HomeDocsIETF standardsRFCRFC 3800 — 3899
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Network Working Group
Request for Comments: 3809
Category: Informational
A. Nagarajan, Ed.
Juniper Networks
June 2004

Generic Requirements for Provider Provisioned

Virtual Private Networks (PPVPN)

Status of this Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2004).

Abstract

This document describes generic requirements for Provider Provisioned Virtual Private Networks (PPVPN). The requirements are categorized into service requirements, provider requirements and engineering requirements. These requirements are not specific to any particular type of PPVPN technology, but rather apply to all PPVPN technologies. All PPVPN technologies are expected to meet the umbrella set of requirements described in this document.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
       1.1. Problem Statement . . . . . . . . . . . . . . . . . . . .  3
       1.2. Deployment Scenarios. . . . . . . . . . . . . . . . . . .  4
       1.3. Outline of this document. . . . . . . . . . . . . . . . .  5
   2.  Contributing Authors . . . . . . . . . . . . . . . . . . . . .  6
   3.  Definitions and Taxonomy . . . . . . . . . . . . . . . . . . .  7
   4.  Service Requirements . . . . . . . . . . . . . . . . . . . . .  7
       4.1. Availability  . . . . . . . . . . . . . . . . . . . . . .  7
       4.2. Stability . . . . . . . . . . . . . . . . . . . . . . . .  8
       4.3. Traffic types . . . . . . . . . . . . . . . . . . . . . .  8
       4.4. Data Isolation. . . . . . . . . . . . . . . . . . . . . .  9
       4.5. Security  . . . . . . . . . . . . . . . . . . . . . . . .  9
            4.5.1. User data security . . . . . . . . . . . . . . . . 10
            4.5.2. Access Control . . . . . . . . . . . . . . . . . . 10
            4.5.3. Site authentication and authorization. . . . . . . 10
            4.5.4. Inter domain security. . . . . . . . . . . . . . . 10
       4.6. Topology  . . . . . . . . . . . . . . . . . . . . . . . . 11
       4.7. Addressing. . . . . . . . . . . . . . . . . . . . . . . . 11
       4.8. Quality of Service  . . . . . . . . . . . . . . . . . . . 11
       4.9. Service Level Agreement and Service Level Specification
            Monitoring and Reporting. . . . . . . . . . . . . . . . . 13
       4.10.Network Resource Partitioning and Sharing between VPNs. . 14
   5.  Provider requirements. . . . . . . . . . . . . . . . . . . . . 14
       5.1. Scalability . . . . . . . . . . . . . . . . . . . . . . . 14
            5.1.1. Service Provider Capacity Sizing Projections . . . 15
            5.1.2. VPN Scalability aspects. . . . . . . . . . . . . . 15
            5.1.3. Solution-Specific Metrics. . . . . . . . . . . . . 17
       5.2. Management  . . . . . . . . . . . . . . . . . . . . . . . 18
            5.2.1. Customer Management of a VPN . . . . . . . . . . . 18
   6.  Engineering requirements . . . . . . . . . . . . . . . . . . . 19
       6.1. Forwarding plane requirements . . . . . . . . . . . . . . 19
       6.2. Control plane requirements. . . . . . . . . . . . . . . . 20
       6.3. Control Plane Containment . . . . . . . . . . . . . . . . 20
       6.4. Requirements related to commonality of PPVPN mechanisms
            with each other and with generic Internet mechanisms. . . 21
       6.5. Interoperability  . . . . . . . . . . . . . . . . . . . . 21
   7.  Security Considerations. . . . . . . . . . . . . . . . . . . . 22
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
       8.1. Normative References. . . . . . . . . . . . . . . . . . . 23
       8.2. Informative References. . . . . . . . . . . . . . . . . . 23
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
   10. Editor's Address . . . . . . . . . . . . . . . . . . . . . . . 24
   11. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 25

1. Introduction

This document is an output of the design team formed to develop requirements for PPVPNs in the Provider Provisioned Virtual Private Networks (PPVPN) working group and provides requirements that are generic to both Layer 2 Virtual Private Networks (L2VPN) and Layer 3 Virtual Private Networks (L3VPN). This document discusses generic PPVPN requirements categorized as service, provider and engineering requirements. These are independent of any particular type of PPVPN technology. In other words, all PPVPN technologies are expected to meet the umbrella set of requirements described in this document. PPVPNs may be constructed across single or multiple provider networks and/or Autonomous Systems (ASes). In most cases the generic requirements described in this document are independent of the deployment scenario. However, specific requirements that differ based on whether the PPVPN is deployed across single or multiple providers (and/or ASes) will be pointed out in the document. Specific requirements related to Layer 3 PPVPNs are described in [L3REQTS]. Similarly, requirements that are specific to layer 2 PPVPNs are described in [L2REQTS].

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

1.1. Problem Statement

Corporations and other organizations have become increasingly dependent on their networks for telecommunications and data communication. The data communication networks were originally built as Local Area Networks (LAN). Over time the possibility to interconnect the networks on different sites has become more and more important. The connectivity for corporate networks has been supplied by service providers, mainly as Frame Relay (FR) or Asynchronous Transfer Mode (ATM) connections, and more recently as Ethernet and IP-based tunnels. This type of network, interconnecting a number of sites over a shared network infrastructure is called Virtual Private Network (VPN). If the sites belong to the same organization, the VPN is called an Intranet. If the sites belong to different organizations that share a common interest, the VPN is called an Extranet.

