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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 Obsoletes: rfc6706 (if approved) June 26, 2014 5 Intended status: Standards Track 6 Expires: December 28, 2014 8 Transmission of IP Packets over AERO Links 9 draft-templin-aerolink-27.txt 11 Abstract 13 This document specifies the operation of IP over tunnel virtual Non- 14 Broadcast, Multiple Access (NBMA) links using Asymmetric Extended 15 Route Optimization (AERO). Nodes attached to AERO links can exchange 16 packets via trusted intermediate routers that provide forwarding 17 services to reach off-link destinations and redirection services for 18 route optimization. AERO provides an IPv6 link-local address format 19 known as the AERO address that supports operation of the IPv6 20 Neighbor Discovery (ND) protocol and links IPv6 ND to IP routing. 21 Admission control and provisioning are supported by the Dynamic Host 22 Configuration Protocol for IPv6 (DHCPv6), and node mobility is 23 naturally supported through dynamic neighbor cache updates. Although 24 IPv6 ND messaging is used in the control plane, both IPv4 and IPv6 25 are supported in the data plane. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at http://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on December 28, 2014. 44 Copyright Notice 46 Copyright (c) 2014 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (http://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 62 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 3. Asymmetric Extended Route Optimization (AERO) . . . . . . . . 5 64 3.1. AERO Node Types . . . . . . . . . . . . . . . . . . . . . 5 65 3.2. AERO Addresses . . . . . . . . . . . . . . . . . . . . . 6 66 3.3. AERO Interface Characteristics . . . . . . . . . . . . . 7 67 3.3.1. Coordination of Multiple Underlying Interfaces . . . 9 68 3.4. AERO Interface Neighbor Cache Maintenace . . . . . . . . 9 69 3.5. AERO Interface Data Origin Authentication . . . . . . . . 11 70 3.6. AERO Interface MTU Considerations . . . . . . . . . . . . 11 71 3.7. AERO Interface Encapsulation, Re-encapsulation and 72 Decapsulation . . . . . . . . . . . . . . . . . . . . . . 13 73 3.8. AERO Router Discovery, Prefix Delegation and Address 74 Configuration . . . . . . . . . . . . . . . . . . . . . . 14 75 3.8.1. AERO Client Behavior . . . . . . . . . . . . . . . . 14 76 3.8.2. AERO Server Behavior . . . . . . . . . . . . . . . . 16 77 3.9. AERO Redirection . . . . . . . . . . . . . . . . . . . . 17 78 3.9.1. Reference Operational Scenario . . . . . . . . . . . 17 79 3.9.2. Classical Redirection Approaches . . . . . . . . . . 19 80 3.9.3. Concept of Operations . . . . . . . . . . . . . . . . 20 81 3.9.4. Message Format . . . . . . . . . . . . . . . . . . . 20 82 3.9.5. Sending Predirects . . . . . . . . . . . . . . . . . 21 83 3.9.6. Processing Predirects and Sending Redirects . . . . . 23 84 3.9.7. Re-encapsulating and Relaying Redirects . . . . . . . 24 85 3.9.8. Processing Redirects . . . . . . . . . . . . . . . . 25 86 3.9.9. Server-Oriented Redirection . . . . . . . . . . . . . 25 87 3.10. Neighbor Reachability Maintenance . . . . . . . . . . . . 25 88 3.11. Mobility Management . . . . . . . . . . . . . . . . . . . 26 89 3.12. Encapsulation Protocol Version Considerations . . . . . . 28 90 3.13. Multicast Considerations . . . . . . . . . . . . . . . . 28 91 3.14. Operation on AERO Links Without DHCPv6 Services . . . . . 29 92 3.15. Operation on Server-less AERO Links . . . . . . . . . . . 29 93 4. Implementation Status . . . . . . . . . . . . . . . . . . . . 29 94 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 95 6. Security Considerations . . . . . . . . . . . . . . . . . . . 29 96 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30 97 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 31 98 8.1. Normative References . . . . . . . . . . . . . . . . . . 31 99 8.2. Informative References . . . . . . . . . . . . . . . . . 32 100 Appendix A. AERO Server and Relay Interworking . . . . . . . . . 34 101 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 36 103 1. Introduction 105 This document specifies the operation of IP over tunnel virtual Non- 106 Broadcast, Multiple Access (NBMA) links using Asymmetric Extended 107 Route Optimization (AERO). The AERO link can be used for tunneling 108 to neighboring nodes on either IPv6 or IPv4 networks, i.e., AERO 109 views the IPv6 and IPv4 networks as equivalent links for tunneling. 110 Nodes attached to AERO links can exchange packets via trusted 111 intermediate routers that provide forwarding services to reach off- 112 link destinations and redirection services for route optimization 113 that addresses the requirements outlined in [RFC5522]. 115 AERO provides an IPv6 link-local address format known as the AERO 116 address that supports operation of the IPv6 Neighbor Discovery (ND) 117 [RFC4861] protocol and links IPv6 ND to IP routing. Admission 118 control and provisioning are supported by the Dynamic Host 119 Configuration Protocol for IPv6 (DHCPv6) [RFC3315], and node mobility 120 is naturally supported through dynamic neighbor cache updates. 121 Although IPv6 ND message signalling is used in the control plane, 122 both IPv4 and IPv6 are supported in the data plane. The remainder of 123 this document presents the AERO specification. 125 2. Terminology 127 The terminology in the normative references applies; the following 128 terms are defined within the scope of this document: 130 AERO link 131 a Non-Broadcast, Multiple Access (NBMA) tunnel virtual overlay 132 configured over a node's attached IPv6 and/or IPv4 networks. All 133 nodes on the AERO link appear as single-hop neighbors from the 134 perspective of the overlay IP layer. 136 AERO interface 137 a node's attachment to an AERO link. 139 AERO address 140 an IPv6 link-local address constructed as specified in Section 3.2 141 and assigned to a Client's AERO interface. 143 AERO node 144 a node that is connected to an AERO link and that participates in 145 IPv6 ND over the link. 147 AERO Client ("Client") 148 a node that assigns an AERO address on an AERO interface and 149 receives an IP prefix delegation. 151 AERO Server ("Server") 152 a node that configures a router interface on an AERO link over 153 which it can provide default forwarding and redirection services 154 for AERO Clients. 156 AERO Relay ("Relay") 157 a node that relays IP packets between Servers on the same AERO 158 link, and/or that forwards IP packets between the AERO link and 159 the native Internetwork. An AERO Relay may or may not also be 160 configured as an AERO Server. 162 ingress tunnel endpoint (ITE) 163 an AERO interface endpoint that injects tunneled packets into an 164 AERO link. 166 egress tunnel endpoint (ETE) 167 an AERO interface endpoint that receives tunneled packets from an 168 AERO link. 170 underlying network 171 a connected IPv6 or IPv4 network routing region over which AERO 172 nodes tunnel IP packets. 174 underlying interface 175 an AERO node's interface point of attachment to an underlying 176 network. 178 link-layer address 179 an IP address assigned to an AERO node's underlying interface. 180 When UDP encapsulation is used, the UDP port number is also 181 considered as part of the link-layer address. Link-layer 182 addresses are used as the encapsulation header source and 183 destination addresses. 185 network layer address 186 the source or destination address of the encapsulated IP packet. 188 end user network (EUN) 189 an internal virtual or external edge IP network that an AERO 190 Client connects to the AERO interface. 192 end user network prefix 193 an IP prefix delegated to an end user network. 195 aggregated prefix 196 an IP prefix assigned to the AERO link and from which end user 197 network prefixes are derived. (For example, and end user network 198 prefix 2001:db8:1:2::/64 is derived from the aggregated prefix 199 2001:db8::/32.) 201 Throughout the document, the simple terms "Client", "Server" and 202 "Relay" refer to "AERO Client", "AERO Server" and "AERO Relay", 203 respectively. Capitalization is used to distinguish these terms from 204 DHCPv6 client/server/relay. This is an important distinction, since 205 an AERO Server may be a DHCPv6 relay, and an AERO Relay may be a 206 DHCPv6 server. 208 Also throughout the document, the term "IP" is used to generically 209 refer to either of IPv4 or IPv6. 211 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 212 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 213 document are to be interpreted as described in [RFC2119]. 215 3. Asymmetric Extended Route Optimization (AERO) 217 The following sections specify the operation of IP over Asymmetric 218 Extended Route Optimization (AERO) links: 220 3.1. AERO Node Types 222 AERO Relays relay packets between nodes connected to the same AERO 223 link and also forward packets between the AERO link and the native 224 Internetwork. The relaying process entails re-encapsulation of IP 225 packets that were received from a first AERO node and are to be 226 forwarded without modification to a second AERO node. 228 AERO Servers configure their AERO interfaces as router interfaces, 229 and provide default routing services to AERO Clients. AERO Servers 230 configure a DHCPv6 relay or server function and facilitate DHCPv6 231 Prefix Delegation (PD) exchanges. An AERO Server may also act as an 232 AERO Relay. 234 AERO Clients act as requesting routers to receive IP prefixes through 235 a DHCPv6 PD exchange via AERO Servers over the AERO link. (Each 236 client MAY associate with multiple Servers, but associating with many 237 Servers may result in excessive control message overhead.) Each IPv6 238 AERO Client receives at least a /64 IPv6 prefix delegation, and may 239 receive even shorter prefixes. Similarly, each IPv4 AERO Client 240 receives at least a /32 IPv4 prefix delegation (i.e., a singleton 241 IPv4 address), and may receive even shorter prefixes. 