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Moskowitz 3 Internet-Draft ICSAlabs, a Division of TruSecure 4 Expires: February 2, 2006 Corporation 5 P. Nikander 6 Ericsson Research Nomadic Lab 7 August 1, 2005 9 Host Identity Protocol Architecture 10 draft-ietf-hip-arch-03 12 Status of this Memo 14 By submitting this Internet-Draft, each author represents that any 15 applicable patent or other IPR claims of which he or she is aware 16 have been or will be disclosed, and any of which he or she becomes 17 aware will be disclosed, in accordance with Section 6 of BCP 79. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that 21 other groups may also distribute working documents as Internet- 22 Drafts. 24 Internet-Drafts are draft documents valid for a maximum of six months 25 and may be updated, replaced, or obsoleted by other documents at any 26 time. It is inappropriate to use Internet-Drafts as reference 27 material or to cite them other than as "work in progress." 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt. 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 35 This Internet-Draft will expire on February 2, 2006. 37 Copyright Notice 39 Copyright (C) The Internet Society (2005). 41 Abstract 43 This memo describes a snapshot of the reasoning behind a proposed new 44 namespace, the Host Identity namespace, and a new protocol layer, the 45 Host Identity Protocol, between the internetworking and transport 46 layers. Herein are presented the basics of the current namespaces, 47 their strengths and weaknesses, and how a new namespace will add 48 completeness to them. The roles of this new namespace in the 49 protocols are defined. The memo describes the thinking of the 50 authors as of Fall 2003. The architecture may have evolved since. 51 This document represents one stable point in that evolution of 52 understanding. 54 Table of Contents 56 1. Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . 3 57 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 58 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4 59 3.1 Terms common to other documents . . . . . . . . . . . . . . 4 60 3.2 Terms specific to this and other HIP documents . . . . . . . 4 61 4. Background . . . . . . . . . . . . . . . . . . . . . . . . . 6 62 4.1 A desire for a namespace for computing platforms . . . . . . 6 63 5. Host Identity namespace . . . . . . . . . . . . . . . . . . 8 64 5.1 Host Identifiers . . . . . . . . . . . . . . . . . . . . . . 9 65 5.2 Storing Host Identifiers in DNS . . . . . . . . . . . . . . 9 66 5.3 Host Identity Tag (HIT) . . . . . . . . . . . . . . . . . . 10 67 5.4 Local Scope Identifier (LSI) . . . . . . . . . . . . . . . . 10 68 6. New stack architecture . . . . . . . . . . . . . . . . . . . 10 69 6.1 Transport associations and end-points . . . . . . . . . . . 11 70 7. End-host mobility and multi-homing . . . . . . . . . . . . . 12 71 7.1 Rendezvous mechanism . . . . . . . . . . . . . . . . . . . . 12 72 7.2 Protection against flooding attacks . . . . . . . . . . . . 13 73 8. HIP and IPsec . . . . . . . . . . . . . . . . . . . . . . . 13 74 9. HIP and NATs . . . . . . . . . . . . . . . . . . . . . . . . 14 75 9.1 HIP and TCP checksums . . . . . . . . . . . . . . . . . . . 15 76 10. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . 15 77 11. HIP policies . . . . . . . . . . . . . . . . . . . . . . . . 15 78 12. Benefits of HIP . . . . . . . . . . . . . . . . . . . . . . 16 79 12.1 HIP's answers to NSRG questions . . . . . . . . . . . . . . 17 80 13. Security considerations . . . . . . . . . . . . . . . . . . 19 81 13.1 HITs used in ACLs . . . . . . . . . . . . . . . . . . . . . 20 82 13.2 Non-security considerations . . . . . . . . . . . . . . . . 21 83 14. IANA considerations . . . . . . . . . . . . . . . . . . . . 21 84 15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 21 85 16. Informative references . . . . . . . . . . . . . . . . . . . 22 86 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 23 87 Intellectual Property and Copyright Statements . . . . . . . 24 89 1. Disclaimer 91 The purpose of this memo is to provide a stable reference point in 92 the development of the Host Identity Protocol architecture. This 93 memo describes the thinking of the authors as of Fall 2003; their 94 thinking may have evolved since then. Occasionally, this memo may be 95 confusing or self-contradicting. That is (partially) intentional, 96 and reflects the snapshot nature of this memo. 98 This RFC is not a candidate for any level of Internet Standard. The 99 IETF disclaims any knowledge of the fitness of this RFC for any 100 purpose and notes that the decision to publish is not based on IETF 101 review. However, the ideas put forth in this RFC have generated 102 significant interest, including the formation of the IETF HIP Working 103 Group and the IRTF HIP Research Group. There groups are expected to 104 generate further documents, sharing their findings with the whole 105 Internet Community. 107 2. Introduction 109 The Internet has two important global namespaces: Internet Protocol 110 (IP) addresses and Domain Name Service (DNS) names. These two 111 namespaces have a set of features and abstractions that have powered 112 the Internet to what it is today. They also have a number of 113 weaknesses. Basically, since they are all we have, we try and do too 114 much with them. Semantic overloading and functionality extensions 115 have greatly complicated these namespaces. 117 The proposed Host Identity namespace fills an important gap between 118 the IP and DNS namespaces. The Host Identity namespace consists of 119 Host Identifiers (HI). A Host Identifier is cryptographic in its 120 nature; it is the public key of an asymmetric key-pair. Each host 121 will have at least one Host Identity, but it will typically have more 122 than one. Each Host Identity uniquely identifies a single host, 123 i.e., no two hosts have the same Host Identity. The Host Identity, 124 and the corresponding Host Identifier, can either be public (e.g. 125 published in the DNS), or unpublished. Client systems will tend to 126 have both public and unpublished Identities. 128 There is a subtle but important difference between Host Identities 129 and Host Identifiers. An Identity refers to the abstract entity that 130 is identified. An Identifier, on the other hand, refers to the 131 concrete bit pattern that is used in the identification process. 