idnits 2.17.1 draft-templin-v6ops-isops-06.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (May 24, 2011) is 4714 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- == Unused Reference: 'RFC1918' is defined on line 1051, 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reference: RFC 3315 (Obsoleted by RFC 8415) ** Obsolete normative reference: RFC 3633 (Obsoleted by RFC 8415) == Outdated reference: A later version (-13) exists of draft-ietf-6man-addr-select-opt-00 == Outdated reference: A later version (-12) exists of draft-templin-aero-00 -- Obsolete informational reference (is this intentional?): RFC 3484 (Obsoleted by RFC 6724)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 24, 2011 5 Expires: November 25, 2011 7 Operational Guidance for IPv6 Deployment in IPv4 Sites using ISATAP 8 draft-templin-v6ops-isops-06.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 25, 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 . . . . . . . . . . . . . . . . . . . . . . 20 78 5. Scaling Considerations . . . . . . . . . . . . . . . . . . . . 21 79 6. Site Renumbering Considerations . . . . . . . . . . . . . . . 21 80 7. Path MTU Considerations . . . . . . . . . . . . . . . . . . . 22 81 8. Anycast Considerations . . . . . . . . . . . . . . . . . . . . 22 82 9. Alternative Approaches . . . . . . . . . . . . . . . . . . . . 23 83 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 84 11. Security Considerations . . . . . . . . . . . . . . . . . . . 23 85 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23 86 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24 87 13.1. Normative References . . . . . . . . . . . . . . . . . . . 24 88 13.2. Informative References . . . . . . . . . . . . . . . . . . 24 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 would be better served to continue to use legacy IPv4 services 382 in 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 would be better served to continue to use legacy IPv4 services 530 in 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 with L=0 so that clients will not 562 consider any IPv6 addresses derived from the prefix as on-link. 564 Site administrators can then institute a policy that prefers native 565 IPv4 addresses over ISATAP addresses for communications between 566 clients covered by the same sliver prefix. Site administrators 567 implement this policy by configuring address selection policy rules 568 [RFC3484] in each ISATAP client in order to give preference to IPv4 569 destination addresses over destination addresses derived from one of 570 the client's IPv6 sliver prefixes. 572 For example, each ISATAP client associated with the sliver prefix 573 2001:db8::5efe:192.0.2.64/124 can add the prefix to its address 574 selection policy table with a lower precedence than the prefix 575 ::ffff:0:0/96. In this way, IPv4 addresses are preferred over IPv6 576 addresses from within the same sliver. The prefix could be added to 577 each ISATAP client either manually, or through an automated service 578 such as a DHCP option [I-D.ietf-6man-addr-select-opt]. In this way, 579 clients will use IPv4 communications to reach correspondents within 580 the same IPv4 site partition, and will use IPv6 communications to 581 reach correspondents in other partitions and/or outside of the site. 583 It should be noted that sliver prefixes longer than /64 cannot be 584 advertised for SLAAC purposes. Also, sliver prefixes longer than /64 585 do not allow for interface identifier rewriting by address 586 translators. These factors may favor the individual prefix model in 587 some deployment scenarios, while the flexibility afforded by the 588 shared prefix model may be more desirable in others. 590 3.7. Loop Avoidance 592 In sites that provide IPv6 services through ISATAP with SLAAC as 593 described in this section, advertising ISATAP routers must take 594 operational precautions to avoid routing loops. For example, with 595 reference to Figure 2 an IPv6 packet that enters the site via 596 advertising ISATAP router 'A' must not be allowed to exit the site 597 via advertising ISATAP router 'B' based on an invalid SLAAC address. 