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Baseline Encoding and Transport of Pre-Congestion Information

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 5696.
Authors Bob Briscoe , T Moncaster , Michael Menth
Last updated 2015-10-14 (Latest revision 2009-09-25)
Replaces draft-moncaster-pcn-baseline-encoding
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
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IESG IESG state Became RFC 5696 (Proposed Standard)
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Consensus boilerplate Unknown
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Responsible AD Lars Eggert
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Congestion and Pre Congestion                               T. Moncaster
Internet-Draft                                                B. Briscoe
Intended status: Standards Track                                      BT
Expires: March 29, 2010                                         M. Menth
                                                 University of Wuerzburg
                                                      September 25, 2009

     Baseline Encoding and Transport of Pre-Congestion Information

Status of This Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  This document may contain material
   from IETF Documents or IETF Contributions published or made publicly
   available before November 10, 2008.  The person(s) controlling the
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Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (
   Please review these documents carefully, as they describe your rights
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   The objective of the pre-congestion notification (PCN) architecture
   is to protect the QoS of inelastic flows within a Diffserv domain.
   It achieves this by marking packets belonging to PCN-flows when the
   rate of traffic exceeds certain configured thresholds on links in the
   domain.  These marks can then be evaluated to determine how close the
   domain is to being congested.  This document specifies how such marks
   are encoded into the IP header by redefining the Explicit Congestion
   Notification (ECN) codepoints within such domains.  The baseline
   encoding described here provides only two PCN encoding states: not-
   marked and PCN-marked.  Future extensions to this encoding may be
   needed in order to provide more than one level of marking severity.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  6
   3.  Terminology and Abbreviations  . . . . . . . . . . . . . . . .  6
     3.1.  List of Abbreviations  . . . . . . . . . . . . . . . . . .  7
   4.  Encoding two PCN States in IP  . . . . . . . . . . . . . . . .  8
     4.1.  Marking Packets  . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Valid and Invalid Codepoint Transitions  . . . . . . . . .  9
     4.3.  Rationale for Encoding . . . . . . . . . . . . . . . . . . 10
     4.4.  PCN-Compatible Diffserv Codepoints . . . . . . . . . . . . 10
       4.4.1.  Co-existence of PCN and not-PCN traffic  . . . . . . . 11
   5.  Rules for Experimental Encoding Schemes  . . . . . . . . . . . 11
   6.  Backwards Compatibility  . . . . . . . . . . . . . . . . . . . 12
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   9.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 13
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 13
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     12.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  PCN Deployment Considerations (Informational) . . . . 15
     A.1.  Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 15
     A.2.  Rationale for Using ECT(0) for Not-marked  . . . . . . . . 16
   Appendix B.  Co-existence of PCN and ECN (Informational) . . . . . 17

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1.  Introduction

   The objective of the Pre-Congestion Notification (PCN) Architecture
   [RFC5559] is to protect the quality of service (QoS) of inelastic
   flows within a Diffserv domain, in a simple, scalable and robust
   fashion.  The overall rate of the PCN-traffic is metered on every
   link in the PCN-domain, and PCN-packets are appropriately marked when
   certain configured rates are exceeded.  These configured rates are
   below the rate of the link thus providing notification before any
   congestion occurs (hence "pre-congestion notification").  The level
   of marking allows the boundary nodes to make decisions about whether
   to admit or block a new flow request, and (in abnormal circumstances)
   whether to terminate some of the existing flows, thereby protecting
   the QoS of previously admitted flows.

   This document specifies how these PCN-marks are encoded into the IP
   header by re-using the bits of the Explicit Congestion Notification
   (ECN) field [RFC3168].  It also describes how packets are identified
   as belonging to a PCN-flow.  Some deployment models require two PCN
   encoding states, others require more.  The baseline encoding
   described here only provides for two PCN encoding states.  However
   the encoding can be easily extended to provide more states.  Rules
   for such extensions are given in Section 5.

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

   From -06 to -07:

      Changes made following IESG review comments.

      Changed Section 4 and added Appendix B to clarify the correct
      behaviour when handling packets that already have values other
      than not-ECT in their ECN field.

