TEAS Working Group J. Dong
Internet-Draft Huawei
Intended status: Informational S. Bryant
Expires: August 14, 2021 Futurewei
Z. Li
China Mobile
T. Miyasaka
KDDI Corporation
Y. Lee
Samsung
February 10, 2021
A Framework for Enhanced Virtual Private Network (VPN+) Services
draft-ietf-teas-enhanced-vpn-07
Abstract
This 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 and above
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.
Compared to traditional VPNs, it is not envisaged that 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+ techniques.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on August 14, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Overview of the Requirements . . . . . . . . . . . . . . . . 6
3.1. Performance Guarantees . . . . . . . . . . . . . . . . . 6
3.2. Isolation between Enhanced VPN Services . . . . . . . . . 8
3.2.1. A Pragmatic Approach to Isolation . . . . . . . . . . 10
3.3. Integration . . . . . . . . . . . . . . . . . . . . . . . 11
3.3.1. Abstraction . . . . . . . . . . . . . . . . . . . . . 11
3.4. Dynamic Changes . . . . . . . . . . . . . . . . . . . . . 12
3.5. Customized Control . . . . . . . . . . . . . . . . . . . 12
3.6. Applicability . . . . . . . . . . . . . . . . . . . . . . 13
3.7. Inter-Domain and Inter-Layer Network . . . . . . . . . . 13
4. Architecture of Enhanced VPNs . . . . . . . . . . . . . . . . 14
4.1. Layered Architecture . . . . . . . . . . . . . . . . . . 15
4.2. Multi-Point to Multi-Point (MP2MP) Connectivity . . . . . 18
4.3. Application Specific Data Types . . . . . . . . . . . . . 18
4.4. Scaling Considerations . . . . . . . . . . . . . . . . . 18
5. Candidate Technologies . . . . . . . . . . . . . . . . . . . 19
5.1. Layer-Two Data Plane . . . . . . . . . . . . . . . . . . 19
5.1.1. Flexible Ethernet . . . . . . . . . . . . . . . . . . 19
5.1.2. Dedicated Queues . . . . . . . . . . . . . . . . . . 20
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5.1.3. Time Sensitive Networking . . . . . . . . . . . . . . 20
5.2. Layer-Three Data Plane . . . . . . . . . . . . . . . . . 21
5.2.1. Deterministic Networking . . . . . . . . . . . . . . 21
5.2.2. MPLS Traffic Engineering (MPLS-TE) . . . . . . . . . 21
5.2.3. Segment Routing . . . . . . . . . . . . . . . . . . . 21
5.3. Non-Packet Data Plane . . . . . . . . . . . . . . . . . . 22
5.4. Control Plane . . . . . . . . . . . . . . . . . . . . . . 22
5.5. Management Plane . . . . . . . . . . . . . . . . . . . . 23
5.6. Applicability of Service Data Models to Enhanced VPN . . 24
5.6.1. An Example of Enhanced VPN Delivery . . . . . . . . . 25
6. Scalability Considerations . . . . . . . . . . . . . . . . . 26
6.1. Maximum Stack Depth of SR . . . . . . . . . . . . . . . . 27
6.2. RSVP-TE Scalability . . . . . . . . . . . . . . . . . . . 27
6.3. SDN Scaling . . . . . . . . . . . . . . . . . . . . . . . 27
7. OAM Considerations . . . . . . . . . . . . . . . . . . . . . 28
8. Telemetry Considerations . . . . . . . . . . . . . . . . . . 28
9. Enhanced Resiliency . . . . . . . . . . . . . . . . . . . . . 29
10. Operational Considerations . . . . . . . . . . . . . . . . . 30
11. Security Considerations . . . . . . . . . . . . . . . . . . . 30
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 31
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
15.1. Normative References . . . . . . . . . . . . . . . . . . 32
15.2. Informative References . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
1. Introduction
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 stricter 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 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 [NGMN-NS-Concept] [TS23501] [TS28530]
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[BBF-SD406]. According to [TS28530], 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 connectivity between different
entities in RAN and CN segments of a 5G end-to-end network slice,
with specific performance commitment.
An IETF network slice [I-D.ietf-teas-ietf-network-slice-definition]
is a virtual (logical) network with its own 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 a specific Service Level Objective (SLO).
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
consumer'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) [RFC7149],
network abstraction [RFC7926] and Network Function Virtualization
(NFV) [RFC8172] [RFC8568] 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,
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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.