599 As a simple mitigation, each advertising ISATAP router should drop 600 any packets coming from the IPv6 Internet that would be forwarded 601 back to the Internet via another advertising router. Additionally, 602 each advertising ISATAP router should drop any encapsulated packets 603 received from another advertising router that would be forwarded to 604 the IPv6 Internet. (Note that IPv6 packets with link-local ISATAP 605 addresses are excluded from these checks, since they cannot be 606 forwarded by an IPv6 router and may be necessary for router-to-router 607 coordinations.) This corresponds to the mitigation documented in 608 Section 3.2.3 of [I-D.ietf-v6ops-tunnel-loops], but other mitigations 609 specified in that document can also be employed. 611 Again with reference to Figure 2, when 'A' receives a packet coming 612 from the IPv6 Internet with destination address 2001:db8:1::5efe: 613 192.0.2.2, it drops the packet since the IPv4 address 192.0.2.2 614 corresponds to advertising ISATAP router 'B'. Similarly, when 'B' 615 receives a packet coming from the tunnel with an IPv6 destination 616 address that would cause the packet to be forwarded back out to the 617 IPv6 Internet and with an IPv4 source address 192.0.2.1, it drops the 618 packet since 192.0.2.1 corresponds to advertising ISATAP router 'A'. 620 4. DHCPv6 Services 622 Whether or not advertising ISATAP routers make stateless IPv6 623 services available using SLAAC, they can also provide managed IPv6 624 services to ISATAP clients (i.e., both hosts and non-advertising 625 ISATAP routers) using the Dynamic Host Configuration Protocol for 626 IPv6 (DHCPv6). Any addresses/prefixes obtained via DHCPv6 are 627 distinct from any IPv6 prefixes advertised on the ISATAP interface 628 for SLAAC purposes, however. In this way, DHCPv6 addresses/prefixes 629 are reached by viewing the ISATAP tunnel interface as a "transit" 630 rather than viewing it as an ordinary IPv6 host interface. We refer 631 to this as the "no prefix" model. 633 ISATAP nodes employ the source address verification checks specified 634 in Section 7.3 of [RFC5214] as a prerequisite for decapsulation of 635 packets received on an ISATAP interface. In order to accommodate 636 direct communications with hosts and non-advertising ISATAP routers 637 that use DHCPv6, ISATAP nodes that support route optimization must 638 employ an additional source address verification check. Namely, the 639 node also considers the outer IPv4 source address correct for the 640 inner IPv6 source address if: 642 o a forwarding table entry exists that lists the packet's IPv4 643 source address as the link-layer address corresponding to the 644 inner IPv6 source address via the ISATAP interface. 646 The following sections discuss operational considerations for 647 enabling ISATAP DHCPv6 services within predominantly IPv4 sites. 649 4.1. Advertising ISATAP Router Behavior 651 Advertising ISATAP routers that support DHCPv6 services send RA 652 messages in response to RS messages received on an advertising ISATAP 653 interface. Advertising ISATAP routers also configure either a DHCPv6 654 relay or server function to service DHCPv6 requests received from 655 ISATAP clients. 657 4.2. Non-Advertising ISATAP Router Behavior 659 Non-advertising ISATAP routers can acquire IPv6 prefixes, e.g., 660 through the use of DHCPv6 Prefix Delegation [RFC3633] via an 661 advertising router in the same fashion as described for host-based 662 DHCPv6 stateful address autoconfiguration in Section 4.3. The 663 advertising router in turn maintains IPv6 forwarding table entries 664 that list the IPv4 address of the non-advertising router as the link- 665 layer address of the next hop toward the delegated IPv6 prefixes. 