CONEX                                                         B. Briscoe
Internet-Draft                                                        BT
Intended status: Informational                                 R. Woundy
Expires: May 19, 2011                                            Comcast
                                                       T. Moncaster, Ed.
                                                          J. Leslie, Ed.
                                                       November 15, 2010

                      ConEx Concepts and Use Cases


   Internet Service Providers (operators) are facing problems where
   localized congestion prevents full utilization of the path between
   sender and receiver at today's "broadband" speeds.  Operators desire
   to control this congestion, which often appears to be caused by a
   small number of users consuming a large amount of bandwidth.
   Building out more capacity along all of the path to handle this
   congestion can be expensive and may not result in improvements for
   all users so network operators have sought other ways to manage
   congestion.  The current mechanisms all suffer from difficulty
   measuring the congestion (as distinguished from the total traffic).

   The ConEx Working Group is designing a mechanism to make congestion
   along any path visible at the Internet Layer.  This document
   describes example cases where this mechanism would be useful.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 19, 2011.

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Copyright Notice

   Copyright (c) 2010 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.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Congestion Management  . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Existing Approaches  . . . . . . . . . . . . . . . . . . .  7
   4.  Exposing Congestion  . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  ECN - a Step in the Right Direction  . . . . . . . . . . .  9
   5.  ConEx Use Cases  . . . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  ConEx as a basis for traffic management  . . . . . . . . . 10
     5.2.  ConEx to incentivise scavenger transports  . . . . . . . . 10
     5.3.  Accounting for Congestion Volume . . . . . . . . . . . . . 11
     5.4.  ConEx as a form of differential QoS  . . . . . . . . . . . 11
     5.5.  Partial vs. Full Deployment  . . . . . . . . . . . . . . . 12
   6.  Other issues . . . . . . . . . . . . . . . . . . . . . . . . . 13
     6.1.  Congestion as a Commercial Secret  . . . . . . . . . . . . 13
     6.2.  Information Security . . . . . . . . . . . . . . . . . . . 14
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     10.2. Informative References . . . . . . . . . . . . . . . . . . 16

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1.  Introduction

   The growth of "always on" broadband connections, coupled with the
   steady increase in access speeds [OfCom], have caused unforeseen
   problems for network operators and users alike.  Users are
   increasingly seeing congestion at peak times and changes in usage
   patterns (with the growth of real-time streaming) simply serve to
   exacerbate this.  Operators want all their users to see a good
   service but are unable to see where congestion problems originate.
   But congestion results from sharing network capacity with others, not
   merely from using it.  In general, today's "DSL" and cable-internet
   users cannot "cause" congestion in the absence of competing traffic.
   (Wireless operators and cellular internet have different tradeoffs
   which we will not discuss here.)

   Congestion generally results from the interaction of traffic from one
   network operator's users with traffic from other users.  The tools
   currently available don't allow an operator to identify which traffic
   contributes most to the congestion and so they are powerless to
   properly control it.

   While building out more capacity to handle increased traffic is
   always good, the expense and lead-time can be prohibitive, especially
   for network operators that charge flat-rate feeds to subscribers and
   are thus unable to charge heavier users more for causing more
   congestion [BB-incentive].  For an operator facing congestion caused
   by other operators' networks, building out its own capacity is
   unlikely to solve the congestion problem.  Operators are thus facing
   increased pressure to find effective solutions to dealing with the
   increasing bandwidth demands of all users.

   The growth of "scavenger" behaviour (e.g.  [LEDBAT]) helps to reduce
   congestion, but can actually make the problem less tractable.  These
   users are trying to make good use of the capacity of the path while
   minimising their own costs.  Thus, users of such services may show
   very heavy total traffic up until the moment congestion is detected
   (at the Transport Layer), but then will immediately back off.
   Monitoring (at the Internet Layer) cannot detect this congestion
   avoidance if the congestion in question is in a different domain
   further along the path; and must treat such users as congestion-
   causing users.

   The ConEx working group proposes that Internet Protocol (IP) packets
   will carry additional ConEx information.  The exact protocol details
   are not described in this document, but the ConEx information will be
   sufficient to allow any node in the network to see how much
   congestion is attributable to a given traffic flow.  See
   [ConEx-Abstract-Mech] for further details.

