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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) March 25, 2014 5 Intended status: Standards Track 6 Expires: September 26, 2014 8 Transmission of IPv6 Packets over AERO Links 9 draft-templin-aerolink-08.txt 11 Abstract 13 This document specifies the operation of IPv6 over tunnel virtual 14 Non-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 on the link that provide 17 forwarding services to reach off-link destinations and/or redirection 18 services to inform the node of an on-link neighbor that is closer to 19 the final destination. Operation of the IPv6 Neighbor Discovery (ND) 20 protocol over AERO links is based on an IPv6 link local address 21 format known as the AERO address. 23 Status of this Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at http://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on September 26, 2014. 40 Copyright Notice 42 Copyright (c) 2014 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (http://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 58 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 59 3. Asymmetric Extended Route Optimization (AERO) . . . . . . . . 5 60 3.1. AERO Node Types . . . . . . . . . . . . . . . . . . . . . 5 61 3.2. AERO Interface Characteristics . . . . . . . . . . . . . . 6 62 3.3. AERO Interface MTU Considerations . . . . . . . . . . . . 8 63 3.4. AERO Interface Encapsulation, Re-encapsulation and 64 Decapsulation . . . . . . . . . . . . . . . . . . . . . . 9 65 3.5. AERO Addresses . . . . . . . . . . . . . . . . . . . . . . 10 66 3.6. AERO Reference Operational Scenario . . . . . . . . . . . 11 67 3.7. AERO Router Discovery and Prefix Delegation . . . . . . . 13 68 3.7.1. AERO Client Behavior . . . . . . . . . . . . . . . . . 13 69 3.7.2. AERO Server Behavior . . . . . . . . . . . . . . . . . 13 70 3.8. AERO Neighbor Solicitation and Advertisement . . . . . . . 14 71 3.9. AERO Redirection . . . . . . . . . . . . . . . . . . . . . 15 72 3.9.1. Classical Redirection Approaches . . . . . . . . . . . 15 73 3.9.2. AERO Redirection Concept of Operations . . . . . . . . 16 74 3.9.3. Conceptual Data Structures and Protocol Constants . . 17 75 3.9.4. AERO Redirection Message Format . . . . . . . . . . . 18 76 3.9.5. Sending Predirects . . . . . . . . . . . . . . . . . . 19 77 3.9.6. Processing Predirects and Sending Redirects . . . . . 20 78 3.9.7. Re-encapsulating and Relaying Redirects . . . . . . . 21 79 3.9.8. Processing Redirects . . . . . . . . . . . . . . . . . 21 80 3.10. Neighbor Reachability Maintenance . . . . . . . . . . . . 22 81 3.11. Mobility and Link-Layer Address Change Considerations . . 23 82 3.12. Underlying Protocol Version Considerations . . . . . . . . 23 83 3.13. Multicast Considerations . . . . . . . . . . . . . . . . . 23 84 3.14. Operation on Server-less AERO Links . . . . . . . . . . . 23 85 3.15. Other Considerations . . . . . . . . . . . . . . . . . . . 24 86 4. Implementation Status . . . . . . . . . . . . . . . . . . . . 24 87 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 88 6. Security Considerations . . . . . . . . . . . . . . . . . . . 24 89 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25 90 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25 91 8.1. Normative References . . . . . . . . . . . . . . . . . . . 25 92 8.2. Informative References . . . . . . . . . . . . . . . . . . 26 93 Appendix A. AERO Server and Relay Interworking . . . . . . . . . 28 94 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 29 96 1. Introduction 98 This document specifies the operation of IPv6 over tunnel virtual 99 Non-Broadcast, Multiple Access (NBMA) links using Asymmetric Extended 100 Route Optimization (AERO). Nodes attached to AERO links can exchange 101 packets via trusted intermediate routers on the link that provide 102 forwarding services to reach off-link destinations and/or redirection 103 services to inform the node of an on-link neighbor that is closer to 104 the final destination. 106 Nodes on AERO links use an IPv6 link-local address format known as 107 the AERO Address. This address type has properties that statelessly 108 link IPv6 Neighbor Discovery (ND) to IPv6 routing. The AERO link can 109 be used for tunneling to neighboring nodes on either IPv6 or IPv4 110 networks, i.e., AERO views the IPv6 and IPv4 networks as equivalent 111 links for tunneling. The remainder of this document presents the 112 AERO specification. 114 2. Terminology 116 The terminology in the normative references applies; the following 117 terms are defined within the scope of this document: 119 AERO link 120 a Non-Broadcast, Multiple Access (NBMA) tunnel virtual overlay 121 configured over a node's attached IPv6 and/or IPv4 networks. All 122 nodes on the AERO link appear as single-hop neighbors from the 123 perspective of IPv6. Note that the AERO link Maximum Transmission 124 Unit (MTU) is 64KB minus the encapsulation overhead for IPv4 and 125 4GB minus the encapsulation overhead for IPv6. 127 AERO interface 128 a node's attachment to an AERO link. The AERO interface MTU is 129 less than or equal to the AERO link MTU. 131 AERO address 132 an IPv6 link-local address assigned to an AERO interface and 133 constructed as specified in Section 3.5. 135 AERO node 136 a node that is connected to an AERO link and that participates in 137 IPv6 Neighbor Discovery over the link. 139 AERO Client ("client") 140 a node that configures either a host interface or a router 141 interface on an AERO link. 143 AERO Server ("server") 144 a node that configures a router interface on an AERO link over 145 which it can provide default forwarding and redirection services 146 for other AERO nodes. 148 AERO Relay ("relay") 149 a node that relays IPv6 packets between Servers on the same AERO 150 link, and/or that forwards IPv6 packets between the AERO link and 151 the IPv6 Internet. An AERO Relay may or may not also be 152 configured as an AERO Server. 154 ingress tunnel endpoint (ITE) 155 an AERO interface endpoint that injects tunneled packets into an 156 AERO link. 158 egress tunnel endpoint (ETE) 159 an AERO interface endpoint that receives tunneled packets from an 160 AERO link. 162 underlying network 163 a connected IPv6 or IPv4 network routing region over which AERO 164 nodes tunnel IPv6 packets. 166 underlying interface 167 an AERO node's interface point of attachment to an underlying 168 network. 170 underlying address 171 an IPv6 or IPv4 address assigned to an AERO node's underlying 172 interface. When UDP encapsulation is used, the UDP port number is 173 also considered as part of the underlying address. Underlying 174 addresses are used as the source and destination addresses of the 175 AERO encapsulation header. 177 link-layer address 178 the same as defined for "underlying address" above. 180 network layer address 181 an IPv6 address used as the source or destination address of the 182 inner IPv6 packet header. 184 end user network (EUN) 185 an IPv6 network attached to a downstream interface of an AERO 186 Client (where the AERO interface is seen as the upstream 187 interface). 189 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 190 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 191 document are to be interpreted as described in [RFC2119]. 