Transport Area Working Group                             B. Briscoe, Ed.
Internet-Draft                                                A. Jacquet
Intended status: Historic                                             BT
Expires: January 17, 2014                                   T. Moncaster
                                                                A. Smith
                                                           July 16, 2013

     Re-ECN: Adding Accountability for Causing Congestion to TCP/IP


   This document introduces re-ECN (re-inserted explicit congestion
   notification), which is intended to make a simple but far-reaching
   change to the Internet architecture.  The sender uses the IP header
   to reveal the congestion that it expects on the end-to-end path.  The
   protocol works by arranging an extended ECN field in each packet so
   that, as it crosses any interface in an internetwork, it will carry a
   truthful prediction of congestion on the remainder of its path.  It
   can be deployed incrementally around unmodified routers.  The purpose
   of this document is to specify the re-ECN protocol at the IP layer
   and to give guidelines on any consequent changes required to
   transport protocols.  It includes the changes required to TCP both as
   an example and as a specification.  It briefly gives examples of
   mechanisms that can use the protocol to ensure data sources respond
   sufficiently to congestion, but these are described more fully in a
   companion document.

   Note concerning Intended Status: If this draft were ever published as
   an RFC it would probably have historic status.  There is limited
   space in the IP header, so re-ECN had to compromise by requiring the
   receiver to be ECN-enabled otherwise the sender could not use re-ECN.
   Re-ECN was a precursor to chartering of the IETF's Congestion
   Exposure (ConEx) working group, but during chartering there were
   still too few ECN receivers enabled, therefore it was decided to
   pursue other compromises in order to fit a similar capability into
   the IP header.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute

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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 17, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  6
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Simplified Re-ECN Protocol . . . . . . . . . . . . . . . .  7
       4.1.1.  Congestion Control and Policing the Protocol . . . . .  8
       4.1.2.  Background and Applicability . . . . . . . . . . . . .  8
     4.2.  Re-ECN Abstracted Network Layer Wire Protocol (IPv4 or
           v6)  . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.3.  Re-ECN Protocol Operation  . . . . . . . . . . . . . . . . 11
     4.4.  Positive and Negative Flows  . . . . . . . . . . . . . . . 13
   5.  Network Layer  . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Re-ECN IPv4 Wire Protocol  . . . . . . . . . . . . . . . . 14
     5.2.  Re-ECN IPv6 Wire Protocol  . . . . . . . . . . . . . . . . 16
     5.3.  Router Forwarding Behaviour  . . . . . . . . . . . . . . . 17
     5.4.  Justification for Setting the First SYN to FNE . . . . . . 18
     5.5.  Control and Management . . . . . . . . . . . . . . . . . . 19
       5.5.1.  Negative Balance Warning . . . . . . . . . . . . . . . 19
       5.5.2.  Rate Response Control  . . . . . . . . . . . . . . . . 20
     5.6.  IP in IP Tunnels . . . . . . . . . . . . . . . . . . . . . 20
     5.7.  Non-Issues . . . . . . . . . . . . . . . . . . . . . . . . 21

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   6.  Transport Layers . . . . . . . . . . . . . . . . . . . . . . . 22
     6.1.  TCP  . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
       6.1.1.  RECN mode: Full Re-ECN capable transport . . . . . . . 23
       6.1.2.  RECN-Co mode: Re-ECT Sender with a RFC3168
               compliant ECN Receiver . . . . . . . . . . . . . . . . 25
       6.1.3.  Capability Negotiation . . . . . . . . . . . . . . . . 27
       6.1.4.  Extended ECN (EECN) Field Settings during Flow
               Start or after Idle Periods  . . . . . . . . . . . . . 28
       6.1.5.  Pure ACKS, Retransmissions, Window Probes and
               Partial ACKs . . . . . . . . . . . . . . . . . . . . . 32
     6.2.  Other Transports . . . . . . . . . . . . . . . . . . . . . 33
       6.2.1.  General Guidelines for Adding Re-ECN to Other
               Transports . . . . . . . . . . . . . . . . . . . . . . 33
       6.2.2.  Guidelines for adding Re-ECN to RSVP or NSIS . . . . . 33
       6.2.3.  Guidelines for adding Re-ECN to DCCP . . . . . . . . . 34
       6.2.4.  Guidelines for adding Re-ECN to SCTP . . . . . . . . . 34
   7.  Incremental Deployment . . . . . . . . . . . . . . . . . . . . 34
   8.  Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 35
     8.1.  Congestion Notification Integrity  . . . . . . . . . . . . 36
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 37
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
   11. Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 39
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 39
   13. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 39
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 39
     14.2. Informative References . . . . . . . . . . . . . . . . . . 40
   Appendix A.  Precise Re-ECN Protocol Operation . . . . . . . . . . 42
   Appendix B.  Justification for Two Codepoints Signifying Zero
                Worth Packets . . . . . . . . . . . . . . . . . . . . 44
   Appendix C.  ECN Compatibility . . . . . . . . . . . . . . . . . . 45
   Appendix D.  Packet Marking with FNE During Flow Start . . . . . . 47
   Appendix E.  Argument for holding back the ECN nonce . . . . . . . 49
   Appendix F.  Alternative Terminology Used in Other Documents . . . 51

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Authors' Statement: (to be removed by the RFC Editor)

   The most immediate priority for the authors is to delay any move of
   the ECN nonce to Proposed Standard status, in order to leave options
   open for the future.  The argument for this position is developed in
   Appendix E.

Changes from previous drafts (to be removed by the RFC Editor)

   Full diffs from all previous versions (created using the rfcdiff
   tool) are available at <>

   From draft-briscoe-conex-...-01 to -02 (current version):  Re-issued
      to keep alive; updated references

   From draft-briscoe-conex-...-00 to -01:  Re-issued to keep alive;
      updated references

   From draft-briscoe-tsvwg-...-08 to draft-briscoe-conex-...-00:

      Re-issued to keep alive for reference by ConEx working group

      Changed working group tag in filename from tsvwg to conex

      Changed intended status to historic and added explanatory note

      Updated references.  Also, now that RFC6040 has been published,
      the section on tunnelling required a re-write

      Corrected name of CE(0) to Cancelled in Table 2

      Noted errors and omissions (rather than spending time correcting

      *  Made a few 'ToDo' comments visible that had previously been
         comments within the document source

      *  Identified errors with 'ToDo' comments, referring to correct
         material where possible.

   From -08 to -09:

      Re-issued to keep alive for reference by ConEx working group.

      Hardly any changes to content, even where it is out of date,
      except references updated.

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   From -07 to -08:

      Minor changes and consistency checks.

      References updated.

   From -06 to -07:

      Major changes made following splitting this protocol document from
      the related motivations document [].

      Significant re-ordering of remaining text.

      New terminology introduced for clarity.

      Minor editorial changes throughout.

1.  Introduction

   This document provides a complete specification for the addition of
   the re-ECN protocol to IP and guidelines on how to add it to
   transport layer protocols, including a complete specification of re-
   ECN in TCP as an example.  The motivation behind this proposal is
   given in [], but we include a brief summary here.

   Re-ECN is intended to allow senders to inform the network of the
   level of congestion they expect their flows to see.  This information
   is currently only visible at the transport layer.  ECN [RFC3168]
   reveals the upstream congestion state of any path by monitoring the
   rate of CE marks.  The receiver then informs the sender when they
   have seen a marked packet.  Re-ECN builds on ECN by providing new
   codepoints that allow the sender to declare the level of congestion
   they expect on the forward path.  It is closely related to ECN and
   indeed we define a compatibility mode to allow a re-ECN sender to
   communicate with an ECN receiver.

   If a sender understates expected congestion compared to actual
   congestion then the network could discard packets or enact some other
   sanction.  A policer can also be introduced at the ingress of
   networks that can limit the level of congestion being caused.

   A general statement of the problem solved by re-ECN is to provide
   sufficient information in each IP datagram to be able to hold senders
   and whole networks accountable for the congestion they cause
   downstream, before they cause it.  But the every-day problems that
   re-ECN can solve are much more recognisable than this rather generic
   statement: mitigating distributed denial of service (DDoS);
   simplifying differentiation of quality of service (QoS); policing

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   compliance to congestion control; and so on.

   It is important to add a few key points.

   o  In any standard network it always takes one round trip before any
      feedback is received.  For this reason a sender must make a
      conservative prediction by transmitting IP packets with a special
      Cautious marking when it is unsure of the state of the network.

   o  It should be noted that the prediction is carried in-band in
      normal data packets and for many transports feedback can be
      carried in the normal acknowledgements or control packets.

   o  The re-ECN protocol is independent of the transport.  In TCP,
      acknowledgments are used to convey the feedback from receiver to
      sender.  This memo concentrates on TCP as an example transport
      protocol, however the re-ECN protocol is compatible with any
      transport where feedback can be sent from receiver to sender.

   This document is structured as follows.  First an overview of the re-
   ECN protocol is given (Section 4), outlining its attributes and
   explaining conceptually how it works as a whole.  The two main parts
   of the document follow.  That is, the protocol specification divided
   into network (Section 5) and transport (Section 6) layers.
   Deployment issues discussed throughout the document are brought
   together in Section 7.  Related work is discussed in (Section 8).

2.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Terminology

   {ToDo: No attempt has been made to bring terminology into line with
   that agreed within the ConEx working group.  For instance the term
   dropper remains unchanged, even though the ConEx w-g has decided to
   call it an audit function (which is actually a much better term).}

   The following terminology is used throughout this memo.  Some of this
   terminology has changed as this draft has been revised.  Therefore,
   to help avoid confusion, Appendix F sets out all the alternative
   terminology that has been used in other re-ECN related documents.

   o  Neutral packet - a packet that is able to be congestion marked by
      an ECN or re-ECN queue.

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   o  Negative packet - a Neutral packet that has been congestion marked
      by an ECN or re-ECN queue.

   o  Positive packet - a packet that has been marked by the sender to
      indicate the expected level of congestion along its path.  In
      general Positive packets should only be sent in response to
      feedback received from the receiver.*

   o  Cancelled packet - a Positive Packet that has been congestion
      marked by an ECN or re-ECN queue.

   o  Cautious packet - a packet that has been marked by the sender to
      indicate the expected level of congestion along its path.  In
      general Cautious packets should be used when there is insufficient
      feedback to be confident about the congestion state of the

      * the difference between positive and cautious packets is
      explained in detail later in the document along with guidelines on
      the use of Cautious packets.

   All the above terms have related IP codepoints as defined in
   (Section 5).

4.  Protocol Overview

4.1.  Simplified Re-ECN Protocol

   We describe here the simplified re-ECN protocol.  To simplify the
   description we assume packets and segments are synonymous.

   Packets are sent from a sender to a receiver.  In Figure 1 the queues
   (Q1 and Q2) are ECN enabled as per RFC 3168 [RFC3168].  If congestion
   occurs then packets are marked with the congestion experienced (CE)
   flag exactly as in the ECN protocol [RFC3168]; the routers do not
   need to be modified and do not need to know the re-ECN protocol.  The
   receiver constantly informs the sender of the current count of
   Negative packets it has seen.  The sender uses this information
   determine how many Positive packets it must send into the network.
   The receiver's aim is to balance the number of bytes that have been
   congestion marked with the number of Positive bytes it has sent.

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          +--------- Feedback----------+
          |                            |
          v                            |
        +---+    +----+    +----+    +---+
        |   |    |    |    |    |    |   |
        | S |--->| Q1 |--->| Q2 |--->| R |
        |   |    |    |    |    |    |   |
        +---+    +----+    +----+    +---+

                          Figure 1: Simple Re-ECN

4.1.1.  Congestion Control and Policing the Protocol

   The arrangement of the protocol ensures that packets carry a
   declaration of the amount of congestion that will be experienced on
   the path.  The re-ECN protocol is orthogonal to any congestion
   control algorithms, but can be used to ensure that congestion control
   is being applied by the sender.

   In general we assume that there will be a policer at the network
   ingress which can rate limit traffic based on the amount of
   congestion declared.

   At the network egress there is a dropper which can impose sanctions
   on flows that incorrectly declare congestion.

   Policers and droppers are explained in more detail in

4.1.2.  Background and Applicability

   The re-ECN protocol makes no changes and has no effect on the TCP
   congestion control algorithm or on other rate responses to
   congestion.  Re-ECN is not a new congestion control protocol, rather
   it is orthogonal to congestion control itself.  Re-ECN is concerned
   with revealing information about congestion so that users and
   networks can be held accountable for the congestion they cause, or
   allow to be caused.

   Re-ECN builds on ECN so we briefly recap the essentials of the ECN
   protocol [RFC3168].  Two bits in the IP protocol (v4 or v6) are
   assigned to the ECN field.  The sender clears the field to "00" (Not-
   ECT) if either end-point transport is not ECN-capable.  Otherwise it
   indicates an ECN-capable transport (ECT) using either of the two
   code-points "10" or "01" (ECT(0) and ECT(1) resp.).