243 AERO Clients that act as routers configure their AERO interfaces as 244 router interfaces and sub-delegate portions of their received prefix 245 delegations to links on EUNs. End system applications on AERO 246 Clients that act as routers bind to EUN interfaces (i.e., and not the 247 AERO interface). 249 AERO Clients that act as ordinary hosts configure their AERO 250 interfaces as host interfaces and assign one or more IP addresses 251 taken from their received prefix delegations to the AERO interface 252 but DO NOT assign the delegated prefix itself to the AERO interface. 253 Instead, the host assigns the delegated prefix to a "black hole" 254 route so that unused portions of the prefix are nullified. End 255 system applications on AERO Clients that act as hosts bind directly 256 to the AERO interface. 258 3.2. AERO Addresses 260 An AERO address is an IPv6 link-local address with an embedded IP 261 prefix and assigned to a Client's AERO interface. The AERO address 262 is formatted as follows: 264 fe80::[IP prefix] 266 For IPv6, the AERO address begins with the prefix fe80::/64 and 267 includes in its interface identifier the base prefix taken from the 268 Client's delegated IPv6 prefix. The base prefix is determined by 269 masking the delegated prefix with the prefix length. For example, if 270 the AERO Client receives the IPv6 prefix delegation: 272 2001:db8:1000:2000::/56 274 it constructs its AERO address as: 276 fe80::2001:db8:1000:2000 278 For IPv4, the AERO address begins with the prefix fe80::/96 and 279 includes in its interface identifier the base prefix taken from the 280 Client's delegated IPv4 prefix. For example, if the AERO Client 281 receives the IPv4 prefix delegation: 283 192.0.2.32/28 285 it constructs its AERO address as: 287 fe80::192.0.2.32 289 The AERO address remains stable as the Client moves between 290 topological locations, i.e., even if its link-layer addresses change. 292 NOTE: In some cases, prospective neighbors may not have a priori 293 knowledge of the Client's delegated prefix length and may therefore 294 send initial IPv6 ND messages with an AERO destination address that 295 matches the delegated prefix but does not correspond to the base 296 prefix. In that case, the Client MUST accept the address as 297 equivalent to the base address, but then use the base address as the 298 source address of any IPv6 ND message replies. For example, if the 299 Client receives the IPv6 prefix delegation 2001:db8:1000:2000::/56 300 then subsequently receives an IPv6 ND message with destination 301 address fe80::2001:db8:1000:2001, it accepts the message but uses 302 fe80::2001:db8:1000:2000 as the source address of any IPv6 ND 303 replies. 305 3.3. AERO Interface Characteristics 307 AERO interfaces use IP-in-IPv6 encapsulation [RFC2473] to exchange 308 tunneled packets with AERO neighbors attached to an underlying IPv6 309 network, and use IP-in-IPv4 encapsulation [RFC2003][RFC4213] to 310 exchange tunneled packets with AERO neighbors attached to an 311 underlying IPv4 network. AERO interfaces can also operate over 312 secured tunnel types such as IPsec [RFC4301] or TLS [RFC5246]. When 313 Network Address Translator (NAT) traversal and/or filtering middlebox 314 traversal may be necessary, a UDP header is further inserted 315 immediately above the IP encapsulation header. 317 Servers assign the address fe80:: to their AERO interfaces as a link- 318 local Subnet Router Anycast address. Servers and Relays also assign 319 a link-local address fe80::ID to support the operation of the IPv6 ND 320 protocol and the inter-Server/Relay routing system (see: Appendix A). 321 Each fe80::ID address MUST be unique among all Servers and Relays on 322 the AERO link, and MUST NOT collide with any potential AERO addresses 323 (e.g., the addresses for Servers and Relays on the link could be 324 assigned as fe80::1, fe80::2, fe80::3, etc.). Servers accept IPV6 ND 325 messages with either fe80::ID or fe80:: as the IPv6 destination 326 address, but MUST use the fe80::ID address as the IPv6 source address 327 of any IPv6 ND messages they generate. 329 When a Client does not know the fe80::ID address of a Server, it can 330 use fe80:: as a temporary destination address in IPv6 ND messages. 331 The Client may also use fe80::, e.g., as the link-local address in a 332 neighbor cache entry for a Server when the Server's fe80::ID address 333 is not known in advance. 335 When a Client enables an AERO interface, it invokes DHCPv6 PD using 336 the temporary IPv6 link-local source address 337 fe80::ffff:ffff:ffff:ffff. After the Client receives a prefix 338 delegation, it assigns the corresponding AERO address to the AERO 339 interface and deprecates the temporary address, i.e., the Client 340 invokes DHCPv6 to bootstrap the provisioning of a unique link-local 341 address before invoking IPv6 ND. 343 AERO interfaces maintain a neighbor cache and use an adaptation of 344 standard unicast IPv6 ND messaging. AERO interfaces use unicast 345 Neighbor Solicitation (NS), Neighbor Advertisement (NA), Router 346 Solicitation (RS) and Router Advertisement (RA) messages the same as 347 for any IPv6 link. AERO interfaces use two redirection message types 348 -- the first known as a Predirect message and the second being the 349 standard Redirect message (see Section 3.9). AERO links further use 350 link-local-only addressing; hence, Clients ignore any Prefix 351 Information Options (PIOs) they may receive in RA messages. 353 AERO interface Redirect/Predirect messages include Target Link-Layer 354 Address Options (TLLAOs) formatted as shown in Figure 1: 356 0 1 2 3 357 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 1 358 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 359 | Type = 2 | Length = 3 | Reserved | 360 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 361 | Link ID | Preference | UDP Port Number (or 0) | 362 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 363 | | 364 +-- --+ 365 | | 366 +-- IP Address --+ 367 | | 368 +-- --+ 369 | | 370 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 372 Figure 1: AERO Target Link-Layer Address Option (TLLAO) Format 374 In this format, Link ID is an integer value between 0 and 255 375 corresponding to an underlying interface of the target node, and 376 Preference is an integer value between 0 and 255 indicating the 377 node's preference for this underlying interface, with 0 being highest 378 preference and 255 being lowest. UDP Port Number and IP Address are 379 set to the addresses used by the target node when it sends 380 encapsulated packets over the underlying interface. When no UDP 381 encapsulation is used, UDP Port Number is set to 0. When the 382 encapsulation IP address family is IPv4, IP Address is formed as an 383 IPv4-compatible IPv6 address [RFC4291], i.e., 96 bits of leading 0's 384 followed by a 32-bit IPv4 address 386 AERO interface Redirect/Predirect messages can both update and create 387 neighbor cache entries, including link-layer address information. 388 Redirect/Predirect messages SHOULD include a Timestamp option (see 389 Section 5.3 of [RFC3971]) that other AERO nodes can use to verify the 390 message time of origin. 392 AERO interface NS/NA/RS/RA messages update timers in existing 393 neighbor cache entires but do not update link-layer addresses nor 394 create new neighbor cache entries. NS/RS messages SHOULD include a 395 Nonce option (see Section 5.3 of [RFC3971]) that the recipient echoes 396 back in the corresponding NA/RA response. Unsolicited NA/RA messages 397 are not used on AERO interfaces, and SHOULD be ignored on receipt. 399 3.3.1. Coordination of Multiple Underlying Interfaces 401 AERO interfaces may be configured over multiple underlying 402 interfaces. For example, common handheld devices have both wireless 403 local area network ("WLAN") and cellular wireless links. These links 404 are typically used "one at a time" with low-cost WLAN preferred and 405 highly-available cellular wireless as a standby. In a more complex 406 example, aircraft frequently have many wireless data link types (e.g. 407 satellite-based, terrestrial, air-to-air directional, etc.) with 408 diverse performance and cost properties. 410 If a Client's multiple underlying interfaces are used "one at a time" 411 (i.e., all other interfaces are in standby mode while one interface 412 is active), then Predirect/Redirect messages include only a single 413 TLLAO with Link ID set to 0. 415 If the Client has multiple active underlying interfaces, then from 416 the perspective of IPv6 ND it would appear to have a single link- 417 local address with multiple link-layer addresses. In that case, 418 Predirect/Redirect messages MAY include multiple TLLAOs -- each with 419 a different Link ID that corresponds to an underlying interface of 420 the Client. Further details on coordination of multiple active 421 underlying interfaces are outside the scope of this specification. 423 3.4. AERO Interface Neighbor Cache Maintenace 425 Each AERO interface maintains a conceptual neighbor cache that 426 includes an entry for each neighbor it communicates with on the AERO 427 link, the same as for any IPv6 interface [RFC4861]. Neighbor cache 428 entries are created and maintained as follows: 430 When an AERO Server relays a DHCPv6 Reply message to an AERO Client, 431 it creates or updates a neighbor cache entry for the Client based on 432 the AERO address corresponding to the Client's prefix delegation as 433 the network-layer address and with the Client's encapsulation IP 434 address and UDP port number as the link-layer address. 436 When an AERO Client receives a DHCPv6 Reply message from an AERO 437 Server, it creates or updates a neighbor cache entry for the Server 438 based on the Reply message link-local source address as the network- 439 layer address, and the encapsulation IP source address and UDP source 440 port number as the link-layer address. 