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Bellovin 3 Internet-Draft Columbia University 4 Expires: December 3, 2006 June 2006 6 Key Change Strategies for TCP-MD5 7 draft-bellovin-keyroll2385-01.txt 9 Status of this Memo 11 By submitting this Internet-Draft, each author represents that any 12 applicable patent or other IPR claims of which he or she is aware 13 have been or will be disclosed, and any of which he or she becomes 14 aware will be disclosed, in accordance with Section 6 of BCP 79. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This Internet-Draft will expire on December 3, 2006. 34 Copyright Notice 36 Copyright (C) The Internet Society (2006). 38 Abstract 40 The TCP-MD5 option is most commonly used to secure BGP sessions 41 between routers. However, changing the long-term key is difficult, 42 since the change needs to be synchronized between different 43 organizations. We describe single-ended strategies that will permit 44 (mostly) unsynchronized key changes. 46 1. Introduction 47 The TCP-MD5 option [RFC2385] is most commonly used to secure BGP 48 sessions between routers. However, changing the long-term key is 49 difficult, since the change needs to be synchronized between 50 different organizations. Worse yet, if the keys are out of sync, it 51 may break the connection between the two routers, rendering repair 52 attempts difficult. 54 The proper solution involves some sort of key management protocol. 55 Apart from the complexity of such things, RFC 2385 was not written 56 with key changes in mind. In particular, there is no KeyID field in 57 the option, which means that even a key management protocol would run 58 into the same problem. 60 Fortunately, a heuristic permits key change despite this protocol 61 deficiency. The change can be installed unilaterally at one end of a 62 connection; it is fully compatible with the existing protocol. 64 1.1. Terminology 66 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 67 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 68 document are to be interpreted as described in [RFC2119]. 70 2. The Algorithm 72 Separate algorithms are necessary for transmission and reception. 73 Reception is easier; we explain it first. 75 2.1. Reception 77 A receiver has a list of valid keys. Each key has a (conceptual) 78 timestamp associated with it. When a segment arrives, each key is 79 tried in turn. The segment is discarded if and only if it cannot be 80 validated by any key in the list. 82 In principle, there is no need to test keys in any particular order. 83 For performance reasons, though, a simple LRU strategy -- try the 84 last valid key first -- should work well. More complex mechanisms, 85 such as examining the TCP sequence number of an arriving segment to 86 see whether it fits in a hole, are almost certainly unnecessary. On 87 the other hand, validating that a received segment is putatively 88 legal, by checking its sequence number against the advertised window, 89 can help avoid denial of service attacks. 91 The newest key that has successfully validated a segment is marked as 92 the "preferred" key; see below. 94 Implicit in this scheme is the assumption that older keys will 95 eventually be unneeded and can be removed. Accordingly, 96 implementations SHOULD provide an indication of when a key was last 97 used successfully. 99 2.2. Transmission 101 Transmission is more complex, because the sender does not know which 102 keys can be accepted at the far end. Accordingly, the conservative 103 strategy is to delay using any new keys for a considerable amount of 104 time, probably measured in days. This time interval is the amount of 105 asynchronicity the parties wish to permit; it is agreed-upon out of 106 band and configured manually. 108 Some automation is possible, however. If a key has been used 109 successfully to validate an incoming segment, clearly the other side 110 knows it. Accordingly, any key marked as "preferred" by the 111 receiving part of a stack SHOULD be used for transmissions. 113 A sophisticated implementation could try alternate keys if the TCP 114 retransmission counter gets too high. (This is analogous to dead 115 gateway detection.) In particular, if a key change has just been 116 attempted but such segments are not acknowledged, it is reasonable to 117 fall back to the previous key and issue an alert of some sort. 118 Similarly, an implementation with a new but unused key could 119 occasionally try to use it, much in the way that TCP implementations 120 probe closed windows. Doing this avoid the "silent host" problem 121 discusssed in Section 3.1. This should be done at a moderately slow 122 rate. 124 Note that there is an ambiguity when an acknowledgment is received 125 for a segment transmitted with two different keys. The TCP Timestamp 126 option [RFC1323] can be used for disambiguation. 