   ECN-capable queues probabilistically set this field to "11" if

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   congestion is experienced (CE).  In general this marking probability
   will increase with the length of the queue at its egress link
   (typically using the RED algorithm [RFC2309]).  However, they still
   drop rather than mark Not-ECT packets.  With multiple ECN-capable
   queues on a path, a flow of packets accumulates the fraction of CE
   marking that each queue adds.  The combined effect of the packet
   marking of all the queues along the path signals congestion of the
   whole path to the receiver.  So, for example, if one queue early in a
   path is marking 1% of packets and another later in a path is marking
   2%, flows that pass through both queues will experience approximately
   3% marking (see Appendix A for a precise treatment).

   The choice of two ECT code-points in the ECN field [RFC3168]
   permitted future flexibility, optionally allowing the sender to
   encode the experimental ECN nonce [RFC3540] in the packet stream.
   The nonce is designed to allow a sender to check the integrity of
   congestion feedback.  But Section 8.1 explains that it still gives no
   control over how fast the sender transmits as a result of the
   feedback.  On the other hand, re-ECN is designed both to ensure that
   congestion is declared honestly and that the sender's rate responds

   Re-ECN is based on a feedback arrangement called `re-
   feedback' [Re-fb].  The word is short for either receiver-aligned,
   re-inserted or re-echoed feedback.  But it actually works even when
   no feedback is available.  In fact it has been carefully designed to
   work for single datagram flows.  It also encourages aggregation of
   single packet flows by congestion control proxies.  Then, even if the
   traffic mix of the Internet were to become dominated by short
   messages, it would still be possible to control congestion
   effectively and efficiently.

   Changing the Internet's feedback architecture seems to imply
   considerable upheaval.  But re-ECN can be deployed incrementally at
   the transport layer around unmodified queues using existing fields in
   IP (v4 or v6).  However it does also require the last undefined bit
   in the IPv4 header, which it uses in combination with the 2-bit ECN
   field to create four new codepoints.  Nonetheless, we RECOMMEND
   adding optional preferential drop to IP queues based on the re-ECN
   fields in order to improve resilience against DoS attacks.
   Similarly, re-ECN works best if both the sender and receiver
   transports are re-ECN-capable, but it can work with just sender
   support(Section 6.1.2).

4.2.  Re-ECN Abstracted Network Layer Wire Protocol (IPv4 or v6)

   The re-ECN wire protocol uses the two bit ECN field broadly as in
   RFC3168 [RFC3168] as described above, but with five differences of

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   detail (brought together in a list in Section 7).  This specification
   defines a new re-ECN extension (RE) flag.  We will defer the
   definition of the actual position of the RE flag in the IPv4 & v6
   headers until Section 5.  When we don't need to choose between IPv4
   and v6 wire protocols it will suffice call it the RE flag.

   Unlike the ECN field, the RE flag is intended to be set by the sender
   and SHOULD remain unchanged along the path, although it can be read
   by network elements that understand the re-ECN protocol.  It is
   feasible that a network element MAY change the setting of the RE
   flag, perhaps acting as a proxy for an end-point, but such a protocol
   would have to be defined in another specification
   (e.g. []).

   Although the RE flag is a separate, single bit field, it can be read
   as an extension to the two-bit ECN field; the three concatenated bits
   in what we will call the extended ECN field (EECN) giving eight
   codepoints.  We will use the RFC3168 names of the ECN codepoints to
   describe settings of the ECN field when the RE flag setting is "don't
   care", but we also define the following six extended ECN codepoint
   names for when we need to be more specific.

   One of re-ECN's codepoints is an alternative use of the codepoint set
   aside in RFC3168 for the ECN nonce (ECT(1)).  Transports using re-ECN
   do not need to use the ECN nonce as long as the sender is also
   checking for transport protocol compliance [tcp-rcv-cheat].  The case
   for doing this is given in Appendix E.  Two re-ECN codepoints are
   given compatible uses to those defined in RFC3168 (Not-ECT and CE).
   The other codepoint used by RFC3168 (ECT(0)) isn't used for re-ECN.
   Altogether this leave one codepoint of the eight unused by ECN or re-
   ECN and available for future use.

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   |   ECN  |   RFC3168   |   RE  |    EECN   |     re-ECN meaning     |
   |  field |  codepoint  |  flag | codepoint |                        |
   |   00   |   Not-ECT   |   0   |  Not-ECT  |   Not re-ECN-capable   |
   |        |             |       |           |   transport (Legacy)   |
   |   00   |     ---     |   1   |    FNE    |      Feedback not      |
   |        |             |       |           | established (Cautious) |
   |   01   |    ECT(1)   |   0   |  Re-Echo  |  Re-echoed congestion  |
   |        |             |       |           |   and RECT (Positive)  |
   |   01   |     ---     |   1   |    RECT   |     Re-ECN capable     |
   |        |             |       |           |   transport (Neutral)  |
   |   10   |    ECT(0)   |   0   |   ECT(0)  |  RFC3168 ECN use only  |
   |        |             |       |           |                        |
   |   10   |     ---     |   1   |   --CU--  |    Currently unused    |
   |        |             |       |           |                        |
   |   11   |      CE     |   0   |   CE(0)   |  Re-Echo cancelled by  |
   |        |             |       |           |     CE (Cancelled)     |
   |   11   |     ---     |   1   |   CE(-1)  | Congestion Experienced |
   |        |             |       |           |       (Negative)       |

                     Table 1: Extended ECN Codepoints

4.3.  Re-ECN Protocol Operation

   In this section we will give an overview of the operation of the re-
   ECN protocol for TCP/IP, leaving a detailed specification to the
   following sections.  Other transports will be discussed later.

   {ToDo: This section to be updated to explain that the sender re-
   echoes losses in the same way as ECN markings.}

   In summary, the protocol adds a third `re-echo' stage to the existing
   TCP/IP ECN protocol.  Whenever the network adds CE congestion
   signalling to the IP header on the forward data path, the receiver
   feeds it back to the ingress using TCP, then the sender re-echoes it
   into the forward data path using the RE flag in the next packet.

   Prior to receiving any feedback a sender will not know which setting
   of the RE flag to use, so it sends Cautious packets by setting the
   FNE codepoint.  The network reads the FNE codepoint conservatively as
   equivalent to re-echoed congestion.

   Specifically, once feedback from an ECN or re-ECN capable flow is
   established, a re-ECN sender always initialises the ECN field to
   ECT(1).  And it usually sets the RE flag to "1" indicating a Neutral
   packet.  Whenever a queue marks a packet to CE, the receiver feeds

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   back this event to the sender.  On receiving this feedback, the re-
   ECN sender will clear the RE flag to "0" in the next packet it sends
   (indicating a Positive packet).

   We chose to set and clear the RE flag this way round to ease
   incremental deployment (see Section 7).  To avoid confusion we will
   use the term `blanking' (rather than marking) when the RE flag is
   cleared to "0".  So, over a stream of packets, we will talk of the
   `RE blanking fraction' as the fraction of octets in packets with the
   RE flag cleared to "0".

       +---+  +----+                +----+  +---+
       | S |--| Q1 |----------------| Q2 |--| R |
       +---+  +----+                +----+  +---+
         .      .                      .      .
       ^ .      .                      .      .
       | .      .                      .      .
       | .     RE blanking fraction    .      .
    3% |-------------------------------+=======
       | .      .                      |      .
    2% | .      .                      |      .
       | .      .  CE marking fraction |      .
    1% | .      +----------------------+      .
       | .      |                      .      .
    0% +--------------------------------------->
         ^          ^                      ^
         L          M                      N    Observation points

                  Figure 2: A 2-Queue Example (Imprecise)

   Figure 2 uses a simple network to illustrate how re-ECN allows queues
   to measure downstream congestion.  The receiver views a CE marking
   fraction of 3% which is fed back to the sender.  The sender sets an
   RE blanking fraction of 3% to match this.  This RE blanking fraction
   can be observed along the path as the RE flag is not changed by
   network nodes once set by the sender.  This is shown by the
   horizontal line at 3% in the figure.  The CE marked fraction is shown
   by the stepped line which rises to meet the RE blanking fraction line
   with steps at each queue where packets are marked.  Two queues are
   shown (Q1 and Q2) that are currently congested.  Each time packets
   pass through a fraction are marked; 1% at Q1 and 2% at Q2).  The
   approximate downstream congestion can be measured at the observation
   points shown along the path by subtracting the CE marking fraction
   from the RE blanking fraction, as shown in the table below
   (Appendix A derives these approximations from a precise analysis).
   NB due to the unary nature of ECN marking and the equivalent unary

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   nature of re-ECN blanking, the precise fraction of marked bytes must
   be calculated by maintaining a moving average of the number of
   packets that have been marked as a proportion of the total number of

   Along the path the fraction of packets that had their RE field
   cleared remains unchanged so it can be used as a reference against
   which to compare upstream congestion.  The difference predicts
   downstream congestion for the rest of the path.  Therefore, measuring
   the fractions of each codepoint at any point in the Internet will
   reveal upstream, downstream and whole path congestion.

   Note that we have introduced discussion of marking and blanking
   fractions solely for illustration.  We are not saying any protocol
   handler will work with these average fractions directly.  In fact the
   protocol actually requires the number of marked and blanked bytes to
   balance by the time the packet reaches the receiver.

4.4.  Positive and Negative Flows

   In Section 3 we introduced the terms Positive, Neutral, Negative,
   Cautious and Cancelled.  This terminology is based on the requirement
   to balance the proportion of bytes marked as CE with the proportion
   of bytes that are re-echo marked.  In the rest of this memo we will
   loosely talk of positive or negative flows, meaning flows where the
   moving average of the downstream congestion metric is persistently
   positive or negative.  A negative flow is one where more CE marked
   packets than re-ECN blanked packets arrive.  Likewise in positive
   flows more re-ECN blanked packets arrive than CE marked packets.  The
   notion of a negative metric arises because it is derived by
   subtracting one metric from another.  Of course actual downstream
   congestion cannot be negative, only the metric can (whether due to
   time lags or deliberate malice).

   Therefore we will talk of packets having `worth' of +1, 0 or -1,
   which, when multiplied by their size, indicates their contribution to
   the downstream congestion metric.  The worth of each type of packet
   is given below in Table 2.  The idea is that most flows start with
   zero worth.  Every time the network decrements the worth of a packet,
   the sender increments the worth of a later packet.  Then, over time,
   as many positive octets should arrive at the receiver as negative.
   Note we have said octets not packets, so if packets are of different
   sizes, the worth should be incremented on enough octets to balance
   the octets in negative packets arriving at the receiver.  It is this
   balance that will allow the network to hold the sender accountable
   for the congestion it causes.

   If a packet carrying re-echoed congestion happens to also be

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   congestion marked, the +1 worth added by the sender will be cancelled
   out by the -1 network congestion marking.  Although the two worth
   values correctly cancel out, neither the congestion marking nor the
   re-echoed congestion are lost, because the RE bit and the ECN field
   are orthogonal.  So, whenever this happens, the receiver will
   correctly detect and re-echo the new congestion event as well.

   The table below specifies unambiguously the worth of each extended
   ECN codepoint.  Note the order is different from the previous table
   to better show how the worth increments and decrements.

   |   ECN   |   RE  | Extended ECN  | Worth |       Re-ECN Term       |
   |  field  |  bit  | codepoint     |       |                         |
   |    00   |   0   | Not-RECT      | ...   |           ---           |
   |    00   |   1   | FNE           | +1    |         Cautious        |
   |    01   |   0   | Re-Echo       | +1    |         Positive        |
   |    10   |   0   | Legacy        | ...   |   RFC3168 ECN use only  |
   |         |       |               |       |                         |
   |    11   |   0   | CE(0)         |  0    |        Cancelled        |
   |    01   |   1   | RECT          |  0    |         Neutral         |
   |    10   |   1   | --CU--        | ...   |     Currently unused    |
   |         |       |               |       |                         |
   |    11   |   1   | CE(-1)        | -1    |         Negative        |

                Table 2: 'Worth' of Extended ECN Codepoints

5.  Network Layer

5.1.  Re-ECN IPv4 Wire Protocol

   The wire protocol of the ECN field in the IP header remains largely
   unchanged from [RFC3168].  However, an extension to the ECN field we
   call the RE (Re-ECN extension) flag (Section 4.2) is defined in this
   document.  It doubles the extended ECN codepoint space, giving 8
   potential codepoints.  The semantics of the extra codepoints are
   backward compatible with the semantics of the 4 original codepoints
   [RFC3168] (Section 7 collects together and summarises all the changes
   defined in this document).

   For IPv4, this document proposes that the new RE control flag will be
   positioned where the `reserved' control flag was at bit 48 of the
   IPv4 header (counting from 0).  Alternatively, some would call this
   bit 0 (counting from 0) of byte 7 (counting from 1) of the IPv4
   header (Figure 3).

