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 ................................... 62
1.0 Introduction
This document describes a framework for Virtual Private Networks
(VPNs) running across IP backbones. It discusses the various
different types of VPNs, their respective requirements, and proposes
specific mechanisms that could be used to implement each type of VPN
using existing or proposed specifications. The objective of this
document is to serve as a framework for related protocol development
in order to develop the full set of specifications required for
widespread deployment of interoperable VPN solutions.
There is currently significant interest in the deployment of virtual
private networks across IP backbone facilities. The widespread
deployment of VPNs has been hampered, however, by the lack of
interoperable implementations, which, in turn, derives from the lack
of general agreement on the definition and scope of VPNs and
confusion over the wide variety of solutions that are all described
by the term VPN. In the context of this document, a VPN is simply
defined as the 'emulation of a private Wide Area Network (WAN)
facility using IP facilities' (including the public Internet, or
private IP backbones). As such, there are as many types of VPNs as
there are types of WANs, hence the confusion over what exactly
constitutes a VPN.
In this document a VPN is modeled as a connectivity object. Hosts
may be attached to a VPN, and VPNs may be interconnected together, in
the same manner as hosts today attach to physical networks, and
physical networks are interconnected together (e.g., via bridges or
routers). Many aspects of networking, such as addressing, forwarding
mechanism, learning and advertising reachability, quality of service
(QoS), security, and firewalling, have common solutions across both
physical and virtual networks, and many issues that arise in the
discussion of VPNs have direct analogues with those issues as
implemented in physical networks. The introduction of VPNs does not
create the need to reinvent networking, or to introduce entirely new
paradigms that have no direct analogue with existing physical
networks. Instead it is often useful to first examine how a
particular issue is handled in a physical network environment, and
then apply the same principle to an environment which contains
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.
4.0 VPN Types: Virtual Leased Lines
The simplest form of a VPN is a 'Virtual Leased Line' (VLL) service.
In this case a point-to-point link is provided to a customer,
connecting two CPE devices, as illustrated below. The link layer
type used to connect the CPE devices to the ISP nodes can be any link
layer type, for example an ATM VCC or a Frame Relay circuit. The CPE
devices can be either routers bridges or hosts.
The two ISP nodes are both connected to an IP network, and an IP
tunnel is set up between them. Each ISP node is configured to bind
the stub link and the IP tunnel together at layer 2 (e.g., an ATM VCC
and the IP tunnel). Frames are relayed between the two links. For
example the ATM Adaptation Layer 5 (AAL5) payload is taken and
encapsulated in an IPSec tunnel, and vice versa. The contents of the
AAL5 payload are opaque to the ISP node, and are not examined there.
+--------+ ----------- +--------+
+---+ | ISP | ( IP ) | ISP | +---+
|CPE|-------| edge |-----( backbone ) -----| edge |------|CPE|
+---+ ATM | node | ( ) | node | ATM +---+
VCC +--------+ ----------- +--------+ VCC
<--------- IP Tunnel -------->
10.1.1.5 subnet = 10.1.1.4/30 10.1.1.6
Addressing used by customer (transparent to provider)
Figure 4.1: VLL Example
To a customer it looks the same as if a single ATM VCC or Frame Relay
circuit were used to interconnect the CPE devices, and the customer
could be unaware that part of the circuit was in fact implemented
over an IP backbone. This may be useful, for example, if a provider
wishes to provide a LAN interconnect service using ATM as the network
interface, but does not have an ATM network that directly
interconnects all possible customer sites.
It is not necessary that the two links used to connect the CPE
devices to the ISP nodes be of the same media type, but in this case
the ISP nodes cannot treat the traffic in an opaque manner, as
described above. Instead the ISP nodes must perform the functions of
an interworking device between the two media types (e.g., ATM and
Frame Relay), and perform functions such as LLC/SNAP to NLPID
conversion, mapping between ARP protocol variants and performing any