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Network Working Group                                   R. Callon (Ed.)
Internet Draft                                         Juniper Networks
Expires: August 2002                                    M. Suzuki (Ed.)
                                                        NTT Corporation
                                                           J. De Clercq
                                                                Alcatel
                                                             B. Gleeson
                                                             Consultant
                                                               A. Malis
                                                  Vivace Networks, Inc.
                                                       K. Muthukrishnan
                                                    Lucent Technologies
                                                             Eric Rosen
                                                          Cisco Systems
                                                         Chandru Sargor
                                                  CoSine Communications
                                                      Jieyun Jessica Yu

                                                      February 15, 2002


 A Framework for Layer 3 Provider Provisioned Virtual Private Networks
                  <draft-ietf-ppvpn-framework-04.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.







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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 which 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.


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.

   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 a VPN,
   and there are therefore multiple different types of PPVPNs.  The
   framework document discusses layer 3 VPNs (as defined in section
   1.3).  It also describes technical issues related to VPNs in which
   the provider participates in provisioning for provider edge and
   customer edge based devices.

   First, this document discusses reference models for layer 3 PPVPNs.
   Then technical aspects of layer 3 PPVPN operation from the customers
   point of view are discussed.  Next, technical aspects of layer 3
   PPVPNs from the providers point of view are also discussed.
   Specifically, this includes discussion of the technical issues which
   are important in the design of standards and mechanisms for support
   of layer 3 PPVPNs.  Furthermore, technical aspects of layer 3 PPVPNs
   interworking are clarified.  Finally, security issues as they apply
   to layer 3 PPVPNs are addressed.

   This document will take a "horizontal description" approach, in that
   it describes issues, technology, and the possible solutions to each
   problem.  We will therefore describe multiple possible solutions
   which may be used for each particular issue which arises in the
   design of VPN solutions.  This document does not make choices, and
   does not select any particular approach to support VPNs.



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   Other documents will be needed to take a "vertical description"
   approach, in that they will specify one or more complete protocols
   for support of VPNs.  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 the communication
   between a set of sites, making use of a shared network
   infrastructure.  Multiple sites of a private network may therefore
   communicate via the public infrastructure, in order to facilitate the
   operation of the private network.  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 some cases, one SP may offer VPN services to another SP.  This
   case is generally known as a "carrier of carrier" 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 support intranets, in which the multiple sites are under the
   control of a single customer administration, such as multiple sites
   of a single company.  VPNs may also support 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 a 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|----|  PE2  |       |  PE2  |      :          .
      .        |   |    |       |       |       |    +---+        .
      .   r4---|   |    |       |       |       |----|CE3|---r6   .
      .        +---+    +-------+       +-------+    +---+        .
      . Customer .          .    Service    .          . Customer .
      .  site 1  .          .  provider(s)  .          .  site 2  .
      ............          .................          ............

                Figure 1.1: VPN interconnecting two sites.


   In general Provider Edge (PE) and Customer Edge (CE) devices may be
   either routers, LSRs, or IP switches.  Some approaches may limit the
   type of PE and/or CE device that can be used.  For example, in some
   approaches the PE devices may be required to be routers.

   In this document, scope of the SP network is an IP or MPLS network.
   It is desired to interconnect the customer network sites via the SP
   network.

   In many cases, customer networks will make use of private IP
   addresses [RFC1918] or non-unique IP address (e.g., unregistered
   addresses).  This implies that there is no guarantee that the IP
   addresses used in the customer network are globally unique.  In the
   case that a single PE device provides services to multiple different
   customer networks, this implies that the addresses used in the
   different customer networks may overlap.  The internal operation of
   the PE device needs to maintain a level of isolation between the
   packets from different customer networks.  This also implies that the
   IP packets from the customer network cannot be transmitted in their
   native form across the SP network.  Instead, some form of
   encapsulation/tunneling must be used.

   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



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   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
   premise 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 much configuration burden to
   manually configure each CE device, and in particular to manually
   configure the links between CE devices.  Instead, the SP might decide
   to make use of 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/MPLS backbone).

   After a while the SP might decide that it is too much trouble to be
   managing a large number of CE-based devices, and might instead
   physically move these routers to be on the provider premise.  Each
   edge router at the provider premise 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 instead of MAN
   links).  The service provided by the SP has now become essentially a
   layer 3 VPN service, between dedicated provider edge routers.

   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]
   [VPN-2917BIS].  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



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   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
   use 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 of the possible 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 (ignoring management systems)
   knowledge of the customer network is limited to customer edge
   devices.  In a customer provisioned CE-based VPN, the SP is oblivious
   to the existence of the customer network.  The provider may be
   offering a simple IP service.  However, it is common for an SP to
   take on the task of managing and provisioning the CE devices, in
   order to reduce the management requirements of the customer.  This
   results in provider provisioned CE-based VPNs [VPN-CE].

   In CE-based VPNs, the customer network is supported by tunnels which
   are set up between CE devices.  The tunnels may make use of various
   encapsulations to send traffic over IP (such as MPLS, GRE, IPsec, IP-
   in-IP, or L2TP tunnels).

   For customer provisioned CE-based VPNs, provisioning and management
   of the tunnels is up to the customer network administration.
   Typically, this may make use of manual configuration of the tunnels.
   In this case the customers is also responsible for operation of the
   routing protocol between CE devices.  For provider provisioned CE-
   based VPNs, provisioning and management of the tunnels is up to 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.







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   For CE-based VPNs (whether customer or provider provisioned) routing
   in the customer network views the tunnels as simple point to point
   links, or in some cases as broadcast LANs.  (Note that discussion of
   customer provisioned CE-based VPNs is out of scope of the document).

   A PE-based VPN (or Provider Edge-based Virtual Private Network) is
   one in which PE devices in the SP network provide the VPN.  This
   allows the existence of the VPN to be hidden from the CE devices,
   which can operate as if part of a normal customer network.

   In PE-based VPNs, the customer network is supported by tunnels which
   are set up between PE devices.  The tunnels may make use of various
   encapsulations to sent traffic over the SP network (such as MPLS,
   GRE, IPsec, or IP-in-IP tunnels).

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

     Provider forwards packets based on layer 3 information, as well as
     on the basis of the incoming link.

   o Layer 2 service

     Provider forwards packets based on layer 2 (such as FR, ATM, or
     MAC) identifiers and/or on the basis of the incoming link.  (Note
     that discussion of layer 2 service is out of scope of the
     document).

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 device which is 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.




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   Layer 3 PE-based VPNs have the advantage that they offload some
   aspects of VPN management from the customer network.  Scaling of
   customer network routing might also be improved, since some layer 3
   PE-based VPN approaches avoid the need for on the order of "N
   squared" (actually N*(N-1)/2) point to point duplex links between N
   customer sites.  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).

   However, these advantages come along with other consequences.
   Specifically, the SP network has to know about the customer network.
   This adds work to the SP network, and limits the protocols which can
   be supported by the VPN.  Given that PE device needs to forward
   packets directly from the customer network, using the customer
   network's address space, this implies that PE device needs to
   participate in some manner in routing for as many customer networks
   as the PE device supports.  The protocols supported are limited to
   those which are understood by the PE device, which typically means
   only IP is supported.

