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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: rfc5320, rfc5558, rfc5720, January 31, 2020 5 rfc6179, rfc6706 (if 6 approved) 7 Intended status: Standards Track 8 Expires: August 3, 2020 10 Asymmetric Extended Route Optimization (AERO) 11 draft-templin-intarea-6706bis-23 13 Abstract 15 This document specifies the operation of IP over tunnel virtual links 16 using Asymmetric Extended Route Optimization (AERO). AERO interfaces 17 use an IPv6 link-local address format that supports operation of the 18 IPv6 Neighbor Discovery (ND) protocol and links ND to IP forwarding. 19 Prefix delegation/registration services are employed for network 20 admission and to manage the routing system. Multilink operation, 21 mobility management, quality of service (QoS) signaling and route 22 optimization are naturally supported through dynamic neighbor cache 23 updates. Standard IP multicasting services are also supported. AERO 24 is a widely-applicable mobile internetworking service especially 25 well-suited to aviation services, mobile Virtual Private Networks 26 (VPNs) and many other applications. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at https://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on August 3, 2020. 45 Copyright Notice 47 Copyright (c) 2020 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (https://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 63 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 64 3. Asymmetric Extended Route Optimization (AERO) . . . . . . . . 10 65 3.1. AERO Link Reference Model . . . . . . . . . . . . . . . . 10 66 3.2. AERO Node Types . . . . . . . . . . . . . . . . . . . . . 12 67 3.3. AERO Routing System . . . . . . . . . . . . . . . . . . . 13 68 3.3.1. IPv4 Compatibility Routing . . . . . . . . . . . . . 15 69 3.4. AERO Addresses . . . . . . . . . . . . . . . . . . . . . 15 70 3.5. Spanning Partitioned AERO Networks (SPAN) . . . . . . . . 17 71 3.5.1. SPAN Compatibility Addressing . . . . . . . . . . . . 21 72 3.6. AERO Interface Characteristics . . . . . . . . . . . . . 21 73 3.7. AERO Interface Initialization . . . . . . . . . . . . . . 25 74 3.7.1. AERO Server/Gateway Behavior . . . . . . . . . . . . 25 75 3.7.2. AERO Proxy Behavior . . . . . . . . . . . . . . . . . 26 76 3.7.3. AERO Client Behavior . . . . . . . . . . . . . . . . 26 77 3.7.4. AERO Relay Behavior . . . . . . . . . . . . . . . . . 26 78 3.8. AERO Interface Neighbor Cache Maintenance . . . . . . . . 26 79 3.9. AERO Interface Encapsulation and Re-encapsulation . . . . 28 80 3.10. AERO Interface Decapsulation . . . . . . . . . . . . . . 29 81 3.11. AERO Interface Data Origin Authentication . . . . . . . . 30 82 3.12. AERO Interface Forwarding Algorithm . . . . . . . . . . . 30 83 3.12.1. Client Forwarding Algorithm . . . . . . . . . . . . 31 84 3.12.2. Proxy Forwarding Algorithm . . . . . . . . . . . . . 31 85 3.12.3. Server/Gateway Forwarding Algorithm . . . . . . . . 32 86 3.12.4. Relay Forwarding Algorithm . . . . . . . . . . . . . 34 87 3.13. AERO Interface MTU and Fragmentation . . . . . . . . . . 34 88 3.13.1. AERO MTU Requirements . . . . . . . . . . . . . . . 37 89 3.14. AERO Interface Error Handling . . . . . . . . . . . . . . 37 90 3.15. AERO Router Discovery, Prefix Delegation and 91 Autoconfiguration . . . . . . . . . . . . . . . . . . . . 40 92 3.15.1. AERO ND/PD Service Model . . . . . . . . . . . . . . 40 93 3.15.2. AERO Client Behavior . . . . . . . . . . . . . . . . 40 94 3.15.3. AERO Server Behavior . . . . . . . . . . . . . . . . 42 95 3.16. The AERO Proxy . . . . . . . . . . . . . . . . . . . . . 45 96 3.16.1. Detecting and Responding to Server Failures . . . . 47 97 3.17. AERO Route Optimization . . . . . . . . . . . . . . . . . 48 98 3.17.1. Route Optimization Initiation . . . . . . . . . . . 48 99 3.17.2. Relaying the NS . . . . . . . . . . . . . . . . . . 49 100 3.17.3. Processing the NS and Sending the NA . . . . . . . . 49 101 3.17.4. Relaying the NA . . . . . . . . . . . . . . . . . . 50 102 3.17.5. Processing the NA . . . . . . . . . . . . . . . . . 50 103 3.17.6. Route Optimization Maintenance . . . . . . . . . . . 50 104 3.18. Neighbor Unreachability Detection (NUD) . . . . . . . . . 51 105 3.19. Mobility Management and Quality of Service (QoS) . . . . 52 106 3.19.1. Mobility Update Messaging . . . . . . . . . . . . . 53 107 3.19.2. Announcing Link-Layer Address and/or QoS Preference 108 Changes . . . . . . . . . . . . . . . . . . . . . . 54 109 3.19.3. Bringing New Links Into Service . . . . . . . . . . 54 110 3.19.4. Removing Existing Links from Service . . . . . . . . 54 111 3.19.5. Moving to a New Server . . . . . . . . . . . . . . . 55 112 3.20. Multicast . . . . . . . . . . . . . . . . . . . . . . . . 56 113 3.20.1. Source-Specific Multicast (SSM) . . . . . . . . . . 56 114 3.20.2. Any-Source Multicast (ASM) . . . . . . . . . . . . . 58 115 3.20.3. Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . . 58 116 3.21. Operation over Multiple AERO Links (VLANs) . . . . . . . 59 117 3.22. DNS Considerations . . . . . . . . . . . . . . . . . . . 60 118 3.23. Transition Considerations . . . . . . . . . . . . . . . . 60 119 3.24. Detecting and Reacting to Server and Relay Failures . . . 61 120 4. Implementation Status . . . . . . . . . . . . . . . . . . . . 61 121 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 61 122 6. Security Considerations . . . . . . . . . . . . . . . . . . . 62 123 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 64 124 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 65 125 8.1. Normative References . . . . . . . . . . . . . . . . . . 65 126 8.2. Informative References . . . . . . . . . . . . . . . . . 66 127 Appendix A. AERO Alternate Encapsulations . . . . . . . . . . . 73 128 Appendix B. Non-Normative Considerations . . . . . . . . . . . . 75 129 B.1. Implementation Strategies for Route Optimization . . . . 75 130 B.2. Implicit Mobility Management . . . . . . . . . . . . . . 76 131 B.3. Direct Underlying Interfaces . . . . . . . . . . . . . . 76 132 B.4. AERO Clients on the Open Internetwork . . . . . . . . . . 76 133 B.5. Operation on AERO Links with /64 ASPs . . . . . . . . . . 77 134 B.6. AERO Adaptations for SEcure Neighbor Discovery (SEND) . . 77 135 B.7. AERO Critical Infrastructure Considerations . . . . . . . 78 136 B.8. AERO Server Failure Implications . . . . . . . . . . . . 79 137 B.9. AERO Client / Server Architecture . . . . . . . . . . . . 79 138 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 81 139 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 87 141 1. Introduction 143 Asymmetric Extended Route Optimization (AERO) fulfills the 144 requirements of Distributed Mobility Management (DMM) [RFC7333] and 145 route optimization [RFC5522] for aeronautical networking and other 146 network mobility use cases. AERO is based on a Non-Broadcast, 147 Multiple Access (NBMA) virtual link model known as the AERO link. 148 The AERO link is a virtual overlay configured over one or more 149 underlying Internetworks, and nodes on the link can exchange IP 150 packets via tunneling. Multilink operation allows for increased 151 reliability, bandwidth optimization and traffic path diversity. 153 The AERO service comprises Clients, Proxys, Servers and Gateways that 154 are seen as AERO link neighbors. Each node's AERO interface uses an 155 IPv6 link-local address format (known as the AERO address) that 156 supports operation of the IPv6 Neighbor Discovery (ND) protocol 157 [RFC4861] and links ND to IP forwarding. A node's AERO interface can 158 be configured over multiple underlying interfaces, and may therefore 159 may appear as a single interface with multiple link-layer addresses. 160 Each link-layer address is subject to change due to mobility and/or 161 QoS fluctuations, and link-layer address changes are signaled by ND 162 messaging the same as for any IPv6 link. 164 AERO links provide a cloud-based service where mobile nodes may use 165 any Server acting as a Mobility Anchor Point (MAP) and fixed nodes 166 may use any Gateway on the link for efficient communications. Fixed 167 nodes forward packets destined to other AERO nodes to the nearest 168 Gateway, which forwards them through the cloud. A mobile node's 169 initial packets are forwarded through the MAP, while direct routing 170 is supported through asymmetric extended route optimization while 171 data packets are flowing. Both unicast and multicast communications 172 are supported, and mobile nodes may efficiently move between 173 locations while maintaining continuous communications with 174 correspondents and without changing their IP Address. 176 AERO Relays are interconnected in a secured private BGP overlay 177 routing instance known as the "SPAN". The SPAN provides a hybrid 178 routing/bridging service to join the underlying Internetworks of 179 multiple disjoint administrative domains into a single unified AERO 180 link. Each AERO link instance is characterized by the set of 181 Mobility Service Prefixes (MSPs) common to all mobile nodes. The 182 link extends to the point where a Gateway/MAP is on the optimal route 183 from any correspondent node on the link, and provides a gateway 184 between the underlying Internetwork and the SPAN. To the underlying 185 Internetwork, the Gateway/MAP is the source of a route to its MSP, 186 and hence uplink traffic to the mobile node is naturally routed to 187 the nearest Gateway/MAP. 189 AERO assumes the use of PIM Sparse Mode in support of multicast 190 communication. In support of Source Specific Multicast (SSM) when a 191 Mobile Node is the source, AERO route optimization ensures that a 192 shortest-path multicast tree is established with provisions for 193 mobility and multilink operation. In all other multicast scenarios 194 there are no AERO dependencies. 196 AERO was designed for aeronautical networking for both manned and 197 unmanned aircraft, where the aircraft is treated as a mobile node 198 that can connect an Internet of Things (IoT). AERO is also 199 applicable to a wide variety of other use cases. For example, it can 200 be used to coordinate the Virtual Private Network (VPN) links of 201 mobile nodes (e.g., cellphones, tablets, laptop computers, etc.) that 202 connect into a home enterprise network via public access networks 203 using services such as OpenVPN [OVPN]. Other applicable use cases 204 are also in scope. 206 The following numbered sections present the AERO specification. The 207 appendices at the end of the document are non-normative. 209 2. Terminology 211 The terminology in the normative references applies; the following 212 terms are defined within the scope of this document: 214 IPv6 Neighbor Discovery (ND) 215 an IPv6 control message service for coordinating neighbor 216 relationships between nodes connected to a common link. AERO 217 interfaces use the ND service specified in [RFC4861]. 219 IPv6 Prefix Delegation (PD) 220 a networking service for delegating IPv6 prefixes to nodes on the 221 link. The nominal PD service is DHCPv6 [RFC8415], however 222 alternate services (e.g., based on ND messaging) are also in scope 223 [I-D.templin-v6ops-pdhost][I-D.templin-6man-dhcpv6-ndopt]. Most 224 notably, a minimal form of PD known as "prefix registration" can 225 be used if the Client knows its prefix in advance and can 226 represent it in the IPv6 source address of an ND message. 228 Access Network (ANET) 229 a node's first-hop data link service network, e.g., a radio access 230 network, cellular service provider network, corporate enterprise 231 network, or the public Internet itself. For secured ANETs, link- 232 layer security services such as IEEE 802.1X and physical-layer 233 security prevent unauthorized access internally while border 234 network-layer security services such as firewalls and proxies 235 prevent unauthorized outside access. 237 ANET interface 238 a node's attachment to a link in an ANET. 240 ANET address 241 an IP address assigned to a node's interface connection to an 242 ANET. 244 Internetwork (INET) 245 a connected IP network topology with a coherent routing and 246 addressing plan and that provides a transit backbone service for 247 ANET end systems. INETs also provide an underlay service over 248 which the AERO virtual link is configured. Example INETs include 249 corporate enterprise networks, aviation networks, and the public 250 Internet itself. When there is no administrative boundary between 251 an ANET and the INET, the ANET and INET are one and the same. 253 INET Partition 254 frequently, INETs such as large corporate enterprise networks are 255 sub-divided internally into separate isolated partitions. Each 256 partition is fully connected internally but disconnected from 257 other partitions, and there is no requirement that separate 258 partitions maintain consistent Internet Protocol and/or addressing 259 plans. (An INET partition is the same as a SPAN segment discussed 260 below.) 262 INET interface 263 a node's attachment to a link in an INET. 265 INET address 266 an IP address assigned to a node's interface connection to an 267 INET. 269 AERO link 270 a Non-Broadcast, Multiple Access (NBMA) tunnel virtual overlay 271 configured over one or more underlying INETs. Nodes on the AERO 272 link appear as single-hop neighbors from the perspective of the 273 virtual overlay even though they may be separated by many 274 underlying INET hops. AERO links may be configured over multiple 275 underlying SPAN segments (see below). 277 AERO interface 278 a node's attachment to an AERO link. Since the addresses assigned 279 to an AERO interface are managed for uniqueness, AERO interfaces 280 do not require Duplicate Address Detection (DAD) and therefore set 281 the administrative variable 'DupAddrDetectTransmits' to zero 282 [RFC4862]. 284 underlying interface 285 an ANET or INET interface over which an AERO interface is 286 configured. 288 AERO address 289 an IPv6 link-local address assigned to an AERO interface and 290 constructed as specified in Section 3.4. 292 base AERO address 293 the lowest-numbered AERO address aggregated by the MNP (see 294 Section 3.4). 296 Mobility Service Prefix (MSP) 297 an IP prefix assigned to the AERO link and from which more- 298 specific Mobile Network Prefixes (MNPs) are derived. 300 Mobile Network Prefix (MNP) 301 an IP prefix allocated from an MSP and delegated to an AERO Client 302 or Gateway. 304 AERO node 305 a node that is connected to an AERO link, or that provides 306 services to other nodes on an AERO link. 308 AERO Client ("Client") 309 an AERO node that connects to one or more ANETs and requests MNP 310 PDs from AERO Servers. The Client assigns a Client AERO address 311 to the AERO interface for use in ND exchanges with other AERO 312 nodes and forwards packets to correspondents according to AERO 313 interface neighbor cache state. 315 AERO Server ("Server") 316 an INET node that configures an AERO interface to provide default 317 forwarding services and a Mobility Anchor Point (MAP) for AERO 318 Clients. The Server assigns an administratively-provisioned AERO 319 address to its AERO interface to support the operation of the ND/ 320 PD services, and advertises all of its associated MNPs via BGP 321 peerings with Relays. 323 AERO Gateway ("Gateway") 324 an AERO Server that also provides forwarding services between 325 nodes reached via the AERO link and correspondents on other links. 326 AERO Gateways are provisioned with MNPs (i.e., the same as for an 327 AERO Client) and run a dynamic routing protocol to discover any 328 non-MNP IP routes. In both cases, the Gateway advertises the 329 MSP(s) over INET interfaces, and distributes all of its associated 330 MNPs and non-MNP IP routes via BGP peerings with Relays (i.e., the 331 same as for an AERO Server). 333 AERO Relay ("Relay") 334 a node that provides hybrid routing/bridging services (as well as 335 a security trust anchor) for nodes on an AERO link. As a router, 336 the Relay forwards packets using standard IP forwarding. As a 337 bridge, the Relay forwards packets over the AERO link without 338 decrementing the IPv6 Hop Limit. AERO Relays peer with Servers 339 and other Relays to discover the full set of MNPs for the link as 340 well as any non-MNPs that are reachable via Gateways. 342 AERO Proxy ("Proxy") 343 a node that provides proxying services between Clients in an ANET 344 and Servers in external INETs. The AERO Proxy is a conduit 345 between the ANET and external INETs in the same manner as for 346 common web proxies, and behaves in a similar fashion as for ND 347 proxies [RFC4389]. 349 Spanning Partitioned AERO Networks (SPAN) 350 a means for bridging disjoint INET partitions as segments of a 351 unified AERO link the same as for a bridged campus LAN. The SPAN 352 is a mid-layer IPv6 encapsulation service in the AERO routing 353 system that supports a unified AERO link view for all segments. 354 Each segment in the SPAN is a distinct INET partition. 356 SPAN Service Prefix (SSP) 357 a global or unique local /96 IPv6 prefix assigned to the AERO link 358 to support SPAN services. 360 SPAN Partition Prefix (SPP) 361 a sub-prefix of the SPAN Service Prefix uniquely assigned to a 362 single SPAN segment. 364 SPAN Address 365 a global or unique local IPv6 address taken from a SPAN Partition 366 Prefix and constructed as specified in Section 3.5. SPAN 367 addresses are statelessly derived from AERO addresses, and vice- 368 versa. 370 ingress tunnel endpoint (ITE) 371 an AERO interface endpoint that injects encapsulated packets into 372 an AERO link. 374 egress tunnel endpoint (ETE) 375 an AERO interface endpoint that receives encapsulated packets from 376 an AERO link. 378 link-layer address 379 an IP address used as an encapsulation header source or 380 destination address from the perspective of the AERO interface. 382 When an upper layer protocol (e.g., UDP) is used as part of the 383 encapsulation, the port number is also considered as part of the 384 link-layer address. From the perspective of the AERO interface, 385 the link-layer address is either an INET address for intra-segment 386 encapsulation or a SPAN address for inter-segment encapsulation. 388 network layer address 389 the source or destination address of an encapsulated IP packet 390 presented to the AERO interface. 392 end user network (EUN) 393 an internal virtual or external edge IP network that an AERO 394 Client or Gateway connects to the rest of the network via the AERO 395 interface. The Client/Gateway sees each EUN as a "downstream" 396 network, and sees the AERO interface as the point of attachment to 397 the "upstream" network. 399 Mobile Node (MN) 400 an AERO Client and all of its downstream-attached networks that 401 move together as a single unit, i.e., an end system that connects 402 an Internet of Things. 404 Mobile Router (MR) 405 a MN's on-board router that forwards packets between any 406 downstream-attached networks and the AERO link. 408 Mobility Anchor Point (MAP) 409 an AERO Server that is currently tracking and reporting the 410 mobility events of its associated Mobile Node Clients. 412 Route Optimization Source (ROS) 413 the AERO node nearest the source that initiates route 414 optimization. The ROS may be a Server or Proxy acting on behalf 415 of the source Client. 417 Route Optimization responder (ROR) 418 the AERO node nearest the target destination that responds to 419 route optimization requests. The ROR may be a Server acting as a 420 MAP on behalf of a target MNP Client, or a Gateway for a non-MNP 421 destination. 423 MAP List 424 a geographically and/or topologically referenced list of AERO 425 addresses of all MAPs within the same AERO link. There is a 426 single MAP list for the entire AERO link. 428 ROS List 429 a list of AERO/SPAN-to-INET address mappings of all ROSes within 430 the same SPAN segment. There is a distinct ROS list for each 431 segment. 433 Distributed Mobility Management (DMM) 434 a BGP-based overlay routing service coordinated by Servers and 435 Relays that tracks all MAP-to-Client associations. 437 Throughout the document, the simple terms "Client", "Server", 438 "Relay", "Proxy" and "Gateway" refer to "AERO Client", "AERO Server", 439 "AERO Relay", "AERO Proxy" and "AERO Gateway", respectively. 440 Capitalization is used to distinguish these terms from other common 441 Internetworking uses in which they appear without capitalization. 443 The terminology of DHCPv6 [RFC8415] and IPv6 ND [RFC4861] (including 444 the names of node variables, messages and protocol constants) is used 445 throughout this document. Also, the term "IP" is used to generically 446 refer to either Internet Protocol version, i.e., IPv4 [RFC0791] or 447 IPv6 [RFC8200]. 449 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 450 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 451 "OPTIONAL" in this document are to be interpreted as described in BCP 452 14 [RFC2119][RFC8174] when, and only when, they appear in all 453 capitals, as shown here. 455 3. Asymmetric Extended Route Optimization (AERO) 457 The following sections specify the operation of IP over Asymmetric 458 Extended Route Optimization (AERO) links: 460 3.1. AERO Link Reference Model 461 +----------------+ 462 | AERO Relay R1 | 463 | Nbr: S1, S2, P1| 464 |(X1->S1; X2->S2)| 465 | MSP M1 | 466 +-+---------+--+-+ 467 +--------------+ | Secured | | +--------------+ 468 |AERO Server S1| | tunnels | | |AERO Server S2| 469 | Nbr: C1, R1 +-----+ | +-----+ Nbr: C2, R1 | 470 | default->R1 | | | default->R1 | 471 | X1->C1 | | | X2->C2 | 472 +-------+------+ | +------+-------+ 473 | AERO Link | | 474 X===+===+===================+==)===============+===+===X 475 | | | | 476 +-----+--------+ +--------+--+-----+ +--------+-----+ 477 |AERO Client C1| | AERO Proxy P1 | |AERO Client C2| 478 | Nbr: S1 | |(Proxy Nbr Cache)| | Nbr: S2 | 479 | default->S1 | +--------+--------+ | default->S2 | 480 | MNP X1 | | | MNP X2 | 481 +------+-------+ .--------+------. +-----+--------+ 482 | (- Proxyed Clients -) | 483 .-. `---------------' .-. 484 ,-( _)-. ,-( _)-. 485 .-(_ IP )-. +-------+ +-------+ .-(_ IP )-. 486 (__ EUN )--|Host H1| |Host H2|--(__ EUN ) 487 `-(______)-' +-------+ +-------+ `-(______)-' 489 Figure 1: AERO Link Reference Model 491 Figure 1 presents the AERO link reference model. In this model: 493 o the AERO link is an overlay network service configured over one or 494 more underlying INET partitions which may be managed by different 495 administrative authorities and have incompatible protocols and/or 496 addressing plans. 498 o AERO Relay R1 aggregates Mobility Service Prefix (MSP) M1, 499 discovers Mobile Network Prefixes (MNPs) X* and advertises the MSP 500 via BGP peerings over secured tunnels to Servers (S1, S2). Relays 501 use the SPAN service to bridge disjoint segments of a partitioned 502 AERO link. 504 o AERO Servers S1 and S2 configure secured tunnels with Relay R1 and 505 also act as Mobility Anchor Points (MAPs) and default routers for 506 their associated Clients C1 and C2. 508 o AERO Clients C1 and C2 associate with Servers S1 and S2, 509 respectively. They receive Mobile Network Prefix (MNP) 510 delegations X1 and X2, and also act as default routers for their 511 associated physical or internal virtual EUNs. Simple hosts H1 and 512 H2 attach to the EUNs served by Clients C1 and C2, respectively. 514 o AERO Proxy P1 configures a secured tunnel with Relay R1 and 515 provides proxy services for AERO Clients in secured enclaves that 516 cannot associate directly with other AERO link neighbors. 518 Each node on the AERO link maintains an AERO interface neighbor cache 519 and an IP forwarding table the same as for any link. Although the 520 figure shows a limited deployment, in common operational practice 521 there will normally be many additional Relays, Servers, Clients and 522 Proxys. 524 3.2. AERO Node Types 526 AERO Relays provide hybrid routing/bridging services (as well as a 527 security trust anchor) for nodes on an AERO link. Relays use 528 standard IPv6 routing to forward packets both within the same INET 529 partitions and between disjoint INET partitions based on a mid-layer 530 IPv6 encapsulation known as the SPAN header. The inner IP layer 531 experiences a virtual bridging service since the inner IP TTL/Hop 532 Limit is not decremented during forwarding. Each Relay also peers 533 with Servers and other Relays in a dynamic routing protocol instance 534 to provide a Distributed Mobility Management (DMM) service for the 535 list of active MNPs (see Section 3.3). Relays present the AERO link 536 as a set of one or more Mobility Service Prefixes (MSPs) but as link- 537 layer devices need not connect directly to the AERO link themselves 538 unless an administrative interface is desired. Relays configure 539 secured tunnels with Servers, Proxys and other Relays; they further 540 maintain IP forwarding table entries for each Mobile Network Prefix 541 (MNP) and any other reachable non-MNP prefixes. 543 AERO Servers provide default forwarding services and a Mobility 544 Anchor Point (MAP) for AERO Client Mobile Nodes (MNs). Each Server 545 also peers with Relays in a dynamic routing protocol instance to 546 advertise its list of associated MNPs (see Section 3.3). Servers 547 facilitate PD exchanges with Clients, where each delegated prefix 548 becomes an MNP taken from an MSP. Servers forward packets between 549 AERO interface neighbors and track each Client's mobility profiles. 