Congestion Exposure (ConEx) Working                            M. Mathis
Group                                                        Google, Inc
Internet-Draft                                                B. Briscoe
Intended status: Informational                                        BT
Expires: September 8, 2011                                 March 7, 2011

      Congestion Exposure (ConEx) Concepts and Abstract Mechanism


   This document describes an abstract mechanism by which senders inform
   the network about the congestion encountered by packets earlier in
   the same flow.  Today, the network may signal congestion to the
   receiver by ECN markings or by dropping packets, and the receiver may
   pass this information back to the sender in transport-layer feedback.
   The mechanism to be developed by the ConEx WG will enable the sender
   to also relay this congestion information back into the network in-
   band at the IP layer, such that the total level of congestion is
   visible to all IP devices along the path, from where it could, for
   example, be provided as input to traffic management.

Status of This Memo

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Requirements for the ConEx Signal  . . . . . . . . . . . . . .  4
   3.  Representing Congestion Exposure . . . . . . . . . . . . . . .  6
     3.1.  Strawman Encoding  . . . . . . . . . . . . . . . . . . . .  7
     3.2.  ECN Based Encoding . . . . . . . . . . . . . . . . . . . .  7
       3.2.1.  ECN Changes  . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Abstract Encoding  . . . . . . . . . . . . . . . . . . . .  8
       3.3.1.  Independent Bits . . . . . . . . . . . . . . . . . . .  9
       3.3.2.  Codepoint Encoding . . . . . . . . . . . . . . . . . .  9
   4.  Congestion Exposure Components . . . . . . . . . . . . . . . .  9
     4.1.  Modified Senders . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Receivers (Optionally Modified)  . . . . . . . . . . . . . 10
     4.3.  Audit  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.4.  Policy Devices . . . . . . . . . . . . . . . . . . . . . . 11
       4.4.1.  Congestion Policers  . . . . . . . . . . . . . . . . . 11
       4.4.2.  Other Policy Devices . . . . . . . . . . . . . . . . . 11
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   7.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   9.  Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 12
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
     10.2. Informative References . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   One of the required functions of a transport protocol is controlling
   congestion in the network.  There are three techniques in use today
   for the network to signal congestion to a transport:
   o  The most common congestion signal is packet loss.  When congested,
      the network simply discards some packets either as part of an
      explicit control function [RFC2309] or as the consequence of a
      queue overflow or other resource starvation.  The transport
      receiver detects that some data is missing and signals such
      through transport acknowledgments to the transport sender (e.g.
      TCP SACK options).  The sender performs the appropriate congestion
      control rate reduction (e.g.  [RFC5681] for TCP) and, if it is a
      reliable transport, it retransmits the missing data.
   o  If the transport supports explicit congestion notification (ECN)
      [RFC3168] or pre-congestion notification (PCN) [RFC5670] , the
      transport sender indicates this by setting an ECN-capable
      transport (ECT) codepoint in every packet.  Network devices can
      then explicitly signal congestion to the receiver by setting ECN
      bits in the IP header of such packets.  The transport receiver
      communicates these ECN signals back to the sender, which then
      performs the appropriate congestion control rate reduction.
   o  Some experimental transport protocols and TCP variants [Vegas]
      sense queuing delays in the network and reduce their rate before
      the network has to signal congestion using loss or ECN.  A purely
      delay-sensing transport will tend to be pushed out by other
      competing transports that do not back off until they have driven
      the queue into loss.  Therefore, modern delay-sensing algorithms
      use delay in some combination with loss to signal congestion (e.g.
      LEDBAT [I-D.ietf-ledbat-congestion], Compound
      [I-D.sridharan-tcpm-ctcp]).  In the rest of this document, we will
      confine the discussion to concrete signals of congestion such as
      loss and ECN.  We will not discuss delay-sensing further, because
      it can only avoid these more concrete signals of congestion in
      some circumstances.

