Transport Area Working Group                                  B. Briscoe
Internet-Draft                                                        BT
Intended status: Standards Track                            Oct 27, 2008
Expires: April 30, 2009

            Layered Encapsulation of Congestion Notification

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   This document redefines how the explicit congestion notification
   (ECN) field of the outer IP header of a tunnel should be constructed.
   It brings all IP in IP tunnels (v4 or v6) into line with the way
   IPsec tunnels now construct the ECN field.  It includes a thorough
   analysis of the reasoning for this change and the implications.  It
   also gives guidelines on the encapsulation of IP congestion
   notification by any outer header, whether encapsulated in an IP
   tunnel or in a lower layer header.  Following these guidelines should
   help interworking, if the IETF or other standards bodies specify any
   new encapsulation of congestion notification.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  The Need for Rationalisation . . . . . . . . . . . . . . .  5
     1.2.  Document Roadmap . . . . . . . . . . . . . . . . . . . . .  6
     1.3.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  8
   3.  Design Constraints . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Security Constraints . . . . . . . . . . . . . . . . . . .  9
     3.2.  Control Constraints  . . . . . . . . . . . . . . . . . . . 11
     3.3.  Management Constraints . . . . . . . . . . . . . . . . . . 12
   4.  Design Principles  . . . . . . . . . . . . . . . . . . . . . . 13
     4.1.  Design Guidelines for New Encapsulations of Congestion
           Notification . . . . . . . . . . . . . . . . . . . . . . . 14
   5.  Default ECN Tunnelling Rules . . . . . . . . . . . . . . . . . 16
   6.  Backward Compatibility . . . . . . . . . . . . . . . . . . . . 17
   7.  Changes from Earlier RFCs  . . . . . . . . . . . . . . . . . . 19
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   10. Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 22
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
   12. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 23
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     13.2. Informative References . . . . . . . . . . . . . . . . . . 24
   Editorial Comments . . . . . . . . . . . . . . . . . . . . . . . .
   Appendix A.  Why resetting CE on encapsulation harms PCN . . . . . 26
   Appendix B.  Contribution to Congestion across a Tunnel  . . . . . 27
   Appendix C.  Comprehensive Decapsulation Rules . . . . . . . . . . 28
     C.1.  Ways to Introduce the Comprehensive Decapsulation Rules  . 31
   Appendix D.  Non-Dependence of Tunnelling on In-path Load
                Regulation  . . . . . . . . . . . . . . . . . . . . . 32
     D.1.  Dependence of In-Path Load Regulation on Tunnelling  . . . 33
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 36
   Intellectual Property and Copyright Statements . . . . . . . . . . 37

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

   Full text differences between IETF draft versions are available at
   <>, and
   between earlier individual draft versions at

   From ietf-00 to ietf-01 (current):

      *  Identified two additional alarm states in the decapsulation
         rules (Figure 3) if ECT(X) in outer and inner contradict each

      *  Altered Comprehensive Decapsulation Rules (Appendix C) so that
         ECT(0) in the outer no longer overrides ECT(1) in the inner.
         Used the term 'Comprehensive' instead of 'Ideal'.  And
         considerably updated the text in this appendix.

      *  Added Appendix C.1 to weigh up the various ways the
         Comprehensive Decapsulation Rules might be introduced.  This
         replaces the previous contradictory statements saying complex
         backwards compatibility interactions would be introduced while
         also saying there would be no backwards compatibility issues.

      *  Updated references.

   From briscoe-01 to ietf-00:

      *  Re-wrote Appendix B giving much simpler technique to measure
         contribution to congestion across a tunnel.

      *  Added discussion of backward compatibility of the ideal
         decapsulation scheme in Appendix C

      *  Updated references.  Minor corrections & clarifications

   From -00 to -01:

      *  Related everything conceptually to the uniform and pipe models
         of RFC2983 on Diffserv Tunnels, and completely removed the
         dependence of tunnelling behaviour on the presence of any in-
         path load regulation by using the [1 - Before] [2 - Outer]
         function placement concepts from RFC2983;

      *  Added specific cases where the existing standards limit new
         proposals, particularly Appendix A;

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      *  Added sub-structure to Introduction (Need for Rationalisation,
         Roadmap), added new Introductory subsection on "Scope" and
         improved clarity;

      *  Added Design Guidelines for New Encapsulations of Congestion
         Notification (Section 4.1);

      *  Considerably clarified the Backward Compatibility section
         (Section 6);

      *  Considerably extended the Security Considerations section
         (Section 9);

      *  Summarised the primary rationale much better in the

      *  Added numerous extra acknowledgements;

      *  Added Appendix A.  "Why resetting CE on encapsulation harms
         PCN", Appendix B.  "Contribution to Congestion across a Tunnel"
         and Appendix C.  "Ideal Decapsulation Rules";

      *  Re-wrote Appendix D, explaining how tunnel encapsulation no
         longer depends on in-path load-regulation (changed title from
         "In-path Load Regulation" to "Non-Dependence of Tunnelling on
         In-path Load Regulation"), but explained how an in-path load
         regulation function must be carefully placed with respect to
         tunnel encapsulation (in a new sub-section entitled "Dependence
         of In-Path Load Regulation on Tunnelling").

1.  Introduction

   This document redefines how the explicit congestion notification
   (ECN) field [RFC3168] of the outer IP header of a tunnel should be
   constructed.  It brings all IP in IP tunnels (v4 or v6) into line
   with the way IPsec tunnels [RFC4301] now construct the ECN field,
   ensuring that the outer header reveals any congestion experienced so
   far on the whole path, not just since the last tunnel ingress.

   ECN allows a congested resource to notify the onset of congestion
   without having to drop packets, by explicitly marking a proportion of
   packets with the congestion experienced (CE) codepoint.  Because
   congestion is exhaustion of a physical resource, if the transport
   layer is to deal with congestion, congestion notification must
   propagate upwards; from the physical layer to the transport layer.
   The transport layer can directly detect loss of a packet (or frame)
   by a lower layer.  But if a lower layer marks rather than drops a

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   forward-travelling data packet (or frame) in order to notify
   incipient congestion, this marking has to be explicitly copied up the
   layers at every header decapsulation.  So, at each decapsulation of
   an outer (lower layer) header a congestion marking has to be arranged
   to propagate into the forwarded (upper layer) header.  It must
   continue upwards until it reaches the destination transport.  Then
   typically the destination feeds this congestion notification back to
   the source transport.  Given encapsulation by lower layer headers is
   functionally similar to tunnelling, it is necessary to arrange
   similar propagation of congestion notification up the layers.  For
   instance, ECN and its propagation up the layers has recently been
   specified for MPLS [RFC5129].

   As packets pass up the layers, current specifications of
   decapsulation behaviours are largely all consistent and correct.
   However, as packets pass down the layers, specifications of
   encapsulation behaviours are not consistent.  This document is
   primarily aimed at rationalising encapsulation.  (Nevertheless,
   Appendix C explains why the consistency of decapsulation solutions
   will not last for long and proposes a fix to decapsulation rules as
   well.  The IETF can then discuss whether to rationalise decapsulation
   at the same time as encapsulation.)

1.1.  The Need for Rationalisation

   IPsec tunnel mode is a specific form of tunnelling that can hide the
   inner headers.  Because the ECN field has to be mutable, it cannot be
   covered by IPsec encryption or authentication calculations.
   Therefore concern has been raised in the past that the ECN field
   could be used as a low bandwidth covert channel to communicate with
   someone on the unprotected public Internet even if an end-host is
   restricted to only communicate with the public Internet through an
   IPsec gateway.  However, the updated version of IPsec [RFC4301] chose
   not to block this covert channel, deciding that the threat could be
   managed given the channel bandwidth is so limited (ECN is a 2-bit

   An unfortunate sequence of standards actions leading up to this
   latest change in IPsec has left us with nearly the worst of all
   possible combinations of outcomes, despite the best endeavours of
   everyone concerned.  The controversy has been over whether to reveal
   information about congestion experienced on the path upstream of the
   tunnel ingress.  Even though this has various uses if it is revealed
   in the outer header of a tunnel, when ECN was standardised [RFC3168]
   it was decided that all IP in IP tunnels should hide this upstream
   congestion simply to avoid the extra complexity of two different
   mechanisms for IPsec and non-IPsec tunnels.  However, now that
   [RFC4301] IPsec tunnels deliberately no longer hide this information,

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   we are left in the perverse position where non-IPsec tunnels still
   hide congestion information unnecessarily.  This document is designed
   to correct that anomaly.

