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Versions: (draft-bryant-rtgwg-enhanced-vpn) 00

Routing Area Working Group                                       J. Dong
Internet-Draft                                                 S. Bryant
Intended status: Informational                                    Huawei
Expires: January 3, 2019                                           Z. Li
                                                            China Mobile
                                                             T. Miyasaka
                                                        KDDI Corporation
                                                            July 2, 2018


                Enhanced Virtual Private Networks (VPN+)
                    draft-dong-teas-enhanced-vpn-00

Abstract

   This draft describes a number of enhancements that need to be made to
   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 may form the
   underpin of network slicing, but will also be of use in its own
   right.

   Editor's Note: This is draft-bryant-rtgwg-enhanced-vpn moved to the
   TEAS WG.

Status of This Memo

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

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

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

   This Internet-Draft will expire on January 3, 2019.

Copyright Notice

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





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include 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
     3.2.  Diverse Performance Guarantees  . . . . . . . . . . . . .   6
     3.3.  A Pragmatic Approach to Isolation . . . . . . . . . . . .   7
     3.4.  Integration . . . . . . . . . . . . . . . . . . . . . . .   8
     3.5.  Dynamic Configuration . . . . . . . . . . . . . . . . . .   9
     3.6.  Customized Control Plane  . . . . . . . . . . . . . . . .   9
   4.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Architecture and Components of Enhanced VPN . . . . . . . . .  10
     5.1.  Communications Layering . . . . . . . . . . . . . . . . .  10
     5.2.  Multi-Point to Multi-point  . . . . . . . . . . . . . . .  13
     5.3.  Candidate Underlay Technologies . . . . . . . . . . . . .  13
       5.3.1.  FlexE . . . . . . . . . . . . . . . . . . . . . . . .  14
       5.3.2.  Dedicated Queues  . . . . . . . . . . . . . . . . . .  14
       5.3.3.  Time Sensitive Networking . . . . . . . . . . . . . .  15
       5.3.4.  Deterministic Networking  . . . . . . . . . . . . . .  15
       5.3.5.  MPLS Traffic Engineering (MPLS-TE)  . . . . . . . . .  15
       5.3.6.  Segment Routing . . . . . . . . . . . . . . . . . . .  16
     5.4.  Control Plane Considerations  . . . . . . . . . . . . . .  19
     5.5.  Application Specific Network Types  . . . . . . . . . . .  19
     5.6.  Integration with Service Functions  . . . . . . . . . . .  20
   6.  Scalability Considerations  . . . . . . . . . . . . . . . . .  20
     6.1.  Maximum Stack Depth . . . . . . . . . . . . . . . . . . .  21
     6.2.  RSVP Scalability  . . . . . . . . . . . . . . . . . . . .  21
   7.  OAM and Instrumentation . . . . . . . . . . . . . . . . . . .  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  . . . . . . . . . . . . . . . . . . . . . . .  25





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1.  Introduction

   Virtual networks, often referred to as 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.  There is a need to create a VPN with
   enhanced characteristics.  Specifically there is a need for a
   transport network supporting a set of virtual networks each of which
   provides the client with dedicated (private) networking, computing
   and storage resources drawn from a shared pool.
   The tenant of such a 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
   of 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 traditional VPN, an enhanced VPN can achieve greater isolation
   and guaranteed performance.

   These new network layer properties, which have general applicability,
   may also be of interest as part of a network slicing solution.

   This document specifies a framework for using the 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 VPN data-plane

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

   o  The mechanisms to achieve integration between overlay and underlay

   o  The necessary method of monitoring an enhanced VPN

   o  The methods of instrumenting an enhanced VPN to ensure that the
      required tenant Service Level Agreement (SLA) is maintained




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   The required layer structure necessary to achieve this is shown in
   Section 5.1.

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

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

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 might be a network supporting both
   emergency services and public broadband multi-media services.

   During a major incident the VPNs supporting these services would both
   be expected to experience high data volumes, and it is important that
   both make progress in the transmission of their data.  In these
   circumstances the VPNs would require an appropriate degree of
   isolation to be able to continue to operate acceptably.

