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RFC 4110
Network Working Group R. Callon
Request for Comments: 4110 Juniper Networks
Category: Informational M. Suzuki
NTT Corporation
July 2005
A Framework for Layer 3
Provider-Provisioned Virtual Private Networks (PPVPNs)
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 (2005).
Abstract
This document provides a framework for Layer 3 Provider-Provisioned
Virtual Private Networks (PPVPNs). This framework is intended to aid
in the standardization of protocols and mechanisms for support of
layer 3 PPVPNs. It is the intent of this document to produce a
coherent description of the significant technical issues that are
important in the design of layer 3 PPVPN solutions. Selection of
specific approaches, making choices regarding engineering tradeoffs,
and detailed protocol specification, are outside of the scope of this
framework document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Objectives of the Document . . . . . . . . . . . . . . . 3
1.2. Overview of Virtual Private Networks . . . . . . . . . . 4
1.3. Types of VPNs. . . . . . . . . . . . . . . . . . . . . . 7
1.3.1. CE- vs PE-based VPNs . . . . . . . . . . . . . . 8
1.3.2. Types of PE-based VPNs . . . . . . . . . . . . . 9
1.3.3. Layer 3 PE-based VPNs. . . . . . . . . . . . . . 10
1.4. Scope of the Document. . . . . . . . . . . . . . . . . . 10
1.5. Terminology. . . . . . . . . . . . . . . . . . . . . . . 11
1.6. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 13
2. Reference Models . . . . . . . . . . . . . . . . . . . . . . . 14
2.1. Reference Model for Layer 3 PE-based VPN . . . . . . . . 14
2.1.1. Entities in the Reference Model. . . . . . . . . 16
2.1.2. Relationship Between CE and PE . . . . . . . . . 18
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2.1.3. Interworking Model . . . . . . . . . . . . . . . 19
2.2. Reference Model for Layer 3 Provider-Provisioned
CE-based VPN . . . . . . . . . . . . . . . . . . . . . . 21
2.2.1. Entities in the Reference Model. . . . . . . . . 22
3. Customer Interface . . . . . . . . . . . . . . . . . . . . . . 23
3.1. VPN Establishment at the Customer Interface. . . . . . . 23
3.1.1. Layer 3 PE-based VPN . . . . . . . . . . . . . . 23
3.1.1.1. Static Binding . . . . . . . . . . . . 24
3.1.1.2. Dynamic Binding. . . . . . . . . . . . 24
3.1.2. Layer 3 Provider-Provisioned CE-based VPN. . . . 25
3.2. Data Exchange at the Customer Interface. . . . . . . . . 25
3.2.1. Layer 3 PE-based VPN . . . . . . . . . . . . . . 25
3.2.2. Layer 3 Provider-Provisioned CE-based VPN. . . . 26
3.3. Customer Visible Routing . . . . . . . . . . . . . . . . 26
3.3.1. Customer View of Routing for Layer 3 PE-based
VPNs . . . . . . . . . . . . . . . . . . . . . . 26
3.3.1.1. Routing for Intranets . . . . . . . . 27
3.3.1.2. Routing for Extranets . . . . . . . . 28
3.3.1.3. CE and PE Devices for Layer 3
PE-based VPNs. . . . . . . . . . . . . 29
3.3.2. Customer View of Routing for Layer 3 Provider-
Provisioned CE-based VPNs. . . . . . . . . . . . 29
3.3.3. Options for Customer Visible Routing . . . . . . 30
4. Network Interface and SP Support of VPNs . . . . . . . . . . . 32
4.1. Functional Components of a VPN . . . . . . . . . . . . . 32
4.2. VPN Establishment and Maintenance. . . . . . . . . . . . 34
4.2.1. VPN Discovery . . . . . . . . . . . . . . . . . 35
4.2.1.1. Network Management for Membership
Information. . . . . . . . . . . . . . 35
4.2.1.2. Directory Servers. . . . . . . . . . . 36
4.2.1.3. Augmented Routing for Membership
Information. . . . . . . . . . . . . . 36
4.2.1.4. VPN Discovery for Inter-SP VPNs. . . . 37
4.2.2. Constraining Distribution of VPN Routing
Information . . . . . . . . . . . . . . . . . . 38
4.2.3. Controlling VPN Topology . . . . . . . . . . . . 38
4.3. VPN Tunneling . . . . . . . . . . . . . . . . . . . . . 40
4.3.1. Tunnel Encapsulations. . . . . . . . . . . . . . 40
4.3.2. Tunnel Multiplexing. . . . . . . . . . . . . . . 41
4.3.3. Tunnel Establishment . . . . . . . . . . . . . . 42
4.3.4. Scaling and Hierarchical Tunnels . . . . . . . . 43
4.3.5. Tunnel Maintenance . . . . . . . . . . . . . . . 45
4.3.6. Survey of Tunneling Techniques . . . . . . . . . 46
4.3.6.1. GRE . . . . . . . . . . . . . . . . . 46
4.3.6.2. IP-in-IP Encapsulation . . . . . . . . 47
4.3.6.3. IPsec. . . . . . . . . . . . . . . . . 48
4.3.6.4. MPLS . . . . . . . . . . . . . . . . . 49
4.4. PE-PE Distribution of VPN Routing Information. . . . . . 51
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4.4.1. Options for VPN Routing in the SP. . . . . . . . 52
4.4.2. VPN Forwarding Instances . . . . . . . . . . . . 52
4.4.3. Per-VPN Routing . . . . . . . . . . . . . . . . 53
4.4.4. Aggregated Routing Model . . . . . . . . . . . . 54
4.4.4.1. Aggregated Routing with OSPF or IS-IS. 55
4.4.4.2. Aggregated Routing with BGP. . . . . . 56
4.4.5. Scalability and Stability of Routing with Layer
3 PE-based VPNs. . . . . . . . . . . . . . . . . 59
4.5. Quality of Service, SLAs, and IP Differentiated Services 61
4.5.1. IntServ/RSVP . . . . . . . . . . . . . . . . . . 61
4.5.2. DiffServ . . . . . . . . . . . . . . . . . . . . 62
4.6. Concurrent Access to VPNs and the Internet . . . . . . . 62
4.7. Network and Customer Management of VPNs. . . . . . . . . 63
4.7.1. Network and Customer Management. . . . . . . . . 63
4.7.2. Segregated Access of VPN Information . . . . . . 64
5. Interworking Interface . . . . . . . . . . . . . . . . . . . . 66
5.1. Interworking Function. . . . . . . . . . . . . . . . . . 66
5.2. Interworking Interface . . . . . . . . . . . . . . . . . 66
5.2.1. Tunnels at the Interworking Interface. . . . . . 67
5.3. Support of Additional Services . . . . . . . . . . . . . 68
5.4. Scalability Discussion . . . . . . . . . . . . . . . . . 69
6. Security Considerations. . . . . . . . . . . . . . . . . . . . 69
6.1. System Security. . . . . . . . . . . . . . . . . . . . . 70
6.2. Access Control . . . . . . . . . . . . . . . . . . . . . 70
6.3. Endpoint Authentication . . . . . . . . . . . . . . . . 70
6.4. Data Integrity . . . . . . . . . . . . . . . . . . . . . 71
6.5. Confidentiality. . . . . . . . . . . . . . . . . . . . . 71
6.6. User Data and Control Data . . . . . . . . . . . . . . . 72
6.7. Security Considerations for Inter-SP VPNs . . . . . . . 72
Appendix A: Optimizations for Tunnel Forwarding. . . . . . . . . . 73
A.1. Header Lookups in the VFIs . . . . . . . . . . . . . . . 73
A.2. Penultimate Hop Popping for MPLS . . . . . . . . . . . . 73
A.3. Demultiplexing to Eliminate the Tunnel Egress VFI Lookup 74
Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . 75
Normative References . . . . . . . . . . . . . . . . . . . . . . . 76
Informative References . . . . . . . . . . . . . . . . . . . . . . 76
Contributors' Addresses. . . . . . . . . . . . . . . . . . . . . . 80
1. Introduction
1.1. Objectives of the Document
This document provides a framework for Layer 3 Provider-Provisioned
Virtual Private Networks (PPVPNs). This framework is intended to aid
in standardizing protocols and mechanisms to support interoperable
layer 3 PPVPNs.
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The term "provider-provisioned VPNs" refers to Virtual Private
Networks (VPNs) for which the Service Provider (SP) participates in
management and provisioning of the VPN, as defined in section 1.3.
There are multiple ways in which a provider can participate in
managing and provisioning a VPN; therefore, there are multiple
different types of PPVPNs. The framework document discusses layer 3
VPNs (as defined in section 1.3).
First, this document provides a reference model for layer 3 PPVPNs.
Then technical aspects of layer 3 PPVPN operation are discussed,
first from the customer's point of view, then from the providers
point of view. Specifically, this includes discussion of the
technical issues which are important in the design of standards and
mechanisms for the operation and support of layer 3 PPVPNs.
Furthermore, technical aspects of layer 3 PPVPN interworking are
clarified. Finally, security issues as they apply to layer 3 PPVPNs
are addressed.
This document takes a "horizontal description" approach. For each
technical issue, it describes multiple approaches. To specify a
particular PPVPN strategy, one must choose a particular way of
solving each problem, but this document does not make choices, and
does not select any particular approach to support VPNs.
The "vertical description" approach is taken in other documents,
viz., in the documents that describe particular PPVPN solutions.
Note that any specific solution will need to make choices based on SP
requirements, customer needs, implementation cost, and engineering
tradeoffs. Solutions will need to chose between flexibility
(supporting multiple options) and conciseness (selection of specific
options in order to simplify implementation and deployment). While a
framework document can discuss issues and criteria which are used as
input to these choices, the specific selection of a solution is
outside of the scope of a framework document.
1.2. Overview of Virtual Private Networks
The term "Virtual Private Network" (VPN) refers to a set of
communicating sites, where (a) communication between sites outside
the set and sites inside the set is restricted, but (b) communication
between sites in the VPN takes place over a network infrastructure
that is also used by sites which are not in the VPN. The fact that
the network infrastructure is shared by multiple VPNs (and possibly
also by non-VPN traffic) is what distinguishes a VPN from a private
network. We will refer to this shared network infrastructure as the
"VPN Backbone".
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The logical structure of the VPN, such as addressing, topology,
connectivity, reachability, and access control, is equivalent to part
of or all of a conventional private network using private facilities
[RFC2764] [VPN-2547BIS].
In this document, we are concerned only with the case where the
shared network infrastructure (VPN backbone) is an IP and/or MPLS
network. Further, we are concerned only with the case where the
Service Provider's edge devices, whether at the provider edge (PE) or
at the Customer Edge (CE), determine how to route VPN traffic by
looking at the IP and/or MPLS headers of the packets they receive
from the customer's edge devices; this is the distinguishing feature
of Layer 3 VPNs.
In some cases, one SP may offer VPN services to another SP. The
former SP is known as a carrier of carriers, and the service it
offers is known as "carrier of carriers" service. In this document,
in cases where the customer could be either an enterprise or SP
network, we will make use of the term "customer" to refer to the user
of the VPN services. Similarly we will use the term "customer
network" to refer to the user's network.
VPNs may be intranets, in which the multiple sites are under the
control of a single customer administration, such as multiple sites
of a single company. Alternatively, VPNs may be extranets, in which
the multiple sites are controlled by administrations of different
customers, such as sites corresponding to a company, its suppliers,
and its customers.
Figure 1.1. illustrates an example network, which will be used in
the discussions below. PE1 and PE2 are Provider Edge devices within
an SP network. CE1, CE2, and CE3 are Customer Edge devices within a
customer network. Routers r3, r4, r5, and r6 are IP routers internal
to the customer sites.
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............ ................. ............
. . . . . .
. +---+ +-------+ +-------+ +---+ .
. r3---| | | | | |----|CE2|---r5 .
. | | | | | | +---+ .
. |CE1|----| PE1 | | PE2 | : .
. | | | | | | +---+ .
. r4---| | | | | |----|CE3|---r6 .
. +---+ +-------+ +-------+ +---+ .
. Customer . . Service . . Customer .
. site 1 . . provider(s) . . site 2 .
............ ................. ............
Figure 1.1.: VPN interconnecting two sites.
In many cases, Provider Edge (PE) and Customer Edge (CE) devices may
be either routers or LSRs.
In this document, the Service Providers' network is an IP or MPLS
network. It is desired to interconnect the customer network sites
via the Service Providers' network. Some VPN solutions require that
the VPN service be provided either over a single SP network, or over
a small set of closely cooperating SP networks. Other VPN solutions
are intended to allow VPN service to be provided over an arbitrary
set of minimally cooperating SP networks (i.e., over the public
Internet).
In many cases, customer networks will make use of private IP
addresses [RFC1918] or other non-unique IP address (i.e.,
unregistered addresses); there is no guarantee that the IP addresses
used in the customer network are globally unique. The addresses used
in one customer's network may overlap the addresses used in others.
However, a single PE device can be used to provide VPN service to
multiple customer networks, even if those customer networks have
overlapping addresses. In PE-based layer 3 VPNs, the PE devices may
route the VPN traffic based on the customer addresses found in the IP
headers; this implies that the PE devices need to maintain a level of
isolation between the packets from different customer networks. In
CE-based layer 3 VPNs, the PEs do not make routing decisions based on
the customer's private addresses, so this issue does not arise. For
either PE or CE-based VPNs, the fact that the VPNs do not necessarily
use globally unique address spaces also implies that IP packets from
a customer network cannot be transmitted over the SP network in their
native form. Instead, some form of encapsulation/tunneling must be
used.
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Tunneling is also important for other reasons, such as providing
isolation between different customer networks, allowing a wide range
of protocols to be carried over an SP network, etc. Different QoS
and security characteristics may be associated with different
tunnels.
1.3. Types of VPNs
This section describes multiple types of VPNs, and some of the
engineering tradeoffs between different types. It is not up to this
document to decide between different types of VPNs. Different types
of VPNs may be appropriate in different situations.
There is a wide spectrum of types of possible VPNs, and it is
difficult to split the types of VPNs into clearly distinguished
categories.
As an example, consider a company making use of a private network,
with several sites interconnected via leased lines. All routing is
done via routers which are internal to the private network.
At some point, the administrator of the private network might decide
to replace the leased lines by ATM links (using an ATM service from
an SP). Here again all IP-level routing is done between customer
premises routers, and managed by the private network administrator.
In order to reduce the network management burden on the private
network, the company may decide to make use of a provider-provisioned
CE devices [VPN-CE]. Here the operation of the network might be
unchanged, except that the CE devices would be provided by and
managed by an SP.
The SP might decide that it is too difficult to manually configure
each CE-CE link. This might lead the SP to replace the ATM links
with a layer 2 VPN service between CE devices [VPN-L2]. Auto-
discovery might be used to simplify configuration of links between CE
devices, and an MPLS service might be used between CE devices instead
of an ATM service (for example, to take advantage of the provider's
high speed IP or MPLS backbone).
After a while the SP might decide that it is too much trouble to be
managing a large number of devices at the customers' premises, and
might instead physically move these routers to be on the provider
premises. Each edge router at the provider premises might
nonetheless be dedicated to a single VPN. The operation might remain
unchanged (except that links from the edge routers to other routers
in the private network become MAN links instead of LAN links, and the
link from the edge routers to provider core routers become LAN links
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instead of MAN links). The routers in question can now be considered
to be provider edge routers, and the service provided by the SP has
now become essentially a layer 3 VPN service.
In order to minimize the cost of equipment, the provider might decide
to replace several dedicated PE devices with a single physical router
with the capability of running virtual routers (VR) [VPN-VR].
Protocol operation may remain unchanged. In this case the provider
is offering a layer 3 VPN service making use of a VR capability.
Note that autodiscovery might be used in a manner which is very
similar to how it had been done in the layer 2 VPN case described
above (for example, BGP might be used between VRs for discovery of
other VRs supporting the same VPN).
Finally, in order to simplify operation of routing protocols for the
private network over the SP network, the provider might decide to
aggregate multiple instances of routing into a single instance of BGP
[VPN-2547BIS].
In practice it is highly unlikely that any one network would actually
evolve through all of these approaches at different points in time.
However, this example illustrates that there is a continuum of
possible approaches, and each approach is relatively similar to at
least some of the other possible approaches for supporting VPN
services. Some techniques (such as auto-discovery of VPN sites) may
be common between multiple approaches.
1.3.1. CE- vs PE-based VPNs
The term "CE-based VPN" (or Customer Edge-based Virtual Private
Network) refers to an approach in which the PE devices do not know
anything about the routing or the addressing of the customer
networks. The PE devices offer a simple IP service, and expect to
receive IP packets whose headers contain only globally unique IP
addresses. What makes a CE-based VPN into a Provider-Provisioned VPN
is that the SP takes on the task of managing and provisioning the CE
devices [VPN-CE].
In CE-based VPNs, the backbone of the customer network is a set of
tunnels whose endpoints are the CE devices. Various kinds of tunnels
may be used (e.g., GRE, IP-in-IP, IPsec, L2TP, MPLS), the only
overall requirement being that sending a packet through the tunnel
requires encapsulating it with a new IP header whose addresses are
globally unique.
For customer provisioned CE-based VPNs, provisioning and management
of the tunnels is the responsibility of the customer network
administration. Typically, this makes use of manual configuration of
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the tunnels. In this case the customer is also responsible for
operation of the routing protocol between CE devices. (Note that
discussion of customer provisioned CE-based VPNs is out of scope of
the document).
For provider-provisioned CE-based VPNs, provisioning and management
of the tunnels is the responsibility of the SP. In this case the
provider may also configure routing protocols on the CE devices.
This implies that routing in the private network is partially under
the control of the customer, and partially under the control of the
SP.
For CE-based VPNs (whether customer or provider-provisioned) routing
in the customer network treats the tunnels as layer 2 links.
In a PE-based VPN (or Provider Edge-based Virtual Private Network),
customer packets are carried through the SP networks in tunnels, just
as they are in CE-based VPNs. However, in a PE-based VPN, the tunnel
endpoints are the PE devices, and the PE devices must know how to
route the customer packets, based on the IP addresses that they
carry. In this case, the CE devices themselves do not have to have
any special VPN capabilities, and do not even have to know that they
are part of a VPN.
