One document matched: draft-briscoe-conex-re-ecn-motiv-01.xml
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<rfc category="historic" docName="draft-briscoe-conex-re-ecn-motiv-01"
ipr="trust200902">
<front>
<title abbrev="Re-ECN: Framework">Re-ECN: A Framework for adding
Congestion Accountability to TCP/IP</title>
<author fullname="Bob Briscoe" initials="B." role="editor"
surname="Briscoe">
<organization>BT</organization>
<address>
<postal>
<street>B54/77, Adastral Park</street>
<street>Martlesham Heath</street>
<city>Ipswich</city>
<code>IP5 3RE</code>
<country>UK</country>
</postal>
<phone>+44 1473 645196</phone>
<email>bob.briscoe@bt.com</email>
<uri>http://bobbriscoe.net/</uri>
</address>
</author>
<author fullname="Arnaud Jacquet" initials="A." surname="Jacquet">
<organization>BT</organization>
<address>
<postal>
<street>B54/70, Adastral Park</street>
<street>Martlesham Heath</street>
<city>Ipswich</city>
<code>IP5 3RE</code>
<country>UK</country>
</postal>
<phone>+44 1473 647284</phone>
<email>arnaud.jacquet@bt.com</email>
<uri></uri>
</address>
</author>
<author fullname="Toby Moncaster" initials="T." surname="Moncaster">
<organization>Moncaster.com</organization>
<address>
<postal>
<street>Dukes</street>
<street>Layer Marney</street>
<city>Colchester</city>
<code>CO5 9UZ</code>
<country>UK</country>
</postal>
<email>toby@moncaster.com</email>
</address>
</author>
<author fullname="Alan Smith" initials="A." surname="Smith">
<organization>BT</organization>
<address>
<postal>
<street>B54/76, Adastral Park</street>
<street>Martlesham Heath</street>
<city>Ipswich</city>
<code>IP5 3RE</code>
<country>UK</country>
</postal>
<phone>+44 1473 640404</phone>
<email>alan.p.smith@bt.com</email>
<!-- <uri>?</uri> -->
</address>
</author>
<date day="22" month="October" year="2012" />
<area>Transport</area>
<workgroup>Transport Area Working Group</workgroup>
<keyword>Quality of Service</keyword>
<keyword>QoS</keyword>
<keyword>Congestion Control</keyword>
<keyword>Differentiated Services</keyword>
<keyword>Integrated Services</keyword>
<keyword>Admission Control</keyword>
<keyword>Signalling</keyword>
<keyword>Protocol</keyword>
<keyword>Pre-emption</keyword>
<abstract>
<t>This document describes a framework for using a new protocol called
re-ECN (re-inserted explicit congestion notification), which can be
deployed incrementally around unmodified routers. Re-ECN allows accurate
congestion monitoring throughout the network thus enabling the upstream
party at any trust boundary in the internetwork to be held responsible
for the congestion they cause, or allow to be caused. So, networks can
introduce straightforward accountability for congestion and policing
mechanisms for incoming traffic from end-customers or from neighbouring
network domains. As well as giving the motivation for re-ECN this
document also gives examples of mechanisms that can use the protocol to
ensure data sources respond correctly to congestion. And it describes
example mechanisms that ensure the dominant selfish strategy of both
network domains and end-points will be to use the protocol honestly.</t>
<t>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.</t>
</abstract>
</front>
<middle>
<!-- ================================================================ -->
<note title="Authors' Statement: Status (to be removed by the RFC Editor)">
<t>Although the re-ECN protocol is intended to make a simple but
far-reaching change to the Internet architecture, the most immediate
priority for the authors is to delay any move of the ECN nonce to
Proposed Standard status. The argument for this position is developed in
<xref target="retcp_Nonce_Limitation" />.</t>
</note>
<!-- ================================================================ -->
<section anchor="retcp_Introduction" title="Introduction">
<t>This document aims to: <list style="symbols">
<t>Describe the motivation for wanting to introduce re-ECN;</t>
<t>Provide a very brief description of the protocol;</t>
<t>The framework within which the protocol sits;</t>
<t>To show how a number of hard problems become much easier to solve
once re-ECN is available in IP.</t>
</list></t>
<t>This introduction starts with a run through of these 4 points.</t>
<section anchor="retcp_Introduction_motivation" title="Motivation">
<t>Re-ECN is proposed as a means of allowing accurate monitoring of
congestion throughout the Internet. The current Internet relies on the
vast majority of end-systems running TCP and reacting to detected
congestion by reducing their sending rates. Thus congestion control is
conducted by the collaboration of the majority of end-systems.</t>
<t>In this situation it is possible for applications that are
unresponsive to congestion to take whatever share of bottleneck
resources they want from responsive flows, the responsive flows reduce
their sending rate in face of congestion and effectively get out of
the way of unresponsive flows. An increasing proportion of such
applications could lead to congestion collapse being more common <xref
target="RFC3714" />. Each network has no visibility of whole path
congestion and can only respond to congestion on a local basis.</t>
<t>Using re-ECN will allow any point along a path to calculate
congestion both upstream and downstream of that point. As a
consequence of this policing of congestion /could/ be carried out in
the network if end-systems fail to do so. Re-ECN enables flows and
users to be policed and for policing to happen at network ingress and
at network borders.</t>
</section>
<section anchor="retcp_Introduction_protocol_in_brief"
title="Re-ECN Protocol in Brief">
<t>In re-ECN each sender makes a prediction of the congestion that
each flow will cause and signals that prediction within the IP headers
of that flow. The prediction is based on, but not limited to, feedback
received from the receiver. Sending a prediction of the congestion
gives network equipment a view of the congestion downstream and
upstream.</t>
<t>In order to explain this mechanism we introduce the notion of IP
packets carrying different, notional values dependent on the state of
their header flags: <list style="symbols">
<t>Negative - are those marked by queues when incipient congestion
is detected. This is exactly the same as ECN <xref
target="RFC3168" />;</t>
<t>Positive - are sent by the sender in proportion to the number
of bytes in packets that have been marked negative according to
feedback received from the receiver;</t>
<t>Cautious - are sent whenever the sender cannot be sure of the
correct amount of positive bytes to inject into the network for
example, at the start of a flow to indicate that feedback has not
been established;</t>
<t>Cancelled - packets sent by the sender as positive that get
marked as negative by queues in the network due to incipient
congestion;</t>
<t>Neutral - normal IP packets but show queues that they can be
marked negative.</t>
</list></t>
<t>A flow starts to transmit packets. No feedback has been established
so a number of cautious packets are sent (see the protocol definition
<xref target="Re-TCP" /> for an analysis of how many cautious packets
should be sent at flow start). The rest are sent as neutral.</t>
<t>The packets traverse a congested queue. A fraction are marked
negative as an indication of incipient congestion.</t>
<t>The packets are received by the receiver. The receiver feeds back
to the sender a count of the number of packets that have been marked
negative. This feedback can be provided either by the transport (e.g.
TCP) or by higher-layer control messages.</t>
<t>The sender receives the feedback and then sends a number of
positive packets in proportion to the bytes represented by packets
that have been marked negative. It is important to note that
congestion is revealed by the fraction of marked packets rather than a
field in the IP header. This is due to the limited code points
available and includes use of the last unallocated bit (sometimes
called the evil bit <xref target="RFC3514" />). Full details of the
code points used is given in <xref target="Re-TCP" />. This lack of
codepoints is, however, the case with IPv4. ECN is similarly
restricted.</t>
<t>The number of bytes inside the negative packets and positive
packets should therefore be approximately equal at the termination
point of the flow. To put it another way, the balance of negative and
positive should be zero.</t>
</section>
<section anchor="retcp_Introduction_re_ecn_framework"
title="The Re-ECN Framework">
<t>The introducion of the protocol enables 3 things:<list
style="symbols">
<t>Gives a view of whole path congestion;</t>
<t>Enables policing of flows;</t>
<t>It allows networks to monitor the flow of congestion across
their borders.</t>
</list></t>
<t>At any point in the network a device can calculate the upstream
congestion by calculating the fraction of bytes in negative packets to
total packets. This it could do using ECN by calculating the fraction
of packets marked Congestion Experienced.</t>
<t>Using re-ECN a device in the network can calculate downstream
congestion by subtracting the fraction of negative packets from the
fraction of positive packets.</t>
<t>A user can be restricted to only causing a certain amount of
congestion. A Policer could be introduced at the ingress of a network
that counts the number of positive packets being sent and limits the
sender if that sender ties to transmit more positive packets than
their allowance.</t>
<t>A user could deliberately ignore some or all of the feedback and
transmit packets with a zero or much lower proportion of positive
packets than negative packets. To solve this a Dropper is proposed.
This would be placed at the egress of a network. If the number of
negative packets exceeds the number of positive packets then the flow
could be dropped or some other sanction enacted.</t>
<t>Policers and droppers could be used between networks in order to
police bulk traffic. A whole network harbouring users causing
congestion in downstream networks can be held responsible or policed
by its downstream neighbour.</t>
</section>
<section anchor="retcp_Introduction_hard_problems"
title="Solving Hard Problems">
<t>We have already shown that by making flows declare the level of
congestion they are causing that they can be policed, more
specifically these are the kind of problems that can be solved: <list
style="symbols">
<t>mitigating distributed denial of service (DDoS);</t>
<t>simplifying differentiation of quality of service (QoS);</t>
<t>policing compliance to congestion control;</t>
<t>inter-provider service monitoring;</t>
<t>etc.</t>
</list></t>
<t>Uniquely, re-ECN manages to enable solutions to these problems
without unduly stifling innovative new ways to use the Internet. This
was a hard balance to strike, given it could be argued that DDoS is an
innovative way to use the Internet. The most valuable insight was to
allow each network to choose the level of constraint it wishes to
impose. Also re-ECN has been carefully designed so that networks that
choose to use it conservatively can protect themselves against the
congestion caused in their network by users on other networks with
more liberal policies.</t>
<t>For instance, some network owners want to block applications like
voice and video unless their network is compensated for the extra
share of bottleneck bandwidth taken. These real-time applications tend
to be unresponsive when congestion arises. Whereas elastic TCP-based
applications back away quickly, ending up taking a much smaller share
of congested capacity for themselves. Other network owners want to
invest in large amounts of capacity and make their gains from
simplicity of operation and economies of scale.</t>
<t>While we have designed re-ECN so that networks can choose to deploy
stringent policing, this does not imply we advocate that every network
should introduce tight controls on those that cause congestion. Re-ECN
has been specifically designed to allow different networks to choose
how conservative or liberal they wish to be with respect to policing
congestion. But those that choose to be conservative can protect
themselves from the excesses that liberal networks allow their
users.</t>
<t>Re-ECN allows the more conservative networks to police out flows
that have not asked to be unresponsive to congestion---not because
they are voice or video---just because they don't respond to
congestion. But it also allows other networks to choose not to police.
Crucially, when flows from liberal networks cross into a conservative
network, re-ECN enables the conservative network to apply penalties to
its neighbouring networks for the congestion they allow to be caused.
And these penalties can be applied to bulk data, without regard to
flows.</t>
<t>Then, if unresponsive applications become so dominant that some of
the more liberal networks experience congestion collapse <xref
target="RFC3714" />, they can change their minds and use re-ECN to
apply tighter controls in order to bring congestion back under
control.</t>
<t>Re-ECN reduces the need for complex network equipment to perform
these functions.</t>
</section>
<section anchor="retcp_Introduction_compass"
title="The Rest of this Document">
<t>This document is structured as follows. First the motivation for
the new protocol is given (<xref target="retcp_Motivation" />)
followed by the incentive framework that is possible with the protocol
<xref target="retcp_Incentive_Framework" />. <xref
target="retcp_Other_Applications" /> then describes other important
applications re-ECN, such as policing DDoS, QoS and congestion
control. Although these applications do not require standardisation
themselves, they are described in a fair degree of detail in order to
explain how re-ECN can be used. Given re-ECN proposes to use the last
undefined bit in the IPv4 header, we felt it necessary to outline the
potential that re-ECN could release in return for being given that
bit.</t>
<t>Deployment issues discussed throughout the document are brought
together in <xref target="retcp_Incremental_Deployment" />, which is
followed by a brief section explaining the somewhat subtle rationale
for the design from an architectural perspective (<xref
target="retcp_Architectural_Rationale" />). We end by describing
related work (<xref target="retcp_Related_Work" />), listing security
considerations (<xref target="retcp_Security_Considerations" />) and
finally drawing conclusions (<xref target="retcp_Conclusions" />).</t>
</section>
</section>
<!-- ================================================================ -->
<section anchor="retcp_Reqs_notation" title="Requirements notation">
<t>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 <xref
target="RFC2119" />.</t>
<t>This document first specifies a protocol, then describes a framework
that creates the right incentives to ensure compliance to the protocol.
