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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 INTERNET-DRAFT E. Nordmark 2 August 25, 2004 Sun Microsystems, Inc. 3 Obsoletes: 2893 R. E. Gilligan 4 Intransa, Inc. 6 Basic Transition Mechanisms for IPv6 Hosts and Routers 7 9 Status of this Memo 11 By submitting this Internet-Draft, I certify that any applicable 12 patent or other IPR claims of which I am aware have been disclosed, 13 and any of which I become aware will be disclosed, in accordance with 14 RFC 3668. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This draft expires on February 25, 2004. 34 Abstract 36 This document specifies IPv4 compatibility mechanisms that can be 37 implemented by IPv6 hosts and routers. Two mechanisms are specified, 38 "dual stack" and configured tunneling. Dual stack implies providing 39 complete implementations of both versions of the Internet Protocol 40 (IPv4 and IPv6) and configured tunneling provides a means to carry 41 IPv6 packets over unmodified IPv4 routing infrastructures. 43 This document obsoletes RFC 2893. 45 Contents 47 Status of this Memo.......................................... 1 49 1. Introduction............................................. 3 50 1.1. Terminology......................................... 3 52 2. Dual IP Layer Operation.................................. 5 53 2.1. Address Configuration............................... 5 54 2.2. DNS................................................. 5 56 3. Configured Tunneling Mechanisms.......................... 6 57 3.1. Encapsulation....................................... 8 58 3.2. Tunnel MTU and Fragmentation........................ 8 59 3.2.1. Static Tunnel MTU.............................. 9 60 3.2.2. Dynamic Tunnel MTU............................. 10 61 3.3. Hop Limit........................................... 11 62 3.4. Handling ICMPv4 errors.............................. 12 63 3.5. IPv4 Header Construction............................ 14 64 3.6. Decapsulation....................................... 15 65 3.7. Link-Local Addresses................................ 18 66 3.8. Neighbor Discovery over Tunnels..................... 19 68 4. Threat Related to Source Address Spoofing................ 20 70 5. Security Considerations.................................. 21 72 6. Acknowledgments.......................................... 22 74 7. References............................................... 23 75 7.1. Normative References................................ 23 76 7.2. Informative References.............................. 23 78 8. Authors' Addresses....................................... 25 80 9. Changes from RFC 2893.................................... 25 81 9.1. Changes from draft-ietf-v6ops-mech-v2-00............ 28 82 9.2. Changes from draft-ietf-v6ops-mech-v2-01............ 29 83 9.3. Changes from draft-ietf-v6ops-mech-v2-02............ 30 84 9.4. Changes from draft-ietf-v6ops-mech-v2-03............ 31 85 9.5. Changes from draft-ietf-v6ops-mech-v2-04............ 31 87 1. Introduction 89 The key to a successful IPv6 transition is compatibility with the 90 large installed base of IPv4 hosts and routers. Maintaining 91 compatibility with IPv4 while deploying IPv6 will streamline the task 92 of transitioning the Internet to IPv6. This specification defines 93 two mechanisms that IPv6 hosts and routers may implement in order to 94 be compatible with IPv4 hosts and routers. 96 The mechanisms in this document are designed to be employed by IPv6 97 hosts and routers that need to interoperate with IPv4 hosts and 98 utilize IPv4 routing infrastructures. We expect that most nodes in 99 the Internet will need such compatibility for a long time to come, 100 and perhaps even indefinitely. 102 The mechanisms specified here are: 104 - Dual IP layer (also known as Dual Stack): A technique for 105 providing complete support for both Internet protocols -- IPv4 106 and IPv6 -- in hosts and routers. 108 - Configured tunneling of IPv6 over IPv4: A technique for 109 establishing point-to-point tunnels by encapsulating IPv6 110 packets within IPv4 headers to carry them over IPv4 routing 111 infrastructures. 113 The mechanisms defined here are intended to be the core of a 114 "transition toolbox" -- a growing collection of techniques which 115 implementations and users may employ to ease the transition. The 116 tools may be used as needed. Implementations and sites decide which 117 techniques are appropriate to their specific needs. 119 This document defines the basic set of transition mechanisms, but 120 these are not the only tools available. Additional transition and 121 compatibility mechanisms are specified in other documents. 123 1.1. Terminology 125 The following terms are used in this document: 127 Types of Nodes 129 IPv4-only node: 131 A host or router that implements only IPv4. An IPv4- 132 only node does not understand IPv6. The installed base 133 of IPv4 hosts and routers existing before the transition 134 begins are IPv4-only nodes. 136 IPv6/IPv4 node: 138 A host or router that implements both IPv4 and IPv6. 140 IPv6-only node: 142 A host or router that implements IPv6, and does not 143 implement IPv4. The operation of IPv6-only nodes is not 144 addressed in this memo. 146 IPv6 node: 148 Any host or router that implements IPv6. IPv6/IPv4 and 149 IPv6-only nodes are both IPv6 nodes. 151 IPv4 node: 153 Any host or router that implements IPv4. IPv6/IPv4 and 154 IPv4-only nodes are both IPv4 nodes. 156 Techniques Used in the Transition 158 IPv6-over-IPv4 tunneling: 160 The technique of encapsulating IPv6 packets within IPv4 161 so that they can be carried across IPv4 routing 162 infrastructures. 164 Configured tunneling: 166 IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint 167 address(es) are determined by configuration information 168 on tunnel endpoints. All tunnels are assumed to be 169 bidirectional. At the IPv6 layer the tunnel provides a 170 (virtual) point-to-point link, the lower layer endpoints 171 of which are the configured IPv4 addresses. 173 Other transition mechanisms, including other tunneling mechanisms, 174 are outside the scope of this document. 176 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, 177 SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this 178 document, are to be interpreted as described in [RFC2119]. 180 2. Dual IP Layer Operation 182 The most straightforward way for IPv6 nodes to remain compatible with 183 IPv4-only nodes is by providing a complete IPv4 implementation. IPv6 184 nodes that provide complete IPv4 and IPv6 implementations are called 185 "IPv6/IPv4 nodes." IPv6/IPv4 nodes have the ability to send and 186 receive both IPv4 and IPv6 packets. They can directly interoperate 187 with IPv4 nodes using IPv4 packets, and also directly interoperate 188 with IPv6 nodes using IPv6 packets. 