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   Changes from previous drafts (to be removed by the RFC Editor):

   From draft-moncaster-conex-concepts-uses-02 to
   draft-ietf-conex-concepts-uses-00 (per decisions of working group):

      Removed section on DDoS mitigation use case.

      Removed appendix on ConEx Architectural Elements.  PLEASE NOTE:
      Alignment of terminology with the Abstract Mechanism draft has
      been deferred to the next version.

   From draft-moncaster-conex-concepts-uses-01 to

      Updated document to take account of the new Abstract Mechanism
      draft [ConEx-Abstract-Mech].

      Updated the definitions section.

      Removed sections on Requirements and Mechanism.

      Moved section on ConEx Architectural Elements to appendix.

      Minor changes throughout.

   From draft-moncaster-conex-concepts-uses-00 to

      Changed end of Abstract to better reflect new title

      Created new section describing the architectural elements of
      ConEx.  Added Edge Monitors and Border Monitors (other elements
      are Ingress, Egress and Border Policers).

      Extensive re-write of Section 5 partly in response to suggestions
      from Dirk Kutscher

      Improved layout of Section 2 and added definitions of Whole Path
      Congestion, ConEx-Enabled and ECN-Enabled.  Re-wrote definition of
      Congestion Volume.  Renamed Ingress and Egress Router to Ingress
      and Egress Node as these nodes may not actually be routers.

      Improved document structure.  Merged sections on Exposing
      Congestion and ECN.

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      Added new section on ConEx requirements with a ConEx Issues
      subsection.  Text for these came from the start of the old ConEx
      Use Cases section

      Added a sub-section on Partial vs Full Deployment Section 5.5

      Added a discussion on ConEx as a Business Secret Section 6.1

   From draft-conex-mechanism-00 to

      Changed filename to draft-moncaster-conex-concepts-uses.

      Changed title to ConEx Concepts and Use Cases.

      Chose uniform capitalisation of ConEx.

      Moved definition of Congestion Volume to list of definitions.

      Clarified mechanism section.  Changed section title.

      Modified text relating to conex-aware policing and policers (which
      are NOT defined terms).

      Re-worded bullet on distinguishing ConEx and non-ConEx traffic in
      Section 5.

2.  Definitions

   In this section we define a number of terms that are used throughout
   the document.  The key definition is that of congestion, which has a
   number of meanings depending on context.  The definition we use in
   this document is based on the definition in [Padhye] where congestion
   is viewed as a probability that a packet will be dropped.  This list
   of definitions is supplementary to that in [ConEx-Abstract-Mech].

   Congestion:  Congestion is a measure of the probability that a packet
      will be marked or dropped as it traverses a queue.

   flow:  a series of packets from a single sender to a single receiver
      that are treated by that sender as belonging to a single stream
      for the purposes of congestion control.  NB in general this is not
      the same as the aggregate of all traffic between the sender and

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   Congestion-rate:  For any granularity of traffic (packet, flow,
      aggregate, etc.), the instantaneous rate of traffic discarded or
      marked due to congestion.  Conceptually, the instantaneous bit-
      rate of the traffic multiplied by the instantaneous congestion it
      is experiencing.

   Congestion-volume:  For any granularity of traffic (packet, flow,
      aggregate, etc.), the volume of bytes dropped or marked in a given
      period of time.  Conceptually, congestion-rate multipled by time.

   Upstream Congestion:  the accumulated level of congestion experienced
      by a traffic flow thus far along its path.  In other words, at any
      point the Upstream Congestion is the accmulated level of
      congestion the traffic flow has experienced as it travels from the
      sender to that point.  At the receiver this is equivalent to the
      end-to-end congestion level that (usually) is reported back to the

   Downstream Congestion:  the level of congestion a flow of traffic is
      expected to experience on the remainder of its path.  In other
      words, at any point the Downstream Congestion is the level of
      congestion the traffic flow is yet to experience as it travels
      from that point to the receiver.

   Ingress:  the first node a packet traverses that is outside the
      source's own network.  In a domestic network that will be the
      first node downstream from the home access equipment.  In an
      enterprise network this is the provider edge router.

