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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Downref: Normative reference to an Informational RFC: RFC 2367 (ref. '3') ** Obsolete normative reference: RFC 2401 (ref. '4') (Obsoleted by RFC 4301) == Outdated reference: A later version (-10) exists of draft-ietf-ipsec-esp-v3-05 == Outdated reference: A later version (-03) exists of draft-ietf-hip-arch-01 == Outdated reference: A later version (-10) exists of draft-ietf-hip-base-00 Summary: 6 errors (**), 0 flaws (~~), 7 warnings (==), 8 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group P. Nikander 3 Internet-Draft J. Melen 4 Expires: May 5, 2006 Ericsson Research Nomadic Lab 5 Nov 2005 7 A Bound End-to-End Tunnel (BEET) mode for ESP 8 draft-nikander-esp-beet-mode-04 10 Status of this Memo 12 By submitting this Internet-Draft, each author represents that any 13 applicable patent or other IPR claims of which he or she is aware 14 have been or will be disclosed, and any of which he or she becomes 15 aware will be disclosed, in accordance with Section 6 of BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt. 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 This Internet-Draft will expire on May 5, 2006. 35 Copyright Notice 37 Copyright (C) The Internet Society (2005). 39 Abstract 41 This document specifies a new mode, called Bound End-to-End Tunnel 42 (BEET) mode, for IPsec ESP. The new mode augments the existing ESP 43 tunnel and transport modes. For end-to-end tunnels, the new mode 44 provides limited tunnel mode semantics without the regular tunnel 45 mode overhead. The mode is intended to support new uses of ESP, 46 including mobility and multi-address multi-homing. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 51 2. Conventions used in this document . . . . . . . . . . . . . . 4 52 2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 53 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5 54 3.1. Related work . . . . . . . . . . . . . . . . . . . . . . . 5 55 4. Use scenarios . . . . . . . . . . . . . . . . . . . . . . . . 6 56 4.1. NAT traversal . . . . . . . . . . . . . . . . . . . . . . 6 57 4.2. Mobile IP . . . . . . . . . . . . . . . . . . . . . . . . 7 58 4.2.1. Mobile IPv4 . . . . . . . . . . . . . . . . . . . . . 7 59 4.2.2. Mobile IPv4 route optimization . . . . . . . . . . . . 9 60 4.2.3. Mobile IPv6 . . . . . . . . . . . . . . . . . . . . . 9 61 4.3. End-node multi-address multi-homing . . . . . . . . . . . 10 62 4.4. Host Identity Protocol . . . . . . . . . . . . . . . . . . 11 63 5. Protocol definition . . . . . . . . . . . . . . . . . . . . . 12 64 5.1. Changes to Security Association data structures . . . . . 12 65 5.2. Packet format . . . . . . . . . . . . . . . . . . . . . . 12 66 5.3. Cryptographic processing . . . . . . . . . . . . . . . . . 14 67 5.4. IP header processing . . . . . . . . . . . . . . . . . . . 14 68 5.5. Handling of outgoing packets . . . . . . . . . . . . . . . 15 69 5.6. Handling of incoming packets . . . . . . . . . . . . . . . 16 70 5.7. IPv4 options handling . . . . . . . . . . . . . . . . . . 16 71 6. Policy considerations . . . . . . . . . . . . . . . . . . . . 18 72 7. PF_KEY extensions . . . . . . . . . . . . . . . . . . . . . . 19 73 8. New requirements on Key Management protocols . . . . . . . . . 20 74 9. Implementing the functionality with other means . . . . . . . 21 75 10. Security Considerations . . . . . . . . . . . . . . . . . . . 23 76 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 77 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26 78 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27 79 13.1. Normative references . . . . . . . . . . . . . . . . . . . 27 80 13.2. Informative references . . . . . . . . . . . . . . . . . . 27 81 Appendix A. Implementation experiences . . . . . . . . . . . . . 28 82 Appendix B. Garden beets . . . . . . . . . . . . . . . . . . . . 29 83 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30 84 Intellectual Property and Copyright Statements . . . . . . . . . . 31 86 1. Introduction 88 The current IPsec ESP specification [5] defines two modes of 89 operation: tunnel mode and transport mode. The tunnel mode is mainly 90 intended for non-end-to-end use where one or both of the ends of the 91 ESP Security Associations (SAs) are located in security gateways, 92 separate from the actual end-nodes. The transport mode is intended 93 for end-to-end use, where both ends of the security association are 94 terminated at the end-nodes themselves. 96 This document defines a new mode for ESP, called Bound End-to-End 97 Tunnel (BEET) mode. The purpose of the mode is to provide limited 98 tunnel mode semantics without the overhead associated with the 99 regular tunnel mode. As the name states, the BEET mode is intended 100 solely for end-to-end use. It provides tunnel mode semantics in the 101 sense that the IP addresses seen by the applications and the IP 102 addresses used on the wire are distinct from each other, providing 103 the illusion that the application level IP addresses are tunneled 104 over the network level IP addresses. However, the mode does not 105 support full tunnel semantics. More specifically, the IP addresses 106 as seen by the application are strictly bound, and only one pair of 107 bound inner addresses can be used on any given BEET mode Security 108 Association. This is in contrast to the regular tunnel mode, where 109 the inner IP addresses can be any addresses from a defined range. 111 A BEET mode Security Associations records two pairs of IP addresses, 112 called inner addresses and outer addresses. The inner addresses are 113 what the applications see. The outer addresses are what appear on 114 the wire. Since the inner addresses are fixed for the lifetime of 115 the Security Association, they need not to be sent in individual 116 packets. Instead, they are set up as the Security Associations are 117 created, they are verified when packets are sent, and they are 118 restored as packets are received. 