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Versions: 00 01 02 03 draft-moncaster-conex-problem

Congestion Exposure                                    T. Moncaster, Ed.
Internet-Draft                                                   L. Krug
Intended status: Informational                                        BT
Expires: April 29, 2010                                         M. Menth
                                                 University of Wuerzburg
                                                               J. Araujo
                                                                S. Blake
                                                        Extreme Networks
                                                               R. Woundy
                                                        October 26, 2009

            The Need for Congestion Exposure in the Internet

Status of This Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on April 29, 2010.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).

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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.


   Today's Internet is a product of its history.  TCP is the main
   transport protocol responsible for sharing out bandwidth and
   preventing a recurrence of congestion collapse while packet drop is
   the primary signal of congestion at bottlenecks.  Since packet drop
   (and increased delay) impacts all their customers negatively, network
   operators would like to be able to distinguish between overly
   aggressive congestion control and a confluence of many low-bandwidth,
   low-impact flows.  But they are unable to see the actual congestion
   signal and thus, they have to implement bandwidth and/or usage limits
   based on the only information they can see or measure (the contents
   of the packet headers and the rate of the traffic).  Such measures
   don't solve the packet-drop problems effectively and are leading to
   calls for government regulation (which also won't solve the problem).

   We propose congestion exposure as a possible solution.  This allows
   packets to carry an accurate prediction of the congestion they expect
   to cause downstream thus allowing it to be visible to ISPs and
   network operators.  This memo sets out the motivations for congestion
   exposure and introduces a strawman protocol designed to achieve
   congestion exposure.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Changes from previous versions . . . . . . . . . . . . . .  6
   2.  The Problem  . . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.1.  Congestion is not the problem  . . . . . . . . . . . . . .  7
     2.2.  Increase capacity or manage traffic? . . . . . . . . . . .  7
       2.2.1.  Making Congestion Visible  . . . . . . . . . . . . . .  8
       2.2.2.  ECN - a Step in the Right Directions . . . . . . . . .  8
   3.  Existing Approaches to Traffic Control . . . . . . . . . . . .  9
     3.1.  Layer 3 Measurement  . . . . . . . . . . . . . . . . . . .  9
       3.1.1.  Volume Accounting  . . . . . . . . . . . . . . . . . .  9
       3.1.2.  Rate Measurement . . . . . . . . . . . . . . . . . . . 10
     3.2.  Higher Layer Discrimination  . . . . . . . . . . . . . . . 10
       3.2.1.  Bottleneck Rate Policing . . . . . . . . . . . . . . . 10
       3.2.2.  DPI and Application Rate Policing  . . . . . . . . . . 11
   4.  Why Now? . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   5.  Requirements for a Solution  . . . . . . . . . . . . . . . . . 12
   6.  A Strawman Congestion Exposure Protocol  . . . . . . . . . . . 14
   7.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     7.1.  Improved Policing  . . . . . . . . . . . . . . . . . . . . 16
       7.1.1.  Per Aggregate Policing . . . . . . . . . . . . . . . . 16
       7.1.2.  Per customer policing  . . . . . . . . . . . . . . . . 16
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   10. Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 18
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   12. Informative References . . . . . . . . . . . . . . . . . . . . 19

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1.  Introduction

   The Internet has grown from humble origins to become a global
   phenomenon with billions of end-users able to share the network and
   exchange data and more.  One of the key elements in this success has
   been the use of distributed algorithms such as TCP that share
   capacity while avoiding congestion collapse.  These algorithms rely
   on the end-systems altruistically reducing their transmission rate in
   response to any congestion they see.

   In recent years ISPs have seen a minority of users taking a larger
   share of the network by using applications that transfer data
   continuously for hours or even days at a time and even opening
   multiple simultaneous TCP connections.  This issue became prevalent
   with the advent of "always on" broadband connections.  Frequently
   peer to peer protocols have been held responsible [RFC5594] but
   streaming video traffic is becoming increasingly significant.  In
   order to improve the network experience for the majority of their
   customers, many ISPs have chosen to impose controls on how their
   network's capacity is shared rather than continually buying more
   capacity.  They calculate that most customers will be unwilling to
   contribute to the cost of extra shared capacity if that will only
   really benefit a minority of users.  Approaches include volume
   counting or charging, and application rate limiting.  Typically these
   traffic controls, whilst not impacting most customers, set a
   restriction on a customer's level of network usage, as defined in a
   "fair usage policy".

