A Framework for Enhanced Virtual Private
Network (VPN+) ServicesHuaweijie.dong@huawei.comUniversity of Surreystewart.bryant@gmail.comChina Mobilelizhenqiang@chinamobile.comKDDI Corporationta-miyasaka@kddi.comSamsungyounglee.tx@gmail.comTEAS Working GroupThis document describes the framework for Enhanced Virtual Private
Network (VPN+) services. The purpose of enhanced VPNs is to support the
needs of new applications, particularly applications that are associated
with 5G services, by utilizing an approach that is based on existing VPN
and Traffic Engineering (TE) technologies and adds characteristics that
specific services require over those provided by traditional VPNs.Typically, VPN+ will be used to underpin network slicing, but could
also be of use in its own right providing enhanced connectivity services
between customer sites.It is envisaged that enhanced VPNs will be delivered using a
combination of existing, modified, and new networking technologies. This
document provides an overview of relevant technologies and identifies
some areas for potential new work.Virtual private networks (VPNs) have served the industry well as a
means of providing different groups of users with logically isolated
connectivity over a common network. The common or base network that is
used to provide the VPNs is often referred to as the underlay, and the
VPN is often called an overlay.Customers of a network operator may request a connectivity services
with advanced characteristics such as low latency guarantees, bounded
jitter, or isolation from other services or customers so that changes in
some other service (such as changes in network load, or events such as
congestion or outages) have no or only acceptable effect on the
throughput or latency of the services provided to the customer. These
services are referred to as "enhanced VPNs" (known as VPN+) in that they
are similar to VPN services providing the customer with the required
connectivity, but in addition they have enhanced characteristics.The concept of network slicing has gained traction driven largely by
needs surfacing from 5G .
According to , a 5G end-to-end network slice
consists of three major types of network segments: Radio Access Network
(RAN), Transport Network (TN), and Mobile Core Network (CN). The
transport network provides the connectivity between different entities
in RAN and CN segments of a 5G end-to-end network slice, with specific
performance commitment. introduces the
concept and the general framework of IETF network slices. An IETF
Network Slice is a logical network topology connecting a number of
endpoints using a set of shared or dedicated network resources that are
used to satisfy specific Service Level Objectives (SLOs) and Service
Level Expectations (SLEs). In this document (which is solely about IETF
technologies) we refer to an "IETF network slice" simply as a "network
slice": a network slice is considered one possible use case of an
enhanced VPN.A network slice could span multiple technologies (such as IP or
Optical) and multiple administrative domains. Depending on the
customer's requirement, a network slice could be isolated from other
network slices in terms of data plane, control plane, and management
plane resources.Network slicing builds on the concepts of resource management,
network virtualization, and abstraction to provide performance
assurance, flexibility, programmability, and modularity. It may use
techniques such as Software Defined Networking (SDN) , network abstraction and
Network Function Virtualization (NFV) to create multiple logical (virtual) networks, each
tailored for use by a set of services or by a particular tenant or a
group of tenants that share the same or similar requirements. These
logical networks are created on top of a common underlay network. How
the network slices are engineered can be deployment-specific.VPN+ can be used to instantiate a network slice, but the technique
can also be of use in general cases to provide enhanced connectivity
services between customer sites.The requirements of enhanced VPN services cannot be met by simple
overlay networks, as these services require tighter coordination and
integration between the underlay and the overlay network. VPN+ is built
from a VPN overlay and an underlying Virtual Transport Network (VTN)
which has a customized network topology and a set of dedicated or shared
resources in the underlay network. The enhanced VPN may also include a
set of invoked service functions located within the underlay network.
Thus, an enhanced VPN can achieve greater isolation with strict
performance guarantees. These new properties, which have general
applicability, are also of interest as part of a network slicing
solution.It is not envisaged that VPN+ services will replace traditional VPN
services. Traditional VPN services will continue to be delivered using
pre-existing mechanisms and can co-exist with VPN+ services. In fact,
compared to traditional VPNs, it is not envisaged that large numbers of
VPN+ services will be deployed in a network. In other words, it is not
intended that all existing VPNs supported by a network will use VPN+
techniques.This document describes a framework for using existing, modified, and
potential new technologies as components to provide a VPN+ service.
