A Framework for Enhanced Virtual Private Networks (VPN+) Service
draft-ietf-teas-enhanced-vpn-03
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| Authors | Jie Dong , Stewart Bryant , Zhenqiang Li , Takuya Miyasaka , Young Lee | ||
| Last updated | 2019-09-12 | ||
| Replaces | draft-dong-teas-enhanced-vpn | ||
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draft-ietf-teas-enhanced-vpn-03
TEAS Working Group J. Dong
Internet-Draft Huawei
Intended status: Informational S. Bryant
Expires: March 15, 2020 Futurewei
Z. Li
China Mobile
T. Miyasaka
KDDI Corporation
Y. Lee
Sung Kyun Kwan University
September 12, 2019
A Framework for Enhanced Virtual Private Networks (VPN+) Service
draft-ietf-teas-enhanced-vpn-03
Abstract
This document specifies a framework for using existing, modified and
potential new networking technologies as components to provide an
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 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. It is not
envisaged that large numbers of VPN+ instances will be deployed in a
network and, in particular, it is not intended that all 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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 15, 2020.
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Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview of the Requirements . . . . . . . . . . . . . . . . 6
2.1. Isolation between Virtual Networks . . . . . . . . . . . 6
2.1.1. A Pragmatic Approach to Isolation . . . . . . . . . . 7
2.2. Performance Guarantee . . . . . . . . . . . . . . . . . . 8
2.3. Integration . . . . . . . . . . . . . . . . . . . . . . . 10
2.3.1. Abstraction . . . . . . . . . . . . . . . . . . . . . 11
2.4. Dynamic Management . . . . . . . . . . . . . . . . . . . 11
2.5. Customized Control . . . . . . . . . . . . . . . . . . . 12
2.6. Applicability . . . . . . . . . . . . . . . . . . . . . . 12
2.7. Inter-Domain and Inter-Layer Network . . . . . . . . . . 12
3. Architecture of Enhanced VPN . . . . . . . . . . . . . . . . 13
3.1. Layered Architecture . . . . . . . . . . . . . . . . . . 15
3.2. Multi-Point to Multi-Point (MP2MP) . . . . . . . . . . . 16
3.3. Application Specific Network Types . . . . . . . . . . . 16
3.4. Scaling Considerations . . . . . . . . . . . . . . . . . 16
4. Candidate Technologies . . . . . . . . . . . . . . . . . . . 17
4.1. Layer-Two Data Plane . . . . . . . . . . . . . . . . . . 17
4.1.1. FlexE . . . . . . . . . . . . . . . . . . . . . . . . 18
4.1.2. Dedicated Queues . . . . . . . . . . . . . . . . . . 18
4.1.3. Time Sensitive Networking . . . . . . . . . . . . . . 19
4.2. Layer-Three Data Plane . . . . . . . . . . . . . . . . . 19
4.2.1. Deterministic Networking . . . . . . . . . . . . . . 19
4.2.2. MPLS Traffic Engineering (MPLS-TE) . . . . . . . . . 20
4.2.3. Segment Routing . . . . . . . . . . . . . . . . . . . 20
4.3. Non-Packet Data Plane . . . . . . . . . . . . . . . . . . 21
4.4. Control Plane . . . . . . . . . . . . . . . . . . . . . . 21
4.5. Management Plane . . . . . . . . . . . . . . . . . . . . 22
4.6. Applicability of Service Data Models to Enhanced VPN . . 23
4.6.1. Enhanced VPN Delivery in ACTN Architecture . . . . . 24
4.6.2. Enhanced VPN Features with Service Data Models . . . 25
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4.6.3. 5G Transport Service Delivery via Coordinated Data
Modules . . . . . . . . . . . . . . . . . . . . . . . 28
5. Scalability Considerations . . . . . . . . . . . . . . . . . 30
5.1. Maximum Stack Depth of SR . . . . . . . . . . . . . . . . 31
5.2. RSVP Scalability . . . . . . . . . . . . . . . . . . . . 31
5.3. SDN Scaling . . . . . . . . . . . . . . . . . . . . . . . 31
6. OAM Considerations . . . . . . . . . . . . . . . . . . . . . 31
7. Telemetry Considerations . . . . . . . . . . . . . . . . . . 32
8. Enhanced Resiliency . . . . . . . . . . . . . . . . . . . . . 32
9. Operational Considerations . . . . . . . . . . . . . . . . . 33
10. Security Considerations . . . . . . . . . . . . . . . . . . . 33
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 34
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 35
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
14.1. Normative References . . . . . . . . . . . . . . . . . . 35
14.2. Informative References . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction
Virtual private networks (VPNs) have served the industry well as a
means of providing different groups of users with logically isolated
access to a common network. The common or base network that is used
to provide the VPNs is often referred to as the underlay, and the VPN
is often called an overlay.
Customers of a network operator may request enhanced overlay services
with advanced characteristics such as complete isolation from other
services so that changes in one service (such as changes in network
load, or events such as congestion or outages) have no effect on the
throughput or latency of other services provided to the customer.
Driven largely by needs surfacing from 5G, the concept of network
slicing has gained traction [NGMN-NS-Concept] [TS23501] [TS28530]
[BBF-SD406]. In [TS23501], Network Slice is defined as "a logical
network that provides specific network capabilities and network
characteristics", and Network Slice Instance is defined as "A set of
Network Function instances and the required resources (e.g. compute,
storage and networking resources) which form a deployed Network
Slice". According to [TS28530], an end-to-end network slice consists
of three major network segments: Radio Access Network (RAN),
Transport Network (TN) and Core Network (CN). Transport network
provides the required connectivity within and between RAN and CN
parts, with specific performance commitment. For each end-to-end
network slice, the topology and performance requirement on transport
network can be very different, which requires transport network to
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have the capability of supporting multiple different transport
network slices.
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 Agreement (SLA). A transport network slice could span multiple
technology (IP, Optical) and multiple administrative domains.
Depends 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.
In the following sections of this document, network slice refers to
transport network slice, and is interchangable with enhanced VPN.
End-to-end network slice is used to refer to the 5G network slice.
Network abstraction is a technique that can be applied to a network
domain to select network resources by policy to obtain a view of
potential connectivity and a set of service functions.
