TEAS working group                                            J. Dong
Internet-Draft                                                 Huawei
Intended status: Informational                              S. Bryant
Expires: July 2020                                          Futurewei
                                                                Z. Li
                                                         China Mobile
                                                          T. Miyasaka
                                                     KDDI Corporation
                                            Sung Kyun Kwan University
                                                    February 18, 2020

    A Framework for Enhanced Virtual Private Networks (VPN+) Services



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

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

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

   It is not envisaged that quite large numbers of VPN+ services will be
   deployed in a network and, in particular, it is not intended that
   all VPNs supported by a network will use VPN+ related techniques.

Status of This Memo

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

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

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

   This Internet-Draft will expire on August 18, 2020.

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   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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Table of Contents

   1. Introduction ................................................ 3
   2. Terminologies ............................................... 6
   3. Overview of the Requirements  ............................... 7
      3.1. Isolation between Enhanced VPN Services ................ 7
         3.1.1. A Pragmatic Approach to Isolation ................. 8
      3.2. Performance Guarantee .................................. 9
      3.3. Integration ........................................... 11
         3.3.1. Abstraction ...................................... 11
      3.4. Dynamic Management .................................... 12
      3.5. Customized Control .................................... 12
      3.6. Applicability ......................................... 13
      3.7. Inter-Domain and Inter-Layer Network .................. 13
   4. Architecture of Enhanced VPN ............................... 13
      4.1. Layered Architecture  ................................. 15
      4.2. Multi-Point to Multi-Point (MP2MP) Connectivity ....... 17
      4.3. Application Specific Network Types .................... 18
      4.4. Scaling Considerations ................................ 18
   5. Candidate Technologies ..................................... 19
      5.1. Layer-Two Data Plane .................................. 19
         5.1.1. Flexible Ethernet ................................ 19
         5.1.2. Dedicated Queues ................................. 20

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         5.1.3. Time Sensitive Networking ........................ 20
      5.2. Layer-Three Data Plane ................................ 21
         5.2.1. Deterministic Networking ......................... 21
         5.2.2. MPLS Traffic Engineering (MPLS-TE) ............... 21
         5.2.3. Segment Routing .................................. 21
      5.3. Non-Packet Data Plane ................................. 22
      5.4. Control Plane ......................................... 22
      5.5. Management Plane ...................................... 23
      5.6. Applicability of Service Data Models to Enhanced VPN .. 23
         5.6.1. Enhanced VPN Delivery in the ACTN Architecture ... 24
         5.6.2. Enhanced VPN Features with Service Data Models ... 25
         5.6.3. 5G Transport Service Delivery via Coordinated Data
         Modules ................................................. 27
   6. Scalability Considerations ................................. 29
      6.1. Maximum Stack Depth of SR ............................. 30
      6.2. RSVP Scalability ...................................... 30
      6.3. SDN Scaling ........................................... 30
   7. OAM Considerations ......................................... 30
   8. Telemetry Considerations ................................... 31
   9. Enhanced Resiliency ........................................ 31
   10. Operational Considerations ................................ 33
   11. Security Considerations ................................... 33
   12. IANA Considerations ....................................... 33
   13. Contributors .............................................. 34
   14. Acknowledgments ........................................... 34
   15. References ................................................ 34
      15.1. Normative References ................................. 34
      15.2. Informative References ............................... 36
   Authors' Addresses ............................................ 40

1. Introduction

   Virtual private networks (VPNs) have served the industry well as a
   means of providing different groups of users with logically isolated
   connectivity over a common network.  The common or base network that
   is used to provide the VPNs is often referred to as the underlay,
   and the VPN is often called an overlay.

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

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   the customer with required connectivity, but have enhanced

   Driven largely by needs surfacing from 5G, the concept of network
   slicing has gained traction [NGMN-NS-Concept] [TS23501] [TS28530]
   [BBF-SD406]. According to [TS28530], a 5G end-to-end network slice
   consists of three major types network segments: Radio Access Network
   (RAN), Transport Network (TN) and Mobile Core Network (CN).  The
   transport network provides the required connectivity between
   different entities in RAN and CN segments of an end-to-end network
   slice, with specific performance commitment.

   A transport network slice is a virtual (logical) network with a
   particular network topology and a set of shared or dedicated network
   resources, which are used to provide the network slice consumer with
   the required connectivity, appropriate isolation and specific
   Service Level Agreement (SLA) or Service Level Objective (SLO).

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

   In this document the term "network slice" refers to a transport
   network slice, and is considered as one typical use case of enhanced

   Network slicing builds on the concept 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 a set of services or a particular tenant
   or a group of tenants that share the same or similar set of
   requirements, on top of a common network.  How the network slices
   are engineered can be deployment-specific.