   In all 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 proposes to extend the capabilities of the Internet
   protocol suite with the addition of a ConEx Signal that, to a first
   approximation, relays the congestion information from the transport
   sender back through the internetwork layer.  That signal is shown in
   Figure 1.  It would be visible to all internetwork layer devices
   along the forward (data) path and is intended to support a number of

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   new policy-controlled mechanisms that might be used to manage
   +---------+                                               +---------+
   |         |<==Feedback Path==============================<|         |
   |         |<--Transport Layer returned Congestion Signal-<|         |
   |         |                                               |         |
   |Transport|                                               |Transport|
   | Sender  |>---------(new)-IP layer ConEx Signal--------->| Receiver|
   |         |        (Carried in Data Packet Headers)       |         |
   |         |             +-----------+                     |         |
   |         |>=Data=Path=>|(Congested)|>=====Data=Path=====>|         |
   |         |             |  Network  |>-Congestion-Signal->|         |
   |         |             |   Device  |                     |         |
   +---------+             +-----------+                     +---------+

   Not shown are policy devices along the data path that observe the
   ConEx Signal, and use the information to monitor or manage traffic.
   These are discussed in Section 4.4.

                                 Figure 1

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   ConEx signals in IP packet headers from the sender to the network
   {ToDo: These are placeholders for whatever words we decide to use}:
   Not-ConEx:  The transport is not ConEx-capable
   ConEx-Capable:  The transport is ConEx-Capable.  This is the opposite
      of Not-ConEx and implies one of the following signals
      Re-Echo-Loss:  (aka Purple) The transport has experienced a loss
      Re-Echo-ECN:  (aka Black) The transport has experienced an ECN
      Credit:  (aka Green) The transport is building up credit to allow
         for any future delay in expected ConEx signals
      ConEx-Not-Marked:  The transport is ConEx-capable but is signaling
         none of Re-Echo-Loss, Re-Echo-ECN or Credit
      ConEx-Marked:  At least one of Re-Echo-Loss, Re-Echo-ECN or

2.  Requirements for the ConEx Signal

   Ideally, all the following requirements would be met by a Congestion
   Exposure Signal.  However it is already known that some compromises
   will be necessary, therefore all the requirements are expressed with

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   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 any of these requirements:
   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.  The ConEx Signal SHOULD be immutable once set by the
       transport sender.  A corollary of these requirements is that
       existing (legacy) networking gear SHOULD pass the Congestion
       Exposure Signal silently without modification.
   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,
       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 accurate.  In potentially hostile
       environments such as the public Internet, it SHOULD be possible
       for techniques to be deployed to audit the Congestion Exposure
       Signal by comparing it to the actual congestion signals on the
       forward data path.  The auditing mechanism must have a capability
       for providing sufficient disincentives against misreported
       congestion, such as by throttling traffic that reports less
       congestion than it is actually experiencing.
   d.  The ConEx Signal SHOULD be timely.  There will be a delay between
       the time when an auditing device sees an actual congestion signal
       and when it sees the subsequent Congestion Exposure Signal from
       the sender.  The minimum delay will be one round trip, but it may
       be much longer depending on the transport's choice of feedback
       delay (consider RTCP [RFC3550] for example).  It is not practical
       to expect auditing devices in the network to make allowance for
       such feedback delays.  Instead, the sender SHOULD be able to send
       ConEx signals in advance, as 'credit' for any audit device to
       hold as a balance against the risk of congestion during the
       feedback delay.  This design choice simplifies auditing devices
       and correctly makes the transport responsible for both minimizing
       feedback delay and minimizing sharp increases in packets in
       flight that would risk causing excessive congestion to others.
       This issue is discussed in more detail in Section 4.3.

