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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: June 23, 2018 Ericsson 6 December 20, 2017 8 Native NAT Traversal Mode for the Host Identity Protocol 9 draft-ietf-hip-native-nat-traversal-27 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 for all NAT traversal procedures. 20 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on June 23, 2018. 37 Copyright Notice 39 Copyright (c) 2017 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 55 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 56 3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 7 57 4. Protocol Description . . . . . . . . . . . . . . . . . . . . 9 58 4.1. Relay Registration . . . . . . . . . . . . . . . . . . . 9 59 4.2. Transport Address Candidate Gathering at the Relay Client 13 60 4.3. NAT Traversal Mode Negotiation . . . . . . . . . . . . . 15 61 4.4. Connectivity Check Pacing Negotiation . . . . . . . . . . 17 62 4.5. Base Exchange via Control Relay Server . . . . . . . . . 17 63 4.6. Connectivity Checks . . . . . . . . . . . . . . . . . . . 20 64 4.6.1. Connectivity Check Procedure . . . . . . . . . . . . 21 65 4.6.2. Rules for Connectivity Checks . . . . . . . . . . . . 24 66 4.6.3. Rules for Concluding Connectivity Checks . . . . . . 26 67 4.7. NAT Traversal Optimizations . . . . . . . . . . . . . . . 27 68 4.7.1. Minimal NAT Traversal Support . . . . . . . . . . . . 27 69 4.7.2. Base Exchange without Connectivity Checks . . . . . . 27 70 4.7.3. Initiating a Base Exchange both with and without UDP 71 Encapsulation . . . . . . . . . . . . . . . . . . . . 29 72 4.8. Sending Control Packets after the Base Exchange . . . . . 29 73 4.9. Mobility Handover Procedure . . . . . . . . . . . . . . . 30 74 4.10. NAT Keepalives . . . . . . . . . . . . . . . . . . . . . 34 75 4.11. Closing Procedure . . . . . . . . . . . . . . . . . . . . 34 76 4.12. Relaying Considerations . . . . . . . . . . . . . . . . . 35 77 4.12.1. Forwarding Rules and Permissions . . . . . . . . . . 35 78 4.12.2. HIP Data Relay and Relaying of Control Packets . . . 36 79 4.12.3. Handling Conflicting SPI Values . . . . . . . . . . 37 80 5. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 38 81 5.1. HIP Control Packets . . . . . . . . . . . . . . . . . . . 38 82 5.2. Connectivity Checks . . . . . . . . . . . . . . . . . . . 39 83 5.3. Keepalives . . . . . . . . . . . . . . . . . . . . . . . 39 84 5.4. NAT Traversal Mode Parameter . . . . . . . . . . . . . . 39 85 5.5. Connectivity Check Transaction Pacing Parameter . . . . . 40 86 5.6. Relay and Registration Parameters . . . . . . . . . . . . 41 87 5.7. LOCATOR_SET Parameter . . . . . . . . . . . . . . . . . . 42 88 5.8. RELAY_HMAC Parameter . . . . . . . . . . . . . . . . . . 44 89 5.9. Registration Types . . . . . . . . . . . . . . . . . . . 44 90 5.10. Notify Packet Types . . . . . . . . . . . . . . . . . . . 44 91 5.11. ESP Data Packets . . . . . . . . . . . . . . . . . . . . 45 92 5.12. RELAYED_ADDRESS and MAPPED_ADDRESS Parameters . . . . . . 45 93 5.13. PEER_PERMISSION Parameter . . . . . . . . . . . . . . . . 46 94 5.14. HIP Connectivity Check Packets . . . . . . . . . . . . . 47 95 5.15. NOMINATE parameter . . . . . . . . . . . . . . . . . . . 48 96 6. Security Considerations . . . . . . . . . . . . . . . . . . . 48 97 6.1. Privacy Considerations . . . . . . . . . . . . . . . . . 48 98 6.2. Opportunistic Mode . . . . . . . . . . . . . . . . . . . 49 99 6.3. Base Exchange Replay Protection for Control Relay Server 49 100 6.4. Demultiplexing Different HIP Associations . . . . . . . . 50 101 6.5. Reuse of Ports at the Data Relay Server . . . . . . . . . 50 102 6.6. Amplification attacks . . . . . . . . . . . . . . . . . . 50 103 6.7. Attacks against Connectivity Checks and Candidate 104 Gathering . . . . . . . . . . . . . . . . . . . . . . . . 50 105 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51 106 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 52 107 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 52 108 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 52 109 10.1. Normative References . . . . . . . . . . . . . . . . . . 52 110 10.2. Informative References . . . . . . . . . . . . . . . . . 53 111 Appendix A. Selecting a Value for Check Pacing . . . . . . . . . 54 112 Appendix B. Differences with respect to ICE . . . . . . . . . . 55 113 Appendix C. Differences to Base Exchange and UPDATE procedures . 56 114 Appendix D. Multihoming Considerations . . . . . . . . . . . . . 59 115 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 60 117 1. Introduction 119 The Host Identity Protocol (HIP) [RFC7401] is specified to run 120 directly on top of IPv4 or IPv6. However, many middleboxes found in 121 the Internet, such as NATs and firewalls, often allow only UDP or TCP 122 traffic to pass [RFC5207]. Also, especially NATs usually require the 123 host behind a NAT to create a forwarding state in the NAT before 124 other hosts outside of the NAT can contact the host behind the NAT. 125 To overcome this problem, different methods, commonly referred to as 126 NAT traversal techniques, have been developed. 128 As one solution, the HIP experiment report [RFC6538] mentions that 129 Teredo based NAT traversal for HIP and related ESP traffic (with 130 double tunneling overhead). Another solution is specified in 131 [RFC5770], which will be referred as "Legacy ICE-HIP" in this 132 document. The experimental Legacy ICE-HIP specification combines 133 Interactive Connectivity Establishment (ICE) protocol 134 [I-D.ietf-ice-rfc5245bis] with HIP, so that basically ICE is 135 responsible of NAT traversal and connectivity testing, while HIP is 136 responsible of end-host authentication and IPsec key management. The 137 resulting protocol uses HIP, STUN and ESP messages tunneled over a 138 single UDP flow. The benefit of using ICE and its STUN/TURN 139 messaging formats is that one can re-use the NAT traversal 140 infrastructure already available in the Internet, such as STUN and 141 TURN servers. Also, some middleboxes may be STUN-aware and may be 142 able to do something "smart" when they see STUN being used for NAT 143 traversal. 145 Implementing a full ICE/STUN/TURN protocol stack as specified in 146 Legacy ICE-HIP results in a considerable amount of effort and code 147 which could be avoided by re-using and extending HIP messages and 148 state machines for the same purpose. Thus, this document specifies 149 an alternative NAT traversal mode referred as "Native ICE-HIP" that 150 employs HIP messaging format instead of STUN or TURN for the 151 connectivity checks, keepalives and data relaying. Native ICE-HIP 152 also specifies how mobility management works in the context of NAT 153 traversal, which is missing from the Legacy ICE-HIP specification. 154 The native specification is also based on HIPv2, whereas legacy 155 specification is based on HIPv1. 157 Similarly as Legacy ICE-HIP, also this specification builds on the 158 HIP registration extensions [RFC8003] and the base exchange procedure 159 [RFC7401] and its closing procedures, so the reader is recommended to 160 get familiar with the relevant specifications. In a nutshell, the 161 registration extensions allow a HIP Initiator (usually a "client" 162 host) to ask for specific services from a HIP Responder (usually a 163 "server" host). The registration parameters are included in a base 164 exchange, which is essentially a four-way Diffie-Hellman key exchange 165 authenticated using the public keys of the end-hosts. When the hosts 166 negotiate support for ESP [RFC7402] during the base exchange, they 167 can deliver ESP protected application payload to each other. When 168 either of the hosts moves and changes its IP address, the two hosts 169 re-establish connectivity using the mobility extensions [RFC8046]. 170 The reader is also recommended to get familiar with the mobility 171 extensions, but basically it is a three-way procedure, where the 172 mobile host first announces its new location to the peer, and then 173 the peer tests for connectivity (so called return routability check), 174 for which the mobile hosts must respond in order to activate its new 175 location. This specification builds on the mobility procedures, but 176 modifies it to be compatible with ICE. The differences to the 177 mobility extensions specified in Appendix C. It is worth noting that 178 multihoming support as specified in [RFC8047] is left for further 179 study. 181 This specification builds heavily on the ICE methodology, so it is 182 recommended that the reader is familiar with the ICE specification 183 [I-D.ietf-ice-rfc5245bis] (especially the overview). However, native 184 ICE-HIP does not implement all the features in ICE, and, hence, the 185 different features of ICE are cross referenced using [RFC2119] 186 terminology for clarity. Appendix B explains the differences to ICE. 188 2. Terminology 190 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 191 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 192 document are to be interpreted as described in [RFC2119]. 194 This document borrows terminology from [RFC5770], [RFC7401], 195 [RFC8046], [RFC4423], [I-D.ietf-ice-rfc5245bis], and [RFC5389]. The 196 following terms recur in the text: 198 ICE: 199 Interactive Connectivity Establishment (ICE) protocol as specified 200 in [I-D.ietf-ice-rfc5245bis] 202 Legacy ICE-HIP: 203 Refers to the "Basic Host Identity Protocol (HIP) Extensions for 204 Traversal of Network Address Translators" as specified in 205 [RFC5770]. The protocol specified in this document offers an 206 alternative to Legacy ICE-HIP. 208 Native ICE-HIP: 209 The protocol specified in this document (Native NAT Traversal Mode 210 for HIP). 212 Initiator: 213 The Initiator is the host that initiates the base exchange using 214 I1 message. 216 Responder: 217 The Responder is the host that receives the I1 packet from the 218 Initiator. 220 Control Relay Server 221 A registrar host that forwards any kind of HIP control plane 222 packets between the Initiator and the Responder. This host is 223 critical because it relays the locators between the Initiator and 224 the Responder, so that they can try to establish a direct 225 communication path with each other. This host is used to replace 226 HIP rendezvous servers [RFC8004] for hosts operating in private 227 address realms. In the Legacy ICE-HIP specification, this host is 228 denoted as "HIP Relay Server". 230 Control Relay Client: 231 A requester host that registers to a Control Relay Server 232 requesting it to forward control-plane traffic (i.e. HIP control 233 messages). In the Legacy ICE-HIP specification, this is denoted 234 as "HIP Relay Client". 236 Data Relay Server: 237 A registrar host that forwards HIP related data plane packets, 238 such as Encapsulating Security Payload (ESP) [RFC7402], between 239 two hosts. This host implements similar functionality as TURN 240 servers. 242 Data Relay Client: 243 A requester host that registers to a Data Relay Server requesting 244 it to forward data-plane traffic (e.g. ESP traffic). 246 Locator: 247 As defined in [RFC8046]: "A name that controls how the packet is 248 routed through the network and demultiplexed by the end-host. It 249 may include a concatenation of traditional network addresses such 250 as an IPv6 address and end-to-end identifiers such as an ESP SPI. 251 It may also include transport port numbers or IPv6 Flow Labels as 252 demultiplexing context, or it may simply be a network address." 254 LOCATOR_SET (written in capital letters): 255 Denotes a HIP control packet parameter that bundles multiple 256 locators together. 258 ICE offer: 259 The Initiator's LOCATOR_SET parameter in a HIP I2 control packet. 260 Corresponds to the ICE offer parameter, but is HIP specific. 262 ICE answer: 263 The Responder's LOCATOR_SET parameter in a HIP R2 control packet. 264 Corresponds to the ICE answer parameter, but is HIP specific. 266 HIP connectivity checks: 267 In order to obtain a direct end-to-end communication path (without 268 employing a Data Relay Server), two communicating HIP hosts try to 269 "punch holes" through their NAT boxes using this mechanism. It is 270 similar to the ICE connectivity checks, but implemented using HIP 271 return routability checks. 273 Controlling host: 274 The controlling host is the Initiator. It nominates the candidate 275 pair to be used with the controlled host. 277 Controlled host: 278 The controlled host is the Responder. It waits for the 279 controlling to nominate an address candidate pair. 281 Checklist: 282 A list of address candidate pairs that need to be tested for 283 connectivity. 285 Transport address: 286 Transport layer port and the corresponding IPv4/v6 address. 288 Candidate: 290 A transport address that is a potential point of contact for 291 receiving data. 293 Host candidate: 294 A candidate obtained by binding to a specific port from an IP 295 address on the host. 297 Server reflexive candidate: 298 A translated transport address of a host as observed by a Control 299 or Data Relay Server. 301 Peer reflexive candidate: 302 A translated transport address of a host as observed by its peer. 304 Relayed candidate: 305 A transport address that exists on a Data Relay Server. Packets 306 that arrive at this address are relayed towards the Data Relay 307 Client. 309 Permission: 310 In the context of Data Relay Server, permission refers to a 311 concept similar to TURN's channels. Before a host can use a 312 relayed candidate to forward traffic through a Data Relay Server, 313 the host must activate the relayed candidate with a specific peer 314 host. 316 Base: 317 The base of an candidate is the local source address a host uses 318 to send packets for the associated candidate. For example, the 319 base of a server reflexive address is the local address the host 320 used for registering itself to the associated Control or Data 321 Relay Server. The base of a host candidate is equal to the host 322 candidate itself. 324 3. Overview of Operation 325 +--------------+ 326 | Control | 327 +--------+ | Relay Server | +--------+ 328 | Data | +----+-----+---+ | Data | 329 | Relay | / \ | Relay | 330 | Server | / \ | Server | 331 +--------+ / \ +--------+ 332 / \ 333 / \ 334 / \ 335 / <- Signaling -> \ 336 / \ 337 +-------+ +-------+ 338 | NAT | | NAT | 339 +-------+ +-------+ 340 / \ 341 / \ 342 +-------+ +-------+ 343 | Init- | | Resp- | 344 | iator | | onder | 345 +-------+ +-------+ 347 Figure 1: Example Network Configuration 349 In the example configuration depicted in Figure 1, both Initiator and 350 Responder are behind one or more NATs, and both private networks are 351 connected to the public Internet. To be contacted from behind a NAT, 352 at least the Responder must be registered with a Control Relay Server 353 reachable on the public Internet. The Responder may have also 354 registered to a Data Relay Server that can forward the data plane in 355 case NAT traversal fails. While, strictly speaking, the Initiator 356 does not need any Relay Servers, it may act in the other role for 357 other hosts and connectivity with the Data Relay Server of the 358 Responder may fail, so it is the Initiator may also have registered 359 to a Control and/or Data Relay Server. It is worth noting that a 360 Control and Data Relay does not forge the source address of a passing 361 packet, but always translates the source address and source port of a 362 packet to be forwarded (to its own). 364 We assume, as a starting point, that the Initiator knows both the 365 Responder's Host Identity Tag (HIT) and the address(es) of the 366 Responder's Control Relay Server(s) (how the Initiator learns of the 367 Responder's Control Relay Server is outside of the scope of this 368 document, but may be through DNS or another name service). The first 369 steps are for both the Initiator and Responder to register with a 370 Control Relay Server (need not be the same one) and gather a set of 371 address candidates. The hosts use either Control Relay Servers or 372 Data Relay Servers (or other infrastructure including STUN or TURN 373 servers) for gathering the candidates. Next, the HIP base exchange 374 is carried out by encapsulating the HIP control packets in UDP 375 datagrams and sending them through the Responder's Control Relay 376 Server. As part of the base exchange, each HIP host learns of the 377 peer's candidate addresses through the HIP offer/answer procedure 378 embedded in the base exchange. 380 Once the base exchange is completed, two HIP hosts have established a 381 working communication session (for signaling) via a Control Relay 382 Server, but the hosts still have to find a better path, preferably 383 without a Data Relay Server, for the ESP data flow. For this, 384 connectivity checks are carried out until a working pair of addresses 385 is discovered. At the end of the procedure, if successful, the hosts 386 will have established a UDP-based tunnel that traverses both NATs, 387 with the data flowing directly from NAT to NAT or via a Data Relay 388 Server. At this point, also the HIP signaling can be sent over the 389 same address/port pair, and is demultiplexed from IPsec as described 390 in the UDP encapsulation standard for IPsec [RFC3948]. Finally, the 391 two hosts send NAT keepalives as needed in order keep their UDP- 392 tunnel state active in the associated NAT boxes. 