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Transport Area Working Group                                  B. Briscoe
Internet-Draft                                                  BT & UCL
Expires: September 7, 2006                                    A. Jacquet
                                                            A. Salvatori
                                                                      BT
                                                          March 06, 2006


     Re-ECN: Adding Accountability for Causing Congestion to TCP/IP
                   draft-briscoe-tsvwg-re-ecn-tcp-01

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   This Internet-Draft will expire on September 7, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document introduces a new protocol for explicit congestion
   notification (ECN), termed re-ECN, which can be deployed
   incrementally around unmodified routers.  The protocol arranges 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.  Then the upstream party at



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   any trust boundary in the internetwork can be held responsible for
   the congestion they cause, or allow to be caused.  So, networks can
   introduce straightforward accountability and policing mechanisms for
   incoming traffic from end-customers or from neighbouring network
   domains.  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 also
   gives examples of mechanisms that can use the protocol to ensure data
   sources respond correctly to congestion.  And it describes example
   mechanisms that ensure the dominant selfish strategy of both network
   domains and end-points will be to set the extended ECN field
   honestly.

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

   This document is posted as an Internet-Draft with the intent (at
   least that of the authors) to eventually progress to standards track.

   Although the re-ECN protocol is intended to make a simple but far-
   reaching change to the Internet architecture, the most immediate
   priority for the authors is to delay any move of the ECN nonce to
   Proposed Standard status.

   The ECN nonce is an experimental RFC that allows /senders/ to check
   the integrity of congestion feedback from /networks/.  Therefore the
   nonce only helps in scenarios where the sender is trusted to control
   network congestion.  On the other hand, the re-ECN protocol aims to
   allow networks themselves to be able to police cheating senders and
   receivers and to police 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 by two factors:

   o  The ECN nonce would permanently consumes a two-bit codepoint in
      the IP header for a purpose specific to a limited trust model.
      Although the nonce is a neat idea, its applicability seems too
      limited to warrant space in the IP header;

   o  Although 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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  6
   3.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Background and Applicability . . . . . . . . . . . . . . .  7
     3.2.  Re-ECN Abstracted Network Layer Wire Protocol (IPv4 or
           v6)  . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Re-ECN Protocol Operation  . . . . . . . . . . . . . . . .  9
     3.4.  Informal Terminology . . . . . . . . . . . . . . . . . . . 11
   4.  Transport Layers . . . . . . . . . . . . . . . . . . . . . . . 13
     4.1.  TCP  . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
       4.1.1.  RECN mode: Full re-ECN capable transport . . . . . . . 14
       4.1.2.  RECN-Co mode: Re-ECT Sender with a Vanilla or
               Nonce ECT Receiver . . . . . . . . . . . . . . . . . . 17
       4.1.3.  Capability Negotiation . . . . . . . . . . . . . . . . 18
       4.1.4.  Extended ECN (EECN) Field Settings during Flow
               Start or after Idle Periods  . . . . . . . . . . . . . 20
       4.1.5.  Pure ACKS, Retransmissions, Window Probes and
               Partial ACKs . . . . . . . . . . . . . . . . . . . . . 23
     4.2.  Other Transports . . . . . . . . . . . . . . . . . . . . . 24
       4.2.1.  Guidelines for Adding Re-ECN to Other Transports . . . 24
   5.  Network Layer  . . . . . . . . . . . . . . . . . . . . . . . . 24
     5.1.  Re-ECN IPv4 Wire Protocol  . . . . . . . . . . . . . . . . 24
     5.2.  Re-ECN IPv6 Wire Protocol  . . . . . . . . . . . . . . . . 26
     5.3.  Router Forwarding Behaviour  . . . . . . . . . . . . . . . 26
     5.4.  Justification for Setting the First SYN to FNE . . . . . . 27
     5.5.  Control and Management . . . . . . . . . . . . . . . . . . 28
       5.5.1.  Negative Balance Warning . . . . . . . . . . . . . . . 28
       5.5.2.  Rate Response Control  . . . . . . . . . . . . . . . . 28
     5.6.  Tunnels  . . . . . . . . . . . . . . . . . . . . . . . . . 29
     5.7.  Non-Issues . . . . . . . . . . . . . . . . . . . . . . . . 29
   6.  Applications . . . . . . . . . . . . . . . . . . . . . . . . . 29
     6.1.  Policing Congestion Response . . . . . . . . . . . . . . . 29
       6.1.1.  The Policing Problem . . . . . . . . . . . . . . . . . 29
       6.1.2.  Incentive Framework  . . . . . . . . . . . . . . . . . 30
       6.1.3.  Egress Dropper . . . . . . . . . . . . . . . . . . . . 36
       6.1.4.  Rate Policing  . . . . . . . . . . . . . . . . . . . . 37
       6.1.5.  Inter-domain Policing  . . . . . . . . . . . . . . . . 39
       6.1.6.  Simulations  . . . . . . . . . . . . . . . . . . . . . 39



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     6.2.  Other Applications . . . . . . . . . . . . . . . . . . . . 40
       6.2.1.  DDoS Mitigation  . . . . . . . . . . . . . . . . . . . 40
       6.2.2.  End-to-end QoS . . . . . . . . . . . . . . . . . . . . 41
       6.2.3.  Traffic Engineering  . . . . . . . . . . . . . . . . . 41
       6.2.4.  Inter-Provider Service Monitoring  . . . . . . . . . . 41
     6.3.  Limitations  . . . . . . . . . . . . . . . . . . . . . . . 41
   7.  Incremental Deployment . . . . . . . . . . . . . . . . . . . . 41
     7.1.  Incremental Deployment Features  . . . . . . . . . . . . . 42
     7.2.  Incremental Deployment Incentives  . . . . . . . . . . . . 42
   8.  Architectural Rationale  . . . . . . . . . . . . . . . . . . . 47
   9.  Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 50
     9.1.  Policing Rate Response to Congestion . . . . . . . . . . . 50
     9.2.  Congestion Notification Integrity  . . . . . . . . . . . . 50
     9.3.  Identifying Upstream and Downstream Congestion . . . . . . 51
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 51
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 52
   12. Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 52
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 53
   14. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 53
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 53
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 53
     15.2. Informative References . . . . . . . . . . . . . . . . . . 54
   Appendix A.  Precise Re-ECN Protocol Operation . . . . . . . . . . 56
   Appendix B.  ECN Compatibility . . . . . . . . . . . . . . . . . . 57
   Appendix C.  Packet Marking During Flow Start  . . . . . . . . . . 58
   Appendix D.  Example Egress Dropper Algorithm  . . . . . . . . . . 59
   Appendix E.  Re-TTL  . . . . . . . . . . . . . . . . . . . . . . . 59
   Appendix F.  Policer Designs to ensure Congestion
                Responsiveness  . . . . . . . . . . . . . . . . . . . 59
     F.1.  Per-user Policing  . . . . . . . . . . . . . . . . . . . . 59
     F.2.  Per-flow Rate Policing . . . . . . . . . . . . . . . . . . 61
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 64
   Intellectual Property and Copyright Statements . . . . . . . . . . 65


















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1.  Introduction

   This document aims:

   o  To provide a complete specification of 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;

   o  To show how a number of hard problems become much easier to solve
      once re-ECN is available in IP.

   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
   compliance to congestion control; and so on.

   Uniquely, re-ECN manages to enable solutions to these problems
   without unduly stifling innovative new ways to use the Internet.
   This was a hard balance to strike, given it could be argued that DDoS
   is an innovative way to use the Internet.  The most valuable insight
   was to allow each network to choose the level of constraint it wishes
   to impose.  Also re-ECN has been carefully designed so that networks
   that choose to use it conservatively can protect themselves against
   the congestion caused in their network by users on other networks
   with more liberal policies.

   For instance, some network owners want to block applications like
   voice and video unless their network is compensated for the extra
   share of bottleneck bandwidth taken.  These real-time applications
   tend to be unresponsive when congestion arises.  Whereas elastic TCP-
   based applications back away quickly, ending up taking a much smaller
   share of congested capacity for themselves.  Other network owners
   want to invest in large amounts of capacity and make their gains from
   simplicity of operation and economies of scale.

   Re-ECN allows the more conservative networks to police out flows that
   have not asked to be unresponsive to congestion---not because they
   are voice or video---just because they don't respond to congestion.
   But it also allows other networks to choose not to police.
   Crucially, when flows from liberal networks cross into a conservative
   network, re-ECN enables the conservative network to apply penalties
   to its neighbouring networks for the congestion they cause.  And
   these penalties can be applied to bulk data, without regard to flows.



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   Then, if unresponsive applications become so dominant that some of
   the more liberal networks experience congestion collapse [RFC3714],
   they can change their minds and use re-ECN to apply tighter controls
   in order to bring congestion back under control.

   Re-ECN works by arranging that each packet arrives at each network
   element carrying a view of expected congestion on its own downstream
   path, albeit averaged over multiple packets.  Most usefully,
   congestion on the remainder of the path becomes visible in the IP
   header at the first ingress.  Many of the applications of re-ECN
   involve a policer at this ingress using the view of downstream
   congestion arriving in packets to police or control the packet rate.

   Importantly, the scheme is recursive: a whole network harbouring
   users causing congestion in downstream networks can be held
   responsible or policed by its downstream neighbour.

   This document is structured as follows.  First an overview of the re-
   ECN protocol is given (Section 3), outlining its attributes and
   explaining conceptually how it works as a whole.  The two main parts
   of the document follow, as described above.  That is, the protocol
   specification divided into transport (Section 4) and network
   (Section 5) layers, then the applications it can be put to, such as
   policing DDoS, QoS and congestion control (Section 6).  Although
   these applications do not require standardisation themselves, they
   are described in a fair degree of detail in order to explain how re-
   ECN can be used.  Given, re-ECN proposes to use the last undefined
   bit in the IPv4 header, we felt it necessary to outline the potential
   that re-ECN could release in return for being given that bit.

   Deployment issues discussed throughout the document are brought
   together in Section 7, which is followed by a brief section
   explaining the somewhat subtle rationale for the design, from an
   architectural perspective (Section 8).  We end by describing related
   work (Section 9), listing security considerations (Section 10) and
   finally drawing conclusions (Section 12).


2.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   This document first specifies a protocol, then describes a framework
   that creates the right incentives to ensure compliance to the
   protocol.  This could cause confusion because the second part of the
   document considers many cases where malicious nodes may not comply



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   with the protocol.  When such contingencies are described, if any of
   the above keywords are not capitalised, that is deliberate.  So, for
   instance, the following two apparently contradictory sentences would
   be perfectly consistent: i) x MUST do this; ii) x may not do this.


3.  Protocol Overview

3.1.  Background and Applicability

   First 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 routers probabilistically set "11" if congestion is
   experienced (CE), the marking probability increasing with the length
   of the queue at its egress link (the RED algorithm [RFC2309]).
   However, they still drop rather than mark Not-ECT packets.  With
   multiple ECN-capable routers on a path, a flow of packets accumulates
   the fraction of CE marking that each router adds.  The combined
   effect of the packet marking of all the routers along the path
   signals congestion of the whole path to the receiver.  So, for
   example, if one router early in a path is marking 1% of packets and
   another later in a path is marking 2%, flows that pass through both
   routers 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 9.2 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
   appropriately.

