path: root/doc/rfc/rfc6789.txt

Internet Engineering Task Force (IETF)                   B. Briscoe, Ed.
Request for Comments: 6789                                            BT
Category: Informational                                   R. Woundy, Ed.
ISSN: 2070-1721                                                  Comcast
                                                          A. Cooper, Ed.
                                                                     CDT
                                                           December 2012


           Congestion Exposure (ConEx) Concepts and Use Cases

Abstract

   This document provides the entry point to the set of documentation
   about the Congestion Exposure (ConEx) protocol.  It explains the
   motivation for including a ConEx marking at the IP layer: to expose
   information about congestion to network nodes.  Although such
   information may have a number of uses, this document focuses on how
   the information communicated by the ConEx marking can serve as the
   basis for significantly more efficient and effective traffic
   management than what exists on the Internet today.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6789.














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Copyright Notice

   Copyright (c) 2012 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Congestion . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Congestion-Volume  . . . . . . . . . . . . . . . . . . . .  5
     2.3.  Rest-of-Path Congestion  . . . . . . . . . . . . . . . . .  6
     2.4.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Core Use Case: Informing Traffic Management  . . . . . . . . .  7
     3.1.  Use Case Description . . . . . . . . . . . . . . . . . . .  7
     3.2.  Additional Benefits  . . . . . . . . . . . . . . . . . . .  9
     3.3.  Comparison with Existing Approaches  . . . . . . . . . . .  9
   4.  Other Use Cases  . . . . . . . . . . . . . . . . . . . . . . . 11
   5.  Deployment Arrangements  . . . . . . . . . . . . . . . . . . . 12
   6.  Experimental Considerations  . . . . . . . . . . . . . . . . . 13
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   9.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 15
   10. Informative References . . . . . . . . . . . . . . . . . . . . 15

1.  Introduction

   The power of Internet technology comes from multiplexing shared
   capacity with packets rather than circuits.  Network operators aim to
   provide sufficient shared capacity, but when too much packet load
   meets too little shared capacity, congestion results.  Congestion
   appears as either increased delay, dropped packets, or packets
   explicitly marked with Explicit Congestion Notification (ECN)
   markings [RFC3168].  As described in Figure 1, congestion control
   currently relies on the transport receiver detecting these
   'Congestion Signals' and informing the transport sender in
   'Congestion Feedback Signals'.  The sender is then expected to reduce
   its rate in response.



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   This document provides the entry point to the set of documentation
   about the Congestion Exposure (ConEx) protocol.  It focuses on the
   motivation for including a ConEx marking at the IP layer.  (A
   companion document, [CONEX-ABS], focuses on the mechanics of the
   protocol.)  Briefly, the idea is for the sender to continually signal
   expected congestion in the headers of any data it sends.  To a first
   approximation, the sender does this by relaying the 'Congestion
   Feedback Signals' back into the IP layer.  They then travel unchanged
   across the network to the receiver (shown as 'IP-Layer-ConEx-Signals'
   in Figure 1).  This enables IP-layer devices on the path to see
   information about the whole-path congestion.

   ,---------.                                               ,---------.
   |Transport|                                               |Transport|
   | Sender  |   .                                           |Receiver |
   |         |  /|___________________________________________|         |
   |     ,-<---------------Congestion-Feedback-Signals--<--------.     |
   |     |   |/                                              |   |     |
   |     |   |\           Transport Layer Feedback Flow      |   |     |
   |     |   | \  ___________________________________________|   |     |
   |     |   |  \|                                           |   |     |
   |     |   |   '         ,-----------.               .     |   |     |
   |     |   |_____________|           |_______________|\    |   |     |
   |     |   |    IP Layer |           |  Data Flow      \   |   |     |
   |     |   |             |(Congested)|                  \  |   |     |
   |     |   |             |  Network  |--Congestion-Signals--->-'     |
   |     |   |             |  Device   |                    \|         |
   |     |   |             |           |                    /|         |
   |     `----------->--(new)-IP-Layer-ConEx-Signals-------->|         |
   |         |             |           |                  /  |         |
   |         |_____________|           |_______________  /   |         |
   |         |             |           |               |/    |         |
   `---------'             `-----------'               '     `---------'

         Figure 1: The ConEx Protocol in the Internet Architecture

   One of the key benefits of exposing this congestion information at
   the IP layer is that it makes the information available to network
   operators for use as input into their traffic management procedures.
   A ConEx-enabled sender signals expected whole-path congestion, which
   is approximately the congestion at least a round-trip time earlier as
   reported by the receiver to the sender (Figure 1).  The ConEx signal
   is a mark in the IP header that is easy for any IP device to read.
   Therefore, a node performing traffic management can count congestion
   as easily as it might count data volume today by simply counting the
   volume of packets with ConEx markings.