   VPN+ could be used to form the underpinning of transport network
   slice, but could also be of use in general cases providing enhanced
   connectivity services between customer sites.

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   The requirement of enhanced VPN services cannot be met by simple
   overlay networks, as they require tighter coordination and
   integration between the underlay and the overlay network. VPN+ is
   built from a VPN overlay and a underlying Virtual Transport Network
   (VTN) which has a customized network topology and a set of dedicated
   or shared network resources.  It may optionally include a set of
   invoked service functions allocated from the underlay network.  Thus
   an enhanced VPN can achieve greater isolation with strict
   performance guarantees.  These new properties, which have general
   applicability, may also be of interest as part of a network slicing
   solution. It is not envisaged that VPN+ services will replace
   traditional VPN services that can continue to be deployed using pre-
   existing mechanisms.

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

      o  The design of the enhanced data plane.

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

      o  The mechanisms to achieve integration between overlay and

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

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

   Note that, in this document, the relationship of the four terms
   "VPN", "Enhanced VPN" (or "VPN+"), "Virtual Transport Network (VTN)",
   and "Network Slice" are described as below:

      o  An enhanced VPN (VPN+) can be considered as an evolution of
   VPN service, but with additional service-specific commitments.  Thus,
   care must be taken with the term "VPN" to distinguish normal or
   legacy VPNs from VPN+ services.

      o  A Virtual Transport Network (VTN) is a virtual underlay
   network that connects customer edge points with the additional
   capability of providing the isolation and performance
   characteristics required by an enhanced VPN customer.

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      o  An enhanced VPN (VPN+) is made by integrating an overlay VPN
   and an VTN with a set of network resources allocated in the underlay

      o  A network slice in transport network could be provided with an
   enhanced VPN (VPN+).

2. Terminologies

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

   ACTN:  Abstraction and Control of TE Networks [RFC8453]

   Detnet:  Deterministic Networking [DETNET]

   FlexE:  Flexible Ethernet [FLEXE]

   TSN:  Time Sensitive Networking [TSN]

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

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

   VPN+:  Enhanced VPN service. An enhanced VPN service (VPN+) can be
   considered as an evolution of VPN service, but with additional
   service-specific commitments such as enhanced isolation and
   performance guarantee.

   VTP:  Virtual Transport Path. A VTP is a virtual underlay path which
   connects two customer edge points with the capability of providing
   the isolation and performance characteristics required by an
   enhanced VPN customer. A VTP usually has a customized path with a
   set of reserved network resources along the path.

   VTN:  Virtual Transport Network. A VTN is a virtual underlay network
   that connects customer edge points with the capability of providing
   the isolation and performance characteristics required by an
   enhanced VPN customer. A VTN usually has a customized topology and a
   set of dedicated or shared network resources.

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3. Overview of the Requirements

   In this section we provide an overview of the requirements of an
   enhanced VPN service.

3.1. Isolation between Enhanced VPN Services

   One element of the SLA demanded for an enhanced VPN is a guarantee
   that the service offered to the customer will not be perturbed by
   any other traffic flows in the network.  One way for a service
   provider to guarantee the customer's SLA is by controlling the
   degree of isolation from other services in the network.  Isolation
   is a feature that can be requested by customers. There are different
   grades of how isolation may be enabled by a network operator and
   that may result in different levels of service perceived by the
   customer.  These range from simple separation of service traffic on
   delivery (ensuring that traffic is not delivered to the wrong
   customer), all the way to complete separation within the underlay so
   that the traffic from different services use distinct network

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

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

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   appropriate degree of isolation to be able to continue to operate
   acceptably.  On the other hand, VPNs servicing ordinary bulk data
   may expect to contest for network resources and queue packets so
   that traffic is delivered within SLAs, but with some potential
   delays and interference.

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

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

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

   Section 3.1.1 explores pragmatic approaches to isolation in packet

3.1.1. A Pragmatic Approach to Isolation

   A key question is whether it is possible to achieve hard isolation
   in packet networks that were never designed to support hard
   isolation. On the contrary, they were designed to provide
   statistical multiplexing, a significant economic advantage when
   compared to a dedicated, or a Time Division Multiplexing (TDM)
   network.  However, there is no need to provide any harder isolation
   than is required by the applications.  An approximation to this
   requirement is sufficient in most cases.  Pseudowires [RFC3985]
   emulate services that would have had hard isolation in their native

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      This spectrum of isolation is shown in Figure 1:

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

                    Figure 1 The Spectrum of Isolation

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

3.2. Performance Guarantee

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

   Guaranteed maximum packet loss is a common parameter, and is usually
   addressed by setting packet priorities, queue 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 the Deterministic
   Networking work that the IETF [DETNET] and IEEE [TSN] are pursuing.
   In modern optical networks, loss due to transmission errors already
   approaches zero, but there are the possibilities of failure of the
   interface or the fiber itself.  This can only be addressed by some
   form of signal duplication and transmission over diverse paths.