Customers are looking for service providers to deliver data and telecom connectivity over one or more shared networks, with service level assurances in the form of security, QoS and other parameters.

In order to provide isolation between the traffic belonging to different customers, mechanisms such as Layer 2 connections or Layer 2/3 tunnels are necessary. When the shared infrastructure is an IP network, the tunneling technologies that are typically used are IPsec, MPLS, L2TP, GRE, IP-in-IP etc.

Traditional Internet VPNs have been based on IPsec to provide security over the Internet. Service providers are now beginning to deploy enhanced VPN services that provide features such as service differentiation, traffic management, Layer 2 and Layer 3 connectivity, etc. in addition to security. Newer tunneling mechanisms have certain features that allow the service providers to provide these enhanced VPN services.

The VPN solutions we define now MUST be able to accommodate the traditional types of VPNs as well as the enhanced services now being deployed. They need to be able to run in a single service provider's network, as well as between a set of service providers and across the Internet. In doing so the VPNs SHOULD NOT be allowed to violate basic Internet design principles or overload the Internet core routers or accelerate the growths of the Internet routing tables. Specifically, Internet core routers SHALL NOT be required to maintain VPN-related information, regardless of whether the Internet routing protocols are used to distribute this information or not. In order to achieve this, the mechanisms used to develop various PPVPN solutions SHALL be as common as possible with generic Internet infrastructure mechanisms like discovery, signaling, routing and management. At the same time, existing Internet infrastructure mechanisms SHALL NOT be overloaded.

Another generic requirement from a standardization perspective is to limit the number of different solution approaches. For example, for service providers that need to support multiple types of VPN services, it may be undesirable to require a completely different solution approach for each type of VPN service.

1.2. Deployment Scenarios

There are three different deployment scenarios that need to be considered for PPVPN services: 

   1. Single-provider, single-AS:  This is the least complex scenario,
      where the PPVPN service is offered across a single service
      provider network spanning a single Autonomous System.
   
   2. Single-provider, multi-AS: In this scenario, a single provider may
      have multiple Autonomous Systems (for e.g., a global Tier-1 ISP
      with different ASes depending on the global location, or an ISP

that has been created by mergers and acquisitions of multiple networks). This scenario involves the constrained distribution of routing information across multiple Autonomous Systems.

  1. Multi-provider: This scenario is the most complex, wherein trust negotiations need to be made across multiple service provider backbones in order to meet the security and service level agreements for the PPVPN customer. This scenario can be generalized to cover the Internet, which comprises of multiple service provider networks. It should be noted that customers can construct their own VPNs across multiple providers. However such VPNs are not considered here as they would not be "Provider- provisioned".

A fourth scenario, "Carrier's carrier" VPN may also be considered. In this scenario, a service provider (for example, a Tier 1 service provider) provides VPN service to another service provider (for example, a Tier 2 service provider), which in turn provides VPN service on its VPN to its customers. In the example given above, the Tier 2 provider's customers are contained within the Tier 2 provider's network, and the Tier 2 provider itself is a customer of the Tier 1 provider's network. Thus, this scenario is not treated separately in the document, because all of the single provider requirements would apply equally to this case.

It is expected that many of the generic requirements described in this document are independent of the three deployment scenarios listed above. However, specific requirements that are indeed dependent on the deployment scenario will be pointed out in this document.

1.3. Outline of this document

This document describes generic requirements for Provider Provisioned Virtual Private Networks (PPVPN). The document contains several sections, with each set representing a significant aspect of PPVPN requirements.

Section 2 lists authors who contributed to this document. Section 3 defines terminology and presents a taxonomy of PPVPN technologies. The taxonomy contains two broad classes, representing Layer 2 and Layer 3 VPNs. Each top level VPN class contains subordinate classes. For example, the Layer 3 VPN class contains a subordinate class of PE-based Layer 3 VPNs.

Sections 4, 5, 6 describe generic PPVPN requirements.

The requirements are broadly classified under the following categories:

  1. Service requirements - Service attributes that the customer can observe or measure. For example, does the service forward frames or route datagrams? What security guarantees does the service provide? Availability and stability are key requirements in this category.
  1. Provider requirements - Characteristics that Service Providers use to determine the cost-effectiveness of a PPVPN service. Scaling and management are examples of Provider requirements.
  1. Engineering requirements - Implementation characteristics that make service and provider requirements achievable. These can be further classified as:
      3a) Forwarding plane requirements - e.g., requirements related to
          router forwarding behavior.
      
      3b) Control plane requirements - e.g., requirements related to
          reachability and distribution of reachability information.

3c) Requirements related to the commonality of PPVPN mechanisms

with each other and with generic Internet mechanisms.

2. Contributing Authors

This document was the combined effort of several individuals that were part of the Service Provider focus group whose intentions were to present Service Provider view on the general requirements for PPVPN. A significant set of requirements were directly taken from previous work by the PPVPN WG to develop requirements for Layer 3 PPVPN [L3REQTS]. The existing work in the L2 requirements area has also influenced the contents of this document [L2REQTS].

