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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) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Dynamic Host Configuration L. Colitti 3 Internet-Draft J. Linkova 4 Updates: 2563 (if approved) Google 5 Intended status: Standards Track M. Richardson 6 Expires: December 19, 2020 Sandelman 7 T. Mrugalski 8 ISC 9 June 17, 2020 11 IPv6-Only-Preferred Option for DHCPv4 12 draft-ietf-dhc-v6only-03 14 Abstract 16 This document specifies a DHCPv4 option to indicate that a host 17 supports an IPv6-only mode and willing to forgo obtaining an IPv4 18 address if the network provides IPv6 connectivity. It also updates 19 RFC2563 to specify the DHCPv4 server behavior when the server 20 receives a DHCPDISCOVER not containing the Auto-Configure option. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at https://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on December 19, 2020. 39 Copyright Notice 41 Copyright (c) 2020 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (https://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 This document may contain material from IETF Documents or IETF 55 Contributions published or made publicly available before November 56 10, 2008. The person(s) controlling the copyright in some of this 57 material may not have granted the IETF Trust the right to allow 58 modifications of such material outside the IETF Standards Process. 59 Without obtaining an adequate license from the person(s) controlling 60 the copyright in such materials, this document may not be modified 61 outside the IETF Standards Process, and derivative works of it may 62 not be created outside the IETF Standards Process, except to format 63 it for publication as an RFC or to translate it into languages other 64 than English. 66 Table of Contents 68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 69 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 70 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 71 2. Reasons to Signal IPv6-Only Support in DHCPv4 Packets . . . . 5 72 3. IPv6-Only Preferred Option . . . . . . . . . . . . . . . . . 5 73 3.1. Option format . . . . . . . . . . . . . . . . . . . . . . 5 74 3.2. DHCPv4 Client Behavior . . . . . . . . . . . . . . . . . 6 75 3.3. DHCPv4 Server Behavior . . . . . . . . . . . . . . . . . 8 76 3.3.1. Interaction with RFC2563 . . . . . . . . . . . . . . 8 77 3.4. Constants and Configuration Variables . . . . . . . . . . 10 78 4. IPv6-Only Transition Technologies Considerations . . . . . . 10 79 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 80 6. Security Considerations . . . . . . . . . . . . . . . . . . . 11 81 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 82 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 83 8.1. Normative References . . . . . . . . . . . . . . . . . . 11 84 8.2. Informative References . . . . . . . . . . . . . . . . . 12 85 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 87 1. Introduction 89 One of the biggest challenges of deploying IPv6-only LANs is that 90 such networks might contain rather heterogeneous collection of hosts. 91 While some hosts are capable of operating in IPv6-only mode (either 92 because the OS and all applications are IPv6-only capable or because 93 the host has some form of 464XLAT [RFC6877] deployed), others might 94 still have IPv4 dependencies and need IPv4 addresses to operate 95 properly. To incrementally rollout IPv6-only, network operators 96 might need to provide IPv4 on demand whereby a host receives an IPv4 97 address if it needs it, while IPv6-only capable hosts (such as modern 98 mobile devices) are not allocated IPv4 addresses. Traditionally that 99 goal is achieved by placing IPv6-only capable devices into a 100 dedicated IPv6-only network segment or WiFi SSID, while dual-stack 101 devices reside in another network with IPv4 and DHCPv4 enabled. 102 However such approach has a number of drawbacks, including but not 103 limited to: 105 o Doubling the number of network segments leads to operational 106 complexity and performance impact, for instance due to high memory 107 utilization caused by an increased number of ACL entries. 109 o Placing a host into the correct network segment is problematic. 