   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
   connectivity between sites, including the use and placement of
   firewalls, based on administrative requirements [PPVPN-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



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   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 (ignoring management systems)
   knowledge of the customer network is limited to customer premise
   equipment.

   Customer: A single organization, corporation, or enterprise that
   administratively controls a set of sites.

   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
   might optionally also be an IP router.

   PE-Based VPNs: The customer network is supported by tunnels which are
   set up between PE devices.  The tunnels may make use of various
   encapsulations to sent traffic over the SP network (such as, but not
   restricted to, MPLS, GRE, IPsec, or IP-in-IP tunnels).

   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 and not the end customer.

   Route Reflectors: An SP-owned network element that is used to
   distribute BGP routes to the SP's BGP-enabled routers; usually used
   to allow a larger number of routers to receive BGP routes within an
   SP network than would otherwise be possible by using a full mesh of
   BGP-enabled routers.







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   Virtual Private Network (VPN): Private communication between a set of
   sites, making use of a shared network infrastructure.

   Virtual Router (VR): An instance of one of a number of logical
   routers located within a single physical router.  These logical
   routers have exactly the same mechanisms as a physical router, and
   therefore inherit all 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 Tunnels: A logical link between two PE or two CE entities which
   is created by encapsulating packets within an encapsulating header
   for purpose of transmission between those two entities to support
   VPNs.

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



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   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


2. Reference Models

   This section describes reference models for PPVPN that provides
   services as mentioned in section 1.  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.












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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

     A CE device is attached via an access connection to a PE device.
     It may be a router, LSR, IP switch, or host usually located at the
     edge of a customer site or colocated on an SP premises.

   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

     A PE device is attached via an access connection to one or more CE
     devices.  It may be a router, LSR, IP switch or other device that
     includes provider edge VPN functionality such as provisioning,
     management, and traffic classification and separation.  In the
     context of supporting layer 3 VPNs, a PE device implements one or
     more VFIs and maintains per-VPN state.  (Note that access
     connections are terminated by VFIs from the functional point of
     view.)







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   o Customer site

     A customer site is a set of users that have mutual IP reachability
     without use of a specific SP network.

   o SP networks

     An SP network is a network administered by a single service
     provider.  It is an IP or MPLS network.

   o Access connection

     An access connection represents an isolated layer 2 connectivity
     between a CE device and a PE device.  This includes dedicated
     physical circuits, logical circuits (such as FR, ATM, and MAC), and
     IP tunnels (e.g., using MPLS, IPsec, or L2TP).

   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 MPLS or L2TP [RFC2661]).

   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.  VFIs are typically interconnected via (per-)VPN tunnels.
     This is illustrated in Figure 2.2.

     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.,
     MPLS, GRE, IPsec, or IP-in-IP tunnel).  This is illustrated in
     Figure 2.3.  See section 4.3 for details.

   o VPN forwarding instance (VFI)

     A VFI is a logical entity that resides in a PE, that includes the
     router information base and forwarding information base for a VPN.
     A VFI enables router functions dedicated to serving a particular
     VPN.  In general a VFI terminates VPN tunnels for interconnection
     with other VFIs and also terminates access connections for
     accommodating CEs.  The interaction between routing and VFIs is
     discussed in section 4.4.2.




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   o Customer management function

     Customer management function administrates 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.

     Customer management function may use a combination of SNMP manager,
     directory service (e.g., LDAP [RFC1777] [RFC2251]), or proprietary
     network management system.

   o Network management function

     Network management function administrates devices attributes and
     their relationship, covering PE devices and other devices
     constructing the concerned PE-based VPN.

     Network management function may use a combination of SNMP manager,
     directory service (e.g., LDAP [RFC1777] [RFC2251]), or proprietary
     network management system.

2.1.2 Relationship between CE and PE

   A CE device is usually connected to a single PE device.  However,
   four types of double-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 differently implemented
   SP networks of layer 3 PE-based VPNs owned by one or more SPs co-
   exist, since it follows the expectation that (1) each SP chooses its
   best layer 3 PE-based VPN implementation out of multiple vendor's



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   implementations, and (2) an SP may deploy multiple networks of layer
   3 PE-based VPNs (e.g., an old network and a new network).  Thus layer
   3 PE-based VPNs spanning multiple differently implemented SP networks
   are required [PPVPN-REQ].

   There are two scenarios that enable layer 3 PE-based VPNs
   interworking among different approaches.

   o Interworking function

     This scenario enables interworking using a PE that is located at
     one or more points between SP networks implemented with different
     approaches and is supporting both 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  |      |
          |      |==============|      |==============|      |
          +------+              +------+              +------+
               |                  |  |                  |
               +------------------+  +------------------+
               |<-- SP Network -->|  |<-- SP Network -->|

                   Figure 2.5: Interworking function.


   o Interworking interface

     This scenario enables interworking using tunnels between PEs
     supported by different 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     +------------------+
       |<-- SP Network -->|      interface      |<-- SP Network -->|

                  Figure 2.6: Interworking interface.


2.3 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.

   Note that, in this model, a CE device maintains one or more VPN
   tunnels endpoints and a PE device has no VPN-specific functionality.
   Thus, there may be no interworking issues of layer 3 provider
   provisioned CE-based VPN spanning multiple SP networks.



















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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

     A CE device is attached via an access connection to a PE device.
     It provides layer 3 connectivity to the customer site and may be a
     router, LSR, IP switch, or host usually located at the edge of a
     customer site or colocated on an SP premises.  In the context of
     layer 3 provider provisioned CE-based VPNs, a CE device maintains
     one or more VPN tunnel endpoints.

   o P router (see section 2.1.1)

   o Provider edge (PE) device

     A PE device is attached via an access connection to one or more CE
     devices.  In the context of layer 3 provider provisioned CE-based
     VPNs, the PE device has no VPN-specific functionality.




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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, IPsec, IP-in-
     IP 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

     Network management function administrates device attributes and
     their relationship, covering PE and CE devices that define the VPN
     connectivity of the customer VPNs.

     Network management function may use a combination of SNMP manager,
     directory service (e.g., LDAP [RFC1777] [RFC2251]), 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.






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   VPN membership refers to the association of VPNs, CEs, and PEs.  A
   given CE belongs to one or more VPNs.  Each PE is therefore
   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 sites for
   new VPNs are added, or all sites for a VPN are removed.  Distributing
   VPN membership information thus refers to distributing information
   about which devices are associated with which VPNs.  This information
   may be used in different ways by different VPN schemes, for example,
   to constrain VPN route distribution or to establish VPN tunnels.

   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

   An example of a static binding between a permanent PE-CE access
   connection and the VPN associated with the access connection is where
   a network administrator sets up a dedicated link layer connection,
   such as an ATM VCC or a FR DLCI, between a PE and a CE device at the
   customer site.  In this case the binding between the PE-CE access
   connection and the VPN to be used is fixed at provisioning time, and
   remains the same until another provisioning action that changes the
   binding.

3.1.1.2 Dynamic binding

   It is also possible for the PE-CE access connection to VPN binding to
   be dynamic.  For example, a mobile user may dial up the provider
   network and carry out user authentication and VPN selection
   procedures.  Thus 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
   possible.  For example a user's HTTP traffic may be initially



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   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.  As
   such there must be a way to dynamically distribute membership
   information to all devices that need it.