394 If either one of the hosts knows that it is not behind a NAT, hosts 395 can negotiate during the base exchange a different mode of NAT 396 traversal that does not use HIP connectivity checks, but only UDP 397 encapsulation of HIP and ESP. Also, it is possible for the Initiator 398 to simultaneously try a base exchange with and without UDP 399 encapsulation. If a base exchange without UDP encapsulation 400 succeeds, no HIP connectivity checks or UDP encapsulation of ESP are 401 needed. 403 4. Protocol Description 405 This section describes the normative behavior of the "Native ICE-HIP" 406 protocol extension. Most of the procedures are similar to what is 407 defined in [RFC5770] but with different, or additional, parameter 408 types and values. In addition, a new type of relaying server, Data 409 Relay Server, is specified. Also, it should be noted that HIP 410 version 2 [RFC7401] instead of HIPv1 is expected to be used with this 411 NAT traversal mode. 413 4.1. Relay Registration 415 In order for two hosts to communicate over NATted environments, they 416 need a reliable way to exchange information. To achieve this, "HIP 417 Relay Server" is defined in [RFC5770]. It supports relaying of HIP 418 control plane traffic over UDP in NATted environments, and forwards 419 HIP control packets between the Initiator and the Responder. In this 420 document, the HIP Relay Server is denoted as "Control Relay Server" 421 for better alignment with the rest of the terminology. The 422 registration to the Control Relay Server can be achieved using 423 RELAY_UDP_ESP parameter as explained later in this section. 425 To guarantee also data plane delivery over varying types of NAT 426 devices, a host MAY also register for UDP encapsulated ESP relaying 427 using Registration Type RELAY_UDP_ESP (value [TBD by IANA: 3]). This 428 service may be coupled with the Control Relay Server or offered 429 separately on another server. If the server supports relaying of UDP 430 encapsulated ESP, the host is allowed to register for a data relaying 431 service using the registration extensions in Section 3.3 of 432 [RFC8003]). If the server has sufficient relaying resources (free 433 port numbers, bandwidth, etc.) available, it opens a UDP port on one 434 of its addresses and signals the address and port to the registering 435 host using the RELAYED_ADDRESS parameter (as defined in Section 5.12 436 in this document). If the Data Relay Server would accept the data 437 relaying request but does not currently have enough resources to 438 provide data relaying service, it MUST reject the request with 439 Failure Type "Insufficient resources" [RFC8003]. 441 A Control Relay Server MUST silently drop packets to a Control Relay 442 Client that has not previously registered with the HIP relay. The 443 registration process follows the generic registration extensions 444 defined in [RFC8003]. The HIP control plane relaying registration 445 follows [RFC5770], but the data plane registration is different. It 446 is worth noting that if the HIP control and data plane relay services 447 reside on different hosts, the client has to register separately to 448 each of them. In the example shown in Figure 2, the two services are 449 coupled on a single host. The text uses "Relay Client" and "Relay 450 Server" as a shorthand when the procedures apply both to control and 451 data cases. 453 Control/Data Control/Data 454 Relay Client (Initiator) Relay Server (Responder) 455 | 1. UDP(I1) | 456 +---------------------------------------------------------------->| 457 | | 458 | 2. UDP(R1(REG_INFO(RELAY_UDP_HIP,[RELAY_UDP_ESP]))) | 459 |<----------------------------------------------------------------+ 460 | | 461 | 3. UDP(I2(REG_REQ(RELAY_UDP_HIP),[RELAY_UDP_ESP])) | 462 +---------------------------------------------------------------->| 463 | | 464 | 4. UDP(R2(REG_RES(RELAY_UDP_HIP,[RELAY_UDP_ESP]), REG_FROM, | 465 | [RELAYED_ADDRESS])) | 466 |<----------------------------------------------------------------+ 467 | | 469 Figure 2: Example Registration with a HIP Relay 471 In step 1, the Relay Client (Initiator) starts the registration 472 procedure by sending an I1 packet over UDP to the Relay Server. It 473 is RECOMMENDED that the Relay Client select a random port number from 474 the ephemeral port range 49152-65535 for initiating a base exchange. 475 Alternatively, a host MAY also use a single fixed port for initiating 476 all outgoing connections. However, the allocated port MUST be 477 maintained until all of the corresponding HIP Associations are 478 closed. It is RECOMMENDED that the Relay Server listen to incoming 479 connections at UDP port 10500. If some other port number is used, it 480 needs to be known by potential Relay Clients. 482 In step 2, the Relay Server (Responder) lists the services that it 483 supports in the R1 packet. The support for HIP control plane over 484 UDP relaying is denoted by the Registration Type value RELAY_UDP_HIP 485 (see Section 5.9). If the server supports also relaying of ESP 486 traffic over UDP, it includes also Registration type value 487 RELAY_UDP_ESP. 489 In step 3, the Relay Client selects the services for which it 490 registers and lists them in the REG_REQ parameter. The Relay Client 491 registers for the Control Data Relay service by listing the 492 RELAY_UDP_HIP value in the request parameter. If the Relay Client 493 requires also ESP relaying over UDP, it lists also RELAY_UDP_ESP. 495 In step 4, the Relay Server concludes the registration procedure with 496 an R2 packet and acknowledges the registered services in the REG_RES 497 parameter. The Relay Server denotes unsuccessful registrations (if 498 any) in the REG_FAILED parameter of R2. The Relay Server also 499 includes a REG_FROM parameter that contains the transport address of 500 the Relay Client as observed by the Relay Server (Server Reflexive 501 candidate). If the Relay Client registered to ESP relaying service, 502 the Relay Server includes RELAYED_ADDRESS parameter that describes 503 the UDP port allocated to the Relay Client for ESP relaying. It is 504 worth noting that the Data Relay Client must first activate this UDP 505 port by sending an UPDATE message to the Data Relay Server that 506 includes a PEER_PERMISSION parameter as described in Section 4.12.1 507 both after base exchange and handover procedures. Also, the Data 508 Relay Server should follow the port allocation recommendations in 509 Section 6.5. 511 After the registration, the Relay Client sends periodically NAT 512 keepalives to the Relay Server in order to keep the NAT bindings 513 between the Relay Client and the relay alive. The keepalive 514 extensions are described in Section 4.10. 516 The Data Relay Client MUST maintain an active HIP association with 517 the Data Relay Server as long as it requires the data relaying 518 service. When the HIP association is closed (or times out), or the 519 registration lifetime passes without the Data Relay Client refreshing 520 the registration, the Data Relay Server MUST stop relaying packets 521 for that host and close the corresponding UDP port (unless other Data 522 Relay Clients are still using it). 524 The Data Relay Server SHOULD offer a different relayed address and 525 port for each Data Relay Client because this can cause problems with 526 stateful firewalls (see Section 6.5). 528 When a Control Relay Client sends an UPDATE (e.g., due to host 529 movement or to renew service registration), the Control Relay Server 530 MUST follow the general guidelines defined in [RFC8003], with the 531 difference that all UPDATE messages are delivered on top of UDP. In 532 addition to this, the Control Relay Server MUST include the REG_FROM 533 parameter in all UPDATE responses sent to the Control Relay Client. 534 This applies both renewals of service registration but also to host 535 movement, where especially the latter requires the Control Relay 536 Client to learn its new server reflexive address candidate. 538 A Data Relay Client can request multiple relayed candidates from the 539 Data Relay Server (e.g., for the reasons described in 540 Section 4.12.3). After the base exchange with registration, the Data 541 Relay Client can request additional relayed candidates similarly as 542 during the base exchange. The Data Relay Client sends an UPDATE 543 message REG_REQ parameter requesting for the RELAY_UDP_ESP service. 544 The UPDATE message MUST also include a SEQ and a ECHO_REQUEST_SIGNED 545 parameter. The Data Relay Server MUST respond with an UPDATE message 546 that includes the corresponding response parameters: REG_RES, ACK and 547 ECHO_REQUEST_SIGNED . In case the Data Relay Server admitted a new 548 relayed UDP port for the Data Relay Client, the REG_RES parameter 549 MUST list RELAY_UDP_ESP as a service and the UPDATE message MUST also 550 include a RELAYED_ADDRESS parameter describing the relayed UDP port. 551 The Data Relay Server MUST also include the Server Reflexive 552 candidate in a REG_FROM parameter. It is worth mentioning that Data 553 Relay Client MUST activate the UDP port as described in 554 Section 4.12.1 before it can be used for any ESP relaying. 556 A Data Relay Client may unregister a relayed candidate in two ways. 557 It can wait for its lifetime to expire or it can explicitly request 558 it with zero lifetime using the UPDATE mechanism. The Data Relay 559 Client can send an REG_REQ parameter with zero lifetime to the Data 560 Relay Server in order to expire all relayed candidates. To expire a 561 specific relayed candidate, the Data Relay Client MUST also include 562 RELAYED_ADDRESS parameter as sent by the server in the UPDATE 563 message. Upon closing the HIP association (CLOSE-CLOSE-ACK procedure 564 initiated by either party), the Data Relay Server MUST also expire 565 all relayed candidates. 567 4.2. Transport Address Candidate Gathering at the Relay Client 569 An Initiator needs to gather a set of address candidates before 570 contacting a (non-relay) Responder. The candidates are needed for 571 connectivity checks that allow two hosts to discover a direct, non- 572 relayed path for communicating with each other. One server reflexive 573 candidate can be discovered during the registration with the Control 574 Relay Server from the REG_FROM parameter (and another from Data Relay 575 Server if one is employed). 577 The candidate gathering can be done at any time, but it needs to be 578 done before sending an I2 or R2 in the base exchange if ICE-HIP-UDP 579 mode is to be used for the connectivity checks. It is RECOMMENDED 580 that all three types of candidates (host, server reflexive, and 581 relayed) are gathered to maximize the probability of successful NAT 582 traversal. However, if no Data Relay Server is used, and the host 583 has only a single local IP address to use, the host MAY use the local 584 address as the only host candidate and the address from the REG_FROM 585 parameter discovered during the Control Relay Server registration as 586 a server reflexive candidate. In this case, no further candidate 587 gathering is needed. 589 A Data Relay Client MAY register only a single relayed candidate to 590 be used with multiple other peers. However, it is RECOMMENDED that a 591 Data Relay Client registers a new server reflexive candidate for each 592 its peer for the reasons described in Section 4.12.3. The procedures 593 for registering multiple relayed candidates are described in 594 Section 4.1. 596 If a Relay Client has more than one network interface, it can 597 discover additional server reflexive candidates by sending UPDATE 598 messages from each of its interfaces to the Relay Server. Each such 599 UPDATE message MUST include the following parameters: registration 600 request (REG_REQ) parameter with Registration Type 601 CANDIDATE_DISCOVERY (value [TBD by IANA: 4]) and ECHO_REQUEST_SIGNED 602 parameter. When a Control Relay Server receives an UPDATE message 603 with registration request containing a CANDIDATE_DISCOVERY type, it 604 MUST include a REG_FROM parameter, containing the same information as 605 if this were a Control Relay Server registration, to the response (in 606 addition to the mandatory ECHO_RESPONSE_SIGNED parameter). This 607 request type SHOULD NOT create any state at the Control Relay Server. 609 ICE guidelines [I-D.ietf-ice-rfc5245bis] for candidate gathering are 610 followed here. A number of host candidates (loopback, anycast and 611 others) should be excluded as described in the ICE specification 612 [I-D.ietf-ice-rfc5245bis]. Relayed candidates SHOULD be gathered in 613 order to guarantee successful NAT traversal, and implementations 614 SHOULD support this functionality even if it will not be used in 615 deployments in order to enable it by software configuration update if 616 needed at some point. A host SHOULD employ only a single server for 617 gathering the candidates for a single HIP association; either one 618 server providing both Control and Data Relay Server functionality, or 619 one Control Relay Server and also Data Relay Server if the 620 functionality is offered by another server. When the relay service 621 is split between two hosts, the server reflexive candidate from the 622 Control Relay Server SHOULD be used instead of the one provided by 623 the Data Relay Server. If a relayed candidate is identical to a host 624 candidate, the relayed candidate must be discarded. NAT64 625 considerations in [I-D.ietf-ice-rfc5245bis] apply as well. 627 HIP based connectivity can be utilized by IPv4 applications using 628 LSIs and by IPv6 based applications using HITs. The LSIs and HITs of 629 the local virtual interfaces MUST be excluded in the candidate 630 gathering phase as well to avoid creating unnecessary loopback 631 connectivity tests. 633 Gathering of candidates MAY also be performed by other means than 634 described in this section. For example, the candidates could be 635 gathered as specified in Section 4.2 of [RFC5770] if STUN servers are 636 available, or if the host has just a single interface and no STUN or 637 Data Relay Server are available. 639 Each local address candidate MUST be assigned a priority. The 640 following recommended formula (as described in 641 [I-D.ietf-ice-rfc5245bis]) SHOULD be used: 643 priority = (2^24)*(type preference) + (2^8)*(local preference) + 644 (2^0)*(256 - component ID) 646 In the formula, type preference follows the ICE specification 647 guidelines: the RECOMMENDED values are 126 for host candidates, 100 648 for server reflexive candidates, 110 for peer reflexive candidates, 649 and 0 for relayed candidates. The highest value is 126 (the most 650 preferred) and lowest is 0 (last resort). For all candidates of the 651 same type, the preference type value MUST be identical, and, 652 correspondingly, the value MUST be different for different types. 653 For peer reflexive values, the type preference value MUST be higher 654 than for server reflexive types. It should be noted that peer 655 reflexive values are learned later during connectivity checks, so a 656 host cannot employ it during candidate gathering stage yet. 658 Following the ICE specification, the local preference MUST be an 659 integer from 0 (lowest preference) to 65535 (highest preference) 660 inclusive. In the case the host has only a single address candidate, 661 the value SHOULD be 65535. In the case of multiple candidates, each 662 local preference value MUST be unique. Dual-stack considerations for 663 IPv6 in ICE apply also here. 665 Unlike ICE, this protocol only creates a single UDP flow between the 666 two communicating hosts, so only a single component exists. Hence, 667 the component ID value MUST always be set to 1. 669 As defined in ICE , the retransmission timeout (RTO) for address 670 gathering from a Control/Data Relay Server SHOULD be calculated as 671 follows: 673 RTO = MAX (500ms, Ta * (Num-Of-Pairs)) 675 where Ta is the value used for Ta is the value used for the 676 connectivity check pacing and Num-Of-Pairs is number of pairs of 677 candidates with Control and Data Relay Servers (e.g. in the case of a 678 single server, it would be 1). A smaller value than 500 ms for the 679 RTO MUST NOT be used. 681 4.3. NAT Traversal Mode Negotiation 683 This section describes the usage of a new non-critical parameter 684 type. The presence of the parameter in a HIP base exchange means 685 that the end-host supports NAT traversal extensions described in this 686 document. As the parameter is non-critical (as defined in 687 Section 5.2.1 of [RFC7401]), it can be ignored by a end-host, which 688 means that the host is not required to support it or may decline to 689 use it. 691 With registration with a Control/Data Relay Server, it is usually 692 sufficient to use the UDP-ENCAPSULATION mode of NAT traversal since 693 the Relay Server is assumed to be in public address space. Thus, the 694 Relay Server SHOULD propose the UDP-ENCAPSULATION mode as the 695 preferred or only mode. The NAT traversal mode negotiation in a HIP 696 base exchange is illustrated in Figure 3. It is worth noting that 697 the Relay Server could be located between the hosts, but is omitted 698 here for simplicity. 