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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: 'MUST' is mentioned on line 177, but not defined ** Obsolete normative reference: RFC 2460 (Obsoleted by RFC 8200) -- Obsolete informational reference (is this intentional?): RFC 3315 (Obsoleted by RFC 8415) -- Obsolete informational reference (is this intentional?): RFC 4941 (Obsoleted by RFC 8981) Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IPv6 Maintenance F. Baker 3 Internet-Draft Cisco Systems 4 Updates: 4861 (if approved) B. Carpenter 5 Intended status: Standards Track Univ. of Auckland 6 Expires: May 6, 2016 November 3, 2015 8 Routing packets from hosts in a multi-prefix network 9 draft-ietf-6man-multi-homed-host-02 11 Abstract 13 This document describes expected IPv6 host behavior in a network that 14 has more than one prefix, each allocated by an upstream network that 15 implements BCP 38 ingress filtering, when the host has multiple 16 routers to choose from. It also applies to other scenarios such as 17 the usage of stateful firewalls that effectively act as address-based 18 filters. 20 This host behavior may interact with source address selection in a 21 given implementation, but logically follows it. Given that the 22 network or host is, or appears to be, multihomed with multiple 23 provider-allocated addresses, that the host has elected to use a 24 source address in a given prefix, and that some but not all 25 neighboring routers are advertising that prefix in their Router 26 Advertisement Prefix Information Options, this document specifies to 27 which router a host should present its transmission. It updates RFC 28 4861. 30 Status of This Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at http://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on May 6, 2016. 47 Copyright Notice 49 Copyright (c) 2015 IETF Trust and the persons identified as the 50 document authors. All rights reserved. 52 This document is subject to BCP 78 and the IETF Trust's Legal 53 Provisions Relating to IETF Documents 54 (http://trustee.ietf.org/license-info) in effect on the date of 55 publication of this document. Please review these documents 56 carefully, as they describe your rights and restrictions with respect 57 to this document. Code Components extracted from this document must 58 include Simplified BSD License text as described in Section 4.e of 59 the Trust Legal Provisions and are provided without warranty as 60 described in the Simplified BSD License. 62 Table of Contents 64 1. Introduction and Applicability . . . . . . . . . . . . . . . 2 65 1.1. Host Model . . . . . . . . . . . . . . . . . . . . . . . 3 66 1.2. Requirements Language . . . . . . . . . . . . . . . . . . 5 67 2. Sending context expected by the host . . . . . . . . . . . . 5 68 2.1. Expectations the host has of the network . . . . . . . . 5 69 2.2. Expectations of multihomed networks . . . . . . . . . . . 7 70 3. Reasonable expectations of the host . . . . . . . . . . . . . 7 71 3.1. Interpreting Router Advertisements . . . . . . . . . . . 7 72 3.2. Default Router Selection . . . . . . . . . . . . . . . . 8 73 3.3. Source Address Selection . . . . . . . . . . . . . . . . 8 74 3.4. Redirects . . . . . . . . . . . . . . . . . . . . . . . . 9 75 3.5. History . . . . . . . . . . . . . . . . . . . . . . . . . 9 76 4. Residual issues . . . . . . . . . . . . . . . . . . . . . . . 9 77 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 78 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 79 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 80 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 81 8.1. Normative References . . . . . . . . . . . . . . . . . . 10 82 8.2. Informative References . . . . . . . . . . . . . . . . . 11 83 Appendix A. Change Log (RFC Editor: please delete) . . . . . . . 12 84 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 86 1. Introduction and Applicability 88 This document describes the expected behavior of an IPv6 [RFC2460] 89 host in a network that has more than one prefix, each allocated by an 90 upstream network that implements BCP 38 [RFC2827] ingress filtering, 91 and in which the host is presented with a choice of routers. It 92 expects that the network will implement some form of egress routing, 93 so that packets sent to a host outside the local network from a given 94 ISP's prefix will go to that ISP. If the packet is sent to the wrong 95 egress, it is liable to be discarded by the BCP 38 filter. However, 96 the mechanics of egress routing once the packet leaves the host are 97 out of scope. The question here is how the host interacts with that 98 network. 100 Various aspects of this issue, and possible solution approaches, are 101 discussed in the document IPv6 Multihoming without Network Address 102 Translation [RFC7157]. 104 BCP 38 filtering by ISPs is not the only scenario where such behavior 105 is valuable. Implementations that combine existing recommendations, 106 such as [RFC6092] [RFC7084] can also result in such filtering. 