The Benefits of Using Explicit Congestion Notification (ECN)
RFC 8087

Internet Engineering Task Force (IETF)                      G. Fairhurst
Request for Comments: 8087                        University of Aberdeen
Category: Informational                                         M. Welzl
ISSN: 2070-1721                                       University of Oslo
                                                              March 2017

      The Benefits of Using Explicit Congestion Notification (ECN)

Abstract

   The goal of this document is to describe the potential benefits of
   applications using a transport that enables Explicit Congestion
   Notification (ECN).  The document outlines the principal gains in
   terms of increased throughput, reduced delay, and other benefits when
   ECN is used over a network path that includes equipment that supports
   Congestion Experienced (CE) marking.  It also discusses challenges
   for successful deployment of ECN.  It does not propose new algorithms
   to use ECN nor does it describe the details of implementation of ECN
   in endpoint devices (Internet hosts), routers, or other network
   devices.

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 7841.

   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/rfc8087.

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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Benefit of Using ECN to Avoid Congestion Loss . . . . . . . .   5
     2.1.  Improved Throughput . . . . . . . . . . . . . . . . . . .   5
     2.2.  Reduced Head-of-Line Blocking . . . . . . . . . . . . . .   6
     2.3.  Reduced Probability of RTO Expiry . . . . . . . . . . . .   6
     2.4.  Applications That Do Not Retransmit Lost Packets  . . . .   7
     2.5.  Making Incipient Congestion Visible . . . . . . . . . . .   8
     2.6.  Opportunities for New Transport Mechanisms  . . . . . . .   8
   3.  Network Support for ECN . . . . . . . . . . . . . . . . . . .   9
     3.1.  The ECN Field . . . . . . . . . . . . . . . . . . . . . .  10
     3.2.  Forwarding ECN-Capable IP Packets . . . . . . . . . . . .  10
     3.3.  Enabling ECN in Network Devices . . . . . . . . . . . . .  11
     3.4.  Coexistence of ECN and Non-ECN Flows  . . . . . . . . . .  11
     3.5.  Bleaching and Middlebox Requirements to Deploy ECN  . . .  11
     3.6.  Tunneling ECN and the Use of ECN by Lower-Layer Networks   12
   4.  Using ECN across the Internet . . . . . . . . . . . . . . . .  12
     4.1.  Partial Deployment  . . . . . . . . . . . . . . . . . . .  13
     4.2.  Detecting Whether a Path Really Supports ECN  . . . . . .  13
     4.3.  Detecting ECN-Receiver Feedback Cheating  . . . . . . . .  14
   5.  Summary: Enabling ECN in Network Devices and Hosts  . . . . .  14
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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1.  Introduction

   Internet transports (such as TCP and Stream Control Transmission
   Protocol (SCTP)) are implemented in endpoints (Internet hosts) and
   are designed to detect and react to network congestion.  Congestion
   may be detected by loss of an IP packet or, if Explicit Congestion
   Notification (ECN) [RFC3168] is enabled, by the reception of a packet
   with a Congestion Experienced (CE) marking in the IP header.  Both of
   these are treated by transports as indications of congestion.  ECN
   may also be enabled by other transports: UDP applications that
   provide congestion control may enable ECN when they are able to
   correctly process the ECN signals [RFC8085] (e.g., ECN with RTP
   [RFC6679]).

   Active Queue Management (AQM) [RFC7567] is a class of techniques that
   can be used by network devices (a router, middlebox, or other device
   that forwards packets through the network) to manage the size of
   queues in network buffers.

   A network device that does not support AQM typically uses a drop-tail
   policy to drop excess IP packets when its queue becomes full.  The
   discard of packets is treated by transport protocols as a signal that
   indicates congestion on the end-to-end network path.  End-to-end
   transports, such as TCP, can cause a low level of loss while seeking
   to share capacity with other flows.  Although losses are not always
   due to congestion (loss may be due to link corruption, receiver
   overrun, etc.), endpoints have to conservatively presume that all
   loss is potentially due to congestion and reduce their rate.
   Observed loss therefore results in a congestion control reaction by
   the transport to reduce the maximum rate permitted by the sending
   endpoint.

