Network Working Group                                         M. Bagnulo
Internet-Draft                                                      UC3M
Obsoletes: 5562 (if approved)                                 B. Briscoe
Intended status: Experimental                                  CableLabs
Expires: April 1, 2018                                September 28, 2017

  ECN++: Adding Explicit Congestion Notification (ECN) to TCP Control


   This document describes an experimental modification to ECN when used
   with TCP.  It allows the use of ECN on the following TCP packets:
   SYNs, pure ACKs, Window probes, FINs, RSTs and retransmissions.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 1, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Experiment Goals  . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Document Structure  . . . . . . . . . . . . . . . . . . .   5
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Network (e.g. Firewall) Behaviour . . . . . . . . . . . .   6
     3.2.  Endpoint Behaviour  . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  SYN . . . . . . . . . . . . . . . . . . . . . . . . .   8
       3.2.2.  SYN-ACK . . . . . . . . . . . . . . . . . . . . . . .  11
       3.2.3.  Pure ACK  . . . . . . . . . . . . . . . . . . . . . .  13
       3.2.4.  Window Probe  . . . . . . . . . . . . . . . . . . . .  14
       3.2.5.  FIN . . . . . . . . . . . . . . . . . . . . . . . . .  14
       3.2.6.  RST . . . . . . . . . . . . . . . . . . . . . . . . .  15
       3.2.7.  Retransmissions . . . . . . . . . . . . . . . . . . .  15
       3.2.8.  General Fall-back for any Control Packet or
               Retransmission  . . . . . . . . . . . . . . . . . . .  16
   4.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     4.1.  The Reliability Argument  . . . . . . . . . . . . . . . .  16
     4.2.  SYNs  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
       4.2.1.  Argument 1a: Unrecognized CE on the SYN . . . . . . .  17
       4.2.2.  Argument 1b: Unrecognized ECT on the SYN  . . . . . .  19
       4.2.3.  Argument 2: DoS Attacks . . . . . . . . . . . . . . .  21
     4.3.  SYN-ACKs  . . . . . . . . . . . . . . . . . . . . . . . .  22
       4.3.1.  Response to Congestion on a SYN-ACK . . . . . . . . .  22
       4.3.2.  Fall-Back if ECT SYN-ACK Fails  . . . . . . . . . . .  23
     4.4.  Pure ACKs . . . . . . . . . . . . . . . . . . . . . . . .  23
       4.4.1.  Cwnd Response to CE-Marked Pure ACKs  . . . . . . . .  25
       4.4.2.  ACK Rate Response to CE-Marked Pure ACKs  . . . . . .  26
       4.4.3.  Summary: Enabling ECN on Pure ACKs  . . . . . . . . .  26
     4.5.  Window Probes . . . . . . . . . . . . . . . . . . . . . .  27
     4.6.  FINs  . . . . . . . . . . . . . . . . . . . . . . . . . .  28
     4.7.  RSTs  . . . . . . . . . . . . . . . . . . . . . . . . . .  28
     4.8.  Retransmitted Packets.  . . . . . . . . . . . . . . . . .  29
     4.9.  General Fall-back for any Control Packet  . . . . . . . .  30
   5.  Interaction with popular variants or derivatives of TCP . . .  31
     5.1.  IW10  . . . . . . . . . . . . . . . . . . . . . . . . . .  31
     5.2.  TFO . . . . . . . . . . . . . . . . . . . . . . . . . . .  32
     5.3.  TCP Derivatives . . . . . . . . . . . . . . . . . . . . .  33
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  33
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  33
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  33
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  33
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  34
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

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1.  Introduction

   RFC 3168 [RFC3168] specifies support of Explicit Congestion
   Notification (ECN) in IP (v4 and v6).  By using the ECN capability,
   network elements (e.g. routers, switches) performing Active Queue
   Management (AQM) can use ECN marks instead of packet drops to signal
   congestion to the endpoints of a communication.  This results in
   lower packet loss and increased performance.  RFC 3168 also specifies
   support for ECN in TCP, but solely on data packets.  For various
   reasons it precludes the use of ECN on TCP control packets (TCP SYN,
   TCP SYN-ACK, pure ACKs, Window probes) and on retransmitted packets.
   RFC 3168 is silent about the use of ECN on RST and FIN packets.  RFC
   5562 [RFC5562] is an experimental modification to ECN that enables
   ECN support for TCP SYN-ACK packets.

   This document defines an experimental modification to ECN [RFC3168]
   that shall be called ECN++. It enables ECN support on all the
   aforementioned types of TCP packet.

   ECN++ is a sender-side change.  It works whether the two ends of the
   TCP connection use classic ECN feedback [RFC3168] or experimental
   Accurate ECN feedback (AccECN [I-D.ietf-tcpm-accurate-ecn]).
   Nonetheless, if the client does not implement AccECN, it cannot use
   ECN++ on the one packet that offers most benefit from it - the
   initial SYN.  Therefore, implementers of ECN++ are RECOMMENDED to
   also implement AccECN.

   ECN++ is designed for compatibility with a number of latency
   improvements to TCP such as TCP Fast Open (TFO [RFC7413]), initial
   window of 10 SMSS (IW10 [RFC6928]) and Low latency Low Loss Scalable
   Transport (L4S [I-D.ietf-tsvwg-l4s-arch]), but they can all be
   implemented and deployed independently.
   [I-D.ietf-tsvwg-ecn-experimentation] is a standards track procedural
   device that relaxes requirements in RFC 3168 and other standards
   track RFCs that would otherwise preclude the experimental
   modifications needed for ECN++ and other ECN experiments.

1.1.  Motivation

   The absence of ECN support on TCP control packets and retransmissions
   has a potential harmful effect.  In any ECN deployment, non-ECN-
   capable packets suffer a penalty when they traverse a congested
   bottleneck.  For instance, with a drop probability of 1%, 1% of
   connection attempts suffer a timeout of about 1 second before the SYN
   is retransmitted, which is highly detrimental to the performance of
   short flows.  TCP control packets, particularly TCP SYNs and SYN-
   ACKs, are important for performance, so dropping them is best

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   Non-ECN control packets particularly harm performance in environments
   where the ECN marking level is high.  For example, [judd-nsdi] shows
   that in a controlled private data centre (DC) environment where ECN
   is used (in conjunction with DCTCP [I-D.ietf-tcpm-dctcp]), the
   probability of being able to establish a new connection using a non-
   ECN SYN packet drops to close to zero even when there are only 16
   ongoing TCP flows transmitting at full speed.  The issue is that
   DCTCP exhibits a much more aggressive response to packet marking
   (which is why it is only applicable in controlled environments).
   This leads to a high marking probability for ECN-capable packets, and
   in turn a high drop probability for non-ECN packets.  Therefore non-
   ECN SYNs are dropped aggressively, rendering it nearly impossible to
   establish a new connection in the presence of even mild traffic load.

   Finally, there are ongoing experimental efforts to promote the
   adoption of a slightly modified variant of DCTCP (and similar
   congestion controls) over the Internet to achieve low latency, low
   loss and scalable throughput (L4S) for all communications
   [I-D.ietf-tsvwg-l4s-arch].  In such an approach, L4S packets identify
   themselves using an ECN codepoint [I-D.ietf-tsvwg-ecn-l4s-id].  With
   L4S and potentially other similar cases, preventing TCP control
   packets from obtaining the benefits of ECN would not only expose them
   to the prevailing level of congestion loss, but it would also
   classify control packets into a different queue with different
   network treatment, which may also lead to reordering, further
   degrading TCP performance.

1.2.  Experiment Goals

   The goal of the experimental modifications defined in this document
   is to allow the use of ECN on all TCP packets.  Experiments are
   expected in the public Internet as well as in controlled environments
   to understand the following issues:

   o  How SYNs, Window probes, pure ACKs, FINs, RSTs and retransmissions
      that carry the ECT(0), ECT(1) or CE codepoints are processed by
      the TCP endpoints and the network (including routers, firewalls
      and other middleboxes).  In particular we would like to learn if
      these packets are frequently blocked or if these packets are
      usually forwarded and processed.

   o  The scale of deployment of the different flavours of ECN,
      including [RFC3168], [RFC5562], [RFC3540] and

   o  How much the performance of TCP communications is improved by
      allowing ECN marking of each packet type.

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   o  To identify any issues (including security issues) raised by
      enabling ECN marking of these packets.

   The data gathered through the experiments described in this document,
   particularly under the first 2 bullets above, will help in the design
   of the final mechanism (if any) for adding ECN support to the
   different packet types considered in this document.  Whenever data
   input is needed to assist in a design choice, it is spelled out
   throughout the document.

   Success criteria: The experiment will be a success if we obtain
   enough data to have a clearer view of the deployability and benefits
   of enabling ECN on all TCP packets, as well as any issues.  If the
   results of the experiment show that it is feasible to deploy such
   changes; that there are gains to be achieved through the changes
   described in this specification; and that no other major issues may
   interfere with the deployment of the proposed changes; then it would
   be reasonable to adopt the proposed changes in a standards track
   specification that would update RFC 3168.

