PCN Working Group B. Briscoe
Internet-Draft BT & UCL
Intended status: Standards Track September 13, 2008
Expires: March 17, 2009
Emulating Border Flow Policing using Re-PCN on Bulk Data
draft-briscoe-re-pcn-border-cheat-02
Status of this Memo
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Abstract
Scaling per flow admission control to the Internet is a hard problem.
The approach of combining Diffserv and pre-congestion notification
(PCN) provides a service slightly better than Intserv controlled load
that scales to networks of any size without needing Diffserv's usual
overprovisioning, but only if domains trust each other to comply with
admission control and rate policing. This memo claims to solve this
trust problem without losing scalability. It provides a sufficient
emulation of per-flow policing at borders but with only passive bulk
metering rather than per-flow processing. Measurements are
sufficient to apply penalties against cheating neighbour networks.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 8
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 11
3. The Problem . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1. The Traditional Per-flow Policing Problem . . . . . . . . 11
3.2. Generic Scenario . . . . . . . . . . . . . . . . . . . . . 14
4. Re-ECN Protocol in IP with Two Congestion Marking Levels . . . 17
4.1. Protocol Overview . . . . . . . . . . . . . . . . . . . . 17
4.2. Re-PCN Abstracted Network Layer Wire Protocol (IPv4 or
v6) . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2.1. Re-ECN Recap . . . . . . . . . . . . . . . . . . . . . 18
4.2.2. Re-ECN Combined with Pre-Congestion Notification
(re-PCN) . . . . . . . . . . . . . . . . . . . . . . . 20
4.3. Protocol Operation . . . . . . . . . . . . . . . . . . . . 22
4.3.1. Protocol Operation for an Established Flow . . . . . . 23
4.3.2. Aggregate Bootstrap . . . . . . . . . . . . . . . . . 24
4.3.3. Flow Bootstrap . . . . . . . . . . . . . . . . . . . . 26
4.3.4. Router Forwarding Behaviour . . . . . . . . . . . . . 26
4.3.5. Extensions . . . . . . . . . . . . . . . . . . . . . . 28
5. Emulating Border Policing with Re-ECN . . . . . . . . . . . . 28
5.1. Informal Terminology . . . . . . . . . . . . . . . . . . . 28
5.2. Policing Overview . . . . . . . . . . . . . . . . . . . . 30
5.3. Pre-requisite Contractual Arrangements . . . . . . . . . . 31
5.4. Emulation of Per-Flow Rate Policing: Rationale and
Limits . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.5. Sanctioning Dishonest Marking . . . . . . . . . . . . . . 36
5.6. Border Mechanisms . . . . . . . . . . . . . . . . . . . . 38
5.6.1. Border Accounting Mechanisms . . . . . . . . . . . . . 38
5.6.2. Competitive Routing . . . . . . . . . . . . . . . . . 41
5.6.3. Fail-safes . . . . . . . . . . . . . . . . . . . . . . 42
6. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
7. Incremental Deployment . . . . . . . . . . . . . . . . . . . . 46
8. Design Choices and Rationale . . . . . . . . . . . . . . . . . 47
9. Security Considerations . . . . . . . . . . . . . . . . . . . 49
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 50
11. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 50
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 51
13. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 52
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 52
14.1. Normative References . . . . . . . . . . . . . . . . . . . 52
14.2. Informative References . . . . . . . . . . . . . . . . . . 53
Appendix A. Implementation . . . . . . . . . . . . . . . . . . . 55
A.1. Ingress Gateway Algorithm for Blanking the RE flag . . . . 55
A.2. Downstream Congestion Metering Algorithms . . . . . . . . 56
A.2.1. Bulk Downstream Congestion Metering Algorithm . . . . 56
A.2.2. Inflation Factor for Persistently Negative Flows . . . 56
A.3. Algorithm for Sanctioning Negative Traffic . . . . . . . . 57
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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 57
Intellectual Property and Copyright Statements . . . . . . . . . . 59
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Status (to be removed by the RFC Editor)
The IETF PCN working group is initially chartered to consider PCN
domains only under a single trust authority. However, after its
initial work is complete the charter says the working group may re-
charter to consider concatenated Diffserv domains, amongst other new
work items. The charter ends by stating "The details of these work
items are outside the scope of the initial phase; but the WG may
consider their requirements to design components that are
sufficiently general to support such extensions in the future."
This memo is therefore contributed to describe how PCN could be
extended to inter-domain. We wanted to document the solution to
reduce the chances that something else eats up the codepoint space
needed before PCN re-charters to consider inter-domain. Losing the
chance to standardise this simple, scalable solution to the problem
of inter-domain flow admission control would be unfortunate
(understatement), given it took years to find, and even then it was
very difficult to find codepoint space for it.
The scheme described here (Section 4) requires the PCN ingress
gateway to re-echo any PCN feedback it receives back into the forward
stream of IP packets (hence we call this scheme re-PCN). Re-PCN
works in a very similar way to the re-ECN proposal on which it is
based [I-D.briscoe-tsvwg-re-ecn-tcp], the only difference being that
PCN might encode three states of congestion, whereas ECN encodes two.
This document is written to stand alone from re-ECN, so that readers
do not have to read [I-D.briscoe-tsvwg-re-ecn-tcp].
The authors seek comments from the Internet community on whether
combining PCN and re-ECN to create re-PCN in this way is a sufficient
solution to the problem of scaling microflow admission control to the
Internet as a whole. Here we emphasise that scaling is not just an
issue of numbers of flows, but also the number of security entities--
networks and users--who may all have conflicting interests.
This memo is posted as an Internet-Draft with the intent to
eventually be broken down in two documents; one for the standards
track and one for informational status. But until it becomes an item
of IETF working group business the whole proposal has been kept
together to aid understanding. Only the text of Section 4 of this
document is intended to be normative (requiring standardisation).
