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Network Working Group                              Paul Knight (Editor)
Internet Draft                                        Hamid Ould-Brahim
draft-ietf-l3vpn-vpn-vr-03.txt                          Nortel Networks
Expires: September 6, 2006
                                                          Bryan Gleeson
                                                                  Nokia

                                                          March 6, 2006



                   Network based IP VPN Architecture
                         Using Virtual Routers



Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes a network-based Virtual Private Network
   (VPN) architecture using the virtual router (VR) concept. Multiple
   VRs can exist in a single physical device. A VR emulates all the
   functionality of a physical router, and therefore inherits all
   existing mechanisms and tools for configuration, operation,
   accounting, and maintenance. Any routing protocol can be used to

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   distribute VPN reachability information among VRs, and no VPN-
   related modifications or extensions are needed to the routing
   protocol for achieving VPN reachability. Direct VR-to-VR
   connectivity may be configured through layer-2 links or through IP-
   or MPLS-based tunnels. Traffic from VRs belonging to different VPNs
   may be aggregated over a "backbone VR" network, which greatly
   simplifies VPN provisioning. This architecture accommodates various
   backbone deployment scenarios, both where the VPN service provider
   owns the backbone, and where the VPN service provider obtains
   backbone service from one or more other service providers.


Table of Contents

   1     Introduction  ........................................  3
   2     Virtual Router VPN Architecture Guidelines ...........  4
   2.1   Membership  ..........................................  5
   2.2   Scalability ..........................................  5
   2.3   Quality of Service ...................................  5
   2.4   Auto-Discovery .......................................  5
   2.5   Routing ..............................................  6
   2.5.1 Routing between CE and PE ............................  6
   2.5.2 Routing in the Service Provider Network ..............  6
   2.5.3 Routing between PEs...................................  6
   2.5.4 Multicast routing using VRs ..........................  6
   2.6   Security .............................................  6
   2.7   Topology .............................................  7
   2.8   Tunneling ............................................  7
   2.9   Management ...........................................  7
   2.10  Additional Characteristics ...........................  7
   3     Network Reference Model ..............................  8
   3.1   Backbone  ............................................  8
   4     Virtual Router Definition ............................  9
   5     How VPNs are Built and Deployed using VRs ............  9
   5.1   VR to VR Connectivity over layer-2 Connections........ 10
   5.2   VR to VR Connectivity through IP or MPLS Tunnels...... 10
   5.3   Virtual Router Backbone Aggregation .................. 11
   5.3.1 Tunneling ............................................ 12
   5.3.1.1  MPLS Tunnels ...................................... 13
   5.3.1.2  IPsec Tunnels ..................................... 13
   5.3.2 Routing .............................................. 13
   5.3.3 Relationship between the VRs and the Backbone VR ..... 13
   5.3.4 Multiple Backbones Connected to a Single PE .......... 14
   6     VPN Membership and Topology Auto-Discovery ........... 14
   7     VRs and Extranets .................................... 15
   8     VPNs across Domains .................................. 16
   9     Internet Access ...................................... 16
   10    Carrier's Carrier Case................................ 17
   11    Operations and Management ............................ 17
   11.1  Backbone Migration ................................... 18
   11.2  Troubleshooting ...................................... 18
   12    Quality of Service  .................................. 18

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   13    Scalability .......................................... 18
   14    Interoperability ..................................... 19
   15    Security Considerations .............................. 20
   16    IANA Considerations .................................. 21
   17    Normative References ................................. 21
   18    Informative References ............................... 22
   19    Contributors and Acknowledgments ..................... 23
   20    Authors' Addresses  .................................. 23


1. Introduction

   This document describes a network-based VPN architecture using
   virtual routers. The objective is to provide per-VPN routing,
   forwarding, quality of service, and service management capabilities.
   The VPN service is based on the virtual router concept. The VR VPN
   architecture is compatible with the Layer 3 PPVPN framework
   described in [RFC-4110], as well as the generic PPVPN requirements
   [RFC-3809] and the service requirements for Layer 3 PPVPNs [RFC-
   4031].

   A virtual router (VR) has exactly the same mechanisms as a physical
   router, and therefore can inherit all existing mechanisms and tools
   for configuration, deployment, operation, troubleshooting,
   monitoring, and accounting. Multiple VRs can exist in a single
   physical device. Virtual routers can be deployed in various VPN
   configurations. Direct VR to VR connectivity may be configured
   through layer-2 links or through a variety of tunnel mechanisms,
   using IP- or MPLS-based tunnels. Multiple VRs may be aggregated over
   a "backbone VR." This architecture accommodates various backbone
   deployment scenarios, including where the VPN service provider owns
   the backbone, and where the VPN service provider obtains backbone
   service from one or more other service providers.

   This informational document does not specify a protocol, and
   therefore does not specify the manner in which VRs interoperate.
   Instead, VRs are interconnected using existing IETF specifications
   for tunneling mechanisms such as IPsec [RFC-4301], GRE [RFC-2784]
   (optionally with key and sequence number extensions [RFC-2890]), IP-
   in-IP [RFC-2003], and MPLS [RFC-3031] [RFC-3035] tunnels.
   Interaction between VRs across these tunnels make use of the same
   mechanisms and routing protocol standards  which would normally be
   used for interoperation between physical routers [RFC-1812], such as
   BGP [RFC-4271], OSPF [STD-54], IS-IS [RFC-1195] [RFC-3787], as well
   as others.

   There are several ways of implementing the VR architecture.
   Although this document does not guarantee interoperability by
   specifying explicit requirements, the nature of the VR approach
   makes it likely that interoperability can be achieved among various
   VR implementations, just as interoperability of VRs with normal


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   routers is straightforward. Section 14, "Interoperability,"
   addresses this in more detail.