   ECN makes it possible for the network to signal the presence of
   incipient congestion without incurring packet loss; it 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
   receiving a CE marking instead of dropping packets.  Several benefits
   reduce latency (e.g., reduced head-of-line blocking).

   The use of ECN is indicated in the ECN field [RFC3168], which is
   carried in the packet header of all IPv4 and IPv6 packets.  This
   field may be set to one of the four values shown in Figure 1.  The
   Not-ECT codepoint '00' indicates a packet that is not using ECN.  The
   ECT(0) codepoint '01' and the ECT(1) codepoint '10' both indicate

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   that the transport protocol using the IP layer supports the use of
   ECN.  The CE codepoint '11' is set by an ECN-capable network device
   to indicate congestion to the transport endpoint.

   +-----+-----+---------+
   | ECN FIELD |  Name   |
   +-----+-----+---------+
   |  0  |  0  | Not-ECT |
   |  0  |  1  | ECT(1)  |
   |  1  |  0  | ECT(0)  |
   |  1  |  1  | CE      |
   +-----+-----+---------+

   Figure 1: The ECN Field in the IP Packet Header (based on [RFC3168])

   When an application uses a transport that enables use of ECN
   [RFC3168], 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, the ECN-capable IP
   packets.  An ECN-capable network device can then signal incipient
   congestion (network queuing) at a point before a transport
   experiences congestion loss or high queuing delay.  The marking is
   generally performed as the result of various AQM algorithms [RFC7567]
   where the exact combination of AQM/ECN algorithms does not need to be
   known by the transport endpoints.

   The focus of the document is on usage of ECN by transport- and
   application-layer flows, not its implementation in endpoint hosts,
   routers, and other network devices.

1.1.  Terminology

   The following terms are used:

   AQM: Active Queue Management.

   CE: Congestion Experienced; a codepoint value '11' marked in the ECN
   field of the IP packet header.

   ECN-capable IP Packet: A packet where the ECN field is set to a non-
   zero ECN value (i.e., with ECT(0), ECT(1), or the CE codepoint).

   ECN-capable network device: An ECN-capable network device may
   forward, drop, or queue an ECN-capable packet and may choose to CE
   mark this packet when there is incipient congestion.

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   ECN-capable transport/application: A transport that sends ECN-capable
   IP Packets, monitors reception of the ECN field, and generates
   appropriate feedback to control the rate of the sending endpoint to
   provide end-to-end congestion control.

   ECN field: A 2-bit field specified for use with explicit congestion
   signaling in the IPv4 and IPv6 packet headers.

   Endpoint: An Internet host that terminates a transport protocol
   connection across an Internet path.

   Incipient Congestion: The detection of congestion when it is
   starting, perhaps by a network device noting that the arrival rate
   exceeds the forwarding rate.

   Network device: A router, middlebox, or other device that forwards IP
   packets through the network.

   non-ECN-capable: A network device or endpoint that does not interpret
   the ECN field.  Such a device is not permitted to change the ECN
   codepoint.

   not-ECN-capable IP Packet: An IP packet with the ECN field set to a
   value of zero ('00').  A not-ECN-capable packet may be forwarded,
   dropped, or queued by a network device.

2.  Benefit of Using ECN to Avoid Congestion Loss

   An ECN-capable network device is expected to CE mark an ECN-capable
   IP packet as a CE when an AQM method detects incipient congestion
   rather than drop the packet [RFC7567].  An application can benefit
   from this marking in several ways, which are detailed in the rest of
   this section.

2.1.  Improved Throughput

   ECN seeks to avoid the inefficiency of dropping data that has already
   made it across at least part of the network path.

   ECN can improve the throughput of an application, although this
   increase in throughput is often not the most significant gain.  When
   an application uses a lightly 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
   an RTT of 0.1 seconds, a packet size of 1500 bytes, and an average
   throughput of 1 Mbps, the average packet-drop ratio would be 0.02
   (i.e., 1 in 50 packets).  This translates into an approximate 2%

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   throughput gain if ECN is enabled.  (Note that in heavy congestion,
   packet loss may be unavoidable with or without ECN.)

