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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 HIP Working Group A. Keranen 3 Internet-Draft J. Melen 4 Intended status: Standards Track M. Komu, Ed. 5 Expires: August 24, 2020 Ericsson 6 February 21, 2020 8 Native NAT Traversal Mode for the Host Identity Protocol 9 draft-ietf-hip-native-nat-traversal-30 11 Abstract 13 This document specifies a new Network Address Translator (NAT) 14 traversal mode for the Host Identity Protocol (HIP). The new mode is 15 based on the Interactive Connectivity Establishment (ICE) methodology 16 and UDP encapsulation of data and signaling traffic. The main 17 difference from the previously specified modes is the use of HIP 18 messages instead of ICE for all NAT traversal procedures due to its 19 kernel-space dependencies. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at https://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on August 24, 2020. 38 Copyright Notice 40 Copyright (c) 2020 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (https://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 56 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 57 3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 8 58 4. Protocol Description . . . . . . . . . . . . . . . . . . . . 10 59 4.1. Relay Registration . . . . . . . . . . . . . . . . . . . 10 60 4.2. Transport Address Candidate Gathering at the Relay Client 13 61 4.3. NAT Traversal Mode Negotiation . . . . . . . . . . . . . 15 62 4.4. Connectivity Check Pacing Negotiation . . . . . . . . . . 17 63 4.5. Base Exchange via Control Relay Server . . . . . . . . . 17 64 4.6. Connectivity Checks . . . . . . . . . . . . . . . . . . . 20 65 4.6.1. Connectivity Check Procedure . . . . . . . . . . . . 21 66 4.6.2. Rules for Connectivity Checks . . . . . . . . . . . . 24 67 4.6.3. Rules for Concluding Connectivity Checks . . . . . . 26 68 4.7. NAT Traversal Optimizations . . . . . . . . . . . . . . . 27 69 4.7.1. Minimal NAT Traversal Support . . . . . . . . . . . . 27 70 4.7.2. Base Exchange without Connectivity Checks . . . . . . 27 71 4.7.3. Initiating a Base Exchange both with and without UDP 72 Encapsulation . . . . . . . . . . . . . . . . . . . . 29 73 4.8. Sending Control Packets after the Base Exchange . . . . . 29 74 4.9. Mobility Handover Procedure . . . . . . . . . . . . . . . 30 75 4.10. NAT Keepalives . . . . . . . . . . . . . . . . . . . . . 34 76 4.11. Closing Procedure . . . . . . . . . . . . . . . . . . . . 35 77 4.12. Relaying Considerations . . . . . . . . . . . . . . . . . 35 78 4.12.1. Forwarding Rules and Permissions . . . . . . . . . . 35 79 4.12.2. HIP Data Relay and Relaying of Control Packets . . . 36 80 4.12.3. Handling Conflicting SPI Values . . . . . . . . . . 37 81 5. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 38 82 5.1. HIP Control Packets . . . . . . . . . . . . . . . . . . . 38 83 5.2. Connectivity Checks . . . . . . . . . . . . . . . . . . . 39 84 5.3. Keepalives . . . . . . . . . . . . . . . . . . . . . . . 39 85 5.4. NAT Traversal Mode Parameter . . . . . . . . . . . . . . 40 86 5.5. Connectivity Check Transaction Pacing Parameter . . . . . 41 87 5.6. Relay and Registration Parameters . . . . . . . . . . . . 41 88 5.7. LOCATOR_SET Parameter . . . . . . . . . . . . . . . . . . 42 89 5.8. RELAY_HMAC Parameter . . . . . . . . . . . . . . . . . . 44 90 5.9. Registration Types . . . . . . . . . . . . . . . . . . . 45 91 5.10. Notify Packet Types . . . . . . . . . . . . . . . . . . . 45 92 5.11. ESP Data Packets . . . . . . . . . . . . . . . . . . . . 46 93 5.12. RELAYED_ADDRESS and MAPPED_ADDRESS Parameters . . . . . . 46 94 5.13. PEER_PERMISSION Parameter . . . . . . . . . . . . . . . . 47 95 5.14. HIP Connectivity Check Packets . . . . . . . . . . . . . 48 96 5.15. NOMINATE parameter . . . . . . . . . . . . . . . . . . . 49 98 6. Security Considerations . . . . . . . . . . . . . . . . . . . 49 99 6.1. Privacy Considerations . . . . . . . . . . . . . . . . . 50 100 6.2. Opportunistic Mode . . . . . . . . . . . . . . . . . . . 50 101 6.3. Base Exchange Replay Protection for Control Relay Server 50 102 6.4. Demultiplexing Different HIP Associations . . . . . . . . 51 103 6.5. Reuse of Ports at the Data Relay Server . . . . . . . . . 51 104 6.6. Amplification attacks . . . . . . . . . . . . . . . . . . 51 105 6.7. Attacks against Connectivity Checks and Candidate 106 Gathering . . . . . . . . . . . . . . . . . . . . . . . . 52 107 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 52 108 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 53 109 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 53 110 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 53 111 10.1. Normative References . . . . . . . . . . . . . . . . . . 53 112 10.2. Informative References . . . . . . . . . . . . . . . . . 55 113 Appendix A. Selecting a Value for Check Pacing . . . . . . . . . 56 114 Appendix B. Differences with respect to ICE . . . . . . . . . . 57 115 Appendix C. Differences to Base Exchange and UPDATE procedures . 59 116 Appendix D. Multihoming Considerations . . . . . . . . . . . . . 62 117 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 63 119 1. Introduction 121 The Host Identity Protocol (HIP) [RFC7401] is specified to run 122 directly on top of IPv4 or IPv6. However, many middleboxes found in 123 the Internet, such as NATs and firewalls, often allow only UDP or TCP 124 traffic to pass [RFC5207]. Also, especially NATs usually require the 125 host behind a NAT to create a forwarding state in the NAT before 126 other hosts outside of the NAT can contact the host behind the NAT. 127 To overcome this problem, different methods, commonly referred to as 128 NAT traversal techniques, have been developed. 130 As one solution, the HIP experiment report [RFC6538] mentions Teredo- 131 based NAT traversal for HIP and related ESP traffic (with double 132 tunneling overhead). Another solution is specified in [RFC5770], 133 which will be referred as "Legacy ICE-HIP" in this document. The 134 experimental Legacy ICE-HIP specification combines Interactive 135 Connectivity Establishment (ICE) protocol [RFC5245] with HIP, so that 136 basically ICE is responsible for NAT traversal and connectivity 137 testing, while HIP is responsible for end-host authentication and 138 IPsec key management. The resulting protocol uses HIP, STUN and ESP 139 messages tunneled over a single UDP flow. The benefit of using ICE 140 and its STUN/TURN messaging formats is that one can re-use the NAT 141 traversal infrastructure already available in the Internet, such as 142 STUN and TURN servers. Also, some middleboxes may be STUN-aware and 143 may be able to do something "smart" when they see STUN being used for 144 NAT traversal. 146 HIP poses a unique challenge to using standard ICE, due not only to 147 its kernel-space implementation, but also due to its close 148 integration with kernel-space IPSec; and, that while [RFC5770] 149 provides a technically workable path, it incurs unacceptable 150 performance drawbacks for kernel-space implementations. Also, 151 implementing and integrating a full ICE/STUN/TURN protocol stack as 152 specified in Legacy ICE-HIP results in a considerable amount of 153 effort and code which could be avoided by re-using and extending HIP 154 messages and state machines for the same purpose. Thus, this 155 document specifies an alternative NAT traversal mode referred as 156 "Native ICE-HIP" that employs HIP messaging format instead of STUN or 157 TURN for the connectivity checks, keepalives and data relaying. 158 Native ICE-HIP also specifies how mobility management works in the 159 context of NAT traversal, which is missing from the Legacy ICE-HIP 160 specification. The native specification is also based on HIPv2, 161 whereas legacy specification is based on HIPv1. The differences to 162 the Legacy ICE-HIP are further elaborated in Appendix B. 164 Similarly as Legacy ICE-HIP, also this specification builds on the 165 HIP registration extensions [RFC8003] and the base exchange procedure 166 [RFC7401] and its closing procedures, so the reader is recommended to 167 get familiar with the relevant specifications. In a nutshell, the 168 registration extensions allow a HIP Initiator (usually a "client" 169 host) to ask for specific services from a HIP Responder (usually a 170 "server" host). The registration parameters are included in a base 171 exchange, which is essentially a four-way Diffie-Hellman key exchange 172 authenticated using the public keys of the end-hosts. When the hosts 173 negotiate support for ESP [RFC7402] during the base exchange, they 174 can deliver ESP protected application payload to each other. When 175 either of the hosts moves and changes its IP address, the two hosts 176 re-establish connectivity using the mobility extensions [RFC8046]. 177 The reader is also recommended to get familiar with the mobility 178 extensions, but basically it is a three-way procedure, where the 179 mobile host first announces its new location to the peer, and then 180 the peer tests for connectivity (so called return routability check), 181 for which the mobile hosts must respond in order to activate its new 182 location. This specification builds on the mobility procedures, but 183 modifies it to be compatible with ICE. The differences to the 184 mobility extensions specified in Appendix C. It is worth noting that 185 multihoming support as specified in [RFC8047] is left for further 186 study. 188 This specification builds heavily on the ICE methodology, so it is 189 recommended that the reader is familiar with the ICE specification 190 [RFC8445] (especially the overview). However, native ICE-HIP does 191 not implement all the features in ICE, and, hence, the different 192 features of ICE are cross referenced using [RFC2119] terminology for 193 clarity. Appendix B explains the differences to ICE, and it is 194 recommended that the reader would read also this section in addition 195 to the ICE specification. 197 2. Terminology 199 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 200 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 201 "OPTIONAL" in this document are to be interpreted as described in BCP 202 14 [RFC2119] [RFC8174] when, and only when, they appear in all 203 capitals, as shown here. 205 This document borrows terminology from [RFC5770], [RFC7401], 206 [RFC8046], [RFC4423], [RFC8445], and [RFC5389]. The following terms 207 recur in the text: 209 ICE: 210 Interactive Connectivity Establishment (ICE) protocol as specified 211 in [RFC8445] 213 Legacy ICE-HIP: 214 Refers to the "Basic Host Identity Protocol (HIP) Extensions for 215 Traversal of Network Address Translators" as specified in 216 [RFC5770]. The protocol specified in this document offers an 217 alternative to Legacy ICE-HIP. 219 Native ICE-HIP: 220 The protocol specified in this document (Native NAT Traversal Mode 221 for HIP). 223 Initiator: 224 The Initiator is the host that initiates the base exchange using 225 I1 message [RFC7401]. 227 Responder: 228 The Responder is the host that receives the I1 packet from the 229 Initiator [RFC7401]. 231 Control Relay Server 232 A registrar host that forwards any kind of HIP control plane 233 packets between the Initiator and the Responder. This host is 234 critical because it relays the locators between the Initiator and 235 the Responder, so that they can try to establish a direct 236 communication path with each other. This host is used to replace 237 HIP rendezvous servers [RFC8004] for hosts operating in private 238 address realms. In the Legacy ICE-HIP specification [RFC5770], 239 this host is denoted as "HIP Relay Server". 241 Control Relay Client: 243 A requester host that registers to a Control Relay Server 244 requesting it to forward control-plane traffic (i.e. HIP control 245 messages). In the Legacy ICE-HIP specification [RFC5770], this is 246 denoted as "HIP Relay Client". 248 Data Relay Server: 249 A new entity introduced in this document; a registrar host that 250 forwards HIP related data plane packets, such as Encapsulating 251 Security Payload (ESP) [RFC7402], between two hosts. This host 252 implements similar functionality as TURN servers. 254 Data Relay Client: 255 A requester host that registers to a Data Relay Server requesting 256 it to forward data-plane traffic (e.g. ESP traffic). This 257 functionality is a new and introduced in this document. 259 Locator: 260 As defined in [RFC8046]: "A name that controls how the packet is 261 routed through the network and demultiplexed by the end-host. It 262 may include a concatenation of traditional network addresses such 263 as an IPv6 address and end-to-end identifiers such as an ESP 264 Security Parameter Index (SPI). It may also include transport 265 port numbers or IPv6 Flow Labels as demultiplexing context, or it 266 may simply be a network address." 268 LOCATOR_SET (written in capital letters): 269 Denotes a HIP control packet parameter that bundles multiple 270 locators together [RFC8046]. 272 HIP offer: 273 Before two end-hosts can establish a communication channel using 274 the NAT traversal procedures defined in this document, they need 275 exchange their locators (i.e. candidates) with each other. In 276 ICE, this procedure is called Candidate Exchange and it does 277 specify how the candidates are exchanged but Session Description 278 Protocol (SDP) "offer/answer" is mentioned as an example. In 279 contrast, the Candidate Exchange in HIP is the base exchange 280 itself or a subsequent UPDATE prodecure occurring after a 281 handover. Following [RFC5770] and Session Description Protocol 282 (SDP) [RFC3264] naming conventions, "HIP offer" is the the 283 Initiator's LOCATOR_SET parameter in a HIP I2 or in an UPDATE 284 control packet. 286 HIP answer: 287 The Responder's LOCATOR_SET parameter in a HIP R2 or UPDATE 288 control packet. Corresponds to the SDP answer parameter 290 [RFC3264], but is HIP specific. Please refer also to the longer 291 description of the "HIP offer" term above. 293 HIP connectivity checks: 294 In order to obtain a direct end-to-end communication path (without 295 employing a Data Relay Server), two communicating HIP hosts try to 296 "punch holes" through their NAT boxes using this mechanism. It is 297 similar to the ICE connectivity checks, but implemented using HIP 298 return routability checks. 300 Controlling host: 301 The controlling host [RFC8445] is always the Initiator in the 302 context of this specification. It nominates the candidate pair to 303 be used with the controlled host. 305 Controlled host: 306 The controlled host [RFC8445] is always the Responder in the 307 context of this specification. It waits for the controlling to 308 nominate an address candidate pair. 310 Checklist: 311 A list of address candidate pairs that need to be tested for 312 connectivity (same as in [RFC8445]). 314 Transport address: 315 Transport layer port and the corresponding IPv4/v6 address (same 316 as in [RFC8445]). 318 Candidate: 319 A transport address that is a potential point of contact for 320 receiving data (same as in [RFC8445]). 322 Host candidate: 323 A candidate obtained by binding to a specific port from an IP 324 address on the host (same as in [RFC8445]). 326 Server reflexive candidate: 327 A translated transport address of a host as observed by a Control 328 or Data Relay Server (same as in [RFC8445]). 330 Peer reflexive candidate: 331 A translated transport address of a host as observed by its peer 332 (same as in [RFC8445]). 334 Relayed candidate: 335 A transport address that exists on a Data Relay Server. Packets 336 that arrive at this address are relayed towards the Data Relay 337 Client. The concept is the same as in [RFC8445], but a Data Relay 338 Server is used instead of a TURN server. 340 Permission: 341 In the context of Data Relay Server, permission refers to a 342 concept similar to TURN's ([RFC5766]) channels. Before a host can 343 use a relayed candidate to forward traffic through a Data Relay 344 Server, the host must activate the relayed candidate with a 345 specific peer host. 347 Base: 348 Similarly as in [RFC8445], the base of a candidate is the local 349 source address a host uses to send packets for the associated 350 candidate. For example, the base of a server reflexive address is 351 the local address the host used for registering itself to the 352 associated Control or Data Relay Server. The base of a host 353 candidate is equal to the host candidate itself. 355 3. Overview of Operation 357 +--------------+ 358 | Control | 359 +--------+ | Relay Server | +--------+ 360 | Data | +----+-----+---+ | Data | 361 | Relay | / \ | Relay | 362 | Server | / \ | Server | 363 +--------+ / \ +--------+ 364 / \ 365 / \ 366 / \ 367 / <- Signaling -> \ 368 / \ 369 +-------+ +-------+ 370 | NAT | | NAT | 371 +-------+ +-------+ 372 / \ 373 / \ 374 +-------+ +-------+ 375 | Init- | | Resp- | 376 | iator | | onder | 377 +-------+ +-------+ 379 Figure 1: Example Network Configuration 381 In the example configuration depicted in Figure 1, both Initiator and 382 Responder are behind one or more NATs, and both private networks are 383 connected to the public Internet. To be contacted from behind a NAT, 384 at least the Responder must be registered with is own Control Relay 385 Server reachable on the public Internet. The Responder may have also 386 registered to a Data Relay Server that can forward the data plane in 387 case NAT traversal fails. While, strictly speaking, the Initiator 388 does not need a Data Relay Server, it may act in the other role with 389 other hosts, and connectivity with the Data Relay Server of the 390 Responder may fail, so the Initiator may also need to register to a 391 Cotrol and/or Data Relay Server. It is worth noting that a Control 392 and Data Relay does not forge the source address of a passing packet, 393 but always translates the source address and source port of a packet 394 to be forwarded (to its own). 