A Framework for Enhanced Virtual Private Network (VPN+) Services
draft-ietf-teas-enhanced-vpn-07

Document Type Active Internet-Draft (teas WG)
Authors Jie Dong  , Stewart Bryant  , Zhenqiang Li  , Takuya Miyasaka  , Young Lee 
Last updated 2021-02-09
Replaces draft-dong-teas-enhanced-vpn
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TEAS Working Group                                               J. Dong
Internet-Draft                                                    Huawei
Intended status: Informational                                 S. Bryant
Expires: August 14, 2021                                       Futurewei
                                                                   Z. Li
                                                            China Mobile
                                                             T. Miyasaka
                                                        KDDI Corporation
                                                                  Y. Lee
                                                                 Samsung
                                                       February 10, 2021

    A Framework for Enhanced Virtual Private Network (VPN+) Services
                    draft-ietf-teas-enhanced-vpn-07

Abstract

   This document describes the framework for Enhanced Virtual Private
   Network (VPN+) services.  The purpose of enhanced VPNs is to support
   the needs of new applications, particularly applications that are
   associated with 5G services, by utilizing an approach that is based
   on existing VPN and Traffic Engineering (TE) technologies and adds
   characteristics that specific services require over and above
   traditional VPNs.

   Typically, VPN+ will be used to underpin network slicing, but could
   also be of use in its own right providing enhanced connectivity
   services between customer sites.

   It is envisaged that enhanced VPNs will be delivered using a
   combination of existing, modified, and new networking technologies.
   This document provides an overview of relevant technologies and
   identifies some areas for potential new work.

   Compared to traditional VPNs, it is not envisaged that large numbers
   of VPN+ services will be deployed in a network.  In other word, it is
   not intended that all existing VPNs supported by a network will use
   VPN+ techniques.

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

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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on August 14, 2021.

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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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Overview of the Requirements  . . . . . . . . . . . . . . . .   6
     3.1.  Performance Guarantees  . . . . . . . . . . . . . . . . .   6
     3.2.  Isolation between Enhanced VPN Services . . . . . . . . .   8
       3.2.1.  A Pragmatic Approach to Isolation . . . . . . . . . .  10
     3.3.  Integration . . . . . . . . . . . . . . . . . . . . . . .  11
       3.3.1.  Abstraction . . . . . . . . . . . . . . . . . . . . .  11
     3.4.  Dynamic Changes . . . . . . . . . . . . . . . . . . . . .  12
     3.5.  Customized Control  . . . . . . . . . . . . . . . . . . .  12
     3.6.  Applicability . . . . . . . . . . . . . . . . . . . . . .  13
     3.7.  Inter-Domain and Inter-Layer Network  . . . . . . . . . .  13
   4.  Architecture of Enhanced VPNs . . . . . . . . . . . . . . . .  14
     4.1.  Layered Architecture  . . . . . . . . . . . . . . . . . .  15
     4.2.  Multi-Point to Multi-Point (MP2MP) Connectivity . . . . .  18
     4.3.  Application Specific Data Types . . . . . . . . . . . . .  18
     4.4.  Scaling Considerations  . . . . . . . . . . . . . . . . .  18
   5.  Candidate Technologies  . . . . . . . . . . . . . . . . . . .  19
     5.1.  Layer-Two Data Plane  . . . . . . . . . . . . . . . . . .  19
       5.1.1.  Flexible Ethernet . . . . . . . . . . . . . . . . . .  19
       5.1.2.  Dedicated Queues  . . . . . . . . . . . . . . . . . .  20

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       5.1.3.  Time Sensitive Networking . . . . . . . . . . . . . .  20
     5.2.  Layer-Three Data Plane  . . . . . . . . . . . . . . . . .  21
       5.2.1.  Deterministic Networking  . . . . . . . . . . . . . .  21
       5.2.2.  MPLS Traffic Engineering (MPLS-TE)  . . . . . . . . .  21
       5.2.3.  Segment Routing . . . . . . . . . . . . . . . . . . .  21
     5.3.  Non-Packet Data Plane . . . . . . . . . . . . . . . . . .  22
     5.4.  Control Plane . . . . . . . . . . . . . . . . . . . . . .  22
     5.5.  Management Plane  . . . . . . . . . . . . . . . . . . . .  23
     5.6.  Applicability of Service Data Models to Enhanced VPN  . .  24
       5.6.1.  An Example of Enhanced VPN Delivery . . . . . . . . .  25
   6.  Scalability Considerations  . . . . . . . . . . . . . . . . .  26
     6.1.  Maximum Stack Depth of SR . . . . . . . . . . . . . . . .  27
     6.2.  RSVP-TE Scalability . . . . . . . . . . . . . . . . . . .  27
     6.3.  SDN Scaling . . . . . . . . . . . . . . . . . . . . . . .  27
   7.  OAM Considerations  . . . . . . . . . . . . . . . . . . . . .  28
   8.  Telemetry Considerations  . . . . . . . . . . . . . . . . . .  28
   9.  Enhanced Resiliency . . . . . . . . . . . . . . . . . . . . .  29
   10. Operational Considerations  . . . . . . . . . . . . . . . . .  30
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  30
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  31
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  31
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     15.2.  Informative References . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

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.  The common or base network that
   is used to provide the VPNs is often referred to as the underlay, and
   the VPN is often called an overlay.

   Customers of a network operator may request a connectivity services
   with advanced characteristics such as low latency guarantees, bounded
   jitter, or stricter isolation from other services or customers so
   that changes in some other service (such as changes in network load,
   or events such as congestion or outages) have no or acceptable effect
   on the throughput or latency of the services provided to the
   customer.  These services are referred to as "enhanced VPNs" (known
   as VPN+) in that they are similar to VPN services providing the
   customer with the required connectivity, but in addition they have
   enhanced characteristics.

   The concept of network slicing has gained traction driven largely by
   needs surfacing from 5G [NGMN-NS-Concept] [TS23501] [TS28530]

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   [BBF-SD406].  According to [TS28530], a 5G end-to-end network slice
   consists of three major types network segments: Radio Access Network
   (RAN), Transport Network (TN), and Mobile Core Network (CN).  The
   transport network provides the connectivity between different
   entities in RAN and CN segments of a 5G end-to-end network slice,
   with specific performance commitment.

   An IETF network slice [I-D.ietf-teas-ietf-network-slice-definition]
   is a virtual (logical) network with its own network topology and a
   set of shared or dedicated network resources, which are used to
   provide the network slice consumer with the required connectivity,
   appropriate isolation, and a specific Service Level Objective (SLO).
   In this document (which is solely about IETF technologies) we refer
   to an "IETF network slice" simply as a "network slice": a network
   slice is considered one possible use case of an enhanced VPN.

   A network slice could span multiple technologies (such as IP or
   Optical) and multiple administrative domains.  Depending on the
   consumer's requirement, a network slice could be isolated from other
   network slices in terms of data plane, control plane, and management
   plane resources.

   Network slicing builds on the concepts of resource management,
   network virtualization, and abstraction to provide performance
   assurance, flexibility, programmability, and modularity.  It may use
   techniques such as Software Defined Networking (SDN) [RFC7149],
   network abstraction [RFC7926] and Network Function Virtualization
   (NFV) [RFC8172] [RFC8568] to create multiple logical (virtual)
   networks, each tailored for use by a set of services or by a
   particular tenant or a group of tenants that share the same or
   similar requirements.  These logical networks are created on top of a
   common underlay network.  How the network slices are engineered can
   be deployment-specific.

   VPN+ can be used to instantiate a network slice, but the technique
   can also be of use in general cases to provide enhanced connectivity
   services between customer sites.

   The requirements of enhanced VPN services cannot be met by simple
   overlay networks, as these services require tighter coordination and
   integration between the underlay and the overlay network.  VPN+ is
   built from a VPN overlay and an underlying Virtual Transport Network
   (VTN) which has a customized network topology and a set of dedicated
   or shared resources in the underlay network.  The enhanced VPN may
   also include a set of invoked service functions located within the
   underlay network.  Thus, an enhanced VPN can achieve greater
   isolation with strict performance guarantees.  These new properties,

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   which have general applicability, are also of interest as part of a
   network slicing solution.

   It is not envisaged that VPN+ services will replace traditional VPN
   services.  Traditional VPN services will continue to be delivered
   using pre-existing mechanisms and can co-exist with VPN+ services.

   This document describes a framework for using existing, modified, and
   potential new technologies as components to provide a VPN+ service.
   Specifically, we are concerned with:

   o  The functional requirements and service characteristics of an
      enhanced VPN.

   o  The design of the enhanced data plane.

   o  The necessary protocols in both the underlay and the overlay of
      the enhanced VPN.

   o  The mechanisms to achieve integration between overlay and
      underlay.

   o  The necessary Operation, Administration, and Management (OAM)
      methods to instrument an enhanced VPN to make sure that the
      required Service Level Agreement (SLA) between the customer and
      the network operator is met, and to take any corrective action
      (such as switching traffic to an alternate path) to avoid SLA
      violation.