Specifically, we are concerned with:The functional requirements and service characteristics of an
enhanced VPN.The design of the enhanced VPN data plane.The necessary control and management protocols in both the
underlay and the overlay of the enhanced VPN.The mechanisms to achieve integration between overlay and
underlay.The necessary Operation, Administration, and Management (OAM)
methods to instrument an enhanced VPN to make sure that the required
Service Level Agreement (SLA) between the customer and the network
operator is met, and to take any corrective action (such as
switching traffic to an alternate path) to avoid SLA violation.The required layered network structure to achieve this is shown in
.In this document, the relationship of the four terms "VPN", "VPN+",
"VTN", and "Network Slice" are as follows:A Virtual Private Network (VPN) refers to the overlay network
service that provides the connectivity between different customer
sites, and that maintains traffic separation between different
customers. IPVPN is defined in , L2VPN is
defined in , L3VPN is defined in , and EVPN is defined in .An enhanced VPN (VPN+) is an evolution of the VPN service that
makes additional service-specific commitments. An enhanced VPN is
made by integrating an overlay VPN with a set of network resources
allocated in the underlay network.A Virtual Transport Network (VTN) is a virtual underlay network
which consists of a set of dedicated or shared network resources
allocated from the physical underlay network, and is associated with
a customized logical network topology. VTN has the capability to
deliver the performance characteristics required by the VPN+
customers and to provide isolation between different VPN+
services.A network slice could be provided by provisioning an enhanced VPN
in the network. Other mechanisms for delivering network slices may
exist but are not in scope for this document.The term "tenant" is used in this document to refer to the customers
and all of their associated enhanced VPNs.The following terms are also used in this document. Some of them are
newly defined, some others reference existing definitions. Abstraction and Control of Traffic Engineered
Networks Deterministic Networking. See and Flexible Ethernet Time Sensitive Networking Virtual Network Virtual Transport Path. A VTP is a path through
the VTN which provides the required connectivity and performance
between two or more customer sites.This section provides an overview of the requirements of an enhanced
VPN service.Performance guarantees are made by network operators to their
customers in relation to the services provided to the customers. They
are usually expressed in SLAs as a set of SLOs.There are several kinds of performance guarantee, including
guaranteed maximum packet loss, guaranteed maximum delay, and
guaranteed delay variation. Note that these guarantees apply to
conformance traffic, out-of-profile traffic will be handled according
to a separate agreement with the customer.Guaranteed maximum packet loss is usually addressed by setting
packet priorities, queue size, and discard policy. However this
becomes more difficult when the requirement is combined with latency
requirements. The limiting case is zero congestion loss, and that is
the goal of DetNet and TSN . In modern optical networks, loss due to transmission
errors already approaches zero, but there is the possibility of
failure of the interface or the fiber itself. This type of fault can
only be addressed by some form of signal duplication and transmission
over diverse paths.Guaranteed maximum latency is required by a number of applications
particularly real-time control applications and some types of virtual
reality applications. DetNet is relevant,
however additional methods of enhancing the underlay to better support
the delay guarantees may be needed, and these methods will need to be
integrated with the overall service provisioning mechanisms.Guaranteed maximum delay variation is a performance guarantee that
may also be needed. calls up a number of
cases that need this guarantee, for example in electrical utilities.
Time transfer is an example service that needs a performance
guarantee, although it is in the nature of time that the service might
be delivered by the underlay as a shared service and not provided
through different enhanced VPNs. Alternatively, a dedicated enhanced
VPN might be used to provide this as a shared service.This suggests that a spectrum of service guarantees need to be
considered when deploying an enhanced VPN. As a guide to understanding
the design requirements we can consider four types of service:Best effortAssured bandwidthGuaranteed latencyEnhanced deliveryThe best effort service is the basic service as provided by current
VPNs.An assured bandwidth service is one in which the bandwidth over
some period of time is assured. This can be achieved either simply
based on a best effort service with over-capacity provisioning, or it
can be based on MPLS traffic engineered label switching paths
(TE-LSPs) with bandwidth reservations. Depending on the technique
used, however, the bandwidth is not necessarily assured at any
instant. Providing assured bandwidth to VPNs, for example by using
per-VPN TE-LSPs, is not widely deployed at least partially due to
scalability concerns. VPN+ aims to provide a more scalable approach
for such services.A guaranteed latency service has an upper bound to edge-to-edge
latency. Assuring the upper bound is sometimes more important than
minimizing latency. There are several new technologies that provide
some assistance with this performance guarantee. Firstly, the IEEE TSN
project introduces the concept of scheduling of
delay- and loss-sensitive packets. The DetNet work is also of relevance in assuring an upper bound of
end-to-end packet latency. FlexE is also useful
to help provide these guarantees. The use of such underlying
technologies to deliver VPN+ services needs to be considered.An enhanced delivery service is one in which the underlay network
(at Layer 3) attempts to deliver the packet through multiple paths in
the hope of eliminating packet loss due to equipment or media
failures. Such a mechanism may need to be used for VPN+ service.One element of the SLA demanded for an enhanced VPN may be a
guarantee that the service offered to the customer will not be
affected by any other traffic flows in the network. This is termed
"isolation" and a customer may express the requirement for isolation
as an SLE .One way for a network operator to meet the requirement for
isolation is simply by setting and conforming to all the SLOs. For
example, traffic congestion (interference from other services) might
impact on the latency experienced by a VPN+ customer. Thus, in this
example, conformance to a latency SLO would be the primary requirement
for delivery of the VPN+ service, and isolation from other services
might be only a means to that end.Another way for a service provider to meet this SLE is to control
the degree to which traffic from one service is isolated from other
services in the network.There is a fine distinction between how isolation is requested by a
customer and how it is delivered by the service provider. In general,
the customer is interested in service performance and not how it is
delivered. Thus, for example, the customer wants specific quality
guarantees and is not concerned about how the service provider
delivers them. However, it should be noted that some aspects of
isolation might be directly measurable by a customer if they have
information about the traffic patterns on a number services supported
by the same service provider. Furthermore, a customer may be nervous
about disruption caused by other services, contamination by other
traffic, or delivery of their traffic to the wrong destinations. In
this way, the customer may want to specify (and pay for) the level of
isolation provided by the service provider.Isolation is achieved in the realization of a VPN+ through existing
technologies that may be supplemented by new mechanisms. The service
provider chooses which processes to use to meet this SLE just as they
choose how to meet all other SLOs and SLEs. Isolation may be achieved
in the network by various forms of resource partitioning ranging from
simple separation of service traffic on delivery (ensuring that
traffic is not delivered to the wrong customer), through sharing of
resources with some form of safeguards, to dedicated allocation of
resources for a specific enhanced VPN. For example, interference
avoidance may be achieved by network capacity planning, allocating
dedicated network resources, traffic policing or shaping, prioritizing
in using shared network resources, etc.The terms hard and soft isolation are used to indicate different
levels of isolation. A service has soft isolation if the traffic of
one service cannot be received by the customers of another service.