2.2.  Reduced Head-of-Line Blocking

   Many Internet transports provide in-order delivery of received data
   segments to the applications they support.  For these applications,
   use of ECN can reduce the delay that can result when these
   applications experience packet loss.

   Packet loss may occur for various reasons.  One cause arises when an
   AQM scheme drops a packet as a signal of incipient congestion.
   Whatever the cause of loss, a missing packet needs to trigger a
   congestion control response.  A reliable transport also triggers
   retransmission to recover the lost data.  For a transport providing
   in-order delivery, this requires that the transport receiver stall
   (or wait) for all data that was sent ahead of a lost segment to be
   correctly received before it can forward any later data to the
   application.  A loss therefore creates a delay of at least one RTT
   after a loss event before data can be delivered to an application.
   We call this head-of-line blocking.  This is the usual requirement
   for TCP and SCTP.  Partially Reliable SCTP (PR-SCTP) [RFC3758], UDP
   [RFC768] [RFC8085], and the Datagram Congestion Control Protocol
   (DCCP) [RFC4340] provide a transport that does not provide
   reordering.

   By enabling ECN, a transport continues to receive in-order data when
   there is incipient congestion and can pass this data to the receiving
   application.  Use of ECN avoids the additional reordering delay in a
   reliable transport.  The sender still needs to make an appropriate
   congestion response to reduce the maximum transmission rate for
   future traffic, which usually will require a reduction in the sending
   rate [RFC8085].

2.3.  Reduced Probability of RTO Expiry

   Some patterns of packet loss can result in a Retransmission Timeout
   (RTO), which causes a sudden and significant change in the allowed
   rate at which a transport/application can forward packets.  Because
   ECN provides an alternative to drop for network devices to signal
   incipient congestion, this can reduce the probability of loss and
   hence reduce the likelihood of RTO expiry.

   Internet transports/applications generally use an RTO timer as a last
   resort to detect and recover loss [RFC8085] [RFC5681].  Specifically,
   an RTO timer detects loss of a packet that is not followed by other
   packets, such as at the end of a burst of data segments or when an
   application becomes idle (either because the application has no

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   further data to send or the network prevents sending further data,
   e.g., flow or congestion control at the transport layer).  This loss
   of the last segment (or last few segments) of a traffic burst is also
   known as a "tail loss".  Standard transport recovery methods, such as
   Fast Recovery [RFC5681], are often unable to recover from a tail
   loss.  This is because the endpoint receiver is unaware that the lost
   segments were actually sent and therefore generates no feedback
   [Fla13].  Retransmission of these segments relies on expiry of a
   transport retransmission timer.  This timer is also used to detect a
   lack of forwarding along a path.  Expiry of the RTO results in the
   consequent loss of state about the network path being used.  This
   typically includes resetting path estimates such as the RTT,
   reinitializing the congestion window, and possibly making updates to
   other transport state.  This can reduce the performance of the
   transport until it again adapts to the path.

   An ECN-capable network device cannot eliminate the possibility of
   tail loss because a drop may occur due to a traffic burst exceeding
   the instantaneous available capacity of a network buffer or as a
   result of the AQM algorithm (e.g., overload protection mechanisms
   [RFC7567]).  However, an ECN-capable network device that observes
   incipient congestion may be expected to buffer the IP packets of an
   ECN-capable flow and set a CE mark in one or more packet(s) rather
   than triggering packet drop.  Setting a CE mark signals incipient
   congestion without forcing the transport/application to enter
   retransmission timeout.  This reduces application-level latency and
   can improve the throughput for applications that send intermittent
   bursts of data.

   The benefit of avoiding retransmission loss is expected to be
   significant when ECN is used on TCP SYN/ACK packets [RFC5562] where
   the RTO interval may be large because TCP cannot base the timeout
   period on prior RTT measurements from the same connection.

2.4.  Applications That Do Not Retransmit Lost Packets

   A transport that enables ECN can receive timely congestion signals
   without the need to retransmit packets each time it receives a
   congestion signal.

