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RFC 7713

Informational
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Congestion Exposure (ConEx) Concepts, Abstract Mechanism, and Requirements

 


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Internet Engineering Task Force (IETF)                         M. Mathis
Request for Comments: 7713                                  Google, Inc.
Category: Informational                                       B. Briscoe
ISSN: 2070-1721                                                       BT
                                                           December 2015


       Congestion Exposure (ConEx) Concepts, Abstract Mechanism,
                            and Requirements

Abstract

   This document describes an abstract mechanism by which senders inform
   the network about the congestion recently encountered by packets in
   the same flow.  Today, network elements at any layer may signal
   congestion to the receiver by dropping packets or by Explicit
   Congestion Notification (ECN) markings, and the receiver passes this
   information back to the sender in transport-layer feedback.  The
   mechanism described here enables the sender to also relay this
   congestion information back into the network in-band at the IP layer,
   such that the total amount of congestion from all elements on the
   path is revealed to all IP elements along the path, where it could,
   for example, be used to provide input to traffic management.  This
   mechanism is called Congestion Exposure, or ConEx.  The companion
   document, "Congestion Exposure (ConEx) Concepts and Use Cases"
   (RFC 6789), provides the entry point to the set of ConEx
   documentation.

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/rfc7713.

Page 2 
Copyright Notice

   Copyright (c) 2015 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Requirements for the ConEx Abstract Mechanism . . . . . . . .   7
     3.1.  Requirements for ConEx Signals  . . . . . . . . . . . . .   7
     3.2.  Constraints on the Audit Function . . . . . . . . . . . .   8
     3.3.  Requirements for Non-abstract ConEx Specifications  . . .   9
   4.  Encoding Congestion Exposure  . . . . . . . . . . . . . . . .  12
     4.1.  Naive Encoding  . . . . . . . . . . . . . . . . . . . . .  12
     4.2.  Null Encoding . . . . . . . . . . . . . . . . . . . . . .  13
     4.3.  ECN-Based Encoding  . . . . . . . . . . . . . . . . . . .  13
     4.4.  Independent Bits  . . . . . . . . . . . . . . . . . . . .  14
     4.5.  Codepoint Encoding  . . . . . . . . . . . . . . . . . . .  14
     4.6.  Units Implied by an Encoding  . . . . . . . . . . . . . .  15
   5.  Congestion Exposure Components  . . . . . . . . . . . . . . .  16
     5.1.  Network Devices (Not Modified)  . . . . . . . . . . . . .  16
     5.2.  Modified Senders  . . . . . . . . . . . . . . . . . . . .  16
     5.3.  Receivers (Optionally Modified) . . . . . . . . . . . . .  17
     5.4.  Policy Devices  . . . . . . . . . . . . . . . . . . . . .  17
       5.4.1.  Congestion Monitoring Devices . . . . . . . . . . . .  18
       5.4.2.  Rest-of-Path Congestion Monitoring  . . . . . . . . .  18
       5.4.3.  Congestion Policers . . . . . . . . . . . . . . . . .  18
     5.5.  Audit . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   6.  Support for Incremental Deployment  . . . . . . . . . . . . .  23
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  27
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  27
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

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1.  Introduction

   This document describes an abstract mechanism by which, to a first
   approximation, senders inform the network about the congestion
   encountered by packets earlier in the same flow.  It is not a
   complete protocol specification because it is known that designing an
   encoding (e.g., packet formats, codepoint allocations, etc.) is
   likely to entail compromises that preclude some uses of the protocol.
   The goal of this document is to provide a framework for developing
   and testing algorithms to evaluate the benefits of the ConEx protocol
   and to evaluate the consequences of the compromises in various
   different encoding designs.  This document lays out requirements for
   concrete protocol specifications.

   A companion document [RFC6789] provides the entry point to the set of
   ConEx documentation.  It outlines concepts that are prerequisites to
   understanding why ConEx is useful, and it outlines various ways that
   ConEx might be used.

2.  Overview

   As typical end-to-end transport protocols continually seek out more
   network capacity, network elements signal whenever congestion
   results, and the transports are responsible for controlling this
   network congestion [RFC5681].  The more a transport tries to use
   capacity that others want to use, the more congestion signals will be
   attributable to that transport.  Likewise, the more transport
   sessions sustained by a user and the longer the user sustains them,
   the more congestion signals will be attributable to that user.  The
   goal of ConEx is to ensure that the resulting congestion signals are
   sufficiently visible and robust, because they are an ideal metric for
   networks to use as the basis of traffic management or other related
   functions.

   Networks indicate congestion by three possible signals: packet loss,
   ECN marking, or queueing delay.  ECN marking and some packet loss may
   be the outcome of Active Queue Management (AQM), which the network
   uses to warn senders to reduce their rates.  Packet loss is also the
   natural consequence of complete exhaustion of a buffer or other
   network resource.  Some experimental transport protocols and TCP
   variants infer impending congestion from increasing queuing delay.
   However, delay is too amorphous to use as a congestion metric.  In
   this and other ConEx documents, the term 'congestion signals' is
   generally used solely for ECN markings and packet losses because they
   are unambiguous signals of congestion.

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   In both cases, the congestion signals follow the route indicated in
   Figure 1.  A congested network device sends a signal in the data
   stream on the forward path to the transport receiver, the receiver
   passes it back to the sender through transport-level feedback, and
   the sender makes some congestion control adjustment.

   This document extends the capabilities of the Internet protocol suite
   with the addition of a new Congestion Exposure signal.  To a first
   approximation, this signal (also shown in Figure 1) relays the
   congestion information from the transport sender back through the
   internetwork layer where it is visible to any interested
   internetwork-layer devices along the forward path.  This document
   frames the engineering problem of designing the ConEx Signal.  The
   requirements are described in Section 3 and some example encodings
   are presented in Section 4.  Section 5 describes all of the protocol
   components.

   This new signal is expressly designed to support a variety of new
   policy mechanisms that might be used to instrument, monitor, or
   manage traffic.  The policy devices are not shown in Figure 1 but
   might be placed anywhere along the forward data path (see
   Section 5.4).

   ,---------.                                               ,---------.
   |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 Flow of Congestion and ConEx Signals

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   Since the policy devices can affect how traffic is treated, it is
   assumed that there is an intrinsic motivation for users,
   applications, or operating systems to understate the congestion that
   they are causing.  Therefore, it is important to be able to audit
   ConEx Signals and to be able to apply sufficient sanction to
   discourage cheating of congestion policies.  The general approach to
   auditing is to count signals on the forward path to confirm that
   there are never fewer ConEx Signals than congestion signals.  Many
   ConEx design constraints come from the need to assure that the audit
   function is sufficiently robust.  The audit function is described in
   Section 5.5; however, significant portions of this document (and
   prior research [Refb-dis]) are motivated by issues relating to the
   audit function and making it robust.

