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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group R. Moskowitz 3 Internet-Draft ICSAlabs, a Division of TruSecure 4 Expires: April 16, 2005 Corporation 5 P. Nikander 6 Ericsson Research Nomadic Lab 7 October 16, 2004 9 Host Identity Protocol Architecture 10 draft-ietf-hip-arch-00 12 Status of this Memo 14 This document is an Internet-Draft and is subject to all provisions 15 of section 3 of RFC 3667. By submitting this Internet-Draft, each 16 author represents that any applicable patent or other IPR claims of 17 which he or she is aware have been or will be disclosed, and any of 18 which he or she become aware will be disclosed, in accordance with 19 RFC 3668. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF), its areas, and its working groups. Note that 23 other groups may also distribute working documents as 24 Internet-Drafts. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 The list of current Internet-Drafts can be accessed at 32 http://www.ietf.org/ietf/1id-abstracts.txt. 34 The list of Internet-Draft Shadow Directories can be accessed at 35 http://www.ietf.org/shadow.html. 37 This Internet-Draft will expire on April 16, 2005. 39 Copyright Notice 41 Copyright (C) The Internet Society (2004). 43 Abstract 45 This memo describes a snapshot of the reasoning behind a proposed new 46 namespace, the Host Identity namespace, and a new protocol layer, the 47 Host Identity Protocol, between the internetworking and transport 48 layers. Herein are presented the basics of the current namespaces, 49 strengths and weaknesses, and how a new namespace will add 50 completeness to them. The roles of this new namespace in the 51 protocols are defined. The memo describes the thinking of the 52 authors as of Fall 2003. 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 . . . . . . 7 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 Checksum . . . . . . . . . . . . . . . . . . . . 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. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 21 84 15. Informative References . . . . . . . . . . . . . . . . . . . 22 85 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 23 86 Intellectual Property and Copyright Statements . . . . . . . 24 88 1. Disclaimer 90 The purpose of this memo is to provide a stable reference point in 91 the development of the Host Identity Protocol architecture. This 92 memo describes the thinking of the authors as of Fall 2003; their 93 thinking may have evolved since then. In occasions, this memo may be 94 confusing or self-contradicting. That is (partially) intentional, 95 and reflects the snapshot nature of this memo. 97 2. Introduction 99 The Internet has created two important global namespaces: Internet 100 Protocol (IP) addresses and Domain Name Service (DNS) names. These 101 two namespaces have a set of features and abstractions that have 102 powered the Internet to what it is today. They also have a number of 103 weaknesses. Basically, since they are all we have, we try and do too 104 much with them. Semantic overloading and functionality extensions 105 have greatly complicated these namespaces. 107 The proposed Host Identity namespace fills an important gap between 108 the IP and DNS namespaces. The Host Identity namespace consists of 109 Host Identifiers (HI). A Host Identifier is cryptographic in its 110 nature; it is the public key of an asymmetric key-pair. A Host 111 Identity is assigned to each host. Each host will have at least one 112 Host Identity and a corresponding Host Identifier, which can either 113 be public (e.g. published in DNS), or unpublished. Client systems 114 will tend to have both public and unpublished Identities. 116 There is a subtle but important difference between Host Identities 117 and Host Identifiers. An Identity refers to the abstract entity that 118 is identified. An Identifier, on the other hand, refers to the 119 concrete bit pattern that is used in the identification process. 121 Although the Host Identifiers could be used in many authentication 122 systems, such as IKEv2 [9], the presented architecture introduces a 123 new protocol, called the Host Identity Protocol (HIP), and a 124 cryptographic exchange, called the HIP base exchange [4]. The new 125 protocol provides for limited forms of trust between systems. It 126 enhances mobility, multi-homing and dynamic IP renumbering [7], aids 127 in protocol translation / transition [4], and reduces certain types 128 of denial-of-service (DoS) attacks [4]. 130 When HIP is used, the actual payload traffic between two HIP hosts is 131 typically, but not necessarily, protected with IPsec. The Host 132 Identities are used to create the needed IPsec Security Associations 133 (SA) and to authenticate the hosts. When IPsec is used, the actual 134 payload IP packets do not differ in any way from standard IPsec 135 protected IP packets. 137 3. Terminology 139 3.1 Terms common to other documents 141 +--------------+----------------------------------------------------+ 142 | Term | Explanation | 143 +--------------+----------------------------------------------------+ 144 | Public key | The public key from an asymmetric cryptographic | 145 | | key pair. Used as a publicly known identifier for | 146 | | cryptographic identity authentication. | 147 | | | 148 | Private key | The private or secret key from an asymmetric | 149 | | cryptographic key pair. Assumed to be known only | 150 | | to the party identified by the corresponding | 151 | | public key. Used by the identified party to | 152 | | authenticate its identity to other parties. | 153 | | | 154 | Public key | An asymmetric cryptographic key pair consisting of | 155 | pair | a public and private keys. For example, | 156 | | Rivest-Shamir-Adelman (RSA) and Digital Signature | 157 | | Algorithm (DSA) key pairs are such key pairs. | 158 | | | 159 | End-point | A communicating entity. For historical reasons, | 160 | | the term 'computing platform' is used in this | 161 | | document as a (rough) synonym for end-point. | 162 +--------------+----------------------------------------------------+ 164 3.2 Terms specific to this and other HIP documents 166 It should be noted that many of the terms defined herein are 167 tautologous, self-referential or defined through circular reference 168 to other terms. This is due to the succinct nature of the 169 definitions. See the text elsewhere in this document for more 170 elaborate explanations. 