Problem Statement and Requirements for Increased Accuracy in Explicit Congestion Notification (ECN) Feedback
RFC 7560

Internet Engineering Task Force (IETF)                M. Kuehlewind, Ed.
Request for Comments: 7560                                    ETH Zurich
Category: Informational                                 R. Scheffenegger
ISSN: 2070-1721                                             NetApp, Inc.
                                                              B. Briscoe
                                                                      BT
                                                             August 2015

       Problem Statement and Requirements for Increased Accuracy
           in Explicit Congestion Notification (ECN) Feedback

Abstract

   Explicit Congestion Notification (ECN) is a mechanism where network
   nodes can mark IP packets, instead of dropping them, to indicate
   congestion to the endpoints.  An ECN-capable receiver will feed this
   information back to the sender.  ECN is specified for TCP in such a
   way that it can only feed back one congestion signal per Round-Trip
   Time (RTT).  In contrast, ECN for other transport protocols, such as
   RTP/UDP and SCTP, is specified with more accurate ECN feedback.
   Recent new TCP mechanisms (like Congestion Exposure (ConEx) or Data
   Center TCP (DCTCP)) need more accurate ECN feedback in the case where
   more than one marking is received in one RTT.  This document
   specifies requirements for an update to the TCP protocol to provide
   more accurate ECN feedback.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7560.

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Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Recap of Classic ECN and ECN Nonce in IP/TCP  . . . . . . . .   5
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Design Approaches . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  Redefinition of ECN/NS Header Bits  . . . . . . . . . . .  11
     5.2.  Using Other Header Bits . . . . . . . . . . . . . . . . .  13
     5.3.  Using a TCP Option  . . . . . . . . . . . . . . . . . . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Ambiguity of the More Accurate ECN Feedback in DCTCP  16
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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1.  Introduction

   Explicit Congestion Notification (ECN) [RFC3168] is a mechanism where
   network nodes can mark IP packets instead of dropping them to
   indicate congestion to the endpoints.  An ECN-capable receiver will
   feed this information back to the sender.  ECN is specified for TCP
   in such a way that only one feedback signal can be transmitted per
   Round-Trip Time (RTT).  This is sufficient for preexisting TCP
   congestion control mechanisms that perform only one reduction in
   sending rate per RTT, independent of the number of ECN congestion
   marks.  But recently proposed or deployed mechanisms like Congestion
   Exposure (ConEx) [RFC6789] or Data Center TCP (DCTCP) [DCTCP] need
   more accurate ECN feedback than 'classic ECN' [RFC3168] to work
   correctly in the case where more than one marking is received in any
   one RTT.

   For an in-depth discussion of the application benefits of using ECN
   (including with sufficiently granular feedback), see [ECN-BENEFITS].

   ECN is also defined for transport protocols beside TCP.  ECN feedback
   as defined for RTP/UDP [RFC6679] provides a very detailed level of
   information, delivering individual counters for all four ECN
   codepoints as well as lost and duplicate segments, but at the cost of
   high signalling overhead.  ECN feedback for SCTP has been proposed in
   [SCTP-ECN].  This delivers a counter for the number of ECN-capable
   packets that were marked due to congestion (since the last sender-
   side window reduction), but it comes at the cost of increased
   overhead.

   Today, implementations of DCTCP already exist that alter TCP's ECN
   feedback protocol in proprietary ways (DCTCP was released in
   Microsoft Windows 8, and implementations exist for Linux and
   FreeBSD).  However, the changes DCTCP makes to TCP omit capability
   negotiation, relying instead on uniform configuration across all
   hosts and network devices with ECN capability.  A primary motivation
   for this document is to intervene before each proprietary
   implementation invents its own non-interoperable handshake, which
   could lead to _de facto_ consumption of the few flags or codepoints
   that remain available for standardizing capability negotiation.

   This document lists requirements for a robust and interoperable TCP/
   ECN feedback protocol that is more accurate than classic ECN
   [RFC3168] and that all implementations of new TCP extensions, like
   ConEx and/or DCTCP, can use.  While a new feedback scheme should
   still deliver as much information as classic ECN, this document also
   clarifies what has to be taken into consideration in addition.  Thus,
   the listed requirements should be addressed in the specification of a
   more accurate ECN feedback scheme.  A few solutions have already been

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   proposed.  Section 5 demonstrates how to use the requirements to
   compare them, by briefly sketching their high-level design choices
   and discussing the benefits and drawbacks of each.

