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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Unused Reference: 'RFC1918' is defined on line 1074, but no explicit reference was found in the text ** Obsolete normative reference: RFC 3315 (Obsoleted by RFC 8415) ** Obsolete normative reference: RFC 3633 (Obsoleted by RFC 8415) == Outdated reference: A later version (-13) exists of draft-ietf-6man-addr-select-opt-00 == Outdated reference: A later version (-12) exists of draft-templin-aero-00 -- Obsolete informational reference (is this intentional?): RFC 3484 (Obsoleted by RFC 6724) Summary: 2 errors (**), 0 flaws (~~), 4 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group F. Templin 3 Internet-Draft Boeing Research & Technology 4 Intended status: Informational May 26, 2011 5 Expires: November 27, 2011 7 Operational Guidance for IPv6 Deployment in IPv4 Sites using ISATAP 8 draft-templin-v6ops-isops-07.txt 10 Abstract 12 Many end user sites in the Internet today still have predominantly 13 IPv4 internal infrastructures. These sites range in size from small 14 home/office networks to large corporate enterprise networks, but 15 share the commonality that IPv4 continues to provide satisfactory 16 internal routing and addressing services for most applications. As 17 more and more IPv6-only services are deployed in the Internet, 18 however, end user devices within such sites will increasingly require 19 at least basic IPv6 functionality for external access. It is also 20 expected that more and more IPv6-only devices will be deployed within 21 the site over time. This document therefore provides operational 22 guidance for deployment of IPv6 within predominantly IPv4 sites using 23 the Intra-Site Automatic Tunnel Addressing Protocol (ISATAP). 25 Status of this Memo 27 This Internet-Draft is submitted in full conformance with the 28 provisions of BCP 78 and BCP 79. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF). Note that other groups may also distribute 32 working documents as Internet-Drafts. The list of current Internet- 33 Drafts is at http://datatracker.ietf.org/drafts/current/. 35 Internet-Drafts are draft documents valid for a maximum of six months 36 and may be updated, replaced, or obsoleted by other documents at any 37 time. It is inappropriate to use Internet-Drafts as reference 38 material or to cite them other than as "work in progress." 40 This Internet-Draft will expire on November 27, 2011. 42 Copyright Notice 44 Copyright (c) 2011 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (http://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 Table of Contents 59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 60 2. Enabling IPv6 Services using ISATAP . . . . . . . . . . . . . 3 61 3. SLAAC Services . . . . . . . . . . . . . . . . . . . . . . . . 5 62 3.1. Advertising ISATAP Router Behavior . . . . . . . . . . . . 5 63 3.2. Non-Advertising ISATAP Router Behavior . . . . . . . . . . 5 64 3.3. ISATAP Host Behavior . . . . . . . . . . . . . . . . . . . 6 65 3.4. Reference Operational Scenario - Shared Prefix Model . . . 6 66 3.5. Reference Operational Scenario - Individual Prefix 67 Model . . . . . . . . . . . . . . . . . . . . . . . . . . 9 68 3.6. SLAAC Site Administration Guidance . . . . . . . . . . . . 12 69 3.7. Loop Avoidance . . . . . . . . . . . . . . . . . . . . . . 14 70 4. DHCPv6 Services . . . . . . . . . . . . . . . . . . . . . . . 14 71 4.1. Advertising ISATAP Router Behavior . . . . . . . . . . . . 15 72 4.2. Non-Advertising ISATAP Router Behavior . . . . . . . . . . 15 73 4.3. ISATAP Host Behavior . . . . . . . . . . . . . . . . . . . 16 74 4.4. Reference Operational Scenario - No Prefix Model . . . . . 16 75 4.5. DHCPv6 Site Administration Guidance . . . . . . . . . . . 19 76 4.6. On-Demand Dynamic Routing for DHCP . . . . . . . . . . . . 20 77 4.7. Loop Avoidance . . . . . . . . . . . . . . . . . . . . . . 21 78 5. Scaling Considerations . . . . . . . . . . . . . . . . . . . . 21 79 6. Site Renumbering Considerations . . . . . . . . . . . . . . . 21 80 7. Path MTU Considerations . . . . . . . . . . . . . . . . . . . 22 81 8. Anycast Considerations . . . . . . . . . . . . . . . . . . . . 23 82 9. Alternative Approaches . . . . . . . . . . . . . . . . . . . . 23 83 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 84 11. Security Considerations . . . . . . . . . . . . . . . . . . . 24 85 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24 86 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24 87 13.1. Normative References . . . . . . . . . . . . . . . . . . . 24 88 13.2. Informative References . . . . . . . . . . . . . . . . . . 25 89 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 26 91 1. Introduction 93 End user sites in the Internet today currently use IPv4 routing and 94 addressing internally for core operating functions such as web 95 browsing, filesharing, network printing, e-mail, teleconferencing and 96 numerous other site-internal networking services. Such sites 97 typically have an abundance of public or private IPv4 addresses for 98 internal networking, and are separated from the public Internet by 99 firewalls, packet filtering gateways, proxies, address translators 100 and other site border demarcation devices. To date, such sites have 101 had little incentive to enable IPv6 services internally [RFC1687]. 103 End-user sites that currently use IPv4 services internally come in 104 endless sizes and varieties. For example, a home network behind a 105 Network Address Translator (NAT) may consist of a single link 106 supporting a few laptops, printers etc. As a larger example, a small 107 business may consist of one or a few offices with several networks 108 connecting considerably larger numbers of computers, routers, 109 handheld devices, printers, faxes, etc. Moving further up the scale, 110 large banks, restaurants, major retailers, large corporations, etc. 111 may consist of hundreds or thousands of branches worldwide that are 112 tied together in a complex global enterprise network. Additional 113 examples include personal-area networks, mobile vehicular networks, 114 disaster relief networks, tactical military networks, and various 115 forms of Mobile Ad-hoc Networks (MANETs). These cases and more are 116 discussed in RANGERS[RFC6139]. 118 With the proliferation of IPv6 devices in the public Internet, 119 however, existing IPv4 sites will increasingly require a means for 120 enabling IPv6 services so that hosts within the site can communicate 121 with IPv6-only correspondents. Such services must be deployable with 122 minimal configuration, and in a fashion that will not cause 123 disruptions to existing IPv4 services. The Intra-Site Automatic 124 Tunnel Addressing Protocol (ISATAP) [RFC5214] provides a simple-to- 125 use service that sites can deploy in the near term to meet these 126 requirements. This document therefore provides operational guidance 127 for using ISATAP to enable IPv6 services within predominantly IPv4 128 sites while causing no disruptions to existing IPv4 services. 130 2. Enabling IPv6 Services using ISATAP 132 Many existing sites within the Internet predominantly use IPv4-based 133 services for their internal networking needs, but there is a growing 134 requirement for enabling IPv6 services to support communications with 135 IPv6-only correspondents. Smaller sites that wish to enable IPv6 136 typically arrange to obtain public IPv6 prefixes from an Internet 137 Service Provider (ISP), where the prefixes may be either purely 138 native or the near-native prefixes offered by 6rd [RFC5969]. Larger 139 sites typically obtain provider independent IPv6 prefixes from an 140 Internet registry and advertise the prefixes into the IPv6 routing 141 system on their own behalf, i.e., they act as an ISP unto themselves. 