   Specifically, RFC3168 says that, if a tunnel fully supports ECN
   (termed a 'full-functionality' ECN tunnel in [RFC3168]), the tunnel
   ingress must not copy a CE marking from the inner header into the
   outer header that it creates.  Instead the tunnel ingress has to set
   the ECN field of the outer header to ECT(0) (i.e. codepoint 10).  We
   term this 'resetting' a CE codepoint.  However, RFC4301 reverses
   this, stating that the tunnel ingress must simply copy the ECN field
   from the inner to the outer header.  The main purpose of this
   document is to carry the new behaviour of IPsec over to all IP in IP
   tunnels, so all tunnel ingress nodes consistently copy the ECN field.

   Why does it matter if we have different ECN encapsulation behaviours
   for IPsec and non-IPsec tunnels?  The general argument is that
   gratuitous inconsistency constrains the available design space and
   makes it harder to design networks and new protocols that work

   Already complicated constraints have had to be added to a standards
   track congestion marking proposal.  The section of the pre-congestion
   notification (PCN) architecture [I-D.ietf-pcn-architecture] on
   tunnelling says PCN works correctly in the presence of RFC4301 IPsec
   encapsulation (and RFC5129 MPLS encapsulation).  However it doesn't
   work with RFC3168 IP in IP encapsulation (Appendix A explains why).

   To ensure we do not cause any unintended side-effects, Section 3
   assesses whether copying or resetting CE would harm any security,
   control or management functions.  It finds that resetting CE makes
   life difficult in a number of directions, while copying CE harms
   nothing (other than opening a low bit-rate covert channel
   vulnerability which the IETF Security Area deems is manageable).

1.2.  Document Roadmap

   Most of the document gives a thorough analysis of the knock-on
   effects of the apparently minor change to tunnel encapsulation.  The
   reader may jump to Section 5 if only interested in standards actions
   impacting implementation.  The whole document is organised as

   o  S.5 of RFC3168 permits the Diffserv codepoint (DSCP)[RFC2474] to
      'switch in' different behaviours for marking the ECN field, just
      as it switches in different per-hop behaviours (PHBs) for
      scheduling.  Therefore we cannot only discuss the ECN protocol
      that RFC3168 gives as a default.  Instead, Section 3 lays out the

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      design constraints when tunnelling congestion notification without
      assuming a particular congestion marking scheme.

   o  Then in Section 4 we resolve the tensions between these
      constraints to give general design principles and guidelines on
      how a tunnel should process congestion notification; principles
      that could apply to any marking behaviour for any PHB, not just
      the default in RFC3168.  In particular, we examine the underlying
      principles behind whether CE should be reset or copied into the
      outer header at the ingress to a tunnel--or indeed at the ingress
      of any layered encapsulation of headers with congestion
      notification fields.  We end this section with a bulleted list of
      design guidelines for new encapsulations of congestion

   o  Section 5 then uses precise standards terminology to confirm the
      rules for the default ECN tunnelling behaviour based on the above
      design principles.

   o  Extending the new IPsec tunnel ingress behaviour to all IP in IP
      tunnels requires consideration of backwards compatibility, which
      is covered in Section 6 and changes from earlier RFCs are brought
      together in Section 7.

   o  Finally, a number of security considerations are discussed and
      conclusions are drawn.

1.3.  Scope

   This document only concerns wire protocol processing at tunnel
   endpoints and makes no changes or recommendations concerning
   algorithms for congestion marking or congestion response.

   This document specifies a common, default congestion encapsulation
   for any IP in IP tunnelling, based on that now specified for IPsec.
   It applies irrespective of whether IPv4 or IPv6 is used for either of
   the inner and outer headers.  It applies to all PHBs, unless stated
   otherwise in the specification of a PHB.  It is intended to be a good
   trade off between somewhat conflicting security, control and
   management requirements.

   Nonetheless, if necessary, an alternate congestion encapsulation
   behaviour can be introduced as part of the definition of an alternate
   congestion marking scheme used by a specific Diffserv PHB (see S.5 of
   [RFC3168] and [RFC4774]).  When designing such new encapsulation
   schemes, the principles in Section 4 should be followed as closely as
   possible.  There is no requirement for a PHB to state anything about
   ECN tunnelling behaviour if the default behaviour is sufficient.

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   Often lower layer resources (e.g. a point-to-point Ethernet link) are
   arranged to be protected by higher layer buffers, so instead of
   congestion occurring at the lower layer, it merely causes the queue
   from the higher layer to overflow.  Such non-blocking link and
   physical layer technologies do not have to implement congestion
   notification, which can be introduced solely in the active queue
   management (AQM) from the IP layer.  However, not all link layer
   technologies are always protected from congestion by buffers at
   higher layers (e.g. a subnetwork of Ethernet links and switches can
   congest internally).  In these cases, when adding congestion
   notification to lower layers, we have to arrange for it to be
   explicitly copied up the layers, just as when IP is tunnelled in IP.

   As well as guiding alternate IP in IP tunnelling schemes, the design
   guidelines of Section 4 are intended to be followed when IP packets
   are encapsulated by any connectionless datagram/packet/frame where
   the outer header is designed to support a congestion notification
   capability.  [RFC5129] already deals with handling ECN for IP in MPLS
   and MPLS in MPLS, and S.9.3 of [RFC3168] lists IP encapsulated in
   L2TP [RFC2661], GRE [RFC1701] or PPTP [RFC2637] as possible examples
   where ECN may be added in future.

   Of course, the IETF does not have standards authority over every link
   or tunnel protocol, so this document merely aims to guide the
   interface between IP ECN and lower layer congestion notification.
   Then the IETF or the relevant standards body can be free to define
   the specifics of each lower layer scheme, but a common interface
   should ensure interworking across all technologies.

   Note that just because there is forward congestion notification in a
   lower layer protocol, if the lower layer has its own feedback and
   load regulation, there is no need to propagate it up the layers.  For
   instance, FECN (forward ECN) has been present in Frame Relay and EFCI
   (explicit forward congestion indication) in ATM [ITU-T.I.371] for a
   long time.  But so far they have been used for internal management
   rather than being propagated to endpoint transports for them to
   control end-to-end congestion.

   [RFC2983] is a comprehensive primer on differentiated services and
   tunnels.  Given ECN raises similar issues to differentiated services
   when interacting with tunnels, useful concepts introduced in RFC2983
   are used throughout, with brief recaps of the explanations where

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Design Constraints

   Tunnel processing of a congestion notification field has to meet
   congestion control needs without creating new information security
   vulnerabilities (if information security is required).

3.1.  Security Constraints

   Information security can be assured by using various end to end
   security solutions (including IPsec in transport mode [RFC4301]), but
   a commonly used scenario involves the need to communicate between two
   physically protected domains across the public Internet.  In this
   case there are certain management advantages to using IPsec in tunnel
   mode solely across the publicly accessible part of the path.  The
   path followed by a packet then crosses security 'domains'; the ones
   protected by physical or other means before and after the tunnel and
   the one protected by an IPsec tunnel across the otherwise unprotected
   domain.  We will use the scenario in Figure 1 where endpoints 'A' and
   'B' communicate through a tunnel.  The tunnel ingress 'I' and egress
   'E' are within physically protected edge domains, while the tunnel
   spans an unprotected internetwork where there may be 'men in the
   middle', M.

             physically       unprotected     physically
         <-protected domain-><--domain--><-protected domain->
         +------------------+            +------------------+
         |                  |      M     |                  |
         |    A-------->I=========>==========>E-------->B   |
         |                  |            |                  |
         +------------------+            +------------------+
                        <----IPsec secured---->

                      Figure 1: IPsec Tunnel Scenario

   IPsec encryption is typically used to prevent 'M' seeing messages
   from 'A' to 'B'.  IPsec authentication is used to prevent 'M'
   masquerading as the sender of messages from 'A' to 'B' or altering
   their contents.  But 'I' can also use IPsec tunnel mode to allow 'A'
   to communicate with 'B', but impose encryption to prevent 'A' leaking
   information to 'M'.  Or 'E' can insist that 'I' uses tunnel mode
   authentication to prevent 'M' communicating information to 'B'.
   Mutable IP header fields such as the ECN field (as well as the TTL/
   Hop Limit and DS fields) cannot be included in the cryptographic

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   calculations of IPsec.  Therefore, if 'I' copies these mutable fields
   into the outer header that is exposed across the tunnel it will have
   allowed a covert channel from 'A' to M that bypasses its encryption
   of the inner header.  And if 'E' copies these fields from the outer
   header to the inner, even if it validates authentication from 'I', it
   will have allowed a covert channel from 'M' to 'B'.