133 Although the Host Identifiers could be used in many authentication 134 systems, such as IKEv2 [9], the presented architecture introduces a 135 new protocol, called the Host Identity Protocol (HIP), and a 136 cryptographic exchange, called the HIP base exchange; see also 137 Section 8. The IETF HIP Working Group is chartered to develop 138 specifications on these protocol exchanges. The HIP protocols under 139 development provide for limited forms of trust between systems, 140 enhance mobility, multi-homing and dynamic IP renumbering, aid in 141 protocol translation / transition, and reduce certain types of 142 denial-of-service (DoS) attacks. 144 When HIP is used, the actual payload traffic between two HIP hosts is 145 typically, but not necessarily, protected with IPsec. The Host 146 Identities are used to create the needed IPsec Security Associations 147 (SAs) and to authenticate the hosts. When IPsec is used, the actual 148 payload IP packets do not differ in any way from standard IPsec 149 protected IP packets. 151 3. Terminology 153 3.1 Terms common to other documents 155 +--------------+----------------------------------------------------+ 156 | Term | Explanation | 157 +--------------+----------------------------------------------------+ 158 | Public key | The public key of an asymmetric cryptographic key | 159 | | pair. Used as a publicly known identifier for | 160 | | cryptographic identity authentication. | 161 | | | 162 | Private key | The private or secret key of an asymmetric | 163 | | cryptographic key pair. Assumed to be known only | 164 | | to the party identified by the corresponding | 165 | | public key. Used by the identified party to | 166 | | authenticate its identity to other parties. | 167 | | | 168 | Public key | An asymmetric cryptographic key pair consisting of | 169 | pair | public and private keys. For example, | 170 | | Rivest-Shamir-Adelman (RSA) and Digital Signature | 171 | | Algorithm (DSA) key pairs are such key pairs. | 172 | | | 173 | End-point | A communicating entity. For historical reasons, | 174 | | the term 'computing platform' is used in this | 175 | | document as a (rough) synonym for end-point. | 176 +--------------+----------------------------------------------------+ 178 3.2 Terms specific to this and other HIP documents 180 It should be noted that many of the terms defined herein are 181 tautologous, self-referential or defined through circular reference 182 to other terms. This is due to the succinct nature of the 183 definitions. See the text elsewhere in this document for more 184 elaborate explanations. 186 +--------------+----------------------------------------------------+ 187 | Term | Explanation | 188 +--------------+----------------------------------------------------+ 189 | Computing | An entity capable of communicating and computing, | 190 | platform | for example, a computer. See the definition of | 191 | | 'End-point', above. | 192 | | | 193 | HIP base | A cryptographic protocol; see also Section 8. | 194 | exchange | | 195 | | | 196 | HIP packet | An IP packet that carries a 'Host Identity | 197 | | Protocol' message. | 198 | | | 199 | Host | An abstract concept assigned to a 'computing | 200 | Identity | platform'. See 'Host Identifier', below. | 201 | | | 202 | Host | A name space formed by all possible Host | 203 | Identity | Identifiers. | 204 | namespace | | 205 | | | 206 | Host | A protocol used to carry and authenticate Host | 207 | Identity | Identifiers and other information. | 208 | Protocol | | 209 | | | 210 | Host | A 128-bit datum created by taking a cryptographic | 211 | Identity Tag | hash over a Host Identifier. | 212 | | | 213 | Host | A public key used as a name for a Host Identity. | 214 | Identifier | | 215 | | | 216 | Local Scope | A 32-bit datum denoting a Host Identity. | 217 | Identifier | | 218 | | | 219 | Public Host | A published or publicly known Host Identfier used | 220 | Identifier | as a public name for a Host Identity, and the | 221 | and Identity | corresponding Identity. | 222 | | | 223 | Unpublished | A Host Identifier that is not placed in any public | 224 | Host | directory, and the corresponding Host Identity. | 225 | Identifier | Unpublished Host Identities are typically short | 226 | and Identity | lived in nature, being often replaced and possibly | 227 | | used just once. | 228 | | | 229 | Rendezvous | A mechanism used to locate mobile hosts based on | 230 | Mechanism | their HIT. | 231 +--------------+----------------------------------------------------+ 233 4. Background 235 The Internet is built from three principal components: computing 236 platforms (end-points), packet transport (i.e., internetworking) 237 infrastructure, and services (applications). The Internet exists to 238 service two principal components: people and robotic services 239 (silicon based people, if you will). All these components need to be 240 named in order to interact in a scalable manner. Here we concentrate 241 on naming computing platforms and packet transport elements. 243 There are two principal namespaces in use in the Internet for these 244 components: IP numbers, and Domain Names. Domain Names provide 245 hierarchically assigned names for some computing platforms and some 246 services. Each hierarchy is delegated from the level above; there is 247 no anonymity in Domain Names. Email, HTTP, and SIP addresses all 248 reference Domain Names. 250 IP numbers are a confounding of two namespaces, the names of a host's 251 networking interfaces and the names of the locations ('confounding' 252 is a term used in statistics to discuss metrics that are merged into 253 one with a gain in indexing, but a loss in informational value). The 254 names of locations should be understood as denoting routing direction 255 vectors, i.e., information that is used to deliver packets to their 256 destinations. 258 IP numbers name networking interfaces, and typically only when the 259 interface is connected to the network. Originally, IP numbers had 260 long-term significance. Today, the vast number of interfaces use 261 ephemeral and/or non-unique IP numbers. That is, every time an 262 interface is connected to the network, it is assigned an IP number. 264 In the current Internet, the transport layers are coupled to the IP 265 addresses. Neither can evolve separately from the other. IPng 266 deliberations were strongly shaped by the decision that a 267 corresponding TCPng would not be created. 269 There are three critical deficiencies with the current namespaces. 270 Firstly, dynamic readdressing cannot be directly managed. Secondly, 271 anonymity is not provided in a consistent, trustable manner. 272 Finally, authentication for systems and datagrams is not provided. 273 All of these deficiencies arise because computing platforms are not 274 well named with the current namespaces. 