Network Working Group                                       G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Informational                                  M. Welzl
Expires: September 23, 2015                           University of Oslo
                                                          March 22, 2015

      The Benefits of using Explicit Congestion Notification (ECN)


   This document describes the potential benefits when applications
   enable Explicit Congestion Notification (ECN).  It outlines the
   principal gains in terms of increased throughput, reduced delay and
   other benefits when ECN is used over network paths that include
   equipment that supports ECN-marking.  It also identifies some
   potential problems that might occur when ECN is used.  The document
   does not propose new algorithms that may be able to use ECN or
   describe the details of implementation of ECN in endpoint devices,
   routers and other network devices.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on September 23, 2015.

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
   ( in effect on the date of
   publication of this document.  Please review these documents

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   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.

1.  Introduction

   Internet Transports (such as TCP and SCTP) have two ways to detect
   congestion: the loss of a packet and, if Explicit Congestion
   Notification (ECN) [RFC3168] is enabled, by reception of a packet
   with a Congestion Experienced (CE)-marking in the IP header.  Both of
   these are treated by transports as indications of (potential)
   congestion.  ECN may also be enabled by other transports: UDP
   applications may enable ECN when they are able to correctly process
   the ECN signals (e.g.  ECN with RTP [RFC6679]).

   A network device (router, middlebox, or other device that forwards
   packets through the network) that does not support AQM, typically
   uses a drop-tail policy to discard excess IP packets when its queue
   becomes full.  The discard of packets serves as a signal to the end-
   to-end transport that there may be congestion on the network path
   being used.  This triggers a congestion control reaction to reduce
   the maximum rate permitted by the sending endpoint.

   When an application uses a transport that enables the use of ECN, the
   transport layer sets the ECT(0) or ECT(1) codepoint in the IP header
   of packets that it sends.  This indicates to network devices that
   they may mark, rather than drop, packets in periods of congestion.
   This marking is generally performed by Active Queue Management (AQM)
   [RFC2309.bis] and may be the result of various AQM algorithms, where
   the exact combination of AQM/ECN algorithms does not need to be known
   by the transport endpoints.  The focus of this document is on usage
   of ECN by transport and application layer flows, not its
   implementation in hosts, routers and other network devices.

   ECN makes it possible for the network to signal the presence of
   congestion without incurring packet loss.  This lets the network
   deliver some packets to an application that would otherwise have been
   dropped if the application or transport did not support ECN.  This
   packet loss reduction is the most obvious benefit of ECN, but it is
   often relatively modest.  However, enabling ECN can also result in a
   number of beneficial side-effects, some of which may be much more
   significant than the immediate packet loss reduction from ECN-marking
   instead of dropping packets.  Several of these benefits have to do
   with reducing latency in some way (e.g., reduced Head-of-Line
   Blocking and potentially smaller queuing delay, depending on the
   marking rules in network devices).

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   The remainder of this document discusses the potential for ECN to
   positively benefit an application without making specific assumptions
   about configuration or implementation.

   [RFC3168] describes a method in which a network device sets the CE
   codepoint of an ECN-Capable packet at the time that the router would
   otherwise have dropped the packet.  While it has often been assumed
   that network devices should CE-mark packets at the same level of
   congestion at which they would otherwise have dropped them, separate
   configuration of the drop and mark thresholds is known to be
   supported in some network devices and this is recommended
   [RFC2309.bis].  Some benefits of ECN that are discussed rely upon
   network devices marking packets at a lower level of congestion,
   before they would otherwise drop packets from queue overflow [KH13].

   The ability to use ECN relies upon using a transport that can support
   ECN.  Some benefits are also only realised when the transport
   endpoint behaviour is also updated, this is discussed further in
   Section 5.

2.  ECN Deployment

   For an application to use ECN requires that the endpoint first
   enables ECN within the transport.