   Guaranteed maximum latency is required in a number of applications
   particularly real-time control applications and some types of

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   virtual reality applications.  The work of the IETF Deterministic
   Networking (DetNet) Working Group [DETNET] is relevant; however
   additional methods of enhancing the underlay to better support the
   delay guarantees may be needed, and these methods will need to be
   integrated with the overall service provisioning mechanisms.

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

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

      o  Best effort

      o  Assured bandwidth

      o  Guaranteed latency

      o  Enhanced delivery

   Best effort service is the basic service that current VPNs provide.

   An assured bandwidth service is one in which the bandwidth over some
   period of time is assured.  This can be achieved either simply based
   on best effort with over-capacity provisioning, or it can be based
   on TE-LSPs with bandwidth reservation.  The instantaneous bandwidth
   is however, not necessarily assured, depending on the technique used.
   Providing assured bandwidth to VPNs, for example by using per-VPN
   TE-LSPs, is not widely deployed at least partially due to
   scalability concerns.

   Guaranteed latency and enhanced delivery are not yet integrated with
   VPNs. A guaranteed latency service has a latency upper bound
   provided by the network.  Assuring the upper bound is sometimes more
   important than minimizing latency.

   There are several new technologies that provide some assistance with
   performance guarantee.  Firstly there is the IEEE project on Time
   Sensitive Networking [TSN] which introduces the concept of packet

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   scheduling of delay and loss sensitive packets. Then the DetNet work
   is also of greater relevance in assuring upper bound of end-to-end
   packet latency. Flex Ethernet [FLEXE] is also useful to provide
   these guarantees.

   An enhanced delivery service is one in which the underlay network
   (at Layer 3) attempts to deliver the packet through multiple paths
   in the hope of eliminating packet loss due to equipment or media

   It is these last two characteristics (guaranteed upper bound to
   latency and elimination of packet loss) that an enhanced VPN adds to
   a VPN service.

3.3. Integration

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

   Taking mobile networks and in particular 5G into consideration, the
   integration of network and the service functions is a likely
   requirement.  The work in IETF SFC working group [SFC] provides a
   foundation for this integration.

3.3.1. Abstraction

   Integration of the overlay VPN and the underlay network resources
   does not need to be a tight mapping.  As described in [RFC7926],
   abstraction is the process of applying policy to a set of
   information about a TE network to produce selective information that
   represents the potential ability to connect across the network.  The
   process of abstraction presents the connectivity graph in a way that
   is independent of the underlying network technologies, capabilities,
   and topology so that the graph can be used to plan and deliver
   network services in a uniform way.

   Virtual networks can be built on top of an abstracted topology that
   represents the connectivity capabilities of the underlay network as
   described in the framework for Abstraction and Control of TE Networks
   (ACTN) [RFC8453] as discussed further in Section 5.5.

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3.4. Dynamic Management

   Enhanced VPNs need to be created, modified, and removed from the
   network according to service demand.  An enhanced VPN that requires
   hard isolation (section 3.1) must not be disrupted by the
   instantiation or modification of another enhanced VPN.  Determining
   whether modification of an enhanced VPN can be disruptive to that
   VPN, and in particular whether the traffic in flight will be
   disrupted can be a difficult problem.

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

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

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

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

   In addition to non-disruptively managing the network as a result of
   gross change such as the inclusion of a new VPN endpoint or a change
   to a link, VPN traffic might need to be moved as a result of traffic
   volume changes.

3.5. Customized Control

   In some cases it is desirable that an enhanced VPN has a customized
   control plane, so that the tenant of the enhanced VPN can have some
   control of how the resources and functions allocated to this
   enhanced VPN are used.  For example, the tenant may be able to
   specify the service paths in his own enhanced VPN.  Depending on the
   requirement, an enhanced VPN may have its own dedicated controller,
   which may be provided with an interface to the control system
   provided by the network operator. Note that such control is within
   the scope of the tenant's enhanced VPN, any change beyond that would
   require some intervention of the 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.