276 4.1 A desire for a namespace for computing platforms 278 An independent namespace for computing platforms could be used in 279 end-to-end operations independent of the evolution of the 280 internetworking layer and across the many internetworking layers. 282 This could support rapid readdressing of the internetworking layer 283 because of mobility, rehoming, or renumbering. 285 If the namespace for computing platforms is based on public-key 286 cryptography, it can also provide authentication services. If this 287 namespace is locally created without requiring registration, it can 288 provide anonymity. 290 Such a namespace (for computing platforms) and the names in it should 291 have the following characteristics: 293 o The namespace should be applied to the IP 'kernel'. The IP kernel 294 is the 'component' between applications and the packet transport 295 infrastructure. 297 o The namespace should fully decouple the internetworking layer from 298 the higher layers. The names should replace all occurrences of IP 299 addresses within applications (like in the Transport Control 300 Block, TCB). This may require changes to the current APIs. In 301 the long run, it is probable that some new APIs are needed. 303 o The introduction of the namespace should not mandate any 304 administrative infrastructure. Deployment must come from the 305 bottom up, in a pairwise deployment. 307 o The names should have a fixed length representation, for easy 308 inclusion in datagram headers and existing programming interfaces 309 (e.g the TCB). 311 o Using the namespace should be affordable when used in protocols. 312 This is primarily a packet size issue. There is also a 313 computational concern in affordability. 315 o Name collisions should be avoided as much as possible. The 316 mathematics of the birthday paradox can be used to estimate the 317 chance of a collision in a given population and hash space. In 318 general, for a random hash space of size n bits, we would expect 319 to obtain a collision after approximately 1.2*sqrt(2**n) hashes 320 were obtained. For 64 bits, this number is roughly 4 billion. A 321 hash size of 64 bits may be too small to avoid collisions in a 322 large population; for example, there is a 1% chance of collision 323 in a population of 640M. For 100 bits (or more), we would not 324 expect a collision until approximately 2**50 (1 quadrillion) 325 hashes were generated. 327 o The names should have a localized abstraction so that it can be 328 used in existing protocols and APIs. 330 o It must be possible to create names locally. This can provide 331 anonymity at the cost of making resolvability very difficult. 333 * Sometimes the names may contain a delegation component. This 334 is the cost of resolvability. 336 o The namespace should provide authentication services. 338 o The names should be long lived, but replaceable at any time. This 339 impacts access control lists; short lifetimes will tend to result 340 in tedious list maintenance or require a namespace infrastructure 341 for central control of access lists. 343 In this document, a new namespace approaching these ideas is called 344 the Host Identity namespace. Using Host Identities requires its own 345 protocol layer, the Host Identity Protocol, between the 346 internetworking and transport layers. The names are based on public- 347 key cryptography to supply authentication services. Properly 348 designed, it can deliver all of the above stated requirements. 350 5. Host Identity namespace 352 A name in the Host Identity namespace, a Host Identifier (HI), 353 represents a statistically globally unique name for naming any system 354 with an IP stack. This identity is normally associated with, but not 355 limited to, an IP stack. A system can have multiple identities, some 356 'well known', some unpublished or 'anonymous'. A system may self- 357 assert its own identity, or may use a third-party authenticator like 358 DNSSEC [2], PGP, or X.509 to 'notarize' the identity assertion. It 359 is expected that the Host Identifiers will initially be authenticated 360 with DNSSEC and that all implementations will support DNSSEC as a 361 minimal baseline. 363 In theory, any name that can claim to be 'statistically globally 364 unique' may serve as a Host Identifier. However, in the authors' 365 opinion, a public key of a 'public key pair' makes the best Host 366 Identifier. As will be specified in the Host Identity Protocol 367 specification, a public-key-based HI can authenticate the HIP packets 368 and protect them for man-in-the-middle attacks. Since authenticated 369 datagrams are mandatory to provide much of HIP's denial-of-service 370 protection, the Diffie-Hellman exchange in HIP has to be 371 authenticated. Thus, only public-key HI and authenticated HIP 372 messages are supported in practice. In this document, the non- 373 cryptographic forms of HI and HIP are presented to complete the 374 theory of HI, but they should not be implemented as they could 375 produce worse denial-of-service attacks than the Internet has without 376 Host Identity. 378 5.1 Host Identifiers 380 Host Identity adds two main features to Internet protocols. The 381 first is a decoupling of the internetworking and transport layers; 382 see Section 6. This decoupling will allow for independent evolution 383 of the two layers. Additionally, it can provide end-to-end services 384 over multiple internetworking realms. The second feature is host 385 authentication. Because the Host Identifier is a public key, this 386 key can be used for authentication in security protocols like IPsec. 388 The only completely defined structure of the Host Identity is that of 389 a public/private key pair. In this case, the Host Identity is 390 referred to by its public component, the public key. Thus, the name 391 representing a Host Identity in the Host Identity namespace, i.e., 392 the Host Identifier, is the public key. In a way, the possession of 393 the private key defines the Identity itself. If the private key is 394 possessed by more than one node, the Identity can be considered to be 395 a distributed one. 397 Architecturally, any other Internet naming convention might form a 398 usable base for Host Identifiers. However, non-cryptographic names 399 should only be used in situations of high trust - low risk. That is 400 any place where host authentication is not needed (no risk of host 401 spoofing) and no use of IPsec. However, at least for interconnected 402 networks spanning several operational domains, the set of 403 environments where the risk of host spoofing allowed by non- 404 cryptographic Host Identifiers is acceptable is the null set. Hence, 405 the current HIP documents do not specify how to use any other types 406 of Host Identifiers but public keys. 408 The actual Host Identities are never directly used in any Internet 409 protocols. The corresponding Host Identifiers (public keys) may be 410 stored in various DNS or LDAP directories as identified elsewhere in 411 this document, and they are passed in the HIP base exchange. A Host 412 Identity Tag (HIT) is used in other protocols to represent the Host 413 Identity. Another representation of the Host Identities, the Local 414 Scope Identifier (LSI), can also be used in protocols and APIs. 416 5.2 Storing Host Identifiers in DNS 418 The public Host Identifiers should be stored in DNS; the unpublished 419 Host Identifiers should not be stored anywhere (besides the 420 communicating hosts themselves). The (public) HI is stored in a new 421 RR type, to be defined. This RR type is likely to be quite similar 422 to the IPSECKEY RR [6]. 