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Network Working Group                                        S. BellovinRequest for Comments: 4808                           Columbia UniversityCategory: Informational                                       March 2007Key Change Strategies for TCP-MD5Status 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 20071.  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 inSection 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.  Operations3.1.  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 willBellovin                     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 inSection 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] orSection 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 inSection 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 20078.  References8.1.  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 2007Author'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.orgBellovin                     Informational                      [Page 7]

RFC 4808                   TCP-MD5 Key Change                 March 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Bellovin                     Informational                      [Page 8]