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rfc:rfc2764

Network Working Group B. Gleeson Request for Comments: 2764 A. Lin Category: Informational Nortel Networks

                                                           J. Heinanen
                                                         Telia Finland
                                                           G. Armitage
                                                              A. Malis
                                                   Lucent Technologies
                                                         February 2000
         A Framework for IP Based Virtual Private Networks

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 (C) The Internet Society (2000).  All Rights Reserved.

IESG Note

 This document is not the product of an IETF Working Group.  The IETF
 currently has no effort underway to standardize a specific VPN
 framework.

Abstract

 This document describes a framework for Virtual Private Networks
 (VPNs) running across IP backbones.  It discusses the various
 different types of VPNs, their respective requirements, and proposes
 specific mechanisms that could be used to implement each type of VPN
 using existing or proposed specifications.  The objective of this
 document is to serve as a framework for related protocol development
 in order to develop the full set of specifications required for
 widespread deployment of interoperable VPN solutions.

Gleeson, et al. Informational [Page 1] RFC 2764 IP Based Virtual Private Networks February 2000

Table of Contents

 1.0 Introduction ................................................  4
 2.0 VPN Application and Implementation Requirements .............  5
 2.1 General VPN Requirements ....................................  5
 2.1.1 Opaque Packet Transport:  .................................  6
 2.1.2 Data Security .............................................  7
 2.1.3 Quality of Service Guarantees .............................  7
 2.1.4 Tunneling Mechanism .......................................  8
 2.2 CPE and Network Based VPNs ..................................  8
 2.3 VPNs and Extranets ..........................................  9
 3.0 VPN Tunneling ............................................... 10
 3.1 Tunneling Protocol Requirements for VPNs .................... 11
 3.1.1 Multiplexing .............................................. 11
 3.1.2 Signalling Protocol ....................................... 12
 3.1.3 Data Security ............................................. 13
 3.1.4 Multiprotocol Transport ................................... 14
 3.1.5 Frame Sequencing .......................................... 14
 3.1.6 Tunnel Maintenance ........................................ 15
 3.1.7 Large MTUs ................................................ 16
 3.1.8 Minimization of Tunnel Overhead ........................... 16
 3.1.9 Flow and congestion control ............................... 17
 3.1.10 QoS / Traffic Management ................................. 17
 3.2 Recommendations ............................................. 18
 4.0 VPN Types:  Virtual Leased Lines ............................ 18
 5.0 VPN Types:  Virtual Private Routed Networks ................. 20
 5.1 VPRN Characteristics ........................................ 20
 5.1.1 Topology .................................................. 23
 5.1.2 Addressing ................................................ 24
 5.1.3 Forwarding ................................................ 24
 5.1.4 Multiple concurrent VPRN connectivity ..................... 24
 5.2 VPRN Related Work ........................................... 24
 5.3 VPRN Generic Requirements ................................... 25
 5.3.1 VPN Identifier ............................................ 26
 5.3.2 VPN Membership Information Configuration .................. 27
 5.3.2.1 Directory Lookup ........................................ 27
 5.3.2.2 Explicit Management Configuration ....................... 28
 5.3.2.3 Piggybacking in Routing Protocols ....................... 28
 5.3.3 Stub Link Reachability Information ........................ 30
 5.3.3.1 Stub Link Connectivity Scenarios ........................ 30
 5.3.3.1.1 Dual VPRN and Internet Connectivity ................... 30
 5.3.3.1.2 VPRN Connectivity Only ................................ 30
 5.3.3.1.3 Multihomed Connectivity ............................... 31
 5.3.3.1.4 Backdoor Links ........................................ 31
 5.3.3.1 Routing Protocol Instance ............................... 31
 5.3.3.2 Configuration ........................................... 33
 5.3.3.3 ISP Administered Addresses .............................. 33
 5.3.3.4 MPLS Label Distribution Protocol ........................ 33

Gleeson, et al. Informational [Page 2] RFC 2764 IP Based Virtual Private Networks February 2000

 5.3.4 Intra-VPN Reachability Information ........................ 34
 5.3.4.1 Directory Lookup ........................................ 34
 5.3.4.2 Explicit Configuration .................................. 34
 5.3.4.3 Local Intra-VPRN Routing Instantiations ................. 34
 5.3.4.4 Link Reachability Protocol .............................. 35
 5.3.4.5 Piggybacking in IP Backbone Routing Protocols ........... 36
 5.3.5 Tunneling Mechanisms ...................................... 36
 5.4 Multihomed Stub Routers ..................................... 37
 5.5 Multicast Support ........................................... 38
 5.5.1 Edge Replication .......................................... 38
 5.5.2 Native Multicast Support .................................. 39
 5.6 Recommendations ............................................. 40
 6.0 VPN Types:  Virtual Private Dial Networks ................... 41
 6.1 L2TP protocol characteristics ............................... 41
 6.1.1 Multiplexing .............................................. 41
 6.1.2 Signalling ................................................ 42
 6.1.3 Data Security ............................................. 42
 6.1.4 Multiprotocol Transport ................................... 42
 6.1.5 Sequencing ................................................ 42
 6.1.6 Tunnel Maintenance ........................................ 43
 6.1.7 Large MTUs ................................................ 43
 6.1.8 Tunnel Overhead ........................................... 43
 6.1.9 Flow and Congestion Control ............................... 43
 6.1.10 QoS / Traffic Management ................................. 43
 6.1.11 Miscellaneous ............................................ 44
 6.2 Compulsory Tunneling ........................................ 44
 6.3 Voluntary Tunnels ........................................... 46
 6.3.1 Issues with Use of L2TP for Voluntary Tunnels ............. 46
 6.3.2 Issues with Use of IPSec for Voluntary Tunnels ............ 48
 6.4 Networked Host Support ...................................... 49
 6.4.1 Extension of PPP to Hosts Through L2TP .................... 49
 6.4.2 Extension of PPP Directly to Hosts:  ...................... 49
 6.4.3 Use of IPSec .............................................. 50
 6.5 Recommendations ............................................. 50
 7.0 VPN Types:  Virtual Private LAN Segment ..................... 50
 7.1 VPLS Requirements ........................................... 51
 7.1.1 Tunneling Protocols ....................................... 51
 7.1.2 Multicast and Broadcast Support ........................... 52
 7.1.3 VPLS Membership Configuration and Topology ................ 52
 7.1.4 CPE Stub Node Types ....................................... 52
 7.1.5 Stub Link Packet Encapsulation ............................ 53
 7.1.5.1 Bridge CPE .............................................. 53
 7.1.5.2 Router CPE .............................................. 53
 7.1.6 CPE Addressing and Address Resolution ..................... 53
 7.1.6.1 Bridge CPE .............................................. 53
 7.1.6.2 Router CPE .............................................. 54
 7.1.7 VPLS Edge Node Forwarding and Reachability Mechanisms ..... 54
 7.1.7.1 Bridge CPE .............................................. 54

Gleeson, et al. Informational [Page 3] RFC 2764 IP Based Virtual Private Networks February 2000

 7.1.7.2 Router CPE .............................................. 54
 7.2 Recommendations ............................................. 55
 8.0 Summary of Recommendations .................................. 55
 9.0 Security Considerations ..................................... 56
 10.0 Acknowledgements ........................................... 56
 11.0 References ................................................. 56
 12.0 Author Information ......................................... 61
 13.0 Full Copyright Statement ................................... 62

1.0 Introduction

 This document describes a framework for Virtual Private Networks
 (VPNs) running across IP backbones.  It discusses the various
 different types of VPNs, their respective requirements, and proposes
 specific mechanisms that could be used to implement each type of VPN
 using existing or proposed specifications.  The objective of this
 document is to serve as a framework for related protocol development
 in order to develop the full set of specifications required for
 widespread deployment of interoperable VPN solutions.
 There is currently significant interest in the deployment of virtual
 private networks across IP backbone facilities.  The widespread
 deployment of VPNs has been hampered, however, by the lack of
 interoperable implementations, which, in turn, derives from the lack
 of general agreement on the definition and scope of VPNs and
 confusion over the wide variety of solutions that are all described
 by the term VPN.  In the context of this document, a VPN is simply
 defined as the 'emulation of a private Wide Area Network (WAN)
 facility using IP facilities' (including the public Internet, or
 private IP backbones).  As such, there are as many types of VPNs as
 there are types of WANs, hence the confusion over what exactly
 constitutes a VPN.
 In this document a VPN is modeled as a connectivity object.  Hosts
 may be attached to a VPN, and VPNs may be interconnected together, in
 the same manner as hosts today attach to physical networks, and
 physical networks are interconnected together (e.g., via bridges or
 routers).  Many aspects of networking, such as addressing, forwarding
 mechanism, learning and advertising reachability, quality of service
 (QoS), security, and firewalling, have common solutions across both
 physical and virtual networks, and many issues that arise in the
 discussion of VPNs have direct analogues with those issues as
 implemented in physical networks.  The introduction of VPNs does not
 create the need to reinvent networking, or to introduce entirely new
 paradigms that have no direct analogue with existing physical
 networks.  Instead it is often useful to first examine how a
 particular issue is handled in a physical network environment, and
 then apply the same principle to an environment which contains

Gleeson, et al. Informational [Page 4] RFC 2764 IP Based Virtual Private Networks February 2000

 virtual as well as physical networks, and to develop appropriate
 extensions and enhancements when necessary.  Clearly having
 mechanisms that are common across both physical and virtual networks
 facilitates the introduction of VPNs into existing networks, and also
 reduces the effort needed for both standards and product development,
 since existing solutions can be leveraged.
 This framework document proposes a taxonomy of a specific set of VPN
 types, showing the specific applications of each, their specific
 requirements, and the specific types of mechanisms that may be most
 appropriate for their implementation.  The intent of this document is
 to serve as a framework to guide a coherent discussion of the
 specific modifications that may be needed to existing IP mechanisms
 in order to develop a full range of interoperable VPN solutions.
 The document first discusses the likely expectations customers have
 of any type of VPN, and the implications of these for the ways in
 which VPNs can be implemented.  It also discusses the distinctions
 between Customer Premises Equipment (CPE) based solutions, and
 network based solutions.  Thereafter it presents a taxonomy of the
 various VPN types and their respective requirements.  It also
 outlines suggested approaches to their implementation, hence also
 pointing to areas for future standardization.
 Note also that this document only discusses implementations of VPNs
 across IP backbones, be they private IP networks, or the public
 Internet.  The models and mechanisms described here are intended to
 apply to both IPV4 and IPV6 backbones.  This document specifically
 does not discuss means of constructing VPNs using native mappings
 onto switched backbones - e.g., VPNs constructed using the LAN
 Emulation over ATM (LANE) [1] or Multiprotocol over ATM (MPOA) [2]
 protocols operating over ATM backbones.  Where IP backbones are
 constructed using such protocols, by interconnecting routers over the
 switched backbone, the VPNs discussed operate on top of this IP
 network, and hence do not directly utilize the native mechanisms of
 the underlying backbone.  Native VPNs are restricted to the scope of
 the underlying backbone, whereas IP based VPNs can extend to the
 extent of IP reachability.  Native VPN protocols are clearly outside
 the scope of the IETF, and may be tackled by such bodies as the ATM
 Forum.

2.0 VPN Application and Implementation Requirements

2.1 General VPN Requirements

 There is growing interest in the use of IP VPNs as a more cost
 effective means of building and deploying private communication
 networks for multi-site communication than with existing approaches.

Gleeson, et al. Informational [Page 5] RFC 2764 IP Based Virtual Private Networks February 2000

 Existing private networks can be generally categorized into two types
 - dedicated WANs that permanently connect together multiple sites,
 and dial networks, that allow on-demand connections through the
 Public Switched Telephone Network (PSTN) to one or more sites in the
 private network.
 WANs are typically implemented using leased lines or dedicated
 circuits - for instance, Frame Relay or ATM connections - between the
 multiple sites.  CPE routers or switches at the various sites connect
 these dedicated facilities together and allow for connectivity across
 the network.  Given the cost and complexity of such dedicated
 facilities and the complexity of CPE device configuration, such
 networks are generally not fully meshed, but instead have some form
 of hierarchical topology.  For example remote offices could be
 connected directly to the nearest regional office, with the regional
 offices connected together in some form of full or partial mesh.
 Private dial networks are used to allow remote users to connect into
 an enterprise network using PSTN or Integrated Services Digital
 Network (ISDN) links.  Typically, this is done through the deployment
 of Network Access Servers (NASs) at one or more central sites.  Users
 dial into such NASs, which interact with Authentication,
 Authorization, and Accounting (AAA) servers to verify the identity of
 the user, and the set of services that the user is authorized to
 receive.
 In recent times, as more businesses have found the need for high
 speed Internet connections to their private corporate networks, there
 has been significant interest in the deployment of CPE based VPNs
 running across the Internet.  This has been driven typically by the
 ubiquity and distance insensitive pricing of current Internet
 services, that can result in significantly lower costs than typical
 dedicated or leased line services.
 The notion of using the Internet for private communications is not
 new, and many techniques, such as controlled route leaking, have been
 used for this purpose [3].  Only in recent times, however, have the
 appropriate IP mechanisms needed to meet customer requirements for
 VPNs all come together.  These requirements include the following:

2.1.1 Opaque Packet Transport:

 The traffic carried within a VPN may have no relation to the traffic
 on the IP backbone, either because the traffic is multiprotocol, or
 because the customer's IP network may use IP addressing unrelated to
 that of the IP backbone on which the traffic is transported.  In
 particular, the customer's IP network may use non-unique, private IP
 addressing [4].

Gleeson, et al. Informational [Page 6] RFC 2764 IP Based Virtual Private Networks February 2000

2.1.2 Data Security

 In general customers using VPNs require some form of data security.
 There are different trust models applicable to the use of VPNs.  One
 such model is where the customer does not trust the service provider
 to provide any form of security, and instead implements a VPN using
 CPE devices that implement firewall functionality and that are
 connected together using secure tunnels.  In this case the service
 provider is used solely for IP packet transport.
 An alternative model is where the customer trusts the service
 provider to provide a secure managed VPN service.  This is similar to
 the trust involved when a customer utilizes a public switched Frame
 Relay or ATM service, in that the customer trusts that packets will
 not be misdirected, injected into the network in an unauthorized
 manner, snooped on, modified in transit, or subjected to traffic
 analysis by unauthorized parties.
 With this model providing firewall functionality and secure packet
 transport services is the responsibility of the service provider.
 Different levels of security may be needed within the provider
 backbone, depending on the deployment scenario used.  If the VPN
 traffic is contained within a single provider's IP backbone then
 strong security mechanisms, such as those provided by the IP Security
 protocol suite (IPSec) [5], may not be necessary for tunnels between
 backbone nodes.  If the VPN traffic traverses networks or equipment
 owned by multiple administrations then strong security mechanisms may
 be appropriate.  Also a strong level of security may be applied by a
 provider to customer traffic to address a customer perception that IP
 networks, and particularly the Internet, are insecure.  Whether or
 not this perception is correct it is one that must be addressed by
 the VPN implementation.

2.1.3 Quality of Service Guarantees

 In addition to ensuring communication privacy, existing private
 networking techniques, building upon physical or link layer
 mechanisms, also offer various types of quality of service
 guarantees.  In particular, leased and dial up lines offer both
 bandwidth and latency guarantees, while dedicated connection
 technologies like ATM and Frame Relay have extensive mechanisms for
 similar guarantees.  As IP based VPNs become more widely deployed,
 there will be market demand for similar guarantees, in order to
 ensure end to end application transparency.  While the ability of IP
 based VPNs to offer such guarantees will depend greatly upon the
 commensurate capabilities of the underlying IP backbones, a VPN
 framework must also address the means by which VPN systems can
 utilize such capabilities, as they evolve.

Gleeson, et al. Informational [Page 7] RFC 2764 IP Based Virtual Private Networks February 2000

2.1.4 Tunneling Mechanism

 Together, the first two of the requirements listed above imply that
 VPNs must be implemented through some form of IP tunneling mechanism,
 where the packet formats and/or the addressing used within the VPN
 can be unrelated to that used to route the tunneled packets across
 the IP backbone.  Such tunnels, depending upon their form, can
 provide some level of intrinsic data security, or this can also be
 enhanced using other mechanisms (e.g., IPSec).
 Furthermore, as discussed later, such tunneling mechanisms can also
 be mapped into evolving IP traffic management mechanisms.  There are
 already defined a large number of IP tunneling mechanisms.  Some of
 these are well suited to VPN applications, as discussed in section
 3.0.