172 +--------------+----------------------------------------------------+ 173 | Term | Explanation | 174 +--------------+----------------------------------------------------+ 175 | Computing | An entity capable of communicating and computing, | 176 | platform | for example, a computer. See the definition of | 177 | | 'End-point', above. | 178 | | | 179 | HIP base | A cryptographic protocol defined in [4]. See also | 180 | exchange | Section 8. | 181 | | | 182 | HIP packet | An IP packet that carries a 'Host Identity | 183 | | Protocol' message. | 184 | | | 185 | Host | An abstract concept assigned to a 'computing | 186 | Identity | platform'. See 'Host Identifier', below. | 187 | | | 188 | Host | A name space formed by all possible Host | 189 | Identity | Identifiers. | 190 | namespace | | 191 | | | 192 | Host | A protocol used to carry and authenticate Host | 193 | Identity | Identifiers and other information. | 194 | Protocol | | 195 | | | 196 | Host | A 128-bit datum created by taking a cryptographic | 197 | Identity Tag | hash over a Host Identifier. | 198 | | | 199 | Host | A public key used as a name for a Host Identity. | 200 | Identifier | | 201 | | | 202 | Local Scope | A 32-bit datum denoting a Host Identity. | 203 | Identifier | | 204 | | | 205 | Public Host | A published or publicly known Host Identfier used | 206 | Identifier | as a public name for a Host Identity, and the | 207 | and Identity | corresponding Identity. | 208 | | | 209 | Unpublished | A Host Identifier that is not placed in any public | 210 | Host | directory, and the corresponding Host Identity. | 211 | Identifier | Unpublished Host Identities are typically short | 212 | and Identity | living in nature, being often replaced and | 213 | | possibly used just once. | 214 | | | 215 | Rendezvous | A mechanism used to locate mobile hosts based on | 216 | Mechanism | their HIT. | 217 +--------------+----------------------------------------------------+ 219 4. Background 221 The Internet is built from three principal components: computing 222 platforms (end-points), packet transport (i.e. internetworking) 223 infrastructure, and services (applications). The Internet exists to 224 service two principal components: people and robotic services 225 (silicon based people, if you will). All these components need to be 226 named in order to interact in a scalable manner. Here we concentrate 227 on naming computing platforms and packet transport elements. 229 There are two principal namespaces in use in the Internet for these 230 components: IP numbers, and Domain Names. Email, HTTP, and SIP 231 addresses are really only extensions of Domain Names. 233 IP numbers are a confounding of two namespaces, the names of a host's 234 networking interfaces and the names of the locations ('confounding' 235 is a term used in statistics to discuss metrics that are merged into 236 one with a gain in indexing, but a loss in informational value). The 237 names of locations should be understood as denoting routing direction 238 vectors, i.e., information that is used to deliver packets to their 239 destinations. 241 IP numbers name networking interfaces, and typically only when the 242 interface is connected to the network. Originally IP numbers had 243 long-term significance. Today, the vast number of interfaces use 244 ephemeral and/or non-unique IP numbers. That is, every time an 245 interface is connected to the network, it is assigned an IP number. 247 In the current Internet, the transport layers are coupled to the IP 248 addresses. Neither can evolve separately from the other. IPng 249 deliberations were strongly shaped by the decision that a 250 corresponding TCPng would not be created. 252 Domain Names provide hierarchically assigned names for some computing 253 platforms and some services. Each hierarchy is delegated from the 254 level above; there is no anonymity in Domain Names. 256 Email, SIP and WWW addresses provide naming for humans, autonomous 257 applications, and documents. Email, SIP and WWW addresses are 258 extensions of Domain Names. 260 There are three critical deficiencies with the current namespaces. 261 Firstly, dynamic readdressing cannot be directly managed. Secondly, 262 anonymity is not provided in a consistent, trustable manner. 263 Finally, authentication for systems and datagrams is not provided. 264 All of these deficiencies arise because computing platforms are not 265 well named with the current namespaces. 267 4.1 A Desire for a Namespace for Computing Platforms 269 An independent namespace for computing platforms could be used in 270 end-to-end operations independent of the evolution of the 271 internetworking layer and across the many internetworking layers. 272 This could support rapid readdressing of the internetworking layer 273 because of mobility, rehoming, or renumbering. 275 If the namespace for computing platforms is based on public key 276 cryptography, it can also provide authentication services. If this 277 namespace is locally created without requiring registration, it can 278 provide anonymity. 280 Such a namespace (for computing platforms) and the names in it should 281 have the following characteristics: 283 o The namespace should be applied to the IP 'kernel'. The IP kernel 284 is the 'component' between applications and the packet transport 285 infrastructure. 287 o The namespace should fully decouple the internetworking layer from 288 the higher layers. The names should replace all occurrences of IP 289 addresses within applications (like in the TCB). This may require 290 changes to the current APIs. In the long run, it is probable that 291 some new APIs are needed. 293 o The introduction of the namespace should not mandate any 294 administrative infrastructure. Deployment must come from the 295 bottom up, in a pairwise deployment. 297 o The names should have a fixed length representation, for easy 298 inclusion in datagram headers and existing programming interfaces 299 (e.g the TCB). 301 o Using the namespace should be affordable when used in protocols. 302 This is primarily a packet size issue. There is also a 303 computational concern in affordability. 