190 Even though a node may be equipped to support both protocols, one or 191 the other stack may be disabled for operational reasons. Here we use 192 a rather loose notion of "stack". A stack being enabled has IP 193 addresses assigned etc, but whether or not any particular application 194 is available on the stacks is explicitly not defined. Thus IPv6/IPv4 195 nodes may be operated in one of three modes: 197 - With their IPv4 stack enabled and their IPv6 stack disabled. 199 - With their IPv6 stack enabled and their IPv4 stack disabled. 201 - With both stacks enabled. 203 IPv6/IPv4 nodes with their IPv6 stack disabled will operate like 204 IPv4-only nodes. Similarly, IPv6/IPv4 nodes with their IPv4 stacks 205 disabled will operate like IPv6-only nodes. IPv6/IPv4 nodes MAY 206 provide a configuration switch to disable either their IPv4 or IPv6 207 stack. 209 The configured tunneling technique, which is described in section 3, 210 may or may not be used in addition to the dual IP layer operation. 212 2.1. Address Configuration 214 Because the nodes support both protocols, IPv6/IPv4 nodes may be 215 configured with both IPv4 and IPv6 addresses. IPv6/IPv4 nodes use 216 IPv4 mechanisms (e.g., DHCP) to acquire their IPv4 addresses, and 217 IPv6 protocol mechanisms (e.g., stateless address autoconfiguration 218 and/or DHCPv6) to acquire their IPv6 addresses. 220 2.2. DNS 222 The Domain Naming System (DNS) is used in both IPv4 and IPv6 to map 223 between hostnames and IP addresses. A new resource record type named 224 "AAAA" has been defined for IPv6 addresses [RFC3596]. Since 225 IPv6/IPv4 nodes must be able to interoperate directly with both IPv4 226 and IPv6 nodes, they must provide resolver libraries capable of 227 dealing with IPv4 "A" records as well as IPv6 "AAAA" records. Note 228 that the lookup of A versus AAAA records is independent of whether 229 the DNS packets are carried in IPv4 or IPv6 packets, and that there 230 is no assumption that the DNS servers know the IPv4/IPv6 capabilities 231 of the requesting node. 233 The issues and operational guidelines for using IPv6 with DNS are 234 described at more length in other documents [DNSOPV6]. 236 DNS resolver libraries on IPv6/IPv4 nodes MUST be capable of handling 237 both AAAA and A records. However, when a query locates an AAAA 238 record holding an IPv6 address, and an A record holding an IPv4 239 address, the resolver library MAY order the results returned to the 240 application in order to influence the version of IP packets used to 241 communicate with that specific node -- IPv6 first, or IPv4 first. 243 The applications SHOULD be able to specify whether they want IPv4, 244 IPv6 or both records [RFC3493]. That defines which address families 245 the resolver looks up. If there isn't an application choice, or if 246 the application has requested both, the resolver library MUST NOT 247 filter out any records. 249 Since most applications try the addresses in the order they are 250 returned by the resolver, this can affect the IP version "preference" 251 of applications. 253 The actual ordering mechanisms are out of scope of this memo. 254 Address selection is described at more length in [RFC3484]. 256 3. Configured Tunneling Mechanisms 258 In most deployment scenarios, the IPv6 routing infrastructure will be 259 built up over time. While the IPv6 infrastructure is being deployed, 260 the existing IPv4 routing infrastructure can remain functional, and 261 can be used to carry IPv6 traffic. Tunneling provides a way to 262 utilize an existing IPv4 routing infrastructure to carry IPv6 263 traffic. 265 IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions of 266 IPv4 routing topology by encapsulating them within IPv4 packets. 267 Tunneling can be used in a variety of ways: 269 - Router-to-Router. IPv6/IPv4 routers interconnected by an IPv4 270 infrastructure can tunnel IPv6 packets between themselves. In 271 this case, the tunnel spans one segment of the end-to-end path 272 that the IPv6 packet takes. 274 - Host-to-Router. IPv6/IPv4 hosts can tunnel IPv6 packets to an 275 intermediary IPv6/IPv4 router that is reachable via an IPv4 276 infrastructure. This type of tunnel spans the first segment of 277 the packet's end-to-end path. 279 - Host-to-Host. IPv6/IPv4 hosts that are interconnected by an 280 IPv4 infrastructure can tunnel IPv6 packets between themselves. 281 In this case, the tunnel spans the entire end-to-end path that 282 the packet takes. 284 - Router-to-Host. IPv6/IPv4 routers can tunnel IPv6 packets to 285 their final destination IPv6/IPv4 host. This tunnel spans only 286 the last segment of the end-to-end path. 288 Configured tunneling can be used in all of the above cases, but is 289 most likely to be used router-to-router due to the need to explicitly 290 configure the tunneling endpoints. 292 The underlying mechanisms for tunneling are: 294 - The entry node of the tunnel (the encapsulator) creates an 295 encapsulating IPv4 header and transmits the encapsulated packet. 297 - The exit node of the tunnel (the decapsulator) receives the 298 encapsulated packet, reassembles the packet if needed, removes 299 the IPv4 header, and processes the received IPv6 packet. 301 - The encapsulator may need to maintain soft state information for 302 each tunnel recording such parameters as the MTU of the tunnel 303 in order to process IPv6 packets forwarded into the tunnel. 305 In configured tunneling, the tunnel endpoint addresses are determined 306 in the encapsulator from configuration information stored for each 307 tunnel. When an IPv6 packet is transmitted over a tunnel, the 308 destination and source addresses for the encapsulating IPv4 header 309 are set based on that information as described in Section 3.5. 311 The determination of which packets to tunnel is usually made by 312 routing information on the encapsulator. This is usually done via a 313 routing table, which directs packets based on their destination 314 address using the prefix mask and match technique. 316 The decapsulator matches the received protocol-41 packets to the 317 tunnels it has configured, and allows only the packets where IPv4 318 source addresses match the tunnels configured on the decapsulator. 319 Therefore the operator must ensure that the tunnel's IPv4 address 320 configuration is the same both at the encapsulator and the 321 decapsulator. 323 3.1. Encapsulation 325 The encapsulation of an IPv6 datagram in IPv4 is shown below: 327 +-------------+ 328 | IPv4 | 329 | Header | 330 +-------------+ +-------------+ 331 | IPv6 | | IPv6 | 332 | Header | | Header | 333 +-------------+ +-------------+ 334 | Transport | | Transport | 335 | Layer | ===> | Layer | 336 | Header | | Header | 337 +-------------+ +-------------+ 338 | | | | 339 ~ Data ~ ~ Data ~ 340 | | | | 341 +-------------+ +-------------+ 343 Encapsulating IPv6 in IPv4 345 In addition to adding an IPv4 header, the encapsulator also has to 346 handle some more complex issues: 348 - Determine when to fragment and when to report an ICMPv6 "packet 349 too big" error back to the source. 351 - How to reflect ICMPv4 errors from routers along the tunnel path 352 back to the source as ICMPv6 errors. 354 Those issues are discussed in the following sections. 