442 When an AERO Client receives a valid Predirect message it creates or 443 updates a neighbor cache entry for the Predirect target network-layer 444 and link-layer addresses, and also creates an IP forwarding table 445 entry for the predirected (source) prefix. The node then sets an 446 "ACCEPT" timer for the neighbor and uses this timer to determine 447 whether messages received from the predirected neighbor can be 448 accepted. 450 When an AERO Client receives a valid Redirect message it creates or 451 updates a neighbor cache entry for the Redirect target network-layer 452 and link-layer addresses, and also creates an IP forwarding table 453 entry for the redirected (destination) prefix. The node then sets a 454 "FORWARD" timer for the neighbor and uses this timer to determine 455 whether packets can be sent directly to the redirected neighbor. The 456 node also maintains a constant value MAX_RETRY to limit the number of 457 keepalives sent when a neighbor may have gone unreachable. 459 When an AERO Client receives a valid NS message it (re)sets the 460 ACCEPT timer for the neighbor to ACCEPT_TIME. 462 When an AERO Client receives a valid NA message, it (re)sets the 463 FORWARD timer for the neighbor to FORWARD_TIME. 465 It is RECOMMENDED that FORWARD_TIME be set to the default constant 466 value 30 seconds to match the default REACHABLE_TIME value specified 467 for IPv6 ND [RFC4861]. 469 It is RECOMMENDED that ACCEPT_TIME be set to the default constant 470 value 40 seconds to allow a 10 second window so that the AERO 471 redirection procedure can converge before the ACCEPT timer decrements 472 below FORWARD_TIME. 474 It is RECOMMENDED that MAX_RETRY be set to 3 the same as described 475 for IPv6 ND address resolution in Section 7.3.3 of [RFC4861]. 477 Different values for FORWARD_TIME, ACCEPT_TIME, and MAX_RETRY MAY be 478 administratively set, if necessary, to better match the AERO link's 479 performance characteristics; however, if different values are chosen, 480 all nodes on the link MUST consistently configure the same values. 481 In particular, ACCEPT_TIME SHOULD be set to a value that is 482 sufficiently longer than FORWARD_TIME to allow the AERO redirection 483 procedure to converge. 485 3.5. AERO Interface Data Origin Authentication 487 AERO nodes use a simple data origin authentication for encapsulated 488 packets they receive from other nodes. In particular, AERO nodes 489 accept encapsulated packets with a link-layer source address 490 belonging to one of their current AERO Servers and accept 491 encapsulated packets with a link-layer source address that is correct 492 for the network-layer source address. 494 The AERO node considers the link-layer source address correct for the 495 network-layer source address if there is an IP forwarding table entry 496 that matches the network-layer source address as well as a neighbor 497 cache entry corresponding to the next hop that includes the link- 498 layer address and the ACCEPT timer is non-zero. 500 Note that this simple data origin authentication only applies to 501 environments in which link-layer addresses cannot be spoofed. 502 Additional security mitigations may be necessary in other 503 environments. 505 3.6. AERO Interface MTU Considerations 507 The AERO link Maximum Transmission Unit (MTU) is 64KB minus the 508 encapsulation overhead for IPv4 as the link-layer [RFC0791] and 4GB 509 minus the encapsulation overhead for IPv6 as the link layer 510 [RFC2675]. This is the most that IPv4 and IPv6 (respectively) can 511 convey within the constraints of protocol constants, but actual sizes 512 available for tunneling will frequently be much smaller. 514 The base tunneling specifications for IPv4 and IPv6 typically set a 515 static MTU on the tunnel interface to 1500 bytes minus the 516 encapsulation overhead or smaller still if the tunnel is likely to 517 incur additional encapsulations on the path. This can result in path 518 MTU related black holes when packets that are too large to be 519 accommodated over the AERO link are dropped, but the resulting ICMP 520 Packet Too Big (PTB) messages are lost on the return path. As a 521 result, AERO nodes use the following MTU mitigations to accommodate 522 larger packets. 524 AERO nodes set their AERO interface MTU to the larger of the 525 underlying interface MTU minus the encapsulation overhead, and 1500 526 bytes. (If there are multiple underlying interfaces, the node sets 527 the AERO interface MTU according to the largest underlying interface 528 MTU, or 64KB /4G minus the encapsulation overhead if the largest MTU 529 cannot be determined.) AERO nodes optionally cache other per- 530 neighbor MTU values in the underlying IP path MTU discovery cache 531 initialized to the underlying interface MTU. 533 AERO nodes admit packets that are no larger than 1280 bytes minus the 534 encapsulation overhead (*) as well as packets that are larger than 535 1500 bytes into the tunnel without fragmentation, i.e., as long as 536 they are no larger than the AERO interface MTU before encapsulation 537 and also no larger than the cached per-neighbor MTU following 538 encapsulation. For IPv4, the node sets the "Don't Fragment" (DF) bit 539 to 0 for packets no larger than 1280 bytes minus the encapsulation 540 overhead (*) and sets the DF bit to 1 for packets larger than 1500 541 bytes. If a large packet is lost in the path, the node may 542 optionally cache the MTU reported in the resulting PTB message or may 543 ignore the message, e.g., if there is a possibility that the message 544 is spurious. 546 For packets destined to an AERO node that are larger than 1280 bytes 547 minus the encapsulation overhead (*) but no larger than 1500 bytes, 548 the node uses IP fragmentation to fragment the encapsulated packet 549 into two pieces (where the first fragment contains 1024 bytes of the 550 original IP packet) then admits the fragments into the tunnel. If 551 the link-layer protocol is IPv4, the node admits each fragment into 552 the tunnel with DF set to 0 and subject to rate limiting to avoid 553 reassembly errors [RFC4963][RFC6864]. For both IPv4 and IPv6, the 554 node also sends a 1500 byte probe message (**) to the neighbor, 555 subject to rate limiting. 557 To construct a probe, the node prepares an NS message with a Nonce 558 option plus trailing padding octets added to a length of 1500 bytes 559 without including the length of the padding in the IPv6 Payload 560 Length field. The node then encapsulates the NS in the encapsulation 561 headers (while including the length of the padding in the 562 encapsulation header length fields), sets DF to 1 (for IPv4) and 563 sends the padded NS message to the neighbor. If the neighbor returns 564 an NA message with a correct Nonce value, the node may then send 565 whole packets within this size range and (for IPv4) relax the rate 566 limiting requirement. (Note that the trailing padding SHOULD NOT be 567 included within the Nonce option itself but rather as padding beyond 568 the last option in the NS message; otherwise, the (large) Nonce 569 option would be echoed back in the solicited NA message and may be 570 lost at a link with a small MTU along the reverse path.) 571 AERO nodes MUST be capable of reassembling packets up to 1500 bytes 572 plus the encapsulation overhead length. It is therefore RECOMMENDED 573 that AERO nodes be capable of reassembling at least 2KB. 575 (*) Note that if it is known without probing that the minimum Path 576 MTU to an AERO node is MINMTU bytes (where 1280 < MINMTU < 1500) then 577 MINMTU can be used instead of 1280 in the fragmentation threshold 578 considerations listed above. 580 (**) It is RECOMMENDED that no probes smaller than 1500 bytes be used 581 for MTU probing purposes, since smaller probes may be fragmented if 582 there is a nested tunnel somewhere on the path to the neighbor. 583 Probe sizes larger than 1500 bytes MAY be used, but may be 584 unnecessary since original sources are expected to implement 585 [RFC4821] when sending large packets. 587 3.7. AERO Interface Encapsulation, Re-encapsulation and Decapsulation 589 AERO interfaces encapsulate IP packets according to whether they are 590 entering the AERO interface for the first time or if they are being 591 forwarded out the same AERO interface that they arrived on. This 592 latter form of encapsulation is known as "re-encapsulation". 594 AERO interfaces encapsulate packets per the specifications in 595 [RFC2003][RFC2473][RFC4213][RFC4301][RFC5246] except that the 596 interface copies the "TTL/Hop Limit", "Type of Service/Traffic Class" 597 and "Congestion Experienced" values in the packet's IP header into 598 the corresponding fields in the encapsulation header. For packets 599 undergoing re-encapsulation, the AERO interface instead copies the 600 "TTL/Hop Limit", "Type of Service/Traffic Class" and "Congestion 601 Experienced" values in the original encapsulation header into the 602 corresponding fields in the new encapsulation header (i.e., the 603 values are transferred between encapsulation headers and *not* copied 604 from the encapsulated packet's network-layer header). 606 When AERO UDP encapsulation is used, the AERO interface encapsulates 607 the packet per the specifications in [RFC2003][RFC2473][RFC4213] 608 except that it inserts a UDP header between the encapsulation header 609 and the packet's IP header. The AERO interface sets the UDP source 610 port to a constant value that it will use in each successive packet 611 it sends, sets the UDP checksum field to zero (see: 612 [RFC6935][RFC6936]) and sets the UDP length field to the length of 613 the IP packet plus 8 bytes for the UDP header itself. For packets 614 sent via a Server, the AERO interface sets the UDP destination port 615 to 8060 (i.e., the IANA-registered port number for AERO) when AERO- 616 only encapsulation is used. For packets sent to a neighboring 617 Client, the AERO interface sets the UDP destination port to the port 618 value stored in the neighbor cache entry for this neighbor. 