305 o The names must be statistically globally unique. 64 bits is 306 inadequate to make the probability of collisions sufficiently low 307 (1% chance of collision in a population of 640M); thus, 308 approximately 100 or more bits should be used. 310 o The names should have a localized abstraction so that it can be 311 used in existing protocols and APIs. 313 o It must be possible to create names locally. This can provide 314 anonymity at the cost of making resolvability very difficult. 316 Sometimes the names may contain a delegation component. This 317 is the cost of resolvability. 319 o The namespace should provide authentication services. 321 o The names should be long lived, but replaceable at any time. This 322 impacts access control lists; short lifetimes will tend to result 323 in tedious list maintenance or require a namespace infrastructure 324 for central control of access lists. 326 In this document, a new namespace approaching these ideas is called 327 the Host Identity namespace. Using Host Identities requires its own 328 protocol layer, the Host Identity Protocol, between the 329 internetworking and transport layers. The names are based on public 330 key cryptography to supply authentication services. Properly 331 designed, it can deliver all of the above stated requirements. 333 5. Host Identity Namespace 335 A name in the Host Identity namespace, a Host Identifier (HI), 336 represents a statistically globally unique name for naming any system 337 with an IP stack. This identity is normally associated with, but not 338 limited to, an IP stack. A system can have multiple identities, some 339 'well known', some unpublished or 'anonymous'. A system may self 340 assert its own identity, or may use a third-party authenticator like 341 DNSSEC, PGP, or X.509 to 'notarize' the identity assertion. It is 342 expected that the Host Identifiers will initially be authenticated 343 with DNSSEC and that all implementations will support DNSSEC as a 344 minimal baseline. 346 In theory, any name that can claim to be 'statistically globally 347 unique' may serve as a Host Identifier. However, in the authors' 348 opinion, a public key of a 'public key pair' makes the best Host 349 Identifier. As documented in the Host Identity Protocol 350 specification [4], a public key based HI can authenticate the HIP 351 packets and protect them for man-in-the-middle attacks. Since 352 authenticated datagrams are mandatory to provide much of HIP's 353 denial-of-service protection, the Diffie-Hellman exchange in HIP has 354 to be authenticated. Thus, only public key HI and authenticated HIP 355 messages are supported in practice. In this document, the 356 non-cryptographic forms of HI and HIP are presented to complete the 357 theory of HI, but they should not be implemented as they could 358 produce worse denial-of-service attacks than the Internet has without 359 Host Identity. 361 5.1 Host Identifiers 363 Host Identity adds two main features to Internet protocols. The 364 first is a decoupling of the internetworking and transport layers; 365 see Section 6. This decoupling will allow for independent evolution 366 of the two layers. Additionally, it can provide end-to-end services 367 over multiple internetworking realms. The second feature is host 368 authentication. Because the Host Identifier is a public key, this 369 key can be used to authenticate security protocols like IPsec. 371 The only completely defined structure of the Host Identity is that of 372 a public/private key pair. In this case, the Host Identity is 373 referred to by its public component, the public key. Thus, the name 374 representing a Host Identity in the Host Identity namespace, i.e. 375 the Host Identifier, is the public key. In a way, the possession of 376 the private key defines the Identity itself. If the private key is 377 possessed by more than one node, the Identity can be considered to be 378 a distributed one. 380 Architecturally, any other Internet naming convention might form a 381 usable base for Host Identifiers. However, non-cryptographic names 382 should only be used in situations of high trust - low risk. That is 383 any place where host authentication is not needed (no risk of host 384 spoofing) and no use of IPsec. However, at least for interconnected 385 networks spanning several operational domains, the set of 386 environments where the risk of host spoofing allowed by 387 non-cryptographic Host Identifiers is acceptable is the null set. 388 Hence, the current HIP documents do not specify how to use any other 389 types of Host Identifiers but public keys. 391 The actual Host Identities are never directly used in any Internet 392 protocols. The corresponding Host Identifiers (public keys) may be 393 stored in various DNS or LDAP directories as identified elsewhere in 394 this document, and they are passed in the HIP base exchange. A Host 395 Identity Tag (HIT) is used in other protocols to represent the Host 396 Identity. Another representation of the Host Identities, the Local 397 Scope Identifier (LSI), can also be used in protocols and APIs. 399 5.2 Storing Host Identifiers in DNS 401 The public Host Identifiers should be stored in DNS; the unpublished 402 Host Identifiers should not be stored anywhere (besides the 403 communicating hosts themselves). The (public) HI is stored in a new 404 RR type, to be defined. This RR type is likely to be quite similar 405 to the IPSECKEY RR [5]. 407 Alternatively, or in addition to storing Host Identifiers in the DNS, 408 they may be stored in various kinds of Public Key Infrastructure 409 (PKI). Such a practice may allow them to be used for purposes other 410 than pure host identification. 412 5.3 Host Identity Tag (HIT) 414 A Host Identity Tag is an 128-bit representation for a Host Identity. 