   We introduce the terms hard (static) and soft (dynamic) 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



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   called hard isolation, and this property is required by some critical
   applications.  Although these isolation requirements are triggered by
   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.

   It is of course possible to achieve high degrees of isolation in the
   optical layer.  However this is done at the cost of allocating
   resources on a long term basis 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 the resources.  On the other hand,
   isolation at the packet layer allows the resources to be shared
   amongst many services and only dedicated to a service on a temporary
   basis.  This allows greater statistical multiplexing of network
   resources and amortizes the cost over many services, leading to
   better economy.  However, the degree of isolation required by network
   slicing cannot easily be met with MPLS-TE packet LSPs as they
   guarantee long-term bandwidth, but not latency.

   Thus some trade-off between the two approaches needs to be considered
   to provide the required isolation between virtual networks while
   still allows reasonable sharing inside each VPN.

   The work of the IEEE project on Time Sensitive Networking is
   introducing 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.  Such a scheduling mechanism may be usable directly,
   or with extension to achieve isolation between multiple VPNs.

   One of the key areas in which isolation needs to be provided is at
   the interfaces.  If nothing is done the system falls back to the
   router queuing system in which the ingress places it on a selected
   output queue.  Modern routers have quite sophisticated output queuing
   systems, traditionally these have not provided the type of scheduling
   system needed to support the levels of isolation required by the
   applications that are the target of VPN+ networks.  However some of
   the more modern approaches to queuing allow the construction of
   virtual channelized sub-interfaces (VCSI).  With VCSIs there is only
   one physical interface, but the queuing system is used to provide
   virtual interfaces with dedicated resources.  Sophisticated queuing
   systems of this type may be used to provide end-to-end virtual
   isolation between tenant's traffic in an otherwise homogeneous
   network.





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   [FLEXE] provides the ability to multiplex multiple channels over an
   Ethernet link in a way that provides hard isolation.  However it is a
   only a link technology.  When packets are received by the downstream
   node they need to be processed in a way that preserves that
   isolation.  This in turn requires a queuing and forwarding
   implementation that preserves the isolation end-to-end.

3.2.  Diverse Performance Guarantees

   There are several aspects to guaranteed performance: 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 than is the goal of the Deterministic Networking
   work that the IETF and IEEE are pursuing.  In modern optical networks
   loss due to transmission errors is already asymptotic to zero due,
   but there is always the possibility of failure of the interface and
   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
   reality applications.  The work of the IETF Deterministic Networking
   (DetNet) Working Group 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.  Time transfer is one example of a service that needs this,
   although the fungible nature of time means that it 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.  The need
   for guaranteed maximum delay variation as a general requirement is
   for further study.

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

   o  Guaranteed latency




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   o  Enhanced delivery

   o  Assured bandwidth

   o  Best effort

   Best effort is the service that current VPNs provide.  Providing
   assured bandwidth to VPNs, for example by using an RSVP-TE is not
   widely deployed at least partially due to scalability concerns.
   Guaranteed latency and enhanced delivery are not yet integrated with
   VPNs.  It is these later two design requirements that enhanced VPNs
   provide.

   In Section 3.1 we considered the work of the IEEE Time Sensitive
   Networking (TSN) project and the work of the IETF DetNet Working
   group in the context of isolation.  However this work is of greater
   relevance in assuring end-to-end packet latency.  It is also of
   importance in considering enhanced delivery.

   A service that is guaranteed latency has a latency upper bound
   provided by the network.  It is important to note that assuring the
   upper bound is more important than achieving the minimum latency.

   A service that is offered enhanced delivery is one in which the
   network (at layer 3) attempts to deliver the packet through multiple
   paths in the hope of avoiding transient congestion
   [I-D.ietf-detnet-dp-sol].

   A useful mechanism to provide these guarantees is to use Flex
   Ethernet [FLEXE] as the underlay.  This is a method of bonding
   Ethernets together and 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 at the cost of making the
   reclamation of unused bandwidth harder.