   In the VR architecture, an instance of routing is used to distribute
   VPN reachability information among the VRs supporting each VPN. Any
   routing protocol can be used, and no VPN-related modifications or
   extensions are needed to the routing protocol for achieving VPN
   reachability. VPN reachability information to and from customer
   sites can be dynamically learned from the CE using standard routing
   protocols, or it can be statically provisioned on the VR. The
   routing protocol between the virtual routers and CEs is independent
   of the routing used in the VPN backbone, between the VRs. That is,
   the routing protocol between the VRs may be the same or it might be
   different than the routing mechanism used between the CE and VR, or
   it may be a different instance of the same protocol. Likewise, since
   the VR-to-VR connectivity can use tunnels, the inter-VR routing
   protocol can be independent of the routing used in the backbone
   network(s) over which the VR-based VPN runs.

   There are two fundamental architectures for implementing network-
   based IP VPNs: virtual routers (VR) and aggregated routing. The main
   difference between the two architectures resides in the model used
   to achieve VPN reachability and membership functions.

   In the VR model, each VR in the VPN domain is running an instance of
   routing protocol responsible for disseminating VPN reachability
   information between VRs. Therefore, VPN membership and VPN
   reachability are treated as separate functions, and separate
   mechanisms are used to implement these functions. In [RFC-4110],
   this is referred to as using per-VPN routing. VPN reachability is
   carried out by a per-VPN instance of routing, and a range of
   mechanisms is possible for determining membership (see section 6.0).

   In the aggregated routing model [RFC-4110], the VPN site's routing
   protocol is terminated at the edge of the backbone, and a backbone
   routing protocol (i.e., extended BGP-4) is responsible for
   disseminating the VPN membership and reachability information
   between provider edge routers (PE) for all the VPNs configured on
   the PE. "BGP/MPLS VPNs" [RFC-4364] describes an example of an
   aggregated routing VPN architecture.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC-2119].


2. Virtual Router VPN Architecture Guidelines

   The following guidelines are intended for designers of VR
   implementations.  Since this document does not describe a specific
   interoperable protocol, there may be alternative ways to achieve
   these guidelines.

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2.1 Membership

   The VR Architecture requires a way to determine VPN membership.
   This can be accomplished by configuration, by the use of an
   autodiscovery mechanism such as [VPN-BGP], or by other means. This
   is discussed in detail in Section 6, "VPN Membership and Topology
   Auto-Discovery".

   The use of a VPN identifier (VPN-ID) provides a way to simplify most
   VPN configuration and operational tasks.  All virtual routers that
   are members of a specific VPN should share the same VPN-ID. This may
   be the VPN-ID format defined in [RFC-2685].

2.2 Scalability

   In this architecture, the backbone internal nodes (e.g., P routers
   [RFC-4110]) are not VPN- or VR-aware, and therefore they don't keep
   any VPN state within the backbone. Thus the VR architecture avoids
   any significant contribution to problems of backbone scalability.

   The PE on which the VRs run (and the VRs themselves) should be able
   to accommodate rapid growth in the number of routes per VR, since
   this number can change suddenly as membership changes. The PE should
   be able to accommodate substantial growth in the number of VRs and
   CEs supported, to avoid reconfiguration that could disrupt existing
   connectivity.

   The optional use of the "backbone VR" improves the scalability of
   the VR approach, since multiple VRs on a PE may share a single
   backbone VR connection to their peer VRs on another PE, rather than
   establishing multiple separate per-VR or per-VPN connections between
   PEs. The backbone VR is described in more detail in section 5.3.

2.3 Quality of Service

   Existing quality of service mechanisms developed for physical
   routers should all be available to be used on a per-VR basis.
   Therefore, quality of service (policing, shaping, classification,
   and scheduling) should be configurable on a per-VPN basis.

2.4 Auto-discovery

   It should be possible for the VRs to automatically discover each
   other, set up tunnels to each other, and exchange private routing
   information across the backbone. The auto-discovery mechanism must
   take into consideration the case where the VPNs are implemented
   across administrative domains. We assume in this document that an
   auto-discovery mechanism which provides services similar to BGP (as
   described in [VPN-BGP]) is used as the mechanism to distribute
   membership, topology, and tunnel information among VRs which are
   members of the same VPN.

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2.5 Routing

2.5.1 Routing between CE and PE

   Any existing routing protocol may be used between the CE and the VR
   running on the PE. Typically, the routing protocol of the specific
   VPN site will be used. Static routes may be used. The routing
   protocol between the CE and the VR running on the PE may be
   independent of the PE-to-PE routing.  That is, they may be different
   routing protocols, or different instances of the same routing
   protocol.

2.5.2 Routing in the Service Provider Network (Backbone)

   The choice of the backbone routing protocol should not be
   constrained by the VPNs. This is one of the key attractions of the
   VR model, since it does not require VPN service providers to run and
   manage any specific backbone protocol or technology.

2.5.3 Routing between VRs in a VPN

   Any existing routing protocol may be used between VRs in a VPN. The
   routing protocol between the VRs may be independent of the CE-to-PE
   routing.

   VRs belonging to the same VPN may construct tunnels providing
   connections to each other, using information from the backbone
   routing protocol. They may then exchange routing information and VPN
   traffic over these tunnels.

   A backbone VR network may be constructed among some or all PEs. VRs
   of customer VPNs may use the backbone VR for routing across the
   backbone.

   It is strongly recommended that care be taken when multiple routing
   protocols are used, due to differences in metrics, detail of
   information, etc.

2.5.4 Multicast routing using VRs

   VR-based VPNs should provide the same mechanisms for IP multicast
   routing as ordinary routers.  The processes of multicast tree
   construction and packet replication should be configurable in the
   same way as for ordinary routers.  In addition, VR-based VPNs may be
   able to employ mechanisms to optimize multicast, depending on the
   specific VPN configuration.

2.6 Security

   The VR architecture accommodates a variety of approaches to ensure
   security for VPN data, routing, and other control information.

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   Different levels of security are possible, using the security
   features of routing and tunneling protocols. The architecture should
   provide authentication and encryption services for VPNs requiring
   strong security capabilities. VR-based VPN implementations should
   support the VPN Security Framework [RFC-4111].

2.7 Topology

   VPN topologies such as a hub and spoke, and full mesh can be
   supported by the VR architecture. It is also possible to build
   arbitrary VPN topologies.

   For example, a PE device with VRs supporting certain VPNs may be
   able to act as a P (Provider backbone) device with respect to other
   VPNs. This increases provisioning flexibility in many topologies.

