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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 5389 (Obsoleted by RFC 8489) ** Obsolete normative reference: RFC 1063 (Obsoleted by RFC 1191) ** Downref: Normative reference to an Experimental RFC: RFC 5770 -- Obsolete informational reference (is this intentional?): RFC 5245 (Obsoleted by RFC 8445, RFC 8839) == Outdated reference: A later version (-22) exists of draft-ietf-tsvwg-datagram-plpmtud-17 -- Obsolete informational reference (is this intentional?): RFC 5766 (Obsoleted by RFC 8656) Summary: 3 errors (**), 0 flaws (~~), 8 warnings (==), 3 comments (--). 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: October 25, 2020 Ericsson 6 April 23, 2020 8 Native NAT Traversal Mode for the Host Identity Protocol 9 draft-ietf-hip-native-nat-traversal-31 11 Abstract 13 This document specifies a new Network Address Translator (NAT) 14 traversal mode for the Host Identity Protocol (HIP). The new mode is 15 based on the Interactive Connectivity Establishment (ICE) methodology 16 and UDP encapsulation of data and signaling traffic. The main 17 difference from the previously specified modes is the use of HIP 18 messages instead of ICE for all NAT traversal procedures due to the 19 kernel-space dependencies of HIP. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at https://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on October 25, 2020. 38 Copyright Notice 40 Copyright (c) 2020 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (https://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 56 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 57 3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 8 58 4. Protocol Description . . . . . . . . . . . . . . . . . . . . 10 59 4.1. Relay Registration . . . . . . . . . . . . . . . . . . . 10 60 4.2. Transport Address Candidate Gathering at the Relay Client 13 61 4.3. NAT Traversal Mode Negotiation . . . . . . . . . . . . . 15 62 4.4. Connectivity Check Pacing Negotiation . . . . . . . . . . 17 63 4.5. Base Exchange via Control Relay Server . . . . . . . . . 17 64 4.6. Connectivity Checks . . . . . . . . . . . . . . . . . . . 20 65 4.6.1. Connectivity Check Procedure . . . . . . . . . . . . 21 66 4.6.2. Rules for Connectivity Checks . . . . . . . . . . . . 24 67 4.6.3. Rules for Concluding Connectivity Checks . . . . . . 26 68 4.7. NAT Traversal Optimizations . . . . . . . . . . . . . . . 27 69 4.7.1. Minimal NAT Traversal Support . . . . . . . . . . . . 27 70 4.7.2. Base Exchange without Connectivity Checks . . . . . . 27 71 4.7.3. Initiating a Base Exchange both with and without UDP 72 Encapsulation . . . . . . . . . . . . . . . . . . . . 29 73 4.8. Sending Control Packets after the Base Exchange . . . . . 29 74 4.9. Mobility Handover Procedure . . . . . . . . . . . . . . . 30 75 4.10. NAT Keepalives . . . . . . . . . . . . . . . . . . . . . 34 76 4.11. Closing Procedure . . . . . . . . . . . . . . . . . . . . 35 77 4.12. Relaying Considerations . . . . . . . . . . . . . . . . . 35 78 4.12.1. Forwarding Rules and Permissions . . . . . . . . . . 35 79 4.12.2. HIP Data Relay and Relaying of Control Packets . . . 36 80 4.12.3. Handling Conflicting SPI Values . . . . . . . . . . 37 81 5. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 38 82 5.1. HIP Control Packets . . . . . . . . . . . . . . . . . . . 38 83 5.2. Connectivity Checks . . . . . . . . . . . . . . . . . . . 40 84 5.3. Keepalives . . . . . . . . . . . . . . . . . . . . . . . 40 85 5.4. NAT Traversal Mode Parameter . . . . . . . . . . . . . . 40 86 5.5. Connectivity Check Transaction Pacing Parameter . . . . . 41 87 5.6. Relay and Registration Parameters . . . . . . . . . . . . 42 88 5.7. LOCATOR_SET Parameter . . . . . . . . . . . . . . . . . . 43 89 5.8. RELAY_HMAC Parameter . . . . . . . . . . . . . . . . . . 45 90 5.9. Registration Types . . . . . . . . . . . . . . . . . . . 45 91 5.10. Notify Packet Types . . . . . . . . . . . . . . . . . . . 45 92 5.11. ESP Data Packets . . . . . . . . . . . . . . . . . . . . 46 93 5.12. RELAYED_ADDRESS and MAPPED_ADDRESS Parameters . . . . . . 46 94 5.13. PEER_PERMISSION Parameter . . . . . . . . . . . . . . . . 47 95 5.14. HIP Connectivity Check Packets . . . . . . . . . . . . . 48 96 5.15. NOMINATE parameter . . . . . . . . . . . . . . . . . . . 49 98 6. Security Considerations . . . . . . . . . . . . . . . . . . . 49 99 6.1. Privacy Considerations . . . . . . . . . . . . . . . . . 50 100 6.2. Opportunistic Mode . . . . . . . . . . . . . . . . . . . 50 101 6.3. Base Exchange Replay Protection for Control Relay Server 50 102 6.4. Demultiplexing Different HIP Associations . . . . . . . . 51 103 6.5. Reuse of Ports at the Data Relay Server . . . . . . . . . 51 104 6.6. Amplification attacks . . . . . . . . . . . . . . . . . . 51 105 6.7. Attacks against Connectivity Checks and Candidate 106 Gathering . . . . . . . . . . . . . . . . . . . . . . . . 52 107 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 52 108 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 53 109 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 53 110 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 54 111 10.1. Normative References . . . . . . . . . . . . . . . . . . 54 112 10.2. Informative References . . . . . . . . . . . . . . . . . 55 113 Appendix A. Selecting a Value for Check Pacing . . . . . . . . . 57 114 Appendix B. Differences with respect to ICE . . . . . . . . . . 58 115 Appendix C. Differences to Base Exchange and UPDATE procedures . 60 116 Appendix D. Multihoming Considerations . . . . . . . . . . . . . 62 117 Appendix E. DNS Considerations . . . . . . . . . . . . . . . . . 63 118 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 65 120 1. Introduction 122 The Host Identity Protocol (HIP) [RFC7401] is specified to run 123 directly on top of IPv4 or IPv6. However, many middleboxes found in 124 the Internet, such as NATs and firewalls, often allow only UDP or TCP 125 traffic to pass [RFC5207]. Also, NATs usually require the host 126 behind a NAT to create a forwarding state in the NAT before other 127 hosts outside of the NAT can contact the host behind the NAT. To 128 overcome this problem, different methods, commonly referred to as NAT 129 traversal techniques, have been developed. 131 As one solution, the HIP experiment report [RFC6538] mentions Teredo- 132 based NAT traversal for HIP and related ESP traffic (with double 133 tunneling overhead). Another solution is specified in [RFC5770], 134 which will be referred to "Legacy ICE-HIP" in this document. The 135 experimental Legacy ICE-HIP specification combines Interactive 136 Connectivity Establishment (ICE) protocol [RFC5245] with HIP, so that 137 basically ICE is responsible for NAT traversal and connectivity 138 testing, while HIP is responsible for end-host authentication and 139 IPsec key management. The resulting protocol uses HIP, STUN and ESP 140 messages tunneled over a single UDP flow. The benefit of using ICE 141 and its STUN/TURN messaging formats is that one can re-use the NAT 142 traversal infrastructure already available in the Internet, such as 143 STUN and TURN servers. Also, some middleboxes may be STUN-aware and 144 may be able to do something "smart" when they see STUN being used for 145 NAT traversal. 147 HIP poses a unique challenge to using standard ICE, due not only to 148 kernel-space dependencies of HIP, but also due to its close 149 integration with kernel-space IPSec; and, that while [RFC5770] 150 provides a technically workable path, it incurs unacceptable 151 performance drawbacks for kernel-space implementations. Also, 152 implementing and integrating a full ICE/STUN/TURN protocol stack as 153 specified in Legacy ICE-HIP results in a considerable amount of 154 effort and code which could be avoided by re-using and extending HIP 155 messages and state machines for the same purpose. Thus, this 156 document specifies an alternative NAT traversal mode referred as 157 "Native ICE-HIP" that employs HIP messaging format instead of STUN or 158 TURN for the connectivity checks, keepalives and data relaying. 159 Native ICE-HIP also specifies how mobility management works in the 160 context of NAT traversal, which is missing from the Legacy ICE-HIP 161 specification. The native specification is also based on HIPv2, 162 whereas legacy specification is based on HIPv1. The differences to 163 the Legacy ICE-HIP are further elaborated in Appendix B. 165 Similarly as Legacy ICE-HIP, also this specification builds on the 166 HIP registration extensions [RFC8003] and the base exchange procedure 167 [RFC7401] and its closing procedures, so the reader is recommended to 168 get familiar with the relevant specifications. In a nutshell, the 169 registration extensions allow a HIP Initiator (usually a "client" 170 host) to ask for specific services from a HIP Responder (usually a 171 "server" host). The registration parameters are included in a base 172 exchange, which is essentially a four-way Diffie-Hellman key exchange 173 authenticated using the public keys of the end-hosts. When the hosts 174 negotiate support for ESP [RFC7402] during the base exchange, they 175 can deliver ESP protected application payload to each other. When 176 either of the hosts moves and changes its IP address, the two hosts 177 re-establish connectivity using the mobility extensions [RFC8046]. 178 The reader is also recommended to get familiar with the mobility 179 extensions, but basically it is a three-way procedure, where the 180 mobile host first announces its new location to the peer, and then 181 the peer tests for connectivity (so called return routability check), 182 for which the mobile hosts must respond in order to activate its new 183 location. This specification builds on the mobility procedures, but 184 modifies it to be compatible with ICE. The differences to the 185 mobility extensions specified in Appendix C. It is worth noting that 186 multihoming support as specified in [RFC8047] is left for further 187 study. 189 This specification builds heavily on the ICE methodology, so it is 190 recommended that the reader is familiar with the ICE specification 191 [RFC8445] (especially the overview). However, native ICE-HIP does 192 not implement all the features in ICE, and, hence, the different 193 features of ICE are cross referenced using [RFC2119] terminology for 194 clarity. Appendix B explains the differences to ICE, and it is 195 recommended that the reader would read also this section in addition 196 to the ICE specification. 198 2. Terminology 200 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 201 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 202 "OPTIONAL" in this document are to be interpreted as described in BCP 203 14 [RFC2119] [RFC8174] when, and only when, they appear in all 204 capitals, as shown here. 206 This document borrows terminology from [RFC5770], [RFC7401], 207 [RFC8046], [I-D.ietf-hip-rfc4423-bis], [RFC8445], and [RFC5389]. The 208 following terms recur in the text: 210 ICE: 211 Interactive Connectivity Establishment (ICE) protocol as specified 212 in [RFC8445] 214 Legacy ICE-HIP: 215 Refers to the "Basic Host Identity Protocol (HIP) Extensions for 216 Traversal of Network Address Translators" as specified in 217 [RFC5770]. The protocol specified in this document offers an 218 alternative to Legacy ICE-HIP. 220 Native ICE-HIP: 221 The protocol specified in this document (Native NAT Traversal Mode 222 for HIP). 224 Initiator: 225 The Initiator is the host that initiates the base exchange using 226 I1 message [RFC7401]. 228 Responder: 229 The Responder is the host that receives the I1 packet from the 230 Initiator [RFC7401]. 232 Control Relay Server 233 A registrar host that forwards any kind of HIP control plane 234 packets between the Initiator and the Responder. This host is 235 critical because it relays the locators between the Initiator and 236 the Responder, so that they can try to establish a direct 237 communication path with each other. This host is used to replace 238 HIP rendezvous servers [RFC8004] for hosts operating in private 239 address realms. In the Legacy ICE-HIP specification [RFC5770], 240 this host is denoted as "HIP Relay Server". 242 Control Relay Client: 244 A requester host that registers to a Control Relay Server 245 requesting it to forward control-plane traffic (i.e. HIP control 246 messages). In the Legacy ICE-HIP specification [RFC5770], this is 247 denoted as "HIP Relay Client". 249 Data Relay Server: 250 A new entity introduced in this document; a registrar host that 251 forwards HIP related data plane packets, such as Encapsulating 252 Security Payload (ESP) [RFC7402], between two hosts. This host 253 implements similar functionality as TURN servers. 255 Data Relay Client: 256 A requester host that registers to a Data Relay Server requesting 257 it to forward data-plane traffic (e.g. ESP traffic). This 258 functionality is a new and introduced in this document. 260 Locator: 261 As defined in [RFC8046]: "A name that controls how the packet is 262 routed through the network and demultiplexed by the end-host. It 263 may include a concatenation of traditional network addresses such 264 as an IPv6 address and end-to-end identifiers such as an ESP 265 Security Parameter Index (SPI). It may also include transport 266 port numbers or IPv6 Flow Labels as demultiplexing context, or it 267 may simply be a network address." 269 LOCATOR_SET (written in capital letters): 270 Denotes a HIP control packet parameter that bundles multiple 271 locators together [RFC8046]. 273 HIP offer: 274 Before two end-hosts can establish a communication channel using 275 the NAT traversal procedures defined in this document, they need 276 exchange their locators (i.e. candidates) with each other. In 277 ICE, this procedure is called Candidate Exchange and it does not 278 specify how the candidates are exchanged but Session Description 279 Protocol (SDP) "offer/answer" is mentioned as an example. In 280 contrast, the Candidate Exchange in HIP is the base exchange 281 itself or a subsequent UPDATE prodecure occurring after a 282 handover. Following [RFC5770] and SDP [RFC3264] naming 283 conventions, "HIP offer" is the the Initiator's LOCATOR_SET 284 parameter in a HIP I2 or in an UPDATE control packet. 286 HIP answer: 287 The Responder's LOCATOR_SET parameter in a HIP R2 or UPDATE 288 control packet. Corresponds to the SDP answer parameter 290 [RFC3264], but is HIP specific. Please refer also to the longer 291 description of the "HIP offer" term above. 293 HIP connectivity checks: 294 In order to obtain a direct end-to-end communication path (without 295 employing a Data Relay Server), two communicating HIP hosts try to 296 "punch holes" through their NAT boxes using this mechanism. It is 297 similar to the ICE connectivity checks, but implemented using HIP 298 return routability checks. 300 Controlling host: 301 The controlling host [RFC8445] is always the Initiator in the 302 context of this specification. It nominates the candidate pair to 303 be used with the controlled host. 305 Controlled host: 306 The controlled host [RFC8445] is always the Responder in the 307 context of this specification. It waits for the controlling to 308 nominate an address candidate pair. 310 Checklist: 311 A list of address candidate pairs that need to be tested for 312 connectivity (same as in [RFC8445]). 314 Transport address: 315 Transport layer port and the corresponding IPv4/v6 address (same 316 as in [RFC8445]). 318 Candidate: 319 A transport address that is a potential point of contact for 320 receiving data (same as in [RFC8445]). 322 Host candidate: 323 A candidate obtained by binding to a specific port from an IP 324 address on the host (same as in [RFC8445]). 326 Server reflexive candidate: 327 A translated transport address of a host as observed by a Control 328 or Data Relay Server (same as in [RFC8445]). 330 Peer reflexive candidate: 331 A translated transport address of a host as observed by its peer 332 (same as in [RFC8445]). 334 Relayed candidate: 335 A transport address that exists on a Data Relay Server. Packets 336 that arrive at this address are relayed towards the Data Relay 337 Client. The concept is the same as in [RFC8445], but a Data Relay 338 Server is used instead of a TURN server. 340 Permission: 341 In the context of Data Relay Server, permission refers to a 342 concept similar to TURN's ([RFC5766]) channels. Before a host can 343 use a relayed candidate to forward traffic through a Data Relay 344 Server, the host must activate the relayed candidate with a 345 specific peer host. 347 Base: 348 Similarly as in [RFC8445], the base of a candidate is the local 349 source address a host uses to send packets for the associated 350 candidate. For example, the base of a server reflexive address is 351 the local address the host used for registering itself to the 352 associated Control or Data Relay Server. The base of a host 353 candidate is equal to the host candidate itself. 355 3. Overview of Operation 357 +--------------+ 358 | Control | 359 +--------+ | Relay Server | +--------+ 360 | Data | +----+-----+---+ | Data | 361 | Relay | / \ | Relay | 362 | Server | / \ | Server | 363 +--------+ / \ +--------+ 364 / \ 365 / \ 366 / \ 367 / <- Signaling -> \ 368 / \ 369 +-------+ +-------+ 370 | NAT | | NAT | 371 +-------+ +-------+ 372 / \ 373 / \ 374 +-------+ +-------+ 375 | Init- | | Resp- | 376 | iator | | onder | 377 +-------+ +-------+ 379 Figure 1: Example Network Configuration 381 In the example configuration depicted in Figure 1, both Initiator and 382 Responder are behind one or more NATs, and both private networks are 383 connected to the public Internet. To be contacted from behind a NAT, 384 at least the Responder must be registered with a Control Relay Server 385 reachable on the public Internet. The Responder may have also 386 registered to a Data Relay Server that can forward the data plane in 387 case NAT traversal fails. While, strictly speaking, the Initiator 388 does not need a Data Relay Server, it may act in the other role with 389 other hosts, and connectivity with the Data Relay Server of the 390 Responder may fail, so the Initiator may also need to register to a 391 Cotrol and/or Data Relay Server. It is worth noting that a Control 392 and Data Relay does not forge the source address of a passing packet, 393 but always translates the source address and source port of a packet 394 to be forwarded (to its own). 396 We assume, as a starting point, that the Initiator knows both the 397 Responder's Host Identity Tag (HIT) and the address(es) of the 398 Responder's Control Relay Server(s) (how the Initiator learns of the 399 Responder's Control Relay Server is outside of the scope of this 400 document, but may be through DNS or another name service). The first 401 steps are for both the Initiator and Responder to register with a 402 Control Relay Server (need not be the same one) and gather a set of 403 address candidates. The hosts use either Control Relay Servers or 404 Data Relay Servers for gathering the candidates. Next, the HIP base 405 exchange is carried out by encapsulating the HIP control packets in 406 UDP datagrams and sending them through the Responder's Control Relay 407 Server. As part of the base exchange, each HIP host learns of the 408 peer's candidate addresses through the HIP offer/answer procedure 409 embedded in the base exchange. 