193 3. Asymmetric Extended Route Optimization (AERO) 195 The following sections specify the operation of IPv6 over Asymmetric 196 Extended Route Optimization (AERO) links: 198 3.1. AERO Node Types 200 AERO Relays relay packets between nodes connected to the same AERO 201 link and also forward packets between the AERO link and the native 202 IPv6 network. The relaying process entails re-encapsulation of IPv6 203 packets that were received from a first AERO node and are to be 204 forwarded without modification to a second AERO node. 206 AERO Servers configure their AERO interfaces as router interfaces, 207 and provide default routing services to AERO Clients. AERO Servers 208 configure a DHCPv6 Relay or Server function and facilitate DHCPv6 209 Prefix Delegation (PD) exchanges. An AERO Server may also act as an 210 AERO Relay. 212 AERO Clients act as requesting routers to receive IPv6 prefixes 213 through a DHCPv6 PD exchange via an AERO Server over the AERO link. 214 Each AERO Client receives at least a /64 prefix delegation, and may 215 receive even shorter prefixes. 217 AERO Clients that act as routers configure their AERO interfaces as 218 router interfaces, i.e., even if the AERO Client otherwise displays 219 the outward characteristics of an ordinary host (for example, the 220 Client may internally configure both an AERO interface and (internal 221 virtual) End User Network (EUN) interfaces). AERO Clients that act 222 as routers sub-delegate portions of their received prefix delegations 223 to links on EUNs. 225 AERO Clients that act as ordinary hosts configure their AERO 226 interfaces as host interfaces and assign one or more IPv6 addresses 227 taken from their received prefix delegations to the AERO interface 228 but DO NOT assign the delegated prefix itself to the AERO interface. 229 Instead, the host assigns the delegated prefix to a "black hole" 230 route so that unused portions of the prefix are nullified. 232 End system applications on AERO hosts bind directly to the AERO 233 interface, while applications on AERO routers (or IPv6 hosts served 234 by an AERO router) bind to EUN interfaces. 236 3.2. AERO Interface Characteristics 238 AERO interfaces use IPv6-in-IPv6 encapsulation [RFC2473] to exchange 239 tunneled packets with AERO neighbors attached to an underlying IPv6 240 network, and use IPv6-in-IPv4 encapsulation [RFC4213] to exchange 241 tunneled packets with AERO neighbors attached to an underlying IPv4 242 network. AERO interfaces can also use IPsec encapsulation [RFC4301] 243 (either IPv6-in-IPsec-in-IPv6 or IPv6-in-IPsec-in-IPv4) in 244 environments where strong authentication and confidentiality are 245 required. When NAT traversal and/or filtering middlebox traversal is 246 necessary, a UDP header is further inserted between the outer IP 247 encapsulation header and the inner packet. 249 AERO interfaces maintain a neighbor cache and use a variation of 250 standard unicast IPv6 ND messaging. AERO interfaces use Neighbor 251 Solicitation (NS), Neighbor Advertisement (NA) and Redirect messages 252 the same as for any IPv6 link. They do not use Router Solicitation 253 (RS) and Router Advertisement (RA) messages for several reasons. 254 First, default router discovery is supported through other means more 255 appropriate for AERO links as described below. Second, discovery of 256 more-specific routes is through the receipt of Redirect messages. 257 Finally, AERO nodes are pre-provisioned with IPv6 prefixes that they 258 register using DHCPv6 PD; hence, there is no need for RA-based prefix 259 discovery. 261 AERO Neighbor Solicitation (NS) and Neighbor Advertisement (NA) 262 messages do not include Source/Target Link Layer Address Options 263 (S/TLLAO). Instead, AERO nodes determine the link-layer addresses of 264 neighbors by examining the encapsulation source address of any NS/NA 265 messages they receive and ignore any S/TLLAOs included in these 266 messages. This is vital to the operation of AERO links for which 267 neighbors are separated by Network Address Translators (NATs) - 268 either IPv4 or IPv6. 270 AERO Redirect messages include a TLLAO the same as for any IPv6 link. 271 The TLLAO includes the link-layer address of the target node, 272 including both the IP address and the UDP source port number used by 273 the target when it sends UDP-encapsulated packets over the AERO 274 interface (the TLLAO instead encodes the value 0 when the target does 275 not use UDP encapsulation). TLLAOs for target nodes that use an IPv6 276 underlying address include the full 16 bytes of the IPv6 address as 277 shown in Figure 1, while TLLAOs for target nodes that use an IPv4 278 underlying address include only the 4 bytes of the IPv4 address as 279 shown in Figure 2. 281 0 1 2 3 282 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 1 283 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 284 | Type = 2 | Length = 3 | UDP Source Port (or 0) | 285 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 286 | Reserved | 287 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 288 | | 289 +-- --+ 290 | | 291 +-- IPv6 Address --+ 292 | | 293 +-- --+ 294 | | 295 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 297 Figure 1: AERO TLLAO Format for IPv6 299 0 1 2 3 300 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 1 301 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 302 | Type = 2 | Length = 1 | UDP Source Port (or 0) | 303 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 304 | IPv4 Address | 305 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 307 Figure 2: AERO TLLAO Format for IPv4 309 Nodes on AERO interfaces use a simple data origin authentication for 310 encapsulated packets they receive from other nodes. In particular, 311 AERO Clients accept encapsulated packets with a link-layer source 312 address belonging to their current AERO Server. AERO nodes also 313 accept encapsulated packets with a link-layer source address that is 314 correct for the network-layer source address. The AERO node 315 considers the link-layer source address correct for the network-layer 316 source address if there is an IPv6 route that matches the network- 317 layer source address as well as a neighbor cache entry corresponding 318 to the next hop that includes the link-layer address. (An exception 319 is that NS, NA and Redirect messages may include a different link- 320 layer address than the one currently in the neighbor cache, and the 321 new link-layer address updates the neighbor cache entry.) 323 Finally, AERO interface NS/NA messages only update existing neighbor 324 cache entires and do not create new neighbor cache entries. This 325 represents a departure from the normal operation of IPv6 ND over 326 common link types, but is consistent with the spirit of IPv6 over 327 NBMA links as discussed in [RFC4861]. Note however that this 328 restriction may be relaxed and/or redefined on AERO links that 329 participate in a fully distributed mobility management model 330 coordinated in a manner outside the scope of this document. 332 3.3. AERO Interface MTU Considerations 334 The base tunneling specifications for IPv4 and IPv6 typically set a 335 static MTU on the tunnel interface to 1500 bytes minus the 336 encapsulation overhead or smaller still if the tunnel is likely to 337 incur additional encapsulations such as IPsec on the path. This can 338 result in path MTU related black holes when packets that are too 339 large to be accommodated over the AERO link are dropped, but the 340 resulting ICMP Packet Too Big (PTB) messages are lost on the return 341 path. As a result, AERO nodes use the following MTU mitigations to 342 accommodate larger packets. 344 AERO nodes set their AERO interface MTU to the larger of 1500 bytes 345 and the underlying interface MTU minus the encapsulation overhead. 