356 3.2. Tunnel MTU and Fragmentation 358 Naively the encapsulator could view encapsulation as IPv6 using IPv4 359 as a link layer with a very large MTU (65535-20 bytes at most; 20 360 bytes "extra" are needed for the encapsulating IPv4 header). The 361 encapsulator would only need to report ICMPv6 "packet too big" errors 362 back to the source for packets that exceed this MTU. However, such a 363 scheme would be inefficient or non-interoperable for three reasons 364 and therefore MUST NOT be used: 366 1) It would result in more fragmentation than needed. IPv4 layer 367 fragmentation should be avoided due to the performance problems 368 caused by the loss unit being smaller than the retransmission 369 unit [KM97]. 371 2) Any IPv4 fragmentation occurring inside the tunnel, i.e. between 372 the encapsulator and the decapsulator, would have to be 373 reassembled at the tunnel endpoint. For tunnels that terminate 374 at a router, this would require additional memory and other 375 resources to reassemble the IPv4 fragments into a complete IPv6 376 packet before that packet could be forwarded onward. 378 3) The encapsulator has no way of knowing that the decapsulator is 379 able to defragment such IPv4 packets (see Section 3.7 for 380 details), and has no way of knowing that the decapsulator is 381 able to handle such a large IPv6 Maximum Receive Unit (MRU). 383 Hence, the encapsulator MUST NOT treat the tunnel as an interface 384 with an MTU of 64 kilobytes, but instead either use the fixed static 385 MTU or OPTIONAL dynamic MTU determination based on the IPv4 path MTU 386 to the tunnel endpoint. 388 If both the mechanisms are implemented, the decision which to use 389 SHOULD be configurable on a per-tunnel endpoint basis. 391 3.2.1. Static Tunnel MTU 393 A node using static tunnel MTU treats the tunnel interface as having 394 a fixed interface MTU. By default, the MTU MUST be between 1280 and 395 1480 bytes (inclusive), but it SHOULD be 1280 bytes. If the default 396 is not 1280 bytes, the implementation MUST have a configuration knob 397 which can be used to change the MTU value. 399 A node must be able to accept a fragmented IPv6 packet that, after 400 reassembly, is as large as 1500 octets [RFC2460]. This memo also 401 includes requirements (see Section 3.6) for the amount of IPv4 402 reassembly and IPv6 MRU that MUST be supported by all the 403 decapsulators. These ensure correct interoperability with any fixed 404 MTUs between 1280 and 1480 bytes. 406 A larger fixed MTU than supported by these requirements, must not be 407 configured unless it has been administratively ensured that the 408 decapsulator can reassemble or receive packets of that size. 410 The selection of a good tunnel MTU depends on many factors; at least: 412 - Whether the IPv4 protocol-41 packets will be transported over 413 media which may have a lower path MTU (e.g., IPv4 Virtual 414 Private Networks); then picking too high a value might lead to 415 IPv4 fragmentation. 417 - Whether the tunnel is used to transport IPv6 tunneled packets 418 (e.g., a mobile node with an IPv4-in-IPv6 configured tunnel, and 419 an IPv6-in-IPv6 tunnel interface); then picking too low a value 420 might lead to IPv6 fragmentation. 422 If layered encapsulation is believed to be present, it may be prudent 423 to consider supporting dynamic MTU determination instead as it is 424 able to minimize fragmentation and optimize packet sizes. 426 When using the static tunnel MTU the Don't Fragment bit MUST NOT be 427 set in the encapsulating IPv4 header. As a result the encapsulator 428 should not receive any ICMPv4 "packet too big" messages as a result 429 of the packets it has encapsulated. 431 3.2.2. Dynamic Tunnel MTU 433 The dynamic MTU determination is OPTIONAL. However, if it is 434 implemented, it SHOULD have the behavior described in this document. 436 The fragmentation inside the tunnel can be reduced to a minimum by 437 having the encapsulator track the IPv4 Path MTU across the tunnel, 438 using the IPv4 Path MTU Discovery Protocol [RFC1191] and recording 439 the resulting path MTU. The IPv6 layer in the encapsulator can then 440 view a tunnel as a link layer with an MTU equal to the IPv4 path MTU, 441 minus the size of the encapsulating IPv4 header. 443 Note that this does not eliminate IPv4 fragmentation in the case when 444 the IPv4 path MTU would result in an IPv6 MTU less than 1280 bytes. 445 (Any link layer used by IPv6 has to have an MTU of at least 1280 446 bytes [RFC2460].) In this case the IPv6 layer has to "see" a link 447 layer with an MTU of 1280 bytes and the encapsulator has to use IPv4 448 fragmentation in order to forward the 1280 byte IPv6 packets. 450 The encapsulator SHOULD employ the following algorithm to determine 451 when to forward an IPv6 packet that is larger than the tunnel's path 452 MTU using IPv4 fragmentation, and when to return an ICMPv6 "packet 453 too big" message per [RFC1981]: 455 if (IPv4 path MTU - 20) is less than 1280 456 if packet is larger than 1280 bytes 457 Send ICMPv6 "packet too big" with MTU = 1280. 458 Drop packet. 459 else 460 Encapsulate but do not set the Don't Fragment 461 flag in the IPv4 header. The resulting IPv4 462 packet might be fragmented by the IPv4 layer 463 on the encapsulator or by some router along 464 the IPv4 path. 465 endif 466 else 467 if packet is larger than (IPv4 path MTU - 20) 468 Send ICMPv6 "packet too big" with 469 MTU = (IPv4 path MTU - 20). 470 Drop packet. 471 else 472 Encapsulate and set the Don't Fragment flag 473 in the IPv4 header. 474 endif 475 endif 477 Encapsulators that have a large number of tunnels may choose between 478 dynamic versus static tunnel MTU on a per-tunnel endpoint basis. In 479 cases where the number of tunnels that any one node is using is 480 large, it is helpful to observe that this state information can be 481 cached and discarded when not in use. 483 Note that using dynamic tunnel MTU is subject to IPv4 PMTU blackholes 484 should the ICMPv4 "packet too big" messages be dropped by firewalls 485 or not generated by the routers. [RFC1435, RFC2923] 487 3.3. Hop Limit 489 IPv6-over-IPv4 tunnels are modeled as "single-hop" from the IPv6 490 perspective. The tunnel is opaque to users of the network, and is not 491 detectable by network diagnostic tools such as traceroute. 493 The single-hop model is implemented by having the encapsulators and 494 decapsulators process the IPv6 hop limit field as they would if they 495 were forwarding a packet on to any other datalink. That is, they 496 decrement the hop limit by 1 when forwarding an IPv6 packet. (The 497 originating node and final destination do not decrement the hop 498 limit.) 500 The TTL of the encapsulating IPv4 header is selected in an 501 implementation dependent manner. The current suggested value is 502 published in the "Assigned Numbers" RFC [RFC3232][ASSIGNED]. The 503 implementations MAY also consider using the value 255. 504 Implementations MAY provide a mechanism to allow the administrator to 505 configure the IPv4 TTL as the IP Tunnel MIB [RFC2667]. 