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Advertising S-BFD Discriminators in BGP
draft-wang-bess-sbfd-discriminator-04

Document Type Active Internet-Draft (individual)
Authors Haibo Wang , Jie Dong , Greg Mirsky , Yang Huang
Last updated 2022-12-03
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draft-wang-bess-sbfd-discriminator-04
Network Working Group                                            H. Wang
Internet-Draft                                                   J. Dong
Intended status: Standards Track                                  Huawei
Expires: 6 June 2023                                           G. Mirsky
                                                                Ericsson
                                                                Y. Huang
                                                                  Huawei
                                                         3 December 2022

                Advertising S-BFD Discriminators in BGP
                 draft-wang-bess-sbfd-discriminator-04

Abstract

   Seamless Bidirectional Forwarding Detection (S-BFD) is a simplified
   BFD mechanism.  It eliminates most negotiation aspects and provides
   advantages such as fast configuration injection.  S-BFD is especially
   useful in multi-homing PE scenarios and reduces resource overheads on
   the dual-homing PEs.  Although S-BFD is simpler than BFD, a large
   number of manual configurations are required when there are a large
   number of PEs.

   This document provides the mechanism of distributing S-BFD
   discriminators with VPN service routes, which simplifies S-BFD
   deployment for VPN services.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://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."

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   This Internet-Draft will expire on 6 June 2023.

Copyright Notice

   Copyright (c) 2022 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 (https://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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Typical Scenarios . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  EVPN Layer 3 Service Over SRv6 BE Use Case  . . . . . . .   4
     3.2.  EVPN Layer 3 Service Over SPv6 Policy Use Case  . . . . .   5
   4.  Procedure . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  BGP Encoding  . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Router Procedure  . . . . . . . . . . . . . . . . . . . .   8
       4.2.1.  Egress Node Process . . . . . . . . . . . . . . . . .   8
       4.2.2.  Transit Node Process  . . . . . . . . . . . . . . . .   8
       4.2.3.  Ingress Node Process  . . . . . . . . . . . . . . . .   8
   5.  Error handling  . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  References  . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   In deploying some network services, carriers usually deploy active
   and standby nodes to improve network reliability.  In this way, a
   protection switchover can be performed quickly when there is a fault
   at the active node or a failure on the working path between PEs.  To
   accelerate fault detection, BFD is usually used.  Traditionally, BFD
   sessions need to be provisioned on both ends of the BFD session,
   which occupies the resources of both PEs.

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   [RFC7880] defines Seamless Bidirectional Forwarding Detection
   (S-BFD), a simplified mechanism for using BFD with a large proportion
   of negotiation aspects eliminated, thus providing benefits such as
   quick provisioning, as well as improved control and flexibility for
   network nodes initiating path monitoring.  This mechanism is
   especially useful in asymmetric scenarios, such as the 3PE scenarios.
   At the dual-homing PE nodes, BFD does not need to be used to detect
   the single-homed PE node.  In this scenario, S-BFD greatly saves
   resources on the dual-homing nodes.

   To deploy S-BFD, the initiator needs to know the reflector's ip
   address and discriminator . When a large number of PEs need to be
   deployed, even the deployment of S-BFD can become complicated.
   [RFC7883] and [RFC7884] introduced an IGP-based S-BFD discriminator
   advertisement mechanism to simplify S-BFD deployment.  Since VPN
   services may be deployed across an area or AS boundaries, the IGP-
   based S-BFD mechanism does not apply in this case.

   This document proposes a mechanism to distribute S-BFD discriminator
   information with BGP service routes.  It allows advertising S-BFD
   discriminator across multiple domains and achieves on-demand end-to-
   end S-BFD session provisioning for specific BGP service routes.