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3.6. Applicability

   The technologies described in this document should be applicable to
   a number of types of VPN services such as:

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

      o  Layer 2 VPNs [RFC4664]

      o  Ethernet VPNs [RFC7209]

      o  Layer 3 VPNs [RFC4364], [RFC2764]

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

3.7. Inter-Domain and Inter-Layer Network

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

4. Architecture of Enhanced VPN

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

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

    o  An enhanced data plane

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

    o  A management plane for enhanced VPN service life-cycle

   These required characteristics are expanded below:

    o  Enhanced data plane

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

       *  Provides the required packet latency and jitter

       *  Provides the required packet loss characteristics.

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

    o  Control plane

       *  Collect information about the underlying network topology
   and resources available and export this to nodes in the network
   and/or the centralized controller as required.

       *  Create the required virtual transport networks (VTNs) with
   the resource and properties needed by the enhanced VPN services that
   are assigned to them.

       *  Determine the risk of SLA violation and take appropriate
   avoiding action.

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

   o  Management plane

       *  Provides an interface between the enhanced VPN provider
   (e.g., the Transport Network Manager) and the enhanced VPN clients
   (e.g., the 3GPP Management System) such that some of the operation
   requests can be met without interfering with the enhanced VPN of
   other clients.

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

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

   o  Operations, Administration, and Maintenance (OAM)

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

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

   o  Telemetry

       *  Provides the mechanism to collect data plane, control plane,
   and management plane information about the network.  More

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

          +  Provides the mechanism to collect network data about
   each enhanced VPN for monitoring and analytics of the
   characteristics and SLA fulfilment of enhanced VPN services.

4.1. Layered Architecture

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

   Underpinning everything is the physical network infrastructure layer
   which provide the underlying resources used to provision the
   separated virtual transport networks (VTNs). This includes the
   partitioning of link and/or node resources. Each subset of link or
   node resource can be considered as a virtual link or virtual node
   used to build the VTNs.

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                             | |
                    +-------------------+       Centralized
                    | Network Controller|    Control& Management

                  /       o----o----o       /
                 /       /         /       /      VTN-1
                / o-----o-----o----o----o /
                  /       o----o            /
                 /       /    /  \         /      VTN-2
                / o-----o----o----o-----o /
                         ......                    ...
                 /             o----o       /
                /             /    /       /      VTN-3
               /  o-----o----o----o-----o /

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

       O    Virtual Node

       --   Virtual Link

      +--+ Physical Node with resource partition

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       ==  Physical Link with resource partition

                     Figure 2 The Layered Architecture

   Various components and techniques discussed in Section 5 can be used
   to enable resource partition, such as FlexE, Time Sensitive
   Networking, Deterministic Networking, Dedicated queues, etc. These
   partitions may be physical, or virtual so long as the SLA required
   by the higher layers is met.

   Based on the network resources provided by the physical network
   infrastructure, multiple VTNs can be provisioned, each with
   customized topology and other attributes to meet the requirement of
   different enhanced VPNs or different groups of enhanced VPNs. To get
   the required characteristic, each VTN needs to be mapped to a set of
   network nodes and links in the network infrastructure. And on each
   node or link, the VTN is associated with a set of resources which
   are allocated for the processing of traffic in the VTN. VTN provides
   the integration between the virtual network topology and the
   required underlying network resources.

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

   The process used to create VTNs and to allocate network resources
   for use by VTNs needs to take a holistic view of the needs of all of
   its tenants (i.e., of all customers and their associated VTNs), 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.

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

   At the VPN service level, the required connectivity is usually mesh
   or partial-mesh.  To support such kinds of VPN service, the
   corresponding VTN in underlay is also an abstract MP2MP medium.
   Other service requirements may be expressed at different granularity,
   some of which can be applicable to the whole service, while some
   others may be only applicable to some pairs of end points. For
   example,    when particular level of performance guarantee is

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   required, the point-to-point path through the underlay of the
   enhanced VPN may need to be specifically engineered to meet the
   required performance guarantee.

4.3. Application Specific Network Types

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

4.4. Scaling Considerations

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

   Enhanced VPNs may need to install some additional state within the
   network to achieve the additional features that they require.
   Solutions must consider minimizing and controlling the scale of such
   state, and deployment architectures should constrain the number of
   enhanced VPNs that would exist where such services would place
   additional state in the network.  It is expected that the number of
   enhanced VPN would be small in the beginning, and even in future the
   number of enhanced VPN will be much fewer than traditional VPNs,
   because pre-existing VPN techniques are be 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 of VPN+ services so that they
   share the same VTN and the same set of network resources (much in
   the way that current VPNs are aggregated over transport tunnels) so
   that collections of enhanced VPNs that require the same behaviour
   from the network in terms of resource reservation, latency bounds,
   resiliency, etc. are able to be grouped together.  This is an

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   important feature to assist with the scaling characteristics of VPN+

   See Section 6 for a further discussion of scalability considerations.

