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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references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 2460 (Obsoleted by RFC 8200) == Outdated reference: A later version (-06) exists of draft-ietf-savi-framework-04 == Outdated reference: A later version (-40) exists of draft-templin-intarea-vet-24 -- Obsolete informational reference (is this intentional?): RFC 3315 (Obsoleted by RFC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 June 23, 2011 5 Expires: December 25, 2011 7 Asymmetric Extended Route Optimization (AERO) 8 draft-templin-aero-01.txt 10 Abstract 12 Nodes (i.e., gateways, routers and hosts) attached to link types such 13 as multicast-capable, shared media and non-broadcast multiple access 14 (NBMA), etc. can exchange packets as neighbors on the link. Each 15 node should therefore be able to discover a neighboring gateway that 16 can provide default routing services to reach off-link destinations, 17 and should also accept redirection messages from the gateway 18 informing it of a neighbor that is closer to the final destination. 19 This redirect function can provide a useful route optimization, since 20 the triangular path from the ingress link neighbor, to the gateway, 21 and finally to the egress link neighbor may be considerably longer 22 than the direct path between the neighbors. However, ordinary 23 redirection may lead to operational issues on certain link types 24 and/or in certain deployment scenarios. This document therefore 25 introduces an Asymmetric Extended Route Optimization (AERO) 26 capability that 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 December 25, 2011. 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 . . . . . . . . . . . . . . . . . 14 79 4.4.5. Sending Periodic Predirect Keepalives . . . . . . . . 15 80 4.4.6. Reachability Considerations . . . . . . . . . . . . . 16 81 4.5. Mobility Considerations . . . . . . . . . . . . . . . . . 17 82 4.6. Scaling Considerations . . . . . . . . . . . . . . . . . . 18 83 4.7. Proxy Chaining . . . . . . . . . . . . . . . . . . . . . . 18 84 4.8. Backward Compatibility . . . . . . . . . . . . . . . . . . 19 85 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 86 6. Security Considerations . . . . . . . . . . . . . . . . . . . 19 87 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19 88 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 89 8.1. Normative References . . . . . . . . . . . . . . . . . . . 19 90 8.2. Informative References . . . . . . . . . . . . . . . . . . 20 91 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20 93 1. Introduction 95 Nodes (i.e., routers and hosts) attached to link types such as 96 multicast-capable, shared media, non-broadcast multiple access 97 (NBMA), etc. can exchange packets with each other as neighbors on the 98 link. Each node should therefore be able to discover a neighboring 99 gateway that can provide default routing services to reach off-link 100 destinations, and should also accept redirection messages from the 101 gateway informing it of a different link neighbor that is closer to 102 the final destination. This redirect function can provide a useful 103 route optimization, since the triangular path from the ingress link 104 neighbor, to the gateway, and finally to the egress link neighbor may 105 be considerably longer than the direct path between the neighbors. 106 However, ordinary redirection may lead to operational issues on 107 certain link types and/or in certain deployment scenarios. 109 For example, when an ingress link neighbor accepts an ordinary 110 redirect message from a gateway, it has no way of knowing whether the 111 egress link neighbor is ready and willing to accept packets directly 112 without involving the gateway. In particular, without involvement 113 from the gateway, the egress would have no way of knowing that the 114 ingress is authorized to forward packets from a given source address. 115 (This is especially important for very large links, since any node on 116 the link can spoof the network-layer source address with low 117 probability of detection even if the link-layer source address cannot 118 be spoofed.) Additionally, the ingress would have no way of knowing 119 whether the direct path to the egress has failed, nor whether the 120 final destination has moved away from the egress to some other 121 network attachment point. 