path: root/doc/rfc/rfc4797.txt

Network Working Group                                         Y. Rekhter
Request for Comments: 4797                                     R. Bonica
Category: Informational                                 Juniper Networks
                                                                E. Rosen
                                                     Cisco Systems, Inc.
                                                            January 2007


             Use of Provider Edge to Provider Edge (PE-PE)
               Generic Routing Encapsulation (GRE) or IP
                in BGP/MPLS IP Virtual Private Networks

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

IESG Note

   This document proposes an automated mechanism for establishing
   tunnels between provider-edge routers in a VPN, but does not provide
   an automated mechanism for establishing security associations for
   these tunnels.  Without such a mechanism, this document is not
   appropriate for publication on the Internet standards track.

Abstract

   This document describes an implementation strategy for BGP/MPLS IP
   Virtual Private Networks (VPNs) in which the outermost MPLS label
   (i.e., the tunnel label) is replaced with either an IP header or an
   IP header with Generic Routing Encapsulation (GRE).

   The implementation strategy described herein enables the deployment
   of BGP/MPLS IP VPN technology in networks whose edge devices are MPLS
   and VPN aware, but whose interior devices are not.











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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Conventions Used In This Document . . . . . . . . . . . . . . . 4
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Specification . . . . . . . . . . . . . . . . . . . . . . . . . 5
     4.1.  MPLS-in-IP/MPLS-in-GRE Encapsulation by Ingress PE  . . . . 5
     4.2.  MPLS-in-IP/MPLS-in-GRE Decapsulation by Egress PE . . . . . 6
   5.  Implications on Packet Spoofing . . . . . . . . . . . . . . . . 7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
   8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 8







































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1.  Introduction

   A "conventional" BGP/MPLS IP VPN [2] is characterized as follows:

      Each Provider Edge (PE) router maintains one or more Virtual
      Routing and Forwarding (VRF) instances.  A VRF instances is a VPN-
      specific forwarding table.

      PE routers exchange reachability information with one another
      using BGP [3] with multi-protocol extensions [4].

      MPLS Label Switching Paths (LSPs) [5] connect PE routers to one
      another.

   In simple configurations, the VPN service is offered by a single
   Autonomous System (AS).  All service provider routers are contained
   by a single AS and all VPN sites attach to that AS.  When an ingress
   PE router receives a packet from a VPN site, it looks up the packet's
   destination IP address in a VRF that is associated with the packet's
   ingress attachment circuit.  As a result of this lookup, the ingress
   PE router determines an MPLS label stack, a data link header, and an
   output interface.  The label stack is prepended to the packet, the
   data link header is prepended to that, and the resulting frame is
   queued for the output interface.

   The innermost label in the MPLS label stack is called the VPN route
   label.  The VPN route label is significant and visible to the egress
   PE router only.  It controls forwarding of the packet by the egress
   PE router.

   The outermost label in the MPLS label stack is called the tunnel
   label.  The tunnel label causes the packet to be delivered to the
   egress PE router that understands the VPN route label.  Specifically,
   the tunnel label identifies an MPLS LSP that connects the ingress PE
   router to the egress PE router.  In the context of BGP/MPLS IP VPNs,
   this LSP is called a tunnel LSP.

   The tunnel LSP provides a forwarding path between the ingress and
   egress PE routers.  Quality of service (QoS) information can be
   mapped from the VPN packet to the tunnel LSP header so that required
   forwarding behaviors can be maintained at each hop along the
   forwarding path.

   Sections 9 and 10 of reference [2] define more complex configurations
   (i.e., carriers' carrier and multi-AS backbones) in which service
   providers offer L3VPN services across multiple autonomous systems.
   In these configurations, VPN route labels can be stitched together




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   across AS boundaries.  Within each AS, tunnel LSPs carry VPN packets
   from network edge to network edge.

   In most configurations, tunnel LSPs never traverse AS boundaries.
   The tunnel LSP is always contained within a single AS.  In one
   particular configuration (i.e., Inter-provider Option C), tunnel LSPs
   may traverse AS boundaries.

   This memo describes procedures for creating an MPLS packet that
   carries the VPN route label, but does not carry the tunnel label.
   Then, using either GRE or IP encapsulation, the ingress PE router
   sends the MPLS packet across the network to the egress PE router.

