





Internet Engineering Task Force (IETF)                       S. Bellovin
Request for Comments: 7353                           Columbia University
Category: Informational                                          R. Bush
ISSN: 2070-1721                                Internet Initiative Japan
                                                                 D. Ward
                                                           Cisco Systems
                                                             August 2014


             Security Requirements for BGP Path Validation

Abstract

   This document describes requirements for a BGP security protocol
   design to provide cryptographic assurance that the origin Autonomous
   System (AS) has the right to announce the prefix and to provide
   assurance of the AS Path of the announcement.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see 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/rfc7353.

Copyright Notice

   Copyright (c) 2014 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.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Recommended Reading . . . . . . . . . . . . . . . . . . . . .   2
   3.  General Requirements  . . . . . . . . . . . . . . . . . . . .   3
   4.  BGP UPDATE Security Requirements  . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Origin validation based on Resource Public Key Infrastructure (RPKI)
   [RFC6811] provides a measure of resilience to accidental
   mis-origination of prefixes; however, it provides neither
   cryptographic assurance (announcements are not signed) nor assurance
   of the AS Path of the announcement.

   This document describes requirements to be placed on a BGP security
   protocol, herein termed "BGPsec", intended to rectify these gaps.

   The threat model assumed here is documented in [RFC4593] and
   [RFC7132].

   As noted in the threat model [RFC7132], this work is limited to
   threats to the BGP protocol.  Issues of business relationship
   conformance, while quite important to operators, are not security
   issues per se and are outside the scope of this document.  It is
   hoped that these issues will be better understood in the future.

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, without normative meaning.

2.  Recommended Reading

   This document assumes knowledge of the RPKI [RFC6480] and the RPKI
   Repository Structure [RFC6481].






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   This document assumes ongoing incremental deployment of Route Origin
   Authorizations (ROAs) [RFC6482], the RPKI to the Router Protocol
   [RFC6810], and RPKI-based Prefix Validation [RFC6811].

   And, of course, a knowledge of BGP [RFC4271] is required.

3.  General Requirements

   The following are general requirements for a BGPsec protocol:

   3.1   A BGPsec design MUST allow the receiver of a BGP announcement
         to determine, to a strong level of certainty, that the
         originating AS in the received PATH attribute possessed the
         authority to announce the prefix.

   3.2   A BGPsec design MUST allow the receiver of a BGP announcement
         to determine, to a strong level of certainty, that the received
         PATH attribute accurately represents the sequence of External
         BGP (eBGP) exchanges that propagated the prefix from the origin
         AS to the receiver, particularly if an AS has added or deleted
         any AS number other than its own in the PATH attribute.  This
         includes modification to the number of AS prepends.

   3.3   BGP attributes other than the AS_PATH are used only locally, or
         have meaning only between immediate neighbors, may be modified
         by intermediate systems and figure less prominently in the
         decision process.  Consequently, it is not appropriate to try
         to protect such attributes in a BGPsec design.

   3.4   A BGPsec design MUST be amenable to incremental deployment.
         This implies that incompatible protocol capabilities MUST be
         negotiated.

   3.5   A BGPsec design MUST provide analysis of the operational
         considerations for deployment and particularly of incremental
         deployment, e.g., contiguous islands, non-contiguous islands,
         universal deployment, etc.

   3.6   As proofs of possession and authentication may require
         cryptographic payloads and/or storage and computation, likely
         increasing processing and memory requirements on routers, a
         BGPsec design MAY require use of new hardware.  That is,
         compatibility with current hardware abilities is not a
         requirement that this document imposes on a solution.

   3.7   A BGPsec design need not prevent attacks on data-plane traffic.
         It need not provide assurance that the data plane even follows
         the control plane.