396 We assume, as a starting point, that the Initiator knows both the 397 Responder's Host Identity Tag (HIT) and the address(es) of the 398 Responder's Control Relay Server(s) (how the Initiator learns of the 399 Responder's Control Relay Server is outside of the scope of this 400 document, but may be through DNS or another name service). The first 401 steps are for both the Initiator and Responder to register with a 402 Control Relay Server (need not be the same one) and gather a set of 403 address candidates. The hosts use either Control Relay Servers or 404 Data Relay Servers for gathering the candidates. Next, the HIP base 405 exchange is carried out by encapsulating the HIP control packets in 406 UDP datagrams and sending them through the Responder's Control Relay 407 Server. As part of the base exchange, each HIP host learns of the 408 peer's candidate addresses through the HIP offer/answer procedure 409 embedded in the base exchange. 411 Once the base exchange is completed, two HIP hosts have established a 412 working communication session (for signaling) via a Control Relay 413 Server, but the hosts still have to find a better path, preferably 414 without a Data Relay Server, for the ESP data flow. For this, 415 connectivity checks are carried out until a working pair of addresses 416 is discovered. At the end of the procedure, if successful, the hosts 417 will have established a UDP-based tunnel that traverses both NATs, 418 with the data flowing directly from NAT to NAT or via a Data Relay 419 Server. At this point, also the HIP signaling can be sent over the 420 same address/port pair, and is demultiplexed (or, in other words, 421 separated) from IPsec as described in the UDP encapsulation standard 422 for IPsec [RFC3948]. Finally, the two hosts send NAT keepalives as 423 needed in order keep their UDP-tunnel state active in the associated 424 NAT boxes. 426 If either one of the hosts knows that it is not behind a NAT, hosts 427 can negotiate during the base exchange a different mode of NAT 428 traversal that does not use HIP connectivity checks, but only UDP 429 encapsulation of HIP and ESP. Also, it is possible for the Initiator 430 to simultaneously try a base exchange with and without UDP 431 encapsulation. If a base exchange without UDP encapsulation 432 succeeds, no HIP connectivity checks or UDP encapsulation of ESP are 433 needed. 435 4. Protocol Description 437 This section describes the normative behavior of the "Native ICE-HIP" 438 protocol extension. Most of the procedures are similar to what is 439 defined in [RFC5770] but with different, or additional, parameter 440 types and values. In addition, a new type of relaying server, Data 441 Relay Server, is specified. Also, it should be noted that HIP 442 version 2 [RFC7401] MUST be used instead of HIPv1 with this NAT 443 traversal mode. 445 4.1. Relay Registration 447 In order for two hosts to communicate over NATted environments, they 448 need a reliable way to exchange information. To achieve this, "HIP 449 Relay Server" is defined in [RFC5770]. It supports relaying of HIP 450 control plane traffic over UDP in NATted environments, and forwards 451 HIP control packets between the Initiator and the Responder. In this 452 document, the HIP Relay Server is denoted as "Control Relay Server" 453 for better alignment with the rest of the terminology. The 454 registration to the Control Relay Server can be achieved using 455 RELAY_UDP_HIP parameter as explained later in this section. 457 To guarantee also data plane delivery over varying types of NAT 458 devices, a host MAY also register for UDP encapsulated ESP relaying 459 using Registration Type RELAY_UDP_ESP (value [TBD by IANA: 3]). This 460 service may be coupled with the Control Relay Server or offered 461 separately on another server. If the server supports relaying of UDP 462 encapsulated ESP, the host is allowed to register for a data relaying 463 service using the registration extensions in Section 3.3 of 464 [RFC8003]). If the server has sufficient relaying resources (free 465 port numbers, bandwidth, etc.) available, it opens a UDP port on one 466 of its addresses and signals the address and port to the registering 467 host using the RELAYED_ADDRESS parameter (as defined in Section 5.12 468 in this document). If the Data Relay Server would accept the data 469 relaying request but does not currently have enough resources to 470 provide data relaying service, it MUST reject the request with 471 Failure Type "Insufficient resources" [RFC8003]. 473 The registration process follows the generic registration extensions 474 defined in [RFC8003]. The HIP control plane relaying registration 475 follows [RFC5770], but the data plane registration is different. It 476 is worth noting that if the HIP control and data plane relay services 477 reside on different hosts, the client has to register separately to 478 each of them. In the example shown in Figure 2, the two services are 479 coupled on a single host. The text uses "Relay Client" and "Relay 480 Server" as a shorthand when the procedures apply both to control and 481 data cases. 483 Control/Data Control/Data 484 Relay Client (Initiator) Relay Server (Responder) 485 | 1. UDP(I1) | 486 +---------------------------------------------------------------->| 487 | | 488 | 2. UDP(R1(REG_INFO(RELAY_UDP_HIP,[RELAY_UDP_ESP]))) | 489 |<----------------------------------------------------------------+ 490 | | 491 | 3. UDP(I2(REG_REQ(RELAY_UDP_HIP),[RELAY_UDP_ESP])) | 492 +---------------------------------------------------------------->| 493 | | 494 | 4. UDP(R2(REG_RES(RELAY_UDP_HIP,[RELAY_UDP_ESP]), REG_FROM, | 495 | [RELAYED_ADDRESS])) | 496 |<----------------------------------------------------------------+ 497 | | 499 Figure 2: Example Registration with a HIP Relay 501 In step 1, the Relay Client (Initiator) starts the registration 502 procedure by sending an I1 packet over UDP to the Relay Server. It 503 is RECOMMENDED that the Relay Client select a random source port 504 number from the ephemeral port range 49152-65535 for initiating a 505 base exchange. Alternatively, a host MAY also use a single fixed 506 port for initiating all outgoing connections. However, the allocated 507 port MUST be maintained until all of the corresponding HIP 508 Associations are closed. It is RECOMMENDED that the Relay Server 509 listen to incoming connections at UDP port 10500. If some other port 510 number is used, it needs to be known by potential Relay Clients. 512 In step 2, the Relay Server (Responder) lists the services that it 513 supports in the R1 packet. The support for HIP control plane over 514 UDP relaying is denoted by the Registration Type value RELAY_UDP_HIP 515 (see Section 5.9). If the server supports also relaying of ESP 516 traffic over UDP, it includes also Registration type value 517 RELAY_UDP_ESP. 519 In step 3, the Relay Client selects the services for which it 520 registers and lists them in the REG_REQ parameter. The Relay Client 521 registers for the Control Data Relay service by listing the 522 RELAY_UDP_HIP value in the request parameter. If the Relay Client 523 requires also ESP relaying over UDP, it lists also RELAY_UDP_ESP. 525 In step 4, the Relay Server concludes the registration procedure with 526 an R2 packet and acknowledges the registered services in the REG_RES 527 parameter. The Relay Server denotes unsuccessful registrations (if 528 any) in the REG_FAILED parameter of R2. The Relay Server also 529 includes a REG_FROM parameter that contains the transport address of 530 the Relay Client as observed by the Relay Server (Server Reflexive 531 candidate). If the Relay Client registered to ESP relaying service, 532 the Relay Server includes RELAYED_ADDRESS parameter that describes 533 the UDP port allocated to the Relay Client for ESP relaying. It is 534 worth noting that the Data Relay Client must first activate this UDP 535 port by sending an UPDATE message to the Data Relay Server that 536 includes a PEER_PERMISSION parameter as described in Section 4.12.1 537 both after base exchange and handover procedures. Also, the Data 538 Relay Server should follow the port allocation recommendations in 539 Section 6.5. 541 After the registration, the Relay Client sends periodically NAT 542 keepalives to the Relay Server in order to keep the NAT bindings 543 between the Relay Client and the relay alive. The keepalive 544 extensions are described in Section 4.10. 546 The Data Relay Client MUST maintain an active HIP association with 547 the Data Relay Server as long as it requires the data relaying 548 service. When the HIP association is closed (or times out), or the 549 registration lifetime passes without the Data Relay Client refreshing 550 the registration, the Data Relay Server MUST stop relaying packets 551 for that host and close the corresponding UDP port (unless other Data 552 Relay Clients are still using it). 554 The Data Relay Server SHOULD offer a different relayed address and 555 port for each Data Relay Client because not doing so can cause 556 problems with stateful firewalls (see Section 6.5). 558 When a Control Relay Client sends an UPDATE (e.g., due to host 559 movement or to renew service registration), the Control Relay Server 560 MUST follow the general guidelines defined in [RFC8003], with the 561 difference that all UPDATE messages are delivered on top of UDP. In 562 addition to this, the Control Relay Server MUST include the REG_FROM 563 parameter in all UPDATE responses sent to the Control Relay Client. 564 This applies to both renewals of service registration and to host 565 movement. It is especially important for the case of host movement, 566 as this is the mechanism that allows the Control Relay Client to 567 learn its new server reflexive address candidate. 569 A Data Relay Client can request multiple relayed candidates from the 570 Data Relay Server (e.g., for the reasons described in 571 Section 4.12.3). After the base exchange with registration, the Data 572 Relay Client can request additional relayed candidates similarly as 573 during the base exchange. The Data Relay Client sends an UPDATE 574 message REG_REQ parameter requesting for the RELAY_UDP_ESP service. 576 The UPDATE message MUST also include a SEQ and a ECHO_REQUEST_SIGNED 577 parameter. The Data Relay Server MUST respond with an UPDATE message 578 that includes the corresponding response parameters: REG_RES, ACK and 579 ECHO_REQUEST_SIGNED . In case the Data Relay Server admitted a new 580 relayed UDP port for the Data Relay Client, the REG_RES parameter 581 MUST list RELAY_UDP_ESP as a service and the UPDATE message MUST also 582 include a RELAYED_ADDRESS parameter describing the relayed UDP port. 583 The Data Relay Server MUST also include the Server Reflexive 584 candidate in a REG_FROM parameter. It is worth mentioning that Data 585 Relay Client MUST activate the UDP port as described in 586 Section 4.12.1 before it can be used for any ESP relaying. 588 A Data Relay Client may unregister a relayed candidate in two ways. 589 It can wait for its lifetime to expire or it can explicitly request 590 it with zero lifetime using the UPDATE mechanism. The Data Relay 591 Client can send an REG_REQ parameter with zero lifetime to the Data 592 Relay Server in order to expire all relayed candidates. To expire a 593 specific relayed candidate, the Data Relay Client MUST also include 594 RELAYED_ADDRESS parameter as sent by the server in the UPDATE 595 message. Upon closing the HIP association (CLOSE-CLOSE-ACK procedure 596 initiated by either party), the Data Relay Server MUST also expire 597 all relayed candidates. 599 4.2. Transport Address Candidate Gathering at the Relay Client 601 An Initiator needs to gather a set of address candidates before 602 contacting a (non-relay) Responder. The candidates are needed for 603 connectivity checks that allow two hosts to discover a direct, non- 604 relayed path for communicating with each other. One server reflexive 605 candidate can be discovered during the registration with the Control 606 Relay Server from the REG_FROM parameter (and another from Data Relay 607 Server if one is employed). 609 The candidate gathering can be done at any time, but it needs to be 610 done before sending an I2 or R2 in the base exchange if ICE-HIP-UDP 611 mode is to be used for the connectivity checks. It is RECOMMENDED 612 that all three types of candidates (host, server reflexive, and 613 relayed) are gathered to maximize the probability of successful NAT 614 traversal. However, if no Data Relay Server is used, and the host 615 has only a single local IP address to use, the host MAY use the local 616 address as the only host candidate and the address from the REG_FROM 617 parameter discovered during the Control Relay Server registration as 618 a server reflexive candidate. In this case, no further candidate 619 gathering is needed. 621 A Data Relay Client MAY register only a single relayed candidate that 622 it uses with multiple other peers. However, it is RECOMMENDED that a 623 Data Relay Client registers a new server relayed candidate for each 624 of its peer for the reasons described in Section 4.12.3. The 625 procedures for registering multiple relayed candidates are described 626 in Section 4.1. 628 If a Relay Client has more than one network interface, it can 629 discover additional server reflexive candidates by sending UPDATE 630 messages from each of its interfaces to the Relay Server. Each such 631 UPDATE message MUST include the following parameters: registration 632 request (REG_REQ) parameter with Registration Type 633 CANDIDATE_DISCOVERY (value [TBD by IANA: 4]) and ECHO_REQUEST_SIGNED 634 parameter. When a Control Relay Server receives an UPDATE message 635 with registration request containing a CANDIDATE_DISCOVERY type, it 636 MUST include a REG_FROM parameter, containing the same information as 637 if this were a Control Relay Server registration, to the response (in 638 addition to the mandatory ECHO_RESPONSE_SIGNED parameter). This 639 request type SHOULD NOT create any state at the Control Relay Server. 641 The rules in section 5.1.1 in [RFC8445] for candidate gathering are 642 followed here. A number of host candidates (loopback, anycast and 643 others) should be excluded as described in section 5.1.1.1 of the ICE 644 specification [RFC8445]. Relayed candidates SHOULD be gathered in 645 order to guarantee successful NAT traversal, and implementations 646 SHOULD support this functionality even if it will not be used in 647 deployments in order to enable it by software configuration update if 648 needed at some point. Similarly as explained in section 5.1.1.2 of 649 the ICE specification [RFC8445], if an IPv6-only host is in a network 650 that utilizes NAT64 [RFC6146] and DNS64 [RFC6147] technologies, it 651 may also gather IPv4 server- reflexive and/or relayed candidates from 652 IPv4-only Control or Data Relay Servers. IPv6-only hosts SHOULD also 653 utilize IPv6 prefix discovery [RFC7050] to discover the IPv6 prefix 654 used by NAT64 (if any) and generate server-reflexive candidates for 655 each IPv6-only interface, accordingly. The NAT64 server-reflexive 656 candidates are prioritized like IPv4 server-reflexive candidates. 658 HIP based connectivity can be utilized by IPv4 applications using 659 Local Scope Identifiers (LSIs) and by IPv6 based applications using 660 HITs. The LSIs and HITs of the local virtual interfaces MUST be 661 excluded in the candidate gathering phase as well to avoid creating 662 unnecessary loopback connectivity tests. 664 Gathering of candidates MAY also be performed by other means than 665 described in this section. For example, the candidates could be 666 gathered as specified in Section 4.2 of [RFC5770] if STUN servers are 667 available, or if the host has just a single interface and no STUN or 668 Data Relay Server are available. 670 Each local address candidate MUST be assigned a priority. The 671 following recommended formula (as described in [RFC8445]) SHOULD be 672 used: 674 priority = (2^24)*(type preference) + (2^8)*(local preference) + 675 (2^0)*(256 - component ID) 677 In the formula, the type preference follows the ICE specification (as 678 defined in section 5.1.2.2 in [RFC8445]): the RECOMMENDED values are 679 126 for host candidates, 100 for server reflexive candidates, 110 for 680 peer reflexive candidates, and 0 for relayed candidates. The highest 681 value is 126 (the most preferred) and lowest is 0 (last resort). For 682 all candidates of the same type, the preference type value MUST be 683 identical, and, correspondingly, the value MUST be different for 684 different types. For peer reflexive values, the type preference 685 value MUST be higher than for server reflexive types. It should be 686 noted that peer reflexive values are learned later during 687 connectivity checks, so a host cannot employ it during candidate 688 gathering stage yet. 690 Following the ICE specification, the local preference MUST be an 691 integer from 0 (lowest preference) to 65535 (highest preference) 692 inclusive. In the case the host has only a single address candidate, 693 the value SHOULD be 65535. In the case of multiple candidates, each 694 local preference value MUST be unique. Dual-stack considerations for 695 IPv6 apply also here as defined in [RFC8445] in section 5.1.2.2. 697 Unlike with SDP used in conjunction with ICE, this protocol only 698 creates a single UDP flow between the two communicating hosts, so 699 only a single component exists. Hence, the component ID value MUST 700 always be set to 1. 702 As defined in section 14.3 in [RFC8445], the retransmission timeout 703 (RTO) for address gathering from a Control/Data Relay Server SHOULD 704 be calculated as follows: 706 RTO = MAX (500ms, Ta * (Num-Of-Cands)) 708 where Ta is the value used for the connectivity check pacing and Num- 709 Of-Cands is the sum of server-reflexive and relay candidates. A 710 smaller value than 500 ms for the RTO MUST NOT be used. 712 4.3. NAT Traversal Mode Negotiation 714 This section describes the usage of a non-critical parameter type 715 called NAT_TRAVERSAL_MODE with a new mode called ICE-HIP-UDP. The 716 presence of the parameter in a HIP base exchange means that the end- 717 host supports NAT traversal extensions described in this document. 