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rfc:rfc4808

Network Working Group S. Bellovin Request for Comments: 4808 Columbia University Category: Informational March 2007

                 Key Change Strategies for TCP-MD5

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 IETF Trust (2007).

Abstract

 The TCP-MD5 option is most commonly used to secure BGP sessions
 between routers.  However, changing the long-term key is difficult,
 since the change needs to be synchronized between different
 organizations.  We describe single-ended strategies that will permit
 (mostly) unsynchronized key changes.

Bellovin Informational [Page 1] RFC 4808 TCP-MD5 Key Change March 2007

1. Introduction

 The TCP-MD5 option [RFC2385] is most commonly used to secure BGP
 sessions between routers.  However, changing the long-term key is
 difficult, since the change needs to be synchronized between
 different organizations.  Worse yet, if the keys are out of sync, it
 may break the connection between the two routers, rendering repair
 attempts difficult.
 The proper solution involves some sort of key management protocol.
 Apart from the complexity of such things, RFC 2385 was not written
 with key changes in mind.  In particular, there is no KeyID field in
 the option, which means that even a key management protocol would run
 into the same problem.
 Fortunately, a heuristic permits key change despite this protocol
 deficiency.  The change can be installed unilaterally at one end of a
 connection; it is fully compatible with the existing protocol.

1.1. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

2. The Algorithm

 Separate algorithms are necessary for transmission and reception.
 Reception is easier; we explain it first.

2.1. Reception

 A receiver has a list of valid keys.  Each key has a (conceptual)
 timestamp associated with it.  When a segment arrives, each key is
 tried in turn.  The segment is discarded if and only if it cannot be
 validated by any key in the list.
 In principle, there is no need to test keys in any particular order.
 For performance reasons, though, a simple most-recently-used (MRU)
 strategy -- try the last valid key first -- should work well.  More
 complex mechanisms, such as examining the TCP sequence number of an
 arriving segment to see whether it fits in a hole, are almost
 certainly unnecessary.  On the other hand, validating that a received
 segment is putatively legal, by checking its sequence number against
 the advertised window, can help avoid denial of service attacks.
 The newest key that has successfully validated a segment is marked as
 the "preferred" key; see below.

Bellovin Informational [Page 2] RFC 4808 TCP-MD5 Key Change March 2007

 Implicit in this scheme is the assumption that older keys will
 eventually be unneeded and can be removed.  Accordingly,
 implementations SHOULD provide an indication of when a key was last
 used successfully.

2.2. Transmission

 Transmission is more complex, because the sender does not know which
 keys can be accepted at the far end.  Accordingly, the conservative
 strategy is to delay using any new keys for a considerable amount of
 time, probably measured in days.  This time interval is the amount of
 asynchronicity the parties wish to permit; it is agreed upon out of
 band and configured manually.
 Some automation is possible, however.  If a key has been used
 successfully to validate an incoming segment, clearly the other side
 knows it.  Accordingly, any key marked as "preferred" by the
 receiving part of a stack SHOULD be used for transmissions.
 A sophisticated implementation could try alternate keys if the TCP
 retransmission counter gets too high.  (This is analogous to dead
 gateway detection.)  In particular, if a key change has just been
 attempted but such segments are not acknowledged, it is reasonable to
 fall back to the previous key and issue an alert of some sort.
 Similarly, an implementation with a new but unused key could
 occasionally try to use it, much in the way that TCP implementations
 probe closed windows.  Doing this avoids the "silent host" problem
 discussed in Section 3.1.  This should be done at a moderately slow
 rate.
 Note that there is an ambiguity when an acknowledgment is received
 for a segment transmitted with two different keys.  The TCP Timestamp
 option [RFC1323] can be used for disambiguation.

3. Operations

3.1. Single-Ended Operations

 Suppose only one end of the connection has this algorithm
 implemented.  The new key is provisioned on that system, with a start
 time far in the future -- sufficiently far, in fact, that it will not
 be used spontaneously.  After the key is ready, the other end is
 notified, out-of-band, that a key change can commence.
 At some point, the other end is upgraded.  Because it does not have
 multiple keys available, it will start using the new key immediately
 for its transmission, and will drop all segments that use the old
 key.  As soon as it tries to transmit, the upgraded side will

Bellovin Informational [Page 3] RFC 4808 TCP-MD5 Key Change March 2007

 designate the new key as preferred, and will use it for all of its
 transmissions.  Note specifically that this will include
 retransmissions of any segments rejected because they used the old
 key.
 There is a problem if the unchanged machine is a "silent host" -- a
 host that has nothing to say, and hence does not transmit.  The best
 way to avoid this is for an upgraded machine to try a variety of keys
 in the event of repeated unacknowledged packets, and to probe for new
 unused keys during silent periods, as discussed in Section 2.2.
 Alternatively, application-level KeepAlive messages may be used to
 ensure that neither end of the connection is completely silent.  See,
 for example, Section 4.4 of [RFC4271] or Section 3.5.4 of [RFC3036].