The existing IP and MPLS VPNs are examples of services with soft
isolation: the network delivers the traffic only to the required
customer endpoints. However, with soft isolation, as the network
resources are shared, traffic from some services may congest the
network, resulting in packet loss and delay for other services. The
ability for a service or a group of services to be sheltered from this
effect is called hard isolation. Hard isolation may be needed so that
applications with exacting requirements can function correctly,
despite other demands (perhaps a burst of traffic in another service)
competing for the underlying resources. A customer may request
different degrees of isolation ranging from soft isolation to hard
isolation. In practice isolation may be delivered on a spectrum
between soft and hard, and in some cases soft and hard isolation may
be used in a hierarchical manner with one enhanced VPN being built on
another.To provide the required level of isolation, resources may need to
be reserved in the data plane of the underlay network and dedicated to
traffic from a specific enhanced VPN or a specific group of enhanced
VPNs. This may introduce scalability concerns both in the
implementation (as each enhanced VPN would need to be tracked in the
network) and in how many resources need to be reserved and may be
under-used (see ). Thus, some trade-off needs
to be considered to provide the isolation between enhanced VPNs while
still allowing reasonable resource sharing.An optical underlay can offer a high degree of isolation, at the
cost of allocating resources on a long-term and end-to-end basis. On
the other hand, where adequate isolation can be achieved at the packet
layer, this permits the resources to be shared amongst a group of
services and only dedicated to a service on a temporary basis.The next section explores a pragmatic approach to isolation in
packet networks.A key question is whether it is possible to achieve hard
isolation in packet networks that were designed to provide
statistical multiplexing through sharing of data plane resources, a
significant economic advantage when compared to a dedicated, or a
Time Division Multiplexing (TDM) network. Clearly, there is no need
to provide more isolation than is required by the applications, and
an approximation to full hard isolation is sufficient in most cases.
For example, pseudowires emulate services
that would have had hard isolation in their native form. shows a spectrum of isolation that may be
delivered by a network. At one end of the spectrum, we see
statistical multiplexing technologies that support traditional VPNs.
This is a service type that has served the industry well and will
continue to do so. At the opposite end of the spectrum, we have the
absolute isolation provided by dedicated transport networks. The
goal of enhanced VPNs is "pragmatic isolation". This is isolation
that is better than what is obtainable from pure statistical
multiplexing, more cost effective and flexible than a dedicated
network, but is a practical solution that is good enough for the
majority of applications. Mechanisms for both soft isolation and
hard isolation are needed to meet different levels of service
requirement.The way to achieve the characteristics demanded by an enhanced VPN
(such as guaranteed or predictable performance) is by integrating the
overlay VPN with a particular set of resources in the underlay network
which are allocated to meet the service requirement. This needs be
done in a flexible and scalable way so that it can be widely deployed
in operators' networks to support a reasonable number of enhanced VPN
customers.Taking mobile networks and in particular 5G into consideration, the
integration of the network with service functions is likely a
requirement. The IETF's work on service function chaining (SFC) provides a foundation for this. Service functions can
be considered as part of enhanced VPN services. The detailed
mechanisms about the integration between service functions and
enhanced VPNs are out of the scope of this document.Integration of the overlay VPN and the underlay network resources
does not need to be a tight mapping. As described in , abstraction is the process of applying policy to
a set of information about a traffic engineered (TE) network to
produce selective information that represents the potential ability
to connect across the network. The process of abstraction presents
the connectivity graph in a way that is independent of the
underlying network technologies, capabilities, and topology so that
the graph can be used to plan and deliver network services in a
uniform way.Virtual networks can be built on top of an abstracted topology
that represents the connectivity capabilities of the underlay
network as described in the framework for Abstraction and Control of
TE Networks (ACTN) as discussed further in
. describes the
applicability of ACTN to network slicing and is, therefore, relevant
to the consideration of using ACTN to enable enhanced VPNs.Enhanced VPNs need to be created, modified, and removed from the
network according to service demands. 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 whether the traffic in flight will be disrupted can be a difficult
problem.The data plane aspects of this problem are discussed further in
,, and .The control plane aspects of this problem are discussed further in
.The management plane aspects of this problem are discussed further
in .Dynamic changes both to the enhanced VPN and to the underlay
transport network need to be managed to avoid disruption to services
that are sensitive to changes in network performance.In addition to non-disruptively managing the network during changes
such as the inclusion of a new VPN endpoint or a change to a link, VPN
traffic might need to be moved because of changes to traffic patterns
and volumes.In many cases the customers are delivered with enhanced VPN
services without knowing the information about the underlying VTNs.