346 AERO nodes optionally cache other per-neighbor MTU values in the 347 underlying IP path MTU discovery cache initialized to the underlying 348 interface MTU. 350 AERO nodes admit packets that are no larger than 1280 bytes minus the 351 encapsulation overhead (*) as well as packets that are larger than 352 1500 bytes into the tunnel without fragmentation, i.e., as long as 353 they are no larger than the AERO interface MTU before encapsulation 354 and also no larger than the cached per-neighbor MTU following 355 encapsulation. For IPv4, the node sets the "Don't Fragment" (DF) bit 356 to 0 for packets no larger than 1280 bytes minus the encapsulation 357 overhead (*) and sets the DF bit to 1 for packets larger than 1500 358 bytes. If a large packet is lost in the path, the node may 359 optionally cache the MTU reported in the resulting PTB message or may 360 ignore the message, e.g., if there is a possibility that the message 361 is spurious. 363 For packets destined to an AERO node that are larger than 1280 bytes 364 minus the encapsulation overhead (*) but no larger than 1500 bytes, 365 the node uses outer IP fragmentation to fragment the packet into two 366 pieces (where the first fragment contains 1024 bytes of the 367 fragmented inner packet) then admits the fragments into the tunnel. 368 If the outer protocol is IPv4, the node admits the packet into the 369 tunnel with DF set to 0 and subject to rate limiting to avoid 370 reassembly errors [RFC4963][RFC6864]. For both IPv4 and IPv6, the 371 node also sends a 1500 byte probe message (**) to the neighbor, 372 subject to rate limiting. To construct a probe, the node prepares an 373 ICMPv6 Neighbor Solicitation (NS) message with trailing padding 374 octets added to a length of 1500 bytes but does not include the 375 length of the padding in the IPv6 Payload Length field. The node 376 then encapsulates the NS in the outer encapsulation headers (while 377 including the length of the padding in the outer length fields), sets 378 DF to 1 (for IPv4) and sends the padded NS message to the neighbor. 379 If the neighbor returns an NA message, the node may then send whole 380 packets within this size range and (for IPv4) relax the rate limiting 381 requirement. 383 AERO nodes MUST be capable of reassembling packets up to 1500 bytes 384 plus the encapsulation overhead length. It is therefore RECOMMENDED 385 that AERO nodes be capable of reassembling at least 2KB. 387 (*) Note that if it is known that the minimum Path MTU to an AERO 388 node is MINMTU bytes (where 1280 < MINMTU < 1500) then MINMTU can be 389 used instead of 1280 in the fragmentation threshold considerations 390 listed above. 392 (**) It is RECOMMENDED that no probes smaller than 1500 bytes be used 393 for MTU probing purposes, since smaller probes may be fragmented if 394 there is a nested tunnel somewhere on the path to the neighbor. 396 3.4. AERO Interface Encapsulation, Re-encapsulation and Decapsulation 398 AERO interfaces encapsulate IPv6 packets according to whether they 399 are entering the AERO interface for the first time or if they are 400 being forwarded out the same AERO interface that they arrived on. 401 This latter form of encapsulation is known as "re-encapsulation". 403 AERO interfaces encapsulate packets per the specifications in , 404 [RFC2473], [RFC4213] except that the interface copies the "TTL/Hop 405 Limit", "Type of Service/Traffic Class" and "Congestion Experienced" 406 values in the inner network layer header into the corresponding 407 fields in the outer IP header. For packets undergoing re- 408 encapsulation, the AERO interface instead copies the "TTL/Hop Limit", 409 "Type of Service/Traffic Class" and "Congestion Experienced" values 410 in the original outer IP header into the corresponding fields in the 411 new outer IP header (i.e., the values are transferred between outer 412 headers and *not* copied from the inner network layer header). 414 When UDP encapsulation is used, the AERO interface inserts a UDP 415 header between the inner packet and outer IP header. If the outer 416 header is IPv6 and is followed by an IPv6 Fragment header, the AERO 417 interface inserts the UDP header between the outer IPv6 header and 418 IPv6 Fragment header. The AERO interface sets the UDP source port to 419 a constant value that it will use in each successive packet it sends, 420 sets the UDP checksum field to zero (see: [RFC6935][RFC6936]) and 421 sets the UDP length field to the length of the inner packet plus 8 422 bytes for the UDP header itself. For packets sent via a Server, the 423 AERO interface sets the UDP destination port to 8060 (i.e., the IANA- 424 registerd port number for AERO). For packets sent to a neighboring 425 Client, the AERO interface sets the UDP destination port to the port 426 value stored in the neighbor cache entry for this neighbor. 428 The AERO interface next sets the outer IP protocol number to the 429 appropriate value for the first protocol layer within the 430 encapsulation (e.g., IPv6, IPv6 Fragment Header, UDP, etc.). When 431 IPv6 is used as the outer IP protocol, the ITE then sets the flow 432 label value in the outer IPv6 header the same as described in 433 [RFC6438]. When IPv4 is used as the outer IP protocol, the AERO 434 interface sets the DF bit as discussed in Section 3.2. 436 AERO interfaces decapsulate packets destined either to the node 437 itself or to a destination reached via an interface other than the 438 receiving AERO interface per the specifications in , [RFC2473], 439 [RFC4213]. When the encapsulated packet includes a UDP header, the 440 AERO interfaces examines the first octet of data following the UDP 441 header to determine the inner header type. If the most significant 442 four bits of the first octet encode the value '0110', the inner 443 header is an IPv6 header. Otherwise, the interface considers the 444 first octet as an IP protocol type that encodes the value '44' for 445 IPv6 fragment header, the value '50' for Encapsulating Security 446 Payload, the value '51' for Authentication Header etc. (If the first 447 octet encodes the value '0', the interface instead discards the 448 packet, since this is the value reserved for experimentation under , 449 [RFC6706]). During the decapsulation, the AERO interface records the 450 UDP source port in the neighbor cache entry for this neighbor then 451 discards the UDP header. 453 3.5. AERO Addresses 455 An AERO address is an IPv6 link-local address assigned to an AERO 456 interface and with an IPv6 prefix embedded within the interface 457 identifier. The AERO address is formatted as: 459 fe80::[IPv6 prefix] 461 Each AERO Client configures an AERO address based on the delegated 462 prefix it has received from the AERO link prefix delegation 463 authority. The address begins with the prefix fe80::/64 and includes 464 in its interface identifier the base /64 prefix taken from the 465 Client's delegated IPv6 prefix. The base prefix is determined by 466 masking the delegated prefix with the prefix length. For example, if 467 an AERO Client has received the prefix delegation: 469 2001:db8:1000:2000::/56 471 it would construct its AERO address as: 473 fe80::2001:db8:1000:2000 475 An AERO Client may have multiple non-contiguous IPv6 prefix 476 delegations, in which case it would configure multiple AERO addresses 477 - one for each prefix. Note that, in order for the DHCPv6 PD 478 function to operate correctly, the AERO Client must already hold a 479 delegated IPv6 prefix so that it can construct an AERO address to use 480 as the source address in the DHCPv6 exchange. This means that the 481 DHCPv6 PD function is actually a registration of a pre-provisioned 482 prefix. 