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Versions: 00                                                            
Internet Engineering Task Force                 Karthik Muthukrishnan
INTERNET-DRAFT                                  Andrew Malis
Expires April 5, 1999                           Ascend Communications
<draft-muthukrishnan-corevpn-arch-00.txt>       October 5 1998

                        Core IP VPN Architecture

1. Status of this Memo

   This document is an Internet-Draft. 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.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or
   ftp.isi.edu (US West Coast).

2. Acronyms

      LSP     Label Switched Path
      PNA     Private Network Administrator
      SP      Service Provider
      SPED    Service Provider Edge Device
      SPNA    SP Network Administrator
      VMA     VPN Multicast Address
      VPNID   VPN Identifier
      VR      Virtual Router
      VRC     Virtual Router Console

3. Abstract

      This draft presents an approach for building core VPN services in
      the service provider backbone, as described in [Heinanen].  This
      approach does not depend on MPLS running in the backbone but will
      benefit from it. The central vision is for the service provider to
      provide a virtual router service to their customers. Ease of
      configuration, dynamic neighbor discovery, scaling and the use of
      existing routing protocols as they exist today without any
      modifications are the keystones of this architecture.

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4. Introduction

      This draft describes how VPN services in the backbone of the SP's
      network could be built. The predominant emphasis is on providing a
      virtual router service and every effort has been made to make the
      virtual router as equivalent to a physical router as possible. The
      aspects of a router that a virtual router needs to emulate are
      configuration of any combination of routing protocols, monitoring
      of the network and troubleshooting. Providing a logically
      independent routing domain to every VPN enhances the SP's ability
      to offer a fully flexible virtual router service that can fully
      serve the SP's customer without requiring physical per-VPN

      The approach presented here meets most of the requirements set
      forth in [Heinanen] but differs significantly in that we have
      strived to not require or depend on any modifications of any
      existing routing protocols. Neighbor discovery is aided by the use
      of  an emulated LAN and is achieved by the use of ARP. This draft
      has made a concerted effort to draw the line between the SP and
      the PNA: layer 1 and layer 2 services belong and are managed by
      the SP while layer 3 services belong to and are managed by the
      PNA. By the provisioning of fully logically independent routing
      domains the PNA has been given the flexibility to use private and
      unregistered addresses. Data security is not an issue given the
      use of private LSPs and the use of VPNID encapsulation when
      forwarding on shared LSPs.

      The approach espoused in this draft differs from that described in
      [Jamieson] in the following ways: No routing protocol is modified
      or used to aid in the neighbor discovery mechanism. No VPN subnet
      from the SP's address space is required to be allocated. No PNL to
      PNL direct peering is used. It is not required for the CPE gear to
      be also MPLS compliant, thus allowing existing enterprise routers
      to not have to be upgraded.

5. Objectives

      1. Easy, scalable configuration of VPN endpoints in the service
      provider network.

      2. No use of globally unique SP IP resources such as IP subnets.

      3. Dynamic discovery of VRs (Virtual Routers) in the SP's cloud.

      4. Virtual Routers fully configured and monitored by network
      administrator of the VPN.

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      5. Forwarding quality fully configurable; at the lowest end best
      effort internet LSP.

      6. Differentiated services on a VPN by VPN basis based on private

      7. Security of internet routers extended to Virtual Routers.

      8. Specific routing protocols not mandated between Virtual

      9. No special extensions to existing routing protocols such as
      BGP, RIP, OSPF, ISIS etc.

6. Requirements

      The service provider network must run some form of multicast
      routing to all nodes that will have VPN connections and to nodes
      that have to forward Virtual Router discovery multicast datagrams.

7. Architectural Outline

      1. Every VPN is assigned a 16 bit VPNID which is unique within the
      SP's network. The choice of 16 bits for VPNID (rather than 32
      bits) allows 65k VPNs to be built in a SP's network and
      simultaneously keeps this ID small enough to be transmitted in
      encapsulation headers.

      2. The VPN service is offered in the form of a Virtual Router
      service. These VRs reside in the SPED and are as such confined to
      the edge of the SP's cloud.  The VRs will use the SP's network for
      data and control packet forwarding but are otherwise invisible
      outside the SPEDs.

      3. The "size" of the VR contracted to the VPN in a given SPED is
      the quantity of IP resources such as routing interfaces, route
      filters, routing entries etc. This is entirely under the control
      of the SP and provides the fine granularity required to empower
      the SP to offer virtually infinite grades of VR service on a per-
      SPED level. [Example: one SPED may be the aggregating point (say
      headquarters of the corporation) for a given VPN and a number of
      other SPEDs may be access points (branch offices). In this case,
      the SPED connected to the headquarters may be contracted to
      provide a large VR while the SPEDs connected to the branch offices
      may house small, perhaps stub VRs].

      4. One of the indicators of  the size of the VPN is the number of

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      SPEDs in the SP’s network that have connections to CPE routers. As
      globally unique IP resources do not have to be dedicated/assigned
      to VPNs, the number of SPEDs is not limited by any artificial
      configuration limits.