   The required layered network structure to achieve this is shown in
   Section 4.1.

   Note that, in this document, the relationship of the four terms
   "VPN", "VPN+", "VTN", and "Network Slice" are as follows:

   o  A VPN refers to the overlay network that provides the connectivity
      between different VPN sites, and that maintains traffic separation
      between different VPN customers.

   o  An enhanced VPN (VPN+) is an evolution of the VPN service that
      makes additional service-specific commitments.  An enhanced VPN is
      made by integrating an overlay VPN with a set of network resources
      allocated in the underlay network.

   o  A VTN is a virtual underlay network that connects customer edge
      points.  The VTN has the capability to deliver the performance
      characteristics required by an enhanced VPN customer and to
      provide isolation between separate VPN+ instances.

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   o  A network slice could be provided by building an enhanced VPN.

2.  Terminology

   The following terms are used in this document.  Some of them are
   newly defined, some others reference existing definitions.

   ACTN: Abstraction and Control of Traffic Engineered Networks
   [RFC8453]

   DetNet: Deterministic Networking.  See [DETNET] and [RFC8655]

   FlexE: Flexible Ethernet [FLEXE]

   TSN: Time Sensitive Networking [TSN]

   VN: Virtual Network [I-D.ietf-teas-actn-vn-yang]

   VPN: Virtual Private Network.  IPVPN is defined in [RFC2764], L2VPN
   is defined in [RFC4664], and L3VPN is defined in [RFC4364].

   VPN+: Enhanced VPN.

   VTN: Virtual Transport Network.

   VTP: Virtual Transport Path.  A VTP is a path through the VTN which
   connects two customer edge points.

3.  Overview of the Requirements

   This section provides an overview of the requirements of an enhanced
   VPN service.

3.1.  Performance Guarantees

   Performance guarantees are made by network operators to their
   customers in relation to the services provided to the customers.
   They are usually expressed in SLAs as a set of SLOs.

   There are several kinds of performance guarantee, including
   guaranteed maximum packet loss, guaranteed maximum delay, and
   guaranteed delay variation.  Note that these guarantees apply to
   conformance traffic, out-of-profile traffic will be handled according
   to a separate agreement with the customer.

   Guaranteed maximum packet loss is usually addressed by setting packet
   priorities, queue size, and discard policy.  However this becomes
   more difficult when the requirement is combined with latency

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   requirements.  The limiting case is zero congestion loss, and that is
   the goal of DetNet [DETNET] and TSN [TSN].  In modern optical
   networks, loss due to transmission errors already approaches zero,
   but there is the possibilities of failure of the interface or the
   fiber itself.  This type of fault can only be addressed by some form
   of signal duplication and transmission over diverse paths.

   Guaranteed maximum latency is required by a number of applications
   particularly real-time control applications and some types of virtual
   reality applications.  DetNet [DETNET] is relevant, however
   additional methods of enhancing the underlay to better support the
   delay guarantees may be needed, and these methods will need to be
   integrated with the overall service provisioning mechanisms.

   Guaranteed maximum delay variation is a performance guarantee that
   may also be needed.  [RFC8578] calls up a number of cases where that
   need this guarantee, for example in electrical utilities.  Time
   transfer is an example service that needs a performance guarantee,
   although it is in the nature of time that the service might be
   delivered by the underlay as a shared service and not provided
   through different enhanced VPNs.  Alternatively, a dedicated enhanced
   VPN might be used to provide this as a shared service.

   This suggests that a spectrum of service guarantees need to be
   considered when deploying an enhanced VPN.  As a guide to
   understanding the design requirements we can consider four types of
   service:

   o  Best effort

   o  Assured bandwidth

   o  Guaranteed latency

   o  Enhanced delivery

   The best effort service is the basic service as provided by current
   VPNs.

   An assured bandwidth service is one in which the bandwidth over some
   period of time is assured.  This can be achieved either simply based
   on a best effort service with over-capacity provisioning, or it can
   be based on MPLS traffic engineered label switching paths (TE-LSPs)
   with bandwidth reservations.  Depending on the technique used,
   however, the bandwidth is not necessarily assured at any instant.
   Providing assured bandwidth to VPNs, for example by using per-VPN TE-
   LSPs, is not widely deployed at least partially due to scalability

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   concerns.  VPN+ aims to provide a more scalable approach for such
   services.

   A guaranteed latency service has an upper bound to edge-to-edge
   latency.  Assuring the upper bound is sometimes more important than
   minimizing latency.  There are several new technologies that provide
   some assistance with this performance guarantee.  Firstly, the IEEE
   TSN project introduces the concept of scheduling of delay- and loss-
   sensitive packets.  The DetNet work is also of relevance in assuring
   an upper bound of end-to-end packet latency.  FlexE is also useful to
   help provide these guarantees.  The use of such underlying
   technologies to deliver VPN+ services needs to be considered.

   An enhanced delivery service is one in which the underlay network (at
   Layer 3) attempts to deliver the packet through multiple paths in the
   hope of eliminating packet loss due to equipment or media failures.
   Such a mechanism may need to be used for VPN+ service.

3.2.  Isolation between Enhanced VPN Services

   One element of the SLA demanded for an enhanced VPN may be a
   guarantee that the service offered to the customer will not be
   affected by any other traffic flows in the network.  This is termed
   "isolation" and a customer may express the requirement for isolation
   as an SLO.

   One way for a network operator to meet the requirement for isolation
   is simply by setting and conforming to other SLOs.  For example,
   traffic congestion (interference from other services) might impact on
   the latency experienced by a VPN+ customer.  Thus, in this example,
   conformance to a latency SLO would be the primary requirement for
   delivery of the VPN+ service, and isolation from other services might
   be only a means to that end.

   Another way for a service provider to meet this SLA is to control the
   degree to which traffic from one service is isolated from other
   services in the network.  There are different grades of how isolation
   may be enabled by a network operator and this may result in different
   levels of service perceived by the customer.  These range from simple
   separation of service traffic on delivery (ensuring that traffic is
   not delivered to the wrong customer, which is a basic requirement of
   all existing VPN services), all the way to complete separation within
   the underlay so that the traffic from different services use distinct
   network resources.

   There is a fine distinction between how isolation is requested by a
   customer and how it is delivered by the service provider.  In
   general, the customer is interested in service performance and not

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   how it is delivered.  Thus, for example, the customer wants specific
   quality guarantees and is not concerned about how the service
   provider delivers them.  However, it should be noted that some
   aspects of isolation may be directly measurable by a customer if they
   have information about the traffic patterns on a number services
   supported by the same service provider.  Furthermore, a customer may
   be nervous about disruption caused by other services, contamination
   by other traffic, or delivery of their traffic to the wrong
   destinations.  In this way, the customer may want to specify (and pay
   for!) the level of isolation provided by the service provider.

   Delivery of isolation is achieved in the realization of a VPN+
   through existing technologies that may be supplemented by future
   mechanisms.  The service provider chooses which processes to use to
   deliver this service requirement just as they choose how to meet all
   other SLOs.  Isolation may be achieved in the underlying network by
   various forms of resource partitioning ranging from dedicated
   allocation of resources for a specific enhanced VPN, to sharing of
   resources with some form of safeguards.  For example, interference
   avoidance may be achieved by network capacity planning, allocating
   dedicated network resources, traffic policing or shaping,
   prioritizing in using shared network resources, etc.

   The terms hard and soft isolation are used to indicate different
   levels of isolation.  A VPN has soft isolation if the traffic of one
   VPN cannot be received by the customers of another VPN.  Both IP and
   MPLS VPNs are examples of VPNs with soft isolation: the network
   delivers the traffic only to the required VPN endpoints.  However,
   with soft isolation, as the network resources are shared, traffic
   from VPNs and regular non-VPN traffic may congest the network
   resulting in packet loss and delay for other VPNs.  The ability for a
   VPN service or a group of VPN services to be sheltered from this
   effect is called hard isolation.  Hard isolation may be needed so
   that applications with exacting requirements can function correctly,
   despite other demands (perhaps a burst of traffic in another VPN)
   competing for the underlying resources.  An operator may offer its
   customers a choice of different degrees of isolation ranging from
   soft isolation to hard isolation.  In practice isolation may be
   offered as a spectrum between soft and hard, and in some cases soft
   and hard isolation may be used in a hierarchical manner with one
   enhanced VPN being built on another.