484 Each AERO Server configures the special AERO address fe80::1 to 485 support the operation of IPv6 Neighbor Discovery over the AERO link; 486 the address therefore has the properties of an IPv6 Anycast address. 487 While all Servers configure the same AERO address and therefore 488 cannot be distinguished from one another at the network layer, 489 Clients can still distinguish Servers at the link layer by examining 490 the Servers' link-layer addresses. 492 Nodes that are configured as pure AERO Relays (i.e., and that do not 493 also act as Servers) do not configure an IPv6 address of any kind on 494 their AERO interfaces. The Relay's AERO interface is therefore used 495 purely for transit and does not participate in IPv6 ND message 496 exchanges. 498 3.6. AERO Reference Operational Scenario 500 Figure 3 depicts the AERO reference operational scenario. The figure 501 shows an AERO Server('A'), two AERO Clients ('B', 'D') and three 502 ordinary IPv6 hosts ('C', 'E', 'F'): 504 .-(::::::::) 505 .-(::: IPv6 :::)-. +-------------+ 506 (:::: Internet ::::)--| Host F | 507 `-(::::::::::::)-' +-------------+ 508 `-(::::::)-' 2001:db8:3::1 509 | 510 +--------------+ 511 | AERO Server A| 512 | (C->B; E->D) | 513 +--------------+ 514 fe80::1 515 L2(A) 516 | 517 X-----+-----------+-----------+--------X 518 | AERO Link | 519 L2(B) L2(D) 520 fe80::2001:db8:0:0 fe80::2001:db8:1:0 .-. 521 +--------------+ +--------------+ ,-( _)-. 522 | AERO Client B| | AERO Client D| .-(_ IPv6 )-. 523 | (default->A) | | (default->A) |--(__ EUN ) 524 +--------------+ +--------------+ `-(______)-' 525 2001:DB8:0::/48 2001:DB8:1::/48 | 526 | 2001:db8:1::1 527 .-. +-------------+ 528 ,-( _)-. 2001:db8:0::1 | Host E | 529 .-(_ IPv6 )-. +-------------+ +-------------+ 530 (__ EUN )--| Host C | 531 `-(______)-' +-------------+ 533 Figure 3: AERO Reference Operational Scenario 535 In Figure 3, AERO Server ('A') connects to the AERO link and connects 536 to the IPv6 Internet, either directly or via an AERO Relay (not 537 shown). Server ('A') assigns the address fe80::1 to its AERO 538 interface with link-layer address L2(A). Server ('A') next arranges 539 to add L2(A) to a published list of valid Servers for the AERO link. 541 AERO Client ('B') assigns the address fe80::2001:db8:0:0 to its AERO 542 interface with link-layer address L2(B) and registers the IPv6 prefix 543 2001:db8:0::/48 in a DHCPv6 PD exchange via Server ('A'). Client 544 ('B') configures a default route via the AERO interface with next-hop 545 address fe80::1 and link-layer address L2(A), then sub-delegates the 546 prefix 2001:db8:0::/48 to its attached EUNs. IPv6 host ('C') 547 connects to the EUN, and configures the address 2001:db8:0::1. 549 AERO Client ('D') assigns the address fe80::2001:db8:1:0 to its AERO 550 interface with link-layer address L2(D) and registers the IPv6 prefix 551 2001:db8:1::/48 in a DHCPv6 PD exchange via Server ('A'). Client 552 ('D') configures a default route via the AERO interface with next-hop 553 address fe80::1 and link-layer address L2(A), then sub-delegates the 554 prefix 2001:db8:1::/48 to its attached EUNs. IPv6 host ('E') 555 connects to the EUN, and configures the address 2001:db8:1::1. 557 Finally, IPv6 host ('F') connects to an IPv6 network outside of the 558 AERO link domain. Host ('F') configures its IPv6 interface in a 559 manner specific to its attached IPv6 link, and assigns the address 560 2001:db8:3::1 to its IPv6 link interface. 562 3.7. AERO Router Discovery and Prefix Delegation 564 3.7.1. AERO Client Behavior 566 AERO Clients observe the IPv6 router requirements defined in 567 [RFC6434]. AERO Clients first discover the link-layer address of an 568 AERO Server via static configuration, or through an automated means 569 such as DNS name resolution. In the absence of other information, 570 the Client resolves the Fully-Qualified Domain Name (FQDN) 571 "linkupnetworks.domainname", where "domainname" is the DNS domain 572 appropriate for the Client's attached underlying network. The Client 573 then creates a neighbor cache entry with the IPv6 link-local address 574 fe80::1 and the discovered address as the link-layer address. The 575 Client further creates a default route with the link-local address 576 fe80::1 as the next hop. 578 Next, the Client assigns the link-local AERO address(es) taken from 579 its delegated prefix(es) to the AERO interface (see: Section 3.5). 580 It then acts as a requesting router to register its IPv6 prefixes 581 through DHCPv6 PD [RFC3633] via the Server. After the Client 582 registers its prefixes, it sub-delegates them to nodes and links 583 within its attached EUNs. 585 After configuring a default route and registering its prefixes, the 586 Client sends periodic NS messages to the Server to obtain new NAs in 587 order to keep neighbor cache entries alive. The Client can also 588 forward IPv6 packets destined to networks beyond its local EUNs via 589 the Server as an IPv6 default router. The Server may in turn return 590 a Redirect message informing the Client of a neighbor on the AERO 591 link that is topologically closer to the final destination as 592 specified in Section 3.9. 594 3.7.2. AERO Server Behavior 596 AERO Servers observe the IPv6 router requirements defined in 597 [RFC6434]. They further configure a DHCPv6 relay/server function on 598 their AERO links. When the Server facilitates prefix delegation, it 599 also establishes forwarding table and neighbor cache entries that 600 list the AERO address of the Client as the next hop toward the 601 delegated IPv6 prefixes (where the AERO address is constructed as 602 specified in Section 3.5). 604 Servers respond to NS messages from Clients on their AERO interfaces 605 by returning an NA message. When the Server receives an NS message, 606 it updates the neighbor cache entry using the network layer source 607 address as the neighbor's network layer address and using the link- 608 layer source address of the NS message as the neighbor's link-layer 609 address. 611 When the Server forwards a packet via the same AERO interface on 612 which it arrived, it initiates an AERO route optimization procedure 613 as specified in Section 3.9. 615 3.8. AERO Neighbor Solicitation and Advertisement 617 Each AERO node uses its delegated prefix to create an AERO address as 618 specified in Section 3.5. It can then send NS messages to elicit NA 619 messages from other AERO nodes. When the AERO node sends NS/NA 620 messages, however, it must also include the length of the prefix 621 corresponding to the AERO address. AERO NS/NA messages therefore 622 include an 8-bit "Prefix Length" field take from the low-order 8 bits 623 of the Reserved field as shown in Figure 4 and Figure 5. 625 0 1 2 3 626 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 1 627 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 628 | Type (=135) | Code | Checksum | 629 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 630 | Reserved | Prefix Length | 631 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 632 | | 633 + + 634 | | 635 + Target Address + 636 | | 637 + + 638 | | 639 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 640 | Options ... 641 +-+-+-+-+-+-+-+-+-+-+-+- 643 Figure 4: AERO Neighbor Solicitation (NS) Message Format 645 0 1 2 3 646 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 1 647 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 648 | Type (=136) | Code | Checksum | 649 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 650 | R|S|O| Reserved | Prefix Length | 651 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 652 | | 653 + + 654 | | 655 + Target Address + 656 | | 657 + + 658 | | 659 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 660 | Options ... 