Congestion and Pre Congestion                               T. Moncaster
Internet-Draft                                                        BT                                                B. Briscoe
Intended status: Standards Track                              B. Briscoe
Expires: November 20, 2009                                      BT & UCL
Expires: February 21, 2010                                      M. Menth
                                                 University of Wuerzburg
                                                            May 19,
                                                         August 20, 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 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 Explicit
   Congestion Notification (ECN) codepoints within this controlled
   domain.  The baseline encoding described here provides for only two
   PCN encoding states, Not-marked and PCN-marked.

Table of Contents

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

1.  Introduction

   The objective of Pre-Congestion Notification (PCN) [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-
   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 "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 -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 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 [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 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  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
   [PCN-architecture]. [RFC5559].

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)
    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 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, such as the Voice-Admit codepoint defined in
      traffic.  Appendix A.1 gives guidance to implementiors on suitable
      DSCPs.  Guidelines for mixing traffic-types within a PCN-
      domain PCN-domain
      are given in [PCN-metering-marking]. [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 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 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 [PCN-metering-marking]) [I-D.ietf-pcn-marking-behaviour]) and from EXP to
   PM (which MAY be allowed by some future experimental extensions).
   The following table gives the full set of valid and invalid codepoint

                  |                  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 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 Tunnel.  [ECN-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 guaranteeed 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
   explains how we have minimised this still further by reusing pre-existing 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.  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 and the
   PCN working group will compile a list of such DSCPs. 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 the default ECN marking
   behaviour introduced in [RFC3168]).  All traffic metering and marking
   behaviours are discussed in [PCN-metering-marking]. [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
   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.  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  All Interior-nodes within a PCN-domain MUST interpret the 00
      codepoint in the ECN field SHALL indicate as not-PCN and MUST NOT be changed change it to any other codepoint within a PCN-domain.
      another value.  Therefore an ingress node wishing to disable PCN
      marking for a packet within a PCN-compatible Diffserv Codepoint
      MUST set the ECN field to 00.

   o  The 11 codepoint in the ECN field SHALL MUST 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 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 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 the 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 wishes to allow ECN to exist end-to-end ensures they must ensure
   there are no tunnel end-points within the
   PCN-domain. PCN-domain to prevent any
   risk of PCN-markings being exposed to endpoints.

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 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 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 trust.  However such considerations are beyond the scope of
   this document.

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

12.1.  Normative References


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

   [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.

   [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.

   [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 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 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, the
   appropriate DSCPs would currently be those for the Real Time Traffic
   Class [RFC5127].  If  To be clear the PCN domain includes lower speed links it Working Group recommends using
   admission control for the following service classes:

   o  Telephony (EF)

   o  Real-time interactive (CS4)

   o  Broadcast Video (CS3)

   o  Multimedia Conferencing (AF4)

   PCN marking is intended to provide a scalable admission control
   mechanism for traffic with a high degree of statistical multiplexing.
   PCN marking would also therefore be appropriate to use apply to traffic in the DSCPs of
   above classes, but only within a PCN region containing highly
   aggregated traffic.  In such cases, the other 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 that [Voice-Admit] defines may be defined for use with which admission control,
   such as control
   is appropriate, whether through some future standards action or
   through local use by certain operators, e.g. the three video classes CS4, CS3 and AF4 and Multimedia Streaming
   service class (AF3).  This document does not preclude the Admitted
   Telephony Class. 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 working group will maintain a list as their mechanism of PCN-
   compatible Diffserv Codepoints.

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
   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