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RFC 6811 - BGP Prefix Origin Validation


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Internet Engineering Task Force (IETF)                      P. Mohapatra
Request for Comments: 6811                                 Cisco Systems
Category: Standards Track                                     J. Scudder
ISSN: 2070-1721                                         Juniper Networks
                                                                 D. Ward
                                                           Cisco Systems
                                                                 R. Bush
                                               Internet Initiative Japan
                                                              R. Austein
                                                    Dragon Research Labs
                                                            January 2013

                      BGP Prefix Origin Validation

Abstract

   To help reduce well-known threats against BGP including prefix mis-
   announcing and monkey-in-the-middle attacks, one of the security
   requirements is the ability to validate the origination Autonomous
   System (AS) of BGP routes.  More specifically, one needs to validate
   that the AS number claiming to originate an address prefix (as
   derived from the AS_PATH attribute of the BGP route) is in fact
   authorized by the prefix holder to do so.  This document describes a
   simple validation mechanism to partially satisfy this requirement.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6811.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . . . 4
   2.  Prefix-to-AS Mapping Database . . . . . . . . . . . . . . . . . 4
     2.1.  Pseudo-Code . . . . . . . . . . . . . . . . . . . . . . . . 6
   3.  Policy Control  . . . . . . . . . . . . . . . . . . . . . . . . 6
   4.  Interaction with Local Cache  . . . . . . . . . . . . . . . . . 7
   5.  Deployment Considerations . . . . . . . . . . . . . . . . . . . 7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     8.2.  Informational References  . . . . . . . . . . . . . . . . . 9

1.  Introduction

   A BGP route associates an address prefix with a set of Autonomous
   Systems (ASes) that identify the interdomain path the prefix has
   traversed in the form of BGP announcements.  This set is represented
   as the AS_PATH attribute in BGP [RFC4271] and starts with the AS that
   originated the prefix.  To help reduce well-known threats against BGP
   including prefix mis-announcing and monkey-in-the-middle attacks, one
   of the security requirements is the ability to validate the
   origination AS of BGP routes.  More specifically, one needs to
   validate that the AS number claiming to originate an address prefix
   (as derived from the AS_PATH attribute of the BGP route) is in fact
   authorized by the prefix holder to do so.  This document describes a
   simple validation mechanism to partially satisfy this requirement.

   The Resource Public Key Infrastructure (RPKI) describes an approach
   to build a formally verifiable database of IP addresses and AS
   numbers as resources.  The overall architecture of RPKI as defined in
   [RFC6480] consists of three main components:

   o  a public key infrastructure (PKI) with the necessary certificate
      objects,

   o  digitally signed routing objects, and

   o  a distributed repository system to hold the objects that would
      also support periodic retrieval.

   The RPKI system is based on resource certificates that define
   extensions to X.509 to represent IP addresses and AS identifiers
   [RFC3779], thus the name RPKI.  Route Origin Authorizations (ROAs)
   [RFC6482] are separate digitally signed objects that define
   associations between ASes and IP address blocks.  Finally, the
   repository system is operated in a distributed fashion through the
   IANA, Regional Internet Registry (RIR) hierarchy, and ISPs.

   In order to benefit from the RPKI system, it is envisioned that
   relying parties at either the AS or organization level obtain a local
   copy of the signed object collection, verify the signatures, and
   process them.  The cache must also be refreshed periodically.  The
   exact access mechanism used to retrieve the local cache is beyond the
   scope of this document.

   Individual BGP speakers can utilize the processed data contained in
   the local cache to validate BGP announcements.  The protocol details
   to retrieve the processed data from the local cache to the BGP
   speakers is beyond the scope of this document (refer to [RFC6810] for
   such a mechanism).  This document proposes a means by which a BGP
   speaker can make use of the processed data in order to assign a
   "validation state" to each prefix in a received BGP UPDATE message.

   Note that the complete path attestation against the AS_PATH attribute
   of a route is outside the scope of this document.

   Like the DNS, the global RPKI presents only a loosely consistent
   view, depending on timing, updating, fetching, etc.  Thus, one cache
   or router may have different data about a particular prefix than
   another cache or router.  There is no 'fix' for this; it is the
   nature of distributed data with distributed caches.

   Although RPKI provides the context for this document, it is equally
   possible to use any other database that is able to map prefixes to
   their authorized origin ASes.  Each distinct database will have its

   own particular operational and security characteristics; such
   characteristics are beyond the scope of this document.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to
   be interpreted as described in RFC 2119 [RFC2119] only when they
   appear in all upper case.  They may also appear in lower or mixed
   case as English words, without special meaning.

2.  Prefix-to-AS Mapping Database

   The BGP speaker loads validated objects from the cache into local
   storage.  The objects loaded have the content (IP address, prefix
   length, maximum length, origin AS number).  We refer to such a
   locally stored object as a "Validated ROA Payload" or "VRP".

