SR Policies Extensions for Path Segment and Bidirectional Path in BGP-LS
draft-ietf-idr-bgp-ls-sr-policy-path-segment-10

Interdomain Routing Working Group                                  C. Li
Internet-Draft                                                     Z. Li
Intended status: Standards Track                     Huawei Technologies
Expires: 17 April 2026                                            Y. Zhu
                                                           China Telecom
                                                                W. Cheng
                                                            China Mobile
                                                           K. Talaulikar
                                                           Cisco Systems
                                                         14 October 2025

SR Policies Extensions for Path Segment and Bidirectional Path in BGP-LS
            draft-ietf-idr-bgp-ls-sr-policy-path-segment-10

Abstract

   This document specifies the way of collecting configuration and
   states of SR policies carrying Path Segment and bidirectional path
   information by using BPG-LS.  Such information can be used by
   external conponents for many use cases such as performance
   measurement, path re-optimization and end-to-end protection.

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 RFC 2119 [RFC2119].

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
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   This Internet-Draft will expire on 17 April 2026.

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Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Carrying SR Path Sub-TLVs in BGP-LS . . . . . . . . . . . . .   4
     3.1.  SR Path Segment Sub-TLV . . . . . . . . . . . . . . . . .   5
     3.2.  Reverse Segment List Sub-TLV  . . . . . . . . . . . . . .   7
   4.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Error Handling and Fault Management . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  BGP-LS TLVs . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Segment routing (SR) [RFC8402] is a source routing paradigm that
   allows the ingress node steers packets into a specific path according
   to the Segment Routing Policy [RFC9256].

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   However, the SR Policies defined in [RFC9256] only supports
   unidirectional SR paths and there is no path ID in a Segment List to
   identify an SR path.  For identifying an SR path and supporting
   bidirectional path [RFC9545], the Path Segment and Reverse Path
   Segment List Sub-TLVs are defined for the Tunnel Encapsulation
   Attribute [RFC9012] for the SR Policy tunnel in
   [I-D.ietf-idr-sr-policy-path-segment].  The Path Segment identifier
   can be a Path Segment in SR-MPLS [RFC9545] and SRv6
   [I-D.ietf-spring-srv6-path-segment], or other IDs that can identify
   the SR path.

   In many network scenarios, the configuration and state of each TE
   Policy is required by a controller which allows the network operator
   to optimize several functions and operations through the use of a
   controller aware of both topology and state information
   [I-D.ietf-idr-bgp-ls-sr-policy].

   To collect the TE Policy information that is locally available in a
   router, [I-D.ietf-idr-bgp-ls-sr-policy] describes a new mechanism by
   using BGP-LS update messages.

   Based on the mechanism defined in [I-D.ietf-idr-bgp-ls-sr-policy],
   this document describes a mechanism to distribute configuration and
   states of the new SR policies defined in
   [I-D.ietf-idr-sr-policy-path-segment] to external components using
   BGP-LS.

2.  Terminology

   This document makes use of the terms defined in [RFC8402] and
   [I-D.ietf-idr-bgp-ls-sr-policy].  Some existing and new terms are
   listed below for reference.

   *  SR: Segment Routing.

   *  SR-MPLS: Segment Routing over MPLS data plane.

   *  SRv6: Segment Routing over IPv6 data plane.

   *  PSID: Path Segment Identifier.

   *  SRPM: SR Policy Module [RFC9830].

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3.  Carrying SR Path Sub-TLVs in BGP-LS

   A mechanism to collect states of SR Policies via BGP-LS is proposed
   by [I-D.ietf-idr-bgp-ls-sr-policy].  The characteristics of an SR
   policy can be described by a TE Policy State TLV, which is carried in
   the optional non-transitive BGP Attribute "LINK_STATE Attribute"
   defined in [RFC9552].  The TE Policy State TLV contains several sub-
   TLVs such as SR TE Policy sub-TLVs.

   [I-D.ietf-idr-sr-policy-path-segment] defines the BGP extensions for
   Path Segment.  The encoding is shown below.

      SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
      Attributes:
         Tunnel Encaps Attribute (23)
            Tunnel Type: SR Policy
                Binding SID
                Preference
                Priority
                Policy Name
                Explicit NULL Label Policy (ENLP)
                Segment List
                    Weight
                    Path Segment
                    Segment
                    Segment
                    ...
                Segment List
                    Weight
                    Path Segment
                    Segment
                    Segment
                    ...
                ...