However, depends on the agreement between the operator and the
customer, in some cases the customer may also be provided with some
information about the underlying VTNs. Such information can be
filtered or aggregated according to the operator's policy. This allows
the customer of the enhanced VPN to have some visibility and even
control over how the underlying topology and resources of the VTN are
used. For example, the customers may be able to specify the service
paths within the VTN for specific traffic flows of their enhanced
VPNs. Depending on the requirements, an enhanced VPN customer may have
his own network controller, which may be provided with an interface to
the control or management system run by the network operator. Note
that such control is within the scope of the customer's enhanced VPN,
any additional changes beyond this would require some intervention by
the network operator.A description of the control plane aspects of this problem are
discussed further in . A description of
the management plane aspects of this feature can be found in .The concept of enhanced VPN can be applied to any existing and
future multi-tenancy overlay technologies including but not limited to
:Layer-2 point-to-point services such as pseudowires Layer-2 VPNs Ethernet VPNs Layer-3 VPNs , Where such VPN service types need enhanced isolation and delivery
characteristics, the technologies described in
can be used to provide an underlay with the required enhanced
performance.In some scenarios, an enhanced VPN service may span multiple
network domains. A domain is considered to be any collection of
network elements within a common realm of address space or path
computation responsibility for example, an
Autonomous System. In some domains the network operator may manage a
multi-layered network, for example, a packet network over an optical
network. When enhanced VPNs are provisioned in such network scenarios,
the technologies used in different network planes (data plane, control
plane, and management plane) need to provide mechanisms to support
multi-domain and multi-layer coordination and integration, so as to
provide the required service characteristics for different enhanced
VPNs, and improve network efficiency and operational simplicity.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 VTN with a specific set of network resources and service
functions allocated in the underlay to satisfy the needs of the VPN
customer. One VTN may support one of more enhanced VPNs. The integration
between overlay and various underlay resources ensures the required
isolation between different enhanced VPNs, and achieves the guaranteed
performance for different services.An enhanced VPN needs to be designed with consideration given to:An enhanced data plane.A control plane to create enhanced VPNs, making use of the data
plane isolation and performance guarantee techniques.A management plane for enhanced VPN service life-cycle
management. These topics are expanded below.The enhanced data plane:Provides the required packet latency and jitter
characteristics.Provides the required packet loss characteristics.Provides the required resource isolation capability, e.g.,
bandwidth guarantee.Provides the mechanism to associate a packet with the set of
resources allocated to the enhanced VPN to which the packet
belongs.The control plane:Collects information about the underlying network topology
and available resources, and exports this to nodes in the
network and/or a centralized controller as required.Creates the required VTNs with the resources and properties
needed by the enhanced VPN services that are they support.Determines the risk of SLA violation and takes appropriate
avoiding action.Determines the right balance of per-packet and per-node state
according to the needs of the enhanced VPN services to scale to
the required size.The management plane:Provides an interface between the enhanced VPN provider
(e.g., operator's network management system ) and the enhanced
VPN customer (e.g., an organization or a service with enhanced
VPN requirement) such that some of the operation requests can be
met without interfering with the enhanced VPN of other
customers.Provides an interface between the enhanced VPN provider and
the enhanced VPN customers to expose the network capability
information toward the enhanced VPN customer.Provides the service life-cycle management and operation of
enhanced VPNs (e.g., creation, modification,
assurance/monitoring, and decommissioning).Operations, Administration, and Maintenance (OAM) Provides the tools to verify the connectivity and performance
of the enhanced VPN.Provides the tools to verify whether the underlay network
resources are correctly allocated and operating properly.Telemetry Provides the mechanisms to collect network information about
the operation of the data plane, control plane, and management
plane. More specifically, telemetry provides the mechanisms to
collect network data: from the underlay network for overall performance
evaluation and for the planning enhanced VPN services.from each enhanced VPN and for monitoring and analytics
of the characteristics and SLA fulfillment of the enhanced
VPN services.The layered architecture of an enhanced VPN is shown in .Underpinning everything is the physical network infrastructure
layer which provide the underlying resources used to provision the
separated virtual transport networks (VTNs). This includes the
partitioning of link and/or node resources. Each subset of link or
node resource can be considered as a virtual link or virtual node used
to build the VTNs.Various components and techniques discussed in can be used to enable resource partitioning, such as
FlexE, TSN, DetNet, dedicated queues, etc. These partitions may be
physical or virtual so long as the SLA required by the higher layers
is met.Based on the network resources provided by the physical network
infrastructure, multiple VTNs can be provisioned, each with customized
topology and other attributes to meet the requirements of different
enhanced VPNs or different groups of enhanced VPNs. To get the
required characteristics, each VTN needs to be mapped to a set of
network nodes and links in the network infrastructure. And on each
node or link, the VTN is associated with a set of resources which are
allocated for the processing of traffic in the VTN. The VTN provides
the integration between the virtual network topology and the required
underlying network resources. The VTN is an essential scaling
technique, as it has the potential of eliminating per-path state from
the network. In addition, when a group of enhanced VPNs is supported
by a single VTN, there is need only to maintain network state for the
single VTN (see for more information).The centralized network controller is used to create the VTN, and
to instruct the network nodes to allocate the required resources to
each VTN and to provision the enhanced VPN services on the VTNs. A
distributed control plane may also be used for the distribution of the
VTN topology and attribute information between nodes within the
VTNs.The process used to create VTNs and to allocate network resources
for use by VTNs needs to take a holistic view of the needs of all of
its customers and to partition the resources accordingly. However,
within a VTN these resources can, if required, be managed via a
dynamic control plane. This provides the required scalability and
isolation.At the level of a overlay VPN service, the required connectivity
for an MP2MP service is usually full or partial mesh. To support such
VPN services, the corresponding VTN connectivity also needs to have an
abstracted MP2MP connectivity.Other service requirements may be expressed at different
granularities, some of which can be applicable to the whole service,
while some others may only be applicable to some pairs of end points.