424 Alternatively, or in addition to storing Host Identifiers in the DNS, 425 they may be stored in various kinds of Public Key Infrastructure 426 (PKI). Such a practice may allow them to be used for purposes other 427 than pure host identification. 429 5.3 Host Identity Tag (HIT) 431 A Host Identity Tag is a 128-bit representation for a Host Identity. 432 It is created by taking a cryptographic hash over the corresponding 433 Host Identifier. There are two advantages of using a hash over using 434 the Host Identifier in protocols. Firstly, its fixed length makes 435 for easier protocol coding and also better manages the packet size 436 cost of this technology. Secondly, it presents the identity in a 437 consistent format to the protocol independent of the cryptographic 438 algorithms used. 440 In the HIP packets, the HITs identify the sender and recipient of a 441 packet. Consequently, a HIT should be unique in the whole IP 442 universe as long as it is being used. In the extremely rare case of 443 a single HIT mapping to more than one Host Identity, the Host 444 Identifiers (public keys) will make the final difference. If there 445 is more than one public key for a given node, the HIT acts as a hint 446 for the correct public key to use. 448 5.4 Local Scope Identifier (LSI) 450 An LSI is a 32-bit localized representation for a Host Identity. The 451 purpose of an LSI is to facilitate using Host Identities in existing 452 protocols and APIs. LSI's advantage over HIT is its size; its 453 disadvantage is its local scope. 455 Examples of how LSIs can be used include: as the address in an FTP 456 command and as the address in a socket call. Thus, LSIs act as a 457 bridge for Host Identities into IPv4-based protocols and APIs. 459 6. New stack architecture 461 One way to characterize Host Identity is to compare the proposed new 462 architecture with the current one. As discussed above, the IP 463 addresses can be seen to be a confounding of routing direction 464 vectors and interface names. Using the terminology from the IRTF 465 Name Space Research Group Report [7] and, e.g., the unpublished 466 Internet-Draft Endpoints and Endpoint Names [10] by Noel Chiappa, the 467 IP addresses currently embody the dual role of locators and end-point 468 identifiers. That is, each IP address names a topological location 469 in the Internet, thereby acting as a routing direction vector, or 470 locator. At the same time, the IP address names the physical network 471 interface currently located at the point-of-attachment, thereby 472 acting as a end-point name. 474 In the HIP architecture, the end-point names and locators are 475 separated from each other. IP addresses continue to act as locators. 476 The Host Identifiers take the role of end-point identifiers. It is 477 important to understand that the end-point names based on Host 478 Identities are slightly different from interface names; a Host 479 Identity can be simultaneously reachable through several interfaces. 481 The difference between the bindings of the logical entities are 482 illustrated in Figure 1. 484 Service ------ Socket Service ------ Socket 485 | | 486 | | 487 | | 488 | | 489 End-point | End-point --- Host Identity 490 \ | | 491 \ | | 492 \ | | 493 \ | | 494 Location --- IP address Location --- IP address 496 Figure 1 498 6.1 Transport associations and end-points 500 Architecturally, HIP provides for a different binding of transport- 501 layer protocols. That is, the transport-layer associations, i.e., 502 TCP connections and UDP associations, are no longer bound to IP 503 addresses but to Host Identities. 505 It is possible that a single physical computer hosts several logical 506 end-points. With HIP, each of these end-points would have a distinct 507 Host Identity. Furthermore, since the transport associations are 508 bound to Host Identities, HIP provides for process migration and 509 clustered servers. That is, if a Host Identity is moved from one 510 physical computer to another, it is also possible to simultaneously 511 move all the transport associations without breaking them. 512 Similarly, if it is possible to distribute the processing of a single 513 Host Identity over several physical computers, HIP provides for 514 cluster based services without any changes at the client end-point. 516 7. End-host mobility and multi-homing 518 HIP decouples the transport from the internetworking layer, and binds 519 the transport associations to the Host Identities (through actually 520 either the HIT or LSI). Consequently, HIP can provide for a degree 521 of internetworking mobility and multi-homing at a low infrastructure 522 cost. HIP mobility includes IP address changes (via any method) to 523 either party. Thus, a system is considered mobile if its IP address 524 can change dynamically for any reason like PPP, DHCP, IPv6 prefix 525 reassignments, or a NAT device remapping its translation. Likewise, 526 a system is considered multi-homed if it has more than one globally 527 routable IP address at the same time. HIP links IP addresses 528 together, when multiple IP addresses correspond to the same Host 529 Identity, and if one address becomes unusable, or a more preferred 530 address becomes available, existing transport associations can easily 531 be moved to another address. 533 When a node moves while communication is already on-going, address 534 changes are rather straightforward. The peer of the mobile node can 535 just accept a HIP or an integrity protected IPsec packet from any 536 address and ignore the source address. However, as discussed in 537 Section 7.2 below, a mobile node must send a HIP readdress packet to 538 inform the peer of the new address(es), and the peer must verify that 539 the mobile node is reachable through these addresses. This is 540 especially helpful for those situations where the peer node is 541 sending data periodically to the mobile node (that is re-starting a 542 connection after the initial connection). 