| TEAS Working Group | J. Dong |
| Internet-Draft | Huawei |
| Intended status: Informational | S. Bryant |
| Expires: January 14, 2021 | Futurewei |
| Z. Li | |
| China Mobile | |
| T. Miyasaka | |
| KDDI Corporation | |
| Y. Lee | |
| Samsung | |
| July 13, 2020 |
A Framework for Enhanced Virtual Private Networks (VPN+) Service
draft-ietf-teas-enhanced-vpn-06
This document describes the framework for Enhanced Virtual Private Network (VPN+) service. The purpose is to support the needs of new applications, particularly applications that are associated with 5G services, by utilizing an approach that is based on existing VPN and Traffic Engineering (TE) technologies and adds features that specific services require over and above traditional VPNs.
Typically, VPN+ will be used to form the underpinning of network slicing, but could also be of use in its own right providing enhanced connectivity services between customer sites.
It is envisaged that enhanced VPNs will be delivered using a combination of existing, modified, and new networking technologies. This document provides an overview of relevant technologies and identifies some areas for potential new work.
Comparing to traditional VPNs, It is not envisaged that quite large numbers of VPN+ services will be deployed in a network. In other word, it is not intended that all existing VPNs supported by a network will use VPN+ related techniques.
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/.
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This Internet-Draft will expire on January 14, 2021.
Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.
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Virtual private networks (VPNs) have served the industry well as a means of providing different groups of users with logically isolated connectivity over a common network. The common or base network that is used to provide the VPNs is often referred to as the underlay, and the VPN is often called an overlay.
Customers of a network operator may request a connectivity services with advanced characteristics such as enhanced isolation from other services so that changes in some other service (such as changes in network load, or events such as congestion or outages) have no or acceptable effect on the throughput or latency of the services provided to the customer. These services are "enhanced VPNs" (known as VPN+) in that they are similar to VPN services as they provide the customer with required connectivity, but have enhanced characteristics.
Driven largely by needs surfacing from 5G, the concept of network slicing has gained traction [NGMN-NS-Concept] [TS23501] [TS28530] [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 required connectivity between different entities in RAN and CN segments of an end-to-end network slice, with specific performance commitment.
A transport network slice is a virtual (logical) network with a particular network topology and a set of shared or dedicated network resources, which are used to provide the network slice consumer with the required connectivity, appropriate isolation and specific Service Level Objective (SLO).
A transport network slice could span multiple technologies (such as IP or Optical) and multiple administrative domains. Depending on the consumer's requirement, a transport network slice could be isolated from other, often concurrent transport network slices in terms of data plane, control plane, and management plane resources.
In this document the term "network slice" refers to a transport network slice, and is considered as one typical use case of enhanced VPN.
Network slicing builds on the concept of resource management, network virtualization, and abstraction to provide performance assurance, flexibility, programmability and modularity. It may use techniques such as Software Defined Networking (SDN) [RFC7149], network abstraction [RFC7926] 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 or similar set of requirements, on top of a common network. How the network slices are engineered can be deployment-specific.
VPN+ could be used to form the underpinning of transport network slice, but could also be of use in general cases providing enhanced connectivity services between customer sites.
The requirement of enhanced VPN services cannot be met by simple overlay networks, as they require tighter coordination and integration between the underlay and the overlay network. VPN+ is built from a VPN overlay and a underlying Virtual Transport Network (VTN) which has a customized network topology and a set of dedicated or shared network resources. It may optionally include a set of invoked service functions allocated from the underlay network. Thus an enhanced VPN can achieve greater isolation with strict performance guarantees. These new properties, which have general applicability, may also be of interest as part of a network slicing solution. It is not envisaged that VPN+ services will replace traditional VPN services that can continue to be deployed using pre- existing mechanisms.
This document specifies a framework for using existing, modified, and potential new technologies as components to provide a VPN+ service. Specifically we are concerned with:
The 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 "Transport Network Slice" are described as below:
The following terms are used in this document. Some of them are newly defined, some others reference existing definitions:
ACTN: Abstraction and Control of TE Networks [RFC8453]
Detnet: Deterministic Networking [DETNET]
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].
VPN+: Enhanced VPN service. An enhanced VPN service (VPN+) can be considered as an evolution of VPN service, but with additional service-specific commitments such as enhanced isolation and performance guarantee.