3.1.2 Layer 3 provider provisioned CE-based VPN

   In the context of layer 3 provider provisioned CE-based VPNs, the PE
   devices have no knowledge of the establishment of VPNs at their
   customer interface.

   CE devices have IP/MPLS connectivity via a connection to a PE device.
   The IP connectivity may be via a static binding, or via some kind of
   dynamic binding.

   The establishment of the VPNs is done at the CE devices, making use
   of the IP/MPLS connectivity to the SP network.

   For the VPN establishment in the context of CE-based VPNs, it is
   necessary for the CE devices to know which other CE devices belong to
   a specific VPN (or at least one other CE device, depending on the VPN
   topology).  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.  Also, the data exchange may use MPLS to carry the IP
   packets, in which case the PE has provided the proper labels to the
   CE for each VPN, so that they IP packets may be properly labeled
   across the interface.




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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 in the SP's network or MPLS frames that can
   be label-switched across the SP network.  The PE device does not
   assign any VPN meaning to IP or MPLS packets on the access
   connection; all VPN meaning is confined to the CE devices.

3.3 Customer Visible Routing

   Once VPN tunnels are set up (between CE devices or between PE
   devices) it is necessary for the private customer network to make use
   of these tunnels.  It is therefore necessary for the customer network
   to know which addresses within the customer network are reachable
   over which tunnels.  This is a routing function.

3.3.1 Customer view of routing for layer 3 PE-based VPNs

   PE-CE routing interaction involves a PE obtaining customer prefixes
   reachable at its attached customer site via the local CE device and
   providing reachability information about other sites in the same VPN
   to the customer site.

   The possible PE-CE route distribution mechanisms are: static routing,
   IGP, such as RIP, OSPF, IS-IS, or BGP.  The extent of this routing
   instance generally involves the PE and CE devices.  However, if the
   customer site is running the same IGP as that used in its
   corresponding PE-PE routing instance, the domain may extend to the
   routing instance of the entire VPN.  If a different routing protocol
   runs in the customer site, the CE device redistributes the routes
   between the PE-CE routing instance and the customer site routing
   instance [VPN-BGP-OSPF] [VPN-BGP-MCAST].

   For layer 3 PE-based VPNs, the PE devices are routers that forward IP
   packets for the customer network.  This implies that a PE device
   participates in some manner in routing for each customer network that
   it supports.  Thus, each PE device looks to the customer network as
   if it were a single router in the customer network.  The access
   connections from CE device to PE device are normal links between
   within the customer network.  The routing information exchanged on
   the customer interface is routing information of the customer
   network.  The options for routing across the customer interface are
   therefore the same as those available on any link within a customer
   IP network.  However, the manner in which routing for the VPN is
   accomplished across the SP network may have an impact on the choice
   of routing for the customer network.  For example, if BGP or static
   routing is used across the customer interface, then the routing in
   the customer network will need to be aware of this.



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   In the case of layer 3 PE-based VPNs a single PE device is likely to
   provide service for multiple different VPNs, implying that it is
   interconnected with multiple CE devices, including multiple CEs with
   one VPN as well as multiple different VPNs.  The PE device must
   therefore support independent forwarding of user data for each VPN
   which it supports.  The forwarding table used for each VPN will in
   general be different.  This implies that the PE device will therefore
   need to maintain multiple separate forwarding instances.  These will
   be referred to as VPN Forwarding Instances (VFIs).  Each VFI is
   therefore 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.

   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 for small
   networks.  In this case, all routers within the customer network
   (including VFIs, for layer 3 PE-based VPNs) appear within a single
   area.  Links between routers also appear as normal links (including
   tunnels between VFIs).

   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



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   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
   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.

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.




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   The other options make a distinction between routing within a site,
   and routing between sites.  In this case, the CE devices would
   normally be considered as part of the site where they are 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, the PE devices (or more precisely the VFI within a PE
   device) may be considered to be internal to the same area or routing
   domain as the site to which they are attached.  This simplifies the
   management responsibilities of the customer network administration,
   since inter-area routing would be handled by the provider.

   For example, suppose that either static routes or BGP are used
   between sites.  With this approach each site could operate as a
   single IGP area, and the access connection would simply be configured
   as internal links within that area.  Static routes or BGP for inter-
   site routing can be handled by the provider.

   In other cases, from the perspective of routing within the customer
   network, the CE devices may be the "edge" 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 device and P
   router are not aware of the reachability within the customer sites.
   The CE and PE devices don't exchange customer's routing information.
   The routing in the customer VPN is transparent to the SP's network.

   This means no VPN routes need to be maintained in any of the SP's
   PE/P devices.

   Customer sites that belong to the same VPN may exchange routing
   information through the VPN tunnels that are seen as CE to CE
   interconnecting links from the customer's perspective.
   Alternatively, instead of exchanging routing information through the
   VPN tunnels, the SP's management system may take care of the
   distribution of the routing 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).






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   As the CE device receives an IP/MPLS service from the SP, the CE and
   PE devices may exchange global 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.

   Note that this does not impose the adoption of routing protocols
   between the PE and CE devices, as in lots of scenarios, the use of
   static/default routes might be sufficient.

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.

   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



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     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.

   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,



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     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
     [RFC1965].  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.


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 across the provider network, and to advertise
     reachable destinations to customer sites.

   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.

   Before data can be exchanged across a VPN, the sites involved in the
   VPN must learn of one another, acquire information on VPN membership,
   determine or know the type of VPN that will be set up, and finally
   invoke mechanisms that will establish the tunnels and disseminate the
   routing information among the sites.  The establishment of a VPN can
   be thought of as composed of the following three stages.  It is worth



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   noting that separation of the VPN setup into three stages is logical.
   Depending on the actual means used to determine such information, one
   or more stages can be combined.

   In the membership/capability discovery stage, membership and
   capability information needs to be acquired to determine if any two
   PEs (or CEs for layer 3 provider provisioned CE-based VPN) support
   common VPNs.  This can be accomplished, for instance, by exchanging
   VPN identifications of the configured VPNs at each PE (or CE) for
   determining if there are any common VPNs between them.  The
   capabilities of the PEs (or CEs) need to be determined to be able to
   agree on a common mechanism to use 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, the mechanisms for tunneling 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.

   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 via
   routing protocols such as BGP, central management system such as LDAP
   or manual configuration.  Manual configuration does not scale and is
   error prone, and therefore is discouraged.

   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.



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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 When a new site is added to a PE, how does the PE find out about
     the existing parts of the VPN and vice versa?  In analogy, when a
     new site is added to a CE-based VPN, how does it find out about the
     existing sites of the VPN and vice versa?

   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 to only those devices that need it?

   o An administrator may wish to use different topologies for different
     VPNs (e.g., a full mesh or a hub & spoke topology).  How can this
     be 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 PEs (or CEs for layer 3 provider provisioned CE-based VPN).  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 PEs (or CEs) act
   as clients of a centralized directory or database server that



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   contains VPN discovery information.  Another is where VPN discovery
   information is piggybacked onto a routing protocol running between
   the PEs (or CEs) [VPN-DISC].

