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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 behave X. Li 3 Internet-Draft C. Bao 4 Obsoletes: 2765 (if approved) CERNET Center/Tsinghua University 5 Intended status: Standards Track F. Baker 6 Expires: October 5, 2010 Cisco Systems 7 April 3, 2010 9 IP/ICMP Translation Algorithm 10 draft-ietf-behave-v6v4-xlate-16 12 Abstract 14 This document forms a replacement of the Stateless IP/ICMP 15 Translation Algorithm (SIIT) described in RFC 2765. The algorithm 16 translates between IPv4 and IPv6 packet headers (including ICMP 17 headers). 19 Status of this Memo 21 This Internet-Draft is submitted in full conformance with the 22 provisions of BCP 78 and BCP 79. 24 Internet-Drafts are working documents of the Internet Engineering 25 Task Force (IETF). Note that other groups may also distribute 26 working documents as Internet-Drafts. The list of current Internet- 27 Drafts is at http://datatracker.ietf.org/drafts/current/. 29 Internet-Drafts are draft documents valid for a maximum of six months 30 and may be updated, replaced, or obsoleted by other documents at any 31 time. It is inappropriate to use Internet-Drafts as reference 32 material or to cite them other than as "work in progress." 34 This Internet-Draft will expire on October 5, 2010. 36 Copyright Notice 38 Copyright (c) 2010 IETF Trust and the persons identified as the 39 document authors. All rights reserved. 41 This document is subject to BCP 78 and the IETF Trust's Legal 42 Provisions Relating to IETF Documents 43 (http://trustee.ietf.org/license-info) in effect on the date of 44 publication of this document. Please review these documents 45 carefully, as they describe your rights and restrictions with respect 46 to this document. Code Components extracted from this document must 47 include Simplified BSD License text as described in Section 4.e of 48 the Trust Legal Provisions and are provided without warranty as 49 described in the Simplified BSD License. 51 This document may contain material from IETF Documents or IETF 52 Contributions published or made publicly available before November 53 10, 2008. The person(s) controlling the copyright in some of this 54 material may not have granted the IETF Trust the right to allow 55 modifications of such material outside the IETF Standards Process. 56 Without obtaining an adequate license from the person(s) controlling 57 the copyright in such materials, this document may not be modified 58 outside the IETF Standards Process, and derivative works of it may 59 not be created outside the IETF Standards Process, except to format 60 it for publication as an RFC or to translate it into languages other 61 than English. 63 Table of Contents 65 1. Introduction and Motivation . . . . . . . . . . . . . . . . . 3 66 1.1. IPv4-IPv6 Translation Model . . . . . . . . . . . . . . . 3 67 1.2. Applicability and Limitations . . . . . . . . . . . . . . 3 68 1.3. Stateless vs. Stateful Mode . . . . . . . . . . . . . . . 4 69 1.4. Path MTU Discovery and Fragmentation . . . . . . . . . . . 5 70 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5 71 3. Translating from IPv4 to IPv6 . . . . . . . . . . . . . . . . 5 72 3.1. Translating IPv4 Headers into IPv6 Headers . . . . . . . . 7 73 3.2. Translating ICMPv4 Headers into ICMPv6 Headers . . . . . . 9 74 3.3. Translating ICMPv4 Error Messages into ICMPv6 . . . . . . 13 75 3.4. Translator Sending ICMPv4 Error Message . . . . . . . . . 14 76 3.5. Transport-layer Header Translation . . . . . . . . . . . . 14 77 3.6. Knowing When to Translate . . . . . . . . . . . . . . . . 14 78 4. Translating from IPv6 to IPv4 . . . . . . . . . . . . . . . . 14 79 4.1. Translating IPv6 Headers into IPv4 Headers . . . . . . . . 17 80 4.2. Translating ICMPv6 Headers into ICMPv4 Headers . . . . . . 19 81 4.3. Translating ICMPv6 Error Messages into ICMPv4 . . . . . . 22 82 4.4. Translator Sending ICMPv6 Error Message . . . . . . . . . 23 83 4.5. Transport-layer Header Translation . . . . . . . . . . . . 23 84 4.6. Knowing When to Translate . . . . . . . . . . . . . . . . 24 85 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 86 6. Security Considerations . . . . . . . . . . . . . . . . . . . 24 87 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25 88 8. Appendix: Stateless translation workflow example . . . . . . . 25 89 8.1. H6 establishes communication with H4 . . . . . . . . . . . 26 90 8.2. H4 establishes communication with H6 . . . . . . . . . . . 27 91 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 92 9.1. Normative References . . . . . . . . . . . . . . . . . . . 28 93 9.2. Informative References . . . . . . . . . . . . . . . . . . 29 94 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30 96 1. Introduction and Motivation 98 This document is a product of the 2008-2010 effort to define a 99 replacement for NAT-PT [RFC2766]. It is directly derivative from 100 Erik Nordmark's "Stateless IP/ICMP Translation Algorithm (SIIT)" 101 [RFC2765], which provides stateless translation between IPv4 102 [RFC0791] and IPv6 [RFC2460], and between ICMPv4 [RFC0792] and ICMPv6 103 [RFC4443]. 105 Readers of this document are expected to have read and understood the 106 framework described in [I-D.ietf-behave-v6v4-framework]. 107 Implementations of this IPv4/IPv6 translation specification MUST also 108 support the address translation algorithms in 109 [I-D.ietf-behave-address-format]. Implementations MAY also support 110 stateful translation [I-D.ietf-behave-v6v4-xlate-stateful]. 112 1.1. IPv4-IPv6 Translation Model 114 The translation model consists of two or more network domains 115 connected by one or more IP/ICMP translators (XLATs) as shown in 116 Figure 1. 118 --------- --------- 119 // \\ // \\ 120 / +----+ \ 121 | |XLAT| | XLAT: IPv6/IPv4 122 | IPv4 +----+ IPv6 | Translator 123 | Domain | | Domain | 124 | | | | 125 \ | | / 126 \\ // \\ // 127 -------- --------- 129 Figure 1: IPv4-IPv6 Translation Model 131 The scenarios of the translation model are discussed in 132 [I-D.ietf-behave-v6v4-framework]. 134 1.2. Applicability and Limitations 136 This document specifies the translation algorithms between IPv4 137 packets and IPv6 packets. 139 As with [RFC2765], the translating function specified in this 140 document does not translate any IPv4 options and it does not 141 translate IPv6 extension headers except fragmentation header. 143 The issues and algorithms in the translation of datagrams containing 144 TCP segments are described in [RFC5382]. 146 Fragmented IPv4 UDP packets that do not contain a UDP checksum (i.e., 147 the UDP checksum field is zero) are not of significant use in the 148 Internet and will not be translated by the IP/ICMP translator. 150 Fragmented ICMP/ICMPv6 packets will not be translated by the IP/ICMP 151 translator. 153 The IP/ICMP header translation specified in this document is 154 consistent with requirements of multicast IP/ICMP headers. However 155 IPv4 multicast addresses [RFC5771] cannot be mapped to IPv6 multicast 156 addresses [RFC3307] based on the unicast mapping rule 157 [I-D.ietf-behave-address-format]. 159 Translator SHOULD make sure that the packets belonging to the same 160 flow leave the translator in the same order in which they arrived. 162 1.3. Stateless vs. Stateful Mode 164 An IP/ICMP translator has two possible modes of operation: stateless 165 and stateful [I-D.ietf-behave-v6v4-framework]. In both cases, we 166 assume that a system (a node or an application) that has an IPv4 167 address but not an IPv6 address is communicating with a system that 168 has an IPv6 address but no IPv4 address, or that the two systems do 169 not have contiguous routing connectivity and hence are forced to have 170 their communications translated. 