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

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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 2019-09-12
Replaces draft-dong-teas-enhanced-vpn
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TEAS Working Group                                               J. Dong
Internet-Draft                                                    Huawei
Intended status: Informational                                 S. Bryant
Expires: March 15, 2020                                        Futurewei
                                                                   Z. Li
                                                            China Mobile
                                                             T. Miyasaka
                                                        KDDI Corporation
                                                                  Y. Lee
                                               Sung Kyun Kwan University
                                                      September 12, 2019

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


   This document specifies a framework for using existing, modified and
   potential new networking technologies as components to provide an
   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.  It is not
   envisaged that large numbers of VPN+ instances will be deployed in a
   network and, in particular, it is not intended that all VPNs
   supported by a network will use VPN+ techniques.

Status of This Memo

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

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

   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 March 15, 2020.

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Copyright Notice

   Copyright (c) 2019 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview of the Requirements  . . . . . . . . . . . . . . . .   6
     2.1.  Isolation between Virtual Networks  . . . . . . . . . . .   6
       2.1.1.  A Pragmatic Approach to Isolation . . . . . . . . . .   7
     2.2.  Performance Guarantee . . . . . . . . . . . . . . . . . .   8
     2.3.  Integration . . . . . . . . . . . . . . . . . . . . . . .  10
       2.3.1.  Abstraction . . . . . . . . . . . . . . . . . . . . .  11
     2.4.  Dynamic Management  . . . . . . . . . . . . . . . . . . .  11
     2.5.  Customized Control  . . . . . . . . . . . . . . . . . . .  12
     2.6.  Applicability . . . . . . . . . . . . . . . . . . . . . .  12
     2.7.  Inter-Domain and Inter-Layer Network  . . . . . . . . . .  12
   3.  Architecture of Enhanced VPN  . . . . . . . . . . . . . . . .  13
     3.1.  Layered Architecture  . . . . . . . . . . . . . . . . . .  15
     3.2.  Multi-Point to Multi-Point (MP2MP)  . . . . . . . . . . .  16
     3.3.  Application Specific Network Types  . . . . . . . . . . .  16
     3.4.  Scaling Considerations  . . . . . . . . . . . . . . . . .  16
   4.  Candidate Technologies  . . . . . . . . . . . . . . . . . . .  17
     4.1.  Layer-Two Data Plane  . . . . . . . . . . . . . . . . . .  17
       4.1.1.  FlexE . . . . . . . . . . . . . . . . . . . . . . . .  18
       4.1.2.  Dedicated Queues  . . . . . . . . . . . . . . . . . .  18
       4.1.3.  Time Sensitive Networking . . . . . . . . . . . . . .  19
     4.2.  Layer-Three Data Plane  . . . . . . . . . . . . . . . . .  19
       4.2.1.  Deterministic Networking  . . . . . . . . . . . . . .  19
       4.2.2.  MPLS Traffic Engineering (MPLS-TE)  . . . . . . . . .  20
       4.2.3.  Segment Routing . . . . . . . . . . . . . . . . . . .  20
     4.3.  Non-Packet Data Plane . . . . . . . . . . . . . . . . . .  21
     4.4.  Control Plane . . . . . . . . . . . . . . . . . . . . . .  21
     4.5.  Management Plane  . . . . . . . . . . . . . . . . . . . .  22
     4.6.  Applicability of Service Data Models to Enhanced VPN  . .  23
       4.6.1.  Enhanced VPN Delivery in ACTN Architecture  . . . . .  24
       4.6.2.  Enhanced VPN Features with Service Data Models  . . .  25

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       4.6.3.  5G Transport Service Delivery via Coordinated Data
               Modules . . . . . . . . . . . . . . . . . . . . . . .  28
   5.  Scalability Considerations  . . . . . . . . . . . . . . . . .  30
     5.1.  Maximum Stack Depth of SR . . . . . . . . . . . . . . . .  31
     5.2.  RSVP Scalability  . . . . . . . . . . . . . . . . . . . .  31
     5.3.  SDN Scaling . . . . . . . . . . . . . . . . . . . . . . .  31
   6.  OAM Considerations  . . . . . . . . . . . . . . . . . . . . .  31
   7.  Telemetry Considerations  . . . . . . . . . . . . . . . . . .  32
   8.  Enhanced Resiliency . . . . . . . . . . . . . . . . . . . . .  32
   9.  Operational Considerations  . . . . . . . . . . . . . . . . .  33
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  33
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  34
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  35
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  35
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  35
     14.2.  Informative References . . . . . . . . . . . . . . . . .  36
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

1.  Introduction

   Virtual private networks (VPNs) have served the industry well as a
   means of providing different groups of users with logically isolated
   access to 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 enhanced overlay services
   with advanced characteristics such as complete isolation from other
   services so that changes in one service (such as changes in network
   load, or events such as congestion or outages) have no effect on the
   throughput or latency of other services provided to the customer.