415 It is created by taking a cryptographic hash over the corresponding 416 Host Identifier. There are two advantages of using a hash over using 417 the Host Identifier in protocols. Firstly, its fixed length makes 418 for easier protocol coding and also better manages the packet size 419 cost of this technology. Secondly, it presents the identity in a 420 consistent format to the protocol independent of the cryptographic 421 algorithms used. 423 In the HIP packets, the HITs identify the sender and recipient of a 424 packet. Consequently, a HIT should be unique in the whole IP 425 universe as long as it is being used. In the extremely rare case 426 that a single HIT happens to map to more than one Host Identity, the 427 Host Identifiers (public keys) will make the final difference. If 428 there is more than one public key for a given node, the HIT acts as a 429 hint for the correct public key to use. 431 5.4 Local Scope Identifier (LSI) 433 An LSI is a 32-bit localized representation for a Host Identity. The 434 purpose of an LSI is to facilitate using Host Identities in existing 435 protocols and APIs. LSI's advantage over HIT is its size; its 436 disadvantage is its local scope. The generation of LSIs is defined 437 in the Host Identity Protocol specification [4]. 439 Examples of how LSIs can be used include: as the address in a FTP 440 command and as the address in a socket call. Thus, LSIs act as a 441 bridge for Host Identities into IPv4-based protocols and APIs. 443 6. New Stack Architecture 445 One way to characterize Host Identity is to compare the proposed new 446 architecture with the current one. As discussed above, the IP 447 addresses can be seen to be a confounding of routing direction 448 vectors and interface names. Using the terminology from the IRTF 449 Name Space Research Group Report [6] and, e.g., the unpublished 450 Internet-Draft Endpoints and Endpoint Names [10] by Noel Chiappa, the 451 IP addresses currently embody the dual role of locators and end-point 452 identifiers. That is, each IP address names a topological location 453 in the Internet, thereby acting as a routing direction vector, or 454 locator. At the same time, the IP address names the physical network 455 interface currently located at the point-of-attachment, thereby 456 acting as a end-point name. 458 In the HIP architecture, the end-point names and locators are 459 separated from each other. IP addresses continue to act as locators. 460 The Host Identifiers take the role of end-point identifiers. It is 461 important to understand that the end-point names based on Host 462 Identities are slightly different from interface names; a Host 463 Identity can be simultaneously reachable through several interfaces. 465 The difference between the bindings of the logical entities are 466 illustrated in Figure 1. 468 Service ------ Socket Service ------ Socket 469 | | 470 | | 471 | | 472 | | 473 End-point | End-point --- Host Identity 474 \ | | 475 \ | | 476 \ | | 477 \ | | 478 Location --- IP address Location --- IP address 480 Figure 1 482 6.1 Transport associations and end-points 484 Architecturally, HIP provides for a different binding of transport 485 layer protocols. That is, the transport layer associations, i.e., 486 TCP connections and UDP associations, are no longer bound to IP 487 addresses but to Host Identities. 489 It is possible that a single physical computer hosts several logical 490 end-points. With HIP, each of these end-points would have a distinct 491 Host Identity. Furthermore, since the transport associations are 492 bound to Host Identities, HIP provides for process migration and 493 clustered servers. That is, if a Host Identity is moved from one 494 physical computer to another, it is also possible to simultaneously 495 move all the transport associations without breaking them. 496 Similarly, if it is possible to distribute the processing of a single 497 Host Identity over several physical computers, HIP provides for 498 cluster based services without any changes at the client end-point. 500 7. End-Host Mobility and Multi-Homing 502 HIP decouples the transport from the internetworking layer, and binds 503 the transport associations to the Host Identities (through actually 504 either the HIT or LSI). Consequently, HIP can provide for a degree 505 of internetworking mobility and multi-homing at a low infrastructure 506 cost. HIP mobility includes IP address changes (via any method) to 507 either party. Thus, a system is considered mobile if its IP address 508 can change dynamically for any reason like PPP, DHCP, IPv6 prefix 509 reassignments, or a NAT device remapping its translation. Likewise, 510 a system is considered multi-homed if it has more than one globally 511 routable IP address at the same time. HIP links IP addresses 512 together, when multiple IP addresses correspond to the same Host 513 Identity, and if one address becomes unusable, or a more preferred 514 address becomes available, existing transport associations can easily 515 be moved to another address. 517 When a node moves while communication is already on-going, address 518 changes are rather straightforward. The peer of the mobile node can 519 just accept a HIP or an integrity protected IPsec packet from any 520 address and totally ignore the source address. However, as discussed 521 in Section 7.2 below, a mobile node must send a HIP readdress packet 522 to inform the peer of the new address(es), and the peer must verify 523 that the mobile node is reachable through these addresses. This is 524 especially helpful for those situations where the peer node is 525 sending data periodically to the mobile node (that is re-starting a 526 connection after the initial connection). 528 7.1 Rendezvous mechanism 530 Making a contact to a mobile node is slightly more involved. In 531 order to start the HIP exchange, the initiator node has to know how 532 to reach the mobile node. Although infrequently moving HIP nodes 533 could use Dynamic DNS to update their reachability information in the 534 DNS, an alternative to using DNS in this fashion is to use a piece of 535 new static infrastructure to facilitate rendezvous between HIP nodes. 537 The mobile node keeps the rendezvous infrastructure continuously 538 updated with its current IP address(es). The mobile nodes must trust 539 the rendezvous mechanism to properly maintain their HIT and IP 540 address mappings. 542 The rendezvous mechanism is also needed if both of the nodes happen 543 to change their address at the same time, either because they are 544 mobile and happen to move at the same time, because one of them is 545 off-line for a while, or because of some other reason. In such a 546 case, the HIP readdress packets will cross each other in the network 547 and never reach the peer node. 549 A separate document will specify the details of the HIP rendezvous 550 mechanism. 