719 As the parameter is non-critical (as defined in Section 5.2.1 of 720 [RFC7401]), it can be ignored by a end-host, which means that the 721 host is not required to support it or may decline to use it. 723 With registration with a Control/Data Relay Server, it is usually 724 sufficient to use the UDP-ENCAPSULATION mode of NAT traversal since 725 the Relay Server is assumed to be in public address space. Thus, the 726 Relay Server SHOULD propose the UDP-ENCAPSULATION mode as the 727 preferred or only mode. The NAT traversal mode negotiation in a HIP 728 base exchange is illustrated in Figure 3. It is worth noting that 729 the Relay Server could be located between the hosts, but is omitted 730 here for simplicity. 732 Initiator Responder 733 | 1. UDP(I1) | 734 +----------------------------------------------------------------->| 735 | | 736 | 2. UDP(R1(.., NAT_TRAVERSAL_MODE(ICE-HIP-UDP), ..)) | 737 |<-----------------------------------------------------------------+ 738 | | 739 | 3. UDP(I2(.., NAT_TRAVERSAL_MODE(ICE-HIP-UDP), ENC(LOC_SET), ..))| 740 +----------------------------------------------------------------->| 741 | | 742 | 4. UDP(R2(.., ENC(LOC_SET), ..)) | 743 |<-----------------------------------------------------------------+ 744 | | 746 Figure 3: Negotiation of NAT Traversal Mode 748 In step 1, the Initiator sends an I1 to the Responder. In step 2, 749 the Responder responds with an R1. As specified in [RFC5770], the 750 NAT_TRAVERSAL_MODE parameter in R1 contains a list of NAT traversal 751 modes the Responder supports. The mode specified in this document is 752 ICE-HIP-UDP (value [TBD by IANA: 3]). 754 In step 3, the Initiator sends an I2 that includes a 755 NAT_TRAVERSAL_MODE parameter. It contains the mode selected by the 756 Initiator from the list of modes offered by the Responder. If ICE- 757 HIP-UDP mode was selected, the I2 also includes the "Transport 758 address" locators (as defined in Section 5.7) of the Initiator in a 759 LOCATOR_SET parameter (denoted here with LOC_SET). With ICE-HIP-UDP 760 mode, the LOCATOR_SET parameter MUST be encapsulated within an 761 ENCRYPTED parameter (denoted here with ENC) according to the 762 procedures in sections 5.2.18 and 6.5 in [RFC7401]. The locators in 763 I2 are the "HIP offer". 765 In step 4, the Responder concludes the base exchange with an R2 766 packet. If the Initiator chose ICE NAT traversal mode, the Responder 767 includes a LOCATOR_SET parameter in the R2 packet. With ICE-HIP-UDP 768 mode, the LOCATOR_SET parameter MUST be encapsulated within an 769 ENCRYPTED parameter according to the procedures in sections 5.2.18 770 and 6.5 in [RFC7401]. The locators in R2, encoded like the locators 771 in I2, are the "ICE answer". If the NAT traversal mode selected by 772 the Initiator is not supported by the Responder, the Responder SHOULD 773 reply with a NOTIFY packet with type 774 NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER and abort the base exchange. 776 4.4. Connectivity Check Pacing Negotiation 778 As explained in Legacy ICE-HIP [RFC5770], when a NAT traversal mode 779 with connectivity checks is used, new transactions should not be 780 started too fast to avoid congestion and overwhelming the NATs. For 781 this purpose, during the base exchange, hosts can negotiate a 782 transaction pacing value, Ta, using a TRANSACTION_PACING parameter in 783 R1 and I2 packets. The parameter contains the minimum time 784 (expressed in milliseconds) the host would wait between two NAT 785 traversal transactions, such as starting a new connectivity check or 786 retrying a previous check. The value that is used by both of the 787 hosts is the higher of the two offered values. 789 The minimum Ta value SHOULD be configurable, and if no value is 790 configured, a value of 50 ms MUST be used. Guidelines for selecting 791 a Ta value are given in Appendix A. Hosts MUST NOT use values 792 smaller than 5 ms for the minimum Ta, since such values may not work 793 well with some NATs (as explained in [RFC8445]). The Initiator MUST 794 NOT propose a smaller value than what the Responder offered. If a 795 host does not include the TRANSACTION_PACING parameter in the base 796 exchange, a Ta value of 50 ms MUST be used as that host's minimum 797 value. 799 4.5. Base Exchange via Control Relay Server 801 This section describes how the Initiator and Responder perform a base 802 exchange through a Control Relay Server. Connectivity pacing 803 (denoted as TA_P here) was described in Section 4.4 and is not 804 repeated here. Similarly, the NAT traversal mode negotiation process 805 (denoted as NAT_TM in the example) was described in Section 4.3 and 806 is neither repeated here. If a Control Relay Server receives an R1 807 or I2 packet without the NAT traversal mode parameter, it MUST drop 808 it and SHOULD send a NOTIFY error packet with type 809 NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER to the sender of the R1 or I2. 811 It is RECOMMENDED that the Initiator send an I1 packet encapsulated 812 in UDP when it is destined to an IPv4 address of the Responder. 814 Respectively, the Responder MUST respond to such an I1 packet with a 815 UDP-encapsulated R1 packet, and also the rest of the communication 816 related to the HIP association MUST also use UDP encapsulation. 818 Figure 4 illustrates a base exchange via a Control Relay Server. We 819 assume that the Responder (i.e. a Control Relay Client) has already 820 registered to the Control Relay Server. The Initiator may have also 821 registered to another (or the same Control Relay Server), but the 822 base exchange will traverse always through the Control Relay Server 823 of the Responder. 825 Initiator Control Relay Server Responder 826 | 1. UDP(I1) | | 827 +--------------------------------->| 2. UDP(I1(RELAY_FROM)) | 828 | +------------------------------->| 829 | | | 830 | | 3. UDP(R1(RELAY_TO, NAT_TM, | 831 | | TA_P)) | 832 | 4. UDP(R1(RELAY_TO, NAT_TM, |<-------------------------------+ 833 | TA_P)) | | 834 |<---------------------------------+ | 835 | | | 836 | 5. UDP(I2(ENC(LOC_SET)), | | 837 | NAT_TM, TA_P)) | | 838 +--------------------------------->| 6. UDP(I2(ENC(LOC_SET), | 839 | | RELAY_FROM, NAT_TM, TA_P))| 840 | +------------------------------->| 841 | | | 842 | | 7. UDP(R2(ENC(LOC_SET), | 843 | 8. UDP(R2(ENC(LOC_SET), | RELAY_TO)) | 844 | RELAY_TO)) |<-------------------------------+ 845 |<---------------------------------+ | 846 | | | 848 Figure 4: Base Exchange via a HIP Relay Server 850 In step 1 of Figure 4, the Initiator sends an I1 packet over UDP via 851 the Control Relay Server to the Responder. In the HIP header, the 852 source HIT belongs to the Initiator and the destination HIT to the 853 Responder. The initiator sends the I1 packet from its IP address to 854 the IP address of the Control Relay Server over UDP. 856 In step 2, the Control Relay Server receives the I1 packet. If the 857 destination HIT belongs to a registered Responder, the Control Relay 858 Server processes the packet. Otherwise, the Control Relay Server 859 MUST drop the packet silently. The Control Relay Server appends a 860 RELAY_FROM parameter to the I1 packet, which contains the transport 861 source address and port of the I1 as observed by the Control Relay 862 Server. The Control Relay Server protects the I1 packet with 863 RELAY_HMAC as described in [RFC8004], except that the parameter type 864 is different (see Section 5.8). The Control Relay Server changes the 865 source and destination ports and IP addresses of the packet to match 866 the values the Responder used when registering to the Control Relay 867 Server, i.e., the reverse of the R2 used in the registration. The 868 Control Relay Server MUST recalculate the transport checksum and 869 forward the packet to the Responder. 871 In step 3, the Responder receives the I1 packet. The Responder 872 processes it according to the rules in [RFC7401]. In addition, the 873 Responder validates the RELAY_HMAC according to [RFC8004] and 874 silently drops the packet if the validation fails. The Responder 875 replies with an R1 packet to which it includes RELAY_TO and NAT 876 traversal mode parameters. The responder MUST include ICE-HIP-UDP in 877 the NAT traversal modes. The RELAY_TO parameter MUST contain the 878 same information as the RELAY_FROM parameter, i.e., the Initiator's 879 transport address, but the type of the parameter is different. The 880 RELAY_TO parameter is not integrity protected by the signature of the 881 R1 to allow pre-created R1 packets at the Responder. 883 In step 4, the Control Relay Server receives the R1 packet. The 884 Control Relay Server drops the packet silently if the source HIT 885 belongs to a Control Relay Client that has not successfully 886 registered. The Control Relay Server MAY verify the signature of the 887 R1 packet and drop it if the signature is invalid. Otherwise, the 888 Control Relay Server rewrites the source address and port, and 889 changes the destination address and port to match RELAY_TO 890 information. Finally, the Control Relay Server recalculates the 891 transport checksum and forwards the packet. 893 In step 5, the Initiator receives the R1 packet and processes it 894 according to [RFC7401]. The Initiator MAY use the address in the 895 RELAY_TO parameter as a local peer-reflexive candidate for this HIP 896 association if it is different from all known local candidates. The 897 Initiator replies with an I2 packet that uses the destination 898 transport address of R1 as the source address and port. The I2 899 packet contains a LOCATOR_SET parameter inside an ENCRYPTED parameter 900 that lists all the HIP candidates (HIP offer) of the Initiator. The 901 candidates are encoded using the format defined in Section 5.7. The 902 I2 packet MUST also contain a NAT traversal mode parameter that 903 includes ICE-HIP-UDP mode. The ENCRYPTED parameter along with its 904 key material generation are described in detail in sections 5.2.18 905 and 6.5 in [RFC7401]. 907 In step 6, the Control Relay Server receives the I2 packet. The 908 Control Relay Server appends a RELAY_FROM and a RELAY_HMAC to the I2 909 packet similarly as explained in step 2, and forwards the packet to 910 the Responder. 912 In step 7, the Responder receives the I2 packet and processes it 913 according to [RFC7401]. The Responder validates the RELAY_HMAC 914 according to [RFC8004] and silently drops the packet if the 915 validation fails. It replies with an R2 packet and includes a 916 RELAY_TO parameter as explained in step 3. The R2 packet includes a 917 LOCATOR_SET parameter inside an ENCRYPTED parameter that lists all 918 the HIP candidates (ICE answer) of the Responder. The RELAY_TO 919 parameter is protected by the HMAC. The ENCRYPTED parameter along 920 with its key material generation are described in detail in sections 921 5.2.18 and 6.5 in [RFC7401]. 923 In step 8, the Control Relay Server processes the R2 as described in 924 step 4. The Control Relay Server forwards the packet to the 925 Initiator. After the Initiator has received the R2 and processed it 926 successfully, the base exchange is completed. 928 Hosts MUST include the address of one or more Control Relay Servers 929 (including the one that is being used for the initial signaling) in 930 the LOCATOR_SET parameter in I2 and R2 if they intend to use such 931 servers for relaying HIP signaling immediately after the base 932 exchange completes. The traffic type of these addresses MUST be "HIP 933 signaling" and they MUST NOT be used as HIP candidates. If the 934 Control Relay Server locator used for relaying the base exchange is 935 not included in I2 or R2 LOCATOR_SET parameters, it SHOULD NOT be 936 used after the base exchange. Instead, further HIP signaling SHOULD 937 use the same path as the data traffic. It is RECOMMENDED to use the 938 same Control Relay Server throughout the lifetime of the host 939 association that was used for forwarding the base exchange if the 940 Responder includes it in the locator parameter of the R2 message. 942 4.6. Connectivity Checks 944 When the Initiator and Responder complete the base exchange through 945 the Control Relay Server, both of them employ the IP address of the 946 Control Relay Server as the destination address for the packets. The 947 address of the Control Relay Server MUST NOT be used as a destination 948 for data plane traffic unless the server supports also Data Relay 949 Server functionality, and the Client has successfully registered to 950 use it. When NAT traversal mode with ICE-HIP-UDP was successfully 951 negotiated and selected, the Initiator and Responder MUST start the 952 connectivity checks in order to attempt to obtain direct end-to-end 953 connectivity through NAT devices. It is worth noting that the 954 connectivity checks MUST be completed even though no ESP_TRANSFORM 955 would be negotiated and selected. 957 The connectivity checks follow the ICE methodology [MMUSIC-ICE], but 958 UDP encapsulated HIP control messages are used instead of ICE 959 messages. As stated in the ICE specification, the basic procedure 960 for connectivity checks has three phases: sorting the candidate pairs 961 according their priority, sending checks in the prioritized order and 962 acknowledging the checks from the peer host. 964 The Initiator MUST take the role of controlling host and the 965 Responder acts as the controlled host. The roles MUST persist 966 throughout the HIP associate lifetime (to be reused in the possibly 967 mobility UPDATE procedures). In the case both communicating nodes 968 are initiating the communications to each other using an I1 packet, 969 the conflict is resolved as defined in section 6.7 in [RFC7401]: the 970 host with the "larger" HIT changes to its Role to Responder. In such 971 a case, the host changing its role to Responder MUST also switch to 972 controlled role. 974 The protocol follows standard HIP UPDATE sending and processing rules 975 as defined in section 6.11 and 6.12 in [RFC7401], but some new 976 parameters are introduced (CANDIDATE_PRIORITY, MAPPED_ADDRESS and 977 NOMINATE). 979 4.6.1. Connectivity Check Procedure 981 Figure 5 illustrates connectivity checks in a simplified scenario, 982 where the Initiator and Responder have only a single candidate pair 983 to check. Typically, NATs drop messages until both sides have sent 984 messages using the same port pair. In this scenario, the Responder 985 sends a connectivity check first but the NAT of the Initiator drops 986 it. However, the connectivity check from the Initiator reaches the 987 Responder because it uses the same port pair as the first message. 988 It is worth noting that the message flow in this section is 989 idealistic, and, in practice, more messages would be dropped, 990 especially in the beginning. For instance, connectivity tests always 991 start with the candidates with the highest priority, which would be 992 host candidates (which would not reach the recipient in this 993 scenario). 995 Initiator NAT1 NAT2 Responder 996 | | 1. UDP(UPDATE(SEQ, CAND_PRIO, | | 997 | | ECHO_REQ_SIGN)) | | 998 | X<-----------------------------------+----------------+ 999 | | | | 1000 | 2. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO)) | | 1001 +-------------+------------------------------------+--------------->| 1002 | | | | 1003 | 3. UDP(UPDATE(ACK, ECHO_RESP_SIGN, MAPPED_ADDR)) | | 1004 |<------------+------------------------------------+----------------+ 1005 | | | | 1006 | 4. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO)) | | 1007 |<------------+------------------------------------+----------------+ 1008 | | | | 1009 | 5. UDP(UPDATE(ACK, ECHO_RESP_SIGN, MAPPED_ADDR)) | | 1010 +-------------+------------------------------------+--------------->| 1011 | | | | 1012 | 6. Other connectivity checks using UPDATE over UDP | 1013 |<------------+------------------------------------+----------------> 1014 | | | | 1015 | 7. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO, NOMINATE)) | 1016 +-------------+------------------------------------+--------------->| 1017 | | | | 1018 | 8. UDP(UPDATE(SEQ, ACK, ECHO_REQ_SIGN, ECHO_RESP_SIGN, | 1019 | NOMINATE)) | | 1020 |<------------+------------------------------------+----------------+ 1021 | | | | 1022 | 9. UDP(UPDATE(ACK, ECHO_RESP_SIGN)) | | 1023 +-------------+------------------------------------+--------------->+ 1024 | | | | 1025 | 10. ESP data traffic over UDP | | 1026 +<------------+------------------------------------+--------------->+ 1027 | | | | 1029 Figure 5: Connectivity Checks 1031 In step 1, the Responder sends a connectivity check to the Initiator 1032 that the NAT of the Initiator drops. The message includes a number 1033 of parameters. As specified in [RFC7401]), the SEQ parameter 1034 includes a running sequence identifier for the connectivity check. 1035 The candidate priority (denoted "CAND_PRIO" in the figure) describes 1036 the priority of the address candidate being tested. The 1037 ECHO_REQUEST_SIGNED (denoted ECHO_REQ_SIGN in the figure) includes a 1038 nonce that the recipient must sign and echo back as it is. 1040 In step 2, the Initiator sends a connectivity check, using the same 1041 address pair candidate as in the previous step, and the message 1042 traverses successfully the NAT boxes. The message includes the same 1043 parameters as in the previous step. It should be noted that the 1044 sequence identifier is locally assigned by the Initiator, so it can 1045 be different than in the previous step. 1047 In step 3, the Responder has successfully received the previous 1048 connectivity check from the Initiator and starts to build a response 1049 message. Since the message from the Initiator included a SEQ, the 1050 Responder must acknowledge it using an ACK parameter. Also, the 1051 nonce contained in the echo request must be echoed back in an 1052 ECHO_RESPONSE_SIGNED (denoted ECHO_RESP_SIGN) parameter. The 1053 Responder includes also a MAPPED_ADDRESS parameter (denoted 1054 MAPPED_ADDR in the figure) that contains the transport address of the 1055 Initiator as observed by the Responder (i.e. peer reflexive 1056 candidate). This message is successfully delivered to the Initiator, 1057 and upon reception the Initiator marks the candidate pair as valid. 