Besides the editor, the following are the authors that contributed to this document: 

      Loa Andersson (loa@pi.se)
      Ron Bonica (ronald.p.bonica@mci.com)
      Dave McDysan (dave.mcdysan@mci.com)
      Junichi Sumimoto (j.sumimoto@ntt.com)
      Muneyoshi Suzuki (suzuki.muneyoshi@lab.ntt.co.jp)
      David Meyer (dmm@1-4-5.net)
      Marco Carugi (marco.carugi@nortelnetworks.com)
      
      Yetik Serbest (yetik_serbest@labs.sbc.com)
      Luyuan Fang (luyuanfang@att.com)
      Javier Achirica (achirica@telefonica.net)

3. Definitions and Taxonomy

The terminology used in this document is defined in [TERMINOLOGY]. In addition the following terminology is used:

Site: a geographical location with one or more users or one or more servers or a combination of servers and users.

User:

the end user equipment (hosts), e.g., a workstation.

                        PPVPN
          ________________|__________________
          |                                 |
       Layer 2 (L2)                     Layer 3 (L3)
    ______|_____                      ______|________
    |          |                      |             |
   PE-based   CE-based             PE-based       CE-based
    |__________|
    ______|_____
    |          |
   P2P        P2MP

The figure above presents a taxonomy of PPVPN technologies. PE-based and CE-based Layer 2 VPNs may also be further classified as point-to- point (P2P) or point-to-multipoint (P2MP). It is also the intention of the working group to have a limited number of solutions, and this goal must be kept in mind when proposing solutions that meet the requirements specified in this document. Definitions for CE-based and PE-based PPVPNs can be obtained from [L3FRAMEWORK]. Layer 2 specific definitions can be obtained from [L2FRAMEWORK].

4. Service requirements

These are the requirements that a customer can observe or measure, in order to verify if the PPVPN service that the Service Provider (SP) provides is satisfactory. As mentioned before, each of these requirements apply equally across each of the three deployment scenarios unless stated otherwise.

4.1. Availability

VPN services MUST have high availability. VPNs that are distributed over several sites require connectivity to be maintained even in the event of network failures or degraded service.

This can be achieved via various redundancy techniques such as:

  1. Physical Diversity

A single site connected to multiple CEs (for CE-based PPVPNs) or PEs (for PE-based PPVPNs), or different POPs, or even different service providers.

  1. Tunnel redundancy

Redundant tunnels may be set up between the PEs (in a PE-based PPVPN) or the CEs (in a CE-based PPVPN) so that if one tunnel fails, VPN traffic can continue to flow across the other tunnel that has already been set-up in advance.

Tunnel redundancy may be provided over and above physical diversity. For example, a single site may be connected to two CEs (for CE-based PPVPNs) or two PEs (for PE-based PPVPNs). Tunnels may be set up between each of the CEs (or PEs as the case may be) across different sites.

Of course, redundancy means additional resources being used, and consequently, management of additional resources, which would impact the overall scaling of the service.

It should be noted that it is difficult to guarantee high availability when the VPN service is across multiple providers, unless there is a negotiation between the different service providers to maintain the service level agreement for the VPN customer.

4.2. Stability

In addition to availability, VPN services MUST also be stable. Stability is a function of several components such as VPN routing, signaling and discovery mechanisms, in addition to tunnel stability. For example, in the case of routing, route flapping or routing loops MUST be avoided in order to ensure stability. Stability of the VPN service is directly related to the stability of the mechanisms and protocols used to establish the service. It SHOULD also be possible to allow network upgrades and maintenance procedures without impacting the VPN service.

4.3. Traffic types

VPN services MUST support unicast (or point to point) traffic and SHOULD support any-to-any or point-to-multipoint traffic including multicast and broadcast traffic. In the broadcast model, the network delivers a stream to all members of a subnetwork, regardless of their interest in that stream. In the multicast model, the network delivers a stream to a set of destinations that have registered interest in the stream. All destinations need not belong to the same subnetwork. Multicast is more applicable to L3 VPNs while broadcast is more applicable to L2VPNs. It is desirable to support multicast limited in scope to an intranet or extranet. The solution SHOULD be able to support a large number of such intranet or extranet specific multicast groups in a scalable manner.

All PPVPN approaches SHALL support both IPv4 and IPv6 traffic. Specific L2 traffic types (e.g., ATM, Frame Relay and Ethernet) SHALL be supported via encapsulation in IP or MPLS tunnels in the case of L2VPNs.

4.4. Data isolation

The PPVPN MUST support forwarding plane isolation. The network MUST never deliver user data across VPN boundaries unless the two VPNs participate in an intranet or extranet.

Furthermore, if the provider network receives signaling or routing information from one VPN, it MUST NOT reveal that information to another VPN unless the two VPNs participate in an intranet or extranet. It should be noted that the disclosure of any signaling/routing information across an extranet MUST be filtered per the extranet agreement between the organizations participating in the extranet.

4.5. Security

A range of security features SHOULD be supported by the suite of PPVPN solutions in the form of securing customer flows, providing authentication services for temporary, remote or mobile users, and the need to protect service provider resources involved in supporting a PPVPN. These security features SHOULD be implemented based on the framework outlined in [VPN-SEC]. Each PPVPN solution SHOULD state which security features it supports and how such features can be configured on a per customer basis. Protection against Denial of Service (DoS) attacks is a key component of security mechanisms. Examples of DoS attacks include attacks to the PE or CE CPUs, access connection congestion, TCP SYN attacks and ping attacks.

