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Versions: 00 01 02

IDR Working Group                                                J. Dong
Internet-Draft                                                     Z. Hu
Intended status: Standards Track                                   Z. Li
Expires: December 25, 2020                           Huawei Technologies
                                                                 X. Tang
                                                                 R. Pang
                                                            China Unicom
                                                           June 23, 2020


        BGP-LS Extensions for Segment Routing based Enhanced VPN
                draft-dong-idr-bgpls-sr-enhanced-vpn-02

Abstract

   Enhanced VPN (VPN+) is an enhancement to VPN services to support the
   needs of new applications, particularly including 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.  An enhanced
   VPN may be used for 5G transport network slicing, and will also be of
   use in more generic scenarios.  To meet the requirement of enhanced
   VPN services, a number of Virtual Transport Networks (VTNs) 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.

   This document specifies the BGP-LS mechanisms with necessary
   extensions to advertise the information of Segment Routing (SR) based
   VTNs.  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.

   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/.



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

   This Internet-Draft will expire on December 25, 2020.

Copyright Notice

   Copyright (c) 2020 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Advertisement of VTN Definition . . . . . . . . . . . . . . .   3
   3.  Advertisement of VTN Topology Attribute . . . . . . . . . . .   5
     3.1.  Intra-domain Topology Advertisement . . . . . . . . . . .   5
       3.1.1.  MTR based Topology Advertisement  . . . . . . . . . .   5
       3.1.2.  Flex-Algo based Topology Advertisement  . . . . . . .   6
     3.2.  Inter-Domain Topology Advertisement . . . . . . . . . . .   6
       3.2.1.  VTN ID TLV  . . . . . . . . . . . . . . . . . . . . .   8
   4.  Advertisement of VTN Resource Attribute . . . . . . . . . . .   9
     4.1.  Link Attribute Flags TLV  . . . . . . . . . . . . . . . .   9
   5.  Advertisement of VTN specific Data Plane Identifiers  . . . .  10
     5.1.  VTN-specific SIDs for SR-MPLS . . . . . . . . . . . . . .  10
       5.1.1.  VTN-specific Prefix-SID TLV . . . . . . . . . . . . .  10
       5.1.2.  VTN specific Adj-SID TLV  . . . . . . . . . . . . . .  11
     5.2.  VTN-specific Locators for SRv6  . . . . . . . . . . . . .  12
     5.3.  Dedicated VTN ID in Data Plane  . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15




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1.  Introduction

   Enhanced VPN (VPN+) is an enhancement to VPN services to support the
   needs of new applications, particularly including 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 cannot be met with pure overlay networks, as they 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
   transport network slicing, and will also be of use in more generic
   scenarios.

   To meet the requirement of enhanced VPN services, a number of Virtual
   Transport Networks (VTNs) 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.

   [I-D.dong-spring-sr-for-enhanced-vpn] specifies how segment routing
   (SR) [RFC8402] can be used to build virtual transport networks (VTNs)
   with the required network topology and network resources, which could
   be used as the underlay of enhanced VPN services.
   [I-D.dong-lsr-sr-enhanced-vpn] specifies the IGP mechanism and
   extensions to build a set of SR based VTNs.  When a VTN spans
   multiple IGP areas or multiple Autonomous Systems (ASes), BGP-LS is
   needed to advertise the VTN information in each IGP area or AS to the
   network controller, so that the controller could use the collected
   information to build the inter-area or inter-AS SR VTNs.

   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 VTNs.  The definition of VTN
   is advertised as a node attribute using BGP-LS.  The associated
   network topology and resources attributes of a VTN are advertised as
   link attribute using BGP-LS.

2.  Advertisement of VTN Definition

   According to [I-D.ietf-teas-enhanced-vpn], a VTN has a customized
   network topology and a set of dedicated or shared network resources.
   Thus a VTN 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.





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   Virtual Transport Network Definition (VTND) TLV is a new TLV of the
   optional BGP-LS Attribute which is associated with the node NLRI.

   The format of VTND TLV 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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            VTN ID                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             MT-ID             |    Algorithm  |     Flags     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Sub-TLVs                             |
      ~                            ...                                ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

   o  Type: TBD

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

   o  VTN ID: A global significant 32-bit identifier which is used to
      identify a virtual transport network.

   o  MT-ID: 16-bit identifier which indicates the multi-topology
      identifier of the IGP topology.

   o  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].

   o  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.

   o  Sub-TLVs: optional sub-TLVs to specify the additional attributes
      of a virtual transport network.  Currently no sub-TLV is defined
      in this document.







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3.  Advertisement of VTN Topology Attribute

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

3.1.  Intra-domain Topology Advertisement

3.1.1.  MTR based Topology Advertisement

   In section 3.2.1.5 of [RFC7752], 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.