552 7.2 Protection against Flooding Attacks 554 While the idea of informing about address changes by simply sending 555 packets with a new source address appears appealing, it is not secure 556 enough. That is, even if HIP does not rely on the source address for 557 anything (once the base exchange has been completed), it appears to 558 be necessary to check a mobile node's reachability at the new address 559 before actually sending any larger amounts of traffic to the new 560 address. 562 Blindly accepting new addresses would potentially lead to flooding 563 Denial-of-Service attacks against third parties [8]. In a 564 distributed flooding attack an attacker opens high volume HIP 565 connections with a large number of hosts (using unpublished HIs), and 566 then claims to all of these hosts that it has moved to a target 567 node's IP address. If the peer hosts were to simply accept the move, 568 the result would be a packet flood to the target node's address. To 569 close this attack, HIP includes an address check mechanism where the 570 reachability of a node is separately checked at each address before 571 using the address for larger amounts of traffic. 573 Whenever HIP is used between two hosts that fully trust each other, 574 the hosts may optionally decide to skip the address tests. However, 575 such performance optimization must be restricted to peers that are 576 known to be trustworthy and capable of protecting themselves from 577 malicious software. 579 8. HIP and IPsec 581 The preferred way of implementing HIP is to use IPsec to carry the 582 actual data traffic. As of today, the only completely defined method 583 is to use IPsec Encapsulated Security Payload (ESP) to carry the data 584 packets. In the future, other ways of transporting payload data may 585 be developed, including ones that do not use cryptographic 586 protection. 588 In practise, the HIP base exchange uses the cryptographic Host 589 Identifiers to set up a pair of ESP Security Associations (SAs) to 590 enable ESP in an end-to-end manner. This is implemented in a way 591 that can span addressing realms. 593 While it would be possible, at least in theory, to use some existing 594 cryptographic protocol, such as IKEv2 together with Host Identifiers, 595 to establish the needed SAs, HIP defines a new protocol. There are a 596 number of historical reasons for this, and there are also a few 597 architectural reasons. First, IKE (and IKEv2) were not design with 598 middle boxes in mind. As adding a new naming layer allows one to 599 potentially add a new forwarding layer (see Section 9, below), it is 600 very important that the HIP protocols are friendly towards any middle 601 boxes. 603 Second, from a conceptual point of view, the IPsec Security Parameter 604 Index (SPI) in ESP provides a simple compression of the HITs. This 605 does require per-HIT-pair SAs (and SPIs), and a decrease of policy 606 granularity over other Key Management Protocols, such as IKE and 607 IKEv2. In particular, the current thinking is limited to a situation 608 where, conceptually, there is only one pair of SAs between any given 609 pair of HITs. In other words, from an architectural point of view, 610 HIP only supports host-to-host (or endpoint-to-endpoint) Security 611 Associations. If two hosts need more pairs of parallel SAs, they 612 should use separate HITs for that. However, future HIP extensions 613 may provide for more granularity and creation of several ESP SAs 614 between a pair of HITs. 616 Since HIP is designed for host usage, not for gateways or so called 617 Bump-in-the-Wire (BITW) implementations, only ESP transport mode is 618 supported. An ESP SA pair is indexed by the SPIs and the two HITs 619 (both HITs since a system can have more than one HIT). The SAs need 620 not to be bound to IP addresses; all internal control of the SA is by 621 the HITs. Thus, a host can easily change its address using Mobile 622 IP, DHCP, PPP, or IPv6 readdressing and still maintain the SAs. 623 Since the transports are bound to the SA (via an LSI or a HIT), any 624 active transport is also maintained. Thus, real world conditions 625 like loss of a PPP connection and its re-establishment or a mobile 626 handover will not require a HIP negotiation or disruption of 627 transport services. [12] 629 Since HIP does not negotiate any SA lifetimes, all lifetimes are 630 local policy. The only lifetimes a HIP implementation MUST support 631 are sequence number rollover (for replay protection), and SA 632 timeout[4]. An SA times out if no packets are received using that 633 SA. Implementations MAY support lifetimes for the various ESP 634 transforms. 636 9. HIP and NATs 638 Passing packets between different IP addressing realms requires 639 changing IP addresses in the packet header. This may happen, for 640 example, when a packet is passed between the public Internet and a 641 private address space, or between IPv4 and IPv6 networks. The 642 address translation is usually implemented as Network Address 643 Translation (NAT) [2] or NAT Protocol translation (NAT-PT) [1]. 645 In a network environment where the identification is based on the IP 646 addresses, identifying the communicating nodes is difficult when NAT 647 is used. With HIP, the transport layer end-points are bound to the 648 Host Identities. Thus, a connection between two hosts can traverse 649 many addressing realm boundaries. The IP addresses are used only for 650 routing purposes; the IP addresses may be changed freely during 651 packet traversal. 653 For a HIP based flow, a HIP-aware NAT or NAT-PT system tracks the 654 mapping of HITs and the corresponding IPsec SPIs to an IP address. 655 Many HITs can map to a single IP address on a NAT, simplifying 656 connections on address poor NAT interfaces. The NAT can gain much of 657 its knowledge from the HIP packets themselves; however, some NAT 658 configuration may be necessary. 660 The NAT systems cannot touch the datagrams within the IPsec envelope, 661 thus application specific address translation must be done in the end 662 systems. HIP provides for 'Distributed NAT', and uses the HIT or the 663 LSI as a place holder for embedded IP addresses. 