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BGP-LS Extensions for Scalable Segment Routing based Enhanced VPN
draft-dong-idr-bgpls-sr-enhanced-vpn-04

Document Type Active Internet-Draft (individual)
Authors Jie Dong , Zhibo Hu, Zhenbin Li , Xiongyan Tang, Ran Pang
Last updated 2022-03-04
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draft-dong-idr-bgpls-sr-enhanced-vpn-04
IDR Working Group                                                J. Dong
Internet-Draft                                                     Z. Hu
Intended status: Standards Track                                   Z. Li
Expires: 5 September 2022                            Huawei Technologies
                                                                 X. Tang
                                                                 R. Pang
                                                            China Unicom
                                                            4 March 2022

   BGP-LS Extensions for Scalable Segment Routing based Enhanced VPN
                draft-dong-idr-bgpls-sr-enhanced-vpn-04

Abstract

   Enhanced VPN (VPN+) aims to provide enhanced VPN services to support
   some applications' needs of enhanced isolation and stringent
   performance requirements.  VPN+ requires integration between the
   overlay VPN connectivity and the resources and characteristics
   provided by the underlay network.  A Virtual Transport Network (VTN)
   is a virtual underlay network which can be used to support one or a
   group of VPN+ services.  In the context of network slicing, a VTN
   could be instantiated as a network resource partition (NRP).

   This document specifies the BGP-LS mechanisms with necessary
   extensions to advertise the information of scalable Segment Routing
   (SR) based NRPs to a centralized network controller.  Each NRP can
   have a customized topology and a set of network resources allocated
   from the physical network.  Multiple NRPs may shared the same
   topology, and multiple NRPs may share the same set of network
   resources on specific network segments.  This allows flexible
   combination of network topology and network resource attributes to
   build a large number of NRPs with a relatively small number of
   logical topologies.  The proposed mechanism is applicable to both
   segment routing with MPLS data plane (SR-MPLS) and segment routing
   with IPv6 data plane (SRv6).

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.

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   This Internet-Draft will expire on 5 September 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Advertisement of NRP Definition . . . . . . . . . . . . . . .   4
   3.  Advertisement of NRP Topology Attribute . . . . . . . . . . .   5
     3.1.  Intra-domain Topology Advertisement . . . . . . . . . . .   6
       3.1.1.  MTR based Topology Advertisement  . . . . . . . . . .   6
       3.1.2.  Flex-Algo based Topology Advertisement  . . . . . . .   7
     3.2.  Inter-Domain Topology Advertisement . . . . . . . . . . .   8
       3.2.1.  NRP IDs TLV . . . . . . . . . . . . . . . . . . . . .   9
   4.  Advertisement of NRP Resource Attribute . . . . . . . . . . .  10
     4.1.  Option 1: L2 Bundle based Approach  . . . . . . . . . . .  11
     4.2.  Option 2: Per-NRP Link TE Attributes  . . . . . . . . . .  12
   5.  Advertisement of NRP specific Data Plane Identifiers  . . . .  13
     5.1.  NRP-specific SR-MPLS SIDs . . . . . . . . . . . . . . . .  13
       5.1.1.  NRP-specific Prefix-SID TLV . . . . . . . . . . . . .  13
       5.1.2.  NRP-specific Adj-SID TLV  . . . . . . . . . . . . . .  14
     5.2.  NRP-specific SRv6 SIDs  . . . . . . . . . . . . . . . . .  15
       5.2.1.  NRP-specific SRv6 Locators and End SIDs . . . . . . .  15
       5.2.2.  NRP-specific SRv6 End.X SID . . . . . . . . . . . . .  16
     5.3.  Dedicated NRP ID in Data Plane  . . . . . . . . . . . . .  17
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17

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   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  18
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   Enhanced VPN (VPN+) is an enhancement to VPN services to support the
   needs of new applications, particularly the applications that are
   associated with 5G services.  These applications require enhanced
   isolation and have more stringent performance requirements than that
   can be provided with traditional overlay VPNs.  These properties
   require integration between the underlay and the overlay networks.
   [I-D.ietf-teas-enhanced-vpn] specifies the framework of enhanced VPN
   and describes the candidate component technologies in different
   network planes and layers.  An enhanced VPN can be used for 5G
   network slicing, and will also be of use in more generic scenarios.

   To meet the requirement of enhanced VPN services, a number of virtual
   underlay networks need to be created, each with a subset of the
   underlay network topology and a set of network resources allocated to
   meet the requirement of a specific VPN+ service or a group of VPN+
   services.  Such a virtual underlay network is called Virtual
   Transport Network (VTN) in [I-D.ietf-teas-enhanced-vpn].
   [I-D.ietf-teas-ietf-network-slices] introduces the concept Network
   Resource Partition (NRP) as a set of network resources that are
   available to carry traffic and meet the SLOs and SLEs.  In order to
   allocate network resources to an NRP, the NRP is associated with a
   network topology to define the set of links and nodes.  Thus VTN and
   NRP are similar concepts, and NRP can be seen as an instantiation of
   VTN in the context of network slicing.  For clarity, the rest of this
   document uses NRP in the description of the proposed mechanisms and
   protocol extensions.

