Congestion and Pre Congestion                               T. Moncaster
Internet-Draft                                                        BT
Intended status: Standards Track                              B. Briscoe
Expires: April 17, 2009                                         BT & UCL
                                                                M. Menth
                                                 University of Wuerzburg
                                                        October 14, 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 extended 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  . . . . . . . . . . . . . . . .  5
     4.1.  Rationale for Encoding . . . . . . . . . . . . . . . . . .  5
     4.2.  PCN-Compatible DiffServ Codepoints . . . . . . . . . . . .  6
   5.  Rules for Experimental Encoding Schemes  . . . . . . . . . . .  6
   6.  Backwards Compatibility  . . . . . . . . . . . . . . . . . . .  6
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   9.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
   11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . .  8
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     12.1. Normative References . . . . . . . . . . . . . . . . . . .  8
     12.2. Informative References . . . . . . . . . . . . . . . . . .  8
   Appendix A.  Tunnelling Constraints  . . . . . . . . . . . . . . .  9
   Appendix B.  PCN Node Behaviours . . . . . . . . . . . . . . . . . 10
   Appendix C.  Deployment Scenarios for PCN Using Baseline
                Encoding  . . . . . . . . . . . . . . . . . . . . . . 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 -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.

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      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.

   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 than [RFC3168]

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

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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.

                        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
      [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.

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].  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 carry the 11 codepoint in
   the ECN field.  If the packet is being 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

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   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

   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].

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.

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

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   ECN semantics.  As such this document is compatible with BCP 124.  It
   should be noted that this baseline encoding blocks 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 (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
   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
   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.

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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.

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

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.

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

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   [pcn-marking-behaviour]  Eardley, P., "Marking behaviour of PCN-
                            nodes", draft-ietf-pcn-marking-behaviour-00
                            (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, "DSCPs
                            for Capacity-Admitted Traffic",
                            (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 always 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 the PCN working group needs to decide whether to
   advise against the use of full functionality RFC3168 IP in IP tunnels
   within a PCN-domain to support the ongoing work 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

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   tunnels [ecn-tunnelling].

Appendix B.  PCN Node Behaviours

   The following table of valid and invalid transitions, while necessary
   for the correct functioning of PCN they is not strictly part of the
   encoding scheme.  The PCN working group needs to decide whether to
   include this in this baseline encoding or whether to transfer it to
   an alternative document.

   | 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        |
   | interior  |   Not-PCN   |      Not-PCN    |  Any other codepoint  |
   |  egress   |     Any     |        00       | Any other codepoint * |
    * 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 Transitions at PCN nodes

   It is also necessary to define a safe behaviour for baseline-
   compliant nodes to follow should they unexpectedly encounter a packet
   carrying the EXP (01) codepoint.  The obvious safe behaviour would be
   to treat this as if it were a NM packet but to raise an alarm at a
   higher layer to check why the packet was there.  An alternative safe
   approach is to treat it as a not-PCN packet but this might jeopardise
   partial deployment of any future experimental encoding scheme.

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 requires only admission control.  Then admission
       control is triggered from PCN-packets that are threshold-marked
       and this baseline encdoding scheme suffices.

   2.  an operator requires only flow termination.  Then flow
       termination is triggered from PCN-packets that are excess-
       traffic-marked and this baseline encdoding scheme suffices.

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   3.  an operator requires both admission control and flow termination.
       If both admission control and flow termination are triggered from
       PCN-packets that are excess-traffic-marked then this baseline
       encoding scheme suffices.

   4.  an operator requires both admission control triggered by packets
       that are threshold-marked and flow termination triggered by
       packets that are excess-traffic-marked.  In this case the
       baseline encoding provides insufficient encoding states to
       achieve this.

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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