Some security mechanisms (such as use of IPsec on a CE-to-CE basis) may be equally useful regardless of the scope of the VPN. Other mechanisms may be more applicable in some scopes than in others. For example, in some cases of single-provider single-AS VPNs, the VPN service may be isolated from some forms of attack by isolating the infrastructure used for supporting VPNs from the infrastructure used for other services. However, the requirements for security are common regardless of the scope of the VPN service.

4.5.1. User data security

PPVPN solutions that support user data security SHOULD use standard methods (e.g., IPsec) to achieve confidentiality, integrity, authentication and replay attack prevention. Such security methods MUST be configurable between different end points, such as CE-CE, PE-PE, and CE-PE. It is also desirable to configure security on a per-route or per-VPN basis. User data security using encryption is especially desirable in the multi-provider scenario.

4.5.2. Access control

A PPVPN solution may also have the ability to activate the appropriate filtering capabilities upon request of a customer. A filter provides a mechanism so that access control can be invoked at the point(s) of communication between different organizations involved in an extranet. Access control can be implemented by a firewall, access control lists on routers, cryptographic mechanisms or similar mechanisms to apply policy-based access control. Access control MUST also be applicable between CE-CE, PE-PE and CE-PE. Such access control mechanisms are desirable in the multi-provider scenario.

4.5.3. Site authentication and authorization

A PPVPN solution requires authentication and authorization of the following:

  • temporary and permanent access for users connecting to sites (authentication and authorization BY the site)
  • the site itself (authentication and authorization FOR the site)

4.5.4. Inter domain security

The VPN solution MUST have appropriate security mechanisms to prevent the different kinds of Distributed Denial of Service (DDoS) attacks mentioned earlier, misconfiguration or unauthorized accesses in inter domain PPVPN connections. This is particularly important for multi- service provider deployment scenarios. However, this will also be important in single-provider multi-AS scenarios.

4.6. Topology

A VPN SHOULD support arbitrary, customer-defined inter-site connectivity, ranging, for example, from hub-and-spoke, partial mesh to full mesh topology. These can actually be different from the topology used by the service provider. To the extent possible, a PPVPN service SHOULD be independent of the geographic extent of the deployment.

Multiple VPNs per customer site SHOULD be supported without requiring additional hardware resources per VPN. This SHOULD also include a free mix of L2 and L3 VPNs.

To the extent possible, the PPVPN services SHOULD be independent of access network technology.

4.7. Addressing

Each customer resource MUST be identified by an address that is unique within its VPN. It need not be identified by a globally unique address.

Support for private addresses as described in [RFC1918], as well as overlapping customer addresses SHALL be supported. One or more VPNs for each customer can be built over the same infrastructure without requiring any of them to renumber. The solution MUST NOT use NAT on the customer traffic to achieve that goal. Interconnection of two networks with overlapping IP addresses is outside the scope of this document.

A VPN service SHALL be capable of supporting non-IP customer addresses via encapsulation techniques, if it is a Layer 2 VPN (e.g., Frame Relay, ATM, Ethernet). Support for non-IP Layer 3 addresses may be desirable in some cases, but is beyond the scope of VPN solutions developed in the IETF, and therefore, this document.

4.8. Quality of Service

A technical approach for supporting VPNs SHALL be able to support QoS via IETF standardized mechanisms such as Diffserv. Support for best-effort traffic SHALL be mandatory for all PPVPN types. The extent to which any specific VPN service will support QoS is up to the service provider. In many cases single-provider single-AS VPNs will offer QoS guarantees. Support of QoS guarantees in the multi- service-provider case will require cooperation between the various service providers involved in offering the service.

It should be noted that QoS mechanisms in the multi-provider scenario REQUIRES each of the participating providers to support the mechanisms being used, and as such, this is difficult to achieve.

Note that all cases involving QoS may require that the CE and/or PE perform shaping and/or policing.

The need to provide QoS will occur primarily in the access network, since that will often be the bottleneck. This is likely to occur since the backbone effectively statistically multiplexes many users, and is traffic engineered or includes capacity for restoration and growth. Hence in most cases PE-PE QoS is not a major issue. As far as access QoS is concerned, there are two directions of QoS management that may be considered in any PPVPN service regarding QoS:

- From the CE across the access network to the PE
- From the PE across the access network to CE

PPVPN CE and PE devices SHOULD be capable of supporting QoS across at least the following subset of access networks, as applicable to the specific type of PPVPN (L2 or L3). However, to the extent possible, the QoS capability of a PPVPN SHOULD be independent of the access network technology: 

   -  ATM Virtual Connections (VCs)
   -  Frame Relay Data Link Connection Identifiers (DLCIs)
   -  802.1d Prioritized Ethernet
   -  MPLS-based access
   -  Multilink Multiclass PPP
   -  QoS-enabled wireless (e.g., LMDS, MMDS)
   -  Cable modem
   -  QoS-enabled Digital Subscriber Line (DSL)

Different service models for QoS may be supported. Examples of PPVPN QoS service models are:

  • Managed access service: Provides QoS on the access connection between CE and the customer facing ports of the PE. No QoS support is required in the provider core network in this case.
   -  Edge-to-edge QoS: Provides QoS across the provider core, either
      between CE pairs or PE pairs, depending on the tunnel demarcation
      points.  This scenario requires QoS support in the provider core
      network.  As mentioned above, this is difficult to achieve in a
      multi-provider VPN offering.

