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Versions: 00 01 02

TEAS Working Group                                               J. Dong
Internet-Draft                                                     Z. Li
Intended status: Informational                       Huawei Technologies
Expires: August 26, 2021                                          F. Qin
                                                            China Mobile
                                                                 G. Yang
                                                           China Telecom
                                                             J. Guichard
                                                  Futurewei Technologies
                                                       February 22, 2021


           Scalability Considerations for Enhanced VPN (VPN+)
            draft-dong-teas-enhanced-vpn-vtn-scalability-02

Abstract

   Enhanced VPN (VPN+) aims to provide enhancements to existing VPN
   services to support the needs of new applications, particularly
   including the applications that are associated with 5G services.
   VPN+ could be used to provide network slicing, and may also be of use
   in more generic scenarios, such as enterprise services which have
   demanding requirement.  With the requirement for VPN+ services
   increase, scalability would become an important factor for the
   deployment of VPN+.  This document describes the scalability
   considerations in the control plane and data plane to enable VPN+
   services, some optimization mechanisms are also described.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 26, 2021.







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Copyright Notice

   Copyright (c) 2021 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  VPN+ Scalability Requirements . . . . . . . . . . . . . . . .   3
   3.  VPN+ Scalability Considerations . . . . . . . . . . . . . . .   5
     3.1.  Control Plane Scalability . . . . . . . . . . . . . . . .   5
       3.1.1.  Distributed Control Plane . . . . . . . . . . . . . .   5
       3.1.2.  Centralized Control Plane . . . . . . . . . . . . . .   6
     3.2.  Data Plane Scalability  . . . . . . . . . . . . . . . . .   6
     3.3.  Gap Analysis of Existing Mechanisms . . . . . . . . . . .   7
   4.  Possible Scalability Optimizations  . . . . . . . . . . . . .   7
     4.1.  Control Plane Optimizations . . . . . . . . . . . . . . .   7
     4.2.  Data Plane Optimizations  . . . . . . . . . . . . . . . .   9
   5.  Solution Evolution for Improved Scalability . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   10. Informative References  . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Virtual Private Networks (VPNs) have served the industry well as a
   means of providing different groups of users with logically isolated
   connectivity over a common network infrastructure.  The VPN service
   is provided with two network layers: the overlay and the underlay.
   The underlay is responsible for establishing network connectivity and
   managing network resources to meet the service requirement.  The
   overlay is used to distribute the membership and reachability
   information of the tenants, and provide logical separation of service
   delivery between different tenants.




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   Enhanced VPN service (VPN+) [I-D.ietf-teas-enhanced-vpn] is targeted
   at new applications which require better isolation between tenants
   and/or services, and have more stringent performance requirements
   than can be provided with existing VPNs.  To meet the requirement of
   VPN+ services, Virtual Transport Networks (VTN) need to be created,
   each has a subset of the underlay network topology and a set of
   network resources allocated to meet the requirements of one or a
   group of VPN+ services.  The VPN together with the corresponding VTN
   in the underlay provide the VPN+ service.

   [I-D.ietf-teas-enhanced-vpn] provides some general analysis of the
   scalability of VPN+. This document gives detailed analysis of the
   scalability considerations when enabling VPN+ services.  The focus of
   this document is mainly on the scalability of the underlay of VPN+,
   i.e. the VTN.

2.  VPN+ Scalability Requirements

   As described in [I-D.ietf-teas-enhanced-vpn], VPN+ services may
   require additional state to be introduced into the network to take
   advantage of the enhanced functionality.  This introduces some
   scalability considerations to the network.  This section gives some
   analysis of the number of VPN+ services that might be needed in a
   network.