   We believe that such traffic controls seek to control the wrong
   quantity.  What matters in the network is neither the volume of
   traffic nor the rate of traffic, it is the contribution to congestion
   over time - congestion means that your traffic impacts other users,
   and conversely that their traffic impacts you.  So if there is no
   congestion there need not be any restriction on the amount a user can
   send; restrictions only need to apply when others are sending traffic
   such that there is congestion.  In fact some of the current work at
   the IETF [LEDBAT] and IRTF [CC-open-research] already reflects this
   thinking.  For example, an application intending to transfer large
   amounts of data could use LEDBAT to try to reduce its transmission
   rate before any competing TCP flows do, by detecting an increase in
   end-to-end delay (as a measure of incipient congestion).  However
   these techniques rely on voluntary, altruistic action by end users
   and their application providers.  ISPs cannot enforce their use.
   This leads to our second point.

   The Internet was designed so that end-hosts detect and control
   congestion.  We believe that congestion needs to be visible to
   network nodes as well, not just to the end hosts.  More specifically,

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   a network needs to be able to measure how much congestion traffic
   causes between the monitoring point in the network and the
   destination ("rest-of-path congestion").  This would be a new
   capability; today a network can use explicit congestion notification
   (ECN) [RFC3168] to detect how much congestion traffic has suffered
   between the source and a monitoring point in the network, but not
   beyond.  Such a capability would enable an ISP to give incentives for
   the use, without restrictions, of LEDBAT-like applications whilst
   perhaps restricting excessive use of TCP and UDP ones.

   So we propose a new approach which we call congestion exposure.  We
   propose that congestion information should be made visible at the IP
   layer, so that any network node can measure the contribution to
   congestion of an aggregate of traffic as easily as straight volume
   can be measured today.  Once the information is exposed in this way,
   it is then possible to use it to measure the true impact of any
   traffic on the network.  Lacking the ability to see congestion, some
   ISPs count the volume each user transfers.  On this basis LEDBAT
   applications would get blamed for hogging the network given the large
   amount of volume they transfer.  However, because they yield rather
   than hog, they actually contribute very little to congestion.  One
   use of exposed congestion information would be to measure the
   congestion attributable to a given user. and thereby incentivise the
   use of protocols such as [LEDBAT] which aim to reduce the congestion
   caused by bulk data transfers.

   Creating the incentive to deploy low-congestion protocols such as
   LEDBAT is just one of many motivations for congestion exposure.  In
   general, congestion exposure gives ISPs a principled way to hold
   their customers accountable for the impact on others of their network
   usage and their choices of applications.  It can measure the impact
   of an individual consumer, a large enterprise network or the traffic
   crossing a border from another ISP - anywhere where volume is used
   today as a (poor) measure of usage.  In Section 7, a range of
   potential use cases for congestion exposure are given, showing it is
   possible to imagine a wide range of other ways to use the exposed
   congestion information.

1.1.  Definitions

   Throughout this document we refer to congestion repeatedly.
   Congestion has a wide range of definitions.  For the purposes of this
   document it is defined using the simplest way that it can be measured
   - the instantaneous fraction of loss.  More precisely, congestion is
   bits lost divided by bits sent, taken over any brief period.  By
   extension, if explicit congestion notification (ECN) is being used,
   the fraction of bits marked (rather than lost) gives a useful metric
   that can be thought of as analagous to congestion.  Strictly

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   congestion should measure impairment, whereas ECN aims to avoid any
   loss or delay impairments due to congestion.  But for the purposes of
   this document, the two will both be called congestion.

   We also need to define two specific terms carefully:

   Upstream Congestion

      Upstream congestion is defined as the congestion that has already
      been experienced by a packet as it travels along its path.  In
      other words at any point on the path it is the congestion between
      that point and the source of the packet.

   Downstream Congestion

      Downstream congestion is defined as the congestion that a packet
      still has to experience on the remainder of its path.  In other
      words at any point it is the congestion still to be experienced as
      the packet travels between that point and ts destination.

1.2.  Changes from previous versions

   From -02 to -03 (current version):

      Abstract re-written again following comments from John Leslie.

      Use Cases Section re-written.

      Security Considerations section improved.

      This ChangeLog added.

      NOTE: there are still sections needing more work, especially the
      Use Cases.  The whole document also needs trimming in places and
      checking for repetition or omission.

   From -01 to -02:

      Extensive changes throughout the document:

      *  Abstract and Introduction re-written.

      *  The Problem section re-written and extended significantly.

      *  Why Now?  Section re-written and extended.

      *  Requirements extended.

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      *  Security Considerations expanded.

      Other less major changes throughout.

   From -00 -01:

      Significant changes throughout including re-organising the main

      New Abstract and changes to Introduction.

2.  The Problem

2.1.  Congestion is not the problem

   The problem is not congestion itself.  The problem is how best to
   share available capacity.  When too much traffic meets too little
   capacity, congestion occurs.  Then we have to share out what capacity
   there is.  But we should not (and cannot) solve the capacity sharing
   problem by trying to make it go away - by saying there should somehow
   be no congestion, slower traffic or more capacity.  That misses the
   whole point of the Internet: to multiplex or share available capacity
   at maximum utilisation.