5. Candidate Technologies

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

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

5.1. Layer-Two Data Plane

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

5.1.1. Flexible Ethernet

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

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

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

5.1.2. Dedicated Queues

   DiffServ based queuing systems are described in [RFC2475] and
   [RFC4594].  This is considered insufficient to provide isolation for
   enhanced VPNs because DiffServ does not always provide enough
   markers to differentiate between traffic of many enhanced VPNs, or
   offer the range of service classes that each VPN needs to provide to
   its tenants.  This problem is particularly acute with an MPLS
   underlay, because MPLS only provides eight Traffic Classes.

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

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

5.1.3. Time Sensitive Networking

   Time Sensitive Networking (TSN) [TSN] is an IEEE project that is
   designing 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.  The mechanisms
   defined in TSN can be used to meet the requirements of time
   sensitive services of an enhanced VPN.

   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

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5.2. Layer-Three Data Plane

   We now consider the problem of slice differentiation and resource
   representation in the network layer.

5.2.1. Deterministic Networking

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

5.2.2. MPLS Traffic Engineering (MPLS-TE)

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

5.2.3. Segment Routing

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

   An SR traffic engineered path operates with a granularity of a link
   with hints about priority provided through the use of the traffic
   class (TC) or Differentiated Services Code Point (DSCP) field in the
   header.  However to achieve the latency and isolation
   characteristics that are sought by the enhanced VPN users, steering

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   packets through specific queues and resources will likely be
   required.  With SR, it is possible to introduce such fine-grained
   packet steering by specifying the queues and resources through an SR
   instruction list.

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

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

5.3. Non-Packet Data Plane

   Non-packet underlay data plane technologies often have TE properties
   and behaviours, 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 the resources are
   allocated on a long term and end-to-end basis.  Such an arrangement
   means that the full cost of the resources has be borne by the
   service that is allocated with the resources.

5.4. Control Plane

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

   RSVP-TE [RFC3209] provides the signaling mechanism for establishing
   a TE-LSP in an MPLS network with end-to-end resource reservation.
   This can be seen as 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 mentioned in
   Section 5.2.2.

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   The control plane of SR [RFC8665] [RFC8667] [I-D.ietf-idr-bgp-ls-
   segment-routing-ext] does not have the capability of signaling
   resource reservations along the path.  On the other hand, the SR
   approach provides a potential way of binding the underlay network
   resource and the enhanced VPN service without requiring per-path
   state to be maintained in the network.  A centralized controller can
   perform resource planning and reservation for enhanced VPNs, while
   it needs to ensure that resources are correctly allocated in network
   nodes for the enhanced VPN service.

5.5. Management Plane

   The management plane provides the interface between the enhanced VPN
   provider and the clients for the service life-cycle management (e.g.
   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.

   In the context of 5G end-to-end network slicing [TS28530], the
   management of enhanced VPNs is considered as the management of the
   transport network part of the end-to-end network slice. 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 capability and status of the transport network slice.
   Thus, an interface between the enhanced VPN management plane and the
   3GPP network slice management system, and relevant service data
   models are needed for the coordination of end-to-end network slice

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

5.6. Applicability of Service Data Models to Enhanced VPN

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

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

   o  Underlying resources are virtualized allocating those resources
   for the customer, application, or service.

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

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

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

5.6.1. Enhanced VPN Delivery in the ACTN Architecture

   ACTN provides VPN connections between multiple sites as requested
   via the Customer Network Controller (CNC).  The CNC is managed by
   the customer themselves, and interacts with the network provider's
   Multi-Domain Service Controller (MDSC).  The Provisioning Network
   Controllers (PNC) are responsible for network resource management,
   thus the PNCs are remain entirely under the management of the
   network provider and are not visible to the customer so that
   management is mostly performed by the network provider, with some
   flexibility delegated to the customer-managed CNC.

   Figure 3 presents a more general representation of how multiple
   enhanced VPNs may be created from the resources of multiple physical
   networks using the CNC, MDSC, and PNC components of the ACTN
   architecture.  Each enhanced VPN is controlled by its own CNC.  The
   CNCs send requests to the provider's MDSC.  The provider manages two
   different physical networks each under the control of PNC.  The MDSC
   asks the PNCs to allocate and provision resources to achieve the
   enhanced VPNs.  In this figure, one enhanced VPN is constructed
   solely from the resources of one of the physical networks, while the
   the VPN uses resources from both physical networks.