2.  Terminology

   BFD : Bidirectional Forwarding Detection

   S-BFD : Seamless Bidirectional Forwarding Detection

   3PE : One PE connect to dual-homed PEs scenario

   APE : Access PE, used to access users

   SPE: Service PE, used to support service for users

   UCE: User CE

   SCE: Service CE

3.  Typical Scenarios

   In some inter-domain VPN service deployments, only one of a pair of
   interconnected PEs benefits from monitoring the status of the
   connection.  In such a case, using S-BFD [RFC7880] is advantageous as
   it reduces the load on one of the PEs while providing the benefit of
   faster convergence.  The following subsections provide some examples
   of SRv6-based VPN services to show the benefits of using S-BFD.

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   For SRv6 services, two types of paths are used to support the
   service.  One is service over SRv6 BE, the other is service over SRv6
   Policy.  For the service over SRv6 BE case, the VPNSID is used to
   resolve the forwarding information.  Thus an S-BFD session is needed
   to detect the reachability of the VPNSID.  The session is an IP-
   routed S-BFD, and the SRv6 locator of the remote VPN SID can be used
   as the destination identifier of the S-BFD session.  For the service
   over SRv6 Policy, the <nexthop, color> in the service route is used
   to resolve to an SRv6 Policy.  Then an S-BFD session is needed to
   detect the reachability of the SRv6 Policy.

3.1.  EVPN Layer 3 Service Over SRv6 BE Use Case

            /---------------------\  /--------------------\
            |                     |  |                    |
   +----+   | +----+      +-----+ |  |+-----+      +----+ |
   |UCE1|---|-|APE1|------|ASBR1|-|--||ASBR3|------|SPE1| |
   +----+   | +----+ \    ,-----+ |  |+-----+\    /+----+ |
            |         \  /        |  |        \  /       `|
            | ....     \/         |  |         \/         |', +----+
            |          /\         |  |         /\         |  .|SCE1|
            |         /  \        |  |        /  \        |-` +----+
   +----+   | +----+ /    '-----+ |  |+-----+-    '+-----`|
   |UCEn|---|-|APEn|------|ASBR2|-|--||ASBR4|------|SPE2| |
   +----+   | +----+      +-----+ |  |+-----+      +----+ |
            |                     |  |                    |
            |      AS65001        |  |       AS65002      |
            \---------------------/  \--------------------/
   Figure 1: EVPN Layer 3 Service Over SRv6 BE

   Figure 1 shows a SRv6 BE based seamless scenario.  UCE is single-
   homed to APE, and SCE is dual-homed to SPE1 and SPE2.  The service is
   across multiple ASes.

   SCE1 accesses SPE1 and SE2 through Layer 3 and advertises its private
   network routes to them.  SPE1 and SPE2 encapsulate the routes into
   Type 5 routes in the EVPN format and sends them to APE1.  After
   receiving Type 5 routes advertised by SPE1 and SPE2, APE1 generates
   primary and backup entries for the routes to speed up service
   switchover.  In this scenario, the SRv6 BE service mode is used.
   APE1 will resolve SPE1's VPN routes reachability through the VPN SID.
   To ensure that APE1 can properly route to PE1, PE1 needs to advertise
   its own locator route.  The advertisement of the locator route is not
   in the scope of this document.

   To speed up the fault detection, we may configure an S-BFD session on
   APE1 to detect SPE1 or SPE2's reachability.  In traditional mode, a
   discriminator needs to be assigned by SPE1 and SPE2, and two S-BFD

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   sessions need to be configured on APE1 to detect the VPN SID's
   reachability of SPE1 and SPE2.  It needs to generate an S-BFD session
   with the destination set to the VPN SID.  To reduce the number of
   S-BFD sessions, locator-based S-BFD sessions can be used instead of
   S-BFD sessions for VPNSIDs.

   There are a large number of such APEs that exist on the network.
   Each APE is configured with several S-BFD sessions to detect PE1 and
   PE2, which increases the deployment complexity.