This could cause confusion because the second part of the document
considers many cases where malicious nodes may not comply with the
protocol. When such contingencies are described, if any of the above
keywords are not capitalised, that is deliberate. So, for instance, the
following two apparently contradictory sentences would be perfectly
consistent: i) x MUST do this; ii) x may not do this.</t>
</section>
<!-- ================================================================ -->
<section anchor="retcp_Motivation" title="Motivation">
<!-- ________________________________________________________________ -->
<section anchor="retcp_Policing_Congestion_Response"
title="Policing Congestion Response">
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<section anchor="retcp_Policing_Problem" title="The Policing Problem">
<t>The current Internet architecture trusts hosts to respond
voluntarily to congestion. Limited evidence shows that the large
majority of end-points on the Internet comply with a TCP-friendly
response to congestion. But telephony (and increasingly video)
services over the best effort Internet are attracting the interest
of major commercial operations. Most of these applications do not
respond to congestion at all. Those that can switch to lower rate
codecs.</t>
<t>Of course, the Internet is intended to support many different
application behaviours. But the problem is that this freedom can be
exercised irresponsibly. The greater problem is that we will never
be able to agree on where the boundary is between responsible and
irresponsible. Therefore re-ECN is designed to allow different
networks to set their own view of the limit to irresponsibility, and
to allow networks that choose a more conservative limit to push back
against congestion caused in more liberal networks.</t>
<t>As an example of the impossibility of setting a standard for
fairness, mandating TCP-friendliness would set the bar too high for
unresponsive streaming media, but still some would say the bar was
too low <xref target="relax-fairness" />. Even though all known
peer-to-peer filesharing applications are TCP-compatible, they can
cause a disproportionate amount of congestion, simply by using
multiple flows and by transferring data continuously relative to
other short-lived sessions. On the other hand, if we swung the other
way and set the bar low enough to allow streaming media to be
unresponsive, we would also allow denial of service attacks, which
are typically unresponsive to congestion and consist of multiple
continuous flows.</t>
<t>Applications that need (or choose) to be unresponsive to
congestion can effectively take (some would say steal) whatever
share of bottleneck resources they want from responsive flows.
Whether or not such free-riding is common, inability to prevent it
increases the risk of poor returns for investors in network
infrastructure, leading to under-investment. An increasing
proportion of unresponsive or free-riding demand coupled with
persistent under-supply is a broken economic cycle. Therefore, if
the current, largely co-operative consensus continues to erode,
congestion collapse could become more common in more areas of the
Internet <xref target="RFC3714" />.</t>
<t>While we have designed re-ECN so that networks can choose to
deploy stringent policing, this does not imply we advocate that
every network should introduce tight controls on those that cause
congestion. Re-ECN has been specifically designed to allow different
networks to choose how conservative or liberal they wish to be with
respect to policing congestion. But those that choose to be
conservative can protect themselves from the excesses that liberal
networks allow their users.</t>
</section>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<section anchor="retcp_Case_Against_Bottleneck_Policing"
title="The Case Against Bottleneck Policing">
<t>The state of the art in rate policing is the bottleneck policer,
which is intended to be deployed at any forwarding resource that may
become congested. Its aim is to detect flows that cause
significantly more local congestion than others. Although operators
might solve their immediate problems by deploying bottleneck
policers, we are concerned that widespread deployment would make it
extremely hard to evolve new application behaviours. We believe the
IETF should offer re-ECN as the preferred protocol on which to base
solutions to the policing problems of operators, because it would
not harm evolvability and, frankly, it would be far more effective
(see later for why).</t>
<t>Approaches like <xref target="XCHOKe" /> & <xref
target="pBox" /> are nice approaches for rate policing traffic
without the benefit of whole path information (such as could be
provided by re-ECN). But they must be deployed at bottlenecks in
order to work. Unfortunately, a large proportion of traffic
traverses at least two bottlenecks (in two access networks),
particularly with the current traffic mix where peer-to-peer
file-sharing is prevalent. If ECN were deployed, we believe it would
be likely that these bottleneck policers would be adapted to combine
ECN congestion marking from the upstream path with local congestion
knowledge. But then the only useful placement for such policers
would be close to the egress of the internetwork.</t>
<t>But then, if these bottleneck policers were widely deployed
(which would require them to be more effective than they are now),
the Internet would find itself with one universal rate adaptation
policy (probably TCP-friendliness) embedded throughout the network.
Given TCP's congestion control algorithm is already known to be
hitting its scalability limits and new algorithms are being
developed for high-speed congestion control, embedding TCP policing
into the Internet would make evolution to new algorithms extremely
painful. If a source wanted to use a different algorithm, it would
have to first discover then negotiate with all the policers on its
path, particularly those in the far access network. The IETF has
already traveled that path with the Intserv architecture and found
it constrains scalability <xref target="RFC2208" />.</t>
<t>Anyway, if bottleneck policers were ever widely deployed, they
would be likely to be bypassed by determined attackers. They
inherently have to police fairness per flow or per
source-destination pair. Therefore they can easily be circumvented
either by opening multiple flows (by varying the end-point port
number); or by spoofing the source address but arranging with the
receiver to hide the true return address at a higher layer.</t>
</section>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
</section>
<!-- ________________________________________________________________ -->
</section>
<!-- ================================================================ -->
<section anchor="retcp_Incentive_Framework"
title="Re-ECN Incentive Framework">
<t>The aim is to create an incentive environment that ensures optimal
sharing of capacity despite everyone acting selfishly (including lying
and cheating). Of course, the mechanisms put in place for this can lie
dormant wherever co-operation is the norm.</t>
<section anchor="retcp_revealing_congestion"
title="Revealing Congestion Along the Path">
<t>Throughout this document we focus on path congestion. But some
forms of fairness, particularly TCP's, also depend on round trip time.
If TCP-fairness is required, we also propose to measure downstream
path delay using re-feedback. We give a simple outline of how this
could work in <xref target="retcp_Re-TTL" />. However, we do not
expect this to be necessary, as researchers tend to agree that only
congestion control dynamics need to depend on RTT, not the rate that
the algorithm would converge on after a period of stability.</t>
<t>Recall that re-ECN can be used to measure path congestion at any
point on the path. End-systems know the whole path congestion. The
receiver knows this by the ratio of negative packets to all other
packets it observes. The sender knows this same information via the
feedback.</t>
<?rfc needLines="19" ?>
<figure anchor="retcp_Fig_Up_Down_Congestion_Imprecise"
title="A 2-Queue Example (Imprecise)">
<artwork><![CDATA[
+---+ +----+ +----+ +---+
| S |--| Q1 |----------------| Q2 |--| R |
+---+ +----+ +----+ +---+
. . . .
^ . . . .
| . . . .
| . positive fraction . .
3% |-------------------------------+=======
| . . | .
2% | . . | .
| . . negative fraction | .
1% | . +----------------------+ .
| . | . .
0% +--------------------------------------->
^ ^ ^
L M N Observation points
]]></artwork>
</figure>
<t><xref target="retcp_Fig_Up_Down_Congestion_Imprecise" /> uses a
simple network to illustrate how re-ECN allows queues to measure
downstream congestion. The receiver counts negative packets as being
3% of all received packets. This fraction is fed back to the sender.
The sender sets 3% of its packets to be positive to match this. This
fraction of positive packets can be observed along the path. This is
shown by the horizontal line at 3% in the figure. The negative
fraction is shown by the stepped line which rises to meet the positive
fraction line with steps at at each queue where packets are marked
negative. Two queues are shown (Q1 and Q2) that are currently
congested. Each time packets pass through a fraction are marked red;
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 negative fraction from the positive fraction, as shown in the
table below. <xref target="Re-TCP" /> [ref other document] derives
these approximations from a precise analysis).</t>
<?rfc needLines="9" ?>
<texttable anchor="retcp_Tab_Downstream_Congestion_Example"
title="Downstream Congestion Measured at Example Observation Points">
<ttcol align="center">Observation point</ttcol>
<ttcol align="center">Approx downstream congestion</ttcol>
<c>L</c>
<c>3% - 0% = 3%</c>
<c>M</c>
<c>3% - 1% = 2%</c>
<c>N</c>
<c>3% - 3% = 0%</c>
</texttable>
<t>All along the path, whole-path congestion 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 negative and
positive packets at any point in the Internet will reveal upstream,
downstream and whole path congestion.</t>
<t>Note: to be absolutely clear these fractions are averages that
would result from the behaviour of the protocol handler mechanically
sending positive packets in direct response to incoming feedback---we
are not saying any protocol handler has to work with these average
fractions directly.</t>
<!--<t>{ToDo: Consider whether this para is necessary.} Indeed, it would actually be incorrect for the protocol handlers to work with marking fractions, because TCP congestion control typically halves the packet rate every time there is congestion feedback. Too few packets would re-echo congestion if 3% of the halved packet rate was re-echoed in response to 3% of the earlier, higher packet rate being marked. The re-ECN algorithm for TCP specified by this document balances congestion markings and re-echoed markings octet for octet (which for a TCP with constant size packets also implies packet for packet).
</t> -->
<!--______________________________________________________________________-->
<section anchor="retcp_pos_neg_flows"
title="Positive and Negative Flows">
<t>In section <xref target="retcp_Introduction_protocol_in_brief" />
we introduced the notion of IP packets having different values
(negative, positive, cautious, cancelled and neutral). So positive
and cautious packets have a value of +1, negative -1, and cancelled
and neutral have zero value.</t>
<t>In the rest of this document we will loosely talk of positive or
negative flows. A negative flow is one where more negative bytes
than positive bytes arrive at the reciever. Likewise positive flows
are where more positive bytes arrive than negative bytes. Both of
these indicate that the wrong amount of positive bytes have been
sent.</t>
</section>
</section>
<section anchor="retcp_re_ecn_framework_detail"
title="Incentive Framework Overview">
<t><xref target="retcp_Fig_Incentive_Framework" /> sketches the
incentive framework that we will describe piece by piece throughout
this section. We will do a first pass in overview, then return to each
piece in detail. We re-use the earlier example of how downstream
congestion is derived by subtracting upstream congestion from path
congestion (<xref target="retcp_Fig_Up_Down_Congestion_Imprecise" />)
but depict multiple trust boundaries to turn it into an internetwork.
For clarity, only downstream congestion is shown (the difference
between the two earlier plots). The graph displays downstream path
congestion seen in a typical flow as it traverses an example path from
sender S to receiver R, across networks N1, N2 & N3. Everyone is
shown using re-ECN correctly, but we intend to show why everyone would
/choose/ to use it correctly, and honestly.</t>
<t>Three main types of self-interest can be identified: <list
style="symbols">
<t>Users want to transmit data across the network as fast as
possible, paying as little as possible for the privilege. In this
respect, there is no distinction between senders and receivers,
but we must be wary of potential malice by one on the other;</t>
<t>Network operators want to maximise revenues from the resources
they invest in. They compete amongst themselves for the custom of
users.</t>
<t>Attackers (whether users or networks) want to use any
opportunity to subvert the new re-ECN system for their own gain or
to damage the service of their victims, whether targeted or
random.</t>
</list></t>
<?rfc needLines="13" ?>
<figure anchor="retcp_Fig_Incentive_Framework"
title="Incentive Framework">
<artwork><![CDATA[
policer dropper
| |
| |
S <-----N1----> <---N2---> <---N3--> R domain
| |
| |
Border Gateways
]]></artwork>
</figure>
<t>
<list style="hanging">
<t hangText="Source congestion control:">We want to ensure that
the sender will throttle its rate as downstream congestion
increases. Whatever the agreed congestion response (whether
TCP-compatible or some enhanced QoS), to some extent it will
always be against the sender's interest to comply.</t>
<t hangText="Ingress policing:">But it is in all the network
operators' interests to encourage fair congestion response, so
that their investments are employed to satisfy the most valuable
demand. The re-ECN protocol ensures packets carry the necessary
information about their own expected downstream congestion so that
N1 can deploy a policer at its ingress to check that S1 is
complying with whatever congestion control it should be using
(<xref target="retcp_Rate_Policing" />). If N1 is extremely
conservative it could police each flow, but it is likely to just
police the bulk amount of congestion each customer causes without
regard to flows, or if it is extremely liberal it need not police
congestion control at all. Whatever, it is always preferable to
police traffic at the very first ingress into an internetwork,
before non-compliant traffic can cause any damage.</t>
<t hangText="Edge egress dropper:">If the policer ensures the
source has less right to a high rate the higher it declares
downstream congestion, the source has a clear incentive to
understate downstream congestion. But, if flows of packets are
understated when they enter the internetwork, they will have
become negative by the time they leave. So, we introduce a dropper
at the last network egress, which drops packets in flows that
persistently declare negative downstream congestion (see <xref
target="retcp_Dropper" /> for details).</t>
<t hangText="Inter-domain traffic policing:">But next we must ask,
if congestion arises downstream (say in N3), what is the ingress
network's (N1's) incentive to police its customers' response? If
N1 turns a blind eye, its own customers benefit while other
networks suffer. This is why all inter-domain QoS architectures
(e.g. Intserv, Diffserv) police traffic each time it crosses
a trust boundary. We have already shown that re-ECN gives a
trustworthy measure of the expected downstream congestion that a
flow will cause by subtracting negative volume from positive at
any intermediate point on a path. N3 (say) can use this measure to
police all the responses to congestion of all the sources beyond
its upstream neighbour (N2), but in bulk with one very simple
passive mechanism, rather than per flow, as we will now
explain.</t>
<t
hangText="Emulating policing with inter-domain congestion penalties:">Between
high-speed networks, we would rather avoid per-flow policing, and
we would rather avoid holding back traffic while it is policed.
