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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 2460 (Obsoleted by RFC 8200) == Outdated reference: A later version (-06) exists of draft-ietf-savi-framework-05 == Outdated reference: A later version (-16) exists of draft-templin-ironbis-05 -- Obsolete informational reference (is this intentional?): RFC 3315 (Obsoleted by RFC 8415) -- Obsolete informational reference (is this intentional?): RFC 3633 (Obsoleted by RFC 8415) Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group F. Templin, Ed. 3 Internet-Draft Boeing Research & Technology 4 Intended status: Standards Track October 19, 2011 5 Expires: April 21, 2012 7 Asymmetric Extended Route Optimization (AERO) 8 draft-templin-aero-04.txt 10 Abstract 12 Nodes attached to link types such as multicast-capable, shared media 13 and non-broadcast multiple access (NBMA), etc. can exchange packets 14 as neighbors on the link. Each node should therefore be able to 15 discover a trusted neighboring gateway that can provide default 16 routing services to reach off-link destinations, and should also 17 accept redirection messages from the gateway informing it of a 18 different neighbor that is closer to the final destination. This 19 redirect function can provide a useful route optimization, since the 20 triangular path from the ingress link neighbor, to the gateway, and 21 finally to the egress link neighbor may be considerably longer than 22 the direct path between the neighbors. However, ordinary redirection 23 may lead to operational issues on certain link types and/or in 24 certain deployment scenarios. This document therefore introduces an 25 Asymmetric Extended Route Optimization (AERO) capability that 26 addresses the issues. 28 Status of this Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at http://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on April 21, 2012. 45 Copyright Notice 47 Copyright (c) 2011 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 64 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4 65 4. Asymmetric Extended Route Optimization (AERO) . . . . . . . . 5 66 4.1. AERO Link Dynamic Routing . . . . . . . . . . . . . . . . 5 67 4.2. AERO Node Behavior . . . . . . . . . . . . . . . . . . . . 6 68 4.2.1. AERO Node Types . . . . . . . . . . . . . . . . . . . 6 69 4.2.2. Source Address Verification . . . . . . . . . . . . . 6 70 4.2.3. AERO Host Behavior . . . . . . . . . . . . . . . . . . 7 71 4.2.4. AERO Router Behavior . . . . . . . . . . . . . . . . . 7 72 4.2.5. AERO Gateway Behavior . . . . . . . . . . . . . . . . 7 73 4.3. AERO Reference Operational Scenario . . . . . . . . . . . 8 74 4.4. AERO Specification . . . . . . . . . . . . . . . . . . . . 9 75 4.4.1. Sending Predirects . . . . . . . . . . . . . . . . . . 11 76 4.4.2. Processing Predirects and Sending Redirects . . . . . 13 77 4.4.3. Proxying Redirects . . . . . . . . . . . . . . . . . . 14 78 4.4.4. Processing Redirects . . . . . . . . . . . . . . . . . 15 79 4.4.5. Sending Periodic Predirect Keepalives . . . . . . . . 15 80 4.4.6. Reachability Considerations . . . . . . . . . . . . . 17 81 4.5. Mobility Considerations . . . . . . . . . . . . . . . . . 17 82 4.6. Scaling Considerations . . . . . . . . . . . . . . . . . . 18 83 4.7. Proxy Chaining . . . . . . . . . . . . . . . . . . . . . . 19 84 4.8. Backward Compatibility . . . . . . . . . . . . . . . . . . 19 85 4.9. Alternate Means of Source Address Verification . . . . . . 19 86 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 87 6. Security Considerations . . . . . . . . . . . . . . . . . . . 20 88 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20 89 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 90 8.1. Normative References . . . . . . . . . . . . . . . . . . . 20 91 8.2. Informative References . . . . . . . . . . . . . . . . . . 21 92 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 21 94 1. Introduction 96 Nodes attached to link types such as multicast-capable, shared media, 97 non-broadcast multiple access (NBMA), etc. can exchange packets with 98 each other as neighbors on the link. Each node should therefore be 99 able to discover a trusted neighboring gateway that can provide 100 default routing services to reach off-link destinations, and should 101 also accept redirection messages from the gateway informing it of a 102 different neighbor that is closer to the final destination. This 103 redirect function can provide a useful route optimization, since the 104 triangular path from the ingress link neighbor, to the gateway, and 105 finally to the egress link neighbor may be considerably longer than 106 the direct path between the neighbors. However, ordinary redirection 107 may lead to operational issues on certain link types and/or in 108 certain deployment scenarios. 110 For example, when an ingress link neighbor accepts an ordinary 111 redirect message from a gateway, it has no way of knowing whether the 112 egress link neighbor is ready and willing to accept packets directly 113 without involving the gateway. In particular, without involvement 114 from the gateway, the egress would have no way of knowing that the 115 ingress is authorized to forward packets from a given source address. 