   Driven largely by needs surfacing from 5G, the concept of network
   slicing has gained traction [NGMN-NS-Concept] [TS23501] [TS28530]
   [BBF-SD406].  In [TS23501], Network Slice is defined as "a logical
   network that provides specific network capabilities and network
   characteristics", and Network Slice Instance is defined as "A set of
   Network Function instances and the required resources (e.g. compute,
   storage and networking resources) which form a deployed Network
   Slice".  According to [TS28530], an end-to-end network slice consists
   of three major network segments: Radio Access Network (RAN),
   Transport Network (TN) and Core Network (CN).  Transport network
   provides the required connectivity within and between RAN and CN
   parts, with specific performance commitment.  For each end-to-end
   network slice, the topology and performance requirement on transport
   network can be very different, which requires transport network to

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   have the capability of supporting multiple different transport
   network slices.

   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).  A transport network slice could span multiple
   technology (IP, Optical) and multiple administrative domains.
   Depends 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.

   In the following sections of this document, network slice refers to
   transport network slice, and is interchangable with enhanced VPN.
   End-to-end network slice is used to refer to the 5G network slice.

   Network abstraction is a technique that can be applied to a network
   domain to select network resources by policy to obtain a view of
   potential connectivity and a set of service functions.

   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] 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 set of
   requirements, on top of a common network.  How the network slices are
   engineered can be deployment-specific.

   Thus, there is a need to create virtual networks with enhanced
   characteristics.  The tenant of such a virtual network can require a
   degree of isolation and performance that previously could not be
   satisfied by traditional overlay VPNs.  Additionally, the tenant may
   ask for some level of control to their virtual networks, e.g., to
   customize the service paths in a network slice.

   These enhanced properties cannot be met with pure overlay networks,
   as they require tighter coordination and integration between the
   underlay and the overlay network.  This document introduces a new
   network service called Enhanced VPN: VPN+. VPN+ is built from a
   virtual network which has a customized network topology and a set of
   dedicated or shared network resources, including invoked service
   functions, allocated from the underlay network.  Unlike a traditional
   VPN, 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

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   solution, but it is not envisaged that VPN+ techniques will be
   applied to normal VPN services that can continue to be deployed using
   pre-existing mechanisms.  Furthermore, it is not intended that large
   numbers of VPN+ instances will be deployed within a single network.
   See Section 5 for a discussion of scalability considerations.

   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 path.

   The required layered network structure to achieve this is shown in
   Section 3.1.

   Note that, in this document, the four terms "VPN", "Enhanced VPN" (or
   "VPN+"), "Virtual Network (VN)", and "Network Slice" may be
   considered as describing similar concepts dependent on the viewpoint
   from which they are used.

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

   o  A Virtual Network is a type of service that connects customer edge
      points with the additional possibility of requesting further
      service characteristics in the manner of an enhanced VPN.

   o  An enhanced VPN or VN is made by creating a slice through the
      resources of the underlay network.

   o  The general concept of network slicing in a TE network is a larger
      problem space than is addressed by VPN+ or VN, but those concepts
      are tools to address some aspects or realizations of network

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2.  Overview of the Requirements

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

2.1.  Isolation between Virtual Networks

   One element of the SLA demanded for an enhanced VPN is the degree of
   isolation from other services in the network.  Isolation is a feature
   requested by some particular customers in the network.  Such a
   feature is offered by a network operator where the traffic from one
   service instance is isolated from the traffic of other services.
   There are different grades of isolation that range from simple
   separation of 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 resources.

   The terms hard and soft isolation are introduced to identify
   different isolation cases.  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 soft isolated VPNs because the
   network delivers the traffic only to the required VPN endpoints.
   However, with soft isolation, traffic from one or more 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 or a group of VPNs to be sheltered from this effect is called
   hard isolation, and this property is required by some critical

   The requirement is for an operator to offer its customers a choice of
   different degrees of isolation ranging from soft isolation up to hard
   isolation so that the traffic of tenants/applications using one
   enhanced VPN can be separated from the traffic of tenants/
   applications using another enhanced VPN appropriately.  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 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.

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   In these circumstances the VPNs would require an appropriate degree
   of isolation to be able to continue to operate acceptably.

   In order to provide the required 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 network slices in the underlay network.  This may introduce
   scalability concerns, thus some trade-off needs to be considered to
   provide the required isolation between network slices while still
   allowing reasonable sharing inside each network slice.

   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.  Such an
   arrangement means that the full cost of the resources must be borne
   by the service that is allocated with the resources.  On the other
   hand, where adequate isolation can be achieved at the packet layer,
   this permits the resources to be shared amongst many services and
   only dedicated to a service on a temporary basis.  This in turn,
   allows greater statistical multiplexing of network resources and thus
   amortizes the cost over many services, leading to better economy.
   However, the different degrees of isolation required by network
   slicing cannot be entirely met with existing mechanisms such as
   Traffic Engineered Label Switched Paths (TE-LSPs).  This is because
   most implementations enforce the bandwidth in the data-plane only at
   the PEs, but at the P routers the bandwidth is only reserved in the
   control plane, thus bursts of data can accidentally occur at a P
   router with higher than committed data rate.