   Some latency-critical applications do not retransmit lost packets,
   yet they may be able to adjust their sending rate following detection
   of incipient 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 the transport/application
   may therefore employ mechanisms (e.g., packet forward error
   correction, data duplication, or media codec error concealment) to

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   mitigate the immediate effect of congestion loss on the application.
   Some mechanisms consume additional network capacity, some require
   additional processing, and some contribute additional path latency
   when congestion is experienced.  By decoupling congestion control
   from loss, ECN can allow transports that support these applications
   to reduce their rate before the application experiences loss from
   congestion.  This can reduce the negative impact of triggering loss-
   hiding mechanisms with a direct positive impact on the quality
   experienced by the users of these applications.

2.5.  Making Incipient 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,
   thus allowing information to be collected about the presence of
   incipient congestion.

   Recording the presence of CE-marked packets can provide information
   about the current congestion level experienced on a network path.  A
   network flow that only experiences CE marking and no loss implies
   that the sending endpoint is experiencing only congestion.  A network
   flow may also experience loss (e.g., due to queue overflow, AQM
   methods that protect other flows, link corruption, or loss in
   middleboxes).  When a mixture of CE marking and packet loss is
   experienced, transports and measurements need to assume there is
   congestion [RFC7567].  Therefore, an absence of CE marks does not
   indicate a path has not experienced congestion.

   The reception of CE-marked packets can be used to monitor the level
   of congestion by a transport/application or a network operator.  For
   example, ECN measurements are used by Congestion Exposure (ConEx)
   [RFC6789].  In contrast, metering packet loss is harder.

2.6.  Opportunities for New Transport Mechanisms

   ECN can enable design and deployment of new algorithms in network
   devices and Internet transports.  Internet transports need to regard
   both loss and CE marking as an indication of congestion.  However,
   while the amount of feedback provided by drop ought naturally be
   minimized, this is not the case for ECN.  In contrast, an ECN-capable
   network device could provide richer (more frequent and fine-grained)
   indication of its congestion state to the transport.

   For any ECN-capable transport (ECT), the receiving endpoint needs to
   provide feedback to the transport sender to indicate that CE marks
   have been received.  [RFC3168] provides one method that signals once
   each round-trip time (RTT) that CE-marked packets have been received.

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   A receiving endpoint may provide more detailed feedback to the
   congestion controller at the sender (e.g., describing the set of
   received ECN codepoints or indicating each received CE-marked
   packet).  Precise feedback about the number of CE marks encountered
   is supported by RTP when used over UDP [RFC6679] and has been
   proposed for SCTP [ST14] and TCP [ECN-FEEDBACK].

   More detailed feedback is expected to enable evolution of transport
   protocols allowing the congestion control mechanism to make a more
   appropriate decision on how to react to congestion.  Designers of
   transport protocols need to consider not only how network devices
   CE-mark packets but also how the control loop in the application/
   transport reacts to reception of these CE-marked packets.

   Benefit has been noted when packets are CE marked early using an
   instantaneous queue, and if the receiving endpoint provides feedback
   about the number of packet marks encountered, an improved sender
   behavior has been shown to be possible, e.g, Data Center TCP (DCTCP)
   [AL10].  DCTCP is targeted at controlled environments such as a data
   center.  This is a work in progress, and it is currently unknown
   whether or how such behavior could be safely introduced into the
   Internet.  Any update to an Internet transport protocol requires
   careful consideration of the robustness of the behavior when working
   with endpoints or network devices that were not designed for the new
   congestion reaction.

3.  Network Support for ECN

   For an application to use ECN requires that the endpoints enable ECN
   within the transport being used.  It also requires that all network
   devices along the path at least forward IP packets that set a
   non-zero ECN codepoint.

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

   o  ECN can be incrementally deployed in the general Internet.  The
      IETF has provided guidance on configuration and usage in
      [RFC7567].

   o  ECN may be deployed within a controlled environment, for example,
      within a data center or within a well-managed private network.
      This use of ECN may be tuned to the specific use case.  An example
      is DCTCP [AL10] [DCTCP].

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   Early experience of using ECN across the general Internet encountered
   a number of operational difficulties when the network path either
   failed to transfer ECN-capable packets or inappropriately changed the
   ECN codepoints [BA11].  A recent survey reported a growing support
   for network paths to pass ECN codepoints [TR15].