123 Therefore, a new redirection mechanism is required that can enable a 124 reliable one-way handshake from the egress to the ingress link node 125 under the mediation of trusted gateways. The mechanism is asymmetric 126 (since only the forward direction from the ingress to the egress is 127 optimized) and extended (since the redirection extends forward to the 128 egress before reaching back to the ingress). This document therefore 129 introduces an Asymmetric Extended Route Optimization (AERO) 130 capability that addresses the issues. 132 While the AERO mechanisms were initially designed for the specific 133 purpose of NBMA tunnel interfaces (e.g., see: 134 [RFC2529][RFC5214][RFC5569][I-D.templin-intarea-vet]) they can also 135 be applied to any link types that support multiple access and 136 redirection [RFC0792][RFC4861]. The rest of this document refers to 137 this class of links collectively as "AERO links". 139 The AERO techniques apply to both the IPv4 [RFC0791] and IPv6 140 [RFC2460] protocols, as well as any other network layer protocol that 141 includes multiple access link models that can support redirection. 143 2. Terminology 145 The terminology in the normative references apply; the following 146 terms are defined within the scope of this document: 148 AERO link 149 any multiple access link over which the AERO mechanisms can be 150 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 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 scaling issues. 237 When a proactive dynamic routing protocol cannot be used, the on- 238 demand dynamic routing capabilities specified in this section should 239 be used. 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 forwarding table entry exists that lists the packet's link-layer 276 source address as the link layer address corresponding to the next 277 hop toward the network-layer source address on the AERO link. 279 The latter of these checks can be established through static 280 configuration, a proactive dynamic routing protocol, or through the 281 AERO mechanisms specified in Section 4.4. 283 4.2.3. AERO Host Behavior 285 AERO hosts send Router Solicitation (RS) messages to obtain RA 286 messages from an AERO gateway. Whether or not non-link-local 287 prefixes for stateless address autoconfiguration are advertised, the 288 host can acquire addresses taken from prefixes aggregated by the 289 gateway through the use of stateful mechanisms, e.g., DHCP 290 [RFC2131][RFC3315], manual configuration, etc. 292 After the host receives addresses, it assigns them to its AERO 293 interface and forwards any of its outbound packets via the gateway as 294 a default router. The host may subsequently receive redirection 295 messages from the gateway listing a better next hop. 297 4.2.4. AERO Router Behavior 299 AERO routers send RS messages to obtain RA messages from an AERO 300 gateway, i.e., they act as "hosts" on their non-advertising AERO link 301 router interfaces. 303 The router can then acquire prefixes aggregated by the gateway 304 through the use of, e.g., DHCPv6 Prefix Delegation [RFC3633], manual 305 configuration, etc. in the same fashion as described above for host- 306 based autoconfiguration. 308 After the router acquires prefixes, it can sub-delegate them to nodes 309 and links within its attached End User Networks (EUNs), then can 310 forward any outbound packets coming from its EUNs via the gateway. 311 The router may subsequently receive redirection messages from the 312 gateway listing a better next hop. 314 4.2.5. AERO Gateway Behavior 316 AERO gateways respond to RS messages from hosts and routers on the 317 AERO link by returning an RA message. Gateways further configure a 318 DHCP relay or server function on their AERO links and/or provide an 319 administrative interface for manual configuration of address/ 320 prefix-to-client forwarding table entries. 322 When the gateway completes a DHCP address or prefix delegation 323 transaction (i.e., either as a DHCP relay or a DHCP server), it 324 establishes forwarding table entries that list the link-layer address 325 of the DHCP client as the link-layer address of the next hop toward 326 the delegated addresses/prefixes. 328 When the gateway forwards a packet out the same AERO interface it 329 arrived on, it initiates an AERO route optimization procedure as 330 specified in Section 4.4. 332 4.3. AERO Reference Operational Scenario 334 Figure 1 depicts a reference AERO network topology. IPv6 is used 335 only as an example network layer protocol, and the same fundamental 336 AERO techniques can be applied for other network layer protocols. 