Internet DRAFT - draft-briscoe-tsvwg-re-ecn-border-cheat
draft-briscoe-tsvwg-re-ecn-border-cheat
Transport Area Working Group B. Briscoe
Internet-Draft BT & UCL
Expires: December 28, 2006 June 26, 2006
Emulating Border Flow Policing using Re-ECN on Bulk Data
draft-briscoe-tsvwg-re-ecn-border-cheat-01
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Scaling per flow admission control to the Internet is a hard problem.
A recently proposed approach combines Diffserv and pre-congestion
notification (PCN) to provide a service slightly better than Intserv
controlled load. It scales to networks of any size, 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 describes bulk border policing that provides
a sufficient emulation of per-flow policing with the help of another
recently proposed extension to ECN, involving re-echoing ECN feedback
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(re-ECN). With only passive bulk measurements at borders, sanctions
can be applied against cheating networks.
Status (to be removed by the RFC Editor)
This memo is posted as an Internet-Draft with the intent to
eventually progress to informational status. It is envisaged that
the necessary standards actions to realise the system described would
sit in three other documents currently being discussed (but not on
the standards track) in the IETF Transport Area [Re-TCP], [RSVP-ECN]
& [PCN]. The authors seek comments from the Internet community on
whether combining PCN and re-ECN is a sufficient solution to the
admission control problem.
Changes from previous drafts (to be removed by the RFC Editor)
From -00 to -01:
Added subsection on Border Accounting Mechanisms (Section 5.6.1)
Section 4.2 on the re-ECN wire protocol clarified and re-organised
to separately discuss re-ECN for default ECN marking and for pre-
congestion marking (PCN).
Router Forwarding Behaviour subsection added to re-organised
section on Protocol Operation (Section 4.3). Extensions section
moved within Protocol Operations.
Emulating Border Policing (Section 5) reorganised, starting with a
new Terminology subsection heading, and a simplified overview
section. Added a large new subsection on Border Accounting
Mechanisms within a new section bringing together other
subsections on Border Mechanisms generally (Section 5.6). Some
text moved from old subsections into these new ones.
Added section on Incremental Deployment (Section 7), drawing
together relevant points about deployment made throughout.
Sections on Design Rationale (Section 8) and Security
Considerations (Section 9) expanded with some new material,
including new attacks and their defences.
Suggested Border Metering Algorithms improved (Appendix A.2) for
resilience to newly identified attacks.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 7
3. The Problem . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. The Traditional Per-flow Policing Problem . . . . . . . . 7
3.2. Generic Scenario . . . . . . . . . . . . . . . . . . . . . 9
4. Re-ECN Protocol for an RSVP (or similar) Transport . . . . . . 11
4.1. Protocol Overview . . . . . . . . . . . . . . . . . . . . 11
4.2. Re-ECN Abstracted Network Layer Wire Protocol (IPv4 or
v6) . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2.1. Re-ECN Recap . . . . . . . . . . . . . . . . . . . . . 13
4.2.2. Re-ECN Combined with Pre-Congestion Notification
(re-PCN) . . . . . . . . . . . . . . . . . . . . . . . 14
4.3. Protocol Operation . . . . . . . . . . . . . . . . . . . . 17
4.3.1. Protocol Operation for an Established Flow . . . . . . 17
4.3.2. Aggregate Bootstrap . . . . . . . . . . . . . . . . . 18
4.3.3. Flow Bootstrap . . . . . . . . . . . . . . . . . . . . 19
4.3.4. Router Forwarding Behaviour . . . . . . . . . . . . . 20
4.3.5. Extensions . . . . . . . . . . . . . . . . . . . . . . 22
5. Emulating Border Policing with Re-ECN . . . . . . . . . . . . 22
5.1. Informal Terminology . . . . . . . . . . . . . . . . . . . 22
5.2. Policing Overview . . . . . . . . . . . . . . . . . . . . 23
5.3. Pre-requisite Contractual Arrangements . . . . . . . . . . 25
5.4. Emulation of Per-Flow Rate Policing: Rationale and
Limits . . . . . . . . . . . . . . . . . . . . . . . . . . 28
5.5. Sanctioning Dishonest Marking . . . . . . . . . . . . . . 29
5.6. Border Mechanisms . . . . . . . . . . . . . . . . . . . . 31
5.6.1. Border Accounting Mechanisms . . . . . . . . . . . . . 31
5.6.2. Competitive Routing . . . . . . . . . . . . . . . . . 35
5.6.3. Fail-safes . . . . . . . . . . . . . . . . . . . . . . 35
6. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
7. Incremental Deployment . . . . . . . . . . . . . . . . . . . . 39
8. Design Choices and Rationale . . . . . . . . . . . . . . . . . 40
9. Security Considerations . . . . . . . . . . . . . . . . . . . 41
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 43
11. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 43
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 44
13. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 44
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 44
14.1. Normative References . . . . . . . . . . . . . . . . . . . 44
14.2. Informative References . . . . . . . . . . . . . . . . . . 45
Appendix A. Implementation . . . . . . . . . . . . . . . . . . . 46
A.1. Ingress Gateway Algorithm for Blanking the RE flag . . . . 47
A.2. Downstream Congestion Metering Algorithms . . . . . . . . 47
A.2.1. Bulk Downstream Congestion Metering Algorithm . . . . 47
A.2.2. Inflation Factor for Persistently Negative Flows . . . 48
A.3. Algorithm for Sanctioning Negative Traffic . . . . . . . . 49
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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 50
Intellectual Property and Copyright Statements . . . . . . . . . . 51
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1. Introduction
The Internet community largely lost interest in the Intserv
architecture after it was clarified that it would be unlikely to
scale to the whole Internet [RFC2208]. Although Intserv mechanisms
proved impractical, the bandwidth reservation service it aimed to
offer is still very much required.
A recently proposed approach [CL-deploy] combines Diffserv and pre-
congestion notification (PCN) to provide a service slightly better
than Intserv controlled load [RFC2211]. It scales to any size
network, but only if domains trust their neighbours to have checked
that upstream customers aren't taking more bandwidth than they
reserved, either accidentally or deliberately. This memo describes
border policing measures so that one network can protect its
interests, even if networks around it are deliberately trying to
cheat. The approach provides a sufficient emulation of flow rate
policing at trust boundaries but without per-flow processing. The
emulation is not perfect, but it is sufficient to ensure that the
punishment is at least proportionate to the severity of the cheat.
The aim is to be able to scale controlled load service to any number
of endpoints, even though such scaling must take account of the
increasing numbers of networks and users who may all have conflicting
interests. To achieve such scaling, this memo combines two recent
proposals, both of which it briefly recaps:
o A deployment model for admission control over Diffserv using pre-
congestion notification [CL-deploy] describes how bulk pre-
congestion notification on routers within an edge-to-edge Diffserv
region can emulate the precision of per-flow admission control to
provide controlled load service without unscalable per-flow
processing;
o Re-ECN: Adding Accountability to TCP/IP [Re-TCP]. The trick that
addresses cheating at borders is to recognise that border policing
is mainly necessary because cheating upstream networks will admit
traffic when they shouldn't only as long as they don't directly
experience the downstream congestion their misbehaviour can cause.
The re-ECN protocol requires upstream nodes to declare expected
downstream congestion in all forwarded packets and it makes it in
their interests to declare it honestly. Operators can then
monitor downstream congestion in bulk at borders to emulate
policing.
Rather than the end-to-end arrangement used when re-ECN was specified
for the TCP transport [Re-TCP], this memo specifies re-ECN in an
edge-to-edge arrangement, making it applicable to the above
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deployment model for admission control over Diffserv. Also, rather
than using a TCP transport for regular congestion feedback, this memo
specifies re-ECN using RSVP as the transport for feedback [RSVP-ECN].
A similar deployment model, but with a different transport for
signalling congestion feedback could be used (e.g. RMD [NSIS-RMD]
uses NSIS).
This memo aims to do two things: i) define how to apply the re-ECN
protocol to the admission control over Diffserv scenario; and ii)
explain why re-ECN sufficiently emulates border policing in that
scenario. Most of the memo is taken up with the second aim;
explaining why it works. Applying re-ECN to the scenario actually
involves quite a trivial modification to the ingress gateway. Our
immediate goal is to convince everyone to build that modification in
to ingress gateways from the start, whether first deployments require
policing or not. Otherwise, when we want to add policing, we will
have built ourselves a legacy problem. In other words, we aim to
convince people to "Build in security from the start."
The body of this memo is structured as follows:
Section 3 describes the border policing problem. We recap the
traditional, unscalable view of how to solve the problem, and we
recap the admission control solution which has the scalability we
do not want to lose when we add border policing;
Section 4 specifies the re-ECN protocol solution in detail;
Section 5 explains how to use the protocol to emulate border
policing, and why it works;
Section 6 analyses the security of the proposed solution;
Section 8 explains the sometimes subtle rationale behind our
design decisions;
Section 9 comments on the overall robustness of the security
assumptions and lists specific security issues.
It must be emphasised that we are not evangelical about removing per-
flow processing from borders. Network operators may choose to do
per-flow processing at their borders for their own reasons, such as
to support business models that require per-flow accounting. Our aim
is to show that per-flow processing at borders is no longer
/necessary/ in order to provide end-to-end QoS using flow admission
control. Indeed, we are absolutely opposed to standardisation of
technology that embeds particular business models into the Internet.
Our aim is merely to provide a new useful metric (downstream
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congestion) at trust boundaries. Given the well-known significance
of congestion in economics, operators can then use this new metric in
their interconnection contracts if they choose. This will enable
competitive evolution of new business models (for examples
see [IXQoS]), alongside more traditional models that depend on more
costly per-flow processing at borders.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. The Problem
3.1. The Traditional Per-flow Policing Problem
If we claim to be able to emulate per-flow policing with bulk
policing at trust boundaries, we need to know exactly what we are
emulating. So, even though we expect it to become a historic
practice, we will start from the traditional scenario with per-flow
policing at trust boundaries to explain why it has always been
considered necessary.
To be able to take advantage of a reservation-based service such as
controlled load, a source must reserve resources using a signalling
protocol such as RSVP [RFC2205]. An RSVP signalling request refers
to a flow of packets by its flow ID tuple (filter spec [RFC2205]) (or
its security parameter index (SPI) [RFC2207] if port numbers are
hidden by IPSec encryption). Other signalling protocols use similar
flow identifiers. But, it is insufficient to merely authorise and
admit a flow based on its identifiers, for instance merely opening a
pin-hole for packets with identifiers that match an admitted flow ID.
Once a flow is admitted, it cannot necessarily be trusted to send
packets within the rate profile it requested.
The packet rate must also be policed to keep the flow within the
requested flow spec [RFC2205]. For instance, without data rate
policing, a source could reserve resources for an 8kbps audio flow
but transmit a 6Mbps video (theft of service). More subtly, the
sender could generate bursts that were outside the profile it had
requested.
In traditional architectures, per-flow packet rate-policing is
expensive and unscalable but, without it, a network is vulnerable to
such theft of service (whether malicious or accidental). Perhaps
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more importantly, if flows are allowed to send more data than they
were permitted, the ability of admission control to give assurances
to other flows will break.
