TEAS Working Group                                               J. Dong
Internet-Draft                                                 S. Bryant
Intended status: Informational                                    Huawei
Expires: February 16, 2019                                         Z. Li
                                                            China Mobile
                                                             T. Miyasaka
                                                        KDDI Corporation
                                                         August 15, 2018

        A Framework for Enhanced Virtual Private Networks (VPN+)


   This document specifies a framework for using existing, modified and
   potential new networking technologies as components to provide an
   enhanced VPN (VPN+) service.  The purpose is to enable virtual
   private networks (VPNs) to support the needs of new applications,
   particularly applications that are associated with 5G services.  A
   network enhanced with these properties can form the underpinning of
   network slicing, but will also be of use in its own right.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on February 16, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of

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   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.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview of the Requirements  . . . . . . . . . . . . . . . .   4
     3.1.  Isolation between Virtual Networks  . . . . . . . . . . .   4
       3.1.1.  A Pragmatic Approach to Isolation . . . . . . . . . .   5
     3.2.  Performance Guarantee . . . . . . . . . . . . . . . . . .   6
     3.3.  Integration . . . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Dynamic Configuration . . . . . . . . . . . . . . . . . .   8
     3.5.  Customized Control  . . . . . . . . . . . . . . . . . . .   9
     3.6.  Applicability . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Architecture of Enhanced VPN  . . . . . . . . . . . . . . . .   9
     4.1.  Layered Architecture  . . . . . . . . . . . . . . . . . .  11
     4.2.  Multi-Point to Multi-Point  . . . . . . . . . . . . . . .  12
     4.3.  Application Specific Network Types  . . . . . . . . . . .  12
   5.  Candidate Technologies  . . . . . . . . . . . . . . . . . . .  13
     5.1.  Underlay Data plane . . . . . . . . . . . . . . . . . . .  14
       5.1.1.  FlexE . . . . . . . . . . . . . . . . . . . . . . . .  14
       5.1.2.  Dedicated Queues  . . . . . . . . . . . . . . . . . .  14
       5.1.3.  Time Sensitive Networking . . . . . . . . . . . . . .  15
     5.2.  Network Layer . . . . . . . . . . . . . . . . . . . . . .  15
       5.2.1.  Deterministic Networking  . . . . . . . . . . . . . .  15
       5.2.2.  MPLS Traffic Engineering (MPLS-TE)  . . . . . . . . .  16
       5.2.3.  Segment Routing . . . . . . . . . . . . . . . . . . .  16
     5.3.  Control Plane . . . . . . . . . . . . . . . . . . . . . .  19
   6.  Scalability Considerations  . . . . . . . . . . . . . . . . .  20
     6.1.  Maximum Stack Depth of SR . . . . . . . . . . . . . . . .  21
     6.2.  RSVP Scalability  . . . . . . . . . . . . . . . . . . . .  21
   7.  OAM Considerations  . . . . . . . . . . . . . . . . . . . . .  21
   8.  Enhanced Resiliency . . . . . . . . . . . . . . . . . . . . .  22
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     11.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

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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.

   Driven largely by needs surfacing from 5G, the concept of network
   slicing has gained traction.  Network slicing requires the transport
   network to support a set of virtual networks, each of which can
   provide the client with dedicated (private) networking, computing and
   storage resources drawn from a shared pool.  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 only be satisfied by dedicated
   networks.  Additionally the tenant may ask for some level of control
   to their virtual network e.g. to customize the service paths in the
   network slice.

   These 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+ refers to a virtual network which
   has dedicated network resources allocated from the underlay network.
   Unlike a traditional VPN, an enhanced VPN can achieve greater
   isolation and guaranteed performance.  These new properties, which
   have general applicability, may also be of interest as part of a
   network slicing solution.

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

   o  The design of the enhanced data plane

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

   o  The mechanisms to achieve integration between overlay and underlay

   o  The necessary OAM methods to instrument an enhanced VPN to make
      sure that the required SLA are met, and to take any required
      action to avoid SLA violation, such as switching to an alternate

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

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   One use for enhanced VPNs is to create network slices with different
   isolation requirements.  Such network slices may be used to provide
   different tenants of vertical industrial markets with their own
   virtual network with the explicit characteristics required.  These
   network slices may be "hard" slices providing a high degree of
   confidence that the VPN+ characteristics will be maintained over the
   slice life cycle, or they may be "soft" slices in which case some
   interaction may be experienced.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

3.  Overview of the Requirements

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

3.1.  Isolation between Virtual Networks

   The requirement is to provide both hard and soft isolation between
   the tenants/applications using one enhanced VPN and the tenants/
   applications using another enhanced VPN.  Hard isolation is needed so
   that applications with exacting requirements can function correctly,
   despite a flash demand being created on another VPN competing for the
   underlying resources.  An example of hard isolation is a network
   supporting both emergency services and public broadband multi-media

   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 VPNs would require an appropriate degree of
   isolation to be able to continue to operate acceptably.