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   ConEx-based traffic management can make highly efficient use of
   capacity.  In times of no congestion, all traffic management
   restraints can be removed, leaving the network's full capacity
   available to all its users.  If some users on the network cause
   disproportionate congestion, the traffic management function can
   learn about this and directly limit those users' traffic in order to
   protect the service of other users sharing the same capacity.  ConEx-
   based traffic management thus presents a step change in terms of the
   options available to network operators for managing traffic on their
   networks.

   The remainder of this document explains the concepts behind ConEx and
   how exposing congestion can significantly improve Internet traffic
   management, among other benefits.  Section 2 introduces a number of
   concepts that are fundamental to understanding how ConEx-based
   traffic management works.  Section 3 shows how ConEx can be used for
   traffic management, discusses additional benefits from such usage,
   and compares ConEx-based traffic management to existing traffic
   management approaches.  Section 4 discusses other related use cases.
   Section 5 briefly discusses deployment arrangements.  Section 6
   suggests open issues that experiments in the use of ConEx could
   usefully be designed to answer.  The final sections are standard RFC
   back matter.

   The remainder of the core ConEx document suite consists of:

      [CONEX-ABS], which provides an abstract encoding of ConEx signals,
      explains the ConEx audit and security mechanisms, and describes
      incremental deployment features;

      [CONEX-DESTOPT], which specifies the IPv6 destination option
      encoding for ConEx;

      [TCP-MOD], which specifies TCP-sender modifications for use of
      ConEx;

      and the following documents, which describe some feasible
      scenarios for deploying ConEx:

         [CONEX-DEPLOY], which describes a scenario around a fixed
         broadband access network;

         [CONEX-MOBILE], which describes a scenario around a mobile
         communications provider;

         [CONEX-DATA], which describes how ConEx could be used for
         performance isolation between tenants of a data centre.




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2.  Concepts

   ConEx relies on a precise definition of congestion and a number of
   newer concepts that are introduced in this section.  Definitions are
   summarized in Section 2.4.

2.1.  Congestion

   Despite its central role in network control and management,
   congestion is a remarkably difficult concept to define.  Experts in
   different disciplines and with different perspectives define
   congestion in a variety of ways [Bauer09].

   The definition used for the purposes of ConEx is expressed as the
   probability of packet loss (or the probability of packet marking if
   ECN is in use).  This definition focuses on how congestion is
   measured, rather than describing congestion as a condition or state.

2.2.  Congestion-Volume

   The metric that ConEx exposes is congestion-volume: the volume of
   bytes dropped or ECN-marked in a given period of time.  Counting
   congestion-volume allows each user to be held responsible for his or
   her contribution to congestion.  Congestion-volume can only be a
   property of traffic, whereas congestion can be a property of traffic
   or a property of a link or a path.

   To understand congestion-volume, consider a simple example.  Imagine
   Alice sends 1 GB of a file while the loss-probability is a constant
   0.2%.  Her contribution to congestion -- her congestion-volume -- is
   1 GB x 0.2% = 2 MB.  If she then sends another 3 GB of the file while
   the loss-probability is 0.1%, this adds 3 MB to her congestion-
   volume.  Her total contribution to congestion is then 2 MB + 3 MB = 5
   MB.

   Fortunately, measuring Alice's congestion-volume on a real network
   does not require the kind of arithmetic shown above, because
   congestion-volume can be directly measured by counting the total
   volume of Alice's traffic that gets discarded or ECN-marked.  (A
   queue with varying percentage loss does these multiplications and
   additions inherently.)  With ConEx, network operators can count
   congestion-volume using techniques very similar to those they use for
   counting volume.








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2.3.  Rest-of-Path Congestion

   At a particular measurement point within a network, "rest-of-path
   congestion" (also known as "downstream congestion") is the level of
   congestion that a traffic flow is expected to experience between the
   measurement point and its final destination.  "Upstream congestion"
   is the congestion experienced up to the measurement point.