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

3.3.  A Pragmatic Approach to Isolation

   A key question to consider is whether it is possible to achieve hard
   isolation in packet networks.  Packet networks were never designed to
   support hard isolation, just the opposite, they were designed to
   provide a high degree of statistical multiplexing and hence a
   significant economic advantage when compared to a dedicated, or a



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   Time Division Multiplexing (TDM) network.  However the key thing to
   bear in mind is that the concept of hard isolation needs to be viewed
   from the perspective of the application, and there is no need to
   provide any harder isolation than is required by the application.
   From a historical perspective it is good to think about pseudowires
   [RFC3985] which emulate services that in many would have had hard
   isolation in their native form.  However experience has shown that in
   most cases an approximation to this requirement is sufficient for
   most uses.

   Thus, for example, using FlexE or channelized sub-interface,together
   with packet scheduling as interface slicing, and optionally, also
   together with the slicing of node resources (Network Processor Unit
   (NPU), etc.), it may be possible to provide a type of hard isolation
   that is adequate for many applications.  Other applications may be
   satisfied with a classical VPN with or without reserved bandwidth,
   but yet others may require dedicated point to point fiber.  The
   requirement is thus to qualify the needs of each application and
   provide an economic solution that satisfies those needs without over-
   engineering.

   This spectrum of isolation is shown below:

        O=============================O===================O
        |                             |                   |
    Statistical                    Pragmatic          Absolute
    Multiplexing                   Isolation          Isolation
   (Traditional                 (Enhanced VPN)      (Dedicated
    VPNs)                                             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.

3.4.  Integration

   A solution to the enhanced VPN problem will need to provide seamless
   integration of both overlay VPN and the underlay network resources.
   This needs be done in a flexible and scalable way so that it can be
   widely deployed in operator networks.  Given the targeting of both
   this technology and service function chaining at mobile networks and




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   in particular 5G the co-integration of service functions is a likely
   requirement.

3.5.  Dynamic Configuration

   It is necessary that new enhanced VPNs can be introduced to the
   network, modified, and removed from the network according to service
   demand.  In doing so due regard must be given to the impact of other
   enhanced VPNs that are operational.  An enhanced VPN that requires
   hard isolation must not be disrupted by the installation or
   modification of another enhanced VPN.

   Whether modification of an enhanced VPN can be disruptive to that
   VPN, and in particular the traffic in flight is to be determined, but
   is likely to be a difficult problem to address.

   The data-plane aspect of this are discussed further in Section 5.3.

   The control-plane and management-plane aspects of this, particularly
   the garbage collection are likely to be challenging and are for
   further study.

   As well as managing dynamic changes to the VPN in a seamless way,
   dynamic changes to the underlay and its transport network need to be
   managed 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, consideration has to be given to the need to move VPN
   traffic as a result of traffic volume changes.

3.6.  Customized Control Plane

   In some cases it is desirable that an enhanced VPN has a custom
   control-plane, so that the tenant of the enhanced VPN can have some
   control to the resources and functions partitioned for 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
   operator.

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







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4.  Applicability

   The technologies described in this document is applicable to a number
   types of VPN technology 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.

5.  Architecture and Components of Enhanced VPN

   Normally a number of enhanced VPN services will 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 underlay ensures the
   isolation between different enhanced VPNs, and facilitates the
   guaranteed performance for different services.

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

   o  Isolation of enhanced VPN data plane.

   o  A scalable control plane to match the data plane isolation.

   o  The amount of state in the packet vs the amount of state in the
      control plane.

   o  Mechanism for diverse performance guarantee within an enhanced VPN

   o  Support of the required integration between network functions and
      service functions.

5.1.  Communications Layering

   The communications layering model use to build an enhanced VPN is
   shown in Figure 1.






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   Tenant          Tenant  connection             Tenant
   CE1    ----------------------------------------CE2
     \                                            /
   AC \   OP        Provider VPN           OP    /AC
       +- PE1------------------------------PE1 -+
                    Enhanced Paths
             ==============================
                       Underlay
             ++++++++++++++++++++++++++++++

                     Figure 1: Communication Layering

   The network operator is required to provide a tenant connection
   between the tenant's Customer Equipment (CE) (CE1 and CE2).  These
   CEs attach to the Operator's Provider Edge Equipments (PE) (PE1 and
   PE2 respectively).  The attachment circuits (AC) are outside the
   scope of this document other than to note that they obviously need to
   provide a connection of sufficient quality in terms of isolation,
   latency etc.  so as to satisfy the needs of the user.  The subtlety
   to be aware of is that the ACs are often provided by a network rather
   than a fixed point to point connection and thus the considerations in
   this document may apply to the network that provides the AC.