700 Initiator Responder 701 | 1. UDP(I1) | 702 +--------------------------------------------------------------->| 703 | | 704 | 2. UDP(R1(.., NAT_TRAVERSAL_MODE(ICE-HIP-UDP), ..)) | 705 |<---------------------------------------------------------------+ 706 | | 707 | 3. UDP(I2(.., NAT_TRAVERSAL_MODE(ICE-HIP-UDP), LOC_SET, ..)) | 708 +--------------------------------------------------------------->| 709 | | 710 | 4. UDP(R2(.., LOC_SET, ..)) | 711 |<---------------------------------------------------------------+ 712 | | 714 Figure 3: Negotiation of NAT Traversal Mode 716 In step 1, the Initiator sends an I1 to the Responder. In step 2, 717 the Responder responds with an R1. As specified in [RFC5770], the 718 NAT_TRAVERSAL_MODE parameter in R1 contains a list of NAT traversal 719 modes the Responder supports. The mode specified in this document is 720 ICE-HIP-UDP (value [TBD by IANA: 3]). 722 In step 3, the Initiator sends an I2 that includes a 723 NAT_TRAVERSAL_MODE parameter. It contains the mode selected by the 724 Initiator from the list of modes offered by the Responder. If ICE- 725 HIP-UDP mode was selected, the I2 also includes the "Transport 726 address" locators (as defined in Section 5.7) of the Initiator in a 727 LOCATOR_SET parameter (denoted here LOC_SET). The locators in I2 are 728 the "ICE offer". 730 In step 4, the Responder concludes the base exchange with an R2 731 packet. If the Initiator chose ICE NAT traversal mode, the Responder 732 includes a LOCATOR_SET parameter in the R2 packet. The locators in 733 R2, encoded like the locators in I2, are the "ICE answer". If the 734 NAT traversal mode selected by the Initiator is not supported by the 735 Responder, the Responder SHOULD reply with a NOTIFY packet with type 736 NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER and abort the base exchange. 738 4.4. Connectivity Check Pacing Negotiation 740 As explained in Legacy ICE-HIP [RFC5770], when a NAT traversal mode 741 with connectivity checks is used, new transactions should not be 742 started too fast to avoid congestion and overwhelming the NATs. For 743 this purpose, during the base exchange, hosts can negotiate a 744 transaction pacing value, Ta, using a TRANSACTION_PACING parameter in 745 R1 and I2 packets. The parameter contains the minimum time 746 (expressed in milliseconds) the host would wait between two NAT 747 traversal transactions, such as starting a new connectivity check or 748 retrying a previous check. The value that is used by both of the 749 hosts is the higher of the two offered values. 751 The minimum Ta value SHOULD be configurable, and if no value is 752 configured, a value of 50 ms MUST be used. Guidelines for selecting 753 a Ta value are given in Appendix A. Hosts SHOULD NOT use values 754 smaller than 5 ms for the minimum Ta, since such values may not work 755 well with some NATs (as explained in [I-D.ietf-ice-rfc5245bis]). The 756 Initiator MUST NOT propose a smaller value than what the Responder 757 offered. If a host does not include the TRANSACTION_PACING parameter 758 in the base exchange, a Ta value of 50 ms MUST be used as that host's 759 minimum value. 761 4.5. Base Exchange via Control Relay Server 763 This section describes how the Initiator and Responder perform a base 764 exchange through a Control Relay Server. Connectivity pacing 765 (denoted as TA_P here) was described in Section 4.4 and is not 766 repeated here. Similarly, the NAT traversal mode negotiation process 767 (denoted as NAT_TM in the example) was described in Section 4.3 and 768 is neither repeated here. If a Control Relay Server receives an R1 769 or I2 packet without the NAT traversal mode parameter, it MUST drop 770 it and SHOULD send a NOTIFY error packet with type 771 NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER to the sender of the R1 or I2. 773 It is RECOMMENDED that the Initiator send an I1 packet encapsulated 774 in UDP when it is destined to an IPv4 address of the Responder. 775 Respectively, the Responder MUST respond to such an I1 packet with a 776 UDP-encapsulated R1 packet, and also the rest of the communication 777 related to the HIP association MUST also use UDP encapsulation. 779 Figure 4 illustrates a base exchange via a Control Relay Server. We 780 assume that the Responder (i.e. a Control Relay Client) has already 781 registered to the Control Relay Server. The Initiator may have also 782 registered to another (or the same Control Relay Server), but the 783 base exchange will traverse always through the Control Relay Server 784 of the Responder. 786 Initiator Control Relay Server Responder 787 | 1. UDP(I1) | | 788 +--------------------------------->| 2. UDP(I1(RELAY_FROM)) | 789 | +------------------------------->| 790 | | | 791 | | 3. UDP(R1(RELAY_TO, NAT_TM, | 792 | | TA_P)) | 793 | 4. UDP(R1(RELAY_TO, NAT_TM, |<-------------------------------+ 794 | TA_P)) | | 795 |<---------------------------------+ | 796 | | | 797 | 5. UDP(I2(LOC_SET, NAT_TM, | | 798 | TA_P)) | | 799 +--------------------------------->| 6. UDP(I2(LOC_SET, RELAY_FROM, | 800 | | NAT_TM, TA_P)) | 801 | +------------------------------->| 802 | | | 803 | | 7. UDP(R2(LOC_SET, RELAY_TO)) | 804 | 8. UDP(R2(LOC_SET, RELAY_TO)) |<-------------------------------+ 805 |<---------------------------------+ | 806 | | | 808 Figure 4: Base Exchange via a HIP Relay Server 810 In step 1 of Figure 4, the Initiator sends an I1 packet over UDP via 811 the Control Relay Server to the Responder. In the HIP header, the 812 source HIT belongs to the Initiator and the destination HIT to the 813 Responder. The initiator sends the I1 packet from its IP address to 814 the IP address of the Control Relay Server over UDP. 816 In step 2, the Control Relay Server receives the I1 packet. If the 817 destination HIT belongs to a registered Responder, the Control Relay 818 Server processes the packet. Otherwise, the Control Relay Server 819 MUST drop the packet silently. The Control Relay Server appends a 820 RELAY_FROM parameter to the I1 packet, which contains the transport 821 source address and port of the I1 as observed by the Control Relay 822 Server. The Control Relay Server protects the I1 packet with 823 RELAY_HMAC as described in [RFC8004], except that the parameter type 824 is different (see Section 5.8). The Control Relay Server changes the 825 source and destination ports and IP addresses of the packet to match 826 the values the Responder used when registering to the Control Relay 827 Server, i.e., the reverse of the R2 used in the registration. The 828 Control Relay Server MUST recalculate the transport checksum and 829 forward the packet to the Responder. 831 In step 3, the Responder receives the I1 packet. The Responder 832 processes it according to the rules in [RFC7401]. In addition, the 833 Responder validates the RELAY_HMAC according to [RFC8004] and 834 silently drops the packet if the validation fails. The Responder 835 replies with an R1 packet to which it includes RELAY_TO and NAT 836 traversal mode parameters. The responder MUST include ICE-HIP-UDP in 837 the NAT traversal modes. The RELAY_TO parameter MUST contain the 838 same information as the RELAY_FROM parameter, i.e., the Initiator's 839 transport address, but the type of the parameter is different. The 840 RELAY_TO parameter is not integrity protected by the signature of the 841 R1 to allow pre-created R1 packets at the Responder. 843 In step 4, the Control Relay Server receives the R1 packet. The 844 Control Relay Server drops the packet silently if the source HIT 845 belongs to a Control Relay Client that has not successfully 846 registered. The Control Relay Server MAY verify the signature of the 847 R1 packet and drop it if the signature is invalid. Otherwise, the 848 Control Relay Server rewrites the source address and port, and 849 changes the destination address and port to match RELAY_TO 850 information. Finally, the Control Relay Server recalculates the 851 transport checksum and forwards the packet. 853 In step 5, the Initiator receives the R1 packet and processes it 854 according to [RFC7401]. The Initiator MAY use the address in the 855 RELAY_TO parameter as a local peer-reflexive candidate for this HIP 856 association if it is different from all known local candidates. The 857 Initiator replies with an I2 packet that uses the destination 858 transport address of R1 as the source address and port. The I2 859 packet contains a LOCATOR_SET parameter that lists all the HIP 860 candidates (ICE offer) of the Initiator. The candidates are encoded 861 using the format defined in Section 5.7. The I2 packet MUST also 862 contain a NAT traversal mode parameter that includes ICE-HIP-UDP 863 mode. 865 In step 6, the Control Relay Server receives the I2 packet. The 866 Control Relay Server appends a RELAY_FROM and a RELAY_HMAC to the I2 867 packet similarly as explained in step 2, and forwards the packet to 868 the Responder. 870 In step 7, the Responder receives the I2 packet and processes it 871 according to [RFC7401]. It replies with an R2 packet and includes a 872 RELAY_TO parameter as explained in step 3. The R2 packet includes a 873 LOCATOR_SET parameter that lists all the HIP candidates (ICE answer) 874 of the Responder. The RELAY_TO parameter is protected by the HMAC. 876 In step 8, the Control Relay Server processes the R2 as described in 877 step 4. The Control Relay Server forwards the packet to the 878 Initiator. After the Initiator has received the R2 and processed it 879 successfully, the base exchange is completed. 881 Hosts MUST include the address of one or more Control Relay Servers 882 (including the one that is being used for the initial signaling) in 883 the LOCATOR_SET parameter in I2 and R2 if they intend to use such 884 servers for relaying HIP signaling immediately after the base 885 exchange completes. The traffic type of these addresses MUST be "HIP 886 signaling" and they MUST NOT be used as HIP candidates. If the 887 Control Relay Server locator used for relaying the base exchange is 888 not included in I2 or R2 LOCATOR_SET parameters, it SHOULD NOT be 889 used after the base exchange. Instead, further HIP signaling SHOULD 890 use the same path as the data traffic. It is RECOMMENDED to use the 891 same Control Relay Server throughout the lifetime of the host 892 association that was used for forwarding the base exchange if the 893 Responder includes it in the locator parameter of the R2 message. 895 4.6. Connectivity Checks 897 When the Initiator and Responder complete the base exchange through 898 the Control Relay Server, both of them employ the IP address of the 899 Control Relay Server as the destination address for the packets. 900 This address MUST NOT be used as a destination for ESP traffic (i.e., 901 the corresponding Control Relay Client cannot advertise it to its 902 peer) unless the server supports also Data Relay Server 903 functionality, for which the client has successfully registered to. 904 When NAT traversal mode with ICE-HIP-UDP was successfully negotiated 905 and selected, the Initiator and Responder MUST start the connectivity 906 checks in order to attempt to obtain direct end-to-end connectivity 907 through NAT devices. It is worth noting that the connectivity checks 908 MUST be completed even though no ESP_TRANSFORM would be negotiated 909 and selected. 911 The connectivity checks follow the ICE methodology [MMUSIC-ICE], but 912 UDP encapsulated HIP control messages are used instead of ICE 913 messages. Only normal nomination MUST be used for the connectivity 914 checks, i.e., aggressive nomination MUST NOT be employed. As stated 915 in the ICE specification, the basic procedure for connectivity checks 916 has three phases: sorting the candidate pairs according their 917 priority, sending checks in the prioritized order and acknowledging 918 the checks from the peer host. 920 The Initiator MUST take the role of controlling host and the 921 Responder acts as the controlled host. The roles MUST persist 922 throughout the HIP associate lifetime (to be reused in the possibly 923 mobility UPDATE procedures). In the case both communicating nodes 924 are initiating the communications to each other using an I1 packet, 925 the conflict is resolved as defined in section in 6.7 in [RFC7401]: 926 the host with the "larger" HIT changes to its Role to Responder. In 927 such a case, the host changing its role to Responder MUST also switch 928 to controlling role. 930 The protocol follows standard HIP UPDATE sending and processing rules 931 as defined in section 6.11 and 6.12 in [RFC7401], but some new 932 parameters are introduced (CANDIDATE_PRIORITY, MAPPED_ADDRESS and 933 NOMINATE). 935 4.6.1. Connectivity Check Procedure 937 Figure 5 illustrates connectivity checks in a simplified scenario, 938 where the Initiator and Responder have only a single candidate pair 939 to check. Typically, NATs drop messages until both sides have sent 940 messages using the same port pair. In this scenario, the Responder 941 sends a connectivity check first but the NAT of the Initiator drops 942 it. However, the connectivity check from the Initiator reaches the 943 Responder because it uses the same port pair as the first message. 944 It is worth noting that the message flow in this section is 945 idealistic, and, in practice, more messages would be dropped, 946 especially in the beginning. For instance, connectivity tests always 947 start with the candidates with the highest priority, which would be 948 host candidates (which would not reach the recipient in this 949 scenario). 951 Initiator NAT1 NAT2 Responder 952 | | 1. UDP(UPDATE(SEQ, CAND_PRIO, | | 953 | | ECHO_REQ_SIGN)) | | 954 | X<-----------------------------------+----------------+ 955 | | | | 956 | 2. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO)) | | 957 +-------------+------------------------------------+--------------->| 958 | | | | 959 | 3. UDP(UPDATE(ACK, ECHO_RESP_SIGN, MAPPED_ADDR)) | | 960 |<------------+------------------------------------+----------------+ 961 | | | | 962 | 4. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO)) | | 963 |<------------+------------------------------------+----------------+ 964 | | | | 965 | 5. UDP(UPDATE(ACK, ECHO_RESP_SIGN, MAPPED_ADDR)) | | 966 +-------------+------------------------------------+--------------->| 967 | | | | 968 | 6. Other connectivity checks using UPDATE over UDP | 969 |<------------+------------------------------------+----------------> 970 | | | | 971 | 7. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO, NOMINATE)) | 972 +-------------+------------------------------------+--------------->| 973 | | | | 974 | 8. UDP(UPDATE(SEQ, ACK, ECHO_REQ_SIGN, ECHO_RESP_SIGN, | 975 | NOMINATE)) | | 976 |<------------+------------------------------------+----------------+ 977 | | | | 978 | 9. UDP(UPDATE(ACK, ECHO_RESP_SIGN)) | | 979 +-------------+------------------------------------+--------------->+ 980 | | | | 981 | 10. ESP data traffic over UDP | | 982 +<------------+------------------------------------+--------------->+ 983 | | | | 985 Figure 5: Connectivity Checks 987 In step 1, the Responder sends a connectivity check to the Initiator 988 that the NAT of the Initiator drops. The message includes a number 989 of parameters. As specified in [RFC7401]), the SEQ parameter 990 includes a running sequence identifier for the connectivity check. 991 The candidate priority (denoted "CAND_PRIO" in the figure) describes 992 the priority of the address candidate being tested. The 993 ECHO_REQUEST_SIGNED (denoted ECHO_REQ_SIGN in the figure) includes a 994 nonce that the recipient must sign and echo back as it is. 996 In step 2, the Initiator sends a connectivity check, using the same 997 address pair candidate as in the previous step, and the message 998 traverses successfully the NAT boxes. The message includes the same 999 parameters as in the previous step. It should be noted that the 1000 sequence identifier is locally assigned by the Responder, so it can 1001 be different than in the previous step. 1003 In step 3, the Responder has successfully received the previous 1004 connectivity check from the Initiator and starts to build a response 1005 message. Since the message from the Initiator included a SEQ, the 1006 Responder must acknowledge it using an ACK parameter. Also, the 1007 nonce contained in the echo request must be echoed back in an 1008 ECHO_RESPONSE_SIGNED (denoted ECHO_RESP_SIGN) parameter. The 1009 Responder includes also a MAPPED_ADDRESS parameter (denoted 1010 MAPPED_ADDR in the figure) that contains the transport address of the 1011 Initiator as observed by the Responder (i.e. peer reflexive 1012 candidate). This message is successfully delivered to the Initiator, 1013 and upon reception the Initiator marks the candidate pair as valid. 