544 7.1 Rendezvous mechanism 546 Making a contact to a mobile node is slightly more involved. In 547 order to start the HIP exchange, the initiator node has to know how 548 to reach the mobile node. Although infrequently moving HIP nodes 549 could use Dynamic DNS [1] to update their reachability information in 550 the DNS, an alternative to using DNS in this fashion is to use a 551 piece of new static infrastructure to facilitate rendezvous between 552 HIP nodes. 554 The mobile node keeps the rendezvous infrastructure continuously 555 updated with its current IP address(es). The mobile nodes must trust 556 the rendezvous mechanism to properly maintain their HIT and IP 557 address mappings. 559 The rendezvous mechanism is also needed if both of the nodes happen 560 to change their address at the same time, either because they are 561 mobile and happen to move at the same time, because one of them is 562 off-line for a while, or because of some other reason. In such a 563 case, the HIP readdress packets will cross each other in the network 564 and never reach the peer node. 566 A separate document will specify the details of the HIP rendezvous 567 mechanism. 569 7.2 Protection against flooding attacks 571 Although the idea of informing about address changes by simply 572 sending packets with a new source address appears appealing, it is 573 not secure enough. That is, even if HIP does not rely on the source 574 address for anything (once the base exchange has been completed), it 575 appears to be necessary to check a mobile node's reachability at the 576 new address before actually sending any larger amounts of traffic to 577 the new address. 579 Blindly accepting new addresses would potentially lead to flooding 580 Denial-of-Service attacks against third parties [8]. In a 581 distributed flooding attack an attacker opens high volume HIP 582 connections with a large number of hosts (using unpublished HIs), and 583 then claims to all of these hosts that it has moved to a target 584 node's IP address. If the peer hosts were to simply accept the move, 585 the result would be a packet flood to the target node's address. To 586 close this attack, HIP includes an address check mechanism where the 587 reachability of a node is separately checked at each address before 588 using the address for larger amounts of traffic. 590 Whenever HIP is used between two hosts that fully trust each other, 591 the hosts may optionally decide to skip the address tests. However, 592 such performance optimization must be restricted to peers that are 593 known to be trustworthy and capable of protecting themselves from 594 malicious software. 596 8. HIP and IPsec 598 The preferred way of implementing HIP is to use IPsec to carry the 599 actual data traffic. As of today, the only completely defined method 600 is to use IPsec Encapsulated Security Payload (ESP) to carry the data 601 packets. In the future, other ways of transporting payload data may 602 be developed, including ones that do not use cryptographic 603 protection. 605 In practice, the HIP base exchange uses the cryptographic Host 606 Identifiers to set up a pair of ESP Security Associations (SAs) to 607 enable ESP in an end-to-end manner. This is implemented in a way 608 that can span addressing realms. 610 While it would be possible, at least in theory, to use some existing 611 cryptographic protocol, such as IKEv2 together with Host Identifiers, 612 to establish the needed SAs, HIP defines a new protocol. There are a 613 number of historical reasons for this, and there are also a few 614 architectural reasons. First, IKE (and IKEv2) were not designed with 615 middle boxes in mind. As adding a new naming layer allows one to 616 potentially add a new forwarding layer (see Section 9, below), it is 617 very important that the HIP protocols are friendly towards any middle 618 boxes. 620 Second, from a conceptual point of view, the IPsec Security Parameter 621 Index (SPI) in ESP provides a simple compression of the HITs. This 622 does require per-HIT-pair SAs (and SPIs), and a decrease of policy 623 granularity over other Key Management Protocols, such as IKE and 624 IKEv2. In particular, the current thinking is limited to a situation 625 where, conceptually, there is only one pair of SAs between any given 626 pair of HITs. In other words, from an architectural point of view, 627 HIP only supports host-to-host (or endpoint-to-endpoint) Security 628 Associations. If two hosts need more pairs of parallel SAs, they 629 should use separate HITs for that. However, future HIP extensions 630 may provide for more granularity and creation of several ESP SAs 631 between a pair of HITs. 633 Since HIP is designed for host usage, not for gateways or so called 634 Bump-in-the-Wire (BITW) implementations, only ESP transport mode is 635 supported. An ESP SA pair is indexed by the SPIs and the two HITs 636 (both HITs since a system can have more than one HIT). The SAs need 637 not to be bound to IP addresses; all internal control of the SA is by 638 the HITs. Thus, a host can easily change its address using Mobile 639 IP, DHCP, PPP, or IPv6 readdressing and still maintain the SAs. 640 Since the transports are bound to the SA (via an LSI or a HIT), any 641 active transport is also maintained. Thus, real-world conditions 642 like loss of a PPP connection and its re-establishment or a mobile 643 handover will not require a HIP negotiation or disruption of 644 transport services [12]. 646 Since HIP does not negotiate any SA lifetimes, all lifetimes are 647 local policy. The only lifetimes a HIP implementation must support 648 are sequence number rollover (for replay protection), and SA timeout. 649 An SA times out if no packets are received using that SA. 650 Implementations may support lifetimes for the various ESP transforms. 652 9. HIP and NATs 654 Passing packets between different IP addressing realms requires 655 changing IP addresses in the packet header. This may happen, for 656 example, when a packet is passed between the public Internet and a 657 private address space, or between IPv4 and IPv6 networks. The 658 address translation is usually implemented as Network Address 659 Translation (NAT) [4] or NAT Protocol translation (NAT-PT) [3]. 