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) [RFC7149] and Network
Function Virtualization (NFV) [RFC8172][RFC8568] 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 set of
requirements, on top of a common network. How the network slices are
engineered can be deployment-specific.
Thus, there is a need to create virtual networks with enhanced
characteristics. The tenant of such a virtual network can require a
degree of isolation and performance that previously could not be
satisfied by traditional overlay VPNs. Additionally, the tenant may
ask for some level of control to their virtual networks, e.g., to
customize the service paths in a network slice.
These enhanced properties cannot be met with pure overlay networks,
as they require tighter coordination and integration between the
underlay and the overlay network. This document introduces a new
network service called Enhanced VPN: VPN+. VPN+ is built from a
virtual network which has a customized network topology and a set of
dedicated or shared network resources, including invoked service
functions, allocated from the underlay network. Unlike a traditional
VPN, 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
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solution, but it is not envisaged that VPN+ techniques will be
applied to normal VPN services that can continue to be deployed using
pre-existing mechanisms. Furthermore, it is not intended that large
numbers of VPN+ instances will be deployed within a single network.
See Section 5 for a discussion of scalability considerations.
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:
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) 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
Section 3.1.
Note that, in this document, the four terms "VPN", "Enhanced VPN" (or
"VPN+"), "Virtual Network (VN)", and "Network Slice" may be
considered as describing similar concepts dependent on the viewpoint
from which they are used.
o An enhanced VPN can be considered as a form of VPN, but with
additional service-specific commitments. Thus, care must be taken
with the term "VPN" to distinguish normal or legacy VPNs from VPN+
instances.
o A Virtual Network is a type of service that connects customer edge
points with the additional possibility of requesting further
service characteristics in the manner of an enhanced VPN.
o An enhanced VPN or VN is made by creating a slice through the
resources of the underlay network.
o The general concept of network slicing in a TE network is a larger
problem space than is addressed by VPN+ or VN, but those concepts
are tools to address some aspects or realizations of network
slicing.
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2. Overview of the Requirements
In this section we provide an overview of the requirements of an
enhanced VPN.
2.1. Isolation between Virtual Networks
One element of the SLA demanded for an enhanced VPN is the degree of
isolation from other services in the network. Isolation is a feature
requested by some particular customers in the network. Such a
feature is offered by a network operator where the traffic from one
service instance is isolated from the traffic of other services.
There are different grades of isolation that range from simple
separation of 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 introduced to identify
different isolation cases. 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 soft isolated VPNs because the
network delivers the traffic only to the required VPN endpoints.
However, with soft isolation, traffic from one or more 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 or a group of VPNs to be sheltered from this effect is called
hard isolation, and this property is required by some critical
applications.
The requirement is for an operator to offer its customers a choice of
different degrees of isolation ranging from soft isolation up to hard
isolation so that the traffic of tenants/applications using one
enhanced VPN can be separated from the traffic of tenants/
applications using another enhanced VPN appropriately. 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 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.
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In these circumstances the VPNs would require an appropriate degree
of isolation to be able to continue to operate acceptably.
In order to provide the required 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 network slices in the underlay network. This may introduce
scalability concerns, thus some trade-off needs to be considered to
provide the required isolation between network slices while still
allowing reasonable sharing inside each network slice.
An optical layer can offer a high degree of isolation, at the cost of
allocating resources on a long term and end-to-end basis. Such an
arrangement means that the full cost of the resources must be borne
by the service that is allocated with the resources. On the other
hand, where adequate isolation can be achieved at the packet layer,
this permits the resources to be shared amongst many services and
only dedicated to a service on a temporary basis. This in turn,
allows greater statistical multiplexing of network resources and thus
amortizes the cost over many services, leading to better economy.
However, the different degrees of isolation required by network
slicing cannot be entirely met with existing mechanisms such as
Traffic Engineered Label Switched Paths (TE-LSPs). This is because
most implementations enforce the bandwidth in the data-plane only at
the PEs, but at the P routers the bandwidth is only reserved in the
control plane, thus bursts of data can accidentally occur at a P
router with higher than committed data rate.
There are several new technologies that provide some assistance with
these data plane issues. Firstly there is the IEEE project on Time
Sensitive Networking [TSN] which introduces the concept of packet
scheduling of delay and loss sensitive packets. Then there is
[FLEXE] 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.
In the remainder of this section we explore how isolation may be
achieved in packet networks.
2.1.1. A Pragmatic Approach to Isolation
A key question is whether it is possible to achieve hard isolation in
packet networks, which were never designed to support hard isolation.
On the contrary, they were designed to provide statistical
multiplexing, a significant economic advantage when compared to a
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dedicated, or a Time Division Multiplexing (TDM) network. However
there is no need to provide any harder isolation than is required by
the application. Pseudowires [RFC3985] emulate services that would
have had hard isolation in their native form. An approximation to
this requirement is sufficient in most cases.
Thus, for example, using FlexE or a virtual sub-interface together
with packet scheduling as the isolation mechanism of interface
resources, optionally along with the partitioning of node resources,
a type of hard isolation can be provided that is adequate for many
enhanced VPN applications. Other applications may be either
satisfied with a classical VPN with or without reserved bandwidth, or
may need a dedicated point to point underlay connection. The needs
of each application must be quantified in order to provide an
economic solution that satisfies those needs without over-
engineering.
This spectrum of isolation is shown in Figure 1:
O=================================================O
| \---------------v---------------/
Statistical Pragmatic Absolute
Multiplexing Isolation Isolation
(Traditional VPNs) (Enhanced VPN) (Dedicated Network)
Figure 1: The Spectrum of Isolation
At one end of the above figure, we have traditional statistical
multiplexing technologies that support VPNs. This is a service type
that has served the industry well and will continue to do so. At the
opposite end of the spectrum, we have the absolute isolation provided
by dedicated transport networks. The goal of enhanced VPN is
pragmatic isolation. This is isolation that is better than is
obtainable from pure statistical multiplexing, more cost effective
and flexible than a dedicated network, but which is a practical
solution that is good enough for the majority of applications.
Mechanisms for both soft isolation and hard isolation would be needed
to meet different levels of service requirement.
2.2. Performance Guarantee
There are several kinds of performance guarantees, including
guaranteed maximum packet loss, guaranteed maximum delay and
guaranteed delay variation. Note that these guarantees apply to the
conformance traffic, the out-of-profile traffic will be handled
following other requirements.