   That is, a GRE or IP tunnel replaces the tunnel LSP that was present
   in "conventional" BGP/MPLS IP VPNs.  Like the tunnel LSP, the GRE/IP
   tunnel provides a forwarding path between the ingress and egress PE
   routers.  QoS information can be copied from the VPN packet to the
   GRE/IP tunnel header so that required forwarding behaviors can be
   maintained at each hop along the forwarding path.  However, because
   the GRE/IP tunnel lacks traffic engineering capabilities, any traffic
   engineering features provided by the tunnel LSP are lost.

2.   Conventions Used In This Document

   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 [1].

3.  Motivation

   "Conventional" BGP/MPLS IP VPNs require an MPLS Label Switched Path
   (LSP) between a packet's ingress PE router and its egress PE router.
   This means that a BGP/MPLS IP VPN cannot be implemented if there is a
   part of the path between the ingress and egress PE routers that does
   not support MPLS.

   In order to enable BGP/MPLS IP VPNs to be deployed even when there
   are non-MPLS routers along the path between the ingress and egress PE
   routers, it is desirable to have an alternative, which allows the
   tunnel label to be replaced with either an IP or (IP + GRE) header.
   The encapsulation header would have the address of the egress PE
   router in its destination IP address field, and this would cause the
   packet to be delivered to the egress PE router.

   In this procedure, the ingress and egress PE routers themselves must
   support MPLS, but that is not an issue, as those routers must
   necessarily have BGP/MPLS IP VPN support, whereas the transit routers
   need not support MPLS or BGP/MPLS VPNs.



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4.  Specification

   In short, the technical approach specified here is:

   1.  Continue to use MPLS to identify a VPN route, by continuing to
       add an MPLS label stack to the VPN packets.  Between the ingress
       and egress PE router, the outermost member of the label stack
       will represent the VPN route label.

   2.  An MPLS-in-GRE or MPLS-in-IP [6] encapsulation will be used to
       turn the MPLS packet, described above, back into an IP packet.
       This, in effect, creates a GRE or an IP tunnel between the
       ingress PE router and the egress PE router.

   The net effect is that an MPLS packet gets sent through a GRE or an
   IP tunnel.

   Service providers must protect the above-mentioned IP or GRE tunnel
   as recommended in Section 8.2 of reference [6].  As stated in that
   document:

      "If the tunnel lies entirely within a single administrative
      domain, address filtering at the boundaries can be used to ensure
      that no packet with the IP source address of a tunnel endpoint or
      with the IP destination address of a tunnel endpoint can enter the
      domain from outside.

      However, when the tunnel head and the tunnel tail are not in the
      same administrative domain, this may become difficult, and
      filtering based on the destination address can even become
      impossible if the packets must traverse the public Internet.

      Sometimes only source address filtering (but not destination
      address filtering) is done at the boundaries of an administrative
      domain.  If this is the case, the filtering does not provide
      effective protection at all unless the decapsulator of an
      MPLS-in-IP or MPLS-in-GRE validates the IP source address of the
      packet.  This document does not require that the decapsulator
      validate the IP source address of the tunneled packets, but it
      should be understood that failure to do so presupposes that there
      is effective destination-based (or a combination of source-based
      and destination-based) filtering at the boundaries."

4.1.  MPLS-in-IP/MPLS-in-GRE Encapsulation by Ingress PE

   The following description is not meant to specify an implementation
   strategy; any implementation procedure that produces the same result
   is acceptable.



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   When an ingress PE router receives a packet from a Customer Edge (CE)
   router, it looks up the packet's destination IP address in a VRF that
   is associated with the packet's ingress attachment circuit.  This
   enables the (ingress) PE router to find a VPN-IP route.  The VPN-IP
   route will have an associated VPN route label and an associated BGP
   Next Hop.  The label is pushed on the packet.  Then an IP (or IP+GRE)
   encapsulation header is prepended to the packet, creating an
   MPLS-in-IP (or MPLS-in-GRE) encapsulated packet.  The IP source
   address field of the encapsulation header will be an address of the
   ingress PE router itself.  The IP destination address field of the
   encapsulation header will contain the value of the associated BGP
   Next Hop; this will be an IP address of the egress PE router.  QoS
   information can be copied from the VPN packet to the GRE/IP tunnel
   header so that required forwarding behaviors can be maintained at
   each hop along the forwarding path.