1015 In step 4, the Responder retransmits the connectivity check sent in 1016 the first step, since it was not acknowledged yet. 1018 In step 5, the Initiator responds to the previous connectivity check 1019 message from the Responder. The Initiator acknowledges the SEQ 1020 parameter from the previous message using ACK parameter and the 1021 ECHO_REQUEST_SIGNED parameter with ECHO_RESPONSE_SIGNED. In 1022 addition, it includes MAPPED_ADDR parameter that includes the peer 1023 reflexive candidate. This response message is successfully delivered 1024 to the Responder, and upon reception the Initiator marks the 1025 candidate pair as valid. 1027 In step 6, despite the two hosts now having valid address candidates, 1028 the hosts still test the remaining address candidates in a similar 1029 way as in the previous steps (due to the use of normal nomination). 1030 It should be noted that each connectivity check has a unique sequence 1031 number in the SEQ parameter. 1033 In step 7, the Initiator has completed testing all address candidates 1034 and nominates one address candidate to be used. It sends an UPDATE 1035 message using the selected address candidates that includes a number 1036 of parameters: SEQ, ECHO_REQUEST_SIGNED, CANDIDATE_PRIORITY and the 1037 NOMINATE parameter. 1039 In step 8, the Responder receives the message with NOMINATE parameter 1040 from the Initiator. It sends a response that includes the NOMINATE 1041 parameter in addition to a number of other parameters. The ACK and 1042 ECHO_RESPONSE_SIGNED parameters acknowledge the SEQ and 1043 ECHO_REQUEST_SIGNED parameters from previous message from the 1044 Initiator. The Responder includes SEQ and ECHO_REQUEST_SIGNED 1045 parameters in order to receive an acknowledgment from the Responder. 1047 In step 9, the Initiator completes the candidate nomination process 1048 by confirming the message reception to the Responder. In the 1049 confirmation message, the ACK and ECHO_RESPONSE_SIGNED parameters 1050 correspond to the SEQ and ECHO_REQUEST_SIGNED parameters in the 1051 message sent by the Responder in the previous step. 1053 In step 10, the Initiator and Responder can start sending application 1054 payload over the successfully nominated address candidates. 1056 It is worth noting that if either host has registered a relayed 1057 address candidate from a Data Relay Server, the host MUST activate 1058 the address before connectivity checks by sending an UPDATE message 1059 containing PEER_PERMISSION parameter as described in Section 4.12.1. 1060 Otherwise, the Data Relay Server drops ESP packets using the relayed 1061 address. 1063 It should be noted that in the case both Initiator and Responder both 1064 advertising their own relayed address candidates, it is possible that 1065 the two hosts choose the two relayed addresses as a result of the ICE 1066 nomination algorithm. While this is possible (and even could be 1067 desirable for privacy reasons), it can be unlikely due to low 1068 priority assigned for the relayed address candidates. In such a 1069 event, the nominated address pair is always symmetric; the nomination 1070 algorithm prevents asymmetric address pairs (i.e. each side choosing 1071 different pair), such as a Data Relay Client using its own Data Relay 1072 Server to send data directly to its peer while receiving data from 1073 the Data Relay Server of its peer. 1075 4.6.2. Rules for Connectivity Checks 1077 The HITs of the two communicating hosts MUST be used as credentials 1078 in this protocol (in contrast to ICE which employs username-password 1079 fragments). A HIT pair uniquely identifies the corresponding HIT 1080 association, and a SEQ number in an UPDATE message identifies a 1081 particular connectivity check. 1083 All of the connectivity check packets MUST be protected with HMACs 1084 and signatures (even though the illustrations in this specification 1085 omit them for simplicity). Each connectivity check sent by a host 1086 MUST include a SEQ parameter and ECHO_REQUEST_SIGNED parameter, and 1087 correspondingly the peer MUST respond to these using ACK and 1088 ECHO_RESPONSE_SIGNED according to the rules specified in [RFC7401]. 1090 The host sending a connectivity check MUST validate that the response 1091 uses the same pair of UDP ports, and drop the packet if this is not 1092 the case. 1094 A host may receive a connectivity check before it has received the 1095 candidates from its peer. In such a case, the host MUST immediately 1096 generate a response, and then continue waiting for the candidates. A 1097 host MUST NOT select a candidate pair until it has verified the pair 1098 using a connectivity check as defined in Section 4.6.1. 1100 [RFC7401] states that UPDATE packets have to include either a SEQ or 1101 ACK parameter (or both). According to the RFC, each SEQ parameter 1102 should be acknowledged separately. In the context of NATs, this 1103 means that some of the SEQ parameters sent in connectivity checks 1104 will be lost or arrive out of order. From the viewpoint of the 1105 recipient, this is not a problem since the recipient will just 1106 "blindly" acknowledge the SEQ. However, the sender needs to be 1107 prepared for lost sequence identifiers and ACKs parameters that 1108 arrive out of order. 1110 As specified in [RFC7401], an ACK parameter may acknowledge multiple 1111 sequence identifiers. While the examples in the previous sections do 1112 not illustrate such functionality, it is also permitted when 1113 employing ICE-HIP-UDP mode. 1115 In ICE-HIP-UDP mode, a retransmission of a connectivity check SHOULD 1116 be sent with the same sequence identifier in the SEQ parameter. Some 1117 tested address candidates will never produce a working address pair, 1118 and thus may cause retransmissions. Upon successful nomination an 1119 address pair, a host MAY immediately stop sending such 1120 retransmissions. 1122 ICE procedures for prioritizing candidates, eliminating redundant 1123 candidates and forming check lists (including pruning) must be 1124 followed (as specified in [I-D.ietf-ice-rfc5245bis]), with the 1125 exception that the foundation, frozen candidates and default 1126 candidates are not used. From viewpoint of the ICE specification 1127 [I-D.ietf-ice-rfc5245bis], the protocol specified in this document 1128 operates using Component ID of 1 on all candidates, and the 1129 foundation of all candidates is unique. This specification defines 1130 only "full ICE" mode, and the "lite ICE" is not supported. The 1131 reasoning behind the missing features is described in Appendix B. 1133 The connectivity check messages MUST be paced by the Ta value 1134 negotiated during the base exchange as described in Section 4.4. If 1135 neither one of the hosts announced a minimum pacing value, a value of 1136 20 ms SHOULD be used. 1138 Both hosts MUST form a priority ordered checklist and begin to check 1139 transactions every Ta milliseconds as long as the checks are running 1140 and there are candidate pairs whose tests have not started. The 1141 retransmission timeout (RTO) for the connectivity check UPDATE 1142 packets SHOULD be calculated as follows: 1144 RTO = MAX (500ms, Ta * (Num-Waiting + Num-In-Progress)) 1146 In the RTO formula, Ta is the value used for the connectivity check 1147 pacing, Num-Waiting is the number of pairs in the checklist in the 1148 "Waiting" state, and Num-In-Progress is the number of pairs in the 1149 "In-Progress" state. This is identical to the formula in 1150 [I-D.ietf-ice-rfc5245bis] when there is only one checklist. A 1151 smaller value than 500 ms for the RTO MUST NOT be used. 1153 Each connectivity check request packet MUST contain a 1154 CANDIDATE_PRIORITY parameter (see Section 5.14) with the priority 1155 value that would be assigned to a peer reflexive candidate if one was 1156 learned from the corresponding check. An UPDATE packet that 1157 acknowledges a connectivity check request MUST be sent from the same 1158 address that received the check and delivered to the same address 1159 where the check was received from. Each acknowledgment UPDATE packet 1160 MUST contain a MAPPED_ADDRESS parameter with the port, protocol, and 1161 IP address of the address where the connectivity check request was 1162 received from. 1164 Following the ICE guidelines [I-D.ietf-ice-rfc5245bis], it is 1165 RECOMMENDED to restrict the total number of connectivity checks to 1166 100 for each host association. This can be achieved by limiting the 1167 connectivity checks to the 100 candidate pairs with the highest 1168 priority. 1170 4.6.3. Rules for Concluding Connectivity Checks 1172 The controlling agent may find multiple working candidate pairs. To 1173 conclude the connectivity checks, it SHOULD nominate the pair with 1174 the highest priority. The controlling agent MUST nominate a 1175 candidate pair essentially by repeating a connectivity check using an 1176 UPDATE message that contains a SEQ parameter (with new sequence 1177 number), a ECHO_REQUEST_SIGNED parameter, the priority of the 1178 candidate in a CANDIDATE_PRIORITY parameter and NOMINATE parameter to 1179 signify conclusion of the connectivity checks. Since the nominated 1180 address pair has already been tested for reachability, the controlled 1181 host should be able to receive the message. Upon reception, the 1182 controlled host SHOULD select the nominated address pair. The 1183 response message MUST include a SEQ parameter with a new sequence id, 1184 acknowledgment of the sequence from the controlling host in an ACK 1185 parameter, a new ECHO_REQUEST_SIGNED parameter, ECHO_RESPONSE_SIGNED 1186 parameter corresponding to the ECHO_REQUEST_SIGNED parameter from the 1187 controlling host and the NOMINATE parameter. After sending this 1188 packet, the controlled host can create IPsec security associations 1189 using the nominated address candidate for delivering application 1190 payload to the controlling host. Since the message from the 1191 controlled host included a new sequence id and echo request for 1192 signature, the controlling host MUST acknowledge this with a new 1193 UPDATE message that includes an ACK and ECHO_RESPONSE_SIGNED 1194 parameters. After this final concluding message, the controlling 1195 host also can create IPsec security associations for delivering 1196 application payload to the controlled host. 1198 It is possible that packets are delayed by the network. Both hosts 1199 MUST continue to respond to any connectivity checks despite an 1200 address pair having been nominated. 1202 If all the connectivity checks have failed, the hosts MUST NOT send 1203 ESP traffic to each other but MAY continue communicating using HIP 1204 packets and the locators used for the base exchange. Also, the hosts 1205 SHOULD notify each other about the failure with a 1206 CONNECTIVITY_CHECKS_FAILED NOTIFY packet (see Section 5.10). 1208 4.7. NAT Traversal Optimizations 1210 4.7.1. Minimal NAT Traversal Support 1212 If the Responder has a fixed and publicly reachable IPv4 address and 1213 does not employ a Control Relay Server, the explicit NAT traversal 1214 mode negotiation MAY be omitted, and thus even the UDP-ENCAPSULATION 1215 mode does not have to be negotiated. In such a scenario, the 1216 Initiator sends an I1 message over UDP and the Responder responds 1217 with an R1 message over UDP without including any NAT traversal mode 1218 parameter. The rest of the base exchange follows the procedures 1219 defined in [RFC7401], except that the control and data plane use UDP 1220 encapsulation. Here, the use of UDP for NAT traversal is agreed 1221 implicitly. This way of operation is still subject to NAT timeouts, 1222 and the hosts MUST employ NAT keepalives as defined in Section 4.10. 1224 When UDP-ENCAPSULATION mode is chosen either explicitly or 1225 implicitly, the connectivity checks as defined in this document MUST 1226 NOT be used. When hosts lose connectivity, they MUST instead utilize 1227 [RFC8046] or [RFC8047] procedures, but with the difference being that 1228 UDP-based tunneling MUST be employed for the entire lifetime of the 1229 corresponding Host Association. 1231 4.7.2. Base Exchange without Connectivity Checks 1233 It is possible to run a base exchange without any connectivity checks 1234 as defined in Legacy ICE-HIP section 4.8 [RFC5770]. The procedure is 1235 applicable also in the context of this specification, so it is 1236 repeated here for completeness. 1238 In certain network environments, the connectivity checks can be 1239 omitted to reduce initial connection set-up latency because a base 1240 exchange acts as an implicit connectivity test itself. For this to 1241 work, the Initiator MUST be able to reach the Responder by simply UDP 1242 encapsulating HIP and ESP packets sent to the Responder's address. 1243 Detecting and configuring this particular scenario is prone to 1244 failure unless carefully planned. 1246 In such a scenario, the Responder MAY include UDP-ENCAPSULATION NAT 1247 traversal mode as one of the supported modes in the R1 packet. If 1248 the Responder has registered to a Control Relay Server, it MUST also 1249 include a LOCATOR_SET parameter in R1 that contains a preferred 1250 address where the Responder is able to receive UDP-encapsulated ESP 1251 and HIP packets. This locator MUST be of type "Transport address", 1252 its Traffic type MUST be "both", and it MUST have the "Preferred bit" 1253 set (see Table 1). If there is no such locator in R1, the source 1254 address of R1 is used as the Responder's preferred address. 1256 The Initiator MAY choose the UDP-ENCAPSULATION mode if the Responder 1257 listed it in the supported modes and the Initiator does not wish to 1258 use the connectivity checks defined in this document for searching 1259 for a more optimal path. In this case, the Initiator sends the I2 1260 with UDP-ENCAPSULATION mode in the NAT traversal mode parameter 1261 directly to the Responder's preferred address (i.e., to the preferred 1262 locator in R1 or to the address where R1 was received from if there 1263 was no preferred locator in R1). The Initiator MAY include locators 1264 in I2 but they MUST NOT be taken as address candidates, since 1265 connectivity checks defined in this document will not be used for 1266 connections with UDP-ENCAPSULATION NAT traversal mode. Instead, if 1267 R2 and I2 are received and processed successfully, a security 1268 association can be created and UDP-encapsulated ESP can be exchanged 1269 between the hosts after the base exchange completes. However, the 1270 Responder SHOULD NOT send any ESP to the Initiator's address before 1271 it has received data from the Initiator, as specified in Sections 1272 4.4.3. and 6.9 of [RFC7401] and in Sections 3.2.9 and 5.4 of 1273 [RFC8046]. 1275 Since an I2 packet with UDP-ENCAPSULATION NAT traversal mode selected 1276 MUST NOT be sent via a Control Relay Server, the Responder SHOULD 1277 reject such I2 packets and reply with a 1278 NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER NOTIFY packet (see 1279 Section 5.10). 1281 If there is no answer for the I2 packet sent directly to the 1282 Responder's preferred address, the Initiator MAY send another I2 via 1283 the Control Relay Server, but it MUST NOT choose UDP-ENCAPSULATION 1284 NAT traversal mode for that I2. 1286 4.7.3. Initiating a Base Exchange both with and without UDP 1287 Encapsulation 1289 It is possible to run a base exchange in parallel both with and 1290 without UDP encapsulation as defined in Legacy ICE-HIP section 4.9 in 1291 [RFC5770]. The procedure is applicable also in the context of this 1292 specification, so it is repeated here for completeness. 1294 The Initiator MAY also try to simultaneously perform a base exchange 1295 with the Responder without UDP encapsulation. In such a case, the 1296 Initiator sends two I1 packets, one without and one with UDP 1297 encapsulation, to the Responder. The Initiator MAY wait for a while 1298 before sending the other I1. How long to wait and in which order to 1299 send the I1 packets can be decided based on local policy. For 1300 retransmissions, the procedure is repeated. 1302 The I1 packet without UDP encapsulation may arrive directly, without 1303 passing any Control Data Relays, at the Responder. When this 1304 happens, the procedures in [RFC7401] are followed for the rest of the 1305 base exchange. The Initiator may receive multiple R1 packets, with 1306 and without UDP encapsulation, from the Responder. However, after 1307 receiving a valid R1 and answering it with an I2, further R1 packets 1308 that are not retransmissions of the original R1 message MUST be 1309 ignored. 1311 The I1 packet without UDP encapsulation may also arrive at a HIP- 1312 capable middlebox. When the middlebox is a HIP rendezvous server and 1313 the Responder has successfully registered with the rendezvous 1314 service, the middlebox follows rendezvous procedures in [RFC8004]. 