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Guaranteed maximum packet loss is a common parameter, and is usually
addressed by setting the packet priorities, queue size and discard
policy. However this becomes more difficult when the requirement is
combined with the latency requirement. The limiting case is zero
congestion loss, and that is the goal of the Deterministic Networking
work that the IETF [DETNET] and IEEE [TSN] are pursuing. In modern
optical networks, loss due to transmission errors 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 [DETNET] is relevant; however the scope needs
to be extended to methods of enhancing the underlay to better support
the delay guarantee, and to integrate these enhancements with the
overall service provision.
Guaranteed maximum delay variation is a service that may also be
needed. [RFC8578] calls up a number of cases where this is needed,
for example electrical utilities have an operational need for this.
Time transfer is one example of a service that needs this, although
it is in the nature of time that the service might be delivered by
the underlay as a shared service and not provided through different
virtual networks. Alternatively a dedicated virtual network may be
used to provide this as a shared service.
This suggests that a spectrum of service guarantee be considered when
deploying an enhanced VPN. As a guide to understanding the design
requirements we can consider four types:
o Best effort
o Assured bandwidth
o Guaranteed latency
o Enhanced delivery
Best effort 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.
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Providing assured bandwidth to VPNs, for example by using TE-LSPs, is
not widely deployed at least partially due to scalability concerns.
Guaranteed latency and enhanced delivery are not yet integrated with
VPNs.
A guaranteed latency service has a latency upper bound provided by
the network. Assuring the upper bound is more important than
achieving the minimum latency.
In Section 2.1 we considered the work of the IEEE Time Sensitive
Networking (TSN) project [TSN] and the work of the IETF DetNet
Working group [DETNET] in the context of isolation. The TSN and
DetNet work is of greater relevance in assuring end-to-end packet
latency. It is also of importance in considering enhanced delivery.
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.
It is these last two characteristics that an enhanced VPN adds to a
VPN service.
Flex Ethernet [FLEXE] is a useful underlay to provide these
guarantees. This is a method of providing time-slot based
channelization over an Ethernet bearer. Such channels are fully
isolated from other channels running over the same Ethernet bearer.
As noted elsewhere this produces hard isolation but makes the
reclamation of unused bandwidth more difficult.
These approaches can be used in tandem. It is possible to use FlexE
to provide tenant isolation, and then to use the TSN/Detnet approach
to provide a performance guarantee inside the a slice or tenant VPN.
2.3. Integration
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. This needs be done in a flexible
and scalable way so that it can be widely deployed in operator
networks to support a reasonable number of enhanced VPN customers.
Taking mobile networks and in particular 5G into consideration, the
integration of network and the service functions is a likely
requirement. The work in IETF SFC working group [SFC] provides a
foundation for this integration.
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2.3.1. 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
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) described in [RFC8453] as discussed further in Section 4.5.
2.4. Dynamic Management
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
Section 4.
The control plane aspects of this problem are discussed further in
Section 4.4.
The management plane aspects of this problem are discussed further in
Section 4.5
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 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.
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2.5. Customized Control
In some cases it is desirable that an enhanced VPN has a customized
control plane, so that the tenant of the enhanced VPN can have some
control to the resources and functions allocated to this enhanced
VPN. 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, or it may be
provided with an interface to a control system which is shared with a
set of other tenants, or it may be provided with an interface to the
control system provided by the network operator.
Further detail on this requirement will be provided in a future
version of the draft.
A description of the control plane aspects of this problem are
discussed further in Section 4.4. A description of the management
plane aspects of this feature can be found in Section 4.5.
2.6. Applicability
The technologies described in this document should be applicable to a
number types of VPN services such as:
o Layer 2 point to point services such as pseudowires [RFC3985]
o Layer 2 VPNs [RFC4664]
o Ethernet VPNs [RFC7209]
o Layer 3 VPNs [RFC4364], [RFC2764]
Where such VPN types need enhanced isolation and delivery
characteristics, the technology described here can be used to provide
an underlay with the required enhanced performance.
2.7. Inter-Domain and Inter-Layer Network
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[RFC5151]. And in some domains the operator may own 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 plane (data
plane, control plane and management plane) need to provide necessary
mechanisms to support multi-domain and multi-layer coordination and
integration, so as to provide the required service characteristics
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for different enhanced VPNs, and improve network efficiency and
operational simplicity.
3. Architecture of Enhanced VPN
A number of enhanced VPN services will typically be provided by a
common network infrastructure. Each enhanced VPN consists of both
the overlay and a specific set of dedicated network resources and
functions allocated in the underlay to satisfy the needs of the VPN
tenant. The integration between overlay and various underlay
resources ensures the isolation between different enhanced VPNs, and
achieves the guaranteed performance for different services.
An enhanced VPN needs to be designed with consideration given to:
o A enhanced data plane
o A control plane to create enhanced VPN, making use of the data
plane isolation and guarantee techniques
o A management plane for enhanced VPN service life-cycle management
These required characteristics are expanded below:
o Enhanced data plane
* Provides the required resource isolation capability, e.g.
bandwidth guarantee.
* Provides the required packet latency and jitter
characteristics.
* Provides the required packet loss characteristics.
* Provides the mechanism to identify network slice and the
associated resources.
o Control plane
* Collect the underlying network topology and resources available
and export this to other nodes and/or the centralized
controller as required.
* Create the required virtual networks with the resource and
properties needed by the enhanced VPN services that are
assigned to it.
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* Determine the risk of SLA violation and take appropriate
avoiding action.
* Determine the right balance of per-packet and per-node state
according to the needs of enhanced VPN service to scale to the
required size.
o Management plane
* Provides 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).
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 mechanism to collect the data plane, control plane
and management plane data of the network, more specifically:
*
+ Provides the mechanism to collect network data of 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.
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3.1. Layered Architecture
The layered architecture of enhanced VPN is shown in Figure 2.
+-------------------+ }
| Network Controller| } Centralized
+-------------------+ } Control
. . . . .
. . . . .
