feedback not established) packets have two functions. Their
main role is to announce the start of a new flow when feedback has
not yet been established. However they also have the role of
balancing the expected feedback and can be used where there are
sudden changes in the rate of transmission. Whilst this should not
happen under TCP their use as speculative marking is used in building
the following argument as to why the first and third packets should
be set to FNE.
The proportion of FNE packets in each round-trip should be a high
estimate of the potential error in the balance of number of
congestion marked packets versus number of re-echo packets already
issued.
Let's call:
S: the number of the TCP segments sent so far
F: the number of FNE packets sent so far
R: the number of Re-Echo packets sent so far
A: the number of acknowledgments received so far
C: the number of acknowledgments echoing a CE packet
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In normal operation, when we want to send packet S+1, we first need
to check that enough Re-Echo packets have been issued:
If R<C, then S+1 will be a Re-echo packet
Next we need to estimate the amount of congestion observed so far.
If congestion was stationary, it could be estimated as C/A. A
pessimistic bound is (C+1)/(A+1) which assumes that the next
acknowledgment will echo a CE packet; we'll use that more pessimistic
estimate to drive the generation of FNE packets.
The number of CE packets expected when (S+1) will be acknowledged is
therefore (S+1)*(C+1)/(A+1). Packet S+1 should be set to FNE if that
expected value exceeds the sum of FNE and Re-Echo packets sent so
far.
If (F+R)<(S+1)*(C+1)/(A+1),
then S+1 will be set to FNE
else S+1 will be set to RECT
So the full test should be:
When packet (S+1) is about to be sent...
If R<C,
then S+1 will be set to Re-Echo
Else if (F+R)<(S+1)*(C+1)/(A+1),
then S+1 will be set to FNE
Else S+1 will be set to RECT
This means that at any point, given A, R, F, C, the source could send
another k RECT packets, so that k < (F+R)*(A+1)/(C+1)-S
The above scheme is independent of the actions of both the dropper
and policer and doesn't depend on the rate adaptation discipline of
the source. It only defines Re-Echo packets as notification of
effective end-to-end congestion (as witnessed at the previous round-
trip), and FNE packets as notification of speculative end-to-end
congestion based on a high estimate of congestion
In practice, for any source:
o for the first packet, A=R=F=C=S=0 ==> 1 FNE
o if the acknowledgment doesn't echo a mark
* for the second packet, A=F=S=1 R=C=0 ==> 1 RECT
* for the third packet, S=2 A=F=1 R=C=0 ==> 1 FNE
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o if no acknowledgement for these two packets echoes a congestion
mark, then {A=S=3 F=2 R=C=0} which gives k<2*4/1-3, so the source
o if no acknowledgement for these four packets echoes a congestion
mark, then {A=S=7 F=2 R=C=0} which gives k<2*8/1-7, so the source
could send another 8 RECT packets. ==> 8 RECT
This behaviour happens to match TCP's congestion window control in
slow start, which is why for TCP sources, only the first and third
packet need be FNE packets.
A source that would open the congestion window any quicker would have
to insert more FNE packets. As another example a UDP source sending
VBR traffic might need to send several FNE packets ahead of the
traffic peaks it generates.
Appendix E. Argument for holding back the ECN nonce
The ECN nonce is a mechanism that allows a /sending/ transport to
detect if drop or ECN marking at a congested router has been
suppressed by a node somewhere in the feedback loop---another router
or the receiver.
Space for the ECN nonce was set aside in [RFC3168] (currently
proposed standard) while the full nonce mechanism is specified in
[RFC3540] (currently experimental). The specifications for [RFC4340]
(currently proposed standard) requires that "Each DCCP sender SHOULD
set ECN Nonces on its packets...". It also mandates as a requirement
for all CCID profiles that "Any newly defined acknowledgement
mechanism MUST include a way to transmit ECN Nonce Echoes back to the
sender.", therefore:
o The CCID profile for TCP-like Congestion Control [RFC4341]
(currently proposed standard) says "The sender will use the ECN
Nonce for data packets, and the receiver will echo those nonces in
its Ack Vectors."
o The CCID profile for TCP-Friendly Rate Control (TFRC) [RFC4342]
recommends that "The sender [use] Loss Intervals options' ECN
Nonce Echoes (and possibly any Ack Vectors' ECN Nonce Echoes) to
probabilistically verify that the receiver is correctly reporting
all dropped or marked packets."