   The remainder of this section identifies what is needed for network
   devices to effectively support ECN.

3.1.  The ECN Field

   The current IPv4 and IPv6 specifications assign usage of 2 bits in
   the IP header to carry the ECN codepoint.  This 2-bit field was
   reserved in [RFC2474] and assigned in [RFC3168].

   [RFC4774] discusses some of the issues in defining alternate
   semantics for the ECN field and specifies requirements for a safe
   coexistence in an Internet that could include routers that do not
   understand the defined alternate semantics.

   Some network devices were configured to use a routing hash that
   included the set of 8 bits forming the now deprecated Type of Service
   (TOS) field [RFC1349].  The present use of this field assigns 2 of
   these bits to carry the ECN field.  This is incompatible with use in
   a routing hash because it could lead to IP packets that carry a CE
   mark being routed over a different path to those packets that carried
   an ECT mark.  The resultant reordering would impact the performance
   of transport protocols (such as TCP or SCTP) and UDP-based
   applications that are sensitive to reordering.  A network device that
   conforms to this older specification needs to be updated to the
   current specifications [RFC2474] to support ECN.  Configuration of
   network devices must note that the ECN field may be updated by any
   ECN-capable network device along a path.

3.2.  Forwarding ECN-Capable IP Packets

   Not all network devices along a path need to be ECN-capable (i.e.,
   perform CE marking).  However, all network devices need to be
   configured not to drop packets solely because the ECT(0) or ECT(1)
   codepoints are used.

   Any network device that does not perform CE marking of an ECN-capable
   packet can be expected to drop these packets under congestion.
   Applications that experience congestion at these network devices do
   not see any benefit from enabling ECN.  However, they may see benefit
   if the congestion were to occur within a network device that did
   support ECN.

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3.3.  Enabling ECN in Network Devices

   Network devices should use an AQM algorithm that CE-marks ECN-capable
   traffic when making decisions about the response to congestion
   [RFC7567].  An ECN method should set a CE mark on ECN-capable packets
   in the presence of incipient congestion.  A CE-marked packet will be
   interpreted as an indication of incipient congestion by the transport
   endpoints.

   There is an opportunity to design an AQM method for an ECN-capable
   network device that differs from an AQM method designed to drop
   packets.  [RFC7567] states that the network device should allow this
   behavior to be configurable.

   [RFC3168] describes a method in which a network device sets the CE
   mark at the time that the network device 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, [RFC7567] recommends that network
   devices allow independent configuration of the settings for AQM
   dropping and ECN marking.  Such separate configuration of the drop
   and mark policies is supported in some network devices.

3.4.  Coexistence of ECN and Non-ECN Flows

   Network devices need to be able to forward all IP flows and provide
   appropriate treatment for both ECN and non-ECN traffic.

   The design considerations for an AQM scheme supporting ECN needs to
   consider the impact of queueing during incipient congestion.  For
   example, a simple AQM scheme could choose to queue ECN-capable and
   non-ECN-capable flows in the same queue with an ECN scheme that
   CE-marks packets during incipient congestion.  The CE-marked packets
   that remain in the queue during congestion can continue to contribute
   to queueing delay.  In contrast, non-ECN-capable packets would
   normally be dropped by an AQM scheme under incipient congestion.
   This difference in queueing is one motivation for consideration of
   more advanced AQM schemes and may provide an incentive for enabling
   flow isolation using scheduling [RFC7567].  The IETF is defining
   methods to evaluate the suitability of AQM schemes for deployment in
   the general Internet [RFC7928].

3.5.  Bleaching and Middlebox Requirements to Deploy ECN

   Network devices should not be configured to change the ECN codepoint
   in the packets that they forward, except to set the CE codepoint to
   signal incipient congestion.

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   Cases have been noted where an endpoint sends a packet with a
   non-zero ECN mark, but the packet is received by the remote endpoint
   with a zero ECN codepoint [TR15].  This could be a result of a policy
   that erases or "bleaches" the ECN codepoint values at a network edge
   (resetting the codepoint to zero).  Bleaching may occur for various
   reasons (including normalizing packets to hide which equipment
   supports ECN).  This policy prevents use of ECN by applications.

