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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 shim6 Working Group J. Abley 3 Internet-Draft Afilias Canada 4 Intended status: Informational M. Bagnulo 5 Expires: March 24, 2011 A. Garcia-Martinez 6 UC3M 7 September 20, 2010 9 Applicability Statement for the Level 3 Multihoming Shim Protocol 10 (Shim6) 11 draft-ietf-shim6-applicability-07 13 Abstract 15 This document discusses the applicability of the Shim6 IPv6 protocol 16 and associated support protocols and mechanisms to provide site 17 multihoming capabilities in IPv6. 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 March 24, 2011. 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 . . . . . . . . . . . . . . . . . . . . . . . . . 4 66 2. Deployment Scenarios . . . . . . . . . . . . . . . . . . . . . 4 67 3. Address Configuration . . . . . . . . . . . . . . . . . . . . 6 68 3.1. Protocol Version (IPv4 vs. IPv6) . . . . . . . . . . . . . 6 69 3.2. Prefix Lengths . . . . . . . . . . . . . . . . . . . . . . 7 70 3.3. Address Generation . . . . . . . . . . . . . . . . . . . . 7 71 3.4. Use of CGA vs. HBA . . . . . . . . . . . . . . . . . . . . 7 72 4. Shim6 and Ingress Filtering . . . . . . . . . . . . . . . . . 8 73 5. Shim6 Capabilities . . . . . . . . . . . . . . . . . . . . . . 10 74 5.1. Fault Tolerance . . . . . . . . . . . . . . . . . . . . . 10 75 5.1.1. Establishing Communications After an Outage . . . . . 10 76 5.1.2. Short-Lived Communications . . . . . . . . . . . . . . 10 77 5.1.3. Long-Lived Communications . . . . . . . . . . . . . . 11 78 5.2. Load Balancing . . . . . . . . . . . . . . . . . . . . . . 11 79 5.3. Traffic Engineering . . . . . . . . . . . . . . . . . . . 12 80 6. Application Considerations . . . . . . . . . . . . . . . . . . 12 81 7. Interaction with Other Protocols . . . . . . . . . . . . . . . 13 82 7.1. Shim6 and Mobile IPv6 . . . . . . . . . . . . . . . . . . 13 83 7.1.1. Multihomed Home Network . . . . . . . . . . . . . . . 13 84 7.1.2. Shim6 Between the HA and the MN . . . . . . . . . . . 16 85 7.2. Shim6 and SEND . . . . . . . . . . . . . . . . . . . . . . 16 86 7.3. Shim6 and SCTP . . . . . . . . . . . . . . . . . . . . . . 17 87 7.4. Shim6 and NEMO . . . . . . . . . . . . . . . . . . . . . . 17 88 7.5. Shim6 and HIP . . . . . . . . . . . . . . . . . . . . . . 18 89 8. Security Considerations . . . . . . . . . . . . . . . . . . . 18 90 8.1. Privacy Considerations . . . . . . . . . . . . . . . . . . 19 91 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 92 10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 20 93 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20 94 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 95 12.1. Normative References . . . . . . . . . . . . . . . . . . . 21 96 12.2. Informative References . . . . . . . . . . . . . . . . . . 22 97 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23 99 1. Introduction 101 Site multihoming is an arrangement by which a site may use multiple 102 paths to the rest of the Internet to provide better reliability for 103 traffic passing in and out of the site than would be possible with a 104 single path. Some of the motivations for operators to multi-home 105 their network are described in [RFC3582]. 107 In IPv4, site multihoming is achieved by injecting into the global 108 Internet routing system (sometimes referred to as the Default-Free 109 Zone, or DFZ) the additional state required to allow session 110 resilience over re-homing events [RFC4116]. There is concern that 111 this approach will not scale [RFC3221], [RFC4984]. 113 In IPv6, site multihoming in the style of IPv4 is not generally 114 available to end sites due to a strict policy of route aggregation in 115 the DFZ. Site multihoming for sites without provider-independent 116 (PI) addresses is achieved by assigning multiple addresses to each 117 host, one or more from each provider. This multihoming approach 118 provides no transport-layer stability across re-homing events. 120 Shim6 provides layer-3 support for making re-homing events 121 transparent to the transport layer by means of a shim approach. 122 State information relating to the multihoming of two endpoints 123 exchanging unicast traffic is retained on the endpoints themselves, 124 rather than in the network. Communications between Shim6-capable 125 hosts and Shim6-incapable hosts proceed as normal, but without the 126 benefit of transport-layer stability. The Shim6 approach is thought 127 to have better scaling properties with respect to the state held in 128 the DFZ than the IPv4 approach. 130 This note describes the applicability of the Level 3 multihoming 131 (hereafter Shim6) protocol defined in [RFC5533] and the failure 132 detection mechanisms defined in [RFC5534]. 134 The terminology used in this document, including terms like locator, 135 and ULID, is defined in [RFC5533]. 137 2. Deployment Scenarios 139 The goal of the Shim6 protocol is to support locator agility in 140 established communications: different layer-3 endpoint addresses may 141 be used to exchange packets belonging to the same transport-layer 142 session, all the time presenting a consistent identifier pair to 143 upper-layer protocols. 145 In order to be useful, the Shim6 protocol requires that at least one 146 of the peers has more than one address which could be used on the 147 wire (as locators). In the event of communications failure between 148 an active pair of addresses, the Shim6 protocol will attempt to 149 reestablish communication by trying different combinations of 150 locators. 