   [I-D.ietf-idr-bgp-ls-segment-routing-ext] 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 and 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
   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.

   [RFC7752] 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 need to be
   generated where each NLRI contains an unique MT-ID.  In the BGP-LS
   attribute of a Node NLRI, one MT-ID TLV containing the array of MT-
   IDs of all topologies where the node is reachable is allowed."




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   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 participates in multiple topologies, multiple NLRIs needs to
   be generated to report all the topologies a link or prefix
   participates in, together with the topology-specific segment routing
   information.  This would increase the number of BGP Updates 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 introduced for the reporting of multi-topology information and the
   associated segment routing information in BGP-LS.  Based on the WG's
   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 topological constraints for path computation on a
   network topology.  As specified in [I-D.dong-lsr-sr-enhanced-vpn],
   the topology of a VTN can be determined by applying Flex-Algo on a
   default 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
   [I-D.ietf-idr-bgp-ls-segment-routing-ext] and SRv6
   [I-D.ietf-idr-bgpls-srv6-ext] provide the mechanism to advertise the
   algorithm-specific segment routing information.

   In[I-D.ietf-idr-bgp-ls-segment-routing-ext], 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 the Prefix NLRI with the SRv6
   Locator TLV carried in the associated BGP-LS Attribute, 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

   [I-D.ietf-idr-bgpls-segment-routing-epe] and
   [I-D.ietf-idr-bgpls-srv6-ext] defines the BGP-LS extensions for
   advertisement of BGP topology information between ASes and the 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.





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   In some network scenarios, VTNs which span multiple ASes need to be
   created.  The multi-domain VTNs 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 VTNs using segment routing, it is
   necessary to advertise the topology and resource attribute of VTN on
   the inter-domain links and the associated BGP Peering SIDs.

   Depending on the requirement of inter-domain VTNs, different
   mechanism can be used on the inter-domain connection:

   o  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 VTNs 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 several 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 VTN identifier associated with each BGP Peer-Adj-
      SID.

   o  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 multi-domain VTN, while some other BGP sessions are used
      for another multi-domain VTN.  In this case, different BGP peer-
      node-SIDs are allocated to each BGP session, and ASBRs SHOULD
      advertise the VTN identifier associated with each BGP Peer-node-
      SIDs.

   o  At the AS-level topology, different multi-domain VTNs 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 multi-domain VTN.

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

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




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   o  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
   VTN-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.  VTN ID TLV

   A new VTN ID TLV is defined to describe the identifiers of one or
   more VTNs 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 VTN ID 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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            VTN ID-1                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                              ...                              ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            VTN ID-n                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

   o  Type: TBD

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

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

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





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   o  VTN IDs: One or more 32-bit identifiers to specify the VTNs this
      link or member link belongs to.

4.  Advertisement of VTN Resource Attribute

   [I-D.dong-lsr-sr-enhanced-vpn] specifies the mechanism to advertise
   the resource information associated with each VTN.  It is based on
   the extensions to the advertisement of L2 bundle member links
   information[RFC8668].  This section defines the corresponding BGP-LS
   extensions.  Two new TLVs are defined to carry the VTN ID and the
   link attribute flags of either a Layer 3 link or L2 bundle member
   link.  The new VTN ID TLV is defined in section 3.2.1 of this
   document, and the new Link Attribute Flags TLV is defined in this
   section.  The TE attributes of each Layer 3 link or Layer 2 bundle
   member link, such as the bandwidth and adj-SIDs, can be advertised
   using the mechanism as defined in [I-D.ietf-idr-bgp-ls-segment-routin
   g-ext][I-D.ietf-idr-bgpls-segment-routing-epe] and
   [I-D.ietf-idr-bgpls-srv6-ext].

4.1.  Link Attribute Flags TLV

   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 V is defined in this
      document.  When the V flag is set, it indicates this link is a
      virtual link.





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5.  Advertisement of VTN specific Data Plane Identifiers

   In network scenarios where each VTN is associated with an independent
   network topology or Flex-Algo, the topology or Flex-Algo specific
   SIDs or Locators could be used as the identifier of the VTN in data
   plane.  In network scenarios where multiple VTNs share the same
   topology or Flex-Algo, additional data plane identifiers would be
   needed to identify different VTNs.

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

5.1.  VTN-specific SIDs for SR-MPLS

   With SR-MPLS data plane, the VTN identification information is
   implicitly carried in the SR SIDs of the corresponding VTN.  Each
   node SHOULD allocate VTN-specific Prefix-SIDs for each VTN it
   participates in.  Similarly, VTN-specific Adj-SIDs MAY be allocated
   for each link which participates in the VTN.

