A Framework for Enhanced Virtual Private
Networks (VPN+) ServiceHuaweijie.dong@huawei.comFutureweistewart.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+) service. The purpose is to support the needs of new
applications, particularly applications that are associated with 5G
services, by utilizing an approach that is based on existing VPN and
Traffic Engineering (TE) technologies and adds features that specific
services require over and above traditional VPNs.Typically, VPN+ will be used to form the underpinning of network
slicing, but could also be of use in its own right providing enhanced
connectivity services between customer sites.It is envisaged that enhanced VPNs will be delivered using a
combination of existing, modified, and new networking technologies. This
document provides an overview of relevant technologies and identifies
some areas for potential new work.Comparing to traditional VPNs, It is not envisaged that quite large
numbers of VPN+ services will be deployed in a network. In other word,
it is not intended that all existing VPNs supported by a network will
use VPN+ related techniques.Virtual private networks (VPNs) have served the industry well as a
means of providing different groups of users with logically isolated
connectivity over a common network. The common or base network that is
used to provide the VPNs is often referred to as the underlay, and the
VPN is often called an overlay.Customers of a network operator may request a connectivity services
with advanced characteristics such as enhanced isolation from other
services so that changes in some other service (such as changes in
network load, or events such as congestion or outages) have no or
acceptable effect on the throughput or latency of the services provided
to the customer. These services are "enhanced VPNs" (known as VPN+) in
that they are similar to VPN services as they provide the customer with
required connectivity, but have enhanced characteristics.Driven largely by needs surfacing from 5G, the concept of network
slicing has gained traction .
According to , a 5G end-to-end network slice
consists of three major types network segments: Radio Access Network
(RAN), Transport Network (TN) and Mobile Core Network (CN). The
transport network provides the required connectivity between different
entities in RAN and CN segments of an end-to-end network slice, with
specific performance commitment.A transport network slice is a virtual (logical) network with a
particular network topology and a set of shared or dedicated network
resources, which are used to provide the network slice consumer with the
required connectivity, appropriate isolation and specific Service Level
Objective (SLO).A transport network slice could span multiple technologies (such as
IP or Optical) and multiple administrative domains. Depending on the
consumer's requirement, a transport network slice could be isolated from
other, often concurrent transport network slices in terms of data plane,
control plane, and management plane resources.In this document the term "network slice" refers to a transport
network slice, and is considered as one typical use case of enhanced
VPN.Network slicing builds on the concept of resource management, network
virtualization, and abstraction to provide performance assurance,
flexibility, programmability and modularity. It may use techniques such
as Software Defined Networking (SDN) , network
abstraction and Network Function Virtualization
(NFV) to create
multiple logical (virtual) networks, each tailored for a set of services
or a particular tenant or a group of tenants that share the same or
similar set of requirements, on top of a common network. How the network
slices are engineered can be deployment-specific.VPN+ could be used to form the underpinning of transport network
slice, but could also be of use in general cases providing enhanced
connectivity services between customer sites.The requirement of enhanced VPN services cannot be met by simple
overlay networks, as they require tighter coordination and integration
between the underlay and the overlay network. VPN+ is built from a VPN
overlay and a underlying Virtual Transport Network (VTN) which has a
customized network topology and a set of dedicated or shared network
resources. It may optionally include a set of invoked service functions
allocated from the underlay network. Thus an enhanced VPN can achieve
greater isolation with strict performance guarantees. These new
properties, which have general applicability, may also be of interest as
part of a network slicing solution. It is not envisaged that VPN+
services will replace traditional VPN services that can continue to be
deployed using pre- existing mechanisms.This document specifies a framework for using existing, modified, and
potential new technologies as components to provide a VPN+ service.