4.9. Service Level Agreement and Service Level Specification Monitoring

and Reporting

A Service Level Specification (SLS) may be defined per access network connection, per VPN, per VPN site, and/or per VPN route. The service provider may define objectives and the measurement interval for at least the SLS using the following Service Level Objective (SLO) parameters:

  • QoS and traffic parameters for the Intserv flow or Diffserv class [Y.1541]
  • Availability for the site, VPN, or access connection
  • Duration of outage intervals per site, route or VPN
  • Service activation interval (e.g., time to turn up a new site)
  • Trouble report response time interval
  • Time to repair interval
  • Total traffic offered to the site, route or VPN
  • Measure of non-conforming traffic for the site, route or VPN
  • Delay and delay variation (jitter) bounds
  • Packet ordering, at least when transporting L2 services sensitive to reordering (e.g., ATM).

The above list contains items from [Y.1241], as well as other items typically part of SLAs for currently deployed VPN services [FRF.13]. See [RFC3198] for generic definitions of SLS, SLA, and SLO.

The provider network management system SHALL measure, and report as necessary, whether measured performance meets or fails to meet the above SLS objectives.

In many cases the guaranteed levels for Service Level Objective (SLO) parameters may depend upon the scope of the VPN. For example, one level of guarantee might be provided for service within a single AS. A different (generally less stringent) guarantee might be provided within multiple ASs within a single service provider. At the current time, in most cases specific guarantees are not offered for multi- provider VPNs, and if guarantees were offered they might be expected to be less stringent still.

The service provider and the customer may negotiate a contractual arrangement that includes a Service Level Agreement (SLA) regarding compensation if the provider does not meet an SLS performance objective. Details of such compensation are outside the scope of this document.

4.10. Network Resource Partitioning and Sharing between VPNs

Network resources such as memory space, FIB table, bandwidth and CPU processing SHALL be shared between VPNs and, where applicable, with non-VPN Internet traffic. Mechanisms SHOULD be provided to prevent any specific VPN from taking up available network resources and causing others to fail. SLAs to this effect SHOULD be provided to the customer.

Similarly, resources used for control plane mechanisms are also shared. When the service provider's control plane is used to distribute VPN specific information and provide other control mechanisms for VPNs, there SHALL be mechanisms to ensure that control plane performance is not degraded below acceptable limits when scaling the VPN service, or during network events such as failure, routing instabilities etc. Since a service provider's network would also be used to provide Internet service, in addition to VPNs, mechanisms to ensure the stable operation of Internet services and other VPNs SHALL be made in order to avoid adverse effects of resource hogging by large VPN customers.

5. Provider requirements

This section describes operational requirements for a cost-effective, profitable VPN service offering.

5.1. Scalability

The scalability for VPN solutions has many aspects. The list below is intended to comprise of the aspects that PPVPN solutions SHOULD address. Clearly these aspects in absolute figures are very different for different types of VPNs - i.e., a point to point service has only two sites, while a VPLS or L3VPN may have a larger number of sites. It is also important to verify that PPVPN solutions not only scales on the high end, but also on the low end - i.e., a VPN with three sites and three users should be as viable as a VPN with hundreds of sites and thousands of users.

5.1.1. Service Provider Capacity Sizing Projections

A PPVPN solution SHOULD be scalable to support a very large number of VPNs per Service Provider network. The estimate is that a large service provider will require support for O(10^4) VPNs within four years.

A PPVPN solution SHOULD be scalable to support a wide range of number of site interfaces per VPN, depending on the size and/or structure of the customer organization. The number of site interfaces SHOULD range from a few site interfaces to over 50,000 site interfaces per VPN.

A PPVPN solution SHOULD be scalable to support of a wide range of number of routes per VPN. The number of routes per VPN may range from just a few to the number of routes exchanged between ISPs (O(10^5)), with typical values being in the O(10^3) range. The high end number is especially true considering the fact that many large ISPs may provide VPN services to smaller ISPs or large corporations. Typically, the number of routes per VPN is at least twice the number of site interfaces.

A PPVPN solution SHOULD support high values of the frequency of configuration setup and change, e.g., for real-time provisioning of an on-demand videoconferencing VPN or addition/deletion of sites. 

   Approaches SHOULD articulate scaling and performance limits for more
   complex deployment scenarios, such as single-provider multi-AS VPNs,
   multi-provider VPNs and carriers' carrier.  Approaches SHOULD also
   describe other dimensions of interest, such as capacity requirements
   or limits, number of interworking instances supported  as well as any
   scalability implications on management systems.

A PPVPN solution SHOULD support a large number of customer interfaces on a single PE (for PE-based PPVPN) or CE (for CE-based PPVPN) with current Internet protocols.

5.1.2. VPN Scalability aspects

This section describes the metrics for scaling PPVPN solutions, points out some of the scaling differences between L2 and L3 VPNs. It should be noted that the scaling numbers used in this document must be treated as typical examples as seen by the authors of this document. These numbers are only representative and different service providers may have different requirements for scaling. Further discussion on service provider sizing projections is in Section 5.1.1. Please note that the terms "user" and "site" are as defined in Section 3. It should also be noted that the numbers given below would be different depending on whether the scope of the VPN is single-provider single-AS, single-provider multi-AS, or multi- provider. Clearly, the larger the scope, the larger the numbers that may need to be supported. However, this also means more management issues. The numbers below may be treated as representative of the single-provider case.

5.1.2.1. Number of users per site

The number of users per site follows the same logic as for users per VPN. Further, it must be possible to have single user sites connected to the same VPN as very large sites are connected to.