Instead, once re-ECN has arranged headers to carry downstream
congestion honestly, N2 can contract to pay N3 penalties in
proportion to a single bulk count of the congestion metrics
crossing their mutual trust boundary (<xref
target="retcp_Inter-domain_Policing" />). In this way, N3 puts
pressure on N2 to suppress downstream congestion, for every flow
passing through the border interface, even though they will all
start and end in different places, and even though they may all be
allowed different responses to congestion. The figure depicts this
downward pressure on N2 by the solid downward arrow at the egress
of N2. Then N2 has an incentive either to police the congestion
response of its own ingress traffic (from N1) or to emulate
policing by applying penalties to N1 in turn on the basis of
congestion counted at their mutual boundary. In this recursive
way, the incentives for each flow to respond correctly to
congestion trace back with each flow precisely to each source,
despite the mechanism not recognising flows (see <xref
target="retcp_E2e_QoS" />).</t>
<!-- <t hangText="A digression: Ingress edge synthesis of end-to-end QoS:"> In fact, by deliberately allowing some customers a more lax response to congestion, an ingress network can synthesise differentiated service. If the ingress network charges a higher but /flat/ subscription to its customers for this privilege, it would recover the higher but unpredictable downstream charges (see <xref target="retcp_Per-user_Policing" />). The ingress effectively brokers the risk of downstream congestion charges. Charging to reduce the risk of having to respond to congestion is equivalent to offering enhanced quality of service (see <xref target="retcp_Rate_Policing" /> and the caveat in <xref target="retcp_E2e_QoS" />).
</t>
-->
<t hangText="Inter-domain congestion charging diversity:">Any two
networks are free to agree any of a range of penalty regimes
between themselves but they would only provide the right
incentives if they were within the following reasonable
constraints. N2 should expect to have to pay penalties to N3 where
penalties monotonically increase with the volume of congestion and
negative penalties are not allowed. For instance, they may agree
an SLA with tiered congestion thresholds, where higher penalties
apply the higher the threshold that is broken. But the most
obvious (and useful) form of penalty is where N3 levies a charge
on N2 proportional to the volume of downstream congestion N2 dumps
into N3. In the explanation that follows, we assume this specific
variant of volume charging between networks - charging
proportionate to the volume of congestion.</t>
<t>We must make clear that we are not advocating that everyone
should use this form of contract. We are well aware that the IETF
tries to avoid standardising technology that depends on a
particular business model. And we strongly share this desire to
encourage diversity. But our aim is merely to show that border
policing can at least work with this one model, then we can assume
that operators might experiment with the metric in other models
(see <xref target="retcp_Inter-domain_Policing" /> for examples).
Of course, operators are free to complement this usage element of
their charges with traditional capacity charging, and we expect
they will as predicted by economics.</t>
<t hangText="No congestion charging to users:">Bulk congestion
penalties at trust boundaries are passive and extremely simple,
and lose none of their per-packet precision from one boundary to
the next (unlike Diffserv all-address traffic conditioning
agreements, which dissipate their effectiveness across long
topologies). But at any trust boundary, there is no imperative to
use congestion charging. Traditional traffic policing can be used,
if the complexity and cost is preferred. In particular, at the
boundary with end customers (e.g. between S and N1), traffic
policing will most likely be more appropriate. Policer complexity
is less of a concern at the edge of the network. And end-customers
are known to be highly averse to the unpredictability of
congestion charging.</t>
<t hangText="NOTE WELL:">This document neither advocates nor
requires congestion charging for end customers and advocates but
does not require inter-domain congestion charging.</t>
<t
hangText="Competitive discipline of inter-domain traffic engineering:">With
inter-domain congestion charging, a domain seems to have a
perverse incentive to fake congestion; N2's profit depends on the
difference between congestion at its ingress (its revenue) and at
its egress (its cost). So, overstating internal congestion seems
to increase profit. However, smart border routing <xref
target="Smart_rtg" /> by N1 will bias its routing towards the
least cost routes. So, N2 risks losing all its revenue to
competitive routes if it overstates congestion (see <xref
target="retcp_Traffic_Engineering" />). In other words, if N2 is
the least congested route, its ability to raise excess profits is
limited by the congestion on the next least congested route.</t>
<t hangText="Closing the loop:">All the above elements conspire to
trap everyone between two opposing pressures, ensuring the
downstream congestion metric arrives at the destination neither
above nor below zero. So, we have arrived back where we started in
our argument. The ingress edge network can rely on downstream
congestion declared in the packet headers presented by the sender.
So it can police the sender's congestion response accordingly.</t>
<t hangText="Evolvability of congestion control:">We have seen
that re-ECN enables policing at the very first ingress. We have
also seen that, as flows continue on their path through further
networks downstream, re-ECN removes the need for further
per-domain ingress policing of all the different congestion
responses allowed to each different flow. This is why the
evolvability of re-ECN policing is so superior to bottleneck
policing or to any policing of different QoS for different flows.
Even if all access networks choose to conservatively police
congestion per flow, each will want to compete with the others to
allow new responses to congestion for new types of application.
With re-ECN, each can introduce new controls independently,
without coordinating with other networks and without having to
standardise anything. But, as we have just seen, by making
inter-domain penalties proportionate to bulk downtream congestion,
downstream networks can be agnostic to the specific congestion
response for each flow, but they can still apply more penalty the
more liberal the ingress access network has been in the response
to congestion it allowed for each flow.</t>
</list>
</t>
<!-- {ToDo: Leads to optimality. Proportional fairness between
networks, but they may choose the congestion response they make their
users keep to.} -->
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<t>We now take a second pass over the incentive framework, filling in
the detail.</t>
</section>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<section anchor="retcp_Dropper" title="Egress Dropper">
<t>As traffic leaves the last network before the receiver (domain N3
in <xref target="retcp_Fig_Incentive_Framework" />), the fraction of
positive octets in a flow should match the fraction of negative octets
introduced by congestion marking (red packets), leaving a balance of
zero. If it is less (a negative flow), it implies that the source is
understating path congestion (which will reduce the penalties that N2
owes N3).</t>
<t>If flows are positive, N3 need take no action---this simply means
its upstream neighbour is paying more penalties than it needs to, and
the source is going slower than it needs to. But, to protect itself
against persistently negative flows, N3 will need to install a dropper
at its egress. <xref target="retcp_Alg_Sanction_Negative" /> gives a
suggested algorithm for this dropper. There is no intention that the
dropper algorithm needs to be standardised, it is merely provided to
show that an efficient, robust algorithm is possible. But whatever
algorithm is used must meet the criteria below: <list style="symbols">
<t>It SHOULD introduce minimal false positives for honest
flows;</t>
<t>It SHOULD quickly detect and sanction dishonest flows (minimal
false negatives);</t>
<t>It SHOULD be invulnerable to state exhaustion attacks from
malicious sources. For instance, if the dropper uses flow-state,
it should not be possible for a source to send numerous packets,
each with a different flow ID, to force the dropper to exhaust its
memory capacity (rationale for SHOULD: Continuously sending
keep-alive packets might be perfectly reasonable behaviour, so we
can't distinguish a deliberate attack from reasonable levels of
such behaviour. Therefore it is strictly impossible to be
invulnerable to such an attack);</t>
<t>It MUST introduce sufficient loss in goodput so that malicious
sources cannot play off losses in the egress dropper against
higher allowed throughput. Salvatori <xref target="CLoop_pol" />
describes this attack, which involves the source understating path
congestion then inserting forward error correction (FEC) packets
to compensate expected losses;</t>
<t>It MUST NOT be vulnerable to `identity whitewashing', where a
transport can label a flow with a new ID more cheaply than paying
the cost of continuing to use its current ID.</t>
</list></t>
<t>Note that the dropper operates on flows but we would like it not to
require per-flow state. This is why we have been careful to ensure
that all flows MUST start with a cautious packet. If a flow does not
start with a cautious packet, a dropper is likely to treat it
unfavourably. This risk makes it worth sending a cautious packet at
the start of a flow, even though there is a cost to the sender of
doing so (positive `worth'). Indeed, with cautious packets, the rate
at which a sender can generate new flows can be limited (<xref
target="retcp_Policer_Implementations" />). In this respect, cautious
packets work like Handley's state set-up bit <xref
target="Steps_DoS" />.</t>
<t><xref target="retcp_Alg_Sanction_Negative" /> also gives an example
dropper implementation that aggregates flow state. Dropper algorithms
will often maintain a moving average across flows of the fraction of
positive packets. When maintaining an average across flows, a dropper
SHOULD only allow flows into the average if they start with a cautious
packet, but it SHOULD NOT include cautious packets in the positive
packet average. A sender sends cautious packets when it does not have
the benefit of feedback from the receiver. So, counting cautious
packets would be likely to make the average unnecessarily positive,
providing headroom (or should we say footroom?) for dishonest
(negative) traffic.</t>
<t>If the dropper detects a persistently negative flow, it SHOULD drop
sufficient negative and neutral packets to force the flow to not be
negative. Drops SHOULD be focused on just sufficient packets in
misbehaving flows to remove the negative bias while doing minimal
extra harm.</t>
</section>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<section anchor="retcp_Rate_Policing" title="Ingress Policing">
<t>Access operators who wish to limit the congeston that a sender is
able to cause can deploy policers at the very first ingress to the
internetwork. Re-ECN has been designed to avoid the need for
bottleneck policing so that we can avoid a future where a single rate
adaptation policy is embedded throughout the network. Instead, re-ECN
allows the particular rate adaptation policy to be solely agreed
bilaterally between the sender and its ingress access provider ([ref
other document] discusses possible ways to signal between them), which
allows congestion control to be policed, but maintains its
evolvability, requiring only a single, local box to be updated.</t>
<t><xref target="retcp_Policer_Implementations" /> gives examples of
per-user policing algorithms. But there is no implication that these
algorithms are to be standardised, or that they are ideal. The ingress
rate policer is the part of the re-ECN incentive framework that is
intended to be the most flexible. Once endpoint protocol handlers for
re-ECN and egress droppers are in place, operators can choose exactly
which congestion response they want to police, and whether they want
to do it per user, per flow or not at all.</t>
<t>The re-ECN protocol allows these ingress policers to easily perform
bulk per-user policing (<xref target="retcp_Per_User_Policing" />).
This is likely to provide sufficient incentive to the user to
correctly respond to congestion without needing the policing function
to be overly complex. If an access operator chose they could use
per-flow policing according to the widely adopted TCP rate adaptation
( <xref target="retcp_Per_Flow_Policing" />) or other alternatives,
however this would introduce extra complexity to the system.</t>
<t>If a per-flow rate policer is used, it should use path (not
downstream) congestion as the relevant metric, which is represented by
the fraction of octets in packets with positive (positive and cautious
packets) and cancelled packets. Of course, re-ECN provides all the
information a policer needs directly in the packets being policed. So,
even policing TCP's AIMD algorithm is relatively straightforward
(<xref target="retcp_Per_Flow_Policing" />).</t>
<t>Note that we have included cancelled packets in the measure of path
congestion. cancelled packets arise when the sender sends a positive
packet in response to feedback, but then this positive packet just
happens to be congestion marked itself. One would not normally expect
many cancelled packets at the first ingress because one would not
normally expect much congestion marking to have been necessary that
soon in the path. However, a home network or campus network may well
sit between the sending endpoint and the ingress policer, so some
congestion may occur upstream of the policer. And if congestion does
occur upstream, some cancelled packets should be visible, and should
be taken into account in the measure of path congestion.</t>
<t>But a much more important reason for including cancelled packets in
the measure of path congestion at an ingress policer is that a sender
might otherwise subvert the protocol by sending cancelled packets
instead of neutral packets. Like neutral, cancelled packets are worth
zero, so the sender knows they won't be counted against any quota it
might have been allowed. But unlike neutral packets, cancelled packets
are immune to congestion marking, because they have already been
congestion marked. So, it is both correct and useful that cancelled
packets should be included in a policer's measure of path congestion,
as this removes the incentive the sender would otherwise have to mark
more packets as cancelled than it should.</t>
<t>An ingress policer should also ensure that flows are not already
negative when they enter the access network. As with cancelled
packets, the presence of negative packets will typically be unusual.