551 AERO Clients register their MNPs through PD exchanges with AERO 552 Servers over the AERO link, and distribute the MNPs to nodes on EUNs. 553 A Client may also be co-resident on the same physical or virtual 554 platform as a Server; in that case, the Client and Server behave as a 555 single functional unit. 557 AERO Proxys provide a conduit for ANET AERO Clients to associate with 558 AERO Servers in external INETs. Client and Servers exchange control 559 plane messages via the Proxy acting as a bridge between the ANET/INET 560 boundary. The Proxy forwards data packets between Clients and the 561 AERO link according to forwarding information in the neighbor cache. 562 The Proxy function is specified in Section 3.16. 564 AERO Gateways are Servers that provide forwarding services between 565 the AERO interface and INET/EUN interfaces. Gateways are provisioned 566 with MNPs the same as for an AERO Client, and also run a dynamic 567 routing protocol to discover any non-MNP IP routes. The Gateway 568 advertises the MSP(s) to INETs, and distributes all of its associated 569 MNPs and non-MNP IP routes via BGP peerings with Relays. 571 AERO Relays, Servers, Proxys and Gateways are critical infrastructure 572 elements in fixed (i.e., non-mobile) INET deployments and hence have 573 permanent and unchanging INET addresses. AERO Clients are MNs that 574 connect via ANET interfaces, i.e., their ANET addresses may change 575 when the Client moves to a new ANET connection. 577 3.3. AERO Routing System 579 The AERO routing system comprises a private instance of the Border 580 Gateway Protocol (BGP) [RFC4271] that is coordinated between Relays 581 and Servers and does not interact with either the public Internet BGP 582 routing system or any underlying INET routing systems. 584 In a reference deployment, each Server is configured as an Autonomous 585 System Border Router (ASBR) for a stub Autonomous System (AS) using 586 an AS Number (ASN) that is unique within the BGP instance, and each 587 Server further uses eBGP to peer with one or more Relays but does not 588 peer with other Servers. Each INET of a multi-segment AERO link must 589 include one or more Relays, which peer with the Servers and Proxys 590 within that INET. All Relays within the same INET are members of the 591 same hub AS using a common ASN, and use iBGP to maintain a consistent 592 view of all active MNPs currently in service. The Relays of 593 different INETs peer with one another using eBGP. 595 Relays advertise the AERO link's MSPs and any non-MNP routes to each 596 of their Servers. This means that any aggregated non-MNPs (including 597 "default") are advertised to all Servers. Each Relay configures a 598 black-hole route for each of its MSPs. By black-holing the MSPs, the 599 Relay will maintain forwarding table entries only for the MNPs that 600 are currently active, and packets destined to all other MNPs will 601 correctly incur Destination Unreachable messages due to the black- 602 hole route. In this way, Servers have only partial topology 603 knowledge (i.e., they know only about the MNPs of their directly 604 associated Clients) and they forward all other packets to Relays 605 which have full topology knowledge. 607 Servers maintain a working set of associated MNPs, and dynamically 608 announce new MNPs and withdraw departed MNPs in eBGP updates to 609 Relays. Servers that are configured as Gateways also redistribute 610 non-MNP routes learned from non-AERO interfaces via their eBGP Relay 611 peerings. 613 Clients are expected to remain associated with their current Servers 614 for extended timeframes, however Servers SHOULD selectively suppress 615 updates for impatient Clients that repeatedly associate and 616 disassociate with them in order to dampen routing churn. Servers 617 that are configured as Gateways advertise the MSPs via INET/EUN 618 interfaces, and forward packets between INET/EUN interfaces and the 619 AERO interface using standard IP forwarding. 621 Scaling properties of the AERO routing system are limited by the 622 number of BGP routes that can be carried by Relays. As of 2015, the 623 global public Internet BGP routing system manages more than 500K 624 routes with linear growth and no signs of router resource exhaustion 625 [BGP]. More recent network emulation studies have also shown that a 626 single Relay can accommodate at least 1M dynamically changing BGP 627 routes even on a lightweight virtual machine, i.e., and without 628 requiring high-end dedicated router hardware. 630 Therefore, assuming each Relay can carry 1M or more routes, this 631 means that at least 1M Clients can be serviced by a single set of 632 Relays. A means of increasing scaling would be to assign a different 633 set of Relays for each set of MSPs. In that case, each Server still 634 peers with one or more Relays, but institutes route filters so that 635 BGP updates are only sent to the specific set of Relays that 636 aggregate the MSP. For example, if the MSP for the AERO link is 637 2001:db8::/32, a first set of Relays could service the MSP 638 2001:db8::/40, a second set of Relays could service 639 2001:db8:0100::/40, a third set could service 2001:db8:0200::/40, 640 etc. 642 Assuming up to 1K sets of Relays, the AERO routing system can then 643 accommodate 1B or more MNPs with no additional overhead (for example, 644 it should be possible to service 1B /64 MNPs taken from a /34 MSP and 645 even more for shorter prefixes). In this way, each set of Relays 646 services a specific set of MSPs that they advertise to the native 647 Internetwork routing system, and each Server configures MSP-specific 648 routes that list the correct set of Relays as next hops. This 649 arrangement also allows for natural incremental deployment, and can 650 support small scale initial deployments followed by dynamic 651 deployment of additional Clients, Servers and Relays without 652 disturbing the already-deployed base. 654 Server and Relays can use the Bidirectional Forwarding Detection 655 (BFD) protocol [RFC5880] to quickly detect link failures that don't 656 result in interface state changes, BGP peer failures, and 657 administrative state changes. BFD is important in environments where 658 rapid response to failures is required for routing reconvergence and, 659 hence, communications continuity. 661 A full discussion of the BGP-based routing system used by AERO is 662 found in [I-D.ietf-rtgwg-atn-bgp]. The system provides for 663 Distributed Mobility Management (DMM) per the distributed mobility 664 anchoring architecture [I-D.ietf-dmm-distributed-mobility-anchoring]. 666 3.3.1. IPv4 Compatibility Routing 668 For IPv6 MNPs, the AERO routing system includes ordinary IPv6 routes. 669 For IPv4 MNPs, the AERO routing system includes IPv6 routes based on 670 an IPv4-embedded IPv6 address format discussed in Section 3.5.1. 672 3.4. AERO Addresses 674 A Client's AERO address is an IPv6 link-local address with an 675 interface identifier based on the Client's delegated MNP. Relay, 676 Server and Proxy AERO addresses are assigned from the range fe80::/96 677 and include an administratively-provisioned value in the lower 32 678 bits. 680 For IPv6, Client AERO addresses begin with the prefix fe80::/64 and 681 include in the interface identifier (i.e., the lower 64 bits) a 682 64-bit prefix taken from one of the Client's IPv6 MNPs. For example, 683 if the AERO Client receives the IPv6 MNP: 685 2001:db8:1000:2000::/56 687 it constructs its corresponding AERO addresses as: 689 fe80::2001:db8:1000:2000 691 fe80::2001:db8:1000:2001 693 fe80::2001:db8:1000:2002 695 ... etc. ... 697 fe80::2001:db8:1000:20ff 699 For IPv4, Client AERO addresses are based on an IPv4-mapped IPv6 700 address [RFC4291] formed from an IPv4 MNP and with a Prefix Length of 701 96 plus the MNP prefix length. For example, for the IPv4 MNP 702 192.0.2.32/28 the IPv4-mapped IPv6 MNP is: 704 0:0:0:0:0:FFFF:192.0.2.16/124 (also written as 705 0:0:0:0:0:FFFF:c000:0210/124) 707 The Client then constructs its AERO addresses with the prefix 708 fe80::/64 and with the lower 64 bits of the IPv4-mapped IPv6 address 709 in the interface identifier as: 711 fe80::FFFF:192.0.2.16 713 fe80::FFFF:192.0.2.17 715 fe80::FFFF:192.0.2.18 717 ... etc. ... 719 fe80:FFFF:192.0.2.31 721 Relay, Server and Proxy AERO addresses are allocated from the range 722 fe80::/96, and MUST be managed for uniqueness. The lower 32 bits of 723 the AERO address includes a unique integer value between 1 and 724 0xfffffffe (e.g., fe80::1, fe80::2, fe80::3, etc., fe80::ffff:fffe) 725 as assigned by the administrative authority for the link. If the 726 link spans multiple SPAN segments, the AERO addresses are assigned to 727 each segment in 1x1 correspondence with SPAN addresses (see: 728 Section 3.5). The address fe80:: is the IPv6 link-local Subnet 729 Router Anycast address, and the address fe80::ffff:ffff is reserved 730 as the unspecified AERO address. 732 The lowest-numbered AERO address from a Client's MNP delegation 733 serves as the "base" AERO address (for example, for the MNP 734 2001:db8:1000:2000::/56 the base AERO address is 735 fe80::2001:db8:1000:2000). The Client then assigns the base AERO 736 address to the AERO interface and uses it for the purpose of 737 maintaining the neighbor cache entry. The Server likewise uses the 738 AERO address as its index into the neighbor cache for this Client. 740 If the Client has multiple AERO addresses (i.e., when there are 741 multiple MNPs and/or MNPs with prefix lengths shorter than /64), the 742 Client originates ND messages using the base AERO address as the 743 source address and accepts and responds to ND messages destined to 744 any of its AERO addresses as equivalent to the base AERO address. In 745 this way, the Client maintains a single neighbor cache entry that may 746 be indexed by multiple AERO addresses. 748 The Client's Subnet Router Anycast address can be statelessly 749 determined from its AERO address by simply transposing the AERO 750 address into the upper N bits of the Anycast address followed by 751 128-N bits of zeroes. For example, for the AERO address 752 fe80::2001:db8:1:2 the subnet router anycast address is 753 2001:db8:1:2::. 755 AERO addresses for mobile node Clients embed a MNP as discussed 756 above, while AERO addresses for non-MNP destinations are constructed 757 in exactly the same way. A Client AERO address therefore encodes 758 either an MNP if the prefix is reached via the SPAN or a non-MNP if 759 the prefix is reached via a Gateway. 761 3.5. Spanning Partitioned AERO Networks (SPAN) 763 An AERO link configured over a single INET appears as a single 764 unified link with a consistent underlying network addressing plan. 765 In that case, all nodes on the link can exchange packets via simple 766 INET encapsulation, since the underlying INET is connected. In 767 common practice, however, an AERO link may be partitioned into 768 multiple "segments", where each segment is a distinct INET 769 potentially managed under a different administrative authority (e.g., 770 as for worldwide aviation service providers such as ARINC, SITA, 771 Inmarsat, etc.). Individual INETs may also themselves be partitioned 772 internally, in which case each internal partition is seen as a 773 separate segment. 775 The addressing plan of each segment is consistent internally but will 776 often bear no relation to the addressing plans of other segments. 777 Each segment is also likely to be separated from others by network 778 security devices (e.g., firewalls, proxies, packet filtering 779 gateways, etc.), and in many cases disjoint segments may not even 780 have any common physical link connections at all. Therefore, nodes 781 can only be assured of exchanging packets directly with 782 correspondents in the same segment, and not with those in other 783 segments. The only means for joining the segments therefore is 784 through inter-domain peerings between AERO Relays. 786 The same as for traditional campus LANs, multiple AERO link segments 787 can be joined into a single unified link via a virtual bridging 788 service termed the "SPAN". The SPAN performs link-layer packet 789 forwarding between segments (i.e., bridging) without decrementing the 790 network-layer TTL/Hop Limit. The SPAN model is depicted in Figure 2: 792 . . . . . . . . . . . . . . . . . . . . . . . 793 . . 794 . .-(::::::::) . 795 . .-(::::::::::::)-. +-+ . 796 . (:::: Segment A :::)--|R|---+ . 797 . `-(::::::::::::)-' +-+ | . 798 . `-(::::::)-' | . 799 . | . 800 . .-(::::::::) | . 801 . .-(::::::::::::)-. +-+ | . 802 . (:::: Segment B :::)--|R|---+ . 803 . `-(::::::::::::)-' +-+ | . 804 . `-(::::::)-' | . 805 . | . 806 . .-(::::::::) | . 807 . .-(::::::::::::)-. +-+ | . 808 . (:::: Segment C :::)--|R|---+ . 809 . `-(::::::::::::)-' +-+ | . 810 . `-(::::::)-' | . 811 . | . 812 . ..(etc).. x . 813 . . 814 . . 815 . <- AERO Link Bridged by the SPAN -> . 816 . . . . . . . . . . . . . .. . . . . . . . . 818 Figure 2: The SPAN 820 To support the SPAN, AERO links require a reserved /64 IPv6 "SPAN 821 Service Prefix (SSP)". Although any routable IPv6 prefix can be 822 used, a Unique Local Address (ULA) prefix (e.g., fd00::/64) [RFC4389] 823 is recommended since border routers are commonly configured to 824 prevent packets with ULAs from being injected into the AERO link by 825 an external IPv6 node and from leaking out of the AERO link to the 826 outside world. 828 Each segment in the SPAN assigns a unique sub-prefix of SSP::/96 829 termed a "SPAN Partition Prefix (SPP)". For example, a first segment 830 could assign fd00::1000/116, a second could assign fd00::2000/116, a 831 third could assign fd00::3000/116, etc. The administrative 832 authorities for each segment must therefore coordinate to assure 833 mutually-exclusive SPP assignments, but internal provisioning of the 834 SPP is an independent local consideration for each administrative 835 authority. 837 A "SPAN address" is an address taken from a SPP and assigned to a 838 Relay, Server, Gateway or Proxy interface. SPAN addresses are formed 839 by simply replacing the upper portion of an administratively-assigned 840 AERO address with the SPP. For example, if the SPP is 841 fd00::1000/116, the SPAN address formed from the AERO address 842 fe80::1001 is simply fd00::1001. 844 An "INET address" is an address of a node's interface connection to 845 an INET. AERO/SPAN/INET address mappings are maintained as permanent 846 neighbor cache entires as discussed in Section 3.8. 848 AERO Relays serve as bridges to join multiple segments into a unified 849 AERO link over multiple diverse administrative domains. They support 850 the bridging function by first establishing forwarding table entries 851 for their SPPs either via standard BGP routing or static routes. For 852 example, if three Relays ('A', 'B' and 'C') from different segments 853 serviced the SPPs fd00::1000/116, fd00::2000/116 and fd00::3000/116 854 respectively, then the forwarding tables in each Relay are as 855 follows: 857 A: fd00::1000/116->local, fd00::2000/116->B, fd00::3000/116->C 859 B: fd00::1000/116->A, fd00::2000/116->local, fd00::3000/116->C 861 C: fd00::1000/116->A, fd00::2000/116->B, fd00::3000/116->local 863 These forwarding table entries are permanent and never change, since 864 they correspond to fixed infrastructure elements in their respective 865 segments. This provides the basis for a link-layer forwarding 866 service that cannot be disrupted by routing updates due to node 867 mobility. 869 With the SPPs in place in each Relay's forwarding table, control and 870 data packets sent between AERO nodes in different segments can 871 therefore be carried over the SPAN via encapsulation. For example, 872 when a source AERO node in segment A forwards a packet with IPv6 873 address 2001:db8:1:2::1 to a target AERO node in segment C with IPv6 874 address 2001:db8:1000:2000::1, it first encapsulates the packet in a 875 SPAN header with source SPAN address taken from fd00::1000/116 (e.g., 876 fd00::1001) and destination SPAN address taken from fd00::3000/116 877 (e.g., fd00::3001). Next, it encapsulates the SPAN message in an 878 INET header with source address set to its own INET address (e.g., 879 192.0.2.100) and destination set to the INET address of a Relay 880 (e.g., 192.0.2.1). 882 SPAN encapsulation is based on Generic Packet Tunneling in IPv6 883 [RFC2473]; the encapsulation format in the above example is shown in 884 Figure 3: 886 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 887 | INET Header | 888 | src = 192.0.2.100 | 889 | dst = 192.0.2.1 | 890 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 891 | SPAN Header | 892 | src = fd00::1001 | 893 | dst = fd00::3001 | 894 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 895 | Inner IP Header | 896 | src = 2001:db8:1:2::1 | 897 | dst = 2001:db8:1000:2000::1 | 898 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 899 | | 900 ~ ~ 901 ~ Inner Packet Body ~ 902 ~ ~ 903 | | 904 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 906 Figure 3: SPAN Encapsulation 908 In this format, the inner IP header and packet body are the original 909 IP packet, the SPAN header is an IPv6 header prepared according to 910 [RFC2473], and the INET header is prepared according to Section 3.9. 911 A packet is said to be "forwarded/sent into the SPAN" when it is 912 encapsulated as described above then forwarded via a secured tunnel 913 to a neighboring Relay. 915 This gives rise to a routing system that contains both MNP routes 916 that may change dynamically due to regional node mobility and SPAN 917 routes that never change. The Relays can therefore provide link- 918 layer bridging by sending packets into the SPAN instead of network- 919 layer routing according to MNP routes. As a result, opportunities 920 for packet loss due to node mobility between different segments are 921 mitigated. 923 With reference to Figure 3, for a Client's AERO address the SPAN 924 address is simply set to the Subnet Router Anycast address. For non- 925 link-local addresses, the destination SPAN address may not be known 926 in advance for the first few packets of a flow sent via the SPAN. In 927 that case, the SPAN destination address is set to the original 928 packet's destination, and the SPAN routing system will direct the 929 packet to the correct SPAN egress node. (In the above example, the 930 SPAN destination address is simply 2001:db8:1000:2000::1.) 932 3.5.1. SPAN Compatibility Addressing 934 For IPv4 MNPs, Servers inject a "SPAN Compatibility Prefix (SCP)" 935 that embeds the MNP into the BGP routing system. The SCP begins with 936 the upper 64 bits of the SSP, followed by the constant string 937 "0000:FFFF" followed by the IPv4 MNP. For example, if the SSP is 938 fd00::/64 and the MNP is 192.0.2.0/24 then the SCP is 939 fd00::FFFF:192.0.2.0/120. 941 This allows for encapsulation of IPv4 packets in IPv6 headers with 942 "SPAN Compatibility Addresses (SCAs)". In this example, the SCA 943 corresponding to the SCP is simply fd00::FFFF:192.0.2.0, and can be 944 used as the SPAN destination address for packets forwarded via the 945 SPAN. This allows for forwarding of initial IPv4 packets over IPv6 946 SPAN routes, followed by route optimization for direct 947 communications. 949 3.6. AERO Interface Characteristics 951 AERO interfaces are virtual interfaces configured over one or more 952 underlying interfaces classified as follows: 954 o Native interfaces have global IP addresses that are reachable from 955 any INET correspondent. All Server, Gateway and Relay interfaces 956 are native interfaces, as are INET-facing interfaces of Proxys. 958 o NATed interfaces connect to a private network behind a Network 959 Address Translator (NAT). The NAT does not participate in any 960 AERO control message signaling, but the Server can issue control 961 messages on behalf of the Client. Clients that are behind a NAT 962 are required to send periodic keepalive messages to keep NAT state 963 alive when there are no data packets flowing. If no other 964 periodic messaging service is available, the Client can send RS 965 messages to receive RA replies from its Server(s). 967 o VPNed interfaces use security encapsulation to a Virtual Private 968 Network (VPN) server that also acts as an AERO Server. As with 969 NATed links, the Server can issue control messages on behalf of 970 the Client, but the Client need not send periodic keepalives in 971 addition to those already used to maintain the VPN connection. 973 o Proxyed interfaces connect to an ANET that is separated from the 974 open INET by an AERO Proxy. Unlike NATed and VPNed interfaces, 975 the Proxy can actively issue control messages on behalf of the 976 Client. 978 o Direct interfaces connect a Client directly to a neighbor without 979 crossing any ANET/INET paths. An example is a line-of-sight link 980 between a remote pilot and an unmanned aircraft. 982 AERO interfaces use encapsulation (see: Section 3.9) to exchange 983 packets with AERO link neighbors over Native, NATed or VPNed 984 interfaces. AERO interfaces do not use encapsulation over Proxyed 985 and Direct underlying interfaces. 987 AERO interfaces maintain a neighbor cache for tracking per-neighbor 988 state the same as for any interface. AERO interfaces use ND messages 989 including Router Solicitation (RS), Router Advertisement (RA), 990 Neighbor Solicitation (NS) and Neighbor Advertisement (NA) for 991 neighbor cache management. 993 AERO interfaces send ND messages with an Overlay Multilink Network 994 Interface (OMNI) option formatted as specified in 995 [I-D.templin-atn-aero-interface]. The OMNI option includes prefix 996 registration information and "ifIndex-tuples" containing link quality 997 information for the AERO interface's underlying interfaces. 999 When encapsulation is used, AERO interface ND messages MAY also 1000 include an AERO Source/Target Link-Layer Address Option (S/TLLAO) 1001 formatted as shown in Figure 4: 1003 0 1 2 3 1004 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 1005 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1006 | Type | Length | ifIndex[1] |V| Reserved[1] | 1007 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1008 ~ Link Layer Address [1] ~ 1009 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1010 | Port Number [1] | ifIndex[2] |V| Reserved[2] | 1011 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1012 ~ Link Layer Address [2] ~ 1013 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1014 | Port Number [2] | ~ 1015 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ 1016 ~ ~ 1017 ~ ... ~ 1018 ~ ~ 1019 ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1020 ~ | ifIndex[N] |V| Reserved[N] | 1021 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1022 ~ Link Layer Address [N] ~ 1023 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1024 | Port Number [N] | Trailing zero padding | 1025 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1026 | Trailing zero padding (if necessary) | 1027 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1029 Figure 4: AERO Source/Target Link-Layer Address Option (S/TLLAO) 1030 Format 1032 In this format, Type and Length are set the same as specified for S/ 1033 TLLAOs in [RFC4861], with trailing zero padding octets added as 1034 necessary to produce an integral number of 8 octet blocks. The S/ 1035 TLLAO includes N ifIndex-tuples in correspondence to ifIndex-tuples 1036 that appear in the OMNI option. Each ifIndex-tuple includes the 1037 folllowing information: 1039 o ifIndex[i] - the same value as in the corresponding ifIndex-tuple 1040 included in the OMNI option. 1042 o V[i] - a bit that identifies the IP protocol version of the 1043 address found in the Link Layer Address [i] field. The bit is set 1044 to 0 for IPv4 or 1 for IPv6. 1046 o Reserved[i] - MUST encode the value 0 on transmission, and ignored 1047 on reception. 1049 o Link Layer Address [i] - the IPv4 or IPv6 address used as the 1050 encapsulation source address. The field is 4 bytes in length for 1051 IPv4 or 16 bytes in length for IPv6. 1053 o Port Number [i] - the upper layer protocol port number used as the 1054 encapsulation source port, or 0 when no upper layer protocol 1055 encapsulation is used. The field is 2 bytes in length. 1057 If an S/TLLAO is included, the first S/TLLAO ifIndex-tuple MUST 1058 correspond to the first OMNI option ifIndex-tuple, and any additional 1059 S/TLLAO ifIndex-tuples MUST correspond to a proper subset of the 1060 remaining OMNI option ifIndex-tuples. Any S/TLLAO ifIndex-tuple 1061 having an ifIndex value that does not appear in an OMNI option 1062 ifindex-tuple is ignored. If the same ifIndex value appears in 1063 multiple ifIndex-tuples, the first tuple is processed and the 1064 remaining tuples are ignored. Any S/TLLAO ifIndex-tuples can 1065 therefore be viewed as inter-dependent extensions of their 1066 corresponidng OMNI option ifIndex-tuples. 1068 A Client's AERO interface may be configured over multiple underlying 1069 interface connections. For example, common mobile handheld devices 1070 have both wireless local area network ("WLAN") and cellular wireless 1071 links. These links are typically used "one at a time" with low-cost 1072 WLAN preferred and highly-available cellular wireless as a standby. 