661 +-+-+-+-+-+-+-+-+-+-+-+- 663 Figure 5: AERO Neighbor Advertisement (NA) Message Format 665 When an AERO node sends an NS/NA message, it MUST use its AERO 666 address as the IPv6 source address and MUST include its AERO address 667 prefix length in the Prefix Length field. 669 When an AERO node receives an NS/NA message, it accepts the message 670 if the Prefix Length applied to the source address is correct for the 671 neighbor; otherwise, it ignores the message. 673 3.9. AERO Redirection 675 Section 3.6 describes the AERO reference operational scenario. We 676 now discuss the operation and protocol details of AERO Redirection 677 with respect to this reference scenario. 679 3.9.1. Classical Redirection Approaches 681 With reference to Figure 3, when the IPv6 source host ('C') sends a 682 packet to an IPv6 destination host ('E'), the packet is first 683 forwarded via the EUN to AERO Client ('B'). Client ('B') then 684 forwards the packet over its AERO interface to AERO Server ('A'), 685 which then re-encapsulates and forwards the packet to AERO Client 686 ('D'), where the packet is finally forwarded to the IPv6 destination 687 host ('E'). When Server ('A') re-encapsulates and forwards the 688 packet back out on its advertising AERO interface, it must arrange to 689 redirect Client ('B') toward Client ('D') as a better next-hop node 690 on the AERO link that is closer to the final destination. However, 691 this redirection process applied to AERO interfaces must be more 692 carefully orchestrated than on ordinary links since the parties may 693 be separated by potentially many underlying network routing hops. 695 Consider a first alternative in which Server ('A') informs Client 696 ('B') only and does not inform Client ('D') (i.e., "classical 697 redirection"). In that case, Client ('D') has no way of knowing that 698 Client ('B') is authorized to forward packets from their claimed 699 network-layer source addresses, and it may simply elect to drop the 700 packets. Also, Client ('B') has no way of knowing whether Client 701 ('D') is performing some form of source address filtering that would 702 reject packets arriving from a node other than a trusted default 703 router, nor whether Client ('D') is even reachable via a direct path 704 that does not involve Server ('A'). 706 Consider a second alternative in which Server ('A') informs both 707 Client ('B') and Client ('D') separately, via independent redirection 708 control messages (i.e., "augmented redirection"). In that case, if 709 Client ('B') receives the redirection control message but Client 710 ('D') does not, subsequent packets sent by Client ('B') could be 711 dropped due to filtering since Client ('D') would not have a route to 712 verify their source network-layer addresses. Also, if Client ('D') 713 receives the redirection control message but Client ('B') does not, 714 subsequent packets sent in the reverse direction by Client ('D') 715 would be lost. 717 Since both of these alternatives have shortcomings, a new redirection 718 technique (i.e., "AERO redirection") is needed. 720 3.9.2. AERO Redirection Concept of Operations 722 Again, with reference to Figure 3, when source host ('C') sends a 723 packet to destination host ('E'), the packet is first forwarded over 724 the source host's attached EUN to Client ('B'), which then forwards 725 the packet via its AERO interface to Server ('A'). 727 Server ('A') then re-encapsulates forwards the packet out the same 728 AERO interface toward Client ('D') and also sends an AERO "Predirect" 729 message forward to Client ('D') as specified in Section 3.9.5. The 730 Predirect message includes Client ('B')'s network- and link-layer 731 addresses as well as information that Client ('D') can use to 732 determine the IPv6 prefix used by Client ('B') . After Client ('D') 733 receives the Predirect message, it process the message and returns an 734 AERO Redirect message destined for Client ('B') via Server ('A') as 735 specified in Section 3.9.6. During the process, Client ('D') also 736 creates or updates a neighbor cache entry for Client ('B'), and 737 creates an IPv6 route for Client ('B')'s IPv6 prefix. 739 When Server ('A') receives the Redirect message, it re-encapsulates 740 the message and forwards it on to Client ('B') as specified in 741 Section 3.9.7. The message includes Client ('D')'s network- and 742 link-layer addresses as well as information that Client ('B') can use 743 to determine the IPv6 prefix used by Client ('D'). After Client 744 ('B') receives the Redirect message, it processes the message as 745 specified in Section 3.9.8. During the process, Client ('B') also 746 creates or updates a neighbor cache entry for Client ('D'), and 747 creates an IPv6 route for Client ('D')'s IPv6 prefix. 749 Following the above Predirect/Redirect message exchange, forwarding 750 of packets from Client ('B') to Client ('D') without involving Server 751 ('A) as an intermediary is enabled. The mechanisms that support this 752 exchange are specified in the following sections. 754 3.9.3. Conceptual Data Structures and Protocol Constants 756 Each AERO node maintains a per-AERO interface conceptual neighbor 757 cache that includes an entry for each neighbor it communicates with 758 on the AERO link, the same as for any IPv6 interface (see [RFC4861]). 759 Each AERO interface neighbor cache entry further maintains an ACCEPT 760 timer and a FORWARD timer. 762 When an AERO node receives a valid Predirect message (See Section 763 3.9.6) it creates a neighbor cache entry for the Predirect target L3 764 and L2 addresses, and also creates an IPv6 route for the Predirected 765 (source) prefix. The node then sets an ACCEPT timer and uses this 766 timer to validate any messages received from the Predirected 767 neighbor. 769 When an AERO node receives a valid Redirect message (see Section 770 3.9.8) it creates a neighbor cache entry for the Redirect target L3 771 and L2 addresses, and also creates an IPv6 route for the Redirected 772 (destination) prefix. The node then sets a FORWARD timer and uses 773 this timer to determine whether packets can be sent directly to the 774 Redirected neighbor. The node also maintains a constant value 775 MAX_RETRY to limit the number of keepalives sent when a neighbor has 776 gone unreachable. 778 It is RECOMMENDED that FORWARD_TIME be set to the default constant 779 value 30 seconds to match the default REACHABLE_TIME value specified 780 for IPv6 neighbor discovery [RFC4861]. 782 It is RECOMMENDED that ACCEPT_TIME be set to the default constant 783 value 40 seconds to allow a 10 second window so that the AERO 784 redirection procedure can converge before the ACCEPT_TIME timer 785 decrements below FORWARD_TIME. 787 It is RECOMMENDED that MAX_RETRY be set to 3 the same as described 788 for IPv6 neighbor discovery address resolution in Section 7.3.3 of 790 [RFC4861]. 792 Different values for FORWARD_TIME, ACCEPT_TIME, and MAX_RETRY MAY be 793 administratively set, if necessary, to better match the AERO link's 794 performance characteristics; however, if different values are chosen, 795 all nodes on the link MUST consistently configure the same values. 796 ACCEPT_TIME SHOULD further be set to a value that is sufficiently 797 longer than FORWARD time to allow the AERO redirection procedure to 798 converge. 