      Added paragraph to end of Section 6 clarifying that a PCN-domain
      has "hard" edges.

   From -05 to -06:

      Extensive changes to the text following IETF Last Call and Gen-ART
      review comments.

      Abstract updated following mailing list discussions after Gen-ART
      review by Spencer Dawkins.

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      Added list of abbreviations

      New [section 4.1] added to explain the required action when a node
      indicates the need to mark a packet.

      Clarified text and Table 2 in Section 4.2.

      Improved explanation of rules for experimental encoding schemes in
      Section 5.  Removed any ambiguity about meaning of PCN-marked in
      such a context.  Added requirements for experimental schemes to
      define which meter causes which mark.

      Clarified text in Section 6 relating to support for e2e ECN.

      Added text in Section 8 relating to injection of PCN-marks into
      the PCN-domain.

      Changed text of Appendix A.1 to reflect comments from Spencer
      Dawkins and Philip Eardley.

   From -04 to -05:

      Clarified throughout that the PCN WG is not requesting a specific
      DSCP for PCN.  Rather we are recommending a set of DSCPs that
      might be suitable.  Appendix A.1 has been re-written to reflect
      this.  References to maintaining a list of PCN-compatible DSCPs
      have also been removed.

      Last sentence of Section 6 altered.

      Several spelling corrections.

      References updated throughout.

   From -03 to -04:

      Major WGLC comments addressed:

      *  Added Section 4.4.1 to clarify why we need the not-PCN

      *  Stated that the PCN WG will maintain a list of PCN-compatible
         DSCPs.  This should help avoid inter-operability issues.

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      Also addressed a number of WGLC nits.

   From -02 to -03:

      Extensive changes to address comments made by Gorry Fairhurst

      *  Abstract re-written.

      *  Clarified throughout that this re-uses the ECN bits in the IP

      *  Re-arranged order of terminology section for clarity.

      *  Table 2 replaced with new table and text.

      *  Security considerations re-written.

      *  Appendixes re-written to improve clarity.

      *  Numerous minor nits and language changes throughout.

      Extensive other minor changes throughout.

   From -01 to -02:

      Removed Appendix A and replaced with reference to

      Moved Appendix B into main body of text.

      Changed Appendix C to give deployment advice.

      Minor changes throughout including checking consistency of
      capitalisation of defined terms.

      Clarified that LU was deliberately excluded from encoding.

   From -00 to -01:

      Added section on restrictions for extension encoding schemes.

      Included table in Appendix showing encoding transitions at
      different PCN nodes.

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      Checked for consistency of terminology.

      Minor language changes for clarity.

   Changes from previous filename

      Filename changed from draft-moncaster-pcn-baseline-encoding.

      Terminology changed for clarity (PCN-compatible DSCP and PCN-
      enabled packet).

      Minor changes throughout.

      Modified meaning of ECT(1) state to EXP.

      Moved text relevant to behaviour of nodes into appendix for later
      transfer to new document on edge behaviours.

   From draft-moncaster -01 to -02:

      Minor changes throughout including tightening up language to
      remain consistent with the PCN Architecture terminology.

   From draft-moncaster -00 to -01:

      Change of title from "Encoding and Transport of (Pre-)Congestion
      Information from within a Diffserv Domain to the Egress"

      Extensive changes to Introduction and abstract.

      Added a section on the implications of re-using a DSCP.

      Added appendix listing possible operator scenarios for using this
      baseline encoding.

      Minor changes throughout.

2.  Requirements notation

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

3.  Terminology and Abbreviations

   The following terms are defined in this document:

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   o  PCN-compatible Diffserv codepoint - a Diffserv codepoint for which
      the ECN field is used to carry PCN markings rather than [RFC3168]

   o  PCN-marked - codepoint indicating packets that have been marked at
      a PCN-interior-node using some PCN marking behaviour
      [I-D.ietf-pcn-marking-behaviour].  Abbreviated to PM.

   o  Not-marked - codepoint indicating packets that are PCN-capable,
      but are not PCN-marked.  Abbreviated to NM.

   o  PCN-enabled codepoints - collective term for all NM and PM
      codepoints.  By definition, packets carrying such codepoints are

   o  not-PCN - packets that are not PCN-enabled.