142 In either case, after obtaining IPv6 prefixes the site can 143 automatically enable IPv6 services internally by configuring ISATAP. 145 The ISATAP service uses a Non-Broadcast, Multiple Access (NBMA) 146 tunnel virtual interface model [RFC2491][RFC2529] based on IPv6-in- 147 IPv4 encapsulation [RFC4213]. The encapsulation format can further 148 use Differentiated Service (DS) [RFC2983] and Explicit Congestion 149 Notification (ECN) [RFC3168] mapping between the inner and outer IP 150 headers to ensure expected per-hop behavior within well-managed 151 sites. 153 The ISATAP service is based on three basic node types known as 154 advertising ISATAP routers, non-advertising ISATAP routers and ISATAP 155 hosts. Advertising ISATAP routers configure their site-facing ISATAP 156 interfaces as advertising router interfaces (see: [RFC4861], Section 157 6.2.2). Non-advertising ISATAP routers configure their site-facing 158 ISATAP interfaces as non-advertising router interfaces and obtain 159 IPv6 addresses/prefixes via autoconfiguration exchanges with 160 advertising ISATAP routers. Finally, ISATAP hosts configure their 161 site-facing ISATAP interfaces as simple host interfaces and also 162 coordinate their autoconfiguration operations with advertising ISATAP 163 routers. In this sense, advertising ISATAP routers are "servers" 164 while non-advertising ISATAP routers and ISATAP hosts are "clients" 165 in the service model. 167 Advertising ISATAP routers arrange to add their IPv4 address to the 168 Potential Router List (PRL) within the site name service. The name 169 service could be either the DNS or some other site-internal name 170 resolution system, but the PRL should be published in such a way that 171 ISATAP clients can resolve the name "isatap.domainname" for the 172 "domainname" suffix associated with their attached link. For 173 example, if the domainname suffix associated with an ISATAP client's 174 attached link is "example.com", then the name of the PRL for that 175 link attachment point is "isatap.example.com". Alternatively, if the 176 site name service is operating without a domainname suffix, then the 177 name of the PRL is simply "isatap". (In either case, however, site 178 administrators should ensure that the name resolution returns IPv4 179 addresses of advertising ISATAP routers that are topologically close 180 to each ISATAP client depending on their locations.) 182 After the PRL is published, ISATAP clients within the site will 183 automatically perform a standard IPv6 Neighbor Discovery Router 184 Solicitation (RS) / Router Advertisement (RA) exchange with 185 advertising ISATAP routers [RFC4861][RFC5214]. Each ISATAP client 186 can also test the round-trip delays to multiple advertising ISATAP 187 routers listed in the PRL during an initial exchange, and select 188 those routers with the smallest delays. Alternatively, site 189 administrators could include an IPv4 anycast address in the PRL and 190 assign the address to multiple advertising ISATAP routers. In that 191 case, IPv4 routing within the site would direct the ISATAP client to 192 the nearest advertising ISATAP router. 194 Following router discovery, ISATAP clients initiate Stateless Address 195 AutoConfiguration (SLAAC) [RFC4862][RFC5214], the Dynamic Host 196 Configuration Protocol for IPv6 (DHCPv6) [RFC3315] or both. 198 3. SLAAC Services 200 Predominantly IPv4 sites can enable SLAAC services for ISATAP clients 201 that need to communicate with IPv6 correspondents. SLAAC services 202 are enabled using either the "shared" or "individual" prefix model. 203 In the shared prefix model, all advertising ISATAP routers advertise 204 a common prefix (e.g., 2001:db8::/64) to ISATAP clients within the 205 site. In the individual prefix model, advertising ISATAP router 206 advertise individual prefixes (e.g., 2001:db8:0:1::/64, 2001:db8:0: 207 2::/64, 2001:db8:0:3::/64, etc.) to ISATAP clients within the site. 208 Note that combinations of the shared and individual prefix models are 209 also possible, in which some of the site's ISATAP routers advertise 210 shared prefixes and others advertise individual prefixes 212 The following sections discuss operational considerations for 213 enabling ISATAP SLAAC services within predominantly IPv4 sites. 215 3.1. Advertising ISATAP Router Behavior 217 Advertising ISATAP routers that support SLAAC services send RA 218 messages in response to RS messages received on an advertising ISATAP 219 interface. SLAAC services are enabled when advertising ISATAP 220 routers advertise non-link-local IPv6 prefixes in Prefix Information 221 Options (PIOs) with the A flag set to 1[RFC4861]. When there are 222 multiple advertising ISATAP routers, the routers can advertise a 223 shared IPv6 prefix or individual IPv6 prefixes. 225 3.2. Non-Advertising ISATAP Router Behavior 227 Non-advertising ISATAP routers that engage in SLAAC behave the same 228 as for hosts (see below). 230 3.3. ISATAP Host Behavior 232 ISATAP hosts resolve the PRL and send RS messages to obtain RA 233 messages from an advertising ISATAP router. When the host receives 234 RA messages, it uses SLAAC to configure IPv6 addresses from any 235 advertised prefixes with the A flag set to 1 as specified in 236 [RFC4862][RFC5214] then assigns the addresses to the ISATAP 237 interface. The host also assigns any of the advertised prefixes with 238 the L flag set to 1 to the ISATAP interface. 240 Any IPv6 addresses configured in this fashion and assigned to an 241 ISATAP interface are known as ISATAP addresses. 243 3.4. Reference Operational Scenario - Shared Prefix Model 245 Figure 1 depicts a reference ISATAP network topology for allowing 246 hosts within a predominantly IPv4 site to configure ISATAP services 247 using SLAAC with the shared prefix model. The scenario shows two 248 advertising ISATAP routers ('A', 'B'), two ISATAP hosts ('C', 'D'), 249 and an ordinary IPv6 host ('E') outside of the site in a typical 250 deployment configuration. In this model, routers 'A' and 'B' both 251 advertise the same (shared) IPv6 prefix 2001:db8::/64 into the IPv6 252 routing system, and also advertise the prefix to ISATAP clients 253 within the site for SLAAC purposes. 255 .-(::::::::) 2001:db8:1::1 256 .-(::: IPv6 :::)-. +-------------+ 257 (:::: Internet ::::) | IPv6 Host E | 258 `-(::::::::::::)-' +-------------+ 259 `-(::::::)-' 260 +------------+ +------------+ 261 | Router A |---.---| Router B |. 262 ,| (isatap) | | (isatap) | `\ 263 . | 192.0.2.1 | | 192.0.2.1 | \ 264 / +------------+ +------------+ \ 265 : fe80::*:192.0.2.17 fe80::*:192.0.2.33 : 266 \ 2001:db8::/64 2001:db8::/64 / 267 : : 268 : : 269 +- IPv4 Site -+ 270 ; (PRL: 192.0.2.1) : 271 | ; 272 : -+-' 273 `-. .) 274 \ _) 275 `-----+--------)----+'----' 276 fe80::*:192.0.2.18 fe80::*:192.0.2.34 277 2001:db8::*:192.0.2.18 2001:db8::*:192.0.2.34 278 +--------------+ +--------------+ 279 | (isatap) | | (isatap) | 280 | Host C | | Host D | 281 +--------------+ +--------------+ 283 (* == "5efe") 285 Figure 1: Reference ISATAP Network Topology using Shared Prefix Model 287 With reference to Figure 1, advertising ISATAP routers 'A' and 'B' 288 within the IPv4 site connect to the IPv6 Internet either directly or 289 via a companion gateway (e.g., as shown in Figure 3). The routers 290 advertise the shared prefix 2001:db8::/64 into the IPv6 Internet 291 routing system either as a singleton /64 or as part of a shorter 292 aggregated IPv6 prefix if the routing system will not accept prefixes 293 as long as a /64. For the purpose of this example, we also assume 294 that the IPv4 site is configured within multiple IPv4 subnets - each 295 with an IPv4 prefix length of /28. 