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   3.8   A BGPsec design MUST resist attacks by an enemy who has access
         to the inter-router link layer, per Section 3.1.1.2 of
         [RFC4593].  In particular, such a design MUST provide
         mechanisms for authentication of all data, including protecting
         against message insertion, deletion, modification, or replay.
         Mechanisms that suffice include TCP sessions authenticated with
         the TCP Authentication Option (TCP-AO) [RFC5925], IPsec
         [RFC4301], or Transport Layer Security (TLS) [RFC5246].

   3.9   It is assumed that a BGPsec design will require information
         about holdings of address space and Autonomous System Numbers
         (ASNs), and assertions about binding of address space to ASNs.
         A BGPsec design MAY make use of a security infrastructure
         (e.g., a PKI) to distribute such authenticated data.

   3.10  It is entirely OPTIONAL to secure AS SETs and prefix
         aggregation.  The long-range solution to this is the
         deprecation of AS_SETs; see [RFC6472].

   3.11  If a BGPsec design uses signed prefixes, given the difficulty
         of splitting a signed message while preserving the signature,
         it need not handle multiple prefixes in a single UPDATE PDU.

   3.12  A BGPsec design MUST enable each BGPsec speaker to configure
         use of the security mechanism on a per-peer basis.

   3.13  A BGPsec design MUST provide backward compatibility in the
         message formatting, transmission, and processing of routing
         information carried through a mixed security environment.
         Message formatting in a fully secured environment MAY be
         handled in a non-backward compatible manner.

   3.14  While the formal validity of a routing announcement should be
         determined by the BGPsec protocol, local routing policy MUST be
         the final arbiter of the best path and other routing decisions.

   3.15  A BGPsec design MUST support 'transparent' route servers,
         meaning that the AS of the route server is not counted in
         downstream BGP AS-path-length tie-breaking decisions.

   3.16  A BGPsec design MUST support AS aliasing.  This technique is
         not well defined or universally implemented but is being
         documented in [AS-MIGRATION].  A BGPsec design SHOULD
         accommodate AS 'migration' techniques such as common
         proprietary and non-standard methods that allow a router to
         have two AS identities, without lengthening the effective AS
         Path.




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   3.17  If a BGPsec design makes use of a security infrastructure, that
         infrastructure SHOULD enable each network operator to select
         the entities it will trust when authenticating data in the
         security infrastructure.  See, for example, [LTA-USE-CASES].

   3.18  A BGPsec design MUST NOT require operators to reveal more than
         is currently revealed in the operational inter-domain routing
         environment, other than the inclusion of necessary security
         credentials to allow others to ascertain for themselves the
         necessary degree of assurance regarding the validity of Network
         Layer Reachability Information (NLRI) received via BGPsec.
         This includes peering, customer/provider relationships, an
         ISP's internal infrastructure, etc.  It is understood that some
         data are revealed to the savvy seeker by BGP, traceroute, etc.,
         today.

   3.19  A BGPsec design MUST signal (e.g., via logging or SNMP)
         security exceptions that are significant to the operator.  The
         specific data to be signaled are an implementation matter.

   3.20  Any routing information database MUST be re-authenticated
         periodically or in an event-driven manner, especially in
         response to events such as, for example, PKI updates.

   3.21  Any inter-AS use of cryptographic hashes or signatures MUST
         provide mechanisms for algorithm agility.  For a discussion,
         see [ALG-AGILITY].

   3.22  A BGPsec design SHOULD NOT presume to know the intent of the
         originator of a NLRI, nor that of any AS on the AS Path, other
         than that they intend to pass it to the next AS in the path.

   3.23  A BGPsec listener SHOULD NOT trust non-BGPsec markings, such as
         communities, across trust boundaries.

4.  BGP UPDATE Security Requirements

   The following requirements MUST be met in the processing of BGP
   UPDATE messages:

   4.1  A BGPsec design MUST enable each recipient of an UPDATE to
        formally validate that the origin AS in the message is
        authorized to originate a route to the prefix(es) in the
        message.