   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.  Indeed, it even
   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



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   control congestion 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 routers 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.  Changes to IP routers are
   RECOMMENDED 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 7 summarises the incremental deployment strategy.

   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 only concerned with enabling the ingress
   network to police that a source is complying with a congestion
   control algorithm, which is orthogonal to congestion control itself.

   Before re-ECN can be considered worthy of using up the last bit in
   the IP header, we must be sure that all our claims are robust.  We
   have gradually been reducing the list of outstanding issues, but the
   few that still remain are listed in Section 6.3.  We expect others
   may find new attacks, but we offer the re-ECN protocol on the basis
   that it is built on fairly solid theoretical foundations and, so far,
   it has proved possible to keep it relatively robust.

3.2.  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 three differences of
   detail (see Section 5.3).  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.
   Until then it will suffice to use an abstraction of the IPv4 and v6
   wire protocols by just calling it the RE flag.

   Unlike the ECN field, the RE flag is intended to be set by the sender
   and 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. [Re-PCN]).

   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) making eight
   codepoints.  We will use the RFC3168 names of the ECN codepoints to



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   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.

   +-------+------------+------+---------------+-----------------------+
   |  ECN  | RFC3168    |  RE  | Extended ECN  |     Re-ECN meaning    |
   | field | codepoint  | flag | codepoint     |                       |
   +-------+------------+------+---------------+-----------------------+
   |   00  | Not-ECT    |   0  | Not-RECT      |   Not re-ECN-capable  |
   |       |            |      |               |       transport       |
   |   00  | Not-ECT    |   1  | FNE           |      Feedback not     |
   |       |            |      |               |      established      |
   |   01  | ECT(1)     |   0  | Re-Echo       |  Re-echoed congestion |
   |       |            |      |               |        and RECT       |
   |   01  | ECT(1)     |   1  | RECT          |     Re-ECN capable    |
   |       |            |      |               |       transport       |
   |   10  | ECT(0)     |   0  | ---           |  Legacy ECN use only  |
   |       |            |      |               |                       |
   |   10  | ECT(0)     |   1  | --CU--        |    Currently unused   |
   |       |            |      |               |                       |
   |   11  | CE         |   0  | CE(0)         |       Congestion      |
   |       |            |      |               |    experienced with   |
   |       |            |      |               |        Re-Echo        |
   |   11  | CE         |   1  | CE(-1)        |       Congestion      |
   |       |            |      |               |      experienced      |
   +-------+------------+------+---------------+-----------------------+

                     Table 1: Extended ECN Codepoints

3.3.  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.

   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 sets the feedback not established (FNE)
   codepoint.  The network reads the FNE codepoint conservatively as
   equivalent to re-echoed congestion.

   Specifically, once a flow is established, a re-ECN sender always
   initialises the ECN field to ECT(1).  And it usually sets the RE flag



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   to "1".  Whenever a router re-marks a packet to CE, the receiver
   feeds 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.

   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".

       ^
       |
       |       RE blanking fraction
    3% |--------------------------------+=====
       |                                |
    2% |                                |
       |            CE marking fraction |
    1% |        +-----------------------+
       |        |
    0% +---------------------------------------->
          ^     0     ^                 i    ^    resource index
          |     ^     |                 ^    |
          0     |     1                 |    2     observation points
              1.00%                  2.00%         marking fraction

   Figure 1: A 2-Router Example (Imprecise)

   Figure 1 uses the two router example introduced earlier to illustrate
   why re-ECN allows routers to measure downstream congestion.  The
   horizontal axis represents the index of each congestible resource
   (typically queues) along a path through the Internet.  There may be
   many routers on the path, but we assume only two are currently
   congested (those with resource index 0 and i).  The two superimposed
   plots show the fraction of each extended ECN codepoint in a flow
   observed along this path.  Given about 3% of packets reaching the
   destination are marked CE, in response to feedback the sender will
   blank the RE flag in about 3% of packets it sends.  Then 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).








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           +-------------------+------------------------------+
           | Observation point | Approx downstream congestion |
           +-------------------+------------------------------+
           |         0         |         3% - 0% = 3%         |
           |         1         |         3% - 1% = 2%         |
           |         2         |         3% - 3% = 0%         |
           +-------------------+------------------------------+

   Table 2: Downstream Congestion Measured at Example Observation Points

   All along the path, whole-path congestion 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.  To be absolutely clear, these
   fractions are averages that would result from the behaviour of a TCP
   protocol handler mechanically blanking outgoing packets in direct
   response to incoming feedback---we are not saying any protocol
   handler works with these average fractions directly.

3.4.  Informal Terminology

   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.  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).

   Just as we will loosely talk of positive and negative flows, we will
   also talk of positive or negative packets, meaning packets that
   contribute positively or negatively to downstream congestion.

   Therefore packets can be considered to have a `worth' of +1, 0 or -1,
   which, when multiplied by their size, indicates their contribution to
   downstream congestion.  Figure 2 shows the main state transitions of
   the system once a flow is established, showing the worth of packets
   in each state.  When the network congestion marks a packet it
   decrements its worth.  When the sender blanks the RE flag in order to
   re-echo congestion it increments the worth of a packet.






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   Sender state         Sent    Worth  Network    Received Worth
                        packet         Congestion packet
            +----------------------------------------------------+
            |                                                    ^
            V                                                    |
   Congestion echoed -->Re-Echo  +1      -->      CE(0)      0 --+
                          /                                      |
        No congestion___/                                        |
                   /    \                                        |
                  V       \                                      |
   Flow established --> RECT      0      -->      CE(-1)    -1 --+

   Figure 2: Re-ECN System State Diagram (bootstrap not shown)

   The idea is that 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 packets should arrive at the receiver as
   negative.  It is this balance that will allow the network to hold the
   sender accountable for the congestion it causes, as we shall see.

   If we start with the sender in `flow established' state, normally it
   goes round the tight sub-loop, sending RECT packets (worth nothing)
   and returning to the flow established state to send another one.  But
   if one of the packets is congestion marked, its worth is decremented.
   The sender will have been continuing round its tight sending loop.
   But when congestion feedback returns from one of the packets in
   flight (the largest loop in the figure) the sender jumps to the
   congestion echoed state in order to re-echo the congestion,
   incrementing the worth of the next packet by blanking its RE bit.
   The sender then returns to the flow established state and continues
   in the tight loop sending zero worth.

   If a packet carrying re-echoed congestion happens to also be
   congestion marked, the worth added by the sender will be cancelled
   out by the 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 top sub-
   loop).

   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.  The FNE
   codepoint is an exception.  It is used in the bootstrap process
   (explained later) and has the same positive worth as a packet with
   the Re-Echo codepoint.




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   +--------+------+----------------+-------+--------------------------+
   |   ECN  |  RE  | Extended ECN   | Worth |      Re-ECN meaning      |
   |  field |  bit | codepoint      |       |                          |
   +--------+------+----------------+-------+--------------------------+
   |   00   |   0  | Not-RECT       | ...   |    Not re-ECN-capable    |
   |        |      |                |       |         transport        |
   |   01   |   0  | Re-Echo        | +1    | Re-echoed congestion and |
   |        |      |                |       |           RECT           |
   |   10   |   0  | ---            | ...   |  Legacy ECN use only     |
   |   11   |   0  | CE(0)          |  0    |  Congestion experienced  |
   |        |      |                |       |       with Re-Echo       |
   |   00   |   1  | FNE            | +1    | Feedback not established |
   |   01   |   1  | RECT           |  0    | Re-ECN capable transport |
   |   10   |   1  | --CU--         | ...   |     Currently unused     |
   |        |      |                |       |                          |
   |   11   |   1  | CE(-1)         | -1    |  Congestion experienced  |
   +--------+------+----------------+-------+--------------------------+

                Table 3: 'Worth' of Extended ECN Codepoints


4.  Transport Layers

4.1.  TCP

   Re-ECN capability at the sender is essential.  At the receiver it is
   optional, as long as the receiver has a basic (`vanilla flavour')
   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 handshakes.

   +--------+---------------+-----------+---------+--------------------+
   | 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



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

   RECN: Full re-ECN capable transport

   RECN-Co: Re-ECN sender in compatibility mode with a vanilla [RFC3168]
      ECN receiver or an [RFC3540] ECN nonce-capable receiver.

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

   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.

4.1.1.  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, modulo 8,
   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
   previous versions of ECN are used as a 3-bit field for the receiver
   to repeatedly tell the sender the current value of ECC 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 4.  The actual definition of the
   TCP header, including the addition of support for the ECN nonce, is
   shown for comparison in Figure 3.  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 3: The (post-ECN Nonce) definition of bytes 13 and 14 of the
   TCP Header




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        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 4: 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 modulo 8
      and MUST echo its value 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 again.


      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
      implemented.

   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 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 (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, 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 4.1.4.




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      As we have already emphasised, the re-ECN protocol makes no
      changes and has no effect on the TCP congestion control algorithm.
      So, each increment of ECI (or detection of a drop) also triggers
      the standard TCP congestion response, but with no more than one
      congestion response per round trip, as usual.


      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 data packet 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
      originally.

4.1.1.1.  Safety against Long Pure ACK Loss Sequences

   The ECI method was chosen for echoing congestion marking because a
   re-ECN sender needs to know about every CE mark arriving at the
   receiver, not just whether at least one arrives within a round trip
   time (which is all the ECE/CWR mechanism supported).  But pure ACKs
   are not protected by TCP reliable delivery, so we repeat the same ECI
   value in every ACK until it changes.  Even if many ACKs in a row are
   lost, as soon as one gets through, the ECI field it repeats from
   previous ACKs that didn't get through will update the sender on how
   many CE marks arrived since the last ACK got through.

   The sender will only lose a record of the arrival of a CE mark if all
   the ACKS are lost (and all of them were pure ACKs) for a stream of
   data long enough to contain 8 or more CE marks.  So, if the marking
   fraction was p, at least 8/p pure ACKs would have to be lost.  For
   example, if p was 5%, a sequence of 160 pure ACKs would all have to
   be lost.  To protect against such extremely unlikely events, if a re-
   ECN sender detects a sequence of pure ACKs has been lost it SHOULD
   assume the ECI field wrapped as many times as possible within the
   sequence.

   Specifically, if a re-ECN sender receives an ACK with an
   acknowledgement number that acknowledges L segments since the
   previous ACK but with a sequence number unchanged from the previously
   received ACK, it SHOULD conservatively assume that the ECI field
   incremented by D' = L - ((L-D) mod 8), where D is the apparent
   increase in the ECI field.  For example if the ACK arriving after 9
   pure ACK losses apparently increased ECI by 2, the assumed increment
   of ECI would still be 2.  But if ECI apparently increased by 2 after
   11 pure ACK losses, ECI should be assumed to have increased by 10.