Just as sources need not be trusted to keep within their requested
flow spec, whole networks might also try to cheat. We will now set
up a concrete scenario to illustrate such cheats. Imagine
reservations for unidirectional flows from senders, through at least
two networks, an edge network and its downstream transit provider.
Imagine the edge network charges its retail customers per reservation
but also has to pay its transit provider a charge per reservation.
Typically, both its selling and buying charges might depend on the
duration and rate of each reservation. The level of the actual
selling and buying prices are irrelevant to our discussion (most
likely the network will sell at a higher price than it buys, of
course).
A cheating ingress network could systematically reduce the size of
its retail customers' reservation signalling requests before
forwarding them to its transit provider (and systematically reinstate
the responses on the way back). It would then receive an honest
income from its upstream retail customer but only pay for
fraudulently smaller reservations downstream. Equivalently, a
cheating ingress network may feed the traffic from a number of flows
into an aggregate reservation over the transit that is smaller than
the total of all the flows. Because of these fraud possibilities, in
traditional QoS reservation architectures the downstream network
polices at each border. The policer checks that the actual sent data
rate of each flow is within the signalled reservation.
Reservation signalling could be authenticated end to end, but this
wouldn't prevent the aggregation cheat just described. For this
reason, and to avoid the need for a global PKI, signalling integrity
is typically only protected on a hop-by-hop basis [RFC2747].
A variant of the above cheat is where a router in an honest
downstream network denies admission to a new reservation, but a
cheating upstream network still admits the flow. For instance, the
networks may be using Diffserv internally, but Intserv admission
control at their borders [RFC2998]. The cheat would only work if
they were using bulk Diffserv traffic policing at their borders,
perhaps to avoid the cost/complexity of Intserv border policing. As
far as the cheating upstream network is concerned, it gets the
revenue from the reservation, but it doesn't have to pay any
downstream wholesale charges and the congestion is in someone else's
network. The cheating network may calculate that most of the flows
affected by congestion in the downstream network aren't likely to be
its own. It may also calculate that the downstream router has been
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configured to deny admission to new flows in order to protect
bandwidth assigned to other network services (e.g. enterprise VPNs).
So the cheating network can steal capacity from the downstream
operator's VPNs that are probably not actually congested.
To summarise, in traditional reservation signalling architectures, if
a network cannot trust a neighbouring upstream network to rate-police
each reservation, it has to check for itself that the data rate fits
within each of the reservations it has admitted.
3.2. Generic Scenario
We will now describe a generic internetworking scenario that we will
use to describe and to test our bulk policing proposal. It consists
of a number of networks and endpoints that do not fully trust each
other to behave. In Section 6 we will tie down exactly what we mean
by partial trust, and we will consider the various combinations where
some networks do not trust each other and others are colluding
together.
_ ___ _____________________________________ ___ _
| | | | _|__ ______ ______ ______ _|__ | | | |
| | | | | | | | | | | | | | | | | |
| | | | | | |Inter-| |Inter-| |Inter-| | | | | | |
| | | | | | | ior | | ior | | ior | | | | | | |
| | | | | | |Domain| |Domain| |Domain| | | | | | |
| | | | | | | A | | B | | C | | | | | | |
| | | | | | | | | | | | | | | | | |
| | | | +----+ +-+ +-+ +-+ +-+ +-+ +-+ +----+ | | | |
| | | | | | |B| |B| |B| |B| |B| |B| | | | |\ | |
| |==| |==|Ingr|==|R| |R|==|R| |R|==|R| |R|==|Egr |==| |=>| |
| | | | |G/W | | | | | | | | | | | | | |G/W | | |/ | |
| | | | +----+ +-+ +-+ +-+ +-+ +-+ +-+ +----+ | | | |
| | | | | | | | | | | | | | | | | |
| | | | |____| |______| |______| |______| |____| | | | |
|_| |___| |_____________________________________| |___| |_|
Sx Ingress Diffserv region Egress Rx
End Access Access End
Host Network Network Host
<-------- edge-to-edge signalling ------->
(for admission control)
<-------------------end-to-end QoS signalling protocol------------->
Figure 1: Generic Scenario (see text for explanation of terms)
An ingress and egress gateway (Ingr G/W and Egr G/W in Figure 1)
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connect the interior Diffserv region to the edge access networks
where routers (not shown) use per-flow reservation processing.
Within the Diffserv region are three interior domains, A, B and C, as
well as the inward facing interfaces of the ingress and egress
gateways. An ingress and egress border router (BR) is shown
interconnecting each interior domain with the next. There may be
other interior routers (not shown) within each interior domain.
In two paragraphs we now briefly recap how pre-congestion
notification is intended to be used to control flow admission to a
large Diffserv region. The first paragraph describes data plane
functions and the second describes signalling in the control plane.
We omit many details from [CL-deploy] including behaviour during
routing changes. For brevity here we assume other flows are already
in progress across a path through the Diffserv region before a new
one arrives, but how bootstrap works is described in Section 4.3.2.
Figure 1 shows a single simplex reserved flow from the sending (Sx)
end host to the receiving (Rx) end host. The ingress gateway polices
incoming traffic within its admitted reservation and remarks it to
turn on an ECN-capable codepoint [RFC3168] and the controlled load
(CL) Diffserv codepoint. Together, these codepoints define which
traffic is entitled to the enhanced scheduling of the CL behaviour
aggregate on routers within the Diffserv region. The CL PHB of
interior routers consists of a scheduling behaviour and a new ECN
marking behaviour that we call `pre-congestion notification' [PCN].
The CL PHB simply re-uses the definition of expedited forwarding
(EF) [RFC3246] for its scheduling behaviour. But it incorporates a
new ECN marking behaviour, which sets the ECN field of an increasing
number of CL packets to the admission marked (AM) codepoint as they
approach a threshold rate that is lower than the line rate. The use
of virtual queues ensures real queues have hardly built up any
congestion delay. The level of marking detected at the egress of the
Diffserv region is then used by the signalling system in order to
determine admission control as follows.
The end-to-end QoS signalling (e.g. RSVP) for a new reservation
takes one giant hop from ingress to egress gateway, because interior
routers within the Diffserv region are configured to ignore RSVP.
The egress gateway holds flow state because it takes part in the end-
to-end reservation. So it can classify all packets by flow and it
can identify all flows that have the same previous RSVP hop (a CL-
region-aggregate). For each CL-region-aggregate of flows in
progress, the egress gateway maintains a per-packet moving average of
the fraction of pre-congestion-marked traffic. Once an RSVP PATH
message for a new reservation has hopped across the Diffserv region
and reached the destination, an RSVP RESV message is returned. As
the RESV message passes, the egress gateway piggy-backs the relevant
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pre-congestion level onto it [RSVP-ECN]. Again, interior routers
ignore the RSVP message, but the ingress gateway strips off the pre-
congestion level. If the pre-congestion level is above a threshold,
the ingress gateway denies admission to the new reservation,
otherwise it returns the original RESV signal back towards the data
sender.
Once a reservation is admitted, its traffic will always receive low
delay service for the duration of the reservation. This is because
ingress gateways ensure that traffic not under a reservation cannot
pass into the Diffserv region with the CL DSCP set. So non-reserved
traffic will always be treated with a lower priority PHB at each
interior router. And even if some disaster re-routes traffic after
it has been admitted, if the traffic through any resource tips over a
fail-safe threshold, pre-congestion notification will trigger flow-
pre-emption to very quickly bring every router within the whole
Diffserv region back below its operating point.
The whole admission control system just described deliberately
confines per-flow processing to the access edges of the network,
where it will not limit the system's scalability. But ideally we
want to extend this approach to multiple networks, to take even more
advantage of its scaling potential. We would still need per-flow
processing at the access edges of each network, but not at the high
speed interfaces where they interconnect. Even though such an
admission control system would work technically, it would gain us no
scaling advantage if each network also wanted to police the rate of
each admitted flow for itself---border routers would still have to do
complex packet operations per-flow anyway, given they don't trust
upstream networks to do their policing for them.
This memo describes how to emulate per-flow rate policing using bulk
mechanisms at border routers, so the full scalability potential of
pre-congestion notification is not limited by the need for per-flow
policing mechanisms at borders, which would make borders the most
cost-critical pinch-points. Then we can achieve the long sought-for
vision of secure Internet-wide bandwidth reservations without needing
per-flow processing at all in core and border routers---where
scalability is most critical.
4. Re-ECN Protocol for an RSVP (or similar) Transport
4.1. Protocol Overview
First we need to recap the way routers accumulate congestion marking
along a path. Each ECN-capable router marks some packets with CE,
the marking probability increasing with the length of the queue at
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its egress link. The only difference with pre-congestion
marking [PCN] is that marking is based on the length of a virtual
queue, so that the real queue occupancy can remain very low. We will
use the terms congestion and pre-congestion interchangeably in the
following unless it is important to distinguish between them.
With multiple ECN-capable routers on a path, the ECN field
accumulates the fraction of CE marking that each router adds. The
combined effect of the packet marking of all the routers along the
path signals congestion of the whole path to the receiver. So, for
example, if one router early in a path is marking 1% of packets and
another later in a path is marking 2%, flows that pass through both
routers will experience approximately 3% marking.
The packets crossing an inter-domain trust boundary within the
Diffserv region will all have come from different ingress gateways
and will all be destined for different egress gateways. We will show
that the key to policing against theft of service is for a border
router to be able to directly measure the congestion that is about to
be caused by the traffic it forwards. That is, it can measure
locally the congestion on each of the downstream paths between itself
and the egress gateways that its traffic is destined for.
With the original ECN protocol, if CE markings crossing the border
had been counted over a period, they would have represented the
accumulated upstream congestion that had already been experienced by
those packets. The general idea of re-ECN is for the ingress gateway
to continuously encode path congestion into the IP header where, in
this case, `path' means from ingress to egress gateway. Then at any
point on that path (e.g. between domains A & B in Figure 2 below), IP
headers can be monitored to subtract upstream congestion from
expected path congestion in order to give the expected downstream
congestion still to be experienced until the egress gateway.
Importantly, it turns out that there is no need to monitor downstream
congestion on a per-flow basis. We will show that accounting for it
in bulk across all flows will be sufficient.
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_____________________________________
_|__ ______ ______ ______ _|__
| | | A | | B | | C | | |
+----+ +-+ +-+ +-+ +-+ +-+ +-+ +----+
| | |B| |B| |B| |B| |B| |B| | |
|Ingr|==|R| |R|==|R| |R|==|R| |R|==|Egr |
|G/W | | | | |: | | | | | | | | |G/W |
+----+ +-+ +-+: +-+ +-+ +-+ +-+ +----+
| | | |: | | | | | |
|____| |______|: |______| |______| |____|
|_____________:_______________________|
:
| : |
|<-upstream-->:<-expected downstream->|
| congestion : congestion |
| u v ~= p - u |
| |
|<--- expected path congestion, p --->|
Figure 2: Re-ECN concept
4.2. Re-ECN Abstracted Network Layer Wire Protocol (IPv4 or v6)
In this section we define the names of the various codepoints of the
re-ECN protocol when used with pre-congestion notification, deferring
description of their semantics to the following sections. But first
we recap the re-ECN wire protocol proposed in [Re-TCP].
4.2.1. Re-ECN Recap
Re-ECN uses the two bit ECN field broadly as in RFC3168 [RFC3168].