120 This all gives the BEET mode the efficiency of transport mode with a 121 limited set of end-to-end tunnel semantics. The efficiency is 122 accomplished by removing the inner IP header from the packet that is 123 transported on the wire. Due to removal of inner IP header, the TTL 124 of tunneled packet is reduced by every router on the path as the TTL 125 value is copied from inner to outer header by the sender and vice 126 versa by the receiver. The semantics of BEET mode is limited in the 127 sense that only one fixed pair of inner addresses are allowed. The 128 outer addresses may change over the life time of the SA, but the 129 inner addresses cannot. If a new pair of inner addresses is needed, 130 a new pair of BEET mode Security Associations must be established, or 131 the regular tunnel mode must be used. However, in the cases 132 considered, a single pair of security associations is usually 133 sufficient between any single pair of nodes. 135 2. Conventions used in this document 137 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 138 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 139 document are to be interpreted as described in RFC2119 [2]. 141 This document contains both normative and informative sections. The 142 normative sections define the BEET mode. The informative sections 143 provide background information that aim to motivate the need for the 144 new mode. Whenever it may not be clear from the context whether a 145 given major section is normative or informative, it is defined in the 146 beginning of the section. 148 2.1. Terminology 150 In this section we define the terms specific to this document. This 151 section is normative. 153 Inner IP address 154 An IP address as seen by applications, stored in TCB or other 155 upper layer data structures, and processed by the IP stack prior 156 to ESP processing in the output side and after ESP processing in 157 the input side. 159 Outer IP address 160 An IP address seen in the wire and processed by the IP stack after 161 ESP processing in the output side and before ESP processing in the 162 input side. 164 Inner IP header 165 An IP header that contains inner IP addresses. In some cases an 166 inner IP header may be represented as an internal data structure 167 containing the data equivalent to an IP header. 169 Outer IP header 170 An IP header that contains outer IP addresses. In some cases an 171 outer IP header may be represented as an internal data structure 172 containing the data equivalent to an IP header. 174 3. Background 176 For a number of years people have been talking about using IPsec for 177 other purposes than VPN. In fact, the current specifications do 178 provide support for end-to-end protection of data. However, that 179 mode is rarely used, for a number of reasons [6], [7]. One of the 180 reasons, though, seems to be address agility. That is, due to NAT, 181 mobility, multi-address multi-homing, etc., the addresses that are 182 used actually on the wire do not necessarily match with the addresses 183 that the applications expect to see. In the NAT case the addresses 184 are changed on the fly, thereby invalidating any transport mode 185 checksums (unless, of course, a tunnel is used). Mobile nodes change 186 their addresses periodically, and the existing applications rarely 187 survive the address changes without some help, e.g., Mobile IP. 188 Multi-addressing based multi-homed nodes would prefer to keep their 189 connections active even when the primary (or currently used) IP 190 address becomes unusable in the face of an network outage. 192 Based on the reasons above, there is clearly a need for a mode of 193 communication where the addresses that the applications see are 194 distinct from the addresses that are actually used in the wire. The 195 current IPsec tunnel mode provides the required functionality, but at 196 the cost of additional overhead in terms of larger packets and more 197 complicated processing. 199 3.1. Related work 201 The basic idea captured by this draft has been floating around for a 202 long time. Steven Bellovin's HostNAT talk [8] at the Los Angeles 203 IETF is an early example. After that, basically the same idea has 204 surfaced several times. Perhaps the most concrete current proposal 205 is the Host Identity Protocol (HIP) [11], where BEET mode ESP 206 processing is an integral part of the overall protocol. 208 4. Use scenarios 210 In this section we describe a number of possible use scenarios. None 211 of these use scenarios are meant to be complete specifications on how 212 exactly to support the functionality. Separate specifications are 213 needed for that. Instead, the purpose of this section is to discuss 214 the overall benefits of the BEET mode, and to lay out a road map for 215 possible future documents. This section is informative. 217 4.1. NAT traversal 219 NAT traversal is currently a major problem in IPsec. It is not 220 sufficient to encapsulate the packets into UDP; additionally, tunnel 221 mode must be used. Tunnel mode is required since the outer IP 222 addresses at the ends of the protected connection differ. If 223 transport mode was used, the differing IP addresses would lead to 224 failing upper layer TCP/UDP checksums. 226 The BEET mode provides sufficient tunnel mode semantics without the 227 packet overhead of the tunnel mode. A pair of BEET mode SAs can be 228 effectively used to "un-NAT" packets that have been NATed during 229 their travel through the network. Figure 1 illustrates the process. 231 Packet contents on a client -> server packet 232 +--------+ 233 | Client | src = 131.160.175.2 dst = 129.15.6.1 clear text 234 +--------+ ^ 235 | 10.0.0.1 | 236 | | src = 10.0.0.1 dst = 129.15.6.1 ESP 237 | | 238 +-----+ | 239 | NAT | SAs 240 +-----+ | 241 | 131.160.175.2 | 242 | | src = 131.160.175.2 dst = 129.15.6.1 ESP 243 | 129.15.5.1 | 244 +--------+ v 245 | Server | src = 131.160.175.2 dst = 129.15.6.1 clear text 246 +--------+ address from unicast SA lookup 248 Figure 1 250 A drawback in this scheme is that the Client must either know its 251 public IP address, or it must rely on the Server to tell what address 252 to use. It must be noticed that if the NAT box is mapping several 253 internal IP addresses into a single public address, the public 254 address cannot be directly used. In that case the client and server 255 need to agree on a unique address, to be used to internally represent 256 the client. It must be pointed out that such an address is 257 semantically very similar to a Mobile IP home address. The details 258 of such address agreement are beyond the scope of this document. 