   The congestion and ConEx Signals shown in Figure 1 represent a series
   of discrete events: ECN marks or lost packets, carried by the forward
   data stream and fed back into the internetwork layer.  The policy and
   audit functions are most likely to act on the accumulated values of
   these signals, for which we use the term "volume".  For example,
   "traffic volume" is the total number of bytes delivered optionally
   over a specified time interval and over some aggregate of traffic
   (e.g., all traffic from a site), while "loss volume" is the total
   amount of bytes discarded from some aggregate over an interval.  The
   term "congestion-volume" is defined precisely in [RFC6789].  Note
   that volume per unit time is average rate.

   A design goal of the ConEx protocol is that the important policy
   mechanisms can be implemented per logical link without per-flow state
   (see Section 5.4).  However, the trade-off is that per-flow state
   could be needed to audit ConEx Signals (Section 5.5).  This is
   justified in that i) auditing at the edges, with a limited number of
   flows, enables policy elsewhere, including in the core, without any
   per-flow state; ii) auditing can use soft flow state, which does not
   require route pinning.

   There is a long standing argument over units of congestion: bytes vs
   packets (see [RFC7141] and its references).  Section 4.6 explains why
   this problem must be addressed carefully.  However, this document
   does not take a strong position on this issue.  Nonetheless, it does
   require that the units of congestion must be an explicitly stated
   property of any proposed encoding, and the consequences of that
   design decision must be evaluated along with other aspects of the
   design.

   To be successful, the ConEx protocol needs to have the property that
   the relevant stakeholders each have the incentive to unilaterally
   start on each stage of partial deployment, which in turn creates

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   incentives for further deployment.  Furthermore, legacy systems that
   will never be upgraded do not become a barrier to deploying ConEx.
   Issues relating to partial deployment are described in Section 6.

   Note that ConEx Signals are not intended to be used for fine-grained
   congestion control.  They are anticipated to be most useful at longer
   time scales and/or at coarser granularity than single microflows.
   For example, the total congestion caused by a user might serve as an
   input to higher-level policy or accountability functions designed to
   create incentives for improving user behavior, such as choosing to
   send large quantities of data at off-peak times, at lower data rates,
   or with less aggressive protocols such as Low Extra Delay Background
   Transport (LEDBAT) [RFC6817]; see [RFC6789].

   Ultimately, ConEx Signals have the potential to provide a mechanism
   to regulate global Internet congestion.  From the earliest days of
   research on congestion control, there has been a concern that there
   is no mechanism to prevent transport designers from incrementally
   making protocols more aggressive without bound and spiraling to a
   "tragedy of the commons" Internet congestion collapse.  The "TCP
   friendly" paradigm was created in part to forestall this failure.
   However, it no longer commands any authority because it has little to
   say about the Internet of today, which has moved beyond the scaling
   range of standard TCP.  As a consequence, many transports and
   applications are opening arbitrarily large numbers of connections or
   using arbitrary levels of aggressiveness.  ConEx represents a
   recognition that the IETF cannot regulate this space directly because
   it concerns the behaviour of users and applications, not individual
   transport protocols.  Instead, the IETF can give network operators
   the protocol tools to arbitrate the space themselves with better bulk
   traffic management.  This, in turn, should create incentives for
   users and designers of applications and of transport protocols to be
   more mindful about contributing to congestion.

2.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   ConEx Signals in IP packet headers from the sender to the network:

   Not-ConEx:  The transport (or at least this packet) is not using
      ConEx.

   ConEx-Capable:  The transport is using ConEx.  This is the opposite
      of Not-ConEx.

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   ConEx Signal:  A signal in a packet sent by a ConEx-capable
      transport.  It carries at least one of the following signals:

      Re-Echo-Loss:  The transport has experienced a loss.

      Re-Echo-ECN:  The transport has detected an ECN Congestion
         Experienced (CE) mark.

      Credit:  The transport is building up credit to signal advance
         notice of the risk of packets contributing to congestion, in
         contrast to signalling only after inherently delayed feedback
         of actual congestion.

      ConEx-Not-Marked:  The transport is ConEx-capable but is not
         signaling Re-Echo-Loss, Re-Echo-ECN, or Credit.

   ConEx-Marked:  At least one of Re-Echo-Loss, Re-Echo-ECN, or Credit.

   ConEx-Re-Echo:  At least one of Re-Echo-Loss or Re-Echo-ECN.

3.  Requirements for the ConEx Abstract Mechanism

   First-time readers may wish to skim this section, since it is more
   understandable having read the entire document.

3.1.  Requirements for ConEx Signals

   Ideally, all the following requirements would be met by a Congestion
   Exposure Signal:

   a.  The ConEx Signal SHOULD be visible to internetwork-layer devices
       along the entire path from the transport sender to the transport
       receiver.  Equivalently, it SHOULD be present in the IPv4 or IPv6
       header and in the outermost IP header if using IP-in-IP
       tunneling.  It MAY need to be visible if other encapsulating
       headers are used to interconnect networks.  The ConEx Signal
       SHOULD be immutable once set by the transport sender.  A
       corollary of these requirements is that the chosen ConEx encoding
       SHOULD pass silently without modification through preexisting
       networking gear.

   b.  The ConEx Signal SHOULD be useful under only partial deployment.
       A minimal deployment SHOULD only require changes to transport
       senders.  Furthermore, partial deployment SHOULD create
       incentives for additional deployment, both in terms of enabling
       ConEx on more devices and adding richer features to existing
       devices.  Nonetheless, ConEx deployment need never be universal,

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       and it is anticipated that some hosts and some transports may
       never support the ConEx protocol and some networks may never use
       the ConEx Signals.

   c.  The ConEx Signal SHOULD be timely.  There will be a minimum delay
       of one RTT and often longer if the transport protocol sends
       infrequent feedback (consider Real-time Transport Control
       Protocol (RTCP) [RFC3550] [RFC6679], for example).

   d.  The ConEx Signal SHOULD be accurate and auditable.  The general
       approach for auditing is to observe the volume of congestion
       signals and ConEx Signals on the forward data path and verify
       that the ConEx Signals do not underrepresent the congestion
       signals (see Section 5.5).

   e.  The ConEx Signals for packet loss and ECN marking SHOULD have
       distinct encodings because they are likely to require different
       auditing techniques.

   f.  Additionally, there SHOULD be an auditable ConEx Credit signal.
       A sender can use Credit to indicate potential future congestion,
       for example, as is often seen during startup.  ConEx Credit is
       intended to overestimate congestion actually experienced across
       the network.

   It is already known that implementing ConEx Signals is likely to
   entail some compromises, and therefore, all the requirements above
   are expressed with the keyword "SHOULD" rather than "MUST".  The only
   mandatory requirement is that a concrete protocol description MUST
   give sound reasoning if it chooses not to meet some requirement.

3.2.  Constraints on the Audit Function

   The role of the audit function and constraints on it are described in
   Section 5.5.  There is no intention to standardise the audit
   function.  However, it is necessary to lay down the following
   normative constraints on audit behaviour so that transport designers
   will know what to design against and implementers of audit devices
   will know what pitfalls to avoid:

   Minimal False Hits:  Audit SHOULD introduce minimal false hits for
      honest flows.

   Minimal False Misses:  Audit SHOULD quickly detect and sanction
      dishonest flows, ideally on the first dishonest packet.