620 The AERO interface next sets the IP protocol number in the 621 encapsulation header to the appropriate value for the first protocol 622 layer within the encapsulation (e.g., IPv4, IPv6, UDP, IPsec, etc.). 623 When IPv6 is used as the encapsulation protocol, the interface then 624 sets the flow label value in the encapsulation header the same as 625 described in [RFC6438]. When IPv4 is used as the encapsulation 626 protocol, the AERO interface sets the DF bit as discussed in 627 Section 3.6. 629 AERO interfaces decapsulate packets destined either to the node 630 itself or to a destination reached via an interface other than the 631 receiving AERO interface. When AERO UDP encapsulation is used (i.e., 632 when a UDP header with destination port 8060 is present) the 633 interface examines the first octet of the encapsulated packet. If 634 the most significant four bits of the first octet encode the value 635 '0110' (i.e., the version number value for IPv6) or the value '0100' 636 (i.e., the version number value for IPv4), the packet is accepted and 637 the encapsulating UDP header is discarded; otherwise, the packet is 638 discarded. 640 Further decapsulation then proceeds according to the appropriate 641 tunnel type [RFC2003][RFC2473][RFC4213][RFC4301][RFC5246]. 643 3.8. AERO Router Discovery, Prefix Delegation and Address Configuration 645 3.8.1. AERO Client Behavior 647 AERO Clients discover the link-layer addresses of AERO Servers via 648 static configuration, or through an automated means such as DNS name 649 resolution. In the absence of other information, the Client resolves 650 the Fully-Qualified Domain Name (FQDN) "linkupnetworks.domainname", 651 where "domainname" is the DNS domain appropriate for the Client's 652 attached underlying network. After discovering the link-layer 653 addresses, the Client associates with one or more of the 654 corresponding Servers. 656 To associate with a Server, the Client acts as a requesting router to 657 request an IP prefix through DHCPv6 PD [RFC3315][RFC3633][RFC6355] 658 using fe80::ffff:ffff:ffff:ffff as the IPv6 source address (see 659 Section 3.3), 'All_DHCP_Relay_Agents_and_Servers' as the IPv6 660 destination address and the link-layer address of the Server as the 661 link-layer destination address. The Client includes a DHCPv6 Unique 662 Identifier (DUID) in the Client Identifier option of its DHCPv6 663 messages (as well as a DHCPv6 authentication option if necessary) to 664 identify itself to the DHCPv6 server. If the Client is pre- 665 provisioned with an IP prefix associated with the AERO service, it 666 MAY also include the prefix in its DHCPv6 PD Request to indicate its 667 preferred prefix to the DHCPv6 server. The Client then sends the 668 encapsulated DHCPv6 request via an underlying interface. 670 When the Client receives its prefix delegation via a Reply from the 671 DHCPv6 server, it creates a neighbor cache entry with the Server's 672 link-local address (i.e., fe80::ID) as the network-layer address and 673 the Server's encapsulation address as the link-layer addresses. 674 Next, the Client assigns the AERO address derived from the delegated 675 prefix to the AERO interface and sub-delegates the prefix to nodes 676 and links within its attached EUNs (the AERO address thereafter 677 remains stable as the Client moves). The Client also sets both the 678 ACCEPT and FORWARD timers for each Server to the constant value 679 REACHABLE_TIME. The Client further renews its prefix delegation by 680 performing DHCPv6 Renew/Reply exchanges with its AERO address as the 681 IPv6 source address, 'All_DHCP_Relay_Agents_and_Servers' as the IPv6 682 destination address, the link-layer address of a Server as the link- 683 layer destination address and the same DUID and authentication 684 information. If the Client wishes to associate with multiple 685 Servers, it can perform DHCPv6 Renew/Reply exchanges via each of the 686 Servers. 688 The Client then sends an RS message to each of its associated Servers 689 to receive an RA message with a default router lifetime and any other 690 link-specific parameters. When the Client receives an RA message, it 691 configures or updates a default route according to the default router 692 lifetime but ignores any Prefix Information Options (PIOs) included 693 in the RA message since the AERO link is link-local-only. The Client 694 further ignores any RS messages it might receive, since only Servers 695 may process RS messages. 697 The Client then sends periodic RS messages to each Server before the 698 ACCEPT/FORWARD timers expire to obtain new RA messages for Neighbor 699 Unreachability Detection (NUD), to refresh any network state, and to 700 update the default router lifetime and any other link-specific 701 parameters. When the Client receives a new RA message, it resets the 702 ACCEPT/FORWARD timers to REACHABLE_TIME. The Client can also forward 703 IP packets destined to networks beyond its local EUNs via a Server as 704 a default router. The Server may in turn return a redirection 705 message informing the Client of a neighbor on the AERO link that is 706 topologically closer to the final destination (see Section 3.9). 708 Note that, since the Client's AERO address is configured from the 709 unique DHCPv6 prefix delegation it receives, there is no need for 710 Duplicate Address Detection (DAD) on AERO links. Other nodes 711 maliciously attempting to hijack an authorized Client's AERO address 712 will be denied access to the network by the DHCPv6 server due to an 713 unacceptable link-layer address and/or security parameters (see: 714 Security Considerations). 716 3.8.2. AERO Server Behavior 718 AERO Servers configure a DHCPv6 relay function on their AERO links. 719 AERO Servers arrange to add their encapsulation layer IP addresses 720 (i.e., their link-layer addresses) to the DNS resource records for 721 the FQDN "linkupnetworks.domainname" before entering service. 723 When an AERO Server relays a prospective Client's DHCPv6 PD messages 724 to the DHCPv6 server, it wraps each message in a "Relay-forward" 725 message per [RFC3315] and includes a DHCPv6 Interface Identifier 726 option that encodes a value that identifies the AERO link to the 727 DHCPv6 server. Without creating internal state, the Server then 728 includes the Client's link-layer address in a DHCPv6 Client Link 729 Layer Address Option (CLLAO) [RFC6939] with the link-layer address 730 format shown in Figure 1 (i.e., Link ID followed by Preference 731 followed by UDP Port Number followed by IP Address). The Server sets 732 the CLLAO 'option-length' field to 22 (2 plus the length of the link- 733 layer address) and sets the 'link-layer type' field to TBD (see: IANA 734 Considerations). The Server finally includes a DHCPv6 Echo Request 735 Option (ERO) [RFC4994] that encodes the option code for the CLLAO in 736 a 'requested-option-code-n' field, then relays the message to the 737 DHCPv6 server. The CLLAO information will therefore subsequently be 738 echoed back in the DHCPv6 server's "Relay-reply" message. 740 When the DHCPv6 server issues the prefix delegation in a "Relay- 741 reply" message via the AERO Server (acting as a DHCPv6 relay), the 742 Server obtains the Client's link-layer address from the echoed CLLAO 743 option and also obtains the Client's delegated prefix from the 744 message. The Server then creates a neighbor cache entry for the 745 Client's AERO address with the Client's link-layer address as the 746 link-layer address for the neighbor cache entry. The neighbor cache 747 entry is created with both ACCEPT and FORWARD timers set to 748 REACHABLE_TIME, since the Client will continue to send RS messages 749 within REACHABLE_TIME seconds as long as it wishes to remain 750 associated with this Server. 752 The Server also configures an IP forwarding table entry that lists 753 the Client's AERO address as the next hop toward the delegated IP 754 prefix with a lifetime derived from the DHCPv6 lease lifetime. The 755 Server finally injects the Client's prefix as an IP route into the 756 inter-Server/Relay routing system (see: Appendix A) then relays the 757 DHCPv6 message to the Client while using fe80::ID as the IPv6 source 758 address, the link-local address found in the "peer address" field of 759 the Relay-reply message as the IPv6 destination address, and the 760 Client's link-layer address as the destination link-layer address. 762 Servers respond to NS/RS messages from Clients on their AERO 763 interfaces by returning an NA/RA message. The Server SHOULD NOT 764 include PIOs in the RA messages it sends to Clients, since the Client 765 will ignore any such options. When the Server receives an NS/RS 766 message from the Client, it resets the ACCEPT/FORWARD timers to 767 REACHABLE_TIME. 769 Servers ignore any RA messages they may receive from a Client, but 770 they MAY examine RA messages received from other Servers for 771 consistency verification purposes. Servers do not send NS messages 772 for the purpose of updating Client neighbor cache timers, since 773 Clients are responsible for refreshing neighbor cache state. 775 When the Server forwards a packet via the same AERO interface on 776 which it arrived, it initiates an AERO route optimization procedure 777 as specified in Section 3.9. 779 3.9. AERO Redirection 781 3.9.1. Reference Operational Scenario 783 Figure 2 depicts the AERO redirection reference operational scenario, 784 using IPv6 addressing as the example (while not shown, a 785 corresponding example for IPv4 addressing can be easily constructed). 786 The figure shows an AERO Server('A'), two AERO Clients ('B', 'C') and 787 three ordinary IPv6 hosts ('D', 'E', 'F'): 789 .-(::::::::) 790 .