The rest of the sections are merely informative, describing how a
system might be built from these protocols by the operators of an
internetwork. Note in particular that the policing and monitoring
functions proposed for the trust boundaries between operators would
not need standardisation by the IETF. They simply represent one
possible way that the proposed protocols could be used to extend the
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PCN architecture [I-D.ietf-pcn-architecture] to span multiple domains
without mutual trust between the operators.
Dependencies (to be removed by the RFC Editor)
To realise the system described, this document also depends on other
documents chartered in the IETF Transport Area progressing along the
standards track:
o Pre-congestion notification (PCN) marking on interior nodes
[I-D.eardley-pcn-marking-behaviour], chartered for standardisation
in the PCN w-g;
o The baseline encoding of pre-congestion notification in the IP
header [I-D.moncaster-pcn-baseline-encoding], also chartered for
standardisation in the PCN w-g;
o Feedback of aggregate PCN measurements by suitably extending the
admission control signalling protocol (e.g. RSVP extension
[RSVP-ECN] or NSIS extension [I-D.arumaithurai-nsis-pcn]).
The baseline encoding makes no new demands on codepoint space in the
IP header but provides just two PCN encoding states (not marked and
marked). The PCN architecture recognises that operators might want
PCN marking to trigger two functions (admission control and flow
termination) at different levels of pre-congestion, which seems to
require three encoding states. A scheme has been proposed
[I-D.charny-pcn-single-marking] that can do both functions with just
two encoding states, but simulations have shown it performs poorly
under certain conditions that might be typical. As it seems likely
that PCN might need three encoding states to be fully operational, we
want to be sure that three encoding states can be extended to work
inter-domain. Therefore, we have defined a three-state extension
encoding scheme in this document, then we have added the re-PCN
scheme to it. The three-state encoding we have chosen depends on
standardisation of yet another document in the IETF Transport Area:
o Propagation beyond the tunnel decapsulator of any changes in the
ECN field to ECT(0) or ECT(1) made within a tunnel (the ideal
decapsulation rules of [I-D.briscoe-tsvwg-ecn-tunnel]);
Changes from previous drafts (to be removed by the RFC Editor)
Full diffs of incremental changes between drafts are available at
URL: <http://www.cs.ucl.ac.uk/staff/B.Briscoe/pubs.html#repcn>
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Changes from <draft-briscoe-re-pcn-border-cheat-01> to
<draft-briscoe-re-pcn-border-cheat-02> (current version):
Considerably updated the 'Status' note to explain the
relationship of this draft to other documents in the IETF
process (or not) and to chartered PCN w-g activity.
Split out the dependencies into a separate note and added
dependencies on new PCN documents in progress.
Made scalability motivation in the introduction clearer,
explaining why Diffserv over-provisioning doesn't scale unless
PCN is used.
Clarified that the standards action in Section 4 is to define
the meanings of the combination of fields in the IP header: the
RE flag and 2-level congestion marking in the ECN field. And
that it is not characterised by a particular feedback style in
the transport.
Switched round the two ECT codepoints to be compatible with the
new PCN baseline encoding and used less confusing naming for
re-PCN codepoints (Section 4).
Generalised rules for encoding probes when bootstrapping or re-
starting aggregates & flows (Section 4.3.2).
Downgraded drop sanction behaviour from MUST to conditional
SHOULD (Section 5.5).
Added incremental deployment safety justification for choice of
which way round the RE flag works (Section 7).
Added possible vulnerability to brief attacks and possible
solution to security considerations (Section 9).
Updated references and terminology, particularly taking account
of recent new PCN w-g documents;
Replaced suggested Ingress Gateway Algorithm for Blanking the
RE flag (Appendix A.1)
Clarifications throughout;
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Changes from <draft-briscoe-re-pcn-border-cheat-00> to
<draft-briscoe-re-pcn-border-cheat-01>:
Updated references.
Changes from <draft-briscoe-tsvwg-re-ecn-border-cheat-01>
to <draft-briscoe-re-pcn-border-cheat-00>:
Changed filename to associate it with the new IETF PCN w-g,
rather than the TSVWG w-g.
Introduction: Clarified that bulk policing only replaces per-
flow policing at interior inter-domain borders, while per-flow
policing is still needed at the access interface to the
internetwork. Also clarified that the aim is to neutralise any
gains from cheating using local bilateral contracts between
neighbouring networks, rather than merely identifying remote
cheaters.
Section 3.1: Described the traditional per-flow policing
problem with inter-domain reservations more precisely,
particularly with respect to direction of reservations and of
traffic flows.
Clarified status of Section 5 onwards, in particular that
policers and monitors would not need standardisation, but that
the protocol in Section 4 would require standardisation.
Section 5.6.2 on competitive routing: Added discussion of
direct incentives for a receiver to switch to a different
provider even if the provider has a termination monopoly.
Clarified that "Designing in security from the start" merely
means allowing codepoint space in the PCN protocol encoding.
There is no need to actually implement inter-domain security
mechanisms for solutions confined to a single domain.
Updated some references and added a ref to the Security
Considerations, as well as other minor corrections and
improvements.
Changes from <draft-briscoe-tsvwg-re-ecn-border-cheat-00> to
<draft-briscoe-tsvwg-re-ecn-border-cheat-01>:
Added subsection on Border Accounting Mechanisms
(Section 5.6.1)
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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.
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 [I-D.ietf-pcn-architecture] combines
Diffserv and pre-congestion notification (PCN) to provide a service
slightly better than Intserv controlled load [RFC2211]. PCN does not
require the considerable over-provisioning that is normally required
for admission control over Diffserv [RFC2998] to be robust against
re-routes or variation in the traffic matrix. It has been proved
that Diffserv's over-provisioning requirement grows linearly with the
network diameter in hops [QoS_scale].
A number of PCN domains can be concatenated into a larger PCN region
without any per-flow processing between them, but only if each domain
trusts the ingress network to have checked that upstream customers
aren't taking more bandwidth than they reserved, either accidentally
or deliberately. Unfortunately, networks can gain considerably by
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breaking this trust. One way for a network to protect itself against
others is to handle flow signalling at its own border and police
traffic against reservations itself. However, this reintroduces the
per-flow unscalability at borders that Intserv over Diffserv suffers
from.