   The scope of these requirements is not limited to any specific
   environment and is intended for general deployment over public and
   private IP networks.  Candidate solutions should try to adhere to all
   these requirements, but, where this is not possible, they should
   justify the deviation.  The ordering of the requirements listed in
   this document is not to be taken as an order of importance, because
   each requirement might have different weight in different deployment
   scenarios.

   These requirements are only concerned with the type and quality of
   the ECN feedback signal.  The requirements do not stipulate how a TCP
   sender might react to the improved ECN signal.  The requirements also
   do not imply that any modifications to TCP senders or receivers are
   obligatory.

1.1.  Terminology

   We use the following terminology from [RFC3168] and [RFC3540]:

   The ECN field in the IP header:

      Not-ECT: the not ECN-Capable Transport codepoint,

      CE:      the Congestion Experienced codepoint,

      ECT(0):  the first ECN-Capable Transport codepoint, and

      ECT(1):  the second ECN-Capable Transport codepoint.

   The ECN flags in the TCP header:

      CWR:     the Congestion Window Reduced flag,

      ECE:     the ECN-Echo flag, and

      NS:      ECN Nonce Sum.

   In this document, the ECN feedback scheme as specified in [RFC3168]
   is called 'classic ECN' and any new proposal is called a 'more
   accurate ECN feedback' scheme.  A 'congestion mark' is defined as an
   IP packet where the CE codepoint is set.  A 'congestion episode'
   refers to one or more congestion marks that belong to the same
   overload situation in the network (usually during one RTT).  A TCP
   segment with the acknowledgement flag set is simply called an ACK.

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2.  Recap of Classic ECN and ECN Nonce in IP/TCP

   ECN requires two bits in the IP header.  The ECN capability of a
   packet is indicated when either one of the two bits is set.  A
   network node can set both bits simultaneously when it experiences
   congestion.  This leads to the four codepoints (Not-ECT, ECT(0),
   ECT(1), and CE) as listed above.

   In the TCP header, the first two bits in byte 14 are defined as ECN
   feedback for each half-connection.  A TCP receiver signals the
   reception of a congestion mark using the ECN-Echo (ECE) flag in the
   TCP header.  For reliability, the receiver continues to set the ECE
   flag on every ACK.  To enable the TCP receiver to determine when to
   stop setting the ECE flag, the sender sets the CWR flag upon
   reception of an ECE feedback signal.  This always leads to a full RTT
   of ACKs with ECE set.  Thus, the receiver cannot signal back any
   additional CE markings arriving within the same RTT.

   The ECN Nonce [RFC3540] is an experimental addition to ECN that the
   TCP sender can use to protect itself against accidental or malicious
   concealment of CE-marked or dropped packets.  This addition defines
   the last bit of byte 13 in the TCP header as the Nonce Sum (NS) flag.
   The receiver maintains a nonce sum that counts the occurrence of
   ECT(1) packets and signals the least significant bit of this sum on
   the NS flag.  There are no known deployments of a TCP stack that
   makes use of the ECN Nonce extension.

       0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
     |               |           | N | C | E | U | A | P | R | S | F |
     | Header Length | Reserved  | S | W | C | R | C | S | S | Y | I |
     |               |           |   | R | E | G | K | H | T | N | N |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

     Figure 1: The (Post-ECN Nonce) Definition of the TCP Header Flags

   An alternative for a sender to assure feedback integrity has been
   proposed where the sender itself occasionally inserts a CE mark or
   reorders packets, and checks that the receiver feeds these back
   faithfully [TEST-RCV].  This alternative consumes no header bits or
   codepoints, and it releases the ECT(1) codepoint in the IP header and
   the NS flag in the TCP header for other uses.

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3.  Use Cases

   The following two examples serve to show where existing mechanisms
   would already benefit from more accurate ECN feedback information.
   However, as it is hard to predict the future, once a more accurate
   ECN feedback mechanism that adheres to the requirements stated in
   this document is widely deployed, it's very likely that additional
   uses will be found.  The examples listed below are in no particular
   order.