VTP: Virtual Transport Path. A VTP is a virtual underlay path which connects two customer edge points with the capability of providing the isolation and performance characteristics required by an enhanced VPN customer. A VTP usually has a customized path with a set of reserved network resources along the path.
VTN: Virtual Transport Network. A VTN is a virtual underlay network that connects customer edge points with the capability of providing the isolation and performance characteristics required by an enhanced VPN customer. A VTN usually has a customized topology and a set of dedicated or shared network resources.
In this section we provide an overview of the requirements of an enhanced VPN service.
One element of the SLA demanded for an enhanced VPN is a guarantee that the service offered to the customer will not be perturbed by any other traffic flows in the network. One way for a service provider to guarantee the customer's SLA is by controlling the degree of isolation from other services in the network. Isolation is a feature that can be requested by customers. There are different grades of how isolation may be enabled by a network operator and that may result in different levels of service perceived by the customer. These range from simple separation of service traffic on delivery (ensuring that traffic is not delivered to the wrong customer), all the way to complete separation within the underlay so that the traffic from different services use distinct network resources.
The terms hard and soft isolation are used to illustrate different levels of isolation. A VPN has soft isolation if the traffic of one VPN cannot be received by the customers of another VPN. Both IP and MPLS VPNs are examples of VPNs with soft isolation: the network delivers the traffic only to the required VPN endpoints. However, with soft isolation, traffic from VPNs and regular non-VPN traffic may congest the network resulting in packet loss and delay for other VPNs operating normally. The ability for a VPN service or a group of VPN services to be sheltered from this effect is called hard isolation, and this property is required by some applications. Hard isolation is needed so that applications with exacting requirements can function correctly, despite other demands (perhaps a burst of traffic in another VPN) competing for the underlying resources. In practice isolation may be offered as a spectrum between soft and hard, and in some cases soft and hard isolation may be used in a hierarchical manner. An operator may offer its customers a choice of different degrees of isolation ranging from soft isolation up to hard isolation.
An example of the requirement for hard isolation is a network supporting both emergency services and public broadband multi-media services. During a major incident the VPNs supporting these services would both be expected to experience high data volumes, and it is important that both make progress in the transmission of their data. In these circumstances the VPN services would require an appropriate degree of isolation to be able to continue to operate acceptably. On the other hand, VPNs servicing ordinary bulk data may expect to contest for network resources and queue packets so that traffic is delivered within SLAs, but with some potential delays and interference.
In order to provide the required level of isolation, resources may have to be reserved in the data plane of the underlay network and dedicated to traffic from a specific VPN or a specific group of VPNs to form different enhanced VPNs in the network. This may introduce scalability concerns, thus some trade-off needs to be considered to provide the required isolation between some enhanced VPNs while still allowing reasonable sharing.
An optical layer can offer a high degree of isolation, at the cost of allocating resources on a long term and end-to-end basis. On the other hand, where adequate isolation can be achieved at the packet layer, this permits the resources to be shared amongst a group of services and only dedicated to a service on a temporary basis.
There are several new technologies that provide some assistance with these data plane issues. Firstly there is the IEEE project on Time Sensitive Networking [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.
Section 3.1.1 explores pragmatic approaches to isolation in packet networks.
A key question is whether it is possible to achieve hard isolation in packet networks that were never designed to support hard isolation. On the contrary, they were designed to provide statistical multiplexing, a significant economic advantage when compared to a dedicated, or a Time Division Multiplexing (TDM) network. However, there is no need to provide any harder isolation than is required by the applications. An approximation to this requirement is sufficient in most cases. Pseudowires[RFC3985] emulate services that would have had hard isolation in their native form.
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
Figure 1 shows the spectrum of isolation that may be delivered by a network. At one end of the figure, we have traditional statistical multiplexing technologies that support VPNs. This is a service type that has served the industry well and will continue to do so. At the opposite end of the spectrum, we have the absolute isolation provided by dedicated transport networks. The goal of enhanced VPNs is "pragmatic isolation". This is isolation that is better than is obtainable from pure statistical multiplexing, more cost effective and flexible than a dedicated network, but is a practical solution that is good enough for the majority of applications. Mechanisms for both soft isolation and hard isolation would be needed to meet different levels of service requirement.
There are several kinds of performance guarantee, including guaranteed maximum packet loss, guaranteed maximum delay, and guaranteed delay variation. Note that these guarantees apply to conformance traffic, out-of-profile traffic will be handled according to other requirements.