116 (This is especially important for very large links, since any node on 117 the link can spoof the network-layer source address with low 118 probability of detection even if the link-layer source address cannot 119 be spoofed.) Additionally, the ingress would have no way of knowing 120 whether the direct path to the egress has failed, nor whether the 121 final destination has moved away from the egress to some other 122 network attachment point. 124 Therefore, a new redirection approach is required that can enable a 125 reliable one-way handshake from the egress to the ingress link node 126 under the mediation of trusted gateways. The mechanism is asymmetric 127 (since only the forward direction from the ingress to the egress is 128 optimized) and extended (since the redirection extends forward to the 129 egress before reaching back to the ingress). This document therefore 130 introduces an Asymmetric Extended Route Optimization (AERO) 131 capability that addresses the issues. 133 While the AERO mechanisms were initially designed for the specific 134 purpose of NBMA tunnel virtual interfaces (e.g., see: 135 [RFC2529][RFC5214][RFC5569][I-D.templin-ironbis]) they can also be 136 applied to any link types that support redirection 137 [RFC0792][RFC4861]. The rest of this document refers to this class 138 of links collectively as "AERO links". 140 The AERO techniques apply to both the IPv4 [RFC0791] and IPv6 141 [RFC2460] protocols, as well as any other network layer protocol that 142 includes link models that can support redirection. 144 2. Terminology 146 The terminology in the normative references apply; the following 147 terms are defined within the scope of this document: 149 AERO link 150 any link over which the AERO mechanisms can be applied. 152 AERO node 153 a gateway, router or host connected to an AERO link. 155 AERO gateway 156 a router that configures an advertising router interface on the 157 AERO link, and that can provide default routing services for 158 forwarding packets toward their final destinations. 160 AERO router 161 a router that configures a non-advertising router interface on the 162 AERO link, and that connects End User Networks to the AERO link. 164 AERO host 165 a simple host on the AERO link. 167 ingress AERO node ("ingress") 168 a node that injects packets into the AERO link. 170 egress AERO node ("egress") 171 a node that removes packets from the AERO link. 173 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 174 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 175 document are to be interpreted as described in [RFC2119]. When used 176 in lower case (e.g., must, must not, etc.), these words MUST NOT be 177 interpreted as described in [RFC2119], but are rather interpreted as 178 they would be in common English. 180 3. Requirements 182 The route optimization mechanism must satisfy the following 183 requirements: 185 Req 1: Off-load traffic from performance-critical gateways 186 The mechanism must offload sustained transit though a gateway that 187 would otherwise become a traffic concentrator. 189 Req 2: Support route optimization 190 Ingress nodes must be able to send packets directly to egress 191 nodes without involving a gateway as an intermediary hop. 193 Req 3: Support multiple levels of hierarchy 194 For scaling purposes, allow multiple levels of hierarchy in which 195 gateways in higher levels have progressively more topology 196 knowledge than those in lower levels. 198 Req 4: Do not circumvent ingress filtering 199 The mechanism must not open an attack vector where network-layer 200 source address spoofing is enabled even when link-layer source 201 address spoofing is disabled. 203 Req 5: Do not expose packets to loss due to filtering 204 The ingress node must have a way of knowing that the egress node 205 will accept its forwarded packets. 207 Req 6: Do not expose packets to loss due to path failure 208 The ingress node must have a way of discovering whether the egress 209 has gone unreachable on the route optimized path. 211 Req 7: Do not introduce routing loops 212 The gateway must not perform a route optimization that would cause 213 a routing loop to form. 215 Req 8: Support mobility 216 The mechanism must continue to work even if the final destination 217 node/network moves from a first egress node and re-associates with 218 a second egress node. 220 4. Asymmetric Extended Route Optimization (AERO) 222 The following sections specify an Asymmetric Extended Route 223 Optimization (AERO) capability that fulfills the requirements 224 specified in Section 3. 226 4.1. AERO Link Dynamic Routing 228 In many AERO link use case scenarios (e.g., small enterprise 229 networks, small and stable MANETs, etc.), AERO gateways and routers 230 can engage in a proactive dynamic routing protocol (e.g., OSPF, RIP, 231 IS-IS, etc.) so that routing/forwarding tables can be populated and 232 standard forwarding between routers can be used. In other scenarios 233 (e.g., large enterprise/ISP networks, cellular service provider 234 networks, dynamic MANETs, etc.), this might be impractical due to 235 routing protocol control message scaling issues. 237 When a proactive dynamic routing protocol cannot be used, the 238 mechanisms specified in this section can provide a useful on-demand 239 dynamic routing capability. 241 4.2. AERO Node Behavior 243 The following sections discuss characteristics of nodes attached to 244 links over which AERO can be used: 246 4.2.1. AERO Node Types 248 AERO gateways configure their AERO link interfaces as advertising 249 router interfaces (see: [RFC4861], Section 6.2.2), and therefore may 250 send Router Advertisement (RA) messages that include non-zero Router 251 Lifetimes. 