   A provider VPN is constructed between PE1 and PE2 to carry tenant
   traffic.  This is a normal VPN, and provides one stage of isolation
   between tenants.

   An enhanced path is constructed to carry the provider VPN using
   dedicated resources drawn from the underlay.

   This layered architecture is shown in more detail in Figure 2.




















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            Network Controller                     ] Network
         =======================                   } 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 Layers Architecture

   Underpinning everything is the physical layer consisting of
   partitioned links and nodes which provide the underlying resources
   used to provision the logical networks.  Various components and
   techniques as discussed in Section 5.3 are used to provide these
   resources, such as FlexE links, 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 resources 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 network controller is used to create the virtual networks, to
   allocate the resources to each virtual network and to control and
   manage these networks.




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   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.

5.2.  Multi-Point to Multi-point

   At a VPN level connections are frequently multi-point-to-multi-point
   (MP2MP).  As far as such services are concerned the underlay is an
   abstract MP2MP medium.  However when service guarantees are provided,
   such as with an enhanced VPN, each point to point path through the
   underlay needs to be specifically engineered to meet the required
   performance guarantees.

5.3.  Candidate Underlay 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
   further.

   In an enhanced VPN different subsets of the underlay resources are
   dedicated to different VPNs.  Any enhanced VPN solution thus needs
   tighter coupling with underlay than is the case with classical VPNs.
   We cannot for example share the tunnel between enhanced VPNs which
   require hard isolation.

   In the following sections we consider a number of candidate underlay
   solutions for proving the required VPN separation.

   o  FlexE

   o  Time Sensitive Networking

   o  Deterministic Networking

   o  Dedicated Queues





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   We then consider the problem of slice differentiation and resource
   representation.  Candidate technologies are:

   o  MPLS

   o  MPLS-SR

   o  Segment Routing over IPv6 (SRv6)

5.3.1.  FlexE

   FlexE [FLEXE] is a method of creating a point-to-point Ethernet with
   a specific fixed bandwidth.  FlexE supports the bonding of multiple
   links, which supports creating 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.  FlexE also supports the
   channelization of links, which allows one link to carry several
   lower-speed or sub-rated links from different sources.

   If different FlexE channels are used for different services, then no
   sharing is possible between the services.  This in turn means that it
   is not possible to dynamically re-distribute unused bandwidth to
   lower priority services increasing the cost of operation of the
   network.  FlexE can on the other hand be used to provide hard
   isolation between different tenants by providing hard isolation on an
   interface.  The tenant can then use other methods to manage the
   relative priority of their own traffic.

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

5.3.2.  Dedicated Queues

   In an enhanced VPN providing multiple isolated virtual networks the
   conventional Diff-Serv based queuing system is insufficient for our
   purposes due to the limited number of queues which cannot
   differentiate between traffic of different VPNs and the range of
   service classes that each need to provide 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 thus reduce the interference between VPNs, it is likely
   to be necessary to steer traffic of VPNs to dedicated input and
   output queues.







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5.3.3.  Time Sensitive Networking

   Time Sensitive Networking (TSN) is an IEEE project that is designing
   a method of carrying time sensitive information over Ethernet.  As
   Ethernet this can obviously be tunneled over a Layer 3 network in 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.

   The mechanisms defined in TSN can be used to meet the requirements of
   time sensitive services of an enhanced VPN.

5.3.4.  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 classical re-transmission
   techniques such as TCP since can add delay that is above the maximum
   tolerated by the applications.  Even the delay improvements that are
   achieved with SCTP-PR are outside the bounds set by application
   demands.  The approach is to pre-emptively send copies of the packet
   over various paths in the expectation that this minimizes the chance
   of all packets being lost, but to 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.  Note that it is not the goal to minimize latency,
   and the optimum upper bound paths may not be the minimum latency
   paths.