In this document we will use the generic term "VPN Edge Device" to
refer to the device, attached to both the customer network and the
VPN backbone, that performs the VPN-specific functions. In the case
of CE-based VPNs, the VPN Edge Device is a CE device. In the case of
PE-based VPNs, the VPN Edge Device is a PE device.
1.3.2. Types of PE-based VPNs
Different types of PE-based VPNs may be distinguished by the service
offered.
o Layer 3 service
When a PE receives a packet from a CE, it determines how to forward
the packet by considering both the packet's incoming link, and the
layer 3 information in the packet's header.
o Layer 2 service
When a PE receives a frame from a CE, it determines how to forward
the packet by considering both the packet's incoming link, and the
layer 2 information in the frame header (such as FR, ATM, or MAC
header). (Note that discussion of layer 2 service is out of scope
of the document).
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1.3.3. Layer 3 PE-based VPNs
A layer 3 PE-based VPN is one in which the SP takes part in IP level
forwarding based on the customer network's IP address space. In
general, the customer network is likely to make use of private and/or
non-unique IP addresses. This implies that at least some devices in
the provider network needs to understand the IP address space as used
in the customer network. Typically this knowledge is limited to the
PE devices which are directly attached to the customer.
In a layer 3 PE-based VPN, the provider will need to participate in
some aspects of management and provisioning of the VPNs, such as
ensuring that the PE devices are configured to support the correct
VPNs. This implies that layer 3 PE-based VPNs are by definition
provider-provisioned VPNs.
Layer 3 PE-based VPNs have the advantage that they offload some
aspects of VPN management from the customer network. From the
perspective of the customer network, it looks as if there is just a
normal network; specific VPN functionality is hidden from the
customer network. Scaling of the customer network's routing might
also be improved, since some layer 3 PE-based VPN approaches avoid
the need for the customer's routing algorithm to see "N squared"
(actually N*(N-1)/2) point to point duplex links between N customer
sites.
However, these advantages come along with other consequences.
Specifically, the PE devices must have some knowledge of the routing,
addressing, and layer 3 protocols of the customer networks to which
they attach. One consequence is that the set of layer 3 protocols
which can be supported by the VPN is limited to those supported by
the PE (which in practice means, limited to IP). Another consequence
is that the PE devices have more to do, and the SP has more
per-customer management to do.
An SP may offer a range of layer 3 PE-based VPN services. At one end
of the range is a service limited to simply providing connectivity
(optionally including QoS support) between specific customer network
sites. This is referred to as "Network Connectivity Service". There
is a spectrum of other possible services, such as firewalls, user or
site of origin authentication, and address assignment (e.g., using
Radius or DHCP).
1.4. Scope of the Document
This framework document will discuss methods for providing layer 3
PE-based VPNs and layer 3 provider-provisioned CE-based VPNs. This
may include mechanisms which will can be used to constrain
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connectivity between sites, including the use and placement of
firewalls, based on administrative requirements [PPVPN-REQ]
[L3VPN-REQ]. Similarly the use and placement of NAT functionality is
discussed. However, this framework document will not discuss methods
for additional services such as firewall administration and address
assignment. A discussion of specific firewall mechanisms and
policies, and detailed discussion of NAT functionality, are outside
of the scope of this document.
This document does not discuss those forms of VPNs that are outside
of the scope of the IETF Provider-Provisioned VPN working group.
Specifically, this document excludes discussion of PPVPNs using VPN
native (non-IP, non-MPLS) protocols as the base technology used to
provide the VPN service (e.g., native ATM service provided using ATM
switches with ATM signaling). However, this does not mean to exclude
multiprotocol access to the PPVPN by customers.
1.5. Terminology
Backdoor Links: Links between CE devices that are provided by the end
customer rather than the SP; may be used to interconnect CE devices
in multiple-homing arrangements.
CE-based VPN: An approach in which all the VPN-specific procedures
are performed in the CE devices, and the PE devices are not aware in
any way that some of the traffic they are processing is VPN traffic.
Customer: A single organization, corporation, or enterprise that
administratively controls a set of sites belonging to a VPN.
Customer Edge (CE) Device: The equipment on the customer side of the
SP-customer boundary (the customer interface).
IP Router: A device which forwards IP packets, and runs associated IP
routing protocols (such as OSPF, IS-IS, RIP, BGP, or similar
protocols). An IP router might optionally also be an LSR. The term
"IP router" is often abbreviated as "router".
Label Switching Router: A device which forwards MPLS packets and runs
associated IP routing and signaling protocols (such as LDP, RSVP-TE,
CR-LDP, OSPF, IS-IS, or similar protocols). A label switching router
is also an IP router.
PE-Based VPNs: The PE devices know that certain traffic is VPN
traffic. They forward the traffic (through tunnels) based on the
destination IP address of the packet, and optionally on based on
other information in the IP header of the packet. The PE devices are
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themselves the tunnel endpoints. The tunnels may make use of various
encapsulations to send traffic over the SP network (such as, but not
restricted to, GRE, IP-in-IP, IPsec, or MPLS tunnels).
Private Network: A network which allows communication between a
restricted set of sites, over an IP backbone that is used only to
carry traffic to and from those sites.
Provider Edge (PE) Device: The equipment on the SP side of the
SP-customer boundary (the customer interface).
Provider-Provisioned VPNs (PPVPNs): VPNs, whether CE-based or
PE-based, that are actively managed by the SP rather than by the end
customer.
Route Reflectors: An SP-owned network element that is used to
distribute BGP routes to the SP's BGP-enabled routers.
Virtual Private Network (VPN): Restricted communication between a set
of sites, making use of an IP backbone which is shared by traffic
that is not going to or coming from those sites.
Virtual Router (VR): An instance of one of a number of logical
routers located within a single physical router. Each logical router
emulates a physical router using existing mechanisms and tools for
configuration, operation, accounting, and maintenance.
VPN Forwarding Instance (VFI): A logical entity that resides in a PE
that includes the router information base and forwarding information
base for a VPN.
VPN Backbone: IP and/or MPLS network which is used to carry VPN
traffic between the customer sites of a particular VPN.
VPN Edge Device: Device, attached to both the VPN backbone and the
customer network, which performs VPN-specific functions. For
PE-based VPNs, this is the PE device; for CE-based VPNs, this is the
CE device.
VPN Routing: Routing that is specific to a particular VPN.
VPN Tunnel: A logical link between two PE or two CE entities, used to
carry VPN traffic, and implemented by encapsulating packets that are
transmitted between those two entities.
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RFC 4110 A Framework for L3 PPVPNs July 2005
1.6. Acronyms
ATM Asynchronous Transfer Mode
BGP Border Gateway Protocol
CE Customer Edge
CLI Command Line Interface
CR-LDP Constraint-based Routing Label Distribution Protocol
EBGP External Border Gateway Protocol
FR Frame Relay
GRE Generic Routing Encapsulation
IBGP Internal Border Gateway Protocol
IKE Internet Key Exchange
IGP Interior Gateway Protocol
(e.g., RIP, IS-IS and OSPF are all IGPs)
IP Internet Protocol (same as IPv4)
IPsec Internet Protocol Security protocol
IPv4 Internet Protocol version 4 (same as IP)
IPv6 Internet Protocol version 6
IS-IS Intermediate System to Intermediate System routing
protocol
L2TP Layer 2 Tunneling Protocol
LAN Local Area Network
LDAP Lightweight Directory Access Protocol
LDP Label Distribution Protocol
LSP Label Switched Path
LSR Label Switching Router
MIB Management Information Base
MPLS Multi Protocol Label Switching
NBMA Non-Broadcast Multi-Access
NMS Network Management System
OSPF Open Shortest Path First routing protocol
P Provider equipment
PE Provider Edge
PPVPN Provider-Provisioned VPN
QoS Quality of Service
RFC Request For Comments
RIP Routing Information Protocol
RSVP Resource Reservation Protocol
RSVP-TE Resource Reservation Protocol with Traffic
Engineering Extensions
SNMP Simple Network Management Protocol
SP Service Provider
VFI VPN Forwarding Instance
VPN Virtual Private Network
VR Virtual Router
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2. Reference Models
This section describes PPVPN reference models. The purpose of
discussing reference models is to clarify the common components and
pieces that are needed to build and deploy a PPVPN. Two types of
VPNs, layer 3 PE-based VPN and layer 3 provider-provisioned CE-based
VPN are covered in separated sections below.
2.1. Reference Model for Layer 3 PE-based VPN
This subsection describes functional components and their
relationship for implementing layer 3 PE-based VPN.
Figure 2.1 shows the reference model for layer 3 PE-based VPNs and
Figures 2.2 and 2.3 show relationship between entities in the
reference model.
As shown in Figure 2.1, the customer interface is defined as the
interface which exists between CE and PE devices, and the network
interface is defined as the interface which exists between a pair of
PE devices.
Figure 2.2 illustrates a single logical tunnel between each pair of
VFIs supporting the same VPN. Other options are possible. For
example, a single tunnel might occur between two PEs, with multiple
per-VFI tunnels multiplexed over the PE to PE tunnel. Similarly,
there may be multiple tunnels between two VFIs, for example to
optimize forwarding within the VFI. Other possibilities will be
discussed later in this framework document.
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+---------+ +------------------------------------+ +---------+
| | | | | |
| | | +------+ +------+ : +------+
+------+ : | | | | | | : | CE |
| CE | : | | | P | | PE | : |device|
|device| : +------+ VPN tunnel : |router| |device| : | of |
| of |-:--| |================:===============| |--:-|VPN A|
|VPN A| : | | : +------+ +------+ : +------+
+------+ : | PE | : | | : |
+------+ : |device| Network interface | | : |
| CE | : | | : +------+ : +------+
|device|-:--| |================:===============| |--:-| CE |
| of | : +------+ : VPN tunnel | PE | : |device|
|VPN B| : | | |device| : | of |
+------+ : | | +------------+ +------------+ | | : |VPN B|
| : | | | Customer | | Network | +------+ : +------+
|Customer | | | management | | management | | | : |
|interface| | | function | | function | | |Customer |
| | | +------------+ +------------+ | |interface|
| | | | | |
+---------+ +------------------------------------+ +---------+
| Access | |<---------- SP network(s) --------->| | Access |
| network | | single or multiple SP domains | | network |
Figure 2.1: Reference model for layer 3 PE-based VPN.
+----------+ +----------+
+-----+ |PE device | |PE device | +-----+
| CE | | | | | | CE |
| dev | Access | +------+ | | +------+ | Access | dev |
| of | conn. | |VFI of| | VPN tunnel | |VFI of| | conn. | of |
|VPN A|----------|VPN A |======================|VPN A |----------|VPN A|
+-----+ | +------+ | | +------+ | +-----+
| | | |
+-----+ Access | +------+ | | +------+ | Access +-----+
| CE | conn. | |VFI of| | VPN tunnel | |VFI of| | conn. | CE |
| dev |----------|VPN B |======================|VPN B |----------| dev |
| of | | +------+ | | +------+ | | of |
|VPN B| | | | | |VPN B|
+-----+ +----------+ +----------+ +-----+
Figure 2.2: Relationship between entities in reference model (1).
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+----------+ +----------+
+-----+ |PE device | |PE device | +-----+
| CE | | | | | | CE |
| dev | Access | +------+ | | +------+ | Access | dev |
| of | conn. | |VFI of| | | |VFI of| | conn. | of |
|VPN A|----------|VPN A | | | |VPN A |----------|VPN A|
+-----+ | +------+\| Tunnel |/+------+ | +-----+
| >==================< |
+-----+ Access | +------+/| |\+------+ | Access +-----+
| CE | conn. | |VFI of| | | |VFI of| | conn. | CE |
| dev |----------|VPN B | | | |VPN B |----------| dev |
| of | | +------+ | | +------+ | | of |
|VPN B| | | | | |VPN B|
+-----+ +----------+ +----------+ +-----+
Figure 2.3: Relationship between entities in reference model (2).
2.1.1. Entities in the Reference Model
The entities in the reference model are described below.
o Customer edge (CE) device
In the context of layer 3 provider-provisioned PE-based VPNs, a CE
device may be a router, LSR, or host that has no VPN-specific
functionality. It is attached via an access connection to a PE
device.
o P router
A router within a provider network which is used to interconnect PE
devices, but which does not have any VPN state and does not have
any direct attachment to CE devices.
o Provider edge (PE) device
In the context of layer 3 provider-provisioned PE-based VPNs, a PE
device implements one or more VFIs and maintains per-VPN state for
the support of one or more VPNs. It may be a router, LSR, or other
device that includes VFIs and provider edge VPN functionality such
as provisioning, management, and traffic classification and
separation. (Note that access connections are terminated by VFIs
from the functional point of view). A PE device is attached via an
access connection to one or more CE devices.
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o Customer site
A customer site is a set of users that have mutual IP reachability
without use of a VPN backbone that goes beyond the site.
o SP networks
An SP network is an IP or MPLS network administered by a single
service provider.
o Access connection
An access connection represents an isolated layer 2 connectivity
between a CE device and a PE device. Access connections can be,
e.g., dedicated physical circuits, logical circuits (such as FR,
ATM, and MAC), or IP tunnels (e.g., using IPsec, L2TP, or MPLS).
o Access network
An access network provides access connections between CE and PE
devices. It may be a TDM network, layer 2 network (e.g., FR, ATM,
and Ethernet), or IP network over which access is tunneled (e.g.,
using L2TP [RFC2661] or MPLS).
o VPN tunnel
A VPN tunnel is a logical link between two VPN edge devices. A VPN
packet is carried on a tunnel by encapsulating it before
transmitting it over the VPN backbone.
Multiple VPN tunnels at one level may be hierarchically multiplexed
into a single tunnel at another level. For example, multiple per-
VPN tunnels may be multiplexed into a single PE to PE tunnel (e.g.,
GRE, IP-in-IP, IPsec, or MPLS tunnel). This is illustrated in
Figure 2.3. See section 4.3 for details.
o VPN forwarding instance (VFI)
A single PE device is likely to be connected to a number of CE
devices. The CE devices are unlikely to all be in the same VPN.
The PE device must therefore maintain a separate forwarding
instances for each VPN to which it is connected. A VFI is a
logical entity, residing in a PE, that contains the router
information base and forwarding information base for a VPN. The
interaction between routing and VFIs is discussed in section 4.4.2.
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o Customer management function
The customer management function supports the provisioning of
customer specific attributes, such as customer ID, personal
information (e.g., name, address, phone number, credit card number,
and etc.), subscription services and parameters, access control
policy information, billing and statistical information, and etc.
The customer management function may use a combination of SNMP
manager, directory service (e.g., LDAP [RFC3377]), or proprietary
network management system.
o Network management function
The network management function supports the provisioning and
monitoring of PE or CE device attributes and their relationships.
The network management function may use a combination of SNMP
manager, directory service (e.g., LDAP [RFC3377]), or proprietary
network management system.
2.1.2. Relationship Between CE and PE
For robustness, a CE device may be connected to more than one PE
device, resulting in a multi-homing arrangement. Four distinct types
of multi-homing arrangements, shown in Figure 2.4, may be supported.
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+---------------- +---------------
| |
+------+ +------+
+---------| PE | +---------| PE |
| |device| | |device| SP network
| +------+ | +------+
+------+ | +------+ |
| CE | | | CE | +---------------
|device| | SP network |device| +---------------
+------+ | +------+ |
| +------+ | +------+
| | PE | | | PE |
+---------|device| +---------|device| SP network
+------+ +------+
| |
+---------------- +---------------
This type includes a CE device connected
to a PE device via two access connections.
(a) (b)
+---------------- +---------------
| |
+------+ +------+ +------+ +------+
| CE |-----| PE | | CE |-----| PE |
|device| |device| |device| |device| SP network
+------+ +------+ +------+ +------+
| | | |
| Backdoor | | Backdoor +---------------
| link | SP network | link +---------------
| | | |
+------+ +------+ +------+ +------+
| CE | | PE | | CE | | PE |
|device|-----|device| |device|-----|device| SP network
+------+ +------+ +------+ +------+
| |
+---------------- +---------------
(c) (d)
Figure 2.4: Four types of double-homing arrangements.
2.1.3. Interworking Model
It is quite natural to assume that multiple different layer 3 VPN
approaches may be implemented, particularly if the VPN backbone
includes more than one SP network. For example, (1) each SP chooses
one or more layer 3 PE-based VPN approaches out of multiple vendor's
implementations, implying that different SPs may choose different
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approaches; and (2) an SP may deploy multiple networks of layer 3
PE-based VPNs (e.g., an old network and a new network). Thus it is
important to allow interworking of layer 3 PE-based VPNs making use
of multiple different layer 3 VPN approaches.
There are three scenarios that enable layer 3 PE-based VPN
interworking among different approaches.
o Interworking function
This scenario enables interworking using a PE that is located at
one or more points which are logically located between VPNs based
on different layer 3 VPN approaches. For example, this PE may be
located on the boundary between SP networks which make use of
different layer 3 VPN approaches [VPN-DISC]. A PE at one of these
points is called an interworking function (IWF), and an example
configuration is shown in Figure 2.5.
+------------------+ +------------------+
| | | |
+------+ VPN tunnel +------+ VPN tunnel +------+
| |==============| |==============| |
| | | | | |
| PE | | PE | | PE |
| | |device| | |
|device| |(IWF) | |device|
| | VPN tunnel | | VPN tunnel | |
| |==============| |==============| |
+------+ +------+ +------+
| | | |
+------------------+ +------------------+
|<-VPN approach 1->| |<-VPN approach 2->|
Figure 2.5: Interworking function.
o Interworking interface
This scenario enables interworking using tunnels between PEs
supporting by different layer 3 VPN approaches. As shown in Figure
2.6, interworking interface is defined as the interface which
exists between a pair of PEs and connects two SP networks
implemented with different approaches. This interface is similar
to the customer interface located between PE and CE, but the
interface is supported by tunnels to identify VPNs, while the
customer interface is supported by access connections.