   In addition, the document uses the terminology defined in [RFC5559].

3.1.  List of Abbreviations

   The following abbreviations are used in this document:

   o  AF = Assured Forwarding [RFC2597]

   o  CE = Congestion Experienced [RFC3168]

   o  CS = Class Selector [RFC2474]

   o  DSCP = Diffserv codepoint

   o  ECN = Explicit Congestion Notification [RFC3168]

   o  ECT = ECN Capable Transport [RFC3168]

   o  EF = Expedited Forwarding [RFC3246]

   o  EXP = Experimental

   o  NM = Not-marked

   o  PCN = Pre-Congestion Notification

   o  PM = PCN-marked

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4.  Encoding two PCN States in IP

   The PCN encoding states are defined using a combination of the DSCP
   and ECN fields within the IP header.  The baseline PCN encoding
   closely follows the semantics of ECN [RFC3168].  It allows the
   encoding of two PCN states: Not-marked and PCN-marked.  It also
   allows for traffic that is not PCN-capable to be marked as such (not-
   PCN).  Given the scarcity of codepoints within the IP header the
   baseline encoding leaves one codepoint free for experimental use.
   The following table defines how to encode these states in IP:

   | ECN codepoint |   Not-ECT   | ECT(0) (10) | ECT(1) (01) | CE (11) |
   |               |     (00)    |             |             |         |
   |     DSCP n    |   not-PCN   |      NM     |     EXP     |    PM   |

   Where DSCP n is a PCN-compatible Diffserv codepoint (see Section 4.4)
    and EXP means available for Experimental use.  N.B. we deliberately
   reserve this codepoint for experimental use only (and not local use)
                  to prevent future compatibility issues.

                        Table 1: Encoding PCN in IP

   The following rules apply to all PCN traffic:

   o  PCN-traffic MUST be marked with a PCN-compatible Diffserv
      Codepoint.  To conserve DSCPs, Diffserv Codepoints SHOULD be
      chosen that are already defined for use with admission controlled
      traffic.  Appendix A.1 gives guidance to implementors on suitable
      DSCPs.  Guidelines for mixing traffic-types within a PCN-domain
      are given in [I-D.ietf-pcn-marking-behaviour].

   o  Any packet arriving at the PCN-ingress that shares a PCN-
      compatible DSCP and is not-PCN MUST be marked as not-PCN within
      the PCN-domain.

   o  If a packet arrives at the PCN-ingress with its ECN field already
      set to a value other than not-ECT, then appropriate action MUST be
      taken to meet the requirements of [RFC3168].  The simplest
      appropriate action is to just drop such packets.  However this is
      a drastic action that an operator may feel is undesirable.
      Appendix B provides more information and summarises other
      alternative actions that might be taken.

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4.1.  Marking Packets

   [I-D.ietf-pcn-marking-behaviour] states that any encoding scheme
   document must specify the required action to take if one of the
   marking algorithms indicates that a packet needs to be marked.  For
   the baseline encoding scheme the required action is simply as

   o  If a marking algorithm indicates the need to mark a PCN-packet
      then that packet MUST have its PCN codepoint set to 11, PCN-

4.2.  Valid and Invalid Codepoint Transitions

   A PCN-ingress-node MUST set the Not-marked (10) codepoint on any
   arriving packet that belongs to a PCN-flow.  It MUST set the not-PCN
   (00) codepoint on all other packets sharing a PCN-compatible Diffserv

   The only valid codepoint transitions within a PCN-interior-node are
   from NM to PM (which should occur if either meter indicates a need to
   PCN-mark a packet [I-D.ietf-pcn-marking-behaviour]) and from EXP to
   PM.  PCN-nodes that only implement the baseline encoding MUST be able
   to PCN mark packets that arrive with the EXP codepoint.  This should
   ease the design of experimental schemes that want to allow partial
   deployment of experimental nodes alongside nodes that only implement
   the baseline encoding.  The following table gives the full set of
   valid and invalid codepoint transitions.

                  |                  Codepoint Out                  |
   | Codepoint in | not-PCN(00) |   NM(10)  |  EXP(01)  |   PM(11)  |
   |  not-PCN(00) |    Valid    | Not valid | Not valid | Not valid |
   |       NM(10) |  Not valid  |   Valid   | Not valid |   Valid   |
   |     EXP(01)* |  Not valid  | Not valid |   Valid   |   Valid   |
   |       PM(11) |  Not valid  | Not valid | Not valid |   Valid   |
     * This MAY cause an alarm to be raised at a management layer.
       See paragraph above for an explanation of this transition.