667 In many use case scenarios (e.g., small enterprise networks, MANETs, 668 etc.), advertising and non-advertising ISATAP routers can engage in a 669 proactive dynamic IPv6 routing protocol (e.g., OSPFv3, RIPng, etc.) 670 over their ISATAP interfaces so that IPv6 routing/forwarding tables 671 can be populated and standard IPv6 forwarding between ISATAP routers 672 can be used. In other scenarios (e.g., large enterprise networks, 673 highly mobile MANETs, etc.), this might be impractical dues to 674 scaling issues. When a proactive dynamic routing protocol cannot be 675 used, non-advertising ISATAP routers send RS messages to obtain RA 676 messages from an advertising ISATAP router, i.e., they act as "hosts" 677 on their non-advertising ISATAP interfaces. 679 After the non-advertising ISATAP router acquires IPv6 prefixes, it 680 can sub-delegate them to routers and links within its attached IPv6 681 edge networks, then can forward any outbound IPv6 packets coming from 682 its edge networks via other ISATAP nodes on the link. 684 4.3. ISATAP Host Behavior 686 ISATAP hosts resolve the PRL and send RS messages to obtain RA 687 messages from an advertising ISATAP router. Whether or not IPv6 688 prefixes for SLAAC are advertised, the host can acquire IPv6 689 addresses, e.g., through the use of DHCPv6 stateful address 690 autoconfiguration [RFC3315]. To acquire addresses, the host performs 691 standard DHCPv6 exchanges while mapping the IPv6 692 "All_DHCP_Relay_Agents_and_Servers" link-scoped multicast address to 693 the IPv4 address of an advertising ISATAP router. 695 After the host receives IPv6 addresses, it assigns them to its ISATAP 696 interface and forwards any of its outbound IPv6 packets via the 697 advertising router as a default router. The advertising router in 698 turn maintains IPv6 forwarding table entries that list the IPv4 699 address of the host as the link-layer address of the delegated IPv6 700 addresses. Note that IPv6 addresses acquired from DHCPv6 therefore 701 need not be ISATAP addresses, i.e., even though the addresses are 702 assigned to the ISATAP interface. 704 4.4. Reference Operational Scenario - No Prefix Model 706 Figure 3 depicts a reference ISATAP network topology that uses 707 DHCPv6. The scenario shows two advertising ISATAP routers ('A', 708 'B'), two non-advertising ISATAP routers ('C', 'E'), an ISATAP host 709 ('G'), and three ordinary IPv6 hosts ('D', 'F', 'H') in a typical 710 deployment configuration: 712 .-(::::::::) 2001:db8:3::1 713 .-(::: IPv6 :::)-. +-------------+ 714 (:::: Internet ::::) | IPv6 Host H | 715 `-(::::::::::::)-' +-------------+ 716 `-(::::::)-' 717 ,~~~~~~~~~~~~~~~~~, 718 ,----|companion gateway|--. 719 / '~~~~~~~~~~~~~~~~~' : 720 / |. 721 ,-' `. 722 ; +------------+ +------------+ ) 723 : | Router A | | Router B | / 724 : | (isatap) | | (isatap) | : fe80::*192.0.2.6 725 : | 192.0.2.1 | | 192.0.2.1 | ; 2001:db8:2::1 726 + +------------+ +------------+ \ +--------------+ 727 fe80::*:192.0.2.2 fe80::*:192.0.2.3 | (isatap) | 728 | ; | Host G | 729 : IPv4 Site -+-' +--------------+ 730 `-. (PRL: 192.0.2.1) .) 731 \ _) 732 `-----+--------)----+'----' 733 fe80::*:192.0.2.4 fe80::*:192.0.2.5 .-. 734 +--------------+ +--------------+ ,-( _)-. 735 | (isatap) | | (isatap) | .-(_ IPv6 )-. 736 | Router C | | Router E |--(__Edge Network ) 737 +--------------+ +--------------+ `-(______)-' 738 2001:db8:0::/48 2001:db8:1::/48 | 739 | 2001:db8:1::1 740 .-. +-------------+ 741 ,-( _)-. 2001:db8::1 | IPv6 Host F | 742 .-(_ IPv6 )-. +-------------+ +-------------+ 743 (__Edge Network )--| IPv6 Host D | 744 `-(______)-' +-------------+ 746 (* == "5efe") 748 Figure 3: Reference ISATAP Network Topology using No Prefix Model 750 In Figure 3, advertising ISATAP routers 'A' and 'B' within the IPv4 751 site connect to the IPv6 Internet via a companion gateway. (Note 752 that the routers may instead connect to the IPv6 Internet directly as 753 shown in Figure 1. For the purpose of this example, we also assume 754 that the IPv4 site is configured within a single IPv4 subnet. 