RFC 2764
A Framework for IP Based Virtual Private Networks
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.
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
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
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 ................................... 621.0 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
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.
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].
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.
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.
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)
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.
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
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.
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.
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
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.
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.
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
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. - the transport of a VPN-ID when establishing an SA (3.1.2) - a null encryption and null authentication option (3.1.3) - multiprotocol operation (3.1.4) - 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.
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