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'5') (Obsoleted by RFC 8200) Summary: 6 errors (**), 0 flaws (~~), 5 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group M. Bagnulo 3 Internet-Draft A. Garcia-Martinez 4 Expires: December 1, 2003 I. Soto 5 UC3M 6 June 2, 2003 8 Preserving MIPv6 communications when the HoA becomes unreachable 9 draft-bagnulo-mobileip-unreachable-hoa-00 11 Status of this Memo 13 This document is an Internet-Draft and is in full conformance with 14 all provisions of Section 10 of RFC2026. 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 19 Internet-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 http:// 27 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 1, 2003. 34 Copyright Notice 36 Copyright (C) The Internet Society (2003). All Rights Reserved. 38 Abstract 40 This note proposes a modification to the MIPv6 specification in order 41 to allow the preservation of established communications when the path 42 between the MN and the CN through the HoA becomes unavailable. The 43 proposed modification essentially consists on allowing the extension 44 of BCE lifetime upon the reception of ICMP Destination Unreachable 45 message as reply to a Binding Refresh Request (BRR) message. 47 1. Introduction 49 The MIPv6 [1] specification defines that Binding Cache Entries (BCE) 50 that have been authorized using the Return Routability (RR) procedure 51 have a maximum lifetime of MAX_RR_BINDING_LIFE (420 seconds). This 52 means that the BCE linking a Home Address (HoA) and a Care-of Address 53 (CoA) at the Correspondent Node (CN) will only remain valid for 7 54 minutes after the Binding Update (BU) reception. If this CoA is to 55 be used to reach the HoA after this period, a new BU message binding 56 the HoA and the CoA has to be sent. In order to be able to do this, 57 the Mobile Node (MN) has to acquire new BU authorization data using 58 the RR procedure, implying communication through both the CoA and the 59 HoA. This implies that if the HoA becomes unreachable from the CN, 60 the established communication will be interrupted because the BCE has 61 expired, even if the path that is actually being used for the 62 communication is still available. Summarizing, as currently defined, 63 MIPv6 communication is vulnerable not only to outages along the 64 communication path used to carry data packets, but also to outages 65 along the path between the MN and the Home Agent (HA), and along the 66 path between the HA and the CN. This behavior not only introduces 67 additional points of failure in MIPv6 communications but it also 68 limits the potential usage of MIPv6 to provide multi-homing support 69 as described in [2]. 71 This note proposes a modification to the MIPv6 specification in order 72 to allow the preservation of established communications when the path 73 between the MN and the CN through the HoA becomes unavailable. The 74 proposed modification essentially consists on allowing the extension 75 of BCE lifetime upon the reception of ICMP Destination Unreachable 76 message as reply to a Binding Refresh Request (BRR) message. 78 2. Security Concerns that lead to reduced BCE lifetime. 80 In order to propose a modification to the defined behavior, we must 81 first analyze the security concerns that lead to the current design. 83 BCE lifetime has been limited to a few minutes in order to limit the 84 possibility of time shifting attacks, as it is presented in [3]. 86 The goal of MIPv6 security is to avoid the introduction of new 87 security hazards which are not present in non-MIPv6 enabled 88 environments. In particular, the RR procedure limits the set of 89 potential attackers to those who can intercept packets flowing 90 between the CN and the HA. This procedure forces the attacker to be 91 present somewhere along the path between the CN and the HA in order 92 to acquire the valid authorization data needed to generate forged BU 93 messages. 