   So as we say, the problem is not congestion in itself.  Every elastic
   data transfer should (and usually will) congest a healthy data
   network.  If it doesn't, its transport protocol is broken.  There
   should always be periods approaching 100% utilisation at some link
   along every data path through the Internet, implying that frequent
   periods of congestion are a healthy sign.  If transport protocols are
   too weak to congest capacity, they are under-utilising it and hanging
   around longer than they need to, reducing the capacity available for
   the next data transfers that might be about to start.

2.2.  Increase capacity or manage traffic?

   Some say the problem is that ISPs should invest in more capacity.
   Certainly increasing capacity should make the congested periods
   during data transfers shorter and the non-congested gaps between them
   longer.  The argument goes that if capacity were large enough it
   would make the periods when there is a capacity sharing problem
   insignificant and not worth solving.

   Yet, ISPs are facing a quandary - traffic is growing rapidly and
   traffic patterns are changing significantly (see Section 4 and
   [Cisco-VNI]) They know that any increases in capacity will have to be
   paid for by all their customers but capacity growth will be of most
   benefit to the heaviest users.  Faced with these problems, some ISPs

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   are seeking to reduce what they regard as "heavy usage" in order to
   improve the service experienced by the majority of their customers.

   If done properly, managing traffic should be a valid alternative to
   increasing capacity.  An ISP's customers can vote with their feet if
   the ISP chooses the wrong balance between making the service more
   expensive or managing heavy traffic.  Current traffic management
   techniques Section 3 fight against the capacity shares that TCP is
   aiming for.  Ironically, they try to impose something approaching
   LEDBAT-like behaviour on heavier flows.  But as we have seen, they
   cannot give LEDBAT the credit for doing this itself - the network
   just sees a LEDBAT flow as a large amount of volume.

   Thus the problem for the IETF is to ensure that ISPs and their
   equipment suppliers have appropriate protocol support - not just to
   impose good capacity sharing themselves, but to encourage end-to-end
   protocols to share out capacity in everyone's best interests.

2.2.1.  Making Congestion Visible

   Unfortunately ISPs are only able to see limited information about the
   traffic they forward.  As we will see in section 3 they are forced to
   use the only information they do have available which leads to myopic
   control that has scant regard for the actual impact of the traffic or
   the underlying network conditions.  All their approaches are unsound
   because they cannot measure the most useful metric.  The volume or
   rate of a given flow or aggregate doesn't directly affect other
   users, but the congestion it causes does.  This can be seen with a
   simple illustration.  A 5Mbps flow in an otherwise empty 10Mbps
   bottleneck causes no congestion and so affects no other users.  By
   contrast a 1Mbps flow entering a 10Mbps bottleneck that is already
   fully occupied causes significant congestion and impacts every other
   user sharing that bottleneck as well as suffering impairment itself.
   So the real problem that needs to be addressed is how to close this
   information gap.  How can we expose congestion at the IP layer so
   that it can be used as the basis for measuring the impact of any
   traffic on the network as a whole?

2.2.2.  ECN - a Step in the Right Directions

   Explicit Congestion Notification [RFC3168] allows routers to
   explicitly tell end-hosts that they are approaching the point of
   congestion.  ECN builds on Active Queue Mechanisms such as random
   early discard (RED) [RFC2309] by allowing the router to mark a packet
   with a Congestion Experienced (CE) codepoint, rather than dropping
   it.  The probability of a packet being marked increases with the
   length of the queue and thus the rate of CE marks is a guide to the
   level of congestion at that queue.  This CE codepoint travels forward

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   through the network to the receiver which then informs the sender
   that it has seen congestion.  The sender is then required to respond
   as if it had experienced a packet loss.  Because the CE codepoint is
   visible in the IP layer, this approach reveals the upstream
   congestion level for a packet.

   So Is ECN the Solution?  Alas not - ECN does allow downstream nodes
   to measure the upstream congestion for any flow, but this is not
   enough.  This can make a receiver accountable for the congestion
   caused by incoming traffic.  But a receiver can only control incoming
   congestion indirectly, by politely asking the sender to control it.
   A receiver cannot make a sender install an adaptive codec, or install
   LEDBAT instead of TCP.  And a receiver cannot ask an attacker to stop
   flooding it with traffic.  What is needed is knowledge of the
   downstream congestion level for which you need additional information
   that is still concealed from the network - by design.

3.  Existing Approaches to Traffic Control

   Existing approaches intended to address the problems outlined above
   can be broadly divided into two groups - those that passively monitor
   traffic and can thus measure the apparent impact of a given flow of
   packets and those that can actively discriminate against certain
   packets, flows, applications or users based on various
   characteristics or metrics.