107 Another case is when the connections to the upstream networks include 108 stateful firewalls, such that return packets in a stream will be 109 discarded if they do not return via the firewall that created state 110 for the outgoing packets. A similar cause of such discards is 111 unicast reverse path forwarding (uRPF) [RFC3704]. 113 In this document, the term "filter" is used for simplicity to cover 114 all such cases. In any case, one cannot assume the host to be aware 115 whether an ingress filter, a stateful firewall, or any other type of 116 filter is in place. Therefore, the only consistent solution is to 117 implement the features defined in this document. 119 Note that, apart from ensuring that a message with a given source 120 address is given to a first-hop router that appears to know about the 121 prefix in question, this specification is consistent with [RFC4861]. 122 Nevertheless, implementers of Sections 5.2, 6.2.3, 6.3.4 and 8 of RFC 123 4861 will need to extend their implementations accordingly. This 124 specification is fully consistent with [RFC6724] and implementers 125 will need to add support for its Rule 5.5. Hosts that do not support 126 these features may fail to communicate in the presence of filters as 127 described above. 129 1.1. Host Model 131 It could be argued that the proposal of this document, which is to 132 send messages using a source address in a given prefix to the router 133 that advertised the prefix in its RA, is a form of [RFC1122]'s Strong 134 ES (e.g. Host) Model, discussed in section 3.3.4.2 of that document. 135 In short, [RFC1122] identifies two basic models, in which the "strong 136 host" model models the host as a set of hosts in one chassis, each of 137 which uses a single address on a single interface, and always both 138 sends and receives on that interface, and the "weak host" model 139 treats the host as one system with zero or more addresses on every 140 interface, and capable of using any interface for any communication. 141 As noted there, neither model is completely satisfactory. For 142 example, a host with an addressless interface and a default route 143 pointing to the interface will necessarily send packets using any 144 address using that interface, and therefore be a de facto weak host. 145 If the router upstream from such a host implements BCP 38 Ingress 146 Filtering [RFC2827], such as by implementing uRPF on each interface, 147 the router will prevent communication by weak hosts. 149 +-----------------+ 150 | | 151 | MIF Router +---/--- other interfaces 152 | | 153 +---+---------+---+ 154 | | Two interfaces sharing a prefix 155 --+-+-- --+-+-- 156 | | 157 +--+---------+--+ 158 | MIF Host | 159 +---------------+ 161 Figure 1: Hypothetical MIF interconnection 163 The proposal also differs slightly from [RFC1122]'s language of the 164 Strong Host Model. The statement is that the packet will go to the 165 router that advertised a given prefix, but doesn't state what 166 interface that might happen on. Hence, if the router is a MIF router 167 and is using the same prefix on two or more LANs shared by the host 168 (as in Figure 1), the host might use each of those LANs and meet the 169 requirement. The Strong Host Model is not stated in those terms, but 170 in terms of the interface used, and would find a MIF router quite 171 confusing: 173 (A) A host [MUST] silently discard an incoming datagram whose 174 destination address does not correspond to the physical interface 175 through which it is received. 177 (B) A host [MUST] restrict itself to sending (non-source- routed) 178 IP datagrams only through the physical interface that corresponds 179 to the IP source address of the datagrams. 181 However, comparing the presumptive route lookup mechanisms in each 182 model, this proposal is indeed most similar to the Strong Host Model, 183 as is any source/destination routing paradigm. 185 Strong: route(src IP addr, dest IP addr, TOS) -> gateway 187 Weak: route(dest IP addr, TOS) -> gateway, interface 188 In the hypothetical MIF model suggested in Figure 1, the address 189 fails to identify a single interface, but it does identify a single 190 gateway. 192 1.2. Requirements Language 194 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 195 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 196 document are to be interpreted as described in [RFC2119]. 