   An example of the requirement for hard isolation is a network
   supporting both emergency services and public broadband multi-media
   services.  During a major incident, the VPNs supporting these
   services would both be expected to experience high data volumes, and
   it is important that both make progress in the transmission of their
   data.  In these circumstances the VPN services would require an

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   appropriate degree of isolation to be able to continue to operate
   acceptably.  On the other hand, VPNs servicing ordinary bulk data may
   expect to contest for network resources and queue packets so that
   traffic is delivered within SLAs, but with some potential delays and
   interference.  While the VPN for the emergency service could be
   provided by specifying hard SLOs (for bandwidth, latency, etc.) the
   customer may feel more comfortable with an SLO that specifies hard
   isolation, and the service provider may decided that the best way to
   ensure that the SLA is met is to utilize hard isolation.

   To provide the required level of isolation, resources may need to be
   reserved in the data plane of the underlay network and dedicated to
   traffic from a specific VPN or a specific group of VPNs to form
   different enhanced VPNs in the network.  This may introduce
   scalability concerns, thus some trade-off needs to be considered to
   provide the required isolation between some enhanced VPNs while still
   allowing reasonable sharing.

   An optical underlay can offer a high degree of isolation, at the cost
   of allocating resources on a long term and end-to-end basis.  On the
   other hand, where adequate isolation can be achieved at the packet
   layer, this permits the resources to be shared amongst a group of
   services and only dedicated to a service on a temporary basis.

   There are also several new technologies that provide some assistance
   with these data plane issues.  Firstly, there is the IEEE's TSN
   project which introduces the concept of packet scheduling of delay
   and loss sensitive packets.  Then there is FlexE which provides the
   ability to multiplex multiple channels over one or more Ethernet
   links in a way that provides hard isolation.  Finally, there are
   advanced queuing approaches which allow the construction of virtual
   sub-interfaces, each of which is provided with dedicated resource in
   a shared physical interface.  These approaches are described in more
   detail later in this document.

   The next section explores a pragmatic approach to isolation in packet
   networks.

3.2.1.  A Pragmatic Approach to Isolation

   A key question is whether it is possible to achieve hard isolation in
   packet networks that were designed to provide statistical
   multiplexing through sharing of data plane resources, a significant
   economic advantage when compared to a dedicated, or a Time Division
   Multiplexing (TDM) network.  Clearly, there is no need to provide
   more isolation than is required by the applications, and an
   approximation to full hard isolation is sufficient in most cases.

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   For example, pseudowires [RFC3985] emulate services that would have
   had hard isolation in their native form.

        O=================================================O
        |          \---------------v---------------/
    Statistical                Pragmatic             Absolute
    Multiplexing               Isolation            Isolation
   (Traditional VPNs)        (Enhanced VPN)     (Dedicated Network)

                    Figure 1: The Spectrum of Isolation

   Figure 1 shows a spectrum of isolation that may be delivered by a
   network.  At one end of the spectrum, we see statistical multiplexing
   technologies that support traditional VPNs.  This is a service type
   that has served the industry well and will continue to do so.  At the
   opposite end of the spectrum, we have the absolute isolation provided
   by dedicated transport networks.  The goal of enhanced VPNs is
   "pragmatic isolation".  This is isolation that is better than what is
   obtainable from pure statistical multiplexing, more cost effective
   and flexible than a dedicated network, but is a practical solution
   that is good enough for the majority of applications.  Mechanisms for
   both soft isolation and hard isolation would be needed to meet
   different levels of service requirement.

3.3.  Integration

   The way to achieve the characteristics demanded by an enhanced VPN
   (such as guaranteed or predictable performance) is by integrating the
   overlay VPN with a particular set of resources in the underlay
   network which are allocated to meet the service requirement.  This
   needs be done in a flexible and scalable way so that it can be widely
   deployed in operators' networks to support a reasonable number of
   enhanced VPN customers.

   Taking mobile networks and in particular 5G into consideration, the
   integration of the network with service functions is likely a
   requirement.  The IETF's work on service function chaining (SFC)
   [SFC] provides a foundation for this.  Service functions can be
   considered as part of enhanced VPN services.  The detailed mechanisms
   about the integration between service functions and enhanced VPNs are
   out of the scope of this document.

3.3.1.  Abstraction

   Integration of the overlay VPN and the underlay network resources
   does not need to be a tight mapping.  As described in [RFC7926],
   abstraction is the process of applying policy to a set of information

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   about a traffic engineered (TE) network to produce selective
   information that represents the potential ability to connect across
   the network.  The process of abstraction presents the connectivity
   graph in a way that is independent of the underlying network
   technologies, capabilities, and topology so that the graph can be
   used to plan and deliver network services in a uniform way.

   Virtual networks can be built on top of an abstracted topology that
   represents the connectivity capabilities of the underlay network as
   described in the framework for Abstraction and Control of TE Networks
   (ACTN) [RFC8453] as discussed further in Section 5.5.
   [I-D.king-teas-applicability-actn-slicing] describes the
   applicability of ACTN to network slicing and is, therefore, relevant
   to the consideration of using ACTN to enable enhanced VPNs.

3.4.  Dynamic Changes

   Enhanced VPNs need to be created, modified, and removed from the
   network according to service demand.  An enhanced VPN that requires
   hard isolation (Section 3.2) must not be disrupted by the
   instantiation or modification of another enhanced VPN.  Determining
   whether modification of an enhanced VPN can be disruptive to that
   VPN, and whether the traffic in flight will be disrupted can be a
   difficult problem.

   The data plane aspects of this problem are discussed further in
   Section 5.1,Section 5.2, and Section 5.3.

   The control plane aspects of this problem are discussed further in
   Section 5.4.

   The management plane aspects of this problem are discussed further in
   Section 5.5.

   Dynamic changes both to the VPN and to the underlay transport network
   need to be managed to avoid disruption to services that are sensitive
   to changes in network performance.

   In addition to non-disruptively managing the network during changes
   such as the inclusion of a new VPN endpoint or a change to a link,
   VPN traffic might need to be moved because of changes to traffic
   patterns and volumes.

3.5.  Customized Control

   In some cases it is desirable that an enhanced VPN has a customized
   control plane, so that the customer of the enhanced VPN can have some
   control over how the resources allocated to this enhanced VPN are

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   used.  For example, the customer may be able to specify the service
   paths in their own enhanced VPN.  Depending on the requirements, an
   enhanced VPN may have its own dedicated controller, which may be
   provided with an interface to the control system run by the network
   operator.  Note that such control is within the scope of the tenant's
   enhanced VPN: any additional changes beyond this would require some
   intervention by the network operator.

   A description of the control plane aspects of this problem are
   discussed further in Section 5.4.  A description of the management
   plane aspects of this feature can be found in Section 5.5.

3.6.  Applicability

   The concept of an enhanced VPN can be applied to any pre-existing VPN
   overlay services including:

   o  Layer-2 point-to-point services such as pseudowires [RFC3985]

   o  Layer-2 VPNs [RFC4664]

   o  Ethernet VPNs [RFC7209]

   o  Layer-3 VPNs [RFC4364], [RFC2764]

   Where such VPN service types need enhanced isolation and delivery
   characteristics, the technologies described in Section 5 can be used
   to provide an underlay with the required enhanced performance.

3.7.  Inter-Domain and Inter-Layer Network

   In some scenarios, an enhanced VPN service may span multiple network
   domains.  A domain is considered to be any collection of network
   elements within a common realm of address space or path computation
   responsibility [RFC5151] for example, an Autonomous System.  In some
   domains the network operator may manage a multi- layered network, for
   example, a packet network over an optical network.  When enhanced
   VPNs are provisioned in such network scenarios, the technologies used
   in different network planes (data plane, control plane, and
   management plane) need to provide mechanisms to support multi-domain
   and multi-layer coordination and integration, so as to provide the
   required service characteristics for different enhanced VPNs, and
   improve network efficiency and operational simplicity.

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4.  Architecture of Enhanced VPNs

   A number of enhanced VPN services will typically be provided by a
   common network infrastructure.  Each enhanced VPN consists of both
   the overlay and a VTN with a specific set of network resources and
   service functions allocated in the underlay to satisfy the needs of
   the VPN customer.  The integration between overlay and various
   underlay resources ensures the required isolation between different
   enhanced VPNs, and achieves the guaranteed performance for different
   services.

   An enhanced VPN needs to be designed with consideration given to:

   o  An enhanced data plane.

   o  A control plane to create enhanced VPNs, making use of the data
      plane isolation and performance guarantee techniques.

   o  A management plane for enhanced VPN service life-cycle management.

   These topics are expanded below:

   o  Enhanced data plane

      *  Provides the required resource isolation capability, e.g.
         bandwidth guarantee.

      *  Provides the required packet latency and jitter
         characteristics.