110 For example, in the case of 802.11 Wi-Fi the user might select the 111 wrong SSID. In the case of wired 802.1x authentication the 112 authentication server might not have all the information required 113 to make the correct decision and choose between an IPv6-only and a 114 dual-stack VLAN. 116 It would be beneficial for IPv6 deployment if operators could 117 implement IPv6-mostly (or IPv4-on-demand) segments where IPv6-only 118 hosts co-exist with legacy dual-stack devices. The trivial solution 119 of disabling IPv4 stack on IPv6-only capable hosts is not feasible as 120 those clients must be able to operate on IPv4-only networks as well. 121 While IPv6-only capable devices might use a heuristic approach to 122 learning if the network provides IPv6-only functionality and stop 123 using IPv4 if it does, such approach might be practically 124 undesirable. One important reason is that when a host connects to a 125 network, it does not know if the network is IPv4-only, dual-stack or 126 IPv6-only. To ensure that the connectivity over whatever protocol is 127 present becomes available as soon as possible the host usually starts 128 configuring both IPv4 and IPv6 immediately. If hosts were to delay 129 requesting IPv4 until IPv6 reachability is confirmed, that would 130 penalize IPv4-only and dual-stack networks, which does not seem 131 practical. Requesting IPv4 and then releasing it later, after IPv6 132 reachability is confirmed, might cause user-visible errors as it 133 would be disruptive for applications which have started using the 134 assigned IPv4 address already. Instead it would be useful to have a 135 mechanism which would allow a host to indicate that its request for 136 an IPv4 address is optional and a network to signal that IPv6-only 137 functionality (such as NAT64, [RFC6146]) is available. The proposed 138 solution is to introduce a new DHCPv4 option which a client uses to 139 indicate that it does not need an IPv4 address if the network 140 provides IPv6-only connectivity (as NAT64 and DNS64). If the 141 particular network segment provides IPv4-on-demand such clients would 142 not be supplied with IPv4 addresses, while on IPv4-only or dual-stack 143 segments without NAT64 services IPv4 addresses will be provided. 145 [RFC2563] introduces the Auto-Configure DHCPv4 option and describes 146 DHCPv4 servers behavior if no address is chosen for a host. This 147 document updates [RFC2563] to modify the server behavior if the 148 DHCPOFFER contains the IPv6-only Preferred option. 150 1.1. Requirements Language 152 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 153 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 154 "OPTIONAL" in this document are to be interpreted as described in BCP 155 14 [RFC2119] [RFC8174] when, and only when, they appear in all 156 capitals, as shown here. 158 1.2. Terminology 160 IPv6-only capable host: a host which does not require an IPv4 address 161 and can operate on IPv6-only networks. Strictly speaking IPv6-only 162 capability is specific to a given interface of the host: if some 163 application on a host require IPv4 and 464XLAT CLAT [RFC6877] is only 164 enabled on one interface, the host is IPv6-only capable if connected 165 to a NAT64 network via that interface. 167 IPv4-requiring host: a host which is not IPv6-only capable and can 168 not operate in IPv6-only network providing NAT64 service. 170 IPv4-on-demand: a deployment scenario when end hosts are expected to 171 operate in IPv6-only mode by default and IPv4 addresses can be 172 assigned to some hosts if those hosts explicitly opt-in to receiving 173 IPv4 addresses. 175 IPv6-mostly network: a network which provides NAT64 (possibly with 176 DNS64) service as well as IPv4 connectivity and allows coexistence of 177 IPv6-only, dual-stack and IPv4-only hosts on the same segment. Such 178 deployment scenario allows operators to incrementally turn off IPv4 179 on end hosts, while still providing IPv4 to devices which require 180 IPv4 to operate. But, IPv6-only capable devices need not be assigned 181 IPv4 addresses. 183 IPv6-Only network: a network which does not provide routing 184 functionality for IPv4 packets. Such networks may or may not allow 185 intra-LAN IPv4 connectivity. IPv6-Only network usually provides 186 access to IPv4-only resources via NAT64 [RFC6146]. 188 NAT64: Network Address and Protocol Translation from IPv6 Clients to 189 IPv4 Servers [RFC6146]. 