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+------------------+ +------------------+
| | : | |
+------+ VPN tunnel +------+Tunnel: +------+ VPN tunnel +------+
| |============| |======:======| |============| |
| | | | : | | | |
| PE | | PE | : | PE | | PE |
| | | | : | | | |
|device| |device| : |device| |device|
| | VPN tunnel | |Tunnel: | | VPN tunnel | |
| |============| |======:======| |============| |
+------+ +------+ : +------+ +------+
| | : | |
+------------------+ Interworking +------------------+
|<-VPN approach 1->| interface |<-VPN approach 2->|
Figure 2.6: Interworking interface.
o Customer-based interworking
If some customer site has a CE attached to one kind of VPN, and a
CE attached to another kind, communication between the two kinds of
VPN occurs automatically.
2.2. Reference Model for Layer 3 Provider-Provisioned CE-based VPN
This subsection describes functional components and their
relationship for implementing layer 3 provider-provisioned CE-based
VPN.
Figure 2.7 shows the reference model for layer 3 provider-provisioned
CE-based VPN. As shown in Figure 2.7, the customer interface is
defined as the interface which exists between CE and PE devices.
In this model, a CE device maintains one or more VPN tunnel
endpoints, and a PE device has no VPN-specific functionality. As a
result, the interworking issues of section 2.1.3 do not arise.
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+---------+ +------------------------------------+ +---------+
| | | | | |
| | | +------+ +------+ : +------+
+------+ : | | | | | | : | CE |
| CE | : | | | P | | PE | : |device|
|device| : +------+ VPN tunnel |router| |device| : | of |
| of |=:====================================================:=|VPN A|
|VPN A| : | | +------+ +------+ : +------+
+------+ : | PE | | | : |
+------+ : |device| | | : |
| CE | : | | VPN tunnel +------+ : +------+
|device|=:====================================================:=| CE |
| of | : +------+ | PE | : |device|
|VPN B| : | | |device| : | of |
+------+ : | | +------------+ +------------+ | | : |VPN B|
| : | | | Customer | | Network | +------+ : +------+
|Customer | | | management | | management | | | : |
|interface| | | function | | function | | |Customer |
| | | +------------+ +------------+ | |interface|
| | | | | |
+---------+ +------------------------------------+ +---------+
| Access | |<---------- SP network(s) --------->| | Access |
| network | | | | network |
Figure 2.7: Reference model for layer 3
provider-provisioned CE-based VPN.
2.2.1. Entities in the Reference Model
The entities in the reference model are described below.
o Customer edge (CE) device
In the context of layer 3 provider-provisioned CE-based VPNs, a CE
device provides layer 3 connectivity to the customer site. It may
be a router, LSR, or host that maintains one or more VPN tunnel
endpoints. A CE device is attached via an access connection to a
PE device and usually located at the edge of a customer site or
co-located on an SP premises.
o P router (see section 2.1.1)
o Provider edge (PE) device
In the context of layer 3 provider-provisioned CE-based VPNs, a PE
device may be a router, LSR, or other device that has no
VPN-specific functionality. It is attached via an access
connection to one or more CE devices.
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o Customer Site (see section 2.1.1)
o SP networks
An SP network is a network administrated by a single service
provider. It is an IP or MPLS network. In the context of layer 3
provider-provisioned CE-based VPNs, the SP network consists of the
SP's network and the SP's management functions that manage both its
own network and the customer's VPN functions on the CE device.
o Access connection (see section 2.1.1)
o Access network (see section 2.1.1)
o VPN tunnel
A VPN tunnel is a logical link between two entities which is
created by encapsulating packets within an encapsulating header for
purpose of transmission between those two entities for support of
VPNs. In the context of layer 3 provider-provisioned CE-based
VPNs, a VPN tunnel is an IP tunnel (e.g., using GRE, IP-in-IP,
IPsec, or L2TP) or an MPLS tunnel between two CE devices over the
SP's network.
o Customer management function (see section 2.1.1)
o Network management function
The network management function supports the provisioning and
monitoring of PE or CE device attributes and their relationships,
covering PE and CE devices that define the VPN connectivity of the
customer VPNs.
The network management function may use a combination of SNMP
manager, directory service (e.g., LDAP [RFC3377]), or proprietary
network management system.
3. Customer Interface
3.1. VPN Establishment at the Customer Interface
3.1.1. Layer 3 PE-based VPN
It is necessary for each PE device to know which CEs it is attached
to, and what VPNs each CE is associated with.
VPN membership refers to the association of VPNs, CEs, and PEs. A
given CE belongs to one or more VPNs. Each PE is therefore
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associated with a set of VPNs, and a given VPN has a set of
associated PEs which are supporting that VPN. If a PE has at least
one attached CE belonging to a given VPN, then state information for
that VPN (e.g., the VPN routes) must exist on that PE. The set of
VPNs that exist on a PE may change over time as customer sites are
added to or removed from the VPNs.
In some layer 3 PE-based PPVPN schemes, VPN membership information
(i.e., information about which PEs are attached to which VPNs) is
explicitly distributed. In others, the membership information is
inferred from other information that is distributed. Different
schemes use the membership information in different ways, e.g., some
to determine what set of tunnels to set up, some to constrain the
distribution of VPN routing information.
A VPN site may be added or deleted as a result of a provisioning
operation carried out by the network administrator, or may be
dynamically added or deleted as a result of a subscriber initiated
operation; thus VPN membership information may be either static or
dynamic, as discussed below.
3.1.1.1. Static Binding
Static binding occurs when a provisioning action binds a particular
PE-CE access link to a particular VPN. For example, a network
administrator may set up a dedicated link layer connection, such as
an ATM VCC or a FR DLCI, between a PE device and a CE device. In
this case the binding between a PE-CE access connection and a
particular VPN to fixed at provisioning time, and remains the same
until another provisioning action changes the binding.
3.1.1.2. Dynamic Binding
Dynamic binding occurs when some real-time protocol interaction
causes a particular PE-CE access link to be temporarily bound to a
particular VPN. For example, a mobile user may dial up the provider
network and carry out user authentication and VPN selection
procedures. Then the PE to which the user is attached is not one
permanently associated with the user, but rather one that is
typically geographically close to where the mobile user happens to
be. Another example of dynamic binding is that of a permanent access
connection between a PE and a CE at a public facility such as a hotel
or conference center, where the link may be accessed by multiple
users in turn, each of which may wish to connect to a different VPN.
To support dynamically connected users, PPP and RADIUS are commonly
used, as these protocols provide for user identification,
authentication and VPN selection. Other mechanisms are also
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possible. For example a user's HTTP traffic may be initially
intercepted by a PE and diverted to a provider hosted web server.
After a dialogue that includes user authentication and VPN selection,
the user can then be connected to the required VPN. This is
sometimes referred to as a "captive portal".
Independent of the particular mechanisms used for user authentication
and VPN selection, an implication of dynamic binding is that a user
for a given VPN may appear at any PE at any time. Thus VPN
membership may change at any time as a result of user initiated
actions, rather than as a result of network provisioning actions.
This suggests that there needs to be a way to distribute membership
information rapidly and reliably when these user-initiated actions
take place.
3.1.2. Layer 3 Provider-Provisioned CE-based VPN
In layer 3 provider-provisioned CE-based VPNs, the PE devices have no
knowledge of the VPNs. A PE device attached to a particular VPN has
no knowledge of the addressing or routing information of that
specific VPN.
CE devices have IP or MPLS connectivity via a connection to a PE
device, which just provides ordinary connectivity to the global IP
address space or to an address space which is unique in a particular
SPs network. The IP connectivity may be via a static binding, or via
some kind of dynamic binding.
The establishment of the VPNs is done at each CE device, making use
of the IP or MPLS connectivity to the others. Therefore, it is
necessary for a given CE device to know which other CE devices belong
to the same VPN. In this context, VPN membership refers to the
association of VPNs and CE devices.
3.2. Data Exchange at the Customer Interface
3.2.1. Layer 3 PE-based VPN
For layer 3 PE-based VPNs, the exchange is normal IP packets,
transmitted in the same form which is available for interconnecting
routers in general. For example, IP packets may be exchanged over
Ethernet, SONET, T1, T3, dial-up lines, and any other link layer
available to the router. It is important to note that those link
layers are strictly local to the interface for the purpose of
carrying IP packets, and are terminated at each end of the customer
interface. The IP packets may contain addresses which, while unique
within the VPN, are not unique on the VPN backbone. Optionally, the
data exchange may use MPLS to carry the IP packets.
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3.2.2. Layer 3 Provider-Provisioned CE-based VPN
The data exchanged at the customer interface are always normal IP
packets that are routable on the VPN backbone, and whose addresses
are unique on the VPN backbone. Optionally, MPLS frames can be used,
if the appropriate label-switched paths exist across the VPN
backbone. The PE device does not know whether these packets are VPN
packets or not. At the current time, MPLS is not commonly offered as
a customer-visible service, so that CE-based VPNs most commonly make
use of IP services.
3.3. Customer Visible Routing
Once VPN tunnels are set up between pairs of VPN edge devices, it is
necessary to set up mechanisms which ensure that packets from the
customer network get sent through the proper tunnels. This routing
function must be performed by the VPN edge device.
3.3.1. Customer View of Routing for Layer 3 PE-based VPNs
There is a PE-CE routing interaction which enables a PE to obtain
those addresses, from the customer network, that are reachable via
the CE. The PE-CE routing interaction also enables a CE device to
obtain those addresses, from the customer network, which are
reachable via the PE; these will generally be addresses that are at
other sites in the customer network.
The PE-CE routing interaction can make use of static routing, an IGP
(such as RIP, OSPF, IS-IS, etc.), or BGP.
If the PE-CE interaction is done via an IGP, the PE will generally
maintain at least several independent IGP instances; one for the
backbone routing, and one for each VPN. Thus the PE participates in
the IGP of the customer VPNs, but the CE does not participate in the
backbone's IGP.
If the PE-CE interaction is done via BGP, the PE MAY support one
instance of BGP for each VPN, as well as an additional instance of
BGP for the public Internet routes. Alternatively, the PE might
support a single instance of BGP, using, e.g., different BGP Address
Families to distinguish the public Internet routes from the VPN
routes.
Routing information which a PE learns from a CE in a particular VPN
must be forwarded to the other PEs that are attached to the same VPN.
Those other PEs must then forward the information in turn to the
other CEs of that VPN.
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The PE-PE routing distribution can be done as part of the same
routing instance to which the PE-CE interface belongs.
Alternatively, it can be done via a different routing instance,
possibly using a different routing algorithm. In this case, the PE
must redistribute VPN routes from one routing instance to another.
Note that VPN routing information is never distributed to the P
routers. VPN routing information is known at the edge of the VPN
backbone, but not in the core.
If the VPN's IGP is different than the routing algorithm running on
the CE-PE link, then the CE must support two routing instances, and
must redistribute the VPN's routes from one instance to the other
(e.g., [VPN-BGP-OSPF]).
In the case of layer 3 PE-based VPNs a single PE device is likely to
provide service for several different VPNs. Since different VPNs may
have address spaces which are not mutually unique, a PE device must
have several forwarding tables, in general one for each VPN to which
it is attached. These will be referred to as VPN Forwarding
Instances (VFIs). Each VFI is a logical entity internal to the PE
device. VFIs are defined in section 2.1.1, and discussed in more
detail in section 4.4.2.
The scaling and management of the customer network (as well as the
operation of the VPN) will depend upon the implementation approach
and the manner in which routing is done.
3.3.1.1. Routing for Intranets
In the intranet case all of the sites to be interconnected belong to
the same administration (for example, the same company). The options
for routing within a single customer network include:
o A single IGP area (using OSPF, IS-IS, or RIP)
o Multiple areas within a single IGP
o A separate IGP within each site, with routes redistributed from
each site to backbone routing (i.e., to a backbone as seen by the
customer network).
Note that these options look at routing from the perspective of the
overall routing in the customer network. This list does not specify
whether PE device is considered to be in a site or not. This issue
is discussed below.
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A single IGP area (such as a single OSPF area, a single IS-IS area,
or a single instance of RIP between routers) may be used. One could
have, all routers within the customer network (including the PEs, or
more precisely, including a VFI within each PE) appear within a
single area. Tunnels between the PEs could also appear as normal
links.
In some cases the multi-level hierarchy of OSPF or IS-IS may be used.
One way to apply this to VPNs would be to have each site be a single
OSPF or IS-IS area. The VFIs will participate in routing within each
site as part of that area. The VFIs may then be interconnected as
the backbone (OSPF area 0 or IS-IS level 2). If OSPF is used, the
VFIs therefore appear to the customer network as area border routers.
If IS-IS is used, the VFIs therefore participate in level 1 routing
within the local area, and appear to the customer network as if they
are level 2 routers in the backbone.
Where an IGP is used across the entire network, it is straightforward
for VPN tunnels, access connections, and backdoor links to be mixed
in a network. Given that OSPF or IS-IS metrics will be assigned to
all links, paths via alternate links can be compared and the shortest
cost path will be used regardless of whether it is via VPN tunnels,
access connections, or backdoor links. If multiple sites of a VPN do
not use a common IGP, or if the backbone does not use the same common
IGP as the sites, then special procedures may be needed to ensure
that routes to/from other sites are treated as intra-area routes,
rather than as external routes (depending upon the VPN approach
taken).
Another option is to operate each site as a separate routing domain.
For example each site could operate as a single OSPF area, a single
IS-IS area, or a RIP domain. In this case the per-site routing
domains will need to redistribute routes into a backbone routing
domain (Note: in this context the "backbone routing domain" refers to
a backbone as viewed by the customer network). In this case it is
optional whether or not the VFIs participate in the routing within
each site.
3.3.1.2. Routing for Extranets
In the extranet case the sites to be interconnected belong to
multiple different administrations. In this case IGPs (such as OSPF,
IS-IS, or RIP) are normally not used across the interface between
organizations. Either static routes or BGP may be used between
sites. If the customer network administration wishes to maintain
control of routing between its site and other networks, then either
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static routing or BGP may be used across the customer interface. If
the customer wants to outsource all such control to the provider,
then an IGP or static routes may be used at this interface.
The use of BGP between sites allows for policy based routing between
sites. This is particularly useful in the extranet case. Note that
private IP addresses or non-unique IP address (e.g., unregistered
addresses) should not be used for extranet communication.
3.3.1.3. CE and PE Devices for Layer 3 PE-based VPNs
When using a single IGP area across an intranet, the entire customer
network participates in a single area of an IGP. In this case, for
layer 3 PE-based VPNs both CE and PE devices participate as normal
routers within the area.
The other options make a distinction between routing within a site,
and routing between sites. In this case, a CE device would normally
be considered as part of the site where it is located. However,
there is an option regarding how the PE devices should be considered.
In some cases, from the perspective of routing within the customer
network, a PE device (or more precisely a VFI within a PE device) may
be considered to be internal to the same area or routing domain as
the site to which it is attached. This simplifies the management
responsibilities of the customer network administration, since
inter-area routing would be handled by the provider.
For example, from the perspective of routing within the customer
network, the CE devices may be the area border or AS boundary routers
of the IGP area. In this case, static routing, BGP, or whatever
routing is used in the backbone, may be used across the customer
interface.
3.3.2. Customer View of Routing for Layer 3 Provider-Provisioned
CE-based VPNs
For layer 3 provider-provisioned CE-based VPNs, the PE devices are
not aware of the set of addresses which are reachable at particular
customer sites. The CE and PE devices do not exchange the customer's
routing information.
Customer sites that belong to the same VPN may exchange routing
information through the CE-CE VPN tunnels that appear, to the
customers IGP, as router adjacencies. Alternatively, instead of
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exchanging routing information through the VPN tunnels, the SP's
management system may take care of the configuration of the static
route information of one site towards the other sites in the VPN.
Routing within the customer site may be done in any possible way,
using any kind of routing protocols (see section 3.3.3).
As the CE device receives an IP or MPLS service from the SP, the CE
and PE devices may exchange routing information that is meaningful
within the SP routing realm.
Moreover, as the forwarding of tunneled customer packets in the SP
network will be based on global IP forwarding, the routes to the
various CE devices must be known in the entire SP's network.
This means that a CE device may need to participate in two different
routing processes:
o routing in its own private network (VPN routing), within its own
site and with the other VPN sites through the VPN tunnels, possibly
using private addresses.
o routing in the SP network (global routing), as such peering with
its PE.
However, in many scenarios, the use of static/default routes at the
CE-PE interface might be all the global routing that is required.
3.3.3. Options for Customer Visible Routing
The following technologies are available for the exchange of routing
information.
o Static routing
Routing tables may be configured through a management system.
o RIP (Routing Information Protocol) [RFC2453]
RIP is an interior gateway protocol and is used within an
autonomous system. It sends out routing updates at regular
intervals and whenever the network topology changes. Routing
information is then propagated by the adjacent routers to their
neighbors and thus to the entire network. A route from a source to
a destination is the path with the least number of routers. This
number is called the "hop count" and its maximum value is 15. This
implies that RIP is suitable for a small- or medium-sized networks.
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o OSPF (Open Shortest Path First) [RFC2328]
OSPF is an interior gateway protocol and is applied to a single
autonomous system. Each router distributes the state of its
interfaces and neighboring routers as a link state advertisement,
and maintains a database describing the autonomous system's
topology. A link state is advertised every 30 minutes or when the
topology is reconfigured.
Each router maintains an identical topological database, from which
it constructs a tree of shortest paths with itself as the root.
The algorithm is known as the Shortest Path First or SPF. The
router generates a routing table from the tree of shortest paths.
OSPF supports a variable length subnet mask, which enables
effective use of the IP address space.
OSPF allows sets of networks to be grouped together into an area.
Each area has its own topological database. The topology of the
area is invisible from outside its area. The areas are
interconnected via a "backbone" network. The backbone network
distributes routing information between the areas. The area
routing scheme can reduce the routing traffic and compute the
shortest path trees and is indispensable for larger scale networks.