      *  Provides the required packet loss characteristics.

      *  Provides the mechanism to associate a packet with the set of
         resources allocated to the enhanced VPN to which the packet
         belongs.

   o  Control plane

      *  Collects information about the underlying network topology and
         available resources, and exports this to nodes in the network
         and/or a centralized controller as required.

      *  Creates the required VTNs with the resources and properties
         needed by the enhanced VPN services that are they support.

      *  Determines the risk of SLA violation and takes appropriate
         avoiding action.

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      *  Determines the right balance of per-packet and per-node state
         according to the needs of the enhanced VPN services to scale to
         the required size.

   o  Management plane

      *  Provides an interface between the enhanced VPN provider (e.g.,
         operator's network management system ) and the enhanced VPN
         clients (e.g. a customer or service with enhanced VPN
         requirement) such that some of the operation requests can be
         met without interfering with the enhanced VPN of other clients.

      *  Provides an interface between the enhanced VPN provider and the
         enhanced VPN clients to expose the network capability
         information toward the enhanced VPN client.

      *  Provides the service life-cycle management and operation of
         enhanced VPNs (e.g., creation, modification, assurance/
         monitoring, and decommissioning).

   o  Operations, Administration, and Maintenance (OAM)

      *  Provides the OAM tools to verify the connectivity and
         performance of the enhanced VPN.

      *  Provide the OAM tools to verify whether the underlay network
         resources are correctly allocated and operated properly.

   o  Telemetry

      *  Provides the mechanisms to collect network information about
         the operation of the data plane, control plane, and management
         plane.  More specifically:

         +  Provides the mechanisms to collect network data from the
            underlay network for overall performance evaluation and for
            planning enhanced VPN services.

         +  Provides the mechanisms to collect network data for each
            enhanced VPN and for monitoring and analytics of the
            characteristics and SLA fulfillment of enhanced VPN service.

4.1.  Layered Architecture

   The layered architecture of an enhanced VPN is shown in Figure 2.

   Underpinning everything is the physical network infrastructure layer
   which provide the underlying resources used to provision the

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   separated virtual transport networks (VTNs).  This includes the
   partitioning of link and/or node resources.  Each subset of link or
   node resource can be considered as a virtual link or virtual node
   used to build the VTNs.

                                 /\
                                 ||
                       +-------------------+       Centralized
                       | Network Controller|   Control & Management
                       +-------------------+
                                 ||
                                 \/
                   o---------------------------o
                                 /-------------o    VPN Service 1
                   o____________/______________o
                              _________________o
                        _____/
                   o___/     \_________________o    VPN Service 2
                       \_______________________o
                            ......
                   o-----------\ /-------------o
                   o____________X______________o    VPN Service n

                      __________________________
                     /       o----o-----o      /
                    /       /          /      /       VTN-1
                   / o-----o-----o----o----o /
                  /_________________________/
                      __________________________
                     /       o----o            /
                    /       /    / \          /       VTN-2
                   / o-----o----o---o------o /
                  /_________________________/
                            ......                     ...
                     ___________________________
                    /             o----o       /
                   /             /    /       /       VTN-n
                  /  o-----o----o----o-----o /
                 /__________________________/

                    ++++   ++++   ++++
                    +--+===+--+===+--+
                    +--+===+--+===+--+
                    ++++   +++\\  ++++
                     ||     || \\  ||             Physical
                     ||     ||  \\ ||              Network
             ++++   ++++   ++++  \\+++   ++++  Infrastructure

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             +--+===+--+===+--+===+--+===+--+
             +--+===+--+===+--+===+--+===+--+
             ++++   ++++   ++++   ++++   ++++

        o    Virtual Node     ++++
                              +--+ Physical Node with resource partition
        --   Virtual Link     +--+
                              ++++
        ==  Physical Link with resource partition

                Figure 2: The Layered Architecture of VPN+

   Various components and techniques discussed in Section 5 can be used
   to enable resource partitioning, such as FlexE, TSN, DetNet,
   dedicated queues, etc.  These partitions may be physical or virtual
   so long as the SLA required by the higher layers is met.

   Based on the network resources provided by the physical network
   infrastructure, multiple VTNs can be provisioned, each with
   customized topology and other attributes to meet the requirement of
   different enhanced VPNs or different groups of enhanced VPNs.  To get
   the required characteristic, each VTN needs to be mapped to a set of
   network nodes and links in the network infrastructure.  And on each
   node or link, the VTN is associated with a set of resources which are
   allocated for the processing of traffic in the VTN.  VTN provides the
   integration between the virtual network topology and the required
   underlying network resources.  The VTN is an essential scaling
   technique, as it has the potential of eliminating per-path state from
   the network.  In addition, when a group of enhanced VPNs is supported
   by a single VTN, there is need only to maintain network state for the
   single VTN (see Section 4.4 for more information).

   The centralized network controller is used to create the VTN, and to
   instruct the network nodes to allocate the required resources to each
   VTN and to provision the enhanced VPN services on the VTNs.  A
   distributed control plane may also be used for the distribution of
   the VTN topology and attribute information between nodes within the
   VTNs.

   The process used to create VTNs and to allocate network resources for
   use by VTNs needs to take a holistic view of the needs of all of its
   tenants (i.e., of all customers and their associated enhanced VPNs),
   and to partition the resources accordingly.  However, within a VTN
   these resources can, if required, be managed via a dynamic control
   plane.  This provides the required scalability and isolation.

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4.2.  Multi-Point to Multi-Point (MP2MP) Connectivity

   At the VPN service level, the required connectivity for an MP2MP
   service is usually full or partial mesh.  To support such VPN
   services, the corresponding VTN connectivity also needs to have an
   abstracted MP2MP connectivity.

   Other service requirements may be expressed at different
   granularities, some of which can be applicable to the whole service,
   while some others may only be applicable to some pairs of end points.
   For example, when a particular level of performance guarantee is
   required, the point-to-point path through the underlay of the
   enhanced VPN may need to be specifically engineered to meet the
   required performance guarantee.

4.3.  Application Specific Data Types

   Although a lot of the traffic that will be carried over the enhanced
   VPN will likely be IPv4 or IPv6, the design must be capable of
   carrying other traffic types, in particular Ethernet traffic.  This
   is easily accomplished through the various pseudowire (PW) techniques
   [RFC3985].  Where the underlay is MPLS, Ethernet can be carried over
   the enhanced VPN encapsulated according to the method specified in
   [RFC4448].  Where the underlay is IP, Layer Two Tunneling Protocol -
   Version 3 (L2TPv3) [RFC3931] can be used with Ethernet traffic
   carried according to [RFC4719].  Encapsulations have been defined for
   most of the common Layer-2 types for both PW over MPLS and for
   L2TPv3.

4.4.  Scaling Considerations

   VPNs are instantiated as overlays on top of an operator's network and
   offered as services to the operator's customers.  An important
   feature of overlays is that they can deliver services without placing
   per-service state in the core of the underlay network.

   Enhanced VPNs may need to install some additional state within the
   network to achieve the features that they require.  Solutions must
   consider minimizing and controlling the scale of such state, and
   deployment architectures should constrain the number of enhanced VPNs
   that would exist where such services would place additional state in
   the network.  It is expected that the number of enhanced VPNs will be
   small at the beginning, and even in future the number of enhanced
   VPNs will be much fewer than traditional VPNs because pre-existing
   VPN techniques are good enough to meet the needs of most existing
   VPN-type services.

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   In general, it is not required that the state in the network be
   maintained in a 1:1 relationship with the VPN+ services.  It will
   usually be possible to aggregate a set or group of VPN+ services so
   that they share the same VTN and the same set of network resources
   (much in the same way that current VPNs are aggregated over transport
   tunnels) so that collections of enhanced VPNs that require the same
   behavior from the network in terms of resource reservation, latency
   bounds, resiliency, etc. can be grouped together.  This is an
   important feature to assist with the scaling characteristics of VPN+
   deployments.

   [I-D.dong-teas-enhanced-vpn-vtn-scalability] provides more details of
   scalability considerations for enhanced VPNs, and Section 6 includes
   a greater discussion of scalability considerations.

5.  Candidate Technologies

   A VPN is a network created by applying a demultiplexing technique to
   the underlying network (the underlay) to distinguish the traffic of
   one VPN from that of another.  A VPN path that travels by other than
   the shortest path through the underlay normally requires state in the
   underlay to specify that path.  State is normally applied to the
   underlay through the use of the RSVP-TE signaling protocol, or
   directly through the use of an SDN controller, although other
   techniques may emerge as this problem is studied.  This state gets
   harder to manage as the number of VPN paths increases.  Furthermore,
   as we increase the coupling between the underlay and the overlay to
   support the enhanced VPN service, this state will increase further.