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   [I-D.ietf-spring-resource-aware-segments] introduces resource-
   awareness to Segment Routing (SR) [RFC8402] by associating existing
   type of SIDs with network resource attributes (e.g.  bandwidth,
   processing or storage resources).  These resource-aware SIDs retain
   their original functionality, with the additional semantics of
   identifying the set of network resources available for the packet
   processing action.  [I-D.ietf-spring-sr-for-enhanced-vpn] describes
   the use of resource-aware segments to build SR based NRPs.  To allow
   the network controller and network nodes to perform NRP-specific
   explicit path computation and/or shortest path computation, the group
   of resource-aware SIDs allocated by network nodes to each NRP and the
   associated topology and resource attributes need to be distributed in
   the control plane.

   When an NRP spans multiple IGP areas or multiple Autonomous Systems
   (ASes), BGP-LS is needed to advertise the NRP information in each IGP
   area or AS to the network controller, so that the controller could
   use the collected information to build the view of inter-area or
   inter-AS SR NRPs.

   This document describes BGP-LS [RFC7752] based mechanism with
   necessary extensions to advertise the topology and resource attribute
   of inter-area and inter-domain SR based NRPs.  Each NRP can have a
   customized topology and a set of network resources allocated.
   Multiple NRPs may shared the same topology, and some of the NRPs may
   share the same set of network resources on specific network segments.
   This allows flexible combination of network topology and network
   resource attributes to build a large number of NRPs with a relatively
   small number of logical topologies.  The definition of NRP is
   advertised as a node attribute using BGP-LS.  The associated network
   topology and resources attributes of a NRP are advertised as link
   attributes using BGP-LS.

2.  Advertisement of NRP Definition

   According to [I-D.ietf-teas-ietf-network-slices], an NRP consists of
   a set of dedicated or shared network resources, and is associated
   with a customized network topology.  Thus a NRP can be defined as the
   combination of a set of network attributes, which include the
   topology attribute and other attributes, such as the associated
   network resources.

   The Network Resource Partition Definition (NRPD) TLV is a new TLV of
   the optional BGP-LS Attribute which is associated with the node NLRI.

   The format of NRPD TLV is as follows:

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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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            NRP ID                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             MT-ID             |    Algorithm  |     Flags     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Sub-TLVs                             |
      ~                            ...                                ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

   *  Type: To be assigned by IANA.

   *  Length: the length of the value field of the TLV.  It is variable
      dependent on the included Sub-TLVs.

   *  NRP ID: A global significant 32-bit identifier which is used to
      identify an NRP.

   *  MT-ID: 16-bit identifier which contains the multi-topology
      identifier of the IGP topology.

   *  Algorithm: 8-bit identifier which indicates the algorithm which
      applies to this virtual transport network.  It can be either a
      normal algorithm in [RFC8402] or a Flex-Algorithm
      [I-D.ietf-lsr-flex-algo].

   *  Flags: 8-bit flags.  Currently all the flags are reserved for
      future use.  They SHOULD be set to zero on transmission and MUST
      be ignored on receipt.

   *  Sub-TLVs: optional sub-TLVs to specify the additional attributes
      of an NRP.  Currently no sub-TLV is defined in this document.

3.  Advertisement of NRP Topology Attribute

   [I-D.dong-lsr-sr-enhanced-vpn] describes the IGP mechanisms to
   distribute the topology attributes of SR based NRPs.  This section
   describes the BGP-LS mechanism to distribute both the intra-domain
   and inter-domain topology attributes of SR based NRPs.

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3.1.  Intra-domain Topology Advertisement

   The intra-domain topology attribute of an NRP can be determined by
   the MT-ID and/or the algorithm ID included in the NRP definition.  In
   practice, it could be described using two optional approaches.

   The first approach is to use Multi-Topology Routing (MTR) [RFC4915]
   [RFC5120] with the segment routing extensions to advertise the
   topology associated with the SR based NRPs.  Different algorithms MAY
   be used to further specify the computation algorithm or the metric
   type used for path computation within the topology.  Multiple NRPs
   can be associated with the same <topology, algorithm> tupple, and the
   IGP computation with the <topology, algorithm> tuple can be shared by
   these NRPs.