95 However, this mechanism by itself only imposes that the attacker has 96 to be present on the path the time needed to intercept the messages 97 that carry authorization information. Once that the attacker has 98 intercepted the valid authorization information, he can leave his 99 position along the path and still perform attacks using such 100 information. These are called time shifting attacks, since an 101 attacker that once was on-path intercepting packets can perform 102 attacks in the future when he is no longer on the communication path. 104 The limitation of the BCE lifetime to a few minutes limits the 105 effects of the following time shifting attack: the attacker placed 106 along the communication path intercepts authorization information and 107 generates a forged BU message. The attacker leaves the position but 108 the attack continues since the traffic is still diverted to the CoA 109 contained in the fake BU message. The effect of this attack is 110 limited by reducing BCE lifetime in the CN to 7 minutes, imposing the 111 generation of a new BU message in order to restore the BCE. Since 112 the attacker is no longer along the communication path, he will not 113 be able to generate new BU messages. 115 3. Proposed modifications to the MIPv6 specification 117 Currently time shifting attacks are prevented by imposing periodical 118 message exchange which imply that the attacker has to be present 119 along the path between the CN and the MN's HoA in order to continue 120 with the attack. So, the currently available mechanism assumes that 121 an attack is being perpetrated when no information can be exchanged 122 with the other end through the HoA. However, it is not really 123 necessary to perform a message exchange with the MN to prevent a time 124 shifting attack. The only thing that is really needed is a mechanism 125 that requires the presence of the attacker along the path between the 126 CN and the MN's HoA in order to continue with the attack. This can 127 be achieved through a message exchange with any device along the path 128 which does not has to be the communicating end-points. This note 129 proposes the exchange of messages between the CN and the first router 130 with no route to the final destination address as a time shifting 131 attack prevention mechanism when the HoA is unreachable from the CN. 133 3.1 Proposed mechanism 135 3.1.1 Correspondent Node Part 137 When the remaining lifetime of an existent BCE reaches 32 seconds, 138 the CN sends a Binding Refresh Request (BRR) to the MN's HoA for this 139 binding. The timeout for this request is set to 1 second. If no 140 response is obtained within this interval, the CN retransmits the BRR 141 until a response is received or the BCE lifetime expires. 143 The BRR message contains a Cookie Mobility option as defined in 144 section 3.2. This option contains a 64-bit randomly generated cookie 145 which will be copied to the response packets in order to verify that 146 the replying party has received (or intercepted) the BRR. 148 If the MN is reachable through the HoA, and it is interested in 149 preserving the BCE valid, it will send a BU message, extending the 150 BCE lifetime. 152 However, if an outage has occurred along the path between the CN and 153 the MN's HoA, an ICMP Destination Unreachable message containing a No 154 Route to Destination Code will be generated by a router along the 155 path according to [4]. The ICMP message contains the ICMP header and 156 it will will be completed with as much of the invoking packet as it 157 will fit within the MTU defined for IPv6 [5], which is 1280 bytes. 158 This means that the complete BRR message, including the newly defined 159 cookie option will be included within the ICMP message. 161 When the CN receives an ICMP Destination Unreachable message 162 containing a No Route to Destination Code, it verifies that the ICMP 163 message was generated as a reply to the BRR. It does so by verifying 164 that the packet included in the ICMP message is a BRR message and 165 that the cookie included in the Cookie Mobility Option matches with 166 the one included in the initial BRR message. If the verification 167 succeeds, the CN detects that an outage has occurred and extends the 168 BCE lifetime for 180 seconds, preserving the established 169 communication through the CoA. After 150 seconds, a new BRR message 170 will be sent. 172 The BCE lifetime can only be renewed 60 times, limiting to 3 hours 173 the maximum time that an BCE entry can be valid without performing 174 the RR procedure. 