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Versions: 00                                                            
L3VPN WG                                                    M. Behringer
Internet-Draft                                               J. Guichard
Expires: September 23, 2005                            Cisco Systems Inc
                                                              P. Marques
                                                        Juniper Networks
                                                          March 22, 2005

                 Layer-3 VPN Import/Export Verification

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on September 23, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   Configuration errors on Provider Edge (PE) routers in Layer-3 VPN
   networks based on [RFC2547] can lead to security breaches of the
   connected VPNs.  For example, the PE router could be mistakenly
   configured such that a connected Customer Edge (CE) router belongs to

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   an incorrect VPN.  Here we propose a scheme that verifies local and
   remote routing information received by the PE router before it
   installs new VPN routes into the Virtual Routing & Forwarding
   Instance (VRF).  The proposed changes affect only the PE routers.

Table of Contents

   1.  Changes to previous version, and to-do-list  . . . . . . . . .  3
   2.  Conventions used in this document  . . . . . . . . . . . . . .  3
   3.  Problem Statement and Overview . . . . . . . . . . . . . . . .  3
   4.  CE-CE Authentication . . . . . . . . . . . . . . . . . . . . .  4
     4.1   PE-CE Authentication Behavior  . . . . . . . . . . . . . .  5
     4.2   Behaviour of PE sending the UPDATE-authenticator . . . . .  5
     4.3   Behaviour of PE receiving the UPDATE-authenticator . . . .  6
   5.  Extranet VPN Processing  . . . . . . . . . . . . . . . . . . .  7
   6.  The UPDATE-authenticator attribute . . . . . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   10.   Informative References . . . . . . . . . . . . . . . . . . .  9
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
       Intellectual Property and Copyright Statements . . . . . . . . 11

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1.  Changes to previous version, and to-do-list

   This document is not much changed from the previous version,
   draft-behringer-mpls-vpn-auth-04.txt.  Some editorial changes were
   made, and the title adapted to reflect better the intentions of the

   Issues to be addressed in the next version:
   o  Address differences to draft-ietf-l3vpn-l3vpn-auth-01.txt.
   o  Refer to the requirements draft, and clarify which requirements
      are met by the approach described here.
   o  Address how this scheme works with Inter-AS.
   o  Look into alternatives to CE-PE routing, for deployments where
      PE-CE routing is not feasible.

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC-2119].

3.  Problem Statement and Overview

   The current Layer-3 VPN architecture as defined in [RFC2547] does not
   provide any mechanism to determine whether an imported route on a PE
   router originated from the correct VPN.  This opens a potential
   security hole where the VPN Service Provider could mistakenly assign
   on a PE router the incorrect "route-target" values, thus
   inadvertently bringing a connected CE router, with the network/s
   behind it, into a wrong VPN.

   [RFC2547] does not require that PE-CE sessions or PE-PE sessions be
   authenticated.  However, in the cases where this is deployed, route
   authentication relies on a three-step configuration process; From the
   CE router to the PE router, from that PE router to other PE routers
   in the same VPN provider network, and from the other PE routers to
   the corresponding CE routers.

   Correct access control between VPNs relies on the accurate
   configuration of "route-targets" on the PE routers.  Because the 3
   authentication steps above are essentially disjoint, the linkage
   necessary to "glue" them together is the correct configuration of the
   VPN provider network, and the corresponding "route-target" values.  .

   If the Service Provider has assigned the wrong "route-target" values
   then this is hard to detect from within the customer's network, and a
   real issue in [RFC2547] networks.  One possible solution to this
   problem is to mount IPsec [RFC2401] on all CE routers, but this is

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   often perceived as too "heavy-weight".  Therefore, a mechanism is
   required which prevents routes from being passed into a PE router's
   Virtual Routing & Forwarding Instance (VRF), unless they have been
   verified to belong to the associated VPN.  Also, in the case of such
   configuration errors, the Service Provider must be alerted so that
   the mistake can be rectified.

   This proposal aims to solve the problem of accidental
   misconfiguration of VPN parameters on PE routers.  The approach is to
   associate one or more authentication keys to a VPN, and use existing
   routing protocol authentication mechanisms [RFC2082, 2154, 2385], to
   provide PE-CE authentication.  PE-PE routing exchanges are validated
   via routing update signatures.  Since a PE router can hold several
   VRF's, the authentication between PEs will use the different MD5
   keys, based on which VRF's routes need to be verified.

   BGP UPDATE messages between PE routers will include a new BGP
   attribute, hereby referred to as the "UPDATE-authenticator".  This
   attribute contains a keyed HMAC MD5 signature of a locally generated
   per-VRF random number, using the MD5 key that is also used on this PE
   router for the PE-CE routing authentication of that VPN.

   The receiving PE router generates a keyed HMAC MD5 signature using
   information from the "UPDATE-authenticator" attribute contained
   within the BGP UPDATE message, and the routing key of the CE router
   that is to receive the routes contained within the update.  If the
   result is different from the signature value transmitted in the
   UPDATE-authenticator attribute, the routes within the UPDATE are not
   imported and a warning is logged.