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:
o The functional requirements and service characteristics of an
enhanced VPN.
o The design of the enhanced data plane.
o The necessary protocols in both the underlay and the overlay of
the enhanced VPN.
o The mechanisms to achieve integration between overlay and
underlay.
o 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
Section 4.1.
Note that, in this document, the relationship of the four terms
"VPN", "VPN+", "VTN", and "Network Slice" are as follows:
o A VPN refers to the overlay network that provides the connectivity
between different VPN sites, and that maintains traffic separation
between different VPN customers.
o 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.
o A VTN is a virtual underlay network that connects customer edge
points. The VTN has the capability to deliver the performance
characteristics required by an enhanced VPN customer and to
provide isolation between separate VPN+ instances.
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o A network slice could be provided by building an enhanced VPN.
2. Terminology
The following terms are used in this document. Some of them are
newly defined, some others reference existing definitions.
ACTN: Abstraction and Control of Traffic Engineered Networks
[RFC8453]
DetNet: Deterministic Networking. See [DETNET] and [RFC8655]
FlexE: Flexible Ethernet [FLEXE]
TSN: Time Sensitive Networking [TSN]
VN: Virtual Network [I-D.ietf-teas-actn-vn-yang]
VPN: Virtual Private Network. IPVPN is defined in [RFC2764], L2VPN
is defined in [RFC4664], and L3VPN is defined in [RFC4364].
VPN+: Enhanced VPN.
VTN: Virtual Transport Network.
VTP: Virtual Transport Path. A VTP is a path through the VTN which
connects two customer edge points.
3. Overview of the Requirements
This section provides an overview of the requirements of an enhanced
VPN service.
3.1. Performance Guarantees
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
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requirements. The limiting case is zero congestion loss, and that is
the goal of DetNet [DETNET] and TSN [TSN]. In modern optical
networks, loss due to transmission errors already approaches zero,
but there is the possibilities 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 [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. [RFC8578] calls up a number of cases where 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:
o Best effort
o Assured bandwidth
o Guaranteed latency
o Enhanced delivery
The 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
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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.
3.2. Isolation between Enhanced VPN Services
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 SLO.
One way for a network operator to meet the requirement for isolation
is simply by setting and conforming to other 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 SLA is to control the
degree to which traffic from one service is isolated from other
services in the network. There are different grades of how isolation
may be enabled by a network operator and this 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, which is a basic requirement of
all existing VPN services), all the way to complete separation within
the underlay so that the traffic from different services use distinct
network resources.
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
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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 may 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.
Delivery of isolation is achieved in the realization of a VPN+
through existing technologies that may be supplemented by future
mechanisms. The service provider chooses which processes to use to
deliver this service requirement just as they choose how to meet all
other SLOs. Isolation may be achieved in the underlying network by
various forms of resource partitioning ranging from dedicated
allocation of resources for a specific enhanced VPN, to sharing of
resources with some form of safeguards. 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 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, as the network resources are shared, traffic
from VPNs and regular non-VPN traffic may congest the network
resulting in packet loss and delay for other VPNs. The ability for a
VPN service or a group of VPN 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 VPN)
competing for the underlying resources. An operator may offer its
customers a choice of different degrees of isolation ranging from
soft isolation to hard isolation. 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 with one
enhanced VPN being built on another.
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
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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. While the VPN for the emergency service could be
provided by specifying hard SLOs (for bandwidth, latency, etc.) the
customer may feel more comfortable with an SLO that specifies hard
isolation, and the service provider may decided that the best way to
ensure that the SLA is met is to utilize hard isolation.
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 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 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.
There are also several new technologies that provide some assistance
with these data plane issues. Firstly, there is the IEEE's TSN
project which introduces the concept of packet scheduling of delay
and loss sensitive packets. Then there is FlexE which provides the
ability to multiplex multiple channels over one or more Ethernet
links in a way that provides hard isolation. Finally, there are
advanced queuing 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.
The next section explores a pragmatic approach to isolation in packet
networks.
3.2.1. A Pragmatic Approach to Isolation
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.
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For example, pseudowires [RFC3985] emulate services that would have
had hard isolation in their native form.