   In an enhanced VPN, different subsets of the underlay resources can
   be dedicated to different enhanced VPNs or different groups of
   enhanced VPNs.  Thus, an enhanced VPN solution needs tighter coupling
   with the underlay than is the case with existing VPN techniques.  We
   cannot, for example, share the network resource between enhanced VPNs
   which require hard isolation.

5.1.  Layer-Two Data Plane

   Several candidate Layer 2 packet- or frame-based data plane solutions
   which can be used provide the required isolation and guarantees are
   described in the following sections.

5.1.1.  Flexible Ethernet

   FlexE [FLEXE] provides the ability to multiplex channels over an
   Ethernet link to create point-to-point fixed- bandwidth connections
   in a way that provides hard isolation.  FlexE also supports bonding
   links to create larger links out of multiple low capacity links.

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   However, FlexE is only a link level technology.  When packets are
   received by the downstream node, they need to be processed in a way
   that preserves that isolation in the downstream node.  This in turn
   requires a queuing and forwarding implementation that preserves the
   end-to-end isolation.

   If different FlexE channels are used for different services, then no
   sharing is possible between the FlexE channels.  This means that it
   may be difficult to dynamically redistribute unused bandwidth to
   lower priority services in another FlexE channel.  If one FlexE
   channel is used by one customer, the customer can use some methods to
   manage the relative priority of their own traffic in the FlexE
   channel.

5.1.2.  Dedicated Queues

   DiffServ based queuing systems are described in [RFC2475] and
   [RFC4594].  This approach is not sufficient to provide isolation for
   enhanced VPNs because DiffServ does not provide enough markers to
   differentiate between traffic of a large number of enhanced VPNs.
   Nor does DiffServ offer the range of service classes that each VPN
   needs to provide to its tenants.  This problem is particularly acute
   with an MPLS underlay, because MPLS only provides eight traffic
   classes.

   In addition, DiffServ, as currently implemented, mainly provides per-
   hop priority-based scheduling, and it is difficult to use it to
   achieve quantitative resource reservation.

   To address these problems and to reduce the potential interference
   between enhanced VPNs, it would be necessary to steer traffic to
   dedicated input and output queues per enhanced VPN: some routers have
   a large number of queues and sophisticated queuing systems which
   could support this, while some routers may struggle to provide the
   granularity and level of isolation required by the applications of
   enhanced VPN.

5.1.3.  Time Sensitive Networking

   Time Sensitive Networking (TSN) [TSN] is an IEEE project to provide a
   method of carrying time sensitive information over Ethernet.  It
   introduces the concept of packet scheduling where a packet stream may
   be given a time slot guaranteeing that it experiences no queuing
   delay or increase in latency beyond the very small scheduling delay.
   The mechanisms defined in TSN can be used to meet the requirements of
   time sensitive services of an enhanced VPN.

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   Ethernet can be emulated over a Layer 3 network using an IP or MPLS
   pseudowire.  However, a TSN Ethernet payload would be opaque to the
   underlay and thus not treated specifically as time sensitive data.
   The preferred method of carrying TSN over a Layer 3 network is
   through the use of deterministic networking as explained in
   Section 5.2.1.

5.2.  Layer-Three Data Plane

   This section considers the problem of enhanced VPN differentiation
   and resource representation in the network layer.

5.2.1.  Deterministic Networking

   Deterministic Networking (DetNet) [RFC8655] is a technique being
   developed in the IETF to enhance the ability of Layer-3 networks to
   deliver packets more reliably and with greater control over the
   delay.  The design cannot use re-transmission techniques such as TCP
   since that can exceed the delay tolerated by the applications.  Even
   the delay improvements that are achieved with Stream Control
   Transmission Protocol Partial Reliability Extension (SCTP-PR)
   [RFC3758] may not meet the bounds set by application demands.  DetNet
   pre-emptively sends copies of the packet over various paths to
   minimize the chance of all copies of a packet being lost.  It also
   seeks to set an upper bound on latency, but the goal is not to
   minimize latency.

5.2.2.  MPLS Traffic Engineering (MPLS-TE)

   MPLS-TE [RFC2702][RFC3209] introduces the concept of reserving end-
   to-end bandwidth for a TE-LSP, which can be used to provide a point-
   to-point Virtual Transport Path (VTP) across the underlay network to
   support VPNs.  VPN traffic can be carried over dedicated TE-LSPs to
   provide reserved bandwidth for each specific connection in a VPN, and
   VPNs with similar behavior requirements may be multiplexed onto the
   same TE-LSPs.  Some network operators have concerns about the
   scalability and management overhead of MPLS-TE system especially with
   regard to those systems that use an active control plane, and this
   has lead them to consider other solutions for their networks.

5.2.3.  Segment Routing

   Segment Routing (SR) [RFC8402] is a method that prepends instructions
   to packets at the head-end of a path.  These instructions are used to
   specify the nodes and links to be traversed, and allow the packets to
   be routed on paths other than the shortest path.  By encoding the
   state in the packet, per-path state is transitioned out of the
   network.

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   An SR traffic engineered path operates with a granularity of a link.
   Hints about priority provided using the Traffic Class (TC) or
   Differentiated Services Code Point (DSCP) field in the header.
   However, to achieve the latency and isolation characteristics that
   are sought by enhanced VPN customers, it will probably be necessary
   to steer packets through specific virtual links and/or queues on the
   same link and direct them to use specific resources.  With SR, it is
   possible to introduce such fine-grained packet steering by specifying
   the queues and resources through an SR instruction list.

   Note that the concept of a queue is a useful abstraction for
   different types of underlay mechanism that may be used to provide
   enhanced isolation and latency support.  How the queue satisfies the
   requirement is implementation specific and is transparent to the
   layer-3 data plane and control plane mechanisms used.

   With Segment Routing, the SR instruction list could be used to build
   a P2P path, and a group of SR SIDs could also be used to represent an
   MP2MP network.  Thus, the SR based mechanism could be used to provide
   both a Virtual Transport Path (VTP) and a Virtual Transport Network
   (VTN) for enhanced VPN services.

5.3.  Non-Packet Data Plane

   Non-packet underlay data plane technologies often have TE properties
   and behaviors, and meet many of the key requirements in particular
   for bandwidth guarantees, traffic isolation (with physical isolation
   often being an integral part of the technology), highly predictable
   latency and jitter characteristics, measurable loss characteristics,
   and ease of identification of flows.  The cost is that the resources
   are allocated on a long-term and end-to-end basis.  Such an
   arrangement means that the full cost of the resources has be borne by
   the service that is allocated with the resources.

5.4.  Control Plane

   An enhanced VPN would likely be based on a hybrid control mechanism
   that takes advantage of the logically centralized controller for on-
   demand provisioning and global optimization, whilst still relying on
   a distributed control plane to provide scalability, high reliability,
   fast reaction, automatic failure recovery, etc.  Extension to and
   optimization of the centralized and distributed control plane is
   needed to support the enhanced properties of VPN+.

   RSVP-TE [RFC3209] provides the signaling mechanism for establishing a
   TE-LSP in an MPLS network with end-to-end resource reservation.  This
   can be seen as an approach of providing a Virtual Transport Path
   (VTP) which could be used to bind the VPN to specific network

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   resources allocated within the underlay, but there remain scalability
   concerns as mentioned in Section 5.2.2.

   The control plane of SR [RFC8665] [RFC8667]
   [I-D.ietf-idr-bgp-ls-segment-routing-ext] does not have the
   capability of signaling resource reservations along the path.  On the
   other hand, the SR approach provides a potential way of binding the
   underlay network resource and the enhanced VPN service without
   requiring per-path state to be maintained in the network.  A
   centralized controller can perform resource planning and reservation
   for enhanced VPNs, while it needs to ensure that resources are
   correctly allocated in network nodes for the enhanced VPN service.
   The controller could also compute the SR paths based on the planned
   or collected network resource and other attributes, and provision the
   SR paths based on the mechanism in
   [I-D.ietf-spring-segment-routing-policy] to the ingress nodes of the
   enhanced VPN services.  The distributed control plane may be used to
   advertise the network attributes associated with enhanced VPNs, and
   compute the SR paths with specific constraints of enhanced VPN
   services.

5.5.  Management Plane

   The management plane provides the interface between the enhanced VPN
   provider and the clients for life-cycle management of the service
   (i.e., creation, modification, assurance/monitoring and
   decommissioning).  It relies on a set of service data models for the
   description of the information and operations needed on the
   interface.

   As an example, in the context of 5G end-to-end network slicing
   [TS28530], the management of enhanced VPNs is considered as the
   management of the transport network part of the 5G end-to-end network
   slice.  The 3GPP management system may provide the connectivity and
   performance related parameters as requirements to the management
   plane of the transport network.  It may also require the transport
   network to expose the capabilities and status of the network slice.
   Thus, an interface between the enhanced VPN management plane and the
   5G network slice management system, and relevant service data models
   are needed for the coordination of 5G end-to-end network slice
   management.