   The second approach is to use Flex-Algo [I-D.ietf-lsr-flex-algo] to
   describe the topological constraints of SR based NRPs on a network
   topology (e.g. the default topology).  Multiple NRPs can be
   associated with the same Flex-Algo, and the IGP computation result
   with this Flex-Algo can be shared.

   This section describes the two optional approaches to advertise the
   intra-domain topology of an NRP using BGP-LS.

3.1.1.  MTR based Topology Advertisement

   In section 4.2.2.1 of [I-D.ietf-idr-rfc7752bis], Multi-Topology
   Identifier (MT-ID) TLV is defined, which can contain one or more IS-
   IS or OSPF Multi-Topology IDs.  The MT-ID TLV MAY be present in a
   Link Descriptor, a Prefix Descriptor, or the BGP-LS Attribute of a
   Node NLRI.

   [RFC9085] defines the BGP-LS extensions to carry the segment routing
   information using TLVs of BGP-LS Attribute.  When MTR is used with
   SR-MPLS data plane, topology-specific prefix-SIDs and topology-
   specific Adj-SIDs can be carried in the BGP-LS Attribute associated
   with the prefix NLRI and link NLRI respectively, the MT-ID TLV is
   carried in the prefix descriptor or link descriptor to identify the
   corresponding topology of the SIDs.

   [I-D.ietf-idr-bgpls-srv6-ext] defines the BGP-LS extensions to
   advertise SRv6 segments along with their functions and attributes.
   When MTR is used with SRv6 data plane, the SRv6 Locator TLV is
   carried in the BGP-LS Attribute associated with the prefix-NLRI, the
   MT-ID TLV can be carried in the prefix descriptor to identify the
   corresponding topology of the SRv6 Locator.  The SRv6 End.X SIDs are
   carried in the BGP-LS Attribute associated with the link NLRI, the
   MT-ID TLV can be carried in the link descriptor to identify the

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   corresponding topology of the End.X SIDs.  The SRv6 SID NLRI is
   defined to advertise other types of SRv6 SIDs, in which the SRv6 SID
   Descriptors can include the MT-ID TLV so as to advertise topology-
   specific SRv6 SIDs.

   [I-D.ietf-idr-rfc7752bis] also defines the rules of the usage of MT-
   ID TLV:

   "In a Link or Prefix Descriptor, only a single MT-ID TLV containing
   the MT-ID of the topology where the link or the prefix is reachable
   is allowed.  In case one wants to advertise multiple topologies for a
   given Link Descriptor or Prefix Descriptor, multiple NLRIs MUST be
   generated where each NLRI contains a single unique MT-ID."

   Editor's note: the above rules indicates that only one MT-ID is
   allowed to be carried the Link or Prefix descriptors.  When a link or
   prefix needs to be advertised in multiple topologies, multiple NLRIs
   needs to be generated to report all the topologies the link or prefix
   participates in, together with the topology-specific segment routing
   information and link attributes.  This may increase the number of BGP
   Updates needed for advertising MT-specific topology attributes, and
   may introduce additional processing burden to both the sending BGP
   speaker and the receiving network controller.  When the number of
   topologies in a network is not a small number, some optimization may
   be needed for the reporting of multi-topology information and the
   associated segment routing information in BGP-LS.  Based on the WG's
   opinion, this will be elaborated in a future version.

3.1.2.  Flex-Algo based Topology Advertisement

   The Flex-Algo definition [I-D.ietf-lsr-flex-algo] can be used to
   describe the calculation-type, the metric-type and the topological
   constraints for path computation on a network topology.  As specified
   in [I-D.dong-lsr-sr-enhanced-vpn], the topology of a NRP can be
   determined by applying Flex-Algo constraints on a network topology.

   BGP-LS extensions for Flex-Algo [I-D.ietf-idr-bgp-ls-flex-algo]
   provide the mechanisms to advertise the Flex-Algo definition
   information.  BGP-LS extensions for SR-MPLS [RFC9085] and SRv6
   [I-D.ietf-idr-bgpls-srv6-ext] provide the mechanism to advertise the
   algorithm-specific segment routing information.

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   In [RFC9085], algorithm-specific prefix-SIDs can be advertised in
   BGP-LS attribute associated with Prefix NLRI.  In
   [I-D.ietf-idr-bgpls-srv6-ext], algorithm-specific SRv6 Locators can
   be advertised in BGP-LS Attribute associated with the corresponding
   Prefix NLRI, and algorithm-specific End.X SID can be advertised in
   BGP-LS Attribute associated with the corresponding Link NLRI.  Other
   types of SRv6 SIDs can also be algorithm-specific and are advertised
   using the SRv6 SID NLRI.

