Congestion and Pre Congestion                               T. Moncaster
Internet-Draft                                                        BT
Intended status: Standards Track                              B. Briscoe
Expires: April 3, 2009                                          BT & UCL
                                                                M. Menth
                                                 University of Wuerzburg
                                                      September 30, 2008

     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 extensible to provide more encoding
   states but such schemes will be described in other documents.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Encoding two PCN States in IP  . . . . . . . . . . . . . . . .  4
     4.1.  Rationale for Encoding . . . . . . . . . . . . . . . . . .  5
     4.2.  PCN-Compatible DiffServ Codepoints . . . . . . . . . . . .  6
   5.  Backwards Compatability  . . . . . . . . . . . . . . . . . . .  6
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   8.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
   10. Comments Solicited . . . . . . . . . . . . . . . . . . . . . .  7
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     11.1. Normative References . . . . . . . . . . . . . . . . . . .  7
     11.2. Informative References . . . . . . . . . . . . . . . . . .  8
   Appendix A.  Tunnelling Constraints  . . . . . . . . . . . . . . .  9
   Appendix B.  PCN Node Behvaiours . . . . . . . . . . . . . . . . .  9
     B.1.  Valid and Invalid Encoding Transitions at a PCN Node . . . 10
   Appendix C.  Deployment Scenarios for PCN Using Baseline
                Encoding  . . . . . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12

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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.

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

   From -02 to -03:

      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 -01 to -02:

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

   From -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.

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      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 capable.

   o  PCN-marked - codepoint indicating packets that have been marked at
      a PCN-interior-node using some 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

   o  PCN-compatible Diffserv codepoint - a Diffserv codepoint for which
      the ECN field is used to carry PCN markings rather thatn [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:

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   |  DSCP \ RFC3168 ECN |   not-ECT  |   ECT(0)  |   ECT(1)  |   CE   |
   |      codepoint      |    (00)    |    (10)   |    (01)   |  (11)  |
   |        DSCP n       |   not-PCN  |     NM    |    EXP    |   PM   |

    Where DSCP n is a PCN-enabled DiffServ codepoint (see Section 4.2)
               and EXP means available for Experimental use.

                        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.  That is a DiffServ codepoint that indicates that PCN
      could be enabled by setting the appropriate value in the ECN
      field.  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

   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 set to 00.

4.1.  Rationale for Encoding

   The exact choice of encoding was dictated by the constraints imposed
   by existing IETF RFCs, in particular [RFC3168] and [RFC4774].  Full
   details are contained in [pcn-enc-compare].  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.
   Appendix A explains this in detail.  The main effect of this
   constraint is that any PCN marking has to use the ECN field set to 11
   (CE codepoint).  If the packet is being tunneled then only the CE
   codepoint gets copied into the inner header upon decapsulation.  An
   additional constraint is the need to minimise the use of DiffServ
   codepoints as these are in increasingly short supply.  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.

   The encoding scheme (Table 1) that best addresses the above
   constraints ends up looking very similar to ECN.  This is perhaps not
   surprising given the similarity in architectural intent between PCN
   and ECN.

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4.2.  PCN-Compatible DiffServ Codepoints

   Equipment complying with the baseline PCN encoding MUST allow PCN to
   be enabled for a certain Diffserv codepoint or 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]).  The scheduling behaviour is discussed in

5.  Backwards Compatability

   BCP 124 [RFC4774] gives guidelines for specifying alternative
   semantics for the ECN field.  It sets out a number of factors that
   must 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.

6.  IANA Considerations

   This document makes no request to IANA.

7.  Security Considerations

   Packets claim entitlement to be PCN marked by carrying a PCN-enabled
   DSCP and a PCN-Capable 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
   (see Appendix B).

   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

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   be described as part of the edge-node behaviour document as above.

   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.

8.  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 traffic that is not PCN-capable within the same DSCP so
   long as theat traffic doesn't require end-to-end ECN support.  The
   encoding scheme is conformant with [RFC4774].

9.  Acknowledgements

   This document builds extensively on work done in the PCN working
   group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley and
   others.  Full details of the alternative schemes that were considered
   for adoption can be found in the document [pcn-enc-compare].  Thanks
   to Ruediger Geib for providing detailed comments on this document.

10.  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.

11.  References

11.1.  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.

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11.2.  Informative References

              Moncaster, T., Briscoe, B., and M. Menth, "A three state
              extended PCN encoding scheme",
              draft-moncaster-pcn-3-state-encoding-00 (work in
              progress), June 2008.

              Eardley, P., "Pre-Congestion Notification Architecture",
              draft-ietf-pcn-architecture-03 (work in progress),
              February 2008.

              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.

              Briscoe, B., "Layered Encapsulation of Congestion
              Notification", draft-briscoe-tsvwg-ecn-tunnel-01 (work in
              progress), July 2008.

              Chan, K., Karagiannis, G., Moncaster, T., Menth, M.,
              Eardley, P., and B. Briscoe, "Pre-Congestion Notification
              Encoding Comparison",
              draft-chan-pcn-encoding-comparison-03 (work in progress),
              February 2008.