This memo describes a protocol called re-PCN that enables bulk border
measurements 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. Per-flow rate policing
for each reservation is still expected to be used at the access edge
of the internetwork, but at the borders between networks bulk
policing can be used to emulate per-flow policing. The emulation is
not perfect, but it is sufficient to ensure that the punishment is at
least proportionate to the severity of the cheat. Re-PCN neither
requires the unscalable over-provisioning of Diffserv nor the per-
flow processing at borders of Intserv over Diffserv.
It should therefore scale controlled load service to the whole
internetwork without the cost of Diffserv's linearly increasing over-
provisioning, or the cost of per-flow policing at each border. To
achieve such scaling, this memo combines two recent proposals, both
of which it briefly recaps:
o The pre-congestion notification (PCN)
architecture[I-D.ietf-pcn-architecture] 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 [I-D.briscoe-tsvwg-re-ecn-tcp].
We coin the term re-PCN for the combination of PCN and re-ECN.
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 ensures a network can be
made to experience the congestion it causes in other networks. Re-
ECN requires the sending node to declare expected downstream
congestion in all packets and it makes it in its interest to declare
this honestly. At the border between upstream network 'A' and
downstream network 'B' (say), both networks can monitor packets
crossing the border to measure how much congestion 'A' is causing in
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'B' and beyond. 'B' can then include a limit or penalty based on
this metric in its contract with 'A'. This is how 'A' experiences
the effect of congestion it causes in other networks. 'A' no longer
gains by admitting traffic when it shouldn't, which is why we can say
re-PCN emulates flow policing, even though it doesn't measure flows.
The aim is not to enable a network to _identify_ some remote cheating
party, which would rarely be useful given the victim network would be
unlikely to be able to seek redress from a cheater in some remote
part of the world with whom no direct contractual relationship
exists. Rather the aim is to ensure that any gain from cheating will
be cancelled out by penalties applied to the cheating party by its
local network. Further, the solution ensures each of the chain of
networks between the cheater and the victim will lose out if it
doesn't apply penalties to its neighbour. Thus the solution builds
on the local bilateral contractual relationships that already exist
between neighbouring networks.
Rather than the end-to-end arrangement used when re-ECN was specified
for the TCP transport [I-D.briscoe-tsvwg-re-ecn-tcp], this memo
specifies re-ECN in an edge-to-edge arrangement, making it applicable
to deployment models where admission control over Diffserv is based
on pre-congestion notification. 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]. RSVP is used to
be concrete, but a similar deployment model, but with a different
transport for signalling congestion feedback could be used (e.g.
Arumaithurai [I-D.arumaithurai-nsis-pcn] and RMD [I-D.ietf-nsis-rmd]
both use NSIS).
This memo aims to do two things: i) define how to apply the re-PCN
protocol to the admission control over Diffserv scenario; and ii)
explain why re-PCN sufficiently emulates border policing in that
scenario. Most of the memo is taken up with the second aim;
explaining why it works. Applying re-PCN to the scenario actually
involves quite a trivial modification to the ingress gateway. That
modification can be added to gateways later, so our immediate goal is
to convince everyone to have the foresight to define the PCN wire
protocol encoding to accommodate the extended codepoints defined in
this document, whether first deployments require border 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 "Design 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
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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-PCN 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
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]), even for sets of flows running alongside another set
across the same border but using the more traditional model that
depends on more costly per-flow processing at each border.
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, we will start from the traditional scenario with per-
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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-destination pair 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. Because, 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-destination pair could reserve resources for an
8kbps audio flow but the source could transmit a 6Mbps video (theft
of service). More subtly, the sender could generate bursts that were
outside the profile 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
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 the 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, 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
the charges for buying from the transit and selling to the retail
customer 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 (e.g. the
SENDER_TSPEC object in RSVP's PATH message) before forwarding them to
its transit provider and systematically reinstate the responses on
the way back (e.g. the FLOWSPEC object in RSVP's RESV message). It
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would then receive an honest income from its upstream retail customer
but only pay for fraudulently smaller reservations downstream. A
similar but opposite trick (increasing the TSPEC and decreasing the
FLOWSPEC) could be perpetrated by the receiver's access network if
the reservation was paid for by the receiver.
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 traffic 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
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.
All the above cheats are framed in the context of RSVP's receiver
confirmed reservation model, but similar cheats are possible with
sender-initiated and other models.
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.
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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.
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| | | | _|__ ______ ______ ______ _|__ | | | |
| | | | | | | | | | | | | | | | | |
| | | | | | |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)
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 will
typically be other interior routers (not shown) within each interior
domain.
In two paragraphs we now briefly recap how pre-congestion
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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 [I-D.ietf-pcn-architecture] 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 and colours conforming traffic within an admitted
reservation to a combination of Diffserv codepoint and ECN field that
defines the traffic as 'PCN-enabled'. This redefines the meaning of
the ECN field as a PCN field, which is largely the same as ECN
[RFC3168], but with slightly different semantics defined in
[I-D.moncaster-pcn-baseline-encoding] (or various extensions that are
currently experimental). The Diffserv region is called a PCN-region
because all the queues within it are PCN-enabled. This means the
per-hop behaviour they apply to PCN-enabled traffic consists of both
a scheduling behaviour and a new ECN marking behaviour that we call
`pre-congestion notification' [I-D.eardley-pcn-marking-behaviour]. A
PCN-enabled queue typically re-uses the definition of expedited
forwarding (EF) [RFC3246] for its scheduling behaviour. The new
congestion marking behaviour sets the PCN field of an increasing
proportion of PCN packets to the PCN-marked (PM) codepoint
[I-D.moncaster-pcn-baseline-encoding] as their load approaches a
threshold rate that is lower than the line rate
[I-D.eardley-pcn-marking-behaviour]. This can be achieved with an
algorithm similar to a token-bucket called a virtual queue. The aim
is for a queue to start marking PCN traffic to trigger admission
control before the real queue builds up any congestion delay. The
level of a queue's pre-congestion marking is detected at the egress
of the Diffserv region and used by the signalling system to control
admission of further traffic that would otherwise overload that
queue, as follows.