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   Transport Oblivious:  Audit SHOULD NOT be designed around one
      particular rate response, such as any particular TCP congestion
      control algorithm or one particular resource-sharing regime such
      as TCP friendliness [RFC5348].  An important goal is to give
      ingress networks the freedom to unilaterally allow different rate
      responses to congestion and different resource sharing regimes
      [Evol_cc] without having to coordinate with other networks over
      details of individual flow behaviour.

   Sufficient Sanction:  Audit SHOULD introduce sufficient sanction
      (e.g., loss in goodput) such that senders cannot gain from
      understating congestion.

   Proportionate Sanction:  To the extent that the audit might be
      subject to false hits, the sanction SHOULD be proportionate to the
      degree to which congestion is understated.  If the audit over-
      punishes, attackers will find ways to harness it into amplifying
      attacks on others.  Ideally the audit should, in the long run,
      cause the user to get no better performance than they would get by
      being accurate.

   Manage Memory Exhaustion:  Audit SHOULD be able to counter state-
      exhaustion attacks.  For instance, if the audit function uses flow
      state, it should not be possible for senders to exhaust its memory
      capacity by gratuitously sending numerous packets, each with a
      different flow ID.

   Identifier Accountability:  Audit SHOULD NOT be vulnerable to
      'identity whitewashing', where a transport can label a flow with a
      new ID more cheaply than paying the cost of continuing to use its
      current ID [CheapPseud].

3.3.  Requirements for Non-abstract ConEx Specifications

   An experimental ConEx specification SHOULD describe the following
   protocol details:

   Network Layer:

      A.  the specific ConEx Signal encodings with packet formats, bit
          fields, and/or codepoints;

      B.  an inventory of invalid combinations of flags or invalid
          codepoints in the encoding, as well as whether security
          gateways should normalise, discard, or ignore such invalid
          encodings, and what values they should be considered
          equivalent to by ConEx-aware elements;

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      C.  an inventory of any conflated signals or any other effects
          that are known to compromise signal integrity;

      D.  whether the source is responsible for allowing for the round-
          trip delay in ConEx Signals (e.g., using a Credit marking),
          and if so, whether Credit is maintained for the duration of a
          flow or degrades over time, and what defines the end of the
          duration of a flow;

      E.  a specification for signal units (bytes vs. packets, etc.),
          any approximations allowed, and the algorithms to do any
          implied conversions or accounting;

      F.  if the units are bytes, a definition of which headers are
          included in the size of the packet;

      G.  how tunnels should propagate the ConEx encoding;

      H.  whether the encoding fields are mutable or not, to ensure that
          header authentication, checksum calculation, etc., process
          them correctly; a ConEx encoding field SHOULD be immutable
          end-to-end, then endpoints can detect if it has been tampered
          with in transit;

      I.  if a specific encoding allows mutability (e.g., at proxies),
          then an inventory of invalid transitions between codepoints;
          in all encodings, transitions from any ConEx marking to Not-
          ConEx MUST be invalid;

      J.  a statement that the ConEx encoding is only applicable to
          unicast and anycast and that forwarding elements should
          silently ignore any ConEx signalling on multicast packets
          (they should be forwarded unchanged);

      K.  the definition of any extensibility;

      L.  backward and forward compatibility and potential migration
          strategies; in all cases, a ConEx encoding MUST be arranged so
          that legacy transport senders implicitly send Not-ConEx;

      M.  any (optional) modification to data-plane forwarding dependent
          on the encoding (e.g., preferential discard, interaction with
          Diffserv, ECN, etc.); and

      N.  any warning or error messages relevant to the encoding.

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      Note regarding item J on multicast: A multicast tree may involve
      different levels of congestion on each leg.  Any traffic
      management can only monitor or control multicast congestion at or
      near each receiver.  It would make no sense for the sender to try
      to expose "whole-path congestion" in sent packets because it
      cannot hope to describe all the differing congestion levels on
      every leg of the tree.

   Transport Layer:

      A.  a specification of any required changes to congestion feedback
          in particular transport protocols;

      B.  a specification (or, minimally, a recommendation) for how a
          transport should estimate credits at the beginning of a
          connection and while it is in progress;

      C.  a specification of whether any other protocol options should
          (or must) be enabled along with an implementation of ConEx
          (e.g., at least attempting to negotiate ECN and Selective
          Acknowledgement (SACK) capability);

      D.  a specification of any configuration that a ConEx stack may
          require (or, preferably, confirmation that it requires no
          configuration); and

      E.  a specification of the statistics that a protocol stack should
          log for each type of marking on a per-flow or aggregate basis.

   Security:

      A.  an example of a strong audit algorithm suitable for detecting
          if a single flow is misstating congestion; this algorithm
          should present minimal false results but need not have optimal
          scaling properties (e.g., may need per-flow state).

      B.  an example of an audit algorithm suitable for detecting
          misstated congestion in a large aggregate (e.g., no per-flow
          state).

      C.  a definition of the level of ConEx-Re-Echo and ConEx-Credit
          signals that will be sufficient to pass audit (see
          Section 5.5).

   The possibility exists that these specifications overconstrain the
   ConEx design and can not be fully satisfied.  An important part of
   the evaluation of any particular design will be a thorough inventory
   of all ways in which it might fail to satisfy these specifications.

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4.  Encoding Congestion Exposure

   Most protocol specifications start with a description of packet
   formats and codepoints with their associated meanings.  This document
   does not: It is already known that choosing the encoding for ConEx is
   likely to entail some engineering compromises that have the potential
   to reduce the protocol's usefulness in some settings.  For instance,
   the experimental ConEx encoding chosen for IPv6 [CONEX-DESTOPT] had
   to make compromises on tunnelling.  Rather than making these
   engineering choices prematurely, this document sidesteps the encoding
   problem by making it abstract.  It describes several different
   representations of ConEx Signals, none of which are specified to the
   level of specific bits or codepoints.

   The goal of this approach is to be as complete as possible for
   discovering the potential usage and capabilities of the ConEx
   protocol, so we have some hope of making optimal design decisions
   when choosing the encoding.  Even if experiments reveal particular
   problems due to the encoding, then this document will still serve as
   a reference model.

4.1.  Naive Encoding

   For tutorial purposes, it is helpful to describe a naive encoding of
   the ConEx protocol for TCP and similar protocols: set a bit (not
   specified here) in the IP header on each retransmission and on each
   ECN-signalled window reduction.  Network devices along the forward
   path can see this bit and act on it.  For example, any device along
   the path might limit the rate of all traffic if the rate of marked
   (congested) packets exceeds a threshold.

   This simple encoding is sufficient to illustrate many of the benefits
   envisioned for ConEx.  At first glance, it looks like it might
   motivate people to deploy and use it.  It is a one-line code change
   that a small number of OS developers and content providers could
   unilaterally deploy across a significant fraction of all Internet
   traffic.  However, this encoding does not support auditing so it
   would also motivate users and/or applications to misrepresent the
   congestion that they are causing [RFC3514].  As a consequence, the
   naive encoding is not likely to be trusted and thus creates its own
   disincentives for deployment.