   ConEx is an experimental approach that allows a sender to relay
   congestion feedback provided by the receiver into the network along
   the forward data path.  ConEx information can be used for traffic
   management to limit traffic proportionate to the actual congestion
   being caused, rather than limiting traffic based on rate or volume
   [RFC6789].  A ConEx sender uses selective acknowledgements (SACK)
   [RFC2018] for accurate feedback of loss signals, but until now TCP
   has offered no equivalent accurate feedback for ECN.

   DCTCP offers very low and predictable queuing delay.  DCTCP changes
   the reaction to congestion of a TCP sender and additionally requires
   switches/routers to have ECN enabled and configured with a low step
   threshold and no signal smoothing, so it is currently only used in
   private networks, e.g., internal to data centers.  DCTCP was released
   in Microsoft Windows 8, and implementations exist for Linux and
   FreeBSD.  To retrieve sufficient congestion information, the
   different DCTCP implementations use a proprietary ECN feedback
   protocol, but they omit capability negotiation.  Moreover, the
   feedback protocol proposed in [DCTCP] only works if there are no
   losses at all, and otherwise it gets very confused (see Appendix A).
   Therefore, if a generic, more accurate ECN feedback scheme were
   available, it would solve two problems for DCTCP: i) the need for a
   consistent variant of DCTCP to be deployed network-wide and ii) the
   inability to cope with ACK loss.

   Classic ECN-TCP would not benefit from more accurate ECN feedback,
   but it would not suffer either.  The same signal that is currently
   conveyed with ECN following the specification given in [RFC3168]
   would be available.

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   The following scenarios should briefly show where accurate ECN
   feedback is needed or adds value:

   A sender with standardized TCP congestion control that supports
   ConEx:
      In this case, the ConEx mechanism uses the extra information per
      RTT to re-echo the precise congestion information, but the
      congestion control algorithm still ignores multiple marks per RTT
      [RFC5681].

   A sender using DCTCP congestion control without ConEx:
      The congestion control algorithm uses the extra info per RTT to
      perform its decrease depending on the number of congestion marks.

   A sender using DCTCP congestion control and supporting ConEx:
      Both the congestion control algorithm and ConEx use the more
      accurate ECN feedback mechanism.

   As-yet-unspecified sender mechanisms:
      The above are two examples of more general interest in sender
      mechanisms that respond to the extent of congestion feedback, not
      just its existence.  It will greatly simplify incremental
      deployment if the sender can unilaterally deploy new behaviours
      and rely on the presence of generic receivers that have already
      implemented more accurate feedback.

   A TCP sender using congestion control as specified in RFC 5681
   without ConEx:
      No accurate feedback is necessary here.  The congestion control
      algorithm still reacts to only one signal per RTT.  But, it is
      best to feed back all the information the receiver gets, whether
      or not the sender uses it -- at least as long as overhead is low
      or zero.

   Using CE for checking integrity:
      If a more accurate ECN feedback scheme feeds all occurrences of CE
      marks back, a sender could perform integrity checking by
      occasionally injecting CE marks itself.  Specifically, a sender
      can send packets that it randomly marks with CE (at low
      frequency), then check if feedback is received for these packets.
      The congestion notification feedback for these self-injected
      markings would not require a congestion control reaction
      [TEST-RCV].

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4.  Requirements

   The requirements of the accurate ECN feedback protocol are to have
   fairly accurate (not necessarily perfect), timely, and protected
   signalling.  This leads to the following requirements, which should
   be discussed for any proposed more accurate ECN feedback scheme:

   Resilience
      The ECN feedback signal is carried within the ACK.  Pure TCP ACKs
      can get lost without recovery (not just due to congestion but also
      due to deliberate ACK thinning).  Moreover, delayed ACKs are
      commonly used with TCP.  Typically, an ACK is triggered after two
      data segments (or more, e.g., due to receive segment coalescing,
      ACK compression, ACK congestion control [RFC5690], or other
      phenomena; see [RFC3449]).  In a high-congestion situation where
      most of the packets are marked with CE, an accurate feedback
      mechanism should still be able to signal sufficient congestion
      information.  Thus, the accurate ECN feedback extension has to
      take delayed ACKs and ACK loss into account.  Also, a more
      accurate feedback protocol should still provide more accurate
      feedback than classic ECN when delayed ACKs cover more than two
      segments, or when a thin stream disables Nagle's algorithm
      [RFC896].  Finally, the feedback mechanism should not be impacted
      by reordering of ACKs, even when the ACKed sequence number does
      not increase.