Each multi-access network with multiple routers attached has a
designated router. The designated router generates a link state
advertisement for the multi-access network and synchronizes the
topological database with other adjacent routers in the area. The
concept of designated router can thus reduce the routing traffic
and compute shortest path trees. To achieve high availability, a
backup designated router is used.
o IS-IS (intermediate system to intermediate system) [RFC1195]
IS-IS is a routing protocol designed for the OSI (Open Systems
Interconnection) protocol suites. Integrated IS-IS is derived from
IS-IS in order to support the IP protocol. In the Internet
community, IS-IS means integrated IS-IS. In this, a link state is
advertised over a connectionless network service. IS-IS has the
same basic features as OSPF. They include: link state
advertisement and maintenance of a topological database within an
area, calculation of a tree of shortest paths, generation of a
routing table from a tree of shortest paths, the area routing
scheme, a designated router, and a variable length subnet mask.
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o BGP-4 (Border Gateway Protocol version 4) [RFC1771]
BGP-4 is an exterior gateway protocol and is applied to the routing
of inter-autonomous systems. A BGP speaker establishes a session
with other BGP speakers and advertises routing information to them.
A session may be an External BGP (EBGP) that connects two BGP
speakers within different autonomous systems, or an internal BGP
(IBGP) that connects two BGP speakers within a single autonomous
system. Routing information is qualified with path attributes,
which differentiate routes for the purpose of selecting an
appropriate one from possible routes. Also, routes are grouped by
the community attribute [RFC1997] [BGP-COM].
The IBGP mesh size tends to increase dramatically with the number
of BGP speakers in an autonomous system. BGP can reduce the number
of IBGP sessions by dividing the autonomous system into smaller
autonomous systems and grouping them into a single confederation
[RFC3065]. Route reflection is another way to reduce the number of
IBGP sessions [RFC1966]. BGP divides the autonomous system into
clusters. Each cluster establishes the IBGP full mesh within
itself, and designates one or more BGP speakers as "route
reflectors," which communicate with other clusters via their route
reflectors. Route reflectors in each cluster maintain path and
attribute information across the autonomous system. The autonomous
system still functions like a fully meshed autonomous system. On
the other hand, confederations provide finer control of routing
within the autonomous system by allowing for policy changes across
confederation boundaries, while route reflection requires the use
of identical policies.
BGP-4 has been extended to support IPv6, IPX, and others as well as
IPv4 [RFC2858]. Multiprotocol BGP-4 carries routes from multiple
"address families".
4. Network Interface and SP Support of VPNs
4.1. Functional Components of a VPN
The basic functional components of an implementation of a VPN are:
o A mechanism to acquire VPN membership/capability information
o A mechanism to tunnel traffic between VPN sites
o For layer 3 PE-based VPNs, a means to learn customer routes,
distribute them between the PEs, and to advertise reachable
destinations to customer sites.
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Based on the actual implementation, these functions could be
implemented on a per-VPN basis or could be accomplished via a common
mechanism shared by all VPNs. For instance, a single process could
handle the routing information for all the VPNs or a separate process
may be created for each VPN.
Logically, the establishment of a VPN can be thought of as composed
of the following three stages. In the first stage, the VPN edge
devices learn of each other. In the second stage, they establish
tunnels to each other. In the third stage, they exchange routing
information with each other. However, not all VPN solutions need be
decomposed into these three stages. For example, in some VPN
solutions, tunnels are not established after learning membership
information; rather, pre-existing tunnels are selected and used.
Also, in some VPN solutions, the membership information and the
routing information are combined.
In the membership/capability discovery stage, membership and
capability information needs to be acquired to determine whether two
particular VPN edge devices support any VPNs in common. This can be
accomplished, for instance, by exchanging VPN identifiers of the
configured VPNs at each VPN edge device. The capabilities of the VPN
edge devices need to be determined, in order to be able to agree on a
common mechanism for tunneling and/or routing. For instance, if site
A supports both IPsec and MPLS as tunneling mechanisms and site B
supports only MPLS, they can both agree to use MPLS for tunneling.
In some cases the capability information may be determined
implicitly, for example some SPs may implement a single VPN solution.
Likewise, the routing information for VPNs can be distributed using
the methods discussed in section 4.4.
In the tunnel establishment stage, mechanisms may need to be invoked
to actually set up the tunnels. With IPsec, for instance, this could
involve the use of IKE to exchange keys and policies for securing the
data traffic. However, if IP tunneling, e.g., is used, there may not
be any need to explicitly set up tunnels; if MPLS tunnels are used,
they may be pre-established as part of normal MPLS functioning.
In the VPN routing stage, routing information for the VPN sites must
be exchanged before data transfer between the sites can take place.
Based on the VPN model, this could involve the use of static routes,
IGPs such as OSPF/ISIS/RIP, or an EGP such as BGP.
VPN membership and capability information can be distributed from a
central management system, using protocols such as, e.g., LDAP.
Alternatively, it can be distributed manually. However, as manual
configuration does not scale and is error prone, its use is
discouraged. As a third alternative, VPN information can be
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distributed via protocols that ensure automatic and consistent
distribution of information in a timely manner, much as routing
protocols do for routing information. This may suggest that the
information be carried in routing protocols themselves, though only
if this can be done without negatively impacting the essential
routing functions.
It can be seen that quite a lot of information needs to be exchanged
in order to establish and maintain a VPN. The scaling and stability
consequences need to be analyzed for any VPN approach.
While every VPN solution must address the functionality of all three
components, the combinations of mechanisms used to provide the needed
functionality, and the order in which different pieces of
functionality are carried out, may differ.
For layer 3 provider-provisioned CE-based VPNs, the VPN service is
offering tunnels between CE devices. IP routing for the VPN is done
by the customer network. With these solutions, the SP is involved in
the operation of the membership/capability discovery stage and the
tunnel establishment stage. The IP routing functional component may
be entirely up to the customer network, or alternatively, the SP's
management system may be responsible for the distribution of the
reachability information of the VPN sites to the other sites of the
same VPN.
4.2. VPN Establishment and Maintenance
For a layer 3 provider-provisioned VPN the SP is responsible for the
establishment and maintenance of the VPNs. Many different approaches
and schemes are possible in order to provide layer 3 PPVPNs, however
there are some generic problems that any VPN solution must address,
including:
o For PE-based VPNs, when a new site is added to a PE, how do the
other PEs find out about it? When a PE first gets attached to a
given VPN, how does it determine which other PEs are attached to
the same VPN. For CE-based VPNs, when a new site is added, how
does its CE find out about all the other CEs at other sites of the
same VPN?
o In order for layer 3 PE-based VPNs to scale, all routes for all
VPNs cannot reside on all PEs. How is the distribution of VPN
routing information constrained so that it is distributed to only
those devices that need it?
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o An administrator may wish to provision different topologies for
different VPNs (e.g., a full mesh or a hub & spoke topology). How
is this achieved?
This section looks at some of these generic problems and at some of
the mechanisms that can be used to solve them.
4.2.1. VPN Discovery
Mechanisms are needed to acquire information that allows the
establishment and maintenance of VPNs. This may include, for
example, information on VPN membership, topology, and VPN device
capabilities. This information may be statically configured, or
distributed by an automated protocol. As a result of the operation
of these mechanisms and protocols, a device is able to determine
where to set up tunnels, and where to advertise the VPN routes for
each VPN.
With a physical network, the equivalent problem can by solved by the
control of the physical interconnection of links, and by having a
router run a discovery/hello protocol over its locally connected
links. With VPNs both the routers and the links (tunnels) may be
logical entities, and thus some other mechanisms are needed.
A number of different approaches are possible for VPN discovery. One
scheme uses the network management system to configure and provision
the VPN edge devices. This approach can also be used to distribute
VPN discovery information, either using proprietary protocols or
using standard management protocols and MIBs. Another approach is
where the VPN edge devices act as clients of a centralized directory
or database server that contains VPN discovery information. Another
possibility is where VPN discovery information is piggybacked onto a
routing protocol running between the VPN edge devices [VPN-DISC].
4.2.1.1. Network Management for Membership Information
SPs use network management extensively to configure and monitor the
various devices that are spread throughout their networks. This
approach could be also used for distributing VPN related information.
A network management system (either centralized or distributed) could
be used by the SP to configure and provision VPNs on the VPN edge
devices at various locations. VPN configuration information could be
entered into a network management application and distributed to the
remote sites via the same means used to distribute other network
management information. This approach is most natural when all the
devices that must be provisioned are within a single SP's network,
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since the SP has access to all VPN edge devices in its domain.
Security and access control are important, and could be achieved for
example using SNMPv3, SSH, or IPsec tunnels.
4.2.1.2. Directory Servers
An SP typically needs to maintain a database of VPN
configuration/membership information, regardless of the mechanisms
used to distribute it. LDAPv3 [RFC3377] is a standard directory
protocol which makes it possible to use a common mechanism for both
storing such information and distributing it.
To facilitate interoperability between different implementations, as
well as between the management systems of different SPs, a standard
schema for representing VPN membership and configuration information
would have to be developed.
LDAPv3 supports authentication of messages and associated access
control, which can be used to limit access to VPN information to
authorized entities.
4.2.1.3. Augmented Routing for Membership Information
Extensions to the use of existing BGP mechanisms, for distribution of
VPN membership information, are proposed in [VPN-2547BIS]. In that
scheme, BGP is used to distribute VPN routes, and each route carries
a set of attributes which indicate the VPN (or VPNs) to which the
route belongs. This allows the VPN discovery information and routing
information to be combined in a single protocol. Information needed
to establish per-VPN tunnels can also be carried as attributes of the
routes. This makes use of the BGP protocol's ability to effectively
carry large amounts of routing information.
It is also possible to use BGP to distribute just the
membership/capability information, while using a different technique
to distribute the routing. BGP's update message would be used to
indicate that a PE is attached to a particular VPN; BGP's withdraw
message would be used to indicate that a PE has ceased to be attached
to a particular VPN. This makes use of the BGP protocol's ability to
dynamically distribute real-time changes in a reliable and fairly
rapid manner. In addition, if a BGP route reflector is used, PEs
never have to be provisioned with each other's IP addresses at all.
Both cases make use of BGP's mechanisms, such as route filters, for
constraining the distribution of information.
Augmented routing may be done in combination with aggregated routing,
as discussed in section 4.4.4. Of course, when using BGP for
distributing any kind of VPN-specific information, one must ensure
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that one is not disrupting the classical use of BGP for distributing
public Internet routing information. For further discussion of this,
see the discussion of aggregated routing, section 4.4.4.
4.2.1.4. VPN Discovery for Inter-SP VPNs
When two sites of a VPN are connected to different SP networks, the
SPs must support a common mechanism for exchanging
membership/capability information. This might make use of manual
configuration or automated exchange of information between the SPs.
Automated exchange may be facilitated if one or more mechanisms for
VPN discovery are standardized and supported across the multiple SPs.
Inter-SP trust relationships will need to be established, for example
to determine which information and how much information about the
VPNs may be exchanged between SPs.
In some cases different service providers may deploy different
approaches for VPN discovery. Where this occurs, this implies that
for multi-SP VPNs, some manual coordination and configuration may be
necessary.
The amount of information which needs to be shared between SPs may
vary greatly depending upon the number of size of the multi-SP VPNs.
The SPs will therefore need to determine and agree upon the expected
amount of membership information to be exchanged, and the dynamic
nature of this information. Mechanisms may also be needed to
authenticate the VPN membership information.
VPN information should be distributed only to places where it needs
to go, whether that is intra-provider or inter-provider. In this
way, the distribution of VPN information is unlike the distribution
of inter-provider routing information, as the latter needs to be
distributed throughout the Internet. In addition, the joint support
of a VPN by two SPs should not require any third SP to maintain state
for that VPN. Again, notice the difference with respect to
inter-provider routing; in inter-provider routing: sending traffic
from one SP to another may indeed require routing state in a third
SP.
As one possible example: Suppose that there are two SPs A and C,
which want to support a common VPN. Suppose that A and C are
interconnected via SP B. In this case B will need to know how to
route traffic between A and C, and therefore will need to know
something about A and C (such as enough routing information to
forward IP traffic and/or connect MPLS LSPs between PEs or route
reflectors in A and C). However, for scaling purposes it is
desirable that B not need to know VPN-specific information about the
VPNs which are supported by A and C.
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4.2.2. Constraining Distribution of VPN Routing Information
In layer 3 provider-provisioned CE-based VPNs, the VPN tunnels
connect CE devices. In this case, distribution of IP routing
information occurs between CE devices on the customer sites. No
additional constraints on the distribution of VPN routing information
are necessary.
In layer 3 PE-based VPNs, however, the PE devices must be aware of
VPN routing information (for the VPNs to which they are attached).
For scalability reasons, one does not want a scheme in which all PEs
contain all routes for all VPNs. Rather, only the PEs that are
attached to sites in a given VPN should contain the routing
information for that VPN. This means that the distribution of VPN
routing information between PE devices must be constrained.
As VPN membership may change dynamically, it is necessary to have a
mechanism that allows VPN route information to be distributed to any
PE where there is an attached user for that VPN, and allows for the
removal of this information when it is no longer needed.
In the Virtual Router scheme, per-VPN tunnels must be established
before any routes for a VPN are distributed, and the routes are then
distributed through those tunnels. Thus by establishing the proper
set of tunnels, one implicitly constrains and controls the
distribution of per-VPN routing information. In this scheme, the
distribution of membership information consists of the set of VPNs
that exists on each PE, as well as information about the desired
topology. This enables a PE to determine the set of remote PEs to
which it must establish tunnels for a particular VPN.
In the aggregated routing scheme (see section 4.4.4), the
distribution of VPN routing information is constrained by means of
route filtering. As VPN membership changes on a PE, the route
filters in use between the PE and its peers can be adjusted. Each
peer may then adjust the filters in use with each of its peers in
turn, and thus the changes propagate across the network. When BGP is
used, this filtering may take place at route reflectors as discussed
in section 4.4.4.
4.2.3. Controlling VPN Topology
The topology for a VPN consists of a set of nodes interconnected via
tunnels. The topology may be a full mesh, a hub and spoke topology,
or an arbitrary topology. For a VPN the set of nodes will include
all VPN edge devices that have attached sites for that VPN.
Naturally, whatever the topology, all VPN sites are reachable from
each other; the topology simply constrains the way traffic is routed
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among the sites. For example, in one topology traffic between site A
and site B goes from one to the other directly over the VPN backbone;
in another topology, traffic from site A to site B must traverse site
C before reaching site B.
The simplest topology is a full mesh, where a tunnel exists between
every pair of VPN edge devices. If we assume the use of point-to-
point tunnels (rather than multipoint-to-point), then with a full
mesh topology there are N*(N-1)/2 duplex tunnels or N*(N-1) simplex
tunnels for N VPN edge devices. Each tunnel consumes some resources
at a VPN edge device, and depending on the type of tunnel, may or may
not consume resources in intermediate routers or LSRs. One reason
for using a partial mesh topology is to reduce the number of tunnels
a VPN edge device, and/or the network, needs to support. Another
reason is to support the scenario where an administrator requires all
traffic from certain sites to traverse some particular site for
policy or control reasons, such as to force traffic through a
firewall, or for monitoring or accounting purposes. Note that the
topologies used for each VPN are separate, and thus the same VPN edge
device may be part of a full mesh topology for one VPN, and of a
partial mesh topology for another VPN.
An example of where a partial mesh topology could be suitable is for
a VPN that supports a large number of telecommuters and a small
number of corporate sites. Most traffic will be between
telecommuters and the corporate sites, not between pairs of
telecommuters. A hub and spoke topology for the VPN would thus map
onto the underlying traffic flow, with the telecommuters attached to
spoke VPN edge devices and the corporate sites attached to hub VPN
edge devices. Traffic between telecommuters is still supported, but
this traffic traverses a hub VPN edge device.
The selection of a topology for a VPN is an administrative choice,
but it is useful to examine protocol mechanisms that can be used to
automate the construction of the desired topology, and thus reduce
the amount of configuration needed. To this end it is useful for a
VPN edge device to be able to advertise per-VPN topology information
to other VPN edge devices. It may be simplest to advertise this at
the same time as the membership information is advertised, using the
same mechanisms.
A simple scheme is where a VPN edge device advertises itself either
as a hub or as a spoke, for each VPN that it has. When received by
other VPN edge devices this information can be used when determining
whether to establish a tunnel. A more comprehensive scheme allows a
VPN edge device to advertise a set of topology groups, with tunnels
established between a pair of VPN edge devices if they have a group
in common.
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4.3. VPN Tunneling
VPN solutions use tunneling in order to transport VPN packets across
the VPN backbone, from one VPN edge device to another. There are
different types of tunneling protocols, different ways of
establishing and maintaining tunnels, and different ways to associate
tunnels with VPNs (e.g., shared versus dedicated per-VPN tunnels).
Sections 4.3.1 through 4.3.5 discusses some common characteristics
shared by all forms of tunneling, and some common problems to which
tunnels provide a solution. Section 4.3.6 provides a survey of
available tunneling techniques. Note that tunneling protocol issues
are generally independent of the mechanisms used for VPN membership
and VPN routing.
One motivation for the use of tunneling is that the packet addressing
used in a VPN may have no relation to the packet addressing used
between the VPN edge devices. For example the customer VPN traffic
could use non-unique or private IP addressing [RFC1918]. Also an
IPv6 VPN could be implemented across an IPv4 provider backbone. As
such the packet forwarding between the VPN edge devices must use
information other than that contained in the VPN packets themselves.
A tunneling protocol adds additional information, such an extra
header or label, to a VPN packet, and this additional information is
then used for forwarding the packet between the VPN edge devices.
Another capability optionally provided by tunneling is that of
isolation between different VPN traffic flows. The QoS and security
requirements for these traffic flows may differ, and can be met by
using different tunnels with the appropriate characteristics. This
allows a provider to offer different service characteristics for
traffic in different VPNs, or to subsets of traffic flows within a
single VPN.
The specific tunneling protocols considered in this section are GRE,
IP-in-IP, IPsec, and MPLS, as these are the most suitable for
carrying VPN traffic across the VPN backbone. Other tunneling
protocols, such as L2TP [RFC2661], may be used as access tunnels,
carrying traffic between a PE and a CE. As backbone tunneling is
independent of and orthogonal to access tunneling, protocols for the
latter are not discussed here.