Therefore it will be easy to detect negative flows at the ingress by
just detecting negative packets then monitoring the flow they belong
to.</t>
<t>Of course, even if the sender does operate its own network, it may
arrange not to congestion mark traffic. Whether the sender does this
or not is of no concern to anyone else except the sender. Such a
sender will not be policed against its own network's contribution to
congestion, but the only resulting problem would be overload in the
sender's own network.</t>
<t>Finally, we must not forget that an easy way to circumvent re-ECN's
defences is for the source to turn off re-ECN support, by setting the
Not-RECT codepoint, implying RFC3168 compliant traffic. Therefore an
ingress policer should put a general rate-limit on Not-RECT traffic,
which SHOULD be lax during early, patchy deployment, but will have to
become stricter as deployment widens. Similarly, flows starting
without a cautious packet can be confined by a strict rate-limit used
for the remainder of flows that haven't proved they are well-behaved
by starting correctly (therefore they need not consume any flow
state---they are just confined to the `misbehaving' bin if they carry
an unrecognised flow ID).</t>
<!-- {ToDo:
Weighted Policer} -->
</section>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<section anchor="retcp_Inter-domain_Policing"
title="Inter-domain Policing">
<t>One of the main design goals of re-ECN is for border security
mechanisms to be as simple as possible, otherwise they will become the
pinch-points that limit scalability of the whole internetwork. We want
to avoid per-flow processing at borders and to keep to passive
mechanisms that can monitor traffic in parallel to forwarding, rather
than having to filter traffic inline---in series with forwarding. Such
passive, off-line mechanisms are essential for future high-speed
all-optical border interconnection where packets cannot be buffered
while they are checked for policy compliance.</t>
<t>So far, we have been able to keep the border mechanisms simple,
despite having had to harden them against some subtle attacks on the
re-ECN design. The mechanisms are still passive and avoid per-flow
processing.</t>
<t>The basic accounting mechanism at each border interface simply
involves accumulating the volume of packets with positive worth
(positive and cautious packets), and subtracting the volume of those
with negative worth (red packets). Even though this mechanism takes no
regard of flows, over an accounting period (say a month) this
subtraction will account for the downstream congestion caused by all
the flows traversing the interface, wherever they come from, and
wherever they go to. The two networks can agree to use this metric
however they wish to determine some congestion-related penalty against
the upstream network. Although the algorithm could hardly be simpler,
it is spelled out using pseudo-code in <xref
target="retcp_Bulk_Alg_Metering" />.</t>
<!-- {ToDo:
Replace the xml from here to just before "Note that the guiding
principle..." with that in
draft-briscoe-tsvwg-re-ecn-border-cheat-02a_fragment.xml} -->
<t>Various attempts to subvert the re-ECN design have been made. In
all cases their root cause is persistently negative flows. But, after
describing these attacks we will show that we don't actually have to
get rid of all persistently negative flows in order to thwart the
attacks.</t>
<t>In honest flows, downstream congestion is measured as positive
minus negative volume. So if all flows are honest (i.e. not
persistently negative), adding all positive volume and all negative
volume without regard to flows will give an aggregate measure of
downstream congestion. But such simple aggregation is only possible if
no flows are persistently negative. Unless persistently negative flows
are completely removed, they will reduce the aggregate measure of
congestion. The aggregate may still be positive overall, but not as
positive as it would have been had the negative flows been
removed.</t>
<t>In <xref target="retcp_Dropper" /> we discussed how to sanction
traffic to remove, or at least to identify, persistently negative
flows. But, even if the sanction for negative traffic is to discard
it, unless it is discarded at the exact point it goes negative, it
will wrongly subtract from aggregate downstream congestion, at least
at any borders it crosses after it has gone negative but before it is
discarded.</t>
<t>We rely on sanctions to deter dishonest understatement of
congestion. But even the ultimate sanction of discard can only be
effective if the sender is bothered about the data getting through to
its destination. A number of attacks have been identified where a
sender gains from sending dummy traffic or it can attack someone or
something using dummy traffic even though it isn't communicating any
information to anyone: <list style="symbols">
<t>A host can send traffic with no positive packets towards its
intended destination, aiming to transmit as much traffic as any
dropper will allow <xref target="Bauer06" />. It may add
forward error correction (FEC) to repair as much drop as it
experiences.</t>
<t>A host can send dummy traffic into the network with no positive
packets and with no intention of communicating with anyone, but
merely to cause higher levels of congestion for others who do want
to communicate (DoS). So, to ride over the extra congestion,
everyone else has to spend more of whatever rights to cause
congestion they have been allowed.</t>
<t>A network can simply create its own dummy traffic to congest
another network, perhaps causing it to lose business at no cost to
the attacking network. This is a form of denial of service
perpetrated by one network on another. The preferential drop
measures in [ref other document] provide crude protection against
such attacks, but we are not overly worried about more accurate
prevention measures, because it is already possible for networks
to DoS other networks on the general Internet, but they generally
don't because of the grave consequences of being found out. We are
only concerned if re-ECN increases the motivation for such an
attack, as in the next example.</t>
<t>A network can just generate negative traffic and send it over
its border with a neighbour to reduce the overall penalties that
it should pay to that neighbour. It could even initialise the TTL
so it expired shortly after entering the neighbouring network,
reducing the chance of detection further downstream. This attack
need not be motivated by a desire to deny service and indeed need
not cause denial of service. A network's main motivator would most
likely be to reduce the penalties it pays to a neighbour. But, the
prospect of financial gain might tempt the network into mounting a
DoS attack on the other network as well, given the gain would
offset some of the risk of being detected.</t>
</list></t>
<!-- {ToDo: Consider adding Bauer's other (estoeric) attack:
Bauer et al argue there may even be cases where it is in a user's direct
self-interest to send dummy traffic if they are playing a strategic game
based on the assumption that a network operator will eventually upgrade
capacity if congestion persists. If a user knows she will have high
future demand, she may calculate it is worth sending a small amount of
dummy traffic to push up congestion for other users with current demand.
This may lead the network to upgrade earlier than it otherwise would,
resulting in her higher future demand not causing any congestion, so
that overall she spends less of her rights to cause congestion by
strategically sending dummy traffic. } -->
<t>The first step towards a solution to all these problems with
negative flows is to be able to estimate the contribution they make to
downstream congestion at a border and to correct the measure
accordingly. Although ideally we want to remove negative flows
themselves, perhaps surprisingly, the most effective first step is to
cancel out the polluting effect negative flows have on the measure of
downstream congestion at a border. It is more important to get an
unbiased estimate of their effect, than to try to remove them all. A
suggested algorithm to give an unbiased estimate of the contribution
from negative flows to the downstream congestion measure is given in
<xref target="retcp_Inflation_Negative_Flows" />.</t>
<t>Although making an accurate assessment of the contribution from
negative flows may not be easy, just the single step of neutralising
their polluting effect on congestion metrics removes all the gains
networks could otherwise make from mounting dummy traffic attacks on
each other. This puts all networks on the same side (only with respect
to negative flows of course), rather than being pitched against each
other. The network where this flow goes negative as well as all the
networks downstream lose out from not being reimbursed for any
congestion this flow causes. So they all have an interest in getting
rid of these negative flows. Networks forwarding a flow before it goes
negative aren't strictly on the same side, but they are disinterested
bystanders---they don't care that the flow goes negative downstream,
but at least they can't actively gain from making it go negative. The
problem becomes localised so that once a flow goes negative, all the
networks from where it happens and beyond downstream each have a small
problem, each can detect it has a problem and each can get rid of the
problem if it chooses to. But negative flows can no longer be used for
any new attacks.</t>
<t>Once an unbiased estimate of the effect of negative flows can be
made, the problem reduces to detecting and preferably removing flows
that have gone negative as soon as possible. But importantly, complete
eradication of negative flows is no longer critical---best endeavours
will be sufficient.</t>
<t>For instance, let us consider the case where a source sends traffic
with no positive packets at all, hoping to at least get as much
traffic delivered as network-based droppers will allow. The flow is
likely to go at least slightly negative in the first network on the
path (N1 if we use the example network layout in <xref
target="retcp_Fig_Incentive_Framework" />). If all networks use the
algorithm in <xref target="retcp_Inflation_Negative_Flows" /> to
inflate penalties at their border with an upstream network, they will
remove the effect of negative flows. So, for instance, N2 will not be
paying a penalty to N1 for this flow. Further, because the flow
contributes no positive packets at all, a dropper at the egress will
completely remove it.</t>
<t>The remaining problem is that every network is carrying a flow that
is causing congestion to others but not being held to account for the
congestion it is causing. Whenever the fail-safe border algorithm
(<xref target="retcp_Fail-safes" />) or the border algorithm to
compensate for negative flows (<xref
target="retcp_Inflation_Negative_Flows" />) detects a negative flow,
it can instantiate a focused dropper for that flow locally. It may be
some time before the flow is detected, but the more strongly negative
the flow is, the more quickly it will be detected by the fail-safe
algorithm. But, in the meantime, it will not be distorting border
incentives. Until it is detected, if it contributes to drop anywhere,
its packets will tend to be dropped before others if queues use the
preferential drop rules in [ref other document], which discriminate
against non-positive packets. All networks below the point where a
flow goes negative (N1, N2 and N3 in this case) have an incentive to
remove this flow, but the queue where it first goes negative (in N1)
can of course remove the problem for everyone downstream.</t>
<t>In the case of DDoS attacks, <xref
target="retcp_DDoS_Mitigation" /> describes how re-ECN mitigates their
force.</t>
</section>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<section anchor="retcp_Fail-safes" title="Inter-domain Fail-safes">
<t>The mechanisms described so far create incentives for rational
network operators to behave. That is, one operator aims to make
another behave responsibly by applying penalties and expects a
rational response (i.e. one that trades off costs against benefits).
It is usually reasonable to assume that other network operators will
behave rationally (policy routing can avoid those that might not). But
this approach does not protect against the misconfigurations and
accidents of other operators.</t>
<t>Therefore, we propose the following two mechanisms at a network's
borders to provide "defence in depth". Both are similar: <list
style="hanging">
<t hangText="Highly positive flows:">A small sample of positive
packets should be picked randomly as they cross a border
interface. Then subsequent packets matching the same source and
destination address and DSCP should be monitored. If the fraction
of positive packets is well above a threshold (to be determined by
operational practice), a management alarm SHOULD be raised, and
the flow MAY be automatically subject to focused drop.</t>
<t hangText="Persistently negative flows:">A small sample of
congestion marked (red) packets should be picked randomly as they
cross a border interface. Then subsequent packets matching the
same source and destination address and DSCP should be monitored.
If the balance of positive packets minus negative packets
(measured in bytes) is persistently negative, a management alarm
SHOULD be raised, and the flow MAY be automatically subject to
focused drop.</t>
</list></t>
<t>Both these mechanisms rely on the fact that highly positive (or
negative) flows will appear more quickly in the sample by selecting
randomly solely from positive (or negative) packets.</t>
</section>
<section anchor="retcp_Case_against_Classic_Feedback"
title="The Case against Classic Feedback">
<t>A system that produces an optimal outcome as a result of everyone's
selfish actions is extremely powerful. Especially one that enables
evolvability of congestion control. But why do we have to change to
re-ECN to achieve it? Can't classic congestion feedback (as used
already by standard ECN) be arranged to provide similar incentives and
similar evolvability? Superficially it can. Kelly's seminal work
showed how we can allow everyone the freedom to evolve whatever
congestion control behaviour is in their application's best interest
but still optimise the whole system of networks and users by placing a
price on congestion to ensure responsible use of this
freedom <xref target="Evol_cc" />). Kelly used ECN with its
classic congestion feedback model as the mechanism to convey
congestion price information. The mechanism could be thought of as
volume charging; except only the volume of packets marked with
congestion experienced (CE) was counted.</t>
<t>However, below we explain why relying on classic feedback
/required/ congestion charging to be used, while re-ECN achieves the
same powerful outcome (given it is built on Kelly's foundations), but
does not /require/ congestion charging. In brief, the problem with
classic feedback is that the incentives have to trace the indirect
path back to the sender---the long way round the feedback loop. For
example, if classic feedback were used in <xref
target="retcp_Fig_Incentive_Framework" />, N2 would have had to
influence N1 via all of N3, R & S rather than directly. <list
style="hanging">
<t hangText="Inability to agree what is happening downstream:">In
order to police its upstream neighbour's congestion response, the
neighbours should be able to agree on the congestion to be
responded to. Whatever the feedback regime, as packets change
hands at each trust boundary, any path metrics they carry are
verifiable by both neighbours. But, with a classic path metric,
they can only agree on the /upstream/ path congestion.</t>
<t hangText="Inaccessible back-channel:">The network needs a
whole-path congestion metric if it wants to control the source.