4.2.1.1 Network management for membership information

   SPs use network management extensively to configure and monitor
   various devices in their network, which may be distributed across
   geographically separate sites.  The same approach could be used for
   distributing VPN related information as well.  A network management
   system (either centralized or distributed) could be used by the SP to
   configure and provision VPNs on PE devices (or CEs devices for layer
   3 provider provisioned CE-based VPN) at various locations.  VPN
   configuration information could be entered into the network
   management application and distributed via SNMP, XML, CLI, or other
   means to the remote sites.  This approach can be very effectively
   used within an SP network, since the SP has access to all PEs (or
   CEs) in its domain.  Security and access control are important, and
   could be achieved for example using SNMPv3, SSH, or IPsec tunnels.
   Standardized MIBs will need to be developed before this approach can
   be used to configure PEs (or CEs) across SP boundaries.  Furthermore,
   a means for per-VPN access may be necessary if an SP wishes to allow
   customers to access the managed objects in these MIBs, or if they
   wish to allow more segregated access to this information.

4.2.1.2 Directory servers

   An SP typically needs to maintain a database of its customer's
   configuration/membership information regardless of the mechanisms
   used to distribute it.  LDAP [RFC1777] is a standard directory
   protocol which makes it possible to use a common mechanism for both
   storing such information and distributing it.

   LDAP defines a schema, which is a standard format for representing
   information that will be stored in an LDAP server.  Having a standard
   schema ensures interoperability between different implementations of
   LDAP servers and clients.  Moreover, LDAPv3 [RFC2251] 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

   BGP supports extensions which allows it to carry VPN information.
   This allows the VPN discovery information and routing information to
   be combined in a single protocol.  BGP is also widely deployed by SPs
   [VPN-2547BIS].





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   BGP also contains mechanisms to control route distribution.  Route
   filters can be used to constraining the distribution of routing
   information.  Information needed to establish per-VPN tunnels can
   also be carried by this routing instance.

   Augmented routing may be done in combination with aggregated routing,
   as discussed in section 4.4.4.

4.2.1.4 Multi-SP VPNs

   When two sites of a VPN are connected to different SP networks, there
   must be a common mechanism for exchanging membership/capability
   information.  At least one mechanism for VPN information discovery
   must be standardized and supported across multiple SPs.  Inter-SP
   trust relationships will need to be established that will allow for
   exchange of membership information across SP boundaries.  Also, some
   mechanisms will be needed to control which membership information is
   exchanged between SPs.

4.2.2 Constraining distribution of VPN routing information

   With layer 3 provider provisioned CE-based VPNs, the VPN service
   provides tunnels between CE devices.  In this case, distribution of
   IP routing information occurs between CE devices on the customer
   sites and is therefore outside of the scope of the provider aspects
   of VPN support.  A possibility for layer 3 provider provisioned CE-
   based VPNs though, is that the SP takes care of the inter-site
   distribution of routing information, while the intra-site
   distribution remains outside of the scope of the provider's control.

   With layer 3 PE-based VPNs, the PE devices are routers, and the SP
   participates directly in routing for the customer network.  In this
   case, it is necessary to control the distribution of VPN routes
   between PE devices.

   In order to provide a scalable solution it is not possible to use a
   scheme where all PEs contain all routes for all VPNs.  Instead only
   PEs that have attached sites for a given VPN should contain the
   routing information for that VPN.  As VPN membership may change
   dynamically, it is necessary to have a mechanism that allows for VPN
   route information to be distributed to any PE where there is an
   attached user for that VPN, and to allow for the removal of this
   information when it is no longer needed.

   With the Virtual Router scheme (see section 4.4), per-VPN tunnels
   must be established before any routes for that VPN are distributed,
   and controlling the distribution of route information is thus
   achieved by controlling the establishment of these tunnels.  In this



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   scheme, the distribution of membership information consists of the
   set of VPNs that exists on each PE, and also topology information, to
   allow a PE to determine its neighbors.  When a PE receives this
   information it checks to see if it has VPNs in common with its
   neighbors, and if so it establishes tunnels for those VPNs.

   With the aggregated routing scheme (see section 4.4.4), the
   distribution of VPN routing information can be 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 PEs (or CEs for layer 3 provider provisioned CE-based VPN) that
   have attached sites for that VPN, and may also include non-PE
   devices.  (Note that in this section topology is used to indicate the
   interconnectivity between PEs (or CEs), (e.g., traffic between PE A
   and PE B traverses PE C), rather than restricted reachability between
   VPN sites (e.g., A can talk to B, and B can talk to C, but A cannot
   talk to C)).

   The simplest topology is a full mesh, where a tunnel exists between
   every pair of PEs (or CEs).  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 PEs (or CEs).  Each tunnel consumes some resources at a
   PE (or CE), and depending on the type of tunnel, may or may not
   consume resources in intermediate routers or switches.  One reason
   for using a partial mesh topology is to reduce the number of tunnels
   a PE (or CE), 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 PE (or
   CE) may be part of a full mesh topology for one VPN, and of a partial
   mesh topology for another VPN.







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   An example of where a partial mesh topology could be suitable is for
   a VPN that supports a large number of telecommuters and one, or 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 PEs (or CEs) and the corporate sites attached to hub PEs (or
   CEs).  Traffic between telecommuters is still supported, but this
   traffic traverses a hub PE (or CE).

   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
   PE (or CE) to be able to advertise per-VPN topology information to
   other PEs (or CEs).  Typically this per-VPN topology information is
   advertised using the same mechanism that is used to advertise
   membership information.  The topology information may be associated
   with a PE (or CE), or with subsets of routes reachable via that PE
   (or CE).

   A simple scheme is where a PE (or CE) advertises itself either as a
   hub or as a spoke, for each VPN that it has.  When received by other
   PEs (or CEs) this information can be used when determining whether to
   establish a tunnel.  A more comprehensive scheme allows a PE (or CE)
   to advertise a set of topology groups, with tunnels established
   between a pair of PEs (or CEs) if they have a group in common.

   For layer 3 provider provisioned CE-based VPNs, as it is not common
   to have inter-CE distribution protocols, the VPN topology is
   restricted by configuring every CE only with the other CEs it has to
   establish tunnels to.  As such, when a new CE is added to an existing
   VPN, the VPN topology will dictate which other CEs need to be
   notified.

4.3 VPN Tunneling

   VPN solutions use tunneling in order to transport VPN packets across
   the SP network.  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.




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   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
   across the SP network.  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 across the SP network 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 across the SP network.

   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 MPLS,
   GRE, IPsec, and IP-in-IP as these are the most suitable for carrying
   VPN traffic across an SP backbone.  Other tunneling protocols, such
   as L2TP [RFC2661], are used primarily to tunnel users across an
   access network to a PE or access server, or are used in a CE-based
   VPN.  As the tunneling protocol used across the SP network between
   PEs is orthogonal to how sites and subscribers access the VPN, these
   access side tunneling protocols are not discussed here.

4.3.1 Tunnel encapsulations

   All tunneling protocols use an encapsulation that adds additional
   information to the packet that is used for forwarding across the SP
   network.  Examples are provided in section 4.3.6.

   One characteristic of a tunneling protocol is whether per-tunnel
   state is needed in the SP network in order to forward the tunneled
   packets.  For IP tunneling schemes (GRE, IPsec, and IP-in-IP) no such
   per-tunnel state is needed since forwarding is carried out using the
   outer IP header.  When forwarding packets core routers make no
   distinction between tunneled and non-tunneled packets.  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.