411 Once the base exchange is completed, two HIP hosts have established a 412 working communication session (for signaling) via a Control Relay 413 Server, but the hosts still have to find a better path, preferably 414 without a Data Relay Server, for the ESP data flow. For this, 415 connectivity checks are carried out until a working pair of addresses 416 is discovered. At the end of the procedure, if successful, the hosts 417 will have established a UDP-based tunnel that traverses both NATs, 418 with the data flowing directly from NAT to NAT or via a Data Relay 419 Server. At this point, also the HIP signaling can be sent over the 420 same address/port pair, and is demultiplexed (or, in other words, 421 separated) from IPsec as described in the UDP encapsulation standard 422 for IPsec [RFC3948]. Finally, the two hosts send NAT keepalives as 423 needed in order keep their UDP-tunnel state active in the associated 424 NAT boxes. 426 If either one of the hosts knows that it is not behind a NAT, hosts 427 can negotiate during the base exchange a different mode of NAT 428 traversal that does not use HIP connectivity checks, but only UDP 429 encapsulation of HIP and ESP. Also, it is possible for the Initiator 430 to simultaneously try a base exchange with and without UDP 431 encapsulation. If a base exchange without UDP encapsulation 432 succeeds, no HIP connectivity checks or UDP encapsulation of ESP are 433 needed. 435 4. Protocol Description 437 This section describes the normative behavior of the "Native ICE-HIP" 438 protocol extension. Most of the procedures are similar to what is 439 defined in [RFC5770] but with different, or additional, parameter 440 types and values. In addition, a new type of relaying server, Data 441 Relay Server, is specified. Also, it should be noted that HIP 442 version 2 [RFC7401] MUST be used instead of HIPv1 with this NAT 443 traversal mode. 445 4.1. Relay Registration 447 In order for two hosts to communicate over NATted environments, they 448 need a reliable way to exchange information. To achieve this, "HIP 449 Relay Server" is defined in [RFC5770]. It supports relaying of HIP 450 control plane traffic over UDP in NATted environments, and forwards 451 HIP control packets between the Initiator and the Responder. In this 452 document, the HIP Relay Server is denoted as "Control Relay Server" 453 for better alignment with the rest of the terminology. The 454 registration to the Control Relay Server can be achieved using 455 RELAY_UDP_HIP parameter as explained later in this section. 457 To guarantee also data plane delivery over varying types of NAT 458 devices, a host MAY also register for UDP encapsulated ESP relaying 459 using Registration Type RELAY_UDP_ESP (value [TBD by IANA: 3]). This 460 service may be coupled with the Control Relay Server or offered 461 separately on another server. If the server supports relaying of UDP 462 encapsulated ESP, the host is allowed to register for a data relaying 463 service using the registration extensions in Section 3.3 of 464 [RFC8003]). If the server has sufficient relaying resources (free 465 port numbers, bandwidth, etc.) available, it opens a UDP port on one 466 of its addresses and signals the address and port to the registering 467 host using the RELAYED_ADDRESS parameter (as defined in Section 5.12 468 in this document). If the Data Relay Server would accept the data 469 relaying request but does not currently have enough resources to 470 provide data relaying service, it MUST reject the request with 471 Failure Type "Insufficient resources" [RFC8003]. 473 The registration process follows the generic registration extensions 474 defined in [RFC8003]. The HIP control plane relaying registration 475 follows [RFC5770], but the data plane registration is different. It 476 is worth noting that if the HIP control and data plane relay services 477 reside on different hosts, the client has to register separately to 478 each of them. In the example shown in Figure 2, the two services are 479 coupled on a single host. The text uses "Relay Client" and "Relay 480 Server" as a shorthand when the procedures apply both to control and 481 data cases. 483 Control/Data Control/Data 484 Relay Client (Initiator) Relay Server (Responder) 485 | 1. UDP(I1) | 486 +---------------------------------------------------------------->| 487 | | 488 | 2. UDP(R1(REG_INFO(RELAY_UDP_HIP,[RELAY_UDP_ESP]))) | 489 |<----------------------------------------------------------------+ 490 | | 491 | 3. UDP(I2(REG_REQ(RELAY_UDP_HIP),[RELAY_UDP_ESP])) | 492 +---------------------------------------------------------------->| 493 | | 494 | 4. UDP(R2(REG_RES(RELAY_UDP_HIP,[RELAY_UDP_ESP]), REG_FROM, | 495 | [RELAYED_ADDRESS])) | 496 |<----------------------------------------------------------------+ 497 | | 499 Figure 2: Example Registration with a HIP Relay 501 In step 1, the Relay Client (Initiator) starts the registration 502 procedure by sending an I1 packet over UDP to the Relay Server. It 503 is RECOMMENDED that the Relay Client select a random source port 504 number from the ephemeral port range 49152-65535 for initiating a 505 base exchange. Alternatively, a host MAY also use a single fixed 506 port for initiating all outgoing connections. However, the allocated 507 port MUST be maintained until all of the corresponding HIP 508 Associations are closed. It is RECOMMENDED that the Relay Server 509 listen to incoming connections at UDP port 10500. If some other port 510 number is used, it needs to be known by potential Relay Clients. 512 In step 2, the Relay Server (Responder) lists the services that it 513 supports in the R1 packet. The support for HIP control plane over 514 UDP relaying is denoted by the Registration Type value RELAY_UDP_HIP 515 (see Section 5.9). If the server supports also relaying of ESP 516 traffic over UDP, it includes also Registration type value 517 RELAY_UDP_ESP. 519 In step 3, the Relay Client selects the services for which it 520 registers and lists them in the REG_REQ parameter. The Relay Client 521 registers for the Control Relay service by listing the RELAY_UDP_HIP 522 value in the request parameter. If the Relay Client requires also 523 ESP relaying over UDP, it lists also RELAY_UDP_ESP. 525 In step 4, the Relay Server concludes the registration procedure with 526 an R2 packet and acknowledges the registered services in the REG_RES 527 parameter. The Relay Server denotes unsuccessful registrations (if 528 any) in the REG_FAILED parameter of R2. The Relay Server also 529 includes a REG_FROM parameter that contains the transport address of 530 the Relay Client as observed by the Relay Server (Server Reflexive 531 candidate). If the Relay Client registered to ESP relaying service, 532 the Relay Server includes RELAYED_ADDRESS parameter that describes 533 the UDP port allocated to the Relay Client for ESP relaying. It is 534 worth noting that the Data Relay Client must first activate this UDP 535 port by sending an UPDATE message to the Data Relay Server that 536 includes a PEER_PERMISSION parameter as described in Section 4.12.1 537 both after base exchange and handover procedures. Also, the Data 538 Relay Server should follow the port allocation recommendations in 539 Section 6.5. 541 After the registration, the Relay Client sends periodically NAT 542 keepalives to the Relay Server in order to keep the NAT bindings 543 between the Relay Client and the relay alive. The keepalive 544 extensions are described in Section 4.10. 546 The Data Relay Client MUST maintain an active HIP association with 547 the Data Relay Server as long as it requires the data relaying 548 service. When the HIP association is closed (or times out), or the 549 registration lifetime passes without the Data Relay Client refreshing 550 the registration, the Data Relay Server MUST stop relaying packets 551 for that host and close the corresponding UDP port (unless other Data 552 Relay Clients are still using it). 554 The Data Relay Server SHOULD offer a different relayed address and 555 port for each Data Relay Client because not doing so can cause 556 problems with stateful firewalls (see Section 6.5). 558 When a Control Relay Client sends an UPDATE (e.g., due to host 559 movement or to renew service registration), the Control Relay Server 560 MUST follow the general guidelines defined in [RFC8003], with the 561 difference that all UPDATE messages are delivered on top of UDP. In 562 addition to this, the Control Relay Server MUST include the REG_FROM 563 parameter in all UPDATE responses sent to the Control Relay Client. 564 This applies to both renewals of service registration and to host 565 movement. It is especially important for the case of host movement, 566 as this is the mechanism that allows the Control Relay Client to 567 learn its new server reflexive address candidate. 569 A Data Relay Client can request multiple relayed candidates from the 570 Data Relay Server (e.g., for the reasons described in 571 Section 4.12.3). After the base exchange with registration, the Data 572 Relay Client can request additional relayed candidates similarly as 573 during the base exchange. The Data Relay Client sends an UPDATE 574 message REG_REQ parameter requesting for the RELAY_UDP_ESP service. 576 The UPDATE message MUST also include a SEQ and a ECHO_REQUEST_SIGNED 577 parameter. The Data Relay Server MUST respond with an UPDATE message 578 that includes the corresponding response parameters: REG_RES, ACK and 579 ECHO_REQUEST_SIGNED . In case the Data Relay Server allocated a new 580 relayed UDP port for the Data Relay Client, the REG_RES parameter 581 MUST list RELAY_UDP_ESP as a service and the UPDATE message MUST also 582 include a RELAYED_ADDRESS parameter describing the relayed UDP port. 583 The Data Relay Server MUST also include the Server Reflexive 584 candidate in a REG_FROM parameter. It is worth mentioning that Data 585 Relay Client MUST activate the UDP port as described in 586 Section 4.12.1 before it can be used for any ESP relaying. 588 A Data Relay Client may unregister a relayed candidate in two ways. 589 It can wait for its lifetime to expire or it can explicitly request 590 it with zero lifetime using the UPDATE mechanism. The Data Relay 591 Client can send an REG_REQ parameter with zero lifetime to the Data 592 Relay Server in order to expire all relayed candidates. To expire a 593 specific relayed candidate, the Data Relay Client MUST also include 594 RELAYED_ADDRESS parameter as sent by the server in the UPDATE 595 message. Upon closing the HIP association (CLOSE-CLOSE-ACK procedure 596 initiated by either party), the Data Relay Server MUST also expire 597 all relayed candidates. 599 To avoid cross-protocol attacks associated with RELAY_HMAC and 600 RVS_HMAC, a Control Relay Server MUST NOT offer RENDEZVOUS 601 registration service [RFC8004] to any Control Relay Client. 603 4.2. Transport Address Candidate Gathering at the Relay Client 605 An Initiator needs to gather a set of address candidates before 606 contacting a (non-relay) Responder. The candidates are needed for 607 connectivity checks that allow two hosts to discover a direct, non- 608 relayed path for communicating with each other. One server reflexive 609 candidate can be discovered during the registration with the Control 610 Relay Server from the REG_FROM parameter (and another from Data Relay 611 Server if one is employed). 613 The candidate gathering can be done at any time, but it needs to be 614 done before sending an I2 or R2 in the base exchange if ICE-HIP-UDP 615 mode is to be used for the connectivity checks. It is RECOMMENDED 616 that all three types of candidates (host, server reflexive, and 617 relayed) are gathered to maximize the probability of successful NAT 618 traversal. However, if no Data Relay Server is used, and the host 619 has only a single local IP address to use, the host MAY use the local 620 address as the only host candidate and the address from the REG_FROM 621 parameter discovered during the Control Relay Server registration as 622 a server reflexive candidate. In this case, no further candidate 623 gathering is needed. 625 A Data Relay Client MAY register only a single relayed candidate that 626 it uses with multiple other peers. However, it is RECOMMENDED that a 627 Data Relay Client registers a new server relayed candidate for each 628 of its peer for the reasons described in Section 4.12.3. The 629 procedures for registering multiple relayed candidates are described 630 in Section 4.1. 632 If a Relay Client has more than one network interface, it can 633 discover additional server reflexive candidates by sending UPDATE 634 messages from each of its interfaces to the Relay Server. Each such 635 UPDATE message MUST include the following parameters: registration 636 request (REG_REQ) parameter with Registration Type 637 CANDIDATE_DISCOVERY (value [TBD by IANA: 4]) and ECHO_REQUEST_SIGNED 638 parameter. When a Control Relay Server receives an UPDATE message 639 with registration request containing a CANDIDATE_DISCOVERY type, it 640 MUST include a REG_FROM parameter, containing the same information as 641 if this were a Control Relay Server registration, to the response (in 642 addition to the mandatory ECHO_RESPONSE_SIGNED parameter). This 643 request type SHOULD NOT create any state at the Control Relay Server. 645 The rules in section 5.1.1 in [RFC8445] for candidate gathering are 646 followed here. A number of host candidates (loopback, anycast and 647 others) should be excluded as described in section 5.1.1.1 of the ICE 648 specification [RFC8445]. Relayed candidates SHOULD be gathered in 649 order to guarantee successful NAT traversal, and implementations 650 SHOULD support this functionality even if it will not be used in 651 deployments in order to enable it by software configuration update if 652 needed at some point. Similarly as explained in section 5.1.1.2 of 653 the ICE specification [RFC8445], if an IPv6-only host is in a network 654 that utilizes NAT64 [RFC6146] and DNS64 [RFC6147] technologies, it 655 may also gather IPv4 server- reflexive and/or relayed candidates from 656 IPv4-only Control or Data Relay Servers. IPv6-only hosts SHOULD also 657 utilize IPv6 prefix discovery [RFC7050] to discover the IPv6 prefix 658 used by NAT64 (if any) and generate server-reflexive candidates for 659 each IPv6-only interface, accordingly. The NAT64 server-reflexive 660 candidates are prioritized like IPv4 server-reflexive candidates. 662 HIP based connectivity can be utilized by IPv4 applications using 663 Local Scope Identifiers (LSIs) and by IPv6 based applications using 664 HITs. The LSIs and HITs of the local virtual interfaces MUST be 665 excluded in the candidate gathering phase as well to avoid creating 666 unnecessary loopback connectivity tests. 668 Gathering of candidates MAY also be performed by other means than 669 described in this section. For example, the candidates could be 670 gathered as specified in Section 4.2 of [RFC5770] if STUN servers are 671 available, or if the host has just a single interface and no STUN or 672 Data Relay Server are available. 674 Each local address candidate MUST be assigned a priority. The 675 following recommended formula (as described in [RFC8445]) SHOULD be 676 used: 678 priority = (2^24)*(type preference) + (2^8)*(local preference) + 679 (2^0)*(256 - component ID) 681 In the formula, the type preference follows the ICE specification (as 682 defined in section 5.1.2.1 in [RFC8445]): the RECOMMENDED values are 683 126 for host candidates, 100 for server reflexive candidates, 110 for 684 peer reflexive candidates, and 0 for relayed candidates. The highest 685 value is 126 (the most preferred) and lowest is 0 (last resort). For 686 all candidates of the same type, the preference type value MUST be 687 identical, and, correspondingly, the value MUST be different for 688 different types. For peer reflexive values, the type preference 689 value MUST be higher than for server reflexive types. It should be 690 noted that peer reflexive values are learned later during 691 connectivity checks, so a host cannot employ it during candidate 692 gathering stage yet. 694 Following the ICE specification, the local preference MUST be an 695 integer from 0 (lowest preference) to 65535 (highest preference) 696 inclusive. In the case the host has only a single address candidate, 697 the value SHOULD be 65535. In the case of multiple candidates, each 698 local preference value MUST be unique. Dual-stack considerations for 699 IPv6 apply also here as defined in [RFC8445] in section 5.1.2.2. 701 Unlike with SDP used in conjunction with ICE, this protocol only 702 creates a single UDP flow between the two communicating hosts, so 703 only a single component exists. Hence, the component ID value MUST 704 always be set to 1. 706 As defined in section 14.3 in [RFC8445], the retransmission timeout 707 (RTO) for address gathering from a Control/Data Relay Server SHOULD 708 be calculated as follows: 710 RTO = MAX (500ms, Ta * (Num-Of-Cands)) 712 where Ta is the value used for the connectivity check pacing and Num- 713 Of-Cands is the number of server-reflexive and relay candidates. A 714 smaller value than 500 ms for the RTO MUST NOT be used. 716 4.3. NAT Traversal Mode Negotiation 718 This section describes the usage of a non-critical parameter type 719 called NAT_TRAVERSAL_MODE with a new mode called ICE-HIP-UDP. The 720 presence of the new mode in the NAT_TRAVERSAL_MODE parameter in a HIP 721 base exchange means that the end-host supports NAT traversal 722 extensions described in this document. As the parameter is non- 723 critical (as defined in Section 5.2.1 of [RFC7401]), it can be 724 ignored by a end-host, which means that the host is not required to 725 support it or may decline to use it. 727 With registration with a Control/Data Relay Server, it is usually 728 sufficient to use the UDP-ENCAPSULATION mode of NAT traversal since 729 the Relay Server is assumed to be in public address space. Thus, the 730 Relay Server SHOULD propose the UDP-ENCAPSULATION mode as the 731 preferred or only mode. The NAT traversal mode negotiation in a HIP 732 base exchange is illustrated in Figure 3. It is worth noting that 733 the Relay Server could be located between the hosts, but is omitted 734 here for simplicity. 736 Initiator Responder 737 | 1. UDP(I1) | 738 +----------------------------------------------------------------->| 739 | | 740 | 2. UDP(R1(.., NAT_TRAVERSAL_MODE(ICE-HIP-UDP), ..)) | 741 |<-----------------------------------------------------------------+ 742 | | 743 | 3. UDP(I2(.., NAT_TRAVERSAL_MODE(ICE-HIP-UDP), ENC(LOC_SET), ..))