128 3. Operations 130 3.1. Single-Ended Operations 132 Suppose only one end of the connection has this algorithm 133 implemented. The new key is provisioned on that system, with a start 134 time far in the future -- sufficiently far, in fact, that it will not 135 be used spontaneously. After the key is ready, the other end is 136 notified, out-of-band, that a key change can commence. 138 At some point, the other end is upgraded. Because it does not have 139 multiple keys available, it will start using the new key immediately 140 for its transmission, and will drop all segments that use the old 141 key. As soon as it tries to transmit, the upgraded side will 142 designate the new key as preferred, and will use it for all of its 143 transmissions. Note specifically that this will include 144 retransmissions of any segments rejected because they used the old 145 key. 147 There is a problem if the unchanged machine is a "silent host" -- a 148 host that has nothing to say, and hence does not transmit. The best 149 way to avoid this is for an upgraded machine to try a variety of keys 150 in event of repeated unacknowledged packets, as discussed earlier. 152 3.2. Double-Ended Operations 154 Double-ended operations are similar, save that both sides deploy the 155 new key at about the same time. One should be configured to start 156 using the new key at a point where it is reasonably certain that the 157 other side would have it installed, too. Assuming that that has in 158 fact happened, the new key will be marked "preferred" on both sides. 160 3.3. Monitoring 162 As noted, implementations should monitor when a key was last used for 163 transmission or reception. Any monitoring mechanism can be used; 164 most likely, it will be a combination of a MIB entry and a command- 165 line display. Regardless, the network operations center should keep 166 track of this. When a new key has been used successfully for both 167 transmission and reception for a reasonable amount of time -- the 168 exact value isn't crucial, but it should probably be longer than 169 twice the maximum segment lifetime -- the old key can be marked for 170 deletion. There is an implicit assumption here that there will not 171 be substantial overlap in the usage period of such keys; monitoring 172 systems should look for any such anomalies, of course. 174 4. Security Considerations 176 In theory, accepting multiple keys simultaneously makes life easier 177 for an attacker. In practice, if the recommendations in [RFC3562] 178 are followed, this should not be a problem. 180 New keys must be communicated securely. Specifically, new key 181 messages must be kept confidential and must be properly 182 authenticated. 184 Having multiple keys makes CPU denial of service attacks easier. 185 This suggests that keeping the overlap period reasonably short is a 186 good idea. In addition, the Generalized TTL Security Mechanism 187 [RFC3682], if applicable to the local topology, can help. Note that 188 there would almost never be more than two keys in existence at any 189 one time in any event. 191 5. Acknowledgments 193 I'd like to thank Ron Bonica, Randy Bush, Ross Callon, Eric Rescorla, 194 and Sam Weiler for their comments and inspiration. 196 6. References 198 [RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions 199 for High Performance", RFC 1323, May 1992. 201 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 202 Requirement Levels", BCP 14, RFC 2119, March 1997. 204 [RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 205 Signature Option", RFC 2385, August 1998. 207 [RFC3562] Leech, M., "Key Management Considerations for the TCP MD5 208 Signature Option", RFC 3562, July 2003. 210 [RFC3682] Gill, V., Heasley, J., and D. Meyer, "The Generalized TTL 211 Security Mechanism (GTSM)", RFC 3682, February 2004. 213 Author's Address 215 Steven M. Bellovin 216 Columbia University 217 1214 Amsterdam Avenue 218 MC 0401 219 New York, NY 10027 220 US 222 Phone: +1 212 939 7149 223 Email: bellovin@acm.org 225 Intellectual Property Statement 227 The IETF takes no position regarding the validity or scope of any 228 Intellectual Property Rights or other rights that might be claimed to 229 pertain to the implementation or use of the technology described in 230 this document or the extent to which any license under such rights 231 might or might not be available; nor does it represent that it has 232 made any independent effort to identify any such rights. 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