661 In a network environment where identification is based on the IP 662 addresses, identifying the communicating nodes is difficult when NAT 663 is used. With HIP, the transport-layer end-points are bound to the 664 Host Identities. Thus, a connection between two hosts can traverse 665 many addressing realm boundaries. The IP addresses are used only for 666 routing purposes; they may be changed freely during packet traversal. 668 For a HIP-based flow, a HIP-aware NAT or NAT-PT system tracks the 669 mapping of HITs, and the corresponding IPsec SPIs, to an IP address. 670 The NAT system has to learn mappings both from HITs and from SPIs to 671 IP addresses. Many HITs (and SPIs) can map to a single IP address on 672 a NAT, simplifying connections on address poor NAT interfaces. The 673 NAT can gain much of its knowledge from the HIP packets themselves; 674 however, some NAT configuration may be necessary. 676 NAT systems cannot touch the datagrams within the IPsec envelope, 677 thus application-specific address translation must be done in the end 678 systems. HIP provides for 'Distributed NAT', and uses the HIT or the 679 LSI as a placeholder for embedded IP addresses. 681 9.1 HIP and TCP checksums 683 There is no way for a host to know if any of the IP addresses in an 684 IP header are the addresses used to calculate the TCP checksum. That 685 is, it is not feasible to calculate the TCP checksum using the actual 686 IP addresses in the pseudo header; the addresses received in the 687 incoming packet are not necessarily the same as they were on the 688 sending host. Furthermore, it is not possible to recompute the 689 upper-layer checksums in the NAT/NAT-PT system, since the traffic is 690 IPsec protected. Consequently, the TCP and UDP checksums are 691 calculated using the HITs in the place of the IP addresses in the 692 pseudo header. Furthermore, only the IPv6 pseudo header format is 693 used. This provides for IPv4 / IPv6 protocol translation. 695 10. Multicast 697 Back in the fall of 2003, there was little if any concrete thoughts 698 about how HIP might affect IP-layer or application-layer multicast. 700 11. HIP policies 702 There are a number of variables that will influence the HIP exchanges 703 that each host must support. All HIP implementations should support 704 at least 2 HIs, one to publish in DNS and an unpublished one for 705 anonymous usage. Although unpublished HIs will be rarely used as 706 responder HIs, they are likely be common for initiators. Support for 707 multiple HIs is recommended. 709 Many initiators would want to use a different HI for different 710 responders. The implementations should provide for a policy of 711 initiator HIT to responder HIT. This policy should also include 712 preferred transforms and local lifetimes. 714 Responders would need a similar policy, describing the hosts allowed 715 to participate in HIP exchanges, and the preferred transforms and 716 local lifetimes. 718 12. Benefits of HIP 720 In the beginning, the network layer protocol (i.e., IP) had the 721 following four "classic" invariants: 723 o Non-mutable: The address sent is the address received. 725 o Non-mobile: The address doesn't change during the course of an 726 "association". 728 o Reversible: A return header can always be formed by reversing the 729 source and destination addresses. 731 o Omniscient: Each host knows what address a partner host can use to 732 send packets to it. 734 Actually, the fourth can be inferred from 1 and 3, but it is worth 735 mentioning for reasons that will be obvious soon if not already. 737 In the current "post-classic" world, we are intentionally trying to 738 get rid of the second invariant (both for mobility and for multi- 739 homing), and we have been forced to give up the first and the fourth. 740 Realm Specific IP [5] is an attempt to reinstate the fourth invariant 741 without the first invariant. IPv6 is an attempt to reinstate the 742 first invariant. 744 Few systems on the Internet have DNS names that are meaningful. That 745 is, if they have a Fully Qualified Domain Name (FQDN), that name 746 typically belongs to a NAT device or a dial-up server, and does not 747 really identify the system itself but its current connectivity. 748 FQDNs (and their extensions as email names) are application-layer 749 names; more frequently naming services than a particular system. 750 This is why many systems on the Internet are not registered in the 751 DNS; they do not have services of interest to other Internet hosts. 753 DNS names are references to IP addresses. This only demonstrates the 754 interrelationship of the networking and application layers. DNS, as 755 the Internet's only deployed, distributed database is also the 756 repository of other namespaces, due in part to DNSSEC and application 757 specific key records. Although each namespace can be stretched (IP 758 with v6, DNS with KEY records), neither can adequately provide for 759 host authentication or act as a separation between internetworking 760 and transport layers. 762 The Host Identity (HI) namespace fills an important gap between the 763 IP and DNS namespaces. An interesting thing about the HI is that it 764 actually allows one to give up all but the 3rd network-layer 765 invariant. That is to say, as long as the source and destination 766 addresses in the network-layer protocol are reversible, then things 767 work ok because HIP takes care of host identification, and 768 reversibility allows one to get a packet back to one's partner host. 769 You do not care if the network-layer address changes in transit 770 (mutable) and you don't care what network-layer address the partner 771 is using (non-omniscient). 773 12.1 HIP's answers to NSRG questions 775 The IRTF Name Space Research Group has posed a number of evaluating 776 questions in their report [7]. In this section, we provide answers 777 to these questions. 779 1. How would a stack name improve the overall functionality of the 780 Internet? 782 HIP decouples the internetworking layer from the transport 783 layer, allowing each to evolve separately. The decoupling 784 makes end-host mobility and multi-homing easier, also across 785 IPv4 and IPv6 networks. HIs make network renumbering easier, 786 and they also make process migration and clustered servers 787 easier to implement. Furthermore, being cryptographic in 788 nature, they provide the basis for solving the security 789 problems related to end-host mobility and multi-homing. 791 2. What does a stack name look like? 793 A HI is a cryptographic public key. However, instead of using 794 the keys directly, most protocols use a fixed size hash of the 795 public key. 797 3. What is its lifetime? 799 HIP provides both stable and temporary Host Identifiers. 