| 744 +----------------------------------------------------------------->| 745 | | 746 | 4. UDP(R2(.., ENC(LOC_SET), ..)) | 747 |<-----------------------------------------------------------------+ 748 | | 750 Figure 3: Negotiation of NAT Traversal Mode 752 In step 1, the Initiator sends an I1 to the Responder. In step 2, 753 the Responder responds with an R1. As specified in [RFC5770], the 754 NAT_TRAVERSAL_MODE parameter in R1 contains a list of NAT traversal 755 modes the Responder supports. The mode specified in this document is 756 ICE-HIP-UDP (value [TBD by IANA: 3]). 758 In step 3, the Initiator sends an I2 that includes a 759 NAT_TRAVERSAL_MODE parameter. It contains the mode selected by the 760 Initiator from the list of modes offered by the Responder. If ICE- 761 HIP-UDP mode was selected, the I2 also includes the "Transport 762 address" locators (as defined in Section 5.7) of the Initiator in a 763 LOCATOR_SET parameter (denoted here with LOC_SET). With ICE-HIP-UDP 764 mode, the LOCATOR_SET parameter MUST be encapsulated within an 765 ENCRYPTED parameter (denoted here with ENC) according to the 766 procedures in sections 5.2.18 and 6.5 in [RFC7401]. The locators in 767 I2 are the "HIP offer". 769 In step 4, the Responder concludes the base exchange with an R2 770 packet. If the Initiator chose ICE-HIP-UDP traversal mode, the 771 Responder includes a LOCATOR_SET parameter in the R2 packet. With 772 ICE-HIP-UDP mode, the LOCATOR_SET parameter MUST be encapsulated 773 within an ENCRYPTED parameter according to the procedures in sections 774 5.2.18 and 6.5 in [RFC7401]. The locators in R2, encoded like the 775 locators in I2, are the "ICE answer". If the NAT traversal mode 776 selected by the Initiator is not supported by the Responder, the 777 Responder SHOULD reply with a NOTIFY packet with type 778 NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER and abort the base exchange. 780 4.4. Connectivity Check Pacing Negotiation 782 As explained in Legacy ICE-HIP [RFC5770], when a NAT traversal mode 783 with connectivity checks is used, new transactions should not be 784 started too fast to avoid congestion and overwhelming the NATs. For 785 this purpose, during the base exchange, hosts can negotiate a 786 transaction pacing value, Ta, using a TRANSACTION_PACING parameter in 787 R1 and I2 packets. The parameter contains the minimum time 788 (expressed in milliseconds) the host would wait between two NAT 789 traversal transactions, such as starting a new connectivity check or 790 retrying a previous check. The value that is used by both of the 791 hosts is the higher of the two offered values. 793 The minimum Ta value SHOULD be configurable, and if no value is 794 configured, a value of 50 ms MUST be used. Guidelines for selecting 795 a Ta value are given in Appendix A. Hosts MUST NOT use values 796 smaller than 5 ms for the minimum Ta, since such values may not work 797 well with some NATs (as explained in [RFC8445]). The Initiator MUST 798 NOT propose a smaller value than what the Responder offered. If a 799 host does not include the TRANSACTION_PACING parameter in the base 800 exchange, a Ta value of 50 ms MUST be used as that host's minimum 801 value. 803 4.5. Base Exchange via Control Relay Server 805 This section describes how the Initiator and Responder perform a base 806 exchange through a Control Relay Server. Connectivity pacing 807 (denoted as TA_P here) was described in Section 4.4 and is not 808 repeated here. Similarly, the NAT traversal mode negotiation process 809 (denoted as NAT_TM in the example) was described in Section 4.3 and 810 is also not repeated here. If a Control Relay Server receives an R1 811 or I2 packet without the NAT traversal mode parameter, it MUST drop 812 it and SHOULD send a NOTIFY error packet with type 813 NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER to the sender of the R1 or I2. 815 It is RECOMMENDED that the Initiator send an I1 packet encapsulated 816 in UDP when it is destined to an IP address of the Responder. 818 Respectively, the Responder MUST respond to such an I1 packet with a 819 UDP-encapsulated R1 packet, and also the rest of the communication 820 related to the HIP association MUST also use UDP encapsulation. 822 Figure 4 illustrates a base exchange via a Control Relay Server. We 823 assume that the Responder (i.e. a Control Relay Client) has already 824 registered to the Control Relay Server. The Initiator may have also 825 registered to another (or the same Control Relay Server), but the 826 base exchange will traverse always through the Control Relay Server 827 of the Responder. 829 Initiator Control Relay Server Responder 830 | 1. UDP(I1) | | 831 +--------------------------------->| 2. UDP(I1(RELAY_FROM)) | 832 | +------------------------------->| 833 | | | 834 | | 3. UDP(R1(RELAY_TO, NAT_TM, | 835 | | TA_P)) | 836 | 4. UDP(R1(RELAY_TO, NAT_TM, |<-------------------------------+ 837 | TA_P)) | | 838 |<---------------------------------+ | 839 | | | 840 | 5. UDP(I2(ENC(LOC_SET)), | | 841 | NAT_TM, TA_P)) | | 842 +--------------------------------->| 6. UDP(I2(ENC(LOC_SET), | 843 | | RELAY_FROM, NAT_TM, TA_P))| 844 | +------------------------------->| 845 | | | 846 | | 7. UDP(R2(ENC(LOC_SET), | 847 | 8. UDP(R2(ENC(LOC_SET), | RELAY_TO)) | 848 | RELAY_TO)) |<-------------------------------+ 849 |<---------------------------------+ | 850 | | | 852 Figure 4: Base Exchange via a HIP Relay Server 854 In step 1 of Figure 4, the Initiator sends an I1 packet over UDP via 855 the Control Relay Server to the Responder. In the HIP header, the 856 source HIT belongs to the Initiator and the destination HIT to the 857 Responder. The initiator sends the I1 packet from its IP address to 858 the IP address of the Control Relay Server over UDP. 860 In step 2, the Control Relay Server receives the I1 packet. If the 861 destination HIT belongs to a successfully registered Control Relay 862 Client (i.e., the host marked "Responder" in Figure 4), the Control 863 Relay Server processes the packet. Otherwise, the Control Relay 864 Server MUST drop the packet silently. The Control Relay Server 865 appends a RELAY_FROM parameter to the I1 packet, which contains the 866 transport source address and port of the I1 as observed by the 867 Control Relay Server. The Control Relay Server protects the I1 868 packet with RELAY_HMAC, except that the parameter type is different 869 as described in Section 5.8. The Control Relay Server changes the 870 source and destination ports and IP addresses of the packet to match 871 the values the Responder used when registering to the Control Relay 872 Server, i.e., the reverse of the R2 used in the registration. The 873 Control Relay Server MUST recalculate the transport checksum and 874 forward the packet to the Responder. 876 In step 3, the Responder receives the I1 packet. The Responder 877 processes it according to the rules in [RFC7401]. In addition, the 878 Responder validates the RELAY_HMAC according to Section 5.8 and 879 silently drops the packet if the validation fails. The Responder 880 replies with an R1 packet to which it includes RELAY_TO and NAT 881 traversal mode parameters. The responder MUST include ICE-HIP-UDP in 882 the NAT traversal modes. The RELAY_TO parameter MUST contain the 883 same information as the RELAY_FROM parameter, i.e., the Initiator's 884 transport address, but the type of the parameter is different. The 885 RELAY_TO parameter is not integrity protected by the signature of the 886 R1 to allow pre-created R1 packets at the Responder. 888 In step 4, the Control Relay Server receives the R1 packet. The 889 Control Relay Server drops the packet silently if the source HIT 890 belongs to a Control Relay Client that has not successfully 891 registered. The Control Relay Server MAY verify the signature of the 892 R1 packet and drop it if the signature is invalid. Otherwise, the 893 Control Relay Server rewrites the source address and port, and 894 changes the destination address and port to match RELAY_TO 895 information. Finally, the Control Relay Server recalculates the 896 transport checksum and forwards the packet. 898 In step 5, the Initiator receives the R1 packet and processes it 899 according to [RFC7401]. The Initiator MAY use the address in the 900 RELAY_TO parameter as a local peer-reflexive candidate for this HIP 901 association if it is different from all known local candidates. The 902 Initiator replies with an I2 packet that uses the destination 903 transport address of R1 as the source address and port. The I2 904 packet contains a LOCATOR_SET parameter inside an ENCRYPTED parameter 905 that lists all the HIP candidates (HIP offer) of the Initiator. The 906 candidates are encoded using the format defined in Section 5.7. The 907 I2 packet MUST also contain a NAT traversal mode parameter that 908 includes ICE-HIP-UDP mode. The ENCRYPTED parameter along with its 909 key material generation are described in detail in sections 5.2.18 910 and 6.5 in [RFC7401]. 912 In step 6, the Control Relay Server receives the I2 packet. The 913 Control Relay Server appends a RELAY_FROM and a RELAY_HMAC to the I2 914 packet similarly as explained in step 2, and forwards the packet to 915 the Responder. 917 In step 7, the Responder receives the I2 packet and processes it 918 according to [RFC7401]. The Responder validates the RELAY_HMAC 919 according to Section 5.8 and silently drops the packet if the 920 validation fails. It replies with an R2 packet and includes a 921 RELAY_TO parameter as explained in step 3. The R2 packet includes a 922 LOCATOR_SET parameter inside an ENCRYPTED parameter that lists all 923 the HIP candidates (ICE answer) of the Responder. The RELAY_TO 924 parameter is protected by the HMAC. The ENCRYPTED parameter along 925 with its key material generation are described in detail in sections 926 5.2.18 and 6.5 in [RFC7401]. 928 In step 8, the Control Relay Server processes the R2 as described in 929 step 4. The Control Relay Server forwards the packet to the 930 Initiator. After the Initiator has received the R2 and processed it 931 successfully, the base exchange is completed. 933 Hosts MUST include the address of one or more Control Relay Servers 934 (including the one that is being used for the initial signaling) in 935 the LOCATOR_SET parameter in I2 and R2 messages if they intend to use 936 such servers for relaying HIP signaling immediately after the base 937 exchange completes. The traffic type of these addresses MUST be "HIP 938 signaling" (see Section 5.7) and they MUST NOT be used for the 939 connectivity tests described in Section 4.6. If the Control Relay 940 Server locator used for relaying the base exchange is not included in 941 I2 or R2 LOCATOR_SET parameters, it SHOULD NOT be used after the base 942 exchange. Instead, further HIP signaling SHOULD use the same path as 943 the data traffic. It is RECOMMENDED to use the same Control Relay 944 Server throughout the lifetime of the host association that was used 945 for forwarding the base exchange if the Responder includes it in the 946 locator parameter of the R2 message. 948 4.6. Connectivity Checks 950 When the Initiator and Responder complete the base exchange through 951 the Control Relay Server, both of them employ the IP address of the 952 Control Relay Server as the destination address for the packets. The 953 address of the Control Relay Server MUST NOT be used as a destination 954 for data plane traffic unless the server supports also Data Relay 955 Server functionality, and the Client has successfully registered to 956 use it. When NAT traversal mode with ICE-HIP-UDP was successfully 957 negotiated and selected, the Initiator and Responder MUST start the 958 connectivity checks in order to attempt to obtain direct end-to-end 959 connectivity through NAT devices. It is worth noting that the 960 connectivity checks MUST be completed even though no ESP_TRANSFORM 961 would be negotiated and selected. 963 The connectivity checks follow the ICE methodology 964 [I-D.rosenberg-mmusic-ice-nonsip], but UDP encapsulated HIP control 965 messages are used instead of ICE messages. As stated in the ICE 966 specification, the basic procedure for connectivity checks has three 967 phases: sorting the candidate pairs according their priority, sending 968 checks in the prioritized order and acknowledging the checks from the 969 peer host. 971 The Initiator MUST take the role of controlling host and the 972 Responder acts as the controlled host. The roles MUST persist 973 throughout the HIP associate lifetime (to be reused in the possibly 974 mobility UPDATE procedures). In the case both communicating nodes 975 are initiating the communications to each other using an I1 packet, 976 the conflict is resolved as defined in section 6.7 in [RFC7401]: the 977 host with the "larger" HIT changes to its Role to Responder. In such 978 a case, the host changing its role to Responder MUST also switch to 979 controlled role. 981 The protocol follows standard HIP UPDATE sending and processing rules 982 as defined in section 6.11 and 6.12 in [RFC7401], but some new 983 parameters are introduced (CANDIDATE_PRIORITY, MAPPED_ADDRESS and 984 NOMINATE). 986 4.6.1. Connectivity Check Procedure 988 Figure 5 illustrates connectivity checks in a simplified scenario, 989 where the Initiator and Responder have only a single candidate pair 990 to check. Typically, NATs drop messages until both sides have sent 991 messages using the same port pair. In this scenario, the Responder 992 sends a connectivity check first but the NAT of the Initiator drops 993 it. However, the connectivity check from the Initiator reaches the 994 Responder because it uses the same port pair as the first message. 995 It is worth noting that the message flow in this section is 996 idealistic, and, in practice, more messages would be dropped, 997 especially in the beginning. For instance, connectivity tests always 998 start with the candidates with the highest priority, which would be 999 host candidates (which would not reach the recipient in this 1000 scenario). 1002 Initiator NAT1 NAT2 Responder 1003 | | 1. UDP(UPDATE(SEQ, CAND_PRIO, | | 1004 | | ECHO_REQ_SIGN)) | | 1005 | X<-----------------------------------+----------------+ 1006 | | | | 1007 | 2. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO)) | | 1008 +-------------+------------------------------------+--------------->| 1009 | | | | 1010 | 3. UDP(UPDATE(ACK, ECHO_RESP_SIGN, MAPPED_ADDR)) | | 1011 |<------------+------------------------------------+----------------+ 1012 | | | | 1013 | 4. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO)) | | 1014 |<------------+------------------------------------+----------------+ 1015 | | | | 1016 | 5. UDP(UPDATE(ACK, ECHO_RESP_SIGN, MAPPED_ADDR)) | | 1017 +-------------+------------------------------------+--------------->| 1018 | | | | 1019 | 6. Other connectivity checks using UPDATE over UDP | 1020 |<------------+------------------------------------+----------------> 1021 | | | | 1022 | 7. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO, NOMINATE)) | 1023 +-------------+------------------------------------+--------------->| 1024 | | | | 1025 | 8. UDP(UPDATE(SEQ, ACK, ECHO_REQ_SIGN, ECHO_RESP_SIGN, | 1026 | NOMINATE)) | | 1027 |<------------+------------------------------------+----------------+ 1028 | | | | 1029 | 9. UDP(UPDATE(ACK, ECHO_RESP_SIGN)) | | 1030 +-------------+------------------------------------+--------------->+ 1031 | | | | 1032 | 10. ESP data traffic over UDP | | 1033 +<------------+------------------------------------+--------------->+ 1034 | | | | 1036 Figure 5: Connectivity Checks 1038 In step 1, the Responder sends a connectivity check to the Initiator 1039 that the NAT of the Initiator drops. The message includes a number 1040 of parameters. As specified in [RFC7401]), the SEQ parameter 1041 includes a running sequence identifier for the connectivity check. 1042 The candidate priority (denoted "CAND_PRIO" in the figure) describes 1043 the priority of the address candidate being tested. The 1044 ECHO_REQUEST_SIGNED (denoted ECHO_REQ_SIGN in the figure) includes a 1045 nonce that the recipient must sign and echo back as it is. 1047 In step 2, the Initiator sends a connectivity check, using the same 1048 address pair candidate as in the previous step, and the message 1049 traverses successfully the NAT boxes. The message includes the same 1050 parameters as in the previous step. It should be noted that the 1051 sequence identifier is locally assigned by the Initiator, so it can 1052 be different than in the previous step. 1054 In step 3, the Responder has successfully received the previous 1055 connectivity check from the Initiator and starts to build a response 1056 message. Since the message from the Initiator included a SEQ, the 1057 Responder must acknowledge it using an ACK parameter. Also, the 1058 nonce contained in the echo request must be echoed back in an 1059 ECHO_RESPONSE_SIGNED (denoted ECHO_RESP_SIGN) parameter. The 1060 Responder includes also a MAPPED_ADDRESS parameter (denoted 1061 MAPPED_ADDR in the figure) that contains the transport address of the 1062 Initiator as observed by the Responder (i.e. peer reflexive 1063 candidate). This message is successfully delivered to the Initiator, 1064 and upon reception the Initiator marks the candidate pair as valid. 1066 In step 4, the Responder retransmits the connectivity check sent in 1067 the first step, since it was not acknowledged yet. 1069 In step 5, the Initiator responds to the previous connectivity check 1070 message from the Responder. The Initiator acknowledges the SEQ 1071 parameter from the previous message using ACK parameter and the 1072 ECHO_REQUEST_SIGNED parameter with ECHO_RESPONSE_SIGNED. In 1073 addition, it includes MAPPED_ADDR parameter that includes the peer 1074 reflexive candidate. This response message is successfully delivered 1075 to the Responder, and upon reception the Initiator marks the 1076 candidate pair as valid. 1078 In step 6, despite the two hosts now having valid address candidates, 1079 the hosts still test the remaining address candidates in a similar 1080 way as in the previous steps. It should be noted that each 1081 connectivity check has a unique sequence number in the SEQ parameter. 1083 In step 7, the Initiator has completed testing all address candidates 1084 and nominates one address candidate to be used. It sends an UPDATE 1085 message using the selected address candidates that includes a number 1086 of parameters: SEQ, ECHO_REQUEST_SIGNED, CANDIDATE_PRIORITY and the 1087 NOMINATE parameter. 1089 In step 8, the Responder receives the message with NOMINATE parameter 1090 from the Initiator. It sends a response that includes the NOMINATE 1091 parameter in addition to a number of other parameters. The ACK and 1092 ECHO_RESPONSE_SIGNED parameters acknowledge the SEQ and 1093 ECHO_REQUEST_SIGNED parameters from previous message from the 1094 Initiator. The Responder includes SEQ and ECHO_REQUEST_SIGNED 1095 parameters in order to receive an acknowledgment from the Responder. 