   It is important to note that the auditing requirement implies a
   number of additional constraints: The basic auditing technique is to
   count both actual congestion signals and ConEx Signals someplace
   along the data path:

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   o  For congestion signaled by ECN, auditing is most accurate when
      located near the transport receiver.  Within any flow or aggregate
      of flows, the total volume of ECN marked data seen near the
      receiver should always be equal to or less than the volume of data
      tagged with ConEx Signals.
   o  For congestion signaled 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
      necessarily see the transport protocol sequence numbers and when
      the missing packets might simply be taking a different route.  But
      there are common cases where sufficient audit accuracy should be
      *  For non-IPsec traffic conforming to standard TCP sequence
         numbering on a single path, an auditor could detect losses by
         observing both the original transmission and the retransmission
         after the loss.  Such auditing would be most accurate near the
      *  For networks designed so that losses predominantly occur under
         the management of one IP-aware node on the path, the auditor
         could be located at this bottleneck.  It could simply compare
         ConEx Signals with actual local losses.  This is a good model
         for most consumer access networks and audit accuracy could well
         be sufficient even if losses occasionally occurred at other
         nodes in the network, such as border gateways (see Section 4.3
         for details).

   Given that loss-based and ECN-based ConEx might sometimes be best
   audited at different locations, having distinct encodings would widen
   the design space for the auditing function.

3.  Representing 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 the
   ConEx Signal is likely to entail some engineering compromises that
   have the potential to reduce the protocol's usefulness in some
   settings.  Rather than making these engineering choices prematurely,
   this document side steps the encoding problem by describing an
   abstract representation of ConEx Signals.  All of the elements of the
   protocol can be defined in terms of this abstract representation.
   Most important, the preliminary use cases for the protocol are
   described in terms of the abstract representation in companion
   documents [I-D.conex-concepts-uses].

   Once we have some example use cases we can evaluate different
   encoding schemes.  Since these schemes are likely to include some
   conflated code points, some information will be lost resulting in

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   weakening or disabling some of the algorithms and eliminating some
   use cases.

   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.

3.1.  Strawman Encoding

   As an aid to the reader, it might be helpful to describe a naive
   strawman encoding of the ConEx protocol described solely in terms of
   TCP: set the Reserved bit in the IPv4 header (bit 48 counting from
   zero [RFC0791]--aka the "evil bit" [RFC3514]) on all retransmissions
   or once per ECN signaled window reduction.  Clearly network devices
   along the forward path can see this bit and act on it.  For example
   they can count marked and unmarked packets to estimate the congestion
   levels along the path.

   However, the IESG has chartered the ConEx working group to establish
   that there is sufficient demand for an IPv6 ConEx protocol before
   using the last available bit in the IPv4 header.  Furthermore this
   encoding, by itself, does not sufficiently support partial deployment
   or strong auditing and might motivate users and/or applications to
   misrepresent the congestion that they are causing.

   Nonetheless, this strawman encoding does present a clear mental model
   of how the ConEx protocol might function under various uses.

3.2.  ECN Based Encoding

   Ideally ConEx and ECN are orthogonal signals and SHOULD be entirely
   independent.  However, given the limited number of header bit and/or
   code points, these signals may have to share code points, at least

   The re-ECN specification [I-D.briscoe-tsvwg-re-ecn-tcp] presents an
   implementation of ConEx that is tightly integrated with the encoding
   of ECN in the IP header.  The central theme of this work is an audit
   mechanism that can provide sufficient disincentives against
   misrepresenting congestion [I-D.briscoe-tsvwg-re-ecn-motiv], which is
   analyzed extensively in Briscoe's PhD dissertation [Refb-dis].

   Re-ECN is a good example of one chosen set of compromises attempting
   to meet the requirements of Section 2.  However, the present document
   takes a step back, aiming to state the ideal requirements in order to
   allow the Internet community to assess whether other compromises are

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   In particular, different incremental deployment choices may be
   desirable to meet the partial deployment requirement of Section 2.
   Re-ECN requires the receiver to be at least ECN-capable as well as
   requiring an update to the sender.  Although ConEx will inherently
   require change at the sender, it would be preferable if it could
   work, even partially, with any receiver.