297 Advertising ISATAP routers 'A' and 'B' both configure the IPv4 298 anycast address 192.0.2.1, e.g., on a loopback interface, and the 299 site administrator places the single IPv4 address 192.0.2.1 in the 300 PRL for the site. 'A' and 'B' then both advertise the anycast 301 address/prefix into the site's IPv4 routing system so that ISATAP 302 clients can locate the router that is topologically closest. 304 Advertising ISATAP router 'A' next configures a site-interior IPv4 305 interface with address 192.0.2.17 and netmask /28, then configures an 306 advertising ISATAP router interface with link-local ISATAP address 307 fe80::5efe:192.0.2.17 over the IPv4 interface. In the same fashion, 308 'B' configures a site-interior IPv4 interface with address 309 192.0.2.33/28, then configures its advertising ISATAP router 310 interface with link-local ISATAP address fe80::5efe:192.0.2.33. 312 ISATAP host 'C' connects to the site via an IPv4 interface with 313 address 192.0.2.18/28, and also configures an ISATAP host interface 314 with link-local ISATAP address fe80::5efe:192.0.2.18 over the IPv4 315 interface. 'C' next resolves the PRL, and sends an IPv6-in-IPv4 316 encapsulated RS message to the IPv4 address 192.0.2.1, where IPv4 317 routing will direct it to the closest of either 'A' or 'B'. Assuming 318 'A' is closest, 'C' receives an RA from 'A' then configures a default 319 IPv6 route with next-hop address fe80::5efe:192.0.2.17 via the ISATAP 320 interface and processes the IPv6 prefix 2001:db8::/64 advertised in 321 the PIO. If the A flag is set in the PIO, 'C' uses SLAAC to 322 automatically configure the ISATAP address 2001:db8::5efe:192.0.2.18 323 and assigns the address to the ISATAP interface. If the L flag is 324 set, 'C' also assigns the prefix 2001:db8::/64 to the ISATAP 325 interface. 327 In the same fashion, ISATAP host 'D' configures its IPv4 interface 328 with address 192.0.2.34/28 and configures its ISATAP interface with 329 link-local ISATAP address fe80::5efe:192.0.2.34. 'D' next performs 330 an anycast RS/RA exchange that is serviced by 'B', then uses SLAAC to 331 autoconfigure the ISATAP address 2001:db8::5efe:192.0.2.34 and a 332 default IPv6 route with next-hop address fe80::5efe:192.0.2.33. 333 Finally, IPv6 host 'E' connects to an IPv6 network outside of the 334 site. 'E' configures its IPv6 interface in a manner specific to its 335 attached IPv6 link, and autoconfigures the IPv6 address 336 2001:db8:1::1. 338 Following this autoconfiguration, when host 'C' inside the site has 339 an IPv6 packet to send to host 'E' outside the site, it prepares the 340 packet with source address 2001:db8::5efe:192.0.2.18 and destination 341 address 2001:db8:1::1. 'C' then uses IPv6-in-IPv4 encapsulation to 342 forward the packet to the link-local address of its default router 343 'A' (i.e., fe80::5efe:192.0.2.17). 'A' in turn decapsulates the 344 packet and forwards it into the public IPv6 Internet where it will be 345 conveyed to 'E' via normal IPv6 routing. In the same fashion, host 346 'D' uses IPv6-in-IPv4 encapsulation via its default router 'B' to 347 send IPv6 packets to IPv6 Internet hosts such as 'E'. 349 When host 'E' outside the site sends IPv6 packets to ISATAP host 'C' 350 inside the site, the IPv6 routing system may direct the packet to 351 either of 'A' or 'B'. If the site is not partitioned internally, the 352 router that receives the packet can use ISATAP to statelessly forward 353 the packet directly to 'C'. If the site may be partitioned 354 internally, however, the packet must first be forwarded to 'C's 355 serving router based on IPv6 routing information. This implies that, 356 in a partitioned site, the advertising ISATAP routers must connect 357 within a full or partial mesh of IPv6 links, and must either run a 358 dynamic IPv6 routing protocol or configure static routes so that 359 incoming IPv6 packets can be forwarded to the correct serving router. 361 In this example, 'A' can configure the IPv6 route 2001:db8::5efe: 362 192.0.2.32/124 with the IPv6 address of the next hop toward 'B' in 363 the mesh network as the next hop, and 'B' can configure the IPv6 364 route 2001:db8::5efe:192.0.2.16/124 with the IPv6 address of the next 365 hop toward 'A' as the next hop. (Notice that the /124 prefixes 366 properly cover the /28 prefix of the IPv4 address that is embedded 367 within the IPv6 ISATAP address.) In that case, when 'A' receives a 368 packet from the IPv6 Internet with destination address 2001:db8:: 369 5efe:192.0.2.34, it first forwards the packet toward 'B' over an IPv6 370 mesh link. 'B' in turn uses ISATAP to forward the packet into the 371 site, where IPv4 routing will direct it to 'D'. In the same fashion, 372 when 'B' receives a packet from the IPv6 Internet with destination 373 address 2001:db8::5efe:192.0.2.18, it first forwards the packet 374 toward 'A' over an IPv6 mesh link. 'A' then uses ISATAP to forward 375 the packet into the site, where IPv4 routing will direct it to 'C'. 377 Finally, when host 'C' inside the site connects to host 'D' inside 378 the site, it has the option of using the native IPv4 service or the 379 ISATAP IPv6-in-IPv4 encapsulation service. When there is operational 380 assurance that IPv4 services between the two hosts are available, the 381 hosts may be better served to continue to use legacy IPv4 services in 382 order to avoid encapsulation overhead and to avoid any IPv4 383 protocol-41 filtering middleboxes that may be in the path. If 'C' 384 and 'D' may be in different IPv4 network partitions, however, IPv6- 385 in-IPv4 encapsulation should be used with one or both of routers 'A' 386 and 'B' serving as intermediate gateways. 388 3.5. Reference Operational Scenario - Individual Prefix Model 390 Figure 2 depicts a reference ISATAP network topology for allowing 391 hosts within a predominantly IPv4 site to configure ISATAP services 392 using SLAAC with the individual prefix model. The scenario shows two 393 advertising ISATAP routers ('A', 'B'), two ISATAP hosts ('C', 'D'), 394 and an ordinary IPv6 host ('E') outside of the site in a typical 395 deployment configuration. In the figure, ISATAP routers 'A' and 'B' 396 both advertise different prefixes taken from the aggregated prefix 397 2001:db8::/48, with 'A' advertising 2001:db8:0:1::/64 and 'B' 398 advertising 2001:db8:0:2::/64. 400 .-(::::::::) 2001:db8:1::1 401 .-(::: IPv6 :::)-. +-------------+ 402 (:::: Internet ::::) | IPv6 Host E | 403 `-(::::::::::::)-' +-------------+ 404 `-(::::::)-' 405 +------------+ +------------+ 406 | Router A |---.---| Router B |. 407 ,| (isatap) | | (isatap) | `\ 408 . | 192.0.2.1 | | 192.0.2.1 | \ 409 / +------------+ +------------+ \ 410 : fe80::*:192.0.2.17 fe80::*:192.0.2.33 : 411 \ 2001:db8:0:1::/64 2001:db8:0:2::/64 / 412 : : 413 : : 414 +- IPv4 Site -+ 415 ; (PRL: 192.0.2.1) : 416 | ; 417 : -+-' 418 `-. .) 419 \ _) 420 `-----+--------)----+'----' 421 fe80::*:192.0.2.18 fe80::*:192.0.2.34 422 2001:db8:0:1::*:192.0.2.18 2001:db8:0:2::*:192.0.2.34 423 +--------------+ +--------------+ 424 | (isatap) | | (isatap) | 425 | Host C | | Host D | 426 +--------------+ +--------------+ 428 (* == "5efe") 430 Figure 2: Reference ISATAP Network Topology using Individual Prefix 431 Model 433 With reference to Figure 2, advertising ISATAP routers 'A' and 'B' 434 within the IPv4 site connect to the IPv6 Internet either directly or 435 via a companion gateway (e.g., as shown in Figure 3). Router 'A' 436 advertises the individual prefix 2001:db8:0:1::/64 into the IPv6 437 Internet routing system, and router 'B' advertises the individual 438 prefix 2001:db8:0:2::/64. The routers could instead both advertise a 439 shorter shared prefix such as 2001:db8::/48 into the IPv6 routing 440 system, but in that case they would need to configure a mesh of IPv6 441 links between themselves in the same fashion as described for the 442 shared prefix model in Section 3.4. For the purpose of this example, 443 we also assume that the IPv4 site is configured within multiple IPv4 444 subnets - each with an IPv4 prefix length of /28. 