1097 In step 9, the Initiator completes the candidate nomination process 1098 by confirming the message reception to the Responder. In the 1099 confirmation message, the ACK and ECHO_RESPONSE_SIGNED parameters 1100 correspond to the SEQ and ECHO_REQUEST_SIGNED parameters in the 1101 message sent by the Responder in the previous step. 1103 In step 10, the Initiator and Responder can start sending application 1104 payload over the successfully nominated address candidates. 1106 It is worth noting that if either host has registered a relayed 1107 address candidate from a Data Relay Server, the host MUST activate 1108 the address before connectivity checks by sending an UPDATE message 1109 containing PEER_PERMISSION parameter as described in Section 4.12.1. 1110 Otherwise, the Data Relay Server drops ESP packets using the relayed 1111 address. 1113 It should be noted that in the case both Initiator and Responder both 1114 advertising their own relayed address candidates, it is possible that 1115 the two hosts choose the two relayed addresses as a result of the ICE 1116 nomination algorithm. While this is possible (and even could be 1117 desirable for privacy reasons), it can be unlikely due to low 1118 priority assigned for the relayed address candidates. In such a 1119 event, the nominated address pair is always symmetric; the nomination 1120 algorithm prevents asymmetric address pairs (i.e. each side choosing 1121 different pair), such as a Data Relay Client using its own Data Relay 1122 Server to send data directly to its peer while receiving data from 1123 the Data Relay Server of its peer. 1125 4.6.2. Rules for Connectivity Checks 1127 The HITs of the two communicating hosts MUST be used as credentials 1128 in this protocol (in contrast to ICE which employs username-password 1129 fragments). A HIT pair uniquely identifies the corresponding HIT 1130 association, and a SEQ number in an UPDATE message identifies a 1131 particular connectivity check. 1133 All of the connectivity check messages MUST be protected with 1134 HIP_HMAC and signatures (even though the illustrations in this 1135 specification omit them for simplicity) according to [RFC7401]. Each 1136 connectivity check sent by a host MUST include a SEQ parameter and 1137 ECHO_REQUEST_SIGNED parameter, and correspondingly the peer MUST 1138 respond to these using ACK and ECHO_RESPONSE_SIGNED according to the 1139 rules specified in [RFC7401]. 1141 The host sending a connectivity check MUST validate that the response 1142 uses the same pair of UDP ports, and drop the packet if this is not 1143 the case. 1145 A host may receive a connectivity check before it has received the 1146 candidates from its peer. In such a case, the host MUST immediately 1147 queue a response by placing it in the triggered-check queue, and then 1148 continue waiting for the candidates. A host MUST NOT select a 1149 candidate pair until it has verified the pair using a connectivity 1150 check as defined in Section 4.6.1. 1152 [RFC7401] section 5.3.5 states that UPDATE packets have to include 1153 either a SEQ or ACK parameter (but can include both). According to 1154 the RFC, each SEQ parameter should be acknowledged separately. In 1155 the context of NATs, this means that some of the SEQ parameters sent 1156 in connectivity checks will be lost or arrive out of order. From the 1157 viewpoint of the recipient, this is not a problem since the recipient 1158 will just "blindly" acknowledge the SEQ. However, the sender needs 1159 to be prepared for lost sequence identifiers and ACKs parameters that 1160 arrive out of order. 1162 As specified in [RFC7401], an ACK parameter may acknowledge multiple 1163 sequence identifiers. While the examples in the previous sections do 1164 not illustrate such functionality, it is also permitted when 1165 employing ICE-HIP-UDP mode. 1167 In ICE-HIP-UDP mode, a retransmission of a connectivity check SHOULD 1168 be sent with the same sequence identifier in the SEQ parameter. Some 1169 tested address candidates will never produce a working address pair, 1170 and thus may cause retransmissions. Upon successful nomination of an 1171 address pair, a host SHOULD immediately stop sending such 1172 retransmissions. 1174 Full ICE procedures for prioritizing candidates, eliminating 1175 redundant candidates, forming check lists (including pruning) and 1176 triggered check-queues MUST be followed as specified in section 6.1 1177 [RFC8445], with the exception of that the foundation, frozen 1178 candidates and default candidates are not used. From viewpoint of 1179 the ICE specification [RFC8445], the protocol specified in this 1180 document operates using Component ID of 1 on all candidates, and the 1181 foundation of all candidates is unique. This specification defines 1182 only "full ICE" mode, and the "lite ICE" is not supported. The 1183 reasoning behind the missing features is described in Appendix B. 1185 The connectivity check messages MUST be paced by the Ta value 1186 negotiated during the base exchange as described in Section 4.4. If 1187 neither one of the hosts announced a minimum pacing value, a value of 1188 50 ms MUST be used. 1190 Both hosts MUST form a priority ordered checklist and begin to check 1191 transactions every Ta milliseconds as long as the checks are running 1192 and there are candidate pairs whose tests have not started. The 1193 retransmission timeout (RTO) for the connectivity check UPDATE 1194 packets SHOULD be calculated as follows: 1196 RTO = MAX (500ms, Ta * (Num-Waiting + Num-In-Progress)) 1198 In the RTO formula, Ta is the value used for the connectivity check 1199 pacing, Num-Waiting is the number of pairs in the checklist in the 1200 "Waiting" state, and Num-In-Progress is the number of pairs in the 1201 "In-Progress" state. This is identical to the formula in [RFC8445] 1202 when there is only one checklist. A smaller value than 500 ms for 1203 the RTO MUST NOT be used. 1205 Each connectivity check request packet MUST contain a 1206 CANDIDATE_PRIORITY parameter (see Section 5.14) with the priority 1207 value that would be assigned to a peer reflexive candidate if one was 1208 learned from the corresponding check. An UPDATE packet that 1209 acknowledges a connectivity check request MUST be sent from the same 1210 address that received the check and delivered to the same address 1211 where the check was received from. Each acknowledgment UPDATE packet 1212 MUST contain a MAPPED_ADDRESS parameter with the port, protocol, and 1213 IP address of the address where the connectivity check request was 1214 received from. 1216 Following the ICE guidelines [RFC8445], it is RECOMMENDED to restrict 1217 the total number of connectivity checks to 100 for each host 1218 association. This can be achieved by limiting the connectivity 1219 checks to the 100 candidate pairs with the highest priority. 1221 4.6.3. Rules for Concluding Connectivity Checks 1223 The controlling agent may find multiple working candidate pairs. To 1224 conclude the connectivity checks, it SHOULD nominate the pair with 1225 the highest priority. The controlling agent MUST nominate a 1226 candidate pair essentially by repeating a connectivity check using an 1227 UPDATE message that contains a SEQ parameter (with new sequence 1228 number), a ECHO_REQUEST_SIGNED parameter, the priority of the 1229 candidate in a CANDIDATE_PRIORITY parameter and NOMINATE parameter to 1230 signify conclusion of the connectivity checks. Since the nominated 1231 address pair has already been tested for reachability, the controlled 1232 host should be able to receive the message. Upon reception, the 1233 controlled host SHOULD select the nominated address pair. The 1234 response message MUST include a SEQ parameter with a new sequence id, 1235 acknowledgment of the sequence from the controlling host in an ACK 1236 parameter, a new ECHO_REQUEST_SIGNED parameter, ECHO_RESPONSE_SIGNED 1237 parameter corresponding to the ECHO_REQUEST_SIGNED parameter from the 1238 controlling host and the NOMINATE parameter. After sending this 1239 packet, the controlled host can create IPsec security associations 1240 using the nominated address candidate for delivering application 1241 payload to the controlling host. Since the message from the 1242 controlled host included a new sequence id and echo request for 1243 signature, the controlling host MUST acknowledge this with a new 1244 UPDATE message that includes an ACK and ECHO_RESPONSE_SIGNED 1245 parameters. After this final concluding message, the controlling 1246 host also can create IPsec security associations for delivering 1247 application payload to the controlled host. 1249 It is possible that packets are delayed by the network. Both hosts 1250 MUST continue to respond to any connectivity checks despite an 1251 address pair having been nominated. 1253 If all the connectivity checks have failed, the hosts MUST NOT send 1254 ESP traffic to each other but MAY continue communicating using HIP 1255 packets and the locators used for the base exchange. Also, the hosts 1256 SHOULD notify each other about the failure with a 1257 CONNECTIVITY_CHECKS_FAILED NOTIFY packet (see Section 5.10). 1259 4.7. NAT Traversal Optimizations 1261 4.7.1. Minimal NAT Traversal Support 1263 If the Responder has a fixed and publicly reachable IPv4 address and 1264 does not employ a Control Relay Server, the explicit NAT traversal 1265 mode negotiation MAY be omitted, and thus even the UDP-ENCAPSULATION 1266 mode does not have to be negotiated. In such a scenario, the 1267 Initiator sends an I1 message over UDP and the Responder responds 1268 with an R1 message over UDP without including any NAT traversal mode 1269 parameter. The rest of the base exchange follows the procedures 1270 defined in [RFC7401], except that the control and data plane use UDP 1271 encapsulation. Here, the use of UDP for NAT traversal is agreed 1272 implicitly. This way of operation is still subject to NAT timeouts, 1273 and the hosts MUST employ NAT keepalives as defined in Section 4.10. 1275 When UDP-ENCAPSULATION mode is chosen either explicitly or 1276 implicitly, the connectivity checks as defined in this document MUST 1277 NOT be used. When hosts lose connectivity, they MUST instead utilize 1278 [RFC8046] or [RFC8047] procedures, but with the difference being that 1279 UDP-based tunneling MUST be employed for the entire lifetime of the 1280 corresponding Host Association. 1282 4.7.2. Base Exchange without Connectivity Checks 1284 It is possible to run a base exchange without any connectivity checks 1285 as defined in Legacy ICE-HIP section 4.8 [RFC5770]. The procedure is 1286 applicable also in the context of this specification, so it is 1287 repeated here for completeness. 1289 In certain network environments, the connectivity checks can be 1290 omitted to reduce initial connection set-up latency because a base 1291 exchange acts as an implicit connectivity test itself. For this to 1292 work, the Initiator MUST be able to reach the Responder by simply UDP 1293 encapsulating HIP and ESP packets sent to the Responder's address. 1294 Detecting and configuring this particular scenario is prone to 1295 failure unless carefully planned. 1297 In such a scenario, the Responder MAY include UDP-ENCAPSULATION NAT 1298 traversal mode as one of the supported modes in the R1 packet. If 1299 the Responder has registered to a Control Relay Server in order to 1300 discover its address candidates, it MUST also include a LOCATOR_SET 1301 parameter encapsulated inside an ENCRYPTED parameter in R1 message 1302 that contains a preferred address where the Responder is able to 1303 receive UDP-encapsulated ESP and HIP packets. This locator MUST be 1304 of type "Transport address", its Traffic type MUST be "both", and it 1305 MUST have the "Preferred bit" set (see Table 1). If there is no such 1306 locator in R1, the Initiator MUST use the source address of the R1 as 1307 the Responder's preferred address. 1309 The Initiator MAY choose the UDP-ENCAPSULATION mode if the Responder 1310 listed it in the supported modes and the Initiator does not wish to 1311 use the connectivity checks defined in this document for searching 1312 for a more optimal path. In this case, the Initiator sends the I2 1313 with UDP-ENCAPSULATION mode in the NAT traversal mode parameter 1314 directly to the Responder's preferred address (i.e., to the preferred 1315 locator in R1 or to the address where R1 was received from if there 1316 was no preferred locator in R1). The Initiator MAY include locators 1317 in I2 but they MUST NOT be taken as address candidates, since 1318 connectivity checks defined in this document will not be used for 1319 connections with UDP-ENCAPSULATION NAT traversal mode. Instead, if 1320 R2 and I2 are received and processed successfully, a security 1321 association can be created and UDP-encapsulated ESP can be exchanged 1322 between the hosts after the base exchange completes according to the 1323 rules in Section 4.4 in [RFC7401]. 1325 The Control Relay Server can be used for discovering address 1326 candidates but it is not intended to be used for relaying end-host 1327 packets using the UDP-ENCAPSULATION NAT mode. Since an I2 packet 1328 with UDP-ENCAPSULATION NAT traversal mode selected MUST NOT be sent 1329 via a Control Relay Server, the Responder SHOULD reject such I2 1330 packets and reply with a NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER NOTIFY 1331 packet (see Section 5.10). 1333 If there is no answer for the I2 packet sent directly to the 1334 Responder's preferred address, the Initiator MAY send another I2 via 1335 the Control Relay Server, but it MUST NOT choose UDP-ENCAPSULATION 1336 NAT traversal mode for that I2. 1338 4.7.3. Initiating a Base Exchange both with and without UDP 1339 Encapsulation 1341 It is possible to run a base exchange in parallel both with and 1342 without UDP encapsulation as defined in Legacy ICE-HIP section 4.9 in 1343 [RFC5770]. The procedure is applicable also in the context of this 1344 specification, so it is repeated here for completeness. 1346 The Initiator MAY also try to simultaneously perform a base exchange 1347 with the Responder without UDP encapsulation. In such a case, the 1348 Initiator sends two I1 packets, one without and one with UDP 1349 encapsulation, to the Responder. The Initiator MAY wait for a while 1350 before sending the other I1. How long to wait and in which order to 1351 send the I1 packets can be decided based on local policy. For 1352 retransmissions, the procedure is repeated. 1354 The I1 packet without UDP encapsulation may arrive directly, without 1355 passing any a Control Relay Server, at the Responder. When this 1356 happens, the procedures in [RFC7401] are followed for the rest of the 1357 base exchange. The Initiator may receive multiple R1 packets, with 1358 and without UDP encapsulation, from the Responder. However, after 1359 receiving a valid R1 and answering it with an I2, further R1 packets 1360 that are not retransmissions of the R1 message received first MUST be 1361 ignored. 1363 The I1 packet without UDP encapsulation may also arrive at a HIP- 1364 capable middlebox. When the middlebox is a HIP rendezvous server and 1365 the Responder has successfully registered with the rendezvous 1366 service, the middlebox follows rendezvous procedures in [RFC8004]. 1368 If the Initiator receives a NAT traversal mode parameter in R1 1369 without UDP encapsulation, the Initiator MAY ignore this parameter 1370 and send an I2 without UDP encapsulation and without any selected NAT 1371 traversal mode. When the Responder receives the I2 without UDP 1372 encapsulation and without NAT traversal mode, it will assume that no 1373 NAT traversal mechanism is needed. The packet processing will be 1374 done as described in [RFC7401]. The Initiator MAY store the NAT 1375 traversal modes for future use, e.g., in case of a mobility or 1376 multihoming event that causes NAT traversal to be used during the 1377 lifetime of the HIP association. 1379 4.8. Sending Control Packets after the Base Exchange 1381 The same considerations of sending control packets after the base 1382 exchange described in legacy ICE-HIP section 5.10 in [RFC5770] apply 1383 also here, so they are repeated here for completeness. 1385 After the base exchange, the two end-hosts MAY send HIP control 1386 packets directly to each other using the transport address pair 1387 established for a data channel without sending the control packets 1388 through any Control Relay Servers . When a host does not receive 1389 acknowledgments, e.g., to an UPDATE or CLOSE packet after a timeout 1390 based on local policies, a host SHOULD resend the packet through the 1391 associated Data Relay Server of the peer (if the peer listed it in 1392 its LOCATOR_SET parameter in the base exchange according the rules 1393 specified in section 4.4.2 in [RFC7401]. 1395 If Control Relay Client sends a packet through a Control Relay 1396 Server, the Control Relay Client MUST always utilize the RELAY_TO 1397 parameter. The Control Relay Server SHOULD forward HIP control 1398 packets originating from a Control Relay Client to the address 1399 denoted in the RELAY_TO parameter. In the other direction, the 1400 Control Relay Server SHOULD forward HIP control packets to the 1401 Control Relay Clients, and MUST add a RELAY_FROM parameter to the 1402 control packets it relays to the Control Relay Clients. 1404 If the Control Relay Server is not willing or able to relay a HIP 1405 packet, it MAY notify the sender of the packet with 1406 MESSAGE_NOT_RELAYED error notification (see Section 5.10). 1408 4.9. Mobility Handover Procedure 1410 A host may move after base exchange and connectivity checks. 1411 Mobility extensions for HIP [RFC8046] define handover procedures 1412 without NATs. In this section, we define how two hosts interact with 1413 handover procedures in scenarios involving NATs. The specified 1414 extensions define only simple mobility using a pair of security 1415 associations, and multihoming extensions are left to be defined in 1416 later specifications. The procedures in this section offer the same 1417 functionality as "ICE restart" specified in [RFC8445]. The example 1418 described in this section shows only a Control Relay Server for the 1419 peer host for the sake of simplicity, but the mobile host may also 1420 have a Control Relay Server. 1422 The assumption here is that the two hosts have successfully 1423 negotiated and chosen the ICE-HIP-UDP mode during the base exchange 1424 as defined in Section 4.3. The Initiator of the base exchange MUST 1425 store information that it was the controlling host during the base 1426 exchange. Similarly, the Responder MUST store information that it 1427 was the controlled host during the base exchange. 1429 Prior to starting the handover procedures with all peer hosts, the 1430 mobile host SHOULD first send its locators in UPDATE messages to its 1431 Control and Data Relay Servers if it has registered to such. It 1432 SHOULD wait for all of them to respond for a configurable time, by 1433 default two minutes, and then continue with the handover procedure 1434 without information from the Relay Server that did not respond. As 1435 defined in Section 4.1, a response message from a Control Relay 1436 Server includes a REG_FROM parameter that describes the server 1437 reflexive candidate of the mobile host to be used in the candidate 1438 exchange during the handover. Similarly, an UPDATE to a Data Relay 1439 Server is necessary to make sure the Data Relay Server can forward 1440 data to the correct IP address after a handoff. 