-(:::: IP ::::)-. +-------------+ 791 (:: Internetwork ::)--| Host F | 792 `-(::::::::::::)-' +-------------+ 793 `-(::::::)-' 2001:db8:2::1 794 | 795 +--------------+ 796 | AERO Server A| 797 | (D->B; E->C) | 798 +--------------+ 799 fe80::ID 800 L2(A) 801 | 802 X-----+-----------+-----------+--------X 803 | AERO Link | 804 L2(B) L2(C) 805 fe80::2001:db8:0:0 fe80::2001:db8:1:0 .-. 806 +--------------+ +--------------+ ,-( _)-. 807 | AERO Client B| | AERO Client C| .-(_ IP )-. 808 | (default->A) | | (default->A) |--(__ EUN ) 809 +--------------+ +--------------+ `-(______)-' 810 2001:DB8:0::/48 2001:DB8:1::/48 | 811 | 2001:db8:1::1 812 .-. +-------------+ 813 ,-( _)-. 2001:db8:0::1 | Host E | 814 .-(_ IP )-. +-------------+ +-------------+ 815 (__ EUN )--| Host D | 816 `-(______)-' +-------------+ 818 Figure 2: AERO Reference Operational Scenario 820 In Figure 2, AERO Server ('A') connects to the AERO link and connects 821 to the IP Internetwork, either directly or via an AERO Relay (not 822 shown). Server ('A') assigns the address fe80::ID to its AERO 823 interface with link-layer address L2(A). Server ('A') next arranges 824 to add L2(A) to a published list of valid Servers for the AERO link. 826 AERO Client ('B') receives the prefix 2001:db8:0::/48 in a DHCPv6 PD 827 exchange via AERO Server ('A') then assigns the address 828 fe80::2001:db8:0:0 to its AERO interface with link-layer address 829 L2(B). Client ('B') configures a default route and neighbor cache 830 entry via the AERO interface with next-hop address fe80::ID and link- 831 layer address L2(A), then sub-delegates the prefix 2001:db8:0::/48 to 832 its attached EUNs. IPv6 host ('D') connects to the EUN, and 833 configures the address 2001:db8:0::1. 835 AERO Client ('C') receives the prefix 2001:db8:1::/48 in a DHCPv6 PD 836 exchange via AERO Server ('A') then assigns the address 837 fe80::2001:db8:1:0 to its AERO interface with link-layer address 838 L2(C). Client ('C') configures a default route and neighbor cache 839 entry via the AERO interface with next-hop address fe80::ID and link- 840 layer address L2(A), then sub-delegates the prefix 2001:db8:1::/48 to 841 its attached EUNs. IPv6 host ('E') connects to the EUN, and 842 configures the address 2001:db8:1::1. 844 Finally, IPv6 host ('F') connects to a network outside of the AERO 845 link domain. Host ('F') configures its IPv6 interface in a manner 846 specific to its attached IPv6 link, and assigns the address 847 2001:db8:2::1 to its IPv6 link interface. 849 3.9.2. Classical Redirection Approaches 851 With reference to Figure 2, when the source host ('D') sends a packet 852 to destination host ('E'), the packet is first forwarded via the EUN 853 to AERO Client ('B'). Client ('B') then forwards the packet over its 854 AERO interface to AERO Server ('A'), which then re-encapsulates and 855 forwards the packet to AERO Client ('C'), where the packet is finally 856 forwarded to destination host ('E'). When Server ('A') re- 857 encapsulates and forwards the packet back out on its advertising AERO 858 interface, it must arrange to redirect Client ('B') toward Client 859 ('C') as a better next-hop node on the AERO link that is closer to 860 the final destination. However, this redirection process applied to 861 AERO interfaces must be more carefully orchestrated than on ordinary 862 links since the parties may be separated by potentially many 863 underlying network routing hops. 865 Consider a first alternative in which Server ('A') informs Client 866 ('B') only and does not inform Client ('C') (i.e., "classical 867 redirection"). In that case, Client ('C') has no way of knowing that 868 Client ('B') is authorized to forward packets from the claimed source 869 address, and it may simply elect to drop the packets. Also, Client 870 ('B') has no way of knowing whether Client ('C') is performing some 871 form of source address filtering that would reject packets arriving 872 from a node other than a trusted default router, nor whether Client 873 ('C') is even reachable via a direct path that does not involve 874 Server ('A'). 876 Consider a second alternative in which Server ('A') informs both 877 Client ('B') and Client ('C') separately, via independent redirection 878 control messages (i.e., "augmented redirection"). In that case, if 879 Client ('B') receives the redirection control message but Client 880 ('C') does not, subsequent packets sent by Client ('B') could be 881 dropped due to filtering since Client ('C') would not have a route to 882 verify the claimed source address. Also, if Client ('C') receives 883 the redirection control message but Client ('B') does not, subsequent 884 packets sent in the reverse direction by Client ('C') would be lost. 886 Since both of these alternatives have shortcomings, a new redirection 887 technique (i.e., "AERO redirection") is needed. 889 3.9.3. Concept of Operations 891 Again, with reference to Figure 2, when source host ('D') sends a 892 packet to destination host ('E'), the packet is first forwarded over 893 the source host's attached EUN to Client ('B'), which then forwards 894 the packet via its AERO interface to Server ('A'). 896 Server ('A') then re-encapsulates and forwards the packet out the 897 same AERO interface toward Client ('C') and also sends an AERO 898 "Predirect" message forward to Client ('C') as specified in 899 Section 3.9.5. The Predirect message includes Client ('B')'s 900 network- and link-layer addresses as well as information that Client 901 ('C') can use to determine the IP prefix used by Client ('B') . After 902 Client ('C') receives the Predirect message, it process the message 903 and returns an AERO Redirect message destined for Client ('B') via 904 Server ('A') as specified in Section 3.9.6. During the process, 905 Client ('C') also creates or updates a neighbor cache entry for 906 Client ('B') and creates an IP forwarding table entry for Client 907 ('B')'s prefix. 909 When Server ('A') receives the Redirect message, it re-encapsulates 910 the message and forwards it on to Client ('B') as specified in 911 Section 3.9.7. The message includes Client ('C')'s network- and 912 link-layer addresses as well as information that Client ('B') can use 913 to determine the IP prefix used by Client ('C'). After Client ('B') 914 receives the Redirect message, it processes the message as specified 915 in Section 3.9.8. During the process, Client ('B') also creates or 916 updates a neighbor cache entry for Client ('C') and creates an IP 917 forwarding table entry for Client ('C')'s prefix. 919 Following the above Predirect/Redirect message exchange, forwarding 920 of packets from Client ('B') to Client ('C') without involving Server 921 ('A) as an intermediary is enabled. The mechanisms that support this 922 exchange are specified in the following sections. 924 3.9.4. Message Format 926 AERO Redirect/Predirect messages use the same format as for ICMPv6 927 Redirect messages depicted in Section 4.5 of [RFC4861], but also 928 include a new "Prefix Length" field taken from the low-order 8 bits 929 of the Redirect message Reserved field. (For IPv6, valid values for 930 the Prefix Length field are 0 through 64; for IPv4, valid values are 931 0 through 32.) The Redirect/Predirect messages are formatted as 932 shown in Figure 3: 934 0 1 2 3 935 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 1 936 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 937 | Type (=137) | Code (=0/1) | Checksum | 938 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 939 | Reserved | Prefix Length | 940 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 941 | | 942 + + 943 | | 944 + Target Address + 945 | | 946 + + 947 | | 948 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 949 | | 950 + + 951 | | 952 + Destination Address + 953 | | 954 + + 955 | | 956 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 957 | Options ... 958 +-+-+-+-+-+-+-+-+-+-+-+- 960 Figure 3: AERO Redirect/Predirect Message Format 962 3.9.5. Sending Predirects 964 When a Server forwards a packet from one of its associated Clients 965 toward another AERO Client connected to the same AERO link, the 966 Server sends a Predirect message forward toward the destination 967 Client instead of sending a Redirect message back to the source 968 Client. 970 In the reference operational scenario, when Server ('A') forwards a 971 packet sent by Client ('B') toward Client ('C'), it also sends a 972 Predirect message forward toward Client ('C'), subject to rate 973 limiting (see Section 8.2 of [RFC4861]). Server ('A') prepares the 974 Predirect message as follows: 976 o the link-layer source address is set to 'L2(A)' (i.e., the link- 977 layer address of Server ('A')). 979 o the link-layer destination address is set to 'L2(C)' (i.e., the 980 link-layer address of Client ('C')). 982 o the network-layer source address is set to fe80::2001:db8:0:0 983 (i.e., the AERO address of Client ('B')). 985 o the network-layer destination address is set to fe80::2001:db8:1:0 986 (i.e., the AERO address of Client ('C')). 988 o the Type is set to 137. 990 o the Code is set to 1 to indicate "Predirect". 992 o the Prefix Length is set to the length of the prefix to be applied 993 to the Target Address. 995 o the Target Address is set to fe80::2001:db8:0:0 (i.e., the AERO 996 address of Client ('B')). 998 o the Destination Address is set to the source address of the 999 originating packet that triggered the Predirection event. (If the 1000 originating packet is an IPv4 packet, the address is constructed 1001 in IPv4-compatible IPv6 address format). 1003 o the message includes a TLLAO with Link ID and Preference set to 1004 appropriate values for Client ('B')'s underlying interface, and 1005 with UDP Port Number and IP Address set to 'L2(B)'. 1007 o the message includes a Timestamp option. 1009 o the message includes a Redirected Header Option (RHO) that 1010 contains the originating packet truncated to ensure that at least 1011 the network-layer header is included but the size of the message 1012 does not exceed 1280 bytes. 