   The ability to use ECN requires network devices along the path to at
   least pass IP packets that set ECN codepoints, and do not drop
   packets because these codepoints are used Section 2.2.  This is the
   recommended behaviour for network devices [RFC2309.bis] [RFC3168].
   Applications and transports (such as TCP or SCTP) can be designed to
   fall-back to not using ECN when they discover they are using a path
   that does not allow use of ECN (e.g., a firewall or other network
   device configured to drop the ECN codepoint) Section 6.1.

   For an application to gain benefit from using a transport that
   enables ECN, network devices need to enable ECN marking.  However,
   not all network devices along the path need to enable ECN, for the
   application to benefit.  Any network device that does not mark an
   ECN-enabled packet with a CE-codepoint can be expected to drop
   packets under congestion.  Applications that experience congestion in
   these network devices do not see any benefit from using ECN, but
   would see benefit if the congestion were to occur within a network
   device that did support ECN.

   ECN can be deployed both in the general Internet and in controlled

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   o  ECN can be incrementally deployed in the general Internet.  The
      IETF has provided guidance on configuration and usage in
      [RFC2309.bis].  A recent survey reported growing support for ECN
      on common network paths [TR15].

   o  ECN may also be deployed within a controlled environment, for
      example within a data centre or within a well-managed private
      network.  In this case, the use of ECN may be tuned to the
      specific use-case.  An example is Datacenter TCP (DCTCP) [AL10].

   Some mechanisms that can assist in using ECN across paths that only
   partially supports ECN are noted in Section 6.

2.1.  Enabling ECN in network devices

   Network deployment needs also to consider the requirements for
   processing ECN at tunnel endpoints of network tunnels, and guidance
   on the treatment of ECN is provided in [RFC6040].

   Further guidance on the encapsulation and use of ECN by non-IP
   network devices is provided in [ID.ECN-Encap].

2.2.  Bleaching and middlebox requirements to deploy ECN

   Cases have been noted where a sending endpoint marks a packet with a
   non-zero ECN mark, but the packet is received with a zero ECN value
   by the remote endpoint.

   The current IPv4 and IPv6 specifications assign usage of 2 bits in
   the IP header to carry the ECN codepoint[RFC2474] [RFC3168].  A
   previous usage assigned these bits as a part of the now deprecated
   Type of Service (ToS) field [RFC1349].  Network devices that conform
   to this older specification may still remark or erase the ECN
   codepoints, and such equipment needs to be updated to the current
   specifications to support ECN . This remarking has also been called
   "ECN bleaching".

   Some network devices have been observed to implement a policy that
   erases or "bleaches" the ECN marks at a network edge (resetting these
   to zero).  This may be implemented for various reasons (including
   normalising packets to hide which equipment supports ECN).  This
   policy prevents use of ECN by applications.  A network device should
   therefore not remark an ECT(0) or ECT(1) mark to zero.

   A network device must not change a packet with a CE mark to a zero
   codepoint (if the CE marking is not propagated, the packet must be
   discarded).  Such a packet has already received ECN treatment in the

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   network, and remarking it would then hide the congestion signal from
   the endpoints.

   Some networks may use ECN internally or tunnel ECN for traffic
   engineering or security.  Guidance on the correct use of ECN in this
   case is provided in [RFC6040].

3.  Benefit of using ECN to avoid congestion loss

   When packet loss is a result of (mild) congestion, an ECN-enabled
   router may be expected to CE-mark, rather than drop an ECN-enabled
   packet [RFC2309.bis].  An application can benefit from this marking
   in several ways:

3.1.  Improved Throughput

   ECN can improve the throughput performance of applications, although
   this increase in throughput offered by ECN is often not the most
   significant gain.

   When an application uses a light to moderately loaded network path,
   the number of packets that are dropped due to congestion is small.
   Using an example from Table 1 of [RFC3649], for a standard TCP sender
   with a Round Trip Time, RTT, of 0.1 seconds, a packet size of 1500
   bytes and an average throughput of 1 Mbps, the average packet drop
   ratio is 0.02.  This translates into an approximate 2% throughput
   gain if ECN is enabled.  In heavy congestion, packet loss may be
   unavoidable with, or without, ECN.