2.8 Tunneling

   The VR architecture should not be limited to a single tunneling
   mechanism. It may allow the use of IPsec [RFC-4301], GRE [RFC-2784],
   IP in IP [RFC-2003], and MPLS [RFC-3031], [RFC-3035] tunnels. It
   should also allow multiple VPNs to share a tunnel across a backbone.
   Within a single VPN, different types of tunnels should be allowed.

2.9 Management

   The VR architecture should provide mechanisms to make it easy to
   configure, deploy, operate and troubleshoot each VPN independently,
   using existing mechanisms and tools. Tools commonly used for
   operating, managing and debugging IP networks should be able to be
   used without any modification.

   Most aspects of the management of the multiple VRs on the PE by the
   Service Provider are implementation-specific, and beyond the scope
   of this document.

2.10 Additional Characteristics

   The following are some additional general characteristics of the VR
   architecture:
   1) The architecture accommodates different sizes of VPNs, and one
     VPN should not impact other VPNs on the PE.
   2) The architecture supports overlapping VPN address spaces in
     separate VPNs.
   3) The architecture supports direct paths between VPN sites that
     bypass the service provider backbone (backdoor links). Traffic can
     be directed to the backdoor link, or injected to the backbone with
     the flexibility of using both the backbone access, and the
     backdoor link as internal or external paths.
   4) The architecture works over different deployment scenarios, e.g.
     where the service provider owns its own backbone, and where the


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     service provider obtains backbone service from one or more other
     service providers.

3. Network Reference Model

   A VPN customer site is connected to the provider backbone by means
   of a connection between a Customer Edge (CE) device, (which can be
   one or more hosts and/or routers) and a virtual router (VR). CE
   devices are preconfigured to connect to one or more VRs. Multiple VRs
   may coexist on the same service provider edge device (PE).

   CE devices can be attached to VRs over any type of access link (e.g.
   ATM, frame relay, Ethernet, PPP [STD-51], L2TP [RFC-2661] or IP
   tunneling mechanisms such as IPsec or GRE tunnels).


                           +---+    +---+
                           | P |....| P |
                           +---+    +---+
                     PE   /              \  PE
          +----+  +------+               +------+  +---+
          | CEs|--|-{VRs}|               |{VRs}-|--|CEs|
          +----+  +------+               +------+  +---+
                          \              /
                           +---+    +---+
                           | P |....| P |
                           +---+    +---+

                Figure 1: Network Reference Model

   CE sites can be statically connected to the provider network via
   dedicated circuits, or can use dial-up links. Routing tables
   associated with each virtual router define the site-to-site
   reachability for each VPN. The internal backbone provider routers
   (P) are not VPN aware and do not keep VPN state.

3.1 Backbone

   In general the backbone is a shared network infrastructure, which
   represents either:
   1) A layer-2 ATM or frame relay network.
   2) An IP network.
   3) An MPLS network.

   Not all VPNs existing on the same PE are necessarily connected via
   the same backbone. A single PE can be connected to multiple
   backbones. Individual VRs on the PE may also connect to multiple
   backbones. Thus a single VPN can be built from multiple transport
   technologies in the VR architecture.




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4. Virtual Router Definition

   A virtual router (VR) is an emulation of a physical router at the
   software and/or hardware levels. Virtual routers have independent IP
   routing and forwarding tables, and they are isolated from each
   other. This means that two VRs on a PE can serve two different VPNs
   which may have overlapping address space. The addresses need only be
   unique within a VPN domain.

   A virtual router has two main functions:
   1) Constructing routing tables for the paths between VPN sites using
     any routing technologies (e.g., static, OSPF, RIP, or BGP).
   2) Forwarding packets to the next hops within the VPN domain.

   From the VPN user point of view, a virtual router provides the same
   functionality as a physical router. Separate routing, and forwarding
   capabilities provide each VR with the appearance of a dedicated
   router that guarantees isolation from the traffic of other VPNs,
   while running on shared forwarding and transmission resources.

   Virtual routers belonging to the same VPN domain should have the
   same Virtual Private Network Identifier (VPN-ID). The VPN-ID may use
   the format described in [RFC-2685]. As noted in [VPN-BGP], when the
   VRs in a given VPN use BGP as the backbone routing protocol, the
   VPN-ID can be carried in the NLRI to make the addresses of VRs
   globally unique. Since globally unique addresses are necessary if
   BGP is used for auto-discovery, the use of a consistent VPN-ID is a
   key element in supporting auto-discovery and improving scalability
   of VR-based VPN services.

   To the CE access device, the virtual router appears as a neighbor
   router in the CE based network. The CE sends all traffic for non-
   local VPN destinations to the VR, unless the specific VPN topology
   provides alternate routes. Each CE access device must learn the set
   of destinations reachable through its connection to the virtual
   router; this may be as simple as a default route. Virtual routers
   participating in a single VPN domain are responsible for learning
   and disseminating VPN reachability information among themselves. A
   given VR holds the routes only for the specific VPN of which that VR
   is a member. Any routing protocol can be used between the VRs and
   the CEs.

5. How VPNs are Built and Deployed using VRs

   Three main VR deployment scenarios can be used for building VPNs:
   1) VR to VR connectivity over a layer 2 connection.
   2) VR to VR connectivity tunneled over an IP or MPLS network.
   3) Aggregating multiple virtual routers over a "backbone virtual
     router," which will provide connectivity over a layer 2, IP, or
     MPLS network.



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   These VR deployment scenarios can coexist on a single PE or within a
   single VPN.

5.1 VR to VR Connectivity over Layer 2 Connections

   As illustrated in Figure 2, virtual routers can be deployed over
   direct layer-2 frame relay or ATM connections or other layer-2
   transport technology.