1.3.  Document Structure

   The remainder of this document is structured as follows.  In
   Section 2, we present the terminology used in the rest of the
   document.  In Section 3, we specify the modifications to provide ECN
   support to TCP SYNs, pure ACKs, Window probes, FINs, RSTs and
   retransmissions.  We describe both the network behaviour and the
   endpoint behaviour.  Section 5 discusses variations of the
   specification that will be necessary to interwork with a number of
   popular variants or derivatives of TCP.  RFC 3168 provides a number
   of specific reasons why ECN support is not appropriate for each
   packet type.  In Section 4, we revisit each of these arguments for
   each packet type to justify why it is reasonable to conduct this

2.  Terminology

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

   Pure ACK: A TCP segment with the ACK flag set and no data payload.

   SYN: A TCP segment with the SYN (synchronize) flag set.

   Window probe: Defined in [RFC0793], a window probe is a TCP segment
   with only one byte of data sent to learn if the receive window is
   still zero.

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   FIN: A TCP segment with the FIN (finish) flag set.

   RST: A TCP segment with the RST (reset) flag set.

   Retransmission: A TCP segment that has been retransmitted by the TCP

   ECT: ECN-Capable Transport.  One of the two codepoints ECT(0) or
   ECT(1) in the ECN field [RFC3168] of the IP header (v4 or v6).  An
   ECN-capable sender sets one of these to indicate that both transport
   end-points support ECN.  When this specification says the sender sets
   an ECT codepoint, by default it means ECT(0).  Optionally, it could
   mean ECT(1), which is in the process of being redefined for use by
   L4S experiments [I-D.ietf-tsvwg-ecn-experimentation]

   Not-ECT: The ECN codepoint set by senders that indicates that the
   transport is not ECN-capable.

   CE: Congestion Experienced.  The ECN codepoint that an intermediate
   node sets to indicate congestion [RFC3168].  A node sets an
   increasing proportion of ECT packets to CE as the level of congestion

3.  Specification

3.1.  Network (e.g.  Firewall) Behaviour

   Previously the specification of ECN for TCP [RFC3168] required the
   sender to set not-ECT on TCP control packets and retransmissions.
   Some readers of RFC 3168 might have erroneously interpreted this as a
   requirement for firewalls, intrusion detection systems, etc. to check
   and enforce this behaviour.  Section 4.3 of
   [I-D.ietf-tsvwg-ecn-experimentation] updates RFC 3168 to remove this
   ambiguity.  It require firewalls or any intermediate nodes not to
   treat certain types of ECN-capable TCP segment differently (except
   potentially in one attack scenario).  This is likely to only involve
   a firewall rule change in a fraction of cases (at most 0.4% of paths
   according to the tests reported in Section 4.2.2).

   In case a TCP sender encounters a middlebox blocking ECT on certain
   TCP segments, the specification below includes behaviour to fall back
   to non-ECN.  However, this loses the benefit of ECN on control
   packets.  So operators are RECOMMENDED to alter their firewall rules
   to comply with the requirement referred to above (section 4.3 of

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3.2.  Endpoint Behaviour

   The changes to the specification of TCP over ECN [RFC3168] defined
   here solely alter the behaviour of the sending host for each half-
   connection.  All changes can be deployed at each end-point
   independently of others and independent of any network behaviour.

   The feedback behaviour at the receiver depends on whether classic ECN
   TCP feedback [RFC3168] or Accurate ECN (AccECN) TCP feedback
   [I-D.ietf-tcpm-accurate-ecn] has been negotiated.  Nonetheless,
   neither receiver feedback behaviour is altered by the present

   For each type of control packet or retransmission, the following
   sections detail changes to the sender's behaviour in two respects: i)
   whether it sets ECT; and ii) its response to congestion feedback.
   Table 1 summarises these two behaviours for each type of packet, but
   the relevant subsection below should be referred to for the detailed
   behaviour.  The subsection on the SYN is more complex than the
   others, because it has to include fall-back behaviour if the ECT
   packet appears not to have got through, and caching of the outcome to
   detect persistent failures.

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   | TCP     | ECN field if    | ECN field if     | Congestion         |
   | packet  | AccECN f/b      | RFC3168 f/b      | Response           |
   | type    | negotiated*     | negotiated*      |                    |
   | SYN     | ECT             | not-ECT          | Reduce IW          |
   |         |                 |                  |                    |
   | SYN-ACK | ECT             | ECT              | Reduce IW          |
   |         |                 |                  |                    |
   | Pure    | ECT             | ECT              | Usual cwnd         |
   | ACK     |                 |                  | response and       |
   |         |                 |                  | optionally         |
   |         |                 |                  | [RFC5690]          |
   |         |                 |                  |                    |
   | W Probe | ECT             | ECT              | Usual cwnd         |
   |         |                 |                  | response           |
   |         |                 |                  |                    |
   | FIN     | ECT             | ECT              | None or optionally |
   |         |                 |                  | [RFC5690]          |
   |         |                 |                  |                    |
   | RST     | ECT             | ECT              | N/A                |
   |         |                 |                  |                    |
   | Re-XMT  | ECT             | ECT              | Usual cwnd         |
   |         |                 |                  | response           |

   Window probe and retransmission are abbreviated to W Probe an Re-XMT.
               * For a SYN, "negotiated" means "requested".

     Table 1: Summary of sender behaviour.  In each case the relevant
      section below should be referred to for the detailed behaviour

   It can be seen that the sender can set ECT in all cases, except if it
   is not requesting AccECN feedback on the SYN.  Therefore it is
   RECOMMENDED that the experimental AccECN specification
   [I-D.ietf-tcpm-accurate-ecn] is implemented (as well as the present
   specification), because it is expected that ECT on the SYN will give
   the most significant performance gain, particularly for short flows.
   Nonetheless, this specification also caters for the case where AccECN
   feedback is not implemented.

3.2.1.  SYN  Setting ECT on the SYN

   With classic [RFC3168] ECN feedback, the SYN was never expected to be
   ECN-capable, so the flag provided to feed back congestion was put to
   another use (it is used in combination with other flags to indicate

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   that the responder supports ECN).  In contrast, Accurate ECN (AccECN)
   feedback [I-D.ietf-tcpm-accurate-ecn] provides two codepoints in the
   SYN-ACK for the responder to feed back whether or not the SYN arrived
   marked CE.

   Therefore, a TCP initiator MUST NOT set ECT on a SYN unless it also
   attempts to negotiate Accurate ECN feedback in the same SYN.

   For the experiments proposed here, if the SYN is requesting AccECN
   feedback, the TCP sender will also set ECT on the SYN.  It can ignore
   the prohibition in section 6.1.1 of RFC 3168 against setting ECT on
   such a SYN.

   The following subsections about the SYN solely apply to this case
   where the initiator sent an ECT SYN.  Caching Lack of AccECN Support for ECT on SYNs

   Until AccECN servers become widely deployed, a TCP initiator that
   sets ECT on a SYN (which implies the same SYN also requests AccECN,
   as required above) SHOULD also maintain a cache entry per server to
   record that the server does not support AccECN and therefore has no
   logic for congestion markings on the SYN.  Mobile hosts MAY maintain
   a cache entry per access network to record lack of AccECN support by
   proxies (see Section 4.2.1).

   The initiator will record any server's SYN-ACK response that does not
   support AccECN.  Subsequently the initiator will not set ECT on a SYN
   to such a server, but it can still always request AccECN support
   (because the response will state any earlier stage of ECN evolution
   that the server supports with no performance penalty).  The initiator
   will discover a server that has upgraded to support AccECN as soon as
   it next connects, then it can remove the server from its cache and
   subsequently always set ECT for that server.

   If the initiator times out without seeing a SYN-ACK, it will also
   cache this fact (see fall-back in Section for details).

   There is no need to cache successful attempts, because the default
   ECT SYN behaviour performs optimally on success anyway.  Servers that
   do not support ECN as a whole probably do not need to be recorded
   separately from non-support of AccECN because the response to a
   request for AccECN immediately states which stage in the evolution of
   ECN the server supports (AccECN [I-D.ietf-tcpm-accurate-ecn], classic
   ECN [RFC3168] or no ECN).

   The above strategy is named "optimistic ECT and cache failures".  It
   is believed to be sufficient based on initial measurements and

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   assumptions detailed in Section 4.2.1, which also gives alternative
   strategies in case larger scale measurements uncover different
   scenarios.  SYN Congestion Response

   If the SYN-ACK returned to the TCP initiator confirms that the server
   supports AccECN, it will also indicate whether or not the SYN was CE-
   marked.  If the SYN was CE-marked, the initiator MUST reduce its
   Initial Window (IW) and SHOULD reduce it to 1 SMSS (sender maximum
   segment size).