   Timeliness
      A CE mark can be induced by the sending host, or more commonly a
      network node on the transmission path, and is then echoed by the
      receiver in the TCP ACK.  Thus, when this information arrives at
      the sender, it is naturally already about one RTT old.  With a
      sufficient ACK rate, a further delay of a small number of packets
      can be tolerated.  However, this information will become stale
      with large delays, given the dynamic nature of networks.  TCP
      congestion control (which itself partly introduces these dynamics)
      operates on a time scale of one RTT.  Thus, to be timely,
      congestion feedback information should be delivered within about
      one RTT.

   Integrity
      The integrity of the feedback in a more accurate ECN feedback
      scheme should be assured, at least as well as the ECN Nonce.
      Alternatively, it should at least be possible to give strong
      incentives for the receiver and network nodes to cooperate
      honestly.

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      Given there are known problems with ECN Nonce deployment, this
      document only requires that the integrity of the more accurate ECN
      feedback can be assured; it does not require that the ECN Nonce
      mechanism is employed to achieve this.  Indeed, if integrity could
      be provided in another manner, a more accurate ECN feedback
      protocol might repurpose the nonce sum (NS) flag in the TCP
      header.

      If the more accurate ECN feedback scheme provides sufficient
      information, the integrity check could be performed by, e.g.,
      deterministically setting the CE in the sender and monitoring the
      respective feedback (similar to ECT(1) and the ECN Nonce sum).
      Whether a sender should enforce when it detects wrong feedback
      information, and what kind of enforcement it should apply, are
      policy issues that need not be specified as part of the more
      accurate ECN feedback signal scheme itself, but rather when
      specifying an update to core TCP mechanisms like congestion
      control that make use of the more accurate ECN signal.

   Accuracy
      Classic ECN feeds back one congestion notification per RTT; this
      is sufficient for classic TCP congestion control, which reduces
      the sending rate at most once per RTT.  Thus, the more accurate
      ECN feedback scheme should ensure that, if a congestion episode
      occurs, at least one congestion notification is echoed and
      received per RTT as classic ECN would do.  Of course, the goal of
      a more accurate ECN extension is to reconstruct the number of CE
      markings more accurately.  In the best case, the new scheme should
      even allow reconstruction of the exact number of payload bytes
      that a CE-marked packet was carrying.  However, it is accepted
      that it may be too complex for a sender to get the exact number of
      congestion markings or marked bytes in all situations.  Ideally,
      the feedback scheme should preserve the order in which any (of the
      four) ECN signals were received.  And, ideally, it would even be
      possible for the sender to determine which of the packets covered
      by one delayed ACK were congestion marked, e.g., if the flow
      consists of packets of different sizes, or to allow for future
      protocols where the order of the markings may be important.

      In the best case, a sender that sees more accurate ECN feedback
      information would be able to reconstruct the occurrence of any of
      the four codepoints (Not-ECT, CE, ECT(0), ECT(1)).  However,
      assuming the sender marks all data packets as ECN-capable and uses
      a default setting of ECT(0) (as with [RFC3168]), solely feeding
      back the occurrence of CE and ECT(1) might be sufficient.  Because
      the sender can keep account of the transmitted segments with any
      of the three ECN codepoints, conveying any two of these back to
      the sender is sufficient for it to reconstruct the third as

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      observed by the receiver.  Thus, a more accurate ECN feedback
      scheme should at least provide information on two of these
      signals, e.g., CE and ECT(1).

      If a more accurate ECN scheme can reliably deliver feedback in
      most but not all circumstances, ideally the scheme should at least
      not introduce bias.  In other words, undetected loss of some ACKs
      should be as likely to increase as decrease the sender's estimate
      of the probability of ECN marking.

   Complexity
      Implementation should be as simple as possible, and only a minimum
      of additional state information should be needed.  This will
      enable more accurate ECN feedback to be used as the default
      feedback mechanism, even if only one ECN feedback signal per RTT
      is needed.