2.2 CPE and Network Based VPNs

 Most current VPN implementations are based on CPE equipment.  VPN
 capabilities are being integrated into a wide variety of CPE devices,
 ranging from firewalls to WAN edge routers and specialized VPN
 termination devices.  Such equipment may be bought and deployed by
 customers, or may be deployed (and often remotely managed) by service
 providers in an outsourcing service.
 There is also significant interest in 'network based VPNs', where the
 operation of the VPN is outsourced to an Internet Service Provider
 (ISP), and is implemented on network as opposed to CPE equipment.
 There is significant interest in such solutions both by customers
 seeking to reduce support costs and by ISPs seeking new revenue
 sources.  Supporting VPNs in the network allows the use of particular
 mechanisms which may lead to highly efficient and cost effective VPN
 solutions, with common equipment and operations support amortized
 across large numbers of customers.
 Most of the mechanisms discussed below can apply to either CPE based
 or network based VPNs.  However particular mechanisms are likely to
 prove applicable only to the latter, since they leverage tools (e.g.,
 piggybacking on routing protocols) which are accessible only to ISPs
 and which are unlikely to be made available to any customer, or even
 hosted on ISP owned and operated CPE, due to the problems of
 coordinating joint management of the CPE gear by both the ISP and the
 customer.  This document will indicate which techniques are likely to
 apply only to network based VPNs.

Gleeson, et al. Informational [Page 8] RFC 2764 IP Based Virtual Private Networks February 2000

2.3 VPNs and Extranets

 The term 'extranet' is commonly used to refer to a scenario whereby
 two or more companies have networked access to a limited amount of
 each other's corporate data.  For example a manufacturing company
 might use an extranet for its suppliers to allow it to query
 databases for the pricing and availability of components, and then to
 order and track the status of outstanding orders.  Another example is
 joint software development, for instance, company A allows one
 development group within company B to access its operating system
 source code, and company B allows one development group in company A
 to access its security software.  Note that the access policies can
 get arbitrarily complex.  For example company B may internally
 restrict access to its security software to groups in certain
 geographic locations to comply with export control laws, for example.
 A key feature of an extranet is thus the control of who can access
 what data, and this is essentially a policy decision.  Policy
 decisions are typically enforced today at the interconnection points
 between different domains, for example between a private network and
 the Internet, or between a software test lab and the rest of the
 company network.  The enforcement may be done via a firewall, router
 with access list functionality, application gateway, or any similar
 device capable of applying policy to transit traffic.  Policy
 controls may be implemented within a corporate network, in addition
 to between corporate networks.  Also the interconnections between
 networks could be a set of bilateral links, or could be a separate
 network, perhaps maintained by an industry consortium.  This separate
 network could itself be a VPN or a physical network.
 Introducing VPNs into a network does not require any change to this
 model.  Policy can be enforced between two VPNs, or between a VPN and
 the Internet, in exactly the same manner as is done today without
 VPNs.  For example two VPNs could be interconnected, which each
 administration locally imposing its own policy controls, via a
 firewall, on all traffic that enters its VPN from the outside,
 whether from another VPN or from the Internet.
 This model of a VPN provides for a separation of policy from the
 underlying mode of packet transport used.  For example, a router may
 direct voice traffic to ATM Virtual Channel Connections (VCCs) for
 guaranteed QoS, non-local internal company traffic to secure tunnels,
 and other traffic to a link to the Internet.  In the past the secure
 tunnels may have been frame relay circuits, now they may also be
 secure IP tunnels or MPLS Label Switched Paths (LSPs)

Gleeson, et al. Informational [Page 9] RFC 2764 IP Based Virtual Private Networks February 2000

 Other models of a VPN are also possible.  For example there is a
 model whereby a set of application flows is mapped into a VPN.  As
 the policy rules imposed by a network administrator can get quite
 complex, the number of distinct sets of application flows that are
 used in the policy rulebase, and hence the number of VPNs, can thus
 grow quite large, and there can be multiple overlapping VPNs.
 However there is little to be gained by introducing such new
 complexity into a network.  Instead a VPN should be viewed as a
 direct analogue to a physical network, as this allows the leveraging
 of existing protocols and procedures, and the current expertise and
 skill sets of network administrators and customers.

3.0 VPN Tunneling

 As noted above in section 2.1, VPNs must be implemented using some
 form of tunneling mechanism.  This section looks at the generic
 requirements for such VPN tunneling mechanisms.  A number of
 characteristics and aspects common to any link layer protocol are
 taken and compared with the features offered by existing tunneling
 protocols.  This provides a basis for comparing different protocols
 and is also useful to highlight areas where existing tunneling
 protocols could benefit from extensions to better support their
 operation in a VPN environment.
 An IP tunnel connecting two VPN endpoints is a basic building block
 from which a variety of different VPN services can be constructed.
 An IP tunnel operates as an overlay across the IP backbone, and the
 traffic sent through the tunnel is opaque to the underlying IP
 backbone.  In effect the IP backbone is being used as a link layer
 technology, and the tunnel forms a point-to-point link.
 A VPN device may terminate multiple IP tunnels and forward packets
 between these tunnels and other network interfaces in different ways.
 In the discussion of different types of VPNs, in later sections of
 this document, the primary distinguishing characteristic of these
 different types is the manner in which packets are forwarded between
 interfaces (e.g., bridged or routed).  There is a direct analogy with
 how existing networking devices are characterized today.  A two-port
 repeater just forwards packets between its ports, and does not
 examine the contents of the packet.  A bridge forwards packets using
 Media Access Control (MAC) layer information contained in the packet,
 while a router forwards packets using layer 3 addressing information
 contained in the packet.  Each of these three scenarios has a direct
 VPN analogue, as discussed later.  Note that an IP tunnel is viewed
 as just another sort of link, which can be concatenated with another
 link, bound to a bridge forwarding table, or bound to an IP
 forwarding table, depending on the type of VPN.

Gleeson, et al. Informational [Page 10] RFC 2764 IP Based Virtual Private Networks February 2000

 The following sections look at the requirements for a generic IP
 tunneling protocol that can be used as a basic building block to
 construct different types of VPNs.

3.1 Tunneling Protocol Requirements for VPNs

 There are numerous IP tunneling mechanisms, including IP/IP [6],
 Generic Routing Encapsulation (GRE) tunnels [7], Layer 2 Tunneling
 Protocol (L2TP) [8], IPSec [5], and Multiprotocol Label Switching
 (MPLS) [9].  Note that while some of these protocols are not often
 thought of as tunneling protocols, they do each allow for opaque
 transport of frames as packet payload across an IP network, with
 forwarding disjoint from the address fields of the encapsulated
 packets.
 Note, however, that there is one significant distinction between each
 of the IP tunneling protocols mentioned above, and MPLS.  MPLS can be
 viewed as a specific link layer for IP, insofar as MPLS specific
 mechanisms apply only within the scope of an MPLS network, whereas IP
 based mechanisms extend to the extent of IP reachability.  As such,
 VPN mechanisms built directly upon MPLS tunneling mechanisms cannot,
 by definition, extend outside the scope of MPLS networks, any more so
 than, for instance, ATM based mechanisms such as LANE can extend
 outside of ATM networks.  Note however, that an MPLS network can span
 many different link layer technologies, and so, like an IP network,
 its scope is not limited by the specific link layers used.  A number
 of proposals for defining a set of mechanisms to allow for
 interoperable VPNs specifically over MPLS networks have also been
 produced ([10] [11] [12] [13], [14] and [15]).
 There are a number of desirable requirements for a VPN tunneling
 mechanism, however, that are not all met by the existing tunneling
 mechanisms.  These requirements include:

3.1.1 Multiplexing

 There are cases where multiple VPN tunnels may be needed between the
 same two IP endpoints.  This may be needed, for instance, in cases
 where the VPNs are network based, and each end point supports
 multiple customers.  Traffic for different customers travels over
 separate tunnels between the same two physical devices.  A
 multiplexing field is needed to distinguish which packets belong to
 which tunnel.  Sharing a tunnel in this manner may also reduce the
 latency and processing burden of tunnel set up.  Of the existing IP
 tunneling mechanisms, L2TP (via the tunnel-id and session-id fields),
 MPLS (via the label) and IPSec (via the Security Parameter Index
 (SPI) field) have a multiplexing mechanism.  Strictly speaking GRE
 does not have a multiplexing field.  However the key field, which was

Gleeson, et al. Informational [Page 11] RFC 2764 IP Based Virtual Private Networks February 2000

 intended to be used for authenticating the source of a packet, has
 sometimes been used as a multiplexing field.  IP/IP does not have a
 multiplexing field.
 The IETF [16] and the ATM Forum [17] have standardized on a single
 format for a globally unique identifier used to identify a VPN (a
 VPN-ID).  A VPN-ID can be used in the control plane, to bind a tunnel
 to a VPN at tunnel establishment time, or in the data plane, to
 identify the VPN associated with a packet, on a per-packet basis.  In
 the data plane a VPN encapsulation header can be used by MPLS, MPOA
 and other tunneling mechanisms to aggregate packets for different
 VPNs over a single tunnel.  In this case an explicit indication of
 VPN-ID is included with every packet, and no use is made of any
 tunnel specific multiplexing field.  In the control plane a VPN-ID
 field can be included in any tunnel establishment signalling protocol
 to allow for the association of a tunnel (e.g., as identified by the
 SPI field) with a VPN.  In this case there is no need for a VPN-ID to
 be included with every data packet.  This is discussed further in
 section 5.3.1.

3.1.2 Signalling Protocol

 There is some configuration information that must be known by an end
 point in advance of tunnel establishment, such as the IP address of
 the remote end point, and any relevant tunnel attributes required,
 such as the level of security needed.  Once this information is
 available, the actual tunnel establishment can be completed in one of
 two ways - via a management operation, or via a signalling protocol
 that allows tunnels to be established dynamically.
 An example of a management operation would be to use an SNMP
 Management Information Base (MIB) to configure various tunneling
 parameters, e.g., MPLS labels, source addresses to use for IP/IP or
 GRE tunnels, L2TP tunnel-ids and session-ids, or security association
 parameters for IPSec.
 Using a signalling protocol can significantly reduce the management
 burden however, and as such, is essential in many deployment
 scenarios.  It reduces the amount of configuration needed, and also
 reduces the management co-ordination needed if a VPN spans multiple
 administrative domains.  For example, the value of the multiplexing
 field, described above, is local to the node assigning the value, and
 can be kept local if distributed via a signalling protocol, rather
 than being first configured into a management station and then
 distributed to the relevant nodes.  A signalling protocol also allows
 nodes that are mobile or are only intermittently connected to
 establish tunnels on demand.

Gleeson, et al. Informational [Page 12] RFC 2764 IP Based Virtual Private Networks February 2000

 When used in a VPN environment a signalling protocol should allow for
 the transport of a VPN-ID to allow the resulting tunnel to be
 associated with a particular VPN.  It should also allow tunnel
 attributes to be exchanged or negotiated, for example the use of
 frame sequencing or the use of multiprotocol transport.  Note that
 the role of the signalling protocol need only be to negotiate tunnel
 attributes, not to carry information about how the tunnel is used,
 for example whether the frames carried in the tunnel are to be
 forwarded at layer 2 or layer 3. (This is similar to Q.2931 ATM
 signalling - the same signalling protocol is used to set up Classical
 IP logical subnetworks as well as for LANE emulated LANs.
 Of the various IP tunneling protocols, the following ones support a
 signalling protocol that could be adapted for this purpose: L2TP (the
 L2TP control protocol), IPSec (the Internet Key Exchange (IKE)
 protocol [18]), and GRE (as used with mobile-ip tunneling [19]). Also
 there are two MPLS signalling protocols that can be used to establish
 LSP tunnels. One uses extensions to the MPLS Label Distribution
 Protocol (LDP) protocol [20], called Constraint-Based Routing LDP
 (CR-LDP) [21], and the other uses extensions to the Resource
 Reservation Protocol (RSVP) for LSP tunnels [22].

3.1.3 Data Security

 A VPN tunneling protocol must support mechanisms to allow for
 whatever level of security may be desired by customers, including
 authentication and/or encryption of various strengths.  None of the
 tunneling mechanisms discussed, other than IPSec, have intrinsic
 security mechanisms, but rely upon the security characteristics of
 the underlying IP backbone.  In particular, MPLS relies upon the
 explicit labeling of label switched paths to ensure that packets
 cannot be misdirected, while the other tunneling mechanisms can all
 be secured through the use of IPSec.  For VPNs implemented over non-
 IP backbones (e.g., MPOA, Frame Relay or ATM virtual circuits), data
 security is implicitly provided by the layer two switch
 infrastructure.
 Overall VPN security is not just a capability of the tunnels alone,
 but has to be viewed in the broader context of how packets are
 forwarded onto those tunnels.  For example with VPRNs implemented
 with virtual routers, the use of separate routing and forwarding
 table instances ensures the isolation of traffic between VPNs.
 Packets on one VPN cannot be misrouted to a tunnel on a second VPN
 since those tunnels are not visible to the forwarding table of the
 first VPN.

Gleeson, et al. Informational [Page 13] RFC 2764 IP Based Virtual Private Networks February 2000

 If some form of signalling mechanism is used by one VPN end point to
 dynamically establish a tunnel with another endpoint, then there is a
 requirement to be able to authenticate the party attempting the
 tunnel establishment.  IPSec has an array of schemes for this
 purpose, allowing, for example, authentication to be based on pre-
 shared keys, or to use digital signatures and certificates.  Other
 tunneling schemes have weaker forms of authentication.  In some cases
 no authentication may be needed, for example if the tunnels are
 provisioned, rather than dynamically established, or if the trust
 model in use does not require it.
 Currently the IPSec Encapsulating Security Payload (ESP) protocol
 [23] can be used to establish SAs that support either encryption or
 authentication or both.  However the protocol specification precludes
 the use of an SA where neither encryption or authentication is used.
 In a VPN environment this "null/null" option is useful, since other
 aspects of the protocol (e.g., that it supports tunneling and
 multiplexing) may be all that is required.  In effect the "null/null"
 option can be viewed as just another level of data security.

3.1.4 Multiprotocol Transport

 In many applications of VPNs, the VPN may carry opaque, multiprotocol
 traffic.  As such, the tunneling protocol used must also support
 multiprotocol transport.  L2TP is designed to transport Point-to-
 Point Protocol (PPP) [24] packets, and thus can be used to carry
 multiprotocol traffic since PPP itself is multiprotocol.  GRE also
 provides for the identification of the protocol being tunneled.
 IP/IP and IPSec tunnels have no such protocol identification field,
 since the traffic being tunneled is assumed to be IP.
 It is possible to extend the IPSec protocol suite to allow for the
 transport of multiprotocol packets.  This can be achieved, for
 example, by extending the signalling component of IPSec - IKE, to
 indicate the protocol type of the traffic being tunneled, or to carry
 a packet multiplexing header (e.g., an LLC/SNAP header or GRE header)
 with each tunneled packet.  This approach is similar to that used for
 the same purpose in ATM networks, where signalling is used to
 indicate the encapsulation used on the VCC, and where packets sent on
 the VCC can use either an LLC/SNAP header or be placed directly into
 the AAL5 payload, the latter being known as VC-multiplexing (see
 [25]).

3.1.5 Frame Sequencing

 One quality of service attribute required by customers of a VPN may
 be frame sequencing, matching the equivalent characteristic of
 physical leased lines or dedicated connections.  Sequencing may be

Gleeson, et al. Informational [Page 14] RFC 2764 IP Based Virtual Private Networks February 2000

 required for the efficient operation of particular end to end
 protocols or applications.  In order to implement frame sequencing,
 the tunneling mechanism must support a sequencing field.  Both L2TP
 and GRE have such a field.  IPSec has a sequence number field, but it
 is used by a receiver to perform an anti-replay check, not to
 guarantee in-order delivery of packets.
 It is possible to extend IPSec to allow the use of the existing
 sequence field to guarantee in-order delivery of packets.  This can
 be achieved, for example, by using IKE to negotiate whether or not
 sequencing is to be used, and to define an end point behaviour which
 preserves packet sequencing.

