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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Routing Working Group F. Baker 3 Internet-Draft 4 Intended status: Informational C. Bowers 5 Expires: January 3, 2018 Juniper Networks 6 J. Linkova 7 Google 8 July 2, 2017 10 Enterprise Multihoming using Provider-Assigned Addresses without Network 11 Prefix Translation: Requirements and Solution 12 draft-ietf-rtgwg-enterprise-pa-multihoming-01 14 Abstract 16 Connecting an enterprise site to multiple ISPs using provider- 17 assigned addresses is difficult without the use of some form of 18 Network Address Translation (NAT). Much has been written on this 19 topic over the last 10 to 15 years, but it still remains a problem 20 without a clearly defined or widely implemented solution. Any 21 multihoming solution without NAT requires hosts at the site to have 22 addresses from each ISP and to select the egress ISP by selecting a 23 source address for outgoing packets. It also requires routers at the 24 site to take into account those source addresses when forwarding 25 packets out towards the ISPs. 27 This document attempts to define a complete solution to this problem. 28 It covers the behavior of routers to forward traffic taking into 29 account source address, and it covers the behavior of host to select 30 appropriate source addresses. It also covers any possible role that 31 routers might play in providing information to hosts to help them 32 select appropriate source addresses. In the process of exploring 33 potential solutions, this documents also makes explicit requirements 34 for how the solution would be expected to behave from the perspective 35 of an enterprise site network administrator . 37 Status of This Memo 39 This Internet-Draft is submitted in full conformance with the 40 provisions of BCP 78 and BCP 79. 42 Internet-Drafts are working documents of the Internet Engineering 43 Task Force (IETF). Note that other groups may also distribute 44 working documents as Internet-Drafts. The list of current Internet- 45 Drafts is at http://datatracker.ietf.org/drafts/current/. 47 Internet-Drafts are draft documents valid for a maximum of six months 48 and may be updated, replaced, or obsoleted by other documents at any 49 time. It is inappropriate to use Internet-Drafts as reference 50 material or to cite them other than as "work in progress." 52 This Internet-Draft will expire on January 3, 2018. 54 Copyright Notice 56 Copyright (c) 2017 IETF Trust and the persons identified as the 57 document authors. All rights reserved. 59 This document is subject to BCP 78 and the IETF Trust's Legal 60 Provisions Relating to IETF Documents 61 (http://trustee.ietf.org/license-info) in effect on the date of 62 publication of this document. Please review these documents 63 carefully, as they describe your rights and restrictions with respect 64 to this document. Code Components extracted from this document must 65 include Simplified BSD License text as described in Section 4.e of 66 the Trust Legal Provisions and are provided without warranty as 67 described in the Simplified BSD License. 69 Table of Contents 71 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 72 2. Enterprise Multihoming Requirements . . . . . . . . . . . . . 6 73 2.1. Simple ISP Connectivity with Connected SERs . . . . . . . 6 74 2.2. Simple ISP Connectivity Where SERs Are Not Directly 75 Connected . . . . . . . . . . . . . . . . . . . . . . . . 7 76 2.3. Enterprise Network Operator Expectations . . . . . . . . 8 77 2.4. More complex ISP connectivity . . . . . . . . . . . . . . 11 78 2.5. ISPs and Provider-Assigned Prefixes . . . . . . . . . . . 13 79 2.6. Simplified Topologies . . . . . . . . . . . . . . . . . . 14 80 3. Generating Source-Prefix-Scoped Forwarding Tables . . . . . 14 81 4. Mechanisms For Hosts To Choose Good Source Addresses In A 82 Multihomed Site . . . . . . . . . . . . . . . . . . . . . . . 21 83 4.1. Source Address Selection Algorithm on Hosts . . . . . . . 23 84 4.2. Selecting Source Address When Both Uplinks Are Working . 26 85 4.2.1. Distributing Address Selection Policy Table with 86 DHCPv6 . . . . . . . . . . . . . . . . . . . . . . . 26 87 4.2.2. Controlling Source Address Selection With Router 88 Advertisements . . . . . . . . . . . . . . . . . . . 26 89 4.2.3. Controlling Source Address Selection With ICMPv6 . . 28 90 4.2.4. Summary of Methods For Controlling Source Address 91 Selection To Implement Routing Policy . . . . . . . . 30 92 4.3. Selecting Source Address When One Uplink Has Failed . . . 31 93 4.3.1. Controlling Source Address Selection With DHCPv6 . . 32 94 4.3.2. Controlling Source Address Selection With Router 95 Advertisements . . . . . . . . . . . . . . . . . . . 33 96 4.3.3. Controlling Source Address Selection With ICMPv6 . . 34 97 4.3.4. Summary Of Methods For Controlling Source Address 98 Selection On The Failure Of An Uplink . . . . . . . . 34 99 4.4. Selecting Source Address Upon Failed Uplink Recovery . . 35 100 4.4.1. Controlling Source Address Selection With DHCPv6 . . 35 101 4.4.2. Controlling Source Address Selection With Router 102 Advertisements . . . . . . . . . . . . . . . . . . . 35 103 4.4.3. Controlling Source Address Selection With ICMP . . . 36 104 4.4.4. Summary Of Methods For Controlling Source Address 105 Selection Upon Failed Uplink Recovery . . . . . . . . 36 106 4.5. Selecting Source Address When All Uplinks Failed . . . . 37 107 4.5.1. Controlling Source Address Selection With DHCPv6 . . 37 108 4.5.2. Controlling Source Address Selection With Router 109 Advertisements . . . . . . . . . . . . . . . . . . . 37 110 4.5.3. Controlling Source Address Selection With ICMPv6 . . 38 111 4.5.4. Summary Of Methods For Controlling Source Address 112 Selection When All Uplinks Failed . . . . . . . . . . 38 113 4.6. Summary Of Methods For Controlling Source Address 114 Selection . . . . . . . . . . . . . . . . . . . . . . . . 38 115 4.7. Other Configuration Parameters . . . . . . . . . . . . . 40 116 4.7.1. DNS Configuration . . . . . . . . . . . . . . . . . . 40 117 5. Other Solutions . . . . . . . . . . . . . . . . . . . . . . . 41 118 5.1. Shim6 . . . . . . . . . . . . . . . . . . . . . . . . . . 41 119 5.2. IPv6-to-IPv6 Network Prefix Translation . . . . . . . . . 42 120 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42 121 7. Security Considerations . . . . . . . . . . . . . . . . . . . 42 122 7.1. Privacy Considerations . . . . . . . . . . . . . . . . . 42 123 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 42 124 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 42 125 9.1. Normative References . . . . . . . . . . . . . . . . . . 42 126 9.2. Informative References . . . . . . . . . . . . . . . . . 44 127 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 47 128 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 47 130 1. Introduction 132 Site multihoming, the connection of a subscriber network to multiple 133 upstream networks using redundant uplinks, is a common enterprise 134 architecture for improving the reliability of its Internet 135 connectivity. If the site uses provider-independent (PI) addresses, 136 all traffic originating from the enterprise can use source addresses 137 from the PI address space. Site multihoming with PI addresses is 138 commonly used with both IPv4 and IPv6, and does not present any new 139 technical challenges. 141 It may be desirable for an enterprise site to connect to multiple 142 ISPs using provider-assigned (PA) addresses, instead of PI addresses. 143 Multihoming with provider-assigned addresses is typically less 144 expensive for the enterprise relative to using provider-independent 145 addresses. PA multihoming is also a practice that should be 146 facilitated and encouraged because it does not add to the size of the 147 Internet routing table, whereas PI multihoming does. Note that PA is 148 also used to mean "provider-aggregatable". In this document we 149 assume that provider-assigned addresses are always provider- 150 aggregatable. 152 With PA multihoming, for each ISP connection, the site is assigned a 153 prefix from within an address block allocated to that ISP by its 154 National or Regional Internet Registry. In the simple case of two 155 ISPs (ISP-A and ISP-B), the site will have two different prefixes 156 assigned to it (prefix-A and prefix-B). This arrangement is 157 problematic. First, packets with the "wrong" source address may be 158 dropped by one of the ISPs. In order to limit denial of service 159 attacks using spoofed source addresses, BCP38 [RFC2827] recommends 160 that ISPs filter traffic from customer sites to only allow traffic 161 with a source address that has been assigned by that ISP. So a 162 packet sent from a multihomed site on the uplink to ISP-B with a 163 source address in prefix-A may be dropped by ISP-B. 165 However, even if ISP-B does not implement BCP38 or ISP-B adds 166 prefix-A to its list of allowed source addresses on the uplink from 167 the multihomed site, two-way communication may still fail. If the 168 packet with source address in prefix-A was sent to ISP-B because the 169 uplink to ISP-A failed, then if ISP-B does not drop the packet and 170 the packet reaches its destination somewhere on the Internet, the 171 return packet will be sent back with a destination address in prefix- 172 A. The return packet will be routed over the Internet to ISP-A, but 173 it will not be delivered to the multihomed site because its link with 174 ISP-A has failed. Two-way communication would require some 175 arrangement for ISP-B to advertise prefix-A when the uplink to ISP-A 176 fails. 178 Note that the same may be true with a provider that does not 179 implement BCP 38, if his upstream provider does, or has no 180 corresponding route. The issue is not that the immediate provider 181 implements ingress filtering; it is that someone upstream does, or 182 lacks a route. 184 With IPv4, this problem is commonly solved by using [RFC1918] private 185 address space within the multi-homed site and Network Address 186 Translation (NAT) or Network Address/Port Translation (NAPT) on the 187 uplinks to the ISPs. However, one of the goals of IPv6 is to 188 eliminate the need for and the use of NAT or NAPT. Therefore, 189 requiring the use of NAT or NAPT for an enterprise site to multihome 190 with provider-assigned addresses is not an attractive solution. 192 [RFC6296] describes a translation solution specifically tailored to 193 meet the requirements of multi-homing with provider-assigned IPv6 194 addresses. With the IPv6-to-IPv6 Network Prefix Translation (NPTv6) 195 solution, within the site an enterprise can use Unique Local 196 Addresses [RFC4193] or the prefix assigned by one of the ISPs. As 197 traffic leaves the site on an uplink to an ISP, the source address 198 gets translated to an address within the prefix assigned by the ISP 199 on that uplink in a predictable and reversible manner. [RFC6296] is 200 currently classified as Experimental, and it has been implemented by 201 several vendors. See Section 5.2, for more discussion of NPTv6. 203 This document defines routing requirements for enterprise multihoming 204 using provider-assigned IPv6 addresses. We have made no attempt to 205 write these requirements in a manner that is agnostic to potential 206 solutions. Instead, this document focuses on the following general 207 class of solutions. 209 Each host at the enterprise has multiple addresses, at least one from 210 each ISP-assigned prefix. Each host, as discussed in Section 4.1 and 211 [RFC6724], is responsible for choosing the source address applied to 212 each packet it sends. A host SHOULD be able respond dynamically to 213 the failure of an uplink to a given ISP by no longer sending packets 214 with the source address corresponding to that ISP. Potential 215 mechanisms for the communication of changes in the network to the 216 host are Neighbor Discovery Router Advertisements, DHCPv6, and 217 ICMPv6. 219 The routers in the enterprise network are responsible for ensuring 220 that packets are delivered to the "correct" ISP uplink based on 221 source address. This requires that at least some routers in the site 222 network are able to take into account the source address of a packet 223 when deciding how to route it. That is, some routers must be capable 224 of some form of Source Address Dependent Routing (SADR), if only as 225 described in [RFC3704]. At a minimum, the routers connected to the 226 ISP uplinks (the site exit routers or SERs) must be capable of Source 227 Address Dependent Routing. Expanding the connected domain of routers 228 capable of SADR from the site exit routers deeper into the site 229 network will generally result in more efficient routing of traffic 230 with external destinations. 232 The document first looks in more detail at the enterprise networking 233 environments in which this solution is expected to operate. It then 234 discusses existing and proposed mechanisms for hosts to select the 235 source address applied to packets. Finally, it looks at the 236 requirements for routing that are needed to support these enterprise 237 network scenarios and the mechanisms by which hosts are expected to 238 select source addresses dynamically based on network state. 