   Egress:  the last node a packet traverses before reaching the
      receiver's network.

   ConEx-enabled:  Any piece of equipment (end-system, router, tunnel
      end-point, firewall, policer, etc) that complies with the core
      ConEx protocol, which is to be defined by the ConEx working group.
      By extension a ConEx-enabled network is a network whose edge nodes
      are all ConEx-enabled.

3.  Congestion Management

   Since 1988 the Internet architecture has made congestion management
   the responsibility of the end-systems.  The network signals
   congestion to the receiver, the receiver feeds this back to the
   sender and the sender is expected to try and reduce the traffic it

   Any network that is persistently highly congested is inefficient.
   However the total absence of congestion is equally bad as it means

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   there is spare capacity in the network that hasn't been used.  The
   long-standing aim of congestion control has been to find the point
   where these two things are in balance.

   Over recent years, some network operators have come to the view that
   end-system congestion management is insufficient.  Because of the
   heuristics used by TCP, a relatively small number of end-machines can
   get a disproportionately high share of network resources.  They have
   sought to "correct" this perceived problem by using middleboxes that
   try and reduce traffic that is causing congestion or by artificially
   starving some traffic classes to create stronger congestion signals.

3.1.  Existing Approaches

   The authors have chosen not to exhaustively list current approaches
   to congestion management.  Broadly these approaches can be divided
   into those that happen at Layer 3 of the OSI model and those that use
   information gathered from higher layers.  In general these approaches
   attempt to find a "proxy" measure for congestion.  Layer 3 approaches

   o  Volume accounting -- the overall volume of traffic a given user or
      network sends is measured.  Users may be subject to an absolute
      volume cap (e.g. 10Gbytes per month) or the "heaviest" users may
      be sanctioned in some manner.

   o  Rate measurement -- the traffic rate per user or per network can
      be measured.  The absolute rate a given user sends at may be
      limited at peak hours or the average rate may be used as the basis
      for inter-network billing.

   Higher layer approaches include:

   o  Bottleneck rate policing -- bottleneck flow rate policers aim to
      share the available capacity at a given bottleneck between all
      concurrent users.

   o  DPI and application rate policing -- deep packet inspection and
      other techniques can be used to determine what application a given
      traffic flow is associated with.  Operators may then use this
      information to rate-limit or otherwise sanction certain
      applications at peak hours.

   All of these current approaches suffer from some general limitations.
   First, they introduce performance uncertainty.  Flat-rate pricing
   plans are popular because users appreciate the certainty of having
   their monthly bill amount remain the same for each billing period,
   allowing them to plan their costs accordingly.  But while flat-rate

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   pricing avoids billing uncertainty, it creates performance
   uncertainty: users cannot know whether the performance of their
   connection is being altered or degraded based on how the network
   operator manages congestion.

   Second, none of the approaches is able to make use of what may be the
   most important factor in managing congestion: the amount that a given
   endpoint contributes to congestion on the network.  This information
   simply is not available to network nodes, and neither volume nor rate
   nor application usage is an adequate proxy for congestion volume,
   because none of these metrics measures a user or network's actual
   contribution to congestion on the network.

   Finally, none of these solutions accounts for inter-network
   congestion.  Mechanisms may exist that allow an operator to identify
   and mitigate congestion in their own network, but the design of the
   Internet means that only the end-hosts have full visibility of
   congestion information along the whole path.  ConEx allows this
   information to be visible to everyone on the path and thus allows
   operators to make better-informed decisions about controlling

4.  Exposing Congestion

   We argue that current traffic-control mechanisms 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.

   For example, an application intending to transfer large amounts of
   data could use a congestion control mechanism like [LEDBAT] to reduce
   its transmission rate before any competing TCP flows do, by detecting
   an increase in end-to-end delay (as a measure of impending
   congestion).  However such techniques rely on voluntary, altruistic
   action by end users and their application providers.  Operators can
   neither enforce their use nor avoid penalizing them for congestion
   they avoid.

