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.

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.

   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

   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.

   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.

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.

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

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