   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.

   In the remainder of this section we explore how isolation may be
   achieved in packet networks.

2.1.1.  A Pragmatic Approach to Isolation

   A key question is whether it is possible to achieve hard isolation in
   packet networks, which 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

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   dedicated, or a Time Division Multiplexing (TDM) network.  However
   there is no need to provide any harder isolation than is required by
   the application.  Pseudowires [RFC3985] emulate services that would
   have had hard isolation in their native form.  An approximation to
   this requirement is sufficient in most cases.

   Thus, for example, using FlexE or a virtual sub-interface together
   with packet scheduling as the isolation mechanism of interface
   resources, optionally along with the partitioning of node resources,
   a type of hard isolation can be provided that is adequate for many
   enhanced VPN applications.  Other applications may be either
   satisfied with a classical VPN with or without reserved bandwidth, or
   may need a dedicated point to point underlay connection.  The needs
   of each application must be quantified in order to provide an
   economic solution that satisfies those needs without over-

   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

   At one end of the above 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 VPN 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.

2.2.  Performance Guarantee

   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 the
   conformance traffic, the out-of-profile traffic will be handled
   following other requirements.

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   Guaranteed maximum packet loss is a common parameter, and is usually
   addressed by setting the packet priorities, queue size and discard
   policy.  However this becomes more difficult when the requirement is
   combined with the latency requirement.  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 virtual
   reality applications.  The work of the IETF Deterministic Networking
   (DetNet) Working Group [DETNET] is relevant; however the scope needs
   to be extended to methods of enhancing the underlay to better support
   the delay guarantee, and to integrate these enhancements with the
   overall service provision.

   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 electrical utilities have an operational need for this.
   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
   virtual networks.  Alternatively a dedicated virtual network may be
   used to provide this as a shared service.

   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 can

   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.

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   Providing assured bandwidth to VPNs, for example by using TE-LSPs, is
   not widely deployed at least partially due to scalability concerns.
   Guaranteed latency and enhanced delivery are not yet integrated with

   A guaranteed latency service has a latency upper bound provided by
   the network.  Assuring the upper bound is more important than
   achieving the minimum latency.

   In Section 2.1 we considered the work of the IEEE Time Sensitive
   Networking (TSN) project [TSN] and the work of the IETF DetNet
   Working group [DETNET] in the context of isolation.  The TSN and
   DetNet work is of greater relevance in assuring end-to-end packet
   latency.  It is also of importance in considering enhanced delivery.

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

   It is these last two characteristics that an enhanced VPN adds to a
   VPN service.

   Flex Ethernet [FLEXE] is a useful underlay to provide these
   guarantees.  This is a method of providing time-slot based
   channelization over an Ethernet bearer.  Such channels are fully
   isolated from other channels running over the same Ethernet bearer.
   As noted elsewhere this produces hard isolation but makes the
   reclamation of unused bandwidth more difficult.

   These approaches can be used in tandem.  It is possible to use FlexE
   to provide tenant isolation, and then to use the TSN/Detnet approach
   to provide a performance guarantee inside the a slice or tenant VPN.

2.3.  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.  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.

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2.3.1.  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) described in [RFC8453] as discussed further in Section 4.5.

2.4.  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 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
   Section 4.

   The control plane aspects of this problem are discussed further in
   Section 4.4.

   The management plane aspects of this problem are discussed further in
   Section 4.5

   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.

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2.5.  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 to the resources and functions allocated to this enhanced
   VPN.  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, or it may be
   provided with an interface to a control system which is shared with a
   set of other tenants, or it may be provided with an interface to the
   control system provided by the network operator.

   Further detail on this requirement will be provided in a future
   version of the draft.

   A description of the control plane aspects of this problem are
   discussed further in Section 4.4.  A description of the management
   plane aspects of this feature can be found in Section 4.5.

2.6.  Applicability

   The technologies described in this document should be applicable to a
   number 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 technology described here can be used to provide
   an underlay with the required enhanced performance.

2.7.  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].  And in some domains the operator may own 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 plane (data
   plane, control plane and management plane) need to provide necessary
   mechanisms to support multi-domain and multi-layer coordination and
   integration, so as to provide the required service characteristics

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   for different enhanced VPNs, and improve network efficiency and
   operational simplicity.

3.  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 specific set of dedicated 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 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  A enhanced data plane

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

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

   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 identify network slice and the
         associated resources.

   o  Control plane

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

      *  Create the required virtual networks with the resource and
         properties needed by the enhanced VPN services that are
         assigned to it.

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      *  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 (TN) 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.