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   A re-ECN sender MAY implement a heuristic algorithm to predict beyond
   reasonable doubt that the ECI field probably did not wrap within a
   sequence of lost pure ACKs.  But such an algorithm is NOT REQUIRED.
   Such an algorithm MUST NOT be used unless it is proven to work even
   in the presence of correlation between high ACK loss rate on the back
   channel and high CE marking rate on the forward channel.

   Whatever assumption a re-ECN sender makes about potentially lost CE
   marks, both its congestion control and its re-echoing behaviour
   SHOULD be consistent with the assumption it makes.

4.1.2.  RECN-Co mode: Re-ECT Sender with a Vanilla or Nonce ECT Receiver

   If the half-connection is in RECN-Co mode, ECN feedback proceeds no
   differently to that of vanilla 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 vanilla ECN feedback to set or clear the RE
   flag reasonably well.  Essentially, 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 sets the IP header
   the same as it would do in full RECN mode.  Specifically, the re-ECN
   sender MUST clear the RE flag to "0" in the next packet.  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
   (Section 6.1.3).  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 vanilla 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
   deployment.

   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 vanilla ECT mode in the presence
   of an ECN-capable receiver.  It MAY choose to fall back to the ECT-



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   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.

4.1.2.1.  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
   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.

4.1.3.  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 options 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.  A Re-ECT server (B) can use either setting of the
   NS flag combined with this type of SYN ACK in response to a SYN from
   a Re-ECT client (A).  Normally a Re-ECT server will reply to a Re-ECT
   client with NS=0, but under special circumstances described in
   Section 4.1.4 it can return a SYN ACK with NS=1.

   These handshakes are summarised in Table 5 below, with X meaning



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   `don't care'.  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.

   +----+---+---+---+------------+-------------+-----------+-----------+
   | 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 SYNs within a connection.}

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




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   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
   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.

4.1.4.  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
   (Section 3.2).  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.

   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 [I-D.ietf-tsvwg-ecnsyn] has shown
   this caution was unnecessary, and proposes to allow a SYN ACK to be
   ECN-capable to improve performance.  However, our use of FNE on the
   initial SYN seems to comply with this aim in word but not in spirit,
   so a justification for choosing to set RE to 1 for a SYN is given in
   Section 5.4.

   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-
   connection:

   RECN mode: Given the constraints on TCP's initial window [RFC3390]
      and its exponential window increase during slow start
      phase [RFC2581], it turns out that the sender SHOULD set FNE on
      the first and third data packets in its flow, assuming equal sized



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      data packets once a flow is established.  Appendix C 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.

   RECN-Co mode: A re-ECT sender that switches into re-ECN compatibility
      mode (because it has detected the corresponding host is ECN-
      capable but 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 A sets in the
   TCP header.  The IP columns show the setting of the extended ECN
   (EECN) field.



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

   After an idle period of more than 1 second, a re-ECN sender MUST set
   the EECN field of the next packet it sends to FNE.  In order that the
   design of network policers can be deterministic, this specification
   deliberately puts an absolute lower limit on how long a connection
   can be idle before the next packet must be FNE, rather than relating
   it to the connection round trip time.  We use the lower bound of the
   retransmission timeout (RTO) [RFC2988], which is commonly used as the
   idle period before TCP must reduce to the restart window [RFC2581].



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   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 legacy modes if packets
   don't appear to be getting through (to work round firewalls
   discarding packets they consider unusual).}

4.1.5.  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 routers.

   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 (NRECN) 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.

   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



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   of packets being sent (typically the MSS).}

4.2.  Other Transports

4.2.1.  Guidelines for Adding Re-ECN to Other Transports

   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 in packets carrying 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.

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

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

   o  UDP streaming with no feedback

   o  UDP streaming with feedback

   o  DCCP}

   o  RSVP and/or NSIS: A separate I-D has been submitted [Re-PCN]
      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 when downstream congestion is too high, even though
      the congestion is in other operators' downstream networks.  This
      relates to current work in progress on Admission Control over
      Diffserv using Pre-Congestion Notification, being reported to the
      IETF TSVWG [CL-arch].


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



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   call the RE (re-ECN extension) flag (Section 3.2) 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 5).

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

   Figure 5: New Definition of the Re-ECN Extension (RE) Control Flag at
   the Start of Byte 7 of the IPv4 Header

   It is believed that the RE flag 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 by David Clark and
   documented by Handley and Greenhalgh [Steps_DoS].  The RE flag 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 lost).

   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 former.

   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,



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   because they declare that they are good less often than they should.

5.2.  Re-ECN IPv6 Wire Protocol

   {ToDo: Include the IPv6 extension header design, including support
   for the FNE flag.  Also its integrated support for a future multi-bit
   congestion notification field, with a TTL hop count scheme to check
   that all routers on the path support it (similar to Quick-Start).
   So, if the whole path of routers doesn't support the extension, the
   end-points can fall back to re-ECN (or drop).}

5.3.  Router Forwarding Behaviour

   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.  [Re-PCN] is one example.

   FNE indicates ECT:

      The FNE codepoint indicates to a router that the packet was sent
      and will be received by an ECN-capable transport.  Therefore an
      FNE packet MAY be marked rather than dropped.  Note that the FNE
      codepoint has been intentionally chosen so that, to legacy routers
      (which do not inspect the RE flag), an FNE packet appears to be
      Not-ECT, so will be dropped by legacy AQM algorithms.

      A network operator MUST NOT configure a router 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 Section 6.1.4 would count as rate limiters
      for this purpose.

   Preferential Drop: If a re-ECN capable router 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 7.  Preferential dropping is difficult to implement, but if
      feasible it would discriminate against attack traffic, if done as
      part of the overall policing framework of Section 6.1.2.  If
      nowhere else, routers at the egress of a network SHOULD implement
      preferential drop (stronger than the MAY above).  For simplicity,
      preferences 3,4 & 5 MAY be merged into one preference level.



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   +-------+-----+------------+-------+-------------+------------------+
   |  ECN  |  RE | Extended   | Worth | Drop Pref   |  Re-ECN meaning  |
   | field | bit | ECN        |       | (1 = drop   |                  |
   |       |     | codepoint  |       | 1st)        |                  |
   +-------+-----+------------+-------+-------------+------------------+
   |   01  |  0  | Re-Echo    | +1    | 7           |     Re-echoed    |
   |       |     |            |       |             |  congestion and  |
   |       |     |            |       |             |       RECT       |
   |   00  |  1  | FNE        | +1    | 6           |   Feedback not   |
   |       |     |            |       |             |    established   |
   |   11  |  0  | CE(0)      | 0     | 5           |    Congestion    |
   |       |     |            |       |             | experienced with |
   |       |     |            |       |             |      Re-Echo     |
   |   01  |  1  | RECT       | 0     | 4           |  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           |  Legacy ECN use  |
   |       |     |            |       |             |       only       |
   |   00  |  0  | Not-RECT   | n/a   | 1           |        Not       |
   |       |     |            |       |             |  re-ECN-capable  |
   |       |     |            |       |             |     transport    |
   +-------+-----+------------+-------+-------------+------------------+

       Table 7: Drop Preference of EECN Codepoints (Sorted by `Worth')

      The above drop preferences are arranged to preserve packets with
      more positive worth (Section 3.4), given senders of positive
      packets must have honestly declared downstream congestion.  This
      is explained fully in Section 6 on applications.

5.4.  Justification for Setting the First SYN to FNE

   We require clients to consider the first SYN as congestion marked if
   they find out at the end of the handshake that the server was not Re-
   ECT capable.  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.  Malicious clients may think they can use the advantage
   that ECN-marking gives over drop in launching classic SYN-flood
   attacks.  But the rate limit on FNE codepoints performed by the
   ingress policer should be a sufficient countermeasure.

   If the server is re-ECN capable, provision is made for it to echo a
   possible congestion marking.  Congested routers may mark an FNE
   packet to CE (see Section 5.3), in which case the packet will arrive
   at B with an extended ECN codepoint of CE(-1).  So, if the initial



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   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 (see Table 5).

   It might seem pedantic worrying about these single packets, but this
   behaviour ensures the system is safe, even if the application mix on
   the Internet 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.

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
   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

   The framework of Section 6.1.2 implies the need for a sender to send
   a request to an ingress policer asking that it be allowed to apply a
   non-default response to congestion (where TCP-friendly is assumed to
   be the default).  This would require the sender to be able to
   discover how to address the policer.  And message format(s) would
   have to be defined.  The required control protocol(s) are outside the
   scope of this document, but will require definition elsewhere.



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   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) 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.  Tunnels

   For tunnels to work correctly, re-ECN largely requires no more than
   the tunnel handling of regular ECN [RFC3168].  The RE flag raises an
   extra issue, but it is more straightforward than the ECN field
   because it is not intended to change along the path.  Therefore a
   tunnel entry point only needs to copy the RE flag into the
   encapsulating header, without any need to negotiate whether the
   tunnel exit supports RE flag handling.

   {ToDo: However, there are some issues to discuss concerning tunnels,
   which will be included in a future version of this draft}

5.7.  Non-Issues

   {ToDo: This section will explain why the addition of re-ECN does not
   interact with any of the following:

   o  Integration with congestion notification in various link layers
      (Ethernet, ATM (and MPLS if it had a congestion notification
      capability added, which is not precluded for the EXP field
      [RFC3270])

   o  Tunnels, and Overlays that wish to support congestion notification
      (see also the brief discussion of edge-to-edge support for re-ECN
      in RSVP or NSIS transports earlier)

   o  Encryption and IPSec

   }


6.  Applications

6.1.  Policing Congestion Response

6.1.1.  The Policing Problem

   The current Internet architecture trusts hosts to respond voluntarily



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   to congestion.  Limited evidence shows that the large majority of
   end-points on the Internet comply with a TCP-friendly response to
   congestion.  But telephony (and increasingly video) services over the
   best efforts Internet are attracting the interest of major commercial
   operations.  Most of these applications do not respond to congestion
   at all.  Those that can switch to lower rate codecs, still have a
   lower bound below which they must become unresponsive to congestion.

   Even TCP-friendly applications can cause a disproportionate amount of
   congestion, simply by using multiple flows or by transferring data
   continuously.  Also the Internet Architecture has few defences
   against distributed denial of service attacks that combine both
   problems: unresponsiveness to congestion and flooding with multiple
   flows.

   Applications that need (or choose) to be unresponsive to congestion
   can effectively steal whatever share of bottleneck resources they
   want from responsive flows.  Whether or not such free-riding is
   common, inability to prevent it increases the risk of poor returns
   for investors in network infrastructure, leading to under-investment.
   An increasing proportion of unresponsive, free-riding demand coupled
   with persistent under-supply is a broken economic cycle.  Therefore,
   if the current, largely co-operative consensus continues to erode,
   congestion collapse could become more common in more areas of the
   Internet [RFC3714].

   However, while we have designed re-ECN to provide a way to solve
   these problems, this does not imply we advocate that every network
   should introduce tight controls on those that cause congestion.  Re-
   ECN has been specifically designed to allow different networks to
   choose how conservative or liberal they wish to be with respect to
   policing congestion.  But those that choose to be conservative can
   protect themselves from the excesses that liberal networks allow
   their users.