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   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.

4.3.2 Tunnel multiplexing

   In order to support multiple VPNs on the same PE (for Layer-3 PE-
   based VPNs), a tunneling protocol must support a multiplexing field
   that allows a particular tunnel to be associated with a particular
   VPN.  Some tunneling protocols have a field explicitly designed for
   multiplexing, while others have a field that wasn't originally
   designed for this but can be pushed into service as a multiplexing
   field.  For example:

   o MPLS: Label.

   o GRE: Key field, originally intended for authentication.

   o IPsec: SPI field.

   o IP-in-IP: IP address in outer header.

   Note that IP-in-IP tunneling does not have a real multiplexing field
   but if a different IP address is used for every VPN then the IP
   address field can be used for this purpose.  This solution has the
   significant disadvantage that it requires the allocation and
   assignment of a potentially large number of IP addresses, and that
   all these addresses have to be reachable via backbone routing.

4.3.3 Tunnel establishment

   In order to be to able associate a tunnel with a VPN, it is necessary
   to determine and distribute values for the multiplexing field used in
   the tunneling protocol.  There are two main approaches to this-the
   use of an explicit signaling protocol used between the two tunnel
   endpoints, or distribution without an explicit signaling exchange.

   With explicit signaling there is a protocol exchange between the
   tunnel endpoints which, among other things, determines a value for
   the multiplexing field.  For example, IKE signaling is used to
   determine SPI values used with IPsec, and CR-LDP and RSVP-TE can be
   used to determine MPLS label values.  Thus, given the identity of the



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   remote party (e.g., IP address) the multiplexing values are generated
   automatically as a result of the protocol exchanges.  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).  One
   advantage of this approach is that per-tunnel security, QoS and other
   characteristics may also be negotiable via the signaling protocol
   used.  A disadvantage is that there may be scalability constraints,
   discussed further below.

   Multiplexing field values can also be exchanged without the use of an
   explicit signaling protocol.  For example MPLS labels can be
   piggybacked on the protocol used for the distribution of VPN routes,
   or on a protocol used for VPN membership.  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 or VPN routing protocol.

   The resources used by the different tunneling establishment
   mechanisms may vary.  With a full mesh VPN topology, and explicit
   signaling, each PE (or CE for layer 3 provider provisioned CE-based
   VPNs) has to establish a tunnel to all the other PEs (or CEs) for
   every VPN.  The resources needed for this on a PE (or CE) may be
   significant and issues such as the time needed to recover following a
   device failure may also be taken into account, due to the need to
   have to reestablish a large number of tunnels.

4.3.4 Scaling and hierarchical tunnels

   For tunnels that require state to be maintained in the core of the
   network, simply using per-VPN tunnels between all adjacent devices in
   the VPN topology may not scale to large numbers of tunnels.  Such a
   scheme also breaks the principle that there should be no per-VPN
   state in the core of the network.  For example, MPLS tunnels require
   that core network devices maintain state for the topmost label in the
   label stack.  If per-VPN tunnels are visible in the core then this
   will not scale, particularly as the core devices can act as
   aggregation points and handle tunnels originating from many PEs (or
   CEs for layer 3 provider provisioned CE-based VPNs).

   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 PE (or CE) 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 a VPN
   topology other than a full mesh.  However this may not always be



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   desirable, and even with hub and spoke topologies the hubs PEs (or
   CEs) may still need to handle large numbers of tunnels.

   Scaling in layer-3 PE-based VPNs can be improved by using
   hierarchical tunnels.  One tunnel is established between a pair of
   PEs, which is then used to carry multiple VPN-specific tunnels inside
   this tunnel.  Many different ways of using hierarchical tunnels are
   possible.  A single PE-PE tunnel could be established, which is used
   by all the per-VPN tunnels, or multiple PE-PE tunnels (perhaps with
   different QoS or security characteristics) could be established,
   which are then used by other groups of tunnels.  The tunnels used in
   the hierarchy may be of the same type (e.g., an MPLS label stack) or
   may be different (e.g., GRE carried over IPsec).

   One example using hierarchical tunnels 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).  Another example is
   the establishment of a number of different IPsec security
   associations, providing different levels of security between PEs.
   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.

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.  For example, where
   appropriate tunnels may be rerouted around failures.

   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.






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   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.

   A tunneling protocol may have a built-in keep alive mechanism that
   can be used to detect loss connectivity.  The base IPsec standard
   does not contain such a mechanism but there are proposals to extended
   IPsec in this manner.  GRE and IP-in-IP tunneling have no such
   mechanism.  MPLS detects failures as part of the signaling protocols.

   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 and secure communication
   between two CE/PE devices.  Available tunneling mechanisms include
   (but are not limited to): MPLS [RFC3031] [RFC3035], GRE [RFC2784]
   [RFC2890], IPsec [RFC2401] [RFC2402], and IP-in-IP encapsulation
   [RFC2003] [RFC2473].

4.3.6.1 MPLS [RFC3031] [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 into another LSP.

   o Multiprotocol transport

     MPLS can carry data packets other than IP.

   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 [MPLS-DIFF]
     [MPLS-DIFF-TE].

   o Tunnel setup and maintenance

     LSPs are set up and maintained by LDP (Label Distribution Protocol)
     or RSVP (Resource Reservation Protocol) [RFC3209].

   o Large MTUs, minimization of tunnel overhead, and frame sequencing

     MPLS does not restrict the MTU size.  The overhead of label
     switching should be minimal.

4.3.6.2 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 may encapsulate an IP
   payload packet over IP.  An endpoint encapsulates and decapsulates
   GRE packets.  A GRE tunnel is a communication path between two
   endpoints established by the use of 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 Multiprotocol transport

     GRE is assumed to support any type of payload protocol.

   o QoS/SLA

     GRE itself does not have intrinsic QoS/SLA capabilities.  These
     capabilities depend on the delivery protocol.  Other mechanism such
     as Diffserv or RSVP extensions [RFC2746] may be used with it.

   o Tunnel setup and maintenance

     GRE is not equipped with standard ways for setting up and
     maintaining GRE tunnels.

   o Large MTUs, minimization of tunnel overhead, and frame sequencing

     These capabilities depend on the delivery protocol, but the GRE
     header overhead is designed to be minimal.  The sequence field
     proposed in [RFC2890] may be used to achieve in-order delivery.

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 between the
   IPv4 header and the transport protocol 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
   unidirectional secure communication path between two endpoints, which
   is called a security association.  In tunnel mode, two security
   associations compose a tunnel between PE devices.  The IKE protocol
   is used to set up IPsec tunnels.

   o Multiplexing

     The SPI field of AH and ESP is used to multiplex security
     associations (or tunnels) between two peer devices.




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   o Multiprotocol transport

     IPsec needs extensions to carry packets other than IP.
     Alternatively, GRE may be used with it.

   o QoS/SLA

     IPsec itself does not have intrinsic QoS/SLA capabilities.  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

     IKE is used for the setup and maintenance of tunnels.

   o Large MTUs, minimization of tunnel overhead, and frame sequencing

     IPsec does not restrict the MTU size.  IPsec may impose its own
     overhead.  IPsec has a sequence number field that is used by a
     receiver to perform an anti-replay check, not to guarantee in-
     order delivery of packets.