   ECN at the IP layer is designed to carry information about congestion
   from a congested resource towards downstream nodes.  Typically a
   downstream transport might feed the information back somehow to the
   point upstream of the congestion that can regulate the load on the
   congested resource, but other actions are possible (see [RFC3168]
   S.6).  In terms of the above unicast scenario, ECN is typically
   intended to create an information channel from 'M' to 'B' (for 'B' to
   feed back to 'A').  Therefore the goals of IPsec and ECN are mutually

   With respect to the DS or ECN fields, S.5.1.2 of RFC4301 says,
   "controls are provided to manage the bandwidth of this [covert]
   channel".  Using the ECN processing rules of RFC4301, the channel
   bandwidth is two bits per datagram from 'A' to 'M' and one bit per
   datagram from 'M' to 'A' (because 'E' limits the combinations of the
   2-bit ECN field that it will copy).  In both cases the covert channel
   bandwidth is further reduced by noise from any real congestion
   marking.  RFC4301 therefore implies that these covert channels are
   sufficiently limited to be considered a manageable threat.  However,
   with respect to the larger (6b) DS field, the same section of RFC4301
   says not copying is the default, but a configuration option can allow
   copying "to allow a local administrator to decide whether the covert
   channel provided by copying these bits outweighs the benefits of
   copying".  Of course, an administrator considering copying of the DS
   field has to take into account that it could be concatenated with the
   ECN field giving an 8b per datagram covert channel.

   Thus, for tunnelling the 6b Diffserv field two conceptual models have
   had to be defined so that administrators can trade off security
   against the needs of traffic conditioning [RFC2983]:

   The uniform model:  where the DIffserv field is preserved end-to-end
      by copying into the outer header on encapsulation and copying from
      the outer header on decapsulation.

   The pipe model:  where the outer header is independent of that in the
      inner header so it hides the Diffserv field of the inner header
      from any interaction with nodes along the tunnel.

   However, for ECN, the new IPsec security architecture in RFC4301 only
   standardised one tunnelling model equivalent to the uniform model.

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   It deemed that simplicity was more important than allowing
   administrators the option of a tiny increment in security, especially
   given not copying congestion indications could seriously harm
   everyone's network service.

3.2.  Control Constraints

   Congestion control requires that any congestion notification marked
   into packets by a resource will be able to traverse a feedback loop
   back to a function capable of controlling the load on that resource.
   To be precise, rather than calling this function the data source, we
   will call it the Load Regulator.  This will allow us to deal with
   exceptional cases where load is not regulated by the data source, but
   usually the two terms will be synonymous.  Note the term "a function
   _capable of_ controlling the load" deliberately includes a source
   application that doesn't actually control the load but ought to (e.g.
   an application without congestion control that uses UDP).


                     Figure 2: Simple Tunnel Scenario

   We now consider a similar tunnelling scenario to the IPsec one just
   described, but without the different security domains so we can just
   focus on ensuring the control loop and management monitoring can work
   (Figure 2).  If we want resources in the tunnel to be able to
   explicitly notify congestion and the feedback path is from 'B' to
   'A', it will certainly be necessary for 'E' to copy any CE marking
   from the outer header to the inner header for onward transmission to
   'B', otherwise congestion notification from resources like 'M' cannot
   be fed back to the Load Regulator ('A').  But it doesn't seem
   necessary for 'I' to copy CE markings from the inner to the outer
   header.  For instance, if resource 'R' is congested, it can send
   congestion information to 'B' using the congestion field in the inner
   header without 'I' copying the congestion field into the outer header
   and 'E' copying it back to the inner header.  'E' can still write any
   additional congestion marking introduced across the tunnel into the
   congestion field of the inner header.

   It might be useful for the tunnel egress to be able to tell whether
   congestion occurred across a tunnel or upstream of it.  If outer
   header congestion marking was reset by the tunnel ingress ('I'), at
   the end of a tunnel ('E') the outer headers would indicate congestion
   experienced across the tunnel ('I' to 'E'), while the inner header
   would indicate congestion upstream of 'I'.  But similar information
   can be gleaned even if the tunnel ingress copies the inner to the

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   outer headers.  At the end of the tunnel ('E'), any packet with an
   _extra_ mark in the outer header relative to the inner header
   indicates congestion across the tunnel ('I' to 'E'), while the inner
   header would still indicate congestion upstream of ('I').  Appendix B
   gives a simple and precise method for a tunnel egress to infer the
   congestion level introduced across a tunnel.

   All this shows that 'E' can preserve the control loop irrespective of
   whether 'I' copies congestion notification into the outer header or
   resets it.

   That is the situation for existing control arrangements but, because
   copying reveals more information, it would open up possibilities for
   better control system designs.  For instance, Appendix A describes
   how resetting CE marking at a tunnel ingress confuses a proposed
   congestion marking scheme on the standards track.  It ends up
   removing excessive amounts of traffic unnecessarily.  Whereas copying
   CE markings at ingress leads to the correct control behaviour.

3.3.  Management Constraints

   As well as control, there are also management constraints.
   Specifically, a management system may monitor congestion markings in
   passing packets, perhaps at the border between networks as part of a
   service level agreement.  For instance, monitors at the borders of
   autonomous systems may need to measure how much congestion has
   accumulated since the original source, perhaps to determine between
   them how much of the congestion is contributed by each domain.

   Therefore, when monitoring the middle of a path, it should be
   possible to establish how far back in the path congestion markings
   have accumulated from.  In this document we term this the baseline of
   congestion marking (or the Congestion Baseline), i.e. the source of
   the layer that last reset (or created) the congestion notification
   field.  Given some tunnels cross domain borders (e.g. consider M in
   Figure 2 is monitoring a border), it would therefore be desirable for
   'I' to copy congestion accumulated so far into the outer headers
   exposed across the tunnel.

   Appendix D discusses various scenarios where the Load Regulator lies
   in-path, not at the source host as we would typically expect.  It
   concludes that a Congestion Baseline is determined by where the Load
   Regulator function is, which should be identified in the transport
   layer, not by addresses in network layer headers.  This applies
   whether the Load Regulator is at the source host or within the path.
   The appendix also discusses where a Load Regulator function should be
   located relative to a local tunnel encapsulation function.

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4.  Design Principles

   The constraints from the three perspectives of security, control and
   management in Section 3 are somewhat in tension as to whether a
   tunnel ingress should copy congestion markings into the outer header
   it creates or reset them.  From the control perspective either
   copying or resetting works for existing arrangements, but copying has
   more potential for simplifying control.  From the management
   perspective copying is preferable.  From the security perspective
   resetting is preferable but copying is now considered acceptable
   given the bandwidth of a 2-bit covert channel can be managed.

   Therefore an outer encapsulating header capable of carrying
   congestion markings SHOULD reflect accumulated congestion since the
   last interface designed to regulate load (the Load Regulator).  This
   implies congestion notification SHOULD be copied into the outer
   header of each new encapsulating header that supports it.

   We have said that a tunnel ingress SHOULD (as opposed to MUST) copy
   incoming congestion notification into an outer encapsulating header
   that supports it.  In the case of 2-bit ECN, the IETF security area
   has deemed the benefit always outweighs the risk.  Therefore for
   2-bit ECN we can and we will say 'MUST' (Section 5).  But in this
   section where we are setting down general design principles, we leave
   it as a 'SHOULD'.  This allows for future multi-bit congestion
   notification fields where the risk from the covert channel created by
   copying congestion notification might outweigh the congestion control
   benefit of copying.

   The Load Regulator is the node to which congestion feedback should be
   returned by the next downstream node with a transport layer feedback
   function (typically but not always the data receiver).  The Load
   Regulator is not always (or even typically) the same thing as the
   node identified by the source address of the outermost exposed
   header.  In general the addressing of the outermost encapsulation
   header says nothing about the identifiers of either the upstream or
   the downstream transport layer functions.  As long as the transport
   functions know each other's addresses, they don't have to be
   identified in the network layer or in any link layer.  It was only a
   convenience that a TCP receiver assumed that the address of the
   source transport is the same as the network layer source address of
   an IP packet it receives.

   More generally, the return transport address for feedback could be
   identified solely in the transport layer protocol.  For instance, a
   signalling protocol like RSVP [RFC2205] breaks up a path into
   transport layer hops and informs each hop of the address of its
   transport layer neighbour without any need to identify these hops in

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   the network layer.  RSVP can be arranged so that these transport
   layer hops are bigger than the underlying network layer hops.  The
   host identity protocol (HIP) architecture [RFC4423] also supports the
   same principled separation (for mobility amongst other things), where
   the transport layer sender identifies its transport address for
   feedback to be sent to, using an identifier provided by a shim below
   the transport layer.