337 The figure shows an AERO gateway ('A'), two non-advertising AERO 338 routers ('B', 'D'), an AERO host ('F'), and three ordinary IPv6 hosts 339 ('C', 'E', 'G') in a typical operational scenario: 340 .-(::::::::) 2001:db8:3::1 341 .-(::: IPv6 :::)-. +-------------+ 342 (:::: Internet ::::) | IPv6 Host G | 343 `-(::::::::::::)-' +-------------+ 344 `-(::::::)-' 345 ,................., 346 |companion gateway| 347 '.................' +---------------+ 348 +--------------+ | Host F | 349 | Gateway A | +---------------+ 350 +--------------+ 2001:db8:2:1 351 L3(A) L3(F) 352 L2(A) L2(F) 353 | AERO Link | 354 X-----+--+-----------------+--+---X 355 | | 356 L2(B) L2(D) .-. 357 L3(B) L3(D) ,-( _)-. 358 +--------------+ +--------------+ .-(_ IPv6 )-. 359 | Router B | | Router D |--(__ EUN ) 360 +--------------+ +--------------+ `-(______)-' 361 2001:db8:0::/48 2001:db8:1::/48 | 362 | 2001:db8:1::1 363 .-. +-------------+ 364 ,-( _)-. 2001:db8:0::1 | IPv6 Host E | 365 .-(_ IPv6 )-. +-------------+ +-------------+ 366 (__ EUN )--| IPv6 Host C | 367 `-(______)-' +-------------+ 369 Figure 1: Reference AERO Network Topology 371 In Figure 1, Gateway 'A' connects to the AERO link and also connects 372 to the IPv6 Internet, either directly or via a companion gateway. 373 'A' configures an AERO link interface with a link-local network-layer 374 address L3(A) and with link-layer address L2(A). 'A' next arranges 375 to add the link-layer address L2(A) to the list of valid gateways for 376 the link. 378 Router 'B' connects to one or more IPv6 End User Networks (EUNs) and 379 also connects to the AERO link via an interface with a link-local 380 network-layer address L3(B) and with link-layer address L2(B). 'B' 381 next configures a default IPv6 route with next-hop address L3(A) via 382 the AERO interface, then receives the IPv6 prefix 2001:db8:0::/48 383 through a DHCPv6 prefix delegation exchange via 'A'. 'B' finally 384 sub-delegates the prefix to its attached EUNs, where IPv6 host 'C' 385 autoconfigures the address 2001:db8:0::1. 387 Router 'D' connects to the AERO link via an interface with addresses 388 L3(D)/L2(D), configures a default IPv6 route with next-hop address 389 L3(A) via the AERO interface, and receives a DHCPv6 prefix delegation 390 of 2001:db8:1::/48 in the same fashion as for router 'B'. 'D' 391 finally sub-delegates the prefix to its attached EUNs, where IPv6 392 host 'E' autoconfigures IPv6 address 2001:db8:1::1. 394 Host 'F' connects to the AERO link via an interface with addresses 395 L3(F)/L2(F). 'F' next configures a default IPv6 route with next-hop 396 address L3(A) via the AERO interface, then receives the IPv6 address 397 2001:db8:2::1 from a DHCPv6 address configuration exchange via 'A'. 398 When 'F' receives the IPv6 address, it assigns the address to the 399 AERO interface but does not assign a non-link-local IPv6 prefix to 400 the interface. 402 Finally, IPv6 host 'G' connects to an IPv6 network outside of the 403 AERO link domain. 'G' configures its IPv6 interface in a manner 404 specific to its attached IPv6 link, and autoconfigures the IPv6 405 address 2001:db8:3::1. 407 In these arrangements, 'A' must maintain routes that associate the 408 delegated IPv6 addresses/prefixes with the correct routers and/or 409 hosts on the AERO link. The routers and hosts must maintain at least 410 a default route that points to gateway 'A', and can discover more- 411 specific routes either via a proactive dynamic routing protocol or 412 via the AERO mechanisms specified in Section 4.4. 414 4.4. AERO Specification 416 Figure 1 depicts a reference operational scenario including an AERO 417 link. The figure shows an AERO gateway ('A'), two AERO routers ('B', 418 'D'), an AERO host ('F') and three ordinary IPv6 hosts ('C', 'E', 419 'G') in a typical deployment configuration. We now discuss the 420 operation of AERO with respect to this reference scenario. 422 With reference to Figure 1, when host 'C' sends a packet with source 423 address 'C' and destination address 'E', the packet is first 424 forwarded over 'C's attached EUN to router 'B'. 'B' then forwards 425 the packet over the AERO interface to gateway 'A', which then 426 forwards the packet to router 'D', where the packet is finally 427 forwarded to host 'E' over 'E's attached EUN. When 'A' forwards the 428 packet back out on its advertising AERO interface, it must arrange to 429 redirect 'B' toward 'D' as a better next hop node on the AERO link 430 that is closer to the final destination 'E'. However, this 431 redirection process should only take place if there is assurance that 432 both 'B' and 'D' are willing participants. 