It also uses a new re-ECN extension (RE) flag. The actual position
of the RE flag is different between IPv4 & v6 headers so we will use
an abstraction of the IPv4 and v6 wire protocols by just calling it
the RE flag. [Re-TCP] proposes using bit 48 (currently unused) in
the IPv4 header for the RE flag, while for IPv6 it proposes an ECN
extension header.
Unlike the ECN field, the RE flag is intended to be set by the sender
and remain unchanged along the path, although it can be read by
network elements that understand the re-ECN protocol. In the
scenario used in this memo, the ingress gateway acts as a proxy for
the sender, setting the RE flag as permitted in the specification of
re-ECN.
Note that general-purpose routers do not have to read the RE flag,
only special policing elements at borders do. And no general-purpose
routers have to change the RE flag, although the ingress and egress
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gateways do because in the edge-to-edge deployment model we are
using, they act as proxies for the endpoints. Therefore the RE flag
does not even have to be visible to interior routers. So the RE flag
has no implications on protocols like MPLS. Congested label
switching routers (LSRs) would have to be able to notify their
congestion with an ECN/PCN codepoint in the MPLS shim [ECN-MPLS], but
like any interior IP router, they can be oblivious to the RE flag,
which need only be read by border policing functions.
Although the RE flag is a separate, single bit field, it can be read
as an extension to the two-bit ECN field; the three concatenated bits
in what we will call the extended ECN field (EECN) make eight
codepoints available. When the RE flag setting is "don't care", we
use the RFC3168 names of the ECN codepoints, but [Re-TCP] proposes
the following six codepoint names for when there is a need to be more
specific.
+-------+------------+------+---------------+-----------------------+
| ECN | RFC3168 | RE | Extended ECN | Re-ECN meaning |
| field | codepoint | flag | codepoint | |
+-------+------------+------+---------------+-----------------------+
| 00 | Not-ECT | 0 | Not-RECT | Not re-ECN-capable |
| | | | | transport |
| 00 | Not-ECT | 1 | FNE | Feedback not |
| | | | | established |
| 01 | ECT(1) | 0 | Re-Echo | Re-echoed congestion |
| | | | | and RECT |
| 01 | ECT(1) | 1 | RECT | Re-ECN capable |
| | | | | transport |
| 10 | ECT(0) | 0 | --- | Legacy ECN use |
| | | | | only |
| 10 | ECT(0) | 1 | --CU-- | Currently unused |
| | | | | |
| 11 | CE | 0 | CE(0) | Congestion |
| | | | | experienced with |
| | | | | Re-Echo |
| 11 | CE | 1 | CE(-1) | Congestion |
| | | | | experienced |
+-------+------------+------+---------------+-----------------------+
Table 1: Re-cap of Default Extended ECN Codepoints Proposed for Re-
ECN
4.2.2. Re-ECN Combined with Pre-Congestion Notification (re-PCN)
As permitted by the ECN specification [RFC3168], a proposal is
currently being advanced in the IETF to define different semantics
for how routers might mark the ECN field of certain packets. The
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idea is to be able to notify congestion when the router's load
approaches a logical limit, rather than the physical limit of the
line. This new marking is called pre-congestion notification [PCN]
and we will use the term PCN-enabled router for a router that can
apply pre-congestion notification marking to the ECN fields of
packets.
[RFC3168] recommends that a packet's Diffserv codepoint should
determine which type of ECN marking it receives. A Diffserv per-hop
behaviour (PHB) can specify that routers should apply pre-congestion
notification marking to PCN-capable packets. We will call this a
PCN-enhanced PHB. A PCN-capable packet must meet two conditions, it
must carry a DSCP that maps to a PCN-enhanced PHB and it must carry
an ECN field that turns on PCN marking.
As an example, the controlled load (CL) PHB might specify expedited
forwarding as its scheduling behaviour and PCN marking as its
congestion marking behaviour. Then we would say the CL PHB is a PCN-
enhanced PHB, and that packets with a DSCP that maps to the CL PHB
and with ECN turned on are PCN-capable packets.
[PCN] actually proposes that two logical limits should be used for
pre-congestion notification, with the higher limit as a back-stop for
dealing with anomalous events. It envisages PCN will be used to
admission control inelastic real-time traffic, so marking at the
lower limit will trigger admission control, while at the higher limit
it will trigger flow pre-emption.
Because it needs two types of congestion marking, PCN seems to need
five states: Not-ECT, ECT (ECN-capable transport), the ECN Nonce,
Admission Marking (AM) and Flow Pre-emption Marking (PM). [PCN]
proposes various alternative encodings of the ECN field, attempting
various compromises to fit these five states into the four available
ECN codepoints.
One of the five states to make room for is the ECN Nonce [RFC3540],
but the capability we describe in this memo supersedes any need for
the Nonce. The ECN Nonce is an elegant scheme, but it only allows a
sending node (or its proxy) to detect suppression of congestion
marking in the feedback loop. Thus the Nonce requires the sender or
its proxy to be trusted to respond correctly to congestion. But this
is precisely the main cheat we want to protect against (as well as
many others).
One of the compromise protocol encodings that [PCN] explores
("Alternative 5") leaves out support for the ECN Nonce. Therefore we
use that one. This encoding of PCN markings is shown on the left of
Table 2. Note that these codepoints of the ECN field only take on
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the semantics of pre-congestion noticiation if they are combined with
a Diffserv codepoint that the operator has configured to cause PCN
marking, by mapping it to a PCN-enhanced PHB.
For the rest of this memo, we will not distinguish between Admission
Marking and Pre-emption Marking unless we need to be specific. We
will call both "congestion marking". With the above encoding,
congestion marking can be read to mean any packet with the left-most
bit of the ECN field set.
The re-ECN protocol can be used to control misbehaving sources
whether congestion is with respect to a logical threshold (PCN) or
the physical line rate (ECN). In either case the RE flag can be used
to create an extended ECN field. For PCN-capable packets, the 8
possible encodings of this 3-bit extended ECN (EECN) field are
defined on the right of Table 2 below. The purposes of these
different codepoints will be introduced in subsequent sections.
+-------+-----------------+------+-------------+--------------------+
| ECN | PCN codepoint | RE | Extended | Re-ECN meaning |
| field | (Alternative 5) | flag | ECN | |
| | | | codepoint | |
+-------+-----------------+------+-------------+--------------------+
| 00 | Not-ECT | 0 | Not-RECT | Not re-ECN-capable |
| | | | | transport |
| 00 | Not-ECT | 1 | FNE | Feedback not |
| | | | | established |
| 01 | ECT(1) | 0 | Re-Echo | Re-echoed |
| | | | | congestion and |
| | | | | RECT |
| 01 | ECT(1) | 1 | RECT | Re-ECN capable |
| | | | | transport |
| 10 | AM | 0 | AM(0) | Admission Marking |
| | | | | with Re-Echo |
| 10 | AM | 1 | AM(-1) | Admission Marking |
| | | | | |
| 11 | PM | 0 | PM(0) | Pre-emption |
| | | | | Marking with |
| | | | | Re-Echo |
| 11 | PM | 1 | PM(-1) | Pre-emption |
| | | | | Marking |
+-------+-----------------+------+-------------+--------------------+
Table 2: Extended ECN Codepoints if the Diffserv codepoint uses Pre-
congestion Notification (PCN)
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4.3. Protocol Operation
4.3.1. Protocol Operation for an Established Flow
The re-ECN protocol involves a simple tweak to the action of the
gateway at the ingress edge of the CL region. In the deployment
model just described [CL-deploy], for each active traffic aggregate
across the CL region (CL-region-aggregate) the ingress gateway will
hold a fairly recent Congestion-Level-Estimate that the egress
gateway will have fed back to it, piggybacked on the signalling that
sets up each flow. For instance, one aggregate might have been
experiencing 3% pre-congestion (that is, congestion marked octets
whether Admission Marked or Pre-emption Marked). In this case, the
ingress gateway MUST clear the RE flag to "0" for the same percentage
of octets of CL-packets (3%) and set it to "1" in the rest (97%).
Appendix A.1 gives a simple pseudo-code algorithm that the ingress
gateway may use to do this.
The RE flag is set and cleared this way round for incremental
deployment reasons (see [Re-TCP]). To avoid confusion we will use
the term `blanking' (rather than marking) when the RE flag is cleared
to "0", so we will talk of the `RE blanking fraction' as the fraction
of octets with the RE flag cleared to "0".
^
|
| RE blanking fraction
3% | +----------------------------+====+
| | | |
2% | | | |
| | congestion marking fraction| |
1% | | +----------------------+ |
| | | |
0% +----+=====+---------------------------+------>
^ <--A---> <---B---> <---C---> ^ domain
| ^ ^ |
ingress | | egress
1.00% 2.00% marking fraction
Figure 3: Example Extended ECN codepoint Marking fractions
(Imprecise)
Figure 3 illustrates our example. The horizontal axis represents the
index of each congestible resource (typically queues) along a path
through the Internet. The two superimposed plots show the fraction
of each ECN codepoint observed along this path, assuming there are
two congested routers somewhere within domains A and C. And Table 3
below shows the downstream pre-congestion measured at various border
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observation points along the path. Figure 4 (later) shows the same
results of these subtractions, but in graphical form like the above
figure. The tabulated figures are actually reasonable approximations
derived from more precise formulae given in Appendix A of [Re-TCP].
The RE flag is not changed by interior routers, so it can be seen
that it acts as a reference against which the congestion marking
fraction can be compared along the path.
+--------------------------+---------------------------------------+
| Border observation point | Approximate Downstream pre-congestion |
+--------------------------+---------------------------------------+
| ingress -- A | 3% - 0% = 3% |
| A -- B | 3% - 1% = 2% |
| B -- C | 3% - 1% = 2% |
| C -- egress | 3% - 3% = 0% |
+--------------------------+---------------------------------------+
Table 3: Downstream Congestion Measured at Example Observation Points
Note that the ingress determines the RE blanking fraction for each
aggregate using the most recent feedback from the relevant egress,
arriving with each new reservation, or each refresh. These updates
arrive relatively infrequently compared to the speed with which
congestion changes. Although this feedback will always be out of
date, on average positive errors should cancel out negative over a
sufficiently long duration.
In summary, the network adds pre-congestion marking in the forward
data path, the egress feeds its level back to the ingress in RSVP (or
similar signalling), then the ingress gateway re-echoes it into the
forward data path by blanking the RE flag. Hence the name re-ECN.
Then at any border within the Diffserv region, the pre-congestion
marking that every passing packet will be expected to experience
downstream can be measured to be the RE blanking fraction minus the
congestion marking fraction.
4.3.2. Aggregate Bootstrap
When a new reservation PATH message arrives at the egress, if there
are currently no flows in progress from the same ingress, there will
be no state maintaining the current level of pre-congestion marking
for the aggregate. While the reservation signalling continues onward
towards the receiving host, the egress gateway returns an RSVP
message to the ingress with a flag [RSVP-ECN] asking the ingress to
send a specified number of data probes between them. This bootstrap
behaviour is all described in the deployment model [CL-deploy].
However, with our new re-ECN scheme, the ingress does not know what
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proportion of the data probes should have the RE flag blanked,
because it has no estimate yet of pre-congestion for the path across
the Diffserv region.