   There are several use cases where VPN+ may be needed, and these
   determine how many VPN+ will be required in a network.  One typical
   use case of VPN+ is to deliver IETF network slice
   [I-D.ietf-teas-ietf-network-slice-definition] for applications or
   services in 5G and other scenarios, thus the number of IETF network
   slices needed could reflect the number of VPN+ services.  With the
   development and evolution of 5G, it is expected that more and more
   network slices will be deployed.  The number of network slices
   required is relevant to how network slicing will be used, and the
   progress of 5G for the vertical industrial services.  The potential
   number of network slices is analyzed by classifying the network
   slicing deployment into three typical scenarios:

   1.  Network slicing can be used by a network operator internally to
       isolate different types of services.  For example, in a converged
       multi-service network, different network slices can be created to
       carry mobile transport service, fixed broadband service and
       enterprise services respectively, each type of service could be
       managed by a separate department or management team.  Some
       service types, such as multicast service may also be deployed in
       a dedicated network slice.  It is also possible that an
       infrastructure network operator provides network slices to other
       network operators as a wholesale service.  In this scenario, the



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       number of network slices in a network would be relatively small,
       such as on the order of 10 or so.  This could be the typical case
       in the beginning of the network slicing deployment.

   2.  Network slicing can be used to provide isolated and customized
       virtual networks for tenants in different vertical industries.
       At the early stage of the vertical industrial service deployment,
       a few top tenants in some typical industries will begin to use
       network slicing to support their business, such as smart grid,
       manufacturing, public safety, on-line gaming, etc.  Considering
       the number of the vertical industries, and the number of top
       tenants in each industry, the number of network slices may
       increase to the order of 100.

   3.  With the evolution of 5G, network slicing could be widely used by
       both vertical industrial tenants and enterprise tenants which
       require guaranteed or predictable service performance.  The total
       amount of network slices may increase to the order of 1000 or
       more.  However, it is expected that the number of network slices
       would still be less than the number of traditional VPN services
       in the network.

   In 3GPP [TS23501], a 5G network slice is identified using Single
   Network Slice Selection Assistance Information (S-NSSAI), which is a
   32-bit identifier comprised of 8-bit Slice/Service Type (SST) and
   24-bit Slice Differentiator (SD).  This allows the mobile networks
   (RAN and CN) to provide a large number of network slices.  Although
   it is possible that multiple network slices in RAN and CN can be
   mapped to the same IETF network slice, the number of IETF network
   slices may still be comparable with the number of 5G network slices.
   Thus the scalability of IETF network slices needs to be taken into
   consideration.

                      8-bit              24-bit
                  +------------+-------------------------+
                  |    SST     |   Slice Differentiator  |
                  +------------+-------------------------+

                   Figure 1. Format of S-NSSAI in 3GPP

   VPN+ needs to meet the scalability requirement of network slicing in
   different scenarios.  The increased number of VPN+ will introduce
   additional complexity and overhead to both the control plane and data
   plane, especially in the aspects related to the underlying VTNs.
   Although multiple VPN+ services can be mapped to the same VTN as the
   underlay, there still can be scalability challenges with the
   increased number of VTNs.




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3.  VPN+ Scalability Considerations

   In this section, the scalability in the control plane and data plane
   is analyzed to understand the possible gaps in meeting the
   scalability requirement of VPN+.

3.1.  Control Plane Scalability

   As described in [I-D.ietf-teas-enhanced-vpn], the control plane of
   VPN+ could be based on the hybrid of a centralized controller and the
   distributed control plane.

3.1.1.  Distributed Control Plane

   At part of the construction of VPN+ services, it is necessary to
   create different VTNs that provide customized topology and resource
   attributes.  The attributes and state information of each VTN needs
   to be exchanged in the control plane.  The scalability of the
   distributed control plane for the establishment and maintenance of
   VTNs needs to be considered in the following aspects:

   o  The number of control protocol instances maintained on each node

   o  The number of protocol sessions maintained on each link

   o  The number of routes advertised by each node

   o  The amount of attributes associated with each route

   o  The number of route computation (i.e.  SPF computation) executed
      on each node

   As the number of VTNs increases, it is expected that for some of the
   above aspects, the overhead in the control plane may increase
   dramatically.  For example, the overhead of maintaining separated
   control protocol instances (e.g.  IGP instances) for different VTNs
   is considered higher than maintaining the information of separated
   VTNs in the same control protocol instance, and the overhead of
   maintaining separate protocol sessions for different VTNs is
   considered higher than using a shared protocol session for the
   information exchange of multiple VTNs.  To meet the requirement of
   the increasing number of VTNs, It is suggested to choose the control
   plane mechanisms which could improve the scalability while still
   provide the required functionality.