152 While other multi-addressing scenarios are not precluded, the 153 scenario in which the Shim6 protocol is expected to operate is that 154 of a multihomed site which is connected to multiple transit 155 providers, and which receives an IPv6 prefix from each of them. This 156 configuration is intended to provide protection for the end-site in 157 the event of a failure in some subset of the available transit 158 providers, without requiring the end-site to acquire PI address space 159 or requiring any particular cooperation between the transit 160 providers. 162 ,------------------------------------. ,----------------. 163 | Rest of the Internet +-------+ Remote Host R | 164 `--+-----------+------------------+--' `----------------' 165 | | | LR[1] ... LR[m] 166 ,---+----. ,---+----. ,----+---. 167 | ISP[1] | | ISP[2] | ...... | ISP[n] | 168 `---+----' `---+----' `----+---' 169 | | | 170 ,---+-----------+------------------+---. 171 | Multi-Homed Site S assigned | 172 | prefixes P[1], P[2], ..., P[n] | 173 | | 174 | ,--------. L[1] = P[1]:iid[1], | 175 | | Host H | L[2] = P[2]:iid[2], ... | 176 | `--------' L[n] = P[n]:iid[n] | 177 `--------------------------------------' 179 Figure 1 181 In the scenario illustrated in Figure 1 host H communicates with some 182 remote host R. Each of the addresses L[i] configured on host H in the 183 multihomed site S can be reached through provider ISP[i] only, since 184 ISP[i] is solely responsible for advertising a covering prefix for 185 P[i] to the rest of the Internet. 187 The use of locator L[i] on H hence causes inbound traffic towards H 188 to be routed through ISP[i]. Changing the locator from L[i] to L[j] 189 will have the effect of re-routing inbound traffic to H from ISP[i] 190 to ISP[j]. This is the central mechanism by which the Shim6 protocol 191 aims to provide multihoming functionality: by changing locators, host 192 H can change the upstream ISP used to route inbound packets towards 193 itself. Regarding to the outbound traffic to H, the path taken in 194 this case depends on both the actual locator LR[j] chosen by R, and 195 the administrative exit selection policy of site S. 197 The Shim6 protocol has other potential applications beyond site 198 multihoming. For example, since Shim6 is a host-based protocol, it 199 can also be used to support host multihoming. In this case, a 200 failure in communication between a multihomed host and some other 201 remote host might be repaired by selecting a locator associated with 202 a different interface. 204 3. Address Configuration 206 3.1. Protocol Version (IPv4 vs. IPv6) 208 The Shim6 protocol is defined only for IPv6. However, there is no 209 fundamental reason why a Shim6-like approach could not support IPv4 210 addresses as locators, either to provide multihoming support to IPv4- 211 numbered sites, or as part of an IPv4/IPv6 transition strategy. Some 212 extensions to the Shim6 protocol for supporting IPv4 locators have 213 been proposed in [I-D.nordmark-shim6-esd]. 215 The Shim6 protocol, as specified for IPv6, incorporates cryptographic 216 elements in the construction of locators (see [RFC3972], [RFC5535]). 217 Since IPv4 addresses are insufficiently large to contain addresses 218 constructed in this fashion, direct implementation of Shim6 as 219 specified for IPv6 for use with IPv4 addresses might require protocol 220 modifications. 222 In addition, there are other factors to take into account when 223 considering the support of IPv4 addresses, in particular IPv4 224 locators. Using multiple IPv4 addresses in a single host in order to 225 support Shim6 style of multihoming would result in an increased IPv4 226 address consumption, which with the current rate of IPv4 addresses 227 would be problematic. Besides, Shim6 may suffer additional problems 228 if locators become translated on the wire. Address translation is 229 more likely to involve IPv4 addresses. IPv4 addressed can be 230 translated to other IPv4 addresses (for example, private IPv4 address 231 into public IPv4 address and vice versa) or to/from IPv6 addresses 232 (for example, as defined by NAT64 233 [I-D.ietf-behave-v6v4-xlate-stateful]). When address translation 234 occurs, a locator exchanged by Shim6 could be different to the 235 address needed to reach the corresponding host, either because the 236 translated version of the locator exchanged by Shim6 is not known or 237 because the translation state does not exist any more in the 238 translator device. Supporting these scenarios would require NAT 239 traversal mechanisms which are not defined yet and which would imply 240 additional complexity (as any other NAT traversal mechanism). 242 3.2. Prefix Lengths 244 The Shim6 protocol does not assume that all the prefixes assigned to 245 the multihomed site have the same prefix length. 247 However, the use of CGA [RFC3972] and HBA [RFC5535] involve encoding 248 information in the lower 64 bits of the locators. This imposes the 249 requirement on address assignment to Shim6-capable hosts that all 250 interface addresses should be able to accommodate 64-bit interface 251 identifiers. It should be noted that this is imposed by RFC4291 252 [RFC4291]. 254 3.3. Address Generation 256 The security of the Shim6 protocol is based on the use of CGA and HBA 257 addresses. 259 CGA and HBA generation process can use the information provided by 260 the stateless auto-configuration mechanism defined in [RFC4862] with 261 the additional considerations presented in [RFC3972] and [RFC5535]. 