240 2. Enterprise Multihoming Requirements 242 2.1. Simple ISP Connectivity with Connected SERs 244 We start by looking at a scenario in which a site has connections to 245 two ISPs, as shown in Figure 1. The site is assigned the prefix 246 2001:db8:0:a000::/52 by ISP-A and prefix 2001:db8:0:b000::/52 by ISP- 247 B. We consider three hosts in the site. H31 and H32 are on a LAN 248 that has been assigned subnets 2001:db8:0:a010::/64 and 249 2001:db8:0:b010::/64. H31 has been assigned the addresses 250 2001:db8:0:a010::31 and 2001:db8:0:b010::31. H32 has been assigned 251 2001:db8:0:a010::32 and 2001:db8:0:b010::32. H41 is on a different 252 subnet that has been assigned 2001:db8:0:a020::/64 and 253 2001:db8:0:b020::/64. 255 2001:db8:0:1234::101 H101 256 | 257 | 258 2001:db8:0:a010::31 -------- 259 2001:db8:0:b010::31 ,-----. / \ 260 +--+ +--+ +----+ ,' `. : : 261 +---|R1|---|R4|---+---|SERa|-+ ISP-A +--+-- : 262 H31--+ +--+ +--+ | +----+ `. ,' : : 263 | | `-----' : Internet : 264 | | : : 265 | | : : 266 | | : : 267 | | ,-----. : : 268 H32--+ +--+ | +----+ ,' `. : : 269 +---|R2|----------+---|SERb|-+ ISP-B +--+-- : 270 +--+ | +----+ `. ,' : : 271 | `-----' : : 272 | : : 273 +--+ +--+ +--+ \ / 274 H41------|R3|--|R5|--|R6| -------- 275 +--+ +--+ +--+ 277 2001:db8:0:a020::41 278 2001:db8:0:b020::41 280 Figure 1: Simple ISP Connectivity With Connected SERs 282 We refer to a router that connects the site to an ISP as a site edge 283 router(SER). Several other routers provide connectivity among the 284 internal hosts (H31, H32, and H41), as well as connecting the 285 internal hosts to the Internet through SERa and SERb. In this 286 example SERa and SERb share a direct connection to each other. In 287 Section 2.2, we consider a scenario where this is not the case. 289 For the moment, we assume that the hosts are able to make good 290 choices about which source addresses through some mechanism that 291 doesn't involve the routers in the site network. Here, we focus on 292 primary task of the routed site network, which is to get packets 293 efficiently to their destinations, while sending a packet to the ISP 294 that assigned the prefix that matches the source address of the 295 packet. In Section 4, we examine what role the routed network may 296 play in helping hosts make good choices about source addresses for 297 packets. 299 With this solution, routers will need form of Source Address 300 Dependent Routing, which will be new functionality. It would be 301 useful if an enterprise site does not need to upgrade all routers to 302 support the new SADR functionality in order to support PA multi- 303 homing. We consider if this is possible and what are the tradeoffs 304 of not having all routers in the site support SADR functionality. 306 In the topology in Figure 1, it is possible to support PA multihoming 307 with only SERa and SERb being capable of SADR. The other routers can 308 continue to forward based only on destination address, and exchange 309 routes that only consider destination address. In this scenario, 310 SERa and SERb communicate source-scoped routing information across 311 their shared connection. When SERa receives a packet with a source 312 address matching prefix 2001:db8:0:b000::/52 , it forwards the packet 313 to SERb, which forwards it on the uplink to ISP-B. The analogous 314 behaviour holds for traffic that SERb receives with a source address 315 matching prefix 2001:db8:0:a000::/52. 317 In Figure 1, when only SERa and SERb are capable of source address 318 dependent routing, PA multi-homing will work. However, the paths 319 over which the packets are sent will generally not be the shortest 320 paths. The forwarding paths will generally be more efficient as more 321 routers are capable of SADR. For example, if R4, R2, and R6 are 322 upgraded to support SADR, then can exchange source-scoped routes with 323 SERa and SERb. They will then know to send traffic with a source 324 address matching prefix 2001:db8:0:b000::/52 directly to SERb, 325 without sending it to SERa first. 327 2.2. Simple ISP Connectivity Where SERs Are Not Directly Connected 329 In Figure 2, we modify the topology slightly by inserting R7, so that 330 SERa and SERb are no longer directly connected. With this topology, 331 it is not enough to just enable SADR routing on SERa and SERb to 332 support PA multi-homing. There are two solutions to ways to enable 333 PA multihoming in this topology. 335 2001:db8:0:1234::101 H101 336 | 337 | 338 2001:db8:0:a010::31 -------- 339 2001:db8:0:b010::31 ,-----. / \ 340 +--+ +--+ +----+ ,' `. : : 341 +---|R1|---|R4|---+---|SERa|-+ ISP-A +--+-- : 342 H31--+ +--+ +--+ | +----+ `. ,' : : 343 | | `-----' : Internet : 344 | +--+ : : 345 | |R7| : : 346 | +--+ : : 347 | | ,-----. : : 348 H32--+ +--+ | +----+ ,' `. : : 349 +---|R2|----------+---|SERb|-+ ISP-B +--+-- : 350 +--+ | +----+ `. ,' : : 351 | `-----' : : 352 | : : 353 +--+ +--+ +--+ \ / 354 H41------|R3|--|R5|--|R6| -------- 355 +--+ +--+ +--+ | 356 | 357 2001:db8:0:a020::41 2001:db8:0:5678::501 H501 358 2001:db8:0:b020::41 360 Figure 2: Simple ISP Connectivity Where SERs Are Not Directly 361 Connected 363 One option is to effectively modify the topology by creating a 364 logical tunnel between SERa and SERb, using GRE for example. 365 Although SERa and SERb are not directly connected physically in this 366 topology, they can be directly connected logically by a tunnel. 368 The other option is to enable SADR functionality on R7. In this way, 369 R7 will exchange source-scoped routes with SERa and SERb, making the 370 three routers act as a single SADR domain. This illustrates the 371 basic principle that the minimum requirement for the routed site 372 network to support PA multi-homing is having all of the site exit 373 routers be part of a connected SADR domain. Extending the connected 374 SADR domain beyond that point can produce more efficient forwarding 375 paths. 377 2.3. Enterprise Network Operator Expectations 379 Before considering a more complex scenario, let's look in more detail 380 at the reasonably simple multihoming scenario in Figure 2 to 381 understand what can reasonably be expected from this solution. As a 382 general guiding principle, we assume an enterprise network operator 383 will expect a multihomed network to behave as close as to a single- 384 homed network as possible. So a solution that meets those 385 expectations where possible is a good thing. 387 For traffic between internal hosts and traffic from outside the site 388 to internal hosts, an enterprise network operator would expect there 389 to be no visible change in the path taken by this traffic, since this 390 traffic does not need to be routed in a way that depends on source 391 address. It is also reasonable to expect that internal hosts should 392 be able to communicate with each other using either of their source 393 addresses without restriction. For example, H31 should be able to 394 communicate with H41 using a packet with S=2001:db8:0:a010::31, 395 D=2001:db8:0:b010::41, regardless of the state of uplink to ISP-B. 397 These goals can be accomplished by having all of the routers in the 398 network continue to originate normal unscoped destination routes for 399 their connected networks. If we can arrange so that these unscoped 400 destination routes get used for forwarding this traffic, then we will 401 have accomplished the goal of keeping forwarding of traffic destined 402 for internal hosts, unaffected by the multihoming solution. 404 For traffic destined for external hosts, it is reasonable to expect 405 that traffic with an source address from the prefix assigned by ISP-A 406 to follow the path to that the traffic would follow if there is no 407 connection to ISP-B. This can be accomplished by having SERa 408 originate a source-scoped route of the form (S=2001:db8:0:a000::/52, 409 D=::/0) . If all of the routers in the site support SADR, then the 410 path of traffic exiting via ISP-A can match that expectation. If 411 some routers don't support SADR, then it is reasonable to expect that 412 the path for traffic exiting via ISP-A may be different within the 413 site. This is a tradeoff that the enterprise network operator may 414 decide to make. 416 It is important to understand how this multihoming solution behaves 417 when an uplink to one of the ISPs fails. To simplify this 418 discussion, we assume that all routers in the site support SADR. We 419 first start by looking at how the network operates when the uplinks 420 to both ISP-A and ISP-B are functioning properly. SERa originates a 421 source-scoped route of the form (S=2001:db8:0:a000::/52, D=::/0), and 422 SERb is originates a source-scoped route of the form 423 (S=2001:db8:0:b000::/52, D=::/0). These routes are distributed 424 through the routers in the site, and they establish within the 425 routers two set of forwarding paths for traffic leaving the site. 426 One set of forwarding paths is for packets with source address in 427 2001:db8:0:a000::/52. The other set of forwarding paths is for 428 packets with source address in 2001:db8:0:b000::/52. The normal 429 destination routes which are not scoped to these two source prefixes 430 play no role in the forwarding. Whether a packet exits the site via 431 SERa or via SERb is completely determined by the source address 432 applied to the packet by the host. So for example, when host H31 433 sends a packet to host H101 with (S=2001:db8:0:a010::31, 434 D=2001:db8:0:1234::101), the packet will only be sent out the link 435 from SERa to ISP-A. 437 Now consider what happens when the uplink from SERa to ISP-A fails. 438 The only way for the packets from H31 to reach H101 is for H31 to 439 start using the source address for ISP-B. H31 needs to send the 440 following packet: (S=2001:db8:0:b010::31, D=2001:db8:0:1234::101). 442 This behavior is very different from the behavior that occurs with 443 site multihoming using PI addresses or with PA addresses using NAT. 444 In these other multi-homing solutions, hosts do not need to react to 445 network failures several hops away in order to regain Internet 446 access. Instead, a host can be largely unaware of the failure of an 447 uplink to an ISP. When multihoming with PA addresses and NAT, 448 existing sessions generally need to be re-established after a failure 449 since the external host will receive packets from the internal host 450 with a new source address. However, new sessions can be established 451 without any action on the part of the hosts. 453 Another example where the behavior of this multihoming solution 454 differs significantly from that of multihoming with PI address or 455 with PA addresses using NAT is in the ability of the enterprise 456 network operator to route traffic over different ISPs based on 457 destination address. We still consider the fairly simple network of 458 Figure 2 and assume that uplinks to both ISPs are functioning. 459 Assume that the site is multihomed using PA addresses and NAT, and 460 that SERa and SERb each originate a normal destination route for 461 D=::/0, with the route origination dependent on the state of the 462 uplink to the respective ISP. 464 Now suppose it is observed that an important application running 465 between internal hosts and external host H101 experience much better 466 performance when the traffic passes through ISP-A (perhaps because 467 ISP-A provides lower latency to H101.) When multihoming this site 468 with PI addresses or with PA addresses and NAT, the enterprise 469 network operator can configure SERa to originate into the site 470 network a normal destination route for D=2001:db8:0:1234::/64 (the 471 destination prefix to reach H101) that depends on the state of the 472 uplink to ISP-A. When the link to ISP-A is functioning, the 473 destination route D=2001:db8:0:1234::/64 will be originated by SERa, 474 so traffic from all hosts will use ISP-A to reach H101 based on the 475 longest destination prefix match in the route lookup. 477 Implementing the same routing policy is more difficult with the PA 478 multihoming solution described in this document since it doesn't use 479 NAT. By design, the only way to control where a packet exits this 480 network is by setting the source address of the packet. Since the 481 network cannot modify the source address without NAT, the host must 482 set it. To implement this routing policy, each host needs to use the 483 source address from the prefix assigned by ISP-A to send traffic 484 destined for H101. Mechanisms have been proposed to allow hosts to 485 choose the source address for packets in a fine grained manner. We 486 will discuss these proposals in Section 4. However, interacting with 487 host operating systems in some manner to ensure a particular source 488 address is chosen for a particular destination prefix is not what an 489 enterprise network administrator would expect to have to do to 490 implement this routing policy. 492 2.4. More complex ISP connectivity 494 The previous sections considered two variations of a simple 495 multihoming scenario where the site is connected to two ISPs offering 496 only Internet connectivity. It is likely that many actual enterprise 497 multihoming scenarios will be similar to this simple example. 