253 AERO routers configure their AERO link interfaces as non-advertising 254 router interfaces. 256 AERO hosts configure their AERO link interfaces as simple host 257 interfaces. 259 4.2.2. Source Address Verification 261 AERO nodes must employ a source address verification check for the 262 packets they receive on an AERO interface in a manner that is 263 consistent with the Source Address Validation Improvement (SAVI) 264 Framework [I-D.ietf-savi-framework]. In order to perform source 265 address verification, the node considers the network-layer source 266 address correct for the link-layer source address if: 268 o the link-layer source address is the address of an AERO gateway, 269 or 271 o the link-layer source address is explicitly linked to the network- 272 layer source address (i.e., through stateless or stateful address 273 mapping), or 275 o a ingress filtering and/or forwarding table entry exists that 276 lists the packet's link-layer source address as the link layer 277 address corresponding to the next hop toward the network-layer 278 source address on the AERO link, or 280 o the packet includes a signature that the node can use to 281 authenticate the source. 283 The latter of these checks can be established through static 284 configuration, via a proactive dynamic routing protocol, or through 285 the AERO mechanisms specified in Section 4.4. 287 4.2.3. AERO Host Behavior 289 AERO hosts send Router Solicitation (RS) messages to obtain RA 290 messages from an AERO gateway. When the RA contains Prefix 291 Information Options with non-link-local prefixes, the host 292 autoconfigures addresses from the prefixes using Stateless Address 293 Autoconfiguration (SLAAC) [RFC4861][RFC4862]. When managed address 294 delegation services are available, the host can also (or instead) 295 acquire addresses taken from prefixes aggregated by the gateway 296 through the use of stateful mechanisms, e.g., DHCP 297 [RFC2131][RFC3315], manual configuration, etc. 299 After the host receives addresses, it assigns them to its AERO 300 interface and forwards any of its outbound packets via the gateway as 301 a default router. The host may subsequently receive redirection 302 messages from the gateway listing a better next hop. 304 4.2.4. AERO Router Behavior 306 AERO routers send RS messages to obtain RA messages from an AERO 307 gateway, i.e., they act as "hosts" on their non-advertising AERO link 308 router interfaces for the purpose of default router discovery. 310 The router can then acquire managed prefix delegations aggregated by 311 the gateway through the use of, e.g., DHCPv6 Prefix Delegation 312 [RFC3633], manual configuration, etc. in the same fashion as 313 described above for host-based autoconfiguration. 315 After the router acquires prefixes, it can sub-delegate them to nodes 316 and links within its attached End User Networks (EUNs), then can 317 forward any outbound packets coming from its EUNs via the gateway. 318 The router may subsequently receive redirection messages from the 319 gateway listing a better next hop. 321 4.2.5. AERO Gateway Behavior 323 AERO gateways respond to RS messages from hosts and routers on the 324 AERO link by returning an RA message. Gateways may further configure 325 a DHCP relay or server function on their AERO links and/or provide an 326 administrative interface for manual configuration of address/ 327 prefix-to-client forwarding table entries. 329 When the gateway completes a stateful address or prefix delegation 330 transaction (e.g., as a DHCP relay/server, etc.), it establishes 331 forwarding table entries that list the link-layer address of client 332 as the link-layer address of the next hop toward the delegated 333 addresses/prefixes. 335 When the gateway forwards a packet out the same AERO interface it 336 arrived on, it initiates an AERO route optimization procedure as 337 specified in Section 4.4. 339 4.3. AERO Reference Operational Scenario 341 Figure 1 depicts an example AERO network topology. IPv6 is used only 342 as an example network layer protocol, and the same fundamental AERO 343 techniques can be applied for other network layer protocols. The 344 figure shows an AERO gateway ('A'), two non-advertising AERO routers 345 ('B', 'D'), an AERO host ('F'), and three ordinary IPv6 hosts ('C', 346 'E', 'G') in a typical operational scenario: 347 .-(::::::::) 2001:db8:3::1 348 .-(::: IPv6 :::)-. +-------------+ 349 (:::: Internet ::::) | IPv6 Host G | 350 `-(::::::::::::)-' +-------------+ 351 `-(::::::)-' 352 ,................., 353 |companion gateway| 354 '.................' +---------------+ 355 +--------------+ | Host F | 356 | Gateway A | +---------------+ 357 +--------------+ 2001:db8:2:1 358 L3(A) L3(F) 359 L2(A) L2(F) 360 | AERO Link | 361 X-----+--+-----------------+--+---X 362 | | 363 L2(B) L2(D) .-. 364 L3(B) L3(D) ,-( _)-. 365 +--------------+ +--------------+ .-(_ IPv6 )-. 366 | Router B | | Router D |--(__ EUN ) 367 +--------------+ +--------------+ `-(______)-' 368 2001:db8:0::/48 2001:db8:1::/48 | 369 | 2001:db8:1::1 370 .-. +-------------+ 371 ,-( _)-. 2001:db8:0::1 | IPv6 Host E | 372 .-(_ IPv6 )-. +-------------+ +-------------+ 373 (__ EUN )--| IPv6 Host C | 374 `-(______)-' +-------------+ 376 Figure 1: Reference AERO Network Topology 378 In Figure 1, Gateway 'A' connects to the AERO link and also connects 379 to the IPv6 Internet, either directly or via a companion gateway. 