1014 Note that the reference operational scenario applies to the case when 1015 the source and destination Clients are associated with the same 1016 Server. When the source and destination Clients are associated with 1017 different Servers, the source Client's Server forwards the packets 1018 and Predirect messages to a Relay which in turn forwards them toward 1019 the destination Client. In that case, the Server sets the Predirect 1020 link-layer destination address to the link-layer address of the 1021 Relay. 1023 Servers therefore require knowledge of all aggregated IP prefixes 1024 associated with the AERO link so that they can determine when a 1025 prospective destination Client is on-link. See Appendix A for a 1026 discussion of AERO Server/Relay interworking. 1028 3.9.6. Processing Predirects and Sending Redirects 1030 When Client ('C') receives a Predirect message, it accepts the 1031 message only if the message has a link-layer source address of the 1032 Server, i.e. 'L2(A)'. Client ('C') further accepts the message only 1033 if it is willing to serve as a redirection target. Next, Client 1034 ('C') validates the message according to the ICMPv6 Redirect message 1035 validation rules in Section 8.1 of [RFC4861], except that it accepts 1036 the message even though the network-layer source address is not that 1037 of it's current first-hop router. 1039 In the reference operational scenario, when Client ('C') receives a 1040 valid Predirect message, it either creates or updates a neighbor 1041 cache entry that stores the Target Address of the message as the 1042 network-layer address of Client ('B') and stores the link-layer 1043 address found in the TLLAO as the link-layer address(es) of Client 1044 ('B'). Client ('C') then sets the neighbor cache entry ACCEPT timer 1045 with timeout value ACCEPT_TIME. Next, Client ('C') applies the 1046 Prefix Length to the Destination Address and records the resulting 1047 prefix in its IP forwarding table. 1049 After processing the message, Client ('C') prepares a Redirect 1050 message response as follows: 1052 o the link-layer source address is set to 'L2(C)' (i.e., the link- 1053 layer address of Client ('C')). 1055 o the link-layer destination address is set to 'L2(A)' (i.e., the 1056 link-layer address of Server ('A')). 1058 o the network-layer source address is set to fe80::2001:db8:1:0 1059 (i.e., the AERO address of Client ('C')). 1061 o the network-layer destination address is set to fe80::2001:db8:0:0 1062 (i.e., the AERO address of Client ('B')). 1064 o the Type is set to 137. 1066 o the Code is set to 0 to indicate "Redirect". 1068 o the Prefix Length is set to the length of the prefix to be applied 1069 to the Target Address. 1071 o the Target Address is set to fe80::2001:db8:1:0 (i.e., the AERO 1072 address of Client ('C')). 1074 o the Destination Address is set to the destination address of the 1075 originating packet that triggered the Redirection event. (If the 1076 originating packet is an IPv4 packet, the address is constructed 1077 in IPv4-compatible IPv6 address format). 1079 o the message includes a TLLAO with Link ID and Preference set to 1080 appropriate values for Client ('C')'s underlying interface, and 1081 with UDP Port Number and IP Address set to '0'. 1083 o the message includes a Timestamp option. 1085 o the message includes as much of the RHO copied from the 1086 corresponding AERO Predirect message as possible such that at 1087 least the network-layer header is included but the size of the 1088 message does not exceed 1280 bytes. 1090 After Client ('C') prepares the Redirect message, it sends the 1091 message to Server ('A'). 1093 3.9.7. Re-encapsulating and Relaying Redirects 1095 When Server ('A') receives a Redirect message from Client ('C'), it 1096 validates the message according to the ICMPv6 Redirect message 1097 validation rules in Section 8.1 of [RFC4861] and also verifies that 1098 Client ('C') is authorized to use the Prefix Length in the Redirect 1099 message when applied to the AERO address in the network-layer source 1100 of the Redirect message by searching for the AERO address' embedded 1101 prefix in the IP routing table. If validation fails, Server ('A') 1102 discards the message; otherwise, it copies the correct UDP Port 1103 number and IP Address for Client ('C') into the (previously empty) 1104 TLLAO. 1106 Server ('A') then examines the network-layer destination address of 1107 the message to determine the next hop toward the prefix of Client 1108 ('B') by searching for the AERO address' embedded prefix in the IP 1109 routing table. If the next hop is reached via the AERO interface, 1110 Server ('A') re-encapsulates the Redirect and relays it on to Client 1111 ('B') by changing the link-layer source address of the message to 1112 'L2(A)' and changing the link-layer destination address to 'L2(B)'. 1113 Server ('A') finally forwards the re-encapsulated message to Client 1114 ('B') without decrementing the network-layer TTL/Hop Limit field. 1116 While not shown in Figure 2, AERO Relays relay Redirect and Predirect 1117 messages in exactly this same fashion described above (see: 1118 Appendix A). 1120 3.9.8. Processing Redirects 1122 When Client ('B') receives the Redirect message, it accepts the 1123 message only if it has a link-layer source address of the Server, 1124 i.e. 'L2(A)'. Next, Client ('B') validates the message according to 1125 the ICMPv6 Redirect message validation rules in Section 8.1 of 1126 [RFC4861], except that it accepts the message even though the 1127 network-layer source address is not that of it's current first-hop 1128 router. Following validation, Client ('B') then processes the 1129 message as follows. 1131 In the reference operational scenario, when Client ('B') receives the 1132 Redirect message, it either creates or updates a neighbor cache entry 1133 that stores the Target Address of the message as the network-layer 1134 address of Client ('C') and stores the link-layer address found in 1135 the TLLAO as the link-layer address of Client ('C'). Client ('B') 1136 then sets the neighbor cache entry FORWARD timer with timeout value 1137 FORWARD_TIME. Next, Client ('B') applies the Prefix Length to the 1138 Destination Address and records the resulting IP prefix in its IP 1139 forwarding table. 1141 Now, Client ('B') has an IP forwarding table entry for Client('C')'s 1142 prefix and a neighbor cache entry with a valid FORWARD time, while 1143 Client ('C') has an IP forwarding table entry for Client ('B')'s 1144 prefix with a valid ACCEPT time. Thereafter, Client ('B') may 1145 forward ordinary network-layer data packets directly to Client ("C") 1146 without involving Server ('A') and Client ('C') can verify that the 1147 packets came from an acceptable source. (In order for Client ('C') 1148 to forward packets to Client ('B') a corresponding Predirect/Redirect 1149 message exchange is required in the reverse direction.) 1151 3.9.9. Server-Oriented Redirection 1153 In some environments, the Server nearest the destination Client may 1154 need to serve as the redirection target, e.g., if direct Client-to- 1155 Client communications are not possible. In that case, the Server 1156 prepares the Redirect message the same as if it were the destination 1157 Client (see: Section 3.9.6), except that it writes its own link-layer 1158 address in the TLLAO option. 1160 3.10. Neighbor Reachability Maintenance 1162 AERO nodes send unicast NS messages to elicit NA messages from 1163 neighbors the same as described for Neighbor Unreachability Detection 1164 (NUD) in [RFC4861]. When an AERO node sends an NS/NA message, it 1165 MUST use its link-local address as the IPv6 source address and the 1166 link-local address of the neighbor as the IPv6 destination address. 1167 When an AERO node receives an NS/NA message, it accepts the message 1168 if it has a neighbor cache entry for the neighbor; otherwise, it 1169 ignores the message. 1171 When a source Client is redirected to a target Client it SHOULD test 1172 the direct path by sending an initial NS message to elicit a 1173 solicited NA response. While testing the path, the source Client can 1174 optionally continue sending packets via the Server, maintain a small 1175 queue of packets until target reachability is confirmed, or 1176 (optimistically) allow packets to flow directly to the target. The 1177 source Client SHOULD thereafter continue to test the direct path to 1178 the target Client (see Section 7.3 of [RFC4861]) periodically in 1179 order to keep neighbor cache entries alive. For example, while the 1180 source Client is actively sending packets to the target Client it can 1181 also send NS messages separated by a minimum probe interval (e.g., 1 1182 second) in order to receive solicited NA messages. If the source 1183 Client is unable to elicit an NA response from the target Client 1184 after MAX_RETRY attempts, it SHOULD consider the direct path unusable 1185 for forwarding purposes and can resume sending packets via the Server 1186 which may or may not result in a new redirection event. Otherwise, 1187 the source Client considers the path usable and SHOULD thereafter 1188 process any link-layer errors as a hint that the direct path to the 1189 target Client has either failed or has become intermittent. 1191 When a target Client receives an NS message from a source Client, it 1192 resets the ACCEPT timer to ACCEPT_TIME if a neighbor cache entry 1193 exists; otherwise, it discards the NS message. 1195 When a source Client receives a solicited NA message from a target 1196 Client, it resets the FORWARD timer to FORWARD_TIME if a neighbor 1197 cache entry exists; otherwise, it discards the NA message. 1199 When the FORWARD timer on a neighbor cache entry expires, the source 1200 Client resumes sending any subsequent packets via the Server and may 1201 (eventually) receive a new Redirect message. When the ACCEPT timer 1202 on a neighbor cache entry expires, the target Client discards any 1203 subsequent packets received directly from the source Client. When 1204 both the FORWARD and ACCEPT timers on a neighbor cache entry expire, 1205 the Client deletes both the neighbor cache entry and the 1206 corresponding IP forwarding table entry. 