   Keeping to this layering principle deliberately doesn't require a
   network layer packet header to reveal the origin address from where
   congestion notification accumulates (its Congestion Baseline).  It is
   not necessary for the network and lower layers to know the address of
   the Load Regulator.  Only the destination transport needs to know
   that.  With forward congestion notification, the network and link
   layers only notify congestion forwards; they aren't involved in
   feeding it backwards.  If they are (e.g. backward congestion
   notification (BCN) in Ethernet [IEEE802.1au] or EFCI in ATM
   [ITU-T.I.371]), that should be considered as a transport function
   added to the lower layer, which must sort out its own addressing.
   Indeed, this is one reason why ICMP source quench is now deprecated
   [RFC1254]; when congestion occurs within a tunnel it is complex
   (particularly in the case of IPsec tunnels) to return the ICMP
   messages beyond the tunnel ingress back to the Load Regulator.

   Similarly, if a management system is monitoring congestion and needs
   to know the Congestion Baseline, the management system has to find
   this out from the transport; in general it cannot tell solely by
   looking at the network or link layer headers.

4.1.  Design Guidelines for New Encapsulations of Congestion

   The following guidelines are for specifications of new schemes for
   encapsulating congestion notification (e.g. for specialised Diffserv
   PHBs in IP, or for lower layer technologies):

   1.  Congestion notification in outer headers SHOULD be relative to a
       Congestion Baseline at the node expected to regulate the load on
       the link in question (the Load Regulator).  This implies incoming
       congestion notifications from the higher layer SHOULD be copied
       into encapsulating headers.  This guideline is particularly
       important where outer headers might cross trust boundaries, but
       less important otherwise.

   2.  Congestion notification MUST NOT simply be copied from outer
       headers to the forwarded header on decapsulation.  The forwarded
       congestion notification field SHOULD be calculated from the inner
       and outer headers, taking account of the following, in the order

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       1.  If the inner header does not support congestion notification,
           or indicates that the transport does not support congestion
           notification, any explicit congestion notifications in the
           outer header will not be understood if propagated further, so
           if the only way to indicate congestion to onward nodes is to
           drop the packet, it MUST be dropped.

       2.  If the outer header does not support explicit congestion
           notification, but the inner header does, the inner header
           SHOULD be forwarded unchanged.

       3.  Congestion indications may be ranked by strength.  For
           instance no congestion would be the weakest indication, with
           possibly increasing levels of congestion given increasingly
           stronger indications.

       4.  Where the inner and outer headers carry indications of
           congestion of different strengths, the stronger indication
           SHOULD be forwarded in preference to the weaker.  Obviously,
           if the strengths in both inner and outer are the same, the
           same strength should be forwarded.

       5.  If the outer header carries a weaker indication of congestion
           than the inner, it MAY be appropriate to raise a warning, as
           this would be in illegal combination if Guideline Paragraph 1
           had been followed.

   3.  Where framing boundaries are different between the two layers,
       congestion indications SHOULD be propagated on the basis that a
       congestion indication in a packet or frame applies to all the
       octets in the frame/packet.  On average, a tunnel endpoint SHOULD
       approximately preserve the number of marked octets arriving and
       leaving.  An algorithm for spreading congestion indications over
       multiple smaller `fragments' SHOULD propagate congestion
       indications as soon as they arrive, and SHOULD NOT hold them back
       for later frames.

   4.  Assumptions on incremental deployment MUST be stated.

   Regarding incremental deployment, the Per-Domain ECT Checking
   of[RFC5129] is a good example to follow.  In this example, header
   space in the lower layer protocol (MPLS) was extremely limited, so no
   ECN-capable transport codepoint was added to the MPLS header.
   Interior nodes in a domain were allowed to set explicit congestion
   indications without checking whether the frame was destined for a
   transport that would understand them.  This was made safe by

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   emphasising repeatedly that all the decapsulating edges of a whole
   domain had to be upgraded at once, so there would always be a check
   that the higher layer transport was ECN-capable on decapsulation.  If
   the decapsulator discovered that the higher layer showed the
   transport would not understand ECN, it dropped the packet on behalf
   of the earlier congestion node (see Guideline Paragraph 2.1).

   Note that such a deployment strategy that assumes a savvy operator
   was only appropriate because MPLS is targeted solely at professional
   operators.  This strategy would not be appropriate for other link
   technologies (e.g.  Ethernet) targeted at deployment by the general

5.  Default ECN Tunnelling Rules

   The following ECN tunnel processing rules are the default for a
   packet with any DSCP.  If required, different ECN encapsulation rules
   MAY be defined as part of the definition of an appropriate Diffserv
   PHB using the guidelines in Section 4.  However, the burden of
   handling exceptional PHBs in implementations of all affected tunnels
   and lower layer link protocols should not be underestimated.

   A tunnel ingress compliant with this specification MUST copy the
   2-bit ECN field of the arriving IP header into the outer
   encapsulating IP header, for all types of IP in IP tunnel.  This
   encapsulation behaviour MUST only be used if the tunnel ingress is in
   `normal state'.  A `compatibility state' with a different
   encapsulation behaviour is also specified in Section 6 for backward
   compatibility with legacy tunnel egresses that do not understand ECN.

   To decapsulate the inner header at the tunnel egress, a compliant
   tunnel egress MUST set the outgoing ECN field to the codepoint at the
   intersection of the appropriate incoming inner header (row) and outer
   header (column) in Figure 3.

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                       |             Incoming Outer Header            |
    |  Incoming Inner  | Not-ECT |   ECT(0)   |   ECT(1)   |    CE    |
    |      Header      |         |            |            |          |
    |     Not-ECT      | Not-ECT |   drop(!!!)|   drop(!!!)| drop(!!!)|
    |      ECT(0)      |  ECT(0) | ECT(0)     | ECT(0)(!!!)|   CE     |
    |      ECT(1)      |  ECT(1) | ECT(1)(!!!)| ECT(1)     |   CE     |
    |        CE        |      CE |     CE     |     CE(!!!)|   CE     |
                       |                Outgoing Header               |

                     Figure 3: IP in IP Decapsulation

   The exclamation marks '(!!!)' in Figure 3 indicate that this
   combination of inner and outer headers should not be possible if only
   legal transitions have taken place.  So, the decapsulator should drop
   or mark the ECN field as the table in Figure 3 specifies, but it MAY
   also raise an appropriate alarm.  It MUST NOT raise an alarm so often
   that the illegal combinations would amplify into a flood of alarm

6.  Backward Compatibility

   Note: in RFC3168, a tunnel was in one of two modes: limited
   functionality or full functionality.  Rather than working with modes
   of the tunnel as a whole, this specification uses the term `state' to
   refer separately to the state of each tunnel end point, which is how
   implementations have to work.

   If one end of an IPsec tunnel is compliant with [RFC4301], the other
   end can be guaranteed to also be [RFC4301] compliant (there could be
   corner cases where manual keying is used, but they will be ignored
   here).  So there is no backward compatibility problem with IKEv2
   RFC4301 IPsec tunnels.  But once we extend our scope to any IP in IP
   tunnel, we have to cater for the possibility that a legacy tunnel
   egress may not know how to process an ECN field, so if ECN capable
   outer headers were sent towards a legacy (e.g.  [RFC2003]) egress, it
   would most likely simply disregard the outer headers, dangerously
   discarding information about congestion experienced within the
   tunnel.  ECN-capable traffic sources would not see any congestion
   feedback and instead continually ratchet up their share of the
   bandwidth without realising that cross-flows from other ECN sources
   were continually having to ratchet down.

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   To be compliant with this specification a tunnel ingress that does
   not always know the ECN capability of its tunnel egress MUST
   implement a 'normal' state and a 'compatibility' state, and it MUST
   initiate each negotiated tunnel in the compatibility state.

   However, a tunnel ingress can be compliant even if it only implements
   the 'normal state' of encapsulation behaviour, but only as long as it
   is designed or configured so that all possible tunnel egress nodes it
   will ever talk to will have full ECN functionality (RFC3168 full
   functionality mode, RFC4301 and this present specification).  The
   `normal state' is that defined in Section 5 (i.e. header copying).
   Note that a [RFC4301] tunnel ingress that has used IKEv2 key
   management [RFC4306] can guarantee that its tunnel egress is also
   RFC4301-compliant and therefore need not further negotiate ECN

   Before switching to normal state, a compliant tunnel ingress that
   does not know the egress ECN capability MUST negotiate with the
   tunnel egress.  If the egress says it is in full functionality state
   (or mode), the ingress puts itself into normal state.  In normal
   state the ingress follows the encapsulation rule in Section 5 (i.e.
   header copying).  If the egress says it is not in full-functionality
   state/mode or doesn't understand the question, the tunnel ingress
   MUST remain in compatibility state.

   A tunnel ingress in compatibility state MUST set all outer headers to
   Not-ECT.  This is the same per packet behaviour as the ingress end of
   RFC3168's limited functionality mode.