   We introduce the terms hard and soft isolation to cover cases such as
   the above.  A VPN has soft isolation if the traffic of one VPN cannot
   be inspected by the traffic of another.  Both IP and MPLS VPNs are
   examples of soft isolated VPNs because the network delivers the
   traffic only to the required VPN endpoints.  However, the traffic
   from one or more VPNs and regular network traffic may congest the
   network resulting in delays for other VPNs operating normally.  The
   ability for a VPN to be sheltered from this effect is called hard
   isolation, and this property is required by some critical
   applications.  Although these isolation requirements are triggered by

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   the needs of 5G networks, they have general utility.  In the
   remainder of this section we explore how isolation may be achieved in
   packet networks.

   In order to provide the required isolation, resources has to be
   reserved in the data plane.  This may introduce scalability concerns,
   thus some trade-off needs to be considered to provide the required
   isolation between network slices while still allows 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 degree of isolation required by network slicing cannot
   be entirely met with existing mechanisms such as 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] 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.

3.1.1.  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
   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] emulates services that would
   have had hard isolation in their native form.  An approximation to
   this requirement is sufficient in most cases.

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   Thus, for example, using FlexE or a channelized sub-interface
   together with packet scheduling as interface slicing, optionally
   along with the slicing of node resources (Network Processor Unit
   (NPU), etc.), a type of hard isolation can be provided that is
   adequate for many VPN+ applications.  Other applications may be
   either satisfied with a classical VPN with or without reserved
   bandwidth, or may need dedicated point to point fiber.  The needs of
   each application must be quantified in order to provide an economic
   solution that satisfies those needs without over-engineering.

   This spectrum of isolation is shown below:

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

   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 traditional 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.

3.2.  Performance Guarantee

   There are several kinds of performance guarantees, including
   guaranteed maximum packet loss, guaranteed maximum delay and
   guaranteed delay variation.

   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
   combine 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 is 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 packet 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

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   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.  [I-D.ietf-detnet-use-cases] 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 is the service that current VPNs provide.  An assured
   bandwidth service is one in which the bandwidth over some period of
   time is assured.  The instantaneous bandwidth is however, not
   necessarily assured, depending on the technique used.  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 3.1 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.

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   An enhanced delivery service is one in which the 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.

3.3.  Integration

   A solution to the enhanced VPN problem has to provide seamless
   integration of both overlay VPN and the underlay network resource.
   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.

3.4.  Dynamic Configuration

   New 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 the traffic in flight will be disrupted can be a difficult

   The data plane aspects of this problem are discussed further in
   Section 5.

   The control-plane and management-plane aspects of this (particularly
   garbage collection) are for further study.

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   Dynamic changes both to the VPN and to the underlay transport network
   need to be managed in a seamless way in order to avoid disruption to
   sensitive services.

   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 to some of the resources and functions allocated to this VPN.
   Each enhanced VPN may have its own dedicated controller, it may be
   provided with an interface to a control plane that is shared with a
   set of other tenants, or it may be provided with an interface to the
   control plane of the underlay provided by the underlay network

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

3.6.  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.

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 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

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   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 suitable 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

      *  Provides the required packet latency and jitter characteristics

      *  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 set of virtual topologies with the resource
         and properties needed by the enhanced VPN services that are
         assigned to it.

      *  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 the life-cycle management (creation, modification,
         decommissioning) of enhanced VPN

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      *  Provide a interface between the enhanced VPN provider and the
         enhanced VPN clients such that some of the operation requests
         can be met without interfering other enhanced VPN clients.

   This document will focus on the data plane and control plane of the
   enhanced VPN.  The details of the management plane is outside the
   scope of this document.

4.1.  Layered Architecture

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

                   +-------------------+              }
                   | 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 1: The Layered Architecture

   Underpinning everything is the physical 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

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   Section 5 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
   dedicated resources that they need.  To get the required
   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.

4.2.  Multi-Point to Multi-Point

   At a VPN service level, the connectivity are usually multi-point-to-
   multi-point (MP2MP).  For such kind of services, 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.

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 the 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 two type for both PW over
   MPLS and for L2TPv3.

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5.  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 are
   dedicated to different enhanced VPNs.  Any enhanced VPN solution thus
   needs tighter coupling with underlay than is the case with existing
   VPNs.  We cannot for example share the tunnel between enhanced VPNs
   which require hard isolation.