6.1.2.  Incentive Framework

   The aim is to create an incentive environment that ensures optimal
   sharing of capacity despite everyone acting selfishly (including
   lying and cheating).  Of course, the mechanisms put in place for this
   can lie dormant wherever co-operation is the norm.

   Throughout this document we focus on path congestion.  But most forms
   of fairness, including TCP's, also depend on round trip time.  So, we
   also propose to measure downstream path delay using re-feedback.
   This proposal will be published in a very simple future draft, but
   for now we give an outline in Appendix E.




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   Figure 6 sketches the incentive framework that we will describe piece
   by piece throughout this section.  We will do a first pass in
   overview, then return to each piece in detail.  An internetwork with
   multiple trust boundaries is depicted.  The difference between the
   two plots in the example we used earlier Figure 1 is plotted below.
   The graph displays downstream path congestion seen in a typical flow
   as it traverses an example path from sender S to receiver R, across
   networks N1, N2 & N4.  Everyone is shown using re-ECN, but we intend
   to show why everyone would /choose/ to use it, correctly and
   honestly.

   Two main types of self-interest can be identified:

   o  Users want to transmit data across the network as fast as
      possible, paying as little as possible for the privilege.  In this
      respect, there is no distinction between senders and receivers,
      but we must be wary of potential malice by one on the other;

   o  Network operators want to maximise revenues from the resources
      they invest in.  They compete amongst themselves for the custom of
      users.


         policer
       A  |
       |  |
       |S <-----N1----> <---N2---> <---N4--> R         domain
       |: :                                :
       |V :                                :
    3% |--------+                          :
       |  :     |                          :
    2% |  :     +-----------------------+  :
       |  :    downstream congestion    |  :
    1% |  :                             |  :
       |  :                             |  :
    0% +--------------------------------+=====-->
                0                       i  ^      resource index
                |                       | /|\
              1.00%                  2.00% |       marking fraction
                                           |
                                        dropper

   Figure 6: Incentive Framework, showing creation of opposing pressures
   to under-declare and over-declare downstream congestion, using a
   policer and a dropper






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   Source congestion control: We want to ensure that the sender will
      throttle its rate as downstream congestion increases.  Whatever
      the agreed congestion response (whether TCP-compatible or some
      enhanced QoS), to some extent it will always be against the
      sender's interest to comply.

   Ingress policing: But it is in all the network operators' interests
      to encourage fair congestion response, so that their investments
      are employed to satisfy the most valuable demand.  N1 is in the
      best position to deploy a policer at its ingress to check that S1
      is complying with congestion control (Section 6.1.4).  But ingress
      policing is not the only possible arrangement.  Re-ECN provides
      the necessary information for dual control of congestion either by
      the sender or by the network ingress.  So, in some scenarios (e.g.
      sensing devices with minimal capabilities) the network ingress
      might do the congestion control as a proxy for the sender.

   Edge egress dropper: If the policer ensures the source has less right
      to a high rate the higher it declares downstream congestion, the
      source has a clear incentive to understate downstream congestion.
      But, if packets are understated when they enter the internetwork,
      they will be negative when they leave.  So, we introduce a dropper
      at the last network egress, which drops packets in flows that
      persistently declare negative downstream congestion (see
      Section 6.1.3 for details).  Incidentally, a network can trivially
      prevent negative traffic from being sent in the first place by not
      permitting a sender to send any CE packets, which would clearly
      contravene the ECN protocol.























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               ..competitive routing
             .'         :      '.
           .'  p e n a l:t i e s '.
          :           | :       \  :
       A  :           | :        | :
       |S <-----N1----> <---N2---> <---N4--> R         domain
       |  :           | :        | :
       |  V           | :        | :
    3% |--------+     | :        | :
       |        |     V V        V V
    2% |        +-----------------------+
       |       downstream congestion    |
    1% |          :                     |
       |          :                     |
    0% +--------------------------------+=====-->
                0                ^      i         resource index
                |               /|\     |
              1.00%              |   2.00%         marking fraction
                                 |
                             sanctions

      Figure 7: Incentives at Inter-domain Borders

   Inter-domain traffic policing: But next we must ask, if congestion
   arises downstream (say in N4), what is the ingress network's (N1's)
   incentive to police its customers' response?  If N1 turns a blind
   eye, its own customers benefit while other networks suffer.  This is
   why all inter-domain QoS architectures (e.g. Intserv, Diffserv)
   police traffic each time it crosses a trust boundary.  Re-ECN gives
   trustworthy information at each trust boundary, which N4 (say) can
   use in bulk to police all the responses to congestion of all the
   sources beyond its upstream neighbour (N2) with one very simple
   passive mechanism, as we will now explain using Figure 7.

   But before we do, we need to make a very important point.  In the
   explanation that follows, we assume a very specific variant of volume
   charging between networks.  We must make clear that we are not
   advocating that everyone should use this form of contract.  We are
   well aware that the IETF tries to avoid standardising technology that
   depends on a particular business model.  And we strongly share this
   desire to encourage diversity.  But our aim is merely to show that
   border policing can at least work with this one model, then we can
   assume that operators might experiment with the metric in other
   models (see Section 6.1.5 for examples).  Of course, operators are
   free to complement this usage element of their charges with
   traditional capacity charging, and we expect they will.





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   Emulating policing with inter-domain congestion charging: Between
      high-speed networks, we would rather avoid holding back traffic
      while it is policed.  Instead, once re-ECN has arranged headers to
      carry downstream congestion honestly, N2 can contract to pay N4
      penalties in proportion to a single bulk count of the congestion
      metrics crossing their mutual trust boundary (Section 6.1.5).  In
      this way, N4 puts pressure on N2 to suppress downstream
      congestion, as shown by the solid downward arrow at the egress of
      N2.  Then N2 has an incentive either to police the congestion
      response of its own ingress traffic (from N1) or to charge N1 in
      turn on the basis of congestion counted at their mutual boundary.
      In this recursive way, the incentives for each flow to respond
      correctly to congestion trace back with each flow precisely to
      each source, despite the mechanism not recognising flows (see
      Section 6.2.2).  If N1 turns a blind eye to its own upstream
      customers' congestion response, it will still have to pay its
      downstream neighbours.

   No congestion charging to users: Bulk congestion charging at trust
      boundaries is passive and extremely simple, and loses none of its
      per-packet precision from one boundary to the next (unlike
      Diffserv all-address traffic conditioning agreements, which
      dissipate their effectiveness across long topologies).  But at any
      trust boundary, there is no imperative to use congestion charging.
      Traditional traffic policing can be used, if the complexity and
      cost is preferred.  In particular, at the boundary with end
      customers (e.g. between S and N1), traffic policing will most
      likely be far more appropriate.  Policer complexity is less of a
      concern at the edge of the network.  And end-customers are known
      to be highly averse to the unpredictability of congestion
      charging.

      So, NOTE WELL: this document neither advocates nor requires
      congestion charging for end customers and advocates but does not
      require inter-domain congestion charging.

   Competitive discipline of inter-domain traffic engineering: With
      inter-domain congestion charging, a domain seems to have a
      perverse incentive to fake congestion; N2's profit depends on the
      difference between congestion at its ingress (its revenue) and at
      its egress (its cost).  So, overstating internal congestion seems
      to increase profit.  However, smart border routing [Smart_rtg] by
      N1 will bias its multipath routing towards the least cost routes.
      So, N2 risks losing all its revenue to competitive routes if it
      overstates congestion (see Section 6.2.3).  In other words, if N2
      is the least congested route, its ability to raise excess profits
      is limited by the congestion on the next least congested route.
      This pressure on N2 to remain competitive is represented by the



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      dotted downward arrow at the ingress to N2 in Figure 7.

   Closing the loop: All the above elements conspire to trap everyone
      between two opposing pressures (upper half of Figure 6), ensuring
      the downstream congestion metric arrives at the destination
      neither above nor below zero.  So, we have arrived back where we
      started in our argument.  The ingress edge network can rely on
      downstream congestion declared in the packet headers presented by
      the sender.  So it can police the sender's congestion response
      accordingly.

6.1.2.1.  The Case against Classic Feedback

   A system that produces an optimal outcome as a result of everyone's
   selfish actions is extremely powerful.  But why do we have to change
   to re-ECN to achieve it?  Can't classic congestion feedback (as used
   already by standard ECN) be arranged to provide similar incentives?
   Superficially it can.  Given ECN already existed, this was the
   deployment path Kelly proposed for his seminal work that used self-
   interest to optimise a system of networks and users (summarised in
   [Evol_cc]).  The mechanism was nearly identical to volume charging;
   except only the volume of packets marked with congestion experienced
   (CE) was counted.

   However, below we explain why relying on classic feedback /required/
   congestion charging to be used, while re-ECN achieves the same
   powerful outcome, but does not /require/ congestion charging.  In
   brief, the problem with classic feedback is that the incentives have
   to trace the indirect path back to the sender---the long way round
   the feedback loop.  For example, if classic feedback were used in
   Figure 6, N2 would have had to influence N1 via N4, R & S rather than
   directly.

   Inability to agree what is happening downstream: In order to police
      its upstream neighbour's congestion response, the neighbours
      should be able to agree on the congestion to be responded to.
      Whatever the feedback regime, as packets change hands at each
      trust boundary, any path metrics they carry are verifiable by both
      neighbours.  But, with a classic path metric, they can only agree
      on the /upstream/ path congestion.

   Inaccessible back-channel: The network needs a whole-path congestion
      metric to control the source.  Classically, whole path congestion
      emerges at the destination, to be fed back from receiver to sender
      in a back-channel.  But, in any data network, back-channels need
      not be visible to relays, as they are essentially communications
      between the end-points.  They may be encrypted, asymmetrically
      routed or simply omitted, so no network element can reliably



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      intercept them.  The congestion charging literature solves this
      problem by charging the receiver and assuming this will cause the
      receiver to refer the charges to the sender.  But, of course, this
      creates unintended side-effects...

   `Receiver pays' unacceptable: In connectionless datagram networks,
      receivers and receiving networks cannot prevent reception from
      malicious senders, so `receiver pays' opens them to `denial of
      funds' attacks.

   End-user congestion charging unacceptable: Even if 'denial of funds'
      were not a problem, we know that end-users are highly averse to
      the unpredictability of congestion charging and anyway, we want to
      avoid restricting network operators to just one retail tariff.
      But with classic feedback only an upstream metric is available, so
      we cannot avoid having to wrap the `receiver pays' money flow
      around the feedback loop, necessarily forcing end-users to be
      subjected to congestion charging.

   To summarise so far, with classic feedback, policing congestion
   response /requires/ congestion charging of end-users and a `receiver
   pays' model, whereas, with re-ECN, incentives can be fashioned either
   by technical policing mechanisms (more appropriate for end users) or
   by congestion charging using the safer `sender pays' model (more
   appropriate inter-domain).

   We now take a second pass over the incentive framework, filling in
   the detail.

6.1.3.  Egress Dropper

   As traffic leaves the last network before the receiver (domain N4 in
   Figure 6), the RE blanking fraction in a flow should match the CE
   congestion marking fraction.  If it is less (a negative flow), it
   implies that the source is understating path congestion (which will
   reduce the penalties that N2 owes N4).