      *  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 decommissioning).


      *  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 the data plane, control plane
         and management plane data of the network, more specifically:


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

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

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3.1.  Layered Architecture

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

                   +-------------------+              }
                   | Network Controller|              } Centralized
                   +-------------------+              }   Control
                   .    .    .     .  .
                  .     .    .     .  .
                 .      N----N----N  .                }
                .      /         /    .               }
               N-----N-----N----N-----N               }
                       N----N                         }
                      /    /  \                       }  Virtual
               N-----N----N----N-----N                } Networks
                             N----N                   }
                            /    /                    }
               N-----N-----N----N-----N               }

       +----+ ===== +----+ =====  +----+ ===== +----+  }
       +----+ ===== +----+ =====  +----+ ===== +----+  } Physical
       +----+ ===== +----+ =====  +----+ ===== +----+  } Network
       +----+       +----+        +----+       +----+  }
         N      L     N      L      N      L      N

       N = Partitioned node
       L = Partitioned link

       +----+ = Partition within a node

       ====== = Partition within a link

                    Figure 2: The Layered Architecture

   Underpinning everything is the physical network infrastructure layer
   consisting of partitioned links and nodes which provide the
   underlying resources used to provision the separated virtual
   networks.  Various components and techniques as discussed in
   Section 4 can be used to provide the resource partition, such as
   FlexE, Time Sensitive Networking, Deterministic Networking, etc.
   These partitions may be physical, or virtual so long as the SLA
   required by the higher layers is met.

   These techniques can be used to provision the virtual networks with
   the dedicated resources that they need.  To get the required

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   functionality there needs to be integration between these overlays
   and the underlay providing the physical resources.

   The centralized controller is used to create the virtual networks, to
   allocate the resources to each virtual network and to provision the
   enhanced VPN services within the virtual networks.  A distributed
   control plane may also be used for the distribution of the topology
   and attribute information of the virtual networks.

   The creation and allocation process needs to take a holistic view of
   the needs of all of its tenants, and to partition the resources
   accordingly.  However within a virtual network these resources can,
   if required, be managed via a dynamic control plane.  This provides
   the required scalability and isolation.

3.2.  Multi-Point to Multi-Point (MP2MP)

   At the VPN service level, the connectivity is usually mesh or
   partial-mesh.  To support such kinds of VPN service, the
   corresponding underlay is also an abstract MP2MP medium.  However
   when service guarantees are provided, the point-to-point path through
   the underlay of the enhanced VPN needs to be specifically engineered
   to meet the required performance guarantees.

3.3.  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

3.4.  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 network.

   Enhanced VPNs may need to install some additional state within the
   network to achieve the additional features that they require.
   Solutions must consider minimising and controlling the scale of such

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   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 a small number in the beginning, and even in
   future the number of enhanced VPN will be much less than traditional
   VPNs, because pre-existing VPN techniques would 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+ instances.  It will
   usually be possible to aggregate a set of VPN+ services so that they
   share the same virtual network 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
   important feature to assist with the scaling characteristics of VPN+

   See Section 5 for a greater discussion of scalability considerations.

4.  Candidate Technologies

   A VPN is a network created by applying a multiplexing 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

   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.

4.1.  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
   guarantee are described in following sections.

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   o  FlexE

   o  Time Sensitive Networking

   o  Dedicated Queues

4.1.1.  FlexE

   FlexE [FLEXE] is a method of creating a point-to-point Ethernet with
   a specific fixed bandwidth.  FlexE provides the ability to multiplex
   multiple channels over an Ethernet link in a way that provides hard
   isolation.  FlexE also supports the bonding of multiple links, which
   can be used to create larger links out of multiple low capacity links
   in a more efficient way that traditional link aggregation.  FlexE
   also supports the sub-rating of links, which allows an operator to
   only use a portion of a link.  However it is a only a link level
   technology.  When packets are received by the downstream node, they
   need to be processed in a way that preserves that isolation in the
   downstream node.  This in turn requires a queuing and forwarding
   implementation that preserves the end-to-end isolation.

   If different FlexE channels are used for different services, then no
   sharing is possible between the FlexE channels.  This in turn means
   that it may be difficult to dynamically redistribute unused bandwidth
   to lower priority services.  This may increase the cost of providing
   services on the network.  On the other hand, FlexE can be used to
   provide hard isolation between different tenants on a shared
   interface.  The tenant can then use other methods to manage the
   relative priority of their own traffic in each FlexE channel.

   Methods of dynamically re-sizing FlexE channels and the implication
   for enhanced VPN are for further study.