Specifically we are concerned with:The design of the enhanced data plane.The necessary 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) is met, and to take any corrective
action to avoid SLA violation, such as switching to an alternate
path.The required layered network structure to achieve this is shown in
.Note that, in this document, the relationship of the four terms
"VPN", "VPN+", "VTN", and "Transport Network Slice" are described as
below:A VPN refers to the overlay virtual private network which
provides the required service connectivity and traffic separation
between different VPN customers.A Virtual Transport Network (VTN) is a virtual underlay network
that connects customer edge points with the additional capability of
providing the isolation and performance characteristics required by
an enhanced VPN customer.An enhanced VPN (VPN+) can be considered as an evolution of VPN
service, but with additional service-specific commitments. An
enhanced VPN (VPN+) is made by integrating an overlay VPN and a VTN
with a set of network resources allocated in the underlay
network.A transport network slice could be provided with an enhanced VPN
(VPN+).The following terms are used in this document. Some of them are newly
defined, some others reference existing definitions:ACTN: Abstraction and Control of TE Networks Detnet: Deterministic Networking FlexE: Flexible Ethernet TSN: Time Sensitive Networking VN: Virtual Network VPN: Virtual Private Network. IPVPN is defined in , L2VPN is defined in .VPN+: Enhanced VPN service. An enhanced VPN service (VPN+) can be
considered as an evolution of VPN service, but with additional
service-specific commitments such as enhanced isolation and performance
guarantee.VTP: Virtual Transport Path. A VTP is a virtual underlay path which
connects two customer edge points with the capability of providing the
isolation and performance characteristics required by an enhanced VPN
customer. A VTP usually has a customized path with a set of reserved
network resources along the path.VTN: Virtual Transport Network. A VTN is a virtual underlay network
that connects customer edge points with the capability of providing the
isolation and performance characteristics required by an enhanced VPN
customer. A VTN usually has a customized topology and a set of dedicated
or shared network resources.In this section we provide an overview of the requirements of an
enhanced VPN service.One element of the SLA demanded for an enhanced VPN is a guarantee
that the service offered to the customer will not be perturbed by any
other traffic flows in the network. One way for a service provider to
guarantee the customer's SLA is by controlling the degree of isolation
from other services in the network. Isolation is a feature that can be
requested by customers. There are different grades of how isolation
may be enabled by a network operator and that may result in different
levels of service perceived by the customer. These range from simple
separation of service traffic on delivery (ensuring that traffic is
not delivered to the wrong customer), all the way to complete
separation within the underlay so that the traffic from different
services use distinct network resources.The terms hard and soft isolation are used to illustrate different
levels of isolation. A VPN has soft isolation if the traffic of one
VPN cannot be received by the customers of another VPN. Both IP and
MPLS VPNs are examples of VPNs with soft isolation: the network
delivers the traffic only to the required VPN endpoints. However, with
soft isolation, traffic from VPNs and regular non-VPN traffic may
congest the network resulting in packet loss and delay for other VPNs
operating normally. The ability for a VPN service or a group of VPN
services to be sheltered from this effect is called hard isolation,
and this property is required by some applications. Hard isolation is
needed so that applications with exacting requirements can function
correctly, despite other demands (perhaps a burst of traffic in
another VPN) competing for the underlying resources. In practice
isolation may be offered as a spectrum between soft and hard, and in
some cases soft and hard isolation may be used in a hierarchical
manner. An operator may offer its customers a choice of different
degrees of isolation ranging from soft isolation up to hard
isolation.An example of the requirement for hard isolation is a network
supporting both emergency services and public broadband multi-media
services. During a major incident the VPNs supporting these services
would both be expected to experience high data volumes, and it is
important that both make progress in the transmission of their data.