O=================================================O
| \---------------v---------------/
Statistical Pragmatic Absolute
Multiplexing Isolation Isolation
(Traditional VPNs) (Enhanced VPN) (Dedicated Network)
Figure 1: The Spectrum of Isolation
Figure 1 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 would be needed to meet
different levels of service requirement.
3.3. Integration
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)
[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.
3.3.1. Abstraction
Integration of the overlay VPN and the underlay network resources
does not need to be a tight mapping. As described in [RFC7926],
abstraction is the process of applying policy to a set of information
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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) [RFC8453] as discussed further in Section 5.5.
[I-D.king-teas-applicability-actn-slicing] describes the
applicability of ACTN to network slicing and is, therefore, relevant
to the consideration of using ACTN to enable enhanced VPNs.
3.4. Dynamic Changes
Enhanced VPNs need to be created, modified, and removed from the
network according to service demand. An enhanced VPN that requires
hard isolation (Section 3.2) 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
Section 5.1,Section 5.2, and Section 5.3.
The control plane aspects of this problem are discussed further in
Section 5.4.
The management plane aspects of this problem are discussed further in
Section 5.5.
Dynamic changes both to the VPN and to the underlay transport network
need to be managed to avoid disruption to services that are sensitive
to 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.
3.5. Customized Control
In some cases it is desirable that an enhanced VPN has a customized
control plane, so that the customer of the enhanced VPN can have some
control over how the resources allocated to this enhanced VPN are
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used. For example, the customer may be able to specify the service
paths in their own enhanced VPN. Depending on the requirements, an
enhanced VPN may have its own dedicated controller, which may be
provided with an interface to the control system run by the network
operator. Note that such control is within the scope of the tenant'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 Section 5.4. A description of the management
plane aspects of this feature can be found in Section 5.5.
3.6. Applicability
The concept of an enhanced VPN can be applied to any pre-existing VPN
overlay services including:
o Layer-2 point-to-point services such as pseudowires [RFC3985]
o Layer-2 VPNs [RFC4664]
o Ethernet VPNs [RFC7209]
o Layer-3 VPNs [RFC4364], [RFC2764]
Where such VPN service types need enhanced isolation and delivery
characteristics, the technologies described in Section 5 can be used
to provide an underlay with the required enhanced performance.
3.7. Inter-Domain and Inter-Layer Network
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 [RFC5151] 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.
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4. Architecture of Enhanced VPNs
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. 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:
o An enhanced data plane.
o A control plane to create enhanced VPNs, making use of the data
plane isolation and performance guarantee techniques.
o A management plane for enhanced VPN service life-cycle management.
These topics are expanded below:
o Enhanced data plane
* Provides 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 to which the packet
belongs.
o 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.
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* 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.
o Management plane
* Provides an interface between the enhanced VPN provider (e.g.,
operator's network management system ) and the enhanced VPN
clients (e.g. a customer or service with enhanced VPN
requirement) 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 the network capability
information toward the enhanced VPN client.
* Provides the service life-cycle management and operation of
enhanced VPNs (e.g., creation, modification, assurance/
monitoring, and decommissioning).
o 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.
o Telemetry
* Provides the mechanisms to collect network information about
the operation of the data plane, control plane, and management
plane. More specifically:
+ Provides the mechanisms to collect network data from the
underlay network for overall performance evaluation and for
planning enhanced VPN services.
+ Provides the mechanisms to collect network data for each
enhanced VPN and for monitoring and analytics of the
characteristics and SLA fulfillment of enhanced VPN service.
4.1. Layered Architecture
The layered architecture of an enhanced VPN is shown in Figure 2.
Underpinning everything is the physical network infrastructure layer
which provide the underlying resources used to provision the
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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.
/\
||
+-------------------+ Centralized
| Network Controller| Control & Management
+-------------------+
||
\/
o---------------------------o
/-------------o VPN Service 1
o____________/______________o
_________________o
_____/
o___/ \_________________o VPN Service 2
\_______________________o
......
o-----------\ /-------------o
o____________X______________o VPN Service n
__________________________
/ o----o-----o /
/ / / / VTN-1
/ o-----o-----o----o----o /
/_________________________/
__________________________
/ o----o /
/ / / \ / VTN-2
/ o-----o----o---o------o /
/_________________________/
...... ...