498 However, there are more complex multihoming scenarios that we would 499 like this solution to address as well. 501 It is fairly common for an ISP to offer a service in addition to 502 Internet access over the same uplink. Two variation of this are 503 reflected in Figure 3. In addition to Internet access, ISP-A offers 504 a service which requires the site to access host H51 at 505 2001:db8:0:5555::51. The site has a single physical and logical 506 connection with ISP-A, and ISP-A only allows access to H51 over that 507 connection. So when H32 needs to access the service at H51 it needs 508 to send packets with (S=2001:db8:0:a010::32, D=2001:db8:0:5555::51) 509 and those packets need to be forward out the link from SERa to ISP-A. 511 2001:db8:0:1234::101 H101 512 | 513 | 514 2001:db8:0:a010::31 -------- 515 2001:db8:0:b010::31 ,-----. / \ 516 +--+ +--+ +----+ ,' `. : : 517 +---|R1|---|R4|---+---|SERa|-+ ISP-A +--+-- : 518 H31--+ +--+ +--+ | +----+ `. ,' : : 519 | | `-----' : Internet : 520 | | | : : 521 | | H51 : : 522 | | 2001:db8:0:5555::51 : : 523 | +--+ : : 524 | |R7| : : 525 | +--+ : : 526 | | : : 527 | | ,-----. : : 528 H32--+ +--+ | +-----+ ,' `. : : 529 +---|R2|-----+----+--|SERb1|-+ ISP-B +--+-- : 530 +--+ | +-----+ `. ,' : : 531 +--+ `--|--' : : 532 2001:db8:0:a010::32 |R8| | \ / 533 +--+ ,--|--. -------- 534 | +-----+ ,' `. | 535 +-------|SERb2|-+ ISP-B | | 536 | +-----+ `. ,' H501 537 | `-----' 2001:db8:0:5678 538 | | ::501 539 +--+ +--+ H61 540 H41------|R3|--|R5| 2001:db8:0:6666::61 541 +--+ +--+ 543 2001:db8:0:a020::41 544 2001:db8:0:b020::41 546 Figure 3: Internet access and services offered by ISP-A and ISP-B 548 ISP-B illustrates a variation on this scenario. In addition to 549 Internet access, ISP-B also offers a service which requires the site 550 to access host H61. The site has two connections to two different 551 parts of ISP-B (shown as SERb1 and SERb2 in Figure 3). ISP-B expects 552 Internet traffic to use the uplink from SERb1, while it expects it 553 expects traffic destined for the service at H61 to use the uplink 554 from SERb2. For either uplink, ISP-B expects the ingress traffic to 555 have a source address matching the prefix it assigned to the site, 556 2001:db8:0:b000::/52. 558 As discussed before, we rely completely on the internal host to set 559 the source address of the packet properly. In the case of a packet 560 sent by H31 to access the service in ISP-B at H61, we expect the 561 packet to have the following addresses: (S=2001:db8:0:b010::31, 562 D=2001:db8:0:6666::61). The routed network has two potential ways of 563 distributing routes so that this packet exits the site on the uplink 564 at SERb2. 566 We could just rely on normal destination routes, without using 567 source-prefix scoped routes. If we have SERb2 originate a normal 568 unscoped destination route for D=2001:db8:0:6666::/64, the packets 569 from H31 to H61 will exit the site at SERb2 as desired. We should 570 not have to worry about SERa needing to originate the same route, 571 because ISP-B should choose a globally unique prefix for the service 572 at H61. 574 The alternative is to have SERb2 originate a source-prefix-scoped 575 destination route of the form (S=2001:db8:0:b000::/52, 576 D=2001:db8:0:6666::/64). From a forwarding point of view, the use of 577 the source-prefix-scoped destination route would result in traffic 578 with source addresses corresponding only to ISP-B being sent to 579 SERb2. Instead, the use of the unscoped destination route would 580 result in traffic with source addresses corresponding to ISP-A and 581 ISP-B being sent to SERb2, as long as the destination address matches 582 the destination prefix. It seems like either forwarding behavior 583 would be acceptable. 585 However, from the point of view of the enterprise network 586 administrator trying to configure, maintain, and trouble-shoot this 587 multihoming solution, it seems much clearer to have SERb2 originate 588 the source-prefix-scoped destination route correspond to the service 589 offered by ISP-B. In this way, all of the traffic leaving the site 590 is determined by the source-prefix-scoped routes, and all of the 591 traffic within the site or arriving from external hosts is determined 592 by the unscoped destination routes. Therefore, for this multihoming 593 solution we choose to originate source-prefix-scoped routes for all 594 traffic leaving the site. 596 2.5. ISPs and Provider-Assigned Prefixes 598 While we expect that most site multihoming involves connecting to 599 only two ISPs, this solution allows for connections to an arbitrary 600 number of ISPs to be supported. However, when evaluating scalable 601 implementations of the solution, it would be reasonable to assume 602 that the maximum number of ISPs that a site would connect to is five. 604 It is also useful to note that the prefixes assigned to the site by 605 different ISPs will not overlap. This must be the case , since the 606 provider-assigned addresses have to be globally unique. 608 2.6. Simplified Topologies 610 The topologies of many enterprise sites using this multihoming 611 solution may in practice be simpler than the examples that we have 612 used. The topology in Figure 1 could be further simplified by having 613 all hosts directly connected to the LAN connecting the two site exit 614 routers, SERa and SERb. The topology could also be simplified by 615 having the uplinks to ISP-A and ISP-B both connected to the same site 616 exit router. However, it is the aim of this draft to provide a 617 solution that applies to a broad a range of enterprise site network 618 topologies, so this draft focuses on providing a solution to the more 619 general case. The simplified cases will also be supported by this 620 solution, and there may even be optimizations that can be made for 621 simplified cases. This solution however needs to support more 622 complex topologies. 624 We are starting with the basic assumption that enterprise site 625 networks can be quite complex from a routing perspective. However, 626 even a complex site network can be multihomed to different ISPs with 627 PA addresses using IPv4 and NAT. It is not reasonable to expect an 628 enterprise network operator to change the routing topology of the 629 site in order to deploy IPv6. 631 3. Generating Source-Prefix-Scoped Forwarding Tables 633 So far we have described in general terms how the routers in this 634 solution that are capable of Source Address Dependent Routing will 635 forward traffic using both normal unscoped destination routes and 636 source-prefix-scoped destination routes. Here we give a precise 637 method for generating a source-prefix-scoped forwarding table on a 638 router that supports SADR. 640 1. Compute the next-hops for the source-prefix-scoped destination 641 prefixes using only routers in the connected SADR domain. These 642 are the initial source-prefix-scoped forwarding table entries. 644 2. Compute the next-hops for the unscoped destination prefixes using 645 all routers in the IGP. This is the unscoped forwarding table. 647 3. Augment each source-prefix-scoped forwarding table with unscoped 648 forwarding table entries based on the following rule. If the 649 destination prefix of the unscoped forwarding entry exactly 650 matches the destination prefix of an existing source-prefix- 651 scoped forwarding entry (including destination prefix length), 652 then do not add the unscoped forwarding entry. If the 653 destination prefix does NOT match an existing entry, then add the 654 entry to the source-prefix-scoped forwarding table. 656 The forward tables produced by this process are used in the following 657 way to forward packets. 659 1. If the source address of the packet matches one of the source 660 prefixes, then look up the destination address of the packet in 661 the corresponding source-prefix-scoped forwarding table to 662 determine the next-hop for the packet. 664 2. If the source address of the packet does NOT match one of the 665 source prefixes, then look up the destination address of the 666 packet in unscoped forwarding table to determine the next-hop for 667 the packet. 669 The following example illustrates how this process is used to create 670 a forwarding table for each provider-assigned source prefix. We 671 consider the multihomed site network in Figure 3. Initially we 672 assume that all of the routers in the site network support SADR. 673 Figure 4 shows the routes that are originated by the routers in the 674 site network. 676 Routes originated by SERa: 677 (S=2001:db8:0:a000::/52, D=2001:db8:0:5555/64) 678 (S=2001:db8:0:a000::/52, D=::/0) 679 (D=2001:db8:0:5555::/64) 680 (D=::/0) 682 Routes originated by SERb1: 683 (S=2001:db8:0:b000::/52, D=::/0) 684 (D=::/0) 686 Routes originated by SERb2: 687 (S=2001:db8:0:b000::/52, D=2001:db8:0:6666::/64) 688 (D=2001:db8:0:6666::/64) 690 Routes originated by R1: 691 (D=2001:db8:0:a010::/64) 692 (D=2001:db8:0:b010::/64) 694 Routes originated by R2: 695 (D=2001:db8:0:a010::/64) 696 (D=2001:db8:0:b010::/64) 698 Routes originated by R3: 699 (D=2001:db8:0:a020::/64) 700 (D=2001:db8:0:b020::/64) 702 Figure 4: Routes Originated by Routers in the Site Network 704 Each SER originates destination routes which are scoped to the source 705 prefix assigned by the ISP that the SER connects to. Note that the 706 SERs also originate the corresponding unscoped destination route. 707 This is not needed when all of the routers in the site support SADR. 708 However, it is required when some routers do not support SADR. This 709 will be discussed in more detail later. 711 We focus on how R8 constructs its source-prefix-scoped forwarding 712 tables from these route advertisements. R8 computes the next hops 713 for destination routes which are scoped to the source prefix 714 2001:db8:0:a000::/52. The results are shown in the first table in 715 Figure 5. (In this example, the next hops are computed assuming that 716 all links have the same metric.) Then, R8 computes the next hops for 717 destination routes which are scoped to the source prefix 718 2001:db8:0:b000::/52. The results are shown in the second table in 719 Figure 5 . Finally, R8 computes the next hops for the unscoped 720 destination prefixes. The results are shown in the third table in 721 Figure 5. 723 forwarding entries scoped to 724 source prefix = 2001:db8:0:a000::/52 725 ============================================ 726 D=2001:db8:0:5555/64 NH=R7 727 D=::/0 NH=R7 729 forwarding entries scoped to 730 source prefix = 2001:db8:0:b000::/52 731 ============================================ 732 D=2001:db8:0:6666/64 NH=SERb2 733 D=::/0 NH=SERb1 735 unscoped forwarding entries 736 ============================================ 737 D=2001:db8:0:a010::/64 NH=R2 738 D=2001:db8:0:b010::/64 NH=R2 739 D=2001:db8:0:a020::/64 NH=R5 740 D=2001:db8:0:b020::/64 NH=R5 741 D=2001:db8:0:5555::/64 NH=R7 742 D=2001:db8:0:6666::/64 NH=SERb2 743 D=::/0 NH=SERb1 745 Figure 5: Forwarding Entries Computed at R8 747 The final step is for R8 to augment the source-prefix-scoped 748 forwarding entries with unscoped forwarding entries. If an unscoped 749 forwarding entry has the exact same destination prefix as an source- 750 prefix-scoped forwarding entry (including destination prefix length), 751 then the source-prefix-scoped forwarding entry wins. 753 As as an example of how the source scoped forwarding entries are 754 augmented with unscoped forwarding entries, we consider how the two 755 entries in the first table in Figure 5 (the table for source prefix = 756 2001:db8:0:a000::/52) are augmented with entries from the third table 757 in Figure 5 (the table of unscoped forwarding entries). The first 758 four unscoped forwarding entries (D=2001:db8:0:a010::/64, 759 D=2001:db8:0:b010::/64, D=2001:db8:0:a020::/64, and 760 D=2001:db8:0:b020::/64) are not an exact match for any of the 761 existing entries in the forwarding table for source prefix 762 2001:db8:0:a000::/52. Therefore, these four entries are added to the 763 final forwarding table for source prefix 2001:db8:0:a000::/52. The 764 result of adding these entries is reflected in first four entries the 765 first table in Figure 6. 767 The next unscoped forwarding table entry is for 768 D=2001:db8:0:5555::/64. This entry is an exact match for the 769 existing entry in the forwarding table for source prefix 770 2001:db8:0:a000::/52. Therefore, we do not replace the existing 771 entry with the entry from the unscoped forwarding table. This is 772 reflected in the fifth entry in the first table in Figure 6. (Note 773 that since both scoped and unscoped entries have R7 as the next hop, 774 the result of applying this rule is not visible.) 776 The next unscoped forwarding table entry is for 777 D=2001:db8:0:6666::/64. This entry is not an exact match for any 778 existing entries in the forwarding table for source prefix 779 2001:db8:0:a000::/52. Therefore, we add this entry. This is 780 reflected in the sixth entry in the first table in Figure 6. 782 The next unscoped forwarding table entry is for D=::/0. This entry 783 is an exact match for the existing entry in the forwarding table for 784 source prefix 2001:db8:0:a000::/52. Therefore, we do not overwrite 785 the existing source-prefix-scoped entry, as can be seen in the last 786 entry in the first table in Figure 6. 