172 In the stateless mode, a specific IPv6 address range will represent 173 IPv4 systems (IPv4-converted addresses), and the IPv6 systems have 174 addresses (IPv4-translatable addresses) that can be algorithmically 175 mapped to a subset of the service provider's IPv4 addresses. Note 176 that IPv4-translatable addresses is a subset of IPv4-converted 177 addresses. In general, there is no need to concern oneself with 178 translation tables, as the IPv4 and IPv6 counterparts are 179 algorithmically related. 181 In the stateful mode, a specific IPv6 address range will represent 182 IPv4 systems (IPv4-converted addresses), but the IPv6 systems may use 183 any IPv6 addresses [RFC4291] except in that range. In this case, a 184 translation table is required to bind the IPv6 systems' addresses to 185 the IPv4 addresses maintained in the translator. 187 The address translation mechanisms for the stateless and the stateful 188 translations are defined in [I-D.ietf-behave-address-format]. 190 1.4. Path MTU Discovery and Fragmentation 192 Due to the different sizes of the IPv4 and IPv6 header, which are 20+ 193 octets and 40 octets respectively, handling the maximum packet size 194 is critical for the operation of the IPv4/IPv6 translator. There are 195 three mechanisms to handle this issue: path MTU discovery (PMTUD), 196 fragmentation, and transport-layer negotiation such as the TCP MSS 197 option [RFC0879]. Note that the translator MUST behave as a router, 198 i.e. the translator MUST send a "Packet Too Big" error message or 199 fragment the packet when the packet size exceeds the MTU of the next 200 hop interface. 202 "Don't Fragment", ICMP "Packet Too Big", and packet fragmentation are 203 discussed in sections 3 and 4 of this document. The reassembling of 204 fragmented packets in the stateful translator is discussed in 205 [I-D.ietf-behave-v6v4-xlate-stateful], since it requires state 206 maintenance in the translator. 208 2. Conventions 210 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 211 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 212 document are to be interpreted as described in [RFC2119]. 214 3. Translating from IPv4 to IPv6 216 When an IP/ICMP translator receives an IPv4 datagram addressed to a 217 destination towards the IPv6 domain, it translates the IPv4 header of 218 that packet into an IPv6 header. The original IPv4 header on the 219 packet is removed and replaced by an IPv6 header. Since the ICMPv6 220 [RFC4443], TCP [RFC0793], UDP [RFC0768] and DCCP [RFC4340] headers 221 contain checksums that cover IP header information, if the address 222 mapping algorithm is not checksum-neutral, the checksum MUST be 223 evaluated before translation and the ICMPv6 and transport-layer 224 headers MUST be updated. The data portion of the packet is left 225 unchanged. The IP/ICMP translator then forwards the packet based on 226 the IPv6 destination address. 228 +-------------+ +-------------+ 229 | IPv4 | | IPv6 | 230 | Header | | Header | 231 +-------------+ +-------------+ 232 | Transport | | Fragment | 233 | Layer | ===> | Header | 234 | Header | | (if needed) | 235 +-------------+ +-------------+ 236 | | | Transport | 237 ~ Data ~ | Layer | 238 | | | Header | 239 +-------------+ +-------------+ 240 | | 241 ~ Data ~ 242 | | 243 +-------------+ 245 Figure 2: IPv4-to-IPv6 Translation 247 Path MTU discovery is mandatory in IPv6 but it is optional in IPv4. 248 IPv6 routers never fragment a packet - only the sender can do 249 fragmentation. 251 When an IPv4 node performs path MTU discovery (by setting the Don't 252 Fragment (DF) bit in the header), path MTU discovery can operate end- 253 to-end, i.e., across the translator. In this case either IPv4 or 254 IPv6 routers (including the translator) might send back ICMP "Packet 255 Too Big" messages to the sender. When the IPv6 routers send these 256 ICMPv6 errors they will pass through a translator that will translate 257 the ICMPv6 error to a form that the IPv4 sender can understand. As a 258 result, an IPv6 fragment header is only included if the IPv4 packet 259 is already fragmented. 261 However, when the IPv4 sender does not set the Don't Fragment (DF) 262 bit, the translator MUST ensure that the packet does not exceed the 263 path MTU on the IPv6 side. This is done by fragmenting the IPv4 264 packet so that it fits in 1280-byte IPv6 packets, since that is the 265 minimum IPv6 MTU. Also, when the IPv4 sender does not set the DF bit 266 the translator MUST always include an IPv6 fragment header to 267 indicate that the sender allows fragmentation. 269 The rules in section 3.1 ensure that when packets are fragmented, 270 either by the sender or by IPv4 routers, the low-order 16 bits of the 271 fragment identification are carried end-to-end, ensuring that packets 272 are correctly reassembled. In addition, the rules in section 3.1 use 273 the presence of an IPv6 fragment header to indicate that the sender 274 might not be using path MTU discovery (i.e., the packet should not 275 have the DF flag set should it later be translated back to IPv4). 277 Other than the special rules for handling fragments and path MTU 278 discovery, the actual translation of the packet header consists of a 279 simple translation as defined below. Note that ICMPv4 packets 280 require special handling in order to translate the content of ICMPv4 281 error messages and also to add the ICMPv6 pseudo-header checksum. 283 3.1. Translating IPv4 Headers into IPv6 Headers 285 If the DF flag is not set and the IPv4 packet will result in an IPv6 286 packet larger than 1280 bytes, the packet MUST be fragmented so the 287 resulting IPv6 packet (with Fragment header added to each fragment) 288 will be less than or equal to 1280 bytes. For example, if the packet 289 is fragmented prior to the translation, the IPv4 packets must be 290 fragmented so that their length, excluding the IPv4 header, is at 291 most 1232 bytes (1280 minus 40 for the IPv6 header and 8 for the 292 Fragment header). The resulting fragments are then translated 293 independently using the logic described below. 295 If the DF bit is set and the MTU of the next-hop interface is less 296 than the total length value of the IPv4 packet plus 20, the 297 translator MUST send an ICMPv4 "Fragmentation Needed" error message 298 to the IPv4 source address. 300 If the DF bit is set and the packet is not a fragment (i.e., the MF 301 flag is not set and the Fragment Offset is equal to zero) then the 302 translator SHOULD NOT add a Fragment header to the resulting packet. 303 The IPv6 header fields are set as follows: 305 Version: 6 307 Traffic Class: By default, copied from IP Type Of Service (TOS) 308 octet. According to [RFC2474] the semantics of the bits are 309 identical in IPv4 and IPv6. However, in some IPv4 environments 310 these fields might be used with the old semantics of "Type Of 311 Service and Precedence". An implementation of a translator SHOULD 312 support an administratively-configurable option to ignore the IPv4 313 TOS and always set the IPv6 traffic class (TC) to zero. In 314 addition, if the translator is at an administrative boundary, the 315 filtering and update considerations of [RFC2475] may be 316 applicable. 318 Flow Label: 0 (all zero bits) 320 Payload Length: Total length value from IPv4 header, minus the size 321 of the IPv4 header and IPv4 options, if present. 323 Next Header: For ICMPv4 (1) changed to ICMPv6 (58), otherwise 324 protocol field MUST be copied from IPv4 header. 326 Hop Limit: The hop limit is derived from the TTL value in the IPv4 327 header. Since the translator is a router, as part of forwarding 328 the packet it needs to decrement either the IPv4 TTL (before the 329 translation) or the IPv6 Hop Limit (after the translation). As 330 part of decrementing the TTL or Hop Limit the translator (as any 331 router) MUST check for zero and send the ICMPv4 "TTL Exceeded" or 332 ICMPv6 "Hop Limit Exceeded" error. 