   Nonetheless, this Naive encoding does present a clear mental model of
   how the ConEx protocol might function under various uses.  It is
   useful for thought experiments where it can be stipulated that all
   participants are honest and it does illustrate some of the incentives
   that might be introduced by ConEx.

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4.2.  Null Encoding

   In limited contexts, it is possible to implement ConEx-like functions
   without any signals at all by measuring rest-of-path congestion
   directly from TCP headers.  The algorithm is to keep at least one RTT
   of past TCP headers and match each new header against the history to
   count duplicate data.

   This could implement many ConEx policies, without any explicit
   protocol.  It is fairly easy to implement, at least at low rate
   (e.g., in a software-based edge router).  However, it would only be
   useful in cases where the network operator can see the TCP headers.
   At the time of writing (2014), those cases are the majority of
   traffic because UDP, IPsec, and VPN tunnels are used far less than
   Secure Socket Layer (SSL) or Transport Layer Security (TLS) over
   TCP/IP, which do not hide TCP sequence numbers from network devices.
   However, anyone specifically intending to avoid the attention of a
   congestion policy device would only have to hide their TCP headers
   from the network operator (e.g., by using a VPN tunnel).

4.3.  ECN-Based Encoding

   The re-ECN specification [RE-ECN-TCP] presents an encoding of ConEx
   in IPv4 and IPv6 that was tightly integrated with ECN encoding in
   order to fit into the IPv4 header.  Any individual packet may need to
   represent any ECN codepoint and any ConEx Signal value independently.
   So, ideally, their encoding should be entirely independent.  However,
   given the limited number of header bits and/or codepoints, re-ECN
   chooses to partially share codepoints and to re-echo both losses and
   ECN with just one codepoint.

   The central theme of the re-ECN work is an audit mechanism that
   provides sufficient disincentives against misrepresenting congestion
   [RE-ECN-MOTIVATION].  It is analyzed extensively in Briscoe's PhD
   dissertation [Refb-dis].  For a tutorial background on re-ECN
   motivation and techniques, see [Re-fb] and [FairerFaster].

   Re-ECN is an example of one chosen set of compromises attempting to
   meet the requirements of Section 3.  The present document takes a
   step back, aiming to state the ideal requirements in order to allow
   the Internet community to assess whether different compromises might
   be better.

   The problem with re-ECN is that it requires that receivers be ECN
   enabled in addition to sender changes.  Newer encodings
   [CONEX-DESTOPT] overcome this problem by being able to represent loss
   and ECN-based congestion separately.

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4.4.  Independent Bits

   This encoding involves flag bits, each of which the sender can set
   independently to indicate to the network one of the following four
   signals:

   ConEx (Not-ConEx):  The transport is (or is not) using ConEx with
      this packet (network-layer encoding requirement L in Section 3.3
      says the protocol must be arranged so that legacy transport
      senders implicitly send Not-ConEx).

   Re-Echo-Loss (Not-Re-Echo-Loss):  The transport has (or has not)
      experienced a loss.

   Re-Echo-ECN (Not-Re-Echo-ECN):  The transport has (or has not)
      experienced ECN-signalled congestion.

   Credit (Not-Credit):  The transport is (or is not) building up
      congestion credit (see Section 5.5 on the audit function).

   A packet with ConEx set, combined with all the three other flags
   cleared, implies ConEx-Not-Marked.

   This encoding does not imply any exclusion property among the
   signals.  Multiple types of congestion (ECN, loss) can be signalled
   on the same ACK.  So, ideally, a ConEx sender would be able to
   reflect these in the next packet.  However, there will be many
   invalid combinations of flags (e.g., Not-ConEx combined with any of
   the ConEx-Marked flags), which a malicious sender could use to
   advantage against naive policy devices that only check each flag
   separately.

   As long as the packets in a flow have uniform sizes, it does not
   matter whether the units of congestion are packets or bytes.
   However, if an application sends very irregular packet sizes, it may
   be necessary for the sender to mark multiple packets to avoid being
   in technical violation of an audit function measuring in bytes (see
   Section 4.6).

4.5.  Codepoint Encoding

   This encoding involves signaling one of the following five
   codepoints:

   ENUM {Not-ConEx, ConEx-Not-Marked, Re-Echo-Loss, Re-Echo-ECN, Credit}

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   Each named codepoint has the same meaning as in the encoding using
   independent bits in the previous section.  The use of any one
   codepoint implies the negative of all the others.

   Inherently, the semantics of most of the enumerated codepoints are
   mutually exclusive.  'Credit' is the only one that might need to be
   used in combination with either Re-Echo-Loss or Re-Echo-ECN, but even
   that requirement is questionable.  It must not be forgotten that the
   enumerated encoding loses the flexibility to signal these two
   combinations, whereas the encoding with four independent bits is not
   so limited.  Alternatively, two extra codepoints could be assigned to
   these two combinations of semantics.  The comment in the previous
   section about units also applies.

4.6.  Units Implied by an Encoding

   The following comments apply generally to all the other encodings.

   Congestion can be due to exhaustion of bit-carrying capacity or
   exhaustion of packet-processing power.  When a packet is discarded or
   marked to indicate congestion, there is no easy way to know whether
   the lost or marked packet signifies bit congestion or packet
   congestion.  The above ConEx encodings that rely on marking packets
   suffer from the same ambiguity.

   This problem is most acute when audit needs to check that one count
   of markings matches another.  For example, if there are ConEx
   markings on three large (1500 B) packets, is that sufficient to match
   the loss of five small (60 B) packets?  If a packet marking is
   defined to mean all the bytes in the packet are marked, then we have
   4500 B of ConEx-Marked data against 300 B of lost data, which is
   easily sufficient.  If instead we are counting packets, then we have
   three ConEx packets against five lost packets, which is not
   sufficient.  This problem will not arise when all the packets in a
   flow are the same size, but a choice needs to be made for flows in
   which packet sizes vary, such as BGP, SPDY, and some variable-rate
   video encoding schemes.

   Whether to use bytes or packets is not obvious.  For instance, the
   most expensive links in the Internet, in terms of cost per bit, are
   all at lower data rates, where transmission times are large and
   packet sizes are important.  In order for a policy to consider wire
   time, it needs to know the number of congested bytes.  However, high
   speed networking equipment and the transport protocols themselves
   sometimes gauge resource consumption and congestion in terms of
   packets.

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   [RFC7141] advises that congestion indications should be interpreted
   in units of bytes when responding to congestion, at least on today's
   Internet.  [RFC6789] takes the same view in its definition of
   congestion-volume, again, for today's Internet.

   In any TCP implementation, this is simple to achieve for varying size
   packets given that TCP SACK tracks losses in bytes.  If an encoding
   is specified in units of bytes, the encoding should also specify
   which headers to include in the size of a packet (see network-layer
   requirement F in Section 3.3).

   RFC 7141 constructs an argument for why equipment tends to be built
   so that the bottleneck will be the bit-carrying capacity of its
   interfaces, not its packet-processing capacity.  However, RFC 7141
   acknowledges that the position may change in future and notes that
   new techniques will need to be developed to distinguish packet and
   bit congestion.