   The chosen ConEx protocol certainly must not require ECN to be
   deployed in any network.  In this respect re-ECN is already a good
   example--it acts perfectly well as a loss-based ConEx protocol it the
   loss-based audit techniques in Section 4.3 are used.  However, it
   would still be desirable to avoid the dependence on an ECN receiver.

   For a tutorial background on Re-Feedback techniques, see [Re-fb,

3.2.1.  ECN Changes

   Although the re-ECN protocol requires no changes to the network side
   of the ECN protocol, it is important to note that it does propose
   some relatively minor modifications to the host-to-host aspects of
   the ECN protocol specified in RFC 3168.  They include: redefining the
   ECT(1) code point (the change is consistent with RFC3168 but requires
   deprecating the experimental ECN nonce [RFC3540]); modifications to
   the ECN negotiations carried on the SYN and SYN-ACK; and using a
   different state machine to carry ECN signals in the transport
   acknowledgments from the Receiver to the Sender.  This last change
   permits the transport protocol to carry multiple congestion signals
   per round trip, and greatly simplifies accurate auditing.

   All of these adjustments to RFC 3168 may also be needed in a future
   standardized ConEx protocol.  There will need to be very careful
   consideration of any proposed changes to ECN or other existing
   protocols, because any such changes increase the cost of deployment.

3.3.  Abstract Encoding

   The ConEx protocol could take one of two different encodings:
   independently settable bits or an enumerated set of mutually
   exclusive codepoints.

   In both cases, the amount of congestion is signaled by the volume of
   marked data--just as the volume of lost data or ECN marked data
   signals the amount of congestion experienced.  Thus the size of each
   packet carrying a ConEx Signal is significant.

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3.3.1.  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
   ConEx (Not-ConEx)  The transport is (or is not) using ConEx with this
      packet (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 signaled congestion
   Credit (Not-Credit)  The transport is (or is not) building up
      congestion credit (see Section 4.3 on audit devices)

3.3.2.  Codepoint Encoding

   This encoding involves signaling one of the following five

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

   Each named codepoint has the same meaning as in the encoding using
   independent bits (Section 3.3.1).  The use of any one codepoint
   implies the negative of all the others, except the last three
   codepoints (Re-Echo-Loss, Re-Echo-ECN and Credit) obviously also
   imply ConEx is supported.

   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.

4.  Congestion Exposure Components

   {ToDo: Picture of the components, similar to that in the last
   slideset about conex-concepts-uses?}

4.1.  Modified Senders

   The sending transport needs to be modified to send Congestion
   Exposure Signals in response to congestion feedback signals.

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4.2.  Receivers (Optionally Modified)

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

   However, a TCP receiver feeds back ECN congestion signals no more
   than once within a round trip.  The sender may require more precise
   feedback from the receiver otherwise it will appear to be
   understating its ConEx Signals (see Section 3.2.1).

   Ideally, ConEx should be added to a transport like TCP without
   mandatory modifications to the receiver.  But an optional
   modification to the receiver could be recommended for precision.
   This was the approach taken when adding re-ECN to TCP

4.3.  Audit

   To audit ConEx Signals against actual losses an auditor could use one
   of the following techniques:
   TCP-specific approach:  The auditor could monitor TCP flows or
      aggregates of flows, only holding state on a flow if it first
      sends a Credit or a Re-Echo-Loss marking.  The auditor could
      detect retransmissions by monitoring sequence numbers.  It would
      assure that (volume of retransmitted data) <= (volume of data
      marked Re-Echo-Loss).  Traffic would only be auditable in this way
      if it conformed to the standard TCP protocol and the IP payload
      was not encrypted (e.g. with IPsec).
   Predominant bottleneck approach:  Unlike the above TCP-specific
      solution, this technique would work for IP packets carrying any
      transport layer protocol, and whether encrypted or not.  But it
      only works well for networks designed so that losses predominantly
      occur under the management of one IP-aware node on the path.  The
      auditor could then be located at this bottleneck.  It could simply
      compare ConEx Signals with actual local losses.  Most consumer
      access networks are design to this model, e.g. the radio network
      controller (RNC) in a cellular network or the broadband remote
      access server (BRAS) in a digital subscriber line (DSL) network.

