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A Framework for Enhanced Virtual Private Network (VPN+)
draft-ietf-teas-enhanced-vpn-14

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Jie Dong , Stewart Bryant , Zhenqiang Li , Takuya Miyasaka , Young Lee
Last updated 2023-09-19 (Latest revision 2023-07-28)
Replaces draft-dong-teas-enhanced-vpn
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draft-ietf-teas-enhanced-vpn-14
TEAS Working Group                                               J. Dong
Internet-Draft                                                    Huawei
Intended status: Informational                                 S. Bryant
Expires: 29 January 2024                            University of Surrey
                                                                   Z. Li
                                                            China Mobile
                                                             T. Miyasaka
                                                        KDDI Corporation
                                                                  Y. Lee
                                                                 Samsung
                                                            28 July 2023

        A Framework for Enhanced Virtual Private Network (VPN+)
                    draft-ietf-teas-enhanced-vpn-14

Abstract

   This document describes the framework for Enhanced Virtual Private
   Network (VPN+) to support the needs of applications with specific
   traffic performance requirements (e.g., low latency, bounded jitter).
   VPN+ leverages the VPN and Traffic Engineering (TE) technologies and
   adds characteristics that specific services require beyond those
   provided by conventional 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.
   This document also provides an overview of relevant technologies in
   different network layers, and identifies some areas for potential new
   work.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on 29 January 2024.

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Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Overview of the Requirements  . . . . . . . . . . . . . . . .   7
     3.1.  Performance Guarantees  . . . . . . . . . . . . . . . . .   7
     3.2.  Interaction between VPN+ Services . . . . . . . . . . . .   9
       3.2.1.  Requirements on Traffic Isolation . . . . . . . . . .   9
       3.2.2.  Limited Interaction with Other Services . . . . . . .  10
       3.2.3.  Realization of Limited Interaction Between VPN+
               Services  . . . . . . . . . . . . . . . . . . . . . .  11
     3.3.  Integration with Network Resources and Service
           Functions . . . . . . . . . . . . . . . . . . . . . . . .  12
       3.3.1.  Abstraction . . . . . . . . . . . . . . . . . . . . .  12
     3.4.  Dynamic Changes . . . . . . . . . . . . . . . . . . . . .  13
     3.5.  Customized Control  . . . . . . . . . . . . . . . . . . .  13
     3.6.  Applicability to Overlay Technologies . . . . . . . . . .  14
     3.7.  Inter-Domain and Inter-Layer Network  . . . . . . . . . .  14
   4.  The Architecture of VPN+  . . . . . . . . . . . . . . . . . .  15
     4.1.  Layered Architecture  . . . . . . . . . . . . . . . . . .  17
     4.2.  Connectivity Types  . . . . . . . . . . . . . . . . . . .  19
     4.3.  Application Specific Data Types . . . . . . . . . . . . .  19
     4.4.  Scalable Service Mapping  . . . . . . . . . . . . . . . .  20
   5.  Candidate Technologies  . . . . . . . . . . . . . . . . . . .  21
     5.1.  Forwarding Resource Partitioning  . . . . . . . . . . . .  21
       5.1.1.  Flexible Ethernet . . . . . . . . . . . . . . . . . .  21
       5.1.2.  Dedicated Queues  . . . . . . . . . . . . . . . . . .  22
       5.1.3.  Time Sensitive Networking . . . . . . . . . . . . . .  22
     5.2.  Data Plane Encapsulation and Forwarding . . . . . . . . .  23
       5.2.1.  Deterministic Networking  . . . . . . . . . . . . . .  23
       5.2.2.  MPLS Traffic Engineering (MPLS-TE)  . . . . . . . . .  23
       5.2.3.  Segment Routing . . . . . . . . . . . . . . . . . . .  23
     5.3.  Non-Packet Data Plane . . . . . . . . . . . . . . . . . .  24
     5.4.  Control Plane . . . . . . . . . . . . . . . . . . . . . .  24

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     5.5.  Management Plane  . . . . . . . . . . . . . . . . . . . .  26
     5.6.  Applicability of Service Data Models to VPN+  . . . . . .  27
   6.  Applicability in Network Slice Realization  . . . . . . . . .  27
     6.1.  VTN Planning  . . . . . . . . . . . . . . . . . . . . . .  28
     6.2.  VTN Instantiation . . . . . . . . . . . . . . . . . . . .  28
     6.3.  VPN+ Service Provisioning . . . . . . . . . . . . . . . .  29
     6.4.  Network Slice Traffic Steering and Forwarding . . . . . .  29
   7.  Scalability Considerations  . . . . . . . . . . . . . . . . .  29
     7.1.  Maximum Stack Depth of SR . . . . . . . . . . . . . . . .  30
     7.2.  RSVP-TE Scalability . . . . . . . . . . . . . . . . . . .  31
     7.3.  SDN Scaling . . . . . . . . . . . . . . . . . . . . . . .  31
   8.  Manageability Considerations  . . . . . . . . . . . . . . . .  31
     8.1.  OAM Considerations  . . . . . . . . . . . . . . . . . . .  31
     8.2.  Telemetry Considerations  . . . . . . . . . . . . . . . .  32
   9.  Enhanced Resiliency . . . . . . . . . . . . . . . . . . . . .  32
   10. Operational Considerations  . . . . . . . . . . . . . . . . .  34
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  34
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  35
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  35
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  35
   15. Informative References  . . . . . . . . . . . . . . . . . . .  36
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  43

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 (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 connectivity services
   with advanced characteristics, such as low latency guarantees,
   bounded jitter, or isolation from other services or customers so that
   changes in some other services (e.g., changes in network load, or
   events such as congestion or outages) have no or only acceptable
   effect on the observed throughput or latency of the services
   delivered 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.

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   The concept of network slicing has gained traction driven largely by
   needs surfacing from 5G [NGMN-NS-Concept] [TS23501] [TS28530].
   According to [TS28530], a 5G end-to-end network slice consists of
   three major types of 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 commitments.

   [I-D.ietf-teas-ietf-network-slices] defines the terminologies and the
   characteristics of IETF Network Slices.  It also discusses the
   general framework, the components and interfaces for requesting and
   operating IETF Network Slices.  An IETF Network Slice Service enables
   connectivity between a set of Service Demarcation Points (SDPs) with
   specific Service Level Objectives (SLOs) and Service Level
   Expectations (SLEs) over a common underlay network.  An IETF Network
   Slice can be realized as a logical network connecting a number of
   endpoints and is associated with a set of shared or dedicated network
   resources that are used to satisfy the Service Level Objectives
   (SLOs) and Service Level Expectations (SLEs) requirements.  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 as one target use case of VPN+.

