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Versions: (draft-briscoe-pcn-3-in-1-encoding) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 6660

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


       Encoding 3 PCN-States in the IP header using a single DSCP
                   draft-ietf-pcn-3-in-1-encoding-03

Abstract

   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-
   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 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
   encoding.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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



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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


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
   4.  Definition of 3-in-1 PCN Encoding  . . . . . . . . . . . . . .  7
   5.  Behaviour of a PCN Node Compliant with the 3-in-1 PCN
       Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   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
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   9.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 10
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 11
     12.2. Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12





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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 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 congestion occurs (hence "pre-
   congestion notification").

   Two metering and marking functions are proposed in [RFC5670] that are
   configured with 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 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],
   [I-D.ietf-pcn-sm-edge-behaviour].

   The baseline encoding defined in [RFC5696] describes how two PCN
   marking states can be encoded using a single Diffserv codepoint.
   However, to support the application of two different marking
   algorithms in a 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 a third PCN marking state in the IP header, still
   using a single Diffserv codepoint.  This encoding scheme is called
   ao˛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.

   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.



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

   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
         [I-D.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.







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2.  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].

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

   The PCN architecture [RFC5559] defines that PCN-ingress-nodes of a
   PCN-domain control incoming packets.  Packets belonging to PCN-
   controlled flows are subject to PCN metering and marking, they are
   termed PCN-packets, 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 schemes: threshold-marking and 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
   and excess-traffic-marking start marking packets at different load
   conditions, 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.  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 a fourth codepoint is required to indicate packets that are
   not PCN-capable (termed 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



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   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.

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

   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 in all outer IP headers after encapsulation and
   decapsulation.  This property is violated by legacy encapsulation and
   decapsulation rules [RFC3168], [RFC4301] due to the way they treat
   the ECN field.  This led to strong limitations regarding how PCN-
   marks can be encoded using the ECN field of the IP header
   [I-D.ietf-pcn-encoding-comparison].  Therefore, baseline encoding
   [RFC5696] was defined which works well with legacy tunnels but
   supports only two PCN marking states.



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   Since then, new rules have been defined for IP-in-IP tunneling
   [I-D.ietf-tsvwg-ecn-tunnel] so that the present 3-in-1 PCN encoding
   has more freedom to accommodate PCN-marks using the ECN field.  From
   this follows that 3-in-1 PCN encoding may be applied only in PCN-
   domains that comply with [I-D.ietf-tsvwg-ecn-tunnel] or do not use
   tunneling.


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:




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

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

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






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6.2.  Recommendations for the Use of PCN Encoding Schemes

   This sub-section is informative not normative.
         +------------------------+------------------------------------+
         |  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 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
   document.

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 solely use threshold-marking.
   However, if only threshold-marking is enabled in a PCN-domain,
   baseline encoding SHOULD be used.  This is because threshold marking
   will work in combination with legacy tunnel decapsulators within the
   PCN-domain, while using threshold marking with the 3-in-1 encoding
   requires that tunnel decapsulators within a PCN-domain comply with
   [I-D.ietf-tsvwg-ecn-tunnel].


7.  IANA Considerations

   This memo includes no request to IANA.




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


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
   other.

   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].


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 <pcn@ietf.org>, and/or to
   the authors.


12.  References





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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.

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-06 (work in progress),
              June 2010.

   [I-D.ietf-pcn-encoding-comparison]
              Chan, K., Karagiannis, G., Moncaster, T., Menth, M.,
              Eardley, P., and B. Briscoe, "Pre-Congestion Notification
              Encoding Comparison",
              draft-ietf-pcn-encoding-comparison-02 (work in progress),
              March 2010.

   [I-D.ietf-pcn-sm-edge-behaviour]
              Charny, A., Karagiannis, G., Menth, M., and T. Taylor,



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              "PCN Boundary Node Behaviour for the Single Marking (SM)
              Mode of Operation", draft-ietf-pcn-sm-edge-behaviour-03
              (work in progress), June 2010.

   [I-D.ietf-tsvwg-ecn-tunnel]
              Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", draft-ietf-tsvwg-ecn-tunnel-08 (work in
              progress), March 2010.


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
   Independent

   Email: toby@moncaster.com


   Michael Menth
   University of Wuerzburg
   room B206, Institute of Computer Science
   Am Hubland
   Wuerzburg  97074
   Germany

   Phone: +49 931 31 86644
   Email: menth@informatik.uni-wuerzburg.de












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