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                  ---------------           (           )
                  |    CNC      |---------->(    VPN+   )
                  --------^------           (           )
                          |                _(_________ _)
               ---------------            (           ) ^
               |    CNC      |----------->(    VPN+   ) :
               ------^--------            (           ) :
                     |    |               (___________) :
                     |    |                   ^    ^    :
   Boundary          |    |                   :    :    :
   Between ==========|====|===================:====:====:========
   Customer &        |    |                   :    :    :
   Network Provider  |    |                   :    :    :
                     v    v                   :    :    :
               ---------------                :    :....:
               |    MDSC     |                :         :
               ---------------                :         :
                     ^                     ---^------    ...
                     |                    (          )      .
                     v                   (  Physical  )      .
                ----------------         ( Network  )        .
                |     PNC      |<-------->(        )      ---^------
                  ---------------- |           --------     (          )
                |              |--                        (  Physical  )
                |    PNC       |<------------------------->( Network  )
                ---------------                             (        )

          Figure 3 Generic VPN+ Delivery in the ACTN Architecture

5.6.2. Enhanced VPN Features with Service Data Models

   This section discusses how the service data models can fulfil the
   enhanced VPN requirements described earlier in this document within
   the scope of the ACTN architecture. Isolation Between VPNs

   The VN YANG model [I-D.ietf-teas-actn-vn-yang] and the TE-service
   mapping model [I-D.ietf-teas-te-service-mapping-yang] fulfil the VPN
   isolation requirement by providing the following features for the

      o  Each VPN is identified with a unique identifier (vpn-id) and
   can be mapped to a specific VN.  Multiple VPNs may mapped to the
   same VN according to service requirements and operator's policy.

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      o  Each VPN is managed and controlled independent of other VPNs.

      o  Each VPN is instantiated with an isolation requirement
   described by the TE-service mapping model [I-D.ietf-teas-te-service-
   mapping-yang].  This mapping supports all levels of isolation (hard
   isolation with deterministic characteristics, hard isolation, soft
   isolation, or no isolation). Guaranteed Performance

   Performance objectives of a VPN [RFC8299][RFC8466] are expressed
   through a QoS profile including the following properties:

      o  Rate-limit

      o  Bandwidth

      o  Latency

      o  Jitter

   [I-D.ietf-teas-actn-vn-yang] and [I-D.ietf-teas-yang-te-topo] allow
   configuration of several TE parameters that may help to meet the VPN
   performance objectives as follows:

      o  Bandwidth

      o  Objective function (e.g., min cost path, min load path, etc.)

      o  Metric Types and their threshold:

         *  TE cost, IGP cost, Hop count, or Unidirectional Delay (e.g.,
   can set all path delay <= threshold)

   Once these requests are instantiated, the resources are committed
   and guaranteed through the life cycle of the VPN. Integration

   The L3VPN network model provides mechanism to correlate customer's
   VPN and the VPN service related resources (e.g., RT and RD)
   allocated in the provider's network.

   The VPN/Network performance monitoring model [I-D.www-bess-yang-vpn-
   service-pm] provides mechanisms to monitor and manage network

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   Performance on the topology at different layer or the overlay
   topology between VPN sites.

   These two models provide mechanisms to correlate the customer's VPN
   and the actual TE tunnels instantiated in the provider's network.

   Service function integration with network topology (L3 and TE
   topology) is in progress in [I-D.ietf-teas-sf-aware-topo-model]
   which addresses a number of use-cases that show how TE topology
   supports various service functions. Dynamic and Customized Management

   The ACTN architecture allows the CNC to interact with the provider's
   MDSC.  This gives the customer dynamic control of their VPNs.

   For example, the ACTN VN model [I-D.ietf-teas-actn-vn-yang] allows
   life-cycle management to create, modify, and delete VNs on demand.
   Customers may also be allowed more customized control of the VN
   topology by provisioning tunnels to connect their endpoints, and
   even configuring the paths of those tunnels.

   Another example is the L3VPN service model [RFC8299] which allows
   VPN lifecycle management such as VPN creation, modification, and
   deletion on demand.

5.6.3. 5G Transport Service Delivery via Coordinated Data Modules

   The overview of network slice structure as defined in the 3GPP 5GS
   is shown in Figure 4.  The terms are described in specific 3GPP
   documents [TS23501] [TS28530].

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   <==================          E2E-NSI       =======================>
                :                 :                  :           :  :
                :                 :                  :           :  :
   <======  RAN-NSSI  ======><=TN-NSSI=><====== CN-NSSI  ======>VL[APL]
       :        :        :        :         :       :        :   :  :
       :        :        :        :         :       :        :   :  :
   RW[NFs ]<=TRN-NSSI=>[NFs ]<=TN-NSSI=>[NFs ]<=TRN-NSSI=>[NFs ]VL[APL]
      . . . . . . . . . . . . ..         . . . . . . . . . . . . ..
      .,----.   ,----.   ,----..  ,----. .,----.  ,----.  ,----..
   UE--|RAN |---| TN |---|RAN |---| TN |--|CN  |--| TN |--|CN  |--[APL]
      .|NFs |   `----'   |NFs |.  `----' .|NFs |  `----'  |NFs |.
        .`----'            `----'.       .`----'          `----'.
        . . . . . . . . . . . . ..       . . . . . . . . . . . ..

