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Versions: (draft-bagnulo-tcpm-generalized-ecn) 00 01 02 03

Network Working Group                                         M. Bagnulo
Internet-Draft                                                      UC3M
Obsoletes: 5562 (if approved)                                 B. Briscoe
Intended status: Experimental                                  CableLabs
Expires: April 25, 2019                                 October 22, 2018


  ECN++: Adding Explicit Congestion Notification (ECN) to TCP Control
                                Packets
                   draft-ietf-tcpm-generalized-ecn-03

Abstract

   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

   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
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   This Internet-Draft will expire on April 25, 2019.

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   Copyright (c) 2018 IETF Trust and the persons identified as the
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   described in the Simplified BSD License.



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   This document may contain material from IETF Documents or IETF
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   material may not have granted the IETF Trust the right to allow
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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Experiment Goals  . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Document Structure  . . . . . . . . . . . . . . . . . . .   6
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Network (e.g. Firewall) Behaviour . . . . . . . . . . . .   7
     3.2.  Sender Behaviour  . . . . . . . . . . . . . . . . . . . .   8
       3.2.1.  SYN (Send)  . . . . . . . . . . . . . . . . . . . . .   9
       3.2.2.  SYN-ACK (Send)  . . . . . . . . . . . . . . . . . . .  12
       3.2.3.  Pure ACK (Send) . . . . . . . . . . . . . . . . . . .  13
       3.2.4.  Window Probe (Send) . . . . . . . . . . . . . . . . .  14
       3.2.5.  FIN (Send)  . . . . . . . . . . . . . . . . . . . . .  15
       3.2.6.  RST (Send)  . . . . . . . . . . . . . . . . . . . . .  15
       3.2.7.  Retransmissions (Send)  . . . . . . . . . . . . . . .  16
       3.2.8.  General Fall-back for any Control Packet or
               Retransmission  . . . . . . . . . . . . . . . . . . .  16
     3.3.  Receiver Behaviour  . . . . . . . . . . . . . . . . . . .  16
       3.3.1.  Receiver Behaviour for Any TCP Control Packet or
               Retransmission  . . . . . . . . . . . . . . . . . . .  17
       3.3.2.  SYN (Receive) . . . . . . . . . . . . . . . . . . . .  17
       3.3.3.  Pure ACK (Receive)  . . . . . . . . . . . . . . . . .  18
       3.3.4.  FIN (Receive) . . . . . . . . . . . . . . . . . . . .  18
       3.3.5.  RST (Receive) . . . . . . . . . . . . . . . . . . . .  18
       3.3.6.  Retransmissions (Receive) . . . . . . . . . . . . . .  19
   4.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     4.1.  The Reliability Argument  . . . . . . . . . . . . . . . .  19
     4.2.  SYNs  . . . . . . . . . . . . . . . . . . . . . . . . . .  20
       4.2.1.  Argument 1a: Unrecognized CE on the SYN . . . . . . .  20
       4.2.2.  Argument 1b: ECT Considered Invalid on the SYN  . . .  21
       4.2.3.  Caching Strategies for ECT on SYNs  . . . . . . . . .  23
       4.2.4.  Argument 2: DoS Attacks . . . . . . . . . . . . . . .  25
     4.3.  SYN-ACKs  . . . . . . . . . . . . . . . . . . . . . . . .  26
       4.3.1.  Possibility of Unrecognized CE on the SYN-ACK . . . .  26



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       4.3.2.  Response to Congestion on a SYN-ACK . . . . . . . . .  26
       4.3.3.  Fall-Back if ECT SYN-ACK Fails  . . . . . . . . . . .  28
     4.4.  Pure ACKs . . . . . . . . . . . . . . . . . . . . . . . .  28
       4.4.1.  Mechanisms to Respond to CE-Marked Pure ACKs  . . . .  29
       4.4.2.  Summary: Enabling ECN on Pure ACKs  . . . . . . . . .  32
     4.5.  Window Probes . . . . . . . . . . . . . . . . . . . . . .  33
     4.6.  FINs  . . . . . . . . . . . . . . . . . . . . . . . . . .  34
     4.7.  RSTs  . . . . . . . . . . . . . . . . . . . . . . . . . .  34
     4.8.  Retransmitted Packets.  . . . . . . . . . . . . . . . . .  35
     4.9.  General Fall-back for any Control Packet  . . . . . . . .  36
   5.  Interaction with popular variants or derivatives of TCP . . .  37
     5.1.  IW10  . . . . . . . . . . . . . . . . . . . . . . . . . .  37
     5.2.  TFO . . . . . . . . . . . . . . . . . . . . . . . . . . .  38
     5.3.  TCP Derivatives . . . . . . . . . . . . . . . . . . . . .  39
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  39
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  39
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  39
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  40
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  40
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  40
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  43

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++ uses a sender-only deployment model.  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.



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   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.  [RFC8311] 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
   avoided.