      5. Layer 1 and Layer 2 entities belong to and are managed by the
      SP. To be specific, physical switches/routers, physical links,
      logical layer 2 connections (such as DLCI in Frame Relay and
      VPI/VCI in ATM) and LSPs (and their assignment to specific VPNs)
      are under the control of the SP. In the context of VPNs, it is the
      SP's responsibility to contract and assign layer 2 entities to
      specific VPNs.

      6. Layer 3 entities belong to and are managed by the PNA. Examples
      of these entities include IP interfaces, choice of dynamic routing
      protocols or static routes, and routing interfaces. This provides
      a virtual routing domain to the PNA and empowers the PNA to design
      the network to achieve intranet, extranet and traffic engineering

      7. The PNA can manage and monitor the VPN using the methods that
      would have been used if physical routers rather than VRs were
      used. Therefore, management may be performed using SNMP or other
      similar methods or directly at the console, the VR console (VRC).
      Monitoring and troubleshooting may be performed using SNMP or
      similar, but may also include the use of standard tools such as
      ping, traceroute etc. Again, the VRC may be used for these
      purposes just like any physical router.

      8.  The VRs in the SPEDs form the VPN in the SP's network.
      Together, they  represent a virtual routing domain. They
      dynamically discover each other by utilizing an emulated LAN
      resident in the SP's network. Each VPN in the SP's network is
      assigned a multicast address. Subscription to this multicast
      address allows a VR to discover and be discovered by other VRs.

      9. Data forwarding may be done in one of several ways: hop-by-hop
      using some form of tunneling SPED-to-SPED, a public LSP with
      best-effort characteristics or a traffic engineered private LSP
      with differentiated characteristics. The choice of which LSP is
      configurable by the SP. The default is the public LSP with best-
      effort characteristics. The hop-by-hop mechanism is available to
      route packets during periods of LSP establishment and failure.

8. Scalable Configuration

      A VPN is expected to have 100s to 1000s of endpoints within the SP
      cloud. Configuration should therefore scale at most linearly with

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      the number of end points. Anything worse will make this task too
      daunting for the service provider.  To this end, all that the
      service provider needs to allocate/assign are physical/logical
      links from the private network to the service provider edge

9. Dynamic Neighbor Discovery

      The VRs in a given VPN reside in a number of SPEDs in the network.
      The problem is that these VRs need to be connected together. One
      way to do this is to require the configuration of tunnels between
      these VRs in a fully meshed fashion. This is obviously not
      scalable from a configuration and network resource standpoint.
      Hence the need arises to allow these VRs to dynamically discover
      each other.  Neighbor discovery is facilitated as follows: each
      VPN is given a limited emulated LAN. This emulated LAN is used in
      several ways:

      1. Address resolution uses this LAN to resolve next-hop (private)
      IP addresses associated with the other VRs.

      2. Routing protocols such as RIP and OSPF use this limited
      emulated LAN for neighbor discovery and to send routing updates.

      The per-VPN LAN is emulated using an IP multicast address.  In the
      interest of conserving public address space and because this
      multicast address needs to be visible only in the SP network
      space, we would use an address from the Organizationally scoped
      multicast addresses (239.192/14) as described in [Meyer]. Each VPN
      is allocated an address from this range. To completely eliminate
      configuration in this regard, this address could be computed given
      the VPNID.

10. Virtual Router Configuration

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            172.150/18                          172.150.128/18
    -----------------------             ---------------------------|
                |                                       |          |
                |                                       |
                |                                       |          Parts DB
                |           ---------------             |
                |    OSPF   |             |     OSPF    |
                ------------|   VR - A    |--------------
                             )             (  RIP
                      RIP   )               (
                           )                 (
              |--------------|             -----------------|
              |              |             |                |
              |    VR - B    |             |    VR - C      |
              |---------------             |-----------------
                     |                              |   Extranet
         -------------------------                  |
               172.150.64/18                        V

                                Figure 1

   Each Virtual Router is configurable by the PNA as though it were a
   private physical router. The resources that this Virtual Router may
   consume is of course limited by the bounds set by the SP on a SPED by
   SPED basis. Each VPN has a number of physical connections (to CPE
   routers) and a number of logical connections (to the emulated LAN).
   Each of these connections is IP capable and can be configured to
   utilize any combination of the standard routing protocols and routing
   policies to achieve specific corporate network goals.

   To illustrate, in Figure 1, there are 3 VRs on 3 SPEDs. VR-C and VR-B
   have a physical connection each to CPE equipment while  VR-A has 2
   physical connections. Each of the VRs has a fully IP capable logical
   connection to the emulated LAN.  VR-A has the (physical) connections
   to the headquarters of the company and runs OSPF over those
   connections. It can therefore route packets to 172.150/18 and
   172.150.128/18. VR-B runs RIP in the branch office(over the physical
   connection) and uses RIP (over the logical connection) to export
   172.150.64/18 to VR-A. VR-A advertises a default route to VR-B over
   the logical connection. VR-C is the extranet connection for vendors
   to use to connect to the parts database at Hence VR-C
   advertises a default route to VR-A over the logical connection. VR-A
   exports only to VR-C. This keeps the rest of the
   corporate network from being subjected to a security problem.