434 Consider a first alternative in which 'A' informs 'B' only and does 435 not inform 'D' (i.e., "classic redirection"). In that case, 'D' has 436 no way of knowing that 'B' is authorized to forward packets from a 437 given source address. Also, 'B' has no way of knowing whether 'D' is 438 willing to accept its packets, nor whether 'D' is even reachable via 439 a direct path that does not involve 'A'. Finally, 'B' has no way of 440 knowing whether the final destination has moved away from 'D'. 442 Consider also a second alternative in which 'A' informs both 'B' and 443 'D' separately via independent redirection messages (i.e., "augmented 444 redirection"). In that case, several conditions can occur that could 445 result in communications failures. First, if 'B' receives the 446 redirection message but 'D' does not, subsequent packets sent by 'B' 447 would be dropped due to filtering since 'D' would not have a 448 forwarding table entry to verify their source addresses. Second, if 449 'D' receives the redirection message but 'B' does not, subsequent 450 packets sent in the reverse direction by 'D' would be lost. Finally, 451 timing issues surrounding the establishment and garbage collection of 452 forwarding table entries at 'B' and 'D' could yield unpredictable 453 behavior. For example, unless the timing were carefully coordinated 454 through some form of synchronization loop, there would invariably be 455 instances in which one node has the correct forwarding table state 456 and the other node does not resulting in non-deterministic packet 457 loss. 459 Since neither of these alternatives can satisfy the requirements 460 listed in Section 3, a new redirection technique is needed. In 461 particular, when 'A' forwards a packet from 'B' out the same AERO 462 interface toward 'D', 'A' must first send a "Predirect" message 463 forward to 'D' to inform it that 'B' is authorized to produce packets 464 using source address 'C'. After 'D' receives the Predirect, it sends 465 a "Redirect" message back to 'B' via 'A' as a trusted intermediary. 466 When 'B' receives the Redirect, it knows that 'D' will accept the 467 packets it sends with source address 'C' as long as 'D' retains the 468 forwarding table entry. This process is known as Asymmetric Extended 469 Route Optimization (AERO), which stands in contrast to the classical 470 and augmented redirection approaches. The following subsections 471 therefore specify the AERO redirection steps necessary to support the 472 reference operational scenario. 474 4.4.1. Sending Predirects 476 When a gateway forwards a packet out the same AERO interface that it 477 arrived on, the gateway sends a "Predirect" message forward to the 478 egress AERO node instead of sending a Redirect message back to the 479 ingress node. The Predirect message is simply an AERO-specific 480 version of an ordinary Redirect message. In the case of IPv6 as the 481 network layer protocol, the Predirect format is the same as depicted 482 in Section 4.5 of [RFC4861], and is identified by two new backward- 483 compatible bits taken from the Reserved field as shown in Figure 2: 485 0 1 2 3 486 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 1 487 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 488 | Type (=137) | Code (=0) | Checksum | 489 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 490 |A|P| Reserved | 491 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 492 | | 493 + + 494 | | 495 + Target Address + 496 | | 497 + + 498 | | 499 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 500 | | 501 + + 502 | | 503 + Destination Address + 504 | | 505 + + 506 | | 507 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 508 | Options ... 509 +-+-+-+-+-+-+-+-+-+-+-+- 511 Figure 2: AERO-Specific IPv6 Redirect Message Format 513 Where the new bits are defined as: 515 A (1) the "AERO" bit. Set to 1 to indicate an AERO-specific 516 Redirect message, and set to 0 to indicate an ordinary IPv6 517 Redirect message. 519 P (1) the "Predirect" bit. Set to 1 to indicate a Predirect 520 message, and set to 0 to indicate a Redirect response to a 521 Predirect message. (This bit is valid only when the A bit is set 522 to 1.) 524 In the reference operational scenario, when gateway 'A' forwards a 525 packet sent by 'B' toward 'D', it also sends a Predirect message 526 forward toward 'D', subject to rate limiting (see Section 8.2 of 527 [RFC4861]). 