   A tunnel ingress that only implements compatibility state is at least
   safe with the ECN behaviour of any egress it may encounter (any of
   RFC2003, RFC2401, either mode of RFC2481 and RFC3168's limited
   functionality mode).  But an ingress cannot claim compliance with
   this specification simply by disabling ECN processing across the
   tunnel.  A compliant tunnel ingress MUST at least implement `normal
   state' and, if it might be used with arbitrary tunnel egress nodes,
   it MUST also implement `compatibility state'.

   A compliant tunnel egress on the other hand merely needs to implement
   the one behaviour in Section 5, which we term 'full-functionality'
   state, as it is the same as the egress end of the full-functionality
   mode of [RFC3168].  It is also the same as the [RFC4301] egress

   The decapsulation rules for the egress of the tunnel in Section 5
   have been defined in such a way that congestion control will still
   work safely if any of the earlier versions of ECN processing are used
   unilaterally at the encapsulating ingress of the tunnel (any of

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   RFC2003, RFC2401, either mode of RFC2481, either mode of RFC3168,
   RFC4301 and this present specification).  If a tunnel ingress tries
   to negotiate to use limited functionality mode or full functionality
   mode [RFC3168], a decapsulating tunnel egress compliant with this
   specification MUST agree to either request, as its behaviour will be
   the same in both cases.

   For 'forward compatibility', a compliant tunnel egress SHOULD raise a
   warning about any requests to enter states or modes it doesn't
   recognise, but it can continue operating.  If no ECN-related state or
   mode is requested, a compliant tunnel egress need not raise an error
   or warning as its egress behaviour is compatible with all the legacy
   ingress behaviours that don't negotiate capabilities.

   Implementation note: if a compliant node is the ingress for multiple
   tunnels, a state setting will need to be stored for each tunnel
   ingress.  However, if a node is the egress for multiple tunnels, none
   of the tunnels will need to store a state setting, because a
   compliant egress can only be in one state.

7.  Changes from Earlier RFCs

   The rule that a normal state tunnel ingress MUST copy any ECN field
   into the outer header is a change to the ingress behaviour of
   RFC3168, but it is the same as the rules for IPsec tunnels in

   The rules for calculating the outgoing ECN field on decapsulation at
   a tunnel egress are in line with the full functionality mode of ECN
   in RFC3168 and with RFC4301, except that neither identified the
   following illegal combinations: outer ECT(1) with inner ECT(0) or
   with CE; outer ECT(0) with inner ECT(1).

   The rules for how a tunnel establishes whether the egress has full
   functionality ECN capabilities are an update to RFC3168.  For all the
   typical cases, RFC4301 is not updated by the ECN capability check in
   this specification, because a typical RFC4301 tunnel ingress will
   have already established that it is talking to an RFC4301 tunnel
   egress (e.g. if it uses IKEv2).  However, there may be some corner
   cases (e.g. manual keying) where an RFC4301 tunnel ingress talks with
   an egress with limited functionality ECN handling.  Strictly, for
   such corner cases, the requirement to use compatibility mode in this
   specification updates RFC4301.

   The optional ECN Tunnel field in the IPsec security association
   database (SAD) and the optional ECN Tunnel Security Association
   Attribute defined in RFC3168 are no longer needed.  The security

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   association (SA) has no policy on ECN usage, because all RFC4301
   tunnels now support ECN without any policy choice.

   RFC3168 defines a (required) limited functionality mode and an
   (optional) full functionality mode for a tunnel, but RFC4301 doesn't
   need modes.  In this specification only the ingress might need two
   states: a normal state (required) and a compatibility state (required
   in some scenarios, optional in others).  The egress needs only full-
   functionality state which handles ECN the same as either mode of
   RFC3168 or RFC4301.

Additional changes to the RFC Index (to be removed by the RFC Editor):

   In the RFC index, RFC3168 should be identified as an update to
   RFC2003 and RFC4301 should be identified as an update to RFC3168.

   This specification updates RFC3168.  It also suggests a minor
   optional warning and a corner-case change to RFC4301, but these don't
   really count as an update.

8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   Section 3.1 discusses the security constraints imposed on ECN tunnel
   processing.  The Design Principles of Section 4 trade-off between
   security (covert channels) and congestion monitoring & control.  In
   fact, ensuring congestion markings are not lost is itself another
   aspect of security, because if we allowed congestion notification to
   be lost, any attempt to enforce a response to congestion would be
   much harder.

   If alternate congestion notification semantics are defined for a
   certain PHB (e.g. the pre-congestion notification architecture
   [I-D.ietf-pcn-architecture]), the scope of the alternate semantics
   might typically be bounded by the limits of a Diffserv region or
   regions, as envisaged in [RFC4774].  The inner headers in tunnels
   crossing the boundary of such a Diffserv region but ending within the
   region can potentially leak the external congestion notification
   semantics into the region, or leak the internal semantics out of the
   region.  [RFC2983] discusses the need for Diffserv traffic
   conditioning to be applied at these tunnel endpoints as if they are
   at the edge of the Diffserv region.  Similar concerns apply to any
   processing or propagation of the ECN field at the edges of a Diffserv

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   region with alternate ECN semantics.  Such edge processing must also
   be applied at the endpoints of tunnels with ends both inside and
   outside the domain.  [I-D.ietf-pcn-architecture] gives specific
   advice on this for the PCN case, but other definitions of alternate
   semantics will need to discuss the specific security implications in
   their case.

   With the rules as they stand in RFC3168 and RFC4301, a small part of
   the protection of the ECN nonce [RFC3540] is compromised.  One reason
   two ECT codepoints were defined was to enable the data source to
   detect if a CE marking had been applied then subsequently removed.
   The source could detect this by weaving a pseudo-random sequence of
   ECT(0) and ECT(1) values into a stream of packets, which is termed an
   ECN nonce.  By the decapsulation rules in RFC3168 and RFC4301, if the
   inner and outer headers carry contradictory ECT values only the inner
   header is preserved for onward forwarding.  So if a CE marking added
   to the outer ECN field has been illegally (or accidentally)
   suppressed by a subsequent node in the tunnel, the decapsulator will
   revert the ECN field to its value before tampering, hiding all
   evidence of the crime from the onward feedback loop.  To close this
   minor loophole, we could have specified that an outer header value of
   ECT should overwrite a contradictory ECT value in the inner header.
   But currently we choose to keep the 'broken' behaviour defined in
   RFC3168 & RFC4301 for all the following reasons:

   1.  We wanted to avoid any changes to IPsec tunnelling behaviour;

   2.  Allowing ECT values in the outer header to override the inner
       header would have increased the bandwidth of the covert channel
       through the egress gateway from 1 to 1.5 bit per datagram,
       potentially threatening to upset the consensus established in the
       security area that says that the bandwidth of this covert channel
       can now be safely managed;

   3.  This loophole is only applicable in the corner case where the
       attacker is a network node downstream of a congested node in the
       same tunnel;

   4.  In tunnelling scenarios, the ECN nonce is already vulnerable to
       suppression by nodes downstream of a congested node in the same
       tunnel, if they can copy the ECT value in the inner header to the
       outer header (any node in the tunnel can do this if the inner
       header is not encrypted, and an IPsec tunnel egress can do it
       whether or not the tunnel is encrypted);

   5.  Although the 'broken' decapsulation behaviour removes evidence of
       congestion suppression from the onward feedback loop, the
       decapsulator itself can at least detect that congestion within

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       the tunnel has been suppressed;

   6.  The ECN nonce [RFC3540] currently has experimental status and
       there has been no evidence that anyone has implemented it beyond
       the author's prototype.

   If a legacy security policy configures a legacy tunnel ingress to
   negotiate to turn off ECN processing, a compliant tunnel egress will
   agree to a request to turn off ECN processing but it will actually
   still copy CE markings from the outer to the forwarded header.
   Although the tunnel ingress 'I' in Figure 1 will set all ECN fields
   in outer headers to Not-ECT, 'M' could still toggle CE on and off to
   communicate covertly with 'B', because we have specified that 'E'
   only has one mode regardless of what mode it says it has negotiated.
   We could have specified that 'E' should have a limited functionality
   mode and check for such behaviour.  But we decided not to add the
   extra complexity of two modes on a compliant tunnel egress merely to
   cater for a legacy security concern that is now considered

10.  Conclusions

   This document updates the ingress tunnelling encapsulation of RFC3168
   ECN for all IP in IP tunnels to bring it into line with the new
   behaviour in the IPsec architecture of RFC4301.