3.2.  Inter-Domain Topology Advertisement

   In some network scenarios, an NRP which spans multiple areas or ASes
   needs to be created.  The multi-domain NRP could have different
   inter-domain connectivity, and may be associated with different set
   of network resources in each domain and also on the inter-domain
   links.  In order to build the multi-domain NRPs using segment
   routing, it is necessary to advertise the topology and resource
   attribute of NRP on the inter-domain links and the associated BGP
   Peering SIDs.

   [RFC9086] and [I-D.ietf-idr-bgpls-srv6-ext] defines the BGP-LS
   extensions for advertisement of BGP topology information between ASes
   and the associated BGP Peering Segment Identifiers.  Such information
   could be used by a network controller for the computation and
   instantiation of inter-AS traffic engineering SR paths.

   Depending on the network scenarios and the requirement of inter-
   domain NRPs, different mechanisms can be used to specify the inter-
   domain connections of NRPs.

   *  One EBGP session between two ASes can be established over multiple
      underlying links.  In this case, different underlying links can be
      used for different inter-domain NRPs which requires link isolation
      between each other.  In another similar case, the EBGP session is
      established over a single link, while the network resource (e.g.
      bandwidth) on this link can be partitioned into different pieces,
      each of which can be considered as a virtual member link.  In both
      cases, different BGP Peer-Adj-SIDs SHOULD be allocated to each
      underlying physical or virtual member link, and ASBRs SHOULD
      advertise the NRP identifier associated with each BGP Peer-Adj-
      SID.

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   *  For inter-domain connection between two ASes, multiple EBGP
      sessions can be established between different set of peering
      ASBRs.  It is possible that some of these BGP sessions are used
      for one inter-domain NRP, while some other BGP sessions are used
      for another inter-domain NRP.  In this case, different BGP peer-
      node-SIDs are allocated to each BGP session, and ASBRs SHOULD
      advertise the NRP identifier associated with each BGP Peer-node-
      SIDs.

   *  At the AS-level topology, different inter-domain NRPs may have
      different inter-domain connectivity.  Different BGP Peer-Set-SIDs
      can be allocated to represent the groups of BGP peers which can be
      used for load-balancing in each inter-domain NRP.

   In network scenarios where the MT-ID or Flex-Algo is used
   consistently in multiple areas or ASes covered by a NRP. the
   approaches to advertise topology-specific BGP peering SIDs are
   described as below:

   *  Using MT-based mechanism, the topology-specific BGP peering SIDs
      can be advertised with the MT-ID associated with the NRP carried
      in the corresponding link NLRI.  This can be supported with the
      existing mechanisms defined in [RFC7752][RFC9086] and
      [I-D.ietf-idr-bgpls-srv6-ext].

   *  Using Flex-Algo based mechanism, the topology-specific BGP peering
      SIDs can be advertised together with the Admin Group (color) of
      the corresponding Flex-Algo in the BGP-LS attribute.

   In network scenarios where consistent usage of MT-ID or Flex-Algo
   among multiple ASes can not be expected, then the global-significant
   NRP-ID can be used to define the AS level topologies.  Within each
   domain, the MT or Flex-Algo based mechanism could still be used for
   topology advertisement.

3.2.1.  NRP IDs TLV

   A new NRP IDs TLV is defined to describe the identifiers of one or
   more NRPs an intra-domain or inter-domain link belongs to.  It can be
   carried in BGP-LS attribute which is associated with a Link NLRI, or
   it could be carried as a sub-TLV in the L2 Bundle Member Attribute
   TLV.

   The format of NRP IDs TLV is as 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             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Flags            |           Reserved            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            NRP ID-1                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                              ...                              ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            NRP ID-n                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

   *  Type: To be assigned by IANA.

   *  Length: The length of the value field of the sub-TLV.  It is
      variable dependent on the number of NRP IDs included.

   *  Flags: 16 bit flags.  All the bits are reserved, which MUST be set
      to 0 on transmission and SHOULD be ignored on receipt.

   *  Reserved: this field is reserved for future use.  MUST be set to 0
      on transmission and SHOULD be ignored on receipt.

   *  NRP IDs: One or more 32-bit identifiers to specify the NRPs this
      link belongs to.

4.  Advertisement of NRP Resource Attribute

   [I-D.dong-lsr-sr-enhanced-vpn] specifies the optional mechanism to
   advertise the resource information associated with each NRP.  One
   approach is to use the L2 bundle mechanism [RFC8668] to advertise the
   set of link resources allocated to an NRP as a L2 physical or virtual
   member link.  Another approach is to advertise the set of network
   resources as per NRP link TE attributes.  This section defines the
   corresponding BGP-LS extensions for both approaches.