Guaranteed maximum packet loss is a common parameter, and is usually addressed by setting packet priorities, queue size, and discard policy. However this becomes more difficult when the requirement is combined with latency requirements. The limiting case is zero congestion loss, and that is the goal of the Deterministic Networking work that the IETF [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 additional methods of enhancing the underlay to better support the delay guarantees may be needed, and these methods will need to be integrated with the overall service provisioning mechanisms.
Guaranteed maximum delay variation is a service that may also be needed. [RFC8578] calls up a number of cases where this is needed, for example in electrical utilities. Time transfer is one example of a service that needs this, although it is in the nature of time that the service might be delivered by the underlay as a shared service and not provided through different enhanced VPNs. Alternatively a dedicated enhanced VPN may be used to provide this as a shared service.
This suggests that a spectrum of service guarantee be considered when deploying an enhanced VPN. As a guide to understanding the design requirements we can consider four types:
Best effort service is the basic service that current VPNs can provide.
An assured bandwidth service is one in which the bandwidth over some period of time is assured. This can be achieved either simply based on best effort with over-capacity provisioning, or it can be based on TE-LSPs with bandwidth reservation. The instantaneous bandwidth is however, not necessarily assured, depending on the technique used. Providing assured bandwidth to VPNs, for example by using per-VPN TE-LSPs, is not widely deployed at least partially due to scalability concerns. VPN+ aims to provide a more scalable approach for such kind of service.
A guaranteed latency service has a latency upper bound provided by the network. Assuring the upper bound is sometimes more important than minimizing latency. There are several new technologies that provide some assistance with performance guarantee. Firstly there is the IEEE project on Time Sensitive Networking [TSN] which introduces the concept of packet scheduling of delay and loss sensitive packets. Then the DetNet work is also of greater relevance in assuring upper bound of end-to-end packet latency. Flex Ethernet [FLEXE] is also useful to provide these guarantees. The usage of such underlying technologies for VPN+ service needs to be considered.
An enhanced delivery service is one in which the underlay network (at Layer 3) attempts to deliver the packet through multiple paths in the hope of eliminating packet loss due to equipment or media failures. Such mechanism may need to be used for VPN+ service.
The only way to achieve the enhanced characteristics provided by an enhanced VPN (such as guaranteed or predicted performance) is by integrating the overlay VPN with a particular set of network resources in the underlay network which are allocated to meet the service requirement. This needs be done in a flexible and scalable way so that it can be widely deployed in operator networks to support a reasonable number of enhanced VPN customers.
Taking mobile networks and in particular 5G into consideration, the integration of network and the service functions is a likely requirement. The work in IETF SFC working group [SFC] provides a foundation for this integration.
Integration of the overlay VPN and the underlay network resources does not need to be a tight mapping. As described in [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) [RFC8453] as discussed further in Section 5.5.
Enhanced VPNs need to be created, modified, and removed from the network according to service demand. An enhanced VPN that requires hard isolation (Section 3.1) must not be disrupted by the instantiation or modification of another enhanced VPN. Determining whether modification of an enhanced VPN can be disruptive to that VPN, and in particular whether the traffic in flight will be disrupted can be a difficult problem.
The data plane aspects of this problem are discussed further in Sections 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 the change of network performance.
In addition to non-disruptively managing the network as a result of gross change such as the inclusion of a new VPN endpoint or a change to a link, VPN traffic might need to be moved as a result of traffic volume changes.
In some cases it is desirable that an enhanced VPN has a customized control plane, so that the tenant of the enhanced VPN can have some control of how the resources and functions allocated to this enhanced VPN are used. For example, the tenant may be able to specify the service paths in his own enhanced VPN. Depending on the requirement, an enhanced VPN may have its own dedicated controller, which may be provided with an interface to the control system provided by the network operator. Note that such control is within the scope of the tenant's enhanced VPN, any change beyond that would require some intervention of the operator.
A description of the control plane aspects of this problem are discussed further in Section 5.4. A description of the management plane aspects of this feature can be found in Section 5.5.
The technologies described in this document should be applicable to a number types of VPN overlay services such as:
Where such VPN 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.