263 Stateful address auto-configuration using DHCP [RFC3315] is not 264 currently supported, because there is no defined mechanism to convey 265 the CGA Parameter Data Structure and other relevant information from 266 the DHCP server to the host. The definition of such mechanism seems 267 to be quite straightforward in the case of the HBA, since only the 268 CGA Parameter Data Structure needs to be delivered from the DHCP 269 server to the Shim6 host, and this data structure does not contain 270 any secret information. In the case of CGAs, the difficulty is 271 increased, since private key information should be exchanged as well 272 as the CGA Parameter Data Structure. However, with appropriate 273 extensions a DHCP server could inform to a host about the SEC value 274 to use when generating an address, or DHCP could even be used by the 275 host to delegate to the server the CPU-intensive task of computing a 276 Modifier for a given combination 277 [I-D.ietf-csi-dhcpv6-cga-ps]. 279 3.4. Use of CGA vs. HBA 281 The choice between CGA and HBA is a trade-off between flexibility and 282 performance. 284 The use of HBA is more efficient in the sense that addresses require 285 less computation than CGA, involving only hash operations for both 286 the generation and the verification of locator sets. However, the 287 locators of an HBA set are determined during the generation process, 288 and cannot be subsequently changed; the addition of new locators to 289 that initial set is not supported, except by re-generation of the 290 entire set which will in turn cause all addresses to change. 292 The use of CGA is more computationally expensive, involving public 293 key cryptography in the verification of locator sets. However, CGAs 294 are more flexible in the sense that they support the dynamic 295 modification of locator sets. 297 Therefore, CGAs are well suited to support dynamic environments such 298 as mobile hosts, where the locator set must be changed frequently. 299 HBAs are better suited for sites where the prefix set remains 300 relatively stable. 302 It should be noted that, since HBAs are defined as a CGA extension, 303 it is possible to generate hybrid HBA/CGA structures that incorporate 304 the strengths of both: i.e. that a single address can be used as an 305 HBA, enabling computationally-cheap validation amongst a fixed set of 306 addresses, and also as a CGA, enabling dynamic manipulation of the 307 locator set. For additional details, see [RFC5535]. 309 4. Shim6 and Ingress Filtering 311 Ingress filtering [RFC2827] prevents address spoofing by dropping 312 packets which come from customer networks with source addresses not 313 belonging to the prefix assigned to them. The problem of deploying 314 ingress filters with multihomed customers is discussed in [RFC3704], 315 in particular considering the case in which non-PI addresses are used 316 by customer networks. This is the case for IPv6 hosts in multihomed 317 networks with PA, and also for a Shim6 host in a multihomed network. 318 Note that this is also the case for other solutions supporting 319 multihoming, such as SCTP [RFC4960], HIP [RFC4423], etc. 321 One solution to this problem is to make the providers aware of the 322 alternative prefixes that can be used by a multihomed site, so that 323 ingress filtering would not be applied to packets with source 324 addresses belonging to these prefixes. This may be possible in some 325 cases, but it cannot be assumed as the general case. 327 [RFC3704] proposes that non-PI addresses should ensure that each 328 packet is delivered to the provider related with the prefix of its 329 source address. To deliver packets to the appropriate outgoing ISP, 330 some routers of the site must consider source addresses in their 331 forwarding decisions, in addition to the usual destination-based 332 forwarding. These routers maintain as many parallel routing tables 333 as valid source prefixes are, and choose a route that is a function 334 of both the source and the destination address. The way these 335 routing tables are populated is out of the scope of this document. 337 Site exit routers are required (at least) to be part of a single 338 connected source based routing domain: 340 Multiple site exits 341 | | | | 342 -----+-----+-----+-----+----- 343 ( ) 344 ( Source based routing domain ) 345 ( ) 346 ----+----+----+----+----+---- 347 ( ) 348 ( Generic routing domain ) 349 ( ) 350 ----------------------------- 352 Figure 2 354 In this way, packets arriving to this connected source based routing 355 domain would be delivered to the appropriate exit router. 357 Some particular cases of this generic deployment scenario are: 359 - a single exit router, in which the router chooses the exit provider 360 according to the source address of the packet to be forwarded 362 - a site in which all routers perform source address based forwarding 364 - a site in which only site-exit routers perform source address based 365 forwarding, and these site-exit routers are connected through point- 366 to-point tunnels, so that packets can be tunneled to the appropriate 367 exit router according to its source address 369 For hosts attached directly to networks of different providers, a 370 host solution to ensure that packets are forwarded to the appropriate 371 interface according to its source address must be provided. This 372 problem is discussed in the Multiple Interfaces (MIF) IETF Working 373 Group. 375 Shim6 has no means to enforce neither host nor network forwarding for 376 a given locator to be used as source address. If any notification is 377 received from the router dropping the packets with legitimate source 378 addresses as a result of ingress filtering, the affected locator 379 could be associated to a low preference (or not being used at all). 