1442 The mobility extensions for NAT traversal are illustrated in 1443 Figure 6. The mobile host is the host that has changed its locators, 1444 and the peer host is the host it has a host association with. The 1445 mobile host may have multiple peers and it repeats the process with 1446 all of its peers. In the figure, the Control Relay Server belongs to 1447 the peer host, i.e., the peer host is a Control Relay Client for the 1448 Control Relay Server. Note that the figure corresponds to figure 3 1449 in [RFC8046], but the difference is that the main UPDATE procedure is 1450 carried over the relay and the connectivity is tested separately. 1451 Next, we describe the procedure in the figure in detail. 1453 Mobile Host Control Relay Server Peer Host 1454 | 1. UDP(UPDATE(ESP_INFO, | | 1455 | ENC(LOC_SET), SEQ)) | | 1456 +--------------------------------->| 2. UDP(UPDATE(ESP_INFO, | 1457 | | ENC(LOC_SET), SEQ, | 1458 | | RELAY_FROM)) | 1459 | +------------------------------->| 1460 | | | 1461 | | 3. UDP(UPDATE(ESP_INFO, SEQ, | 1462 | | ACK, ECHO_REQ_SIGN, | 1463 | | RELAY_TO)) | 1464 | 4. UDP(UPDATE(ESP_INFO, SEQ, |<-------------------------------+ 1465 | ACK, ECHO_REQ_SIGN, | | 1466 | RELAY_TO)) | | 1467 |<---------------------------------+ | 1468 | | | 1469 | 5. UDP(UPDATE(ACK, | | 1470 | ECHO_RESP_SIGNED)) | | 1471 +--------------------------------->| 6. UDP(UPDATE(ACK, | 1472 | | ECHO_RESP_SIGNED, | 1473 | | RELAY_FROM)) | 1474 | +------------------------------->| 1475 | | | 1476 | 7. connectivity checks over UDP | 1477 +<----------------------------------------------------------------->+ 1478 | | | 1479 | 8. ESP data over UDP | 1480 +<----------------------------------------------------------------->+ 1481 | | | 1483 Figure 6: HIP UPDATE procedure 1485 In step 1, the mobile host has changed location and sends a location 1486 update to its peer through the Control Relay Server of the peer. It 1487 sends an UPDATE packet with source HIT belonging to itself and 1488 destination HIT belonging to the peer host. In the packet, the 1489 source IP address belongs to the mobile host and the destination to 1490 the Control Relay Server. The packet contains an ESP_INFO parameter, 1491 where, in this case, the OLD SPI and NEW SPI parameters both contain 1492 the pre-existing incoming SPI. The packet also contains the locators 1493 of the mobile host in a LOCATOR_SET parameter, encapsulated inside an 1494 ENCRYPTED parameter (see sections 5.2.18 and 6.5 in [RFC7401] for 1495 details on the ENCRYPTED parameter). The packet contains also a SEQ 1496 number to be acknowledged by the peer. As specified in [RFC8046], 1497 the packet may also include a HOST_ID (for middlebox inspection) and 1498 DIFFIE_HELLMAN parameter for rekeying. 1500 In step 2, the Control Relay Server receives the UPDATE packet and 1501 forwards it to the peer host (i.e. Control Relay Client). The 1502 Control Relay Server rewrites the destination IP address and appends 1503 a RELAY_FROM parameter to the message. 1505 In step 3, the peer host receives the UPDATE packet, processes it and 1506 responds with another UPDATE message. The message is destined to the 1507 HIT of mobile host and to the IP address of the Control Relay Server. 1508 The message includes an ESP_INFO parameter where, in this case, the 1509 OLD SPI and NEW SPI parameters both contain the pre-existing incoming 1510 SPI. The peer includes a new SEQ and ECHO_REQUEST_SIGNED parameters 1511 to be acknowledged by the mobile host. The message acknowledges the 1512 SEQ parameter of the earlier message with an ACK parameter. The 1513 RELAY_TO parameter specifies the address of the mobile host where the 1514 Control Relay Server should forward the message. 1516 In step 4, the Control Relay Server receives the message, rewrites 1517 the destination IP address and UDP port according to the RELAY_TO 1518 parameter, and then forwards the modified message to the mobile host. 1520 In step 5, the mobile host receives the UPDATE packet and processes 1521 it. The mobile host concludes the handover procedure by 1522 acknowledging the received SEQ parameter with an ACK parameter and 1523 the ECHO_REQUEST_SIGNED parameter with ECHO_RESPONSE_SIGNED 1524 parameter. The mobile host delivers the packet to the HIT of the 1525 peer and to the address of the HIP relay. The mobile host can start 1526 connectivity checks after this packet. 1528 In step 6, HIP relay receives the UPDATE packet and forwards it to 1529 the peer host (i.e. Relay Client). The HIP relay rewrites the 1530 destination IP address and port, and then appends a RELAY_FROM 1531 parameter to the message. When the peer host receives this 1532 concluding UPDATE packet, it can initiate the connectivity checks. 1534 In step 7, the two hosts test for connectivity across NATs according 1535 to procedures described in Section 4.6. The original Initiator of 1536 the communications is the controlling and the original Responder is 1537 the controlled host. 1539 In step 8, the connectivity checks are successfully completed and the 1540 controlling host has nominated one address pair to be used. The 1541 hosts set up security associations to deliver the application 1542 payload. 1544 It is worth noting that the Control and Data Relay Client do not have 1545 to re-register for the related services after a handoff. However, if 1546 a Data Relay Client has registered a relayed address candidate from a 1547 Data Relay Server, the Data Relay Client MUST reactivate the address 1548 before the connectivity checks by sending an UPDATE message 1549 containing PEER_PERMISSION parameter as described in Section 4.12.1. 1550 Otherwise, the Data Relay Server drops ESP packets sent to the 1551 relayed address. 1553 In so called "double jump" or simultaneous mobility scenario both 1554 peers change their location simultaneously. In such a case, both 1555 peers trigger the procedure described earlier in this section at the 1556 same time. In other words, both of the communicating hosts send an 1557 UPDATE packet carrying locators at the same time or with some delay. 1558 When the locators are exchanged almost simultaneously (reliably via 1559 Control Relay Servers), the two hosts can continue with connectivity 1560 checks after both have completed separately the steps in Figure 6. 1561 The problematic case occurs when one of the hosts (referred here as 1562 host "M") moves later during the connectivity checks. In such a 1563 case, host M sends a locator to the peer which is in the middle of 1564 connectivity checks. Upon receiving the UPDATE message, the peer 1565 responds with an UPDATE with ECHO_REQ_SIGN as described in step 3 in 1566 Figure 6. Upon receiving the valid response from host M as described 1567 in step 6, the peer host MUST restart the connectivity checks with 1568 host M. This way, both hosts start the connectivity checks roughly 1569 in a synchronized way. It is also important that peer host does not 1570 restart the connectivity checks until the step 6 is successfully 1571 completed because the UPDATE message carrying locators in step 1 1572 could be replayed by an attacker. 1574 4.10. NAT Keepalives 1576 To prevent NAT states from expiring, communicating hosts MUST send 1577 periodic keepalives to other hosts with which they have established a 1578 Host Association every 15 seconds (the so called Tr value in ICE). 1579 Other values MAY be used, but a Tr value smaller than 15 seconds MUST 1580 NOT be used. Both a Control/Data Relay Client and Control/Data Relay 1581 Server, as well as two peers employing UDP-ENCAPSULATION or ICE-HIP- 1582 UDP mode, SHOULD send HIP NOTIFY packets unless they have exchanged 1583 some other traffic over the used UDP ports. However, the Data Relay 1584 Client and Data Relay Server MUST employ only HIP NOTIFY packets in 1585 order to keep the server reflexive candidates alive. The keepalive 1586 message encoding format is defined in Section 5.3. If the base 1587 exchange or mobility handover procedure occurs during an extremely 1588 slow path, a host (with a Host Association with the peer) MAY also 1589 send HIP NOTIFY packets every 15 seconds to keep the path active with 1590 the recipient. 1592 4.11. Closing Procedure 1594 The two-way procedure for closing a HIP association and the related 1595 security associations is defined in [RFC7401]. One host initiates 1596 the procedure by sending a CLOSE message and the recipient confirms 1597 it with CLOSE_ACK. All packets are protected using HMACs and 1598 signatures, and the CLOSE messages includes a ECHO_REQUEST_SIGNED 1599 parameter to protect against replay attacks. 1601 The same procedure for closing HIP associations applies also here, 1602 but the messaging occurs using the UDP encapsulated tunnel that the 1603 two hosts employ. A host sending the CLOSE message SHOULD first send 1604 the message over a direct link. After a number of retransmissions, 1605 it MUST send over a Control Relay Server of the recipient if one 1606 exists. The host receiving the CLOSE message directly without a 1607 Control Relay Server SHOULD respond directly. If CLOSE message came 1608 via a Control Relay Server, the host SHOULD respond using the same 1609 Control Relay Server. 1611 4.12. Relaying Considerations 1613 4.12.1. Forwarding Rules and Permissions 1615 The Data Relay Server uses a similar permission model as a TURN 1616 server: before the Data Relay Server forwards any ESP data packets 1617 from a peer to a Data Relay Client (or the other direction), the 1618 client MUST set a permission for the peer's address. The permissions 1619 also install a forwarding rule for each direction, similar to TURN's 1620 channels, based on the Security Parameter Index (SPI) values in the 1621 ESP packets. 1623 Permissions are not required for HIP control packets. However, if a 1624 relayed address (as conveyed in the RELAYED_ADDRESS parameter from 1625 the Data Relay Server) is selected to be used for data, the Control 1626 Relay Client MUST send an UPDATE message to the Data Relay Server 1627 containing a PEER_PERMISSION parameter (see Section 5.13) with the 1628 following information: the UDP port and address for the server 1629 reflexive address, the UDP port and address of the peer, and the 1630 inbound and outbound SPIs used for ESP. The packet MUST be sent to 1631 the same UDP tunnel the Client employed in the base exchange to 1632 contact the Server (i.e., not to the port occupied by the server 1633 reflexive candidate). To avoid packet dropping of ESP packets, the 1634 Control Relay Client SHOULD send the PEER_PERMISSION parameter before 1635 connectivity checks both in the case of base exchange and a mobility 1636 handover. It is worth noting that the UPDATE message includes a SEQ 1637 parameter (as specified in [RFC7401]) that the Data Relay Server must 1638 acknowledge, so that the Control Relay Client can resend the message 1639 with PEER_PERMISSION parameter if it gets lost. 1641 When a Data Relay Server receives an UPDATE with a PEER_PERMISSION 1642 parameter, it MUST check if the sender of the UPDATE is registered 1643 for data relaying service, and drop the UPDATE if the host was not 1644 registered. If the host was registered, the Data Relay Server checks 1645 if there is a permission with matching information (protocol, 1646 addresses, ports and SPI values). If there is no such permission, a 1647 new permission MUST be created and its lifetime MUST be set to 5 1648 minutes. If an identical permission already existed, it MUST be 1649 refreshed by setting the lifetime to 5 minutes. A Data Relay Client 1650 SHOULD refresh permissions 1 minute before the expiration when the 1651 permission is still needed. 1653 When a Data Relay Server receives an UPDATE from a registered client 1654 but without a PEER_PERMISSION parameter and with a new locator set, 1655 the Data Relay Server can assume that the mobile host has changed its 1656 location and, thus, is not reachable in its previous location. In 1657 such an event, the Data Relay Server SHOULD deactivate the permission 1658 and stop relaying data plane traffic to the client. 1660 The relayed address MUST be activated with the PEER_PERMISSION 1661 parameter both after a base exchange and after a handover procedure 1662 with another ICE-HIP-UDP capable host. Unless activated, the Data 1663 Relay Server MUST drop all ESP packets. It is worth noting that a 1664 Data Relay Client does not have to renew its registration upon a 1665 change of location UPDATE, but only when the lifetime of the 1666 registration is close to end. 1668 4.12.2. HIP Data Relay and Relaying of Control Packets 1670 When a Data Relay Server accepts to relay UDP encapsulated ESP 1671 between a Data Relay Client and its peer, the Data Relay Server opens 1672 a UDP port (relayed address) for this purpose as described in 1673 Section 4.1. This port can be used for delivering also control 1674 packets because connectivity checks also cover the path through the 1675 Data Relay Server. If the Data Relay Server receives a UDP 1676 encapsulated HIP control packet on that port, it MUST forward the 1677 packet to the Data Relay Client and add a RELAY_FROM parameter to the 1678 packet as if the Data Relay Server were acting as a Control Relay 1679 Server. When the Data Relay Client replies to a control packet with 1680 a RELAY_FROM parameter via its Data Relay Server, the Data Relay 1681 Client MUST add a RELAY_TO parameter containing the peer's address 1682 and use the address of its Data Relay Server as the destination 1683 address. Further, the Data Relay Server MUST send this packet to the 1684 peer's address from the relayed address. 1686 If the Data Relay Server receives a UDP packet that is not a HIP 1687 control packet to the relayed address, it MUST check if it has a 1688 permission set for the peer the packet is arriving from (i.e., the 1689 sender's address and SPI value matches to an installed permission). 1690 If permissions are set, the Data Relay Server MUST forward the packet 1691 to the Data Relay Client that created the permission. The Data Relay 1692 Server MUST also implement the similar checks for the reverse 1693 direction (i.e. ESP packets from the Data Relay Client to the peer). 1694 Packets without a permission MUST be dropped silently. 1696 4.12.3. Handling Conflicting SPI Values 1698 From the viewpoint of a host, its remote peers can have overlapping 1699 inbound SPI numbers because the IPsec uses also the destination IP 1700 address to index the remote peer host. However, a Data Relay Server 1701 can represent multiple remote peers, thus masquerading the actual 1702 destination. Since a Data Relay Server may have to deal with a 1703 multitude of Relay Clients and their peers, a Data Relay Server may 1704 experience collisions in the SPI namespace, thus being unable forward 1705 datagrams to the correct destination. Since the SPI space is 32 bits 1706 and the SPI values should be random, the probability for a 1707 conflicting SPI value is fairly small, but could occur on a busy Data 1708 Relay Server. The two problematic cases are described in this 1709 section. 1711 In the first scenario, the SPI collision problems occurs if two hosts 1712 have registered to the same Data Relay Server and a third host 1713 initiates base exchange with both of them. Here, the two Responders 1714 (i.e. Data Relay Clients) claim the same inbound SPI number with the 1715 same Initiator (peer). However, in this case, the Data Relay Server 1716 has allocated separate UDP ports for the two Data Relay Clients 1717 acting now as Responders (as recommended in Section 6.5). When the 1718 third host sends an ESP packet, the Data Relay Server is able to 1719 forward the packet to the correct Data Relay Client because the 1720 destination UDP port is different for each of the clients. 1722 In the second scenario, an SPI collision may occur when two 1723 Initiators run a base exchange to the same Responder (i.e. Data 1724 Relay Client), and both of the Initiators claim the same inbound SPI 1725 at the Data Relay Server using PEER_PERMISSION Parameter. In this 1726 case, the Data Relay Server cannot disambiguate the correct 1727 destination of an ESP packet originating from the Data Relay Client 1728 because the SPI could belong to either of the peers (and destination 1729 IP and UDP port belonging to the Data Relay Server are not unique 1730 either). The recommended way and a contingency plan to solve this 1731 issue are described below. 1733 The recommend way to mitigate the problem is as follows. For each 1734 new Host Association, A Data Relay Client acting as a Responder 1735 SHOULD register a new server reflexive candidate as described in 1736 Section 4.2. Similarly, the Data Relay Server SHOULD NOT re-use the 1737 port numbers as described in Section 6.5. This way, each server 1738 reflexive candidate for the Data Relay Client has a separate UDP port 1739 that the Data Relay Server can use to disambiguate packet 1740 destinations in case of SPI collisions. 1742 When the Data Relay Client is not registering or failed to register a 1743 new relay candidate for a new peer, the Data Relay Client MUST follow 1744 a contingency plan as follows. Upon receiving an I2 with a colliding 1745 SPI, the Data Relay client acting as the Responder MUST NOT include 1746 the relayed address candidate in the R2 message because the Data 1747 Relay Server would not be able demultiplex the related ESP packet to 1748 the correct Initiator. The same applies also the handover 1749 procedures; the Data Relay Client MUST NOT include the relayed 1750 address candidate when sending its new locator set in an UPDATE to 1751 its peer if it would cause a SPI conflict with another peer. 1753 5. Packet Formats 1755 The following subsections define the parameter and packet encodings 1756 for the HIP and ESP packets. All values MUST be in network byte 1757 order. 1759 It is worth noting that all of the parameters are shown for the sake 1760 of completeness even though they are specified already in Legacy ICE- 1761 HIP [RFC5770]. New parameters are explicitly described as new. 1763 5.1. HIP Control Packets 1765 Figure 7 illustrates the packet format for UDP-encapsulated HIP. The 1766 format is identical to Legacy ICE-HIP [RFC5770]. 