334 Source Address: The IPv4-converted address derived from the IPv4 335 source address per [I-D.ietf-behave-address-format] section 2.1. 337 If the translator gets an illegal source address (e.g. 0.0.0.0, 338 127.0.0.1, etc.), the translator SHOULD silently drop the packet 339 (as discussed in Section 5.3.7 of [RFC1812]). 341 Destination Address: In the stateless mode, which is to say that if 342 the IPv4 destination address is within a range of configured IPv4 343 stateless translation prefix, the IPv6 destination address is the 344 IPv4-translatable address derived from the IPv4 destination 345 address per [I-D.ietf-behave-address-format] section 2.1. A 346 workflow example of stateless translation is shown in the Appendix 347 of this document. 349 In the stateful mode, which is to say that if the IPv4 destination 350 address is not within the range of any configured IPv4 stateless 351 translation prefix, the IPv6 destination address and corresponding 352 transport-layer destination port are derived from the Binding 353 Information Bases (BIBs) reflecting current session state in the 354 translator as described in [I-D.ietf-behave-v6v4-xlate-stateful]. 356 If any IPv4 options are present in the IPv4 packet, the IPv4 options 357 MUST be ignored and the packet translated normally; there is no 358 attempt to translate the options. However, if an unexpired source 359 route option is present then the packet MUST instead be discarded, 360 and an ICMPv4 "Destination Unreachable/Source Route Failed" (Type 361 3/Code 5) error message SHOULD be returned to the sender. 363 If there is a need to add a Fragment header (the DF bit is not set or 364 the packet is a fragment) the header fields are set as above with the 365 following exceptions: 367 IPv6 fields: 369 Payload Length: Total length value from IPv4 header, plus 8 for 370 the fragment header, minus the size of the IPv4 header and IPv4 371 options, if present. 373 Next Header: Fragment header (44). 375 Fragment header fields: 377 Next Header: For ICMPv4 (1) changed to ICMPv6 (58), otherwise 378 protocol field MUST be copied from IPv4 header. 380 Fragment Offset: Fragment Offset copied from the IPv4 header. 382 M flag: More Fragments bit copied from the IPv4 header. 384 Identification: The low-order 16 bits copied from the 385 Identification field in the IPv4 header. The high-order 16 386 bits set to zero. 388 3.2. Translating ICMPv4 Headers into ICMPv6 Headers 390 All ICMPv4 messages that are to be translated require that the ICMPv6 391 checksum field be calculated as part of the translation since ICMPv6, 392 unlike ICMPv4, has a pseudo-header checksum just like UDP and TCP. 394 In addition, all ICMPv4 packets MUST have the Type value translated 395 and, for ICMPv4 error messages, the included IP header also MUST be 396 translated. 398 The actions needed to translate various ICMPv4 messages are as 399 follows: 401 ICMPv4 query messages: 403 Echo and Echo Reply (Type 8 and Type 0): Adjust the Type values 404 to 128 and 129, respectively, and adjust the ICMP checksum both 405 to take the type change into account and to include the ICMPv6 406 pseudo-header. 408 Information Request/Reply (Type 15 and Type 16): Obsoleted in 409 ICMPv6. Silently drop. 411 Timestamp and Timestamp Reply (Type 13 and Type 14): Obsoleted in 412 ICMPv6. Silently drop. 414 Address Mask Request/Reply (Type 17 and Type 18): Obsoleted in 415 ICMPv6. Silently drop. 417 ICMP Router Advertisement (Type 9): Single hop message. Silently 418 drop. 420 ICMP Router Solicitation (Type 10): Single hop message. Silently 421 drop. 423 Unknown ICMPv4 types: Silently drop. 425 IGMP messages: While the MLD messages [RFC2710][RFC3590][RFC3810] 426 are the logical IPv6 counterparts for the IPv4 IGMP messages 427 all the "normal" IGMP messages are single-hop messages and 428 SHOULD be silently dropped by the translator. Other IGMP 429 messages might be used by multicast routing protocols and, 430 since it would be a configuration error to try to have router 431 adjacencies across IP/ICMP translators those packets SHOULD 432 also be silently dropped. 434 ICMPv4 error messages: 436 Destination Unreachable (Type 3): Translate the Code field as 437 described below, set the Type field to 1, and adjust the 438 ICMP checksum both to take the type/code change into account 439 and to include the ICMPv6 pseudo-header. 441 Translate the Code field as follows: 443 Code 0, 1 (Net, host unreachable): Set Code value to 0 (no 444 route to destination). 446 Code 2 (Protocol unreachable): Translate to an ICMPv6 447 Parameter Problem (Type 4, Code value 1) and make the 448 Pointer point to the IPv6 Next Header field. 450 Code 3 (Port unreachable): Set Code value to 4 (port 451 unreachable). 453 Code 4 (Fragmentation needed and DF set): Translate to an 454 ICMPv6 Packet Too Big message (Type 2) with Code value 455 set to 0. The MTU field MUST be adjusted for the 456 difference between the IPv4 and IPv6 header sizes, i.e. 457 minimum(advertised MTU+20, MTU_of_IPv6_nexthop, 458 (MTU_of_IPv4_nexthop)+20). Note that if the IPv4 router 459 set the MTU field to zero, i.e., the router does not 460 implement [RFC1191], then the translator MUST use the 461 plateau values specified in [RFC1191] to determine a 462 likely path MTU and include that path MTU in the ICMPv6 463 packet. (Use the greatest plateau value that is less 464 than the returned Total Length field.) The translator 465 MUST provide a configuration function to adjust the MTU 466 from a value smaller than 1280 to 1280, which is required 467 for the workaround ([RFC2460] related issue) discussed in 468 Section 4. 470 Code 5 (Source route failed): Set Code value to 0 (No route 471 to destination). Note that this error is unlikely since 472 source routes are not translated. 474 Code 6, 7, 8: Set Code value to 0 (No route to 475 destination). 477 Code 9, 10 (Communication with destination host 478 administratively prohibited): Set Code value to 1 479 (Communication with destination administratively 480 prohibited) 482 Code 11, 12: Set Code value to 0 (no route to destination). 484 Code 13 (Communication Administratively Prohibited): Set 485 Code value to 1 (Communication with destination 486 administratively prohibited). 488 Code 14 (Host Precedence Violation): Silently drop. 490 Code 15 (Precedence cutoff in effect): Set Code value to 1 491 (Communication with destination administratively 492 prohibited). 494 Other Code values: Silently drop. 496 Redirect (Type 5): Single hop message. Silently drop. 498 Alternative Host Address (Type 6): Silently drop. 500 Source Quench (Type 4): Obsoleted in ICMPv6. Silently drop. 502 Time Exceeded (Type 11): Set the Type field to 3, and adjust 503 the ICMP checksum both to take the type change into account 504 and to include the ICMPv6 pseudo-header. The Code field is 505 unchanged. 507 Parameter Problem (Type 12): Set the Type field to 4, and 508 adjust the ICMP checksum both to take the type/code change 509 into account and to include the ICMPv6 pseudo-header. 511 Translate the Code field as follows: 513 Code 0 (Pointer indicates the error): Set the Code value to 514 0 (Erroneous header field encountered) and update the 515 pointer as defined in Figure 3 (If the Original IPv4 516 Pointer Value is not listed or the Translated IPv6 517 Pointer Value is listed as "n/a", silently drop the 518 packet). 