   If traffic is ECN-capable, ECN signals monitored in the middle of a
   network will indicate the congestion experienced so far on the path
   (upstream congestion).  In contrast, the ConEx signals inserted into
   IP headers as shown in Figure 1 indicate the congestion along a whole
   path from transport source to transport destination.  Therefore, if a
   measurement point detects both of these signals, it can subtract the
   level of ECN (upstream congestion) from the level of ConEx (whole
   path) to derive a measure of the congestion that packets are likely
   to experience between the monitoring point and their destination
   (rest-of-path congestion).  A measurement point can calculate this
   measurement in the aggregate, across all flows.

   A network monitor can usually accurately measure upstream congestion
   only if the traffic it observes is ECN-capable.  [CONEX-ABS] further
   discusses the constraints around the network's ability to measure
   upstream and rest-of-path congestion in these circumstances.
   However, there are a number of initial deployment arrangements that
   benefit from ConEx but work without ECN (see Section 5).

2.4.  Definitions

   Congestion:  In general, congestion occurs when any user's traffic
      suffers loss, ECN marking, or increased delay as a result of one
      or more network resources becoming overloaded.  For the purposes
      of ConEx, congestion is measured using the concrete signals
      provided by loss and ECN markings (delay is not considered).
      Congestion is measured as the probability of loss or the
      probability of ECN marking, usually expressed as a dimensionless
      percentage.

   Congestion-volume:  For any granularity of traffic (packet, flow,
      aggregate, link, etc.), the volume of bytes dropped or ECN-marked
      in a given period of time.  Conceptually, data volume multiplied
      by the congestion each packet of the volume experienced.  This is
      usually expressed in bytes (or kB, MB, etc.).

   Congestion policer:  A logical entity that allows a network operator
      to monitor each user's congestion-volume and enforce congestion-
      volume limits (discussed in Section 3.1).




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   Rest-of-path congestion (or downstream congestion):  The congestion a
      flow of traffic is expected to experience on the remainder of its
      path.  In other words, at a measurement point in the network, the
      rest-of-path congestion is the congestion the traffic flow has yet
      to experience as it travels from that point to the receiver.
      Upstream congestion is usually expressed as a dimensionless
      percentage.

   Upstream congestion:  The accumulated congestion experienced by a
      traffic flow thus far, relative to a point along its path.  In
      other words, at a measurement point in the network, the upstream
      congestion is the accumulated 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 sender.  This is usually
      expressed as a dimensionless percentage.

   Network operator (or provider):  Operator of a residential,
      commercial, enterprise, campus, or other network.

   User:  The contractual entity that represents an individual,
      household, business, or institution that uses the service of a
      network operator.  There is no implication that the contract has
      to be commercial; for instance, the users of a university or
      enterprise network service could be students or employees who do
      not pay for access, but may be required to comply with some form
      of contract or acceptable use policy.  There is also no
      implication that every user is an end user.  Where two networks
      form a customer-provider relationship, the term "user" applies to
      the customer network.

   [CONEX-ABS] gives further definitions for aspects of ConEx related to
   protocol mechanisms.

3.  Core Use Case: Informing Traffic Management

   This section explains how ConEx could be used as the basis for
   traffic management, highlights additional benefits derived from
   having ConEx-aware nodes on the network, and compares ConEx-based
   traffic management to existing approaches.

3.1.  Use Case Description

   One of the key benefits that ConEx can deliver is in helping network
   operators to improve how they manage traffic on their networks.
   Consider the common case of a commercial broadband network where a
   relatively small number of users place disproportionate demand on
   network resources, at times resulting in congestion.  The network



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   operator seeks a way to manage traffic such that the traffic that
   contributes more to congestion bears more of the brunt of the
   management.

   Assuming ConEx signals are visible at the IP layer, the network
   operator can accomplish this by placing a congestion policer at an
   enforcement point within the network and configuring it with a
   traffic management policy that monitors each user's contribution to
   congestion.  As described in [CONEX-ABS] and elaborated in [CongPol],
   one way to implement a congestion policer is in a similar way to a
   bit-rate policer, except that it monitors congestion-volume (based on
   IP-layer ConEx signals) rather than bit rate.  When implemented as a
   token bucket, the tokens provide users with the right to cause bits
   of congestion-volume, rather than to send bits of data volume.  The
   fill rate represents each user's congestion-volume quota.