756 Advertising ISATAP routers 'A' and 'B' both configure the IPv4 757 anycast address 192.0.2.1, e.g., on a loopback interface, and the 758 site administrator places the single IPv4 address 192.0.2.1 in the 759 PRL for the site. 'A' and 'B' then both advertise the anycast 760 address/prefix into the site's IPv4 routing system so that ISATAP 761 clients can locate the router that is topologically closest. 763 Advertising ISATAP router 'A' next configures a site-interior IPv4 764 interface with address 192.0.2.2, then configures an advertising 765 ISATAP router interface with link-local ISATAP address fe80::5efe: 766 192.0.2.2 over the IPv4 interface. In the same fashion, 'B' 767 configures the IPv4 interface address 192.0.2.3, then configures its 768 advertising ISATAP router interface with link-local ISATAP address 769 fe80::5efe:192.0.2.3. 771 Non-advertising ISATAP router 'C' connects to one or more IPv6 edge 772 networks and also connects to the site via an IPv4 interface with 773 address 192.0.2.4, but it does not advertise the site's IPv4 anycast 774 address/prefix. 'C' next configures a non-advertising ISATAP router 775 interface with link-local ISATAP address fe80::5efe:192.0.2.4, then 776 discovers router 'A' via an IPv6-in-IPv4 encapsulated RS/RA exchange. 777 'C' next receives the IPv6 prefix 2001:db8::/48 through a DHCPv6 778 prefix delegation exchange via 'A', then engages in an IPv6 routing 779 protocol over its ISATAP interface and announces the delegated IPv6 780 prefix. 'C' finally sub-delegates the prefix to its attached edge 781 networks, where IPv6 host 'D' autoconfigures the address 2001:db8::1. 783 Non-advertising ISATAP router 'E' connects to the site, configures 784 its ISATAP interface, performs an RS/RA exchange, receives a DHCPv6 785 prefix delegation, and engages in the IPv6 routing protocol the same 786 as for 'C'. In particular, 'E' configures the IPv4 address 192.0.2.5 787 and the link-local ISATAP address fe80::5efe:192.0.2.5. 'E' then 788 receives the delegated IPv6 prefix 2001:db8:1::/48 and sub-delegates 789 the prefix to its attached edge networks, where IPv6 host 'F' 790 autoconfigures IPv6 address 2001:db8:1::1. 792 ISATAP host 'G' connects to the site via an IPv4 interface with 793 address 192.0.2.6, and also configures an ISATAP host interface with 794 link-local ISATAP address fe80::5efe:192.0.2.6 over the IPv4 795 interface. 'G' next performs an anycast RS/RA exchange to discover 796 'B" and configure a default IPv6 route with next-hop address fe80:: 797 5efe:192.0.2.3. 'G' then receives the IPv6 address 2001:db8:2::1 798 from a DHCPv6 address configuration exchange via 'B'; it then assigns 799 the address to the ISATAP interface but does not assign a non-link- 800 local IPv6 prefix to the interface. 802 Finally, IPv6 host 'H' connects to an IPv6 network outside of the 803 ISATAP domain. 'H' configures its IPv6 interface in a manner 804 specific to its attached IPv6 link, and autoconfigures the IPv6 805 address 2001:db8:3::1. 807 Following this autoconfiguration, when host 'D' has an IPv6 packet to 808 send to host 'F', it prepares the packet with source address 2001: 809 db8::1 and destination address 2001:db8:1::1, then sends the packet 810 into the edge network where IPv6 forwarding will eventually convey it 811 to router 'C'. 'C' then uses IPv6-in-IPv4 encapsulation to forward 812 the packet to router 'E', since it has discovered a route to 2001: 813 db8:1::/48 with next hop 'E' via dynamic routing over the ISATAP 814 interface. Router 'E' finally sends the packet into the edge network 815 where IPv6 forwarding will eventually convey it to host 'F'. 817 In a second scenario, when 'D' has a packet to send to ISATAP host 818 'G', it prepares the packet with source address 2001:db8::1 and 819 destination address 2001:db8:2::1, then sends the packet into the 820 edge network where it will eventually be forwarded to router 'C' the 821 same as above. 