4.3.1. Tunnel Encapsulations
All tunneling protocols use an encapsulation that adds additional
information to the encapsulated packet; this information is used for
forwarding across the VPN backbone. Examples are provided in section
4.3.6.
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One characteristic of a tunneling protocol is whether per-tunnel
state is needed in the SP network in order to forward the
encapsulated packets. For IP tunneling schemes (GRE, IP-in-IP, and
IPsec) per-tunnel state is completely confined to the VPN edge
devices. Other routers are unaware of the tunnels, and forward
according to the IP header. For MPLS, per-tunnel state is needed,
since the top label in the label stack must be examined and swapped
by intermediate LSRs. The amount of state required can be minimized
by hierarchical multiplexing, and by use of multi-point to point
tunnels, as discussed below.
Another characteristic is the tunneling overhead introduced. With
IPsec the overhead may be considerable as it may include, for
example, an ESP header, ESP trailer and an additional IP header. The
other mechanisms listed use less overhead, with MPLS being the most
lightweight. The overhead inherent in any tunneling mechanism may
result in additional IP packet fragmentation, if the resulting packet
is too large to be carried by the underlying link layer. As such it
is important to report any reduced MTU sizes via mechanisms such as
path MTU discovery in order to avoid fragmentation wherever possible.
Yet another characteristic is something we might call "transparency
to the Internet". IP-based encapsulation can carry be used to carry
a packet anywhere in the Internet. MPLS encapsulation can only be
used to carry a packet on IP networks that support MPLS. If an
MPLS-encapsulated packet must cross the networks of multiple SPs, the
adjacent SPs must bilateral agreements to accept MPLS packets from
each other. If only a portion of the path across the backbone lacks
MPLS support, then an MPLS-in-IP encapsulation can be used to move
the MPLS packets across that part of the backbone. However, this
does add complexity. On the other hand, MPLS has efficiency
advantages, particularly in environments where encapsulations may
need to be nested.
Transparency to the Internet is sometimes a requirement, but
sometimes not. This depends on the sort of service which a SP is
offering to its customer.
4.3.2. Tunnel Multiplexing
When a tunneled packet arrives at the tunnel egress, it must be
possible to infer the packet's VPN from its encapsulation header. In
MPLS encapsulations, this must be inferred from the packet's label
stack. In IP-based encapsulations, this can be inferred from some
combination of the IP source address, the IP destination address, and
a "multiplexing field" in the encapsulation header. The multiplexing
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field might be one which was explicitly designed for multiplexing, or
one that wasn't originally designed for this but can be pushed into
service as a multiplexing field. For example:
o GRE: Packets associated to VPN by source IP address, destination IP
address, and Key field, although the key field was originally
intended for authentication.
o IP-in-IP: Packets associated to VPN by IP destination address in
outer header.
o IPsec: Packets associated to VPN by IP source address, IP
destination address, and SPI field.
o MPLS: Packets associated to VPN by label stack.
Note that IP-in-IP tunneling does not have a real multiplexing field,
so a different IP destination address must be used for every VPN
supported by a given PE. In the other IP-based encapsulations, a
given PE need have only a single IP address, and the multiplexing
field is used to distinguish the different VPNs supported by a PE.
Thus the IP-in-IP solution has the significant disadvantage that it
requires the allocation and assignment of a potentially large number
of IP addresses, all of which have to be reachable via backbone
routing.
In the following, we will use the term "multiplexing field" to refer
to whichever field in the encapsulation header must is used to
distinguish different VPNs at a given PE. In the IP-in-IP
encapsulation, this is the destination IP address field, in the other
encapsulations it is a true multiplexing field.
4.3.3. Tunnel Establishment
When tunnels are established, the tunnel endpoints must agree on the
multiplexing field values which are to be used to indicate that
particular packets are in particular VPNs. The use of "well known"
or explicitly provisioned values would not scale well as the number
of VPNs increases. So it is necessary to have some sort of protocol
interaction in which the tunnel endpoints agree on the multiplexing
field values.
For some tunneling protocols, setting up a tunnel requires an
explicit exchange of signaling messages. Generally the multiplexing
field values would be agreed upon as part of this exchange. For
example, if an IPsec encapsulation is used, the SPI field plays the
role of the multiplexing field, and IKE signaling is used to
distribute the SPI values; if an MPLS encapsulation is used, LDP,
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CR-LDP or RSVP-TE can be used to distribute the MPLS label value used
as the multiplexing field. Information about the identity of the VPN
with which the tunnel is to be associated needs to be exchanged as
part of the signaling protocol (e.g., a VPN-ID can be carried in the
signaling protocol). An advantage of this approach is that
per-tunnel security, QoS and other characteristics may also be
negotiable via the signaling protocol. A disadvantage is that the
signaling imposes overhead, which may then lead to scalability
considerations, discussed further below.
For some tunneling protocols, there is no explicit protocol
interaction that sets up the tunnel, and the multiplexing field
values must be exchanged in some other way. For example, for MPLS
tunnels, MPLS labels can be piggybacked on the protocols used to
distribute VPN routes or VPN membership information. GRE and
IP-in-IP have no associated signaling protocol, and thus by necessity
the multiplexing values are distributed via some other mechanism,
such as via configuration, control protocol, or piggybacked in some
manner on a VPN membership protocol.
The resources used by the different tunneling establishment
mechanisms may vary. With a full mesh VPN topology, and explicit
signaling, each VPN edge device has to establish a tunnel to all the
other VPN edge devices for in each VPN. The resources needed for
this on a VPN edge device may be significant, and issues such as the
time needed to recover following a device failure may need to be
taken into account, as the time to recovery includes the time needed
to reestablish a large number of tunnels.
4.3.4. Scaling and Hierarchical Tunnels
If tunnels require state to be maintained in the core of the network,
it may not be feasible to set up per-VPN tunnels between all adjacent
devices that are adjacent in some VPN topology. This would violate
the principle that there is no per-VPN state in the core of the
network, and would make the core scale poorly as the number of VPNs
increases. For example, MPLS tunnels require that core network
devices maintain state for the topmost label in the label stack. If
every core router had to maintain one or more labels for every VPN,
scaling would be very poor.
There are also scaling considerations related to the use of explicit
signaling for tunnel establishment. Even if the tunneling protocol
does not maintain per tunnel state in the core, the number of tunnels
that a single VPN edge device needs to handle may be large, as this
grows according to the number of VPNs and the number of neighbors per
VPN. One way to reduce the number of tunnels in a network is to use
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a VPN topology other than a full mesh. However this may not always
be desirable, and even with hub and spoke topologies the hubs VPN
edge devices may still need to handle large numbers of tunnels.
If the core routers need to maintain any per-tunnel state at all,
scaling can be greatly improved by using hierarchical tunnels. One
tunnel can be established between each pair of VPN edge devices, and
multiple VPN-specific tunnels can then be carried through the single
"outer" tunnel. Now the amount of state is dependent only on the
number of VPN edge devices, not on the number of VPNs. Scaling can
be further improved by having the outer tunnels be
multipoint-to-point "merging" tunnels. Now the amount of state to be
maintained in the core is on the order of the number of VPN edge
devices, not on the order of the square of that number. That is, the
amount of tunnel state is roughly equivalent to the amount of state
needed to maintain IP routes to the VPN edge devices. This is almost
(if not quite) as good as using tunnels which do not require any
state to be maintained in the core.
Using hierarchical tunnels may also reduce the amount of state to be
maintained in the VPN edge devices, particularly if maintaining the
outer tunnels requires more state than maintaining the per-VPN
tunnels that run inside the outer tunnels.
There are other factors relevant to determining the number of VPN
edge to VPN edge "outer" tunnels to use. While using a single such
tunnel has the best scaling properties, using more than one may allow
different QoS capabilities or different security characteristics to
be used for different traffic flows (from the same or from different
VPNs).
When tunnels are used hierarchically, the tunnels in the hierarchy
may all be of the same type (e.g., an MPLS label stack) or they may
be of different types (e.g., a GRE tunnel carried inside an IPsec
tunnel).
One example using hierarchical tunnels is the establishment of a
number of different IPsec security associations, providing different
levels of security between a given pair of VPN edge devices. Per-VPN
GRE tunnels can then be grouped together and then carried over the
appropriate IPsec tunnel, rather than having a separate IPsec tunnel
per-VPN. Another example is the use of an MPLS label stack. A
single PE-PE LSP is used to carry all the per-VPN LSPs. The
mechanisms used for label establishment are typically different. The
PE-PE LSP could be established using LDP, as part or normal backbone
operation, with the per-VPN LSP labels established by piggybacking on
VPN routing (e.g., using BGP) discussed in sections 3.3.1.3 and 4.1.
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4.3.5. Tunnel Maintenance
Once a tunnel is established it is necessary to know that the tunnel
is operational. Mechanisms are needed to detect tunnel failures, and
to respond appropriately to restore service.
There is a potential issue regarding propagation of failures when
multiple tunnels are multiplexed hierarchically. Suppose that
multiple VPN-specific tunnels are multiplexed inside a single PE to
PE tunnel. In this case, suppose that routing for the VPN is done
over the VPN-specific tunnels (as may be the case for CE-based and VR
approaches). Suppose that the PE to PE tunnel fails. In this case
multiple VPN-specific tunnels may fail, and layer 3 routing may
simultaneously respond for each VPN using the failed tunnel. If the
PE to PE tunnel is subsequently restored, there may then be multiple
VPN-specific tunnels and multiple routing protocol instances which
also need to recover. Each of these could potentially require some
exchange of control traffic.
When a tunnel fails, if the tunnel can be restored quickly, it might
therefore be preferable to restore the tunnel without any response by
high levels (such as other tunnels which were multiplexed inside the
failed tunnels). By having high levels delay response to a lower
level failed tunnel, this may limit the amount of control traffic
needed to completely restore correct service. However, if the failed
tunnel cannot be quickly restored, then it is necessary for the
tunnels or routing instances multiplexed over the failed tunnel to
respond, and preferable for them to respond quickly and without
explicit action by network operators.
With most layer 3 provider-provisioned CE-based VPNs and the VR
scheme, a per-VPN instance of routing is running over the tunnel,
thus any loss of connectivity between the tunnel endpoints will be
detected by the VPN routing instance. This allows rapid detection of
tunnel failure. Careful adjustment of timers might be needed to
avoid failure propagation as discussed the above. With the
aggregated routing scheme, there isn't a per-VPN instance of routing
running over the tunnel, and therefore some other scheme to detect
loss of connectivity is needed in the event that the tunnel cannot be
rapidly restored.
Failure of connectivity in a tunnel can be very difficult to detect
reliably. Among the mechanisms that can be used to detect failure
are loss of the underlying connectivity to the remote endpoint (as
indicated, e.g., by "no IP route to host" or no MPLS label), timeout
of higher layer "hello" mechanisms (e.g., IGP hellos, when the tunnel
is an adjacency in some IGP), and timeout of keep alive mechanisms in
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the tunnel establishment protocols (if any). However, none of these
techniques provides completely reliable detection of all failure
modes. Additional monitoring techniques may also be necessary.
With hierarchical tunnels it may suffice to only monitor the
outermost tunnel for loss of connectivity. However there may be
failure modes in a device where the outermost tunnel is up but one of
the inner tunnels is down.
4.3.6. Survey of Tunneling Techniques
Tunneling mechanisms provide isolated communication between two CE-PE
devices. Available tunneling mechanisms include (but are not limited
to): GRE [RFC2784] [RFC2890], IP-in-IP encapsulation [RFC2003]
[RFC2473], IPsec [RFC2401] [RFC2402], and MPLS [RFC3031] [RFC3035].
Note that the following subsections address tunnel overhead to
clarify the risk of fragmentation. Some SP networks contain layer 2
switches that enforce the standard/default MTU of 1500 bytes. In
this case, any encapsulation whatsoever creates a significant risk of
fragmentation. However, layer 2 switch vendors are in general aware
of IP tunneling as well as stacked VLAN overhead, thus many switches
practically allow an MTU of approximately 1512 bytes now. In this
case, up to 12 bytes of encapsulation can be used before there is any
risk of fragmentation. Furthermore, to improve TCP and NFS
performance, switches that support 9K bytes "jumbo frames" are also
on the market. In this case, there is no risk of fragmentation.
4.3.6.1. GRE [RFC2784] [RFC2890]
Generic Routing Encapsulation (GRE) specifies a protocol for
encapsulating an arbitrary payload protocol over an arbitrary
delivery protocol [RFC2784]. In particular, it can be used where
both the payload and the delivery protocol are IP as is the case in
layer 3 VPNs. A GRE tunnel is a tunnel whose packets are
encapsulated by GRE.
o Multiplexing
The GRE specification [RFC2784] does not explicitly support
multiplexing. But the key field extension to GRE is specified in
[RFC2890] and it may be used as a multiplexing field.
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o QoS/SLA
GRE itself does not have intrinsic QoS/SLA capabilities, but it
inherits whatever capabilities exist in the delivery protocol (IP).
Additional mechanisms, such as Diffserv or RSVP extensions
[RFC2746], can be applied.
o Tunnel setup and maintenance
There is no standard signaling protocol for setting up and
maintaining GRE tunnels.
o Large MTUs and minimization of tunnel overhead
When GRE encapsulation is used, the resulting packet consists of a
delivery protocol header, followed by a GRE header, followed by the
payload packet. When the delivery protocol is IPv4, and if the key
field is not present, GRE encapsulation adds at least 28 bytes of
overhead (36 bytes if key field extension is used.)
o Security
GRE encapsulation does not provide any significant security. The
optional key field can be used as a clear text password to aid in
the detection of misconfigurations, but it does not provide
integrity or authentication. An SP network which supports VPNs
must do extensive IP address filtering at its borders to prevent
spoofed packets from penetrating the VPNs. If multi-provider VPNs
are being supported, it may be difficult to set up these filters.
4.3.6.2. IP-in-IP Encapsulation [RFC2003] [RFC2473]
IP-in-IP specifies the format and procedures for IP-in-IP
encapsulation. This allows an IP datagram to be encapsulated within
another IP datagram. That is, the resulting packet consists of an
outer IP header, followed immediately by the payload packet. There
is no intermediate header as in GRE. [RFC2003] and [RFC2473] specify
IPv4 and IPv6 encapsulations respectively. Once the encapsulated
datagram arrives at the intermediate destination (as specified in the
outer IP header), it is decapsulated, yielding the original IP
datagram, which is then delivered to the destination indicated by the
original destination address field.
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o Multiplexing
The IP-in-IP specifications don't explicitly support multiplexing.
But if a different IP address is used for every VPN then the IP
address field can be used for this purpose. (See section 4.3.2 for
detail).
o QoS/SLA
IP-in-IP itself does not have intrinsic QoS/SLA capabilities, but
of course it inherits whatever capabilities exist for IP.
Additional mechanisms, such as RSVP extensions [RFC2764] or
DiffServ extensions [RFC2983], may be used with it.
o Tunnel setup and maintenance
There is no standard setup and maintenance protocol for IP-in-IP.
o Large MTUs and minimization of tunnel overhead
When the delivery protocol is IPv4, IP-in-IP adds at least 20 bytes
of overhead.
o Security
IP-in-IP encapsulation does not provide any significant security.
An SP network which supports VPNs must do extensive IP address
filtering at its borders to prevent spoofed packets from
penetrating the VPNs. If multi-provider VPNs are being supported,
it may be difficult to set up these filters.
4.3.6.3. IPsec [RFC2401] [RFC2402] [RFC2406] [RFC2409]
IP Security (IPsec) provides security services at the IP layer
[RFC2401]. It comprises authentication header (AH) protocol
[RFC2402], encapsulating security payload (ESP) protocol [RFC2406],
and Internet key exchange (IKE) protocol [RFC2409]. AH protocol
provides data integrity, data origin authentication, and an
anti-replay service. ESP protocol provides data confidentiality and
limited traffic flow confidentiality. It may also provide data
integrity, data origin authentication, and an anti-replay service.
AH and ESP may be used in combination.
IPsec may be employed in either transport or tunnel mode. In
transport mode, either an AH or ESP header is inserted immediately
after the payload packet's IP header. In tunnel mode, an IP packet
is encapsulated with an outer IP packet header. Either an AH or ESP
header is inserted between them. AH and ESP establish a
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unidirectional secure communication path between two endpoints, which
is called a security association. In tunnel mode, PE-PE tunnel (or a
CE-CE tunnel) consists of a pair of unidirectional security
associations. The IPsec and IKE protocols are used for setting up
IPsec tunnels.
o Multiplexing
The SPI field of AH and ESP is used to multiplex security
associations (or tunnels) between two peer devices.
o QoS/SLA
IPsec itself does not have intrinsic QoS/SLA capabilities, but it
inherits whatever mechanisms exist for IP. Other mechanisms such
as "RSVP Extensions for IPsec Data Flows" [RFC2207] or DiffServ
extensions [RFC2983] may be used with it.
o Tunnel setup and maintenance
The IPsec and IKE protocols are used for the setup and maintenance
of tunnels.
o Large MTUs and minimization of tunnel overhead
IPsec transport mode adds at least 8 bytes of overhead. IPsec
tunnel mode adds at least 28 bytes of overhead. IPsec transport
mode adds minimal overhead. In PE-based PPVPNs, the processing
overhead of IPsec (due to its cryptography) may limit the PE's
performance, especially if privacy is being provided; this is not
generally an issue in CE-based PPVPNs.
o Security
When IPsec tunneling is used in conjunction with IPsec's
cryptographic capabilities, excellent authentication and integrity
functions can be provided. Privacy can also be optionally
provided.
4.3.6.4. MPLS [RFC3031] [RFC3032] [RFC3035]
Multiprotocol Label Switching (MPLS) is a method for forwarding
packets through a network. Routers at the edge of a network apply
simple labels to packets. A label may be inserted between the data
link and network headers, or may be carried in the data link header
(e.g., the VPI/VCI field in an ATM header). Routers in the network
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switch packets according to the labels, with minimal lookup overhead.