   Given this document describes an abstract ConEx mechanism, it is
   intended to be timeless.  Therefore, it does not take a strong
   position on this issue.  However, a ConEx encoding will need to
   explicitly specify whether it assumes units of bytes or packets
   consistently for both congestion indications and ConEx markings (see
   network-layer requirement E in Section 3.3).  It may help to refer to
   the guidance in [RFC7141].

5.  Congestion Exposure Components

   The components shown in Figure 1 as well as policy and audit are
   described in more detail.

5.1.  Network Devices (Not Modified)

   Congestion signals originate from network devices as they do today.
   A congested router, switch, or other network device can discard or
   ECN-mark packets when it is congested.

5.2.  Modified Senders

   The sending transport needs to be modified to send Congestion
   Exposure signals in response to congestion feedback signals (e.g.,
   for the case of a TCP transport, see [TCP-MODIFICATION]).  We want to
   permit ConEx without ECN (e.g., if the receiver does not support
   ECN).  However, we want to encourage a ConEx sender to at least
   attempt to negotiate ECN (a ConEx transport protocol specification
   may require this) because it is believed that ConEx without ECN is
   harder to audit and thus potentially exposed to cheating.  Since
   honest users have the potential to benefit from stronger mechanisms

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   to manage traffic, they have an incentive to deploy ConEx and ECN
   together.  This incentive is not sufficient to prevent a dishonest
   user from constructing (or configuring) a sender that enables ConEx
   after choosing not to negotiate ECN, but it should be sufficient to
   prevent this from being the sustained default case for any
   significant pool of users.

   Permitting ConEx without ECN is necessary to facilitate bootstrapping
   other parts of ConEx deployment.

5.3.  Receivers (Optionally Modified)

   Any receiving transport may already feedback sufficiently useful
   signals to the sender so that it does not need to be altered.

   The native loss or ECN signaling mechanism required for compliance
   with existing congestion control standards (e.g., RTCP, Stream
   Control Transmission Protocol (SCTP)) will typically be sufficient
   for the Sender to generate ConEx Signals.

   TCP's loss feedback is sufficient for ConEx if SACK is used
   [RFC2018].  However, the original specification for ECN in TCP
   [RFC3168] signals congestion no more than once per round trip.  The
   sender may require more precise feedback from the receiver otherwise
   it is at risk of appearing to be understating its ConEx Signals.

   Ideally, ConEx should be added to a transport like TCP without
   mandatory modifications to the receiver.  But in the TCP-ECN case, an
   optional modification to the receiver could be recommended for
   precision (see [RFC7560], which is based on the approach originally
   taken when adding re-ECN to TCP [RE-ECN-TCP]).

5.4.  Policy Devices

   Policy devices are characterised by a need to be configured with a
   policy related to the users or neighboring networks being served.  In
   contrast, auditing devices solely enforce compliance with the ConEx
   protocol and do not need to be configured with any client-specific
   policy.

   One of the design goals of the ConEx protocol is that none of the
   important policy mechanisms requires per-flow state and that policy
   mechanisms can even be implemented for heavily aggregated traffic in
   the core of the Internet with complexity akin to accumulating marking
   volumes per logical link.  Of course, policy mechanisms may sometimes
   choose to focus down on individual flows, but ConEx aims to make
   aggregate policy devices feasible.

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5.4.1.  Congestion Monitoring Devices

   Policy devices can typically be decomposed into two functions:
   i) monitoring the ConEx Signal to compare it with a policy; then ii)
   acting in some way on the result.  Various actions might be invoked
   against 'out of contract' traffic, such as policing (see
   Section 5.4.3), re-routing, or downgrading the class of service.

   Alternatively, a policy device might not act directly on the traffic,
   but instead report to management systems that are designed to control
   congestion indirectly.  For instance, the reports might trigger
   capacity upgrades, penalty clauses in contracts, levy charges based
   on congestion, or merely send warnings to clients who are causing
   excessive congestion.

   Nonetheless, whatever action is invoked, the congestion monitoring
   function will always be a necessary part of any policy device.

5.4.2.  Rest-of-Path Congestion Monitoring

   ConEx Signals indicate the level of congestion along a whole path
   from source to destination.  In contrast, ECN signals monitored in
   the middle of a network indicate the level of congestion experienced
   so far on the path (of course, only in ECN-capable traffic).

   If a monitor in the middle of a network (e.g., at a network border)
   measures both of these signals, it can subtract the level of ECN
   (path so far) 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).

   It will often be preferable for policy devices to monitor rest-of-
   path congestion if they can, because it is a measure of the
   downstream congestion that the policy device can directly influence
   by controlling the traffic passing through it.

5.4.3.  Congestion Policers

   A congestion policer can be implemented in a very similar way to a
   bit-rate policer, but its effect can be focused solely on traffic of
   users causing congestion downstream, which ConEx Signals make
   visible.  Without ConEx Signals, the only way to mitigate congestion
   is to blindly limit the traffic bit-rate on the assumption that high
   bit-rate is more likely to cause congestion.

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   A congestion policer monitors all ConEx traffic entering a network or
   some identifiable subset.  Using ConEx Signals and/or Credit signals
   (and preferably subtracting ECN signals to yield rest-of-path
   congestion), it measures the amount of congestion that this traffic
   is contributing somewhere downstream.  If this persistently exceeds a
   policy-configured 'congestion-bit-rate', the congestion policer can
   limit all the monitored ConEx traffic.

   A congestion policer can be implemented by a simple token bucket
   applied to an aggregate.  But unlike a bit-rate policer, it removes
   tokens only when it forwards packets that are ConEx-Marked,
   effectively treating Not-ConEx-Marked packets as invisible.
   Consequently, because tokens give the right to send congested bits,
   the fill rate of the token bucket will represent the allowed
   congestion-bit-rate.  This should provide sufficient traffic
   management without having to additionally constrain the straight bit-
   rate at all.  See [ISOLATION-POLICING] for details.

   Note that the policing action could be to introduce a throttle
   (discard some traffic) immediately upstream of the congestion
   monitor.  Alternatively, this throttle could introduce delay using a
   queue with its own AQM, which potentially increases the whole path
   congestion.  In effect, the congestion policer has moved the
   congestion earlier in the path and focused it on one user to protect
   downstream resources by reducing the congestion in the rest of the
   path.

5.5.  Audit

   The most critical aspect of ConEx is the capability to support robust
   auditing.  It can be assumed that sanctions based on ConEx Signals
   will create an intrinsic motivation for users to understate the
   congestion that they are causing.  So, without strong audit
   functions, the ConEx Signal would become understated to the point of
   being useless.  Therefore, the most important feature of an encoding
   design is likely to be the robustness of the auditing it supports.

   The general goal of an auditor is to make sure that any ConEx-enabled
   traffic is sent with sufficient ConEx-Re-Echo and ConEx-Credit
   signals.  A concrete definition of the ConEx protocol MUST define
   what sufficient means.

   If a ConEx-enabled transport does not carry sufficient ConEx Signals,
   then an auditor is likely to apply some sanction to that traffic.
   Although sanctions are beyond the scope of this document, an example
   sanction might be to throttle the traffic immediately upstream of the

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   auditor to prevent the user from getting any advantage by
   understating congestion.  Such a throttle would likely include some
   combination of delaying or dropping traffic.