'C' then uses IPv6-in-IPv4 encapsulation to forward 822 the packet to router 'A' (i.e., 'C's default router), which in turn 823 forwards the packet to 'G'. Note that this operation entails two 824 hops across the ISATAP link (i.e., one from 'C' to 'A', and a second 825 from 'A' to 'G'). If 'G' also participates in the dynamic IPv6 826 routing protocol, however, 'C' could instead forward the packet 827 directly to 'G' without involving 'A'. 829 In a third scenario, when 'D' has a packet to send to host 'H' in the 830 IPv6 Internet, the packet is forwarded to 'C' the same as above. 'C' 831 then forwards the packet to 'A', which forwards the packet into the 832 IPv6 Internet. 834 In a final scenario, when 'G' has a packet to send to host 'H' in the 835 IPv6 Internet, the packet is forwarded directly to 'B', which 836 forwards the packet into the IPv6 Internet. 838 4.5. DHCPv6 Site Administration Guidance 840 As discussed in Section 3.5, gateways and packet filtering devices of 841 various forms are often deployed in order to divide the site into 842 separate partitions. Although the purely DHCPv6 model does not 843 involve the advertisement of non-link-local IPv6 prefixes on ISATAP 844 interfaces, alignment of IPv6 prefixes used for DHCPv6 address 845 assignment with IPv4 site partitions is still recommended so that 846 ISATAP clients can prefer native IPv4 communications over ISATAP IPv6 847 services for correspondents within their contiguous IPv4 partition. 849 For example, if the site is assigned the aggregate prefix 2001: 850 db8::/48, then the site administrators can assign the sliver prefixes 851 2001:db8:0:0::/64, 2001:db8:0:1::/64, 2001:db8:0:2::/64, etc. to the 852 different IPv4 partitions within the site. The administrators can 853 then institute a policy that prefers native IPv4 addresses for 854 communications between clients covered by the same IPv6 sliver 855 prefix. Site administrators implement this policy by configuring 856 address selection policy rules [RFC3484] in each ISATAP client in 857 order to give preference to IPv4 destination addresses over 858 destination addresses derived from one of the client's IPv6 sliver 859 prefixes. 861 For example, each ISATAP client associated with the sliver prefix 862 2001:db8:0:0::/64 can add the prefix to its address selection policy 863 table with a lower precedence than the prefix ::ffff:0:0/96. In this 864 way, IPv4 addresses are preferred over IPv6 addresses from within the 865 same sliver. The prefix could be added to each ISATAP client either 866 manually, or through an automated service such as a DHCP option 867 [I-D.ietf-6man-addr-select-opt]. In this way, clients will use IPv4 868 communications to reach correspondents within the same IPv4 site 869 partition, and will use IPv6 communications to reach correspondents 870 in other partitions and/or outside of the site. 872 4.6. On-Demand Dynamic Routing for DHCP 874 With respect to the reference operational scenarios depicted in 875 Figure 3, there may be use cases in which a proactive dynamic IPv6 876 routing protocol cannot be used. For example, in large enterprise 877 network deployments it would be impractical for all ISATAP routers to 878 engage in a common routing protocol instance due to scaling 879 considerations. 881 In those cases, an on-demand routing capability can be enabled in 882 which ISATAP nodes send initial packets via an advertising ISATAP 883 router and receive redirection messages back. For example, when a 884 non-advertising ISATAP router 'C' has a packet to send to a host 885 located behind non-advertising ISATAP router 'E', it can send the 886 initial packets via advertising router 'A' which will return 887 redirection messages to inform 'C' that 'E' is a better first hop. 888 Protocol details for this redirection procedure (including a means 889 for detecting whether the direct path is usable) are specified in 890 [I-D.templin-aero]. 892 4.7. Loop Avoidance 894 In a purely DHCPv6-based ISATAP deployment, no non-link-local IPv6 895 prefixes are assigned to ISATAP router interfaces. Therefore, an 896 ISATAP router cannot mistake another router for an ISATAP host due to 897 an address that matches an on-link prefix. This corresponds to the 898 mitigation documented in Section 3.2.4 of 899 [I-D.ietf-v6ops-tunnel-loops]. 