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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, May 28, 2020 5 rfc6179, rfc6706 (if 6 approved) 7 Intended status: Standards Track 8 Expires: November 29, 2020 10 Asymmetric Extended Route Optimization (AERO) 11 draft-templin-intarea-6706bis-52 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, intelligent transportation systems, 26 mobile Virtual Private Networks (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 November 29, 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 Node Types . . . . . . . . . . . . . . . . . . . . . 10 66 3.2. The AERO Link . . . . . . . . . . . . . . . . . . . . . . 11 67 3.2.1. AERO Link Reference Model . . . . . . . . . . . . . . 11 68 3.2.2. AERO Link-Local Addresses (LLAs) . . . . . . . . . . 14 69 3.2.3. AERO Unique Local Addresses (ULAs) . . . . . . . . . 15 70 3.2.4. AERO Routing System . . . . . . . . . . . . . . . . . 16 71 3.2.5. AERO Link Encapsulation . . . . . . . . . . . . . . . 17 72 3.2.6. Segment Routing Topologies (SRTs) . . . . . . . . . . 19 73 3.2.7. Segment Routing To the AERO Link . . . . . . . . . . 19 74 3.2.8. Segment Routing Within the AERO Link . . . . . . . . 20 75 3.2.9. Segment Routing Header Compression . . . . . . . . . 22 76 3.3. AERO Interface Characteristics . . . . . . . . . . . . . 22 77 3.4. AERO Interface Initialization . . . . . . . . . . . . . . 27 78 3.4.1. AERO Server/Relay Behavior . . . . . . . . . . . . . 27 79 3.4.2. AERO Proxy Behavior . . . . . . . . . . . . . . . . . 27 80 3.4.3. AERO Client Behavior . . . . . . . . . . . . . . . . 27 81 3.4.4. AERO Bridge Behavior . . . . . . . . . . . . . . . . 28 82 3.5. AERO Interface Neighbor Cache Maintenance . . . . . . . . 28 83 3.6. AERO Interface Encapsulation and Re-encapsulation . . . . 30 84 3.7. AERO Interface Decapsulation . . . . . . . . . . . . . . 31 85 3.8. AERO Interface Data Origin Authentication . . . . . . . . 31 86 3.9. AERO Interface MTU and Fragmentation . . . . . . . . . . 32 87 3.9.1. Fragmentation Security Implications . . . . . . . . . 34 88 3.10. AERO Interface Forwarding Algorithm . . . . . . . . . . . 34 89 3.10.1. Client Forwarding Algorithm . . . . . . . . . . . . 35 90 3.10.2. Proxy Forwarding Algorithm . . . . . . . . . . . . . 36 91 3.10.3. Server/Relay Forwarding Algorithm . . . . . . . . . 37 92 3.10.4. Bridge Forwarding Algorithm . . . . . . . . . . . . 38 94 3.11. AERO Interface Error Handling . . . . . . . . . . . . . . 39 95 3.12. AERO Router Discovery, Prefix Delegation and 96 Autoconfiguration . . . . . . . . . . . . . . . . . . . . 41 97 3.12.1. AERO ND/PD Service Model . . . . . . . . . . . . . . 41 98 3.12.2. AERO Client Behavior . . . . . . . . . . . . . . . . 42 99 3.12.3. AERO Server Behavior . . . . . . . . . . . . . . . . 44 100 3.13. The AERO Proxy . . . . . . . . . . . . . . . . . . . . . 47 101 3.13.1. Detecting and Responding to Server Failures . . . . 49 102 3.13.2. Point-to-Multipoint Server Coordination . . . . . . 50 103 3.14. AERO Route Optimization . . . . . . . . . . . . . . . . . 50 104 3.14.1. Route Optimization Initiation . . . . . . . . . . . 51 105 3.14.2. Relaying the NS . . . . . . . . . . . . . . . . . . 51 106 3.14.3. Processing the NS and Sending the NA . . . . . . . . 51 107 3.14.4. Relaying the NA . . . . . . . . . . . . . . . . . . 53 108 3.14.5. Processing the NA . . . . . . . . . . . . . . . . . 53 109 3.14.6. Route Optimization Maintenance . . . . . . . . . . . 53 110 3.15. Neighbor Unreachability Detection (NUD) . . . . . . . . . 54 111 3.16. Mobility Management and Quality of Service (QoS) . . . . 55 112 3.16.1. Mobility Update Messaging . . . . . . . . . . . . . 56 113 3.16.2. Announcing Link-Layer Address and/or QoS Preference 114 Changes . . . . . . . . . . . . . . . . . . . . . . 57 115 3.16.3. Bringing New Links Into Service . . . . . . . . . . 57 116 3.16.4. Removing Existing Links from Service . . . . . . . . 57 117 3.16.5. Moving to a New Server . . . . . . . . . . . . . . . 58 118 3.17. Multicast . . . . . . . . . . . . . . . . . . . . . . . . 59 119 3.17.1. Source-Specific Multicast (SSM) . . . . . . . . . . 59 120 3.17.2. Any-Source Multicast (ASM) . . . . . . . . . . . . . 60 121 3.17.3. Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . . 61 122 3.18. Operation over Multiple AERO Links (VLANs) . . . . . . . 61 123 3.19. DNS Considerations . . . . . . . . . . . . . . . . . . . 62 124 3.20. Transition Considerations . . . . . . . . . . . . . . . . 63 125 3.21. Detecting and Reacting to Server and Bridge Failures . . 63 126 3.22. AERO Clients on the Open Internet . . . . . . . . . . . . 64 127 3.22.1. Use of SEND and CGA . . . . . . . . . . . . . . . . 66 128 3.23. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . . 68 129 4. Implementation Status . . . . . . . . . . . . . . . . . . . . 68 130 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 68 131 6. Security Considerations . . . . . . . . . . . . . . . . . . . 69 132 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 71 133 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 72 134 8.1. Normative References . . . . . . . . . . . . . . . . . . 72 135 8.2. Informative References . . . . . . . . . . . . . . . . . 74 136 Appendix A. AERO Alternate Encapsulations . . . . . . . . . . . 82 137 Appendix B. Non-Normative Considerations . . . . . . . . . . . . 83 138 B.1. Implementation Strategies for Route Optimization . . . . 84 139 B.2. Implicit Mobility Management . . . . . . . . . . . . . . 84 140 B.3. Direct Underlying Interfaces . . . . . . . . . . . . . . 85 141 B.4. Operation on AERO Links with /64 ASPs . . . . . . . . . . 85 142 B.5. AERO Critical Infrastructure Considerations . . . . . . . 86 143 B.6. AERO Server Failure Implications . . . . . . . . . . . . 86 144 B.7. AERO Client / Server Architecture . . . . . . . . . . . . 87 145 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 89 146 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 98 148 1. Introduction 150 Asymmetric Extended Route Optimization (AERO) fulfills the 151 requirements of Distributed Mobility Management (DMM) [RFC7333] and 152 route optimization [RFC5522] for aeronautical networking and other 153 network mobility use cases such as intelligent transportation 154 systems. AERO is based on a Non-Broadcast, Multiple Access (NBMA) 155 virtual link model known as the AERO link. The AERO link is a 156 virtual overlay configured over one or more underlying Internetworks, 157 and nodes on the link can exchange IP packets via tunneling. 158 Multilink operation allows for increased reliability, bandwidth 159 optimization and traffic path diversity. 161 The AERO service comprises Clients, Proxys, Servers and Relays that 162 are seen as AERO link neighbors as well as Bridges that interconnect 163 AERO link segments. Each node's AERO interface uses an IPv6 link- 164 local address format that supports operation of the IPv6 Neighbor 165 Discovery (ND) protocol [RFC4861] and links ND to IP forwarding. A 166 node's AERO interface can be configured over multiple underlying 167 interfaces, and may therefore appear as a single interface with 168 multiple link-layer addresses. Each link-layer address is subject to 169 change due to mobility and/or QoS fluctuations, and link-layer 170 address changes are signaled by ND messaging the same as for any IPv6 171 link. 173 AERO links provide a cloud-based service where mobile nodes may use 174 any Server acting as a Mobility Anchor Point (MAP) and fixed nodes 175 may use any Relay on the link for efficient communications. Fixed 176 nodes forward packets destined to other AERO nodes to the nearest 177 Relay, which forwards them through the cloud. A mobile node's 178 initial packets are forwarded through the Server, while direct 179 routing is supported through asymmetric extended route optimization 180 while data packets are flowing. Both unicast and multicast 181 communications are supported, and mobile nodes may efficiently move 182 between locations while maintaining continuous communications with 183 correspondents and without changing their IP Address. 185 AERO Bridges are interconnected in a secured private BGP overlay 186 routing instance using encapsulation to provide a hybrid routing/ 187 bridging service that joins the underlying Internetworks of multiple 188 disjoint administrative domains into a single unified AERO link. 189 Each AERO link instance is characterized by the set of Mobility 190 Service Prefixes (MSPs) common to all mobile nodes. The link extends 191 to the point where a Relay/Server is on the optimal route from any 192 correspondent node on the link, and provides a conduit between the 193 underlying Internetwork and the AERO link. To the underlying 194 Internetwork, the Relay/Server is the source of a route to the MSP, 195 and hence uplink traffic to the mobile node is naturally routed to 196 the nearest Relay/Server. 198 AERO assumes the use of PIM Sparse Mode in support of multicast 199 communication. In support of Source Specific Multicast (SSM) when a 200 Mobile Node is the source, AERO route optimization ensures that a 201 shortest-path multicast tree is established with provisions for 202 mobility and multilink operation. In all other multicast scenarios 203 there are no AERO dependencies. 205 AERO was designed for aeronautical networking for both manned and 206 unmanned aircraft, where the aircraft is treated as a mobile node 207 that can connect an Internet of Things (IoT). AERO is also 208 applicable to a wide variety of other use cases. For example, it can 209 be used to coordinate the Virtual Private Network (VPN) links of 210 mobile nodes (e.g., cellphones, tablets, laptop computers, etc.) that 211 connect into a home enterprise network via public access networks 212 using services such as OpenVPN [OVPN]. It can also be used to 213 facilitate vehicular and pedestrian communications services for 214 intelligent transportation systems. Other applicable use cases are 215 also in scope. 217 The following numbered sections present the AERO specification. The 218 appendices at the end of the document are non-normative. 220 2. Terminology 222 The terminology in the normative references applies; the following 223 terms are defined within the scope of this document: 225 IPv6 Neighbor Discovery (ND) 226 an IPv6 control message service for coordinating neighbor 227 relationships between nodes connected to a common link. AERO 228 interfaces use the ND service specified in [RFC4861]. 230 IPv6 Prefix Delegation (PD) 231 a networking service for delegating IPv6 prefixes to nodes on the 232 link. The nominal PD service is DHCPv6 [RFC8415], however 233 alternate services (e.g., based on ND messaging) are also in scope 234 [I-D.templin-v6ops-pdhost][I-D.templin-6man-dhcpv6-ndopt]. Most 235 notably, a minimal form of PD known as "prefix registration" can 236 be used if the Client knows its prefix in advance and can 237 represent it in the IPv6 source address of an ND message. 239 Access Network (ANET) 240 a node's first-hop data link service network (e.g., a radio access 241 network, cellular service provider network, corporate enterprise 242 network, etc.) that often provides link-layer security services 243 such as IEEE 802.1X and physical-layer security prevent 244 unauthorized access internally and with border network-layer 245 security services such as firewalls and proxies that prevent 246 unauthorized outside access. 248 ANET interface 249 a node's attachment to a link in an ANET. 251 Internetwork (INET) 252 a connected IP network topology with a coherent routing and 253 addressing plan and that provides a transit backbone service for 254 ANET end systems. INETs also provide an underlay service over 255 which the AERO virtual link is configured. Example INETs include 256 corporate enterprise networks, aviation networks, and the public 257 Internet itself. When there is no administrative boundary between 258 an ANET and the INET, the ANET and INET are one and the same. 260 INET Partition 261 frequently, INETs such as large corporate enterprise networks are 262 sub-divided internally into separate isolated partitions. Each 263 partition is fully connected internally but disconnected from 264 other partitions, and there is no requirement that separate 265 partitions maintain consistent Internet Protocol and/or addressing 266 plans. (Each INET partition is seen as a separate AERO link 267 segment as discussed below.) 269 INET interface 270 a node's attachment to a link in an INET. 272 INET address 273 an IP address assigned to a node's interface connection to an 274 INET. 276 INET encapsulation 277 the encapsulation of a packet in an outer header or headers that 278 can be routed within the scope of the local INET partition. 280 AERO link 281 a Non-Broadcast, Multiple Access (NBMA) virtual overlay over one 282 or more underlying INETs manifested by IPv6 encapsulation 283 [RFC2473]. The AERO link spans underlying INET segments joined by 284 virtual bridges in a spanning tree the same as a bridged campus 285 LAN. Nodes on the AERO link appear as single-hop neighbors even 286 though they may be separated by multiple underlying INET hops, and 287 can use Segment Routing [RFC8402] to cause packets to visit 288 selected waypoints on the link. 290 AERO interface 291 a node's attachment to an AERO link. Since the addresses assigned 292 to an AERO interface are managed for uniqueness, AERO interfaces 293 do not require Duplicate Address Detection (DAD) and therefore set 294 the administrative variable 'DupAddrDetectTransmits' to zero 295 [RFC4862]. 297 underlying interface 298 an ANET or INET interface over which an AERO interface is 299 configured. 301 AERO Link-Local Address (LLA) 302 a link local IPv6 address per [RFC4291] constructed as specified 303 in Section 3.2.2. 305 AERO Unique-Local Address (ULA) 306 a unique local IPv6 address per [RFC4193] constructed as specified 307 in Section 3.2.3. AERO ULAs are statelessly derived from AERO 308 LLAs, and vice-versa. 310 Mobility Service Prefix (MSP) 311 an IP prefix assigned to the AERO link and from which more- 312 specific Mobile Network Prefixes (MNPs) are derived. 314 Mobile Network Prefix (MNP) 315 an IP prefix allocated from an MSP and delegated to an AERO Client 316 or Relay. 318 AERO node 319 a node that is connected to an AERO link, or that provides 320 services to other nodes on an AERO link. 322 AERO Client ("Client") 323 an AERO node that connects over one or more underlying interfaces 324 and requests MNP PDs from AERO Servers. The Client assigns a 325 Client LLA to the AERO interface for use in ND exchanges with 326 other AERO nodes and forwards packets to correspondents according 327 to AERO interface neighbor cache state. 329 AERO Server ("Server") 330 an INET node that configures an AERO interface to provide default 331 forwarding and mobility/multilink services for AERO Clients. The 332 Server assigns an administratively-provisioned LLA to its AERO 333 interface to support the operation of the ND/PD services, and 334 advertises all of its associated MNPs via BGP peerings with 335 Bridges. 337 AERO Relay ("Relay") 338 an AERO Server that also provides forwarding services between 339 nodes reached via the AERO link and correspondents on other links. 340 AERO Relays are provisioned with MNPs (i.e., the same as for an 341 AERO Client) and run a dynamic routing protocol to discover any 342 non-MNP IP routes. In both cases, the Relay advertises the MSP(s) 343 to its downstream networks, and distributes all of its associated 344 MNPs and non-MNP IP routes via BGP peerings with Bridges (i.e., 345 the same as for an AERO Server). 347 AERO Bridge ("Bridge") 348 a node that provides hybrid routing/bridging services (as well as 349 a security trust anchor) for nodes on an AERO link. As a router, 350 the Bridge forwards packets using standard IP forwarding. As a 351 bridge, the Bridge forwards packets over the AERO link without 352 decrementing the IPv6 Hop Limit. AERO Bridges peer with Servers 353 and other Bridges to discover the full set of MNPs for the link as 354 well as any non-MNPs that are reachable via Relays. 356 AERO Proxy ("Proxy") 357 a node that provides proxying services between Clients in an ANET 358 and Servers in external INETs. The AERO Proxy is a conduit 359 between the ANET and external INETs in the same manner as for 360 common web proxies, and behaves in a similar fashion as for ND 361 proxies [RFC4389]. 363 ingress tunnel endpoint (ITE) 364 an AERO interface endpoint that injects encapsulated packets into 365 an AERO link. 367 egress tunnel endpoint (ETE) 368 an AERO interface endpoint that receives encapsulated packets from 369 an AERO link. 371 link-layer address 372 an IP address used as an encapsulation header source or 373 destination address from the perspective of the AERO interface. 374 When an upper layer protocol (e.g., UDP) is used as part of the 375 encapsulation, the port number is also considered as part of the 376 link-layer address. 378 network layer address 379 the source or destination address of an encapsulated IP packet 380 presented to the AERO interface. 