Congestion and Pre-Congestion                                 B. Briscoe
Notification                                                          BT
Internet-Draft                                              T. Moncaster
Obsoletes: 5696 (if approved)              Moncaster Internet Consulting
Intended status: Standards Track                                M. Menth
Expires: January 11, 31, 2012                        University of Tuebingen
                                                           July 10, 30, 2011

       Encoding 3 PCN-States in the IP header using a single DSCP
                   draft-ietf-pcn-3-in-1-encoding-06
                   draft-ietf-pcn-3-in-1-encoding-07

Abstract

   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.  Egress nodes pass information about
   these PCN-marks to decision points which then decide whether to admit
   or block new flow requests or to terminate some already-admitted
   flows during serious pre-congestion.

   This document specifies how PCN-marks are to be encoded into the IP
   header by re-using the Explicit Congestion Notification (ECN)
   codepoints within a PCN-domain.  This encoding provides for up to
   three different PCN marking states using a single DSCP: not-marked
   (NM), threshold-marked (ThM) and excess-traffic-marked (ETM).  Hence,
   it is called the 3-in-1 PCN encoding.  This document obsoletes
   RFC5696.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on January 11, 31, 2012.

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   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  5
     1.2.  Changes in This Version (to be removed by RFC Editor)  . .  5
   2.  Definitions and Abbreviations  . . . . . . . . . . . . . . . .  7
     2.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.2.  List of Abbreviations  . . . . . . . . . . . . . . . . . .  7  8
   3.  Definition of 3-in-1 PCN Encoding  . . . . . . . . . . . . . .  8
   4.  Requirements for and Applicability of 3-in-1 PCN Encoding  . .  9
     4.1.  PCN Requirements . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Requirements Imposed by Tunnelling . . . . . . . . . . . .  9 10
     4.3.  Applicability of 3-in-1 PCN Encoding . . . . . . . . . . . 10
   5.  Behaviour of a PCN-node to Comply with the 3-in-1 PCN
       Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 11
     5.1.  PCN-ingress Node Behaviour . . . . . . . . . . . . . . . . 10 11
     5.2.  PCN-interior Node Behaviour  . . . . . . . . . . . . . . . 11
       5.2.1.  Behaviour Common to all PCN-interior Nodes . . . . . . 11
       5.2.2.  Behaviour of PCN-interior Nodes Using Two
               PCN-markings . . . . . . . . . . . . . . . . . . . . . 11 12
       5.2.3.  Behaviour of PCN-interior Nodes Using One
               PCN-marking  . . . . . . . . . . . . . . . . . . . . . 11 12
     5.3.  Behaviour of PCN-egress Nodes  . . . . . . . . . . . . . . 12 13
   6.  Backward Compatibility . . . . . . . . . . . . . . . . . . . . 13
     6.1.  Backward Compatibility with ECN  . . . . . . . . . . . . . 13
     6.2.  Backward Compatibility with the Baseline Encoding  . . . . 13 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13 14
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   9.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 14 15
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14 15
   11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 15
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     12.2. Informative References . . . . . . . . . . . . . . . . . . 15 16
   Appendix A.  Choice of Suitable DSCPs  . . . . . . . . . . . . . . 16 17
   Appendix B.  Co-existence of ECN and PCN . . . . . . . . . . . . . 18
   Appendix C.  Example Mapping between Encoding of PCN-Marks in
                IP and in MPLS Shim Headers . . . . . . . . . . . . . 20
   Appendix D.  Rationale for different behaviours for single
                marking schemes . . . Discrepancy Between the Schemes
                using One PCN-Marking . . . . . . . . . . . . . . . . 21 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21 22

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.  Two
   mechanisms are used: admission control, to decide whether to admit or
   block a new flow request, and flow termination to terminate some
   existing flows during serious pre-congestion.  To achieve this, the
   overall rate of PCN-traffic is metered on every link in the 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 to boundary nodes about overloads
   before any real congestion occurs (hence "pre-congestion
   notification").

   [RFC5670] provides for two metering and marking functions that are
   generally configured with different reference rates.  Threshold-
   marking marks all PCN packets once their traffic rate on a link
   exceeds the configured reference rate (PCN-threshold-rate).  Excess-
   traffic-marking marks only those PCN packets that exceed the
   configured reference rate (PCN-excess-rate).  The PCN-excess-rate is
   typically larger than the PCN-threshold-rate [RFC5559].  Egress nodes
   monitor the PCN-marks of received PCN-packets and pass information
   about these PCN-marks to decision points which then decide whether to
   admit new flows or terminate existing flows
   [I-D.ietf-pcn-cl-edge-behaviour], [I-D.ietf-pcn-sm-edge-behaviour].