   If ECT has been set on the SYN and if the SYN-ACK shows that the
   server does not support AccECN, the TCP initiator MUST conservatively
   reduce its Initial Window and SHOULD reduce it to 1 SMSS.  A
   reduction to greater than 1 SMSS MAY be appropriate (see
   Section 4.2.1).  Conservatism is necessary because a non-AccECN SYN-
   ACK cannot show whether the SYN was CE-marked.

   If the TCP initiator (host A) receives a SYN from the remote end
   (host B) after it has sent a SYN to B, it indicates the (unusual)
   case of a simultaneous open.  Host A will respond with a SYN-ACK.
   Host A will probably then receive a SYN-ACK in response to its own
   SYN, after which it can follow the appropriate one of the two
   paragraphs above.

   In all the above cases, the initiator does not have to back off its
   retransmission timer as it would in response to a timeout following
   no response to its SYN [RFC6298], because both the SYN and the SYN-
   ACK have been successfully delivered through the network.  Also, the
   initiator does not need to exit slow start or reduce ssthresh, which
   is not even required when a SYN is lost [RFC5681].

   If an initial window of 10 (IW10 [RFC6928]) is implemented, Section 5
   gives additional recommendations.  Fall-Back Following No Response to an ECT SYN

   An ECT SYN might be lost due to an over-zealous path element (or
   server) blocking ECT packets that do not conform to RFC 3168.  Some
   evidence of this was found in a 2014 study [ecn-pam], but in a more
   recent 2017 study {ToDo: Add reference (under submission)} extensive
   measurements found no case where ECT on TCP control packets was
   treated any differently from ECT on TCP data packets.  Loss is
   commonplace for numerous other reasons, e.g. congestion loss at a
   non-ECN queue on the forward or reverse path, transmission errors,
   etc.  Alternatively, the cause of the loss might be the attempt to
   negotiate AccECN, or possibly other unrelated options on the SYN.

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   Therefore, if the timer expires after the TCP initiator has sent the
   first ECT SYN, it SHOULD make one more attempt to retransmit the SYN
   with ECT set (backing off the timer as usual).  If the retransmission
   timer expires again, it SHOULD retransmit the SYN with the not-ECT
   codepoint in the IP header, to expedite connection set-up.  If other
   experimental fields or options were on the SYN, it will also be
   necessary to follow their specifications for fall-back too.  It would
   make sense to coordinate all the strategies for fall-back in order to
   isolate the specific cause of the problem.

   If the TCP initiator is caching failed connection attempts, it SHOULD
   NOT give up using ECT on the first SYN of subsequent connection
   attempts until it is clear that a blockage persistently and
   specifically affects ECT on SYNs.  This is because loss is so
   commonplace for other reasons.  Even if it does eventually decide to
   give up setting ECT on the SYN, it will probably not need to give up
   on AccECN on the SYN.  In any case, if a cache is used, it SHOULD be
   arranged to expire so that the initiator will infrequently attempt to
   check whether the problem has been resolved.

   Other fall-back strategies MAY be adopted where applicable (see
   Section 4.2.2 for suggestions, and the conditions under which they
   would apply).

3.2.2.  SYN-ACK  Setting ECT on the SYN-ACK

   For the experiments proposed here, the TCP implementation will set
   ECT on SYN-ACKs.  It can ignore the requirement in section 6.1.1 of
   RFC 3168 to set not-ECT on a SYN-ACK.

   The feedback behaviour by the initiator in response to a CE-marked
   SYN-ACK from the responder depends on whether classic ECN feedback
   [RFC3168] or AccECN feedback [I-D.ietf-tcpm-accurate-ecn] has been
   negotiated.  In either case no change is required to RFC 3168 or the
   AccECN specification.

   Some classic ECN implementations might ignore a CE-mark on a SYN-ACK,
   or even ignore a SYN-ACK packet entirely if it is set to ECT or CE.
   This is a possibility because an RFC 3168 implementation would not
   necessarily expect a SYN-ACK to be ECN-capable.

      FOR DISCUSSION: To eliminate this problem, the WG could decide to
      prohibit setting ECT on SYN-ACKs unless AccECN has been
      negotiated.  However, this issue already came up when the IETF
      first decided to experiment with ECN on SYN-ACKs [RFC5562] and it
      was decided to go ahead without any extra precautionary measures

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      because the risk was low.  This was because the probability of
      encountering the problem was believed to be low and the harm if
      the problem arose was also low (see Appendix B of RFC 5562).

      MEASUREMENTS NEEDED: Server-side experiments could determine
      whether this specific problem is indeed rare across the current
      installed base of clients that support ECN.  SYN-ACK Congestion Response

   A host that sets ECT on SYN-ACKs MUST reduce its initial window in
   response to any congestion feedback, whether using classic ECN or
   AccECN.  It SHOULD reduce it to 1 SMSS.  This is different to the
   behaviour specified in an earlier experiment that set ECT on the SYN-
   ACK [RFC5562].  This is justified in Section 4.3.

   The responder does not have to back off its retransmission timer
   because the ECN feedback proves that the network is delivering
   packets successfully and is not severely overloaded.  Also the
   responder does not have to leave slow start or reduce ssthresh, which
   is not even required when a SYN-ACK has been lost.

   The congestion response to CE-marking on a SYN-ACK for a server that
   implements either the TCP Fast Open experiment (TFO [RFC7413]) or the
   initial window of 10 experiment (IW10 [RFC6928]) is discussed in
   Section 5.  Fall-Back Following No Response to an ECT SYN-ACK

   After the responder sends a SYN-ACK with ECT set, if its
   retransmission timer expires it SHOULD retransmit one more SYN-ACK
   with ECT set (and back-off its timer as usual).  If the timer expires
   again, it SHOULD retransmit the SYN-ACK with not-ECT in the IP
   header.  If other experimental fields or options were on the initial
   SYN-ACK, it will also be necessary to follow their specifications for
   fall-back.  It would make sense to co-ordinate all the strategies for
   fall-back in order to isolate the specific cause of the problem.

   This fall-back strategy attempts to use ECT one more time than the
   strategy for ECT SYN-ACKs in [RFC5562] (which is made obsolete, being
   superseded by the present specification).  Other fall-back strategies
   MAY be adopted if found to be more effective, e.g. fall-back to not-
   ECT on the first retransmission attempt.

   The server MAY cache failed connection attempts, e.g. per client
   access network.  An client-based alternative to caching at the server
   is given in Section 4.3.2.  If the TCP server is caching failed
   connection attempts, it SHOULD NOT give up using ECT on the first

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   SYN-ACK of subsequent connection attempts until it is clear that the
   blockage persistently and specifically affects ECT on SYN-ACKs.  This
   is because loss is so commonplace for other reasons (see
   Section  If a cache is used, it SHOULD be arranged to
   expire so that the server will infrequently attempt to check whether
   the problem has been resolved.

3.2.3.  Pure ACK

   For the experiments proposed here, the TCP implementation will set
   ECT on pure ACKs.  It can ignore the requirement in section 6.1.4 of
   RFC 3168 to set not-ECT on a pure ACK.

   A host that sets ECT on pure ACKs MUST reduce its congestion window
   in response to any congestion feedback, in order to regulate any data
   segments it might be sending amongst the pure ACKs. {ToDo: Reconsider
   this requirement in the light of WG comments.} It MAY also implement
   AckCC [RFC5690] to regulate the pure ACK rate, but this is not
   required.  Note that, in comparison, TCP Congestion Control [RFC5681]
   does not require a TCP to detect or respond to loss of pure ACKs at
   all; it requires no reduction in congestion window or ACK rate.

   The question of whether the receiver of pure ACKs is required to feed
   back any CE marks on them is a matter for the relevant feedback
   specification ([RFC3168] or [I-D.ietf-tcpm-accurate-ecn]).  It is
   outside the scope of the present specification.  Currently AccECN
   feedback is required to count CE marking of any control packet
   including pure ACKs.  Whereas RFC 3168 is silent on this point, so
   feedback of CE-markings might be implementation specific (see
   Section 4.4.1).

      DISCUSSION: An AccECN deployment or an implementation of RFC 3168
      that feeds back CE on pure ACKs will be at a disadvantage compared
      to an RFC 3168 implementation that does not.  To solve this, the
      WG could decide to prohibit setting ECT on pure ACKs unless AccECN
      has been negotiated.  If it does, the penultimate sentence of the
      Introduction will need to be modified.

      MEASUREMENTS NEEDED: Measurements are needed to learn how the
      deployed base of network elements and RFC 3168 servers react to
      pure ACKs marked with the ECT(0)/ECT(1)/CE codepoints, i.e.
      whether they are dropped, codepoint cleared or processed and the
      congestion indication fed back on a subsequent packet.

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3.2.4.  Window Probe

   For the experiments proposed here, the TCP sender will set ECT on
   window probes.  It can ignore the prohibition in section 6.1.6 of RFC
   3168 against setting ECT on a window probe.