191 RA: Router Advertisement, a message used by IPv6 routers to advertise 192 their presence together with various link and Internet parameters 193 [RFC4861]. 195 DNS64: a mechanism for synthesizing AAAA records from A records 196 [RFC6147]. 198 2. Reasons to Signal IPv6-Only Support in DHCPv4 Packets 200 For networks which contain both IPv6-only capable and IPv4-requiring 201 hosts and utilize DHCPv4 for configuring the IPv4 network stack on 202 hosts, it seems only natural to leverage the same protocol to signal 203 that IPv4 is discretional on a given segment. An ability to remotely 204 disable IPv4 on a host can be seen as a new denial-of-service attack 205 vector. The proposed approach limits the attack surface to 206 DHCPv4-related attacks without introducing new vulnerable elements. 208 Another benefit of using DHCPv4 for signaling is that IPv4 will be 209 disabled only if both the client and the server indicate IPv6-only 210 capability. It allows IPv6-only capable hosts to turn off IPv4 only 211 upon receiving an explicit signal from the network and operate in 212 dual-stack or IPv4-only mode otherwise. In addition, the proposed 213 mechanism does not introduce any additional delays to the process of 214 configuring IP stack on hosts. If the network does not support IPv6- 215 only/IPv4-on-demand mode, an IPv6-only capable host would configure 216 an IPv4 address as quickly as on any other host. 218 Being a client/server protocol, DHCPv4 allows IPv4 to be selectively 219 disabled on a per-host basis on a given network segment. Coexistence 220 of IPv6-only, dual-stack and even IPv4-only hosts on the same LAN 221 would not only allow network administrators to preserve scarce IPv4 222 addresses but would also drastically simplify incremental deployment 223 of IPv6-only networks, positively impacting IPv6 adoption. 225 3. IPv6-Only Preferred Option 227 3.1. Option format 228 0 1 2 3 229 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 1 230 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 231 | Code | Length | Value | 232 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 233 | Value (contd) | 234 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 236 Figure 1: IPv6-Only Preferred Option Format 238 Fields: 240 Code: 8-bit identifier of the IPv6-Only Preferred option code as 241 assigned by IANA: TBD 242 Length: 8-bit unsigned integer. The length of the option excluding 243 the Code and Length Fields. The server MUST set the length 244 field to 4. The client MUST ignore the IPv6-Only Preferred 245 option if the length field value is not 4. 246 Value: 32-bit unsigned integer. 247 The number of seconds the client should disable DHCPv4 for 248 (V6ONLY_WAIT configuration variable). 249 If the server pool is explicitly configured with a 250 V6ONLY_WAIT timer the server MUST set the field to that 251 configured value. Otherwise the server MUST set it to zero. 252 The client MUST process that field as described in 253 Section 3.2. 255 3.2. DHCPv4 Client Behavior 257 A DHCPv4 client SHOULD allow a device administrator to configure 258 IPv6-only preferred mode either for a specific interface (to indicate 259 that the device is IPv6-only capable if connected to a NAT64 network 260 via that interface) or for all interfaces. If only a specific 261 interface is configured as IPv6-only capable the DHCPv4 client MUST 262 NOT consider the host to be an IPv6-only capable for the purpose of 263 sending/receiving DHCPv4 packets over any other interfaces. 265 The DHCPv4 client on an IPv4-requiring host MUST NOT include the 266 IPv6-only Preferred option in the Parameter Request List of any 267 DHCPv4 packets and MUST ignore that option in packets received from 268 DHCPv4 servers. 270 DHCPv4 clients running on IPv6-only capable hosts SHOULD include the 271 IPv6-only Preferred option code in the Parameter Request List in 272 DHCPDISCOVER and DHCPREQUEST messages for interfaces so enabled and 273 follow the processing as described below on a per interface enabled 274 basis. 276 If the client did not include the IPv6-only Preferred option code in 277 the Parameter Request List option in the DHCPDISCOVER or DHCPREQUEST 278 message it MUST ignore the IPv6-only Preferred option in any messages 279 received from the server. 