446 Advertising ISATAP routers 'A' and 'B' both configure the IPv4 447 anycast address 192.0.2.1, e.g., on a loopback interface, and the 448 site administrator places the single IPv4 address 192.0.2.1 in the 449 PRL for the site. 'A' and 'B' then both advertise the anycast 450 address/prefix into the site's IPv4 routing system so that ISATAP 451 clients can locate the router that is topologically closest. 453 Advertising ISATAP router 'A' next configures a site-interior IPv4 454 interface with address 192.0.2.17/28, then configures an advertising 455 ISATAP router interface with link-local ISATAP address fe80::5efe: 456 192.0.2.17 over the IPv4 interface. In the same fashion, 'B' 457 configures the IPv4 interface address 192.0.2.33/28, then configures 458 its advertising ISATAP router interface with link-local ISATAP 459 address fe80::5efe:192.0.2.33. 461 ISATAP host 'C' connects to the site via an IPv4 interface with 462 address 192.0.2.18/28, and also configures an ISATAP host interface 463 with link-local ISATAP address fe80::5efe:192.0.2.18 over the IPv4 464 interface. 'C' next resolves the PRL, and sends an IPv6-in-IPv4 465 encapsulated RS message to the IPv4 address 192.0.2.1, where IPv4 466 routing will direct it to the closest of either 'A' or 'B'. Assuming 467 'A' is closest, 'C' receives an RA from 'A' then configures a default 468 IPv6 route with next-hop address fe80::5efe:192.0.2.17 via the ISATAP 469 interface and processes the IPv6 prefix 2001:db8:0:1::/64 advertised 470 in the PIO. If the A flag is set in the PIO, 'C' uses SLAAC to 471 automatically configure the ISATAP address 2001:db8:0:1::5efe: 472 192.0.2.18 and assigns the address to the ISATAP interface. If the L 473 flag is set, 'C' also assigns the prefix 2001:db8:0:1::/64 to the 474 ISATAP interface. 476 In the same fashion, ISATAP host 'D' configures its IPv4 interface 477 with address 192.0.2.34/28 and configures its ISATAP interface with 478 link-local ISATAP address fe80::5efe:192.0.2.34. 'D' next performs 479 an anycast RS/RA exchange that is serviced by 'B', then uses SLAAC to 480 autoconfigure the ISATAP address 2001:db8:0:2::5efe:192.0.2.34 and a 481 default IPv6 route with next-hop address fe80::5efe:192.0.2.33. 482 Finally, IPv6 host 'E' connects to an IPv6 network outside of the 483 site. 'E' configures its IPv6 interface in a manner specific to its 484 attached IPv6 link, and autoconfigures the IPv6 address 485 2001:db8:1::1. 487 Following this autoconfiguration, when host 'C' inside the site has 488 an IPv6 packet to send to host 'E' outside the site, it prepares the 489 packet with source address 2001:db8:0:1::5efe:192.0.2.18 and 490 destination address 2001:db8:1::1. 'C' then uses IPv6-in-IPv4 491 encapsulation to forward the packet to the link-local ISATAP address 492 of 'A' (fe80::5efe:192.0.2.17), where 'A' in turn decapsulates the 493 packet and forwards it into the public IPv6 Internet where it will be 494 conveyed to 'E' via normal IPv6 routing. In the same fashion, host 495 'D' uses IPv6-in-IPv4 encapsulation via its default router 'B' to 496 send IPv6 packets to IPv6 Internet hosts such as 'E'. 498 When host 'E' outside the site sends IPv6 packets to ISATAP host 'C' 499 inside the site, the IPv6 routing system will direct the packet to 500 'A' since 'A' advertises the individual prefix that matches 'C's 501 destination address. 'A' can then use ISATAP to statelessly forward 502 the packet directly to 'C'. If 'A' and 'B' both advertise the shared 503 shorter prefix 2001:db8::/48 into the IPv6 routing system, however 504 packets coming from 'E' may be directed to either 'A' or 'B'. In 505 that case, the advertising ISATAP routers must connect within a full 506 or partial mesh of IPv6 links the same as for the shared prefix 507 model, and must either run a dynamic IPv6 routing protocol or 508 configure static routes so that incoming IPv6 packets can be 509 forwarded to the correct serving router. 511 In this example, 'A' can configure the IPv6 route 2001:db8:0:2::/64 512 with the IPv6 address of the next hop toward 'B' in the mesh network 513 as the next hop, and 'B' can configure the IPv6 route 2001:db8: 514 0.1::/64 with the IPv6 address of the next hop toward 'A' as the next 515 hop. Then, when 'A' receives a packet from the IPv6 Internet with 516 destination address 2001:db8:0:2::5efe:192.0.2.34, it first forwards 517 the packet toward 'B' over an IPv6 mesh link. 'B' in turn uses 518 ISATAP to forward the packet into the site, where IPv4 routing will 519 direct it to 'D'. In the same fashion, when 'B' receives a packet 520 from the IPv6 Internet with destination address 2001:db8:0:1::5efe: 521 192.0.2.18, it first forwards the packet toward 'A' over an IPv6 mesh 522 link. 'A' then uses ISATAP to forward the packet into the site, 523 where IPv4 routing will direct it to 'C'. 525 Finally, when host 'C' inside the site connects to host 'D' inside 526 the site, it has the option of using the native IPv4 service or the 527 ISATAP IPv6-in-IPv4 encapsulation service. When there is operational 528 assurance that IPv4 services between the two hosts are available, the 529 hosts may be better served to continue to use legacy IPv4 services in 530 order to avoid encapsulation overhead and to avoid any IPv4 531 protocol-41 filtering middleboxes that may be in the path. If 'C' 532 and 'D' may be in different IPv4 network partitions, however, IPv6- 533 in-IPv4 encapsulation should be used with one or both of routers 'A' 534 and 'B' serving as intermediate gateways. 536 3.6. SLAAC Site Administration Guidance 538 In common practice, firewalls, gateways and packet filtering devices 539 of various forms are often deployed in order to divide the site into 540 separate partitions. In both the shared and individual prefix models 541 described above, the entire site can be represented by the aggregate 542 IPv6 prefix assigned to the site, while each site partition can be 543 represented by "sliver" IPv6 prefixes taken from the aggregate. In 544 order to provide a simple service that does not interact poorly with 545 the site topology, site administrators should therefore institute an 546 address plan to align IPv6 sliver prefixes with IPv4 site partition 547 boundaries. 549 For example, in the shared prefix model in Section 3.4, the aggregate 550 prefix is 2001:db8::/64, and the sliver prefixes are 2001:db8::5efe: 551 192.0.2.0/124, 2001:db8::5efe:192.0.2.16/124, 2001:db8::5efe: 552 192.0.2.32/124, etc. In the individual prefix model in Section 3.5, 553 the aggregate prefix is 2001:db8::/48 and the sliver prefixes are 554 2001:db8:0:0::/64, 2001:db8:0:1::/64, 2001:db8:0:2::/64, etc. 556 When individual prefixes are used, site administrators can configure 557 advertising ISATAP routers to advertise different individual (sliver) 558 prefixes to different sets of clients, e.g., based on the client's 559 IPv4 subnet prefix. When a shared prefix is used, the site 560 administrator could instead configure the ISATAP routers to advertise 561 the shared (aggregate) prefix to all clients. 