   The congestion policer monitors the ConEx signals of the traffic
   entering the network.  As long as the network remains uncongested and
   users stay within their quotas, no action is taken.  When the network
   becomes congested and a user exhausts his quota, some action is taken
   against the traffic that breached the quota in accordance with the
   network operator's traffic management policy.  For example, the
   traffic may be dropped, delayed, or marked with a lower QoS class.
   In this way, traffic is managed according to its contribution to
   congestion -- not some application- or flow-specific policy -- and is
   not managed at all during times of no congestion.

   As an example of how a network operator might employ a ConEx-based
   traffic management system, consider a typical DSL network
   architecture (as elaborated in [TR-059] and [TR-101]).  Traffic is
   routed from regional and global IP networks to an operator-controlled
   IP node, the Broadband Remote Access Server (BRAS).  From the BRAS,
   traffic is delivered to access nodes.  The BRAS carries enhanced
   functionality, including IP QoS and traffic management capabilities.

   By deploying a congestion policer at the BRAS location, the network
   operator can measure the congestion-volume created by users within
   the access nodes and police misbehaving users before their traffic
   affects others on the access network.  The policer would be
   provisioned with a traffic management policy, perhaps directing the
   BRAS to drop packets from users that exceed their congestion-volume
   quotas during times of congestion.  Those users' apps would be likely
   to react in the typical way to drops, backing off (assuming at least
   some use TCP), and thereby lowering the users' congestion-volumes
   back within the quota limits.  If none of a user's apps responds, the
   policer would continue to increase focused drops and effectively
   enforce its own congestion control.




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3.2.  Additional Benefits

   The ConEx-based approach to traffic management has a number of
   benefits in addition to efficient management of traffic.  It provides
   incentives for users to make use of "scavenger" transport protocols,
   such as the Low Extra Delay Background Transport [LEDBAT], that
   provide ways for bulk-transfer applications to rapidly yield when
   interactive applications require capacity (thereby "scavenging"
   remaining bandwidth).  With a congestion policer in place as
   described in Section 3.1, users of these protocols will be less
   likely to run afoul of the network operator's traffic management
   policy than those whose bulk-transfer applications generate the same
   volume of traffic without being sensitive to congestion.  In short,
   two users who produce similar traffic volumes over the same time
   interval may produce different congestion-volumes if one of them is
   using a scavenger transport protocol and the other is not; in that
   situation, the scavenger user's traffic is less likely to be managed
   by the network operator.

   ConEx-based traffic management also makes it possible for a user to
   control the relative performance among its own traffic flows.  If a
   user wants some flows to have more bandwidth than others, it can
   reduce the rate of some traffic so that it consumes less congestion-
   volume "budget", leaving more congestion-volume "budget" for the user
   to "spend" on making other traffic go faster.  This approach is most
   relevant if congestion is signalled by ECN, because no impairment due
   to loss is involved and delay can remain low.

3.3.  Comparison with Existing Approaches

   A variety of approaches already exist for network operators to manage
   congestion, traffic, and the disproportionate usage of scarce
   capacity by a small number of users.  Common approaches can be
   categorized as rate-based, volume-based, or application-based.

   Rate-based approaches constrain the traffic rate per user or per
   network.  A user's peak and average (or "committed") rate may be
   limited.  These approaches have the potential to either over- or
   under-constrain the network, suppressing rates even when the network
   is uncongested or not suppressing them enough during heavy usage
   periods.

   Round-robin scheduling and fair queuing were developed to address
   these problems.  They equalize relative rates between active users
   (or flows) at a known bottleneck.  The bit rate allocated to any one
   user depends on the number of active users at each instant.  The
   drawback of these approaches is that they favor heavy users over
   light users over time, because they do not have any memory of usage.



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   These approaches share bit rate instant by instant; however, heavy
   users are active at every instant, whereas light users only occupy
   their share of the link occasionally.

   Volume-based approaches measure the overall volume of traffic a user
   sends (and/or receives) over time.  Users may be subject to an
   absolute volume cap (for example, 10GB per month) or the "heaviest"
   users may be sanctioned in some other manner.  Many providers use
   monthly volume limits, and count volume regardless of whether the
   network is congested or not, creating the potential for over- or
   under-constraining problems, as with the original rate-based
   approaches.

