Congestion and Pre-Congestion                                 B. Briscoe
Notification                                                          BT
Internet-Draft                                              T. Moncaster
Intended status: Experimental                                Independent              Moncaster Internet Consulting
Expires: January 13, July 15, 2011                                          M. Menth
                                                 University of Wuerzburg
                                                           July 12, 2010 Tuebingen
                                                        January 11, 2011

       Encoding 3 PCN-States in the IP header using a single DSCP


   The objective of Pre-Congestion Notification (PCN) is to protect the
   quality of service (QoS) of inelastic flows within a Diffserv domain.
   On every link in the PCN domain, the overall rate of the PCN-traffic
   is metered, and PCN-packets are appropriately marked when certain
   configured rates are exceeded.  Egress nodes provide decision points
   with information about the PCN-marks of PCN-packets which allows them
   to take decisions about whether to admit or block a new flow request,
   and to terminate some already admitted flows during serious pre-

   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 builds on the baseline
   encoding of RFC5696 and provides for 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

Status of this Memo

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   This Internet-Draft will expire on January 13, July 15, 2011.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Changes in This Version (to be removed by RFC Editor)  . .  4
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Requirements for and Applicability of 3-in-1 PCN Encoding  . .  5
     3.1.  PCN Requirements . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Requirements Imposed by Baseline Encoding  . . . . . . . .  6
     3.3.  Applicability of 3-in-1 PCN Encoding . . . . . . . . . . .  6  7
   4.  Definition of 3-in-1 PCN Encoding  . . . . . . . . . . . . . .  7
   5.  Behaviour of a PCN Node Compliant with the 3-in-1 PCN
       Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . .  7  8
   6.  Backward Compatibility . . . . . . . . . . . . . . . . . . . .  8
     6.1.  Backward Compatibility with Pre-existing PCN
           Implementations  . . . . . . . . . . . . . . . . . . . . .  8
     6.2.  Recommendations for the Use of PCN Encoding Schemes  . . .  9
       6.2.1.  Use of Both Excess-Traffic-Marking and
               Threshold-Marking  . . . . . . . . . . . . . . . . . .  9
       6.2.2.  Unique Use of Excess-Traffic-Marking . . . . . . . . .  9
       6.2.3.  Unique Use of Threshold-Marking  . . . . . . . . . . .  9 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   9.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 10 11
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 11
     12.2. Informative References . . . . . . . . . . . . . . . . . . 11 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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 decide whether to terminate some already admitted
   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 "pre-congestion


   [RFC5670] provides for two metering and marking functions are proposed in [RFC5670] that are
   configured with reference rates.  Threshold- marking  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 provide information about the PCN-marks to
   decision points which take decisions about flow admission and
   termination on this basis [I-D.ietf-pcn-cl-edge-behaviour],

   The baseline encoding defined in [RFC5696] describes how two PCN
   marking states (Not-marked and PCN-Marked) can be encoded using a
   single Diffserv codepoint.
   However, to  It also provides an experimental
   codepoint (EXP), along with guidelines for use of that codepoint.  To
   support the application of two different marking algorithms in a PCN-domain, PCN-
   domain, for example as required in [I-D.ietf-pcn-cl-edge-behaviour],
   three PCN marking states are needed.  This document describes an
   extension to the baseline encoding that adds uses the EXP codepoint to
   provide a third PCN marking state in the IP header, still using a
   single Diffserv codepoint.  This encoding scheme is called
   ao˛3-in-1 "3-in-1
   PCN encodingaoˇ.

   All PCN encoding schemes require an additional marking state to
   indicate non-PCN traffic.  Therefore, four codepoints are required to
   encode three PCN marking states. encoding".

   This document only concerns the PCN wire protocol encoding for all IP
   headers, whether IPv4 or IPv6.  It makes no changes or
   recommendations concerning algorithms for congestion marking or
   congestion response.  Other documents define the PCN wire protocol
   for other header types.  For example, the MPLS encoding is defined in
   [RFC5129].  Appendix A provides an informative example for a mapping
   between the encodings in IP and in MPLS.

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

   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

      *  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

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

      *  Introduction altered to include new template description of

      *  References updated.