563 Advertising ISATAP routers can set the L flag in each advertised 564 prefix as an indication to clients as to when ISATAP IPv6 services 565 should be preferred or de-preferred with respect to native IPv4 566 services. For example, if an advertsing router advertises a prefix 567 to multiple clients which might not be able to send IPv6-in-IPv4 568 encapsulated packets to each other directly within the site, the 569 router should set the L flag to 0 as an indication that IPv4 should 570 be preferred over IPv6 destinations that configure addresses from the 571 same prefix. (Otherwise, the clients would be obliged to use the 572 advertising ISATAP router as an IPv6 first-hop toward the destination 573 even though the destination could be reached directly via IPv4.) 575 Site administrators can instead (or in addition) implement address 576 selection policy rules [RFC3484] through explicit configurations in 577 each ISATAP client. Site administrators implement this policy by 578 configuring address selection policy rules [RFC3484] in each ISATAP 579 client in order to give preference to IPv4 destination addresses over 580 destination addresses derived from one of the client's IPv6 sliver 581 prefixes. 583 For example, site administrators can configure each ISATAP client 584 associated with a sliver prefix such as 2001:db8::5efe:192.0.2.64/124 585 to add the prefix to its address selection policy table with a lower 586 precedence than the prefix ::ffff:0:0/96. In this way, IPv4 587 addresses are preferred over IPv6 addresses from within the same 588 sliver. The prefix could be added to each ISATAP client either 589 manually, or through an automated service such as a DHCP option 590 [I-D.ietf-6man-addr-select-opt]. In this way, clients will use IPv4 591 communications to reach correspondents within the same IPv4 site 592 partition, and will use IPv6 communications to reach correspondents 593 in other partitions and/or outside of the site. 595 It should be noted that sliver prefixes longer than /64 cannot be 596 advertised for SLAAC purposes. Also, sliver prefixes longer than /64 597 do not allow for interface identifier rewriting by address 598 translators. These factors may favor the individual prefix model in 599 some deployment scenarios, while the flexibility afforded by the 600 shared prefix model may be more desirable in others. 602 3.7. Loop Avoidance 604 In sites that provide IPv6 services through ISATAP with SLAAC as 605 described in this section, advertising ISATAP routers must take 606 operational precautions to avoid routing loops. For example, with 607 reference to Figure 2 an IPv6 packet that enters the site via 608 advertising ISATAP router 'A' must not be allowed to exit the site 609 via advertising ISATAP router 'B' based on an invalid SLAAC address. 611 As a simple mitigation, each advertising ISATAP router should drop 612 any packets coming from the IPv6 Internet that would be forwarded 613 back to the Internet via another advertising router. Additionally, 614 each advertising ISATAP router should drop any encapsulated packets 615 received from another advertising router that would be forwarded to 616 the IPv6 Internet. (Note that IPv6 packets with link-local ISATAP 617 addresses are excluded from these checks, since they cannot be 618 forwarded by an IPv6 router and may be necessary for router-to-router 619 coordinations.) This corresponds to the mitigation documented in 620 Section 3.2.3 of [I-D.ietf-v6ops-tunnel-loops], but other mitigations 621 specified in that document can also be employed. 623 Again with reference to Figure 2, when 'A' receives a packet coming 624 from the IPv6 Internet with destination address 2001:db8:1::5efe: 625 192.0.2.2, it drops the packet since the IPv4 address 192.0.2.2 626 corresponds to advertising ISATAP router 'B'. Similarly, when 'B' 627 receives a packet coming from the tunnel with an IPv6 destination 628 address that would cause the packet to be forwarded back out to the 629 IPv6 Internet and with an IPv4 source address 192.0.2.1, it drops the 630 packet since 192.0.2.1 corresponds to advertising ISATAP router 'A'. 632 4. DHCPv6 Services 634 Whether or not advertising ISATAP routers make stateless IPv6 635 services available using SLAAC, they can also provide managed IPv6 636 services to ISATAP clients (i.e., both hosts and non-advertising 637 ISATAP routers) using the Dynamic Host Configuration Protocol for 638 IPv6 (DHCPv6). Any addresses/prefixes obtained via DHCPv6 are 639 distinct from any IPv6 prefixes advertised on the ISATAP interface 640 for SLAAC purposes, however. In this way, DHCPv6 addresses/prefixes 641 are reached by viewing the ISATAP tunnel interface as a "transit" 642 rather than viewing it as an ordinary IPv6 host interface. We refer 643 to this as the "no prefix" model. 645 ISATAP nodes employ the source address verification checks specified 646 in Section 7.3 of [RFC5214] as a prerequisite for decapsulation of 647 packets received on an ISATAP interface. In order to accommodate 648 direct communications with hosts and non-advertising ISATAP routers 649 that use DHCPv6, ISATAP nodes that support route optimization must 650 employ an additional source address verification check. Namely, the 651 node also considers the outer IPv4 source address correct for the 652 inner IPv6 source address if: 654 o a forwarding table entry exists that lists the packet's IPv4 655 source address as the link-layer address corresponding to the 656 inner IPv6 source address via the ISATAP interface. 658 The following sections discuss operational considerations for 659 enabling ISATAP DHCPv6 services within predominantly IPv4 sites. 661 4.1. Advertising ISATAP Router Behavior 663 Advertising ISATAP routers that support DHCPv6 services send RA 664 messages in response to RS messages received on an advertising ISATAP 665 interface. Advertising ISATAP routers also configure either a DHCPv6 666 relay or server function to service DHCPv6 requests received from 667 ISATAP clients. 669 4.2. Non-Advertising ISATAP Router Behavior 671 Non-advertising ISATAP routers can acquire IPv6 prefixes, e.g., 672 through the use of DHCPv6 Prefix Delegation [RFC3633] via an 673 advertising router in the same fashion as described for host-based 674 DHCPv6 stateful address autoconfiguration in Section 4.3. The 675 advertising router in turn maintains IPv6 forwarding table entries 676 that list the IPv4 address of the non-advertising router as the link- 677 layer address of the next hop toward the delegated IPv6 prefixes. 679 In many use case scenarios (e.g., small enterprise networks, MANETs, 680 etc.), advertising and non-advertising ISATAP routers can engage in a 681 proactive dynamic IPv6 routing protocol (e.g., OSPFv3, RIPng, etc.) 682 over their ISATAP interfaces so that IPv6 routing/forwarding tables 683 can be populated and standard IPv6 forwarding between ISATAP routers 684 can be used. In other scenarios (e.g., large enterprise networks, 685 highly mobile MANETs, etc.), this might be impractical dues to 686 scaling issues. When a proactive dynamic routing protocol cannot be 687 used, non-advertising ISATAP routers send RS messages to obtain RA 688 messages from an advertising ISATAP router, i.e., they act as "hosts" 689 on their non-advertising ISATAP interfaces. 691 After the non-advertising ISATAP router acquires IPv6 prefixes, it 692 can sub-delegate them to routers and links within its attached IPv6 693 edge networks, then can forward any outbound IPv6 packets coming from 694 its edge networks via other ISATAP nodes on the link. 696 4.3. ISATAP Host Behavior 698 ISATAP hosts resolve the PRL and send RS messages to obtain RA 699 messages from an advertising ISATAP router. Whether or not IPv6 700 prefixes for SLAAC are advertised, the host can acquire IPv6 701 addresses, e.g., through the use of DHCPv6 stateful address 702 autoconfiguration [RFC3315]. To acquire addresses, the host performs 703 standard DHCPv6 exchanges while mapping the IPv6 704 "All_DHCP_Relay_Agents_and_Servers" link-scoped multicast address to 705 the IPv4 address of an advertising ISATAP router. 