3.2.  Reduced Head-of-Line Blocking

   Many transports provide in-order delivery of received data segments
   to the applications they support.  This requires that the transport
   stalls (or waits) for all data that was sent ahead of a particular
   segment to be correctly received before it can forward any later
   data.  This is the usual requirement for TCP and SCTP.  PR-SCTP
   [RFC3758], UDP, and DCCP [RFC4340] provide a transport that does not
   have this requirement.

   Delaying data to provide in-order transmission to an application
   results in additional latency when segments are dropped as
   indications of congestion.  The congestive loss creates a delay of at
   least one RTT for a loss event before data can be delivered to an
   application.  We call this Head-of-Line (HOL) blocking.

   In contrast, using ECN can remove the resulting delay following a
   loss that is a result of congestion:

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   o  First, the application receives the data normally.  This also
      avoids the inefficiency of dropping data that has already made it
      across at least part of the network path.  It also avoids the
      additional delay of waiting for recovery of the lost segment.

   o  Second, the transport receiver notes that it has received CE-
      marked packets, and then requests the sender to make an
      appropriate congestion-response to reduce the maximum transmission
      rate for future traffic.

3.3.  Reduced Probability of RTO Expiry

   In some situations, ECN can help reduce the chance of a
   retransmission timer expiring (e.g., expiry of the TCP or SCTP
   retransmission timeout, RTO [RFC5681].  When an application sends a
   burst of segments and then becomes idle (either because the
   application has no further data to send or the network prevents
   sending further data - e.g., flow or congestion control at the
   transport layer), the last segment of the burst may be lost.  It is
   often not possible to recover this last segment (or last few
   segments) using standard methods such as Fast Recovery [RFC5681],
   since the receiver generates no feedback because it is unaware that
   the lost segments were actually sent.

   In addition to avoiding HOL blocking, this allows the transport to
   avoid the consequent loss of state about the network path it is
   using, which would have arisen had there been a retransmission
   timeout.  Typical impacts of a transport timeout are to reset path
   estimates such as the RTT, the congestion window, and possibly other
   transport state that can reduce the performance of the transport
   until it again adapts to the path.

   Avoiding timeouts can hence improve the throughput of the
   application.  This benefits applications that send intermittent
   bursts of data, and rely upon timer-based recovery of packet loss.
   It can be especially significant when ECN is used on TCP SYN/ACK
   packets [RFC5562] where the RTO interval may be large because in this
   case TCP cannot base the timeout period on prior RTT measurements
   from the same connection.

3.4.  Applications that do not retransmit lost packets

   Some latency-critical applications do not retransmit lost packets,
   yet they may be able to adjust the sending rate in the presence of
   congestion.  Examples of such applications include UDP-based services
   that carry Voice over IP (VoIP), interactive video or real-time data.
   The performance of many such applications degrades rapidly with
   increasing packet loss, and many therefore employ loss-hiding

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   mechanisms (e.g., packet forward error correction, or data
   duplication) to mitigate the effect of congestion loss on the
   application.  However, such mechanisms add complexity and can
   themselves consume additional network capacity reducing the capacity
   for application data and contributing to the path latency when
   congestion is experienced.

   By decoupling congestion control from loss, ECN can allow the
   transports supporting these applications to reduce their rate before
   the application experiences loss from congestion, especially when the
   congestion is mild and the application/transport can react promptly
   to reception of a CE-marked packet.  Because this reduces the
   negative impact of using loss-hiding mechanisms, ECN can have a
   direct positive impact on the quality experienced by the users of
   these applications.

4.  Benefit from Early Congestion Detection

   An application can further benefit from using ECN, when the network
   devices are configured such that they mark packets at a lower level
   of congestion before they would otherwise have dropped packets from
   queue overflow:

4.1.  Avoiding Capacity Overshoot

   Internet transports do not know apriori how much capacity exists
   along a network path.  Transports therefore try to measure the
   capacity available to an application by probing the network path with
   increasing traffic to the point where they detect the onset of
   congestion (such as TCP or SCTP Slow Start).