   A network slice may involve multiple technologies (e.g., IP or
   Optical) and may span multiple administrative domains.  Depending on
   the customer's requirements, the traffic that belongs to a network
   slice could be isolated from other network slices in terms of data
   plane, control plane, and management plane resources.

   Network slicing can build 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 one tenant
   or a group of tenants that share the same or similar service
   requirements.  These logical networks are created on top of a common
   underlay network.  How the network slices are engineered is
   deployment-specific.

   The requirements of VPN+ services cannot simply be met by overlay
   networks, as VPN+ services require tighter coordination and
   integration between the overlay and the underlay networks.

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   In the overlay network, VPN has been defined as the network construct
   to provide the required connectivity for different services or
   customers.  Multiple VPN flavors can be considered to create that
   construct [RFC4026].  In the underlay network, this document
   introduces the concept Virtual Transport Network (VTN).  A VTN is a
   virtual underlay network that is associated with a network topology,
   and is allocated with a set of dedicated or shared resources from the
   underlay physical network.

   A VPN+ service is realized by integrating a VPN in the overlay and a
   VTN in the underlay.  In doing so, a VPN+ service can provide
   enhanced properties, such as guaranteed resources and assured or
   predictable performance.  A VPN+ service may also involve a set of
   service functions (Section 1.4 of [RFC7665]).  VPN+ techniques can be
   used to instantiate a network slice service, and they can also be of
   use in general cases to provide enhanced connectivity services
   between customer sites or service endpoints.

   [I-D.ietf-teas-ietf-network-slices] introduces the concept of Network
   Resource Partition (NRP) as a subset of resources and associated
   policies in the underlay network that can reliably support specific
   IETF Network Slice Service Level Agreements (SLAs).  An NRP can be
   associated with a network topology to select or specify the set of
   links and nodes involved.  NRP can be seen as an instantiation of VTN
   in the context of network slicing.

   It is not envisaged that VPN+ services will replace conventional VPN
   services.  VPN services will continue to be delivered using existing
   mechanisms and can co-exist with VPN+ services.  Whether enriched
   VPN+ features are added to an active VPN service is deployment
   specific.

   This document describes a framework for using existing, modified, and
   potential new technologies as components to provide VPN+ services.
   Specifically, this document provides:

   *  The functional requirements and service characteristics of a VPN+
      service.

   *  The design of the data plane for VPN+.

   *  The necessary control and management protocols in both the
      underlay and the overlay of VPN+.

   *  The mechanisms to achieve integration between overlay and
      underlay.

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   *  The necessary Operation, Administration, and Management (OAM)
      methods to instrument a VPN+ to make sure that the required 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 these objectives is
   shown in Section 4.1.

2.  Terminology

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

   *  A Virtual Private Network (VPN) refers to the overlay network
      service that provides connectivity between different customer
      sites, and that maintains traffic separation between different
      customers.  Examples of VPN technologies are: IPVPN [RFC2764],
      L2VPN [RFC4664], L3VPN [RFC4364], and EVPN [RFC7432].

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

   *  A Virtual Transport Network (VTN) is a virtual underlay network
      which is associated with a logical network topology, and is
      allocated with a set of dedicated or shared network resources from
      the underlay physical network.  A VTN is designed to meet the
      network resources and performance characteristics required by the
      VPN+ customers.

   *  A network slice service could be delivered by provisioning one or
      more VPN+ services in the network.  Other mechanisms for realizing
      network slices may exist but are not in scope for this document.

   The term "tenant" is used in this document to refer to the customers
   of the VPN+ services.

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

   SLA:  Service Level Agreement.  See
      [I-D.ietf-teas-ietf-network-slices].

   SLO:  Service Level Objective.  See
      [I-D.ietf-teas-ietf-network-slices].

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   SLE:  Service Level Expectation.  See
      [I-D.ietf-teas-ietf-network-slices].

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

   DetNet:  Deterministic Networking.  See [RFC8655].

   FlexE:  Flexible Ethernet [FLEXE].

   TSN:  Time Sensitive Networking [TSN].

   VN:  Virtual Network.  See [RFC8453].

   VTP:  Virtual Transport Path.  A VTP is a path through the VTN which
      provides the required connectivity and performance between two or
      more customer sites.

3.  Overview of the Requirements

   This section provides an overview of the requirements of a VPN+
   service.

3.1.  Performance Guarantees

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

   There are several kinds of performance guarantees, 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 (see, for example,
   Section 3.6 of [RFC7297]).

   Guaranteed maximum packet loss is usually addressed by setting packet
   priorities, queues size, and discard policy.  However, this becomes
   more difficult when the requirement is combined with latency
   requirements.  The limiting case is zero congestion loss, and that is
   the goal of Deterministic Networking (DetNet) [RFC8655] and Time-
   Sensitive Networking (TSN) [TSN].  In modern optical networks, loss
   due to transmission errors already approaches zero, but there is the
   possibility of failure of the interface or the fiber itself.  This
   type of fault can be addressed by some form of signal duplication and
   transmission over diverse paths.

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   Guaranteed maximum latency is required by a number of applications,
   particularly real-time control applications and some types of
   augumented reality and virtual reality (AR/VR) applications.  DetNet
   techniques may be considered [RFC8655], 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 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
   VPN+s.  Alternatively, a dedicated VPN+ might be used to provide time
   transfer as a shared service.

   This suggests that a spectrum of service guarantees need to be
   considered when designing and deploying a VPN+. For illustration
   purposes and without claiming to be exhaustive, four types of
   services are considered:

   *  Best effort

   *  Assured bandwidth

   *  Guaranteed latency

   *  Enhanced delivery

   It is noted that some service may have mixed requirements of the
   above, e.g., both assured bandwidth and guaranteed latency can be
   required.

   The best effort service is the basic connectivity service that can be
   provided by current VPNs.