    Table 2: Valid and Invalid Codepoint Transitions for PCN-packets
                        at PCN-interior-nodes

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   The codepoint transition constraints given here apply only to the
   baseline encoding scheme.  Constraints on codepoint transitions for
   future experimental schemes are discussed in Section 5.

   A PCN-egress-node SHOULD set the not-PCN (00) codepoint on all
   packets it forwards out of the PCN-domain.  The only exception to
   this is if the PCN-egress-node is certain that revealing other
   codepoints outside the PCN-domain won't contravene the guidance given
   in [RFC4774].  For instance if the PCN-ingress-node has explicitly
   informed the PCN-egress-node that this flow is ECN-capable then it
   might be safe to expose other codepoints.

4.3.  Rationale for Encoding

   The exact choice of encoding was dictated by the constraints imposed
   by existing IETF RFCs, in particular [RFC3168], [RFC4301] and
   [RFC4774].  One of the tightest constraints was the need for any PCN
   encoding to survive being tunnelled through either an IP in IP tunnel
   or an IPsec Tunnel.  [I-D.ietf-tsvwg-ecn-tunnel] explains this in
   more detail.  The main effect of this constraint is that any PCN
   marking has to carry the 11 codepoint in the ECN field since this is
   the only codepoint that is guaranteed to be copied down into the
   inner header upon decapsulation.  An additional constraint is the
   need to minimise the use of Diffserv codepoints because there is a
   limited supply of standards track codepoints remaining.  Section 4.4
   explains how we have minimised this still further by reusing pre-
   existing Diffserv codepoint(s) such that non-PCN traffic can still be
   distinguished from PCN traffic.

   There are a number of factors that were considered before choosing to
   set 10 as the NM state instead of 01.  These included similarity to
   ECN, presence of tunnels within the domain, leakage into and out of
   PCN-domain and incremental deployment (see Appendix A.2).

   The encoding scheme above seems to meet all these constraints and
   ends up looking very similar to ECN.  This is perhaps not surprising
   given the similarity in architectural intent between PCN and ECN.

4.4.  PCN-Compatible Diffserv Codepoints

   Equipment complying with the baseline PCN encoding MUST allow PCN to
   be enabled for certain Diffserv codepoints.  This document defines
   the term "PCN-compatible Diffserv codepoint" for such a DSCP.  To be
   clear, any packets with such a DSCP will be PCN enabled only if they
   are within a PCN-domain and have their ECN field set to indicate a
   codepoint other than not-PCN.

   Enabling PCN marking behaviour for a specific DSCP disables any other

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   marking behaviour (e.g. enabling PCN replaces the default ECN marking
   behaviour introduced in [RFC3168]) with the PCN metering and marking
   behaviours described in [I-D.ietf-pcn-marking-behaviour]).  This
   ensures compliance with the BCP guidance set out in [RFC4774].

   The PCN Working Group has chosen not to define a single DSCP for use
   with PCN for several reasons.  Firstly the PCN mechanism is
   applicable to a variety of different traffic classes.  Secondly
   standards track DSCPs are in increasingly short supply.  Thirdly PCN
   is not a scheduling behaviour - rather it should be seen as being
   essentially a marking behaviour similar to ECN but intended for
   inelastic traffic.  More details are given in the informational
   Appendix A.1.

4.4.1.  Co-existence of PCN and not-PCN traffic

   The scarcity of pool 1 DSCPs coupled with the fact that PCN is
   envisaged as a marking behaviour that could be applied to a number of
   different DSCPs makes it essential that we provide a not-PCN state.
   As stated above (and expanded in Appendix A.1) the aim is for PCN to
   re-use existing DSCPs.  Because PCN re-defines the meaning of the ECN
   field for such DSCPs it is important to allow an operator to still
   use the DSCP for traffic that isn't PCN-enabled.  This is achieved by
   providing a not-PCN state within the encoding scheme.  S3.5 of
   [RFC5559] discusses how competing-non-PCN-traffic should be handled.