380 'A' configures an AERO link interface with a link-local network-layer 381 address L3(A) and with link-layer address L2(A). 'A' next arranges 382 to add the link-layer address L2(A) to the list of valid gateways for 383 the link. 385 Router 'B' connects to one or more IPv6 EUNs and also connects to the 386 AERO link via an interface with a link-local network-layer address 387 L3(B) and with link-layer address L2(B). 'B' next configures a 388 default IPv6 route with next-hop address L3(A) via the AERO 389 interface, then receives the IPv6 prefix 2001:db8:0::/48 through a 390 stateful prefix delegation exchange via 'A'. 'B' finally sub- 391 delegates the prefix to its attached EUNs, where IPv6 host 'C' 392 autoconfigures the address 2001:db8:0::1. 394 Router 'D' connects to the AERO link via an interface with addresses 395 L3(D)/L2(D), configures a default IPv6 route with next-hop address 396 L3(A) via the AERO interface, and receives a stateful prefix 397 delegation of 2001:db8:1::/48 in the same fashion as for router 'B'. 398 'D' finally sub-delegates the prefix to its attached EUNs, where IPv6 399 host 'E' autoconfigures IPv6 address 2001:db8:1::1. 401 Host 'F' connects to the AERO link via an interface with addresses 402 L3(F)/L2(F). 'F' next configures a default IPv6 route with next-hop 403 address L3(A) via the AERO interface, then receives the IPv6 address 404 2001:db8:2::1 from a stateful address configuration exchange via 'A'. 405 When 'F' receives the IPv6 address, it assigns the address to the 406 AERO interface but does not assign a non-link-local IPv6 prefix to 407 the interface. 409 Finally, IPv6 host 'G' connects to an IPv6 network outside of the 410 AERO link domain. 'G' configures its IPv6 interface in a manner 411 specific to its attached IPv6 link, and autoconfigures the IPv6 412 address 2001:db8:3::1. 414 In these arrangements, 'A' must maintain routes that associate the 415 delegated IPv6 addresses/prefixes with the correct routers and/or 416 hosts on the AERO link. The routers and hosts must maintain at least 417 a default route that points to gateway 'A', and can discover more- 418 specific routes either via a proactive dynamic routing protocol or 419 via the AERO mechanisms specified in Section 4.4. 421 4.4. AERO Specification 423 Figure 1 depicts a reference operational scenario including an AERO 424 link. The figure shows an AERO gateway ('A'), two AERO routers ('B', 425 'D'), an AERO host ('F') and three ordinary IPv6 hosts ('C', 'E', 426 'G') in a typical deployment configuration. We now discuss the 427 operation of AERO with respect to this reference scenario. 429 With reference to Figure 1, when host 'C' sends a packet with source 430 address 'C' and destination address 'E', the packet is first 431 forwarded over 'C's attached EUN to router 'B'. 'B' then forwards 432 the packet over the AERO interface to gateway 'A', which then 433 forwards the packet to router 'D', where the packet is finally 434 forwarded to host 'E' over 'E's attached EUN. When 'A' forwards the 435 packet back out on its advertising AERO interface, it must arrange to 436 redirect 'B' toward 'D' as a better next hop node on the AERO link 437 that is closer to the final destination 'E'. However, this 438 redirection process should only take place if there is assurance that 439 both 'B' and 'D' are willing participants. 441 Consider a first alternative in which 'A' informs 'B' only and does 442 not inform 'D' (i.e., "classic redirection"). In that case, 'D' has 443 no way of knowing that 'B' is authorized to forward packets from a 444 given source address. Also, 'B' has no way of knowing whether 'D' is 445 willing to accept its packets, nor whether 'D' is even reachable via 446 a direct path that does not involve 'A'. Finally, 'B' has no way of 447 knowing whether the final destination has moved away from 'D'. 449 Consider also a second alternative in which 'A' informs both 'B' and 450 'D' separately via independent redirection messages (i.e., "augmented 451 redirection"). In that case, several conditions can occur that could 452 result in communications failures. First, if 'B' receives the 453 redirection message but 'D' does not, subsequent packets sent by 'B' 454 would be dropped due to filtering since 'D' would not have a 455 forwarding table entry to verify their source addresses. Second, if 456 'D' receives the redirection message but 'B' does not, subsequent 457 packets sent in the reverse direction by 'D' would be lost. Finally, 458 timing issues surrounding the establishment and garbage collection of 459 forwarding table entries at 'B' and 'D' could yield unpredictable 460 behavior. For example, unless the timing were carefully coordinated 461 through some form of synchronization loop, there would invariably be 462 instances in which one node has the correct forwarding table state 463 and the other node does not resulting in non-deterministic packet 464 loss. 466 Since neither of these alternatives can satisfy the requirements 467 listed in Section 3, a new redirection technique is needed. In this 468 new method (i.e., "AERO redirection"), when 'A' forwards a packet 469 from 'B' out the same AERO interface toward 'D', 'A' must first send 470 a "predirect" message forward to 'D' to inform it that 'B' is 471 authorized to produce packets using source address 'C'. After 'D' 472 receives the predirect, it sends a Redirect message back to 'B' via 473 'A' as a trusted intermediary. When 'B' receives the Redirect, it 474 knows that 'D' will accept the packets it sends with source address 475 'C' as long as 'D' retains the forwarding table entry. This process 476 stands in contrast to the classical and augmented redirection 477 approaches; the following subsections therefore specify the AERO 478 redirection steps necessary to support the reference operational 479 scenario. 