4.1.2.  Dedicated Queues

   In order to provide multiple isolated virtual networks for enhanced
   VPN, the conventional DiffServ based queuing system [RFC2475]
   [RFC4594] is considered insufficient, as DiffServ does not always
   provide enough queues to differentiate between traffic of different
   enhanced VPNs, or the range of service classes that each need to
   provide to their tenants.  This problem is particularly acute with an
   MPLS underlay, because MPLS only provides 8 Traffic Classes (TC), and
   it's highly likely that there will be more than eight enhanced VPN
   instances supported by a network.  In addition, DiffServ, as
   currently implemented, mainly provides relative priority-based
   scheduling, and is difficult to achieve quantitive resource
   reservation.  In order to address this problem and reduce the
   interference between enhanced VPNs, it is necessary to steer traffic

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   of enhanced VPNs to dedicated input and output queues.  Some routers
   have large amount of queues and sophisticated queuing systems, which
   could be used or enhanced to provide the granularity and level of
   isolation required by the applications of enhanced VPN.  For example,
   on one physical interface, the queuing system can provide a set of
   virtual sub-interfaces, each allocated with dedicated queueing and
   buffer resources.  Sophisticated queuing systems of this type may be
   used to provide end-to-end virtual isolation between traffic of
   different enhanced VPNs.

4.1.3.  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
   high priority packet stream may be given a scheduled time slot
   thereby guaranteeing that it experiences no queuing delay and hence a
   reduced latency.  However, when no scheduled packet arrives, its
   reserved time-slot is handed over to best effort traffic, thereby
   improving the economics of the network.  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 a pseudowire.
   However the TSN 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 the following section of
   this document.

4.2.  Layer-Three Data Plane

   We now consider the problem of slice differentiation and resource
   representation in the network layer.  The candidate technologies are:

   o  Deterministic Networking

   o  MPLS-TE

   o  Segment Routing

4.2.1.  Deterministic Networking

   Deterministic Networking (DetNet) [I-D.ietf-detnet-architecture] 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

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   the applications.  Even the delay improvements that are achieved with
   Stream Control Transmission Protocol Partial Reliability Extenstion
   (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, and trims duplicate packets to prevent excessive flooding of
   the network and to prevent multiple packets being delivered to the
   destination.  It also seeks to set an upper bound on latency.  The
   goal is not to minimize latency; the optimum upper bound paths may
   not be the minimum latency paths.

   DetNet is based on flows.  It currently does not specify the use of
   underlay topology other than the base topology.  To be of use for
   enhanced VPN, DetNet needs to be integrated with different virtual
   topologies of enhanced VPNs.

   The detailed design that allows the use DetNet in a multi-tenant
   network, and how to improve the scalability of DetNet in a multi-
   tenant network are topics for further study.

4.2.2.  MPLS Traffic Engineering (MPLS-TE)

   MPLS-TE introduces the concept of reserving end-to-end bandwidth for
   a TE-LSP, which can be used as the underlay of VPNs.  It also
   introduces the concept of non-shortest path routing through the use
   of the Explicit Route Object [RFC3209].  VPN traffic can be run over
   dedicated TE-LSPs to provide reserved bandwidth for each specific
   connection in a VPN.  Some network operators have concerns about the
   scalability and management overhead of RSVP-TE system, and this has
   lead them to consider other solutions for their networks.

4.2.3.  Segment Routing

   Segment Routing [RFC8402] is a method that prepends instructions to
   packets at the head-end node and optionally at various points as it
   passes though the network.  These instructions allow the packets to
   be routed on paths other than the shortest path for various traffic
   engineering reasons.  With SR, a path needs to be dynamically created
   through a set of segments by simply specifying the Segment
   Identifiers (SIDs), i.e. instructions rooted at a particular point in
   the network.  By encoding the state in the packet, per-path state is
   transitioned out of the network.

   With current segment routing, the instructions are used to specify
   the nodes and links to be traversed.  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

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   the latency and isolation characteristics that are sought by the
   enhanced VPN users, steering 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 a queue is a useful abstraction for many
   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.

   Both SR-MPLS and SRv6 are candidate data plane technologies for
   enhanced VPN.  In some cases they can further be used for DetNet to
   meet the packet loss, delay and jitter requirement of particular
   service.  How to provide the DetNet enhanced delivery in an SRv6
   environment is specified in [I-D.geng-spring-srv6-for-detnet].

4.3.  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 (and hence slices).

   The control and management planes for non-packet data plane
   technologies have most in common with MPLS-TE (Section 4.2.2) and
   offer the same set of advanced features [RFC3945].  Furthermore,
   management techniques such as ACTN ([RFC8453] and Section 4.6 can be
   used to aid in the reporting of underlying network topologies, and
   the creation of virtual networks with the resource and properties
   needed by the enhanced VPN services.

4.4.  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 relies on
   distributed control plane to provide scalability, high reliability,
   fast reaction, automatic failure recovery etc.  Extension and
   optimization to the distributed control plane is needed to support
   the enhanced properties of VPN+.

   RSVP-TE provides the signaling mechanism of establishing a TE-LSP
   with end-to-end resource reservation.  It can be used to bind the VPN

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   to specific network resource allocated within the underlay, but there
   are the above mentioned scalability concerns.

