Congestion and Pre Congestion                               T. Moncaster
Internet-Draft                                                        BT
Intended status: Standards Track                              B. Briscoe
Expires: October 9, November 20, 2009                                      BT & UCL
                                                                M. Menth
                                                 University of Wuerzburg
                                                           April 7,
                                                            May 19, 2009

     Baseline Encoding and Transport of Pre-Congestion Information

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   The objective of Pre-Congestion Notification (PCN) is to protect the
   quality of service (QoS) of inelastic flows within a Diffserv domain.
   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.  The level of marking allows the
   boundary nodes to make decisions about whether t o 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 such marks
   are to be encoded into the IP header by re-using the ECN Explicit
   Congestion Notification (ECN) codepoints within this controlled
   domain.  The baseline encoding described here provides for only two
   PCN encoding states, unmarked Not-marked and marked. PCN-marked.

Table of Contents

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

1.  Introduction

   The objective of Pre-Congestion Notification (PCN) 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

   This document specifies how these PCN marks are encoded into the IP
   header by re-using the bits of the ECN field. 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 and rules states.  Rules
   for such extensions are given in this document. Section 5.

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

   From -03 to -04:

      Major WGLC comments addressed:

      *  Added Section 4.3.1 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.

      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
      [I-D.ietf-tsvwg-ecn-tunnel] [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 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  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
      [PCN-metering-marking].  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

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 Diffserv codepoint (see Section 4.3)
    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 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 Diffserv
      Codepoint.  To conserve DSCPs, DiffServ Diffserv Codepoints SHOULD be
      chosen that are already defined for use with admission controlled
      traffic, such as the Voice-Admit codepoint defined in
      [Voice-Admit].  Guidelines for mixing traffic-types within a PCN-domain PCN-
      domain are given in
      [I-D.ietf-pcn-marking-behaviour]. [PCN-metering-marking].

   o  Any packet that is not-PCN but which shares the same DiffServ Diffserv
      codepoint as PCN-enabled traffic MUST have the ECN field of its
      outermost IP header equal to 00.

4.1.  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.

   A packets sharing a PCN-compatible Diffserv

   The only valid codepoint transitions within a PCN-interior-node MUST observe are
   from NM to PM (which should occur if either meter indicates a need to
   PCN-mark a packet [PCN-metering-marking]) and from EXP to PM (which
   MAY be allowed by some future experimental extensions).  The
   following table gives the rules for full set of valid and invalid codepoint transitions as set out in the following table.  The precise
   rules governing which valid transition to use are set out in

                  |                  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 cause an alarm to be raised at a higher layer. The
       packet MUST be treated as if it carried the NM codepoint.

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

   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.  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 IPsec Tunnel.  [I-D.ietf-tsvwg-ecn-tunnel]  [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 guaranteeed to be copied down into the inner header upon
   decapsulation.  An additional constraint is the need to minimise the
   use of DiffServ Diffserv codepoints as because there is a limited supply of
   standards track codepoints remaining.  Section 4.3 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 choosing to set 10 as the NM state. 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.  PCN-Compatible DiffServ 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. DSCP and the
   PCN working group will compile a list of such DSCPs.  To be clear,
   any packets with such a DSCP will be PCN enabled only if they
   also are
   within a PCN-domain and have their ECN field set to indicate a
   codepoint other than not-
   PCN. not-PCN.

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

4.3.1.  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.

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 SHALL indicate not-PCN and MUST
      NOT be changed to any otehr other codepoint within a PCN-domain.
      Therefore an ingress node wishing to disable PCN marking for a
      packet within a PCN-compatible DiffServ Diffserv Codepoint MUST set the ECN
      field to 00.

   o  The 11 codepoint in the ECN field SHALL indicate PCN-marked
      (though this does not exclude the 01 Experimental codepoint from
      carrying the same meaning).

   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 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 unless mechanisms are put in place to tunnel such traffic
   across the PCN-domain.  Standard IP-in-IP or IPsec tunnels will
   always copy the CE codepoint from teh outer header into the inner
   header in decapsulation (unless the inner packet is not-ECT).  If an
   operator it is essential that any operator wishing to allow ECN to
   exist end-to-end ensures there are no tunnel end-points within the

7.  IANA Considerations

   This document makes no direct request to IANA.  However this document
   allows for a set of Diffserv Codepoints to be assigned different ECN
   semantics within a controlled domain as described in [RFC4774].  A
   list of such DSCPs will be maintained by the PCN working group.

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 PCN-compatible DSCP and a PCN-Enabled ECN codepoint.
   This encoding document is intended to stand independently of the
   architecture used to determine whether how specific packets are authorised to
   be PCN-marked, which will be described in separate documents on PCN edge-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 but such considerations are beyond the scope of this

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].

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

12.1.  Normative References


   [PCN-metering-marking]  Eardley, P., "Marking "Metering and marking behaviour
                           of PCN-nodes", dra
                           draft-ietf-pcn-marking-behaviour-03 (work in
                           progress), March May 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


   [ECN-tunnel]            Briscoe, B., "Tunnelling of Explicit
                           Congestion Notification",
                           draft-ietf-tsvwg-ecn-tunnel-02 (work in
                           progress), March 2009.

   [PCN-architecture]      Eardley, P., "Pre-Congestion Notification
                           (PCN) Architecture", draft-ietf-
                           draft-ietf-pcn-architecture-11 (work in
                           progress), March April 2009.

   [I-D.ietf-tsvwg-admitted-realtime-dscp]  Baker, F., Polk, J.,

   [RFC3540]               Spring, N., Wetherall, D., and M.
                                            Dolly, "DSCP for Capacity-
                                            Admitted Traffic", draft-
                                            realtime-dscp-05 (work in
                                            progress), November 2008.

   [I-D.ietf-tsvwg-ecn-tunnel]              Briscoe, B., "Tunnelling of D. Ely,
                           "Robust Explicit Congestion
                                            Notification", draft-ietf-
                                            tsvwg-ecn-tunnel-02 (work in
                                            progress), March 2009. 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.

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

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

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 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 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 [I-D.ietf-tsvwg-admitted-realtime-dscp] [Voice-Admit] defines for use with admission control,
   such as the three video classes CS4, CS3 and AF4 and the Admitted
   Telephony Class.  The PCN working group will maintain a list of PCN-
   compatible Diffserv Codepoints.

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

   The choice of which ECT codepoint to use for the Not Marked 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 slightly 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
   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