198 2. Sending context expected by the host 200 2.1. Expectations the host has of the network 202 A host receives prefixes in a Router Advertisement [RFC4861], which 203 goes on to identify whether they are usable by SLAAC [RFC4862] 204 [RFC4941] [RFC7217]. When no prefixes are usable for SLAAC, the 205 Router Advertisement would normally signal the availability of DHCPv6 206 [RFC3315] and the host would use it to configure its addresses. In 207 the latter case (or if both SLAAC and DHCPv6 are used on the same 208 link for some reason) it will be generally the case that the 209 configured addresses match one of the prefixes advertised in a Router 210 Advertisement that are supposed to be on-link for that link. 212 The simplest multihomed network implementation in which a host makes 213 choices among routers might be a LAN with one or more hosts on it and 214 two or more routers, one for each upstream network, or a host that is 215 served by disjoint networks on separate interfaces. In such a 216 network, especially the latter, there is not necessarily a routing 217 protocol, and the two routers may not even know that the other is a 218 router as opposed to a host, or may be configured to ignore its 219 presence. One might expect that the routers may or may not receive 220 each other's RAs and form an address in the other router's prefix 221 (which is not per [RFC4862], but is implemented by some stub router 222 implementations). However, all hosts in such a network might be 223 expected to create an address in each prefix so advertised. 225 +---------+ +---------+ +---------+ +---------+ 226 | ISP | | ISP | | ISP | | ISP | 227 +----+----+ +----+----+ +----+----+ +----+----+ 228 | | | | 229 | | | | 230 +----+----+ +----+----+ +----+----+ +----+----+ 231 | Router | | Router | | Router | | Router | 232 +----+----+ +----+----+ +----+----+ +----+----+ 233 | | | | 234 +------+------+ | +--------+ | 235 | +--+ Host +--+ 236 +----+----+ +--------+ 237 | Host | 238 +---------+ 239 Common LAN Case Disjoint LAN Case 240 (Multihomed Network) (Multihomed Host) 242 Figure 2: Two simple networks 244 If there is no routing protocol among those routers, there is no 245 mechanism by which packets can be deterministically forwarded between 246 the routers (as described in BCP 84 [RFC3704]) in order to avoid 247 filters. Even if there was routing, it would result in an indirect 248 route, rather than a direct route originating with the host; this is 249 not "wrong", but can be inefficient. Therefore the host would do 250 well to select the appropriate router itself. 252 Since the host derives fundamental default routing information from 253 the Router Advertisement, this implies that, in any network with 254 hosts using multiple prefixes, each prefix SHOULD be advertised via a 255 Prefix Information Option (PIO) [RFC4861] by one of the attached 256 routers, even if addresses are being assigned using DHCPv6. A router 257 that advertises a prefix indicates that it is able to appropriately 258 route packets with source addresses within that prefix, regardless of 259 the setting of the L and A flags in the PIO. 261 In some circumstances both L and A might be zero. If SLAAC is not 262 wanted (A=0) and there is no reason to announce an on-link prefix 263 (L=0), a PIO SHOULD be sent to inform hosts that the prefix is 264 source-routed by the router in question. Although this does not 265 violate the existing standard [RFC4861], such a PIO has not 266 previously been common, and it is possible that existing host 267 implementations simply ignore such a PIO or that a router 268 implementation rejects such a PIO as a configuration error. Newer 269 implementations that support this mechanism will need to be updated 270 accordingly: a host SHOULD NOT ignore a PIO simply because both L and 271 A flags are cleared; a router SHOULD be able to send such a PIO. 273 2.2. Expectations of multihomed networks 275 The direct implication of Section 2.1 is that, if the network uses a 276 routing protocol, the routing protocols used in multihomed networks 277 SHOULD implement source-prefix based egress routing. Network designs 278 exist that can usefully limit themselves to static routing (such as a 279 simple tree network), or may internally use no routers at all, such 280 as a single LAN with two CE routers, each of which leads to a 281 different upstream network. 283 3. Reasonable expectations of the host 285 3.1. Interpreting Router Advertisements 287 As described in [RFC4191] and [RFC4861], a Router Advertisement may 288 contain zero or more Prefix information Options (PIOs), zero or more 289 Route Information Options (RIOs), or a Default Router List. In their 290 original intent, these indicate general information to a host: "these 291 are your default routers", "you might create or be configured with an 292 address in this prefix", or "this router would be a good place to 293 send traffic directed to a given destination prefix". In a multi- 294 homed network implementing source/destination routing, the 295 interpretation of a default router list or an RIO has to be modified 296 with the words "if the source address is in one of the prefixes I 297 advertise in a PIO". Additionally, the PIO must be reinterpreted to 298 also imply that the advertising router would be a reasonable first 299 hop for any packet using a source address in any advertised prefix. 