   A window probe contains a single octet, so it is no different from a
   regular TCP data segment.  Therefore a TCP receiver will feed back
   any CE marking on a window probe as normal (either using classic ECN
   feedback or AccECN feedback).  The sender of the probe will then
   reduce its congestion window as normal.

   A receive window of zero indicates that the application is not
   consuming data fast enough and does not imply anything about network
   congestion.  Once the receive window opens, the congestion window
   might become the limiting factor, so it is correct that CE-marked
   probes reduce the congestion window.  This complements cwnd
   validation [RFC7661], which reduces cwnd as more time elapses without
   having used available capacity.  However, CE-marking on window probes
   does not reduce the rate of the probes themselves.  This is unlikely
   to present a problem, given the duration between window probes
   doubles [RFC1122] as long as the receiver is advertising a zero
   window (currently minimum 1 second, maximum at least 1 minute

      MEASUREMENTS NEEDED: Measurements are needed to learn how the
      deployed base of network elements and servers react to Window
      probes marked with the ECT(0)/ECT(1)/CE codepoints, i.e. whether
      they are dropped, codepoint cleared or processed.

3.2.5.  FIN

   A TCP implementation can set ECT on a FIN.

   The TCP data receiver MUST ignore the CE codepoint on incoming FINs
   that fail any validity check.  The validity check in section 5.2 of
   [RFC5961] is RECOMMENDED.

   A congestion response to a CE-marking on a FIN is not required.

   After sending a FIN, the endpoint will not send any more data in the
   connection.  Therefore, even if the FIN-ACK indicates that the FIN
   was CE-marked (whether using classic or AccECN feedback), reducing
   the congestion window will not affect anything.

   After sending a FIN, a host might send one or more pure ACKs.  If it
   is using one of the techniques in Section 3.2.3 to regulate the

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   delayed ACK ratio for pure ACKs, it could equally be applied after a
   FIN.  But this is not required.

      MEASUREMENTS NEEDED: Measurements are needed to learn how the
      deployed base of network elements and servers react to FIN packets
      marked with the ECT(0)/ECT(1)/CE codepoints, i.e.  whether they
      are dropped, codepoint cleared or processed.

3.2.6.  RST

   A TCP implementation can set ECT on a RST.

   The "challenge ACK" approach to checking the validity of RSTs
   (section 3.2 of [RFC5961] is RECOMMENDED at the data receiver.

   A congestion response to a CE-marking on a RST is not required (and
   actually not possible).

      MEASUREMENTS NEEDED: Measurements are needed to learn how the
      deployed base of network elements and servers react to RST packets
      marked with the ECT(0)/ECT(1)/CE codepoints, i.e.  whether they
      are dropped, codepoint cleared or processed.

3.2.7.  Retransmissions

   For the experiments proposed here, the TCP sender will set ECT on
   retransmitted segments.  It can ignore the prohibition in section
   6.1.5 of RFC 3168 against setting ECT on retransmissions.

   Nonetheless, the TCP data receiver MUST ignore the CE codepoint on
   incoming segments that fail any validity check.  The validity check
   in section 5.2 of [RFC5961] is RECOMMENDED.  This will effectively
   mitigate an attack that uses spoofed data packets to fool the
   receiver into feeding back spoofed congestion indications to the
   sender, which in turn would be fooled into continually halving its
   congestion window.

   If the TCP sender receives feedback that a retransmitted packet was
   CE-marked, it will react as it would to any feedback of CE-marking on
   a data packet.

      MEASUREMENTS NEEDED: Measurements are needed to learn how the
      deployed base of network elements and servers react to
      retransmissions marked with the ECT(0)/ECT(1)/CE codepoints, i.e.
      whether they are dropped, codepoint cleared or processed.

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3.2.8.  General Fall-back for any Control Packet or Retransmission

   Extensive measurements in fixed and mobile networks {ToDo: reference
   (under submission)} have found no evidence of blockages due to ECT
   being set on any type of TCP control packet.

   In case traversal problems arise in future, fall-back measures have
   been specified above, but only for the cases where ECT on the initial
   packet of a half-connection (SYN or SYN-ACK) is persistently failing
   to get through.

   Fall-back measures for blockage of ECT on other TCP control packets
   MAY be implemented.  However they are not specified here given the
   lack of any evidence they will be needed.  Section 4.9 justifies this
   advice in more detail.

4.  Rationale

   This section is informative, not normative.  It presents counter-
   arguments against the justifications in the RFC series for disabling
   ECN on TCP control segments and retransmissions.  It also gives
   rationale for why ECT is safe on control segments that have not, so
   far, been mentioned in the RFC series.  First it addresses over-
   arching arguments used for most packet types, then it addresses the
   specific arguments for each packet type in turn.

4.1.  The Reliability Argument

   Section 5.2 of RFC 3168 states:

      "To ensure the reliable delivery of the congestion indication of
      the CE codepoint, an ECT codepoint MUST NOT be set in a packet
      unless the loss of that packet [at a subsequent node] in the
      network would be detected by the end nodes and interpreted as an
      indication of congestion."

   We believe this argument is misplaced.  TCP does not deliver most
   control packets reliably.  So it is more important to allow control
   packets to be ECN-capable, which greatly improves reliable delivery
   of the control packets themselves (see motivation in Section 1.1).
   ECN also improves the reliability and latency of delivery of any
   congestion notification on control packets, particularly because TCP
   does not detect the loss of most types of control packet anyway.
   Both these points outweigh by far the concern that a CE marking
   applied to a control packet by one node might subsequently be dropped
   by another node.

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   The principle to determine whether a packet can be ECN-capable ought
   to be "do no extra harm", meaning that the reliability of a
   congestion signal's delivery ought to be no worse with ECN than
   without.  In particular, setting the CE codepoint on the very same
   packet that would otherwise have been dropped fulfills this
   criterion, since either the packet is delivered and the CE signal is
   delivered to the endpoint, or the packet is dropped and the original
   congestion signal (packet loss) is delivered to the endpoint.

   The concern about a CE marking being dropped at a subsequent node
   might be motivated by the idea that ECN-marking a packet at the first
   node does not remove the packet, so it could go on to worsen
   congestion at a subsequent node.  However, it is not useful to reason
   about congestion by considering single packets.  The departure rate
   from the first node will generally be the same (fully utilized) with
   or without ECN, so this argument does not apply.

4.2.  SYNs

   RFC 5562 presents two arguments against ECT marking of SYN packets
   (quoted verbatim):

      "First, when the TCP SYN packet is sent, there are no guarantees
      that the other TCP endpoint (node B in Figure 2) is ECN-Capable,
      or that it would be able to understand and react if the ECN CE
      codepoint was set by a congested router.

      Second, the ECN-Capable codepoint in TCP SYN packets could be
      misused by malicious clients to "improve" the well-known TCP SYN
      attack.  By setting an ECN-Capable codepoint in TCP SYN packets, a
      malicious host might be able to inject a large number of TCP SYN
      packets through a potentially congested ECN-enabled router,
      congesting it even further."

   The first point actually describes two subtly different issues.  So
   below three arguments are countered in turn.

4.2.1.  Argument 1a: Unrecognized CE on the SYN

   This argument certainly applied at the time RFC 5562 was written,
   when no ECN responder mechanism had any logic to recognize or feed
   back a CE marking on a SYN.  The problem was that, during the 3WHS,
   the flag in the TCP header for ECN feedback (called Echo Congestion
   Experienced) had been overloaded to negotiate the use of ECN itself.
   So there was no space for feedback in a SYN-ACK.

   The accurate ECN (AccECN) protocol [I-D.ietf-tcpm-accurate-ecn] has
   since been designed to solve this problem, using a two-pronged

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   approach.  First AccECN uses the 3 ECN bits in the TCP header as 8
   codepoints, so there is space for the responder to feed back whether
   there was CE on the SYN.  Second a TCP initiator can always request
   AccECN support on every SYN, and any responder reveals its level of
   ECN support: AccECN, classic ECN, or no ECN.  Therefore, if a
   responder does indicate that it supports AccECN, the initiator can be
   sure that, if there is no CE feedback on the SYN-ACK, then there
   really was no CE on the SYN.

   An initiator can combine AccECN with three possible strategies for
   setting ECT on a SYN:

   (S1):  Pessimistic ECT and cache successes: The initiator always
          requests AccECN in the SYN, but without setting ECT.  Then it
          records those servers that confirm that they support AccECN in
          a cache.  On a subsequent connection to any server that
          supports AccECN, the initiator can then set ECT on the SYN.

   (S2):  Optimistic ECT: The initiator always sets ECT optimistically
          on the initial SYN and it always requests AccECN support.
          Then, if the server response shows it has no AccECN logic (so
          it cannot feed back a CE mark), the initiator conservatively
          behaves as if the SYN was CE-marked, by reducing its initial

          A.  No cache: The optimistic ECT strategy ought to work fairly
              well without caching any responses.

          B.  Cache failures: The optimistic ECT strategy can be
              improved by recording solely those servers that do not
              support AccECN.  On subsequent connections to these non-
              AccECN servers, the initiator will still request AccECN
              but not set ECT on the SYN.  Then, the initiator can use
              its full initial window (if it has enough request data to
              need it).  Longer term, as servers upgrade to AccECN, the
              initiator will remove them from the cache and use ECT on
              subsequent SYNs to that server.