                 PE                             PE
           +---------------+            +---------------+
   +-----+ |               |            |               | +-----+
   |VPN-A| | +----+        Layer-2 connections   +----+ | |VPN-A|
   |sites|-|-|VR-A|<---------------------------->|VR-A|-|-|sites|
   +-----+ | +----+        |  --------  |        +----+ | +-----+
           |               |-( Layer-2)-|               |
   +-----+ | +----+        | (Backbone) |        +----+ | +-----+
   |VPN-B|-|-|VR-B|        |  --------  |        |VR-B|-|-|VPN-B|
   |sites| | +----+<--------------------|------->+----+ | |sites|
   +-----+ |               |            |               | +-----+
           +---------------+            +---------------+

        Figure 2: VR to VR connectivity over a layer-2 backbone

   This type of VR deployment allows direct quality of service
   engineering on a per-VPN connection basis. The connections can be
   statically configured or dynamically established.

5.2 VR to VR Connectivity through IP or MPLS tunnels

   In addition to connecting via layer-2 transport technologies,
   virtual routers can connect over an IP or MPLS backbone. In a manner
   analogous to layer-2 transport, they can use the backbone to support
   tunneled connections among the VRs. The topology can be described
   similar to that for layer-2 transport, as in figure 2.

   VPN data and routing information is tunneled through the use of IP
   or MPLS based tunnels (e.g., IPsec, GRE, IP in IP, MPLS). The use of
   tunnels between VRs is addressed in more detail in the discussion of
   backbone VRs in the following section of this document.

   Although it is clearly possible to use a topology similar to the
   layer-2 model over an IP or MPLS backbone, the VR capability also
   provides a highly scalable alternative to the use of individual
   tunnels between VRs. This alternative is the creation (on each
   participating PE) of another VR facing into the backbone network,
   which is used to build a kind of backbone VPN that may be shared
   among multiple customer VPNs. This is described below as the
   "backbone VR."



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5.3 Virtual Router Backbone Aggregation

   Another typical VPN configuration consists of connecting multiple
   virtual routers to the backbone through the use of a single virtual
   router in each PE (figure 3). In the following sections we call this
   single virtual router "the backbone virtual router" or "the backbone
   VR." The backbone VR is a mechanism to enhance scalability.  The use
   of backbone VRs is optional in VR-based VPNs. When backbone VRs are
   used, they should be configured on all PEs which participate in VPNs
   carried over the backbone VRs.

   The backbone virtual router is not functionally different than other
   virtual routers.  It is only a virtual router that is configured and
   deployed in a special configuration.

   The backbone VR connects each PE to a shared backbone
   infrastructure. Backbone VRs can be deployed over ATM, FR, IP, or
   MPLS networks. Since the backbone VR allows the aggregation of VRs
   from multiple VPNs, backbone configuration can remain unaffected as
   new VPNs or VPN sites are added. The relationship between the VRs
   and the backbone VR is an overlay relationship.

                     PE-1                          PE-2
              +---------------+            +---------------+
              |               |            |               |
      +-----+ | +----+     MPLS/IP based Tunnels    +----+ | +-----+
      |VPN-A| | |VR-A|........|<---------->|........|VR-A| | |VPN-A|
      |sites|-|-|(1) |        |            |        |(2) |-|-|sites|
      +-----+ | +----+\+----+ | ---------  | +----+/+----+ | +-----+
              |        |VR-1|-|-(IP/MPLS )-|-|VR-2|        |
      +-----+ | +----+/+----+ |(Backbones) | +----+\+----+ | +-----+
      |VPN-B|-|-|VR-B|        | ---------  |        |VR-B|-|-|VPN-B|
      |sites| | |(1) |        |            |        |(2) | | |sites|
      +-----+ | +----+........|<---------->|........+----+ | +-----+
              |               |            |               |
              +---------------+            +---------------+

               Figure 3: VR-1 and VR-2 used as backbone VRs

   The relationship between the "ordinary" VPN VRs and the backbone VRs
   is conceptually similar to the relationship between separate
   routers, even though they coexist in the same device. The individual
   VRs in a PE, representing different VPNs, can relate to the backbone
   VR as if they were the CEs of a single VPN, with the backbone VR
   acting as a PE to them. Thus the VPNs can be multiplexed in a
   hierarchical fashion, using IP encapsulation or stacked labels,
   depending on the tunnel technology used between the backbone VRs.

   The use of the backbone VR provides multiplexing across the backbone
   for multiple VPNs, while still allowing individually-engineered
   connections where desired. Note that Figure 3 depicts both a
   backbone connection between backbone VRs (VR-1 to VR-2) and also

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   connections between the customer VPN VRs (VR-A(1) to VR-A(2) and VR-
   B(1) to VR-B(2) ) which do not pass through the backbone VRs. Both
   types of connections may be used simultaneously, e.g., to provide
   differentiated services to different classes of traffic.  Best-
   effort traffic between VR-A(1) and VR-A(2) may be routed through the
   shared backbone VRs, while high-priority traffic between these same
   VRs might be routed through the direct connection, which could be
   engineered with higher Quality-of-Service parameters. This
   illustrates how a service provider can trade off greater scalability
   offered by the backbone VR against higher value "personalized
   service" for VPN customers.

   Note that although the backbone VR concept is described above using
   a single backbone VR per PE, there may be multiple backbone VRs per
   PE.

5.3.1 Tunneling

   VPN data and routing information is tunneled through the use of IP
   or MPLS based tunnels (e.g., IPsec, GRE, IP in IP, MPLS). Depending
   on the tunnel technology used, the tunnels can be statically
   configured or dynamically established. The tunnel appears to VRs as
   a point-to-point link. Traffic sent through the tunnel and forwarded
   by the backbone VR is opaque to the underlying backbone technology
   used.

   A tunnel can be established per VPN or shared among many VPNs (VRs).
   The tunnel can originate from the backbone virtual router or from
   the VRs. This can provide an opportunity for service
   differentiation, in which a service provider can offer a higher
   level of service (at a higher price point) for individually mapped
   VPN connections among a customer's VRs.

   The backbone VR makes it appear as if each VR within a VPN is
   directly connected. It supports both full and partial mesh
   configurations. Each VR within the VPN exchanges routing information
   directly with the adjacent VRs in the VPN. Note that adjacency in
   this case is determined by the overlay topology of the particular
   VPN, as determined by configuration or discovery.

