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
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.
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.
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 6.3
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].
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", RFC2401, November 1998.
[6] Perkins, C., "IP Encapsulation within IP", RFC2003, October
1996.
[7] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing
Encapsulation (GRE)", RFC1701, October 1994.
[8] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and
B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC2661,
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", RFC2547, March 1999.
[16] Fox, B. and B. Gleeson, "Virtual Private Networks Identifier",
RFC2685, 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)",
RFC2409, 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)", RFC2406, November 1998.
[24] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
51, RFC1661, July 1994.
[25] Perez, M., Liaw, F., Mankin, A., Hoffman, E., Grossman, D. and
A. Malis, "ATM Signalling Support for IP over ATM", RFC1755,
February 1995.
[26] Malkin, G. "RIP Version 2 Carrying Additional Information",
RFC1723, November 1994.
[27] Moy, J., "OSPF Version 2", STD 54, RFC2328, April 1998.
[28] Shacham, A., Monsour, R., Pereira, R. and M. Thomas, "IP Payload
Compression Protocol (IPComp)", RFC2393, 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", RFC2598, 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),"
RFC1771, March 1995.
[33] Grossman, D. and J. Heinanen, "Multiprotocol Encapsulation over
ATM Adaptation Layer 5", RFC2684, September 1999.
[34] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory Access
Protocol (v3)", RFC2251, December 1997.
[35] Boyle, J., et al., "The COPS (Common Open Policy Service)
Protocol", RFC2748, January 2000.
[36] MacRae, M. and S. Ayandeh, "Using COPS for VPN Connectivity"
Work in Progress.
[37] Droms, R., "Dynamic Host Configuration Protocol", RFC2131,
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", RFC2362, June 1998.
[40] Waitzman, D., Partridge, C., and S. Deering, "Distance Vector
Multicast Routing Protocol", RFC1075, 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", RFC2627, 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)", RFC2138, April
1997.
[45] Valencia, A., Littlewood, M. and T. Kolar, "Cisco Layer Two
Forwarding (Protocol) "L2F"", RFC2341, May 1998.
[46] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W. and
G. Zorn, "Point-to-Point Tunneling Protocol (PPTP)", RFC2637,
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", RFC2477, 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)", RFC1990, August
1996.
[54] Richards, C. and K. Smith, "The PPP Bandwidth Allocation
Protocol (BAP) The PPP Bandwidth Allocation Control Protocol
(BACP)", RFC2125, 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",