   Overhead
      A more accurate ECN feedback signal should limit the additional
      network load, because ECN feedback is ultimately not critical
      information (in the worst case, loss will still be available as a
      congestion signal of last resort).  As feedback information has to
      be provided frequently and in a timely fashion, potentially all or
      a large fraction of TCP acknowledgements might carry this
      information.  Ideally, no additional segments should be exchanged
      compared to a TCP session as specified in RFC 3168, and the
      overhead in each segment should be minimized.

   Backward and forward compatibility
      Given more accurate ECN feedback will involve a change to the TCP
      protocol, it should be negotiated between the two TCP endpoints.
      If either end does not support the more accurate feedback, they
      should both be able to fall back to classic ECN feedback.

      A more accurate ECN feedback extension should aim to traverse most
      middleboxes, including firewalls and Network Address Translators
      (NATs).  Further, a feedback mechanism should provide a method to
      fall back to classic ECN signalling if the new signal is
      suppressed by certain middleboxes.

      In order to avoid a fork in the TCP protocol specifications, if
      experiments with the new ECN feedback protocol are successful, the
      intention is to eventually update RFC 3168 for any TCP/ECN sender,
      not just for ConEx or DCTCP senders.  Then, future senders will be
      able to unilaterally deploy new behaviours that exploit the
      existence of more accurate ECN feedback in receivers (forward

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      compatibility).  Conversely, even if another sender only needs one
      ECN feedback signal per RTT, it should be able to use more
      accurate ECN feedback and simply ignore the excess information.

   Furthermore, the receiver should not make assumptions about the
   mechanism that was used to set the markings nor about any
   interpretation or reaction to the congestion signal.  The receiver
   only needs to faithfully reflect congestion information back to the
   sender.

5.  Design Approaches

   This section introduces some possible design approaches for TCP ECN
   feedback.  The purpose of this section is to give examples of how
   trade-offs might be needed between the requirements, as input to
   future IETF work to specify a protocol.  The order is not
   significant, and there is no intention to endorse any particular
   approach.

   All approaches presented below (and proposed so far) are able to
   provide accurate ECN feedback information as long as no ACK loss
   occurs and the congestion rate is reasonable.  In the case of a high
   ACK loss rate or very high congestion (CE-marking) rate, the proposed
   schemes have different resilience characteristics depending on the
   number of bits used for the encoding.  While classic ECN provides
   reliable (but inaccurate) feedback of a maximum of one congestion
   signal per RTT, the proposed schemes do not implement an explicit
   acknowledgement mechanism for the feedback (as, e.g., the ECE/CWR
   exchange of [RFC3168]).

5.1.  Redefinition of ECN/NS Header Bits

   Schemes in this category can additionally use the NS bit for
   capability negotiation during the TCP handshake exchange.  Thus a
   more accurate ECN could be negotiated without changing the classic
   ECN negotiation and thus being backwards compatible.

   Schemes in this category can simply redefine the ECN header flags,
   ECE and CWR, to encode the occurrence of a CE marking at the
   receiver.  This approach provides very limited resilience against
   loss of ACK, particularly pure ACKs (no payload and therefore
   delivered unreliably).

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   A couple of schemes have been proposed so far:

   o  A naive 1-bit scheme that sends one ECE for each CE received could
      use CWR to increase robustness against ACK loss by introducing
      redundant information on the next ACK, but this is still
      vulnerable to ACK loss.

   o  The scheme defined for DCTCP [DCTCP], which toggles the ECE
      feedback on an immediate ACK whenever the CE marking changes, and
      otherwise feeds back delayed ACKs with the ECE value unchanged.
      Appendix A demonstrates that this scheme is still ambiguous to the
      sender if the ACKs are pure ACKs, and if some may have been lost.

   Alternatively, the receiver uses the three ECN/NS header flags, ECE,
   CWR, and NS, to represent a counter that signals the accumulated
   number of CE markings it has received.  Resilience against loss is
   better than the flag-based schemes but may not suffice in the
   presence of extended ACK loss that otherwise would not affect the TCP
   sender's performance.

   A number of coding schemes have been proposed so far in this
   category:

   o  A 3-bit counter scheme continuously feeds back the three least
      significant bits of a CE counter;

   o  A scheme that defines a standardized lookup table to map the eight
      codepoints onto either a CE counter or an ECT(1) counter.