   DetNet is based on flows.  It currently makes no comment on the
   underlay, and so at this stage must be assumed to use the base
   topology.  To be of use in this application DetNet there needs to be
   a description of how to deal with the concept of flows within an
   enhanced VPN.

   How we use DetNet in a multi-tenant (VPN) network, and how to improve
   the scalability of DetNet in a multi-tenant (VPN) network is for
   further study.

5.3.5.  MPLS Traffic Engineering (MPLS-TE)

   Normal MPLS runs on the base topology and has the concepts of
   reserving end to end bandwidth for an LSP, and of creating VPNs.  VPN
   traffic can be run over dedicated RSVP-TE tunnels to provide reserved




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   bandwidth for a specific VPN connection.  This is rarely deployed in
   practice due to scaling and management overhead concerns.

5.3.6.  Segment Routing

   Segment Routing [I-D.ietf-spring-segment-routing] is a method that
   prepends instructions to packets at entry and sometimes at various
   points as it passes though the network.  These instructions allow
   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 ECMP).

   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.

   A-------B-----E
   |       |     |
   |       |     |
   C-------D-----+

                     Figure 3: An SR Network Fragment

   Consider the network fragment shown in Figure 3.  To send a packet
   from A to E via B, D & E: Node A prepends the ordered list of SIDs
   (B, D, E) to the packet and pushes 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.

   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.






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   A---P---B---S---E
   |       |       |
   |       Q       |
   |       |       |
   C---R---D-------+

                   Figure 4: Another SR Network Fragment

   Consider a further network fragment shown in Figure 4, and further
   consider VPN A+D+E.

   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}

   To create a new VPN C+D+B the following list are introduced:

   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
   endpoints.

   There are a number of limitations in SR as it is currently defined
   that limit its applicability to enhanced VPNs:

   o  Segments are shared between different VPNs,

   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 core in anticipation of its future use by
   authorized services.  We therefore need to evaluate the balance
   between this additional state and the performance delivered by the
   network.

   Both MPLS Segment Routing and SRv6 Segment Routing are candidate
   technologies for enhanced VPN.

   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



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   can be created which can be prepended to a packet to steer it through
   specific dedicated network resources and functions, e.g. links,
   queues, processors, services etc.

   Clearly we can use traditional constructs to create a VPN, but there
   are advantages to the use of other constructs such as Segment Routing
   (SR) in the creation of virtual networks with enhanced properties.

   Traditionally a traffic engineered path operates with a granularity
   of a link with hints about priority provided through the use of the
   traffic class 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 a universal
   construct.  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 time sensitive
   networking packet scheduling slot are abstracted to the same concept
   and bound to the data plane in a common manner.

   We can introduce the specification of finer, deterministic,
   granularity to path selection through extensions to traditional path
   construction techniques such as RSVP-TE and MPLS-TP.

   We can also introduce it 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 such as TCAMs



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5.4.  Control Plane Considerations

   It is expected that enhanced VPN would 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+.

   Where SR is used as a the data-plane construct it needs to be noted
   that it does not have the capability of reserving resources along the
   path nor do its currently specified distributed control plane (the
   link state routing protocols).  An SDN controller can clearly do
   this, from the controllers point of view, and no resource reservation
   is done on the device.  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 to the correct service, and no allocated to
   multiple services causing unintended resource conflict.  This needs
   further study.

   On the other hand an advantage of using an SR approach is that it
   provides a way of efficiently binding the network underlay and the
   enhanced VPN overlay.  With a technology such as RSVP-TE LSPs, each
   virtual path in the VPN is bound to the underlay with a dedicated TE-
   LSP.
   RSVP-TE could be enhanced to bind the VPN to specific resources
   within the underlay, but as noted elsewhere in this document there
   are concerns as to the scalability of this approach.  With an SR-
   based approach to resource reservation (per-slice reservation), it is
   straightforward to create dedicated SR network slices, and the VPN
   can be bound to a particular SR network slice.