           Figure 1. Path Segment in SR policy

   Also, [I-D.ietf-idr-sr-policy-path-segment] defines SR policy
   extensions for bidirectional SR path, the encoding is shown below:

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       SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
           Attributes: Tunnel Encaps Attribute (23)
           Tunnel Type: SR Policy
               Binding SID
               Preference
               Priority
               Policy Name
               Explicit NULL Label Policy (ENLP)
               Segment List
                   Weight
                   Path Segment
                   Segment
                   Segment
                   ...
                   Reverse Segment List
                       Weight
                       Path Segment
                       Segment
                       Segment
                       ...

             Figure 2. SR policy for Bidirectional path

   In order to collect configuration and states of unidirectional and
   bidirectional SR policies defined in
   [I-D.ietf-idr-sr-policy-path-segment], this document defines new sub-
   TLVs in SR TE Policy sub-TLVs.

3.1.  SR Path Segment Sub-TLV

   This section defines the SR Path Segment sub-TLV to describe a Path
   Segment, and it can be included in the Segment List sub-TLV as
   defined in [I-D.ietf-idr-bgp-ls-sr-policy] . An SR Path Segment sub-
   TLV can be associated with an SR path specified by a Segment List
   sub-TLV.  Multiple Path Segment MAY be included in a Segment List for
   different use cases.  When all the SID Lists within a candidate path
   share the same Path Segment ID, the Path Segment can be used to
   collect the aggregated information of the candidate path.  The format
   of Path Segment TLV is shown below.

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Type           |             Length              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Flag           |             Reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Path Segment ID (4 or 16 octets)            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //   Sub-TLVs (variable)                                       //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      Figure 3. Path Segment sub-TLV

   Where,

   *  Type: to be assigned by IANA.

   *  Length: the total length of the value field not including Type and
      Length fields.

   *  Flags: 2 octet field that indicates attribute and status of the
      Path Segment.  The following bit positions are defined.  Other
      bits SHOULD be cleared by originator and MUST be ignored by
      receiver.

          0                   1
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |D|B| |L|                       |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

   *  -  D-Flag : Indicates the dataplane for the BSIDs.  This flag is
         set when Path Segment ID is a 16-octet SRv6 SID.  This flag is
         unset when the Path Segment ID is a 4-octet SR/MPLS label
         value.

      -  B-Flag: This flag when set indicates the presence of the SRv6
         Endpoint Behavior and SID Structure encoding specified in
         [RFC9514].  The B-Flag when unset (clear) means that SRv6
         Endpoint Behavior and SID Structure are not included.  The
         B-Flag MUST be ignored when D-flag is unset.  The B-Flag and
         D-Flag indicate the SRv6 Endpoint behavior and SID structure
         for the Path Segment ID value in the TLV.

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      -  L-Flag: Local flag.  Set when the Path Segment has local
         significance on an SR node.  Unset when the Path Segment does
         not have local significance on an SR node

   *  RESERVED: 2 octets.  SHOULD be set to 0 by originator and MUST be
      ignored by receiver.

   *  Path Segment ID: It indicates the Path Segment ID value based on
      the status flags.

   The SRv6 Endpoint Behavior TLV (1250) and the SRv6 SID Structure TLV
   (1252) defined in [RFC9514] MAY be used as sub-TLVs of the SR Path
   Segment Sub-TLV.  These optional sub-TLVS indicate the SRv6 Endpoint
   behavior and SID structure for the Path Segment ID value in the TLV
   when the Path Segment is an SRv6 Path Segment.

3.2.  Reverse Segment List Sub-TLV

   In some scenarios like mobile backhaul transport network, there are
   requirements to support bidirectional path.  In SR, a bidirectional
   path can be represented as a binding of two unidirectional SR paths
   [RFC9545].  An SR policy carrying SR bidirectional path information
   is expressed in Figure 2.  [I-D.ietf-idr-sr-policy-path-segment]
   defines a new sub-TLV to describe a reversed SR path of an SID list.

   This section defines a Reverse Segment List sub-TLV to specify a
   reverse SR path associated with the path specified by the Segment
   List, and it reuses the format of SR Segment List TLV defined in
   [I-D.ietf-idr-bgp-ls-sr-policy]:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Type             |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Flags            |           RESERVED            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             MTID              |   Algorithm   |    RESERVED   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Weight (4 octets)                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   sub-TLVs (variable)                                        //
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 5. Reverse Segment List Sub-TLV

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   All fields, except the type are defined in
   [I-D.ietf-idr-bgp-ls-sr-policy], and this TLV reuses it directly.
   The Type of this TLV is TBA.