   A number of candidate underlay data plane solutions which can be used
   provide the required isolation and guarantee are described in
   following sections.

   o  FlexE

   o  Time Sensitive Networking

   o  Dedicated Queues

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

   o  MPLS

   o  MPLS-SR

   o  Segment Routing over IPv6 (SRv6)

   o  Deterministic Networking

   The considerations about the control plane is also described.

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5.1.  Underlay Data plane

5.1.1.  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 slower 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 under study.

5.1.2.  Dedicated Queues

   In order to provide multiple isolated virtual networks for enhanced
   VPN, the conventional Diff-Serv based queuing system is insufficient,
   due to the limited number of queues which cannot differentiate
   between traffic of different enhanced VPNs, and the range of service
   classes that each need to provide to their tenants.  This problem is
   particularly acute with an MPLS underlay due to the small number of
   traffic class services available.  In order to address this problem
   and reduce the interference between enhanced VPNs, it is necessary to
   steer traffic of VPNs to dedicated input and output queues.  Routers
   usually have large amount of queues and sophisticated queuing
   systems, which could be used or enhanced to provide the levels 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.

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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
   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 where 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.

5.2.  Network Layer

5.2.1.  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 the
   applications.  Even the delay improvements that are achieved with
   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 packets being lost, and trim 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

   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.

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5.2.2.  MPLS Traffic Engineering (MPLS-TE)

   MPLS-TE introduces the concept of reserving end-to-end bandwidth for
   an 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.  This is not widely deployed in practice due to
   scaling and management overhead concerns.

5.2.3.  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.  These paths can be strict or loose paths,
   depending on the compactness required of the instruction list and the
   degree of autonomy granted to the network, for example to support
   Equal Cost Multipath load-balancing (ECMP) [RFC2992].

   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.  Thus if a
   path is to be provisioned from some ingress point A to some egress
   point B in the underlay, A is provided with the A..B SID list and
   instructions on how to identify the packets to which the SID list is
   to be prepended.

   By encoding the state in the packet, as is done in Segment Routing,
   per-path state is transitioned out of the network.

                               |       |  /
                               |       | /

                     Figure 2: An SR Network Fragment

   Consider a further network fragment shown in Figure 2.  To send a
   packet from A to E via B and D: Node A prepends the ordered SID list
   {B, D, E} to the packet and sends the packet to B.  SID list {B, D,
   E} can be used as a VPN path.  Thus, to create a VPN, a set of SID
   Lists is created and provided to each ingress node of the VPN
   together with packet selection criteria.  In this way it is possible
   to create a VPN with no state in the core.  However this is at the
   expense of creating a larger packet with possible MTU and hardware
   restriction limits that need to be overcome.

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   Note in the above if A and E support multiple VPN an additional VPN
   identifier will need to be added to the packet, but this is omitted
   from this text for simplicity.

                             |       |      /
                             |       Q     /
                             |       |    /

                   Figure 3: Another SR Network Fragment

   Consider a further network fragment shown in Figure 3, and further
   consider VPN A+D+E (i.e. the VPN connecting together nodes A, D and E
   with the requires VPN properties.)  This requires the nodes to be
   configured with, or otherwise learn the following path lists to
   provide complete connectivity:

   A has lists: {P, B, Q, D}, {P, B, S, E}
   D has lists: {Q, B, P, A}, {E}
   E has lists: {S, B, P, A}, {D}

   Similarly, to create a new VPN C+D+B the following list are

   C lists: {R, D}, {A, P, B}
   D lists: {R, C}, {Q, B}
   B lists: {Q, D}, {P, A, C}

   Thus VPN C+D+B was created without touching the settings of the core
   routers, indeed it is possible to add endpoints to the VPNs, and move
   the paths around simply by providing new lists to the affected

   However, there are a number of limitations in current SR, which limit
   its applicability to enhanced VPNs:

   o  Segments are shared between different VPNs paths

   o  There is no reservation of bandwidth

   o  There is limited differentiation in the data plane.

   Thus some extensions to SR are needed to provide isolation between
   different enhanced VPNs.  This can be achieved by including a finer
   granularity of state in the network in anticipation of its future use
   by authorized services.  We therefore need to evaluate the balance

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   between this additional state and the performance delivered by the

   With current segment routing, the instructions are used to specify
   the nodes and links to be traversed.  However, in order to achieve
   the required isolation between different services, new instructions
   can be created which can be prepended to a packet to steer it through
   specific network resources and functions.

   Traditionally 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) field in the header.  However to
   achieve the latency and isolation characteristics that are sought by
   the enhanced VPN users, steering packets through specific queues and
   resources will likely be required.  The extent to which these needs
   can be satisfied through existing QoS mechanisms is to be determined.
   What is clear is that a fine control of which services wait for
   which, with a fine granularity of queue management policy is needed.
   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.