In these circumstances the VPN services would require an appropriate
degree of isolation to be able to continue to operate acceptably. On
the other hand, VPNs servicing ordinary bulk data may expect to
contest for network resources and queue packets so that traffic is
delivered within SLAs, but with some potential delays and
interference.In order to provide the required level of isolation, resources may
have to be reserved in the data plane of the underlay network and
dedicated to traffic from a specific VPN or a specific group of VPNs
to form different enhanced VPNs in the network. This may introduce
scalability concerns, thus some trade-off needs to be considered to
provide the required isolation between some enhanced VPNs while still
allowing reasonable sharing.An optical layer can offer a high degree of isolation, at the cost
of allocating resources on a long term and end-to-end basis. On the
other hand, where adequate isolation can be achieved at the packet
layer, this permits the resources to be shared amongst a group of
services and only dedicated to a service on a temporary basis.There are several new technologies that provide some assistance
with these data plane issues. Firstly there is the IEEE project on
Time Sensitive Networking which introduces the
concept of packet scheduling of delay and loss sensitive packets. Then
there is which provides the ability to
multiplex multiple channels over one or more Ethernet links in a way
that provides hard isolation. Finally there are advanced queueing
approaches which allow the construction of virtual sub-interfaces,
each of which is provided with dedicated resource in a shared physical
interface. These approaches are described in more detail later in this
document. explores
pragmatic approaches to isolation in packet networks.A key question is whether it is possible to achieve hard
isolation in packet networks that were never designed to support
hard isolation. On the contrary, they were designed to provide
statistical multiplexing, a significant economic advantage when
compared to a dedicated, or a Time Division Multiplexing (TDM)
network. However, there is no need to provide any harder isolation
than is required by the applications. An approximation to this
requirement is sufficient in most cases. Pseudowires emulate services that would have had hard
isolation in their native form.This spectrum of isolation is shown in :Figure 1 shows the spectrum of isolation that may be delivered by
a network. At one end of the figure, we have traditional statistical
multiplexing technologies that support VPNs. This is a service type
that has served the industry well and will continue to do so. At the
opposite end of the spectrum, we have the absolute isolation
provided by dedicated transport networks. The goal of enhanced VPNs
is "pragmatic isolation". This is isolation that is better than is
obtainable from pure statistical multiplexing, more cost effective
and flexible than a dedicated network, but is a practical solution
that is good enough for the majority of applications. Mechanisms for
both soft isolation and hard isolation would be needed to meet
different levels of service requirement.There are several kinds of performance guarantee, including
guaranteed maximum packet loss, guaranteed maximum delay, and
guaranteed delay variation. Note that these guarantees apply to
conformance traffic, out-of-profile traffic will be handled according
to other requirements.Guaranteed maximum packet loss is a common parameter, and is
usually addressed by setting packet priorities, queue size, and
discard policy. However this becomes more difficult when the
requirement is combined with latency requirements. The limiting case
is zero congestion loss, and that is the goal of the Deterministic
Networking work that the IETF and IEEE are pursuing. In modern optical networks, loss due to
transmission errors already approaches zero, but there are the
possibilities of failure of the interface or the fiber itself. This
can only be addressed by some form of signal duplication and
transmission over diverse paths.Guaranteed maximum latency is required in a number of applications
particularly real-time control applications and some types of virtual
reality applications. The work of the IETF Deterministic Networking
(DetNet) Working Group is relevant, however
additional methods of enhancing the underlay to better support the
delay guarantees may be needed, and these methods will need to be
integrated with the overall service provisioning mechanisms.Guaranteed maximum delay variation is a service that may also be
needed. calls up a number of cases where this
is needed, for example in electrical utilities. Time transfer is one
example of a service that needs this, although it is in the nature of
time that the service might be delivered by the underlay as a shared
service and not provided through different enhanced VPNs.