A path, or a tunnel in the PPVPN, is called a "label switched path
(LSP)".
o Multiplexing
LSPs may be multiplexed within other LSPs.
o QoS/SLA
MPLS does not have intrinsic QoS or SLA management mechanisms, but
bandwidth may be allocated to LSPs, and their routing may be
explicitly controlled. Additional techniques such as DiffServ and
DiffServ aware traffic engineering may be used with it [RFC3270]
[MPLS-DIFF-TE]. QoS capabilities from IP may be inherited.
o Tunnel setup and maintenance
LSPs are set up and maintained by LDP (Label Distribution
Protocol), RSVP (Resource Reservation Protocol) [RFC3209], or BGP.
o Large MTUs and minimization of tunnel overhead.
MPLS encapsulation adds four bytes per label. VPN-2547BIS's
[VPN-2547BIS] approach uses at least two labels for encapsulation
and adds minimal overhead.
o Encapsulation
MPLS packets may optionally be encapsulated in IP or GRE, for cases
where it is desirable to carry MPLS packets over an IP-only
infrastructure.
o Security
MPLS encapsulation does not provide any significant security. An
SP which is providing VPN service can refuse to accept MPLS packets
from outside its borders. This provides the same level of
assurance as would be obtained via IP address filtering when
IP-based encapsulations are used. If a VPN is jointly provided by
multiple SPs, care should be taken to ensure that a labeled packet
is accepted from a neighboring router in another SP only if its top
label is one which was actually distributed to that router.
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o Applicability
MPLS is the only one of the encapsulation techniques that cannot be
guaranteed to run over any IP network. Hence it would not be
applicable when transparency to the Internet is a requirement.
If the VPN backbone consists of several cooperating SP networks
which support MPLS, then the adjacent networks may support MPLS at
their interconnects. If two cooperating SP networks which support
MPLS are separated by a third which does not support MPLS, then
MPLS-in-IP or MPLS-in-IPsec tunneling may be done between them.
4.4. PE-PE Distribution of VPN Routing Information
In layer 3 PE-based VPNs, PE devices examine the IP headers of
packets they receive from the customer networks. Forwarding is based
on routing information received from the customer network. This
implies that the PE devices need to participate in some manner in
routing for the customer network. Section 3.3 discussed how routing
would be done in the customer network, including the customer
interface. In this section, we discuss ways in which the routing
information from a particular VPN may be passed, over the shared VPN
backbone, among the set of PEs attaching to that VPN.
The PEs needs to distribute two types of routing information to each
other: (i) Public Routing: routing information which specifies how to
reach addresses on the VPN backbone (i.e., "public addresses"); call
this "public routing information" (ii) VPN Routing: routing
information obtained from the CEs, which specifies how to reach
addresses ("private addresses") that are in the VPNs.
The way in which routing information in the first category is
distributed is outside the scope of this document; we discuss only
the distribution of routing information in the second category. Of
course, one of the requirements for distributing VPN routing
information is that it be kept separate and distinct from the public
information. Another requirement is that the distribution of VPN
routing information not destabilize or otherwise interfere with the
distribution of public routing information.
Similarly, distribution of VPN routing information associated with
one VPN should not destabilize or otherwise interfere with the
operation of other VPNs. These requirements are, for example,
relevant in the case that a private network might be suffering from
instability or other problems with its internal routing, which might
be propagated to the VPN used to support that private network.
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Note that this issue does not arise in CE-based VPNs, as in CE-based
VPNs, the PE devices do not see packets from the VPN until after the
packets haven been encapsulated in an outer header that has only
public addresses.
4.4.1. Options for VPN Routing in the SP
The following technologies can be used for exchanging VPN routing
information discussed in sections 3.3.1.3 and 4.1.
o Static routing
o RIP [RFC2453]
o OSPF [RFC2328]
o BGP-4 [RFC1771]
4.4.2. VPN Forwarding Instances (VFIs)
In layer 3 PE-based VPNs, the PE devices receive unencapsulated IP
packets from the CE devices, and the PE devices use the IP
destination addresses in these packets to help make their forwarding
decisions. In order to do this properly, the PE devices must obtain
routing information from the customer networks. This implies that
the PE device participates in some manner in the customer network's
routing.
In layer 3 PE-based VPNs, a single PE device connected to several CE
devices that are in the same VPN, and it may also be connected to CE
devices of different VPNs. The route which the PE chooses for a
given IP destination address in a given packet will depend on the VPN
from which the packet was received. A PE device must therefore have
a separate forwarding table for each VPN to which it is attached. We
refer to these forwarding tables as "VPN Forwarding Instances"
(VFIs), as defined in section 2.1.
A VFI contains routes to locally attached VPN sites, as well as
routes to remote VPN sites. Section 4.4 discusses the way in which
routes to remote sites are obtained.
Routes to local sites may be obtained in several ways. One way is to
explicitly configure static routes into the VFI. This can be useful
in simple deployments, but it requires that one or more devices in
the customer's network be configured with static routes (perhaps just
a default route), so that traffic will be directed from the site to
the PE device.
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Another way is to have the PE device be a routing peer of the CE
device, in a routing algorithm such as RIP, OSPF, or BGP. Depending
on the deployment scenario, the PE might need to advertise a large
number of routes to each CE (e.g., all the routes which the PE
obtained from remote sites in the CE's VPN), or it might just need to
advertise a single default route to the CE.
A PE device uses some resources in proportion to the number of VFIs
that it has, particularly if a distinct dynamic routing protocol
instance is associated with each VFI. A PE device also uses some
resources in proportion to the total number of routes it supports,
where the total number of routes includes all the routes in all its
VFIs, and all the public routes. These scaling factors will limit
the number of VPNs which a single PE device can support.
When dynamic routing is used between a PE and a CE, it is not
necessarily the case that each VFI is associated with a single
routing protocol instance. A single routing protocol instance may
provide routing information for multiple VFIs, and/or multiple
routing protocol instances might provide information for a single
VFI. See sections 4.4.3, 4.4.4, 3.3.1, and 3.3.1.3 for details.
There are several options for how VPN routes are carried between the
PEs, as discussed below.
4.4.3. Per-VPN Routing
One option is to operate separate instances of routing protocols
between the PEs, one instance for each VPN. When this is done,
routing protocol packets for each customer network need to be
tunneled between PEs. This uses the same tunneling method, and
optionally the same tunnels, as is used for transporting VPN user
data traffic between PEs.
With per-VPN routing, a distinct routing instance corresponding to
each VPN exists within the corresponding PE device. VPN-specific
tunnels are set up between PE devices (using the control mechanisms
that were discussed in sections 3 and 4). Logically these tunnels
are between the VFIs which are within the PE devices. The tunnels
then used as if they were normal links between normal routers.
Routing protocols for each VPN operate between VFIs and the routers
within the customer network.
This approach establishes, for each VPN, a distinct "control plane"
operating across the VPN backbone. There is no sharing of control
plane by any two VPNs, nor is there any sharing of control plane by
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the VPN routing and the public routing. With this approach each PE
device can logically be thought of as consisting of multiple
independent routers.
The multiple routing instances within the PE device may be separate
processes, or may be in the same process with different data
structures. Similarly, there may be mechanisms internal to the PE
devices to partition memory and other resources between routing
instances. The mechanisms for implementing multiple routing
instances within a single physical PE are outside of the scope of
this framework document, and are also outside of the scope of other
standards documents.
This approach tends to minimize the explicit interactions between
different VPNs, as well as between VPN routing and public routing.
However, as long as the independent logical routers share the same
hardware, there is some sharing of resources, and interactions are
still possible. Also, each independent control plane has its
associated overheads, and this can raise issues of scale. For
example, the PE device must run a potentially large number of
independent routing "decision processes," and must also maintain a
potentially very large number of routing adjacencies.
4.4.4. Aggregated Routing Model
Another option is to use one single instance of a routing protocol
for carrying VPN routing information between the PEs. In this
method, the routing information for multiple different VPNs is
aggregated into a single routing protocol.
This approach greatly reduces the number of routing adjacencies which
the PEs must maintain, since there is no longer any need to maintain
more than one such adjacency between a given pair of PEs. If the
single routing protocol supports a hierarchical route distribution
mechanism (such as BGP's "route reflectors"), the PE-PE adjacencies
can be completely eliminated, and the number of backbone adjacencies
can be made into a small constant which is independent of the number
of PE devices. This improves the scaling properties.
Additional routing instances may still be needed to support the
exchange of routing information between the PE and its locally
attached CEs. These can be eliminated, with a consequent further
improvement in scalability, by using static routing on the PE-CE
interfaces, or possibly by having the PE-CE routing interaction use
the same protocol instance that is used to distribute VPN routes
across the VPN backbone (see section 4.4.4.2 for a way to do this).
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With this approach, the number of routing protocol instances in a PE
device does not depend on the number of CEs supported by the PE
device, if the routing between PE and CE devices is static or BGP-4.
However, CE and PE devices in a VPN exchange route information inside
a VPN using a routing protocol except for BGP-4, the number of
routing protocol entities in a PE device depends on the number of CEs
supported by the PE device.
In principle it is possible for routing to be aggregated using either
BGP or on an IGP.
4.4.4.1. Aggregated Routing with OSPF or IS-IS
When supporting VPNs, it is likely that there can be a large number
of VPNs supported within any given SP network. In general only a
small number of PE devices will be interested in the operation of any
one VPN. Thus while the total amount of routing information related
to the various customer networks will be very large, any one PE needs
to know about only a small number of such networks.
Generally SP networks use OSPF or IS-IS for interior routing within
the SP network. There are very good reasons for this choice, which
are outside of the scope of this document.
Both OSPF and IS-IS are link state routing protocols. In link state
routing, routing information is distributed via a flooding protocol.
The set of routing peers is in general not fully meshed, but there is
a path from any router in the set to any other. Flooding ensures
that routing information from any one router reaches all the others.
This requires all routers in the set to maintain the same routing
information. One couldn't withhold any routing information from a
particular peer unless it is known that none of the peers further
downstream will need that information, and in general this cannot be
known.
As a result, if one tried to do aggregated routing by using OSPF,
with all the PEs in the set of routing peers, all the PEs would end
up with the exact same routing information; there is no way to
constrain the distribution of routing information to a subset of the
PEs. Given the potential magnitude of the total routing information
required for supporting a large number of VPNs, this would have
unfortunate scaling implications.
In some cases VPNs may span multiple areas within a provider, or span
multiple providers. If VPN routing information were aggregated into
the IGP used within the provider, then some method would need to be
used to extend the reach of IGP routing information between areas and
between SPs.
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4.4.4.2. Aggregated Routing with BGP
In order to use BGP for aggregated routing, the VPN routing
information must be clearly distinguished from the public Internet
routing information. This is typically done by making use of BGP's
capability of handling multiple address families, and treating the
VPN routes as being in a different address family than the public
Internet routes. Typically a VPN route also carries attributes which
depend on the particular VPN or VPNs to which that route belongs.
When BGP is used for carrying VPN information, the total amount of
information carried in BGP (including the Internet routes and VPN
routes) may be quite large. As noted above, there may be a large
number of VPNs which are supported by any particular provider, and
the total amount of routing information associated with all VPNs may
be quite large. However, any one PE will in general only need to be
aware of a small number of VPNs. This implies that where VPN routing
information is aggregated into BGP, it is desirable to be able to
limit which VPN information is distributed to which PEs.
In "Interior BGP" (IBGP), routing information is not flooded; it is
sent directly, over a TCP connection, to the peer routers (or to a
route reflector). These peer routers (unless they are route
reflectors) are then not even allowed to redistribute the information
to each other. BGP also has a comprehensive set of mechanisms for
constraining the routing information that any one peer sends to
another, based on policies established by the network administration.
Thus IBGP satisfies one of the requirements for aggregated routing
within a single SP network - it makes it possible to ensure that
routing information relevant to a particular VPN is processed only by
the PE devices that attach to that VPN. All that is necessary is
that each VPN route be distributed with one or more attributes which
identify the distribution policies. Then distribution can be
constrained by filtering against these attributes.
In "Exterior BGP" (EBGP), routing peers do redistribute routing
information to each other. However, it is very common to constrain
the distribution of particular items of routing information so that
they only go to those exterior peers who have a "need to know,"
although this does require a priori knowledge of which paths may
validly lead to which addresses. In the case of VPN routing, if a
VPN is provided by a small set of cooperating SPs, such constraints
can be applied to ensure that the routing information relevant to
that VPN does not get distributed anywhere it doesn't need to be. To
the extent that a particular VPN is supported by a small number of
cooperating SPs with private peering arrangements, this is
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particularly straightforward, as the set of EBGP neighbors which need
to know the routing information from a particular VPN is easier to
determine.
BGP also has mechanisms (such as "Outbound Route Filtering," ORF)
which enable the proper set of VPN routing distribution constraints
to be dynamically distributed. This reduces the management burden of
setting up the constraints, and hence improves scalability.
Within a single routing domain (in the layer 3 VPN context, this
typically means within a single SP's network), it is common to have
the IBGP routers peer directly with one or two route reflectors,
rather than having them peer directly with each other. This greatly
reduces the number of IBGP adjacencies which any one router must
support. Further, a route reflector does not merely redistribute
routing information, it "digests" the information first, by running
its own decision processes. Only routes which survive the decision
process are redistributed.
As a result, when route reflectors are used, the amount of routing
information carried around the network, and in particular, the amount
of routing information which any given router must receive and
process, is greatly reduced. This greatly increases the scalability
of the routing distribution system.
It has already been stated that a given PE has VPN routing
information only for those PEs to which it is directly attached. It
is similarly important, for scalability, to ensure that no single
route reflector should have to have all the routing information for
all VPNs. It is after all possible for the total number of VPN
routes (across all VPNs supported by an SP) to exceed the number
which can be supported by a single route reflector. Therefore, the
VPN routes may themselves be partitioned, with some route reflectors
carrying one subset of the VPN routes and other route reflectors
carrying a different subset. The route reflectors which carry the
public Internet routes can also be completely separate from the route
reflectors that carry the VPN routes.
The use of outbound route filters allows any one PE and any one route
reflector to exchange information about only those VPNs which the PE
and route reflector are both interested in. This in turn ensures
that each PE and each route reflector receives routing information
only about the VPNs which it is directly supporting. Large SPs which
support a large number of VPNs therefore can partition the
information which is required for support of those VPNs.
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Generally a PE device will be restricted in the total number of
routes it can support, whether those are public Internet routes or
VPN routes. As a result, a PE device may be able to be attached to a
larger number of VPNs if it does not also need to support Internet
routes.
The way in which VPN routes are partitioned among PEs and/or route
reflectors is a deployment issue. With suitable deployment
procedures, the limited capacity of these devices will not limit the
number of VPNs that can be supported.
Similarly, whether a given PE and/or route reflector contains
Internet routes as well as VPN routes is a deployment issue. If the
customer networks served by a particular PE do not need the Internet
access, then that PE does not need to be aware of the Internet
routes. If some or all of the VPNs served by a particular PE do need
the Internet access, but the PE does not contain Internet routes,
then the PE can maintain a default route that routes all the Internet
traffic from that PE to a different router within the SP network,
where that other router holds the full the Internet routing table.
With this approach the PE device needs only a single default route
for all the Internet routes.
For the reasons given above, the BGP protocol seems to be a
reasonable protocol to use for distributing VPN routing information.
Additional reasons for the use of BGP are:
o BGP has been proven to be useful for distributing very large
amounts of routing information; there isn't any routing
distribution protocol which is known to scale any better.
o The same BGP instance that is used for PE-PE distribution of VPN
routes can be used for PE-CE route distribution, if CE-PE routing
is static or BGP. PEs and CEs are really parts of distinct
Autonomous Systems, and BGP is particularly well-suited for
carrying routing information between Autonomous Systems.
On the other hand, BGP is also used for distributing public Internet
routes, and it is crucially important that VPN route distributing not
compromise the distribution of public Internet routes in any way.
This issue is discussed in the following section.
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4.4.5. Scalability and Stability of Routing with Layer 3 PE-based VPNs
For layer 3 PE-based VPNs, there are likely to be cases where a
service provider supports Internet access over the same link that is
used for VPN service. Thus, a particular CE to PE link may carry
both private network IP packets (for transmission between sites of
the private network using VPN services) as well as public Internet
traffic (for transmission from the private site to the Internet, and
for transmission to the private site from the Internet). This
section looks at the scalability and stability of routing in this
case. It is worth noting that this sort of issue may be applicable
where per-VPN routing is used, as well as where aggregated routing is
used.
For layer 3 PE-based VPNs, it is necessary for the PE devices to be
able to forward IP packets using the addresses spaces of the
supported private networks, as well as using the full Internet
address space. This implies that PE devices might in some cases
participate in routing for the private networks, as well as for the
public Internet.
In some cases the routing demand on the PE might be low enough, and
the capabilities of the PE, might be great enough, that it is
reasonable for the PE to participate fully in routing for both
private networks and the public Internet. For example, the PE device
might participate in normal operation of BGP as part of the global
Internet. The PE device might also operate routing protocols (or in
some cases use static routing) to exchange routes with CE devices.
For large installations, or where PE capabilities are more limited,
it may be undesirable for the PE to fully participate in routing for
both VPNs as well as the public Internet. For example, suppose that
the total volume of routes and routing instances supported by one PE
across multiple VPNs is very large. Suppose furthermore that one or
more of the private networks suffers from routing instabilities, for
example resulting in a large number of routing updates being
transmitted to the PE device. In this case it is important to
prevent such routing from causing any instability in the routing used
in the global Internet.
In these cases it may be necessary to partition routing, so that the
PE does not need to maintain as large a collection of routes, and so
that the PE is not able to adversely effect Internet routing. Also,
given that the total number of route prefixes and the total number of
routing instances which the PE needs to maintain might be very large,
it may be desirable to limit the participation in Internet routing
for those PEs which are supporting a large number of VPNs or which
are supporting large VPNs.