. N----N----N . }
. / / . }
N-----N-----N----N-----N }
N----N }
/ / \ } Virtual
N-----N----N----N-----N } Networks
N----N }
/ / }
N-----N-----N----N-----N }
+----+ ===== +----+ ===== +----+ ===== +----+ }
+----+ ===== +----+ ===== +----+ ===== +----+ } Physical
+----+ ===== +----+ ===== +----+ ===== +----+ } Network
+----+ +----+ +----+ +----+ }
N L N L N L N
N = Partitioned node
L = Partitioned link
+----+ = Partition within a node
+----+
====== = Partition within a link
Figure 2: The Layered Architecture
Underpinning everything is the physical network infrastructure layer
consisting of partitioned links and nodes which provide the
underlying resources used to provision the separated virtual
networks. Various components and techniques as discussed in
Section 4 can be used to provide the resource partition, such as
FlexE, Time Sensitive Networking, Deterministic Networking, etc.
These partitions may be physical, or virtual so long as the SLA
required by the higher layers is met.
These techniques can be used to provision the virtual networks with
the dedicated resources that they need. To get the required
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functionality there needs to be integration between these overlays
and the underlay providing the physical resources.
The centralized controller is used to create the virtual networks, to
allocate the resources to each virtual network and to provision the
enhanced VPN services within the virtual networks. A distributed
control plane may also be used for the distribution of the topology
and attribute information of the virtual networks.
The creation and allocation process needs to take a holistic view of
the needs of all of its tenants, and to partition the resources
accordingly. However within a virtual network these resources can,
if required, be managed via a dynamic control plane. This provides
the required scalability and isolation.
3.2. Multi-Point to Multi-Point (MP2MP)
At the VPN service level, the connectivity is usually mesh or
partial-mesh. To support such kinds of VPN service, the
corresponding underlay is also an abstract MP2MP medium. However
when service guarantees are provided, the point-to-point path through
the underlay of the enhanced VPN needs to be specifically engineered
to meet the required performance guarantees.
3.3. Application Specific Network Types
Although a lot of the traffic that will be carried over the enhanced
VPN will likely be IPv4 or IPv6, the design has to be capable of
carrying other traffic types, in particular 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.
3.4. 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 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 minimising and controlling the scale of such
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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 a small number in the beginning, and even in
future the number of enhanced VPN will be much less than traditional
VPNs, because pre-existing VPN techniques would 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+ instances. It will
usually be possible to aggregate a set of VPN+ services so that they
share the same virtual network 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 Section 5 for a greater discussion of scalability considerations.
4. Candidate Technologies
A VPN is a network created by applying a multiplexing technique to
the underlying network (the underlay) in order to distinguish the
traffic of one VPN from that of another. A VPN path that travels by
other than the shortest path through the underlay normally requires
state in the underlay to specify that path. State is normally
applied to the underlay through the use of the RSVP Signaling
protocol, or directly through the use of an SDN controller, although
other techniques may emerge as this problem is studied. This state
gets harder to manage as the number of VPN paths increases.
Furthermore, as we increase the coupling between the underlay and the
overlay to support the enhanced VPN service, this state will increase
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.
4.1. Layer-Two Data Plane
A number of candidate Layer-2 packet or frame-based data plane
solutions which can be used provide the required isolation and
guarantee are described in following sections.
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o FlexE
o Time Sensitive Networking
o Dedicated Queues
4.1.1. FlexE
FlexE [FLEXE] is a method of creating a point-to-point Ethernet with
a specific fixed bandwidth. FlexE provides the ability to multiplex
multiple channels over an Ethernet link in a way that provides hard
isolation. FlexE also supports the bonding of multiple links, which
can be used to create larger links out of multiple low capacity links
in a more efficient way that traditional link aggregation. FlexE
also supports the sub-rating of links, which allows an operator to
only use a portion of a link. However it is a only a link level
technology. When packets are received by the downstream node, they
need to be processed in a way that preserves that isolation in the
downstream node. This in turn requires a queuing and forwarding
implementation that preserves the end-to-end isolation.
If different FlexE channels are used for different services, then no
sharing is possible between the FlexE channels. This in turn means
that it may be difficult to dynamically redistribute unused bandwidth
to lower priority services. This may increase the cost of providing
services on the network. On the other hand, FlexE can be used to
provide hard isolation between different tenants on a shared
interface. The tenant can then use other methods to manage the
relative priority of their own traffic in each FlexE channel.
Methods of dynamically re-sizing FlexE channels and the implication
for enhanced VPN are for further study.
4.1.2. Dedicated Queues
In order to provide multiple isolated virtual networks for enhanced
VPN, the conventional DiffServ based queuing system [RFC2475]
[RFC4594] is considered insufficient, as DiffServ does not always
provide enough queues to differentiate between traffic of different
enhanced VPNs, or the range of service classes that each need to
provide to their tenants. This problem is particularly acute with an
MPLS underlay, because MPLS only provides 8 Traffic Classes (TC), and
it's highly likely that there will be more than eight enhanced VPN
instances supported by a network. In addition, DiffServ, as
currently implemented, mainly provides relative priority-based
scheduling, and is difficult to achieve quantitive resource
reservation. In order to address this problem and reduce the
interference between enhanced VPNs, it is necessary to steer traffic
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of enhanced VPNs to dedicated input and output queues. Some routers
have large amount of queues and sophisticated queuing systems, which
could be used or enhanced to provide the granularity and level of
isolation required by the applications of enhanced VPN. For example,
on one physical interface, the queuing system can provide a set of
virtual sub-interfaces, each allocated with dedicated queueing and
buffer resources. Sophisticated queuing systems of this type may be
used to provide end-to-end virtual isolation between traffic of
different enhanced VPNs.
4.1.3. Time Sensitive Networking
Time Sensitive Networking (TSN) [TSN] is an IEEE project that is
designing a method of carrying time sensitive information over
Ethernet. It introduces the concept of packet scheduling where a
high priority packet stream may be given a scheduled time slot
thereby guaranteeing that it experiences no queuing delay and hence a
reduced latency. However, when no scheduled packet arrives, its
reserved time-slot is handed over to best effort traffic, thereby
improving the economics of the network. The mechanisms defined in
TSN can be used to meet the requirements of time sensitive services
of an enhanced VPN.
Ethernet can be emulated over a Layer 3 network using a pseudowire.