800 3.9.4. AERO Redirection Message Format 802 AERO Redirect/Predirect messages use the same format as for ICMPv6 803 Redirect messages depicted in Section 4.5 of [RFC4861], but also 804 include a new "Prefix Length" field taken from the low-order 8 bits 805 of the Redirect message Reserved field. The Redirect/Predirect 806 messages are formatted as shown in Figure 6: 807 0 1 2 3 808 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 1 809 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 810 | Type (=137) | Code (=0/1) | Checksum | 811 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 812 | Reserved | Prefix Length | 813 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 814 | | 815 + + 816 | | 817 + Target Address + 818 | | 819 + + 820 | | 821 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 822 | | 823 + + 824 | | 825 + Destination Address + 826 | | 827 + + 828 | | 829 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 830 | Options ... 831 +-+-+-+-+-+-+-+-+-+-+-+- 833 Figure 6: AERO Redirect/Predirect Message Format 835 3.9.5. Sending Predirects 837 When an AERO Server forwards a packet out the same AERO interface 838 that it arrived on, the Server sends a Predirect message forward 839 toward the AERO Client nearest the destination instead of sending a 840 Redirect message back to AERO Client nearest the source. 842 In the reference operational scenario, when Server ('A') forwards a 843 packet sent by Client ('B') toward Client ('D'), it also sends a 844 Predirect message forward toward Client ('D'), subject to rate 845 limiting (see Section 8.2 of [RFC4861]). Server ('A') prepares the 846 Predirect message as follows: 848 o the link-layer source address is set to 'L2(A)' (i.e., the 849 underlying address of Server ('A')). 851 o the link-layer destination address is set to 'L2(D)' (i.e., the 852 underlying address of Client ('D')). 854 o the network-layer source address is set to fe80::1 (i.e., the AERO 855 address of Server ('A')). 857 o the network-layer destination address is set to fe80::2001:db8:1:0 858 (i.e., the AERO address of Client ('D')). 860 o the Type is set to 137. 862 o the Code is set to 1 to indicate "Predirect". 864 o the Prefix Length is set to the length of the prefix to be applied 865 to Target address. 867 o the Target Address is set to fe80::2001:db8:0::0 (i.e., the AERO 868 address of Client ('B')). 870 o the Destination Address is set to the IPv6 source address of the 871 packet that triggered the Predirection event. 873 o the message includes a TLLAO set to 'L2(B)' (i.e., the underlying 874 address of Client ('B')). 876 o the message includes a Redirected Header Option (RHO) that 877 contains the originating packet truncated to ensure that at least 878 the network-layer header is included but the size of the message 879 does not exceed 1280 bytes. 881 Server ('A') then sends the message forward to Client ('D'). 883 3.9.6. Processing Predirects and Sending Redirects 885 When Client ('D') receives a Predirect message, it accepts the 886 message only if it has a link-layer source address of the Server, 887 i.e. 'L2(A)'. Client ('D') further accepts the message only if it 888 is willing to serve as a redirection target. Next, Client ('D') 889 validates the message according to the ICMPv6 Redirect message 890 validation rules in Section 8.1 of [RFC4861]. 892 In the reference operational scenario, when the Client ('D') receives 893 a valid Predirect message, it either creates or updates a neighbor 894 cache entry that stores the Target Address of the message as the 895 network-layer address of Client ('B') and stores the link-layer 896 address found in the TLLAO as the link-layer address of Client ('B'). 897 Client ('D') then applies the Prefix Length to the Interface 898 Identifier portion of the Target Address and records the resulting 899 IPv6 prefix in its IPv6 forwarding table. 901 After processing the message, Client ('D') prepares a Redirect 902 message response as follows: 904 o the link-layer source address is set to 'L2(D)' (i.e., the link- 905 layer address of Client ('D')). 907 o the link-layer destination address is set to 'L2(A)' (i.e., the 908 link-layer address of Server ('A')). 910 o the network-layer source address is set to 'L3(D)' (i.e., the AERO 911 address of Client ('D')). 913 o the network-layer destination address is set to 'L3(B)' (i.e., the 914 AERO address of Client ('B')). 916 o the Type is set to 137. 918 o the Code is set to 0 to indicate "Redirect". 920 o the Prefix Length is set to the length of the prefix to be applied 921 to the Target and Destination address. 923 o the Target Address is set to fe80::2001:db8:1::1 (i.e., the AERO 924 address of Client ('D')). 926 o the Destination Address is set to the IPv6 destination address of 927 the packet that triggered the Redirection event. 929 o the message includes a TLLAO set to 'L2(D)' (i.e., the underlying 930 address of Client ('D')). 932 o the message includes as much of the RHO copied from the 933 corresponding AERO Predirect message as possible such that at 934 least the network-layer header is included but the size of the 935 message does not exceed 1280 bytes. 937 After Client ('D') prepares the Redirect message, it sends the 938 message to Server ('A'). 940 3.9.7. Re-encapsulating and Relaying Redirects 942 When Server ('A') receives a Redirect message, it accepts the message 943 only if it has a neighbor cache entry that associates the message's 944 link-layer source address with the network-layer source address. 945 Next, Server ('A') validates the message according to the ICMPv6 946 Redirect message validation rules in Section 8.1 of [RFC4861]. 947 Following validation, Server ('A') re-encapsulates the Redirect then 948 relays the re-encapsulated Redirect on to Client ('B') as follows. 950 In the reference operational scenario, Server ('A') receives the 951 Redirect message from Client ('D') and prepares to re-encapsulate and 952 forward the message to Client ('B'). Server ('A') first verifies 953 that Client ('D') is authorized to use the Prefix Length in the 954 Redirect message when applied to the AERO address in the network- 955 layer source of the Redirect message, and discards the message if 956 verification fails. Otherwise, Server ('A') re-encapsulates the 957 message by changing the link-layer source address of the message to 958 'L2(A)', changing the network-layer source address of the message to 959 fe80::1, and changing the link-layer destination address to 'L2(B)' . 960 Server ('A') finally relays the re-encapsulated message to the 961 ingress node ('B') without decrementing the network-layer IPv6 header 962 Hop Limit field. 964 While not shown in Figure 3, AERO Relays relay Redirect and Predirect 965 messages in exactly this same fashion described above. See Figure 7 966 in Appendix A for an extension of the reference operational scenario 967 that includes Relays. 969 3.9.8. Processing Redirects 971 When Client ('B') receives the Redirect message, it accepts the 972 message only if it has a link-layer source address of the Server, 973 i.e. 'L2(A)'. Next, Client ('B') validates the message according to 974 the ICMPv6 Redirect message validation rules in Section 8.1 of 975 [RFC4861]. Following validation, Client ('B') then processes the 976 message as follows. 978 In the reference operational scenario, when Client ('B') receives the 979 Redirect message, it either creates or updates a neighbor cache entry 980 that stores the Target Address of the message as the network-layer 981 address of Client ('D') and stores the link-layer address found in 982 the TLLAO as the link-layer address of Client ('D'). Client ('B') 983 then applies the Prefix Length to the Interface Identifier portion of 984 the Target Address and records the resulting IPv6 prefix in its IPv6 985 forwarding table. 