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
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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.
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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
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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.
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RFC 4278 Standards Maturity Variance: RFC 2385 and BGP-4 January 2006
[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.
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RFC 4278 Standards Maturity Variance: RFC 2385 and BGP-4 January 2006
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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Acknowledgement
Funding for the RFC Editor function is provided by the IETF
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