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.
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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.
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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
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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.
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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.
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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.
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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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