665 9.1 HIP and TCP Checksum 667 There is no way for a host to know if any of the IP addresses in the 668 IP header are the addresses used to calculate the TCP checksum. That 669 is, it is not feasible to calculate the TCP checksum using the actual 670 IP addresses in the pseudo header; the addresses received in the 671 incoming packet are not necessarily the same as they were on the 672 sending host. Furthermore, it is not possible to recompute the upper 673 layer checksums in the NAT/NAT-PT system, since the traffic is IPsec 674 protected. Consequently, the TCP and UDP checksums are calculated 675 using the HITs in the place of the IP addresses in the pseudo header. 676 Furthermore, only the IPv6 pseudo header format is used. This 677 provides for IPv4 / IPv6 protocol translation. 679 10. Multicast 681 Back in fall 2003, there was little if any concrete thoughts about 682 how HIP might affect IP or application layer multi-cast. 684 11. HIP Policies 686 There are a number of variables that will influence the HIP exchanges 687 that each host must support. All HIP implementations should support 688 at least 2 HIs, one to publish in DNS and an unpublished one for 689 anonymous usage. Although unpublished HIs will be rarely used as 690 responder HIs, they are likely be common for initiators. Support for 691 multiple HIs is recommended. 693 Many initiators would want to use a different HI for different 694 responders. The implementations should provide for a policy of 695 initiator HIT to responder HIT. This policy should also include 696 preferred transforms and local lifetimes. 698 Responders would need a similar policy, describing the hosts allowed 699 to participate in HIP exchanges, and the preferred transforms and 700 local lifetimes. 702 12. Benefits of HIP 704 In the beginning, the network layer protocol (i.e. IP) had the 705 following four "classic" invariants: 707 o Non-mutable: The address sent is the address received. 709 o Non-mobile: The address doesn't change during the course of an 710 "association". 712 o Reversible: A return header can always be formed by reversing the 713 source and destination addresses. 715 o Omniscient: Each host knows what address a partner host can use to 716 send packets to it. 718 Actually, the fourth can be inferred from 1 and 3, but it is worth 719 mentioning for reasons that will be obvious soon if not already. 721 In the current "post-classic" world, we are trying intentionally to 722 get rid of the second invariant (both for mobility and for 723 multi-homing), and we have been forced to give up the first and the 724 fourth. Realm Specific IP [3] is an attempt to reinstate the fourth 725 invariant without the first invariant. IPv6 is an attempt to 726 reinstate the first invariant. 728 Few systems on the Internet have DNS names that are meaningful to 729 them. That is, if they have a Fully Qualified Domain Name (FQDN), 730 that typically belongs to a NAT device or a dial-up server, and does 731 not really identify the system itself but its current connectivity. 732 FQDN names (and their extensions as email names) are Application 733 Layer names; more frequently naming services than a particular 734 system. This is why many systems on the internet are not registered 735 in DNS; they do not have services of interest to other Internet 736 hosts. 738 DNS names are references to IP addresses. This only demonstrates the 739 interrelationship of the networking and application layers. DNS, as 740 the Internet's only deployed, distributed, database is also the 741 repository of other namespaces, due in part to DNSSEC and application 742 specific key records. Although each namespace can be stretched (IP 743 with v6, DNS with KEY records), neither can adequately provide for 744 host authentication or act as a separation between internetworking 745 and transport layers. 747 The Host Identity (HI) namespace fills an important gap between the 748 IP and DNS namespaces. An interesting thing about the HI is that it 749 actually allows one to give-up all but the 3rd Network Layer 750 invariant. That is to say, as long as the source and destination 751 addresses in the network layer protocol are reversible, then things 752 work ok because HIP takes care of host identification, and 753 reversibility allows one to get a packet back to one's partner host. 754 You don't care if the network layer address changes in transit 755 (mutable) and you don't care what network layer address the partner 756 is using (non-omniscient). 758 12.1 HIP's Answers to NSRG questions 760 The IRTF Name Space Research Group has posed a number of evaluating 761 questions in their report [6]. In this section, we provide answers 762 to these questions. 764 1. How would a stack name improve the overall functionality of the 765 Internet? 767 HIP decouples the internetworking layer from the transport 768 layer, allowing each to evolve separately. The decoupling 769 makes end-host mobility and multi-homing easier, also across 770 IPv4 and IPv6 networks. HIs make network renumbering easier, 771 and they also make process migration and clustered servers 772 easier to implement. Furthermore, being cryptographic in 773 nature, they provide the basis for solving the security 774 problems related to end-host mobility and multi-homing. 776 2. What does a stack name look like? 778 A HI is a cryptographic public key. However, instead of using 779 the keys directly, most protocols use a fixed size hash of the 780 public key. 782 3. What is its lifetime? 784 HIP provides both stable and temporary Host Identifiers. 785 Stable HIs are typically long lived, with a lifetime of years 786 or more. The lifetime of temporary HIs depends on how long 787 the upper layer connections and applications need them, and 788 can range from a few seconds to years. 790 4. Where does it live in the stack? 792 The HIs live between the transport and internetworking layers. 794 5. How is it used on the end points 796 The Host Identifiers may be used directly or indirectly (in 797 the form of HITs or LSIs) by applications when they access 798 network services. Additionally, the Host Identifiers, as 799 public keys, are used in the built in key agreement protocol, 800 called the HIP base exchange, to authenticate the hosts to 801 each other. 