    RW         RAN                MBH               CN              DN

   UE: User Equipment
   RAN: Radio Access Network
   CN: Core Network
   DN: Data Network
   TN: Transport Network
   MBH: Mobile Backhaul
   RW: Radio Wave
   NF: Network Function
   APL: Application Server
   NSI: Network Slice Instance
   NSSI: Network Slice Subnet Instance

        Figure 4 Overview of Structure of Network Slice in 3GPP 5GS

   The L3VPN service model [RFC8299] and TEAS VN model [I-D.ietf-teas-
   actn-vn-yang] can both be used to describe the 5G MBB Transport
   Service or connectivity service.  The L3VPN service model is used to
   describe end-to-end IP connectivity service, while the TEAS VN model
   is used to describe TE connectivity service between VPN sites or
   between RAN NFs and Core network NFs.

   A VN in the TEAS VN model with its support of point-to-point or
   multipoint-to-multipoint connectivity services can be seen as one
   example of a network slice.

   The TE Service mapping model can be used to map L3VPN service
   requests onto underlying network resource and TE models to get the
   TE network provisioned.

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   For IP VPN service provisioning, the service parameters in the L3VPN
   service model [RFC8299] can be decomposed into a set of
   configuration parameters described in the L3VPN network model [I-
   D.ietf-opsawg-l3sm-l3nm] which will get the VPN network provisioned.

6. Scalability Considerations

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

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

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

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

   Reducing the state in the network is important to enhanced VPN, as
   it requires the overlay to be more closely integrated with the
   underlay than with traditional VPNs.  This tighter coupling would
   normally mean that more state needed to be created and maintained in
   the network, as the state about fine granularity processing would

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   need to be loaded and maintained in the routers.  However, a segment
   routed approach allows much of this state to be spread amongst the
   network ingress nodes, and transiently carried in the packets as

6.1. Maximum Stack Depth of SR

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

6.2. RSVP Scalability

   The traditional method of creating a resource allocated path through
   an MPLS network is to use the RSVP 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 as the number of paths through an LSR grows. [RFC8577]
   proposes to address this by employing SR within a tunnel established
   by RSVP-TE.

6.3. SDN Scaling

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

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

7. OAM Considerations

   The enhanced VPN OAM design needs to consider the following

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

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

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

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

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

8. Telemetry Considerations

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

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

   How the telemetry mechanisms could be used or extended for the
   enhanced VPN service will be described in a separate document.

9. Enhanced Resiliency

   Each enhanced VPN has a life-cycle, and may need modification during
   deployment as the needs of its tenant change.  Additionally, as the
   network as a whole evolves, there may need to be garbage collection
   performed 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.
   This is a single action by the head-end 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 up and meets the required SLA before traffic is
   transitioned to it.  It is possible for deadlocks to arise as a
   result of the network becoming fragmented over time, such that it is
   impossible to create a new path or to modify an existing path
   without impacting the SLA of other paths.  Resolution of this
   situation is as much a commercial issue as it is a technical issue
   and is outside the scope of this document.

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

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

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

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

   In addition to the approach used to protect high priority packets,
   consideration has to be given to the impact of best effort traffic
   on the high priority packets during a transient.  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.

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10. Operational Considerations

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

11. Security Considerations

   All types of virtual network require special consideration to be
   given to the isolation of traffic belonging to different tenants.
   That is, traffic belonging to one VPN must not be delivered to end
   points outside that VPN.  In this regard enhanced VPNs neither
   introduce, no experience a greater security risks than other VPNs.

   However, in an enhanced Virtual Private Network service the
   additional service requirements need to be considered.  For example,
   if a service requires a specific upper bound to latency then it can
   be damaged by simply delaying the packets through the activities of
   another tenant, i.e., by introducing bursts of traffic for other

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

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

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

12. IANA Considerations

   There are no requested IANA actions.

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13. Contributors

   Daniel King
   Email: daniel@olddog.co.uk

   Adrian Farrel
   Email: adrian@olddog.co.uk

   Jeff Tansura
   Email: jefftant.ietf@gmail.com

   Qin Wu
   Email: bill.wu@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

   Zhenbin Li
   Email: lizhenbin@huawei.com

14. Acknowledgments

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

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

15. References

15.1. Normative References

   [I-D.ietf-teas-actn-vn-yang] Lee, Y., Dhody, D., Ceccarelli, D.,
             Bryskin, I., and B. Yoon, "A Yang Data Model for VN
             Operation", draft-ietf-teas-actn-vn-yang-07 (work in
             progress), October 2019.