507 3.4. Handling ICMPv4 errors 509 In response to encapsulated packets it has sent into the tunnel, the 510 encapsulator might receive ICMPv4 error messages from IPv4 routers 511 inside the tunnel. These packets are addressed to the encapsulator 512 because it is the IPv4 source of the encapsulated packet. 514 ICMPv4 error handling is only applicable to dynamic MTU 515 determination, even though the functions could be used with static 516 MTU tunnels as well. 518 The ICMPv4 "packet too big" error messages are handled according to 519 IPv4 Path MTU Discovery [RFC1191] and the resulting path MTU is 520 recorded in the IPv4 layer. The recorded path MTU is used by IPv6 to 521 determine if an ICMPv6 "packet too big" error has to be generated as 522 described in section 3.2.2. 524 The handling of other types of ICMPv4 error messages depends on how 525 much information is available from the encapsulated packet that 526 caused the error. 528 Many older IPv4 routers return only 8 bytes of data beyond the IPv4 529 header of the packet in error, which is not enough to include the 530 address fields of the IPv6 header. More modern IPv4 routers are 531 likely to return enough data beyond the IPv4 header to include the 532 entire IPv6 header and possibly even the data beyond that. 534 If sufficient data bytes from the offending packet are available, the 535 encapsulator MAY extract the encapsulated IPv6 packet and use it to 536 generate an ICMPv6 message directed back to the originating IPv6 537 node, as shown below: 539 +--------------+ 540 | IPv4 Header | 541 | dst = encaps | 542 | node | 543 +--------------+ 544 | ICMPv4 | 545 | Header | 546 - - +--------------+ 547 | IPv4 Header | 548 | src = encaps | 549 IPv4 | node | 550 +--------------+ - - 551 Packet | IPv6 | 552 | Header | Original IPv6 553 in +--------------+ Packet - 554 | Transport | Can be used to 555 Error | Header | generate an 556 +--------------+ ICMPv6 557 | | error message 558 ~ Data ~ back to the source. 559 | | 560 - - +--------------+ - - 562 ICMPv4 Error Message Returned to Encapsulating Node 564 When receiving ICMPv4 errors as above and the errors are not "packet 565 too big" it would be useful to log the error as an error related to 566 the tunnel. Also, if sufficient headers are available, then the 567 originating node MAY send an ICMPv6 error of type "unreachable" with 568 code "address unreachable" to the IPv6 source. (The "address 569 unreachable" code is appropriate since, from the perspective of IPv6, 570 the tunnel is a link and that code is used for link-specific errors 571 [RFC2463]). 573 Note that when the IPv4 path MTU is exceeded, and sufficient bytes of 574 payload associated with the ICMPv4 errors are not available, or 575 ICMPv4 errors do not cause the generation of ICMPv6 errors in case 576 there is enough payload, there will be at least two packet drops 577 instead of at least one (the case of a single layer of MTU 578 discovery). Consider a case where an IPv6 host is connected to an 579 IPv4/IPv6 router, which is connected to a network where an ICMPv4 580 error about too big packet size is generated. First the router needs 581 to learn the tunnel (IPv4) MTU which causes at least one packet loss, 582 and then the host needs to learn the (IPv6) MTU from the router which 583 causes at least one packet loss. Still, in all cases there can be 584 more than one packet loss if there are multiple large packets in 585 flight at the same time. 587 3.5. IPv4 Header Construction 589 When encapsulating an IPv6 packet in an IPv4 datagram, the IPv4 590 header fields are set as follows: 592 Version: 594 4 596 IP Header Length in 32-bit words: 598 5 (There are no IPv4 options in the encapsulating 599 header.) 601 Type of Service: 603 0 unless otherwise specified. (See [RFC2983] and 604 [RFC3168] section 9.1 for issues relating to the Type- 605 of-Service byte and tunneling.) 607 Total Length: 609 Payload length from IPv6 header plus length of IPv6 and 610 IPv4 headers (i.e., IPv6 payload length plus a constant 611 60 bytes). 613 Identification: 615 Generated uniquely as for any IPv4 packet transmitted by 616 the system. 618 Flags: 620 Set the Don't Fragment (DF) flag as specified in section 621 3.2. Set the More Fragments (MF) bit as necessary if 622 fragmenting. 624 Fragment offset: 626 Set as necessary if fragmenting. 628 Time to Live: 630 Set in an implementation-specific manner, as described 631 in section 3.3. 633 Protocol: 635 41 (Assigned payload type number for IPv6). 637 Header Checksum: 639 Calculate the checksum of the IPv4 header. [RFC791] 641 Source Address: 643 An IPv4 address of the encapsulator: either manually 644 configured or an address of the outgoing interface. 646 Destination Address: 648 IPv4 address of the tunnel endpoint. 650 When encapsulating the packets, the node must ensure that it will use 651 the correct source address so that the packets are acceptable to the 652 decapsulator as described in Section 3.6. This may be a problem with 653 multi-addressed, and in particular, multi-interface nodes, especially 654 when the routing is changed from a stable condition, as the source 655 address selection may be adversely affected. Therefore, it SHOULD be 656 possible to administratively specify the source address of a tunnel. 658 3.6. Decapsulation 660 When an IPv6/IPv4 host or a router receives an IPv4 datagram that is 661 addressed to one of its own IPv4 addresses or a joined multicast 662 group address, and the value of the protocol field is 41, the packet 663 is potentially a tunnel packet and needs to be verified to belong to 664 one of the configured tunnel interfaces (by checking 665 source/destination addresses), reassembled (if fragmented at the IPv4 666 level), have the IPv4 header removed and the resulting IPv6 datagram 667 be submitted to the IPv6 layer code on the node. 669 The decapsulator MUST verify that the tunnel source address is 670 correct before further processing packets, to mitigate the problems 671 with address spoofing (see section 4). This check also applies to 672 packets which are delivered to transport protocols on the 673 decapsulator. This is done by verifying that the source address is 674 the IPv4 address of the encapsulator, as configured on the 675 decapsulator. Packets for which the IPv4 source address does not 676 match MUST be discarded and an ICMP message SHOULD NOT be generated; 677 however, if the implementation normally sends an ICMP message when 678 receiving an unknown protocol packet, such an error message MAY be 679 sent (e.g., ICMPv4 Protocol 41 Unreachable). 681 A side effect of this address verification is that the node will 682 silently discard packets with a wrong source address, and packets 683 which were received by the node but not directly addressed to it 684 (e.g., broadcast addresses). 686 Independent of any other forms of IPv4 ingress filtering the 687 administrator of the node may have configured, the implementation MAY 688 perform ingress filtering, i.e., check that the packet is arriving 689 from the interface in the direction of the route towards the tunnel 690 end-point, similar to a Strict Reverse Path Forwarding (RPF) check 691 [RFC3704]. As this may cause problems on tunnels which are routed 692 through multiple links, it is RECOMMENDED that this check, if done, 693 is disabled by default. The packets caught by this check SHOULD be 694 discarded; an ICMP message SHOULD NOT be generated by default. 