   If flows are positive, N4 need take no action---this simply means its
   upstream neighbour is paying more penalties than it needs to, and the
   source is going slower than it needs to.  But, to protect itself
   against persistently negative flows, N4 should install a dropper at
   its egress.  Appendix D gives a suggested algorithm for the dropper,
   meeting the criteria below.

   o  It SHOULD introduce minimal false positives for honest flows;

   o  It SHOULD quickly detect and sanction dishonest flows (minimal
      false negatives);



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   o  It MUST be invulnerable to state exhaustion attacks from malicious
      sources.  For instance, if the dropper uses flow-state, it should
      not be possible for a source to send numerous packets, each with a
      different flow ID, to force the dropper to exhaust its memory
      capacity.;

   o  It MUST introduce sufficient loss in goodput so that malicious
      sources cannot play off losses in the egress dropper against
      higher allowed throughput.  Salvatori [CLoop_pol] describes this
      attack, which involves the source understating path congestion
      then inserting forward error correction (FEC) packets to
      compensate expected losses.

   Note that the dropper operates on flows but we would like it not to
   require per-flow state.  This is why we have been careful to ensure
   that all flows MUST start with a packet marked with the FNE
   codepoint.  If a flow does not start with the FNE codepoint, a
   dropper is likely to treat it unfavourably.  This risk makes it worth
   setting the FNE codepoint at the start of a flow, even though there
   is a cost to the sender of setting FNE (positive `worth').  Indeed,
   with the FNE codepoint, the rate at which a sender can generate new
   flows can be limited (Appendix F).  In this respect, the FNE
   codepoint works like Clark's state set-up bit [Steps_DoS].

   Appendix F also gives an example dropper implementation that
   aggregates flow state.  Dropper algorithms will often maintain a
   moving average across flows of the fraction of RE blanked packets.
   When maintaining an average across flows, a dropper SHOULD only allow
   flows into the average if they start with FNE, but it SHOULD not
   include packets with the FNE codepoint set in the average.  An
   ingress gateway sets the FNE codepoint when it does not have the
   benefit of feedback from the ingress.  So, counting packets with FNE
   cleared would be likely to make the average unnecessarily positive,
   providing headroom (or should we say footroom?) for dishonest
   (negative) traffic.

   If the dropper detects a persistently negative flow, it SHOULD drop
   sufficient negative and neutral packets to force the flow to not be
   negative.  Drops SHOULD be focused on just sufficient packets in
   misbehaving flows to remove the negative bias while doing minimal
   harm.

6.1.4.  Rate Policing

   Approaches like [XCHOKe] & [pBox] are nice approaches for rate
   policing traffic without the benefit of whole path information, such
   as could be provided by re-ECN.  But they must be deployed at
   bottlenecks in order to work.  Unfortunately, a large proportion of



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   traffic traverses at least two bottlenecks (in the two access
   networks), particularly with the current traffic mix where peer-to-
   peer file-sharing is prevalent.  These `bottleneck policers' could be
   adapted to combine ECN congestion marking from the upstream path with
   local congestion knowledge.  But then the only useful placement for
   them would be close to the egress of the network.

   But then, if these bottleneck policers were widely deployed, the
   Internet would find itself with one universal rate adaptation policy
   (TCP-friendliness) embedded throughout the network.  Given TCP's
   congestion control algorithm is already known to be hitting its
   scalability limits and new algorithms are being developed for high-
   speed congestion control, embedding TCP policing into the Internet
   would make evolution to new algorithms extremely painful.  If a
   source wanted to use a different algorithm, it would have to both
   discover and negotiate with a policer in some remote access network,
   as well as possibly others on its path.

   Therefore, re-ECN has been designed to avoid the need for bottleneck
   policing so that we can avoid the threat of a single rate adaptation
   policy throughout the network.  Instead, re-ECN allows the access
   network operator at the ingress to choose which rate adaptation to
   enforce.  If desired, the re-ECN wire protocol allows these ingress
   policers to perform per-flow policing according to the widely adopted
   TCP rate adaptation, but it also allows new rate adaptation policies
   beyond TCP to be enforced.  Further, it also allows the flexibility
   for networks to choose to police users as a whole, rather than flows
   (see Appendix F for example designs).

   o  The particular rate adaptation may be agreed bilaterally between
      the sender and its ingress provider (Section 5.5.2), which would
      greatly improve the evolvability of congestion control, requiring
      only a single, local box to be updated upon changes.  Of course,
      one would currently expect TCP to be the default of choice.

   o  Bottleneck policing can easily be circumvented, opening multiple
      flows by varying the active end-point port number; or by spoofing
      the source address but arranging with the receiver to hide the
      true return address at a higher layer.

   A useful feature of re-ECN is that it provides all the information a
   policer needs directly in the packets being policed.  Re-Echo packets
   represent congestion echoes as far as an ingress policer is
   concerned.  So, even policing TCP's AIMD algorithm is relatively
   straightforward.  Appendix F presents an example design, but the
   choice of the preferred mechanism is up to the implementer.

   Finally, we must not forget that an easy way to circumvent re-ECN's



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   defences is for the source to turn off re-ECN support, by setting the
   Not-RECT codepoint, implying legacy traffic.  Therefore an ingress
   policer must put a general rate-limit on Not-RECT traffic, which
   SHOULD be lax during early, patchy deployment, but will have to
   become stricter as deployment widens.  Similarly, flows starting
   without an FNE packet can be confined by a strict rate-limit used for
   the remainder of flows that haven't proved they are well-behaved by
   starting correctly (therefore they need not consume any flow state---
   they are just confined to the `misbehaving' bin if they carry an
   unrecognised flow ID).  Also, as already pointed out, an ingress rate
   policer MUST block both CE codepoints, as traffic that is already
   negative as soon as it is sent must be invalid.

6.1.5.  Inter-domain Policing

   Section 6.1.2 outlining the whole the Incentive Framework above has
   already explained how neighbouring domains can arrange their contract
   with each other so that a network can penalises its upstream
   neighbour in proportion to the total downstream congestion that
   crosses the interface between them over an accounting period.  That
   is, a simple count of the volume of data in packets with RE blanked
   minus the volume with CE marked over, say, a month.

   Full details of how this can be done, why it works and a security
   analysis are available in a sister Internet Draft entitled `Emulating
   Border Flow Policing using Re-ECN on Bulk Data' [Re-PCN].  That I-D
   gives examples of how downstream networks can police the aggregate
   congestion response of their upstream neighbours, against different
   contractual arrangements.  The goal is to ensure an upstream network
   in turn polices its upstream networks, eventually ensuring upstream
   networks will suffer if they do not police the rate response to
   congestion of their users.

   The scenario used in [Re-PCN] is one where re-ECN is used edge-to-
   edge rather than end-to-end as in the present document.  However, the
   position at inter-domain borders is nearly identical. {ToDo: A
   summary of the relevant aspects of that I-D will be included here,
   but due to lack of time this has had to be deferred for the next
   version.}

6.1.6.  Simulations

   Simulations of policer and dropper performance done for the multi-bit
   version of re-feedback have been included in section 5 "Dropper
   Performance" of [Re-fb].  Simulations of policer and dropper for the
   re-ECN version described in this document are work in progress.





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6.2.  Other Applications

   {ToDo: Other applications of re-ECN will be briefly outlined here
   (largely drawing from section 3 of [Re-fb]), such as: }

6.2.1.  DDoS Mitigation

   A flooding attack is inherently about congestion of a resource.
   Because re-ECN ensures the sources causing network congestion
   experience the cost of their own actions, it acts as a first line of
   defence against DDoS.  As load focuses on a victim, upstream queues
   grow, requiring honest sources to pre-load packets with a higher
   fraction of positive packets.  Once downstream routers are so
   congested that they are dropping traffic, they will be CE marking the
   traffic they do forward 100%.  Honest sources will therefore be
   sending Re-Echo 100% (and therefore being severely rate-limited at
   the ingress).

   Malicious sources can either do the same as honest sources, and be
   rate-limited at ingress, or they can understate congestion by sending
   more neutral RECT packets than they should.  If sources understate
   congestion (i.e. do not re-echo sufficient positive packets) and the
   preferential drop ranking is implemented on routers (Section 5.3),
   these routers will preserve positive traffic until last.  So, the
   neutral traffic from malicious sources will all be automatically
   dropped first.  Either way, the malicious sources cannot send more
   than honest sources.

   Further, DDoS sources will tend to be re-used by different
   controllers for different attacks.  They will therefore build up a
   long term history of causing congestion.  Therefore, as long as the
   population of potentially compromisable hosts around the Internet is
   limited, the per-user policing algorithms in Appendix F.1 will
   gradually throttle down the zombies.  Therefore, widespread
   deployment of re-ECN could considerably dampen the force of DDoS.
   Zombie armies could hold back from attacking for long enough to be
   able to build up enough credit in the per-user policers to launch an
   attack.  But they would then still be limited to no more throughput
   than other, honest users.

   Inter-domain traffic policing (see Section 6.1.5)ensures that any
   network that harbours compromised `zombie' hosts will have to bear
   the cost of the congestion caused by the packets of the zombies in
   downstream networks.  Such network will be incentivised to deploy
   per-user policers that rate-limit hosts unresponsive to congestion so
   they can only send very slowly into congested paths.  As well as
   protecting other networks, the extremely poor performance at any sign
   of congestion will incentivise the zombie's owner to clean it up.



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   However, the host should behave normally when using uncongested
   paths.

6.2.2.  End-to-end QoS

   {ToDo: }

6.2.3.  Traffic Engineering

   {ToDo: }

6.2.4.  Inter-Provider Service Monitoring

   {ToDo: }

6.3.  Limitations

   This section will discuss the limitations of the re-ECN approach,
   particularly:

   o  Malicious users have the ability to turn off ECT.  Given Not-ECT
      traffic cannot be efficiently policed, users would be able to get
      a considerable advantage that would not be simply compensated by
      their being the preferential candidates for drops in case of
      sustained congestion.  For this reason, we recommend that while
      accommodating a smooth initial transition to re-ECN policers
      should gradually be tuned to rate limit Not-ECT traffic in the
      long term.

   o  Re-feedback for TTL (re-TTL) would also be desirable at the same
      time as re-ECN.  Unfortunately this requires a further agreement
      to standardise the mechanisms briefly described in Appendix E

   o  We are considering the issue of whether it would be useful to
      truncate rather than drop packets that appear to be malicious, so
      that the feedback loop is not broken but useful data can be
      removed.

   o  The inability to police excessive congestion when it causes an
      ECN-capable router to drop ECT traffic rather than marking it.
      Re-ECN allows policing of downstream explicit congestion
      notifications, not drops.


7.  Incremental Deployment






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7.1.  Incremental Deployment Features

   We chose to use ECT(1) for Re-ECN traffic deliberately.  Existing ECN
   sources set ECT(0) at either 50% (the nonce) or 100% (the default).
   So they will appear to a re-ECN policer as very highly congested
   paths.  When policers are first deployed they can be configured
   permissively, allowing through both `legacy' ECN and misbehaving re-
   ECN flows.  Then, as the threshold is set more strictly, the more
   legacy ECN sources will gain by upgrading to re-ECN.  Thus, towards
   the end of the voluntary incremental deployment period, legacy
   transports can be given progressively stronger encouragement to
   upgrade.