3.1.  Layer 3 Measurement

   L3 measurement of traffic relies on using the information that can be
   measured directly or is revealed in the IP header of the packet (or
   lower layers).  Architecturally, L3 measurement is best since it fits
   with the idea of the hourglass design of the Internet [RFC3439].
   This asserts that "the complexity of the Internet belongs at the
   edges, and the IP layer of the Internet should remain as simple as

3.1.1.  Volume Accounting

   Volume accounting is a technique that is often used to discriminate
   between heavy and light users.  The volume of traffic sent by a given
   user or network is one of the easiest pieces of information to
   monitor in a network.  Measuring the size of every packet from the
   header and adding them up is a simple operation.  Consequently this
   has long been a favoured measure used by operators to control their

   The precise manner in which this volume information is used may vary.
   Typically ISPs may impose an overall volume cap on their customers

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   (perhaps 10Gbytes a month).  Alternatively they may decide that the
   heaviest users each month are subjected to some sanction.

   Volume is naively thought to indicate the impact that one party's
   traffic has on others.  But the same volume can cause very different
   impacts on others if it is transferred at slightly different times,
   or between slightly different endpoints.  Also the impact on others
   greatly depends on how responsive the transport is to congestion,
   whether responsive (TCP), very responsive (LEDBAT), aggressive
   (multiple TCPs) or totally unresponsive.

3.1.2.  Rate Measurement

   Traffic rates are often used as the basis of accounting at borders
   between ISPs.  For instance a contract might specify a charge based
   on the 95th percentile of the peak rate of traffic crossing the
   border every month.  Such bulk rate measurements are usually
   simplified by measuring traffic volume over a short period, such as
   15mins.  With a little extra effort it is possible to measure the
   rate of a given flow by using the 3-tuple of source and destination
   IP address and protocol number.

   Rate measurements might be thought indicative of the impact of one
   aggregate of traffic on others, and rate is often limited to avoid
   impact on others.  However such limits generally constrain everyone
   much more than they need to, just in case most parties send fast at
   the same time.  And such limits constrain everyone too little at
   other times, when everyone actually does send fast at the same time.

3.2.  Higher Layer Discrimination

   Over recent years a number of traffic management techniques have
   emerged that explicitly differentiate between different traffic
   types, applications and even users.  This is done because ISPs and
   operators feel they have a need to use such techniques to better
   control a new raft of applications that break some of the implicit
   design assumptions behind TCP (short-lived flows, limited flows per
   connection, generally between server and client).

3.2.1.  Bottleneck Rate Policing

   Bottleneck flow rate policers such as [XCHOKe] and [pBox] have been
   proposed as approaches for rate policing traffic.  But they must be
   deployed at bottlenecks in order to work.  Unfortunately, a large
   proportion of traffic traverses at least two bottlenecks which limits
   the utility of this approach.  Such rate policers also make an
   assumption about what constitutes acceptable per-flow behaviour.  If
   these bottleneck policers were widely deployed, the Internet could

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   find itself with one universal rate adaptation policy embedded
   throughout the network.  With TCP's congestion control algorithm
   approaching its scalability limits as the network bandwidth continues
   to increase, new algorithms are being developed for high-speed
   congestion control.  Embedding assumptions about acceptable rate
   adaptation would make evolution to such new algorithms extremely

3.2.2.  DPI and Application Rate Policing

   Some operators use deep packet inspection (DPI) and traffic analysis
   to identify certain applications they believe to have an excessive
   impact on the network.  ISPs generally pick on applications that that
   they judge as low value to the customer in question and high impact
   on other customers.  A common example is peer-to-peer file-sharing.
   Having identified a flow as belonging to such an application, the
   operator uses differential scheduling to limit the impact of that
   flow on others, which usually limits its throughput as well.  This
   has fuelled the on-going battle between application developers and
   DPI vendors.

   When operators first started to limit the throughput of P2P, it soon
   became common knowledge that turning on encryption could boost your
   throughput.  The DPI vendors then improved their equipment so that it
   could identify P2P traffic by the pattern of packets it sends.  This
   risks becoming an endless vicious cycle - an arms race that neither
   side can win.  Furthermore such techniques may put the operator in
   direct conflict with the customers, regulators and content providers.

4.  Why Now?

   The accountability and capacity sharing problems highlighted so far
   have always characterised the Internet to some extent.  In 1988 Van
   Jacobson coded capacity sharing into TCP's e2e congestion control
   algorithms [TCPcc].  But fair queuing algorithms were already being
   written for network operators to ensure each active user received an
   equal share of a link and couldn't game the system [RFC0970].  The
   two approaches have divergent objectives, but they have co-existed
   ever since.