For example, when a particular level of performance guarantee is
required, the point-to-point path through the underlying VTN of the
enhanced VPN may need to be specifically engineered to meet the
required performance guarantee.Although a lot of the traffic that will be carried over the
enhanced VPN will likely be IPv4 or IPv6, the design must be capable
of carrying other traffic types, in particular Ethernet traffic. This
is easily accomplished through the various pseudowire (PW) techniques
. Where the underlay is MPLS, Ethernet can be
carried over the enhanced VPN encapsulated according to the method
specified in . Where the underlay is IP, Layer
Two Tunneling Protocol - Version 3 (L2TPv3)
can be used with Ethernet traffic carried according to . Encapsulations have been defined for most of the
common Layer-2 types for both PW over MPLS and for L2TPv3.VPNs are instantiated as overlays on top of an operator's network
and offered as services to the operator's customers. An important
feature of overlays is that they can deliver services without placing
per-service state in the core of the underlay network.Enhanced VPNs may need to install some additional state within the
network to achieve the features that they require. Solutions must
consider minimizing and controlling the scale of such state, and
deployment architectures should constrain the number of enhanced VPNs
that would exist where such services would place additional state in
the network. It is expected that the number of enhanced VPNs will be
small at the beginning, and even in future the number of enhanced VPNs
will be much fewer than traditional VPNs because pre-existing VPN
techniques are good enough to meet the needs of most existing VPN-type
services.In general, it is not required that the state in the network be
maintained in a 1:1 relationship with the VPN+ services. It will
usually be possible to aggregate a set or group of VPN+ services so
that they share the same VTN and the same set of network resources
(much in the same way that current VPNs are aggregated over transport
tunnels) so that collections of enhanced VPNs that require the same
behavior from the network in terms of resource reservation, latency
bounds, resiliency, etc. can be grouped together. This is an important
feature to assist with the scaling characteristics of VPN+
deployments.
provides more details of scalability considerations for enhanced VPNs,
and includes a greater
discussion of scalability considerations.A VPN is a network created by applying a demultiplexing technique to
the underlying network (the underlay) 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-TE 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. Thus, an enhanced VPN solution needs
tighter coupling with the underlay than is the case with existing VPN
techniques. We cannot, for example, share the network resource between
enhanced VPNs which require hard isolation.In an enhanced VPN, different subsets of the underlay resources can
be dedicated to different enhanced VPNs or different groups of enhanced
VPNs through the use of VTNs.Several candidate Layer 2 packet- or frame-based data plane
solutions which provide the required isolation and guarantees are
described in the following sections.FlexE provides the ability to multiplex
channels over an Ethernet link to create point-to-point fixed-
bandwidth connections in a way that provides hard isolation. FlexE
also supports bonding links to create larger links out of multiple
low capacity links.However, FlexE is only a link level technology. When packets are
received by the downstream node, they need to be processed in a way
that preserves that isolation in the downstream node. This in turn
requires a queuing and forwarding implementation that preserves the
end-to-end isolation.If different FlexE channels are used for different services, then
no sharing is possible between the FlexE channels. This means that
it may be difficult to dynamically redistribute unused bandwidth to
lower priority services in another FlexE channel. If one FlexE
channel is used by one customer, the customer can use some methods
to manage the relative priority of their own traffic in the FlexE
channel.DiffServ based queuing systems are described in and . This approach is
not sufficient to provide isolation for enhanced VPNs because
DiffServ does not provide enough markers to differentiate between
traffic of a large number of enhanced VPNs. Nor does DiffServ offer
the range of service classes that each VPN needs to provide to its
tenants. This problem is particularly acute with an MPLS underlay,
because MPLS only provides eight traffic classes.In addition, DiffServ, as currently implemented, mainly provides
per-hop priority-based scheduling, and it is difficult to use it to
achieve quantitative resource reservation.To address these problems and to reduce the potential
interference between enhanced VPNs, it would be necessary to steer
traffic to dedicated input and output queues per enhanced VPN: some
routers have a large number of queues and sophisticated queuing
systems which could support this, while some routers may struggle to
provide the granularity and level of isolation required by the
applications of enhanced VPN.Time Sensitive Networking (TSN) is an IEEE
project to provide a method of carrying time sensitive information
over Ethernet. It introduces the concept of packet scheduling where
a packet stream may be given a time slot guaranteeing that it
experiences no queuing delay or increase in latency beyond the very
small scheduling delay. The mechanisms defined in TSN can be used to
meet the requirements of time sensitive services of an enhanced
VPN.Ethernet can be emulated over a Layer 3 network using an IP or
MPLS pseudowire. However, a TSN Ethernet payload would be opaque to
the underlay and thus not treated specifically as time sensitive
data. The preferred method of carrying TSN over a Layer 3 network is
through the use of deterministic networking as explained in .This section considers the problem of enhanced VPN differentiation
and resource representation in the network layer.Deterministic Networking (DetNet) is a
technique being developed in the IETF to enhance the ability of
Layer-3 networks to deliver packets more reliably and with greater
control over the delay. The design cannot use re-transmission
techniques such as TCP since that can exceed the delay tolerated by
the applications. Even the delay improvements that are achieved with
Stream Control Transmission Protocol Partial Reliability Extension
(SCTP-PR) may not meet the bounds set by
application demands. DetNet pre-emptively sends copies of the packet
over various paths to minimize the chance of all copies of a packet
being lost. It also seeks to set an upper bound on latency, but the
goal is not to minimize latency.MPLS-TE
introduces the concept of reserving end-to-end bandwidth for a
TE-LSP, which can be used to provide a point-to-point Virtual
Transport Path (VTP) across the underlay network to support VPNs.
VPN traffic can be carried over dedicated TE-LSPs to provide
reserved bandwidth for each specific connection in a VPN, and VPNs
with similar behavior requirements may be multiplexed onto the same
TE-LSPs. Some network operators have concerns about the scalability
and management overhead of MPLS-TE system especially with regard to
those systems that use an active control plane, and this has lead
them to consider other solutions for their networks.Segment Routing (SR) is a method that
prepends instructions to packets at the head-end of a path. These
instructions are used to specify the nodes and links to be
traversed, and allow the packets to be routed on paths other than
the shortest path. By encoding the state in the packet, per-path
state is transitioned out of the network.An SR traffic engineered path operates with a granularity of a
link. Hints about priority are provided using the Traffic Class (TC)
or Differentiated Services Code Point (DSCP) field in the header.