   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 [RFC8257]), 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, 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 them into a different queue.  Then
   only L4S data packets would enjoy ultra-low latency forwarding, while
   the packets controlling and retransmitting these data packets would




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   still get stuck behind the queue induced by legacy ('Classic') TCP
   traffic.

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
      [I-D.ietf-tcpm-accurate-ecn].

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

   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
   redesign of the final mechanism (if needed) 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.








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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
   experiment.

2.  Terminology

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL in this
   document, are to be interpreted as described in BCP 14 [RFC2119] when
   and only when they appear in all capitals.

   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.

   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
   sender.

   TCP client: The initiating end of a TCP connection.  Also called the
   initiator.

   TCP server: The responding end of a TCP connection.  Also called the
   responder.

   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



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   mean ECT(1), which is in the process of being redefined for use by
   L4S experiments [RFC8311] [I-D.ietf-tsvwg-ecn-l4s-id].

   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
   increases.

3.  Specification

   The experimental ECN++ changes to the specification of TCP over ECN
   [RFC3168] defined here primarily alter the behaviour of the sending
   host for each half-connection.  However, there are subsections for
   forwarding elements and receivers below, which recommend that they
   accept the new packets - they should do already, but might not.  This
   will allow implementers to check the receive side code while they are
   altering the send-side code.  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
   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 [RFC8311] updates RFC
   3168 to remove this ambiguity.  It requires 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




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   to comply with the requirement referred to above (section 4.3 of
   [RFC8311]).

3.2.  Sender Behaviour

   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.

   +---------+----------------+-----------------+----------------------+
   | TCP     | ECN field if   | ECN field if    | Congestion Response  |
   | packet  | AccECN f/b     | RFC3168 f/b     |                      |
   | type    | negotiated*    | negotiated*     |                      |
   +---------+----------------+-----------------+----------------------+
   | SYN     | ECT            | not-ECT         | If AccECN, reduce IW |
   |         |                |                 |                      |
   | SYN-ACK | ECT            | ECT             | Reduce IW            |
   |         |                |                 |                      |
   | Pure    | ECT            | not-ECT         | If AccECN, usual     |
   | ACK     |                |                 | cwnd 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 cannot set ECT on the SYN if it is not
   requesting AccECN feedback.  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



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   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 an
   ECN++ TCP sender is not using AccECN.  This could be because it does
   not support AccECN or because the other end of the TCP connection
   does not (AccECN can only be used for a connection if both ends
   support it).

3.2.1.  SYN (Send)

3.2.1.1.  Setting ECT on the SYN

   With classic [RFC3168] ECN feedback, the SYN was not 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
   that the responder supports ECN).  In contrast, Accurate ECN (AccECN)
   feedback [I-D.ietf-tcpm-accurate-ecn] provides a codepoint in the
   SYN-ACK for the responder to feed back whether the SYN arrived marked
   CE.  Therefore the setting of the IP/ECN field on the SYN is
   specified separately for each case in the following two subsections.

3.2.1.1.1.  ECN++ TCP Client also Supports AccECN

   For the ECN++ experiment, 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, as per Section 4.3 of [RFC8311].

3.2.1.1.2.  ECN++ TCP Client does not Support AccECN

   A TCP initiator MUST NOT set ECT on a SYN if it does not also attempt
   to negotiate Accurate ECN feedback in the same SYN.

   If the TCP initiator does not support AccECN, the rest of
   Section 3.2.1 does not apply.  It solely applies to the case where
   the TCP initiator supports AccECN as well as ECN++.

3.2.1.2.  Caching where to use ECT on SYNs

   As explained above, this subsection only applies if the ECN++ TCP
   client also supports AccECN.

   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 servers that it is not worth sending an ECT SYN to, e.g.
   because they do not support AccECN and therefore have no logic for



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   congestion markings on the SYN.  Mobile hosts MAY maintain a cache
   entry per access network to record 'non-ECT SYN' entries against
   proxies (see Section 4.2.1).

   Subsequently the initiator will not set ECT on a SYN to such a server
   or proxy, 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.

   The client can limit the size of its cache of 'non-ECT SYN' servers.
   Then, while AccECN is not widely deployed, it will only cache the
   'non-ECT SYN' servers that are most used and most recently used by
   the client.  As the client accesses servers that have been expelled
   from its cache, it will simply use ECT on the SYN by default.

   Servers that do not support ECN as a whole 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 three measurement studies and
   assumptions detailed in Section 4.2.1.  However, Section 4.2.1 gives
   two other strategies and the choice between them depends on the
   implementer's goals and the deployment prevalence of ECN variants in
   the network and on servers, not to mention the prevalence of some
   significant bugs.

   If the initiator times out without seeing a SYN-ACK, it will
   separately cache this fact (see fall-back in Section 3.2.1.4 for
   details).

3.2.1.3.  SYN Congestion Response

   As explained above, this subsection only applies if the ECN++ TCP
   client also supports AccECN.