3.1.6 Tunnel Maintenance

 The VPN end points must monitor the operation of the VPN tunnels to
 ensure that connectivity has not been lost, and to take appropriate
 action (such as route recalculation) if there has been a failure.
 There are two approaches possible.  One is for the tunneling protocol
 itself to periodically check in-band for loss of connectivity, and to
 provide an explicit indication of failure.  For example L2TP has an
 optional keep-alive mechanism to detect non-operational tunnels.
 The other approach does not require the tunneling protocol itself to
 perform this function, but relies on the operation of some out-of-
 band mechanism to determine loss of connectivity.  For example if a
 routing protocol such as Routing Information Protocol (RIP) [26] or
 Open Shortest Path First (OSPF) [27] is run over a tunnel mesh, a
 failure to hear from a neighbor within a certain period of time will
 result in the routing protocol declaring the tunnel to be down.
 Another out-of-band approach is to perform regular ICMP pings with a
 peer.  This is generally sufficient assurance that the tunnel is
 operational, due to the fact the tunnel also runs across the same IP
 backbone.
 When tunnels are established dynamically a distinction needs to be
 drawn between the static and dynamic tunnel information needed.
 Before a tunnel can be established some static information is needed
 by a node, such as the identify of the remote end point and the
 attributes of the tunnel to propose and accept.  This is typically
 put in place as a result of a configuration operation.  As a result
 of the signalling exchange to establish a tunnel, some dynamic state
 is established in each end point, such as the value of the
 multiplexing field or keys to be used.  For example with IPSec, the
 establishment of a Security Association (SA) puts in place the keys
 to be used for the lifetime of that SA.

Gleeson, et al. Informational [Page 15] RFC 2764 IP Based Virtual Private Networks February 2000

 Different policies may be used as to when to trigger the
 establishment of a dynamic tunnel.  One approach is to use a data-
 driven approach and to trigger tunnel establishment whenever there is
 data to be transferred, and to timeout the tunnel due to inactivity.
 This approach is particularly useful if resources for the tunnel are
 being allocated in the network for QoS purposes.  Another approach is
 to trigger tunnel establishment whenever the static tunnel
 configuration information is installed, and to attempt to keep the
 tunnel up all the time.

3.1.7 Large MTUs

 An IP tunnel has an associated Maximum Transmission Unit (MTU), just
 like a regular link. It is conceivable that this MTU may be larger
 than the MTU of one or more individual hops along the path between
 tunnel endpoints. If so, some form of frame fragmentation will be
 required within the tunnel.
 If the frame to be transferred is mapped into one IP datagram, normal
 IP fragmentation will occur when the IP datagram reaches a hop with
 an MTU smaller than the IP tunnel's MTU. This can have undesirable
 performance implications at the router performing such mid-tunnel
 fragmentation.
 An alternative approach is for the tunneling protocol itself to
 incorporate a segmentation and reassembly capability that operates at
 the tunnel level, perhaps using the tunnel sequence number and an
 end-of-message marker of some sort.  (Note that multilink PPP uses a
 mechanism similar to this to fragment packets).  This avoids IP level
 fragmentation within the tunnel itself. None of the existing
 tunneling protocols support such a mechanism.

3.1.8 Minimization of Tunnel Overhead

 There is clearly benefit in minimizing the overhead of any tunneling
 mechanisms.  This is particularly important for the transport of
 jitter and latency sensitive traffic such as packetized voice and
 video.  On the other hand, the use of security mechanisms, such as
 IPSec, do impose their own overhead, hence the objective should be to
 minimize overhead over and above that needed for security, and to not
 burden those tunnels in which security is not mandatory with
 unnecessary overhead.
 One area where the amount of overhead may be significant is when
 voluntary tunneling is used for dial-up remote clients connecting to
 a VPN, due to the typically low bandwidth of dial-up links.  This is
 discussed further in section 6.3.

Gleeson, et al. Informational [Page 16] RFC 2764 IP Based Virtual Private Networks February 2000

3.1.9 Flow and congestion control

 During the development of the L2TP protocol procedures were developed
 for flow and congestion control.  These were necessitated primarily
 because of the need to provide adequate performance over lossy
 networks when PPP compression is used, which, unlike IP Payload
 Compression Protocol (IPComp) [28], is stateful across packets.
 Another motivation was to accommodate devices with very little
 buffering, used for example to terminate low speed dial-up lines.
 However the flow and congestion control mechanisms defined in the
 final version of the L2TP specification are used only for the control
 channels, and not for data traffic.
 In general the interactions between multiple layers of flow and
 congestion control schemes can be very complex.  Given the
 predominance of TCP traffic in today's networks and the fact that TCP
 has its own end-to-end flow and congestion control mechanisms, it is
 not clear that there is much benefit to implementing similar
 mechanisms within tunneling protocols.  Good flow and congestion
 control schemes, that can adapt to a wide variety of network
 conditions and deployment scenarios are complex to develop and test,
 both in themselves and in understanding the interaction with other
 schemes that may be running in parallel.  There may be some benefit,
 however, in having the capability whereby a sender can shape traffic
 to the capacity of a receiver in some manner, and in providing the
 protocol mechanisms to allow a receiver to signal its capabilities to
 a sender.  This is an area that may benefit from further study.
 Note also the work of the Performance Implications of Link
 Characteristics (PILC) working group of the IETF, which is examining
 how the properties of different network links can have an impact on
 the performance of Internet protocols operating over those links.

3.1.10 QoS / Traffic Management

 As noted above, customers may require that VPNs yield similar
 behaviour to physical leased lines or dedicated connections with
 respect to such QoS parameters as loss rates, jitter, latency and
 bandwidth guarantees.  How such guarantees could be delivered will,
 in general, be a function of the traffic management characteristics
 of the VPN nodes themselves, and the access and backbone networks
 across which they are connected.
 A full discussion of QoS and VPNs is outside the scope of this
 document, however by modeling a VPN tunnel as just another type of
 link layer, many of the existing mechanisms developed for ensuring
 QoS over physical links can also be applied.  For example at a VPN
 node, the mechanisms of policing, marking, queuing, shaping and

Gleeson, et al. Informational [Page 17] RFC 2764 IP Based Virtual Private Networks February 2000

 scheduling can all be applied to VPN traffic with VPN-specific
 parameters, queues and interfaces, just as for non-VPN traffic.  The
 techniques developed for Diffserv, Intserv and for traffic
 engineering in MPLS are also applicable.  See also [29] for a
 discussion of QoS and VPNs.
 It should be noted, however, that this model of tunnel operation is
 not necessarily consistent with the way in which specific tunneling
 protocols are currently modeled.  While a model is an aid to
 comprehension, and not part of a protocol specification, having
 differing models can complicate discussions, particularly if a model
 is misinterpreted as being part of a protocol specification or as
 constraining choice of implementation method.  For example, IPSec
 tunnel processing can be modeled both as an interface and as an
 attribute of a particular packet flow.

3.2 Recommendations

 IPSec is needed whenever there is a requirement for strong encryption
 or strong authentication.  It also supports multiplexing and a
 signalling protocol - IKE.  However extending the IPSec protocol
 suite to also cover the following areas would be beneficial, in order
 to better support the tunneling requirements of a VPN environment.
  1. the transport of a VPN-ID when establishing an SA (3.1.2)
  1. a null encryption and null authentication option (3.1.3)
  1. multiprotocol operation (3.1.4)
  1. frame sequencing (3.1.5)
 L2TP provides no data security by itself, and any PPP security
 mechanisms used do not apply to the L2TP protocol itself, so that in
 order for strong security to be provided L2TP must run over IPSec.
 Defining specific modes of operation for IPSec when it is used to
 support L2TP traffic will aid interoperability.  This is currently a
 work item for the proposed L2TP working group.

4.0 VPN Types: Virtual Leased Lines

 The simplest form of a VPN is a 'Virtual Leased Line' (VLL) service.
 In this case a point-to-point link is provided to a customer,
 connecting two CPE devices, as illustrated below.  The link layer
 type used to connect the CPE devices to the ISP nodes can be any link
 layer type, for example an ATM VCC or a Frame Relay circuit.  The CPE
 devices can be either routers bridges or hosts.

Gleeson, et al. Informational [Page 18] RFC 2764 IP Based Virtual Private Networks February 2000

 The two ISP nodes are both connected to an IP network, and an IP
 tunnel is set up between them.  Each ISP node is configured to bind
 the stub link and the IP tunnel together at layer 2 (e.g., an ATM VCC
 and the IP tunnel).  Frames are relayed between the two links.  For
 example the ATM Adaptation Layer 5 (AAL5) payload is taken and
 encapsulated in an IPSec tunnel, and vice versa.  The contents of the
 AAL5 payload are opaque to the ISP node, and are not examined there.
             +--------+      -----------       +--------+
 +---+       | ISP    |     ( IP        )      | ISP    |      +---+
 |CPE|-------| edge   |-----( backbone  ) -----| edge   |------|CPE|
 +---+ ATM   | node   |     (           )      | node   |  ATM +---+
       VCC   +--------+      -----------       +--------+  VCC
                    <--------- IP Tunnel -------->
 10.1.1.5                subnet = 10.1.1.4/30              10.1.1.6
        Addressing used by customer (transparent to provider)
                        Figure 4.1: VLL Example
 To a customer it looks the same as if a single ATM VCC or Frame Relay
 circuit were used to interconnect the CPE devices, and the customer
 could be unaware that part of the circuit was in fact implemented
 over an IP backbone.  This may be useful, for example, if a provider
 wishes to provide a LAN interconnect service using ATM as the network
 interface, but does not have an ATM network that directly
 interconnects all possible customer sites.
 It is not necessary that the two links used to connect the CPE
 devices to the ISP nodes be of the same media type, but in this case
 the ISP nodes cannot treat the traffic in an opaque manner, as
 described above.  Instead the ISP nodes must perform the functions of
 an interworking device between the two media types (e.g., ATM and
 Frame Relay), and perform functions such as LLC/SNAP to NLPID
 conversion, mapping between ARP protocol variants and performing any
 media specific processing that may be expected by the CPE devices
 (e.g., ATM OAM cell handling or Frame Relay XID exchanges).
 The IP tunneling protocol used must support multiprotocol operation
 and may need to support sequencing, if that characteristic is
 important to the customer traffic.  If the tunnels are established
 using a signalling protocol, they may be set up in a data driven
 manner, when a frame is received from a customer link and no tunnel
 exists, or the tunnels may be established at provisioning time and
 kept up permanently.

Gleeson, et al. Informational [Page 19] RFC 2764 IP Based Virtual Private Networks February 2000

 Note that the use of the term 'VLL' in this document is different to
 that used in the definition of the Diffserv Expedited Forwarding Per
 Hop Behaviour (EF-PHB) [30].  In that document a VLL is used to mean
 a low latency, low jitter, assured bandwidth path, which can be
 provided using the described PHB. Thus the focus there is primarily
 on link characteristics that are temporal in nature. In this document
 the term VLL does not imply the use of any specific QoS mechanism,
 Diffserv or otherwise.  Instead the focus is primarily on link
 characteristics that are more topological in nature, (e.g., such as
 constructing a link which includes an IP tunnel as one segment of the
 link). For a truly complete emulation of a link layer both the
 temporal and topological aspects need to be taken into account.

5.0 VPN Types: Virtual Private Routed Networks

5.1 VPRN Characteristics

 A Virtual Private Routed Network (VPRN) is defined to be the
 emulation of a multi-site wide area routed network using IP
 facilities.  This section looks at how a network-based VPRN service
 can be provided.  CPE-based VPRNs are also possible, but are not
 specifically discussed here.  With network-based VPRNs many of the
 issues that need to be addressed are concerned with configuration and
 operational issues, which must take into account the split in
 administrative responsibility between the service provider and the
 service user.
 The distinguishing characteristic of a VPRN, in comparison to other
 types of VPNs, is that packet forwarding is carried out at the
 network layer.  A VPRN consists of a mesh of IP tunnels between ISP
 routers, together with the routing capabilities needed to forward
 traffic received at each VPRN node to the appropriate destination
 site.  Attached to the ISP routers are CPE routers connected via one
 or more links, termed 'stub' links.  There is a VPRN specific
 forwarding table at each ISP router to which members of the VPRN are
 connected.  Traffic is forwarded between ISP routers, and between ISP
 routers and customer sites, using these forwarding tables, which
 contain network layer reachability information (in contrast to a
 Virtual Private LAN Segment type of VPN (VPLS) where the forwarding
 tables contain MAC layer reachability information - see section 7.0).
 An example VPRN is illustrated in the following diagram, which shows
 3 ISP edge routers connected via a full mesh of IP tunnels, used to
 interconnect 4 CPE routers.  One of the CPE routers is multihomed to
 the ISP network.  In the multihomed case, all stub links may be
 active, or, as shown, there may be one primary and one or more backup
 links to be used in case of failure of the primary.  The term '
 backdoor' link is used to refer to a link between two customer sites

Gleeson, et al. Informational [Page 20] RFC 2764 IP Based Virtual Private Networks February 2000

 that does not traverse the ISP network.
 10.1.1.0/30 +--------+                       +--------+ 10.2.2.0/30
 +---+       | ISP    |     IP tunnel         | ISP    |       +---+
 |CPE|-------| edge   |<--------------------->| edge   |-------|CPE|
 +---+ stub  | router |     10.9.9.4/30       | router |  stub +---+
       link  +--------+                       +--------+  link   :
              |   ^  |                         |   ^             :
              |   |  |     ---------------     |   |             :
              |   |  +----(               )----+   |             :
              |   |       ( IP BACKBONE   )        |             :
              |   |       (               )        |             :
              |   |        ---------------         |             :
              |   |               |                |             :
              |   |IP tunnel  +--------+  IP tunnel|             :
              |   |           | ISP    |           |             :
              |   +---------->| edge   |<----------+             :
              |   10.9.9.8/30 | router | 10.9.9.12/30            :
        backup|               +--------+                 backdoor:
         link |                |      |                    link  :
              |      stub link |      |  stub link               :
              |                |      |                          :
              |             +---+    +---+                       :
              +-------------|CPE|    |CPE|.......................:
              10.3.3.0/30   +---+    +---+      10.4.4.0/30
                       Figure 5.1: VPRN Example
 The principal benefit of a VPRN is that the complexity and the
 configuration of the CPE routers is minimized.  To a CPE router, the
 ISP edge router appears as a neighbor router in the customer's
 network, to which it sends all traffic, using a default route.  The
 tunnel mesh that is set up to transfer traffic extends between the
 ISP edge routers, not the CPE routers.  In effect the burden of
 tunnel establishment and maintenance and routing configuration is
 outsourced to the ISP.  In addition other services needed for the
 operation of a VPN such as the provision of a firewall and QoS
 processing can be handled by a small number of ISP edge routers,
 rather than a large number of potentially heterogeneous CPE devices.
 The introduction and management of new services can also be more
 easily handled, as this can be achieved without the need to upgrade
 any CPE equipment.  This latter benefit is particularly important
 when there may be large numbers of residential subscribers using VPN
 services to access private corporate networks.  In this respect the
 model is somewhat akin to that used for telephony services, whereby
 new services (e.g., call waiting) can be introduced with no change in
 subscriber equipment.

Gleeson, et al. Informational [Page 21] RFC 2764 IP Based Virtual Private Networks February 2000

 The VPRN type of VPN is in contrast to one where the tunnel mesh
 extends to the CPE routers, and where the ISP network provides layer
 2 connectivity alone.  The latter case can be implemented either as a
 set of VLLs between CPE routers (see section 4.0), in which case the
 ISP network provides a set of layer 2 point-to-point links, or as a
 VPLS (see section 7.0), in which case the ISP network is used to
 emulate a multiaccess LAN segment.  With these scenarios a customer
 may have more flexibility (e.g., any IGP or any protocol can be run
 across all customer sites) but this usually comes at the expense of a
 more complex configuration for the customer.  Thus, depending on
 customer requirements, a VPRN or a VPLS may be the more appropriate
 solution.
 Because a VPRN carries out forwarding at the network layer, a single
 VPRN only directly supports a single network layer protocol.  For
 multiprotocol support, a separate VPRN for each network layer
 protocol could be used, or one protocol could be tunneled over
 another (e.g., non-IP protocols tunneled over an IP VPRN) or
 alternatively the ISP network could be used to provide layer 2
 connectivity only, such as with a VPLS as mentioned above.
 The issues to be addressed for VPRNs include initial configuration,
 determination by an ISP edge router of the set of links that are in
 each VPRN, the set of other routers that have members in the VPRN,
 and the set of IP address prefixes reachable via each stub link,
 determination by a CPE router of the set of IP address prefixes to be
 forwarded to an ISP edge router, the mechanism used to disseminate
 stub reachability information to the correct set of ISP routers, and
 the establishment and use of the tunnels used to carry the data
 traffic.  Note also that, although discussed first for VPRNs, many of
 these issues also apply to the VPLS scenario described later, with
 the network layer addresses being replaced by link layer addresses.
 Note that VPRN operation is decoupled from the mechanisms used by the
 customer sites to access the Internet.  A typical scenario would be
 for the ISP edge router to be used to provide both VPRN and Internet
 connectivity to a customer site.  In this case the CPE router just
 has a default route pointing to the ISP edge router, with the latter
 being responsible for steering private traffic to the VPRN and other
 traffic to the Internet, and providing firewall functionality between
 the two domains.  Alternatively a customer site could have Internet
 connectivity via an ISP router not involved in the VPRN, or even via
 a different ISP.  In this case the CPE device is responsible for
 splitting the traffic into the two domains and providing firewall
 functionality.