382 end user network (EUN) 383 an internal virtual or external edge IP network that an AERO 384 Client or Relay connects to the rest of the network via the AERO 385 interface. The Client/Relay sees each EUN as a "downstream" 386 network, and sees the AERO interface as the point of attachment to 387 the "upstream" network. 389 Mobile Node (MN) 390 an AERO Client and all of its downstream-attached networks that 391 move together as a single unit, i.e., an end system that connects 392 an Internet of Things. 394 Mobile Router (MR) 395 a MN's on-board router that forwards packets between any 396 downstream-attached networks and the AERO link. 398 Route Optimization Source (ROS) 399 the AERO node nearest the source that initiates route 400 optimization. The ROS may be a Server or Proxy acting on behalf 401 of the source Client. 403 Route Optimization responder (ROR) 404 the AERO node nearest the target destination that responds to 405 route optimization requests. The ROR may be a Server acting on 406 behalf of a target MNP Client, or a Relay for a non-MNP 407 destination. 409 MAP List 410 a geographically and/or topologically referenced list of addresses 411 of all Servers within the same AERO link. There is a single MAP 412 list for the entire AERO link. 414 Distributed Mobility Management (DMM) 415 a BGP-based overlay routing service coordinated by Servers and 416 Bridges that tracks all Server-to-Client associations. 418 Mobility Service (MS) 419 the collective set of all Servers, Proxys, Bridges and Relays that 420 provide the AERO Service to Clients. 422 Mobility Service Endpoint MSE) 423 an individual Server, Proxy, Bridge or Relay in the Mobility 424 Service. 426 Throughout the document, the simple terms "Client", "Server", 427 "Bridge", "Proxy" and "Relay" refer to "AERO Client", "AERO Server", 428 "AERO Bridge", "AERO Proxy" and "AERO Relay", respectively. 430 Capitalization is used to distinguish these terms from other common 431 Internetworking uses in which they appear without capitalization. 433 The terminology of DHCPv6 [RFC8415] and IPv6 ND [RFC4861] (including 434 the names of node variables, messages and protocol constants) is used 435 throughout this document. The terms "All-Routers multicast", "All- 436 Nodes multicast", "Solicited-Node multicast" and "Subnet-Router 437 anycast" are defined in [RFC4291] (with Link-Local scope assumed). 438 Also, the term "IP" is used to generically refer to either Internet 439 Protocol version, i.e., IPv4 [RFC0791] or IPv6 [RFC8200]. 441 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 442 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 443 "OPTIONAL" in this document are to be interpreted as described in BCP 444 14 [RFC2119][RFC8174] when, and only when, they appear in all 445 capitals, as shown here. 447 3. Asymmetric Extended Route Optimization (AERO) 449 The following sections specify the operation of IP over Asymmetric 450 Extended Route Optimization (AERO) links: 452 3.1. AERO Node Types 454 AERO Bridges provide hybrid routing/bridging services (as well as a 455 security trust anchor) for nodes on an AERO link. Bridges use 456 standard IPv6 routing to forward packets both within the same INET 457 partitions and between disjoint INET partitions based on a mid-layer 458 IPv6 encapsulation per [RFC2473]. The inner IP layer experiences a 459 virtual bridging service since the inner IP TTL/Hop Limit is not 460 decremented during forwarding. Each Bridge also peers with Servers 461 and other Bridges in a dynamic routing protocol instance to provide a 462 Distributed Mobility Management (DMM) service for the list of active 463 MNPs (see Section 3.2.4). Bridges present the AERO link as a set of 464 one or more Mobility Service Prefixes (MSPs) but as link-layer 465 devices need not connect directly to the AERO link themselves unless 466 an administrative interface is desired. Bridges configure secured 467 tunnels with Servers, Relays, Proxys and other Bridges; they further 468 maintain IP forwarding table entries for each Mobile Network Prefix 469 (MNP) and any other reachable non-MNP prefixes. 471 AERO Servers provide default forwarding and mobility/multilink 472 services for AERO Client Mobile Nodes (MNs). Each Server also peers 473 with Bridges in a dynamic routing protocol instance to advertise its 474 list of associated MNPs (see Section 3.2.4). Servers facilitate PD 475 exchanges with Clients, where each delegated prefix becomes an MNP 476 taken from an MSP. Servers forward packets between AERO interface 477 neighbors and track each Client's mobility profiles. 479 AERO Clients register their MNPs through PD exchanges with AERO 480 Servers over the AERO link, and distribute the MNPs to nodes on EUNs. 481 A Client may also be co-resident on the same physical or virtual 482 platform as a Server; in that case, the Client and Server behave as a 483 single functional unit. 485 AERO Proxys provide a conduit for ANET Clients to associate with 486 Servers in external INETs. Client and Servers exchange control plane 487 messages via the Proxy acting as a bridge between the ANET/INET 488 boundary. The Proxy forwards data packets between Clients and the 489 AERO link according to forwarding information in the neighbor cache. 490 The Proxy function is specified in Section 3.13. 492 AERO Relays are Servers that provide forwarding services between the 493 AERO interface and INET/EUN interfaces. Relays are provisioned with 494 MNPs the same as for an AERO Client, and also run a dynamic routing 495 protocol to discover any non-MNP IP routes. The Relay advertises the 496 MSP(s) to its connected networks, and distributes all of its 497 associated MNPs and non-MNP IP routes via BGP peerings with Bridges. 499 AERO Bridges, Servers, Proxys and Relays are critical infrastructure 500 elements in fixed (i.e., non-mobile) INET deployments and hence have 501 permanent and unchanging INET addresses. AERO Clients are MNs that 502 connect via underlying interfaces with addresses that may change when 503 the Client moves to a new network connection point. 505 3.2. The AERO Link 507 3.2.1. AERO Link Reference Model 509 Figure 1 presents the basic AERO link reference model: 511 +----------------+ 512 | AERO Bridge B1 | 513 | Nbr: S1, S2, P1| 514 |(X1->S1; X2->S2)| 515 | MSP M1 | 516 +-+---------+--+-+ 517 +--------------+ | Secured | | +--------------+ 518 |AERO Server S1| | tunnels | | |AERO Server S2| 519 | Nbr: C1, B1 +-----+ | +-----+ Nbr: C2, B1 | 520 | default->B1 | | | default->B1 | 521 | X1->C1 | | | X2->C2 | 522 +-------+------+ | +------+-------+ 523 | AERO Link | | 524 X===+===+===================+==)===============+===+===X 525 | | | | 526 +-----+--------+ +--------+--+-----+ +--------+-----+ 527 |AERO Client C1| | AERO Proxy P1 | |AERO Client C2| 528 | Nbr: S1 | |(Proxy Nbr Cache)| | Nbr: S2 | 529 | default->S1 | +--------+--------+ | default->S2 | 530 | MNP X1 | | | MNP X2 | 531 +------+-------+ .--------+------. +-----+--------+ 532 | (- Proxyed Clients -) | 533 .-. `---------------' .-. 534 ,-( _)-. ,-( _)-. 535 .-(_ IP )-. +-------+ +-------+ .-(_ IP )-. 536 (__ EUN )--|Host H1| |Host H2|--(__ EUN ) 537 `-(______)-' +-------+ +-------+ `-(______)-' 539 Figure 1: AERO Link Reference Model 541 In this model: 543 o the AERO link is an overlay network service configured over one or 544 more underlying INET partitions which may be managed by different 545 administrative authorities and have incompatible protocols and/or 546 addressing plans. 548 o AERO Bridge B1 aggregates Mobility Service Prefix (MSP) M1, 549 discovers Mobile Network Prefixes (MNPs) X* and advertises the MSP 550 via BGP peerings over secured tunnels to Servers (S1, S2). 551 Bridges connect the disjoint segments of a partitioned AERO link. 553 o AERO Servers/Relays S1 and S2 configure secured tunnels with 554 Bridge B1 and also provide mobility, multilink and default router 555 services for their associated Clients C1 and C2. 557 o AERO Clients C1 and C2 associate with Servers S1 and S2, 558 respectively. They receive Mobile Network Prefix (MNP) 559 delegations X1 and X2, and also act as default routers for their 560 associated physical or internal virtual EUNs. Simple hosts H1 and 561 H2 attach to the EUNs served by Clients C1 and C2, respectively. 563 o AERO Proxy P1 configures a secured tunnel with Bridge B1 and 564 provides proxy services for AERO Clients in secured enclaves that 565 cannot associate directly with other AERO link neighbors. 567 An AERO link configured over a single INET appears as a single 568 unified link with a consistent underlying network addressing plan. 569 In that case, all nodes on the link can exchange packets via simple 570 INET encapsulation, since the underlying INET is connected. In 571 common practice, however, an AERO link may be partitioned into 572 multiple "segments", where each segment is a distinct INET 573 potentially managed under a different administrative authority (e.g., 574 as for worldwide aviation service providers such as ARINC, SITA, 575 Inmarsat, etc.). Individual INETs may also themselves be partitioned 576 internally, in which case each internal partition is seen as a 577 separate segment. 579 The addressing plan of each segment is consistent internally but will 580 often bear no relation to the addressing plans of other segments. 581 Each segment is also likely to be separated from others by network 582 security devices (e.g., firewalls, proxies, packet filtering 583 gateways, etc.), and in many cases disjoint segments may not even 584 have any common physical link connections. Therefore, nodes can only 585 be assured of exchanging packets directly with correspondents in the 586 same segment, and not with those in other segments. The only means 587 for joining the segments therefore is through inter-domain peerings 588 between AERO Bridges. 590 The same as for traditional campus LANs, multiple AERO link segments 591 can be joined into a single unified link via a virtual bridging 592 service using a mid-layer IPv6 encpasulation per [RFC2473] known as 593 the "SPAN header" that supports inter-segment forwarding (i.e., 594 bridging) without decrementing the network-layer TTL/Hop Limit. This 595 bridging of AERO link segments is shown in Figure 2: 597 . . . . . . . . . . . . . . . . . . . . . . . 598 . . 599 . .-(::::::::) . 600 . .-(::::::::::::)-. +-+ . 601 . (:::: Segment A :::)--|B|---+ . 602 . `-(::::::::::::)-' +-+ | . 603 . `-(::::::)-' | . 604 . | . 605 . .-(::::::::) | . 606 . .-(::::::::::::)-. +-+ | . 607 . (:::: Segment B :::)--|B|---+ . 608 . `-(::::::::::::)-' +-+ | . 609 . `-(::::::)-' | . 610 . | . 611 . .-(::::::::) | . 612 . .-(::::::::::::)-. +-+ | . 613 . (:::: Segment C :::)--|B|---+ . 614 . `-(::::::::::::)-' +-+ | . 615 . `-(::::::)-' | . 616 . | . 617 . ..(etc).. x . 618 . . 619 . . 620 . <- AERO Link Bridged by encapsulation -> . 621 . . . . . . . . . . . . . .. . . . . . . . . 623 Figure 2: Bridging AERO Link Segments 625 Bridges, Servers, Relays and Proxys connect via secured INET tunnels 626 over their respecitve segments in a spanning tree topology rooted at 627 the Bridges. The secured spanning tree supports strong 628 authentication for IPv6 ND control messages and may also be used to 629 convey the initial data packets in a flow. Route optimization can 630 then be employed to cause data packets to take more direct paths 631 between AERO link neighbors without having to strictly follow the 632 spanning tree. 634 3.2.2. AERO Link-Local Addresses (LLAs) 636 Nodes on AERO links use the Link-Local Address (LLA) prefix fe80::/10 637 [RFC4193] to assign LLAs used for network-layer addresses in IPv6 ND 638 and data messages. A Client's LLA is an IPv6 link-local address 639 formed from the Client's delegated MNP. Bridge, Server, Relay and 640 Proxy LLAs are assigned from the range fe80::/96 and include an 641 administratively-provisioned value in the lower 32 bits. 643 IPv6 Client LLAs encode the Subnet-Router anycast address of a MNP 644 (or non-MNP globally routable prefix) within the least-significant 645 112 bits of the IPv6 link-local prefix fe80::/16. For example, for 646 the MNP 2001:db8:1000:2000::/56 the corresponding LLA is 647 fe80:2001:db8:1000:2000::/72. 649 IPv4-compatible Client LLAs are based on an IPv4-mapped IPv6 address 650 [RFC4291] formed from an IPv4 MNP and with a prefix length of 96 plus 651 the MNP prefix length. For example, for the IPv4 MNP 192.0.2.16/28 652 the IPv4-mapped IPv6 MNP is: 654 0:0:0:0:0:ffff:192.0.2.16/124 (also written as 655 0:0:0:0:0:ffff:c000:0210/124) 657 The Client then constructs its LLA with the prefix fe80::/64 and with 658 the lower 64 bits of the IPv4-mapped IPv6 address in the interface 659 identifier as: fe80::ffff:192.0.2.16. 661 Mobility Service (MS) LLAs (used by Bridges, Servers, Relays and 662 Proxys) are allocated from the range fe80::/96, and MUST be managed 663 for uniqueness. The lower 32 bits of the LLA includes a unique 664 integer value between 1 and 0xfeffffff (e.g., fe80::1, fe80::2, 665 fe80::3, etc., fe80::feff:ffff) as assigned by the administrative 666 authority for the link. The address fe80:: is the IPv6 link-local 667 Subnet-Router anycast address, and the address range 668 fe80::ff00:0000/104 is reserved for future use. 670 Finally, the address range fe80::/32 is used as the Teredo service 671 prefix for AERO according to the format in Section 4 of [RFC4380] 672 (see Section 3.22 for further discussion). 674 3.2.3. AERO Unique Local Addresses (ULAs) 676 Nodes on AERO links use the Unique Local Address (ULA) prefix 677 fd80::/10 [RFC4193] to form ULAs used for SPAN header source and 678 desitnation addresses. The prefix length intentionally matches the 679 IPv6 link-local prefix (fe80::/10), and enables a simple stateless 680 translation between LLAs and ULAs. 682 AERO ULAs are formed by simply rewriting the upper bits of the 683 corresponding LLA as follows: 685 o the ULA formed from the IPv6 Client LLA fe80:2001:db8:1000:2000:: 686 is simply fd80:2001:db8:1000:2000:: 688 o the ULA formed from the IPv4-compatible Client LLA 689 fe80::ffff:192.0.2.1 is simply fd80::ffff:192.0.2.1 691 o the ULA formed from the MS LLA fe80::1001 is simply fd80::1001 692 o the ULA prefix fd80::/32 is used as the Teredo service prefix the 693 same as for LLAs above. 695 For routing system organization (see Section 3.2.4), MS ULAs are 696 organized in partition prefixes, e.g., fd80::1000/116. For each such 697 partition prefix, the Bridge(s) that connect that segment assign the 698 :: address of the prefix as a Subnet Router Anycast address. For 699 example, the Subnet Router Anycast address for fd80::1000/116 is 700 simply fd80::1000. 702 3.2.4. AERO Routing System 704 The AERO routing system comprises a private instance of the Border 705 Gateway Protocol (BGP) [RFC4271] that is coordinated between Bridges 706 and Servers and does not interact with either the public Internet BGP 707 routing system or any underlying INET routing systems. 709 In a reference deployment, each Server is configured as an Autonomous 710 System Border Router (ASBR) for a stub Autonomous System (AS) using 711 an AS Number (ASN) that is unique within the BGP instance, and each 712 Server further uses eBGP to peer with one or more Bridges but does 713 not peer with other Servers. Each INET of a multi-segment AERO link 714 must include one or more Bridges, which peer with the Servers and 715 Proxys within that INET. All Bridges within the same INET are 716 members of the same hub AS using a common ASN, and use iBGP to 717 maintain a consistent view of all active MNPs currently in service. 718 The Bridges of different INETs peer with one another using eBGP. 720 Bridges advertise the AERO link's MSPs and any non-MNP routes to each 721 of their Servers. This means that any aggregated non-MNPs (including 722 "default") are advertised to all Servers. Each Bridge configures a 723 black-hole route for each of its MSPs. By black-holing the MSPs, the 724 Bridge will maintain forwarding table entries only for the MNPs that 725 are currently active, and packets destined to all other MNPs will 726 correctly incur Destination Unreachable messages due to the black- 727 hole route. In this way, Servers have only partial topology 728 knowledge (i.e., they know only about the MNPs of their directly 729 associated Clients) and they forward all other packets to Bridges 730 which have full topology knowledge. 732 Each AERO link segment assigns a unique sub-prefix of fd80::/96 known 733 as the ULA partition prefix. For example, a first segment could 734 assign fd80::1000/116, a second could assign fd80::2000/116, a third 735 could assign fd80::3000/116, etc. The administrative authorities for 736 each segment must therefore coordinate to assure mutually-exclusive 737 partiton prefix assignments, but internal provisioning of each prefix 738 is an independent local consideration for each administrative 739 authority. 741 ULA partition prefixes are statitcally represented in Bridge 742 forwarding tables. Bridges join multiple segments into a unified 743 AERO link over multiple diverse administrative domains. They support 744 a bridging function by first establishing forwarding table entries 745 for their partiion prefixes either via standard BGP routing or static 746 routes. For example, if three Bridges ('A', 'B' and 'C') from 747 different segments serviced fd80::1000/116, fd80::2000/116 and 748 fd80::3000/116 respectively, then the forwarding tables in each 749 Bridge are as follows: 751 A: fd80::1000/116->local, fd80::2000/116->B, fd80::3000/116->C 753 B: fd80::1000/116->A, fd80::2000/116->local, fd80::3000/116->C 755 C: fd80::1000/116->A, fd80::2000/116->B, fd80::3000/116->local 757 These forwarding table entries are permanent and never change, since 758 they correspond to fixed infrastructure elements in their respective 759 segments. 