The end-to-end QoS signalling for a new reservation (to be concrete
we will use RSVP) 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 (an ingress-egress-aggregate). For each ingress-egress-
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
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message is returned. As the RESV message passes, the egress gateway
piggy-backs the relevant 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 PCN-region with a Diffserv codepoint that gives it
priority over the capacity used for PCN traffic.
Even if some disaster re-routes traffic after it has been admitted,
if the PCN traffic through any PCN resource tips over a higher, fail-
safe threshold, pre-congestion notification can trigger flow
termination to very quickly bring every router within the whole PCN-
region back below its operating point. The same marking process and
ECN codepoint can be used for both admission control and flow
termination, by simply triggering them at different fractions of
marking [I-D.charny-pcn-single-marking]. However simulations have
confirmed that this approach is not robust in all circumstances that
might typically be encountered, so approaches with two thresholds and
two congestion encodings are expected to be required in production
networks.
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. Otherwise the full scalability
potential of pre-congestion notification would be limited by the need
for per-flow policing mechanisms at borders, which would make borders
the most cost-critical pinch-points. Instead we can achieve the long
sought-for vision of secure Internet-wide bandwidth reservations
without over-generous provisioning or per-flow processing. We still
use per-flow processing at the edge routers closest to the end-user,
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but we need no per-flow processing at all in core _or border
routers_--where scalability is most critical.
4. Re-ECN Protocol in IP with Two Congestion Marking Levels
4.1. Protocol Overview
First we need to recap the way routers accumulate PCN congestion
marking along a path (it accumulates the same way as ECN). Each PCN-
capable queue into a link might mark some packets with a PCN-marked
(PM) codepoint, the marking probability increasing with the length of
the queue [I-D.eardley-pcn-marking-behaviour]. With a series of PCN-
capable routers on a path, a stream of packets accumulates the
fraction of PCN markings that each queue adds. The combined effect
of the packet marking of all the queues along the path signals
congestion of the whole path to the receiver. So, for example, if
one queue early in a path is marking 1% of packets and another later
in a path is marking 2%, flows that pass through both queues will
experience approximately 3% marking over a sequence of packets.
(Note: Whenever the word 'congestion' is used in this document it
should be taken to mean congestion of the virtual resource assigned
for use by PCN-traffic. This avoids cumbersome repetition of the
strictly correct term 'pre-congestion'.)
The packets crossing an inter-domain trust boundary within the PCN-
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 packets it forwards into any of the downstream paths between
itself and the egress gateways that each packet is destined for. The
purpose of the re-PCN protocol is to make packets automatically carry
this information, which then merely needs to be counted locally at
the border.
With the original PCN protocol, if a border router, e.g. that between
domains 'A' & 'B' Figure 2), counts PCN markings crossing the border
over a period, they represent the accumulated congestion that has
already been experienced by those packets (congestion upstream of the
border, u). The idea of re-PCN is to make the ingress gateway
continuously encode the path congestion it knows into a new field in
the IP header (in this case, `path' means the path from the ingress
to the egress gateway). This new field is _not_ altered by queues
along the path. Then at any point on that path (e.g. between domains
'A' & 'B'), IP headers can be monitored to measure both expected path
congestion, p and upstream congestion, u. Then congestion expected
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downstream of the border, v, can be derived simply by subtracting
upstream congestion from expected path congestion. That is v ~= p -
u.
Importantly, it turns out that there is no need to monitor downstream
congestion on a per-flow, per-path or per-aggregate basis. We will
show that accounting for it in bulk by counting the volume of all
marked packet will be sufficient.
_____________________________________
_|__ ______ ______ ______ _|__
| | | 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-PCN Abstracted Network Layer Wire Protocol (IPv4 or v6)
In this section we define the names of the various codepoints of the
extended ECN field 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
[I-D.briscoe-tsvwg-re-ecn-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. [I-D.briscoe-tsvwg-re-ecn-tcp] proposes using bit 48
(currently unused) in the IPv4 header for the RE flag, while for IPv6
it proposes an congestion extension header.
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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 is the 'sender' as
far as the scope of the PCN region is concerned, so it sets the RE
flag (as permitted for sender proxies 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
gateways do because in the edge-to-edge deployment model we are
using, they act as the endpoints of the PCN region. 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 [RFC5129], 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
[I-D.briscoe-tsvwg-re-ecn-tcp] proposes the following six codepoint
names for when there is a need to be more specific.
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+--------+-------------+-------+-------------+----------------------+
| ECN | RFC3168 | RE | Extended | Re-ECN meaning |
| field | codepoint | flag | ECN | |
| | | | codepoint | |
+--------+-------------+-------+-------------+----------------------+
| 00 | Not-ECT | 0 | Not-RECT | Not re-ECN-capable |
| | | | | transport |
| 00 | Not-ECT | 1 | FNE | Feedback not |
| | | | | established |
| 10 | ECT(0) | 0 | --- | Legacy ECN use |
| | | | | only |
| 10 | ECT(0) | 1 | --CU-- | Currently unused |
| | | | | |
| 01 | ECT(1) | 0 | Re-Echo | Re-echoed congestion |
| | | | | and RECT |
| 01 | ECT(1) | 1 | RECT | Re-ECN capable |
| | | | | transport |
| 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] and by the guidelines
for specifying alternative semantics for the ECN field [RFC4774], a
proposal is currently being advanced in the IETF to define different
semantics for how queues might mark the ECN field of certain packets.
The idea is to be able to notify congestion when the queue's load
approaches a logical limit, rather than the physical limit of the
line. This new marking is called pre-congestion
notification [I-D.eardley-pcn-marking-behaviour] and we will use the
term PCN-enabled queue for a queue 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 PCN-capable
packet must meet two conditions; it must carry a DSCP that has been
associated with PCN marking and it must carry an ECN field that turns
on PCN marking.