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Consider a case where a PE is supporting a very large number of VPNs,
some of which have a large number of sites. To pick a VERY large
example, let's suppose 1000 VPNs, with an average of 100 sites each,
plus 10 prefixes per site on average. Consider that the PE also
needs to be able to route traffic to the Internet in general. In
this example the PE might need to support approximately 1,000,000
prefixes for the VPNs, plus more than 100,000 prefixes for the
Internet. If augmented and aggregated routing is used, then this
implies a large number of routes which may be advertised in a single
routing protocol (most likely BGP). If the VR approach is used, then
there are also 100,000 neighbor adjacencies in the various per-VPN
routing protocol instances. In some cases this number of routing
prefixes and/or this number of adjacencies might be difficult to
support in one device.
In this case, an alternate approach is to limit the PE's
participation in Internet routing to the absolute minimum required:
Specifically the PE will need to know which Internet address prefixes
are reachable via directly attached CE devices. All other Internet
routes may be summarized into a single default route pointing to one
or more P routers. In many cases the P routers to which the default
routes are directed may be the P routers to which the PE device is
directly attached (which are the ones which it needs to use for
forwarding most Internet traffic). Thus if there are M CE devices
directly connected to the PE, and if these M CE devices are the next
hop for a total of N globally addressable Internet address prefixes,
then the PE device would maintain N+1 routes corresponding to
globally routable Internet addresses.
In this example, those PE devices which provide VPN service run
routing to compute routes for the VPNs, but don't operate Internet
routing, and instead use only a default route to route traffic to all
Internet destinations (not counting the addresses which are reachable
via directly attached CE devices). The P routers need to maintain
Internet routes, and therefore take part in Internet routing
protocols. However, the P routers don't know anything about the VPN
routes.
In some cases the maximum number of routes and/or routing instances
supportable via a single PE device may limit the number of VPNs which
can be supported by that PE. For example, in some cases this might
require that two different PE devices be used to support VPN services
for a set of multiple CEs, even if one PE might have had sufficient
throughput to handle the data traffic from the full set of CEs.
Similarly, the amount of resources which any one VPN is permitted to
use in a single PE might be restricted.
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There will be cases where it is not necessary to partition the
routing, since the PEs will be able to maintain all VPN routes and
all Internet routes without a problem. However, it is important that
VPN approaches allow partitioning to be used where needed in order to
prevent future scaling problems. Again, making the system scalable
is a matter of proper deployment.
It may be wondered whether it is ever desirable to have both Internet
routing and VPN routing running in a single PE device or route
reflector. In fact, if there is even a single system running both
Internet routing and VPN routing, doesn't that raise the possibility
that a disruption within the VPN routing system will cause a
disruption within the Internet routing system?
Certainly this possibility exists in theory. To minimize that
possibility, BGP implementations which support multiple address
families should be organized so as to minimize the degree to which
the processing and distribution of one address family affects the
processing and distribution of another. This could be done, for
example, by suitable partitioning of resources. This partitioning
may be helpful both to protect Internet routing from VPN routing, and
to protect well behaved VPN customers from "mis-behaving" VPNs. Or
one could try to protect the Internet routing system from the VPN
routing system by giving preference to the Internet routing. Such
implementation issues are outside the scope of this document. If one
has inadequate confidence in an implementation, deployment procedures
can be used, as explained above, to separate the Internet routing
from the VPN routing.
4.5. Quality of Service, SLAs, and IP Differentiated Services
The following technologies for QoS/SLA may be applicable to PPVPNs.
4.5.1. IntServ/RSVP [RFC2205] [RFC2208] [RFC2210] [RFC2211] [RFC2212]
Integrated services, or IntServ for short, is a mechanism for
providing QoS/SLA by admission control. RSVP is used to reserve
network resources. The network needs to maintain a state for each
reservation. The number of states in the network increases in
proportion to the number of concurrent reservations.
In some cases, IntServ on the edge of a network (e.g., over the
customer interface) may be mapped to DiffServ in the SP network.
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4.5.2. DiffServ [RFC2474] [RFC2475]
IP differentiated service, or DiffServ for short, is a mechanism for
providing QoS/SLA by differentiating traffic. Traffic entering a
network is classified into several behavior aggregates at the network
edge and each is assigned a corresponding DiffServ codepoint. Within
the network, traffic is treated according to its DiffServ codepoint.
Some behavior aggregates have already been defined. Expedited
forwarding behavior [RFC3246] guarantees the QoS, whereas assured
forwarding behavior [RFC2597] differentiates traffic packet
precedence values.
When DiffServ is used, network provisioning is done on a
per-traffic-class basis. This ensures a specific class of service
can be achieved for a class (assuming that the traffic load is
controlled). All packets within a class are then treated equally
within an SP network. Policing is done at input to prevent any one
user from exceeding their allocation and therefore defeating the
provisioning for the class as a whole. If a user exceeds their
traffic contract, then the excess packets may optionally be
discarded, or may be marked as "over contract". Routers throughout
the network can then preferentially discard over contract packets in
response to congestion, in order to ensure that such packets do not
defeat the service guarantees intended for in contract traffic.
4.6. Concurrent Access to VPNs and the Internet
In some scenarios, customers will need to concurrently have access to
their VPN network and to the public Internet.
Two potential problems are identified in this scenario: the use of
private addresses and the potential security threads.
o The use of private addresses
The IP addresses used in the customer's sites will possibly belong
to a private routing realm, and as such be unusable in the public
Internet. This means that a network address translation function
(e.g., NAT) will need to be implemented to allow VPN customers to
access the Public Internet.
In the case of layer 3 PE-based VPNs, this translation function
will be implemented in the PE to which the CE device is connected.
In the case of layer 3 provider-provisioned CE-based VPNs, this
translation function will be implemented on the CE device itself.
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o Potential security threat
As portions of the traffic that flow to and from the public
Internet are not necessarily under the SP's nor the customer's
control, some traffic analyzing function (e.g., a firewall
function) will be implemented to control the traffic entering and
leaving the VPN.
In the case of layer 3 PE-based VPNs, this traffic analyzing
function will be implemented in the PE device (or in the VFI
supporting a specific VPN), while in the case of layer 3 provider
provisioned CE-based VPNs, this function will be implemented in the
CE device.
o Handling of a customer IP packet destined for the Internet
In the case of layer 3 PE-based VPNs, an IP packet coming from a
customer site will be handled in the corresponding VFI. If the IP
destination address in the packet's IP header belongs to the
Internet, multiple scenarios are possible, based on the adapted
policy. As a first possibility, when Internet access is not
allowed, the packet will be dropped. As a second possibility, when
(controlled) Internet access is allowed, the IP packet will go
through the translation function and eventually through the traffic
analyzing function before further processing in the PE's global
Internet forwarding table.
Note that different implementation choices are possible. One can
choose to implement the translation and/or the traffic analyzing
function in every VFI (or CE device in the context of layer 3
provider-provisioned CE-based VPNs), or alternatively in a subset or
even in only one VPN network element. This would mean that the
traffic to/from the Internet from/to any VPN site needs to be routed
through that single network element (this is what happens in a hub
and spoke topology for example).
4.7. Network and Customer Management of VPNs
4.7.1. Network and Customer Management
Network and customer management systems responsible for managing VPN
networks have several challenges depending on the type of VPN network
or networks they are required to manage.
For any type of provider-provisioned VPN it is useful to have one
place where the VPN can be viewed and optionally managed as a whole.
The NMS may therefore be a place where the collective instances of a
VPN are brought together into a cohesive picture to form a VPN. To
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be more precise, the instances of a VPN on their own do not form the
VPN; rather, the collection of disparate VPN sites together forms the
VPN. This is important because VPNs are typically configured at the
edges of the network (i.e., PEs) either through manual configuration
or auto-configuration. This results in no state information being
kept in within the "core" of the network. Sometimes little or no
information about other PEs is configured at any particular PE.
Support of any one VPN may span a wide range of network equipment,
potentially including equipment from multiple implementors. Allowing
a unified network management view of the VPN therefore is simplified
through use of standard management interfaces and models. This will
also facilitate customer self-managed (monitored) network devices or
systems.
In cases where significant configuration is required whenever a new
service is provisioned, it is important for scalability reasons that
the NMS provide a largely automated mechanism for this operation.
Manual configuration of VPN services (i.e., new sites, or
re-provisioning existing ones), could lead to scalability issues, and
should be avoided. It is thus important for network operators to
maintain visibility of the complete picture of the VPN through the
NMS system. This must be achieved using standard protocols such as
SNMP, XML, or LDAP. Use of proprietary command-line interfaces has
the disadvantage that proprietary interfaces do not lend themselves
to standard representations of managed objects.
To achieve the goals outlined above for network and customer
management, device implementors should employ standard management
interfaces to expose the information required to manage VPNs. To
this end, devices should utilize standards-based mechanisms such as
SNMP, XML, or LDAP to achieve this goal.
4.7.2. Segregated Access of VPN Information
Segregated access of VPNs information is important in that customers
sometimes require access to information in several ways. First, it
is important for some customers (or operators) to access PEs, CEs or
P devices within the context of a particular VPN on a per-VPN-basis
in order to access statistics, configuration or status information.
This can either be under the guise of general management,
operator-initiated provisioning, or SLA verification (SP, customer or
operator).
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Where users outside of the SP have access to information from PE or P
devices, managed objects within the managed devices must be
accessible on a per-VPN basis in order to provide the customer, the
SP or the third party SLA verification agent with a high degree of
security and convenience.
Security may require authentication or encryption of network
management commands and information. Information hiding may use
encryption or may isolate information through a mechanism that
provides per-VPN access. Authentication or encryption of both
requests and responses for managed objects within a device may be
employed. Examples of how this can be achieved include IPsec
tunnels, SNMPv3 encryption for SNMP-based management, or encrypted
telnet sessions for CLI-based management.
In the case of information isolation, any one customer should only be
able to view information pertaining to its own VPN or VPNs.
Information isolation can also be used to partition the space of
managed objects on a device in such a way as to make it more
convenient for the SP to manage the device. In certain deployments,
it is also important for the SP to have access to information
pertaining to all VPNs, thus it may be important for the SP to create
virtual VPNs within the management domain which overlap across
existing VPNs.
If the user is allowed to change the configuration of their VPN, then
in some cases customers may make unanticipated changes or even
mistakes, thereby causing their VPN to mis-behave. This in turn may
require an audit trail to allow determination of what went wrong and
some way to inform the carrier of the cause.
The segregation and security access of information on a per-VPN basis
is also important when the carrier of carrier's paradigm is employed.
In this case it may be desirable for customers (i.e., sub-carriers or
VPN wholesalers) to manage and provision services within their VPNs
on their respective devices in order to reduce the management
overhead cost to the carrier of carrier's SP. In this case, it is
important to observe the guidelines detailed above with regard to
information hiding, isolation and encryption. It should be noted
that there may be many flavors of information hiding and isolation
employed by the carrier of carrier's SP. If the carrier of carriers
SP does not want to grant the sub-carrier open access to all of the
managed objects within their PEs or P routers, it is necessary for
devices to provide network operators with secure and scalable per-VPN
network management access to their devices. For the reasons outlined
above, it therefore is desirable to provide standard mechanisms for
achieving these goals.
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5. Interworking Interface
This section describes interworking between different layer 3 VPN
approaches. This may occur either within a single SP network, or at
an interface between SP networks.
5.1. Interworking Function
Figure 2.5 (see section 2.1.3) illustrates a case where one or more
PE devices sits at the logical interface between two different layer
3 VPN approaches. With this approach the interworking function
occurs at a PE device which participates in two or more layer 3 VPN
approaches. This might be physically located at the boundary between
service providers, or might occur at the logical interface between
different approaches within a service provider.
With layer 3 VPNs, the PE devices are in general layer 3 routers, and
are able to forward layer 3 packets on behalf of one or more private
networks. For example, it may be common for a PE device supporting
layer 3 VPNs to contain multiple logical VFIs (sections 1, 2, 3.3.1,
4.4.2) each of which supports forwarding and routing for a private
network.
The PE which implements an interworking function needs to participate
in the normal manner in the operation of multiple approaches for
supporting layer 3 VPNs. This involves the functions discussed
elsewhere in this document, such as VPN establishment and
maintenance, VPN tunneling, routing for the VPNs, and QoS
maintenance.
VPN establishment and maintenance information, as well as VPN routing
information will need to be passed between VPN approaches. This
might involve passing of information between approaches as part of
the interworking function. Optionally this might involve manual
configuration so that, for example, all of the participants in the
VPN on one side of the interworking function considers the PE
performing the interworking function to be the point to use to
contact a large number of systems (comprising all systems supported
by the VPN located on the other side of the interworking function).
5.2. Interworking Interface
Figure 2.6 (see section 2.1.3) illustrates a case where interworking
is performed by use of tunnels between PE devices. In this case each
PE device participates in the operation of one layer 3 VPN approach.
Interworking between approaches makes use of per-VPN tunnels set up
between PE. Each PEs operates as if it is a normal PEs, and
considers each tunnel to be associated with a particular VPN.
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Information can then be transmitted over the interworking interface
in the same manner that it is transmitted over a CE to PE interface.
In some cases establishment of the interworking interfaces may
require manual configuration, for example to allow each PE to
determine which tunnels should be set up, and which private network
is associated with each tunnel.
5.2.1. Tunnels at the Interworking Interface
In order to implement an interworking interface between two SP
networks for supporting one or more PPVPN spanning both SP networks,
a mechanism for exchanging customer data as well as associated
control data (e.g., routing data) should be provided.
Since PEs of SP networks to be interworked may only communicate over
a network cloud, an appropriate tunnel established through the
network cloud will be used for exchanging data associated with the
PPVPN realized by interworked SP networks.
In this way, each interworking tunnel is assigned to an associated
layer 3 PE-based VPN; in other words, a tunnel is terminated by a VFI
(associated with the PPVPN) in a PE device. This scenario results in
implementation of traffic isolation for PPVPNs supported by an
Interworking Interface and spanning multiple SP networks (in each SP
network, there is no restriction in applied technology for providing
PPVPN so that both sides may adopt different technologies). The way
of the assignment of each tunnel for a PE-based VPN is specific to
implementation technology used by the SP network that is
inter-connected to the tunnel at the PE device.
The identifier of layer 3 PE-based VPN at each end is meaningful only
in the context of the specific technology of an SP network and need
not be understood by another SP network interworking through the
tunnel.
The following tunneling mechanisms may be used at the interworking
interface. Available tunneling mechanisms include (but are not
limited to): GRE, IP-in-IP, IP over ATM, IP over FR, IPsec, and MPLS.
o GRE
The tunnels at interworking interface may be provided by GRE
[RFC2784] with key and sequence number extensions [RFC2890].
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o IP-in-IP
The tunnels at interworking interface may be provided by IP-in-IP
[RFC2003] [RFC2473].
o IP over ATM AAL5
The tunnels at interworking interface may be provided by IP over
ATM AAL5 [RFC2684] [RFC2685].
o IP over FR
The tunnels at interworking interface may be provided by IP over
FR.
o IPsec
The tunnels at interworking interface may be provided by IPsec
[RFC2401] [RFC2402].
o MPLS
The tunnels at interworking interface may be provided by MPLS
[RFC3031] [RFC3035].
5.3. Support of Additional Services
This subsection describes additional usages for supporting QoS/SLA,
customer visible routing, and customer visible multicast routing, as
services of layer 3 PE-based VPNs spanning multiple SP networks.
o QoS/SLA
QoS/SLA management mechanisms for GRE, IP-in-IP, IPsec, and MPLS
tunnels were discussed in sections 4.3.6 and 4.5. See these
sections for details. FR and ATM are capable of QoS guarantee.
Thus, QoS/SLA may also be supported at the interworking interface.
o Customer visible routing
As described in section 3.3, customer visible routing enables the
exchange of unicast routing information between customer sites
using a routing protocol such as OSPF, IS-IS, RIP, and BGP-4. On
the interworking interface, routing packets, such as OSPF packets,
are transmitted through a tunnel associated with a layer 3 PE-based
VPN in the same manner as that for user data packets within the
VPN.
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o Customer visible multicast routing
Customer visible multicast routing enables the exchange of
multicast routing information between customer sites using a
routing protocol such as DVMRP and PIM. On the interworking
interface, multicast routing packets are transmitted through a
tunnel associated with a layer 3 PE-based VPN in the same manner as
that for user data packets within the VPN. This enables a
multicast tree construction within the layer 3 PE-based VPN.
5.4. Scalability Discussion
This subsection discusses scalability aspect of the interworking
scenario.
o Number of routing protocol instances
In the interworking scenario discussed in this section, the number
of routing protocol instances and that of layer 3 PE-based VPNs are
the same. However, the number of layer 3 PE-based VPNs in a PE
device is limited due to resource amount and performance of the PE
device. Furthermore, each tunnel is expected to require some
bandwidth, but total of the bandwidth is limited by the capacity of
a PE device; thus, the number of the tunnels is limited by the
capabilities of the PE. This limit is not a critical drawback.
o Performance of packet transmission
The interworking scenario discussed in this section does not place
any additional burden on tunneling technologies used at
interworking interface. Since performance of packet transmission
depends on a tunneling technology applied, it should be carefully
selected when provisioning interworking. For example, IPsec places
computational requirements for encryption/decryption.
6. Security Considerations
Security is one of the key requirements concerning VPNs. In network
environments, the term security currently covers many different
aspects of which the most important from a networking perspective are
shortly discussed hereafter.
Note that the Provider-Provisioned VPN requirements document explains
the different security requirements for Provider-Provisioned VPNs in
more detail.
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6.1. System Security
Like in every network environment, system security is the most
important security aspect that must be enforced. Care must be taken
that no unauthorized party can gain access to the network elements
that control the VPN functionality (e.g., PE and CE devices).
As the VPN customers are making use of the shared SP's backbone, the
SP must ensure the system security of its network elements and
management systems.
6.2. Access Control
When a network or parts of a network are private, one of the
requirements is that access to that network (part) must be restricted
to a limited number of well-defined customers. To accomplish this
requirement, the responsible authority must control every possible
access to the network.
In the context of PE-based VPNs, the access points to a VPN must be
limited to the interfaces that are known by the SP.
6.3. Endpoint Authentication
When one receives data from a certain entity, one would like to be
sure of the identity of the sending party. One would like to be sure
that the sending entity is indeed whom he or she claims to be, and
that the sending entity is authorized to reach a particular
destination.