1059 In step 4, the Responder retransmits the connectivity check sent in 1060 the first step, since it was not acknowledged yet. 1062 In step 5, the Initiator responds to the previous connectivity check 1063 message from the Responder. The Initiator acknowledges the SEQ 1064 parameter from the previous message using ACK parameter and the 1065 ECHO_REQUEST_SIGNED parameter with ECHO_RESPONSE_SIGNED. In 1066 addition, it includes MAPPED_ADDR parameter that includes the peer 1067 reflexive candidate. This response message is successfully delivered 1068 to the Responder, and upon reception the Initiator marks the 1069 candidate pair as valid. 1071 In step 6, despite the two hosts now having valid address candidates, 1072 the hosts still test the remaining address candidates in a similar 1073 way as in the previous steps. It should be noted that each 1074 connectivity check has a unique sequence number in the SEQ parameter. 1076 In step 7, the Initiator has completed testing all address candidates 1077 and nominates one address candidate to be used. It sends an UPDATE 1078 message using the selected address candidates that includes a number 1079 of parameters: SEQ, ECHO_REQUEST_SIGNED, CANDIDATE_PRIORITY and the 1080 NOMINATE parameter. 1082 In step 8, the Responder receives the message with NOMINATE parameter 1083 from the Initiator. It sends a response that includes the NOMINATE 1084 parameter in addition to a number of other parameters. The ACK and 1085 ECHO_RESPONSE_SIGNED parameters acknowledge the SEQ and 1086 ECHO_REQUEST_SIGNED parameters from previous message from the 1087 Initiator. The Responder includes SEQ and ECHO_REQUEST_SIGNED 1088 parameters in order to receive an acknowledgment from the Responder. 1090 In step 9, the Initiator completes the candidate nomination process 1091 by confirming the message reception to the Responder. In the 1092 confirmation message, the ACK and ECHO_RESPONSE_SIGNED parameters 1093 correspond to the SEQ and ECHO_REQUEST_SIGNED parameters in the 1094 message sent by the Responder in the previous step. 1096 In step 10, the Initiator and Responder can start sending application 1097 payload over the successfully nominated address candidates. 1099 It is worth noting that if either host has registered a relayed 1100 address candidate from a Data Relay Server, the host MUST activate 1101 the address before connectivity checks by sending an UPDATE message 1102 containing PEER_PERMISSION parameter as described in Section 4.12.1. 1103 Otherwise, the Data Relay Server drops ESP packets using the relayed 1104 address. 1106 It should be noted that in the case both Initiator and Responder both 1107 advertising their own relayed address candidates, it is possible that 1108 the two hosts choose the two relayed addresses as a result of the ICE 1109 nomination algorithm. While this is possible (and even could be 1110 desirable for privacy reasons), it can be unlikely due to low 1111 priority assigned for the relayed address candidates. In such a 1112 event, the nominated address pair is always symmetric; the nomination 1113 algorithm prevents asymmetric address pairs (i.e. each side choosing 1114 different pair), such as a Data Relay Client using its own Data Relay 1115 Server to send data directly to its peer while receiving data from 1116 the Data Relay Server of its peer. 1118 4.6.2. Rules for Connectivity Checks 1120 The HITs of the two communicating hosts MUST be used as credentials 1121 in this protocol (in contrast to ICE which employs username-password 1122 fragments). A HIT pair uniquely identifies the corresponding HIT 1123 association, and a SEQ number in an UPDATE message identifies a 1124 particular connectivity check. 1126 All of the connectivity check packets MUST be protected with HMACs 1127 and signatures (even though the illustrations in this specification 1128 omit them for simplicity). Each connectivity check sent by a host 1129 MUST include a SEQ parameter and ECHO_REQUEST_SIGNED parameter, and 1130 correspondingly the peer MUST respond to these using ACK and 1131 ECHO_RESPONSE_SIGNED according to the rules specified in [RFC7401]. 1133 The host sending a connectivity check MUST validate that the response 1134 uses the same pair of UDP ports, and drop the packet if this is not 1135 the case. 1137 A host may receive a connectivity check before it has received the 1138 candidates from its peer. In such a case, the host MUST immediately 1139 queue a response by placing it in the triggered-check queue, and then 1140 continue waiting for the candidates. A host MUST NOT select a 1141 candidate pair until it has verified the pair using a connectivity 1142 check as defined in Section 4.6.1. 1144 [RFC7401] states that UPDATE packets have to include either a SEQ or 1145 ACK parameter (or both). According to the RFC, each SEQ parameter 1146 should be acknowledged separately. In the context of NATs, this 1147 means that some of the SEQ parameters sent in connectivity checks 1148 will be lost or arrive out of order. From the viewpoint of the 1149 recipient, this is not a problem since the recipient will just 1150 "blindly" acknowledge the SEQ. However, the sender needs to be 1151 prepared for lost sequence identifiers and ACKs parameters that 1152 arrive out of order. 1154 As specified in [RFC7401], an ACK parameter may acknowledge multiple 1155 sequence identifiers. While the examples in the previous sections do 1156 not illustrate such functionality, it is also permitted when 1157 employing ICE-HIP-UDP mode. 1159 In ICE-HIP-UDP mode, a retransmission of a connectivity check SHOULD 1160 be sent with the same sequence identifier in the SEQ parameter. Some 1161 tested address candidates will never produce a working address pair, 1162 and thus may cause retransmissions. Upon successful nomination of an 1163 address pair, a host SHOULD immediately stop sending such 1164 retransmissions. 1166 Full ICE procedures for prioritizing candidates, eliminating 1167 redundant candidates, forming check lists (including pruning) and 1168 triggered check-queues MUST be followed as specified in section 6.1 1169 [RFC8445], with the exception that the foundation, frozen candidates 1170 and default candidates are not used. From viewpoint of the ICE 1171 specification [RFC8445], the protocol specified in this document 1172 operates using Component ID of 1 on all candidates, and the 1173 foundation of all candidates is unique. This specification defines 1174 only "full ICE" mode, and the "lite ICE" is not supported. The 1175 reasoning behind the missing features is described in Appendix B. 1177 The connectivity check messages MUST be paced by the Ta value 1178 negotiated during the base exchange as described in Section 4.4. If 1179 neither one of the hosts announced a minimum pacing value, a value of 1180 50 ms MUST be used. 1182 Both hosts MUST form a priority ordered checklist and begin to check 1183 transactions every Ta milliseconds as long as the checks are running 1184 and there are candidate pairs whose tests have not started. The 1185 retransmission timeout (RTO) for the connectivity check UPDATE 1186 packets SHOULD be calculated as follows: 1188 RTO = MAX (500ms, Ta * (Num-Waiting + Num-In-Progress)) 1190 In the RTO formula, Ta is the value used for the connectivity check 1191 pacing, Num-Waiting is the number of pairs in the checklist in the 1192 "Waiting" state, and Num-In-Progress is the number of pairs in the 1193 "In-Progress" state. This is identical to the formula in [RFC8445] 1194 when there is only one checklist. A smaller value than 500 ms for 1195 the RTO MUST NOT be used. 1197 Each connectivity check request packet MUST contain a 1198 CANDIDATE_PRIORITY parameter (see Section 5.14) with the priority 1199 value that would be assigned to a peer reflexive candidate if one was 1200 learned from the corresponding check. An UPDATE packet that 1201 acknowledges a connectivity check request MUST be sent from the same 1202 address that received the check and delivered to the same address 1203 where the check was received from. Each acknowledgment UPDATE packet 1204 MUST contain a MAPPED_ADDRESS parameter with the port, protocol, and 1205 IP address of the address where the connectivity check request was 1206 received from. 1208 Following the ICE guidelines [RFC8445], it is RECOMMENDED to restrict 1209 the total number of connectivity checks to 100 for each host 1210 association. This can be achieved by limiting the connectivity 1211 checks to the 100 candidate pairs with the highest priority. 1213 4.6.3. Rules for Concluding Connectivity Checks 1215 The controlling agent may find multiple working candidate pairs. To 1216 conclude the connectivity checks, it SHOULD nominate the pair with 1217 the highest priority. The controlling agent MUST nominate a 1218 candidate pair essentially by repeating a connectivity check using an 1219 UPDATE message that contains a SEQ parameter (with new sequence 1220 number), a ECHO_REQUEST_SIGNED parameter, the priority of the 1221 candidate in a CANDIDATE_PRIORITY parameter and NOMINATE parameter to 1222 signify conclusion of the connectivity checks. Since the nominated 1223 address pair has already been tested for reachability, the controlled 1224 host should be able to receive the message. Upon reception, the 1225 controlled host SHOULD select the nominated address pair. The 1226 response message MUST include a SEQ parameter with a new sequence id, 1227 acknowledgment of the sequence from the controlling host in an ACK 1228 parameter, a new ECHO_REQUEST_SIGNED parameter, ECHO_RESPONSE_SIGNED 1229 parameter corresponding to the ECHO_REQUEST_SIGNED parameter from the 1230 controlling host and the NOMINATE parameter. After sending this 1231 packet, the controlled host can create IPsec security associations 1232 using the nominated address candidate for delivering application 1233 payload to the controlling host. Since the message from the 1234 controlled host included a new sequence id and echo request for 1235 signature, the controlling host MUST acknowledge this with a new 1236 UPDATE message that includes an ACK and ECHO_RESPONSE_SIGNED 1237 parameters. After this final concluding message, the controlling 1238 host also can create IPsec security associations for delivering 1239 application payload to the controlled host. 1241 It is possible that packets are delayed by the network. Both hosts 1242 MUST continue to respond to any connectivity checks despite an 1243 address pair having been nominated. 1245 If all the connectivity checks have failed, the hosts MUST NOT send 1246 ESP traffic to each other but MAY continue communicating using HIP 1247 packets and the locators used for the base exchange. Also, the hosts 1248 SHOULD notify each other about the failure with a 1249 CONNECTIVITY_CHECKS_FAILED NOTIFY packet (see Section 5.10). 1251 4.7. NAT Traversal Optimizations 1253 4.7.1. Minimal NAT Traversal Support 1255 If the Responder has a fixed and publicly reachable IPv4 address and 1256 does not employ a Control Relay Server, the explicit NAT traversal 1257 mode negotiation MAY be omitted, and thus even the UDP-ENCAPSULATION 1258 mode does not have to be negotiated. In such a scenario, the 1259 Initiator sends an I1 message over UDP and the Responder responds 1260 with an R1 message over UDP without including any NAT traversal mode 1261 parameter. The rest of the base exchange follows the procedures 1262 defined in [RFC7401], except that the control and data plane use UDP 1263 encapsulation. Here, the use of UDP for NAT traversal is agreed 1264 implicitly. This way of operation is still subject to NAT timeouts, 1265 and the hosts MUST employ NAT keepalives as defined in Section 4.10. 1267 When UDP-ENCAPSULATION mode is chosen either explicitly or 1268 implicitly, the connectivity checks as defined in this document MUST 1269 NOT be used. When hosts lose connectivity, they MUST instead utilize 1270 [RFC8046] or [RFC8047] procedures, but with the difference being that 1271 UDP-based tunneling MUST be employed for the entire lifetime of the 1272 corresponding Host Association. 1274 4.7.2. Base Exchange without Connectivity Checks 1276 It is possible to run a base exchange without any connectivity checks 1277 as defined in Legacy ICE-HIP section 4.8 [RFC5770]. The procedure is 1278 applicable also in the context of this specification, so it is 1279 repeated here for completeness. 1281 In certain network environments, the connectivity checks can be 1282 omitted to reduce initial connection set-up latency because a base 1283 exchange acts as an implicit connectivity test itself. For this to 1284 work, the Initiator MUST be able to reach the Responder by simply UDP 1285 encapsulating HIP and ESP packets sent to the Responder's address. 1286 Detecting and configuring this particular scenario is prone to 1287 failure unless carefully planned. 1289 In such a scenario, the Responder MAY include UDP-ENCAPSULATION NAT 1290 traversal mode as one of the supported modes in the R1 packet. If 1291 the Responder has registered to a Control Relay Server in order to 1292 discover its address candidates, it MUST also include a LOCATOR_SET 1293 parameter in R1 that contains a preferred address where the Responder 1294 is able to receive UDP-encapsulated ESP and HIP packets. This 1295 locator MUST be of type "Transport address", its Traffic type MUST be 1296 "both", and it MUST have the "Preferred bit" set (see Table 1). If 1297 there is no such locator in R1, the source address of R1 is used as 1298 the Responder's preferred address. 1300 The Initiator MAY choose the UDP-ENCAPSULATION mode if the Responder 1301 listed it in the supported modes and the Initiator does not wish to 1302 use the connectivity checks defined in this document for searching 1303 for a more optimal path. In this case, the Initiator sends the I2 1304 with UDP-ENCAPSULATION mode in the NAT traversal mode parameter 1305 directly to the Responder's preferred address (i.e., to the preferred 1306 locator in R1 or to the address where R1 was received from if there 1307 was no preferred locator in R1). The Initiator MAY include locators 1308 in I2 but they MUST NOT be taken as address candidates, since 1309 connectivity checks defined in this document will not be used for 1310 connections with UDP-ENCAPSULATION NAT traversal mode. Instead, if 1311 R2 and I2 are received and processed successfully, a security 1312 association can be created and UDP-encapsulated ESP can be exchanged 1313 between the hosts after the base exchange completes according to the 1314 rules in Section 4.4 in [RFC7401]. 1316 The Control Relay Server can be used for discovering address 1317 candidates but it is not intended to be used for relaying end-host 1318 packets using the UDP-ENCAPSULATION NAT mode. Since an I2 packet 1319 with UDP-ENCAPSULATION NAT traversal mode selected MUST NOT be sent 1320 via a Control Relay Server, the Responder SHOULD reject such I2 1321 packets and reply with a NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER NOTIFY 1322 packet (see Section 5.10). 1324 If there is no answer for the I2 packet sent directly to the 1325 Responder's preferred address, the Initiator MAY send another I2 via 1326 the Control Relay Server, but it MUST NOT choose UDP-ENCAPSULATION 1327 NAT traversal mode for that I2. 1329 4.7.3. Initiating a Base Exchange both with and without UDP 1330 Encapsulation 1332 It is possible to run a base exchange in parallel both with and 1333 without UDP encapsulation as defined in Legacy ICE-HIP section 4.9 in 1334 [RFC5770]. The procedure is applicable also in the context of this 1335 specification, so it is repeated here for completeness. 1337 The Initiator MAY also try to simultaneously perform a base exchange 1338 with the Responder without UDP encapsulation. In such a case, the 1339 Initiator sends two I1 packets, one without and one with UDP 1340 encapsulation, to the Responder. The Initiator MAY wait for a while 1341 before sending the other I1. How long to wait and in which order to 1342 send the I1 packets can be decided based on local policy. For 1343 retransmissions, the procedure is repeated. 1345 The I1 packet without UDP encapsulation may arrive directly, without 1346 passing any Control Data Relays, at the Responder. When this 1347 happens, the procedures in [RFC7401] are followed for the rest of the 1348 base exchange. The Initiator may receive multiple R1 packets, with 1349 and without UDP encapsulation, from the Responder. However, after 1350 receiving a valid R1 and answering it with an I2, further R1 packets 1351 that are not retransmissions of the R1 message received first MUST be 1352 ignored. 1354 The I1 packet without UDP encapsulation may also arrive at a HIP- 1355 capable middlebox. When the middlebox is a HIP rendezvous server and 1356 the Responder has successfully registered with the rendezvous 1357 service, the middlebox follows rendezvous procedures in [RFC8004]. 1359 If the Initiator receives a NAT traversal mode parameter in R1 1360 without UDP encapsulation, the Initiator MAY ignore this parameter 1361 and send an I2 without UDP encapsulation and without any selected NAT 1362 traversal mode. When the Responder receives the I2 without UDP 1363 encapsulation and without NAT traversal mode, it will assume that no 1364 NAT traversal mechanism is needed. The packet processing will be 1365 done as described in [RFC7401]. The Initiator MAY store the NAT 1366 traversal modes for future use, e.g., in case of a mobility or 1367 multihoming event that causes NAT traversal to be used during the 1368 lifetime of the HIP association. 1370 4.8. Sending Control Packets after the Base Exchange 1372 The same considerations of sending control packets after the base 1373 exchange described in legacy ICE-HIP section 5.10 in [RFC5770] apply 1374 also here, so they are repeated here for completeness. 