788 if source address matches 2001:db8:0:a000::/52 789 then use this forwarding table 790 ============================================ 791 D=2001:db8:0:a010::/64 NH=R2 792 D=2001:db8:0:b010::/64 NH=R2 793 D=2001:db8:0:a020::/64 NH=R5 794 D=2001:db8:0:b020::/64 NH=R5 795 D=2001:db8:0:5555::/64 NH=R7 796 D=2001:db8:0:6666::/64 NH=SERb2 797 D=::/0 NH=R7 799 else if source address matches 2001:db8:0:b000::/52 800 then use this forwarding table 801 ============================================ 802 D=2001:db8:0:a010::/64 NH=R2 803 D=2001:db8:0:b010::/64 NH=R2 804 D=2001:db8:0:a020::/64 NH=R5 805 D=2001:db8:0:b020::/64 NH=R5 806 D=2001:db8:0:5555::/64 NH=R7 807 D=2001:db8:0:6666::/64 NH=SERb2 808 D=::/0 NH=SERb1 810 else use this forwarding table 811 ============================================ 812 D=2001:db8:0:a010::/64 NH=R2 813 D=2001:db8:0:b010::/64 NH=R2 814 D=2001:db8:0:a020::/64 NH=R5 815 D=2001:db8:0:b020::/64 NH=R5 816 D=2001:db8:0:5555::/64 NH=R7 817 D=2001:db8:0:6666::/64 NH=SERb2 818 D=::/0 NH=SERb1 820 Figure 6: Complete Forwarding Tables Computed at R8 822 The forwarding tables produced by this process at R8 have the desired 823 properties. A packet with a source address in 2001:db8:0:a000::/52 824 will be forwarded based on the first table in Figure 6. If the 825 packet is destined for the Internet at large or the service at 826 D=2001:db8:0:5555/64, it will be sent to R7 in the direction of SERa. 827 If the packet is destined for an internal host, then the first four 828 entries will send it to R2 or R5 as expected. Note that if this 829 packet has a destination address corresponding to the service offered 830 by ISP-B (D=2001:db8:0:5555::/64), then it will get forwarded to 831 SERb2. It will be dropped by SERb2 or by ISP-B, since it the packet 832 has a source address that was not assigned by ISP-B. However, this 833 is expected behavior. In order to use the service offered by ISP-B, 834 the host needs to originate the packet with a source address assigned 835 by ISP-B. 837 In this example, a packet with a source address that doesn't match 838 2001:db8:0:a000::/52 or 2001:db8:0:b000::/52 must have originated 839 from an external host. Such a packet will use the unscoped 840 forwarding table (the last table in Figure 6). These packets will 841 flow exactly as they would in absence of multihoming. 843 We can also modify this example to illustrate how it supports 844 deployments where not all routers in the site support SADR. 845 Continuing with the topology shown in Figure 3, suppose that R3 and 846 R5 do not support SADR. Instead they are only capable of 847 understanding unscoped route advertisements. The SADR routers in the 848 network will still originate the routes shown in Figure 4. However, 849 R3 and R5 will only understand the unscoped routes as shown in 850 Figure 7. 852 Routes originated by SERa: 853 (D=2001:db8:0:5555::/64) 854 (D=::/0) 856 Routes originated by SERb1: 857 (D=::/0) 859 Routes originated by SERb2: 860 (D=2001:db8:0:6666::/64) 862 Routes originated by R1: 863 (D=2001:db8:0:a010::/64) 864 (D=2001:db8:0:b010::/64) 866 Routes originated by R2: 867 (D=2001:db8:0:a010::/64) 868 (D=2001:db8:0:b010::/64) 870 Routes originated by R3: 871 (D=2001:db8:0:a020::/64) 872 (D=2001:db8:0:b020::/64) 874 Figure 7: Routes Advertisements Understood by Routers that do no 875 Support SADR 877 With these unscoped route advertisements, R5 will produce the 878 forwarding table shown in Figure 8. 880 forwarding table 881 ============================================ 882 D=2001:db8:0:a010::/64 NH=R8 883 D=2001:db8:0:b010::/64 NH=R8 884 D=2001:db8:0:a020::/64 NH=R3 885 D=2001:db8:0:b020::/64 NH=R3 886 D=2001:db8:0:5555::/64 NH=R8 887 D=2001:db8:0:6666::/64 NH=SERb2 888 D=::/0 NH=R8 890 Figure 8: Forwarding Table For R5, Which Doesn't Understand Source- 891 Prefix-Scoped Routes 893 Any traffic that needs to exit the site will eventually hit a SADR- 894 capable router. Once that traffic enters the SADR-capable domain, 895 then it will not leave that domain until it exits the site. This 896 property is required in order to guarantee that there will not be 897 routing loops involving SADR-capable and non-SADR-capable routers. 899 Note that the mechanism described here for converting source-prefix- 900 scoped destination prefix routing advertisements into forwarding 901 state is somewhat different from that proposed in 902 [I-D.ietf-rtgwg-dst-src-routing]. The method described in this 903 document is intended to be easy to understand for network enterprise 904 operators while at the same time being functionally correct. Another 905 difference is that the method in this document assumes that source 906 prefix will not overlap. Other differences between the two 907 approaches still need to be understood and reconciled. 909 An interesting side-effect of deploying SADR is if all routers in a 910 given network support SADR and have a scoped forwarding table, then 911 the unscoped forwarding table can be eliminated which ensures that 912 packets with legitimate source addresses only can leave the network 913 (as there are no scoped forwarding tables for spoofed/bogon source 914 addresses). It would prevent accidental leaks of ULA/reserved/link- 915 local sources to the Internet as well as ensures that no spoofing is 916 possible from the SADR-enabled network. 918 4. Mechanisms For Hosts To Choose Good Source Addresses In A Multihomed 919 Site 921 Until this point, we have made the assumption that hosts are able to 922 choose the correct source address using some unspecified mechanism. 923 This has allowed us to just focus on what the routers in a multihomed 924 site network need to do in order to forward packets to the correct 925 ISP based on source address. Now we look at possible mechanisms for 926 hosts to choose the correct source address. We also look at what 927 role, if any, the routers may play in providing information that 928 helps hosts to choose source addresses. 930 Any host that needs to be able to send traffic using the uplinks to a 931 given ISP is expected to be configured with an address from the 932 prefix assigned by that ISP. The host will control which ISP is used 933 for its traffic by selecting one of the addresses configured on the 934 host as the source address for outgoing traffic. It is the 935 responsibility of the site network to ensure that a packet with the 936 source address from an ISP is now sent on an uplink to that ISP. 938 If all of the ISP uplinks are working, the choice of source address 939 by the host may be driven by the desire to load share across ISP 940 uplinks, or it may be driven by the desire to take advantage of 941 certain properties of a particular uplink or ISP. If any of the ISP 942 uplinks is not working, then the choice of source address by the host 943 can determine if packets get dropped. 945 How a host should make good decisions about source address selection 946 in a multihomed site is not a solved problem. We do not attempt to 947 solve this problem in this document. Instead we discuss the current 948 state of affairs with respect to standardized solutions and 949 implementation of those solutions. We also look at proposed 950 solutions for this problem. 952 An external host initiating communication with a host internal to a 953 PA multihomed site will need to know multiple addresses for that host 954 in order to communicate with it using different ISPs to the 955 multihomed site. These addresses are typically learned through DNS. 956 (For simplicity, we assume that the external host is single-homed.) 957 The external host chooses the ISP that will be used at the remote 958 multihomed site by setting the destination address on the packets it 959 transmits. For a sessions originated from an external host to an 960 internal host, the choice of source address used by the internal host 961 is simple. The internal host has no choice but to use the 962 destination address in the received packet as the source address of 963 the transmitted packet. 965 For a session originated by a host internal to the multi-homed site, 966 the decision of what source address to select is more complicated. 967 We consider three main methods for hosts to get information about the 968 network. The two proactive methods are Neighbor Discovery Router 969 Advertisements and DHCPv6. The one reactive method we consider is 970 ICMPv6. Note that we are explicitly excluding the possibility of 971 having hosts participate in or even listen directly to routing 972 protocol advertisements. 974 First we look at how a host is currently expected to select the 975 source and destination address with which it sends a packet. 977 4.1. Source Address Selection Algorithm on Hosts 979 [RFC6724] defines the algorithms that hosts are expected to use to 980 select source and destination addresses for packets. It defines an 981 algorithm for selecting a source address and a separate algorithm for 982 selecting a destination address. Both of these algorithms depend on 983 a policy table. [RFC6724] defines a default policy which produces 984 certain behavior. 986 The rules in the two algorithms in [RFC6724] depend on many different 987 properties of addresses. While these are needed for understanding 988 how a host should choose addresses in an arbitrary environment, most 989 of the rules are not relevant for understanding how a host should 990 choose among multiple source addresses in mulitihomed envinronment 991 when sending a packet to a remote host. Returning to the example in 992 Figure 3, we look at what the default algorithms in [RFC6724] say 993 about the source address that internal host H31 should use to send 994 traffic to external host H101, somewhere on the Internet. Let's look 995 at what rules in [RFC6724] are actually used by H31 in this case. 997 There is no choice to be made with respect to destination address. 998 H31 needs to send a packet with D=2001:db8:0:1234::101 in order to 999 reach H101. So H31 have to choose between using 1000 S=2001:db8:0:a010::31 or S=2001:db8:0:b010::31 as the source address 1001 for this packet. We go through the rules for source address 1002 selection in Section 5 of [RFC6724]. Rule 1 (Prefer same address) is 1003 not useful to break the tie between source addresses, because neither 1004 the candidate source addresses equals the destination address. Rule 1005 2 (Prefer appropriate scope) is also not used in this scenario, 1006 because both source addresses and the destination address have global 1007 scope. 1009 Rule 3 (Avoid deprecated addresses) applies to an address that has 1010 been autoconfigured by a host using stateless address 1011 autoconfiguration as defined in [RFC4862]. An address autoconfigured 1012 by a host has a preferred lifetime and a valid lifetime. The address 1013 is preferred until the preferred lifetime expires, after which it 1014 becomes deprecated. A deprecated address is not used if there is a 1015 preferred address of the appropriate scope available. When the valid 1016 lifetime expires, the address cannot be used at all. The preferred 1017 and valid lifetimes for an autoconfigured address are set based on 1018 the corresponding lifetimes in the Prefix Information Option in 1019 Neighbor Discovery Router Advertisements. So a possible tool to 1020 control source address selection in this scenario would be for a host 1021 to make an address deprecated by having routers on that link, R1 and 1022 R2 in Figure 3, send a Router Advertisement message contaning a 1023 Prefix Information Option for the source prefix to be discouraged (or 1024 prohibited) with the preferred lifetime set to zero. This is a 1025 rather blunt tool, because it discourages or prohibits the use of 1026 that source prefix for all destinations. However, it may be useful 1027 in some scenarios. For example, if all uplinks to a particular ISP 1028 fail, it is desirable to prevent hosts from using source addresses 1029 from that ISP address space. 1031 Rule 4 (Avoid home addresses) does not apply here because we are not 1032 considering Mobile IP. 1034 Rule 5 (Prefer outgoing interface) is not useful in this scenario, 1035 because both source addresses are assigned to the same interface. 1037 Rule 5.5 (Prefer addresses in a prefix advertised by the next-hop) is 1038 not useful in the scenario when both R1 and R2 will advertise both 1039 source prefixes. However potentially this rule may allow a host to 1040 select the correct source prefix by selecting a next-hop. The most 1041 obvious way would be to make R1 to advertise itself as a default 1042 router and send PIO for 2001:db8:0:a010::/64, while R2 is advertising 1043 itself as a default router and sending PIO for 2001:db8:0:b010::/64. 1044 We'll discuss later how Rule 5.5 can be used to influence a source 1045 address selection in single-router topologies (e.g. when H41 is 1046 sending traffic using R3 as a default gateway). 1048 Rule 6 (Prefer matching label) refers to the Label value determined 1049 for each source and destination prefix as a result of applying the 1050 policy table to the prefix. With the default policy table defined in 1051 Section 2.1 of [RFC6724], Label(2001:db8:0:a010::31) = 5, 1052 Label(2001:db8:0:b010::31) = 5, and Label(2001:db8:0:1234::101) = 5. 