   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).





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   If the initiator has set ECT 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.

3.2.1.4.  Fall-Back Following No Response to an ECT SYN

   As explained above, this subsection only applies if the ECN++ TCP
   client also supports AccECN.

   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 study using 2017 data [Mandalari18] 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.

   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




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   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 (Send)

3.2.2.1.  Setting ECT on the SYN-ACK

   For the ECN++ experiment, 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, as per Section 4.3 of [RFC8311].

3.2.2.2.  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 (see Section 4.3.1).  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.








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3.2.2.3.  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.  A client-based alternative to caching at the server
   is given in Section 4.3.3.  If the TCP server is caching failed
   connection attempts, it SHOULD NOT give up using ECT on the first
   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 3.2.1.4).  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 (Send)

   A Pure ACK is an ACK packet that does not carry data, which includes
   the Pure ACK at the end of TCP's 3-way handshake.

   For the ECN++ experiment, whether a TCP implementation sets ECT on a
   Pure ACK depends on whether or not Accurate ECN TCP feedback
   [I-D.ietf-tcpm-accurate-ecn] has been successfully negotiated for a
   particular TCP connection, as specified in the following two
   subsections.

3.2.3.1.  Pure ACK without AccECN Feedback

   If AccECN has not been successfully negotiated for a connection, ECT
   MUST NOT be set on Pure ACKs by either end.








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3.2.3.2.  Pure ACK with AccECN Feedback

   For the ECN++ experiment, if AccECN has been successfully negotiated,
   either end of the connection will set ECT on Pure ACKs.  They can
   ignore the requirement in section 6.1.4 of RFC 3168 to set not-ECT on
   a pure ACK, as per Section 4.3 of [RFC8311].

      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.

   See Section 3.3.3 for the implications if a host receives a CE-marked
   Pure ACK.

3.2.3.2.1.  Pure ACK Congestion Response

   As explained above, this subsection only applies if AccECN has been
   successfully negotiated for the TCP connection.

   A host that sets ECT on pure ACKs SHOULD respond to the congestion
   signal resulting from pure ACKs being marked with the CE codepoint.
   The specific response will need to be defined as an update to each
   congestion control specification.  Possible responses to congestion
   feedback include reducing the congestion window (CWND) and/or
   regulating the pure ACK rate (see Section 4.4.1.1).

   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.

3.2.4.  Window Probe (Send)

   For the ECN++ experiment, 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, as per Section 4.3 of
   [RFC8311].

   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



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   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
   [RFC6298]).

      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 (Send)

   A TCP implementation can set ECT on a FIN.

   See Section 3.3.4 for the implications if a host receives a CE-marked
   FIN.

   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
   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 (Send)

   A TCP implementation can set ECT on a RST.

   See Section 3.3.5 for the implications if a host receives a CE-marked
   RST.

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



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      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 (Send)

   For the ECN++ experiment, 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, as per
   Section 4.3 of [RFC8311].

   See Section 3.3.6 for the implications if a host receives a CE-marked
   retransmission.

   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.

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

   Extensive measurements in fixed and mobile networks [Mandalari18]
   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.

3.3.  Receiver Behaviour

   The present ECN++ specification primarily concerns the behaviour for
   sending TCP control packets or retransmissions.  Below are a few
   changes to the receive side of an implementation that are recommended
   while updating its send side.  Nonetheless, where deployment is
   concerned, ECN++ is still a sender-only deployment, because it does
   not depend on receivers complying with any of these recommendations.



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3.3.1.  Receiver Behaviour for Any TCP Control Packet or Retransmission

   RFC8311 is a standards track update to RFC 3168 in order to (amongst
   other things) "...allow the use of ECT codepoints on SYN packets,
   pure acknowledgement packets, window probe packets, and
   retransmissions of packets..., provided that the changes from RFC
   3168 are documented in an Experimental RFC in the IETF document
   stream."

   Section 4.3 of RFC 8311 amends every statement in RFC 3168 that
   precludes the use of ECT on control packets and retransmissions to
   add "unless otherwise specified by an Experimental RFC in the IETF
   document stream".  The present specification is such an Experimental
   RFC.  Therefore, In order for this experiment to be useful, the
   following requirements follow from RFC8311:

   o  Any TCP implementation SHOULD accept receipt of any valid TCP
      control packet or retransmission irrespective of its IP/ECN field.
      If any existing implementation does not, it SHOULD be updated to
      do so.

   o  A TCP implementation taking part in the experiments proposed here
      MUST accept receipt of any valid TCP control packet or
      retransmission irrespective of its IP/ECN field.

   These measures are derived from the robustness principle of "... be
   liberal in what you accept from others", in order to ensure
   compatibility with any future protocol changes that allow ECT on any
   TCP packet.