1376 After the base exchange, the two end-hosts MAY send HIP control 1377 packets directly to each other using the transport address pair 1378 established for a data channel without sending the control packets 1379 through any Control Relay Servers . When a host does not receive 1380 acknowledgments, e.g., to an UPDATE or CLOSE packet after a timeout 1381 based on local policies, a host SHOULD resend the packet through the 1382 associated Data Relay Server of the peer (if the peer listed it in 1383 its LOCATOR_SET parameter in the base exchange. 1385 If Control Relay Client sends a packet through a Control Relay 1386 Server, the Control Relay Client MUST always utilize the RELAY_TO 1387 parameter. The Control Relay Server SHOULD forward HIP control 1388 packets originating from a Control Relay Client to the address 1389 denoted in the RELAY_TO parameter. In the other direction, the 1390 Control Relay Server SHOULD forward HIP control packets to the 1391 Control Relay Clients, and MUST add a RELAY_FROM parameter to the 1392 control packets it relays to the Control Relay Clients. 1394 If the Control Relay Server is not willing or able to relay a HIP 1395 packet, it MAY notify the sender of the packet with 1396 MESSAGE_NOT_RELAYED error notification (see Section 5.10). 1398 4.9. Mobility Handover Procedure 1400 A host may move after base exchange and connectivity checks. 1401 Mobility extensions for HIP [RFC8046] define handover procedures 1402 without NATs. In this section, we define how two hosts interact with 1403 handover procedures in scenarios involving NATs. The specified 1404 extensions define only simple mobility using a pair of security 1405 associations, and multihoming extensions are left to be defined in 1406 later specifications. The procedures in this section offer the same 1407 functionality as "ICE restart" specified in [RFC8445]. The example 1408 described in this section shows only a Control Relay Server for the 1409 peer host for the sake of simplicity, but the mobile host may also 1410 have a Control Relay Server. 1412 The assumption here is that the two hosts have successfully 1413 negotiated and chosen the ICE-HIP-UDP mode during the base exchange 1414 as defined in Section 4.3. The Initiator of the base exchange MUST 1415 store information that it was the controlling host during the base 1416 exchange. Similarly, the Responder MUST store information that it 1417 was the controlled host during the base exchange. 1419 Prior to starting the handover procedures with all peer hosts, the 1420 mobile host SHOULD first send its locators in UPDATE messages to its 1421 Control and Data Relay Servers if it has registered to such. It 1422 SHOULD wait for all of them to respond for a configurable time, by 1423 default two minutes, and then continue with the handover procedure 1424 without information from the Relay Server that did not respond. As 1425 defined in Section 4.1, a response message from a Control Relay 1426 Server includes a REG_FROM parameter that describes the server 1427 reflexive candidate of the mobile host to be used in the candidate 1428 exchange during the handover. Similarly, an UPDATE to a Data Relay 1429 Server is necessary to make sure the Data Relay Server can forward 1430 data to the correct IP address after a handoff. 1432 The mobility extensions for NAT traversal are illustrated in 1433 Figure 6. The mobile host is the host that has changed its locators, 1434 and the peer host is the host it has a host association with. The 1435 mobile host may have multiple peers and it repeats the process with 1436 all of its peers. In the figure, the Control Relay Server belongs to 1437 the peer host, i.e., the peer host is a Control Relay Client for the 1438 Control Relay Server. Note that the figure corresponds to figure 3 1439 in [RFC8046], but the difference is that the main UPDATE procedure is 1440 carried over the relay and the connectivity is tested separately. 1441 Next, we describe the procedure in the figure in detail. 1443 Mobile Host Control Relay Server Peer Host 1444 | 1. UDP(UPDATE(ESP_INFO, | | 1445 | ENC(LOC_SET), SEQ)) | | 1446 +--------------------------------->| 2. UDP(UPDATE(ESP_INFO, | 1447 | | ENC(LOC_SET), SEQ, | 1448 | | RELAY_FROM)) | 1449 | +------------------------------->| 1450 | | | 1451 | | 3. UDP(UPDATE(ESP_INFO, SEQ, | 1452 | | ACK, ECHO_REQ_SIGN, | 1453 | | RELAY_TO)) | 1454 | 4. UDP(UPDATE(ESP_INFO, SEQ, |<-------------------------------+ 1455 | ACK, ECHO_REQ_SIGN, | | 1456 | RELAY_TO)) | | 1457 |<---------------------------------+ | 1458 | | | 1459 | 5. UDP(UPDATE(ACK, | | 1460 | ECHO_RESP_SIGNED)) | | 1461 +--------------------------------->| 6. UDP(UPDATE(ACK, | 1462 | | ECHO_RESP_SIGNED, | 1463 | | RELAY_FROM)) | 1464 | +------------------------------->| 1465 | | | 1466 | 7. connectivity checks over UDP | 1467 +<----------------------------------------------------------------->+ 1468 | | | 1469 | 8. ESP data over UDP | 1470 +<----------------------------------------------------------------->+ 1471 | | | 1473 Figure 6: HIP UPDATE procedure 1475 In step 1, the mobile host has changed location and sends a location 1476 update to its peer through the Control Relay Server of the peer. It 1477 sends an UPDATE packet with source HIT belonging to itself and 1478 destination HIT belonging to the peer host. In the packet, the 1479 source IP address belongs to the mobile host and the destination to 1480 the Control Relay Server. The packet contains an ESP_INFO parameter, 1481 where, in this case, the OLD SPI and NEW SPI parameters both contain 1482 the pre-existing incoming SPI. The packet also contains the locators 1483 of the mobile host in a LOCATOR_SET parameter, encapsulated inside an 1484 ENCRYPTED parameter (see sections 5.2.18 and 6.5 in [RFC7401] for 1485 details on the ENCRYPTED parameter). The packet contains also a SEQ 1486 number to be acknowledged by the peer. As specified in [RFC8046], 1487 the packet may also include a HOST_ID (for middlebox inspection) and 1488 DIFFIE_HELLMAN parameter for rekeying. 1490 In step 2, the Control Relay Server receives the UPDATE packet and 1491 forwards it to the peer host (i.e. Control Relay Client). The 1492 Control Relay Server rewrites the destination IP address and appends 1493 a RELAY_FROM parameter to the message. 1495 In step 3, the peer host receives the UPDATE packet, processes it and 1496 responds with another UPDATE message. The message is destined to the 1497 HIT of mobile host and to the IP address of the Control Relay Server. 1498 The message includes an ESP_INFO parameter where, in this case, the 1499 OLD SPI and NEW SPI parameters both contain the pre-existing incoming 1500 SPI. The peer includes a new SEQ and ECHO_REQUEST_SIGNED parameters 1501 to be acknowledged by the mobile host. The message acknowledges the 1502 SEQ parameter of the earlier message with an ACK parameter. The 1503 RELAY_TO parameter specifies the address of the mobile host where the 1504 Control Relay Server should forward the message. 1506 In step 4, the Control Relay Server receives the message, rewrites 1507 the destination IP address and UDP port according to the RELAY_TO 1508 parameter, and then forwards the modified message to the mobile host. 1510 In step 5, the mobile host receives the UPDATE packet and processes 1511 it. The mobile host concludes the handover procedure by 1512 acknowledging the received SEQ parameter with an ACK parameter and 1513 the ECHO_REQUEST_SIGNED parameter with ECHO_RESPONSE_SIGNED 1514 parameter. The mobile host delivers the packet to the HIT of the 1515 peer and to the address of the HIP relay. The mobile host can start 1516 connectivity checks after this packet. 1518 In step 6, HIP relay receives the UPDATE packet and forwards it to 1519 the peer host (i.e. Relay Client). The HIP relay rewrites the 1520 destination IP address and port, and then appends a RELAY_FROM 1521 parameter to the message. When the peer host receives this 1522 concluding UPDATE packet, it can initiate the connectivity checks. 1524 In step 7, the two hosts test for connectivity across NATs according 1525 to procedures described in Section 4.6. The original Initiator of 1526 the communications is the controlling and the original Responder is 1527 the controlled host. 1529 In step 8, the connectivity checks are successfully completed and the 1530 controlling host has nominated one address pair to be used. The 1531 hosts set up security associations to deliver the application 1532 payload. 1534 It is worth noting that the Control and Data Relay Client do not have 1535 to re-register for the related services after a handoff. However, if 1536 a Data Relay Client has registered a relayed address candidate from a 1537 Data Relay Server, the Data Relay Client MUST reactivate the address 1538 before the connectivity checks by sending an UPDATE message 1539 containing PEER_PERMISSION parameter as described in Section 4.12.1. 1540 Otherwise, the Data Relay Server drops ESP packets sent to the 1541 relayed address. 1543 In so called "double jump" or simultaneous mobility scenario both 1544 peers change their location simultaneously. In such a case, both 1545 peers trigger the procedure described earlier in this section at the 1546 same time. In other words, both of the communicating hosts send an 1547 UPDATE packet carrying locators at the same time or with some delay. 1548 When the locators are exchanged almost simultaneously (reliably via 1549 Control Relay Servers), the two hosts can continue with connectivity 1550 checks after both have completed separately the steps in Figure 6. 1551 The problematic case occurs when the one of the hosts (referred here 1552 as host "M") moves later during the connectivity checks. In such a 1553 case, host M sends a locator to the peer which is in the middle of 1554 connectivity checks. Upon receiving the UPDATE message, the peer 1555 responds with an UPDATE with ECHO_REQ_SIGN as described in step 3 in 1556 Figure 6. Upon receiving the valid response from host M as described 1557 in step 6, the peer host MUST restart the connectivity checks with 1558 host M. This way, both hosts start the connectivity checks roughly 1559 in a synchronized way. It is also important that peer host does not 1560 restart the connectivity checks until it has received a valid "fresh" 1561 confirmation from host M because the UPDATE message carrying locators 1562 could be replayed by an attacker. 1564 4.10. NAT Keepalives 1566 To prevent NAT states from expiring, communicating hosts MUST send 1567 periodic keepalives to other hosts with which they have established a 1568 Host Association every 15 seconds (the so called Tr value in ICE). 1569 Other values MAY be used, but a Tr value smaller than 15 seconds MUST 1570 NOT be used. Both a Control/Data Relay Client and Control/Data Relay 1571 Server, as well as two peers employing UDP-ENCAPSULATION or ICE-HIP- 1572 UDP mode, SHOULD send HIP NOTIFY packets unless they have exchanged 1573 some other traffic over the used UDP ports. However, the Data Relay 1574 Client and Data Relay Server MUST employ only HIP NOTIFY packets in 1575 order to keep the server reflexive candidates alive. The keepalive 1576 message encoding format is defined in Section 5.3. If the base 1577 exchange or mobility handover procedure occurs during an extremely 1578 slow path, a host (with a Host Association with the peer) MAY also 1579 send HIP NOTIFY packets every 15 seconds to keep the path active with 1580 the recipient. 1582 4.11. Closing Procedure 1584 The two-way procedure for closing a HIP association and the related 1585 security associations is defined in [RFC7401]. One host initiates 1586 the procedure by sending a CLOSE message and the recipient confirms 1587 it with CLOSE_ACK. All packets are protected using HMACs and 1588 signatures, and the CLOSE messages includes a ECHO_REQUEST_SIGNED 1589 parameter to protect against replay attacks. 1591 The same procedure for closing HIP associations applies also here, 1592 but the messaging occurs using the UDP encapsulated tunnel that the 1593 two hosts employ. A host sending the CLOSE message SHOULD first send 1594 the message over a direct link. After a number of retransmissions, 1595 it MUST send over a Control Relay Server of the recipient if one 1596 exists. The host receiving the CLOSE message directly without a 1597 Control Data Relay SHOULD respond directly. If CLOSE message came 1598 via a Control Data Relay, the host SHOULD respond using the same 1599 Control Data Relay. 1601 4.12. Relaying Considerations 1603 4.12.1. Forwarding Rules and Permissions 1605 The Data Relay Server uses a similar permission model as a TURN 1606 server: before the Data Relay Server forwards any ESP data packets 1607 from a peer to a Data Relay Client (or the other direction), the 1608 client MUST set a permission for the peer's address. The permissions 1609 also install a forwarding rule for each direction, similar to TURN's 1610 channels, based on the Security Parameter Index (SPI) values in the 1611 ESP packets. 1613 Permissions are not required for HIP control packets. However, if a 1614 relayed address (as conveyed in the RELAYED_ADDRESS parameter from 1615 the Data Relay Server) is selected to be used for data, the Control 1616 Relay Client MUST send an UPDATE message to the Data Relay Server 1617 containing a PEER_PERMISSION parameter (see Section 5.13) with the 1618 following information: the UDP port and address for the server 1619 reflexive address, the UDP port and address of the peer, and the 1620 inbound and outbound SPIs used for ESP. The packet MUST be sent to 1621 the same UDP tunnel the Client employed in the base exchange to 1622 contact the Server (i.e., not to the port occupied by the server 1623 reflexive candidate). To avoid packet dropping of ESP packets, the 1624 Control Relay Client SHOULD send the PEER_PERMISSION parameter before 1625 connectivity checks both in the case of base exchange and a mobility 1626 handover. It is worth noting that the UPDATE message includes a SEQ 1627 parameter (as specified in [RFC7401]) that the Data Relay Server must 1628 acknowledge, so that the Control Relay Client can resend the message 1629 with PEER_PERMISSION parameter if it gets lost. 1631 When a Data Relay Server receives an UPDATE with a PEER_PERMISSION 1632 parameter, it MUST check if the sender of the UPDATE is registered 1633 for data relaying service, and drop the UPDATE if the host was not 1634 registered. If the host was registered, the Data Relay Server checks 1635 if there is a permission with matching information (protocol, 1636 addresses, ports and SPI values). If there is no such permission, a 1637 new permission MUST be created and its lifetime MUST be set to 5 1638 minutes. If an identical permission already existed, it MUST be 1639 refreshed by setting the lifetime to 5 minutes. A Data Relay Client 1640 SHOULD refresh permissions 1 minute before the expiration when the 1641 permission is still needed. 1643 When a Data Relay Server receives an UPDATE from a registered client 1644 but without a PEER_PERMISSION parameter and with a new locator set, 1645 the Data Relay Server can assume that the mobile host has changed its 1646 location and, thus, is not reachable in its previous location. In 1647 such an event, the Data Relay Server SHOULD deactivate the permission 1648 and stop relaying data plane traffic to the client. 1650 The relayed address MUST be activated with the PEER_PERMISSION 1651 parameter both after a base exchange and after a handover procedure 1652 with another ICE-HIP-UDP capable host. Unless activated, the Data 1653 Relay Server MUST drop all ESP packets. It is worth noting that a 1654 Data Relay Client does not have to renew its registration upon a 1655 change of location UPDATE, but only when the lifetime of the 1656 registration is close to end. 1658 4.12.2. HIP Data Relay and Relaying of Control Packets 1660 When a Data Relay Server accepts to relay UDP encapsulated ESP 1661 between a Data Relay Client and its peer, the Data Relay Server opens 1662 a UDP port (relayed address) for this purpose as described in 1663 Section 4.1. This port can be used for delivering also control 1664 packets because connectivity checks also cover the path through the 1665 Data Relay Server. If the Data Relay Server receives a UDP 1666 encapsulated HIP control packet on that port, it MUST forward the 1667 packet to the Data Relay Client and add a RELAY_FROM parameter to the 1668 packet as if the Data Relay Server were acting as a Control Relay 1669 Server. When the Data Relay Client replies to a control packet with 1670 a RELAY_FROM parameter via its Data Relay Server, the Data Relay 1671 Client MUST add a RELAY_TO parameter containing the peer's address 1672 and use the address of its Data Relay Server as the destination 1673 address. Further, the Data Relay Server MUST send this packet to the 1674 peer's address from the relayed address. 1676 If the Data Relay Server receives a UDP packet that is not a HIP 1677 control packet to the relayed address, it MUST check if it has a 1678 permission set for the peer the packet is arriving from (i.e., the 1679 sender's address and SPI value matches to an installed permission). 1680 If permissions are set, the Data Relay Server MUST forward the packet 1681 to the Data Relay Client that created the permission. The Data Relay 1682 Server MUST also implement the similar checks for the reverse 1683 direction (i.e. ESP packets from the Data Relay Client to the peer). 1684 Packets without a permission MUST be dropped silently. 1686 4.12.3. Handling Conflicting SPI Values 1688 From the viewpoint of a host, its remote peers can have overlapping 1689 inbound SPI numbers because the IPsec uses also the destination IP 1690 address to index the remote peer host. However, a Data Relay Server 1691 can represent multiple remote peers, thus masquerading the actual 1692 destination. Since a Data Relay Server may have to deal with a 1693 multitude of Relay Clients and their peers, a Data Relay Server may 1694 experience collisions in the SPI namespace, thus being unable forward 1695 datagrams to the correct destination. Since the SPI space is 32 bits 1696 and the SPI values should be random, the probability for a 1697 conflicting SPI value is fairly small, but could occur on a busy Data 1698 Relay Server. The two problematic cases are described in this 1699 section. 