As an example, a packet carrying the VOICE-ADMIT
[I-D.ietf-tsvwg-admitted-realtime-dscp] DSCP would be associated with
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expedited forwarding [RFC3246] as its scheduling behaviour and pre-
congestion notification as its congestion marking behaviour. PCN
would only be turned on within a PCN-region by an ECN codepoint other
than Not-ECT (00). Then we would describe packets with the VOICE-
ADMIT DSCP and with ECN turned on as PCN-capable packets.
[I-D.eardley-pcn-marking-behaviour] actually proposes that two
logical limits can 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 termination.
Because it needs two types of congestion marking, PCN needs four
states: Not PCN-capable (Not-PCN), PCN-capable but not PCN-marked
(NM), Admission Marked (AM) and Flow Termination Marked (TM). A
proposed encoding of the four required PCN states is shown on the
left of Table 2. Note that these codepoints of the ECN field only
take on the semantics of pre-congestion notification if they are
combined with a Diffserv codepoint that the operator has configured
to be associated with PCN marking.
This encoding only correctly traverses an IP in IP tunnel if the
ideal decapsulation rules in [I-D.briscoe-tsvwg-ecn-tunnel] are
followed when combining the ECN fields of the outer and inner
headers. If instead the decapsulation rules in [RFC3168] or
[RFC4301] are followed, any admission marking applied to an outer
header will be incorrectly removed on decapsulation at the tunnel
egress.
The RFC3168 ECN field includes space for the experimental ECN
Nonce [RFC3540], which seems to require a fifth state if it is also
needed with re-PCN. But re-PCN supersedes any need for the Nonce
within the PCN-region. 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 in our case the PCN ingress) 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). Also, the ECN
nonce only works once the receiver has placed packets in the same
order as they left the ingress, which cannot be done by an edge node
without adding unnecessary edge-edge packet ordering. Nonetheless,
if the ECN nonce were in use outside the PCN region (end-to-end), the
ingress would have to tunnel the arriving IP header across the PCN
region ([I-D.ietf-pcn-architecture]).
For the rest of this memo, to mean either Admission Marking or
Termination Marking we will call both "congestion marking" or "PCN
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marking" unless we need to be specific. With the above encoding,
congestion marking can be read to mean any packet with the right-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 PCN (EPCN) field are
defined on the right of Table 2 below. The purposes of these
different codepoints will be introduced in subsequent sections.
+--------+-----------+-------+-----------------+--------------------+
| ECN | PCN | RE | Extended PCN | Re-PCN meaning |
| field | codepoint | flag | codepoint | |
+--------+-----------+-------+-----------------+--------------------+
| 00 | Not-PCN | 0 | Not-PCN | Not PCN-capable |
| | | | | transport |
| 00 | Not-PCN | 1 | FNE | Feedback not |
| | | | | established |
| 10 | NM | 0 | Re-PCT-Echo | Re-echoed |
| | | | | congestion and |
| | | | | Re-PCT |
| 10 | NM | 1 | Re-PCT | Re-PCN capable |
| | | | | transport |
| 01 | AM | 0 | AM(0) | Admission Marking |
| | | | | with Re-Echo |
| 01 | AM | 1 | AM(-1) | Admission Marking |
| | | | | |
| 11 | TM | 0 | TM(0) | Termination |
| | | | | Marking with |
| | | | | Re-Echo |
| 11 | TM | 1 | TM(-1) | Termination |
| | | | | Marking |
+--------+-----------+-------+-----------------+--------------------+
Table 2: Extended ECN Codepoints if the Diffserv codepoint uses Pre-
congestion Notification (PCN)
Note that Table 2 shows re-PCN uses ECT(0) but Table 1 shows re-ECN
uses ECT(1) for the unmarked state. The difference is intended--
although it makes it harder to remember the two schemes, it makes
them both safer during incremental deployment.
4.3. Protocol Operation
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4.3.1. Protocol Operation for an Established Flow
The re-PCN protocol involves a simple addition to the action of the
gateway at the ingress edge of the PCN region (the PCN-ingress-node).
But first we will recap how PCN works without the addition. For each
active traffic aggregate across a PCN region (ingress-egress-
aggregate) the egress gateway measures the level of PCN marking and
feeds it back to the ingress piggy-backed as 'PCN-feedback-
information' on any control signal passing between the nodes (e.g.
every flow set-up, refresh or tear-down). Therefore the ingress
gateway will always hold a fairly recent (typically at most 30sec)
estimate of the ingress-egress-aggregate congestion level. For
instance, one aggregate might have been experiencing 3% pre-
congestion (that is, congestion marked octets whether Admission
Marked or Termination Marked).
To comply with the re-PCN protocol, for all PCN packets in each
ingress-egress-aggregate the ingress gateway MUST clear the RE flag
to "0" for the same percentage of octets as its current estimate of
congestion on the aggregate (e.g. 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 Section 7). 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
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index of each congestible resource (typically queues) along a path
through the Internet. The two superimposed plots show the fraction
of each extended PCN 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 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 [I-D.briscoe-tsvwg-re-ecn-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. Then at any border within
the PCN-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. In the case of RSVP reservation signalling, while
the signal continues onward towards the receiving host, the egress
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gateway can return an RSVP message to the ingress with a
flag [RSVP-ECN] asking the ingress to send a specified number of data
probes between them. The more general possibilities for bootstrap
behaviour are described in the PCN
architecture [I-D.ietf-pcn-architecture], including using the
reservation signal itself as a probe.
However, with our new re-PCN scheme, the ingress does not know what
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 PCN-region.
To be conservative, following the guidance for specifying other re-
ECN transports in [I-D.briscoe-tsvwg-re-ecn-tcp], the ingress SHOULD
set the FNE codepoint of the extended PCN header in all probe packets
(Table 2). As per the PCN 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 carry the FNE
codepoint.
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 PCN-region has to
be explicitly configured to support the admission controlled service.