260 4.2. Mobile IP 262 In Mobile IP, the BEET mode could be used instead of the currently 263 defined wire formats. If the hosts would be using end-to-end ESP 264 anyway, this has the benefit of saving the space that would otherwise 265 be taken by the standard Mobile IP wire formats. Furthermore, in 266 BEET the inner IP header does not actually appear in the wire format. 267 Effectively, this makes BEET as space efficient for mobile nodes as 268 the standard ESP transport mode is today between fixed hosts. 270 Instead of having a separate Binding Cache, the nodes could include 271 the address translation information into a pair of BEET mode security 272 associations. 274 4.2.1. Mobile IPv4 276 In the current Mobile IPv4, two different wire formats are used, 277 depending on whether there is a NAT device between the communicating 278 hosts or not. See Figure 2, below. 280 Mobile IPv4 wire format without NAT traversal 282 IP(CoA->HA) | IP(HoA->CN) | payload 284 Mobile IPv4 wire format with NAT traversal 286 IP(CoA->HA) | UDP(any->434) | MIP header | IP(HoA->CN) | payload 288 (where the MIP header is a minimal 4 octet header) 290 [Figure courtesy to Sami Vaarala.] 292 Figure 2 294 It is required that the inner address representing the mobile node, 295 as seen by the application, is always the home address. That is, 296 from the application point of view, the packets flow between the home 297 address and the correspondent node address. 299 If IPsec is used to protect the traffic between the Mobile Node and 300 the Correspondent node, ESP transport mode can be used. However, the 301 transport mode ESP packet is enclosed into an IP-over-IP wrapper at 302 the home agent, see Figure 3. 304 Current Mobile IPv4 wire format with end-to-end ESP transport mode: 306 CN -> HA: IP(CN->HoA) | ESP | payload | ESP trailer 307 HA -> MN: IP(HA->CoA) | IP(CN->HoA) | ESP | payload | ESP trailer 309 MN -> HA: IP(CoA->HA) | IP(HoA->CN) | ESP | payload | ESP trailer 310 HA -> CN: IP(HoA->CN) | ESP | payload | ESP trailer 312 Proposed Mobile IPv4 wire format with ESP BEET mode: 314 CN -> HA: IP(CN->HoA) | ESP | payload | ESP trailer 315 HA -> MN: IP(HA->CoA) | ESP | payload | ESP trailer 317 MN -> HA: IP(CoA->HA) | ESP | payload | ESP trailer 318 HA -> CN: IP(HoA->CN) | ESP | payload | ESP trailer 320 Figure 3 322 In this scenario, the correspondent node does not need to be aware 323 that the security association is in fact using the BEET mode. If the 324 home agent and the mobile node co-operate, and the mobile node 325 implements the BEET semantics, the change could be implemented 326 transparently to the correspondent node. 328 The SPI towards the MN MUST be selected so that the HA can 329 differentiate MNs from each other if they are communicating towards 330 the same CN. As the SA is anyway end-to-end the HA MAY check during 331 the key exchange that the selected SPI will not collide with other 332 MNs. 334 As multiple CNs may choose same SPI for receiving data from HA, the 335 HA must implement SPINAT [9] towards MN. Thus, the SPI used to 336 receive packets from MN at HA would uniquely identify the real 337 destination CN. The SPI must be negotiated per CN basis but as it is 338 assumed that there would be a end-to-end SA anyway the amount of 339 signaling doesn't need to be increased 341 It should be noticed that the space savings are even larger in the 342 NAT traversal situation, as is illustrated in Figure Figure 4, below. 344 Current Mobile IPv4 NAT-traversal wire format with end-to-end transport ESP: 346 CN -> HA: IP(CN->HoA) | ESP | payload | ESP trailer 347 HA -> MN: IP(HA->CoA) | UDP | MIP | IP(CN->HoA) | ESP | payload | ESP trailer 349 MN -> HA: IP(CoA->HA) | UDP | MIP | IP(HoA->CN) | ESP | payload | ESP trailer 350 HA -> CN: IP(HoA->CN) | ESP | payload | ESP trailer 352 Proposed Mobile IPv4 NAT-traversal wire format with BEET ESP: 354 CN -> HA: IP(CN->HoA) | ESP | payload | ESP trailer 355 HA -> MN: IP(HA->CoA) | UDP | ESP | payload | ESP trailer 357 MN -> HA: IP(CoA->HA) | UDP | ESP | payload | ESP trailer 358 HA -> CN: IP(HoA->CN) | ESP | payload | ESP trailer 360 Figure 4 362 In the NAT traversal case the HA doesn't have to implement the SPINAT 363 because the CN MAY be piggy packed in the UDP source and destination 364 IP and port information. Two separate UDP connections MAY not have 365 the same source and destination IP and port pairs thus the UDP 366 connection will identify the CN uniquely. 368 4.2.2. Mobile IPv4 route optimization 370 BEET can be used for route optimization purposes as the outer IP 371 address can always be set to as the current CoA of the MN. Likewise 372 the MN can set the outer address as the address of the CN instead of 373 the HA's address, although this would require that both ends support 374 BEET mode. Binding updates would be sent to the CN as well instead 375 of just updating the location on HA. Revealing MN's real location to 376 the CN might not always be desirable. 378 4.2.3. Mobile IPv6 380 Triangular routing in Mobile IPv6 is similar to that of Mobile IPv4. 381 However, the tunnel between the home agent and the mobile node is an 382 ESP tunnel instead of being a plain IP-over-IP tunnel. However, if 383 BEET mode was used between the correspondent node and the mobile 384 node, the ESP tunnel between the home agent and the mobile node would 385 not bring any additional protection to the payload data. Thus, in 386 that case BEET could replace the ESP tunnel, similar to the IPv4 387 case, illustrated in Figure 3 above. 389 Mobile IPv6 Route Optimization uses a Type 2 Routing Header (RH) and 390 Home Address Option (HAO) in the packet wire format. However, it can 391 be argued that the semantics of these options is equivalent to a 392 optimized point-to-point tunnel. That is, the Type 2 RH defines the 393 real destination address of a packet, thereby effectively creating a 394 partial tunnel where the inner and outer source addresses are 395 identical but the destination addresses differ. Similarly, the Home 396 Address Option defines the real source address of the packet, again 397 creating a partial tunnel. The only difference is that this time the 398 inner and outer destination addresses are identical but the source 399 addresses differ. 