281 If the client includes the IPv6-only Preferred option in the 282 Parameter Request List and the DHCPOFFER message from the server 283 contains a valid IPv6-only Preferred option, the client SHOULD NOT 284 request the IPv4 address provided in the DHCPOFFER. If the IPv6-only 285 Preferred option returned by the server contains a value greater or 286 equal to MIN_V6ONLY_WAIT, the client SHOULD set the V6ONLY_WAIT timer 287 to that value. Otherwise, the client SHOULD set the V6ONLY_WAIT 288 timer to MIN_V6ONLY_WAIT. The client SHOULD stop the DHCPv4 289 configuration process for at least V6ONLY_WAIT seconds or until a 290 network attachment event happens. The host MAY disable the IPv4 291 stack completely for V6ONLY_WAIT seconds or until the network 292 disconnection event happens. 294 The IPv6-only Preferred option SHOULD be included in the Parameter 295 Request List option in DHCPREQUEST messages (after receiving a 296 DHCPOFFER without this option, for a INIT-REBOOT, or when renewing or 297 rebinding a leased address). If the DHCPv4 server responds with a 298 DHCPACK that includes the IPv6-only Preferred option, the client MAY 299 send a DHCPRELEASE message and MAY either stop the DHCPv4 300 configuration process or disable IPv4 stack completely for 301 V6ONLY_WAIT seconds or until the network disconnection event happens. 302 Alternatively the client MAY continue to use the assigned IPv4 303 address until further DHCPv4 reconfiguration events. 305 If the client includes the IPv6-only Preferred option in the 306 Parameter Request List and the server responds with DHCPOFFER message 307 without a valid IPv6-only Preferred option, the client MUST proceed 308 as normal with a DHCPREQUEST. 310 If the client waits for multiple DHCPOFFER responses and selects one 311 of them, it MUST follow the processing for the IPv6-only Preferred 312 option based on the selected response. A client MAY use the presence 313 of the IPv6-only Preferred option as a selection criteria. 315 When an IPv6-only capable client receives the IPv6-Only Preferred 316 option from the server, the client MAY configure IPv4 link-local 317 address [RFC3927]. In that case IPv6-Only capable devices might 318 still be able to communicate over IPv4 to other devices on the link. 319 The Auto-Configure Option [RFC2563] can be used to control IPv4 link- 320 local addresses autoconfiguration. Section 3.3.1 discusses the 321 interaction between the IPv6-only Preferred and the Auto-Configure 322 options. 324 3.3. DHCPv4 Server Behavior 326 The DHCPv4 server SHOULD be able to configure certain pools to 327 include the IPv6-only preferred option in DHCPv4 responses if the 328 client included the option code in the Parameter Request List option. 329 The DHCPv4 server MAY have a configuration option to specify 330 V6ONLY_WAIT timer for all or individual IPv6-mostly pools. 332 The server MUST NOT include the IPv6-only Preferred option in the 333 DHCPOFFER or DHCPACK message if the YIADDR field in the message does 334 not belong to a pool configured as IPv6-mostly. The server MUST NOT 335 include the IPv6-only Preferred option in the DHCPOFFER or DHCPACK 336 message if the option was not present in the Parameter Request List 337 sent by the client. 339 If the IPv6-only Preferred option is present in the Parameter Request 340 List received from the client and the corresponding DHCPv4 pool is 341 explicitly configured as belonging to an IPv6-mostly network segment, 342 the server MUST include the IPv6-only Preferred option when 343 responding with the DHCPOFFER or DHCPACK message. If the server 344 responds with the IPv6-only Preferred option and the V6ONLY_WAIT 345 timer is configured for the pool, the server MUST copy the configured 346 value to the IPv6-only Preferred option value field. Otherwise it 347 MUST set the field to zero. The server SHOULD NOT assign an address 348 for the pool. Instead it SHOULD return 0.0.0.0 as the offered 349 address. Alternatively, the server MAY include an available IPv4 350 address from the pool into the DHCPOFFER as per recommendations in 351 [RFC2131]. In this case, the offered address MUST be a valid address 352 that is not committed to any other client. Because the client is not 353 expected ever to request this address, the server SHOULD NOT reserve 354 the address and SHOULD NOT verify its uniqueness. If the client then 355 issues a DHCPREQUEST for the address, the server MUST process it per 356 [RFC2131], including replying with a DHCPACK for the address if in 357 the meantime it has not been committed to another client. 