520 Code 1 (Missing a required option): Silently drop 522 Code 2 (Bad length): Set the Code value to 0 (Erroneous 523 header field encountered) and update the pointer as 524 defined in Figure 3 (If the Original IPv4 Pointer Value 525 is not listed or the Translated IPv6 Pointer Value is 526 listed as "n/a", silently drop the packet). 528 Other Code values: Silently drop 530 Unknown ICMPv4 types: Silently drop. 532 | Original IPv4 Pointer Value | Translated IPv6 Pointer Value | 533 +--------------------------------+--------------------------------+ 534 | 0 | Version/IHL | 0 | Version/Traffic Class | 535 | 1 | Type Of Service | 1 | Traffic Class/Flow Label | 536 | 2,3 | Total Length | 4 | Payload Length | 537 | 4,5 | Identification | n/a | | 538 | 6 | Flags/Fragment Offset | n/a | | 539 | 7 | Fragment Offset | n/a | | 540 | 8 | Time to Live | 7 | Hop Limit | 541 | 9 | Protocol | 6 | Next Header | 542 |10,11| Header Checksum | n/a | | 543 |12-15| Source Address | 8 | Source Address | 544 |16-19| Destination Address | 24 | Destination Address | 545 +--------------------------------+--------------------------------+ 547 Figure 3: Pointer value for translating from IPv4 to IPv6 549 ICMP Error Payload: If the received ICMPv4 packet contains an 550 ICMPv4 Extension [RFC4884], the translation of the ICMPv4 551 packet will cause the ICMPv6 packet to change length. When 552 this occurs, the ICMPv6 Extension length attribute MUST be 553 adjusted accordingly (e.g., longer due to the translation 554 from IPv4 to IPv6). If the ICMPv4 Extension exceeds the 555 maximum size of an ICMPv6 message on the outgoing interface, 556 the ICMPv4 extension SHOULD be simply truncated. For 557 extensions not defined in [RFC4884], the translator passes 558 the extensions as opaque bit strings and those containing 559 IPv4 address literals will not have those addresses 560 translated to IPv6 address literals; this may cause problems 561 with processing of those ICMP extensions. 563 3.3. Translating ICMPv4 Error Messages into ICMPv6 565 There are some differences between the ICMPv4 and the ICMPv6 error 566 message formats as detailed above. In addition, the ICMP error 567 messages contain the packet in error, which MUST be translated just 568 like a normal IP packet. If the translation of this "packet in 569 error" changes the length of the datagram, the Total Length field in 570 the outer IPv6 header MUST be updated. 572 +-------------+ +-------------+ 573 | IPv4 | | IPv6 | 574 | Header | | Header | 575 +-------------+ +-------------+ 576 | ICMPv4 | | ICMPv6 | 577 | Header | | Header | 578 +-------------+ +-------------+ 579 | IPv4 | ===> | IPv6 | 580 | Header | | Header | 581 +-------------+ +-------------+ 582 | Partial | | Partial | 583 | Transport | | Transport | 584 | Layer | | Layer | 585 | Header | | Header | 586 +-------------+ +-------------+ 588 Figure 4: IPv4-to-IPv6 ICMP Error Translation 590 The translation of the inner IP header can be done by invoking the 591 function that translated the outer IP headers. This process MUST 592 stop at the first embedded header and drop the packet if it contains 593 more. 595 3.4. Translator Sending ICMPv4 Error Message 597 If the IPv4 packet is discarded, then the translator SHOULD be able 598 to send back an ICMPv4 error message to the original sender of the 599 packet, unless the discarded packet is itself an ICMPv4 message. The 600 ICMPv4 message, if sent, has a Type value of 3 (Destination 601 Unreachable) and a Code value of 13 (Communication Administratively 602 Prohibited), unless otherwise specified in this document or in 603 [I-D.ietf-behave-v6v4-xlate-stateful]. The translator SHOULD allow 604 an administrator to configure whether the ICMPv4 error messages are 605 sent, rate-limited, or not sent. 607 3.5. Transport-layer Header Translation 609 If the address translation algorithm is not checksum neutral, the 610 recalculation and updating of the transport-layer headers which 611 contain pseudo headers (e.g. of TCP, UDP and DCCP) MUST be performed. 613 When a translator receives an unfragmented UDP IPv4 packet and the 614 checksum field is zero, the translator SHOULD compute the missing UDP 615 checksum as part of translating the packet. Also, the translator 616 SHOULD maintain a counter of how many UDP checksums are generated in 617 this manner. 619 When a stateless translator receives the first fragment of a 620 fragmented UDP IPv4 packet and the checksum field is zero, the 621 translator SHOULD drop the packet and generate a system management 622 event specifying at least the IP addresses and port numbers in the 623 packet. When it receives fragments other than the first, it SHOULD 624 silently drop the packet, since there is no port information to log. 626 For stateful translator, the handling of fragmented UDP IPv4 packets 627 with a zero checksum is discussed in 628 [I-D.ietf-behave-v6v4-xlate-stateful] section 3.1. 630 3.6. Knowing When to Translate 632 If the IP/ICMP translator also provides normal forwarding function, 633 and the destination IPv4 address is reachable by a more specific 634 route without translation, the translator MUST forward it without 635 translating it. Otherwise, when an IP/ICMP translator receives an 636 IPv4 datagram addressed to an IPv4 destination representing a host in 637 the IPv6 domain, the packet MUST be translated to IPv6. 639 4. Translating from IPv6 to IPv4 641 When an IP/ICMP translator receives an IPv6 datagram addressed to a 642 destination towards the IPv4 domain, it translates the IPv6 header of 643 the received IPv6 packet into an IPv4 header. The original IPv6 644 header on the packet is removed and replaced by an IPv4 header. 645 Since the ICMPv6 [RFC4443], TCP [RFC0793], UDP [RFC0768] and DCCP 646 [RFC4340] headers contain checksums that cover the IP header, if the 647 address mapping algorithm is not checksum-neutral, the checksum MUST 648 be evaluated before translation and the ICMP and transport-layer 649 headers MUST be updated. The data portion of the packet is left 650 unchanged. The IP/ICMP translator then forwards the packet based on 651 the IPv4 destination address. 653 +-------------+ +-------------+ 654 | IPv6 | | IPv4 | 655 | Header | | Header | 656 +-------------+ +-------------+ 657 | Fragment | | Transport | 658 | Header | ===> | Layer | 659 |(if present) | | Header | 660 +-------------+ +-------------+ 661 | Transport | | | 662 | Layer | ~ Data ~ 663 | Header | | | 664 +-------------+ +-------------+ 665 | | 666 ~ Data ~ 667 | | 668 +-------------+ 670 Figure 5: IPv6-to-IPv4 Translation 672 There are some differences between IPv6 and IPv4 in the area of 673 fragmentation and the minimum link MTU that affect the translation. 674 An IPv6 link has to have an MTU of 1280 bytes or greater. The 675 corresponding limit for IPv4 is 68 bytes. Thus, unless there were 676 special measures, it would not be possible to do end-to-end path MTU 677 discovery when the path includes a translator, since the IPv6 node 678 might receive ICMPv6 "Packet Too Big" messages originated by an IPv4 679 router that report an MTU less than 1280. However, [RFC2460] section 680 5 requires that IPv6 nodes handle such an ICMPv6 "Packet Too Big" 681 message by reducing the path MTU to 1280 and including an IPv6 682 fragment header with each packet. In this case, the translator 683 SHOULD set DF to 0 and take the identification value from the IPv6 684 fragment header when a fragmentation header with (MF=0; Offset=0) is 685 present or set DF to 1 otherwise. This allows end-to-end path MTU 686 discovery across the translator as long as the path MTU is 1280 bytes 687 or greater. When the path MTU drops below the 1280 limit, the IPv6 688 sender will originate 1280-byte packets that will be fragmented by 689 IPv4 routers along the path after being translated to IPv4. 