401 Thus, for Mobile IPv6, BEET mode would define a different wire format 402 for the payload packets. Instead of using Type 2 RH and HAO, the 403 packets could be encapsulated into a BEET mode ESP tunnel. In the 404 case that ESP is used anyway, this has the advantage that the 405 standard Mobile IPv6 extra headers are not needed, thereby saving 406 bytes in the headers. Compared to tunnel mode ESP, BEET mode has the 407 advantage that the inner IP header is not needed. 409 In Mobile IPv6, mobility management can be implemented just as 410 before, using the HoTI/CoTI, HoT/CoT and Binding Update (BU) 411 messages. The difference would lay in handling Binding Updates. If 412 BEET mode was used, processing Binding Updates would change the outer 413 IP addresses in the BEET mode Security Associations instead of 414 changing the Binding Cache. 416 4.3. End-node multi-address multi-homing 418 The BEET mode provides for limited end-node multi-address multi- 419 homing. It semantically provides a tunnel between the end-hosts, 420 with fixed inner IP addresses. This allows a multi-homed host to use 421 different outer IP addresses in different packets, without any notice 422 by the upper layer protocols. The upper layer protocols see the 423 inner IP address at all times. Thus, this limited form of multi- 424 homing has no affect on the applications, which seemingly communicate 425 over fixed IP addresses all the time. 427 Implementing this kind of limited multi-homing support would require 428 a small change to the current IPsec SPD and SA implementations. 429 Currently the incoming SA selection is based on the SPI and 430 destination address, with the implicit assumption that there is only 431 one possible destination address for each incoming SA. In a multi- 432 homed host it would be desirable to have multiple destination 433 addresses associated with the SA, thereby allowing the same SA to be 434 used independent on the actual destination address in the packets. 436 Removing the destination address from unicast SA lookup is already 437 being proposed in the current ESP draft [5]. 439 If it is considered undesirable to change the implementations to 440 support multiple alternative destination addresses, it would still be 441 possible to support limited multi-homing by creating several parallel 442 SAs, one for each destination address. Each of these SAs would have 443 identical inner addresses. Effectively, this would distribute the 444 tunnel over multiple SAs. 446 In this latter implementation, the outgoing SA processing becomes 447 more complex. Selecting the outgoing SA does not depend only on the 448 inner IP addresses but also on the outer destination address. 449 Selecting the outer destination address depends on the current multi- 450 homing situation. This creates a situation where the SA processing 451 must be deferred after selecting the actual outer address to be used. 452 This might be difficult in some implementations. 454 4.4. Host Identity Protocol 456 The Host Identity Protocol (HIP) is a piece of more recent 457 development. Its aim is to explore the possibilities created by 458 separating the end-host identifier and locator of IP addresses. 459 There are currently five implementations, and the specifications are 460 being finalized. [10] [11] 462 In HIP, the TCP and UDP sockets are not bound to IP addresses but to 463 Host Identifiers (HI). The Host Identifiers create a new independent 464 name space. 466 The BEET mode supports HIP by defining the inner tunnel in terms of 467 Host Identifiers and the outer tunnel in terms of standard IP 468 addresses. In that way all processing prior to outgoing ESP and 469 after incoming ESP uses Host Identifiers. The wire format packets 470 use standard IP addresses and ESP transport packet format. 472 5. Protocol definition 474 In this section we define the exact protocol formats and operations. 475 This section is normative. 477 5.1. Changes to Security Association data structures 479 A BEET mode Security Association contains the same data as a regular 480 tunnel mode Security Association, with the exception that the inner 481 selectors must be single addresses and cannot be subnets. The data 482 includes the following: 484 A pair of inner IP addresses. 486 A pair of outer IP addresses. 488 Cryptographic keys and other data as defined in RFC2401 [4] 489 Section 4.4.3. 491 A conforming implementation MAY store the data in a way different 492 than or similar to a regular tunnel mode Security Association. 494 Note that in a conforming implementation the inner and outer 495 addresses MAY belong to different address families. All 496 implementations that support both IPv4 and IPv6 SHOULD support both 497 IPv4-over-IPv6 and IPv6-over-IPv4 tunneling. 499 5.2. Packet format 501 The wire packet format is identical to the ESP transport mode wire 502 format as defined in [5] Section 3.1.1. However, the resulting 503 packet contains outer IP addresses instead of the inner IP addresses 504 received from the upper layer. The construction of the outer headers 505 is defined in RFC2401 [4] Section 5.1.2. The following diagram 506 illustrates ESP BEET mode positioning for typical IPv4 and IPv6 507 packets. 