4.3.6.4 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.  [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.

   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 Multiprotocol transport

     IP-in-IP needs extensions to carry packets other than IP.

   o QoS/SLA

     IP-in-IP itself does not have intrinsic QoS/SLA capabilities.  But,
     mechanisms such as RSVP extensions [RFC2764] or DiffServ extensions



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     [RFC2983] may be used with it.

   o Tunnel setup and maintenance

     IP-in-IP itself does not have intrinsic setup and maintenance
     capabilities.  Additional technique [GMNCL] may be used with it.

   o Large MTUs, minimization of tunnel overhead, and frame sequencing

     IP-in-IP does not restrict the MTU size.  The IP-in-IP header
     overhead is designed to be minimal.  IP-in-IP does not guarantee
     in-order delivery of packets.

4.4 Routing for VPNs Across the SP Network

   For layer 3 PE-based VPNs, PE devices forward network layer packets
   (IP packets) on behalf of 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.  However,
   there are also significant issues regarding how routing is done in
   the SP network, which are discussed here.

   The SP network needs to carry two types of information: (i) Routing
   information about the public network (including routes to the public
   Internet); (ii) Routing information about routes within the customer
   networks served by the VPNs.  Routing for the Internet or for public
   IP networks are outside of the scope of this document.

   For layer 3 provider provisioned CE-based VPNs, the SP does not
   consciously participate in the forwarding of VPN packets.  As the SP
   offers IP or MPLS connectivity to the CE devices connected to its PE
   devices, tunneled VPN packets are forwarded through the SP network
   based on the outer global encapsulating headers, as if it were global
   Internet traffic.

   As such, the PE devices don't need to participate in the routing for
   the customer network.  Note that the CE devices may need to
   participate in the routing of the SP network, as the VPN tunnel
   endpoints need to be know by the SP network.

4.4.1 Options for VPN routing in the SP

   The following technologies can be used for exchanging routing
   information within an SP network:






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   o Static routing (see section 3.3.3)

   o RIP (see section 3.3.3)

   o OSPF (see section 3.3.3)

   o BGP (see section 3.3.3)

   o Multiprotocol BGP-4 [RFC2858]

     BGP-4 has been extended to support IPv6, IPX, and others as well as
     IPv4 [RFC2283].  Multiprotocol BGP-4 carries routes from multiple
     "address families," such as the "VPN-IPv4 address family" discussed
     in [VPN-2547BIS].

4.4.2 VPN forwarding instances (VFIs)

   For layer 3 PE-based VPNs, the PE devices are routers which forward
   IP packets for the customer network.  This implies that PE devices
   must obtain routes for the customer networks.  This in turn implies
   that the PE device participates in some manner in routing for the
   customer network.  Thus each PE device looks to the customer network
   as if it were a router in the customer network.  The access
   connections from CE device to PE device are normal links between
   routers within the customer network.

   In layer 3 PE-based VPNs, a single PE device is likely to be
   interconnected with multiple CE devices, including multiple CEs
   within one VPN as well as CE devices from multiple different VPNs.
   The PE device must therefore support independent forwarding of user
   data for each VPN which it supports.  The forwarding table used for
   each VPN will in general be different.  This implies that the PE
   device will therefore need to maintain multiple separate forwarding
   instances.  These will be referred to as VPN Forwarding Instances
   (VFIs), as defined in section 2.1.

   Note that each VFI will need to obtain routes from the customer
   network that is supports, implying that it needs to participate in
   the operation of routing within each customer network.  This implies
   that from the PE perspective, routing towards the edge of the network
   (on the customer interfaces) must be separated on a per-VPN basis.
   However, note that in some cases routing across the customer
   interface may be very simple.  For example, a static route may be
   used.  Alternatively, BGP may be used, but with the provider
   advertising only a simple default route to the CE device, and with
   the CE device advertising only a single address prefix or a very
   small list of address prefixes to the provider.




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   PE devices may end up supporting a large number of VPNs, and
   therefore a large number of VFIs.  This implies that scaling may
   potentially be difficult in PE devices.  On the other hand, the
   resource load on a particular PE is largely linearly proportional to
   the number of VPNs that the PE device supports, and to the size of
   the VPNs.

   In general, a routing protocol instance may populate multiple VFIs,
   or a single VFI.  Also, a VFI may be populated by a single routing
   protocol, or multiple routing protocols.  Therefore there is not
   necessarily a one to one correspondence between VFI and routing
   protocol instance.

   There are several options for how VPN routes are carried across the
   SP network, as discussed below.

4.4.3 Per-VPN routing

   One option is to operate separate instances of routing protocols
   across the SP network, 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 minimizes the interactions between multiple different
   VPNs, in that routing is done independently for each VPN.  However,
   with this approach each PE device implements the capabilities of
   multiple different routers.  This implies that some sharing of
   resources may occur within the PE device.

   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.



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4.4.4 Aggregated routing model

   Another option is to use one single instance of a routing protocol
   for carrying VPN routing information across the SP network.  With
   this method the routing information for multiple different VPNs is
   aggregated into a single routing protocol.  This implies that
   whichever routing protocol is used in the SP network needs to be
   enhanced to allow routes from different VPNs to be distinguished.

   In 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.  Also, in general only
   a small number of PE devices will be interested in the operation of
   any one VPN.  Thus the total amount of routing information related to
   the various customer networks will be very large.  However, 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.  With link
   state routing, the information used for routing is broadcast
   throughout all routers within an area of the network.  This implies
   that all routers within an area have identical information about the
   status of the network.

   In general, any given PE will only support a subset of the VPNs
   supported by the SP.  It is important therefore to be able to
   restrict the distribution of routing information for any one VPN to
   those PEs which support that VPN.

   With link state routing protocols there is no provision for limiting
   the distribution of routing information to only a small number of
   routers within an area.  Thus if the VPN routing information is
   aggregated into OSPF or IS-IS, the information would need to be



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   broadcast to all routers in the area, even routers which don't want
   to know about any one particular VPN.  Given the potential magnitude
   of the total routing information required for supporting a large
   number of VPNs, this broadcast may have unfortunate scaling
   implications.

   In some cases VPNs may span multiple areas within a provider, or span
   multiple providers.  If VPN routing information is 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.

4.4.4.2 Aggregated routing with BGP

   BGP is a path vector routing protocol.  This implies that any router
   participating in BGP has a great deal of flexibility concerning which
   routes it gives to any particular neighbor.  Specifically, a router x
   passes to a peer router y whatever routes x would like y to be able
   to use.  Each route includes a specification of what IP addresses are
   reachable via the route, and the path for the route (summarized to be
   on a routing domain by routing domain basis).  Flexible and
   configurable export policies control which routes x decides to pass
   to y.  Similarly, import policies control which routes y is willing
   to accept from x.

   In many cases, all routers within a routing domain, or at least all
   border routers within the domain (i.e., all routers which have
   neighbors outside the domain) may want to hear about the same routes.
   It is not efficient to have each router exchange routes directly with
   every other router in the domain.  Instead BGP allows certain routers
   (or devices running routing software) to operate as route reflectors.
   A route reflector can then receive routes from certain routers, and
   distribute those routes as desired to other routers.