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]. In some domains the operator may manage a multi-layered network, for example, a packet network over an optical network. When enhanced VPNs are provisioned in such network scenarios, the technologies used in different network planes (data plane, control plane, and management plane) need to provide mechanisms to support multi-domain and multi-layer coordination and integration, so as to provide the required service characteristics for different enhanced VPNs, and improve network efficiency and operational simplicity.
A number of enhanced VPN services will typically be provided by a common network infrastructure. Each enhanced VPN consists of both the overlay and a corresponding VTN with a specific set of network resources and functions allocated in the underlay to satisfy the needs of the VPN tenant. The integration between overlay and various underlay resources ensures the required isolation between different enhanced VPNs, and achieves the guaranteed performance for different services.
An enhanced VPN needs to be designed with consideration given to:
These required characteristics are expanded below:
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 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.
A
| |
+-------------------+ Centralized
| Network Controller| Control & Management
+-------------------+
||
\/
o---------------------------o
/-------------o
o____________/______________o VPN Services
...... (P2P,P2MP,MP2MP...)
o-----------\ /-------------o
o____________X______________o
__________________________
/ o----o----o /
/ / / / VTN-1
/ o-----o-----o----o----o /
/_________________________/
__________________________
/ o----o /
/ / / \ / VTN-2
/ o-----o----o----o-----o /
/_________________________/
...... ...
___________________________
/ o----o /
/ / / / VTN-3
/ o-----o----o----o-----o /
/__________________________/
++++ ++++ ++++
+--+===+--+===+--+
+--+===+--+===+--+
++++ +++\\ ++++ Physical
|| || \\ ||
|| || \\ || Network
++++ ++++ ++++ \\+++ ++++
+--+===+--+===+--+===+--+===+--+ Infrastructure
+--+===+--+===+--+===+--+===+--+
++++ ++++ ++++ ++++ ++++
O Virtual Node
-- Virtual Link
++++
+--+ Physical Node with resource partition
+--+
++++
== 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 partition, such as FlexE, Time Sensitive Networking, Deterministic Networking, Dedicated queues, etc. These partitions may be physical, or virtual so long as the SLA required by the higher layers is met.
Based on the network resources provided by the physical network infrastructure, multiple VTNs can be provisioned, each with customized topology and other attributes to meet the requirement of different enhanced VPNs or different groups of enhanced VPNs. To get the required characteristic, each VTN needs to be mapped to a set of network nodes and links in the network infrastructure. And on each node or link, the VTN is associated with a set of resources which are allocated for the processing of traffic in the VTN. VTN provides the integration between the virtual network topology and the required underlying network resources.
The centralized controller is used to create the VTN, and to instruct the network nodes to allocate the required resources to each VTN and to provision the enhanced VPN services on the VTNs. A distributed control plane may also be used for the distribution of the VTN topology and attribute information between nodes within the VTNs.
The process used to create VTNs and to allocate network resources for use by VTNs needs to take a holistic view of the needs of all of its tenants (i.e., of all customers and their associated VTNs), and to partition the resources accordingly. However, within a VTN these resources can, if required, be managed via a dynamic control plane. This provides the required scalability and isolation.
At the VPN service level, the required connectivity is usually mesh or partial-mesh. To support such kinds of VPN service, the corresponding VTN in underlay is also an abstract MP2MP medium. Other service requirements may be expressed at different granularity, some of which can be applicable to the whole service, while some others may be only applicable to some pairs of end points. For example, when particular level of performance guarantee is required, the point-to-point path through the underlay of the enhanced VPN may need to be specifically engineered to meet the required performance guarantee.
Although a lot of the traffic that will be carried over the enhanced VPN will likely be IPv4 or IPv6, the design has to be capable of carrying other traffic types, in particular Ethernet traffic. This is easily accomplished through the various pseudowire (PW) techniques [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.
VPNs are instantiated as overlays on top of an operator's network and offered as services to the operator's customers. An important feature of overlays is that they are able to deliver services without placing per-service state in the core of the underlay network.
Enhanced VPNs may need to install some additional state within the network to achieve the additional features that they require. Solutions must consider minimizing and controlling the scale of such state, and deployment architectures should constrain the number of enhanced VPNs that would exist where such services would place additional state in the network. It is expected that the number of enhanced VPN would be small in the beginning, and even in future the number of enhanced VPN will be much fewer than traditional VPNs, because pre-existing VPN techniques are be good enough to meet the needs of most existing VPN-type services.