Alternatively a dedicated enhanced VPN may be used to provide this as
a shared service.This suggests that a spectrum of service guarantee be considered
when deploying an enhanced VPN. As a guide to understanding the design
requirements we can consider four types:Best effortAssured bandwidthGuaranteed latencyEnhanced deliveryBest effort service is the basic service that current VPNs can
provide.An assured bandwidth service is one in which the bandwidth over
some period of time is assured. This can be achieved either simply
based on best effort with over-capacity provisioning, or it can be
based on TE-LSPs with bandwidth reservation. The instantaneous
bandwidth is however, not necessarily assured, depending on the
technique used. Providing assured bandwidth to VPNs, for example by
using per-VPN TE-LSPs, is not widely deployed at least partially due
to scalability concerns. VPN+ aims to provide a more scalable approach
for such kind of service.A guaranteed latency service has a latency upper bound provided by
the network. Assuring the upper bound is sometimes more important than
minimizing latency. There are several new technologies that provide
some assistance with performance guarantee. Firstly there is the IEEE
project on Time Sensitive Networking which
introduces the concept of packet scheduling of delay and loss
sensitive packets. Then the DetNet work is also of greater relevance
in assuring upper bound of end-to-end packet latency. Flex Ethernet
is also useful to provide these guarantees. The
usage of such underlying technologies for VPN+ service 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 mechanism may need to be used for VPN+ service.The only way to achieve the enhanced characteristics provided by an
enhanced VPN (such as guaranteed or predicted performance) is by
integrating the overlay VPN with a particular set of network resources
in the underlay network which are allocated to meet the service
requirement. This needs be done in a flexible and scalable way so that
it can be widely deployed in operator networks to support a reasonable
number of enhanced VPN customers.Taking mobile networks and in particular 5G into consideration, the
integration of network and the service functions is a likely
requirement. The work in IETF SFC working group
provides a foundation for this integration.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 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
.Enhanced VPNs need to be created, modified, and removed from the
network according to service demand. An enhanced VPN that requires
hard isolation () must not be disrupted by
the instantiation or modification of another enhanced VPN. Determining
whether modification of an enhanced VPN can be disruptive to that VPN,
and in particular whether the traffic in flight will be disrupted can
be a difficult problem.The data plane aspects of this problem are discussed further in
Sections , 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 VPN and to the underlay transport
network need to be managed to avoid disruption to services that are
sensitive to the change of network performance.In addition to non-disruptively managing the network as a result of
gross change such as the inclusion of a new VPN endpoint or a change
to a link, VPN traffic might need to be moved as a result of traffic
volume changes.In some cases it is desirable that an enhanced VPN has a customized
control plane, so that the tenant of the enhanced VPN can have some
control of how the resources and functions allocated to this enhanced
VPN are used. For example, the tenant may be able to specify the
service paths in his own enhanced VPN. Depending on the requirement,
an enhanced VPN may have its own dedicated controller, which may be
provided with an interface to the control system provided by the
network operator. Note that such control is within the scope of the
tenant's enhanced VPN, any change beyond that would require some
intervention of the operator.A description of the control plane aspects of this problem are
discussed further in . A description of
the management plane aspects of this feature can be found in .The technologies described in this document should be applicable to
a number types of VPN overlay services such as:Layer 2 point-to-point services such as pseudowires Layer 2 VPNs Ethernet VPNs Layer 3 VPNs , Where such VPN 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 services may span multiple
network domains. A domain is considered to be any collection of
network elements within a common realm of address space or path
computation responsibility . In some domains
the operator may manage a multi-layered network, for example, a packet
network over an optical network. When enhanced VPNs are provisioned in
such network scenarios, the technologies used in different network
planes (data plane, control plane, and management plane) need to
provide mechanisms to support multi-domain and multi-layer
coordination and integration, so as to provide the required service
characteristics for different enhanced VPNs, and improve network
efficiency and operational simplicity.A number of enhanced VPN services will typically be provided by a
common network infrastructure. Each enhanced VPN consists of both the
overlay and a corresponding VTN with a specific set of network resources
and functions allocated in the underlay to satisfy the needs of the VPN
tenant. The integration between overlay and various underlay resources
ensures the required isolation between different enhanced VPNs, and
achieves the guaranteed performance for different services.An enhanced VPN needs to be designed with consideration given to:A enhanced data planeA 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 required characteristics are expanded below:Enhanced data planeProvides the required resource isolation capability, e.g.