696 The decapsulator MUST be capable of having, on the tunnel interfaces, 697 an IPv6 MRU of at least the maximum of of 1500 bytes and the largest 698 (IPv6) interface MTU on the decapsulator. 700 The decapsulator MUST be capable of reassembling an IPv4 packet that 701 is (after the reassembly) the maximum of 1500 bytes and the largest 702 (IPv4) interface MTU on the decapsulator. The 1500 byte number is a 703 result of encapsulators that use the static MTU scheme in section 704 3.2.1, while encapsulators that use the dynamic scheme in section 705 3.2.2 can cause up to the largest interface MTU on the decapsulator 706 to be received. (Note that it is strictly the interface MTU on the 707 last IPv4 router *before* the decapsulator that matters, but for most 708 links the MTU is the same between all neighbors.) 710 This reassembly limit allows dynamic tunnel MTU determination by the 711 encapsulator to take advantage of larger IPv4 path MTUs. An 712 implementation MAY have a configuration knob which can be used to set 713 a larger value of the tunnel reassembly buffers than the above 714 number, but it MUST NOT be set below the above number. 716 The decapsulation is shown below: 718 +-------------+ 719 | IPv4 | 720 | Header | 721 +-------------+ +-------------+ 722 | IPv6 | | IPv6 | 723 | Header | | Header | 724 +-------------+ +-------------+ 725 | Transport | | Transport | 726 | Layer | ===> | Layer | 727 | Header | | Header | 728 +-------------+ +-------------+ 729 | | | | 730 ~ Data ~ ~ Data ~ 731 | | | | 732 +-------------+ +-------------+ 734 Decapsulating IPv6 from IPv4 736 The decapsulator performs IPv4 reassembly before decapsulating the 737 IPv6 packet. 739 When decapsulating the packet, the IPv6 header is not modified. 740 (However, see [RFC2983] and [RFC3168] section 9.1 for issues relating 741 to the Type of Service byte and tunneling.) If the packet is 742 subsequently forwarded, its hop limit is decremented by one. 744 If the payload is not at least 40 bytes in length (i.e., the minimum 745 IPv6 packet), the packet MUST be discarded. Likewise, if the the 746 version encoded in the first 4 bits of the encapsulated packet is not 747 "6", the packet MUST be discarded. 749 The encapsulating IPv4 header is discarded. When reconstructing the 750 IPv6 packet the length MUST be determined from the IPv6 payload 751 length since the IPv4 packet might be padded (thus have a length 752 which is larger than the IPv6 packet plus the IPv4 header being 753 removed). 755 After the decapsulation the node MUST silently discard a packet with 756 an invalid IPv6 source address. The list of invalid source addresses 757 SHOULD include at least: 759 - all multicast addresses (FF00::/8) 761 - the loopback address (::1) 763 - all the IPv4-compatible IPv6 addresses [RFC3513] (::/96), 764 excluding the unspecified address for Duplicate Address 765 Detection (::/128) 767 - all the IPv4-mapped IPv6 addresses (::ffff:0:0/96) 769 In addition, the node should be configured to perform ingress 770 filtering [RFC2827][RFC3704] on the IPv6 source address, similar to 771 on any of its interfaces, e.g.: 773 1) if the tunnel is towards the Internet, the node should be 774 configured to check that the site's IPv6 prefixes are not used 775 as the source addresses, or 777 2) if the tunnel is towards an edge network, the node should be 778 configured to check that the source address belongs to that edge 779 network. 781 The prefix lists in the former typically need to be manually 782 configured; the latter could be verified automatically, e.g., by 783 using a strict unicast RPF check, as long as an interface can be 784 designated to be towards an edge. 786 It is RECOMMENDED that the implementations provide a single knob to 787 make it easier to for the administrators to enable strict ingress 788 filtering towards edge networks. 790 3.7. Link-Local Addresses 792 The configured tunnels are IPv6 interfaces (over the IPv4 "link 793 layer") and thus MUST have link-local addresses. The link-local 794 addresses are used by, e.g., routing protocols operating over the 795 tunnels. 797 The interface identifier [RFC3513] for such an interface may be based 798 on the 32-bit IPv4 address of an underlying interface, or formed 799 using some other means, as long as it's unique from the other tunnel 800 endpoint with a reasonably high probability. 802 Note that it may be desirable to form the link-local address in a 803 fashion that minimizes the probability and the effect of having to 804 renumber the link-local address in the event of a topology or 805 hardware change. 807 If an IPv4 address is used for forming the IPv6 link-local address, 808 the interface identifier is the IPv4 address, prepended by zeros. 809 Note that the "Universal/Local" bit is zero, indicating that the 810 interface identifier is not globally unique. The link-local address 811 is formed by appending the interface identifier to the prefix 812 FE80::/64. 814 When the host has more than one IPv4 address in use on the physical 815 interface concerned, a choice of one of these IPv4 addresses is made 816 by the administrator or the implementation when forming the link- 817 local address. 819 +-------+-------+-------+-------+-------+-------+------+------+ 820 | FE 80 00 00 00 00 00 00 | 821 +-------+-------+-------+-------+-------+-------+------+------+ 822 | 00 00 00 00 | IPv4 Address | 823 +-------+-------+-------+-------+-------+-------+------+------+ 825 3.8. Neighbor Discovery over Tunnels 827 Configured tunnel implementations MUST at least accept and respond to 828 the probe packets used by Neighbor Unreachability Detection (NUD) 829 [RFC2461]. The implementations SHOULD also send NUD probe packets to 830 detect when the configured tunnel fails at which point the 831 implementation can use an alternate path to reach the destination. 832 Note that Neighbor Discovery allows that the sending of NUD probes be 833 omitted for router to router links if the routing protocol tracks 834 bidirectional reachability. 836 For the purposes of Neighbor Discovery the configured tunnels 837 specified in this document are assumed to NOT have a link-layer 838 address, even though the link-layer (IPv4) does have an address. 839 This means that: 841 - the sender of Neighbor Discovery packets SHOULD NOT include 842 Source Link Layer Address options or Target Link Layer Address 843 options on the tunnel link. 845 - the receiver MUST, while otherwise processing the Neighbor 846 Discovery packet, silently ignore the content of any Source Link 847 Layer Address options or Target Link Layer Address options 848 received on the tunnel link. 850 Not using a link layer address options is consistent with how 851 Neighbor Discovery is used on other point-to-point links. 