'A' prepares the Predirect message in a similar fashion 528 as for an ordinary IPv6 Redirect message as follows: 530 o the link-layer source address is set to 'L2(A)' 532 o the link-layer destination address is set to 'L2(D)' 534 o the network-layer source address is set to 'L3(B)'. 536 o the network-layer destination address is set to 'L3(D)'. 538 o the Predirect Target and Destination Addresses are both set to 539 'L3(B)'. 541 o on links that require stateful address mapping, the Predirect 542 message includes a Target Link Layer Address Option (TLLAO) set to 543 'L2(B)'. 545 o the Predirect message includes Route Information Options (RIOs) 546 [RFC4191] that encode prefixes taken from 'B's address/prefix 547 delegations, including one that covers the source address of the 548 originating packet. 550 o the Predirect message includes a Redirected Header Option (RHO) 551 that contains the first 128 bytes of the originating packet 552 beginning with the network-layer header (or up to the remainder of 553 the packet if there are fewer than 128 bytes). 555 o the A and P bits in the Predirect message header are both set to 556 1. 558 'A' then sends the Predirect message forward to 'D'. 560 4.4.2. Processing Predirects and Sending Redirects 562 When an AERO router or host receives a Predirect message, it 563 validates the message according to the appropriate redirect message 564 validation rules (e.g., Section 8.1 of [RFC4861] for IPv6). The node 565 further accepts the message only if it came from the link-layer 566 address of either a trusted gateway or the current previous hop on 567 the AERO link for the source address of the originating packet 568 encapsulated in the RHO. Finally, the node only processes the 569 message if the destination address of the originating packet 570 encapsulated in the RHO is covered by one of its CURRENT delegated 571 addresses/prefixes (see Section 4.5). 573 In the reference operational scenario, when router 'D' receives the 574 Predirect message from the gateway it creates forwarding table 575 entries with the prefixes encoded in RIO options as the target 576 prefixes, and associates the forwarding table entries with a node 577 structure (e.g., in a neighbor cache) that stores the source address 578 of the Predirect message (i.e., 'L3(B)'). 'D' places the node 579 structure in the FILTERING state, then sets/resets a filtering 580 expiration timer value of 40 seconds. If the filtering timer later 581 expires, 'D' clears the FILTERING state. If the node structure is 582 not in the FORWARDING state, 'D' then deletes the node structure and 583 all of its associated forwarding table entries. (This suggests that 584 'D's AERO interface should maintain a private forwarding table 585 separate from the primary forwarding table, since the node structure 586 and forwarding table entries must be managed by the AERO interface 587 itself.) 589 After processing the Predirect message and establishing the 590 forwarding table entry, 'D' prepares a Redirect message in response 591 to the Predirect as follows: 593 o the link-layer source address is set to 'L2(D)' 595 o the link-layer destination address is set to 'L2(A)' 597 o the network-layer source address is set to 'L3(D)'. 599 o the network-layer destination address is set to 'L3(B)'. 601 o the Redirect Target and the Redirect Destination Addresses are 602 both set to 'L3(D)'. 604 o on links that require stateful address mapping, the Redirect 605 message includes a TLLAO set to 'L2(D)'. 607 o the Redirect message includes an RHO copied from the corresponding 608 Predirect message. 610 o the (A, P) bits in the Redirect message header are set to (1, 0). 612 After 'D' prepares the Redirect message, it sends the message to 'A'. 613 (Note that the Redirect message does not include RIOs, since the 614 gateway is the only authoritative source of routing information and 615 will supply RIOs when it proxies the message.) 617 4.4.3. Proxying Redirects 619 When an AERO gateway receives a Redirect message, it accepts the 620 message only if it satisfies the redirect message validation rules. 621 The gateway further accepts the message only if it came from the 622 link-layer address of the current next hop toward the destination 623 address of the originating packet encapsulated in the RHO. The 624 gateway then "proxies" the Redirect message back to the original 625 ingress AERO node as described below. 627 In the reference operational scenario, 'A' receives the Redirect 628 message from 'D' then proxies the message back toward 'B'. In 629 particular, 'A' adds RIOs to the message that encode 'D's address/ 630 prefix delegations. Without decrementing the hopcount in the 631 Redirect message, 'A' next changes the link-layer source address of 632 the message to 'L2(A)' and changes the link-layer destination address 633 to 'L2(B)'. 