   ConEx-based approaches, by comparison, only react during times of
   congestion and in proportion to each user's congestion contribution,
   making more efficient use of capacity and more proportionate
   management decisions.

   Unlike ConEx-based approaches, neither rate-based nor volume-based
   approaches provide incentives for applications to use scavenger
   transport protocols.  They may even penalize users of applications
   that employ scavenger transports for the large amount of volume they
   send, rather than rewarding them for carefully avoiding congestion
   while sending it.  While the volume-based approach described in
   "Comcast's Protocol-Agnostic Congestion Management System" [RFC6057]
   aims to overcome the over-/under-constraining problem by only
   measuring volume and triggering traffic management action during
   periods of high utilization, it still does not provide incentives to
   use scavenger transports, because congestion-causing volume cannot be
   distinguished from volume overall.  ConEx provides this ability.

   Application-based approaches use deep packet inspection or other
   techniques to determine what application a given traffic flow is
   associated with.  Network operators may then use this information to
   rate-limit or otherwise sanction certain applications, in some cases
   only during peak hours.  These approaches suffer from being at odds
   with IPsec and some application-layer encryption, and they may raise
   additional policy concerns.  In contrast, ConEx offers an
   application-agnostic metric to serve as the basis for traffic
   management decisions.

   The existing types of approaches share a further limitation that
   ConEx can help to overcome: 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 pricing avoids billing uncertainty, it creates performance
   uncertainty: users cannot know whether the performance of their



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   connections is being altered or degraded based on how the network
   operator is attempting to manage congestion.  By exposing congestion
   information at the IP layer, ConEx instead provides a metric that can
   serve as an open, transparent basis for traffic management policies
   that both providers and their customers can measure and verify.  It
   can be used to reduce the performance uncertainty that some users
   currently experience, without having to sacrifice their billing
   certainty.

4.  Other Use Cases

   ConEx information can be put to a number of uses other than informing
   traffic management.  These include:

   Informing inter-operator contracts:  ConEx information is made
      visible to every IP node, including border nodes between networks.
      Network operators can use ConEx combined with ECN markings to
      measure how much traffic from each network contributes to
      congestion in the other.  As such, congestion-volume could be
      included as a metric in inter-operator contracts, just as volume
      or bit rate are included today.  This would not be an initial
      deployment scenario, unless ECN became widely deployed.

   Enabling more efficient capacity provisioning:  Section 3.2 explains
      how operators can use ConEx-based traffic management to encourage
      use of scavenger transport protocols, which significantly improves
      the performance of interactive applications while still allowing
      heavy users to transfer high volumes.  Here we explain how this
      can also benefit network operators.

      Today, when loss, delay, or average utilization exceeds a certain
      threshold, some operators just buy more capacity without
      attempting to manage the traffic.  Other operators prefer to limit
      a minority of heavy users at peak times, but they still eventually
      buy more capacity when utilization rises.

      With ConEx-based traffic management, a network operator should be
      able to provision capacity more efficiently.  An operator could
      benefit from this in a variety of ways.  For example, the operator
      could add capacity as it would do without ConEx, but deliver
      better quality of service for its users.  Or, the operator could
      delay adding capacity while delivering similar quality of service
      to what it currently provides.








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5.  Deployment Arrangements

   ConEx is designed so that it can be incrementally deployed in the
   Internet and still be valuable for early adopters.  As long as some
   senders are ConEx-enabled, a network on the path can unilaterally use
   ConEx-aware policy devices for traffic management; no changes to
   network forwarding elements are needed, and ConEx still works if
   there are other networks on the path that are unaware of ConEx marks.

   The above two steps seem to represent a stand-off where neither step
   is useful until the other has made the first move: i) some sending
   hosts must be modified to give information to the network, and ii) a
   network must deploy policy devices to monitor this information and
   act on it.  Nonetheless, the developer of a scavenger transport
   protocol like LEDBAT does stand to benefit from deploying ConEx.  In
   this case, the developer makes the first move, expecting it will
   prompt at least some networks to move in response, using the ConEx
   information to reward users of the scavenger transport protocol.