1701 In the first scenario, the SPI collision problems occurs if two hosts 1702 have registered to the same Data Relay Server and a third host 1703 initiates base exchange with both of them. Here, the two Responders 1704 (i.e. Data Relay Clients) claim the same inbound SPI number with the 1705 same Initiator (peer). However, in this case, the Data Relay Server 1706 has allocated separate UDP ports for the two Data Relay Clients 1707 acting now as Responders (as recommended in Section 6.5). When the 1708 third host sends an ESP packet, the Data Relay Server is able to 1709 forward the packet to the correct Data Relay Client because the 1710 destination UDP port is different for each of the clients. 1712 In the second scenario, an SPI collision may occur when two 1713 Initiators run a base exchange to the same Responder (i.e. Data 1714 Relay Client), and both of the Initiators claim the same inbound SPI 1715 at the Data Relay Server using PEER_PERMISSION Parameter. In this 1716 case, the Data Relay Server cannot disambiguate the correct 1717 destination of an ESP packet originating from the Data Relay Client 1718 because the SPI could belong to either of the peers (and destination 1719 IP and UDP port belonging to the Data Relay Server are not unique 1720 either). The recommended way and a contingency plan to solve this 1721 issue are described below. 1723 The recommend way to mitigate the problem is as follows. For each 1724 new Host Association, A Data Relay Client acting as a Responder 1725 SHOULD register a new server reflexive candidate as described in 1726 Section 4.2. Similarly, the Data Relay Server SHOULD NOT re-use the 1727 port numbers as described in Section 6.5. This way, each server 1728 reflexive candidate for the Data Relay Client has a separate UDP port 1729 that the Data Relay Server can use to disambiguate packet 1730 destinations in case of SPI collisions. 1732 When the Data Relay Client is not registering or failed to register a 1733 new relay candidate for a new peer, the Data Relay Client MUST follow 1734 a contingency plan as follows. Upon receiving an I2 with a colliding 1735 SPI, the Data Relay client acting as the Responder MUST NOT include 1736 the relayed address candidate in the R2 message because the Data 1737 Relay Server would not be able demultiplex the related ESP packet to 1738 the correct Initiator. The same applies also the handover 1739 procedures; the Data Relay Client MUST NOT include the relayed 1740 address candidate when sending its new locator set in an UPDATE to 1741 its peer if it would cause a SPI conflict with another peer. 1743 5. Packet Formats 1745 The following subsections define the parameter and packet encodings 1746 for the HIP and ESP packets. All values MUST be in network byte 1747 order. 1749 It is worth noting that most of the parameters are shown for the sake 1750 of completeness even though they are specified already in Legacy ICE- 1751 HIP [RFC5770]. New parameters are explicitly described as new. 1753 5.1. HIP Control Packets 1755 Figure 7 illustrates the packet format for UDP-encapsulated HIP. The 1756 format is identical to Legacy ICE-HIP [RFC5770]. 1758 0 1 2 3 1759 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1760 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1761 | Source Port | Destination Port | 1762 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1763 | Length | Checksum | 1764 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1765 | 32 bits of zeroes | 1766 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1767 | | 1768 ~ HIP Header and Parameters ~ 1769 | | 1770 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1772 Figure 7: Format of UDP-Encapsulated HIP Control Packets 1774 HIP control packets are encapsulated in UDP packets as defined in 1775 Section 2.2 of [RFC3948], "IKE Header Format for Port 4500", except 1776 that a different port number is used. Figure 7 illustrates the 1777 encapsulation. The UDP header is followed by 32 zero bits that can 1778 be used to differentiate HIP control packets from ESP packets. The 1779 HIP header and parameters follow the conventions of [RFC7401] with 1780 the exception that the HIP header checksum MUST be zero. The HIP 1781 header checksum is zero for two reasons. First, the UDP header 1782 already contains a checksum. Second, the checksum definition in 1783 [RFC7401] includes the IP addresses in the checksum calculation. The 1784 NATs that are unaware of HIP cannot recompute the HIP checksum after 1785 changing IP addresses. 1787 A Control/Data Relay Server or a non-relay Responder SHOULD listen at 1788 UDP port 10500 for incoming UDP-encapsulated HIP control packets. If 1789 some other port number is used, it needs to be known by potential 1790 Initiators. 1792 It is worth noting that UDP encapsulation of HIP packets reduces the 1793 Maximum Transfer Unit (MTU) size of the control plane by 12 bytes. 1795 5.2. Connectivity Checks 1797 HIP connectivity checks are HIP UPDATE packets. The format is 1798 specified in [RFC7401]. 1800 5.3. Keepalives 1802 The RECOMMENDED encoding format for keepalives is HIP NOTIFY packets 1803 as specified in [RFC7401] with Notify message type field set to 1804 NAT_KEEPALIVE [TBD by IANA: 16385] and with an empty Notification 1805 data field. It is worth noting that sending of such a HIP NOTIFY 1806 message SHOULD be omitted if the host is actively (or passively) 1807 sending some other traffic (HIP or ESP) to the peer host over the 1808 related UDP tunnel during the Tr period. For instance, the host MAY 1809 actively send ICMPv6 requests (or respond with an ICMPv6 response) 1810 inside the ESP tunnel to test the health of the associated IPsec 1811 security association. Alternatively, the host MAY use UPDATE packets 1812 as a substitute. A minimal UPDATE packet would consist of a SEQ and 1813 ECHO_REQ_SIGN parameters, and a more complex would involve rekeying 1814 procedures as specified in section 6.8 in [RFC7402]. It is worth 1815 noting that a host actively sending periodic UPDATE packets to a busy 1816 server may increase the computational load of the server since it has 1817 to verify HMACs and signatures in UPDATE messages. 1819 5.4. NAT Traversal Mode Parameter 1821 The format of NAT traversal mode parameter is borrowed from Legacy 1822 ICE-HIP [RFC5770]. The format of the NAT_TRAVERSAL_MODE parameter is 1823 similar to the format of the ESP_TRANSFORM parameter in [RFC7402] and 1824 is shown in Figure 8. The Native ICE-HIP extension specified in this 1825 document defines the new NAT traversal mode identifier for ICE-HIP- 1826 UDP and reuses the UDP-ENCAPSULATION mode from Legacy ICE-HIP 1827 [RFC5770]. The identifier named RESERVED is reserved for future use. 1828 Future specifications may define more traversal modes. 1830 0 1 2 3 1831 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1832 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1833 | Type | Length | 1834 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1835 | Reserved | Mode ID #1 | 1836 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1837 | Mode ID #2 | Mode ID #3 | 1838 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1839 | Mode ID #n | Padding | 1840 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1842 Type 608 1843 Length length in octets, excluding Type, Length, and padding 1844 Reserved zero when sent, ignored when received 1845 Mode ID defines the proposed or selected NAT traversal mode(s) 1847 The following NAT traversal mode IDs are defined: 1849 ID name Value 1850 RESERVED 0 1851 UDP-ENCAPSULATION 1 1852 ICE-STUN-UDP 2 1853 ICE-HIP-UDP 3 1855 Figure 8: Format of the NAT_TRAVERSAL_MODE Parameter 1857 The sender of a NAT_TRAVERSAL_MODE parameter MUST make sure that 1858 there are no more than six (6) Mode IDs in one NAT_TRAVERSAL_MODE 1859 parameter. Conversely, a recipient MUST be prepared to handle 1860 received NAT traversal mode parameters that contain more than six 1861 Mode IDs by accepting the first six Mode IDs and dropping the rest. 1862 The limited number of Mode IDs sets the maximum size of the 1863 NAT_TRAVERSAL_MODE parameter. The modes MUST be in preference order, 1864 most preferred mode(s) first. 1866 Implementations conforming to this specification MUST implement UDP- 1867 ENCAPSULATION and SHOULD implement ICE-HIP-UDP modes. 1869 5.5. Connectivity Check Transaction Pacing Parameter 1871 The TRANSACTION_PACING is defined in [RFC5770], but repeated in 1872 Figure 9 for completeness. It contains only the connectivity check 1873 pacing value, expressed in milliseconds, as a 32-bit unsigned 1874 integer. 1876 0 1 2 3 1877 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1878 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1879 | Type | Length | 1880 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1881 | Min Ta | 1882 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1884 Type 610 1885 Length 4 1886 Min Ta the minimum connectivity check transaction pacing 1887 value the host would use (in milliseconds) 1889 Figure 9: Format of the TRANSACTION_PACING Parameter 1891 5.6. Relay and Registration Parameters 1893 The format of the REG_FROM, RELAY_FROM, and RELAY_TO parameters is 1894 shown in Figure 10. All parameters are identical except for the 1895 type. Of the three, only REG_FROM is covered by the signature. 1897 0 1 2 3 1898 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1899 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1900 | Type | Length | 1901 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1902 | Port | Protocol | Reserved | 1903 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1904 | | 1905 | Address | 1906 | | 1907 | | 1908 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1910 Type REG_FROM: 950 1911 RELAY_FROM: 63998 1912 RELAY_TO: 64002 1913 Length 20 1914 Port transport port number; zero when plain IP is used 1915 Protocol IANA assigned, Internet Protocol number. 1916 17 for UDP, 0 for plain IP 1917 Reserved reserved for future use; zero when sent, ignored 1918 when received 1919 Address an IPv6 address or an IPv4 address in "IPv4-Mapped 1920 IPv6 address" format 1922 Figure 10: Format of the REG_FROM, RELAY_FROM, and RELAY_TO 1923 Parameters 1925 REG_FROM contains the transport address and protocol from which the 1926 Control Relay Server sees the registration coming. RELAY_FROM 1927 contains the address from which the relayed packet was received by 1928 the Control Relay Server and the protocol that was used. RELAY_TO 1929 contains the same information about the address to which a packet 1930 should be forwarded. 1932 5.7. LOCATOR_SET Parameter 1934 This specification reuses the format for UDP-based locators as 1935 specified in Legacy ICE-HIP [RFC5770] to be used for communicating 1936 the address candidates between two hosts. The generic and NAT- 1937 traversal-specific locator parameters are illustrated in Figure 11. 1939 0 1 2 3 1940 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1941 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1942 | Type | Length | 1943 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1944 | Traffic Type | Locator Type | Locator Length| Reserved |P| 1945 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1946 | Locator Lifetime | 1947 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1948 | Locator | 1949 | | 1950 | | 1951 | | 1952 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1953 . . 1954 . . 1955 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1956 | Traffic Type | Loc Type = 2 | Locator Length| Reserved |P| 1957 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1958 | Locator Lifetime | 1959 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1960 | Transport Port | Transp. Proto| Kind | 1961 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1962 | Priority | 1963 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1964 | SPI | 1965 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1966 | Address | 1967 | | 1968 | | 1969 | | 1970 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1972 Figure 11: LOCATOR_SET Parameter 1974 The individual fields in the LOCATOR_SET parameter are described in 1975 Table 1. 1977 +-----------+----------+--------------------------------------------+ 1978 | Field | Value(s) | Purpose | 1979 +-----------+----------+--------------------------------------------+ 1980 | Type | 193 | Parameter type | 1981 | Length | Variable | Length in octets, excluding Type and | 1982 | | | Length fields and padding | 1983 | Traffic | 0-2 | Is the locator for HIP signaling (1), for | 1984 | Type | | ESP (2), or for both (0) | 1985 | Locator | 2 | "Transport address" locator type | 1986 | Type | | | 1987 | Locator | 7 | Length of the fields after Locator | 1988 | Length | | Lifetime in 4-octet units | 1989 | Reserved | 0 | Reserved for future extensions | 1990 | Preferred | 0 or 1 | Set to 1 for a Locator in R1 if the | 1991 | (P) bit | | Responder can use it for the rest of the | 1992 | | | base exchange, otherwise set to zero | 1993 | Locator | Variable | Locator lifetime in seconds, see Section 4 | 1994 | Lifetime | | in [RFC8046] | 1995 | Transport | Variable | Transport layer port number | 1996 | Port | | | 1997 | Transport | Variable | IANA assigned, transport layer Internet | 1998 | Protocol | | Protocol number. Currently only UDP (17) | 1999 | | | is supported. | 2000 | Kind | Variable | 0 for host, 1 for server reflexive, 2 for | 2001 | | | peer reflexive (currently unused) or 3 for | 2002 | | | relayed address | 2003 | Priority | Variable | Locator's priority as described in | 2004 | | | [RFC8445]. It is worth noting that while | 2005 | | | the priority of a single locator candidate | 2006 | | | is 32-bits, but an implementation should | 2007 | | | use a 64-bit integer to calculate the | 2008 | | | priority of a candidate pair for the ICE | 2009 | | | priority algorithm. | 2010 | SPI | Variable | Security Parameter Index (SPI) value that | 2011 | | | the host expects to see in incoming ESP | 2012 | | | packets that use this locator | 2013 | Address | Variable | IPv6 address or an "IPv4-Mapped IPv6 | 2014 | | | address" format IPv4 address [RFC4291] | 2015 +-----------+----------+--------------------------------------------+ 2017 Table 1: Fields of the LOCATOR_SET Parameter 2019 5.8. RELAY_HMAC Parameter 2021 As specified in Legacy ICE-HIP [RFC5770], the RELAY_HMAC parameter 2022 value has the TLV type 65520. It has the same semantics as RVS_HMAC 2023 as specified in section 4.2.1 in [RFC8004]. Similarly as with 2024 RVS_HMAC, also RELAY_HMAC is keyed with the HIP integrity key (HIP-lg 2025 or HIP-gl as specified in section 6.5 in [RFC7401]), established 2026 during the relay registration procedure as described in Section 4.1. 2028 5.9. Registration Types 2030 The REG_INFO, REG_REQ, REG_RESP, and REG_FAILED parameters contain 2031 Registration Type [RFC8003] values for Control Relay Server 2032 registration. The value for RELAY_UDP_HIP is 2 as specified in 2033 Legacy ICE-HIP [RFC5770]. The value for RELAY_UDP_ESP is (value [TBD 2034 by IANA: 3]). 2036 5.10. Notify Packet Types 2038 A Control/Data Relay Server and end-hosts can use NOTIFY packets to 2039 signal different error conditions. The NOTIFY packet types are the 2040 same as in Legacy ICE-HIP [RFC5770] except for last one, which is 2041 new. 2043 The Notify Packet Types [RFC7401] are shown below. The Notification 2044 Data field for the error notifications SHOULD contain the HIP header 2045 of the rejected packet and SHOULD be empty for the 2046 CONNECTIVITY_CHECKS_FAILED type. 2048 NOTIFICATION PARAMETER - ERROR TYPES Value 2049 ------------------------------------ ----- 2051 NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER 60 2053 If a Control Relay Server does not forward a base exchange packet 2054 due to missing NAT traversal mode parameter, or the Initiator 2055 selects a NAT traversal mode that the (non-relay) Responder did 2056 not expect, the Control Relay Server or the Responder may send 2057 back a NOTIFY error packet with this type. 2059 CONNECTIVITY_CHECKS_FAILED 61 2061 Used by the end-hosts to signal that NAT traversal connectivity 2062 checks failed and did not produce a working path. 2064 MESSAGE_NOT_RELAYED 62 2066 Used by a Control Relay Server to signal that is was not able or 2067 willing to relay a HIP packet. 2069 SERVER_REFLEXIVE_CANDIDATE_ALLOCATION_FAILED 63 2070 Used by a Data Relay Server to signal that is was not able or 2071 willing to allocate a new server reflexive candidate for the Data 2072 Relay Client 2074 5.11. ESP Data Packets 2076 The format for ESP data packets is identical to Legacy ICE-HIP 2077 [RFC5770]. 2079 [RFC3948] describes the UDP encapsulation of the IPsec ESP transport 2080 and tunnel mode. On the wire, the HIP ESP packets do not differ from 2081 the transport mode ESP, and thus the encapsulation of the HIP ESP 2082 packets is same as the UDP encapsulation transport mode ESP. 2083 However, the (semantic) difference to Bound End-to-End Tunnel (BEET) 2084 mode ESP packets used by HIP is that IP header is not used in BEET 2085 integrity protection calculation. 2087 During the HIP base exchange, the two peers exchange parameters that 2088 enable them to define a pair of IPsec ESP security associations (SAs) 2089 as described in [RFC7402]. When two peers perform a UDP-encapsulated 2090 base exchange, they MUST define a pair of IPsec SAs that produces 2091 UDP-encapsulated ESP data traffic. 2093 The management of encryption/authentication protocols and SPIs is 2094 defined in [RFC7402]. The UDP encapsulation format and processing of 2095 HIP ESP traffic is described in Section 6.1 of [RFC7402]. 2097 It is worth noting that UDP encapsulation of ESP reduces the MTU size 2098 of data plane by 8 bytes. 2100 5.12. RELAYED_ADDRESS and MAPPED_ADDRESS Parameters 2102 While the type values are new, the format of the RELAYED_ADDRESS and 2103 MAPPED_ADDRESS parameters (Figure 12) is identical to REG_FROM, 2104 RELAY_FROM and RELAY_TO parameters. This document specifies only the 2105 use of UDP relaying, and, thus, only protocol 17 is allowed. 