      *  Terminology brought into line with [RFC5670].

      *  Minor corrections.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.1.  Terminology

   General PCN-related terminology is defined in the PCN architecture
   [RFC5559], and terminology specific to packet encoding is defined in
   the PCN baseline encoding [RFC5696].  Additional terminology is
   defined below.

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

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

3.1.  PCN Requirements


   In accordance with the PCN architecture [RFC5559] defines that [RFC5559], PCN-ingress-nodes of a
   control incoming packets. packets entering a PCN-domain.  Packets belonging to PCN-
   controlled flows are subject to PCN metering PCN-metering and marking, they are
   termed PCN-packets, -marking, and PCN-ingress-nodes PCN-
   ingress-nodes mark them as not-marked Not-marked (PCN-colouring).  Any node in
   the PCN-domain may perform PCN metering PCN-metering and marking -marking and mark PCN-packets PCN-
   packets if needed.  There are two different metering and marking
   schemes: threshold-marking and excess-traffic-
   marking excess-traffic-marking [RFC5670].
   Some edge behaviors require only a single marking scheme
   [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].  As threshold-marking  Threshold-marking and
   excess-traffic-marking are configured to start marking packets at
   different load conditions, so one marking scheme indicates more
   severe pre-congestion than the other in terms of higher load.  If a packet has been marked
   by both a threshold-marker and an excess-traffic-marker, it is marked
   with the more severe state. other.  Therefore, a fourth PCN
   marking state indicating that a packet is marked by both markers is
   not needed.

   Nonetheless, in addition to codepoints for the three PCN marking
   states  However a fourth codepoint is required to indicate
   packets that are not PCN-capable (termed the (the not-PCN codepoint).

   In all current PCN edge behaviors that use two marking schemes
   [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.

3.2.  Requirements Imposed by Baseline Encoding

   The baseline encoding scheme [RFC5696] was defined so that it could
   be extended to accommodate an additional marking state.  It provides
   rules to embed the encoding of two PCN states in the IP header.
   Figure 1 shows the structure of the former type-of-service field.  It
   contains the 6-bit Differentiated Services (DS) field that holds the
   DS codepoint (DSCP) [RFC2474] and the 2-bit ECN field [RFC3168].

            0     1     2     3     4     5     6     7
         |              DS FIELD             | ECN FIELD |

       Figure 1: Structure of the former type-of-service field in IP

   Baseline encoding defines that the DSCP must be set to a PCN-
   compatible DSCP n and the ECN-field [RFC3168] indicates the specific
   PCN-mark.  Baseline encoding offers four possible encoding states
   within a single DSCP with the following restrictions.

   o  Codepoint `00' (not-ECT) is used to indicate non-PCN traffic as
      "not-PCN".  This allows the use of a DSCP for both PCN and non-PCN
      traffic. traffic to use the
      same DSCP.

   o  Codepoint `10' (ECT(0)) is used to indicate Not-marked PCN

   o  Codepoint `11' (CE) is used to indicate the most severe PCN-mark.

   o  Codepoint `01' (ECT(1)) is available for experimental use and may
      be re-used by other PCN encodings such as the presently defined
      3-in-1 PCN encoding.

3.3.  Applicability of 3-in-1 PCN Encoding

   When PCN traffic is tunneled IP-in-IP within a PCN-domain, PCN-marks
   must be preserved encoding (subject to the rules defined in all outer IP headers after encapsulation and
   decapsulation.  This property is violated by legacy [RFC5696]).

   [RFC6040] defines rules for the encapsulation and decapsulation rules [RFC3168], [RFC4301] due to the way they treat
   the of
   ECN field. markings within IP-in-IP tunnels.  This led to strong limitations regarding how PCN-
   marks can be encoded using the ECN field RFC removes some of the IP header
   [I-D.ietf-pcn-encoding-comparison].  Therefore, baseline encoding
   constraints that existed when [RFC5696] was defined which works well with legacy tunnels but
   supports only two PCN marking states.