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             0   1   2
           | R | D | M |
           | E | F | F |

   Figure 3: New Definition of the Re-ECN Extension (RE) Control Flag at
                  the Start of Byte 7 of the IPv4 Header

   The semantics of the RE flag are described in outline in Section 4
   and specified fully in Section 6.  The RE flag is always considered
   in conjunction with the 2-bit ECN field, as if they were concatenated
   together to form a 3-bit extended ECN field.  If the ECN field is set
   to either the ECT(1) or CE codepoint, when the RE flag is blanked
   (cleared to "0") it represents a re-echo of congestion experienced by
   an early packet.  If the ECN field is set to the Not-ECT codepoint,
   when the RE flag is set to "1" it represents the feedback not
   established (FNE) codepoint, which signals that the packet was sent
   without the benefit of congestion feedback.

   It is believed that the FNE codepoint can simultaneously serve other
   purposes, particularly where the start of a flow needs distinguishing
   from packets later in the flow.  For instance it would have been
   useful to identify new flows for tag switching and might enable
   similar developments in the future if it were adopted.  It is similar
   to the state set-up bit idea designed to protect against memory
   exhaustion attacks.  This idea was proposed informally by David Clark
   and documented by Handley and Greenhalgh  [Steps_DoS].  The FNE
   codepoint can be thought of as a `soft-state set-up flag', because it
   is idempotent (i.e. one occurrence of the flag is sufficient but
   further occurrences achieve the same effect if previous ones were

   We are sure there will probably be other claims pending on the use of
   bit 48.  We know of at least two  [ARI05], [RFC3514] but neither have
   been pursued in the IETF, so far, although the present proposal would
   meet the needs of the latter.

   The security flag proposal (commonly known as the evil bit) was
   published on 1 April 2003 as Informational RFC 3514, but it was not
   adopted due to confusion over whether evil-doers might set it
   inappropriately.  The present proposal is backward compatible with
   RFC3514 because if re-ECN compliant senders were benign they would
   correctly clear the evil bit to honestly declare that they had just
   received congestion feedback.  Whereas evil-doers would hide
   congestion feedback by setting the evil bit continuously, or at least
   more often than they should.  So, evil senders can be identified,
   because they declare that they are good less often than they should.

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5.2.  Re-ECN IPv6 Wire Protocol

   For IPv6, this document proposes that the new RE control flag will be
   positioned as the first bit of the option field of a new Congestion
   hop by hop option header (Figure 4).

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |  Next Header  |  Hdr ext Len  |  Option Type  | Opt Length =4 |
       |R|                     Reserved for future use                 |
       |E|                                                             |

      Figure 4: Definition of a New IPv6 Congestion Hop by Hop Option
         Header containing the re-ECN Extension (RE) Control Flag

               0 1 2 3 4 5 6 7 8
               |AIU|C|Option ID|

           Figure 5: Congestion Hop by Hop Option Type Encoding

   The Hop-by-Hop Options header enables packets to carry information to
   be examined and processed by routers or nodes along the packet's
   delivery path, including the source and destination nodes.  For re-
   ECN, the two bits of the Action If Unrecognized (AIU) flag of the
   Congestion extension header MUST be set to "00" meaning if
   unrecognized `skip over option and continue processing the header'.
   Then, any routers or a receiver not upgraded with the optional re-ECN
   features described in this memo will simply ignore this header.  But
   routers with these optional re-ECN features or a re-ECN policing
   function, will process this Congestion extension header.

   The `C' flag MUST be set to "1" to specify that the Option Data
   (currently only the RE control flag) can change en-route to the
   packet's final destination.  This ensures that, when an
   Authentication header (AH [RFC4302]) is present in the packet, for
   any option whose data may change en-route, its entire Option Data
   field will be treated as zero-valued octets when computing or
   verifying the packet's authenticating value.

   Although the RE control flag should not be changed along the path, we
   expect that the rest of this option field that is currently `Reserved
   for future use' could be used for a multi-bit congestion notification

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   field which we would expect to change en route.  Therefore, as
   changes to the RE flag could be detected end-to-end without
   authentication (see Section 9), we set the C flag to '1'.

5.3.  Router Forwarding Behaviour

   {ToDo: Consider a section on how whole protocol interworks with drop.
   Perhaps in Protocol Overview.}

   Re-ECN works well without modifying the forwarding behaviour of any
   routers.  However, below, two OPTIONAL changes to forwarding
   behaviour are defined which respectively enhance performance and
   improve a router's discrimination against flooding attacks.  They are
   both OPTIONAL additions that we propose MAY apply by default to all
   Diffserv per-hop scheduling behaviours (PHBs) [RFC2475] and ECN
   marking behaviours [RFC3168].  Specifications for PHBs MAY define
   different forwarding behaviours from this default, but this is not
   required.  [] is one example.

   FNE indicates ECT:

      The FNE codepoint tells a router to assume that the packet was
      sent by an ECN-capable transport (see Section 5.4).  Therefore an
      FNE packet MAY be marked rather than dropped.  Note that the FNE
      codepoint has been intentionally chosen so that, to RFC3168
      compliant routers (which do not inspect the RE flag) an FNE packet
      appears to be Not-ECT so it will be dropped by legacy AQM

      A network operator MUST NOT configure a queue to ECN mark rather
      than drop FNE packets unless it can guarantee that FNE packets
      will be rate limited, either locally or upstream.  The ingress
      policers discussed in [] would count as rate
      limiters for this purpose.

   Preferential Drop:  If a re-ECN capable router queue experiences very
      high load so that it has to drop arriving packets (e.g. a DoS
      attack), it MAY preferentially drop packets within the same
      Diffserv PHB using the preference order for extended ECN
      codepoints given in Table 3.  Preferential dropping can be
      difficult to implement on some hardware, but if feasible it would
      discriminate against attack traffic if done as part of the overall
      policing framework of [].  If nowhere else,
      routers at the egress of a network SHOULD implement preferential
      drop (stronger than the MAY above).  For simplicity, preferences 4
      & 5 MAY be merged into one preference level.

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      The tabulated drop preferences are arranged to preserve packets
      with more positive worth (Section 4.4), given senders of positive
      packets must have honestly declared downstream congestion.  A full
      treatment of this is provided in the companion document describing
      the motivation and architecture for re-ECN []
      particularly when the application of re-ECN to protect against
      DDoS attacks is described.

   |  ECN  |  RE | Extended   | Worth | Drop Pref  |   Re-ECN meaning  |
   | field | bit | ECN        |       | (1 = drop  |                   |
   |       |     | codepoint  |       | 1st)       |                   |
   |   01  |  0  | Re-Echo    | +1    | 5/4        |     Re-echoed     |
   |       |     |            |       |            |   congestion and  |
   |       |     |            |       |            |        RECT       |
   |   00  |  1  | FNE        | +1    | 4          |    Feedback not   |
   |       |     |            |       |            |    established    |
   |   11  |  0  | CE(0)      | 0     | 3          |  Re-Echo canceled |
   |       |     |            |       |            |   by congestion   |
   |       |     |            |       |            |    experienced    |
   |   01  |  1  | RECT       | 0     | 3          |   Re-ECN capable  |
   |       |     |            |       |            |     transport     |
   |   11  |  1  | CE(-1)     | -1    | 3          |     Congestion    |
   |       |     |            |       |            |    experienced    |
   |   10  |  1  | --CU--     | n/a   | 2          |  Currently Unused |
   |   10  |  0  | ---        | n/a   | 2          |  RFC3168 ECN use  |
   |       |     |            |       |            |        only       |
   |   00  |  0  | Not-RECT   | n/a   | 1          |        Not        |
   |       |     |            |       |            |   Re-ECN-capable  |
   |       |     |            |       |            |     transport     |

      Table 3: Drop Preference of EECN Codepoints (Sorted by `Worth')

5.4.  Justification for Setting the First SYN to FNE

   the initial SYN MUST be set to FNE by Re-ECT client A (Section 6.1.4)
   and (Section 5.3) says a queue MAY optionally treat an FNE packet as
   ECN capable, so an initial SYN may be marked CE(-1) rather than
   dropped.  This seems dangerous, because the sender has not yet
   established whether the receiver is a RFC3168 one that does not
   understand congestion marking.  It also seems to allow malicious
   senders to take advantage of ECN marking to avoid so much drop when
   launching SYN flooding attacks.  Below we explain the features of the
   protocol design that remove both these dangers.

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   ECN-capable initial SYN with a Not-ECT server:  If the TCP server B
      is re-ECN capable, provision is made for it to feedback a possible
      congestion marked SYN in the SYN ACK (Section 6.1.4).  But if the
      TCP client A finds out from the SYN ACK that the server was not
      ECN-capable, the TCP client MUST conservatively consider the first
      SYN as congestion marked before setting itself into Not-ECT mode.
      Section 6.1.4 mandates that such a TCP client MUST also set its
      initial window to 1 segment.  In this way we remove the need to
      cautiously avoid setting the first SYN to Not-RECT.  This will
      give worse performance while deployment is patchy, but better
      performance once deployment is widespread.

   SYN flooding attacks can't exploit ECN-capability:  Malicious hosts
      may think they can use the advantage that ECN-marking gives over
      drop in launching classic SYN-flood attacks.  But Section 5.3
      mandates that a router MUST only be configured to treat packets
      with the FNE codepoint as ECN-capable if FNE packets are rate
      limited somewhere.  Introduction of the FNE codepoint was a
      deliberate move to enable transport-neutral handling of flow-start
      and flow state set-up in the IP layer where it belongs.  It then
      becomes possible to protect against flooding attacks of all forms
      (not just SYN flooding) without transport-specific inspection for
      things like the SYN flag in TCP headers.  Then, for instance, SYN
      flooding attacks using IPsec ESP encryption can also be rate
      limited at the IP layer.

   It might seem pedantic going to all this trouble to enable ECN on the
   initial packet of a flow, but it is motivated by a much wider concern
   to ensure safe congestion control will still be possible even if the
   application mix evolves to the point where the majority of flows
   consist of a single window or even a single packet.  It also allows
   denial of service attacks to be more easily isolated and prevented.

   {ToDo: Give alternative where initial packet is Not-RECT and last ACK
   of three-way handshake is FNE.  Explain this will give better
   performance while deployment is patchy, but worse performance once
   deployment is high.}

5.5.  Control and Management

5.5.1.  Negative Balance Warning

   A new ICMP message type is being considered so that a dropper can
   warn the apparent sender of a flow that it has started to sanction
   the flow.  The message would have similar semantics to the `Time
   exceeded' ICMP message type.  To ensure the sender has to invest some
   work before the network will generate such a message, a dropper
   SHOULD only send such a message for flows that have demonstrated that

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   they have started correctly by establishing a positive record, but
   have later gone negative.  The threshold is up to the implementation.
   The purpose of the message is to deconfuse the cause of drops from
   other causes, such as congestion or transmission losses.  The dropper
   would send the message to the sender of the flow, not the receiver.
   If we did define this message type, it would be REQUIRED for all re-
   ECT senders to parse and understand it.  Note that a sender MUST only
   use this message to explain why losses are occurring.  A sender MUST
   NOT take this message to mean that losses have occurred that it was
   not aware of.  Otherwise, spoof messages could be sent by malicious
   sources to slow down a sender (c.f.  ICMP source quench).

   However, the need for this message type is not yet confirmed, as we
   are considering how to prevent it being used by malicious senders to
   scan for droppers and to test their threshold settings. {ToDo:
   Complete this section.}

5.5.2.  Rate Response Control

   As discussed in [] the sender's access operator will
   be expected to use bulk per-user policing, but they might choose to
   introduce a per-flow policer.  In cases where operators do introduce
   per-flow policing, there may be a need for a sender to send a request
   to the ingress policer asking for permission to apply a non-default
   response to congestion (where TCP-friendly is assumed to be the
   default).  This would require the sender to know what message
   format(s) to use and to be able to discover how to address the
   policer.  The required control protocol(s) are outside the scope of
   this document, but will require definition elsewhere.

   The policer is likely to be local to the sender and inline, probably
   at the ingress interface to the internetwork.  So, discovery should
   not be hard.  A variety of control protocols already exist for some
   widely used rate-responses to congestion.  For instance DCCP
   congestion control identifiers (CCIDs [RFC4340]) fulfil this role and
   so does QoS signalling (e.g. and RSVP request for controlled load
   service is equivalent to a request for no rate response to
   congestion, but with admission control).

