Network Working Group                                           M. Welzl
Internet-Draft                                        University of Oslo
Intended status: Informational                              G. Fairhurst
Expires: September 10, 2015                       University of Aberdeen
                                                          March 09, 2015

 The Benefits to Applications of using Explicit Congestion Notification


   This document describes the potential benefits to applications when
   they 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.  The focus of this document is
   on usage of ECN, not its implementation in hosts, routers and other
   network devices.

Status of This Memo

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   This Internet-Draft will expire on September 10, 2015.

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   document authors.  All rights reserved.

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   carefully, as they describe your rights and restrictions with respect

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   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 [RFC5405] (e.g.  ECN with RTP [RFC6679]).

   A network device (router, middlebox, or other device that forward
   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 this packet 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 indicate 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 is generally not known by
   the transport endpoints.  The focus of this document is on usage of
   ECN, not its implementation in hosts, routers and other network

   ECN makes it possible for the network to signal congestion without
   packet loss.  This lets the network deliver some packets to an
   application that would otherwise have been dropped.  This reduction
   in packet loss 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 reduction in packet loss from ECN-
   marking instead of dropping packets.

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

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

   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.  This 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.  This is
   the recommended behaviour for network devices [RFC2309.bis].
   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).

   XXX NOTE: A future revision could include some words and reference a
   paper on the current state of network support for transparently
   passing the ECN codepoints.

   For an application at an endpoint to gain benefit from ECN, network
   devices need to enable ECN marking.  Not all network devices along
   the path need to enable ECN, for the application to benefit.  Any
   network devices that does not set a CE-codepoint can be expected to
   drop packets under congestion.  Applications that experience
   congestion in such endpoints 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 incrementally deployed in the general Internet.  The IETF
   has provided guidance on configuration and usage in [RFC2309.bis].

   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].

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   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

3.  Benefit of using ECN to avoid congestion loss

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

3.1.  Improved Throughput

   ECN can improve the throughput performance of an application,
   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 [RFC2309.bis].

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 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:

   o  First, the application receives the data normally - this also
      avoids dropping data that has already made it across the network

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      path.  It avoids the additional delay of waiting for recovery of
      the lost segment when using a reliable transport.

   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 as specified in [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 use transports that do not
   retransmit lost packets, yet these applications 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 mechanisms (e.g., packet
   forward error correction, or data duplication) to mitigate the effect

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   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 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 transport mechanisms 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 to the point where it builds a queue in a network
   device that induces congestion loss.  This benefits an application

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   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 could be used for monitoring the 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.  This is 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

   The ECN mechanism defines both how packets are CE-marked and how
   transports need to react to reception of marked packets.  This
   section describes the benefits when updated methods are used.

   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 RTP over UDP [RFC6679] and proposed for SCTP [ST14] and
   TCP [KU13].  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 introduced into the

6.  Conclusion

   Network devices should enable ECN and people configuring host stacks
   should also enable ECN.  These are pre-requisites to allow
   applications to gain the benefits of ECN.

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   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

   Table 1 summarizes some of these benefits.

   | Section | Benefit                                             |
   |   2.1   | Improved Throughput                                 |
   |   2.2   | Reduced Head-of-Line                                |
   |   2.3   | Reduced Probability of RTO Expiry                   |
   |   2.4   | Applications that do not retransmit lost packets    |
   |   3.1   | Avoiding Capacity Overshoot                         |
   |   3.2   | Making Congestion Visible                           |

   Table 1: Summary of Key Benefits from using ECN

7.  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.

8.  IANA Considerations


   This memo includes no request to IANA.

9.  Security Considerations

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

10.  Revision Information

   RFC-Ed please remove this section prior to publication.

   Revision 00 was the first WG draft.

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   Revision 01 includes updates to complete sections and improve
   readability.  Added section 2.

   Comments are welcome to the authors or via the IETF AQM or TSVWG
   mailing lists.

11.  References

11.1.  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.

11.2.  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.

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

   [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

   [KU13]     Kuehlewind, M. and R. Scheffenegger, "Problem Statement
              and Requirements for a More Accurate ECN Feedback",
              Internet-draft draft-ietf-tcpm-accecn-reqs-04.txt, October

   [RFC2884]  Hadi Salim, J. and U. Ahmed, "Performance Evaluation of
              Explicit Congestion Notification (ECN) in IP Networks",
              RFC 2884, July 2000.

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

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   [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.

   [RFC5405]  Eggert, Lars. and Gorry. Fairhurst, "Unicast UDP Usage
              Guidelines for Application Designers", November 2008.

   [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]  "TCP Congestion Control", .

   [RFC6040]  "Tunnelling of Explicit Congestion Notification", .

   [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.

   [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.

Authors' Addresses

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

   Phone: +47 22 85 24 20

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


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