901 Any routing loops introduced in the DHCPv6 scenario would therefore 902 be due to a misconfiguration in IPv6 routing the same as for any IPv6 903 router, and hence are out of scope for this document. 905 5. Scaling Considerations 907 Sections 3 and 4 depict ISATAP network topologies with only two 908 advertising ISATAP routers within the site. In order to support 909 larger numbers of ISATAP clients (and/or multiple site partitions), 910 the site can deploy more advertising ISATAP routers to support load 911 balancing and generally shortest-path routing. 913 Such an arrangement requires that the advertising ISATAP routers 914 participate in an IPv6 routing protocol instance so that IPv6 915 addresses/prefixes can be mapped to the correct ISATAP router. The 916 routing protocol instance can be configured as either a full mesh 917 topology involving all advertising ISATAP routers, or as a partial 918 mesh topology with each advertising ISATAP router associating with 919 one or more companion gateways. Each such companion gateway would in 920 turn participate in a full mesh between all companion gateways. 922 6. Site Renumbering Considerations 924 Advertising ISATAP routers distribute IPv6 prefixes to ISATAP clients 925 within the site via DHCPv6 and/or SLAAC. If the site subsequently 926 reconnects to a different ISP, however, the site must renumber to use 927 addresses derived from the new IPv6 prefixes 928 [RFC1900][RFC4192][RFC5887]. 930 For IPv6 services provided by SLAAC, site renumbering in the event of 931 a change in an ISP-served IPv6 prefix entails a simple renumbering of 932 IPv6 addresses and/or prefixes that are assigned to the ISATAP 933 interfaces of clients within the site. In some cases, filtering 934 rules (e.g., within site border firewall filtering tables) may also 935 require renumbering, but this operation can be automated and limited 936 to only one or a few administrative "touch points". 938 In order to renumber the ISATAP interfaces of clients within the site 939 using SLAAC, advertising ISATAP routers need only schedule the 940 services offered by the old ISP for deprecation and begin to 941 advertise the IPv6 prefixes provided by the new ISP. ISATAP client 942 interface address lifetimes will eventually expire, and the host will 943 renumber its interfaces with addresses derived from the new prefixes. 944 ISATAP clients should also eventually remove any deprecated SLAAC 945 prefixes from their address selection policy tables, but this action 946 is not time-critical. 948 Finally, site renumbering in the event of a change in an ISP-served 949 IPv6 prefix further entails locating and rewriting all IPv6 addresses 950 in naming services, databases, configuration files, packet filtering 951 rules, documentation, etc. If the site has published the IPv6 952 addresses of any site-internal nodes within the public Internet DNS 953 system, then the corresponding resource records will also need to be 954 updated during the renumbering operation. This can be accomplished 955 via secure dynamic updates to the DNS. 957 7. Path MTU Considerations 959 IPv6-in-IPv4 encapsulation overhead effectively reduces the size of 960 IPv6 packets that can traverse the tunnel in relation to the actual 961 Maximum Transmission Unit (MTU) of the underlying IPv4 network path 962 between the encapsulator and decapsulator. Two methods for 963 accommodating IPv6 path MTU discovery over IPv6-in-IPv4 tunnels 964 (i.e., the static and dynamic methods) are documented in Section 3.2 965 of [RFC4213]. 967 The static method places a "safe" upper bound on the size of IPv6 968 packets permitted to enter the tunnel, however the method can be 969 overly conservative when larger IPv4 path MTUs are available. The 970 dynamic method can accommodate much larger IPv6 packet sizes in some 971 cases, but can fail silently if the underlying IPv4 network path does 972 not return the necessary error messages. 