987 Now, Client ('B') has an IPv6 forwarding table entry for 988 Client('D')'s prefix, and Client ('D') has an IPv6 forwarding table 989 entry for Client ('B')'s prefix. Thereafter, the clients may 990 exchange ordinary network-layer data packets directly without 991 forwarding through Server ('A'). 993 3.10. Neighbor Reachability Maintenance 995 When a source Client is redirected to a target neighbor it MUST test 996 the direct path to the target by sending an initial NS message to 997 elicit a solicited NA response. While testing the path, the Client 998 SHOULD continue sending packets via the Server until target 999 reachability has been confirmed. The Client MUST thereafter continue 1000 to test the forward path to the neighbor using Neighbor 1001 Unreachability Detection (NUD) (see Section 7.3 of [RFC4861]) in 1002 order to keep neighbor cache entries alive. In particular, the 1003 Client sends NS messages to the neighbor subject to rate limiting in 1004 order to receive solicited NA messages. If at any time the direct 1005 path to the neighbor appears to be failing, the Client can resume 1006 sending packets via the Server which may or may not result in a new 1007 redirection event. 1009 When a target Client receives an NS message from a source Client, it 1010 resets the ACCEPT_TIME timer if a neighbor cache entry exists; 1011 otherwise, it discards the NS message. 1013 When a source Client receives a solicited NA message form a target 1014 Client, it resets the FORWARD_TIME timer if a neighbor cache entry 1015 exists; otherwise, it discards the NA message. 1017 When both the FORWARD_TIME and ACCEPT_TIME timers on a neighbor cache 1018 entry expire, the Client discards the neighbor cache entry. 1020 If the source Client is unable to elicit an NA response after 1021 MAX_RETRY attempts, it SHOULD consider the direct path unusable for 1022 forwarding purposes. If reachability is confirmed, the Client SHOULD 1023 thereafter process any link-layer errors as a hint that the direct 1024 path to the target has either failed or has become intermittent. 1026 3.11. Mobility and Link-Layer Address Change Considerations 1028 When a Client needs to change its link-layer address (e.g., due to a 1029 mobility event, due to a change in underlying network interface, 1030 etc.), it sends an immediate NS message forward to any of its 1031 correspondents (including the Server and any other Clients) which 1032 then discover the new link-layer address. 1034 If two Clients change their link-layer addresses simultaneously, the 1035 NS/NA messages may be lost. In that case, the Clients SHOULD delete 1036 their respective neighbor cache entries and allow packets to again 1037 flow through their Server(s), which MAY result in a new AERO 1038 redirection exchange. 1040 When a Client needs to change to a new Server, it performs a DHCPv6 1041 "Release" message exchange with the delegating router via the old 1042 Server then sends a DHCPv6 "Request" message to the delegating router 1043 via the new Server. Note that this may result in a temporary service 1044 outage during Server "handovers". 1046 3.12. Underlying Protocol Version Considerations 1048 A source Client may connect only to an IPvX underlying network, while 1049 the target Client connects only to an IPvY underlying network. In 1050 that case, the source Client has no means for reaching the target 1051 directly (since they connect to underlying networks of different IP 1052 protocol versions) and so must ignore any Redirects and continue to 1053 send packets via the Server. 1055 3.13. Multicast Considerations 1057 When the underlying network does not support multicast, AERO nodes 1058 map IPv6 link-scoped multicast addresses (including 1059 "All_DHCP_Relay_Agents_and_Servers") to the underlying IP address of 1060 the AERO Server. 1062 When the underlying network supports multicast, AERO nodes use the 1063 multicast address mapping specification found in [RFC2529] for IPv4 1064 underlying networks and use a direct multicast mapping for IPv6 1065 underlying networks. (In the latter case, "direct multicast mapping" 1066 means that if the IPv6 multicast destination address of the inner 1067 packet is "M", then the IPv6 multicast destination address of the 1068 encapsulating header is also "M".) 1070 3.14. Operation on Server-less AERO Links 1072 In some AERO link scenarios, there may be no Server on the link 1073 and/or no need for Clients to use a Server as an intermediary trust 1074 anchor. In that case, Clients can establish neighbor cache entries 1075 and IPv6 routes by performing direct Client-to-Client exchanges, and 1076 some other form of trust basis must be applied so that each Client 1077 can verify that the prospective neighbor is authorized to use its 1078 claimed prefix. 1080 When there is no Server on the link, Clients must arrange to receive 1081 prefix delegations and publish the delegations via a secure prefix 1082 discovery service through some means outside the scope of this 1083 document. 1085 3.15. Other Considerations 1087 IPv6 hosts serviced by an AERO Client can reach IPv4-only services 1088 via a NAT64 gateway [RFC6146] within the IPv6 network. 1090 AERO nodes can use the Default Address Selection Policy with DHCPv6 1091 option [RFC7078] the same as on any IPv6 link. 1093 All other (non-multicast) functions that operate over ordinary IPv6 1094 links operate in the same fashion over AERO links. 1096 4. Implementation Status 1098 An early implementation is available at: 1099 http://linkupnetworks.com/aero/aerov2-0.1.tgz. 1101 5. IANA Considerations 1103 This document uses the UDP Service Port Number 8060 reserved by IANA 1104 for AERO in [RFC6706]. Therefore, there are no new IANA actions 1105 required for this document. 1107 6. Security Considerations 1109 AERO link security considerations are the same as for standard IPv6 1110 Neighbor Discovery [RFC4861] except that AERO improves on some 1111 aspects. In particular, AERO is dependent on a trust basis between 1112 AERO Clients and Servers, where the Clients only engage in the AERO 1113 mechanism when it is facilitated by a trust anchor. 1115 AERO links must be protected against link-layer address spoofing 1116 attacks in which an attacker on the link pretends to be a trusted 1117 neighbor. Links that provide link-layer securing mechanisms (e.g., 1118 WiFi networks) and links that provide physical security (e.g., 1119 enterprise network LANs) provide a first line of defense that is 1120 often sufficient. In other instances, securing mechanisms such as 1121 Secure Neighbor Discovery (SeND) [RFC3971] or IPsec [RFC4301] may be 1122 necessary. 1124 AERO Clients MUST ensure that their connectivity is not used by 1125 unauthorized nodes to gain access to a protected network. (This 1126 concern is no different than for ordinary hosts that receive an IP 1127 address delegation but then "share" the address with unauthorized 1128 nodes via an IPv6/IPv6 NAT function.) 1130 On some AERO links, establishment and maintenance of a direct path 1131 between neighbors requires secured coordination such as through the 1132 Internet Key Exchange (IKEv2) protocol [RFC5996]. 1134 7. Acknowledgements 1136 Discussions both on the v6ops list and in private exchanges helped 1137 shape some of the concepts in this work. Individuals who contributed 1138 insights include Mikael Abrahamsson, Fred Baker, Stewart Bryant, 1139 Brian Carpenter, Brian Haberman, Joel Halpern, Sascha Hlusiak, Lee 1140 Howard and Joe Touch. Members of the IESG also provided valuable 1141 input during their review process that greatly improved the document. 1142 Special thanks go to Stewart Bryant, Joel Halpern and Brian Haberman 1143 for their shepherding guidance. 