___________________________
/ o----o /
/ / / / VTN-n
/ o-----o----o----o-----o /
/__________________________/
++++ ++++ ++++
+--+===+--+===+--+
+--+===+--+===+--+
++++ +++\\ ++++
|| || \\ || Physical
|| || \\ || Network
++++ ++++ ++++ \\+++ ++++ Infrastructure
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+--+===+--+===+--+===+--+===+--+
+--+===+--+===+--+===+--+===+--+
++++ ++++ ++++ ++++ ++++
o Virtual Node ++++
+--+ Physical Node with resource partition
-- Virtual Link +--+
++++
== Physical Link with resource partition
Figure 2: The Layered Architecture of VPN+
Various components and techniques discussed in Section 5 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 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 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 Section 4.4 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
tenants (i.e., of all customers and their associated enhanced VPNs),
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.
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4.2. Multi-Point to Multi-Point (MP2MP) Connectivity
At the VPN service level, 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 underlay of the
enhanced VPN may need to be specifically engineered to meet the
required performance guarantee.
4.3. Application Specific Data Types
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
[RFC3985]. Where the underlay is MPLS, Ethernet can be carried over
the enhanced VPN encapsulated according to the method specified in
[RFC4448]. Where the underlay is IP, Layer Two Tunneling Protocol -
Version 3 (L2TPv3) [RFC3931] can be used with Ethernet traffic
carried according to [RFC4719]. Encapsulations have been defined for
most of the common Layer-2 types for both PW over MPLS and for
L2TPv3.
4.4. Scaling Considerations
VPNs are instantiated as overlays on top of an operator's network and
offered as services to the operator's customers. An important
feature of overlays is that they 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.
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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.
[I-D.dong-teas-enhanced-vpn-vtn-scalability] provides more details of
scalability considerations for enhanced VPNs, and Section 6 includes
a greater discussion of scalability considerations.
5. Candidate Technologies
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.
In an enhanced VPN, different subsets of the underlay resources can
be dedicated to different enhanced VPNs or different groups of
enhanced VPNs. 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.
5.1. Layer-Two Data Plane
Several candidate Layer 2 packet- or frame-based data plane solutions
which can be used provide the required isolation and guarantees are
described in the following sections.
5.1.1. Flexible Ethernet
FlexE [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.
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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.
5.1.2. Dedicated Queues
DiffServ based queuing systems are described in [RFC2475] and
[RFC4594]. 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.
5.1.3. Time Sensitive Networking
Time Sensitive Networking (TSN) [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.
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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
Section 5.2.1.
5.2. Layer-Three Data Plane
This section considers the problem of enhanced VPN differentiation
and resource representation in the network layer.
5.2.1. Deterministic Networking
Deterministic Networking (DetNet) [RFC8655] 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)
[RFC3758] 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.
5.2.2. MPLS Traffic Engineering (MPLS-TE)
MPLS-TE [RFC2702][RFC3209] 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.
5.2.3. Segment Routing
Segment Routing (SR) [RFC8402] 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.
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An SR traffic engineered path operates with a granularity of a link.
Hints about priority 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 probably 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.
5.3. Non-Packet Data Plane
Non-packet underlay data plane technologies often have TE properties
and behaviors, and meet many of the key requirements in particular
for bandwidth guarantees, traffic isolation (with physical isolation
often being an integral part of the technology), highly predictable
latency and jitter characteristics, measurable loss characteristics,
and ease of identification of flows. 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 be borne by
the service that is allocated with the resources.
5.4. Control Plane
An enhanced VPN would likely be based on a hybrid control mechanism
that 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 centralized and distributed control plane is
needed to support the enhanced properties of VPN+.
RSVP-TE [RFC3209] 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
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resources allocated within the underlay, but there remain scalability
concerns as mentioned in Section 5.2.2.
The control plane of SR [RFC8665] [RFC8667]
[I-D.ietf-idr-bgp-ls-segment-routing-ext] 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
[I-D.ietf-spring-segment-routing-policy] 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.
5.5. Management Plane
The management plane provides the interface between the enhanced VPN
provider and the clients 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
[TS28530], 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 Section 5.6.
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
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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 tenant (the VPN+ 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.
5.6. Applicability of Service Data Models to Enhanced VPN
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[RFC8453] supports operators in viewing and
controlling different domains and presenting virtualized networks to
their customers. It highlights how:
o Abstraction of the underlying network resources is provided to
higher-layer applications and customers.
o Underlying resources are virtualized and allocated for the
customer, application, or service.