   The SR Segment sub-TLV [I-D.ietf-idr-bgp-ls-sr-policy] MUST be
   included as an ordered set of sub-TLVs within the SR Segment List TLV
   when the SID-List is not empty.  A SID-List may be empty in certain
   cases (e.g.  for a dynamic path) where the headend has not yet
   performed the computation and hence not derived the segments required
   for the path; in such cases, the SR Segment List TLV SHOULD NOT
   include any SR Segment sub-TLVs [I-D.ietf-idr-bgp-ls-sr-policy].  In
   this case, the Path Segment Sub-TLV SHOULD NOT be included in the
   sub-TLVs field.

   Note: currently, only one reverse SID list is supported, so the
   weight field CAN be ignored when processing.  However, multiple
   reverse SID list MAY be supported in the future, and the use case of
   supporting this still need to be discussed.

4.  Operations

   The operations procedures of [RFC9552] can apply to this document.

   Typically but not limited to, the uni/bidirectional SR policies
   carrying path identification information can be distributed by the
   ingress node.

   Generally, BGP-LS is used for collecting link states and
   synchronizing with the external component.  The consumer of the uni/
   bidirectional SR policies carrying path identification information is
   not BGP LS process by itself.  This consumer can be any applications
   such as performance measurement [I-D.ietf-spring-stamp-srpm], path
   re- coputation or re-optimization.  The operation of sending
   information to other precesses is out of scope of this document

5.  Error Handling and Fault Management

   This document defines a new SR Path Segment sub-TLV included in the
   Segment List sub-TLV as defined in [I-D.ietf-idr-bgp-ls-sr-policy],
   therefore, the error handling defined in
   [I-D.ietf-idr-bgp-ls-sr-policy] can apply to this document.  The
   error handling as defined in [RFC7606] applies to new Sub-TLVs as
   well as SAFI context, therefore, the error handling in [RFC7606] also
   applies to this document.

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   Specifically, a BGP Speaker MUST perform Syntax validation of the SR
   Path Segment sub-TLV following the error handling defined in
   [RFC7606] and [I-D.ietf-idr-bgp-ls-sr-policy], to determine if it is
   malformed.  This includes the validation of the length of the Sub-TLV
   and the range of the value fileds.  If any validation check fails,
   the Update message MUST be handle as 'Treat-as-withdraw'.

   In addition, the validation of the individual fields of the TLVs/Sub-
   TLVs of the associated segment list are beyond the scope of BGP and
   out of the scope of this document.  A BGP implementation MUST NOT
   perform semantic verification of such fields nor consider the SR
   Policy update to be invalid or not usable based on such validation.
   An implementation SHOULD log any errors found during the above
   validation for further analysis.

6.  IANA Considerations

6.1.  BGP-LS TLVs

   IANA maintains a registry called "Border Gateway Protocol - Link
   State (BGP-LS) Parameters" with a sub-registry called "Node Anchor,
   Link Descriptor and Link Attribute TLVs".  The following TLV
   codepoints are suggested (for early allocation by IANA):

          Codepoint   Description                           Reference
          -------------------------------------------------------------
          TBA        Path Segment sub-TLV                  This document
          TBA        Reverse Segment List sub-TLV          This document

7.  Security Considerations

   Similar to [I-D.ietf-idr-bgp-ls-sr-policy], the security mechanisms
   of the base BGP security model [RFC4271] apply to the extensions
   described in this document.  Also, the new security considerations
   defined in [I-D.ietf-idr-bgp-ls-sr-policy] also apply to this
   document.

   The Path Segment extension is included in the SR Policy extension
   [I-D.ietf-idr-bgp-ls-sr-policy], so it does not introduce extra
   security problems comparing the existing SR policy entension.  The
   Path Segment information is critical to the path, and a wrong Path
   Segment ID may cause unexpected forwarding actions and results.

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   An implementation needs to make sure that the value of Path Segment
   ID is correct to avoid unexpected forwarding actions and results,
   especially in an SR-MPLS network.  In addition, the Path Segment
   information distribution from a router to an controller has to be
   protected.  The security considereations in
   [I-D.ietf-idr-bgp-ls-sr-policy] apply to this distribution procedure.

8.  Contributors

      Mach(Guoyi) Chen
      Huawei Technologies
      Huawei Campus, No. 156 Beiqing Rd.
      Beijing  100095
      China

      Email: Mach.chen@huawei.com

      Jie Dong
      Huawei Technologies
      Huawei Campus, No. 156 Beiqing Rd.
      Beijing  100095
      China

      Email: jie.dong@huawei.com

      James N Guichard
      Futurewei Technologies
      2330 Central Express Way
      Santa Clara
      USA

      Email: james.n.guichard@futurewei.com

9.  Acknowledgements

   Many thanks to Shraddha Hedge for her detailed review and
   professional comments.