1073 In a more complex example, aircraft frequently have many wireless 1074 data link types (e.g. satellite-based, cellular, terrestrial, air-to- 1075 air directional, etc.) with diverse performance and cost properties. 1077 If a Client's multiple underlying interfaces are used "one at a time" 1078 (i.e., all other interfaces are in standby mode while one interface 1079 is active), then ND message OMNI options include only a single 1080 ifIndex-tuple and set to a constant value. In that case, the Client 1081 would appear to have a single interface but with a dynamically 1082 changing link-layer address. 1084 If the Client has multiple active underlying interfaces, then from 1085 the perspective of ND it would appear to have multiple link-layer 1086 addresses. In that case, ND message OMNI options MAY include 1087 multiple ifIndex-tuples - each with a value that corresponds to a 1088 specific interface. The OMNI option MUST include a first ifIndex- 1089 tuple that corresponds to the interface over which the ND message is 1090 sent. Every ND message need not include all OMNI and/or S/TLLAO 1091 ifIndex-tuples; for any ifIndex-tuple not included, the neighbor 1092 considers the status as unchanged. 1094 Relay, Server and Proxy AERO interfaces may be configured over one or 1095 more secured tunnel interfaces. The AERO interface configures both 1096 an AERO address and its corresponding SPAN address, while the 1097 underlying secured tunnel interfaces are either unnumbered or 1098 configure the same SPAN address. The AERO interface encapsulates 1099 each IP packet in a SPAN header and presents the packet to the 1100 underlying secured tunnel interface. For Relays that do not 1101 configure an AERO interface, the secured tunnel interfaces themselves 1102 are exposed to the IP layer with each interface configuring the 1103 Relay's SPAN address. Routing protocols such as BGP therefore run 1104 directly over the Relay's secured tunnel interfaces. For nodes that 1105 configure an AERO interface, routing protocols such as BGP run over 1106 the AERO interface but do not employ SPAN encapsulation. Instead, 1107 the AERO interface presents the routing protocol messages directly to 1108 the underlying secured tunnels without applying encapsulation and 1109 while using the SPAN address as the source address. This distinction 1110 must be honored consistently according to each node's configuration 1111 so that the IP forwarding table will associate discovered IP routes 1112 with the correct interface. 1114 3.7. AERO Interface Initialization 1116 AERO Servers, Proxys and Clients configure AERO interfaces as their 1117 point of attachment to the AERO link. AERO nodes assign the MSPs for 1118 the link to their AERO interfaces (i.e., as a "route-to-interface") 1119 to ensure that packets with destination addresses covered by an MNP 1120 not explicitly assigned to a non-AERO interface are directed to the 1121 AERO interface. 1123 AERO interface initialization procedures for Servers, Proxys, Clients 1124 and Relays are discussed in the following sections. 1126 3.7.1. AERO Server/Gateway Behavior 1128 When a Server enables an AERO interface, it assigns AERO/SPAN 1129 addresses and configures permanent neighbor cache entries for 1130 neighbors in the same SPAN segment by consulting the ROS list for the 1131 segment. The Server also configures secured tunnels with one or more 1132 neighboring Relays and engages in a BGP routing protocol session with 1133 each Relay. 1135 The AERO interface provides a single interface abstraction to the IP 1136 layer, but internally comprises multiple secured tunnels as well as 1137 an NBMA nexus for sending encapsulated data packets to AERO interface 1138 neighbors. The Server further configures a service to facilitate ND/ 1139 PD exchanges with AERO Clients and manages per-Client neighbor cache 1140 entries and IP forwarding table entries based on control message 1141 exchanges. 1143 Gateways are simply Servers that run a dynamic routing protocol 1144 between the AERO interface and INET/EUN interfaces (see: 1146 Section 3.3). The Gateway provisions MNPs to networks on the INET/ 1147 EUN interfaces (i.e., the same as a Client would do) and advertises 1148 the MSP(s) for the AERO link over the INET/EUN interfaces. The 1149 Gateway further provides an attachment point of the AERO link to the 1150 non-MNP-based global topology. 1152 3.7.2. AERO Proxy Behavior 1154 When a Proxy enables an AERO interface, it assigns AERO/SPAN 1155 addresses and configures permanent neighbor cache entries the same as 1156 for Servers. The Proxy also configures secured tunnels with one or 1157 more neighboring Relays and maintains per-Client neighbor cache 1158 entries based on control message exchanges. 1160 3.7.3. AERO Client Behavior 1162 When a Client enables an AERO interface, it sends an RS message with 1163 ND/PD parameters over an ANET interface to a Server in the MAP list, 1164 which returns an RA message with corresponding parameters. (The RS/ 1165 RA messages may pass through a Proxy in the case of a Client's 1166 Proxyed interface.) 1168 After the initial ND/PD message exchange, the Client assigns AERO 1169 addresses to the AERO interface based on the delegated prefix(es). 1170 The Client can then register additional ANET interfaces with the 1171 Server by sending an RS message over each ANET interface. 1173 3.7.4. AERO Relay Behavior 1175 AERO Relays need not connect directly to the AERO link, since they 1176 operate as link-layer forwarding devices instead of network layer 1177 routers. Configuration of AERO interfaces on Relays is therefore 1178 OPTIONAL, e.g., if an administrative interface is needed. Relays 1179 configure secured tunnels with Servers, Proxys and other Relays; they 1180 also configure AERO/SPAN addresses and permanent neighbor cache 1181 entries the same as Servers. Relays engage in a BGP routing protocol 1182 session with a subset of the Servers on the local SPAN segment, and 1183 with other Relays on the SPAN (see: Section 3.3). 1185 3.8. AERO Interface Neighbor Cache Maintenance 1187 Each AERO interface maintains a conceptual neighbor cache that 1188 includes an entry for each neighbor it communicates with on the AERO 1189 link per [RFC4861]. AERO interface neighbor cache entries are said 1190 to be one of "permanent", "symmetric", "asymmetric" or "proxy". 1192 Permanent neighbor cache entries are created through explicit 1193 administrative action; they have no timeout values and remain in 1194 place until explicitly deleted. AERO Servers and Proxys maintain 1195 permanent neighbor cache entries for all other Servers and Proxys 1196 within the same SPAN segment. Each entry maintains the mapping 1197 between the neighbor's network-layer AERO address and corresponding 1198 INET address. The list of all permanent neighbor cache entries for 1199 the SPAN segment is maintained in the segment's ROS list. 1201 Symmetric neighbor cache entries are created and maintained through 1202 RS/RA exchanges as specified in Section 3.15, and remain in place for 1203 durations bounded by ND/PD lifetimes. AERO Servers maintain 1204 symmetric neighbor cache entries for each of their associated 1205 Clients, and AERO Clients maintain symmetric neighbor cache entries 1206 for each of their associated Servers. The list of all Servers on the 1207 AERO link is maintained in the link's MAP list. 1209 Asymmetric neighbor cache entries are created or updated based on 1210 route optimization messaging as specified in Section 3.17, and are 1211 garbage-collected when keepalive timers expire. AERO route 1212 optimization sources (ROSs) maintain asymmetric neighbor cache 1213 entries for active targets with lifetimes based on ND messaging 1214 constants. Asymmetric neighbor cache entries are unidirectional 1215 since only the ROS and not the target (e.g., a Client's MAP) creates 1216 an entry. 1218 Proxy neighbor cache entries are created and maintained by AERO 1219 Proxys when they process Client/Server ND/PD exchanges, and remain in 1220 place for durations bounded by ND/PD lifetimes. AERO Proxys maintain 1221 proxy neighbor cache entries for each of their associated Clients. 1222 Proxy neighbor cache entries track the Client state and the address 1223 of the Client's associated Server. 1225 To the list of neighbor cache entry states in Section 7.3.2 of 1226 [RFC4861], Proxy and Server AERO interfaces add an additional state 1227 DEPARTED that applies to symmetric and proxy neighbor cache entries 1228 for Clients that have recently departed. The interface sets a 1229 "DepartTime" variable for the neighbor cache entry to "DEPARTTIME" 1230 seconds. DepartTime is decremented unless a new ND message causes 1231 the state to return to REACHABLE. While a neighbor cache entry is in 1232 the DEPARTED state, packets destined to the target Client are 1233 forwarded to the Client's new location instead of being dropped. 1234 When DepartTime decrements to 0, the neighbor cache entry is deleted. 1235 It is RECOMMENDED that DEPARTTIME be set to the default constant 1236 value 40 seconds to allow for packets in flight to be delivered while 1237 stale route optimization state may be present. 1239 When a target Server (acting as a MAP) receives a valid NS message 1240 used for route optimization, it searches for a symmetric neighbor 1241 cache entry for the target Client. The MAP then returns a solicited 1242 NA message without creating a neighbor cache entry for the ROS, but 1243 creates or updates a target Client "Report List" entry for the ROS 1244 and sets a "ReportTime" variable for the entry to REPORTTIME seconds. 1245 The MAP resets ReportTime when it receives a new authentic NS 1246 message, and otherwise decrements ReportTime while no NS messages 1247 have been received. It is RECOMMENDED that REPORTTIME be set to the 1248 default constant value 40 seconds to allow a 10 second window so that 1249 route optimization can converge before ReportTime decrements below 1250 REACHABLETIME. 1252 When the ROS receives a solicited NA message response to its NS 1253 message, it creates or updates an asymmetric neighbor cache entry for 1254 the target network-layer and link-layer addresses. The ROS then 1255 (re)sets ReachableTime for the neighbor cache entry to REACHABLETIME 1256 seconds and uses this value to determine whether packets can be 1257 forwarded directly to the target, i.e., instead of via a default 1258 route. The ROS otherwise decrements ReachableTime while no further 1259 solicited NA messages arrive. It is RECOMMENDED that REACHABLETIME 1260 be set to the default constant value 30 seconds as specified in 1261 [RFC4861]. 1263 The ROS also uses the value MAX_UNICAST_SOLICIT to limit the number 1264 of NS keepalives sent when a correspondent may have gone unreachable, 1265 the value MAX_RTR_SOLICITATIONS to limit the number of RS messages 1266 sent without receiving an RA and the value MAX_NEIGHBOR_ADVERTISEMENT 1267 to limit the number of unsolicited NAs that can be sent based on a 1268 single event. It is RECOMMENDED that MAX_UNICAST_SOLICIT, 1269 MAX_RTR_SOLICITATIONS and MAX_NEIGHBOR_ADVERTISEMENT be set to 3 the 1270 same as specified in [RFC4861]. 1272 Different values for DEPARTTIME, REPORTTIME, REACHABLETIME, 1273 MAX_UNICAST_SOLICIT, MAX_RTR_SOLCITATIONS and 1274 MAX_NEIGHBOR_ADVERTISEMENT MAY be administratively set; however, if 1275 different values are chosen, all nodes on the link MUST consistently 1276 configure the same values. Most importantly, DEPARTTIME and 1277 REPORTTIME SHOULD be set to a value that is sufficiently longer than 1278 REACHABLETIME to avoid packet loss due to stale route optimization 1279 state. 1281 3.9. AERO Interface Encapsulation and Re-encapsulation 1283 Client AERO interfaces avoid encapsulation over Direct underlying 1284 interfaces and Proxyed underlying interfaces for which the first-hop 1285 access router is AERO-aware. Other AERO interfaces encapsulate 1286 packets according to whether they are entering the AERO interface 1287 from the network layer or if they are being re-admitted into the same 1288 AERO link they arrived on. This latter form of encapsulation is 1289 known as "re-encapsulation". 1291 For packets entering the AERO interface from the network layer, the 1292 AERO interface copies the "TTL/Hop Limit", "Type of Service/Traffic 1293 Class" [RFC2983], "Flow Label"[RFC6438] (for IPv6) and "Congestion 1294 Experienced" [RFC3168] values in the packet's IP header into the 1295 corresponding fields in the encapsulation header(s). 1297 For packets undergoing re-encapsulation, the AERO interface instead 1298 copies these values from the original encapsulation header into the 1299 new encapsulation header, i.e., the values are transferred between 1300 encapsulation headers and *not* copied from the encapsulated packet's 1301 network-layer header. (Note especially that by copying the TTL/Hop 1302 Limit between encapsulation headers the value will eventually 1303 decrement to 0 if there is a (temporary) routing loop.) For IPv4 1304 encapsulation/re-encapsulation, the AERO interface sets the DF bit as 1305 discussed in Section 3.13. 1307 AERO interfaces configured over INET underlying interfaces 1308 encapsulate each packet in a SPAN header, then encapsulate the 1309 resulting SPAN packet in an INET header according to the next hop 1310 determined in the forwarding algorithm in Section 3.12. If the next 1311 hop is reached via a secured tunnel, the AERO interface uses an INET 1312 encapsulation format specific to the secured tunnel type (see: 1313 Section 6). If the next hop is reached via an unsecured underlying 1314 interface, the AERO interface instead uses Generic UDP Encapsulation 1315 (GUE) [I-D.ietf-intarea-gue] or an alternate minimal encapsulation 1316 format Appendix A. 1318 When GUE encapsulation is used, the AERO interface next sets the UDP 1319 source port to a constant value that it will use in each successive 1320 packet it sends, and sets the UDP length field to the length of the 1321 SPAN packet plus 8 bytes for the UDP header itself plus the length of 1322 the GUE header (or 0 if GUE direct IP encapsulation is used). For 1323 packets sent to a Server or Relay, the AERO interface sets the UDP 1324 destination port to 8060, i.e., the IANA-registered port number for 1325 AERO. For packets sent to a Client, the AERO interface sets the UDP 1326 destination port to the port value stored in the neighbor cache entry 1327 for this Client. The AERO interface then either includes or omits 1328 the UDP checksum according to the GUE specification. 1330 AERO interfaces observes the packet sizing and fragmentation 1331 considerations found in Section 3.13. 1333 3.10. AERO Interface Decapsulation 1335 AERO interfaces decapsulate packets destined either to the AERO node 1336 itself or to a destination reached via an interface other than the 1337 AERO interface the packet was received on. When the encapsulated 1338 packet arrives in multiple fragments, the AERO interface reassembles 1339 as discussed in Section 3.13. Further decapsulation steps are 1340 performed according to the appropriate encapsulation format 1341 specification. 1343 3.11. AERO Interface Data Origin Authentication 1345 AERO nodes employ simple data origin authentication procedures. In 1346 particular: 1348 o AERO Relays, Servers and Proxys accept encapsulated data packets 1349 and control messages received from secured tunnels. 1351 o AERO Servers and Proxys accept encapsulated data packets and NS 1352 messages used for Neighbor Unreachability Detection (NUD) received 1353 from a source found in the ROS list. 1355 o AERO Proxys and Clients accept packets that originate from within 1356 the same secured ANET. 1358 o AERO Clients and Gateways accept packets from downstream network 1359 correspondents based on ingress filtering. 1361 AERO nodes silently drop any packets that do not satisfy the above 1362 data origin authentication procedures. Further security 1363 considerations are discussed Section 6. 1365 3.12. AERO Interface Forwarding Algorithm 1367 IP packets enter a node's AERO interface either from the network 1368 layer (i.e., from a local application or the IP forwarding system) or 1369 from the link layer (i.e., from an AERO interface neighbor). All 1370 packets entering a node's AERO interface first undergo data origin 1371 authentication as discussed in Section 3.11. Packets that satisfy 1372 data origin authentication are processed further, while all others 1373 are dropped silently. 1375 Packets that enter the AERO interface from the network layer are 1376 forwarded to an AERO interface neighbor. Packets that enter the AERO 1377 interface from the link layer are either re-admitted into the AERO 1378 link or forwarded to the network layer where they are subject to 1379 either local delivery or IP forwarding. In all cases, the AERO 1380 interface itself MUST NOT decrement the network layer TTL/Hop-count 1381 since its forwarding actions occur below the network layer. 1383 AERO interfaces may have multiple underlying interfaces and/or 1384 neighbor cache entries for neighbors with multiple ifIndex-tuple 1385 registrations (see Section 3.6). The AERO interface uses each 1386 packet's DSCP value (and/or other traffic discriminators such as port 1387 number) to select an outgoing underlying interface based on the 1388 node's own QoS preferences, and also to select a destination link- 1389 layer address based on the neighbor's underlying interface with the 1390 highest preference. AERO implementations SHOULD allow for QoS 1391 preference values to be modified at runtime through network 1392 management. 1394 If multiple outgoing interfaces and/or neighbor interfaces have a 1395 preference of "high", the AERO node replicates the packet and sends 1396 one copy via each of the (outgoing / neighbor) interface pairs; 1397 otherwise, the node sends a single copy of the packet via an 1398 interface with the highest preference. AERO nodes keep track of 1399 which underlying interfaces are currently "reachable" or 1400 "unreachable", and only use "reachable" interfaces for forwarding 1401 purposes. 1403 The following sections discuss the AERO interface forwarding 1404 algorithms for Clients, Proxys, Servers and Relays. In the following 1405 discussion, a packet's destination address is said to "match" if it 1406 is the same as a cached address, or if it is covered by a cached 1407 prefix (which may be encoded in an AERO address). 1409 3.12.1. Client Forwarding Algorithm 1411 When an IP packet enters a Client's AERO interface from the network 1412 layer the Client searches for an asymmetric neighbor cache entry that 1413 matches the destination. If there is a match, the Client uses one or 1414 more "reachable" neighbor interfaces in the entry for packet 1415 forwarding. If there is no asymmetric neighbor cache entry, the 1416 Client instead forwards the packet toward a Server (the packet is 1417 intercepted by a Proxy if there is a Proxy on the path). 1419 When an IP packet enters a Client's AERO interface from the link- 1420 layer, if the destination matches one of the Client's MNPs or link- 1421 local addresses the Client decapsulates the packet (if necessary) and 1422 delivers it to the network layer. Otherwise, the Client drops the 1423 packet and MAY return a network-layer ICMP Destination Unreachable 1424 message subject to rate limiting (see: Section 3.14). 1426 3.12.2. Proxy Forwarding Algorithm 1428 For control messages originating from or destined to a Client, the 1429 Proxy intercepts the message and updates its proxy neighbor cache 1430 entry for the Client. The Proxy then forwards a (proxyed) copy of 1431 the control message. (For example, the Proxy forwards a proxied 1432 version of a Client's NS/RS message to the target neighbor, and 1433 forwards a proxied version of the NA/RA reply to the Client.) 1434 When the Proxy receives a data packet from a Client within the ANET, 1435 the Proxy searches for an asymmetric neighbor cache entry that 1436 matches the destination and forwards the packet as follows: 1438 o if the destination matches an asymmetric neighbor cache entry, the 1439 Proxy uses one or more "reachable" neighbor interfaces in the 1440 entry for packet forwarding via encapsulation. If the neighbor 1441 interface is in the same SPAN segment, the Proxy forwards the 1442 packet directly to the neighbor; otherwise, it forwards the packet 1443 to a Relay. 1445 o else, the Proxy encapsulates and forwards the packet to a Relay 1446 while using the packet's destination address as the SPAN 1447 destination address. (If the destination is an AERO address, the 1448 Proxy instead uses the corresponding Subnet Router Anycast address 1449 for Client AERO addresses and the SPAN address for 1450 administratively-provisioned AERO addresses.). 1452 When the Proxy receives an encapsulated data packet from an INET 1453 neighbor or from a secured tunnel, it accepts the packet only if data 1454 origin authentication succeeds and the SPAN destination address is 1455 its own address. If the packet is a SPAN fragment, the Proxy then 1456 adds the fragment to the reassembly buffer and returns if the 1457 reassembly is still incomplete. Otherwise, the Proxy reassembles the 1458 packet (if necessary) and continues processing. 1460 Next, the Proxy searches for a proxy neighbor cache entry that 1461 matches the destination. If there is a proxy neighbor cache entry in 1462 the REACHABLE state, the Proxy decapsulates and forwards the packet 1463 to the Client. If the neighbor cache entry is in the DEPARTED state, 1464 the Proxy instead re-encapsulates the packet and forwards it to a 1465 Relay. If there is no neighbor cache entry, the Proxy instead 1466 discards the packet. 1468 3.12.3. Server/Gateway Forwarding Algorithm 1470 For control messages destined to a target Client's AERO address that 1471 are received from a secured tunnel, the Server (acting as a MAP) 1472 intercepts the message and sends an appropriate response on behalf of 1473 the Client. (For example, the Server sends an NA message reply in 1474 response to an NS message directed to one of its associated Clients.) 1475 If the Client's neighbor cache entry is in the DEPARTED state, 1476 however, the Server instead forwards the packet to the Client's new 1477 Server as discussed in Section 3.19. 1479 When the Server receives an encapsulated data packet from an INET 1480 neighbor or from a secured tunnel, it accepts the packet only if data 1481 origin authentication succeeds. If the SPAN destination address is 1482 its own address, the Server reassembles if necessary and discards the 1483 SPAN header (if the reassembly is incomplete, the Server instead adds 1484 the fragment to the reassembly buffer and returns). The Server then 1485 continues processing as follows: 1487 o if the destination matches a symmetric neighbor cache entry in the 1488 REACHABLE state the Server prepares the packet for forwarding to 1489 the destination Client. If the current header is a SPAN header, 1490 the Server reassembles if necessary and discards the SPAN header. 1491 The Server then forwards the packet according to the cached link- 1492 layer information, while using SPAN encapsulation for the Client's 1493 Proxyed/Native interfaces, simple INET encapsulation for NATed/ 1494 VPNed interfaces, or no encapsulation for Direct interfaces. 1496 o else, if the destination matches a symmetric neighbor cache entry 1497 in the DEPARETED state the Server re-encapsulates the packet and 1498 forwards it using the SPAN address of the Client's new Server as 1499 the destination. 1501 o else, if the destination matches an asymmetric neighbor cache 1502 entry, the Server uses one or more "reachable" neighbor interfaces 1503 in the entry for packet forwarding via the local INET if the 1504 neighbor is in the same SPAN segment or via a Relay otherwise. 1506 o else, if the destination is an AERO address that is not assigned 1507 on the AERO interface the Server drops the packet. 1509 o else, the Server (acting as a Gateway) releases the packet to the 1510 network layer for local delivery or IP forwarding. Based on the 1511 information in the forwarding table, the network layer may return 1512 the packet to the same AERO interface in which case further 1513 processing occurs as below. (Note that this arrangement 1514 accommodates common implementations in which the IP forwarding 1515 table is not accessible from within the AERO interface. If the 1516 AERO interface can directly access the IP forwarding table, the 1517 forwarding table lookup can instead be performed internally from 1518 within the AERO interface itself.) 1520 When the Server's AERO interface receives a data packet from the 1521 network layer or from a NATed/VPNed/Direct Client, it processes the 1522 packet according to the network-layer destination address as follows: 1524 o if the destination matches a symmetric or asymmetric neighbor 1525 cache entry the Server processes the packet as above. 1527 o else, the Server encapsulates the packet and forwards it to a 1528 Relay. For administratively-assigned AERO address destinations, 1529 the Server uses the SPAN address corresponding to the destination 1530 as the SPAN destination address. For Client AERO address 1531 destinations, the Server uses the Subnet Router Anycast address 1532 corresponding to the destination as the SPAN destination address. 