   ECN can help capacity probing algorithms (such as Slow Start) from
   significantly exceeding the bottleneck capacity of a network path.
   Since a transport that enables ECN can receive congestion signals
   before there is significant congestion, an early-marking method in
   network devices can help a transport respond before it induces
   significant congestion with resultant loss to itself or other
   applications sharing a common bottleneck.  For example, an
   application/transport can avoid incurring significant congestion
   during Slow Start, or a bulk application that tries to increase its
   rate as fast as possible, may quickly detect the presence of
   congestion, causing it to promptly reduce its rate.

   Use of ECN is more effective than schemes such as Limited Slow-Start
   [RFC3742] because it provides direct information about the state of
   the network path.  An ECN-enabled application/transport that probes
   for capacity can reduce its rate as soon as it discovers CE-marked
   packets are received, and before the applications increases its rate

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   to the point where it builds a queue in a network device that induces
   congestion loss.  This benefits an application seeking to increase
   its rate - but perhaps more significantly, it eliminates the often
   unwanted loss and queueing delay that otherwise may be inflicted on
   flows that share a common bottleneck.

4.2.  Making Congestion Visible

   A characteristic of using ECN is that it exposes the presence of
   congestion on a network path to the transport and network layers.
   This information can be used for monitoring performance of the path,
   and could be used to directly meter the amount of congestion that has
   been encountered upstream on a path; metering packet loss is harder.
   ECN measurements are used by Congestion Exposure (CoNex) [RFC6789].

   A network flow that only experiences CE-marks and no loss implies
   that the sending endpoint is experiencing only congestion and not
   other sources of packet loss (e.g., link corruption or loss in
   middleboxes).  The converse is not true - a flow may experience a
   mixture of ECN-marks and loss when there is only congestion or when
   there is a combination of packet loss and congestion [RFC2309.bis].
   Recording the presence of CE-marked packets can therefore provide
   information about the performance of the network path.

5.  Other forms of ECN-Marking/Reactions

   ECN requires a definition of both how packets are CE-marked and how
   applications/transports need to react to reception of CE-marked
   packets.  This section describes the benefits when updated methods
   are used.

   ECN-capable receiving endpoints may provide more detailed feedback
   describing the ECN codepoints that they observe using [ID.Acc-ECN].
   This can provide more information to a sending endpoint's congestion
   control mechanism.

   Benefit has been noted when packets are CE-marked earlier than they
   would otherwise be dropped, using an instantaneous queue, and if the
   receiver provides precise feedback about the number of packet marks
   encountered, a better sender behavior is possible.  This has been
   shown by Datacenter TCP (DCTCP) [AL10].

   Precise feedback about the number of packet marks encountered is
   supported by the Real Time Protocol (RTP) when used over UDP
   [RFC6679] and proposed for SCTP [ST14] and TCP [ID.Acc-ECN].  An
   underlying assumption of DCTCP is that it is deployed in confined
   environments such as a datacenter.  It is currently unknown whether
   or how such behaviour could be safely introduced into the Internet.

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6.  ECN transport mechanisms for paths with partial ECN support

   Early deployment of ECN encountered a number of operational
   difficulties when the network only partially supports the use of ECN,
   or to respond to the challenges due to misbehaving network devices
   and/or endpoints.  These problems have been observed to diminish with
   time, but may still be encountered on some Internet paths [TR15].

   This section describes transport mechanisms that allow ECN-enabled
   endpoints to continue to work effectively over a path with partial
   ECN support.

6.1.  Verifying whether a path really supports ECN

   ECN transport and applications need to implement mechanisms to verify
   ECN support on the path that they use and fallback to not using ECN
   when it would not work.  This is expected to be a normal feature of
   IETF-defined transports supporting ECN.

   Before a transport relies on the presence or absence of CE-marked
   packets, it may need to verify that any ECN marks applied to packets
   passed by the path are indeed delivered to the remote endpoint.  This
   may be achieved by the sender setting known ECN codepoints into
   specific packets in a network flow and then verifying that these
   reach the remote endpoint [ID.Fallback], [TR15].