5.1.1.  VTN-specific Prefix-SID TLV

   A new VTN-specific Prefix-SID TLV is defined to advertise the prefix-
   SID and its associated VTN.  It is derived from VTN 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               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            VTN ID                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      SID/Index/Label(Variable)                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

   o  Type: TBD

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

   o  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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   o  VTN ID: A 32-bit local identifier to identify the VTN this prefix-
      SID associates with.

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

   Multiple VTN-specific Prefix-SID TLV could 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
   these VTNs.

5.1.2.  VTN specific Adj-SID TLV

   A new VTN-specific Adj-SID TLV is defined to advertise the Adj-SID
   and its associated VTN.  It is derived from VTN 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               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            VTN ID                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      SID/Index/Label(Variable)                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

   o  Type: TBD

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

   o  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.

   o  VTN ID: A 32-bit local identifier to identify the VTN this Adj-SID
      associates with.

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

   Multiple VTN-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 VTNs.




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5.2.  VTN-specific Locators for SRv6

   With SRv6 data plane, the VTN identification information can be
   implicitly or explicitly carried in the SRv6 Locator of the
   corresponding VTN.  Network nodes SHOULD allocate VTN-specific
   Locators for each VTN it participates in.  The VTN-specific Locators
   are used as the covering prefix of VTN-specific SRv6 End SIDs, End.X
   SIDs and their variants.

   Each VTN-specific SRv6 Locator MAY be advertised in a separate Prefix
   NLRI.  If multiple VTNs share the same topology, the topology/
   algorithm specific Locator is the covering prefix of a group of VTN-
   specific Locators.  Then the advertisement of VTN-specific locators
   can be optimized to reduce the amount of information advertised in
   the control plane.  More details about this mechanism will be
   provided in a future version of this document.

5.3.  Dedicated VTN ID in Data Plane

   As the number of VTNs increases, some data plane optimization is
   needed to reduce the amount of SR SIDs and locators allocated for
   VTNs.  Thus a dedicated VTN ID could be encapsulated in the packet as
   proposed in [I-D.dong-6man-enhanced-vpn-vtn-id].

   In that case, the VTN ID encapsulated in data plane can has the same
   value as the VTN ID in control plane, so that the overhead of
   advertising the mapping between the VTN IDs in 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.

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.






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9.  References

9.1.  Normative References

   [I-D.dong-spring-sr-for-enhanced-vpn]
              Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., and
              Z. Li, "Segment Routing for Resource Guaranteed Virtual
              Networks", draft-dong-spring-sr-for-enhanced-vpn-08 (work
              in progress), June 2020.

   [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", draft-ietf-idr-bgp-ls-flex-algo-02 (work in
              progress), January 2020.

   [I-D.ietf-idr-bgp-ls-segment-routing-ext]
              Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
              and M. Chen, "BGP Link-State extensions for Segment
              Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-16
              (work in progress), June 2019.

   [I-D.ietf-idr-bgpls-segment-routing-epe]
              Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray,
              S., and J. Dong, "BGP-LS extensions for Segment Routing
              BGP Egress Peer Engineering", draft-ietf-idr-bgpls-
              segment-routing-epe-19 (work in progress), May 2019.

   [I-D.ietf-idr-bgpls-srv6-ext]
              Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
              daniel.bernier@bell.ca, d., and B. Decraene, "BGP Link
              State Extensions for SRv6", draft-ietf-idr-bgpls-
              srv6-ext-02 (work in progress), January 2020.

   [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>.



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   [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>.

9.2.  Informative References

   [I-D.dong-6man-enhanced-vpn-vtn-id]
              Dong, J. and Z. Li, "Carrying Virtual Transport Network
              (VTN) Identifier in IPv6 Extensison Header for Enhanced
              VPN", draft-dong-6man-enhanced-vpn-vtn-id-00 (work in
              progress), February 2020.

   [I-D.dong-lsr-sr-enhanced-vpn]
              Dong, J., Hu, Z., Li, Z., and S. Bryant, "IGP Extensions
              for Segment Routing based Enhanced VPN", draft-dong-lsr-
              sr-enhanced-vpn-03 (work in progress), March 2020.

   [I-D.ietf-lsr-flex-algo]
              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
              algo-07 (work in progress), April 2020.

   [I-D.ietf-lsr-isis-srv6-extensions]
              Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
              Z. Hu, "IS-IS Extension to Support Segment Routing over
              IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-08
              (work in progress), April 2020.

   [I-D.ietf-teas-enhanced-vpn]
              Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
              Framework for Enhanced Virtual Private Networks (VPN+)
              Services", draft-ietf-teas-enhanced-vpn-05 (work in
              progress), February 2020.

   [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>.






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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

   Email: tangxy@chinaunicom.cn


   Ran Pang
   China Unicom

   Email: pangran@chinaunicom.cn





















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