800 Stable HIs are typically long lived, with a lifetime of years 801 or more. The lifetime of temporary HIs depends on how long 802 the upper-layer connections and applications need them, and 803 can range from a few seconds to years. 805 4. Where does it live in the stack? 807 The HIs live between the transport and internetworking layers. 809 5. How is it used on the end points 811 The Host Identifiers may be used directly or indirectly (in 812 the form of HITs or LSIs) by applications when they access 813 network services. Additionally, the Host Identifiers, as 814 public keys, are used in the built in key agreement protocol, 815 called the HIP base exchange, to authenticate the hosts to 816 each other. 818 6. What administrative infrastructure is needed to support it? 820 In some environments, it is possible to use HIP 821 opportunistically, without any infrastructure. However, to 822 gain full benefit from HIP, the HIs must be stored in the DNS 823 or a PKI, and a new rendezvous mechanism is needed. Such a 824 new rendezvous mechanism may need new infrastructure to be 825 deployed. 827 7. If we add an additional layer would it make the address list in 828 SCTP unnecessary? 830 Yes 832 8. What additional security benefits would a new naming scheme 833 offer? 835 HIP reduces dependency on IP addresses, making the so called 836 address ownership [11] problems easier to solve. In practice, 837 HIP provides security for end-host mobility and multi-homing. 838 Furthermore, since HIP Host Identifiers are public keys, 839 standard public key certificate infrastructures can be applied 840 on the top of HIP. 842 9. What would the resolution mechanisms be, or what characteristics 843 of a resolution mechanisms would be required? 845 For most purposes, an approach where DNS names are resolved 846 simultaneously to HIs and IP addresses is sufficient. 847 However, if it becomes necessary to resolve HIs into IP 848 addresses or back to DNS names, a flat resolution 849 infrastructure is needed. Such an infrastructure could be 850 based on the ideas of Distributed Hash Tables, but would 851 require significant new development and deployment. 853 13. Security considerations 855 HIP takes advantage of the new Host Identity paradigm to provide 856 secure authentication of hosts and to provide a fast key exchange for 857 IPsec. HIP also attempts to limit the exposure of the host to 858 various denial-of-service (DoS) and man-in-the-middle (MitM) attacks. 859 In so doing, HIP itself is subject to its own DoS and MitM attacks 860 that potentially could be more damaging to a host's ability to 861 conduct business as usual. 863 Resource exhausting denial-of-service attacks take advantage of the 864 cost of setting up a state for a protocol on the responder compared 865 to the 'cheapness' on the initiator. HIP allows a responder to 866 increase the cost of the start of state on the initiator and makes an 867 effort to reduce the cost to the responder. This is done by having 868 the responder start the authenticated Diffie-Hellman exchange instead 869 of the initiator, making the HIP base exchange 4 packets long. There 870 are more details on this process in the Host Identity Protocol under 871 development. 873 HIP optionally supports opportunistic negotiation. That is, if a 874 host receives a start of transport without a HIP negotiation, it can 875 attempt to force a HIP exchange before accepting the connection. 876 This has the potential for DoS attacks against both hosts. If the 877 method to force the start of HIP is expensive on either host, the 878 attacker need only spoof a TCP SYN. This would put both systems into 879 the expensive operations. HIP avoids this attack by having the 880 responder send a simple HIP packet that it can pre-build. Since this 881 packet is fixed and easily replayed, the initiator only reacts to it 882 if it has just started a connection to the responder. 884 Man-in-the-middle attacks are difficult to defend against, without 885 third-party authentication. A skillful MitM could easily handle all 886 parts of the HIP base exchange, but HIP indirectly provides the 887 following protection from a MitM attack. If the responder's HI is 888 retrieved from a signed DNS zone or secured by some other means, the 889 initiator can use this to authenticate the signed HIP packets. 890 Likewise, if the initiator's HI is in a secure DNS zone, the 891 responder can retrieve it and validate the signed HIP packets. 892 However, since an initiator may choose to use an unpublished HI, it 893 knowingly risks a MitM attack. The responder may choose not to 894 accept a HIP exchange with an initiator using an unknown HI. 896 In HIP, the Security Association for IPsec is indexed by the SPI; the 897 source address is always ignored, and the destination address may be 898 ignored as well. Therefore, HIP-enabled IPsec Encapsulated Security 899 Payload (ESP) is IP address independent. This might seem to make it 900 easier for an attacker, but ESP with replay protection is already as 901 well protected as possible, and the removal of the IP address as a 902 check should not increase the exposure of IPsec ESP to DoS attacks. 904 Since not all hosts will ever support HIP, ICMPv4 'Destination 905 Unreachable, Protocol Unreachable' and ICMPv6 'Parameter Problem, 906 Unrecognized Next Header' messages are to be expected and present a 907 DoS attack. Against an initiator, the attack would look like the 908 responder does not support HIP, but shortly after receiving the ICMP 909 message, the initiator would receive a valid HIP packet. Thus, to 910 protect against this attack, an initiator should not react to an ICMP 911 message until a reasonable time has passed, allowing it to get the 912 real responder's HIP packet. A similar attack against the responder 913 is more involved. 915 Another MitM attack is simulating a responder's administrative 916 rejection of a HIP initiation. This is a simple ICMP 'Destination 917 Unreachable, Administratively Prohibited' message. A HIP packet is 918 not used because it would either have to have unique content, and 919 thus difficult to generate, resulting in yet another DoS attack, or 920 just as spoofable as the ICMP message. Like in the previous case, 921 the defense against this attack is for the initiator to wait a 922 reasonable time period to get a valid HIP packet. If one does not 923 come, then the initiator has to assume that the ICMP message is 924 valid. Since this is the only point in the HIP base exchange where 925 this ICMP message is appropriate, it can be ignored at any other 926 point in the exchange. 