481 4.4.1. Sending Predirects 483 When a gateway forwards a packet out the same AERO interface that it 484 arrived on, the gateway sends a predirect message forward to the 485 egress AERO node instead of sending a Redirect message back to the 486 ingress node. If there is some way of marking the data packet itself 487 as a predirect, then the data packet itself serves as a "predirect" 488 without the need for a separate message (e.g., see: 489 [I-D.templin-ironbis]). 491 If there is no means for signaling a predirect in the data plane, the 492 gateway instead sends an explicit Predirect message which is simply 493 an AERO-specific version of an ordinary Redirect message. In the 494 case of IPv6 as the network layer protocol, the Predirect format is 495 the same as depicted in Section 4.5 of [RFC4861], and is identified 496 by two new backward-compatible bits taken from the Reserved field as 497 shown in Figure 2: 499 0 1 2 3 500 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 501 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 502 | Type (=137) | Code (=0) | Checksum | 503 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 504 |A|P| Reserved | 505 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 506 | | 507 + + 508 | | 509 + Target Address + 510 | | 511 + + 512 | | 513 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 514 | | 515 + + 516 | | 517 + Destination Address + 518 | | 519 + + 520 | | 521 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 522 | Options ... 523 +-+-+-+-+-+-+-+-+-+-+-+- 525 Figure 2: AERO-Specific IPv6 Redirect Message Format 527 Where the new bits are defined as: 529 A (1) the "AERO" bit. Set to 1 to indicate an AERO-specific 530 Redirect message, and set to 0 to indicate an ordinary IPv6 531 Redirect message. 533 P (1) the "Predirect" bit. Set to 1 to indicate a Predirect 534 message, and set to 0 to indicate a Redirect response to a 535 Predirect message. (This bit is valid only when the A bit is set 536 to 1.) 538 In the reference operational scenario, when gateway 'A' forwards a 539 packet sent by 'B' toward 'D', it marks the packet as a "predirect" 540 if possible. Otherwise, it also sends an explicit Predirect message 541 forward toward 'D', subject to rate limiting (see Section 8.2 of 542 [RFC4861]). 'A' prepares the Predirect message in a similar fashion 543 as for an ordinary IPv6 Redirect message as follows: 545 o the link-layer source address is set to 'L2(A)' 547 o the link-layer destination address is set to 'L2(D)' 549 o the network-layer source address is set to 'L3(B)'. 551 o the network-layer destination address is set to 'L3(D)'. 553 o the Predirect Target and Destination Addresses are both set to 554 'L3(B)'. 556 o on links that require stateful address mapping, the Predirect 557 message includes a Target Link Layer Address Option (TLLAO) set to 558 'L2(B)'. 560 o the Predirect message includes Route Information Options (RIOs) 561 [RFC4191] that encode prefixes taken from 'B's address/prefix 562 delegations, including one that covers the source address of the 563 originating packet. 565 o the Predirect message includes a Redirected Header Option (RHO) 566 that contains as much of the originating packet as possible 567 beginning with the network-layer header such that the total length 568 of the Predirect message does not exceed 512 bytes. 570 o the A and P bits in the Predirect message header are both set to 571 1. 573 'A' then sends the Predirect message forward to 'D'. 575 4.4.2. Processing Predirects and Sending Redirects 577 When an AERO router or host receives a either a data packet marked as 578 a predirect or an explicit Predirect message, it validates the 579 message according to the appropriate redirect message validation 580 rules (e.g., Section 8.1 of [RFC4861] for IPv6). The node further 581 accepts the message only if it came from the link-layer address of a 582 trusted gateway. Finally, the node only processes the message if the 583 destination address of the originating packet encapsulated in the RHO 584 is covered by one of its CURRENT delegated addresses/prefixes (see 585 Section 4.5). 587 In the reference operational scenario, when router 'D' receives the 588 predirect it creates forwarding table entries with the prefixes 589 encoded in RIO options as the target prefixes, and associates the 590 forwarding table entries with a node structure (e.g., in a neighbor 591 cache) that stores the source address of the Predirect message (i.e., 592 'L3(B)'). 'D' places the node structure in the FILTERING state, then 593 sets/resets a filtering expiration timer value of 40 seconds. If the 594 filtering timer later expires, 'D' clears the FILTERING state. If 595 the node structure is not in the FORWARDING state, 'D' then deletes 596 the node structure and all of its associated forwarding table 597 entries. (This suggests that 'D's AERO interface should maintain a 598 private forwarding table separate from the primary forwarding table, 599 since the node structure and forwarding table entries must be managed 600 by the AERO interface itself.) 