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3.1.2.  Centralized Control Plane

   Although the SDN approach can reduce the amount of control plane
   overhead in the distributed control plane, it may transfer some of
   the scalability concerns from network nodes to the centralized
   controller, thus the scalability of the controller also needs to be
   considered.

   To provide global optimization for the Traffic Engineered (TE) paths
   in different VTNs, the controller needs to keep the topology and
   resource information of all the VTNs up to date.  To achieve this,
   the controller may need to maintain a communication channel with each
   network node in the network.  When there is significant change in the
   network, or multiple VTNs requires global optimization concurrently,
   there may be a heavy processing burden at the controller, and a heavy
   load in the network surrounding the controller for the distribution
   of the updated network state.

3.2.  Data Plane Scalability

   To provide different VPN+ services with the required isolation and
   performance characteristics, it is necessary to allocate different
   sets of network resources to different VTNs.  As the number of VPN+
   increases, the number of VTNs will increase accordingly.  This
   requires the underlying network to provide finer-granular network
   resource partitioning, which means the amount of state about the
   reserved network resources to be maintained on network nodes will
   also increase.

   In data plane, traffic of different VPN+ services need to be
   processed separately according to the topology and resource
   constraints of the associated VTN , thus the identifier of VTN needs
   to be carried either directly or implicitly in the data packet.
   Different representations of the VTN information in data packet can
   have different scalability implications.

   One approach is to reuse some existing fields in the data packet to
   additionally identify the VTN the packet belongs to.  This avoids the
   cost of defining new fields in the data packet, while since it
   introduces additional semantics to an existing field, it may change
   the processing of the existing field in packet forwarding.  To
   distinguish different VTNs, the number of identifiers which were used
   to identify a node or link may be increased in proportion to the
   number of the VTNs, which may cause scalability problem in some
   networks.

   An alternative approach is to introduce a dedicated field in the
   packet for VTN identification.  This could avoid the impact to the



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   existing fields in the packet.  And if this new field carries a
   global-significant VTN identifier, it could be used together with the
   existing fields to determine the VTN-specific packet forwarding.  The
   potential issue with this approach is the difficulty in introducing a
   new field in some types of the data plane.

   In addition, the introduction of per VTN packet forwarding has impact
   on the scalability of the forwarding entries on network nodes, as a
   network node needs to maintain separate forwarding entries for a
   target node in each VTN it participates.

3.3.  Gap Analysis of Existing Mechanisms

   One candidate approach to build VTN is to use Segment Routing (either
   SR-MPLS or SRv6) as the data plane, and define and distribute the
   customized topology and resource attribute of each VTN based on
   Multi-topology [RFC4915] [RFC5120], Flex-Algo
   [I-D.ietf-lsr-flex-algo] or the combination of these mechanisms in
   the control plane.  As the number of VTNs increases, there may be
   several scalability concerns with this approach:

   1.  The number of SR SIDs needed will increase dependent upon the
       number of VTNs in the network, which will bring challenges both
       to the SID information distribution in the control plane and to
       the installation of forwarding entries for the SIDs in data
       plane.

   2.  The number of SPF computation will increase in proportion to the
       number of VTNs in the network, which can introduce significant
       overhead of the computing resources on network nodes.

   3.  The maximum number of network topology supported by OSPF Multi-
       topology is 128, and the maximum number of Flex-Algo is 128,
       which may not meet the required number of VTNs in some networks.

4.  Possible Scalability Optimizations

4.1.  Control Plane Optimizations

   For the distributed control plane, several optimizations can be
   considered to reduce the overhead and improve the scalability.

   The first optimization mechanism is to reduce the amount of control
   plane sessions used for the establishment and maintenance of the
   VTNs.  For multiple VTNs which have the same peering relationship
   between two adjacent network nodes, it is proposed that one single
   control session is used for the establishment of multiple VTNs.
   Information of different VTNs can be exchanged over the same control



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   session, with necessary identification information to distinguish
   them in the control messages.  This could reduce the overhead of
   maintaining a large number of control protocol sessions, and could
   also reduce the amount of control plane message flooding in the
   network.