1768 0 1 2 3 1769 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 1770 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1771 | Source Port | Destination Port | 1772 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1773 | Length | Checksum | 1774 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1775 | 32 bits of zeroes | 1776 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1777 | | 1778 ~ HIP Header and Parameters ~ 1779 | | 1780 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1782 Figure 7: Format of UDP-Encapsulated HIP Control Packets 1784 HIP control packets are encapsulated in UDP packets as defined in 1785 Section 2.2 of [RFC3948], "IKE Header Format for Port 4500", except 1786 that a different port number is used. Figure 7 illustrates the 1787 encapsulation. The UDP header is followed by 32 zero bits that can 1788 be used to differentiate HIP control packets from ESP packets. The 1789 HIP header and parameters follow the conventions of [RFC7401] with 1790 the exception that the HIP header checksum MUST be zero. The HIP 1791 header checksum is zero for two reasons. First, the UDP header 1792 already contains a checksum. Second, the checksum definition in 1793 [RFC7401] includes the IP addresses in the checksum calculation. The 1794 NATs that are unaware of HIP cannot recompute the HIP checksum after 1795 changing IP addresses. 1797 A Control/Data Relay Server or a non-relay Responder SHOULD listen at 1798 UDP port 10500 for incoming UDP-encapsulated HIP control packets. If 1799 some other port number is used, it needs to be known by potential 1800 Initiators. 1802 UDP encapsulation of HIP packets reduces the Maximum Transfer Unit 1803 (MTU) size of the control plane by 12 bytes (8-byte UDP header plus 1804 4-byte zero SPI marker), and the data plane by 8 bytes. Additional 1805 HIP relay parameters, such as RELAY_HMAC, RELAY_UDP_HIP, 1806 RELAY_UDP_ESP, etc., further increase the size of certain HIP 1807 packets. In regard to MTU, the following aspects need to be 1808 considered in an implementation: 1810 o A HIP host SHOULD implement ICMP message handling to support path 1811 MTU discovery (PMTUD) discovery as described in [RFC1063] 1812 [RFC8201] 1814 o Reliance on IP fragmentation is unlikely to be a viable strategy 1815 through NATs. If ICMP MTU discovery is not working, MTU related 1816 path black holes may occur. 1818 o A mitigation strategy is to constrain the MTU, especially for 1819 virtual interfaces, to expected safe MTU values, e.g., 1400 bytes 1820 for the underlying interfaces that support 1500 bytes MTU. 1822 o Further extensions to this specification may define a HIP-based 1823 mechanism to find a working path MTU without unnecessary 1824 constraining that size using Packetization Layer Path MTU 1825 Discovery for Datagram Transports 1826 [I-D.ietf-tsvwg-datagram-plpmtud]. For instance, such mechanism 1827 could be implemented between a HIP Relay Client and HIP Relay 1828 Server. 1830 o It is worth noting that further HIP extensions can trim off 8 1831 bytes in the ESP header by negotiating implicit IV support in the 1832 ESP_TRANSFORM parameter as described in [RFC8750]. 1834 5.2. Connectivity Checks 1836 HIP connectivity checks are HIP UPDATE packets. The format is 1837 specified in [RFC7401]. 1839 5.3. Keepalives 1841 The RECOMMENDED encoding format for keepalives is HIP NOTIFY packets 1842 as specified in [RFC7401] with Notify message type field set to 1843 NAT_KEEPALIVE [TBD by IANA: 16385] and with an empty Notification 1844 data field. It is worth noting that sending of such a HIP NOTIFY 1845 message SHOULD be omitted if the host is actively (or passively) 1846 sending some other traffic (HIP or ESP) to the peer host over the 1847 related UDP tunnel during the Tr period. For instance, the host MAY 1848 actively send ICMPv6 requests (or respond with an ICMPv6 response) 1849 inside the ESP tunnel to test the health of the associated IPsec 1850 security association. Alternatively, the host MAY use UPDATE packets 1851 as a substitute. A minimal UPDATE packet would consist of a SEQ and 1852 ECHO_REQ_SIGN parameters, and a more complex would involve rekeying 1853 procedures as specified in section 6.8 in [RFC7402]. It is worth 1854 noting that a host actively sending periodic UPDATE packets to a busy 1855 server may increase the computational load of the server since it has 1856 to verify HMACs and signatures in UPDATE messages. 1858 5.4. NAT Traversal Mode Parameter 1860 The format of NAT traversal mode parameter is defined in Legacy ICE- 1861 HIP [RFC5770] but repeated here for completeness. The format of the 1862 NAT_TRAVERSAL_MODE parameter is similar to the format of the 1863 ESP_TRANSFORM parameter in [RFC7402] and is shown in Figure 8. The 1864 Native ICE-HIP extension specified in this document defines the new 1865 NAT traversal mode identifier for ICE-HIP-UDP and reuses the UDP- 1866 ENCAPSULATION mode from Legacy ICE-HIP [RFC5770]. The identifier 1867 named RESERVED is reserved for future use. Future specifications may 1868 define more traversal modes. 1870 0 1 2 3 1871 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 1872 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1873 | Type | Length | 1874 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1875 | Reserved | Mode ID #1 | 1876 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1877 | Mode ID #2 | Mode ID #3 | 1878 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1879 | Mode ID #n | Padding | 1880 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1882 Type 608 1883 Length length in octets, excluding Type, Length, and padding 1884 Reserved zero when sent, ignored when received 1885 Mode ID defines the proposed or selected NAT traversal mode(s) 1887 The following NAT traversal mode IDs are defined: 1889 ID name Value 1890 RESERVED 0 1891 UDP-ENCAPSULATION 1 1892 ICE-STUN-UDP 2 1893 ICE-HIP-UDP 3 1895 Figure 8: Format of the NAT_TRAVERSAL_MODE Parameter 1897 The sender of a NAT_TRAVERSAL_MODE parameter MUST make sure that 1898 there are no more than six (6) Mode IDs in one NAT_TRAVERSAL_MODE 1899 parameter. Conversely, a recipient MUST be prepared to handle 1900 received NAT traversal mode parameters that contain more than six 1901 Mode IDs by accepting the first six Mode IDs and dropping the rest. 1902 The limited number of Mode IDs sets the maximum size of the 1903 NAT_TRAVERSAL_MODE parameter. The modes MUST be in preference order, 1904 most preferred mode(s) first. 1906 Implementations conforming to this specification MUST implement UDP- 1907 ENCAPSULATION and SHOULD implement ICE-HIP-UDP modes. 1909 5.5. Connectivity Check Transaction Pacing Parameter 1911 The TRANSACTION_PACING is defined in [RFC5770], but repeated in 1912 Figure 9 for completeness. It contains only the connectivity check 1913 pacing value, expressed in milliseconds, as a 32-bit unsigned 1914 integer. 1916 0 1 2 3 1917 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 1918 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1919 | Type | Length | 1920 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1921 | Min Ta | 1922 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1924 Type 610 1925 Length 4 1926 Min Ta the minimum connectivity check transaction pacing 1927 value the host would use (in milliseconds) 1929 Figure 9: Format of the TRANSACTION_PACING Parameter 1931 5.6. Relay and Registration Parameters 1933 The format of the REG_FROM, RELAY_FROM, and RELAY_TO parameters is 1934 shown in Figure 10. All parameters are identical except for the 1935 type. Of the three, only REG_FROM is covered by the signature. 1937 0 1 2 3 1938 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 1939 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1940 | Type | Length | 1941 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1942 | Port | Protocol | Reserved | 1943 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1944 | | 1945 | Address | 1946 | | 1947 | | 1948 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1950 Type REG_FROM: 950 1951 RELAY_FROM: 63998 1952 RELAY_TO: 64002 1953 Length 20 1954 Port transport port number; zero when plain IP is used 1955 Protocol IANA assigned, Internet Protocol number. 1956 17 for UDP, 0 for plain IP 1957 Reserved reserved for future use; zero when sent, ignored 1958 when received 1959 Address an IPv6 address or an IPv4 address in "IPv4-Mapped 1960 IPv6 address" format 1962 Figure 10: Format of the REG_FROM, RELAY_FROM, and RELAY_TO 1963 Parameters 1965 REG_FROM contains the transport address and protocol from which the 1966 Control Relay Server sees the registration coming. RELAY_FROM 1967 contains the address from which the relayed packet was received by 1968 the Control Relay Server and the protocol that was used. RELAY_TO 1969 contains the same information about the address to which a packet 1970 should be forwarded. 1972 5.7. LOCATOR_SET Parameter 1974 This specification reuses the format for UDP-based locators as 1975 specified in Legacy ICE-HIP [RFC5770] to be used for communicating 1976 the address candidates between two hosts. The generic and NAT- 1977 traversal-specific locator parameters are illustrated in Figure 11. 1979 0 1 2 3 1980 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 1981 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1982 | Type | Length | 1983 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1984 | Traffic Type | Locator Type | Locator Length| Reserved |P| 1985 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1986 | Locator Lifetime | 1987 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1988 | Locator | 1989 | | 1990 | | 1991 | | 1992 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1993 . . 1994 . . 1995 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1996 | Traffic Type | Loc Type = 2 | Locator Length| Reserved |P| 1997 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1998 | Locator Lifetime | 1999 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2000 | Transport Port | Transp. Proto| Kind | 2001 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2002 | Priority | 2003 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2004 | SPI | 2005 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2006 | Address | 2007 | | 2008 | | 2009 | | 2010 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2012 Figure 11: LOCATOR_SET Parameter 2014 The individual fields in the LOCATOR_SET parameter are described in 2015 Table 1. 2017 +-----------+----------+--------------------------------------------+ 2018 | Field | Value(s) | Purpose | 2019 +-----------+----------+--------------------------------------------+ 2020 | Type | 193 | Parameter type | 2021 | Length | Variable | Length in octets, excluding Type and | 2022 | | | Length fields and padding | 2023 | Traffic | 0-2 | Is the locator for HIP signaling (1), for | 2024 | Type | | ESP (2), or for both (0) | 2025 | Locator | 2 | "Transport address" locator type | 2026 | Type | | | 2027 | Locator | 7 | Length of the fields after Locator | 2028 | Length | | Lifetime in 4-octet units | 2029 | Reserved | 0 | Reserved for future extensions | 2030 | Preferred | 0 or 1 | Set to 1 for a Locator in R1 if the | 2031 | (P) bit | | Responder can use it for the rest of the | 2032 | | | base exchange, otherwise set to zero | 2033 | Locator | Variable | Locator lifetime in seconds, see Section 4 | 2034 | Lifetime | | in [RFC8046] | 2035 | Transport | Variable | Transport layer port number | 2036 | Port | | | 2037 | Transport | Variable | IANA assigned, transport layer Internet | 2038 | Protocol | | Protocol number. Currently only UDP (17) | 2039 | | | is supported. | 2040 | Kind | Variable | 0 for host, 1 for server reflexive, 2 for | 2041 | | | peer reflexive (currently unused) or 3 for | 2042 | | | relayed address | 2043 | Priority | Variable | Locator's priority as described in | 2044 | | | [RFC8445]. It is worth noting that while | 2045 | | | the priority of a single locator candidate | 2046 | | | is 32-bits, but an implementation should | 2047 | | | use a 64-bit integer to calculate the | 2048 | | | priority of a candidate pair for the ICE | 2049 | | | priority algorithm. | 2050 | SPI | Variable | Security Parameter Index (SPI) value that | 2051 | | | the host expects to see in incoming ESP | 2052 | | | packets that use this locator | 2053 | Address | Variable | IPv6 address or an "IPv4-Mapped IPv6 | 2054 | | | address" format IPv4 address [RFC4291] | 2055 +-----------+----------+--------------------------------------------+ 2057 Table 1: Fields of the LOCATOR_SET Parameter 2059 The LOCATOR parameter MUST be encapsulated inside an ENCRYPTED 2060 parameter. 2062 5.8. RELAY_HMAC Parameter 2064 As specified in Legacy ICE-HIP [RFC5770], the RELAY_HMAC parameter 2065 value has the TLV type 65520. It has the same semantics as RVS_HMAC 2066 as specified in section 4.2.1 in [RFC8004]. Similarly as with 2067 RVS_HMAC, also RELAY_HMAC is keyed with the HIP integrity key (HIP-lg 2068 or HIP-gl as specified in section 6.5 in [RFC7401]), established 2069 during the relay registration procedure as described in Section 4.1. 2071 5.9. Registration Types 2073 The REG_INFO, REG_REQ, REG_RESP, and REG_FAILED parameters contain 2074 Registration Type [RFC8003] values for Control Relay Server 2075 registration. The value for RELAY_UDP_HIP is 2 as specified in 2076 Legacy ICE-HIP [RFC5770]. The value for RELAY_UDP_ESP is (value [TBD 2077 by IANA: 3]). 2079 5.10. Notify Packet Types 2081 A Control/Data Relay Server and end-hosts can use NOTIFY packets to 2082 signal different error conditions. The NOTIFY packet types are the 2083 same as in Legacy ICE-HIP [RFC5770] except for last one, which is 2084 new. 2086 The Notify Packet Types [RFC7401] are shown below. The Notification 2087 Data field for the error notifications SHOULD contain the HIP header 2088 of the rejected packet and SHOULD be empty for the 2089 CONNECTIVITY_CHECKS_FAILED type. 2091 NOTIFICATION PARAMETER - ERROR TYPES Value 2092 ------------------------------------ ----- 2094 NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER 60 2096 If a Control Relay Server does not forward a base exchange packet 2097 due to missing NAT traversal mode parameter, or the Initiator 2098 selects a NAT traversal mode that the (non-relay) Responder did 2099 not expect, the Control Relay Server or the Responder may send 2100 back a NOTIFY error packet with this type. 2102 CONNECTIVITY_CHECKS_FAILED 61 2104 Used by the end-hosts to signal that NAT traversal connectivity 2105 checks failed and did not produce a working path. 2107 MESSAGE_NOT_RELAYED 62 2108 Used by a Control Relay Server to signal that is was not able or 2109 willing to relay a HIP packet. 2111 SERVER_REFLEXIVE_CANDIDATE_ALLOCATION_FAILED 63 2113 Used by a Data Relay Server to signal that is was not able or 2114 willing to allocate a new server reflexive candidate for the Data 2115 Relay Client 2117 5.11. ESP Data Packets 2119 The format for ESP data packets is identical to Legacy ICE-HIP 2120 [RFC5770]. 2122 [RFC3948] describes the UDP encapsulation of the IPsec ESP transport 2123 and tunnel mode. On the wire, the HIP ESP packets do not differ from 2124 the transport mode ESP, and thus the encapsulation of the HIP ESP 2125 packets is same as the UDP encapsulation transport mode ESP. 2126 However, the (semantic) difference to Bound End-to-End Tunnel (BEET) 2127 mode ESP packets used by HIP is that IP header is not used in BEET 2128 integrity protection calculation. 2130 During the HIP base exchange, the two peers exchange parameters that 2131 enable them to define a pair of IPsec ESP security associations (SAs) 2132 as described in [RFC7402]. When two peers perform a UDP-encapsulated 2133 base exchange, they MUST define a pair of IPsec SAs that produces 2134 UDP-encapsulated ESP data traffic. 2136 The management of encryption/authentication protocols and SPIs is 2137 defined in [RFC7402]. The UDP encapsulation format and processing of 2138 HIP ESP traffic is described in Section 6.1 of [RFC7402]. 2140 5.12. RELAYED_ADDRESS and MAPPED_ADDRESS Parameters 2142 While the type values are new, the format of the RELAYED_ADDRESS and 2143 MAPPED_ADDRESS parameters (Figure 12) is identical to REG_FROM, 2144 RELAY_FROM and RELAY_TO parameters. This document specifies only the 2145 use of UDP relaying, and, thus, only protocol 17 is allowed. 2146 However, future documents may specify support for other protocols. 2148 0 1 2 3 2149 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 2150 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2151 | Type | Length | 2152 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2153 | Port | Protocol | Reserved | 2154 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2155 | | 2156 | Address | 2157 | | 2158 | | 2159 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2161 Type [TBD by IANA; 2162 RELAYED_ADDRESS: 4650 2163 MAPPED_ADDRESS: 4660] 2164 Length 20 2165 Port the UDP port number 2166 Protocol IANA assigned, Internet Protocol number (17 for UDP) 2167 Reserved reserved for future use; zero when sent, ignored 2168 when received 2169 Address an IPv6 address or an IPv4 address in "IPv4-Mapped 2170 IPv6 address" format 2172 Figure 12: Format of the RELAYED_ADDRESS and MAPPED_ADDRESS 2173 Parameters 2175 5.13. PEER_PERMISSION Parameter 2177 The format of the new PEER_PERMISSION parameter is shown in 2178 Figure 13. The parameter is used for setting up and refreshing 2179 forwarding rules and the permissions for data packets at the Data 2180 Relay Server. The parameter contains one or more sets of Port, 2181 Protocol, Address, Outbound SPI (OSPI), and Inbound SPI (ISPI) 2182 values. One set defines a rule for one peer address. 2184 0 1 2 3 2185 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 2186 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2187 | Type | Length | 2188 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2189 | RPort | PPort | 2190 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2191 | Protocol | Reserved | 2192 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2193 | | 2194 | RAddress | 2195 | | 2196 | | 2197 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2198 | | 2199 | PAddress | 2200 | | 2201 | | 2202 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2203 | OSPI | 2204 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2205 | ISPI | 2206 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2208 Type [TBD by IANA; 4680] 2209 Length 48 2210 RPort the transport layer (UDP) port at the Data Relay Server 2211 (i.e. the port of the server reflexive candidate) 2212 PPort the transport layer (UDP) port number of the peer 2213 Protocol IANA assigned, Internet Protocol number (17 for UDP) 2214 Reserved reserved for future use; zero when sent, ignored 2215 when received 2216 RAddress an IPv6 address, or an IPv4 address in "IPv4-Mapped 2217 IPv6 address" format, of the server reflexive candidate 2218 PAddress an IPv6 address, or an IPv4 address in "IPv4-Mapped 2219 IPv6 address" format, of the peer 2220 OSPI the outbound SPI value the Data Relay Client is using for 2221 the peer 2222 ISPI the inbound SPI value the Data Relay Client is using for 2223 the peer 2225 Figure 13: Format of the PEER_PERMISSION Parameter 2227 5.14. HIP Connectivity Check Packets 2229 The connectivity request messages are HIP UPDATE packets containing a 2230 new CANDIDATE_PRIORITY parameter (Figure 14). Response UPDATE 2231 packets contain a MAPPED_ADDRESS parameter (Figure 12). 