1316 If the Initiator receives a NAT traversal mode parameter in R1 1317 without UDP encapsulation, the Initiator MAY ignore this parameter 1318 and send an I2 without UDP encapsulation and without any selected NAT 1319 traversal mode. When the Responder receives the I2 without UDP 1320 encapsulation and without NAT traversal mode, it will assume that no 1321 NAT traversal mechanism is needed. The packet processing will be 1322 done as described in [RFC7401]. The Initiator MAY store the NAT 1323 traversal modes for future use, e.g., in case of a mobility or 1324 multihoming event that causes NAT traversal to be used during the 1325 lifetime of the HIP association. 1327 4.8. Sending Control Packets after the Base Exchange 1329 The same considerations of sending control packets after the base 1330 exchange described in legacy ICE-HIP section 5.10 in [RFC5770] apply 1331 also here, so they are repeated here for completeness. 1333 After the base exchange, the two end-hosts MAY send HIP control 1334 packets directly to each other using the transport address pair 1335 established for a data channel without sending the control packets 1336 through any Control Relay Servers . When a host does not receive 1337 acknowledgments, e.g., to an UPDATE or CLOSE packet after a timeout 1338 based on local policies, a host SHOULD resend the packet through the 1339 associated Data Relay Server of the peer (if the peer listed it in 1340 its LOCATOR_SET parameter in the base exchange. 1342 If Control Relay Client sends a packet through a Control Relay 1343 Server, the Control Relay Client MUST always utilize the RELAY_TO 1344 parameter. The Control Relay Server SHOULD forward HIP control 1345 packets originating from a Control Relay Client to the address 1346 denoted in the RELAY_TO parameter. In the other direction, the 1347 Control Relay Server SHOULD forward HIP control packets to the 1348 Control Relay Clients, and MUST add a RELAY_FROM parameter to the 1349 control packets it relays to the Control Relay Clients. 1351 If the Control Relay Server is not willing or able to relay a HIP 1352 packet, it MAY notify the sender of the packet with 1353 MESSAGE_NOT_RELAYED error notification (see Section 5.10). 1355 4.9. Mobility Handover Procedure 1357 A host may move after base exchange and connectivity checks. 1358 Mobility extensions for HIP [RFC8046] define handover procedures 1359 without NATs. In this section, we define how two hosts interact with 1360 handover procedures in scenarios involving NATs. The specified 1361 extensions define only simple mobility using a pair of security 1362 associations, and multihoming extensions are left to be defined in 1363 later specifications. The procedures in this section offer the same 1364 functionality as "ICE restart" specified in 1365 [I-D.ietf-ice-rfc5245bis]. The example described in this section 1366 shows only a Control Relay Server for the peer host for the sake of 1367 simplicity, but the mobile host may also have a Control Relay Server. 1369 The assumption here is that the two hosts have successfully 1370 negotiated and chosen the ICE-HIP-UDP mode during the base exchange 1371 as defined in Section 4.3. The Initiator of the base exchange MUST 1372 store information that it was the controlling host during the base 1373 exchange. Similarly, the Responder MUST store information that it 1374 was the controlled host during the base exchange. 1376 Prior to starting the handover procedures with all peer hosts, the 1377 mobile host SHOULD first send its locators in UPDATE messages to its 1378 Control and Data Relay Servers if it has registered to such. It 1379 SHOULD wait for all of them to respond for a configurable time, by 1380 default two minutes, and then continue with the handover procedure 1381 without information from the Relay Server that did not respond. As 1382 defined in Section 4.1, a response message from a Control Relay 1383 Server includes a REG_FROM parameter that describes the server 1384 reflexive candidate of the mobile host to be used in the candidate 1385 exchange during the handover. Similarly, an UPDATE to a Data Relay 1386 Server is necessary to make sure the Data Relay Server can forward 1387 data to the correct IP address after a handoff. 1389 The mobility extensions for NAT traversal are illustrated in 1390 Figure 6. The mobile host is the host that has changed its locators, 1391 and the peer host is the host it has a host association with. The 1392 mobile host may have multiple peers and it repeats the process with 1393 all of its peers. In the figure, the Control Relay Server belongs to 1394 the peer host, i.e., the peer host is a Control Relay Client for the 1395 Control Relay Server. Note that the figure corresponds to figure 3 1396 in [RFC8046], but the difference is that the main UPDATE procedure is 1397 carried over the relay and the connectivity is tested separately. 1398 Next, we describe the procedure in the figure in detail. 1400 Mobile Host Control Relay Server Peer Host 1401 | 1. UDP(UPDATE(ESP_INFO, | | 1402 | LOC_SET, SEQ)) | | 1403 +--------------------------------->| 2. UDP(UPDATE(ESP_INFO, | 1404 | | LOC_SET, SEQ, | 1405 | | RELAY_FROM)) | 1406 | +------------------------------->| 1407 | | | 1408 | | 3. UDP(UPDATE(ESP_INFO, SEQ, | 1409 | | ACK, ECHO_REQ_SIGN, | 1410 | | RELAY_TO)) | 1411 | 4. UDP(UPDATE(ESP_INFO, SEQ, |<-------------------------------+ 1412 | ACK, ECHO_REQ_SIGN, | | 1413 | RELAY_TO)) | | 1414 |<---------------------------------+ | 1415 | | | 1416 | 5. UDP(UPDATE(ACK, | | 1417 | ECHO_RESP_SIGNED)) | | 1418 +--------------------------------->| 6. UDP(UPDATE(ACK, | 1419 | | ECHO_RESP_SIGNED, | 1420 | | RELAY_FROM)) | 1421 | +------------------------------->| 1422 | | | 1423 | 7. connectivity checks over UDP | 1424 +<----------------------------------------------------------------->+ 1425 | | | 1426 | 8. ESP data over UDP | 1427 +<----------------------------------------------------------------->+ 1428 | | | 1430 Figure 6: HIP UPDATE procedure 1432 In step 1, the mobile host has changed location and sends a location 1433 update to its peer through the Control Relay Server of the peer. It 1434 sends an UPDATE packet with source HIT belonging to itself and 1435 destination HIT belonging to the peer host. In the packet, the 1436 source IP address belongs to the mobile host and the destination to 1437 the Control Relay Server. The packet contains an ESP_INFO parameter, 1438 where, in this case, the OLD SPI and NEW SPI parameters both contain 1439 the pre-existing incoming SPI. The packet also contains the locators 1440 of the mobile host in a LOCATOR_SET parameter. The packet contains 1441 also a SEQ number to be acknowledged by the peer. As specified in 1442 [RFC8046], the packet may also include a HOST_ID (for middlebox 1443 inspection) and DIFFIE_HELLMAN parameter for rekeying. 1445 In step 2, the Control Relay Server receives the UPDATE packet and 1446 forwards it to the peer host (i.e. Control Relay Client). The 1447 Control Relay Server rewrites the destination IP address and appends 1448 a RELAY_FROM parameter to the message. 1450 In step 3, the peer host receives the UPDATE packet, processes it and 1451 responds with another UPDATE message. The message is destined to the 1452 HIT of mobile host and to the IP address of the Control Relay Server. 1453 The message includes an ESP_INFO parameter where, in this case, the 1454 OLD SPI and NEW SPI parameters both contain the pre-existing incoming 1455 SPI. The peer includes a new SEQ and ECHO_REQUEST_SIGNED parameters 1456 to be acknowledged by the mobile host. The message acknowledges the 1457 SEQ parameter of the earlier message with an ACK parameter. The 1458 RELAY_TO parameter specifies the address of the mobile host where the 1459 Control Relay Server should forward the message. 1461 In step 4, the Control Relay Server receives the message, rewrites 1462 the destination IP address and UDP port according to the RELAY_TO 1463 parameter, and then forwards the modified message to the mobile host. 1465 In step 5, the mobile host receives the UPDATE packet and processes 1466 it. The mobile host concludes the handover procedure by 1467 acknowledging the received SEQ parameter with an ACK parameter and 1468 the ECHO_REQUEST_SIGNED parameter with ECHO_RESPONSE_SIGNED 1469 parameter. The mobile host delivers the packet to the HIT of the 1470 peer and to the address of the HIP relay. The mobile host can start 1471 connectivity checks after this packet. 1473 In step 6, HIP relay receives the UPDATE packet and forwards it to 1474 the peer host (i.e. Relay Client). The HIP relay rewrites the 1475 destination IP address and port, and then appends a RELAY_FROM 1476 parameter to the message. When the peer host receives this 1477 concluding UPDATE packet, it can initiate the connectivity checks. 1479 In step 7, the two hosts test for connectivity across NATs according 1480 to procedures described in Section 4.6. The original Initiator of 1481 the communications is the controlling and the original Responder is 1482 the controlled host. 1484 In step 8, the connectivity checks are successfully completed and the 1485 controlling host has nominated one address pair to be used. The 1486 hosts set up security associations to deliver the application 1487 payload. 1489 It is worth noting that the Control and Data Relay Client do not have 1490 to re-register for the related services after a handoff. However, if 1491 a Data Relay Client has registered a relayed address candidate from a 1492 Data Relay Server, the Data Relay Client MUST reactivate the address 1493 before the connectivity checks by sending an UPDATE message 1494 containing PEER_PERMISSION parameter as described in Section 4.12.1. 1496 Otherwise, the Data Relay Server drops ESP packets sent to the 1497 relayed address. 1499 In so called "double jump" or simultaneous mobility scenario both 1500 peers change their location simultaneously. In such a case, both 1501 peers trigger the procedure described earlier in this section at the 1502 same time. In other words, both of the communicating hosts send an 1503 UPDATE packet carrying locators at the same time or with some delay. 1504 When the locators are exchanged almost simultaneously (reliably via 1505 Control Relay Servers), the two hosts can continue with connectivity 1506 checks after both have completed separately the steps in Figure 6. 1507 The problematic case occurs when the one of the hosts (referred here 1508 as host "M") moves later during the connectivity checks. In such a 1509 case, host M sends a locator to the peer which is in the middle of 1510 connectivity checks. Upon receiving the UPDATE message, the peer 1511 responds with an UPDATE with ECHO_REQ_SIGN as described in step 3 in 1512 Figure 6. Upon receiving the valid response from host M as described 1513 in step 6, the peer host MUST restart the connectivity checks with 1514 host M. This way, both hosts start the connectivity checks roughly 1515 in a synchronized way. It is also important that peer host does not 1516 restart the connectivity checks until it has received a valid "fresh" 1517 confirmation from host M because the UPDATE message carrying locators 1518 could be replayed by an attacker. 1520 4.10. NAT Keepalives 1522 To prevent NAT states from expiring, communicating hosts MUST send 1523 periodic keepalives to other hosts with which they have established a 1524 Host Association every 15 seconds (the so called Tr value in ICE). 1525 Other values MAY be used, but a Tr value smaller than 15 seconds MUST 1526 NOT be used. Both a Control/Data Relay Client and Control/Data Relay 1527 Server, as well as two peers employing UDP-ENCAPSULATION or ICE-HIP- 1528 UDP mode, SHOULD send HIP NOTIFY packets unless they have exchanged 1529 some other traffic over the used UDP ports. However, the Data Relay 1530 Client and Data Relay Server MUST employ only HIP NOTIFY packets in 1531 order to keep the server reflexive candidates alive. The keepalive 1532 message encoding format is defined in Section 5.3. If the base 1533 exchange or mobility handover procedure occurs during an extremely 1534 slow path, a host (with a Host Association with the peer) MAY also 1535 send HIP NOTIFY packets every 15 seconds to keep the path active with 1536 the recipient. 1538 4.11. Closing Procedure 1540 The two-way procedure for closing a HIP association and the related 1541 security associations is defined in [RFC7401]. One host initiates 1542 the procedure by sending a CLOSE message and the recipient confirms 1543 it with CLOSE_ACK. All packets are protected using HMACs and 1544 signatures, and the CLOSE messages includes a ECHO_REQUEST_SIGNED 1545 parameter to protect against replay attacks. 1547 The same procedure for closing HIP associations applies also here, 1548 but the messaging occurs using the UDP encapsulated tunnel that the 1549 two hosts employ. A host sending the CLOSE message SHOULD first send 1550 the message over a direct link. After a number of retransmissions, 1551 it MUST send over a Control Relay Server of the recipient if one 1552 exists. The host receiving the CLOSE message directly without a 1553 Control Data Relay SHOULD respond directly. If CLOSE message came 1554 via a Control Data Relay, the host SHOULD respond using the same 1555 Control Data Relay. 1557 4.12. Relaying Considerations 1559 4.12.1. Forwarding Rules and Permissions 1561 The Data Relay Server uses a similar permission model as a TURN 1562 server: before the Data Relay Server forwards any ESP data packets 1563 from a peer to a Data Relay Client (or the other direction), the 1564 client MUST set a permission for the peer's address. The permissions 1565 also install a forwarding rule for each direction, similar to TURN's 1566 channels, based on the Security Parameter Index (SPI) values in the 1567 ESP packets. 1569 Permissions are not required for HIP control packets. However, if a 1570 relayed address (as conveyed in the RELAYED_ADDRESS parameter from 1571 the Data Relay Server) is selected to be used for data, the Control 1572 Relay Client MUST send an UPDATE message to the Data Relay Server 1573 containing a PEER_PERMISSION parameter (see Section 5.13) with the 1574 following information: the UDP port and address for the server 1575 reflexive address, the UDP port and address of the peer, and the 1576 inbound and outbound SPIs used for ESP. The packet MUST be sent to 1577 the same UDP tunnel the Client employed in the base exchange to 1578 contact the Server (i.e., not to the port occupied by the server 1579 reflexive candidate). To avoid packet dropping of ESP packets, the 1580 Control Relay Client SHOULD send the PEER_PERMISSION parameter before 1581 connectivity checks both in the case of base exchange and a mobility 1582 handover. It is worth noting that the UPDATE message includes a SEQ 1583 parameter (as specified in [RFC7401]) that the Data Relay Server must 1584 acknowledge, so that the Control Relay Client can resend the message 1585 with PEER_PERMISSION parameter if it gets lost. 1587 When a Data Relay Server receives an UPDATE with a PEER_PERMISSION 1588 parameter, it MUST check if the sender of the UPDATE is registered 1589 for data relaying service, and drop the UPDATE if the host was not 1590 registered. If the host was registered, the Data Relay Server checks 1591 if there is a permission with matching information (protocol, 1592 addresses, ports and SPI values). If there is no such permission, a 1593 new permission MUST be created and its lifetime MUST be set to 5 1594 minutes. If an identical permission already existed, it MUST be 1595 refreshed by setting the lifetime to 5 minutes. A Data Relay Client 1596 SHOULD refresh permissions 1 minute before the expiration when the 1597 permission is still needed. 1599 When a Data Relay Server receives an UPDATE from a registered client 1600 but without a PEER_PERMISSION parameter and with a new locator set, 1601 the Data Relay Server can assume that the mobile host has changed its 1602 location and, thus, is not reachable in its previous location. In 1603 such an event, the Data Relay Server SHOULD deactivate the permission 1604 and stop relaying data plane traffic to the client. 