To be conservative, following the guidance for specifying other re-
ECN transports in [Re-TCP], the ingress SHOULD set the FNE codepoint
of the extended ECN header in all probe packets (Table 2). As per
the deployment model, the egress gateway measures the fraction of
congestion-marked probe octets and feeds back the resulting pre-
congestion level to the ingress, piggy-backed on the returning
reservation response (RESV) for the new flow. Probe packets are
identifiable by the egress because they have the ingress as the
source and the egress as the destination in the IP header.
It may seem inadvisable to expect the FNE codepoint to be set on
probes, given legacy firewalls etc. might discard such packets
(because this flag had no previous legitimate use). However, in the
deployment scenarios envisaged, each domain in the Diffserv region
has to be explicitly configured to support the controlled load
service. So, before deploying the service, the operator MUST
reconfigure such a misbehaving middlebox to allow through packets
with the RE flag set.
Note that we have said SHOULD rather than MUST for the FNE setting
behaviour of the ingress for probe packets. This entertains the
possibility of an ingress implementation having the benefit of other
knowledge of the path, which it re-uses for a newly starting
aggregate. For instance, it may hold cached information from a
recent use of the aggregate that is still sufficiently current to be
useful.
It might seem pedantic worrying about these few probe packets, but
this behaviour ensures the system is safe, even if the proportion of
probe packets becomes large.
4.3.3. Flow Bootstrap
It might be expected that a new flow within an active aggregate would
need no special bootstrap behaviour. If there was an aggregate
already in progress between the gateways the new flow was about to
use, it would inherit the prevailing RE blanking fraction. And if
there were no active aggregate, the bootstrap behaviour for an
aggregate would be appropriate and sufficient for the new flow.
However, for a number of reasons, at least the first packet of each
new flow SHOULD be set to the FNE codepoint, irrespective of whether
it is joining an active aggregate or not. If the first packet is
unlikely to be reliably delivered, a number of FNE packets MAY be
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sent to increase the probability that at least one is delivered to
the egress gateway.
If each flow does not start with an FNE packet, it will be seen later
that sanctions may be too strict at the interface before the egress
gateway. It will often be possible to apply sanctions at the
granularity of aggregates rather than flows, but in an internetworked
environment it cannot be guaranteed that aggregates will be
identifiable in remote networks. So setting FNE at the start of each
flow is a safe strategy. For instance, a remote network may have
equal cost multi-path (ECMP) routing enabled, causing different flows
between the same gateways to traverse different paths.
After an idle period of more than 1 second, the ingress gateway
SHOULD set the EECN field of the next packet it sends to FNE. This
allows the design of network policers to be deterministic (see [Re-
TCP]).
However, if the ingress gateway can guarantee that the network(s)
that will carry the flow to its egress gateway all use a common
identifier for the aggregate (e.g. a single MPLS network without ECMP
routing), it MAY NOT set FNE when it adds a new flow to an active
aggregate. And an FNE packet need only be sent if a whole aggregate
has been idle for more than 1 second.
4.3.4. Router Forwarding Behaviour
Adding re-ECN works well without modifying the forwarding behaviour
of any routers. However, below, two changes are proposed when
forwarding packets with a per-hop-behaviour that requires pre-
congestion notification:
Preferential drop: When a router cannot avoid dropping ECN-capable
packets, preferential dropping of packets with different extended
ECN codepoints SHOULD be implemented between packets within a PHB
that uses PCN marking. The drop preference order to use is
defined in Table 4. Note that to reduce configuration complexity,
Re-Echo and FNE MAY be given the same drop preference, but if
feasible, FNE should be dropped in preference to Re-Echo.
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+--------+------+----------------+---------+------------------------+
| ECN | RE | Extended ECN | Drop | Re-ECN meaning |
| field | flag | codepoint | Pref | |
+--------+------+----------------+---------+------------------------+
| 01 | 0 | Re-Echo | 5/4 | Re-echoed congestion |
| | | | | and RECT |
| 00 | 1 | FNE | 4 | Feedback not |
| | | | | established |
| 01 | 1 | RECT | 3 | Re-ECN capable |
| | | | | transport |
| 10 | 0 | AM(0) | 3 | Admission Marking with |
| | | | | Re-Echo |
| 10 | 1 | AM(-1) | 3 | Admission Marking |
| | | | | |
| 11 | 0 | PM(0) | 2 | Pre-emption Marking |
| | | | | with Re-Echo |
| 11 | 1 | PM(-1) | 2 | Pre-emption Marking |
| | | | | |
| 00 | 0 | Not-RECT | 1 | Not re-ECN-capable |
| | | | | transport |
+--------+------+----------------+---------+------------------------+
Table 4: Drop Preference of Extended ECN Codepoints (1 = drop 1st)
Given this proposal is being advanced at the same time as PCN
itself, we strongly RECOMMEND that preferential drop based on
extended ECN codepoint is added to router forwarding at the same
time as PCN marking. Preferential dropping can be difficult to
implement, but we strongly RECOMMEND this security-related re-ECN
improvement where feasible as it is an effective defence against
flooding attacks.
Marking vs. Drop: We propose that PCN-routers SHOULD inspect the RE
flag as well as the ECN field to decide whether to drop or mark
PCN DSCPs. They MUST choose drop if the codepoint of this
extended ECN field is Not-RECT. Otherwise they SHOULD mark
(unless, of course, buffer space is exhausted).
A PCN-capable router MUST NOT ever congestion mark a packet
carrying the Not-RECT codepoint because the transport will only
understand drop, not congestion marking. But a PCN-capable router
can mark rather than drop an FNE packet, even though its ECN field
when looked at in isolation is '00' which appears to be a legacy
Not-ECT packet. Therefore, if a packet's RE flag is '1', even if
its ECN field is '00', a PCN-enabled router SHOULD use congestion
marking. This allows the `feedback not established' (FNE)
codepoint to be used for probe packets, in order to pick up PCN
marking when bootstrapping an aggregate.
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ECN marking rather than dropping of FNE packets MUST only be
deployed in controlled environments, such as that in [CL-deploy],
where the presence of an egress node that understands ECN marking
is assured. Congestion events might otherwise be ignored if the
receiver only understands drop, rather than ECN marking. This is
because there is no guarantee that ECN capability has been
negotiated if feedback is not established (FNE). Also, [Re-TCP]
places the strong condition that a router MUST apply drop rather
than marking to FNE packets unless it can guarantee that FNE
packets are rate limited either locally or upstream.
4.3.5. Extensions
If a different signalling system, such as NSIS, were used, but it
provided admission control in a similar way, using pre-congestion
notification (e.g. with RMD [NSIS-RMD]) we believe re-ECN could be
used to protect against misbehaving networks in the same way as
proposed above.
5. Emulating Border Policing with Re-ECN
5.1. Informal Terminology
In the rest of this memo, where the context makes it clear, we will
sometimes loosely use the term `congestion' rather than using the
stricter `downstream pre-congestion'. Also we will loosely talk of
positive or negative flows, meaning flows where the moving average of
the downstream pre-congestion metric is persistently positive or
negative. The notion of a negative metric arises because it is
derived by subtracting one metric from another. Of course actual
downstream congestion cannot be negative, only the metric can
(whether due to time lags or deliberate malice).
Just as we will loosely talk of positive and negative flows, we will
also talk of positive or negative packets, meaning packets that
contribute positively or negatively to downstream pre-congestion.
Therefore packets can be considered to have a `worth' of +1, 0 or -1,
which, when multiplied by their size, indicates their contribution to
downstream congestion. Packets will usually be sent with a worth of
0. Blanking the RE flag increments the worth of a packet to +1.
Congestion marking a packet decrements its worth (whether admission
marking or pre-emption marking). Congestion marking a previously
blanked packet cancel out the positive and negative worth of each
marking (a worth of 0). The FNE codepoint is an exception. It has
the same positive worth as a packet with the Re-Echo codepoint. The
table below specifies unambiguously the worth of each extended ECN
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codepoint. Note the order is different from the previous table to
emphasise how congestion marking processes decrement the worth.
+--------+------+------------------+-------+------------------------+
| ECN | RE | Extended ECN | Worth | Re-ECN meaning |
| field | flag | codepoint | | |
+--------+------+------------------+-------+------------------------+
| 00 | 0 | Not-RECT | n/a | Not re-ECN-capable |
| | | | | transport |
| 01 | 0 | Re-Echo | +1 | Re-echoed congestion |
| | | | | and RECT |
| 10 | 0 | AM(0) | 0 | Admission Marking with |
| | | | | Re-Echo |
| 11 | 0 | PM(0) | 0 | Pre-emption Marking |
| | | | | with Re-Echo |
| 00 | 1 | FNE | +1 | Feedback not |
| | | | | established |
| 01 | 1 | RECT | 0 | Re-ECN capable |
| | | | | transport |
| 10 | 1 | AM(-1) | -1 | Admission Marking |
| | | | | |
| 11 | 1 | PM(-1) | -1 | Pre-emption Marking |
+--------+------+------------------+-------+------------------------+
Table 5: 'Worth' of Extended ECN Codepoints
5.2. Policing Overview
It will be recalled that downstream congestion can be found by
subtracting upstream congestion from path congestion. Figure 4
displays the difference between the two plots in Figure 3 to show
downstream pre-congestion across the same path through the Internet.
To emulate border policing, the general idea is for each domain to
apply penalties to its upstream neighbour in proportion to the amount
of downstream pre-congestion that the upstream network sends across
the border. That is, the penalties should be in proportion to the
height of the plot. Downward arrows in the figure show the resulting
pressure for each domain to under-declare downstream pre-congestion
in traffic they pass to the next domain, because of the penalties.
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p e n a l t i e s
/ | \
A : : :
| | <--A---> <---B---> <---C---> domain
| V : : :
3% | +-----+ | | :
| | | V V :
2% | | +----------------------+ :
| | downstream pre-congestion | :
1% | | : | :
| | : | :
0% +----+----------------------------+====+------>
: : : A :
: : : | :
ingress : : : egress
1.00% 2.00%: pre-congestion
|
sanctions
Figure 4: Policing Framework, showing creation of opposing pressures
to under-declare and over-declare downstream pre-congestion, using
penalties and sanctions
These penalties seem to encourage everyone to understate downstream
congestion in order to reduce the penalties they incur. But a
balancing pressure is introduced by the last domain, which applies
sanctions to flows if downstream congestion goes negative before the
egress gateway. The upward arrow at Domain C's border with the
egress gateway represents the incentive the sanctions would create to
prevent negative traffic. The same upward pressure can be applied at
any domain border (arrows not shown).
Any flow that persistently goes negative by the time it leaves a
domain must not have been marked correctly in the first place. A
domain that discovers such a flow can adopt a range of strategies to
protect itself. Which strategy it uses will depend on policy,
because it cannot immediately assume malice---there may be an
innocent configuration error somewhere in the system.
This memo does not propose to standardise any particular mechanism to
detect persistently negative flows, but Section 5.5 does give
examples. Note that we have used the term flow, but there will be no
need to bury into the transport layer for port numbers; identifiers
visible in the network layer will be sufficient (IP address pair,
DSCP, protocol ID). The appendix also gives a mechanism to bound the
required flow state, preventing state exhaustion attacks.