bandwidth guarantee.Provides the required packet latency and jitter
characteristics.Provides the required packet loss characteristics.Provides the mechanism to associate a packet with the set of
resources allocated to the enhanced VPN which the packet
belongs.Control planeCollect information about the underlying network topology and
resources available and export this to nodes in the network
and/or the centralized controller as required.Create the required virtual transport networks (VTNs) with
the resource and properties needed by the enhanced VPN services
that are assigned to them.Determine the risk of SLA violation and take appropriate
avoiding action.Determine the right balance of per-packet and per-node state
according to the needs of enhanced VPN service to scale to the
required size.Management planeProvides an interface between the enhanced VPN provider (e.g.
the Transport Network (TN) Manager) and the enhanced VPN clients
(e.g. the 3GPP Management System) such that some of the
operation requests can be met without interfering with the
enhanced VPN of other clients.Provides an interface between the enhanced VPN provider and
the enhanced VPN clients to expose transport network capability
information toward the enhanced VPN client.Provides the service life-cycle management and operation of
enhanced VPN (e.g. creation, modification, assurance/monitoring
and decommissioning).Operations, Administration, and Maintenance (OAM) Provides the OAM tools to verify the connectivity and
performance of the enhanced VPN.Provide the OAM tools to verify whether the underlay network
resources are correctly allocated and operated properly.TelemetryProvides the mechanism to collect the data plane, control
plane and management plane data of the network, more
specifically:Provides the mechanism to collect network data from the
underlay network for overall performance evaluation and the
enhanced VPN service planning.Provides the mechanism to collect network data of each
enhanced VPN for the monitoring and analytics of the
characteristics and SLA fulfilment of enhanced VPN
services.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 partition, such as
FlexE, Time Sensitive Networking, Deterministic Networking, Dedicated
queues, etc. These partitions may be physical, or virtual so long as
the SLA required by the higher layers is met.Based on the network resources provided by the physical network
infrastructure, multiple VTNs can be provisioned, each with customized
topology and other attributes to meet the requirement of different
enhanced VPNs or different groups of enhanced VPNs. To get the
required characteristic, each VTN needs to be mapped to a set of
network nodes and links in the network infrastructure. And on each
node or link, the VTN is associated with a set of resources which are
allocated for the processing of traffic in the VTN. VTN provides the
integration between the virtual network topology and the required
underlying network resources.The centralized controller is used to create the VTN, and to
instruct the network nodes to allocate the required resources to each
VTN and to provision the enhanced VPN services on the VTNs. A
distributed control plane may also be used for the distribution of the
VTN topology and attribute information between nodes within the
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 tenants (i.e., of all customers and their associated VTNs), and to
partition the resources accordingly. However, within a VTN these
resources can, if required, be managed via a dynamic control plane.
This provides the required scalability and isolation.At the VPN service level, the required connectivity is usually mesh
or partial-mesh. To support such kinds of VPN service, the
corresponding VTN in underlay is also an abstract MP2MP medium. Other
service requirements may be expressed at different granularity, some
of which can be applicable to the whole service, while some others may
be only applicable to some pairs of end points. For example, when
particular level of performance guarantee is required, the
point-to-point path through the underlay of the enhanced VPN may need
to be specifically engineered to meet the required performance
guarantee.Although a lot of the traffic that will be carried over the
enhanced VPN will likely be IPv4 or IPv6, the design has to be capable
of carrying other traffic types, in particular Ethernet traffic. This
is easily accomplished through the various pseudowire (PW) techniques
. 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 are able to deliver services without
placing per-service state in the core of the underlay network.Enhanced VPNs may need to install some additional state within the
network to achieve the additional features that they require.