   When ECN-capable IP packets, marked as ECT(0) or ECT(1), are
   re-marked to non-ECN-capable (i.e., the ECN field is set to the zero
   codepoint), this could result in the packets being dropped by
   ECN-capable network devices further along the path.  This eliminates
   the advantage of using of ECN.

   A network device must not change a packet with a CE mark to a zero
   codepoint; if the network device decides not to forward the packet
   with the CE mark, it has to instead drop the packet and not bleach
   the marking.  This is because a CE-marked packet has already received
   ECN treatment in the network, and re-marking it would then hide the
   congestion signal from the receiving endpoint.  This eliminates the
   benefits of ECN.  It can also slow down the response to congestion
   compared to using AQM because the transport will only react if it
   later discovers congestion by some other mechanism.

   Prior to [RFC2474], a previous usage assigned the bits now forming
   the ECN field as a part of the now deprecated TOS field [RFC1349].  A
   network device that conforms to this older specification was allowed
   to re-mark or erase the ECN codepoints, and such equipment needs to
   be updated to the current specifications in order to support ECN.

3.6.  Tunneling ECN and the Use of ECN by Lower-Layer Networks

   Some networks may use ECN internally or tunnel ECN (e.g., for traffic
   engineering or security).  These methods need to ensure that the ECN
   field of the tunnel packets is handled correctly at the ingress and
   egress of the tunnel.  Guidance on the correct use of ECN is provided
   in [RFC6040].

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

4.  Using ECN across the Internet

   A receiving endpoint needs to report the loss it experiences when it
   uses loss-based congestion control.  So also, when ECN is enabled, a
   receiving endpoint must correctly report the presence of CE marks by
   providing a mechanism to feed this congestion information back to the
   sending endpoint [RFC3168] [RFC8085], thus enabling the sender to

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   react to experienced congestion.  This mechanism needs to be designed
   to operate robustly across a wide range of Internet path
   characteristics.  This section describes partial deployment, that is,
   how ECN-enabled endpoints can continue to work effectively over a
   path that experiences misbehaving network devices or when an endpoint
   does not correctly provide feedback of ECN information.

4.1.  Partial Deployment

   Use of ECN is negotiated between the endpoints prior to using the
   mechanism.

   ECN has been designed to allow incremental partial deployment
   [RFC3168].  Any network device can choose to use either ECN or some
   other loss-based policy to manage its traffic.  Similarly, transport/
   application negotiation allows sending and receiving endpoints to
   choose whether ECN will be used to manage congestion for a particular
   network flow.

4.2.  Detecting Whether a Path Really Supports ECN

   Internet transports and applications need to be robust to the variety
   and sometimes varying path characteristics that are encountered in
   the general Internet.  They need to monitor correct forwarding of ECN
   over the entire path and duration of a session.

   To be robust, applications and transports need to be designed with
   the expectation of heterogeneous forwarding (e.g., where some IP
   packets are CE marked by one network device and some by another,
   possibly using a different AQM algorithm, or when a combination of CE
   marking and loss-based congestion indications are used).  Note that
   [RFC7928] describes methodologies for evaluating AQM schemes.

   A transport/application also needs to be robust to path changes.  A
   change in the set of network devices along a path could 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 (see
   Section 3.5).

   A sending endpoint can check that any CE marks applied to packets
   received over the path are indeed delivered to the remote receiving
   endpoint and that appropriate feedback is provided.  (This could be
   done by a sender setting a known CE codepoint for specific packets in
   a network flow and then checking whether the remote endpoint
   correctly reports these marks [ECN-FALLBACK] [TR15].)  If a sender
   detects persistent misuse of ECN, it needs to fall back to using
   loss-based recovery and congestion control.  Guidance on a suitable
   transport reaction is provided in [ECN-FALLBACK].

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4.3.  Detecting ECN-Receiver Feedback Cheating

   Appropriate feedback requires that the endpoint receiver not try to
   conceal reception of CE-marked packets in the ECN feedback
   information provided to the sending endpoint [RFC7567].  Designers of
   applications/transports are therefore encouraged to include
   mechanisms that can detect this misbehavior.  If a sending endpoint
   detects that a receiver is not correctly providing this feedback, it
   needs to fall back to using loss-based recovery instead of ECN.