   The network administrator will configure the following:

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   1. OSPF connections to the 172.150/18 and 172.150.128/18  network in

   2. RIP connections to VR-B and VR-C on VR-A.

   3. Route policies on VR-A to advertise only the default route to VR-

   4. Route policies on VR-A to advertise only to VR-C.

   5. RIP on VR-B to VR-A.

   6. RIP on VR-C to advertise a default route to VR-A.

11. Forwarding

   As mentioned in the architectural outline, data forwarding may be
   done in one of four ways. The actual method in all but the first
   outlined here is configurable. At the high end the private LSP is
   preferred for data forwarding and at the other end hop-by-hop
   forwarding is used. The order of forwarding preference is therefore:
   optionally configured private LSP, best effort public LSP and lastly,

   11.1  Private LSP

   This LSP is optionally configured on a per-VPN basis. This LSP is
   usually associated with non-zero bandwidth reservation and/or a
   specific differentiated service or QOS class. If this LSP is
   available it is used for user data and for VPN private control data

   11.2 Best Effort Public LSP

   VPN data packets are forwarded using this LSP if a private LSP with
   specified bandwidth and/or QOS characteristics is either not
   configured or not presently available. The LSP that is used is that
   destined for the egress router in VPN 0. The VPNID in the shim header
   is used to de-multiplex data packets from various VPNs at the egress

   11.3 Hop-by-hop

   This method of forwarding is used when no LSP is currently available
   to carry the traffic. This could happen when the LSP is going through
   a down transient. To confine the VPN routing tables to the edges of
   the SP network, the VPNID and the egress SPED's ID need to carried

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   all the way. An approach is to tunnel the packet to the egress SPED
   with the IP protocol set to IPVPN  (protocol number to be allocated
   by IANA) and with a label pushed to represent the VPNID [TBD].

12.  Differentiated Services

   The configuration of private LSPs for VPNs allows the SP to offer
   differentiated services to paying customers. These private LSPs could
   be associated with any available QOS class. Multiple private LSPs
   with different QOS classes could be configured in a VPN with flow
   profiles used to sort the packets among the LSPs. This feature
   together with the ability to size the virtual routers allows the SP
   to offer truly differentiated services to the VPN customer.

13.  Virtual Router Security Considerations

   13.1  Data Security

   This allows the SP to assure the VPN customer that data packets in
   one VPN never has the opportunity wander into another. From a routing
   standpoint, this is achieved by maintaining separate instances of
   routing protocols and routing tables for each virtual router. From a
   data forwarding standpoint, the use of VPN encapsulation headers (in
   the case of shared LSPs or hop-by-hop forwarding) or the use of
   private LSPs guarantees data privacy.

   13.2  Configuration Security

   Virtual routers appear as real routers to the PNA. This means that
   they may be configured by the PNA to achieve connectivity between
   offices of a corporation. Obviously, the SP has to guarantee that the
   PNA and the PNA's designees are the only ones to have access to the
   VRs on the SPEDs the private network has connections to. Since the
   virtual router console is functionally equivalent to a physical
   router, all of the authentication methods available on a physical
   console such as password, RADIUS, etc. are available to the PNA.  By
   allowing only authenticated PNAs to access the VR console, the SP
   guarantees that the VPN is in full control of its destiny.

14.  Physical Network Security

   When a PNA logs in to a SPED to configure or monitor the VPN, the PNA
   is logged into the VR for the VPN. The PNA has layer 3 configuration
   and monitoring privileges for the VR. Specifically the PNA has no
   configuration privileges for the physical network. This provides the
   guarantee to the SP that a VPN administrator will not be able to
   inadvertently or otherwise adversely affect the SP's network.

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15.  Virtual Router Monitoring

   All of the router monitoring features available on a physical router
   is available on the virtual router. This includes utilities such as
   "ping" and "traceroute". In addition, the ability to display private
   routing tables, link state databases, etc. are available.

16.  Acknowledgements

   Thanks to Sridhar Komandur and Peter Fetterolf of Ascend
   Communications for their helpful review and feedback.

17.  References

   [Callon] Callon R., et al, "A framework for Multiprotocol Label
       Switching, draft-ietf-mpls-framework-02.txt".

   [Rosen] Rosen E., et al, "Multiprotocol Label Switching
       Architecture", draft-ietf-mpls-arch-02.txt.

   [Heinanen] Heinanen J., et al, "MPLS Mappings of generic VPN
       mechanisms", draft-heinanen-generic-vpn-mpls-00.txt.

   [Jamieson] Jamieson D., et al, "MPLS VPN Architecture", draft-

   [Meyer] Meyer D., "Administratively Scoped IP Multicast". RFC 2365.

18. Authors' addresses

   Karthik Muthukrishnan
   Ascend Communications
   1 Robbins Road
   Phone: (978) 952-1368
   Westford, MA 01886
   Email: karthikm@ascend.com

   Andrew Malis
   Ascend Communications
   1 Robbins Road
   Westford, MA 01886
   Phone: (978)-952-7414
   Email: malis@ascend.com

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