853 4. Threat Related to Source Address Spoofing 855 The specification above contains rules that apply tunnel source 856 address verification in particular and ingress filtering 857 [RFC2827][RFC3704] in general to packets before they are 858 decapsulated. When IP-in-IP tunneling (independent of IP versions) 859 is used it is important that this can not be used to bypass any 860 ingress filtering in use for non-tunneled packets. Thus the rules in 861 this document are derived based on should ingress filtering be used 862 for IPv4 and IPv6, the use of tunneling should not provide an easy 863 way to circumvent the filtering. 865 In this case, without specific ingress filtering checks in the 866 decapsulator, it would be possible for an attacker to inject a packet 867 with: 869 - Outer IPv4 source: real IPv4 address of attacker 871 - Outer IPv4 destination: IPv4 address of decapsulator 873 - Inner IPv6 source: Alice which is either the decapsulator or a 874 node close to it. 876 - Inner IPv6 destination: Bob 878 Even if all IPv4 routers between the attacker and the decapsulator 879 implement IPv4 ingress filtering, and all IPv6 routers between the 880 decapsulator and Bob implement IPv6 ingress filtering, the above 881 spoofed packets will not be filtered out. As a result Bob will 882 receive a packet that looks like it was sent from Alice even though 883 the sender was some unrelated node. 885 The solution to this is to have the decapsulator only accept 886 encapsulated packets from the explicitly configured source address 887 (i.e., the other end of the tunnel) as specified in section 3.6. 888 While this does not provide complete protection in the case ingress 889 filtering has not been deployed, it does provide a significant 890 increase in security. The issue and the remainder threats are 891 discussed at more length in Security Considerations. 893 5. Security Considerations 895 Generic security considerations of using IPv6 are discussed in a 896 separate document [V6SEC]. 898 An implementation of tunneling needs to be aware that while a tunnel 899 is a link (as defined in [RFC2460]), the threat model for a tunnel 900 might be rather different than for other links, since the tunnel 901 potentially includes all of the Internet. 903 Several mechanisms (e.g., Neighbor Discovery) depend on Hop Count 904 being 255 and/or the addresses being link-local for ensuring that a 905 packet originated on-link, in a semi-trusted environment. Tunnels 906 are more vulnerable to a breach of this assumption than physical 907 links, as an attacker anywhere in the Internet can send an IPv6-in- 908 IPv4 packet to the tunnel decapsulator, causing injection of an 909 encapsulted IPv6 packet to the configured tunnel interface unless the 910 decapsulation checks are able to discard packets injected in such a 911 manner. 913 Therefore, this memo specifies that the decapsulators make these 914 steps (as described in Section 3.6) to mitigate this threat: 916 - IPv4 source address of the packet MUST be the same as configured 917 for the tunnel end-point, 919 - Independent of any IPv4 ingress filtering the administrator may 920 have configured, the implementation MAY perform IPv4 ingress 921 filtering to check that the IPv4 packets are received from an 922 expected interface (but as this may cause some problems, it may 923 be disabled by default), 925 - IPv6 packets with several, obviously invalid IPv6 source 926 addresses received from the tunnel MUST be discarded (see 927 Section 3.6 for details), and 929 - IPv6 ingress filtering should be performed (typically requiring 930 configuration from the operator), to check that the tunneled 931 IPv6 packets are received from an expected interface. 933 Especially the first verification is vital: to avoid this check, the 934 attacker must be able to know the source of the tunnel (ranging from 935 difficult to predictable) and be able to spoof it (easier). 937 If the remainder threats of tunnel source verification are considered 938 to be significant, a tunneling scheme with authentication should be 939 used instead, for example IPsec [RFC2401] (preferable) or Generic 940 Routing Encapsulation with a pre-configured secret key [RFC2890]. As 941 the configured tunnels are set up more or less manually, setting up 942 the keying material is probably not a problem. However, setting up 943 secure IPsec IPv6-in-IPv4 tunnels is described in another document 944 [V64IPSEC]. 946 If the tunneling is done inside an administrative domain, proper 947 ingress filtering at the edge of the domain can also eliminate the 948 threat from outside of the domain. Therefore shorter tunnels are 949 preferable to longer ones, possibly spanning the whole Internet. 951 Additionally, an implementation MUST treat interfaces to different 952 links as separate, e.g., to ensure that Neighbor Discovery packets 953 arriving on one link does not effect other links. This is especially 954 important for tunnel links. 956 When dropping packets due to failing to match the allowed IPv4 source 957 addresses for a tunnel the node should not "acknowledge" the 958 existence of a tunnel, otherwise this could be used to probe the 959 acceptable tunnel endpoint addresses. For that reason, the 960 specification says that such packets MUST be discarded, and an ICMP 961 error message SHOULD NOT be generated, unless the implementation 962 normally sends ICMP destination unreachable messages for unknown 963 protocols; in such a case, the same code MAY be sent. As should be 964 obvious, the not returning the same ICMP code if an error is returned 965 for other protocols may hint that the IPv6 stack (or the protocol 41 966 tunneling processing) has been enabled -- the behaviour should be 967 consistent on how the implementation otherwise behaves to be 968 transparent to probing. 970 6. Acknowledgments 972 We would like to thank the members of the IPv6 working group, the 973 Next Generation Transition (ngtrans) working group, and the v6ops 974 working group for their many contributions and extensive review of 975 this document. Special thanks are due to (in alphabetical order) Jim 976 Bound, Ross Callon, Tim Chown, Alex Conta, Bob Hinden, Bill Manning, 977 John Moy, Mohan Parthasarathy, Chirayu Patel, Pekka Savola, and Fred 978 Templin for many helpful suggestions. Pekka Savola helped in editing 979 the final revisions of the specification. 981 7. References 983 7.1. Normative References 985 [RFC791] J. Postel, "Internet Protocol", RFC 791, September 1981. 987 [RFC1191] Mogul, J., and S. Deering., "Path MTU Discovery", RFC 1191, 988 November 1990. 990 [RFC1981] McCann, J., S. Deering, and J. Mogul. "Path MTU Discovery 991 for IP version 6", RFC 1981, August 1996. 993 [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate 994 Requirement Levels", RFC 2119, March 1997. 996 [RFC2460] Deering, S., and Hinden, R. "Internet Protocol, Version 6 997 (IPv6) Specification", RFC 2460, December 1998. 999 [RFC2463] A. Conta, S. Deering, "Internet Control Message Protocol 1000 (ICMPv6) for the Internet Protocol Version 6 (IPv6) 1001 Specification", RFC 2463, December 1998. 