However the TSN payload would be opaque to the underlay and thus not
treated specifically as time sensitive data. The preferred method of
carrying TSN over a layer 3 network is through the use of
deterministic networking as explained in the following section of
this document.
4.2. Layer-Three Data Plane
We now consider the problem of slice differentiation and resource
representation in the network layer. The candidate technologies are:
o Deterministic Networking
o MPLS-TE
o Segment Routing
4.2.1. Deterministic Networking
Deterministic Networking (DetNet) [I-D.ietf-detnet-architecture] is a
technique being developed in the IETF to enhance the ability of
layer-3 networks to deliver packets more reliably and with greater
control over the delay. The design cannot use re-transmission
techniques such as TCP since that can exceed the delay tolerated by
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the applications. Even the delay improvements that are achieved with
Stream Control Transmission Protocol Partial Reliability Extenstion
(SCTP-PR) [RFC3758] do not meet the bounds set by application
demands. DetNet pre-emptively sends copies of the packet over
various paths to minimize the chance of all copies of a packet being
lost, and trims duplicate packets to prevent excessive flooding of
the network and to prevent multiple packets being delivered to the
destination. It also seeks to set an upper bound on latency. The
goal is not to minimize latency; the optimum upper bound paths may
not be the minimum latency paths.
DetNet is based on flows. It currently does not specify the use of
underlay topology other than the base topology. To be of use for
enhanced VPN, DetNet needs to be integrated with different virtual
topologies of enhanced VPNs.
The detailed design that allows the use DetNet in a multi-tenant
network, and how to improve the scalability of DetNet in a multi-
tenant network are topics for further study.
4.2.2. MPLS Traffic Engineering (MPLS-TE)
MPLS-TE introduces the concept of reserving end-to-end bandwidth for
a TE-LSP, which can be used as the underlay of VPNs. It also
introduces the concept of non-shortest path routing through the use
of the Explicit Route Object [RFC3209]. VPN traffic can be run over
dedicated TE-LSPs to provide reserved bandwidth for each specific
connection in a VPN. Some network operators have concerns about the
scalability and management overhead of RSVP-TE system, and this has
lead them to consider other solutions for their networks.
4.2.3. Segment Routing
Segment Routing [RFC8402] is a method that prepends instructions to
packets at the head-end node and optionally at various points as it
passes though the network. These instructions allow the packets to
be routed on paths other than the shortest path for various traffic
engineering reasons. With SR, a path needs to be dynamically created
through a set of segments by simply specifying the Segment
Identifiers (SIDs), i.e. instructions rooted at a particular point in
the network. By encoding the state in the packet, per-path state is
transitioned out of the network.
With current segment routing, the instructions are used to specify
the nodes and links to be traversed. 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
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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 a queue is a useful abstraction for many
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.
Both SR-MPLS and SRv6 are candidate data plane technologies for
enhanced VPN. In some cases they can further be used for DetNet to
meet the packet loss, delay and jitter requirement of particular
service. How to provide the DetNet enhanced delivery in an SRv6
environment is specified in [I-D.geng-spring-srv6-for-detnet].
4.3. 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 (and hence slices).
The control and management planes for non-packet data plane
technologies have most in common with MPLS-TE (Section 4.2.2) and
offer the same set of advanced features [RFC3945]. Furthermore,
management techniques such as ACTN ([RFC8453] and Section 4.6 can be
used to aid in the reporting of underlying network topologies, and
the creation of virtual networks with the resource and properties
needed by the enhanced VPN services.
4.4. Control Plane
Enhanced VPN would likely be based on a hybrid control mechanism,
which takes advantage of the logically centralized controller for on-
demand provisioning and global optimization, whilst still relies on
distributed control plane to provide scalability, high reliability,
fast reaction, automatic failure recovery etc. Extension and
optimization to the distributed control plane is needed to support
the enhanced properties of VPN+.
RSVP-TE provides the signaling mechanism of establishing a TE-LSP
with end-to-end resource reservation. It can be used to bind the VPN
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to specific network resource allocated within the underlay, but there
are the above mentioned scalability concerns.
SR does not have the capability of signaling the resource reservation
along the path, nor do its currently specified distributed link state
routing protocols. On the other hand, the SR approach provides a way
of efficiently binding the network underlay and the enhanced VPN
overlay, as it reduces the amount of state to be maintained in the
network. An SR-based approach with per-slice resource reservation
can easily create dedicated SR network slices, and the VPN services
can be bound to a particular SR network slice. A centralized
controller can perform resource planning and reservation from the
controller's point of view, but this does not ensure resource
reservation is actually done in the network nodes. Thus, if a
distributed control plane is needed, either in place of an SDN
controller or as an assistant to it, the design of the control system
needs to ensure that resources are uniquely allocated in the network
nodes for the correct services, and not allocated to other services
causing unintended resource conflict.
In addition, in multi-domain and multi-layer networks, the
centralized and distributed control mechanisms will be used for
inter-domain coordination and inter-layer optimization, so that the
diverse and customized enhanced VPN service requirement could be met.
The detailed mechanisms will be described in a future version.
4.5. Management Plane
In the context of 5G end-to-end network slicing, the management of
enhanced VPN is considered as the management of transport network
part of the end-to-end network slice. 3GPP management system may
provide the topology and QoS parameters as requirement to the
management plane of transport network. It may also require the
transport network to expose the capability and status of the
transport network slice. Thus an interface between enhanced VPN
management plane and 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 such as:
o VPN service models defined in [RFC8299] and [RFC8466]
o Possible augmentations and extensions
(e.g.,[I-D.ietf-teas-te-service-mapping-yang]) to VPN service
models
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o ACTN related service models such as [I-D.ietf-teas-actn-vn-yang]
and [I-D.ietf-teas-actn-pm-telemetry-autonomics].
o VPN network model as defined in [I-D.aguado-opsawg-l3sm-l3nm].
o TE Tunnel model as defined in [I-D.ietf-teas-yang-te].
These data models can be applicable in the provisioning of enhanced
VPN service. The details are described in Section 4.6.
4.6. Applicability of Service Data Models to Enhanced VPN
ACTN supports operators in viewing and controlling different domains
and presenting virtualized networks to their customers. The ACTN
framework [RFC8453] highlights how:
o Abstraction of the underlying network resources are provided to
higher-layer applications and customers.
o Virtualization of underlying resources, whose selection criterion
is the allocation of those resources for the customer,
application, or service.
o Creation of a virtualized environment allowing operators to view
and control multi-domain networks as a single virtualized network.
o The presentation to customers of networks as a virtual network via
open and programmable interfaces.