1606 The relayed address MUST be activated with the PEER_PERMISSION 1607 parameter both after a base exchange and after a handover procedure 1608 with another ICE-HIP-UDP capable host. Unless activated, the Data 1609 Relay Server MUST drop all ESP packets. It is worth noting that a 1610 Data Relay Client does not have to renew its registration upon a 1611 change of location UPDATE, but only when the lifetime of the 1612 registration is close to end. 1614 4.12.2. HIP Data Relay and Relaying of Control Packets 1616 When a Data Relay Server accepts to relay UDP encapsulated ESP 1617 between a Data Relay Client and its peer, the Data Relay Server opens 1618 a UDP port (relayed address) for this purpose as described in 1619 Section 4.1. This port can be used for delivering also control 1620 packets because connectivity checks also cover the path through the 1621 Data Relay Server. If the Data Relay Server receives a UDP 1622 encapsulated HIP control packet on that port, it MUST forward the 1623 packet to the Data Relay Client and add a RELAY_FROM parameter to the 1624 packet as if the Data Relay Server were acting as a Control Relay 1625 Server. When the Data Relay Client replies to a control packet with 1626 a RELAY_FROM parameter via its Data Relay Server, the Data Relay 1627 Client MUST add a RELAY_TO parameter containing the peer's address 1628 and use the address of its Data Relay Server as the destination 1629 address. Further, the Data Relay Server MUST send this packet to the 1630 peer's address from the relayed address. 1632 If the Data Relay Server receives a UDP packet that is not a HIP 1633 control packet to the relayed address, it MUST check if it has a 1634 permission set for the peer the packet is arriving from (i.e., the 1635 sender's address and SPI value matches to an installed permission). 1636 If permissions are set, the Data Relay Server MUST forward the packet 1637 to the Data Relay Client that created the permission. The Data Relay 1638 Server MUST also implement the similar checks for the reverse 1639 direction (i.e. ESP packets from the Data Relay Client to the peer). 1640 Packets without a permission MUST be dropped silently. 1642 4.12.3. Handling Conflicting SPI Values 1644 From the viewpoint of a host, its remote peers can have overlapping 1645 inbound SPI numbers because the IPsec uses also the destination IP 1646 address to index the remote peer host. However, a Data Relay Server 1647 can represent multiple remote peers, thus masquerading the actual 1648 destination. Since a Data Relay Server may have to deal with a 1649 multitude of Relay Clients and their peers, a Data Relay Server may 1650 experience collisions in the SPI namespace, thus being unable forward 1651 datagrams to the correct destination. Since the SPI space is 32 bits 1652 and the SPI values should be random, the probability for a 1653 conflicting SPI value is fairly small, but could occur on a busy Data 1654 Relay Server. The two problematic cases are described in this 1655 section. 1657 In the first scenario, the SPI collision problems occurs if two hosts 1658 have registered to the same Data Relay Server and a third host 1659 initiates base exchange with both of them. Here, the two Responders 1660 (i.e. Data Relay Clients) claim the same inbound SPI number with the 1661 same Initiator (peer). However, in this case, the Data Relay Server 1662 has allocated separate UDP ports for the two Data Relay Clients 1663 acting now as Responders (as recommended in Section 6.5). When the 1664 third host sends an ESP packet, the Data Relay Server is able to 1665 forward the packet to the correct Data Relay Client because the 1666 destination UDP port is different for each of the clients. 1668 In the second scenario, an SPI collision may occur when two 1669 Initiators run a base exchange to the same Responder (i.e. Data 1670 Relay Client), and both of the Initiators claim the same inbound SPI 1671 at the Data Relay Server using PEER_PERMISSION Parameter. In this 1672 case, the Data Relay Server cannot disambiguate the correct 1673 destination of an ESP packet originating from the Data Relay Client 1674 because the SPI could belong to either of the peers (and destination 1675 IP and UDP port belonging to the Data Relay Server are not unique 1676 either). The recommended way and a contingency plan to solve this 1677 issue are described below. 1679 The recommend way to mitigate the problem is as follows. For each 1680 new Host Association, A Data Relay Client acting as a Responder 1681 SHOULD register a new server reflexive candidate as described in 1682 Section 4.2. Similarly, the Data Relay Server SHOULD NOT re-use the 1683 port numbers as described in Section 6.5. This way, each server 1684 reflexive candidate for the Data Relay Client has a separate UDP port 1685 that the Data Relay Server can use to disambiguate packet 1686 destinations in case of SPI collisions. 1688 When the Data Relay Client is not registering or failed to register a 1689 new relay candidate for a new peer, the Data Relay Client MUST follow 1690 a contingency plan as follows. Upon receiving an I2 with a colliding 1691 SPI, the Data Relay client acting as the Responder MUST NOT include 1692 the relayed address candidate in the R2 message because the Data 1693 Relay Server would not be able demultiplex the related ESP packet to 1694 the correct Initiator. The same applies also the handover 1695 procedures; the Data Relay Client MUST NOT include the relayed 1696 address candidate when sending its new locator set in an UPDATE to 1697 its peer if it would cause a SPI conflict with another peer. 1699 5. Packet Formats 1701 The following subsections define the parameter and packet encodings 1702 for the HIP and ESP packets. All values MUST be in network byte 1703 order. 1705 It is worth noting that most of the parameters are shown for the sake 1706 of completeness even though they are specified already in Legacy ICE- 1707 HIP [RFC5770]. New parameters are explicitly described as new. 1709 5.1. HIP Control Packets 1711 Figure 7 illustrates the packet format for UDP-encapsulated HIP. The 1712 format is identical to Legacy ICE-HIP [RFC5770]. 1714 0 1 2 3 1715 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 1716 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1717 | Source Port | Destination Port | 1718 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1719 | Length | Checksum | 1720 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1721 | 32 bits of zeroes | 1722 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1723 | | 1724 ~ HIP Header and Parameters ~ 1725 | | 1726 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1728 Figure 7: Format of UDP-Encapsulated HIP Control Packets 1730 HIP control packets are encapsulated in UDP packets as defined in 1731 Section 2.2 of [RFC3948], "IKE Header Format for Port 4500", except 1732 that a different port number is used. Figure 7 illustrates the 1733 encapsulation. The UDP header is followed by 32 zero bits that can 1734 be used to differentiate HIP control packets from ESP packets. The 1735 HIP header and parameters follow the conventions of [RFC7401] with 1736 the exception that the HIP header checksum MUST be zero. The HIP 1737 header checksum is zero for two reasons. First, the UDP header 1738 already contains a checksum. Second, the checksum definition in 1739 [RFC7401] includes the IP addresses in the checksum calculation. The 1740 NATs that are unaware of HIP cannot recompute the HIP checksum after 1741 changing IP addresses. 1743 A Control/Data Relay Server or a non-relay Responder SHOULD listen at 1744 UDP port 10500 for incoming UDP-encapsulated HIP control packets. If 1745 some other port number is used, it needs to be known by potential 1746 Initiators. 1748 5.2. Connectivity Checks 1750 HIP connectivity checks are HIP UPDATE packets. The format is 1751 specified in [RFC7401]. 1753 5.3. Keepalives 1755 The RECOMMENDED encoding format for keepalives is HIP NOTIFY packets 1756 as specified in [RFC7401] with Notify message type field set to 1757 NAT_KEEPALIVE [TBD by IANA: 16385] and with an empty Notification 1758 data field. It is worth noting that sending of such a HIP NOTIFY 1759 message MAY be omitted if the host is actively (or passively) sending 1760 some other traffic (HIP or ESP) to the peer host over the related UDP 1761 tunnel during the Tr period. For instance, the host MAY actively 1762 send ICMPv6 requests (or respond with an ICMPv6 response) inside the 1763 ESP tunnel to test the health of the associated IPsec security 1764 association. Alternatively, the host MAY use UPDATE packets as a 1765 substitute. A minimal UPDATE packet would consist of a SEQ and 1766 ECHO_REQ_SIGN parameters, and a more complex would involve rekeying 1767 procedures as specified in section 6.8 in [RFC7402]. It is worth 1768 noting that a host actively sending periodic UPDATE packets to a busy 1769 server may increase the computational load of the server since it has 1770 to verify HMACs and signatures in UPDATE messages. 1772 5.4. NAT Traversal Mode Parameter 1774 The format of NAT traversal mode parameter is borrowed from Legacy 1775 ICE-HIP [RFC5770]. The format of the NAT_TRAVERSAL_MODE parameter is 1776 similar to the format of the ESP_TRANSFORM parameter in [RFC7402] and 1777 is shown in Figure 8. The Native ICE-HIP extension specified in this 1778 document defines the new NAT traversal mode identifier for ICE-HIP- 1779 UDP and reuses the UDP-ENCAPSULATION mode from Legacy ICE-HIP 1780 [RFC5770]. The identifier named RESERVED is reserved for future use. 1781 Future specifications may define more traversal modes. 1783 0 1 2 3 1784 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 1785 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1786 | Type | Length | 1787 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1788 | Reserved | Mode ID #1 | 1789 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1790 | Mode ID #2 | Mode ID #3 | 1791 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1792 | Mode ID #n | Padding | 1793 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1795 Type 608 1796 Length length in octets, excluding Type, Length, and padding 1797 Reserved zero when sent, ignored when received 1798 Mode ID defines the proposed or selected NAT traversal mode(s) 1800 The following NAT traversal mode IDs are defined: 1802 ID name Value 1803 RESERVED 0 1804 UDP-ENCAPSULATION 1 1805 ICE-HIP-UDP 3 1807 Figure 8: Format of the NAT_TRAVERSAL_MODE Parameter 1809 The sender of a NAT_TRAVERSAL_MODE parameter MUST make sure that 1810 there are no more than six (6) Mode IDs in one NAT_TRAVERSAL_MODE 1811 parameter. Conversely, a recipient MUST be prepared to handle 1812 received NAT traversal mode parameters that contain more than six 1813 Mode IDs by accepting the first six Mode IDs and dropping the rest. 1814 The limited number of Mode IDs sets the maximum size of the 1815 NAT_TRAVERSAL_MODE parameter. The modes MUST be in preference order, 1816 most preferred mode(s) first. 1818 Implementations conforming to this specification MUST implement UDP- 1819 ENCAPSULATION and SHOULD implement ICE-HIP-UDP modes. 1821 5.5. Connectivity Check Transaction Pacing Parameter 1823 The TRANSACTION_PACING is a new parameter, and it shown in Figure 9 1824 contains only the connectivity check pacing value, expressed in 1825 milliseconds, as a 32-bit unsigned integer. 1827 0 1 2 3 1828 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 1829 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1830 | Type | Length | 1831 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1832 | Min Ta | 1833 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1835 Type 610 1836 Length 4 1837 Min Ta the minimum connectivity check transaction pacing 1838 value the host would use (in milliseconds) 1840 Figure 9: Format of the TRANSACTION_PACING Parameter 1842 5.6. Relay and Registration Parameters 1844 The format of the REG_FROM, RELAY_FROM, and RELAY_TO parameters is 1845 shown in Figure 10. All parameters are identical except for the 1846 type. REG_FROM is the only parameter covered with the signature. 1848 0 1 2 3 1849 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 1850 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1851 | Type | Length | 1852 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1853 | Port | Protocol | Reserved | 1854 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1855 | | 1856 | Address | 1857 | | 1858 | | 1859 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1861 Type REG_FROM: 950 1862 RELAY_FROM: 63998 1863 RELAY_TO: 64002 1864 Length 20 1865 Port transport port number; zero when plain IP is used 1866 Protocol IANA assigned, Internet Protocol number. 1867 17 for UDP, 0 for plain IP 1868 Reserved reserved for future use; zero when sent, ignored 1869 when received 1870 Address an IPv6 address or an IPv4 address in "IPv4-Mapped 1871 IPv6 address" format 1873 Figure 10: Format of the REG_FROM, RELAY_FROM, and RELAY_TO 1874 Parameters 1876 REG_FROM contains the transport address and protocol from which the 1877 Control Relay Server sees the registration coming. RELAY_FROM 1878 contains the address from which the relayed packet was received by 1879 the Control Relay Server and the protocol that was used. RELAY_TO 1880 contains the same information about the address to which a packet 1881 should be forwarded. 1883 5.7. LOCATOR_SET Parameter 1885 This specification reuses the format for UDP-based locators as 1886 specified in Legacy ICE-HIP [RFC5770] to be used for communicating 1887 the address candidates between two hosts. The generic and NAT- 1888 traversal-specific locator parameters are illustrated in Figure 11. 1890 0 1 2 3 1891 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 1892 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1893 | Type | Length | 1894 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1895 | Traffic Type | Locator Type | Locator Length| Reserved |P| 1896 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1897 | Locator Lifetime | 1898 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1899 | Locator | 1900 | | 1901 | | 1902 | | 1903 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1904 . . 1905 . . 1906 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1907 | Traffic Type | Loc Type = 2 | Locator Length| Reserved |P| 1908 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1909 | Locator Lifetime | 1910 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1911 | Transport Port | Transp. Proto| Kind | 1912 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1913 | Priority | 1914 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1915 | SPI | 1916 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1917 | Address | 1918 | | 1919 | | 1920 | | 1921 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1923 Figure 11: LOCATOR_SET Parameter 1925 The individual fields in the LOCATOR_SET parameter are described in 1926 Table 1. 1928 +-----------+----------+--------------------------------------------+ 1929 | Field | Value(s) | Purpose | 1930 +-----------+----------+--------------------------------------------+ 1931 | Type | 193 | Parameter type | 1932 | Length | Variable | Length in octets, excluding Type and | 1933 | | | Length fields and padding | 1934 | Traffic | 0-2 | Is the locator for HIP signaling (1), for | 1935 | Type | | ESP (2), or for both (0) | 1936 | Locator | 2 | "Transport address" locator type | 1937 | Type | | | 1938 | Locator | 7 | Length of the fields after Locator | 1939 | Length | | Lifetime in 4-octet units | 1940 | Reserved | 0 | Reserved for future extensions | 1941 | Preferred | 0 or 1 | Set to 1 for a Locator in R1 if the | 1942 | (P) bit | | Responder can use it for the rest of the | 1943 | | | base exchange, otherwise set to zero | 1944 | Locator | Variable | Locator lifetime in seconds | 1945 | Lifetime | | | 1946 | Transport | Variable | Transport layer port number | 1947 | Port | | | 1948 | Transport | Variable | IANA assigned, transport layer Internet | 1949 | Protocol | | Protocol number. Currently only UDP (17) | 1950 | | | is supported. | 1951 | Kind | Variable | 0 for host, 1 for server reflexive, 2 for | 1952 | | | peer reflexive or 3 for relayed address | 1953 | Priority | Variable | Locator's priority as described in | 1954 | | | [I-D.ietf-ice-rfc5245bis]. It is worth | 1955 | | | noting that while the priority of a single | 1956 | | | locator candidate is 32-bits, but an | 1957 | | | implementation should use a 64-bit integer | 1958 | | | to calculate the priority of a candidate | 1959 | | | pair for the ICE priority algorithm. | 1960 | SPI | Variable | Security Parameter Index (SPI) value that | 1961 | | | the host expects to see in incoming ESP | 1962 | | | packets that use this locator | 1963 | Address | Variable | IPv6 address or an "IPv4-Mapped IPv6 | 1964 | | | address" format IPv4 address [RFC4291] | 1965 +-----------+----------+--------------------------------------------+ 1967 Table 1: Fields of the LOCATOR_SET Parameter 1969 5.8. RELAY_HMAC Parameter 1971 As specified in Legacy ICE-HIP [RFC5770], the RELAY_HMAC parameter 1972 value has the TLV type 65520. It has the same semantics as RVS_HMAC 1973 [RFC8004]. 1975 5.9. Registration Types 1977 The REG_INFO, REG_REQ, REG_RESP, and REG_FAILED parameters contain 1978 Registration Type [RFC8003] values for Control Relay Server 1979 registration. The value for RELAY_UDP_HIP is 2 as specified in 1980 Legacy ICE-HIP [RFC5770]. The value for RELAY_UDP_ESP is (value [TBD 1981 by IANA: 3]). 1983 5.10. Notify Packet Types 1985 A Control/Data Relay Server and end-hosts can use NOTIFY packets to 1986 signal different error conditions. The NOTIFY packet types are the 1987 same as in Legacy ICE-HIP [RFC5770]. 