1533 For all others, the Server uses the packet's destination IP 1534 address as the SPAN destination address. 1536 3.12.4. Relay Forwarding Algorithm 1538 Relays forward packets over secured tunnels the same as any IP 1539 router. When the Relay receives an encapsulated packet via a secured 1540 tunnel, it removes the INET header and searches for a forwarding 1541 table entry that matches the destination address in the next header. 1542 The Relay then processes the packet as follows: 1544 o if the destination matches one of the Relay's own addresses, the 1545 Relay submits the packet for local delivery. 1547 o else, if the destination matches a forwarding table entry the 1548 Relay forwards the packet via a secured tunnel to the next hop. 1549 If the destination matches an MSP without matching an MNP, 1550 however, the Relay instead drops the packet and returns an ICMP 1551 Destination Unreachable message subject to rate limiting (see: 1552 Section 3.14). 1554 o else, the Relay drops the packet and returns an ICMP Destination 1555 Unreachable as above. 1557 As for any IP router, the Relay decrements the TTL/Hop Limit when it 1558 forwards the packet. If the packet is encapsulated in a SPAN header, 1559 only the Hop Limit in the SPAN header is decremented, and not the 1560 TTL/Hop Limit in the inner packet header. 1562 3.13. AERO Interface MTU and Fragmentation 1564 The AERO interface is the node's attachment to the AERO link. For 1565 AERO link neighbor underlying interface paths that do not require 1566 encapsulation, the AERO interface sends unencapsulated IP packets. 1567 For other paths, the AERO interface acts as a tunnel ingress when it 1568 sends packets to the neighbor and as a tunnel egress when it receives 1569 packets from the neighbor. 1571 AERO interfaces configure an MTU the same as for any IP interface, 1572 however the MTU does not reflect the physical size of any links in 1573 the path but rather determines the maximum size for reassembly. AERO 1574 interfaces observe the packet sizing considerations for tunnels 1575 discussed in [I-D.ietf-intarea-tunnels] and as specified below. 1577 The Internet Protocol expects that IP packets will either be 1578 delivered to the destination or a suitable Packet Too Big (PTB) 1579 message returned to support the process known as IP Path MTU 1580 Discovery (PMTUD) [RFC1191][RFC8201]. However, PTB messages may be 1581 crafted for malicious purposes or lost in the network [RFC2923]. 1582 This can be especially problematic for tunnels, where a condition 1583 known as a PMTUD "black hole" can result. For these reasons, AERO 1584 interfaces employ operational procedures that avoid interactions with 1585 PMTUD, including the use of fragmentation when necessary. 1587 AERO interfaces observe three different types of fragmentation. 1588 Source fragmentation occurs when the AERO interface (acting as a 1589 tunnel ingress) fragments the encapsulated packet into multiple 1590 fragments before admitting each fragment into the tunnel. Network 1591 fragmentation occurs when an encapsulated packet admitted into the 1592 tunnel by the ingress is fragmented by an IPv4 router on the path to 1593 the egress. Finally, link-layer fragmentation (aka link adaptation) 1594 occurs at a layer below IP and is coordinated between underlying data 1595 link endpoints. 1597 IPv6 specifies a minimum link Maximum Transmission Unit (MTU) of 1280 1598 bytes [RFC8200]. Although IPv4 specifies a smaller minimum link MTU 1599 of 68 bytes [RFC0791], AERO interfaces also observe the IPv6 minimum 1600 for IPv4 even if encapsulated packets may incur network 1601 fragmentation. 1603 IPv6 specifies a minimum Maximum Reassembly Unit (MRU) of 1500 bytes 1604 [RFC8200], while the minimum MRU for IPv4 is only 576 bytes [RFC1122] 1605 (but, note that many standard IPv6 over IPv4 tunnel types already 1606 assume a larger MRU than the IPv4 minimum). 1608 AERO interfaces therefore configure an MTU that MUST NOT be smaller 1609 than 1280 bytes, MUST NOT be larger than the minimum MRU among all 1610 nodes on the AERO link minus the encapsulation overhead ("ENCAPS"), 1611 and SHOULD NOT be smaller than 1500 bytes. AERO interfaces also 1612 configure a Maximum Segment Unit (MSU) as the maximum-sized 1613 encapsulated packet that the ingress can inject into the tunnel 1614 without source fragmentation. The MSU value SHOULD NOT be larger 1615 than 1280 bytes if there is no operational assurance that a larger 1616 size can traverse the link along all paths. 1618 The network layer proceeds as follows when it forwards an IP packet 1619 to the AERO interface. For each IPv4 packet that is larger than the 1620 AERO interface MTU and with DF set to 0, the network layer uses IPv4 1621 fragmentation to break the packet into a minimum number of non- 1622 overlapping fragments where the first fragment is no larger than the 1623 MTU and the remaining fragments are no larger than the first. For 1624 all other IP packets, if the packet is larger than the AERO interface 1625 MTU, the network layer drops the packet and returns a PTB message to 1626 the original source. Otherwise, the network layer admits each IP 1627 packet or fragment into the AERO interface. 1629 For each IP packet admitted into AERO interface, if the neighbor is 1630 reached via an underlying interface that does not require 1631 encapsulation the AERO interface proceeds according to the underlying 1632 interface MTU. If the packet is no larger than the underlying 1633 interface MTU, the AERO interface presents the packet to the 1634 underlying interface. Otherwise, for IPv4 packets with DF set to 0 1635 the AERO interface uses IPv4 fragmentation to break the packet into 1636 fragments no larger than the underlying interface MTU. For other 1637 packets, the AERO interface either performs link adaptation or drops 1638 the packet and returns a PTB message to the original source. (If the 1639 original source corresponds to a local application, the PTB would 1640 appear to have originated from a router on the path when in fact it 1641 was locally generated from within the AERO interface.) In the same 1642 way, when a packet that has been admitted into the AERO link reaches 1643 a target neighbor but is too large to be delivered over the final-hop 1644 underlying interface, the target either performs link adaptation or 1645 drops the packet and returns a PTB. Link adaptation is preferred in 1646 both cases when possible to avoid packet loss. 1648 For underlying interfaces that require encapsulation, the AERO 1649 interface (acting as a tunnel ingress) instead encapsulates the 1650 packet and performs path MTU procedures according to the specific 1651 encapsulation format. For INET interfaces, the ingress encapsulates 1652 the packet in a SPAN header. If the SPAN packet is larger than the 1653 MSU, the ingress source fragments the SPAN packet into a minimum 1654 number of non-overlapping fragments where the first fragment is no 1655 larger than the MSU and the remaining fragments are no larger than 1656 the first. The ingress then encapsulates each SPAN packet/fragment 1657 in an INET header and admits them into the tunnel. For IPv4, the 1658 ingress sets the DF bit to 0 in the INET header in case any network 1659 fragmentation is necessary. The encapsulated packets will be 1660 delivered to the egress, which reassembles them into a whole packet 1661 if necessary. 1663 By fragmenting at the SPAN layer instead of lower layers, standard 1664 IPv6 fragmentation and reassembly [RFC8200] ensures that IPv4 issues 1665 such as data corruption due to reassembly misassociations will not 1666 occur [RFC6864][RFC4963]. The ingress sends each fragment with 1667 minimal delay (i.e., in a multi-fragment burst) so that individual 1668 fragments are unlikely to be diverted to different destinations due 1669 to routing fluctuations. 1671 Since the SPAN header and IPv6 fragment extension header reduces the 1672 room available for packet data, but the original source has no way to 1673 control its insertion, the ingress MUST include their lengths in 1674 ENCAPS even for packets in which the header is absent. 1676 3.13.1. AERO MTU Requirements 1678 In light of the above considerations, AERO interfaces SHOULD 1679 configure an MTU of 9180 bytes. This means that the AERO interface 1680 MUST be capable of reassembling original IP packets up to 9180 bytes 1681 in length. When an IP packet is admitted into an AERO interface, the 1682 interface encapsulates the packet using SPAN encapsulation and 1683 fragments the encapsulated packet into fragments that are no larger 1684 than 1280 bytes. The fragments will be reassembled by the tunnel 1685 egress that services the final destination. 1687 AERO Clients behind Proxys MAY configure an MTU smaller than 9180 1688 (but no smaller than IP minimum link MTU). If Clients configure a 1689 diversity of MTUs (e.g., 1280, 1500, 4KB, 8KB, etc.) then neighbors 1690 on the link would appear to have multiple MTUs. IPv6 Path MTU 1691 Discovery [RFC8201] accounts for this possibility since MTU discovery 1692 must be performed even between nodes that appear to be connected to 1693 the same link. 1695 Applications that cannot tolerate loss in the network due to MTU 1696 restrictions should restrict themselves to sending packets no larger 1697 than the IP minimum link MTU, i.e., even if the current path MTU 1698 would appear to support a larger size. This is due to the fact that 1699 routing changes could cause the path to traverse links with smaller 1700 MTUs at any given point in time. 1702 3.14. AERO Interface Error Handling 1704 When an AERO node admits a packet into the AERO interface, it may 1705 receive link-layer or network-layer error indications. 1707 A link-layer error indication is an ICMP error message generated by a 1708 router in the INET on the path to the neighbor or by the neighbor 1709 itself. The message includes an IP header with the address of the 1710 node that generated the error as the source address and with the 1711 link-layer address of the AERO node as the destination address. 1713 The IP header is followed by an ICMP header that includes an error 1714 Type, Code and Checksum. Valid type values include "Destination 1715 Unreachable", "Time Exceeded" and "Parameter Problem" 1716 [RFC0792][RFC4443]. (AERO interfaces ignore all link-layer IPv4 1717 "Fragmentation Needed" and IPv6 "Packet Too Big" messages since they 1718 only emit packets that are guaranteed to be no larger than the IP 1719 minimum link MTU as discussed in Section 3.13.) 1720 The ICMP header is followed by the leading portion of the packet that 1721 generated the error, also known as the "packet-in-error". For 1722 ICMPv6, [RFC4443] specifies that the packet-in-error includes: "As 1723 much of invoking packet as possible without the ICMPv6 packet 1724 exceeding the minimum IPv6 MTU" (i.e., no more than 1280 bytes). For 1725 ICMPv4, [RFC0792] specifies that the packet-in-error includes: 1726 "Internet Header + 64 bits of Original Data Datagram", however 1727 [RFC1812] Section 4.3.2.3 updates this specification by stating: "the 1728 ICMP datagram SHOULD contain as much of the original datagram as 1729 possible without the length of the ICMP datagram exceeding 576 1730 bytes". 1732 The link-layer error message format is shown in Figure 5 (where, "L2" 1733 and "L3" refer to link-layer and network-layer, respectively): 1735 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1736 ~ ~ 1737 | L2 IP Header of | 1738 | error message | 1739 ~ ~ 1740 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1741 | L2 ICMP Header | 1742 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --- 1743 ~ ~ P 1744 | IP and other encapsulation | a 1745 | headers of original L3 packet | c 1746 ~ ~ k 1747 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ e 1748 ~ ~ t 1749 | IP header of | 1750 | original L3 packet | i 1751 ~ ~ n 1752 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1753 ~ ~ e 1754 | Upper layer headers and | r 1755 | leading portion of body | r 1756 | of the original L3 packet | o 1757 ~ ~ r 1758 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --- 1760 Figure 5: AERO Interface Link-Layer Error Message Format 1762 The AERO node rules for processing these link-layer error messages 1763 are as follows: 1765 o When an AERO node receives a link-layer Parameter Problem message, 1766 it processes the message the same as described as for ordinary 1767 ICMP errors in the normative references [RFC0792][RFC4443]. 1769 o When an AERO node receives persistent link-layer Time Exceeded 1770 messages, the IP ID field may be wrapping before earlier fragments 1771 awaiting reassembly have been processed. In that case, the node 1772 should begin including integrity checks and/or institute rate 1773 limits for subsequent packets. 1775 o When an AERO node receives persistent link-layer Destination 1776 Unreachable messages in response to encapsulated packets that it 1777 sends to one of its asymmetric neighbor correspondents, the node 1778 should process the message as an indication that a path may be 1779 failing, and optionally initiate NUD over that path. If it 1780 receives Destination Unreachable messages over multiple paths, the 1781 node should allow future packets destined to the correspondent to 1782 flow through a default route and re-initiate route optimization. 1784 o When an AERO Client receives persistent link-layer Destination 1785 Unreachable messages in response to encapsulated packets that it 1786 sends to one of its symmetric neighbor Servers, the Client should 1787 mark the path as unusable and use another path. If it receives 1788 Destination Unreachable messages on many or all paths, the Client 1789 should associate with a new Server and release its association 1790 with the old Server as specified in Section 3.19.5. 1792 o When an AERO Server receives persistent link-layer Destination 1793 Unreachable messages in response to encapsulated packets that it 1794 sends to one of its symmetric neighbor Clients, the Server should 1795 mark the underlying path as unusable and use another underlying 1796 path. 1798 o When an AERO Server or Proxy receives link-layer Destination 1799 Unreachable messages in response to an encapsulated packet that it 1800 sends to one of its permanent neighbors, it treats the messages as 1801 an indication that the path to the neighbor may be failing. 1802 However, the dynamic routing protocol should soon reconverge and 1803 correct the temporary outage. 1805 When an AERO Relay receives a packet for which the network-layer 1806 destination address is covered by an MSP, if there is no more- 1807 specific routing information for the destination the Relay drops the 1808 packet and returns a network-layer Destination Unreachable message 1809 subject to rate limiting. The Relay writes the network-layer source 1810 address of the original packet as the destination address and uses 1811 one of its non link-local addresses as the source address of the 1812 message. 1814 When an AERO node receives an encapsulated packet for which the 1815 reassembly buffer it too small, it drops the packet and returns a 1816 network-layer Packet Too Big (PTB) message. The node first writes 1817 the MRU value into the PTB message MTU field, writes the network- 1818 layer source address of the original packet as the destination 1819 address and writes one of its non link-local addresses as the source 1820 address. 1822 3.15. AERO Router Discovery, Prefix Delegation and Autoconfiguration 1824 AERO Router Discovery, Prefix Delegation and Autoconfiguration are 1825 coordinated as discussed in the following Sections. 1827 3.15.1. AERO ND/PD Service Model 1829 Each AERO Server on the link configures a PD service to facilitate 1830 Client requests. Each Server is provisioned with a database of MNP- 1831 to-Client ID mappings for all Clients enrolled in the AERO service, 1832 as well as any information necessary to authenticate each Client. 1833 The Client database is maintained by a central administrative 1834 authority for the AERO link and securely distributed to all Servers, 1835 e.g., via the Lightweight Directory Access Protocol (LDAP) [RFC4511], 1836 via static configuration, etc. Clients receive the same service 1837 regardless of the Servers they select. 1839 AERO Clients and Servers use ND messages to maintain neighbor cache 1840 entries. AERO Servers configure their AERO interfaces as advertising 1841 interfaces, and therefore send unicast RA messages with configuration 1842 information in response to a Client's RS message. Thereafter, 1843 Clients send additional RS messages to refresh prefix and/or router 1844 lifetimes. 1846 AERO Clients and Servers include PD parameters in RS/RA messages (see 1847 [I-D.templin-6man-dhcpv6-ndopt] for ND/PD alternatives). The unified 1848 ND/PD messages are exchanged between Client and Server according to 1849 the prefix management schedule required by the PD service. If the 1850 Client knows its MNP in advance, it can employ prefix registration by 1851 including its AERO address as the source address of an RS message and 1852 with an OMNI option with a valid Prefix Length for the MNP. If the 1853 Server (and Proxy) accept the Client's MNP assertion, they inject the 1854 prefix into the routing system and establish the necessary neighbor 1855 cache state. 1857 The following sections specify the Client and Server behavior. 1859 3.15.2. AERO Client Behavior 1861 AERO Clients discover the addresses of Servers in a similar manner as 1862 described in [RFC5214]. Discovery methods include static 1863 configuration (e.g., from a flat-file map of Server addresses and 1864 locations), or through an automated means such as Domain Name System 1865 (DNS) name resolution [RFC1035]. Alternatively, the Client can 1866 discover Server addresses through a layer 2 data link login exchange, 1867 or through a unicast RA response to a multicast/anycast RS as 1868 described below. In the absence of other information, the Client can 1869 resolve the DNS Fully-Qualified Domain Name (FQDN) 1870 "linkupnetworks.[domainname]" where "linkupnetworks" is a constant 1871 text string and "[domainname]" is a DNS suffix for the AERO link 1872 (e.g., "example.com"). 1874 To associate with a Server, the Client acts as a requesting router to 1875 request MNPs. The Client prepares an RS message with PD parameters 1876 and includes a Nonce and Timestamp option if the Client needs to 1877 correlate RA replies. If the Client already knows the Server's AERO 1878 address, it includes the AERO address as the network-layer 1879 destination address; otherwise, it includes all-routers multicast 1880 (ff02::2) or subnet routers anycast (fe80::) as the network-layer 1881 destination address. If the Client already knows its own AERO 1882 address, it uses the AERO address as the network-layer source 1883 address; otherwise, it uses the unspecified AERO address 1884 (fe80::ffff:ffff) as the network-layer source address. 1886 The Client next includes an OMNI option in the RS message to register 1887 its link-layer information with the Server. The first ifIndex-tuple 1888 MUST correspond to the underlying interface over which the Client 1889 will send the RS message. The Client MAY include additional ifIndex- 1890 tuples specific to other underlying interfaces. The Client also 1891 includes an SLLAO with a single link-layer address corresponding to 1892 the first OMNI option ifIndex-tuple. 1894 The Client then sends the RS message (either directly via Direct 1895 interfaces, via INET encapsulation for NATed interfaces, via a VPN 1896 for VPNed interfaces, via a Proxy for proxyed interfaces or via a 1897 Relay for native interfaces) and waits for an RA message reply (see 1898 Section 3.15.3). The Client retries up to MAX_RTR_SOLICITATIONS 1899 times until an RA is received. If the Client receives no RAs, or if 1900 it receives an RA with Router Lifetime set to 0, the Client SHOULD 1901 abandon this Server and try another Server. Otherwise, the Client 1902 processes the PD information found in the RA message. 1904 Next, the Client creates a symmetric neighbor cache entry with the 1905 Server's AERO address as the network-layer address and the Server's 1906 encapsulation and/or link-layer addresses as the link-layer address. 1907 The Client records the RA Router Lifetime field value in the neighbor 1908 cache entry as the time for which the Server has committed to 1909 maintaining the MNP in the routing system. The Client then 1910 autoconfigures AERO addresses for each of the delegated MNPs and 1911 assigns them to the AERO interface. The Client also caches any MSPs 1912 included in Route Information Options (RIOs) [RFC4191] as MSPs to 1913 associate with the AERO link, and assigns the MTU value in the MTU 1914 option to its AERO interface while configuring an appropriate MRU. 1916 The Client then registers additional underlying interfaces with the 1917 Server by sending RS messages via each additional interface. The RS 1918 messages include the same parameters as for the initial RS/RA 1919 exchange, but with destination address set to the Server's AERO 1920 address and with the initial OMNI option ifIndex-tuple corresponding 1921 to the underlying interface. 1923 Following autoconfiguration, the Client sub-delegates the MNPs to its 1924 attached EUNs and/or the Client's own internal virtual interfaces as 1925 described in [I-D.templin-v6ops-pdhost] to support the Client's 1926 downstream attached "Internet of Things (IoT)". The Client 1927 subsequently maintains its MNP delegations through each of its 1928 Servers by sending additional RS messages before Router Lifetime 1929 expires. 1931 After the Client registers its underlying interfaces, it may wish to 1932 change one or more registrations, e.g., if an interface changes 1933 address or becomes unavailable, if QoS preferences change, etc. To 1934 do so, the Client prepares an RS message to send over any available 1935 underlying interface. The RS includes an OMNI option with a first 1936 ifIndex-tuple specific to the selected underlying interface, and MAY 1937 include any additional ifIndex-tuples specific to other underlying 1938 interfaces. The Client includes fresh ifIndex-tuple values to update 1939 the Server's neighbor cache entry. When the Client receives the 1940 Server's RA response, it has assurance that the Server has been 1941 updated with the new information. 1943 If the Client wishes to discontinue use of a Server it issues an RS 1944 message over any underlying interface with an OMNI option with a 1945 release indication. When the Server processes the message, it 1946 releases the MNP, sets the symmetric neighbor cache entry state for 1947 the Client to DEPARTED and returns an RA reply with Router Lifetime 1948 set to 0. After a short delay (e.g., 2 seconds), the Server 1949 withdraws the MNP from the routing system. 1951 3.15.3. AERO Server Behavior 1953 AERO Servers act as IP routers and support a PD service for Clients. 1954 Servers arrange to add their AERO addresses to a static map of Server 1955 addresses for the link and/or the DNS resource records for the FQDN 1956 "linkupnetworks.[domainname]" before entering service. Server 1957 addresses should be geographically and/or topologically referenced, 1958 and made available for discovery by Clients on the AERO link. 1960 When a Server receives a prospective Client's RS message on its AERO 1961 interface, it SHOULD return an immediate RA reply with Router 1962 Lifetime set to 0 if it is currently too busy or otherwise unable to 1963 service the Client. Otherwise, the Server authenticates the RS 1964 message and processes the PD parameters. The Server first determines 1965 the correct MNPs to delegate to the Client by searching the Client 1966 database. When the Server delegates the MNPs, it also creates an IP 1967 forwarding table entry for each MNP so that the MNPs are propagated 1968 into the routing system (see: Section 3.3). For IPv6, the Server 1969 creates an IPv6 forwarding table entry for each MNP. For IPv4, the 1970 Server creates both an IPv4 forwarding table entry and an IPv6 1971 forwarding table entry with the SPAN Compatibility Prefix (SCP) 1972 corresponding to the IPv4 address. 1974 The Server next creates a symmetric neighbor cache entry for the 1975 Client using the base AERO address as the network-layer address and 1976 with lifetime set to no more than the smallest PD lifetime. Next, 1977 the Server updates the neighbor cache entry by recording the 1978 information in each ifIndex-tuple in the RS OMNI option. The Server 1979 also records the actual SPAN/INET addresses in the neighbor cache 1980 entry. If an SLLAO was present, the Server also compares the SLLAO 1981 address information for the first ifIndex-tuple with the SPAN/INET 1982 information to determine if there is a NAT on the path. 1984 Next, the Server prepares an RA message using its AERO address as the 1985 network-layer source address and the network-layer source address of 1986 the RS message as the network-layer destination address. The Server 1987 includes the delegated MNPs, any other PD parameters and an OMNI 1988 option with an ifIndex-tuple with ifIndex set to 0. The Server then 1989 includes one or more RIOs that encode the MSPs for the AERO link, 1990 plus an MTU option for the link MTU (see Section 3.13). The Server 1991 finally forwards the message to the Client using SPAN/INET, INET, or 1992 NULL encapsulation as necessary. 