   At a tunnel egress, header decapsulation for the default ECN marking
   behaviour is broadly unchanged except that one exceptional case has
   been catered for.  At the ingress, for all forms of IP in IP tunnel,
   encapsulation has been brought into line with the new IPsec rules in
   RFC4301 which copy rather than reset CE markings when creating outer

   This change to encapsulation has been motivated by analysis from the
   three perspectives of security, control and management.  They are
   somewhat in tension as to whether a tunnel ingress should copy
   congestion markings into the outer header it creates or reset them.
   From the control perspective either copying or resetting works for
   existing arrangements, but copying has more potential for simplifying
   control and resetting breaks at least one proposal already on the
   standards track.  From the management and monitoring perspective
   copying is preferable.  From the network security perspective (theft
   of service etc) copying is preferable.  From the information security
   perspective resetting is preferable, but the IETF Security Area now
   considers copying acceptable given the bandwidth of a 2-bit covert
   channel can be managed.  Therefore there are no points against
   copying and a number against resetting CE on ingress.

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   The change ensures ECN processing in all IP in IP tunnels reflects
   this slightly more permissive attitude to revealing congestion
   information in the new IPsec architecture.  Once all tunnelling of
   ECN works the same, ECN markings will have a defined meaning when
   measured at any point in a network.  This new certainty will enable
   new uses of the ECN field that would otherwise be confounded by

   Also, this document defines more generic principles to guide the
   design of alternate forms of tunnel processing of congestion
   notification, if required for specific Diffserv PHBs or for other
   lower layer encapsulating protocols that might support congestion
   notification in the future.

11.  Acknowledgements

   Thanks to David Black for explaining a better way to think about
   function placement and to Louise Burness for a better way to think
   about multilayer transports and networks, having read
   [Patterns_Arch].  Also thanks to Arnaud Jacquet for the idea for
   Appendix B.  Thanks to Bruce Davie, Toby Moncaster, Gorry Fairhurst,
   Sally Floyd, Alfred Hoenes and Gabriele Corliano for their thoughts
   and careful review comments.

   Bob Briscoe is partly funded by Trilogy, a research project (ICT-
   216372) supported by the European Community under its Seventh
   Framework Programme.  The views expressed here are those of the
   author only.

12.  Comments Solicited

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

13.  References

13.1.  Normative References

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.

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

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   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              December 1998.

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

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

13.2.  Informative References

              Briscoe, B., "PCN 3-State Encoding Extension in a single
              DSCP", draft-briscoe-pcn-3-in-1-encoding-00 (work in
              progress), October 2008.

              Eardley, P., "Pre-Congestion Notification (PCN)
              Architecture", draft-ietf-pcn-architecture-08 (work in
              progress), October 2008.

              Moncaster, T., Briscoe, B., and M. Menth, "Baseline
              Encoding and Transport of Pre-Congestion Information",
              draft-ietf-pcn-baseline-encoding-01 (work in progress),
              October 2008.

              Eardley, P., "Marking behaviour of PCN-nodes",
              draft-ietf-pcn-marking-behaviour-01 (work in progress),
              October 2008.

              Bryant, S., Davie, B., Martini, L., and E. Rosen,
              "Pseudowire Congestion Control Framework",
              draft-ietf-pwe3-congestion-frmwk-01 (work in progress),
              May 2008.

              Menth, M., Babiarz, J., Moncaster, T., and B. Briscoe,
              "PCN Encoding for Packet-Specific Dual Marking (PSDM)",
              draft-menth-pcn-psdm-encoding-00 (work in progress),
              July 2008.


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              Moncaster, T., Briscoe, B., and M. Menth, "A three state
              extended PCN encoding scheme",
              draft-moncaster-pcn-3-state-encoding-00 (work in
              progress), June 2008.

              IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks--Virtual Bridged Local Area Networks - Amendment
              10: Congestion Notification", 2008,

              (Work in Progress; Access Controlled link within page)

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

              IETF, "Congestion and Pre-Congestion Notification (pcn)",
              IETF w-g charter , Feb 2007,

              Day, J., "Patterns in Network Architecture: A Return to
              Fundamentals", Pub: Prentice Hall ISBN-13: 9780132252423,
              Jan 2008.

   [RFC1254]  Mankin, A. and K. Ramakrishnan, "Gateway Congestion
              Control Survey", RFC 1254, August 1991.

   [RFC1701]  Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
              Routing Encapsulation (GRE)", RFC 1701, October 1994.

   [RFC2205]  Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
              Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
              Functional Specification", RFC 2205, September 1997.

   [RFC2637]  Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little,
              W., and G. Zorn, "Point-to-Point Tunneling Protocol",
              RFC 2637, July 1999.

   [RFC2661]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
              G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
              RFC 2661, August 1999.

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, October 2000.

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   [RFC3426]  Floyd, S., "General Architectural and Policy
              Considerations", RFC 3426, November 2002.

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

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

   [RFC4423]  Moskowitz, R. and P. Nikander, "Host Identity Protocol
              (HIP) Architecture", RFC 4423, May 2006.

   [RFC4774]  Floyd, S., "Specifying Alternate Semantics for the
              Explicit Congestion Notification (ECN) Field", BCP 124,
              RFC 4774, November 2006.

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

   [Shayman]  "Using ECN to Signal Congestion Within an MPLS Domain",
              2000, <


Editorial Comments

   [Note_Nonce_Compr]  Note that even the tentatively proposed
                       Comprehensive Decapsulation Rules in Appendix C
                       do not fix the minor compromise to the protection
                       of the ECN nonce that RFC3168 and RFC4301 both
                       suffer from (described under Security
                       Considerations above). An attacker with control
                       over a tunnel interior node can revert a packet
                       previously marked CE within the same tunnel to
                       its original marking. It can do this by changing
                       CE markings to ECT(0) because the decapsulator
                       rules give precedence to the inner header if the
                       outer is ECT(0). To fix this, we could have
                       specified that the outgoing header should be
                       ECT(0) when the incoming outer is ECT(0) but the
                       inner is ECT(1). Although this would close the
                       minor loophole in the nonce, it would raise a
                       minor safety issue if multilevel ECN or PCN were
                       used. A less severe marking in the inner header
                       would override a more severe one in the outer.
                       Both are corner cases so it is difficult to

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                       decide which is more important: i) the loophole
                       in the nonce is only for a minor case of one
                       tunnel node attacking another in the same tunnel;
                       and ii) the severity inversion would not result
                       from any legal codepoint transition. If the
                       Comprehensive Decapsulation Rules of Appendix C
                       are taken up, we currently believe i) safety
                       against misconfiguration is slightly more
                       important than ii) securing against an attack
                       that has little, if any, clear motivation.

Appendix A.  Why resetting CE on encapsulation harms PCN

   Regarding encapsulation, the section of the PCN architecture
   [I-D.ietf-pcn-architecture] on tunnelling says that header copying
   (RFC4301) allows PCN to work correctly.  Whereas resetting CE
   markings confuses PCN marking.

   The specific issue here concerns PCN excess rate marking
   [I-D.ietf-pcn-marking-behaviour], i.e. the bulk marking of traffic
   that exceeds a configured threshold rate.  One of the goals of excess
   rate marking is to enable the speedy removal of excess admission
   controlled traffic following re-routes caused by link failures or
   other disasters.  This maintains a share of the capacity for
   competing admission controlled traffic and for traffic in lower
   priority classes.  After failures, traffic re-routed onto remaining
   links can often stress multiple links along a path.  Therefore,
   traffic can arrive at a link under stress with some proportion
   already marked for removal by a previous link.  By design, marked
   traffic will be removed by the overall system in subsequent round
   trips.  So when the excess rate marking algorithm decides how much
   traffic to mark for removal, it doesn't include traffic already
   marked for removal by another node upstream (the `Excess traffic
   meter function' of [I-D.ietf-pcn-marking-behaviour]).

   However, if an RFC3168 tunnel ingress intervenes, it resets the ECN
   field in all the outer headers, hiding all the evidence of problems
   upstream.  Thus, although excess rate marking works fine with RFC4301
   IPsec tunnels, with RFC3168 tunnels it typically removes large
   volumes of traffic that it didn't need to remove at all.

Appendix B.  Contribution to Congestion across a Tunnel

   This specification mandates that a tunnel ingress determines the ECN
   field of each new outer tunnel header by copying the arriving header.
   Concern has been expressed that this will make it difficult for the

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   tunnel egress to monitor congestion introduced along a tunnel, which
   is easy if the outer ECN field is reset at a tunnel ingress (RFC3168
   full functionality mode).  However, in fact copying CE marks at
   ingress will still make it easy for the egress to measure congestion
   introduced across a tunnel, as illustrated below.