3.3.2.  SYN (Receive)

   RFC 3168 negotiates the use of ECN for the connection end-to-end
   using the ECN flags in the TCP header.  When RFC3168 says that "A
   host MUST NOT set ECT on SYN ... packets." it is silent as to what a
   TCP server ought to do if it receives a SYN packet with a non-zero
   IP/ECN field.

   Some implementations of TCP servers (e.g. current Linux) assume that,
   if a host receives a SYN with a non-zero IP/ECN field, it must be due
   to network mangling, and they disable ECN for the rest of the
   connection.  Section 4.2.2.2 finds that this type of network mangling
   seems to be virtually non-existent so it would be preferable to
   report any such mangling so it can be fixed.

   For the avoidance of doubt, the normative statements for all TCP
   control packets in Section 3.3.1 are interpreted for the case when a
   SYN is received as follows:



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   o  Any TCP server implementation SHOULD accept receipt of a valid SYN
      that requests ECN support for the connection, irrespective of the
      IP/ECN field of the SYN.  If any existing implementation does not,
      it SHOULD be updated to do so.

   o  A TCP implementation taking part in the ECN++ experiment MUST
      accept receipt of a valid SYN, irrespective of its IP/ECN field.

   o  If the SYN is CE-marked and the server has no logic to feed back a
      CE mark on a SYN-ACK (e.g. it does not support AccECN), it has to
      ignore the CE-mark (the client detects this case and behaves
      conservatively in mitigation - see Section 3.2.1.3).

3.3.3.  Pure ACK (Receive)

   For the avoidance of doubt, the normative statements for all TCP
   control packets in Section 3.3.1 are interpreted for the case when a
   Pure ACK is received as follows:

   o  Any TCP implementation SHOULD accept receipt of a pure ACK with a
      non-zero ECN field, despite current RFCs precluding the sending of
      such packets.

   o  A TCP implementation taking part in the ECN++ experiment MUST
      accept receipt of a pure ACK with a non-zero ECN field.

   The question of whether and how the receiver of pure ACKs is required
   to feed back any CE marks on them is outside the scope of the present
   specification because it is a matter for the relevant feedback
   specification ([RFC3168] or [I-D.ietf-tcpm-accurate-ecn]).  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 4.4.1.1).

3.3.4.  FIN (Receive)

   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.

3.3.5.  RST (Receive)

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






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3.3.6.  Retransmissions (Receive)

   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 reducing its congestion window.

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.

   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.



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   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 a CE
   marking on a SYN and, even if logic were added, there was no field in
   the SYN-ACK to feed it back.  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.

   The accurate ECN (AccECN) protocol [I-D.ietf-tcpm-accurate-ecn] has
   since been designed to solve this problem.  Two features are
   important here:

   1.  An AccECN server uses the 3 'ECN' bits in the TCP header of the
       SYN-ACK to respond to the client. 4 of the possible 8 codepoints
       provide enough space for the server to feed back which of the 4
       IP/ECN codepoints was on the incoming SYN (including CE of
       course).





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   2.  If any of these 4 codepoints are in the SYN-ACK, it confirms that
       the server supports AccECN and, if another codepoint is returned,
       it confirms that the server doesn't support AccECN.

   This still does not seem to allow a client set ECT on a SYN, it only
   finds out whether the server would have supported it afterwards.  The
   trick the client uses for ECN++ is to set ECT on the SYN
   optimistically then, if the SYN-ACK reveals that the server wouldn't
   have understood CE on the SYN, the client responds conservatively as
   if the SYN was marked with CE.

   Happily, the appropriate conservative congestion response is to
   reduce the initial window, and it is extremely rare for a TCP client
   to send more than one packet as its initial request anyway.  Any
   clients that do frequently use a larger initial window for their
   first message to the server can cache which servers will not
   understand ECT on a SYN (see Section 4.2.3 below).

4.2.2.  Argument 1b: ECT Considered Invalid on the SYN

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

4.2.2.1.  ECT on SYN Considered Invalid by Middleboxes

   According to a study using 2014 data [ecn-pam] from a limited range
   of fixed vantage points, for the top 1M Alexa web sites, adding the
   ECN capability to SYNs was increasing connection establishment
   failures by about 0.4%.

   From a wider range of fixed and mobile vantage points, a more recent
   study in Jan-May 2017 [Mandalari18] found no occurrences of blocking
   of ECT on SYNs.  However, in more than half the mobile networks
   tested it found wiping of the ECN codepoint at the first hop.

      MEASUREMENTS NEEDED: As wiping at the first hop is remedied,
      measurements will be needed to check whether SYNs with ECT are
      sometimes blocked deeper into the path.

   Silent failures introduce a retransmission timeout delay (default 1
   second) at the initiator before it attempts any fall back strategy
   (whereas explicit RSTs can be dealt with immediately).  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 should be able to avoid




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   it by caching those sites that do not support ECN-capable SYNs (see
   the last paragraph of Section 3.2.1.2).