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   4.2  A BGPsec design MUST enable the recipient of an UPDATE to
        formally determine that the NLRI has traversed the AS Path
        indicated in the UPDATE.  Note that this is more stringent than
        showing that the path is merely not impossible.

   4.3  Replay of BGP UPDATE messages need not be completely prevented,
        but a BGPsec design SHOULD provide a mechanism to control the
        window of exposure to replay attacks.

   4.4  A BGPsec design SHOULD provide some level of assurance that the
        origin of a prefix is still 'alive', i.e., that a monkey in the
        middle has not withheld a WITHDRAW message or the effects
        thereof.

   4.5  The AS Path of an UPDATE message SHOULD be able to be
        authenticated as the message is processed.

   4.6  Normal sanity checks of received announcements MUST be done,
        e.g., verification that the first element of the AS_PATH list
        corresponds to the locally configured AS of the peer from which
        the UPDATE was received.

   4.7  The output of a router applying BGPsec validation to a received
        UPDATE MUST be unequivocal and conform to a fully specified
        state in the design.

5.  Security Considerations

   If an external "security infrastructure" is used, as mentioned in
   Section 3, paragraphs 9 and 17 above, the authenticity and integrity
   of the data of such an infrastructure MUST be assured.  In addition,
   the integrity of those data MUST be assured when they are used by
   BGPsec, e.g., in transport.

   The requirement of backward compatibility to BGP4 may open an avenue
   to downgrade attacks.

   The data plane might not follow the path signaled by the control
   plane.

   Security for subscriber traffic is outside the scope of this document
   and of BGP security in general.  IETF standards for payload data
   security should be employed.  While adoption of BGP security measures
   may ameliorate some classes of attacks on traffic, these measures are
   not a substitute for use of subscriber-based security.






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6.  Acknowledgments

   The authors wish to thank the authors of [BGP-SECURITY] from whom we
   liberally stole, Roque Gagliano, Russ Housley, Geoff Huston, Steve
   Kent, Sandy Murphy, Eric Osterweil, John Scudder, Kotikalapudi
   Sriram, Sam Weiler, and a number of others.

7.  References

7.1.  Normative References

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

   [RFC4593]  Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
              Routing Protocols", RFC 4593, October 2006.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, June 2010.

   [RFC7132]  Kent, S. and A. Chi, "Threat Model for BGP Path Security",
              RFC 7132, February 2014.

7.2.  Informative References

   [ALG-AGILITY]
              Housley, R., "Guidelines for Cryptographic Algorithm
              Agility", Work in Progress, June 2014.

   [AS-MIGRATION]
              George, W. and S. Amante, "Autonomous System (AS)
              Migration Features and Their Effects on the BGP AS_PATH
              Attribute", Work in Progress, January 2014.

   [BGP-SECURITY]
              Christian, B. and T. Tauber, "BGP Security Requirements",
              Work in Progress, November 2008.

   [LTA-USE-CASES]
              Bush, R., "RPKI Local Trust Anchor Use Cases", Work in
              Progress, June 2014.

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

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.




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   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC6472]  Kumari, W. and K. Sriram, "Recommendation for Not Using
              AS_SET and AS_CONFED_SET in BGP", BCP 172, RFC 6472,
              December 2011.

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

   [RFC6481]  Huston, G., Loomans, R., and G. Michaelson, "A Profile for
              Resource Certificate Repository Structure", RFC 6481,
              February 2012.

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

   [RFC6810]  Bush, R. and R. Austein, "The Resource Public Key
              Infrastructure (RPKI) to Router Protocol", RFC 6810,
              January 2013.

   [RFC6811]  Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
              Austein, "BGP Prefix Origin Validation", RFC 6811, January
              2013.



























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Authors' Addresses

   Steven M. Bellovin
   Columbia University
   1214 Amsterdam Avenue, MC 0401
   New York, New York  10027
   USA

   Phone: +1 212 939 7149
   EMail: bellovin@acm.org


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

   EMail: randy@psg.com


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

   EMail: dward@cisco.com























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