   Endpoints also need to be robust to path changes.  A change in the
   set of network devices along a path may impact the ability to
   effectively signal or use ECN across the path, e.g., when a path
   changes to use a middlebox that bleaches ECN codepoints.  As a
   necessary, but short term fix, transports could implement mechanisms
   that detect this and fall-back to disabling use of ECN [BA11].

6.2.  Detecting ECN receiver feedback cheating

   It is important that receiving endpoints accurately report the loss
   they experience when using a transport that uses loss-based
   congestion control.  So also, when using ECN, a receiver must
   correctly report the congestion marking that it receives and then
   provide a mechanism to feed the congestion information back to the
   sending endpoint.

   The transport at endpoint receivers must not try to conceal reception
   of CE-marked packets in the ECN feedback information that they
   provide to the sending endpoint [RFC2309.bis].  Transport protocols
   are actively encouraged to include mechanisms that can detect and
   appropriately respond to such misbehavior (e.g., disabling use of
   ECN, and relying on loss-based congestion detection [TR15]).

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7.  Conclusion

   Network devices should enable ECN and people configuring host stacks
   should also enable ECN.  Specifically network devices must not change
   a packet with a CE mark to a zero codepoint (if the CE marking is not
   propagated, the packet must be discarded).  These are prerequisites
   to allow applications to gain the benefits of ECN.

   Prerequisites for network devices (including IP routers) to enable
   use of ECN include:

   o  should not reset the ECN codepoint to zero by default Section 2.2.

   o  should correctly update the ECN codepoint in the presence of

   o  should correctly support alternate ECN semantics ([RFC4774]).

   Prerequisites for network endpoints to enable use of ECN include:

   o  should use transports that can set and receive ECN marks.

   o  should correctly return feedback of congestion to the sending

   o  must use transports that react appropriately to received ECN
      feedback Section 6.2.

   o  should use transports that can detect misuse of ECN and detect
      paths that do not support ECN, providing fallback to loss-based
      congestion detection when ECN is not supported Section 6.1.

   Application developers should where possible use transports that
   enable the benefits of ECN.  Applications that directly use UDP need
   to provide support to implement the functions required for ECN.  Once
   enabled, an application that uses a transport that supports ECN will
   experience the benefits of ECN as network deployment starts to enable
   ECN.  The application does not need to be rewritten to gain these
   benefits.  Table 1 summarises some of these benefits.

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   | Section | Benefit                                             |
   |   3.1   | Improved Throughput                                 |
   |   3.2   | Reduced Head-of-Line                                |
   |   3.3   | Reduced Probability of RTO Expiry                   |
   |   3.4   | Applications that do not retransmit lost packets    |
   |   4.1   | Avoiding Capacity Overshoot                         |
   |   4.2   | Making Congestion Visible                           |

   Table 1: Summary of Key Benefits

8.  Acknowledgements

   The authors were part-funded by the European Community under its
   Seventh Framework Programme through the Reducing Internet Transport
   Latency (RITE) project (ICT-317700).  The views expressed are solely
   those of the authors.

   The authors would like to thank the following people for their
   comments on prior versions of this document: Bob Briscoe, David
   Collier-Brown, John Leslie, Colin Perkins, Richard Scheffenegger,
   Dave Taht, Wes Eddy.

9.  IANA Considerations


   This memo includes no request to IANA.

10.  Security Considerations

   This document introduces no new security considerations.  Each RFC
   listed in this document discusses the security considerations of the
   specification it contains.

11.  Revision Information

   XXX RFC-Ed please remove this section prior to publication.

   Revision 00 was the first WG draft.

   Revision 01 includes updates to complete all the sections and a
   rewrite to improve readability.  Added section 2.  Author list
   reversed, since Gorry has become the lead author.  Corrections

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   following feedback from Wes Eddy upon review of an interim version of
   this draft.

   Note: Wes Eddy raised a question about whether discussion of the ECN
   Pitfalls could be improved or restrcutured - this is expected to be
   addressed in the next revision.

   Revision 02 updates the title, and also the description of mechanisms
   that help with partial ECN support.