4.2.2.2.  ECT on SYN Considered Invalid by Servers

   A study conducted in Nov 2017 [Kuehlewind18] found that, of the 82%
   of the Alexa top 50k web servers that supported ECN, 84% disabled ECN
   if the IP/ECN field on the SYN was ECT0, CE or either.  Given most
   web servers use Linux, this behaviour can most likely be traced to a
   patch contributed in May 2012 that was first distributed in v3.5 of
   the Linux kernel [strict-ecn].  The comment says "RFC3168 : 6.1.1 SYN
   packets must not have ECT/ECN bits set.  If we receive a SYN packet
   with these bits set, it means a network is playing bad games with TOS
   bits.  In order to avoid possible false congestion notifications, we
   disable TCP ECN negociation."  Of course, some of the 84% might be
   due to similar code in other OSs.

   For brevity we shall call this the "contra-Postel" ECN test, because
   it is conservative with what it accepts, contrary to Postel's
   robustness principle.  A robust protocol will not usually assume
   network mangling without comparing with the value originally sent,
   and one packet is not sufficient to make an assumption with such
   irreversible consequences anyway.

   Ironically, networks rarely seem to alter the IP/ECN field on a SYN
   from zero to non-zero anyway.  In a study conducted in Jan-May 2017
   over millions of paths from vantage points in a few dozen mobile and
   fixed networks [Mandalari18], no such transition was observed.  With
   such a small or non-existent incidence of this sort of network
   mangling, it would be preferable to report any residual problem paths
   so that they can be fixed.

   Whatever, the widespread presence of this 'contra-Postel' test proves
   that RFC 5562 was correct to expect that ECT would be considered
   invalid on SYNs.  Nonetheless, it is not an insurmountable problem -
   caching can work round it.  The prevalence of these "contra-Postel"
   ECN servers makes it challenging to cache them all.  However,
   Section 4.2.3 below explains how a cache of limited size can
   alleviate this problem for a client's most popular sites.

   For the future, [RFC8311] updates RFC 3168 to clarify that the IP/ECN
   field does not have to be zero on a SYN if documented in an
   experimental RFC such as the present ECN++ specification.








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4.2.3.  Caching Strategies for ECT on SYNs

   Given the server handling of ECN on SYNs outlined in Section 4.2.2.2
   above, an initiator might combine AccECN with three candidate caching
   strategies for setting ECT on a SYN:

   (S1):  Pessimistic ECT and cache successes: The initiator always
          requests AccECN, but by default without ECT on the SYN.  Then
          it caches those servers that confirm that they support AccECN
          as 'ECT SYN OK'.  On a subsequent connection to any server
          that supports AccECN, the initiator can then set ECT on the
          SYN.  When connecting to other servers (non-ECN or classic
          ECN) it will not set ECT on the SYN, so it will not fail the
          'contra-Postel' test.

          Longer term, as servers upgrade to AccECN, the initiator is
          still requesting AccECN, so it will add them to the cache and
          use ECT on subsequent SYNs to those servers.  However,
          assuming it has to cap the size of the cache, the client will
          not have the benefit of ECT SYNs to those less frequently used
          AccECN servers expelled from its cache.

   (S2):  Optimistic ECT: The initiator always requests AccECN and by
          default sets ECT on the SYN.  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 window.

          A.  No cache.

          B.  Cache failures: The optimistic ECT strategy can be
              improved by caching solely those servers that do not
              support AccECN as 'ECT SYN NOK'.  This would include non-
              ECN servers and all Classic ECN servers whether 'contra-
              Postel' or not.  On subsequent connections to these non-
              AccECN servers, the initiator will still request AccECN
              but not set ECT on the SYN.  Then, the connection can
              still fall back to Classic ECN, if the server supports it,
              and 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



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

   (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), (S2A) and (S2B).  The
   normative specification for ECT on a SYN in Section 3.2.1 recommends
   the "optimistic ECT and cache failures" strategy (S2B) but the choice
   depends on the implementer's motivation for using ECN++, and the
   deployment prevalence of different technologies and bug-fixes.  For
   instance, if a user's Internet access bottleneck supported L4S ECN
   but not Classic ECN, strategy (S2A) would make most sense and there
   would be no point trying to avoid the 'contra-Postel' test and
   negotiate Classic ECN.

   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.  If AccECN becomes widely
      deployed on servers, SYNs to those AccECN servers that are less
      frequently used by the client and therefore don't fit in the cache
      will not benefit from ECN protection at all.

   o  The "optimistic ECT without a cache" strategy (S2A) is the
      simplest.  It would satisfy the goal of an implementer who is
      solely interested in ultra-low latency using AccECN and ECN++
      (e.g. accessing L4S servers) and is not concerned about fall-back
      to Classic ECN (e.g. when accessing other servers).

   o  The "optimistic ECT and cache failures" strategy (S2B) exploits
      ECT on SYNs from the very first attempt.  But if the server turns
      out to be 'contra-Postel' it will disable ECN for the connection,
      but only for the first connection if it's one of the client's more
      popular servers that fits in the cache.  If the server turns out
      not to support AccECN, the initiator has to conservatively limit
      its initial window, but again only for the first connection if



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      it's one of the client's more popular servers (and anyway this
      rarely makes any difference when most client requests fit in a
      single packet).