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o A virtualized environment is created allowing operators to view
and control multi-domain networks as a single virtualized network.
o 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 [RFC8299], L2SM [RFC8466]) or an ACTN Virtual
Network (VN) [I-D.ietf-teas-actn-vn-yang] 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 [I-D.ietf-opsawg-l3sm-l3nm] and L2VPN network model
[I-D.ietf-opsawg-l2nm] provide a network view of the ACTN managed
infrastructure presented by the ACTN provider as a set of virtual
networks and the associated resources.
[I-D.king-teas-applicability-actn-slicing] 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.
5.6.1. An Example of Enhanced VPN Delivery
One typical use case of enhanced VPN is to instantiate a network
slice. In order to provide network slices to customers, a
technology-agnostic network slice Northbound Interface (NBI) data
model may be needed for the customers to communicate the requirements
and operations of network slices. These requirements may then be
realized using technology-specific Southbound Interface (SBI) to
instruct the network to instantiate an enhanced VPN service to meet
the requirements of the customer.
As per [RFC8453] and [I-D.ietf-teas-actn-yang], the CNC-MDSC
Interface (CMI) of ACTN can be used to convey the virtual network
service requirements, which is a generic interface to deliver various
TE based VN services. In the context of the network slice NBI, there
may be some gaps in the combination of the L3SM/L2SM and VN models.
The NBI is required to communicate the connectivity of the network
slice, along with the SLO parameters and traffic selection rules, and
provides a way to monitor the state of the network slice. This can
be described in a more abstract manner, so as to reduce the
association with specific technologies used to realize the network
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slice, such as the VPN and TE technologies. The network slice NBI
model as defined in [I-D.wd-teas-ietf-network-slice-nbi-yang]
provides an abstract and generic approach to provide the network
slice NBI functions.
The MDSC-PNC Interface (MPI) models in the ACTN architecture can be
used for the realization of network slices, for example, in a TE
enabled network, and may also be used for cross-layer or cross-domain
implementation of network slice.
6. Scalability Considerations
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 present in the network:
o Introduce the complete state into the packet, as is done in SR.
This allows the controller to specify the detailed series of
forwarding and processing instructions for the packet as it
transits the network. The cost of this is an increase in the
packet header size. The cost is also that systems will have
capabilities enabled in case they are called upon by a service.
This is a type of latent state, and increases as we more precisely
specify the path and resources that need to be exclusively
available to a VPN.
o Introduce the state to the network. This is normally done by
creating a path using RSVP-TE, which can be extended to introduce
any element that needs to be specified along the path, for example
explicitly specifying queuing policy. It is 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 its life cycle. This is more network state than
is needed using SR, but the packets are shorter.
o Provide a hybrid approach. One example is based on using binding
SIDs [RFC8402] 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
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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 [I-D.dong-teas-enhanced-vpn-vtn-scalability].
6.1. Maximum Stack Depth of SR
One of the challenges with SR is the stack depth that nodes are able
to impose on packets [RFC8491]. This leads to a difficult balance
between adding state to the network and minimizing stack depth, or
minimizing state and increasing the stack depth.
6.2. RSVP-TE Scalability
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 [RFC8370].
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. [RFC8577] addresses this by employing SR within a
tunnel established by RSVP-TE.
6.3. SDN Scaling
The centralized approach of SDN requires state to be stored in the
network, but does not have the overhead of also requiring control
plane state to be maintained. Each individual network node may need
to maintain a communication channel with the SDN controller, but that
compares 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.
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7. OAM Considerations
The design of OAM for enhanced VPNs needs to consider the following
requirements:
o Instrumentation of the underlay so that the network operator can
be sure that the resources committed to a tenant are operating
correctly and delivering the required performance.
o Instrumentation of the overlay by the tenant. This is likely to
be transparent to the network operator and to use existing
methods. Particular consideration needs to be given to the need
to verify the isolation and the various committed performance
characteristics.
o Instrumentation of the overlay by the network provider to
proactively demonstrate that the committed performance is being
delivered. This needs to be done in a non-intrusive manner,
particularly when the tenant is deploying a performance sensitive
application.
o Verification of the conformity of the path to the service
requirement. This may need to be done as part of a commissioning
test.
A study of OAM in SR networks has been documented in [RFC8403].
8. Telemetry Considerations
Network visibility is essential for network operation. Network
telemetry has been considered as an ideal means to gain sufficient
network visibility with better flexibility, scalability, accuracy,
coverage, and performance than conventional OAM technologies.