   A single VR-based VPN may use different type of tunnels for inter-VR
   connectivity. Some sites may use MPLS, while other sites (which
   transit through non-secure domains) may choose to use IPsec to
   encrypt their data.

   The scalability and security of dynamic tunnel establishment between
   VRs is enhanced by the ability to exchange a VPN-ID. [VPN-BGP]
   supports auto-discovery of the VPN-ID within BGP-based networks.
   Further work beyond the scope of this document is needed to
   determine the requirements and usage of the VPN-ID exchange within
   most tunneling scenarios.


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5.3.1.1 MPLS Tunnels

   The VR architecture can use MPLS tunneling in various forwarding
   scenarios. Individual VRs of some VPNs may be configured to
   participate in BGP/MPLS IP VPNs as described in [RFC-4364].

   In some scenarios, a hierarchy of two labels can be used. One simple
   forwarding scenario is where the inner label identifies the VR
   intended to receive the private packet (to be forwarded to the CE).

   Another forwarding scenario is to distribute the inner label on a
   per-VPN basis across the tunnels, after the tunnel endpoints (VRs)
   have been discovered. The label and reachability distribution is
   done through the tunnels. In this case the inner label distribution
   process can be achieved using BGP or an existing label distribution
   protocol on a per-VPN basis. The inner label relates to the private
   VPN prefixes. On the egress side traffic will be directed to the
   egress interface by looking up the inner label.

5.3.1.2 IPsec Tunnels

   IPsec [RFC-4301] is needed when there is a requirement for strong
   encryption or strong authentication. It also supports multiplexing
   and a signaling protocol - IKE. IPsec tunnels can be established
   between two VPN sites across the backbone (originating from the
   backbone VRs).

5.3.2 Routing

   The backbone VR exchanges backbone routing information with other
   backbone entities (P routers and possibly other backbone VRs). The
   backbone routing is separated from the customer VPN routing.

   Virtual routers can run any routing protocol on their local VPN
   domain. Both static routes and dynamic routing protocols such as
   RIP, OSPF, and BGP-4 can be used. The VRs of a given VPN exchange
   routing information with adjacent VRs through the tunnels over the
   backbone.

   If a backdoor link is used between VPN sites running any IGP, then
   by adjusting the backdoor link costs appropriately, the backbone
   link can be favored for forwarding VPN traffic. By lowering the
   weight, the backdoor link can be used as a backup link in case the
   backbone path fails.

5.3.3 Relationship between the VRs and the Backbone VR

   The routing domain of a set of VRs participating in a single VPN has
   no relation to the routing domain of the backbone VR. The backbone
   VR is not necessarily aware of the routing instances running on each
   private virtual router. However, because the backbone VR is also a


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   virtual router, it can build routing relationships with other VRs if
   needed.

5.3.4 Multiple Backbones Connected to a Single PE

   Figure 4 illustrates an example where multiple backbones are
   connected to the same PE. This type of configuration can be used
   when the PE is connected to multiple service provider backbones, or
   when the service provider offers different VPN services for
   different types of backbones.

                  PE                            PE
             +---------------+            +---------------+
     +-----+ |               |            |               | +-----+
     |VPN-A|-|-+----+        |            |        +----+-|-|VPN-A|
     |sites| | |VR-A|\       |            |        |VR-A| | |sites|
     +-----+ | +----+ +----+ |  --------- | +----+/+----+ | +-----+
             |        |VR-1|-|-(Backbone )|-|VR-2|        |
     +-----+ | +----+/+----+ | (    1    )| +----+\+----+ | +-----+
     |VPN-B|-|-|VR-B|        |  --------- |        |VR-B|-|-|VPN-B|
     |sites| | +----+        |            |        +----+ | |sites|
     +-----+ |               |            |               | +-----+
             |               |            |               |
     +-----+ |               |            |               | +-----+
     |VPN-C| | +----+        |            |        +----+ | |VPN-C|
     |sites|-|-|VR-C|\       |            |        |VR-C|-|-|sites|
     +-----+ | +----+ +----+ |  --------  | +----+/+----+ | +-----+
             |        |VR-3|-|-(Backbone)-|-|VR-4|        |
     +-----+ | +----+/+----+ | (  2 & 3 ) | +----+\+----+ | +-----+
     |VPN-D|-|-|VR-D|        |  --------  |        |VR-D|-|-|VPN-D|
     |sites| | +----+        |            |        +----+ | |sites|
     +-----+ |               |            |               | +-----+
             +---------------+            +---------------+

            Figure 4: Multiple Backbones Connected to a Single PE


6. VPN Membership and Topology Auto-Discovery

   The virtual router approach explicitly separates the mechanisms used
   for distributing reachability information from mechanisms used for
   distributing VPN topology and membership information. VPN membership
   information refers to the set of PEs (and the VRs on those PEs) that
   have customers in a particular VPN. VPN topology represents the set
   of VRs configured on PEs and their interconnectivity within the VPN.
   The topology can be a full-mesh of VRs, a hub and spoke, or anything
   in between. Dynamic topology due to on-demand VPN customers can also
   be handled.

   VPN discovery can be achieved through a variety of different
   mechanisms, for example:


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   - Directory server approach, in which VRs query a server to
   determine their neighbors.
   - Explicit configuration via a management platform.
   - Piggybacking VPN membership and topology information using
   existing routing protocols (e.g., BGP) [VPN-BGP].
   - Other VPN membership and topology auto-discovery approaches.

   The above mechanisms can be combined on a single PE, with different
   mechanisms used on a per-VPN basis. As an example, for some VPNs
   topology discovery is done only through a management platform. For
   others, dynamic topology discovery is achieved using existing
   routing protocols.

   In this document it is assumed that a mechanism that provides
   services similar to BGP is used to achieve auto-discovery of VPN
   members. A robust auto-discovery mechanism provides the scalability
   needed in large provider-provisioned VPNs. In the approach described
   in [VPN-BGP], VR addresses are exchanged, along with the information
   needed to enable the PEs to determine which VRs are in the same VPN
   ("membership"), and which of those VRs are to have VPN connectivity
   ("topology"). Once the VRs are reachable through the tunnels, routes
   ("reachability") are then exchanged by running existing routing
   protocols on a per-VPN basis across the tunnels.