   The main new factor has been the introduction of residential
   broadband, making 'always-on' available to all, not just campuses and
   enterprises.  Both TCP and approaches like fair queuing only share
   out capacity instantaneously; they don't take account of how much of
   each user's data is occupying a link over time, which can
   significantly reduce the capacity available to lighter usage.
   Therefore residential ISPs have been introducing new traffic
   management equipment that can prioritise based on each customer's

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   usage profile.  Otherwise capacity upgrades get eaten up by transfers
   of large amounts of data, with little gain for interactive usage

   In campus networks, capacity upgrades are the easiest way to mitigate
   the inability of TCP or FQ to take account of activity over time.
   But capacity upgrades are much more expensive in residential
   broadband networks that are spread over large geographic areas and
   customers will only be happy to pay more for their service if they
   can see a significant benefit.

   However, these traffic management techniques fight the capacity
   shares e2e protocols are aiming at, rather than working together in
   unison.  And, the more optimal ISPs try to make their controls, the
   more they need application knowledge within the network - which isn't
   how the Internet was designed to work.  Congestion exposure hasn't
   been considered before, because the depth of the problem has only
   recently been understood.  We now understand that both networks and
   end-systems to focus on contribution to congestion, not volume or
   rate.  Then application knowledge is only needed on the end-system,
   where it should be.  But the reason this isn't happening is because
   the network cannot see the information it needs (congestion).

   As long as ISPs continue to use rate and volume as the key metrics
   for determining when to control traffic there is no incentive to use
   LEDBAT or other low-congestion protocols that improve the performance
   of competing interactive traffic.  We believe that congestion
   exposure gives ISPs the information they need to be able to
   discriminate in favour of such low-congestion transports.  In turn
   this will give users a direct benefit from using such transports and
   so encourage their wider use.

5.  Requirements for a Solution

   This section proposes some requirements for any solution to this
   problem.  We believe that a solution that meets most of these
   requirements is likely to be better than one that doesn't, but we
   recognise that if a working group is established in this area, it may
   have to make tradeoffs.

   o  Allow both upstream and downstream congestion to be visible at the
      IP layer -- visibility at the IP layer allows congestion in the
      heart of the network to be monitored at the edges and without
      deploying complicated and intrusive equipment such as DPI boxes.
      This gives several advantages:

      1.  It enables policing of flows based on the congestion they are
          actually going to cause in the network.

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      2.  It allows the flow of congestion across ISP borders to be

      3.  It supports a diversity of intra-domain and inter-domain
          congestion management practices.

      4.  It allows the contribution to congestion over time to be
          counted as easily as volume can be counted today.

      5.  It supports contractual arrangements for managing traffic
          (acceptable use policies, SLAs etc) between just the two
          parties exchanging traffic across their point of attachment,
          without involving others.

   o  Avoid making assumptions about the behavior of specific
      applications (e.g. be agnostic to application and transport

   o  Support the widest possible range of transport protocols for the
      widest range of data types (elastic, inelastic, real-time,
      background, etc) -- don't force a "universal rate adaptable
      policy" such as TCP-friendliness [RFC3448].

   o  Be responsive to real-time congestion in the network.

   o  Allow incremental deployment of the solution and ideally permit
      permanent partial deployment to increase chances of successful

   o  Ensure packets supporting congestion exposure are distinguishable
      from others, so that each transport can control when it chooses to
      deploy congestion exposure, and ISPs can manage the two types of
      traffic distinctly.

   o  Support mechanisms that ensure the integrity of congestion
      notifications, thus making it hard for a user or network to
      distort the congestion signal.

   o  Be robust in the face of DoS attacks, so that congestion
      information can be used to identify and limit DoS traffic and to
      protect the hosts and network elements implementing congestion

   Many of these requirements are by no means unique to the problem of
   congestion exposure.  Incremental deployment for instance is a
   critical requirement for any new protocol that affects something as
   fundamental as IP.  Being robust under attack is also a pre-requisite
   for any protocol to succeed in the real Internet and this is covered

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   in more detail in Section 9.

6.  A Strawman Congestion Exposure Protocol

   In this section we explore a simple strawman protocol that would
   solve the congestion exposure problem.  This protocol neatly
   illustrates how a solution might work.  A practical implementation of
   this protocol has been produced and both simulations and real-life
   testing show that it works.  The protocol is based on a concept known
   as re-feedback [Re-fb] and builds on existing active queue management
   techniques like RED [RFC2309] and ECN [RFC3168] that network elements
   can already use to measure and expose congestion.

   Re-feedback, standing for re-inserted feedback, is a system designed
   to allow end-hosts to reveal to the network information about their
   network path that they have received via conventional feedback (for
   instance congestion).