691 The drawback with this scheme is that it is not possible to use PMTU 692 discovery to do optimal UDP fragmentation (as opposed to completely 693 avoiding fragmentation) at the sender, since the presence of an IPv6 694 Fragment header is interpreted that it is okay to fragment the packet 695 on the IPv4 side. Thus if a UDP application wants to send large 696 packets independent of the PMTU, the sender will only be able to 697 determine the path MTU on the IPv6 side of the translator. If the 698 path MTU on the IPv4 side of the translator is smaller, then the IPv6 699 sender will not receive any ICMPv6 "Too Big" errors and cannot adjust 700 the size fragments it is sending. 702 On the other hand, a recent study indicates that only 43.46% of IPv6- 703 capable web servers include an IPv6 fragmentation header in their 704 respond packets after they were sent an ICMPv6 "Packet Too Big" 705 message specifying an MTU<1280 bytes. A workaround to this problem 706 (ICMPv6 "Packet Too Big" message with MTU<1280) is that (1) in the 707 IPv4 to IPv6 direction, the translator can adjust MTU in "Packet Too 708 Big" message from a value smaller than 1280 to 1280; (2) in the IPv6 709 to IPv4 direction, if there is no fragmentation header in the IPv6 710 packet, the translator SHOULD set DF to 0 for the packets equal to or 711 smaller than 1280 bytes and set DF to 1 for packets larger than 1280 712 bytes. In addition, the translator SHOULD take the identification 713 value from the IPv6 fragmentation header if present or generate the 714 identification value otherwise. This avoids the introduction of the 715 path MTU discovery black hole. Note that translator generating the 716 IPv4 identification value is tricky in stateless mode. The Internet 717 Protocol standard [RFC0791] specifies: 719 "The choice of the Identifier for a datagram is based on the need 720 to provide a way to uniquely identify the fragments of a 721 particular datagram. The protocol module assembling fragments 722 judges fragments to belong to the same datagram if they have the 723 same source, destination, protocol, and Identifier. Thus, the 724 sender must choose the Identifier to be unique for this source, 725 destination pair and protocol for the time the datagram (or any 726 fragment of it) could be alive in the Internet." 728 Therefore, the translator may require states per three tuple IPv4 729 identification field. However, this does not prevent the deployment 730 of the stateless translator, since as discussed in 731 [I-D.ietf-behave-v6v4-framework], the stateless translation can be 732 used in scenarios 1, 2, 5 and 6. All of these scenarios involve "An 733 IPv6 network" which are managed networks and network firewall, host 734 firewall or host misbehavior can be controlled. In such a controlled 735 environment, it can be assured that hosts and firewalls properly 736 process ICMPv6 messages as described in Section 5 of [RFC2460]. 738 The translator does not translate IPv6 routing headers, hop-by-hop 739 extension headers, destination options headers, source routing 740 headers, or any layer 4 protocol (e.g., TCP header, IPsec 741 authentication header (AH) or encapsulating security payload (ESP) 742 header). However, the translator needs to traverse the IPv6 'next 743 header' chain and copy the next header value (which contains the 744 transport protocol number) in the last known 'next header' to the 745 protocol field in the IPv4 header. This means that the translator 746 MUST forward all protocols to avoid black holing. Some protocols are 747 known to fail when translated (e.g., IPsec AH) and will fail at the 748 receiver. 750 Other than the special rules for handling fragments and path MTU 751 discovery, the actual translation of the packet header consists of a 752 simple translation as defined below. Note that ICMPv6 packets 753 require special handling in order to translate the contents of ICMPv6 754 error messages and also to remove the ICMPv6 pseudo-header checksum. 756 4.1. Translating IPv6 Headers into IPv4 Headers 758 If there is no IPv6 Fragment header, the IPv4 header fields are set 759 as follows: 761 Version: 4 763 Internet Header Length: 5 (no IPv4 options) 765 Type of Service (TOS) Octet: By default, copied from the IPv6 766 Traffic Class (all 8 bits). According to [RFC2474] the semantics 767 of the bits are identical in IPv4 and IPv6. However, in some IPv4 768 environments, these bits might be used with the old semantics of 769 "Type Of Service and Precedence". An implementation of a 770 translator SHOULD provide the ability to ignore the IPv6 traffic 771 class and always set the IPv4 TOS Octet to a specified value. In 772 addition, if the translator is at an administrative boundary, the 773 filtering and update considerations of [RFC2475] may be 774 applicable. 776 Total Length: Payload length value from IPv6 header, plus the size 777 of the IPv4 header. 779 Identification: If the packet size is equal to or smaller than 1280 780 bytes and greater than 88 bytes, generate the identification 781 value. If the packet size is greater than 1280 bytes or smaller 782 than 88 bytes, set the Identification field to all zeros 784 Flags: The More Fragments (MF) flag is set to zero. If the packet 785 size is equal to or smaller than 1280 bytes and greater than 88 786 bytes, the Don't Fragments (DF) flag is set to zero. If the 787 packet size is greater than 1280 bytes or smaller than 88 bytes, 788 the Don't Fragments (DF) flag is set to one. 790 Fragment Offset: All zeros. 792 Time to Live: Time to Live is derived from Hop Limit value in IPv6 793 header. Since the translator is a router, as part of forwarding 794 the packet it needs to decrement either the IPv6 Hop Limit (before 795 the translation) or the IPv4 TTL (after the translation). As part 796 of decrementing the TTL or Hop Limit the translator (as any 797 router) MUST check for zero and send the ICMPv4 "TTL Exceeded" or 798 ICMPv6 "Hop Limit Exceeded" error. 800 Protocol: For ICMPv6 (58) changed to ICMPv4 (1), otherwise skip 801 extension headers, copy the Next Header field (ESP, transport 802 protocol or undefined next header value) in the last known next 803 header. 805 Header Checksum: Computed once the IPv4 header has been created. 807 Source Address: In the stateless mode, which is to say that if the 808 IPv6 source address is within the range of a configured IPv6 809 translation prefix, the IPv4 source address is derived from the 810 IPv6 source address per [I-D.ietf-behave-address-format] section 811 2.1. Note that the original IPv6 source address is an IPv4- 812 translatable address. A workflow example of stateless translation 813 is shown in Appendix of this document. If the translator only 814 supports stateless mode and if the IPv6 source address is not 815 within the range of configured IPv6 prefix(es), the translator 816 SHOULD drop the packet and respond with an ICMPv6 Type=1, Code=5 817 (Destination Unreachable, Source address failed ingress/egress 818 policy). 820 In the stateful mode, which is to say that if the IPv6 source 821 address is not within the range of any configured IPv6 stateless 822 translation prefix, the IPv4 source address and transport-layer 823 source port corresponding to the IPv4-related IPv6 source address 824 and source port are derived from the Binding Information Bases 825 (BIBs) as described in [I-D.ietf-behave-v6v4-xlate-stateful]. 827 In stateless and stateful modes, if the translator gets an illegal 828 source address (e.g. ::1, etc.), the translator SHOULD silently 829 drop the packet. 831 Destination Address: The IPv4 destination address is derived from 832 the IPv6 destination address of the datagram being translated per 833 [I-D.ietf-behave-address-format] section 2.1. Note that the 834 original IPv6 destination address is an IPv4-converted address. 