1003 7.2. Informative References 1005 [ASSIGNED] IANA, "Assigned numbers online database", 1006 http://www.iana.org/numbers.html 1008 [DNSOPV6] Durand, A., Ihren, J., and Savola P., "Operational 1009 Considerations and Issues with IPv6 DNS", draft-ietf-dnsop- 1010 ipv6-dns-issues-09.txt, work-in-progress, August 2004. 1012 [KM97] Kent, C., and J. Mogul, "Fragmentation Considered Harmful". 1013 In Proc. SIGCOMM '87 Workshop on Frontiers in Computer 1014 Communications Technology. August 1987. 1016 [V6SEC] P. Savola, "IPv6 Transition/Co-existence Security 1017 Considerations", draft-savola-v6ops-security-overview- 1018 02.txt, work-in-progress, February 2004. 1020 [V64IPSEC] Graveman, R., et al., "Using IPsec to Secure IPv6-over-IPv4 1021 Tunnels", draft-tschofenig-v6ops-secure-tunnels-01.txt, 1022 work-in-progress, July 2004. 1024 [RFC1122] Braden, R., "Requirements for Internet Hosts - Communication 1025 Layers", STD 3, RFC 1122, October 1989. 1027 [RFC1435] S. Knowles, "IESG Advice from Experience with Path MTU 1028 Discovery", RFC 1435, March 1993. 1030 [RFC1812] F. Baker, "Requirements for IP Version 4 Routers", RFC 1812, 1031 June 1995. 1033 [RFC2401] Kent, S., Atkinson, R., "Security Architecture for the 1034 Internet Protocol", RFC 2401, November 1998. 1036 [RFC2461] Narten, T., Nordmark, E., and Simpson, W. "Neighbor 1037 Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. 1039 [RFC2462] Thomson, S., and Narten, T. "IPv6 Stateless Address 1040 Autoconfiguration," RFC 2462, December 1998. 1042 [RFC2667] D. Thaler, "IP Tunnel MIB", RFC 2667, August 1999. 1044 [RFC2827] Ferguson, P., and Senie, D., "Network Ingress Filtering: 1045 Defeating Denial of Service Attacks which employ IP Source 1046 Address Spoofing", RFC 2827, May 2000. 1048 [RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE", 1049 RFC 2890, September 2000. 1051 [RFC2923] K. Lahey, "TCP Problems with Path MTU Discovery", RFC 2923, 1052 September 2000. 1054 [RFC2983] D. Black, "Differentiated Services and Tunnels", RFC 2983, 1055 October 2000. 1057 [RFC3056] B. Carpenter, and K. Moore, "Connection of IPv6 Domains via 1058 IPv4 Clouds", RFC 3056, February 2001. 1060 [RFC3168] K. Ramakrishnan, S. Floyd, D. Black, "The Addition of 1061 Explicit Congestion Notification (ECN) to IP", RFC 3168, 1062 September 2001. 1064 [RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an 1065 On-line Database", RFC 3232, January 2002. 1067 [RFC3484] R. Draves, "Default Address Selection for IPv6", RFC 3484, 1068 February 2003. 1070 [RFC3493] Gilligan, R., et al, "Basic Socket Interface Extensions for 1071 IPv6", RFC 3493, February 2003. 1073 [RFC3513] Hinden, R., and S. Deering, "IP Version 6 Addressing 1074 Architecture", RFC 3513, April 2003. 1076 [RFC3596] Thomson, S., C. Huitema, V. Ksinant, and M. Souissi, "DNS 1077 Extensions to support IP version 6", RFC 3596, October 2003. 1079 [RFC3704] Baker, F., and Savola P., "Ingress Filtering for Multihomed 1080 Networks", RFC 3704, BCP 84, March 2004. 1082 8. Authors' Addresses 1084 Erik Nordmark 1085 Sun Microsystems Laboratories 1086 180, avenue de l'Europe 1087 38334 SAINT ISMIER Cedex, France 1088 Tel : +33 (0)4 76 18 88 03 1089 Fax : +33 (0)4 76 18 88 88 1090 Email : erik.nordmark@sun.com 1092 Robert E. Gilligan 1093 Intransa, Inc. 1094 2870 Zanker Rd., Suite 100 1095 San Jose, CA 95134 1097 Tel : +1 408 678 8600 1098 Fax : +1 408 678 8800 1099 Email : gilligan@intransa.com, gilligan@leaf.com 1101 9. Changes from RFC 2893 1103 The motivation for the bulk of these changes are to simplify the 1104 document to only contain the mechanisms of wide-spread use. 1106 RFC 2893 contains a mechanism called automatic tunneling. But a much 1107 more general mechanism is specified in RFC 3056 [RFC3056] which gives 1108 each node with a (global) IPv4 address a /48 IPv6 prefix i.e., enough 1109 for a whole site. 1111 The following changes have been performed since RFC 2893: 1113 - Removed references to A6 and retained AAAA. 1115 - Removed automatic tunneling and use of IPv4-compatible 1116 addresses. 1118 - Removed default Configured Tunnel using IPv4 "Anycast Address" 1120 - Removed Source Address Selection section since this is now 1121 covered by another document ([RFC3484]). 1123 - Removed brief mention of 6over4. 1125 - Split into normative and non-normative references and other 1126 reference cleanup. 1128 - Dropped "or equal" in if (IPv4 path MTU - 20) is less than or 1129 equal to 1280 1131 - Dropped this: However, IPv6 may be used in some environments 1132 where interoperability with IPv4 is not required. IPv6 nodes 1133 that are designed to be used in such environments need not use 1134 or even implement these mechanisms. 1136 - Described Static MTU and Dynamic MTU cases separately; clarified 1137 that the dynamic path MTU mechanism is OPTIONAL but if it is 1138 implemented it should follow the rules in section 3.2.2. 1140 - Specified Static MTU to default to a MTU of 1280 to 1480 bytes, 1141 and that this may be configurable. Discussed the issues with 1142 using Static MTU at more length. 1144 - Specified minimal rules for IPv4 reassembly and IPv6 MRU to 1145 enhance interoperability and to minimize blacholes. 1147 - Restated the "currently underway" language about Type-of- 1148 Service, and loosely point at [RFC2983] and [RFC3168]. 1150 - Fixed reference to Assigned Numbers to be to online version 1151 (with proper pointer to "Assigned Numbers is obsolete" RFC). 1153 - Clarified text about ingress filtering e.g. that it applies to 1154 packet delivered to transport protocols on the decapsulator as 1155 well as packets being forwarded by the decapsulator, and how the 1156 decapsulator's checks help when IPv4 and IPv6 ingress filtering 1157 is in place. 1159 - Removed unidirectional tunneling; assume all tunnels are 1160 bidirectional, between endpoint addresses (not nodes). 1162 - Removed the guidelines for advertising addresses in DNS as 1163 slightly out of scope, referring to another document for the 1164 details. 1166 - Removed the SHOULD requirement that the link-local addresses 1167 should be formed based on IPv4 addresses. 1169 - Added a SHOULD for implementing a knob to be able to set the 1170 source address of the tunnel, and add discussion why this is 1171 useful. 1173 - Added stronger wording for source address checks: both IPv4 and 1174 IPv6 source addresses MUST be checked, and RPF-like ingress 1175 filtering is optional. 1177 - Rewrote security considerations to be more precise about the 1178 threats of tunneling. 1180 - Added a note about considering using TTL=255 when encapsulating. 1182 - Added more discussion in Section 3.2 why using an "infinite" 1183 IPv6 MTU leads to likely interoperability problems. 1185 - Added an explicit requirement that if both MTU determination 1186 methods are used, choosing one should be possible on a per- 1187 tunnel basis. 1189 - Clarified that ICMPv4 error handling is only applicable to 1190 dynamic MTU determination. 