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   An assured bandwidth service is a connectivity service in which the
   bandwidth over some period of time is assured.  This could 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 concerns.  The more common
   approach of aggregating multiple VPNs onto common TE-LSPs results in
   shared bandwidth and so may reduce the assurance of bandwidth to any
   one service.  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 [TSN] introduces the concept of scheduling of delay- and
   loss-sensitive packets.  FlexE [FLEXE] is also useful to help provide
   a guaranteed upper bound to latency.  DetNet is also of relevance in
   assuring an upper bound of end-to-end packet latency in network
   layer.  The use of these technologies to deliver VPN+ services needs
   to be considered when a guaranteed latency service is required.

   An enhanced delivery service is a connectivity service in which the
   underlay network (at Layer 3) needs to ensure to eliminate or
   minimize packet loss in the event of equipment or media failures.
   This may be achieved by delivering a copy of the packet through
   multiple paths.  Such a mechanism may need to be used for VPN+
   services.

3.2.  Interaction between VPN+ Services

   There is a fine distinction between how a customer requests limits on
   interaction between VPN+ services, and how that is delivered by the
   service provider.  This section examines the requirements and
   realization of limited interaction between VPN+ services.

3.2.1.  Requirements on Traffic Isolation

   Traffic isolation is a generic term that can be used to describe the
   requirements on separating the services of different customers or
   different service types in the network.  In the context of network
   slicing, traffic isolation is defined as an SLE of the network slice
   service (Section 8.1 of [I-D.ietf-teas-ietf-network-slices]), which
   is one element of the SLA.  A customer may care about disruption
   caused by other services, contamination by other traffic, or delivery

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   of their traffic to the wrong destinations.

   A customer may want to specify (and thus pay for) the traffic
   isolation provided by the service provider.  Some customers (banking,
   for example) may have strict requirements on how their flows are
   handled when delivered over a shared network.  Some professional
   services are used to relying on specific certifications and audits to
   ensure the compliancy of a network with traffic isolation
   requirements, and specifically to prevent data leaks.

   With traffic isolation, a customer expects that the service traffic
   cannot be received by other customers in the same network.  In
   [RFC4176], traffic isolation is mentioned as one of the requirements
   of VPN customers.  Traffic isolation is also described in Section 3.8
   of [RFC7297].

   There can be different expectations on traffic isolation.  For
   example, a customer may further request the protection of their
   traffic by requesting specific encryption schemes at the VPN+ network
   access and also when transported between PEs.

   A VPN+ service customer may request traffic isolation together with
   other operator defined service characteristics.  The exact details
   about the expected behavior need to be specified in the service
   request, so that meaningful service assurance and fulfillment
   feedback can be exposed to the customers.  It is out of the scope of
   this document to elaborate the service modeling considerations.

3.2.2.  Limited Interaction with Other Services

   [RFC2211] describes the Controlled Load Service.  In that document,
   the end-to-end behavior provided to an application by a series of
   network elements providing controlled-load service is described as
   closely approximating to the behavior visible to applications
   receiving best-effort service when those network elements are not
   carrying substantial traffic from other services.

   Thus, a consumer of a Controlled Load Service may assume that:

   *  A very high percentage of transmitted packets will be successfully
      delivered by the network to the receiving end-nodes.

   *  The transit delay experienced by a very high percentage of the
      delivered packets will not greatly exceed the minimum transmit
      delay experienced by any successfully delivered packet.

   A VPN+ customer may request a Controlled Load Service in one of two
   ways:

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   1.  It may configure a set of SLOs (for example, for delay and loss)
       such that the delivered enhanced VPN meets the behavioral
       objectives of the customer.

   2.  As described in [RFC2211], a customer may request the Controlled
       Load Service without reference to or specification of specific
       target values for control parameters such as delay or loss.
       Instead, acceptance of a request for Controlled Load Service is
       defined to imply a commitment by the network element to provide
       the requestor with service closely equivalent to that provided to
       uncontrolled (best-effort) traffic under lightly loaded
       conditions.  This way of requesting the service is an SLE.

   Limited interaction between VPN+ services does not cover service
   degradation due to non-interaction-related causes, such as link
   errors.

3.2.3.  Realization of Limited Interaction Between VPN+ Services

   A service provider may translate the requirements related to limited
   interaction into distinct engineering rules in its network.  Honoring
   the service requirement may involve tweaking a set of QoS, TE,
   security, and planning tools, while traffic isolation will involve
   adequately configuring routing and authorization capabilities.

   Concretely, there are many existing techniques which can be used to
   provide traffic isolation, such as IP and MPLS VPNs or other multi-
   tenant virtual network techniques.  Controlled Load Services may be
   realized as described in [RFC2211].  Other tools may include various
   forms of resource management and reservation techniques, such as
   network capacity planning, allocating dedicated network resources,
   traffic policing or shaping, prioritizing in using shared network
   resources etc., so that a subset of bandwidth, buffers, and queueing
   resources can be available in the underlay network to support the
   VPN+ services.

   To provide the required traffic isolation, or to reduce the
   interaction with other VPN+ services, network resources may need to
   be reserved in the data plane of the underlay network and dedicated
   to traffic from a specific VPN+ service or a specific group of VPN+
   services.  This may introduce scalability concerns both in the
   implementation (as each VPN+ may need to be tracked in the network)
   and in how many resources need to be reserved and how the services
   are mapped to the resources (Section 4.4).  Thus, some trade-off
   needs to be considered to provide the traffic isolation and limited
   interaction between VPN+ services.

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   A dedicated physical network can be used to meet stricter SLO and SLE
   requests, at the cost of allocating resources on a long-term and end-
   to-end basis.  On the other hand, where adequate traffic isolation
   and limited interaction 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.  By combining
   conventional VPNs with TE/QoS/security techniques, VPN+ offers a
   variety of means to honor customer's requirements.

3.3.  Integration with Network Resources and Service Functions

   The way to achieve the characteristics demand of a VPN+ service (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 requirements.  This
   needs to be done in a flexible and scalable way so that it can be
   widely deployed in operators' networks to support a good number of
   VPN+ services.

   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)
   [RFC7665] provides a foundation for this.  Service functions in the
   underlay network can be considered as part of the VPN+ services,
   which means the service functions may need to be an integral part of
   the corresponding VTN.  The details of the integration between
   service functions and VPN+ are out of the scope of this document.