   The baseline encoding defined in [RFC5696] described how two PCN
   marking states (Not-marked and PCN-Marked) could be encoded into the
   IP header using a single Diffserv codepoint.  It also provided an
   experimental codepoint (EXP), along with guidelines for the use of
   that codepoint.  Two PCN marking states are sufficient for the Single
   Marking edge behaviour [I-D.ietf-pcn-sm-edge-behaviour].  However,
   PCN-domains utilising the controlled load edge behaviour
   [I-D.ietf-pcn-cl-edge-behaviour] require three PCN marking states.
   This document extends the baseline encoding by redefining the EXP
   codepoint to provide a third PCN marking state in the IP header,
   still using a single Diffserv codepoint.  This encoding scheme is
   therefore called the "3-in-1 PCN encoding".  It obsoletes the
   baseline encoding [RFC5696], which provides only a sub-set of the
   same capabilities.

   The full version of this encoding requires any tunnel endpoint within
   the PCN-domain to support the normal tunnelling rules defined in
   [RFC6040].  There is one limited exception to this constraint where
   the PCN-domain only uses the excess-traffic-marking behaviour and
   where the threshold-marking behaviour is deactivated.  This is
   discussed in Section 5.2.3.1.

   This document only concerns the PCN wire protocol encoding for IP
   headers, whether IPv4 or IPv6.  It makes no changes or
   recommendations concerning algorithms for congestion marking or
   congestion response.  Other documents will define the PCN wire
   protocol for other header types.  Appendix C discusses a possible
   mapping between IP and MPLS.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

1.2.  Changes in This Version (to be removed by RFC Editor)

   From draft-ietf-pcn-3-in-1-encoding-06 to -07:

      *  Clarified that each operator not the IETF chooses which DSCP(s)
         are PCN-compatible, and made it unambiguous that only PCN-nodes
         recognise that PCN-compatible DSCPs enable the 3-in-1 encoding.

      *  Removed statements about the PCN working group, given RFCs are
         meant to survive beyond the life of a w-g.

      *  Corrected the final para of "Rationale for Different Behaviours
         in Schemes with Only One Marking"

   From draft-ietf-pcn-3-in-1-encoding-05 to -06:

      *  Draft re-written to obsolete baseline encoding [RFC5696].

      *  New section defining utilising this encoding for single
         marking. only one PCN-
         Marking.  Added an appendix explaining an apparent
         inconsistency relating to single marking. within this section.

      *  Moved (and updated) informative appendixes from [RFC5696] to
         this document.  Original Appendix C was omitted as it is now
         redundant.

      *  Significant re-structuring of document.

   From draft-ietf-pcn-3-in-1-encoding-04 to -05:

      *  Draft moved to standards track as per working group
         discussions.

      *  Added Appendix B discussing ECN handling in the PCN-domain.

      *  Clarified that this document modifies [RFC5696].

   From draft-ietf-pcn-3-in-1-encoding-03 to -04:

      *  Updated document to reflect RFC6040.

      *  Re-wrote introduction.

      *  Re-wrote section on applicability.

      *  Re-wrote section on choosing encoding scheme.

      *  Updated author details.

   From draft-ietf-pcn-3-in-1-encoding-02 to -03:

      *  Corrected mistakes in introduction and improved overall
         readability.

      *  Added new terminology.

      *  Rewrote a good part of Section 4 and 5 to achieve more clarity.

      *  Added appendix explaining when to use which encoding scheme and
         how to encode them in MPLS shim headers.

      *  Added new co-author.

   From draft-ietf-pcn-3-in-1-encoding-01 to -02:

      *  Corrected mistake in introduction, which wrongly stated that
         the threshold-traffic rate is higher than the excess-traffic
         rate.  Other minor corrections.

      *  Updated acks & refs.

   From draft-ietf-pcn-3-in-1-encoding-00 to -01:

      *  Altered the wording to make sense if
         draft-ietf-tsvwg-ecn-tunnel moves to proposed standard.

      *  References updated

   From draft-briscoe-pcn-3-in-1-encoding-00 to
   draft-ietf-pcn-3-in-1-encoding-00:

      *  Filename changed to draft-ietf-pcn-3-in-1-encoding.

      *  Introduction altered to include new template description of
         PCN.

      *  References updated.

      *  Terminology brought into line with [RFC5670].

      *  Minor corrections.

2.  Definitions and Abbreviations

2.1.  Terminology

   The terms PCN-domain, PCN-node, PCN-interior-node, PCN-ingress-node,
   PCN-egress-node, PCN-boundary-node, PCN-traffic, PCN-packets and PCN-
   marking are used as defined in [RFC5559].  The following additional
   terms are defined in this document:

   PCN encoding:  mapping of PCN marking states to specific codepoints
      in the packet header.

   PCN-compatible Diffserv codepoint:  a Diffserv codepoint indicating
      packets for which the ECN field is used to carry carries PCN-markings rather than
      [RFC3168] markings (see Appendix A). markings.  Note that an operator configures PCN-nodes to
      recognise PCN-compatible DSCPs, whereas the same DSCP has no PCN-
      specific meaning to a node outside the PCN domain.