Classically, whole path congestion emerges at the destination, to
be fed back from receiver to sender in a back-channel. But, in any
data network, back-channels need not be visible to relays, as they
are essentially communications between the end-points. They may be
encrypted, asymmetrically routed or simply omitted, so no network
element can reliably intercept them. The congestion charging
literature solves this problem by charging the receiver and
assuming this will cause the receiver to refer the charges to the
sender. But, of course, this creates unintended
side-effects...</t>
<t hangText="`Receiver pays' unacceptable:">In connectionless
datagram networks, receivers and receiving networks cannot prevent
reception from malicious senders, so `receiver pays' opens them to
`denial of funds' attacks.</t>
<t
hangText="End-user congestion charging unacceptable in many societies:">Even
if 'denial of funds' were not a problem, we know that end-users
are highly averse to the unpredictability of congestion charging
and anyway, we want to avoid restricting network operators to just
one retail tariff. But with classic feedback only an upstream
metric is available, so we cannot avoid having to wrap the
`receiver pays' money flow around the feedback loop, necessarily
forcing end-users to be subjected to congestion charging.</t>
</list></t>
<t>To summarise so far, with classic feedback, policing congestion
response without losing evolvability /requires/ congestion charging of
end-users and a `receiver pays' model, whereas, with re-ECN, it is
still possible to influence incentives using congestion charging but
using the safer `sender pays' model. However, congestion charging is
only likely to be appropriate between domains. So, without losing
evolvability, re-ECN enables technical policing mechanisms that are
more appropriate for end users than congestion pricing.</t>
</section>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<section anchor="retcp_Simulations" title="Simulations">
<t>Simulations of policer and dropper performance done for the
multi-bit version of re-feedback have been included in section 5
"Dropper Performance" of <xref target="Re-fb" />. Simulations of
policer and dropper for the re-ECN version described in this document
are work in progress.</t>
</section>
</section>
<!-- =============================================================== -->
<section anchor="retcp_Other_Applications"
title="Other Applications of Re-ECN">
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<section anchor="retcp_DDoS_Mitigation" title="DDoS Mitigation">
<t>A flooding attack is inherently about congestion of a resource.
Because re-ECN ensures the sources causing network congestion
experience the cost of their own actions, it acts as a first line of
defence against DDoS. As load focuses on a victim, upstream queues
grow, requiring honest sources to pre-load packets with a higher
fraction of positive packets. Once downstream queues are so congested
that they are dropping traffic, they will be marking to negative the
traffic they do forward 100%. Honest sources will therefore be sending
positive packets 100% (and therefore being severely rate-limited at
the ingress).</t>
<t>Senders under malicious control can either do the same as honest
sources, and be rate-limited at ingress, or they can understate
congestion by sending more neutral RECT packets than they should. If
sources understate congestion (i.e. do not re-echo sufficient positive
packets) and the preferential drop ranking is implemented on queues
([ref othe document]), these queues will preserve positive traffic
until last. So, the neutral traffic from malicious sources will all be
automatically dropped first. Either way, the malicious sources cannot
send more than honest sources.</t>
<t>Further, hosts under malicious control will tend to be re-used for
many different attacks. They will therefore build up a long term
history of causing congestion. Therefore, as long as the population of
potentially compromisable hosts around the Internet is limited, the
per-user policing algorithms in <xref
target="retcp_Per_User_Policing" /> will gradually throttle down
zombies and other launchpads for attacks. Therefore, widespread
deployment of re-ECN could considerably dampen the force of DDoS.
Certainly, zombie armies could hold their fire for long enough to be
able to build up enough credit in the per-user policers to launch an
attack. But they would then still be limited to no more throughput
than other, honest users.</t>
<t>Inter-domain traffic policing (see <xref
target="retcp_Inter-domain_Policing" />)ensures that any network that
harbours compromised `zombie' hosts will have to bear the cost of the
congestion caused by traffic from zombies in downstream networks. Such
networks will be incentivised to deploy per-user policers that
rate-limit hosts that are unresponsive to congestion so they can only
send very slowly into congested paths. As well as protecting other
networks, the extremely poor performance at any sign of congestion
will incentivise the zombie's owner to clean it up. However, the host
should behave normally when using uncongested paths.</t>
<t>Uniquely, re-ECN handles DDoS traffic without relying on the
validity of identifiers in packets. Certainly the egress dropper
relies on uniqueness of flow identifiers, but not their validity. So
if a source spoofs another address, re-ECN works just as well, as long
as the attacker cannot imitate all the flow identifiers of another
active flow passing through the same dropper (see <xref
target="retcp_Limitations" />). Similarly, the ingress policer relies
on uniqueness of flow IDs, not their validity. Because a new flow will
only be allowed any rate at all if it starts with a cautious packet,
and the more cautious packets there are starting new flows, the more
they will be limited. Essentially a re-ECN policer limits the bulk of
all congestion entering the network through a physical interface;
limiting the congestion caused by each flow is merely an optional
extra.</t>
<!-- <t>Note, however, that delay in detecting attacks does leave re-feedback briefly vulnerable. -->
<!-- </t> -->
</section>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<section anchor="retcp_E2e_QoS" title="End-to-end QoS">
<t>{ToDo: (Section 3.3.2 of <xref target="Re-fb" /> entitled `Edge
QoS' gives an outline of the text that will be added here).}</t>
</section>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<section anchor="retcp_Traffic_Engineering" title="Traffic Engineering">
<t>Classic feedback makes congestion-based traffic engineering
inefficient too. Network N3 can see which of its two alternative
upstream networks N2 and N3 are less congested. But it is N1 that
makes the routing decision. This is why current traffic engineering
requires a continuous message stream from congestion monitors to the
routing controller. And even then the monitors can only be trusted for
/intra-/domain traffic engineering. The trustworthiness of re-ECN
enables /inter-/domain traffic engineering without messaging overhead.
{ToDo: Elaborate}</t>
</section>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<section anchor="retcp_Inter-Provider_Monitoring"
title="Inter-Provider Service Monitoring">
<t>{ToDo: }</t>
</section>
<!-- {ToDo: Routing Certain Flag Incentivising Slow-Start} -->
</section>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<section anchor="retcp_Limitations" title="Limitations">
<t>{ToDo:See also: slide of limitations}</t>
<t>The known limitations of the re-ECN approach are: <list
style="symbols">
<t>We still cannot defend against the attack described in <xref
target="retcp_Security_Considerations" /> where a malicious source
sends negative traffic through the same egress dropper as another
flow and imitates its flow identifiers, allowing a malicious source
to cause an innocent flow to experience heavy drop.</t>
<t>Re-feedback for TTL (re-TTL) would also be desirable at the same
time as re-ECN. Unfortunately this requires a further standards
action for the mechanisms briefly described in <xref
target="retcp_Re-TTL" /></t>
<t>Traffic must be ECN-capable for re-ECN to be effective. The only
defence against malicious users who turn off ECN capbility is that
networks are expected to rate limit Not-ECT traffic and to apply
higher drop preference to it during congestion. Although these are
blunt instruments, they at least represent a feasible scenario for
the future Internet where Not-ECT traffic co-exists with re-ECN
traffic, but as a severely hobbled under-class. We recommend (<xref
target="retcp_Deployment_Features" />) that while accommodating a
smooth initial transition to re-ECN, policing policies should
gradually be tightened to rate limit Not-ECT traffic more strictly
in the longer term.</t>
<t>When checking whether a flow is balancing positive packets with
negative packets (measured in bytes), re-ECN can only account for
congestion marking, not drops. So, whenever a sender experiences
drop, it does not have to re-echo the congestion event by sending
positive packet(s). Nonetheless, it is hardly any advantage to be
able to send faster than other flows only if your traffic is dropped
and the other traffic isn't.</t>
<t>We are considering the issue of whether it would be useful to
truncate rather than drop packets that appear to be malicious, so
that the feedback loop is not broken but useful data can be
removed.</t>
</list></t>
<t>{ToDo: Monopolies over Routes}</t>
</section>
<!-- ================================================================ -->
<section anchor="retcp_Incremental_Deployment"
title="Incremental Deployment">
<!-- ________________________________________________________________ -->
<section anchor="retcp_Deployment_Features"
title="Incremental Deployment Features">
<t>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 vanishly small during early deployment,
giving no incentive to start deployment. Also, as there is no new
forwarding behaviour, routers and hosts do not have to signal or
negotiate anything.</t>
<t>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 . 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.</t>
<t>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 amrked with
the unused codepoint as defined in <xref target="Re-TCP" />) 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.</t>
<t>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.</t>
</section>
<!-- ________________________________________________________________ -->
<section anchor="retcp_Deployment_Incentives"
title="Incremental Deployment Incentives">
<t>It would only be worth standardising the re-ECN protocol if there
existed a coherent story for how it might be incrementally deployed.
In order for it to have a chance of deployment, everyone who needs to
act must have a strong incentive to act, and the incentives must arise
in the order that deployment would have to happen. Re-ECN works around
unmodified ECN routers, but we can't just discuss why and how re-ECN
deployment might build on ECN deployment, because there is precious
little to build on in the first place. Instead, we aim to show that
re-ECN deployment could carry ECN with it. We focus on commercial
deployment incentives, although some of the arguments apply equally to
academic or government sectors. <list style="hanging">
<t hangText="ECN deployment:" />
<t>ECN is largely implemented in commercial routers, but generally
not as a supported feature, and it has largely not been deployed
by commercial network operators. ECN has been implemented in most
Unix-based operating systems for some time. Microsoft first
implemented ECN in Windows Vista, but it is only on by default for
the server end of a TCP connection. Unfortunately the client end
had to be turned off by default, because a non-zero ECN field
triggers a bug in a legacy home gateway which makes it crash. For
detailed deployment status, see <xref target="ECN-Deploy" />. We
believe the reason ECN deployment has not happened is twofold:
<list style="symbols">
<t>ECN requires changes to both routers and hosts. If someone
wanted to sell the improvement that ECN offers, they would
have to co-ordinate deployment of their product with others.
An ECN server only gives any improvement on an ECN network. An
ECN network only gives any improvement if used by ECN devices.
Deployment that requires co-ordination adds cost and delay and
tends to dilute any competitive advantage that might be
gained.</t>
<t>ECN `only' gives a performance improvement. Making a
product a bit faster (whether the product is a device or a
network), isn't usually a sufficient selling point to be worth
the cost of co-ordinating across the industry to deploy it.
Network operators tend to avoid re-configuring a working
network unless launching a new product.</t>
</list></t>
<t hangText="ECN and Re-ECN for Edge-to-edge Assured QoS:" />
<t>We believe the proposal to provide assured QoS sessions using a
form of ECN called pre-congestion notification (PCN) <xref
target="RFC5559" /> is most likely to break the deadlock in ECN
deployment first. It only requires edge-to-edge deployment so it
does not require endpoint support. It can be deployed in a single
network, then grow incrementally to interconnected networks. And
it provides a different `product' (internetworked assured QoS),
rather than merely making an existing product a bit faster.</t>
<t>Not only could this assured QoS application kick-start ECN
deployment, it could also carry re-ECN deployment with it; because
re-ECN can enable the assured QoS region to expand to a large
internetwork where neighbouring networks do not trust each other.