              Where an access network operator mediates Internet access
              via a proxy that does not support AccECN, the optimistic
              ECT strategy will always fail.  This scenario is more
              likely in mobile networks.  Therefore, a mobile host could
              cache lack of AccECN support per attached access network
              operator.  Whenever it attached to a new operator, it
              could check a well-known AccECN test server and, if it
              found no AccECN support, it would add a cache entry for
              the attached operator.  It would only use ECT when neither
              network nor server were cached.  It would only populate

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              its per server cache when not attached to a non-AccECN

   (S3):  ECT by configuration: In a controlled environment, the
          administrator can make sure that servers support ECN-capable
          SYN packets.  Examples of controlled environments are single-
          tenant DCs, and possibly multi-tenant DCs if it is assumed
          that each tenant mostly communicates with its own VMs.

   For unmanaged environments like the public Internet, pragmatically
   the choice is between strategies (S1) and (S2B):

   o  The "pessimistic ECT and cache successes" strategy (S1) suffers
      from exposing the initial SYN to the prevailing loss level, even
      if the server supports ECT on SYNs, but only on the first
      connection to each AccECN server.

   o  The "optimistic ECT and cache failures" strategy (S2B) exploits a
      server's support for ECT on SYNs from the very first attempt.  But
      if the server turns out not to support AccECN, the initiator has
      to conservatively limit its initial window - usually
      unnecessarily.  Nonetheless, initiator request data (as opposed to
      server response data) is rarely larger than 1 SMSS anyway {ToDo:
      reference? (this information was given informally by Yuchung

   The normative specification for ECT on a SYN in Section 3.2.1 uses
   the "optimistic ECT and cache failures" strategy (S2B) on the
   assumption that an initial window of 1 SMSS is usually sufficient for
   client requests anyway.  Clients that often initially send more than
   1 SMSS of data could use strategy (S1) during initial deployment, and
   strategy (S2B) later (when the probability of servers supporting
   AccECN and the likelihood of seeing some CE marking is higher).
   Also, as deployment proceeds, caching successes (S1) starts off small
   then grows, while caching failures (S2B) becomes large at first, then

      MEASUREMENTS NEEDED: Measurements are needed to determine whether
      one or the other strategy would be sufficient for any particular
      client, or whether a particular client would need both strategies
      in different circumstances.

4.2.2.  Argument 1b: Unrecognized ECT on the SYN

   Given, until now, ECT-marked SYN packets have been prohibited, it
   cannot be assumed they will be accepted.

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   According to a study using 2014 data [ecn-pam] from a limited range
   of vantage points, out of the top 1M Alexa web sites, 4791 (0.82%)
   IPv4 sites and 104 (0.61%) IPv6 sites failed to establish a
   connection when they received a TCP SYN with any ECN codepoint set in
   the IP header and the appropriate ECN flags in the TCP header.  Of
   these, about 41% failed to establish a connection due to the ECN
   flags in the TCP header even with a Not-ECT ECN field in the IP
   header (i.e. despite full compliance with RFC 3168).  Therefore
   adding the ECN capability to SYNs was increasing connection
   establishment failures by about 0.4%.

   In a study using 2017 data from a wider range of fixed and mobile
   vantage points to the top 500k Alexa servers, no case was found where
   adding the ECN capability to a SYN increased the likelihood of
   connection establishment failure {ToDo: reference (under

      MEASUREMENTS NEEDED: More investigation is needed to understand
      the different outcomes of the 2014 and 2017 studies.

   RFC 3168 says "a host MUST NOT set ECT on SYN [...] packets", but it
   does not say what the responder should do if an ECN-capable SYN
   arrives.  So, in the 2014 study, perhaps some responder
   implementations were checking that the SYN complied with RFC 3168,
   then silently ignoring non-compliant SYNs (or perhaps returning a
   RST).  Also some middleboxes (e.g. firewalls) might have been
   discarding non-compliant SYNs.  For the future,
   [I-D.ietf-tsvwg-ecn-experimentation] updates RFC 3168 to clarify that
   middleboxes "SHOULD NOT" do this, but that does not alter the past.

   Whereas RSTs can be dealt with immediately, silent failures introduce
   a retransmission timeout delay (default 1 second) at the initiator
   before it attempts any fall back strategy.  Ironically, making SYNs
   ECN-capable is intended to avoid the timeout when a SYN is lost due
   to congestion.  Fortunately, if there is any discard of ECN-capable
   SYNs due to policy, it will occur predictably, not randomly like
   congestion.  So the initiator can avoid it by caching those sites
   that do not support ECN-capable SYNs.  This further justifies the use
   of the "optimistic ECT and cache failures" strategy in Section 3.2.1.

      MEASUREMENTS NEEDED: Experiments are needed to determine whether
      blocking of ECT on SYNs is widespread, and how many occurrences of
      problems would be masked by how few cache entries.

   If blocking is too widespread for the "optimistic ECT and cache
   failures" strategy (S2B), the "pessimistic ECT and cache successes"
   strategy (Section 4.2.1) would be better.

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      MEASUREMENTS NEEDED: Then measurements would be needed on whether
      failures were still widespread on the third connection attempt
      after the more careful ("pessimistic") first and second attempts.

   If so, it might be necessary to send a not-ECT SYN a short delay
   after an ECT SYN and only accept the non-ECT connection if it
   returned first.  This would reduce the performance penalty for those
   deploying ECT SYN support.

      FOR DISCUSSION: If this becomes necessary, how much delay ought to
      be required before the second SYN?  Certainly less than the
      standard RTO (1 second).  But more or less than the maximum RTT
      expected over the surface of the earth (roughly 250ms)?  Or even

   However, based on the data above from [ecn-pam], even a cache of a
   dozen or so sites ought to avoid all ECN-related performance problems
   with roughly the Alexa top thousand.  So it is questionable whether
   sending two SYNs will be necessary, particularly given failures at
   well-maintained sites could reduce further once ECT SYNs are

4.2.3.  Argument 2: DoS Attacks

   [RFC5562] says that ECT SYN packets could be misused by malicious
   clients to augment "the well-known TCP SYN attack".  It goes on to
   say "a malicious host might be able to inject a large number of TCP
   SYN packets through a potentially congested ECN-enabled router,
   congesting it even further."

   We assume this is a reference to the TCP SYN flood attack (see, which is an attack against
   a responder end point.  We assume the idea of this attack is to use
   ECT to get more packets through an ECN-enabled router in preference
   to other non-ECN traffic so that they can go on to use the SYN
   flooding attack to inflict more damage on the responder end point.
   This argument could apply to flooding with any type of packet, but we
   assume SYNs are singled out because their source address is easier to
   spoof, whereas floods of other types of packets are easier to block.

   Mandating Not-ECT in an RFC does not stop attackers using ECT for
   flooding.  Nonetheless, if a standard says SYNs are not meant to be
   ECT it would make it legitimate for firewalls to discard them.
   However this would negate the considerable benefit of ECT SYNs for
   compliant transports and seems unnecessary because RFC 3168 already
   provides the means to address this concern.  In section 7, RFC 3168
   says "During periods where ... the potential packet marking rate
   would be high, our recommendation is that routers drop packets rather

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   then set the CE codepoint..." and this advice is repeated in
   [RFC7567] (section 4.2.1).  This makes it harder for flooding packets
   to gain from ECT.

   Further experiments are needed to test how much malicious hosts can
   use ECT to augment flooding attacks without triggering AQMs to turn
   off ECN support (flying "just under the radar").  If it is found that
   ECT can only slightly augment flooding attacks, the risk of such
   attacks will need to be weighed against the performance benefits of
   ECT SYNs.

4.3.  SYN-ACKs

   The proposed approach in Section 3.2.2 for experimenting with ECN-
   capable SYN-ACKs is effectively identical to the scheme called ECN+
   [ECN-PLUS].  In 2005, the ECN+ paper demonstrated that it could
   reduce the average Web response time by an order of magnitude.  It
   also argued that adding ECT to SYN-ACKs did not raise any new
   security vulnerabilities.

4.3.1.  Response to Congestion on a SYN-ACK

   The IETF has already specified an experiment with ECN-capable SYN-ACK
   packets [RFC5562].  It was inspired by the ECN+ paper, but it
   specified a much more conservative congestion response to a CE-marked
   SYN-ACK, called ECN+/TryOnce.  This required the server to reduce its
   initial window to 1 segment (like ECN+), but then the server had to
   send a second SYN-ACK and wait for its ACK before it could continue
   with its initial window of 1 SMSS.  The second SYN-ACK of this 5-way
   handshake had to carry no data, and had to disable ECN, but no
   justification was given for these last two aspects.