1989 The Notify Packet Types [RFC7401] are shown below. The Notification 1990 Data field for the error notifications SHOULD contain the HIP header 1991 of the rejected packet and SHOULD be empty for the 1992 CONNECTIVITY_CHECKS_FAILED type. 1994 NOTIFICATION PARAMETER - ERROR TYPES Value 1995 ------------------------------------ ----- 1997 NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER 60 1999 If a Control Relay Server does not forward a base exchange packet 2000 due to missing NAT traversal mode parameter, or the Initiator 2001 selects a NAT traversal mode that the (non-relay) Responder did 2002 not expect, the Control Relay Server or the Responder may send 2003 back a NOTIFY error packet with this type. 2005 CONNECTIVITY_CHECKS_FAILED 61 2007 Used by the end-hosts to signal that NAT traversal connectivity 2008 checks failed and did not produce a working path. 2010 MESSAGE_NOT_RELAYED 62 2012 Used by a Control Relay Server to signal that is was not able or 2013 willing to relay a HIP packet. 2015 SERVER_REFLEXIVE_CANDIDATE_ALLOCATION_FAILED 63 2016 Used by a Data Relay Server to signal that is was not able or 2017 willing to allocate a new server reflexive candidate for the Data 2018 Relay Client 2020 5.11. ESP Data Packets 2022 The format for ESP data packets is identical to Legacy ICE-HIP 2023 [RFC5770]. 2025 [RFC3948] describes the UDP encapsulation of the IPsec ESP transport 2026 and tunnel mode. On the wire, the HIP ESP packets do not differ from 2027 the transport mode ESP, and thus the encapsulation of the HIP ESP 2028 packets is same as the UDP encapsulation transport mode ESP. 2029 However, the (semantic) difference to Bound End-to-End Tunnel (BEET) 2030 mode ESP packets used by HIP is that IP header is not used in BEET 2031 integrity protection calculation. 2033 During the HIP base exchange, the two peers exchange parameters that 2034 enable them to define a pair of IPsec ESP security associations (SAs) 2035 as described in [RFC7402]. When two peers perform a UDP-encapsulated 2036 base exchange, they MUST define a pair of IPsec SAs that produces 2037 UDP-encapsulated ESP data traffic. 2039 The management of encryption/authentication protocols and SPIs is 2040 defined in [RFC7402]. The UDP encapsulation format and processing of 2041 HIP ESP traffic is described in Section 6.1 of [RFC7402]. 2043 5.12. RELAYED_ADDRESS and MAPPED_ADDRESS Parameters 2045 While the type values are new, the format of the RELAYED_ADDRESS and 2046 MAPPED_ADDRESS parameters (Figure 12) is identical to REG_FROM, 2047 RELAY_FROM and RELAY_TO parameters. This document specifies only the 2048 use of UDP relaying, and, thus, only protocol 17 is allowed. 2049 However, future documents may specify support for other protocols. 2051 0 1 2 3 2052 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 2053 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2054 | Type | Length | 2055 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2056 | Port | Protocol | Reserved | 2057 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2058 | | 2059 | Address | 2060 | | 2061 | | 2062 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2064 Type [TBD by IANA; 2065 RELAYED_ADDRESS: 4650 2066 MAPPED_ADDRESS: 4660] 2067 Length 20 2068 Port the UDP port number 2069 Protocol IANA assigned, Internet Protocol number (17 for UDP) 2070 Reserved reserved for future use; zero when sent, ignored 2071 when received 2072 Address an IPv6 address or an IPv4 address in "IPv4-Mapped 2073 IPv6 address" format 2075 Figure 12: Format of the RELAYED_ADDRESS and MAPPED_ADDRESS 2076 Parameters 2078 5.13. PEER_PERMISSION Parameter 2080 The format of the new PEER_PERMISSION parameter is shown in 2081 Figure 13. The parameter is used for setting up and refreshing 2082 forwarding rules and the permissions for data packets at the Data 2083 Relay Server. The parameter contains one or more sets of Port, 2084 Protocol, Address, Outbound SPI (OSPI), and Inbound SPI (ISPI) 2085 values. One set defines a rule for one peer address. 2087 0 1 2 3 2088 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 2089 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2090 | Type | Length | 2091 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2092 | RPort | PPort | 2093 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2094 | Protocol | Reserved | 2095 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2096 | | 2097 | RAddress | 2098 | | 2099 | | 2100 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2101 | | 2102 | PAddress | 2103 | | 2104 | | 2105 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2106 | OSPI | 2107 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2108 | ISPI | 2109 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2111 Type [TBD by IANA; 4680] 2112 Length length in octets, excluding Type and Length 2113 RPort the transport layer (UDP) port at the Data Relay Server 2114 (i.e. the port of the server reflexive candidate) 2115 PPort the transport layer (UDP) port number of the peer 2116 Protocol IANA assigned, Internet Protocol number (17 for UDP) 2117 Reserved reserved for future use; zero when sent, ignored 2118 when received 2119 RAddress an IPv6 address, or an IPv4 address in "IPv4-Mapped 2120 IPv6 address" format, of the server reflexive candidate 2121 PAddress an IPv6 address, or an IPv4 address in "IPv4-Mapped 2122 IPv6 address" format, of the peer 2123 OSPI the outbound SPI value the Data Relay Client is using for 2124 the peer 2125 ISPI the inbound SPI value the Data Relay Client is using for 2126 the peer 2128 Figure 13: Format of the PEER_PERMISSION Parameter 2130 5.14. HIP Connectivity Check Packets 2132 The connectivity request messages are HIP UPDATE packets containing a 2133 new CANDIDATE_PRIORITY parameter (Figure 14). Response UPDATE 2134 packets contain a MAPPED_ADDRESS parameter (Figure 12). 2136 0 1 2 3 2137 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 2138 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2139 | Type | Length | 2140 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2141 | Priority | 2142 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2144 Type [TBD by IANA; 4700] 2145 Length 4 2146 Priority the priority of a (potential) peer reflexive candidate 2148 Figure 14: Format of the CANDIDATE_PRIORITY Parameter 2150 5.15. NOMINATE parameter 2152 Figure 15 shows the NOMINATE parameter that is used to conclude the 2153 candidate nomination process. 2155 0 1 2 3 2156 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 2157 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2158 | Type | Length | 2159 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2160 | Reserved | 2161 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2163 Type [TBD by IANA; 4710] 2164 Length 4 2165 Reserved Reserved for future extension purposes 2167 Figure 15: Format of the NOMINATE Parameter 2169 6. Security Considerations 2171 The security considerations are the same as in Legacy ICE-HIP 2172 [RFC5770], but are repeated here for the sake of completeness. 2174 6.1. Privacy Considerations 2176 The locators are in plain text format in favor of inspection at HIP- 2177 aware middleboxes in the future. The current document does not 2178 specify encrypted versions of LOCATOR_SETs, even though it could be 2179 beneficial for privacy reasons to avoid disclosing them to 2180 middleboxes. 2182 It is also possible that end-users may not want to reveal all 2183 locators to each other. For example, tracking the physical location 2184 of a multihoming end-host may become easier if it reveals all 2185 locators to its peer during a base exchange. Also, revealing host 2186 addresses exposes information about the local topology that may not 2187 be allowed in all corporate environments. For these two reasons, an 2188 end-host may exclude certain host addresses from its LOCATOR_SET 2189 parameter. However, such behavior creates non-optimal paths when the 2190 hosts are located behind the same NAT. Especially, this could be 2191 problematic with a legacy NAT that does not support routing from the 2192 private address realm back to itself through the outer address of the 2193 NAT. This scenario is referred to as the hairpin problem [RFC5128]. 2194 With such a legacy NAT, the only option left would be to use a 2195 relayed transport address from a TURN server. 2197 The use of Control and Data Relay Servers can be also useful for 2198 privacy purposes. For example, a privacy concerned Responder may 2199 reveal only its Control Relay Server and Relayed candidates to 2200 Initiators. This same mechanism also protects the Responder against 2201 Denial-of-Service (DoS) attacks by allowing the Responder to initiate 2202 new connections even if its relays would be unavailable due to a DoS 2203 attack. 2205 6.2. Opportunistic Mode 2207 In opportunistic HIP mode, an Initiator sends an I1 with without 2208 setting the destination HIT of the Responder (i.e. the Control Relay 2209 Client). A Control Relay Server SHOULD have a unique IP address per 2210 Control Relay Client when the Control Relay Server is serving more 2211 than one Control Relay Client and supports opportunistic mode. 2212 Otherwise, the Control Relay Server cannot guarantee to deliver the 2213 I1 packet to the intended recipient. Future extensions of this 2214 document may allow opportunistic mode to be used with non-unique IP 2215 addresses to be utilized either as a HIP-level anycast or multicast 2216 mechanism. Both of the mentioned cases would require a separate 2217 registration parameters that the Control Relay Server proposes and 2218 the Control Client Server accepts during registration. 2220 6.3. Base Exchange Replay Protection for Control Relay Server 2222 In certain scenarios, it is possible that an attacker, or two 2223 attackers, can replay an earlier base exchange through a Control 2224 Relay Server by masquerading as the original Initiator and Responder. 2225 The attack does not require the attacker(s) to compromise the private 2226 key(s) of the attacked host(s). However, for this attack to succeed, 2227 the legitimate Responder has to be disconnected from the Control 2228 Relay Server. 2230 The Control Relay Server can protect itself against replay attacks by 2231 becoming involved in the base exchange by introducing nonces that the 2232 end-hosts (Initiator and Responder) are required to sign. One way to 2233 do this is to add ECHO_REQUEST_M parameters to the R1 and I2 packets 2234 as described in [HIP-MIDDLE] and drop the I2 or R2 packets if the 2235 corresponding ECHO_RESPONSE_M parameters are not present. 2237 6.4. Demultiplexing Different HIP Associations 2239 Section 5.1 of [RFC3948] describes a security issue for the UDP 2240 encapsulation in the standard IP tunnel mode when two hosts behind 2241 different NATs have the same private IP address and initiate 2242 communication to the same Responder in the public Internet. The 2243 Responder cannot distinguish between two hosts, because security 2244 associations are based on the same inner IP addresses. 2246 This issue does not exist with the UDP encapsulation of HIP ESP 2247 transport format because the Responder uses HITs to distinguish 2248 between different Initiators. 2250 6.5. Reuse of Ports at the Data Relay Server 2252 If the Data Relay Server uses the same relayed address and port (as 2253 conveyed in the RELAYED_ADDRESS parameter) for multiple Data Relay 2254 Clients, it appears to all the peers, and their firewalls, that all 2255 the Data Relay Clients are at the same address. Thus, a stateful 2256 firewall may allow packets pass from hosts that would not normally be 2257 able to send packets to a peer behind the firewall. Therefore, a 2258 Data Relay Server SHOULD NOT re-use the port numbers. If port 2259 numbers need to be re-used, the Data Relay Server SHOULD have a 2260 sufficiently large pool of port numbers and select ports from the 2261 pool randomly to decrease the chances of a Data Relay Client 2262 obtaining the same address that a another host behind the same 2263 firewall is using. 2265 6.6. Amplification attacks 2267 A malicious host may send an invalid list of candidates for its peer 2268 that are used for targeting a victim host by flooding it with 2269 connectivity checks. To mitigate the attack, this protocol adopts 2270 the ICE mechanism to cap the total amount of connectivity checks as 2271 defined in Section 4.7. 2273 6.7. Attacks against Connectivity Checks and Candidate Gathering 2275 [I-D.ietf-ice-rfc5245bis] describes attacks against ICE connectivity 2276 checks. HIP bases its control plane security on Diffie-Hellman key 2277 exchange, public keys and Hashed Message Authentication codes, 2278 meaning that the mentioned security concerns do not apply to HIP 2279 either. The mentioned section discusses also of man-in-the-middle 2280 replay attacks that are difficult to prevent. The connectivity 2281 checks in this protocol are immune against replay attacks because a 2282 connectivity request includes a random nonce that the recipient must 2283 sign and send back as a response. 2285 [I-D.ietf-ice-rfc5245bis] describes attacks on server reflexive 2286 address gathering. Similarly here, if the DNS, a Control Relay 2287 Server or a Data Relay Server has been compromised, not much can be 2288 done. However, the case where attacker can inject fake messages 2289 (located on a shared network segment like Wifi) does not apply here. 2290 HIP messages are integrity and replay protected, so it is not 2291 possible inject fake server reflexive address candidates. 2293 [I-D.ietf-ice-rfc5245bis] describes attacks on relayed candidate 2294 gathering. Similarly to ICE TURN servers, Data Relay Server require 2295 an authenticated base exchange that protects relayed address 2296 gathering against fake requests and responses. Further, replay 2297 attacks are not possible because the HIP base exchange (and also 2298 UPDATE procedure) is protected against replay attacks. 2300 7. IANA Considerations 2302 This section is to be interpreted according to [RFC8126]. 2304 This document updates the IANA Registry for HIP Parameter Types 2305 [RFC7401] by assigning new HIP Parameter Type values for the new HIP 2306 Parameters: RELAYED_ADDRESS, MAPPED_ADDRESS (defined in 2307 Section 5.12), and PEER_PERMISSION (defined in Section 5.13). 2309 This document updates the IANA Registry for HIP NAT traversal modes 2310 specified in Legacy ICE-HIP [RFC5770] by assigning value for the NAT 2311 traversal mode ICE-HIP-UDP (defined in Section 5.4) This 2312 specification introduces a new keepalive Notify message type field 2313 NAT_KEEPALIVE. 2315 This document defines additional registration types for the HIP 2316 Registration Extension [RFC8003] that allow registering with a Data 2317 Relay Server for ESP relaying service: RELAY_UDP_ESP (defined in 2318 Section 4.1, and performing server reflexive candidate discovery: 2319 CANDIDATE_DISCOVERY (defined in Section 4.2). 2321 This document specifies new error values to be used in HIP NOTIFY 2322 messages as described in Section 5.10: 2323 NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER, CONNECTIVITY_CHECKS_FAILED, 2324 MESSAGE_NOT_RELAYED and SERVER_REFLEXIVE_CANDIDATE_ALLOCATION_FAILED. 2326 ICE specification [I-D.ietf-ice-rfc5245bis] discusses "Unilateral 2327 Self-Address Fixing" . This protocol is based on ICE, and thus the 2328 same considerations apply also here with one exception: this protocol 2329 does not hide binary IP addresses from application-level gateways. 2331 8. Contributors 2333 Marcelo Bagnulo, Philip Matthews and Hannes Tschofenig have 2334 contributed to [RFC5770]. This document leans heavily on the work in 2335 the RFC. 2337 9. Acknowledgments 2339 Thanks to Jonathan Rosenberg, Christer Holmberg and the rest of the 2340 MMUSIC WG folks for the excellent work on ICE. In addition, the 2341 authors would like to thank Andrei Gurtov, Simon Schuetz, Martin 2342 Stiemerling, Lars Eggert, Vivien Schmitt, and Abhinav Pathak for 2343 their contributions and Tobias Heer, Teemu Koponen, Juhana Mattila, 2344 Jeffrey M. Ahrenholz, Kristian Slavov, Janne Lindqvist, Pekka 2345 Nikander, Lauri Silvennoinen, Jukka Ylitalo, Juha Heinanen, Joakim 2346 Koskela, Samu Varjonen, Dan Wing, Tom Henderson, Alex Elsayed and 2347 Jani Hautakorpi for their comments to [RFC5770], which is the basis 2348 for this document. 2350 This work has been partially funded by CyberTrust programme by 2351 Digile/Tekes in Finland. 2353 10. References 2355 10.1. Normative References 2357 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2358 Requirement Levels", BCP 14, RFC 2119, 2359 DOI 10.17487/RFC2119, March 1997, . 2362 [RFC7401] Moskowitz, R., Ed., Heer, T., Jokela, P., and T. 2363 Henderson, "Host Identity Protocol Version 2 (HIPv2)", 2364 RFC 7401, DOI 10.17487/RFC7401, April 2015, 2365 . 2367 [RFC8003] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP) 2368 Registration Extension", RFC 8003, DOI 10.17487/RFC8003, 2369 October 2016, . 