   Note that, if AccECN deployment grows, caching successes (S1) starts
   off small then grows, while caching failures (S2B) becomes large at
   first, then shrinks.  At half-way, the size of the cache has to be
   capped with either approach, so the default behaviour for all the
   servers that do not fit in the cache is as important as the behaviour
   for the popular servers that do fit.

      MEASUREMENTS NEEDED: Measurements are needed to determine which
      strategy would be sufficient for any particular client, whether a
      particular client would need different strategies in different
      circumstances and how many occurrences of problems would be masked
      by how few cache entries.

   Another strategy would be to send a not-ECT SYN a short delay (below
   the typical lowest RTT) 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.  However, this 'happy
   eyeballs' approach becomes complex when multiple optional features
   are all tried on the first SYN (or on multiple SYNs), so it is not
   recommended.

4.2.4.  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
   https://en.wikipedia.org/wiki/SYN_flood), 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



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   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
   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.

   [ecn-overload] showed that ECT can only slightly augment flooding
   attacks relative to a non-ECT attack.  It was hard to overload the
   link without causing the queue to grow, which in turn caused the AQM
   to disable ECN and switch to drop, thus negating any advantage of
   using ECT.  This was true even with the switch-over point set to 25%
   drop probability (i.e. the arrival rate was 133% of the link rate).

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.  Possibility of Unrecognized CE on the 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 client 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.  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.  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).

4.3.2.  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



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   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++ experimental specification 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-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.






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4.3.3.  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
   data):

      "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.

      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."



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   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.  This argument actually
   comprises three sub-arguments:

   Mechanism feasibility:  If ECN is enabled on Pure ACKs, are there, or
      could there be, suitable mechanisms to detect, feed back and
      respond to ECN-marked Pure ACKs?

   Do no extra harm:  There has never been a mechanism to respond to
      loss of non-ECN Pure ACKs.  So it seems that adding ECN without a
      response mechanism will do no extra harm to others, while
      improving a connection's own performance (because loss of an ACK
      holds back new data).  However, if the end systems have no
      response mechanism, ECN Pure ACKS do slightly more harm than non-
      ECN, because the AQM doesn't immediately clear ECT packets from
      the queue until it reaches overload and disables ECN.

   Standards policy:  Even if there were no harm to others, does it set
      an undesirable precedent to allow a flow to use ECN to protect its
      Pure ACKs from loss, when there is no mechanism to respond to ECN-
      marking?

   The last two arguments involve value judgements, but they both depend
   on the concrete technical question of mechanism feasibility, which
   will therefore be addressed first in Section 4.4.1 below.  Then
   Section 4.4.2 draws conclusions by addressing the value judgements in
   the other two questions.

4.4.1.  Mechanisms to Respond to CE-Marked Pure ACKs

   The question of whether the receiver of pure ACKs is required to
   detect and feed back any CE-marking is outside the scope of the
   present specification - it is a matter for the relevant feedback
   specification (classic ECN [RFC3168] and AccECN
   [I-D.ietf-tcpm-accurate-ecn]).  The response to congestion feedback
   is also out of scope, because it would be defined in the base TCP
   congestion control specification [RFC5681] or its variants.

   Nonetheless, in order to decide whether the present ECN++
   experimental specification should require a host to set ECT on pure
   ACKs, we only need to know whether a response mechanism would be
   feasible - we do not have to standardize it.  So the bullets below



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   assess, for each type of feedback, whether the three stages of the
   congestion response mechanism could all work.

   Detection:  Can the receiver of a pure ACK detect a CE marking on
      it?:

      *  Classic feedback: RFC 3168 is silent on this point.  The
         implementer of the receiver would not expect CE marks on pure
         ACKs, but the implementation might happen to check for CE marks
         before it looks for the data.  So detection will be
         implementation-dependent.

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

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

      *  Classic feedback: RFC 3168 is silent on this point, so feedback
         of CE-markings might be implementation specific.  If the
         receiver (of the pure ACKs) did generate feedback, it would set
         the echo congestion experienced (ECE) flag in the TCP header of
         subsequent packets in the round, as it would to feed back CE on
         data packets.

      *  AccECN feedback: the receiver continually feeds back a count of
         the number of CE-marked packets that it has received and,
         optionally, a count of CE-marked bytes.  For either metric,
         AccECN includes pure ACKs and indeed all types of packets.

   Congestion response:  In either case (classic or AccECN feedback), if
      the TCP sender does receive feedback about CE-markings on pure
      ACKs, it will be able to reduce the congestion window (cwnd) and/
      or the ACK rate.