   On the host side, we have already shown (Figure 1) how the sender
   piggy-backs ConEx signals on normal data packets to re-insert
   feedback about packet drops (and/or ECN) back into the IP layer.  In
   the case of TCP, [TCP-MOD] proposes the required sender
   modifications.  ConEx works with any TCP receiver as long as it uses
   SACK (selective acknowledgment), which most do.  There is a receiver
   optimisation [TCPM-ECN] that improves ConEx precision when using ECN,
   but ConEx can still use ECN without it.  Networks can make use of
   ConEx even if the implementations of some of the transport protocols
   on a host do not support ConEx (e.g., the implementation of DNS over
   UDP might not support ConEx, while perhaps RTP over UDP and TCP
   will).

   On the network side, the provider solely needs to place ConEx
   congestion policers at each ingress to its network in a similar
   arrangement to the edge-policed architecture of Diffserv [RFC2475].

   A sender can choose whether to send packets that support ConEx or
   packets that don't.  ConEx-enabled packets bring information to the
   policer about congestion expected on the rest of the path beyond the
   policer.  Packets that do not support ConEx bring no such
   information.  Therefore, the network will tend to conservatively
   rate-limit non-ConEx-enabled packets in order to manage the unknown
   risk of congestion.  In contrast, a network doesn't normally need to
   rate-limit ConEx-enabled packets unless they reveal a persistently
   high contribution to congestion.  This natural tendency for networks
   to favour senders that provide ConEx information reinforces ConEx
   deployment.




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   Feasible initial deployment scenarios exist for a broadband access
   network [CONEX-DEPLOY], a mobile communications network
   [CONEX-MOBILE], and a multi-tenant data centre [CONEX-DATA].  The
   first two of these scenarios are believed to work well without ECN
   support, while the data center scenario works best with ECN (and ECN
   can be deployed unilaterally).

   The above gives only the most salient aspects of ConEx deployment.
   For further detail, [CONEX-ABS] describes the incremental deployment
   features of the ConEx protocol and the components that need to be
   deployed for ConEx to work.

6.  Experimental Considerations

   ConEx is initially designed as an experimental protocol because it
   makes an ambitious change at the interoperability (IP) layer, so no
   amount of careful design can foresee all the potential feature
   interactions with other uses of IP.  This section identifies a number
   of questions that would be useful to answer through well-designed
   experiments:

   o  Are the compromises that were made in order to fit the ConEx
      encoding into IP (for example, that the initial design was solely
      for IPv6 and not for IPv4, and that the encoding has limited
      visibility when tunnelled [CONEX-DESTOPT]) the right ones?

   o  Is it possible to combine techniques for distinguishing self-
      congestion from shared congestion with ConEx-based traffic
      management such that users are not penalized for congestion that
      does not impact others on the network?  Are other techniques
      needed?

   o  In practice, how does traffic management using ConEx compare with
      traditional techniques (Section 3.3)?  Does it give the benefits
      claimed in Sections 3.1 and 3.2?

   o  Approaches are proposed for congestion policing of ConEx traffic
      alongside existing management (or lack thereof) of non-ConEx
      traffic, including UDP traffic [CONEX-ABS].  Are they strategy-
      proof against users selectively using both?  Are there better
      transition strategies?

   o  Audit devices have been designed and implemented to assure ConEx
      signal integrity [CONEX-ABS].  Do they achieve minimal false hits
      and false misses in a wide range of traffic scenarios?  Are there
      new attacks?  Are there better audit designs to defend against
      these?




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   o  If ECN deployment remains patchy, are the proposed initial ConEx
      deployment scenarios (Section 5) still useful enough to kick-start
      deployment?  Is auditing effective when based on loss at a primary
      bottleneck?  Can rest-of-path congestion be approximated
      accurately enough without ECN?  Are there other useful deployment
      scenarios?

   ConEx is intended to be a generative technology that might be used
   for unexpected purposes unforeseen by the designers.  Therefore, this
   list of experimental considerations is not intended to be exhaustive.

7.  Security Considerations

   This document does not specify a mechanism, it merely motivates
   congestion exposure at the IP layer.  Therefore, security
   considerations are described in the companion document that gives an
   abstract description of the ConEx protocol and the components that
   would use it [CONEX-ABS].