Gleeson, et al. Informational [Page 22] RFC 2764 IP Based Virtual Private Networks February 2000

5.1.1 Topology

 The topology of a VPRN may consist of a full mesh of tunnels between
 each VPRN node, or may be an arbitrary topology, such as a set of
 remote offices connected to the nearest regional site, with these
 regional sites connected together via a full or partial mesh.  With
 VPRNs using IP tunnels there is much less cost assumed with full
 meshing than in cases where physical resources (e.g., a leased line)
 must be allocated for each connected pair of sites, or where the
 tunneling method requires resources to be allocated in the devices
 used to interconnect the edge routers (e.g., Frame Relay DLCIs).  A
 full mesh topology yields optimal routing, since it precludes the
 need for traffic between two sites to traverse a third.  Another
 attraction of a full mesh is that there is no need to configure
 topology information for the VPRN.  Instead, given the member routers
 of a VPRN, the topology is implicit.  If the number of ISP edge
 routers in a VPRN is very large, however, a full mesh topology may
 not be appropriate, due to the scaling issues involved, for example,
 the growth in the number of tunnels needed between sites, (which for
 n sites is n(n-1)/2), or the number of routing peers per router.
 Network policy may also lead to non full mesh topologies, for example
 an administrator may wish to set up the topology so that traffic
 between two remote sites passes through a central site, rather than
 go directly between the remote sites.  It is also necessary to deal
 with the scenario where there is only partial connectivity across the
 IP backbone under certain error conditions (e.g. A can reach B, and B
 can reach C, but A cannot reach C directly), which can occur if
 policy routing is being used.
 For a network-based VPRN, it is assumed that each customer site CPE
 router connects to an ISP edge router through one or more point-to-
 point stub links (e.g. leased lines, ATM or Frame Relay connections).
 The ISP routers are responsible for learning and disseminating
 reachability information amongst themselves.  The CPE routers must
 learn the set of destinations reachable via each stub link, though
 this may be as simple as a default route.
 The stub links may either be dedicated links, set up via
 provisioning, or may be dynamic links set up on demand, for example
 using PPP, voluntary tunneling (see section 6.3), or ATM signalling.
 With dynamic links it is necessary to authenticate the subscriber,
 and determine the authorized resources that the subscriber can access
 (e.g. which VPRNs the subscriber may join).  Other than the way the
 subscriber is initially bound to the VPRN, (and this process may
 involve extra considerations such as dynamic IP address assignment),
 the subsequent VPRN mechanisms and services can be used for both
 types of subscribers in the same way.

Gleeson, et al. Informational [Page 23] RFC 2764 IP Based Virtual Private Networks February 2000

5.1.2 Addressing

 The addressing used within a VPRN may have no relation to the
 addressing used on the IP backbone over which the VPRN is
 instantiated.  In particular non-unique private IP addressing may be
 used [4].  Multiple VPRNs may be instantiated over the same set of
 physical devices, and they may use the same or overlapping address
 spaces.

5.1.3 Forwarding

 For a VPRN the tunnel mesh forms an overlay network operating over an
 IP backbone.  Within each of the ISP edge routers there must be VPN
 specific forwarding state to forward packets received from stub links
 ('ingress traffic') to the appropriate next hop router, and to
 forward packets received from the core ('egress traffic') to the
 appropriate stub link.  For cases where an ISP edge router supports
 multiple stub links belonging to the same VPRN, the tunnels can, as a
 local matter, either terminate on the edge router, or on a stub link.
 In the former case a VPN specific forwarding table is needed for
 egress traffic, in the latter case it is not.  A VPN specific
 forwarding table is generally needed in the ingress direction, in
 order to direct traffic received on a stub link onto the correct IP
 tunnel towards the core.
 Also since a VPRN operates at the internetwork layer, the IP packets
 sent over a tunnel will have their Time to Live (TTL) field
 decremented in the normal manner, preventing packets circulating
 indefinitely in the event of a routing loop within the VPRN.

5.1.4 Multiple concurrent VPRN connectivity

 Note also that a single customer site may belong concurrently to
 multiple VPRNs and may want to transmit traffic both onto one or more
 VPRNs and to the default Internet, over the same stub link.  There
 are a number of possible approaches to this problem, but these are
 outside the scope of this document.

5.2 VPRN Related Work

 VPRN requirements and mechanisms have been discussed previously in a
 number of different documents.  One of the first was [10], which
 showed how the same VPN functionality can be implemented over both
 MPLS and non-MPLS networks.  Some others are briefly discussed below.
 There are two main variants as regards the mechanisms used to provide
 VPRN membership and reachability functionality, - overlay and
 piggybacking.  These are discussed in greater detail in sections

Gleeson, et al. Informational [Page 24] RFC 2764 IP Based Virtual Private Networks February 2000

 5.3.2, 5.3.3 and 5.3.4 below.  An example of the overlay model is
 described in [14], which discusses the provision of VPRN
 functionality by means of a separate per-VPN routing protocol
 instance and route and forwarding table instantiation, otherwise
 known as virtual routing.  Each VPN routing instance is isolated from
 any other VPN routing instance, and from the routing used across the
 backbone.  As a result any routing protocol (e.g. OSPF, RIP2, IS-IS)
 can be run with any VPRN, independently of the routing protocols used
 in other VPRNs, or in the backbone itself.  The VPN model described
 in [12] is also an overlay VPRN model using virtual routing.  That
 document is specifically geared towards the provision of VPRN
 functionality over MPLS backbones, and it describes how VPRN
 membership dissemination can be automated over an MPLS backbone, by
 performing VPN neighbor discovery over the base MPLS tunnel mesh.
 [31] extends the virtual routing model to include VPN areas, and VPN
 border routers which route between VPN areas.  VPN areas may be
 defined for administrative or technical reasons, such as different
 underlying network infrastructures (e.g. ATM, MPLS, IP).
 In contrast [15] describes the provision of VPN functionality using a
 piggybacking approach for membership and reachability dissemination,
 with this information being piggybacked in Border Gateway Protocol 4
 (BGP) [32] packets.  VPNs are constructed using BGP policies, which
 are used to control which sites can communicate with each other. [13]
 also uses BGP for piggybacking membership information, and piggybacks
 reachability information on the protocol used to establish MPLS LSPs
 (CR-LDP or extended RSVP).  Unlike the other proposals, however, this
 proposal requires the participation on the CPE router to implement
 the VPN functionality.

5.3 VPRN Generic Requirements

 There are a number of common requirements which any network-based
 VPRN solution must address, and there are a number of different
 mechanisms that can be used to meet these requirements.  These
 generic issues are
 1) The use of a globally unique VPN identifier in order to be able to
    refer to a particular VPN.
 2) VPRN membership determination.  An edge router must learn of the
    local stub links that are in each VPRN, and must learn of the set
    of other routers that have members in that VPRN.
 3) Stub link reachability information.  An edge router must learn the
    set of addresses and address prefixes reachable via each stub
    link.

Gleeson, et al. Informational [Page 25] RFC 2764 IP Based Virtual Private Networks February 2000

 4) Intra-VPRN reachability information.  Once an edge router has
    determined the set of address prefixes associated with each of its
    stub links, then this information must be disseminated to each
    other edge router in the VPRN.
 5) Tunneling mechanism.  An edge router must construct the necessary
    tunnels to other routers that have members in the VPRN, and must
    perform the encapsulation and decapsulation necessary to send and
    receive packets over the tunnels.

5.3.1 VPN Identifier

 The IETF [16] and the ATM Forum [17] have standardized on a single
 format for a globally unique identifier used to identify a VPN - a
 VPN-ID.  Only the format of the VPN-ID has been defined, not its
 semantics or usage.  The aim is to allow its use for a wide variety
 of purposes, and to allow the same identifier to used with different
 technologies and mechanisms.  For example a VPN-ID can be included in
 a MIB to identify a VPN for management purposes.  A VPN-ID can be
 used in a control plane protocol, for example to bind a tunnel to a
 VPN at tunnel establishment time.  All packets that traverse the
 tunnel are then implicitly associated with the identified VPN.  A
 VPN-ID can be used in a data plane encapsulation, to allow for an
 explicit per-packet identification of the VPN associated with the
 packet.  If a VPN is implemented using different technologies (e.g.,
 IP and ATM) in a network, the same identifier can be used to identify
 the VPN across the different technologies.  Also if a VPN spans
 multiple administrative domains the same identifier can be used
 everywhere.
 Most of the VPN schemes developed (e.g. [11], [12], [13], [14])
 require the use of a VPN-ID that is carried in control and/or data
 packets, which is used to associate the packet with a particular VPN.
 Although the use of a VPN-ID in this manner is very common, it is not
 universal. [15] describes a scheme where there is no protocol field
 used to identify a VPN in this manner.  In this scheme the VPNs as
 understood by a user, are administrative constructs, built using BGP
 policies.  There are a number of attributes associated with VPN
 routes, such as a route distinguisher, and origin and target "VPN",
 that are used by the underlying protocol mechanisms for
 disambiguation and scoping, and these are also used by the BGP policy
 mechanism in the construction of VPNs, but there is nothing
 corresponding with the VPN-ID as used in the other documents.
 Note also that [33] defines a multiprotocol encapsulation for use
 over ATM AAL5 that uses the standard VPN-ID format.

Gleeson, et al. Informational [Page 26] RFC 2764 IP Based Virtual Private Networks February 2000

5.3.2 VPN Membership Information Configuration and Dissemination

 In order to establish a VPRN, or to insert new customer sites into an
 established VPRN, an ISP edge router must determine which stub links
 are associated with which VPRN.  For static links (e.g. an ATM VCC)
 this information must be configured into the edge router, since the
 edge router cannot infer such bindings by itself.  An SNMP MIB
 allowing for bindings between local stub links and VPN identities is
 one solution.
 For subscribers that attach to the network dynamically (e.g. using
 PPP or voluntary tunneling) it is possible to make the association
 between stub link and VPRN as part of the end user authentication
 processing that must occur with such dynamic links.  For example the
 VPRN to which a user is to be bound may be derived from the domain
 name the used as part of PPP authentication.  If the user is
 successfully authenticated (e.g. using a Radius server), then the
 newly created dynamic link can be bound to the correct VPRN.  Note
 that static configuration information is still needed, for example to
 maintain the list of authorized subscribers for each VPRN, but the
 location of this static information could be an external
 authentication server rather than on an ISP edge router.  Whether the
 link was statically or dynamically created, a VPN-ID can be
 associated with that link to signify to which VPRN it is bound.
 After learning which stub links are bound to which VPRN, each edge
 router must learn either the identity of, or, at least, the route to,
 each other edge router supporting other stub links in that particular
 VPRN.  Implicit in the latter is the notion that there exists some
 mechanism by which the configured edge routers can then use this edge
 router and/or stub link identity information to subsequently set up
 the appropriate tunnels between them.  The problem of VPRN member
 dissemination between participating edge routers, can be solved in a
 variety of ways, discussed below.

5.3.2.1 Directory Lookup

 The members of a particular VPRN, that is, the identity of the edge
 routers supporting stub links in the VPRN, and the set of static stub
 links bound to the VPRN per edge router, could be configured into a
 directory, which edge routers could query, using some defined
 mechanism (e.g. Lightweight Directory Access Protocol (LDAP) [34]),
 upon startup.
 Using a directory allows either a full mesh topology or an arbitrary
 topology to be configured.  For a full mesh, the full list of member
 routers in a VPRN is distributed everywhere.  For an arbitrary
 topology, different routers may receive different member lists.

Gleeson, et al. Informational [Page 27] RFC 2764 IP Based Virtual Private Networks February 2000

 Using a directory allows for authorization checking prior to
 disseminating VPRN membership information, which may be desirable
 where VPRNs span multiple administrative domains.  In such a case,
 directory to directory protocol mechanisms could also be used to
 propagate authorized VPRN membership information between the
 directory systems of the multiple administrative domains.
 There also needs to be some form of database synchronization
 mechanism (e.g. triggered or regular polling of the directory by edge
 routers, or active pushing of update information to the edge routers
 by the directory) in order for all edge routers to learn the identity
 of newly configured sites inserted into an active VPRN, and also to
 learn of sites removed from a VPRN.

5.3.2.2 Explicit Management Configuration

 A VPRN MIB could be defined which would allow a central management
 system to configure each edge router with the identities of each
 other participating edge router and the identity of each of the
 static stub links bound to the VPRN.  Like the use of a directory,
 this mechanism allows both full mesh and arbitrary topologies to be
 configured.  Another mechanism using a centralized management system
 is to use a policy server and use the Common Open Policy Service
 (COPS) protocol [35] to distribute VPRN membership and policy
 information, such as the tunnel attributes to use when establishing a
 tunnel, as described in [36].
 Note that this mechanism allows the management station to impose
 strict authorization control; on the other hand, it may be more
 difficult to configure edge routers outside the scope of the
 management system.  The management configuration model can also be
 considered a subset of the directory method, in that the management
 directories could use MIBs to push VPRN membership information to the
 participating edge routers, either subsequent to, or as part of, the
 local stub link configuration process.

5.3.2.3 Piggybacking in Routing Protocols

 VPRN membership information could be piggybacked into the routing
 protocols run by each edge router across the IP backbone, since this
 is an efficient means of automatically propagating information
 throughout the network to other participating edge routers.
 Specifically, each route advertisement by each edge router could
 include, at a minimum, the set of VPN identifiers associated with
 each edge router, and adequate information to allow other edge
 routers to determine the identity of, and/or, the route to, the
 particular edge router.  Other edge routers would examine received
 route advertisements to determine if any contained information was

Gleeson, et al. Informational [Page 28] RFC 2764 IP Based Virtual Private Networks February 2000

 relevant to a supported (i.e., configured) VPRN; this determination
 could be done by looking for a VPN identifier matching a locally
 configured VPN.  The nature of the piggybacked information, and
 related issues, such as scoping, and the means by which the nodes
 advertising particular VPN memberships will be identified, will
 generally be a function both of the routing protocol and of the
 nature of the underlying transport.
 Using this method all the routers in the network will have the same
 view of the VPRN membership information, and so a full mesh topology
 is easily supported.  Supporting an arbitrary topology is more
 difficult, however, since some form of pruning would seem to be
 needed.
 The advantage of the piggybacking scheme is that it allows for
 efficient information dissemination, but it does require that all
 nodes in the path, and not just the participating edge routers, be
 able to accept such modified route advertisements.  A disadvantage is
 that significant administrative complexity may be required to
 configure scoping mechanisms so as to both permit and constrain the
 dissemination of the piggybacked advertisements, and in itself this
 may be quite a configuration burden, particularly if the VPRN spans
 multiple routing domains (e.g. different autonomous systems / ISPs).
 Furthermore, unless some security mechanism is used for routing
 updates so as to permit only all relevant edge routers to read the
 piggybacked advertisements, this scheme generally implies a trust
 model where all routers in the path must perforce be authorized to
 know this information.  Depending upon the nature of the routing
 protocol, piggybacking may also require intermediate routers,
 particularly autonomous system (AS) border routers, to cache such
 advertisements and potentially also re-distribute them between
 multiple routing protocols.
 Each of the schemes described above have merit in particular
 situations.  Note that, in practice, there will almost always be some
 centralized directory or management system which will maintain VPRN
 membership information, such as the set of edge routers that are
 allowed to support a certain VPRN, the bindings of static stub links
 to VPRNs, or authentication and authorization information for users
 that access the network via dynamics links.  This information needs
 to be configured and stored in some form of database, so that the
 additional steps needed to facilitate the configuration of such
 information into edge routers, and/or, facilitate edge router access
 to such information, may not be excessively onerous.