   {ToDo: As well as introducing the new information above, this section
   is intended to collect together all the snippets of information
   throughout the draft about incremental deployment.  Through lack of
   time, this rationalisation will have to wait until the next version,
   except for the brief list below.  However, a long section describing
   possible deployment scenarios is available in the section following.}

   Re-ECN semantics for use of the two-bit ECN field are different in
   the following minor respects compared to RFC3168:

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

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

   o  Routers MAY preferentially drop different extended ECN codepoints;

   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
      "retcp_Router_Forwarding_Behaviour");

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

   None of these changes REQUIRE any modifications to routers.

7.2.  Incremental Deployment Incentives

   It would only be worth standardising the re-ECN protocol if there
   existed a coherent story for how it might be incrementally deployed.
   In order for it to have a chance of deployment, everyone who needs to
   act, must have a strong incentive to act, and the incentives must
   arise in the order that deployment would have to happen.  Re-ECN



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   works around unmodified ECN routers, but we can't just discuss why
   and how re-ECN deployment might build on ECN deployment, because
   there is precious little to build on in the first place.  Instead, we
   aim to show that re-ECN deployment could carry ECN with it.  We focus
   on commercial deployment incentives, although some of the arguments
   apply equally to academic or government sectors.

   ECN deployment:

      ECN is largely implemented in commercial routers, but generally
      not as a supported feature, and it has largely not been deployed
      by commercial network operators.  It has been released in many
      Unix-based operating systems, but not in proprietary OSs like
      Windows or those in many mobile devices.  For detailed deployment
      status, see [ECN-Deploy].  We believe the reason ECN deployment
      has not happened is twofold:

      *  ECN requires changes to both routers and hosts.  If someone
         wanted to sell the improvement that ECN offers, they would have
         to co-ordinate deployment of their product with others.  An ECN
         server only gives any improvement on an ECN network.  An ECN
         network only gives any improvement if used by ECN devices.
         Deployment that requires co-ordination adds cost and delay and
         tends to dilute any competitive advantage that might be gained.

      *  ECN `only' gives a performance improvement.  Making a product a
         bit faster (whether the product is a device or a network),
         isn't usually a sufficient selling point to be worth the cost
         of co-ordinating across the industry to deploy it.  Network
         operators tend to avoid re-configuring a working network unless
         launching a new product.

   ECN and re-ECN for Edge-to-edge Assured QoS:

      We believe the proposal to provide assured QoS sessions using a
      form of ECN called pre-congestion notification (PCN) [CL-arch] is
      most likely to break the deadlock in ECN deployment first.  It
      only requires edge-to-edge deployment so it does not require
      endpoint support.  It can be deployed in a single network, then
      grow incrementally to interconnected networks.  And it provides a
      different `product' (internetworked assured QoS), rather than
      merely making an existing product a bit faster.

      Not only could this assured QoS application kick-start ECN
      deployment, it could also carry re-ECN deployment with it; because
      re-ECN can enable the assured QoS region to expand to a large
      internetwork where neighbouring networks do not trust each other.
      [Re-PCN] argues that re-ECN security should be built in to the QoS



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      system from the start, explaining why and how.

      If ECN and re-ECN were deployed edge-to-edge for assured QoS,
      operators would gain valuable experience.  They would also clear
      away many technical obstacles such as firewall configurations that
      block all but the legacy settings of the ECN field and the RE
      flag.

   ECN in Access Networks:

      The next obstacle to ECN deployment would be extension to access
      and backhaul networks, where considerable link layer differences
      makes implementation non-trivial, particularly on congested
      wireless links.  ECN and re-ECN work fine during partial
      deployment, but they will not be very useful if the most congested
      elements in networks are the last to support them.  Access network
      support is one of the weakest parts of this deployment story.  All
      we can hope is that, once the benefits of ECN are better
      understood by operators, they will push for the necessary link
      layer implementations as deployment proceeds.

   Policing Unresponsive Flows:

      Re-ECN allows a network to offer differentiated quality of service
      as explained in Section 6.2.2.  But we do not believe this will
      motivate initial deployment of re-ECN, because the industry is
      already set on alternative ways of doing QoS.  Despite being much
      more complicated and expensive, the alternative approaches are
      here and now.

      But re-ECN is critical to QoS deployment in another respect.  It
      can be used to prevent applications from taking whatever bandwidth
      they choose without asking.

      Currently, applications that remain resolute in their lack of
      response to congestion are rewarded by other TCP applications.  In
      other words, TCP is naively friendly, in that it reduces its rate
      in response to congestion whether it is competing with friends
      (other TCPs) or with enemies (unresponsive applications).

      Therefore, those network owners that want to sell QoS will be keen
      to ensure that their users can't help themselves to QoS for free.
      Given the very large revenues at stake, we believe effective
      policing of congestion response will become highly sought after by
      network owners.






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      But this does not necessarily argue for re-ECN deployment.
      Network owners might choose to deploy bottleneck policers rather
      than re-ECN-based policing.  However, under Related Work
      (Section 9) we argue that bottleneck policers are inherently
      vulnerable to circumvention.

      Therefore we believe there will be a strong demand from network
      owners for re-ECN deployment so they can police flows that do not
      ask to be unresponsive to congestion, in order to protect their
      revenues from flows that do ask (QoS).  In particular, we suspect
      that the operators of cellular networks will want to prevent VoIP
      and video applications being used freely on their networks as a
      more open market develops in GPRS and 3G devices.

      Initial deployments are likely to be isolated to single cellular
      networks.  Cellular operators would first place requirements on
      device manufacturers to include re-ECN in the standards for mobile
      devices.  In parallel, they would put out tenders for ingress and
      egress policers.  Then, after a while they would start to tighten
      rate limits on Not-ECT traffic from non-standard devices and they
      would start policing whatever non-accredited applications people
      might install on mobile devices with re-ECN support in the
      operating system.  This would force even independent mobile device
      manufacturers to provide re-ECN support.  Early standardisation
      across the cellular operators is likely, including interconnection
      agreements with penalties for excess downstream congestion.

      We suspect some fixed broadband networks (whether cable or DSL)
      would follow a similar path.  However, we also believe that larger
      parts of the fixed Internet would not choose to police on a per-
      flow basis.  Some might choose to police congestion on a per-user
      basis in order to manage heavy peer-to-peer file-sharing, but it
      seems likely that a sizeable majority would not deploy any form of
      policing.

      This hybrid situation begs the question, "How does re-ECN work for
      networks that choose to using policing if they connect with others
      that don't?"  Traffic from non-ECN capable sources will arrive
      from other networks and cause congestion within the policed, ECN-
      capable networks.  So networks that chose to police congestion
      would rate-limit Not-ECT traffic throughout their network,
      particularly at their borders.  They would probably also set
      higher usage prices in their interconnection contracts for
      incoming Not-ECT and Not-RECT traffic.  We assume that
      interconnection contracts between networks in the same tier will
      include congestion penalties before contracts with provider
      backbones do.




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      A hybrid situation could remain for all time.  As was explained in
      the introduction, we believe in healthy competition between
      policing and not policing, with no imperative to convert the whole
      world to the religion of policing.  Networks that chose not to
      deploy egress droppers would leave themselves open to being
      congested by senders in other networks.  But that would be their
      choice.

      The important aspect of the egress dropper though is that it most
      protects the network that deploys it.  If a network does not
      deploy an egress dropper, sources sending into it from other
      networks will be able to understate the congestion they are
      causing.  Whereas, if a network deploys an egress dropper, it can
      know how much congestion other networks are dumping into it.  And
      apply penalties or charges accordingly.  So, whether or not a
      network polices its own sources at ingress, it is in its interests
      to deploy an egress dropper.

   Host support:

      In the above deployment scenario, host operating system support
      for re-ECN came about through the cellular operators demanding it
      in device standards (i.e. 3GPP).  Of course, increasingly, mobile
      devices are being built to support multiple wireless technologies.
      So, if re-ECN were stipulated for cellular devices, it would
      automatically appear in those devices connected to the wireless
      fringes of fixed networks if they coupled cellular with WiFi or
      Bluetooth technology, for instance.  Also, once implemented in the
      operating system of one mobile device, it would tend to be found
      in other devices using the same family of operating system.

      Therefore, whether or not a fixed network deployed ECN, or
      deployed re-ECN policers and droppers, many of its hosts might
      well be using re-ECN over it.  Indeed, they would be at an
      advantage when communicating with hosts across Re-ECN policed
      networks that rate limited Not-RECT traffic.

   Other possible scenarios:

      The above is thankfully not the only plausible scenario we can
      think of.  One of the many clubs of operators that meet regularly
      around the world might decide to act together to persuade a major
      operating system manufacturer to implement re-ECN.  And they may
      agree between them on an interconnection model that includes
      congestion penalties.






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      Re-ECN provides an interesting opportunity for device
      manufacturers as well as network operators.  Policers can be
      configured loosely when first deployed.  Then as re-ECN take-up
      increases, they can be tightened up, so that a network with re-ECN
      deployed can gradually squeeze down the service provided to legacy
      devices that have not upgraded to re-ECN.  Many device vendors
      rely on replacement sales.  And operating system companies rely
      heavily on new release sales.  Also support services would like to
      be able to force stragglers to upgrade.  So, the ability to
      throttle service to legacy operating systems is quite valuable.

      Also, policing unresponsive sources may not be the only or even
      the first application that drives deployment.  It may be policing
      causes of heavy congestion (e.g. peer-to-peer file-sharing).  Or
      it may be mitigation of denial of service.  Or we may be wrong in
      thinking simpler QoS will not be the initial motivation for re-ECN
      deployment.  Indeed, the combined pressure for all these may be
      the motivator, but it seems optimistic to expect such a level of
      joined-up thinking from today's communications industry.  We
      believe a single application alone must be a sufficient motivator.

      In short, everyone gains from adding accountability to TCP/IP,
      except the selfish or malicious.  So, deployment incentives tend
      to be strong.


8.  Architectural Rationale

   In the Internet's technical community the danger of not responding to
   congestion is well-understood, with its attendant risk of congestion
   collapse [RFC3714].  However, many of the Internet's commercial
   community consider that the very essence of IP is to provide open
   access to the internetwork for all applications.  Congestion is seen
   as a symptom of over-conservative investment.  And the goal of
   application design is to find novel ways to continue working despite
   congestion.  They argue that the Internet was never intended to be
   solely for TCP-friendly applications.  Another side of the Internet's
   commercial community believe that it is no use providing a network
   for novel applications if it has insufficient capacity.  And it will
   always have insufficient capacity unless a greater share of
   application revenues can be /assured/ for the infrastructure
   provider.  Otherwise the major investments required will carry too
   much risk and won't happen.