   A ConEx auditor might use one of the following techniques:

   Generic loss auditing:  For congestion signalled by loss, totally
      accurate auditing is not believed to be possible in the general
      case because it involves a network node detecting the absence of
      some packets when it cannot always necessarily identify
      retransmissions or missing packets.  The missing packet might
      simply be taking a different route, or the IP payload may be
      encrypted.

      It is for this reason that it is desirable to motivate the
      deploying of ECN, even though ECN is not strictly required for
      ConEx.

   ECN auditing:  Directly observe and compare the volume of ECN and
      ConEx marks.  Since the volume of ECN marks rises monotonically
      along a path, ECN auditing is most accurate when located near the
      transport receiver.  For this reason, ECN should be monitored
      downstream of the predominant bottleneck.

   TCP-specific loss auditing:  For non-encrypted standard TCP traffic
      on a single path, a tactical audit approach could be to measure
      losses by detecting retransmissions, which appear as duplicate
      sequence numbers upstream of the loss and out of order data
      downstream of the loss.  Since some reordering is present in the
      Internet, such a loss estimator would be most accurate near the
      sender.  Such an audit device should treat non-ECN-capable packets
      with encrypted IP payload as Not-ConEx, even if they claim to be
      ConEx-capable, unless the operator is also using one of the other
      two techniques below that can audit such packets against losses.

   Predominant bottleneck loss auditing:  For networks designed so that
      losses predominantly occur under the control of one IP-aware
      bottleneck node on the path, the auditor could be located at this
      bottleneck.  It could simply compare ConEx Signals with actual
      local packet discards (and ECN marks).  This is a good model for
      most consumer access networks where audit accuracy could well be
      sufficient even if losses occasionally occur elsewhere in the
      network.

      Although the auditor at the predominant bottleneck would not be
      able to count losses at other nodes, transports would not know
      where losses were occurring either.  Therefore, a transport would

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      not know which losses it could cheat and which ones it couldn't
      without getting caught.

   ECN tunnel loss auditing:  A network operator can arrange IP-in-IP
      tunnels (or IP-in-MPLS, etc.) so that any losses within the
      tunnels are deferred until the tunnel egress.  Then, the audit
      function can be deployed at the egress and be aware of all losses.
      This is possible by enabling ECN marking on switches and routers
      within a tunnel, irrespective of whether end systems support ECN,
      by exploiting a side effect of the way tunnels handle the ECN
      field.  After encapsulation at the tunnel ingress, the network
      should arrange for any non-ECN packets (with '00' in the ECN field
      of the outer) to be set to the ECN-capable transport (ECT(0))
      codepoint.  Then, if they experience congestion at one of the ECN-
      capable switches or routers within the tunnel, some will be ECN-
      marked rather than immediately dropped.  However, when the tunnel
      decapsulator strips the outer from such an ECN-marked packet, if
      it finds the inner header has '00' in the ECN field (meaning that
      the endpoints do not support ECN), it will automatically drop the
      packet, assuming it complies with [RFC6040].  Thus, an audit
      function at the decapsulator can know which packets would have
      been dropped within the tunnel (and even which are genuinely ECN-
      marked for the end-to-end protocol).  Non-ECN end systems outside
      the tunnel see no sign of the use of ECN internally.

   In addition, other audit techniques may be identified in the future.

   [Refb-dis] gives a comprehensive inventory of attacks against audit
   proposed by various people.  It includes pseudocode for both
   deterministic and statistical audit functions designed to thwart
   these attacks and analyses the effectiveness of an implementation.
   Although this work is specific to the re-ECN protocol, most of the
   material is useful for designing and assessing audit of other
   specific ConEx encodings, against both ECN and loss.

   The auditing function should be able to trigger sufficient sanction
   to discourage understating congestion [Salvatori05].  This seems to
   require designing the sanction in concert with the policy functions,
   even though they might be implemented in different parts of the
   network.  However, [Refb-dis] proves audit and policy functions can
   be independent as long as audit drops sufficient traffic to
   'normalise' actual congestion signals to be no greater than ConEx
   Signals.

   Similarly, the job of incentivising the sending of ConEx-enabled
   packets is proper solely to policy devices independent of the audit
   function.  The audit function's job is policy neutral, so it should
   be solely confined to checking for correctness within those packets

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   that have been marked as ConEx-capable.  Even if there are Not-ConEx
   packets mixed with ConEx packets within a flow, audit will not need
   to monitor any Not-ConEx packets.

   Note that in the future it might prove to be desirable to provide
   advice on uniformly implementing sanctions, because otherwise
   insufficient sanctions could impair the ability to implement policy
   elsewhere in the network.

   Some of the audit algorithms require per-flow state.  This cost is
   expected to be tolerable because these techniques are most apropos
   near the edges of the network where traffic is generally much less
   aggregated so the state need not overwhelm any one device.  The flow
   state required for the audit creates itself as it detects new flows.
   Therefore, a flow will not fail if it is re-routed away from the
   audit box currently holding its flow state, so auditing does not
   require route pinning and works fine with multipath flows.

   Holding flow state seems to create a vulnerability to attacks that
   exhaust the auditor's memory by opening numerous new short flows.
   The audit function can protect itself from this attack by not
   allocating new flow state unless a ConEx-Marked packet arrives (e.g.,
   credit at the start of a flow).  Because policy devices rate limit
   ConEx-Marked packets, this sets a natural limit to the rate at which
   a source can create flow state in audit devices.  The auditor would
   treat all the remaining flows without any ConEx-Marked packets as a
   single misbehaving aggregate.

   Auditing can be distributed and redundant.  One flow may be audited
   in multiple places, using multiple techniques.  Some audit techniques
   do not require any per-flow state and can be applied to aggregate
   traffic.  These might be able to detect the presence of understated
   congestion at large scale and support recursively hunting for
   individual flows that are understating their congestion.  Even at
   large scales, flows can be randomly selected for individual auditing.

   Sampling techniques can also be used to bound the total auditing
   memory footprint, although the implementer needs to counter the
   tactic where a source cheats until caught by sampling, then simply
   discards that flow ID and starts cheating with a new one (termed
   'identifier whitewashing when caught').

   For the concrete ConEx protocol encoding defined in [CONEX-DESTOPT],
   ConEx Credit and ConEx-Re-Echo signals are intended to be audited
   separately.  The Credit signal can be audited directly against actual
   congestion (loss and ECN).  However, there will be an inherent delay
   of at least one round trip between a congestion signal and the
   subsequent ConEx-Re-Echo signal it triggers, as shown in Figure 1.

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   Therefore, ConEx-Re-Echo signals will need to be audited with some
   allowance for this delay.  Further discussion of design and
   implementation choices for functions intended to audit this concrete
   ConEx encoding can be found in [CONEX-AUDIT].

6.  Support for Incremental Deployment

   The ConEx abstract protocol described so far is intended to support
   incremental deployment in every possible respect.  For convenience,
   the following list collects together all the features that support
   incremental deployment in the concrete ConEx specifications and
   points to further information on each:

   Packets:  The wire protocol encoding allows each packet to indicate
      whether it is using ConEx or not (see Section 4 on
      Encoding Congestion Exposure).