1053 So with the default policy, Rule 6 does not break the tie. However, 1054 the algorithms in [RFC6724] are defined in such as way that non- 1055 default address selection policy tables can be used. [RFC7078] 1056 defines a way to distribute a non-default address selection policy 1057 table to hosts using DHCPv6. So even though the application of rule 1058 6 to this scenario using the default policy table is not useful, rule 1059 6 may still be a useful tool. 1061 Rule 7 (Prefer temporary addresses) has to do with the technique 1062 described in [RFC4941] to periodically randomize the interface 1063 portion of an IPv6 address that has been generated using stateless 1064 address autoconfiguration. In general, if H31 were using this 1065 technique, it would use it for both source addresses, for example 1066 creating temporary addresses 2001:db8:0:a010:2839:9938:ab58:830f and 1067 2001:db8:0:b010:4838:f483:8384:3208, in addition to 1068 2001:db8:0:a010::31 and 2001:db8:0:b010::31. So this rule would 1069 prefer the two temporary addresses, but it would not break the tie 1070 between the two source prefixes from ISP-A and ISP-B. 1072 Rule 8 (Use longest matching prefix) dictates that between two 1073 candidate source addresses the one which has longest common prefix 1074 length with the destination address. For example, if H31 were 1075 selecting the source address for sending packets to H101, this rule 1076 would not be a tie breaker as for both candidate source addresses 1077 2001:db8:0:a101::31 and 2001:db8:0:b101::31 the common prefix length 1078 with the destination is 48. However if H31 were selecting the source 1079 address for sending packets H41 address 2001:db8:0:a020::41, then 1080 this rule would result in using 2001:db8:0:a101::31 as a source 1081 (2001:db8:0:a101::31 and 2001:db8:0:a020::41 share the common prefix 1082 2001:db8:0:a000::/58, while for `2001:db8:0:b101::31 and 1083 2001:db8:0:a020::41 the common prefix is 2001:db8:0:a000::/51). 1084 Therefore rule 8 might be useful for selecting the correct source 1085 address in some but not all scenarios (for example if ISP-B services 1086 belong to 2001:db8:0:b000::/59 then H31 would always use 1087 2001:db8:0:b010::31 to access those destinations). 1089 So we can see that of the 8 source selection address rules from 1090 [RFC6724], five actually apply to our basic site multihoming 1091 scenario. The rules that are relevant to this scenario are 1092 summarized below. 1094 o Rule 3: Avoid deprecated addresses. 1096 o Rule 5.5: Prefer addresses in a prefix advertised by the next-hop. 1098 o Rule 6: Prefer matching label. 1100 o Rule 8: Prefer longest matching prefix. 1102 The two methods that we discuss for controlling the source address 1103 selection through the four relevant rules above are SLAAC Router 1104 Advertisement messages and DHCPv6. 1106 We also consider a possible role for ICMPv6 for getting traffic- 1107 driven feedback from the network. With the source address selection 1108 algorithm discussed above, the goal is to choose the correct source 1109 address on the first try, before any traffic is sent. However, 1110 another strategy is to choose a source address, send the packet, get 1111 feedback from the network about whether or not the source address is 1112 correct, and try another source address if it is not. 1114 We consider four scenarios where a host needs to select the correct 1115 source address. The first is when both uplinks are working. The 1116 second is when one uplink has failed. The third one is a situation 1117 when one failed uplink has recovered. The last one is failure of 1118 both (all) uplinks. 1120 4.2. Selecting Source Address When Both Uplinks Are Working 1122 Again we return to the topology in Figure 3. Suppose that the site 1123 administrator wants to implement a policy by which all hosts need to 1124 use ISP-A to reach H01 at D=2001:db8:0:1234::101. So for example, 1125 H31 needs to select S=2001:db8:0:a010::31. 1127 4.2.1. Distributing Address Selection Policy Table with DHCPv6 1129 This policy can be implemented by using DHCPv6 to distribute an 1130 address selection policy table that assigns the same label to 1131 destination address that match 2001:db8:0:1234::/64 as it does to 1132 source addresses that match 2001:db8:0:a000::/52. The following two 1133 entries accomplish this. 1135 Prefix Precedence Label 1136 2001:db8:0:1234::/64 50 33 1137 2001:db8:0:a000::/52 50 33 1139 Figure 9: Policy table entries to implement a routing policy 1141 This requires that the hosts implement [RFC6724], the basic source 1142 and destination address framework, along with [RFC7078], the DHCPv6 1143 extension for distributing a non-default policy table. Note that it 1144 does NOT require that the hosts use DHCPv6 for address assignment. 1145 The hosts could still use stateless address autoconfiguration for 1146 address configuration, while using DHCPv6 only for policy table 1147 distribution (see [RFC3736]). However this method has a number of 1148 disadvantages: 1150 o DHCPv6 support is not a mandatory requirement for IPv6 hosts, so 1151 this method might not work for all devices. 1153 o Network administrators are required to explicitly configure the 1154 desired network access policies on DHCPv6 servers. While it might 1155 be feasible in the scenarion of a single multihomed network, such 1156 approach might have some scalability issues, especially if the 1157 centralized DHCPv6 solution is deployed to serve a large number of 1158 multiomed sites. 1160 4.2.2. Controlling Source Address Selection With Router Advertisements 1162 Neighbor Discovery currently has two mechanisms to communicate prefix 1163 information to hosts. The base specification for Neighbor Discovery 1164 (see [RFC4861]) defines the Prefix Information Option (PIO) in the 1165 Router Advertisement (RA) message. When a host receives a PIO with 1166 the A-flag set, it can use the prefix in the PIO as source prefix 1167 from which it assigns itself an IP address using stateless address 1168 autoconfiguration (SLAAC) procedures described in [RFC4862]. In the 1169 example of Figure 3, if the site network is using SLAAC, we would 1170 expect both R1 and R2 to send RA messages with PIOs for both source 1171 prefixes 2001:db8:0:a010::/64 and 2001:db8:0:b010::/64 with the 1172 A-flag set. H31 would then use the SLAAC procedure to configure 1173 itself with the 2001:db8:0:a010::31 and 2001:db8:0:b010::31. 1175 Whereas a host learns about source prefixes from PIO messages, hosts 1176 can learn about a destination prefix from a Router Advertisement 1177 containing Route Information Option (RIO), as specified in [RFC4191]. 1178 The destination prefixes in RIOs are intended to allow a host to 1179 choose the router that it uses as its first hop to reach a particular 1180 destination prefix. 1182 As currently standardized, neither PIO nor RIO options contained in 1183 Neighbor Discovery Router Advertisements can communicate the 1184 information needed to implement the desired routing policy. PIO's 1185 communicate source prefixes, and RIO communicate destination 1186 prefixes. However, there is currently no standardized way to 1187 directly associate a particular destination prefix with a particular 1188 source prefix. 1190 [I-D.pfister-6man-sadr-ra] proposes a Source Address Dependent Route 1191 Information option for Neighbor Discovery Router Advertisements which 1192 would associate a source prefix and with a destination prefix. The 1193 details of [I-D.pfister-6man-sadr-ra] might need tweaking to address 1194 this use case. However, in order to be able to use Neighbor 1195 Discovery Router Advertisements to implement this routing policy, an 1196 extension that allows a R1 and R2 to explicitly communicate to H31 an 1197 association between S=2001:db8:0:a000::/52 D=2001:db8:0:1234::/64 1198 would be needed. 1200 However, Rule 5.5 of the source address selection algorithm 1201 (discussed in Section 4.1 above), together with default router 1202 preference (specified in [RFC4191]) and RIO can be used to influence 1203 a source address selection on a host as described below. Let's look 1204 at source address selection on the host H41. It receives RAs from R3 1205 with PIOs for 2001:db8:0:a020::/64 and 2001:db8:0:b020::/64. At that 1206 point all traffic would use the same next-hop (R3 link-local address) 1207 so Rule 5.5 does not apply. Now let's assume that R3 supports SADR 1208 and has two scoped forwarding tables, one scoped to 1209 S=2001:db8:0:a000::/52 and another scoped to S=2001:db8:0:b000::/52. 1210 If R3 generates two different link-local addresses for its interface 1211 facing H41 (one for each scoped forwarding table, LLA_A and LLA_B) 1212 and starts sending two different RAs: one is sent from LLA_A and 1213 includes PIO for 2001:db8:0:a020::/64, another us sent from LLA_B and 1214 includes PIO for 2001:db8:0:b020::/64. Now it is possible to 1215 influence H41 source address selection for destinations which follow 1216 the default route by setting default router preference in RAs. If it 1217 is desired that H41 reaches H101 (or any destinations in the 1218 Internet) via ISP-A, then RAs sent from LLA_A should have default 1219 router preference set to 01 (high priority), while RAs sent from 1220 LLA_B should have preference set to 11 (low). Then LLA_A would be 1221 chosen as a next-hop for H101 and therefore (as per rule 5.5) 1222 2001:db8:0:a020::41 would be selected as the source address. If, at 1223 the same time, it is desired that H61 is accessible via ISP-B then R3 1224 should include a RIO for 2001:db8:0:6666::/64 to its RA sent from 1225 LLA_B. H41 would chose LLA_B as a next-hop for all traffic to H61 1226 and then as per Rule 5.5, 2001:db8:0:b020::41 would be selected as a 1227 source address. 1229 If in the above mentioned scenario it is desirable that all Internet 1230 traffic leaves the network via ISP-A and the link to ISP-B is used 1231 for accessing ISP-B services only (not as ISP-A link backup), then 1232 RAs sent by R3 from LLA_B should have Router Lifetime set to 0 and 1233 should include RIOs for ISP-B address space. It would instruct H41 1234 to use LLA_A for all Internet traffic but use LLA_B as a next-hop 1235 while sending traffic to ISP-B addresses. 1237 The description of the mechanism above assumes SADR support by the 1238 first-hop routers as well as SERs. However, a first-hop router can 1239 still provide a less flexible version of this mechanism even without 1240 implementing SADR. This could be done by providing configuration 1241 knobs on the first-hop router that allow it to generate different 1242 link-local addresses and to send individual RAs for each prefix. 1244 The mechanism described above relies on Rule 5.5 of the default 1245 source address selection algorithm defined in [RFC6724]. [RFC8028] 1246 recommends that a host SHOULD select default routers for each prefix 1247 in which it is assigned an address. It also recommends that hosts 1248 SHOULD implement Rule 5.5. of [RFC6724]. Hosts following the 1249 recommendations specified in [RFC8028] therefore should be able to 1250 benefit from the solution described in this document. No standards 1251 need to be updated in regards to host behavior. 1253 4.2.3. Controlling Source Address Selection With ICMPv6 1255 We now discuss how one might use ICMPv6 to implement the routing 1256 policy to send traffic destined for H101 out the uplink to ISP-A, 1257 even when uplinks to both ISPs are working. If H31 started sending 1258 traffic to H101 with S=2001:db8:0:b010::31 and 1259 D=2001:db8:0:1234::101, it would be routed through SER-b1 and out the 1260 uplink to ISP-B. SERb1 could recognize that this is traffic is not 1261 following the desired routing policy and react by sending an ICMPv6 1262 message back to H31. 1264 In this example, we could arrange things so that SERb1 drops the 1265 packet with S=2001:db8:0:b010::31 and D=2001:db8:0:1234::101, and 1266 then sends to H31 an ICMPv6 Destination Unreachable message with Code 1267 5 (Source address failed ingress/egress policy). When H31 receives 1268 this packet, it would then be expected to try another source address 1269 to reach the destination. In this example, H31 would then send a 1270 packet with S=2001:db8:0:a010::31 and D=2001:db8:0:1234::101, which 1271 will reach SERa and be forwarded out the uplink to ISP-A. 1273 However, we would also want it to be the case that SERb1 does not 1274 enforce this routing policy when the uplink from SERa to ISP-A has 1275 failed. This could be accomplished by having SERa originate a 1276 source-prefix-scoped route for (S=2001:db8:0:a000::/52, 1277 D=2001:db8:0:1234::/64) and have SERb1 monitor the presence of that 1278 route. If that route is not present (because SERa has stopped 1279 originating it), then SERb1 will not enforce the routing policy, and 1280 it will forward packets with S=2001:db8:0:b010::31 and 1281 D=2001:db8:0:1234::101 out its uplink to ISP-B. 1283 We can also use this source-prefix-scoped route originated by SERa to 1284 communicate the desired routing policy to SERb1. We can define an 1285 EXCLUSIVE flag to be advertised together with the IGP route for 1286 (S=2001:db8:0:a000::/52, D=2001:db8:0:1234::/64). This would allow 1287 SERa to communicate to SERb that SERb should reject traffic for 1288 D=2001:db8:0:1234::/64 and respond with an ICMPv6 Destination 1289 Unreachable Code 5 message, as long as the route for 1290 (S=2001:db8:0:a000::/52, D=2001:db8:0:1234::/64) is present. 1292 Finally, if we are willing to extend ICMPv6 to support this solution, 1293 then we could create a mechanism for SERb1 to tell the host what 1294 source address it should be using to successfully forward packets 1295 that meet the policy. In its current form, when SERb1 sends an 1296 ICMPv6 Destination Unreachable Code 5 message, it is basically 1297 saying, "This source address is wrong. Try another source address." 1298 In the absence of a clear indication which address to try next, the 1299 host will iterate over all addresses assigned to the interface (e.g. 1300 various privacy addresses) which would lead to significant delays and 1301 degraded user experience. It would be better is if the ICMPv6 1302 message could say, "This source address is wrong. Instead use a 1303 source address in S=2001:db8:0:a000::/52.". 