2233 0 1 2 3 2234 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 2235 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2236 | Type | Length | 2237 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2238 | Priority | 2239 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2241 Type [TBD by IANA; 4700] 2242 Length 4 2243 Priority the priority of a (potential) peer reflexive candidate 2245 Figure 14: Format of the CANDIDATE_PRIORITY Parameter 2247 5.15. NOMINATE parameter 2249 Figure 15 shows the NOMINATE parameter that is used to conclude the 2250 candidate nomination process. 2252 0 1 2 3 2253 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 2254 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2255 | Type | Length | 2256 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2257 | Reserved | 2258 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2260 Type [TBD by IANA; 4710] 2261 Length 4 2262 Reserved Reserved for future extension purposes 2264 Figure 15: Format of the NOMINATE Parameter 2266 6. Security Considerations 2268 Since the control plane protocol and Control Relay Server are 2269 essentially the same (with some minor differences) in this document 2270 as in Legacy ICE-HIP [RFC5770], the same security considerations (in 2271 Section 6.1, Section 6.2, Section 6.3 and Section 6.4,) are still 2272 valid, but are repeated here for the sake of completeness. New 2273 security considerations related to the new Data Relay Server are 2274 discussed in Section 6.5, and considerations related to the new 2275 connectivity check protocol are discussed in Section 6.6 and 2276 Section 6.7. 2278 6.1. Privacy Considerations 2280 It is also possible that end-users may not want to reveal all 2281 locators to each other. For example, tracking the physical location 2282 of a multihoming end-host may become easier if it reveals all 2283 locators to its peer during a base exchange. Also, revealing host 2284 addresses exposes information about the local topology that may not 2285 be allowed in all corporate environments. For these two local policy 2286 reasons, it might be tempting exclude certain host addresses from the 2287 LOCATOR_SET parameter of an end-host but this is NOT RECOMMENDED. 2288 For instance, such behavior creates non-optimal paths when the hosts 2289 are located behind the same NAT. Especially, this could be 2290 problematic with a legacy NAT that does not support routing from the 2291 private address realm back to itself through the outer address of the 2292 NAT. This scenario is referred to as the hairpin problem [RFC5128]. 2293 With such a legacy NAT, the only option left would be to use a 2294 relayed transport address from an Data Relay Server. 2296 The use of Control and Data Relay Servers can be also useful for 2297 privacy purposes. For example, a privacy concerned Responder may 2298 reveal only its Control Relay Server and Relayed candidates to 2299 Initiators. This partially protects the Responder against Denial-of- 2300 Service (DoS) attacks by allowing the Responder to initiate new 2301 connections even if its relays would be unavailable due to a DoS 2302 attack. 2304 6.2. Opportunistic Mode 2306 In opportunistic HIP mode (cf. Section 4.1.8 in [RFC7401]), an 2307 Initiator sends an I1 with without setting the destination HIT of the 2308 Responder (i.e. the Control Relay Client). A Control Relay Server 2309 SHOULD have a unique IP address per Control Relay Client when the 2310 Control Relay Server is serving more than one Control Relay Client 2311 and supports opportunistic mode. Otherwise, the Control Relay Server 2312 cannot guarantee to deliver the I1 packet to the intended recipient. 2313 Future extensions of this document may allow opportunistic mode to be 2314 used with non-unique IP addresses to be utilized either as a HIP- 2315 level anycast or multicast mechanism. Both of the mentioned cases 2316 would require a separate registration parameters that the Control 2317 Relay Server proposes and the Control Client Server accepts during 2318 registration. 2320 6.3. Base Exchange Replay Protection for Control Relay Server 2322 In certain scenarios, it is possible that an attacker, or two 2323 attackers, can replay an earlier base exchange through a Control 2324 Relay Server by masquerading as the original Initiator and Responder. 2325 The attack does not require the attacker(s) to compromise the private 2326 key(s) of the attacked host(s). However, for this attack to succeed, 2327 the legitimate Responder has to be disconnected from the Control 2328 Relay Server. 2330 The Control Relay Server can protect itself against replay attacks by 2331 becoming involved in the base exchange by introducing nonces that the 2332 end-hosts (Initiator and Responder) are required to sign. One way to 2333 do this is to add ECHO_REQUEST_M parameters to the R1 and I2 packets 2334 as described in [I-D.heer-hip-middle-auth] and drop the I2 or R2 2335 packets if the corresponding ECHO_RESPONSE_M parameters are not 2336 present. 2338 6.4. Demultiplexing Different HIP Associations 2340 Section 5.1 of [RFC3948] describes a security issue for the UDP 2341 encapsulation in the standard IP tunnel mode when two hosts behind 2342 different NATs have the same private IP address and initiate 2343 communication to the same Responder in the public Internet. The 2344 Responder cannot distinguish between two hosts, because security 2345 associations are based on the same inner IP addresses. 2347 This issue does not exist with the UDP encapsulation of HIP ESP 2348 transport format because the Responder uses HITs to distinguish 2349 between different Initiators. 2351 6.5. Reuse of Ports at the Data Relay Server 2353 If the Data Relay Server uses the same relayed address and port (as 2354 conveyed in the RELAYED_ADDRESS parameter) for multiple Data Relay 2355 Clients, it appears to all the peers, and their firewalls, that all 2356 the Data Relay Clients are at the same address. Thus, a stateful 2357 firewall may allow packets pass from hosts that would not normally be 2358 able to send packets to a peer behind the firewall. Therefore, a 2359 Data Relay Server SHOULD NOT re-use the port numbers. If port 2360 numbers need to be re-used, the Data Relay Server SHOULD have a 2361 sufficiently large pool of port numbers and select ports from the 2362 pool randomly to decrease the chances of a Data Relay Client 2363 obtaining the same address that a another host behind the same 2364 firewall is using. 2366 6.6. Amplification attacks 2368 A malicious host may send an invalid list of candidates to its peer 2369 that are used for targeting a victim host by flooding it with 2370 connectivity checks. To mitigate the attack, this protocol adopts 2371 the ICE mechanism to cap the total amount of connectivity checks as 2372 defined in Section 4.7. 2374 6.7. Attacks against Connectivity Checks and Candidate Gathering 2376 Section 19.2 in [RFC8445] describes attacks against ICE connectivity 2377 checks. HIP bases its control plane security on Diffie-Hellman key 2378 exchange, public keys and Hashed Message Authentication codes, 2379 meaning that the mentioned security concerns do not apply to HIP 2380 either. The mentioned section discusses also of man-in-the-middle 2381 replay attacks that are difficult to prevent. The connectivity 2382 checks in this protocol are effectively immune against replay attacks 2383 because a connectivity request includes a random nonce that the 2384 recipient must sign and send back as a response. 2386 Section 19.3 in [RFC8445] describes attacks on server reflexive 2387 address gathering. Similarly here, if the DNS, a Control Relay 2388 Server or a Data Relay Server has been compromised, not much can be 2389 done. However, the case where attacker can inject fake messages 2390 (located on a shared network segment like Wifi) does not apply here. 2391 HIP messages are integrity and replay protected, so it is not 2392 possible inject fake server reflexive address candidates. 2394 Section 19.4 in [RFC8445] describes attacks on relayed candidate 2395 gathering. Similarly to ICE TURN servers, Data Relay Server require 2396 an authenticated base exchange that protects relayed address 2397 gathering against fake requests and responses. Further, replay 2398 attacks are not possible because the HIP base exchange (and also 2399 UPDATE procedure) is protected against replay attacks. 2401 7. IANA Considerations 2403 This section is to be interpreted according to [RFC8126]. 2405 This document reuses the same default UDP port number 10500 as 2406 specified by Legacy ICE-HIP [RFC5770] for tunneling both HIP control 2407 plane and data plane traffic. The port was was registered separately 2408 for RFC5770 to co-author Ari Keranen but should now be re-assigned 2409 for IESG control. With the permission of Ari Keranen, the new 2410 assignee is IESG and contact "chair@ietf.org". In addition, IANA is 2411 requested to add a reference to this document in the entry for UDP 2412 port 10500 in the Transport Protocol Port Number Registry. The 2413 selection between Legacy ICE-HIP and Native ICE-HIP mode is 2414 negotiated using NAT_TRAVERSAL_MODE parameter during the base 2415 exchange. By default, hosts listen this port for incoming UDP 2416 datagrams and can use it also for sending UDP datagrams. Other 2417 emphemeral port numbers are negotiated and utilized dynamically. 2419 This document updates the IANA Registry for HIP Parameter Types 2420 [RFC7401] by assigning new HIP Parameter Type values for the new HIP 2421 Parameters: RELAYED_ADDRESS (length 20), MAPPED_ADDRESS (length 20, 2422 defined in Section 5.12), PEER_PERMISSION (length 48, defined in 2423 Section 5.13), CANDIDATE_PRIORITY (length 4, defined in Section 5.14) 2424 and NOMINATE (length 4, defined in Section 5.15). 2426 This document updates the IANA Registry for HIP NAT traversal modes 2427 specified in Legacy ICE-HIP [RFC5770] by assigning value for the NAT 2428 traversal mode ICE-HIP-UDP (defined in Section 5.4). 2430 This document updates the IANA Registry for HIP Notify Message Types: 2431 type field NAT_KEEPALIVE in Section 5.3 and a new error type 2432 SERVER_REFLEXIVE_CANDIDATE_ALLOCATION_FAILED in Section 5.10. 2434 This document defines additional registration types for the HIP 2435 Registration Extension [RFC8003] that allow registering with a Data 2436 Relay Server for ESP relaying service: RELAY_UDP_ESP (defined in 2437 Section 5.9, and performing server reflexive candidate discovery: 2438 CANDIDATE_DISCOVERY (defined in Section 4.2). 2440 ICE specification [RFC8445] discusses "Unilateral Self-Address 2441 Fixing" in section 18. This protocol is based on ICE, and thus the 2442 same considerations apply also here. 2444 8. Contributors 2446 Marcelo Bagnulo, Philip Matthews and Hannes Tschofenig have 2447 contributed to [RFC5770]. This document leans heavily on the work in 2448 the RFC. 2450 9. Acknowledgments 2452 Thanks to Jonathan Rosenberg, Christer Holmberg and the rest of the 2453 MMUSIC WG folks for the excellent work on ICE. The authors would 2454 like to thank also Andrei Gurtov, Simon Schuetz, Martin Stiemerling, 2455 Lars Eggert, Vivien Schmitt, and Abhinav Pathak for their 2456 contributions and Tobias Heer, Teemu Koponen, Juhana Mattila, Jeffrey 2457 M. Ahrenholz, Kristian Slavov, Janne Lindqvist, Pekka Nikander, 2458 Lauri Silvennoinen, Jukka Ylitalo, Juha Heinanen, Joakim Koskela, 2459 Samu Varjonen, Dan Wing, Tom Henderson, Alex Elsayed, Jani 2460 Hautakorpi, Tero Kauppinen and Timo Simanainen for their comments to 2461 [RFC5770] and this document. Thanks for Eric Vyncke, Alvaro Retana, 2462 Adam Roach, Ben Campbell, Eric Rescorla, Mirja Kuhlewind, Spencer 2463 Dawkins, Derek Fawcus, Tianran Zhou, Amanda Barber, Colin Perkins, 2464 Roni Even, Alissa Cooper, Carl Wallace and Benjamin Kaduk for 2465 reviewing this document. 2467 This work has been partially funded by CyberTrust programme by 2468 Digile/Tekes in Finland. 2470 10. References 2472 10.1. Normative References 2474 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2475 Requirement Levels", BCP 14, RFC 2119, 2476 DOI 10.17487/RFC2119, March 1997, 2477 . 2479 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2480 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2481 May 2017, . 2483 [RFC7401] Moskowitz, R., Ed., Heer, T., Jokela, P., and T. 2484 Henderson, "Host Identity Protocol Version 2 (HIPv2)", 2485 RFC 7401, DOI 10.17487/RFC7401, April 2015, 2486 . 2488 [RFC8003] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP) 2489 Registration Extension", RFC 8003, DOI 10.17487/RFC8003, 2490 October 2016, . 2492 [RFC8004] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP) 2493 Rendezvous Extension", RFC 8004, DOI 10.17487/RFC8004, 2494 October 2016, . 2496 [RFC8046] Henderson, T., Ed., Vogt, C., and J. Arkko, "Host Mobility 2497 with the Host Identity Protocol", RFC 8046, 2498 DOI 10.17487/RFC8046, February 2017, 2499 . 2501 [RFC8047] Henderson, T., Ed., Vogt, C., and J. Arkko, "Host 2502 Multihoming with the Host Identity Protocol", RFC 8047, 2503 DOI 10.17487/RFC8047, February 2017, 2504 . 2506 [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, 2507 "Session Traversal Utilities for NAT (STUN)", RFC 5389, 2508 DOI 10.17487/RFC5389, October 2008, 2509 . 2511 [RFC7402] Jokela, P., Moskowitz, R., and J. Melen, "Using the 2512 Encapsulating Security Payload (ESP) Transport Format with 2513 the Host Identity Protocol (HIP)", RFC 7402, 2514 DOI 10.17487/RFC7402, April 2015, 2515 . 2517 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 2518 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2519 2006, . 2521 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2522 Writing an IANA Considerations Section in RFCs", BCP 26, 2523 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2524 . 2526 [RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive 2527 Connectivity Establishment (ICE): A Protocol for Network 2528 Address Translator (NAT) Traversal", RFC 8445, 2529 DOI 10.17487/RFC8445, July 2018, 2530 . 2532 [RFC7050] Savolainen, T., Korhonen, J., and D. Wing, "Discovery of 2533 the IPv6 Prefix Used for IPv6 Address Synthesis", 2534 RFC 7050, DOI 10.17487/RFC7050, November 2013, 2535 . 2537 [RFC8005] Laganier, J., "Host Identity Protocol (HIP) Domain Name 2538 System (DNS) Extension", RFC 8005, DOI 10.17487/RFC8005, 2539 October 2016, . 2541 [RFC1063] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP 2542 MTU discovery options", RFC 1063, DOI 10.17487/RFC1063, 2543 July 1988, . 2545 [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., 2546 "Path MTU Discovery for IP version 6", STD 87, RFC 8201, 2547 DOI 10.17487/RFC8201, July 2017, 2548 . 2550 [RFC5770] Komu, M., Henderson, T., Tschofenig, H., Melen, J., and A. 2551 Keranen, Ed., "Basic Host Identity Protocol (HIP) 2552 Extensions for Traversal of Network Address Translators", 2553 RFC 5770, DOI 10.17487/RFC5770, April 2010, 2554 . 2556 10.2. Informative References 2558 [I-D.ietf-hip-rfc4423-bis] 2559 Moskowitz, R. and M. Komu, "Host Identity Protocol 2560 Architecture", draft-ietf-hip-rfc4423-bis-20 (work in 2561 progress), February 2019. 2563 [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., 2564 and W. Weiss, "An Architecture for Differentiated 2565 Services", RFC 2475, DOI 10.17487/RFC2475, December 1998, 2566 . 2568 [RFC5207] Stiemerling, M., Quittek, J., and L. Eggert, "NAT and 2569 Firewall Traversal Issues of Host Identity Protocol (HIP) 2570 Communication", RFC 5207, DOI 10.17487/RFC5207, April 2571 2008, . 2573 [I-D.rosenberg-mmusic-ice-nonsip] 2574 Rosenberg, J., "Guidelines for Usage of Interactive 2575 Connectivity Establishment (ICE) by non Session Initiation 2576 Protocol (SIP) Protocols", draft-rosenberg-mmusic-ice- 2577 nonsip-01 (work in progress), July 2008. 2579 [RFC6538] Henderson, T. and A. Gurtov, "The Host Identity Protocol 2580 (HIP) Experiment Report", RFC 6538, DOI 10.17487/RFC6538, 2581 March 2012, . 2583 [RFC5128] Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to- 2584 Peer (P2P) Communication across Network Address 2585 Translators (NATs)", RFC 5128, DOI 10.17487/RFC5128, March 2586 2008, . 2588 [I-D.heer-hip-middle-auth] 2589 Heer, T., Hummen, R., and M. Komu, "End-Host 2590 Authentication for HIP Middleboxes", draft-heer-hip- 2591 middle-auth-04 (work in progress), October 2011. 2593 [RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M. 2594 Stenberg, "UDP Encapsulation of IPsec ESP Packets", 2595 RFC 3948, DOI 10.17487/RFC3948, January 2005, 2596 . 2598 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 2599 with Session Description Protocol (SDP)", RFC 3264, 2600 DOI 10.17487/RFC3264, June 2002, 2601 . 2603 [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment 2604 (ICE): A Protocol for Network Address Translator (NAT) 2605 Traversal for Offer/Answer Protocols", RFC 5245, 2606 DOI 10.17487/RFC5245, April 2010, 2607 . 2609 [RFC8750] Migault, D., Guggemos, T., and Y. Nir, "Implicit 2610 Initialization Vector (IV) for Counter-Based Ciphers in 2611 Encapsulating Security Payload (ESP)", RFC 8750, 2612 DOI 10.17487/RFC8750, March 2020, 2613 . 2615 [I-D.ietf-tsvwg-datagram-plpmtud] 2616 Fairhurst, G., Jones, T., Tuexen, M., Ruengeler, I., and 2617 T. Voelker, "Packetization Layer Path MTU Discovery for 2618 Datagram Transports", draft-ietf-tsvwg-datagram-plpmtud-17 2619 (work in progress), March 2020. 2621 [RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using 2622 Relays around NAT (TURN): Relay Extensions to Session 2623 Traversal Utilities for NAT (STUN)", RFC 5766, 2624 DOI 10.17487/RFC5766, April 2010, 2625 . 2627 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful 2628 NAT64: Network Address and Protocol Translation from IPv6 2629 Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146, 2630 April 2011, . 2632 [RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van 2633 Beijnum, "DNS64: DNS Extensions for Network Address 2634 Translation from IPv6 Clients to IPv4 Servers", RFC 6147, 2635 DOI 10.17487/RFC6147, April 2011, 2636 . 2638 Appendix A. Selecting a Value for Check Pacing 2640 Selecting a suitable value for the connectivity check transaction 2641 pacing is essential for the performance of connectivity check-based 2642 NAT traversal. The value should not be so small that the checks 2643 cause network congestion or overwhelm the NATs. On the other hand, a 2644 pacing value that is too high makes the checks last for a long time, 2645 thus increasing the connection setup delay. 2647 The Ta value may be configured by the user in environments where the 2648 network characteristics are known beforehand. However, if the 2649 characteristics are not known, it is recommended that the value is 2650 adjusted dynamically. In this case, it is recommended that the hosts 2651 estimate the round-trip time (RTT) between them and SHOULD set the 2652 minimum Ta value so that at most a single connectivity check message 2653 is sent on every RTT. 2655 One way to estimate the RTT is to use the time that it takes for the 2656 Control Relay Server registration exchange to complete; this would 2657 give an estimate on the registering host's access link's RTT. Also, 2658 the I1/R1 exchange could be used for estimating the RTT, but since 2659 the R1 can be cached in the network, or the relaying service can 2660 increase the delay notably, this is not recommended. In general, 2661 estimating RTT can be difficult and error prone, thus the guidelines 2662 for choosing a Ta value in Section 4.4 MUST be followed. 