1145 This work has further been encouraged and supported by Boeing 1146 colleagues including Keith Bartley, Balaguruna Chidambaram, Jeff 1147 Holland, Cam Brodie, Yueli Yang, Wen Fang, Ed King, Mike Slane, Kent 1148 Shuey, Gen MacLean, and other members of the BR&T and BIT mobile 1149 networking teams. 1151 Earlier works on NBMA tunneling approaches are found in 1152 [RFC2529][RFC5214][RFC5569]. 1154 8. References 1156 8.1. Normative References 1158 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 1159 August 1980. 1161 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 1162 September 1981. 1164 [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, 1165 RFC 792, September 1981. 1167 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1168 Requirement Levels", BCP 14, RFC 2119, March 1997. 1170 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 1171 (IPv6) Specification", RFC 2460, December 1998. 1173 [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in 1174 IPv6 Specification", RFC 2473, December 1998. 1176 [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic 1177 Host Configuration Protocol (DHCP) version 6", RFC 3633, 1178 December 2003. 1180 [RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms 1181 for IPv6 Hosts and Routers", RFC 4213, October 2005. 1183 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 1184 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 1185 September 2007. 1187 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 1188 Address Autoconfiguration", RFC 4862, September 2007. 1190 [RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node 1191 Requirements", RFC 6434, December 2011. 1193 8.2. Informative References 1195 [IRON] Templin, F., "The Internet Routing Overlay Network 1196 (IRON)", Work in Progress, June 2012. 1198 [RFC0879] Postel, J., "TCP maximum segment size and related topics", 1199 RFC 879, November 1983. 1201 [RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4 1202 Domains without Explicit Tunnels", RFC 2529, March 1999. 1204 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., 1205 and M. Carney, "Dynamic Host Configuration Protocol for 1206 IPv6 (DHCPv6)", RFC 3315, July 2003. 1208 [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure 1209 Neighbor Discovery (SEND)", RFC 3971, March 2005. 1211 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 1212 Internet Protocol", RFC 4301, December 2005. 1214 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 1215 Discovery", RFC 4821, March 2007. 1217 [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly 1218 Errors at High Data Rates", RFC 4963, July 2007. 1220 [RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site 1221 Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, 1222 March 2008. 1224 [RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4 1225 Infrastructures (6rd)", RFC 5569, January 2010. 1227 [RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen, 1228 "Internet Key Exchange Protocol Version 2 (IKEv2)", 1229 RFC 5996, September 2010. 1231 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful 1232 NAT64: Network Address and Protocol Translation from IPv6 1233 Clients to IPv4 Servers", RFC 6146, April 2011. 1235 [RFC6204] Singh, H., Beebee, W., Donley, C., Stark, B., and O. 1236 Troan, "Basic Requirements for IPv6 Customer Edge 1237 Routers", RFC 6204, April 2011. 1239 [RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label 1240 for Equal Cost Multipath Routing and Link Aggregation in 1241 Tunnels", RFC 6438, November 2011. 1243 [RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)", 1244 RFC 6691, July 2012. 1246 [RFC6706] Templin, F., "Asymmetric Extended Route Optimization 1247 (AERO)", RFC 6706, August 2012. 1249 [RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field", 1250 RFC 6864, February 2013. 1252 [RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and 1253 UDP Checksums for Tunneled Packets", RFC 6935, April 2013. 1255 [RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement 1256 for the Use of IPv6 UDP Datagrams with Zero Checksums", 1257 RFC 6936, April 2013. 1259 [RFC6980] Gont, F., "Security Implications of IPv6 Fragmentation 1260 with IPv6 Neighbor Discovery", RFC 6980, August 2013. 1262 [RFC7078] Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing 1263 Address Selection Policy Using DHCPv6", RFC 7078, 1264 January 2014. 1266 Appendix A. AERO Server and Relay Interworking 1268 Figure 3 depicts a reference AERO operational scenario with a single 1269 Server on the AERO link. In order to support scaling to larger 1270 numbers of nodes, the AERO link can deploy multiple Servers and 1271 Relays, e.g., as shown in Figure 7. 1273 .-(::::::::) 1274 .-(::: IPv6 :::)-. 1275 (:: Internetwork ::) 1276 `-(::::::::::::)-' 1277 `-(::::::)-' 1278 | 1279 +--------------+ +------+-------+ +--------------+ 1280 |AERO Server C | | AERO Relay D | |AERO Server E | 1281 | (default->D) | | (A->C; G->E) | | (default->D) | 1282 | (A->B) | +-------+------+ | (G->F) | 1283 +-------+------+ | +------+-------+ 1284 | | | 1285 X---+---+-------------------+------------------+---+---X 1286 | AERO Link | 1287 +-----+--------+ +--------+-----+ 1288 |AERO Client B | |AERO Client F | 1289 | (default->C) | | (default->E) | 1290 +--------------+ +--------------+ 1291 .-. .-. 1292 ,-( _)-. ,-( _)-. 1293 .-(_ IPv6 )-. .-(_ IPv6 )-. 1294 (__ EUN ) (__ EUN ) 1295 `-(______)-' `-(______)-' 1296 | | 1297 +--------+ +--------+ 1298 | Host A | | Host G | 1299 +--------+ +--------+ 1301 Figure 7: AERO Server/Relay Interworking 1303 In this example, AERO Client ('B') associates with AERO Server ('C'), 1304 while AERO Client ('F') associates with AERO Server ('E'). 1305 Furthermore, AERO Servers ('C') and ('E') do not associate with each 1306 other directly, but rather have an association with AERO Relay ('D') 1307 (i.e., a router that has full topology information concerning its 1308 associated Servers and their Clients). Relay ('D') connects to the 1309 AERO link, and also connects to the native IPv6 Internetwork. 1311 When host ('A') sends a packet toward destination host ('G'), IPv6 1312 forwarding directs the packet through the EUN to Client ('B'), which 1313 forwards the packet to Server ('C') in absence of more-specific 1314 forwarding information. Server ('C') forwards the packet, and it 1315 also generates an AERO Predirect message that is then forwarded 1316 through Relay ('D') to Server ('E'). When Server ('E') receives the 1317 message, it forwards the message to Client ('F'). 1319 After processing the AERO Predirect message, Client ('F') sends an 1320 AERO Redirect message to Server ('E'). Server ('E'), in turn, 1321 forwards the message through Relay ('D') to Server ('C'). When 1322 Server ('C') receives the message, it forwards the message to Client 1323 ('B') informing it that host 'G's EUN can be reached via Client 1324 ('F'), thus completing the AERO redirection. 1326 The network layer routing information shared between Servers and 1327 Relays must be carefully coordinated in a manner outside the scope of 1328 this document. In particular, Relays require full topology 1329 information, while individual Servers only require partial topology 1330 information (i.e., they only need to know the EUN prefixes associated 1331 with their current set of Clients). See [IRON] for an architectural 1332 discussion of routing coordination between Relays and Servers. 1334 Author's Address 1336 Fred L. Templin (editor) 1337 Boeing Research & Technology 1338 P.O. Box 3707 1339 Seattle, WA 98124 1340 USA 1342 Email: fltemplin@acm.org