   Two new TLVs are defined to carry the NRP ID and the link attribute
   flags of either a Layer-3 link or the L2 bundle member links.  The
   NRP ID TLV is defined in section 3.2.1 of this document, and a new
   Link Attribute Flags TLV is defined in this section.  The TE
   attributes of each Layer 3 link or the L2 bundle member link, such as
   the bandwidth and the SR SIDs, can be advertised using the mechanism
   as defined in [RFC9085][RFC9086] and [I-D.ietf-idr-bgpls-srv6-ext].

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4.1.  Option 1: L2 Bundle based Approach

   On an Layer-3 interface, each NRP can be allocated with a subset of
   link resources (e.g. bandwidth).  A subset of link resources may be
   dedicated to an NRP, or may be shared by a group of NRPs.  Each
   subset of link resource can be instantiated as a virtual layer-2
   member link under the Layer-3 interface, and the Layer-3 interface is
   considered as a virtual Layer-2 bundle.  The Layer-3 interface may
   also be a physical Layer 2 link bundle, in this case a subset of link
   resources allocated to an NRP may be provided by one of the physical
   Layer-2 member links.

   The NRP ID TLV defined in section 3.2.1 of this document is used to
   carry the NRP IDs associated with the L2 bundle member links.  The TE
   attributes of the L2 bundle member links, such as the maximum link
   bandwidth, and the SR SIDs, can be advertised using the mechanism as
   defined in [RFC9085][RFC9086] and [I-D.ietf-idr-bgpls-srv6-ext].

   A new Link attribute Flags TLV is defined to specify the
   characteristics of a link.  It can be carried in BGP-LS attribute
   which is associated with a Link NLRI, or it could be carried as a
   sub-TLV in the L2 Bundle Member Attribute TLV.  The format of the
   sub-TLV is as below:

       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             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

      Type: TBD

      Length: 4 octets.

      Flags: 16-bit flags.  This field is consistent with the Flag field
      in IS-IS Link Attribute sub-TLV in [RFC5029].  In addition to the
      flags defined in [RFC5029], A new Flag "E" is defined in this
      document.

      -  Link excluded from load balancing.  When the flag is set, it
         indicates this link is only used for the associated NRPs.

      .

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4.2.  Option 2: Per-NRP Link TE Attributes

   An Layer-3 interface can participate in multiple NRPs, each of which
   is allocated with a subset of the resources of the interface.  For
   each NRP, the associated resources can be described using per-NRP TE
   attributes.  A new NRP-specific TE attribute TLV is defined to
   advertise the link attributes associated with an NRP.  This sub-TLV
   MAY be carried in the BGP-LS Attribute associated with a Link NLRI.
   The format of the NRP-specific TE attribute TLV is shown as below:

        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           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        NRP IDs Sub-TLV                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                        Other Sub-TLVs                         ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

   *  Type: To be assigned by IANA.

   *  Length: The length of the value field of the TLV.  It is variable
      dependent on the length of the Sub-TLVs field.

   *  Flags: 16-bit flags.  All the 16 bits are reserved for future use,
      which SHOULD be set to 0 on transmission and MUST be ignored on
      receipt.

   *  Reserved: 16-bit field reserved for future use, SHOULD be set to 0
      on transmission and MUST be ignored on receipt.

   The NRP IDs TLV as defined in section 3.2.1 is used as the NRP IDs
   Sub-TLV in the per-NRP Link TE Attribute TLV.

   Other Sub-TLVs are optional and can be used to carry the TE
   attributes associated with the NRPs.  The existing Link TE Attribute
   TLVs as defined in [I-D.ietf-idr-rfc7752bis] can be reused as sub-
   TLVs here.  New sub-TLVs may be defined in the future.

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5.  Advertisement of NRP specific Data Plane Identifiers

   In network scenarios where each NRP is associated with an independent
   topology or Flex-Algo, the topology or Flex-Algo specific SR SIDs or
   Locators could be used to identify the NRP in data plane, so that the
   set of network resources associated with the NRP can be determined.
   In network scenarios where multiple NRPs share the same topology or
   Flex-Algo, additional data plane identifiers are needed to identify
   different NRPs.

   This section describes the mechanisms to advertise the NRP
   identifiers with different data plane encapsulations.

5.1.  NRP-specific SR-MPLS SIDs

   With SR-MPLS data plane, the NRP identifier can be implicitly
   determined by the SR SIDs associated with the NRP.  Each node SHOULD
   allocate NRP-specific Prefix-SIDs for each NRP it participates in.
   Similarly, NRP-specific Adj-SIDs MAY be allocated for each link which
   participates in the NRP.

5.1.1.  NRP-specific Prefix-SID TLV

   A new NRP-specific Prefix-SID TLV is defined to advertise the
   relationship between the prefix-SID and its associated NRP.  It is
   derived from NRP-specific Prefix-SID sub-TLV of IS-IS
   [I-D.dong-lsr-sr-enhanced-vpn].  The format of the sub-TLV is as
   below:

        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           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            NRP ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      SID/Index/Label(Variable)                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

   *  Type: TBD

   *  Length: The length of the value field of the sub-TLV.  It is
      variable dependent on the length of the SID/Index/Label field.