602 After processing the Predirect message and establishing the 603 forwarding table entry, 'D' prepares a Redirect message in response 604 to the Predirect as follows: 606 o the link-layer source address is set to 'L2(D)' 608 o the link-layer destination address is set to 'L2(A)' 610 o the network-layer source address is set to 'L3(D)'. 612 o the network-layer destination address is set to 'L3(B)'. 614 o the Redirect Target and the Redirect Destination Addresses are 615 both set to 'L3(D)'. 617 o on links that require stateful address mapping, the Redirect 618 message includes a TLLAO set to 'L2(D)'. 620 o the Redirect message includes an RHO copied from the corresponding 621 Predirect message. 623 o the (A, P) bits in the Redirect message header are set to (1, 0). 625 After 'D' prepares the Redirect message, it sends the message to 'A'. 626 (Note that the Redirect message does not include RIOs, since the 627 gateway is the only authoritative source of routing information and 628 will supply RIOs when it proxies the message.) 630 4.4.3. Proxying Redirects 632 When an AERO gateway receives a Redirect message, it accepts the 633 message only if it satisfies the redirect message validation rules. 634 The gateway further accepts the message only if it came from the 635 link-layer address of the current next hop toward the destination 636 address of the originating packet encapsulated in the RHO. The 637 gateway then "proxies" the Redirect message back to the original 638 ingress AERO node as described below. 640 In the reference operational scenario, 'A' receives the Redirect 641 message from 'D' and adds RIOs to the message that encode 'D's 642 address/prefix delegations. Without decrementing the hopcount in the 643 Redirect message, 'A' next changes the link-layer source address of 644 the message to 'L2(A)' and changes the link-layer destination address 645 to 'L2(B)'. 'A' then sends this proxied Redirect message to 'B'. 647 4.4.4. Processing Redirects 649 When an AERO router or host receives a Redirect message, it validates 650 the message according to the appropriate redirect message validation 651 rules. The node further accepts the message only if it came from the 652 link-layer address of either a trusted gateway or the current next 653 hop on the AERO link for the destination address of the originating 654 packet encapsulated in the RHO. 656 In the reference operational scenario, 'B' receives the (proxied) 657 Redirect message then creates forwarding table entries with the 658 prefixes encoded in RIO options as the target prefixes. 'B' further 659 associates the forwarding table entries with a node structure (e.g., 660 in a neighbor cache) that stores the source address of the Predirect 661 message (i.e., 'L3(D)'). 'B' places the node structure in the 662 FORWARDING state, then sets/resets a filtering expiration timer value 663 of 30 seconds. If the filtering timer later expires, 'B' clears the 664 FORWARDING state. If the node structure is not in the FILTERING 665 state, 'B' then deletes the node structure and all of its associated 666 forwarding table entries. 668 Now, 'B' has a node structure for 'D' in the FORWARDING state, and 669 'D' has a node structure for 'B' in the FILTERING state. Therefore, 670 'B' may forward ordinary network-layer data packets with destination 671 addresses covered by 'D's prefixes directly to 'D' without involving 672 'A'. 'D' will in turn accept the packets since it has a forwarding 673 table entry authorizing 'B' to forward packets with source addresses 674 covered by 'B's prefixes. 676 To enable packet forwarding in the reverse direction, a separate AERO 677 operation is required which is the mirror-image of the forward AERO 678 operation described above, i.e., the forward and reverse AERO 679 operations are asymmetric. Following the reverse operation, packets 680 can be exchanged bidirectionally without involving 'A'. 682 4.4.5. Sending Periodic Predirect Keepalives 684 In order to prevent forwarding table entries from expiring while data 685 packets are actively flowing, the ingress node can periodically send 686 Predirect messages directly to the egress node (subject to rate 687 limiting) to solicit Redirect messages. In the reference operational 688 scenario, when 'B' forwards a packet to 'D' and wishes to update the 689 corresponding FORWARDING state timer, 'B' can also send a Predirect 690 message directly to 'D' prepared as follows: 692 o the link-layer source address is set to 'L2(B)'. 694 o the link-layer destination address is set to 'L2(D)'. 696 o the network-layer source address is set to 'L3(B)'. 698 o the network-layer destination address is set to 'L3(D)'. 700 o the Predirect Target and Destination Addresses are both set to 701 'L3(B)'. 703 o the Predirect message includes a Redirected Header Option (RHO) 704 that contains as much of the originating packet as possible 705 beginning with the network-layer header such that the total length 706 of the Predirect message does not exceed 512 bytes. 708 o the A and P bits in the Predirect message header are both set to 709 1. 