Network Working Group                                        S. Bellovin
Request for Comments: 4278                            AT&T Labs Research
Category: Informational                                         A. Zinin
                                                                 Alcatel
                                                            January 2006

Standards Maturity Variance Regarding the TCP MD5 Signature Option
(RFC 2385) and the BGP-4 Specification

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 Internet Society (2006).

Abstract

   The IETF Standards Process requires that all normative references for
   a document be at the same or higher level of standardization.  RFC
   2026 section 9.1 allows the IESG to grant a variance to the standard
   practices of the IETF.  This document explains why the IESG is
   considering doing so for the revised version of the BGP-4
   specification, which refers normatively to RFC 2385, "Protection of
   BGP Sessions via the TCP MD5 Signature Option".  RFC 2385 will remain
   at the Proposed Standard level.

1. Introduction

   The IETF Standards Process [RFC2026] requires that all normative
   references for a document be at the same or higher level of
   standardization.  RFC 2026 section 9.1 allows the IESG to grant a
   variance to the standard practices of the IETF.  Pursuant to that, it
   is considering publishing the updated BGP-4 specification [RFC4271]
   as Draft Standard, despite the normative reference to [RFC2385],
   "Protection of BGP Sessions via the TCP MD5 Signature Option".  RFC
   2385 will remain a Proposed Standard.  (Note that although the title
   of [RFC2385] includes the word "signature", the technology described
   in it is commonly known as a Message Authentication Code or MAC, and
   should not be confused with digital signature technologies.)

   [RFC2385], which is widely implemented, is the only transmission
   security mechanism defined for BGP-4.  Other possible mechanisms,
   such as IPsec [RFC2401] and TLS [RFC2246], are rarely, if ever, used

   for this purpose.  Given the long-standing requirement for security
   features in protocols, it is not possible to advance BGP-4 without a
   mandated security mechanism.

   The conflict of maturity levels between specifications would normally
   be resolved by advancing the specification being referred to along
   the standards track, to the level of maturity that the referring
   specification needs to achieve.  However, in the particular case
   considered here, the IESG believes that [RFC2385], though adequate
   for BGP deployments at this moment, is not strong enough for general
   use, and thus should not be progressed along the standards track.  In
   this situation, the IESG believes that variance procedure should be
   used to allow the updated BGP-4 specification to be published as
   Draft Standard.

   The following sections of the document give detailed explanations of
   the statements above.

2. Draft Standard Requirements

   The requirements for Proposed Standards and Draft Standards are given
   in [RFC2026].  For Proposed Standards, [RFC2026] warns that:

        Implementors should treat Proposed Standards as immature
        specifications.  It is desirable to implement them in order to
        gain experience and to validate, test, and clarify the
        specification.  However, since the content of Proposed Standards
        may be changed if problems are found or better solutions are
        identified, deploying implementations of such standards into a
        disruption-sensitive environment is not recommended.

   In other words, it is considered reasonable for flaws to be
   discovered in Proposed Standards.

   The requirements for Draft Standards are higher:

        A Draft Standard must be well-understood and known to be quite
        stable, both in its semantics and as a basis for developing an
        implementation.

   In other words, any document that has known deficiencies should not
   be promoted to Draft Standard.

3. The TCP MD5 Signature Option

   [RFC2385], despite its 1998 publication date, describes a Message
   Authentication Code (MAC) that is considerably older.  It utilizes a
   technique known as a "keyed hash function", using MD5 [RFC1321] as
   the hash function.  When the original code was developed, this was
   believed to be a reasonable technique, especially if the key was
   appended (rather than prepended) to the data being protected.  But
   cryptographic hash functions were never intended for use as MACs, and
   later cryptanalytic results showed that the construct was not as
   strong as originally believed [PV1, PV2].  Worse yet, the underlying
   hash function, MD5, has shown signs of weakness [Dobbertin, Wang].
   Accordingly, the IETF community has adopted Hashed Message
   Authentication Code (HMAC) [RFC2104], a scheme with provable security
   properties, as its standard MAC.

   Beyond that, [RFC2385] does not include any sort of key management
   technique.  Common practice is to use a password as a shared secret
   between pairs of sites, but this is not a good idea [RFC3562].

   Other problems are documented in [RFC2385] itself, including the lack
   of a type code or version number, and the inability of systems using
   this scheme to accept certain TCP resets.

   Despite the widespread deployment of [RFC2385] in BGP deployments,
   the IESG has thus concluded that it is not appropriate for use in
   other contexts.  [RFC2385] is not suitable for advancement to Draft
   Standard.

4. Usage Patterns for RFC 2385

   Given the above analysis, it is reasonable to ask why [RFC2385] is
   still used for BGP.  The answer lies in the deployment patterns
   peculiar to BGP.