   These proposed schemes provide accumulated information on CE marking
   feedback, similar to the number of acknowledged bytes in the TCP
   header.  Due to the limited number of bits, the ECN feedback
   information will wrap much more often than the acknowledgement field.
   Thus, feedback information could be lost due to a relatively small
   sequence of pure-ACK losses.  Resilience could be increased by
   introducing redundancy, e.g., send each counter increase two or more
   times.  Of course, any of these additional mechanisms will increase
   the complexity.  If the congestion rate is greater than the ACK rate
   (multiplied by the number of congestion marks that can be signaled
   per ACK), the congestion information cannot correctly be fed back.
   Covering the worst case (where every packet is CE marked) can
   potentially be realized by dynamically adapting the ACK rate and
   redundancy.  This again increases complexity and perhaps the
   signalling overhead as well.  Schemes that do not repurpose the ECN
   NS bit could still support the ECN Nonce.

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5.2.  Using Other Header Bits

   As seen in Figure 1, there are currently three unused flags in the
   TCP header.  The proposed 3-bit counter or codepoint schemes could be
   extended by one or more bits to add higher resilience against ACK
   loss.  The relative gain would be exponentially higher resilience
   against ACK loss, while the respective drawbacks would remain
   identical.

   Alternatively, a new method could standardize the use of the bits in
   the Urgent Pointer field (see [RFC6093]) to signal more bits of its
   congestion signal counter, but only whenever the Urgent Flag is not
   set.  As this is often the case, resilience could be increased
   without additional header overhead.

   Any proposal to use such bits would need to check the likelihood that
   some middleboxes might discard or 'normalize' the currently unused
   flag bits or a non-zero Urgent Pointer when the Urgent Flag is
   cleared.  If during experimentation certain bits have been proven to
   be usable, the assignment of any of these bits would then require an
   IETF standards action.

5.3.  Using a TCP Option

   Alternatively, a new TCP option could be introduced, to help maintain
   the accuracy and integrity of ECN feedback between receiver and
   sender.  Such an option could provide higher resilience and even more
   information, e.g., as much as is provided by a proposal for SCTP that
   counts the number of CE marked packet [SCTP-ECN] since the last CWR
   was observed, or by ECN for RTP/UDP [RFC6679].  The latter explicitly
   provides the total number of packets during a connection where the IP
   ECN field is set to ECT(0), ECT(1), CE, or Not-ECT, as well as the
   number of lost packets.  However, deploying new TCP options has its
   own challenges.  Moreover, to actually achieve high resilience, this
   option would need to be carried by most or all ACKs as the receiver
   cannot know if and when ACKs may be dropped.  Thus, this approach
   would introduce considerable signalling overhead even though ECN
   feedback is not extremely critical information (in the worst case,
   loss will still be available to provide a strong congestion feedback
   signal).  Nevertheless, such a TCP option could be used in addition
   to a more accurate ECN feedback scheme in the TCP header or in
   addition to classic ECN, only when needed and when space is
   available.

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6.  Security Considerations

   ECN feedback information must only be used if the other information
   contained in a received TCP segment indicates that the congestion was
   genuinely part of the flow and not spoofed.  That is, the normal TCP
   acceptance techniques have to be used to verify that the segment is
   part of the flow before returning any contained ECN information, and,
   similarly, ECN feedback is only accepted on valid ACKs.

   Given ECN feedback is used as input for congestion control, the
   respective algorithm would not react appropriately if ECN feedback
   were lost and the resilience mechanism to recover it was inadequate.
   This resilience requirement is articulated in Section 4.  However, it
   should be noted that ECN feedback is not the last resort against
   congestion collapse, because if there is insufficient response to
   ECN, loss will ensue, and TCP will still react appropriately to loss.

   A receiver could suppress ECN feedback information leading to its
   connections consuming excess sender or network resources.  This
   problem is similar to that seen with the classic ECN feedback scheme
   and should be addressed by integrity checking as required in
   Section 4.

7.  References

7.1.  Normative References

   [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>.

   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
              Congestion Notification (ECN) Signaling with Nonces",
              RFC 3540, DOI 10.17487/RFC3540, June 2003,
              <http://www.rfc-editor.org/info/rfc3540>.