2106 However, future documents may specify support for other protocols. 2108 0 1 2 3 2109 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2110 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2111 | Type | Length | 2112 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2113 | Port | Protocol | Reserved | 2114 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2115 | | 2116 | Address | 2117 | | 2118 | | 2119 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2121 Type [TBD by IANA; 2122 RELAYED_ADDRESS: 4650 2123 MAPPED_ADDRESS: 4660] 2124 Length 20 2125 Port the UDP port number 2126 Protocol IANA assigned, Internet Protocol number (17 for UDP) 2127 Reserved reserved for future use; zero when sent, ignored 2128 when received 2129 Address an IPv6 address or an IPv4 address in "IPv4-Mapped 2130 IPv6 address" format 2132 Figure 12: Format of the RELAYED_ADDRESS and MAPPED_ADDRESS 2133 Parameters 2135 5.13. PEER_PERMISSION Parameter 2137 The format of the new PEER_PERMISSION parameter is shown in 2138 Figure 13. The parameter is used for setting up and refreshing 2139 forwarding rules and the permissions for data packets at the Data 2140 Relay Server. The parameter contains one or more sets of Port, 2141 Protocol, Address, Outbound SPI (OSPI), and Inbound SPI (ISPI) 2142 values. One set defines a rule for one peer address. 2144 0 1 2 3 2145 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2146 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2147 | Type | Length | 2148 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2149 | RPort | PPort | 2150 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2151 | Protocol | Reserved | 2152 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2153 | | 2154 | RAddress | 2155 | | 2156 | | 2157 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2158 | | 2159 | PAddress | 2160 | | 2161 | | 2162 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2163 | OSPI | 2164 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2165 | ISPI | 2166 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2168 Type [TBD by IANA; 4680] 2169 Length 48 2170 RPort the transport layer (UDP) port at the Data Relay Server 2171 (i.e. the port of the server reflexive candidate) 2172 PPort the transport layer (UDP) port number of the peer 2173 Protocol IANA assigned, Internet Protocol number (17 for UDP) 2174 Reserved reserved for future use; zero when sent, ignored 2175 when received 2176 RAddress an IPv6 address, or an IPv4 address in "IPv4-Mapped 2177 IPv6 address" format, of the server reflexive candidate 2178 PAddress an IPv6 address, or an IPv4 address in "IPv4-Mapped 2179 IPv6 address" format, of the peer 2180 OSPI the outbound SPI value the Data Relay Client is using for 2181 the peer 2182 ISPI the inbound SPI value the Data Relay Client is using for 2183 the peer 2185 Figure 13: Format of the PEER_PERMISSION Parameter 2187 5.14. HIP Connectivity Check Packets 2189 The connectivity request messages are HIP UPDATE packets containing a 2190 new CANDIDATE_PRIORITY parameter (Figure 14). Response UPDATE 2191 packets contain a MAPPED_ADDRESS parameter (Figure 12). 2193 0 1 2 3 2194 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2195 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2196 | Type | Length | 2197 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2198 | Priority | 2199 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2201 Type [TBD by IANA; 4700] 2202 Length 4 2203 Priority the priority of a (potential) peer reflexive candidate 2205 Figure 14: Format of the CANDIDATE_PRIORITY Parameter 2207 5.15. NOMINATE parameter 2209 Figure 15 shows the NOMINATE parameter that is used to conclude the 2210 candidate nomination process. 2212 0 1 2 3 2213 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2214 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2215 | Type | Length | 2216 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2217 | Reserved | 2218 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2220 Type [TBD by IANA; 4710] 2221 Length 4 2222 Reserved Reserved for future extension purposes 2224 Figure 15: Format of the NOMINATE Parameter 2226 6. Security Considerations 2228 Since the control plane protocol and Control Relay Server are 2229 essentially the same (with some minor differences) in this document 2230 as in Legacy ICE-HIP [RFC5770], the same security considerations (in 2231 Section 6.1, Section 6.2, Section 6.3 and Section 6.4,) are still 2232 valid, but are repeated here for the sake of completeness. New 2233 security considerations related to the new Data Relay Server are 2234 discussed in Section 6.5, and considerations related to the new 2235 connectivity check protocol are discussed in Section 6.6 and 2236 Section 6.7. 2238 6.1. Privacy Considerations 2240 It is also possible that end-users may not want to reveal all 2241 locators to each other. For example, tracking the physical location 2242 of a multihoming end-host may become easier if it reveals all 2243 locators to its peer during a base exchange. Also, revealing host 2244 addresses exposes information about the local topology that may not 2245 be allowed in all corporate environments. For these two local policy 2246 reasons, an end-host MAY exclude certain host addresses from its 2247 LOCATOR_SET parameter, but this requires further experimentation. 2248 However, such behavior creates non-optimal paths when the hosts are 2249 located behind the same NAT. Especially, this could be problematic 2250 with a legacy NAT that does not support routing from the private 2251 address realm back to itself through the outer address of the NAT. 2252 This scenario is referred to as the hairpin problem [RFC5128]. With 2253 such a legacy NAT, the only option left would be to use a relayed 2254 transport address from an Control Relay Server server. 2256 The use of Control and Data Relay Servers can be also useful for 2257 privacy purposes. For example, a privacy concerned Responder may 2258 reveal only its Control Relay Server and Relayed candidates to 2259 Initiators. This partially protects the Responder against Denial-of- 2260 Service (DoS) attacks by allowing the Responder to initiate new 2261 connections even if its relays would be unavailable due to a DoS 2262 attack. 2264 6.2. Opportunistic Mode 2266 In opportunistic HIP mode (cf. Section 4.1.8 in [RFC7401]), an 2267 Initiator sends an I1 with without setting the destination HIT of the 2268 Responder (i.e. the Control Relay Client). A Control Relay Server 2269 SHOULD have a unique IP address per Control Relay Client when the 2270 Control Relay Server is serving more than one Control Relay Client 2271 and supports opportunistic mode. Otherwise, the Control Relay Server 2272 cannot guarantee to deliver the I1 packet to the intended recipient. 2273 Future extensions of this document may allow opportunistic mode to be 2274 used with non-unique IP addresses to be utilized either as a HIP- 2275 level anycast or multicast mechanism. Both of the mentioned cases 2276 would require a separate registration parameters that the Control 2277 Relay Server proposes and the Control Client Server accepts during 2278 registration. 2280 6.3. Base Exchange Replay Protection for Control Relay Server 2282 In certain scenarios, it is possible that an attacker, or two 2283 attackers, can replay an earlier base exchange through a Control 2284 Relay Server by masquerading as the original Initiator and Responder. 2285 The attack does not require the attacker(s) to compromise the private 2286 key(s) of the attacked host(s). However, for this attack to succeed, 2287 the legitimate Responder has to be disconnected from the Control 2288 Relay Server. 2290 The Control Relay Server can protect itself against replay attacks by 2291 becoming involved in the base exchange by introducing nonces that the 2292 end-hosts (Initiator and Responder) are required to sign. One way to 2293 do this is to add ECHO_REQUEST_M parameters to the R1 and I2 packets 2294 as described in [HIP-MIDDLE] and drop the I2 or R2 packets if the 2295 corresponding ECHO_RESPONSE_M parameters are not present. 2297 6.4. Demultiplexing Different HIP Associations 2299 Section 5.1 of [RFC3948] describes a security issue for the UDP 2300 encapsulation in the standard IP tunnel mode when two hosts behind 2301 different NATs have the same private IP address and initiate 2302 communication to the same Responder in the public Internet. The 2303 Responder cannot distinguish between two hosts, because security 2304 associations are based on the same inner IP addresses. 2306 This issue does not exist with the UDP encapsulation of HIP ESP 2307 transport format because the Responder uses HITs to distinguish 2308 between different Initiators. 2310 6.5. Reuse of Ports at the Data Relay Server 2312 If the Data Relay Server uses the same relayed address and port (as 2313 conveyed in the RELAYED_ADDRESS parameter) for multiple Data Relay 2314 Clients, it appears to all the peers, and their firewalls, that all 2315 the Data Relay Clients are at the same address. Thus, a stateful 2316 firewall may allow packets pass from hosts that would not normally be 2317 able to send packets to a peer behind the firewall. Therefore, a 2318 Data Relay Server SHOULD NOT re-use the port numbers. If port 2319 numbers need to be re-used, the Data Relay Server SHOULD have a 2320 sufficiently large pool of port numbers and select ports from the 2321 pool randomly to decrease the chances of a Data Relay Client 2322 obtaining the same address that a another host behind the same 2323 firewall is using. 2325 6.6. Amplification attacks 2327 A malicious host may send an invalid list of candidates to its peer 2328 that are used for targeting a victim host by flooding it with 2329 connectivity checks. To mitigate the attack, this protocol adopts 2330 the ICE mechanism to cap the total amount of connectivity checks as 2331 defined in Section 4.7. 2333 6.7. Attacks against Connectivity Checks and Candidate Gathering 2335 Section 19.2 in [RFC8445] describes attacks against ICE connectivity 2336 checks. HIP bases its control plane security on Diffie-Hellman key 2337 exchange, public keys and Hashed Message Authentication codes, 2338 meaning that the mentioned security concerns do not apply to HIP 2339 either. The mentioned section discusses also of man-in-the-middle 2340 replay attacks that are difficult to prevent. The connectivity 2341 checks in this protocol are immune against replay attacks because a 2342 connectivity request includes a random nonce that the recipient must 2343 sign and send back as a response. 2345 Section 19.3 in [RFC8445] describes attacks on server reflexive 2346 address gathering. Similarly here, if the DNS, a Control Relay 2347 Server or a Data Relay Server has been compromised, not much can be 2348 done. However, the case where attacker can inject fake messages 2349 (located on a shared network segment like Wifi) does not apply here. 2350 HIP messages are integrity and replay protected, so it is not 2351 possible inject fake server reflexive address candidates. 2353 Section 19.4 in [RFC8445] describes attacks on relayed candidate 2354 gathering. Similarly to ICE TURN servers, Data Relay Server require 2355 an authenticated base exchange that protects relayed address 2356 gathering against fake requests and responses. Further, replay 2357 attacks are not possible because the HIP base exchange (and also 2358 UPDATE procedure) is protected against replay attacks. 2360 7. IANA Considerations 2362 This section is to be interpreted according to [RFC8126]. 2364 This document reuses the same default UDP port number 10500 as 2365 specified by Legacy ICE-HIP [RFC5770] for tunneling both HIP control 2366 plane and data plane traffic. IANA is requested to add a reference 2367 to this document in the entry for UDP port 10500 in the Transport 2368 Protocol Port Number Registry. The selection between Legacy ICE-HIP 2369 and Native ICE-HIP mode is negotiated using NAT_TRAVERSAL_MODE 2370 parameter during the base exchange. By default, hosts listen this 2371 port for incoming UDP datagrams and can use it also for sending UDP 2372 datagrams. Other emphemeral port numbers are negotiated and utilized 2373 dynamically. 2375 This document updates the IANA Registry for HIP Parameter Types 2376 [RFC7401] by assigning new HIP Parameter Type values for the new HIP 2377 Parameters: RELAYED_ADDRESS (length 20), MAPPED_ADDRESS (length 20, 2378 defined in Section 5.12), PEER_PERMISSION (length 48, defined in 2379 Section 5.13), CANDIDATE_PRIORITY (length 4, defined in Section 5.14) 2380 and NOMINATE (length 4, defined in Section 5.15). 2382 This document updates the IANA Registry for HIP NAT traversal modes 2383 specified in Legacy ICE-HIP [RFC5770] by assigning value for the NAT 2384 traversal mode ICE-HIP-UDP (defined in Section 5.4). 2386 This document updates the IANA Registry for HIP Notify Message Types: 2387 type field NAT_KEEPALIVE in Section 5.3 and a new error type 2388 SERVER_REFLEXIVE_CANDIDATE_ALLOCATION_FAILED in Section 5.10. 2390 This document defines additional registration types for the HIP 2391 Registration Extension [RFC8003] that allow registering with a Data 2392 Relay Server for ESP relaying service: RELAY_UDP_ESP (defined in 2393 Section 5.9, and performing server reflexive candidate discovery: 2394 CANDIDATE_DISCOVERY (defined in Section 4.2). 2396 ICE specification [RFC8445] discusses "Unilateral Self-Address 2397 Fixing" in section 18. This protocol is based on ICE, and thus the 2398 same considerations apply also here. 2400 8. Contributors 2402 Marcelo Bagnulo, Philip Matthews and Hannes Tschofenig have 2403 contributed to [RFC5770]. This document leans heavily on the work in 2404 the RFC. 2406 9. Acknowledgments 2408 Thanks to Jonathan Rosenberg, Christer Holmberg and the rest of the 2409 MMUSIC WG folks for the excellent work on ICE. In addition, the 2410 authors would like to thank Andrei Gurtov, Simon Schuetz, Martin 2411 Stiemerling, Lars Eggert, Vivien Schmitt, and Abhinav Pathak for 2412 their contributions and Tobias Heer, Teemu Koponen, Juhana Mattila, 2413 Jeffrey M. Ahrenholz, Kristian Slavov, Janne Lindqvist, Pekka 2414 Nikander, Lauri Silvennoinen, Jukka Ylitalo, Juha Heinanen, Joakim 2415 Koskela, Samu Varjonen, Dan Wing, Tom Henderson, Alex Elsayed and 2416 Jani Hautakorpi for their comments to [RFC5770], which is the basis 2417 for this document. 2419 This work has been partially funded by CyberTrust programme by 2420 Digile/Tekes in Finland. 2422 10. References 2424 10.1. Normative References 2426 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2427 Requirement Levels", BCP 14, RFC 2119, 2428 DOI 10.17487/RFC2119, March 1997, 2429 . 2431 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2432 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2433 May 2017, . 2435 [RFC7401] Moskowitz, R., Ed., Heer, T., Jokela, P., and T. 2436 Henderson, "Host Identity Protocol Version 2 (HIPv2)", 2437 RFC 7401, DOI 10.17487/RFC7401, April 2015, 2438 . 2440 [RFC8003] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP) 2441 Registration Extension", RFC 8003, DOI 10.17487/RFC8003, 2442 October 2016, . 2444 [RFC8004] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP) 2445 Rendezvous Extension", RFC 8004, DOI 10.17487/RFC8004, 2446 October 2016, . 2448 [RFC8046] Henderson, T., Ed., Vogt, C., and J. Arkko, "Host Mobility 2449 with the Host Identity Protocol", RFC 8046, 2450 DOI 10.17487/RFC8046, February 2017, 2451 . 2453 [RFC8047] Henderson, T., Ed., Vogt, C., and J. Arkko, "Host 2454 Multihoming with the Host Identity Protocol", RFC 8047, 2455 DOI 10.17487/RFC8047, February 2017, 2456 . 2458 [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, 2459 "Session Traversal Utilities for NAT (STUN)", RFC 5389, 2460 DOI 10.17487/RFC5389, October 2008, 2461 . 2463 [RFC7402] Jokela, P., Moskowitz, R., and J. Melen, "Using the 2464 Encapsulating Security Payload (ESP) Transport Format with 2465 the Host Identity Protocol (HIP)", RFC 7402, 2466 DOI 10.17487/RFC7402, April 2015, 2467 . 2469 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 2470 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2471 2006, . 2473 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2474 Writing an IANA Considerations Section in RFCs", BCP 26, 2475 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2476 . 2478 [RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive 2479 Connectivity Establishment (ICE): A Protocol for Network 2480 Address Translator (NAT) Traversal", RFC 8445, 2481 DOI 10.17487/RFC8445, July 2018, 2482 . 2484 [RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using 2485 Relays around NAT (TURN): Relay Extensions to Session 2486 Traversal Utilities for NAT (STUN)", RFC 5766, 2487 DOI 10.17487/RFC5766, April 2010, 2488 . 2490 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful 2491 NAT64: Network Address and Protocol Translation from IPv6 2492 Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146, 2493 April 2011, . 2495 [RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van 2496 Beijnum, "DNS64: DNS Extensions for Network Address 2497 Translation from IPv6 Clients to IPv4 Servers", RFC 6147, 2498 DOI 10.17487/RFC6147, April 2011, 2499 . 2501 [RFC7050] Savolainen, T., Korhonen, J., and D. Wing, "Discovery of 2502 the IPv6 Prefix Used for IPv6 Address Synthesis", 2503 RFC 7050, DOI 10.17487/RFC7050, November 2013, 2504 . 2506 10.2. Informative References 2508 [RFC5770] Komu, M., Henderson, T., Tschofenig, H., Melen, J., and A. 2509 Keranen, Ed., "Basic Host Identity Protocol (HIP) 2510 Extensions for Traversal of Network Address Translators", 2511 RFC 5770, DOI 10.17487/RFC5770, April 2010, 2512 . 2514 [RFC4423] Moskowitz, R. and P. Nikander, "Host Identity Protocol 2515 (HIP) Architecture", RFC 4423, DOI 10.17487/RFC4423, May 2516 2006, . 2518 [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., 2519 and W. Weiss, "An Architecture for Differentiated 2520 Services", RFC 2475, DOI 10.17487/RFC2475, December 1998, 2521 . 