3.3.1.  Abstraction

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

   With the approach of abstraction, VPN+ may be built on top of an
   abstracted topology that represents the connectivity capabilities of
   the underlay TE based network as described in the framework for
   Abstraction and Control of TE Networks (ACTN) [RFC8453] as discussed
   further in Section 5.5.

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3.4.  Dynamic Changes

   VPN+s need to be created, modified, and removed from the network
   according to service demands (including scheduled requests).  A VPN+
   that requires limited interaction with other services (Section 3.2.2)
   must not be disrupted by the instantiation or modification of another
   VPN+ service.  As discussed in Section 3.1 of [RFC4176], the
   assessment of traffic isolation is part of the management of a VPN
   service.  Determining whether modification of a VPN+ can be
   disruptive to that VPN+ and whether the traffic in flight will be
   disrupted can be a difficult problem.

   Dynamic changes both to the VPN+ and to the underlay 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+ service endpoint or a change to a
   link, VPN+ traffic might need to be moved because of changes to
   traffic patterns and volumes.  This means that during the lifetime of
   a VPN+ service, closed-loop optimization is needed so that the
   delivered service always matches the ordered service SLA.

   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.

3.5.  Customized Control

   In many cases the customers are delivered with VPN+ services without
   information about the underlying VTNs.  However, depending on the
   agreement between the operator and the customer, in some cases the
   customer may also be provided with some information about the
   underlying VTNs.  Such information can be filtered or aggregated
   according to the operator's policy.  This allows the customer of a
   VPN+ service to have some visibility and even control over how the
   underlying topology and resources of the VTN are used.  For example,
   the customers may be able to specify the path or path constraints
   within the VTN for specific traffic flows of their VPN+ service.
   Depending on the requirements, a VPN+ customer may have their own
   network controller, which may be provided with an interface to the
   control or management system run by the network operator.  Note that
   such a control is within the scope of the customer's VPN+ service;

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   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 to Overlay Technologies

   The concept of VPN+ can be applied to any existing and future multi-
   tenancy overlay technologies including but not limited to:

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

   *  Layer-2 VPNs [RFC4664]

   *  Ethernet VPNs [RFC7209], [RFC7432]

   *  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 tweak the underlay to provide the required enhanced performance.

3.7.  Inter-Domain and Inter-Layer Network

   In some scenarios, a VPN+ service may span multiple network domains.
   A domain is considered to be any collection of network elements under
   the responsibility of the same administrative entity, for example, an
   Autonomous System (AS).  In some domains the network operator may
   manage a multi-layered network, for example, a packet network over an
   optical network.  When VPN+ services 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 VPN+ services, and improve network efficiency and
   operational simplicity.  The mechanisms for multi-domain VPNs
   [RFC4364] may be reused, and some enhancement may be needed to meet
   the additional requirements of VPN+ services.

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4.  The Architecture of VPN+

   Multiple VPN+ services can be provided by a common network
   infrastructure.  Each VPN+ service is provisioned with an overlay VPN
   and mapped to a corresponding VTN, which has a specific set of
   network resources and service functions allocated in the underlay to
   satisfy the needs of the customer.  One VTN may support one of more
   VPN+ services.  The integration between the overlay connectivity and
   the underlay resources ensures the required isolation between
   different VPN+ services, and achieves the guaranteed performance for
   different customers.

   The VPN+ architecture needs to be designed with consideration given
   to:

   *  An enhanced data plane.

   *  A control plane to create VPN+ and VTN, making use of the data
      plane isolation and performance guarantee techniques.

   *  A management plane for VPN+ service life-cycle management.

   *  The OAM mechanisms for VPN+ and the underlaying VTN.

   *  Telemetry mechanisms for VPN+ and the underlaying VTN.

   These topics are expanded below.

   *  The enhanced data plane provides:

      -  The required packet latency and jitter characteristics.

      -  The required packet loss characteristics.

      -  The required resource isolation capability, e.g., bandwidth
         guarantee.

      -  The mechanism to associate a packet with the set of resources
         allocated to a VTN which the VPN+ service packet is mapped to.

   *  The control plane:

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

      -  Creates VTNs with the network resource and topology properties
         needed by the VPN+ services.

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      -  Distributes the attributes of VTNs to network nodes which
         participate in the VTNs and/or a centralized controller.

      -  Computes and sets up network paths in each VTN.

      -  Maps VPN+ services to an appropriate VTN.

      -  Determines the risk of SLA violation and takes appropriate
         avoiding/correction actions.

      -  Considers the right balance of per-packet and per-node state
         according to the needs of the VPN+ services to scale to the
         required size.

   *  The management plane provides:

      -  An interface between the VPN+ service provider (e.g.,
         operator's network management system) and the VPN+ customer
         (e.g., an organization or a service with VPN+ requirement) such
         that the operation requests and the related parameters can be
         exchanged without the awareness of other VPN+ customers.

      -  An interface between the VPN+ service provider and the VPN+
         customers to expose the network capability information toward
         the customer.

      -  The service life-cycle management and operation of VPN+
         services (e.g., creation, modification, assurance/monitoring,
         and decommissioning).

   *  Operations, Administration, and Maintenance (OAM) provides:

      -  The tools to verify the connectivity and monitor the
         performance of the VPN+ service.

      -  The tools to verify whether the underlay network resources are
         correctly allocated and operating properly.

   *  Telemetry provides:

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

         o  from the underlay network for overall performance evaluation
            and for the planning of the VPN+ services.

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         o  from each VPN+ service for monitoring and analytics of the
            characteristics and SLA fulfillment of the VPN+ services.

4.1.  Layered Architecture

   The layered architecture of VPN+ is shown in Figure 1.

   Underpinning everything is the physical network infrastructure layer
   which provides the underlying resources used to provision the
   separate VTNs.  This layer is responsible for the partitioning of
   link and/or node resources for different VTNs.  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      VPN+ #1
                               /-------------o
                 o____________/______________o      VPN+ #2
                            _________________o
                      _____/
                 o___/     \_________________o      VPN+ #3
                     \_______________________o
                            ......                  ...
                 o-----------\ /-------------o
                 o____________X______________o      VPN+ #n

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

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

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

                 Figure 1: The Layered Architecture of VPN+

   Various components and techniques discussed in Section 5 can be used
   to enable resource partitioning of the physical network
   infrastructure, such as FlexE, TSN, 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 set of network resource partitions provided by the
   physical network infrastructure, multiple VTNs can be created, each
   with a set of dedicated or shared network resources allocated from
   the physical underlay network, and each can be associated with a
   customized logical network topology, so as to meet the requirements
   of different VPN+ services or different groups of VPN+ services.
   According to the associated logical network topology, each VTN needs
   to be instantiated on a set of network nodes and links which are
   involved in the logical topology.  And on each node or link, each VTN
   is associated with a set of local resources which are allocated for
   the processing of traffic in the VTN.  The VTN provides the
   integration between the logical network topology and the required
   underlying network resources.