3.2.  EVPN Layer 3 Service Over SPv6 Policy Use Case

            /---------------------\  /--------------------\
            |                     |  |                    |
   +----+   | +----+      +-----+ |  |+-----+      +----+ |
   |UCE1|---|-|APE1|------|ASBR1|-|--||ASBR3|------|SPE1| |
   +----+   | +----+ \    ,-----+ |  |+-----+\    /+----+ |
            |         \  /        |  |        \  /       `|
            | ....     \/         |  |         \/         |', +----+
            |          /\         |  |         /\         |  .|SCE1|
            |         /  \        |  |        /  \        |-` +----+
   +----+   | +----+ /    '-----+ |  |+-----+-    '+-----`|
   |UCEn|---|-|APEn|------|ASBR2|-|--||ASBR4|------|SPE2| |
   +----+   | +----+      +-----+ |  |+-----+      +----+ |
            |                     |  |                    |
            |      AS65001        |  |       AS65002      |
            \---------------------/  \--------------------/
   Figure 2: EVPN Layer 3 Service Over SRv6 Policy

   Figure 2 shows an SRv6 Policy scenario.  SCE1 is dual-homed to SPE1
   and SPE2, and UCE1 is accessed to APE1.  SPE1, SPE2, and APE1 are
   cross BGP ASes.

   SCE1 accesses SPE1 and SPE2 through Layer 3 and advertises its
   private network routes to APE1.  SPE1 and SPE2 encapsulate the routes
   into Type 5 routes in the EVPN format and sends them to APE1.

   After receiving Type 5 routes advertised by SPE1 and SPE2, APE1
   generates primary and backup entries for the routes, speeding up
   service switchover.  APE1 parses the tunnel based on the <nexthop,
   color> of the service routes advertised by SPE1 and SPE2, and matches
   an SRv6 Policy.  After receiving the traffic from UCE1 to SCE1, APE1
   encapsulates and forwards the traffic based on the SRv6 Policy.

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   An S-BFD session needs to be established for these SRv6 Policy-based
   forwarding paths to swiftly detect the availability of the paths.
   When detecting a fault on the SRv6 Policy path of the primary service
   route, services can be swiftly switched to the backup path, providing
   more reliable protection for services.

   There are a large number of such PEs that exist on the network.  Each
   PE is configured with several S-BFD sessions to detect PE1 and PE2,
   which increases the deployment complexity.

   Certainly, this scenario may also be implemented in other methods.
   For example, when provisioning an SRv6 policy, an S-BFD session can
   be provisioned.  While in some cases, it would be more efficient if
   the S-BFD session could be provisioned based on the demand of the
   services.

4.  Procedure

4.1.  BGP Encoding

   [RFC9026] specifies the "BFD Discriminators" (38) attribute, which is
   an optional transitive BGP attribute that conveys the Discriminators
   and other optional attributes used to establish BFD sessions.

   In [RFC9026], the BFD Discriminators attribute is used to transmit
   P2MP BFD session creation information MVPN scenarios.  For non-
   multicast services, such as L3VPN services, L2VPN services, EVPN
   services and native IP services, BFD discriminators are also required
   for creating an S-BFD session.  This document reuses the BFD
   Discriminators attribute and defines new BFD modes for some of these
   services.

   The format of BFD Discriminator attribute is shown as follows:

      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
     +-+-+-+-+-+-+-+-+
     |    BFD Mode   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       BFD Discriminator                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                         Optional TLVs                         ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Figure 3: Format of the BFD Discriminator Attribute

   o BFD Mode:

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   The BFD Mode field is 1 octet.  [RFC9026] defines only the P2MP BFD
   session for MVPN.  This document defines two new types of S-BFD
   session types for the scenarios in Section 3.

   As described in the preceding scenarios, there are two types of S-BFD
   sessions for SRv6 services.  For service over SRv6 BE, an IP-routed
   S-BFD session needs to be created to detect the reachability of the
   SRv6 locator.  For service over SRv6 Policy, an S-BFD session for
   SRv6 Policy needs to be created to detect the reachability of the
   SRv6 Policy.  Thus two new BFD modes are introduced:

   *  S-BFD for SRv6 Locator Session Mode, which is dedicated to
      detecting the locator.  The type value is to be allocated as
      described in Section 6.