The primary function of the ECN nonce is to protect the integrity of
the information about congestion: ECN marks and packet drops.
However, when the nonce is used to protect the integrity of
information about packet drops, rather than ECN marks, a transport
layer nonce will always be sufficient (because a drop loses the
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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.
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.
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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.
Appendix F. Alternative Terminology Used in Other Documents
A number of alternative terms have been used in various documents
describing re-feedback and re-ECN. These are set out in the
following table
+---------------------+----------------+------------------+
| Current Terminology | EECN codepoint | Colour |
+---------------------+----------------+------------------+
| Cautious | FNE | Green |
| Positive | Re-Echo | Black |
| Neutral | RECT | Grey |
| Negative | CE(-1) | Red |
| Cancelled | CE(0) | Red-Black |
| Legacy ECN | ECT(0) | White |
| Currently Unused | --CU-- | Currently unused |
| | | |
| Legacy | Not-ECT | White |
+---------------------+----------------+------------------+
Table 7: Alternative re-ECN Terminology
Appendix G. Changes from previous drafts (to be removed by the RFC
Editor)
Full diffs from all previous versions (created using the rfcdiff
tool) are available at <http://www.bobbriscoe.net/pubs.html#retcp>
From draft-briscoe-conex-...-03 to -04 (current version): Re-issued
to keep alive; Updated references (but protocol specification
remains frozen as it was at draft-briscoe-tsvwg-...-08);
reinstated section on "Safety against Long Pure ACK Loss
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Sequences" about wrap of the ECI field that had accidentially been
commented out in draft-briscoe-tsvwg-...-07
From draft-briscoe-conex-...-02 to -03 (current version): Re-issued
to keep alive; updated references
From draft-briscoe-conex-...-01 to -02 (current version): Re-issued
to keep alive; updated references
From draft-briscoe-conex-...-00 to -01: Re-issued to keep alive;
updated references
From draft-briscoe-tsvwg-...-08 to draft-briscoe-conex-...-00:
Re-issued to keep alive for reference by ConEx working group
Changed working group tag in filename from tsvwg to conex
Changed intended status to historic and added explanatory note
Updated references. Also, now that RFC6040 has been published,
the section on tunnelling required a re-write
Corrected name of CE(0) to Cancelled in Table 2
Noted errors and omissions (rather than spending time correcting
them):
* Made a few 'ToDo' comments visible that had previously been
comments within the document source
* Identified errors with 'ToDo' comments, referring to correct
material where possible.
From -08 to -09:
Re-issued to keep alive for reference by ConEx working group.
Hardly any changes to content, even where it is out of date,
except references updated.
From -07 to -08:
Minor changes and consistency checks.
References updated.
From -06 to -07:
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Major changes made following splitting this protocol document from
the related motivations document [I-D.re-ecn-motiv].
Significant re-ordering of remaining text.
New terminology introduced for clarity.
Minor editorial changes throughout.
Authors' Addresses
Bob Briscoe (editor)
BT
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 645196
EMail: bob.briscoe@bt.com
URI: http://bobbriscoe.net/
Arnaud Jacquet
BT
B54/70, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 647284
EMail: arnaud.jacquet@bt.com
Toby Moncaster
Moncaster.com
Dukes
Layer Marney
Colchester CO5 9UZ
UK
EMail: toby@moncaster.com
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Alan Smith
BT
B54/76, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 640404
EMail: alan.p.smith@bt.com
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