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   *  Flags: 16-bit flags.  The high-order 8 bits are the same as in the
      Prefix-SID sub-TLV defined in [RFC8667].  The lower-order 8 bits
      are reserved for future use, which SHOULD be set to 0 on
      transmission and MUST be ignored on receipt.

   *  Reserved: 16-bit field reserved for future use, SHOULD be set to 0
      on transmission and MUST be ignored on receipt.

   *  NRP ID: A 32-bit local identifier to identify the NRP this prefix-
      SID is associated with.

   *  SID/Index/Label: The same as defined in [RFC8667].

   One or more of NRP-specific Prefix-SID TLVs MAY be carried in BGP-LS
   attribute of the associated Prefix NLRI.  The MT-ID in the Prefix
   descriptors SHOULD be the same as the MT-ID in the definition of the
   NRP.

5.1.2.  NRP-specific Adj-SID TLV

   A new NRP-specific Adj-SID TLV is defined to advertise between the
   Adj-SID and its associated NRP.  It is derived from NRP specific Adj-
   SID sub-TLV of IS-IS [I-D.dong-lsr-sr-enhanced-vpn].  The format of
   the sub-TLV is as below:

        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           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            NRP ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      SID/Index/Label(Variable)                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

   *  Type: TBD

   *  Length: The length of the value field of the sub-TLV.  It is
      variable dependent on the length of the SID/Index/Label field.

   *  Flags: 16-bit flags.  The high-order 8 bits are the same as in the
      Adj-SID sub-TLV defined in [RFC8667].  The lower-order 8 bits are
      reserved for future use, which SHOULD be set to 0 on transmission
      and MUST be ignored on receipt.

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   *  Reserved: 16-bit field reserved for future use, SHOULD be set to 0
      on transmission and MUST be ignored on receipt.

   *  NRP ID: A 32-bit global unique identifier to identify the NRP this
      Adj-SID is associated with.

   *  SID/Index/Label: The same as defined in [RFC8667].

   Multiple NRP-specific Adj-SID TLVs MAY be carried in BGP-LS attribute
   of the associated Link NLRI.  The MT-ID in the Link descriptors
   SHOULD be the same as the MT-ID in the definition of these NRPs.

5.2.  NRP-specific SRv6 SIDs

5.2.1.  NRP-specific SRv6 Locators and End SIDs

   With SRv6 data plane, the NRP identifier can be implicitly or
   explicitly determined using the SRv6 Locators associated with the
   NRP, this is to ensure that all network nodes (including both the
   SRv6 End nodes and Transit nodes) can identify the NRP to which a
   packet belongs.  Network nodes SHOULD allocate NRP-specific Locators
   for each NRP it participates in.  The NRP-specific Locators are used
   as the covering prefix of NRP-specific SRv6 End SIDs, End.X SIDs and
   other types of SIDs.

   Each NRP-specific SRv6 Locator MAY be advertised in a separate Prefix
   NLRI.  If multiple NRPs share the same topology/algorithm, the
   topology/algorithm specific Locator is the covering prefix of a group
   of NRP-specific Locators.  Then the advertisement of NRP-specific
   locators can be optimized to reduce the amount of information
   advertised in the control plane.

   A new NRP locator-block sub-TLV under the SRv6 Locator TLV is defined
   to advertise a set of sub-blocks which follows the topology/algorithm
   specific Locator.  Each NRP locator-block value is assigned to one of
   the NRPs which share the same topology/algorithm.

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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             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Number of NRPs|  Block Length |           Reserved            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            NRP ID #1                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                       Locator Block Value                     ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                               ...                             ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            NRP ID #n                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                       Locator Block Value                     ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

   *  Type: TBD

   *  Length: The length of the value field of the sub-TLV.  It is
      variable dependent on the number of NRPs and the Block Length.

   *  Number of NRPs: The number of NRPs which share the same topology/
      algorithm specific Locator as the covering prefix.

   *  Block Length: The length of the NRP locator-block which follows
      the length of the topology/algorithm specific Locator.

   *  NRP ID: A 32-bit identifier to identify the NRP the locator-block
      is associated with.

   *  Block Value: The value of the NRP locator-block for each NRP.

   With the NRP locator-block sub-TLV, the NRP-specific Locator can be
   obtained by concatenating the topology/algorithm specific locator and
   the locator-block value advertised for the NRP.