   It is important to note that, for the VR architecture, the auto-
   discovery mechanism is only used to automatically exchange VPN
   control information between VRs and/or PEs. It is not intended for
   piggybacking VPN private reachability information onto the backbone
   routing instance, as is done in [RFC-4364], for example.

7. VRs and Extranets

   Extranets are commonly used to refer to a scenario whereby a company
   has network access to a limited part of another company's corporate
   network. An important feature of extranets is the control of who can
   access what data, and this is essentially a policy decision. Policy
   decisions are enforced at the interconnection points between
   different domains. The enforcement may be done via a firewall, a
   router with access list functionality, or any device capable of
   applying policy decisions to transit traffic.

   In the VR architecture, policy can be enforced between two VPNs, or
   between a VPN and the Internet, in exactly the same manner as is
   done today without VPNs. For example, two VRs (of different VPNs)
   could be interconnected, with each VR locally imposing its own
   policy controls via a firewall or other enforcement mechanism on all
   traffic that enters its VPN from the outside (whether from another
   VR or from the Internet). Combining firewalls and exchanging private
   routes between VRs (members of different VPNs) provide a flexible
   mechanism to build different flavors of extranets.



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8. VPNs across Domains

   It is possible that a VPN may cross multiple domains administered by
   different service providers. In the VR model, tunnels are used to
   provide intra-VPN connectivity across the backbones. The main
   requirement for the service provider in order to achieve end-to-end
   cross-domain VPN connectivity is the ability for both domains to
   support a common tunnel technology, plus the ability to support a
   common membership and topology discovery technology. Once the tunnel
   is established, private data (e.g., routing information, and private
   customer data) can flow from one domain to the other with the same
   level of security or isolation as that tunnel mechanism provides
   when used within a single service provider network.

   Another scenario for supporting VPNs with multiple service providers
   is to use two virtual routers configured on PEs at the
   interconnection points. Each VR will use policy decisions and
   firewalling to control VPN traffic transiting from one domain to the
   other. The two "gateway VRs" have some similarities to the "backbone
   VRs," specifically with respect to being able to handle multiple
   VPNs.  The individual VPN traffic is not terminated on these
   "gateway VRs".  They provide ingress/egress filtering for any or all
   the bidirectional tunneled VPN traffic crossing the boundary.  The
   VPN traffic will normally be opaque at the boundary, and typical
   inter-provider agreements apply to all traffic within individual
   VPNs, so the inter-provider VPN traffic is typically filtered all-
   or-nothing (by VPN) based on the visible packet identifiers or
   labels.

   When there are VPN links crossing intervening domains which are not
   VPN-aware, tunnels should be configured across the intervening
   domains, and the "gateway VR" approach can be employed at the tunnel
   endpoints to provide security services appropriate to the
   circumstances. Some aspects of this are discussed in more detail in
   the "Carrier's Carrier" section.

   The ability to use a standard, globally-unique VPN-ID format also
   supports the implementation of unambiguous VPN traffic
   identification mechanisms across domains.

9. Internet Access

   The same link attaching the CE to the VR can be used to provide
   Internet access to the VPN sites. The VR operations can be decoupled
   from the mechanisms used by the customer sites to access the
   Internet.

   There are a number of ways to provide Internet access to a VPN using
   the VR model. One way of providing VPN Internet access is to
   configure a "backbone VR" to steer private traffic to the VPN VR,
   and Internet traffic to the normal backbone/Internet forwarding
   table. The backbone VR can hold the Internet routes (so it will not

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   be necessary for the VPN VRs to handle them). Firewall functionality
   should be used to secure the Internet backbone VR access. Network
   address translation services can also be configured on the backbone
   VR or on VPN VRs where needed for Internet access.

   There are a number of other options, since the VR architecture
   reflects the flexibility of normal router architecture. An
   additional approach is to configure a particular VR to handle
   Internet access only (rather than going to the backbone VR). Another
   approach is to use a default route to an Internet gateway (which
   could be a VR).

10. Carrier's Carrier Case

   In some cases, the customer of a VPN is a service provider or
   carrier offering VPN services for its own customers.  We can
   describe this as a VPN hierarchy, with the "carrier's carrier"
   providing backbone services to a "sub-carrier." This is sometimes
   called "VPN wholesaling." The carrier's carrier may support multiple
   sub-carriers within a single PE device. The VR model provides
   several approaches to implement this VPN hierarchy.

   In one approach, tunnels are built from the VRs of the carrier's
   carrier to the CEs of the customers of the sub-carrier ("remote
   CEs"). In this case, the VRs of the carrier's carrier provide VPN
   service to the remote CEs. The sub-carrier provides transport but
   does not participate in the VPN services. This can be particularly
   useful in cases where the sub-carrier's PE or P devices are
   themselves VRs (which may be instantiated within the same device as
   the VRs of the carrier's carrier, handling the connections from the
   remote CEs) and where the sub-carrier is outsourcing the management
   of its customers' VPN services.

   Another approach is where the sub-carrier's VPN services are
   completely transparent to the VRs of the carrier's carrier. This is
   the default case. It is up to the sub-carrier's VPN service to
   distribute VPN reachability among the CEs of its customers.

11. Operations and Management

   Each VR operates independently, and can be individually reconfigured
   without affecting other VRs on the same PE.  In some
   implementations, it may be possible for a VR to be "rebooted"
   without affecting other VRs. In case of PE failure (e.g., migration,
   upgrades, etc.), the service provider may want to control and decide
   what VPN services get reestablished first. This particular point is
   important when a large number of VPNs is supported on the PE where
   each VPN service has different service availability requirements.

   Since each VR operates as an independent router, it is possible for
   the management of the VRs to be outsourced.  VPN customers may
   choose to configure (or perhaps only to monitor) the VRs that make

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   up their VPN.  It is also possible that the backbone VRs could be
   managed by a separate entity.

11.1 Backbone Migration

   One benefit in using multiple backbone virtual routers is the
   ability for the backbone network administrator to migrate its
   backbone from one core technology to another with minimal disruption
   to VPN services. Conversely, a VPN configuration change or a VPN-
   software upgrade is totally transparent to the backbone protocol and
   policies (this is due to decoupling the VPN routing protocol from
   the provider backbone routing protocol).