   This proposal imposes some operational constraints to be workable;
   Regardless of whether a routing protocol is used or not within the
   VRF, at least one authentication key MUST be configured for each VRF
   that wishes to use the mechanisms described within this document.  If
   a dynamic routing protocol is used, then routing with MD5
   authentication [RFC2082, 2154, 2385] SHOULD be configured for all PE-
   CE links of a particular VPN.  All CE routers of the same VPN MAY use
   the same or different MD5 keys and the PE router MUST indicate which
   key has been used when advertising routes from the associated VRF.
   If the Service Provider manages the CE routers on behalf of the
   customer, then downstream C routers MUST also use the same MD5 key.
   MD5 keys SHOULD be chosen to be unique to a VPN.

4.  CE-CE Authentication

   As previously stated, this document proposes to re-use the MD5 key
   that is being used for PE-CE routing authentication.  This has the
   advantage that no changes or software upgrades are necessary at the

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   CE routers or within the VPN site.  For this proposal to work, each
   PE router, on export of the routes from within a given VPN, MUST
   indicate which MD5 key has been used to authenticate the local
   routes.  The MD5 key set SHOULD be unique to each VPN.  The VPN
   customer configures thus all their CE routers with an MD5 key.  The
   VPN Service Provider also configures the PE routers with this local
   key on all links to the customers CE routers.  This proposal does not
   affect the CE-PE routing authentication, but the authentication MUST
   be used for this scheme to work.

   This proposal is orthogonal with MD5 authentication between PE
   routers on the VPN network.  Authentication of peering sessions
   between PEs provides protection of the VPN routing information
   without any validation of its origin.

   While currently, the VPN service provider may choose to configure
   routing authentication between the PE and CE, this information only
   affects the local routing session between the two routers.
   Conceptually, this proposal extends this key verification between the
   local PE and CE to remote PE to CE connections.

   Using the mechanisms described within this document, the BGP UPDATE
   message, as defined in [RFC1771], is sent between PE routers (or BGP
   route reflectors), and carries a new UPDATE-authenticator attribute,
   which is used to verify the source of the routing information.

4.1  PE-CE Authentication Behavior

   If a dynamic routing protocol is used between PE and CE routers, then
   the routing protocol is secured with MD5 authentication.  Routes are
   only put into a VRF that is configured with Layer-3 VPN
   "Import/Export Verification" if the MD5 authentication is successful.

   If a VRF is configured at the PE router for Layer-3 VPN
   "Import/Export Verification" using MD5 authentication, it is OPTIONAL
   to confirm local route authentication prior to any route export from
   the VRF.  Route authentication involves checking whether the PE
   router can confirm route receipt from each CE router that is attached
   to the VRF.

4.2  Behaviour of PE sending the UPDATE-authenticator

   When Layer-3 "Import/Export Verification" is enabled, the PE router
   SHOULD calculate a random number, referred to as the 'Generator', for
   each VRF that is configured for authentication.  Alternatively a
   combination of the local "route-target" values may be used to
   generate this number.  This is implementation specific.

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   Having generated the VRF specific random number, the PE router on
   sending a [RFC2858] BGP UPDATE calculates a keyed HMAC-MD5 signature,
   as defined in [RFC2104], over the 'Generator', using the key of one
   of the CEs that is connected to the corresponding VRF.  The result of
   this calculation is carried, along with the 'Generator' and an
   identification of the key used against the 'Generator', in the "HMAC-
   MD5 Signature" field within the UPDATE-authenticator attribute.

   Each key within a VRF will have a corresponding 'key-identifier',
   which SHOULD be configurable within the VRF, and MUST be unique
   across VPNs.  Every PE router that holds members of the VPN MUST
   carry <key, key-identifier> mappings so that they can verify which
   key to use when authenticating incoming routing updates.  The
   key-identifier MAY be the route-target.

   The PE sending an [RFC2858] UPDATE will add a 'key-identifier' to the
   UPDATE-authenticator attribute to indicate which key should be used
   by a receiving PE router to verify the update.  The UPDATE message is
   sent to any [RFC2858] BGP peers (other PE routers or BGP route
   reflectors).  The "route-targets" in the [RFC2858] update determine
   which VRF/s the UPDATE refers to, and these are used as described in
   [RFC2547] to determine which PE routers will import which routes.