   The second optimization mechanism is to decompose the attributes of a
   VTN into different groups, so that different types of attribute can
   be advertised and processed separately in control plane.  For a VTN,
   there are two basic types of attributes: the topology attribute and
   the associated network resource attribute.  In a network, it is
   possible that multiple VTNs share the same topology, and multiple
   VTNs may share the same set of network resource on particular network
   segments.  It is more efficient if only one copy of the topology
   attribute is advertised, then multiple VTNs sharing the same topology
   could refer to the topology information.  More importantly, the
   result of topology-based route computation could be shared by these
   VTNs, so that the overhead of per-VTN route computation could be
   reduced.  Similarly, information of a subset of network resources
   reserved on network segments could be advertised once and then be
   used by multiple VTNs.  This methodology could also apply to other
   attributes of VTN which may be introduced later and can be processed
   independently.

                        O#####O#####O          O*****O*****O
                        #     #     #          *     *     *
                        #     #     #          *     *     *
                        O#####O#####O          O*****O*****O

                            VTN-1                  VTN-2

                                   O-----O-----O
                                   |     |     |
                                   |     |     |
                                   O-----O-----O

                               Shared Network Topology

          Legend

          O     Virtual node
          ###   Virtual links with a set of reserved resources
          ***   Virtual links with another set of reserved resources

                     Figure 2. Topology Sharing between VTNs

                                   FIG-2




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   Figure 2 gives an example of multiple VTNs which share the same
   topology attribute.  As shown in the figure, VTN-1 and VTN-2 have the
   same topology, while the link resource attributes of each VTN are
   different.  In this case, only one copy of the network topology
   information needs to be advertised, and the topology-based route
   computation result can be used by both VTNs to generate the routing
   tables.

                       O#####O#####O         O-  -O#####O
                       #     #     #           \/ #     #
                       #     #     #           /\ #     #
                       O#####O#####O         O-  -O#####O

                           VTN-1                VTN-2

       Legend

       O     Virtual node
       ###   Virtual links with a set of reserved resource
       ---   Virtual links with another set of reserved resource

                  Figure 3. Resource Sharing between VTNs

   Figure 3 gives another example of multiple VTNs which shares the same
   set of network resources on some links.  In this case, information
   about the reserved resource on each link only needs to be advertised
   once, then both VTN-1 and VTN-2 could refer to the link resource for
   constraint based path computation.

   For the centralized control plane, it is suggested that the
   centralized controller is deployed as a complementary mechanism to
   the distributed control plane rather than a replacement, so that the
   VTN specific path computation burden in control plane could be shared
   by both the centralized controller and the network nodes, thus the
   scalability of both systems could be improved.

4.2.  Data Plane Optimizations

   To support more VPN+ services while keeping the amount of data plane
   state at a reasonable scale, one possible approach is to classify a
   set of VPN+ services which have similar service characteristics and
   performance requirements into a group, and such group of VPN+ is
   mapped to one VTN, which is allocated with an aggregated set of
   network topology and resources to meet the service requirement of the
   whole group of VPN+. Different groups of VPN+ need to be mapped to
   different VTNs with different set of network resources allocated.
   With appropriate grouping of VPN+ services, a reasonable number of




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   VTNs with network resources reservation and aggregation could still
   meet the service requirements.

   Another optimization in the data plane is to decouple the identifier
   used for topology-based forwarding and the identifier used for the
   resource-specific processing introduced by VTN.  One possible
   mechanism is to introduce a dedicated field in the packet header to
   uniquely identify the set of local network resources allocated to a
   VTN on each network node for the processing and forwarding of the
   received packet.  Then the existing identifier in the packet header
   used for topology based forwarding is kept unchanged.  The benefit is
   the number of existing topology-specific identifiers will only
   increase in proportion to the number of topologies rather than the
   number of VTNs, so that its scalability will not be impacted by the
   increase of VTN.  Since this new VTN field will be used together with
   the existing fields to determine the VTN-specific packet forwarding,
   this probably requires network nodes to support a hierarchical
   forwarding table in the data plane.  Figure 4 shows the concept of
   using different data plane identifiers for topology-based and VTN
   resource-based packet processing respectively.