However, to achieve the latency and isolation characteristics that
are sought by enhanced VPN customers, it will be necessary to steer
packets through specific virtual links and/or queues on the same
link and direct them to use specific resources. With SR, it is
possible to introduce such fine-grained packet steering by
specifying the queues and resources through an SR instruction
list.Note that the concept of a queue is a useful abstraction for
different types of underlay mechanism that may be used to provide
enhanced isolation and latency support. How the queue satisfies the
requirement is implementation specific and is transparent to the
layer-3 data plane and control plane mechanisms used.With Segment Routing, the SR instruction list could be used to
build a P2P path, and a group of SR SIDs could also be used to
represent an MP2MP network. Thus, the SR based mechanism could be
used to provide both a Virtual Transport Path (VTP) and a Virtual
Transport Network (VTN) for enhanced VPN services.Non-packet underlay data plane technologies often have TE
properties and behaviors, and meet many of the key requirements in
particular for bandwidth guarantees, traffic isolation (with physical
isolation often being an integral part of the technology), highly
predictable latency and jitter characteristics, measurable loss
characteristics, and ease of identification of flows. The cost is that
the resources are allocated on a long-term and end-to-end basis. Such
an arrangement means that the full cost of the resources has to be
borne by the service that is allocated with the resources.An enhanced VPN would likely be based on a hybrid control mechanism
that takes advantage of a logically centralized controller for
on-demand provisioning and global optimization, whilst still relying
on a distributed control plane to provide scalability, high
reliability, fast reaction, automatic failure recovery, etc. Extension
to and optimization of the centralized and distributed control plane
is needed to support the enhanced properties of VPN+.RSVP-TE provides the signaling mechanism
for establishing a TE-LSP in an MPLS network with end-to-end resource
reservation. This can be seen as an approach of providing a Virtual
Transport Path (VTP) which could be used to bind the VPN to specific
network resources allocated within the underlay, but there remain
scalability concerns as mentioned in .The control plane of SR does not have the
capability of signaling resource reservations along the path. On the
other hand, the SR approach provides a potential way of binding the
underlay network resource and the enhanced VPN service without
requiring per-path state to be maintained in the network. A
centralized controller can perform resource planning and reservation
for VTNs, while it needs to ensure that resources are correctly
allocated in network nodes for the enhanced VPN service. The
controller could also compute the SR paths based on the planned or
collected network resource and other attributes, and provision the SR
paths based on the mechanism in to the ingress nodes
of the enhanced VPN services. The distributed control plane may be
used to advertise the network topology and resource attributes
associated with the VTNs, and compute the SR paths with VTN specific
constraints for the enhanced VPN services.The management plane provides the interface between the enhanced
VPN provider and the customers for life-cycle management of the
service (i.e., creation, modification, assurance/monitoring, and
decommissioning). It relies on a set of service data models for the
description of the information and operations needed on the
interface.As an example, in the context of 5G end-to-end network slicing
, the management of enhanced VPNs is
considered as the management of the transport network part of the 5G
end-to-end network slice. The 3GPP management system may provide the
connectivity and performance related parameters as requirements to the
management plane of the transport network. It may also require the
transport network to expose the capabilities and status of the network
slice. Thus, an interface between the enhanced VPN management plane
and the 5G network slice management system, and relevant service data
models are needed for the coordination of 5G end-to-end network slice
management.The management plane interface and data models for enhanced VPN can
be based on the service models described in .It is important that the management life-cycle supports in-place
modification of enhanced VPNs. That is, it should be possible to add
and remove end points, as well as to change the requested
characteristics of the service that is delivered. The management
system needs to be able to assess the revised VPN+ requests and
determine whether they can be provided by the existing VTN or whether
changes must be made, and it will additionally need to determine
whether those changes to the VTN are possible. If not, then the
customer's modification request may be rejected.When the modification of an enhanced VPN is possible, the
management system should make every effort to make the changes in a
non-disruptive way. That is, the modification of the enhanced VPN or
the underlying VTN should not perturbate traffic on the enhanced VPN
in a way that causes the service level to drop below the agreed
levels. Furthermore, in the spirit of isolation, changes to one
enhanced VPN should not cause disruption to other enhanced VPNs.The network operator for the underlay network (i.e., the provider
of the enhanced VPN) may delegate some operational aspects of the
enhanced VPN to the customer. In this way, the VPN+ is presented to
the customer as a virtual network, and the customer can choose how to
use that network. The customer cannot exceed the capabilities of
virtual links and nodes, but can decide how to load traffic onto the
network, for example, by assigning different metrics to the virtual
links so that the customer can control how traffic is routed through
the overlay. This approach requires a management system for the
overlay network, but does not necessarily require any coordination
between the underlay and overlay management systems, except that the
overlay management system might notice when the enhanced VPN network
is close to capacity or considerably under-used and automatically
request changes in the service provided by the underlay.This section describes the applicability of the existing and
in-progress service data models to enhanced VPN. New service models
may also be introduced for some of the required management
functions.The ACTN framework supports
operators in viewing and controlling different domains and presenting
virtualized networks to their customers. It highlights how:Abstraction of the underlying network resources is provided to
higher-layer applications and customers.Underlying resources are virtualized and allocated for the
customer, application, or service.A virtualized environment is created allowing operators to view
and control multi-domain networks as a single virtualized
network.Networks can be presented to customers as a virtual network via
open and programmable interfaces.The type of network virtualization enabled by ACTN managed
infrastructure provides customers with the capability to utilize and
independently control allocated virtual network resources as if they
were physically their own resources. Service Data models are used to
represent, monitor, and manage the virtual networks and services
enabled by ACTN. The VPN customer service models (e.g., the layer 3
VPN service model (L3SM) , the layer 2 VPN
service model (L2SM) ), or the ACTN Virtual
Network (VN) model ) are a
customer view of the ACTN managed infrastructure, and is presented by
the ACTN provider as a set of abstracted services or resources. The
layer 3 VPN network model (L3NM) and layer 2 VPN network model
(L2NM) provide network views of