301 +---------+ | 302 ( ISP A ) - + Bob-A +--+ +-----+ 303 +---------+ / +---------+ +--+ | 304 | | / | | | 305 | Alice +---/---( The Internet ) | Bob | 306 | | \ | | | 307 +---------+ \ +---------+ +--+ | 308 ( ISP B ) - + Bob-B +--+ +-----+ 309 +---------+ | 311 Figure 3: PIOs, RIOs, and Default Routes 313 The implications bear consideration. Imagine, Figure 3, that hosts 314 Alice and Bob are in communication, Bob's network is multihomed, and 315 Bob's first hop routers are Bob-A (to upstream ISP A advertising 316 prefix A') and Bob-B (to upstream network B and advertising prefix 317 B'). If Bob is responding to a message from Alice, his choice of 318 source address is forced to be the address Alice used as a 319 destination (which we may presume to have been in prefix A'). Hence, 320 Bob created or was assigned an address in A', and can only reasonably 321 send traffic using it to Bob-A as a first hop router. If there were 322 several instances of Bob-A and one had advertised itself as a default 323 router or as having a route to Alice, that is the router Bob should 324 choose. If none of Bob-A have advertised that but Bob-B has, it is 325 irrelevant; Bob is using the address allocated in A' and courts a BCP 326 38 discard if he doesn't send the packet to Bob-A. 328 In the special case that Bob is initiating the conversation, an RIO 329 might, however, influence source address choice. Bob could 330 presumably use any address allocated to him, in this case his address 331 in A' or B'. If Bob-B has advertised an RIO for Alice's prefix and 332 Bob-A has not, Bob MAY take that fact into account in address 333 selection - choosing an address that would allow him to make use of 334 the RIO. 336 3.2. Default Router Selection 338 Default Router Selection is modified as follows: A host SHOULD select 339 default routers for each prefix it is assigned an address in. 340 Routers that have advertised the prefix in its Router Advertisement 341 message SHOULD be preferred over routers that do not advertise the 342 prefix. 344 As a result of doing so, when a host sends a packet using a source 345 address in one of those prefixes and has no history directing it 346 otherwise, it SHOULD send it to the indicated default router. In the 347 "simplest" network described in Section 2.1, that would get it to the 348 only router that is directly capable of getting it to the right ISP. 349 This will also apply in more complex networks, even when more than 350 one physical or virtual interface is involved. 352 In more complex cases, wherein routers advertise RAs for multiple 353 prefixes whether or not they have direct or isolated upstream 354 connectivity, the host is dependent on the routing system already. 355 If the host gives the packet to a router advertising its source 356 prefix, it should be able to depend on the router to do the right 357 thing. 359 3.3. Source Address Selection 361 There is an interaction with Default Address Selection [RFC6724]. A 362 host following the recommendation in the previous section will store 363 information about which next-hops advertised which prefixes. Rule 364 5.5 of RFC 6724 states that the source address used to send to a 365 given destination address should if possible be chosen from a prefix 366 known to be advertised by the next-hop router for that destination. 368 This selection rule would therefore be applicable in a host following 369 the recommendation in the previous section. 371 3.4. Redirects 373 There is potential for adverse interaction with any off-link Redirect 374 (Redirect for a GUA destination that is not on-link) message sent by 375 a router in accordance with Section 8 of [RFC4861]. Hosts SHOULD 376 apply off-link redirects only for the specific pair of source and 377 destination addresses concerned, so the host's Destination Cache may 378 need to contain appropriate source-specific entries. 380 3.5. History 382 Some modern hosts maintain history, in terms of what has previously 383 worked or not worked for a given address or prefix and in some cases 384 the effective window and MSS values for TCP or other protocols. This 385 might include a next hop address for use when a packet is sent to the 386 indicated address. 388 When such a host makes a successful exchange with a remote 389 destination using a particular address pair, and the host has 390 previously received a PIO that matches the source address, then the 391 host SHOULD include the prefix in such history, whatever the setting 392 of the L and A flags in the PIO. On subsequent attempts to 393 communicate with that destination, if it has an address in that 394 prefix at that time, a host MAY use an address in the remembered 395 prefix for the session. 397 4. Residual issues 399 Consider a network where routers on a link run a routing protocol and 400 are configured with the same information. Thus, on each link all 401 routers advertise all prefixes on the link. The assumption that 402 packets will be forwarded to the appropriate egress by the local 403 routing system might cause at least one extra hop in the local 404 network (from the host to the wrong router, and from there to another 405 router on the same link). 407 In a slightly more complex situation such as the disjoint LAN case of 408 Figure 2, which happens to be one of the authors' home plus corporate 409 home-office configuration, the two upstream routers might be on 410 different LANs and therefore different subnets (e.g., the host is 411 itself multi-homed). In that case, there is no way for the "wrong" 412 router to detect the existence of the "right" router, or to route to 413 it. 415 In such a case it is particularly important that hosts take the 416 responsibility to memorize and select the best first-hop as described 417 in Section 3. 