4.3  Behaviour of PE receiving the UPDATE-authenticator

   A PE router that receives a [RFC2858] BGP update that contains the
   UPDATE-authenticator attribute SHOULD verify the contents of the
   update with the following algorithm.  As an OPTIONAL step, the PE
   router MAY perform this comparison only if it has authenticated local
   routes from the CE router:

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   IF target VRF is configured for Layer-3 VPN Import/Exp. Verification
      IF UPDATE-authenticator attribute is present
         subroutine determine_MD5-key
         verify UPDATE-authenticator with MD5-key
         IF result = signature of received UPDATE-authenticator
            import route into VRF
            mark routes as 'not authenticated'; log error
         mark routes as 'not authenticated'; log error
      mark routes as 'not authenticated'; log error

   subroutine determine_MD5-key
      IF key-identifier = 0
         MD5-key = the MD5 key used for routing authentication
                   with one of the routing peers of the VRF.
         MD5-key = lookup_in_config (key-identifier)
   RETURN MD5-key

   A router MAY verify whether all MD5 keys for a given VRF are the
   same.  If it does a warning message MUST be logged if it detects

   In the case where the Service Provider manages the CE routers, the
   Service Provider must also configure the key at the CE routers and
   this should match with any directly connected downstream C routers
   within the customer site.  If the C routers have a different key than
   the CE router then the CE will not authenticate any routes from
   within the site, and will therefore not advertise any routing
   information to the PE router.  The PE router is thus able to use the
   previously described mechanisms and will not import/export any routes
   from/to the customers VRF.

5.  Extranet VPN Processing

   There are typically two types of Extranets that can be defined using
   the [RFC2547] architecture; Central Services Extranet and Distributed

   The Central Services Extranet provides connectivity between spoke VPN
   sites through a central PE router.  This PE router carries routes for
   all members of the extranet whereas spoke PE routers carry only local

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   routes, and a route to the central PE router.  To support this type
   of configuration, the central PE router needs to carry <key, key-
   identifier> mappings for ALL members of the extranet.  On receiving
   an incoming update, the central PE is able to identify which key to
   use on the UPDATE-authenticator attribute by looking at the key-
   identifier carried within the update.

   The Distributed Extranet model provides connectivity directly between
   members of a given VPN.  This means that each PE router that holds
   members of the extranet is configured to import the relevant "route-
   target" values used for export by other members of the VPN.  Using
   the key-identifier, a PE router is able to identify which key to use
   on an incoming update to verify the source.  This means that each PE
   router within the extranet MUST carry <key, key-identifier> mappings
   for all members of the VPN.

6.  The UPDATE-authenticator attribute

   The UPDATE-authenticator attribute is an optional, transitive BGP
   attribute, with an attribute type code value to be assigned.  Its
   length is 24 octets, which is the length of the output of an MD5
   function (16 octets), a 'Generator' field, and a 'Key-identifier', as
   shown in the following figure.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |                          HMAC-MD5 Signature                   |
   |                          HMAC-MD5 (cont)                      |
   |                          HMAC-MD5 (cont)                      |
   |                          HMAC-MD5 (cont)                      |
   |                          Generator                            |
   |                        Key-identifier                         |

7.  IANA Considerations

   The UPDATE-authenticator BGP attribute type will need to be
   registered with IANA, according to the procedures defined in

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8.  Security Considerations

   This modification to the behavior of the PE router aims at detecting
   inadvertent configuration mistakes of the Service Provider, and at
   isolating CE routers that appear not to belong to the VPN they were
   configured for.

   There is no protection against the Service Provider staff maliciously
   adding a CE router to a VPN.  However, the malicious engineer must
   know the MD5 key of the VPN to be intruded.  This threat can be
   avoided with CE-CE IPsec authentication, which is configured by the
   VPN customer, and to which the Service Provider does not have access.

9.  Acknowledgements

   Many thanks to Dan Tappan, David Allan and Eric Vyncke for their
   contributions to this work.

10.  Informative References

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

   [2]  Manning, B., "Registering New BGP Attribute Types", RFC 2042,
        January 1997.

   [3]  Baker, F., Atkinson, R. and G. Malkin, "RIP-2 MD5
        Authentication", RFC 2082, January 1997.

   [4]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
        for Message Authentication", RFC 2104, February 1997.

   [5]  Murphy, S., Badger, M. and B. Wellington, "OSPF with Digital
        Signatures", RFC 2154, June 1997.

   [6]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
        Signature Option", RFC 2385, August 1998.

   [7]  Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547, March 1999.

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

   [9]  Bates, T., Rekhter, Y., Chandra, R. and D. Katz, "Multiprotocol
        Extensions for BGP-4", RFC 2858, June 2000.

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Authors' Addresses

   Michael H. Behringer
   Cisco Systems Inc
   Village d'Entreprises Green Side
   400, Avenue Roumanille, Batiment T 3
   Biot - Sophia Antipolis  06410

   Email: mbehring@cisco.com
   URI:   http://www.cisco.com

   Jim Guichard
   Cisco Systems Inc
   300 Apollo Drive
   Chelmsford, MA  01824

   Email: jguichar@cisco.com
   URI:   http://www.cisco.com

   Pedro Marques
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089

   Email: roque@juniper.net
   URI:   http://www.juniper.net

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Intellectual Property Statement

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Copyright Statement

   Copyright (C) The Internet Society (2005).  This document is subject
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