711 When 'D' receives the Predirect message, it accepts the message only 712 if it satisfies the redirect message validation rules. The node 713 further accepts the message only if it came from the current previous 714 hop on the AERO link for the source address of the originating packet 715 encapsulated in the RHO. 'D' then resets its FILTERING expiration 716 timer for node 'B' to 40 seconds, and sends a Redirect message 717 directly to 'B' prepared as follows: 719 o the link-layer source address is set to 'L2(D)'. 721 o the link-layer destination address is set to 'L2(B)'. 723 o the network-layer source address is set to 'L3(D)'. 725 o the network-layer destination address is set to 'L3(B)'. 727 o the Redirect Target and Destination Addresses are both set to 728 'L3(D)'. 730 o the Redirect message includes an RHO copied from the corresponding 731 Predirect message. 733 o the (A, P) bits in the Redirect message header are set to (1, 0). 735 When 'B' receives the Redirect message, it accepts the message only 736 if it satisfies the redirect message validation rules. The node 737 further accepts the message only if it came from the current next hop 738 on the AERO link for the source address of the originating packet 739 encapsulated in the RHO. 'B' then resets its FORWARDING expiration 740 timer for node 'D' to 30 seconds. 742 Note that in these direct neighbor to neighbor exchanges, neither the 743 Predirect nor Redirect message contain RIOs, since gateways are the 744 only authoritative source of routing information. Therefore, AERO 745 routers and hosts should not include RIOs in the Predirects/Redirects 746 they send, and they must ignore any RIOs included in received 747 Predirect/Redirect messages that did not come from a trusted gateway. 749 4.4.6. Reachability Considerations 751 When an ingress node receives a Redirect message informing it of a 752 direct path to a new egress, there is a question in point as to 753 whether the new egress can be reached directly without involving the 754 gateway as an intermediary. On some AERO links, it may be reasonable 755 for the ingress to (optimistically) assume that the new egress is 756 reachable, and to immediately begin forwarding data packets to the 757 egress without testing reachability. 759 On AERO links in which an optimistic assumption of reachability may 760 be inappropriate, however, the ingress can defer the redirection 761 until it tests the direct path to the egress by sending a direct 762 Predirect message to elicit a Redirect as specified in Section 4.4.5. 763 If the ingress is unable to elicit a Redirect message after a small 764 number of attempts, it should consider the direct path to the egress 765 as unusable. 767 In either case, the ingress can process any link errors corresponding 768 to the data packets sent directly to the egress as a hint that the 769 direct path has either failed or has become intermittent. 771 4.5. Mobility Considerations 773 An AERO link egress router 'D' can configure both a non-advertising 774 router interface on a provider AERO link and advertising router 775 interfaces on EUN links to provide gateway services to nodes in EUNs. 776 When node 'E' in an EUN that has obtained addresses/prefixes moves to 777 a different network point of attachment, however, 'E' can release its 778 address/prefix delegations via 'D' and re-establish them via a 779 different gateway. 781 When 'E' releases its address/prefix delegations via 'D', 'D' marks 782 the forwarding table entries that cover the addresses/prefixes as 783 DEPARTED (i.e., it clears the CURRENT state). 'D' therefore ceases 784 to respond to Predirect messages correlated with the DEPARTED 785 entries, and also schedules a garbage-collection timer of 60 seconds, 786 after which it deletes the DEPARTED entries. 788 When 'D' receives packets destined to an address covered by the 789 DEPARTED forwarding table entries, it forwards them to the last-known 790 EUN link-layer address of 'E' as a means for avoiding mobility- 791 related packet loss during routing changes. 'D' also returns a NULL 792 Redirect message to inform the correspondent 'B' of the departure. 793 The Redirect message is prepared as follows: 795 o the link-layer source address is set to 'L2(D)'. 797 o the link-layer destination address is set to 'L2(B)'. 799 o the network-layer source address is set to 'L3(D)'. 801 o the network-layer destination address is set to 'L3(B)'. 803 o the Redirect Target and Destination Addresses are both set to 804 NULL. 806 o the Redirect message includes an RHO copied from the corresponding 807 Predirect message. 809 o the (A, P) bits in the Redirect message header are set to (1, 0). 811 Eventually, any correspondents will receive such a NULL Redirect 812 message and will cease to use 'D' as a next hop. They will then 813 revert to sending packets destined to 'E' via their gateways and may 814 subsequently receive new Redirect messages to discover that 'E' is 815 now associated with a new egress router. Note that any packets 816 forwarded by 'D' via a forwarding table entry in the DEPARTED state 817 may be lost if the mobile node moves off-link with respect to its 818 previous EUN point of attachment. This should not be a problem for 819 large links (e.g., large cellular network deployments, large ISP 820 networks, etc.) in which all/most mobility events are intra-link. 822 4.6. Scaling Considerations 824 Figure 1 depicts a reference network topology with only a single 825 gateway on the AERO link. In order to support larger numbers of AERO 826 routers and hosts, the AERO link can deploy more gateways to support 827 load balancing and generally shortest-path routing. 