5.  Rules for Experimental Encoding Schemes

   Any experimental encoding scheme MUST follow these rules to ensure
   backward compatibility with this baseline scheme:

   o  All Interior-nodes within a PCN-domain MUST interpret the 00
      codepoint in the ECN field as not-PCN and MUST NOT change it to
      another value.  Therefore an ingress node wishing to disable PCN
      marking for a packet with a PCN-compatible Diffserv Codepoint MUST
      set the ECN field to 00.

   o  The 11 codepoint in the ECN field MUST indicate that the packet
      has been PCN-marked as the result of one or both of the meters
      indicating a need to PCN-mark a packet
      [I-D.ietf-pcn-marking-behaviour].  The experimental scheme MUST
      define which meter(s) trigger this marking.

   o  The 01 Experimental codepoint in the ECN field MAY mean PCN-marked
      or it MAY carry some other meaning.  However any experimental
      scheme MUST define its meaning in the context of that experiment.

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   o  If both the 01 and 11 codepoints are being used to indicate PCN-
      Marked then the 11 codepoint MUST be taken to be the more severe
      marking and the choice of which meter sets which mark MUST be

   o  Once set, the 11 codepoint in the ECN field MUST NOT be changed to
      any other codepoint.

   o  Any experimental scheme MUST include details of all valid and
      invalid codepoint transitions at any PCN nodes.

6.  Backwards Compatibility

   BCP 124 [RFC4774] gives guidelines for specifying alternative
   semantics for the ECN field.  It sets out a number of factors to be
   taken into consideration.  It also suggests various techniques to
   allow the co-existence of default ECN and alternative ECN semantics.
   The baseline encoding specified in this document defines PCN-
   compatible Diffserv codepoints as no longer supporting the default
   ECN semantics.  As such this document is compatible with BCP 124.

   On its own, this baseline encoding cannot support both ECN marking
   end to end and PCN marking within a PCN-domain.  It is possible to do
   this by carrying e2e ECN across a PCN domain within the inner header
   of an IP in IP tunnel, or by using a richer encoding such as the
   proposed experimental scheme in [I-D.ietf-pcn-3-state-encoding].

   In any PCN deployment, traffic can only enter the PCN-Domain through
   PCN-ingress-nodes and leave through PCN-egress-nodes.  PCN-ingress-
   nodes ensure that any packets entering the PCN- domain have the ECN-
   field in their outermost IP header set to the appropriate PCN
   codepoint.  PCN-egress-nodes then guarantee that the ECN-field of any
   packet leaving the PCN-domain has the correct ECN semantics.  This
   prevents leakage of ECN marks into or out of the PCN-domain and thus
   reduces backward compatibility issues.

7.  IANA Considerations

   This document makes no request to IANA.

8.  Security Considerations

   PCN-marking only carries a meaning within the confines of a PCN-
   domain.  This encoding document is intended to stand independently of
   the architecture used to determine how specific packets are
   authorised to be PCN-marked, which will be described in separate
   documents on PCN-boundary-node behaviour.

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   This document assumes the PCN-domain to be entirely under the control
   of a single operator, or a set of operators who trust each other.
   However future extensions to PCN might include inter-domain versions
   where trust cannot be assumed between domains.  If such schemes are
   proposed they must ensure that they can operate securely despite the
   lack of trust.  However such considerations are beyond the scope of
   this document.

   One potential security concern is the injection of spurious PCN-marks
   into the PCN-domain.  However these can only enter the domain if a
   PCN-ingress-node is misconfigured.  The precise impact of any such
   misconfiguration will depend on which of the proposed PCN-boundary-
   node behaviour schemes is used, but in general spurious marks will
   lead to admitting fewer flows into the domain or potentially
   terminating too many flows.  In either case good management should be
   able to quickly spot the problem since the overall utilisation of the
   domain will rapidly fall.