11.2 Troubleshooting

   The service provider or the VPN customer can use all existing
   troubleshooting tools on a per-VPN basis (e.g. ping and traceroute).
   As an example, a VPN customer may be able to perform some
   troubleshooting operations on its own VR. In this particular case,
   the service provider can configure restricted privileges for each
   VPN customer over the VR associated with the customer's VPN network.
   This access may provide only the privilege to monitor (with no
   privilege to change) the layer 3 status of the customer's VPN, as
   seen by the VR. The service provider may be able to offer VPN
   customers an SNMP-based method for read-only access to information
   about their own VPN. However, backbone topology information is
   completely hidden to the VPN VR, and therefore to the service
   provider's customer.

12. Quality of Service

   This architecture can utilize a variety of Quality of Service
   mechanisms. QoS mechanisms developed for physical routers can be
   used with VRs, on a per-VR basis, including classification,
   policing, drop policies, traffic shaping and scheduling/bandwidth
   reservation. The architecture allows separate quality of service
   engineering of the VPNs and the backbone.

13. Scalability

   The VR VPN architecture shares the scalability advantages of other
   provider-provisioned VPN architectures. Only the PEs need to handle
   the VPN type information. The internal backbone routers (the P
   routers) are not VPN aware. Virtual routers allow multiple private
   CE-based networks to connect to a single PE.

   One advantage of the ability to contain the VPN address space and
   VPN routing and forwarding capabilities within the virtual router
   entity is the possibility to distribute PE system resources on a
   per-VPN basis. Indeed, as an example, different scheduling
   mechanisms can be used for processing each VPN activity within the
   PE. This type of per-VPN resource management contributes to

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   establishing a wide range of priority schemes among the VPNs within
   the PE, and contributes to the ability to support a wide range of
   VPN scales (high traffic and/or many member sites) in the VR
   architecture.

   As noted earlier in this document, the use of the "backbone VR"
   provides significant scalability advantages, allowing very
   straightforward multiplexing of multiple VPNs across PE-PE tunnels
   or connections.  The individual VPNs and their VRs need not
   participate in the discovery and maintenance of the topology of the
   backbone network, essentially seeing the backbone as a single large
   router to which they are all connected.

14. Interoperability

   There are several ways of implementing the VR architecture.
   Although this document does not guarantee interoperability by
   specifying explicit requirements, the nature of the VR approach
   makes it likely that interoperability can be achieved among various
   VR implementations, just as interoperability of VRs with normal
   routers is straightforward.

   The VR architecture doesn't specify any protocol. Rather it defines
   how one physical device (a router) can support multiple logical
   devices (virtual routers), where each virtual router is
   interconnected with other normal routers or virtual routers either
   via physical links or by tunnels (of pretty much any kind defined by
   other IETF standards). Thus interoperation between a virtual router
   and other routers (whether virtual or logical) makes use of the same
   IETF standards (and proposed standards) that are used to allow
   interoperation between normal routers.

   VR implementations will either interoperate or not interoperate
   depending upon their implementation of other IETF standards. In
   particular, for two VR implementations to interoperate, they simply
   need a common data link or tunnel technology to link them together
   and common routing protocols. If they are using MPLS, then MPLS has
   to work over the common link or tunnels, and they need compatible
   MPLS signaling.

   Given that normal physical links terminating on a physical router
   can be assigned to a specific VR, and given that routers can in
   general interoperate at the data link level, then finding a common
   link to hook VRs with VRs, or VRs with routers, is straightforward.
   Also, there is a short list of relatively well known tunneling
   techniques (MPLS, IPsec, IP-in-IP tunnels, GRE, etc.) which are
   commonly used in multi-vendor networks.

   Naturally VRs will also interoperate with normal physical routers in
   almost any case. Even the "backbone VR" mechanism can be terminated
   at the other end on normal routers. This would involve one remote
   "backbone router" which would be the last node of a tunnel, inside

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   of which multiple other tunnels are multiplexed, with these multiple
   tunnels terminating on multiple real routers, each of which
   terminates a tunnel which it treats as a normal link. These real
   routers can run routing protocols over the tunnel, with the VR on
   the other end of the tunnel being the peer router for the routing
   protocol.

15. Security Considerations

   From a security viewpoint, the virtual router VPN architecture is an
   extension of existing router architectures in which multiple VRs,
   each with the same mechanisms of a physical router, can be
   configured in a PE device.  Thus the VRs inherit the security
   concerns and security capabilities of individual routers, which are
   largely beyond the scope of this document. Many of those elements
   are discussed in some detail in the routing protocol security
   document [RP-SEC]. The provider-provisioned VPN framework in general
   also has a number of security considerations due to the shared
   infrastructure, which are addressed in the PPVPN security framework
   document [RFC-4111]. This section addresses security considerations
   which are more specific to the VR architecture.

   The VR architecture provides an inherently high level of security
   against many types of attacks against individual VPNs, since
   individual VPN routing information does not propagate throughout the
   backbone network. The VRs usually do not exchange routing
   information directly through the backbone routing protocol, but
   through tunnels, through layer 2 connections, or (in the case of
   backbone VRs supporting ordinary VRs) through communication internal
   to the PE device. The tunnels can use the security mechanisms
   available to the backbone network, such as IPsec in an IP backbone
   network, to protect both the routing exchange and the VPN data.

   Since the VR architecture concentrates multiple VRs in a single
   device, there is a potential for disruption of one VR to affect
   other VRs within the same device. It is highly desirable for
   implementations to provide mechanisms to isolate problems to a
   single VR within a PE, or to a single VPN.

   If physical or logical network links are shared among VRs, it is
   possible that bandwidth depletion attacks against one VPN may affect
   other VPNs. VR implementations should provide mechanisms to mitigate
   the effect of excessive traffic being received for individual VPNs
   on shared links. In addition, VR implementations should provide
   mechanisms to control the bandwidth usage on a per-VPN basis for
   traffic transmitted by the PE device. The VPN service provider
   should ensure that both access networks and backbone networks are
   engineered to reduce the likelihood of this kind of attack.