836 If any of an IPv6 Hop-by-Hop Options header, Destination Options 837 header, or Routing header with the Segments Left field equal to zero 838 are present in the IPv6 packet, those IPv6 extension headers MUST be 839 ignored (i.e., there is no attempt to translate the extension 840 headers) and the packet translated normally. However, the Total 841 Length field and the Protocol field are adjusted to "skip" these 842 extension headers. 844 If a Routing header with a non-zero Segments Left field is present 845 then the packet MUST NOT be translated, and an ICMPv6 "parameter 846 problem/erroneous header field encountered" (Type 4/Code 0) error 847 message, with the Pointer field indicating the first byte of the 848 Segments Left field, SHOULD be returned to the sender. 850 If the IPv6 packet contains a Fragment header, the header fields are 851 set as above with the following exceptions: 853 Total Length: Payload length value from IPv6 header, minus 8 for the 854 Fragment header, plus the size of the IPv4 header. 856 Identification: Copied from the low-order 16-bits in the 857 Identification field in the Fragment header. 859 Flags: The More Fragments (MF) flag is copied from the M flag in the 860 Fragment header. The Don't Fragments (DF) flag is set to zero 861 allowing this packet to be fragmented if required by IPv4 routers. 863 Fragment Offset: Copied from the Fragment Offset field in the 864 Fragment header. 866 Protocol: For ICMPv6 (58) changed to ICMPv4 (1), otherwise skip 867 extension headers, Next Header field copied from the last IPv6 868 header. 870 If a translated packet with DF set to 1 will be larger than the MTU 871 of the next-hop interface, then the translator MUST drop the packet 872 and send the ICMPv6 "Packet Too Big" (Type 2/Code 0) error message to 873 the IPv6 host with an adjusted MTU in the ICMPv6 message. 875 4.2. Translating ICMPv6 Headers into ICMPv4 Headers 877 All ICMPv6 messages that are to be translated require that the ICMPv4 878 checksum field be updated as part of the translation since ICMPv6 879 (unlike ICMPv4) includes a pseudo-header in the checksum just like 880 UDP and TCP. 882 In addition all ICMP packets MUST have the Type value translated and, 883 for ICMP error messages, the included IP header also MUST be 884 translated. Note that the IPv6 addresses in the IPv6 header may not 885 be IPv4-translatable addresses and there will be no corresponding 886 IPv4 addresses representing this IPv6 address. In this case, the 887 translator can do stateful translation. A mechanism by which the 888 translator can instead do stateless translation is left for future 889 work. 891 The actions needed to translate various ICMPv6 messages are: 893 ICMPv6 informational messages: 895 Echo Request and Echo Reply (Type 128 and 129): Adjust the Type 896 values to 8 and 0, respectively, and adjust the ICMP checksum 897 both to take the type change into account and to exclude the 898 ICMPv6 pseudo-header. 900 MLD Multicast Listener Query/Report/Done (Type 130, 131, 132): 901 Single hop message. Silently drop. 903 Neighbor Discover messages (Type 133 through 137): Single hop 904 message. Silently drop. 906 Unknown informational messages: Silently drop. 908 ICMPv6 error messages: 910 Destination Unreachable (Type 1) Set the Type field to 3, and 911 adjust the ICMP checksum both to take the type/code change into 912 account and to exclude the ICMPv6 pseudo-header. 914 Translate the Code field as follows: 916 Code 0 (no route to destination): Set Code value to 1 (Host 917 unreachable). 919 Code 1 (Communication with destination administratively 920 prohibited): Set Code value to 10 (Communication with 921 destination host administratively prohibited). 923 Code 2 (Beyond scope of source address): Set Code value to 1 924 (Host unreachable). Note that this error is very unlikely 925 since an IPv4-translatable source address is typically 926 considered to have global scope. 928 Code 3 (Address unreachable): Set Code value to 1 (Host 929 unreachable). 931 Code 4 (Port unreachable): Set Code value to 3 (Port 932 unreachable). 934 Other Code values: Silently drop. 936 Packet Too Big (Type 2): Translate to an ICMPv4 Destination 937 Unreachable (Type 3) with Code value equal to 4, and adjust the 938 ICMPv4 checksum both to take the type change into account and 939 to exclude the ICMPv6 pseudo-header. The MTU field MUST be 940 adjusted for the difference between the IPv4 and IPv6 header 941 sizes taking into account whether or not the packet in error 942 includes a Fragment header, i.e. minimum(advertised MTU-20, 943 MTU_of_IPv4_nexthop, (MTU_of_IPv6_nexthop)-20) 945 Time Exceeded (Type 3): Set the Type value to 11, and adjust the 946 ICMPv4 checksum both to take the type change into account and 947 to exclude the ICMPv6 pseudo-header. The Code field is 948 unchanged. 950 Parameter Problem (Type 4): Translate the Type and Code field as 951 follows, and adjust the ICMPv4 checksum both to take the type/ 952 code change into account and to exclude the ICMPv6 pseudo- 953 header. 955 Translate the Code field as follows: 957 Code 0 (Erroneous header field encountered): Set Type 12, Code 958 0 and update the pointer as defined in Figure 6 (If the 959 Original IPv6 Pointer Value is not listed or the Translated 960 IPv4 Pointer Value is listed as "n/a", silently drop the 961 packet). 963 Code 1 (Unrecognized Next Header type encountered): Translate 964 this to an ICMPv4 protocol unreachable (Type 3, Code 2). 966 Code 2 (Unrecognized IPv6 option encountered): Silently drop. 968 Unknown error messages: Silently drop. 970 | Original IPv6 Pointer Value | Translated IPv4 Pointer Value | 971 +--------------------------------+--------------------------------+ 972 | 0 | Version/Traffic Class | 0 | Version/IHL, Type Of Ser | 973 | 1 | Traffic Class/Flow Label | 1 | Type Of Service | 974 | 2,3 | Flow Label | n/a | | 975 | 4,5 | Payload Length | 2 | Total Length | 976 | 6 | Next Header | 9 | Protocol | 977 | 7 | Hop Limit | 8 | Time to Live | 978 | 8-23| Source Address | 12 | Source Address | 979 |24-39| Destination Address | 16 | Destination Address | 980 +--------------------------------+--------------------------------+ 982 Figure 6: Pointer Value for translating from IPv6 to IPv4 984 ICMP Error Payload: If the received ICMPv6 packet contains an 985 ICMPv6 Extension [RFC4884], the translation of the ICMPv6 986 packet will cause the ICMPv4 packet to change length. When 987 this occurs, the ICMPv6 Extension length attribute MUST be 988 adjusted accordingly (e.g., shorter due to the translation from 989 IPv6 to IPv4). For extensions not defined in [RFC4884], the 990 translator passes the extensions as opaque bit strings and 991 those containing IPv6 address literals will not have those 992 addresses translated to IPv4 address literals; this may cause 993 problems with processing of those ICMP extensions. 995 4.3. Translating ICMPv6 Error Messages into ICMPv4 997 There are some differences between the ICMPv4 and the ICMPv6 error 998 message formats as detailed above. In addition, the ICMP error 999 messages contain the packet in error, which MUST be translated just 1000 like a normal IP packet. The translation of this "packet in error" 1001 is likely to change the length of the datagram thus the Total Length 1002 field in the outer IPv4 header MUST be updated. 