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 © 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.

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].

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.
   
   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.
   
   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.
   
   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.

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.
   
   [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.

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