1305 However using ICMPv6 for signalling source address information back 1306 to hosts introduces new challenges. Most routers currently have 1307 software or hardware limits on generating ICMP messages. An site 1308 administrator deploying a solution that relies on the SERs generating 1309 ICMP messages could try to improve the performance of SERs for 1310 generating ICMP messages. However, in a large network, it is still 1311 likely that ICMP message generation limits will be reached. As a 1312 result hosts would not receive ICMPv6 back which in turn leads to 1313 traffic blackholing and poor user experience. To improve the 1314 scalability of ICMPv6-based signalling hosts SHOULD cache the 1315 preferred source address (or prefix) for the given destination (which 1316 in turn might cause issues in case of the corresponding ISP uplinks 1317 failure - see Section 4.3). In addition, the same source prefix 1318 SHOULD be used for other destinations in the same /64 as the original 1319 destination address. The source prefix SHOULD have a specific 1320 lifetime. Expiration of the lifetime SHOULD trigger the source 1321 address selection algorithm again. 1323 Using ICMPv6 Code 5 message for influencing source address selection 1324 allows an attacker to exhaust the list of candidate source addresses 1325 on the host by sending spoofed ICMPv6 Code 5 for all prefixes known 1326 on the network (therefore preventing a victim from establishing a 1327 communication with the destination host). To protect from such 1328 attack hosts SHOULD verify that the original packet header included 1329 into ICMPv6 error message was actually sent by the host. 1331 As currently standardized in [RFC4443], the ICMPv6 Destination 1332 Unreachable Message with Code 5 would allow for the iterative 1333 approach to retransmitting packets using different source addresses. 1334 As currently defined, the ICMPv6 message does not provide a mechanism 1335 to communication information about which source prefix should be used 1336 for a retransmitted packet. The current document does not define 1337 such a mechanism. However, we note that this might be a useful 1338 extension to define in a different document. 1340 4.2.4. Summary of Methods For Controlling Source Address Selection To 1341 Implement Routing Policy 1343 So to summarize this section, we have looked at three methods for 1344 implementing a simple routing policy where all traffic for a given 1345 destination on the Internet needs to use a particular ISP, even when 1346 the uplinks to both ISPs are working. 1348 The default source address selection policy cannot distinguish 1349 between the source addresses needed to enforce this policy, so a non- 1350 default policy table using associating source and destination 1351 prefixes using Label values would need to be installed on each host. 1352 A mechanism exists for DHCPv6 to distribute a non-default policy 1353 table but such solution would heavily rely on DHCPv6 support by host 1354 operating system. Moreover there is no mechanism to translate 1355 desired routing/traffic engineering policies into policy tables on 1356 DHCPv6 servers. Therefore using DHCPv6 for controlling address 1357 selection policy table is not recommended and SHOULD NOT be used. 1359 At the same time Router Advertisements provide a reliable mechanism 1360 to influence source address selection process via PIO, RIO and 1361 default router preferences. As all those options have been 1362 standardized by IETF and are supported by various operating systems, 1363 no changes are required on hosts. First-hop routers in the 1364 enterprise network need to be able of sending different RAs for 1365 different SLAAC prefixes (either based on scoped forwarding tables or 1366 based on pre-configured policies). 1368 SERs can enforce the routing policy by sending ICMPv6 Destination 1369 Unreachable messages with Code 5 (Source address failed ingress/ 1370 egress policy) for traffic that is being sent with the wrong source 1371 address. The policy distribution can be automated by defining an 1372 EXCLUSIVE flag for the source-prefix-scoped route which can be set on 1373 the SER that originates the route. As ICMPv6 message generation can 1374 be rate-limited on routers, it SHOULD NOT be used as the only 1375 mechanism to influence source address selection on hosts. While 1376 hosts SHOULD select the correct source address for a given 1377 destination the network SHOULD signal any source address issues back 1378 to hosts using ICMPv6 error messages. 1380 4.3. Selecting Source Address When One Uplink Has Failed 1382 Now we discuss if DHCPv6, Neighbor Discovery Router Advertisements, 1383 and ICMPv6 can help a host choose the right source address when an 1384 uplink to one of the ISPs has failed. Again we look at the scenario 1385 in Figure 3. This time we look at traffic from H31 destined for 1386 external host H501 at D=2001:db8:0:5678::501. We initially assume 1387 that the uplink from SERa to ISP-A is working and that the uplink 1388 from SERb1 to ISP-B is working. 1390 We assume there is no particular routing policy desired, so H31 is 1391 free to send packets with S=2001:db8:0:a010::31 or 1392 S=2001:db8:0:b010::31 and have them delivered to H501. For this 1393 example, we assume that H31 has chosen S=2001:db8:0:b010::31 so that 1394 the packets exit via SERb to ISP-B. Now we see what happens when the 1395 link from SERb1 to ISP-B fails. How should H31 learn that it needs 1396 to start sending the packet to H501 with S=2001:db8:0:a010::31 in 1397 order to start using the uplink to ISP-A? We need to do this in a 1398 way that doesn't prevent H31 from still sending packets with 1399 S=2001:db8:0:b010::31 in order to reach H61 at D=2001:db8:0:6666::61. 1401 4.3.1. Controlling Source Address Selection With DHCPv6 1403 For this example we assume that the site network in Figure 3 has a 1404 centralized DHCP server and all routers act as DHCP relay agents. We 1405 assume that both of the addresses assigned to H31 were assigned via 1406 DHCP. 1408 We could try to have the DHCP server monitor the state of the uplink 1409 from SERb1 to ISP-B in some manner and then tell H31 that it can no 1410 longer use S=2001:db8:0:b010::31 by settings its valid lifetime to 1411 zero. The DHCP server could initiate this process by sending a 1412 Reconfigure Message to H31 as described in Section 19 of [RFC3315]. 1413 Or the DHCP server can assign addresses with short lifetimes in order 1414 to force clients to renew them often. 1416 This approach would prevent H31 from using S=2001:db8:0:b010::31 to 1417 reach the a host on the Internet. However, it would also prevent H31 1418 from using S=2001:db8:0:b010::31 to reach H61 at 1419 D=2001:db8:0:6666::61, which is not desirable. 1421 Another potential approach is to have the DHCP server monitor the 1422 uplink from SERb1 to ISP-B and control the choice of source address 1423 on H31 by updating its address selection policy table via the 1424 mechanism in [RFC7078]. The DHCP server could initiate this process 1425 by sending a Reconfigure Message to H31. Note that [RFC3315] 1426 requires that Reconfigure Message use DHCP authentication. DHCP 1427 authentication could be avoided by using short address lifetimes to 1428 force clients to send Renew messages to the server often. If the 1429 host is not obtaining its IP addresses from the DHCP server, then it 1430 would need to use the Information Refresh Time option defined in 1431 [RFC4242]. 1433 If the following policy table can be installed on H31 after the 1434 failure of the uplink from SERb1, then the desired routing behavior 1435 should be achieved based on source and destination prefix being 1436 matched with label values. 1438 Prefix Precedence Label 1439 ::/0 50 44 1440 2001:db8:0:a000::/52 50 44 1441 2001:db8:0:6666::/64 50 55 1442 2001:db8:0:b000::/52 50 55 1444 Figure 10: Policy Table Needed On Failure Of Uplink From SERb1 1446 The described solution has a number of significant drawbacks, some of 1447 them already discussed in Section 4.2.1. 1449 o DHCPv6 support is not required for an IPv6 host and there are 1450 operating systems which do not support DHCPv6. Besides that, it 1451 does not appear that [RFC7078] has been widely implemented on host 1452 operating systems. 1454 o [RFC7078] does not clearly specify this kind of a dynamic use case 1455 where address selection policy needs to be updated quickly in 1456 response to the failure of a link. In a large network it would 1457 present scalability issues as many hosts need to be reconfigured 1458 in very short period of time. 1460 o Updating DHCPv6 server configuration each time an ISP uplink 1461 changes its state introduces some scalability issues, especially 1462 for mid/large distributed scale enterprise networks. In addition 1463 to that, the policy table needs to be manually configured by 1464 administrators which makes that solution prone to human error. 1466 o No mechanism exists for making DHCPv6 servers aware of network 1467 topology/routing changes in the network. In general DHCPv6 1468 servers monitoring network-related events sounds like a bad idea 1469 as completely new functionality beyond the scope of DHCPv6 role is 1470 required. 1472 4.3.2. Controlling Source Address Selection With Router Advertisements 1474 The same mechanism as discussed in Section 4.2.2 can be used to 1475 control the source address selection in the case of an uplink 1476 failure. If a particular prefix should not be used as a source for 1477 any destinations, then the router needs to send RA with Preferred 1478 Lifetime field for that prefix set to 0. 1480 Let's consider a scenario when all uplinks are operational and H41 1481 receives two different RAs from R3: one from LLA_A with PIO for 1482 2001:db8:0:a020::/64, default router preference set to 11 (low) and 1483 another one from LLA_B with PIO for 2001:db8:0:a020::/64, default 1484 router preference set to 01 (high) and RIO for 2001:db8:0:6666::/64. 1485 As a result H41 is using 2001:db8:0:b020::41 as a source address for 1486 all Internet traffic and those packets are sent by SERs to ISP-B. If 1487 SERb1 uplink to ISP-B failed, the desired behavior is that H41 stops 1488 using 2001:db8:0:b020::41 as a source address for all destinations 1489 but H61. To achieve that R3 should react to SERb1 uplink failure 1490 (which could be detected as the scoped route (S=2001:db8:0:b000::/52, 1491 D=::/0) disappearance) by withdrawing itself as a default router. R3 1492 sends a new RA from LLA_B with Router Lifetime value set to 0 (which 1493 means that it should not be used as default router). That RA still 1494 contains PIO for 2001:db8:0:b020::/64 (for SLAAC purposes) and RIO 1495 for 2001:db8:0:6666::/64 so H41 can reach H61 using LLA_B as a next- 1496 hop and 2001:db8:0:b020::41 as a source address. For all traffic 1497 following the default route, LLA_A will be used as a next-hop and 1498 2001:db8:0:a020::41 as a source address. 1500 If all uplinks to ISP-B have failed and therefore source addresses 1501 from ISP-B address space should not be used at all, the forwarding 1502 table scoped S=2001:db8:0:b000::/52 contains no entries. Hosts can 1503 be instructed to stop using source addresses from that block by 1504 sending RAs containing PIO with Preferred Lifetime set to 0. 1506 4.3.3. Controlling Source Address Selection With ICMPv6 1508 Now we look at how ICMPv6 messages can provide information back to 1509 H31. We assume again that at the time of the failure H31 is sending 1510 packets to H501 using (S=2001:db8:0:b010::31, 1511 D=2001:db8:0:5678::501). When the uplink from SERb1 to ISP-B fails, 1512 SERb1 would stop originating its source-prefix-scoped route for the 1513 default destination (S=2001:db8:0:b000::/52, D=::/0) as well as its 1514 unscoped default destination route. With these routes no longer in 1515 the IGP, traffic with (S=2001:db8:0:b010::31, D=2001:db8:0:5678::501) 1516 would end up at SERa based on the unscoped default destination route 1517 being originated by SERa. Since that traffic has the wrong source 1518 address to be forwarded to ISP-A, SERa would drop it and send a 1519 Destination Unreachable message with Code 5 (Source address failed 1520 ingress/egress policy) back to H31. H31 would then know to use 1521 another source address for that destination and would try with 1522 (S=2001:db8:0:a010::31, D=2001:db8:0:5678::501). This would be 1523 forwarded to SERa based on the source-prefix-scoped default 1524 destination route still being originated by SERa, and SERa would 1525 forward it to ISP-A. As discussed above, if we are willing to extend 1526 ICMPv6, SERa can even tell H31 what source address it should use to 1527 reach that destination. The expected host behaviour has been 1528 discussed in Section 4.2.3. Potential issue with using ICMPv6 for 1529 signalling source address issues back to hosts is that uplink to an 1530 ISP-B failure immediately invalidates source addresses from 1531 2001:db8:0:b000::/52 for all hosts which triggers a large number of 1532 ICMPv6 being sent back to hosts - the same scalability/rate limiting 1533 issues discussed in Section 4.2.3 would apply. 