1004 +-------------+ +-------------+ 1005 | IPv6 | | IPv4 | 1006 | Header | | Header | 1007 +-------------+ +-------------+ 1008 | ICMPv6 | | ICMPv4 | 1009 | Header | | Header | 1010 +-------------+ +-------------+ 1011 | IPv6 | ===> | IPv4 | 1012 | Header | | Header | 1013 +-------------+ +-------------+ 1014 | Partial | | Partial | 1015 | Transport | | Transport | 1016 | Layer | | Layer | 1017 | Header | | Header | 1018 +-------------+ +-------------+ 1020 Figure 7: IPv6-to-IPv4 ICMP Error Translation 1022 The translation of the inner IP header can be done by invoking the 1023 function that translated the outer IP headers. This process MUST 1024 stop at first embedded header and drop the packet if it contains 1025 more. Note that the IPv6 addresses in the IPv6 header may not be 1026 IPv4-translatable addresses and there will be no corresponding IPv4 1027 addresses. In this case, the translator can do stateful translation. 1028 A mechanism by which the translator can instead do stateless 1029 translation is left for future work. 1031 4.4. Translator Sending ICMPv6 Error Message 1033 If the IPv6 packet is discarded, then the translator SHOULD be able 1034 to send back an ICMPv6 error message to the original sender of the 1035 packet, unless the discarded packet is itself an ICMPv6 message. 1037 If the ICMPv6 error message is being sent because the IPv6 source 1038 address is not an IPv4-translatable address and the translator is 1039 stateless, the ICMPv6 message, if sent, has a Type value 1 and Code 1040 value 5 (Source address failed ingress/egress policy). In other 1041 cases, the ICMPv6 message has a Type value of 1 (Destination 1042 Unreachable) and a Code value of 1 (Communication with destination 1043 administratively prohibited), unless otherwise specified in this 1044 document or [I-D.ietf-behave-v6v4-xlate-stateful]. The translator 1045 SHOULD allow an administrator to configure whether the ICMPv6 error 1046 messages are sent, rate-limited, or not sent. 1048 4.5. Transport-layer Header Translation 1050 If the address translation algorithm is not checksum neutral, the 1051 recalculation and updating of the known transport-layer headers which 1052 contain pseudo headers (e.g. of TCP, UDP and DCCP) MUST be performed. 1053 For ESP or undefined transport protocol, the translator MUST forward 1054 the packets to the destination without touching the transport-layer 1055 header. 1057 4.6. Knowing When to Translate 1059 If the IP/ICMP translator also provides a normal forwarding function, 1060 and the destination address is reachable by a more specific route 1061 without translation, the router MUST forward it without translating 1062 it. When an IP/ICMP translator receives an IPv6 datagram addressed 1063 to an IPv6 address representing a host in the IPv4 domain, the IPv6 1064 packet MUST be translated to IPv4. 1066 5. IANA Considerations 1068 This memo adds no new IANA considerations. 1070 Note to RFC Editor: This section will have served its purpose if it 1071 correctly tells IANA that no new assignments or registries are 1072 required, or if those assignments or registries are created during 1073 the RFC publication process. From the author's perspective, it may 1074 therefore be removed upon publication as an RFC at the RFC Editor's 1075 discretion. 1077 6. Security Considerations 1079 The use of stateless IP/ICMP translators does not introduce any new 1080 security issues beyond the security issues that are already present 1081 in the IPv4 and IPv6 protocols and in the routing protocols that are 1082 used to make the packets reach the translator. 1084 There are potential issues that might arise by deriving an IPv4 1085 address from an IPv6 address - particularly addresses like broadcast 1086 or loopback addresses and the non IPv4-translatable IPv6 addresses, 1087 etc. The [I-D.ietf-behave-address-format] addresses these issues. 1089 As the Authentication Header [RFC4302] is specified to include the 1090 IPv4 Identification field and the translating function is not able to 1091 always preserve the Identification field, it is not possible for an 1092 IPv6 endpoint to verify the AH on received packets that have been 1093 translated from IPv4 packets. Thus AH does not work through a 1094 translator. 1096 Packets with ESP can be translated since ESP does not depend on 1097 header fields prior to the ESP header. Note that ESP transport mode 1098 is easier to handle than ESP tunnel mode; in order to use ESP tunnel 1099 mode, the IPv6 node MUST be able to generate an inner IPv4 header 1100 when transmitting packets and remove such an IPv4 header when 1101 receiving packets. 1103 7. Acknowledgements 1105 This is under development by a large group of people. Those who have 1106 posted to the list during the discussion include Andrew Sullivan, 1107 Andrew Yourtchenko, Brian Carpenter, Dan Wing, Dave Thaler, David 1108 Harrington, Ed Jankiewicz, Hiroshi Miyata, Iljitsch van Beijnum, Jari 1109 Arkko, Jerry Huang, John Schnizlein, Jouni Korhonen, Kentaro Ebisawa, 1110 Kevin Yin, Magnus Westerlund, Marcelo Bagnulo Braun, Margaret 1111 Wasserman, Masahito Endo, Phil Roberts, Philip Matthews, Reinaldo 1112 Penno, Remi Denis-Courmont, Remi Despres, Senthil Sivakumar, Simon 1113 Perreault and Zen Cao. 1115 8. Appendix: Stateless translation workflow example 1117 A stateless translation workflow example is depicted in the following 1118 figure. The documentation address blocks 2001:DB8::/32 [RFC3849], 1119 192.0.2.0/24 and 198.51.100.0/24 [RFC5737] are used in this example. 1121 +--------------+ +--------------+ 1122 | IPv4 network | | IPv6 network | 1123 | | +-------+ | | 1124 | +----+ |-----| XLAT |---- | +----+ | 1125 | | H4 |-----| +-------+ |--| H6 | | 1126 | +----+ | | +----+ | 1127 +--------------+ +--------------+ 1129 Figure 8 1131 A translator (XLAT) connects the IPv6 network to the IPv4 network. 1132 This XLAT uses the Network Specific Prefix (NSP) 2001:DB8:100::/40 1133 defined in [I-D.ietf-behave-address-format] to represent IPv4 1134 addresses in the IPv6 address space (IPv4-converted addresses) and to 1135 represent IPv6 addresses (IPv4-translatable addresses) in the IPv4 1136 address space. In this example, 192.0.2.0/24 is the IPv4 block of 1137 the corresponding IPv4-translatable addresses. 1139 Based on the address mapping rule, the IPv6 node H6 has an IPv4- 1140 translatable IPv6 address 2001:DB8:1C0:2:21:: (address mapping from 1141 192.0.2.33). The IPv4 node H4 has IPv4 address 198.51.100.2. 1143 The IPv6 routing is configured in such a way that the IPv6 packets 1144 addressed to a destination address in 2001:DB8:100::/40 are routed to 1145 the IPv6 interface of the XLAT. 1147 The IPv4 routing is configured in such a way that the IPv4 packets 1148 addressed to a destination address in 192.0.2.0/24 are routed to the 1149 IPv4 interface of the XLAT. 1151 8.1. H6 establishes communication with H4 1153 The steps by which H6 establishes communication with H4 are: 1155 1. H6 performs the destination address mapping, so the IPv4- 1156 converted address 2001:DB8:1C6:3364:200:: is formed from 1157 198.51.100.2 based on the address mapping algorithm 1158 [I-D.ietf-behave-address-format]. 1160 2. H6 sends a packet to H4. The packet is sent from a source 1161 address 2001:DB8:1C0:2:21:: to a destination address 1162 2001:DB8:1C6:3364:200::. 1164 3. The packet is routed to the IPv6 interface of the XLAT (since 1165 IPv6 routing is configured that way). 1167 4. The XLAT receives the packet and performs the following actions: 1169 * The XLAT translates the IPv6 header into an IPv4 header using 1170 the IP/ICMP Translation Algorithm defined in this document. 1172 * The XLAT includes 192.0.2.33 as source address in the packet 1173 and 198.51.100.2 as destination address in the packet. Note 1174 that 192.0.2.33 and 198.51.100.2 are extracted directly from 1175 the source IPv6 address 2001:DB8:1C0:2:21:: (IPv4-translatable 1176 address) and destination IPv6 address 2001:DB8:1C6:3364:200:: 1177 (IPv4-converted address) of the received IPv6 packet that is 1178 being translated. 