As defined in [I-D.ietf-opsawg-ntf], 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.
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9. Enhanced Resiliency
Each enhanced VPN has a life cycle, and may need modification during
deployment as the needs of its tenant change. This is discussed in
Section 5.5. 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:
o The problem of packets overtaking one another if a path latency
reduces during a transition.
o 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 [RFC5654]. An alternative is some form of N+1 delivery
such as has been used for many years to support protection from
service disruption. This may be taken to a different level using the
techniques of DetNet with multiple in-network replication and the
culling of later packets [RFC8655].
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
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priority traffic, either this traffic must be shielded from the
micro-loops, or micro-loops must be prevented completely.
10. Operational Considerations
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.
11. Security Considerations
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.
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12. IANA Considerations
There are no requested IANA actions.
13. Contributors
Daniel King
Email: daniel@olddog.co.uk
Adrian Farrel
Email: adrian@olddog.co.uk
Jeff Tansura
Email: jefftant.ietf@gmail.com
Zhenbin Li
Email: lizhenbin@huawei.com
Qin Wu
Email: bill.wu@huawei.com
Bo Wu
Email: lana.wubo@huawei.com
Daniele Ceccarelli
Email: daniele.ceccarelli@ericsson.com
Mohamed Boucadair
Email: mohamed.boucadair@orange.com
Sergio Belotti
Email: sergio.belotti@nokia.com
Haomian Zheng
Email: zhenghaomian@huawei.com
14. Acknowledgements
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).
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15. References
15.1. Normative References
[RFC2764] Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and A.
Malis, "A Framework for IP Based Virtual Private
Networks", RFC 2764, DOI 10.17487/RFC2764, February 2000,
<https://www.rfc-editor.org/info/rfc2764>.
[RFC3985] Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
Edge-to-Edge (PWE3) Architecture", RFC 3985,
DOI 10.17487/RFC3985, March 2005,
<https://www.rfc-editor.org/info/rfc3985>.
[RFC4664] Andersson, L., Ed. and E. Rosen, Ed., "Framework for Layer
2 Virtual Private Networks (L2VPNs)", RFC 4664,
DOI 10.17487/RFC4664, September 2006,
<https://www.rfc-editor.org/info/rfc4664>.
15.2. Informative References
[BBF-SD406]
"BBF SD-406: End-to-End Network Slicing", 2016,
<https://wiki.broadband-forum.org/display/BBF/SD-406+End-
to-End+Network+Slicing>.
[DETNET] "Deterministic Networking", March ,
<https://datatracker.ietf.org/wg/detnet/about/>.
[FLEXE] "Flex Ethernet Implementation Agreement", March 2016,
<http://www.oiforum.com/wp-content/uploads/OIF-FLEXE-
01.0.pdf>.
[I-D.dong-teas-enhanced-vpn-vtn-scalability]
Dong, J., Li, Z., Qin, F., and G. Yang, "Scalability
Considerations for Enhanced VPN (VPN+)", draft-dong-teas-
enhanced-vpn-vtn-scalability-01 (work in progress),
November 2020.
[I-D.ietf-idr-bgp-ls-segment-routing-ext]
Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
and M. Chen, "BGP Link-State extensions for Segment
Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-16
(work in progress), June 2019.
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[I-D.ietf-opsawg-l2nm]
barguil, s., Dios, O., Boucadair, M., Munoz, L., Jalil,
L., and J. Ma, "A Layer 2 VPN Network YANG Model", draft-
ietf-opsawg-l2nm-01 (work in progress), November 2020.
[I-D.ietf-opsawg-l3sm-l3nm]
barguil, s., Dios, O., Boucadair, M., Munoz, L., and A.
Aguado, "A Layer 3 VPN Network YANG Model", draft-ietf-
opsawg-l3sm-l3nm-05 (work in progress), October 2020.
[I-D.ietf-opsawg-ntf]
Song, H., Qin, F., Martinez-Julia, P., Ciavaglia, L., and
A. Wang, "Network Telemetry Framework", draft-ietf-opsawg-
ntf-06 (work in progress), January 2021.
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", draft-
ietf-spring-segment-routing-policy-09 (work in progress),
November 2020.
[I-D.ietf-teas-actn-vn-yang]
Lee, Y., Dhody, D., Ceccarelli, D., Bryskin, I., and B.