   From the perspective of the control plane, and from the perspective
   of the segment routing, the method of steering a packet to a queue
   that provides the required properties is an abstraction that hides
   the details of the underlying implementation.  How the queue
   satisfies the requirement is implementation specific and is
   transparent to the control plane and data plane mechanisms used.
   Thus, for example, a FlexE channel, or a time sensitive networking
   packet scheduling slot are abstracted to the same concept and bound
   to the data plane in a common manner.

   We can also introduce such fine grained packet steering by specifying
   the queues through an SR instruction list.  Thus new SR instructions
   may be created to specify not only which resources are traversed, but
   in some cases how they are traversed.  For example, it may be
   possible to specify not only the queue to be used but the policy to
   be applied when enqueuing and dequeuing.

   This concept can be further generalized, since as well as queuing to
   the output port of a router, it is possible to queue to any resource,
   for example:

   o  A network processor unit (NPU)

   o  A central processing unit (CPU) Core

   o  A Look-up engine

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   Both SR-MPLS and SRv6 are candidate network layer technologies for
   enhanced VPN.  In some cases they can be supported by DetNet to meet
   the packet loss, delay and jitter requirement of particular service.
   However, currently the "pure" IP variant of DetNet
   [I-D.ietf-detnet-dp-sol-ip] does not support the Packet Replication,
   Elimination, and Re-ordering (PREOF) [I-D.ietf-detnet-architecture]
   functions.  How to provide the DetNet enhanced delivery in an SRv6
   environment needs further study.

5.3.  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
   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 reduce 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 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 cannot 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 service,
   and not allocated to multiple services causing unintended resource

   Abstraction and Control of Traffic Engineered Networks (ACTN)
   [I-D.ietf-teas-actn-framework] specifies the SDN based architecture
   for the control of TE networks.  The ACTN approach can be applicable
   to the provisioning of enhanced VPN service.  The details are
   described in [I-D.lee-rtgwg-actn-applicability-enhanced-vpn].

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6.  Scalability Considerations

   Enhanced VPN provides the performance guaranteed services in packet
   networks, with the cost of introducing necessary additional states
   into the network.  There are at least three ways of adding the state
   needed for VPN+:

   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
   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.

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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 [I-D.ietf-isis-segment-routing-msd].  This leads
   to a difficult balance between adding state to the network and
   minimizing stack depth, or minimizing state and increasing the stack

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.  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
   [I-D.sitaraman-mpls-rsvp-shared-labels] proposes to address this by
   employing SR within a tunnel established by RSVP-TE.

7.  OAM Considerations

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

   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.

   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

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   These issues will be discussed in a future version of this document.

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,
   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

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   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.  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
   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.

10.  IANA Considerations

   There are no requested IANA actions.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

11.2.  Informative References

   [DETNET]   "Deterministic Networking", March ,

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

              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", draft-ietf-
              detnet-architecture-07 (work in progress), August 2018.

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              Korhonen, J. and B. Varga, "DetNet IP Data Plane
              Encapsulation", draft-ietf-detnet-dp-sol-ip-00 (work in
              progress), July 2018.

              Grossman, E., "Deterministic Networking Use Cases", draft-
              ietf-detnet-use-cases-17 (work in progress), June 2018.

              Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
              "Signaling MSD (Maximum SID Depth) using IS-IS", draft-
              ietf-isis-segment-routing-msd-13 (work in progress), July

              Ceccarelli, D. and Y. Lee, "Framework for Abstraction and
              Control of Traffic Engineered Networks", draft-ietf-teas-
              actn-framework-15 (work in progress), May 2018.

              King, D., Lee, Y., Tantsura, J., Wu, Q., and D.
              Ceccarelli, "Applicability of Abstraction and Control of
              Traffic Engineered Networks (ACTN) to Enhanced VPN",
              draft-lee-rtgwg-actn-applicability-enhanced-vpn-03 (work
              in progress), July 2018.

              Sitaraman, H., Beeram, V., Parikh, T., and T. Saad,
              "Signaling RSVP-TE tunnels on a shared MPLS forwarding
              plane", draft-sitaraman-mpls-rsvp-shared-labels-03 (work
              in progress), December 2017.

   [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,

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

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   [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,

   [RFC3985]  Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
              Edge-to-Edge (PWE3) Architecture", RFC 3985,
              DOI 10.17487/RFC3985, March 2005,

   [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,

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

   [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>.

   [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,

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   [RFC8370]  Beeram, V., Ed., Minei, I., Shakir, R., Pacella, D., and
              T. Saad, "Techniques to Improve the Scalability of RSVP-TE
              Deployments", RFC 8370, DOI 10.17487/RFC8370, May 2018,

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

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

   [SFC]      "Deterministic Networking", March ,

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

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

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