RFC 2764
A Framework for IP Based Virtual Private Networks
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.
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.
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 6.36.1.9 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].
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.
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.
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
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).
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
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.
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 - asymmetric / legacy user authentication (6.3) - 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.
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.
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
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
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
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: - 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)
- asymmetric / legacy user authentication (6.3)
- host address assignment and configuration (6.3)
For L2TP:
- defining modes of operation of IPSec when used to support L2TP
(3.2)
For VPNs generally:
- defining a VPN membership information configuration and
dissemination mechanism, that uses some form of directory or MIB
(5.3.2)
- 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.
[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.
[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.
[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.
[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.
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
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.