1192 - Removed/clarified DNS record filtering; an API is a SHOULD and 1193 if it does not exist, MUST NOT filter anything. Decree ordering 1194 out of scope, but refer to RFC3484. 1196 - Add a note that the destination IPv4 address could also be a 1197 multicast address. 1199 - Make it RECOMMENDED to provide a toggle to perform strict 1200 ingress filtering on an interface. 1202 - Generalize the text on the data in ICMPv4 messages. 1204 - Require that IP version of the data is 6, and that the payload 1205 is at least 40 bytes. 1207 - Made a lot of miscellaneous editorial cleanups. 1209 9.1. Changes from draft-ietf-v6ops-mech-v2-00 1211 [[ RFC-Editor note: remove the change history between the drafts 1212 before publication. ]] 1214 - Clarified in section 2.2 that there is no assumption that the 1215 DNS server knows the IPv4/IPv6 capabilities of the requesting 1216 node. 1218 - Clarified in section 2.2 that a filtering resolver might want to 1219 take into account external factors e.g., whether IPv6 interfaces 1220 have been configured on the node. 1222 - Clarified in section 2.3 that part of the motivation for the 1223 section is that this is the opposite of common DNS practices in 1224 IPv4; advertising unreachable IPv4 addresses in the DNS is 1225 common. 1227 - Removed the now artificial separation in a section on "common 1228 tunneling techniques" and "configured tunneling" to make one 1229 section on "configured tunneling". 1231 - Restructured the section on tunnel MTU to make the relationship 1232 between static tunnel MTU and dynamic tunnel MTU more clear. 1233 This includes fixing the unclear language about "must be 1280 1234 but may be configurable". 1236 - Added warning about manually configuring large tunnel MTUs 1237 causing excessive fragmentation. 1239 - Added warning about IPv4 PMTU blackholes when using dynamic MTU. 1241 - Clarified that when decapsulating the receiver must be liberal 1242 and allow for padding of the encapsulated packet. 1244 - Added example that when reflecting ICMPv4 errors as ICMPv6 1245 errors it would be appropriate to use ICMPv6 unreachable type 1246 with code "address unreachable" since an error from inside the 1247 tunnel is in effect a link specific problem from IPv6's 1248 perspective. 1250 - Consolidated the text on ingress filtering and created a 1251 separate section on the threat related to source address 1252 spoofing through open decapsulators. 1254 - Clarified "martian" filtering as follows: 0.0.0.0 should be 1255 0.0.0.0/8, same for 127. (per RFC1812), and elaborated that the 1256 broadcast address check includes both the 255.255.255.255 1257 address and all the broadcast addresses of the decapsulator. 1259 - Clarified that packets which fail the checks (such as verifying 1260 the IPv4 source address, martian, and ingress filtering) on the 1261 decapsulator should be silently dropped. 1263 - Clarified that while source link layer address options and 1264 target link layer address options are ignored in received ND 1265 packets, the ND packets themselves are processed as normal. 1267 9.2. Changes from draft-ietf-v6ops-mech-v2-01 1269 - Removed unidirectional tunnels; assume all the tunnels are 1270 bidirectional. 1272 - Removed the definition of IPv4-compatible IPv6 addresses. 1274 - Removed redundant text in the Hop Limit processing rules. 1276 - Removed the guidelines for advertising addresses in DNS as 1277 slightly out of scope, referring to another document for the 1278 details. 1280 - Removed the SHOULD requirement that the link-local addresses 1281 should be formed based on IPv4 addresses. 1283 - Added more discussion on the ICMPv4/6 Path MTU Discovery and the 1284 required number of packet drops. 1286 - Added a SHOULD for implementing a knob to be able to set the 1287 source address of the tunnel, and add discussion why this is 1288 useful. 1290 - Added stronger wording for source address checks: both IPv4 and 1291 IPv6 source addresses MUST be checked, and RPF-like ingress 1292 filtering is optional. 1294 - Rewrote security considerations to be more precise about the 1295 threats of tunneling. 1297 - Added a note that using TTL=255 when encapsulating might be 1298 useful for decapsulation security checks later on. 1300 - Added more discussion in Section 3.2 why using an "infinite" 1301 IPv6 MTU leads to likely interoperability problems. 1303 - Added an explicit requirement that if both MTU determination 1304 methods are used, choosing one should be possible on a per- 1305 tunnel basis. 1307 - Clarified that ICMPv4 error handling is only applicable to 1308 dynamic MTU determination. 1310 - Specified Static MTU to default to a MTU of 1280 to 1480 bytes, 1311 and that this may be configurable. Discussed the issues with 1312 using Static MTU at more length. 1314 - Specified minimal rules for IPv4 reassembly and IPv6 MRU to 1315 enhance interoperability and to minimize blacholes. 1317 - Made a lot of miscellaneous editorial cleanups. 1319 9.3. Changes from draft-ietf-v6ops-mech-v2-02 1321 - Removed DNS record filtering, and made it a SHOULD NOT; 1322 summarized the DNS record ordering. 1324 - Further clarified the usage of IPv6 RPF checks, as they 1325 typically require manual configuration. 1327 - Add a note on link-local address formation strategy to point out 1328 that minimizing the probability of renumbering may be desirable. 1330 - Clarify that with multiple IPv4 addresses, and the link-local 1331 address formed based on one of them, the selection can be done 1332 administratively or by the implementation. 1334 - Refer to separate documents on generic IPv6 security 1335 considerations and IPsec set-up details. 1337 - Clarify that the IPv4 end-point address can be predictable in 1338 some situations. 1340 - Add a note that the destination IPv4 address could also be a 1341 multicast address. 1343 - Make it RECOMMENDED to provide a toggle to perform strict 1344 ingress filtering on an interface. 1346 9.4. Changes from draft-ietf-v6ops-mech-v2-03 1348 - Generalize the text in section 3.4 about the data included in 1349 ICMPv4 messages. 1351 - Decree the address selection ordering mechanism out of scope for 1352 this spec. 1354 9.5. Changes from draft-ietf-v6ops-mech-v2-04 1356 - Reword the definition of a configured tunnel to be more up-to- 1357 todate, and to specify that the tunnel is between addresses, not 1358 hosts. 1360 - Spell out that the source address is expected to be the 1361 encapsulator's IPv4 address. 1363 - Require that IP version of the data is 6, and that the length is 1364 at least 40 bytes. 1366 - Some minor editorial clarifications. 1368 Intellectual Property Statement 1370 The IETF takes no position regarding the validity or scope of any 1371 Intellectual Property Rights or other rights that might be claimed to 1372 pertain to the implementation or use of the technology described in 1373 this document or the extent to which any license under such rights 1374 might or might not be available; nor does it represent that it has 1375 made any independent effort to identify any such rights. 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