8.  Acknowledgments

   Bob Briscoe was 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 the many people that have commented
   on this document: Bernard Aboba, Mikael Abrahamsson, Joao Taveira
   Araujo, Marcelo Bagnulo Braun, Steve Bauer, Caitlin Bestler, Steven
   Blake, Louise Burness, Ken Carlberg, Nandita Dukkipati, Dave McDysan,
   Wes Eddy, Matthew Ford, Ingemar Johansson, Georgios Karagiannis,
   Mirja Kuehlewind, Dirk Kutscher, Zhu Lei, Kevin Mason, Matt Mathis,
   Michael Menth, Chris Morrow, Tim Shepard, Hannes Tschofenig, and
   Stuart Venters.  Please accept our apologies if your name has been
   missed off this list.
















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9.  Contributors

   Philip Eardley and Andrea Soppera made helpful text contributions to
   this document.

   The following co-edited this document through most of its life:

      Toby Moncaster
      Computer Laboratory
      William Gates Building
      JJ Thomson Avenue
      Cambridge, CB3 0FD
      UK
      EMail: toby.moncaster@cl.cam.ac.uk

      John Leslie
      JLC.net
      10 Souhegan Street
      Milford, NH  03055
      US
      EMail: john@jlc.net

10.  Informative References

   [Bauer09]       Bauer, S., Clark, D., and W. Lehr, "The Evolution of
                   Internet Congestion", 2009.

   [CONEX-ABS]     Mathis, M. and B. Briscoe, "Congestion Exposure
                   (ConEx) Concepts and Abstract Mechanism", Work
                   in Progress, October 2012.

   [CONEX-DATA]    Briscoe, B. and M. Sridharan, "Network Performance
                   Isolation in Data Centres using Congestion Exposure
                   (ConEx)", Work in Progress, July 2012.

   [CONEX-DEPLOY]  Briscoe, B., "Initial Congestion Exposure (ConEx)
                   Deployment Examples", Work in Progress, July 2012.

   [CONEX-DESTOPT] Krishnan, S., Kuehlewind, M., and C. Ucendo, "IPv6
                   Destination Option for ConEx", Work in Progress,
                   September 2012.

   [CONEX-MOBILE]  Kutscher, D., Mir, F., Winter, R., Krishnan, S.,
                   Zhang, Y., and C. Bernardos, "Mobile Communication
                   Congestion Exposure Scenario", Work in Progress,
                   July 2012.





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

   [LEDBAT]        Shalunov, S., Hazel, G., Iyengar, J., and M.
                   Kuehlewind, "Low Extra Delay Background Transport
                   (LEDBAT)", Work in Progress, September 2012.

   [RFC2475]       Blake, S., Black, D., Carlson, M., Davies, E., Wang,
                   Z., and W. Weiss, "An Architecture for Differentiated
                   Services", RFC 2475, December 1998.

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

   [RFC6057]       Bastian, C., Klieber, T., Livingood, J., Mills, J.,
                   and R. Woundy, "Comcast's Protocol-Agnostic
                   Congestion Management System", RFC 6057,
                   December 2010.

   [TCP-MOD]       Kuehlewind, M. and R. Scheffenegger, "TCP
                   modifications for Congestion Exposure", Work
                   in Progress, May 2012.

   [TCPM-ECN]      Kuehlewind, M. and R. Scheffenegger, "More Accurate
                   ECN Feedback in TCP", Work in Progress, July 2012.

   [TR-059]        Anschutz, T., Ed., "DSL Forum Technical Report
                   TR-059: Requirements for the Support of QoS-Enabled
                   IP Services", September 2003.

   [TR-101]        Cohen, A., Ed. and E. Schrum, Ed., "DSL Forum
                   Technical Report TR-101: Migration to Ethernet-Based
                   DSL Aggregation", April 2006.















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Authors' Addresses

   Bob Briscoe (editor)
   BT
   B54/77, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE
   UK

   Phone: +44 1473 645196
   EMail: bob.briscoe@bt.com
   URI:   http://bobbriscoe.net/


   Richard Woundy (editor)
   Comcast
   1701 John F Kennedy Boulevard
   Philadelphia, PA  19103
   US

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


   Alissa Cooper (editor)
   CDT
   1634 Eye St. NW, Suite 1100
   Washington, DC  20006
   US

   EMail: acooper@cdt.org




















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