Solutions must consider minimizing and controlling the scale of such
state, and deployment architectures should constrain the number of
enhanced VPNs that would exist where such services would place
additional state in the network. It is expected that the number of
enhanced VPN would be small in the beginning, and even in future the
number of enhanced VPN will be much fewer than traditional VPNs,
because pre-existing VPN techniques are be good enough to meet the
needs of most existing VPN-type services.In general, it is not required that the state in the network be
maintained in a 1:1 relationship with the VPN+ services. It will
usually be possible to aggregate a set of VPN+ services so that they
share the same VTN and the same set of network resources (much in the
way that current VPNs are aggregated over transport tunnels) so that
collections of enhanced VPNs that require the same behaviour from the
network in terms of resource reservation, latency bounds, resiliency,
etc. are able to be grouped together. This is an important feature to
assist with the scaling characteristics of VPN+ deployments.See for a greater
discussion of scalability considerations.A VPN is a network created by applying a demultiplexing technique to
the underlying network (the underlay) in order to distinguish the
traffic of one VPN from that of another. A VPN path that travels by
other than the shortest path through the underlay normally requires
state in the underlay to specify that path. State is normally applied to
the underlay through the use of the RSVP signaling protocol, or directly
through the use of an SDN controller, although other techniques may
emerge as this problem is studied. This state gets harder to manage as
the number of VPN paths increases. Furthermore, as we increase the
coupling between the underlay and the overlay to support the enhanced
VPN service, this state will increase further.In an enhanced VPN different subsets of the underlay resources can be
dedicated to different enhanced VPNs or different groups of enhanced
VPNs. An enhanced VPN solution thus needs tighter coupling with underlay
than is the case with existing VPNs. We cannot, for example, share the
network resource between enhanced VPNs which require hard isolation.A number of candidate Layer 2 packet or frame-based data plane
solutions which can be used provide the required isolation and
guarantees are described in following sections.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 tenant, the tenant can use some methods to
manage the relative priority of his own traffic in the FlexE
channel.DiffServ based queuing systems are described in and . This is considered
insufficient to provide isolation for enhanced VPNs because DiffServ
does not always provide enough markers to differentiate between
traffic of many enhanced VPNs, or offer the range of service classes
that each VPN needs to provide to its tenants. This problem is
particularly acute with an MPLS underlay, because MPLS only provides
eight Traffic Classes.In addition, DiffServ, as currently implemented, mainly provides
per-hop priority-based scheduling, and it is difficult to use it to
achieve quantitive resource reservation.In order to address these problems and to reduce the potential
interference between enhanced VPNs, it would be necessary to steer
traffic to dedicated input and output queues per enhanced VPN: some
routers have a large number of queues and sophisticated queuing
systems, which could support this, while some routers may struggle
to provide the granularity and level of isolation required by the
applications of enhanced VPN.Time Sensitive Networking (TSN) is an IEEE
project that is designing a method of carrying time sensitive
information over Ethernet. It introduces the concept of packet
scheduling where a packet stream may be given a time slot
guaranteeing that it experiences no queuing delay or increase in
latency. The mechanisms defined in TSN can be used to meet the
requirements of time sensitive services of an enhanced VPN.Ethernet can be emulated over a Layer 3 network using an IP or
MPLS pseudowire. However, a TSN Ethernet payload would be opaque to
the underlay and thus not treated specifically as time sensitive
data. The preferred method of carrying TSN over a Layer 3 network is
through the use of deterministic networking as explained in .We now consider the problem of slice 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 point- to-point Virtual
Transport Path (VTP) across the underlay network to support VPNs.