359 If a client includes both a Rapid-Commit option [RFC4039] and 360 IPv6-Only Preferred option in the DHCPDISCOVER message the server 361 SHOULD NOT honor the Rapid-Commit option if the response would 362 contain the IPv6-only Preferred option to the client. It SHOULD 363 instead respond with a DHCPOFFER as indicated above. 365 3.3.1. Interaction with RFC2563 367 [RFC2563] defines an Auto-Configure DHCPv4 option to disable IPv4 368 link-local address configuration for IPv4 clients. Clients can 369 support both, neither or just one of IPv6-Only Preferred and Auto- 370 Configure options. If a client sends both IPv6-Only Preferred and 371 Auto-Configure options the network administrator can prevent the host 372 from configuring an IPv4 link-local address on IPv6-mostly network. 373 To achieve this the server needs to send DHCPOFFER which contains a 374 'yiaddr' of 0x00000000, and the Auto-Configure flag saying 375 "DoNotAutoConfigure". 377 However special care should be taken in a situation when a server 378 supports both options and receives just IPv6-Only Preferred option 379 from a client. Section 2.3 of [RFC2563] states that if no address is 380 chosen for the host (which would be the case for IPv6-only capable 381 clients on IPv6-mostly network) then: "If the DHCPDISCOVER does not 382 contain the Auto-Configure option, it is not answered." Such 383 behavior would be undesirable for clients supporting the IPv6-Only 384 Preferred option w/o supporting the Auto-Configure option as they 385 would not receive any response from the server and would keep asking, 386 instead of disabling DHCPv4 for V6ONLY_WAIT second. Therefore the 387 following update is proposed to Section 2.3 of [RFC2563]" 389 OLD TEXT: 391 --- 393 However, if no address is chosen for the host, a few additional steps 394 MUST be taken. 396 If the DHCPDISCOVER does not contain the Auto-Configure option, it is 397 not answered. 399 --- 401 NEW TEXT: 403 --- 405 However, if no address is chosen for the host, a few additional steps 406 MUST be taken. 408 If the DHCPDISCOVER does not contain the Auto-Configure option and 409 the IPv6-Only Preferred option is not present, it is not answered. 410 If the DHCPDISCOVER does not contain the Auto-Configure option but 411 contains the IPv6-Only Preferred option, the processing rules for the 412 IPv6-Only Preferred option apply. 414 --- 416 3.4. Constants and Configuration Variables 418 V6ONLY_WAIT The minimum time the client SHOULD stop the DHCPv4 419 configuration process for. MUST be no less than 420 MIN_V6ONLY_WAIT seconds. Default: 1800 seconds 421 MIN_V6ONLY_WAIT The lower boundary for V6ONLY_WAIT. Value: 300 422 seconds 424 4. IPv6-Only Transition Technologies Considerations 426 Until IPv6 adoption in the Internet reaches 100%, communication 427 between an IPv6-only host and IPv4-only destination requires some 428 form of transition mechanism deployed in the network. At the time of 429 writing, the only such mechanism that is widely supported by end 430 hosts is NAT64 [RFC6146] (either with or without 464XLAT). Therefore 431 the IPv6-only Preferred option is only sent by hosts capable of 432 operating on NAT64 networks. In a typical deployment scenario, a 433 network administrator would not configure the DHCPv4 server to return 434 the IPv6-only Preferred option unless the network provides NAT64 435 service. 437 Hypothetically it is possible for multiple transition technologies to 438 coexist. In such scenario some form of negotiation would be required 439 between a client and a server to ensure that the transition 440 technology supported by the client is the one the network provides. 441 However it seems unlikely that any new transition technology would 442 arise and be widely adopted in any foreseeable future. Therefore 443 adding support for non-existing technologies seems to be suboptimal 444 and the proposed mechanism implies that NAT64 is used to facilitate 445 connectivity between IPv6 and IPv4. 447 It should be also noted that declaring a host or (strictly speaking, 448 a host interface) IPv6-only capable is a policy decision. For 449 example, 451 o An operating system vendor may make such decision and configure 452 their DHCPv4 clients to send the IPv6-Only Preferred option by 453 default if the OS has 464XLAT CLAT [RFC6877] enabled. 