L3 VPNs SHOULD scale from 1 user per site to O(10^4) per site. L2 VPNs SHOULD scale from 1 user to O(10^3) per site for point-to-point VPNs and to O(10^4) for point-to-multipoint VPNs.

5.1.2.2. Number of sites per VPN

The number of sites per VPN clearly depends on the number of users per site. VPNs SHOULD scale from 2 to O(10^3) sites per VPN. These numbers are usually limited by device memory.

5.1.2.3. Number of PEs and CEs

The number of PEs that supports the same set of VPNs, i.e., the number of PEs that needs to directly exchange information on VPN de- multiplexing information is clearly a scaling factor in a PE-based VPN. Similarly, in a CE-based VPN, the number of CEs is a scaling factor. This number is driven by the type of VPN service, and also by whether the service is within a single AS/domain or involves a multi-SP or multi-AS network. Typically, this number SHOULD be as low as possible in order to make the VPN cost effective and manageable.

5.1.2.4. Number of sites per PE

The number of sites per PE needs to be discussed based on several different scenarios. On the one hand there is a limitation to the number of customer facing interfaces that the PE can support. On the other hand the access network may aggregate several sites connected on comparatively low bandwidth on to one single high bandwidth interface on the PE. The scaling point here is that the PE SHOULD be able to support a few or even a single site on the low end and O(10^4) sites on the high end. This number is also limited by device memory. Implementations of PPVPN solutions may be evaluated based on this requirement, because it directly impacts cost and manageability of a VPN.

5.1.2.5. Number of VPNs in the network

The number of VPNs SHOULD scale linearly with the size of the access network and with the number of PEs. As mentioned in Section 5.1.1, the number of VPNs in the network SHOULD be O(10^4). This requirement also effectively places a requirement on the number of tunnels that SHOULD be supported in the network. For a PE-based VPN, the number of tunnels is of the same order as the number of VPNs. For a CE-based VPN, the number of tunnels in the core network may be fewer, because of the possibility of tunnel aggregation or multiplexing across the core.

5.1.2.6. Number of VPNs per customer

In some cases a service provider may support multiple VPNs for the same customer of that service provider. For example, this may occur due to differences in services offered per VPN (e.g., different QoS, security levels, or reachability) as well as due to the presence of multiple workgroups per customer. It is possible that one customer will run up to O(100) VPNs.

5.1.2.7. Number of addresses and address prefixes per VPN

Since any VPN solution SHALL support private customer addresses, the number of addresses and address prefixes are important in evaluating the scaling requirements. The number of address prefixes used in routing protocols and in forwarding tables specific to the VPN needs to scale from very few (for smaller customers) to very large numbers seen in typical Service Provider backbones. The high end is especially true considering that many Tier 1 SPs may provide VPN services to Tier 2 SPs or to large corporations. For a L2 VPN this number would be on the order of addresses supported in typical native Layer 2 backbones.

5.1.3. Solution-Specific Metrics

Each PPVPN solution SHALL document its scalability characteristics in quantitative terms. A VPN solution SHOULD quantify the amount of state that a PE and P device has to support. This SHOULD be stated in terms of the order of magnitude of the number of VPNs and site interfaces supported by the service provider. Ideally, all VPN- specific state SHOULD be contained in the PE device for a PE-based VPN. Similarly, all VPN-specific state SHOULD be contained in the CE device for a CE-based VPN. In all cases, the backbone routers (P devices) SHALL NOT maintain VPN-specific state as far as possible.

Another metric is that of complexity. In a PE-based solution the PE is more complex in that it has to maintain tunnel-specific information for each VPN, but the CE is simpler since it does not need to support tunnels. On the other hand, in a CE-based solution, the CE is more complex since it has to implement routing across a number of tunnels to other CEs in the VPN, but the PE is simpler since it has only one routing and forwarding instance. Thus, the complexity of the PE or CE SHOULD be noted in terms of their processing and management functions.

5.2. Management

A service provider MUST have a means to view the topology, operational state, service order status, and other parameters associated with each customer's VPN. Furthermore, the service provider MUST have a means to view the underlying logical and physical topology, operational state, provisioning status, and other parameters associated with the equipment providing the VPN service(s) to its customers.

In the multi-provider scenario, it is unlikely that participating providers would provide each other a view to the network topology and other parameters mentioned above. However, each provider MUST ensure via management of their own networks that the overall VPN service offered to the customers are properly managed. In general the support of a single VPN spanning multiple service providers requires close cooperation between the service providers. One aspect of this cooperation involves agreement on what information about the VPN will be visible across providers, and what network management protocols will be used between providers.

VPN devices SHOULD provide standards-based management interfaces wherever feasible.

5.2.1. Customer Management of a VPN

A customer SHOULD have a means to view the topology, operational state, service order status, and other parameters associated with his or her VPN.

All aspects of management information about CE devices and customer attributes of a PPVPN manageable by an SP SHOULD be capable of being configured and maintained by the customer after being authenticated and authorized.

A customer SHOULD be able to make dynamic requests for changes to traffic parameters. A customer SHOULD be able to receive real-time response from the SP network in response to these requests. One example of such as service is a "Dynamic Bandwidth management" capability, that enables real-time response to customer requests for changes of allocated bandwidth allocated to their VPN(s). A possible outcome of giving customers such capabilities is Denial of Service attacks on other VPN customers or Internet users. This possibility is documented in the Security Considerations section.