      The accuracy of an auditor at one predominant bottleneck might
      still be sufficient, even if losses occasionally occurred at other
      nodes in the network (e.g. border gateways).  Although the auditor
      at the predominant bottleneck would not always be able to detect
      losses at other nodes, transports would not know where losses were
      occurring either.  Therefore any transport would not know which
      losses it could cheat on without getting caught, and which ones it

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   To audit ConEx Signals against actual ECN markings or losses, the
   auditor could work as follows: monitor flows or aggregates of flows,
   only holding state on a flow if it first sends a Credit or either Re-
   Echo marking.  Count the number of bytes marked with Credit or Re-
   Echo-ECN.  Separately count the number of bytes marked with ECN.  Use
   Credits to assure that #ECN<=#Re-Echo-ECN+#Credit, even though the
   Re-Echo-ECN markings are delayed by at least one RTT.

4.4.  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, the auditing devices referred to in the previous section
   primarily enforce compliance with the ConEx protocol and do not need
   to be configured with any client-specific policy.

4.4.1.  Congestion Policers

   Note that a congestion policer can be implemented in a very similar
   way to a bit-rate policer, but its effect is focused solely on
   traffic causing congestion downstream, not on all traffic just in
   case it causes congestion.

   It monitors all ConEx traffic entering a network, or some
   identifiable subset.  Using ConEx signals, it measures the amount of
   congestion being caused by this traffic.  If this exceeds a policy-
   configured 'congestion-bit-rate' the congestion policer will limit
   all the monitored ConEx traffic.  A congestion policer can be
   implemented by a simple token bucket.  But unlike a bit-rate policer,
   it only removes tokens when forwarding packets that a ConEx marked.
   See [CongPol] for details.

4.4.2.  Other Policy Devices

   Other policy devices that use ConEx signaling might traffic traffic
   based on ConEx Signals in much the same way as the monitoring element
   of a Congestion Policer.  But the resulting action could be
   different.  It might re-route traffic or downgrade the class of

   It might do nothing directly to the traffic, but instead report
   measurements of ConEx Signals to systems designed to control
   congestion indirectly.  For instance the measurements might be used
   to trigger penalty clauses in contracts, to levy charges between
   networks based on congestion or simply to notify customers who cause
   excessive congestion.

   an auditing device only needs to enforce protocol compliance, it does

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   not need to reflect any policy.

5.  IANA Considerations

   This memo includes no request to IANA.

   Note to RFC Editor: this section may be removed on publication as an

6.  Security Considerations

   Significant parts of this whole document are about the auditability
   of ConEx Signals, in particular Section 4.3.

7.  Conclusions


8.  Acknowledgements

   This document was improved by review comments from Toby Moncaster.

9.  Comments Solicited

   Comments and questions are encouraged and very welcome.  They can be
   addressed to the IETF Congestion Exposure (ConEx) working group
   mailing list <>, and/or to the authors.

10.  References

10.1.  Normative References

   [RFC2119]                         Bradner, S., "Key words for use in
                                     RFCs to Indicate Requirement
                                     Levels", BCP 14, RFC 2119,
                                     March 1997.