   The Internet was designed so that end-hosts detect and control
   congestion.  We argue that congestion needs to be visible to network
   nodes as well, not just to the end hosts.  More specifically, a
   network needs to be able to measure how much congestion any
   particular traffic expects to cause between the monitoring point in
   the network and the destination ("rest-of-path congestion").  This

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   would be a new capability.  Today a network can use Explicit
   Congestion Notification (ECN) [RFC3168] to detect how much congestion
   the traffic has suffered between the source and a monitoring point,
   but not beyond.  This new capability would enable an ISP to give
   incentives for the use of LEDBAT-like applications that seek to
   minimise congestion in the network whilst restricting inappropriate
   uses of traditional TCP and UDP applications.

   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.

   In general, congestion exposure gives operators a principled way to
   hold their customers accountable for the impact on others of their
   network usage and reward them for choosing congestion-sensitive

4.1.  ECN - a Step in the Right Direction

   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
   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.

   Alas, this is not enough - ECN gives downstream nodes an idea of the
   congestion so far for any flow.  This can help hold a receiver
   accountable for the congestion caused by incoming traffic.  But a
   receiver can only indirectly influence incoming congestion, by
   politely asking the sender to control it.  A receiver cannot make a
   sender install an adaptive codec, or install LEDBAT instead of TCP
   congestion-control.  And a receiver cannot cause 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

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   the network.

5.  ConEx Use Cases

   This section sets out some of the use cases for ConEx.  These use
   cases rely on some of the conceptual elements described in
   [ConEx-Abstract-Mech].  The authors don't claim this is an exhaustive
   list of use cases, nor that these have equal merit.  In most cases
   ConEx is not the only solution to achieve these.  But these use cases
   represent a consensus among people that have been working on this
   approach for some years.

5.1.  ConEx as a basis for traffic management

   Currently many operators impose some form of traffic management at
   peak hours.  This is a simple economic necessity -- the only reason
   the Internet works as a commercial concern is that operators are able
   to rely on statistical multiplexing to share their expensive core
   network between large numbers of customers.  In order to ensure all
   customers get some chance to access the network, the "heaviest"
   customers will be subjected to some form of traffic management at
   peak times (typically a rate cap for certain types of traffic)
   [Fair-use].  Often this traffic management is done with expensive
   flow aware devices such as DPI boxes or flow-aware routers.

   ConEx offers a better approach that will actually target the users
   that are causing the congestion.  By using Ingress or Egress
   Policers, an ISP can identify which users are causing the greatest
   Congestion Volume throughout the network.  This can then be used as
   the basis for traffic management decisions.  The Ingress Policer
   described in [Policing-freedom] is one interesting approach that
   gives the user a congestion volume limit.  So long as they stay
   within their limit then their traffic is unaffected.  Once they
   exceed that limit then their traffic will be blocked temporarily.

5.2.  ConEx to incentivise scavenger transports

   Recent work proposes a new approach for QoS where traffic is provided
   with a less than best effort or "scavenger" quality of service.  The
   idea is that low priority but high volume traffic such as OS updates,
   P2P file transfers and view-later TV programs should be allowed to
   use any spare network capacity, but should rapidly get out of the way
   if a higher priority or interactive application starts up.  One
   solution being actively explored is LEDBAT which proposes a new
   congestion control algorithm that is less aggressive in seeking out
   bandwidth than TCP.

   At present most operators assume a strong correlation between the

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   volume of a flow and the impact that flow causes in the network.
   This assumption has been eroded by the growth of interactive
   streaming which behaves in an inelastic manner and hence can cause
   high congestion at relatively low data volumes.  Currently LEDBAT-
   like transports get no incentive from the ISP since they still
   transfer large volumes of data and may reach high transfer speeds if
   the network is uncongested.  Consequently the only current incentive
   for LEDBAT is that it can reduce self-congestion effects.

   If the ISP has deployed a ConEx-aware Ingress Policer then they are
   able to incentivise the use of LEDBAT because a user will be policed
   according to the overall congestion volume their traffic generates,
   not the rate or data volume.  If all background file transfers are
   only generating a low level of congestion, then the sender has more
   "congestion budget" to "spend" on their interactive applications.  It
   can be shown [Kelly] that this approach improves social welfare -- in
   other words if you limit the congestion that all users can generate
   then everyone benefits from a better service.