3.2. Double-Ended Operations

 Double-ended operations are similar, save that both sides deploy the
 new key at about the same time.  One should be configured to start
 using the new key at a point where it is reasonably certain that the
 other side would have it installed, too.  Assuming that has in fact
 happened, the new key will be marked "preferred" on both sides.

3.3. Monitoring

 As noted, implementations should monitor when a key was last used for
 transmission or reception.  Any monitoring mechanism can be used;
 most likely, it will be one or both of a MIB object or objects and
 the vendor's usual command-line mechanism for displaying data of this
 type.  Regardless, the network operations center should keep track of
 this.  When a new key has been used successfully for both
 transmission and reception for a reasonable amount of time -- the
 exact value isn't crucial, but it should probably be longer than
 twice the maximum segment lifetime -- the old key can be marked for
 deletion.  There is an implicit assumption here that there will not
 be substantial overlap in the usage period of such keys; monitoring
 systems should look for any such anomalies, of course.

4. Moving Forward

 As implied in Section 1, this is an interim strategy, intended to
 make TCP-MD5 operationally usable today.  We do not suggest or
 recommend it as a long-term solution.  In this section, we make some
 suggestions about the design of a future TCP authentication option.
 The first and most obvious change is to replace keyed MD5 with a
 stronger MAC [RFC4278].  Today, HMAC-SHA1 [RFC4634] is the preferred
 choice, though others such as UMAC [RFC4418] should be considered as
 well.

Bellovin Informational [Page 4] RFC 4808 TCP-MD5 Key Change March 2007

 A new authentication option should contain some form of a Key ID
 field.  Such an option would permit unambiguous identification of
 which key was used to create the MAC for a given segment, sparing the
 receiver the need to engage in the sort of heuristics described here.
 A Key ID is useful with both manual and automatic key management.
 (Note carefully that we do not prescribe any particular Key ID
 mechanism here.  Rather, we are stating a requirement: there must be
 a simple, low-cost way to select a particular key, and it must be
 possible to rekey without tearing down long-lived connections.)
 Finally, an automated key management mechanism should be defined.
 The general reasoning for that is set forth in [RFC4107]; specific
 issues pertaining to BGP and TCP are given in [RFC3562].

5. Security Considerations

 In theory, accepting multiple keys simultaneously makes life easier
 for an attacker.  In practice, if the recommendations in [RFC3562]
 are followed, this should not be a problem.
 New keys must be communicated securely.  Specifically, new key
 messages must be kept confidential and must be properly
 authenticated.
 Having multiple keys makes CPU denial-of-service attacks easier.
 This suggests that keeping the overlap period reasonably short is a
 good idea.  In addition, the Generalized TTL Security Mechanism
 [RFC3682], if applicable to the local topology, can help.  Note that
 most of the time, only one key will exist; virtually all of the
 remaining time there will be only two keys in existence.

6. IANA Considerations

 There are no IANA actions required.  The TCP-MD5 option number is
 defined in [RFC2385], and is currently listed by IANA.

7. Acknowledgments

 I'd like to thank Ron Bonica, Randy Bush, Ross Callon, Rob Evans,
 Eric Rescorla, and Sam Weiler for their comments and inspiration.

Bellovin Informational [Page 5] RFC 4808 TCP-MD5 Key Change March 2007

8. References

8.1. Normative References

 [RFC1323]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
            for High Performance", RFC 1323, May 1992.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
            Signature Option", RFC 2385, August 1998.

8.2. Informative References

 [RFC3036]  Andersson, L., Doolan, P., Feldman, N., Fredette, A., and
            B. Thomas, "LDP Specification", RFC 3036, January 2001.
 [RFC3562]  Leech, M., "Key Management Considerations for the TCP MD5
            Signature Option", RFC 3562, July 2003.
 [RFC3682]  Gill, V., Heasley, J., and D. Meyer, "The Generalized TTL
            Security Mechanism (GTSM)", RFC 3682, February 2004.
 [RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
            Key Management", BCP 107, RFC 4107, June 2005.
 [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
            Protocol 4 (BGP-4)", RFC 4271, January 2006.
 [RFC4278]  Bellovin, S. and A. Zinin, "Standards Maturity Variance
            Regarding the TCP MD5 Signature Option (RFC 2385) and the
            BGP-4 Specification", RFC 4278, January 2006.
 [RFC4418]  Krovetz, T., "UMAC: Message Authentication Code using
            Universal Hashing", RFC 4418, March 2006.
 [RFC4634]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
            (SHA and HMAC-SHA)", RFC 4634, August 2006.

Bellovin Informational [Page 6] RFC 4808 TCP-MD5 Key Change March 2007

Author's Address

 Steven M. Bellovin
 Columbia University
 1214 Amsterdam Avenue
 MC 0401
 New York, NY  10027
 US
 Phone: +1 212 939 7149
 EMail: bellovin@acm.org

Bellovin Informational [Page 7] RFC 4808 TCP-MD5 Key Change March 2007

Full Copyright Statement

 Copyright (C) The IETF Trust (2007).
 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.
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Bellovin Informational [Page 8]

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