380 But even if such notification is not received, or not processed by 381 the Shim6 layer, defective ingress filtering configuration will be 382 treated as a communication failure, and Shim6 re-homing would finally 383 select a working path in which packets are not filtered, if this path 384 exists. Note that this behavior results from the powerful end-to-end 385 resilience properties exhibited by REAP. 387 5. Shim6 Capabilities 389 5.1. Fault Tolerance 391 5.1.1. Establishing Communications After an Outage 393 If a host within a multihomed site attempts to establish a 394 communication with a remote host and selects a locator which 395 corresponds to a failed transit path, bidirectional communication 396 between the two hosts will not succeed. In order to establish a new 397 communication, the initiating host must try different combinations of 398 (source, destination) locator pairs until it finds a pair that works. 399 The mechanism for this default address selection is described in 400 [RFC3484]. As a result of the use of this mechanism, some failures 401 may not be recovered even if a valid alternative path exists between 402 two communicating hosts. For example, assuming a failure in ISP[1] 403 (see figure 1), and host H initiating a communication with host R, 404 the source address selection algorithm described in [RFC3484] may 405 result in the selection of the source address corresponding to ISP[1] 406 for every destination address being tried by the application. 407 However, note that if R is the node initiating the communication, it 408 will find a valid path provided that the application at R tries every 409 available address for H. 411 Since a Shim6 context is normally established between two hosts only 412 after initial communication has been set up, there is no opportunity 413 for Shim6 to participate in the discovery of a suitable, initial 414 (source, destination) locator pair. The same consideration holds for 415 referrals, as it is described in Section 6. 417 5.1.2. Short-Lived Communications 419 The Shim6 context establishment operation requires a 4-way packet 420 exchange, and involves some overhead on the participating hosts in 421 memory and CPU. 423 For short-lived communications between two hosts, the benefit of 424 establishing a Shim6 context might not exceed the cost, perhaps 425 because the protocols concerned are fault tolerant and can arrange 426 their own recovery (e.g. DNS) or because the frequency of re-homing 427 events is sufficiently low that the probability of such a failure 428 occurring during a short-lived exchange is not considered 429 significant. 431 It is anticipated that the exchange of Shim6 context will provide 432 most benefit for exchanges between hosts which are long-lived. For 433 this reason the default behaviour of Shim6-capable hosts is expected 434 to employ deferred context-establishment. This default behaviour 435 will be able to be overridden by applications which prefer immediate 436 context establishment regardless of transaction longevity. 438 It must be noted that all the above considerations refer to the 439 lifetime of the interaction between the peers and not about the 440 lifetime of a particular connection (e.g. TCP connection). In other 441 words, the Shim6 context is established between ULID pairs and it 442 affects all the communication between these ULIDs. So, two nodes 443 with multiple short-lived communications using the same ULID pair 444 would benefit as much from the Shim6 features as two nodes having a 445 single long-lived communication. One example of such scenario would 446 be a web client software downloading web contents from a server over 447 multiple TCP connections. Each TCP connection is short-lived, but 448 the communication/contact between the two ULID could be long-lived. 450 5.1.3. Long-Lived Communications 452 As discussed in Section 5.1.2, hosts engaged in long-lived 453 communications will suffer lower proportional overhead, and greater 454 probability of benefit than those performing brief transactions. 456 Deferred context setup ensures that session establishment time will 457 not be increased by the use of Shim6. 459 5.2. Load Balancing 461 The Shim6 protocol does not support load balancing within a single 462 context: all packets associated with a particular context are 463 exchanged using a single locator pair per direction, with the 464 exception of forked contexts, which are created upon explicit 465 requests from the upper-layer protocol. 467 It may be possible to extend the Shim6 protocol to use multiple 468 locator pairs in a single context, but the impact of such an 469 extension on upper-layer protocols (e.g. on TCP congestion control) 470 should be considered carefully. 472 When many contexts are considered together in aggregation, e.g. on a 473 single host which participates in many simultaneous contexts or in a 474 site full of hosts, some degree of load sharing should occur 475 naturally due to the selection of different locator pairs in each 476 context. However, there is no mechanism defined to ensure that this 477 natural load sharing is arranged to provide a statistical balance 478 between transit providers. 480 5.3. Traffic Engineering 482 The Shim6 protocol provides some lightweight traffic engineering 483 capabilities in the form of the Locator Preferences option, which 484 allows a host to inform a remote host of local preferences for 485 locator selection. 487 This mechanism is only available after a Shim6 context has been 488 established, and it is a host-based capability rather than a site- 489 based capability. There is no defined mechanism which would allow 490 use of the Locator Preferences option amongst a site full of hosts to 491 be managed centrally. 