5.3.  Accounting for Congestion Volume

   Accountability was one of the original design goals for the Internet
   [Design-Philosophy].  At the time it was ranked low because the
   network was non-commercial and it was assumed users had the best
   interests of the network at heart.  Nowadays users generally treat
   the network as a commodity and the Internet has become highly
   commercialised.  This causes problems for operators and others which
   they have tried to solve and often leads to a tragedy of the commons
   where users end up fighting each other for scarce peak capacity.

   The most elegant solution would be to introduce an Internet-wide
   system of accountability where every actor in the network is held to
   account for the impact they have on others.  If Policers are placed
   at every Network Ingress or Egress and Border Monitors at every
   border, then you have the basis for a system of congestion
   accounting.  Simply by controlling the overall Congestion Volume each
   end-system or stub-network can send you ensure everyone gets a better

5.4.  ConEx as a form of differential QoS

   Most QoS approaches require the active participation of routers to
   control the delay and loss characteristics for the traffic.  For
   real-time interactive traffic it is clear that low delay (and
   predictable jitter) are critical, and thus these probably always need
   different treatment at a router.  However if low loss is the issue
   then ConEx offers an alternative approach.

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   Assuming the ingress ISP has deployed a ConEx Ingress Policer, then
   the only control on a user's traffic is dependent on the congestion
   that user has caused.  Likewise, if they are receiving traffic
   through a ConEx Egress Policer then their ISP will impose traffic
   controls (prioritisation, rate limiting, etc) based on the congestion
   they have caused.  If an end-user (be they the receiver or sender)
   wants to prioritise some traffic over other traffic then they can
   allow that traffic to generate or cause more congestion.  The price
   they will pay will be to reduce the congestion that their other
   traffic causes.

   Streaming video content-delivery is a good candidate for such ConEx-
   mediated QoS.  Such traffic can tolerate moderately high delays, but
   there are strong economic pressures to maintain a high enough data
   rate (as that will directly influence the Quality of Experience the
   end-user receives.  This approach removes the need for bandwidth
   brokers to establish QoS sessions, by removing the need to coordinate
   requests from multiple sources to pre-allocate bandwidth, as well as
   to coordinate which allocations to revoke when bandwidth predictions
   turn out to be wrong.  There is also no need to "rate-police" at the
   boundaries on a per-flow basis, removing the need to keep per-flow
   state (which in turn makes this approach more scalable).

5.5.  Partial vs. Full Deployment

   In a fully-deployed ConEx-enabled internet, [QoS-Models] shows that
   ISP settlements based on congestion volume can allocate money to
   where upgrades are needed.  Fully-deployed implies that ConEx-marked
   packets which have not exhausted their expected congestion would go
   through a congested path in preference to non-ConEx packets, with
   money changing hands to justify that priority.

   In a partial deployment, routers that ignore ConEx markings and let
   them pass unaltered are no problem unless they become congested and
   drop packets.  Since ConEx incentivises the use of lower congestion
   transports, such congestion drops should anyway become rare events.
   ConEx-unaware routers that do drop ConEx-marked packets would cause a
   problem so to minimise this risk ConEx should be designed such that
   ConEx packets will appear valid to any node they traverse.  Failing
   that it could be possible to bypass such nodes with a tunnel.

   If any network is not ConEx enabled then the sender and receiver have
   to rely on ECN-marking or packet drops to establish the congestion
   level.  If the receiver isn't ConEx-enabled then there needs to be
   some form of compatibility mode.  Even in such partial deployments
   the end-users and access networks will benefit from ConEx.  This will
   put create incentives for ConEx to be more widely adopted as access
   networks put pressure on their backhaul providers to use congestion

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   as the basis of their interconnect agreement.

   The actual charge per unit of congestion would be specified in an
   interconnection agreement, with economic pressure driving that charge
   downward to the cost to upgrade whenever alternative paths are
   available.  That charge would most likely be invisible to the
   majority of users.  Instead such users will have a contractual
   allowance to cause congestion, and would see packets dropped when
   that allowance is depleted.