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   According to the service requirements of connectivity, performance
   and isolation, etc., VPN+ services can be mapped to the appropriate
   VTNs in the network.  Different VPN+ services can be mapped to
   different VTNs, while it is also possible that multiple VPN+ services
   are mapped to the same VTN.  Thus, the VTN is an essential scaling
   technique, as it has the potential of eliminating per-service per-
   path state from the network.  In addition, when a group of VPN+
   services are mapped to a single VTN, only the network state of the
   single VTN needs to be maintained in the network (see Section 4.4 for
   more information).

   The network controller is responsible for creating a VTN, instructing
   the involved network nodes to allocate network resources to the VTN,
   and provisioning the VPN+ services on the VTN.  A distributed control
   plane may be used for distributing the VTN resource and topology
   attributes among nodes in the VTN.

   The process used to create VTNs and to allocate network resources for
   use by the VTNs needs to take a holistic view of the needs of all of
   the service provider's customers 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 with some flexibility.

4.2.  Connectivity Types

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

   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 underlying VTN of the
   VPN+ service 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 VPN+ will
   likely be IP based, 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].

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   Where the underlay is MPLS, Ethernet traffic can be carried over 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.  Scalable Service Mapping

   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.

   VPN+ 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 VPN+ services so that
   the additional state introduced to the network is acceptable and
   under control.  It is expected that the number of VPN+ services will
   be small at the beginning, and even in the future the number of VPN+
   services will be fewer than conventional VPNs because existing VPN
   techniques are good enough to meet the needs of most existing VPN-
   type services.

   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 VPN+ services 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.ietf-teas-nrp-scalability] provides more details of scalability
   considerations for the network resource partitions used to
   instantiate VTNs, and Section 7 includes a greater discussion of
   scalability considerations.

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5.  Candidate Technologies

   A VPN is a virtual network created by applying a demultiplexing
   technique to the underlying network (the underlay) to distinguish the
   traffic of one VPN from that of another.  The connections of VPN are
   supported by a set of underlay paths.  A path that travels by other
   than the shortest path through the underlay normally requires state
   to specify that path.  The state of the paths could be applied to the
   underlay through the use of the RSVP-TE signaling protocol, or
   directly through the use of an SDN controller.  Based on Segment
   Routing, state could be maintained at the ingress node of the path,
   and carried in the data packet.  Other techniques may emerge as this
   problem is studied.  This state gets harder to manage as the number
   of paths increases.  Furthermore, as we increase the coupling between
   the underlay and the overlay to support the VPN+ service, this state
   is likely to increase further.

   VTN can be used to provide a group of virtual underlay paths (VTP)
   with a common set of network resources.  Through the use of VTNs, a
   subset of underlay network resource can be either dedicated for a
   particular VPN+ service or shared among a group of VPN+ services.
   This section describes the candidate technologies in different
   network planes which can be used to build VTNs.

5.1.  Forwarding Resource Partitioning

   Several candidate layer-2 packet- or frame-based forwarding plane
   mechanisms which can provide the required traffic isolation and
   performance 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 separation between VPN+ services.  FlexE also
   supports bonding links to create larger links out of multiple low-
   capacity links.

   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 traffic isolation in the downstream node.  This
   in turn requires a queuing and forwarding implementation that
   preserves the end-to-end separation of enhanced VPNs.

   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

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   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 separation of
   VPN+ services because DiffServ does not provide enough markers to
   differentiate between traffic of a large number of VPN+ services.
   Nor does DiffServ offer the range of service classes that each VPN+
   service 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 for different VPN+
   services.

   To address these problems and to reduce the potential interactions
   between VPN+ services, it would be necessary to steer traffic to
   dedicated input and output queues per VPN+ service or per group of
   VPN+ services: 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
   separation required by the applications of 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 traffic flows of VPN+ service.

   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.

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5.2.  Data Plane Encapsulation and Forwarding

   This section considers the problem of VPN+ service differentiation
   and the representation of underlying network resources in the network
   layer.  More specifically, it describes the possible data plane
   mechanisms to determine the network resources and the logical network
   topology or paths associated with a VTN.

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.
   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.  Detnet can be realized over IP data plane
   [RFC8939] or MPLS data plane [RFC8964], and may be used to provide
   Virtual Transport Paths (VTPs) for VPN+ services.

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 VPN services.  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 traffic
   engineering in 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.  SR can be instantiated using MPLS data plane (SR-MPLS) or
   IPv6 data plane (SRv6).

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   An SR traffic engineered path operates with a granularity of a link.
   Hints about priority are provided using the Traffic Class (TC) field
   in the packet header.  However, to achieve the performance and
   isolation characteristics that are sought by VPN+ customers, it will
   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 the associated 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 performance 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 Segment Identifiers (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 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 to be borne
   by the client to which the resources are allocated.  When a VTN built
   with this data plane is used to support multiple VPN+ services, the
   cost could be distributed among such group of services.

5.4.  Control Plane

   The control plane of VPN+ would likely be based on a hybrid control
   mechanism that takes advantage of a 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+.

   As described in Section 4, the VPN+ control plane needs to provide
   the following functions:

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

   *  Create VTNs with the network resource and topology properties
      needed by the VPN+ services.

   *  Distribute the attributes of VTNs to network nodes which
      participate in the VTNs and/or the centralized controller.

   *  Map VPN+ services to an appropriate VTN.

   *  Compute and set up VTPs in each VTN to meet VPN+ service
      requirements.

   The collection of underlying network topology and resource
   information can be done using existing the IGP and Border Gateway
   Protocol - Link State (BGP-LS) [RFC7752] based mechanisms.  The
   creation of VTN and the distribution of VTN attributes may need
   further control protocol extensions.  The computation of VTPs based
   on the attributes and constraints of the VTN can be performed either
   by the headend node of the path or a centralized Path Computation
   Element (PCE) [RFC4655].