In general, it is not required that the state in the network be maintained in a 1:1 relationship with the VPN+ services. It will usually be possible to aggregate a set of VPN+ services so that they share the same VTN and the same set of network resources (much in the way that current VPNs are aggregated over transport tunnels) so that collections of enhanced VPNs that require the same behaviour from the network in terms of resource reservation, latency bounds, resiliency, etc. are able to be grouped together. This is an important feature to assist with the scaling characteristics of VPN+ deployments.
See Section 6 for a greater discussion of scalability considerations.
A VPN is a network created by applying a demultiplexing technique to the underlying network (the underlay) in order to distinguish the traffic of one VPN from that of another. A VPN path that travels by other than the shortest path through the underlay normally requires state in the underlay to specify that path. State is normally applied to the underlay through the use of the RSVP signaling protocol, or directly through the use of an SDN controller, although other techniques may emerge as this problem is studied. This state gets harder to manage as the number of VPN paths increases. Furthermore, as we increase the coupling between the underlay and the overlay to support the enhanced VPN service, this state will increase further.
In an enhanced VPN different subsets of the underlay resources can be dedicated to different enhanced VPNs or different groups of enhanced VPNs. An enhanced VPN solution thus needs tighter coupling with underlay than is the case with existing VPNs. We cannot, for example, share the network resource between enhanced VPNs which require hard isolation.
A number of candidate Layer 2 packet or frame-based data plane solutions which can be used provide the required isolation and guarantees are described in following sections.
FlexE [FLEXE] provides the ability to multiplex channels over an Ethernet link to create point-to-point fixed-bandwidth connections in a way that provides hard isolation. FlexE also supports bonding links to create larger links out of multiple low capacity links.
However, FlexE is only a link level technology. When packets are received by the downstream node, they need to be processed in a way that preserves that isolation in the downstream node. This in turn requires a queuing and forwarding implementation that preserves the end-to-end isolation.
If different FlexE channels are used for different services, then no sharing is possible between the FlexE channels. This means that it may be difficult to dynamically redistribute unused bandwidth to lower priority services in another FlexE channel. If one FlexE channel is used by one tenant, the tenant can use some methods to manage the relative priority of his own traffic in the FlexE channel.
DiffServ based queuing systems are described in [RFC2475] and [RFC4594]. This is considered insufficient to provide isolation for enhanced VPNs because DiffServ does not always provide enough markers to differentiate between traffic of many enhanced VPNs, or offer the range of service classes that each VPN needs to provide to its tenants. This problem is particularly acute with an MPLS underlay, because MPLS only provides eight Traffic Classes.
In addition, DiffServ, as currently implemented, mainly provides per-hop priority-based scheduling, and it is difficult to use it to achieve quantitive resource reservation.
In order to address these problems and to reduce the potential interference between enhanced VPNs, it would be necessary to steer traffic to dedicated input and output queues per enhanced VPN: some routers have a large number of queues and sophisticated queuing systems, which could support this, while some routers may struggle to provide the granularity and level of isolation required by the applications of enhanced VPN.
Time Sensitive Networking (TSN) [TSN] is an IEEE project that is designing a method of carrying time sensitive information over Ethernet. It introduces the concept of packet scheduling where a packet stream may be given a time slot guaranteeing that it experiences no queuing delay or increase in latency. The mechanisms defined in TSN can be used to meet the requirements of time sensitive services of an enhanced VPN.
Ethernet can be emulated over a Layer 3 network using an IP or MPLS pseudowire. However, a TSN Ethernet payload would be opaque to the underlay and thus not treated specifically as time sensitive data. The preferred method of carrying TSN over a Layer 3 network is through the use of deterministic networking as explained in Section 5.2.1.
We now consider the problem of slice differentiation and resource representation in the network layer.
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.
MPLS-TE [RFC2702][RFC3209] introduces the concept of reserving end-to-end bandwidth for a TE-LSP, which can be used to provide point- to-point Virtual Transport Path (VTP) across the underlay network to support VPNs. VPN traffic can be carried over dedicated TE-LSPs to provide reserved bandwidth for each specific connection in a VPN, and VPNs with similar behaviour requirements may be multiplexed onto the same TE-LSPs. Some network operators have concerns about the scalability and management overhead of MPLS-TE system, and this has lead them to consider other solutions for their networks.
Segment Routing (SR) [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.