419 5. IANA Considerations 421 This memo asks the IANA for no new parameters. 423 6. Security Considerations 425 This document does not create any new security or privacy exposures. 426 It is intended to avoid connectivity issues in the presence of BCP 38 427 ingress filters or stateful firewalls combined with multihoming. 429 There might be a small privacy improvement, however: with the current 430 practice, a multihomed host that sends packets with the wrong address 431 to an upstream router or network discloses the prefix of one upstream 432 to the other upstream network. This practice reduces the probability 433 of that occurrence. 435 7. Acknowledgements 437 Comments were received from Jinmei Tatuya and Ole Troan, who have 438 suggested important text, plus Mikael Abrahamsson, Steven Barth, 439 Juliusz Chroboczek, Toerless Eckert, David Farmer, Dusan Mudric, 440 Tadahisa Okimoto, Pierre Pfister, Mark Smith, and James Woodyatt. 442 8. References 444 8.1. Normative References 446 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 447 Requirement Levels", BCP 14, RFC 2119, 448 DOI 10.17487/RFC2119, March 1997, 449 . 451 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 452 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, 453 December 1998, . 455 [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and 456 More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191, 457 November 2005, . 459 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 460 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 461 DOI 10.17487/RFC4861, September 2007, 462 . 464 [RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, 465 "Default Address Selection for Internet Protocol Version 6 466 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012, 467 . 469 8.2. Informative References 471 [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - 472 Communication Layers", STD 3, RFC 1122, 473 DOI 10.17487/RFC1122, October 1989, 474 . 476 [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: 477 Defeating Denial of Service Attacks which employ IP Source 478 Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, 479 May 2000, . 481 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, 482 C., and M. Carney, "Dynamic Host Configuration Protocol 483 for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 484 2003, . 486 [RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed 487 Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March 488 2004, . 490 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 491 Address Autoconfiguration", RFC 4862, 492 DOI 10.17487/RFC4862, September 2007, 493 . 495 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 496 Extensions for Stateless Address Autoconfiguration in 497 IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, 498 . 500 [RFC6092] Woodyatt, J., Ed., "Recommended Simple Security 501 Capabilities in Customer Premises Equipment (CPE) for 502 Providing Residential IPv6 Internet Service", RFC 6092, 503 DOI 10.17487/RFC6092, January 2011, 504 . 506 [RFC7084] Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic 507 Requirements for IPv6 Customer Edge Routers", RFC 7084, 508 DOI 10.17487/RFC7084, November 2013, 509 . 511 [RFC7157] Troan, O., Ed., Miles, D., Matsushima, S., Okimoto, T., 512 and D. Wing, "IPv6 Multihoming without Network Address 513 Translation", RFC 7157, DOI 10.17487/RFC7157, March 2014, 514 . 516 [RFC7217] Gont, F., "A Method for Generating Semantically Opaque 517 Interface Identifiers with IPv6 Stateless Address 518 Autoconfiguration (SLAAC)", RFC 7217, 519 DOI 10.17487/RFC7217, April 2014, 520 . 522 Appendix A. Change Log (RFC Editor: please delete) 524 Initial Version: 2015-08-05 526 Version 01: Update text on PIOs, added text on Redirects, and 527 clarified the concept of a "simple" network, 2015-08-13. 529 Version 02: Clarifications after WG discussions, 2015-08-19. 531 Version 03: More clarifications after more WG discussions, 532 especially adding stateful firewalls, uRPF, and more precise 533 discussion of RFC 4861, 2015-09-03. 535 Version 04: Responds to various comments including 537 * Questions regarding RFC 1122's strong and weak host models. 538 This model is, strictly speaking, neither, but is most similar 539 to the strong host model. 541 * Some wording errors. 543 * Requests for discussion of the handling of the RIO, PIO, and 544 Default Router List in an RA. 546 WG Versions 00-02: More clarifications after more WG discussions, 547 2015-11-03. 549 Authors' Addresses 551 Fred Baker 552 Cisco Systems 553 Santa Barbara, California 93117 554 USA 556 Email: fred@cisco.com 557 Brian Carpenter 558 Department of Computer Science 559 University of Auckland 560 PB 92019 561 Auckland 1142 562 New Zealand 564 Email: brian.e.carpenter@gmail.com