   SR does not have the capability of signaling the resource reservation
   along the path, nor do its currently specified distributed link state
   routing protocols.  On the other hand, the SR approach provides a way
   of efficiently binding the network underlay and the enhanced VPN
   overlay, as it reduces the amount of state to be maintained in the
   network.  An SR-based approach with per-slice resource reservation
   can easily create dedicated SR network slices, and the VPN services
   can be bound to a particular SR network slice.  A centralized
   controller can perform resource planning and reservation from the
   controller's point of view, but this does not ensure resource
   reservation is actually done in the network nodes.  Thus, if a
   distributed control plane is needed, either in place of an SDN
   controller or as an assistant to it, the design of the control system
   needs to ensure that resources are uniquely allocated in the network
   nodes for the correct services, and not allocated to other services
   causing unintended resource conflict.

   In addition, in multi-domain and multi-layer networks, the
   centralized and distributed control mechanisms will be used for
   inter-domain coordination and inter-layer optimization, so that the
   diverse and customized enhanced VPN service requirement could be met.
   The detailed mechanisms will be described in a future version.

4.5.  Management Plane

   In the context of 5G end-to-end network slicing, the management of
   enhanced VPN is considered as the management of transport network
   part of the end-to-end network slice. 3GPP management system may
   provide the topology and QoS parameters as requirement to the
   management plane of transport network.  It may also require the
   transport network to expose the capability and status of the
   transport network slice.  Thus an interface between enhanced VPN
   management plane and 3GPP network slice management system and
   relevant service data models are needed for the coordination of end-
   to-end network slice management.

   The management plane interface and data models for enhanced VPN can
   be based on the service models such as:

   o  VPN service models defined in [RFC8299] and [RFC8466]

   o  Possible augmentations and extensions
      (e.g.,[I-D.ietf-teas-te-service-mapping-yang]) to VPN service

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   o  ACTN related service models such as [I-D.ietf-teas-actn-vn-yang]
      and [I-D.ietf-teas-actn-pm-telemetry-autonomics].

   o  VPN network model as defined in [I-D.aguado-opsawg-l3sm-l3nm].

   o  TE Tunnel model as defined in [I-D.ietf-teas-yang-te].

   These data models can be applicable in the provisioning of enhanced
   VPN service.  The details are described in Section 4.6.

4.6.  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 are provided to
      higher-layer applications and customers.

   o  Virtualization of underlying resources, whose selection criterion
      is the allocation of those resources for the customer,
      application, or service.

   o  Creation of a virtualized environment allowing operators to view
      and control multi-domain networks as a single virtualized network.

   o  The presentation to customers of networks as a virtual network via
      open and programmable interfaces.

   The infrastructure managed through the Service Data models comprises
   traffic engineered network resources (e.g. bandwidth, time slot,
   wavelength) and VPN service related resources (e.g.  Route Target
   (RT) and Route Distinguisher (RD)).

   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.

   The Customer VPN model (e.g.  L3SM) or an ACTN Virtual Network (VN)
   model is a customer view of the ACTN managed infrastructure, and is
   presented by the ACTN provider as a set of abstracted services or

   L3VPN network model or TE tunnel model is a network view of the ACTN
   managed infrastructure, and is presented by the ACTN provider as a
   set of transport resources.

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   Depending on the agreement between customer and provider, various
   VPN/VN operations and VPN/VN views are possible.

   o  Virtual Network Creation: A VPN/VN could be pre-configured and
      created via static or dynamic request and negotiation between
      customer and provider.  It must meet the specified SLA attributes
      which satisfy the customer's objectives.

   o  Virtual Network Operations: The virtual network may be further
      modified and deleted based on customer request to request changes
      in the network resources reserved for the customer, and used to
      construct the network slice.  The customer can further act upon
      the virtual network to manage traffic flow across the virtual

   o  Virtual Network View: The VPN/VN topology from a customer point of
      view.  These may be a variety of tunnels, or an entire VN
      topology, or an VPN service topology.  Such connections may
      comprise of customer end points, access links, intra-domain paths,
      and inter-domain links.

   Dynamic VPN/VN Operations allow a customer to modify or delete the
   VPN/VN.  The customer can further act upon the virtual network to
   create/modify/delete virtual links and nodes or VPN sites.  These
   changes will result in subsequent tunnel management or VPN service
   management in the operator's networks.

4.6.1.  Enhanced VPN Delivery in ACTN Architecture

   ACTN provides VPN connections or VN connections between multiple
   sites as requested via a VPN requestor enabled by 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 responible for network resource management, thus the PNCs
   are remain entirely under the management of the network provider and
   are not visible to the customer.

   The benefits of this model include:

   o  Provision of edge-to-edge VPN multi-access connectivity.

   o  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

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   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.