803 6. What administrative infrastructure is needed to support it? 805 In some environments, it is possible to use HIP 806 opportunistically, without any infrastructure. However, to 807 gain full benefit from HIP, the HIs must be stored in the DNS 808 or a PKI, and a new rendezvous mechanism is needed. Such a 809 new rendezvous mechanism may need new infrastructure to be 810 deployed. 812 7. If we add an additional layer would it make the address list in 813 SCTP unnecessary? 815 Yes 817 8. What additional security benefits would a new naming scheme 818 offer? 820 HIP reduces dependency on IP addresses, making the so called 821 address ownership [11] problems easier to solve. In practice, 822 HIP provides security for end-host mobility and multi-homing. 823 Furthermore, since HIP Host Identifiers are public keys, 824 standard public key certificate infrastructures can be applied 825 on the top of HIP. 827 9. What would the resolution mechanisms be, or what characteristics 828 of a resolution mechanisms would be required? 830 For most purposes, an approach where DNS names are resolved 831 simultaneously to HIs and IP addresses is sufficient. 832 However, if it becomes necessary to resolve HIs into IP 833 addresses or back to DNS names, a flat resolution 834 infrastructure is needed. Such an infrastructure could be 835 based on the ideas of Distributed Hash Tables, but would 836 require significant new development and deployment. 838 13. Security Considerations 840 HIP takes advantage of the new Host Identity paradigm to provide 841 secure authentication of hosts and to provide a fast key exchange for 842 IPsec. HIP also attempts to limit the exposure of the host to 843 various denial-of-service (DoS) and man-in-the-middle (MitM) attacks. 844 In so doing, HIP itself is subject to its own DoS and MitM attacks 845 that potentially could be more damaging to a host's ability to 846 conduct business as usual. 848 Resource exhausting Denial-of-service attacks take advantage of the 849 cost of setting up a state for a protocol on the responder compared 850 to the 'cheapness' on the initiator. HIP allows a responder to 851 increase the cost of the start of state on the initiator and makes an 852 effort to reduce the cost to the responder. This is done by having 853 the responder start the authenticated Diffie-Hellman exchange instead 854 of the initiator, making the HIP base exchange 4 packets long. There 855 are more details on this process in the Host Identity Protocol 856 specification [4]. 858 HIP optionally supports opportunistic negotiation. That is, if a 859 host receives a start of transport without a HIP negotiation, it can 860 attempt to force a HIP exchange before accepting the connection. 861 This has the potential for DoS attacks against both hosts. If the 862 method to force the start of HIP is expensive on either host, the 863 attacker need only spoof a TCP SYN. This would put both systems into 864 the expensive operations. HIP avoids this attack by having the 865 responder send a simple HIP packet that it can pre-build. Since this 866 packet is fixed and easily replayed, the initiator only reacts to it 867 if it has just started a connection to the responder. 869 Man-in-the-middle attacks are difficult to defend against, without 870 third-party authentication. A skillful MitM could easily handle all 871 parts of the HIP base exchange, but HIP indirectly provides the 872 following protection from a MitM attack. If the responder's HI is 873 retrieved from a signed DNS zone or secured by some other means, the 874 initiator can use this to authenticate the signed HIP packets. 875 Likewise, if the initiator's HI is in a secure DNS zone, the 876 responder can retrieve it and validate the signed HIP packets. 877 However, since an initiator may choose to use an unpublished HI, it 878 knowingly risks a MitM attack. The responder may choose not to 879 accept a HIP exchange with an initiator using an unknown HI. 881 In HIP, the Security Association for IPsec is indexed by the SPI; the 882 source address is always ignored, and the destination address may be 883 ignored as well. Therefore, HIP enabled IPsec Encapsulated Security 884 Payload (ESP) is IP address independent. This might seem to make it 885 easier for an attacker, but ESP with replay protection is already as 886 well protected as possible, and the removal of the IP address as a 887 check should not increase the exposure of IPsec ESP to DoS attacks. 889 Since not all hosts will ever support HIP, ICMPv4 'Destination 890 Unreachable, Protocol Unreachable' and ICMPv6 'Parameter Problem, 891 Unrecognized Next Header' messages are to be expected and present a 892 DoS attack. Against an initiator, the attack would look like the 893 responder does not support HIP, but shortly after receiving the ICMP 894 message, the initiator would receive a valid HIP packet. Thus, to 895 protect against this attack, an initiator should not react to an ICMP 896 message until a reasonable time has passed, allowing it to get the 897 real responder's HIP packet. A similar attack against the responder 898 is more involved. 900 Another MitM attack is simulating a responder's administrative 901 rejection of a HIP initiation. This is a simple ICMP 'Destination 902 Unreachable, Administratively Prohibited' message. A HIP packet is 903 not used because it would either have to have unique content, and 904 thus difficult to generate, resulting in yet another DoS attack, or 905 just as spoofable as the ICMP message. Like in the previous case, 906 the defense against this attack is for the initiator to wait a 907 reasonable time period to get a valid HIP packet. If one does not 908 come, then the initiator has to assume that the ICMP message is 909 valid. Since this is the only point in the HIP base exchange where 910 this ICMP message is appropriate, it can be ignored at any other 911 point in the exchange. 913 13.1 HITs used in ACLs 915 It is expected that HITs will be used in ACLs. Future firewalls can 916 use HITs to control egress and ingress to networks, with an assurance 917 level difficult to achieve today. As discussed above in Section 8, 918 once a HIP session has been established, the SPI value in an IPsec 919 packet may be used as an index, indicating the HITs. In practise, 920 the firewalls can inspect the HIP packets to learn of the bindings 921 between HITs, SPI values, and IP addresses. They can even explicitly 922 control IPsec usage, dynamically opening IPsec ESP only for specific 923 SPI values and IP addresses. The signatures in the HIP packets allow 924 a capable firewall to make sure that the HIP exchange is indeed 925 happening between two known hosts. This may increase firewall 926 security. 