509 IPv4 INNER ADDRESSES 510 -------------------- 512 BEFORE APPLYING ESP 513 ------------------------------ 514 | inner IP hdr | | | 515 | | TCP | Data | 516 ------------------------------ 518 AFTER APPLYING ESP, OUTER v4 ADDRESSES 520 ---------------------------------------------------- 521 | outer IP hdr | | | | ESP | ESP | 522 | (any options) | ESP | TCP | Data | Trailer | ICV | 523 ---------------------------------------------------- 524 |<---- encryption ---->| 525 |<-------- integrity ------->| 527 AFTER APPLYING ESP, OUTER v6 ADDRESSES 528 ------------------------------------------------------ 529 | outer | new ext | | | | ESP | ESP | 530 | IP hdr | hdrs. | ESP | TCP | Data | Trailer| ICV | 531 ------------------------------------------------------ 532 |<--- encryption ---->| 533 |<------- integrity ------->| 535 IPv4 INNER ADDRESSES with options 536 --------------------------------- 538 BEFORE APPLYING ESP 539 ------------------------------ 540 | inner IP hdr | | | 541 | + options | TCP | Data | 542 ------------------------------ 544 AFTER APPLYING ESP, OUTER v4 ADDRESSES 545 ---------------------------------------------------------- 546 | outer IP hdr | | | | | ESP | ESP | 547 | (any options) | ESP | PH | TCP | Data | Trailer | ICV | 548 ---------------------------------------------------------- 549 |<------- encryption ------->| 550 |<----------- integrity ---------->| 552 AFTER APPLYING ESP, OUTER v6 ADDRESSES 553 ------------------------------------------------------------ 554 | outer | new ext | | | | | ESP | ESP | 555 | IP hdr | hdrs. | ESP | PH | TCP | Data | Trailer| ICV | 556 ------------------------------------------------------------ 557 |<------ encryption ------->| 558 |<---------- integrity ---------->| 560 PH Pseudo Header for IPv4 options 562 IPv6 INNER ADDRESSES 563 -------------------- 565 BEFORE APPLYING ESP 566 ------------------------------------------ 567 | | ext hdrs | | | 568 | inner IP hdr | if present | TCP | Data | 569 ------------------------------------------ 571 AFTER APPLYING ESP, OUTER v6 ADDRESSES 572 -------------------------------------------------------------- 573 | outer | new ext | | dest | | | ESP | ESP | 574 | IP hdr | hdrs. | ESP | opts.| TCP | Data | Trailer | ICV | 575 -------------------------------------------------------------- 576 |<---- encryption ---->| 577 |<------- integrity ------>| 579 AFTER APPLYING ESP, OUTER v4 ADDRESSES 580 ---------------------------------------------------- 581 | outer | | dest | | | ESP | ESP | 582 | IP hdr | ESP | opts.| TCP | Data | Trailer | ICV | 583 ---------------------------------------------------- 584 |<------- encryption -------->| 585 |<----------- integrity ----------->| 587 5.3. Cryptographic processing 589 The outgoing packets MUST be protected exactly as in ESP transport 590 mode [5]. That is, the upper layer protocol packet is wrapped into 591 an ESP header, encrypted, and authenticated exactly as if regular 592 transport mode was used. The resulting ESP packet is subject to IP 593 header processing as defined in Section 5.4 and Section 5.5. The 594 incoming ESP protected messages are verified and decrypted exactly as 595 if regular transport mode was used. The resulting clear text packet 596 is subject to IP header processing as defined in Section 5.4 and 597 Section 5.6. 599 5.4. IP header processing 601 The biggest difference between the BEET mode and the other two modes 602 is in IP header processing. In the regular transport mode the IP 603 header is kept intact. In the regular tunnel mode an outer IP header 604 is created on output and discarded on input. In the BEET mode the IP 605 header is replaced with another one on both input and output. 607 On the BEET mode output side, the IP header processing MUST first 608 ensure that the IP addresses in the original IP header contain the 609 inner addresses as specified in the SA. This MAY be ensured by 610 proper policy processing, and it is possible that no checks are 611 needed at the SA processing time. Once the IP header has been 612 verified to contain the right IP inner addresses, it is discarded. A 613 new IP header is created, using the discarded inner header as a hint 614 for other fields but the IP addresses. The IP addresses in the new 615 header MUST be the outer tunnel addresses. 617 On input side, the received IP header is simply discarded. Since the 618 packet has been decrypted and verified, no further checks are 619 necessary. A new IP header, corresponding to a tunnel mode inner 620 header, is created, using the discarded outer header as a hint for 621 other fields but the IP addresses. The IP addresses in the new 622 header MUST be the inner addresses. 624 As the outer header fields are used as hint for creating inner 625 header, it must be noted that inner header differs as compared to 626 tunnel-mode inner header. In BEET mode the inner header will have 627 the TTL, DF-bit and other option values from the outer header. The 628 TTL, DF-bit and other option values of the inner header MUST be 629 processed by the stack. 631 5.5. Handling of outgoing packets 633 The outgoing BEET mode packets are processed as follows: 635 1. The system MUST verify that the IP header contains the inner 636 source and destination addresses, exactly as defined in the SA. 637 This verification MAY be explicit, or it MAY be implicit, for 638 example, as a result of prior policy processing. Note that in 639 some implementations there may be no real IP header at this time 640 but the source and destination addresses may be carried out-of- 641 band. In case the source address is still unassigned, it SHOULD 642 be ensured that the designated inner source address would be 643 selected at a later stage. 645 2. The IP payload (the contents of the packet beyond the IP header) 646 is wrapped into an ESP header as defined in [5] Section 3.3. 648 3. A new IP header is constructed, replacing the original one. The 649 new IP header MUST contain the outer source and destination 650 addresses, as defined in the SA. Note that in some 651 implementations there may be no real IP header at this time but 652 the source and destination addresses may be carried out-of-band. 653 In the case where the source address must be left unassigned, it 654 SHOULD be made sure that the right source address is selected at 655 a later stage. Other than the addresses, it is RECOMMENDED that 656 the new IP header copies the fields from the original IP header. 658 4. If there are any IPv4 options in the original packet, it is 659 RECOMMENDED that they are discarded. If the inner header 660 contains one or more options that need to be transported between 661 the tunnel end-points, sender MUST encapsulate the options as 662 defined in Section 5.7 664 Instead of literally discarding the IP header and constructing a new 665 one, a conforming implementation MAY simply replace the addresses in 666 an existing header. However, if the RECOMMENDED feature of allowing 667 the inner and outer addresses from different address families is 668 used, this simple strategy does not work. 670 5.6. Handling of incoming packets 672 The incoming BEET mode packets are processed as follows: 674 1. The system MUST verify and decrypt the incoming packet 675 successfully, as defined in [5] section 3.4. If the verification 676 or decryption fails, the packet MUST be discarded. 