   Consider 100 packets measured at the egress.  It measures that 30 are
   CE marked in the inner and outer headers and 12 have additional CE
   marks in the outer but not the inner.  This means packets arriving at
   the ingress had already experienced 30% congestion.  However, it does
   not mean there was 12% congestion across the tunnel.  The correct
   calculation of congestion across the tunnel is p_t = 12/(100-30) =
   12/70 = 17%.  This is easy for the egress to to measure.  It is the
   packets with additional CE marking in the outer header (12) as a
   proportion of packets not marked in the inner header (70).

   Figure 4 illustrates this in a combinatorial probability diagram.
   The square represents 100 packets.  The 30% division along the bottom
   represents marking before the ingress, and the p_t division up the
   side represents marking along the tunnel.

     |     |         |
     | 30  |         |
     |     |         |       The large square
     |     +---------+p_t    represents 100 packets
     |     |   12    |
     0    30%       100%
     inner header marking

       Figure 4: Tunnel Marking of Packets Already Marked at Ingress

Appendix C.  Comprehensive Decapsulation Rules

   This appendix is not currently normative.  Compliance with this
   appendix is NOT REQUIRED for compliance with the present

   Given this specification requests a standards action to update the
   RFC3168 encapsulation behaviour, this appendix explores a further
   change to decapsulation that we ought to specify at the same time.
   If instead this further change is added later, it will add another
   optional mode to the already complicated change history of ECN

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   Multi-level congestion notification is currently on the IETF's
   standards track agenda in the Congestion and Pre-Congestion
   Notification (PCN) working group.  The PCN working group eventually
   requires three congestion states (not marked and two increasingly
   severe levels of congestion marking) [I-D.ietf-pcn-architecture].
   The aim is for the less severe level of marking to stop admitting new
   traffic and the more severe level to terminate sufficient existing
   flows to bring a network back to its operating point after a serious

   Although the ECN field gives sufficient codepoints for these three
   states, current ECN tunnelling RFCs prevent the PCN working group
   from using them in case any tunnel decapsulations occur within a PCN
   region (see Appendix A of [I-D.ietf-pcn-baseline-encoding]).  If a
   node in a tunnel sets the ECN field to ECT(0) or ECT(1), this change
   will be discarded by a tunnel egress compliant with RFC4301 or
   RFC3168.  This can be seen in Figure 3, where the ECT values in the
   outer header are ignored unless the inner header is the same.
   Effectively the ECT(0) and ECT(1) codepoints have to be treated as
   just one codepoint when they could otherwise have been used for their
   intended purpose of congestion notification.

   As a consequence, the PCN w-g has initially confined itself to two
   encoding states as a baseline encoding
   [I-D.ietf-pcn-baseline-encoding].  And it has had to propose an
   experimental extension using extra Diffserv codepoint(s) to encode
   the extra states [I-D.moncaster-pcn-3-state-encoding], using up the
   rapidly exhausting DSCP space while leaving ECN codepoints unused.
   Another PCN encoding has been proposed that would survive tunnelling
   without an extra DSCP [I-D.menth-pcn-psdm-encoding], but it requires
   the PCN edge gateways to somehow share state so the egress can
   determine which marking a packet started with at the ingress.  Also a
   PCN ingress node can game the system by initiating packets with
   inappropriate markings.

   Although this issue is currently most pressing for the PCN working
   group, it is more general.  The currently standardised tunnel
   decapsulation behaviour unnecessarily wastes a quarter of two bits
   (i.e. half a bit) in the IP (v4 & v6) header.  As explained in
   Section 3.1, the original reason for not copying down outer ECT
   codepoints for onward forwarding was to limit the covert channel
   across a decapsulator to 1 bit per packet.  However, now that the
   IETF Security Area has deemed that a 2-bit covert channel through an
   encapsulator is a manageable risk, the same should be true for a

   Figure 5 proposes a more comprehensive layered decapsulation
   behaviour that would properly support a simpler experimental 3-state

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   ECN encodings such as
   [I-D.briscoe-pcn-3-in-1-encoding].[Note_Nonce_Compr] Note that the
   proposal tabulated in Figure 5 is only to support discussion.  It is
   not currently proposed for standards action.  The only difference
   from Figure 3 (which _is_ proposed for standards action) is the
   change to the cell highlighted as *ECT(1)*.

                      |             Incoming Outer Header            |
   |  Incoming Inner  | Not-ECT |   ECT(0)   |   ECT(1)   |    CE    |
   |      Header      |         |            |            |          |
   |     Not-ECT      | Not-ECT |   drop(!!!)|   drop(!!!)| drop(!!!)|
   |      ECT(0)      |  ECT(0) | ECT(0)     |*ECT(1)*    |   CE     |
   |      ECT(1)      |  ECT(1) | ECT(1)(!!!)| ECT(1)     |   CE     |
   |        CE        |      CE |     CE     |     CE(!!!)|   CE     |
                      |                Outgoing Header               |

         Figure 5: Comprehensive IP in IP Decapsulation (currently
                        informative, not normative)

   The table is derived from the following logic:

   o  On decapsulation, if the inner ECN field is Not-ECT but the outer
      ECN field is anything but Not-ECT the decapsulator must drop the
      packet.  This is because the Not-ECT marking on the inner header
      is set by transports that do not know how to respond to an
      explicit congestion marking;

   o  In all other cases, the outgoing ECN field is set to the more
      severe marking of the outer and inner ECN fields, where the
      ranking of severity from highest to lowest is CE, ECT(1), ECT(0),

   o  There are cases where no legal transition in any current or
      previous ECN tunneling specification would result in certain
      combinations of inner and outer ECN fields.  In these cases
      (indicated in the table by '(!!!)'), the decapsulator may also
      raise an alarm, but not so often that the illegal combinations
      would amplify into a flood of alarm messages.

   If this more comprehensive decapsulation proposal were taken up, it
   would be backwards compatible with all previous encapsulations of ECN
   at the ingress (RFC4301, both modes of RFC3168, both modes of RFC2481

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   and RFC2003).  The outgoing header is different for one combination
   of inner & outer headers, but that combination was previously illegal
   anyway, so no known mechanisms in the Internet rely on the previous
   behaviour.  The proposed tunnel egress requires no additional option
   configuration at the ingress or egress nor any additional negotiation
   with the ingress.

C.1.  Ways to Introduce the Comprehensive Decapsulation Rules

   There would be a number of ways for this more comprehensive
   decapsulation proposal to be introduced:

   o  It could be specified in the present standards track proposal
      (preferred) or in an experimental extension;

   o  it could be specified as a new default for all Diffserv PHBs
      (preferred) or as an option to be configured only for Diffserv
      PHBs requiring it.

   The argument for making this change now, rather than in a separate
   experimental extension, is to avoid the burden of an extra standard
   to be compliant with and to be backwards compatible with--so we don't
   add to the already complex history of ECN tunnelling RFCs.  The
   argument for a separate experimental extension is that we may never
   need this change (if PCN is never successfully deployed and if no-one
   ever needs three ECN or PCN encoding states rather than two).
   However, the change does no harm to existing mechanisms and stops
   tunnels wasting of quarter of a bit (a 2-bit codepoint).

   The argument for making this new decapsulation behaviour the default
   for all PHBs is that it doesn't change any expected behaviour that
   existing mechanisms rely on already.  Also, by ending the present
   waste of a codepoint, in the future a use of that codepoint could be
   proposed for all PHBs, even if PCN isn't successfully deployed.

   In practice, if this comprehensive decapsulation was specified
   straightaway as the normative default for all PHBs, a network
   operator deploying 3-state PCN would be able to request that tunnels
   comply with the latest specification.  Implementers of non-PCN
   tunnels would not need to comply but, if they did, their code would
   be future proofed and no harm would be done to legacy operations.
   Therefore, rather than branching their code base, it would be easiest
   for implementers to make all their new tunnel code comply with this
   specfication, whether or not it was for PCN.  But they could leave
   old code untouched, unless it was for PCN.

   The alternatives are worse.  Implementers would otherwise have to
   provide configurable decapsulation options and operators would have

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   to configure all IPsec and IP in IP tunnel endpoints for the
   exceptional behaviour of certain PHBs.  The rules for tunnel
   endpoints to handle both the Diffserv field and the ECN field should
   'just work' when handling packets with any Diffserv codepoint.

Appendix D.  Non-Dependence of Tunnelling on In-path Load Regulation

   We have said that at any point in a network, the Congestion Baseline
   (where congestion notification starts from zero) should be the
   previous upstream Load Regulator.  We have also said that the ingress
   of an IP in IP tunnel must copy congestion indications to the
   encapsulating outer headers it creates.  If the Load Regulator is in-
   path rather than at the source, and also a tunnel ingress, these two
   requirements seem to be contradictory.  A tunnel ingress must not
   reset incoming congestion, but a Load Regulator must be the
   Congestion Baseline, implying it needs to reset incoming congestion.