9.  Conclusions

   This document defines the baseline PCN encoding utilising a
   combination of a PCN-enabled DSCP and the ECN field in the IP header.
   This baseline encoding allows the existence of two PCN encoding
   states, not-Marked and PCN-marked.  It also allows for the co-
   existence of competing traffic within the same DSCP so long as that
   traffic does not require ECN support within the PCN-domain.  The
   encoding scheme is conformant with [RFC4774].  The Working Group has
   chosen not to define a single DSCP for use with PCN.  The rationale
   for this decision along with advice relating to choice of suitable
   DSCPs can be found in Appendix A.1.

10.  Acknowledgements

   This document builds extensively on work done in the PCN working
   group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Anna
   Charny, Joe Babiarz and others.  Thanks to Ruediger Geib and Gorry
   Fairhurst for providing detailed comments on this document.

11.  Comments Solicited

   (To be removed by the RFC-Editor.)  Comments and questions are
   encouraged and very welcome.  They can be addressed to the IETF
   congestion and pre-congestion working group mailing list
   <>, and/or to the authors.

12.  References

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12.1.  Normative References

   [I-D.ietf-pcn-marking-behaviour]  Eardley, P., "Metering and marking
                                     behaviour of PCN-nodes",
                                     (work in progress), August 2009.

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

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

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

12.2.  Informative References

   [I-D.ietf-pcn-3-state-encoding]   Moncaster, T., Briscoe, B., and M.
                                     Menth, "A PCN encoding using 2
                                     DSCPs to provide 3 or more states",
                                     (work in progress), April 2009.

   [I-D.ietf-tsvwg-ecn-tunnel]       Briscoe, B., "Tunnelling of
                                     Explicit Congestion Notification",
                                     (work in progress), July 2009.

   [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.

   [RFC2597]                         Heinanen, J., Baker, F., Weiss, W.,
                                     and J. Wroclawski, "Assured
                                     Forwarding PHB Group", RFC 2597,
                                     June 1999.

   [RFC3246]                         Davie, B., Charny, A., Bennet, J.,

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                                     Benson, K., Le Boudec, J.,
                                     Courtney, W., Davari, S., Firoiu,
                                     V., and D. Stiliadis, "An Expedited
                                     Forwarding PHB (Per-Hop Behavior)",
                                     RFC 3246, March 2002.

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

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

   [RFC4594]                         Babiarz, J., Chan, K., and F.
                                     Baker, "Configuration Guidelines
                                     for DiffServ Service Classes",
                                     RFC 4594, August 2006.

   [RFC5127]                         Chan, K., Babiarz, J., and F.
                                     Baker, "Aggregation of DiffServ
                                     Service Classes", RFC 5127,
                                     February 2008.

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

Appendix A.  PCN Deployment Considerations (Informational)

A.1.  Choice of Suitable DSCPs

   The PCN Working Group chose not to define a single DSCP for use with
   PCN for several reasons.  Firstly the PCN mechanism is applicable to
   a variety of different traffic classes.  Secondly standards track
   DSCPs are in increasingly short supply.  Thirdly PCN is not a
   scheduling behaviour - rather it should be seen as being a marking
   behaviour similar to ECN but intended for inelastic traffic.  The
   choice of which DSCP is most suitable for a given PCN-domain is
   dependent on the nature of the traffic entering that domain and the
   link rates of all the links making up that domain.  In PCN-domains
   with sufficient aggregation, the appropriate DSCPs would currently be
   those for the Real Time Treatment Aggregate [RFC5127].  The PCN
   Working Group suggests using admission control for the following
   service classes (defined in [RFC4594]):

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   o  Telephony (EF)

   o  Real-time interactive (CS4)

   o  Broadcast Video (CS3)

   o  Multimedia Conferencing (AF4)

   CS5 is excluded from this list since PCN is not expected to be
   applied to signalling traffic.