   *  S-BFD for Common Session Mode, which is for general S-BFD session.
      The type value is to be allocated as described in Section 6.  This
      mode is not only for SRv6, but also can be used for other
      scenarios.

   o BFD Discriminators:

   The field length is 4 octets.  Used to specify the discriminator for
   S-BFD session.

   o Optional TLVs:

   Variable-length fields are optional.  Indicates the additional
   information required for creating a S-BFD session.  The format is as
   follows:

    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    |           Value             ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 4: Format of the Optional TLV

   If a transit node changes the next hop or reassigns a VPN SID when
   advertising a route, the transit node needs to use the locally
   allocated S-BFD discriminator for the S-BFD discriminator attribute.
   Suppose the transit node does not recognize the S-BFD Discriminator
   attribute in the received route and continues to advertise the route
   to the remote PE.  In that case, the receiver may use incorrect
   information when creating an S-BFD session.  Therefore, the
   advertised S-BFD Discriminator attribute also needs to carry the IP
   address of the originator of the discriminator for receiver side
   verification.

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   For the two BFD modes defined in this document, the "Source IP
   Address" TLV as defined in [RFC9026] MUST be carried in the BFD
   Discriminator attribute.  If the mode is "S-BFD for SRv6 Locator
   Session", the SRv6 Locator address MUST be used for the service is
   carried in the TLV.  If the mode is "S-BFD for Common Session", the
   next-hop address MUST be used for the service is carried in the TLV.

4.2.  Router Procedure

   In BGP address families, such as L3VPN or EVPN, routes can carry the
   S-BFD Discriminator attribute as required so that S-BFD sessions can
   be established based on the attribute.  The following uses S-BFD for
   SRv6 Locator as an example.  If mode is set to S-BFD for Common
   Session, the processing method is similar.

4.2.1.  Egress Node Process

   As shown in figure 1, the S-BFD discriminator is configured on PE1.
   After obtaining the information, BGP encapsulates the attribute into
   the EVPN route and sets the BFD Mode to S-BFD for Locator Session,
   when advertising the EVPN route.  The Discriminator value is local
   discriminator value.  The optional TLV carries the local PE's locator
   address used by the VPN.

4.2.2.  Transit Node Process

   Here is the end-to-end SRv6 BE scenario.  The ASBR does not re-
   allocate the VPN SID.  Thus, the ASBR does not require to modify the
   VPN SID, and not to alter the BFD discriminator attribute.

4.2.3.  Ingress Node Process

   After receiving the EVPN Type 5 routes from PE1 and PE2, PE3 imports
   the routes to the VRF of PE3 based on the route targets.  Routes
   triggers establish the S-BFD sessions based on <S-BFD discriminator,
   locator ip>.

   Then, routes with the same prefix from PE1 and PE2 form primary and
   backup paths.  When the primary path or the egress node is in fault,
   S-BFD detects that fault and forms switch to backup path quickly.

   To avoid the waste of redundant resources, assume that the ASBR re-
   assigns the SID in Option B and the ASBR does not recognize the
   attribute.  In this case, the SID and locator carried in the route
   received by PE3 do not match the Source IP carried in the Optional
   TLV in the BFD attribute.  Therefore, PE3 does not need to establish
   an S-BFD session to remote PE, which can avoid resource waste.

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5.  Error handling

   Error handling complies with [RFC7606].  In this document, the BFD
   discriminator information is used only to establish an S-BFD session.
   Therefore, if the BFD discriminator information is invalid, the BFD
   attribute will be discard and not transmit to other devices.

   For BFD discriminator attribute, the following case will be
   processed:

   o The BFD Discriminator value in receiving BFD Discriminator
   attribute is 0, the attribute is invalid.