6. Engineering requirements

These requirements are driven by implementation characteristics that make service and provider requirements achievable.

6.1. Forwarding plane requirements

VPN solutions SHOULD NOT pre-suppose or preclude the use of IETF developed tunneling techniques such as IP-in-IP, L2TP, GRE, MPLS or IPsec. The separation of VPN solution and tunnels will facilitate adaptability with extensions to current tunneling techniques or development of new tunneling techniques. It should be noted that the choice of the tunneling techniques may impact the service and scaling capabilities of the VPN solution.

It should also be noted that specific tunneling techniques may not be feasible depending on the deployment scenario. In particular, there is currently very little use of MPLS in the inter-provider scenario. Thus, native MPLS support may be needed between the service providers, or it would be necessary to run MPLS over IP or GRE. It should be noted that if MPLS is run over IP or GRE, some of the other capabilities of MPLS, such as Traffic Engineering, would be impacted. Also note that a service provider MAY optionally choose to use a different encapsulation for multi-AS VPNs than is used for single AS VPNs. Similarly, a group of service providers may choose to use a different encapsulation for multi-service provider VPNs than for VPNs within a single service provider.

For Layer 2 VPNs, solutions SHOULD utilize the encapsulation techniques defined by the Pseudo-Wire Emulation Edge-to-Edge (PWE3) Working Group, and SHOULD NOT impose any new requirements on these techniques.

PPVPN solutions MUST NOT impose any restrictions on the backbone traffic engineering and management techniques. Conversely, backbone engineering and management techniques MUST NOT affect the basic operation of a PPVPN, apart from influencing the SLA/SLS guarantees associated with the service. The SP SHOULD, however, be REQUIRED to provide per-VPN management, tunnel maintenance and other maintenance required in order to meet the SLA/SLS.

By definition, VPN traffic SHOULD be segregated from each other, and from non-VPN traffic in the network. After all, VPNs are a means of dividing a physical network into several logical (virtual) networks. VPN traffic separation SHOULD be done in a scalable fashion. However, safeguards SHOULD be made available against misbehaving VPNs to not affect the network and other VPNs.

A VPN solution SHOULD NOT impose any hard limit on the number of VPNs provided in the network.

6.2. Control plane requirements

The plug and play feature of a VPN solution with minimum configuration requirements is an important consideration. The VPN solutions SHOULD have mechanisms for protection against customer interface and/or routing instabilities so that they do not impact other customers' services or impact general Internet traffic handling in any way.

A VPN SHOULD be provisioned with minimum number of steps. For instance, a VPN need not be configured in every PE. For this to be accomplished, an auto-configuration and an auto-discovery protocol, which SHOULD be as common as possible to all VPN solutions, SHOULD be defined. However, these mechanisms SHOULD NOT adversely affect the cost, scalability or stability of a service by being overly complex, or by increasing layers in the protocol stack.

Mechanisms to protect the SP network from effects of misconfiguration of VPNs SHOULD be provided. This is especially of importance in the multi-provider case, where misconfiguration could possibly impact more than one network.

6.3. Control Plane Containment

The PPVPN control plane MUST include a mechanism through which the service provider can filter PPVPN related control plane information as it passes between Autonomous Systems. For example, if a service provider supports a PPVPN offering, but the service provider's neighbors do not participate in that offering, the service provider SHOULD NOT leak PPVPN control information into neighboring networks. Neighboring networks MUST be equipped with mechanisms that filter this information should the service provider leak it. This is important in the case of multi-provider VPNs as well as single- provider multi-AS VPNs.

6.4. Requirements related to commonality of PPVPN mechanisms with each

other and with generic Internet mechanisms

As far as possible, the mechanisms used to establish a VPN service SHOULD re-use well-known IETF protocols, limiting the need to define new protocols from scratch. It should, however, be noted that the use of Internet mechanisms for the establishment and running of an Internet-based VPN service, SHALL NOT affect the stability, robustness, and scalability of the Internet or Internet services. In other words, these mechanisms SHOULD NOT conflict with the architectural principles of the Internet, nor SHOULD it put at risk the existing Internet systems. For example, IETF-developed routing protocols SHOULD be used for routing of L3 PPVPN traffic, without adding VPN-specific state to the Internet core routers. Similarly, well-known L2 technologies SHOULD be used in VPNs offering L2 services, without imposing risks to the Internet routers. A solution MUST be implementable without requiring additional functionality to the P devices in a network, and minimal functionality to the PE in a PE-based VPN and CE in a CE-based VPN.

In addition to commonality with generic Internet mechanisms, infrastructure mechanisms used in different PPVPN solutions (both L2 and L3), e.g., discovery, signaling, routing and management, SHOULD be as common as possible.

6.5. Interoperability

Each technical solution is expected to be based on interoperable Internet standards.

Multi-vendor interoperability at network element, network and service levels among different implementations of the same technical solution SHOULD be ensured (that will likely rely on the completeness of the corresponding standard). This is a central requirement for SPs and customers.

The technical solution MUST be multi-vendor interoperable not only within the SP network infrastructure, but also with the customer's network equipment and services making usage of the PPVPN service.

Customer access connections to a PPVPN solution may be different at different sites (e.g., Frame Relay on one site and Ethernet on another).