   In our strawman protocol we imagine that packets have two
   "congestion" fields in their IP header:

   o  The first is a congestion experienced field to record the upstream
      congestion level along the path.  Routers indicate their current
      congestion level by updating this field in every packet.  As the
      packet traverses the network it builds up a record of the overall
      congestion along its path in this field.  This data is sent back
      to the sender who uses it to determine its transmission rate.

   o  The other is a whole-path congestion field that uses re-feedback
      to record the total congestion along the path.  The sender does
      this by re-inserting the current congestion level for the path
      into this field for every packet it transmits.

   Thus at any node downstream of the sender you can see the upstream
   congestion for the packet (the congestion thus far), the whole path
   congestion (with a time lag of 1RTT) and can calculate the downstream
   congestion by subtracting one from the other.

   So congestion exposure can be achieved by coupling congestion
   notification from routers with the re-insertion of this information
   by the sender.  This establishes information symmetry between users
   and network providers.

7.  Use Cases

   Once downstream congestion information is revealed in the IP header
   it can be used for a number of purposes.  Precise details of how the
   information might be used are beyond the scope of this document but

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   this section will give an overview of some possible uses. {ToDo:
   write up the rest of this section properly.  Concentrate on a couple
   of the most useful potential use cases (traffic management and
   accountability?) and mention a couple of more arcane uses (traffic
   engineering and e2e QoS).  The key thing is to clarify that
   Congestion Exposure is a tool that can be used for many other

   It allows an ISP to accurately identify which traffic is having the
   greatest impact on the network and either police directly on that
   basis or use it to determine which users should be policed.  It can
   form the basis of inter-domain contracts between operators.  It could
   even be used as the basis for inter-domain routing, thus encouraging
   operators to invest appropriately in improving their infrastructure.

   From Rich Woundy: "I would add a section about use cases.  The
   primary use case would seem to be an "incentive environment that
   ensures optimal sharing of capacity", although that could use a
   better title.  Other use cases may include "DDoS mitigation", "end-
   to-end QoS", "traffic engineering", and "inter-provider service
   monitoring".  (You can see I am stealing liberally from the
   motivation draft here.  We'll have to see whether the other use cases
   are "core" to this group, or "freebies" that come along with re-ECN
   as a particular protocol.)"

   My take on this is we need to concentrate on one or two major use
   cases.  The most obvious one is using this to control user-behaviour
   and encourage the use of "congestion friendly" protocols such as

   {Comments from Louise Krug:} simply say that operators MUST turn off
   any kind of rate limitation for LEDBAT traffic and what they might
   mean for the amount of bandwidth they see compared to a throttled
   customer?  You could then extend that to say how it leads to better
   QoS differentiation under the assumption that there is a broad
   traffic mix any way?  Not sure how much detail you want to go into
   here though?

   {ToDo: better incorporate this text from Mirja into Michael's text
   below.} Congestion exposure can enable ISPs to give an incentive to
   end-systems to response to congestion in a way that leads to a better
   share of the available capacity.  For example the introduction of a
   per-user congestion volume might motivate "heavy-user" to back off
   with their high-bandwidth traffic (when congestion occurs) to save
   their congestion volume for more time-critical traffic.  If every
   end-system reacts to congestion in such a way that it avoids
   congestion for non-critical traffic and allow a certain level of
   congestion for the more important traffic (from the user's point of

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   view), the all-over user experience will be increased.  More-over the
   network might be utilized more equally when less-important traffic is
   shifted to less congested time slots.

7.1.  Improved Policing

   As described earlier in this document, ISPs throttle traffic not
   because it causes congestion in the network but because users have
   exceeded their traffic profile or because individual applications or
   flows are suspected to cause congestion.  This is done because it is
   not possible to police only the traffic that is causing congestion.
   Congestion exposure allows new possibilities for rate policing.

7.1.1.  Per Aggregate Policing

   A straightforward application of congestion exposure is per-flow or
   per-aggregate congestion policing.  Instead of limiting flows or
   aggregates because they have exceeded certain rate thresholds, they
   can be throttled if they cause too much congestion in the network.
   This is throttling on evidence instead of suspicion.

7.1.2.  Per customer policing

   The assumption is that every customer has an allowance of congestion
   per second.  If he causes more congestion than this throughout the
   network, his traffic can be policed or shaped to ensure he stays
   within his allowance.  The nice features of this approach are that it
   sets incentives for the use of congestion-minimising transport
   protocols such as LEDBAT and allows tariffs that better reflect the
   relative impact of customers each other.