   We define several terms in addition to "VRP".  Where these terms are
   used, they are capitalized:

   o  Prefix: (IP address, prefix length), interpreted as is customary
      (see [RFC4632]).

   o  Route: Data derived from a received BGP UPDATE, as defined in
      [RFC4271], Section 1.1.  The Route includes one Prefix and an
      AS_PATH; it may include other attributes to characterize the
      prefix.

   o  VRP Prefix: The Prefix from a VRP.

   o  VRP ASN: The origin AS number from a VRP.

   o  Route Prefix: The Prefix derived from a route.

   o  Route Origin ASN: The origin AS number derived from a Route as
      follows:

      *  the rightmost AS in the final segment of the AS_PATH attribute
         in the Route if that segment is of type AS_SEQUENCE, or

      *  the BGP speaker's own AS number if that segment is of type
         AS_CONFED_SEQUENCE or AS_CONFED_SET or if the AS_PATH is empty,
         or

      *  the distinguished value "NONE" if the final segment of the
         AS_PATH attribute is of any other type.

   o  Covered: A Route Prefix is said to be Covered by a VRP when the
      VRP prefix length is less than or equal to the Route prefix
      length, and the VRP prefix address and the Route prefix address
      are identical for all bits specified by the VRP prefix length.
      (That is, the Route prefix is either identical to the VRP prefix
      or more specific than the VRP prefix.)

   o  Matched: A Route Prefix is said to be Matched by a VRP when the
      Route Prefix is Covered by that VRP, the Route prefix length is
      less than or equal to the VRP maximum length, and the Route Origin
      ASN is equal to the VRP ASN.

   Given these definitions, any given BGP Route will be found to have
   one of the following validation states:

   o  NotFound: No VRP Covers the Route Prefix.

   o  Valid: At least one VRP Matches the Route Prefix.

   o  Invalid: At least one VRP Covers the Route Prefix, but no VRP
      Matches it.

   We observe that no VRP can have the value "NONE" as its VRP ASN.
   Thus, a Route whose Origin ASN is "NONE" cannot be Matched by any
   VRP.  Similarly, no valid Route can have an Origin ASN of zero [AS0].
   Thus, no Route can be Matched by a VRP whose ASN is zero.

   When a BGP speaker receives an UPDATE from a neighbor, it SHOULD
   perform a lookup as described above for each of the Routes in the
   UPDATE message.  The lookup SHOULD also be applied to routes that are
   redistributed into BGP from another source, such as another protocol
   or a locally defined static route.  An implementation MAY provide
   configuration options to control which routes the lookup is applied
   to.  The validation state of the Route MUST be set to reflect the
   result of the lookup.  The implementation should consider the
   validation state as described in the document as a local property or
   attribute of the Route.  If validation is not performed on a Route,
   the implementation SHOULD initialize the validation state of such a
   route to "NotFound".

   Use of the validation state is discussed in Sections 3 and 5.  An
   implementation MUST NOT exclude a route from the Adj-RIB-In or from
   consideration in the decision process as a side effect of its
   validation state, unless explicitly configured to do so.

   We observe that a Route can be Matched or Covered by more than one
   VRP.  This procedure does not mandate an order in which VRPs must be
   visited; however, the validation state output is fully determined.

2.1.  Pseudo-Code

   The following pseudo-code illustrates the procedure above.  In case
   of ambiguity, the procedure above, rather than the pseudo-code,
   should be taken as authoritative.

   result = BGP_PFXV_STATE_NOT_FOUND;

   //Iterate through all the Covering entries in the local VRP
   //database, pfx_validate_table.
   entry = next_lookup_result(pfx_validate_table, route_prefix);

   while (entry != NULL) {
     prefix_exists = TRUE;

     if (route_prefix_length <= entry->max_length) {
       if (route_origin_as != NONE
           && entry->origin_as != 0
           && route_origin_as == entry->origin_as) {
         result = BGP_PFXV_STATE_VALID;
         return (result);
       }
     }
     entry = next_lookup_result(pfx_validate_table, input.prefix);
   }

   //If one or more VRP entries Covered the route prefix, but
   //none Matched, return "Invalid" validation state.
   if (prefix_exists == TRUE) {
     result = BGP_PFXV_STATE_INVALID;
   }

   return (result);

3.  Policy Control

   An implementation MUST provide the ability to match and set the
   validation state of routes as part of its route policy filtering
   function.  Use of validation state in route policy is elaborated in
   Section 5.  For more details on operational policy considerations,
   see [ORIGIN-OPS].

   An implementation MUST also support four-octet AS numbers [RFC6793].