   Threshold-marked codepoint:  a codepoint that indicates packets that
      have been marked at a PCN-interior-node as a result of an
      indication from the threshold-metering function [RFC5670].
      Abbreviated to ThM.

   Excess-traffic-marked codepoint:  a codepoint that indicates packets
      that have been marked at a PCN-interior-node as a result of an
      indication from the excess-traffic-metering function [RFC5670].
      Abbreviated to ETM.

   Not-marked codepoint:  a codepoint that indicates PCN-packets but
      that are not PCN-marked.  Abbreviated to NM.

   not-PCN codepoint:  a codepoint that indicates packets that are not
      PCN-packets.

2.2.  List of Abbreviations

   The following abbreviations are used in this document:

   o  AF = Assured Forwarding [RFC2597]

   o  CE = Congestion Experienced [RFC3168]

   o  CS = Class Selector [RFC2474]

   o  DSCP = Diffserv codepoint

   o  ECN = Explicit Congestion Notification [RFC3168]

   o  ECT = ECN Capable Transport [RFC3168]

   o  EF = Expedited Forwarding [RFC3246]

   o  ETM = Excess-traffic-marked

   o  EXP = Experimental

   o  IP = Internet protocol

   o  NM = Not-marked

   o  PCN = Pre-Congestion Notification

   o  ThM = Threshold-marked

3.  Definition of 3-in-1 PCN Encoding

   The 3-in-1 PCN encoding scheme allows for two or three PCN-marking
   states to be encoded within the IP header.  The full encoding is
   shown in Figure 1.

   +--------+----------------------------------------------------+
   |        |           Codepoint in ECN field of IP header      |
   |  DSCP  |               <RFC3168 codepoint name>             |
   |        +--------------+-------------+-------------+---------+
   |        | 00 <Not-ECT> | 10 <ECT(0)> | 01 <ECT(1)> | 11 <CE> |
   +--------+--------------+-------------+-------------+---------+
   | DSCP n |    Not-PCN   |      NM     |     ThM     |   ETM   |
   +--------+--------------+-------------+-------------+---------+

                       Figure 1: 3-in-1 PCN Encoding

   As mentioned above, the 3-in-1 PCN encoding is an extension of the
   baseline encoding [RFC5696].  Like the baseline encoding it uses

   A PCN-node (i.e. a
   combination of node within a PCN-compatible DSCP (DSCP n in Figure 1) and the ECN
   field for the encoding of PCN-marks. PCN-domain) will be configured to
   recognise certain DSCPs as PCN-compatible.  Appendix A discusses the
   choice of suitable DSCPs.  The PCN-marks have the following meaning.

   Not-PCN:  In Figure 1 'DSCP n' indicates such a non-PCN-packet, i.e., a PCN-
   compatible DSCP.  Within the PCN-domain, any packet that carrying a PCN-
   compatible DSCP is not
      subject to PCN metering and marking.

   NM:  Not-marked.  Indicates a PCN-packet that has as defined in [RFC5559].

   PCN-nodes MUST interpret the ECN field of a PCN-packet using the
   3-in-1 PCN encoding, rather than [RFC3168].  This does not yet been marked
      by change the
   behaviour for any PCN marker.

   ThM:  Threshold-marked.  Indicates packet with a PCN-packet DSCP that has been marked
      by is not PCN-compatible, or
   for any node outside a PCN-domain.  In all such cases the 3-in-1
   encoding is not applicable and so by default the node will interpret
   the ECN field using [RFC3168].

   When using the 3-in-1 encoding, the codepoints of the ECN field have
   the following meanings:

   Not-PCN:  indicates a non-PCN-packet, i.e., a packet that uses a PCN-
      compatible DSCP but is not subject to PCN metering and marking.

   NM:  Not-marked.  Indicates a PCN-packet that has not yet been marked
      by any PCN marker.

   ThM:  Threshold-marked.  Indicates a PCN-packet that has been marked
      by a threshold-marker [RFC5670].

   ETM:  Excess-traffic-marked.  Indicates a PCN-packet that has been
      marked by an excess-traffic-marker [RFC5670].