1180 5. The XLAT sends the translated packet out its IPv4 interface and 1181 the packet arrives at H4. 1183 6. H4 node responds by sending a packet with destination address 1184 192.0.2.33 and source address 198.51.100.2. 1186 7. The packet is routed to the IPv4 interface of the XLAT (since 1187 IPv4 routing is configured that way). The XLAT performs the 1188 following operations: 1190 * The XLAT translates the IPv4 header into an IPv6 header using 1191 the IP/ICMP Translation Algorithm defined in this document. 1193 * The XLAT includes 2001:DB8:1C0:2:21:: as destination address 1194 in the packet and 2001:DB8:1C6:3364:200:: as source address in 1195 the packet. Note that 2001:DB8:1C0:2:21:: and 1196 2001:DB8:1C6:3364:200:: 1197 are formed directly from the destination IPv4 1198 address 192.0.2.33 and source IPv4 address 198.51.100.2 of the 1199 received IPv4 packet that is being translated. 1201 8. The translated packet is sent out the IPv6 interface to H6. 1203 The packet exchange between H6 and H4 continues until the session is 1204 finished. 1206 8.2. H4 establishes communication with H6 1208 The steps by which H4 establishes communication with H6 are: 1210 1. H4 performs the destination address mapping, so 192.0.2.33 is 1211 formed from IPv4-translatable address 2001:DB8:1C0:2:21:: based 1212 on the address mapping algorithm 1213 [I-D.ietf-behave-address-format]. 1215 2. H4 sends a packet to H6. The packet is sent from a source 1216 address 198.51.100.2 to a destination address 192.0.2.33. 1218 3. The packet is routed to the IPv4 interface of the XLAT (since 1219 IPv4 routing is configured that way). 1221 4. The XLAT receives the packet and performs the following actions: 1223 * The XLAT translates the IPv4 header into an IPv6 header using 1224 the IP/ICMP Translation Algorithm defined in this document. 1226 * The XLAT includes 2001:DB8:1C6:3364:200:: as source address in 1227 the packet and 2001:DB8:1C0:2:21:: as destination address in 1228 the packet. Note that 2001:DB8:1C6:3364:200:: (IPv4-converted 1229 address) and 2001:DB8:1C0:2:21:: (IPv4-translatable address) 1230 are obtained directly from the source IPv4 address 1231 198.51.100.2 and destination IPv4 address 192.0.2.33 of the 1232 received IPv4 packet that is being translated. 1234 5. The XLAT sends the translated packet out its IPv6 interface and 1235 the packet arrives at H6. 1237 6. H6 node responds by sending a packet with destination address 1238 2001:DB8:1C6:3364:200:: and source address 2001:DB8:1C0:2:21::. 1240 7. The packet is routed to the IPv6 interface of the XLAT (since 1241 IPv6 routing is configured that way). The XLAT performs the 1242 following operations: 1244 * The XLAT translates the IPv6 header into an IPv4 header using 1245 the IP/ICMP Translation Algorithm defined in this document. 1247 * The XLAT includes 198.51.100.2 as destination address in the 1248 packet and 192.0.2.33 as source address in the packet. Note 1249 that 198.51.100.2 and 192.0.2.33 are formed directly from the 1250 destination IPv6 address 2001:DB8:1C6:3364:200:: and source 1251 IPv6 address 2001:DB8:1C0:2:21:: of the received IPv6 packet 1252 that is being translated. 1254 8. The translated packet is sent out the IPv4 interface to H4. 1256 The packet exchange between H4 and H6 continues until the session 1257 finished. 1259 9. References 1261 9.1. Normative References 1263 [I-D.ietf-behave-address-format] 1264 Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. 1265 Li, "IPv6 Addressing of IPv4/IPv6 Translators", 1266 draft-ietf-behave-address-format-06 (work in progress), 1267 March 2010. 1269 [I-D.ietf-behave-v6v4-xlate-stateful] 1270 Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful 1271 NAT64: Network Address and Protocol Translation from IPv6 1272 Clients to IPv4 Servers", 1273 draft-ietf-behave-v6v4-xlate-stateful-11 (work in 1274 progress), March 2010. 1276 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 1277 August 1980. 1279 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 1280 September 1981. 1282 [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, 1283 RFC 792, September 1981. 1285 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 1286 RFC 793, September 1981. 1288 [RFC1812] Baker, F., "Requirements for IP Version 4 Routers", 1289 RFC 1812, June 1995. 1291 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1292 Requirement Levels", BCP 14, RFC 2119, March 1997. 1294 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 1295 (IPv6) Specification", RFC 2460, December 1998. 1297 [RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm 1298 (SIIT)", RFC 2765, February 2000. 1300 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 1301 Architecture", RFC 4291, February 2006. 1303 [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram 1304 Congestion Control Protocol (DCCP)", RFC 4340, March 2006. 1306 [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control 1307 Message Protocol (ICMPv6) for the Internet Protocol 1308 Version 6 (IPv6) Specification", RFC 4443, March 2006. 1310 [RFC4884] Bonica, R., Gan, D., Tappan, D., and C. Pignataro, 1311 "Extended ICMP to Support Multi-Part Messages", RFC 4884, 1312 April 2007. 1314 [RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P. 1315 Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142, 1316 RFC 5382, October 2008. 1318 [RFC5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for 1319 IPv4 Multicast Address Assignments", BCP 51, RFC 5771, 1320 March 2010. 1322 9.2. Informative References 1324 [I-D.ietf-behave-v6v4-framework] 1325 Baker, F., Li, X., Bao, C., and K. Yin, "Framework for 1326 IPv4/IPv6 Translation", 1327 draft-ietf-behave-v6v4-framework-08 (work in progress), 1328 March 2010. 1330 [RFC0879] Postel, J., "TCP maximum segment size and related topics", 1331 RFC 879, November 1983. 1333 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 1334 November 1990. 1336 [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, 1337 "Definition of the Differentiated Services Field (DS 1338 Field) in the IPv4 and IPv6 Headers", RFC 2474, 1339 December 1998. 1341 [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., 1342 and W. Weiss, "An Architecture for Differentiated 1343 Services", RFC 2475, December 1998. 1345 [RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast 1346 Listener Discovery (MLD) for IPv6", RFC 2710, 1347 October 1999. 1349 [RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address 1350 Translation - Protocol Translation (NAT-PT)", RFC 2766, 1351 February 2000. 1353 [RFC3307] Haberman, B., "Allocation Guidelines for IPv6 Multicast 1354 Addresses", RFC 3307, August 2002. 1356 [RFC3590] Haberman, B., "Source Address Selection for the Multicast 1357 Listener Discovery (MLD) Protocol", RFC 3590, 1358 September 2003. 1360 [RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery 1361 Version 2 (MLDv2) for IPv6", RFC 3810, June 2004. 1363 [RFC3849] Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix 1364 Reserved for Documentation", RFC 3849, July 2004. 1366 [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, 1367 December 2005. 1369 [RFC5737] Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks 1370 Reserved for Documentation", RFC 5737, January 2010. 1372 Authors' Addresses 1374 Xing Li 1375 CERNET Center/Tsinghua University 1376 Room 225, Main Building, Tsinghua University 1377 Beijing, 100084 1378 China 1380 Phone: +86 10-62785983 1381 Email: xing@cernet.edu.cn 1382 Congxiao Bao 1383 CERNET Center/Tsinghua University 1384 Room 225, Main Building, Tsinghua University 1385 Beijing, 100084 1386 China 1388 Phone: +86 10-62785983 1389 Email: congxiao@cernet.edu.cn 1391 Fred Baker 1392 Cisco Systems 1393 Santa Barbara, California 93117 1394 USA 1396 Phone: +1-408-526-4257 1397 Email: fred@cisco.com