   There are two candidate control plane mechanisms for the setup of
   VTPs in the VTN: RSVP-TE and Segment Routing (SR).

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

   *  The SR control plane [RFC8665] [RFC8667] [RFC9085] 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 VTNs without
      requiring per-path state to be maintained in the network.  A
      centralized controller can perform resource planning and
      reservation for VTNs, and it needs to instruct the network nodes
      to ensure that resources are correctly allocated for the VTN.  The
      controller could provision the SR paths based on the mechanism in
      [RFC9256] to the headend nodes of the paths.

   According to the service requirements for connectivity, performance
   and isolation, one VPN+ service may be mapped a dedicated VTN, or a
   group of VPN+ services may be mapped to the same VTN.  The mapping of

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   VPN+ services to VTN can be achieved using existing control
   mechanisms with possible extensions, and it can be based on either
   the characteristics of the data packet or the attributes of the VPN
   service routes.

5.5.  Management Plane

   The management plane provides the interface between the VPN+ service
   provider and the customers for life-cycle management of the VPN+
   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 the transport network segment of the 5G
   end-to-end network slice can be realized with the management plane of
   VPN+. 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 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 VPN+ services 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 VPN+ services.  That is, it should be possible to add
   and remove end points, as well as to change the requested
   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 VTNs 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 a VPN+ service is possible, the management
   system must make every effort to make the changes in a non-disruptive
   way.  That is, the modification of the VPN+ service or the underlying
   VTN must not perturbate traffic on the VPN+ service in a way that
   causes the service level to drop below the agreed levels.
   Furthermore, changes to one VPN+ service should not cause disruption
   to other VPN+ services.

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   The network operator for the underlay network (i.e., the provider of
   the VPN+ service) may delegate some operational aspects of the
   overlay VPN and the underlying VTN to the 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.  Some mechanisms in the
   operator's network is needed, so that a customer cannot exceed the
   capabilities of the 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 virtual network.  This approach
   requires a management system for the virtual network, but does not
   necessarily require any coordination between the management systems
   of the virtual network and the physical network, except that the
   virtual network management system might notice when the VTN is close
   to capacity or considerably under-used and automatically request
   changes in the service provided by the underlay network.

5.6.  Applicability of Service Data Models to VPN+

   This section describes the applicability of the existing and in-
   progress service data models to VPN+. [RFC8309] describes the scope
   and purpose of service models and shows where a service model might
   fit into an SDN based network management architecture.  New service
   models may also be introduced for some of the required management
   functions.

   Service data models are used to represent, monitor, and manage the
   virtual networks and services enabled by VPN+. The VPN customer
   service models (e.g., the Layer 3 VPN Service Model (L3SM) [RFC8299],
   the Layer 2 VPN Service Model (L2SM) [RFC8466]), or the ACTN Virtual
   Network (VN) model [I-D.ietf-teas-actn-vn-yang]) are service models
   which can provide the customer's view of the VPN+ service.  The
   Layer-3 VPN Network Model (L3NM) [RFC9182], the Layer-2 VPN network
   model (L2NM) [RFC9291] provide the operator's view of the managed
   infrastructure as a set of virtual networks and the associated
   resources.  The Service Attachment Points (SAPs) model [RFC9408]
   provides an abstract view of the service attachment points (SAPs) to
   various network services in the provider network, where VPN+ could be
   one of the service types.  Augmentation to these service models may
   be needed to provide the VPN+ services.  The NRP model
   [I-D.wdbsp-teas-nrp-yang] further provides the management of the NRP
   topology and resources both in the controller and in the network
   devices to instantiate the VTNs needed for the VPN+ services.

6.  Applicability in Network Slice Realization

   This section describes the applicability of VPN+ in network slice
   realization.

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   In order to provide IETF network slices to customers, a technology-
   agnostic network slice service model
   [I-D.ietf-teas-ietf-network-slice-nbi-yang] is needed for the
   customers to communicate the requirements of IETF network slices (end
   points, connectivity, SLOs, and SLEs).  These requirements may be
   realized using technology specified in this document to instruct the
   network to deliver a VPN+ service so as to meet the requirements of
   the IETF network slice customers.

6.1.  VTN Planning

   According to the network operators' network resource planning policy,
   or based on the requirements of one or a group of customers or
   services, a VTN may need to be created to meet the requirements of
   VPN+ services.  In the network slicing context, a VTN could be
   considered as an NRP used to support the IETF network slice services.
   One of the basic requirements for a VTN is to provide a set of
   dedicated network resources to avoid unexpected interference from
   other services in the same network.  Other possible requirements may
   include the required topology and connectivity, bandwidth, latency,
   reliability, etc.

   A centralized network controller can be responsible for calculating a
   subset of the underlay network topology (which is called a logical
   topology) to support the VTN requirement.  And on the network nodes
   and links within the logical topology, the set of network resources
   to be allocated to the VTN can also be determined by the controller.
   Normally such calculation needs to take the underlay network
   connectivity information and the available network resource
   information of the underlay network into consideration.  The network
   controller may also take the status of the existing VTNs into
   consideration in the planning and calculation of a new VTN.

6.2.  VTN Instantiation

   According to the result of the VTN planning, the network nodes and
   links involved in the logical topology of the VTN are instructed to
   allocated the required set of network resources for the VTN.  In the
   network slicing context, a VTN can be instantiated as an NRP.  One or
   multiple mechanisms as specified in section 5.1 can be used to
   partition the forwarding plane network resources and allocate
   different subsets of resources to different VTNs.  In addition, the
   data plane identifiers which are used to identify the set of network
   resources allocated to the VTN are also provisioned on the network
   nodes.  Depending on the data plane technologies used, the set of
   network resources of a VTN can be identified using e.g. either
   resource aware SR segments as specified in
   [I-D.ietf-spring-resource-aware-segments]

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   [I-D.ietf-spring-sr-for-enhanced-vpn], or a dedicated VTN resource ID
   as specified in [I-D.ietf-6man-enhanced-vpn-vtn-id] can be
   introduced.  The network nodes involved in a VTN may distribute the
   logical topology information, the VTN specific network resource
   information and the VTN resource identifiers using the control plane.
   Such information could be used by the controller and the network
   nodes to compute the TE or shortest paths within the VTN, and install
   the VTN specific forwarding entries to network nodes.