the ACTN managed infrastructure presented by the ACTN provider as a
set of virtual networks and the associated resources. The VTN network
model further provides
the management of the virtual underlay network topology and resources
for the mapping of the VPN network models. discusses
the applicability of the ACTN approach in the context of network
slicing. Since there is a strong correlation between network slices
and enhanced VPNs, that document can also give guidance on how ACTN
can be applied to enhanced VPNs.One of the typical use cases of enhanced VPN is to deliver IETF
network slice service. This section describes the applicability of
enhanced VPN to network slice realization.In order to provide network slices to customers, a
technology-agnostic network slice service Northbound Interface (NBI)
data model is
needed for the customers to communicate the requirements of IETF network
slices (end points, connectivity, SLOs, and SLEs). These requirements
may be realized using technology specified in this document to instruct
the network to instantiate an enhanced VPN to meet the requirements of
the IETF network slice customers.According to the network operators' network resource planning
policy, or based on the requirement of one or a group of customers or
services, a VTN may need to be created. One of the basic requirements
for a VTN is to provide a set of dedicated network resources to avoid
unexpected interference from other services in the same network. Other
possible requirements may include the required topology and
connectivity, bandwidth, latency, reliability, etc.A centralized network controller can be responsible for calculating
a subset of the underlay network topology (which is called a logical
topology) to support the VTN requirement. And on the network nodes and
links within the logical topology, the set of network resources to be
allocated to the VTN can also be determined by the controller.
Normally such calculation needs to take the underlay network
connectivity information and the available network resource
information of the underlay network into consideration. The network
controller may also take the status of the existing VTNs into
consideration in the planning and calculation of a new VTN.According to the result of the VTN planning, the network nodes and
links involved in the logical topology of the VTN are instructed to
allocated the required set of network resources for the VTN. One or
multiple mechanisms as specified in section 5.1 can be used to
partition the forwarding plane network resources for different VTNs.
In addition, the data plane VTN identifiers which are used to identify
the set of network resources allocated to the VTN are also provisioned
on the network nodes. Depends on the data plane technologies used, the
set of network resources of a VTN can be identified using either
resource aware SR segments as specified in , or a dedicated VTN
resource ID as specified in can be introduced. The
network nodes involved in a VTN may distribute the logical topology
information, the VTN specific network resource information and the VTN
resource identifiers using the control plane, which could be used by
the controller and the network nodes to compute the TE paths within
the VTN, and install the VTN specific forwarding entries.According to the connectivity requirements of an IETF network slice
service, an overlay VPN can be created using the existing or future
multi-tenancy overlay technologies as described in .Then according to the SLOs and SLEs requirements of the network
slice, the overlay VPN is mapped to an appropriate VTN as the virtual
underlay. The integration of the overlay VPN and the underlay VTN
together provide an enhanced VPN service which can meet the network
slice service requirements.At the edge of the operator's network, traffic of IETF network
slices can be classified based on the matching rules defined by
operator's policy, so that the traffic is mapped to a specific
enhanced VPN, which is further mapped to a underlay VTN. Packets
belonging to the enhanced VPN will be processed and forwarded by
network nodes using the set of network resources allocated to the
corresponding VTN.An enhanced VPN provides performance guaranteed services in packet
networks, but with the potential cost of introducing additional state
into the network. There are at least three ways that this additional
state might be brought into the network:Introduce the complete state into the packet, as is done in SR.
This allows the controller to specify the detailed series of
forwarding and processing instructions for the packet as it transits
the network. The cost of this is an increase in the packet header
size. The cost is also that systems will have capabilities enabled
in case they are called upon by a service. This is a type of latent
state, and increases as the path and resources that need to be
exclusively available to a VPN are specified more precisely.Introduce the state to the network. This is normally done by
creating a path using signaling such as RSVP-TE. This could be
extended to include any element that needs to be specified along the
path, for example explicitly specifying queuing policy. It is also
possible to use other methods to introduce path state, such as via
an SDN controller, or possibly by modifying a routing protocol. With
this approach there is state per path: per-path characteristic that
needs to be maintained over the life of the path. This is more
network state than is needed using SR, but the packets are usually
shorter.Provide a hybrid approach. One example is based on using binding
SIDs to create path fragments, and bind
them together with SR. Dynamic creation of a VPN service path using
SR requires less state maintenance in the network core at the
expense of larger packet headers. The packet size can be lower if a
form of loose source routing is used (using a few nodal SIDs), and
it will be lower if no specific functions or resources on the
routers are 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 needs to be created and maintained in the network, as
the state about fine granularity processing would need to be loaded and
maintained in the routers. However, an SR approach allows much of this
state to be spread amongst the network ingress nodes, and transiently
carried in the packets as SIDs.Further discussion of the scalability considerations of enhanced VPNs
can be found in .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.The traditional method of creating a resource allocated path
through an MPLS network is to use the RSVP-TE protocol. However, there
have been concerns that this requires significant continuous state
maintenance in the network. Work to improve the scalability of RSVP-TE
LSPs in the control plane can be found in .There is also concern at the scalability of the forwarder footprint
of RSVP-TE as the number of paths through a label switching router
(LSR) grows. addresses this by employing SR
within a tunnel established by RSVP-TE.The centralized approach of SDN requires state to be stored in the
network, but does not have the overhead of also requiring control
plane state to be maintained. Each individual network node may need to
maintain a communication channel with the SDN controller, but that
compares favorably with the need for a control plane to maintain
communication with all neighbors.However, SDN may transfer some of the scalability concerns from the
network to the centralized controller. In particular, there may be a
heavy processing burden at the controller, and a heavy load in the
network surrounding the controller. A centralized controller also
presents a single point of failure within the network.The design of OAM for enhanced VPNs needs to consider the following
requirements: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.Instrumentation of the overlay by the tenant. This is likely to
be transparent to the network operator and to use existing methods.