   Also note that BGP can be run between routers which are physically
   adjacent, or alternatively can be run between two routers which are
   interconnected only by a longer path through other routers.  BGP is
   tunneled over IP in order to allow its operation between non-adjacent
   routers.

   When the VPN routing information is piggybacked on BGP, there is
   therefore a considerable amount of flexibility regarding which
   information is exchanged via which routers and route reflectors.
   This flexibility makes BGP a candidate for carrying BGP routes across
   an SP network [VPN-2547BIS].






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   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.  BGP route filters can be used to
   control the distribution of routing information.

   In some cases it may be desirable to simplify the design of PE
   device, and limit the number of systems which needs to be
   reconfigured when VPN membership changes.  It may also be desirable
   to avoid a full mesh of N-squared BGP associations between N PE
   devices within an SP network.  BGP route reflectors can be used to
   control the redistribution of VPN routes between PE devices.  In many
   cases therefore it is likely that route reflectors may be used to
   distribute VPN routing information to PEs, and route filters will be
   used to ensure that information about a particular VPN will be
   distributed only to those PEs which participate in that VPN.

4.4.4.3 Partitioning of routing information with BGP

   BGP is used in most or all public networks for computing inter-domain
   routes to sites throughout the Internet.  If 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.

   In some cases it may be desirable for any one route reflector to
   carry only a subset of the routing information.  For example, a set
   of one or more route reflectors might be used to carry the Internet
   routes.  These route reflectors would therefore not carry any VPN
   routes.  A different set of one or more route reflectors might be
   used to carry all VPN routes.

   It is 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.  For this reason, 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.

   Similarly, each PE device would need to be aware of only those routes
   which it needs (specifically the VPN routes for VPNs which are
   present in that PE device, and optionally the Internet routes).







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   BGP policies need to be configured to control which route reflectors
   and which PE devices need to pay attention to which pieces of routing
   information.

   Whether the Internet routes are carried on the PE devices would again
   depend on configuration.  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 need the Internet access, then there are
   two options: (i) A default route may be used to route all the
   Internet traffic from that PE to a different router within the
   service SP network, and that other router can handle the full the
   Internet routing table.  With this approach the PE device needs only
   a single default route for all the Internet routes; (ii) The PE could
   instead obtain the full the Internet routes from an appropriate route
   reflector.  In this case the PE device may be able to pick more
   optimal routes (avoiding an extra router hop), but at the cost of
   additional memory and CPU usage at the PE device.


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.

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 [RFC2598] guarantees the QoS, whereas assured
   forwarding behavior [RFC2597] differentiates traffic packet
   precedence values.





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   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.

   o Potential security threat

     As portions of the traffic that flow to and from the public
     Internet are not necessarily under nor 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.




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   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
   trough 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
   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.



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   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 is
   highly undesirable for this task, as they 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).

   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 encrypted
   telnet sessions for CLI-based management, IPsec tunnels, or SNMP V3
   encryption for SNMP-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



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   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.


5. Interworking Interface

   This section describes interworking interface between SP networks;
   especially, focuses on customer data exchange on the interface.

5.1 Tunnels at 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.





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   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): MPLS, GRE, IPsec, IP-in-IP, FR, and ATM.

   o MPLS

     The tunnels at interworking interface may be provided by MPLS
     [RFC3031] [RFC3035].

   o GRE

     The tunnels at interworking interface may be provided by GRE
     [RFC2784] with key and sequence number extensions [RFC2890].

   o IPsec

     The tunnels at interworking interface may be provided by IPsec
     [RFC2401] [RFC2402].

   o IP-in-IP

     The tunnels at interworking interface may be provided by IP-in-IP
     [RFC2003] [RFC2473].

   o IP over FR

     The tunnels at interworking interface may be provided by IP over
     FR.







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   o IP over ATM AAL5

     The tunnels at interworking interface may be provided by IP over
     ATM AAL5 [RFC2684] [RFC2685].

5.2 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 MPLS, GRE, IPsec, and IP-in-IP
     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.

   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.

5.3 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, number of
     routing protocol instances and that of layer 3 PE-based VPNs are
     the same.  However, number of layer 3 PE-based VPNs in a PE device



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     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 cannot be so large.  Then, this
     limit is not a critical drawback.

   o Performance of packet transmission

     The interworking scenario discussed in this section does not make
     any additional burden to 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 needs
     certain load 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.

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.






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   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 as the data received by the PEs 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, like in
   temporary access (dial-in) scenarios, a preliminary authentication
   phase might be requested, e.g., when PPP is used.  Or alternatively,
   an authentication prove might need to be present in every data packet
   transmitted (like 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
   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).






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   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 for 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.

   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).




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6.6 User data and Control data

   An important remark is the fact that both the user data as 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 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.


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.




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   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).

   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.



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   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.

   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).



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   The same approach is also possible when other encapsulations are
   used, such as GRE [RFC2784] [RFC2890], IPsec [RFC2401] [RFC2402], or
   IP-in-IP [RFC2003] [RFC2473].  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.  However, sources of this document are based on various
   inputs from colleagues of authors.  We would like to thank Junichi
   Sumimoto, Kosei Suzuki, Hiroshi Kurakami, Takafumi Hamano, Naoto
   Makinae, and Kenichi Kitami of NTT and Rajesh Balay, Anoop Ghanwani,
   Harpreet Chadha, Samir Jain, Lianghwa Jou, Vijay Srinivasan, and
   Abbie Matthews of CoSine Communications.

   We would also like to thank Yakov Rekhter of Juniper Networks, Scott
   Bradner of Harvard University, Dave McDysan of WorldCom, Marco Carugi
   of France Telecom, Pascal Menezes of Terabeam, and Thomas Nadeau of
   Cisco Systems for their valuable comments and suggestions.


Intellectual Property

   Intellectual property rights may have been claimed with regard to
   some of the techniques and mechanisms described in this framework
   document.  For more information consult the online list of claimed
   rights maintained by the IETF at http://www.ietf.org/ipr.html.


References

   [PPVPN-REQ] Carugi, M. et al., "Service Requirements for Provider
   Provisioned Virtual Private Networks," Internet-draft <draft-ietf-
   ppvpn-requirements-03.txt>, November 2001.

   [RFC2764] Gleeson, B. et al., "A Framework for IP Based Virtual
   Private Networks," RFC 2764, February 2000.

   [RFC1918] Rekhter, Y. et al., "Address Allocation for Private
   Internets," RFC 1918, February 1996.

   [VPN-2547BIS] Rosen, E. et al., "BGP/MPLS VPNs," Internet-draft
   <draft-ietf-ppvpn-rfc2547bis-01.txt>, January 2002.

   [VPN-BGP-OSPF] Rosen, E. et al., "OSPF as the PE/CE Protocol in
   BGP/MPLS VPNs," Internet-draft <draft-rosen-vpns-ospf-bgp-



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   mpls-04.txt>, January 2002.

   [VPN-BGP-MCAST] Rosen, E. et al., "Multicast in MPLS/BGP VPNs,"
   Internet-draft <draft-rosen-vpn-mcast-02.txt>, July 2001.

   [VPN-VR] Ould-Brahim, H. et al., "Network based IP VPN Architecture
   Using Virtual Routers," Internet-draft <draft-ietf-ppvpn-vpn-
   vr-01.txt>, November 2001.