493 6. Application Considerations 495 Shim6 provides multihoming support without forcing changes in the 496 applications running on the host. The fact that an address has been 497 generated according to the CGA or HBA specification does not require 498 any specific action from the application, e.g. it can obtain remote 499 CGA or HBA addresses as a result of a getaddrinfo() call to trigger a 500 DNS Request. The storage of CGA or HBA addresses in DNS does not 501 require also any modification of this protocol, since they are 502 recorded using AAAA records. Moreover, neither the ULID/locator 503 management [RFC5533] nor the failure detection and recovery [RFC5534] 504 functions require application awareness. 506 However, a specific API [I-D.ietf-shim6-multihome-shim-api] is 507 developed for those applications which might require additional 508 capabilities in ULID/locator management, such as the locator pair in 509 use for a given context, or the set of local or remote locators 510 available for it. This API can also be used to disable Shim6 511 operation when required. 513 It is worth to note that callbacks can benefit naturally from Shim6 514 support. In a callback, an application in B retrieves IP_A, the IP 515 address of a peer A, and B uses IP_A to establish a new communication 516 with A. As long as the address exchanged, IP_A is the ULID for the 517 initial communication between A and B, and B uses the same address as 518 in the initial communication, and this initial communication is alive 519 (or the context has not been deleted), the new communication could 520 use the locators exchanged by Shim6 for the first communication. In 521 this case, communication could proceed even if the ULID of A is not 522 reachable. 524 However, Shim6 does not provide specific protection to current 525 applications when they use referrals. A referral is the exchange of 526 the IP address IP_A of a party A by party B to party C, so that party 527 C could use IP_A to communicate with party A. In a normal case, the 528 ULID IP_A would be the only information sent by B to C as referral. 529 But if IP_A is no longer valid as locator in A, C could have trouble 530 in establishing a communication with A. Increased failure protection 531 for referrals could be obtained if B exchanged the whole list of 532 alternative locators of A, although in this case the application 533 protocol should be modified. Note that B could send to C the current 534 locator of A, instead of the ULID of A, as a way of using the most 535 recent reachability information about A. While in this case no 536 modification of the application protocol is required, some concerns 537 arise: host A may not accept one of its locator as ULID for 538 initiating a communication, and if CGA are used, the locator may not 539 be a CGA so a Shim6 context among A and C could not be created. 541 7. Interaction with Other Protocols 543 7.1. Shim6 and Mobile IPv6 545 We next consider some scenarios in which the Shim6 protocol and the 546 MIPv6 protocol [RFC3775] might be used simultaneously. 548 7.1.1. Multihomed Home Network 550 In this case, the Home Network of the Mobile Node (MN) is multihomed. 551 This implies the availability of multiple Home Network prefixes, 552 resulting on multiple HoAs for each MN. Since the MN is a node 553 within a multihomed site, it seems reasonable to expect that the MN 554 should be able to benefit from the multihoming capabilities provided 555 by the Shim6 protocol. Moreover, the MN needs to be able to obtain 556 the multihoming benefits even when it is roaming away from the Home 557 Network: if the MN is away from the Home Network while the Home 558 Network suffers a failure in a transit path, the MN should be able to 559 continue communicating using alternate paths to reach the Home 560 Network. 562 The resulting scenario is the following: 564 +------------------------------------+ 565 | Internet | 566 +------------------------------------+ 567 | | 568 +----+ +----+ 569 |ISP1| |ISP2| 570 +----+ +----+ 571 | | 572 +------------------------------------+ 573 | Multihomed Home Network | 574 | Prefixes: P1 and P2 | 575 | | 576 | Home Agent | 577 | // | 578 +------------------//----------------+ 579 // 580 // 581 +-----+ 582 | MN | HoA1, HoA2 583 +-----+ 585 Figure 3 587 So, in this configuration, the Shim6 protocol is used to provide 588 multiple communication paths to all the nodes within the multihomed 589 sites (including the mobile nodes) and the MIPv6 protocol is used to 590 support mobility of the mobile nodes of the multihomed site. 592 The proposed protocol architecture would be the following: 594 +--------------+ 595 | Application | 596 +--------------+ 597 | Transport | 598 +--------------+ 599 | IP | 600 | +----------+ | 601 | | IPSec | | 602 | +----------+<--ULIDs 603 | | Shim6 | | 604 | +----------+<--HoAs 605 | | MIPv6 | | 606 | +----------+<--CoAs 607 | | 608 +--------------+ 610 Figure 4 612 In this architecture, the upper layer protocols and IPSec would use 613 ULIDs of the Shim6 protocol. Only the HoAs will be presented by the 614 upper layers to the Shim6 layer as potential ULIDs. Two Shim6 615 entities will exchange their own available HoAs as locators. 