Particular consideration needs to be given to the need to verify the
isolation and the various committed performance characteristics.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.Verification of the conformity of the path to the service
requirement. This may need to be done as part of a commissioning
test.A study of OAM in SR networks has been documented in .Network visibility is essential for network operation. Network
telemetry has been considered as an ideal means to gain sufficient
network visibility with better flexibility, scalability, accuracy,
coverage, and performance than conventional OAM technologies.As defined in , the objective of
Network Telemetry is to acquire network data remotely for network
monitoring and operation. It is a general term for a large set of
network visibility techniques and protocols. Network telemetry addresses
the current network operation issues and enables smooth evolution toward
intent-driven autonomous networks. Telemetry can be applied on the
forwarding plane, the control plane, and the management plane in a
network.How the telemetry mechanisms could be used or extended for the
enhanced VPN service is out of the scope of this document.Each enhanced VPN has a life cycle, and may need modification during
deployment as the needs of its tenant change. This is discussed in . Additionally, as the network evolves, there
may 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. That
is, a single action by the head-end that changes the path without the
need for coordinated action by the routers along the path. However,
implementations and the monitoring protocols need to make sure that the
new path is operational and meets the required SLA before traffic is
transitioned to it. It is possible for deadlocks to arise as a result of
the network becoming fragmented over time, such that it is impossible to
create a new path or to modify an existing path without impacting the
SLA of other paths. 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:The problem of packets overtaking one another if a path latency
reduces during a transition.The problem of transient variation in latency in either direction
as a path migrates.There is also the matter of what happens during failure in the
underlay infrastructure. Fast reroute is one approach, but that still
produces a transient loss with a normal goal of rectifying this within
50ms . An alternative is some form of N+1
delivery such as has been used for many years to support protection from
service disruption. This may be taken to a different level using the
techniques of DetNet with multiple in-network replication and the
culling of later packets .In addition to the approach used to protect high priority packets,
consideration should be given to the impact of best effort traffic on
the high priority packets during a transition. Specifically, if a
conventional re-convergence process is used there will inevitably be
micro-loops and whilst some form of explicit routing will protect the
high priority traffic, lower priority traffic on best effort shortest
paths will micro-loop without the use of a loop prevention technology.
To provide the highest quality of service to high priority traffic,
either this traffic must be shielded from the micro-loops, or
micro-loops must be prevented completely.It is likely that enhanced VPN services will be introduced in
networks which already have traditional VPN services deployed. Depending
on service requirements, the tenants or the operator may choose to use a
traditional VPN or an enhanced VPN to fulfill a service requirement. The
information and parameters to assist such a decision needs to be
reflected on the management interface between the tenant and the
operator.All types of virtual network require special consideration to be
given to the isolation of traffic belonging to different tenants. That
is, traffic belonging to one VPN must not be delivered to end points
outside that VPN. In this regard enhanced VPNs neither introduce, nor
experience a greater security risks than other VPNs.However, in an enhanced Virtual Private Network service the
additional service requirements need to be considered. For example, if a
service requires a specific upper bound to latency then it can be
damaged by simply delaying the packets through the activities of another
tenant, i.e., by introducing bursts of traffic for other services. In
some respects this makes the enhanced VPN more susceptible to attacks
since the SLA may be broken. But another view is that the operator must,
in any case, preform monitoring of the enhanced VPN to ensure that the
SLA is met, and this means that the operator may be more likely to spot
the early onset of a security attack and be able to take pre-emptive
protective action.The measures to address these dynamic security risks must be
specified as part to the specific solution are form part of the
isolation requirements of a service.While an enhanced VPN service may be sold as offering encryption and
other security features as part of the service, customers would be well
advised to take responsibility for their own security requirements
themselves possibly by encrypting traffic before handing it off to the
service provider.The privacy of enhanced VPN service customers must be preserved. It
should not be possible for one customer to discover the existence of
another customer, nor should the sites that are members of an enhanced
VPN be externally visible.There are no requested IANA actions.The authors would like to thank Charlie Perkins, James N Guichard,
John E Drake, Shunsuke Homma, and Luis M. Contreras for their review and
valuable comments.This work was supported in part by the European Commission funded
H2020-ICT-2016-2 METRO-HAUL project (G.A. 761727).3GPP TS23.5013GPP TS28.530NGMN NS ConceptBBF SD-406: End-to-End Network SlicingFlex Ethernet Implementation AgreementTime-Sensitive NetworkingDeterministic NetworkingService Function Chaining