Gleeson, et al. Informational [Page 29] RFC 2764 IP Based Virtual Private Networks February 2000

5.3.3 Stub Link Reachability Information

 There are two aspects to stub site reachability - the means by which
 VPRN edge routers determine the set of VPRN addresses and address
 prefixes reachable at each stub site, and the means by which the CPE
 routers learn the destinations reachable via each stub link.  A
 number of common scenarios are outlined below.  In each case the
 information needed by the ISP edge router is the same - the set of
 VPRN addresses reachable at the customer site, but the information
 needed by the CPE router differs.

5.3.3.1 Stub Link Connectivity Scenarios

5.3.3.1.1 Dual VPRN and Internet Connectivity

 The CPE router is connected via one link to an ISP edge router, which
 provides both VPRN and Internet connectivity.
 This is the simplest case for the CPE router, as it just needs a
 default route pointing to the ISP edge router.

5.3.3.1.2 VPRN Connectivity Only

 The CPE router is connected via one link to an ISP edge router, which
 provides VPRN, but not Internet, connectivity.
 The CPE router must know the set of non-local VPRN destinations
 reachable via that link.  This may be a single prefix, or may be a
 number of disjoint prefixes.  The CPE router may be either statically
 configured with this information, or may learn it dynamically by
 running an instance of an Interior Gateway Protocol (IGP).  For
 simplicity it is assumed that the IGP used for this purpose is RIP,
 though it could be any IGP.  The ISP edge router will inject into
 this instance of RIP the VRPN routes which it learns by means of one
 of the intra-VPRN reachability mechanisms described in section 5.3.4.
 Note that the instance of RIP run to the CPE, and any instance of a
 routing protocol used to learn intra-VPRN reachability (even if also
 RIP) are separate, with the ISP edge router redistributing the routes
 from one instance to another.

Gleeson, et al. Informational [Page 30] RFC 2764 IP Based Virtual Private Networks February 2000

5.3.3.1.3 Multihomed Connectivity

 The CPE router is multihomed to the ISP network, which provides VPRN
 connectivity.
 In this case all the ISP edge routers could advertise the same VPRN
 routes to the CPE router, which then sees all VPRN prefixes equally
 reachable via all links.  More specific route redistribution is also
 possible, whereby each ISP edge router advertises a different set of
 prefixes to the CPE router.

5.3.3.1.4 Backdoor Links

 The CPE router is connected to the ISP network, which provides VPRN
 connectivity, but also has a backdoor link to another customer site
 In this case the ISP edge router will advertise VPRN routes as in
 case 2 to the CPE device.  However now the same destination is
 reachable via both the ISP edge router and via the backdoor link.  If
 the CPE routers connected to the backdoor link are running the
 customer's IGP, then the backdoor link may always be the favored link
 as it will appear an an 'internal' path, whereas the destination as
 injected via the ISP edge router will appear as an 'external' path
 (to the customer's IGP).  To avoid this problem, assuming that the
 customer wants the traffic to traverse the ISP network, then a
 separate instance of  RIP should be run between the CPE routers at
 both ends of the backdoor link, in the same manner as an instance of
 RIP is run on a stub or backup link between a CPE router and an ISP
 edge router.  This will then also make the backdoor link appear as an
 external path, and by adjusting the link costs appropriately, the ISP
 path can always be favored, unless it goes down, when the backdoor
 link is then used.
 The description of the above scenarios covers what reachability
 information is needed by the ISP edge routers and the CPE routers,
 and discusses some of the mechanisms used to convey this information.
 The sections below look at these mechanisms in more detail.

5.3.3.1 Routing Protocol Instance

 A routing protocol can be run between the CPE edge router and the ISP
 edge router to exchange reachability information.  This allows an ISP
 edge router to learn the VPRN prefixes reachable at a customer site,
 and also allows a CPE router to learn the destinations reachable via
 the provider network.

Gleeson, et al. Informational [Page 31] RFC 2764 IP Based Virtual Private Networks February 2000

 The extent of the routing domain for this protocol instance is
 generally just the ISP edge router and the CPE router although if the
 customer site is also running the same protocol as its IGP, then the
 domain may extend into customer site.  If the customer site is
 running a different routing protocol then the CPE router
 redistributes the routes between the instance running to the ISP edge
 router, and the instance running into the customer site.
 Given the typically restricted scope of this routing instance, a
 simple protocol will generally suffice.  RIP is likely to be the most
 common protocol used, though any routing protocol, such as OSPF, or
 BGP run in internal mode (IBGP), could also be used.
 Note that the instance of the stub link routing protocol is different
 from any instance of a routing protocol used for intra-VPRN
 reachability.  For example, if the ISP edge router uses routing
 protocol piggybacking to disseminate VPRN membership and reachability
 information across the core, then it may redistribute suitably
 labeled routes from the CPE routing instance to the core routing
 instance.  The routing protocols used for each instance are
 decoupled, and any suitable protocol can be used in each case.  There
 is no requirement that the same protocol, or even the same stub link
 reachability information gathering mechanism, be run between each CPE
 router and associated ISP edge router in a particular VPRN, since
 this is a purely local matter.
 This decoupling allows ISPs to deploy a common (across all VPRNs)
 intra-VPRN reachability mechanism, and a common stub link
 reachability mechanism, with these mechanisms isolated both from each
 other, and from the particular IGP used in a customer network.  In
 the first case, due to the IGP-IGP boundary implemented on the ISP
 edge router, the ISP can insulate the intra-VPRN reachability
 mechanism from misbehaving stub link protocol instances.  In the
 second case the ISP is not required to be aware of the particular IGP
 running in a customer site.  Other scenarios are possible, where the
 ISP edge routers are running a routing protocol in the same instance
 as the customer's IGP, but are unlikely to be practical, since it
 defeats the purpose of a VPRN simplifying CPE router configuration.
 In cases where a customer wishes to run an IGP across multiple sites,
 a VPLS solution is more suitable.
 Note that if a particular customer site concurrently belongs to
 multiple VPRNs (or wishes to concurrently communicate with both a
 VPRN and the Internet), then the ISP edge router must have some means
 of unambiguously mapping stub link address prefixes to particular
 VPRNs.  A simple way is to have multiple stub links, one per VPRN.
 It is also possible to run multiple VPRNs over one stub link.  This
 could be done either by ensuring (and appropriately configuring the

Gleeson, et al. Informational [Page 32] RFC 2764 IP Based Virtual Private Networks February 2000

 ISP edge router to know) that particular disjoint address prefixes
 are mapped into separate VPRNs, or by tagging the routing
 advertisements from the CPE router with the appropriate VPN
 identifier.  For example if MPLS was being used to convey stub link
 reachability information, different MPLS labels would be used to
 differentiate the disjoint prefixes assigned to particular VPRNs.  In
 any case, some administrative procedure would be required for this
 coordination.

5.3.3.2 Configuration

 The reachability information across each stub link could be manually
 configured, which may be appropriate if the set of addresses or
 prefixes is small and static.

5.3.3.3 ISP Administered Addresses

 The set of addresses used by each stub site could be administered and
 allocated via the VPRN edge router, which may be appropriate for
 small customer sites, typically containing either a single host, or a
 single subnet.  Address allocation can be carried out using protocols
 such as PPP or DHCP [37], with, for example, the edge router acting
 as a Radius client and retrieving the customer's IP address to use
 from a Radius server, or acting as a DHCP relay and examining the
 DHCP reply message as it is relayed to the customer site.  In this
 manner the edge router can build up a table of stub link reachability
 information.  Although these address assignment mechanisms are
 typically used to assign an address to a single host, some vendors
 have added extensions whereby an address prefix can be assigned,
 with, in some cases, the CPE device acting as a "mini-DHCP" server
 and assigning addresses for the hosts in the customer site.
 Note that with these schemes it is the responsibility of the address
 allocation server to ensure that each site in the VPN received a
 disjoint address space.  Note also that an ISP would typically only
 use this mechanism for small stub sites, which are unlikely to have
 backdoor links.

5.3.3.4 MPLS Label Distribution Protocol

 In cases where the CPE router runs MPLS, LDP can be used to convey
 the set of prefixes at a stub site to a VPRN edge router.  Using the
 downstream unsolicited mode of label distribution the CPE router can
 distribute a label for each route in the stub site.  Note however
 that the processing carried out by the edge router in this case is
 more than just the normal LDP processing, since it is learning new
 routes via LDP, rather than the usual case of learning labels for
 existing routes that it has learned via standard routing mechanisms.

Gleeson, et al. Informational [Page 33] RFC 2764 IP Based Virtual Private Networks February 2000

5.3.4 Intra-VPN Reachability Information

 Once an edge router has determined the set of prefixes associated
 with each of its stub links, then this information must be
 disseminated to each other edge router in the VPRN.  Note also that
 there is an implicit requirement that the set of reachable addresses
 within the VPRN be locally unique that is, each VPRN stub link (not
 performing load sharing) maintain an address space disjoint from any
 other, so as to permit unambiguous routing.  In practical terms, it
 is also generally desirable, though not required, that this address
 space be well partitioned i.e., specific, disjoint address prefixes
 per edge router, so as to preclude the need to maintain and
 disseminate large numbers of host routes.
 The problem of intra-VPN reachability information dissemination can
 be solved in a number of ways, some of which include the following:

5.3.4.1 Directory Lookup

 Along with VPRN membership information, a central directory could
 maintain a listing of the address prefixes associated with each
 customer site.  Such information could be obtained by the server
 through protocol interactions with each edge router.  Note that the
 same directory synchronization issues discussed above in section
 5.3.2 also apply in this case.

5.3.4.2 Explicit Configuration

 The address spaces associated with each edge router could be
 explicitly configured into each other router.  This is clearly a
 non-scalable solution, particularly when arbitrary topologies are
 used, and also raises the question of how the management system
 learns such information in the first place.

5.3.4.3 Local Intra-VPRN Routing Instantiations

 In this approach, each edge router runs an instance of a routing
 protocol (a 'virtual router') per VPRN, running across the VPRN
 tunnels to each peer edge router, to disseminate intra-VPRN
 reachability information.  Both full-mesh and arbitrary VPRN
 topologies can be easily supported, since the routing protocol itself
 can run over any topology.  The intra-VPRN routing advertisements
 could be distinguished from normal tunnel data packets either by
 being addressed directly to the peer edge router, or by a tunnel
 specific mechanism.

Gleeson, et al. Informational [Page 34] RFC 2764 IP Based Virtual Private Networks February 2000

 Note that this intra-VPRN routing protocol need have no relationship
 either with the IGP of any customer site or with the routing
 protocols operated by the ISPs in the IP backbone.  Depending on the
 size and scale of the VPRNs to be supported either a simple protocol
 like RIP or a more sophisticated protocol like OSPF could be used.
 Because the intra-VPRN routing protocol operates as an overlay over
 the IP backbone it is wholly transparent to any intermediate routers,
 and to any edge routers not within the VPRN.  This also implies that
 such routing information can remain opaque to such routers, which may
 be a necessary security requirements in some cases.  Also note that
 if the routing protocol runs directly over the same tunnels as the
 data traffic, then it will inherit the same level of security as that
 afforded the data traffic, for example strong encryption and
 authentication.
 If the tunnels over which an intra-VPRN routing protocol runs are
 dedicated to a specific VPN (e.g. a different multiplexing field is
 used for each VPN) then no changes are needed to the routing protocol
 itself.  On the other hand if shared tunnels are used, then it is
 necessary to extend the routing protocol to allow a VPN-ID field to
 be included in routing update packets, to allow sets of prefixes to
 be associated with a particular VPN.

5.3.4.4 Link Reachability Protocol

 By link reachability protocol is meant a protocol that allows two
 nodes, connected via a point-to-point link, to exchange reachability
 information.  Given a full mesh topology, each edge router could run
 a link reachability protocol, for instance some variation of MPLS
 CR-LDP, across the tunnel to each peer edge router in the VPRN,
 carrying the VPN-ID and the reachability information of each VPRN
 running across the tunnel between the two edge routers.  If VPRN
 membership information has already been distributed to an edge
 router, then the neighbor discovery aspects of a traditional routing
 protocol are not needed, as the set of neighbors is already known.
 TCP connections can be used to interconnect the neighbors, to provide
 reliability.  This approach may reduce the processing burden of
 running routing protocol instances per VPRN, and may be of particular
 benefit where a shared tunnel mechanism is used to connect a set of
 edge routers supporting multiple VPRNs.
 Another approach to developing a link reachability protocol would be
 to base it on IBGP.  The problem that needs to be solved by a link
 reachability protocol is very similar to that solved by IBGP -
 conveying address prefixes reliably between edge routers.

Gleeson, et al. Informational [Page 35] RFC 2764 IP Based Virtual Private Networks February 2000

 Using a link reachability protocol it is straightforward to support a
 full mesh topology - each edge router conveys its own local
 reachability information to all other routers, but does not
 redistribute information received from any other router.  However
 once an arbitrary topology needs to be supported, the link
 reachability protocol needs to develop into a full routing protocol,
 due to the need to implement mechanisms to avoid loops, and there
 would seem little benefit in reinventing another routing protocol to
 deal with this.  Some reasons why partially connected meshes may be
 needed even in a tunneled environment are discussed in section 5.1.1.

5.3.4.5 Piggybacking in IP Backbone Routing Protocols

 As with VPRN membership, the set of address prefixes associated with
 each stub interface could also be piggybacked into the routing
 advertisements from each edge router and propagated through the
 network.  Other edge routers extract this information from received
 route advertisements in the same way as they obtain the VPRN
 membership information (which, in this case, is implicit in the
 identification of the source of each route advertisement).  Note that
 this scheme may require, depending upon the nature of the routing
 protocols involved, that intermediate routers, e.g. border routers,
 cache intra-VPRN routing information in order to propagate it
 further.  This also has implications for the trust model, and for the
 level of security possible for intra-VPRN routing information.
 Note that in any of the cases discussed above, an edge router has the
 option of disseminating its stub link prefixes in a manner so as to
 permit tunneling from remote edge routers directly to the egress stub
 links.  Alternatively, it could disseminate the information so as to
 associate all such prefixes with the edge router, rather than with
 specific stub links.  In this case, the edge router would need to
 implement a VPN specific forwarding mechanism for egress traffic, to
 determine the correct egress stub link.  The advantage of this is
 that it may significantly reduce the number of distinct tunnels or
 tunnel label information which need to be constructed and maintained.
 Note that this choice is purely a local manner and is not visible to
 remote edge routers.

5.3.5 Tunneling Mechanisms

 Once VPRN membership information has been disseminated, the tunnels
 comprising the VPRN core can be constructed.
 One approach to setting up the tunnel mesh is to use point-to-point
 IP tunnels, and the requirements and issues for such tunnels have
 been discussed in section 3.0.  For example while tunnel
 establishment can be done through manual configuration, this is

Gleeson, et al. Informational [Page 36] RFC 2764 IP Based Virtual Private Networks February 2000

 clearly not likely to be a scalable solution, given the O(n^2)
 problem of meshed links.  As such, tunnel set up should use some form
 of signalling protocol to allow two nodes to construct a tunnel to
 each other knowing only each other's identity.
 Another approach is to use the multipoint to point 'tunnels' provided
 by MPLS.  As noted in [38], MPLS can be considered to be a form of IP
 tunneling, since the labels of MPLS packets allow for routing
 decisions to be decoupled from the addressing information of the
 packets themselves.  MPLS label distribution mechanisms can be used
 to associate specific sets of MPLS labels with particular VPRN
 address prefixes supported on particular egress points (i.e., stub
 links of edge routers) and hence allow other edge routers to
 explicitly label and route traffic to particular VPRN stub links.
 One attraction of MPLS as a tunneling mechanism is that it may
 require less processing within each edge router than alternative
 tunneling mechanisms.  This is a function of the fact that data
 security within a MPLS network is implicit in the explicit label
 binding, much as with a connection oriented network, such as Frame
 Relay.  This may hence lessen customer concerns about data security
 and hence require less processor intensive security mechanisms (e.g.,
 IPSec).  However there are other potential security concerns with
 MPLS.  There is no direct support for security features such as
 authentication, confidentiality, and non-repudiation and the trust
 model for MPLS means that intermediate routers, (which may belong to
 different administrative domains), through which membership and
 prefix reachability information is conveyed, must be trusted, not
 just the edge routers themselves.