678 2. The original IP header is simply discarded, without any checks. 679 Since the ESP verification succeeded, the packet can be safely 680 assumed to have arrived from the right sender. 682 3. A new IP header is constructed, replacing the original one. The 683 new IP header MUST contain the inner source and destination 684 addresses, as defined in the SA. If the sender has set the ESP 685 next protocol field to 94 and included the pseudo header as 686 described in Section 5.7, the receiver MUST include the options 687 after the constructed IP header. Note, that in some 688 implementations the real IP header may have already been 689 discarded and the source and destination addresses are carried 690 out-of-band. In such case the out-of-band addresses MUST be the 691 inner addresses. Other than the addresses, it is RECOMMENDED 692 that the new IP header copies the fields from the original IP 693 header. 695 Instead of literally discarding the IP header and constructing a new 696 one a conforming implementation MAY simply replace the addresses in 697 an existing header. However, if the RECOMMENDED feature of allowing 698 the inner and outer addresses from different address families is 699 used, this simple strategy does not work. 701 5.7. IPv4 options handling 703 In BEET mode if IPv4 options are transported inside the tunnel the 704 sender MUST include a pseudo-header after ESP header. The pseudo- 705 header identifies that IPv4 options from the original packet are to 706 be applied on the packet on input side. 708 The sender MUST set the next protocol field on the ESP header as 94. 709 The resulting pseudo header including the IPv4 options MUST be padded 710 to 8 byte boundary. Padding MUST be set to zero which indicates end 711 of option list as defined in Internet Protocol [1] section 3.1 712 Internet header format. This way, the receiver is able to identify 713 where the options ended in the original header. The Header Length 714 field contains the length of the pseudo header, IPv4 options, and 715 padding in 8 bytes units. 717 0 1 2 3 718 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 1 719 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 720 | Next Header | Header Len | IPv4 options ... | 721 +---------------+---------------+ | 722 | (padded with NUL options) | 723 +---------------------------------------------------------------+ 725 Next Header Identifies the data following this header 726 Length in bytes (Header Len + 1) * 8 728 The receiver MUST remove this pseudo-header as a part of BEET 729 processing. The IPv4 options included into the pseudo-header MUST be 730 added after the reconstructed IPv4 (inner) header on the receiving 731 side. 733 6. Policy considerations 735 In this section we describe how the BEET mode affects on IPsec policy 736 processing. This section is normative. 738 A BEET Security Association SHOULD NOT be used with NULL 739 authentication. 741 On the output side, the IPsec policy processing mechanism SHOULD take 742 care that only packets with IP addresses matching with the inner 743 addresses of a Security Association are passed to that Security 744 Association. If the policy mechanism do not provide full assurance 745 on this, the SA processing MUST check the addresses. Further policy 746 distinction may be specified based on IP version, upper layer 747 protocol, and ports. If such restrictions are defined, they MUST be 748 enforced. 750 On the output side, the policy rules SHOULD prevent any packets 751 containing the inner IP addresses pair from escaping to the wire in 752 clear text. 754 On the input side, there is no policy processing necessary on 755 encrypted packets. The SA is found based on the SPI and destination 756 address. A single SA MAY be associated with several destination 757 addresses. Since the outer IPsec addresses are discarded, and since 758 the packet authenticity and integrity is protected by ESP, there is 759 no need to check the outer addresses. Since the inner addresses are 760 fixed and restored from the SA, there is no need to check them. 761 There MAY be further policy rules specifying allowed upper layer 762 protocols and ports. If such restrictions are defined, they MUST be 763 enforced. 765 On the input side, there SHOULD be a policy rule that filters out 766 clear text packets that contain the inner addresses. 768 7. PF_KEY extensions 770 This section defines the necessary extensions to the PF_KEYv2 API [3] 771 to support the BEET mode. This section is informative. 773 A BEET mode Security Association is created by specifying the inner 774 IP addresses in the PF_KEYv2 Identity extensions, using two new 775 identity types. The identity types of the source and destination 776 identity extensions MUST be identical, i.e. either IPv4 or IPv6. 778 #define SADB_X_IDENTTYPE_ADDR 4 779 #define SADB_IDENTTYPE_MAX 4 781 When this new identity type is used, the contents of the identity 782 field in the PF_KEY messages MUST be a socket address. 784 For SADB_X_IDENTTYPE_ADDR the length of AF_INET type of identity MUST 785 be the length of struct sockaddr_in. Thus the length for AF_INET6 786 type of identity MUST be the length of struct sockaddr_in6. 788 Additionally, a new IPsec mode is defined in ipsec.h, and used in the 789 unspecified but commonly used the PF_KEY extension SADB_X_EXT_SA2 790 field sadb_x_sa2_mode. 792 #define IPSEC_MODE_BEET 4 794 If an SA is specified using SADB_X_EXT_SA2, if the sadb_x_sa2_mode is 795 IPSEC_MODE_BEET, and if the source and destination identities are 796 defined in terms of SADB_IDENTTYPE_ADDR, then the BEET mode MUST be 797 used. If an SA is specified using SADB_X_EXT_SA2, if the 798 sadb_x_sa2_mode is IPSEC_MODE_BEET, and if the identities are defined 799 in terms (other than the new type defined above) that exactly match 800 to single IPv4 or IPv6 addresses, then the BEET mode SHOULD be used. 802 8. New requirements on Key Management protocols 804 In this section we discuss the requirements that the new mode places 805 upon existing and new key management protocols. This section is 806 informative. 