1994 After the initial RS/RA exchanges, the Server maintains a timer for 1995 the Client's symmetric neighbor cache entry set to expire after 1996 Router Lifetime seconds. If the Client (or Proxy) issues additional 1997 RS messages, the Server sends an RA response and resets the timer. 1998 If the Client (or Proxy) issues an RS with PD release indication the 1999 Server sets the Client's symmetric neighbor cache entry to the 2000 DEPARTED state and withdraws the MNP from the routing system after a 2001 short delay (e.g., 2 seconds). If the timer expires before a new RS 2002 is received, the Server deletes the neighbor cache entry and 2003 withdraws the MNP without delay. 2005 The Server processes these and any other Client ND/PD messages, and 2006 returns an NA/RA reply. The Server may also issue unsolicited RA 2007 messages, e.g., with PD reconfigure parameters to cause the Client to 2008 renegotiate its PDs, with Router Lifetime set to 0 if it can no 2009 longer service this Client, etc. Finally, If the symmetric neighbor 2010 cache entry is in the DEPARTED state, the Server deletes the entry 2011 after DepartTime expires. 2013 Note: Clients SHOULD notify former Servers of their departures, but 2014 Servers are responsible for expiring neighbor cache entries and 2015 withdrawing routes even if no departure notification is received 2016 (e.g., if the Client leaves the network unexpectedly). Servers 2017 SHOULD therefore include a short Router Lifetime (e.g., 30 seconds) 2018 in solicited RA messages to avoid persistent stale routing 2019 information in the absence of Client departure notifications. A 2020 short Router Lifetime also ensures that proactive Client/Server RS/RA 2021 messaging will keep any NAT state alive (see above). 2023 3.15.3.1. Lightweight DHCPv6 Relay Agent (LDRA) 2025 When DHCPv6 is used as the ND/PD service back end, AERO Clients and 2026 Servers are always on the same link (i.e., the AERO link) from the 2027 perspective of DHCPv6. However, in some implementations the DHCPv6 2028 server and ND function may be located in separate modules. In that 2029 case, the Server's AERO interface module can act as a Lightweight 2030 DHCPv6 Relay Agent (LDRA)[RFC6221] to relay PD messages to and from 2031 the DHCPv6 server module. 2033 When the LDRA receives an authentic RS message, it extracts the PD 2034 message parameters and uses them to construct an IPv6/UDP/DHCPv6 2035 message. It sets the IPv6 source address to the source address of 2036 the RS message, sets the IPv6 destination address to 2037 'All_DHCP_Relay_Agents_and_Servers' and sets the UDP fields to values 2038 that will be understood by the DHCPv6 server. 2040 The LDRA then wraps the message in a DHCPv6 'Relay-Forward' message 2041 header and includes an 'Interface-Id' option that includes enough 2042 information to allow the LDRA to forward the resulting Reply message 2043 back to the Client (e.g., the Client's link-layer addresses, a 2044 security association identifier, etc.). The LDRA also wraps the OMNI 2045 option and SLLAO into the Interface-Id option, then forwards the 2046 message to the DHCPv6 server. 2048 When the DHCPv6 server prepares a Reply message, it wraps the message 2049 in a 'Relay-Reply' message and echoes the Interface-Id option. The 2050 DHCPv6 server then delivers the Relay-Reply message to the LDRA, 2051 which discards the Relay-Reply wrapper and IPv6/UDP headers, then 2052 uses the DHCPv6 message to construct an RA response to the Client. 2053 The Server uses the information in the Interface-Id option to prepare 2054 the RA message and to cache the link-layer addresses taken from the 2055 OMNI option and SLLAO echoed in the Interface-Id option. 2057 3.16. The AERO Proxy 2059 Clients may connect to ANETs that require a perimeter security 2060 gateway to enable communications to Servers in outside INETs. In 2061 that case, the ANET can employ an AERO Proxy. The Proxy is located 2062 at the ANET/INET border and listens for RS messages originating from 2063 or RA messages destined to ANET Clients. The Proxy acts on these 2064 control messages as follows: 2066 o when the Proxy receives an RS message from a new ANET Client, it 2067 first authenticates the message then examines the network-layer 2068 destination address. If the destination address is a Server's 2069 AERO address, the Proxy proceeds to the next step. Otherwise, if 2070 the destination is all-routers multicast or subnet router anycast, 2071 the Proxy selects a "nearby" Server that is likely to be a good 2072 candidate to serve the Client and replaces the destination address 2073 with the Server's AERO address. Next, the Proxy creates a proxy 2074 neighbor cache entry and caches the Client and Server link-layer 2075 addresses along with any identifying information including 2076 Transaction IDs, Client Identifiers, Nonce values, etc. The Proxy 2077 then replaces the SLLAO in the RS message with a new SLLAO with a 2078 single ifIndex-tuple matching the first ifIndex-tuple in the OMNI 2079 option and with the Link Layer Address and Port Number fields set 2080 to the Proxy's SPAN address. The Proxy finally encapsulates the 2081 (proxyed) RS message in a SPAN header with destination set to the 2082 Server's SPAN address then forwards the message into the SPAN. 2084 o when the Server receives the RS message, it authenticates the 2085 message then creates or updates a symmetric neighbor cache entry 2086 for the Client with the Proxy's SPAN address as the link-layer 2087 address. The Server then sends an RA message back to the Proxy 2088 via the SPAN. 2090 o when the Proxy receives the RA message, it matches the message 2091 with the RS that created the proxy neighbor cache entry. The 2092 Proxy then caches the PD route information as a mapping from the 2093 Client's MNPs to the Client's ANET address, and sets the neighbor 2094 cache entry state to REACHABLE. The Proxy then replaces the SLLAO 2095 in the RA with an SLLAO with its own ANET address, sets the P bit 2096 in the RA flags field, and forwards the (proxyed) message to the 2097 Client. 2099 o when the Proxy forwards the (proxyed) RA message, it MAY adjust 2100 Router Lifetime to a larger value. In that case, the Proxy is 2101 responsible for performing periodic RS/RA messaging on the 2102 Client's behalf to refresh Server state lifetimes. This would 2103 allow for higher-frequency RS/RA messaging between the Proxy and 2104 Server without involving the Client, supplemented by lower- 2105 frequency RS/RA messaging between the Client and Server (via the 2106 Proxy). 2108 After the initial RS/RA exchange, the Proxy forwards any Client data 2109 packets for which there is no matching asymmetric neighbor cache 2110 entry to a Relay via the SPAN. The Proxy instead forwards any Client 2111 data destined to an asymmetric neighbor cache target directly to the 2112 target according to the link-layer information - the process of 2113 establishing asymmetric neighbor cache entries is specified in 2114 Section 3.17. 2116 While the Client is still attached to the ANET, the Proxy send RS or 2117 unsolicited NA messages to update the Server's symmetric neighbor 2118 cache entries on behalf of the Client and/or to convey QoS updates. 2119 If the Server ceases to send solicited RA responses, the Proxy marks 2120 the Server as unreachable and sends an unsolicited RA with Router 2121 Lifetime set to zero on the ANET interface to inform Clients that 2122 this Server is no longer able to provide service. Although the Proxy 2123 engages in ND exchanges on behalf of the Client, the Client can also 2124 send ND messages on its own behalf, e.g., if it is in a better 2125 position than the Proxy to convey QoS changes, etc. For this reason, 2126 the Proxy marks any Client-originated solicitation messages (e.g. by 2127 inserting a Nonce option) so that it can return the solicited 2128 advertisement to the Client instead of processsing it locally. 2130 If the Client becomes unreachable, the Proxy sets the neighbor cache 2131 entry state to DEPARTED and retains the entry for DEPARTTIME seconds. 2132 While the state is DEPARTED, the Proxy forwards any packets destined 2133 to the Client to a Relay. The Relay in turn forwards the packets to 2134 the Client's current Server. When DepartTime expires, the Proxy 2135 deletes the neighbor cache entry and discards any further packets 2136 destined to this (now forgotten) Client. 2138 When a neighbor cache entry transitions to DEPARTED, some of the 2139 fragments of a multiple fragment packet may have already arrived at 2140 the Proxy while others are en route to the Client's new location. 2141 However, no special attention in the reassembly algorithm is 2142 necessary when re-routed packets are simply treated as loss. Since 2143 the fragments of a multiple-fragment packet are sent in minimal 2144 inter-packet delay bursts, such occasions will be rare. 2146 In some ANETs that employ a Proxy, the Client's MNP can be injected 2147 into the ANET routing system. In that case, the Client can send data 2148 messages without encapsulation so that the ANET native routing system 2149 transports the unencapsulated packets to the Proxy. This can be very 2150 beneficial, e.g., if the Client connects to the ANET via low-end data 2151 links such as some aviation wireless links. 2153 If the first-hop ANET access router is AERO-aware, the Client can 2154 avoid encapsulation for both its control and data messages. When the 2155 Client connects to the link, it can send an unencapsulated RS message 2156 with source address set to its AERO address and with destination 2157 address set to the AERO address of the Client's selected Server or to 2158 all-routers multicast or subnet router anycast. The Client includes 2159 an OMNI option formatted as specified in 2160 [I-D.templin-atn-aero-interface]. 2162 The Client then sends the unencapsulated RS message, which will be 2163 intercepted by the AERO-Aware access router. The access router then 2164 encapsulates the RS message in an ANET header with its own address as 2165 the source address and the address of a Proxy as the destination 2166 address. The access router further remembers the address of the 2167 Proxy so that it can encapsulate future data packets from the Client 2168 via the same Proxy. If the access router needs to change to a new 2169 Proxy, it simply sends another RS message toward the Server via the 2170 new Proxy on behalf of the Client. 2172 In some cases, the access router and Proxy may be one and the same 2173 node. In that case, the node would be located on the same physical 2174 link as the Client, but its message exchanges with the Server would 2175 need to pass through a security gateway at the ANET/INET border. The 2176 method for deploying access routers and Proxys (i.e. as a single node 2177 or multiple nodes) is an ANET-local administrative consideration. 2179 3.16.1. Detecting and Responding to Server Failures 2181 In environments where fast recovery from Server failure is required, 2182 Proxys SHOULD use proactive Neighbor Unreachability Detection (NUD) 2183 to track Server reachability in a similar fashion as for 2184 Bidirectional Forwarding Detection (BFD) [RFC5880]. Proxys can then 2185 quickly detect and react to failures so that cached information is 2186 re-established through alternate paths. The NUD control messaging is 2187 carried only over well-connected ground domain networks (i.e., and 2188 not low-end aeronautical radio links) and can therefore be tuned for 2189 rapid response. 2191 Proxys perform proactive NUD with Servers for which there are 2192 currently active ANET Clients by sending continuous NS messages in 2193 rapid succession, e.g., one message per second. The Proxy sends the 2194 NS message via the SPAN with the Proxy's AERO address as the source 2195 and the AERO address of the Server as the destination. When the 2196 Proxy is also sending RS messages to the Server on behalf of ANET 2197 Clients, the RS/RA messaging can be considered as equivalent hints of 2198 forward progress. This means that the Proxy need not also send a 2199 periodic NS if it has already sent an RS within the same period. If 2200 the Server fails (i.e., if the Proxy ceases to receive 2201 advertisements), the Proxy can quickly inform Clients by sending RA 2202 messages on the ANET interface. 2204 The Proxy sends RA messages on the ANET interface with source address 2205 set to the Server's address, destination address set to all-nodes 2206 multicast, and Router Lifetime set to 0. The Proxy SHOULD send 2207 MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated by small delays 2208 [RFC4861]. Any Clients on the ANET that have been using the (now 2209 defunct) Server will receive the RA messages and associate with a new 2210 Server. 2212 3.17. AERO Route Optimization 2214 While data packets are flowing between a source and target node, 2215 route optimization SHOULD be used. Route optimization is initiated 2216 by the first eligible Route Optimization Source (ROS) closest to the 2217 source as follows: 2219 o For Clients on VPNed, NATed and Direct interfaces, the Server is 2220 the ROS. 2222 o For Clients on Proxyed interfaces, the Proxy is the ROS. 2224 o For Clients on native interfaces, the Client itself is the ROS. 2226 o For correspondent nodes on INET/EUN interfaces serviced by a 2227 Gateway, the Gateway is the ROS. 2229 The route optimization procedure is conducted between the ROS and the 2230 target Server/Gateway acting as a Route Optimization Responder (ROR) 2231 in the same manner as for IPv6 ND Address Resolution and using the 2232 same NS/NA messaging. The target may either be a MNP Client serviced 2233 by a Server, or a non-MNP correspondent reachable via a Gateway. 2235 The procedures are specified in the following sections. 2237 3.17.1. Route Optimization Initiation 2239 While data packets are flowing from the source node toward a target 2240 node, the ROS performs address resolution by sending an NS message to 2241 receive a solicited NA message from the ROR. 2243 When the ROS sends an NS, it includes the AERO address of the ROS as 2244 the source address (e.g., fe80::1) and the AERO address corresponding 2245 to the data packet's destination address as the destination address 2246 (e.g., if the destination address is 2001:db8:1:2::1 then the 2247 corresponding AERO address is fe80::2001:db8:1:2). The NS message 2248 includes an OMNI option with a single ifIndex-tuple with ifIndex set 2249 to 0, and an SLLAO with the SPAN address of the ROS. The message 2250 also includes a Nonce and Timestamp option if the ROS needs to 2251 correlate NA replies. 2253 The ROS then encapsulates the NS message in a SPAN header with source 2254 set to its own SPAN address and destination set to the data packet's 2255 destination address, then sends the message into the SPAN without 2256 decrementing the network-layer TTL/Hop Limit field. 2258 3.17.2. Relaying the NS 2260 When the Relay receives the NS message from the ROS, it discards the 2261 INET header and determines that the ROR is the next hop by consulting 2262 its standard IPv6 forwarding table for the SPAN header destination 2263 address. The Relay then forwards the SPAN message toward the ROR the 2264 same as for any IPv6 router. The final-hop Relay in the SPAN will 2265 deliver the message via a secured tunnel to the ROR. 2267 3.17.3. Processing the NS and Sending the NA 2269 When the ROR receives the NS message, it examines the AERO 2270 destination address to determine whether it has a neighbor cache 2271 entry and/or route that matches the target. If there is no match, 2272 the ROR drops the NS message. Otherwise, the ROR continues 2273 processing as follows: 2275 o if the target belongs to an MNP Client neighbor in the DEPARTED 2276 state the ROR changes the NS message SPAN destination address to 2277 the SPAN address of the Client's new Server, forwards the message 2278 into the SPAN and returns from processing. 2280 o If the target belongs to an MNP Client neighbor in the REACHABLE 2281 state, the ROR instead adds the AERO source address to the target 2282 Client's Report List with time set to ReportTime. 2284 o If the target belongs to a non-MNP route, the ROR continues 2285 processing without adding an entry to the Report List. 2287 The ROR then prepares a solicited NA message to send back to the ROS 2288 but does not create a neighbor cache entry. The ROR sets the NA 2289 source address to the destination AERO address of the NS, and 2290 includes the Nonce value received in the NS plus the current 2291 Timestamp. 2293 If the target belongs to an MNP Client, the ROR then includes an OMNI 2294 option with prefix information set according to the MNP prefix 2295 length; otherwise, it sets it to the maximum of the non-MNP prefix 2296 length and 64. (Note that a /64 limit is imposed to avoid causing 2297 the ROS to set short prefixes (e.g., "default") that would match 2298 destinations for which the routing system includes more-specific 2299 prefixes.) 2301 The ROR next includes a first ifIndex-tuple in the OMNI option with 2302 ifIndex set to 0. If the target belongs to an MNP Client, the ROR 2303 next includes additional ifIndex-tuples in the OMNI option for the 2304 target Client's underlying interfaces with current information for 2305 each interface 2307 The ROR then includes a TLLAO option with ifIndex-tuples in one-to- 2308 one correspondence with the tuples that appear in the OMNI option. 2309 For NATed, VPNed and Direct interfaces, the link layer addresses are 2310 the SPAN address of the ROR. For Proxyed and native interfaces, the 2311 link-layer addresses are the SPAN addresses of the Proxys and the 2312 Client's native interfaces. 2314 The ROR finally encapsulates the NA message in a SPAN header with 2315 source set to its own SPAN address and destination set to the source 2316 SPAN address of the NS message, then forwards the message into the 2317 SPAN without decrementing the network-layer TTL/Hop Limit field. 2319 3.17.4. Relaying the NA 2321 When the Relay receives the NA message from the ROR, it discards the 2322 INET header and determines that the ROS is the next hop by consulting 2323 its standard IPv6 forwarding table for the SPAN header destination 2324 address. The Relay then forwards the SPAN-encapsulated NA message 2325 toward the ROS the same as for any IPv6 router. The final-hop Relay 2326 in the SPAN will deliver the message via a secured tunnel to the ROS. 2328 3.17.5. Processing the NA 2330 When the ROS receives the solicited NA message, it caches the source 2331 SPAN address then discards the INET and SPAN headers. The ROS next 2332 verifies the Nonce and Timestamp values (if present), then creates an 2333 asymmetric neighbor cache entry for the ROR and caches all 2334 information found in the solicited NA OMNI and TLLAO options. The 2335 ROS finally sets the asymmetric neighbor cache entry lifetime to 2336 ReachableTime seconds. 2338 3.17.6. Route Optimization Maintenance 2340 Following route optimization, the ROS forwards future data packets 2341 destined to the target via the addresses found in the cached link- 2342 layer information. The route optimization is shared by all sources 2343 that send packets to the target via the ROS, i.e., and not just the 2344 source on behalf of which the route optimization was initiated. 2346 While new data packets destined to the target are flowing through the 2347 ROS, it sends additional NS messages to the ROR before ReachableTime 2348 expires to receive a fresh solicited NA message the same as described 2349 in the previous sections (route optimization refreshment strategies 2350 are an implementation matter, with a non-normative example given in 2351 Appendix B.1). The ROS uses the cached SPAN address of the ROR as 2352 the NS SPAN destination address, and sends up to MAX_UNICAST_SOLICIT 2353 NS messages separated by 1 second until an NA is received. If no NA 2354 is received, the ROS assumes that the current ROR has become 2355 unreachable and deletes the neighbor cache entry. Subsequent data 2356 packets will trigger a new route optimization per Section 3.17.1 to 2357 discover a new ROR while initial data packets travel over a 2358 suboptimal route. 2360 If an NA is received, the ROS then updates the asymmetric neighbor 2361 cache entry to refresh ReachableTime, while (for MNP destinations) 2362 the ROR adds or updates the ROS address to the target Client's Report 2363 List and with time set to ReportTime. While no data packets are 2364 flowing, the ROS instead allows ReachableTime for the asymmetric 2365 neighbor cache entry to expire. When ReachableTime expires, the ROS 2366 deletes the asymmetric neighbor cache entry. Future data packets 2367 flowing through the ROS will again trigger a new route optimization. 2369 The ROS may also receive unsolicited NA messages from the ROR at any 2370 time (see: Section 3.19). If there is an asymmetric neighbor cache 2371 entry for the target, the ROS updates the link-layer information but 2372 does not update ReachableTime since the receipt of an unsolicited NA 2373 does not confirm that the forward path is still working. If there is 2374 no asymmetric neighbor cache entry, the ROS simply discards the 2375 unsolicited NA. 2377 In this arrangement, the ROS holds an asymmetric neighbor cache entry 2378 for the ROR, but the ROR does not hold an asymmetric neighbor cache 2379 entry for the ROS. The route optimization neighbor relationship is 2380 therefore asymmetric and unidirectional. If the target node also has 2381 packets to send back to the source node, then a separate route 2382 optimization procedure is performed in the reverse direction. But, 2383 there is no requirement that the forward and reverse paths be 2384 symmetric. 2386 3.18. Neighbor Unreachability Detection (NUD) 2388 AERO nodes perform Neighbor Unreachability Detection (NUD) per 2389 [RFC4861]. NUD is performed either reactively in response to 2390 persistent link-layer errors (see Section 3.14) or proactively to 2391 confirm reachability. The NUD algorithm is based on periodic 2392 authentic NS/NA message exchanges. The algorithm may further be 2393 seeded by ND hints of forward progress, but care must be taken to 2394 avoid inferring reachability based on spoofed information. For 2395 example, authentic RS/RA exchanges may be considered as acceptable 2396 hints of forward progress, while spurious data packets should not be. 2398 When an ROR directs an ROS to a neighbor with one or more target 2399 link-layer addresses, the ROS can proactively test each direct path 2400 by sending an initial NS message to elicit a solicited NA response. 2401 While testing the paths, the ROS can optionally continue sending 2402 packets via the SPAN, maintain a small queue of packets until target 2403 reachability is confirmed, or (optimistically) allow packets to flow 2404 via the direct paths. In any case, the ROS should only consider the 2405 neighbor unreachable if NUD fails over multiple target link-layer 2406 address paths. 2408 When a ROS sends an NS message used for NUD, it uses its AERO 2409 addresses as the IPv6 source address and the AERO address 2410 corresponding to a target link-layer address as the destination. For 2411 each target link-layer address, the source node encapsulates the NS 2412 message in SPAN/INET headers with its own SPAN address as the source 2413 and the SPAN address of the target as the destination, If the target 2414 is located within the same SPAN segment, the source sets the INET 2415 address of the target as the destination; otherwise, it sets the INET 2416 address of a Relay as the destination. The source then forwards the 2417 message into the SPAN. 2419 Paths that pass NUD tests are marked as "reachable", while those that 2420 do not are marked as "unreachable". These markings inform the AERO 2421 interface forwarding algorithm specified in Section 3.12. 2423 Proxys can perform NUD to verify Server reachability on behalf of 2424 their proxyed Clients to reduce Client-initated control messaging 2425 overhead. 2427 3.19. Mobility Management and Quality of Service (QoS) 2429 AERO is a Distributed Mobility Management (DMM) service. Each Server 2430 is responsible for only a subset of the Clients on the AERO link, as 2431 opposed to a Centralized Mobility Management (CMM) service where 2432 there is a single network mobility collective entity for all Clients. 2433 Clients coordinate with their associated Servers via RS/RA exchanges 2434 to maintain the DMM profile, and the AERO routing system tracks all 2435 current Client/Server peering relationships. 2437 Servers provide a Mobility Anchor Point (MAP) for their dependent 2438 Clients. Clients are responsible for maintaining neighbor 2439 relationships with their Servers through periodic RS/RA exchanges, 2440 which also serves to confirm neighbor reachability. When a Client's 2441 underlying interface address and/or QoS information changes, the 2442 Client is responsible for updating the Server with this new 2443 information. Note that for Proxyed interfaces, however, the Proxy 2444 can perform the RS/RA exchanges on the Client's behalf. 