10.2.  Informative References

   [CongPol]                         Jacquet, A., Briscoe, B., and T.
                                     Moncaster, "Policing Freedom to Use
                                     the Internet Resource Pool", Proc
                                     ACM Workshop on Re-Architecting the
                                     Internet (ReArch'08) ,
                                     December 2008, <http://

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   [FairerFaster]                    Briscoe, B., "A Fairer, Faster
                                     Internet Protocol", IEEE
                                     Spectrum Dec 2008:38--43,
                                     December 2008, <http://

   [I-D.briscoe-tsvwg-re-ecn-motiv]  Briscoe, B., Jacquet, A.,
                                     Moncaster, T., and A. Smith, "Re-
                                     ECN: A Framework for adding
                                     Congestion Accountability to
                                     TCP/IP", draft-briscoe-tsvwg-re-
                                     ecn-tcp-motivation-01 (work in
                                     progress), September 2009.

   [I-D.briscoe-tsvwg-re-ecn-tcp]    Briscoe, B., Jacquet, A.,
                                     Moncaster, T., and A. Smith, "Re-
                                     ECN: Adding Accountability for
                                     Causing Congestion to TCP/IP",
                                     (work in progress), October 2010.

   [I-D.conex-concepts-uses]         Briscoe, B., Woundy, R., Moncaster,
                                     T., and J. Leslie, "ConEx Concepts
                                     and Use Cases", draft-moncaster-
                                     conex-concepts-uses-01 (work in
                                     progress), July 2010.

   [I-D.ietf-ledbat-congestion]      Shalunov, S., Hazel, G., and J.
                                     Iyengar, "Low Extra Delay
                                     Background Transport (LEDBAT)",
                                     (work in progress), October 2010.

   [I-D.sridharan-tcpm-ctcp]         Sridharan, M., Tan, K., Bansal, D.,
                                     and D. Thaler, "Compound TCP: A New
                                     TCP Congestion Control for High-
                                     Speed and Long Distance  Networks",
                                     draft-sridharan-tcpm-ctcp-02 (work
                                     in progress), November 2008.

   [RFC0791]                         Postel, J., "Internet Protocol",
                                     STD 5, RFC 791, September 1981.

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

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Internet-Draft    ConEx Concepts and Abstract Mechanism       March 2011

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

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

   [RFC3514]                         Bellovin, S., "The Security Flag in
                                     the IPv4 Header", RFC 3514, April 1

   [RFC3540]                         Spring, N., Wetherall, D., and D.
                                     Ely, "Robust Explicit Congestion
                                     Notification (ECN) Signaling with
                                     Nonces", RFC 3540, June 2003.

   [RFC3550]                         Schulzrinne, H., Casner, S.,
                                     Frederick, R., and V. Jacobson,
                                     "RTP: A Transport Protocol for
                                     Real-Time Applications", STD 64,
                                     RFC 3550, July 2003.

   [RFC5670]                         Eardley, P., "Metering and Marking
                                     Behaviour of PCN-Nodes", RFC 5670,
                                     November 2009.

   [RFC5681]                         Allman, M., Paxson, V., and E.
                                     Blanton, "TCP Congestion Control",
                                     RFC 5681, September 2009.

   [Re-fb]                           Briscoe, B., Jacquet, A., Di
                                     Cairano-Gilfedder, C., Salvatori,
                                     A., Soppera, A., and M. Koyabe,
                                     "Policing Congestion Response in an
                                     Internetwork Using Re-Feedback",
                                     ACM SIGCOMM CCR 35(4)277--288,
                                     August 2005, <

   [Refb-dis]                        Briscoe, B., "Re-feedback: Freedom
                                     with Accountability for Causing
                                     Congestion in a Connectionless

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Internet-Draft    ConEx Concepts and Abstract Mechanism       March 2011

                                     Internetwork", UCL PhD
                                     Dissertation , 2009, <http://

   [Vegas]                           Brakmo, L. and L. Peterson, "TCP
                                     Vegas: End-to-End Congestion
                                     Avoidance on a Global Internet",
                                     IEEE Journal on Selected Areas in
                                     Communications 13(8)1465--80,
                                     October 1995, <http://

Authors' Addresses

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

   EMail: mattmathis at

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

   Phone: +44 1473 645196

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