The infrastructure managed through the Service Data models comprises
traffic engineered network resources (e.g. bandwidth, time slot,
wavelength) and VPN service related resources (e.g. Route Target
(RT) and Route Distinguisher (RD)).
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.
The Customer VPN model (e.g. L3SM) 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.
L3VPN network model or TE tunnel model is a network view of the ACTN
managed infrastructure, and is presented by the ACTN provider as a
set of transport resources.
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Depending on the agreement between customer and provider, various
VPN/VN operations and VPN/VN views are possible.
o Virtual Network Creation: A VPN/VN could be pre-configured and
created via static or dynamic request and negotiation between
customer and provider. It must meet the specified SLA attributes
which satisfy the customer's objectives.
o Virtual Network Operations: The virtual network may be further
modified and deleted based on customer request to request changes
in the network resources reserved for the customer, and used to
construct the network slice. The customer can further act upon
the virtual network to manage traffic flow across the virtual
network.
o Virtual Network View: The VPN/VN topology from a customer point of
view. These may be a variety of tunnels, or an entire VN
topology, or an VPN service topology. Such connections may
comprise of customer end points, access links, intra-domain paths,
and inter-domain links.
Dynamic VPN/VN Operations allow a customer to modify or delete the
VPN/VN. The customer can further act upon the virtual network to
create/modify/delete virtual links and nodes or VPN sites. These
changes will result in subsequent tunnel management or VPN service
management in the operator's networks.
4.6.1. Enhanced VPN Delivery in ACTN Architecture
ACTN provides VPN connections or VN connections between multiple
sites as requested via a VPN requestor enabled by the Customer
Network Controller (CNC). The CNC is managed by the customer
themselves, and interacts with the network provider's Multi-Domain
Service Controller (MDSC). The Provisioning Network Controllers
(PNC) are responible for network resource management, thus the PNCs
are remain entirely under the management of the network provider and
are not visible to the customer.
The benefits of this model include:
o Provision of edge-to-edge VPN multi-access connectivity.
o Management is mostly performed by the network provider, with some
flexibility delegated to the customer-managed CNC.
Figure 3 presents a more general representation of how multiple
enhanced VPNs may be created from the resources of multiple physical
networks using the CNC, MDSC, and PNC components of the ACTN
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architecture. Each enhanced VPN is controlled by its own CNC. The
CNCs send requests to the provider's MDSC. The provider manages two
different physical networks each under the control of PNC. The MDSC
asks the PNCs to allocate and provision resources to achieve the
enhanced VPNs. In this figure, one enhanced VPN is constructed
solely from the resources of one of the physical networks, while the
the VPN uses resources from both physical networks.
___________
--------------- ( )
| CNC |---------->( VPN+ )
--------^------ ( )
| _(_________ _)
--------------- ( ) ^
| CNC |----------->( VPN+ ) :
------^-------- ( ) :
| | (___________) :
| | ^ ^ :
Boundary | | : : :
Between ==========|====|===================:====:====:========
Customer & | | : : :
Network Provider | | : : :
v v : : :
--------------- : :....:
| MDSC | : :
--------------- : :
^ ---^------ ...
| ( ) .
v ( Physical ) .
---------------- ( Network ) .
| PNC |<-------->( ) ---^------
---------------- | -------- ( )
| |-- ( Physical )
| PNC |<------------------------->( Network )
--------------- ( )
--------
Figure 3: Generic VPN+ Delivery in the ACTN Architecture
4.6.2. Enhanced VPN Features with Service Data Models
This section discusses how the service data models can fulfill the
enhanced VPN requirements described earlier in this document. As
previously noted, key requirements of the enhanced VPN include:
1. Isolation between VPNs/VNs
2. Guaranteed Performance
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3. Integration
4. Dynamic Management
5. Customized Control
The subsections that follow outline how each requirement is met using
ACTN.
4.6.2.1. Isolation Between VPN/VNs
The VN YANG model [I-D.ietf-teas-actn-vn-yang] and the TE-service
mapping model [I-D.ietf-teas-te-service-mapping-yang] fulfill the
VPN/VN isolation requirement by providing the following features for
the VPN/VNs:
o Each VN is identified with a unique identifier (vn-id and vn-name)
and so is each VN member that belongs to the VN (vn-member-id).
o Each VPN is identified with a unique identifier (vpn-id) and can
be mapped to one specific VN. While multiple VPNs may mapped to
the same VN according to service requirement and operator's
policy.
o Each VPN and the corresponding VN is managed and controlled
independent of other VPNs/VNs in the network with proper
availability level.
o Each VPN/VN is instantiated with an isolation requirement
described by the TE-service mapping model
[I-D.ietf-teas-te-service-mapping-yang]. This mapping supports:
* Hard isolation with deterministic characteristics (e.g., this
case may need an optical bypass tunnel or a DetNet/TSN tunnel
to guarantee latency with no jitter)
* Hard isolation (i.e., dedicated TE resources in all underlays)
* Soft isolation (i.e., resource in some layer may be shared
while in some other layers is dedicated).
* No isolation (i.e., sharing with other VPN/VN).
4.6.2.2. Guaranteed Performance
Performance objectives of a VN need first to be expressed in order to
assure the performance guarantee.
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Performance objectives of a VPN [RFC8299][RFC8466] are expressed with
QoS profile, either standard profile or customer profile. The
customer QoS profile include the following properties:
o Rate-limit
o Bandwidth
o Latency
o Jitter
[I-D.ietf-teas-actn-vn-yang] and [I-D.ietf-teas-yang-te-topo] allow
configuration of several TE parameters that may affect the VN
performance objectives as follows:
o Bandwidth
o Objective function (e.g., min cost path, min load path, etc.)
o Metric Types and their threshold:
* TE cost, IGP cost, Hop count, or Unidirectional Delay (e.g.,
can set all path delay <= threshold)
Once these requests are instantiated, the resources are committed and
guaranteed through the life cycle of the VPN/VN.