2523 [RFC5207] Stiemerling, M., Quittek, J., and L. Eggert, "NAT and 2524 Firewall Traversal Issues of Host Identity Protocol (HIP) 2525 Communication", RFC 5207, DOI 10.17487/RFC5207, April 2526 2008, . 2528 [RFC6538] Henderson, T. and A. Gurtov, "The Host Identity Protocol 2529 (HIP) Experiment Report", RFC 6538, DOI 10.17487/RFC6538, 2530 March 2012, . 2532 [MMUSIC-ICE] 2533 Rosenberg, J., "Guidelines for Usage of Interactive 2534 Connectivity Establishment (ICE) by non Session Initiation 2535 Protocol (SIP) Protocols", Work in Progress, July 2008. 2537 [RFC5128] Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to- 2538 Peer (P2P) Communication across Network Address 2539 Translators (NATs)", RFC 5128, DOI 10.17487/RFC5128, March 2540 2008, . 2542 [HIP-MIDDLE] 2543 Heer, T., Wehrle, K., and M. Komu, "End-Host 2544 Authentication for HIP Middleboxes", Work in Progress, 2545 February 2009. 2547 [RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M. 2548 Stenberg, "UDP Encapsulation of IPsec ESP Packets", 2549 RFC 3948, DOI 10.17487/RFC3948, January 2005, 2550 . 2552 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 2553 with Session Description Protocol (SDP)", RFC 3264, 2554 DOI 10.17487/RFC3264, June 2002, 2555 . 2557 [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment 2558 (ICE): A Protocol for Network Address Translator (NAT) 2559 Traversal for Offer/Answer Protocols", RFC 5245, 2560 DOI 10.17487/RFC5245, April 2010, 2561 . 2563 Appendix A. Selecting a Value for Check Pacing 2565 Selecting a suitable value for the connectivity check transaction 2566 pacing is essential for the performance of connectivity check-based 2567 NAT traversal. The value should not be so small that the checks 2568 cause network congestion or overwhelm the NATs. On the other hand, a 2569 pacing value that is too high makes the checks last for a long time, 2570 thus increasing the connection setup delay. 2572 The Ta value may be configured by the user in environments where the 2573 network characteristics are known beforehand. However, if the 2574 characteristics are not known, it is recommended that the value is 2575 adjusted dynamically. In this case, it is recommended that the hosts 2576 estimate the round-trip time (RTT) between them and SHOULD set the 2577 minimum Ta value so that at most a single connectivity check message 2578 is sent on every RTT. 2580 One way to estimate the RTT is to use the time that it takes for the 2581 Control Relay Server registration exchange to complete; this would 2582 give an estimate on the registering host's access link's RTT. Also, 2583 the I1/R1 exchange could be used for estimating the RTT, but since 2584 the R1 can be cached in the network, or the relaying service can 2585 increase the delay notably, this is not recommended. In general, 2586 estimating RTT can be difficult and error prone; further 2587 experimentation is required for reliable RTT estimation. 2589 Appendix B. Differences with respect to ICE 2591 Legacy ICE-HIP reuses ICE/STUN/TURN protocol stack as it is. The 2592 benefits of such as an approach include the reuse of STUN/TURN 2593 infrastructure and possibly the reuse of existing software libraries, 2594 but there are also drawbacks with the approach. For example, ICE is 2595 meant for application-layer protocols, whereas HIP operates at layer 2596 3.5 between transport and network layers. This is particularly 2597 problematic because the implementations employ kernelspace IPsec ESP 2598 as their data plane: demultiplexing of incoming ESP, HIP and TURN 2599 messages required capturing of all UDP packets destined to port 10500 2600 to the userspace (due to different, incompatible markers in ESP and 2601 STUN), thus causing additional software complexity and an unnecessary 2602 latency/throughput bottleneck for the dataplane performance. It is 2603 also worth noting that demultiplexing of STUN packets in the kernel 2604 would incur an also a performance impact (albeit smaller than with 2605 userspace demultiplexing), and secure verification of STUN messages 2606 would require communication between the kernelspace STUN detector and 2607 HIP daemon typically residing in the userspace (thus, again 2608 increasing the performance overhead). 2610 Legacy ICE-HIP involves also some other complexities when compared to 2611 the approach taken in this document. Relaying of ESP packets via 2612 TURN relays was not considered that simple because TURN relays 2613 require adding and removing extra TURN framing for the relayed 2614 packets. Finally, the developers of the two Legacy ICE-HIP 2615 implementations concluded that "effort needed for integrating an ICE 2616 library into a HIP implementation turned out to be quite a bit higher 2617 that initially estimated. Also, the amount of extra code (some 10 2618 kLoC) needed for all the new parsers, state machines, etc., is quite 2619 high and by re-using the HIP code one should be able to do with much 2620 less. This should result in smaller binary size, less bugs, and 2621 easier debugging.". Consequently, the HIP working group decided to 2622 follow ICE methodology but reuse HIP messaging format to achieve the 2623 same functionality as ICE, and consequently the result is this 2624 document that specifies the Native ICE-HIP protocol. 2626 The Native ICE-HIP protocol specified in this document follows the 2627 semantics of ICE as close as possible, and most of the differences 2628 are syntactical due to the use of a different protocol. In this 2629 section, we describe the differences to the ICE protocol. 2631 o ICE operates at the application layer, whereas this protocol 2632 operates between transport and network layers, thus hiding the 2633 protocol details from the application. 2635 o The STUN protocol is not employed. Instead, native ICE-HIP reuses 2636 the HIP control plane format in order simplify demultiplexing of 2637 different protocols. For example, the STUN binding response is 2638 replaced with a HIP UPDATE message containing an 2639 ECHO_REQUEST_SIGNED parameter and the STUN binding response with a 2640 HIP UPDATE message containing an ECHO_RESPONSE_SIGNED parameter as 2641 defined in Section 4.6. It is worth noting that a drawback of not 2642 employing STUN is that discovery of the address candidates 2643 requires creating (using HIP base exchange) and maintaining (using 2644 HIP UPDATE procedures) state at the Control Relay Client and 2645 Control Relay Server. Future extensions to this document may 2646 define a stateless, HIP-specific mechanism for an end-host to 2647 discover its address candidates. 2649 o The TURN protocol is not utilized. Instead, native ICE-HIP reuses 2650 Control Relay Servers for the same purpose. 2652 o ICMP errors may be used in ICE to signal failure. In Native ICE- 2653 HIP protocol, HIP NOTIFY messages are used instead. 2655 o Instead of the ICE username fragment and password mechanism for 2656 credentials, native ICE-HIP uses the HIT, derived from a public 2657 key, for the same purpose. The username fragments are "transient 2658 host identifiers, bound to a particular session established as 2659 part of the candidate exchange" [RFC8445]. Generally in HIP, a 2660 local public key and the derived HIT are considered long-term 2661 identifiers, and invariant across different host associations and 2662 different transport-layer flows. 2664 o In ICE, the conflict when two communicating end-points take the 2665 same controlling role is solved using random values (so called 2666 tie-breaker value). In Native ICE-HIP protocol, the conflict is 2667 solved by the standard HIP base exchange procedure, where the host 2668 with the "larger" HIT switches to Responder role, thus changing 2669 also to controlled role. 2671 o The ICE-CONTROLLED and ICE-CONTROLLING attributes are not included 2672 in the connectivity checks. 2674 o The foundation concept is unnecessary in native ICE-HIP because 2675 only a single UDP flow for the IPsec tunnel will be negotiated. 2677 o Frozen candidates are omitted for the same reason as foundation 2678 concept is excluded. 2680 o Components are omitted for the same reason as foundation concept 2681 is excluded. 2683 o Native ICE-HIP supports only "full ICE" where the two 2684 communicating hosts participate actively to the connectivity 2685 checks, and the "lite" mode is not supported. This design 2686 decision follows the guidelines of ICE which recommends full ICE 2687 implementations. However, it should be noted that a publicly 2688 reachable Responder may refuse to negotiate the ICE mode as 2689 described in Section 4.7.2. This would result in a [RFC7401] 2690 based HIP base exchange tunneled over UDP followed ESP traffic 2691 over the same tunnel, without the connectivity check procedures 2692 defined in this document (in some sense, this mode corresponds to 2693 the case where two ICE lite implementations connect since no 2694 connectivity checks are sent). 2696 o As the "ICE lite" is not adopted here and both sides are capable 2697 of ICE-HIP-UDP mode (negotiated during the base exchange), default 2698 candidates are not employed in Native ICE-HIP. 2700 o If the agent is using Diffserv Codepoint markings [RFC2475] in its 2701 media packets, it SHOULD apply those same markings to its 2702 connectivity checks. 2704 o Unlike in ICE, the addresses are not XOR-ed in Native ICE-HIP 2705 protocol but rather encrypted to avoid middlebox tampering. 2707 o Native ICE-HIP protocol does not employ the ICE related address 2708 and related port attributes (that are used for diagnostic or SIP 2709 purposes). 2711 Appendix C. Differences to Base Exchange and UPDATE procedures 2713 This section gives some design guidance for implementers how the 2714 extensions in this protocol extend and differ from [RFC7401] and 2715 [RFC8046]. 2717 o Both control and data plane are operated on top of UDP, not 2718 directly on IP. 2720 o A minimal implementation would conform only to Section 4.7.1 or 2721 Section 4.7.2, thus merely tunneling HIP control and data traffic 2722 over UDP. The drawback here is that it works only in the limited 2723 cases where the Responder has a public address. 2725 o It is worth noting that while a rendezvous server [RFC8004] has 2726 not been designed to be used in NATted scenarios because it just 2727 relays the first I1 packet and does not employ UDP encapsulation, 2728 the Control Relay Server forwards all control traffic and, hence, 2729 is more suitable in NATted environments. Further, the Data Relay 2730 Server guarantees forwarding of data plane traffic also in the 2731 cases when the NAT traversal procedures fail. 2733 o Registration procedures with a Control/Data Relay Server are 2734 similar as with rendezvous server. However, a Control/Data Relay 2735 Server has different registration parameters than rendezvous 2736 because it offers a different service. Also, the Control/Data 2737 Relay Server includes also a REG_FROM parameter that informs the 2738 Control/Data Relay Client about its server reflexive address. A 2739 Data Relay Server includes also a RELAYED_ADDRESS containing the 2740 relayed address for the Data Relay Client. 2742 o In [RFC7401], the Initiator and Responder can start to exchange 2743 application payload immediately after the base exchange. While 2744 exchanging data immediately after a base exchange via a Data 2745 Control Relay would be possible also here, we follow the ICE 2746 methodology to establish a direct path between two hosts using 2747 connectivity checks. This means that there will be some 2748 additional delay after the base exchange before application 2749 payload can be transmitted. The same applies for the UPDATE 2750 procedure as the connectivity checks introduce some additional 2751 delay. 2753 o In HIP without any NAT traversal support, the base exchange acts 2754 as an implicit connectivity check, and the mobility and 2755 multihoming extensions support explicit connectivity checks. 2756 After a base exchange or UPDATE based connectivity checks, a host 2757 can use the associated address pair for transmitting application 2758 payload. In this Native ICE-HIP extension, we follow the ICE 2759 methodology, where one end-point acting in the controlled role 2760 chooses the used address pair also on behalf of the other end- 2761 point acting in controlled role, which is different from HIP 2762 without NAT traversal support. Another difference is that the 2763 process of choosing an address pair is explicitly signaled using 2764 the nomination packets. The nomination process in this protocol 2765 supports only single address pair, and multihoming extensions are 2766 left for further study. 2768 o The UPDATE procedure resembles the mobility extensions defined in 2769 [RFC8046]. The first UPDATE message from the mobile host is 2770 exactly the same as in the mobility extensions. The second UPDATE 2771 message from the peer host and third from the mobile host are 2772 different in the sense that they merely acknowledge and conclude 2773 the reception of the candidates through the Control Relay Server. 2774 In other words, they do not yet test for connectivity (besides 2775 reachability through the Control Relay Server) unlike in the 2776 mobility extensions. The idea is that connectivity check 2777 procedure follows the ICE specification, which is somewhat 2778 different from the HIP mobility extensions. 2780 o The connectivity checks as defined in the mobility extensions 2781 [RFC8046] are triggered only by the peer of the mobile host. 2782 Since successful NAT traversal requires that both end-points test 2783 connectivity, both the mobile host and its peer host have to test 2784 for connectivity. In addition, this protocol validates also the 2785 UDP ports; the ports in the connectivity check must match with the 2786 response, as required by ICE. 2788 o In HIP mobility extensions [RFC8046], an outbound locator has some 2789 associated state: UNVERIFIED mean that the locator has not been 2790 tested for reachability, ACTIVE means that the address has been 2791 verified for reachability and is being used actively, and 2792 DEPRECATED means that the locator lifetime has expired. In the 2793 subset of ICE specifications used by this protocol, an individual 2794 address candidate has only two properties: type and priority. 2795 Instead, the actual state in ICE is associated with candidate 2796 pairs rather than individual addresses. The subset of ICE 2797 specifications utilized by this protocol require the following 2798 attributes for a candidate pair: valid bit, nominated bit, base 2799 and the state of connectivity check. The connectivity checks have 2800 the following states: Waiting, In-progress, Succeeded and Failed. 2801 Handling of this state attribute requires some additional logic 2802 when compared to the mobility extensions since the state is 2803 associated with a local-remote address pair rather just a remote 2804 address, and, thus, the mobility and ICE states do not have an 2805 unambiguous one-to-one mapping. 2807 o Credit-based authorization as defined in [RFC8046] could be used 2808 before candidate nomination has been concluded upon discovering 2809 working candidate pairs. However, this may result in the use of 2810 asymmetric paths for a short time period in the beginning of 2811 communications. Thus, support of credit-based authorization is 2812 left for further study. 2814 Appendix D. Multihoming Considerations 2816 This document allows a host to collect address candidates from 2817 multiple interfaces, but does not support activation and the 2818 simultaneous use of multiple address candidates. While multihoming 2819 extensions to support [RFC8047] like functionality are left for 2820 further study and experimentation, we envision here some potential 2821 compatibility improvements to support multihoming: 2823 o Data Relay Registration: a Data Relay Client acting as an 2824 Initiator with another peer host should register a new server 2825 reflexive candidate for each local transport address candidate. A 2826 Data Relay Client acting as an Responder should register a new 2827 server reflexive candidate for each { local transport address 2828 candidate, new peer host} pair for the reasons described in 2829 Section 4.12.3. In both cases, the Data Relay Client should 2830 request the additional server reflexive candidates by sending 2831 UPDATE messages originating from each of the local address 2832 candidates as described in Section 4.1. As the UPDATE messages 2833 are originating from an unknown location from the viewpoint of the 2834 Data Relay Server, it must include also a ECHO_REQUEST_SIGNED in 2835 the response in order to test for return routability. 2837 o Data Relay unregistration: this follows the procedure in Section 4 2838 but the Data Relay Client should unregister using the particular 2839 transport address to be unregistered. All transport address pair 2840 registrations can be unregistered when no RELAYED_ADDRESS 2841 parameter is included. 2843 o PEER_PERMISSION parameter: this needs to be extended or an 2844 additional parameter is needed to declare the specific local 2845 candidate of the Data Relay Client. Alternatively, the use of the 2846 PEER_PERMISSION could be used as a wild card to open permissions 2847 for a specific peer to all of the candidates of the Data Relay 2848 Client. 2850 o Connectivity checks: the controlling host should be able to 2851 nominate multiple candidates (by repeating step 7 in Figure 5 in 2852 Section 4.6 using the additional candidate pairs). 2854 o Keepalives should be sent for all the nominated candidate pairs. 2855 Similarly, the Control/Data Relay Client should send keepalives 2856 from its local candidates to its Control/Data Relay Server 2857 transport addresses. 2859 Authors' Addresses 2861 Ari Keranen 2862 Ericsson 2863 Hirsalantie 11 2864 02420 Jorvas 2865 Finland 2867 Email: ari.keranen@ericsson.com 2869 Jan Melen 2870 Ericsson 2871 Hirsalantie 11 2872 02420 Jorvas 2873 Finland 2875 Email: jan.melen@ericsson.com 2877 Miika Komu (editor) 2878 Ericsson 2879 Hirsalantie 11 2880 02420 Jorvas 2881 Finland 2883 Email: miika.komu@ericsson.com