707 After the host receives IPv6 addresses, it assigns them to its ISATAP 708 interface and forwards any of its outbound IPv6 packets via the 709 advertising router as a default router. The advertising router in 710 turn maintains IPv6 forwarding table entries that list the IPv4 711 address of the host as the link-layer address of the delegated IPv6 712 addresses. Note that IPv6 addresses acquired from DHCPv6 therefore 713 need not be ISATAP addresses, i.e., even though the addresses are 714 assigned to the ISATAP interface. 716 4.4. Reference Operational Scenario - No Prefix Model 718 Figure 3 depicts a reference ISATAP network topology that uses 719 DHCPv6. The scenario shows two advertising ISATAP routers ('A', 720 'B'), two non-advertising ISATAP routers ('C', 'E'), an ISATAP host 721 ('G'), and three ordinary IPv6 hosts ('D', 'F', 'H') in a typical 722 deployment configuration: 724 .-(::::::::) 2001:db8:3::1 725 .-(::: IPv6 :::)-. +-------------+ 726 (:::: Internet ::::) | IPv6 Host H | 727 `-(::::::::::::)-' +-------------+ 728 `-(::::::)-' 729 ,~~~~~~~~~~~~~~~~~, 730 ,----|companion gateway|--. 731 / '~~~~~~~~~~~~~~~~~' : 732 / |. 733 ,-' `. 734 ; +------------+ +------------+ ) 735 : | Router A | | Router B | / 736 : | (isatap) | | (isatap) | : fe80::*192.0.2.6 737 : | 192.0.2.1 | | 192.0.2.1 | ; 2001:db8:2::1 738 + +------------+ +------------+ \ +--------------+ 739 fe80::*:192.0.2.2 fe80::*:192.0.2.3 | (isatap) | 740 | ; | Host G | 741 : IPv4 Site -+-' +--------------+ 742 `-. (PRL: 192.0.2.1) .) 743 \ _) 744 `-----+--------)----+'----' 745 fe80::*:192.0.2.4 fe80::*:192.0.2.5 .-. 746 +--------------+ +--------------+ ,-( _)-. 747 | (isatap) | | (isatap) | .-(_ IPv6 )-. 748 | Router C | | Router E |--(__Edge Network ) 749 +--------------+ +--------------+ `-(______)-' 750 2001:db8:0::/48 2001:db8:1::/48 | 751 | 2001:db8:1::1 752 .-. +-------------+ 753 ,-( _)-. 2001:db8::1 | IPv6 Host F | 754 .-(_ IPv6 )-. +-------------+ +-------------+ 755 (__Edge Network )--| IPv6 Host D | 756 `-(______)-' +-------------+ 758 (* == "5efe") 760 Figure 3: Reference ISATAP Network Topology using No Prefix Model 762 In Figure 3, advertising ISATAP routers 'A' and 'B' within the IPv4 763 site connect to the IPv6 Internet via a companion gateway. (Note 764 that the routers may instead connect to the IPv6 Internet directly as 765 shown in Figure 1. For the purpose of this example, we also assume 766 that the IPv4 site is configured within a single IPv4 subnet. 768 Advertising ISATAP routers 'A' and 'B' both configure the IPv4 769 anycast address 192.0.2.1, e.g., on a loopback interface, and the 770 site administrator places the single IPv4 address 192.0.2.1 in the 771 PRL for the site. 'A' and 'B' then both advertise the anycast 772 address/prefix into the site's IPv4 routing system so that ISATAP 773 clients can locate the router that is topologically closest. 775 Advertising ISATAP router 'A' next configures a site-interior IPv4 776 interface with address 192.0.2.2, then configures an advertising 777 ISATAP router interface with link-local ISATAP address fe80::5efe: 778 192.0.2.2 over the IPv4 interface. In the same fashion, 'B' 779 configures the IPv4 interface address 192.0.2.3, then configures its 780 advertising ISATAP router interface with link-local ISATAP address 781 fe80::5efe:192.0.2.3. 783 Non-advertising ISATAP router 'C' connects to one or more IPv6 edge 784 networks and also connects to the site via an IPv4 interface with 785 address 192.0.2.4, but it does not advertise the site's IPv4 anycast 786 address/prefix. 'C' next configures a non-advertising ISATAP router 787 interface with link-local ISATAP address fe80::5efe:192.0.2.4, then 788 discovers router 'A' via an IPv6-in-IPv4 encapsulated RS/RA exchange. 789 'C' next receives the IPv6 prefix 2001:db8::/48 through a DHCPv6 790 prefix delegation exchange via 'A', then engages in an IPv6 routing 791 protocol over its ISATAP interface and announces the delegated IPv6 792 prefix. 'C' finally sub-delegates the prefix to its attached edge 793 networks, where IPv6 host 'D' autoconfigures the address 2001:db8::1. 795 Non-advertising ISATAP router 'E' connects to the site, configures 796 its ISATAP interface, performs an RS/RA exchange, receives a DHCPv6 797 prefix delegation, and engages in the IPv6 routing protocol the same 798 as for 'C'. In particular, 'E' configures the IPv4 address 192.0.2.5 799 and the link-local ISATAP address fe80::5efe:192.0.2.5. 'E' then 800 receives the delegated IPv6 prefix 2001:db8:1::/48 and sub-delegates 801 the prefix to its attached edge networks, where IPv6 host 'F' 802 autoconfigures IPv6 address 2001:db8:1::1. 804 ISATAP host 'G' connects to the site via an IPv4 interface with 805 address 192.0.2.6, and also configures an ISATAP host interface with 806 link-local ISATAP address fe80::5efe:192.0.2.6 over the IPv4 807 interface. 'G' next performs an anycast RS/RA exchange to discover 808 'B" and configure a default IPv6 route with next-hop address fe80:: 809 5efe:192.0.2.3. 'G' then receives the IPv6 address 2001:db8:2::1 810 from a DHCPv6 address configuration exchange via 'B'; it then assigns 811 the address to the ISATAP interface but does not assign a non-link- 812 local IPv6 prefix to the interface. 814 Finally, IPv6 host 'H' connects to an IPv6 network outside of the 815 ISATAP domain. 'H' configures its IPv6 interface in a manner 816 specific to its attached IPv6 link, and autoconfigures the IPv6 817 address 2001:db8:3::1. 819 Following this autoconfiguration, when host 'D' has an IPv6 packet to 820 send to host 'F', it prepares the packet with source address 2001: 821 db8::1 and destination address 2001:db8:1::1, then sends the packet 822 into the edge network where IPv6 forwarding will eventually convey it 823 to router 'C'. 'C' then uses IPv6-in-IPv4 encapsulation to forward 824 the packet to router 'E', since it has discovered a route to 2001: 825 db8:1::/48 with next hop 'E' via dynamic routing over the ISATAP 826 interface. Router 'E' finally sends the packet into the edge network 827 where IPv6 forwarding will eventually convey it to host 'F'. 829 In a second scenario, when 'D' has a packet to send to ISATAP host 830 'G', it prepares the packet with source address 2001:db8::1 and 831 destination address 2001:db8:2::1, then sends the packet into the 832 edge network where it will eventually be forwarded to router 'C' the 833 same as above. 'C' then uses IPv6-in-IPv4 encapsulation to forward 834 the packet to router 'A' (i.e., 'C's default router), which in turn 835 forwards the packet to 'G'. Note that this operation entails two 836 hops across the ISATAP link (i.e., one from 'C' to 'A', and a second 837 from 'A' to 'G'). If 'G' also participates in the dynamic IPv6 838 routing protocol, however, 'C' could instead forward the packet 839 directly to 'G' without involving 'A'. 841 In a third scenario, when 'D' has a packet to send to host 'H' in the 842 IPv6 Internet, the packet is forwarded to 'C' the same as above. 'C' 843 then forwards the packet to 'A', which forwards the packet into the 844 IPv6 Internet. 846 In a final scenario, when 'G' has a packet to send to host 'H' in the 847 IPv6 Internet, the packet is forwarded directly to 'B', which 848 forwards the packet into the IPv6 Internet. 850 4.5. DHCPv6 Site Administration Guidance 852 Site administrators configure advertising ISATAP routers that also 853 support the DHCPv6 relay/server function to send RA messages with the 854 M flag set to 1 as an indication to clients that the stateful DHCPv6 855 address autoconfiguration services area available. If stateless 856 DHCPv6 services are also available, the RA messages also set the O 857 flag to 1. 