Of course, some domains may trust other domains to comply with
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admission control without applying sanctions or penalties. In these
cases, the protocol should still be used but no penalties need be
applied. The re-ECN protocol ensures downstream pre-congestion
marking is passed on correctly whether or not penalties are applied
to it, so the system works just as well with a mixture of some
domains trusting each other and others not.
Providers should be free to agree the contractual terms they wish
between themselves, so this memo does not propose to standardise how
these penalties would be applied. It is sufficient to standardise
the re-ECN protocol so the downstream pre-congestion metric is
available if providers choose to use it. However, the next section
(Section 5.3) gives some examples of how these penalties might be
implemented.
5.3. Pre-requisite Contractual Arrangements
The re-ECN protocol has been chosen to solve the policing problem
because it embeds a downstream pre-congestion metric in passing CL
traffic that is difficult to lie about and can be measured in bulk.
The ability to emulate border policing depends on network operators
choosing to use this metric as one of the elements in their contracts
with each other.
Already many inter-domain agreements involve a capacity and a usage
element. The usage element may be based on volume or various
measures of peak demand. We expect that those network operators who
choose to use pre-congestion notification for admission control would
also be willing to consider using this downstream pre-congestion
metric as a usage element in their interconnection contracts for
admission controlled (CL) traffic.
Congestion (or pre-congestion) has the dimension of [octet], being
the product of volume transferred [octet] and the congestion fraction
[dimensionless], which is the fraction of the offered load that the
network isn't able to serve (or would rather not serve in the case of
pre-congestion). Measuring downstream congestion gives a measure of
the volume transferred but modulated by congestion expected
downstream. So volume transferred during off-peak periods counts as
nearly nothing, while volume transferred at peak times counts very
highly. The re-ECN protocol allows one network to measure how much
pre-congestion has been `dumped' into it by another network. And
then in turn how much of that pre-congestion it dumped into the next
downstream network.
Section 5.6 describes mechanisms for calculating border penalties
referring to Appendix A.2 for suggested metering algorithms for
downstream congestion at a border router. Conceptually, it could
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hardly be simpler. It broadly involves accumulating the volume of
packets with the RE flag blanked and the volume of those with
congestion marking then subtracting the two.
Once this downstream pre-congestion metric is available, operators
are free to choose how they incorporate it into their interconnection
contracts [IXQoS]. Some may include a threshold volume of pre-
congestion as a quality measure in their service level agreement,
perhaps with a penalty clause if the upstream network exceeds this
threshold over, say, a month. Others may agree a set of tiered
monthly thresholds, with increasing penalties as each threshold is
exceeded. But, it would be just as easy, and more resistant to
gaming, to do away with discrete thresholds, and instead make the
penalty rise smoothly with the volume of pre-congestion by applying a
price to pre-congestion itself. Then the usage element of the
interconnection contract would directly relate to the volume of pre-
congestion caused by the upstream network.
The direction of penalties and charges relative to the direction of
traffic flow is a constant source of confusion. Typically, where
capacity charges are concerned, lower tier customer networks pay
higher tier provider networks. So money flows from the edges to the
middle of the internetwork, towards greater connectivity,
irrespective of the flow of data. But we advise that penalties or
charges for usage should follow the same direction as the data
flow---the direction of control at the network layer. Otherwise a
network lays itself open to `denial of funds' attacks. So, where a
tier 2 provider sends data into a tier 3 customer network, we would
expect the penalty clauses for sending too much pre-congestion to be
against the tier 2 network, even though it is the provider.
It may help to remember that data will be flowing in the other
direction too. So the provider network has as much opportunity to
levy usage penalties as its customer, and it can set the price or
strength of its own penalties higher if it chooses. Usage charges in
both directions tend to cancel each other out, which confirms that
usage-charging is less to do with revenue raising and more to do with
encouraging load control discipline in order to smooth peaks and
troughs, improving utilisation and quality.
Further, when operators agree penalties in their interconnection
contracts for sending downstream congestion, they should make sure
that any level of negative marking only equates to zero penalty. In
other words, penalties are always paid in the same direction as the
data, and never against the data flow, even if downstream congestion
seems to be negative. This is consistent with the definition of
physical congestion; when a resource is underutilised, it is not
negatively congested. Its congestion is just zero. So, although
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short periods of negative marking can be tolerated to correct
temporary over-declarations due to lags in the feedback system,
persistent downstream negative congestion can have no physical
meaning and therefore must signify a problem. The incentive for
domains not to tolerate persistently negative traffic depends on this
principle that penalties must never be paid against the data flow.
Also note that at the last egress of the Diffserv region, domain C
should not agree to pay any penalties to the egress gateway for pre-
congestion passed to the egress gateway. Downstream pre-congestion
to the egress gateway should have reached zero here. If domain C
were to agree to pay for any remaining downstream pre-congestion, it
would give the egress gateway an incentive to over-declare pre-
congestion feedback and take the resulting profit from domain C.
To focus the discussion, from now on, unless otherwise stated, we
will assume a downstream network charges its upstream neighbour in
proportion to the pre-congestion it sends (V_b in the notation of
Appendix A.2). Effectively tiered thresholds would be just more
coarse-grained approximations of the fine-grained case we choose to
examine. If these neighbours had previously agreed that the (fixed)
price per octet of pre-congestion would be L, then the bill at the
end of the month would simply be the product L*V_b, plus any fixed
charges they may also have agreed.
We are well aware that the IETF tries to avoid standardising
technology that depends on a particular business model. Indeed, this
principle is at the heart of all our own work. Our aim here is to
make a new metric available that we believe is superior to all
existing metrics. Then, our aim is to show that border policing can
at least work with the one model we have just outlined. We assume
that operators might then experiment with the metric in other models.
Of course, operators are free to complement this pre-congestion-based
usage element of their charges with traditional capacity charging,
and we expect they will.
Also note well that everything we discuss in this memo only concerns
interconnection within the Diffserv region. ISPs are free to sell or
give away reservations however they want on the retail market. But
of course, interconnection charges will have a bearing on that.
Indeed, in the present scenario, the ingress gateway effectively
sells reservations on one side and buys congestion penalties on the
other. As congestion rises, one can imagine the gateway discovering
that congestion penalties have risen higher than the (probably fixed)
revenue it will earn from selling the next flow reservation. This
encourages the gateway to cut its losses by blocking new calls, which
is why we believe downstream congestion penalties can emulate per-
flow rate policing at borders, as the next section explains.
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5.4. Emulation of Per-Flow Rate Policing: Rationale and Limits
The important feature of charging in proportion to congestion volume
is that the penalty aggregates and disaggregates correctly along with
packet flows. This is because the penalty rises linearly with bit
rate (unless congestion is absolutely zero) and linearly with
congestion, because it is the product of them both. So if the
packets crossing a border belong to a thousand flows, and one of
those flows doubles its rate, the ingress gateway forwarding that
flow will have to put twice as much congestion marking into the
packets of that flow. And this extra congestion marking will add
proportionately to the penalties levied at every border the flow
crosses in proportion to the amount of pre-congestion remaining on
the path.
Effectively, usage charges will continuously flow from ingress
gateways to the places generating pre-congestion marking, in
proportion to the pre-congestion marking introduced and to the data
rates from those gateways.
As importantly, pre-congestion itself rises super-linearly with
utilisation of a particular resource. So if someone tries to push
another flow into a path that is already signalling enough pre-
congestion to warrant admission control, the penalty will be a lot
greater than it would have been to add the same flow to a less
congested path. This makes the incentive system fairly insensitive
to the actual level of pre-congestion for triggering admission
control that each ingress chooses. The deterrent against exceeding
whatever threshold is chosen rises very quickly with a small amount
of cheating.
These are the properties that allow re-ECN to emulate per-flow border
policing of both rate and admission control. It is not a perfect
emulation of per-flow border policing, but we claim it is sufficient
to at least ensure the cost to others of a cheat is borne by the
cheater, because the penalties are at least proportionate to the
level of the cheat. If an edge network operator is selling
reservations at a large profit over the congestion cost, these pre-
congestion penalties will not be sufficient to ensure networks in the
middle get a share of those profits, but at least they can cover
their costs.
We will now explain with an example. When a whole inter-network is
operating at normal (typically very low) congestion, the pre-
congestion marking from virtual queues will be a little higher than
if the real queues had been used---still low, but more noticeable.
But low congestion levels do not imply that usage /charges/ must also
be low. Usage charges will depend on the /price/ L as well.
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If the metric of the usage element of an interconnection agreement
was changed from pure volume to pre-congested volume, one would
expect the price of pre-congestion to be arranged so that the total
usage charge remained about the same. So, if an average pre-
congestion fraction turned out to be 1/1000, one would expect that
the price L (per octet) of pre-congestion would be about 1000 times
the previously used (per octet) price for volume. We should add that
a switch to pre-congestion is unlikely to exactly maintain the same
overall level of usage charges, but this argument will be
approximately true, because usage charge will rise to at least the
level the market finds necessary to push back against usage.
From the above example it can be seen why a 1000x higher price will
make operators become acutely sensitive to the congestion they cause
in other networks, which is of course the desired effect; to
encourage networks to /control/ the congestion they allow their users
to cause to others.
If any network sends even one flow at higher rate, they will
immediately have to pay proportionately more usage charges. Because
there is no knowledge of reservations within the Diffserv region, no
interior router can police whether the rate of each flow is greater
than each reservation. So the system doesn't truly emulate rate-
policing of each flow. But there is no incentive to pack a higher
rate into a reservation, because the charges are directly
proportional to rate, irrespective of the reservations.
However, if virtual queues start to fill on any path, even though
real queues will still be able to provide low latency service, pre-
congestion marking will rise fairly quickly. It may eventually reach
the threshold where the ingress gateway would deny admission to new
flows. If the ingress gateway cheats and continues to admit new
flows, the affected virtual queues will rapidly fill, even though the
real queues will still be little worse than they were when admission
control should have been invoked. The ingress gateway will have to
pay the penalty for such an extremely high pre-congestion level, so
the pressure to invoke admission control should become unbearable.
The above mechanisms protect against rational operators. In
Section 5.6.3 we discuss how networks can protect themselves from
accidental or deliberate misconfiguration in neighbouring networks.
5.5. Sanctioning Dishonest Marking
As CL traffic leaves the last network before the egress gateway
(domain C) the RE blanking fraction should match the congestion
marking fraction, when averaged over a sufficiently long duration
(perhaps ~10s to allow a few rounds of feedback through regular
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signalling of new and refreshed reservations).
To protect itself, domain C should install a monitor at its egress.
It aims to detect flows of CL packets that are persistently negative.
If flows are positive, domain C need take no action---this simply
means an upstream network must be paying more penalties than it needs
to. Appendix A.3 gives a suggested algorithm for the monitor,
meeting the criteria below.
o It SHOULD introduce minimal false positives for honest flows;
o It SHOULD quickly detect and sanction dishonest flows (minimal
false negatives);
o It MUST be invulnerable to state exhaustion attacks from malicious
sources. For instance, if the dropper uses flow-state, it should
not be possible for a source to send numerous packets, each with a
different flow ID, to force the dropper to exhaust its memory
capacity;
o It MUST introduce sufficient loss in goodput so that malicious
sources cannot play off losses in the egress dropper against
higher allowed throughput. Salvatori [CLoop_pol] describes this
attack, which involves the source understating path congestion
then inserting forward error correction (FEC) packets to
compensate expected losses.