2446 Mobility management considerations are specified in the following 2447 sections. 2449 3.19.1. Mobility Update Messaging 2451 Servers acting as MAPs accommodate Client mobility and/or QoS change 2452 events by sending unsolicited NA messages to each ROS in the target 2453 Client's Report List. When a MAP sends an unsolicited NA message, it 2454 sets the IPv6 source address to the Client's AERO address and sets 2455 the IPv6 destination address to all-nodes multicast (ff02::1). The 2456 MAP also includes an OMNI option with a first ifIndex-tuple with 2457 ifIndex set to 0, and with additional ifIndex-tuples for the target 2458 Client's remaining interfaces. The MAP then includes a TLLAO with 2459 corresponding ifIndex-tuples, with the link layer address of the 2460 first tuple set to the MAP's SPAN address and with link layer 2461 addresses of the remaining tuples set to the corresponding target 2462 SPAN addresses. The MAP finally encapsulates the message in a SPAN 2463 header with source set to its own SPAN address and destination set to 2464 the SPAN address of the ROS, then sends the message to a Relay which 2465 in turn forwards it to the ROS. 2467 As discussed in Section 7.2.6 of [RFC4861], the transmission and 2468 reception of unsolicited NA messages is unreliable but provides a 2469 useful optimization. In well-connected Internetworks with robust 2470 data links unsolicited NA messages will be delivered with high 2471 probability, but in any case the MAP can optionally send up to 2472 MAX_NEIGHBOR_ADVERTISEMENT unsolicited NAs to each ROS to increase 2473 the likelihood that at least one will be received. 2475 When the ROS receives an unsolicited NA message, it ignores the 2476 message if there is no existing neighbor cache entry for the Client. 2477 Otherwise, it uses the included OMNI option and TLLAO information to 2478 update the neighbor cache entry, but does not reset ReachableTime 2479 since the receipt of an unsolicited NA message from the target Server 2480 does not provide confirmation that any forward paths to the target 2481 Client are working. 2483 If unsolicited NA messages are lost, the ROS may be left with stale 2484 address and/or QoS information for the Client for up to REACHABLETIME 2485 seconds. During this time, the ROS can continue sending packets 2486 according to its stale neighbor cache information. When 2487 ReachableTime is close to expiring, the ROS will re-initiate route 2488 optimization and receive fresh state information. 2490 In addition to sending unsolicited NA messages to the current set of 2491 ROSs for the Client, the MAP also sends unsolicited NAs to the former 2492 link-layer address for any ifIndex-tuple for which the link-layer 2493 address has changed. The NA messages update Proxys or Servers that 2494 cannot easily detect (e.g., without active probing) when a formerly- 2495 active Client has departed. 2497 3.19.2. Announcing Link-Layer Address and/or QoS Preference Changes 2499 When a Client needs to change its ANET addresses and/or QoS 2500 preferences (e.g., due to a mobility event), either the Client or its 2501 Proxys send RS messages to the Server via the SPAN with an OMNI 2502 option and SLLAO that include an ifIndex-tuple with the new link 2503 quality and address information. 2505 Up to MAX_RTR_SOLICITATION RS messages MAY be sent in parallel with 2506 sending actual data packets in case one or more RAs are lost. If all 2507 RAs are lost, the Client SHOULD re-associate with a new Server. 2509 When the Server receives the Client's changes, it sends unsolicited 2510 NA messages to all nodes in the Report List the same as described in 2511 the previous section. 2513 3.19.3. Bringing New Links Into Service 2515 When a Client needs to bring new underlying interfaces into service 2516 (e.g., when it activates a new data link), it sends an RS message to 2517 the Server via the underlying interface with an OMNI option with 2518 appropriate link quality values and with an SLLAO (if necessary) with 2519 link-layer address information for the new link.. 2521 3.19.4. Removing Existing Links from Service 2523 When a Client needs to remove existing underlying interfaces from 2524 service (e.g., when it de-activates an existing data link), it sends 2525 an RS message to its Server with an OMNI option with appropriate link 2526 quality values. 2528 If the Client needs to send RS messages over an underlying interface 2529 other than the one being removed from service, it MUST include an 2530 ifIndex-tuple for the sending interface as the first tuple and 2531 include additional ifIndex-tuples with appropriate link quality 2532 values for any underlying interfaces being removed from service. 2534 3.19.5. Moving to a New Server 2536 When a Client associates with a new Server, it performs the Client 2537 procedures specified in Section 3.15.2. The Client also includes a 2538 notification identifier in the RS message OMNI option per 2539 [I-D.templin-atn-aero-interface] if it wants the new Server to notify 2540 the old Server. 2542 When the new Server receives the Client's RS message, it responds by 2543 returning an RA as specified in Section 3.15.3. If the Client's RS 2544 includes a notification identifier, the new Server also prepares an 2545 RS or unsolicited NA message to send to the old Server. The RS/NA 2546 message includes the Client's AERO address as the source address, the 2547 old Server's AERO address as the destination address, and an OMNI 2548 option and S/TLLAO with an ifIndex-tuple with ifIndex set to 0. The 2549 OMNI option includes a release indication, and the S/TLLAO includes 2550 the SPAN address of the new Server. For RS messages, the new Server 2551 retries up to MAX_RTR_SOLICITATIONS attempts until an RA is received. 2552 (Note that the Client can alternatively send RS/NA messages with a 2553 release indication to the old Server on its own behalf, however, this 2554 additional Client messaging may be undesirable in some environments. 2555 Note also that the choice of using RS or unsolicited NA is based on 2556 the need for a reliable acknowledgement; in environments where Router 2557 Lifetimes can be expected to be short, sending an unsolicited NA may 2558 be sufficient.) 2560 When the old Server processes the RS/NA, it changes the symmetric 2561 neighbor cache entry state to DEPARTED, sets the link-layer address 2562 of the Client to the address found in the S/TLLAO, and sets 2563 DepartTime to DEPARTTIME seconds. For RS messages, the old Server 2564 then returns an immediate RA message with Router Lifetime set to 0. 2565 After a short delay (e.g., 2 seconds) the old Server withdraws the 2566 Client's MNP from the routing system. After DepartTime expires, the 2567 old Server deletes the symmetric neighbor cache entry. 2569 The old Server also sends unsolicited NA messages to all ROSs in the 2570 Client's Report List with an OMNI option and TLLAO with a single 2571 ifIndex-tuple with ifIndex set to 0 and with the SPAN address of the 2572 new Server. When the ROS receives the NA, it caches the address of 2573 the new Server in the existing asymmetric neighbor cache entry and 2574 marks the entry as STALE. Subsequent data packets will then flow 2575 according to any existing cached link-layer information and trigger a 2576 new NS/NA exchange via the new Server. 2578 Clients SHOULD NOT move rapidly between Servers in order to avoid 2579 causing excessive oscillations in the AERO routing system. Examples 2580 of when a Client might wish to change to a different Server include a 2581 Server that has gone unreachable, topological movements of 2582 significant distance, movement to a new geographic region, movement 2583 to a new SPAN segment, etc. 2585 When a Client moves to a new Server, some of the fragments of a 2586 multiple fragment packet may have already arrived at the old Server 2587 while others are en route to the new Server. However, no special 2588 attention in the reassembly algorithm is necessary when re-routed 2589 fragments are simply treated as loss. Since the fragments of a 2590 multiple-fragment packet are sent in a minimal inter-packet delay 2591 burst, such occasions will be rare. 2593 3.20. Multicast 2595 The AERO Client provides an IGMP (IPv4) [RFC2236] or MLD (IPv6) 2596 [RFC3810] proxy service for its EUNs and/or hosted applications 2597 [RFC4605]. The Client forwards IGMP/MLD messages over any of its 2598 underlying interfaces for which group membership is required. The 2599 IGMP/MLD messages may be further forwarded by a first-hop ANET access 2600 router acting as an IGMP/MLD-snooping switch [RFC4541], then 2601 ultimately delivered to an AERO Proxy/Server acting as a Protocol 2602 Independent Multicast - Sparse-Mode (PIM-SM, or simply "PIM") 2603 Designated Router (DR) [RFC7761]. AERO Gateways also act as PIM 2604 routers (i.e., the same as AERO Proxys/Servers) on behalf of nodes on 2605 INET/EUN networks. The behaviors identified in the following 2606 sections correspond to Source-Specific Multicast (SSM) and Any-Source 2607 Multicast (ASM) operational modes. 2609 3.20.1. Source-Specific Multicast (SSM) 2611 When an ROS (i.e., an AERO Proxy/Server/Gateway) "X" acting as PIM 2612 router receives a Join/Prune message from a node on its downstream 2613 interfaces containing one or more ((S)ource, (G)roup) pairs, it 2614 updates its Multicast Routing Information Base (MRIB) accordingly. 2615 For each S belonging to a prefix reachable via X's non-AERO 2616 interfaces, X then forwards the (S, G) Join/Prune to any PIM routers 2617 on those interfaces per [RFC7761]. 2619 For each S belonging to a prefix reachable via X's AERO interface, X 2620 originates a separate copy of the Join/Prune for each (S,G) in the 2621 message using its own AERO address as the source address and ALL-PIM- 2622 ROUTERS as the destination address. X then encapsulates each message 2623 in a SPAN header with source address set to the SPAN address of X and 2624 destination address set to S then forwards the message into the SPAN. 2625 The SPAN in turn forwards the message to AERO Server/Gateway "Y" that 2626 services S. At the same time, if the message was a Join, X sends a 2627 route-optimization NS message toward each S the same as discussed in 2628 Section 3.17. The resulting NAs will return the AERO address for the 2629 prefix that matches S as the network-layer source address and TLLAOs 2630 with the SPAN addresses corresponding to any ifIndex-tuples that are 2631 currently servicing S. 2633 When Y processes the Join/Prune message, if S located behind any 2634 Native, Direct, VPNed or NATed interfaces Y acts as a PIM router and 2635 updates its MRIB to list X as the next hop in the reverse path. If S 2636 is located behind any Proxys "Z"*, Y also forwards the message to 2637 each Z* over the SPAN while continuing to use the AERO address of X 2638 as the source address. Each Z* then updates its MRIB accordingly and 2639 maintains the AERO address of X as the next hop in the reverse path. 2640 Since the Relays in the SPAN do not examine network layer control 2641 messages, this means that the (reverse) multicast tree path is simply 2642 from each Z* (and/or Y) to X with no other multicast-aware routers in 2643 the path. If any Z* (and/or Y) is located on the same SPAN segment 2644 as X, the multicast data traffic sent to X directly using SPAN/INET 2645 encapsulation instead of via a Relay. 2647 Following the initial Join/Prune and NS/NA messaging, X maintains an 2648 asymmetric neighbor cache entry for each S the same as if X was 2649 sending unicast data traffic to S. In particular, X performs 2650 additional NS/NA exchanges to keep the neighbor cache entry alive for 2651 up to t_periodic seconds [RFC7761]. If no new Joins are received 2652 within t_periodic seconds, X allows the neighbor cache entry to 2653 expire. Finally, if X receives any additional Join/Prune messages 2654 for (S,G) it forwards the messages to each Y and Z* in the neighbor 2655 cache entry over the SPAN. 2657 At some later time, Client C that holds an MNP for source S may 2658 depart from a first Proxy Z1 and/or connect via a new Proxy Z2. In 2659 that case, Y sends an unsolicited NA message to X the same as 2660 specified for unicast mobility in Section 3.19. When X receives the 2661 unsolicited NA message, it updates its asymmetric neighbor cache 2662 entry for the AERO address for source S and sends new Join messages 2663 to any new Proxys Z2. There is no requirement to send any Prune 2664 messages to old Proxys Z1 since source S will no longer source any 2665 multicast data traffic via Z1. Instead, the multicast state for 2666 (S,G) in Proxy Z1 will soon time out since no new Joins will arrive. 2668 After some later time, C may move to a new Server Y2 and depart from 2669 old Sever Y1. In that case, Y1 sends Join messages for any of C's 2670 active (S,G) groups to Y2 while including its own AERO address as the 2671 source address. This causes Y2 to include Y1 in the multicast 2672 forwarding tree during the interim time that Y1's symmetric neighbor 2673 cache entry for C is in the DEPARTED state. At the same time, Y1 2674 sends an unsolicited NA message to X with an OMNI option and TLLAO 2675 with ifIndex-tuple set to 0 and a release indication to cause X to 2676 release its asymmetric neighbor cache entry. X then sends a new Join 2677 message to S via the SPAN and re-initiates route optimization the 2678 same as if it were receiving a fresh Join message from a node on a 2679 downstream link. 2681 3.20.2. Any-Source Multicast (ASM) 2683 When an ROS X acting as a PIM router receives a Join/Prune from a 2684 node on its downstream interfaces containing one or more (*,G) pairs, 2685 it updates its Multicast Routing Information Base (MRIB) accordingly. 2686 X then forwards a copy of the message to the Rendezvous Point (RP) R 2687 for each G over the SPAN. X uses its own AERO address as the source 2688 address and ALL-PIM-ROUTERS as the destination address, then 2689 encapsulates each message in a SPAN header with source address set to 2690 the SPAN address of X and destination address set to R, then sends 2691 the message into the SPAN. At the same time, if the message was a 2692 Join X initiates NS/NA route optimization the same as for the SSM 2693 case discussed in Section 3.20.1. 2695 For each source S that sends multicast traffic to group G via R, the 2696 Proxy/Server Z* for the Client that aggregates S encapsulates the 2697 packets in PIM Register messages and forwards them to R via the SPAN. 2698 R may then elect to send a PIM Join to Z* over the SPAN. This will 2699 result in an (S,G) tree rooted at Z* with R as the next hop so that R 2700 will begin to receive two copies of the packet; one native copy from 2701 the (S, G) tree and a second copy from the pre-existing (*, G) tree 2702 that still uses PIM Register encapsulation. R can then issue a PIM 2703 Register-stop message to suppress the Register-encapsulated stream. 2704 At some later time, if C moves to a new Proxy/Server Z*, it resumes 2705 sending packets via PIM Register encapsulation via the new Z*. 2707 At the same time, as multicast listeners discover individual S's for 2708 a given G, they can initiate an (S,G) Join for each S under the same 2709 procedures discussed in Section 3.20.1. Once the (S,G) tree is 2710 established, the listeners can send (S, G) Prune messages to R so 2711 that multicast packets for group G sourced by S will only be 2712 delivered via the (S, G) tree and not from the (*, G) tree rooted at 2713 R. All mobility considerations discussed for SSM apply. 2715 3.20.3. Bi-Directional PIM (BIDIR-PIM) 2717 Bi-Directional PIM (BIDIR-PIM) [RFC5015] provides an alternate 2718 approach to ASM that treats the Rendezvous Point (RP) as a Designated 2719 Forwarder (DF). Further considerations for BIDIR-PIM are out of 2720 scope. 2722 3.21. Operation over Multiple AERO Links (VLANs) 2724 An AERO Client can connect to multiple AERO links the same as for any 2725 data link service. In that case, the Client maintains a distinct 2726 AERO interface for each link, e.g., 'aero0' for the first link, 2727 'aero1' for the second, 'aero2' for the third, etc. Each AERO link 2728 would include its own distinct set of Relays, Servers and Proxys, 2729 thereby providing redundancy in case of failures. 2731 The Relays, Servers and Proxys on each AERO link can assign AERO and 2732 SPAN addresses that use the same or different numberings from those 2733 on other links. Since the links are mutually independent there is no 2734 requirement for avoiding inter-link address duplication, e.g., the 2735 same AERO address such as fe80::1000 could be used to number distinct 2736 nodes that connect to different AERO links. 2738 Each AERO link could utilize the same or different ANET connections. 2739 The links can be distinguished at the link-layer via Virtual Local 2740 Area Network (VLAN) tagging (e.g., IEEE 802.1Q) and/or through 2741 assignment of distinct sets of MSPs on each link. This gives rise to 2742 the opportunity for supporting multiple redundant networked paths, 2743 where each VLAN is distinguished by a different label (e.g., colors 2744 such as Red, Green, Blue, etc.). In particular, the Client can tag 2745 its RS messages with the appropriate label to cause the network to 2746 select the desired VLAN. 2748 Clients that connect to multiple AERO interfaces can select the 2749 outgoing interface appropriate for a given Red/Blue/Green/etc. 2750 traffic profile while (in the reverse direction) correspondent nodes 2751 must have some way of steering their packets destined to a target via 2752 the correct AERO link. 2754 In a first alternative, if each AERO link services different MSPs, 2755 then the Client can receive a distinct MNP from each of the links. 2756 IP routing will therefore assure that the correct Red/Green/Blue/etc. 2757 network is used for both outbound and inbound traffic. This can be 2758 accomplished using existing technologies and approaches, and without 2759 requiring any special supporting code in correspondent nodes or 2760 Relays. 2762 In a second alternative, if each AERO link services the same MSP(s) 2763 then each link could assign a distinct "AERO Link Anycast" address 2764 that is configured by all Relays on the link. Correspondent nodes 2765 then include a "type 4" routing header with the Anycast address for 2766 the AERO link as the IPv6 destination and with the address of the 2767 target encoded as the "next segment" in the routing header 2768 [RFC8402][I-D.ietf-6man-segment-routing-header]. Standard IP routing 2769 will then direct the packet to the nearest Relay for the correct AERO 2770 link, which will replace the destination address with the target 2771 address then forward the packet to the target. 2773 3.22. DNS Considerations 2775 AERO Client MNs and INET correspondent nodes consult the Domain Name 2776 System (DNS) the same as for any Internetworking node. When 2777 correspondent nodes and Client MNs use different IP protocol versions 2778 (e.g., IPv4 correspondents and IPv6 MNs), the INET DNS must maintain 2779 A records for IPv4 address mappings to MNs which must then be 2780 populated in Gateway NAT64 mapping caches. In that way, an IPv4 2781 correspondent node can send packets to the IPv4 address mapping of 2782 the target MN, and the Gateway will translate the IPv4 header and 2783 destination address into an IPv6 header and IPv6 destination address 2784 of the MN. 2786 When an AERO Client registers with an AERO Server, the Server can 2787 return the address(es) of DNS servers in RDNSS options [RFC6106]. 2788 The DNS server provides the IP addresses of other MNs and 2789 correspondent nodes in AAAA records for IPv6 or A records for IPv4. 2791 3.23. Transition Considerations 2793 The SPAN ensures that dissimilar INET partitions can be joined into a 2794 single unified AERO link, even though the partitions themselves may 2795 have differing protocol versions and/or incompatible addressing 2796 plans. However, a commonality can be achieved by incrementally 2797 distributing globally routable (i.e., native) IP prefixes to 2798 eventually reach all nodes (both mobile and fixed) in all SPAN 2799 segments. This can be accomplished by incrementally deploying AERO 2800 Gateways on each INET partition, with each Gateway distributing its 2801 MNPs and/or discovering non-MNP prefixes on its INET links. 2803 This gives rise to the opportunity to eventually distribute native IP 2804 addresses to all nodes, and to present a unified AERO link view 2805 (bridged by the SPAN) even if the INET partitions remain in their 2806 current protocol and addressing plans. In that way, the AERO link 2807 can serve the dual purpose of providing a mobility service and a 2808 transition service. Or, if an INET partition is transitioned to a 2809 native IP protocol version and addressing scheme that is compatible 2810 with the AERO link MNP-based addressing scheme, the partition and 2811 AERO link can be joined by Gateways. 2813 Gateways that connect INETs/EUNs with dissimilar IP protocol versions 2814 must employ a network address and protocol translation function such 2815 as NAT64[RFC6146]. 2817 3.24. Detecting and Reacting to Server and Relay Failures 2819 In environments where rapid failure recovery is required, Servers and 2820 Relays SHOULD use Bidirectional Forwarding Detection (BFD) [RFC5880]. 2821 Nodes that use BFD can quickly detect and react to failures so that 2822 cached information is re-established through alternate nodes. BFD 2823 control messaging is carried only over well-connected ground domain 2824 networks (i.e., and not low-end radio links) and can therefore be 2825 tuned for rapid response. 2827 Servers and Relays maintain BFD sessions in parallel with their BGP 2828 peerings. If a Server or Relay fails, BGP peers will quickly re- 2829 establish routes through alternate paths the same as for common BGP 2830 deployments. Similarly, Proxys maintain BFD sessions with their 2831 associated Relays even though they do not establish BGP peerings with 2832 them. 2834 Proxys SHOULD use proactive NUD for Servers for which there are 2835 currently active ANET Clients in a manner that parallels BFD, i.e., 2836 by sending unicast NS messages in rapid succession to receive 2837 solicited NA messages. When the Proxy is also sending RS messages on 2838 behalf of ANET Clients, the RS/RA messaging can be considered as 2839 equivalent hints of forward progress. This means that the Proxy need 2840 not also send a periodic NS if it has already sent an RS within the 2841 same period. If a Server fails, the Proxy will cease to receive 2842 advertisements and can quickly inform Clients of the outage by 2843 sending RA messages on the ANET interface. 2845 The Proxy sends RA messages with source address set to the Server's 2846 address, destination address set to all-nodes multicast, and Router 2847 Lifetime set to 0. The Proxy SHOULD send 2848 MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated by small delays 2849 [RFC4861]. Any Clients on the ANET interface that have been using 2850 the (now defunct) Server will receive the RA messages and associate 2851 with a new Server. 2853 4. Implementation Status 2855 An AERO implementation based on OpenVPN (https://openvpn.net/) was 2856 announced on the v6ops mailing list on January 10, 2018 and an 2857 initial public release of the AERO proof-of-concept source code was 2858 announced on the intarea mailing list on August 21, 2015. 2860 5. IANA Considerations 2862 The IANA has assigned a 4-octet Private Enterprise Number "45282" for 2863 AERO in the "enterprise-numbers" registry. 2865 The IANA has assigned the UDP port number "8060" for an earlier 2866 experimental version of AERO [RFC6706]. This document obsoletes 2867 [RFC6706] and claims the UDP port number "8060" for all future use. 2869 No further IANA actions are required. 2871 6. Security Considerations 2873 AERO Relays configure secured tunnels with AERO Servers and Proxys 2874 within their local SPAN segments. Applicable secured tunnel 2875 alternatives include IPsec [RFC4301], TLS/SSL [RFC8446], DTLS 2876 [RFC6347], etc. The AERO Relays of all SPAN segments in turn 2877 configure secured tunnels for their neighboring AERO Relays across 2878 the SPAN. Therefore, packets that traverse the SPAN between any pair 2879 of AERO link neighbors are already secured. 2881 AERO Servers, Gateways and Proxys targeted by a route optimization 2882 may also receive packets directly from the INET partitions instead of 2883 via the SPAN. For INET partitions that apply effective ingress 2884 filtering to defeat source address spoofing, the simple data origin 2885 authentication procedures in Section 3.11 can be applied. This 2886 implies that the ROS list must be maintained consistently by all 2887 route optimization targets within the same INET partition, and that 2888 the ROS list must be securely managed by the partition administrative 2889 authority. 