                   ---------------           (           )
                   |    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

4.6.2.  Enhanced VPN Features with Service Data Models

   This section discusses how the service data models can fulfill the
   enhanced VPN requirements described earlier in this document.  As
   previously noted, key requirements of the enhanced VPN include:

   1.  Isolation between VPNs/VNs

   2.  Guaranteed Performance

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   3.  Integration

   4.  Dynamic Management

   5.  Customized Control

   The subsections that follow outline how each requirement is met using
   ACTN.  Isolation Between VPN/VNs

   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] fulfill the
   VPN/VN isolation requirement by providing the following features for
   the VPN/VNs:

   o  Each VN is identified with a unique identifier (vn-id and vn-name)
      and so is each VN member that belongs to the VN (vn-member-id).

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

   o  Each VPN and the corresponding VN is managed and controlled
      independent of other VPNs/VNs in the network with proper
      availability level.

   o  Each VPN/VN is instantiated with an isolation requirement
      described by the TE-service mapping model
      [I-D.ietf-teas-te-service-mapping-yang].  This mapping supports:

      *  Hard isolation with deterministic characteristics (e.g., this
         case may need an optical bypass tunnel or a DetNet/TSN tunnel
         to guarantee latency with no jitter)

      *  Hard isolation (i.e., dedicated TE resources in all underlays)

      *  Soft isolation (i.e., resource in some layer may be shared
         while in some other layers is dedicated).

      *  No isolation (i.e., sharing with other VPN/VN).  Guaranteed Performance

   Performance objectives of a VN need first to be expressed in order to
   assure the performance guarantee.

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   Performance objectives of a VPN [RFC8299][RFC8466] are expressed with
   QoS profile, either standard profile or customer profile.  The
   customer QoS profile include 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 affect the VN
   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/VN.  Integration

   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.

   VPN/Network performance monitoring model
   [I-D.www-bess-yang-vpn-service-pm] provides mechanisms to monitor and
   manage network Performance on the topology at different layer or the
   overlay topology between VPN sites.

   VN model and Performance Monitoring Telemetry model provides
   mechanisms to correlate customer's VN and the actual TE tunnels
   instantiated in the provider's network.  Specifically:

   o  Link each VN member to actual TE tunnel.

   o  Each VN can be monitored on a various level such as VN level, VN
      member level, TE-tunnel level, and link/node level.

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

   ACTN provides an architecture that allows the CNC to interact with
   the MDSC which is network provider's SDN controller.  This gives the
   customer control of their VPN or VNs.

   e.g., the ACTN VN model [I-D.ietf-teas-actn-vn-yang] allows the VN to
   life-cycle management such as create, modify, and delete VNs on
   demand.  Another example is L3VPN servicel model [RFC8299] which
   allows the VPN lifecycle management such as VPN creation,
   modification and deletion on demand.  Customized Control

   ACTN provides a YANG model that allows the CNC to control a VN as a
   "Type 2 VN" that allows the customer to provision tunnels that
   connect their endpoints over the customized VN topology.

   For some VN members, the customers are allowed to configure the path
   (i.e., the sequence of virtual nodes and virtual links) over the VN/
   abstract topology.

4.6.3.  5G Transport Service Delivery via Coordinated Data Modules

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

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   <==================          E2E-NSI         =======================>
                 :                 :                  :           :  :
                 :                 :                  :           :  :
    <======  RAN-NSSI  ======><=TRN-NSSI=><====== CN-NSSI  ======>VL[APL]
        :        :        :        :         :       :        :   :  :
        :        :        :        :         :       :        :   :  :
    RW[NFs ]<=TRN-NSSI=>[NFs ]<=TRN-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

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

   VN in TEAS VN model and support point-to-point or multipoint-to-
   multipoint connectivity service and can be seen as one example of
   network slice.

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

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   For IP VPN service provision, 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.aguado-opsawg-l3sm-l3nm] which will get VPN network setup.

5.  Scalability Considerations

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

   o  Introduce the state to the network.  This is normally done by
      creating a path using RSVP-TE, which can be extended to introduce
      any element that needs to be specified along the path, for example
      explicitly specifying queuing policy.  It is of course 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 packet
      are shorter.

   o  Provide a hybrid approach based on using binding SIDs to create
      path fragments, and bind them together with SR.

   Dynamic creation of a VPN path using SR requires less state
   maintenance in the network core at the expense of larger VPN headers
   on the packet.  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 resource on the routers are specified.
   Reducing the state in the network is important to enhanced VPN, as it
   requires the overlay to be more closely integrated with the underlay
   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 need to
   be loaded and maintained in the routers.  However, a segment routed

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   approach allows much of this state to be spread amongst the network
   ingress nodes, and transiently carried in the packets as SIDs.

   These approaches are for further study.

5.1.  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.

5.2.  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.  There are ongoing works to improve the scalability
   of RSVP-TE LSPs in the control plane [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.

5.3.  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.