   Since then, new written.  Happily the
   rules have been defined for IP-in-IP tunneling
   [I-D.ietf-tsvwg-ecn-tunnel] so that use of the present 3-in-1 PCN encoding
   has EXP codepoint are fully compatible with

   [RFC6040].  In particular, the relative severity of each marking is
   the same: CE (PM) is more freedom to accommodate PCN-marks using severe than ECT(1) (EXP) is more severe
   than ECT(0) (NM).  This is discussed in more detail in both the ECN field.  From
   this follows that
   baseline encoding document [RFC5696] and in

3.3.  Applicability of 3-in-1 PCN Encoding

   The 3-in-1 encoding may is applicable in situations where two marking
   schemes are being used in the PCN-domain.  In some circumstances it
   can also be applied used in PCN-domains with only a single marking scheme in PCN-
   domains that comply
   use.  Further guidance on choosing an encoding scheme can be found in
   Section 6.2.  All nodes within the PCN-domain MUST be fully compliant
   with [I-D.ietf-tsvwg-ecn-tunnel] or do the ECN encapsulation rules set out in [RFC6040].  As such the
   encoding is not use
   tunneling. applicable in situations where legacy tunnels might

4.  Definition of 3-in-1 PCN Encoding

   The 3-in-1 PCN encoding scheme is an extension of the baseline
   encoding scheme defined in [RFC5696].  The PCN requirements and the
   extension rules for baseline encoding presented in the previous
   section determine how PCN encoding states are carried in the IP
   headers.  This is shown in Figure 2.

         |        |           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 2: 3-in-1 PCN Encoding

   Like baseline encoding, 3-in-1 PCN encoding also uses a PCN
   compatible DSCP n and the ECN field for the encoding of PCN-marks.
   The PCN-marks have the following meaning.

   Not-PCN:  indicates a non-PCN-packet, i.e., a packet that 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].

5.  Behaviour of a PCN Node Compliant with the 3-in-1 PCN Encoding

   To be compliant with the 3-in-1 PCN Encoding, an PCN interior node
   behaves as follows:

   o  It MUST change NM to ThM if the threshold-meter function indicates
      a need to mark the packet. packet;

   o  It MUST change NM or ThM to ETM if the excess-traffic-meter
      function indicates a need to mark the packet. packet;

   o  It MUST NOT change not-PCN to NM, ThM, or ETM, and ETM;

   o  It MUST NOT change a NM, ThM, or ETM to not-PCN;

   o  It MUST NOT change ThM to NM;

   o  It MUST NOT change ETM to ThM or to NM;

   In other words, a PCN interior node MUST NOT mark PCN-packets into
   non-PCN packets and vice-versa, and it may increase the severity of
   the PCN-mark of a PCN-packet, but it MUST NOT decrease it.

6.  Backward Compatibility

   Discussion of backward compatibility between PCN encoding schemes and
   previous uses of the ECN field is given in Section 6 of [RFC5696].

6.1.  Backward Compatibility with Pre-existing PCN Implementations

   This encoding complies with the rules for extending the baseline PCN
   encoding schemes in Section 5 of [RFC5696].

   The term "compatibility" is meant in the following sense.  It is
   possible to operate nodes with baseline encoding [RFC5696] and 3-in-1
   encoding in the same PCN domain.  The nodes with baseline encoding
   MUST perform excess-traffic-marking because the 11 codepoint of
   3-in-1 encoding also means excess-traffic-marked.  PCN-boundary-nodes
   of such domains are required to interpret the full 3-in-1 encoding
   and not just baseline encoding, otherwise they cannot interpret the
   01 codepoint.

   Using nodes that perform only excess-traffic-marking may make sense
   in networks using the CL edge behavior
   [I-D.ietf-pcn-cl-edge-behaviour].  Such nodes are able to notify the
   egress only about severe pre-congestion when traffic needs to be
   terminated.  This seems reasonable for locations that are not
   expected to see any pre-congestion, but excess-traffic-marking gives
   them a means to terminate traffic if unexpected overload still occurs.

6.2.  Recommendations for the Use of PCN Encoding Schemes

   NOTE: This sub-section is informative not normative.