   PCN marking is intended to provide a scalable admission control
   mechanism for traffic with a high degree of statistical multiplexing.
   PCN marking would therefore be appropriate to apply to traffic in the
   above classes, but only within a PCN-domain containing sufficiently
   aggregated traffic.  In such cases, the above service classes may
   well all be subject to a single forwarding treatment (treatment
   aggregate [RFC5127]).  However, this does not imply all such IP
   traffic would necessarily be identified by one DSCP - each service
   class might keep a distinct DSCP within the highly aggregated region

   Additional service classes may be defined for which admission control
   is appropriate, whether through some future standards action or
   through local use by certain operators, e.g. the Multimedia Streaming
   service class (AF3).  This document does not preclude the use of PCN
   in more cases than those listed above.

   NOTE: The above discussion is informative not normative, as operators
   are ultimately free to decide whether to use admission control for
   certain service classes and whether to use PCN as their mechanism of

A.2.  Rationale for Using ECT(0) for Not-marked

   The choice of which ECT codepoint to use for the Not-marked state was
   based on the following considerations:

   o  [RFC3168] full functionality tunnel within the PCN-domain: Either
      ECT is safe.

   o  Leakage of traffic into PCN-domain: because of the lack of take-up
      of the ECN nonce [RFC3540], leakage of ECT(1) is less likely to
      occur so might be considered safer.

   o  Leakage of traffic out of PCN-domain: Either ECT is equally unsafe
      (since this would incorrectly indicate the traffic was ECN-capable
      outside the controlled PCN-domain).

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   o  Incremental deployment: Either codepoint is suitable providing
      that the codepoints are used consistently.

   o  Conceptual consistency with other schemes: ECT(0) is conceptually
      consistent with [RFC3168].

   Overall this seemed to suggest ECT(0) was most appropriate to use.

Appendix B.  Co-existence of PCN and ECN (Informational)

   This baseline encoding scheme redefines the ECN codepoints within the
   PCN-domain.  As packets with a PCN-compatible DSCP leave the PCN-
   domain, their ECN field is reset to not-ECT (00).  This is a problem
   for the operator if packets with a PCN-compatible DSCP arrive at the
   PCN-domain with any ECN codepoint other than not-ECN.  If the ECN-
   codepoint is ECT(0) (10) or ECT(1) (01), resetting the ECN field to
   00 effectively turns off end-to-end ECN.  This is undesirable as it
   removes the benefits of ECN but [RFC3168] states it is no worse than
   dropping the packet.  However, if a packet was marked with CE (11),
   resetting the ECN field to 00 at the PCN egress node violates the
   rule that CE-marks must never be lost except as a result of packet
   drop [RFC3168].

   A number of options exist to overcome this issue.  The most
   appropriate option will depend on the circumstances and has to be a
   decision for the operator.  The definition of the action is beyond
   the scope of this document but we briefly explain the four broad
   categories of solution below: tunnelling the packets, using an
   extended encoding scheme, signalling to the end-systems to stop using
   ECN or re-marking packets to a different DSCP.

   o  Tunnelling the packets across the PCN-domain (for instance in an
      IP-in-IP tunnel from the PCN-ingress-node to the PCN-egress-node)
      preserves the original ECN marking on the inner header.

   o  An extended encoding scheme can be designed that preserves the
      original ECN codepoints.  For instance if the PCN-egress-node can
      determine from the PCN codepoint what the original ECN codepoint
      was then it can reset the packet to that codepoint.
      [I-D.ietf-pcn-3-state-encoding] partially achieves this but is
      unable to recover ECN markings if the packet is PCN-marked in the

   o  Explicit signalling to the end systems can indicate to the source
      that ECN cannot be used on this path (because it does not support
      ECN & PCN at the same time).  Dropping the packet can be thought
      of as a form of silent signal to the source as it will see any
      ECT-marked packets it sends being dropped.

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   o  Packets that are not-PCN but which share a PCN-compatible DSCP can
      be re-marked to a different local-use DSCP at the PCN-ingress-node
      with the original DSCP restored at the PCN-egress.  This preserves
      the ECN codepoint on the packets, but it relies on there being
      spare local-use DSCPs within the PCN-domain.

Authors' Addresses

   Toby Moncaster
   B54/70, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE

   Phone: +44 1473 648734

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

   Phone: +44 1473 645196

   Michael Menth
   University of Wuerzburg
   room B206, Institute of Computer Science
   Am Hubland
   Wuerzburg  D-97074

   Phone: +49 931 888 6644

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