761 ULA Client prefixes are instead dynamically advertised in the AERO 762 link routing system by Servers and Relays that provide service for 763 their corresponding MNPs. For example, if three Servers ('D', 'E' 764 and 'F') service the MNPs 2001:db8:1000:2000::/56, 765 2001:db8:3000:4000::/56 and 2001:db8:5000:6000::/56 then the routing 766 system would include: 768 D: fd80:2001:db8:1000:2000::/72 770 E: fd80:2001:db8:3000:4000::/72 772 F: fd80:2001:db8:5000:6000::/72 774 A full discussion of the BGP-based routing system used by AERO is 775 found in [I-D.ietf-rtgwg-atn-bgp]. The system provides for 776 Distributed Mobility Management (DMM) per the distributed mobility 777 anchoring architecture [I-D.ietf-dmm-distributed-mobility-anchoring]. 779 3.2.5. AERO Link Encapsulation 781 With the Client and partition prefixes in place in each Bridge's 782 forwarding table, control and data packets sent between AERO nodes in 783 different segments can therefore be carried over the via mid-layer 784 encapsulation using the SPAN header. For example, when a source AERO 785 node forwards a packet with IPv6 address 2001:db8:1:2::1 to a target 786 AERO node with IPv6 address 2001:db8:1000:2000::1, it first 787 encapsulates the packet in a SPAN header with source address set to 788 fd80:2001:db8:1:2:: and destination address set to 789 fd80:2001:db8:1000:2000::. Next, it encapsulates the resulting SPAN 790 packet in an INET header with source address set to its own INET 791 address (e.g., 192.0.2.100) and destination set to the INET address 792 of a Bridge (e.g., 192.0.2.1). 794 SPAN encapsulation is based on Generic Packet Tunneling in IPv6 795 [RFC2473]; the encapsulation format in the above example is shown in 796 Figure 3: 798 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 799 | INET Header | 800 | src = 192.0.2.100 | 801 | dst = 192.0.2.1 | 802 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 803 | SPAN Header | 804 | src = fd80:2001:db8:1:2:: | 805 | dst=fd80:2001:db8:1000:2000:: | 806 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 807 | Inner IP Header | 808 | src = 2001:db8:1:2::1 | 809 | dst = 2001:db8:1000:2000::1 | 810 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 811 | | 812 ~ ~ 813 ~ Inner Packet Body ~ 814 ~ ~ 815 | | 816 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 818 Figure 3: SPAN Encapsulation 820 In this format, the inner IP header and packet body are the original 821 IP packet, the SPAN header is an IPv6 header prepared according to 822 [RFC2473], and the INET header is prepared as discussed in 823 Section 3.6. 825 This gives rise to a routing system that contains both Client prefix 826 routes that may change dynamically due to regional node mobility and 827 partion prefix routes that never change. The Bridges can therefore 828 provide link-layer bridging by sending packets over the SRT instead 829 of network-layer routing according to MNP routes. As a result, 830 opportunities for packet loss due to node mobility between different 831 segments are mitigated. 833 In normal operations, IPv6 ND messages are conveyed over secured 834 paths between AERO link neighbors so that specific Proxys, Servers or 835 Relays can be addressed without being subject to mobility events. 836 Conversely, only the first few packets destined to Clients need to 837 traverse secured paths until route optimization can determine a more 838 direct path. 840 3.2.6. Segment Routing Topologies (SRTs) 842 The 16-bit sub-prefixes of fd80::/10 (e.g., fd80::/16, fd81::/16, 843 fd82::/16, etc.) identify distinct Segment Routing Topologies (SRTs) 844 (see: Section 3.2.6). Each SRT is a mutually-exclusive AERO link 845 overlay instance using a mutually-exclusive set of ULAs, and emulates 846 a Virtual LAN (VLAN) service for the AERO link. In some cases (e.g., 847 when redundant topologies are needed for fault tolerance and 848 reliability) it may be beneficial to deploy multiple SRTs that act as 849 independent overlay instances. A communication failure in one 850 instance therefore will not affect communications in other instances. 852 Each SRT is identified by a distinct value in bits 10-15 of he SSP 853 fd80::10, i.e., as fd80::/16, fd81::/16, fd82::/16, etc. This 854 document asserts that up to four SRTs provide a level of safety 855 sufficient for critical communications such as civil aviation. Each 856 SRT is designated with a color that identifies a different AERO link 857 instance as follows: 859 o Red - corresponds to the SSP fd80::/16 861 o Green - corresponds to the SSP fd81::/16 863 o Blue-1 - corresponds to the SSP fd82::/16 865 o Blue-2 - corresponds to SSP fd83::/16 867 o SSPs fd84::/16 through fdbf::/16 are reserved for future use. 869 Each AERO interface assigns an anycast ULA corresponding to its SRT 870 prefix. For example, the anycast ULA for the Green SRT is simply 871 fd81::. The anycast ULA is used for AERO interface determination in 872 Safety-Based Multilink (SBM) as discussed in 873 [I-D.templin-6man-omni-interface]. Each AERO interface further 874 applies Performance-Based Multilink (PBM) internally. 876 3.2.7. Segment Routing To the AERO Link 878 An original IPv6 source can direct a packet to a specific SRT ingress 879 router for the AERO link by including a Segment Routing Header (SRH) 880 with the anycast ULA for the selected SRT as either the IPv6 881 destination or as an intermediate segment ID within the SRH. This 882 allows the original source to determine the specific topology a 883 packet will traverse when there may be multiple alternatives to 884 choose from. This form of Segment Routing supports Safety-Based 885 Multilink (SBM), and can be exercised through general-purpose SRH 886 types such as [RFC8754]. 888 3.2.8. Segment Routing Within the AERO Link 890 AERO nodes that insert a SPAN header can use Segment Routing within 891 the AERO link when necessary to influence the path of packets 892 destined to INET Clients without causing all packets to traverse the 893 Server. 895 When a Client, Proxy or Server has a packet to send to a target 896 discovered through route optimization located in the same AERO link 897 segment, it encapsulates the packet in a SPAN header with the ULA of 898 the target as the destination address if fragmentation is necessary, 899 then uses the target's Link Layer Address information for INET 900 encapsulation without including an SRH. 902 When a Client, Proxy or Server has a packet to send to a route 903 optimization target located in a different AERO link segment, it 904 encapsulates the packet in a SPAN header with its own ULA as the 905 source address and with destination set according to the target. If 906 the route optimization target is located behind a Proxy or Server, 907 the node sets the SPAN destination address to the ULA of the Proxy/ 908 Server and forwards the packet to a Bridge without including an SRH. 910 If the route optimization target in a foreign segment is an INET 911 Client, the node instead sets the SPAN destination address to the ULA 912 Subnet Router Anycast address of the foreign segment. The node also 913 includes a SRH [RFC8754] with the ULA of the target's Server as the 914 ultimate Segment ID (SID) and with an AERO Route Optimization 915 specification in the SRH TLV section as shown in Figure 4: 917 0 1 2 3 918 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 1 919 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 920 | Type=TBD | Length |FMT|V| Preflen | MNP[1] | 921 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 922 | MNP[2] | MNP[3] | ... | MNP[i] | 923 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 924 ~ Link Layer Address ~ 925 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 926 | Port Number | 927 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 929 Figure 4: AERO Route Optimization SRH TLV 931 In this format: 933 o Type is TBD to be assigned according to the Segment Routing Header 934 TLV registry [RFC8754]. 936 o Length is the length of the body of the TLV in bytes, excluding 937 the Type and Length fields. 939 o FMT is a two bit code that determines the format of the Link Layer 940 Address exactly as specified in Figure 5. 942 o V indicates the IP protocol version of the MNP that follows. V is 943 set to 0 for IPv4 or 1 for IPv6. 945 o Preflen encodes the value 'i' (between 0 and 16) that indicates 946 the number of bytes of the IPv4/IPv6 MNP prefix that follows. 948 o MNP{1], MNP[2], etc. up to MNP[i] encode the leading 'i' octets of 949 the MNP, beginning with the most significant octet followed by the 950 next most significant octet, etc. The number of MNP octets to be 951 included is determined by the number of trailing zero octets in 952 the prefix. For example, for the MNP fd80:2001:db8:1:2::, 'i' 953 would be 10 and only the leftmost 10 octets of the MNP would be 954 included. 956 o Link Layer Address and Port Number are encoded according to FMT 957 exactly as specified in Figure 5. 959 The node then forwards the packet via a local Bridge, which will 960 eventually direct it to a Bridge on the same segment as the target's 961 Server. 963 When a Bridge on the same segment as the target's Server receives a 964 SPAN-encapsulated packet, it examines the SRH. If Segments Left is 1 965 and the ultimate SID is a Server on the local segment, it checks to 966 see if there is an AERO Route Optimization TLV. If so, the Bridge 967 places the MNP into the IPv6 destination address (i.e., as a Subnet 968 Router Anycast address) and encapsulates the SPAN packet in an INET 969 header based on the target's Link Layer Address, then forwards the 970 packet to the target directly while bypassing the target's Server. 972 In this way, the Bridge participates in route optimization to greatly 973 reduce traffic load and suboptimal routing through the target's 974 Server. Note that if the Bridge does not recognize the AERO Route 975 Optimization TLV, it instead places the Server's address in the IPv6 976 destination address and forwards to the Server. This is the same 977 behavior that would occur if the AERO Route Optimization TLV were not 978 present. 980 3.2.9. Segment Routing Header Compression 982 In the Segment Routing use cases discussed above, the segment routing 983 headers must be kept to a minimum size since source and target 984 Clients may be located behind low-end wireless links (e.g., 1Mbps or 985 less). The Compressed Routing Header (CRH) 986 [I-D.bonica-6man-comp-rtg-hdr] provides a compact form that reduces 987 the header size by omitting information that can already be derived 988 by intermediate Bridges. The CRH Helper 989 option[I-D.bonica-6man-crh-helper-opt] can be used to encode the AERO 990 Route Optimization TLV, and the final hop Bridge that performs route 991 optimization may remove the CRH and its helper before encapsulating 992 and forwarding to the target Client. 994 The CRH and its companion helper option are therefore seen as 995 critical architectural elements that should be quickly progressed 996 through the standards process. Implementations SHOULD use the CRH 997 and its companion helper option instead of other Routing Header types 998 whenever possible to conserve bandwidth. 1000 3.3. AERO Interface Characteristics 1002 AERO interfaces are virtual interfaces configured over one or more 1003 underlying interfaces classified as follows: 1005 o INET interfaces connect to an INET either natively or through one 1006 or several IPv4 Network Address Translators (NATs). Native INET 1007 interfaces have global IP addresses that are reachable from any 1008 INET correspondent. All Server, Relay and Bridge interfaces are 1009 native interfaces, as are INET-facing interfaces of Proxys. NATed 1010 INET interfaces connect to a private network behind one or more 1011 NATs that provide INET access. Clients that are behind a NAT are 1012 required to send periodic keepalive messages to keep NAT state 1013 alive when there are no data packets flowing. 1015 o Proxyed interfaces connect to an ANET that is separated from the 1016 open INET by an AERO Proxy. Proxys can actively issue control 1017 messages over the INET on behalf of the Client to reduce ANET 1018 congestion. Clients connected to Proxyed interfaces receive RAs 1019 with the P flag set to 1. 1021 o VPNed interfaces use security encapsulation over the INET to a 1022 Virtual Private Network (VPN) server that also acts as an AERO 1023 Server. Other than the link-layer encapsulation format, VPNed 1024 interfaces behave the same as Direct interfaces. 1026 o Direct interfaces connect a Client directly to a Server without 1027 crossing any ANET/INET paths. An example is a line-of-sight link 1028 between a remote pilot and an unmanned aircraft. The same Client 1029 considerations apply as for VPNed interfaces. 1031 AERO interfaces use SPAN encapsulation as necessary as discussed in 1032 Section 3.2.5. AERO interfaces use link-layer encapsulation (see: 1033 Section 3.6) to exchange packets with AERO link neighbors over INET 1034 or VPNed interfaces. AERO interfaces do not use link-layer 1035 encapsulation over Proxyed and Direct underlying interfaces. 1037 AERO interfaces maintain a neighbor cache for tracking per-neighbor 1038 state the same as for any interface. AERO interfaces use ND messages 1039 including Router Solicitation (RS), Router Advertisement (RA), 1040 Neighbor Solicitation (NS) and Neighbor Advertisement (NA) for 1041 neighbor cache management. 1043 AERO interfaces send ND messages with an Overlay Multilink Network 1044 Interface (OMNI) option formatted as specified in 1045 [I-D.templin-6man-omni-interface]. The OMNI option includes prefix 1046 registration information and "ifIndex-tuples" containing link 1047 information parameters for the AERO interface's underlying 1048 interfaces. 1050 When encapsulation is used, AERO interface ND messages MAY also 1051 include an AERO Source/Target Link-Layer Address Option (S/TLLAO) 1052 formatted as shown in Figure 5: 1054 0 1 2 3 1055 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 1056 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1057 | Type | Length | ifIndex[1] | SRT | LHS |FMT| 1058 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1059 ~ Segment Routing List [1] ~ 1060 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1061 ~ Link Layer Address [1] ~ 1062 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1063 | Port Number [1] | ifIndex[2] | SRT | LHS |FMT| 1064 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1065 ~ Segment Routing List [2] ~ 1066 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1067 ~ Link Layer Address [2] ~ 1068 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1069 | Port Number [2] | .... ~ 1070 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ 1071 ~ ... ~ 1072 ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1073 ~ | ifIndex[N] | SRT | LHS |FMT| 1074 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1075 ~ Segment Routing List [N] ~ 1076 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1077 ~ Link Layer Address [N] ~ 1078 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1079 | Port Number [N] | Zero Padding (if necessary) ... 1080 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+- 1082 Figure 5: AERO Source/Target Link-Layer Address Option (S/TLLAO) 1083 Format 1085 In this format, Type and Length are set the same as specified for S/ 1086 TLLAOs in [RFC4861], with trailing zero padding octets added as 1087 necessary to produce an integral number of 8 octet blocks. The S/ 1088 TLLAO includes N ifIndex-tuples in correspondence to ifIndex-tuples 1089 that appear in the OMNI option. Each ifIndex-tuple includes the 1090 following information: 1092 o ifIndex[i] - the same value as in the corresponding ifIndex-tuple 1093 included in the OMNI option. 1095 o SRT[i] - a 3-bit "Segment Routing Topology" value (see: 1096 Section 3.2.6) coded as follows: 1098 * 000 - Red 1100 * 001 - Green 1101 * 010 - Blue-1 1103 * 011 - Blue-2 1105 * 100 - 111 - Reserved 1107 o LHS[i] - a 3-bit "LookaHead Segments" value that encodes the 1108 number (from 0 to 7) of entries in Segment Routing List [i]. 1110 o FMT[i] - a 2-bit "Format" code. Determines the format of the Link 1111 Layer Address [i] field as follows: 1113 * 00 - Link Layer Address [i] encodes an IPv4 address for a node 1114 behind a NAT. 1116 * 01 - Link Layer Address [i] encodes an IPv4 address for a node 1117 on the open INET. 1119 * 10 - Link Layer Address [i] encodes an IPv6 address for a node 1120 behind a NAT.. 1122 * 11 - Link Layer Address [i] encodes an IPv6 address for a node 1123 on the open INET. 1125 o Segment Routing List [i] - Includes LHS[i]-many 4 byte ULA 1126 suffixes from the SRT corresponding to the Segment IDs (SIDs) that 1127 must be visited prior to forwarding to Link Layer Address [i]. 