1208 3.11. Mobility Management 1210 When a Client needs to change its link-layer address (e.g., due to a 1211 mobility event), it performs an immediate DHCPv6 Renew/Reply via each 1212 of its Servers using the new link-layer address as the source. The 1213 DHCPv6 Server will re-authenticate the Client and (assuming 1214 authentication succeeds) the DHCPv6 Renew/Reply exchange will update 1215 each Server's neighbor cache. 1217 Next, the Client sends a Predirect message to each of its active 1218 neighbors via a Server as follows: 1220 o the link-layer source address is set to the Client's new link- 1221 layer address. 1223 o the link-layer destination address is set to the link-layer 1224 address of the Server. 1226 o the network-layer source address is set to the Client's AERO 1227 address. 1229 o the network-layer destination address is set to the neighbor's 1230 AERO address. 1232 o the Type is set to 137. 1234 o the Code is set to 1 to indicate "Predirect". 1236 o the Prefix Length is set to the length of the prefix to be applied 1237 to the Target address. 1239 o the Target Address is set to the Client's AERO address. 1241 o the Destination Address contains the IPv6 source address of a NULL 1242 packet (i.e., a minimum-length IPv6 packet with Next Header set to 1243 'No Next Header') originating from an address within the Client's 1244 prefix and destined to an address within the neighbor's prefix. 1245 (If the source and destination Clients hold IPv4 prefix 1246 delegations, the NULL packet addresses and Predirect Destination 1247 Address field are constructed in IPv4-compatible IPv6 address 1248 format). 1250 o the message includes a TLLAO with Link ID and Preference set to 1251 appropriate values for the underlying interface, and with UDP Port 1252 Number and IP Address set to 0. 1254 o the message includes a Timestamp option. 1256 o the message includes a Redirected Header Option (RHO) that 1257 contains the leading portion of the NULL packet without exceeding 1258 1280 bytes. 1260 When the Server receives the Predirect message, it copies the correct 1261 UDP port number and IP address into the TLLAO supplied by the Client, 1262 changes the link-layer source address to its own address, changes the 1263 link-layer destination address to the address of the neighbor, then 1264 forwards the Predirect message to the neighbor based on an IP route 1265 matching the AERO address in the network-layer destination address. 1266 When the neighbor receives the Predirect message, it returns a 1267 Redirect message the same as specified in Section 3.9. 1269 When a Client associates with a new Server, it issues a new DHCPv6 1270 Renew message via the new Server as the DHCPv6 relay. The new Server 1271 then relays the message to the DHCPv6 server and processes the 1272 resulting exchange. After the Client receives the resulting DHCPv6 1273 Reply message, it sends an RS message to the new Server to receive a 1274 new RA message. 1276 When a Client disassociates with an existing Server, it sends a 1277 "terminating RS" message to the old Server. The terminating RS 1278 message is prepared exactly the same as for an ordinary RS message, 1279 except that the Code field contains the value '1'. When the old 1280 Server receives the terminating RS message, it withdraws the IP route 1281 from the routing system and deletes the neighbor cache entry and IP 1282 forwarding table entry for the Client. The old Server then returns 1283 an RA message with default router lifetime set to 0 which the Client 1284 can use to verify that the termination signal has been processed. 1285 The client then deletes both the default route and the neighbor cache 1286 entry for the old Server. (Note that the Client and the old Server 1287 MAY impose a small delay before deleting the neighbor cache and IP 1288 forwarding table entries so that any packets already in the system 1289 can still be delivered to the Client.) 1291 3.12. Encapsulation Protocol Version Considerations 1293 A source Client may connect only to an IPvX underlying network, while 1294 the target Client connects only to an IPvY underlying network. In 1295 that case, the target and source Clients have no means for reaching 1296 each other directly (since they connect to underlying networks of 1297 different IP protocol versions) and so must ignore any redirection 1298 messages and continue to send packets via the Server. 1300 3.13. Multicast Considerations 1302 When the underlying network does not support multicast, AERO nodes 1303 map IPv6 link-scoped multicast addresses (including 1304 'All_DHCP_Relay_Agents_and_Servers') to the link-layer address of a 1305 Server. 1307 When the underlying network supports multicast, AERO nodes use the 1308 multicast address mapping specification found in [RFC2529] for IPv4 1309 underlying networks and use a direct multicast mapping for IPv6 1310 underlying networks. (In the latter case, "direct multicast mapping" 1311 means that if the IPv6 multicast destination address of the 1312 encapsulated packet is "M", then the IPv6 multicast destination 1313 address of the encapsulating header is also "M".) 1315 3.14. Operation on AERO Links Without DHCPv6 Services 1317 When the AERO link does not provide DHCPv6 services, operation can 1318 still be accommodated through administrative configuration of 1319 prefixes on AERO Clients. In that case, administrative 1320 configurations of IP routes and AERO interface neighbor cache entries 1321 on both the Server and Client are also necessary. However, this may 1322 preclude the ability for Clients to dynamically change to new 1323 Servers, and can expose the AERO link to misconfigurations unless the 1324 administrative configurations are carefully coordinated. 1326 3.15. Operation on Server-less AERO Links 1328 In some AERO link scenarios, there may be no Servers on the link and/ 1329 or no need for Clients to use a Server as an intermediary trust 1330 anchor. In that case, each Client acts as a Server unto itself to 1331 establish neighbor cache entries and IP forwarding table entries by 1332 performing direct Client-to-Client Predirect/Redirect exchanges, and 1333 some other form of trust basis must be applied so that each Client 1334 can verify that the prospective neighbor is authorized to use its 1335 claimed prefix. 1337 When there is no Server on the link, Clients must arrange to receive 1338 prefix delegations and publish the delegations via a secure alternate 1339 prefix delegation authority through some means outside the scope of 1340 this document. 1342 4. Implementation Status 1344 An application-layer implementation is in progress. 1346 5. IANA Considerations 1348 The IANA is instructed to assign a new 2-octet Hardware Type number 1349 for AERO in the "arp-parameters" registry per Section 2 of [RFC5494]. 1350 The number is assigned from the 2-octet Unassigned range with 1351 Hardware Type "AERO" and with this document as the reference. 1353 6. Security Considerations 1355 AERO link security considerations are the same as for standard IPv6 1356 Neighbor Discovery [RFC4861] except that AERO improves on some 1357 aspects. In particular, AERO uses a trust basis between Clients and 1358 Servers, where the Clients only engage in the AERO mechanism when it 1359 is facilitated by a trust anchor. AERO also uses DHCPv6 1360 authentication for Client authentication and network admission 1361 control. 1363 AERO links must be protected against link-layer address spoofing 1364 attacks in which an attacker on the link pretends to be a trusted 1365 neighbor. Links that provide link-layer securing mechanisms (e.g., 1366 IEEE 802.1X WLANs) and links that provide physical security (e.g., 1367 enterprise network wired LANs) provide a first line of defense that 1368 is often sufficient. In other instances, additional securing 1369 mechanisms such as Secure Neighbor Discovery (SeND) [RFC3971], IPsec 1370 [RFC4301] or TLS [RFC5246] may be necessary. 1372 AERO Clients MUST ensure that their connectivity is not used by 1373 unauthorized nodes on EUNs to gain access to a protected network, 1374 i.e., AERO Clients that act as routers MUST NOT provide routing 1375 services for unauthorized nodes. (This concern is no different than 1376 for ordinary hosts that receive an IP address delegation but then 1377 "share" the address with unauthorized nodes via a NAT function.) 1379 On some AERO links, establishment and maintenance of a direct path 1380 between neighbors requires secured coordination such as through the 1381 Internet Key Exchange (IKEv2) protocol [RFC5996] to establish a 1382 security association. 1384 7. Acknowledgements 1386 Discussions both on IETF lists and in private exchanges helped shape 1387 some of the concepts in this work. Individuals who contributed 1388 insights include Mikael Abrahamsson, Fred Baker, Stewart Bryant, 1389 Brian Carpenter, Wojciech Dec, Brian Haberman, Joel Halpern, Sascha 1390 Hlusiak, Lee Howard, Joe Touch and Bernie Volz. Members of the IESG 1391 also provided valuable input during their review process that greatly 1392 improved the document. Special thanks go to Stewart Bryant, Joel 1393 Halpern and Brian Haberman for their shepherding guidance. 1395 This work has further been encouraged and supported by Boeing 1396 colleagues including Keith Bartley, Dave Bernhardt, Cam Brodie, 1397 Balaguruna Chidambaram, Wen Fang, Anthony Gregory, Jeff Holland, Ed 1398 King, Gen MacLean, Kent Shuey, Mike Slane, Julie Wulff, Yueli Yang, 1399 and other members of the BR&T and BIT mobile networking teams. 1401 Earlier works on NBMA tunneling approaches are found in 1402 [RFC2529][RFC5214][RFC5569]. 1404 8. References 1406 8.1. Normative References 1408 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 1409 August 1980. 