An SR traffic engineered path operates with a granularity of a link with hints about priority provided through the use of the traffic class (TC) or Differentiated Services Code Point (DSCP) field in the header. However to achieve the latency and isolation characteristics that are sought by the enhanced VPN users, steering packets through specific queues and resources will likely be required. With SR, it is possible to introduce such fine-grained packet steering by specifying the queues and resources through an SR instruction list.
Note that the concept of queue is a useful abstraction for different types of underlay mechanism that may be used to provide enhanced isolation and latency support. How the queue satisfies the requirement is implementation specific and is transparent to the layer-3 data plane and control plane mechanisms used.
With Segment Routing, the SR instruction list could be used to build a P2P path, a group of SR SIDs could also be used to represent a MP2MP network. Thus the SR based mechanism could be used to provide both Virtual Transport Path (VTP) and Virtual Transport Network (VTN) for enhanced VPN services.
Non-packet underlay data plane technologies often have TE properties and behaviours, and meet many of the key requirements in particular for bandwidth guarantees, traffic isolation (with physical isolation often being an integral part of the technology), highly predictable latency and jitter characteristics, measurable loss characteristics, and ease of identification of flows. The cost is the resources are allocated on a long term and end-to-end basis. Such an arrangement means that the full cost of the resources has be borne by the service that is allocated with the resources.
Enhanced VPN would likely be based on a hybrid control mechanism, which takes advantage of the logically centralized controller for on-demand provisioning and global optimization, whilst still relying on a distributed control plane to provide scalability, high reliability, fast reaction, automatic failure recovery, etc. Extension to and optimization of the distributed control plane is needed to support the enhanced properties of VPN+.
RSVP-TE [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 Virtual Transport Path (VTP), which could be used to bind the VPN to specific network resources allocated within the underlay, but there remain scalability concerns mentioned in 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.
The management plane provides the interface between the enhanced VPN provider and the clients for the service life-cycle management (e.g. creation, modification, assurance/monitoring and decommissioning). It relies on a set of service data models for the description of the information and operations needed on the interface.
As an example, in the context of 5G end-to-end network slicing [TS28530], the management of enhanced VPNs is considered as the management of the transport network part of the end-to-end network slice. 3GPP management system may provide the connectivity and performance related parameters as requirements to the management plane of the transport network. It may also require the transport network to expose the capability and status of the transport network slice. Thus, an interface between the enhanced VPN management plane and the 3GPP network slice management system, and relevant service data models are needed for the coordination of end-to-end network slice management.
The management plane interface and data models for enhanced VPN can be based on the service models described in Section 5.6
ACTN supports operators in viewing and controlling different domains and presenting virtualized networks to their customers. The ACTN framework [RFC8453] highlights how:
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 [I-D.ietf-opsawg-l2nm] provide a network view of the ACTN managed infrastructure presented by the ACTN provider as a set of transport resources.
In order to support network slice service in transport network, a Transport Slice (TS) Northbound Interface (NBI) data model may be needed for a consumer to express the requirements for transport slices, which can be technology-agnostic. Then these requirements may be realized using technology-specific Southbound Interface (SBI).
As per [RFC8453] and [I-D.ietf-teas-actn-yang], the CNC-MDSC Interface (CMI) of ACTN is used to convey the virtual network service requirements, which is a generic interface to deliver various TE based VN services. In the context of network slice northbound interface, there may be some gaps in L3SM/L2SM or VN model, or the combination of them. The TS NBI is required to communicate the connectivity of the transport slice, along with the service level objective (SLO) parameters and traffic selection rules, and provides a way to monitor the state of the transport slice. This can be described in a more abstracted manner, so as to reduce the association with specific realization technologies of transport network slice, such as the VPN and TE technologies. The transport slice model as defined in [I-D.wd-teas-transport-slice-yang] provides an abstracted and generic approach to meet the transport slice NBI requirement.
The MDSC-PNC Interface (MPI) models in the ACTN architecture can be used for the realization of transport slices, for example, in a TE enabled transport network, and may also be used for cross-layer or cross-domain implementation of transport slice.
