Congestion and Pre Congestion                               T. Moncaster
Internet-Draft                                                        BT
Intended status: Standards Track                              B. Briscoe
Expires: August 14, 2009                                        BT & UCL
                                                                M. Menth
                                                 University of Wuerzburg
                                                       February 10, 2009

     Baseline Encoding and Transport of Pre-Congestion Information

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   Pre-congestion notification (PCN) provides information to support
   admission control and flow termination in order to protect the
   Quality of Service of inelastic flows.  It does this by marking
   packets when traffic load on a link is approaching or has exceeded a
   threshold below the physical link rate.  This document specifies how
   such marks are to be encoded into the IP header.  The baseline
   encoding described here provides for only two PCN encoding states.
   It is designed to be easily extended to provide more encoding states
   but such schemes will be described in other documents.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Encoding two PCN States in IP  . . . . . . . . . . . . . . . .  5
     4.1.  Rationale for Encoding . . . . . . . . . . . . . . . . . .  6
     4.2.  PCN-Compatible DiffServ Codepoints . . . . . . . . . . . .  7
   5.  Rules for Experimental Encoding Schemes  . . . . . . . . . . .  7
   6.  Backwards Compatibility  . . . . . . . . . . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   9.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . .  9
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     12.1. Normative References . . . . . . . . . . . . . . . . . . .  9
     12.2. Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  PCN Deployment Considerations . . . . . . . . . . . . 10
     A.1.  Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 10
     A.2.  Rationale for Using ECT(0) for Not Marked  . . . . . . . . 10

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1.  Introduction

   Pre-congestion notification (PCN) provides information to support
   admission control and flow termination in order to protect the
   quality of service (QoS) of inelastic flows.  This is achieved by
   marking packets according to the level of pre-congestion at nodes
   within a PCN-domain.  These markings are evaluated by the egress
   nodes of the PCN-domain. [pcn-arch] describes how PCN packet markings
   can be used to assure the QoS of inelastic flows within a single
   DiffServ domain.

   This document specifies how these PCN marks are encoded into the IP
   header.  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.  An extension of the baseline
   encoding described in [PCN-3-enc-state] provides for three PCN
   encoding states.  Other extensions have also been suggested all of
   which can build on the baseline encoding.  In order to ensure
   backward compatibility any alternative encoding schemes that claim
   compliance with PCN standards MUST extend this baseline scheme.

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

   From -01 to -02:

      Removed Appendix A and replaced with reference to [ecn-tunneling]

      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.

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      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

   The following terms are used in this document:

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

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   o  PCN-marked - codepoint indicating packets that have been marked at
      a PCN-interior-node using some PCN marking behaviour
      [pcn-marking-behaviour].  Also PM.

   o  Not-marked - codepoint indicating packets that are PCN-capable but
      are not PCN-marked.  Also NM.

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

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

   In addition the document uses the terminology defined in [pcn-arch].

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 4.2)
    and EXP means available for Experimental use.  N.B. we deliberately
   reserve this codepoint for experimental use only (and not local use)
           to prevent any possible future compatability 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, such as the Voice-Admit codepoint defined in

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      [voice-admit].  Guidelines for mixing traffic-types within a PCN-
      domain are given in [pcn-marking-behaviour].

   o  Any packet that is not PCN-enabled (Not-PCN) but which shares the
      same DiffServ codepoint as PCN-enabled traffic MUST have the ECN
      field equal to 00.

   The following table sets out the valid and invalid codepoint
   transitions at PCN-nodes for this baseline encoding.  Extension
   encodings may have different rules regarding the validity of the
   transitions.  Note that this table assumes there is a functional
   separation between a PCN-boundary-node and a PCN-interior-node such
   that PCN-boundary-nodes do not perform packet metering or marking
   functions.  PCN-nodes MUST follow the encoding transition rules set
   out in this table (e.g. they MUST NOT set invalid codepoints on
   packets they forward).  This table only applies to PCN-packets.

   | PCN node  |  Codepoint  | Valid codepoint | Invalid codepoint out |
   |   type    |      in     |        out      |                       |
   |  ingress  |     Any     | NM (or Not-PCN) |          PM           |
   | interior  |     NM      |     NM or PM    |     Not-PCN or EXP    |
   | interior  |     EXP +   |     EXP or PM   |        Not-PCN        |
   | interior  |   Not-PCN   |      Not-PCN    |  Any other codepoint  |
   | interior  |     PM      |        PM       |  Any other codepoint  |
   |  egress   |     Any     |        00       | Any other codepoint * |
    + This SHOULD cause an alarm to be raised at a higher layer. The
      packet MUST be treated as if it were NM.
    * Except where the egress node knows that other marks may be safely
      exposed outside the PCN-domain (e.g. [PCN-3-enc-state]).