808 In order to provide support for the BEET mode, key agreement protocol 809 implementations must understand the existence of such a mode. In 810 some situations it is sufficient that the BEET mode is implemented at 811 the IPsec ESP level only at one end as long as the key management is 812 aware of its usage. For example, the NAT scenario described in 813 Section 4.1 does not require a BEET ESP implementation at the server 814 end. It is sufficient that the client implements the BEET mode; in 815 fact, if the client somehow knows its public IP address it may be 816 able to set up the BEET mode security associations without any 817 explicit concent on the server end. On the other hand, if the client 818 does not know its public IP address, it needs help from the server in 819 order to determine it. 821 More generally, one can get benefit from the BEET mode only to the 822 extend the key management protocol supports it. If the key 823 management protocol is fully aware of mobility and multi-homing 824 issues, and provides facilities for signaling changes in the current 825 connectivity situation, it is relatively easy to implement end-node 826 mobility and multi-address multi-homing with BEET. An example of 827 such usage is HIP [11]. 829 9. Implementing the functionality with other means 831 It is currently possible to implement the equivalent of BEET mode by 832 using transport mode ESP and explicit network address translation at 833 the end-hosts themselves. In this section we briefly compare BEET 834 mode and transport mode ESP with explicit network address translation 835 alternatives. The purpose of this section is to give background 836 information for security considerations. This section is 837 informative. 839 In an implementation using the BEET mode, the input side IP address 840 translation is integrated with the decryption and integrity 841 verification processing. The packet is passed and given the inner 842 addresses if and only if it is correctly decrypted and verified. A 843 typical IPsec SPD implementation would prohibit receiving unprotected 844 IP packets that use the inner addresses on the wire, as it is done in 845 the regular tunnel mode. At the same time, any other uses of the 846 outer addresses would be trivial; passing a packet to the SA requires 847 both that the packet has an ESP header and that the SPI matches. 849 In an implementation based on explicit network address translation 850 and transport mode ESP, the address translation and cryptographic 851 processing are completely separate. In practise, the host must 852 translate the outer IP address into the inner IP addresses before the 853 packet is passed to IPsec. (The other way around may not be secure, 854 since there would be no way for the address translation process to 855 know if the packet was received through IPsec processing or if it was 856 received via some other means.) Using the outer addresses for other 857 purposes may be hard, depending on the implementation of the address 858 translation mechanism. In particular, using the outer addresses on 859 other ESP SAs may be hard, since the typical address translation 860 mechanisms are only configured on protocol level ESP or not ESP and 861 typically do not understand SPIs. 863 At the output side, a BEET mode implementation takes care of 864 translating the inner addresses to outer addresses, as a part of the 865 encryption process. The IPsec SPD contains necessary entries that 866 make sure that the inner addresses never leak. 868 In an implementation based on transport mode ESP and explicit network 869 address translation, the output packets would be passed with inner 870 addresses from IPsec to the address translation mechanism. The 871 address translation mechanism will then translate the inner addresses 872 to outer addresses. While this does not prevent usage of the outer 873 addresses for other purposes, the configuration is brittle and error 874 prone. If there are mistakes at the IPsec configuration, the address 875 translation mechanism may translate unprotected packets, leading to 876 potential confusion. If there are mistakes at the address 877 translation side, the inner addresses may leak to the network. 879 10. Security Considerations 881 In this section we discuss the security properties of the BEET mode, 882 discussing some limitations [12]. This section is normative. 884 There are no known new vulnerabilities that the introduction of the 885 BEET mode would create. 887 It is currently possible to implement the equivalent of BEET mode by 888 using transport mode ESP and explicit network address translation at 889 the end-hosts themselves. However, such an implementation is more 890 complex, less flexible, and potentially more vulnerable to security 891 problems that are caused by misconfigurations; see Section 9. 893 The main security benefit is an operational one. To implement the 894 same functionality without the BEET mode typically requires 895 configuring three different, unrelated components in the hosts. 897 The transport mode ESP SAs must be configured. 899 A host based NAT function must be configured to properly translate 900 between the inner and outer addresses. 902 A host firewall must be configured to properly filter out packets 903 so that inner addresses do not leak in or out. 905 While it may be possible to configure these components to achieve the 906 same functionality, such a configuration is error prone, increasing 907 the probability of security vulnerabilities. An integrated BEET mode 908 implementation is less prone to configuration mistakes. Furthermore, 909 it would be fairly hard to implement portable key management 910 protocols that would be able to configure all of the required 911 components at the same time. On the other hand, it would be easy to 912 provide a portable key management protocol implementation that would 913 be able to configure BEET mode SAs through the specified PF_KEY 914 extensions. 916 Since the BEET security associations have the semantics of a fixed, 917 point-to-point tunnel between two IP addresses, it is possible to 918 place one or both of the tunnel end points into other nodes but those 919 that actually "possess" the inner IP addresses, i.e., to implement a 920 BEET mode proxy. However, since such usage defeats the security 921 benefits of combined ESP and hostNAT processing, as discussed above, 922 the implementations SHOULD NOT support such usage. 