   Therefore a congestion response mechanism is clearly feasible if
   AccECN has been negotiated, but the position is unknown for the
   installed base of classic ECN feedback.

4.4.1.1.  Congestion Window Response to CE-Marked Pure ACKs

   This subsection explores issues that congestion control designers
   will need to consider when defining a cwnd response to CE-marked Pure
   ACKs.





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   A CE-mark on a Pure ACK does not mean that only Pure ACKs are causing
   congestion.  It only means that the marked Pure ACK is part of an
   aggregate that is collectively causing a bottleneck queue to randomly
   CE-mark a fraction of the packets.  A CE-mark on a Pure ACK might be
   due to data packets in other flows through the same bottleneck, due
   to data packets interspersed between Pure ACKs in the same half-
   connection, or just due to the rate of Pure ACKs alone.  (RFC 3168
   only considered the last possibility, which led to the argument that
   ECN-enabled Pure ACKs had to be deferred, because ACK congestion
   control was a research issue.)

   If a host has been sending a mix of Pure ACKs and data, it doesn't
   need to work out whether a particular CE mark was on a Pure ACK or
   not; it just needs to respond to congestion feedback as a whole by
   reducing its congestion window (cwnd), which limits the data it can
   launch into flight through the congested bottleneck.  If it is purely
   receiving data and sending only Pure ACKs, reducing cwnd will have
   caused it no harm, having no effect on its ACK rate (the next
   subsection addresses that).

   However, when a host is sending data as well as Pure ACKs, it would
   not be right for CE-marks on Pure ACKs and on data packets to induce
   the same reduction in cwnd.  A possible way to address this issue
   would be to weight the response by the size of the marked packets.
   For instance, one could calculate the fraction of CE-marked bytes
   (headers and data) over each round trip (say) as follows:

      (CE-marked header bytes + CE-marked data bytes) / (all header
      bytes + all data bytes)

   Header bytes can be calculated by multiplying a packet count by a
   nominal header size, which is possible with AccECN feedback, because
   it gives a count of CE-marked packets (as well as CE-marked bytes).
   The above simple aggregate calculation caters for the full range of
   scenarios; from all Pure ACKs to just a few interspersed with data
   packets.

   Note that any mechanism that reduces cwnd due to CE-marked Pure ACKs
   would need to be integrated with the congestion window validation
   mechanism [RFC7661], which already conservatively reduces cwnd over
   time because cwnd becomes stale if it is not used to fill the pipe.

4.4.1.2.  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




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   fraction of the load was from unresponsive ACKs, leaving little or no
   capacity for the load from responsive data.

   Since RFC 3168 was published, experimental 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 issues
   concerning deployment (e.g. it requires support from both ends, 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 address 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 CE-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.

4.4.2.  Summary: Enabling ECN on Pure ACKs

   In the case when AccECN has been negotiated, it provides a feasible
   congestion response mechanism, so the arguments for ECT on pure ACKs
   heavily outweigh those against.  ECN is always more and never less
   reliable for delivery of congestion notification.  A cwnd reduction
   needs to be considered by congestion control designers as a response
   to congestion on pure ACKs.  Separately, AckCC (or 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.  If it is not, packet discard will still act as the
   "congestion response of last resort" by thinning out the traffic.  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.



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   In the case when Classic ECN has been negotiated, the argument for
   ECT on pure ACKs is less clear-cut.  Some of the installed base of
   RFC 3168 implementations might happen to (unintentionally) provide a
   feedback mechanism to support a cwnd response.  For those that did
   not, setting ECT on pure ACKs would be better for the flow's own
   performance than not setting it.  However, where there was no
   feedback mechanism, setting ECT could do slightly more harm than not
   setting it.  AckCC could provide a complementary response mechanism,
   because it is designed to work with RFC 3168 ECN, but it has
   deployment challenges.  In summary, a congestion response mechanism
   is unlikely to be feasible with the installed base of classic ECN.

   During review of this specification, it was decided that allowing
   hosts to set ECT on Pure ACKs without a feasible response mechanism
   would set an undesirable precedent.  It would certainly improve the
   flow's own performance, but it would slightly increase potential harm
   to others.  Therefore, Section 3.2.3 allows ECT on Pure ACKs if
   AccECN feedback has been negotiated, but not with classic RFC 3168
   ECN feedback.

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
      packets.

      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
   RST.

   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
         scenarios;




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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
         attack;

      *  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
      RFCs;

   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
         function.

   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
   trip(s).

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

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 [Mandalari18].
   Nonetheless, Sections 3.2.1.4 and 3.2.2.3 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
   below.




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   As specified in Section 3.2.1.1, 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 3.2.1.1 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, at least for web sessions, it is extremely rare for a
      TCP initiator (client) to have more than one data segment to send
      at the start of a TCP connection [28; Fig 3] - IW10 is primarily
      exploited by TCP servers.