5.6.  IP in IP Tunnels

   Ideally, for re-ECN to work through IP in IP tunnels, the tunnel
   entry should copy both the RE flag and the ECN field from the inner
   to the outer IP header.  Then at the tunnel exit, any CE marking of
   the outer ECN field should overwrite the inner ECN field (unless the
   inner field is Not-ECT in which case an alarm should be raised).  The
   RE flag shouldn't change along a path, so the outer RE flag should be
   the same as the inner.  If it isn't, a management alarm should be

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   This requirement is satisfied by the latest specification for
   handling ECN through IP tunnels [RFC6040] as well as by IPsec
   [RFC4301].  However, it is not satisfied by the ingress behaviour
   specified in [RFC3168] although at least the full-functionality
   variant of the egress behaviour is fine.  RFC6040 updates RFC3168,
   but it is likely that many legacy non-IPsec IP-in-IP tunnels will

   If legacy tunnels are left as specified in [RFC3168], whether the
   limited or full-functionality variants is used, a problem arises with
   re-ECN if a tunnel crosses an inter-domain boundary, because the
   difference between positive and negative markings will not be
   correctly accounted for.  In a limited functionality ECN tunnel, the
   flow will appear to be RFC3168 compliant traffic, and therefore may
   be wrongly rate limited.  In a full-functionality ECN tunnel, the
   result will depend whether the tunnel entry copies the inner RE flag
   to the outer header or the RE flag in the outer header is always
   cleared.  If the former, the flow will tend to be too positive when
   accounted for at borders.  If the latter, it will be too negative.
   If the rules set out in [RFC6040] are followed then this will not be
   an issue.

5.7.  Non-Issues

   The following issues might seem to cause unfavourable interactions
   with re-ECN, but we will explain why they don't:

   o  Various link layers support explicit congestion notification, such
      as Frame Relay and ATM.  Explicit congestion notification is
      proposed to be added to other link layers, such as Ethernet
      (802.3ar Ethernet congestion management) and MPLS [RFC5129];

   o  Encryption and IPsec.

   In the case of congestion notification at the link layer, each
   particular link layer scheme either manages congestion on the link
   with its own link-level feedback (the usual arrangement in the cases
   of ATM and Frame Relay), or congestion notification from the link
   layer is merged into congestion notification at the IP level when the
   frame headers are decapsulated at the end of the link (the
   recommended arrangement in the Ethernet and MPLS cases).  Given the
   RE flag is not intended to change along the path, this means that
   downstream congestion will still be measurable at any point where IP
   is processed on the path by subtracting positive from negative

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   In the case of encryption, as long as the tunnel issues described in
   Section 5.6 are dealt with, payload encryption itself will not be a
   problem.  The design goal of re-ECN is to include downstream
   congestion in the IP header so that it is not necessary to bury into
   inner headers.  Obfuscation of flow identifiers is not a problem for
   re-ECN policing elements.  Re-ECN doesn't ever require flow
   identifiers to be valid, it only requires them to be unique.  So if
   an IPsec encapsulating security payload (ESP [RFC4835]) or an
   authentication header (AH [RFC4302]) is used, the security parameters
   index (SPI) will be a sufficient flow identifier, as it is intended
   to be unique to a flow without revealing actual port numbers.

   In general, even if endpoints use some locally agreed scheme to hide
   port numbers, re-ECN policing elements can just consider the pair of
   source and destination IP addresses as the flow identifier.  Re-ECN
   encourages endpoints to at least tell the network layer that a
   sequence of packets are all part of the same flow, if indeed they
   are.  The alternative would be for the sender to make each packet
   appear to be a new flow, which would require them all to be marked
   FNE in order to avoid being treated with the bulk of malicious flows
   at the egress dropper.  Given the FNE marking is worth +1 and
   networks are likely to rate limit FNE packets, endpoints are given an
   incentive not to set FNE on each packet.  But if the sender really
   does want to hide the flow relationship between packets it can choose
   to pay the cost of multiple FNE packets, which in the long run will
   compensate for the extra memory required on network policing elements
   to process each flow.

   {ToDo: Add a note about it being useful that the AH header does not
   cover the RE flag, referring to Section 9.}

6.  Transport Layers

6.1.  TCP

   Re-ECN capability at the sender is essential.  At the receiver it is
   optional, as long as the receiver has a basic RFC3168-compliant ECN-
   capable transport (ECT) [RFC3168].  Given re-ECN is not the first
   attempt to define the semantics of the ECN field, we give a table
   below summarising what happens for various combinations of
   capabilities of the sender S and receiver R, as indicated in the
   first four columns below.  The last column gives the mode a half-
   connection should be in after the first two of the three TCP

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   | Re-ECT |   ECT-Nonce  |     ECT    | Not-ECT |         S-R        |
   |        |   (RFC3540)  |  (RFC3168) |         |   Half-connection  |
   |        |              |            |         |        Mode        |
   |   SR   |              |            |         |        RECN        |
   |    S   |       R      |            |         |       RECN-Co      |
   |    S   |              |      R     |         |       RECN-Co      |
   |    S   |              |            |    R    |       Not-ECT      |

       Table 4: Modes of TCP Half-connection for Combinations of ECN
                  Capabilities of Sender S and Receiver R

   We will describe what happens in each mode, then describe how they
   are negotiated.  The abbreviations for the modes in the above table

   RECN:  Full re-ECN capable transport

   RECN-Co:  Re-ECN sender in compatibility mode with a RFC3168
      compliant [RFC3168] ECN receiver or an [RFC3540] ECN nonce-capable
      receiver.  Implementation of this mode is OPTIONAL.

   Not-ECT:  Not ECN-capable transport, as defined in [RFC3168] for when
      at least one of the transports does not understand even basic ECN

   Note that we use the term Re-ECT for a host transport that is re-ECN-
   capable but RECN for the modes of the half connections between hosts
   when they are both Re-ECT.  If a host transport is Re-ECT, this fact
   alone does NOT imply either of its half connections will necessarily
   be in RECN mode, at least not until it has confirmed that the other
   host is Re-ECT.

6.1.1.  RECN mode: Full Re-ECN capable transport

   In full RECN mode, for each half connection, both the sender and the
   receiver each maintain an unsigned integer counter we will call ECC
   (echo congestion counter).  The receiver maintains a count of how
   many times a CE marked packet has arrived during the half-connection.
   Once a RECN connection is established, the three TCP option flags
   (ECE, CWR & NS) used for ECN-related functions in other versions of
   ECN are used as a 3-bit field for the receiver to repeatedly tell the
   sender the current value of ECC, modulo 8, whenever it sends a TCP
   ACK.  We will call this the echo congestion increment (ECI) field.
   This overloaded use of these 3 option flags as one 3-bit ECI field is
   shown in Figure 7.  The actual definition of the TCP header,

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   including the addition of support for the ECN nonce, is shown for
   comparison in Figure 6.  This specification does not redefine the
   names of these three TCP option flags, it merely overloads them with
   another definition once a flow is established.

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

    Figure 6: The (post-ECN Nonce) definition of bytes 13 and 14 of the
                                TCP Header

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

    Figure 7: Definition of the ECI field within bytes 13 and 14 of the
   TCP Header, overloading the current definitions above for established
                                RECN flows.

   Receiver Action in RECN Mode

      Every time a CE marked packet arrives at a receiver in RECN mode,
      the receiver transport increments its local value of ECC and MUST
      echo its value, modulo 8, to the sender in the ECI field of the
      next ACK.  It MUST repeat the same value of ECI in every
      subsequent ACK until the next CE event, when it increments ECI

      The increment of the local ECC values is modulo 8 so the field
      value simply wraps round back to zero when it overflows.  The
      least significant bit is to the right (labelled bit 9).

      A receiver in RECN mode MAY delay the echo of a CE to the next
      delayed-ACK, which would be necessary if ACK-withholding were

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   Sender Action in RECN Mode

      On the arrival of every ACK, the sender compares the ECI field
      with its own ECC value, then replaces its local value with that
      from the ACK.  The difference D (D = (ECI + 8 - ECC mod 8) mod 8)
      is assumed to be the number of CE marked packets that arrived at
      the receiver since it sent the previously received ACK (but see
      below for the sender's safety strategy).  Whenever the ECI field
      increments by D (and/or d drops are detected), the sender MUST
      clear the RE flag to "0" in the IP header of the next D' data
      packets it sends (where D' = D + d), effectively re-echoing each
      single increment of ECI.  Otherwise the data sender MUST send all
      data packets with RE set to "1".

      As a general rule, once a flow is established, as well as setting
      or clearing the RE flag as above, a data sender in RECN mode MUST
      always set the ECN field to ECT(1).  However, the settings of the
      extended ECN field during flow start are defined in Section 6.1.4.

      As we have already emphasised, the re-ECN protocol makes no
      changes and has no effect on the TCP congestion control algorithm.
      So, the first increment of ECI (or detection of a drop) in a RTT
      triggers the standard TCP congestion response, no more than one
      congestion response per round trip, as usual.  However, the sender
      re-echoes every increment of ECI irrespective of RTTs.

      A TCP sender also acts as the receiver for the other half-
      connection.  The host will maintain two ECC values S.ECC and R.ECC
      as sender and receiver respectively.  Every TCP header sent by a
      host in RECN mode will also repeat the prevailing value of R.ECC
      in its ECI field.  If a sender in RECN mode has to retransmit a
      packet due to a suspected loss, the re-transmitted packet MUST
      carry the latest prevailing value of R.ECC when it is re-
      transmitted, which will not necessarily be the one it carried

6.1.2.  RECN-Co mode: Re-ECT Sender with a RFC3168 compliant ECN

   If the half-connection is in RECN-Co mode, ECN feedback proceeds no
   differently to that of RFC3168 compliant ECN.  In other words, the
   receiver sets the ECE flag repeatedly in the TCP header and the
   sender responds by setting the CWR flag.  Although RECN-Co mode is
   used when the receiver has not implemented the re-ECN protocol, the
   sender can infer enough from its RFC3168 compliant ECN feedback to
   set or clear the RE flag reasonably well.  Specifically, every time
   the receiver toggles the ECE field from "0" to "1" (or a loss is
   detected), as well as setting CWR in the TCP flags, the re-ECN sender

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   MUST blank the RE flag of the next packet to "0" as it would do in
   full RECN mode.  Otherwise, the data sender SHOULD send all other
   packets with RE set to "1".  Once a flow is established, a re-ECN
   data sender in RECN-Co mode MUST always set the ECN field to ECT(1).

   If a CE marked packet arrives at the receiver within a round trip
   time of a previous mark, the receiver will still be echoing ECE for
   the last CE mark.  Therefore, such a mark will be missed by the
   sender.  Of course, this isn't of concern for congestion control, but
   it does mean that very occasionally the RE blanking fraction will be
   understated.  Therefore flows in RECN-Co mode may occasionally be
   mistaken for very lightly cheating flows and consequently might
   suffer a small number of packet drops through an egress dropper.  We
   expect re-ECN would be deployed for some time before policers and
   droppers start to enforce it.  So, given there is not much ECN
   deployment yet anyway, this minor problem may affect only a very
   small proportion of flows, reducing to nothing over the years as
   RFC3168 compliant ECN hosts upgrade.  The use of RECN-Co mode would
   need to be reviewed in the light of experience at the time of re-ECN

   RECN-Co mode is OPTIONAL.  Re-ECN implementers who want to keep their
   code simple, MAY choose not to implement this mode.  If they do not,
   a re-ECN sender SHOULD fall back to RFC3168 compliant ECT mode in the
   presence of an ECN-capable receiver.  It MAY choose to fall back to
   the ECT-Nonce mode, but if re-ECN implementers don't want to be
   bothered with RECN-Co mode, they probably won't want to add an ECT-
   Nonce mode either.  Re-ECN support for the ECN Nonce

   A TCP half-connection in RECN-Co mode MUST NOT support the ECN
   Nonce [RFC3540].  This means that the sending code of a re-ECN
   implementation will never need to include ECN Nonce support.  Re-ECN
   is intended to provide wider protection than the ECN nonce against
   congestion control misbehaviour, and re-ECN only requires support
   from the sender, therefore it is preferable to specifically rule out
   the need for dual sender implementations.  As a consequence, a re-ECN
   capable sender will never set ECT(0), so it will be easier for
   network elements to discriminate re-ECN traffic flows from other ECN
   traffic, which will always contain some ECT(0) packets.

   However, a re-ECN implementation MAY OPTIONALLY include receiving
   code that complies with the ECN Nonce protocol when interacting with
   a sender that supports the ECN nonce (rather than re-ECN), but this
   support is not required.

   RFC3540 allows an ECN nonce sender to choose whether to sanction a

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   receiver that does not ever set the nonce sum.  Given re-ECN is
   intended to provide wider protection than the ECN nonce against
   congestion control misbehaviour, implementers of re-ECN receivers MAY
   choose not to implement backwards compatibility with the ECN nonce
   capability.  This may be because they deem that the risk of sanctions
   is low, perhaps because significant deployment of the ECN nonce seems
   unlikely at implementation time.

6.1.3.  Capability Negotiation

   During the TCP hand-shake at the start of a connection, an originator
   of the connection (host A) with a re-ECN-capable transport MUST
   indicate it is Re-ECT by setting the TCP flags NS=1, CWR=1 and ECE=1
   in the initial SYN.