   We think this draft is ready for wider review.  Comments are welcome
   to the authors or via the IETF AQM or TSVWG mailing lists.

12.  References

12.1.  Normative References

              Baker, F. and G. Fairhurst, "IETF Recommendations
              Regarding Active Queue Management", Internet-draft draft-
              ietf-aqm-recommendation-06, October 2014.

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

12.2.  Informative References

   [AL10]     Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel,
              P., Prabhakar, B., Sengupta, S., and M. Sridharan, "Data
              Center TCP (DCTCP)", SIGCOMM 2010, August 2010.

   [BA11]     Bauer, Steven., Beverly, Robert., and Arthur. Berger,
              "Measuring the State of ECN Readiness in Servers, Clients,
              and Routers, ACM IMC", 2011.

              Kuehlewind, Mirja., Scheffenegger, Richard., and Bob.
              Briscoe, "Problem Statement and Requirements for a More
              Accurate ECN Feedback", Internet-draft, IETF work-in-
              progress draft-ietf-tcpm-accecn-reqs, 2015.

              Briscoe, B., Kaippallimalil, J., and P. Thaler,
              "Guidelines for Adding Congestion Notification to
              Protocols that Encapsulate IP", Internet-draft, IETF work-
              in-progress draft-ietf-tsvwg-ecn-encap-guidelines, .

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              Kuehlewind, Mirja. and Brian. Trammell, "A Mechanism for
              ECN Path Probing and Fallback, draft-kuehlewind-tcpm-ecn-
              fallback, Work-in-Progress", .

   [KH13]     Khademi, N., Ros, D., and M. Welzl, "The New AQM Kids on
              the Block: Much Ado About Nothing?", University of Oslo
              Department of Informatics technical report 434, October

   [RFC1349]  "Type of Service in the Internet Protocol Suite", .

   [RFC2474]  "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", .

   [RFC3649]  Floyd, S., "HighSpeed TCP for Large Congestion Windows",
              RFC 3649, December 2003.

   [RFC3742]  Floyd, S., "Limited Slow-Start for TCP with Large
              Congestion Windows", RFC 3742, March 2004.

   [RFC3758]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
              Conrad, "Stream Control Transmission Protocol (SCTP)
              Partial Reliability Extension", RFC 3758, May 2004.

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

   [RFC4774]  Floyd, S., "Specifying Alternate Semantics for the
              Explicit Congestion Notification (ECN) Field", BCP 124,
              RFC 4774, November 2006.

   [RFC5562]  Kuzmanovic, A., Mondal, A., Floyd, S., and K.
              Ramakrishnan, "Adding Explicit Congestion Notification
              (ECN) Capability to TCP's SYN/ACK Packets", RFC 5562, June

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

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, November 2010.

   [RFC6679]  Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
              and K. Carlberg, "Explicit Congestion Notification (ECN)
              for RTP over UDP", RFC 6679, August 2012.

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   [RFC6789]  Briscoe, B., Woundy, R., and A. Cooper, "Congestion
              Exposure (ConEx) Concepts and Use Cases", RFC 6789,
              December 2012.

   [ST14]     Stewart, R., Tuexen, M., and X. Dong, "ECN for Stream
              Control Transmission Protocol (SCTP)", Internet-draft
              draft-stewart-tsvwg-sctpecn-05.txt, January 2014.

   [TR15]     Tranmmel, Brian., Kuehlewind, Mirja., Boppart, Damiano,
              Learmonth, Iain., and Gorry.  Fairhurst, "Enabling
              internet-wide deployment of Explicit Congestion
              Notification Tramwell, B., Kuehlewind, M., Boppart, D.,
              Learmonth, I., Fairhurst, G. & Scheffnegger, Passive and
              Active Measurement Conference (PAM)", March 2015.

Authors' Addresses

   Godred Fairhurst
   University of Aberdeen
   School of Engineering, Fraser Noble Building
   Aberdeen  AB24 3UE


   Michael Welzl
   University of Oslo
   PO Box 1080 Blindern
   Oslo  N-0316

   Phone: +47 22 85 24 20

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