4.  Requirements for and Applicability of 3-in-1 PCN Encoding

4.1.  PCN Requirements

   In accordance with the PCN architecture [RFC5559], PCN-ingress-nodes
   control packets entering a PCN-domain.  Packets belonging to PCN-
   controlled flows are subject to PCN-metering and -marking, and PCN-
   ingress-nodes mark them as Not-marked (PCN-colouring).  Any node in
   the PCN-domain may perform PCN-metering and -marking and mark PCN-
   packets if needed.  There are two different metering and marking
   behaviours: threshold-marking and excess-traffic-marking [RFC5670].
   Some edge behaviors require only a single marking behaviour
   [I-D.ietf-pcn-sm-edge-behaviour], others require both
   [I-D.ietf-pcn-cl-edge-behaviour].  In the latter case, three PCN
   marking states are needed: not-marked (NM) to indicate not-marked
   packets, threshold-marked (ThM) to indicate packets marked by the
   threshold-marker, and excess-traffic-marked (ETM) to indicate packets
   marked by the excess-traffic-marker [RFC5670].  Threshold-marking and
   excess-traffic-marking are configured to start marking packets at
   different load conditions, so one marking behaviour indicates more
   severe pre-congestion than the other.  Therefore, a fourth PCN
   marking state indicating that a packet is marked by both markers is
   not needed.  However a fourth codepoint is required to indicate
   packets that use a PCN-compatible DSCP but do not use PCN PCN-marking
   (the not-PCN codepoint).

   In all current PCN edge behaviors that use two marking behaviours
   [RFC5559], [I-D.ietf-pcn-cl-edge-behaviour], excess-traffic-marking
   is configured with a larger reference rate than threshold-marking.
   We take this as a rule and define excess-traffic-marked as a more
   severe PCN-mark than threshold-marked.

4.2.  Requirements Imposed by Tunnelling

   [RFC6040] defines rules for the encapsulation and decapsulation of
   ECN markings within IP-in-IP tunnels.  The publication of RFC6040
   removed the tunnelling constraints that existed when the baseline
   encoding [RFC5696] was written (see section 3.3.2 of
   [I-D.ietf-pcn-encoding-comparison]).

   Nonetheless, there is still a problem if there are any legacy (pre-
   RFC6040) decapsulating tunnel endpoints within a PCN domain.  If a
   PCN node Threshold-marks the outer header of a tunnelled packet with
   a Not-marked codepoint on the inner header, the legacy decapsulator
   will revert the Threshold-marking to Not-marked.  The rules on
   applicability in Section 4.3 below are designed to avoid this
   problem.

4.3.  Applicability of 3-in-1 PCN Encoding

   The 3-in-1 encoding is applicable in situations where two marking
   behaviours are being used in the PCN-domain.  The 3-in-1 encoding can
   also be used with only one marking behaviour, in which case one of
   the codepoints MUST NOT be used throughout the PCN-domain (see
   Section 5.2.3).

   For the full 3-in-1 encoding to apply, any tunnel endpoints (IP-in-IP
   and IPsec) within the PCN-domain MUST comply with the ECN
   encapsulation and decapsulation rules set out in [RFC6040] (see
   Section 4.2).  There is one exception to this rule outlined next.

   It may not be possible to upgrade every pre-RFC6040 tunnel endpoint
   within a PCN-domain.  In such cirsumstances circumstances a limited version of the
   3-in-1 encoding can still be used but only under the following
   stringent condition.  If any pre-RFC6040 tunnel endpoint exists
   within a PCN-domain then every PCN-node in the PCN-domain MUST be
   configured so that it never sets the ThM codepoint.  The behaviour of
   PCN-interior nodes in this case is defined in Section 5.2.3.1. 5.2.3.1, which
   describes the rules for using only the Excess Traffic marking
   function.  In all other situations where legacy tunnel endpoints
   might be present within the PCN domain, the 3-in-1 encoding is not
   applicable.

5.  Behaviour of a PCN-node to Comply with the 3-in-1 PCN Encoding

   As mentioned in Section 4.3 above, all PCN-nodes MUST comply with
   [RFC6040].

5.1.  PCN-ingress Node Behaviour

   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.
   Appendix A gives guidance to implementors on suitable DSCPs.
   Guidelines for mixing traffic types within a PCN-domain are given in
   [RFC5670].

   If a packet arrives at the PCN-ingress-node that shares a PCN-
   compatible DSCP and is not a PCN-packet, the PCN-ingress MUST mark it
   as not-PCN.

   If a PCN-packet arrives at the PCN-ingress-node, the PCN-ingress MUST
   change the PCN codepoint to Not-marked.

   If a PCN-packet arrives at the PCN-ingress-node with its ECN field
   already set to a value other than not-ECT, then appropriate action
   MUST be taken to meet the requirements of [RFC4774].  The simplest
   appropriate action is to just drop such packets.  However, this is a
   drastic action that an operator may feel is undesirable.  Appendix B
   provides more information and summarises other alternative actions
   that might be taken.

5.2.  PCN-interior Node Behaviour

5.2.1.  Behaviour Common to all PCN-interior Nodes

   Interior nodes MUST NOT change not-PCN to any other codepoint.

   Interior nodes MUST NOT change NM to not-PCN.

   Interior nodes MUST NOT change ThM to NM or not-PCN.

   Interior nodes MUST NOT change ETM to any other codepoint.