   In fact, the two requirements are not contradictory, because a Load
   Regulator and a tunnel ingress are functions within a node that
   typically occur in sequence on a stream of packets, not at the same
   point.  Figure 6 is borrowed from [RFC2983] (which was making a
   similar point about the location of Diffserv traffic conditioning
   relative to the encapsulation function of a tunnel).  An in-path Load
   Regulator can act on packets either at [1 - Before] encapsulation or
   at [2 - Outer] after encapsulation.  Load Regulation does not ever
   need to be integrated with the [Encapsulate] function (but it can be
   for efficiency).  Therefore we can still mandate that the
   [Encapsulate] function always copies CE into the outer header.

     >>-----[1 - Before]--------[Encapsulate]----[3 - Inner]---------->>
                                           +--------[2 - Outer]------->>

     Figure 6: Placement of In-Path Load Regulator Relative to Tunnel

   Then separately, if there is a Load Regulator at location [2 -
   Outer], it might reset CE to ECT(0), say.  Then the Congestion
   Baseline for the lower layer (outer) will be [2 - Outer], while the
   Congestion Baseline of the inner layer will be unchanged.  But how
   encapsulation works has nothing to do with whether a Load Regulator
   is present or where it is.

   If on the other hand a Load Regulator resets CE at [1 - Before], the

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   Congestion Baseline of both the inner and outer headers will be [1 -
   Before].  But again, encapsulation is independent of load regulation.

D.1.  Dependence of In-Path Load Regulation on Tunnelling

   Although encapsulation doesn't need to depend on in-path load
   regulation, the reverse is not true.  The placement of an in-path
   Load Regulator must be carefully considered relative to
   encapsulation.  Some examples are given in the following for

   In the traditional Internet architecture one tends to think of the
   source host as the Load Regulator for a path.  It is generally not
   desirable or practical for a node part way along the path to regulate
   the load.  However, various reasonable proposals for in-path load
   regulation have been made from time to time (e.g. fair queuing,
   traffic engineering, flow admission control).  The IETF has recently
   chartered a working group to standardise admission control across a
   part of a path using pre-congestion notification (PCN) [PCNcharter].
   This is of particular relevance here because it involves congestion
   notification with an in-path Load Regulator, it can involve
   tunnelling and it certainly involves encapsulation more generally.

   We will use the more complex scenario in Figure 7 to tease out all
   the issues that arise when combining congestion notification and
   tunnelling with various possible in-path load regulation schemes.  In
   this case 'I1' and 'E2' break up the path into three separate
   congestion control loops.  The feedback for these loops is shown
   going right to left across the top of the figure.  The 'V's are arrow
   heads representing the direction of feedback, not letters.  But there
   are also two tunnels within the middle control loop: 'I1' to 'E1' and
   'I2' to 'E2'.  The two tunnels might be VPNs, perhaps over two MPLS
   core networks.  M is a congestion monitoring point, perhaps between
   two border routers where the same tunnel continues unbroken across
   the border.
        ______     _______________________________________      _____
       /      \   /                                        \   /     \
      V        \ V                                M         \ V       \

                     Figure 7: complex Tunnel Scenario

   The question is, should the congestion markings in the outer exposed
   headers of a tunnel represent congestion only since the tunnel
   ingress or over the whole upstream path from the source of the inner
   header (whatever that may mean)?  Or put another way, should 'I1' and
   'I2' copy or reset CE markings?

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   Based on the design principles in Section 4, the answer is that the
   Congestion Baseline should be the nearest upstream interface designed
   to regulate traffic load--the Load Regulator.  In Figure 7 'A', 'I1'
   or 'E2' are all Load Regulators.  We have shown the feedback loops
   returning to each of these nodes so that they can regulate the load
   causing the congestion notification.  So the Congestion Baseline
   exposed to M should be 'I1' (the Load Regulator), not 'I2'.
   Therefore I1 should reset any arriving CE markings.  In this case,
   'I1' knows the tunnel to 'E1' is unrelated to its load regulation
   function.  So the load regulation function within 'I1' should be
   placed at [1 - Before] tunnel encapsulation within 'I1' (using the
   terminology of Figure 6).  Then the Congestion Baseline all across
   the networks from 'I1' to 'E2' in both inner and outer headers will
   be 'I1'.

   The following further examples illustrate how this answer might be

   o  We argued in Appendix A that resetting CE on encapsulation could
      harm PCN excess rate marking, which marks excess traffic for
      removal in subsequent round trips.  This marking relies on not
      marking packets if another node upstream has already marked them
      for removal.  If there were a tunnel ingress between the two which
      reset CE markings, it would confuse the downstream node into
      marking far too much traffic for removal.  So why do we say that
      'I1' should reset CE, while a tunnel ingress shouldn't?  The
      answer is that it is the Load Regulator function at 'I1' that is
      resetting CE, not the tunnel encapsulator.  The Load Regulator
      needs to set itself as the Congestion Baseline, so the feedback it
      gets will only be about congestion on links it can relieve itself
      (by regulating the load into them).  When it resets CE markings,
      it knows that something else upstream will have dealt with the
      congestion notifications it removes, given it is part of an end-
      to-end admission control signalling loop.  It therefore knows that
      previous hops will be covered by other Load Regulators.
      Meanwhile, the tunnel ingresses at both 'I1' and 'I2' should
      follow the new rule for any tunnel ingress and copy congestion
      marking into the outer tunnel header.  The ingress at 'I1' will
      happen to copy headers that have already been reset just
      beforehand.  But it doesn't need to know that.

   o  [Shayman] suggested feedback of ECN accumulated across an MPLS
      domain could cause the ingress to trigger re-routing to mitigate
      congestion.  This case is more like the simple scenario of
      Figure 2, with a feedback loop across the MPLS domain ('E' back to
      'I').  I is a Load Regulator because re-routing around congestion
      is a load regulation function.  But in this case 'I' should only
      reset itself as the Congestion Baseline in outer headers, as it is

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      not handling congestion outside its domain, so it must preserve
      the end-to-end congestion feedback loop for something else to
      handle (probably the data source).  Therefore the Load Regulator
      within 'I' should be placed at [2 - Outer] to reset CE markings
      just after the tunnel ingress has copied them from arriving
      headers.  Again, the tunnel encapsulation function at 'I' simply
      copies incoming headers, unaware that the load regulator will
      subsequently reset its outer headers.

   o  The PWE3 working group of the IETF is considering the problem of
      how and whether an aggregate edge-to-edge pseudo-wire emulation
      should respond to congestion [I-D.ietf-pwe3-congestion-frmwk].
      Although the study is still at the requirements stage, some
      (controversial) solution proposals include in-path load regulation
      at the ingress to the tunnel that could lead to tunnel
      arrangements with similar complexity to that of Figure 7.

   These are not contrived scenarios--they could be a lot worse.  For
   instance, a host may create a tunnel for IPsec which is placed inside
   a tunnel for Mobile IP over a remote part of its path.  And around
   this all we may have MPLS labels being pushed and popped as packets
   pass across different core networks.  Similarly, it is possible that
   subnets could be built from link technology (e.g. future Ethernet
   switches) so that link headers being added and removed could involve
   congestion notification in future Ethernet link headers with all the
   same issues as with IP in IP tunnels.

   One reason we introduced the concept of a Load Regulator was to allow
   for in-path load regulation.  In the traditional Internet
   architecture one tends to think of a host and a Load Regulator as
   synonymous, but when considering tunnelling, even the definition of a
   host is too fuzzy, whereas a Load Regulator is a clearly defined
   function.  Similarly, the concept of innermost header is too fuzzy to
   be able to (wrongly) say that the source address of the innermost
   header should be the Congestion Baseline.  Which is the innermost
   header when multiple encapsulations may be in use?  Where do we stop?
   If we say the original source in the above IPsec-Mobile IP case is
   the host, how do we know it isn't tunnelling an encrypted packet
   stream on behalf of another host in a p2p network?

   We have become used to thinking that only hosts regulate load.  The
   end to end design principle advises that this is a good idea
   [RFC3426], but it also advises that it is solely a guiding principle
   intended to make the designer think very carefully before breaking
   it.  We do have proposals where load regulation functions sit within
   a network path for good, if sometimes controversial, reasons, e.g.
   PCN edge admission control gateways [I-D.ietf-pcn-architecture] or
   traffic engineering functions at domain borders to re-route around

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   congestion [Shayman].  Whether or not we want in-path load
   regulation, we have to work round the fact that it will not go away.

Author's Address

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

   Phone: +44 1473 645196

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