6.  OAM Considerations

   The enhanced VPN OAM design needs to consider the following

   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.

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   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

   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].

7.  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], 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 future version.

8.  Enhanced Resiliency

   Each enhanced VPN has a life-cycle, and needs modification during
   deployment as the needs of its tenant change.  Additionally, as the
   network as a whole evolves, there will need to be garbage collection
   performed to consolidate resources into usable quanta.

   Systems in which the path is imposed such as SR, or some form of
   explicit routing tend to do well in these applications, because it is
   possible to perform an atomic transition from one path to another.
   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,

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   implementations and the monitoring protocols need to make sure that
   the new path is up and meet the required SLA before traffic is
   transitioned to it.  It is possible for deadlocks arise as a result
   of the network becoming fragmented over time, such that it is
   impossible to create a new path or modify a 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 the latency transient in either direction as a path

   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

   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.

9.  Operational Considerations

   TBD in a future version.

10.  Security Considerations

   All types of virtual network require special consideration to be
   given to the isolation between the tenants.  In this regard enhanced
   VPNs neither introduce, no experience a greater security risk than

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   another VPN of the same base type.  However, in an enhanced virtual
   network service the isolation requirement needs to be considered.  If
   a service requires a specific latency then it can be damaged by
   simply delaying the packet through the activities of another tenant.
   In a network with virtual functions, depriving a function used by
   another tenant of compute resources can be just as damaging as
   delaying transmission of a packet in the network.  The measures to
   address these dynamic security risks must be specified as part to the
   specific solution.

   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.

11.  IANA Considerations

   There are no requested IANA actions.

12.  Contributors

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      Daniel King

      Adrian Farrel

      Jeff Tansura

      Qin Wu

      Daniele Ceccarelli

      Mohamed Boucadair

      Sergio Belotti

      Haomian Zheng

13.  Acknowledgements

   The authors would like to thank Charlie Perkins, James N Guichard and
   John E Drake 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).

14.  References

14.1.  Normative References

              Lee, Y., Dhody, D., Ceccarelli, D., Bryskin, I., and B.
              Yoon, "A Yang Data Model for VN Operation", draft-ietf-
              teas-actn-vn-yang-06 (work in progress), July 2019.

              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.

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   [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,

   [RFC4664]  Andersson, L., Ed. and E. Rosen, Ed., "Framework for Layer
              2 Virtual Private Networks (L2VPNs)", RFC 4664,
              DOI 10.17487/RFC4664, September 2006,

   [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,

   [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, <>.

   [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,

   [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, <>.

14.2.  Informative References

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

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   [DETNET]   "Deterministic Networking", March ,

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

              Aguado, A., Dios, O., Lopezalvarez, V.,
    , d., and L. Munoz, "Layer 3 VPN
              Network Model", draft-aguado-opsawg-l3sm-l3nm-01 (work in
              progress), July 2019.

              Geng, X., Li, Z., and M. Chen, "SRv6 for Deterministic
              Networking (DetNet)", draft-geng-spring-srv6-for-detnet-00
              (work in progress), July 2019.

              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", draft-ietf-
              detnet-architecture-13 (work in progress), May 2019.

              Korhonen, J. and B. Varga, "DetNet IP Data Plane
              Encapsulation", draft-ietf-detnet-dp-sol-ip-02 (work in
              progress), March 2019.

              Song, H., Qin, F., Martinez-Julia, P., Ciavaglia, L., and
              A. Wang, "Network Telemetry Framework", draft-ietf-opsawg-
              ntf-01 (work in progress), June 2019.

              Lee, Y., Dhody, D., Karunanithi, S., Vilata, R., King, D.,
              and D. Ceccarelli, "YANG models for VN & TE Performance
              Monitoring Telemetry and Scaling Intent Autonomics",
              draft-ietf-teas-actn-pm-telemetry-autonomics-00 (work in
              progress), July 2019.

              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-03 (work
              in progress), March 2019.

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              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-21 (work in
              progress), April 2019.

              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.

              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-03 (work in progress),
              July 2019.

              "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,

   [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,

   [RFC3945]  Mannie, E., Ed., "Generalized Multi-Protocol Label
              Switching (GMPLS) Architecture", RFC 3945,
              DOI 10.17487/RFC3945, October 2004,

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   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <>.

   [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,

   [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,

   [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, <>.

   [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,

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

   [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,

   [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, <>.

   [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,

   [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,

   [SFC]      "Service Function Chaining", March ,

   [TS23501]  "3GPP TS23.501", 2016,

   [TS28530]  "3GPP TS28.530", 2016,

   [TSN]      "Time-Sensitive Networking", March ,

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Authors' Addresses

   Jie Dong


   Stewart Bryant


   Zhenqiang Li
   China Mobile


   Takuya Miyasaka
   KDDI Corporation


   Young Lee
   Sung Kyun Kwan University


Dong, et al.             Expires March 15, 2020                [Page 41]