6.3.  VPN+ Service Provisioning

   According to the connectivity requirements of an IETF network slice
   service, an overlay VPN can be created using the existing or future
   multi-tenancy overlay technologies as described in Section 3.6.

   Then according to the SLO and SLE requirements of a network slice
   service, the overlay VPN is mapped to an appropriate VTN as the
   virtual underlay.  The integration of the overlay VPN and the
   underlay VTN together provide a VPN+ service which can meet the
   network slice service requirements.

6.4.  Network Slice Traffic Steering and Forwarding

   At the edge of the operator's network, traffic of IETF network slices
   can be classified based on the rules defined by the operator's
   policy, so that the traffic is treated as a specific VPN+ service,
   which is further mapped to an underlay VTN.  Packets belonging to the
   VPN+ service will be processed and forwarded by network nodes based
   the TE or shortest path forwarding entries and the set of network
   resources of the corresponding VTN.

7.  Scalability Considerations

   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 brought into the network:

   *  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 to
      provide VTN specific segments in case they are called upon by a
      service.  This is a type of latent state, and increases as the
      segments and resources that need to be exclusively available to
      VPN+ service are specified more precisely.

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   *  Introduce the state to the network.  This is normally done by
      creating a path using signaling such as RSVP-TE.  This could be
      extended to include any element that needs to be specified along
      the path, for example explicitly specifying queuing policy.  It is
      also 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 the life of the
      path.  This is more network state than is needed using SR, but the
      packets are usually shorter.

   *  Provide a hybrid approach.  One example is based on using binding
      SIDs [RFC8402] to represent 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 VPN+, as it
   requires the overlay to be more closely integrated with the underlay
   than with conventional 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.  Aggregation is a well-
   established approach to reduce the amount of state and improve
   scaling, and VTN is considered as the network construct to aggregate
   the states of VPN+ services.  In addition, an SR approach allows much
   of the state to be spread amongst the network ingress nodes, and
   transiently carried in the packets as SIDs.

   The following subsections describe some of the scalability concerns
   that need to be considered.  Further discussion of the scalability
   considerations of the underlaying network construct of VPN+ can be
   found in [I-D.ietf-teas-nrp-scalability].

7.1.  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.

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7.2.  RSVP-TE Scalability

   The established 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.

7.3.  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 an 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 a 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 may
   also present a single point of failure within the network.

8.  Manageability Considerations

   This section describes the considerations about the OAM and Telemetry
   mechanisms used to support the verification, monitoring and
   optimization of the characteristics and SLA fulfillment of the VPN+
   services.

8.1.  OAM Considerations

   The design of OAM for VPN+ services needs to consider the following
   requirements:

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

   *  Instrumentation of the overlay by the customer.  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 various committed performance characteristics.

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   *  Instrumentation of the overlay by the service 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.

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

8.2.  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 [RFC9232], 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 VPN+
   service is out of the scope of this document.

9.  Enhanced Resiliency

   Each VPN+ service 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
   perform garbage collection to consolidate resources into usable
   quanta.

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   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.  The global
   concurrent optimization mechanisms as described in [RFC5557] and
   discussed in [RFC7399] may be helpful, while complete resolution of
   this situation is as much a commercial issue as it is a technical
   issue.

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

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

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

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10.  Operational Considerations

   It is expected that VPN+ services will be introduced in networks
   which already have conventional VPN services deployed.  Depending on
   service requirements, the tenants or the operator may choose to use a
   VPN or a VPN+ to fulfill a service requirement.  The information and
   parameters to assist such a decision needs to be supplied 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 VPN+ neither introduces, nor
   experiences greater security risks than other VPNs.

   However, in a VPN+ 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 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 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 of the specific solution to the isolation
   requirements of a VPN+ service.

   While a 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 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 VPN+ be
   externally visible.

   A VPN+ service (even one with traffic isolation requirements or with
   limited interaction with other enhanced VPNs) does not provide any
   additional guarantees of privacy for customer traffic compared to
   regular VPNs: the traffic within the network may be intercepted and

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   errors may lead to mis-delivery.  Users who wish to ensure the
   privacy of their traffic must take their own precautions including
   end-to-end encryption.

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, Luis M.  Contreras, and Joel Halpern
   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.  Informative References

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

   [I-D.ietf-6man-enhanced-vpn-vtn-id]
              Dong, J., Li, Z., Xie, C., Ma, C., and G. S. Mishra,
              "Carrying Virtual Transport Network (VTN) Information in
              IPv6 Extension Header", Work in Progress, Internet-Draft,
              draft-ietf-6man-enhanced-vpn-vtn-id-05, 6 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-6man-
              enhanced-vpn-vtn-id-05>.

   [I-D.ietf-spring-resource-aware-segments]
              Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., Li,
              Z., and F. Clad, "Introducing Resource Awareness to SR
              Segments", Work in Progress, Internet-Draft, draft-ietf-
              spring-resource-aware-segments-07, 31 May 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              resource-aware-segments-07>.

   [I-D.ietf-spring-sr-for-enhanced-vpn]
              Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., Li,
              Z., and F. Clad, "Segment Routing based Virtual Transport
              Network (VTN) for Enhanced VPN", Work in Progress,
              Internet-Draft, draft-ietf-spring-sr-for-enhanced-vpn-05,
              31 May 2023, <https://datatracker.ietf.org/doc/html/draft-
              ietf-spring-sr-for-enhanced-vpn-05>.

   [I-D.ietf-teas-actn-vn-yang]
              Lee, Y., Dhody, D., Ceccarelli, D., Bryskin, I., and B. Y.
              Yoon, "A YANG Data Model for Virtual Network (VN)
              Operations", Work in Progress, Internet-Draft, draft-ietf-
              teas-actn-vn-yang-18, 2 April 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-teas-
              actn-vn-yang-18>.

   [I-D.ietf-teas-ietf-network-slice-nbi-yang]
              Wu, B., Dhody, D., Rokui, R., Saad, T., Han, L., and J.
              Mullooly, "A YANG Data Model for the IETF Network Slice
              Service", Work in Progress, Internet-Draft, draft-ietf-
              teas-ietf-network-slice-nbi-yang-06, 10 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-teas-
              ietf-network-slice-nbi-yang-06>.