5.4 Multihomed Stub Routers

 The discussion thus far has implicitly assumed that stub routers are
 connected to one and only one VPRN edge router.  In general, this
 restriction should be capable of being relaxed without any change to
 VPRN operation, given general market interest in multihoming for
 reliability and other reasons.  In particular, in cases where the
 stub router supports multiple redundant links, with only one
 operational at any given time, with the links connected either to the
 same VPRN edge router, or to two or more different VPRN edge routers,
 then the stub link reachability mechanisms will both discover the
 loss of an active link, and the activation of a backup link.  In the
 former situation, the previously connected VPRN edge router will
 cease advertising reachability to the stub node, while the VPRN edge
 router with the now active link will begin advertising reachability,
 hence restoring connectivity.

Gleeson, et al. Informational [Page 37] RFC 2764 IP Based Virtual Private Networks February 2000

 An alternative scenario is where the stub node supports multiple
 active links, using some form of load sharing algorithm.  In such a
 case, multiple VPRN edge routers may have active paths to the stub
 node, and may so advertise across the VPRN.  This scenario should not
 cause any problem with reachability across the VPRN providing that
 the intra-VPRN reachability mechanism can accommodate multiple paths
 to the same prefix, and has the appropriate mechanisms to preclude
 looping - for instance, distance vector metrics associated with each
 advertised prefix.

5.5 Multicast Support

 Multicast and broadcast traffic can be supported across VPRNs either
 by edge replication or by native multicast support in the backbone.
 These two cases are discussed below.

5.5.1 Edge Replication

 This is where each VPRN edge router replicates multicast traffic for
 transmission across each link in the VPRN.  Note that this is the
 same operation that would be performed by CPE routers terminating
 actual physical links or dedicated connections.  As with CPE routers,
 multicast routing protocols could also be run on each VPRN edge
 router to determine the distribution tree for multicast traffic and
 hence reduce unnecessary flood traffic.  This could be done by
 running instances of standard multicast routing protocols, e.g.
 Protocol Independent Multicast (PIM) [39] or Distance Vector
 Multicast Routing Protocol (DVMRP) [40], on and between each VPRN
 edge router, through the VPRN tunnels, in the same way that unicast
 routing protocols might be run at each VPRN edge router to determine
 intra-VPN unicast reachability, as discussed in section 5.3.4.
 Alternatively, if a link reachability protocol was run across the
 VPRN tunnels for intra-VPRN reachability, then this could also be
 augmented to allow VPRN edge routers to indicate both the particular
 multicast groups requested for reception at each edge node, and also
 the multicast sources at each edge site.
 In either case, there would need to be some mechanism to allow for
 the VPRN edge routers to determine which particular multicast groups
 were requested at each site and which sources were present at each
 site.  How this could be done would, in general, be a function of the
 capabilities of the CPE stub routers at each site.  If these run
 multicast routing protocols, then they can interact directly with the
 equivalent protocols at each VPRN edge router.  If the CPE device
 does not run a multicast routing protocol, then in the absence of
 Internet Group Management Protocol (IGMP) proxying [41] the customer
 site would be limited to a single subnet connected to the VPRN edge
 router via a bridging device, as the scope of an IGMP message is

Gleeson, et al. Informational [Page 38] RFC 2764 IP Based Virtual Private Networks February 2000

 limited to a single subnet.  However using IGMP-proxying the CPE
 router can engage in multicast forwarding without running a multicast
 routing protocol, in constrained topologies.  On its interfaces into
 the customer site the CPE router performs the router functions of
 IGMP, and on its interface to the VPRN edge router it performs the
 host functions of IGMP.

5.5.2 Native Multicast Support

 This is where VPRN edge routers map intra-VPRN multicast traffic onto
 a native IP multicast distribution mechanism across the backbone.
 Note that intra-VPRN multicast has the same requirements for
 isolation from general backbone traffic as intra-VPRN unicast
 traffic.  Currently the only IP tunneling mechanism that has native
 support for multicast is MPLS.  On the other hand, while MPLS
 supports native transport of IP multicast packets, additional
 mechanisms would be needed to leverage these mechanisms for the
 support of intra-VPRN multicast.
 For instance, each VPRN router could prefix multicast group addresses
 within each VPRN with the VPN-ID of that VPRN and then redistribute
 these, essentially treating this VPN-ID/intra-VPRN multicast address
 tuple as a normal multicast address, within the backbone multicast
 routing protocols, as with the case of unicast reachability, as
 discussed previously.  The MPLS multicast label distribution
 mechanisms could then be used to set up the appropriate multicast
 LSPs to interconnect those sites within each VPRN supporting
 particular multicast group addresses.  Note, however, that this would
 require each of the intermediate LSRs to not only be aware of each
 intra-VPRN multicast group, but also to have the capability of
 interpreting these modified advertisements.  Alternatively,
 mechanisms could be defined to map intra-VPRN multicast groups into
 backbone multicast groups.
 Other IP tunneling mechanisms do not have native multicast support.
 It may prove feasible to extend such tunneling mechanisms by
 allocating IP multicast group addresses to the VPRN as a whole and
 hence distributing intra-VPRN multicast traffic encapsulated within
 backbone multicast packets.  Edge VPRN routers could filter out
 unwanted multicast groups.  Alternatively, mechanisms could also be
 defined to allow for allocation of backbone multicast group addresses
 for particular intra-VPRN multicast groups, and to then utilize
 these, through backbone multicast protocols, as discussed above, to
 limit forwarding of intra-VPRN multicast traffic only to those nodes
 within the group.

Gleeson, et al. Informational [Page 39] RFC 2764 IP Based Virtual Private Networks February 2000

 A particular issue with the use of native multicast support is the
 provision of security for such multicast traffic.  Unlike the case of
 edge replication, which inherits the security characteristics of the
 underlying tunnel, native multicast mechanisms will need to use some
 form of secure multicast mechanism.  The development of architectures
 and solutions for secure multicast is an active research area, for
 example see [42] and [43].  The Secure Multicast Group (SMuG) of the
 IRTF has been set up to develop prototype solutions, which would then
 be passed to the IETF IPSec working group for standardization.
 However considerably more development is needed before scalable
 secure native multicast mechanisms can be generally deployed.

5.6 Recommendations

 The various proposals that have been developed to support some form
 of VPRN functionality can be broadly classified into two groups -
 those that utilize the router piggybacking approach for distributing
 VPN membership and/or reachability information ([13],[15]) and those
 that use the virtual routing approach ([12],[14]).  In some cases the
 mechanisms described rely on the characteristics of a particular
 infrastructure (e.g. MPLS) rather than just IP.
 Within the context of the virtual routing approach it may be useful
 to develop a membership distribution protocol based on a directory or
 MIB.  When combined with the protocol extensions for IP tunneling
 protocols outlined in section 3.2, this would then provide the basis
 for a complete set of protocols and mechanisms that support
 interoperable VPRNs that span multiple administrations over an IP
 backbone.  Note that the other major pieces of functionality needed -
 the learning and distribution of customer reachability information,
 can be performed by instances of standard routing protocols, without
 the need for any protocol extensions.
 Also for the constrained case of a full mesh topology, the usefulness
 of developing a link reachability protocol could be examined, however
 the limitations and scalability issues associated with this topology
 may not make it worthwhile to develop something specific for this
 case, as standard routing will just work.
 Extending routing protocols to allow a VPN-ID to carried in routing
 update packets could also be examined, but is not necessary if VPN
 specific tunnels are used.

Gleeson, et al. Informational [Page 40] RFC 2764 IP Based Virtual Private Networks February 2000

6.0 VPN Types: Virtual Private Dial Networks

 A Virtual Private Dial Network (VPDN) allows for a remote user to
 connect on demand through an ad hoc tunnel into another site.  The
 user is connected to a public IP network via a dial-up PSTN or ISDN
 link, and user packets are tunneled across the public network to the
 desired site, giving the impression to the user of being 'directly'
 connected into that site.  A key characteristic of such ad hoc
 connections is the need for user authentication as a prime
 requirement, since anyone could potentially attempt to gain access to
 such a site using a switched dial network.
 Today many corporate networks allow access to remote users through
 dial connections made through the PSTN, with users setting up PPP
 connections across an access network to a network access server, at
 which point the PPP sessions are authenticated using AAA systems
 running such standard protocols as Radius [44].  Given the pervasive
 deployment of such systems, any VPDN system must in practice allow
 for the near transparent re-use of such existing systems.
 The IETF have developed the Layer 2 Tunneling Protocol (L2TP) [8]
 which allows for the extension of of user PPP sessions from an L2TP
 Access Concentrator (LAC) to a remote L2TP Network Server (LNS).  The
 L2TP protocol itself was based on two earlier protocols, the Layer 2
 Forwarding protocol (L2F) [45], and the Point-to-Point Tunneling
 Protocol (PPTP) [46], and this is reflected in the two quite
 different scenarios for which L2TP can be used - compulsory tunneling
 and voluntary tunneling, discussed further below in sections 6.2 and
 6.3.
 This document focuses on the use of L2TP over an IP network (using
 UDP), but L2TP may also be run directly over other protocols such as
 ATM or Frame Relay.  Issues specifically related to running L2TP over
 non-IP networks, such as how to secure such tunnels, are not
 addressed here.

6.1 L2TP protocol characteristics

 This section looks at the characteristics of the L2TP tunneling
 protocol using the categories outlined in section 3.0.

6.1.1 Multiplexing

 L2TP has inherent support for the multiplexing of multiple calls from
 different users over a single link.  Between the same two IP
 endpoints, there can be multiple L2TP tunnels, as identified by a
 tunnel-id, and multiple sessions within a tunnel, as identified by a
 session-id.

Gleeson, et al. Informational [Page 41] RFC 2764 IP Based Virtual Private Networks February 2000

6.1.2 Signalling

 This is supported via the inbuilt control connection protocol,
 allowing both tunnels and sessions to be established dynamically.

6.1.3 Data Security

 By allowing for the transparent extension of PPP from the user,
 through the LAC to the LNS, L2TP allows for the use of whatever
 security mechanisms, with respect to both connection set up, and data
 transfer, may be used with normal PPP connections.  However this does
 not provide security for the L2TP control protocol itself.  In this
 case L2TP could be further secured by running it in combination with
 IPSec through IP backbones [47], [48], or related mechanisms on non-
 IP backbones [49].
 The interaction of L2TP with AAA systems for user authentication and
 authorization is a function of the specific means by which L2TP is
 used, and the nature of the devices supporting the LAC and the LNS.
 These issues are discussed in depth in [50].
 The means by which the host determines the correct LAC to connect to,
 and the means by which the LAC determines which users to further
 tunnel, and the LNS parameters associated with each user, are outside
 the scope of the operation of a VPDN, but may be addressed, for
 instance, by evolving Internet roaming specifications [51].

6.1.4 Multiprotocol Transport

 L2TP transports PPP packets (and only PPP packets) and thus can be
 used to carry multiprotocol traffic since PPP itself is
 multiprotocol.

6.1.5 Sequencing

 L2TP supports sequenced delivery of packets.  This is a capability
 that can be negotiated at session establishment, and that can be
 turned on and off by an LNS during a session.  The sequence number
 field in L2TP can also be used to provide an indication of dropped
 packets, which is needed by various PPP compression algorithms to
 operate correctly.  If no compression is in use, and the LNS
 determines that the protocols in use (as evidenced by the PPP NCP
 negotiations) can deal with out of sequence packets (e.g. IP), then
 it may disable the use of sequencing.

Gleeson, et al. Informational [Page 42] RFC 2764 IP Based Virtual Private Networks February 2000

6.1.6 Tunnel Maintenance

 A keepalive protocol is used by L2TP in order to allow it to
 distinguish between a tunnel outage and prolonged periods of tunnel
 inactivity.

6.1.7 Large MTUs

 L2TP itself has no inbuilt support for a segmentation and reassembly
 capability, but when run over UDP/IP IP fragmentation will take place
 if necessary.  Note that a LAC or LNS may adjust the Maximum Receive
 Unit (MRU) negotiated via PPP in order to preclude fragmentation, if
 it has knowledge of the MTU used on the path between LAC and LNS.  To
 this end, there is a proposal to allow the use of MTU discovery for
 cases where the L2TP tunnel transports IP frames [52].

6.1.8 Tunnel Overhead

 L2TP as used over IP networks runs over UDP and must be used to carry
 PPP traffic.  This results in a significant amount of overhead, both
 in the data plane with UDP, L2TP and PPP headers, and also in the
 control plane, with the L2TP and PPP control protocols.  This is
 discussed further in section 6.3

6.1.9 Flow and Congestion Control

 L2TP supports flow and congestion control mechanisms for the control
 protocol, but not for data traffic.  See section 3.1.9 for more
 details.

6.1.10 QoS / Traffic Management

 An L2TP header contains a 1-bit priority field, which can be set for
 packets that may need preferential treatment (e.g. keepalives) during
 local queuing and transmission.  Also by transparently extending PPP,
 L2TP has inherent support for such PPP mechanisms as multi-link PPP
 [53] and its associated control protocols [54], which allow for
 bandwidth on demand to meet user requirements.
 In addition L2TP calls can be mapped into whatever underlying traffic
 management mechanisms may exist in the network, and there are
 proposals to allow for requests through L2TP signalling for specific
 differentiated services behaviors [55].

Gleeson, et al. Informational [Page 43] RFC 2764 IP Based Virtual Private Networks February 2000

6.1.11 Miscellaneous

 Since L2TP is designed to transparently extend PPP, it does not
 attempt to supplant the normal address assignment mechanisms
 associated with PPP.  Hence, in general terms the host initiating the
 PPP session will be assigned an address by the LNS using PPP
 procedures.  This addressing may have no relation to the addressing
 used for communication between the LAC and LNS.  The LNS will also
 need to support whatever forwarding mechanisms are needed to route
 traffic to and from the remote host.

6.2 Compulsory Tunneling

 Compulsory tunneling refers to the scenario in which a network node -
 a dial or network access server, for instance - acting as a LAC,
 extends a PPP session across a backbone using L2TP to a remote LNS,
 as illustrated below.  This operation is transparent to the user
 initiating the PPP session to the LAC.  This allows for the
 decoupling of the location and/or ownership of the modem pools used
 to terminate dial calls, from the site to which users are provided
 access.  Support for this scenario was the original intent of the L2F
 specification, upon which the L2TP specification was based.
 There are a number of different deployment scenarios possible. One
 example, shown in the diagram below, is where a subscriber host dials
 into a NAS acting as a LAC, and is tunneled across an IP network
 (e.g. the Internet) to a gateway acting as an LNS. The gateway
 provides access to a corporate network, and could either be a device
 in the corporate network itself, or could be an ISP edge router, in
 the case where a customer has outsourced the maintenance of LNS
 functionality to an ISP.  Another scenario is where an ISP uses L2TP
 to provide a subscriber with access to the Internet. The subscriber
 host dials into a NAS acting as a LAC, and is tunneled across an
 access network to an ISP edge router acting as an LNS. This ISP edge
 router then feeds the subscriber traffic into the Internet.  Yet
 other scenarios are where an ISP uses L2TP to provide a subscriber
 with access to a VPRN, or with concurrent access to both a VPRN and
 the Internet.
 A VPDN, whether using compulsory or voluntary tunneling, can be
 viewed as just another type of access method for subscriber traffic,
 and as such can be used to provide connectivity to different types of
 networks, e.g. a corporate network, the Internet, or a VPRN. The last
 scenario is also an example of how a VPN service as provided to a
 customer may be implemented using a combination of different types of
 VPN.