In the context of layer 3 PE-based VPNs, both the data received by
the PEs from the customer sites via the SP network and destined for a
customer site should be authenticated.
Note that different methods for authentication exist. In certain
circumstances, identifying incoming packets with specific customer
interfaces might be sufficient. In other circumstances, (e.g., in
temporary access (dial-in) scenarios), a preliminary authentication
phase might be requested. For example, when PPP is used. Or
alternatively, an authentication process might need to be present in
every data packet transmitted (e.g., in remote access via IPsec).
For layer 3 PE-based VPNs, VPN traffic is tunneled from PE to PE and
the VPN tunnel endpoint will check the origin of the transmitted
packet. When MPLS is used for VPN tunneling, the tunnel endpoint
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checks whether the correct labels are used. When IPsec is used for
VPN tunneling, the tunnel endpoint can make use of the IPsec
authentication mechanisms.
In the context of layer 3 provider-provisioned CE-based VPNs, the
endpoint authentication is enforced by the CE devices.
6.4. Data Integrity
When information is exchanged over a certain part of a network, one
would like to be sure that the information that is received by the
receiving party of the exchange is identical to the information that
was sent by the sending party of the exchange.
In the context of layer 3 PE-based VPNs, the SP assures the data
integrity by ensuring the system security of every network element.
Alternatively, explicit mechanisms may be implemented in the used
tunneling technique (e.g., IPsec).
In the context of layer 3 provider-provisioned CE-based VPNs, the
underlying network that will tunnel the encapsulated packets will not
always be of a trusted nature, and the CE devices that are
responsible for the tunneling will also ensure the data integrity,
e.g., by making use of the IPsec architecture.
6.5. Confidentiality
One would like that the information that is being sent from one party
to another is not received and not readable by other parties. With
traffic flow confidentiality one would like that even the
characteristics of the information sent is hidden from third parties.
Data privacy is the confidentiality of the user data.
In the context of PPVPNs, confidentiality is often seen as the basic
service offered, as the functionalities of a private network are
offered over a shared infrastructure.
In the context of layer 3 PE-based VPNs, as the SP network (and more
precisely the PE devices) participates in the routing and forwarding
of the customer VPN data, it is the SP's responsibility to ensure
confidentiality. The technique used in PE-based VPN solutions is the
ensuring of PE to PE data separation. By implementing VFI's in the
PE devices and by tunneling VPN packets through the shared network
infrastructure between PE devices, the VPN data is always kept in a
separate context and thus separated from the other data.
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In some situations, this data separation might not be sufficient.
Circumstances where the VPN tunnel traverses other than only trusted
and SP controlled network parts require stronger confidentiality
measures such as cryptographic data encryption. This is the case in
certain inter-SP VPN scenarios or when the considered SP is on itself
a client of a third party network provider.
For layer 3 provider-provisioned CE-based VPNs, the SP network does
not bare responsibility for confidentiality assurance, as the SP just
offers IP connectivity. The confidentiality will then be enforced at
the CE and will lie in the tunneling (data separation) or in the
cryptographic encryption (e.g., using IPsec) by the CE device.
Note that for very sensitive user data (e.g., used in banking
operations) the VPN customer may not outsource his data privacy
enforcement to a trusted SP. In those situations, PE-to-PE
confidentiality will not be sufficient and end-to-end cryptographic
encryption will be implemented by the VPN customer on its own private
equipment (e.g., using CE-based VPN technologies or cryptographic
encryption over the provided VPN connectivity).
6.6. User Data and Control Data
An important remark is the fact that both the user data and the VPN
control data must be protected.
Previous subsections were focused on the protection of the user data,
but all the control data (e.g., used to set up the VPN tunnels, used
to configure the VFI's or the CE devices (in the context of layer 3
provider-provisioned CE-based VPNs)) will also be secured by the SP
to prevent deliberate misconfiguration of provider-provisioned VPNs.
6.7. Security Considerations for Inter-SP VPNs
In certain scenarios, a single VPN will need to cross multiple SPs.
The fact that the edge-to-edge part of the data path does not fall
under the control of the same entity can have security implications,
for example with regards to endpoint authentication.
Another point is that the SPs involved must closely interact to avoid
conflicting configuration information on VPN network elements (such
as VFIs, PEs, CE devices) connected to the different SPs.
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Appendix A: Optimizations for Tunnel Forwarding
A.1. Header Lookups in the VFIs
If layer 3 PE-based VPNs are implemented in the most straightforward
manner, then it may be necessary for PE devices to perform multiple
header lookups in order to forward a single data packet. This
section discusses an example of how multiple lookups might be needed
with the most straightforward implementation. Optimizations which
might optionally be used to reduce the number of lookups are
discussed in the following sections.
As an example, in many cases a tunnel may be set up between VFIs
within PEs for support of a given VPN. When a packet arrives at the
egress PE, the PE may need to do a lookup on the outer header to
determine which VFI the packet belongs to. The PE may then need to
do a second lookup on the packet that was encapsulated across the VPN
tunnel, using the forwarding table specific to that VPN, before
forwarding the packet.
For scaling reasons it may be desired in some cases to set up VPN
tunnels, and then multiplex multiple VPN-specific tunnels within the
VPN tunnels.
This implies that in the most straightforward implementation three
header lookups might be necessary in a single PE device: One lookup
may identify that this is the end of the VPN tunnel (implying the
need to strip off the associated header). A second lookup may
identify that this is the end of the VPN-specific tunnel. This
lookup will result in stripping off the second encapsulating header,
and will identify the VFI context for the final lookup. The last
lookup will make use of the IP address space associated with the VPN,
and will result in the packet being forwarded to the correct CE
within the correct VPN.
A.2. Penultimate Hop Popping for MPLS
Penultimate hop popping is an optimization which is described in the
MPLS architecture document [RFC3031].
Consider the egress node of any MPLS LSP. The node looks at the
label, and discovers that it is the last node. It then strips off
the label header, and looks at the next header in the packet (which
may be an IP header, or which may have another MPLS header in the
case that hierarchical nesting of LSPs is used). For the last node
on the LSP, the outer MPLS header doesn't actually convey any useful
information (except for one situation discussed below).
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For this reason, the MPLS standards allow the egress node to request
that the penultimate node strip the MPLS header. If requested, this
implies that the penultimate node does not have a valid label for the
LSP, and must strip the MPLS header. In this case, the egress node
receives the packet with the corresponding MPLS header already
stripped, and can forward the packet properly without needing to
strip the header for the LSP which ends at that egress node.
There is one case in which the MPLS header conveys useful
information: This is in the case of a VPN-specific LSP terminating at
a PE device. In this case, the value of the label tells the PE which
LSP the packet is arriving on, which in turn is used to determine
which VFI is used for the packet (i.e., which VPN-specific forwarding
table needs to be used to forward the packet).
However, consider the case where multiple VPN-specific LSPs are
multiplexed inside one PE-to-PE LSP. Also, let's suppose that in
this case the egress PE has chosen all incoming labels (for all LSPs)
to be unique in the context of that PE. This implies that the label
associated with the PE-to-PE LSP is not needed by the egress node.
Rather, it can determine which VFI to use based on the VPN-specific
LSP. In this case, the egress PE can request that the penultimate
LSR performs penultimate label popping for the PE-to-PE LSP. This
eliminates one header lookup in the egress LSR.
Note that penultimate node label popping is only applicable for VPN
standards which use multiple levels of LSPs. Even in this case
penultimate node label popping is only done when the egress node
specifically requests it from the penultimate node.
A.3. Demultiplexing to Eliminate the Tunnel Egress VFI Lookup
Consider a VPN standard which makes use of MPLS as the tunneling
mechanism. Any standard for encapsulating VPN traffic inside LSPs
needs to specify what degree of granularity is available in terms of
the manner in which user data traffic is assigned to LSPs. In other
words, for any given LSP, the ingress or egress PE device needs to
know which LSPs need to be set up, and the ingress PE needs to know
which set of VPN packets are allowed to be mapped to any particular
LSP.
Suppose that a VPN standard allows some flexibility in terms of the
mapping of packets to LSPs, and suppose that the standard allows the
egress node to determine the granularity. In this case the egress
node would need to have some way to indicate the granularity to the
ingress node, so that the ingress node will know which packets can be
mapped to each LSP.
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In this case, the egress node might decide to have packets mapped to
LSPs in a manner which simplifies the header lookup function at the
egress node. For example, the egress node could determine which set
of packets it will forward to a particular neighbor CE device. The
egress node can then specify that the set of IP packets which are to
use a particular LSP correspond to that specific set of packets. For
packets which arrive on the specified LSP, the egress node does not
need to do a header lookup on the VPN's customer address space: It
can just pop the MPLS header and forward the packet to the
appropriate CE device. If all LSPs are set up accordingly, then the
egress node does not need to do any lookup for VPN traffic which
arrives on LSPs from other PEs (in other words, the PE device will
not need to do a second lookup in its role as an egress node).
Note that PE devices will most likely also be an ingress routers for
traffic going in the other direction. The PE device will need to do
an address lookup in the customer network's address space in its role
as an ingress node. However, in this direction the PE still needs to
do only a single header lookup.
When used with MPLS tunnels, this optional optimization reduces the
need for header lookups, at the cost of possibly increasing the
number of label values which need to be assigned (since one label
would need to be assigned for each next-hop CE device, rather than
just one label for every VFI).
The same approach is also possible when other encapsulations are
used, such as GRE [RFC2784] [RFC2890], IP-in-IP [RFC2003] [RFC2473],
or IPsec [RFC2401] [RFC2402]. This requires that distinct values are
used for the multiplexing field in the tunneling protocol. See
section 4.3.2 for detail.
Acknowledgments
This document is output of the framework document design team of the
PPVPN WG. The members of the design team are listed in the
"contributors" and "author's addresses" sections below.
However, sources of this document are based on various inputs from
colleagues of authors and contributors. We would like to thank
Junichi Sumimoto, Kosei Suzuki, Hiroshi Kurakami, Takafumi Hamano,
Naoto Makinae, Kenichi Kitami, Rajesh Balay, Anoop Ghanwani, Harpreet
Chadha, Samir Jain, Lianghwa Jou, Vijay Srinivasan, and Abbie
Matthews.
We would also like to thank Yakov Rekhter, Scott Bradner, Dave
McDysan, Marco Carugi, Pascal Menezes, Thomas Nadeau, and Alex Zinin
for their valuable comments and suggestions.
Callon & Suzuki Informational [Page 75]
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Normative References
[PPVPN-REQ] Nagarajan, A., Ed., "Generic Requirements for Provider
Provisioned Virtual Private Networks (PPVPN)", RFC
3809, June 2004.
[L3VPN-REQ] Carugi, M., Ed. and D. McDysan, Ed., "Service
Requirements for Layer 3 Provider Provisioned Virtual
Private Networks (PPVPNs)", RFC 4031, April 2005.
Informative References
[BGP-COM] Sangli, S., et al., "BGP Extended Communities
Attribute", Work In Progress, February 2005.
[MPLS-DIFF-TE] Le Faucheur, F., Ed., "Protocol extensions for support
of Differentiated-Service-aware MPLS Traffic
Engineering", Work In Progress, December 2004.
[VPN-2547BIS] Rosen, E., et al., "BGP/MPLS VPNs", Work In Progress.
[VPN-BGP-OSPF] Rosen, E., et al., "OSPF as the Provider/Customer Edge
Protocol for BGP/MPLS IP VPNs", Work In Progress, May
2005.
[VPN-CE] De Clercq, J., et al., "An Architecture for Provider
Provisioned CE-based Virtual Private Networks using
IPsec", Work In Progress.
[VPN-DISC] Ould-Brahim, H., et al., "Using BGP as an Auto-
Discovery Mechanism for Layer-3 and Layer-2 VPNs,"
Work In Progress.
[VPN-L2] Andersson, L. and E. Rosen, Eds., "Framework for Layer
2 Virtual Private Networks (L2VPNs)", Work In
Progress.
[VPN-VR] Knight, P., et al., "Network based IP VPN Architecture
Using Virtual Routers", Work In Progress, July 2002.
[RFC1195] Callon, R., "Use of OSI IS-IS for Routing in TCP/IP
and Dual Environments", RFC 1195, December 1990.
Callon & Suzuki Informational [Page 76]
RFC 4110 A Framework for L3 PPVPNs July 2005
[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot,
G., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, February 1996.
[RFC1966] Bates, T., "BGP Route Reflection: An alternative to
full mesh IBGP", RFC 1966, June 1996.
[RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities
Attribute", RFC 1997, February 2001.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) --
Version 1 Functional Specification", RFC 2205,
September 1997.
[RFC2208] Mankin, A., Ed., Baker, F., Braden, B., Bradner, S.,
O'Dell, M., Romanow, A., Weinrib, A., and L. Zhang,
"Resource ReSerVation Protocol (RSVP) Version 1
Applicability Statement Some Guidelines on
Deployment", RFC 2208, September 1997.
[RFC2210] Wroclawski, J., "The Use of RSVP with IETF Integrated
Services", RFC 2210, September 1997.
[RFC2211] Wroclawski, J., "Specification of the Controlled-Load
Network Element Service", RFC 2211, September 1997.
[RFC2212] Shenker, S., Partridge, C., and R. Guerin,
"Specification of Guaranteed Quality of Service", RFC
2212, September 1997.
[RFC2207] Berger, L. and T. O'Malley, "RSVP Extensions for IPSEC
Data Flows", RFC 2207, September 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April
1998.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for
the Internet Protocol", RFC 2401, November 1998.
[RFC2402] Kent, S. and R. Atkinson, "IP Authentication Header",
RFC 2402, November 1998.
Callon & Suzuki Informational [Page 77]
RFC 4110 A Framework for L3 PPVPNs July 2005
[RFC2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2453] Malkin, G., "RIP Version 2", STD 56, RFC 2453,
November 1994.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, December 1998.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang,
Z., and W. Weiss, "An architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597, June 1999.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G.,
Zorn, G., and B. Palter, "Layer Two Tunneling Protocol
'L2TP'", RFC 2661, August 1999.
[RFC2684] Grossman, D. and J. Heinanen, "Multiprotocol
Encapsulation Over ATM Adaptation Layer 5", RFC 2684,
September 1999.
[RFC2685] Fox B. and B. Gleeson, "Virtual Private Networks
Identifier," RFC 2685, September 1999.
[RFC2746] Terzis, A., Krawczyk, J., Wroclawski, J., and L.
Zhang, "RSVP Operation Over IP Tunnels", RFC 2746,
January 2000.
[RFC2764] Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and
A. Malis, "A Framework for IP Based Virtual Private
Networks", RFC 2764, February 2000.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC
2784, March 2000.
Callon & Suzuki Informational [Page 78]
RFC 4110 A Framework for L3 PPVPNs July 2005
[RFC2890] Dommety, G., "Key and Sequence Number Extensions to
GRE", RFC 2890, September 2000.
[RFC2858] Bates, T., Rekhter, Y., Chandra, R., and D. Katz,
"Multiprotocol Extensions for BGP-4", RFC 2858, June
2000.
[RFC2983] Black, D., "Differentiated Services and Tunnels", RFC
2983, October 2000.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon,
"Multiprotocol Label Switching Architecture", RFC
3031, January 2001.
[RFC3032] Rosen E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC3035] Davie, B., Lawrence, J., McCloghrie, K., Rosen, E.,
Swallow, G., Rekhter, Y., and P. Doolan, "MPLS using
LDP and ATM VC Switching", RFC 3035, January 2001.
[RFC3065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 3065, June 1996.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
V., and G. Swallow, "RSVP-TE: Extensions to RSVP for
LSP Tunnels", RFC 3209, December 2001.
[RFC3246] Davie, B., Charny, A., Bennet, J.C.R., Benson, K., Le
Boudec, J.Y., Courtney, W., Davari, S., Firoiu, V.,
and D. Stiliadis, "An Expedited Forwarding PHB (Per-
Hop Behavior)", RFC 3246, March 2002.
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S.,
Vaananen, P., Krishnan, R., Cheval, P., and J.
Heinanen, "Multi-Protocol Label Switching (MPLS)
Support of Differentiated Services", RFC 3270, May
2002.
[RFC3377] Hodges, J. and R. Morgan, "Lightweight Directory
Access Protocol (v3): Technical Specification", RFC
3377, September 2002.
Callon & Suzuki Informational [Page 79]
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Contributors' Addresses
Jeremy De Clercq
Alcatel
Fr. Wellesplein 1,
2018 Antwerpen, Belgium
EMail: jeremy.de_clercq@alcatel.be
Bryan Gleeson
Nokia
313 Fairchild Drive,
Mountain View, CA 94043 USA.
EMail: bryan.gleeson@nokia.com
Andrew G. Malis
Tellabs
90 Rio Robles Drive
San Jose, CA 95134 USA
EMail: andy.malis@tellabs.com
Karthik Muthukrishnan
Lucent Technologies
1 Robbins Road
Westford, MA 01886, USA
EMail: mkarthik@lucent.com
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA, 01719, USA
EMail: erosen@cisco.com
Chandru Sargor
Redback Networks
300 Holger Way
San Jose, CA 95134, USA
EMail: apricot+l3vpn@redback.com
Callon & Suzuki Informational [Page 80]
RFC 4110 A Framework for L3 PPVPNs July 2005
Jieyun Jessica Yu
University of California, Irvine
5201 California Ave., Suite 150,
Irvine, CA, 92697 USA
EMail: jyy@uci.edu
Authors' Addresses
Ross Callon
Juniper Networks
10 Technology Park Drive
Westford, MA 01886-3146, USA
EMail: rcallon@juniper.net
Muneyoshi Suzuki
NTT Information Sharing Platform Labs.
3-9-11, Midori-cho,
Musashino-shi, Tokyo 180-8585, Japan
EMail: suzuki.muneyoshi@lab.ntt.co.jp
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Full Copyright Statement
Copyright (C) The Internet Society (2005).
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contained in BCP 78, and except as set forth therein, the authors
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Callon & Suzuki Informational [Page 82]
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