So, before deploying the service, the operator MUST reconfigure such
a badly implemented 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. If not all probe packets are set to FNE, the ingress will
have to ensure probe packets are identifiable by some other means,
perhaps by using the egress as the destination address.
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.
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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
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 EPCN field of the next packet it sends to FNE. This
allows the design of network policers to be deterministic (see
[I-D.briscoe-tsvwg-re-ecn-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-PCN works well with the regular PCN forwarding behaviour of
interior queues. However, below, two optional changes are proposed
when forwarding packets with a per-hop-behaviour that requires pre-
congestion notification:
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Preferential drop: When a router cannot avoid dropping PCN-capable
packets, preferential dropping of packets with different extended
PCN 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-PCT-Echo and FNE MAY be given the same drop preference, but if
feasible, FNE SHOULD be dropped in preference to Re-PCT-Echo.
If this proposal were advanced at the same time as PCN itself, we
would recommend that preferential drop based on extended PCN
codepoint SHOULD be added to router forwarding at the same time as
PCN marking. Preferential dropping can be difficult to implement,
but we RECOMMEND this security-related re-PCN 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-PCN. 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-PCN 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.
PCN marking rather than dropping of FNE packets MUST only be
deployed in controlled environments, such as that in
[I-D.ietf-pcn-architecture], where the presence of an egress node
that understands PCN marking is assured. Congestion events might
otherwise be ignored if the receiver only understands drop, rather
than PCN marking. This is because there is no guarantee that PCN
capability has been negotiated if feedback is not established
(FNE). Also, [I-D.briscoe-tsvwg-re-ecn-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.
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+---------+-------+-----------------+---------+---------------------+
| PCN | RE | Extended PCN | Drop | Re-PCN meaning |
| field | flag | codepoint | Pref | |
+---------+-------+-----------------+---------+---------------------+
| 10 | 0 | Re-PCT-Echo | 5/4 | Re-echoed |
| | | | | congestion and |
| | | | | Re-PCT |
| 00 | 1 | FNE | 4 | Feedback not |
| | | | | established |
| 10 | 1 | Re-PCT | 3 | Re-PCN capable |
| | | | | transport |
| 01 | 0 | AM(0) | 3 | Admission Marking |
| | | | | with Re-Echo |
| 01 | 1 | AM(-1) | 3 | Admission Marking |
| | | | | |
| 11 | 0 | TM(0) | 2 | Termination Marking |
| | | | | with Re-Echo |
| 11 | 1 | TM(-1) | 2 | Termination Marking |
| | | | | |
| 00 | 0 | Not-PCN | 1 | Not PCN-capable |
| | | | | transport |
+---------+-------+-----------------+---------+---------------------+
Table 4: Drop Preference of Extended ECN Codepoints (1 = drop 1st)
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. Arumaithurai [I-D.arumaithurai-nsis-pcn] or
RMD [I-D.ietf-nsis-rmd]), we believe re-PCN could be used to protect
against misbehaving networks in the same way as proposed above.
5. Emulating Border Policing with Re-ECN
The following sections are informative, not normative. The re-PCN
protocol described in Section 4 above would require standardisation,
whereas operators acting in their own interests would be expected to
deploy policing and monitoring functions similar to those proposed in
the sections below without any further need for standardisation by
the IETF. Flexibility is expected in exactly how policing and
monitoring is done.
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
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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 initialised by the
PCN ingress 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 termination marking). Congestion
marking a previously blanked packet cancels out the positive worth
with the negative worth of the congestion marking (resulting in a
packet worth 0). The FNE codepoint is an exception. It has the same
positive worth as a packet with the Re-PCT-Echo codepoint. The table
below specifies unambiguously the worth of each extended PCN
codepoint. Note the order is different from the previous table to
emphasise how congestion marking processes decrement the worth (with
the exception of FNE).
+---------+-------+------------------+-------+----------------------+
| ECN | RE | Extended PCN | Worth | Re-PCN meaning |
| field | flag | codepoint | | |
+---------+-------+------------------+-------+----------------------+
| 00 | 0 | Not-PCN | n/a | Not PCN-capable |
| | | | | transport |
| 10 | 0 | Re-PCT-Echo | +1 | Re-echoed congestion |
| | | | | and Re-PCT |
| 01 | 0 | AM(0) | 0 | Admission Marking |
| | | | | with Re-Echo |
| 11 | 0 | TM(0) | 0 | Termination Marking |
| | | | | with Re-Echo |
| 00 | 1 | FNE | +1 | Feedback not |
| | | | | established |
| 10 | 1 | Re-PCT | 0 | Re-PCN capable |
| | | | | transport |
| 01 | 1 | AM(-1) | -1 | Admission Marking |
| | | | | |
| 11 | 1 | TM(-1) | -1 | Termination Marking |
+---------+-------+------------------+-------+----------------------+
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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.
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 (strictly by any
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).
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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 limit the
required flow state, preventing state exhaustion attacks.
Of course, some domains may trust other domains to comply with
admission control without applying sanctions or penalties. In these
cases, the protocol should still be used but no penalties need be
applied. The re-PCN 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-PCN 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-PCN protocol has been chosen to solve the policing problem
because it embeds a downstream pre-congestion metric in passing PCN
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 (PCN) traffic.
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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 or over
temporarily congested links counts very highly. The re-PCN 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
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
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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
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 negative penalties must never be paid for negative
congestion.
Also note that at the last egress of the PCN-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 bulk border policing
can at least work with the one model we have just outlined. Of
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course, operators are free to complement this pre-congestion-based
usage element of their charges with traditional capacity charging,
and we expect they will. But if operators don't want to use this
business model at all, they don't have to do bulk border policing.
We also assume that operators might experiment with the metric in
other models.
Also note well that everything we discuss in this memo only concerns
interconnection within the PCN-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.
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
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whatever threshold is chosen rises very quickly with a small amount
of cheating.
These are the properties that allow re-PCN 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.
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 _avoid_ 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 PCN-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.
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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 PCN traffic leaves the last network before the egress gateway
(domain 'C' in Figure 4) 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 signalling of new and refreshed reservations).