616 Therefore, Shim6 provides failover between different HoAs and allows 617 preserving established communications when an outage affects the path 618 through the ISP that has delegated the HoA used for initiating the 619 communication (similarly to the case of a host within a multihomed 620 site). The CoAs are not presented to the Shim6 layer and are not 621 included in the local locator set in this case. The CoAs are managed 622 by the MIPv6 layer, which binds each HoA to a CoA. 624 So, in this case, the upper layer protocols select a ULID pair for 625 the communication. The Shim6 protocol translates the ULID pair to an 626 alternative locator in case that is needed. Both the ULIDs and the 627 alternative locators are HoAs. Next, the MIPv6 layer maps the 628 selected HoA to the corresponding CoA, which is the actual address 629 included in the wire. 631 The Shim6 context is established between the MN and the CN, and it 632 would allow the communication to use all the available HoAs to 633 provide fault tolerance. The MIPv6 protocol is used between the MN 634 and the HA in the case of the bidirectional tunnel mode, and between 635 the MN and the CN in case of the RO (Route Optimization) mode. 637 7.1.2. Shim6 Between the HA and the MN 639 Another scenario where a Shim6-MIPv6 interaction may be useful is the 640 case where a Shim6 context is established between the MN and the HA 641 in order to provide fault tolerance capabilities to the bidirectional 642 tunnel between them. 644 Consider the case where the HA has multiple addresses (whether 645 because the Home Network is multihomed or because the HA has multiple 646 interfaces) and/or the MN has multiple addresses (whether because the 647 visited network is multihomed or because the MN has multiple 648 interfaces). In this case, if a failure affects the address pair 649 that is being used to run the tunnel between the MN and HA, 650 additional mechanisms need to be used to preserve the communication. 652 One possibility would be to use MIPv6 capabilities, by simply 653 changing the CoA used as the tunnel endpoint. However, MIPv6 lacks 654 of failure detection mechanisms that would allow the MN and/or the HA 655 to detect the failure and trigger the usage of an alternative 656 address. Shim6 provides such failure detection protocol, so one 657 possibility would be re-using the failure detection function from the 658 Shim6 failure detection protocol in MIPv6. In this case, the Shim6 659 protocol wouldn't be used to create Shim6 context and provide fault 660 tolerance, but just its failure detection functionality would be re- 661 used. 663 The other possibility would be to use the Shim6 protocol to create a 664 Shim6 context between the HA and the MN so that the Shim6 detects any 665 failure and re-homes the communication in a transparent fashion to 666 MIPv6. In this case, the Shim6 protocol would be associated to the 667 tunnel interface. 669 7.2. Shim6 and SEND 671 Secure Neighbor Discovery (SEND) [RFC3971] uses CGAs to prove address 672 ownership for Neighbor Discovery [RFC4861]. The Shim6 protocol can 673 use either CGAs or HBAs to protect locator sets included in Shim6 674 contexts. It is expected that some hosts will need to participate in 675 both SEND and Shim6 simultaneously. 677 In the case that both the SEND and Shim6 protocols are using the CGA 678 technique to generate addresses, then there is no conflict: the host 679 will generate addresses for both purposes as CGAs, and since it will 680 be in control of the associated private key, the same CGA can be used 681 for the different protocols. 683 In the case that a Shim6-capable host is using HBAs to protect its 684 locator sets, the host will need to generate hybrid HBA/CGA addresses 685 as defined in [RFC5535] and discussed briefly in Section 3.4. In 686 this case, the CGA Parameter Data Structure containing a valid public 687 key and the Multi-Prefix extension are included as inputs to the hash 688 function. 690 7.3. Shim6 and SCTP 692 The SCTP [RFC4960] protocol provides a reliable, stream-based 693 communications channel between two hosts which provides a superset of 694 the capabilities of TCP. One of the notable features of SCTP is that 695 it allows the exchange of endpoint addresses between hosts, and is 696 able to recover from the failure of a particular endpoint pair in a 697 manner which is conceptually similar to locator selection in Shim6. 699 SCTP is a transport-layer protocol, higher in the protocol stack than 700 Shim6, and hence there is no fundamental incompatibility which would 701 prevent a Shim6-capable host from communicating using SCTP. 703 However, since SCTP and Shim6 both aim to exchange addressing 704 information between hosts in order to meet the same generic goal, it 705 is possible that their simultaneous use might result in unexpected 706 behaviour, e.g. lead to race conditions. 708 The capabilities of SCTP with respect to path maintenance of a 709 reliable, connection-oriented stream protocol are more extensive than 710 the more general layer-3 locator agility provided by Shim6. 711 Therefore, It is recommended that Shim6 is not used for SCTP 712 sessions, and that path maintenance is provided solely by SCTP. 713 There are at least two ways to enforce this behaviour. One option 714 would be to make the stack, and in particular the Shim6 sublayer, 715 aware of SCTP sockets and in this case refrain from creating a Shim6 716 context. The other option is that the upper layer, SCTP in this 717 case, informs using a Shim6-capable API like the one proposed in 718 [I-D.ietf-shim6-multihome-shim-api] that no Shim6 context must be 719 created for this particular communication. 721 Note that the issues described here for SCTP may also arise for a 722 multipath TCP solution. 724 7.4. Shim6 and NEMO 726 The NEMO [RFC3963] protocol extensions to MIPv6 allow a Mobile 727 Network to communicate through a bidirectional tunnel via a Mobile 728 Router (MR) to a NEMO-compliant Home Agent (HA) located in a Home 729 Network. 