829 Such an arrangement requires that the gateways participate in a 830 routing protocol instance (e.g., an eBGP instance with each gateway 831 configuring a private Autonomous System Number (ASN)) so that 832 address/prefix delegations can be mapped to the correct gateway. The 833 routing protocol instance can be configured as either a full mesh 834 topology involving all gateways, or as a partial mesh topology with 835 each AERO link gateway associating with one or more backhaul network 836 companion gateways and a full mesh between companion gateways. 838 4.7. Proxy Chaining 840 In large AERO link deployments, there may be many gateways - each 841 serving many AERO routers and hosts. The gateways then either 842 require full topology knowledge, or a default route to a companion 843 gateway that does have full topology knowledge. For example, if AERO 844 node 'A' connects to gateway 'B', and AERO node 'E' connects to 845 gateway 'D', then 'B' and 'D' must either have full topology 846 knowledge or have a default route to a companion gateway (e.g., 'C') 847 that does. 849 In that case, when 'A' forwards an initial packet destined to an end 850 system behind 'E', it forwards the packet to 'B'. Next, 'B' forwards 851 the packet toward 'C', which both forwards the packet generates a 852 Predirect message toward 'D'. 'D' then either processes the 853 Predirect message locally or proxies it toward 'E'. 855 In the reverse direction, when 'E'/'D' sends a Redirect response 856 message back to 'A', it first sends the message to 'D'/'C', which 857 proxies the message toward 'B', which finally proxies the message 858 toward 'A'. 860 4.8. Backward Compatibility 862 If a legacy host or router receives an AERO Redirect or Predirect 863 message, it will process the message as if it were an ordinary 864 Redirect. This will cause no harmful effects, since the legacy 865 system will ignore the 'A' and P' bits in the Reserved field, and 866 will also ignore any RIOs that are included. The values encoded in 867 the Redirect message target and destination addresses will also not 868 cause the legacy node to create incorrect routing state. The 869 mechanism therefore causes no harm to legacy systems, and supports 870 natural incremental deployment. 872 4.9. Alternate Means of Source Address Verification 874 On some AERO links, each packet can include a signature that the link 875 egress nodes can use to authenticate the link ingress node (e.g., 876 see: [I-D.templin-ironbis]). On those links, the procedures for 877 maintaining ingress filtering entries described above are not 878 necessary. 880 5. IANA Considerations 882 There are no IANA considerations for this document. 884 6. Security Considerations 886 This document enables ingress filtering, and therefore improves the 887 security of AERO links. 889 7. Acknowledgements 891 Discussions both on the v6ops list and in private exchanges helped 892 shape some of the concepts in this work. Individuals who contributed 893 insights include Mikael Abrahamsson, Fred Baker, Brian Carpenter, 894 Joel Halpern, Lee Howard, 896 8. References 898 8.1. Normative References 900 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 901 September 1981. 903 [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, 904 RFC 792, September 1981. 906 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 907 Requirement Levels", BCP 14, RFC 2119, March 1997. 909 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 910 (IPv6) Specification", RFC 2460, December 1998. 912 [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and 913 More-Specific Routes", RFC 4191, November 2005. 915 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 916 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 917 September 2007. 919 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 920 Address Autoconfiguration", RFC 4862, September 2007. 922 8.2. Informative References 924 [I-D.ietf-savi-framework] 925 Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, 926 "Source Address Validation Improvement Framework", 927 draft-ietf-savi-framework-05 (work in progress), 928 July 2011. 930 [I-D.templin-ironbis] 931 Templin, F., "The Internet Routing Overlay Network 932 (IRON)", draft-templin-ironbis-05 (work in progress), 933 October 2011. 935 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 936 RFC 2131, March 1997. 938 [RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4 939 Domains without Explicit Tunnels", RFC 2529, March 1999. 941 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., 942 and M. Carney, "Dynamic Host Configuration Protocol for 943 IPv6 (DHCPv6)", RFC 3315, July 2003. 945 [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic 946 Host Configuration Protocol (DHCP) version 6", RFC 3633, 947 December 2003. 949 [RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site 950 Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, 951 March 2008. 953 [RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4 954 Infrastructures (6rd)", RFC 5569, January 2010. 956 Author's Address 958 Fred L. Templin (editor) 959 Boeing Research & Technology 960 P.O. Box 3707 MC 7L-49 961 Seattle, WA 98124 962 USA 964 Email: fltemplin@acm.org