   Incentives for congestion minimising transports:

      A user generates foreground and background traffic.  Foreground
      traffic needs to go fast while background traffic can afford to go
      slow.  With per-customer congestion policing, users can optimise
      their network experience by using congestion-minimising transport
      protocols for background traffic and normal TCP-like or even high-
      speed transport protocols for foreground traffic.  Doing so means
      background traffic only causes minimal congestion so that
      foreground traffic can go faster than when both were transmitted
      over the same transport protocols.  Hence, per-customer congestion
      policing sets incentives for selfish users to utilise congestion-
      minimising transport protocols.

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   Improved tariff structures:

      Currently customers are offered tariffs with all manner of
      differentaitors from peak access rate to volume limit and even
      specific application rate limits.  Congestion-policing offers a
      better means of distinguishing between tariffs.  Heavy users and
      light users will get equal access in terms of speed and short-term
      throughput, but customers that cause more congestion and thus have
      a bigger impact on others will have to pay for the privilege or
      suffer reduced throuhgput during periods of heavy congestion.
      However tariffs are a subject best left to the market to
      determine, not the IETF.

8.  IANA Considerations

   This document makes no request to IANA.

9.  Security Considerations

   One intended use of exposed congestion information is to hold the e2e
   transport and the network accountable to each other.  Therefore, any
   congestion exposure protocol will have to provide the necessary hooks
   to mechanisms that can assure the integrity of this information.  The
   network cannot be relied on to report information to the receiver
   against its interest, and the same applies for the information the
   receiver feeds back to the sender, and that the sender reports back
   to the network.  Looking at all each in turn:

   o  The Network.  In general it is not in any network's interest to
      under-declare congestion since this will have potentially negative
      consequences for all users of that network.  It may be in its
      interest to over-declare congestion if, for instance, it wishes to
      force traffic to move away to a different network or indeed simply
      wants to reduce the amonut of traffic it is carrying.  Congestion
      Exposure itself shouldn't significantly alter the incentives for
      and against honest declaration of congestion by a network, but it
      is possible to imagine applications of Congestion Exposure that
      will change these incentives.  There is a general perception among
      networks that their level of congestion is a business secret.
      Actually in the Internet architecture congestion is one of the
      worst-kept secrets a network has, because end-hosts can see
      congestion better than networks can.  Nonetheless, one goal of a
      congestion exposure protocol is to allow networks to pinpoint
      whether congestion is in one side or the other of a border.
      Although this extra transparency should be good for ISPs with low
      congestion, those with underprovisioned networks may try to
      obstruct deployment.

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   o  The Receiver.  Receivers generally have an incentive to under-
      declare congestion since they generally wish to receive the data
      from the sender as rapidly as possible.  [Savage] explains how a
      receiver can significantly improve their throughput my failing to
      declare congestion.  This is a problem with or without Congestion
      Exposure.  [KGao] explains one possible technique to encourage
      receiver's to be honest in their declaration of congestion.

   o  The Sender.  One proposed mechanisms for congestion exposure adds
      a requirement for a sender to let the network know how much
      congestion it has suffered or caused.  Although most senders
      currently respond to congestion they are informed of, one use of
      exposed congestion information might be to encourage sources of
      excessive congestion to respond more than previously.  Then
      clearly there may be an incentive for the sender to under-declare
      congestion.  This will be a particular problem with sources of
      flooding attacks.

   In addition there are potential problems from source spoofing.  A
   malicious sender can pretend to be another user by spoofing the
   source address.  A congestion exposure protocol will need to be
   robust against injection of false congestion information into the
   forward path that could distort or disrupt the integrity of the
   congestion signal.

10.  Conclusions

   Congestion exposure is the idea that traffic itself indicates to all
   nodes on its path how much congestion it causes on the entire path.
   It is useful for network operators to police traffic only if it
   really causes congestion in the Internet instead of doing blind rate
   capping independently of the congestion situation.  This change would
   give incentives to users to adopt new transport protocols such as
   LEDBAT which try to avoid congestion more than TCP does.
   Requirements for congestion exposure in the IP header were
   summarized, one technical solution was presented, and additional use
   cases for congestion exposure were discussed.

11.  Acknowledgements

   A number of people other than authors have provided text and comments
   for this memo.  The document is being produced in support of a BoF on
   Congestion Exposure as discussed extensively on the <re-ecn@ietf.org>
   mailing list.

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12.  Informative References

   [BB-Incentive]      MIT Communications Futures Program (CFP) and
                       Cambridge University Communications Research
                       Network, "The Broadband Incentive Problem",
                       September 2005.

   [CC-open-research]  Welzl, M., Scharf, M., Briscoe, B., and D.
                       Papadimitriou, "Open Research Issues in Internet
                       Congestion Control", draft-irtf-iccrg-welzl-
                       congestion-control-open-research-05 (work in
                       progress), September 2009.

   [Cisco-VNI]         Cisco Systems, inc., "Cisco Visual Networking
                       Index: Forecast and Methodology, 2008-2013",
                       June 2009.