5.2.2.  Behaviour of PCN-interior Nodes Using Two PCN-markings

   If the threshold-meter function indicates a need to mark the packet,
   the PCN-interior node MUST change NM to ThM.

   If the excess-traffic-meter function indicates a need to mark the
   packet:

   o  the PCN-interior node MUST change NM to ETM;

   o  the PCN-interior node MUST change ThM to ETM.

   If both the threshold meter and the excess-traffic meter indicate the
   need to mark a packet, the excess traffic marking rules MUST take
   priority.

5.2.3.  Behaviour of PCN-interior Nodes Using One PCN-marking

   Some PCN edge behaviours require only one PCN-marking within the PCN-
   domain.  The Single Marking edge behaviour
   [I-D.ietf-pcn-sm-edge-behaviour] requires PCN-interior nodes to mark
   packets using the excess-traffic-meter function [RFC5670].  It is
   possible that future schemes may require only the threshold-meter
   function.  Observant readers may spot an apparent inconsistency
   between the two following cases.  Appendix D explains the rationale
   behind this inconsistency.

5.2.3.1.  Marking using only the Excess-traffic-meter Function

   The threshold-traffic-meter function SHOULD be disabled and MUST NOT
   trigger any packet marking.

   The PCN-interior node SHOULD raise a management alarm if it receives
   a ThM packet, but the frequency of such alarms SHOULD be limited.

   If the excess-traffic-meter function indicates a need to mark the
   packet:

   o  the PCN-interior node MUST change NM to ETM;

   o  the PCN-interior node MUST change ThM to ETM.  It SHOULD also
      raise an alarm as above.

5.2.3.2.  Marking using only the Threshold-meter Function

   The excess-traffic-meter function SHOULD be disabled and MUST NOT
   trigger any packet marking.

   The PCN-interior node SHOULD raise a management alarm if it receives
   an ETM packet, but the frequency of such alarms SHOULD be limited.

   If the threshold-meter function indicates a need to mark the packet:

   o  the PCN-interior node MUST change NM to ThM;

   o  the PCN-interior node MUST NOT change ETM to any other codepoint.
      It SHOULD raise an alarm as above.

5.3.  Behaviour of PCN-egress 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].  For instance, if the PCN-ingress-
   node has explicitly informed the PCN-egress-node that this flow is
   ECN-capable, then it might be safe to expose other codepoints.
   Appendix B gives details of how such schemes might work, but such
   schemes are currently experimental. only tentative ideas.

   If the PCN-domain is configured to use only excess-traffic marking,
   the PCN-egress node MUST treat ThM as ETM and if only threshold-
   marking is used it should treat ETM as ThM.  However it SHOULD raise
   a management alarm in either instance since this means there is some
   misconfiguration in the PCN-domain.

6.  Backward Compatibility

6.1.  Backward Compatibility with ECN

   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 encoding specified in this document uses one of those techniques;
   it defines PCN-compatible Diffserv codepoints as no longer supporting
   the default ECN semantics.  As such, this document is compatible with
   BCP 124.

   On its own, the 3-in-1 encoding cannot support both ECN marking end-
   to-end (e2e) and PCN-marking within a PCN-domain.  Appendix B
   discusses possible ways to do this, e.g. by carrying e2e ECN across a
   PCN-domain within the inner header of an IP-in-IP tunnel.  Although
   Appendix B recommends various approaches over others, it is merely
   informative and all such schemes are beyond the normative scope of
   this document.

   In any PCN deployment, traffic can only enter the PCN-domain through
   PCN-ingress-nodes and leave through PCN-egress-nodes.  PCN-ingress-
   nodes ensure that any packets entering the PCN-domain have the ECN
   field in their outermost IP header set to the appropriate PCN
   codepoint.  PCN-egress-nodes then guarantee that the ECN field of any
   packet leaving the PCN-domain has appropriate ECN semantics.  This
   prevents unintended leakage of ECN marks into or out of the PCN-
   domain, and thus reduces backward-compatibility issues.

6.2.  Backward Compatibility with the Baseline Encoding

   A PCN node implemented to use the obsoleted baseline encoding could
   conceivably have been configured so that the Threshold-meter function
   marked what is now defined as the ETM codepoint in the 3-in-1
   encoding.  However, thre is no known deployment of such an
   implementation and no reason to believe that such an implementation
   would ever have been built.  Therefore, it seems safe to ignore this
   issue.

7.  IANA Considerations

   This memo includes no request to IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.

8.  Security Considerations

   PCN-marking only carries a meaning within the confines of a PCN-
   domain.  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.  However, such considerations are beyond the scope of
   this document.

   One potential security concern is the injection of spurious PCN-marks
   into the PCN-domain.  However, these can only enter the domain if a
   PCN-ingress-node is misconfigured.  The precise impact of any such
   misconfiguration will depend on which of the proposed PCN-boundary-
   node behaviours is used, but in general spurious marks will lead to
   admitting fewer flows into the domain or potentially terminating too
   many flows.  In either case, good management should be able to
   quickly spot the problem since the overall utilisation of the domain
   will rapidly fall.