   The present ECN experiment obsoletes RFC 5562 because it uses the
   ECN+ congestion response, not ECN+/TryOnce.  First we argue against
   the rationale for ECN+/TryOnce given in sections 4.4 and 6.2 of
   [RFC5562].  It starts with a rather too literal interpretation of the
   requirement in RFC 3168 that says TCP's response to a single CE mark
   has to be "essentially the same as the congestion control response to
   a *single* dropped packet."  TCP's response to a dropped initial (SYN
   or SYN-ACK) packet is to wait for the retransmission timer to expire
   (currently 1s).  However, this long delay assumes the worst case
   between two possible causes of the loss: a) heavy overload; or b) the
   normal capacity-seeking behaviour of other TCP flows.  When the
   network is still delivering CE-marked packets, it implies that there
   is an AQM at the bottleneck and that it is not overloaded.  This is
   because an AQM under overload will disable ECN (as recommended in
   section 7 of RFC 3168 and repeated in section 4.2.1 of RFC 7567).  So
   scenario (a) can be ruled out.  Therefore, TCP's response to a CE-

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   marked SYN-ACK can be similar to its response to the loss of _any_
   packet, rather than backing off as if the special _initial_ packet of
   a flow has been lost.

   How TCP responds to the loss of any single packet depends what it has
   just been doing.  But there is not really a precedent for TCP's
   response when it experiences a CE mark having sent only one (small)
   packet.  If TCP had been adding one segment per RTT, it would have
   halved its congestion window, but it hasn't established a congestion
   window yet.  If it had been exponentially increasing it would have
   exited slow start, but it hasn't started exponentially increasing yet
   so it hasn't established a slow-start threshold.

   Therefore, we have to work out a reasoned argument for what to do.
   If an AQM is CE-marking packets, it implies there is already a queue
   and it is probably already somewhere around the AQM's operating point
   - it is unlikely to be well below and it might be well above.  So, it
   does not seem sensible to add a number of packets at once.  On the
   other hand, it is highly unlikely that the SYN-ACK itself pushed the
   AQM into congestion, so it will be safe to introduce another single
   segment immediately (1 RTT after the SYN-ACK).  Therefore, starting
   to probe for capacity with a slow start from an initial window of 1
   segment seems appropriate to the circumstances.  This is the approach
   adopted in Section 3.2.2.

4.3.2.  Fall-Back if ECT SYN-ACK Fails

   An alternative to the server caching failed connection attempts would
   be for the server to rely on the client caching failed attempts (on
   the basis that the client would cache a failure whether ECT was
   blocked on the SYN or the SYN-ACK).  This strategy cannot be used if
   the SYN does not request AccECN support.  It works as follows: if the
   server receives a SYN that requests AccECN support but is set to not-
   ECT, it replies with a SYN-ACK also set to not-ECT.  If a middlebox
   only blocks ECT on SYNs, not SYN-ACKs, this strategy might disable
   ECN on a SYN-ACK when it did not need to, but at least it saves the
   server from maintaining a cache.

4.4.  Pure ACKs

   Section 5.2 of RFC 3168 gives the following arguments for not
   allowing the ECT marking of pure ACKs (ACKs not piggy-backed on

      "To ensure the reliable delivery of the congestion indication of
      the CE codepoint, an ECT codepoint MUST NOT be set in a packet
      unless the loss of that packet in the network would be detected by
      the end nodes and interpreted as an indication of congestion.

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      Transport protocols such as TCP do not necessarily detect all
      packet drops, such as the drop of a "pure" ACK packet; for
      example, TCP does not reduce the arrival rate of subsequent ACK
      packets in response to an earlier dropped ACK packet.  Any
      proposal for extending ECN-Capability to such packets would have
      to address issues such as the case of an ACK packet that was
      marked with the CE codepoint but was later dropped in the network.
      We believe that this aspect is still the subject of research, so
      this document specifies that at this time, "pure" ACK packets MUST
      NOT indicate ECN-Capability."

   Later on, in section 6.1.4 it reads:

      "For the current generation of TCP congestion control algorithms,
      pure acknowledgement packets (e.g., packets that do not contain
      any accompanying data) MUST be sent with the not-ECT codepoint.
      Current TCP receivers have no mechanisms for reducing traffic on
      the ACK-path in response to congestion notification.  Mechanisms
      for responding to congestion on the ACK-path are areas for current
      and future research.  (One simple possibility would be for the
      sender to reduce its congestion window when it receives a pure ACK
      packet with the CE codepoint set).  For current TCP
      implementations, a single dropped ACK generally has only a very
      small effect on the TCP's sending rate."

   We next address each of the arguments presented above.

   The first argument is a specific instance of the reliability argument
   for the case of pure ACKs.  This has already been addressed by
   countering the general reliability argument in Section 4.1.

   The second argument says that ECN ought not to be enabled unless
   there is a mechanism to respond to it.  However, actually there _is_
   a mechanism to respond to congestion on a pure ACK that RFC 3168 has
   overlooked - the congestion window mechanism.  When data segments and
   pure ACKs are interspersed, congestion notifications ought to
   regulate the congestion window, whether they are on data segments or
   on pure ACKs.  Otherwise, if ECN is disabled on Pure ACKs, and if
   (say) 70% of the segments in one direction are Pure ACKs, about 70%
   of the congestion notifications will be missed and the data segments
   will not be correctly regulated.

   So RFC 3168 ought to have considered two congestion response
   mechanisms - reducing the congestion window (cwnd) and reducing the
   ACK rate - and only the latter was missing.  Further, RFC 3168 was
   incorrect to assume that, if one ACK was a pure ACK, all segments in
   the same direction would be pure ACKs.  Admittedly a continual stream
   of pure ACKs in one direction is quite a common case (e.g. a file

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   download).  However, it is also common for the pure ACKs to be
   interspersed with data segments (e.g.  HTTP/2 browser requests
   controlling a web application).  Indeed, it is more likely that any
   congestion experienced by pure ACKs will be due to mixing with data
   segments, either within the same flow, or within competing flows.

   This insight swings the argument towards enabling ECN on pure ACKs so
   that CE marks can drive the cwnd response to congestion (whenever
   data segments are interspersed with the pure ACKs).  Then to
   separately decide whether an ACK rate response is also required (when
   they are ECN-enabled).  The two types of response are addressed
   separately in the following two subsections, then a final subsection
   draws conclusions.

4.4.1.  Cwnd Response to CE-Marked Pure ACKs

   If the sender of pure ACKs sets them to ECT, the bullets below assess
   whether the three stages of the congestion response mechanism will
   all work for each type of congestion feedback (classic ECN [RFC3168]
   and AccECN [I-D.ietf-tcpm-accurate-ecn]):

   Detection:  The receiver of a pure ACK can detect a CE marking on it:

      *  Classic feedback: the receiver will not expect CE marks on pure
         ACKs, so it will be implementation-dependent whether it happens
         to check for CE marks on all packets.

      *  AccECN feedback: the AccECN specification requires the receiver
         of any TCP packets to count any CE marks on them (whether or
         not control packets are ECN-capable).

   Feedback:  TCP never ACKs a pure ACK, but the receiver of a CE-mark
      on a pure ACK can feed it back when it sends a subsequent data
      segment (if it ever does):

      *  Classic feedback: the receiver (of the pure ACKs) would set the
         echo congestion experienced (ECE) flag in the TCP header as

      *  AccECN feedback: the receiver continually feeds back a count of
         the number of CE-marked packets that it has received (and, if
         possible, a count of CE-marked bytes).

   Congestion response:  In either case (classic or AccECN feedback), if
      the TCP sender does receive feedback about CE-markings on pure
      ACKs, it will react in the usual way by reducing its congestion
      window accordingly.  This will regulate the rate of any data
      packets it is sending amongst the pure ACKs.  Note that, while a

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      host has no application data to send, any congestion window it has
      attained might also be reduced by the congestion window validation
      mechanism [RFC7661].

4.4.2.  ACK Rate Response to CE-Marked Pure ACKs

   Reducing the congestion window will have no effect on the rate of
   pure ACKs.  The worst case here is if the bottleneck is congested
   solely with pure ACKs, but it could also be problematic if a large
   fraction of the load was from unresponsive ACKs, leaving little or no
   capacity for the load from responsive data.

   Since RFC 3168 was published, Acknowledgement Congestion Control
   (AckCC) techniques have been documented in [RFC5690] (informational).
   So any pair of TCP end-points can choose to agree to regulate the
   delayed ACK ratio in response to lost or CE-marked pure ACKs.
   However, the protocol has a number of open deployment issues (e.g. it
   relies on two new TCP options, one of which is required on the SYN
   where option space is at a premium and, if either option is blocked
   by a middlebox, no fall-back behaviour is specified).  The new TCP
   options addressed two problems, namely that TCP had: i) no mechanism
   to allow ECT to be set on pure ACKs; and ii) no mechanism to feed
   back loss or CE-marking of pure ACKs.  A combination of the present
   specification and AccECN addresses both these problems, at least for
   ECN marking.  So it might now be possible to design an ECN-specific
   ACK congestion control scheme without the extra TCP options proposed
   in RFC 5690.  However, such a mechanism is out of scope of the
   present document.