Gleeson, et al. Informational [Page 44] RFC 2764 IP Based Virtual Private Networks February 2000

 10.0.0.1
 +----+
 |Host|-----    LAC      -------------     LNS        10.0.0.0/8
 +----+   /   +-----+   (             )   +-----+     ---------
         /----| NAS |---( IP Backbone )---| GW  |----( Corp.   )
      dial    +-----+   (             )   +-----+    ( Network )
      connection         -------------                ---------
                 <------- L2TP Tunnel ------->
   <--------------------- PPP Session ------->
               Figure 6.1: Compulsory Tunneling Example
 Compulsory tunneling was originally intended for deployment on
 network access servers supporting wholesale dial services, allowing
 for remote dial access through common facilities to an enterprise
 site, while precluding the need for the enterprise to deploy its own
 dial servers.  Another example of this is where an ISP outsources its
 own dial connectivity to an access network provider (such as a Local
 Exchange Carrier (LEC) in the USA) removing the need for an ISP to
 maintain its own dial servers and allowing the LEC to serve multiple
 ISPs.  More recently, compulsory tunneling mechanisms have also been
 proposed for evolving Digital Subscriber Line (DSL) services [56],
 [57], which also seek to leverage the existing AAA infrastructure.
 Call routing for compulsory tunnels requires that some aspect of the
 initial PPP call set up can be used to allow the LAC to determine the
 identity of the LNS.  As noted in [50], these aspects can include the
 user identity, as determined through some aspect of the access
 network, including calling party number, or some attribute of the
 called party, such as the Fully Qualified Domain Name (FQDN) of the
 identity claimed during PPP authentication.
 It is also possible to chain two L2TP tunnels together, whereby a LAC
 initiates a tunnel to an intermediate relay device, which acts as an
 LNS to this first LAC, and acts as a LAC to the final LNS.  This may
 be needed in some cases due to administrative, organizational or
 regulatory issues pertaining to the split between access network
 provider, IP backbone provider and enterprise customer.

Gleeson, et al. Informational [Page 45] RFC 2764 IP Based Virtual Private Networks February 2000

6.3 Voluntary Tunnels

 Voluntary tunneling refers to the case where an individual host
 connects to a remote site using a tunnel originating on the host,
 with no involvement from intermediate network nodes, as illustrated
 below.  The PPTP specification, parts of which have been incorporated
 into L2TP, was based upon a voluntary tunneling model.
 As with compulsory tunneling there are different deployment scenarios
 possible. The diagram below shows a subscriber host accessing a
 corporate network with either L2TP or IPSec being used as the
 voluntary tunneling mechanism. Another scenario is where voluntary
 tunneling is used to provide a subscriber with access to a VPRN.

6.3.1 Issues with Use of L2TP for Voluntary Tunnels

 The L2TP specification has support for voluntary tunneling, insofar
 as the LAC can be located on a host, not only on a network node.
 Note that such a host has two IP addresses - one for the LAC-LNS IP
 tunnel, and another, typically allocated via PPP, for the network to
 which the host is connecting.  The benefits of using L2TP for
 voluntary tunneling are that the existing authentication and address
 assignment mechanisms used by PPP can be reused without modification.
 For example an LNS could also include a Radius client, and
 communicate with a Radius server to authenticate a PPP PAP or CHAP
 exchange, and to retrieve configuration information for the host such
 as its IP address and a list of DNS servers to use.  This information
 can then be passed to the host via the PPP IPCP protocol.
 10.0.0.1
 +----+
 |Host|-----             -------------                10.0.0.0/8
 +----+   /   +-----+   (             )   +-----+     ---------
         /----| NAS |---( IP Backbone )---| GW  |----( Corp.   )
      dial    +-----+   (             )   +-----+    ( Network )
      connection         -------------                ---------
   <-------------- L2TP Tunnel -------------->
                      with                      LAC on host
   <-------------- PPP Session -------------->  LNS on gateway
                      or
   <-------------- IPSEC Tunnel -------------->
                Figure 6.2: Voluntary Tunneling Example

Gleeson, et al. Informational [Page 46] RFC 2764 IP Based Virtual Private Networks February 2000

 The above procedure is not without its costs, however.  There is
 considerable overhead with such a protocol stack, particularly when
 IPSec is also needed for security purposes, and given that the host
 may be connected via a low-bandwidth dial up link.  The overhead
 consists of both extra headers in the data plane and extra control
 protocols needed in the control plane.  Using L2TP for voluntary
 tunneling, secured with IPSec, means a web application, for example,
 would run over the following stack
   HTTP/TCP/IP/PPP/L2TP/UDP/ESP/IP/PPP/AHDLC
 It is proposed in [58] that IPSec alone be used for voluntary tunnels
 reducing overhead, using the following stack.
   HTTP/TCP/IP/ESP/IP/PPP/AHDLC
 In this case IPSec is used in tunnel mode, with the tunnel
 terminating either on an IPSec edge device at the enterprise site, or
 on the provider edge router connected to the enterprise site.  There
 are two possibilities for the IP addressing of the host.  Two IP
 addresses could be used, in a similar manner to the L2TP case.
 Alternatively the host can use a single public IP address as the
 source IP address in both inner and outer IP headers, with the
 gateway performing Network Address Translation (NAT) before
 forwarding the traffic to the enterprise network.  To other hosts in
 the enterprise network the host appears to have an 'internal' IP
 address.  Using NAT has some limitations and restrictions, also
 pointed out in [58].
 Another area of potential problems with PPP is due to the fact that
 the characteristics of a link layer implemented via an L2TP tunnel
 over an IP backbone are quite different to a link layer run over a
 serial line, as discussed in the L2TP specification itself.  For
 example, poorly chosen PPP parameters may lead to frequent resets and
 timeouts, particularly if compression is in use.  This is because an
 L2TP tunnel may misorder packets, and may silently drop packets,
 neither of which normally occurs on serial lines.  The general packet
 loss rate could also be significantly higher due to network
 congestion.  Using the sequence number field in an L2TP header
 addresses the misordering issue, and for cases where the LAC and LNS
 are coincident with the PPP endpoints, as in voluntary tunneling, the
 sequence number field can also be used to detect a dropped packet,
 and to pass a suitable indication to any compression entity in use,
 which typically requires such knowledge in order to keep the
 compression histories in synchronization at both ends. (In fact this
 is more of an issue with compulsory tunneling since the LAC may have
 to deliberately issue a corrupted frame to the PPP host, to give an
 indication of packet loss, and some hardware may not allow this).

Gleeson, et al. Informational [Page 47] RFC 2764 IP Based Virtual Private Networks February 2000

6.3.2 Issues with Use of IPSec for Voluntary Tunnels

 If IPSec is used for voluntary tunneling, the functions of user
 authentication and host configuration, achieved by means of PPP when
 using L2TP, still need to be carried out.  A distinction needs to be
 drawn here between machine authentication and user authentication.  '
 Two factor' authentication is carried out on the basis of both
 something the user has, such as a machine or smartcard with a digital
 certificate, and something the user knows, such as a password.
 (Another example is getting money from an bank ATM machine - you need
 a card and a PIN number).  Many of the existing legacy schemes
 currently in use to perform user authentication are asymmetric in
 nature, and are not supported by IKE. For remote access the most
 common existing user authentication mechanism is to use PPP between
 the user and access server, and Radius between the access server and
 authentication server.  The authentication exchanges that occur in
 this case, e.g. a PAP or CHAP exchange, are asymmetric.  Also CHAP
 supports the ability for the network to reauthenticate the user at
 any time after the initial session has been established, to ensure
 that the current user is the same person that initiated the session.
 While IKE provides strong support for machine authentication, it has
 only limited support for any form of user authentication and has no
 support for asymmetric user authentication.  While a user password
 can be used to derive a key used as a preshared key, this cannot be
 used with IKE Main Mode in a remote access environment, as the user
 will not have a fixed IP address, and while Aggressive Mode can be
 used instead, this affords no identity protection.  To this end there
 have been a number of proposals to allow for support of legacy
 asymmetric user level authentication schemes with IPSec.  [59]
 defines a new IKE message exchange - the transaction exchange - which
 allows for both Request/Reply and Set/Acknowledge message sequences,
 and it also defines attributes that can be used for client IP stack
 configuration. [60] and [61] describe mechanisms that use the
 transaction message exchange, or a series of such exchanges, carried
 out between the IKE Phase 1 and Phase 2 exchanges, to perform user
 authentication. A different approach, that does not extend the IKE
 protocol itself, is described in [62]. With this approach a user
 establishes a Phase 1 SA with a security gateway and then sets up a
 Phase 2 SA to the gateway, over which an existing authentication
 protocol is run. The gateway acts as a proxy and relays the protocol
 messages to an authentication server.
 In addition there have also been proposals to allow the remote host
 to be configured with an IP address and other configuration
 information over IPSec.  For example [63] describes a method whereby
 a remote host first establishes a Phase 1 SA with a security gateway
 and then sets up a Phase 2 SA to the gateway, over which the DHCP

Gleeson, et al. Informational [Page 48] RFC 2764 IP Based Virtual Private Networks February 2000

 protocol is run. The gateway acts as a proxy and relays the protocol
 messages to the DHCP server.  Again, like [62], this proposal does
 not involve extensions to the IKE protocol itself.
 Another aspect of PPP functionality that may need to supported is
 multiprotocol operation, as there may be a need to carry network
 layer protocols other than IP, and even to carry link layer protocols
 (e.g.  ethernet) as would be needed to support bridging over IPSec.
 This is discussed in section 3.1.4.
 The methods of supporting legacy user authentication and host
 configuration capabilities in a remote access environment are
 currently being discussed in the IPSec working group.

6.4 Networked Host Support

 The current PPP based dial model assumes a host directly connected to
 a connection oriented dial access network.  Recent work on new access
 technologies such as DSL have attempted to replicate this model [57],
 so as to allow for the re-use of existing AAA systems.  The
 proliferation of personal computers, printers and other network
 appliances in homes and small businesses, and the ever lowering costs
 of networks, however, are increasingly challenging the directly
 connected host model.  Increasingly, most hosts will access the
 Internet through small, typically Ethernet, local area networks.
 There is hence interest in means of accommodating the existing AAA
 infrastructure within service providers, whilst also supporting
 multiple networked hosts at each customer site.  The principal
 complication with this scenario is the need to support the login
 dialogue, through which the appropriate AAA information is exchanged.
 A number of proposals have been made to address this scenario:

6.4.1 Extension of PPP to Hosts Through L2TP

 A number of proposals (e.g. [56]) have been made to extend L2TP over
 Ethernet so that PPP sessions can run from networked hosts out to the
 network, in much the same manner as a directly attached host.

6.4.2 Extension of PPP Directly to Hosts:

 There is also a specification for mapping PPP directly onto Ethernet
 (PPPOE) [64] which uses a broadcast mechanism to allow hosts to find
 appropriate access servers with which to connect. Such servers could
 then further tunnel, if needed, the PPP sessions using L2TP or a
 similar mechanism.

Gleeson, et al. Informational [Page 49] RFC 2764 IP Based Virtual Private Networks February 2000

6.4.3 Use of IPSec

 The IPSec based voluntary tunneling mechanisms discussed above can be
 used either with networked or directly connected hosts.
 Note that all of these methods require additional host software to be
 used, which implements either LAC, PPPOE client or IPSec client
 functionality.

6.5 Recommendations

 The L2TP specification has been finalized and will be widely used for
 compulsory tunneling.  As discussed in section 3.2, defining specific
 modes of operation for IPSec when used to secure L2TP would be
 beneficial.
 Also, for voluntary tunneling using IPSec, completing the work needed
 to provide support for the following areas would be useful
  1. asymmetric / legacy user authentication (6.3)
  1. host address assignment and configuration (6.3)
 along with any other issues specifically related to the support of
 remote hosts. Currently as there are many different non-interoperable
 proprietary solutions in this area.

7.0 VPN Types: Virtual Private LAN Segment

 A Virtual Private LAN Segment (VPLS) is the emulation of a LAN
 segment using Internet facilities.  A VPLS can be used to provide
 what is sometimes known also as a Transparent LAN Service (TLS),
 which can be used to interconnect multiple stub CPE nodes, either
 bridges or routers, in a protocol transparent manner.  A VPLS
 emulates a LAN segment over IP, in the same way as protocols such as
 LANE emulate a LAN segment over ATM.  The primary benefits of a VPLS
 are complete protocol transparency, which may be important both for
 multiprotocol transport and for regulatory reasons in particular
 service provider contexts.

Gleeson, et al. Informational [Page 50] RFC 2764 IP Based Virtual Private Networks February 2000

 10.1.1.1    +--------+                       +--------+    10.1.1.2
 +---+       | ISP    |     IP tunnel         | ISP    |       +---+
 |CPE|-------| edge   |-----------------------| edge   |-------|CPE|
 +---+ stub  | node   |                       | node   |  stub +---+
       link  +--------+                       +--------+  link
                  ^  |                         |   ^
                  |  |     ---------------     |   |
                  |  |    (               )    |   |
                  |  +----( IP BACKBONE   )----+   |
                  |       (               )        |
                  |        ---------------         |
                  |               |                |
                  |IP tunnel  +--------+  IP tunnel|
                  |           | ISP    |           |
                  +-----------| edge   |-----------+
                              | node   |
                              +--------+    subnet = 10.1.1.0/24
                                  |
                        stub link |
                                  |
                                +---+
                                |CPE| 10.1.1.3
                                +---+
                       Figure 7.1: VPLS Example

7.1 VPLS Requirements

 Topologically and operationally a VPLS can be most easily modeled as
 being essentially equivalent to a VPRN, except that each VPLS edge
 node implements link layer bridging rather than network layer
 forwarding.  As such, most of the VPRN tunneling and configuration
 mechanisms discussed previously can also be used for a VPLS, with the
 appropriate changes to accommodate link layer, rather than network
 layer, packets and addressing information.  The following sections
 discuss the primary changes needed in VPRN operation to support
 VPLSs.

7.1.1 Tunneling Protocols

 The tunneling protocols employed within a VPLS can be exactly the
 same as those used within a VPRN, if the tunneling protocol permits
 the transport of multiprotocol traffic, and this is assumed below.

Gleeson, et al. Informational [Page 51] RFC 2764 IP Based Virtual Private Networks February 2000

7.1.2 Multicast and Broadcast Support

 A VPLS needs to have a broadcast capability.  This is needed both for
 broadcast frames, and for link layer packet flooding, where a unicast
 frame is flooded because the path to the destination link layer
 address is unknown.  The address resolution protocols that run over a
 bridged network typically use broadcast frames (e.g. ARP).  The same
 set of possible multicast tunneling mechanisms discussed earlier for
 VPRNs apply also to a VPLS, though the generally more frequent use of
 broadcast in VPLSs may increase the pressure for native multicast
 support that reduces, for instance, the burden of replication on VPLS
 edge nodes.

7.1.3 VPLS Membership Configuration and Topology

 The configuration of VPLS membership is analogous to that of VPRNs
 since this generally requires only knowledge of the local VPN link
 assignments at any given VPLS edge node, and the identity of, or
 route to, the other edge nodes in the VPLS; in particular, such
 configuration is independent of the nature of the forwarding at each
 VPN edge node.  As such, any of the mechanisms for VPN member
 configuration and dissemination discussed for VPRN configuration can
 also be applied to VPLS configuration.  Also as with VPRNs, the
 topology of the VPLS could be easily manipulated by controlling the
 configuration of peer nodes at each VPLS edge node, assuming that the
 membership dissemination mechanism was such as to permit this.  It is
 likely that typical VPLSs will be fully meshed, however, in order to
 preclude the need for traffic between two VPLS nodes to transit
 through another VPLS node, which would then require the use of the
 Spanning Tree protocol [65] for loop prevention.

7.1.4 CPE Stub Node Types

 A VPLS can support either bridges or routers as a CPE device.
 CPE routers would peer transparently across a VPLS with each other
 without requiring any router peering with any nodes within the VPLS.
 The same scalability issues that apply to a full mesh topology for
 VPRNs, apply also in this case, only that now the number of peering
 routers is potentially greater, since the ISP edge device is no
 longer acting as an aggregation point.
 With CPE bridge devices the broadcast domain encompasses all the CPE
 sites as well as the VPLS itself.  There are significant scalability
 constraints in this case, due to the need for packet flooding, and

Gleeson, et al. Informational [Page 52] RFC 2764 IP Based Virtual Private Networks February 2000

 the fact that any topology change in the bridged domain is not
 localized, but is visible throughout the domain.  As such this
 scenario is generally only suited for support of non-routable
 protocols.
 The nature of the CPE impacts the nature of the encapsulation,
 addressing, forwarding and reachability protocols within the VPLS,
 and are discussed separately below.