   [Fairness]          Briscoe, B., Moncaster, T., and A. Burness,
                       "Problem Statement: Transport Protocols Don't
                       Have To Do Fairness",
                       draft-briscoe-tsvwg-relax-fairness-01 (work in
                       progress), July 2008.

   [KGao]              Gao, K. and C. Wang, "Incrementally Deployable
                       Prevention to TCP Attack with Misbehaving
                       Receivers", December 2004.

   [Kelly]             Kelly, F., Maulloo, A., and D. Tan, "Rate control
                       for communication networks: shadow prices,
                       proportional fairness and stability", Journal of
                       the Operational Research Society 49(3) 237--252,

   [LEDBAT]            Shalunov, S., "Low Extra Delay Background
                       Transport (LEDBAT)",
                       draft-shalunov-ledbat-congestion-00 (work in
                       progress), March 2009.

   [OfCom]             Ofcom: Office of Communications, "UK Broadband
                       Speeds 2008: Research report", January 2009.

   [RFC0970]           Nagle, J., "On packet switches with infinite
                       storage", RFC 970, December 1985.

   [RFC2309]           Braden, B., Clark, D., Crowcroft, J., Davie, B.,
                       Deering, S., Estrin, D., Floyd, S., Jacobson, V.,
                       Minshall, G., Partridge, C., Peterson, L.,

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                       Ramakrishnan, K., Shenker, S., Wroclawski, J.,
                       and L. Zhang, "Recommendations on Queue
                       Management and Congestion Avoidance in the
                       Internet", RFC 2309, April 1998.

   [RFC3168]           Ramakrishnan, K., Floyd, S., and D. Black, "The
                       Addition of Explicit Congestion Notification
                       (ECN) to IP", RFC 3168, September 2001.

   [RFC3439]           Bush, R. and D. Meyer, "Some Internet
                       Architectural Guidelines and Philosophy",
                       RFC 3439, December 2002.

   [RFC3448]           Handley, M., Floyd, S., Padhye, J., and J.
                       Widmer, "TCP Friendly Rate Control (TFRC):
                       Protocol Specification", RFC 3448, January 2003.

   [RFC4340]           Kohler, E., Handley, M., and S. Floyd, "Datagram
                       Congestion Control Protocol (DCCP)", RFC 4340,
                       March 2006.

   [RFC5594]           Peterson, J. and A. Cooper, "Report from the IETF
                       Workshop on Peer-to-Peer (P2P) Infrastructure,
                       May 28, 2008", RFC 5594, July 2009.

   [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/

   [Savage]            Savage, S., Wetherall, D., and T. Anderson, "TCP
                       Congestion Control with a Misbehaving Receiver",
                       ACM SIGCOMM Computer Communication Review , 1999.

   [TCPcc]             Jacobson, V. and M. Karels, "Congestion Avoidance
                       and Control", Proc. ACM SIGCOMM'88 Symposium,
                       Computer Communication Review 18(4)314--329,
                       Proc. ACM SIGCOMM'88 Symposium, Computer
                       Communication Review 18(4)314--329, August 1988,

   [XCHOKe]            Chhabra, P., Chuig, S., Goel, A., John, A.,
                       Kumar, A., Saran, H., and R. Shorey, "XCHOKe:
                       Malicious Source Control for Congestion Avoidance
                       at Internet Gateways", Proceedings of IEEE
                       International Conference on Network Protocols

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                       (ICNP-02) , November 2002,

   [pBox]              Floyd, S. and K. Fall, "Promoting the Use of End-
                       to-End Congestion Control in the Internet", IEEE/
                       ACM Transactions on Networking 7(4) 458--472,
                       August 1999,

Authors' Addresses

   Toby Moncaster (editor)
   B54/70, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE

   Phone: +44 7918 901170
   EMail: toby.moncaster@bt.com

   Louise Krug
   B54/77, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE

   EMail: louise.burness@bt.com

   Michael Menth
   University of Wuerzburg
   room B206, Institute of Computer Science
   Am Hubland
   Wuerzburg  D-97074

   Phone: +49 931 888 6644
   EMail: menth@informatik.uni-wuerzburg.de

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   Joao Taveira Araujo
   GS206 Department of Electronic and Electrical Engineering
   Torrington Place
   London  WC1E 7JE

   EMail: j.araujo@ee.ucl.ac.uk

   Steven Blake
   Extreme Networks
   Pamlico Building One, Suite 100
   3306/08 E. NC Hwy 54
   RTP, NC 27709

   EMail: sblake@extremenetworks.com

   Richard Woundy
   Comcast Cable Communications
   27 Industrial Avenue
   Chelmsford, MA  01824

   EMail: richard_woundy@cable.comcast.com
   URI:   http://www.comcast.com

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