Yoon, "A YANG Data Model for VN Operation", draft-ietf-
teas-actn-vn-yang-10 (work in progress), November 2020.
[I-D.ietf-teas-actn-yang]
Lee, Y., Zheng, H., Ceccarelli, D., Yoon, B., Dios, O.,
Shin, J., and S. Belotti, "Applicability of YANG models
for Abstraction and Control of Traffic Engineered
Networks", draft-ietf-teas-actn-yang-06 (work in
progress), August 2020.
[I-D.ietf-teas-ietf-network-slice-definition]
Rokui, R., Homma, S., Makhijani, K., Contreras, L., and J.
Tantsura, "Definition of IETF Network Slices", draft-ietf-
teas-ietf-network-slice-definition-00 (work in progress),
January 2021.
[I-D.king-teas-applicability-actn-slicing]
King, D., Drake, J., and H. Zheng, "Applicability of
Abstraction and Control of Traffic Engineered Networks
(ACTN) to Network Slicing", draft-king-teas-applicability-
actn-slicing-08 (work in progress), October 2020.
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[I-D.wd-teas-ietf-network-slice-nbi-yang]
Bo, W., Dhody, D., Han, L., and R. Rokui, "A Yang Data
Model for IETF Network Slice NBI", draft-wd-teas-ietf-
network-slice-nbi-yang-01 (work in progress), November
2020.
[NGMN-NS-Concept]
"NGMN NS Concept", 2016, <https://www.ngmn.org/fileadmin/u
ser_upload/161010_NGMN_Network_Slicing_framework_v1.0.8.pd
f>.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.
[RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
McManus, "Requirements for Traffic Engineering Over MPLS",
RFC 2702, DOI 10.17487/RFC2702, September 1999,
<https://www.rfc-editor.org/info/rfc2702>.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.
[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
Conrad, "Stream Control Transmission Protocol (SCTP)
Partial Reliability Extension", RFC 3758,
DOI 10.17487/RFC3758, May 2004,
<https://www.rfc-editor.org/info/rfc3758>.
[RFC3931] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed.,
"Layer Two Tunneling Protocol - Version 3 (L2TPv3)",
RFC 3931, DOI 10.17487/RFC3931, March 2005,
<https://www.rfc-editor.org/info/rfc3931>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>.
[RFC4448] Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron,
"Encapsulation Methods for Transport of Ethernet over MPLS
Networks", RFC 4448, DOI 10.17487/RFC4448, April 2006,
<https://www.rfc-editor.org/info/rfc4448>.
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[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
DOI 10.17487/RFC4594, August 2006,
<https://www.rfc-editor.org/info/rfc4594>.
[RFC4719] Aggarwal, R., Ed., Townsley, M., Ed., and M. Dos Santos,
Ed., "Transport of Ethernet Frames over Layer 2 Tunneling
Protocol Version 3 (L2TPv3)", RFC 4719,
DOI 10.17487/RFC4719, November 2006,
<https://www.rfc-editor.org/info/rfc4719>.
[RFC5151] Farrel, A., Ed., Ayyangar, A., and JP. Vasseur, "Inter-
Domain MPLS and GMPLS Traffic Engineering -- Resource
Reservation Protocol-Traffic Engineering (RSVP-TE)
Extensions", RFC 5151, DOI 10.17487/RFC5151, February
2008, <https://www.rfc-editor.org/info/rfc5151>.
[RFC5654] Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
Sprecher, N., and S. Ueno, "Requirements of an MPLS
Transport Profile", RFC 5654, DOI 10.17487/RFC5654,
September 2009, <https://www.rfc-editor.org/info/rfc5654>.
[RFC7149] Boucadair, M. and C. Jacquenet, "Software-Defined
Networking: A Perspective from within a Service Provider
Environment", RFC 7149, DOI 10.17487/RFC7149, March 2014,
<https://www.rfc-editor.org/info/rfc7149>.
[RFC7209] Sajassi, A., Aggarwal, R., Uttaro, J., Bitar, N.,
Henderickx, W., and A. Isaac, "Requirements for Ethernet
VPN (EVPN)", RFC 7209, DOI 10.17487/RFC7209, May 2014,
<https://www.rfc-editor.org/info/rfc7209>.