'A' then sends this proxied Redirect message to 'B'. 635 4.4.4. Processing Redirects 637 When an AERO router or host receives a Redirect message, it validates 638 the message according to the appropriate redirect message validation 639 rules. The node further accepts the message only if it came from the 640 link-layer address of either a trusted gateway or the current next 641 hop on the AERO link for the destination address of the originating 642 packet encapsulated in the RHO. 644 In the reference operational scenario, 'B' receives the (proxied) 645 Redirect message then creates forwarding table entries with the 646 prefixes encoded in RIO options as the target prefixes. 'B' further 647 associates the forwarding table entries with a node structure (e.g., 648 in a neighbor cache) that stores the source address of the Predirect 649 message (i.e., 'L3(D)'). 'B' places the node structure in the 650 FORWARDING state, then sets/resets a filtering expiration timer value 651 of 30 seconds. If the filtering timer later expires, 'B' clears the 652 FORWARDING state. If the node structure is not in the FILTERING 653 state, 'B' then deletes the node structure and all of its associated 654 forwarding table entries. 656 Now, 'B' has a node structure for 'D' in the FORWARDING state, and 657 'D' has a node structure for 'B' in the FILTERING state. Therefore, 658 'B' may forward ordinary network-layer data packets with destination 659 addresses covered by 'D's prefixes directly to 'D' without involving 660 'A'. 'D' will in turn accept the packets since it has a forwarding 661 table entry authorizing 'B' to forward packets with source addresses 662 covered by 'B's prefixes. 664 To enable packet forwarding in the reverse direction, a separate AERO 665 operation is required which is the mirror-image of the forward AERO 666 operation described above, i.e., the forward and reverse AERO 667 operations are asymmetric. Following the reverse operation, packets 668 can be exchanged bidirectionally without involving 'A'. 670 4.4.5. Sending Periodic Predirect Keepalives 672 In order to prevent forwarding table entries from expiring while data 673 packets are actively flowing, the ingress node can periodically send 674 Predirect messages directly to the egress node (subject to rate 675 limiting) to solicit Redirect messages. In the reference operational 676 scenario, when 'B' forwards a packet to 'D' and wishes to update the 677 corresponding FORWARDING state timer, 'B' can also send a Predirect 678 message directly to 'D' prepared as follows: 680 o the link-layer source address is set to 'L2(B)'. 682 o the link-layer destination address is set to 'L2(D)'. 684 o the network-layer source address is set to 'L3(B)'. 686 o the network-layer destination address is set to 'L3(D)'. 688 o the Predirect Target and Destination Addresses are both set to 689 'L3(B)'. 691 o the Predirect message includes a Redirected Header Option (RHO) 692 that contains the first 128 bytes of the originating packet 693 beginning with the network-layer header (or up to the remainder of 694 the packet if there are fewer than 128 bytes). 696 o the A and P bits in the Predirect message header are both set to 697 1. 699 When 'D' receives the Predirect message, it accepts the message only 700 if it satisfies the redirect message validation rules. The node 701 further accepts the message only if it came from the current previous 702 hop on the AERO link for the source address of the originating packet 703 encapsulated in the RHO. 'D' then resets its FILTERING expiration 704 timer for node 'B' to 40 seconds, and sends a Redirect message 705 directly to 'B' prepared as follows: 707 o the link-layer source address is set to 'L2(D)'. 709 o the link-layer destination address is set to 'L2(B)'. 711 o the network-layer source address is set to 'L3(D)'. 713 o the network-layer destination address is set to 'L3(B)'. 715 o the Redirect Target and Destination Addresses are both set to 716 'L3(D)'. 718 o the Redirect message includes an RHO copied from the corresponding 719 Predirect message. 721 o the (A, P) bits in the Redirect message header are set to (1, 0). 723 When 'B' receives the Redirect message, it accepts the message only 724 if it satisfies the redirect message validation rules. The node 725 further accepts the message only if it came from the current next hop 726 on the AERO link for the source address of the originating packet 727 encapsulated in the RHO. 