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

   If a pcn-interior-node compliant with this baseline encoding receives

4.1.  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. [ecn-tunneling] 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.  If the packet is being

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   tunneled then only the 11 codepoint gets copied into the inner header
   upon decapsulation.  An additional constraint is the need to minimise
   the use of DiffServ codepoints as there is a limited supply of
   standards track codepoints remaining.  Section 4.2 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 deciding to set 10 as the NM state.  These included similarity
   to ECN, presence of tunnels within the domain, leakage into and out
   of PCN-domain and incremental deployment.

   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.

4.2.  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.
   Enabling PCN for a DSCP switches on PCN marking behaviour for packets
   with that DSCP, but only if those packets also have their ECN field
   set to indicate a codepoint other than Not-PCN.

   Enabling PCN marking behaviour disables any other marking behaviour
   (e.g. enabling PCN disables the default ECN marking behaviour
   introduced in [RFC3168]).  All traffic scheduling and conditioning
   behaviours are discussed in [pcn-marking-behaviour].  This ensures
   compliance with the BCP guidance set out in [RFC4774].

5.  Rules for Experimental Encoding Schemes

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

   o  The 00 codepoint in the ECN field MUST mean Not-PCN.

   o  The 11 codepoint in the ECN field MUST mean PCN-marked (though
      this doesn't exclude other codepoints from carrying the same

   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.

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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 this baseline encoding effectively disables end-
   to-end ECN except where mechanisms are put in place to tunnel such
   traffic across the PCN-domain.

7.  IANA Considerations

   This document makes no request to IANA.

8.  Security Considerations

   Packets claim entitlement to be PCN marked by carrying a PCN-
   Compatible DSCP and a PCN-Enabled ECN codepoint.  This encoding
   document is intended to stand independently of the architecture used
   to determine whether specific packets are authorised to be PCN
   marked, which will be described in a future separate document on PCN
   edge-node behaviour.

   The PCN working group has initially been chartered to only consider a
   PCN-domain to be entirely under the control of one operator, or a set
   of operators who trust each other [PCN-charter].  However there is a
   requirement to keep inter-domain scenarios in mind when defining the
   PCN encoding.  One way to extend to multiple domains would be to
   concatenate PCN-domains and use PCN-boundary-nodes back to back at
   borders.  Then any one domain's security against its neighbours would
   be described as part of the proposed edge-node behaviour document.

   One proposal on the table allows one to extend PCN across multiple
   domains without PCN-boundary-nodes back-to-back at borders [re-PCN].
   It is believed that the encoding described here would be compatible
   with the security framework described there.

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 doesn't require end-to-end ECN support.  The encoding scheme

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   is conformant with [RFC4774].

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 for
   providing detailed comments on this document.

11.  Comments Solicited

   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

12.1.  Normative References

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

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

12.2.  Informative References

   [PCN-3-enc-state]        Moncaster, T., Briscoe, B., and M. Menth, "A
                            three state extended PCN encoding scheme",
                            (work in progress), June 2008.

   [PCN-charter]            IETF, "IETF Charter for Congestion and Pre-
                            Congestion Notification Working Group".

   [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.

   [RFC5127]                Chan, K., Babiarz, J., and F. Baker,

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                            "Aggregation of DiffServ Service Classes",
                            RFC 5127, February 2008.

   [ecn-tunneling]          Briscoe, B., "Layered Encapsulation of
                            Congestion Notification",
                            draft-ietf-tsvwg-ecn-tunnel-01 (work in
                            progress), October 2008.

   [pcn-arch]               Eardley, P., "Pre-Congestion Notification
                            (PCN) Architecture",
                            draft-ietf-pcn-architecture-07 (work in
                            progress), September 2008.

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

   [re-PCN]                 Briscoe, B., "Emulating Border Flow Policing
                            using Re-ECN on Bulk Data",
                            draft-briscoe-re-pcn-border-cheat-00 (work
                            in progress), July 2007.

   [voice-admit]            Baker, F., Polk, J., and M. Dolly, "DSCP for
                            Capacity-Admitted Traffic",
                            (work in progress), November 2008.

Appendix A.  PCN Deployment Considerations

A.1.  Choice of Suitable DSCPs

   The choice of which DSCP is most suitable for the PCN-domain is
   dependant 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, the appropriate DSCPs would currently
   be those for the Real Time Traffic Class [RFC5127].  If the PCN
   domain includes lower speed links it would also be appropriate to use
   the DSCPs of the other traffic classes that [voice-admit] defines for
   use with admission control, such as the three video classes CS4, CS3
   and AF4 and the Admitted Telephony Class.

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 PCN-domain: Either ECT
      is safe.

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   o  Leakage of traffic into PCN-domain: ECT(1) is less often correct.

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

   o  Incremental deployment: Either ECT is suitable as long as they are
      used consistently.

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

Authors' Addresses

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

   Phone: +44 1473 648734

   Bob Briscoe
   BT & UCL
   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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