5.  Summary: Enabling ECN in Network Devices and Hosts

   This section summarizes the benefits of deploying and using ECN
   within the Internet.  It also provides a list of prerequisites to
   achieve ECN deployment.

   Application developers should, where possible, use transports that
   enable ECN.  Applications that directly use UDP need to provide
   support to implement the functions required for ECN [RFC8085].  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.  Figure 2 summarizes the key benefits.

   +---------+-----------------------------------------------------+
   | Section | Benefit                                             |
   +---------+-----------------------------------------------------+
   |   2.1   | Improved Throughput                                 |
   |   2.2   | Reduced Head-of-Line Blocking                       |
   |   2.3   | Reduced Probability of RTO Expiry                   |
   |   2.4   | Applications that do not Retransmit Lost Packets    |
   |   2.5   | Making Incipient Congestion Visible                 |
   |   2.6   | Opportunities for New Transport Mechanisms          |
   +---------+-----------------------------------------------------+

                     Figure 2: Summary of Key Benefits

   Network operators and people configuring network devices should
   enable ECN [RFC7567].

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

   o  A network device that updates the ECN field in IP packets must use
      IETF-specified methods (see Section 3.1).

   o  A network device may support alternate ECN semantics (see
      Section 3.1).

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   o  A network device must not choose a different network path solely
      because a packet carries a CE-codepoint set in the ECN Field;
      CE-marked packets need to follow the same path as packets with an
      ECT(0) or ECT(1) codepoint (see Section 3.1).  Network devices
      need to be configured not to drop packets solely because the
      ECT(0) or ECT(1) codepoints are used (see Section 3.2).

   o  An ECN-capable network device should correctly update the ECN
      codepoint of ECN-capable packets in the presence of incipient
      congestion (see Section 3.3).

   o  Network devices need to be able to forward both ECN-capable and
      not-ECN-capable flows (see Section 3.4).

   o  A network device must not change a packet with a CE mark to a not-
      ECN-capable codepoint ('00'); if the network device decides not to
      forward the packet with the CE mark, it has to instead drop the
      packet and not bleach the marking (see Section 3.5).

   Prerequisites for network endpoints to enable use of ECN include the
   following:

   o  An application should use an Internet transport that can set and
      receive ECN marks (see Section 4).

   o  An ECN-capable transport/application must return feedback
      indicating congestion to the sending endpoint and perform an
      appropriate congestion response (see Section 4).

   o  An ECN-capable transport/application should detect paths where
      there is persistent misuse of ECN and fall back to not sending
      ECT(0) or ECT(1) (see Section 4.2).

   o  Designers of applications/transports are encouraged to include
      mechanisms that can detect and react appropriately to misbehaving
      receivers that fail to report CE-marked packets (see Section 4.3).

6.  Security Considerations

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

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7.  References

7.1.  Normative References

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <http://www.rfc-editor.org/info/rfc2474>.

   [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>.

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

   [RFC7567]  Baker, F., Ed. and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management",
              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
              <http://www.rfc-editor.org/info/rfc7567>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <http://www.rfc-editor.org/info/rfc8085>.

7.2.  Informative References

   [AL10]     Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel,
              P., Prabhakar, B., Sengupta, S., and M. Sridharan, "Data
              Center TCP (DCTCP)", ACM SIGCOMM Computer Communication
              Review, Volume 40, Issue 4, pages 63-74,
              DOI 10.1145/1851182.1851192, October 2010.

   [BA11]     Bauer, Steven., Beverly, Robert., and Arthur. Berger,
              "Measuring the State of ECN Readiness in Servers, Clients,
              and Routers", Proceedings of the 2011 ACM SIGCOMM
              Conference on ICM, pages 171-180,
              DOI 10.1145/2068816.2068833, November 2011.

   [DCTCP]    Bensley, S., Eggert, L., Thaler, D., Balasubramanian, P.,
              and G. Judd, "Microsoft's Datacenter TCP (DCTCP): TCP
              Congestion Control for Datacenters", Work in Progress,
              draft-bensley-tcpm-dctcp-05, July 2015.