1535 4.3.4. Summary Of Methods For Controlling Source Address Selection On 1536 The Failure Of An Uplink 1538 It appears that DHCPv6 is not particularly well suited to quickly 1539 changing the source address used by a host in the event of the 1540 failure of an uplink, which eliminates DHCPv6 from the list of 1541 potential solutions. On the other hand Router Advertisements 1542 provides a reliable mechanism to dynamically provide hosts with a 1543 list of valid prefixes to use as source addresses as well as prevent 1544 particular prefixes to be used. While no additional new features are 1545 required to be implemented on hosts, routers need to be able to send 1546 RAs based on the state of scoped forwarding tables entries and to 1547 react to network topology changes by sending RAs with particular 1548 parameters set. 1550 The use of ICMPv6 Destination Unreachable messages generated by the 1551 SER (or any SADR-capable) routers seem like they have the potential 1552 to provide a support mechanism together with RAs to signal source 1553 address selection errors back to hosts, however scalability issues 1554 may arise in large networks in case of sudden topology change. 1555 Therefore it is highly desirable that hosts are able to select the 1556 correct source address in case of uplinks failure with ICMPv6 being 1557 an additional mechanism to signal unexpected failures back to hosts. 1559 The current behavior of different host operating system when 1560 receiving ICMPv6 Destination Unreachable message with code 5 (Source 1561 address failed ingress/egress policy) is not clear to the authors. 1562 Information from implementers, users, and testing would be quite 1563 helpful in evaluating this approach. 1565 4.4. Selecting Source Address Upon Failed Uplink Recovery 1567 The next logical step is to look at the scenario when a failed uplink 1568 on SERb1 to ISP-B is coming back up, so hosts can start using source 1569 addresses belonging to 2001:db8:0:b000::/52 again. 1571 4.4.1. Controlling Source Address Selection With DHCPv6 1573 The mechanism to use DHCPv6 to instruct the hosts (H31 in our 1574 example) to start using prefixes from ISP-B space (e.g. 1575 S=2001:db8:0:b010::31 for H31) to reach hosts on the Internet is 1576 quite similar to one discussed in Section 4.3.1 and shares the same 1577 drawbacks. 1579 4.4.2. Controlling Source Address Selection With Router Advertisements 1581 Let's look at the scenario discussed in Section 4.3.2. If the 1582 uplink(s) failure caused the complete withdrawal of prefixes from 1583 2001:db8:0:b000::/52 address space by setting Preferred Lifetime 1584 value to 0, then the recovery of the link should just trigger new RA 1585 being sent with non-zero Preferred Lifetime. In another scenario 1586 discussed in Section 4.3.2, the SERb1 uplink to ISP-B failure leads 1587 to disappearance of the (S=2001:db8:0:b000::/52, D=::/0) entry from 1588 the forwarding table scoped to S=2001:db8:0:b000::/52 and, in turn, 1589 caused R3 to send RAs from LLA_B with Router Lifetime set to 0. The 1590 recovery of the SERb1 uplink to ISP-B leads to 1591 (S=2001:db8:0:b000::/52, D=::/0) scoped forwarding entry re- 1592 appearance and instructs R3 that it should advertise itself as a 1593 default router for ISP-B address space domain (send RAs from LLA_B 1594 with non-zero Router Lifetime). 1596 4.4.3. Controlling Source Address Selection With ICMP 1598 It looks like ICMPv6 provides a rather limited functionality to 1599 signal back to hosts that particular source addresses have become 1600 valid again. Unless the changes in the uplink state a particular 1601 (S,D) pair, hosts can keep using the same source address even after 1602 an ISP uplink has come back up. For example, after the uplink from 1603 SERb1 to ISP-B had failed, H31 received ICMPv6 Code 5 message (as 1604 described in Section 4.3.3) and allegedly started using 1605 (S=2001:db8:0:a010::31, D=2001:db8:0:5678::501) to reach H501. Now 1606 when the SERb1 uplink comes back up, the packets with that (S,D) pair 1607 are still routed to SERa1 and sent to the Internet. Therefore H31 is 1608 not informed that it should stop using 2001:db8:0:a010::31 and start 1609 using 2001:db8:0:b010::31 again. Unless SERa has a policy configured 1610 to drop packets (S=2001:db8:0:a010::31, D=2001:db8:0:5678::501) and 1611 send ICMPv6 back if SERb1 uplink to ISP-B is up, H31 will be unaware 1612 of the network topology change and keep using S=2001:db8:0:a010::31 1613 for Internet destinations, including H51. 1615 One of the possible option may be using a scoped route with EXCLUSIVE 1616 flag as described in Section 4.2.3. SERa1 uplink recovery would 1617 cause (S=2001:db8:0:a000::/52, D=2001:db8:0:1234::/64) route to 1618 reappear in the routing table. In the absence of that route packets 1619 to H101 which were sent to ISP-B (as ISP-A uplink was down) with 1620 source addresses from 2001:db8:0:b000::/52. When the route re- 1621 appears SERb1 would reject those packets and sends ICMPv6 back as 1622 discussed in Section 4.2.3. Practically it might lead to scalability 1623 issues which have been already discussed in Section 4.2.3 and 1624 Section 4.4.3. 1626 4.4.4. Summary Of Methods For Controlling Source Address Selection Upon 1627 Failed Uplink Recovery 1629 Once again DHCPv6 does not look like reasonable choice to manipulate 1630 source address selection process on a host in the case of network 1631 topology changes. Using Router Advertisement provides the flexible 1632 mechanism to dynamically react to network topology changes (if 1633 routers are able to use routing changes as a trigger for sending out 1634 RAs with specific parameters). ICMPv6 could be considered as a 1635 supporting mechanism to signal incorrect source address back to hosts 1636 but should not be considered as the only mechanism to control the 1637 address selection in multihomed environments. 1639 4.5. Selecting Source Address When All Uplinks Failed 1641 One particular tricky case is a scenario when all uplinks have 1642 failed. In that case there is no valid source address to be used for 1643 any external destinations while it might be desirable to have intra- 1644 site connectivity. 1646 4.5.1. Controlling Source Address Selection With DHCPv6 1648 From DHCPv6 perspective uplinks failure should be treated as two 1649 independent failures and processed as described in Section 4.3.1. At 1650 this stage it is quite obvious that it would result in quite 1651 complicated policy table which needs to be explicitly configured by 1652 administrators and therefore seems to be impractical. 1654 4.5.2. Controlling Source Address Selection With Router Advertisements 1656 As discussed in Section 4.3.2 an uplink failure causes the scoped 1657 default entry to disappear from the scoped forwarding table and 1658 triggers RAs with zero Router Lifetime. Complete disappearance of 1659 all scoped entries for a given source prefix would cause the prefix 1660 being withdrawn from hosts by setting Preferred Lifetime value to 1661 zero in PIO. If all uplinks (SERa, SERb1 and SERb2) failed, hosts 1662 either lost their default routers and/or have no global IPv6 1663 addresses to use as a source. (Note that 'uplink failure' might mean 1664 'IPv6 connectivity failure with IPv4 still being reachable', in which 1665 case hosts might fall back to IPv4 if there is IPv4 connectivity to 1666 destinations). As a results intra-site connectivity is broken. One 1667 of the possible way to solve it is to use ULAs. 1669 All hosts have ULA addresses assigned in addition to GUAs and used 1670 for intra-site communication even if there is no GUA assigned to a 1671 host. To avoid accidental leaking of packets with ULA sources SADR- 1672 capable routers SHOULD have a scoped forwarding table for ULA source 1673 for internal routes but MUST NOT have an entry for D=::/0 in that 1674 table. In the absence of (S=ULA_Prefix; D=::/0) first-hop routers 1675 will send dedicated RAs from a unique link-local source LLA_ULA with 1676 PIO from ULA address space, RIO for the ULA prefix and Router 1677 Lifetime set to zero. The behaviour is consistent with the situation 1678 when SERb1 lost the uplink to ISP-B (so there is no Internet 1679 connectivity from 2001:db8:0:b000::/52 sources) but those sources can 1680 be used to reach some specific destinations. In the case of ULA 1681 there is no Internet connectivity from ULA sources but they can be 1682 used to reach another ULA destinations. Note that ULA usage could be 1683 particularly useful if all ISPs assign prefixes via DHCP-PD. In the 1684 absence of ULAs uplinks failure hosts would lost all their GUAs upon 1685 prefix lifetime expiration which again makes intra-site communication 1686 impossible. 1688 It should be noted that the Rule 5.5 (prefer a prefix advertised by 1689 the selected next-hop) takes precedence over the Rule 6 (prefer 1690 matching label, which ensures that GUA source addresses are preferred 1691 over ULAs for GUA destinations). Therefore if ULAs are used, the 1692 network adminstrator needs to ensure that while the site has an 1693 Internet connectivity, hosts do not select a roter which advertises 1694 ULA prefixes as their default router. 1696 4.5.3. Controlling Source Address Selection With ICMPv6 1698 In case of all uplinks failure all SERs will drop outgoing IPv6 1699 traffic and respond with ICMPv6 error message. In the large network 1700 when many hosts are trying to reach Internet destinations it means 1701 that SERs need to generate an ICMPv6 error to every packet they 1702 receive from hosts which presents the same scalability issues 1703 discussed in Section 4.3.3 1705 4.5.4. Summary Of Methods For Controlling Source Address Selection When 1706 All Uplinks Failed 1708 Again, combining SADR with Router Advertisements seems to be the most 1709 flexible and scalable way to control the source address selection on 1710 hosts. 1712 4.6. Summary Of Methods For Controlling Source Address Selection 1714 To summarize the scenarios and options discussed above: 1716 While DHCPv6 allows administrators to manipulate source address 1717 selection policy tables, this method has a number of significant 1718 disadvantages which eliminates DHCPv6 from a list of potential 1719 solutions: 1721 1. It required hosts to support DHCPv6 and its extension (RFC7078); 1723 2. DHCPv6 server needs to monitor network state and detect routing 1724 changes. 1726 3. The use of policy tables requires manual configuration and might 1727 be extremely complicated, especially in the case of distributed 1728 network when large number of remote sites are being served by 1729 centralized DHCPv6 servers. 1731 4. Network topology/routing policy changes could trigger 1732 simultaneous re-configuration of large number of hosts which 1733 present serious scalability issues. 1735 The use of Router Advertisements to influence the source address 1736 selection on hosts seem to be the most reliable, flexible and 1737 scalable solution. It has the following benefits: 1739 1. no new (non-standard) functionality needs to be implemented on 1740 hosts (except for [RFC4191] support); 1742 2. no changes in RA format; 1744 3. routers can react to routing table changes by sending RAs which 1745 would minimize the failover time in the case of network topology 1746 changes; 1748 4. information required for source address selection is broadcast to 1749 all affected hosts in case of topology change event which 1750 improves the scalability of the solution (comparing to DHCPv6 1751 reconfiguration or ICMPv6 error messages). 1753 To fully benefit from the RA-based solution, first-hop routers need 1754 to implement SADR and be able to send dedicated RAs per scoped 1755 forwarding table as discussed above, reacting to network changes with 1756 sending new RAs. It should be noted that the proposed solution would 1757 work even if first-hop routers are not SADR-capable but still able to 1758 send individual RAs for each ISP prefix and react to topology changes 1759 as discussed above (e.g. via configuration knobs). 1761 The RA-based solution relies heavily on hosts correctly implementing 1762 default address selection algorith as defined in [RFC6724]. While 1763 the basic (and most common) multihoming scenario (two or more 1764 Internet uplinks, no 'wall gardens') would work for any host 1765 supporting the minimal implementation of [RFC6724], more complex use 1766 cases (such as "wall garden" and other scenarios when some ISP 1767 resources can only be reached from that ISP address space) require 1768 that hosts support Rule 5.5 of the default address selection 1769 algorithm. There is some evidence that not all host OSes have that 1770 rule implemented currently. However it should be noted that 1771 [RFC8028] states that Rule 5.5 SHOULD be implemented. 1773 ICMPv6 Code 5 error message SHOULD be used to complement RA-based 1774 solution to signal incorrect source address selection back to hosts, 1775 but it SHOULD NOT be considered as the stand-alone solution. To 1776 prevent scenarios when hosts in multihomed envinronments incorrectly 1777 identify onlink/offlink destinations, hosts should treat ICMPv6 1778 Redirects as discussed in [RFC8028]. 1780 4.7. Other Configuration Parameters 1782 4.7.1. DNS Configuration 1784 In mutihomed envinronment each ISP might provide their own list of 1785 DNS servers. E.g. in the topology show on Figure 3, ISP-A might 1786 provide recursive DNS server H51 2001:db8:0:5555::51, while ISP-B 1787 might provide H61 2001:db8:0:6666::61 as a recursive DNS server. 1788 [RFC6106] defines IPv6 Router Advertisement options to allow IPv6 1789 routers to advertise a list of DNS recursive server addresses and a 1790 DNS Search List to IPv6 hosts. Using RDNSS together with 'scoped' 1791 RAs as described above would allow a first-hop router (R3 in the 1792 Figure 3) to send DNS server addresses and search lists provided by 1793 each ISP (or the corporate DNS servers addresses if the enterprise is 1794 running its own DNS servers). 