924 As in the BEET mode the outer header source address is not checked at 925 the input handling, there is the potential possibility a DoS attack 926 where the attacker sends random packets that match with the SPI of 927 some BEET mode SA. This kind of attack would cause the victim to 928 perform unnecessary integrity checks that would result in a failure. 929 If this kind of behaviour is detected, the node may request rekeying 930 from the Key Management Protocol, and after rekeying, if the attacker 931 was not on the path, the new SPI value would not be known by the 932 attacker. 934 11. IANA Considerations 936 The PF_KEYv2 interface should probably have an IANA registry. 938 12. Acknowledgments 940 During the 56th IETF meeting in San Francisco and afterwards, the 941 following people made comments on the ideas, helping the author to 942 write the draft: Jari Arkko, Steven Bellovin, Charlie Kaufman, Tero 943 Kivinen, Cheryl Madson, Andrew McGrecor, Robert Moskowitz, Michael 944 Richardson, Timothy Shepard, Jukka Ylitalo, Sami Vaarala, Petri 945 Jokela, Herbert Xu, Miika Komu. 947 The author ows special thanks to Derek Atkins and Steve Kent, who 948 strongly opposed the idea during the San Francisco IETF, and thereby 949 forced writing a high quality initial draft. 951 13. References 953 13.1. Normative references 955 [1] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. 957 [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement 958 Levels", BCP 14, RFC 2119, March 1997. 960 [3] McDonald, D., Metz, C., and B. Phan, "PF_KEY Key Management API, 961 Version 2", RFC 2367, July 1998. 963 [4] Kent, S. and R. Atkinson, "Security Architecture for the 964 Internet Protocol", RFC 2401, November 1998. 966 [5] Kent, S., "IP Encapsulating Security Payload (ESP)", 967 draft-ietf-ipsec-esp-v3-05 (work in progress), April 2003. 969 13.2. Informative references 971 [6] Arkko, J. and P. Nikander, "Limitations in IPsec Policy", 972 Security Protocols 11th International Workshop, Cambridge, UK, 973 April 2-4, 2003, LNCS to be published, Springer, April 2003. 975 [7] Ionnadis, J., "Why we still don't have IPsec", Network and 976 Distributed Systems Security Symposium (NDSS'03), Internet 977 Society, February 2003. 979 [8] Bellovin, S., "EIDs, IPsec, and HostNAT", IETF 41th, 980 March 1998. 982 [9] Ylitalo, J., Melen, J., Nikander, P., and V. Torvinen, "Re- 983 thinking Security in IP based Micro-Mobility", 7th Information 984 Security Conference (ISC'04) , Palo Alto, September 27-29, 985 2004, to be published, Springer, September 2004. 987 [10] Moskowitz, R., "Host Identity Protocol Architecture", 988 draft-ietf-hip-arch-01 (work in progress), December 2004. 990 [11] Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-00 991 (work in progress), June 2004. 993 [12] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on 994 Security Considerations", draft-iab-sec-cons-03 (work in 995 progress), February 2003. 997 Appendix A. Implementation experiences 999 We have implemented the BEET mode to the FreeBSD 5.3 KAME stack. Our 1000 implementation uses the PF_KEYv2 identity extension, as described in 1001 Section 7. 1003 The current implementation is based on four hooks placed at the 1004 strategical locations at the ESP and ip_output processing. We 1005 support full IPv4/IPv6 conversions, allowing both IPv4-over-IPv6 and 1006 IPv6-over-IPv4 tunneling. The number of lines changed in the KAME 1007 policy processing is 36 lines; these changes were necessary to fully 1008 support the identity extension, which was partly unimplemented in the 1009 KAME stack. The hooks themselves take 83 lines, and the protocol 1010 processing code is 450 lines long. About 90% of the protocol 1011 processing code was copied and pasted from the IPsec tunnel mode and 1012 transport mode routines, with minimal changes. About 70% of the code 1013 is needed to implement v4-over-v6 and v6-over-v4 tunneling. The 1014 number of actual functional lines for the simple v4-over-v4 and v6- 1015 over-v6 cases is mere 62 lines. The implementation effort took three 1016 days from two programmers, including writing simple test cases and 1017 performing rudimentary testing on the implementation to see that it 1018 works. 1020 The current implementation is tailored for experimentation. A more 1021 proper implementation would implement all of the processing as an 1022 integral part of the IPsec processing. The current KAME code 1023 supports only two modes. Once the necessary cleanups, such as 1024 replacing "if" statements with "switch" statements, we expect the 1025 extra protocol processing code required by the BEET mode to take less 1026 than 100 lines. 1028 Appendix B. Garden beets 1030 Commonly known as the garden beet, this firm, round root vegetable 1031 has leafy green tops, which are also edible and highly nutritious. 1032 The most common color for beets (called "beetroots" in the British 1033 Isles) is a garnet red. However, they can range in color from deep 1034 red to white, the most intriguing being the Chioggia (also called 1035 "candy cane"), with its concentric rings of red and white. Beets are 1036 available year-round and should be chosen by their firmness and 1037 smooth skins. 1039 Authors' Addresses 1041 Pekka Nikander 1042 Ericsson Research Nomadic Lab 1043 JORVAS FIN-02420 1044 FINLAND 1046 Phone: +358 9 299 1 1047 Email: pekka.nikander@nomadiclab.com 1049 Jan Melen 1050 Ericsson Research Nomadic Lab 1051 JORVAS FIN-02420 1052 FINLAND 1054 Phone: +358 9 299 1 1055 Email: jan.melen@nomadiclab.com 1057 Intellectual Property Statement 1059 The IETF takes no position regarding the validity or scope of any 1060 Intellectual Property Rights or other rights that might be claimed to 1061 pertain to the implementation or use of the technology described in 1062 this document or the extent to which any license under such rights 1063 might or might not be available; nor does it represent that it has 1064 made any independent effort to identify any such rights. 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