928 There has been considerable bad experience with distributed ACLs that 929 contain public key related material, for example, with SSH. If the 930 owner of the key needs to revoke it for any reason, the task of 931 finding all locations where the key is held in an ACL may be 932 impossible. If the reason for the revocation is due to private key 933 theft, this could be a serious issue. 935 A host can keep track of all of its partners that might use its HIT 936 in an ACL by logging all remote HITs. It should only be necessary to 937 log responder hosts. With this information, the host can notify the 938 various hosts about the change to the HIT. There has been no attempt 939 to develop a secure method to issue the HIT revocation notice. 941 HIP-aware NATs, however, are transparent to the HIP aware systems by 942 design. Thus, the host may find it difficult to notify any NAT that 943 is using a HIT in an ACL. Since most systems will know of the NATs 944 for their network, there should be a process by which they can notify 945 these NATs of the change of the HIT. This is mandatory for systems 946 that function as responders behind a NAT. In a similar vein, if a 947 host is notified of a change in a HIT of an initiator, it should 948 notify its NAT of the change. In this manner, NATs will get updated 949 with the HIT change. 951 13.2 Non-security Considerations 953 The definition of the Host Identifier states that the HI need not be 954 a public key. It implies that the HI could be any value; for example 955 an FQDN. This document does not describe how to support such a 956 non-cryptographic HI. A non-cryptographic HI would still offer the 957 services of the HIT or LSI for NAT traversal. It would be possible 958 carry the HITs in HIP packets that had neither privacy nor 959 authentication. Since such a mode would offer so little additional 960 functionality for so much addition to the IP kernel, it has not been 961 defined. Given how little public key cryptography HIP requires, HIP 962 should only be implemented using public key Host Identities. 964 If it is desirable to use HIP in a low security situation where 965 public key computations are considered expensive, HIP can be used 966 with very short Diffie-Hellman and Host Identity keys. Such use 967 makes the participating hosts vulnerable to MitM and connection 968 hijacking attacks. However, it does not cause flooding dangers, 969 since the address check mechanism relies on the routing system and 970 not on cryptographic strength. 972 14. Acknowledgments 974 For the people historically involved in the early stages of HIP, see 975 the Acknowledgements section in the Host Identity Protocol 976 specification [4]. 978 During the later stages of this document, when the editing baton was 979 transfered to Pekka Nikander, the comments from the early 980 implementors and others, including Jari Arkko, Tom Henderson, Petri 981 Jokela, Miika Komu, Mika Kousa, Andrew McGregor, Jan Melen, Tim 982 Shepard, Jukka Ylitalo, and Jorma Wall, were invaluable. Finally, 983 Spencer Dawkins and Dave Crocker provided valuable input during the 984 final stages of publication, most of which was incorporated but some 985 of which the authors decided to ignore in order to get this document 986 published in the first place. 988 15 Informative References 990 [1] Tsirtsis, G. and P. Srisuresh, "Network Address Translation - 991 Protocol Translation (NAT-PT)", RFC 2766, February 2000. 993 [2] Srisuresh, P. and K. Egevang, "Traditional IP Network Address 994 Translator (Traditional NAT)", RFC 3022, January 2001. 996 [3] Borella, M., Lo, J., Grabelsky, D. and G. Montenegro, "Realm 997 Specific IP: Framework", RFC 3102, October 2001. 999 [4] Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-00 1000 (work in progress), June 2004. 1002 [5] Richardson, M., "A method for storing IPsec keying material in 1003 DNS", draft-ietf-ipseckey-rr-10 (work in progress), April 2004. 1005 [6] Lear, E. and R. Droms, "What's In A Name:Thoughts from the 1006 NSRG", draft-irtf-nsrg-report-10 (work in progress), September 1007 2003. 1009 [7] Nikander, P., "End-Host Mobility and Multi-Homing with Host 1010 Identity Protocol", draft-ietf-hip-mm-00 (work in progress), 1011 October 2004. 1013 [8] Nikander, P., "Mobile IP version 6 Route Optimization Security 1014 Design Background", draft-ietf-mip6-ro-sec-00 (work in 1015 progress), April 2004. 1017 [9] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", 1018 draft-ietf-ipsec-ikev2-14 (work in progress), June 2004. 1020 [10] Chiappa, J., "Endpoints and Endpoint Names: A Proposed 1021 Enhancement to the Internet Architecture", URL 1022 http://users.exis.net/~jnc/tech/endpoints.txt, 1999. 1024 [11] Nikander, P., "Denial-of-Service, Address Ownership, and Early 1025 Authentication in the IPv6 World", in Proceesings of Security 1026 Protocols, 9th International Workshop, Cambridge, UK, April 1027 25-27 2001, LNCS 2467, pp. 12-26, Springer, 2002. 1029 [12] Bellovin, S., "EIDs, IPsec, and HostNAT", in Proceesings of 1030 41th IETF, Los Angeles, CA, March 1998. 1032 Authors' Addresses 1034 Robert Moskowitz 1035 ICSAlabs, a Division of TruSecure Corporation 1036 1000 Bent Creek Blvd, Suite 200 1037 Mechanicsburg, PA 1038 USA 1040 EMail: rgm@icsalabs.com 1042 Pekka Nikander 1043 Ericsson Research Nomadic Lab 1044 JORVAS FIN-02420 1045 FINLAND 1047 Phone: +358 9 299 1 1048 EMail: pekka.nikander@nomadiclab.com 1050 Intellectual Property Statement 1052 The IETF takes no position regarding the validity or scope of any 1053 Intellectual Property Rights or other rights that might be claimed to 1054 pertain to the implementation or use of the technology described in 1055 this document or the extent to which any license under such rights 1056 might or might not be available; nor does it represent that it has 1057 made any independent effort to identify any such rights. 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