   Senders:  ConEx requires a modification to the source in order to
      send ConEx packet markings (see Section 5.2).  Although ConEx
      support can be indicated on a packet-by-packet basis, it is likely
      that all the packets in a flow will either consistently support
      ConEx or consistently not.  It is also likely that, if the
      implementation of a transport protocol supports ConEx, all the
      packets sent from that host using that protocol will be ConEx-
      Capable.

      The implementations of some of the transport protocols on a host
      might not support ConEx (e.g., the implementation of DNS over UDP
      might not support ConEx, while perhaps RTP over UDP and TCP will).
      Any non-upgraded transports and non-upgraded hosts will simply
      continue to send regular Not-ConEx packets as always.

      A network operator can create incentives for senders to
      voluntarily reveal ConEx information (see the item on incremental
      deployment by 'Networks' below).

   Receivers:  A ConEx source should be able to work with the regular
      receiver for the transport in question without requiring any
      ConEx-specific modifications.  This is true for modern transport
      protocols (RTCP, SCTP, etc.) and it is even true for TCP, as long
      as the receiver supports SACK, which is widely deployed anyway.
      However, it is not true for ECN feedback in TCP.  The need for
      more precise ECN feedback in TCP is not exclusive to ConEx; for
      instance, Data Centre TCP [DCTCP] uses precise feedback to good
      effect.  Therefore, if a receiver offers precise feedback,
      [RFC7560] it will be best if ConEx uses it (see Section 5.3).

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      Alternatively, without sufficiently precise congestion feedback
      from the receiver, the source may have to conservatively send
      extra ConEx markings in order to avoid understating congestion.

   Proxies:  Although it was stated above that ConEx requires a
      modification to the source, ConEx Signals could theoretically be
      introduced by a proxy for the source as long as it can intercept
      feedback from the receiver.  Similarly, more precise feedback
      could theoretically be provided by a proxy for the receiver rather
      than modifying the receiver itself.

   Forwarding:  No modification to forwarding or queuing is needed for
      ConEx.

      However, once some ConEx is deployed, it is possible that a queue
      implementation could optionally take advantage of the ConEx
      information in packets.  For instance, it has been suggested
      [CONEX-DESTOPT] that a queue would be more robust against flooding
      if it preferentially discarded Not-ConEx packets then Not-Marked
      ConEx packets.

      A ConEx sender re-echoes congestion whether the queues signaling
      congestion are ECN enabled or not.  Nonetheless, an operator
      relying on ConEx Signals is recommended to enable ECN in queues
      wherever possible.  This is because auditing works best if most
      congestion is indicated by ECN rather than loss (see Section 3).
      Also, monitoring rest-of-path congestion is not accurate if there
      are congested non-ECN queues upstream of the monitoring point
      (Section 5.4.2).

   Networks:  If a subset of traffic sources (or proxies) use ConEx
      Signals to reveal congestion in the internetwork layer, a network
      operator can choose (or not) to use this information for traffic
      management.  As long as the end-to-end ConEx Signals are present,
      each network can unilaterally choose to use them -- independently
      of whether other networks do.

      ConEx marked packets may safely traverse a network that ignores
      them.  ConEx Signals are defined to remain unchanged once set by
      the sender, but some encodings may allow changes in transit (e.g.,
      by proxies).  In no circumstances will a network node change
      ConEx-Capable packets to Not-ConEx (network-layer encoding
      requirement I in Section 3.3).  If necessary, endpoints should be
      able to detect if a network is removing ConEx Signals (network-
      layer encoding requirement H in Section 3.3).

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      An operator can deploy policy devices (Section 5.4) wherever
      traffic enters its network in order to monitor the downstream
      congestion that incoming traffic contributes to and control it if
      necessary.  A network operator can create incentives for the
      developers of sending applications and transports to voluntarily
      reveal ConEx information.  Without ConEx information, a network
      operator tends to have to limit the bit-rate or volume from a site
      more than is necessary, just in case it might congest others.
      With ConEx information, the operator can solely limit congestion-
      causing traffic and otherwise allow complete freedom.  This
      greater freedom acts as an inducement for the source to volunteer
      ConEx information.  An operator may also monitor whether a source
      transport has sent ConEx packets and treat the same transport with
      greater suspicion (e.g., a more stringent rate limit) whenever it
      selectively sends packets without ConEx support.  See [RFC6789]
      for further discussion of deployment incentives for networks and
      references to scenarios where some networks use ConEx-based policy
      devices and others don't.

      An operator can deploy audit devices (Section 5.5) unilaterally
      within its own network to verify that traffic sources are not
      understating ConEx information.  From the viewpoint of one network
      operator (say N_a), it only cares that the level of ConEx
      signaling is sufficient to cover congestion in its own network.
      If traffic continues into a congested downstream network (say
      N_b), it is of no concern to the first network (N_a) if the end-
      to-end ConEx signaling is insufficient to cover the congestion in
      N_b as well.  This is N_b's concern, and N_b can both detect such
      anomalous traffic and deal with it using ConEx-based audit devices
      itself.

7.  Security Considerations

   The only known risk associated with ConEx is that users and
   applications are very likely to be motivated to underrepresent the
   congestion that they are causing.  Significant portions of this
   document are about mechanisms to audit the ConEx Signals and create
   sufficient sanction to inhibit such underrepresentation.  In
   particular, see Section 5.5.

   Security attacks and their defences are best discussed against a
   concrete protocol specification, not the abstract mechanism of this
   document.  A concrete ConEx protocol will need to be accompanied by a
   document describing how the protocol and its audit mechanisms defend
   against likely attacks.  [Refb-dis] will be a useful source for such
   a document.  It gives a comprehensive inventory of attacks against
   audit that have been proposed by various parties.  It includes

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   pseudocode for both deterministic and statistical audit functions
   designed to thwart these attacks and analyses the effectiveness of an
   implementation.

   However, [Refb-dis] is specific to the re-ECN protocol, which
   signalled ECN and loss together, whereas the concrete ConEx protocol
   defined in [CONEX-DESTOPT] signals them separately.  Therefore,
   although likely attacks will be similar, there will be more
   combinations of attacks to worry about, and defences and their
   analysis are likely to be a little different for ConEx.

   The main known attacks that a security document for a concrete ConEx
   protocol will need to address are listed below and [Refb-dis] should
   be referred to for how re-ECN was designed to defend against similar
   attacks:

   o  Attacks on the audit function (see Section 7.5 of [Refb-dis]):

      Flow ID Whitewashing:  Designing the audit function so that a
         source cannot gain from starting a new flow once audit has
         detected cheating in a previous flow.

      Dragging Down an Aggregate:  Avoiding audit discarding packets
         from all flows within an aggregate, which would allow one flow
         to pull down the average so that the audit function would
         discard packets from all flows, not just the offending flow.

      Dragging Down a Spoofed Flow ID:  An attacker understates ConEx
         markings in packets that spoof another flow, which fools the
         audit function into dropping the genuine user's packets.

   o  Attacks by networks on other networks (see Section 8.2 of
      [Refb-dis]):

      Dummy Traffic:  Sending dummy traffic across a border with
         understated ConEx markings to bring down the average ConEx
         markings in the aggregate of border traffic.  This attack can
         be combined with a TTL that expires before the packets reach an
         audit function.