   If a responder receives feedback that the SYN-ACK was CE-marked,
   Section 3.2.2.2 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.






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   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
      [RFC7413].

5.3.  TCP Derivatives

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

   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.  Building on the arguments
   in the current draft, a QUIC sender sets ECT(0) on all packets.

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 4.2.4
   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, Gorry
   Fairhurst, Michael Scharf, Yuchung Cheng and Christophe Paasch 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.




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   Bob Briscoe's contribution was partly funded by the Research Council
   of Norway through the TimeIn project.  The views expressed here are
   solely those of the authors.

9.  References

9.1.  Normative References

   [I-D.ietf-tcpm-accurate-ecn]
              Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
              ecn-07 (work in progress), July 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc3168>.

   [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,
              <https://www.rfc-editor.org/info/rfc5961>.

   [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,
              DOI 10.17487/RFC8311, January 2018,
              <https://www.rfc-editor.org/info/rfc8311>.

9.2.  Informative References

   [ecn-overload]
              Steen, H., "Destruction Testing: Ultra-Low Delay using
              Dual Queue Coupled Active Queue Management", Masters
              Thesis, Uni Oslo , May 2017,
              <https://www.duo.uio.no/bitstream/handle/10852/57424/
              thesis-henrste.pdf?sequence=1>.

   [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, <https://link.springer.com/
              chapter/10.1007/978-3-319-15509-8_15>.



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   [ECN-PLUS]
              Kuzmanovic, A., "The Power of Explicit Congestion
              Notification", ACM SIGCOMM 35(4):61--72, 2005,
              <http://dl.acm.org/citation.cfm?id=1080100>.

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-15 (work
              in progress), October 2018.

   [I-D.ietf-tsvwg-ecn-l4s-id]
              Schepper, K. and B. Briscoe, "Identifying Modified
              Explicit Congestion Notification (ECN) Semantics for
              Ultra-Low Queuing Delay (L4S)", draft-ietf-tsvwg-ecn-l4s-
              id-03 (work in progress), July 2018.

   [I-D.ietf-tsvwg-l4s-arch]
              Briscoe, B., Schepper, K., and M. Bagnulo, "Low Latency,
              Low Loss, Scalable Throughput (L4S) Internet Service:
              Architecture", draft-ietf-tsvwg-l4s-arch-02 (work in
              progress), March 2018.

   [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.

   [judd-nsdi]
              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, <https://www.usenix.org/node/188966>.

   [Kuehlewind18]
              Kuehlewind, M., Walter, M., Learmonth, I., and B.
              Trammell, "Tracing Internet Path Transparency", In Proc:
              Network Traffic Measurement and Analysis Conference (TMA)
              2018 , June 2018, <http://tma.ifip.org/2018/wp-
              content/uploads/sites/3/2018/06/tma2018_paper12.pdf>.

   [Mandalari18]
              Mandalari, A., Lutu, A., Briscoe, B., Bagnulo, M., and Oe.
              Alay, "Measuring ECN++: Good News for ++, Bad News for ECN
              over Mobile", IEEE Communications Magazine , March 2018,
              <https://ieeexplore.ieee.org/document/8316790>.






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   [Manzoor17]
              Manzoor, J., Drago, I., and R. Sadre, "How HTTP/2 is
              changing Web traffic and how to detect it", In Proc:
              Network Traffic Measurement and Analysis Conference (TMA)
              2017 pp.1-9, June 2017,
              <https://ieeexplore.ieee.org/document/8002899>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

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

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/info/rfc4960>.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
              <https://www.rfc-editor.org/info/rfc4987>.

   [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,
              <https://www.rfc-editor.org/info/rfc5562>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

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

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,
              <https://www.rfc-editor.org/info/rfc6298>.



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   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928,
              DOI 10.17487/RFC6928, April 2013,
              <https://www.rfc-editor.org/info/rfc6928>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <https://www.rfc-editor.org/info/rfc7413>.

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

   [RFC7661]  Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating
              TCP to Support Rate-Limited Traffic", RFC 7661,
              DOI 10.17487/RFC7661, October 2015,
              <https://www.rfc-editor.org/info/rfc7661>.

   [RFC8257]  Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
              and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
              Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
              October 2017, <https://www.rfc-editor.org/info/rfc8257>.

   [strict-ecn]
              Dumazet, E., "tcp: be more strict before accepting ECN
              negociation", Linux netdev patch list , May 2012,
              <https://github.com/torvalds/linux/commit/
              bd14b1b2e29bd6812597f896dde06eaf7c6d2f24#diff-
              5c7c60ed5f9efb6bce97ff5233f17282>.

Authors' Addresses

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

   Phone: 34 91 6249500
   Email: marcelo@it.uc3m.es
   URI:   http://www.it.uc3m.es









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   Bob Briscoe
   CableLabs
   UK

   Email: ietf@bobbriscoe.net
   URI:   http://bobbriscoe.net/













































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