Enhanced VPN provides performance guaranteed services in packet networks, but with the potential cost of introducing additional states into the network. There are at least three ways that this additional state might be presented in the network:
Reducing the state in the network is important to enhanced VPN, as it requires the overlay to be more closely integrated with the underlay than with traditional VPNs. This tighter coupling would normally mean that more state needed to be created and maintained in the network, as the state about fine granularity processing would need to be loaded and maintained in the routers. However, a segment routed approach allows much of this state to be spread amongst the network ingress nodes, and transiently carried in the packets as SIDs.
One of the challenges with SR is the stack depth that nodes are able to impose on packets [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.
The traditional method of creating a resource allocated path through an MPLS network is to use the RSVP protocol. However there have been concerns that this requires significant continuous state maintenance in the network. Work to improve the scalability of RSVP-TE LSPs in the control plane can be found in [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.
The centralized approach of SDN requires state to be stored in the network, but does not have the overhead of also requiring control plane state to be maintained. Each individual network node may need to maintain a communication channel with the SDN controller, but that compares favourably with the need for a control plane to maintain communication with all neighbors.
However, SDN may transfer some of the scalability concerns from the network to the centralized controller. In particular, there may be a heavy processing burden at the controller, and a heavy load in the network surrounding the controller.
The enhanced VPN OAM design needs to consider the following requirements:
A study of OAM in SR networks has been documented in [RFC8403].
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 is out of the scope of this document.
Each enhanced VPN has a life-cycle, and may need modification during deployment as the needs of its tenant change. Additionally, as the network as a whole evolves, there may need to be garbage collection performed to consolidate resources into usable quanta.
Systems in which the path is imposed such as SR, or some form of explicit routing tend to do well in these applications, because it is possible to perform an atomic transition from one path to another. This is a single action by the head-end changes the path without the need for coordinated action by the routers along the path. However, implementations and the monitoring protocols need to make sure that the new path is up and meets the required SLA before traffic is transitioned to it. It is possible for deadlocks to arise as a result of the network becoming fragmented over time, such that it is impossible to create a new path or to modify an existing path without impacting the SLA of other paths. Resolution of this situation is as much a commercial issue as it is a technical issue and is outside the scope of this document.
There are, however, two manifestations of the latency problem that are for further study in any of these approaches:
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 [RFC8655].
In addition to the approach used to protect high priority packets, consideration has to be given to the impact of best effort traffic on the high priority packets during a transient. Specifically if a conventional re-convergence process is used there will inevitably be micro-loops and whilst some form of explicit routing will protect the high priority traffic, lower priority traffic on best effort shortest paths will micro-loop without the use of a loop prevention technology. To provide the highest quality of service to high priority traffic, either this traffic must be shielded from the micro-loops, or micro-loops must be prevented.
It is likely that enhanced VPN service will be introduced in networks which already have traditional VPN services deployed. Depends on service requirement, the tenants or the operator may choose to use traditional VPN or enhanced VPN to fulfil the service requirement. The information and parameters to assist such decision needs to be reflected on the management interface between the tenants and the operator.
All types of virtual network require special consideration to be given to the isolation of traffic belonging to different tenants. That is, traffic belonging to one VPN must not be delivered to end points outside that VPN. In this regard enhanced VPNs neither introduce, no experience a greater security risks than other VPNs.
However, in an enhanced Virtual Private Network service the additional service requirements need to be considered. For example, if a service requires a specific upper bound to latency then it can be damaged by simply delaying the packets through the activities of another tenant, i.e., by introducing bursts of traffic for other services.
The measures to address these dynamic security risks must be specified as part to the specific solution are form part of the isolation requirements of a service.
While an enhanced VPN service may be sold as offering encryption and other security features as part of the service, customers would be well advised to take responsibility for their own security requirements themselves possibly by encrypting traffic before handing it off to the service provider.
The privacy of enhanced VPN service customers must be preserved. It should not be possible for one customer to discover the existence of another customer, nor should the sites that are members of an enhanced VPN be externally visible.
There are no requested IANA actions.
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
The authors would like to thank Charlie Perkins, James N Guichard, John E Drake and Shunsuke Homma for their review and valuable comments.
This work was supported in part by the European Commission funded H2020-ICT-2016-2 METRO-HAUL project (G.A. 761727).
| [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. |
| [RFC3985] | Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture", RFC 3985, DOI 10.17487/RFC3985, March 2005. |
| [RFC4664] | Andersson, L. and E. Rosen, "Framework for Layer 2 Virtual Private Networks (L2VPNs)", RFC 4664, DOI 10.17487/RFC4664, September 2006. |