'B' then resets its FORWARDING expiration 728 timer for node 'D' to 30 seconds. 730 Note that in these direct neighbor to neighbor exchanges, neither the 731 Predirect nor Redirect message contain RIOs, since gateways are the 732 only authoritative source of routing information. Therefore, AERO 733 routers and hosts should not include RIOs in the Predirects/Redirects 734 they send, and they must ignore any RIOs included in received 735 Predirect/Redirect messages that did not come from a trusted gateway. 737 4.4.6. Reachability Considerations 739 When an ingress node receives a Redirect message informing it of a 740 direct path to a new egress, there is a question in point as to 741 whether the new egress can be reached directly without involving the 742 gateway as an intermediary. In some environments, it may be 743 reasonable for the ingress to (optimistically) assume that the new 744 egress is reachable, and to immediately begin forwarding data packets 745 to the egress without testing reachability. 747 In environments in which an optimistic assumption of reachability may 748 be inappropriate, however, the ingress can defer the redirection 749 until it tests the direct path to the egress by sending a direct 750 Predirect message to elicit a Redirect as specified in Section 4.4.5. 751 If the ingress is unable to elicit a Redirect message after a small 752 number of attempts, it should consider the direct path to the egress 753 as unusable. 755 In either case, the ingress can process any link errors corresponding 756 to the data packets sent directly to the egress as a hint that the 757 direct path has either failed or has become intermittent. 759 4.5. Mobility Considerations 761 An AERO link egress router 'D' can configure both a non-advertising 762 router interface on a provider AERO link and advertising router 763 interfaces on EUN links to provide gateway services to nodes in EUNs. 764 When node 'E' in an EUN that has obtained addresses/prefixes moves to 765 a different network point of attachment, however, 'E' can release its 766 address/prefix delegations via 'D' and re-establish them via a 767 different gateway. 769 When 'E' releases its address/prefix delegations via 'D', 'D' marks 770 the forwarding table entries that cover the addresses/prefixes as 771 DEPARTED (i.e., it clears the CURRENT state). 'D' therefore ceases 772 to respond to Predirect messages correlated with the DEPARTED 773 entries, and also schedules a garbage-collection timer of 60 seconds, 774 after which it deletes the DEPARTED entries. 776 When 'D' receives packets destined to an address covered by the 777 DEPARTED forwarding table entries, it forwards them to the last-known 778 EUN link-layer address of 'E' as a means for avoiding mobility- 779 related packet loss during routing changes. 'D' also returns a NULL 780 Redirect message to inform the correspondent 'B' of the departure. 781 The Redirect message is prepared as follows: 783 o the link-layer source address is set to 'L2(D)'. 785 o the link-layer destination address is set to 'L2(B)'. 787 o the network-layer source address is set to 'L3(D)'. 789 o the network-layer destination address is set to 'L3(B)'. 791 o the Redirect Target and Destination Addresses are both set to 792 NULL. 794 o the Redirect message includes an RHO copied from the corresponding 795 Predirect message. 797 o the (A, P) bits in the Redirect message header are set to (1, 0). 799 Eventually, any correspondents will receive such a NULL Redirect 800 message and will cease to use 'D' as a next hop. They will then 801 revert to sending packets destined to 'E' via their gateways and will 802 receive new Redirect messages to discover that 'E' is now associated 803 with a new egress router. Note that any packets forwarded by 'D' via 804 a forwarding table entry in the DEPARTED state may be lost if the 805 mobile node moves off-link with respect to its previous EUN point of 806 attachment. This should not be a problem for large links (e.g., 807 large cellular network deployments, large ISP networks, etc.) in 808 which all/most mobility events are intra-link. 810 4.6. Scaling Considerations 812 Figure 1 depicts a reference network topology with only a single 813 gateway on the AERO link. In order to support larger numbers of AERO 814 routers and hosts, the AERO link can deploy more gateways to support 815 load balancing and generally shortest-path routing. 