   The management plane interface and data models for enhanced VPN can
   be based on the service models described in Section 5.6.

   It is important that the management life-cycle supports in-place
   modification of enhanced VPNs.  That is, it should be possible to add
   and remove end points, as well as to change the requested

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   characteristics of the service that is delivered.  The management
   system needs to be able to assess the revised VPN+ requests and
   determine whether they can be provided by the existing VTN or whether
   changes must be made, and it will additionally need to determine
   whether those changes to the VTN are possible.  If not, then the
   customer's modification request may be rejected.

   When the modification of an enhanced VPN is possible, the management
   system should make every effort to make the changes in a non-
   disruptive way.  That is, the modification of the enhanced VPN or the
   underlying VTN should not perturbate traffic on the enhanced VPN in a
   way that causes the service level to drop below the agreed levels.
   Furthermore, in the spirit of isolation, changes to one enhanced VPN
   should not cause disruption to other enhanced VPNs.

   The network operator for the underlay network (i.e., the provider of
   the enhanced VPN) may delegate some operational aspects of the
   enhanced VPN to the tenant (the VPN+ customer).  In this way, the
   VPN+ is presented to the customer as a virtual network, and the
   customer can choose how to use that network.  The customer cannot
   exceed the capabilities of virtual links and nodes, but can decide
   how to load traffic onto the network, for example, by assigning
   different metrics to the virtual links so that the customer can
   control how traffic is routed through the overlay.  This approach
   requires a management system for the overlay network, but does not
   necessarily require any coordination between the underlay and overlay
   management systems, except that the overlay management system might
   notice when the enhanced VPN network is close to capacity or
   considerably under-used and automatically request changes in the
   service provided by the underlay.

5.6.  Applicability of Service Data Models to Enhanced VPN

   This section describes the applicability of the existing and in-
   progress service data models to enhanced VPN.  New service models may
   also be introduced for some of the required management functions.

   The ACTN framework[RFC8453] supports operators in viewing and
   controlling different domains and presenting virtualized networks to
   their customers.  It highlights how:

   o  Abstraction of the underlying network resources is provided to
      higher-layer applications and customers.

   o  Underlying resources are virtualized and allocated for the
      customer, application, or service.

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   o  A virtualized environment is created allowing operators to view
      and control multi-domain networks as a single virtualized network.

   o  Networks can be presented to customers as a virtual network via
      open and programmable interfaces.

   The type of network virtualization enabled by ACTN managed
   infrastructure provides customers and applications (tenants) with the
   capability to utilize and independently control allocated virtual
   network resources as if they were physically their own resources.
   Service Data models are used to represent, monitor, and manage the
   virtual networks and services enabled by ACTN.  The Customer VPN
   model (e.g.  L3SM [RFC8299], L2SM [RFC8466]) or an ACTN Virtual
   Network (VN) [I-D.ietf-teas-actn-vn-yang] model is a customer view of
   the ACTN managed infrastructure, and is presented by the ACTN
   provider as a set of abstracted services or resources.  The L3VPN
   network model [I-D.ietf-opsawg-l3sm-l3nm] and L2VPN network model
   [I-D.ietf-opsawg-l2nm] provide a network view of the ACTN managed
   infrastructure presented by the ACTN provider as a set of virtual
   networks and the associated resources.

   [I-D.king-teas-applicability-actn-slicing] discusses the
   applicability of the ACTN approach in the context of network slicing.
   Since there is a strong correlation between network slices and
   enhanced VPNs, that document can also give guidance on how ACTN can
   be applied to enhanced VPNs.

5.6.1.  An Example of Enhanced VPN Delivery

   One typical use case of enhanced VPN is to instantiate a network
   slice.  In order to provide network slices to customers, a
   technology-agnostic network slice Northbound Interface (NBI) data
   model may be needed for the customers to communicate the requirements
   and operations of network slices.  These requirements may then be
   realized using technology-specific Southbound Interface (SBI) to
   instruct the network to instantiate an enhanced VPN service to meet
   the requirements of the customer.

   As per [RFC8453] and [I-D.ietf-teas-actn-yang], the CNC-MDSC
   Interface (CMI) of ACTN can be used to convey the virtual network
   service requirements, which is a generic interface to deliver various
   TE based VN services.  In the context of the network slice NBI, there
   may be some gaps in the combination of the L3SM/L2SM and VN models.
   The NBI is required to communicate the connectivity of the network
   slice, along with the SLO parameters and traffic selection rules, and
   provides a way to monitor the state of the network slice.  This can
   be described in a more abstract manner, so as to reduce the
   association with specific technologies used to realize the network

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   slice, such as the VPN and TE technologies.  The network slice NBI
   model as defined in [I-D.wd-teas-ietf-network-slice-nbi-yang]
   provides an abstract and generic approach to provide the network
   slice NBI functions.

   The MDSC-PNC Interface (MPI) models in the ACTN architecture can be
   used for the realization of network slices, for example, in a TE
   enabled network, and may also be used for cross-layer or cross-domain
   implementation of network slice.

6.  Scalability Considerations

   An enhanced VPN provides performance guaranteed services in packet
   networks, but with the potential cost of introducing additional state
   into the network.  There are at least three ways that this additional
   state might be present in the network:

   o  Introduce the complete state into the packet, as is done in SR.
      This allows the controller to specify the detailed series of
      forwarding and processing instructions for the packet as it
      transits the network.  The cost of this is an increase in the
      packet header size.  The cost is also that systems will have
      capabilities enabled in case they are called upon by a service.
      This is a type of latent state, and increases as we more precisely
      specify the path and resources that need to be exclusively
      available to a VPN.

   o  Introduce the state to the network.  This is normally done by
      creating a path using RSVP-TE, which can be extended to introduce
      any element that needs to be specified along the path, for example
      explicitly specifying queuing policy.  It is possible to use other
      methods to introduce path state, such as via an SDN controller, or
      possibly by modifying a routing protocol.  With this approach
      there is state per path: per path characteristic that needs to be
      maintained over its life cycle.  This is more network state than
      is needed using SR, but the packets are shorter.

   o  Provide a hybrid approach.  One example is based on using binding
      SIDs [RFC8402] to create path fragments, and bind them together
      with SR.  Dynamic creation of a VPN service path using SR requires
      less state maintenance in the network core at the expense of
      larger packet headers.  The packet size can be lower if a form of
      loose source routing is used (using a few nodal SIDs), and it will
      be lower if no specific functions or resources on the routers are
      specified.

   Reducing the state in the network is important to enhanced VPN, as it
   requires the overlay to be more closely integrated with the underlay

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   than with traditional VPNs.  This tighter coupling would normally
   mean that more state needs to be created and maintained in the
   network, as the state about fine granularity processing would need to
   be loaded and maintained in the routers.  However, an SR approach
   allows much of this state to be spread amongst the network ingress
   nodes, and transiently carried in the packets as SIDs.

   Further discussion of the scalability considerations of enhanced VPNs
   can be found in [I-D.dong-teas-enhanced-vpn-vtn-scalability].

6.1.  Maximum Stack Depth of SR

   One of the challenges with SR is the stack depth that nodes are able
   to impose on packets [RFC8491].  This leads to a difficult balance
   between adding state to the network and minimizing stack depth, or
   minimizing state and increasing the stack depth.

6.2.  RSVP-TE Scalability

   The traditional method of creating a resource allocated path through
   an MPLS network is to use the RSVP-TE protocol.  However, there have
   been concerns that this requires significant continuous state
   maintenance in the network.  Work to improve the scalability of RSVP-
   TE LSPs in the control plane can be found in [RFC8370].

   There is also concern at the scalability of the forwarder footprint
   of RSVP-TE as the number of paths through a label switching router
   (LSR) grows.  [RFC8577] addresses this by employing SR within a
   tunnel established by RSVP-TE.

6.3.  SDN Scaling

   The centralized approach of SDN requires state to be stored in the
   network, but does not have the overhead of also requiring control
   plane state to be maintained.  Each individual network node may need
   to maintain a communication channel with the SDN controller, but that
   compares favorably with the need for a control plane to maintain
   communication with all neighbors.

   However, SDN may transfer some of the scalability concerns from the
   network to the centralized controller.  In particular, there may be a
   heavy processing burden at the controller, and a heavy load in the
   network surrounding the controller.  A centralized controller also
   presents a single point of failure within the network.