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   [I-D.ietf-teas-te-service-mapping-yang] Lee, Y., Dhody, D., Fioccola,
             G., Wu, Q., Ceccarelli, D., and J. Tantsura, "Traffic
             Engineering (TE) and Service Mapping Yang Model", draft-
             ietf-teas-te-service-mapping-yang-02 (work in progress),
             September 2019.

   [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,

   [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,

   [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-

   [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-

   [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-

   [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>.

   [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-

   [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>.

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15.2. Informative References

   [BBF-SD406] "BBF SD-406: End-to-End Network Slicing", 2016,

   [DETNET] "Deterministic Networking", March ,

   [FLEXE] "Flex Ethernet Implementation Agreement", March 2016,

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

   [I-D.ietf-opsawg-l3sm-l3nm] Aguado, A., Dios, O., Lopezalvarez, V.,
             daniel.voyer@bell.ca, d., and L. Munoz, "Layer 3 VPN
             Network Model", draft-ietf-opsawg-l3sm-l3nm-01, (work in
             progress), November 2019.

   [I-D.ietf-opsawg-ntf] Song, H., Qin, F., Martinez-Julia, P.,
             Ciavaglia, L., and A. Wang, "Network Telemetry Framework",
             draft-ietf-opsawg-ntf-02 (work in progress), October 2019.

   [I-D.ietf-teas-sf-aware-topo-model] Bryskin, I., Liu, X., Lee, Y.,
             Guichard, J., Contreras, L., Ceccarelli, D., and J.
             Tantsura, "SF Aware TE Topology YANG Model", draft-ietf-
             teas-sf-aware-topo-model-04 (work in progress), November

   [I-D.ietf-teas-yang-te] Saad, T., Gandhi, R., Liu, X., Beeram, V.,
             and I. Bryskin, "A YANG Data Model for Traffic Engineering
             Tunnels and Interfaces", draft-ietf-teas-yang-te-22 (work
             in progress), November 2019.

   [I-D.ietf-teas-yang-te-topo] Liu, X., Bryskin, I., Beeram, V., Saad,
             T., Shah, H., and O. Dios, "YANG Data Model for Traffic
             Engineering (TE) Topologies", draft-ietf-teas-yang-te-
             topo-22 (work in progress), June 2019.

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   [I-D.www-bess-yang-vpn-service-pm] Wang, Z., Wu, Q., Even, R., Wen,
             B., and C. Liu, "A YANG Model for Network and VPN Service
             Performance Monitoring", draft-www-bess-yang-vpn-service-
             pm-04 (work in progress), November 2019.

   [NGMN-NS-Concept] "NGMN NS Concept", 2016,

   [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,

   [RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path
             Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000,

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

   [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-

   [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,

   [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-

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   [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-

   [RFC5151] Farrel, A., Ed., Ayyangar, A., and JP. Vasseur, "Inter-
             Domain MPLS and GMPLS Traffic Engineering - Resource
             Reservation Protocol-Traffic Engineering (RSVP-TE)
             Extensions", RFC 5151, DOI 10.17487/RFC5151, February 2008,

   [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,

   [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,

   [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-

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

   [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,

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   [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>.

   [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-

   [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,

   [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,

   [RFC8578] Grossman, E., Ed., "Deterministic Networking Use Cases",
             RFC 8578, DOI 10.17487/RFC8578, May 2019,

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

   [RFC8665] Psenak, P., Previdi, S., 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., Ginsberg, L., Filsfils, C., Bashandy, A.,
             Gredler, H., and B. Decraene, "IS-IS Extensions for
             Segment Routing", RFC 8667, DOI 10.17487/RFC8667, December
             2019, <https://www.rfc-editor.org/info/rfc8667>.

   [SFC] "Service Function Chaining",

   [TS23501] "3GPP TS23.501", 2019,

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   [TS28530] "3GPP TS28.530", 2019,

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

Authors' Addresses

   Jie Dong

   Email: jie.dong@huawei.com

   Stewart Bryant

   Email: stewart.bryant@gmail.com

   Zhenqiang Li
   China Mobile

   Email: lizhenqiang@chinamobile.com

   Takuya Miyasaka
   KDDI Corporation

   Email: ta-miyasaka@kddi.com

   Young Lee
   Sung Kyun Kwan University

   Email: younglee.tx@gmail.com

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