



TCP Maintenance and Minor Extensions (tcpm)           M. Kuehlewind, Ed.
Internet-Draft                                                ETH Zurich
Intended status: Informational                          R. Scheffenegger
Expires: September 10, 2015                                 NetApp, Inc.
                                                              B. Briscoe
                                                                      BT
                                                           March 9, 2015


  Problem Statement and Requirements for a More Accurate ECN Feedback
                     draft-ietf-tcpm-accecn-reqs-08

Abstract

   Explicit Congestion Notification (ECN) is a mechanism where network
   nodes can mark IP packets instead of dropping them to indicate
   congestion to the end-points.  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 ConEx or 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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Recap of Classic ECN and ECN Nonce in IP/TCP  . . . . . . . .   4
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Design Approaches . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Re-Definition of ECN/NS Header Bits . . . . . . . . . . .  11
     5.2.  Using Other Header Bits . . . . . . . . . . . . . . . . .  12
     5.3.  Using a TCP Option  . . . . . . . . . . . . . . . . . . .  12
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Ambiguity of the More Accurate ECN Feedback in DCTCP  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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 end-points.  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 pre-existing 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)
   [I-D.bensley-tcpm-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.






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   For an in-depth discussion of the application benefits of using ECN
   (including with sufficiently granular feedback) see
   [I-D.welzl-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 signaling overhead.  ECN feedback for SCTP has been proposed in
   [I-D.stewart-tsvwg-sctpecn].  This delivers a counter for the number
   of CE marked segments between CWR chunks, but also 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).  The changes DCTCP makes to TCP are not currently the
   subject of any IETF standardization activity, and they 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
   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



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   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 acknowledgment flag set is simply called an ACK.

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



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   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 ECN-Echo 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 occasionally inserts a CE mark or
   reordering itself, and checks that the receiver feeds it back
   faithfully [I-D.moncaster-tcpm-rcv-cheat].  This alternative consumes
   no header bits or codepoints, as well as releasing the ECT(1)
   codepoint in the IP header and the NS flag in the TCP header for
   other uses.

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 are 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



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   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 currently TCP
   offers 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 [I-D.bensley-tcpm-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) need
   for a consistent variant of DCTCP to be deployed network-wide and ii)
   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.

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

   A sender with standardised 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,



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           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.

   An RFC5681 TCP sender 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 the sender uses it or not -- 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 which 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 [I-D.moncaster-tcpm-rcv-cheat].

4.  Requirements

   The requirements of the accurate ECN feedback protocol are to have
   fairly accurate (not necessarily perfect), timely and protected
   signaling.  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 [RFC0896].  Finally, the feedback mechanism
           should not be impacted by reordering of ACKs, even when the
           ACK'ed sequence number does not increase.



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   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.

           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 else-wise, a more
           accurate ECN feedback protocol might re-purpose the nonce sum
           (NS) flag in the TCP header.

           If the more accurate ECN feedback scheme provides sufficient
           information, the integrity check could e.g. be performed by
           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 more accurate ECN feedback signal scheme itself, but
           rather when specifying an update to core TCP mechanisms like
           congestion control that makes use of the more accurate ECN
           signal.

   Accuracy
           Classic ECN feeds back one congestion notification per RTT,
           which 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



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           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 code points (non-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 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



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           timely fashion, potentially all or a large fraction of TCP
           acknowledgments might carry this information.  Ideally, no
           additional segments should be exchanged compared to an
           RFC3168 TCP session, 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 (NAT).  Further, a feedback mechanism should
           provide a method to fall back to classic ECN signaling 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, it is intended to eventually update RFC3168 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 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 TCP ECN feedback design
   approaches.  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



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   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.  Re-Definition 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 re-define 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).

   A couple of schemes have been proposed so far:

   o  A naive one-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 [I-D.bensley-tcpm-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;





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   o  A scheme that defines a standardised lookup table to map the 8
      codepoints onto either a CE counter or an ECT(1) counter.

   These proposed schemes provide accumulated information on ECN-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
   signaling overhead as well.  Schemes that do not re-purpose the ECN
   NS bit, could still support the ECN Nonce.

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 standardise the use of the bits in
   the Urgent Pointer field (see [RFC6093]) to signal more bits of its
   congestion signal counter, but only whenever it does not set the
   Urgent Flag.  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, perhaps as much as ECN for RTP/UDP [RFC6679], which



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   explicitly provides the number of ECT(0), ECT(1), CE, non-ECT marked
   and lost packets, or as much as a proposal for SCTP that counts the
   number of ECN marks [I-D.stewart-tsvwg-sctpecn] between CWR chunks.
   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
   signaling 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).  Whatever, 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.

6.  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 part-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). he views expressed here are solely those of the
   authors, in the context of the mentioned funding projects

7.  IANA Considerations

   This memo includes no request to IANA.

8.  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 - i.e. 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.





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   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.

9.  References

9.1.  Normative References

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

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

9.2.  Informative References

   [I-D.bensley-tcpm-dctcp]
              sbens@microsoft.com, s., Eggert, L., and D. Thaler,
              "Microsoft's Datacenter TCP (DCTCP): TCP Congestion
              Control for Datacenters", draft-bensley-tcpm-dctcp-02
              (work in progress), January 2015.

   [I-D.moncaster-tcpm-rcv-cheat]
              Moncaster, T., Briscoe, B., and A. Jacquet, "A TCP Test to
              Allow Senders to Identify Receiver Non-Compliance", draft-
              moncaster-tcpm-rcv-cheat-03 (work in progress), July 2014.

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

   [I-D.welzl-ecn-benefits]
              Welzl, M. and G. Fairhurst, "The Benefits to Applications
              of using Explicit Congestion Notification (ECN)", draft-
              welzl-ecn-benefits-01 (work in progress), July 2014.

   [RFC0896]  Nagle, J., "Congestion control in IP/TCP internetworks",
              RFC 896, January 1984.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018, October 1996.





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   [RFC3449]  Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
              Sooriyabandara, "TCP Performance Implications of Network
              Path Asymmetry", BCP 69, RFC 3449, December 2002.

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

   [RFC5690]  Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
              Acknowledgement Congestion Control to TCP", RFC 5690,
              February 2010.

   [RFC6093]  Gont, F. and A. Yourtchenko, "On the Implementation of the
              TCP Urgent Mechanism", RFC 6093, January 2011.

   [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.

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

   As defined in [I-D.bensley-tcpm-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' will represent ECE=0, '1' will represent ECE=1
   and '.' will represent a gap of one segment between delayed ACKs.
   Now imagine that the sender receives the following sequence of
   feedback on 3 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)




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   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, but even then there will still be ambiguity whenever
   there are pure ACK losses.

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
   UK

   Phone: +44 1473 645196
   Email: bob.briscoe@bt.com
   URI:   http://bobbriscoe.net/



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