4.6.2.3. Integration
L3VPN network model provides mechanism to correlate customer's VPN
and the VPN service related resources (e.g.RT and RD) allocated in
the provider's network.
VPN/Network performance monitoring model
[I-D.www-bess-yang-vpn-service-pm] provides mechanisms to monitor and
manage network Performance on the topology at different layer or the
overlay topology between VPN sites.
VN model and Performance Monitoring Telemetry model provides
mechanisms to correlate customer's VN and the actual TE tunnels
instantiated in the provider's network. Specifically:
o Link each VN member to actual TE tunnel.
o Each VN can be monitored on a various level such as VN level, VN
member level, TE-tunnel level, and link/node level.
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Service function integration with network topology (L3 and TE
topology) is in progress in [I-D.ietf-teas-sf-aware-topo-model].
Specifically, [I-D.ietf-teas-sf-aware-topo-model] addresses a number
of use-cases that show how TE topology supports various service
functions.
4.6.2.4. Dynamic Management
ACTN provides an architecture that allows the CNC to interact with
the MDSC which is network provider's SDN controller. This gives the
customer control of their VPN or VNs.
e.g., the ACTN VN model [I-D.ietf-teas-actn-vn-yang] allows the VN to
life-cycle management such as create, modify, and delete VNs on
demand. Another example is L3VPN servicel model [RFC8299] which
allows the VPN lifecycle management such as VPN creation,
modification and deletion on demand.
4.6.2.5. Customized Control
ACTN provides a YANG model that allows the CNC to control a VN as a
"Type 2 VN" that allows the customer to provision tunnels that
connect their endpoints over the customized VN topology.
For some VN members, the customers are allowed to configure the path
(i.e., the sequence of virtual nodes and virtual links) over the VN/
abstract topology.
4.6.3. 5G Transport Service Delivery via Coordinated Data Modules
The overview of network slice structure as defined in the 3GPP 5GS is
shown in Figure 5. The terms are described in specific 3GPP
documents (e.g. [TS23501] and [TS28530].)
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<================== E2E-NSI =======================>
: : : : :
: : : : :
<====== RAN-NSSI ======><=TRN-NSSI=><====== CN-NSSI ======>VL[APL]
: : : : : : : : :
: : : : : : : : :
RW[NFs ]<=TRN-NSSI=>[NFs ]<=TRN-NSSI=>[NFs ]<=TRN-NSSI=>[NFs ]VL[APL]
. . . . . . . . . . . . .. . . . . . . . . . . . . ..
.,----. ,----. ,----.. ,----. .,----. ,----. ,----..
UE--|RAN |---| TN |---|RAN |---| TN |---|CN |---| TN |---|CN |--[APL]
.|NFs | `----' |NFs |. `----' .|NFs | `----' |NFs |.
.`----' `----'. .`----' `----'.
. . . . . . . . . . . . .. . . . . . . . . . . . . ..
RW RAN MBH CN DN
*Legends
UE: User Equipment
RAN: Radio Access Network
CN: Core Network
DN: Data Network
TN: Transport Network
MBH: Mobile Backhaul
RW: Radio Wave
NF: Network Function
APL: Application Server
NSI: Network Slice Instance
NSSI: Network Slice Subnet Instance
Figure 4: Overview of Structure of Network Slice in 3GPP 5GS
To support 5G service (e.g., 5G MBB service), L3VPN service model
[RFC8299] and TEAS VN model [I-D.ietf-teas-actn-vn-yang] can be both
provided to describe 5G MBB Transport Service or connectivity
service. L3VPN service model is used to describe end-to-end IP
connectivity service while TEAS VN model is used to describe TE
connectivity service between VPN sites or between RAN NFs and Core
network NFs.
VN in TEAS VN model and support point-to-point or multipoint-to-
multipoint connectivity service and can be seen as one example of
network slice.
TE Service mapping model can be used to map L3VPN service requests
onto underlying network resource and TE models to get TE network
setup.
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For IP VPN service provision, the service parameters in the L3VPN
service model [RFC8299] can be decomposed into a set of configuration
parameters described in the L3VPN network model
[I-D.aguado-opsawg-l3sm-l3nm] which will get VPN network setup.
5. Scalability Considerations
Enhanced VPN provides the 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
adding state might be presented 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 of course possible to
use other methods to introduce path state, such as via a Software
Defined Network (SDN) controller, or possibly by modifying a
routing protocol. With this approach there is state per path per
path characteristic that needs to be maintained over its life-
cycle. This is more state than is needed using SR, but the packet
are shorter.
o Provide a hybrid approach based on using binding SIDs to create
path fragments, and bind them together with SR.
Dynamic creation of a VPN path using SR requires less state
maintenance in the network core at the expense of larger VPN headers
on the packet. The packet size can be lower if a form of loose
source routing is used (using a few nodal SIDs), and it will be lower
if no specific functions or resource on the routers are specified.
Reducing the state in the network is important to enhanced VPN, as it
requires the overlay to be more closely integrated with the underlay
than with traditional VPNs. This tighter coupling would normally
mean that more state needed to be created and maintained in the
network, as the state about fine granularity processing would need to
be loaded and maintained in the routers. However, a segment routed
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approach allows much of this state to be spread amongst the network
ingress nodes, and transiently carried in the packets as SIDs.
These approaches are for further study.
5.1. 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.
5.2. RSVP Scalability
The traditional method of creating a resource allocated path through
an MPLS network is to use the RSVP protocol. However there have been
concerns that this requires significant continuous state maintenance
in the network. There are ongoing works to improve the scalability
of RSVP-TE LSPs in the control plane [RFC8370].
There is also concern at the scalability of the forwarder footprint
of RSVP as the number of paths through an LSR grows [RFC8577]
proposes to address this by employing SR within a tunnel established
by RSVP-TE.
5.3. 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 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.
6. OAM Considerations
The enhanced VPN OAM design needs to consider the following
requirements:
o Instrumentation of the underlay so that the network operator can
be sure that the resources committed to a tenant are operating
correctly and delivering the required performance.
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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].
7. 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], 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 will be described in a future version.
8. Enhanced Resiliency
Each enhanced VPN has a life-cycle, and needs modification during
deployment as the needs of its tenant change. Additionally, as the
network as a whole evolves, there will need to be garbage collection
performed to consolidate resources into usable quanta.