10.  References

10.1.  Normative References

   [I-D.ietf-idr-bgp-ls-sr-policy]
              Previdi, S., Talaulikar, K., Dong, J., Gredler, H., and J.
              Tantsura, "Advertisement of Segment Routing Policies using

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              BGP Link-State", Work in Progress, Internet-Draft, draft-
              ietf-idr-bgp-ls-sr-policy-17, 6 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
              ls-sr-policy-17>.

   [I-D.ietf-spring-srv6-path-segment]
              Li, C., Cheng, W., Chen, M., Dhody, D., and Y. Zhu, "Path
              Segment Identifier (PSID) in SRv6 (Segment Routing in
              IPv6)", Work in Progress, Internet-Draft, draft-ietf-
              spring-srv6-path-segment-12, 3 April 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              srv6-path-segment-12>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC7606]  Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
              Patel, "Revised Error Handling for BGP UPDATE Messages",
              RFC 7606, DOI 10.17487/RFC7606, August 2015,
              <https://www.rfc-editor.org/info/rfc7606>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC9012]  Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
              "The BGP Tunnel Encapsulation Attribute", RFC 9012,
              DOI 10.17487/RFC9012, April 2021,
              <https://www.rfc-editor.org/info/rfc9012>.

   [RFC9256]  Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
              A., and P. Mattes, "Segment Routing Policy Architecture",
              RFC 9256, DOI 10.17487/RFC9256, July 2022,
              <https://www.rfc-editor.org/info/rfc9256>.

   [RFC9514]  Dawra, G., Filsfils, C., Talaulikar, K., Ed., Chen, M.,
              Bernier, D., and B. Decraene, "Border Gateway Protocol -
              Link State (BGP-LS) Extensions for Segment Routing over
              IPv6 (SRv6)", RFC 9514, DOI 10.17487/RFC9514, December
              2023, <https://www.rfc-editor.org/info/rfc9514>.

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   [RFC9545]  Cheng, W., Ed., Li, H., Li, C., Ed., Gandhi, R., and R.
              Zigler, "Path Segment Identifier in MPLS-Based Segment
              Routing Networks", RFC 9545, DOI 10.17487/RFC9545,
              February 2024, <https://www.rfc-editor.org/info/rfc9545>.

   [RFC9552]  Talaulikar, K., Ed., "Distribution of Link-State and
              Traffic Engineering Information Using BGP", RFC 9552,
              DOI 10.17487/RFC9552, December 2023,
              <https://www.rfc-editor.org/info/rfc9552>.

   [RFC9830]  Previdi, S., Filsfils, C., Talaulikar, K., Ed., Mattes,
              P., and D. Jain, "Advertising Segment Routing Policies in
              BGP", RFC 9830, DOI 10.17487/RFC9830, September 2025,
              <https://www.rfc-editor.org/info/rfc9830>.

10.2.  Informative References

   [I-D.ietf-idr-sr-policy-path-segment]
              Li, C., Li, Z., Yin, Y., Cheng, W., and K. Talaulikar, "SR
              Policy Extensions for Path Segment and Bidirectional
              Path", Work in Progress, Internet-Draft, draft-ietf-idr-
              sr-policy-path-segment-14, 11 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
              policy-path-segment-14>.

   [I-D.ietf-spring-stamp-srpm]
              Gandhi, R., Filsfils, C., Janssens, B., Chen, M., and R.
              F. Foote, "Performance Measurement Using Simple Two-Way
              Active Measurement Protocol (STAMP) for Segment Routing
              Networks", Work in Progress, Internet-Draft, draft-ietf-
              spring-stamp-srpm-19, 20 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              stamp-srpm-19>.

Authors' Addresses

   Cheng Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing
   100095
   China
   Email: c.l@huawei.com

   Zhenbin Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.

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   Beijing
   100095
   China
   Email: lizhenbin@huawei.com

   Yongqing Zhu
   China Telecom
   109 West Zhongshan Ave
   Guangzhou
   China
   Email: zhuyq8@chinatelecom.cn

   Weiqiang Cheng
   China Mobile
   Beijing
   China
   Email: chengweiqiang@chinamobile.com

   Ketan Talaulikar
   Cisco Systems
   Email: ketant.ietf@gmail.com

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