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7.  OAM Considerations

   The design of OAM for enhanced VPNs needs to consider the following
   requirements:

   o  Instrumentation of the underlay so that the network operator can
      be sure that the resources committed to a tenant are operating
      correctly and delivering the required performance.

   o  Instrumentation of the overlay by the tenant.  This is likely to
      be transparent to the network operator and to use existing
      methods.  Particular consideration needs to be given to the need
      to verify the isolation and the various committed performance
      characteristics.

   o  Instrumentation of the overlay by the network provider to
      proactively demonstrate that the committed performance is being
      delivered.  This needs to be done in a non-intrusive manner,
      particularly when the tenant is deploying a performance sensitive
      application.

   o  Verification of the conformity of the path to the service
      requirement.  This may need to be done as part of a commissioning
      test.

   A study of OAM in SR networks has been documented in [RFC8403].

8.  Telemetry Considerations

   Network visibility is essential for network operation.  Network
   telemetry has been considered as an ideal means to gain sufficient
   network visibility with better flexibility, scalability, accuracy,
   coverage, and performance than conventional OAM technologies.

   As defined in [I-D.ietf-opsawg-ntf], the objective of Network
   Telemetry is to acquire network data remotely for network monitoring
   and operation.  It is a general term for a large set of network
   visibility techniques and protocols.  Network telemetry addresses the
   current network operation issues and enables smooth evolution toward
   intent-driven autonomous networks.  Telemetry can be applied on the
   forwarding plane, the control plane, and the management plane in a
   network.

   How the telemetry mechanisms could be used or extended for the
   enhanced VPN service is out of the scope of this document.

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9.  Enhanced Resiliency

   Each enhanced VPN has a life cycle, and may need modification during
   deployment as the needs of its tenant change.  This is discussed in
   Section 5.5.  Additionally, as the network evolves, there may need to
   be garbage collection performed to consolidate resources into usable
   quanta.

   Systems in which the path is imposed, such as SR or some form of
   explicit routing, tend to do well in these applications, because it
   is possible to perform an atomic transition from one path to another.
   That is, a single action by the head-end that changes the path
   without the need for coordinated action by the routers along the
   path.  However, implementations and the monitoring protocols need to
   make sure that the new path is operational and meets the required SLA
   before traffic is transitioned to it.  It is possible for deadlocks
   to arise as a result of the network becoming fragmented over time,
   such that it is impossible to create a new path or to modify an
   existing path without impacting the SLA of other paths.  Resolution
   of this situation is as much a commercial issue as it is a technical
   issue and is outside the scope of this document.

   There are, however, two manifestations of the latency problem that
   are for further study in any of these approaches:

   o  The problem of packets overtaking one another if a path latency
      reduces during a transition.

   o  The problem of transient variation in latency in either direction
      as a path migrates.

   There is also the matter of what happens during failure in the
   underlay infrastructure.  Fast reroute is one approach, but that
   still produces a transient loss with a normal goal of rectifying this
   within 50ms [RFC5654].  An alternative is some form of N+1 delivery
   such as has been used for many years to support protection from
   service disruption.  This may be taken to a different level using the
   techniques of DetNet with multiple in-network replication and the
   culling of later packets [RFC8655].

   In addition to the approach used to protect high priority packets,
   consideration should be given to the impact of best effort traffic on
   the high priority packets during a transition.  Specifically, if a
   conventional re-convergence process is used there will inevitably be
   micro-loops and whilst some form of explicit routing will protect the
   high priority traffic, lower priority traffic on best effort shortest
   paths will micro-loop without the use of a loop prevention
   technology.  To provide the highest quality of service to high

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   priority traffic, either this traffic must be shielded from the
   micro-loops, or micro-loops must be prevented completely.

10.  Operational Considerations

   It is likely that enhanced VPN services will be introduced in
   networks which already have traditional VPN services deployed.
   Depending on service requirements, the tenants or the operator may
   choose to use a traditional VPN or an enhanced VPN to fulfill a
   service requirement.  The information and parameters to assist such a
   decision needs to be reflected on the management interface between
   the tenant and the operator.

11.  Security Considerations

   All types of virtual network require special consideration to be
   given to the isolation of traffic belonging to different tenants.
   That is, traffic belonging to one VPN must not be delivered to end
   points outside that VPN.  In this regard enhanced VPNs neither
   introduce, nor experience a greater security risks than other VPNs.

   However, in an enhanced Virtual Private Network service the
   additional service requirements need to be considered.  For example,
   if a service requires a specific upper bound to latency then it can
   be damaged by simply delaying the packets through the activities of
   another tenant, i.e., by introducing bursts of traffic for other
   services.  In some respects this makes the enhanced VPN more
   susceptible to attacks since the SLA may be broken.  But another view
   is that the operator must, in any case, preform monitoring of the
   enhanced VPN to ensure that the SLA is met, and this means that the
   operator may be more likely to spot the early onset of a security
   attack and be able to take pre-emptive protective action.

   The measures to address these dynamic security risks must be
   specified as part to the specific solution are form part of the
   isolation requirements of a service.

   While an enhanced VPN service may be sold as offering encryption and
   other security features as part of the service, customers would be
   well advised to take responsibility for their own security
   requirements themselves possibly by encrypting traffic before handing
   it off to the service provider.

   The privacy of enhanced VPN service customers must be preserved.  It
   should not be possible for one customer to discover the existence of
   another customer, nor should the sites that are members of an
   enhanced VPN be externally visible.

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12.  IANA Considerations

   There are no requested IANA actions.

13.  Contributors

      Daniel King
      Email: daniel@olddog.co.uk

      Adrian Farrel
      Email: adrian@olddog.co.uk

      Jeff Tansura
      Email: jefftant.ietf@gmail.com

      Zhenbin Li
      Email: lizhenbin@huawei.com

      Qin Wu
      Email: bill.wu@huawei.com

      Bo Wu
      Email: lana.wubo@huawei.com

      Daniele Ceccarelli
      Email: daniele.ceccarelli@ericsson.com

      Mohamed Boucadair
      Email: mohamed.boucadair@orange.com

      Sergio Belotti
      Email: sergio.belotti@nokia.com

      Haomian Zheng
      Email: zhenghaomian@huawei.com

14.  Acknowledgements

   The authors would like to thank Charlie Perkins, James N Guichard,
   John E Drake, Shunsuke Homma and Luis M.  Contreras for their review
   and valuable comments.

   This work was supported in part by the European Commission funded
   H2020-ICT-2016-2 METRO-HAUL project (G.A. 761727).

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15.  References

15.1.  Normative References

   [RFC2764]  Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and A.
              Malis, "A Framework for IP Based Virtual Private
              Networks", RFC 2764, DOI 10.17487/RFC2764, February 2000,
              <https://www.rfc-editor.org/info/rfc2764>.

   [RFC3985]  Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
              Edge-to-Edge (PWE3) Architecture", RFC 3985,
              DOI 10.17487/RFC3985, March 2005,
              <https://www.rfc-editor.org/info/rfc3985>.

   [RFC4664]  Andersson, L., Ed. and E. Rosen, Ed., "Framework for Layer
              2 Virtual Private Networks (L2VPNs)", RFC 4664,
              DOI 10.17487/RFC4664, September 2006,
              <https://www.rfc-editor.org/info/rfc4664>.

15.2.  Informative References

   [BBF-SD406]
              "BBF SD-406: End-to-End Network Slicing", 2016,
              <https://wiki.broadband-forum.org/display/BBF/SD-406+End-
              to-End+Network+Slicing>.

   [DETNET]   "Deterministic Networking", March ,
              <https://datatracker.ietf.org/wg/detnet/about/>.

   [FLEXE]    "Flex Ethernet Implementation Agreement", March 2016,
              <http://www.oiforum.com/wp-content/uploads/OIF-FLEXE-
              01.0.pdf>.

   [I-D.dong-teas-enhanced-vpn-vtn-scalability]
              Dong, J., Li, Z., Qin, F., and G. Yang, "Scalability
              Considerations for Enhanced VPN (VPN+)", draft-dong-teas-
              enhanced-vpn-vtn-scalability-01 (work in progress),
              November 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.

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   [I-D.ietf-opsawg-l2nm]
              barguil, s., Dios, O., Boucadair, M., Munoz, L., Jalil,
              L., and J. Ma, "A Layer 2 VPN Network YANG Model", draft-
              ietf-opsawg-l2nm-01 (work in progress), November 2020.

   [I-D.ietf-opsawg-l3sm-l3nm]
              barguil, s., Dios, O., Boucadair, M., Munoz, L., and A.
              Aguado, "A Layer 3 VPN Network YANG Model", draft-ietf-
              opsawg-l3sm-l3nm-05 (work in progress), October 2020.

   [I-D.ietf-opsawg-ntf]
              Song, H., Qin, F., Martinez-Julia, P., Ciavaglia, L., and
              A. Wang, "Network Telemetry Framework", draft-ietf-opsawg-
              ntf-06 (work in progress), January 2021.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-09 (work in progress),
              November 2020.