   [VPN-2917BIS] Muthukrishnan, K. et al, "A Core MPLS IP VPN
   Architecture," Internet-draft <draft-ietf-ppvpn-rfc2917bis-00.txt>,
   July 2001.

   [VPN-DISC] Ould-Brahim, H. et al., "Using BGP as an Auto-Discovery
   Mechanism for Network-based VPNs," Internet-draft <draft-ietf-ppvpn-
   bgpvpn-auto-02.txt>, January 2002.

   [VPN-L2] Rosen, E., "An Architecture for L2VPNs," Internet-draft
   <draft-ietf-ppvpn-l2vpn-00.txt>, July 2001.

   [VPN-CE] De Clercq, J. et al., "A Framework for Provider Provisioned
   CE-based Virtual Private Networks using IPsec," Internet-draft
   <draft-ietf-ppvpn-ce-based-01.txt>, November 2001.

   [RFC3031] Rosen E. et al., "Multiprotocol Label Switching
   Architecture," RFC 3031, January 2001.

   [RFC3035] Davie, B. et al., "MPLS using LDP and ATM VC Switching,"
   RFC 3035, January 2001.

   [MPLS-DIFF] Le Faucheur, F. et al., "MPLS Support of Differentiated
   Services," Internet-draft <draft-ietf-mpls-diff-ext-09.txt>, April,
   2001.

   [MPLS-DIFF-TE] Le Faucheur, F. (Ed.), "Requirements for support of
   Diff-Serv-aware MPLS Traffic Engineering," Internet-draft <draft-
   ietf-tewg-diff-te-reqts-01.txt> June 2001.

   [RFC2784] Farinacci, D. et al., "Generic Routing Encapsulation
   (GRE)," RFC 2784, March 2000.

   [RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE,"
   RFC 2890, September 2000.

   [RFC2401] Kent, S. and Atkinson, R., "Security Architecture for the
   Internet Protocol," RFC 2401, November 1998.

   [RFC2402] Kent, S. and Atkinson, R., "IP Authentication Header," RFC



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   2402, November 1998.

   [RFC2406] Kent, S. and Atkinson, R., "IP Encapsulating Security
   Payload (ESP)," RFC 2406, November 1998.

   [RFC2409] Harkins, D. and Carrel, D., "The Internet Key Exchange
   (IKE)," RFC 2409, November 1998.

   [RFC2003] Perkins, C., "IP Encapsulation within IP," RFC 2003,
   October 1996.

   [RFC2473] Conta, A. and Deering, S., "Generic Packet Tunneling in
   IPv6 Specification," RFC 2473, December 1998.

   [GMNCL] Kuwahara, T. et al., "Scalable Connectionless Tunneling
   Architecture and Protocols for VPNs," Internet-draft <draft-kuwahara-
   cl-tunneling-vpn-00.txt>, June 2001.

   [RFC2661] Townsley, W. et al., "Layer Two Tunneling Protocol 'L2TP',"
   RFC 2661, August 1999.

   [RFC2684] Grossman, D. and Heinanen, J., "Multiprotocol Encapsulation
   over ATM Adaptation Layer 5," RFC 2684, September 1999.

   [RFC2685] Fox B. and Gleeson, B., "Virtual Private Networks
   Identifier," RFC 2685, September 1999.

   [RFC2453] Malkin, G., "RIP Version 2," RFC 2453, November 1994.

   [RFC2328] Moy, J., "OSPF Version 2," RFC 2328, April 1998.

   [RFC1195] Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and
   Dual Environments," RFC 1195, December 1990.

   [RFC1771] Rekhter, Y. and Li, T., "A Border Gateway Protocol 4
   (BGP-4)," RFC 1771, March 1995.

   [RFC1965] Traina, P., "Autonomous System Confederations for BGP," RFC
   1965, June 1996.

   [RFC1966] Bates, T., "BGP Route Reflection: An alternative to full
   mesh IBGP," RFC 1966, June 1996.

   [RFC1997] Chandra, R., Traina, P., and Li, T., "BGP Communities
   Attribute," RFC 1997, August 1996.

   [RFC2858] Bates, T., Chandra, R., Katz, D., and Rekhter, Y.,
   "Multiprotocol Extensions for BGP-4," RFC 2283, February 1998.



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   [BGP-COM] Sangli, S. et al., "BGP Extended Communities Attribute,"
   Internet-draft <draft-ietf-idr-bgp-ext-communities-02.txt>, October
   2001.

   [RFC2205] Braden, R. et al., "Resource ReSerVation Protocol (RSVP) --
   Version 1 Functional Specification," RFC 2205, September 1997.

   [RFC2208] Mankin, A. et al., "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 Guerin, R., "Specification
   of Guaranteed Quality of Service," RFC 2212, September 1997.

   [RFC2207] Berger, L. and O'Malley, T., "RSVP Extensions for IPSEC
   Data Flows," RFC 2207, September 1997.

   [RFC2746] Terzis, A. et al., "RSVP Operation Over IP Tunnels," RFC
   2746, January 2000.

   [RFC3209] Awduche, D. et al., "RSVP-TE: Extensions to RSVP for LSP
   Tunnels," RFC 3209, December 2001.

   [RFC2474] Nichols, K. et al., "Definition of the Differentiated
   Services Field (DS Field) in the IPv4 and IPv6 Headers," RFC 2474,
   December 1998.

   [RFC2475] Blake S. et al., "An architecture for Differentiated
   Services," RFC 2475, December 1998.

   [RFC2597] Heinanen, J. et al., "Assured Forwarding PHB Group," RFC
   2597, June 1999.

   [RFC2598] Jacobsen, V. et al., "An Expedited Forwarding PHB," RFC
   2598, June 1999.

   [RFC2983] Black, D., "Differentiated Services and Tunnels," RFC 2983,
   October 2000.

   [RFC1777] Yeong, W. et al., "Lightweight Directory Access Protocol,"
   RFC 1777, March 1995.




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INTERNET-DRAFT          A Framework for L3 PPVPNs          February 2002


   [RFC2251] Wahl, M. et al., "Lightweight Directory Access Protocol
   (v3)," RFC 2251, December 1997.


Authors' addresses

   Ross Callon
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA 94089, 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

   Jeremy De Clercq
   Alcatel
   Fr. Wellesplein 1,
   2018 Antwerpen, Belgium
   Email: jeremy.de_clercq@alcatel.be

   Bryan Gleeson
   Email: bryangleeson@yahoo.com

   Andrew G. Malis
   Vivace Networks, Inc.
   2730 Orchard Parkway
   San Jose, CA 95134, USA
   Email: Andy.Malis@vivacenetworks.com

   Karthik Muthukrishnan
   Lucent Technologies
   1 Robbins Road
   Westford, MA 01886, USA
   Email: mkarthik@lucent.com

   Eric C. Rosen
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824
   Email: erosen@cisco.com







Design Team                Expires August 2002                 [Page 67]


INTERNET-DRAFT          A Framework for L3 PPVPNs          February 2002


   Chandru Sargor
   CoSine Communications
   1200 Bridge Parkway
   Redwood City, CA 94065
   Email: Chandramouli.Sargor@cosinecom.com

   Jieyun Jessica Yu
   Email: jyy_99@yahoo.com











































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