<xref target="Re-PCN" /> argues that re-ECN security should be
built in to the QoS system from the start, explaining why and
how.</t>
<t>If ECN and re-ECN were deployed edge-to-edge for assured QoS,
operators would gain valuable experience. They would also clear
away many technical obstacles such as firewall configurations that
block all but the RFC3168 settings of the ECN field and the RE
flag.</t>
<t hangText="ECN in Access Networks:" />
<t>The next obstacle to ECN deployment would be extension to
access and backhaul networks, where considerable link layer
differences makes implementation non-trivial, particularly on
congested wireless links. ECN and re-ECN work fine during partial
deployment, but they will not be very useful if the most congested
elements in networks are the last to support them. Access network
support is one of the weakest parts of this deployment story. All
we can hope is that, once the benefits of ECN are better
understood by operators, they will push for the necessary link
layer implementations as deployment proceeds.</t>
<t hangText="Policing Unresponsive Flows:" />
<t>Re-ECN allows a network to offer differentiated quality of
service as explained in <xref target="retcp_E2e_QoS" />. But we do
not believe this will motivate initial deployment of re-ECN,
because the industry is already set on alternative ways of doing
QoS. Despite being much more complicated and expensive, the
alternative approaches are here and now.</t>
<t>But re-ECN is critical to QoS deployment in another respect. It
can be used to prevent applications from taking whatever bandwidth
they choose without asking.</t>
<t>Currently, applications that remain resolute in their lack of
response to congestion are rewarded by other TCP applications. In
other words, TCP is naively friendly, in that it reduces its rate
in response to congestion whether it is competing with friends
(other TCPs) or with enemies (unresponsive applications).</t>
<t>Therefore, those network owners that want to sell QoS will be
keen to ensure that their users can't help themselves to QoS for
free. Given the very large revenues at stake, we believe effective
policing of congestion response will become highly sought after by
network owners.</t>
<t>But this does not necessarily argue for re-ECN deployment.
Network owners might choose to deploy bottleneck policers rather
than re-ECN-based policing. However, under Related Work (<xref
target="retcp_Related_Work" />) we argue that bottleneck policers
are inherently vulnerable to circumvention.</t>
<t>Therefore we believe there will be a strong demand from network
owners for re-ECN deployment so they can police flows that do not
ask to be unresponsive to congestion, in order to protect their
revenues from flows that do ask (QoS). In particular, we suspect
that the operators of cellular networks will want to prevent VoIP
and video applications being used freely on their networks as a
more open market develops in GPRS and 3G devices.</t>
<t>Initial deployments are likely to be isolated to single
cellular networks. Cellular operators would first place
requirements on device manufacturers to include re-ECN in the
standards for mobile devices. In parallel, they would put out
tenders for ingress and egress policers. Then, after a while they
would start to tighten rate limits on Not-ECT traffic from
non-standard devices and they would start policing whatever
non-accredited applications people might install on mobile devices
with re-ECN support in the operating system. This would force even
independent mobile device manufacturers to provide re-ECN support.
Early standardisation across the cellular operators is likely,
including interconnection agreements with penalties for excess
downstream congestion.</t>
<t>We suspect some fixed broadband networks (whether cable or DSL)
would follow a similar path. However, we also believe that larger
parts of the fixed Internet would not choose to police on a
per-flow basis. Some might choose to police congestion on a
per-user basis in order to manage heavy peer-to-peer file-sharing,
but it seems likely that a sizeable majority would not deploy any
form of policing.</t>
<t>This hybrid situation begs the question, "How does re-ECN work
for networks that choose to using policing if they connect with
others that don't?" Traffic from non-ECN capable sources will
arrive from other networks and cause congestion within the
policed, ECN-capable networks. So networks that chose to police
congestion would rate-limit Not-ECT traffic throughout their
network, particularly at their borders. They would probably also
set higher usage prices in their interconnection contracts for
incoming Not-ECT and Not-RECT traffic. We assume that
interconnection contracts between networks in the same tier will
include congestion penalties before contracts with provider
backbones do.</t>
<t>A hybrid situation could remain for all time. As was explained
in the introduction, we believe in healthy competition between
policing and not policing, with no imperative to convert the whole
world to the religion of policing. Networks that chose not to
deploy egress droppers would leave themselves open to being
congested by senders in other networks. But that would be their
choice.</t>
<t>The important aspect of the egress dropper though is that it
most protects the network that deploys it. If a network does not
deploy an egress dropper, sources sending into it from other
networks will be able to understate the congestion they are
causing. Whereas, if a network deploys an egress dropper, it can
know how much congestion other networks are dumping into it, and
apply penalties or charges accordingly. So, whether or not a
network polices its own sources at ingress, it is in its interests
to deploy an egress dropper.</t>
<t hangText="Host support:" />
<t>In the above deployment scenario, host operating system support
for re-ECN came about through the cellular operators demanding it
in device standards (i.e. 3GPP). Of course, increasingly, mobile
devices are being built to support multiple wireless technologies.
So, if re-ECN were stipulated for cellular devices, it would
automatically appear in those devices connected to the wireless
fringes of fixed networks if they coupled cellular with WiFi or
Bluetooth technology, for instance. Also, once implemented in the
operating system of one mobile device, it would tend to be found
in other devices using the same family of operating system.</t>
<t>Therefore, whether or not a fixed network deployed ECN, or
deployed re-ECN policers and droppers, many of its hosts might
well be using re-ECN over it. Indeed, they would be at an
advantage when communicating with hosts across re-ECN policed
networks that rate limited Not-RECT traffic.</t>
<t hangText="Other possible scenarios:" />
<t>The above is thankfully not the only plausible scenario we can
think of. One of the many clubs of operators that meet regularly
around the world might decide to act together to persuade a major
operating system manufacturer to implement re-ECN. And they may
agree between them on an interconnection model that includes
congestion penalties.</t>
<t>Re-ECN provides an interesting opportunity for device
manufacturers as well as network operators. Policers can be
configured loosely when first deployed. Then as re-ECN take-up
increases, they can be tightened up, so that a network with re-ECN
deployed can gradually squeeze down the service provided to
RFC3168 compliant devices that have not upgraded to re-ECN. Many
device vendors rely on replacement sales. And operating system
companies rely heavily on new release sales. Also support services
would like to be able to force stragglers to upgrade. So, the
ability to throttle service to RFC3168 compliant operating systems
is quite valuable.</t>
<t>Also, policing unresponsive sources may not be the only or even
the first application that drives deployment. It may be policing
causes of heavy congestion (e.g. peer-to-peer file-sharing). Or it
may be mitigation of denial of service. Or we may be wrong in
thinking simpler QoS will not be the initial motivation for re-ECN
deployment. Indeed, the combined pressure for all these may be the
motivator, but it seems optimistic to expect such a level of
joined-up thinking from today's communications industry. We
believe a single application alone must be a sufficient
motivator.</t>
<t>In short, everyone gains from adding accountability to TCP/IP,
except the selfish or malicious. So, deployment incentives tend to
be strong.</t>
</list></t>
</section>
</section>
<!-- ================================================================ -->
<section anchor="retcp_Architectural_Rationale"
title="Architectural Rationale">
<t>In the Internet's technical community, the danger of not responding
to congestion is well-understood, as well as its attendant risk of
congestion collapse <xref target="RFC3714" />. However, one side of
the Internet's commercial community considers that the very essence of
IP is to provide open access to the internetwork for all applications.
They see congestion as a symptom of over-conservative investment, and
rely on revising application designs to find novel ways to keep
applications working despite congestion. They argue that the Internet
was never intended to be solely for TCP-friendly applications.
Meanwhile, another side of the Internet's commercial community believes
that it is worthwhile providing a network for novel applications only if
it has sufficient capacity, which can happen only if a greater share of
application revenues can be /assured/ for the infrastructure provider.
Otherwise the major investments required would carry too much risk and
wouldn't happen.</t>
<t>The lesson articulated in <xref target="Tussle" /> is that we
shouldn't embed our view on these arguments into the Internet at design
time. Instead we should design the Internet so that the outcome of these
arguments can get decided at run-time. Re-ECN is designed in that
spirit. Once the protocol is available, different network operators can
choose how liberal they want to be in holding people accountable for the
congestion they cause. Some might boldly invest in capacity and not
police its use at all, hoping that novel applications will result.
Others might use re-ECN for fine-grained flow policing, expecting to
make money selling vertically integrated services. Yet others might sit
somewhere half-way, perhaps doing coarse, per-user policing. All might
change their minds later. But re-ECN always allows them to interconnect
so that the careful ones can protect themselves from the liberal
ones.</t>
<t>The incentive-based approach used for re-ECN is based on Gibbens and
Kelly's arguments <xref target="Evol_cc" /> on allowing endpoints
the freedom to evolve new congestion control algorithms for new
applications. They ensured responsible behaviour despite everyone's
self-interest by applying pricing to ECN marking, and Kelly had proved
stability and optimality in an earlier paper.</t>
<t>Re-ECN keeps all the underlying economic incentives, but rearranges
the feedback. The idea is to allow a network operator (if it chooses) to
deploy engineering mechanisms like policers at the front of the network
which can be designed to behave /as if/ they are responding to
congestion prices. Rather than having to subject users to congestion
pricing, networks can then use more traditional charging regimes (or
novel ones). But the engineering can constrain the overall amount of
congestion a user can cause. This provides a buffer against completely
outrageous congestion control, but still makes it easy for novel
applications to evolve if they need different congestion control to the
norms. It also allows novel charging regimes to evolve.</t>
<t>Despite being achieved with a relatively minor protocol change,
re-ECN is an architectural change. Previously, Internet congestion could
only be controlled by the data sender, because it was the only one both
in a position to control the load and in a position to see information
on congestion. Re-ECN levels the playing field. It recognises that the
network also has a role to play in moderating (policing) congestion
control. But policing is only truly effective at the first ingress into
an internetwork, whereas path congestion was previously only visible at
the last egress. So, re-ECN democratises congestion information. Then
the choice over who actually controls congestion can be made at
run-time, not design time---a bit like an aircraft with dual controls.
And different operators can make different choices. We believe
non-architectural approaches to this problem are unlikely to offer more
than partial solutions (see <xref target="retcp_Related_Work" />).</t>
<t>Importantly, re-ECN does not require assumptions about specific
congestion responses to be embedded in any network elements, except at
the first ingress to the internetwork if that level of control is
desired by the ingress operator. But such tight policing will be a
matter of agreement between the source and its access network operator.
The ingress operator need not police congestion response at flow
granularity; it can simply hold a source responsible for the aggregate
congestion it causes, perhaps keeping it within a monthly congestion
quota. Or if the ingress network trusts the source, it can do
nothing.</t>
<t>Therefore, the aim of the re-ECN protocol is NOT solely to police
TCP-friendliness. Re-ECN preserves IP as a generic network layer for all
sorts of responses to congestion, for all sorts of transports. Re-ECN
merely ensures truthful downstream congestion information is available
in the network layer for all sorts of accountability applications.</t>
<t>The end to end design principle does not say that all functions
should be moved out of the lower layers---only those functions that are
not generic to all higher layers. Re-ECN adds a function to the network
layer that is generic, but was omitted: accountability for causing
congestion. Accountability is not something that an end-user can provide
to themselves. We believe re-ECN adds no more than is sufficient to hold
each flow accountable, even if it consists of a single datagram.</t>
<t>"Accountability" implies being able to identify who is responsible
for causing congestion. However, at the network layer it would NOT be
useful to identify the cause of congestion by adding individual or
organisational identity information, NOR by using source IP addresses.
Rather than bringing identity information to the point of congestion, we
bring downstream congestion information to the point where the cause can
be most easily identified and dealt with. That is, at any trust boundary
congestion can be associated with the physically connected upstream
neighbour that is directly responsible for causing it (whether
intentionally or not). A trust boundary interface is exactly the place
to police or throttle in order to directly mitigate congestion, rather
than having to trace the (ir)responsible party in order to shut them
down.</t>
<t>Some considered that ECN itself was a layering violation. The
reasoning went that the interface to a layer should provide a service to
the higher layer and hide how the lower layer does it. However, ECN
reveals the state of the network layer and below to the transport layer.
A more positive way to describe ECN is that it is like the return value
of a function call to the network layer. It explicitly returns the
status of the request to deliver a packet, by returning a value
representing the current risk that a packet will not be served. Re-ECN
has similar semantics, except the transport layer must try to guess the
return value, then it can use the actual return value from the network
layer to modify the next guess.</t>
<t>The guiding principle behind all the discussion in <xref
target="retcp_Inter-domain_Policing" /> on Policing is that any gain
from subverting the protocol should be precisely neutralised, rather
than punished. If a gain is punished to a greater extent than is
sufficient to neutralise it, it will most likely open up a new
vulnerability, where the amplifying effect of the punishment mechanism
can be turned on others.</t>
<t>For instance, if possible, flows should be removed as soon as they go
negative, but we do NOT RECOMMEND any attempts to discard such flows
further upstream while they are still positive. Such over-zealous
push-back is unnecessary and potentially dangerous. These flows have
paid their `fare' up to the point they go negative, so there is no harm
in delivering them that far. If someone downstream asks for a flow to be
dropped as near to the source as possible, because they say it is going
to become negative later, an upstream node cannot test the truth of this
assertion. Rather than have to authenticate such messages, re-ECN has
been designed so that flows can be dropped solely based on locally
measurable evidence. A message hinting that a flow should be watched
closely to test for negativity is fine. But not a message that claims
that a positive flow will go negative later, so it should be dropped.