   For the BFD mode type "S-BFD for SRv6 Locator Session", the following
   case will be processed:

   o If the BFD discriminator attribute doesn't contain optional TLV
   with type set to 1, the attribute is invalid.

   o If the optional TLV type is 1 but the length is not 16, the
   attribute is invalid.

   o If the optional TLV type is 1 but the value is all 0, the attribute
   is invalid.

   o If multiple Source IP Optional TLVs are carried, the first source
   IP address should be used as the destination to establish an S-BFD
   session.  For EVPN type 2 MAC-IP routes may use the first and the
   second IP address because it may carry two SRv6 SIDs with different
   locators.  Other source IP addresses should be ignored.

   o If a non-Source IP Optional TLV is carried, the Optional TLV will
   be ignored.

   For the BFD mode type "S-BFD for Common Session", the following case
   will be processed:

   o If the BFD discriminator attribute doesn't contain optional TLV
   with type set to 1, the attribute is invalid.

   o If the optional TLV type is 1 but the length is not 4 or 16, the
   attribute is invalid.

   o It the optional TLV type is 1 but the value is all 0, the attribute
   is invalid.

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   o If multiple Source IP Optional TLVs are carried, only the first
   source IP address should be used as the destination to establish an
   S-BFD session.  Other source IP addresses should be ignored.

   o If a non-Source IP Optional TLV is carried, the Optional TLV will
   be ignored.

6.  IANA Considerations

   IANA is requested to assign two new code points from the "BFD Mode"
   subregistry in the "Border Gateway Protocol (BGP) Parameters"
   registry.

                   Value  Description
                   ----  -------------------------
                   TBA1   S-BFD for SRv6 Locator Session
                   TBA2   S-BFD for Common Session

7.  Security Considerations

   The new S-BFD modes introduced in this document does not introduce
   any new security risks for BGP.

   The BFD attribute is an optional attribute and is mainly used for
   network services within a single administrative domain.  The operator
   SHOULD ensure this attribute does not propagate across the boundary
   of the administrative domain.  For VPN services, the advertisement
   range of this attribute is the same as that of VPN routes.
   Therefore, this attribute is not advertised outside the management
   domain.  For public IPv4 and IPv6 services, the border node of the
   administrative domain SHOULD be configured not to propagate the BFD
   attribute to other domains.

   When creating an S-BFD session, the initiator verifies the S-BFD
   session based on routing information.  This reduces the number of
   invalid S-BFD sessions and avoid attribute attack.

8.  Acknowledgements

   NA

9.  References

9.1.  Normative References

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   [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>.

9.2.  References

   [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>.

   [RFC7880]  Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
              Pallagatti, "Seamless Bidirectional Forwarding Detection
              (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
              <https://www.rfc-editor.org/info/rfc7880>.

   [RFC7883]  Ginsberg, L., Akiya, N., and M. Chen, "Advertising
              Seamless Bidirectional Forwarding Detection (S-BFD)
              Discriminators in IS-IS", RFC 7883, DOI 10.17487/RFC7883,
              July 2016, <https://www.rfc-editor.org/info/rfc7883>.

   [RFC7884]  Pignataro, C., Bhatia, M., Aldrin, S., and T. Ranganath,
              "OSPF Extensions to Advertise Seamless Bidirectional
              Forwarding Detection (S-BFD) Target Discriminators",
              RFC 7884, DOI 10.17487/RFC7884, July 2016,
              <https://www.rfc-editor.org/info/rfc7884>.

   [RFC9026]  Morin, T., Ed., Kebler, R., Ed., and G. Mirsky, Ed.,
              "Multicast VPN Fast Upstream Failover", RFC 9026,
              DOI 10.17487/RFC9026, April 2021,
              <https://www.rfc-editor.org/info/rfc9026>.

Authors' Addresses

   Haibo Wang
   Huawei
   No. 156 Beiqing Road
   Beijing
   100095
   P.R. China
   Email: rainsword.wang@huawei.com

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   Jie Dong
   Huawei
   No. 156 Beiqing Road
   Beijing
   100095
   P.R. China
   Email: jie.dong@huawei.com

   Greg Mirsky
   Ericsson
   Email: gregimirsky@gmail.com

   Yang Huang
   Huawei
   No. 156 Beiqing Road
   Beijing
   100095
   P.R. China
   Email: yang.huang@huawei.com

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