   The lesson articulated in [Tussle] is that we shouldn't embed our
   view on these arguments into the Internet at design time.  Instead we
   should design the Internet so that the outcome of these arguments can
   get decided at run-time.  Re-ECN is designed in that spirit.  Once



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   the protocol is available, different network operators can choose how
   liberal they want to be in holding people accountable for the
   congestion they cause.  Some might boldly invest in capacity and not
   police its use at all, hoping that novel applications will result.
   Others might use re-ECN for fine-grained flow policing, expecting to
   make money selling vertically integrated services.  Yet others might
   sit somewhere half-way, perhaps doing coarse, per-user policing.  All
   might change their minds later.  But re-ECN always allows them to
   interconnect so that the careful ones can protect themselves from the
   liberal ones.

   The incentive-based approach used for re-ECN is based on Gibbens and
   Kelly's arguments [Evol_cc] on allowing endpoints the freedom to
   evolve new congestion control algorithms for new applications.  They
   ensured responsible behaviour despite everyone's self-interest by
   applying pricing to ECN marking, and Kelly had proved stability and
   optimality in an earlier paper.

   Re-ECN keeps all the underlying economic incentives, but rearranges
   the feedback.  The idea is to allow a network operator (if it
   chooses) to deploy engineering mechanisms like policers at the front
   of the network which can be designed to behave /as if/ they are
   responding to congestion prices.  Rather than having to subject users
   to congestion pricing, networks can then use more traditional
   charging regimes (or novel ones).  But the engineering can constrain
   the overall amount of congestion a user can cause.  This provides a
   buffer against completely outrageous congestion control, but still
   makes it easy for novel applications to evolve if they need different
   congestion control to the norms.  It also allows novel charging
   regimes to evolve.

   Despite being achieved with a relatively minor protocol change, re-
   ECN is an architectural change.  Previously, Internet congestion
   could only be controlled by the data sender, because it was the only
   one both in a position to control the load and in a position to see
   information on congestion.  Re-ECN levels the playing field.  It
   recognises that the network also has a role to play in moderating
   (policing) congestion control.  But policing is only truly effective
   at the first ingress into an internetwork, whereas path congestion
   was previously only visible at the last egress.  So, re-ECN
   democratises congestion information.  Then the choice over who
   actually controls congestion can be made at run-time, not design
   time---a bit like an aircraft with dual controls.  And different
   operators can make different choices.  We believe non-architectural
   approaches to this problem are unlikely to offer more than partial
   solutions (see Section 9).

   Importantly, re-ECN does NOT REQUIRE assumptions about specific



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   congestion responses to be embedded in any network elements, except
   at the first ingress to the internetwork if that level of control is
   desired by the ingress operator.  But such tight policing will be a
   matter of agreement between the source and its access network
   operator.  The ingress operator need not police congestion response
   at flow granularity; it can simply hold a source responsible for the
   aggregate congestion it causes, perhaps keeping it within a monthly
   congestion quota.  Or if the ingress network trusts the source, it
   can do nothing.

   Therefore, the aim of the re-ECN protocol is NOT solely to police
   TCP-friendliness.  Re-ECN preserves IP as a generic network layer for
   all sorts of responses to congestion, for all sorts of transports.
   Re-ECN merely ensures truthful downstream congestion information is
   available in the network layer for all sorts of accountability
   applications.

   The end to end design principle does not say that all functions
   should be moved out of the lower layers---only those functions that
   are not generic to all higher layers.  Re-ECN adds a function to the
   network layer that is generic, but was omitted: accountability for
   causing congestion.  Accountability is not something that an end-user
   can provide to themselves.  We believe re-ECN adds no more than is
   sufficient to hold each flow accountable, even if it consists of a
   single datagram.

   "Accountability" implies being able to identify who is responsible
   for causing congestion.  However, at the network layer it would NOT
   be useful to identify the cause of congestion by adding individual or
   organisational identity information, NOR by using source IP
   addresses.  Rather than bringing identity information to the point of
   congestion, we bring downstream congestion information to the point
   where the cause can be most easily identified and dealt with.  That
   is, at any trust boundary, congestion can be associated with the
   physically connected upstream neighbour that is directly responsible
   for causing it (whether intentionally or not).  A trust boundary
   interface is exactly the place to police or throttle in order to
   directly mitigate congestion, rather than having to trace the
   (ir)responsible party in order to shut them down.

   Some considered that ECN itself was a layering violation.  The
   reasoning went that the interface to a layer should provide a service
   to the higher layer and hide how the lower layer does it.  However,
   ECN reveals the state of the network layer and below to the transport
   layer.  A more positive way to describe ECN is that it is like the
   return value of a function call to the network layer.  It explicitly
   returns the status of the request to deliver a packet, by returning a
   value representing the current risk that a packet will not be served.



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   Re-ECN has similar semantics, except the transport layer must try to
   guess the return value, then it can use the actual return value from
   the network layer to modify the next guess.


9.  Related Work

   {Due to lack of time, this section is incomplete.  The reader is
   referred to the Related Work section of [Re-fb] for a brief selection
   of related ideas.}

9.1.  Policing Rate Response to Congestion

   ATM network elements send congestion back-pressure messages [ITU-
   T.I.371] along each connection, duplicating any end to end feedback
   because they don't trust it.  On the other hand, re-ECN ensures
   information in forwarded packets can be used for congestion
   management without requiring a connection-oriented architecture and
   re-using the overhead of fields that are already set aside for end to
   end congestion control (and routing loop detection in the case of re-
   TTL in Appendix E).

   We borrowed ideas from policers in the literature [pBox],[XCHOKe],
   AFD etc. for our rate equation policer.  However, without the benefit
   of re-ECN they don't police the correct rate for the condition of
   their path.  They detect unusually high /absolute/ rates, but only
   while the policer itself is congested, because they work by detecting
   prevalent flows in the discards from the local RED queue.  These
   policers must sit at every potential bottleneck, whereas our policer
   need only be located at each ingress to the internetwork.  As Floyd &
   Fall explain [pBox], the limitation of their approach is that a high
   sending rate might be perfectly legitimate, if the rest of the path
   is uncongested or the round trip time is short.  Commercially
   available rate policers cap the rate of any one flow.  Or they
   enforce monthly volume caps in an attempt to control high volume
   file-sharing.  They limit the value a customer derives.  They might
   also limit the congestion customers can cause, but only as an
   accidental side-effect.  They actually punish traffic that fills
   troughs as much as traffic that causes peaks in utilisation.  In
   practice network operators need to be able to allocate service by
   cost during congestion, and by value at other times.

9.2.  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.




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   The ECN nonce is an elegant scheme that allows the sender to detect
   if someone in the feedback loop tries to claim no congestion was
   experienced when it fact it was (whether drop or ECN marking).  The
   sender chooses between the two ECT codepoints in a pseudo-random
   sequence.  Then, whenever the network marks a packet with CE, to deny
   the congestion happened, the cheater would have to guess which ECT
   codepoint was overwritten, with only a 50:50 chance of being correct
   each time.

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

9.3.  Identifying Upstream and Downstream Congestion

   Purple [Purple] proposes that routers should use the CWR flag in the
   TCP header of ECN-capable flows to work out path congestion and
   therefore downstream congestion in a similar way to re-ECN.  However,
   because CWR is in the transport layer, it is not always visible to
   network layer routers and policers.  Purple's motivation was to
   improve AQM, not policing.  But, of course, nodes trying to avoid a
   policer would not be expected to allow CWR to be visible.


10.  Security Considerations

   This whole memo concerns the deployment of a secure congestion



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   control framework.  There are some specific security issues that we
   are still working on.

   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.

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

   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.
   Section 5.5.1 discusses one of these.

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


11.  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 3.2)

   o  The new extension header for IPv6 (Section 5.2);

   o  The new combinations of flags in the TCP header for capability
      negotiation (Section 4.1.3);

   o  The new ICMP message type (Section 5.5.1).


12.  Conclusions

   {ToDo:}



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13.  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 (BT), Sally Floyd (ICIR), Stephen
   Hailes, Mark Handley, Adam Greenhalgh (UCL), Jon Crowcroft (Uni Cam),
   David Clark, Bill Lehr, Sharon Gillett, Steve Bauer, Liz Maida (MIT),
   and comments from participants in the CRN/CFP Broadband and DoS-
   resistant Internet working groups.


14.  Comments Solicited

   Comments and questions are encouraged and very welcome.  They can be
   addressed to the IETF Transport Area working group's mailing list
   <tsvwg@ietf.org>, and/or to the authors.


15.  References

15.1.  Normative References

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

   [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.

   [RFC2581]  Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
              Control", RFC 2581, April 1999.

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

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



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15.2.  Informative References

   [ARI05]    Adams, J., Roberts, L., and A. IJsselmuiden, "Changing the
              Internet to Support Real-Time Content Supply from a Large
              Fraction of Broadband Residential Users", BT Technology
              Journal (BTTJ) 23(2), April 2005.

   [CL-arch]  Briscoe, B., Eardley, P., Songhurst, D., Le Faucheur, F.,
              Charny, A., Babiarz, J., and K. Chan, "A Framework for
              Admission Control over DiffServ using Pre-Congestion
              Notification", draft-briscoe-tsvwg-cl-architecture-02
              (work in progress), March 2006.

   [CLoop_pol]
              Salvatori, A., "Closed Loop Traffic Policing", Politecnico
              Torino and Institut Eurecom Masters Thesis ,
              September 2005.

   [ECN-Deploy]
              Floyd, S., "ECN (Explicit Congestion Notification) in
              TCP/IP; Implementation and Deployment of ECN", Web-page ,
              May 2004,
              <http://www.icir.org/floyd/ecn.html#implementations>.

   [Evol_cc]  Gibbens, R. and F. Kelly, "Resource pricing and the
              evolution of congestion control", Automatica 35(12)1969--
              1985, December 1999,
              <http://www.statslab.cam.ac.uk/~frank/evol.html>.

   [I-D.ietf-tsvwg-ecnsyn]
              Kuzmanovic, A., "Adding Explicit Congestion Notification
              (ECN) Capability to TCP's SYN/ACK  Packets",
              draft-ietf-tsvwg-ecnsyn-00 (work in progress),
              November 2005.

   [ITU-T.I.371]
              ITU-T, "Traffic Control and Congestion Control in
              {B-ISDN}", ITU-T Rec. I.371 (03/04), March 2004.

   [Jiang02]  Jiang, H. and D. Dovrolis, "The Macroscopic Behavior of
              the TCP Congestion Avoidance Algorithm", ACM SIGCOMM
              CCR 32(3)75-88, July 2002,
              <http://doi.acm.org/10.1145/571697.571725>.

   [Mathis97]
              Mathis, M., Semke, J., Mahdavi, J., and T. Ott, "The
              Macroscopic Behavior of the TCP Congestion Avoidance
              Algorithm", ACM SIGCOMM CCR 27(3)67--82, July 1997,



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              <http://doi.acm.org/10.1145/263932.264023>.

   [Purple]   Pletka, R., Waldvogel, M., and S. Mannal, "PURPLE:
              Predictive Active Queue Management Utilizing Congestion
              Information", Proc. Local Computer Networks (LCN 2003) ,
              October 2003.