5.5.  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 design SHOULD be
   capable of carrying 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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5.6.  Integration with Service Functions

   There is a significant overlap between the problem of routing a
   packet though a set of network resources and the problem of routing a
   packet through a set of compute resources.  Service Function Chain
   technology is designed to forward a packet through a set of compute
   resources.

   A future version of this document will discuss this further.

6.  Scalability Considerations

   For a packet to transit a network, other than on a best effort,
   shortest path basis, it is necessary to introduce additional state,
   either in the packet, or in the network of some combination of both.

   There are at least three ways of doing this:

   o  Introduce the complete state into the packet.  That is how SR does
      this, and this allows the controller to specify the precise series
      of forwarding and processing instructions that will happen to 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 scaling properties will reduce roughly from a
   function of (N/2)^2 to a function of N, where N is the VPN path
   length in intervention points (hops plus network functions).
   Reducing the state in the network is important to VPN+, as VPN+



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   requires the overlay to be more closely integrated with the underlay
   than with traditional VPNs.  This tighter coupling would normally
   mean that significant state needed to be created and maintained in
   the core.  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.

6.1.  Maximum Stack Depth

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

6.2.  RSVP Scalability

   The traditional method of creating a resource allocated path through
   an MPLS network is to use the RSVP protocol.  However there have been
   concerns that this requires significant continuous state maintenance
   in the network.  There are ongoing works to improve the scalability
   of RSVP-TE LSPs in the control plane
   [I-D.ietf-teas-rsvp-te-scaling-rec].  This will be considered further
   in a future version of this document.

   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.  This work will
   be considered in a future version of this document.

7.  OAM and Instrumentation

   A study of OAM in SR networks has been documented in
   [I-D.ietf-spring-oam-usecase].

   The enhanced VPN OAM design needs to consider the following
   requirements:

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

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




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      to verify the isolation and the various committed performance
      characteristics.

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

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

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

8.  Enhanced Resiliency

   Each enhanced VPN, of necessity, has a life-cycle, and needs
   modification during deployment as the needs of its user 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.
   However implementations and the monitoring protocols need to make
   sure that the new path is up before traffic is transitioned to it.

   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
      migrates.

   There is also the matter of what happens during failure in the
   underlay infrastructure.  Fast reroute is one approach, but that
   still produces a transient loss with a normal goal of rectifying this
   within 50ms.  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.





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   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.  Security Considerations

   All types of virtual network require special consideration to be
   given to the isolation between the tenants.  However in an enhanced
   virtual network service hard isolation 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.

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,
              <https://www.rfc-editor.org/info/rfc2119>.

11.2.  Informative References

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

   [I-D.ietf-detnet-architecture]
              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", draft-ietf-
              detnet-architecture-05 (work in progress), May 2018.






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   [I-D.ietf-detnet-dp-sol]
              Korhonen, J., Andersson, L., Jiang, Y., Finn, N., Varga,
              B., Farkas, J., Bernardos, C., Mizrahi, T., and L. Berger,
              "DetNet Data Plane Encapsulation", draft-ietf-detnet-dp-
              sol-04 (work in progress), March 2018.

   [I-D.ietf-spring-oam-usecase]
              Geib, R., Filsfils, C., Pignataro, C., and N. Kumar, "A
              Scalable and Topology-Aware MPLS Dataplane Monitoring
              System", draft-ietf-spring-oam-usecase-10 (work in
              progress), December 2017.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-15 (work
              in progress), January 2018.

   [I-D.ietf-teas-rsvp-te-scaling-rec]
              Beeram, V., Minei, I., Shakir, R., Pacella, D., and T.
              Saad, "Techniques to Improve the Scalability of RSVP
              Traffic Engineering Deployments", draft-ietf-teas-rsvp-te-
              scaling-rec-09 (work in progress), February 2018.

   [I-D.sitaraman-mpls-rsvp-shared-labels]
              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,
              <https://www.rfc-editor.org/info/rfc2764>.

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

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

   [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>.



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

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

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

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

Authors' Addresses

   Jie Dong
   Huawei

   Email: jie.dong@huawei.com


   Stewart Bryant
   Huawei

   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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