              Eardley, P., "Marking behaviour of PCN-nodes",
              draft-eardley-pcn-marking-behaviour-01 (work in progress),
              June 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.

              Baker, F., Polk, J., and M. Dolly, "DSCPs for Capacity-
              Admitted Traffic",

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              draft-ietf-tsvwg-admitted-realtime-dscp-04 (work in
              progress), February 2008.

Appendix A.  Tunnelling Constraints

   The rules that govern the behaviour of the ECN field for IP-in-IP
   tunnels were defined in [RFC3168].  This allowed for two tunnel
   modes.  The limited functionality mode sets the outer header to not-
   ECT, regardless of the value of the inner header, in other words
   disabling ECN within the tunnel.  The full functionality mode copies
   the inner ECN field into the outer header if the inner header is not-
   ECT or either of the 2 ECT codepoints.  If the inner header is CE
   then the outer header is set to ECT(0).  On decapsulation, if the CE
   codepoint is set on the outer header then this is copied into the
   inner header.  Otherwise the inner header is left unchanged.  The
   stated reason for blocking CE from being copied to the outer header
   was to prevent this from being used as a covert channel through IPSec

   The IPSec protocol [RFC4301] changed the ECN tunnelling rule to allow
   IPSec tunnels to simply copy the inner header into the outer header.
   On decapsulation the outer header is discarded and the ECN field is
   only copied down if it is set to CE.

   Because of the possible existence of tunnels, only CE (11) can be
   used as a PCN marking as it is the only mark that will survive
   decapsulation.  However there is a need for caution with all
   tunneling within the PCN-domain.  RFC3168 full functionality IP in IP
   tunnels are expected to set the ECN field to ECT(0) if the inner ECN
   field is set to CE.  This leads to the possibility that some packets
   within the PCN-domain that have already been marked may have that
   mark concealed further into the domain.  This is undesirable for many
   PCN schemes and thus standard IP in IP tunnels SHOULD NOT be used
   within a PCN-domain.  Further work is needed within the Transport
   Area to rationalise the behaviour of IP in IP tunnels in respect to
   the ECN field and bring them in line with the behaviour of IPSec
   tunnels [ecn-tunnelling].

Appendix B.  PCN Node Behvaiours

   Any packet that belongs to a PCN capable flow MUST have the ECN field
   set to indicate a NM state at the PCN-ingress-node.

   Any packet that is PCN capable and has been PCN-marked by a PCN-
   interior-node MUST have the ECN field set to indicate a PM state.

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   Any packet leaving the PCN-domain SHOULD have the ECN field reset to
   00.  The only exception is where the egress node knows the end-hosts
   will react safely to any PCN marks they receive.

B.1.  Valid and Invalid Encoding Transitions at a PCN Node

   o  PCN-interior-nodes MUST NOT change not-PCN to another codepoint
      and they SHOULD NOT change a PCN-Capable codepoint to not-PCN
      except where they need to downgrade the packet to a lower class of

   o  PCN-interior-nodes that are in a pre-congestion state above the
      configured level MUST set a PM codepoint as defined in Table 1 or
      in any local/experimental scheme running within the PCN-domain.

   o  Packets carrying the 01 ECT(1) codepoint are for local/
      experimental use only and their unexpected presence SHOULD cause
      an alarm to be raised at the management level.  However, to allow
      for the possibility of misconfiguration they SHOULD be treated as
      NM packets.

   o  The PM codepoint MUST NOT be changed to NM.

Appendix C.  Deployment Scenarios for PCN Using Baseline Encoding

   This appendix illustrates possible PCN deployment scenarios where the
   baseline encoding can be used and also explain a case for which
   baseline encoding is not sufficient. {Note this appendix is provided
   for information only}.

   1.  An operator may wish to use PCN-based admission control only.  To
       that end, threshold marking based on admissible rates might be
       used as the only PCN metering and marking algorithm.  As a
       consequence, the PM marks on the packets are interpreted as
       meaning the ingress should stop admitting new traffic.

   2.  An operator may wish to use PCN-based flow termination only.  To
       that end, excess rate marking based on supportable rates might be
       used as the only PCN metering and marking algorithm.  As a
       consequence, the PM marks on the packets are interpreted as
       meaning the ingress shoudl start terminating appropriate flows.

   3.  An operator may wish to use both PCN-based admission control and
       flow termination.  To that end, excess rate marking based on
       admissible rates might be used as the only PCN metering and
       marking algorithm.  The level of marks will be used to determine
       when the ingress shoudl stop admitting new traffic and whether

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       the ingress should terminate any flows.

   4.  An operator may wish to implement admission control based on
       threshold marking at admissible rates and flow termination based
       on excess rate marking at supportable rates because these methods
       are believed to work better with small ingress-egress aggregates.
       Then two different markings are needed.  Such a deployment
       scenario is not supported by the PCN baseline encoding.

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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