731 If either or both of the MR or HA are multihomed, then a Shim6 732 context established preserves the integrity of the bidirectional 733 tunnel between them in the event that a transit failure occurs in the 734 connecting path. 736 Once the tunnel between MR and HA is established, hosts within the 737 Mobile Network which are Shim6-capable can establish contexts with 738 remote hosts in order to receive the same multihoming benefits as any 739 host located within the Home Network. 741 7.5. Shim6 and HIP 743 Shim6 and the Host Identity Protocol (HIP [RFC4423]) are 744 architecturally similar in the sense that both solutions allow two 745 hosts to use different locators to support communications between 746 stable ULIDs. The signaling exchange to establish the demultiplexing 747 context on the hosts is very similar for both protocols. However, 748 there are a few key differences. First, Shim6 avoids defining a new 749 namespace for ULIDs, preferring instead to use a routable locator as 750 a ULID, while HIP uses public keys and hashes thereof as ULIDs. The 751 use of a routable locator as ULID better supports deferred context 752 establishment, application callbacks, and application referrals, and 753 avoids management and resolution costs of a new namespace, but 754 requires additional security mechanisms to securely bind the ULID 755 with the locators. Second, Shim6 uses an explicit context header on 756 data packets for which the ULIDs differ from the locators in use 757 (this header is only needed after a failure/rehoming event occurs), 758 while HIP may compress this context-tag function into the ESP SPI 759 field [RFC5201]. Third, HIP as presently defined requires the use of 760 public-key operations in its signaling exchange and ESP encryption in 761 the data plane, while the use of Shim6 requires neither (if only HBA 762 addresses are used). HIP by default provides data protection, while 763 this is a non-goal for Shim6. 765 The Shim6 working group was chartered to provide a solution to a 766 specific problem, multihoming, which minimizes deployment disruption, 767 while HIP is considered more of an experimental approach intended to 768 solve several more general problems (mobility, multihoming and loss 769 of end-to-end addressing transparency) through an explicit 770 identifier/locator split. Communicating hosts that are willing and 771 interested to run HIP (perhaps extended with Shim6's failure 772 detection protocol) likely have no reason to also run Shim6. In this 773 sense, HIP may be viewed as a possible long-term evolution or 774 extension of the Shim6 architecture, or one possible implementation 775 of the extended Shim6 design ESD [I-D.nordmark-shim6-esd]. 777 8. Security Considerations 779 This section considers the applicability of the Shim6 protocol from a 780 security perspective, i.e. which security features can expect 781 applications and users of the Shim6 protocol. 783 First of all, it should be noted that the Shim6 protocol is not a 784 security protocol, like for instance HIP. This means that as opposed 785 to HIP, it is an explicit non-goal of the Shim6 protocol to provide 786 enhanced security for the communications that use the Shim6 protocol. 787 The goal of the Shim6 protocol design in terms of security is not to 788 introduce new vulnerabilities that were not present in the current 789 non-Shim6 enabled communications. In particular, it is an explicit 790 non-goal of the Shim6 protocol security to provide protection from 791 on-path attackers. On-path attackers are able to sniff and spoof 792 packets in the current Internet, and they are able to do the same in 793 Shim6 communications (as long as the communication flows through the 794 path they are located on). So, summarizing, the Shim6 protocol does 795 not provide data packet protection from on-path attackers. 797 However, the Shim6 protocol does use several security techniques. 798 The goal of these security measures is to protect the Shim6 signaling 799 protocol from new attacks resulting from the adoption of the Shim6 800 protocol. In particular, the use of HBA/CGA prevents on-path and 801 off-path attackers to introduce new locators in the locator set of a 802 Shim6 context, preventing redirection attacks [RFC4218]. Moreover, 803 the usage of probes before re-homing to a different locator as a 804 destination address prevents flooding attacks from off-path 805 attackers. 807 In addition, the usage of a 4-way handshake for establishing the 808 Shim6 context protects against DoS attacks, so hosts implementing the 809 Shim6 protocol should not be more vulnerable to DoS attacks than 810 regular IPv6 hosts. 812 Finally, many Shim6 signaling messages contain a Context Tag, meaning 813 that only attackers that know the Context Tag can forge them. As a 814 consequence, only on-path attackers can generate false Shim6 815 signaling packets for an established context. The impact of these 816 attacks would be limited since they would not be able to add 817 additional locators to the locator set (because of the HBA/CGA 818 protection). In general the possible attacks have similar effects to 819 the ones that an on-path attacker can launch on any regular IPv6 820 communication. The residual threats are described in the Security 821 Considerations of the Shim6 protocol specification [RFC5533]. 