Interconnection of a L2VPN over an L3VPN as if it were a customer site SHALL be supported. However, interworking of Layer 2 technologies is not required, and is outside the scope of the working group, and therefore, of this document.

Inter-domain interoperability - It SHOULD be possible to deploy a PPVPN solution across domains, Autonomous Systems, or the Internet.

7. Security Considerations

Security requirements for Provider Provisioned VPNs have been described in Section 4.5. In addition, the following considerations need to be kept in mind when a provider provisioned VPN service is provided across a public network infrastructure that is also used to provide Internet connectivity. In general, the security framework described in [VPN-SEC] SHOULD be used as far as it is applicable to the given type of PPVPN service.

The PE device has a lot of functionality required for the successful operation of the VPN service. The PE device is frequently also part of the backbone providing Internet services, and is therefore susceptible to security and denial of service attacks. The PE control plane CPU is vulnerable from this point of view, and it may impact not only VPN services but also general Internet services if not adequately protected. In addition to VPN configuration, if mechanisms such as QoS are provisioned on the PE, it is possible for attackers to recognize the highest priority traffic or customers and launch directed attacks. Care SHOULD be taken to prevent such attacks whenever any value added services such as QoS are offered.

When a service such as "Dynamic Bandwidth Management" as described in Section 5.2.1 is provided, it allows customers to dynamically request for changes to their bandwidth allocation. The provider MUST take care to authenticate such requests and detect and prevent possible Denial-of-Service attacks. These DoS attacks are possible when a customer maliciously or accidentally may cause a change in bandwidth allocation that may impact the bandwidth allocated to other VPN customers or Internet users.

Different choices of VPN technology have different assurance levels of the privacy of a customer's network. For example, CE-based solutions may enjoy more privacy than PE-based VPNs by virtue of tunnels extending from CE to CE, even if the tunnels are not encrypted. In a PE-based VPN, a PE has many more sites than those attached to a CE in a CE-based VPN. A large number of these sites may use [RFC1918] addresses. Provisioning mistakes and PE software bugs may make traffic more prone to being misdirected as opposed to a CE-based VPN. Care MUST be taken to prevent misconfiguration in all kinds of PPVPNs, but more care MUST be taken in the case of PE-based VPNs, as this could impact other customers and Internet services. Similarly, there SHOULD be mechanisms to prevent the flooding of

Internet routing tables whenever there is a misconfiguration or failure of PPVPN control mechanisms that use Internet routing protocols for relay of VPN-specific information.

Different deployment scenarios also dictate the level of security that may be needed for a VPN. For example, it is easier to control security in a single provider, single AS VPN and therefore, expensive encryption techniques may not be used in this case, as long as VPN traffic is isolated from the Internet. There is a reasonable amount of control possible in the single provider, multi AS case, although care SHOULD be taken to ensure the constrained distribution of VPN route information across the ASes. Security is more of a challenge in the multi-provider case, where it may be necessary to adopt encryption techniques in order to provide the highest level of security.

8. References

8.1. Normative References

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2. Informative References

   [TERMINOLOGY] Andersson, L., Madsen, T., "Terminology for Provider
                 Provisioned Virtual Private Networks", Work in
                 Progress.
   
   [L3FRAMEWORK] Callon, R., Suzuki, M., et al. "A Framework for Layer 3
                 Provider Provisioned Virtual Private Networks", Work in
                 Progress, March 2003.
   
   [L2FRAMEWORK] Andersson, L., et al. "Framework for Layer 2 Virtual
                 Private Networks (L2VPNs)", Work in Progress, March
                 2004.
   
   [L3REQTS]     Carugi, M., McDysan, D. et al., "Service Requirements
                 for Layer 3 Provider Provisioned Virtual Private
                 Networks", Work in Progress, April 2003.
   
   [L2REQTS]     Augustyn, W., Serbest, Y., et al., "Service
                 Requirements for Layer 2 Provider Provisioned Virtual
                 Private Networks", Work in Progress, April 2003.
   
   [Y.1241]      "IP Transfer Capability for the support of IP based
                 Services", Y.1241 ITU-T Draft Recommendation, March
                 2000.
   
   [RFC1918]     Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot,
                 G. and E. Lear, "Address Allocation for Private
                 Internets", BCP 5, RFC 1918, February 1996.
   
   [RFC3198]     Westerinen, A., Schnizlein, J., Strassner, J.,
                 Scherling, M., Quinn, B., Herzog, S., Huynh, A.,
                 Carlson, M., Perry, J. and S. Waldbusser, "Terminology
                 for Policy-Based Management", RFC 3198, November 2001.
   
   [VPN-SEC]     Fang, L., et al., "Security Framework for Provider
                 Provisioned Virtual Private Networks", Work in
                 Progress, February 2004.
   
   [FRF.13]      Frame Relay Forum, "Service Level Definitions
                 Implementation Agreement", August 1998.
   
   [Y.1541]      "Network Performance Objectives for IP-based Services",
                 Y.1541, ITU-T Recommendation.

9. Acknowledgements

This work was done in consultation with the entire design team for PPVPN requirements. A lot of the text was adapted from the Layer 3 requirements document produced by the Layer 3 requirements design team. The authors would also like to acknowledge the constructive feedback from Scott Bradner, Alex Zinin, Steve Bellovin, Thomas Narten and other IESG members, and the detailed comments from Ross Callon.

10. Editor's Address

Ananth Nagarajan
Juniper Networks

EMail: 

          ananth@juniper.net

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