   When deciding which PCN encoding is suitable an operator needs to
   take account of how many PCN states need to be encoded.  The
   following table gives guidelines on which encoding to use with either
   threshold-marking, excess-traffic marking or both.

         |  Used marking schemes  |  Recommended PCN encoding scheme   |
         | Only threshold-marking |   Baseline encoding [RFC5696]  |
         | Only excess-traffic-   |   Baseline encoding [RFC5696]  |
         |       marking          |     or 3-in-1 PCN encoding     |
         | Threshold-marking and  |     3-in-1 PCN encoding        |
         | excess-traffic-marking |                                |

          Figure 3: Use of Guidelines for choosing PCN encoding schemes

   Figure 3 gives guidelines under which conditions baseline encoding
   and 3-in-1 PCN encoding would typically be used.

6.2.1.  Use of Both Excess-Traffic-Marking and Threshold-Marking

   If both excess-traffic-marking and threshold-marking are enabled in a
   PCN-domain, 3-in-1 encoding should be used as described in this

6.2.2.  Unique Use of Excess-Traffic-Marking

   If only excess-traffic-marking is enabled in a PCN-domain, baseline
   encoding or 3-in-1 encoding may be used.  They lead to the same
   encoding because PCN-boundary nodes will interpret baseline "PCN-
   marked (PM)" as "excess-traffic-marked (ETM)".

6.2.3.  Unique Use of Threshold-Marking

   No scheme is currently proposed to that solely use uses threshold-marking.
   However, if only threshold-marking is enabled in
   If such a PCN-domain,
   baseline encoding SHOULD be used.  This scheme is because threshold marking
   will work in combination with legacy tunnel decapsulators within proposed, the
   PCN-domain, while using threshold marking choice of encoding scheme will
   depend on whether nodes are compliant with [RFC6040] or not.  Where
   it is certain that all nodes in the PCN-domain are compliant then
   either 3-in-1 encoding
   requires that or baseline encoding are suitable.  If legacy
   tunnel decapsulators exist within a the PCN-domain comply with
   [I-D.ietf-tsvwg-ecn-tunnel]. then baseline
   encoding SHOULD be used.

7.  IANA Considerations

   This memo includes no request to IANA.

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

8.  Security Considerations

   The security concerns relating to this extended PCN encoding are the
   same as those in [RFC5696].  In summary, PCN-boundary nodes are
   responsible for ensuring inappropriate PCN markings do not leak into
   or out of a PCN domain, and the current phase of the PCN architecture
   assumes that all the nodes of a PCN-domain are entirely under the
   control of a single operator, or a set of operators who trust each

   Given the only difference between the baseline encoding and the
   present 3-in-1 encoding is the use of the 01 codepoint, no new
   security issues are raised, as this codepoint was already available
   for experimental use in the baseline encoding.

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.
   The use of this PCN encoding scheme presupposes that any tunnels in
   the PCN region have been updated to comply with
   [I-D.ietf-tsvwg-ecn-tunnel]. [RFC6040].

10.  Acknowledgements

   Thanks to Phil Eardley, Teco Boot, and Kwok Ho Chan for reviewing
   this 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 <>, 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.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

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

   [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.

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

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

12.2.  Informative References

              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-08 (work in progress),
              December 2010.

              Chan, K., Karagiannis, G., Moncaster, T., Menth, M.,
              Eardley, P., and B. Briscoe, "Pre-Congestion Notification
              Encoding Comparison",
              draft-ietf-pcn-encoding-comparison-03 (work in progress),
              October 2010.

              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-03
              (work in progress), June 2010.

              Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", draft-ietf-tsvwg-ecn-tunnel-08 draft-ietf-pcn-sm-edge-behaviour-05
              (work in progress), March December 2010.

Authors' Addresses

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

   Phone: +44 1473 645196

   Toby Moncaster
   Moncaster Internet Consulting
   Layer Marney
   Colchester  CO5 9UZ

   Phone: +44 7764 185416
   Michael Menth
   University of Wuerzburg
   room B206, Institute of Computer Science
   Am Hubland
   Wuerzburg  97074 Tuebingen
   Sand 13
   72076 Tuebingen

   Phone: +49 931 31 86644 7071 2970505