   A responding Re-ECT host (host B) MUST return a SYN ACK with flags
   CWR=1 and ECE=0.  The responding host MUST NOT set this combination
   of flags unless the preceding SYN has already indicated Re-ECT
   support as above.  Normally a Re-ECT server (B) will reply to a Re-
   ECT client with NS=0, but if the initial SYN from Re-ECT client A is
   marked CE(-1), a Re-ECT server B MUST increment its local value of
   ECC.  But B cannot reflect the value of ECC in the SYN ACK, because
   it is still using the 3 bits to negotiate connection capabilities.
   So, server B MUST set the alternative TCP header flags in its SYN
   ACK: NS=1, CWR=1 and ECE=0.

   These handshakes are summarised in Table 5 below, with X indicating
   NS can be either 1 or 0 depending respectively on whether congestion
   had been experienced or not.  The handshakes used for the other
   flavours of ECN are also shown for comparison.  To compress the width
   of the table, the headings of the first four columns have been
   severely abbreviated, as follows:

      R: *R*e-ECT

      N: ECT-*N*once (RFC3540)

      E: *E*CT (RFC3168)

      I: Not-ECT (*I*mplicit congestion notification).

   These correspond with the same headings used in Table 4.  Indeed, the
   resulting modes in the last two columns of the table below are a more
   comprehensive way of saying the same thing as Table 4.

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   | R  | N | E | I |   SYN A-B  | SYN ACK B-A |  A-B Mode |  B-A Mode |
   |    |   |   |   | NS CWR ECE |  NS CWR ECE |           |           |
   | AB |   |   |   |  1   1   1 |  X   1   0  |    RECN   |    RECN   |
   | A  | B |   |   |  1   1   1 |  1   0   1  |  RECN-Co  | ECT-Nonce |
   | A  |   | B |   |  1   1   1 |  0   0   1  |  RECN-Co  |    ECT    |
   | A  |   |   | B |  1   1   1 |  0   0   0  |  Not-ECT  |  Not-ECT  |
   | B  | A |   |   |  0   1   1 |  0   0   1  | ECT-Nonce |  RECN-Co  |
   | B  |   | A |   |  0   1   1 |  0   0   1  |    ECT    |  RECN-Co  |
   | B  |   |   | A |  0   0   0 |  0   0   0  |  Not-ECT  |  Not-ECT  |

      Table 5: TCP Capability Negotiation between Originator (A) and
                               Responder (B)

   As soon as a re-ECN capable TCP server receives a SYN, it MUST set
   its two half-connections into the modes given in Table 5.  As soon as
   a re-ECN capable TCP client receives a SYN ACK, it MUST set its two
   half-connections into the modes given in Table 5.  The half-
   connections will remain in these modes for the rest of the
   connection, including for the third segment of TCP's three-way hand-
   shake (the ACK).

   {ToDo: Consider delaying mode changes if using SYN cookies (will also
   affect next section).}

   {ToDo: consider RSTs within a connection.}

   Recall that, if the SYN ACK reflects the same flag settings as the
   preceding SYN (because there is a broken RFC3168 compliant
   implementation that behaves this way), RFC3168 specifies that the
   whole connection MUST revert to Not-ECT.

   Also note that, whenever the SYN flag of a TCP segment is set
   (including when the ACK flag is also set), the NS, CWR and ECE flags
   ( i.e the ECI field of the SYN-ACK) MUST NOT be interpreted as the
   3-bit ECI value, which is only set as a copy of the local ECC value
   in non-SYN packets.

6.1.4.  Extended ECN (EECN) Field Settings during Flow Start or after
        Idle Periods

   If the originator (A) of a TCP connection supports re-ECN it MUST set
   the extended ECN (EECN) field in the IP header of the initial SYN
   packet to the feedback not established (FNE) codepoint.

   FNE is a new extended ECN codepoint defined by this specification

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   (Section 4.2).  The feedback not established (FNE) codepoint is used
   when the transport does not have the benefit of ECN feedback so it
   cannot decide whether to set or clear the RE flag.

   If after receiving a SYN the server B has set its sending half-
   connection into RECN mode or RECN-Co mode, it MUST set the extended
   ECN field in the IP header of its SYN ACK to the feedback not
   established (FNE) codepoint.  Note the careful wording here, which
   means that Re-ECT server B MUST set FNE on a SYN ACK whether it is
   responding to a SYN from a Re-ECT client or from a client that is
   merely ECN-capable.  This is because FNE indicates the transport is
   ECN capable as well as re-ECN capable.

   The original ECN specification [RFC3168] required SYNs and SYN ACKs
   to use the Not-ECT codepoint of the ECN field.  The aim was to
   prevent well-known DoS attacks such as SYN flooding being able to
   gain from the advantage that ECN capability afforded over drop at
   ECN-capable routers.

   For a SYN ACK, Kuzmanovic [RFC5562] has shown that this caution was
   unnecessary, and allows a SYN ACK to be ECN-capable to improve
   performance.  By stipulating the FNE codepoint for the initial SYN,
   we comply with RFC3168 in word but not in spirit, because we have
   indeed set the ECN field to Not-ECT, but we have extended the ECN
   field with another bit.  And it will be seen (Section 5.3) that we
   have defined one setting of that bit to mean an ECN-capable
   transport.  Therefore, by proposing that the FNE codepoint MUST be
   used on the initial SYN of a connection, we have gone further by
   proposing to make the initial SYN ECN-capable too.  Section 5.4
   justifies deciding to make the initial SYN ECN-capable.

   Once a TCP half connection is in RECN mode or RECN-Co mode, FNE will
   have already been set on the initial SYN and possibly the SYN ACK as
   above.  But each re-ECN sender will have to set FNE cautiously on a
   few data packets as well, given a number of packets will usually have
   to be sent before sufficient congestion feedback is received.  The
   behaviour will be different depending on the mode of the half-

   RECN mode:  Given the constraints on TCP's initial window [RFC3390]
      and its exponential window increase during slow start
      phase [RFC5681], it turns out that the sender SHOULD set FNE on
      the first and third data packets in its flow after the initial
      3-way handshake, assuming equal sized data packets once a flow is
      established.  Appendix D presents the calculation that led to this
      conclusion.  Below, after running through the start of an example
      TCP session, we give the intuition learned from that calculation.
      {ToDo: unfortunately the calculation was based on erroneous

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      assumptions; see [I-D.conex-tcp-mods] for a better approach.}

   RECN-Co mode:  A re-ECT sender that switches into re-ECN
      compatibility mode or into Not-ECT mode (because it has detected
      the corresponding host is not re-ECN capable) MUST limit its
      initial window to 1 segment.  The reasoning behind this constraint
      is given in Section 5.4.  Having set this initial window, a re-ECN
      sender in RECN-Co mode SHOULD set FNE on the first and third data
      packets in a flow, as for RECN mode.

   |    | Data | TCP A(Re-ECT)  | IP A  | IP B  | TCP B(Re-ECT) | Data |
   |    | Byte |  SEQ  ACK CTL  | EECN  | EECN  |  SEQ  ACK CTL | Byte |
   | -- | ---- | -------------  | ----- | ----- | ------------- | ---- |
   |  1 |      | 0100      SYN  | FNE   | -->   |      R.ECC=0  |      |
   |    |      |    CWR,ECE,NS  |       |       |               |      |
   |  2 |      |      R.ECC=0   | <--   | FNE   | 0300 0101     |      |
   |    |      |                |       |       |   SYN,ACK,CWR |      |
   |  3 |      | 0101 0301 ACK  | RECT  | -->   |      R.ECC=0  |      |
   |  4 | 1000 | 0101 0301 ACK  | FNE   | -->   |      R.ECC=0  |      |
   |  5 |      |      R.ECC=0   | <--   | FNE   | 0301 1102 ACK | 1460 |
   |  6 |      |      R.ECC=0   | <--   | RECT  | 1762 1102 ACK | 1460 |
   |  7 |      |      R.ECC=0   | <--   | FNE   | 3222 1102 ACK | 1460 |
   |  8 |      | 1102 1762 ACK  | RECT  | -->   |      R.ECC=0  |      |
   |  9 |      |      R.ECC=0   | <--   | RECT  | 4682 1102 ACK | 1460 |
   | 10 |      |      R.ECC=0   | <--   | RECT  | 6142 1102 ACK | 1460 |
   | 11 |      | 1102 3222 ACK  | RECT  | -->   |      R.ECC=0  |      |
   | 12 |      |      R.ECC=0   | <--   | RECT  | 7602 1102 ACK | 1460 |
   | 13 |      |      R.ECC=1   | <*-   | RECT  | 9062 1102 ACK | 1460 |
   |    |      | ...            |       |       |               |      |

                      Table 6: TCP Session Example #1

   Table 6 shows an example TCP session, where the server B sets FNE on
   its first and third data packets (lines 5 & 7) as well as on the
   initial SYN ACK as previously described.  The left hand half of the
   table shows the relevant settings of headers sent by client A in
   three layers: the TCP payload size; TCP settings; then IP settings.
   The right hand half gives equivalent columns for server B. The only
   TCP settings shown are the sequence number (SEQ), acknowledgement
   number (ACK) and the relevant control (CTL) flags that the relevant
   sending host sets in the TCP header.  The IP columns show the setting
   of the extended ECN (EECN) field.

   Also shown on the receiving side of the table is the value of the
   receiver's echo congestion counter (R.ECC) after processing the

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   incoming EECN header.  Note that, once a host sets a half-connection
   into RECN mode, it MUST initialise its local value of ECC to zero.

   The intuition that Appendix D gives for why a sender should set FNE
   on the first and third data packets is as follows.  At line 13, a
   packet sent by B is shown with an '*', which means it has been
   congestion marked by an intermediate queue from RECT to CE(-1).  On
   receiving this CE marked packet, client A increments its ECC counter
   to 1 as shown.  This was the 7th data packet B sent, but before
   feedback about this event returns to B, it might well have sent many
   more packets.  Indeed, during exponential slow start, about as many
   packets will be in flight (unacknowledged) as have been acknowledged.
   So, when the feedback from the congestion event on B's 7th segment
   returns, B will have sent about 7 further packets that will still be
   in flight.  At that stage, B's best estimate of the network's packet
   marking fraction will be 1/7.  So, as B will have sent about 14
   packets, it should have already marked 2 of them as FNE in order to
   have marked 1/7; hence the need to have set the first and third data
   packets to FNE.

   Client A's behaviour in Table 6 also shows FNE being set on the first
   SYN and the first data packet (lines 1 & 4), but in this case it
   sends no more data packets, so of course, it cannot, and does not
   need to, set FNE again.  Note that in the A-B direction there is no
   need to set FNE on the third part of the three-way hand-shake (line
   3---the ACK).

   Note that in this section we have used the word SHOULD rather than
   MUST when specifying how to set FNE on data segments before positive
   congestion feedback arrives (but note that the word MUST was used for
   FNE on the SYN and SYN ACK).  FNE is only RECOMMENDED for the first
   and third data segments to entertain the possibility that the TCP
   transport has the benefit of other knowledge of the path, which it
   re-uses from one flow for the benefit of a newly starting flow.  For
   instance, one flow can re-use knowledge of other flows between the
   same hosts if using a Congestion Manager [RFC3124] or when a proxy
   host aggregates congestion information for large numbers of flows.

   {ToDo: There is probably scope for re-writing the above in a
   different way so that it says MUST unless some other knowledge of the
   path is available.  See earlier note pointing out FNE on 1st & 3rd is
   too few.}

   After an idle period of more than 1 second, a re-ECN sender transport
   MUST set the EECN field of the packet that resumes the connection to
   FNE.  Note that this next packet may be sent a very long time later,
   a packet does NOT have to be sent after 1 second of idling.  In order
   that the design of network policers can be deterministic, this

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   specification deliberately puts an absolute lower limit on how long a
   connection can be idle before the packet that resumes the connection
   must be set to FNE, rather than relating it to the connection round
   trip time.  We use the lower bound of the retransmission timeout
   (RTO) [RFC6298], which is commonly used as the idle period before TCP
   must reduce to the restart window [RFC5681].  Note our specification
   of re-ECN's idle period is NOT intended to change the idle period for
   TCP's restart, nor indeed for any other purposes.

   {ToDo: Describe how the sender falls back to RFC3168 modes if packets
   don't appear to be getting through (to work round firewalls
   discarding packets they consider unusual).}

   {ToDo: Possible future capabilities for changing Slow Start}

6.1.5.  Pure ACKS, Retransmissions, Window Probes and Partial ACKs

   A re-ECN sender MUST clear the RE flag to "0" and set the ECN field
   to Not-ECT in pure ACKs, retransmissions and window probes, as
   specified in  [RFC3168].  Our eventual goal is for all packets to be
   sent with re-ECN enabled, and we believe the semantics of the ECI
   field go a long way towards being able to achieve this.  However, we
   have not completed a full security analysis for these cases,
   therefore, currently we merely re-state current practice.