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   [I-D.ietf-teas-ietf-network-slices]
              Farrel, A., Drake, J., Rokui, R., Homma, S., Makhijani,
              K., Contreras, L. M., and J. Tantsura, "A Framework for
              IETF Network Slices", Work in Progress, Internet-Draft,
              draft-ietf-teas-ietf-network-slices-21, 15 June 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-teas-
              ietf-network-slices-21>.

   [I-D.ietf-teas-nrp-scalability]
              Dong, J., Li, Z., Gong, L., Yang, G., Guichard, J.,
              Mishra, G. S., Qin, F., Saad, T., and V. P. Beeram,
              "Scalability Considerations for Network Resource
              Partition", Work in Progress, Internet-Draft, draft-ietf-
              teas-nrp-scalability-02, 2 June 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-teas-
              nrp-scalability-02>.

   [I-D.wdbsp-teas-nrp-yang]
              Wu, B., Dhody, D., Beeram, V. P., Saad, T., and S. Peng,
              "A YANG Data Model for Network Resource Partitions
              (NRPs)", Work in Progress, Internet-Draft, draft-wdbsp-
              teas-nrp-yang-01, 24 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-wdbsp-teas-
              nrp-yang-01>.

   [NGMN-NS-Concept]
              hao ,, "NGMN NS Concept", 2016,
              <https://www.ngmn.org/fileadmin/user_upload/161010_NGMN_Ne
              twork_Slicing_framework_v1.0.8.pdf>.

   [RFC2211]  Wroclawski, J., "Specification of the Controlled-Load
              Network Element Service", RFC 2211, DOI 10.17487/RFC2211,
              September 1997, <https://www.rfc-editor.org/info/rfc2211>.

   [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>.

   [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>.

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   [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>.

   [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>.

   [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>.

   [RFC4026]  Andersson, L. and T. Madsen, "Provider Provisioned Virtual
              Private Network (VPN) Terminology", RFC 4026,
              DOI 10.17487/RFC4026, March 2005,
              <https://www.rfc-editor.org/info/rfc4026>.

   [RFC4176]  El Mghazli, Y., Ed., Nadeau, T., Boucadair, M., Chan, K.,
              and A. Gonguet, "Framework for Layer 3 Virtual Private
              Networks (L3VPN) Operations and Management", RFC 4176,
              DOI 10.17487/RFC4176, October 2005,
              <https://www.rfc-editor.org/info/rfc4176>.

   [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>.

   [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>.

   [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
              Computation Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

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   [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>.

   [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>.

   [RFC5557]  Lee, Y., Le Roux, JL., King, D., and E. Oki, "Path
              Computation Element Communication Protocol (PCEP)
              Requirements and Protocol Extensions in Support of Global
              Concurrent Optimization", RFC 5557, DOI 10.17487/RFC5557,
              July 2009, <https://www.rfc-editor.org/info/rfc5557>.

   [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>.

   [RFC7297]  Boucadair, M., Jacquenet, C., and N. Wang, "IP
              Connectivity Provisioning Profile (CPP)", RFC 7297,
              DOI 10.17487/RFC7297, July 2014,
              <https://www.rfc-editor.org/info/rfc7297>.

   [RFC7399]  Farrel, A. and D. King, "Unanswered Questions in the Path
              Computation Element Architecture", RFC 7399,
              DOI 10.17487/RFC7399, October 2014,
              <https://www.rfc-editor.org/info/rfc7399>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

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   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,
              <https://www.rfc-editor.org/info/rfc7665>.

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

   [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>.

   [RFC8309]  Wu, Q., Liu, W., and A. Farrel, "Service Models
              Explained", RFC 8309, DOI 10.17487/RFC8309, January 2018,
              <https://www.rfc-editor.org/info/rfc8309>.

   [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>.

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   [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>.

   [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>.

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   [RFC8939]  Varga, B., Ed., Farkas, J., Berger, L., Fedyk, D., and S.
              Bryant, "Deterministic Networking (DetNet) Data Plane:
              IP", RFC 8939, DOI 10.17487/RFC8939, November 2020,
              <https://www.rfc-editor.org/info/rfc8939>.

   [RFC8964]  Varga, B., Ed., Farkas, J., Berger, L., Malis, A., Bryant,
              S., and J. Korhonen, "Deterministic Networking (DetNet)
              Data Plane: MPLS", RFC 8964, DOI 10.17487/RFC8964, January
              2021, <https://www.rfc-editor.org/info/rfc8964>.

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

   [RFC9182]  Barguil, S., Gonzalez de Dios, O., Ed., Boucadair, M.,
              Ed., Munoz, L., and A. Aguado, "A YANG Network Data Model
              for Layer 3 VPNs", RFC 9182, DOI 10.17487/RFC9182,
              February 2022, <https://www.rfc-editor.org/info/rfc9182>.

   [RFC9232]  Song, H., Qin, F., Martinez-Julia, P., Ciavaglia, L., and
              A. Wang, "Network Telemetry Framework", RFC 9232,
              DOI 10.17487/RFC9232, May 2022,
              <https://www.rfc-editor.org/info/rfc9232>.

   [RFC9256]  Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
              A., and P. Mattes, "Segment Routing Policy Architecture",
              RFC 9256, DOI 10.17487/RFC9256, July 2022,
              <https://www.rfc-editor.org/info/rfc9256>.

   [RFC9291]  Boucadair, M., Ed., Gonzalez de Dios, O., Ed., Barguil,
              S., and L. Munoz, "A YANG Network Data Model for Layer 2
              VPNs", RFC 9291, DOI 10.17487/RFC9291, September 2022,
              <https://www.rfc-editor.org/info/rfc9291>.

   [RFC9408]  Boucadair, M., Ed., Gonzalez de Dios, O., Barguil, S., Wu,
              Q., and V. Lopez, "A YANG Network Data Model for Service
              Attachment Points (SAPs)", RFC 9408, DOI 10.17487/RFC9408,
              June 2023, <https://www.rfc-editor.org/info/rfc9408>.

   [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>.

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

Authors' Addresses

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

   Stewart Bryant
   University of Surrey
   Email: stewart.bryant@gmail.com

   Zhenqiang Li
   China Mobile
   Email: lizhenqiang@chinamobile.com

   Takuya Miyasaka
   KDDI Corporation
   Email: ta-miyasaka@kddi.com

   Young Lee
   Samsung
   Email: younglee.tx@gmail.com

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