Note that the monitor operates on flows but with careful design we
can avoid per-flow state. This is why we have been careful to ensure
that all flows MUST start with a packet marked with the FNE
codepoint. If a flow does not start with the FNE codepoint, a
monitor is likely to treat it unfavourably. This risk makes it worth
setting the FNE codepoint at the start of a flow, even though there
is a cost to setting FNE (positive `worth').
Starting flows with an FNE packet also means that a monitor will be
resistant to state exhaustion attacks from other networks, as the
monitor can then be designed to never create state unless an FNE
packet arrives. And an FNE packet counts positive, so it will cost a
lot for a network to send many of them.
Monitor algorithms will often maintain a moving average across flows
of the fraction of RE blanked packets. When maintaining an average
across flows, a monitor MUST ignore packets with the FNE codepoint
set. An ingress gateway sets the FNE codepoint when it does not have
the benefit of feedback from the egress. So counting packets with
FNE cleared would be likely to make the average unnecessarily
positive, providing headroom (or should we say footroom?) for
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dishonest (negative) traffic.
If the monitor detects a persistently negative flow, it could drop
sufficient negative and neutral packets to force the flow to not be
negative. This is the approach taken for the `egress dropper' in
[Re-TCP], but for the scenario in this memo, where everyone would
expect everyone else to keep to the protocol, a management alarm
SHOULD be raised on detecting persistently negative traffic and any
automatic sanctions taken SHOULD be logged. Even if the chosen
policy is to take no automatic action, the cause can then be
investigated manually.
Then all ingresses cannot understate downstream pre-congestion
without their action being logged. So network operators can deal
with offending networks at the human level, out of band. As a last
resort, perhaps where the ingress gateway address seems to have been
spoofed in the signalling, packets can be dropped. Drops could be
focused on just sufficient packets in misbehaving flows to remove the
negative bias while doing minimal harm.
A future version of this memo may define a control message that could
be used to notify an offending ingress gateway (possibly via the
egress gateway) that it is sending persistently negative flows.
However, we are aware that such messages could be used to test the
sensitivity of the detection system, so currently we prefer silent
sanctions.
An extreme scenario would be where an ingress gateway (or set of
gateways) mounted a DoS attack against another network. If their
traffic caused sufficient congestion to lead to drop but they
understated path congestion to avoid penalties for causing high
congestion, the preferential drop recommendations in Section 4.3.4
would at least ensure that these flows would always be dropped before
honest flows..
5.6. Border Mechanisms
5.6.1. Border Accounting Mechanisms
One of the main design goals of re-ECN was for border security
mechanisms to be as simple as possible, otherwise they would become
the pinch-points that limit scalability of the whole internetwork.
As the title of this memo suggests, we want to avoid per-flow
processing at borders. We also want to keep to passive mechanisms
that can monitor traffic in parallel to forwarding, rather than
having to filter traffic inline---in series with forwarding. As data
rates continue to rise, we suspect that all-optical interconnection
between networks will soon be a requirement. So we want to avoid any
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new need for buffering (even though border filtering is current
practice for other reasons, we don't want to make it even less likely
that we will ever get rid of it).
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, although we do use filtering as a fail-safe to
temporarily shield against extreme events in other networks, such as
accidental misconfigurations (Section 5.6.3).
The basic accounting mechanism at each border interface simply
involves accumulating the volume of packets with positive worth (Re-
Echo and FNE), and subtracting the volume of those with negative
worth: AM(-1) and PM(-1). Even though this mechanism takes no regard
of flows, over an accounting period (say a month) this subtraction
will account for the downstream congestion caused by all the flows
traversing the interface, wherever they come from, and wherever they
go to. The two networks can agree to use this metric however they
wish to determine some congestion-related penalty against the
upstream network (see Section 5.3 for examples). Although the
algorithm could hardly be simpler, it is spelled out using pseudo-
code in Appendix A.2.1.
Various attempts to subvert the re-ECN design have been made. In all
cases their root cause is persistently negative flows. But, after
describing these attacks we will show that we don't actually have to
get rid of all persistently negative flows in order to thwart the
attacks.
In honest flows, downstream congestion is measured as positive minus
negative volume. So if all flows are honest (i.e. not persistently
negative), adding all positive volume and all negative volume without
regard to flows will give an aggregate measure of downstream
congestion. But such simple aggregation is only possible if no flows
are persistently negative. Unless persistently negative flows are
completely removed, they will reduce the aggregate measure of
congestion. The aggregate may still be positive overall, but not as
positive as it would have been had the negative flows been removed.
In Section 5.5 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.
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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:
o A network can simply create its own dummy traffic to congest
another network, perhaps causing it to lose business at no cost to
the attacking network. This is a form of denial of service
perpetrated by one network on another. The preferential drop
measures in Section 4.3.4 provide crude protection against such
attacks, but we are not overly worried about more accurate
prevention measures, because it is already possible for networks
to DoS other networks on the general Internet, but they generally
don't because of the grave consequences of being found out. We
are only concerned if re-ECN increases the motivation for such an
attack, as in the next example.
o A network can just generate negative traffic and send it over its
border with a neighbour to reduce the overall penalties that it
should pay to that neighbour. It could even initialise the TTL so
it expired shortly after entering the neighbouring network,
reducing the chance of detection further downstream. This attack
need not be motivated by a desire to deny service and indeed need
not cause denial of service. A network's main motivator would
most likely be to reduce the penalties it pays to a neighbour.
But, the prospect of financial gain might tempt the network into
mounting a DoS attack on the other network as well, given the gain
would offset some of the risk of being detected.
Note that we have not included DoS by Internet hosts in the above
list of attacks, because we have restricted ourselves to a scenario
with edge-to-edge admission control across a Diffserv region. In
this case, the edge ingress gateways insulate the Diffserv region
from DoS by Internet hosts. Re-ECN resists more general DoS attacks,
but this is discussed in [Re-TCP].
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
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from negative flows to the downstream congestion measure is given in
Appendix A.2.2.
Although making an accurate assessment of the contribution from
negative flows may not be easy, just the single step of neutralising
their polluting effect on congestion metrics removes all the gains
networks could otherwise make from mounting dummy traffic attacks on
each other. This puts all networks on the same side (only with
respect to negative flows of course), rather than being pitched
against each other. The network where this flow goes negative as
well as all the networks downstream lose out from not being
reimbursed for any congestion this flow causes. So they all have an
interest in getting rid of these negative flows. Networks forwarding
a flow before it goes negative aren't strictly on the same side, but
they are disinterested bystanders---they don't care that the flow
goes negative downstream, but at least they can't actively gain from
making it go negative. The problem becomes localised so that once a
flow goes negative, all the networks from where it happens and beyond
downstream each have a small problem, each can detect it has a
problem and each can get rid of the problem if it chooses to. But
negative flows can no longer be used for any new attacks.
Once an unbiased estimate of the effect of negative flows can be
made, the problem reduces to detecting and preferably removing flows
that have gone negative as soon as possible. But importantly,
complete eradication of negative flows is no longer critical---best
endeavours will be sufficient.
Note that the guiding principle behind all the above discussion is
that any gain from subverting the protocol should be precisely
neutralised, rather than punished. If a gain is punished to a
greater extent than is sufficient to neutralise it, it will most
likely open up a new vulnerability, where the amplifying effect of
the punishment mechanism can be turned on others.
For instance, if possible, flows should be removed as soon as they go
negative, but we do NOT RECOMMEND any attempts to discard such flows
further upstream while they are still positive. Such over-zealous
push-back is unnecessary and potentially dangerous. These flows have
paid their `fare' up to the point they go negative, so there is no
harm in delivering them that far. If someone downstream asks for a
flow to be dropped as near to the source as possible, because they
say it is going to become negative later, an upstream node cannot
test the truth of this assertion. Rather than have to authenticate
such messages, re-ECN has been designed so that flows can be dropped
solely based on locally measurable evidence. A message hinting that
a flow should be watched closely to test for negativity is fine. But
not a message that claims that a positive flow will go negative
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later, so it should be dropped. .
5.6.2. Competitive Routing
With the above penalty system, each domain seems to have a perverse
incentive to fake pre-congestion. For instance domain B profits from
the difference between penalties it receives at its ingress (its
revenue) and those it pays at its egress (its cost). So if B
overstates internal pre-congestion it seems to increase its profit.
However, we can assume that domain A could bypass B, routing through
other domains to reach the egress. So the competitive discipline of
least-cost routing can ensure that any domain tempted to fake pre-
congestion for profit risks losing /all/ its incoming traffic. The
least congested route would eventually be able to win this
competitive game, only as long as it didn't declare more fake pre-
congestion than the next most competitive route.
This memo does not need to standardise any particular mechanism for
routing based on re-ECN. Goldenberg et al [Smart_rtg] refers to
various commercial products and presents its own algorithms for
moving traffic between multi-homed routes based on usage charges.
None of these systems require any changes to standards protocols
because the choice between the available border gateway protocol
(BGP) routes is based on a combination of local knowledge of the
charging regime and local measurement of traffic levels. If, as we
propose, charges or penalties were based on the level of re-ECN
measured in passing traffic, a similar optimisation could be achieved
without requiring any changes to standard routing protocols.
We must be clear that applying pre-congestion-based routing to this
admission control system remains an open research issue. Traffic
engineering based on congestion requires careful damping to avoid
oscillations, and should not be attempted without adult supervision
:) Mortier & Pratt [ECN-BGP] have analysed traffic engineering based
on congestion. But without the benefit of re-ECN, they had to add a
path attribute to BGP to advertise a route's downstream congestion
(actually they proposed that BGP should advertise the charge for
congestion, which we believe wrongly embeds an assumption into BGP
that the only thing to do with congestion is charge for it).
5.6.3. Fail-safes
The mechanisms described so far create incentives for rational
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
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approach does not protect against the misconfigurations and accidents
of other operators.
Therefore, we propose the following two mechanisms at a network's
borders to provide "defence in depth". Both are similar:
Highly positive flows: A small sample of positive packets should be
picked randomly as they cross a border interface. Then subsequent
packets matching the same source and destination address and DSCP
should be monitored. If the fraction of positive marking is well
above a threshold (to be determined by operational practice), a
management alarm SHOULD be raised, and the flow MAY be
automatically subject to focused drop.
Persistently negative flows: A small sample of congestion marked
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 RE
blanking fraction minus the congestion marking fraction is
persistently negative, a management alarm SHOULD be raised, and
the flow MAY be automatically subject to focused drop.
Both these mechanisms rely on the fact that highly positive (or
negative) flows will appear more quickly in the sample by selecting
randomly solely from positive (or negative) packets.
Note that there is no assumption that /users/ behave rationally. The
system is protected from the vagaries of irrational user behaviour by
the ingress gateways, which transform internal penalties into a
deterministic, admission control mechanism that prevents users from
misbehaving, by directly engineered means.
6. Analysis
The domains in Figure 1 are not expected to be completely malicious
towards each other. After all, we can assume that they are all co-
operating to provide an internetworking service to the benefit of
each of them and their customers. Otherwise their routing polices
would not interconnect them in the first place. However, we assume
that they are also competitors of each other. So a network may try
to contravene our proposed protocol if it would gain or make a
competitor lose, or both, but only if it can do so without being
caught. Therefore we do not have to consider every possible random
attack one network could launch on the traffic of another, given
anyway one network can always drop or corrupt packets that it
forwards on behalf of another.