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   [ECN-ENCAP]
              Briscoe, B., Kaippallimalil, J., and P. Thaler,
              "Guidelines for Adding Congestion Notification to
              Protocols that Encapsulate IP", Work in Progress,
              draft-ietf-tsvwg-ecn-encap-guidelines-07, July 2016.

   [ECN-FALLBACK]
              Kuehlewind, M. and B. Trammell, "A Mechanism for ECN Path
              Probing and Fallback", Work in Progress,
              draft-kuehlewind-tcpm-ecn-fallback-01, September 2013.

   [ECN-FEEDBACK]
              Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", Work in Progress,
              draft-ietf-tcpm-accurate-ecn-02, October 2016.

   [Fla13]    Flach, Tobias., Dukkipati, Nandita., Terzis, Andreas.,
              Raghavan, Barath., Cardwell, Neal., Cheng, Yuchung., Jain,
              Ankur., Hao, Shuai., Katz-Bassett, Ethan., and Ramesh.
              Govindan, "Reducing web latency: the virtue of gentle
              aggression", ACM SIGCOMM Computer Communication
              Review, Volume 43, Issue 4, pages 159-170,
              DOI 10.1145/2534169.2486014, October 2013.

   [RFC768]   Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <http://www.rfc-editor.org/info/rfc768>.

   [RFC1349]  Almquist, P., "Type of Service in the Internet Protocol
              Suite", RFC 1349, DOI 10.17487/RFC1349, July 1992,
              <http://www.rfc-editor.org/info/rfc1349>.

   [RFC3649]  Floyd, S., "HighSpeed TCP for Large Congestion Windows",
              RFC 3649, DOI 10.17487/RFC3649, December 2003,
              <http://www.rfc-editor.org/info/rfc3649>.

   [RFC3758]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
              Conrad, "Stream Control Transmission Protocol (SCTP)
              Partial Reliability Extension", RFC 3758,
              DOI 10.17487/RFC3758, May 2004,
              <http://www.rfc-editor.org/info/rfc3758>.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340,
              DOI 10.17487/RFC4340, March 2006,
              <http://www.rfc-editor.org/info/rfc4340>.

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   [RFC4774]  Floyd, S., "Specifying Alternate Semantics for the
              Explicit Congestion Notification (ECN) Field", BCP 124,
              RFC 4774, DOI 10.17487/RFC4774, November 2006,
              <http://www.rfc-editor.org/info/rfc4774>.

   [RFC5562]  Kuzmanovic, A., Mondal, A., Floyd, S., and K.
              Ramakrishnan, "Adding Explicit Congestion Notification
              (ECN) Capability to TCP's SYN/ACK Packets", RFC 5562,
              DOI 10.17487/RFC5562, June 2009,
              <http://www.rfc-editor.org/info/rfc5562>.

   [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>.

   [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>.

   [RFC7928]  Kuhn, N., Ed., Natarajan, P., Ed., Khademi, N., Ed., and
              D. Ros, "Characterization Guidelines for Active Queue
              Management (AQM)", RFC 7928, DOI 10.17487/RFC7928, July
              2016, <http://www.rfc-editor.org/info/rfc7928>.

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

   [TR15]     Tranmmel, Brian., Kuehlewind, Mirja., Boppart, Damiano,
              Learmonth, Iain., and Gorry.  Fairhurst, "Enabling
              Internet-Wide Deployment of Explicit Congestion
              Notification", Lecture Notes in Computer Science, Volume
              8995, pp 193-205, DOI 10.1007/978-3-319-15509-8_15, March
              2015.

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Acknowledgements

   The authors were partly 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 draft versions of this document: Bob Briscoe, David
   Collier-Brown, Colin Perkins, Richard Scheffenegger, Dave Taht, Wes
   Eddy, Fred Baker, Mikael Abrahamsson, Mirja Kuehlewind, John Leslie,
   and other members of the TSVWG and AQM working groups.

Authors' Addresses

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

   Email: gorry@erg.abdn.ac.uk

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

   Phone: +47 22 85 24 20
   Email: michawe@ifi.uio.no

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