2664 Appendix B. Differences with respect to ICE 2666 Legacy ICE-HIP reuses ICE/STUN/TURN protocol stack as it is. The 2667 benefits of such as an approach include the reuse of STUN/TURN 2668 infrastructure and possibly the reuse of existing software libraries, 2669 but there are also drawbacks with the approach. For example, ICE is 2670 meant for application-layer protocols, whereas HIP operates at layer 2671 3.5 between transport and network layers. This is particularly 2672 problematic because the implementations employ kernelspace IPsec ESP 2673 as their data plane: demultiplexing of incoming ESP, HIP and TURN 2674 messages required capturing of all UDP packets destined to port 10500 2675 to the userspace (due to different, incompatible markers in ESP and 2676 STUN), thus causing additional software complexity and an unnecessary 2677 latency/throughput bottleneck for the dataplane performance. It is 2678 also worth noting that demultiplexing of STUN packets in the kernel 2679 would incur an also a performance impact (albeit smaller than with 2680 userspace demultiplexing), and secure verification of STUN messages 2681 would require communication between the kernelspace STUN detector and 2682 HIP daemon typically residing in the userspace (thus, again 2683 increasing the performance overhead). 2685 Legacy ICE-HIP involves also some other complexities when compared to 2686 the approach taken in this document. Relaying of ESP packets via 2687 TURN relays was not considered that simple because TURN relays 2688 require adding and removing extra TURN framing for the relayed 2689 packets. Finally, the developers of the two Legacy ICE-HIP 2690 implementations concluded that "effort needed for integrating an ICE 2691 library into a HIP implementation turned out to be quite a bit higher 2692 that initially estimated. Also, the amount of extra code (some 10 2693 kLoC) needed for all the new parsers, state machines, etc., is quite 2694 high and by re-using the HIP code one should be able to do with much 2695 less. This should result in smaller binary size, less bugs, and 2696 easier debugging.". Consequently, the HIP working group decided to 2697 follow ICE methodology but reuse HIP messaging format to achieve the 2698 same functionality as ICE, and consequently the result is this 2699 document that specifies the Native ICE-HIP protocol. 2701 The Native ICE-HIP protocol specified in this document follows the 2702 semantics of ICE as close as possible, and most of the differences 2703 are syntactical due to the use of a different protocol. In this 2704 section, we describe the differences to the ICE protocol. 2706 o ICE operates at the application layer, whereas this protocol 2707 operates between transport and network layers, thus hiding the 2708 protocol details from the application. 2710 o The STUN protocol is not employed. Instead, native ICE-HIP reuses 2711 the HIP control plane format in order simplify demultiplexing of 2712 different protocols. For example, the STUN binding response is 2713 replaced with a HIP UPDATE message containing an 2714 ECHO_REQUEST_SIGNED parameter and the STUN binding response with a 2715 HIP UPDATE message containing an ECHO_RESPONSE_SIGNED parameter as 2716 defined in Section 4.6. It is worth noting that a drawback of not 2717 employing STUN is that discovery of the address candidates 2718 requires creating (using HIP base exchange) and maintaining (using 2719 HIP UPDATE procedures) state at the Control Relay Client and 2720 Control Relay Server. Future extensions to this document may 2721 define a stateless, HIP-specific mechanism for an end-host to 2722 discover its address candidates. 2724 o The TURN protocol is not utilized. Instead, native ICE-HIP reuses 2725 Control Relay Servers for the same purpose. 2727 o ICMP errors may be used in ICE to signal failure. In Native ICE- 2728 HIP protocol, HIP NOTIFY messages are used instead. 2730 o Instead of the ICE username fragment and password mechanism for 2731 credentials, native ICE-HIP uses the HIT, derived from a public 2732 key, for the same purpose. The username fragments are "transient 2733 host identifiers, bound to a particular session established as 2734 part of the candidate exchange" [RFC8445]. Generally in HIP, a 2735 local public key and the derived HIT are considered long-term 2736 identifiers, and invariant across different host associations and 2737 different transport-layer flows. 2739 o In ICE, the conflict when two communicating end-points take the 2740 same controlling role is solved using random values (so called 2741 tie-breaker value). In Native ICE-HIP protocol, the conflict is 2742 solved by the standard HIP base exchange procedure, where the host 2743 with the "larger" HIT switches to Responder role, thus changing 2744 also to controlled role. 2746 o The ICE-CONTROLLED and ICE-CONTROLLING attributes are not included 2747 in the connectivity checks. 2749 o The foundation concept is unnecessary in native ICE-HIP because 2750 only a single UDP flow for the IPsec tunnel will be negotiated. 2752 o Frozen candidates are omitted for the same reason as foundation 2753 concept is excluded. 2755 o Components are omitted for the same reason as foundation concept 2756 is excluded. 2758 o Native ICE-HIP supports only "full ICE" where the two 2759 communicating hosts participate actively to the connectivity 2760 checks, and the "lite" mode is not supported. This design 2761 decision follows the guidelines of ICE which recommends full ICE 2762 implementations. However, it should be noted that a publicly 2763 reachable Responder may refuse to negotiate the ICE mode as 2764 described in Section 4.7.2. This would result in a [RFC7401] 2765 based HIP base exchange tunneled over UDP followed ESP traffic 2766 over the same tunnel, without the connectivity check procedures 2767 defined in this document (in some sense, this mode corresponds to 2768 the case where two ICE lite implementations connect since no 2769 connectivity checks are sent). 2771 o As the "ICE lite" is not adopted here and both sides are capable 2772 of ICE-HIP-UDP mode (negotiated during the base exchange), default 2773 candidates are not employed in Native ICE-HIP. 2775 o If the agent is using Diffserv Codepoint markings [RFC2475] in its 2776 media packets, it SHOULD apply those same markings to its 2777 connectivity checks. 2779 o Unlike in ICE, the addresses are not XOR-ed in Native ICE-HIP 2780 protocol but rather encrypted to avoid middlebox tampering. 2782 o Native ICE-HIP protocol does not employ the ICE related address 2783 and related port attributes (that are used for diagnostic or SIP 2784 purposes). 2786 Appendix C. Differences to Base Exchange and UPDATE procedures 2788 This section gives some design guidance for implementers how the 2789 extensions in this protocol extend and differ from [RFC7401] and 2790 [RFC8046]. 2792 o Both control and data plane are operated on top of UDP, not 2793 directly on IP. 2795 o A minimal implementation would conform only to Section 4.7.1 or 2796 Section 4.7.2, thus merely tunneling HIP control and data traffic 2797 over UDP. The drawback here is that it works only in the limited 2798 cases where the Responder has a public address. 2800 o It is worth noting that while a rendezvous server [RFC8004] has 2801 not been designed to be used in NATted scenarios because it just 2802 relays the first I1 packet and does not employ UDP encapsulation, 2803 the Control Relay Server forwards all control traffic and, hence, 2804 is more suitable in NATted environments. Further, the Data Relay 2805 Server guarantees forwarding of data plane traffic also in the 2806 cases when the NAT traversal procedures fail. 2808 o Registration procedures with a Control/Data Relay Server are 2809 similar as with rendezvous server. However, a Control/Data Relay 2810 Server has different registration parameters than rendezvous 2811 because it offers a different service. Also, the Control/Data 2812 Relay Server includes also a REG_FROM parameter that informs the 2813 Control/Data Relay Client about its server reflexive address. A 2814 Data Relay Server includes also a RELAYED_ADDRESS containing the 2815 relayed address for the Data Relay Client. 2817 o In [RFC7401], the Initiator and Responder can start to exchange 2818 application payload immediately after the base exchange. While 2819 exchanging data immediately after a base exchange via a Data 2820 Control Relay would be possible also here, we follow the ICE 2821 methodology to establish a direct path between two hosts using 2822 connectivity checks. This means that there will be some 2823 additional delay after the base exchange before application 2824 payload can be transmitted. The same applies for the UPDATE 2825 procedure as the connectivity checks introduce some additional 2826 delay. 2828 o In HIP without any NAT traversal support, the base exchange acts 2829 as an implicit connectivity check, and the mobility and 2830 multihoming extensions support explicit connectivity checks. 2831 After a base exchange or UPDATE based connectivity checks, a host 2832 can use the associated address pair for transmitting application 2833 payload. In this Native ICE-HIP extension, we follow the ICE 2834 methodology, where one end-point acting in the controlled role 2835 chooses the used address pair also on behalf of the other end- 2836 point acting in controlled role, which is different from HIP 2837 without NAT traversal support. Another difference is that the 2838 process of choosing an address pair is explicitly signaled using 2839 the nomination packets. The nomination process in this protocol 2840 supports only single address pair, and multihoming extensions are 2841 left for further study. 2843 o The UPDATE procedure resembles the mobility extensions defined in 2844 [RFC8046]. The first UPDATE message from the mobile host is 2845 exactly the same as in the mobility extensions. The second UPDATE 2846 message from the peer host and third from the mobile host are 2847 different in the sense that they merely acknowledge and conclude 2848 the reception of the candidates through the Control Relay Server. 2849 In other words, they do not yet test for connectivity (besides 2850 reachability through the Control Relay Server) unlike in the 2851 mobility extensions. The idea is that connectivity check 2852 procedure follows the ICE specification, which is somewhat 2853 different from the HIP mobility extensions. 2855 o The connectivity checks as defined in the mobility extensions 2856 [RFC8046] are triggered only by the peer of the mobile host. 2857 Since successful NAT traversal requires that both end-points test 2858 connectivity, both the mobile host and its peer host have to test 2859 for connectivity. In addition, this protocol validates also the 2860 UDP ports; the ports in the connectivity check must match with the 2861 response, as required by ICE. 2863 o In HIP mobility extensions [RFC8046], an outbound locator has some 2864 associated state: UNVERIFIED mean that the locator has not been 2865 tested for reachability, ACTIVE means that the address has been 2866 verified for reachability and is being used actively, and 2867 DEPRECATED means that the locator lifetime has expired. In the 2868 subset of ICE specifications used by this protocol, an individual 2869 address candidate has only two properties: type and priority. 2870 Instead, the actual state in ICE is associated with candidate 2871 pairs rather than individual addresses. The subset of ICE 2872 specifications utilized by this protocol require the following 2873 attributes for a candidate pair: valid bit, nominated bit, base 2874 and the state of connectivity check. The connectivity checks have 2875 the following states: Waiting, In-progress, Succeeded and Failed. 2876 Handling of this state attribute requires some additional logic 2877 when compared to the mobility extensions since the state is 2878 associated with a local-remote address pair rather just a remote 2879 address, and, thus, the mobility and ICE states do not have an 2880 unambiguous one-to-one mapping. 2882 o Credit-based authorization as defined in [RFC8046] could be used 2883 before candidate nomination has been concluded upon discovering 2884 working candidate pairs. However, this may result in the use of 2885 asymmetric paths for a short time period in the beginning of 2886 communications. Thus, support of credit-based authorization is 2887 left for further study. 2889 Appendix D. Multihoming Considerations 2891 This document allows a host to collect address candidates from 2892 multiple interfaces, but does not support activation and the 2893 simultaneous use of multiple address candidates. While multihoming 2894 extensions to support [RFC8047] like functionality are left for 2895 further study and experimentation, we envision here some potential 2896 compatibility improvements to support multihoming: 2898 o Data Relay Registration: a Data Relay Client acting as an 2899 Initiator with another peer host should register a new server 2900 reflexive candidate for each local transport address candidate. A 2901 Data Relay Client acting as an Responder should register a new 2902 server reflexive candidate for each { local transport address 2903 candidate, new peer host} pair for the reasons described in 2904 Section 4.12.3. In both cases, the Data Relay Client should 2905 request the additional server reflexive candidates by sending 2906 UPDATE messages originating from each of the local address 2907 candidates as described in Section 4.1. As the UPDATE messages 2908 are originating from an unknown location from the viewpoint of the 2909 Data Relay Server, it must include also a ECHO_REQUEST_SIGNED in 2910 the response in order to test for return routability. 2912 o Data Relay unregistration: this follows the procedure in Section 4 2913 but the Data Relay Client should unregister using the particular 2914 transport address to be unregistered. All transport address pair 2915 registrations can be unregistered when no RELAYED_ADDRESS 2916 parameter is included. 2918 o PEER_PERMISSION parameter: this needs to be extended or an 2919 additional parameter is needed to declare the specific local 2920 candidate of the Data Relay Client. Alternatively, the use of the 2921 PEER_PERMISSION could be used as a wild card to open permissions 2922 for a specific peer to all of the candidates of the Data Relay 2923 Client. 2925 o Connectivity checks: the controlling host should be able to 2926 nominate multiple candidates (by repeating step 7 in Figure 5 in 2927 Section 4.6 using the additional candidate pairs). 2929 o Keepalives should be sent for all the nominated candidate pairs. 2930 Similarly, the Control/Data Relay Client should send keepalives 2931 from its local candidates to its Control/Data Relay Server 2932 transport addresses. 2934 Appendix E. DNS Considerations 2936 This section updates [RFC5770] Appendix B which will be replaced with 2937 the mechanism described in this section. 2939 [RFC5770] did not specify how an end-host can look up another end- 2940 host via DNS and initiate an UDP-based HIP base exchange with it, so 2941 this section makes an attempt to fill this gap. 2943 [RFC8005] specifies how a HIP end-host and its Rendezvous server is 2944 registered to DNS. Essentially, the public key of the end-host is 2945 stored as HI record and its Rendezvous Server as A or AAAA record. 2947 This way, the Rendezvous Server can act as an intermediary for the 2948 end-host and forward packets to it based on the DNS configuration. 2949 Control Relay Server offers similar functionality as Rendezvous 2950 Server, with the difference that the Control Relay Server forwards 2951 all control messages, not just the first I1 message. 2953 Prior to this document, the A and AAAA records in the DNS refer 2954 either to the HIP end-host itself or a Rendezvous Server [RFC8005], 2955 and control and data plane communication with the associated host has 2956 been assumed to occur directly over IPv4 or IPv6. However, this 2957 specification extends the records to be used for UDP-based 2958 communications. 2960 Let us consider the case of a HIP Initiator with the default policy 2961 to employ UDP encapsulation and the extensions defined in this 2962 document. The Initiator looks up the FQDN of a Responder, and 2963 retrieves its HI, A and AAAA records. Since the default policy is to 2964 use UDP encapsulation, the Initiator MUST send the I1 message over 2965 UDP to destination port 10500 (either over IPv4 in the case of a A 2966 record or over IPv6 in the case of a AAAA record). It MAY send an I1 2967 message both with and without UDP encapsulation in parallel. In the 2968 case the Initiator receives R1 messages both with and without UDP 2969 encapsulation from the Responder, the Initiator SHOULD ignore the R1 2970 messages without UDP encapsulation. 2972 The UDP encapsulated I1 packet could be received by three different 2973 types of hosts: 2975 1. HIP Control Relay Server: in this case the A/AAAA records refers 2976 to a Control Relay Server, and it will forward the packet to the 2977 corresponding Control Relay Client based on the destination HIT 2978 in the I1 packet. 2980 2. HIP Responder supporting UDP encapsulation: in this case, the A/ 2981 AAAA records refers to the end-host. Assuming the destination 2982 HIT belongs to the Responder, it receives and processes it 2983 according to the negotiated NAT traversal mechanism. The support 2984 for the protocol defined in this document vs [RFC5770] is 2985 dynamically negotiated during the base exchange. The details are 2986 specified in Section 4.3. 2988 3. HIP Rendezvous Server: this entity is not listening to UDP port 2989 10500, so it will drop the I1 message. 2991 4. HIP Responder not supporting UDP encapsulation: the targeted end- 2992 host is not listening to UDP port 10500, so it will drop the I1 2993 message. 2995 The A/AAAA-record MUST NOT be configured to refer to a Data Relay 2996 Server unless the host in question supports also Control Relay Server 2997 functionality. 2999 It also worth noting that SRV records are not employed in this 3000 specification. While they could be used for more flexible UDP port 3001 selection, they are not suitable for end-host discovery but rather 3002 would be more suitable for the discovery of HIP-specific 3003 infrastructure. Further extensions to this document may define SRV 3004 records for Control and Data Relay Server discovery within a DNS 3005 domain. 3007 Authors' Addresses 3009 Ari Keranen 3010 Ericsson 3011 Hirsalantie 11 3012 02420 Jorvas 3013 Finland 3015 Email: ari.keranen@ericsson.com 3017 Jan Melen 3018 Ericsson 3019 Hirsalantie 11 3020 02420 Jorvas 3021 Finland 3023 Email: jan.melen@ericsson.com 3025 Miika Komu (editor) 3026 Ericsson 3027 Hirsalantie 11 3028 02420 Jorvas 3029 Finland 3031 Email: miika.komu@ericsson.com