2891 For INET partitions that cannot apply effective ingress filtering, 2892 the two options for securing communications include 1) disable route 2893 optimization so that all traffic is conveyed over secured tunnels via 2894 the SPAN, or 2) enable on-demand secure tunnel creation between INET 2895 partition neighbors. Option 1) would result in longer routes than 2896 necessary and traffic concentration on critical infrastructure 2897 elements. Option 2) could be coordinated by establishing a secured 2898 tunnel on-demand instead of performing an NS/NA exchange in the route 2899 optimization procedures. Procedures for establishing on-demand 2900 secured tunnels are out of scope. 2902 AERO Clients that connect to secured enclaves need not apply security 2903 to their ND messages, since the messages will be intercepted by a 2904 perimeter Proxy that applies security on its outward-facing 2905 interface. AERO Clients located outside of secured enclaves SHOULD 2906 use symmetric network and/or transport layer security services, but 2907 when there are many prospective neighbors with dynamically changing 2908 connectivity an asymmetric security service such as SEND may be 2909 needed (see: Appendix B.6). 2911 Application endpoints SHOULD use application-layer security services 2912 such as TLS/SSL, DTLS or SSH [RFC4251] to assure the same level of 2913 protection as for critical secured Internet services. AERO Clients 2914 that require host-based VPN services SHOULD use symmetric network 2915 and/or transport layer security services such as IPsec, TLS/SSL, 2916 DTLS, etc. AERO Proxys and Servers can also provide a network-based 2917 VPN service on behalf of the Client, e.g., if the Client is located 2918 within a secured enclave and cannot establish a VPN on its own 2919 behalf. 2921 AERO Servers and Relays present targets for traffic amplification 2922 Denial of Service (DoS) attacks. This concern is no different than 2923 for widely-deployed VPN security gateways in the Internet, where 2924 attackers could send spoofed packets to the gateways at high data 2925 rates. This can be mitigated by connecting Servers and Relays over 2926 dedicated links with no connections to the Internet and/or when 2927 connections to the Internet are only permitted through well-managed 2928 firewalls. Traffic amplification DoS attacks can also target an AERO 2929 Client's low data rate links. This is a concern not only for Clients 2930 located on the open Internet but also for Clients in secured 2931 enclaves. AERO Servers and Proxys can institute rate limits that 2932 protect Clients from receiving packet floods that could DoS low data 2933 rate links. 2935 AERO Gateways must implement ingress filtering to avoid a spoofing 2936 attack in which spurious SPAN messages are injected into an AERO link 2937 from an outside attacker. AERO Clients MUST ensure that their 2938 connectivity is not used by unauthorized nodes on their EUNs to gain 2939 access to a protected network, i.e., AERO Clients that act as routers 2940 MUST NOT provide routing services for unauthorized nodes. (This 2941 concern is no different than for ordinary hosts that receive an IP 2942 address delegation but then "share" the address with other nodes via 2943 some form of Internet connection sharing such as tethering.) 2945 The MAP list and ROS lists MUST be well-managed and secured from 2946 unauthorized tampering, even though the list contains only public 2947 information. The MAP list can be conveyed to the Client in a similar 2948 fashion as in [RFC5214] (e.g., through layer 2 data link login 2949 messaging, secure upload of a static file, DNS lookups, etc.). The 2950 ROS list can be conveyed to Servers and Proxys through administrative 2951 action, secured file distribution, etc. 2953 Although public domain and commercial SEND implementations exist, 2954 concerns regarding the strength of the cryptographic hash algorithm 2955 have been documented [RFC6273] [RFC4982]. 2957 Security considerations for accepting link-layer ICMP messages and 2958 reflected packets are discussed throughout the document. 2960 7. Acknowledgements 2962 Discussions in the IETF, aviation standards communities and private 2963 exchanges helped shape some of the concepts in this work. 2964 Individuals who contributed insights include Mikael Abrahamsson, Mark 2965 Andrews, Fred Baker, Bob Braden, Stewart Bryant, Brian Carpenter, 2966 Wojciech Dec, Ralph Droms, Adrian Farrel, Nick Green, Sri Gundavelli, 2967 Brian Haberman, Bernhard Haindl, Joel Halpern, Tom Herbert, Sascha 2968 Hlusiak, Lee Howard, Andre Kostur, Hubert Kuenig, Ted Lemon, Andy 2969 Malis, Satoru Matsushima, Tomek Mrugalski, Madhu Niraula, Alexandru 2970 Petrescu, Behcet Saikaya, Michal Skorepa, Joe Touch, Bernie Volz, 2971 Ryuji Wakikawa, Tony Whyman, Lloyd Wood and James Woodyatt. Members 2972 of the IESG also provided valuable input during their review process 2973 that greatly improved the document. Special thanks go to Stewart 2974 Bryant, Joel Halpern and Brian Haberman for their shepherding 2975 guidance during the publication of the AERO first edition. 2977 This work has further been encouraged and supported by Boeing 2978 colleagues including Kyle Bae, M. Wayne Benson, Dave Bernhardt, Cam 2979 Brodie, John Bush, Balaguruna Chidambaram, Irene Chin, Bruce Cornish, 2980 Claudiu Danilov, Don Dillenburg, Joe Dudkowski, Wen Fang, Samad 2981 Farooqui, Anthony Gregory, Jeff Holland, Seth Jahne, Brian Jaury, 2982 Greg Kimberly, Ed King, Madhuri Madhava Badgandi, Laurel Matthew, 2983 Gene MacLean III, Rob Muszkiewicz, Sean O'Sullivan, Vijay 2984 Rajagopalan, Greg Saccone, Rod Santiago, Kent Shuey, Brian Skeen, 2985 Mike Slane, Carrie Spiker, Katie Tran, Brendan Williams, Amelia 2986 Wilson, Julie Wulff, Yueli Yang, Eric Yeh and other members of the 2987 Boeing mobility, networking and autonomy teams. Kyle Bae, Wayne 2988 Benson, Katie Tran and Eric Yeh are especially acknowledged for 2989 implementing the AERO functions as extensions to the public domain 2990 OpenVPN distribution. 2992 Earlier works on NBMA tunneling approaches are found in 2993 [RFC2529][RFC5214][RFC5569]. 2995 Many of the constructs presented in this second edition of AERO are 2996 based on the author's earlier works, including: 2998 o The Internet Routing Overlay Network (IRON) 2999 [RFC6179][I-D.templin-ironbis] 3001 o Virtual Enterprise Traversal (VET) 3002 [RFC5558][I-D.templin-intarea-vet] 3004 o The Subnetwork Encapsulation and Adaptation Layer (SEAL) 3005 [RFC5320][I-D.templin-intarea-seal] 3007 o AERO, First Edition [RFC6706] 3008 Note that these works cite numerous earlier efforts that are not also 3009 cited here due to space limitations. The authors of those earlier 3010 works are acknowledged for their insights. 3012 This work is aligned with the NASA Safe Autonomous Systems Operation 3013 (SASO) program under NASA contract number NNA16BD84C. 3015 This work is aligned with the FAA as per the SE2025 contract number 3016 DTFAWA-15-D-00030. 3018 This work is aligned with the Boeing Commercial Airplanes (BCA) 3019 Internet of Things (IoT) and autonomy programs. 3021 This work is aligned with the Boeing Information Technology (BIT) 3022 MobileNet program. 3024 8. References 3026 8.1. Normative References 3028 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 3029 DOI 10.17487/RFC0791, September 1981, 3030 . 3032 [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, 3033 RFC 792, DOI 10.17487/RFC0792, September 1981, 3034 . 3036 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 3037 Requirement Levels", BCP 14, RFC 2119, 3038 DOI 10.17487/RFC2119, March 1997, 3039 . 3041 [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in 3042 IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473, 3043 December 1998, . 3045 [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, 3046 "Definition of the Differentiated Services Field (DS 3047 Field) in the IPv4 and IPv6 Headers", RFC 2474, 3048 DOI 10.17487/RFC2474, December 1998, 3049 . 3051 [RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander, 3052 "SEcure Neighbor Discovery (SEND)", RFC 3971, 3053 DOI 10.17487/RFC3971, March 2005, 3054 . 3056 [RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)", 3057 RFC 3972, DOI 10.17487/RFC3972, March 2005, 3058 . 3060 [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and 3061 More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191, 3062 November 2005, . 3064 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 3065 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 3066 DOI 10.17487/RFC4861, September 2007, 3067 . 3069 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 3070 Address Autoconfiguration", RFC 4862, 3071 DOI 10.17487/RFC4862, September 2007, 3072 . 3074 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 3075 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 3076 May 2017, . 3078 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 3079 (IPv6) Specification", STD 86, RFC 8200, 3080 DOI 10.17487/RFC8200, July 2017, 3081 . 3083 [RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., 3084 Richardson, M., Jiang, S., Lemon, T., and T. Winters, 3085 "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", 3086 RFC 8415, DOI 10.17487/RFC8415, November 2018, 3087 . 3089 8.2. Informative References 3091 [BGP] Huston, G., "BGP in 2015, http://potaroo.net", January 3092 2016. 3094 [I-D.ietf-6man-segment-routing-header] 3095 Filsfils, C., Dukes, D., Previdi, S., Leddy, J., 3096 Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header 3097 (SRH)", draft-ietf-6man-segment-routing-header-26 (work in 3098 progress), October 2019. 3100 [I-D.ietf-dmm-distributed-mobility-anchoring] 3101 Chan, A., Wei, X., Lee, J., Jeon, S., and C. Bernardos, 3102 "Distributed Mobility Anchoring", draft-ietf-dmm- 3103 distributed-mobility-anchoring-14 (work in progress), 3104 November 2019. 3106 [I-D.ietf-intarea-gue] 3107 Herbert, T., Yong, L., and O. Zia, "Generic UDP 3108 Encapsulation", draft-ietf-intarea-gue-09 (work in 3109 progress), October 2019. 3111 [I-D.ietf-intarea-gue-extensions] 3112 Herbert, T., Yong, L., and F. Templin, "Extensions for 3113 Generic UDP Encapsulation", draft-ietf-intarea-gue- 3114 extensions-06 (work in progress), March 2019. 3116 [I-D.ietf-intarea-tunnels] 3117 Touch, J. and M. Townsley, "IP Tunnels in the Internet 3118 Architecture", draft-ietf-intarea-tunnels-10 (work in 3119 progress), September 2019. 3121 [I-D.ietf-rtgwg-atn-bgp] 3122 Templin, F., Saccone, G., Dawra, G., Lindem, A., and V. 3123 Moreno, "A Simple BGP-based Mobile Routing System for the 3124 Aeronautical Telecommunications Network", draft-ietf- 3125 rtgwg-atn-bgp-05 (work in progress), January 2020. 3127 [I-D.templin-6man-dhcpv6-ndopt] 3128 Templin, F., "A Unified Stateful/Stateless Configuration 3129 Service for IPv6", draft-templin-6man-dhcpv6-ndopt-09 3130 (work in progress), January 2020. 3132 [I-D.templin-atn-aero-interface] 3133 Templin, F. and T. Whyman, "Transmission of IPv6 Packets 3134 over Overlay Multilink Network (OMNI) Interfaces", draft- 3135 templin-atn-aero-interface-14 (work in progress), January 3136 2020. 3138 [I-D.templin-intarea-grefrag] 3139 Templin, F., "GRE Tunnel Level Fragmentation", draft- 3140 templin-intarea-grefrag-04 (work in progress), July 2016. 3142 [I-D.templin-intarea-seal] 3143 Templin, F., "The Subnetwork Encapsulation and Adaptation 3144 Layer (SEAL)", draft-templin-intarea-seal-68 (work in 3145 progress), January 2014. 3147 [I-D.templin-intarea-vet] 3148 Templin, F., "Virtual Enterprise Traversal (VET)", draft- 3149 templin-intarea-vet-40 (work in progress), May 2013. 3151 [I-D.templin-ironbis] 3152 Templin, F., "The Interior Routing Overlay Network 3153 (IRON)", draft-templin-ironbis-16 (work in progress), 3154 March 2014. 3156 [I-D.templin-v6ops-pdhost] 3157 Templin, F., "IPv6 Prefix Delegation and Multi-Addressing 3158 Models", draft-templin-v6ops-pdhost-25 (work in progress), 3159 January 2020. 3161 [OVPN] OpenVPN, O., "http://openvpn.net", October 2016. 3163 [RFC1035] Mockapetris, P., "Domain names - implementation and 3164 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 3165 November 1987, . 3167 [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - 3168 Communication Layers", STD 3, RFC 1122, 3169 DOI 10.17487/RFC1122, October 1989, 3170 . 3172 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 3173 DOI 10.17487/RFC1191, November 1990, 3174 . 3176 [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers", 3177 RFC 1812, DOI 10.17487/RFC1812, June 1995, 3178 . 3180 [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, 3181 DOI 10.17487/RFC2003, October 1996, 3182 . 3184 [RFC2236] Fenner, W., "Internet Group Management Protocol, Version 3185 2", RFC 2236, DOI 10.17487/RFC2236, November 1997, 3186 . 3188 [RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4 3189 Domains without Explicit Tunnels", RFC 2529, 3190 DOI 10.17487/RFC2529, March 1999, 3191 . 3193 [RFC2764] Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and A. 3194 Malis, "A Framework for IP Based Virtual Private 3195 Networks", RFC 2764, DOI 10.17487/RFC2764, February 2000, 3196 . 3198 [RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. 3199 Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, 3200 DOI 10.17487/RFC2784, March 2000, 3201 . 3203 [RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE", 3204 RFC 2890, DOI 10.17487/RFC2890, September 2000, 3205 . 3207 [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", 3208 RFC 2923, DOI 10.17487/RFC2923, September 2000, 3209 . 3211 [RFC2983] Black, D., "Differentiated Services and Tunnels", 3212 RFC 2983, DOI 10.17487/RFC2983, October 2000, 3213 . 3215 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition 3216 of Explicit Congestion Notification (ECN) to IP", 3217 RFC 3168, DOI 10.17487/RFC3168, September 2001, 3218 . 3220 [RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener 3221 Discovery Version 2 (MLDv2) for IPv6", RFC 3810, 3222 DOI 10.17487/RFC3810, June 2004, 3223 . 3225 [RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D., 3226 Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. 3227 Wood, "Advice for Internet Subnetwork Designers", BCP 89, 3228 RFC 3819, DOI 10.17487/RFC3819, July 2004, 3229 . 3231 [RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms 3232 for IPv6 Hosts and Routers", RFC 4213, 3233 DOI 10.17487/RFC4213, October 2005, 3234 . 3236 [RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) 3237 Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251, 3238 January 2006, . 3240 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 3241 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 3242 DOI 10.17487/RFC4271, January 2006, 3243 . 3245 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 3246 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 3247 2006, . 3249 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 3250 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 3251 December 2005, . 3253 [RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery 3254 Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April 3255 2006, . 3257 [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet 3258 Control Message Protocol (ICMPv6) for the Internet 3259 Protocol Version 6 (IPv6) Specification", STD 89, 3260 RFC 4443, DOI 10.17487/RFC4443, March 2006, 3261 . 3263 [RFC4511] Sermersheim, J., Ed., "Lightweight Directory Access 3264 Protocol (LDAP): The Protocol", RFC 4511, 3265 DOI 10.17487/RFC4511, June 2006, 3266 . 3268 [RFC4541] Christensen, M., Kimball, K., and F. Solensky, 3269 "Considerations for Internet Group Management Protocol 3270 (IGMP) and Multicast Listener Discovery (MLD) Snooping 3271 Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006, 3272 . 3274 [RFC4605] Fenner, B., He, H., Haberman, B., and H. Sandick, 3275 "Internet Group Management Protocol (IGMP) / Multicast 3276 Listener Discovery (MLD)-Based Multicast Forwarding 3277 ("IGMP/MLD Proxying")", RFC 4605, DOI 10.17487/RFC4605, 3278 August 2006, . 3280 [RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for 3281 IP", RFC 4607, DOI 10.17487/RFC4607, August 2006, 3282 . 3284 [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly 3285 Errors at High Data Rates", RFC 4963, 3286 DOI 10.17487/RFC4963, July 2007, 3287 . 3289 [RFC4982] Bagnulo, M. and J. Arkko, "Support for Multiple Hash 3290 Algorithms in Cryptographically Generated Addresses 3291 (CGAs)", RFC 4982, DOI 10.17487/RFC4982, July 2007, 3292 . 3294 [RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano, 3295 "Bidirectional Protocol Independent Multicast (BIDIR- 3296 PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007, 3297 . 3299 [RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site 3300 Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, 3301 DOI 10.17487/RFC5214, March 2008, 3302 . 3304 [RFC5320] Templin, F., Ed., "The Subnetwork Encapsulation and 3305 Adaptation Layer (SEAL)", RFC 5320, DOI 10.17487/RFC5320, 3306 February 2010, . 3308 [RFC5522] Eddy, W., Ivancic, W., and T. Davis, "Network Mobility 3309 Route Optimization Requirements for Operational Use in 3310 Aeronautics and Space Exploration Mobile Networks", 3311 RFC 5522, DOI 10.17487/RFC5522, October 2009, 3312 . 3314 [RFC5558] Templin, F., Ed., "Virtual Enterprise Traversal (VET)", 3315 RFC 5558, DOI 10.17487/RFC5558, February 2010, 3316 . 3318 [RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4 3319 Infrastructures (6rd)", RFC 5569, DOI 10.17487/RFC5569, 3320 January 2010, . 3322 [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 3323 (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, 3324 . 3326 [RFC6106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, 3327 "IPv6 Router Advertisement Options for DNS Configuration", 3328 RFC 6106, DOI 10.17487/RFC6106, November 2010, 3329 . 3331 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful 3332 NAT64: Network Address and Protocol Translation from IPv6 3333 Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146, 3334 April 2011, . 3336 [RFC6179] Templin, F., Ed., "The Internet Routing Overlay Network 3337 (IRON)", RFC 6179, DOI 10.17487/RFC6179, March 2011, 3338 . 3340 [RFC6221] Miles, D., Ed., Ooghe, S., Dec, W., Krishnan, S., and A. 3341 Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221, 3342 DOI 10.17487/RFC6221, May 2011, 3343 . 3345 [RFC6273] Kukec, A., Krishnan, S., and S. Jiang, "The Secure 3346 Neighbor Discovery (SEND) Hash Threat Analysis", RFC 6273, 3347 DOI 10.17487/RFC6273, June 2011, 3348 . 3350 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 3351 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 3352 January 2012, . 3354 [RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label 3355 for Equal Cost Multipath Routing and Link Aggregation in 3356 Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011, 3357 . 3359 [RFC6706] Templin, F., Ed., "Asymmetric Extended Route Optimization 3360 (AERO)", RFC 6706, DOI 10.17487/RFC6706, August 2012, 3361 . 3363 [RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field", 3364 RFC 6864, DOI 10.17487/RFC6864, February 2013, 3365 . 3367 [RFC7269] Chen, G., Cao, Z., Xie, C., and D. Binet, "NAT64 3368 Deployment Options and Experience", RFC 7269, 3369 DOI 10.17487/RFC7269, June 2014, 3370 . 3372 [RFC7333] Chan, H., Ed., Liu, D., Seite, P., Yokota, H., and J. 3373 Korhonen, "Requirements for Distributed Mobility 3374 Management", RFC 7333, DOI 10.17487/RFC7333, August 2014, 3375 . 3377 [RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I., 3378 Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent 3379 Multicast - Sparse Mode (PIM-SM): Protocol Specification 3380 (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March 3381 2016, . 3383 [RFC8086] Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE- 3384 in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086, 3385 March 2017, . 3387 [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., 3388 "Path MTU Discovery for IP version 6", STD 87, RFC 8201, 3389 DOI 10.17487/RFC8201, July 2017, 3390 . 3392 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 3393 Decraene, B., Litkowski, S., and R. Shakir, "Segment 3394 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 3395 July 2018, . 3397 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 3398 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 3399 . 3401 Appendix A. AERO Alternate Encapsulations 3403 When GUE encapsulation is not needed, AERO can use common 3404 encapsulations such as IP-in-IP [RFC2003][RFC2473][RFC4213], Generic 3405 Routing Encapsulation (GRE) [RFC2784][RFC2890] and others. The 3406 encapsulation is therefore only differentiated from non-AERO tunnels 3407 through the application of AERO control messaging and not through, 3408 e.g., a well-known UDP port number. 3410 As for GUE encapsulation, alternate AERO encapsulation formats may 3411 require encapsulation layer fragmentation. For simple IP-in-IP 3412 encapsulation, an IPv6 fragment header is inserted directly between 3413 the inner and outer IP headers when needed, i.e., even if the outer 3414 header is IPv4. The IPv6 Fragment Header is identified to the outer 3415 IP layer by its IP protocol number, and the Next Header field in the 3416 IPv6 Fragment Header identifies the inner IP header version. For GRE 3417 encapsulation, a GRE fragment header is inserted within the GRE 3418 header [I-D.templin-intarea-grefrag]. 3420 Figure 6 shows the AERO IP-in-IP encapsulation format before any 3421 fragmentation is applied: 3423 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3424 | Outer IPv4 Header | | Outer IPv6 Header | 3425 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3426 |IPv6 Frag Header (optional)| |IPv6 Frag Header (optional)| 3427 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3428 | Inner IP Header | | Inner IP Header | 3429 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3430 | | | | 3431 ~ ~ ~ ~ 3432 ~ Inner Packet Body ~ ~ Inner Packet Body ~ 3433 ~ ~ ~ ~ 3434 | | | | 3435 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3437 Minimal Encapsulation in IPv4 Minimal Encapsulation in IPv6 3439 Figure 6: Minimal Encapsulation Format using IP-in-IP 3441 Figure 7 shows the AERO GRE encapsulation format before any 3442 fragmentation is applied: 3444 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3445 | Outer IP Header | 3446 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3447 | GRE Header | 3448 | (with checksum, key, etc..) | 3449 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3450 | GRE Fragment Header (optional)| 3451 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3452 | Inner IP Header | 3453 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3454 | | 3455 ~ ~ 3456 ~ Inner Packet Body ~ 3457 ~ ~ 3458 | | 3459 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3461 Figure 7: Minimal Encapsulation Using GRE 3463 Alternate encapsulation may be preferred in environments where GUE 3464 encapsulation would add unnecessary overhead. For example, certain 3465 low-bandwidth wireless data links may benefit from a reduced 3466 encapsulation overhead. 3468 GUE encapsulation can traverse network paths that are inaccessible to 3469 non-UDP encapsulations, e.g., for crossing Network Address 3470 Translators (NATs). More and more, network middleboxes are also 3471 being configured to discard packets that include anything other than 3472 a well-known IP protocol such as UDP and TCP. It may therefore be 3473 necessary to determine the potential for middlebox filtering before 3474 enabling alternate encapsulation in a given environment. 3476 In addition to IP-in-IP, GRE and GUE, AERO can also use security 3477 encapsulations such as IPsec, TLS/SSL, DTLS, etc. In that case, AERO 3478 control messaging and route determination occur before security 3479 encapsulation is applied for outgoing packets and after security 3480 decapsulation is applied for incoming packets. 3482 AERO is especially well suited for use with VPN system encapsulations 3483 such as OpenVPN [OVPN]. 3485 Appendix B. Non-Normative Considerations 3487 AERO can be applied to a multitude of Internetworking scenarios, with 3488 each having its own adaptations. The following considerations are 3489 provided as non-normative guidance: 3491 B.1. Implementation Strategies for Route Optimization 3493 Route optimization as discussed in Section 3.17 results in the route 3494 optimization source (ROS) creating an asymmetric neighbor cache entry 3495 for the target neighbor. The neighbor cache entry is maintained for 3496 at most REACHABLETIME seconds and then deleted unless updated. In 3497 order to refresh the neighbor cache entry lifetime before the 3498 ReachableTime timer expires, the specification requires 3499 implementations to issue a new NS/NA exchange to reset ReachableTime 3500 to REACHABLETIME seconds while data packets are still flowing. 3501 However, the decision of when to initiate a new NS/NA exchange and to 3502 perpetuate the process is left as an implementation detail. 3504 One possible strategy may be to monitor the neighbor cache entry 3505 watching for data packets for (REACHABLETIME - 5) seconds. If any 3506 data packets have been sent to the neighbor within this timeframe, 3507 then send an NS to receive a new NA. If no data packets have been 3508 sent, wait for 5 additional seconds and send an immediate NS if any 3509 data packets are sent within this "expiration pending" 5 second 3510 window. If no additional data packets are sent within the 5 second 3511 window, delete the neighbor cache entry. 