   We must also reconcile the facts that congestion marking is applied
   to packets but acknowledgements cover octet ranges and acknowledged
   octet boundaries need not match the transmitted boundaries.  The
   general principle we work to is to remain compatible with TCP's
   congestion control which is driven by congestion events at packet
   granularity while at the same time aiming to blank the RE flag on at
   least as many octets in a flow as have been marked CE.

   Therefore, a re-ECN TCP receiver MUST increment its ECC value as many
   times as CE marked packets have been received.  And that value MUST
   be echoed to the sender in the first available ACK using the ECI
   field.  This ensures the TCP sender's congestion control receives
   timely feedback on congestion events at the same packet granularity
   that they were generated on congested queues.

   Then, a re-ECN sender stores the difference D between its own ECC
   value and the incoming ECI field by incrementing a counter R. Then, R
   is decremented by 1 each subsequent packet that is sent with the RE
   flag blanked, until R is no longer positive.  Using this technique,
   whenever a re-ECN transport sends a not re-ECN capable packet (e.g. a
   retransmission), the remaining packets required to have the RE flag
   blanked will be automatically carried over to subsequent packets,
   through the variable R.

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   This does not ensure precisely the same number of octets have RE
   blanked as were CE marked.  But we believe positive errors will
   cancel negative over a long enough period. {ToDo: However, more
   research is needed to prove whether this is so.  If it is not, it may
   be necessary to increment and decrement R in octets rather than
   packets, by incrementing R as the product of D and the size in octets
   of packets being sent (typically the MSS).}

6.2.  Other Transports

6.2.1.  General Guidelines for Adding Re-ECN to Other Transports

   As a general rule, Re-ECT sender transports that have established the
   receiver transport is at least ECN-capable (not necessarily re-ECN
   capable) MUST blank the RE codepoint for at least as many octets as
   arrive at receiver with the CE codepoint set.  Re-ECN-capable sender
   transports should always initialise the ECN field to the ECT(1)
   codepoint once a flow is established.

   If the sender transport does not have sufficient feedback to even
   estimate the path's CE rate, it SHOULD set FNE continuously.  If the
   sender transport has some, perhaps stale, feedback to estimate that
   the path's CE rate is nearly definitely less than E%, the transport
   MAY blank RE in packets for E% of sent octets, and set the RECT
   codepoint for the remainder.

   The following sections give guidelines on how re-ECN support could be
   added to RSVP or NSIS, to DCCP, and to SCTP - although separate
   Internet drafts will be necessary to document the exact mechanics of
   re-ECN in each of these protocols.

   {ToDo: Give a brief outline of what would be expected for each of the

   o  UDP fire and forget (e.g.  DNS)

   o  UDP streaming with no feedback

   o  UDP streaming with feedback


6.2.2.  Guidelines for adding Re-ECN to RSVP or NSIS

   A separate I-D has been submitted []
   describing how re-ECN can be used in an edge-to-edge rather than end-
   to-end scenario.  It can then be used by downstream networks to
   police whether upstream networks are blocking new flow reservations

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   when downstream congestion is too high, even though the congestion is
   in other operators' downstream networks.  This relates to current
   IETF work on Admission Control over Diffserv using Pre-Congestion
   Notification (PCN)  [RFC5559].

6.2.3.  Guidelines for adding Re-ECN to DCCP

   Beside adjusting the initial features negotiation sequence, operating
   re-ECN in DCCP [RFC4340] could be achieved by defining a new option
   to be added to acknowledgments, that would include a multibit field
   where the destination could copy its ECC.

6.2.4.  Guidelines for adding Re-ECN to SCTP

   Appendix A in [RFC4960] gives the specifications for SCTP to support
   ECN.  Similar steps should be taken to support re-ECN.  Beside
   adjusting the initial features negotiation sequence, operating re-ECN
   in SCTP could be achieved by defining a new control chunk, that would
   include a multibit field where the destination could copy its ECC

7.  Incremental Deployment

   The design of the re-ECN protocol started from the fact that the
   current ECN marking behaviour of queues was sufficient and that re-
   feedback could be introduced around these queues by changing the
   sender behaviour but not the routers.  Otherwise, if we had required
   routers to be changed, the chance of encountering a path that had
   every router upgraded would be vanishingly small during early
   deployment, giving no incentive to start deployment.  Also, as there
   is no new forwarding behaviour, routers and hosts do not have to
   signal or negotiate anything.

   However, networks that choose to protect themselves using re-ECN do
   have to add new security functions at their trust boundaries with
   others.  They distinguish legacy traffic by its ECN field.  Traffic
   from Not-ECT transports is distinguishable by its Not-ECT marking.
   Traffic from RFC3168 compliant ECN transports is distinguished from
   re-ECN by which of ECT(0) or ECT(1) is used.  We chose to use ECT(1)
   for re-ECN traffic deliberately.  Existing ECN sources set ECT(0) on
   either 50% (the nonce) or 100% (the default) of packets, whereas re-
   ECN does not use ECT(0) at all.  We can use this distinguishing
   feature of RFC3168 compliant ECN traffic to separate it out for
   different treatment at the various border security functions: egress
   dropping, ingress policing and border policing.

   The general principle we adopt is that an egress dropper will not
   drop any legacy traffic, but ingress and border policers will limit
   the bulk rate of legacy traffic (Not-ECT, ECT(0) and those marked

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   with the unused codepoint) that can enter each network.  Then, during
   early re-ECN deployment, operators can set very permissive (or non-
   existent) rate-limits on legacy traffic, but once re-ECN
   implementations are generally available, legacy traffic can be rate-
   limited increasingly harshly.  Ultimately, an operator might choose
   to block all legacy traffic entering its network, or at least only
   allow through a trickle.

   Then, as the limits are set more strictly, the more RFC3168 ECN
   sources will gain by upgrading to re-ECN.  Thus, towards the end of
   the voluntary incremental deployment period, RFC3168 compliant
   transports can be given progressively stronger encouragement to

   The following list of minor changes, brings together all the points
   where re-ECN semantics for use of the two-bit ECN field are different
   compared to RFC3168:

   o  A re-ECN sender sets ECT(1) by default, whereas an RFC3168 sender
      sets ECT(0) by default (Section 4.3);

   o  No provision is necessary for a re-ECN capable source transport to
      use the ECN nonce (Section;

   o  Routers MAY preferentially drop different extended ECN codepoints
      (Section 5.3);

   o  Packets carrying the feedback not established (FNE) codepoint MAY
      optionally be marked rather than dropped by routers, even though
      their ECN field is Not-ECT (with the important caveat in
      Section 5.3);

   o  Packets may be dropped by policing nodes because of apparent
      misbehaviour, not just because of congestion ;

   o  Tunnel entry behaviour is still to be defined, but may have to be
      different from RFC3168 (Section 5.6).

   None of these changes REQUIRE any modifications to routers.  Also
   none of these changes affect anything about end to end congestion
   control; they are all to do with allowing networks to police that end
   to end congestion control is well-behaved.

8.  Related Work

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8.1.  Congestion Notification Integrity

   The choice of two ECT code-points in the ECN field [RFC3168]
   permitted future flexibility, optionally allowing the sender to
   encode the experimental ECN nonce [RFC3540] in the packet stream.
   This mechanism has since been included in the specifications of DCCP

   {ToDo: DCCP provides nonce support - how does this affect the RFC?}

   The ECN nonce is an elegant scheme that allows the sender to detect
   if someone in the feedback loop - the receiver especially - tries to
   claim no congestion was experienced when in fact congestion led to
   packet drops or ECN marks.  For each packet it sends, the sender
   chooses between the two ECT codepoints in a pseudo-random sequence.
   Then, whenever the network marks a packet with CE, if the receiver
   wants to deny congestion happened, she has to guess which ECT
   codepoint was overwritten.  She has only a 50:50 chance of being
   correct each time she denies a congestion mark or a drop, which
   ultimately will give her away.

   The purpose of a network-layer nonce should primarily be protection
   of the network, while a transport-layer nonce would be better used to
   protect the sender from cheating receivers.  Now, the assumption
   behind the ECN nonce is that a sender will want to detect whether a
   receiver is suppressing congestion feedback.  This is only true if
   the sender's interests are aligned with the network's, or with the
   community of users as a whole.  This may be true for certain large
   senders, who are under close scrutiny and have a reputation to
   maintain.  But we have to deal with a more hostile world, where
   traffic may be dominated by peer-to-peer transfers, rather than
   downloads from a few popular sites.  Often the `natural' self-
   interest of a sender is not aligned with the interests of other
   users.  It often wishes to transfer data quickly to the receiver as
   much as the receiver wants the data quickly.

   In contrast, the re-ECN protocol enables policing of an agreed rate-
   response to congestion (e.g. TCP-friendliness) at the sender's
   interface with the internetwork.  It also ensures downstream networks
   can police their upstream neighbours, to encourage them to police
   their users in turn.  But most importantly, it requires the sender to
   declare path congestion to the network and it can remove traffic at
   the egress if this declaration is dishonest.  So it can police
   correctly, irrespective of whether the receiver tries to suppress
   congestion feedback or whether the sender ignores genuine congestion
   feedback.  Therefore the re-ECN protocol addresses a much wider range
   of cheating problems, which includes the one addressed by the ECN

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   {ToDo: Ensure we address the early ACK problem.}

9.  Security Considerations

   {ToDo: Describe attacks by networks on flows and by spoofing
   sources.} {ToDo: Re-ECN & DNS servers}

   This whole memo concerns the deployment of a secure congestion
   control framework.  However, below we list some specific security
   issues that we are still working on:

   o  Malicious users have ability to launch dynamically changing
      attacks, exploiting the time it takes to detect an attack, given
      ECN marking is binary.  We are concentrating on subtle
      interactions between the ingress policer and the egress dropper in
      an effort to make it impossible to game the system.

   o  There is an inherent need for at least some flow state at the
      egress dropper given the binary marking environment, which leads
      to an apparent vulnerability to state exhaustion attacks.  An
      egress dropper design with bounded flow state is in write-up.

   o  A malicious source can spoof another user's address and send
      negative traffic to the same destination in order to fool the
      dropper into sanctioning the other user's flow.  To prevent or
      mitigate these two different kinds of DoS attack, against the
      dropper and against given flows, we are considering various
      protection mechanisms.

   o  A malicious client can send requests using a spoofed source
      address to a server (such as a DNS server) that tends to respond
      with single packet responses.  This server will then be tricked
      into having to set FNE on the first (and only) packet of all these
      wasted responses.  Given packets marked FNE are worth +1, this
      will cause such servers to consume more of their allowance to
      cause congestion than they would wish to.  In general, re-ECN is
      deliberately designed so that single packet flows have to bear the
      cost of not discovering the congestion state of their path.  One
      of the reasons for introducing re-ECN is to encourage short flows
      to make use of previous path knowledge by moving the cost of this
      lack of knowledge to sources that create short flows.  Therefore,
      we in the long run we might expect services like DNS to aggregate
      single packet flows into connections where it brings benefits.
      However, this attack where DNS requests are made from spoofed
      addresses genuinely forces the server to waste its resources.  The
      only mitigating feature is that the attacker has to set FNE on
      each of its requests if they are to get through an egress dropper
      to a DNS server.  The attacker therefore has to consume as many

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      resources as the victim, which at least implies re-ECN does not
      unwittingly amplify this attack.

   Having highlighted outstanding security issues, we now explain the
   design decisions that were taken based on a security-related
   rationale.  It may seem that the six codepoints of the eight made
   available by extending the ECN field with the RE flag have been used
   rather wastefully to encode just five states.  In effect the RE flag
   has been used as an orthogonal single bit, using up four codepoints
   to encode the three states of positive, neutral and negative worth.
   The mapping of the codepoints in an earlier version of this proposal
   used the codepoint space more efficiently, but the scheme became
   vulnerable to network operators bypassing congestion penalties by
   focusing congestion marking on positive packets.  Appendix B explains
   why fixing that problem while allowing for incremental deployment,
   would have used another codepoint anyway.  So it was better to use
   this orthogonal encoding scheme, which greatly simplified the whole
   protocol and brought with it some subtle security benefits (see the
   last paragraph of Appendix B).

   With the scheme as now proposed, once the RE flag is set or cleared
   by the sender or its proxy, it should not be written by the network,
   only read.  So the endpoints can detect if any network maliciously
   alters the RE flag.  IPsec AH integrity checking does not cover the
   IPv4 option flags (they were considered mutable---even the one we
   propose using for the RE flag that was `currently unused' when IPsec
   was defined).  But it would be sufficient for a pair of endpoints to
   make random checks on whether the RE flag was the same when it
   reached the egress as when it left the ingress.  Indeed, if IPsec AH
   had covered the RE flag, any network intending to alter sufficient RE
   flags to make a gain would have focused its alterations on packets
   without authenticating headers (AHs).