7.1.5 Stub Link Packet Encapsulation

7.1.5.1 Bridge CPE

 In this case, packets sent to and from the VPLS across stub links are
 link layer frames, with a suitable access link encapsulation.  The
 most common case is likely to be ethernet frames, using an
 encapsulation appropriate to the particular access technology, such
 as ATM, connecting the CPE bridges to the VPLS edge nodes.  Such
 frames are then forwarded at layer 2 onto a tunnel used in the VPLS.
 As noted previously, this does mandate the use of an IP tunneling
 protocol which can transport such link layer frames.  Note that this
 does not necessarily mandate, however, the use of a protocol
 identification field in each tunnel packet, since the nature of the
 encapsulated traffic (e.g. ethernet frames) could be indicated at
 tunnel setup.

7.1.5.2 Router CPE

 In this case, typically, CPE routers send link layer packets to and
 from the VPLS across stub links, destined to the link layer addresses
 of their peer CPE routers.  Other types of encapsulations may also
 prove feasible in such a case, however, since the relatively
 constrained addressing space needed for a VPLS to which only router
 CPE are connected, could allow for alternative encapsulations, as
 discussed further below.

7.1.6 CPE Addressing and Address Resolution

7.1.6.1 Bridge CPE

 Since a VPLS operates at the link layer, all hosts within all stub
 sites, in the case of bridge CPE, will typically be in the same
 network layer subnet.  (Multinetting, whereby multiple subnets
 operate over the same LAN segment, is possible, but much less
 common).  Frames are forwarded across and within the VPLS based upon
 the link layer addresses - e.g. IEEE MAC addresses - associated with
 the individual hosts.  The VPLS needs to support broadcast traffic,
 such as that typically used for the address resolution mechanism used

Gleeson, et al. Informational [Page 53] RFC 2764 IP Based Virtual Private Networks February 2000

 to map the host network addresses to their respective link addresses.
 The VPLS forwarding and reachability algorithms also need to be able
 to accommodate flooded traffic.

7.1.6.2 Router CPE

 A single network layer subnet is generally used to interconnect
 router CPE devices, across a VPLS.  Behind each CPE router are hosts
 in different network layer subnets.  CPE routers transfer packets
 across the VPLS by mapping next hop network layer addresses to the
 link layer addresses of a router peer.  A link layer encapsulation is
 used, most commonly ethernet, as for the bridge case.
 As noted above, however, in cases where all of the CPE nodes
 connected to the VPLS are routers, then it may be possible, due to
 the constrained addressing space of the VPLS, to use encapsulations
 that use a different address space than normal MAC addressing.  See,
 for instance, [11], for a proposed mechanism for VPLSs over MPLS
 networks, leveraging earlier work on VPRN support over MPLS [38],
 which proposes MPLS as the tunneling mechanism, and locally assigned
 MPLS labels as the link layer addressing scheme to identify the CPE
 LSR routers connected to the VPLS.

7.1.7 VPLS Edge Node Forwarding and Reachability Mechanisms

7.1.7.1 Bridge CPE

 The only practical VPLS edge node forwarding mechanism in this case
 is likely to be standard link layer packet flooding and MAC address
 learning, as per [65].  As such, no explicit intra-VPLS reachability
 protocol will be needed, though there will be a need for broadcast
 mechanisms to flood traffic, as discussed above.  In general, it may
 not prove necessary to also implement the Spanning Tree protocol
 between VPLS edge nodes, if the VPLS topology is such that no VPLS
 edge node is used for transit traffic between any other VPLS edge
 nodes - in other words, where there is both full mesh connectivity
 and transit is explicitly precluded.  On the other hand, the CPE
 bridges may well implement the spanning tree protocol in order to
 safeguard against 'backdoor' paths that bypass connectivity through
 the VPLS.

7.1.7.2 Router CPE

 Standard bridging techniques can also be used in this case.  In
 addition, the smaller link layer address space of such a VPLS may
 also permit other techniques, with explicit link layer routes between
 CPE routers.  [11], for instance, proposes that MPLS LSPs be set up,
 at the insertion of any new CPE router into the VPLS, between all CPE

Gleeson, et al. Informational [Page 54] RFC 2764 IP Based Virtual Private Networks February 2000

 LSRs.  This then precludes the need for packet flooding.  In the more
 general case, if stub link reachability mechanisms were used to
 configure VPLS edge nodes with the link layer addresses of the CPE
 routers connected to them, then modifications of any of the intra-VPN
 reachability mechanisms discussed for VPRNs could be used to
 propagate this information to each other VPLS edge node.  This would
 then allow for packet forwarding across the VPLS without flooding.
 Mechanisms could also be developed to further propagate the link
 layer addresses of peer CPE routers and their corresponding network
 layer addresses across the stub links to the CPE routers, where such
 information could be inserted into the CPE router's address
 resolution tables.  This would then also preclude the need for
 broadcast address resolution protocols across the VPLS.
 Clearly there would be no need for the support of spanning tree
 protocols if explicit link layer routes were determined across the
 VPLS.  If normal flooding mechanisms were used then spanning tree
 would only be required if full mesh connectivity was not available
 and hence VPLS nodes had to carry transit traffic.

7.2 Recommendations

 There is significant commonality between VPRNs and VPLSs, and, where
 possible, this similarity should be exploited in order to reduce
 development and configuration complexity.  In particular, VPLSs
 should utilize the same tunneling and membership configuration
 mechanisms, with changes only to reflect the specific characteristics
 of VPLSs.

8.0 Summary of Recommendations

 In this document different types of VPNs have been discussed
 individually, but there are many common requirements and mechanisms
 that apply to all types of VPNs, and many networks will contain a mix
 of different types of VPNs.  It is useful to have as much commonality
 as possible across these different VPN types.  In particular, by
 standardizing a relatively small number of mechanisms, it is possible
 to allow a wide variety of VPNs to be implemented.
 The benefits of adding support for the following mechanisms should be
 carefully examined.
 For IKE/IPSec:
  1. the transport of a VPN-ID when establishing an SA (3.1.2)
  1. a null encryption and null authentication option (3.1.3)

Gleeson, et al. Informational [Page 55] RFC 2764 IP Based Virtual Private Networks February 2000

  1. multiprotocol operation (3.1.4)
  1. frame sequencing (3.1.5)
  1. asymmetric / legacy user authentication (6.3)
  1. host address assignment and configuration (6.3)
 For L2TP:
  1. defining modes of operation of IPSec when used to support L2TP

(3.2)

 For VPNs generally:
  1. defining a VPN membership information configuration and

dissemination mechanism, that uses some form of directory or MIB

    (5.3.2)
  1. ensure that solutions developed, as far as possible, are

applicable to different types of VPNs, rather than being specific

    to a single type of VPN.

9.0 Security Considerations

 Security considerations are an integral part of any VPN mechanisms,
 and these are discussed in the sections describing those mechanisms.

10.0 Acknowledgements

 Thanks to Anthony Alles, of Nortel Networks, for his invaluable
 assistance with the generation of this document, and who developed
 much of the material on which early versions of this document were
 based.  Thanks also to Joel Halpern for his helpful review comments.

11.0 References

 [1]  ATM Forum. "LAN Emulation over ATM 1.0", af-lane-0021.000,
      January 1995.
 [2]  ATM Forum. "Multi-Protocol Over ATM Specification v1.0", af-
      mpoa-0087.000, June 1997.
 [3]  Ferguson, P. and Huston, G. "What is a VPN?", Revision 1, April
      1 1998; http://www.employees.org/~ferguson/vpn.pdf.

Gleeson, et al. Informational [Page 56] RFC 2764 IP Based Virtual Private Networks February 2000

 [4]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and E.
      Lear, "Address Allocation for Private Internets", BCP 5, RFC
      1918, February 1996.
 [5]  Kent, S. and R. Atkinson, "Security Architecture for the
      Internet Protocol", RFC 2401, November 1998.
 [6]  Perkins, C., "IP Encapsulation within IP", RFC 2003, October
      1996.
 [7]  Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing
      Encapsulation (GRE)", RFC 1701, October 1994.
 [8]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and
      B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC 2661,
      August 1999.
 [9]  Rosen, E., et al., "Multiprotocol Label Switching Architecture",
      Work in Progress.
 [10] Heinanen, J., et al., "MPLS Mappings of Generic VPN Mechanisms",
      Work in Progress.
 [11] Jamieson, D., et al., "MPLS VPN Architecture", Work in Progress.
 [12] Casey, L., et al., "IP VPN Realization using MPLS Tunnels", Work
      in Progress.
 [13] Li, T. "CPE based VPNs using MPLS", Work in Progress.
 [14] Muthukrishnan, K. and A. Malis, "Core MPLS IP VPN Architecture",
      Work in Progress.
 [15] Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547, March 1999.
 [16] Fox, B. and B. Gleeson, "Virtual Private Networks Identifier",
      RFC 2685, September 1999.
 [17] Petri, B. (editor) "MPOA v1.1 Addendum on VPN support", ATM
      Forum, af-mpoa-0129.000.
 [18] Harkins, D. and C. Carrel, "The Internet Key Exchange (IKE)",
      RFC 2409, November 1998.
 [19] Calhoun, P., et al., "Tunnel Establishment Protocol", Work in
      Progress.

Gleeson, et al. Informational [Page 57] RFC 2764 IP Based Virtual Private Networks February 2000

 [20] Andersson, L., et al., "LDP Specification", Work in Progress.
 [21] Jamoussi, B., et al., "Constraint-Based LSP Setup using LDP"
      Work in Progress.
 [22] Awduche, D., et al., "Extensions to RSVP for LSP Tunnels", Work
      in Progress.
 [23] Kent, S. and R. Atkinson, "IP Encapsulating Security Protocol
      (ESP)", RFC 2406, November 1998.
 [24] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
      51, RFC 1661, July 1994.
 [25] Perez, M., Liaw, F., Mankin, A., Hoffman, E., Grossman, D. and
      A. Malis, "ATM Signalling Support for IP over ATM", RFC 1755,
      February 1995.
 [26] Malkin, G.  "RIP Version 2  Carrying Additional Information",
      RFC 1723, November 1994.
 [27] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
 [28] Shacham, A., Monsour, R., Pereira, R. and M. Thomas, "IP Payload
      Compression Protocol (IPComp)", RFC 2393, December 1998.
 [29] Duffield N., et al., "A Performance Oriented Service Interface
      for Virtual Private Networks", Work in Progress.
 [30] Jacobson, V., Nichols, K. and B. Poduri, "An Expedited
      Forwarding PHB", RFC 2598, June 1999.
 [31] Casey, L., "An extended IP VPN Architecture", Work in Progress.
 [32] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4),"
      RFC 1771, March 1995.
 [33] Grossman, D. and J. Heinanen, "Multiprotocol Encapsulation over
      ATM Adaptation Layer 5", RFC 2684, September 1999.
 [34] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory Access
      Protocol (v3)", RFC 2251, December 1997.
 [35] Boyle, J., et al., "The COPS (Common Open Policy Service)
      Protocol", RFC 2748, January 2000.
 [36] MacRae, M. and S. Ayandeh, "Using COPS for VPN Connectivity"
      Work in Progress.

Gleeson, et al. Informational [Page 58] RFC 2764 IP Based Virtual Private Networks February 2000

 [37] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
      March 1997.
 [38] Heinanen, J. and E. Rosen, "VPN Support with MPLS", Work in
      Progress.
 [39] Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S.,
      Handley, M., Jacobson, V., Liu, C., Sharma, P. and L. Wei,
      "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol
      Specification", RFC 2362, June 1998.
 [40] Waitzman, D., Partridge, C., and S. Deering, "Distance Vector
      Multicast Routing Protocol", RFC 1075, November 1988.
 [41] Fenner, W., "IGMP-based Multicast Forwarding (IGMP Proxying)",
      Work in Progress.
 [42] Wallner, D., Harder, E. and R. Agee, "Key Management for
      Multicast: Issues and Architectures", RFC 2627, June 1999.
 [43] Hardjono, T., et al., "Secure IP Multicast: Problem areas,
      Framework, and Building Blocks", Work in Progress.
 [44] Rigney, C., Rubens, A., Simpson, W. and S. Willens, "Remote
      Authentication Dial In User Service (RADIUS)", RFC 2138, April
      1997.
 [45] Valencia, A., Littlewood, M. and T. Kolar, "Cisco Layer Two
      Forwarding (Protocol) "L2F"", RFC 2341, May 1998.
 [46] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W. and
      G. Zorn, "Point-to-Point Tunneling Protocol (PPTP)", RFC 2637,
      July 1999.
 [47] Patel, B., et al., "Securing L2TP using IPSEC", Work in
      Progress.
 [48] Srisuresh, P., "Secure Remote Access with L2TP", Work in
      Progress.
 [49] Calhoun, P., et al., "Layer Two Tunneling Protocol "L2TP"
      Security Extensions for Non-IP networks", Work in Progress.
 [50] Aboba, B. and Zorn, G. "Implementation of PPTP/L2TP Compulsory
      Tunneling via RADIUS", Work in progress.
 [51] Aboba, B. and G. Zorn, "Criteria for Evaluating Roaming
      Protocols", RFC 2477, January 1999.

Gleeson, et al. Informational [Page 59] RFC 2764 IP Based Virtual Private Networks February 2000

 [52] Shea, R., "L2TP-over-IP Path MTU Discovery (L2TPMTU)", Work in
      Progress.
 [53] Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T.
      Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990, August
      1996.
 [54] Richards, C. and K. Smith, "The PPP Bandwidth Allocation
      Protocol (BAP) The PPP Bandwidth Allocation Control Protocol
      (BACP)", RFC 2125, March 1997.
 [55] Calhoun, P. and K. Peirce, "Layer Two Tunneling Protocol "L2TP"
      IP Differential Services Extension", Work in Progress.
 [56] ADSL Forum. "An Interoperable End-to-end Broadband Service
      Architecture over ADSL Systems (Version 3.0)", ADSL Forum 97-
      215.
 [57] ADSL Forum. "Core Network Architectures for ADSL Access Systems
      (Version 1.01)", ADSL Forum 98-017.
 [58] Gupta, V., "Secure, Remote Access over the Internet using
      IPSec", Work in Progress.
 [59] Pereira, R., et al., "The ISAKMP Configuration Method", Work in
      Progress.
 [60] Pereira, R. and S. Beaulieu, "Extended Authentication Within
      ISAKMP/Oakley", Work in Progress.
 [61] Litvin, M., et al., "A Hybrid Authentication Mode for IKE", Work
      in Progress.
 [62] Kelly, S., et al., "User-level Authentication Mechanisms for
      IPsec", Work in Progress.
 [63] Patel, B., et al., "DHCP Configuration of IPSEC Tunnel Mode",
      Work in Progress.
 [64] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D. and R.
      Wheeler, "A Method for Transmitting PPP Over Ethernet (PPPoE)",
      RFC 2516, February 1999.
 [65] ANSI/IEEE - 10038: 1993 (ISO/IEC) Information technology -
      Telecommunications and information exchange between systems -
      Local area networks - Media access control (MAC) bridges,
      ANSI/IEEE Std 802.1D, 1993 Edition.

Gleeson, et al. Informational [Page 60] RFC 2764 IP Based Virtual Private Networks February 2000

12.0 Author Information

 Bryan Gleeson
 Nortel Networks
 4500 Great America Parkway
 Santa Clara CA 95054
 USA
 Phone: +1 (408) 548 3711
 EMail: bgleeson@shastanets.com
 Juha Heinanen
 Telia Finland, Inc.
 Myyrmaentie 2
 01600 VANTAA
 Finland
 Phone: +358 303 944 808
 EMail: jh@telia.fi
 Arthur Lin
 Nortel Networks
 4500 Great America Parkway
 Santa Clara CA 95054
 USA
 Phone: +1 (408) 548 3788
 EMail: alin@shastanets.com
 Grenville Armitage
 Bell Labs Research Silicon Valley
 Lucent Technologies
 3180 Porter Drive,
 Palo Alto, CA 94304
 USA
 EMail: gja@lucent.com
 Andrew G. Malis
 Lucent Technologies
 1 Robbins Road
 Westford, MA 01886
 USA
 Phone: +1 978 952 7414
 EMail: amalis@lucent.com

Gleeson, et al. Informational [Page 61] RFC 2764 IP Based Virtual Private Networks February 2000

13.0 Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Gleeson, et al. Informational [Page 62]

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