To protect itself, domain 'C' should install a monitor at its egress.
It aims to detect flows of PCN 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 If drop is used as a sanction, it SHOULD 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.
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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
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
[I-D.briscoe-tsvwg-re-ecn-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.
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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-PCN 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
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-PCN 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-
PCT-Echo and FNE), and subtracting the volume of those with negative
worth: AM(-1) and TM(-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
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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.
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-PCN 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
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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 PCN-region. In this
case, the edge ingress gateways insulate the PCN-region from DoS by
Internet hosts. Re-ECN resists more general DoS attacks, but this is
discussed in [I-D.briscoe-tsvwg-re-ecn-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
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 a 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.
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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-PCN has been designed so that flows can be dropped
solely based on locally measurable evidence. A message hinting that
a flow should be watched closely to test for negativity is fine. But
not a message that claims that a positive flow will go negative
later, so it should be dropped.
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.
The competitive effect of interdomain routing might be weaker nearer
to the egress. For instance, 'C' may be the only route 'B' can take
to reach the ultimate receiver. And if 'C' over-penalises 'B', the
egress gateway and the ultimate receiver seem to have no incentive to
move their terminating attachment to another network, because only
'B' and those upstream of 'B' suffer the higher penalties. However,
we must remember that we are only looking at the money flows at the
unidirectional network layer. There are likely to be all sorts of
higher level business models constructed over the top of these low
level 'sender-pays' penalties. For instance, we might expect a
session layer charging model where the session originator pays for a
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pair of duplex flows, one as receiver and one as sender.
Traditionally this has been a common model for telephony and we might
expect it to be used, at least sometimes, for other media such as
video. Wherever such a model is used, the data receiver will be
directly affected if its sessions terminate through a network like
'C' that fakes congestion to over-penalise 'B'. So end-customers
will experience a direct competitive pressure to switch to cheaper
networks, away from networks like 'C' that try to over-penalise 'B'.
This memo does not need to standardise any particular mechanism for
routing based on re-PCN. 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-PCN
measured locally 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 or re-PCN, 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
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:
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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.
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.
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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 PCN-region, we assume
the operators of networks outside the region will be doing per-flow
policing. That is, we assume the networks outside the PCN-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-PCN
protocol has two main effects:
o The more pre-congestion there is on a path across the PCN-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
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.
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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 PCN-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
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.
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7. Incremental Deployment
We believe ECN has so far not been widely deployed because it
requires end system and widespread 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 "Design in
security from the start" when admission control is based on pre-
congestion notification. The proposal has been designed so that
security can be added some time after first deployment, but only if
the PCN wire protocol encoding is defined with the foresight to
accommodate the extended set of codepoints defined in this document.
Given admission control based on pre-congestion notification requires
few changes to standards, it should be deployable fairly soon.
However, re-PCN 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-PCN, 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 [RFC4727] 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. Therefore, for IPv4 at
least, we will need to find a way to run an experiment so that the
header fields it uses can be reclaimed if the experiment is not a
success.
If PCN-based admission control is deployed before re-PCN is
standardised into IP, wherever a network (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-PCN is
eventually standardised into IP, it will be possible for these
separate regions to upgrade all their ingress gateways to support re-
PCN 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 expect incremental deployment of re-PCN
will be feasible on a domain-by domain basis, without needing to
cater for partial deployment of re-PCN in just some of the gateways
around one PCN-domain.
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Nonetheless, if the upgrade of one ingress gateway is accidentally
overlooked, the RE flag has been defined the safe way round for the
default legacy behaviour (leaving RE cleared as "0"). A legacy
ingress will appear to be declaring a high level of pre-congestion
into the aggregate. The fail-safe border mechanism in Section 5.6.3
might trigger management alarms (which would help in tracking down
the need to upgrade the ingress), but all packets would continue to
be delivered safely, as overstatement of downstream congestion
requires no sanction.
Only the ingress edge gateways around a PCN-region have to be
upgraded to add re-PCN support, not interior routers. It is also
necessary to add the mechanisms that monitor re-PCN 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 as it
crosses the physical interface between networks. This is achieved by
bringing information about congestion of resources later on the path
to the interface, rather than trying to deal with congestion where it
happens by examining the notoriously unreliable source address in
packets. 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 makes a source to act conservatively before it gets
feedback.
On the subject of lack of feedback, the feedback not established
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(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 developed 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
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 PCN-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-PCN 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.
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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
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.
o 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.
o Given the congestion feedback system is piggy-backed on flow
signalling, which can be fairly infrequent, sanctions may not be
appropriate until a flow has been persistently negative for
perhaps 20s. This may allow brief attacks to go unpunished.
However, vulnerability to brief attacks may be reduced if the
egress triggers asynchronous feedback when the congestion level on
an aggregate has risen sufficiently since the last feedback,
rather than waiting for the next opportunity to piggy-back on a
signal.
o 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
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termination behaviour. This is because a user will tend to
perceive much greater loss in value if a flow is terminated than
if admission is denied at the start. However, in general the
incentives for correct flow termination 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 (see Appendix B of
[I-D.briscoe-tsvwg-re-ecn-tcp]).
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
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).
Therefore, no cryptographic algorithms have been exploited in the
making of this proposal.
10. IANA Considerations
This memo includes no request to IANA.
11. Conclusions
This memo solves the classic problem of making flow admission control
scale to any size network. It builds on a technique, called PCN,
which involves the use of Diffserv in a domain and uses pre-
congestion notification feedback to control admission into each
network path across the domain [I-D.ietf-pcn-architecture].
Without PCN, Diffserv requires over-provisioning that must grow
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linearly with network diameter to cater for variation in the traffic
matrix. However, even with PCN, multiple network domains can only
join together into one larger PCN region if all domains trust each
other to comply with the protocols, invoking admission control and
flow termination when requested. Domains could join together and
still police flows at their borders by requiring reservation
signalling to touch each border and only use PCN internally to each
domain. But the per-flow processing at borders would still limit
scalability.