   Once an Autonomous System (AS) agrees to pay any congestion charges
   to any other AS it forwards to, it has an economic incentive to
   increase congestion-so-far marking for any congestion within its
   network.  Failure to do this quickly becomes a significant cost,
   giving it an incentive to turn on such marking.

   End users (or the writers of the applications they use) will be given
   an incentive to use a congestion control that back off more
   aggressively than TCP for any elastic traffic.  Indeed they will
   actually have an incentive to use fully weighted congestion controls
   that allow traffic to cause congestion in proportion to its priority.
   Traffic which backs off more aggressively than TCP will see
   congestion charges remain the same (or even drop) as congestion
   increases; traffic which backs off less aggressively will see charges
   rise, but the user may be prepared to accept this if it is high-
   priority traffic; traffic which backs off not at all will see charges
   rise dramatically.

6.  Other issues

6.1.  Congestion as a Commercial Secret

   Network operators have long viewed the congestion levels in their
   network as a business secret.  In some ways this harks back to the
   days of fixed-line telecommunications where congestion manifested as
   failed connections or dropped calls.  But even in modern data-centric
   packet networks congestion is viewed as a secret not to be shared
   with competitors.  It can be debated whether this view is sensible,
   but it may make operators uneasy about deploying ConEx.  The
   following two examples highlight some of the arguments used:

   o  An ISP buys backhaul capacity from an operator.  Most operators
      want their customers to get a decent service and so they want the
      backhaul to be relatively uncongested.  If there is competition,
      operators will seek to reassure their customers (the operators)
      that their network is not congested in order to attract their
      custom.  Some operators may see ConEx as a threat since it will
      enable those operators to see the actual congestion in their

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      network.  On the other hand, operators with low congestion could
      use ConEx to show how well their network performs, and so might
      have an incentive to enable it.

   o  Operators would like to be part of the lucrative content provision
      market.  Currently the ISP can gain a competitive edge as it can
      put its own content in a higher QoS class, whereas traffic from
      content providers has to use the Best Effort class.  The ISP may
      take the view that if they can conceal the congestion level in
      their Best Effort class this will make it harder for the content
      provider to maintain a good level of QoS.  But in reality the
      Content Provider will just use the feedback mechanisms in
      streaming protocols such as Adobe Flash to monitor the congestion.

   Of course some might say that the idea of keeping congestion secret
   is silly.  After all, end-hosts already have knowledge of the
   congestion throughout the network, albeit only along specific paths,
   and operators can work out that there is persistent congestion as
   their customers will be suffering degraded network performance.

6.2.  Information Security

   make a source believe it has seen more congestion than it has

   hijack a user's identity and make it appear they are dishonest at an
   egress policer

   clear or otherwise tamper with the ConEx markings


   {ToDo} Write these up properly...

7.  Security Considerations

   This document proposes a mechanism tagging onto Explicit Congestion
   Notification [RFC3168], and inherits the security issues listed
   therein.  The additional issues from ConEx markings relate to the
   degree of trust each forwarding point places in the ConEx markings it
   receives, which is a business decision mostly orthogonal to the
   markings themselves.

   One expected use of exposed congestion information is to hold the
   end-to-end transport and the network accountable to each other.  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 each in turn:

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   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 simply to
      reduce the amount of traffic it is carrying.  Congestion Exposure
      itself won't significantly alter the incentives for and against
      honest declaration of congestion by a network, but we can imagine
      applications of Congestion Exposure that will change these
      incentives.  There is a perception among network operators that
      their level of congestion is a business secret.  Today, congestion
      is one of the worst-kept secrets a network has, because end-hosts
      can see congestion better than network operators can.  Congestion
      Exposure will enable network operators to pinpoint whether
      congestion is on one side or the other of any border.  It is
      conceivable that forwarders with underprovisioned networks may try
      to obstruct deployment of Congestion Exposure.

   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.

   The Sender  One proposed mechanism for Congestion Exposure deployment
      adds a requirement for a sender to advise the network 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
      persistent congestion to back off more aggressively.  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.  "Policing" mechanisms have been proposed to
      deal with this.