3513 The monitoring of the neighbor data packet traffic therefore becomes 3514 an asymmetric ongoing process during the neighbor cache entry 3515 lifetime. If the neighbor cache entry expires, future data packets 3516 will trigger a new NS/NA exchange while the packets themselves are 3517 delivered over a longer path until route optimization state is re- 3518 established. 3520 B.2. Implicit Mobility Management 3522 AERO interface neighbors MAY provide a configuration option that 3523 allows them to perform implicit mobility management in which no ND 3524 messaging is used. In that case, the Client only transmits packets 3525 over a single interface at a time, and the neighbor always observes 3526 packets arriving from the Client from the same link-layer source 3527 address. 3529 If the Client's underlying interface address changes (either due to a 3530 readdressing of the original interface or switching to a new 3531 interface) the neighbor immediately updates the neighbor cache entry 3532 for the Client and begins accepting and sending packets according to 3533 the Client's new address. This implicit mobility method applies to 3534 use cases such as cellphones with both WiFi and Cellular interfaces 3535 where only one of the interfaces is active at a given time, and the 3536 Client automatically switches over to the backup interface if the 3537 primary interface fails. 3539 B.3. Direct Underlying Interfaces 3541 When a Client's AERO interface is configured over a Direct interface, 3542 the neighbor at the other end of the Direct link can receive packets 3543 without any encapsulation. In that case, the Client sends packets 3544 over the Direct link according to QoS preferences. If the Direct 3545 interface has the highest QoS preference, then the Client's IP 3546 packets are transmitted directly to the peer without going through an 3547 ANET/INET. If other interfaces have higher QoS preferences, then the 3548 Client's IP packets are transmitted via a different interface, which 3549 may result in the inclusion of Proxys, Servers and Relays in the 3550 communications path. Direct interfaces must be tested periodically 3551 for reachability, e.g., via NUD. 3553 B.4. AERO Clients on the Open Internetwork 3555 AERO Clients that connect to the open Internetwork via either a 3556 native or NATed interface can establish a VPN to securely connect to 3557 a Server. Alternatively, the Client can exchange ND messages 3558 directly with other AERO nodes on the same SPAN segment using INET 3559 encapsulation only and without joining the SPAN. In that case, 3560 however, the Client must apply asymmetric security for ND messages to 3561 ensure routing and neighbor cache integrity (see: Section 6). 3563 B.5. Operation on AERO Links with /64 ASPs 3565 IPv6 AERO links typically have MSPs that aggregate many candidate 3566 MNPs of length /64 or shorter. However, in some cases it may be 3567 desirable to use AERO over links that have only a /64 MSP. This can 3568 be accommodated by treating all Clients on the AERO link as simple 3569 hosts that receive /128 prefix delegations. 3571 In that case, the Client sends an RS message to the Server the same 3572 as for ordinary AERO links. The Server responds with an RA message 3573 that includes one or more /128 prefixes (i.e., singleton addresses) 3574 that include the /64 MSP prefix along with an interface identifier 3575 portion to be assigned to the Client. The Client and Server then 3576 configure their AERO addresses based on the interface identifier 3577 portions of the /128s (i.e., the lower 64 bits) and not based on the 3578 /64 prefix (i.e., the upper 64 bits). 3580 For example, if the MSP for the host-only IPv6 AERO link is 3581 2001:db8:1000:2000::/64, each Client will receive one or more /128 3582 IPv6 prefix delegations such as 2001:db8:1000:2000::1/128, 3583 2001:db8:1000:2000::2/128, etc. When the Client receives the prefix 3584 delegations, it assigns the AERO addresses fe80::1, fe80::2, etc. to 3585 the AERO interface, and assigns the global IPv6 addresses (i.e., the 3586 /128s) to either the AERO interface or an internal virtual interface 3587 such as a loopback. In this arrangement, the Client conducts route 3588 optimization in the same sense as discussed in Section 3.17. 3590 This specification has applicability for nodes that act as a Client 3591 on an "upstream" AERO link, but also act as a Server on "downstream" 3592 AERO links. More specifically, if the node acts as a Client to 3593 receive a /64 prefix from the upstream AERO link it can then act as a 3594 Server to provision /128s to Clients on downstream AERO links. 3596 B.6. AERO Adaptations for SEcure Neighbor Discovery (SEND) 3598 SEcure Neighbor Discovery (SEND) [RFC3971] and Cryptographically 3599 Generated Addresses (CGAs) [RFC3972] were designed to secure IPv6 ND 3600 messaging in environments where symmetric network and/or transport- 3601 layer security services are impractical (see: Section 6). AERO nodes 3602 that use SEND/CGA employ the following adaptations. 3604 When a source AERO node prepares a SEND-protected ND message, it uses 3605 a link-local CGA as the IPv6 source address and writes the prefix 3606 embedded in its AERO address (i.e., instead of fe80::/64) in the CGA 3607 parameters Subnet Prefix field. When the neighbor receives the ND 3608 message, it first verifies the message checksum and SEND/CGA 3609 parameters while using the link-local prefix fe80::/64 (i.e., instead 3610 of the value in the Subnet Prefix field) to match against the IPv6 3611 source address of the ND message. 3613 The neighbor then derives the AERO address of the source by using the 3614 value in the Subnet Prefix field as the interface identifier of an 3615 AERO address. For example, if the Subnet Prefix field contains 3616 2001:db8:1:2, the neighbor constructs the AERO address as 3617 fe80::2001:db8:1:2. The neighbor then caches the AERO address in the 3618 neighbor cache entry it creates for the source, and uses the AERO 3619 address as the IPv6 destination address of any ND message replies. 3621 B.7. AERO Critical Infrastructure Considerations 3623 AERO Relays can be either Commercial off-the Shelf (COTS) standard IP 3624 routers or virtual machines in the cloud. Relays must be 3625 provisioned, supported and managed by the INET administrative 3626 authority, and connected to the Relays of other INETs via inter- 3627 domain peerings. Cost for purchasing, configuring and managing 3628 Relays is nominal even for very large AERO links. 3630 AERO Servers can be standard dedicated server platforms, but most 3631 often will be deployed as virtual machines in the cloud. The only 3632 requirements for Servers are that they can run the AERO user-level 3633 code and have at least one network interface connection to the INET. 3634 As with Relays, Servers must be provisioned, supported and managed by 3635 the INET administrative authority. Cost for purchasing, configuring 3636 and managing Servers is nominal especially for virtual Servers hosted 3637 in the cloud. 3639 AERO Proxys are most often standard dedicated server platforms with 3640 one network interface connected to the ANET and a second interface 3641 connected to an INET. As with Servers, the only requirements are 3642 that they can run the AERO user-level code and have at least one 3643 interface connection to the INET. Proxys must be provisioned, 3644 supported and managed by the ANET administrative authority. Cost for 3645 purchasing, configuring and managing Proxys is nominal, and borne by 3646 the ANET administrative authority. 3648 AERO Gateways can be any dedicated server or COTS router platform 3649 connected to INETs and/or EUNs. The Gateway joins the SPAN and 3650 engages in eBGP peering with one or more Relays as a stub AS. The 3651 Gateway then injects its MNPs and/or non-MNP prefixes into the BGP 3652 routing system, and provisions the prefixes to its downstream- 3653 attached networks. The Gateway can perform ROS and MAP services the 3654 same as for any Server, and can route between the MNP and non-MNP 3655 address spaces. 3657 B.8. AERO Server Failure Implications 3659 AERO Servers may appear as a single point of failure in the 3660 architecture, but such is not the case since all Servers on the link 3661 provide identical services and loss of a Server does not imply 3662 immediate and/or comprehensive communication failures. Although 3663 Clients typically associate with a single Server at a time, Server 3664 failure is quickly detected and conveyed by Bidirectional Forward 3665 Detection (BFD) and/or proactive NUD allowing Clients to migrate to 3666 new Servers. 3668 If a Server fails, ongoing packet forwarding to Clients will continue 3669 by virtue of the asymmetric neighbor cache entries that have already 3670 been established in route optimization sources (ROSs). If a Client 3671 also experiences mobility events at roughly the same time the Server 3672 fails, unsolicited NA messages may be lost but proxy neighbor cache 3673 entries in the DEPARTED state will ensure that packet forwarding to 3674 the Client's new locations will continue for up to DEPARTTIME 3675 seconds. 3677 If a Client is left without a Server for an extended timeframe (e.g., 3678 greater than REACHABLETIIME seconds) then existing asymmetric 3679 neighbor cache entries will eventually expire and both ongoing and 3680 new communications will fail. The original source will continue to 3681 retransmit until the Client has established a new Server 3682 relationship, after which time continuous communications will resume. 3684 Therefore, providing many Servers on the link with high availability 3685 profiles provides resilience against loss of individual Servers and 3686 assurance that Clients can establish new Server relationships quickly 3687 in event of a Server failure. 3689 B.9. AERO Client / Server Architecture 3691 The AERO architectural model is client / server in the control plane, 3692 with route optimization in the data plane. The same as for common 3693 Internet services, the AERO Client discovers the addresses of AERO 3694 Servers and selects one Server to connect to. The AERO service is 3695 analogous to common Internet services such as google.com, yahoo.com, 3696 cnn.com, etc. However, there is only one AERO service for the link 3697 and all Servers provide identical services. 3699 Common Internet services provide differing strategies for advertising 3700 server addresses to clients. The strategy is conveyed through the 3701 DNS resource records returned in response to name resolution queries. 3702 As of January 2020 Internet-based 'nslookup' services were used to 3703 determine the following: 3705 o When a client resolves the domainname "google.com", the DNS always 3706 returns one A record (i.e., an IPv4 address) and one AAAA record 3707 (i.e., an IPv6 address). The client receives the same addresses 3708 each time it resolves the domainname via the same DNS resolver, 3709 but may receive different addresses when it resolves the 3710 domainname via different DNS resolvers. But, in each case, 3711 exactly one A and one AAAA record are returned. 3713 o When a client resolves the domainname "ietf.org", the DNS always 3714 returns one A record and one AAAA record with the same addresses 3715 regardless of which DNS resolver is used. 3717 o When a client resolves the domainname "yahoo.com", the DNS always 3718 returns a list of 4 A records and 4 AAAA records. Each time the 3719 client resolves the domainname via the same DNS resolver, the same 3720 list of addresses are returned but in randomized order (i.e., 3721 consistent with a DNS round-robin strategy). But, interestingly, 3722 the same addresses are returned (albeit in randomized order) when 3723 the domainname is resolved via different DNS resolvers. 3725 o When a client resolves the domainname "amazon.com", the DNS always 3726 returns a list of 3 A records and no AAAA records. As with 3727 "yahoo.com", the same three A records are returned from any 3728 worldwide Internet connection point in randomized order. 3730 The above example strategies show differing approaches to Internet 3731 resilience and service distribution offered by major Internet 3732 services. The Google approach exposes only a single IPv4 and a 3733 single IPv6 address to clients. Clients can then select whichever IP 3734 protocol version offers the best response, but will always use the 3735 same IP address according to the current Internet connection point. 3736 This means that the IP address offered by the network must lead to a 3737 highly-available server and/or service distribution point. In other 3738 words, resilience is predicated on high availability within the 3739 network and with no client-initiated failovers expected (i.e., it is 3740 all-or-nothing from the client's perspective). However, Google does 3741 provide for worldwide distributed service distribution by virtue of 3742 the fact that each Internet connection point responds with a 3743 different IPv6 and IPv4 address. The IETF approach is like google 3744 (all-or-nothing from the client's perspective), but provides only a 3745 single IPv4 or IPv6 address on a worldwide basis. This means that 3746 the addresses must be made highly-available at the network level with 3747 no client failover possibility, and if there is any worldwide service 3748 distribution it would need to be conducted by a network element that 3749 is reached via the IP address acting as a service distribution point. 3751 In contrast to the Google and IETF philosophies, Yahoo and Amazon 3752 both provide clients with a (short) list of IP addresses with Yahoo 3753 providing both IP protocol versions and Amazon as IPv4-only. The 3754 order of the list is randomized with each name service query 3755 response, with the effect of round-robin load balancing for service 3756 distribution. With a short list of addresses, there is still 3757 expectation that the network will implement high availability for 3758 each address but in case any single address fails the client can 3759 switch over to using a different address. The balance then becomes 3760 one of function in the network vs function in the end system. 3762 The same implications observed for common highly-available services 3763 in the Internet apply also to the AERO client/server architecture. 3764 When an AERO Client connects to one or more ANETs, it discovers one 3765 or more AERO Server addresses through the mechanisms discussed in 3766 earlier sections. Each Server address presumably leads to a fault- 3767 tolerant clustering arrangement such as supported by Linux-HA, 3768 Extended Virtual Synchrony or Paxos. Such an arrangement has 3769 precedence in common Internet service deployments in lightweight 3770 virtual machines without requiring expensive hardware deployment. 3771 Similarly, common Internet service deployments set service IP 3772 addresses on service distribution points that may relay requests to 3773 many different servers. 3775 For AERO, the expectation is that a combination of the Google/IETF 3776 and Yahoo/Amazon philosophies would be employed. The AERO Client 3777 connects to different ANET access points and can receive 1-2 Server 3778 AERO addresses at each point. It then selects one AERO Server 3779 address, and engages in RS/RA exchanges with the same Server from all 3780 ANET connections. The Client remains with this Server unless or 3781 until the Server fails, in which case it can switch over to an 3782 alternate Server. The Client can likewise switch over to a different 3783 Server at any time if there is some reason for it to do so. So, the 3784 AERO expectation is for a balance of function in the network and end 3785 system, with fault tolerance and resilience at both levels. 3787 Appendix C. Change Log 3789 << RFC Editor - remove prior to publication >> 3791 Changes from draft-templin-intarea-6706bis-22 to draft-templin- 3792 intrea-6706bis-23: 3794 o Choice of using either RS/RA or unsolicited NA for old Server 3795 notification. 3797 o General cleanup. 3799 Changes from draft-templin-intarea-6706bis-21 to draft-templin- 3800 intrea-6706bis-22: 3802 o Tightened up text on Proxy. 3804 o Removed unnecessarily restrictive texts. 3806 o General cleanup. 3808 Changes from draft-templin-intarea-6706bis-20 to draft-templin- 3809 intrea-6706bis-21: 3811 o Clarified relationship between OMNI and S/TLLAO ifIndex-tuples. 3813 o Important text in Section 13.15.3 on Servers timing out Clients 3814 that have gone silent without sending a departure notification. 3816 o New text on RS/RA as "hints of forward progress" for proactive 3817 NUD. 3819 Changes from draft-templin-intarea-6706bis-19 to draft-templin- 3820 intrea-6706bis-20: 3822 o Included new route optimization source and destination addressing 3823 strategy. Now, route optimization maintenance uses the address of 3824 the existing Server instead of the data packet destination address 3825 so that less pressure is placed on the BGP routing system 3826 convergence time and Server constancy is supported. 3828 o Included new method for releasing from old MSE without requiring 3829 Client messaging. 3831 o Included references to new OMNI interface spec (including the OMNI 3832 option). 3834 o New appendix on AERO Client/Server architecture. 3836 Changes from draft-templin-intarea-6706bis-18 to draft-templin- 3837 intrea-6706bis-19: 3839 o Changed Proxy/Server keepalives to use "proactive NUD" in a manner 3840 tha paralles BFD 3842 Changes from draft-templin-intarea-6706bis-17 to draft-templin- 3843 intrea-6706bis-18: 3845 o Discuss how AERO option is used in relation to S/TLLAOs 3847 o New text on Bidirectional Forwarding Detection (BFD) 3849 o Cleaned up usage (and non-usage) of unsolicited NAs 3850 o New appendix on Server failures 3852 Changes from draft-templin-intarea-6706bis-15 to draft-templin- 3853 intrea-6706bis-17: 3855 o S/TLLAO now includes multiple link-layer addresses within a single 3856 option instead of requiring multiple options 3858 o New unsolicited NA message to inform the old link that a Client 3859 has moved to a new link 3861 Changes from draft-templin-intarea-6706bis-14 to draft-templin- 3862 intrea-6706bis-15: 3864 o MTU and fragmentation 3866 o New details in movement to new Server 3868 Changes from draft-templin-intarea-6706bis-13 to draft-templin- 3869 intrea-6706bis-14: 3871 o Security based on secured tunnels, ingress filtering, MAP list and 3872 ROS list 3874 Changes from draft-templin-intarea-6706bis-12 to draft-templin- 3875 intrea-6706bis-13: 3877 o New paragraph in Section 3.6 on AERO interface layering over 3878 secured tunnels 3880 o Removed extraneous text in Section 3.7 3882 o Added new detail to the forwarding algorithm in Section 3.9 3884 o Clarified use of fragmentation 3886 o Route optimization now supported for both MNP and non-MNP-based 3887 prefixes 3889 o Relays are now seen as link-layer elements in the architecture. 3891 o Built out multicast section in detail. 3893 o New Appendix on implementation considerations for route 3894 optimization. 3896 Changes from draft-templin-intarea-6706bis-11 to draft-templin- 3897 intrea-6706bis-12: 3899 o Introduced Gateways as a new AERO element for connecting 3900 Correspondent Nodes on INET links 3902 o Introduced terms "Access Network (ANET)" and "Internetwork (INET)" 3904 o Changed "ASP" to "MSP", and "ACP" to "MNP" 3906 o New figure on the relation of Segments to the SPAN and AERO link 3908 o New "S" bit in S/TLLAO to indicate the "Source" S/TLLAO as opposed 3909 to additional S/TLLAOs 3911 o Changed Interface ID for Servers from 255 to 0xffff 3913 o Significant updates to Route Optimization, NUD, and Mobility 3914 Management 3916 o New Section on Multicast 3918 o New Section on AERO Clients in the open Internetwork 3920 o New Section on Operation over multiple AERO links (VLANs over the 3921 SPAN) 3923 o New Sections on DNS considerations and Transition considerations 3925 o 3927 Changes from draft-templin-intarea-6706bis-10 to draft-templin- 3928 intrea-6706bis-11: 3930 o Added The SPAN 3932 Changes from draft-templin-intarea-6706bis-09 to draft-templin- 3933 intrea-6706bis-10: 3935 o Orphaned packets in flight (e.g., when a neighbor cache entry is 3936 in the DEPARTED state) are now forwarded at the link layer instead 3937 of at the network layer. Forwarding at the network layer can 3938 result in routing loops and/or excessive delays of forwarded 3939 packets while the routing system is still reconverging. 3941 o Update route optimization to clarify the unsecured nature of the 3942 first NS used for route discovery 3944 o Many cleanups and clarifications on ND messaging parameters 3945 Changes from draft-templin-intarea-6706bis-08 to draft-templin- 3946 intrea-6706bis-09: 3948 o Changed PRL to "MAP list" 3950 o For neighbor cache entries, changed "static" to "symmetric", and 3951 "dynamic" to "asymmetric" 3953 o Specified Proxy RS/RA exchanges with Servers on behalf of Clients 3955 o Added discussion of unsolicited NAs in Section 3.16, and included 3956 forward reference to Section 3.18 3958 o Added discussion of AERO Clients used as critical infrastructure 3959 elements to connect fixed networks. 3961 o Added network-based VPN under security considerations 3963 Changes from draft-templin-intarea-6706bis-07 to draft-templin- 3964 intrea-6706bis-08: 3966 o New section on AERO-Aware Access Router 3968 Changes from draft-templin-intarea-6706bis-06 to draft-templin- 3969 intrea-6706bis-07: 3971 o Added "R" bit for release of PDs. Now have a full RS/RA service 3972 that can do PD without requiring DHCPv6 messaging over-the-air 3974 o Clarifications on solicited vs unsolicited NAs 3976 o Clarified use of MAX_NEIGHBOR_ADVERTISEMENTS for the purpose of 3977 increase reliability 3979 Changes from draft-templin-intarea-6706bis-05 to draft-templin- 3980 intrea-6706bis-06: 3982 o Major re-work and simplification of Route Optimization function 3984 o Added Distributed Mobility Management (DMM) and Mobility Anchor 3985 Point (MAP) terminology 3987 o New section on "AERO Critical Infrastructure Element 3988 Considerations" demonstrating low overall cost for the service 3990 o minor text revisions and deletions 3992 o removed extraneous appendices 3993 Changes from draft-templin-intarea-6706bis-04 to draft-templin- 3994 intrea-6706bis-05: 3996 o New Appendix E on S/TLLAO Extensions for special-purpose links. 3997 Discussed ATN/IPS as example. 3999 o New sentence in introduction to declare appendices as non- 4000 normative. 4002 Changes from draft-templin-intarea-6706bis-03 to draft-templin- 4003 intrea-6706bis-04: 4005 o Added definitions for Potential Router List (PRL) and secure 4006 enclave 4008 o Included text on mapping transport layer port numbers to network 4009 layer DSCP values 4011 o Added reference to DTLS and DMM Distributed Mobility Anchoring 4012 working group document 4014 o Reworked Security Considerations 4016 o Updated references. 4018 Changes from draft-templin-intarea-6706bis-02 to draft-templin- 4019 intrea-6706bis-03: 4021 o Added new section on SEND. 4023 o Clarifications on "AERO Address" section. 4025 o Updated references and added new reference for RFC8086. 4027 o Security considerations updates. 4029 o General text clarifications and cleanup. 4031 Changes from draft-templin-intarea-6706bis-01 to draft-templin- 4032 intrea-6706bis-02: 4034 o Note on encapsulation avoidance in Section 4. 4036 Changes from draft-templin-intarea-6706bis-00 to draft-templin- 4037 intrea-6706bis-01: 4039 o Remove DHCPv6 Server Release procedures that leveraged the old way 4040 Relays used to "route" between Server link-local addresses 4042 o Remove all text relating to Relays needing to do any AERO-specific 4043 operations 4045 o Proxy sends RS and receives RA from Server using SEND. Use CGAs 4046 as source addresses, and destination address of RA reply is to the 4047 AERO address corresponding to the Client's ACP. 4049 o Proxy uses SEND to protect RS and authenticate RA (Client does not 4050 use SEND, but rather relies on subnetwork security. When the 4051 Proxy receives an RS from the Client, it creates a new RS using 4052 its own addresses as the source and uses SEND with CGAs to send a 4053 new RS to the Server. 4055 o Emphasize distributed mobility management 4057 o AERO address-based RS injection of ACP into underlying routing 4058 system. 4060 Changes from draft-templin-aerolink-82 to draft-templin-intarea- 4061 6706bis-00: 4063 o Document use of NUD (NS/NA) for reliable link-layer address 4064 updates as an alternative to unreliable unsolicited NA. 4065 Consistent with Section 7.2.6 of RFC4861. 4067 o Server adds additional layer of encapsulation between outer and 4068 inner headers of NS/NA messages for transmission through Relays 4069 that act as vanilla IPv6 routers. The messages include the AERO 4070 Server Subnet Router Anycast address as the source and the Subnet 4071 Router Anycast address corresponding to the Client's ACP as the 4072 destination. 4074 o Clients use Subnet Router Anycast address as the encapsulation 4075 source address when the access network does not provide a 4076 topologically-fixed address. 4078 Author's Address 4080 Fred L. Templin (editor) 4081 Boeing Research & Technology 4082 P.O. Box 3707 4083 Seattle, WA 98124 4084 USA 4086 Email: fltemplin@acm.org