4.  Interaction with Local Cache

   Each BGP speaker supporting prefix validation as described in this
   document is expected to communicate with one or more RPKI caches,
   each of which stores a local copy of the global RPKI database.  The
   protocol mechanisms used to gather and validate these data and
   present them to BGP speakers are described in [RFC6810].

   The prefix-to-AS mappings used by the BGP speaker are expected to be
   updated over time.  When a mapping is added or deleted, the
   implementation MUST re-validate any affected prefixes and run the BGP
   decision process if needed.  An "affected prefix" is any prefix that
   was matched by a deleted or updated mapping, or could be matched by
   an added or updated mapping.

5.  Deployment Considerations

   Once a Route is selected for validation, it is categorized according
   the procedure given in Section 2.  Subsequently, routing policy as
   discussed in Section 3 can be used to take action based on the
   validation state.

   Policies that could be implemented include filtering routes based on
   validation state (for example, rejecting all "invalid" routes) or
   adjusting a route's degree of preference in the selection algorithm
   based on its validation state.  The latter could be accomplished by
   adjusting the value of such attributes as LOCAL_PREF.  Considering
   invalid routes for BGP decision process is a purely local policy
   matter and should be done with utmost care.

   In some cases (particularly when the selection algorithm is
   influenced by the adjustment of a route property that is not
   propagated into Internal BGP (IBGP)) it could be necessary for
   routing correctness to propagate the validation state to the IBGP
   peer.  This can be accomplished on the sending side by setting a
   community or extended community based on the validation state, and on
   the receiving side by matching the (extended) community and setting
   the validation state.

6.  Security Considerations

   Although this specification discusses one portion of a system to
   validate BGP routes, it should be noted that it relies on a database
   (RPKI or other) to provide validation information.  As such, the
   security properties of that database must be considered in order to
   determine the security provided by the overall solution.  If
   "invalid" routes are blocked as this specification suggests, the
   overall system provides a possible denial-of-service vector; for

   example, if an attacker is able to inject (or remove) one or more
   records into (or from) the validation database, it could lead an
   otherwise valid route to be marked as invalid.

   In addition, this system is only able to provide limited protection
   against a determined attacker -- the attacker need only prepend the
   "valid" source AS to a forged BGP route announcement in order to
   defeat the protection provided by this system.

   This mechanism does not protect against "AS-in-the-middle attacks" or
   provide any path validation.  It only attempts to verify the origin.
   In general, this system should be thought of more as a protection
   against misconfiguration than as true "security" in the strong sense.

7.  Acknowledgments

   The authors wish to thank Rex Fernando, Hannes Gredler, Mouhcine
   Guennoun, Russ Housley, Junaid Israr, Miya Kohno, Shin Miyakawa, Taka
   Mizuguchi, Hussein Mouftah, Keyur Patel, Tomoya Yoshida, Kannan
   Varadhan, Wes George, Jay Borkenhagen, and Sandra Murphy.  The
   authors are grateful for the feedback from the members of the SIDR
   working group.

   Junaid Israr's contribution to this specification was part of his PhD
   research work and thesis at University of Ottawa.

8.  References

8.1.  Normative References

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3779]     Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for
                 IP Addresses and AS Identifiers", RFC 3779, June 2004.

   [RFC4271]     Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
                 Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4632]     Fuller, V. and T. Li, "Classless Inter-domain Routing
                 (CIDR): The Internet Address Assignment and Aggregation
                 Plan", BCP 122, RFC 4632, August 2006.

   [RFC6482]     Lepinski, M., Kent, S., and D. Kong, "A Profile for
                 Route Origin Authorizations (ROAs)", RFC 6482,
                 February 2012.

   [RFC6793]     Vohra, Q. and E. Chen, "BGP Support for Four-Octet
                 Autonomous System (AS) Number Space", RFC 6793,
                 December 2012.

8.2.  Informational References

   [AS0]         Kumari, W., Bush, R., Schiller, H., and K. Patel,
                 "Codification of AS 0 processing.", Work in Progress,
                 August 2012.

   [ORIGIN-OPS]  Bush, R., "RPKI-Based Origin Validation Operation",
                 Work in Progress, August 2012.

   [RFC6480]     Lepinski, M. and S. Kent, "An Infrastructure to Support
                 Secure Internet Routing", RFC 6480, February 2012.

   [RFC6810]     Bush, R. and R. Austein, "The RPKI/Router Protocol",
                 RFC 6810, January 2013.

Authors' Addresses

   Pradosh Mohapatra
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   EMail: pmohapat@cisco.com

   John Scudder
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA  94089
   USA

   EMail: jgs@juniper.net

   David Ward
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   EMail: dward@cisco.com

   Randy Bush
   Internet Initiative Japan
   5147 Crystal Springs
   Bainbridge Island, WA  98110
   USA

   EMail: randy@psg.com

   Rob Austein
   Dragon Research Labs

   EMail: sra@hactrn.net

 

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