1128 The ultimate SID appears first, followed by the penultimate SID 1129 second, etc. 1131 o Link Layer Address [i] - Included according to FMT[i], and 1132 identifies the link-layer address of the source/target. The IP 1133 address is recorded in ones-compliment "obfuscated" form per 1134 [RFC4380]. 1136 o Port Number [i] - The field is 2 bytes in length and immediately 1137 follows Link Layer Address [i]. Also recorded in ones-compliment 1138 "obfuscated" form. 1140 If an S/TLLAO is included, any ifIndex-tuples correspond to a proper 1141 subset of the OMNI option ifIndex-tuples. Any S/TLLAO ifIndex-tuple 1142 having an ifIndex value that does not appear in an OMNI option 1143 ifindex-tuple is ignored. If the same ifIndex value appears in 1144 multiple ifIndex-tuples, the first tuple is processed and the 1145 remaining tuples are ignored. Any S/TLLAO ifIndex-tuples can 1146 therefore be viewed as inter-dependent extensions of their 1147 corresponding OMNI option ifIndex-tuples, i.e., the OMNI option and 1148 S/TLLAO are companions that are interpreted in conjunction with each 1149 other. 1151 A Client's AERO interface may be configured over multiple underlying 1152 interface connections. For example, common mobile handheld devices 1153 have both wireless local area network ("WLAN") and cellular wireless 1154 links. These links are often used "one at a time" with low-cost WLAN 1155 preferred and highly-available cellular wireless as a standby, but a 1156 simultaneous-use capability could provide benefits. In a more 1157 complex example, aircraft frequently have many wireless data link 1158 types (e.g. satellite-based, cellular, terrestrial, air-to-air 1159 directional, etc.) with diverse performance and cost properties. 1161 If a Client's multiple underlying interfaces are used "one at a time" 1162 (i.e., all other interfaces are in standby mode while one interface 1163 is active), then ND message OMNI options include only a single 1164 ifIndex-tuple set to constant values. In that case, the Client would 1165 appear to have a single interface but with a dynamically changing 1166 link-layer address. 1168 If the Client has multiple active underlying interfaces, then from 1169 the perspective of ND it would appear to have multiple link-layer 1170 addresses. In that case, ND message OMNI options MAY include 1171 multiple ifIndex-tuples - each with values that correspond to a 1172 specific interface. Every ND message need not include all OMNI and/ 1173 or S/TLLAO ifIndex-tuples; for any ifIndex-tuple not included, the 1174 neighbor considers the status as unchanged. 1176 Bridge, Server and Proxy AERO interfaces may be configured over one 1177 or more secured tunnel interfaces. The AERO interface configures 1178 both an LLA and its corresponding ULA, while the underlying secured 1179 tunnel interfaces are either unnumbered or configure the same ULA. 1180 The AERO interface encapsulates each IP packet in a SPAN header and 1181 presents the packet to the underlying secured tunnel interface. For 1182 Bridges that do not configure an AERO interface, the secured tunnel 1183 interfaces themselves are exposed to the IP layer with each interface 1184 configuring the Bridge's ULA. Routing protocols such as BGP 1185 therefore run directly over the Bridge's secured tunnel interfaces. 1186 For nodes that configure an AERO interface, routing protocols such as 1187 BGP run over the AERO interface but do not employ SPAN encapsulation. 1188 Instead, the AERO interface presents the routing protocol messages 1189 directly to the underlying secured tunnels without applying 1190 encapsulation and while using the ULA as the source address. This 1191 distinction must be honored consistently according to each node's 1192 configuration so that the IP forwarding table will associate 1193 discovered IP routes with the correct interface. 1195 3.4. AERO Interface Initialization 1197 AERO Servers, Proxys and Clients configure AERO interfaces as their 1198 point of attachment to the AERO link. AERO nodes assign the MSPs for 1199 the link to their AERO interfaces (i.e., as a "route-to-interface") 1200 to ensure that packets with destination addresses covered by an MNP 1201 not explicitly assigned to a non-AERO interface are directed to the 1202 AERO interface. 1204 AERO interface initialization procedures for Servers, Proxys, Clients 1205 and Bridges are discussed in the following sections. 1207 3.4.1. AERO Server/Relay Behavior 1209 When a Server enables an AERO interface, it assigns an LLA/ULA 1210 appropriate for the given AERO link segment. The Server also 1211 configures secured tunnels with one or more neighboring Bridges and 1212 engages in a BGP routing protocol session with each Bridge. 1214 The AERO interface provides a single interface abstraction to the IP 1215 layer, but internally comprises multiple secured tunnels as well as 1216 an NBMA nexus for sending encapsulated data packets to AERO interface 1217 neighbors. The Server further configures a service to facilitate ND/ 1218 PD exchanges with AERO Clients and manages per-Client neighbor cache 1219 entries and IP forwarding table entries based on control message 1220 exchanges. 1222 Relays are simply Servers that run a dynamic routing protocol to 1223 redistribute routes between the AERO interface and INET/EUN 1224 interfaces (see: Section 3.2.4). The Relay provisions MNPs to 1225 networks on the INET/EUN interfaces (i.e., the same as a Client would 1226 do) and advertises the MSP(s) for the AERO link over the INET/EUN 1227 interfaces. The Relay further provides an attachment point of the 1228 AERO link to a non-MNP-based global topology. 1230 3.4.2. AERO Proxy Behavior 1232 When a Proxy enables an AERO interface, it assigns an LLA/ULA and 1233 configures permanent neighbor cache entries the same as for Servers. 1234 The Proxy also configures secured tunnels with one or more 1235 neighboring Bridges and maintains per-Client neighbor cache entries 1236 based on control message exchanges. 1238 3.4.3. AERO Client Behavior 1240 When a Client enables an AERO interface, it sends RS messages with 1241 ND/PD parameters over its underlying interfaces to a Server in the 1242 MAP list, which returns an RA message with corresponding parameters. 1244 (The RS/RA messages may pass through a Proxy in the case of a 1245 Client's Proxyed interface, or through one or more NATs in the case 1246 of a Client's INET interface.) 1248 3.4.4. AERO Bridge Behavior 1250 AERO Bridges configure an AERO interface and assign the ULA Subnet 1251 Router Anycast address for each AERO link segment they connect to. 1252 Bridges configure secured tunnels with Servers, Proxys and other 1253 Bridges; they also configure LLAs/ULAs and permanent neighbor cache 1254 entries the same as Servers. Bridges engage in a BGP routing 1255 protocol session with a subset of the Servers and other Bridges on 1256 the spanning tree (see: Section 3.2.4). 1258 3.5. AERO Interface Neighbor Cache Maintenance 1260 Each AERO interface maintains a conceptual neighbor cache that 1261 includes an entry for each neighbor it communicates with on the AERO 1262 link per [RFC4861]. AERO interface neighbor cache entries are said 1263 to be one of "permanent", "symmetric", "asymmetric" or "proxy". 1265 Permanent neighbor cache entries are created through explicit 1266 administrative action; they have no timeout values and remain in 1267 place until explicitly deleted. AERO Bridges maintain permanent 1268 neighbor cache entries for their associated Proxys and Servers (and 1269 vice-versa). Each entry maintains the mapping between the neighbor's 1270 network-layer LLA and corresponding INET address. 1272 Symmetric neighbor cache entries are created and maintained through 1273 RS/RA exchanges as specified in Section 3.12, and remain in place for 1274 durations bounded by ND/PD lifetimes. AERO Servers maintain 1275 symmetric neighbor cache entries for each of their associated 1276 Clients, and AERO Clients maintain symmetric neighbor cache entries 1277 for each of their associated Servers. The list of all Servers on the 1278 AERO link is maintained in the link's MAP list. 1280 Asymmetric neighbor cache entries are created or updated based on 1281 route optimization messaging as specified in Section 3.14, and are 1282 garbage-collected when keepalive timers expire. AERO ROSs maintain 1283 asymmetric neighbor cache entries for active targets with lifetimes 1284 based on ND messaging constants. Asymmetric neighbor cache entries 1285 are unidirectional since only the ROS (and not the ROR) creates an 1286 entry. 1288 Proxy neighbor cache entries are created and maintained by AERO 1289 Proxys when they process Client/Server ND/PD exchanges, and remain in 1290 place for durations bounded by ND/PD lifetimes. AERO Proxys maintain 1291 proxy neighbor cache entries for each of their associated Clients. 1293 Proxy neighbor cache entries track the Client state and the address 1294 of the Client's associated Server(s). 1296 To the list of neighbor cache entry states in Section 7.3.2 of 1297 [RFC4861], Proxy and Server AERO interfaces add an additional state 1298 DEPARTED that applies to symmetric and proxy neighbor cache entries 1299 for Clients that have recently departed. The interface sets a 1300 "DepartTime" variable for the neighbor cache entry to "DEPART_TIME" 1301 seconds. DepartTime is decremented unless a new ND message causes 1302 the state to return to REACHABLE. While a neighbor cache entry is in 1303 the DEPARTED state, packets destined to the target Client are 1304 forwarded to the Client's new location instead of being dropped. 1305 When DepartTime decrements to 0, the neighbor cache entry is deleted. 1306 It is RECOMMENDED that DEPART_TIME be set to the default constant 1307 value REACHABLE_TIME plus 10 seconds (40 seconds by default) to allow 1308 a window for packets in flight to be delivered while stale route 1309 optimization state may be present. 1311 When an ROR receives an authentic NS message used for route 1312 optimization, it searches for a symmetric neighbor cache entry for 1313 the target Client. The ROR then returns a solicited NA message 1314 without creating a neighbor cache entry for the ROS, but creates or 1315 updates a target Client "Report List" entry for the ROS and sets a 1316 "ReportTime" variable for the entry to REPORT_TIME seconds. The ROR 1317 resets ReportTime when it receives a new authentic NS message, and 1318 otherwise decrements ReportTime while no authentic NS messages have 1319 been received. It is RECOMMENDED that REPORT_TIME be set to the 1320 default constant value REACHABLE_TIME plus 10 seconds (40 seconds by 1321 default) to allow a window for route optimization to converge before 1322 ReportTime decrements below REACHABLE_TIME. 1324 When the ROS receives a solicited NA message response to its NS 1325 message used for route optimization, it creates or updates an 1326 asymmetric neighbor cache entry for the target network-layer and 1327 link-layer addresses. The ROS then (re)sets ReachableTime for the 1328 neighbor cache entry to REACHABLE_TIME seconds and uses this value to 1329 determine whether packets can be forwarded directly to the target, 1330 i.e., instead of via a default route. The ROS otherwise decrements 1331 ReachableTime while no further solicited NA messages arrive. It is 1332 RECOMMENDED that REACHABLE_TIME be set to the default constant value 1333 30 seconds as specified in [RFC4861]. 1335 AERO nodes also use the value MAX_UNICAST_SOLICIT to limit the number 1336 of NS keepalives sent when a correspondent may have gone unreachable, 1337 the value MAX_RTR_SOLICITATIONS to limit the number of RS messages 1338 sent without receiving an RA and the value MAX_NEIGHBOR_ADVERTISEMENT 1339 to limit the number of unsolicited NAs that can be sent based on a 1340 single event. It is RECOMMENDED that MAX_UNICAST_SOLICIT, 1341 MAX_RTR_SOLICITATIONS and MAX_NEIGHBOR_ADVERTISEMENT be set to 3 the 1342 same as specified in [RFC4861]. 1344 Different values for DEPART_TIME, REPORT_TIME, REACHABLE_TIME, 1345 MAX_UNICAST_SOLICIT, MAX_RTR_SOLCITATIONS and 1346 MAX_NEIGHBOR_ADVERTISEMENT MAY be administratively set; however, if 1347 different values are chosen, all nodes on the link MUST consistently 1348 configure the same values. Most importantly, DEPART_TIME and 1349 REPORT_TIME SHOULD be set to a value that is sufficiently longer than 1350 REACHABLE_TIME to avoid packet loss due to stale route optimization 1351 state. 1353 3.6. AERO Interface Encapsulation and Re-encapsulation 1355 In some instances, AERO interfaces insert a mid-layer IPv6 header 1356 known as the SPAN header as discussed in the following sections. 1357 After either inserting or omitting the SPAN header, the AERO 1358 interface inserts an outer encapsulation header as discussed below. 1360 AERO interfaces avoid outer encapsulation over Direct underlying 1361 interfaces and Proxyed underlying interfaces for which the first-hop 1362 access router is AERO-aware. Other AERO interfaces encapsulate 1363 packets according to whether they are entering the AERO interface 1364 from the network layer or if they are being re-admitted into the same 1365 AERO link they arrived on. This latter form of encapsulation is 1366 known as "re-encapsulation". 1368 For packets entering the AERO interface from the network layer, the 1369 AERO interface copies the "TTL/Hop Limit", "Type of Service/Traffic 1370 Class" [RFC2983], "Flow Label"[RFC6438] (for IPv6) and "Congestion 1371 Experienced" [RFC3168] values in the inner packet's IP header into 1372 the corresponding fields in the SPAN and outer encapsulation 1373 header(s). 1375 For packets undergoing re-encapsulation, the AERO interface instead 1376 copies these values from the original encapsulation header into the 1377 new encapsulation header, i.e., the values are transferred between 1378 encapsulation headers and *not* copied from the encapsulated packet's 1379 network-layer header. (Note especially that by copying the TTL/Hop 1380 Limit between encapsulation headers the value will eventually 1381 decrement to 0 if there is a (temporary) routing loop.) For IPv4 1382 encapsulation/re-encapsulation, the AERO interface sets the DF bit as 1383 discussed in Section 3.9. 1385 AERO interfaces configured over INET underlying interfaces 1386 encapsulate packets in INET headers according to the next hop 1387 determined in the forwarding algorithm in Section 3.10. If the next 1388 hop is reached via a secured tunnel, the AERO interface uses an 1389 encapsulation format specific to the secured tunnel type (see: 1390 Section 6). If the next hop is reached via an unsecured INET 1391 interface, the AERO interface instead uses UDP/IP encapsulation 1392 according to the Teredo format specified in [RFC4380] and as extended 1393 in [RFC6081]. 1395 When Teredo encapsulation is used, the AERO interface next sets the 1396 UDP source port to a constant value that it will use in each 1397 successive packet it sends, and sets the UDP length field to the 1398 length of the encapsulated packet plus 8 bytes for the UDP header 1399 itself plus the length of any included Teredo extension headers or 1400 trailers. The encapsulated packet may be either IPv6 or IPv4, as 1401 distinguished by the version number found in the first four bits. 1403 For Teredo-encapsulated packets sent to a Server, Relay or Bridge, 1404 the AERO interface sets the UDP destination port to 8060, i.e., the 1405 IANA-registered port number for AERO. For packets sent to a Client, 1406 the AERO interface sets the UDP destination port to the port value 1407 stored in the neighbor cache entry for this Client. The AERO 1408 interface finally includes/omits the UDP checksum according to 1409 [RFC6935][RFC6936]. 1411 AERO interfaces observe the packet sizing and fragmentation 1412 considerations found in Section 3.9. 1414 3.7. AERO Interface Decapsulation 1416 AERO interfaces decapsulate packets destined either to the AERO node 1417 itself or to a destination reached via an interface other than the 1418 AERO interface the packet was received on. When the encapsulated 1419 packet arrives in multiple SPAN fragments, the AERO interface 1420 reassembles as discussed in Section 3.9. Further decapsulation steps 1421 are performed according to the appropriate encapsulation format 1422 specification. 1424 3.8. AERO Interface Data Origin Authentication 1426 AERO nodes employ simple data origin authentication procedures. In 1427 particular: 1429 o AERO Bridges, Servers and Proxys accept encapsulated data packets 1430 and control messages received from the spanning tree. 1432 o AERO Proxys and Clients accept packets that originate from within 1433 the same secured ANET. 1435 o AERO Clients and Relays accept packets from downstream network 1436 correspondents based on ingress filtering. 1438 o AERO Clients, Relays and Servers verify the outer UDP/IP 1439 encapsulation addresses according to the Teredo specification 1440 [RFC4380]. 1442 AERO nodes silently drop any packets that do not satisfy the above 1443 data origin authentication procedures. Further security 1444 considerations are discussed in Section 6. 