176 3.1.2 Mobile Node Part 178 A similar mechanism is to be implemented in the MN in order to 179 preserve the state needed in the MN to maintain the established 180 communication, so that the MN continues to send packets directly to 181 the CN without using the HA. Such state is stored in the Binding 182 Update List (BUL) within the MN and it has a limited lifetime, 183 imposing its periodical refresh. So when a BUL entry is about to 184 expire, the RR procedure is to be performed so that the authorization 185 information needed to send a BU message is acquired. The RR 186 procedure consists on the exchange of the CoTI/CoT messages directly 187 between the CN and the MN and the exchange of the HoTI/HoT messages 188 through the HA. If the path between the CN and the MN's HoA is 189 working properly, the RR procedure will be completed successfully and 190 a new BU message will be issued, and the lifetime of the BUL entry 191 corresponding to that CN will be extended. If the path between the 192 CN and the MN's HoA is not working, the RR procedure will not be 193 completed, preventing the generation of the BU message, implying that 194 the BUL entry corresponding to that CN will expire. This means that 195 forthcoming packets will be sent from the MN to the CN through the HA 196 and since there is no path available, the communication will fail. 198 It is proposed that the BUL lifetime is extended upon the reception 199 of an ICMP Destination Unreachable message containing a No Route to 200 Destination Code as a reply to a HoTI message issued by the MN. The 201 resulting behavior is that when a BUL entry is about to expire, the 202 MN will initiate the RR procedure sending a HoTI and a CoTI message. 203 If there is no route available between the CN and the MN through the 204 HA, an ICMP Destination Unreachable message containing a No Route to 205 Destination Code is be sent back to the MN. Then, when the MN 206 receives such message, it verifies that the ICMP message was 207 generated as a reply to the HoTI message. It does so by verifying 208 that the packet included in the ICMP message is a HoTI message and 209 that the cookie included in the Home Init Cookie field matches with 210 the one included in the initial HoTI message. If the verification 211 succeeds, the MN detects that an outage has occurred and extends the 212 BUL lifetime for a period equal to the initial value of the lifetime 213 (contained in the BUL entry), preserving the established 214 communication. 216 3.2 Cookie Mobility Option 218 The Cookie option has the following format: 220 0 1 2 3 221 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 222 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 223 | Type = TBD | Length = 8 | 224 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 225 | | 226 + Cookie + 227 | | 228 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 230 This Mobility Option contains a 64 bit long randomly generated 231 cookie. 233 4. Security Considerations 235 This note proposes changes to MIPv6 security. The reader is referred 236 to section 2 for the risks that the modified security features 237 prevent and to section 3 for an analysis of the proposed changes. 239 5. Acknowledgments 241 Thanks to Pekka Nikander for suggesting a more general problem for 242 the solution proposed in this document and also for providing many 243 constructive comments. 245 References 247 [1] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in 248 IPv6", Internet Draft, Work in progress 249 draft-ietf-mobileip-ipv6-21.txt, May 2002. 251 [2] Bagnulo, M., Garcia-Martinez, A. and I. Soto, "Application of 252 the MIPv6 protocol to the multi-homing problem", Internet Draft, 253 Work in progress draft-bagnulo-multi6-mnm-00, February 2003. 255 [3] Nikander, P., Aura, T., Arkko, J. and G. Montenegro, "Mobile IP 256 version 6 (MIPv6) Route Optimization Security Design 257 Background", Internet Draft, Work in progress 258 draft-nikander-mobileip-v6-ro-sec-00, March 2003. 260 [4] Conta, A. and S. Deering, "Internet Control Message Protocol 261 (ICMPv6) for the Internet Protocol Version 6 262 (IPv6)Specification", RFC 2463, December 1998. 264 [5] Hinden, R. and S. Deering, "Internet Protocol, version 6(IPv6) 265 Specification", RFC 2460, December 1998. 267 Authors' Addresses 269 Marcelo Bagnulo 270 Universidad Carlos III de Madrid 271 Av. Universidad 30 272 Leganes, Madrid 28911 273 SPAIN 275 Phone: 34 91 6249500 276 EMail: marcelo@it.uc3m.es 277 URI: http://www.it.uc3m.es/marcelo 279 Alberto Garcia-Martinez 280 Universidad Carlos III de Madrid 281 Av. Universidad 30 282 Leganes, Madrid 28911 283 SPAIN 285 Phone: 34 91 6249500 286 EMail: alberto@it.uc3m.es 287 URI: http://www.it.uc3m.es/alberto 288 Ignacio Soto 289 Universidad Carlos III de Madrid 290 Av. 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