Systems in which the path is imposed such as SR, or some form of
explicit routing tend to do well in these applications, because it is
possible to perform an atomic transition from one path to another.
This is a single action by the head-end changes the path without the
need for coordinated action by the routers along the path. However,
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implementations and the monitoring protocols need to make sure that
the new path is up and meet the required SLA before traffic is
transitioned to it. It is possible for deadlocks arise as a result
of the network becoming fragmented over time, such that it is
impossible to create a new path or modify a existing path without
impacting the SLA of other paths. Resolution of this situation is as
much a commercial issue as it is a technical issue and is outside the
scope of this document.
There are however two manifestations of the latency problem that are
for further study in any of these approaches:
o The problem of packets overtaking one and other if a path latency
reduces during a transition.
o The problem of the latency transient in either direction as a path
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 proposed by the IETF deterministic network work with
multiple in-network replication and the culling of later packets
[I-D.ietf-detnet-architecture].
In addition to the approach used to protect high priority packets,
consideration has to be given to the impact of best effort traffic on
the high priority packets during a transient. Specifically if a
conventional re-convergence process is used there will inevitably be
micro-loops and whilst some form of explicit routing will protect the
high priority traffic, lower priority traffic on best effort shortest
paths will micro-loop without the use of a loop prevention
technology. To provide the highest quality of service to high
priority traffic, either this traffic must be shielded from the
micro-loops, or micro-loops must be prevented.
9. Operational Considerations
TBD in a future version.
10. Security Considerations
All types of virtual network require special consideration to be
given to the isolation between the tenants. In this regard enhanced
VPNs neither introduce, no experience a greater security risk than
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another VPN of the same base type. However, in an enhanced virtual
network service the isolation requirement needs to be considered. If
a service requires a specific latency then it can be damaged by
simply delaying the packet through the activities of another tenant.
In a network with virtual functions, depriving a function used by
another tenant of compute resources can be just as damaging as
delaying transmission of a packet in the network. The measures to
address these dynamic security risks must be specified as part to the
specific solution.
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.
11. IANA Considerations
There are no requested IANA actions.
12. Contributors
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Daniel King
Email: daniel@olddog.co.uk
Adrian Farrel
Email: adrian@olddog.co.uk
Jeff Tansura
Email: jefftant.ietf@gmail.com
Qin Wu
Email: bill.wu@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
13. Acknowledgements
The authors would like to thank Charlie Perkins, James N Guichard and
John E Drake 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).
14. References
14.1. Normative References
[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-06 (work in progress), July 2019.
[I-D.ietf-teas-te-service-mapping-yang]
Lee, Y., Dhody, D., Fioccola, G., Wu, Q., Ceccarelli, D.,
and J. Tantsura, "Traffic Engineering (TE) and Service
Mapping Yang Model", draft-ietf-teas-te-service-mapping-
yang-02 (work in progress), September 2019.
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[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
14.2. 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>.
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[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.aguado-opsawg-l3sm-l3nm]
Aguado, A., Dios, O., Lopezalvarez, V.,
daniel.voyer@bell.ca, d., and L. Munoz, "Layer 3 VPN
Network Model", draft-aguado-opsawg-l3sm-l3nm-01 (work in
progress), July 2019.
[I-D.geng-spring-srv6-for-detnet]
Geng, X., Li, Z., and M. Chen, "SRv6 for Deterministic
Networking (DetNet)", draft-geng-spring-srv6-for-detnet-00
(work in progress), July 2019.
[I-D.ietf-detnet-architecture]
Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", draft-ietf-
detnet-architecture-13 (work in progress), May 2019.
[I-D.ietf-detnet-dp-sol-ip]
Korhonen, J. and B. Varga, "DetNet IP Data Plane
Encapsulation", draft-ietf-detnet-dp-sol-ip-02 (work in
progress), March 2019.
[I-D.ietf-opsawg-ntf]
Song, H., Qin, F., Martinez-Julia, P., Ciavaglia, L., and
A. Wang, "Network Telemetry Framework", draft-ietf-opsawg-
ntf-01 (work in progress), June 2019.
[I-D.ietf-teas-actn-pm-telemetry-autonomics]
Lee, Y., Dhody, D., Karunanithi, S., Vilata, R., King, D.,
and D. Ceccarelli, "YANG models for VN & TE Performance
Monitoring Telemetry and Scaling Intent Autonomics",
draft-ietf-teas-actn-pm-telemetry-autonomics-00 (work in
progress), July 2019.
[I-D.ietf-teas-sf-aware-topo-model]
Bryskin, I., Liu, X., Lee, Y., Guichard, J., Contreras,
L., Ceccarelli, D., and J. Tantsura, "SF Aware TE Topology
YANG Model", draft-ietf-teas-sf-aware-topo-model-03 (work
in progress), March 2019.
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[I-D.ietf-teas-yang-te]
Saad, T., Gandhi, R., Liu, X., Beeram, V., and I. Bryskin,
"A YANG Data Model for Traffic Engineering Tunnels and
Interfaces", draft-ietf-teas-yang-te-21 (work in
progress), April 2019.
[I-D.ietf-teas-yang-te-topo]
Liu, X., Bryskin, I., Beeram, V., Saad, T., Shah, H., and
O. Dios, "YANG Data Model for Traffic Engineering (TE)
Topologies", draft-ietf-teas-yang-te-topo-22 (work in
progress), June 2019.
[I-D.www-bess-yang-vpn-service-pm]
Wang, Z., Wu, Q., Even, R., Wen, B., and C. Liu, "A YANG
Model for Network and VPN Service Performance Monitoring",
draft-www-bess-yang-vpn-service-pm-03 (work in progress),
July 2019.
[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>.
[RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path
Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000,
<https://www.rfc-editor.org/info/rfc2992>.
[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>.
[RFC3945] Mannie, E., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Architecture", RFC 3945,
DOI 10.17487/RFC3945, October 2004,
<https://www.rfc-editor.org/info/rfc3945>.
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[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>.
[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>.
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[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>.
[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>.
[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>.
[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>.
[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/>.
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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
Takuya Miyasaka
KDDI Corporation
Email: ta-miyasaka@kddi.com
Young Lee
Sung Kyun Kwan University
Email: younglee.tx@gmail.com
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