823 8.1. Privacy Considerations 825 The Shim6 protocol is designed to provide some basic privacy 826 features. In particular, HBAs are generated in such a way, that the 827 different addresses assigned to a host cannot be trivially linked 828 together as belonging to the same host, since there is nothing in 829 common in the addresses themselves. Similar features are provided 830 when the CGA protection is used. This means that it is not trivial 831 to determine that a set of addresses is assigned to a single Shim6 832 host. 834 However, the Shim6 protocol does exchange the locator set in clear 835 text and it also uses a fixed Context Tag when using different 836 locators in a given context. This implies that an attacker observing 837 the Shim6 context establishment exchange or seeing different payload 838 packets exchanged through different locators, but with the same 839 Context Tag, can determine the set of addresses assigned to a host. 840 However, this requires that the attacker is located along the path 841 and that it can capture the Shim6 signaling packets. 843 9. IANA Considerations 845 This document has no actions for IANA. 847 10. Contributors 849 The analysis on the interaction between the Shim6 protocol and the 850 other protocols presented in this note benefited from the advice of 851 various people including Tom Henderson, Erik Nordmark, Hesham 852 Soliman, Vijay Devarpalli, John Loughney and Dave Thaler. 854 11. Acknowledgements 856 Joe Abley's work was supported in part by the US National Science 857 Foundation (research grant SCI-0427144) and DNS-OARC. 859 Marcelo Bagnulo worked on this document while visiting Ericsson 860 Research Laboratory Nomadiclab. 862 Shinta Sugimoto reviewed this document and provided comments and 863 text. 865 Iljitsch van Beijnum, Brian Carpenter, Sam Xia reviewed this document 866 and provided comments. 868 12. References 869 12.1. Normative References 871 [I-D.ietf-behave-v6v4-xlate-stateful] 872 Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful 873 NAT64: Network Address and Protocol Translation from IPv6 874 Clients to IPv4 Servers", 875 draft-ietf-behave-v6v4-xlate-stateful-12 (work in 876 progress), July 2010. 878 [I-D.ietf-shim6-multihome-shim-api] 879 Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto, 880 "Socket Application Program Interface (API) for 881 Multihoming Shim", draft-ietf-shim6-multihome-shim-api-14 882 (work in progress), August 2010. 884 [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: 885 Defeating Denial of Service Attacks which employ IP Source 886 Address Spoofing", BCP 38, RFC 2827, May 2000. 888 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., 889 and M. Carney, "Dynamic Host Configuration Protocol for 890 IPv6 (DHCPv6)", RFC 3315, July 2003. 892 [RFC3484] Draves, R., "Default Address Selection for Internet 893 Protocol version 6 (IPv6)", RFC 3484, February 2003. 895 [RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed 896 Networks", BCP 84, RFC 3704, March 2004. 898 [RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support 899 in IPv6", RFC 3775, June 2004. 901 [RFC3963] Devarapalli, V., Wakikawa, R., Petrescu, A., and P. 902 Thubert, "Network Mobility (NEMO) Basic Support Protocol", 903 RFC 3963, January 2005. 905 [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure 906 Neighbor Discovery (SEND)", RFC 3971, March 2005. 908 [RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)", 909 RFC 3972, March 2005. 911 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 912 Architecture", RFC 4291, February 2006. 914 [RFC4423] Moskowitz, R. and P. Nikander, "Host Identity Protocol 915 (HIP) Architecture", RFC 4423, May 2006. 917 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 918 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 919 September 2007. 921 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 922 Address Autoconfiguration", RFC 4862, September 2007. 924 [RFC4960] Stewart, R., "Stream Control Transmission Protocol", 925 RFC 4960, September 2007. 927 [RFC5201] Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson, 928 "Host Identity Protocol", RFC 5201, April 2008. 930 [RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming 931 Shim Protocol for IPv6", RFC 5533, June 2009. 933 [RFC5534] Arkko, J. and I. van Beijnum, "Failure Detection and 934 Locator Pair Exploration Protocol for IPv6 Multihoming", 935 RFC 5534, June 2009. 937 [RFC5535] Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535, 938 June 2009. 940 12.2. Informative References 942 [I-D.ietf-csi-dhcpv6-cga-ps] 943 Jiang, S., "DHCPv6 and CGA Interaction: Problem 944 Statement", draft-ietf-csi-dhcpv6-cga-ps-04 (work in 945 progress), September 2010. 947 [I-D.nordmark-shim6-esd] 948 Nordmark, E., "Extended Shim6 Design for ID/loc split and 949 Traffic Engineering", draft-nordmark-shim6-esd-01 (work in 950 progress), February 2008. 952 [RFC3221] Huston, G., "Commentary on Inter-Domain Routing in the 953 Internet", RFC 3221, December 2001. 955 [RFC3582] Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site- 956 Multihoming Architectures", RFC 3582, August 2003. 958 [RFC4116] Abley, J., Lindqvist, K., Davies, E., Black, B., and V. 959 Gill, "IPv4 Multihoming Practices and Limitations", 960 RFC 4116, July 2005. 962 [RFC4218] Nordmark, E. and T. Li, "Threats Relating to IPv6 963 Multihoming Solutions", RFC 4218, October 2005. 965 [RFC4984] Meyer, D., Zhang, L., and K. Fall, "Report from the IAB 966 Workshop on Routing and Addressing", RFC 4984, 967 September 2007. 969 Authors' Addresses 971 Joe Abley 972 Afilias Canada, Inc. 973 Suite 204 974 4141 Yonge Street 975 Toronto, Ontario M2P 2A8 976 Canada 978 Phone: +1 416 673 4176 979 Email: jabley@ca.afilias.info 980 URI: http://afilias.info/ 982 Marcelo Bagnulo 983 U. Carlos III de Madrid 984 Av. Universidad 30 985 Leganes, Madrid 28911 986 Spain 988 Phone: +34 91 6248814 989 Email: marcelo@it.uc3m.es 990 URI: http://www.it.uc3m.es/ 992 Alberto Garcia Martinez 993 U. Carlos III de Madrid 994 Av. Universidad 30 995 Leganes, Madrid 28911 996 Spain 998 Phone: +34 91 6248782 999 Email: alberto@it.uc3m.es 1000 URI: http://www.it.uc3m.es/