.</t>
</section>
<!-- ================================================================ -->
<section anchor="retcp_Related_Work" title="Related Work">
<t>{Due to lack of time, this section is incomplete. The reader is
referred to the Related Work section of <xref target="Re-fb" /> for a
brief selection of related ideas.}</t>
<!-- {ToDo: Ancestry Under perfect competition, charges will tend to cost,
so all prices will be equal. But under less than perfect competition,
/value pricing/ can be applied, particularly at the retail edges of the
network. In this sense, re-ECN can be loosely thought of as a realisation
of MacKie-Mason & Varian's `smart pricing' idea that was intended to
be a thought experiment rather than a practical
proposition~\cite{MacKieVar93:Pricing_the_Internet}.} -->
<!-- ________________________________________________________________ -->
<section anchor="retcp_Policing_Rate_Response_to_Congestion"
title="Policing Rate Response to Congestion">
<t>ATM network elements send congestion back-pressure
messages <xref target="ITU-T.I.371" /> along each connection,
duplicating any end to end feedback because they don't trust it. On
the other hand, re-ECN ensures information in forwarded packets can be
used for congestion management without requiring a connection-oriented
architecture and re-using the overhead of fields that are already set
aside for end to end congestion control (and routing loop detection in
the case of re-TTL in <xref target="retcp_Re-TTL" />).</t>
<t>We borrowed ideas from policers in the literature <xref
target="pBox" />,<xref target="XCHOKe" />, AFD etc. for our rate
equation policer. However, without the benefit of re-ECN they don't
police the correct rate for the condition of their path. They detect
unusually high /absolute/ rates, but only while the policer itself is
congested, because they work by detecting prevalent flows in the
discards from the local RED queue. These policers must sit at every
potential bottleneck, whereas our policer need only be located at each
ingress to the internetwork. As Floyd & Fall explain <xref
target="pBox" />, the limitation of their approach is that a high
sending rate might be perfectly legitimate, if the rest of the path is
uncongested or the round trip time is short. Commercially available
rate policers cap the rate of any one flow. Or they enforce monthly
volume caps in an attempt to control high volume file-sharing. They
limit the value a customer derives. They might also limit the
congestion customers can cause, but only as an accidental side-effect.
They actually punish traffic that fills troughs as much as traffic
that causes peaks in utilisation. In practice network operators need
to be able to allocate service by cost during congestion, and by value
at other times.</t>
</section>
<!-- ________________________________________________________________ -->
<section anchor="retcp_Congestion_Notification_Integrity"
title="Congestion Notification Integrity">
<t>The choice of two ECT code-points in the ECN field <xref
target="RFC3168" /> permitted future flexibility, optionally allowing
the sender to encode the experimental ECN nonce <xref
target="RFC3540" /> in the packet stream. This mechanism has since
been included in the specifications of DCCP <xref
target="RFC4340" />.</t>
<t>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.</t>
<t>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.</t>
<t>In contrast, the re-ECN protocol enables policing of an agreed
rate-response to congestion (e.g. TCP-friendliness) at the
sender's interface with the internetwork. It also ensures downstream
networks can police their upstream neighbours, to encourage them to
police their users in turn. But most importantly, it requires the
sender to declare path congestion to the network and it can remove
traffic at the egress if this declaration is dishonest. So it can
police correctly, irrespective of whether the receiver tries to
suppress congestion feedback or whether the sender ignores genuine
congestion feedback. Therefore the re-ECN protocol addresses a much
wider range of cheating problems, which includes the one addressed by
the ECN nonce.</t>
<!-- {ToDo: Ensure we address the early ACK
problem.} -->
</section>
<!-- ________________________________________________________________ -->
<section anchor="retcp_Identifying_Upstream_Downstream"
title="Identifying Upstream and Downstream Congestion">
<t>Purple <xref target="Purple" /> proposes that queues should
use the CWR flag in the TCP header of ECN-capable flows to work out
path congestion and therefore downstream congestion in a similar way
to re-ECN. However, because CWR is in the transport layer, it is not
always visible to network layer routers and policers. Purple's
motivation was to improve AQM, not policing. But, of course, nodes
trying to avoid a policer would not be expected to allow CWR to be
visible.</t>
<!-- {ToDo: Clark~\cite{Clark96:Combining_sndr+rcvr_payments}
proposed a decrementing field representing payment as a packet traversed a
path, with receiver-initiated messages able to meet it in the middle to
make up any shortfall. We argue that network layer fields should represent
verifiable properties of the path. Then operators can choose to apply
pricing to them to determine cost or value (or choose not
to).} -->
</section>
</section>
<!-- ================================================================ -->
<section anchor="retcp_Security_Considerations"
title="Security Considerations">
<t>{ToDo: enrich this section}{ToDo: Describe attacks by networks on
flows (and by spoofing sources).} {ToDo: Re-ECN & DNS servers}</t>
<t>Nearly the whole of this document concerns security.</t>
</section>
<!-- ================================================================ -->
<section anchor="retcp_IANA_Considerations" title="IANA Considerations">
<t>This memo includes no request to IANA.</t>
</section>
<!-- ================================================================ -->
<section anchor="retcp_Conclusions" title="Conclusions">
<t>{ToDo:}</t>
</section>
<!-- ================================================================ -->
<section anchor="retcp_Acknowledgements" title="Acknowledgements">
<t>Sébastien 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 cancelled packets), Adam Greenhalgh (who
developed the attack on DNS) (UCL), Jon Crowcroft (Uni Cam), David
Clark, Bill Lehr, Sharon Gillett, Steve Bauer (who complemented our own
dummy traffic attacks with others), Liz Maida (MIT), and comments from
participants in the CRN/CFP Broadband and DoS-resistant Internet working
groups.A special thank you to Alessandro Salvatori for coming up with
fiendish attacks on re-ECN.</t>
</section>
<!-- ================================================================ -->
<section anchor="retcp_Comments_Solicited" title="Comments Solicited">
<t>Comments and questions are encouraged and very welcome. They can be
addressed to the IETF Transport Area working group's mailing list
<tsvwg@ietf.org>, and/or to the authors.</t>
</section>
</middle>
<back>
<!-- ================================================================ -->
<references title="Normative References">
<?rfc include="reference.RFC.2119" ?>
<?rfc include="reference.RFC.3168" ?>
</references>
<references title="Informative References">
<?rfc include="localref.Briscoe05d.Re-fb_policing" ?>
<?rfc include="localref.Bauer06.Strat_cong_pric" ?>
<?rfc include="localref.Clark02.Tussle.xml" ?>
<!-- <?rfc include="reference.I-D.briscoe-re-pcn-border-cheat" ?>-->
<reference anchor="Re-PCN">
<front>
<title>Emulating Border Flow Policing using Re-PCN on Bulk
Data</title>
<author fullname="Bob Briscoe" initials="B" surname="Briscoe">
<organization></organization>
</author>
<date day="26" month="October" year="2009" />
<abstract>
<t>Scaling per flow admission control to the Internet is a hard
problem. The approach of combining Diffserv and pre-congestion
notification (PCN) provides a service slightly better than Intserv
controlled load that scales to networks of any size without
needing Diffserv's usual overprovisioning, but only if domains
trust each other to comply with admission control and rate
policing. This memo claims to solve this trust problem without
losing scalability. It provides a sufficient emulation of per-flow
policing at borders but with only passive bulk metering rather
than per-flow processing. Measurements are sufficient to apply
penalties against cheating neighbour networks.</t>
</abstract>
</front>
<seriesInfo name="Internet-Draft"
value="draft-briscoe-re-pcn-border-cheat-03" />
<format target="http://www.ietf.org/internet-drafts/draft-briscoe-re-pcn-border-cheat-03.txt"
type="TXT" />
</reference>
<!-- <?rfc include="localref.I-D.briscoe-tsvwg-re-ecn-tcp" ?>-->
<reference anchor="Re-TCP">
<front>
<title>Re-ECN: Adding Accountability for Causing Congestion to
TCP/IP</title>
<author fullname="Bob Briscoe" initials="B" surname="Briscoe">
<organization></organization>
</author>
<author fullname="Arnaud Jacquet" initials="A" surname="Jacquet">
<organization></organization>
</author>
<author fullname="T Moncaster" initials="T" surname="Moncaster">
<organization></organization>
</author>
<author fullname="Alan Smith" initials="A" surname="Smith">
<organization></organization>
</author>
<date day="22" month="October" year="2012" />
<abstract>
<t>This document introduces a new protocol for explicit congestion
notification (ECN), termed re-ECN, which can be deployed
incrementally around unmodified routers. 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. 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 correctly to congestion,and these are described more fully
in a companion document.</t>
</abstract>
</front>
<seriesInfo name="Internet-Draft"
value="draft-briscoe-conex-re-ecn-tcp-01" />
<format target="http://www.ietf.org/internet-drafts/draft-briscoe-conex-re-ecn-tcp-01.txt"
type="TXT" />
</reference>
<reference anchor="relax-fairness">
<front>
<title>Problem Statement: Transport Protocols Don't Have To Do
Fairness</title>
<author fullname="Bob Briscoe" initials="B" surname="Briscoe">
<organization></organization>
</author>
<author fullname="T Moncaster" initials="T" surname="Moncaster">
<organization></organization>
</author>
<author fullname="Lou Burness" initials="L" surname="Burness">
<organization></organization>
</author>
<date day="" month="July" year="2008" />
</front>
<seriesInfo name="Internet-Draft"
value="draft-briscoe-tsvwg-relax-fairness-01" />
<format target="http://www.ietf.org/internet-drafts/draft-briscoe-tsvwg-relax-fairness-01.txt"
type="TXT" />
</reference>
<?rfc include="localref.Floyd-ecn-deploy" ?>
<?rfc include="localref.Gibbens99.Evol_cc" ?>
<?rfc include="localref.Golden04.Smart_routing_multihome" ?>
<?rfc include="localref.Handley04.Steps_DoS_Arch" ?>
<?rfc include="localref.ITU-T.I.371_ATMTrafficMgmt" ?>
<?rfc include="localref.Pletka03.Purple" ?>
<?rfc include="localref.Floyd99.Penalty_box" ?>
<?rfc include="localref.Chhabra02.XCHOKe" ?>
<?rfc include="localref.Mathis97.TCP_Macroscopic" ?>
<?rfc include="localref.Jiang02.RTT_estimation" ?>
<?rfc include="localref.Salvatori05a.Re-fb_closed_loop_policing" ?>
<?rfc include="localref.Savage99.Mis_rcvr" ?>
<?rfc include="reference.RFC.2208" ?>
<?rfc include="reference.RFC.3514" ?>
<?rfc include="reference.RFC.3540" ?>
<?rfc include="reference.RFC.3714" ?>
<?rfc include="reference.RFC.4340" ?>
<?rfc include="reference.RFC.4341" ?>
<?rfc include="reference.RFC.4342" ?>
<?rfc include="reference.RFC.5559" ?>
</references>
<!-- ================================================================ -->
<section anchor="retcp_Alg_Sanction_Negative"
title="Example Egress Dropper Algorithm">
<t>{ToDo: Write up the basic algorithm with flow state, then the
aggregated one.}</t>
</section>
<!-- ================================================================ -->
<section anchor="retcp_Policer_Implementations"
title="Policer Designs to ensure Congestion Responsiveness">
<!-- ________________________________________________________________ -->
<section anchor="retcp_Per_User_Policing" title="Per-user Policing">
<t>User policing requires a policer on the ingress interface of the
access router associated with the user. At that point, the traffic of
the user hasn't diverged on different routes yet; nor has it mixed
with traffic from other sources.</t>
<t>In order to ensure that a user doesn't generate more congestion in
the network than her due share, a modified bulk token-bucket is
maintained with the following parameter: <list style="symbols">
<t>b_0 the initial token level</t>
<t>r the filling rate</t>
<t>b_max the bucket depth</t>
</list></t>
<t>The same token bucket algorithm is used as in many areas of
networking, but how it is used is very different: <list
style="symbols">
<t>all traffic from a user over the lifetime of their subscription
is policed in the same token bucket.</t>
<t>only positive and cancelled packets (positive, cautious and
cancelled) consume tokens</t>
</list></t>
<t>Such a policer will allow network operators to throttle the
contribution of their users to network congestion. This will require
the appropriate contractual terms to be in place between operators and
users. For instance: a condition for a user to subscribe to a given
network service may be that she should not cause more than a volume
C_user of congestion over a reference period T_user, although she may
carry forward up to N_user times her allowance at the end of each
period. These terms directly set the parameter of the user policer:
<list style="symbols">
<t>b_0 = C_user</t>
<t>r = C_user/T_user</t>
<t>b_max = b_0 * (N_user +1)</t>
</list></t>
<t>Besides the congestion budget policer above, another user policer
may be necessary to further rate-limit cautious packets, if they are
to be marked rather than dropped (see discussion in [ref other
document].). Rate-limiting cautious packets will prevent high bursts
of new flow arrivals, which is a very useful feature in DoS
prevention. A condition to subscribe to a given network service would
have to be that a user should not generate more than C_cautious
cautious packets, over a reference period T_cautious, with no option
to carry forward any of the allowance at the end of each period. These
terms directly set the parameters of the cautious packet policer:
<list style="symbols">
<t>b_0 = C_cautious</t>
<t>r = C_cautious/T_cautious</t>
<t>b_max = b_0</t>
</list></t>
<t>T_cautious should be a much shorter period than T_user: for
instance T_cautious could be in the order of minutes while T_user
could be in order of weeks.</t>
</section>
<!-- ________________________________________________________________ -->
<section anchor="retcp_Per_Flow_Policing"
title="Per-flow Rate Policing ">
<t>Whilst we believe that simple per-user policing would be sufficient
to ensure senders comply with congestion control, some operators may
wish to police the rate response of each flow to congestion as well.