2371 [RFC8004] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP) 2372 Rendezvous Extension", RFC 8004, DOI 10.17487/RFC8004, 2373 October 2016, . 2375 [RFC8046] Henderson, T., Ed., Vogt, C., and J. Arkko, "Host Mobility 2376 with the Host Identity Protocol", RFC 8046, 2377 DOI 10.17487/RFC8046, February 2017, . 2380 [RFC8047] Henderson, T., Ed., Vogt, C., and J. Arkko, "Host 2381 Multihoming with the Host Identity Protocol", RFC 8047, 2382 DOI 10.17487/RFC8047, February 2017, . 2385 [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, 2386 "Session Traversal Utilities for NAT (STUN)", RFC 5389, 2387 DOI 10.17487/RFC5389, October 2008, . 2390 [RFC7402] Jokela, P., Moskowitz, R., and J. Melen, "Using the 2391 Encapsulating Security Payload (ESP) Transport Format with 2392 the Host Identity Protocol (HIP)", RFC 7402, 2393 DOI 10.17487/RFC7402, April 2015, . 2396 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 2397 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2398 2006, . 2400 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2401 Writing an IANA Considerations Section in RFCs", BCP 26, 2402 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2403 . 2405 [I-D.ietf-ice-rfc5245bis] 2406 Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive 2407 Connectivity Establishment (ICE): A Protocol for Network 2408 Address Translator (NAT) Traversal", draft-ietf-ice- 2409 rfc5245bis-15 (work in progress), November 2017. 2411 10.2. Informative References 2413 [RFC5770] Komu, M., Henderson, T., Tschofenig, H., Melen, J., and A. 2414 Keranen, Ed., "Basic Host Identity Protocol (HIP) 2415 Extensions for Traversal of Network Address Translators", 2416 RFC 5770, DOI 10.17487/RFC5770, April 2010, 2417 . 2419 [RFC4423] Moskowitz, R. and P. Nikander, "Host Identity Protocol 2420 (HIP) Architecture", RFC 4423, DOI 10.17487/RFC4423, May 2421 2006, . 2423 [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., 2424 and W. Weiss, "An Architecture for Differentiated 2425 Services", RFC 2475, DOI 10.17487/RFC2475, December 1998, 2426 . 2428 [RFC5207] Stiemerling, M., Quittek, J., and L. Eggert, "NAT and 2429 Firewall Traversal Issues of Host Identity Protocol (HIP) 2430 Communication", RFC 5207, DOI 10.17487/RFC5207, April 2431 2008, . 2433 [RFC6538] Henderson, T. and A. Gurtov, "The Host Identity Protocol 2434 (HIP) Experiment Report", RFC 6538, DOI 10.17487/RFC6538, 2435 March 2012, . 2437 [MMUSIC-ICE] 2438 Rosenberg, J., "Guidelines for Usage of Interactive 2439 Connectivity Establishment (ICE) by non Session Initiation 2440 Protocol (SIP) Protocols", Work in Progress, July 2008. 2442 [RFC5128] Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to- 2443 Peer (P2P) Communication across Network Address 2444 Translators (NATs)", RFC 5128, DOI 10.17487/RFC5128, March 2445 2008, . 2447 [HIP-MIDDLE] 2448 Heer, T., Wehrle, K., and M. Komu, "End-Host 2449 Authentication for HIP Middleboxes", Work in Progress, 2450 February 2009. 2452 [RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M. 2453 Stenberg, "UDP Encapsulation of IPsec ESP Packets", 2454 RFC 3948, DOI 10.17487/RFC3948, January 2005, 2455 . 2457 Appendix A. Selecting a Value for Check Pacing 2459 Selecting a suitable value for the connectivity check transaction 2460 pacing is essential for the performance of connectivity check-based 2461 NAT traversal. The value should not be so small that the checks 2462 cause network congestion or overwhelm the NATs. On the other hand, a 2463 pacing value that is too high makes the checks last for a long time, 2464 thus increasing the connection setup delay. 2466 The Ta value may be configured by the user in environments where the 2467 network characteristics are known beforehand. However, if the 2468 characteristics are not known, it is recommended that the value is 2469 adjusted dynamically. In this case, it is recommended that the hosts 2470 estimate the round-trip time (RTT) between them and set the minimum 2471 Ta value so that only two connectivity check messages are sent on 2472 every RTT. 2474 One way to estimate the RTT is to use the time that it takes for the 2475 Control Relay Server registration exchange to complete; this would 2476 give an estimate on the registering host's access link's RTT. Also, 2477 the I1/R1 exchange could be used for estimating the RTT, but since 2478 the R1 can be cached in the network, or the relaying service can 2479 increase the delay notably, this is not recommended. 2481 Appendix B. Differences with respect to ICE 2483 The Native ICE-HIP protocol specified in this document follows the 2484 semantics of ICE as close as possible, and most of the differences 2485 are syntactical due to the use of a different protocol. In this 2486 section, we describe the differences to the ICE protocol. 2488 o ICE operates at the application layer, whereas this protocol 2489 operates between transport and network layers, thus hiding the 2490 protocol details from the application. 2492 o The STUN protocol is not employed. Instead, native ICE-HIP reuses 2493 the HIP control plane format in order simplify demultiplexing of 2494 different protocols. For example, the STUN binding response is 2495 replaced with a HIP UPDATE message containing an 2496 ECHO_REQUEST_SIGNED parameter and the STUN binding response with a 2497 HIP UPDATE message containing an ECHO_RESPONSE_SIGNED parameter as 2498 defined in Section 4.6. 2500 o The TURN protocol is not utilized. Instead, native ICE-HIP reuses 2501 Control Relay Servers for the same purpose. 2503 o ICMP errors may be used in ICE to signal failure. In Native ICE- 2504 HIP protocol, HIP NOTIFY messages are used instead. 2506 o Instead of the ICE username fragment and password mechanism for 2507 credentials, native ICE-HIP uses the HIT, derived from a public 2508 key, for the same purpose. The username fragments are "transient 2509 host identifiers, bound to a particular session established as 2510 part of the candidate exchange" [I-D.ietf-ice-rfc5245bis]. 2511 Generally in HIP, a local public key and the derived HIT are 2512 considered long-term identifiers, and invariant across different 2513 host associations and different transport-layer flows. 2515 o In ICE, the conflict when two communicating end-points take the 2516 same controlling role is solved using random values (so called 2517 tie-breaker value). In Native ICE-HIP protocol, the conflict is 2518 solved by the standard HIP base exchange procedure, where the host 2519 with the "larger" HIT switches to Responder role, thus changing 2520 also to controlled role. 2522 o The ICE-CONTROLLED and ICE-CONTROLLING attributes are not included 2523 in the connectivity checks. 2525 o The foundation concept is unnecessary in native ICE-HIP because 2526 only a single UDP flow for the IPsec tunnel will be negotiated. 2528 o Frozen candidates are omitted for the same reason as foundation 2529 concept is excluded. 2531 o Components are omitted for the same reason as foundation concept 2532 is excluded. 2534 o Native ICE-HIP supports only "full ICE" where the two 2535 communicating hosts participate actively to the connectivity 2536 checks, and the "lite" mode is not supported. This design 2537 decision follows the guidelines of ICE which recommends full ICE 2538 implementations. However, it should be noted that a publicly 2539 reachable Responder may refuse to negotiate the ICE mode as 2540 described in Section 4.7.2. This would result in a [RFC7401] 2541 based HIP base exchange tunneled over UDP followed ESP traffic 2542 over the same tunnel, without the connectivity check procedures 2543 defined in this document (in some sense, this mode corresponds to 2544 the case where two ICE lite implementations connect since no 2545 connectivity checks are sent). 2547 o As the "ICE lite" is not adopted here and both sides are capable 2548 of ICE-HIP-UDP mode (negotiated during the base exchange), default 2549 candidates are not employed in Native ICE-HIP. 2551 o If the agent is using Diffserv Codepoint markings [RFC2475] in its 2552 media packets, it SHOULD apply those same markings to its 2553 connectivity checks. 2555 o Unlike in ICE, the addresses are not XOR-ed in Native ICE-HIP 2556 protocol in order to avoid middlebox tampering. 2558 o Native ICE-HIP protocol does not employ the ICE related address 2559 and related port attributes (that are used for diagnostic or SIP 2560 purposes). 2562 Appendix C. Differences to Base Exchange and UPDATE procedures 2564 This section gives some design guidance for implementers how the 2565 extensions in this protocol extend and differ from [RFC7401] and 2566 [RFC8046]. 2568 o Both control and data plane are operated on top of UDP, not 2569 directly on IP. 2571 o A minimal implementation would conform only to Section 4.7.1 or 2572 Section 4.7.2, thus merely tunneling HIP control and data traffic 2573 over UDP. The drawback here is that it works only in the limited 2574 cases where the Responder has a public address. 2576 o It is worth noting that while a rendezvous server [RFC8004] has 2577 not been designed to be used in NATted scenarios because it just 2578 relays the first I1 packet and does not employ UDP encapsulation, 2579 the Control Relay Server forwards all control traffic and, hence, 2580 is more suitable in NATted environments. Further, the Data Relay 2581 Server guarantees forwarding of data plane traffic also in the 2582 cases when the NAT traversal procedures fail. 2584 o Registration procedures with a Control/Data Relay Server are 2585 similar as with rendezvous server. However, a Control/Data Relay 2586 Server has different registration parameters than rendezvous 2587 because it offers a different service. Also, the Control/Data 2588 Relay Server includes also a REG_FROM parameter that informs the 2589 Control/Data Relay Client about its server reflexive address. A 2590 Data Relay Server includes also a RELAYED_ADDRESS containing the 2591 relayed address for the Data Relay Client. 2593 o In [RFC7401], the Initiator and Responder can start to exchange 2594 application payload immediately after the base exchange. While 2595 exchanging data immediately after a base exchange via a Data 2596 Control Relay would be possible also here, we follow the ICE 2597 methodology to establish a direct path between two hosts using 2598 connectivity checks. This means that there will be some 2599 additional delay after the base exchange before application 2600 payload can be transmitted. The same applies for the UPDATE 2601 procedure as the connectivity checks introduce some additional 2602 delay. 2604 o In HIP without any NAT traversal support, the base exchange acts 2605 as an implicit connectivity check, and the mobility and 2606 multihoming extensions support explicit connectivity checks. 2607 After a base exchange or UPDATE based connectivity checks, a host 2608 can use the associated address pair for transmitting application 2609 payload. In this Native ICE-HIP extension, we follow the ICE 2610 methodology, where one end-point acting in the controlled role 2611 chooses the used address pair also on behalf of the other end- 2612 point acting in controlled role, which is different from HIP 2613 without NAT traversal support. Another difference is that the 2614 process of choosing an address pair is explicitly signaled using 2615 the nomination packets. The nomination process in this protocol 2616 supports only single address pair, and multihoming extensions are 2617 left for further study. 2619 o The UPDATE procedure resembles the mobility extensions defined in 2620 [RFC8046]. The first UPDATE message from the mobile host is 2621 exactly the same as in the mobility extensions. The second UPDATE 2622 message from the peer host and third from the mobile host are 2623 different in the sense that they merely acknowledge and conclude 2624 the reception of the candidates through the Control Relay Server. 2625 In other words, they do not yet test for connectivity (besides 2626 reachability through the Control Relay Server) unlike in the 2627 mobility extensions. The idea is that connectivity check 2628 procedure follows the ICE specification, which is somewhat 2629 different from the HIP mobility extensions. 2631 o The connectivity checks as defined in the mobility extensions 2632 [RFC8046] are triggered only by the peer of the mobile host. 2633 Since successful NAT traversal requires that both end-points test 2634 connectivity, both the mobile host and its peer host have to test 2635 for connectivity. In addition, this protocol validates also the 2636 UDP ports; the ports in the connectivity check must match with the 2637 response, as required by ICE. 2639 o In HIP mobility extensions [RFC8046], an outbound locator has some 2640 associated state: UNVERIFIED mean that the locator has not been 2641 tested for reachability, ACTIVE means that the address has been 2642 verified for reachability and is being used actively, and 2643 DEPRECATED means that the locator lifetime has expired. In the 2644 subset of ICE specifications used by this protocol, an individual 2645 address candidate has only two properties: type and priority. 2646 Instead, the actual state in ICE is associated with candidate 2647 pairs rather than individual addresses. The subset of ICE 2648 specifications utilized by this protocol require the following 2649 attributes for a candidate pair: valid bit, nominated bit, base 2650 and the state of connectivity check. The connectivity checks have 2651 the following states: Waiting, In-progress, Succeeded and Failed. 2652 Handling of this state attribute requires some additional logic 2653 when compared to the mobility extensions since the state is 2654 associated with a local-remote address pair rather just a remote 2655 address, and, thus, the mobility and ICE states do not have an 2656 unambiguous one-to-one mapping. 2658 o Credit-based authorization as defined in [RFC8046] could be used 2659 before candidate nomination has been concluded upon discovering 2660 working candidate pairs. However, this may result in the use of 2661 asymmetric paths for a short time period in the beginning of 2662 communications (similarly as in aggressive ICE nomination). Thus, 2663 support of credit-based authorization is left for further study. 2665 Appendix D. Multihoming Considerations 2667 This document allows a host to collect address candidates from 2668 multiple interfaces, but does not support activation and the 2669 simultaneous use of multiple address candidates. While multihoming 2670 extensions to support [RFC8047] like functionality are left for 2671 further study and experimentation, we envision here some potential 2672 compatibility improvements to support multihoming: 2674 o Data Relay Registration: a Data Relay Client acting as an 2675 Initiator with another peer host should register a new server 2676 reflexive candidate for each local transport address candidate. A 2677 Data Relay Client acting as an Responder should register a new 2678 server reflexive candidate for each { local transport address 2679 candidate, new peer host} pair for the reasons described in 2680 Section 4.12.3. In both cases, the Data Relay Client should 2681 request the additional server reflexive candidates by sending 2682 UPDATE messages originating from each of the local address 2683 candidates as described in Section 4.1. As the UPDATE messages 2684 are originating from an unknown location from the viewpoint of the 2685 Data Relay Server, it must include also a ECHO_REQUEST_SIGNED in 2686 the response in order to test for return routability. 2688 o Data Relay unregistration: this follows the procedure in Section 4 2689 but the Data Relay Client should unregister using the particular 2690 transport address to be unregistered. All transport address pair 2691 registrations can be unregistered when no RELAYED_ADDRESS 2692 parameter is included. 2694 o PEER_PERMISSION parameter: this needs to be extended or an 2695 additional parameter is needed to declare the specific local 2696 candidate of the Data Relay Client. Alternatively, the use of the 2697 PEER_PERMISSION could be used as a wild card to open permissions 2698 for a specific peer to all of the candidates of the Data Relay 2699 Client. 2701 o Connectivity checks: the controlling host should be able to 2702 nominate multiple candidates (by repeating step 7 in Figure 5 in 2703 Section 4.6 using the additional candidate pairs). 2705 o Keepalives should be sent for all the nominated candidate pairs. 2706 Similarly, the Control/Data Relay Client should send keepalives 2707 from its local candidates to its Control/Data Relay Server 2708 transport addresses. 2710 Authors' Addresses 2712 Ari Keranen 2713 Ericsson 2714 Hirsalantie 11 2715 02420 Jorvas 2716 Finland 2718 Email: ari.keranen@ericsson.com 2720 Jan Melen 2721 Ericsson 2722 Hirsalantie 11 2723 02420 Jorvas 2724 Finland 2726 Email: jan.melen@ericsson.com 2728 Miika Komu (editor) 2729 Ericsson 2730 Hirsalantie 11 2731 02420 Jorvas 2732 Finland 2734 Email: miika.komu@ericsson.com