   The security of re-ECN has been deliberately designed to not rely on

10.  IANA Considerations

   This memo includes no request to IANA (yet).

   If this memo was to progress to standards track, it would list:

   o  The new RE flag in IPv4 (Section 5.1) and its extension with the
      ECN field to create a new set of extended ECN (EECN) codepoints;

   o  The definition of the EECN codepoints for default Diffserv PHBs
      (Section 4.2)

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   o  The Hop-by-Hop option ID for the new extension header for IPv6
      (Section 5.2);

   o  The new combinations of flags in the TCP header for capability
      negotiation (Section 6.1.3);

11.  Conclusions


12.  Acknowledgements

   Sebastien Cazalet and Andrea Soppera contributed to the idea of re-
   feedback.  All the following have given helpful comments: Andrea
   Soppera, David Songhurst, Peter Hovell, Louise Burness, Phil Eardley,
   Steve Rudkin, Marc Wennink, Fabrice Saffre, Cefn Hoile, Steve Wright,
   John Davey, Martin Koyabe, Carla Di Cairano-Gilfedder, Alexandru
   Murgu, Nigel Geffen, Pete Willis, John Adams (BT), Sally Floyd
   (ICIR), Joe Babiarz, Kwok Ho-Chan (Nortel), Stephen Hailes, Mark
   Handley (who developed the attack with canceled packets), Adam
   Greenhalgh (who developed the attack on DNS) (UCL), Jon Crowcroft
   (Uni Cam), David Clark, Bill Lehr, Sharon Gillett, Steve Bauer (who
   complemented our own dummy traffic attacks with others), Liz Maida
   (MIT), Meral Shirazipour (Ericsson) and comments from participants in
   the CRN/CFP Broadband and DoS-resistant Internet working groups.A
   special thank you to Alessandro Salvatori for coming up with fiendish
   attacks on re-ECN.

13.  Comments Solicited

   Comments and questions are encouraged and very welcome.  They can be
   addressed to the IETF Congestion Exposure (ConEx) working group's
   mailing list <>, and/or to the authors.

14.  References

14.1.  Normative References

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

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

   [RFC3390]                  Allman, M., Floyd, S., and C. Partridge,

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                              "Increasing TCP's Initial Window",
                              RFC 3390, October 2002.

   [RFC4302]                  Kent, S., "IP Authentication Header",
                              RFC 4302, December 2005.

   [RFC4340]                  Kohler, E., Handley, M., and S. Floyd,
                              "Datagram Congestion Control Protocol
                              (DCCP)", RFC 4340, March 2006.

   [RFC4341]                  Floyd, S. and E. Kohler, "Profile for
                              Datagram Congestion Control Protocol
                              (DCCP) Congestion Control ID 2: TCP-like
                              Congestion Control", RFC 4341, March 2006.

   [RFC4342]                  Floyd, S., Kohler, E., and J. Padhye,
                              "Profile for Datagram Congestion Control
                              Protocol (DCCP) Congestion Control ID 3:
                              TCP-Friendly Rate Control (TFRC)",
                              RFC 4342, March 2006.

   [RFC4835]                  Manral, V., "Cryptographic Algorithm
                              Implementation Requirements for
                              Encapsulating Security Payload (ESP) and
                              Authentication Header (AH)", RFC 4835,
                              April 2007.

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

   [RFC5562]                  Kuzmanovic, A., Mondal, A., Floyd, S., and
                              K. Ramakrishnan, "Adding Explicit
                              Congestion Notification (ECN) Capability
                              to TCP's SYN/ACK Packets", RFC 5562,
                              June 2009.

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

   [RFC6040]                  Briscoe, B., "Tunnelling of Explicit
                              Congestion Notification", RFC 6040,
                              November 2010.

14.2.  Informative References

   [ARI05]                    Adams, J., Roberts, L., and A.
                              IJsselmuiden, "Changing the Internet to

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                              Support Real-Time Content Supply from a
                              Large Fraction of Broadband Residential
                              Users", BT Technology Journal
                              (BTTJ) 23(2), April 2005.

   [I-D.conex-tcp-mods]       Kuehlewind, M. and R. Scheffenegger, "TCP
                              modifications for Congestion Exposure",
                              (work in progress), July 2013.

   []         Briscoe, B., Jacquet, A., Moncaster, T.,
                              and A. Smith, "Re-ECN: A Framework for
                              adding Congestion Accountability to
                              draft-briscoe-conex-re-ecn-motiv-02 (work
                              in progress), July 2013.

   []  Briscoe, B., "Emulating Border Flow
                              Policing using Re-PCN on Bulk Data",
                              draft-briscoe-re-pcn-border-cheat-03 (work
                              in progress), October 2009.

   [RFC2309]                  Braden, B., Clark, D., Crowcroft, J.,
                              Davie, B., Deering, S., Estrin, D., Floyd,
                              S., Jacobson, V., Minshall, G., Partridge,
                              C., Peterson, L., Ramakrishnan, K.,
                              Shenker, S., Wroclawski, J., and L. Zhang,
                              "Recommendations on Queue Management and
                              Congestion Avoidance in the Internet",
                              RFC 2309, April 1998.

   [RFC2475]                  Blake, S., Black, D., Carlson, M., Davies,
                              E., Wang, Z., and W. Weiss, "An
                              Architecture for Differentiated Services",
                              RFC 2475, December 1998.

   [RFC3124]                  Balakrishnan, H. and S. Seshan, "The
                              Congestion Manager", RFC 3124, June 2001.

   [RFC3514]                  Bellovin, S., "The Security Flag in the
                              IPv4 Header", RFC 3514, April 2003.

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

   [RFC4301]                  Kent, S. and K. Seo, "Security

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                              Architecture for the Internet Protocol",
                              RFC 4301, December 2005.

   [RFC5129]                  Davie, B., Briscoe, B., and J. Tay,
                              "Explicit Congestion Marking in MPLS",
                              RFC 5129, January 2008.

   [RFC5559]                  Eardley, P., "Pre-Congestion Notification
                              (PCN) Architecture", RFC 5559, June 2009.

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

   [Re-fb]                    Briscoe, B., Jacquet, A., Di Cairano-
                              Gilfedder, C., Salvatori, A., Soppera, A.,
                              and M. Koyabe, "Policing Congestion
                              Response in an Internetwork Using Re-
                              Feedback", ACM SIGCOMM CCR 35(4)277--288,
                              August 2005, <

   [Savage99]                 Savage, S., Cardwell, N., Wetherall, D.,
                              and T. Anderson, "TCP congestion control
                              with a misbehaving receiver", ACM SIGCOMM
                              CCR 29(5), October 1999, <http://

   [Steps_DoS]                Handley, M. and A. Greenhalgh, "Steps
                              towards a DoS-resistant Internet
                              Architecture", Proc. ACM SIGCOMM workshop
                              on Future directions in network
                              architecture (FDNA'04) pp 49--56,
                              August 2004.

   [tcp-rcv-cheat]            Moncaster, T., Briscoe, B., and A.
                              Jacquet, "A TCP Test to Allow Senders to
                              Identify Receiver Non-Compliance",
                              draft-moncaster-tcpm-rcv-cheat-02 (work in
                              progress), November 2007.

Appendix A.  Precise Re-ECN Protocol Operation

   The protocol operation in Section 4.3 was described as an
   approximation.  In fact, standard ECN marking at a queue combines 1%
   and 2% marking into slightly less than 3% whole-path marking, because
   queues deliberately mark CE whether or not it has already been marked

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   by another queue upstream.  So the combined marking fraction would
   actually be 100% - (100% - 1%)(100% - 2%) = 2.98%.

   To generalise this we will need some notation.

   o  j represents the index of each resource (typically queues) along a
      path, ranging from 0 at the first queue to n-1 at the last.

   o  m_j represents the fraction of octets to be *m*arked CE by a
      particular queue (whether or not they are already marked) because
      of congestion of resource j.

   o  u_j represents congestion signals arriving from *u*pstream of
      resource j, being the fraction of CE marking in arriving packet
      headers (before marking).

   o  p_j represents *p*ath congestion, being the fraction of packets
      arriving at resource j with the RE flag blanked (excluding Not-
      RECT packets).

   o  v_j denotes expected congestion downstream of resource j, which
      can be thought of as a *v*irtual marking fraction, being derived
      from two other marking fractions.

   Observed fractions of each particular codepoint (u, p and v) and
   queue marking rate m are dimensionless fractions, being the ratio of
   two data volumes (marked and total) over a monitoring period.  All
   measurements are in terms of octets, not packets, assuming that line
   resources are more congestible than packet processing.

   The path congestion (RE blanking fraction) set by the sender should
   reflect upstream congestion (CE marking fraction) from the viewpoint
   of the destination, which it feeds back to the sender.  Therefore in
   the steady state

      p_0  = u_n
           = 1 - (1 - m_1)(1 - m_2)...

   Similarly, at some point j in the middle of the network, given p = 1
   - (1 - u_j)(1 - v_j), then

      v_j  = 1 - (1 - p)/(1 - u_j)

          ~= p - u_j;                      if u_j << 100%

   So, between the two routers in the example in Section 4.3, congestion
   downstream is

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      v_1  = 100.00% - (100% - 2.98%) / (100% - 1.00%)
           = 2.00%,

   or a useful approximation of downstream congestion is

      v_1 ~= 2.98% - 1.00%
          ~= 1.98%.

Appendix B.  Justification for Two Codepoints Signifying Zero Worth

   It may seem a waste of a codepoint to set aside two codepoints of the
   Extended ECN field to signify zero worth (RECT and CE(0) are both
   worth zero).  The justification is subtle, but worth recording.

   The original version of Re-ECN ([Re-fb] and draft-00 of this memo)
   used three codepoints for neutral (ECT(1)), positive (ECT(0)) and
   negative (CE) packets.  The sender set packets to neutral unless re-
   echoing congestion, when it set them positive, in much the same way
   that it blanks the RE flag in the current protocol.  However, routers
   were meant to mark congestion by setting packets negative (CE)
   irrespective of whether they had previously been neutral or positive.

   However, we did not arrange for senders to remember which packet had
   been sent with which codepoint, or for feedback to say exactly which
   packets arrived with which codepoints.  The transport was meant to
   inflate the number of positive packets it sent to allow for a few
   being wiped out by congestion marking.  We (wrongly) assumed that
   routers would congestion mark packets indiscriminately, so the
   transport could infer how many positive packets had been marked and
   compensate accordingly by re-echoing.  But this created a perverse
   incentive for routers to preferentially congestion mark positive
   packets rather than neutral ones.

   We could have removed this perverse incentive by requiring Re-ECN
   senders to remember which packets they had sent with which codepoint.
   And for feedback from the receiver to identify which packets arrived
   as which.  Then, if a positive packet was congestion marked to
   negative, the sender could have re-echoed twice to maintain the
   balance between positive and negative at the receiver.

   Instead, we chose to make re-echoing congestion (blanking RE)
   orthogonal to congestion notification (marking CE), which required a
   second neutral codepoint.  Then the receiver would be able to detect
   and echo a congestion event even if it arrived on a packet that had
   originally been positive.

   If we had added extra complexity to the sender and receiver

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   transports to track changes to individual packets, we could have made
   it work, but then routers would have had an incentive to mark
   positive packets with half the probability of neutral packets.  That
   in turn would have led router algorithms to become more complex.
   Then senders wouldn't know whether a mark had been introduced by a
   simple or a complex router algorithm.  That in turn would have
   required another codepoint to distinguish between RFC3168 ECN and new
   Re-ECN router marking.

   Once the cost of IP header codepoint real-estate was the same for
   both schemes, there was no doubt that the simpler option for
   endpoints and for routers should be chosen.  The resulting protocol
   also no longer needed the tricky inflation/deflation complexity of
   the original (broken) scheme.  It was also much simpler to understand

   A further advantage of the new orthogonal four-codepoint scheme was
   that senders owned sole rights to change the RE flag and routers
   owned sole rights to change the ECN field.  Although we still arrange
   the incentives so neither party strays outside their dominion, these
   clear lines of authority simplify the matter.

   Finally, a little redundancy can be very powerful in a scheme such as
   this.  In one flow, the proportion of packets changed to CE should be
   the same as the proportion of RECT packets changed to CE(-1) and the
   proportion of Re-Echo packets changed to CE(0).  Double checking
   using such redundant relationships can improve the security of a
   scheme (cf. double-entry book-keeping or the ECN Nonce).
   Alternatively, it might be necessary to exploit the redundancy in the
   future to encode an extra information channel.

Appendix C.  ECN Compatibility

   The rationale for choosing the particular combinations of SYN and SYN
   ACK flags in Section 6.1.3 is as follows.