Although we do not believe this will be neceesary, we include this
section to show how one could perform per-flow policing using
enforcement of TCP-fairness as an example. Per-flow policing aims to
enforce congestion responsiveness on the shortest information
timescale on a network path: packet roundtrips.</t>
<t>This again requires that the appropriate terms be agreed between a
network operator and its users, where a congestion responsiveness
policy might be required for the use of a given network service
(perhaps unless the user specifically requests otherwise).</t>
<t>As an example, we describe below how a rate adaptation policer can
be designed when the applicable rate adaptation policy is
TCP-compliance. In that context, the average throughput of a flow will
be expected to be bounded by the value of the TCP throughput during
congestion avoidance, given in Mathis' formula <xref
target="Mathis97"></xref> <list style="empty">
<t>x_TCP = k * s / ( T * sqrt(m) )</t>
</list> where: <list style="symbols">
<t>x_TCP is the throughput of the TCP flow in packets per
second,</t>
<t>k is a constant upper-bounded by sqrt(3/2),</t>
<t>s is the average packet size of the flow,</t>
<t>T is the roundtrip time of the flow,</t>
<t>m is the congestion level experienced by the flow.</t>
</list></t>
<t>We define the marking period N=1/m which represents the average
number of packets between two positive or cancelled packets. Mathis'
formula can be re-written as: <list style="empty">
<t>x_TCP = k*s*sqrt(N)/T</t>
</list></t>
<t>We can then get the average inter-mark time in a compliant TCP
flow, dt_TCP, by solving (x_TCP/s)*dt_TCP = N which gives <list
style="empty">
<t>dt_TCP = sqrt(N)*T/k</t>
</list></t>
<t>We rely on this equation for the design of a rate-adaptation
policer as a variation of a token bucket. In that case a policer has
to be set up for each policed flow. This may be triggered by cautious
packets, with the remainder of flows being all rate limited together
if they do not start with a cautious packet.</t>
<t>Where maintaining per flow state is not a problem, for instance on
some access routers, systematic per-flow policing may be considered.
Should per-flow state be more constrained, rate adaptation policing
could be limited to a random sample of flows exhibiting positive or
cancelled packets.</t>
<t>As in the case of user policing, only positive or cancelled packets
will consume tokens, however the amount of tokens consumed will depend
on the congestion signal.</t>
<t>When a new rate adaptation policer is set up for flow j, the
following state is created: <list style="symbols">
<t>a token bucket b_j of depth b_max starting at level b_0</t>
<t>a timestamp t_j = timenow()</t>
<t>a counter N_j = 0</t>
<t>a roundtrip estimate T_j</t>
<t>a filling rate r</t>
</list></t>
<t>When the policing node forwards a packet of flow j with no positive
packets: <list style="symbols">
<t>. the counter is incremented: N_j += 1</t>
</list></t>
<t>When the policing node forwards a packet of flow j carrying a
negative packet: <list style="symbols">
<t>the counter is incremented: N_j += 1</t>
<t>the token level is adjusted: b_j += r*(timenow()-t_j) -
sqrt(N_j)* T_j/k</t>
<t>the counter is reset: N_j = 0</t>
<t>the timer is reset: t_j = timenow()</t>
</list></t>
<t>An implementation example will be given in a later draft that
avoids having to extract the square root.</t>
<t>Analysis: For a TCP flow, for r= 1 token/sec, on average, <list
style="empty">
<t>r*(timenow()-t_j)-sqrt(N_j)* T_j/k = dt_TCP - sqrt(N)*T/k =
0</t>
</list></t>
<t>This means that the token level will fluctuate around its initial
level. The depth b_max of the bucket sets the timescale on which the
rate adaptation policy is performed while the filling rate r sets the
trade-off between responsiveness and robustness: <list style="symbols">
<t>the higher b_max, the longer it will take to catch greedy
flows</t>
<t>the higher r, the fewer false positives (greedy verdict on
compliant flows) but the more false negatives (compliant verdict
on greedy flows)</t>
</list></t>
<t>This rate adaptation policer requires the availability of a
roundtrip estimate which may be obtained for instance from the
application of re-feedback to the downstream delay <xref
target="retcp_Re-TTL"></xref> or passive estimation <xref
target="Jiang02"></xref>.</t>
<t>When the bucket of a policer located at the access router (whether
it is a per-user policer or a per-flow policer) becomes empty, the
access router SHOULD drop at least all packets causing the token level
to become negative. The network operator MAY take further sanctions if
the token level of the per-flow policers associated with a user
becomes negative.</t>
</section>
</section>
<!-- ================================================================ -->
<section anchor="retcp_Alg_Metering"
title="Downstream Congestion Metering Algorithms">
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<section anchor="retcp_Bulk_Alg_Metering"
title="Bulk Downstream Congestion Metering Algorithm">
<t>To meter the bulk amount of downstream congestion in traffic
crossing an inter-domain border an algorithm is needed that
accumulates the size of positive packets and subtracts the size of
negative packets. We maintain two counters: <list style="empty">
<t>V_b: accumulated congestion volume</t>
<t>B: total data volume (in case it is needed)</t>
</list></t>
<t>A suitable pseudo-code algorithm for a border router is as
follows:</t>
<figure>
<artwork><![CDATA[
====================================================================
V_b = 0
B = 0
for each Re-ECN-capable packet {
b = readLength(packet) /* set b to packet size */
B += b /* accumulate total volume */
if readEECN(packet) == (positive || cautious {
V_b += b /* increment... */
} elseif readEECN(packet) == negative {
V_b -= b /* ...or decrement V_b... */
} /*...depending on EECN field */
}
====================================================================
]]></artwork>
</figure>
<t>At the end of an accounting period this counter V_b represents the
congestion volume that penalties could be applied to, as described in
<xref target="retcp_Inter-domain_Policing"></xref>.</t>
<t>For instance, accumulated volume of congestion through a border
interface over a month might be V_b = 5PB (petabyte = 10^15 byte).
This might have resulted from an average downstream congestion level
of 1% on an accumulated total data volume of B = 500PB.</t>
<t>{ToDo: Include algorithm for precise downstream congestion.}</t>
</section>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<section anchor="retcp_Inflation_Negative_Flows"
title="Inflation Factor for Persistently Negative Flows">
<t>The following process is suggested to complement the simple
algorithm above in order to protect against the various attacks from
persistently negative flows described in <xref
target="retcp_Inter-domain_Policing"></xref>. As explained in that
section, the most important and first step is to estimate the
contribution of persistently negative flows to the bulk volume of
downstream pre-congestion and to inflate this bulk volume as if these
flows weren't there. The process below has been designed to give an
unbiased estimate, but it may be possible to define other processes
that achieve similar ends.</t>
<t>While the above simple metering algorithm is counting the bulk of
traffic over an accounting period, the meter should also select a
subset of the whole flow ID space that is small enough to be able to
realistically measure but large enough to give a realistic sample.
Many different samples of different subsets of the ID space should be
taken at different times during the accounting period, preferably
covering the whole ID space. During each sample, the meter should
count the volume of positive packets and subtract the volume of
negative, maintaining a separate account for each flow in the sample.
It should run a lot longer than the large majority of flows, to avoid
a bias from missing the starts and ends of flows, which tend to be
positive and negative respectively.</t>
<t>Once the accounting period finishes, the meter should calculate the
total of the accounts V_{bI} for the subset of flows I in the sample,
and the total of the accounts V_{fI} excluding flows with a negative
account from the subset I. Then the weighted mean of all these samples
should be taken a_S = sum_{forall I} V_{fI} / sum_{forall I}
V_{bI}.</t>
<t>If V_b is the result of the bulk accounting algorithm over the
accounting period (<xref target="retcp_Bulk_Alg_Metering"></xref>) it
can be inflated by this factor a_S to get a good unbiased estimate of
the volume of downstream congestion over the accounting period
a_S.V_b, without being polluted by the effect of persistently negative
flows.</t>
</section>
</section>
<!-- ================================================================ -->
<section anchor="retcp_Re-TTL" title="Re-TTL">
<t>This Appendix gives an overview of a proposal to be able to overload
the TTL field in the IP header to monitor downstream propagation delay.
This is included to show that it would be possible to take account of
RTT if it was deemed desirable.</t>
<t>Delay re-feedback can be achieved by overloading the TTL field,
without changing IP or router TTL processing. A target value for TTL at
the destination would need standardising, say 16. If the path hop count
increased by more than 16 during a routing change, it would temporarily
be mistaken for a routing loop, so this target would need to be chosen
to exceed typical hop count increases. The TCP wire protocol and
handlers would need modifying to feed back the destination TTL and
initialise it. It would be necessary to standardise the unit of TTL in
terms of real time (as was the original intent in the early days of the
Internet).</t>
<t>In the longer term, precision could be improved if routers
decremented TTL to represent exact propagation delay to the next router.
That is, for a router to decrement TTL by, say, 1.8 time units it would
alternate the decrement of every packet between 1 & 2 at a ratio of
1:4. Although this might sometimes require a seemingly dangerous null
decrement, a packet in a loop would still decrement to zero after 255
time units on average. As more routers were upgraded to this more
accurate TTL decrement, path delay estimates would become increasingly
accurate despite the presence of some RFC3168 compliant routers that
continued to always decrement the TTL by 1.</t>
</section>
<section anchor="retcp_Nonce_Limitation"
title="Argument for holding back the ECN nonce">
<t>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.</t>
<t>Space for the ECN nonce was set aside in <xref
target="RFC3168"></xref> (currently proposed standard) while the full
nonce mechanism is specified in <xref target="RFC3540"></xref>
(currently experimental). The specifications for <xref
target="RFC4340"></xref> (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: <list style="symbols">
<t>The CCID profile for TCP-like Congestion Control <xref
target="RFC4341"></xref> (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."</t>
<t>The CCID profile for TCP-Friendly Rate Control (TFRC) <xref
target="RFC4342"></xref> 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."</t>
</list></t>
<t>The primary function of the ECN nonce is to protect the integrity of
the information about congestion: ECN marks and packet drops. However,
when the nonce is used to protect the integrity of information about
packet drops, rather than ECN marks, a transport layer nonce will always
be sufficient (because a drop loses the transport header as well as the
ECN field in the network header), which would avoid using scarce IP
header codepoint space. Similarly, a transport layer nonce would protect
against a receiver sending early acknowledgements <xref
target="Savage99"></xref>.</t>
<t>If the ECN nonce reveals integrity problems with the information
about congestion, the sending transport can use that knowledge for two
functions: <list style="symbols">
<t>to protect its own resources, by allocating them in proportion to
the rates that each network path can sustain, based on congestion
control,</t>
<t>and to protect congested routers in the network, by slowing down
drastically its connection to the destination with corrupt
congestion information.</t>
</list></t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-22 17:45:41 |