974 This document notes that sites that include well-managed IPv4 links, 975 routers and other network middleboxes are candidates for use of the 976 dynamic MTU determination method, which may provide for a better 977 operational IPv6 experience in the presence of IPv6-in-IPv4 tunnels. 978 The dynamic MTU determination method can potentially also present a 979 larger MTU to IPv6 correspondents outside of the site, since IPv6 980 path MTU discovery is considered robust even over the wide area in 981 the public IPv6 Internet. 983 8. Anycast Considerations 985 When an advertising ISATAP router configures an IPv4 anycast address, 986 and site administrators place the address in the PRL, the router uses 987 the anycast address as the IPv4 source address for all IPv6-in-IPv4 988 encapsulated packets it sends. However, the router must also derive 989 its ISATAP link-local addresses from an IPv4 unicast address assigned 990 to an underlying IPv4 interface instead of from the anycast address. 992 For example, if an advertising ISATAP router configures the IPv4 993 anycast address 192.0.2.1 and also configures an ordinary IPv4 994 interface with IPv4 unicast address 192.0.2.91, the router must 995 configure the ISATAP link-local address fe80::5efe:192.0.2.91 and use 996 this address as the IPv6 source / destination address in link-local 997 messages it exchanges with other ISATAP nodes. 999 This arrangement is necessary so that ISATAP clients can 1000 unambiguously differentiate advertising ISATAP routers. Furthermore, 1001 since the IPv4 anycast source address is a member of the PRL, ISATAP 1002 clients will accept any messages coming from the advertising router 1003 even though the IPv4 source address does not match the IPv4 address 1004 embedded in the IPv6 source address. 1006 9. Alternative Approaches 1008 [RFC4554] proposes a use of VLANs for IPv4-IPv6 coexistence in 1009 enterprise networks. The ISATAP approach provides a more flexible 1010 and broadly-applicable alternative, and with fewer administrative 1011 touch points. 1013 The tunnel broker service [RFC3053] uses point-to-point tunnels that 1014 require end users to establish an explicit administrative 1015 configuration of the tunnel far end, which may be outside of the 1016 administrative boundaries of the site. 1018 6to4 [RFC3056] and Teredo [RFC4380] provide "last resort" unmanaged 1019 automatic tunneling services when no other means for IPv6 1020 connectivity is available. These services are given lower priority 1021 when the ISATAP managed service and/or native IPv6 services are 1022 enabled. 1024 IRON [RFC6179], RANGER [RFC5720], VET [RFC5558] and SEAL [RFC5320] 1025 were developed as the "next-generation" of ISATAP and extend to a 1026 wide variety of use cases [RFC6139]. However, these technologies are 1027 not yet widely implemented or deployed. 1029 10. IANA Considerations 1031 This document has no IANA considerations. 1033 11. Security Considerations 1035 In addition to the security considerations documented in [RFC5214], 1036 sites that use ISATAP should take care to ensure that no routing 1037 loops are enabled [I-D.ietf-v6ops-tunnel-loops]. Additional security 1038 concerns with IP tunneling are documented in [RFC6169]. 1040 12. Acknowledgments 1042 The following are acknowledged for their insights that helped shape 1043 this work: Fred Baker, Brian Carpenter, Thomas Henderson, Philip 1044 Homburg, Lee Howard, Ray Hunter, Joel Jaeggli, Gabi Nakibly, Hemant 1045 Singh, Mark Smith, Ole Troan, Gunter Van de Velde, ... 1047 13. References 1049 13.1. Normative References 1051 [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and 1052 E. Lear, "Address Allocation for Private Internets", 1053 BCP 5, RFC 1918, February 1996. 1055 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., 1056 and M. Carney, "Dynamic Host Configuration Protocol for 1057 IPv6 (DHCPv6)", RFC 3315, July 2003. 