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Versions: 00 01 rfc4278                                                 
Network Working Group                                 Steven M. Bellovin
Internet Draft                                        AT&T Labs Research

Expiration Date: September 2004                               March 2004

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


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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   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.

Bellovin                                                        [Page 1]

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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 [RLH] as
   Draft Standard, despite the normative reference to [RFC2385],
   "Protection of BGP Sessions via the TCP MD5 Signature Option"; that
   protocol will remain a Proposed Standard.

   [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 with no
   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.

Bellovin                                                        [Page 2]

Internet Draft         draft-iesg-tcpmd5app-00.txt            March 2004

   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

   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.  At the time the original code was developed, this
   was believed to be a reasonable technique, especially if the key was
   appended to the data being protected, rather than being prepended.
   But cryptographic hash functions were never intended for use as MACs,
   and later cryptanalytic results showed that the construct was not as
   strong as was originally believed [PV1,PV2].  Worse yet, the
   underlying hash function, MD5, has shown signs of weakness
   [Dobbertin].  Accordingly, the IETF community has adopted 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.  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

Bellovin                                                        [Page 3]

Internet Draft         draft-iesg-tcpmd5app-00.txt            March 2004

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.  Furthermore, though 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].

   It is also the case that 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 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].
   Since LDP connections are always between routers and are always one
   hop long, the threat environment is similar to BGP's.  Accordingly,
   we are not deprecating [RFC2385] for use with LDP.

Bellovin                                                        [Page 4]

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6. Security Considerations

   The IESG believes that the variance described here will not 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
   towards the major threat environment for BGP.

8. 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,

   [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.

   [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,

   [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.

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Internet Draft         draft-iesg-tcpmd5app-00.txt            March 2004

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

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

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

   [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., P. DOolan, N. Feldman, A.
               Fredette, and B. Thomas.  "LDP Specification."  RFC 3036,
               January 2001.

   [RLH]       Rekhter, Y., T. Li, and S. Hares.  "A Border Gateway
               Protocol 4 (BGP-4)."  draft-ietf-idr-bgp4, December 2003,
               work in progress.

9. Author Information

Bellovin                                                        [Page 6]

Internet Draft         draft-iesg-tcpmd5app-00.txt            March 2004

Steven M. Bellovin
AT&T Labs Research
Shannon Laboratory
180 Park Avenue
Florham Park, NJ 07932
Phone: +1 973-360-8656
email: bellovin@acm.org

IPR Disclosure Acknowledgement

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

Copyright Notice

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


   This document and the information contained herein are provided on an

Bellovin                                                        [Page 7]