1796 As discussed in Section 4.5.2, failure of all ISP uplinks would cause 1797 deprecaction of all addresses assigned to a host from the address 1798 space if all ISPs. If any intra-site IPv6 connectivity is still 1799 desirable (most likely to be the case for any mid/large scare 1800 network), then ULAs should be used as discussed in Section 4.5.2. In 1801 such a scenario, the enterprise network should run its own recursive 1802 DNS server(s) and provide its ULA addresses to hosts via RDNSS in RAs 1803 send for ULA-scoped forwarding table as described in Section 4.5.2. 1805 There are some scenarios when the final outcome of the name 1806 resolution might be different depending on: 1808 o which DNS server is used; 1810 o which source address the client uses to send a DNS query to the 1811 server (DNS split horizon). 1813 There is no way currently to instruct a host to use a particular DNS 1814 server out of the configured servers list for resolving a particular 1815 name. Therefore it does not seem feasible to solve the problem of 1816 DNS server selection on the host (it should be noted that this 1817 particular issue is protocol-agnostic and happens for IPv4 as well). 1818 In such a scenario it is recommended that the enterprise run its own 1819 local recursive DNS server. 1821 To influence host source address selection for packets sent to a 1822 particular DNS server the following requirements must be met: 1824 o the host supports RIO as defined in [RFC4191]; 1826 o the routers send RIO for routes to DNS server addresses. 1828 For example, if it is desirable that host H31 reaches the ISP-A DNS 1829 server H51 2001:db8:0:5555::51 using its source address 1830 2001:db8:0:a010::31, then both R1 and R2 should send the RIO 1831 containing the route to 2001:db8:0:5555::51 (or covering route) in 1832 their 'scoped' RAs, containing LLA_A as the default router address 1833 and the PO for SLAAC prefix 2001:db8:0:a010::/64. In that case the 1834 host H31 (if it supports the Rule 5.5) would select LLA_A as a next- 1835 hop and then chose 2001:db8:0:a010::31 as the source address for 1836 packets to the DNS server. 1838 It should be noted that [RFC6106] explicitly prohibits using DNS 1839 information if the RA router Lifetime expired: "An RDNSS address or a 1840 DNSSL domain name MUST be used only as long as both the RA router 1841 Lifetime (advertised by a Router Advertisement message) and the 1842 corresponding option Lifetime have not expired.". Therefore hosts 1843 might ignore RDNSS information provided in ULA-scoped RAs as those 1844 RAs would have router lifetime set to 0. However the updated version 1845 of RFC6106 ([I-D.ietf-6man-rdnss-rfc6106bis]) has that requirement 1846 removed. 1848 5. Other Solutions 1850 5.1. Shim6 1852 The Shim6 working group specified the Shim6 protocol [RFC5533] which 1853 allows a host at a multihomed site to communicate with an external 1854 host and exchange information about possible source and destination 1855 address pairs that they can use to communicate. It also specified 1856 the REAP protocol [RFC5534] to detect failures in the path between 1857 working address pairs and find new working address pairs. A 1858 fundamental requirement for Shim6 is that both internal and external 1859 hosts need to support Shim6. That is, both the host internal to the 1860 multihomed site and the host external to the multihomed site need to 1861 support Shim6 in order for there to be any benefit for the internal 1862 host to run Shim6. The Shim6 protocol specification was published in 1863 2009, but it has not been implemented on widely used operating 1864 systems. 1866 We do not consider Shim6 to be a viable solution. It suffers from 1867 the fact that it requires widespread deployment of Shim6 on hosts all 1868 over the Internet before the host at a PA multihomed site sees 1869 significant benefit. However, there appears to be no motivation for 1870 the vast majority of hosts on the Internet (which are not at PA 1871 multihomed sites) to deploy Shim6. This may help explain why Shim6 1872 has not been widely implemented. 1874 5.2. IPv6-to-IPv6 Network Prefix Translation 1876 IPv6-to-IPv6 Network Prefix Translation (NPTv6) [RFC6296] is not the 1877 focus of this document. This document describes a solution where a 1878 host in a multihomed site determines which ISP a packet will be sent 1879 to based on the source address it applies to the packet. This 1880 solution has many moving parts. It requires some routers in the 1881 enterprise site to support some form of Source Address Dependent 1882 Routing (SADR). It requires a host to be able to learn when the 1883 uplink to an ISP fails so that it can stop using the source address 1884 corresponding to that ISP. Ongoing work to create mechanisms to 1885 accomplish this are discussed in this document, but they are still a 1886 work in progress. 1888 This document attempts to create a PA multihoming solution that is as 1889 easy as possible for an enterprise to deploy. However, the success 1890 of this solution will depend greatly on whether or not the mechanisms 1891 for hosts to select source addresses based on the state of ISP 1892 uplinks gets implemented across a wide range of operating systems as 1893 the default mode of operation. Until that occurs, NPTv6 should still 1894 be considered a viable option to enable PA multihoming for 1895 enterprises. 1897 6. IANA Considerations 1899 This memo asks the IANA for no new parameters. 1901 7. Security Considerations 1903 7.1. Privacy Considerations 1905 8. Acknowledgements 1907 The original outline was suggested by Ole Troan. 1909 9. References 1911 9.1. Normative References 1913 [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - 1914 Communication Layers", STD 3, RFC 1122, 1915 DOI 10.17487/RFC1122, October 1989, 1916 . 1918 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - 1919 Application and Support", STD 3, RFC 1123, 1920 DOI 10.17487/RFC1123, October 1989, 1921 . 1923 [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G., 1924 and E. Lear, "Address Allocation for Private Internets", 1925 BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996, 1926 . 1928 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1929 Requirement Levels", BCP 14, RFC 2119, 1930 DOI 10.17487/RFC2119, March 1997, 1931 . 1933 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 1934 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, 1935 December 1998, . 1937 [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: 1938 Defeating Denial of Service Attacks which employ IP Source 1939 Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, 1940 May 2000, . 1942 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, 1943 C., and M. Carney, "Dynamic Host Configuration Protocol 1944 for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 1945 2003, . 1947 [RFC3582] Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site- 1948 Multihoming Architectures", RFC 3582, 1949 DOI 10.17487/RFC3582, August 2003, 1950 . 1952 [RFC4116] Abley, J., Lindqvist, K., Davies, E., Black, B., and V. 1953 Gill, "IPv4 Multihoming Practices and Limitations", 1954 RFC 4116, DOI 10.17487/RFC4116, July 2005, 1955 . 1957 [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and 1958 More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191, 1959 November 2005, . 1961 [RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast 1962 Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005, 1963 . 1965 [RFC4218] Nordmark, E. and T. Li, "Threats Relating to IPv6 1966 Multihoming Solutions", RFC 4218, DOI 10.17487/RFC4218, 1967 October 2005, . 1969 [RFC4219] Lear, E., "Things Multihoming in IPv6 (MULTI6) Developers 1970 Should Think About", RFC 4219, DOI 10.17487/RFC4219, 1971 October 2005, . 1973 [RFC4242] Venaas, S., Chown, T., and B. Volz, "Information Refresh 1974 Time Option for Dynamic Host Configuration Protocol for 1975 IPv6 (DHCPv6)", RFC 4242, DOI 10.17487/RFC4242, November 1976 2005, . 1978 [RFC6106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, 1979 "IPv6 Router Advertisement Options for DNS Configuration", 1980 RFC 6106, DOI 10.17487/RFC6106, November 2010, 1981 . 1983 [RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix 1984 Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011, 1985 . 1987 [RFC7157] Troan, O., Ed., Miles, D., Matsushima, S., Okimoto, T., 1988 and D. Wing, "IPv6 Multihoming without Network Address 1989 Translation", RFC 7157, DOI 10.17487/RFC7157, March 2014, 1990 . 1992 9.2. Informative References 1994 [I-D.baker-ipv6-isis-dst-src-routing] 1995 Baker, F. and D. Lamparter, "IPv6 Source/Destination 1996 Routing using IS-IS", draft-baker-ipv6-isis-dst-src- 1997 routing-06 (work in progress), October 2016. 1999 [I-D.baker-rtgwg-src-dst-routing-use-cases] 2000 Baker, F., Xu, M., Yang, S., and J. Wu, "Requirements and 2001 Use Cases for Source/Destination Routing", draft-baker- 2002 rtgwg-src-dst-routing-use-cases-02 (work in progress), 2003 April 2016. 2005 [I-D.boutier-babel-source-specific] 2006 Boutier, M. and J. Chroboczek, "Source-Specific Routing in 2007 Babel", draft-boutier-babel-source-specific-02 (work in 2008 progress), June 2017. 2010 [I-D.huitema-shim6-ingress-filtering] 2011 Huitema, C., "Ingress filtering compatibility for IPv6 2012 multihomed sites", draft-huitema-shim6-ingress- 2013 filtering-00 (work in progress), September 2005. 2015 [I-D.ietf-6man-rdnss-rfc6106bis] 2016 Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, 2017 "IPv6 Router Advertisement Options for DNS Configuration", 2018 draft-ietf-6man-rdnss-rfc6106bis-16 (work in progress), 2019 February 2017. 2021 [I-D.ietf-mif-mpvd-arch] 2022 Anipko, D., "Multiple Provisioning Domain Architecture", 2023 draft-ietf-mif-mpvd-arch-11 (work in progress), March 2024 2015. 2026 [I-D.ietf-mptcp-experience] 2027 Bonaventure, O., Paasch, C., and G. Detal, "Use Cases and 2028 Operational Experience with Multipath TCP", draft-ietf- 2029 mptcp-experience-07 (work in progress), October 2016. 2031 [I-D.ietf-rtgwg-dst-src-routing] 2032 Lamparter, D. and A. Smirnov, "Destination/Source 2033 Routing", draft-ietf-rtgwg-dst-src-routing-04 (work in 2034 progress), May 2017. 2036 [I-D.pfister-6man-sadr-ra] 2037 Pfister, P., "Source Address Dependent Route Information 2038 Option for Router Advertisements", draft-pfister-6man- 2039 sadr-ra-01 (work in progress), June 2015. 2041 [I-D.xu-src-dst-bgp] 2042 Xu, M., Yang, S., and J. Wu, "Source/Destination Routing 2043 Using BGP-4", draft-xu-src-dst-bgp-00 (work in progress), 2044 March 2016. 2046 [PATRICIA] 2047 Morrison, D., "Practical Algorithm to Retrieve Information 2048 Coded in Alphanumeric", Journal of the ACM 15(4) 2049 pp514-534, October 1968. 2051 [RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed 2052 Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March 2053 2004, . 2055 [RFC3736] Droms, R., "Stateless Dynamic Host Configuration Protocol 2056 (DHCP) Service for IPv6", RFC 3736, DOI 10.17487/RFC3736, 2057 April 2004, . 2059 [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet 2060 Control Message Protocol (ICMPv6) for the Internet 2061 Protocol Version 6 (IPv6) Specification", RFC 4443, 2062 DOI 10.17487/RFC4443, March 2006, 2063 . 2065 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 2066 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 2067 DOI 10.17487/RFC4861, September 2007, 2068 . 2070 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 2071 Address Autoconfiguration", RFC 4862, 2072 DOI 10.17487/RFC4862, September 2007, 2073 . 2075 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 2076 Extensions for Stateless Address Autoconfiguration in 2077 IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, 2078 . 2080 [RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming 2081 Shim Protocol for IPv6", RFC 5533, DOI 10.17487/RFC5533, 2082 June 2009, . 2084 [RFC5534] Arkko, J. and I. van Beijnum, "Failure Detection and 2085 Locator Pair Exploration Protocol for IPv6 Multihoming", 2086 RFC 5534, DOI 10.17487/RFC5534, June 2009, 2087 . 2089 [RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with 2090 Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April 2091 2012, . 2093 [RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, 2094 "Default Address Selection for Internet Protocol Version 6 2095 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012, 2096 . 2098 [RFC7078] Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing 2099 Address Selection Policy Using DHCPv6", RFC 7078, 2100 DOI 10.17487/RFC7078, January 2014, 2101 . 2103 [RFC7788] Stenberg, M., Barth, S., and P. Pfister, "Home Networking 2104 Control Protocol", RFC 7788, DOI 10.17487/RFC7788, April 2105 2016, . 2107 [RFC8028] Baker, F. and B. Carpenter, "First-Hop Router Selection by 2108 Hosts in a Multi-Prefix Network", RFC 8028, 2109 DOI 10.17487/RFC8028, November 2016, 2110 . 2112 Appendix A. Change Log 2114 Initial Version: July 2016 2116 Authors' Addresses 2118 Fred Baker 2119 Santa Barbara, California 93117 2120 USA 2122 Email: FredBaker.IETF@gmail.com 2124 Chris Bowers 2125 Juniper Networks 2126 Sunnyvale, California 94089 2127 USA 2129 Email: cbowers@juniper.net 2131 Jen Linkova 2132 Google 2133 Mountain View, California 94043 2134 USA 2136 Email: furry@google.com