   Setting aside the practicality of RFC 5690, the need for AckCC has
   not been conclusively demonstrated.  It has been argued that the
   Internet has survived so far with no mechanism to even detect loss of
   pure ACKs.  However, it has also been argued that ECN is not the same
   as loss.  Packet discard can naturally thin the ACK load to whatever
   the bottleneck can support, whereas ECN marking does not (it queues
   the ACKs instead).  Nonetheless, RFC 3168 (section 7) recommends that
   an AQM switches over from ECN marking to discard when the marking
   probability becomes high.  Therefore discard can still be relied on
   to thin out ECN-enabled pure ACKs as a last resort.

4.4.3.  Summary: Enabling ECN on Pure ACKs

   In the case when AccECN has been negotiated, the arguments for ECT
   (and CE) on pure ACKs heavily outweigh those against.  ECN is always
   more and never less reliable for delivery of congestion notification.
   The cwnd response has been overlooked as a mechanism for responding
   to congestion on pure ACKs, so it is incorrect not to set ECT on pure
   ACKs when they are interspersed with data segments.  And when they

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   are not, packet discard still acts as the "congestion response of
   last resort".  In contrast, not setting ECT on pure ACKs is certainly
   detrimental to performance, because when a pure ACK is lost it can
   prevent the release of new data.  Separately, AckCC (or perhaps an
   improved variant exploiting AccECN) could optionally be used to
   regulate the spacing between pure ACKs.  However, it is not clear
   whether AckCC is justified.

   In the case when Classic ECN has been negotiated, there is still an
   argument for ECT (and CE) on pure ACKs, but it is less clear-cut.
   Some existing RFC 3168 implementations might happen to
   (unintentionally) provide the correct feedback to support a cwnd
   response.  Even for those that did not, setting ECT on pure ACKs
   would still be better for performance than not setting it and do no
   extra harm.  If AckCC was required, it is designed to work with RFC
   3168 ECN.

4.5.  Window Probes

   Section 6.1.6 of RFC 3168 presents only the reliability argument for
   prohibiting ECT on Window probes:

      "If a window probe packet is dropped in the network, this loss is
      not detected by the receiver.  Therefore, the TCP data sender MUST
      NOT set either an ECT codepoint or the CWR bit on window probe

      However, because window probes use exact sequence numbers, they
      cannot be easily spoofed in denial-of-service attacks.  Therefore,
      if a window probe arrives with the CE codepoint set, then the
      receiver SHOULD respond to the ECN indications."

   The reliability argument has already been addressed in Section 4.1.

   Allowing ECT on window probes could considerably improve performance
   because, once the receive window has reopened, if a window probe is
   lost the sender will stall until the next window probe reaches the
   receiver, which might be after the maximum retransmission timeout (at
   least 1 minute [RFC6928]).

   On the bright side, RFC 3168 at least specifies the receiver
   behaviour if a CE-marked window probe arrives, so changing the
   behaviour ought to be less painful than for other packet types.

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4.6.  FINs

   RFC 3168 is silent on whether a TCP sender can set ECT on a FIN.  A
   FIN is considered as part of the sequence of data, and the rate of
   pure ACKs sent after a FIN could be controlled by a CE marking on the
   FIN.  Therefore there is no reason not to set ECT on a FIN.

4.7.  RSTs

   RFC 3168 is silent on whether a TCP sender can set ECT on a RST.  The
   host generating the RST message does not have an open connection
   after sending it (either because there was no such connection when
   the packet that triggered the RST message was received or because the
   packet that triggered the RST message also triggered the closure of
   the connection).

   Moreover, the receiver of a CE-marked RST message can either: i)
   accept the RST message and close the connection; ii) emit a so-called
   challenge ACK in response (with suitable throttling) [RFC5961] and
   otherwise ignore the RST (e.g. because the sequence number is in-
   window but not the precise number expected next); or iii) discard the
   RST message (e.g. because the sequence number is out-of-window).  In
   the first two cases there is no point in echoing any CE mark received
   because the sender closed its connection when it sent the RST.  In
   the third case it makes sense to discard the CE signal as well as the

   Although a congestion response following a CE-marking on a RST does
   not appear to make sense, the following factors have been considered
   before deciding whether the sender ought to set ECT on a RST message:

   o  As explained above, a congestion response by the sender of a CE-
      marked RST message is not possible;

   o  So the only reason for the sender setting ECT on a RST would be to
      improve the reliability of the message's delivery;

   o  RST messages are used to both mount and mitigate attacks:

      *  Spoofed RST messages are used by attackers to terminate ongoing
         connections, although the mitigations in RFC 5961 have
         considerably raised the bar against off-path RST attacks;

      *  Legitimate RST messages allow endpoints to inform their peers
         to eliminate existing state that correspond to non existing
         connections, liberating resources e.g. in DoS attacks

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   o  AQMs are advised to disable ECN marking during persistent
      overload, so:

      *  it is harder for an attacker to exploit ECN to intensify an

      *  it is harder for a legitimate user to exploit ECN to more
         reliably mitigate an attack

   o  Prohibiting ECT on a RST would deny the benefit of ECN to
      legitimate RST messages, but not to attackers who can disregard

   o  If ECT were prohibited on RSTs

      *  it would be easy for security middleboxes to discard all ECN-
         capable RSTs;

      *  However, unlike a SYN flood, it is already easy for a security
         middlebox (or host) to distinguish a RST flood from legitimate
         traffic [RFC5961], and even if a some legitimate RSTs are
         accidentally removed as well, legitimate connections still

   So, on balance, it has been decided that it is worth experimenting
   with ECT on RSTs.  During experiments, if the ECN capability on RSTs
   is found to open a vulnerability that is hard to close, this decision
   can be reversed, before it is specified for the standards track.

4.8.  Retransmitted Packets.

   RFC 3168 says the sender "MUST NOT" set ECT on retransmitted packets.
   The rationale for this consumes nearly 2 pages of RFC 3168, so the
   reader is referred to section 6.1.5 of RFC 3168, rather than quoting
   it all here.  There are essentially three arguments, namely:
   reliability; DoS attacks; and over-reaction to congestion.  We
   address them in order below.

   The reliability argument has already been addressed in Section 4.1.

   Protection against DoS attacks is not afforded by prohibiting ECT on
   retransmitted packets.  An attacker can set CE on spoofed
   retransmissions whether or not it is prohibited by an RFC.
   Protection against the DoS attack described in section 6.1.5 of RFC
   3168 is solely afforded by the requirement that "the TCP data
   receiver SHOULD ignore the CE codepoint on out-of-window packets".
   Therefore in Section 3.2.7 the sender is allowed to set ECT on
   retransmitted packets, in order to reduce the chance of them being

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   dropped.  We also strengthen the receiver's requirement from "SHOULD
   ignore" to "MUST ignore".  And we generalize the receiver's
   requirement to include failure of any validity check, not just out-
   of-window checks, in order to include the more stringent validity
   checks in RFC 5961 that have been developed since RFC 3168.

   A consequence is that, for those retransmitted packets that arrive at
   the receiver after the original packet has been properly received
   (so-called spurious retransmissions), any CE marking will be ignored.
   There is no problem with that because the fact that the original
   packet has been delivered implies that the sender's original
   congestion response (when it deemed the packet lost and retransmitted
   it) was unnecessary.

   Finally, the third argument is about over-reacting to congestion.
   The argument goes that, if a retransmitted packet is dropped, the
   sender will not detect it, so it will not react again to congestion
   (it would have reduced its congestion window already when it
   retransmitted the packet).  Whereas, if retransmitted packets can be
   CE tagged instead of dropped, senders could potentially react more
   than once to congestion.  However, we argue that it is legitimate to
   respond again to congestion if it still persists in subsequent round

   Therefore, in all three cases, it is not incorrect to set ECT on

4.9.  General Fall-back for any Control Packet

   Extensive experiments have found no evidence of any traversal
   problems with ECT on any TCP control packet {ToDo: reference (under
   submission)}. Nonetheless, Sections and specify fall-
   back measures if ECT on the first packet of each half-connection (SYN
   or SYN-ACK) appears to be blocking progress.  Here, the question of
   fall-back measures for ECT on other control packets is explored.  It
   supports the advice given in Section 3.2.8; until there's evidence
   that something's broken, don't fix it.

   If an implementation has had to disable ECT to ensure the first
   packet of a flow (SYN or SYN-ACK) gets through, the question arises
   whether it ought to disable ECT on all subsequent control packets
   within the same TCP connection.  Without evidence of any such
   problems, this seems unnecessarily cautious.  Particularly given it
   would be hard to detect loss of most other types of TCP control
   packets that are not ACK'd.  And particularly given that
   unnecessarily removing ECT from other control packets could lead to
   performance problems, e.g. by directing them into an inferior queue
   [I-D.ietf-tsvwg-ecn-l4s-id] or over a different path, because some

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   broken multipath equipment (erroneously) routes based on all 8 bits
   of the Diffserv field.