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Therefore, we only consider new opportunities for /gainful/ attack
that our proposal introduces. But to a certain extent we can also
rely on the in depth defences we have described (Section 5.6.3 )
intended to mitigate the potential impact if one network accidentally
misconfiguring the workings of this protocol.
The ingress and egress gateways are shown in the most generic
arrangement possible in Figure 1, without any surrounding network.
This allows us to consider more specific cases where these gateways
and a neighbouring network are operated by the same player. As well
as cases where the same player operates neighbouring networks, we
will also consider cases where the two gateways collude as one player
and where the sender and receiver collude as one. Collusion of other
sets of domains is less likely, but we will consider such cases. In
the general case, we will assume none of the nine trust domains
across the figure fully trust any of the others.
As we only propose to change routers within the Diffserv region, we
assume the operators of networks outside the region will be doing
per-flow policing. That is, we assume the networks outside the
Diffserv region and the gateways around its edges can protect
themselves. So given we are proposing to remove flow policing from
some networks, our primary concern must be to protect networks that
don't do per-flow policing (the potential `victims') from those that
do (the `enemy'). The ingress and egress gateways are the only way
the outer enemy can get at the middle victim, so we can consider the
gateways as the representatives of the enemy as far as domains A, B
and C are concerned. We will call this trust scenario `edges against
middles'.
Earlier in this memo, we outlined the classic border rate policing
problem (Section 3). It will now be useful to reiterate the
motivations that are the root cause of the problem. The more
reservations a gateway can allow, the more revenue it receives. The
middle networks want the edges to comply with the admission control
protocol when they become so congested that their service to others
might suffer. The middle networks also want to ensure the edges
cannot steal more service from them than they are entitled to.
In the context of this `edges against middles' scenario, the re-ECN
protocol has two main effects:
o The more pre-congestion there is on a path across the Diffserv
region, the higher the ingress gateway must declare downstream
pre-congestion.
o If the ingress gateway does not declare downstream pre-congestion
high enough on average, it will `hit the ground before the
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runway', going negative and triggering sanctions, either directly
against the traffic or against the ingress gateway at a management
level
An executive summary of our security analysis can be stated in three
parts, distinguished by the type of collusion considered.
Neighbour-only Middle-Middle Collusion: Here there is no collusion or
collusion is limited to neighbours in the feedback loop. In other
words, two neighbouring networks can be assumed to act as one. Or
the egress gateway might collude with domain C. Or the ingress
gateway might collude with domain A. Or ingress and egress
gateways might collude with each other.
In these cases where only neighbours in the feedback loop collude,
we concludes that all parties have a positive incentive to declare
downstream pre-congestion truthfully, and the ingress gateway has
a positive incentive to invoke admission control when congestion
rises above the admission threshold in any network in the region
(including its own). No party has an incentive to send more
traffic than declared in reservation signalling (even though only
the gateways read this signalling). In short, no party can gain
at the expense of another.
Non-neighbour Middle-Middle Collusion: In the case of other forms of
collusion between middle networks (e.g. between domain A and C) it
would be possible for say A & C to create a tunnel between
themselves so that A would gain at the expense of B. But C would
then lose the gain that A had made. Therefore the value to A & C
of colluding to mount this attack seems questionable. It is made
more questionable, because the attack can be statistically
detected by B using the second `defence in depth' mechanism
mentioned already. Note that C can defend itself from being
attacked through a tunnel by treating the tunnel end point as a
direct link to a neighbouring network (e.g. as if A were a
neighbour of C, via the tunnel), which falls back to the safety of
the neighbour-only scenario.
Middle-Edge Collusion: Collusion between networks or gateways within
the Diffserv region and networks or users outside the region has
not yet been fully analysed. The presence of full per-flow
policing at the ingress gateway seems to make this a less likely
source of a successful attack.
{ToDo: Due to lack of time, the full write up of the security
analysis is deferred to the next version of this memo.}
Finally, it is well known that the best person to analyse the
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security of a system is not the designer. Therefore, our confident
claims must be hedged with doubt until others with perhaps a greater
incentive to break it have mounted a full analysis.
7. Incremental Deployment
We believe ECN has so far not been widely deployed because it
requires widespread end system and network deployment just to achieve
a marginal improvement in performance. The ability to offer a new
service (admission control) would be a much stronger driver for ECN
deployment.
As stated in the introduction, the aim of this memo is to "build in
security from the start" when admission control is based on pre-
congestion notification. However, the proposal has been designed so
that security can be added some time after first deployment. Given
admission control based on pre-congestion notification requires few
changes to standards, it should be deployable fairly soon. However,
re-ECN requires a change to IP, which may take a little longer.
We expect that initial deployments of PCN-based admission control
will be confined to single networks, or to clubs of networks that
trust each other. The proposal in this memo will only become
relevant once networks with conflicting interests wish to
interconnect their admission controlled services, but without the
scalability constraints of per-flow border policing. It will not be
possible to use re-ECN, even in a controlled environment between
consenting operators, unless it is standardised into IP. Given the
IPv4 header has limited space for further changes, current IESG
policy [{ToDo: ref?}] is not to allow experimental use of codepoints
in the IPv4 header, as whenever an experiment isn't taken up, the
space it used tends to be impossible to reclaim.
If PCN-based admission control is deployed before re-ECN is
standardised into IP, wherever a networks (or club of networks)
connects to another network (or club of networks) with conflicting
interests, they will place a gateway between the two regions that
does per-flow rate policing and admission control. If re-ECN is
eventually standardised into IP, it will be possible for these
separate regions to upgrade all their gateways to use re-ECN before
removing the per-flow policing gateways between them. Given the
edge-to-edge deployment model of PCN-based admission control, it is
reasonable to imagine this incremental deployment model without
needing to cater for partial deployment of re-ECN in just some of the
gateways around one Diffserv region.
Only the edge gateways around a Diffserv region have to be upgraded
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to add re-ECN support, not interior routers. It is also necessary to
add the mechanisms that use re-ECN to secure a network against
misbehaving gateways and networks. Specifically, these are the
border mechanisms (Section 5.6) and the mechanisms to sanction
dishonest marking (Section 5.5).
We also RECOMMEND adding improvements to forwarding on interior
routers (Section 4.3.4). But the system works whether all, some or
none are upgraded, so interior routers may be upgraded in a piecemeal
fashion at any time.
8. Design Choices and Rationale
The primary insight of this work is that downstream congestion is the
metric that would be most useful to control an internetwork, and
particularly to police how one network responds to the congestion it
causes in a remote network. This is the problem that has previously
made it so hard to provide scalable admission control.
The case for using re-feedback (a generalisation of re-ECN) to police
congestion response and provide QoS is made in [Re-fb]. Essentially,
the insight is that congestion is a factor that crosses layers from
the physical upwards. Therefore re-feedback polices congestion where
it emerges from a physical interface between networks. This is
achieved by bringing the congestion information to the interface,
rather than examining packet addressing where there is congestion.
Then congestion crossing the physical interface at a border can be
policed at the interface, rather than policing the congestion on
packets that claim to come from an address (which may be spoofed).
Also, re-feedback works in the network layer independently of other
layers---despite its name re-feedback does not actually require
feedback. It requires a source to act conservatively before it gets
feedback.
On the subject of lack of feedback, the feedback not established
(FNE) codepoint is motivated by arguments for a state set-up bit in
IP to prevent state exhaustion attacks. This idea was first put
forward informally by David Clark and documented by Handley and
Greenhalgh in [Steps_DoS]. The idea is that network layer datagrams
should signal explicitly when they require state to be created in the
network layer or the layer above (e.g. at flow start). Then a node
can refuse to create any state unless a datagram declares this
intent. We believe the proposed FNE codepoint serves the same
purpose as the proposed state-set-up bit, but it has been overloaded
with a more specific purpose, using it on more packets than just the
first in a flow, but never less (i.e. it is idempotent). In effect
the FNE codepoint serves the purpose of a `soft-state set-up
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codepoint'.
The re-feedback paper [Re-fb] also makes the case for converting the
economic interpretation of congestion into hard engineering
mechanism, which is the basis of the approach used in this memo. The
admission control gateways around the Diffserv region use hard
engineering, not incentives, to prevent end users from sending more
traffic than they have reserved. Incentive-based mechanisms are only
used between networks, because they are expected to respond to
incentives more rationally than end-users can be expected to.
However, even then, a network can use fail-safes to protect itself
from excessively unusual behaviour by neighbouring networks, whether
due to an accidental misconfiguration or malicious intent.
The guiding principle behind the incentive-based approach used
between networks 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.
The re-feedback paper also makes the case against the use of
congestion charging to police congestion if it is based on classic
feedback (where only upstream congestion is visible to network
elements). It argues this would open up receiving networks to
`denial of funds' attacks and would require end users to accept
dynamic pricing (which few would).
Re-ECN has been deliberately designed to simplify policing at the
borders between networks. These trust boundaries are the critical
pinch-points that will limit the scalability of the whole
internetwork unless the overall design minimises the complexity of
security functions at these borders. The border mechanisms described
in this memo run passively in parallel to data forwarding and they do
not require per-flow processing.
9. Security Considerations
This whole memo concerns the security of a scalable admission control
system. In particular the analysis section. Below some specific
security issues are mentioned that did not belong elsewhere or which
comment on the overall robustness of the security provided by the
design.
Firstly, we must repeat the statement of applicability in the
analysis: that we only consider new opportunities for /gainful/
attack that our proposal introduces, particularly if the attacker can
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avoid being identified. Despite only involving a few bits, there is
sufficient complexity in the whole system that there are probably
numerous possibilities for other attacks. However, as far as we are
aware, none reap any benefit to the attacker. For instance, it would
be possible for a downstream network to remove the congestion
markings introduced by an upstream network, but it would only lose
out on the penalties it could apply to a downstream network.
When one network forwards a neighbouring network's traffic it will
always be possible to cause damage by dropping or corrupting it.
Therefore we do not believe networks would set their routing policies
to interconnect in the first place if they didn't trust the other
networks not to arbitrarily damage their traffic.
Having said this, we do want to highlight some of the weaker parts of
our argument. We have argued that networks will be dissuaded from
faking congestion marking by the possibility that upstream networks
will route round them. As we have said, these arguments are based on
fairly delicate assumptions and will remain fairly tenuous until
proved in practice, particularly close to the egress where less
competitive routing is likely.
We should also point out that the approach in this memo was only
designed to be robust for admission control. We do not claim the
incentives will always be strong enough to force correct flow pre-
emption behaviour. This is because a user will tend to perceive much
greater loss in value if a flow is pre-empted than if admission is
denied at the start. However, in general the incentives for correct
flow pre-emption are similar to those for admission control.
Finally, it may seem that the 8 codepoints that have been made
available by extending the ECN field with the RE flag have been used
rather wastefully. In effect the RE flag has been used as an
orthogonal single bit in nearly all cases. The only exception being
when the ECN field is cleared to "00". The mapping of the codepoints
in an earlier version of this proposal used the codepoint space more
efficiently, but the scheme became vulnerable to a network operator
focusing its congestion marking to mark more positive than neutral
packets in order to reduce its penalties.