VPN traffic can be carried over dedicated TE-LSPs to provide
reserved bandwidth for each specific connection in a VPN, and VPNs
with similar behaviour requirements may be multiplexed onto the same
TE-LSPs. Some network operators have concerns about the scalability
and management overhead of MPLS-TE system, and this has lead them to
consider other solutions for their networks.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 with hints about priority provided through the use of the
traffic class (TC) or Differentiated Services Code Point (DSCP)
field in the header. However to achieve the latency and isolation
characteristics that are sought by the enhanced VPN users, steering
packets through specific queues and resources will likely be
required. With SR, it is possible to introduce such fine-grained
packet steering by specifying the queues and resources through an SR
instruction list.Note that the concept of queue is a useful abstraction for
different types of underlay mechanism that may be used to provide
enhanced isolation and latency support. How the queue satisfies the
requirement is implementation specific and is transparent to the
layer-3 data plane and control plane mechanisms used.With Segment Routing, the SR instruction list could be used to
build a P2P path, a group of SR SIDs could also be used to represent
a MP2MP network. Thus the SR based mechanism could be used to
provide both Virtual Transport Path (VTP) and Virtual Transport
Network (VTN) for enhanced VPN services.Non-packet underlay data plane technologies often have TE
properties and behaviours, and meet many of the key requirements in
particular for bandwidth guarantees, traffic isolation (with physical
isolation often being an integral part of the technology), highly
predictable latency and jitter characteristics, measurable loss
characteristics, and ease of identification of flows. The cost is the
resources are allocated on a long term and end-to-end basis. Such an
arrangement means that the full cost of the resources has be borne by
the service that is allocated with the resources.Enhanced VPN would likely be based on a hybrid control mechanism,
which takes advantage of the logically centralized controller for
on-demand provisioning and global optimization, whilst still relying
on a distributed control plane to provide scalability, high
reliability, fast reaction, automatic failure recovery, etc. Extension
to and optimization of the distributed control plane is needed to
support the enhanced properties of VPN+.RSVP-TE 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 Virtual
Transport Path (VTP), which could be used to bind the VPN to specific
network resources allocated within the underlay, but there remain
scalability concerns mentioned in .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 enhanced VPNs, 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 attributes associated with enhanced
VPNs, and compute the SR paths with specific constraints of enhanced
VPN services.The management plane provides the interface between the enhanced
VPN provider and the clients for the service life-cycle management
(e.g. creation, modification, assurance/monitoring and
decommissioning). It relies on a set of service data models for the
description of the information and operations needed on the
interface.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
end-to-end network slice. 3GPP management system may provide the
connectivity and performance related parameters as requirements to the
management plane of the transport network. It may also require the
transport network to expose the capability and status of the transport
network slice. Thus, an interface between the enhanced VPN management
plane and the 3GPP network slice management system, and relevant
service data models are needed for the coordination of end-to-end
network slice management.The management plane interface and data models for enhanced VPN can
be based on the service models described in ACTN supports operators in viewing and controlling different
domains and presenting virtualized networks to their customers. The
ACTN framework 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 and applications (tenants) with the
capability to utilize and independently control allocated virtual
network resources as if they were physically their own resources.
Service Data models are used to represent, monitor, and manage the
virtual networks and services enabled by ACTN. The Customer VPN model
(e.g. L3SM , L2SM ) or
an ACTN Virtual Network (VN) model is a customer view of the
ACTN managed infrastructure, and is presented by the ACTN provider as
a set of abstracted services or resources. The L3VPN network model
and provide a network view of the ACTN
managed infrastructure presented by the ACTN provider as a set of
transport resources.In order to support network slice service in transport network, a
Transport Slice (TS) Northbound Interface (NBI) data model may be
needed for a consumer to express the requirements for transport
slices, which can be technology-agnostic. Then these requirements
may be realized using technology-specific Southbound Interface
(SBI).As per and , the CNC-MDSC Interface (CMI) of
ACTN is used to convey the virtual network service requirements,
which is a generic interface to deliver various TE based VN
services. In the context of network slice northbound interface,
there may be some gaps in L3SM/L2SM or VN model, or the combination
of them. The TS NBI is required to communicate the connectivity of
the transport slice, along with the service level objective (SLO)
parameters and traffic selection rules, and provides a way to
monitor the state of the transport slice. This can be described in a
more abstracted manner, so as to reduce the association with
specific realization technologies of transport network slice, such
as the VPN and TE technologies. The transport slice model as defined
in provides an
abstracted and generic approach to meet the transport slice NBI
requirement.The MDSC-PNC Interface (MPI) models in the ACTN architecture can
be used for the realization of transport slices, for example, in a
TE enabled transport network, and may also be used for cross-layer
or cross-domain implementation of transport slice.Enhanced VPN provides performance guaranteed services in packet
networks, but with the potential cost of introducing additional states
into the network. There are at least three ways that this additional
state might be presented in the network:Introduce the complete state into the packet, as is done in SR.