   Choice of SYN flags:  A Re-ECN sender can work with RFC3168 compliant
      ECN receivers so we wanted to use the same flags as would be used
      in an ECN-setup SYN [RFC3168] (CWR=1, ECE=1).  But at the same
      time, we wanted a server (host B) that is Re-ECT to be able to
      recognise that the client (A) is also Re-ECT.  We believe also
      setting NS=1 in the initial SYN achieves both these objectives, as
      it should be ignored by RFC3168 compliant ECT receivers and by
      ECT-Nonce receivers.  But senders that are not Re-ECT should not
      set NS=1.  At the time ECN was defined, the NS flag was not
      defined, so setting NS=1 should be ignored by existing ECT
      receivers (but testing against implementations may yet prove
      otherwise).  The ECN Nonce RFC [RFC3540] is silent on what the NS

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      field might be set to in the TCP SYN, but we believe the intent
      was for a nonce client to set NS=0 in the initial SYN (again only
      testing will tell).  Therefore we define a Re-ECN-setup SYN as one
      with NS=1, CWR=1 & ECE=1

   Choice of SYN ACK flags:  Choice of SYN ACK: The client (A) needs to
      be able to determine whether the server (B) is Re-ECT.  The
      original ECN specification required an ECT server to respond to an
      ECN-setup SYN with an ECN-setup SYN ACK of CWR=0 and ECE=1.  There
      is no room to modify this by setting the NS flag, as that is
      already set in the SYN ACK of an ECT-Nonce server.  So we used the
      only combination of CWR and ECE that would not be used by existing
      TCP receivers: CWR=1 and ECE=0.  The original ECN specification
      defines this combination as a non-ECN-setup SYN ACK, which remains
      true for RFC3168 compliant and Nonce ECTs.  But for Re-ECN we
      define it as a Re-ECN-setup SYN ACK.  We didn't use a SYN ACK with
      both CWR and ECE cleared to 0 because that would be the likely
      response from most Not-ECT receivers.  And we didn't use a SYN ACK
      with both CWR and ECE set to 1 either, as at least one broken
      receiver implementation echoes whatever flags were in the SYN into
      its SYN ACK.  Therefore we define a Re-ECN-setup SYN ACK as one
      with CWR=1 & ECE=0.

   Choice of two alternative SYN ACKs:  the NS flag may take either
      value in a Re-ECN-setup SYN ACK.  Section 5.4 REQUIRES that a Re-
      ECT server MUST set the NS flag to 1 in a Re-ECN-setup SYN ACK to
      echo congestion experienced (CE) on the initial SYN.  Otherwise a
      Re-ECN-setup SYN ACK MUST be returned with NS=0.  The only current
      known use of the NS flag in a SYN ACK is to indicate support for
      the ECN nonce, which will be negotiated by setting CWR=0 & ECE=1.
      Given the ECN nonce MUST NOT be used for a RECN mode connection, a
      Re-ECN-setup SYN ACK can use either setting of the NS flag without
      any risk of confusion, because the CWR & ECE flags will be
      reversed relative to those used by an ECN nonce SYN ACK.

   {ToDo: include the text below, either here, or in the algorithm
   sections} At an egress dropper, well-behaved RFC3168 compliant flows
   will appear to consist mostly of ECT(0) packets, with a few CE(0)
   packet.  And, if the legacy source is setting the ECN nonce, the
   majority of packets will be an equal mix of ECT(0) and ECT(1) packets
   (the latter appearing to be Re-Echo packets in Re-ECN terms).  None
   of these three packet markings is negative, so an egress dropper can
   handle all legacy flows in bulk and, as long as they don't send any
   packets using Re-ECN markings, it need not drop any legacy packets.
   So, as soon as an ECT(0) packet is seen, its flow ID can be added to
   the set of known legacy flows (a single Bloom filter would suffice).
   But, if any packets in flows classified as RFC3168 compliant are
   marked with any other marking than the three expected, the flow can

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   be removed from the RFC3168 set, to be treated in bulk with mis-
   behaving Re-ECN flows---the remainder of flow IDs that require no
   flow state to be held.

   To an ingress Re-ECN policer, legacy ECN flows will appear as very
   highly congested paths.  When policers are first deployed they can be
   configured permissively, allowing through both `RFC3168' ECN and
   misbehaving Re-ECN flows.  Then, as the threshold is set more
   strictly, the more RFC3168 ECN sources will gain by upgrading to Re-
   ECN.  Thus, towards the end of the voluntary incremental deployment
   period, RFC3168 transports can be given progressively stronger
   encouragement to upgrade.

Appendix D.  Packet Marking with FNE During Flow Start

   FNE (feedback not established) packets have two functions.  Their
   main role is to announce the start of a new flow when feedback has
   not yet been established.  However they also have the role of
   balancing the expected feedback and can be used where there are
   sudden changes in the rate of transmission.  Whilst this should not
   happen under TCP their use as speculative marking is used in building
   the following argument as to why the first and third packets should
   be set to FNE.

   The proportion of FNE packets in each round-trip should be a high
   estimate of the potential error in the balance of number of
   congestion marked packets versus number of re-echo packets already

   Let's call:

      S: the number of the TCP segments sent so far

      F: the number of FNE packets sent so far

      R: the number of Re-Echo packets sent so far

      A: the number of acknowledgments received so far

      C: the number of acknowledgments echoing a CE packet

   In normal operation, when we want to send packet S+1, we first need
   to check that enough Re-Echo packets have been issued:

   If R<C, then S+1 will be a Re-echo packet

   Next we need to estimate the amount of congestion observed so far.
   If congestion was stationary, it could be estimated as C/A. A

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   pessimistic bound is (C+1)/(A+1) which assumes that the next
   acknowledgment will echo a CE packet; we'll use that more pessimistic
   estimate to drive the generation of FNE packets.

   The number of CE packets expected when (S+1) will be acknowledged is
   therefore (S+1)*(C+1)/(A+1).  Packet S+1 should be set to FNE if that
   expected value exceeds the sum of FNE and Re-Echo packets sent so

      If  (F+R)<(S+1)*(C+1)/(A+1),
        then S+1 will be set to FNE
        else S+1 will be set to RECT

   So the full test should be:

      When packet (S+1) is about to be sent...
        If R<C,
           then S+1 will be set to Re-Echo
        Else if  (F+R)<(S+1)*(C+1)/(A+1),
          then S+1 will be set to FNE
        Else S+1 will be set to RECT

   This means that at any point, given A, R, F, C, the source could send
   another k RECT packets, so that k < (F+R)*(A+1)/(C+1)-S

   The above scheme is independent of the actions of both the dropper
   and policer and doesn't depend on the rate adaptation discipline of
   the source.  It only defines Re-Echo packets as notification of
   effective end-to-end congestion (as witnessed at the previous round-
   trip), and FNE packets as notification of speculative end-to-end
   congestion based on a high estimate of congestion

   In practice, for any source:

   o  for the first packet, A=R=F=C=S=0 ==> 1 FNE

   o  if the acknowledgment doesn't echo a mark

      *  for the second packet, A=F=S=1 R=C=0 ==> 1 RECT

      *  for the third packet, S=2 A=F=1 R=C=0 ==> 1 FNE

   o  if no acknowledgement for these two packets echoes a congestion
      mark, then {A=S=3 F=2 R=C=0} which gives k<2*4/1-3, so the source

   o  if no acknowledgement for these four packets echoes a congestion
      mark, then {A=S=7 F=2 R=C=0} which gives k<2*8/1-7, so the source
      could send another 8 RECT packets. ==> 8 RECT

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   This behaviour happens to match TCP's congestion window control in
   slow start, which is why for TCP sources, only the first and third
   packet need be FNE packets.

   A source that would open the congestion window any quicker would have
   to insert more FNE packets.  As another example a UDP source sending
   VBR traffic might need to send several FNE packets ahead of the
   traffic peaks it generates.

Appendix E.  Argument for holding back the ECN nonce

   The ECN nonce is a mechanism that allows a /sending/ transport to
   detect if drop or ECN marking at a congested router has been
   suppressed by a node somewhere in the feedback loop---another router
   or the receiver.

   Space for the ECN nonce was set aside in [RFC3168] (currently
   proposed standard) while the full nonce mechanism is specified in
   [RFC3540] (currently experimental).  The specifications for [RFC4340]
   (currently proposed standard) requires that "Each DCCP sender SHOULD
   set ECN Nonces on its packets...".  It also mandates as a requirement
   for all CCID profiles that "Any newly defined acknowledgement
   mechanism MUST include a way to transmit ECN Nonce Echoes back to the
   sender.", therefore:

   o  The CCID profile for TCP-like Congestion Control [RFC4341]
      (currently proposed standard) says "The sender will use the ECN
      Nonce for data packets, and the receiver will echo those nonces in
      its Ack Vectors."

   o  The CCID profile for TCP-Friendly Rate Control (TFRC) [RFC4342]
      recommends that "The sender [use] Loss Intervals options' ECN
      Nonce Echoes (and possibly any Ack Vectors' ECN Nonce Echoes) to
      probabilistically verify that the receiver is correctly reporting
      all dropped or marked packets."

   The primary function of the ECN nonce is to protect the integrity of
   the information about congestion: ECN marks and packet drops.
   However, when the nonce is used to protect the integrity of
   information about packet drops, rather than ECN marks, a transport
   layer nonce will always be sufficient (because a drop loses the
   transport header as well as the ECN field in the network header),
   which would avoid using scarce IP header codepoint space.  Similarly,
   a transport layer nonce would protect against a receiver sending
   early acknowledgements [Savage99].

   If the ECN nonce reveals integrity problems with the information
   about congestion, the sending transport can use that knowledge for

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   two functions:

   o  to protect its own resources, by allocating them in proportion to
      the rates that each network path can sustain, based on congestion

   o  and to protect congested routers in the network, by slowing down
      drastically its connection to the destination with corrupt
      congestion information.

   If the sending transport chooses to act in the interests of congested
   routers, it can reduce its rate if it detects some malicious party in
   the feedback loop may be suppressing ECN feedback.  But it would only
   be useful to congested routers when /all/ senders using them are
   trusted to act in interest of the congested routers.

   In the end, the only essential use of a network layer nonce is when
   sending transports (e.g. large servers) want to allocate their /own/
   resources in proportion to the rates that each network path can
   sustain, based on congestion control.  In that case, the nonce allows
   senders to be assured that they aren't being duped into giving more
   of their own resources to a particular flow.  And if congestion
   suppression is detected, the sending transport can rate limit the
   offending connection to protect its own resources.  Certainly, this
   is a useful function, but the IETF should carefully decide whether
   such a single, very specific case warrants IP header space.

   In contrast, Re-ECN allows all routers to fully protect themselves
   from such attacks, without having to trust anyone - senders,
   receivers, neighbouring networks.  Re-ECN is therefore proposed in
   preference to the ECN nonce on the basis that it addresses the
   generic problem of accountability for congestion of a network's
   resources at the IP layer.

   Delaying the ECN nonce is justified because the applicability of the
   ECN nonce seems too limited for it to consume a two-bit codepoint in
   the IP header.  It therefore seems prudent to give time for an
   alternative way to be found to do the one function the nonce is
   essential for.

   Moreover, while we have re-designed the Re-ECN codepoints so that
   they do not prevent the ECN nonce progressing, the same is not true
   the other way round.  If the ECN nonce started to see some deployment
   (perhaps because it was blessed with proposed standard status),
   incremental deployment of Re-ECN would effectively be impossible,
   because Re-ECN marking fractions at inter-domain borders would be
   polluted by unknown levels of nonce traffic.

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   The authors are aware that Re-ECN must prove it has the potential it
   claims if it is to displace the nonce.  Therefore, every effort has
   been made to complete a comprehensive specification of Re-ECN so that
   its potential can be assessed.  We therefore seek the opinion of the
   Internet community on whether the Re-ECN protocol is sufficiently
   useful to warrant standards action.

Appendix F.  Alternative Terminology Used in Other Documents

   A number of alternative terms have been used in various documents
   describing re-feedback and re-ECN.  These are set out in the
   following table

        | Current Terminology | EECN codepoint |      Colour      |
        |       Cautious      |       FNE      |       Green      |
        |       Positive      |     Re-Echo    |       Black      |
        |       Neutral       |      RECT      |       Grey       |
        |       Negative      |     CE(-1)     |        Red       |
        |      Cancelled      |      CE(0)     |     Red-Black    |
        |      Legacy ECN     |     ECT(0)     |       White      |
        |   Currently Unused  |     --CU--     | Currently unused |
        |                     |                |                  |
        |        Legacy       |     Not-ECT    |       White      |

                  Table 7: Alternative re-ECN Terminology

Authors' Addresses

   Bob Briscoe (editor)
   B54/77, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE

   Phone: +44 1473 645196

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   Arnaud Jacquet
   B54/70, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE

   Phone: +44 1473 647284

   Toby Moncaster
   Layer Marney
   Colchester  CO5 9UZ


   Alan Smith
   B54/76, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE

   Phone: +44 1473 640404

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