   Since the backbone VR(s) may carry traffic from multiple VPNs, the
   implementation of backbone VR mechanisms should provide redundancy
   mechanisms. They should provide protection against hostile or

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   inadvertent resource exhaustion attacks, originating either within
   or outside the VPNs.

   If the auto-discovery mechanism used in determining membership to
   the VPN is subverted, it could potentially be possible for an
   attacker to join a VPN without authorization. Likewise, if the VPN-
   ID of a VR is erroneously configured, a VPN site could potentially
   be joined to the wrong VPN. These issues can both be addressed by
   the use of tunnel mechanisms between VRs which include other means
   of authentication, such as a shared secret. Other proposals for VPN
   membership verification, such as [MPLSVPN-AUTH], offer mechanisms
   which may also be useful to mitigate this potential issue.

   Various levels of data, routing and configuration security can be
   implemented in the VR architecture. Any existing security-related
   mechanisms supported by existing routing protocols (e.g.
   authentication) can be used unmodified. If IPsec tunneling is used
   as the tunneling protocol, then both the control and data traffic
   that travels over the tunnel can be secured; so that routing
   specific security enhancements are not needed. Any private routing,
   forwarding and addressing manipulation is done within the virtual
   router context. Direct layer-2 connections (ATM, FR), or specific
   tunneling mechanisms can also provide various levels of data
   security.

16. IANA Considerations

   This document has no actions for IANA.

17. Normative References

   [RFC-1812] F. Baker, Ed., "Requirements for IP Version 4 Routers",
      RFC 1812, June 1995.

   [RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
      Requirement Levels", RFC 2119, BCP 14, March 1997.

   [RFC-3031] E. Rosen, A. Viswanathan, R. Callon, "Multiprotocol Label
      Switching Architecture", RFC 3031, January 2001.

   [RFC-3035] B. Davie, J. Lawrence, K. McCloghrie, E. Rosen, G.
      Swallow, Y. Rekhter, P. Doolan, "MPLS using LDP and ATM VC
      Switching", RFC 3035, January 2001.

   [RFC-3809] Nagarajan, A., Ed., "Generic Requirements for Provider
      Provisioned Virtual Private Networks (PPVPN)", RFC 3809, June
      2004.

   [RFC-4031] Carugi, M., Ed. and D. McDysan, Ed., "Service
      Requirements for Layer 3 Provider Provisioned Virtual Private
      Networks (PPVPNs)", RFC 4031, April 2005.


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   [RFC-4110] R. Callon, M. Suzuki, "A Framework for Layer 3 Provider-
      Provisioned Virtual Private Networks (PPVPNs)", RFC 4110, July
      2005.

   [RFC-4111] Fang, L., Ed., "Security Framework for Provider
      Provisioned Virtual Private Networks", RFC 4111, July 2005.

   [RFC-4271] Y. Rekhter, Ed., T. Li, Ed., S. Hares, Ed., "A Border
      Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC-4301] Kent, S., Seo, K., "Security Architecture for the
      Internet Protocol", RFC 4301, December 2005.

   [RFC-4364] Rosen, E., et al, "BGP/MPLS IP Virtual Private networks
      (VPNs)", RFC 4364, February, 2006.

   [STD-54] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

18. Informative References

   [MPLSVPN-AUTH] Behringer, M., Guichard, J., Marques, P. R., "Layer-3
      VPN Import/Export Verification" (draft-ietf-l3vpn-vpn-
      verification-00.txt), work in progress.

   [RFC-1195] Callon, R., "OSI IS-IS for IP and Dual Environment," RFC
      1195, December 1990.

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

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

   [RFC-2685] Fox, B., et al, "Virtual Private Networks Identifier",
      RFC 2685, September 1999.

   [RFC-2784] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic
      Routing Encapsulation (GRE)", RFC 2784, March 2000.

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

   [RFC-3787] Parker, J., Ed., "Interoperable IP Networks using IS-IS",
      RFC 3787, May 2004

   [RP-SEC] Barbir, A., Murphy, S., and Yang, Y., "Generic Threats to
      Routing Protocols" (draft-ietf-rpsec-routing-threats-07.txt),
      work in progress.

   [STD-51] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
      RFC 1661, July 1994.


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   [VPN-BGP] Ould-Brahim, H., et al., "Using BGP as an Auto-Discovery
      Mechanism for Layer-3 and Layer-2 VPNs" (draft-ietf-l3vpn-bgpvpn-
      auto-06.txt), work in progress.

19. Contributors and Acknowledgments

   The authors would like to acknowledge the active contributions of
   the following individuals, all of whom would be included as authors
   if allowed by IETF processes:
   Rainer Bach
   Rick Bubenik
   Luyuan Fang
   Isaac Negusse
   Chandru Sargor
   Timon Sloan
   Dr. Christian Weber
   Gregory Wright
   Abraham Young
   Jieyun Jessica Yu

   The authors would also like to acknowledge the following individuals
   for their helpful comments and suggestions: Peter Ashwood-Smith, Ron
   Bonica, Ross Callon, David Drynan, Mark Duffy, Don Fedyk, David
   Hudson, Bilel Jamoussi, Ahmad Khalid, Scott Larrigan, Keerti
   Melkote, Martin Pepin, Benson Schliesser, Jerry Sydir, and Ru
   Wadasinghe.

20. Authors' Addresses

   (change /at/ to @ for email)

   Paul Knight (Editor)
   Nortel Networks
   600 Technology Park Drive
   Billerica, MA  01821  USA
   paul.knight/at/nortel.com
   Phone:  +1 (978) 288 6414

   Hamid Ould-Brahim
   Nortel Networks
   P O Box 3511 Station C
   Ottawa, ON K1Y 4H7  Canada
   Phone: +1 (613) 765 3418
   Email: hbrahim/at/nortel.com

   Bryan Gleeson
   Nokia
   313 Fairchild Drive
   Mountain View CA 94043  USA
   bryan.gleeson/at/nokia.com



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Knight, et al.       Expires September 2006          [Page 24]


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