   In the case where a connection starts without ECT on the SYN (perhaps
   because problems with previous connections had been cached), there
   will have been no test for ECT traversal in the client-server
   direction until the pure ACK that completes the handshake.  It is
   possible that some middlebox might block ECT on this pure ACK or on
   later retransmissions of lost packets.  Similarly, after a route
   change, the new path might include some middlebox that blocks ECT on
   some or all TCP control packets.  However, without evidence of such
   problems, the complexity of a fix does not seem worthwhile.

      MORE MEASUREMENTS NEEDED (?): If further two-ended measurements do
      find evidence for these traversal problems, measurements would be
      needed to check for correlation of ECT traversal problems between
      different control packets.  It might then be necessary to
      introduce a catch-all fall-back rule that disables ECT on certain
      subsequent TCP control packets based on some criteria developed
      from these measurements.

5.  Interaction with popular variants or derivatives of TCP

   The following subsections discuss any interactions between setting
   ECT on all packets and using the following popular variants of TCP:
   IW10 and TFO.  It also briefly notes the possibility that the
   principles applied here should translate to protocols derived from
   TCP.  This section is informative not normative, because no
   interactions have been identified that require any change to
   specifications.  The subsection on IW10 discusses potential changes
   to specifications but recommends that no changes are needed.

   The designs of the following TCP variants have also been assessed and
   found not to interact adversely with ECT on TCP control packets: SYN
   cookies (see Appendix A of [RFC4987] and section 3.1 of [RFC5562]),
   TCP Fast Open (TFO [RFC7413]) and L4S [I-D.ietf-tsvwg-l4s-arch].

5.1.  IW10

   IW10 is an experiment to determine whether it is safe for TCP to use
   an initial window of 10 SMSS [RFC6928].

   This subsection does not recommend any additions to the present
   specification in order to interwork with IW10.  The specifications as
   they stand are safe, and there is only a corner-case with ECT on the
   SYN where performance could be occasionally improved, as explained

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   As specified in Section, a TCP initiator can only set ECT on
   the SYN if it requests AccECN support.  If, however, the SYN-ACK
   tells the initiator that the responder does not support AccECN,
   Section advises the initiator to conservatively reduce its
   initial window to 1 SMSS because, if the SYN was CE-marked, the SYN-
   ACK has no way to feed that back.

   If the initiator implements IW10, it seems rather over-conservative
   to reduce IW from 10 to 1 just in case a congestion marking was
   missed.  Nonetheless, the reduction to 1 SMSS will rarely harm
   performance, because:

   o  as long as the initiator is caching failures to negotiate AccECN,
      subsequent attempts to access the same server will not use ECT on
      the SYN anyway, so there will no longer be any need to
      conservatively reduce IW;

   o  currently it is not common for a TCP initiator (client) to have
      more than one data segment to send {ToDo: evidence/reference?} -
      IW10 is primarily exploited by TCP servers.

   If a responder receives feedback that the SYN-ACK was CE-marked,
   Section mandates that it reduces its initial window to 1
   SMSS.  When the responder also implements IW10, it is particularly
   important to adhere to this requirement in order to avoid overflowing
   a queue that is clearly already congested.

5.2.  TFO

   TCP Fast Open (TFO [RFC7413]) is an experiment to remove the round
   trip delay of TCP's 3-way hand-shake (3WHS).  A TFO initiator caches
   a cookie from a previous connection with a TFO-enabled server.  Then,
   for subsequent connections to the same server, any data included on
   the SYN can be passed directly to the server application, which can
   then return up to an initial window of response data on the SYN-ACK
   and on data segments straight after it, without waiting for the ACK
   that completes the 3WHS.

   The TFO experiment and the present experiment to add ECN-support for
   TCP control packets can be combined without altering either
   specification, which is justified as follows:

   o  The handling of ECN marking on a SYN is no different whether or
      not it carries data.

   o  In response to any CE-marking on the SYN-ACK, the responder adopts
      the normal response to congestion, as discussed in Section 7.2 of

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5.3.  TCP Derivatives

   Stream Control Transmission Protocol (SCTP [RFC4960]) is a standards
   track transport protocol derived from TCP.  SCTP currently does not
   include ECN support, but Appendix A of RFC 4960 broadly describes how
   it would be supported and a (long-expired) draft on the addition of
   ECN to SCTP has been produced [I-D.stewart-tsvwg-sctpecn].  This
   draft avoided setting ECT on control packets and retransmissions,
   closely following the arguments in RFC 3168.

   QUIC [I-D.ietf-quic-transport] is another standards track transport
   protocol offering similar services to TCP but intended to exploit
   some of the benefits of running over UDP.  A way to add ECN support
   to QUIC has been proposed [I-D.johansson-quic-ecn].

   Experience from experiments on adding ECN support to all TCP packets
   ought to be directly transferable to derivatives of TCP, like SCTP or

6.  Security Considerations

   Section 3.2.6 considers the question of whether ECT on RSTs will
   allow RST attacks to be intensified.  There are several security
   arguments presented in RFC 3168 for preventing the ECN marking of TCP
   control packets and retransmitted segments.  We believe all of them
   have been properly addressed in Section 4, particularly Section 4.2.3
   and Section 4.8 on DoS attacks using spoofed ECT-marked SYNs and
   spoofed CE-marked retransmissions.

7.  IANA Considerations

   There are no IANA considerations in this memo.

8.  Acknowledgments

   Thanks to Mirja Kuehlewind, David Black, Padma Bhooma and Gorry
   Fairhurst for their useful reviews.

   The work of Marcelo Bagnulo has been performed in the framework of
   the H2020-ICT-2014-2 project 5G NORMA.  His contribution reflects the
   consortium's view, but the consortium is not liable for any use that
   may be made of any of the information contained therein.

9.  References

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9.1.  Normative References

              Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
              ecn-03 (work in progress), May 2017.

              Black, D., "Explicit Congestion Notification (ECN)
              Experimentation", draft-ietf-tsvwg-ecn-experimentation-06
              (work in progress), September 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [RFC5961]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
              Robustness to Blind In-Window Attacks", RFC 5961,
              DOI 10.17487/RFC5961, August 2010,

9.2.  Informative References

   [ecn-pam]  Trammell, B., Kuehlewind, M., Boppart, D., Learmonth, I.,
              Fairhurst, G., and R. Scheffenegger, "Enabling Internet-
              Wide Deployment of Explicit Congestion Notification",
              Int'l Conf. on Passive and Active Network Measurement
              (PAM'15) pp193-205, 2015.

              Kuzmanovic, A., "The Power of Explicit Congestion
              Notification", ACM SIGCOMM 35(4):61--72, 2005.

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-06 (work
              in progress), September 2017.

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              Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
              and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion
              Control for Datacenters", draft-ietf-tcpm-dctcp-10 (work
              in progress), August 2017.

              Schepper, K. and B. Briscoe, "Identifying Modified
              Explicit Congestion Notification (ECN) Semantics for
              Ultra-Low Queuing Delay", draft-ietf-tsvwg-ecn-l4s-id-00
              (work in progress), May 2017.

              Briscoe, B., Schepper, K., and M. Bagnulo, "Low Latency,
              Low Loss, Scalable Throughput (L4S) Internet Service:
              Architecture", draft-ietf-tsvwg-l4s-arch-00 (work in
              progress), May 2017.

              Johansson, I., "ECN support in QUIC", draft-johansson-
              quic-ecn-03 (work in progress), May 2017.

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

              Judd, G., "Attaining the promise and avoiding the pitfalls
              of TCP in the Datacenter", USENIX Symposium on Networked
              Systems Design and Implementation (NSDI'15) pp.145-157,
              May 2015.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
              Congestion Notification (ECN) Signaling with Nonces",
              RFC 3540, DOI 10.17487/RFC3540, June 2003,

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   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,

   [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,

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,

   [RFC5690]  Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
              Acknowledgement Congestion Control to TCP", RFC 5690,
              DOI 10.17487/RFC5690, February 2010,

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,

   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928,
              DOI 10.17487/RFC6928, April 2013,

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

   [RFC7567]  Baker, F., Ed. and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management",
              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,

   [RFC7661]  Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating
              TCP to Support Rate-Limited Traffic", RFC 7661,
              DOI 10.17487/RFC7661, October 2015,

Bagnulo & Briscoe         Expires April 1, 2018                [Page 36]

Internet-Draft                    ECN++                   September 2017

Authors' Addresses

   Marcelo Bagnulo
   Universidad Carlos III de Madrid
   Av. Universidad 30
   Leganes, Madrid  28911

   Phone: 34 91 6249500

   Bob Briscoe


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