9.  Conclusions

   The 3-in-1 PCN encoding uses a PCN-compatible DSCP and the ECN field
   to encode PCN-marks.  One codepoint allows non-PCN traffic to be
   carried with the same PCN-compatible DSCP and three other codepoints
   support three PCN marking states with different levels of severity.
   In general, the use of this PCN encoding scheme presupposes that any
   tunnel endpoints within the PCN-domain comply with [RFC6040].

10.  Acknowledgements

   Many thanks to Phil Eardley for providing extensive feedback,
   critcism and advice.  Thanks also to Teco Boot, Kwok Ho Chan,
   Ruediger Geib, Georgios Karaginannis and everyone else who has
   commented on the document.

11.  Comments Solicited

   To be removed by 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 <pcn@ietf.org>, 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.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              December 1998.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, September 2001.

   [RFC5559]  Eardley, P., "Pre-Congestion Notification (PCN)
              Architecture", RFC 5559, June 2009.

   [RFC5670]  Eardley, P., "Metering and Marking Behaviour of PCN-
              Nodes", RFC 5670, November 2009.

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, November 2010.

12.2.  Informative References

   [I-D.ietf-pcn-cl-edge-behaviour]
              Charny, A., Huang, F., Karagiannis, G., Menth, M., and T.
              Taylor, "PCN Boundary Node Behaviour for the Controlled
              Load (CL) Mode of Operation",
              draft-ietf-pcn-cl-edge-behaviour-09 (work in progress),
              June 2011.

   [I-D.ietf-pcn-encoding-comparison]
              Karagiannis, G., Chan, K., Moncaster, T., Menth, M.,
              Eardley, P., and B. Briscoe, "Overview of Pre-Congestion
              Notification Encoding",
              draft-ietf-pcn-encoding-comparison-06 (work in progress),
              June 2011.

   [I-D.ietf-pcn-sm-edge-behaviour]
              Charny, A., Karagiannis, G., Menth, M., and T. Taylor,
              "PCN Boundary Node Behaviour for the Single Marking (SM)
              Mode of Operation", draft-ietf-pcn-sm-edge-behaviour-06
              (work in progress), June 2011.

   [RFC2597]  Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
              "Assured Forwarding PHB Group", RFC 2597, June 1999.

   [RFC3246]  Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
              J., Courtney, W., Davari, S., Firoiu, V., and D.
              Stiliadis, "An Expedited Forwarding PHB (Per-Hop
              Behavior)", RFC 3246, March 2002.

   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
              Congestion Notification (ECN) Signaling with Nonces",
              RFC 3540, June 2003.

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              August 2006.

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

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

   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
              Marking in MPLS", RFC 5129, January 2008.

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, February 2009.

   [RFC5696]  Moncaster, T., Briscoe, B., and M. Menth, "Baseline
              Encoding and Transport of Pre-Congestion Information",
              RFC 5696, November 2009.

   [RFC5865]  Baker, F., Polk, J., and M. Dolly, "A Differentiated
              Services Code Point (DSCP) for Capacity-Admitted Traffic",
              RFC 5865, May 2010.

Appendix A.  Choice of Suitable DSCPs

   This appendix is informative, not normative.

   The PCN working group chose not to define a

   A single DSCP has not been defined 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 is not a scheduling
   behaviour -- rather, it should be seen as being 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 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 sufficient
   aggregation, the appropriate DSCPs would currently be those for the
   Real-Time Treatment Aggregate [RFC5127].  The PCN
   working group suggests using  It is suggested that
   admission control could be used for the following service classes
   (defined in [RFC4594]): [RFC4594] unless otherwise stated):

   o  Telephony (EF)

   o  Real-time interactive (CS4)
   o  Broadcast Video (CS3)

   o  Multimedia Conferencing (AF4)

   o  the VOICE-ADMIT codepoint defined in [RFC5865].

   CS5 is excluded from this list since PCN is not expected to be
   applied to signalling traffic.  PCN can also be applied to the VOICE-
   ADMIT codepoint defined in [RFC5865].

   PCN-marking is intended to provide a scalable admission-control
   mechanism for traffic with a high degree of statistical multiplexing.
   PCN-marking would therefore be appropriate to apply to traffic in the
   above classes, but only within a PCN-domain containing sufficiently
   aggregated traffic.  In such cases, the 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
   [RFC5127].

   Additional service classes may be defined for which admission control
   is appropriate, whether through some future standards action or
   through local use by certain operators, e.g., the Multimedia
   Streaming service class (AF3).  This document does not preclude the
   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 as their mechanism of
   choice.

Appendix B.  Co-existence of ECN and PCN

   This appendix is informative, not normative.