1446 3.9. AERO Interface MTU and Fragmentation 1448 The AERO interface observes the link nature of tunnels, including the 1449 Maximum Transmission Unit (MTU) and the role of fragmentation and 1450 reassembly[I-D.ietf-intarea-tunnels]. The AERO interface is 1451 configured over one or more underlying interfaces that may have 1452 diverse MTUs. 1454 IPv6 underlying interfaces are REQUIRED to configure a minimum MTU of 1455 1280 bytes [RFC8200]. The minimum MTU for IPv4 underlying interfaces 1456 is only 68 bytes [RFC1122], meaning that a packet smaller than the 1457 IPv6 minimum MTU may require fragmentation when IPv4 encapsulation is 1458 used. Therefore, the Don't Fragment (DF) bit in the IPv4 1459 encapsulation header MUST be set to 0. 1461 The AERO interface configures an MTU of 9180 bytes [RFC2492]; the 1462 size is therefore not a reflection of the underlying interface MTUs, 1463 but rather determines the largest packet the AERO interface can 1464 forward or reassemble. The AERO interface therefore accommodates 1465 packets as large as the AERO interface MTU while generating IPv6 Path 1466 MTU Discovery (PMTUD) Packet Too Big (PTB) messages [RFC8201] as 1467 necessary (see below). For IPv4 packets with DF=0, the IP layer 1468 applies IPv4 fragmentation if necessary to admit the fragments into 1469 the AERO interface. The interface may then internally apply further 1470 IPv4 fragmentation prior to encapsulation to ensure that the IPv4 1471 fragments are delivered to the final destination. 1473 AERO interfaces internally employ SPAN encapsulation and 1474 fragmentation/reassembly per [RFC2473]. The AERO interface returns 1475 internally-generated PTB messages for packets admitted into the 1476 interface that it deems too large (e.g., according to link 1477 performance characteristics, reassembly cost, etc.) while either 1478 dropping or forwarding the packet as necessary. The AERO interface 1479 performs PMTUD even if the destination appears to be on the same link 1480 since intermediate AERO link nodes may return a PTB. This ensures 1481 that the path MTU is adaptive and reflects the current path used for 1482 a given data flow. 1484 AERO nodes perform SPAN encapsulation and fragmentation/reassembly as 1485 follows: 1487 o When a node's AERO interface sends a packet over a Proxyed, VPNed 1488 or Direct underlying interface, it sends without SPAN 1489 encapsulation if the packet is no larger than the underlying 1490 interface MTU. Otherwise, it inserts a SPAN header with source 1491 address set to the node's own ULA and destination set to the ULA 1492 of the link-layer peer Proxy, Server or Client on the underlying 1493 interface. The AERO interface then uses IPv6 fragmentation to 1494 break the packet into a minimum number of non-overlapping 1495 fragments, where the largest fragment size is determined by the 1496 underlying interface MTU and the smallest fragment is no smaller 1497 than 640 bytes. The AERO interface then sends the fragments to 1498 the link-layer peer, which reassembles before forwarding toward 1499 the final destination. 1501 o When a node's AERO interface sends a packet over an INET 1502 underlying interface, it sends packets no larger than 1280 bytes 1503 (including any INET headers) without a SPAN header if the 1504 destination is reached via an INET address within the same AERO 1505 link segment. Otherwise, it inserts a SPAN header with source 1506 address set to the node's ULA, destination set to the ULA of the 1507 next hop AERO node toward the final destination and (if necessary) 1508 with a SRH with the remaining Segment IDs on the path to the final 1509 destination. The AERO interface then uses IPv6 fragmentation to 1510 break the encapsulated packet into a minimum number of non- 1511 overlapping fragments, where the largest fragment size (including 1512 both SPAN and INET encapsulation) is 1280 bytes and the smallest 1513 fragment is no smaller than 640 bytes. The AERO interface then 1514 encapsulates the SPAN fragments in INET headers and sends them to 1515 the SPAN destination, which reassembles before forwarding toward 1516 the final destination. 1518 AERO interfaces unconditionally drop all SPAN fragments smaller than 1519 640 bytes. In order to set the correct context for reassembly, the 1520 AERO interface that inserts a SPAN header MUST also be the one that 1521 inserts the IPv6 Fragment Header Identification value. While not 1522 strictly required, sending all fragments of the same fragmented SPAN 1523 packet consecutively over the same underlying interface with minimal 1524 inter-fragment delay can in some cases increase the likelihood of 1525 successful reassembly. 1527 Note that the AERO interface can forward large packets via 1528 encapsulation and fragmentation while at the same time returning 1529 advisory PTB messages, e.g., subject to rate limiting. The receiving 1530 node that performs reassembly can also send advisory PTB messages if 1531 reassembly conditions become unfavorable. The AERO interface can 1532 therefore continuously forward large packets without loss while 1533 returning advisory messages recommending a smaller size (but no 1534 smaller than 1280). Advisory PTB messages are differentiated from 1535 PTB messages that report loss by setting the Code field in the ICMPv6 1536 message header to the value 1. This document therefore updates 1537 [RFC4443] and [RFC8201]. 1539 3.9.1. Fragmentation Security Implications 1541 As discussed in Section 3.7 of [I-D.ietf-intarea-frag-fragile], there 1542 are four basic threats concerning IPv6 fragmentation; each of which 1543 is addressed by a suitable mitigation as follows: 1545 1. Overlapping fragment attacks - reassembly of overlapping 1546 fragments is forbidden by [RFC8200]; therefore, this threat does 1547 not apply to AERO interfaces. 1549 2. Resource exhaustion attacks - this threat is mitigated by 1550 providing a sufficiently large AERO interface reassembly cache 1551 and instituting "fast discard" of incomplete reassemblies that 1552 may be part of a buffer exhaustion attack. The reassembly cache 1553 should be sufficiently large so that a sustained attack does not 1554 cause excessive loss of good reassemblies but not so large that 1555 (timer-based) data structure management becomes computationally 1556 expensive. 1558 3. Attacks based on predictable fragment identification values - 1559 this threat is mitigated by selecting a suitably random ID value 1560 per [RFC7739]. 1562 4. Evasion of Network Intrusion Detection Systems (NIDS) - this 1563 threat is mitigated by disallowing "tiny fragments" per the AERO 1564 interface fragmentation procedures specified above. 1566 Additionally, IPv4 fragmentation includes a 16-bit Identification (IP 1567 ID) field with only 65535 unique values, meaning that for even 1568 moderately high data rates the field could wrap and apply to new 1569 packets while the fragments of old packets using the same ID are 1570 still alive in the network [RFC4963]. Since IPv6 provides a 32-bit 1571 Identification value, however, this is not a concern for IPv6 1572 fragmentation. 1574 3.10. AERO Interface Forwarding Algorithm 1576 IP packets enter a node's AERO interface either from the network 1577 layer (i.e., from a local application or the IP forwarding system) or 1578 from the link layer (i.e., from an AERO interface neighbor). All 1579 packets entering a node's AERO interface first undergo data origin 1580 authentication as discussed in Section 3.8. Packets that satisfy 1581 data origin authentication are processed further, while all others 1582 are dropped silently. 1584 Packets that enter the AERO interface from the network layer are 1585 forwarded to an AERO interface neighbor. Packets that enter the AERO 1586 interface from the link layer are either re-admitted into the AERO 1587 link or forwarded to the network layer where they are subject to 1588 either local delivery or IP forwarding. In all cases, the AERO 1589 interface itself MUST NOT decrement the network layer TTL/Hop-count 1590 since its forwarding actions occur below the network layer. 1592 AERO interfaces may have multiple underlying interfaces and/or 1593 neighbor cache entries for neighbors with multiple ifIndex-tuple 1594 registrations (see Section 3.3). The AERO interface uses traffic 1595 classifiers (e.g., DSCP value, port number, etc.) to select an 1596 outgoing underlying interface for each packet based on the node's own 1597 QoS preferences, and also to select a destination link-layer address 1598 based on the neighbor's underlying interface with the highest 1599 preference. AERO implementations SHOULD allow for QoS preference 1600 values to be modified at runtime through network management. 1602 If multiple outgoing interfaces and/or neighbor interfaces have a 1603 preference of "high", the AERO node replicates the packet and sends 1604 one copy via each of the (outgoing / neighbor) interface pairs; 1605 otherwise, the node sends a single copy of the packet via an 1606 interface with the highest preference. AERO nodes keep track of 1607 which underlying interfaces are currently "reachable" or 1608 "unreachable", and only use "reachable" interfaces for forwarding 1609 purposes. 1611 The following sections discuss the AERO interface forwarding 1612 algorithms for Clients, Proxys, Servers and Bridges. In the 1613 following discussion, a packet's destination address is said to 1614 "match" if it is the same as a cached address, or if it is covered by 1615 a cached prefix (which may be encoded in an LLA). 1617 3.10.1. Client Forwarding Algorithm 1619 When an IP packet enters a Client's AERO interface from the network 1620 layer the Client searches for an asymmetric neighbor cache entry that 1621 matches the destination. If there is a match, the Client uses one or 1622 more "reachable" neighbor interfaces in the entry for packet 1623 forwarding. If there is no asymmetric neighbor cache entry, the 1624 Client instead forwards the packet toward a Server (the packet is 1625 intercepted by a Proxy if there is a Proxy on the path). The Client 1626 encapsulates the packet in a SPAN header and fragments if necessary 1627 according to MTU requirements (see: Section 3.9). 1629 When an IP packet enters a Client's AERO interface from the link- 1630 layer, if the destination matches one of the Client's MNPs or link- 1631 local addresses the Client reassembles and decapsulates as necessary 1632 and delivers the inner packet to the network layer. Otherwise, the 1633 Client drops the packet and MAY return a network-layer ICMP 1634 Destination Unreachable message subject to rate limiting (see: 1635 Section 3.11). 1637 3.10.2. Proxy Forwarding Algorithm 1639 For control messages originating from or destined to a Client, the 1640 Proxy intercepts the message and updates its proxy neighbor cache 1641 entry for the Client. The Proxy then forwards a (proxyed) copy of 1642 the control message. (For example, the Proxy forwards a proxied 1643 version of a Client's NS/RS message to the target neighbor, and 1644 forwards a proxied version of the NA/RA reply to the Client.) 1646 When the Proxy receives a data packet from a Client within the ANET, 1647 the Proxy reassembles and re-fragments if necessary then searches for 1648 an asymmetric neighbor cache entry that matches the destination and 1649 forwards as follows: 1651 o if the destination matches an asymmetric neighbor cache entry, the 1652 Proxy uses one or more "reachable" neighbor interfaces in the 1653 entry for packet forwarding using SPAN encapsulation and including 1654 a SRH if necessary according to the cached TLLAO information. If 1655 the neighbor interface is in the same SPAN segment, the Proxy 1656 forwards the packet directly to the neighbor; otherwise, it 1657 forwards the packet to a Bridge. 1659 o else, the Proxy uses SPAN encapsulation and forwards the packet to 1660 a Bridge while using the ULA corresponding to the packet's 1661 destination as the SPAN destination address. 1663 When the Proxy receives an encapsulated data packet from an INET 1664 neighbor or from a secured tunnel from a Bridge, it accepts the 1665 packet only if data origin authentication succeeds and if there is a 1666 proxy neighbor cache entry that matches the inner destination. Next, 1667 the Proxy reassembles the packet (if necessary) and continues 1668 processing. 1670 Next if reassembly is complete and the neighbor cache state is 1671 REACHABLE, the Proxy returns a PTB if necessary (see: Section 3.9) 1672 then either drops or forwards the packet to the Client while 1673 performing SPAN encapsulation and re-fragmentation to the ANET MTU 1674 size if necessary. If the neighbor cache entry state is DEPARTED, 1675 the Proxy instead changes the SPAN destination address to the address 1676 of the new Server and forwards it to a Bridge while performing re- 1677 fragmentation to 1280 bytes if necessary. 1679 3.10.3. Server/Relay Forwarding Algorithm 1681 For control messages destined to a target Client's LLA that are 1682 received from a secured tunnel, the Server intercepts the message and 1683 sends an appropriate response on behalf of the Client. (For example, 1684 the Server sends an NA message reply in response to an NS message 1685 directed to one of its associated Clients.) If the Client's neighbor 1686 cache entry is in the DEPARTED state, however, the Server instead 1687 forwards the packet to the Client's new Server as discussed in 1688 Section 3.16. 1690 When the Server receives an encapsulated data packet from an INET 1691 neighbor or from a secured tunnel, it accepts the packet only if data 1692 origin authentication succeeds. If the SPAN destination address is 1693 its own address, the Server continues processing as follows: 1695 o if the destination matches a symmetric neighbor cache entry in the 1696 REACHABLE state the Server prepares the packet for forwarding to 1697 the destination Client. The Server first reassembles (if 1698 necessary) and forwards the packet (while re-fragmenting if 1699 necessary) as specified in Section 3.9. 1701 o else, if the destination matches a symmetric neighbor cache entry 1702 in the DEPARETED state the Server re-encapsulates the packet and 1703 forwards it using the ULA of the Client's new Server as the 1704 destination. 1706 o else, if the destination matches an asymmetric neighbor cache 1707 entry, the Server uses one or more "reachable" neighbor interfaces 1708 in the entry for packet forwarding via the local INET if the 1709 neighbor is in the same AERO link segment or using SPAN 1710 encapsulation and Segment Routing if necessary with the final 1711 destination set to the neighbor's ULA otherwise. 1713 o else, if the destination is an LLA that is not assigned on the 1714 AERO interface the Server drops the packet. 1716 o else, the Server (acting as a Relay) reassembles if necessary, 1717 decapsulates the packet and releases it to the network layer for 1718 local delivery or IP forwarding. Based on the information in the 1719 forwarding table, the network layer may return the packet to the 1720 same AERO interface in which case further processing occurs as 1721 below. (Note that this arrangement accommodates common 1722 implementations in which the IP forwarding table is not accessible 1723 from within the AERO interface. If the AERO interface can 1724 directly access the IP forwarding table (such as for in-kernel 1725 implementations) the forwarding table lookup can instead be 1726 performed internally from within the AERO interface itself.) 1728 When the Server's AERO interface receives a data packet from the 1729 network layer or from a VPNed or Direct Client, it performs SPAN 1730 encapsulation and fragmentation if necessary, then processes the 1731 packet according to the network-layer destination address as follows: 1733 o if the destination matches a symmetric or asymmetric neighbor 1734 cache entry the Server processes the packet as above. 1736 o else, the Server encapsulates the packet and forwards it to a 1737 Bridge using its own ULA as the source and the ULA corresponding 1738 to the destination as the destination. 1740 3.10.4. Bridge Forwarding Algorithm 1742 Bridges forward SPAN-encapsulated packets over secured tunnels the 1743 same as any IP router. When the Bridge receives a SPAN-encapsulated 1744 packet via a secured tunnel, it removes the outer INET header and 1745 searches for a forwarding table entry that matches the SPAN 1746 destination address. The Bridge then processes the packet as 1747 follows: 1749 o if the destination matches its ULA Subnet Router Anycast address, 1750 the Bridge checks for a SRH. If there is a SRH with Segments 1751 Left=1, with the ULA of a Server on the local segment as the 1752 ultimate segment ID, and with an AERO Route Optimization TLV, the 1753 Bridge examines the FMT to determine if the target is behind a 1754 NAT. If no NAT is indicated, the Bridge copies the MNP Subnet 1755 Router Anycast address into the destination address then forwards 1756 the packet directly to the Link Layer Address using link-layer 1757 (UDP/IP) encapsulation. If a NAT is indicated, the Bridge MAY 1758 perform NAT traversal procedures by sending bubbles per [RFC4380]. 1759 The Bridge then either applies AERO route optimization if NAT 1760 traversal procedures have been successfully applied, or forwards 1761 the packet directly to the Server. 