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

   [RFC2988]  Paxson, V. and M. Allman, "Computing TCP's Retransmission
              Timer", RFC 2988, November 2000.

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

   [RFC3270]  Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
              P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
              Protocol Label Switching (MPLS) Support of Differentiated
              Services", RFC 3270, May 2002.

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

   [RFC3714]  Floyd, S. and J. Kempf, "IAB Concerns Regarding Congestion
              Control for Voice Traffic in the Internet", RFC 3714,
              March 2004.

   [Re-PCN]   Briscoe, B., "Emulating Border Flow Policing using Re-ECN
              on Bulk Data", draft-briscoe-tsvwg-re-ecn-border-cheat-01
              (work in progress), March 2006.

   [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, <http://
              www.acm.org/sigs/sigcomm/sigcomm2005/
              techprog.html#session8>.

   [Smart_rtg]
              Goldenberg, D., Qiu, L., Xie, H., Yang, Y., and Y. Zhang,
              "Optimizing Cost and Performance for Multihoming", ACM
              SIGCOMM CCR 34(4)79--92, October 2004,
              <http://citeseer.ist.psu.edu/698472.html>.

   [Steps_DoS]
              Handley, M. and A. Greenhalgh, "Steps towards a DoS-



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              resistant Internet Architecture", Proc. ACM SIGCOMM
              workshop on Future directions in network architecture
              (FDNA'04) pp 49--56, August 2004.

   [Tussle]   Clark, D., Sollins, K., Wroclawski, J., and R. Braden,
              "Tussle in Cyberspace: Defining Tomorrow's Internet", ACM
              SIGCOMM CCR 32(4)347--356, October 2002,
              <http://www.acm.org/sigcomm/sigcomm2002/papers/
              tussle.pdf>.

   [XCHOKe]   Chhabra, P., Chuig, S., Goel, A., John, A., Kumar, A.,
              Saran, H., and R. Shorey, "XCHOKe: Malicious Source
              Control for Congestion Avoidance at Internet Gateways",
              Proceedings of IEEE International Conference on Network
              Protocols (ICNP-02) , November 2002,
              <http://www.cc.gatech.edu/~akumar/xchoke.pdf>.

   [pBox]     Floyd, S. and K. Fall, "Promoting the Use of End-to-End
              Congestion Control in the Internet", IEEE/ACM Transactions
              on Networking 7(4) 458--472, August 1999,
              <http://www.aciri.org/floyd/end2end-paper.html>.


Appendix A.  Precise Re-ECN Protocol Operation

   The protocol operation described in Section 3.3 was an approximation.
   In fact, standard ECN router marking combines 1% and 2% marking into
   slightly less than 3% whole-path marking, because routers
   deliberately mark CE whether or not it has already been marked by
   another router 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 router to n-1 at the last.

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

   o  u_j represents congestion *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).



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   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
   router 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 the upstream congestion (CE marking fraction) fed back from
   the destination.  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, if 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 3.3, congestion
   downstream is

      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.  ECN Compatibility

   The rationale for choosing the particular combinations of SYN and SYN
   ACK flags in Section 4.1.3 is as follows.

   Choice of SYN flags: A re-ECN sender can work with vanilla 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 vanilla ECT receivers and by ECT-Nonce receivers.  But



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      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 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 vanilla 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.


Appendix C.  Packet Marking During Flow Start

   {ToDo: Write up proof that sender should mark FNE on first and third
   data packets, even with the largest allowed initial window.}






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Appendix D.  Example Egress Dropper Algorithm

   {ToDo: Write up the basic algorithm with flow state, then the
   aggregated one.}


Appendix E.  Re-TTL

   This Appendix gives an overview of a proposal to be able to overload
   the TTL field in the IP header to monitor downstream propagation
   delay.  It is planned to fully write up this proposal in a future
   Internet Draft.

   Delay re-feedback can be achieved by overloading the TTL field,
   without changing IP or router TTL processing.  A target value for TTL
   at the destination would need standardising, say 16.  If the path hop
   count increased by more than 16 during a routing change, it would
   temporarily be mistaken for a routing loop, so this target would need
   to be chosen to exceed typical hop count increases.  The TCP wire
   protocol and handlers would need modifying to feed back the
   destination TTL and initialise it.  It would be necessary to
   standardise the unit of TTL in terms of real time (as was the
   original intent in the early days of the Internet).

   In the longer term, precision could be improved if routers
   decremented TTL to represent exact propagation delay to the next
   router.  That is, for a router to decrement TTL by, say, 1.8 time
   units it would alternate the decrement of every packet between 1 & 2
   at a ratio of 1:4.  Although this might sometimes require a seemingly
   dangerous null decrement, a packet in a loop would still decrement to
   zero after 255 time units on average.  As more routers were upgraded
   to this more accurate TTL decrement, path delay estimates would
   become increasingly accurate despite the presence of some legacy
   routers that continued to always decrement the TTL by 1.


Appendix F.  Policer Designs to ensure Congestion Responsiveness

F.1.  Per-user Policing

   User policing requires a policer on the ingress interface of the
   access router associated with the user.  At that point, the traffic
   of the user hasn't diverged on different routes yet; nor has it mixed
   with traffic from other sources.

   In order to ensure that a user doesn't generate more congestion in
   the network than her due share, a modified bulk token-bucket is
   maintained with the following parameter:



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   o  b_0 the initial token level

   o  r the filling rate

   o  b_max the bucket depth

   The same token bucket algorithm is used as in many areas of
   networking, but how it is used is very different:

   o  all traffic from a user over the lifetime of their subscription is
      policed in the same token bucket.

   o  only Re-Echo packets consume tokens

   Such a policer will allow network operators to throttle the
   contribution of their users to network congestion.  This will require
   the appropriate contractual terms to be in place between operators
   and users.  For instance: a condition for a user to subscribe to a
   given network service may be that she should not cause more than a
   volume C_user of congestion over a reference period T_user, although
   she may carry forward up to N_user times her allowance at the end of
   each period.  These terms directly set the parameter of the user
   policer:

   o  b_0 = C_user

   o  r = C_user/T_user

   o  b_max = b_0 * (N_user +1)

   Besides the congestion budget policer above, another user policer
   will be necessary to rate-limit FNE packets, if they are to be marked
   rather than dropped (see discussion in Section 5.3.).  Rate-limiting
   FNE packets will prevent high bursts of new flow arrivals, which is a
   very useful feature in DoS prevention.  A condition to subscribe to a
   given network service would have to be that a user should not
   generate more than C_FNE FNE packets, over a reference period T_FNE,
   with no option to carry forward any of the allowance at the end of
   each period.  These terms directly set the parameters of the FNE
   policer:

   o  b_0 = C_FNE

   o  r = C_FNE/T_FNE

   o  b_max = b_0

   T_FNE should be a much shorter period than T_user: for instance T_FNE



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   could be in the order of minutes while T_user could be in order of
   weeks.

F.2.  Per-flow Rate Policing

   Per-flow policing aims to enforce congestion responsiveness on the
   shortest information timescale on a network path: packet roundtrips.

   This again requires that the appropriate terms be agreed between a
   network operator and its users, where a congestion responsiveness
   policy might be required for the use of a given network service
   (perhaps unless the user specifically requests otherwise).

   As an example, we describe below how a rate adaptation policer can be
   designed when the applicable rate adaptation policy is TCP-
   compliance.  In that context, the average throughput of a flow will
   be expected to be bounded by the value of the TCP throughput during
   congestion avoidance, given n Mathis' formula [Mathis97]

      x_TCP = k * s / ( T * sqrt(m) )

   where:

   o  x_TCP is the throughput of the TCP flow in packets per second,

   o  k is a constant upper-bounded by sqrt(3/2),

   o  s is the average packet size of the flow,

   o  T is the roundtrip time of the flow,

   o  m is the congestion level experienced by the flow.

   We define the marking period N=1/m which represents the average
   number of packets between two re-echoes.  Mathis' formula can be re-
   written as:

      x_TCP = k*s*sqrt(N)/T

   We can then get the average inter-mark time in a compliant TCP flow,
   dt_TCP, by solving (x_TCP/s)*dt_TCP = N which gives

      dt_TCP = sqrt(N)*T/k

   We rely on this equation for the design of a rate-adaptation policer
   as a variation of a token bucket.  In that case a policer has to be
   set up for each policed flow.  This may be triggered by FNE packets,
   with the remainder of flows being all rate limited together if they



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   do not start with an FNE packet.

   Where maintaining per flow state is not a problem, for instance on
   some access routers, systematic per-flow policing may be considered.
   Should per-flow state be more constrained, rate adaptation policing
   could be limited to a random sample of flows exhibiting Re-Echoes.

   As in the case of user policing, only re-echo packets will consume
   tokens, however the amount of tokens consumed will depend on the
   congestion signal.

   When a new rate adaptation policer is set up for flow j, the
   following state is created:

   o  a token bucket b_j of depth b_max starting at level b_0

   o  a timestamp t_j = timenow()

   o  a counter N_j = 0

   o  a roundtrip estimate T_j

   o  a filling rate r

   When the policing node forwards a packet of flow j with no Re-Echo:

   o  . the counter is incremented: N_j += 1

   When the policing node forwards a packet of flow j carrying a
   congestion mark (CE):

   o  the counter is incremented: N_j += 1

   o  the token level is adjusted: b_j += r*(timenow()-t_j) - sqrt(N_j)*
      T_j/k

   o  the counter is reset: N_j = 0

   o  the timer is reset: t_j = timenow()

   An implementation example will be given in a later draft that avoids
   having to extract the square root.

   Analysis: For a TCP flow, for r= 1 token/sec, on average,

      r*(timenow()-t_j)-sqrt(N_j)* T_j/k = dt_TCP - sqrt(N)*T/k = 0

   This means that the token level will fluctuate around its initial



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   level.  The depth b_max of the bucket sets the timescale on which the
   rate adaptation policy is performed while the filling rate r sets the
   trade-off between responsiveness and robustness:

   o  the higher b_max, the longer it will take to catch greedy flows

   o  the higher r, the fewer false positives (greedy verdict on
      compliant flows) but the more false negatives (compliant verdict
      on greedy flows)

   This rate adaptation policer requires the availability of a roundtrip
   estimate which may be obtained for instance from the application of
   re-feedback to the downstream delay Appendix E or passive estimation
   [Jiang02].

   When the bucket of a policer located at the access router (whether it
   is a per-user policer or a per-flow policer) becomes empty, the
   access router SHOULD drop at least all packets causing the token
   level to become negative.  The network operator MAY take further
   sanctions if the token level of the per-flow policers associated with
   a user becomes negative.






























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Authors' Addresses

   Bob Briscoe
   BT & UCL
   B54/77, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE
   UK

   Phone: +44 1473 645196
   Email: bob.briscoe@bt.com
   URI:   http://www.cs.ucl.ac.uk/staff/B.Briscoe/


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

   Phone: +44 1473 647284
   Email: arnaud.jacquet@bt.com
   URI:


   Alessandro Salvatori
   BT
   B54/77, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE
   UK

   Email: sandr8@gmail.com

















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