   The IP address of the remote tunnel endpoints MAY be inferred from
   the Network Address of the Next Hop field of the MP_REACH_NLRI BGP
   attribute [4].  Note that the set of Next Hop Network Addresses is
   not known in advance, but is learned dynamically via the BGP
   distribution of VPN-IP routes.  Assuming a consistent set of tunnel
   capabilities exist between all the PEs and Autonomous System Border
   Routers (ASBRs), no a priori configuration of the remote tunnel
   endpoints is needed.  The entire set of PE and ASBRs MUST have the
   same tunnel capabilities if the dynamic creation of IP (or GRE)
   tunnels is desired.  The preference to use an IP (or GRE) tunnel MUST
   be configured.  A set of PEs using two or more tunnel mechanisms
   (i.e., LSP, GRE, IP, etc.)  MUST determine the tunnel type on a per-
   peer basis.  The automatic association of tunnel capabilities on a
   per-peer basis is for future study.  Note that these tunnels SHOULD
   NOT be IGP-visible links, and routing adjacencies SHOULD NOT be
   supported across these tunnel.

4.2.  MPLS-in-IP/MPLS-in-GRE Decapsulation by Egress PE

   Every egress PE is also an ingress PE, and hence has the ability to
   decapsulate MPLS-in-IP (or MPLS-in-GRE) packets.  After
   decapsulation, the packets SHOULD be delivered to the routing
   function for ordinary MPLS switching.

   As stated above, if the service provider deploys source-based
   filtering at network edges to protect the IP/GRE tunnel (instead of
   destination-based filtering), the decapsulator must validate the IP
   source address of the tunneled packets.







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5.  Implications on Packet Spoofing

   It should be noted that if the tunnel MPLS labels are replaced with
   an unsecured IP encapsulation, like GRE or IP, it becomes more
   difficult to protect the VPNs against spoofed packets.  This is
   because a Service Provider (SP) can protect against spoofed MPLS
   packets by the simple expedient of not accepting MPLS packets from
   outside its own boundaries (or more generally, by keeping track of
   which labels are validly received over which interfaces, and
   discarding packets that arrive with labels that are not valid for
   their incoming interfaces).

   By contrast, protection against spoofed IP packets requires all SP
   boundary routers to perform filtering; either (a) filtering packets
   from "outside" the SP, which are addressed to PE routers, or (b)
   filtering packets from "outside" the SP, which have source addresses
   that belong "inside" and, in addition, filtering on each PE all
   packets that have source addresses that belong "outside" the SP.

   The maintenance of these filter lists can be management intensive.
   Furthermore, depending upon implementation, these filter lists can be
   performance affecting.  However, such filters may be required for
   reasons other than protection against spoofed VPN packets, in which
   case the additional maintenance overhead of these filters to protect
   (among other things) against spoofing of VPN packets may be of no
   practical significance.  Note that allocating IP addresses used for
   GRE or IP tunnels out of a single (or a small number of) IP block
   could simplify maintenance of the filters.

6.  Security Considerations

   Security considerations in reference [6] apply here as well.
   Additional security issues are discussed in the previous section
   "Implications on Packet Spoofing".

7.  Acknowledgments

   Thanks to Robert Raszuk and Scott Wainner for their contributions to
   this document.












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8.  Normative References

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

   [2]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private Networks
        (VPNs)", RFC 4364, February 2006.

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

   [4]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol
        Extensions for BGP-4", RFC 4760, January 2007.

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

   [6]  Worster, T., Rekhter, Y., and E. Rosen, "Encapsulating MPLS in
        IP or Generic Routing Encapsulation (GRE)", RFC 4023,
        March 2005.































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Authors' Addresses

   Yakov Rekhter
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   EMail: yakov@juniper.net


   Ronald P. Bonica
   Juniper Networks
   2251 Corporate Park Drive
   Herndon, VA  20171
   US

   EMail: rbonica@juniper.net


   Eric C. Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA  01719
   US

   EMail: erosen@cisco.com
























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