455 o An enterprise network administrator may provision the corporate 456 hosts as IPv6-only capable if all applications users are supposed 457 to run have been tested in IPv6-only environment (or if 464XLAT 458 CLAT is enabled on the devices). 460 o IoT devices may be shipped in IPv6-only capable mode if they are 461 designed to connect to IPv6-enabled cloud destination only. 463 5. IANA Considerations 465 The IANA is requested to assign a new DHCPv4 Option code for the 466 IPv6-Only Preferred option from the BOOTP Vendor Extensions and 467 DHCPv4 Options registry, located at https://www.iana.org/assignments/ 468 bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml#options . If 469 possible, please assign option code 108. 471 +----------------------------+-------+ 472 | Option Name | Code | 473 +----------------------------+-------+ 474 | IPv6-only Preferred option | (TBD) | 475 +----------------------------+-------+ 477 Table 1 479 6. Security Considerations 481 The proposed mechanism is not introducing any new security 482 implications. While clients using the IPv6-only Preferred option are 483 vulnerable to attacks related to a rogue DHCPv4 server, enabling 484 IPv6-only Preferred option does not provide an attacker with any 485 additional mechanisms. 487 It should be noted that disabling IPv4 on a host upon receiving the 488 IPv6-only Preferred option from the DHCPv4 server protects the host 489 from IPv4-related attacks and therefore could be considered a 490 security feature. 492 7. Acknowledgements 494 Thanks to the following people (in alphabetical order) for their 495 review and feedback: Mohamed Boucadair, Ted Lemon, Roy Marples, Bjorn 496 Mork, Peng Shuping, Bernie Volz, Eric Vyncke. Authors would like to 497 thank Bob Hinden and Brian Carpenter for the initial idea of 498 signaling IPv6-only capability to hosts. Special thanks to Erik 499 Kline, Mark Townsley and Maciej Zenczykowski for the discussion which 500 led to the idea of signalling IPv6-only capability over DHCPv4. 502 8. References 504 8.1. Normative References 506 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 507 Requirement Levels", BCP 14, RFC 2119, 508 DOI 10.17487/RFC2119, March 1997, 509 . 511 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 512 RFC 2131, DOI 10.17487/RFC2131, March 1997, 513 . 515 [RFC2563] Troll, R., "DHCP Option to Disable Stateless Auto- 516 Configuration in IPv4 Clients", RFC 2563, 517 DOI 10.17487/RFC2563, May 1999, 518 . 520 [RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic 521 Configuration of IPv4 Link-Local Addresses", RFC 3927, 522 DOI 10.17487/RFC3927, May 2005, 523 . 525 [RFC4039] Park, S., Kim, P., and B. Volz, "Rapid Commit Option for 526 the Dynamic Host Configuration Protocol version 4 527 (DHCPv4)", RFC 4039, DOI 10.17487/RFC4039, March 2005, 528 . 530 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 531 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 532 DOI 10.17487/RFC4861, September 2007, 533 . 535 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 536 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 537 May 2017, . 539 8.2. Informative References 541 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful 542 NAT64: Network Address and Protocol Translation from IPv6 543 Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146, 544 April 2011, . 546 [RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van 547 Beijnum, "DNS64: DNS Extensions for Network Address 548 Translation from IPv6 Clients to IPv4 Servers", RFC 6147, 549 DOI 10.17487/RFC6147, April 2011, 550 . 552 [RFC6877] Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT: 553 Combination of Stateful and Stateless Translation", 554 RFC 6877, DOI 10.17487/RFC6877, April 2013, 555 . 557 Authors' Addresses 559 Lorenzo Colitti 560 Google 561 Shibuya 3-21-3 562 Shibuya, Tokyo 150-0002 563 JP 565 Email: lorenzo@google.com 567 Jen Linkova 568 Google 569 1 Darling Island Rd 570 Pyrmont, NSW 2009 571 AU 573 Email: furry@google.com 575 Michael C. Richardson 576 Sandelman Software Works 578 Email: mcr+ietf@sandelman.ca 579 URI: http://www.sandelman.ca/ 581 Tomek Mrugalski 582 Internet Systems Consortium, Inc. 583 950 Charter Street 584 Redwood City, CA 94063 585 USA 587 Email: tomasz.mrugalski@gmail.com