5.2.2.  NRP-specific SRv6 End.X SID

   The SRv6 End.X SIDs are advertised in the BGP-LS attribute with Link
   NLRI.In order to distinguish the End.X SIDs which belong to different
   NRPs, a new "NRP ID Sub-TLV" is introduced under the SRv6 End.X SID
   TLV and SRv6 LAN End.X SID TLV defined in
   [I-D.ietf-idr-bgpls-srv6-ext].  Its format is shown as 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             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           NRP ID                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

   *  Type: TBD.

   *  Length: the length of the Value field of the TLV.  It is set to 4.

   *  NRP ID: A 32-bit global identifier to identify the NRP this End.X
      SID is associated with.

5.3.  Dedicated NRP ID in Data Plane

   As the number of NRPs increases, with the mechanism described in
   [I-D.ietf-spring-sr-for-enhanced-vpn], the number of SR SIDs and SRv6
   Locators allocated for different NRPs would also increase.  In
   network scenarios where the number of SIDs or Locators becomes a
   concern, some data plane optimization may be needed to reduce the
   amount of SR SIDs and Locators allocated.  As described in
   [I-D.dong-teas-nrp-scalability], one approach is to decouple the data
   plane identifiers used for topology based forwarding and the
   identifiers used for the NRP-specific processing.  Thus a new data
   plane global NRP-ID could be introduced and encapsulated in the
   packet.  One possible encapsulation of NRP-ID in IPv6 data plane is
   proposed in [I-D.dong-6man-enhanced-vpn-vtn-id].  One possible
   encapsulation of NRP-ID in MPLS data plane is proposed in
   [I-D.li-mpls-enhanced-vpn-vtn-id].

   In that case, the NRP ID encapsulated in data packet can be the same
   value as the NRP ID used in the control protocols, so that the
   overhead of advertising the mapping relationship between the NRP IDs
   in the control plane and the corresponding data plane identifiers
   could be saved.

6.  Security Considerations

   This document introduces no additional security vulnerabilities to
   BGP-LS.

   The mechanism proposed in this document is subject to the same
   vulnerabilities as any other protocol that relies on BGP-LS.

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7.  IANA Considerations

   TBD

8.  Acknowledgments

   The authors would like to thank Shunwan Zhuang and Zhenbin Li for the
   review and discussion of this document.

9.  References

9.1.  Normative References

   [I-D.ietf-idr-bgp-ls-flex-algo]
              Talaulikar, K., Psenak, P., Zandi, S., and G. Dawra,
              "Flexible Algorithm Definition Advertisement with BGP
              Link-State", Work in Progress, Internet-Draft, draft-ietf-
              idr-bgp-ls-flex-algo-08, 10 November 2021,
              <https://www.ietf.org/archive/id/draft-ietf-idr-bgp-ls-
              flex-algo-08.txt>.

   [I-D.ietf-idr-bgpls-srv6-ext]
              Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
              Bernier, D., and B. Decraene, "BGP Link State Extensions
              for SRv6", Work in Progress, Internet-Draft, draft-ietf-
              idr-bgpls-srv6-ext-09, 10 November 2021,
              <https://www.ietf.org/archive/id/draft-ietf-idr-bgpls-
              srv6-ext-09.txt>.

   [I-D.ietf-idr-rfc7752bis]
              Talaulikar, K., "Distribution of Link-State and Traffic
              Engineering Information Using BGP", Work in Progress,
              Internet-Draft, draft-ietf-idr-rfc7752bis-10, 10 November
              2021, <https://www.ietf.org/archive/id/draft-ietf-idr-
              rfc7752bis-10.txt>.

   [I-D.ietf-spring-resource-aware-segments]
              Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., Li,
              Z., and F. Clad, "Introducing Resource Awareness to SR
              Segments", Work in Progress, Internet-Draft, draft-ietf-
              spring-resource-aware-segments-03, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-spring-
              resource-aware-segments-03.txt>.

   [I-D.ietf-spring-sr-for-enhanced-vpn]
              Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., Li,
              Z., and F. Clad, "Segment Routing based Virtual Transport
              Network (VTN) for Enhanced VPN", Work in Progress,

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              Internet-Draft, draft-ietf-spring-sr-for-enhanced-vpn-01,
              12 July 2021, <https://www.ietf.org/archive/id/draft-ietf-
              spring-sr-for-enhanced-vpn-01.txt>.

   [I-D.ietf-teas-enhanced-vpn]
              Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
              Framework for Enhanced Virtual Private Network (VPN+)
              Services", Work in Progress, Internet-Draft, draft-ietf-
              teas-enhanced-vpn-09, 25 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-teas-enhanced-
              vpn-09.txt>.

   [I-D.ietf-teas-ietf-network-slices]
              Farrel, A., Gray, E., Drake, J., Rokui, R., Homma, S.,
              Makhijani, K., Contreras, L. M., and J. Tantsura,
              "Framework for IETF Network Slices", Work in Progress,
              Internet-Draft, draft-ietf-teas-ietf-network-slices-05, 25
              October 2021, <https://www.ietf.org/archive/id/draft-ietf-
              teas-ietf-network-slices-05.txt>.