1763 o if the destination matches one of the Bridge's own addresses, the 1764 Bridge submits the packet for local delivery. 1766 o else, if the destination matches a forwarding table entry the 1767 Bridge forwards the packet via a secured tunnel to the next hop. 1768 If the destination matches an MSP without matching an MNP, 1769 however, the Bridge instead drops the packet and returns an ICMP 1770 Destination Unreachable message subject to rate limiting (see: 1771 Section 3.11). 1773 o else, the Bridge drops the packet and returns an ICMP Destination 1774 Unreachable as above. 1776 As for any IP router, the Bridge decrements the TTL/Hop Limit when it 1777 forwards the packet. Therefore, only the Hop Limit in the SPAN 1778 header is decremented, and not the TTL/Hop Limit in the inner packet 1779 header. 1781 3.11. AERO Interface Error Handling 1783 When an AERO node admits a packet into the AERO interface, it may 1784 receive link-layer or network-layer error indications. 1786 A link-layer error indication is an ICMP error message generated by a 1787 router in the INET on the path to the neighbor or by the neighbor 1788 itself. The message includes an IP header with the address of the 1789 node that generated the error as the source address and with the 1790 link-layer address of the AERO node as the destination address. 1792 The IP header is followed by an ICMP header that includes an error 1793 Type, Code and Checksum. Valid type values include "Destination 1794 Unreachable", "Time Exceeded" and "Parameter Problem" 1795 [RFC0792][RFC4443]. (AERO interfaces ignore all link-layer IPv4 1796 "Fragmentation Needed" and IPv6 "Packet Too Big" messages since they 1797 only emit packets that are guaranteed to be no larger than the IP 1798 minimum link MTU as discussed in Section 3.9.) 1800 The ICMP header is followed by the leading portion of the packet that 1801 generated the error, also known as the "packet-in-error". For 1802 ICMPv6, [RFC4443] specifies that the packet-in-error includes: "As 1803 much of invoking packet as possible without the ICMPv6 packet 1804 exceeding the minimum IPv6 MTU" (i.e., no more than 1280 bytes). For 1805 ICMPv4, [RFC0792] specifies that the packet-in-error includes: 1806 "Internet Header + 64 bits of Original Data Datagram", however 1807 [RFC1812] Section 4.3.2.3 updates this specification by stating: "the 1808 ICMP datagram SHOULD contain as much of the original datagram as 1809 possible without the length of the ICMP datagram exceeding 576 1810 bytes". 1812 The link-layer error message format is shown in Figure 6 (where, "L2" 1813 and "L3" refer to link-layer and network-layer, respectively): 1815 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1816 ~ ~ 1817 | L2 IP Header of | 1818 | error message | 1819 ~ ~ 1820 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1821 | L2 ICMP Header | 1822 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --- 1823 ~ ~ P 1824 | IP and other encapsulation | a 1825 | headers of original L3 packet | c 1826 ~ ~ k 1827 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ e 1828 ~ ~ t 1829 | IP header of | 1830 | original L3 packet | i 1831 ~ ~ n 1832 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1833 ~ ~ e 1834 | Upper layer headers and | r 1835 | leading portion of body | r 1836 | of the original L3 packet | o 1837 ~ ~ r 1838 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --- 1840 Figure 6: AERO Interface Link-Layer Error Message Format 1842 The AERO node rules for processing these link-layer error messages 1843 are as follows: 1845 o When an AERO node receives a link-layer Parameter Problem message, 1846 it processes the message the same as described as for ordinary 1847 ICMP errors in the normative references [RFC0792][RFC4443]. 1849 o When an AERO node receives persistent link-layer Time Exceeded 1850 messages, the IP ID field may be wrapping before earlier fragments 1851 awaiting reassembly have been processed. In that case, the node 1852 should begin including integrity checks and/or institute rate 1853 limits for subsequent packets. 1855 o When an AERO node receives persistent link-layer Destination 1856 Unreachable messages in response to encapsulated packets that it 1857 sends to one of its asymmetric neighbor correspondents, the node 1858 should process the message as an indication that a path may be 1859 failing, and optionally initiate NUD over that path. If it 1860 receives Destination Unreachable messages over multiple paths, the 1861 node should allow future packets destined to the correspondent to 1862 flow through a default route and re-initiate route optimization. 1864 o When an AERO Client receives persistent link-layer Destination 1865 Unreachable messages in response to encapsulated packets that it 1866 sends to one of its symmetric neighbor Servers, the Client should 1867 mark the path as unusable and use another path. If it receives 1868 Destination Unreachable messages on many or all paths, the Client 1869 should associate with a new Server and release its association 1870 with the old Server as specified in Section 3.16.5. 1872 o When an AERO Server receives persistent link-layer Destination 1873 Unreachable messages in response to encapsulated packets that it 1874 sends to one of its symmetric neighbor Clients, the Server should 1875 mark the underlying path as unusable and use another underlying 1876 path. 1878 o When an AERO Server or Proxy receives link-layer Destination 1879 Unreachable messages in response to an encapsulated packet that it 1880 sends to one of its permanent neighbors, it treats the messages as 1881 an indication that the path to the neighbor may be failing. 1882 However, the dynamic routing protocol should soon reconverge and 1883 correct the temporary outage. 1885 When an AERO Bridge receives a packet for which the network-layer 1886 destination address is covered by an MSP, if there is no more- 1887 specific routing information for the destination the Bridge drops the 1888 packet and returns a network-layer Destination Unreachable message 1889 subject to rate limiting. The Bridge writes the network-layer source 1890 address of the original packet as the destination address and uses 1891 one of its non link-local addresses as the source address of the 1892 message. 1894 When an AERO node receives an encapsulated packet for which the 1895 reassembly buffer it too small, it drops the packet and returns a 1896 network-layer Packet Too Big (PTB) message. The node first writes 1897 the MRU value into the PTB message MTU field, writes the network- 1898 layer source address of the original packet as the destination 1899 address and writes one of its non link-local addresses as the source 1900 address. 1902 3.12. AERO Router Discovery, Prefix Delegation and Autoconfiguration 1904 AERO Router Discovery, Prefix Delegation and Autoconfiguration are 1905 coordinated as discussed in the following Sections. 1907 3.12.1. AERO ND/PD Service Model 1909 Each AERO Server on the link configures a PD service to facilitate 1910 Client requests. Each Server is provisioned with a database of MNP- 1911 to-Client ID mappings for all Clients enrolled in the AERO service, 1912 as well as any information necessary to authenticate each Client. 1913 The Client database is maintained by a central administrative 1914 authority for the AERO link and securely distributed to all Servers, 1915 e.g., via the Lightweight Directory Access Protocol (LDAP) [RFC4511], 1916 via static configuration, etc. Clients receive the same service 1917 regardless of the Servers they select. 1919 AERO Clients and Servers use ND messages to maintain neighbor cache 1920 entries. AERO Servers configure their AERO interfaces as advertising 1921 NBMA interfaces, and therefore send unicast RA messages with a short 1922 Router Lifetime value (e.g., ReachableTime seconds) in response to a 1923 Client's RS message. Thereafter, Clients send additional RS messages 1924 to keep Server state alive. 1926 AERO Clients and Servers include PD parameters in RS/RA messages (see 1927 [I-D.templin-6man-dhcpv6-ndopt] for ND/PD alternatives). The unified 1928 ND/PD messages are exchanged between Client and Server according to 1929 the prefix management schedule required by the PD service. If the 1930 Client knows its MNP in advance, it can instead employ prefix 1931 registration by including its LLA as the source address of an RS 1932 message and with an OMNI option with valid prefix registration 1933 information for the MNP. If the Server (and Proxy) accept the 1934 Client's MNP assertion, they inject the prefix into the routing 1935 system and establish the necessary neighbor cache state. 1937 The following sections specify the Client and Server behavior. 1939 3.12.2. AERO Client Behavior 1941 AERO Clients discover the addresses of Servers in a similar manner as 1942 described in [RFC5214]. Discovery methods include static 1943 configuration (e.g., from a flat-file map of Server addresses and 1944 locations), or through an automated means such as Domain Name System 1945 (DNS) name resolution [RFC1035]. Alternatively, the Client can 1946 discover Server addresses through a layer 2 data link login exchange, 1947 or through a unicast RA response to a multicast/anycast RS as 1948 described below. In the absence of other information, the Client can 1949 resolve the DNS Fully-Qualified Domain Name (FQDN) 1950 "linkupnetworks.[domainname]" where "linkupnetworks" is a constant 1951 text string and "[domainname]" is a DNS suffix for the AERO link 1952 (e.g., "example.com"). 1954 To associate with a Server, the Client acts as a requesting router to 1955 request MNPs. The Client prepares an RS message with PD parameters 1956 and includes a Nonce and Timestamp option if the Client needs to 1957 correlate RA replies. If the Client already knows the Server's LLA, 1958 it includes the LLA as the network-layer destination address; 1959 otherwise, it includes the link-scoped All-Routers multicast 1960 (ff02::2) or Subnet-Router anycast (fe80::) address as the network- 1961 layer destination. If the Client already knows its own LLA, it uses 1962 the LLA as the network-layer source address; otherwise, it uses the 1963 unspecified IPv6 address (::/128) as the network-layer source 1964 address. 1966 The Client next includes an OMNI option in the RS message to register 1967 its link-layer information with the Server. The Client sets the OMNI 1968 option prefix registration information according to the MNP, and 1969 includes an ifIndex-tuple with S set to '1' corresponding to the 1970 underlying interface over which the Client will send the RS message. 1971 The Client MAY include additional ifIndex-tuples specific to other 1972 underlying interfaces. The Client MAY also include an SLLAO 1973 corresponding to the OMNI option ifIndex-tuple with S set to '1'. 1975 The Client then sends the RS message (either directly via Direct 1976 interfaces, via a VPN for VPNed interfaces, via a Proxy for proxyed 1977 interfaces or via INET encapsulation for INET interfaces) and waits 1978 for an RA message reply (see Section 3.12.3). The Client retries up 1979 to MAX_RTR_SOLICITATIONS times until an RA is received. If the 1980 Client receives no RAs, or if it receives an RA with Router Lifetime 1981 set to 0, the Client SHOULD abandon this Server and try another 1982 Server. Otherwise, the Client processes the PD information found in 1983 the RA message. 1985 Next, the Client creates a symmetric neighbor cache entry with the 1986 Server's LLA as the network-layer address and the Server's 1987 encapsulation and/or link-layer addresses as the link-layer address. 1988 The Client records the RA Router Lifetime field value in the neighbor 1989 cache entry as the time for which the Server has committed to 1990 maintaining the MNP in the routing system via this underlying 1991 interface, and caches the other RA configuration information 1992 including Cur Hop Limit, M and O flags, Reachable Time and Retrans 1993 Timer. The Client then autoconfigures LLAs for each of the delegated 1994 MNPs and assigns them to the AERO interface. The Client also caches 1995 any MSPs included in Route Information Options (RIOs) [RFC4191] as 1996 MSPs to associate with the AERO link, and assigns the MTU value in 1997 the MTU option to the underlying interface. 1999 The Client then registers additional underlying interfaces with the 2000 Server by sending RS messages via each additional interface. The RS 2001 messages include the same parameters as for the initial RS/RA 2002 exchange, but with destination address set to the Server's LLA. 2004 Following autoconfiguration, the Client sub-delegates the MNPs to its 2005 attached EUNs and/or the Client's own internal virtual interfaces as 2006 described in [I-D.templin-v6ops-pdhost] to support the Client's 2007 downstream attached "Internet of Things (IoT)". The Client 2008 subsequently sends additional RS messages over each underlying 2009 interface before the Router Lifetime received for that interface 2010 expires. 2012 After the Client registers its underlying interfaces, it may wish to 2013 change one or more registrations, e.g., if an interface changes 2014 address or becomes unavailable, if QoS preferences change, etc. To 2015 do so, the Client prepares an RS message to send over any available 2016 underlying interface. The RS includes an OMNI option with prefix 2017 registration information specific to its MNP, with an ifIndex-tuple 2018 specific to the selected underlying interface with S set to '1', and 2019 with any additional ifIndex-tuples specific to other underlying 2020 interfaces. The Client includes fresh ifIndex-tuple values to update 2021 the Server's neighbor cache entry. When the Client receives the 2022 Server's RA response, it has assurance that the Server has been 2023 updated with the new information. 2025 If the Client wishes to discontinue use of a Server it issues an RS 2026 message over any underlying interface with an OMNI option with a 2027 prefix release indication. When the Server processes the message, it 2028 releases the MNP, sets the symmetric neighbor cache entry state for 2029 the Client to DEPARTED and returns an RA reply with Router Lifetime 2030 set to 0. After a short delay (e.g., 2 seconds), the Server 2031 withdraws the MNP from the routing system. 2033 3.12.3. AERO Server Behavior 2035 AERO Servers act as IP routers and support a PD service for Clients. 2036 Servers arrange to add their LLAs to a static map of Server addresses 2037 for the link and/or the DNS resource records for the FQDN 2038 "linkupnetworks.[domainname]" before entering service. Server 2039 addresses should be geographically and/or topologically referenced, 2040 and made available for discovery by Clients on the AERO link. 2042 When a Server receives a prospective Client's RS message on its AERO 2043 interface, it SHOULD return an immediate RA reply with Router 2044 Lifetime set to 0 if it is currently too busy or otherwise unable to 2045 service the Client. Otherwise, the Server authenticates the RS 2046 message and processes the PD parameters. The Server first determines 2047 the correct MNPs to delegate to the Client by searching the Client 2048 database. When the Server delegates the MNPs, it also creates a 2049 forwarding table entry for each MNP so that the MNPs are propagated 2050 into the routing system (see: Section 3.2.4). For IPv6, the Server 2051 creates an IPv6 forwarding table entry for each MNP. For IPv4, the 2052 Server creates an IPv6 forwarding table entry with the SPAN 2053 Compatibility Prefix (SCP) corresponding to the IPv4 address. 2055 The Server next creates a symmetric neighbor cache entry for the 2056 Client using the base LLA as the network-layer address and with 2057 lifetime set to no more than the smallest PD lifetime. Next, the 2058 Server updates the neighbor cache entry by recording the information 2059 in each ifIndex-tuple in the RS OMNI option. The Server also records 2060 the actual SPAN/INET addresses in the neighbor cache entry. 2062 Next, the Server prepares an RA message using its LLA as the network- 2063 layer source address and the network-layer source address of the RS 2064 message as the network-layer destination address. The Server sets 2065 the Router Lifetime to the time for which it will maintain both this 2066 underlying interface individually and the symmetric neighbor cache 2067 entry as a whole. The Server also sets Cur Hop Limit, M and O flags, 2068 Reachable Time and Retrans Timer to values appropriate for the AERO 2069 link. The Server includes the delegated MNPs, any other PD 2070 parameters and an OMNI option with no ifIndex-tuples. The Server 2071 then includes one or more RIOs that encode the MSPs for the AERO 2072 link, plus an MTU option (see Section 3.9). The Server finally 2073 forwards the message to the Client using SPAN/INET, INET, or NULL 2074 encapsulation as necessary. 