7.2.  Informative References

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

   [ECN-BENEFITS]
              Fairhurst, G. and M. Welzl, "The Benefits of using
              Explicit Congestion Notification (ECN)", Work in Progress
              draft-ietf-aqm-ecn-benefits-06, July 2015.

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   [RFC896]   Nagle, J., "Congestion Control in IP/TCP Internetworks",
              RFC 896, DOI 10.17487/RFC0896, January 1984,
              <http://www.rfc-editor.org/info/rfc896>.

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

   [RFC3449]  Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
              Sooriyabandara, "TCP Performance Implications of Network
              Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
              December 2002, <http://www.rfc-editor.org/info/rfc3449>.

   [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>.

   [RFC5690]  Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
              Acknowledgement Congestion Control to TCP", RFC 5690,
              DOI 10.17487/RFC5690, February 2010,
              <http://www.rfc-editor.org/info/rfc5690>.

   [RFC6093]  Gont, F. and A. Yourtchenko, "On the Implementation of the
              TCP Urgent Mechanism", RFC 6093, DOI 10.17487/RFC6093,
              January 2011, <http://www.rfc-editor.org/info/rfc6093>.

   [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>.

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

   [TEST-RCV] Moncaster, T., Briscoe, B., and A. Jacquet, "A TCP Test to
              Allow Senders to Identify Receiver Non-Compliance", Work
              in Progress, draft-moncaster-tcpm-rcv-cheat-03, July 2014.

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Appendix A.  Ambiguity of the More Accurate ECN Feedback in DCTCP

   As defined in [DCTCP], a DCTCP receiver feeds back ECE=0 on delayed
   ACKs as long as CE remains 0, and also immediately sends an ACK with
   ECE=0 when CE transitions to 1.  Similarly, it continually feeds back
   ECE=1 on delayed ACKs while CE remains 1 and immediately feeds back
   ECE=1 when CE transitions to 0.  A sender can unambiguously decode
   this scheme if there is never any ACK loss, and the sender assumes
   there will never be any ACK loss.

   The following two examples show that the feedback sequence becomes
   highly ambiguous to the sender if either of these conditions is
   broken.  Below, '0' represents ECE=0, '1' represents ECE=1, and '.'
   represents a gap of one segment between delayed ACKs.  Now imagine
   that the sender receives the following sequence of feedback on three
   pure ACKs:

      0.0.0

   When the receiver sent this sequence, it could have been any of the
   following four sequences:

   a.  0.0.0 (0 x CE)

   b.  010.0 (1 x CE)

   c.  0.010 (1 x CE)

   d.  01010 (2 x CE)

   where any of the 1s represent a possible pure ACK carrying ECE
   feedback that could have been lost.  If the sender guesses (a), it
   might be correct, or it might miss 1 or 2 congestion marks over 5
   packets.  Therefore, when confronted with this simple sequence (that
   is not contrived), a sender can guess that congestion might have been
   0%, 20%, or 40%, but it doesn't know which.

   Sequences with a longer gap (e.g., 0...0.0) become far more
   ambiguous.  It helps a little if the sender knows the distance the
   receiver uses between delayed ACKs, and it helps a lot if the
   distance is 1, i.e., no delayed ACKs.  However, even without delayed
   ACKs there will still be ambiguity whenever there are pure ACK
   losses.

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Acknowledgements

   Thanks to Gorry Fairhurst for his review and for ideas on CE-based
   integrity checking and to Mohammad Alizadeh for suggesting the need
   to avoid bias.

   Bob Briscoe was partly funded by the European Community under its
   Seventh Framework Programme through the Reducing Internet Transport
   Latency (RITE) project (ICT-317700) and through the Trilogy 2 project
   (ICT-317756).  The views expressed here are solely those of the
   authors, in the context of the mentioned funding projects.

Authors' Addresses

   Mirja Kuehlewind (editor)
   ETH Zurich
   Gloriastrasse 35
   Zurich  8092
   Switzerland

   Email: mirja.kuehlewind@tik.ee.ethz.ch

   Richard Scheffenegger
   NetApp, Inc.
   Am Euro Platz 2
   Vienna  1120
   Austria

   Phone: +43 1 3676811 3146
   Email: rs@netapp.com

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

   Phone: +44 1473 645196
   Email: ietf@bobbriscoe.net
   URI:   http://bobbriscoe.net/

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