      Signal Poisoning with 'Cancelled' Marking:  Sending high volumes
         of valid packets that are both ConEx-Marked and ECN-marked,
         which seems to represent congestion upstream, but it makes
         these packets immune to being further ECN-marked downstream.

Top      ToC       Page 27 
   It is planned to document all known attacks and their defences
   (including all of the above) in the RFC series against a concrete
   ConEx protocol specification.  In the interim, [Refb-dis] and its
   references should be referred to for details and ways to address
   these attacks in the case of re-ECN.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

8.2.  Informative References

   [CheapPseud]
              Friedman, E. and P. Resnick, "The Social Cost of Cheap
              Pseudonyms", Journal of Economics and Management Strategy,
              Volume 10, Issue 2, pp. 173-199,
              DOI 10.1111/j.1430-9134.2001.00173.x, Summer 2001.

   [CONEX-AUDIT]
              Wagner, D. and M. Kuehlewind, "Auditing of Congestion
              Exposure (ConEx) signals", Work in Progress,
              draft-wagner-conex-audit-01, February 2014.

   [CONEX-DESTOPT]
              Krishnan, S., Kuehlewind, M., and C. Ucendo, "IPv6
              Destination Option for Congestion Exposure (ConEx)", Work
              in Progress, draft-ietf-conex-destopt-11, October 2015.

   [DCTCP]    Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel,
              P., Prabhakar, B., Sengupta, S., and M. Sridharan, "Data
              Center TCP (DCTCP)", ACM SIGCOMM Computer Communication
              Review, Volume 40, Issue 4, pages 63-74,
              DOI 10.1145/1851182.1851192, October 2010,
              <http://portal.acm.org/citation.cfm?id=1851192>.

   [Evol_cc]  Gibbens, R. and F. Kelly, "Resource pricing and the
              evolution of congestion control", Automatica, Volume 35,
              Issue 12, pages 1969-1985,
              DOI 10.1016/S0005-1098(99)00135-1, December 1999,
              <http://www.sciencedirect.com/science/article/pii/
              S0005109899001351>.

Top      ToC       Page 28 
   [FairerFaster]
              Briscoe, B., "A Fairer, Faster Internet Protocol", IEEE
              Spectrum, pages 38-43, DOI 10.1109/MSPEC.2008.4687368,
              December 2008,
              <http://spectrum.ieee.org/telecom/standards/
              a-fairer-faster-internet-protocol>.

   [ISOLATION-POLICING]
              Briscoe, B., "Network Performance Isolation using
              Congestion Policing", Work in Progress,
              draft-briscoe-conex-policing-01, February 2014.

   [RE-ECN-MOTIVATION]
              Briscoe, B., Jacquet, A., Moncaster, T., and A. Smith,
              "Re-ECN: A Framework for adding Congestion Accountability
              to TCP/IP", Work in Progress,
              draft-briscoe-conex-re-ecn-motiv-03, March 2014.

   [RE-ECN-TCP]
              Briscoe, B., Jacquet, A., Moncaster, T., and A. Smith,
              "Re-ECN: Adding Accountability for Causing Congestion to
              TCP/IP", Work in Progress,
              draft-briscoe-conex-re-ecn-tcp-04, July 2014.

   [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 Computer Communication Review, Volume 35,
              Issue 4, pages 277--288, DOI 10.1145/1090191.1080124,
              August 2005,
              <http://portal.acm.org/citation.cfm?id=1080091.1080124>.

   [Refb-dis] Briscoe, B., "Re-feedback: Freedom with Accountability for
              Causing Congestion in a Connectionless Internetwork", PhD
              Dissertation, University College London, May 2009,
              <http://discovery.ucl.ac.uk/16274/>.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018,
              DOI 10.17487/RFC2018, October 1996,
              <http://www.rfc-editor.org/info/rfc2018>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <http://www.rfc-editor.org/info/rfc3168>.

Top      ToC       Page 29 
   [RFC3514]  Bellovin, S., "The Security Flag in the IPv4 Header",
              RFC 3514, DOI 10.17487/RFC3514, April 2003,
              <http://www.rfc-editor.org/info/rfc3514>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <http://www.rfc-editor.org/info/rfc3550>.

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 5348, DOI 10.17487/RFC5348, September 2008,
              <http://www.rfc-editor.org/info/rfc5348>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <http://www.rfc-editor.org/info/rfc5681>.

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <http://www.rfc-editor.org/info/rfc6040>.

   [RFC6679]  Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
              and K. Carlberg, "Explicit Congestion Notification (ECN)
              for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
              2012, <http://www.rfc-editor.org/info/rfc6679>.

   [RFC6789]  Briscoe, B., Ed., Woundy, R., Ed., and A. Cooper, Ed.,
              "Congestion Exposure (ConEx) Concepts and Use Cases",
              RFC 6789, DOI 10.17487/RFC6789, December 2012,
              <http://www.rfc-editor.org/info/rfc6789>.

   [RFC6817]  Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
              DOI 10.17487/RFC6817, December 2012,
              <http://www.rfc-editor.org/info/rfc6817>.

   [RFC7141]  Briscoe, B. and J. Manner, "Byte and Packet Congestion
              Notification", BCP 41, RFC 7141, DOI 10.17487/RFC7141,
              February 2014, <http://www.rfc-editor.org/info/rfc7141>.

   [RFC7560]  Kuehlewind, M., Ed., Scheffenegger, R., and B. Briscoe,
              "Problem Statement and Requirements for Increased Accuracy
              in Explicit Congestion Notification (ECN) Feedback",
              RFC 7560, DOI 10.17487/RFC7560, August 2015,
              <http://www.rfc-editor.org/info/rfc7560>.

Top      ToC       Page 30 
   [Salvatori05]
              Salvatori, A., "Closed Loop Traffic Policing", Politecnico
              Torino and Institut Eurecom Masters Thesis, September
              2005.

   [TCP-MODIFICATION]
              Kuehlewind, M. and R. Scheffenegger, "TCP modifications
              for Congestion Exposure", Work in Progress, draft-ietf-
              conex-tcp-modifications-10, October 2015.

Acknowledgments

   This document was improved by review comments from Toby Moncaster,
   Nandita Dukkipati, Mirja Kuehlewind, Caitlin Bestler, Marcelo Bagnulo
   Braun, John Leslie, Ingemar Johansson, and David Wagner.

   Bob Briscoe's work on this specification received part-funding from
   the European Union's Seventh Framework Programme FP7/2007-2013 under
   the Trilogy 2 project, grant agreement no. 317756.  The views
   expressed here are solely those of the authors.

Authors' Addresses

   Matt Mathis
   Google, Inc.
   1600 Amphitheater Parkway
   Mountain View, California  93117
   United States

   Email: mattmathis@google.com


   Bob Briscoe
   BT (now at Simula Research Laboratory)

   Email: ietf@bobbriscoe.net
   URI:   http://bobbriscoe.net/