Instead, this memo proposes a technique called re-PCN which enables a
PCN region to extend across multiple domains, without unscalable per-
flow processing at borders, and still without the need for linear
growth in capacity over-provisioning as the hop-diameter of the
Diffserv region grows.
We propose that the congestion feedback used for PCN-based admission
control 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 ensuring the PCN wire protocol encoding can
accommodate the extended set of codepoints defined in this document,
even if per-flow policing is used at first rather than the bulk
border policing described here. This way, we will not build
ourselves tomorrow's legacy problem.
Re-echoing congestion feedback is based on a principled technique
called Re-ECN [I-D.briscoe-tsvwg-re-ecn-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 or it uses extension header in
IPv6.
12. Acknowledgements
All the following have given helpful comments either on re-PCN or on
relevant parts of re-ECN that re-PCN uses: Arnaud Jacquet, Alessandro
Salvatori, Steve Rudkin, David Songhurst, John Davey, Ian Self,
Anthony Sheppard, Carla Di Cairano-Gilfedder (BT), Mark Handley (who
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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, Broadband and DoS-Resistant
Internet working groups.
13. Comments Solicited
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF Congestion and Pre-Congestion Notification
working group's mailing list <pcn@ietf.org>, and/or to the author(s).
14. References
14.1. Normative References
[I-D.briscoe-tsvwg-ecn-tunnel]
Briscoe, B., "Layered Encapsulation of Congestion
Notification", draft-briscoe-tsvwg-ecn-tunnel-01 (work in
progress), July 2008.
[I-D.briscoe-tsvwg-re-ecn-tcp]
Briscoe, B., Jacquet, A., Moncaster, T., and A. Smith,
"Re-ECN: Adding Accountability for Causing Congestion to
TCP/IP", draft-briscoe-tsvwg-re-ecn-tcp-06 (work in
progress), August 2008.
[I-D.eardley-pcn-marking-behaviour]
Eardley, P., "Marking behaviour of PCN-nodes",
draft-eardley-pcn-marking-behaviour-01 (work in progress),
June 2008.
[I-D.moncaster-pcn-baseline-encoding]
Moncaster, T., Briscoe, B., and M. Menth, "Baseline
Encoding and Transport of Pre-Congestion Information",
draft-moncaster-pcn-baseline-encoding-02 (work in
progress), July 2008.
[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.
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[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.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, November 2006.
14.2. Informative References
[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>.
[I-D.arumaithurai-nsis-pcn]
Arumaithurai, M., "NSIS PCN-QoSM: A Quality of Service
Model for Pre-Congestion Notification (PCN)",
draft-arumaithurai-nsis-pcn-00 (work in progress),
September 2007.
[I-D.charny-pcn-single-marking]
Charny, A., Zhang, X., Faucheur, F., and V. Liatsos, "Pre-
Congestion Notification Using Single Marking for Admission
and Termination", draft-charny-pcn-single-marking-03
(work in progress), November 2007.
[I-D.ietf-nsis-rmd]
Bader, A., "RMD-QOSM - The Resource Management in Diffserv
QOS Model", draft-ietf-nsis-rmd-12 (work in progress),
November 2007.
[I-D.ietf-pcn-architecture]
Eardley, P., "Pre-Congestion Notification (PCN)
Architecture", draft-ietf-pcn-architecture-06 (work in
progress), September 2008.
[I-D.ietf-tsvwg-admitted-realtime-dscp]
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Baker, F., Polk, J., and M. Dolly, "DSCPs for Capacity-
Admitted Traffic",
draft-ietf-tsvwg-admitted-realtime-dscp-04 (work in
progress), February 2008.
[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>.
[QoS_scale]
Reid, A., "Economics and Scalability of QoS Solutions", BT
Technology Journal (BTTJ) 23(2)97--117, April 2005.
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
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.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.
[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, January 2008.
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[RSVP-ECN]
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-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
A.1. Ingress Gateway Algorithm for Blanking the RE flag
The ingress gateway receives regular feedback 'PCN-feedback-
information' 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 this fraction of octets. A suitable
pseudo-code algorithm for the ingress gateway is as follows:
====================================================================
for each PCN-capable-packet {
if RAND(0,1) <= PCN-feedback-information
writeRE(0);
else
writeRE(1);
}
====================================================================
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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:
====================================================================
V_b = 0
B = 0
for each PCN-capable packet {
b = readLength(packet) /* set b to packet size */
B += b /* accumulate total volume */
if readEPCN(packet) == (Re-PCT-Echo || FNE) {
V_b += b /* increment... */
} elseif readEPCN(packet) == ( AM(-1) || TM(-1) ) {
V_b -= b /* ...or decrement V_b... */
} /*...depending on EPCN 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 = 5TB (terabyte = 10^12 byte).
This might have resulted from an average downstream pre-congestion
level of 0.001% on an accumulated total data volume of B = 500PB
(petabyte = 10^15 byte).
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
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unbiased estimate, but it may be possible to define other processes
that achieve similar ends.
While the above simple metering algorithm (Appendix A.2) is counting
the bulk of traffic over an accounting period, the meter should also
select a subset of the whole flow ID space that is small enough to be
able to realistically measure but large enough to give a realistic
sample. Many different samples of different subsets of the ID space
should be taken at different times during the accounting period,
preferably covering the whole ID space. During each sample, the
meter should count the volume of positive packets and subtract the
volume of negative, maintaining a separate account for each flow in
the sample. It should run a lot longer than the large majority of
flows, to avoid a bias from missing the starts and ends of flows,
which tend to be positive and negative respectively.
Once the accounting period finishes, the meter should calculate the
total of the accounts V_{bI} for the subset of flows I in the sample,
and the total of the accounts V_{fI} excluding flows with a negative
account from the subset I. Then the weighted mean of all these
samples should be taken a_S = sum_{forall I} V_{fI} / sum_{forall I}
V_{bI}.
If V_b is the result of the bulk accounting algorithm over the
accounting period (Appendix 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 E of
[I-D.briscoe-tsvwg-re-ecn-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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