Network Working Group Steven M. Bellovin
Internet Draft AT&T Labs Research
Expiration Date: May 2005 Alex Zinin
Alcatel
September 2004
Standards Maturity Variance Regarding the TCP MD5 Signature Option (RFC
2385) and the BGP-4 Specification
draft-iesg-tcpmd5app-01.txt
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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 [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. (Note that though 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 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.
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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. 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
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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. 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.
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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 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 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,
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.
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[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." RFC 2026. October 1996.
[RFC2104] Krawczyk, H., M. Bellare, 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., 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.
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9. Author Information
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
Alex Zinin
Alcatel
Mountain View, CA
email: zinin@psg.com
Copyright Notice
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