2076 After the initial RS/RA exchange, the Server maintains a 2077 ReachableTime timer for each of the Client's underlying interfaces 2078 individually (and for the Client's symmetric neighbor cache entry 2079 collectively) set to expire after ReachableTime seconds. If the 2080 Client (or Proxy) issues additional RS messages, the Server sends an 2081 RA response and resets ReachableTime. If the Server receives an ND 2082 message with PD release indication it sets the Client's symmetric 2083 neighbor cache entry to the DEPARTED state and withdraws the MNP from 2084 the routing system after a short delay (e.g., 2 seconds). If 2085 ReachableTime expires before a new RS is received on an individual 2086 underlying interface, the Server marks the interface as DOWN. If 2087 ReachableTime expires before any new RS is received on any individual 2088 underlying interface, the Server sets the symmetric neighbor cache 2089 entry state to STALE and sets a 10 second timer. If the Server has 2090 not received a new RS or ND message with PD release indication before 2091 the 10 second timer expires, it deletes the neighbor cache entry and 2092 withdraws the MNP from the routing system. 2094 The Server processes any ND/PD messages pertaining to the Client and 2095 returns an NA/RA reply in response to solicitations. The Server may 2096 also issue unsolicited RA messages, e.g., with PD reconfigure 2097 parameters to cause the Client to renegotiate its PDs, with Router 2098 Lifetime set to 0 if it can no longer service this Client, etc. 2099 Finally, If the symmetric neighbor cache entry is in the DEPARTED 2100 state, the Server deletes the entry after DepartTime expires. 2102 Note: Clients SHOULD notify former Servers of their departures, but 2103 Servers are responsible for expiring neighbor cache entries and 2104 withdrawing routes even if no departure notification is received 2105 (e.g., if the Client leaves the network unexpectedly). Servers 2106 SHOULD therefore set Router Lifetime to ReachableTime seconds in 2107 solicited RA messages to minimize persistent stale cache information 2108 in the absence of Client departure notifications. A short Router 2109 Lifetime also ensures that proactive Client/Server RS/RA messaging 2110 will keep any NAT state alive (see above). 2112 Note: All Servers on an AERO link MUST advertise consistent values in 2113 the RA Cur Hop Limit, M and O flags, Reachable Time and Retrans Timer 2114 fields the same as for any link, since unpredictable behavior could 2115 result if different Servers on the same link advertised different 2116 values. 2118 3.12.3.1. Lightweight DHCPv6 Relay Agent (LDRA) 2120 When DHCPv6 is used as the ND/PD service back end, AERO Clients and 2121 Servers are always on the same link (i.e., the AERO link) from the 2122 perspective of DHCPv6. However, in some implementations the DHCPv6 2123 server and ND function may be located in separate modules. In that 2124 case, the Server's AERO interface module can act as a Lightweight 2125 DHCPv6 Relay Agent (LDRA)[RFC6221] to relay PD messages to and from 2126 the DHCPv6 server module. 2128 When the LDRA receives an authentic RS message, it extracts the PD 2129 message parameters and uses them to construct an IPv6/UDP/DHCPv6 2130 message. It sets the IPv6 source address to the source address of 2131 the RS message, sets the IPv6 destination address to 2132 'All_DHCP_Relay_Agents_and_Servers' and sets the UDP fields to values 2133 that will be understood by the DHCPv6 server. 2135 The LDRA then wraps the message in a DHCPv6 'Relay-Forward' message 2136 header and includes an 'Interface-Id' option that includes enough 2137 information to allow the LDRA to forward the resulting Reply message 2138 back to the Client (e.g., the Client's link-layer addresses, a 2139 security association identifier, etc.). The LDRA also wraps the OMNI 2140 option and SLLAO into the Interface-Id option, then forwards the 2141 message to the DHCPv6 server. 2143 When the DHCPv6 server prepares a Reply message, it wraps the message 2144 in a 'Relay-Reply' message and echoes the Interface-Id option. The 2145 DHCPv6 server then delivers the Relay-Reply message to the LDRA, 2146 which discards the Relay-Reply wrapper and IPv6/UDP headers, then 2147 uses the DHCPv6 message to construct an RA response to the Client. 2148 The Server uses the information in the Interface-Id option to prepare 2149 the RA message and to cache the link-layer addresses taken from the 2150 OMNI option and SLLAO echoed in the Interface-Id option. 2152 3.13. The AERO Proxy 2154 Clients may connect to ANETs that deploy perimeter security services 2155 to facilitate communications to Servers in outside INETs. In that 2156 case, the ANET can employ an AERO Proxy. The Proxy is located at the 2157 ANET/INET border and listens for RS messages originating from or RA 2158 messages destined to ANET Clients. The Proxy acts on these control 2159 messages as follows: 2161 o when the Proxy receives an RS message from a new ANET Client, it 2162 first authenticates the message then examines the network-layer 2163 destination address. If the destination address is a Server's 2164 LLA, the Proxy proceeds to the next step. Otherwise, if the 2165 destination is All-Routers multicast or Subnet-Router anycast, the 2166 Proxy selects a "nearby" Server that is likely to be a good 2167 candidate to serve the Client and replaces the destination address 2168 with the Server's LLA. Next, the Proxy creates a proxy neighbor 2169 cache entry and caches the Client and Server link-layer addresses 2170 along with the OMNI option information and any other identifying 2171 information including Transaction IDs, Client Identifiers, Nonce 2172 values, etc. The Proxy finally encapsulates the (proxyed) RS 2173 message in a SPAN header with source set to the Proxy's ULA and 2174 destination set to the Server's ULA then forwards the message into 2175 the SPAN. 2177 o when the Server receives the RS, it authenticates the message then 2178 creates or updates a symmetric neighbor cache entry for the Client 2179 with the Proxy's ULA as the link-layer address. The Server then 2180 sends an RA message back to the Proxy via the spanning tree. 2182 o when the Proxy receives the RA, it authenticates the message and 2183 matches it with the proxy neighbor cache entry created by the RS. 2184 The Proxy then caches the PD route information as a mapping from 2185 the Client's MNPs to the Client's link-layer address, caches the 2186 Server's advertised Router Lifetime and sets the neighbor cache 2187 entry state to REACHABLE. The Proxy then sets the P bit in the RA 2188 flags field, optionally rewrites the Router Lifetime and forwards 2189 the (proxyed) message to the Client. The Proxy finally includes 2190 an MTU option (if necessary) with an MTU to use for the underlying 2191 ANET interface. 2193 After the initial RS/RA exchange, the Proxy forwards any Client data 2194 packets for which there is no matching asymmetric neighbor cache 2195 entry to a Bridge using SPAN encapsulation with its own ULA as the 2196 source and the ULA corresponding to the Client as the destination. 2198 The Proxy instead forwards any Client data destined to an asymmetric 2199 neighbor cache target directly to the target according to the SPAN/ 2200 link-layer information - the process of establishing asymmetric 2201 neighbor cache entries is specified in Section 3.14. 2203 While the Client is still attached to the ANET, the Proxy sends NS, 2204 RS and/or unsolicited NA messages to update the Server's symmetric 2205 neighbor cache entries on behalf of the Client and/or to convey QoS 2206 updates. This allows for higher-frequency Proxy-initiated RS/RA 2207 messaging over well-connected INET infrastructure supplemented by 2208 lower-frequency Client-initiated RS/RA messaging over constrained 2209 ANET data links. 2211 If the Server ceases to send solicited advertisements, the Proxy 2212 sends unsolicited RAs on the ANET interface with destination set to 2213 All-Nodes multicast (ff02::1) and with Router Lifetime set to zero to 2214 inform Clients that the Server has failed. Although the Proxy 2215 engages in ND exchanges on behalf of the Client, the Client can also 2216 send ND messages on its own behalf, e.g., if it is in a better 2217 position than the Proxy to convey QoS changes, etc. For this reason, 2218 the Proxy marks any Client-originated solicitation messages (e.g. by 2219 inserting a Nonce option) so that it can return the solicited 2220 advertisement to the Client instead of processing it locally. 2222 If the Client becomes unreachable, the Proxy sets the neighbor cache 2223 entry state to DEPARTED and retains the entry for DepartTime seconds. 2224 While the state is DEPARTED, the Proxy forwards any packets destined 2225 to the Client to a Bridge via SPAN encapsulation with the Client's 2226 current Server as the destination. The Bridge in turn forwards the 2227 packets to the Client's current Server. When DepartTime expires, the 2228 Proxy deletes the neighbor cache entry and discards any further 2229 packets destined to this (now forgotten) Client. 2231 In some ANETs that employ a Proxy, the Client's MNP can be injected 2232 into the ANET routing system. In that case, the Client can send data 2233 messages without encapsulation so that the ANET routing system 2234 transports the unencapsulated packets to the Proxy. This can be very 2235 beneficial, e.g., if the Client connects to the ANET via low-end data 2236 links such as some aviation wireless links. 2238 If the first-hop ANET access router is AERO-aware, the Client can 2239 avoid encapsulation for both its control and data messages. When the 2240 Client connects to the link, it can send an unencapsulated RS message 2241 with source address set to its LLA and with destination address set 2242 to the LLA of the Client's selected Server or to All-Routers 2243 multicast or Subnet-Router anycast. The Client includes an OMNI 2244 option formatted as specified in [I-D.templin-6man-omni-interface]. 2246 The Client then sends the unencapsulated RS message, which will be 2247 intercepted by the AERO-Aware access router. The access router then 2248 encapsulates the RS message in an ANET header with its own address as 2249 the source address and the address of a Proxy as the destination 2250 address. The access router further remembers the address of the 2251 Proxy so that it can encapsulate future data packets from the Client 2252 via the same Proxy. If the access router needs to change to a new 2253 Proxy, it simply sends another RS message toward the Server via the 2254 new Proxy on behalf of the Client. 2256 In some cases, the access router and Proxy may be one and the same 2257 node. In that case, the node would be located on the same physical 2258 link as the Client, but its message exchanges with the Server would 2259 need to pass through a security gateway at the ANET/INET border. The 2260 method for deploying access routers and Proxys (i.e. as a single node 2261 or multiple nodes) is an ANET-local administrative consideration. 2263 3.13.1. Detecting and Responding to Server Failures 2265 In environments where fast recovery from Server failure is required, 2266 Proxys SHOULD use proactive Neighbor Unreachability Detection (NUD) 2267 to track Server reachability in a similar fashion as for 2268 Bidirectional Forwarding Detection (BFD) [RFC5880]. Proxys can then 2269 quickly detect and react to failures so that cached information is 2270 re-established through alternate paths. The NUD control messaging is 2271 carried only over well-connected ground domain networks (i.e., and 2272 not low-end aeronautical radio links) and can therefore be tuned for 2273 rapid response. 2275 Proxys perform proactive NUD with Servers for which there are 2276 currently active ANET Clients by sending continuous NS messages in 2277 rapid succession, e.g., one message per second. The Proxy sends the 2278 NS message via the spanning tree with the Proxy's LLA as the source 2279 and the LLA of the Server as the destination. When the Proxy is also 2280 sending RS messages to the Server on behalf of ANET Clients, the 2281 resulting RA responses can be considered as equivalent hints of 2282 forward progress. This means that the Proxy need not also send a 2283 periodic NS if it has already sent an RS within the same period. If 2284 the Server fails (i.e., if the Proxy ceases to receive 2285 advertisements), the Proxy can quickly inform Clients by sending 2286 multicast RA messages on the ANET interface. 2288 The Proxy sends RA messages on the ANET interface with source address 2289 set to the Server's address, destination address set to All-Nodes 2290 multicast, and Router Lifetime set to 0. The Proxy SHOULD send 2291 MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated by small delays 2292 [RFC4861]. Any Clients on the ANET that had been using the failed 2293 Server will receive the RA messages and associate with a new Server. 2295 3.13.2. Point-to-Multipoint Server Coordination 2297 In environments where Client messaging over ANETs is bandwidth- 2298 limited and/or expensive, Clients can enlist the services of the 2299 Proxy to coordinate with multiple Servers in a single RS/RA message 2300 exchange. The Client can send a single RS message to All-Routers 2301 multicast that includes the ID's of multiple Servers in MS-Register 2302 sub-options of the OMNI option. 2304 When the Proxy receives the RS and processes the OMNI option, it 2305 performs a separate RS/RA exchange with each MS-Register Server. 2306 When it has received the RA messages, it creates an "aggregate" RA 2307 message to return to the Client with an OMNI option with each 2308 responding Server's ID recorded in an MS-Register sub-option. 2310 Clients can thereafter employ efficient point-to-multipoint Server 2311 coordination under the assistance of the Proxy to dramatically reduce 2312 the number of messages sent over the ANET while enlisting the support 2313 of multiple Servers for fault tolerance. Clients can further include 2314 MS-Release suboptions in RS messages to request the Proxy to release 2315 from former Servers via the procedures discussed in Section 3.16.5. 2317 The OMNI interface specification [I-D.templin-6man-omni-interface] 2318 provides further discussion of the Client/Proxy RS/RA messaging 2319 involved in point-to-multipoint coordination. 2321 3.14. AERO Route Optimization 2323 While data packets are flowing between a source and target node, 2324 route optimization SHOULD be used. Route optimization is initiated 2325 by the first eligible Route Optimization Source (ROS) closest to the 2326 source as follows: 2328 o For Clients on VPNed and Direct interfaces, the Server is the ROS. 2330 o For Clients on Proxyed interfaces, the Proxy is the ROS. 2332 o For Clients on INET interfaces, the Client itself is the ROS. 2334 o For correspondent nodes on INET/EUN interfaces serviced by a 2335 Relay, the Relay is the ROS. 2337 The route optimization procedure is conducted between the ROS and the 2338 target Server/Relay acting as a Route Optimization Responder (ROR) in 2339 the same manner as for IPv6 ND Address Resolution and using the same 2340 NS/NA messaging. The target may either be a MNP Client serviced by a 2341 Server, or a non-MNP correspondent reachable via a Relay. 2343 The procedures are specified in the following sections. 2345 3.14.1. Route Optimization Initiation 2347 While data packets are flowing from the source node toward a target 2348 node, the ROS performs address resolution by sending an NS message 2349 for Address Resolution (NS(AR)) to receive a solicited NA message 2350 from the ROR. When the ROS sends an NS(AR), it includes: 2352 o the LLA of the ROS as the source address. 2354 o the data packet's destination as the Target Address. 2356 o the Solicited-Node multicast address [RFC4291] formed from the 2357 lower 24 bits of the data packet's destination as the destination 2358 address, e.g., for 2001:db8:1:2::10:2000 the NS destination 2359 address is ff02:0:0:0:0:1:ff10:2000. 2361 The NS(AR) message includes an OMNI option with no ifIndex-tuples and 2362 no SLLAO, such that the target will not create a neighbor cache 2363 entry. 2365 The ROS then encapsulates the NS(AR) message in a SPAN header with 2366 source set to its own ULA and destination set to the ULA 2367 corresponding to the packet's final destination, then sends the 2368 message into the spanning tree without decrementing the network-layer 2369 TTL/Hop Limit field. 2371 3.14.2. Relaying the NS 2373 When the Bridge receives the NS(AR) message from the ROS, it discards 2374 the INET header and determines that the ROR is the next hop by 2375 consulting its standard IPv6 forwarding table for the SPAN header 2376 destination address. The Bridge then forwards the message toward the 2377 ROR via the spanning tree the same as for any IPv6 router. The 2378 final-hop Bridge in the spanning tree will deliver the message via a 2379 secured tunnel to the ROR. 2381 3.14.3. Processing the NS and Sending the NA 2383 When the ROR receives the NS(AR) message, it examines the Target 2384 Address to determine whether it has a neighbor cache entry and/or 2385 route that matches the target. If there is no match, the ROR drops 2386 the message. Otherwise, the ROR continues processing as follows: 2388 o if the target belongs to an MNP Client neighbor in the DEPARTED 2389 state the ROR changes the NS(AR) message SPAN destination address 2390 to the ULA of the Client's new Server, forwards the message into 2391 the spanning tree and returns from processing. 