928 13.1 HITs used in ACLs 930 It is expected that HITs will be used in ACLs. Future firewalls can 931 use HITs to control egress and ingress to networks, with an assurance 932 level difficult to achieve today. As discussed above in Section 8, 933 once a HIP session has been established, the SPI value in an IPsec 934 packet may be used as an index, indicating the HITs. In practice, 935 firewalls can inspect HIP packets to learn of the bindings between 936 HITs, SPI values, and IP addresses. They can even explicitly control 937 IPsec usage, dynamically opening IPsec ESP only for specific SPI 938 values and IP addresses. The signatures in HIP packets allow a 939 capable firewall to ensure that the HIP exchange is indeed happening 940 between two known hosts. This may increase firewall security. 942 There has been considerable bad experience with distributed ACLs that 943 contain public key related material, for example, with SSH. If the 944 owner of a key needs to revoke it for any reason, the task of finding 945 all locations where the key is held in an ACL may be impossible. If 946 the reason for the revocation is due to private key theft, this could 947 be a serious issue. 949 A host can keep track of all of its partners that might use its HIT 950 in an ACL by logging all remote HITs. It should only be necessary to 951 log responder hosts. With this information, the host can notify the 952 various hosts about the change to the HIT. There has been no attempt 953 to develop a secure method to issue the HIT revocation notice. 955 HIP-aware NATs, however, are transparent to the HIP aware systems by 956 design. Thus, the host may find it difficult to notify any NAT that 957 is using a HIT in an ACL. Since most systems will know of the NATs 958 for their network, there should be a process by which they can notify 959 these NATs of the change of the HIT. This is mandatory for systems 960 that function as responders behind a NAT. In a similar vein, if a 961 host is notified of a change in a HIT of an initiator, it should 962 notify its NAT of the change. In this manner, NATs will get updated 963 with the HIT change. 965 13.2 Non-security considerations 967 The definition of the Host Identifier states that the HI need not be 968 a public key. It implies that the HI could be any value; for example 969 a FQDN. This document does not describe how to support such a non- 970 cryptographic HI. A non-cryptographic HI would still offer the 971 services of the HIT or LSI for NAT traversal. It would be possible 972 to carry HITs in HIP packets that had neither privacy nor 973 authentication. Since such a mode would offer so little additional 974 functionality for so much addition to the IP kernel, it has not been 975 defined. Given how little public key cryptography HIP requires, HIP 976 should only be implemented using public key Host Identities. 978 If it is desirable to use HIP in a low security situation where 979 public key computations are considered expensive, HIP can be used 980 with very short Diffie-Hellman and Host Identity keys. Such use 981 makes the participating hosts vulnerable to MitM and connection 982 hijacking attacks. However, it does not cause flooding dangers, 983 since the address check mechanism relies on the routing system and 984 not on cryptographic strength. 986 14. IANA considerations 988 This document has no actions for IANA. 990 15. Acknowledgments 992 For the people historically involved in the early stages of HIP, see 993 the Acknowledgements section in the Host Identity Protocol 994 specification. 996 During the later stages of this document, when the editing baton was 997 transfered to Pekka Nikander, the comments from the early 998 implementors and others, including Jari Arkko, Tom Henderson, Petri 999 Jokela, Miika Komu, Mika Kousa, Andrew McGregor, Jan Melen, Tim 1000 Shepard, Jukka Ylitalo, and Jorma Wall, were invaluable. Finally, 1001 Lars Eggert, Spencer Dawkins and Dave Crocker provided valuable input 1002 during the final stages of publication, most of which was 1003 incorporated but some of which the authors decided to ignore in order 1004 to get this document published in the first place. 1006 16. Informative references 1008 [1] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic 1009 Updates in the Domain Name System (DNS UPDATE)", RFC 2136, 1010 April 1997. 1012 [2] Eastlake, D., "Domain Name System Security Extensions", 1013 RFC 2535, March 1999. 1015 [3] Tsirtsis, G. and P. Srisuresh, "Network Address Translation - 1016 Protocol Translation (NAT-PT)", RFC 2766, February 2000. 1018 [4] Srisuresh, P. and K. Egevang, "Traditional IP Network Address 1019 Translator (Traditional NAT)", RFC 3022, January 2001. 1021 [5] Borella, M., Lo, J., Grabelsky, D., and G. Montenegro, "Realm 1022 Specific IP: Framework", RFC 3102, October 2001. 1024 [6] Richardson, M., "A Method for Storing IPsec Keying Material in 1025 DNS", RFC 4025, March 2005. 1027 [7] Lear, E. and R. Droms, "What's In A Name: Thoughts from the 1028 NSRG", draft-irtf-nsrg-report-10 (work in progress), 1029 September 2003. 1031 [8] Nikander, P., "Mobile IP version 6 Route Optimization Security 1032 Design Background", draft-ietf-mip6-ro-sec-03 (work in 1033 progress), May 2005. 1035 [9] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", 1036 draft-ietf-ipsec-ikev2-17 (work in progress), October 2004. 1038 [10] Chiappa, J., "Endpoints and Endpoint Names: A Proposed 1039 Enhancement to the Internet Architecture", 1040 URL http://users.exis.net/~jnc/tech/endpoints.txt, 1999. 1042 [11] Nikander, P., "Denial-of-Service, Address Ownership, and Early 1043 Authentication in the IPv6 World", in Proceesings of Security 1044 Protocols, 9th International Workshop, Cambridge, UK, April 1045 25-27 2001, LNCS 2467, pp. 12-26, Springer, 2002. 1047 [12] Bellovin, S., "EIDs, IPsec, and HostNAT", in Proceedings 1048 of 41th IETF, Los Angeles, CA, March 1998. 1050 Authors' Addresses 1052 Robert Moskowitz 1053 ICSAlabs, a Division of TruSecure Corporation 1054 1000 Bent Creek Blvd, Suite 200 1055 Mechanicsburg, PA 1056 USA 1058 Email: rgm@icsalabs.com 1060 Pekka Nikander 1061 Ericsson Research Nomadic Lab 1062 JORVAS FIN-02420 1063 FINLAND 1065 Phone: +358 9 299 1 1066 Email: pekka.nikander@nomadiclab.com 1068 Intellectual Property Statement 1070 The IETF takes no position regarding the validity or scope of any 1071 Intellectual Property Rights or other rights that might be claimed to 1072 pertain to the implementation or use of the technology described in 1073 this document or the extent to which any license under such rights 1074 might or might not be available; nor does it represent that it has 1075 made any independent effort to identify any such rights. 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