With the scheme as now proposed, once the RE flag is set or cleared
by the sender or its proxy, it should not be written by the network,
only read. So the gateways can detect if any network maliciously
alters the RE flag. IPSec AH integrity checking does not cover the
IPv4 option flags (they were considered mutable---even the one we
propose using for the RE flag that was `currently unused' when IPSec
was defined). But it would be sufficient for a pair of gateways to
make random checks on whether the RE flag was the same when it
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reached the egress gateway as when it left the ingress. Indeed, if
IPSec AH had covered the RE flag, any network intending to alter
sufficient RE flags to make a gain would have focused its alterations
on packets without authenticating headers (AHs).
No cryptographic algorithms have been harmed in the making of this
proposal.
10. IANA Considerations
This memo includes no request to IANA.
11. Conclusions
This memo builds on a promising technique to solve the classic
problem of making flow admission control scale to any size network.
It involves the use of Diffserv in a deployment model that uses pre-
congestion notification feedback to control admission into a network
path [CL-deploy]. However as it stands, that deployment model
depends on all network domains trusting each other to comply with the
protocols, invoking admission control and flow pre-emption when
requested.
We propose that the congestion feedback used in that deployment model
should be re-echoed into the forward data path, by making a trivial
modification to the ingress gateway. We then explain how the
resulting downstream pre-congestion metric in packets can be
monitored in bulk at borders to sufficiently emulate flow rate
policing.
We claim the result of combining these two approaches is an admission
control system that scales to any size network /and/ any number of
interconnected networks, even if they all act in their own interests.
This proposal aims to convince its readers to "Design in Security
from the start," by building modified ingress gateways from day one,
even if border policing is not needed at first. This way, we will
not build ourselves tomorrow's legacy problem.
Re-echoing congestion feedback is based on a principled technique
called Re-ECN [Re-TCP], designed to add accountability for causing
congestion to the general-purpose IP datagram service. Re-ECN
proposes to consume the last completely unused bit in the basic IPv4
header.
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12. Acknowledgements
All the following have given helpful comments and some may become co-
authors of later drafts: Arnaud Jacquet, Alessandro Salvatori, Steve
Rudkin, David Songhurst, John Davey, Ian Self, Anthony Sheppard,
Carla Di Cairano-Gilfedder (BT), Mark Handley (who identified the
excess canceled packets attack), Stephen Hailes, Adam Greenhalgh
(UCL), Francois Le Faucheur, Anna Charny (Cisco), Jozef Babiarz,
Kwok-Ho Chan, Corey Alexander (Nortel), David Clark, Bill Lehr,
Sharon Gillett, Steve Bauer (MIT) (who publicised various dummy
traffic attacks), Sally Floyd (ICIR) and comments from participants
in the CFP/CRN inter-provider QoS and broadband working groups.
13. Comments Solicited
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF Transport Area working group's mailing list
<tsvwg@ietf.org>, and/or to the authors.
14. References
14.1. Normative References
[PCN] Briscoe, B., Eardley, P., Songhurst, D., Le Faucheur, F.,
Charny, A., Liatsos, V., Babiarz, J., Chan, K., Dudley,
S., Westberg, L., Bader, A., and G. Karagiannis, "Pre-
Congestion Notification Marking",
draft-briscoe-tsvwg-cl-phb-02 (work in progress),
June 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2211] Wroclawski, J., "Specification of the Controlled-Load
Network Element Service", RFC 2211, September 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
J., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, March 2002.
[RSVP-ECN]
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Le Faucheur, F., Charny, A., Briscoe, B., Eardley, P.,
Babiarz, J., and K. Chan, "RSVP Extensions for Admission
Control over Diffserv using Pre-congestion Notification",
draft-lefaucheur-rsvp-ecn-01 (work in progress),
June 2006.
[Re-TCP] Briscoe, B., Jacquet, A., and A. Salvatori, "Re-ECN:
Adding Accountability for Causing Congestion to TCP/IP",
draft-briscoe-tsvwg-re-ecn-tcp-02 (work in progress),
June 2006.
14.2. Informative References
[CL-deploy]
Briscoe, B., Eardley, P., Songhurst, D., Le Faucheur, F.,
Charny, A., Babiarz, J., Chan, K., Westberg, L., Bader,
A., and G. Karagiannis, "A Deployment Model for Admission
Control over DiffServ using Pre-Congestion Notification",
draft-briscoe-tsvwg-cl-architecture-03 (work in progress),
June 2006.
[CLoop_pol]
Salvatori, A., "Closed Loop Traffic Policing", Politecnico
Torino and Institut Eurecom Masters Thesis ,
September 2005.
[ECN-BGP] Mortier, R. and I. Pratt, "Incentive Based Inter-Domain
Routeing", Proc Internet Charging and QoS Technology
Workshop (ICQT'03) pp308--317, September 2003, <http://
research.microsoft.com/users/mort/publications.aspx>.
[ECN-MPLS]
Bruce, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", draft-davie-ecn-mpls-00 (work in
progress), June 2006.
[IXQoS] Briscoe, B. and S. Rudkin, "Commercial Models for IP
Quality of Service Interconnect", BT Technology Journal
(BTTJ) 23(2)171--195, April 2005,
<http://www.cs.ucl.ac.uk/staff/B.Briscoe/pubs.html#ixqos>.
[NSIS-RMD]
Bader, A., Westberg, L., Karagiannis, G., Kappler, C., and
T. Phelan, "RMD-QOSM - The Resource Management in Diffserv
QOS Model", draft-ietf-nsis-rmd-06 (work in progress),
February 2006.
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
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Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC2207] Berger, L. and T. O'Malley, "RSVP Extensions for IPSEC
Data Flows", RFC 2207, September 1997.
[RFC2208] Mankin, A., Baker, F., Braden, B., Bradner, S., O'Dell,
M., Romanow, A., Weinrib, A., and L. Zhang, "Resource
ReSerVation Protocol (RSVP) Version 1 Applicability
Statement Some Guidelines on Deployment", RFC 2208,
September 1997.
[RFC2747] Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
Authentication", RFC 2747, January 2000.
[RFC2998] Bernet, Y., Ford, P., Yavatkar, R., Baker, F., Zhang, L.,
Speer, M., Braden, R., Davie, B., Wroclawski, J., and E.
Felstaine, "A Framework for Integrated Services Operation
over Diffserv Networks", RFC 2998, November 2000.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
RFC 3540, June 2003.
[Re-fb] Briscoe, B., Jacquet, A., Di Cairano-Gilfedder, C.,
Salvatori, A., Soppera, A., and M. Koyabe, "Policing
Congestion Response in an Internetwork Using Re-Feedback",
ACM SIGCOMM CCR 35(4)277--288, August 2005, <http://
www.acm.org/sigs/sigcomm/sigcomm2005/
techprog.html#session8>.
[Smart_rtg]
Goldenberg, D., Qiu, L., Xie, H., Yang, Y., and Y. Zhang,
"Optimizing Cost and Performance for Multihoming", ACM
SIGCOMM CCR 34(4)79--92, October 2004,
<http://citeseer.ist.psu.edu/698472.html>.
[Steps_DoS]
Handley, M. and A. Greenhalgh, "Steps towards a DoS-
resistant Internet Architecture", Proc. ACM SIGCOMM
workshop on Future directions in network architecture
(FDNA'04) pp 49--56, August 2004.
Appendix A. Implementation
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A.1. Ingress Gateway Algorithm for Blanking the RE flag
The ingress gateway receives regular feedback reporting the fraction
of congestion marked octets for each aggregate arriving at the
egress. So for each aggregate it should blank the RE flag on the
same fraction of octets. It is more efficient to calculate the
reciprocal of this fraction when the signalling arrives, Z_0 = (1 /
Congestion-Level-Estimate). Z_0 will be the number of octets of
packets the ingress should send with the RE flag set between those it
sends with the RE flag blanked. Z_0 will also take account of the
sustainable rate reported during the flow pre-emption process, if
necessary.
A suitable pseudo-code algorithm for the ingress gateway is as
follows:
====================================================================
B_i = 0 /* interblank volume */
for each PCN-capable packet {
b = readLength() /* set b to packet size */
B_i += b /* accumulate interblank volume */
if B_i < b * Z_0 { /* test whether interblank volume... */
writeRE(1)
} else { /* ...exceeds blank RE spacing * pkt size*/
writeRE(0) /* ...and if so, clear RE */
B_i = 0 /* ...and re-set interblank volume */
}
}
====================================================================
A.2. Downstream Congestion Metering Algorithms
A.2.1. Bulk Downstream Congestion Metering Algorithm
To meter the bulk amount of downstream pre-congestion in traffic
crossing an inter-domain border, an algorithm is needed that
accumulates the size of positive packets and subtracts the size of
negative packets. We maintain two counters:
V_b: accumulated pre-congestion volume
B: total data volume (in case it is needed)
A suitable pseudo-code algorithm for a border router is as follows:
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====================================================================
V_b = 0
B = 0
for each PCN-capable packet {
b = readLength(packet) /* set b to packet size */
B += b /* accumulate total volume */
if readEECN(packet) == (Re-Echo || FNE) {
V_b += b /* increment... */
} elseif readEECN(packet) == ( AM(-1) || PM(-1) ) {
V_b -= b /* ...or decrement V_b... */
} /*...depending on EECN field */
}
====================================================================
At the end of an accounting period this counter V_b represents the
pre-congestion volume that penalties could be applied to, as
described in Section 5.3.
For instance, accumulated volume of pre-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 pre-congestion
level of 1% on an accumulated total data volume of B = 500PB.
A.2.2. Inflation Factor for Persistently Negative Flows
The following process is suggested to complement the simple algorithm
above in order to protect against the various attacks from
persistently negative flows described in Section 5.6.1. 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
unboased estimate, but it may be possible to define other processes
that achieve similar ends.
While the above simple metering algorithm is counting the bulk of
traffic over an accounting period, the meter should also select a
subset of the whole flow ID space that is small enough to be able to
realistically measure but large enough to give a realistic sample.
Many different samples of different subsets of the ID space should be
taken at different times during the accounting period, preferably
covering the whole ID space. During each sample, the meter should
count the volume of positive packets and subtract the volume of
negative, maintaining a separate account for each flow in the sample.
It should run a lot longer than the large majority of flows, to avoid
a bias from missing the starts and ends of flows, which tend to be
positive and negative respectively.
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Once the accounting period finishes, the meter should calculate the
total of the accounts V_{bI} for the subset of flows I in the sample,
and the total of the accounts V_{fI} excluding flows with a negative
account from the subset I. Then the weighted mean of all these
samples should be taken a_S = sum_{forall I} V_{fI} / sum_{forall I}
V_{bI}.
If V_b is the result of the bulk accounting algorithm over the
accounting period (Appendix A.2.1) it can be inflated by this factor
a_S to get a good unbiased estimate of the volume of downstream
congestion over the accounting period a_S.V_b, without being polluted
by the effect of persistently negative flows.
A.3. Algorithm for Sanctioning Negative Traffic
{ToDo: Write up algorithms similar to Appendix D of [Re-TCP] for the
negative flow monitor with flow management algorithm and the variant
with bounded flow state.}
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Author's Address
Bob Briscoe
BT & UCL
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 645196
Email: bob.briscoe@bt.com
URI: http://www.cs.ucl.ac.uk/staff/B.Briscoe/
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