This allows the controller to specify the detailed series of
forwarding and processing instructions for the packet as it transits
the network. The cost of this is an increase in the packet header
size. The cost is also that systems will have capabilities enabled
in case they are called upon by a service. This is a type of latent
state, and increases as we more precisely specify the path and
resources that need to be exclusively available to a VPN.Introduce the state to the network. This is normally done by
creating a path using RSVP-TE, which can be extended to introduce
any element that needs to be specified along the path, for example
explicitly specifying queuing policy. It is possible to use other
methods to introduce path state, such as via a Software Defined
Network (SDN) controller, or possibly by modifying a routing
protocol. With this approach there is state per path, per path
characteristic that needs to be maintained over its life-cycle. This
is more state than is needed using SR, but the packets are
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 needed to be created and maintained in the network, as
the state about fine granularity processing would need to be loaded and
maintained in the routers. However, a segment routed approach allows
much of this state to be spread amongst the network ingress nodes, and
transiently carried in the packets as SIDs.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 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 as the number of paths through an LSR grows. proposes to address 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 favourably with the need for a control plane to maintain
communication with all neighbors.However, SDN may transfer some of the scalability concerns from the
network to the centralized controller. In particular, there may be a
heavy processing burden at the controller, and a heavy load in the
network surrounding the controller.The enhanced VPN OAM design 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 , 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. Additionally, as the
network as a whole 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. This
is a single action by the head-end changes the path without the need for
coordinated action by the routers along the path. However,
implementations and the monitoring protocols need to make sure that the
new path is up and meets the required SLA before traffic is transitioned
to it. It is possible for deadlocks to arise as a result of the network
becoming fragmented over time, such that it is impossible to create a
new path or to modify an existing path without impacting the SLA of
other paths. Resolution of this situation is as much a commercial issue
as it is a technical issue and is outside the scope of this
document.There are, however, two manifestations of the latency problem that
are for further study in any of these approaches:The problem of packets overtaking one and other 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 proposed by the IETF deterministic network work with multiple
in-network replication and the culling of later packets .In addition to the approach used to protect high priority packets,
consideration has to be given to the impact of best effort traffic on
the high priority packets during a transient. Specifically if a
conventional re-convergence process is used there will inevitably be
micro-loops and whilst some form of explicit routing will protect the
high priority traffic, lower priority traffic on best effort shortest
paths will micro-loop without the use of a loop prevention technology.
To provide the highest quality of service to high priority traffic,
either this traffic must be shielded from the micro-loops, or
micro-loops must be prevented.It is likely that enhanced VPN service will be introduced in networks
which already have traditional VPN services deployed. Depends on service
requirement, the tenants or the operator may choose to use traditional
VPN or enhanced VPN to fulfil the service requirement. The information
and parameters to assist such decision needs to be reflected on the
management interface between the tenants and the operator.All types of virtual network require special consideration to be
given to the isolation of traffic belonging to different tenants. That
is, traffic belonging to one VPN must not be delivered to end points
outside that VPN. In this regard enhanced VPNs neither introduce, no
experience a greater security risks than other VPNs.However, in an enhanced Virtual Private Network service the
additional service requirements need to be considered. For example, if a
service requires a specific upper bound to latency then it can be
damaged by simply delaying the packets through the activities of another
tenant, i.e., by introducing bursts of traffic for other services.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 and Shunsuke Homma 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