   [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>.

   [RFC5029]  Vasseur, JP. and S. Previdi, "Definition of an IS-IS Link
              Attribute Sub-TLV", RFC 5029, DOI 10.17487/RFC5029,
              September 2007, <https://www.rfc-editor.org/info/rfc5029>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [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>.

   [RFC9085]  Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
              H., and M. Chen, "Border Gateway Protocol - Link State
              (BGP-LS) Extensions for Segment Routing", RFC 9085,
              DOI 10.17487/RFC9085, August 2021,
              <https://www.rfc-editor.org/info/rfc9085>.

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   [RFC9086]  Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K.,
              Ray, S., and J. Dong, "Border Gateway Protocol - Link
              State (BGP-LS) Extensions for Segment Routing BGP Egress
              Peer Engineering", RFC 9086, DOI 10.17487/RFC9086, August
              2021, <https://www.rfc-editor.org/info/rfc9086>.

9.2.  Informative References

   [I-D.dong-6man-enhanced-vpn-vtn-id]
              Dong, J., Li, Z., Xie, C., Ma, C., and G. Mishra,
              "Carrying Virtual Transport Network (VTN) Identifier in
              IPv6 Extension Header", Work in Progress, Internet-Draft,
              draft-dong-6man-enhanced-vpn-vtn-id-06, 24 October 2021,
              <https://www.ietf.org/archive/id/draft-dong-6man-enhanced-
              vpn-vtn-id-06.txt>.

   [I-D.dong-lsr-sr-enhanced-vpn]
              Dong, J., Hu, Z., Li, Z., Tang, X., Pang, R., JooHeon, L.,
              and S. Bryant, "IGP Extensions for Scalable Segment
              Routing based Enhanced VPN", Work in Progress, Internet-
              Draft, draft-dong-lsr-sr-enhanced-vpn-07, 29 January 2022,
              <https://www.ietf.org/archive/id/draft-dong-lsr-sr-
              enhanced-vpn-07.txt>.

   [I-D.dong-teas-nrp-scalability]
              Dong, J., Li, Z., Gong, L., Yang, G., Guichard, J. N.,
              Mishra, G., Qin, F., Saad, T., and V. P. Beeram,
              "Scalability Considerations for Network Resource
              Partition", Work in Progress, Internet-Draft, draft-dong-
              teas-nrp-scalability-01, 7 February 2022,
              <https://www.ietf.org/archive/id/draft-dong-teas-nrp-
              scalability-01.txt>.

   [I-D.ietf-lsr-flex-algo]
              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", Work in Progress,
              Internet-Draft, draft-ietf-lsr-flex-algo-18, 25 October
              2021, <https://www.ietf.org/archive/id/draft-ietf-lsr-
              flex-algo-18.txt>.

   [I-D.ietf-lsr-isis-srv6-extensions]
              Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
              Z. Hu, "IS-IS Extensions to Support Segment Routing over
              IPv6 Dataplane", Work in Progress, Internet-Draft, draft-
              ietf-lsr-isis-srv6-extensions-18, 20 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-lsr-isis-srv6-
              extensions-18.txt>.

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   [I-D.li-mpls-enhanced-vpn-vtn-id]
              Li, Z. and J. Dong, "Carrying Virtual Transport Network
              Identifier in MPLS Packet", Work in Progress, Internet-
              Draft, draft-li-mpls-enhanced-vpn-vtn-id-01, 14 April
              2021, <https://www.ietf.org/archive/id/draft-li-mpls-
              enhanced-vpn-vtn-id-01.txt>.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, DOI 10.17487/RFC4915, June 2007,
              <https://www.rfc-editor.org/info/rfc4915>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <https://www.rfc-editor.org/info/rfc5120>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

   [RFC8668]  Ginsberg, L., Ed., Bashandy, A., Filsfils, C., Nanduri,
              M., and E. Aries, "Advertising Layer 2 Bundle Member Link
              Attributes in IS-IS", RFC 8668, DOI 10.17487/RFC8668,
              December 2019, <https://www.rfc-editor.org/info/rfc8668>.

Authors' Addresses

   Jie Dong
   Huawei Technologies
   Email: jie.dong@huawei.com

   Zhibo Hu
   Huawei Technologies
   Email: huzhibo@huawei.com

   Zhenbin Li
   Huawei Technologies
   Email: lizhenbin@huawei.com

   Xiongyan Tang
   China Unicom

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   Email: tangxy@chinaunicom.cn

   Ran Pang
   China Unicom
   Email: pangran@chinaunicom.cn

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