1411 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1412 1981. 1414 [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, 1415 RFC 792, September 1981. 1417 [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, 1418 October 1996. 1420 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1421 Requirement Levels", BCP 14, RFC 2119, March 1997. 1423 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 1424 (IPv6) Specification", RFC 2460, December 1998. 1426 [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in 1427 IPv6 Specification", RFC 2473, December 1998. 1429 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., 1430 and M. Carney, "Dynamic Host Configuration Protocol for 1431 IPv6 (DHCPv6)", RFC 3315, July 2003. 1433 [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic 1434 Host Configuration Protocol (DHCP) version 6", RFC 3633, 1435 December 2003. 1437 [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure 1438 Neighbor Discovery (SEND)", RFC 3971, March 2005. 1440 [RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms 1441 for IPv6 Hosts and Routers", RFC 4213, October 2005. 1443 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 1444 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 1445 September 2007. 1447 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 1448 Address Autoconfiguration", RFC 4862, September 2007. 1450 [RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node 1451 Requirements", RFC 6434, December 2011. 1453 8.2. Informative References 1455 [IRON] Templin, F., "The Internet Routing Overlay Network 1456 (IRON)", Work in Progress, June 2012. 1458 [RFC0879] Postel, J., "TCP maximum segment size and related topics", 1459 RFC 879, November 1983. 1461 [RFC1930] Hawkinson, J. and T. Bates, "Guidelines for creation, 1462 selection, and registration of an Autonomous System (AS)", 1463 BCP 6, RFC 1930, March 1996. 1465 [RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4 1466 Domains without Explicit Tunnels", RFC 2529, March 1999. 1468 [RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms", 1469 RFC 2675, August 1999. 1471 [RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway 1472 Protocol 4 (BGP-4)", RFC 4271, January 2006. 1474 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 1475 Architecture", RFC 4291, February 2006. 1477 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 1478 Internet Protocol", RFC 4301, December 2005. 1480 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 1481 Discovery", RFC 4821, March 2007. 1483 [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly 1484 Errors at High Data Rates", RFC 4963, July 2007. 1486 [RFC4994] Zeng, S., Volz, B., Kinnear, K., and J. Brzozowski, 1487 "DHCPv6 Relay Agent Echo Request Option", RFC 4994, 1488 September 2007. 1490 [RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site 1491 Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, 1492 March 2008. 1494 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1495 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 1497 [RFC5494] Arkko, J. and C. Pignataro, "IANA Allocation Guidelines 1498 for the Address Resolution Protocol (ARP)", RFC 5494, 1499 April 2009. 1501 [RFC5522] Eddy, W., Ivancic, W., and T. Davis, "Network Mobility 1502 Route Optimization Requirements for Operational Use in 1503 Aeronautics and Space Exploration Mobile Networks", RFC 1504 5522, October 2009. 1506 [RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4 1507 Infrastructures (6rd)", RFC 5569, January 2010. 1509 [RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen, 1510 "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC 1511 5996, September 2010. 1513 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful 1514 NAT64: Network Address and Protocol Translation from IPv6 1515 Clients to IPv4 Servers", RFC 6146, April 2011. 1517 [RFC6204] Singh, H., Beebee, W., Donley, C., Stark, B., and O. 1518 Troan, "Basic Requirements for IPv6 Customer Edge 1519 Routers", RFC 6204, April 2011. 1521 [RFC6355] Narten, T. and J. Johnson, "Definition of the UUID-Based 1522 DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355, August 1523 2011. 1525 [RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label 1526 for Equal Cost Multipath Routing and Link Aggregation in 1527 Tunnels", RFC 6438, November 2011. 1529 [RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)", 1530 RFC 6691, July 2012. 1532 [RFC6706] Templin, F., "Asymmetric Extended Route Optimization 1533 (AERO)", RFC 6706, August 2012. 1535 [RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field", 1536 RFC 6864, February 2013. 1538 [RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and 1539 UDP Checksums for Tunneled Packets", RFC 6935, April 2013. 1541 [RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement 1542 for the Use of IPv6 UDP Datagrams with Zero Checksums", 1543 RFC 6936, April 2013. 1545 [RFC6939] Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer 1546 Address Option in DHCPv6", RFC 6939, May 2013. 1548 [RFC6980] Gont, F., "Security Implications of IPv6 Fragmentation 1549 with IPv6 Neighbor Discovery", RFC 6980, August 2013. 1551 [RFC7078] Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing 1552 Address Selection Policy Using DHCPv6", RFC 7078, January 1553 2014. 1555 Appendix A. AERO Server and Relay Interworking 1557 Figure 2 depicts a reference AERO operational scenario with a single 1558 Server on the AERO link. In order to support scaling to larger 1559 numbers of nodes, the AERO link can deploy multiple Servers and 1560 Relays, e.g., as shown in Figure 4. 1562 .-(::::::::) 1563 .-(:::: IP ::::)-. 1564 (:: Internetwork ::) 1565 `-(::::::::::::)-' 1566 `-(::::::)-' 1567 | 1568 +--------------+ +------+-------+ +--------------+ 1569 |AERO Server C | | AERO Relay D | |AERO Server E | 1570 | (default->D) | | (A->C; G->E) | | (default->D) | 1571 | (A->B) | +-------+------+ | (G->F) | 1572 +-------+------+ | +------+-------+ 1573 | | | 1574 X---+---+-------------------+------------------+---+---X 1575 | AERO Link | 1576 +-----+--------+ +--------+-----+ 1577 |AERO Client B | |AERO Client F | 1578 | (default->C) | | (default->E) | 1579 +--------------+ +--------------+ 1580 .-. .-. 1581 ,-( _)-. ,-( _)-. 1582 .-(_ IP )-. .-(_ IP )-. 1583 (__ EUN ) (__ EUN ) 1584 `-(______)-' `-(______)-' 1585 | | 1586 +--------+ +--------+ 1587 | Host A | | Host G | 1588 +--------+ +--------+ 1590 Figure 4: AERO Server/Relay Interworking 1592 In this example, Client ('B') associates with Server ('C'), while 1593 Client ('F') associates with Server ('E'). Furthermore, Servers 1594 ('C') and ('E') do not associate with each other directly, but rather 1595 have an association with Relay ('D') (i.e., a router that has full 1596 topology information concerning its associated Servers and their 1597 Clients). Relay ('D') connects to the AERO link, and also connects 1598 to the native IP Internetwork. 1600 When source host ('A') sends a packet toward destination host ('G'), 1601 IP forwarding directs the packet through the EUN to Client ('B'), 1602 which forwards the packet to Server ('C'). Server ('C') forwards 1603 both the packet and a Predirect message through Relay ('D'). Relay 1604 ('D') then forwards both the original packet and Predirect to Server 1605 ('E'). When Server ('E') receives the packet and Predirect message, 1606 it forwards them to Client ('F'). 1608 After processing the Predirect message, Client ('F') sends a Redirect 1609 message to Server ('E'). Server ('E'), in turn, forwards the message 1610 through Relay ('D') to Server ('C'). When Server ('C') receives the 1611 Redirect message, it forwards the message to Client ('B') informing 1612 it that host 'G's EUN can be reached via Client ('F'), thus 1613 completing the AERO redirection. 1615 The network-layer routing information shared between Servers and 1616 Relays must be carefully coordinated. In particular, Relays require 1617 full topology information, while individual Servers only require 1618 partial topology information, i.e., they only need to know the set of 1619 aggregated prefixes associated with the AERO link and the EUN 1620 prefixes associated with their current set of associated Clients. 1621 This can be accomplished in a number of ways, but a prominent example 1622 is through the use of an internal instance of the Border Gateway 1623 Protocol (BGP) [RFC4271] coordinated between Servers and Relays. 1624 This internal BGP instance does not interact with the public Internet 1625 BGP instance; therefore, the AERO link is presented to the IP 1626 Internetwork as a small set of aggregated prefixes as opposed to the 1627 full set of individual Client prefixes. 1629 In a reference BGP arrangement, each AERO Server is configured as an 1630 Autonomous System Border Router (ASBR) for a stub Autonomous System 1631 (AS) (possibly using a private AS Number (ASN) [RFC1930]), and each 1632 Server further peers with each Relay but does not peer with other 1633 Servers. Each Server maintains a working set of associated Clients, 1634 and dynamically announces new Client prefixes and withdraws departed 1635 Client prefixes in its BGP updates. The Relays therefore discover 1636 the full topology of the AERO link in terms of the working set of 1637 Clients associated with each Server. Since Clients are expected to 1638 remain associated with their current set of Servers for extended 1639 timeframes, the amount of BGP control messaging between Servers and 1640 Relays should be minimal. However, Servers SHOULD dampen any route 1641 oscillations caused by impatient Clients that repeatedly associate 1642 and disassociate with the Server. 1644 See [IRON] for further architectural discussion. 1646 Author's Address 1648 Fred L. Templin (editor) 1649 Boeing Research & Technology 1650 P.O. Box 3707 1651 Seattle, WA 98124 1652 USA 1654 Email: fltemplin@acm.org