   The PCN encoding described in this document re-uses the bits of the
   ECN field in the IP header.  Consequently, this disables ECN within
   the PCN domain.  Appendix B of [RFC5696] (obsoleted) included advice
   on handling ECN traffic within a PCN-domain.  This appendix
   reiterates and clarifies that advice.

   For the purposes of this appendix we define two forms of traffic that
   might arrive at a PCN-ingress node.  These are Admission-controlled
   traffic and Non-admission-controlled traffic.

   Admission-controlled traffic will be remarked re-marked to a PCN-compatible
   DSCP by the PCN-ingress node.  Two mechanisms can be used to identify
   such traffic:

   a.  flow signalling associates a filterspec with a need for admission
       control (e.g. through RSVP or some equivalent message, e.g. from
       a SIP server to the ingress), and the PCN-ingress remarks re-marks
       traffic matching that filterspec to a PCN-compatible DSCP, as its
       chosen admission control mechanism.

   b.  Traffic arrives with a DSCP that implies it requires admission
       control such as VOICE-ADMIT [RFC5865] or Interactive Real-Time,
       Broadcast TV when used for video on demand, and Multimedia
       Conferencing [RFC4594][RFC5865]. [RFC4594][RFC5865] (see Appendix A).

   All other traffic can be thought of as Non-admission-controlled (and
   therefore outside the scope of PCN).  However such traffic may still
   need to share the same DSCP as the Admission-controlled traffic.
   This may be due to policy (for instance if it is high priority voice
   traffic), or may be because there is a shortage of local DSCPs.

   ECN [RFC3168] is an end-to-end congestion notification mechanism.  As
   such it is possible that some traffic entering the PCN-domain may
   also be ECN capable.

   Unless specified otherwise, for any of the cases in the list below,
   an IP-in-IP tunnel can be used to preserve ECN markings across the
   PCN domain.  However the  The tunnelling action should be applied wholly outside
   the PCN-domain as illustrated in the following figure:

                ,  .  .  .  .  .  PCN-domain  .  .  .  .  .  .
               .   ,--------.                   ,--------.    .
              .   _|  PCN-   |___________________|  PCN-  |_   .
              .  / | ingress |                   | egress | \  .
               .|  '---------'                   '--------'  |.
                | .  .  .  .  .  .  .  .  .  .  .  .  .  .  .|
           ,--------.                                     ,--------.
     _____| Tunnel  |                                     | Tunnel |____
          | Ingress | - - ECN preserved inside tunnel - - | Egress |
          '---------'                                     '--------'

             Figure 2: Separation of tunneling and PCN actions

   There are four three cases for how e2e ECN traffic may wish to be treated
   while crossing a PCN domain:

   ECN capable traffic that does

   a) Does not require admission control and does control:

      *  Does not carry a DSCP that the PCN-ingress is using for PCN-capable
   traffic.  This requires no action.

   ECN capable traffic that does not require admission control but
   carries PCN-compatible DSCP: No action required.

      *  Arrives carrying a DSCP that uses the   PCN-ingress is using for PCN-capable
   traffic. same codepoint as a PCN-
         compatible DSCP: There are two options.

      * options:

         1.  The ingress maps the DSCP to a local DSCP with the same
             scheduling PHB as the original DSCP, and the egress re-maps
             it to the original PCN-compatible DSCP.

      *

         2.  The ingress tunnels the traffic, setting not-PCN in the
             outer header; note that this turns off ECN for this traffic
             within the PCN domain.

         The first option is recommended unless the operator is short of
         local DSCPs.

   ECN-capable Admission-controlled traffic:

   b) Requires Admission-control:  There are two options.

      *  The PCN-ingress places this traffic in a tunnel with a PCN-
         compatible DSCP in the outer header.  The PCN-egress zeroes the
         ECN-field before decapsulation.

      *  The PCN-ingress drops CE-marked packets and the PCN-egress
         zeros the ECN field of all PCN packets.

      The second option is emphatically not recommended, unless perhaps
      as a last resort if tunnelling is not possible for some
      insurmountable reason.

   ECN-capable Admission-controlled traffic where the e2e transport
   somehow indicates that it wants

   c) Requires Admission Control and asks to see PCN marks:  NOTE this
      scheme is currently only a tentative only.

      Schemes idea.

      For real-time data generated by an adaptive codec, schemes have
      been suggested where PCN marks may be leaked out of the PCN-domain and used by the
      so that end hosts can drop to modify realtime a lower data
      rates. rate, thus deferring
      the need for admission control.  Currently all such schemes require
      further study and the following is for guidance only.

      The PCN-ingress needs to tunnel the traffic, traffic as in Figure 2, taking
      care to comply with [RFC6040].  In this case the PCN-egress should
      not zero the ECN field, and then the [RFC6040] tunnel egress will
      preserve any PCN-marking.  Note that a PCN interior node may turn
      ECT(0) into ECT(1), which would not be compatible with the
      (currently experimental) ECN nonce [RFC3540].