859 As discussed in Section 3.5, gateways and packet filtering devices of 860 various forms are often deployed in order to divide the site into 861 separate partitions. Although the purely DHCPv6 model does not 862 involve the advertisement of non-link-local IPv6 prefixes on ISATAP 863 interfaces, alignment of IPv6 prefixes used for DHCPv6 address 864 assignment with IPv4 site partitions is still recommended so that 865 ISATAP clients can prefer native IPv4 communications over ISATAP IPv6 866 services for correspondents within their contiguous IPv4 partition. 868 For example, if the site is assigned the aggregate prefix 2001: 869 db8::/48, then the site administrators can assign the sliver prefixes 870 2001:db8:0:0::/64, 2001:db8:0:1::/64, 2001:db8:0:2::/64, etc. to the 871 different IPv4 partitions within the site. The administrators can 872 then institute a policy that prefers native IPv4 addresses for 873 communications between clients covered by the same IPv6 sliver 874 prefix. 876 Site administrators can implement this policy implicitly by 877 configuring advertising ISATAP routers to advertise each sliver 878 prefix with both the A and L flags set to 0 as an indication that 879 IPv4 should be preferred over IPv6 destinations that configure 880 addresses from the same prefix. Site administrators can instead (or 881 in addition) implement address selection policy rules [RFC3484] 882 through explicit configurations in each ISATAP client. 884 For example, each ISATAP client associated with the sliver prefix 885 2001:db8:0:0::/64 can add the prefix to its address selection policy 886 table with a lower precedence than the prefix ::ffff:0:0/96. In this 887 way, IPv4 addresses are preferred over IPv6 addresses from within the 888 same sliver. The prefix could be added to each ISATAP client either 889 manually, or through an automated service such as a DHCP option 890 [I-D.ietf-6man-addr-select-opt]. In this way, clients will use IPv4 891 communications to reach correspondents within the same IPv4 site 892 partition, and will use IPv6 communications to reach correspondents 893 in other partitions and/or outside of the site. 895 4.6. On-Demand Dynamic Routing for DHCP 897 With respect to the reference operational scenarios depicted in 898 Figure 3, there may be use cases in which a proactive dynamic IPv6 899 routing protocol cannot be used. For example, in large enterprise 900 network deployments it would be impractical for all ISATAP routers to 901 engage in a common routing protocol instance due to scaling 902 considerations. 904 In those cases, an on-demand routing capability can be enabled in 905 which ISATAP nodes send initial packets via an advertising ISATAP 906 router and receive redirection messages back. For example, when a 907 non-advertising ISATAP router 'C' has a packet to send to a host 908 located behind non-advertising ISATAP router 'E', it can send the 909 initial packets via advertising router 'A' which will return 910 redirection messages to inform 'C' that 'E' is a better first hop. 911 Protocol details for this redirection procedure (including a means 912 for detecting whether the direct path is usable) are specified in 913 [I-D.templin-aero]. 915 4.7. Loop Avoidance 917 In a purely DHCPv6-based ISATAP deployment, no non-link-local IPv6 918 prefixes are assigned to ISATAP router interfaces. Therefore, an 919 ISATAP router cannot mistake another router for an ISATAP host due to 920 an address that matches an on-link prefix. This corresponds to the 921 mitigation documented in Section 3.2.4 of 922 [I-D.ietf-v6ops-tunnel-loops]. 924 Any routing loops introduced in the DHCPv6 scenario would therefore 925 be due to a misconfiguration in IPv6 routing the same as for any IPv6 926 router, and hence are out of scope for this document. 928 5. Scaling Considerations 930 Sections 3 and 4 depict ISATAP network topologies with only two 931 advertising ISATAP routers within the site. In order to support 932 larger numbers of ISATAP clients (and/or multiple site partitions), 933 the site can deploy more advertising ISATAP routers to support load 934 balancing and generally shortest-path routing. 936 Such an arrangement requires that the advertising ISATAP routers 937 participate in an IPv6 routing protocol instance so that IPv6 938 addresses/prefixes can be mapped to the correct ISATAP router. The 939 routing protocol instance can be configured as either a full mesh 940 topology involving all advertising ISATAP routers, or as a partial 941 mesh topology with each advertising ISATAP router associating with 942 one or more companion gateways. Each such companion gateway would in 943 turn participate in a full mesh between all companion gateways. 945 6. Site Renumbering Considerations 947 Advertising ISATAP routers distribute IPv6 prefixes to ISATAP clients 948 within the site via DHCPv6 and/or SLAAC. If the site subsequently 949 reconnects to a different ISP, however, the site must renumber to use 950 addresses derived from the new IPv6 prefixes 951 [RFC1900][RFC4192][RFC5887]. 953 For IPv6 services provided by SLAAC, site renumbering in the event of 954 a change in an ISP-served IPv6 prefix entails a simple renumbering of 955 IPv6 addresses and/or prefixes that are assigned to the ISATAP 956 interfaces of clients within the site. In some cases, filtering 957 rules (e.g., within site border firewall filtering tables) may also 958 require renumbering, but this operation can be automated and limited 959 to only one or a few administrative "touch points". 961 In order to renumber the ISATAP interfaces of clients within the site 962 using SLAAC, advertising ISATAP routers need only schedule the 963 services offered by the old ISP for deprecation and begin to 964 advertise the IPv6 prefixes provided by the new ISP. ISATAP client 965 interface address lifetimes will eventually expire, and the host will 966 renumber its interfaces with addresses derived from the new prefixes. 967 ISATAP clients should also eventually remove any deprecated SLAAC 968 prefixes from their address selection policy tables, but this action 969 is not time-critical. 971 Finally, site renumbering in the event of a change in an ISP-served 972 IPv6 prefix further entails locating and rewriting all IPv6 addresses 973 in naming services, databases, configuration files, packet filtering 974 rules, documentation, etc. If the site has published the IPv6 975 addresses of any site-internal nodes within the public Internet DNS 976 system, then the corresponding resource records will also need to be 977 updated during the renumbering operation. This can be accomplished 978 via secure dynamic updates to the DNS. 980 7. Path MTU Considerations 982 IPv6-in-IPv4 encapsulation overhead effectively reduces the size of 983 IPv6 packets that can traverse the tunnel in relation to the actual 984 Maximum Transmission Unit (MTU) of the underlying IPv4 network path 985 between the encapsulator and decapsulator. Two methods for 986 accommodating IPv6 path MTU discovery over IPv6-in-IPv4 tunnels 987 (i.e., the static and dynamic methods) are documented in Section 3.2 988 of [RFC4213]. 990 The static method places a "safe" upper bound on the size of IPv6 991 packets permitted to enter the tunnel, however the method can be 992 overly conservative when larger IPv4 path MTUs are available. The 993 dynamic method can accommodate much larger IPv6 packet sizes in some 994 cases, but can fail silently if the underlying IPv4 network path does 995 not return the necessary error messages. 997 This document notes that sites that include well-managed IPv4 links, 998 routers and other network middleboxes are candidates for use of the 999 dynamic MTU determination method, which may provide for a better 1000 operational IPv6 experience in the presence of IPv6-in-IPv4 tunnels. 