817 Such an arrangement requires that the gateways participate in a 818 routing protocol instance (e.g., iBGP) so that address/prefix 819 delegations can be mapped to the correct gateway. The routing 820 protocol instance can be configured as either a full mesh topology 821 involving all gateways, or as a partial mesh topology with each AERO 822 link gateway associating with one or more backhaul network companion 823 gateways and a full mesh between companion gateways. 825 4.7. Proxy Chaining 827 In large AERO link deployments, there may be many gateways - each 828 serving many AERO routers and hosts. The gateways then either 829 require full topology knowledge, or a default route to a companion 830 gateway that does have full topology knowledge. For example, if AERO 831 node 'A' connects to gateway 'B', and AERO node 'E' connects to 832 gateway 'D', then 'B' and 'D' must either have full topology 833 knowledge or have a default route to a companion gateway (e.g., 'C') 834 that does. 836 In that case, when 'A' forwards an initial packet destined to an end 837 system behind 'E', 'B' generates a Predirect message toward 'C', 838 which proxies the message toward 'D' which finally proxies the 839 message toward 'E'. 841 In the reverse direction, when 'E' sends a Redirect response message 842 back to 'A', it first sends the message to 'D', which proxies the 843 message toward 'C', which proxies the message toward 'B', which 844 finally proxies the message toward 'A'. 846 4.8. Backward Compatibility 848 If a legacy host or router receives an AERO Redirect or Predirect 849 message, it will process the message as if it were an ordinary 850 Redirect. This will cause no harmful effects, since the legacy 851 system will ignore the 'A' and P' bits in the Reserved field, and 852 will also ignore any RIOs that are included. The values encoded in 853 the Redirect message target and destination addresses will also not 854 cause the legacy node to create incorrect routing state. The 855 mechanism therefore causes no harm to legacy systems, and supports 856 natural incremental deployment. 858 5. IANA Considerations 860 There are no IANA considerations for this document. 862 6. Security Considerations 864 This document enables ingress filtering, and therefore improves the 865 security of AERO links. 867 7. Acknowledgements 869 Discussions both on the v6ops list and in private exchanges helped 870 shape some of the concepts in this work. Individuals who contributed 871 insights include Mikael Abrahamsson, Fred Baker, Brian Carpenter, 872 Joel Halpern, Lee Howard, 874 8. References 876 8.1. Normative References 878 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 879 September 1981. 881 [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, 882 RFC 792, September 1981. 884 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 885 Requirement Levels", BCP 14, RFC 2119, March 1997. 887 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 888 (IPv6) Specification", RFC 2460, December 1998. 890 [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and 891 More-Specific Routes", RFC 4191, November 2005. 893 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 894 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 895 September 2007. 897 8.2. Informative References 899 [I-D.ietf-savi-framework] 900 Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, 901 "Source Address Validation Improvement Framework", 902 draft-ietf-savi-framework-04 (work in progress), 903 March 2011. 905 [I-D.templin-intarea-vet] 906 Templin, F., "Virtual Enterprise Traversal (VET)", 907 draft-templin-intarea-vet-24 (work in progress), 908 March 2011. 910 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 911 RFC 2131, March 1997. 913 [RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4 914 Domains without Explicit Tunnels", RFC 2529, March 1999. 916 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., 917 and M. Carney, "Dynamic Host Configuration Protocol for 918 IPv6 (DHCPv6)", RFC 3315, July 2003. 920 [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic 921 Host Configuration Protocol (DHCP) version 6", RFC 3633, 922 December 2003. 924 [RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site 925 Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, 926 March 2008. 928 [RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4 929 Infrastructures (6rd)", RFC 5569, January 2010. 931 Author's Address 933 Fred L. Templin (editor) 934 Boeing Research & Technology 935 P.O. Box 3707 MC 7L-49 936 Seattle, WA 98124 937 USA 939 Email: fltemplin@acm.org