   BGP connections inherently tend to travel over short paths.  Indeed,
   most external BGP links are one hop.  Although internal BGP sessions
   are usually multi-hop, the links involved are generally inhabited
   only by routers rather than general-purpose computers; general-
   purpose computers are easier for attackers to use as TCP hijacking
   tools [Joncheray].

   Also, BGP peering associations tend to be long-lived and static.  By
   contrast, many other security situations are more dynamic.

   This is not to say that such attacks cannot happen.  (If they
   couldn't happen at all, there would be no point to any security
   measures.)  Attackers could divert links at layers 1 or 2, or they

   could (in some situations) use Address Resolution Protocol (ARP)
   spoofing at Ethernet-based exchange points.  Still, on balance, BGP
   is employed in an environment that is less susceptible to this sort
   of attack.

   There is another class of attack against which BGP is extremely
   vulnerable:  false route advertisements from more than one autonomous
   system (AS) hop away.  However, neither [RFC2385] nor any other
   transmission security mechanism can block such attacks.  Rather, a
   scheme such as S-BGP [Kent] would be needed.

5. LDP

   The Label Distribution Protocol (LDP) [RFC3036] also uses [RFC2385].
   Deployment practices for LDP are very similar to those of BGP: LDP
   connections are usually confined within a single autonomous system
   and most frequently span a single link between two routers.  This
   makes the LDP threat environment very similar to BGP's.  Given this,
   and a considerable installed base of LDP in service provider
   networks, we are not deprecating [RFC2385] for use with LDP.

6. Security Considerations

The IESG believes that the variance described here will not adversely
affect the security of the Internet.

7. Conclusions

   Given the above analysis, the IESG is persuaded that waiving the
   prerequisite requirement is the appropriate thing to do.  [RFC2385]
   is clearly not suitable for Draft Standard.  Other existing
   mechanisms, such as IPsec, would do its job better.  However, given
   the current operational practices in service provider networks at the
   moment -- and in particular the common use of long-lived standard
   keys, [RFC3562] notwithstanding --  the marginal benefit of such
   schemes in this situation would be low, and not worth the transition
   effort.  We would prefer to wait for a security mechanism tailored to
   the major threat environment for BGP.

8. Informative References

   [Dobbertin]  H. Dobbertin, "The Status of MD5 After a Recent Attack",
                RSA Labs' CryptoBytes, Vol. 2 No. 2, Summer 1996.

   [Joncheray]  Joncheray, L.  "A Simple Active Attack Against TCP."
                Proceedings of the Fifth Usenix Unix Security Symposium,
                1995.

   [Kent]       Kent, S., C. Lynn, and K. Seo.  "Secure Border Gateway
                Protocol (Secure-BGP)."  IEEE Journal on Selected Areas
                in Communications, vol. 18, no. 4, April, 2000, pp.
                582-592.

   [RFC3562]    Leech, M., "Key Management Considerations for the TCP
                MD5 Signature Option", RFC 3562, July 2003.

   [PV1]        B. Preneel and P. van Oorschot, "MD-x MAC and building
                fast MACs from hash functions," Advances in Cryptology
                --- Crypto 95 Proceedings, Lecture Notes in Computer
                Science Vol. 963, D. Coppersmith, ed., Springer-Verlag,
                1995.

   [PV2]        B. Preneel and P. van Oorschot, "On the security of two
                MAC algorithms," Advances in Cryptology --- Eurocrypt 96
                Proceedings, Lecture Notes in Computer Science, U.
                Maurer, ed., Springer-Verlag, 1996.

   [RFC1321]    Rivest, R., "The MD5 Message-Digest Algorithm ", RFC
                1321, April 1992.

   [RFC2026]    Bradner, S., "The Internet Standards Process -- Revision
                3", BCP 9, RFC 2026, October 1996.

   [RFC2104]    Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
                Keyed-Hashing for Message Authentication", RFC 2104,
                February 1997.

   [RFC2246]    Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
                RFC 2246, January 1999.

   [RFC2385]    Heffernan, A., "Protection of BGP Sessions via the TCP
                MD5 Signature Option", RFC 2385, August 1998.

   [RFC2401]    Kent, S. and R. Atkinson, "Security Architecture for the
                Internet Protocol", RFC 2401, November 1998.

   [RFC3036]    Andersson, L., Doolan, P., Feldman, N., Fredette, A.,
                and B. Thomas, "LDP Specification", RFC 3036, January
                2001.

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

   [Wang]       Wang, X. and H. Yu, "How to Break MD5 and Other Hash
                Functions."  Proceedings of Eurocrypt '05, 2005.

Authors' Addresses

   Steven M. Bellovin
   Department of Computer Science
   Columbia University
   1214 Amsterdam Avenue, M.C. 0401
   New York, NY 10027-7003

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

   Alex Zinin
   Alcatel
   701 E Middlefield Rd
   Mountain View, CA 94043

   EMail: zinin@psg.com

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