[RFC7926] Farrel, A., Ed., Drake, J., Bitar, N., Swallow, G.,
Ceccarelli, D., and X. Zhang, "Problem Statement and
Architecture for Information Exchange between
Interconnected Traffic-Engineered Networks", BCP 206,
RFC 7926, DOI 10.17487/RFC7926, July 2016,
<https://www.rfc-editor.org/info/rfc7926>.
[RFC8172] Morton, A., "Considerations for Benchmarking Virtual
Network Functions and Their Infrastructure", RFC 8172,
DOI 10.17487/RFC8172, July 2017,
<https://www.rfc-editor.org/info/rfc8172>.
[RFC8299] Wu, Q., Ed., Litkowski, S., Tomotaki, L., and K. Ogaki,
"YANG Data Model for L3VPN Service Delivery", RFC 8299,
DOI 10.17487/RFC8299, January 2018,
<https://www.rfc-editor.org/info/rfc8299>.
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[RFC8370] Beeram, V., Ed., Minei, I., Shakir, R., Pacella, D., and
T. Saad, "Techniques to Improve the Scalability of RSVP-TE
Deployments", RFC 8370, DOI 10.17487/RFC8370, May 2018,
<https://www.rfc-editor.org/info/rfc8370>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8403] Geib, R., Ed., Filsfils, C., Pignataro, C., Ed., and N.
Kumar, "A Scalable and Topology-Aware MPLS Data-Plane
Monitoring System", RFC 8403, DOI 10.17487/RFC8403, July
2018, <https://www.rfc-editor.org/info/rfc8403>.
[RFC8453] Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for
Abstraction and Control of TE Networks (ACTN)", RFC 8453,
DOI 10.17487/RFC8453, August 2018,
<https://www.rfc-editor.org/info/rfc8453>.
[RFC8466] Wen, B., Fioccola, G., Ed., Xie, C., and L. Jalil, "A YANG
Data Model for Layer 2 Virtual Private Network (L2VPN)
Service Delivery", RFC 8466, DOI 10.17487/RFC8466, October
2018, <https://www.rfc-editor.org/info/rfc8466>.
[RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
"Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
DOI 10.17487/RFC8491, November 2018,
<https://www.rfc-editor.org/info/rfc8491>.
[RFC8568] Bernardos, CJ., Rahman, A., Zuniga, JC., Contreras, LM.,
Aranda, P., and P. Lynch, "Network Virtualization Research
Challenges", RFC 8568, DOI 10.17487/RFC8568, April 2019,
<https://www.rfc-editor.org/info/rfc8568>.
[RFC8577] Sitaraman, H., Beeram, V., Parikh, T., and T. Saad,
"Signaling RSVP-TE Tunnels on a Shared MPLS Forwarding
Plane", RFC 8577, DOI 10.17487/RFC8577, April 2019,
<https://www.rfc-editor.org/info/rfc8577>.
[RFC8578] Grossman, E., Ed., "Deterministic Networking Use Cases",
RFC 8578, DOI 10.17487/RFC8578, May 2019,
<https://www.rfc-editor.org/info/rfc8578>.
[RFC8655] Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", RFC 8655,
DOI 10.17487/RFC8655, October 2019,
<https://www.rfc-editor.org/info/rfc8655>.
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[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.
[RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
Extensions for Segment Routing", RFC 8667,
DOI 10.17487/RFC8667, December 2019,
<https://www.rfc-editor.org/info/rfc8667>.
[SFC] "Service Function Chaining", March ,
<https://datatracker.ietf.org/wg/sfc/about>.
[TS23501] "3GPP TS23.501", 2016,
<https://portal.3gpp.org/desktopmodules/Specifications/
SpecificationDetails.aspx?specificationId=3144>.
[TS28530] "3GPP TS28.530", 2016,
<https://portal.3gpp.org/desktopmodules/Specifications/
SpecificationDetails.aspx?specificationId=3273>.
[TSN] "Time-Sensitive Networking", March ,
<https://1.ieee802.org/tsn/>.
Authors' Addresses
Jie Dong
Huawei
Email: jie.dong@huawei.com
Stewart Bryant
Futurewei
Email: stewart.bryant@gmail.com
Zhenqiang Li
China Mobile
Email: lizhenqiang@chinamobile.com
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Takuya Miyasaka
KDDI Corporation
Email: ta-miyasaka@kddi.com
Young Lee
Samsung
Email: younglee.tx@gmail.com
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