1059 [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic 1060 Host Configuration Protocol (DHCP) version 6", RFC 3633, 1061 December 2003. 1063 [RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms 1064 for IPv6 Hosts and Routers", RFC 4213, October 2005. 1066 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 1067 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 1068 September 2007. 1070 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 1071 Address Autoconfiguration", RFC 4862, September 2007. 1073 [RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site 1074 Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, 1075 March 2008. 1077 13.2. Informative References 1079 [I-D.ietf-6man-addr-select-opt] 1080 Matsumoto, A., Fujisaki, T., and J. Kato, "Distributing 1081 Address Selection Policy using DHCPv6", 1082 draft-ietf-6man-addr-select-opt-00 (work in progress), 1083 December 2010. 1085 [I-D.ietf-v6ops-tunnel-loops] 1086 Nakibly, G. and F. Templin, "Routing Loop Attack using 1087 IPv6 Automatic Tunnels: Problem Statement and Proposed 1088 Mitigations", draft-ietf-v6ops-tunnel-loops-07 (work in 1089 progress), May 2011. 1091 [I-D.templin-aero] 1092 Templin, F., "Asymmetric Extended Route Optimization 1093 (AERO)", draft-templin-aero-00 (work in progress), 1094 March 2011. 1096 [RFC1687] Fleischman, E., "A Large Corporate User's View of IPng", 1097 RFC 1687, August 1994. 1099 [RFC1900] Carpenter, B. and Y. Rekhter, "Renumbering Needs Work", 1100 RFC 1900, February 1996. 1102 [RFC2491] Armitage, G., Schulter, P., Jork, M., and G. Harter, "IPv6 1103 over Non-Broadcast Multiple Access (NBMA) networks", 1104 RFC 2491, January 1999. 1106 [RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4 1107 Domains without Explicit Tunnels", RFC 2529, March 1999. 1109 [RFC2983] Black, D., "Differentiated Services and Tunnels", 1110 RFC 2983, October 2000. 1112 [RFC3053] Durand, A., Fasano, P., Guardini, I., and D. Lento, "IPv6 1113 Tunnel Broker", RFC 3053, January 2001. 1115 [RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains 1116 via IPv4 Clouds", RFC 3056, February 2001. 1118 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition 1119 of Explicit Congestion Notification (ECN) to IP", 1120 RFC 3168, September 2001. 1122 [RFC3484] Draves, R., "Default Address Selection for Internet 1123 Protocol version 6 (IPv6)", RFC 3484, February 2003. 1125 [RFC4192] Baker, F., Lear, E., and R. Droms, "Procedures for 1126 Renumbering an IPv6 Network without a Flag Day", RFC 4192, 1127 September 2005. 1129 [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through 1130 Network Address Translations (NATs)", RFC 4380, 1131 February 2006. 1133 [RFC4554] Chown, T., "Use of VLANs for IPv4-IPv6 Coexistence in 1134 Enterprise Networks", RFC 4554, June 2006. 1136 [RFC5320] Templin, F., "The Subnetwork Encapsulation and Adaptation 1137 Layer (SEAL)", RFC 5320, February 2010. 1139 [RFC5558] Templin, F., "Virtual Enterprise Traversal (VET)", 1140 RFC 5558, February 2010. 1142 [RFC5720] Templin, F., "Routing and Addressing in Networks with 1143 Global Enterprise Recursion (RANGER)", RFC 5720, 1144 February 2010. 1146 [RFC5887] Carpenter, B., Atkinson, R., and H. Flinck, "Renumbering 1147 Still Needs Work", RFC 5887, May 2010. 1149 [RFC5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4 1150 Infrastructures (6rd) -- Protocol Specification", 1151 RFC 5969, August 2010. 1153 [RFC6139] Russert, S., Fleischman, E., and F. Templin, "Routing and 1154 Addressing in Networks with Global Enterprise Recursion 1155 (RANGER) Scenarios", RFC 6139, February 2011. 1157 [RFC6169] Krishnan, S., Thaler, D., and J. Hoagland, "Security 1158 Concerns with IP Tunneling", RFC 6169, April 2011. 1160 [RFC6179] Templin, F., "The Internet Routing Overlay Network 1161 (IRON)", RFC 6179, March 2011. 1163 Author's Address 1165 Fred L. Templin 1166 Boeing Research & Technology 1167 P.O. Box 3707 MC 7L-49 1168 Seattle, WA 98124 1169 USA 1171 Email: fltemplin@acm.org