   In addition there are potential problems from source spoofing.  A
   malicious sender can pretend to be another user by spoofing the
   source address.  Congestion Exposure allows for "Policers" and
   "Traffic Shapers" so as to be robust against injection of false
   congestion information into the forward path.

8.  IANA Considerations

   This document does not require actions by IANA.

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9.  Acknowledgments

   Bob Briscoe is partly funded by Trilogy, a research project (ICT-
   216372) supported by the European Community under its Seventh
   Framework Programme.  The views expressed here are those of the
   author only.

   The authors would like to thank Contributing Authors Bernard Aboba,
   Joao Taveira Araujo, Louise Burness, Alissa Cooper, Philip Eardley,
   Michael Menth, and Hannes Tschofenig for their inputs to this
   document.  Useful feedback was also provided by Dirk Kutscher.

10.  References

10.1.  Normative References

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

10.2.  Informative References

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

   [ConEx-Abstract-Mech]  Briscoe, B., "Congestion Exposure (ConEx)
                          Concepts and Abstract Mechanism",
                          draft-mathis-conex-abstract-mech-00 (work in
                          progress), October 2010.

   [Design-Philosophy]    Clarke, D., "The Design Philosophy of the
                          DARPA Internet Protocols", 1988.

   [Fair-use]             Broadband Choices, "Truth about 'fair usage'
                          broadband", 2009.

   [Fairer-faster]        Briscoe, B., "A Fairer Faster Internet
                          Protocol", IEEE Spectrum Dec 2008 pp38-43,
                          December 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

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                          control for communication networks: shadow
                          prices, proportional fairness and stability",
                          Journal of the Operational Research
                          Society 49(3) 237--252, 1998, <http://

   [LEDBAT]               Shalunov, S., "Low Extra Delay Background
                          Transport (LEDBAT)",
                          draft-ietf-ledbat-congestion-01 (work in
                          progress), March 2010.

   [Malice]               Briscoe, B., "Using Self Interest to Prevent
                          Malice; Fixing the Denial of Service Flaw of
                          the Internet", WESII - Workshop on the
                          Economics of Securing the Information
                          Infrastructure 2006, 2006, <http://

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

   [Padhye]               Padhye, J., Firoiu, V., Towsley, D., and J.
                          Kurose, "Modeling TCP Throughput: A Simple
                          Model and its Empirical Validation", ACM
                          SIGCOMM Computer Communications Review Vol
                          28(4), pages 303-314, May 1998.

   [Policing-freedom]     Briscoe, B., Jacquet, A., and T. Moncaster,
                          "Policing Freedom to Use the Internet Resource
                          Pool", RE-Arch 2008 hosted at the 2008 CoNEXT
                          conference , December 2008.

   [QoS-Models]           Briscoe, B. and S. Rudkin, "Commercial Models
                          for IP Quality of Service Interconnect", BTTJ
                          Special Edition on IP Quality of Service vol
                          23 (2), April 2005.

   [RFC2309]              Braden, B., Clark, D., Crowcroft, J., Davie,
                          B., Deering, S., Estrin, D., Floyd, S.,
                          Jacobson, V., Minshall, G., Partridge, C.,
                          Peterson, L., Ramakrishnan, K., Shenker, S.,
                          Wroclawski, J., and L. Zhang, "Recommendations
                          on Queue Management and Congestion Avoidance
                          in the Internet", RFC 2309, April 1998.

   [Re-Feedback]          Briscoe, B., Jacquet, A., Di Cairano-
                          Gilfedder, C., Salvatori, A., Soppera, A., and
                          M. Koyabe, "Policing Congestion Response in an

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                          Internetwork Using Re-Feedback", ACM SIGCOMM
                          CCR 35(4)277--288, August 2005, <http://

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

   [re-ecn-motive]        Briscoe, B., Jacquet, A., Moncaster, T., and
                          A. Smith, "Re-ECN: A Framework for adding
                          Congestion Accountability to TCP/IP",
                          (work in progress), October 2010.

Authors' Addresses

   Bob Briscoe
   B54/77, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE

   Phone: +44 1473 645196

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


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   Toby Moncaster (editor)
   Layer Marney
   Colchester  CO5 9UZ


   John Leslie (editor)
   10 Souhegan Street
   Milford, NH  03055


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