1001 The dynamic MTU determination method can potentially also present a 1002 larger MTU to IPv6 correspondents outside of the site, since IPv6 1003 path MTU discovery is considered robust even over the wide area in 1004 the public IPv6 Internet. 1006 8. Anycast Considerations 1008 When an advertising ISATAP router configures an IPv4 anycast address, 1009 and site administrators place the address in the PRL, the router uses 1010 the anycast address as the IPv4 source address for all IPv6-in-IPv4 1011 encapsulated packets it sends. However, the router must also derive 1012 its ISATAP link-local addresses from an IPv4 unicast address assigned 1013 to an underlying IPv4 interface instead of from the anycast address. 1015 For example, if an advertising ISATAP router configures the IPv4 1016 anycast address 192.0.2.1 and also configures an ordinary IPv4 1017 interface with IPv4 unicast address 192.0.2.91, the router must 1018 configure the ISATAP link-local address fe80::5efe:192.0.2.91 and use 1019 this address as the IPv6 source / destination address in link-local 1020 messages it exchanges with other ISATAP nodes. 1022 This arrangement is necessary so that ISATAP clients can 1023 unambiguously differentiate advertising ISATAP routers. Furthermore, 1024 since the IPv4 anycast source address is a member of the PRL, ISATAP 1025 clients will accept any messages coming from the advertising router 1026 even though the IPv4 source address does not match the IPv4 address 1027 embedded in the IPv6 source address. 1029 9. Alternative Approaches 1031 [RFC4554] proposes a use of VLANs for IPv4-IPv6 coexistence in 1032 enterprise networks. The ISATAP approach provides a more flexible 1033 and broadly-applicable alternative, and with fewer administrative 1034 touch points. 1036 The tunnel broker service [RFC3053] uses point-to-point tunnels that 1037 require end users to establish an explicit administrative 1038 configuration of the tunnel far end, which may be outside of the 1039 administrative boundaries of the site. 1041 6to4 [RFC3056] and Teredo [RFC4380] provide "last resort" unmanaged 1042 automatic tunneling services when no other means for IPv6 1043 connectivity is available. These services are given lower priority 1044 when the ISATAP managed service and/or native IPv6 services are 1045 enabled. 1047 IRON [RFC6179], RANGER [RFC5720], VET [RFC5558] and SEAL [RFC5320] 1048 were developed as the "next-generation" of ISATAP and extend to a 1049 wide variety of use cases [RFC6139]. However, these technologies are 1050 not yet widely implemented or deployed. 1052 10. IANA Considerations 1054 This document has no IANA considerations. 1056 11. Security Considerations 1058 In addition to the security considerations documented in [RFC5214], 1059 sites that use ISATAP should take care to ensure that no routing 1060 loops are enabled [I-D.ietf-v6ops-tunnel-loops]. Additional security 1061 concerns with IP tunneling are documented in [RFC6169]. 1063 12. Acknowledgments 1065 The following are acknowledged for their insights that helped shape 1066 this work: Fred Baker, Brian Carpenter, Thomas Henderson, Philip 1067 Homburg, Lee Howard, Ray Hunter, Joel Jaeggli, Gabi Nakibly, Hemant 1068 Singh, Mark Smith, Ole Troan, Gunter Van de Velde, ... 1070 13. References 1072 13.1. Normative References 1074 [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and 1075 E. Lear, "Address Allocation for Private Internets", 1076 BCP 5, RFC 1918, February 1996. 1078 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., 1079 and M. Carney, "Dynamic Host Configuration Protocol for 1080 IPv6 (DHCPv6)", RFC 3315, July 2003. 1082 [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic 1083 Host Configuration Protocol (DHCP) version 6", RFC 3633, 1084 December 2003. 1086 [RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms 1087 for IPv6 Hosts and Routers", RFC 4213, October 2005. 1089 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 1090 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 1091 September 2007. 1093 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 1094 Address Autoconfiguration", RFC 4862, September 2007. 1096 [RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site 1097 Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, 1098 March 2008. 1100 13.2. Informative References 1102 [I-D.ietf-6man-addr-select-opt] 1103 Matsumoto, A., Fujisaki, T., and J. Kato, "Distributing 1104 Address Selection Policy using DHCPv6", 1105 draft-ietf-6man-addr-select-opt-00 (work in progress), 1106 December 2010. 1108 [I-D.ietf-v6ops-tunnel-loops] 1109 Nakibly, G. and F. Templin, "Routing Loop Attack using 1110 IPv6 Automatic Tunnels: Problem Statement and Proposed 1111 Mitigations", draft-ietf-v6ops-tunnel-loops-07 (work in 1112 progress), May 2011. 1114 [I-D.templin-aero] 1115 Templin, F., "Asymmetric Extended Route Optimization 1116 (AERO)", draft-templin-aero-00 (work in progress), 1117 March 2011. 1119 [RFC1687] Fleischman, E., "A Large Corporate User's View of IPng", 1120 RFC 1687, August 1994. 1122 [RFC1900] Carpenter, B. and Y. Rekhter, "Renumbering Needs Work", 1123 RFC 1900, February 1996. 1125 [RFC2491] Armitage, G., Schulter, P., Jork, M., and G. Harter, "IPv6 1126 over Non-Broadcast Multiple Access (NBMA) networks", 1127 RFC 2491, January 1999. 1129 [RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4 1130 Domains without Explicit Tunnels", RFC 2529, March 1999. 1132 [RFC2983] Black, D., "Differentiated Services and Tunnels", 1133 RFC 2983, October 2000. 1135 [RFC3053] Durand, A., Fasano, P., Guardini, I., and D. Lento, "IPv6 1136 Tunnel Broker", RFC 3053, January 2001. 1138 [RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains 1139 via IPv4 Clouds", RFC 3056, February 2001. 1141 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition 1142 of Explicit Congestion Notification (ECN) to IP", 1143 RFC 3168, September 2001. 1145 [RFC3484] Draves, R., "Default Address Selection for Internet 1146 Protocol version 6 (IPv6)", RFC 3484, February 2003. 1148 [RFC4192] Baker, F., Lear, E., and R. Droms, "Procedures for 1149 Renumbering an IPv6 Network without a Flag Day", RFC 4192, 1150 September 2005. 1152 [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through 1153 Network Address Translations (NATs)", RFC 4380, 1154 February 2006. 1156 [RFC4554] Chown, T., "Use of VLANs for IPv4-IPv6 Coexistence in 1157 Enterprise Networks", RFC 4554, June 2006. 1159 [RFC5320] Templin, F., "The Subnetwork Encapsulation and Adaptation 1160 Layer (SEAL)", RFC 5320, February 2010. 1162 [RFC5558] Templin, F., "Virtual Enterprise Traversal (VET)", 1163 RFC 5558, February 2010. 1165 [RFC5720] Templin, F., "Routing and Addressing in Networks with 1166 Global Enterprise Recursion (RANGER)", RFC 5720, 1167 February 2010. 1169 [RFC5887] Carpenter, B., Atkinson, R., and H. Flinck, "Renumbering 1170 Still Needs Work", RFC 5887, May 2010. 1172 [RFC5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4 1173 Infrastructures (6rd) -- Protocol Specification", 1174 RFC 5969, August 2010. 1176 [RFC6139] Russert, S., Fleischman, E., and F. Templin, "Routing and 1177 Addressing in Networks with Global Enterprise Recursion 1178 (RANGER) Scenarios", RFC 6139, February 2011. 1180 [RFC6169] Krishnan, S., Thaler, D., and J. Hoagland, "Security 1181 Concerns with IP Tunneling", RFC 6169, April 2011. 1183 [RFC6179] Templin, F., "The Internet Routing Overlay Network 1184 (IRON)", RFC 6179, March 2011. 1186 Author's Address 1188 Fred L. Templin 1189 Boeing Research & Technology 1190 P.O. Box 3707 MC 7L-49 1191 Seattle, WA 98124 1192 USA 1194 Email: fltemplin@acm.org