Congestion and Pre Congestion                               T. Moncaster
Internet-Draft                                                B. Briscoe
Intended status: Standards Track                                      BT
Expires: February 21, March 8, 2010                                          M. Menth
                                                 University of Wuerzburg
                                                         August 20,
                                                       September 4, 2009

     Baseline Encoding and Transport of Pre-Congestion Information
                  draft-ietf-pcn-baseline-encoding-05
                  draft-ietf-pcn-baseline-encoding-06

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Abstract

   The objective of Pre-Congestion Notification the pre-congestion notification (PCN) architecture
   is to protect the
   quality of service (QoS) QoS of inelastic flows within a Diffserv domain.
   The overall
   It achieves this by marking packets belonging to PCN-flows when the
   rate of the PCN-traffic is metered on every link in the
   PCN-domain, and PCN-packets are appropriately marked when traffic exceeds certain configured rates are exceeded.  The level of marking allows thresholds on links in the
   boundary nodes to make decisions about whether to admit or block a
   new flow request, and (in abnormal circumstances) whether
   domain.  These marks can then be evaluated to
   terminate some of the existing flows, thereby protecting determine how close the QoS of
   previously admitted flows.
   domain is to being congested.  This document specifies how such marks
   are to be encoded into the IP header by re-using redefining the Explicit Congestion
   Notification (ECN) codepoints within this controlled
   domain. such domains.  The baseline
   encoding described here provides for only two PCN encoding states, Not-marked 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  . . . . . . . . . . . . . . . . . . . .  5  6
   3.  Terminology and Abbreviations  . . . . . . . . . . . . . . . .  6
     3.1.  List of Abbreviations  . . . . . . . . . .  6 . . . . . . . .  7
   4.  Encoding two PCN States in IP  . . . . . . . . . . . . . . . .  6  7
     4.1.  Marking Packets  . . . . . . . . . . . . . . . . . . . . .  8
     4.2.  Valid and Invalid Codepoint Transitions  . . . . . . . . .  7
     4.2.  8
     4.3.  Rationale for Encoding . . . . . . . . . . . . . . . . . .  8
     4.3.  9
     4.4.  PCN-Compatible Diffserv Codepoints . . . . . . . . . . . .  8
       4.3.1. 10
       4.4.1.  Co-existence of PCN and not-PCN traffic  . . . . . . .  9 10
   5.  Rules for Experimental Encoding Schemes  . . . . . . . . . . .  9 11
   6.  Backwards Compatibility  . . . . . . . . . . . . . . . . . . .  9 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10 12
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10 12
   9.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 10 12
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 12
   11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 11 13
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 13
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 11 13
     12.2. Informative References . . . . . . . . . . . . . . . . . . 11 13
   Appendix A.  PCN Deployment Considerations (Informational) . . . . 12 14
     A.1.  Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 12 14
     A.2.  Rationale for Using ECT(0) for Not-marked  . . . . . . . . 13 15

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, 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 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. 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 -04 -05 to -06:

      Extensive changes to -05:

      Clarified throughout that the PCN WG is not requesting a specific
      DSCP for PCN.  Rather we are recommending a set text following IETF Last Call and Gen-ART
      review comments.

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

      Added list of DSCPs that
      might be 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 Eardey.

   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 4.3.1 4.4.1 to clarify why we need the not-PCN
         codepoint.

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

      Also addressed a number of WGLC nits.

   From -02 to -03:

      Extensive changes to address comments made by Gorry Fairhurst
      including:

      *  Abstract re-written.

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

      *  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
      [I-D.ietf-tsvwg-ecn-tunnel]

      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.

      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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Terminology and Abbreviations

   The following terms are used defined in this document:

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

   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
      PCN-packets.

   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

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 4.3) 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 implementiors 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 that is not-PCN but which shares the same Diffserv
      codepoint as PCN-enabled traffic MUST have the its ECN field of its
      outermost IP header equal set to
      00.

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
   follows:

   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-
      marked.

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
   codepoint.

   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 (which MAY
   PM.  PCN-nodes that only implement the baseline encoding MUST be allowed by some future 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 extensions). 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*   Valid   |
   +--------------+-------------+-----------+-----------+-----------+
   |       PM(11) |  Not valid  | Not valid | Not valid |   Valid   |
   +--------------+-------------+-----------+-----------+-----------+
     * This SHOULD MAY cause an alarm to be raised at a higher management layer. The
       packet MUST be treated as if it carried the NM codepoint.
       See paragraph above for an explanation of this transition.

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

   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].

4.2.  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 4.3 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.

4.3.

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
   marking behaviour (e.g. enabling PCN disables replaces the default ECN marking
   behaviour introduced in [RFC3168]).  All traffic [RFC3168]) with the PCN metering and marking
   behaviours are discussed described in [I-D.ietf-pcn-marking-behaviour]. [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
   Appendix A.1.

4.3.1.

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 within 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 (though
      this does not exclude 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 from carrying in the same meaning). 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.

   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
      defined.

   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.

   o  Any experimental scheme MUST NOT update the meaning of the 00 and
      11 codepoints defined above.

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.  It
   should be noted that

   On its own, this baseline encoding effectively disables end-
   to-end cannot support both ECN unless mechanisms are put in place marking
   end to tunnel such traffic
   across the end and PCN marking within a PCN-domain.  Standard IP-in-IP or IPsec tunnels will
   always copy the CE codepoint from the outer header into the inner
   header in decapsulation (unless the inner packet  It is not-ECT).  If an
   operator wishes possible to allow do
   this by carrying e2e ECN to exist end-to-end they must ensure
   there are no tunnel end-points across a PCN domain within the PCN-domain to prevent any
   risk inner header
   of PCN-markings being exposed to endpoints. 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].

7.  IANA Considerations

   This document makes no direct request to IANA.

8.  Security Considerations

   PCN-marking only carries a meaning within the confines of a PCN-
   domain.  Packets wishing to be treated as belonging to a PCN-flow
   must carry a PCN-compatible DSCP and a PCN-Enabled ECN codepoint.  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 PCN-boundary-node behaviour.

   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
   <pcn@ietf.org>, and/or to the authors.

12.  References

12.1.  Normative References

   [I-D.ietf-pcn-marking-behaviour]  Eardley, P., "Metering and marking
                                     behaviour of PCN-nodes",
                                     draft-ietf-pcn-marking-behaviour-05
                                     (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",
                                     draft-ietf-pcn-3-state-encoding-00
                                     (work in progress), April 2009.

   [I-D.ietf-tsvwg-ecn-tunnel]       Briscoe, B., "Tunnelling of
                                     Explicit Congestion Notification",
                                     draft-ietf-tsvwg-ecn-tunnel-03
                                     (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.,
                                     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 essentially 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 uniformly high link rates, sufficient aggregation, the appropriate DSCPs would currently be
   those for the Real Time Traffic
   Class Treatment Aggregate [RFC5127].  To be clear the  The PCN
   Working Group recommends suggests using admission control for the following
   service classes: classes (defined in [RFC4594]):

   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 region PCN-domain containing highly 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
   [RFC5127].

   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
   choice.

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).

   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.

Authors' Addresses

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

   Phone: +44 1473 648734
   EMail: toby.moncaster@bt.com

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

   Phone: +44 1473 645196
   EMail: bob.briscoe@bt.com
   Michael Menth
   University of Wuerzburg
   room B206, Institute of Computer Science
   Am Hubland
   Wuerzburg  D-97074
   Germany

   Phone: +49 931 888 6644
   EMail: menth@informatik.uni-wuerzburg.de