   [I-D.ietf-teas-actn-vn-yang]
              Lee, Y., Dhody, D., Ceccarelli, D., Bryskin, I., and B.
              Yoon, "A YANG Data Model for VN Operation", draft-ietf-
              teas-actn-vn-yang-10 (work in progress), November 2020.

   [I-D.ietf-teas-actn-yang]
              Lee, Y., Zheng, H., Ceccarelli, D., Yoon, B., Dios, O.,
              Shin, J., and S. Belotti, "Applicability of YANG models
              for Abstraction and Control of Traffic Engineered
              Networks", draft-ietf-teas-actn-yang-06 (work in
              progress), August 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.

   [I-D.king-teas-applicability-actn-slicing]
              King, D., Drake, J., and H. Zheng, "Applicability of
              Abstraction and Control of Traffic Engineered Networks
              (ACTN) to Network Slicing", draft-king-teas-applicability-
              actn-slicing-08 (work in progress), October 2020.

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   [I-D.wd-teas-ietf-network-slice-nbi-yang]
              Bo, W., Dhody, D., Han, L., and R. Rokui, "A Yang Data
              Model for IETF Network Slice NBI", draft-wd-teas-ietf-
              network-slice-nbi-yang-01 (work in progress), November
              2020.

   [NGMN-NS-Concept]
              "NGMN NS Concept", 2016, <https://www.ngmn.org/fileadmin/u
              ser_upload/161010_NGMN_Network_Slicing_framework_v1.0.8.pd
              f>.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
              <https://www.rfc-editor.org/info/rfc2475>.

   [RFC2702]  Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
              McManus, "Requirements for Traffic Engineering Over MPLS",
              RFC 2702, DOI 10.17487/RFC2702, September 1999,
              <https://www.rfc-editor.org/info/rfc2702>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

   [RFC3758]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
              Conrad, "Stream Control Transmission Protocol (SCTP)
              Partial Reliability Extension", RFC 3758,
              DOI 10.17487/RFC3758, May 2004,
              <https://www.rfc-editor.org/info/rfc3758>.

   [RFC3931]  Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed.,
              "Layer Two Tunneling Protocol - Version 3 (L2TPv3)",
              RFC 3931, DOI 10.17487/RFC3931, March 2005,
              <https://www.rfc-editor.org/info/rfc3931>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC4448]  Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron,
              "Encapsulation Methods for Transport of Ethernet over MPLS
              Networks", RFC 4448, DOI 10.17487/RFC4448, April 2006,
              <https://www.rfc-editor.org/info/rfc4448>.

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   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              DOI 10.17487/RFC4594, August 2006,
              <https://www.rfc-editor.org/info/rfc4594>.

   [RFC4719]  Aggarwal, R., Ed., Townsley, M., Ed., and M. Dos Santos,
              Ed., "Transport of Ethernet Frames over Layer 2 Tunneling
              Protocol Version 3 (L2TPv3)", RFC 4719,
              DOI 10.17487/RFC4719, November 2006,
              <https://www.rfc-editor.org/info/rfc4719>.

   [RFC5151]  Farrel, A., Ed., Ayyangar, A., and JP. Vasseur, "Inter-
              Domain MPLS and GMPLS Traffic Engineering -- Resource
              Reservation Protocol-Traffic Engineering (RSVP-TE)
              Extensions", RFC 5151, DOI 10.17487/RFC5151, February
              2008, <https://www.rfc-editor.org/info/rfc5151>.

   [RFC5654]  Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
              Sprecher, N., and S. Ueno, "Requirements of an MPLS
              Transport Profile", RFC 5654, DOI 10.17487/RFC5654,
              September 2009, <https://www.rfc-editor.org/info/rfc5654>.

   [RFC7149]  Boucadair, M. and C. Jacquenet, "Software-Defined
              Networking: A Perspective from within a Service Provider
              Environment", RFC 7149, DOI 10.17487/RFC7149, March 2014,
              <https://www.rfc-editor.org/info/rfc7149>.

   [RFC7209]  Sajassi, A., Aggarwal, R., Uttaro, J., Bitar, N.,
              Henderickx, W., and A. Isaac, "Requirements for Ethernet
              VPN (EVPN)", RFC 7209, DOI 10.17487/RFC7209, May 2014,
              <https://www.rfc-editor.org/info/rfc7209>.

   [RFC7926]  Farrel, A., Ed., Drake, J., Bitar, N., Swallow, G.,
              Ceccarelli, D., and X. Zhang, "Problem Statement and
              Architecture for Information Exchange between
              Interconnected Traffic-Engineered Networks", BCP 206,
              RFC 7926, DOI 10.17487/RFC7926, July 2016,
              <https://www.rfc-editor.org/info/rfc7926>.

   [RFC8172]  Morton, A., "Considerations for Benchmarking Virtual
              Network Functions and Their Infrastructure", RFC 8172,
              DOI 10.17487/RFC8172, July 2017,
              <https://www.rfc-editor.org/info/rfc8172>.

   [RFC8299]  Wu, Q., Ed., Litkowski, S., Tomotaki, L., and K. Ogaki,
              "YANG Data Model for L3VPN Service Delivery", RFC 8299,
              DOI 10.17487/RFC8299, January 2018,
              <https://www.rfc-editor.org/info/rfc8299>.

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   [RFC8370]  Beeram, V., Ed., Minei, I., Shakir, R., Pacella, D., and
              T. Saad, "Techniques to Improve the Scalability of RSVP-TE
              Deployments", RFC 8370, DOI 10.17487/RFC8370, May 2018,
              <https://www.rfc-editor.org/info/rfc8370>.

   [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>.

   [RFC8403]  Geib, R., Ed., Filsfils, C., Pignataro, C., Ed., and N.
              Kumar, "A Scalable and Topology-Aware MPLS Data-Plane
              Monitoring System", RFC 8403, DOI 10.17487/RFC8403, July
              2018, <https://www.rfc-editor.org/info/rfc8403>.

   [RFC8453]  Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for
              Abstraction and Control of TE Networks (ACTN)", RFC 8453,
              DOI 10.17487/RFC8453, August 2018,
              <https://www.rfc-editor.org/info/rfc8453>.

   [RFC8466]  Wen, B., Fioccola, G., Ed., Xie, C., and L. Jalil, "A YANG
              Data Model for Layer 2 Virtual Private Network (L2VPN)
              Service Delivery", RFC 8466, DOI 10.17487/RFC8466, October
              2018, <https://www.rfc-editor.org/info/rfc8466>.

   [RFC8491]  Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
              "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
              DOI 10.17487/RFC8491, November 2018,
              <https://www.rfc-editor.org/info/rfc8491>.

   [RFC8568]  Bernardos, CJ., Rahman, A., Zuniga, JC., Contreras, LM.,
              Aranda, P., and P. Lynch, "Network Virtualization Research
              Challenges", RFC 8568, DOI 10.17487/RFC8568, April 2019,
              <https://www.rfc-editor.org/info/rfc8568>.

   [RFC8577]  Sitaraman, H., Beeram, V., Parikh, T., and T. Saad,
              "Signaling RSVP-TE Tunnels on a Shared MPLS Forwarding
              Plane", RFC 8577, DOI 10.17487/RFC8577, April 2019,
              <https://www.rfc-editor.org/info/rfc8577>.

   [RFC8578]  Grossman, E., Ed., "Deterministic Networking Use Cases",
              RFC 8578, DOI 10.17487/RFC8578, May 2019,
              <https://www.rfc-editor.org/info/rfc8578>.

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,
              <https://www.rfc-editor.org/info/rfc8655>.

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   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, December 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

   [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>.

   [SFC]      "Service Function Chaining", March ,
              <https://datatracker.ietf.org/wg/sfc/about>.

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

   [TS28530]  "3GPP TS28.530", 2016,
              <https://portal.3gpp.org/desktopmodules/Specifications/
              SpecificationDetails.aspx?specificationId=3273>.

   [TSN]      "Time-Sensitive Networking", March ,
              <https://1.ieee802.org/tsn/>.

Authors' Addresses

   Jie Dong
   Huawei

   Email: jie.dong@huawei.com

   Stewart Bryant
   Futurewei

   Email: stewart.bryant@gmail.com

   Zhenqiang Li
   China Mobile

   Email: lizhenqiang@chinamobile.com

Dong, et al.             Expires August 14, 2021               [Page 37]
Internet-Draft               VPN+ Framework                February 2021

   Takuya Miyasaka
   KDDI Corporation

   Email: ta-miyasaka@kddi.com

   Young Lee
   Samsung

   Email: younglee.tx@gmail.com

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