                           +--------------------------+
                           |       Packet Header      |
                           |                          |
                           | +----------------------+ |
                           | | Topology-specific ID | |
                           | +----------------------+ |
                           |                          |
                           | +----------------------+ |
                           | |    VTN Resource ID   | |
                           | +----------------------+ |
                           +--------------------------+

                   Figure 4. Decoupled Data Plane Identifiers

   In an IPv6 [RFC8200] based network, this could be achieved by
   introducing a dedicated field in either the IPv6 fixed header or one
   of the extension headers to carry the VTN identifier for the
   resource-specific forwarding, while keeping the destination IP
   address field used for routing towards the destination prefix in the
   corresponding topology.  Note that the VTN ID needs to be parsed by
   every node along the path which is capable of VTN-specific
   forwarding.  In an MPLS [RFC3032] based network, this may be achieved
   by introducing a dedicated MPLS label to identify the VTN instance,
   while the existing MPLS labels could be used for topology-based
   packet forwarding towards the associated destination prefix.  This
   requires that both labels be parsed by each node along the forwarding
   path of the packet.  Another option with MPLS data plane is to



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   introduce a new VTN header which follows the MPLS label stack.  The
   detailed extensions in IPv6 and MPLS encapsulation are out of the
   scope of this document.

5.  Solution Evolution for Improved Scalability

   Based on the analysis in this document, the control plane and data
   plane for VPN+ needs to evolve to support the increasing number of
   VPN+ services in the network.

   As the first step, by introducing resource-awareness to segment
   routing SIDs [I-D.ietf-spring-resource-aware-segments], and using
   Multi-Topology or Flex-Algo as the control plane, it could provide a
   solution for building a limited number of VTNs in the network to meet
   the requirement of a small number of VPN+ services in the network.
   This mechanism is considered as the basic SR VTN.

   As the number of required VPN+ services increases, more VTNs may need
   to be created, then the control plane scalability could be improved
   by decoupling the topology attribute from other attributes (e.g.
   resource attribute) of VTN, so that multiple VTNs could share the
   same topology or resource attribute.  This mechanism is considered as
   the optimized SR VTN.  Both the basic and the optimized SR VTN
   mechanisms are described in [I-D.ietf-spring-sr-for-enhanced-vpn].

   If the data plane scalability becomes a concern, dedicated data plane
   VTN identifiers can be introduced to decouple the topology-specific
   identifiers from the VTN-specific resource identifier in the data
   plane, this could help to reduce the number of SR SIDs needed to
   support . This mechanism is considered as resource-independent VTNs.

6.  Security Considerations

   TBD

7.  IANA Considerations

   This document makes no request of IANA.

8.  Contributors

   Zhibo Hu
   Email: huzhibo@huawei.com








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9.  Acknowledgments

   The authors would like to thank Adrian Farrel for the review and
   discussion of this document.

10.  Informative References

   [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-13 (work in progress), October 2020.

   [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", draft-ietf-spring-resource-aware-segments-01
              (work in progress), January 2021.

   [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", February 2021,
              <https://tools.ietf.org/html/draft-ietf-spring-sr-for-
              enhanced-vpn>.

   [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+)
              Service", draft-ietf-teas-enhanced-vpn-06 (work in
              progress), July 2020.

   [I-D.ietf-teas-ietf-network-slice-definition]
              Rokui, R., Homma, S., Makhijani, K., Contreras, L., and J.
              Tantsura, "Definition of IETF Network Slices", draft-ietf-
              teas-ietf-network-slice-definition-00 (work in progress),
              January 2021.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <https://www.rfc-editor.org/info/rfc3032>.

   [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>.





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   [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>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [TS23501]  "3GPP TS23.501", 2016,
              <https://portal.3gpp.org/desktopmodules/Specifications/
              SpecificationDetails.aspx?specificationId=3144>.

Authors' Addresses

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

   Email: jie.dong@huawei.com


   Zhenbin Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Road
   Beijing  100095
   China

   Email: lizhenbin@huawei.com


   Fengwei Qin
   China Mobile
   No. 32 Xuanwumenxi Ave., Xicheng District
   Beijing
   China

   Email: qinfengwei@chinamobile.com









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Internet-Draft       VPN+ Scalability Considerations       February 2021


   Guangming Yang
   China Telecom
   No.109 West Zhongshan Ave., Tianhe District
   Guangzhou
   China

   Email: yangguangm@chinatelecom.cn


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

   Email: james.n.guichard@futurewei.com



































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