Appendix C.  Example Mapping between Encoding of PCN-Marks in IP and in
             MPLS Shim Headers

   This appendix is informative not normative.

   The 6 bits of the DS field in the IP header provide for 64
   codepoints.  When encapsulating IP traffic in MPLS, it is useful to
   make the DS field information accessible in the MPLS header.
   However, the MPLS shim header has only a 3-bit traffic class (TC)
   field [RFC5462] providing for 8 codepoints.  The operator has the
   freedom to define a site-local mapping of a subset of the 64 codepoints of the DS
   field to onto the 8 codepoints in the TC field.

   [RFC5129] describes how ECN markings in the IP header can also be
   mapped to codepoints in the MPLS TC field.  Appendix A of [RFC5129]
   gives an informative description of how to support PCN in MPLS by
   extending the way MPLS supports ECN.  But [RFC5129] was written while
   PCN specifications were in early draft stages.  The following
   provides a clearer example of a mapping between PCN in IP and in MPLS
   using the PCN terminology and concepts that have since been
   specified.

   To support PCN in a MPLS domain, a PCN-compatible DSCP ('DSCP n')
   needs codepoints for all used PCN-marks
   need to be provided in the TC field. field for all the PCN-marks
   used.  That means, when e.g. for instance only excess-traffic-marking is
   used for PCN purposes, the operator needs to define a site-local
   mapping to two codepoints in the MPLS TC field for IP headers with:

   o  DSCP n and ECT(0)

   o  DSCP n and CE

   If both excess-traffic-marking and threshold-marking are used, the
   operator needs to define a site-local mapping to codepoints in the
   MPLS TC field for IP headers with all three of the 3-in-1 codepoints:

   o  DSCP n and ECT(0)

   o  DSCP n and ECT(1)

   o  DSCP n and CE

   In either case, if the operator wishes to support the same Diffserv
   PHB but without PCN marking, it will also be necessary to define a
   site-local mapping to an MPLS TC codepoint for IP headers marked
   with:

   o  DSCP n and Not-ECT

   Clearly, given so few TC codepoints are available, it may be
   necessary to compromise by merging together some capabilities.

Appendix D.  Rationale for different behaviours for single marking
             schemes Discrepancy Between the Schemes using One
             PCN-Marking

   Readers may notice an apparent discrepancy between the two single
   marking behaviours
   in Section 5.2.3.1 and Section 5.2.3.2.  For the
   excess-traffic  With only excess-traffic
   marking enabled, an unexpected ThM marked packet is
   remarked as can be re-marked to ETM.  For the threshold
   However, with only threshold marking, an unexpected ETM
   marked packet cannot
   be re-marked to ThM.

   This apparent inconsistency is simply ignored (apart from an optional management
   alarm).

   There are two reasons deliberate, for having these seemingly contradictory
   requirements.  Firstly these behaviours conform with the expected
   behaviour where both metering functions are being two reasons:

   o  If only one type of marking function is meant to be used for marking--
   ETM
      throughout the PCN-domain but the other type unexpectedly appears
      on some packets, it is always a more severe marking than ThM and so should never safest to assume that some link is trying
      to signal that it is pre-congested, but that it is somehow using
      the wrong signal.  This only needs to be
   re-marked.  Secondly corrected if the threshold-metering
      behaviour in [RFC5670]
   uses of other nodes depends on the current marking state of a packet arrives
      with.  In [RFC5670], the excess-traffic-metering behaviour depends
      on the markings on arriving packet packets, whereas threshold-metering
      does not.  Therefore, if ThM should not be present, it seems safe
      to determine
   what action allow it to take.  Consequently, in the be re-marked to ETM, but if ETM should not be
      present there is no need to re-mark it to ThM.

   o  The behaviour with only threshold marking it would keeps to the rule that
      ETM is more severe and must never be potentially unsafe changed to allow ThM packets to propagate forward even though
      ETM is not a valid marking in
   the network as they may adversely affect the threshold-metering
   function. this case.  Otherwise
      implementations would have to allow operators to configure an
      exception to this rule, which would not be safe practice.

Authors' Addresses

   Bob Briscoe
   BT
   B54/77, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE
   UK

   Phone: +44 1473 645196
   Email: bob.briscoe@bt.com
   URI:   http://bobbriscoe.net/
   Toby Moncaster
   Moncaster Internet Consulting
   Dukes
   Layer Marney
   Colchester  CO5 9UZ
   UK

   Phone: +44 7764 185416
   Email: toby@moncaster.com
   URI:   http://www.moncaster.com/

   Michael Menth
   University of Tuebingen
   Sand 13
   Tuebingen  72076
   Germany

   Phone: +49 7071 2970505
   Email: menth@informatik.uni-tuebingen.de