2393 o If the target belongs to an MNP Client neighbor in the REACHABLE 2394 state, the ROR instead adds the AERO source address to the target 2395 Client's Report List with time set to ReportTime. 2397 o If the target belongs to a non-MNP route, the ROR continues 2398 processing without adding an entry to the Report List. 2400 The ROR then prepares a solicited NA message to send back to the ROS 2401 but does not create a neighbor cache entry. The ROR sets the NA 2402 source address to the LLA corresponding to the target, sets the 2403 Target Address to the target of the solicitation, and sets the 2404 destination address to the source of the solicitation. 2406 The ROR then includes an OMNI option with prefix registration length 2407 set to the length of the MNP if the target is an MNP Client; 2408 otherwise, set to the maximum of the non-MNP prefix length and 64. 2409 (Note that a /64 limit is imposed to avoid causing the ROS to set 2410 short prefixes (e.g., "default") that would match destinations for 2411 which the routing system includes more-specific prefixes.) 2413 If the target is an MNP Client, the ROR next includes ifIndex-tuples 2414 in the OMNI option for each of the target Client's underlying 2415 interfaces with current information for each interface and with the S 2416 flag set to 0. The ROR then includes a TLLAO with ifIndex-tuples in 2417 one-to-one correspondence with the tuples that appear in the OMNI 2418 option. 2420 The ROR sets the Link Layer Address and Port Number (if necessary) to 2421 its own INET address for VPNed and Direct interfaces or to the INET 2422 address of the Proxy for Proxyed interface, then includes its own ULA 2423 or the ULA of the Proxy as the ultimate Segment Routing List entry. 2424 For INET interfaces, the ROR instead sets the Link Layer Address and 2425 Port Number (if necessary) to the Client's INET address then sets its 2426 own ULA in the penultimate Segment Routing List entry and sets the 2427 target's ULA in the ultimate Segment Routing List entry. 2429 The ROR then sets the NA message R flag to 1 (as a router), S flag to 2430 1 (as a response to a solicitation), and O flag to 0 (as a proxy). 2431 The ROR finally encapsulates the NA message in a SPAN header with 2432 source set to its own ULA and destination set to the source ULA of 2433 the NS(AR) message, then forwards the message into the spanning tree 2434 without decrementing the network-layer TTL/Hop Limit field. 2436 3.14.4. Relaying the NA 2438 When the Bridge receives the NA message from the ROR, it discards the 2439 INET header and determines that the ROS is the next hop by consulting 2440 its standard IPv6 forwarding table for the SPAN header destination 2441 address. The Bridge then forwards the SPAN-encapsulated NA message 2442 toward the ROS the same as for any IPv6 router. The final-hop Bridge 2443 in the spanning tree will deliver the message via a secured tunnel to 2444 the ROS. 2446 3.14.5. Processing the NA 2448 When the ROS receives the solicited NA message, it processes the 2449 message the same as for standard IPv6 Address Resolution [RFC4861]. 2450 In the process, it caches the source ULA then creates an asymmetric 2451 neighbor cache entry for the ROR and caches all information found in 2452 the OMNI and TLLAO options. The ROS finally sets the asymmetric 2453 neighbor cache entry lifetime to ReachableTime seconds. 2455 3.14.6. Route Optimization Maintenance 2457 Following route optimization, the ROS forwards future data packets 2458 destined to the target via the addresses found in the cached link- 2459 layer information. The route optimization is shared by all sources 2460 that send packets to the target via the ROS, i.e., and not just the 2461 source on behalf of which the route optimization was initiated. 2463 While new data packets destined to the target are flowing through the 2464 ROS, it sends additional NS(AR) messages to the ROR before 2465 ReachableTime expires to receive a fresh solicited NA message the 2466 same as described in the previous sections (route optimization 2467 refreshment strategies are an implementation matter, with a non- 2468 normative example given in Appendix B.1). The ROS uses the cached 2469 ULA of the ROR as the NS(AR) SPAN destination address, and sends up 2470 to MAX_MULTICAST_SOLICIT NS(AR) messages separated by 1 second until 2471 an NA is received. If no NA is received, the ROS assumes that the 2472 current ROR has become unreachable and deletes the neighbor cache 2473 entry. Subsequent data packets will trigger a new route optimization 2474 per Section 3.14.1 to discover a new ROR while initial data packets 2475 travel over a suboptimal route. 2477 If an NA is received, the ROS then updates the asymmetric neighbor 2478 cache entry to refresh ReachableTime, while (for MNP destinations) 2479 the ROR adds or updates the ROS address to the target Client's Report 2480 List and with time set to ReportTime. While no data packets are 2481 flowing, the ROS instead allows ReachableTime for the asymmetric 2482 neighbor cache entry to expire. When ReachableTime expires, the ROS 2483 deletes the asymmetric neighbor cache entry. Any future data packets 2484 flowing through the ROS will again trigger a new route optimization. 2486 The ROS may also receive unsolicited NA messages from the ROR at any 2487 time (see: Section 3.16). If there is an asymmetric neighbor cache 2488 entry for the target, the ROS updates the link-layer information but 2489 does not update ReachableTime since the receipt of an unsolicited NA 2490 does not confirm that any forward paths are working. If there is no 2491 asymmetric neighbor cache entry, the ROS simply discards the 2492 unsolicited NA. 2494 In this arrangement, the ROS holds an asymmetric neighbor cache entry 2495 for the ROR, but the ROR does not hold an asymmetric neighbor cache 2496 entry for the ROS. The route optimization neighbor relationship is 2497 therefore asymmetric and unidirectional. If the target node also has 2498 packets to send back to the source node, then a separate route 2499 optimization procedure is performed in the reverse direction. But, 2500 there is no requirement that the forward and reverse paths be 2501 symmetric. 2503 3.15. Neighbor Unreachability Detection (NUD) 2505 AERO nodes perform Neighbor Unreachability Detection (NUD) per 2506 [RFC4861] either reactively in response to persistent link-layer 2507 errors (see Section 3.11) or proactively to confirm reachability. 2508 The NUD algorithm is based on periodic control message exchanges. 2509 The algorithm may further be seeded by ND hints of forward progress, 2510 but care must be taken to avoid inferring reachability based on 2511 spoofed information. For example, authentic IPv6 ND message 2512 exchanges may be considered as acceptable hints of forward progress, 2513 while spurious data packets should not be. 2515 AERO Servers, Proxys and Relays can use standard NS/NA NUD exchanges 2516 sent over the spanning tree to securely test reachability without 2517 risk of DoS attacks from nodes pretending to be a neighbor; Proxys 2518 can further perform NUD to securely verify Server reachability on 2519 behalf of their proxyed Clients. However, a means for a ROS to test 2520 the unsecured forward directions of target route optimized paths is 2521 also necessary. 2523 When an ROR directs an ROS to a neighbor with one or more target 2524 link-layer addresses, the ROS can proactively test each such 2525 unsecured route optimized path by sending "loopback" NS(NUD) 2526 messages. While testing the paths, the ROS can optionally continue 2527 to send packets via the spanning tree, maintain a small queue of 2528 packets until target reachability is confirmed, or (optimistically) 2529 allow packets to flow via the route optimized paths. 2531 When the ROS sends a loopback NS(NUD) message, it uses its LLA as 2532 both the IPv6 source and destination address, and the MNP Subnet- 2533 Router anycast address as the Target Address. The ROS includes a 2534 Nonce and Timestamp option, then encapsulates the message in SPAN/ 2535 INET headers with its own ULA as the source and the ULA of the route 2536 optimization target as the destination. The ROS then forwards the 2537 message to the target (either directly to the link layer address of 2538 the target if the target is in the same AERO link segment, or via a 2539 Bridge if the target is in a different AERO link segment). 2541 When the route optimization target receives the NS(NUD) message, it 2542 notices that the IPv6 destination address is the same as the source 2543 address. It then reverses the SPAN source and destination addresses 2544 and returns the message to the ROS (either directly or via the 2545 spanning tree). The route optimization target does not decrement the 2546 NS(NUD) message IPv6 Hop-Limit in the process, since the message has 2547 not exited the AERO link. 2549 When the ROS receives the NS(NUD) message, it can determine from the 2550 Nonce, Timestamp and Target Address that the message originated from 2551 itself and that it transited the forward path. The ROS need not 2552 prepare an NA response, since the destination of the response would 2553 be itself and testing the route optimization path again would be 2554 redundant. 2556 The ROS marks route optimization target paths that pass these NUD 2557 tests as "reachable", and those that do not as "unreachable". These 2558 markings inform the AERO interface forwarding algorithm specified in 2559 Section 3.10. 2561 Note that to avoid a DoS vector nodes MUST NOT return loopback 2562 NS(NUD) messages received from an unsecured link-layer source via the 2563 spanning tree. 2565 3.16. Mobility Management and Quality of Service (QoS) 2567 AERO is a Distributed Mobility Management (DMM) service. Each Server 2568 is responsible for only a subset of the Clients on the AERO link, as 2569 opposed to a Centralized Mobility Management (CMM) service where 2570 there is a single network mobility collective entity for all Clients. 2571 Clients coordinate with their associated Servers via RS/RA exchanges 2572 to maintain the DMM profile, and the AERO routing system tracks all 2573 current Client/Server peering relationships. 2575 Servers provide default routing and mobility/multilink services for 2576 their dependent Clients. Clients are responsible for maintaining 2577 neighbor relationships with their Servers through periodic RS/RA 2578 exchanges, which also serves to confirm neighbor reachability. When 2579 a Client's underlying interface address and/or QoS information 2580 changes, the Client is responsible for updating the Server with this 2581 new information. Note that for Proxyed interfaces, however, the 2582 Proxy can also perform some RS/RA exchanges on the Client's behalf. 2584 Mobility management considerations are specified in the following 2585 sections. 2587 3.16.1. Mobility Update Messaging 2589 Servers accommodate Client mobility/multilink and/or QoS change 2590 events by sending unsolicited NA (uNA) messages to each ROS in the 2591 target Client's Report List. When a Server sends a uNA message, it 2592 sets the IPv6 source address to the Client's LLA, sets the 2593 destination address to All-Nodes multicast and sets the Target 2594 Address to the Client's Subnet-Router anycast address. The Server 2595 also includes an OMNI option with prefix registration information and 2596 with ifIndex-tuples for the target Client's remaining interfaces with 2597 S set to 0. The Server then includes a TLLAO with corresponding 2598 ifIndex-tuples prepared the same as for the initial route 2599 optimization event. The Server sets the NA R flag to 1, the S flag 2600 to 0 and the O flag to 0, then encapsulates the message in a SPAN 2601 header with source set to its own ULA and destination set to the ULA 2602 of the ROS and sends the message into the spanning tree. 2604 As discussed in Section 7.2.6 of [RFC4861], the transmission and 2605 reception of uNA messages is unreliable but provides a useful 2606 optimization. In well-connected Internetworks with robust data links 2607 uNA messages will be delivered with high probability, but in any case 2608 the Server can optionally send up to MAX_NEIGHBOR_ADVERTISEMENT uNAs 2609 to each ROS to increase the likelihood that at least one will be 2610 received. 2612 When the ROS receives a uNA message, it ignores the message if there 2613 is no existing neighbor cache entry for the Client. Otherwise, it 2614 uses the included OMNI option and TLLAO information to update the 2615 neighbor cache entry, but does not reset ReachableTime since the 2616 receipt of an unsolicited NA message from the target Server does not 2617 provide confirmation that any forward paths to the target Client are 2618 working. 2620 If uNA messages are lost, the ROS may be left with stale address and/ 2621 or QoS information for the Client for up to ReachableTime seconds. 2622 During this time, the ROS can continue sending packets according to 2623 its stale neighbor cache information. When ReachableTime is close to 2624 expiring, the ROS will re-initiate route optimization and receive 2625 fresh link-layer address information. 2627 In addition to sending uNA messages to the current set of ROSs for 2628 the Client, the Server also sends uNAs to the former link-layer 2629 address for any ifIndex-tuple for which the link-layer address has 2630 changed. The uNA messages update Proxys that cannot easily detect 2631 (e.g., without active probing) when a formerly-active Client has 2632 departed. 2634 3.16.2. Announcing Link-Layer Address and/or QoS Preference Changes 2636 When a Client needs to change its underlying interface addresses and/ 2637 or QoS preferences (e.g., due to a mobility event), either the Client 2638 or its Proxys send RS messages to the Server via the spanning tree 2639 with an OMNI option that includes an ifIndex-tuple with S set to 1 2640 and with the new link quality and address information. 2642 Up to MAX_RTR_SOLICITATIONS RS messages MAY be sent in parallel with 2643 sending actual data packets in case one or more RAs are lost. If all 2644 RAs are lost, the Client SHOULD re-associate with a new Server. 2646 When the Server receives the Client's changes, it sends uNA messages 2647 to all nodes in the Report List the same as described in the previous 2648 section. 2650 3.16.3. Bringing New Links Into Service 2652 When a Client needs to bring new underlying interfaces into service 2653 (e.g., when it activates a new data link), it sends an RS message to 2654 the Server via the underlying interface with an OMNI option that 2655 includes an ifIndex-tuple with S set to 1 and appropriate link 2656 quality values and with link-layer address information for the new 2657 link. 2659 3.16.4. Removing Existing Links from Service 2661 When a Client needs to remove existing underlying interfaces from 2662 service (e.g., when it de-activates an existing data link), it sends 2663 an RS or uNA message to its Server with an OMNI option with 2664 appropriate link quality values. 2666 If the Client needs to send RS/uNA messages over an underlying 2667 interface other than the one being removed from service, it MUST 2668 include ifIndex-tuples with appropriate link quality values for any 2669 underlying interfaces being removed from service. 2671 3.16.5. Moving to a New Server 2673 When a Client associates with a new Server, it performs the Client 2674 procedures specified in Section 3.12.2. The Client also includes MS- 2675 Release identifiers in the RS message OMNI option per 2676 [I-D.templin-6man-omni-interface] if it wants the new Server to 2677 notify any old Servers from which the Client is departing. 2679 When the new Server receives the Client's RS message, it returns an 2680 RA as specified in Section 3.12.3 and sends up to 2681 MAX_NEIGHBOR_ADVERTIISEMENT uNA messages to any old Servers listed in 2682 OMNI option MS-Release identifiers. Each uNA message includes the 2683 Client's LLA as the source address, the old Server's LLA as the 2684 destination address, and an OMNI option with the Register/Release bit 2685 set to 0. The new Server wraps the uNA in a SPAN header with its own 2686 ULA as the source and the old Server's ULA as the destination, then 2687 sends the message into the spanning tree. 2689 When an old Server receives the uNA, it changes the Client's neighbor 2690 cache entry state to DEPARTED, sets the link-layer address of the 2691 Client to the new Server's ULA, and resets DepartTime. After a short 2692 delay (e.g., 2 seconds) the old Server withdraws the Client's MNP 2693 from the routing system. After DepartTime expires, the old Server 2694 deletes the Client's neighbor cache entry. 2696 The old Server also sends unsolicited NA messages to all ROSs in the 2697 Client's Report List with an OMNI option with a single ifIndex-tuple 2698 with ifIndex set to 0 and S set to '1', and with the ULA of the new 2699 Server in a companion TLLAO. When the ROS receives the NA, it caches 2700 the address of the new Server in the existing asymmetric neighbor 2701 cache entry and marks the entry as STALE for a period of 10 seconds 2702 after which the cache entry is deleted. While in the STALE state, 2703 subsequent data packets flow according to any existing cached link- 2704 layer information and trigger a new NS(AR)/NA exchange via the new 2705 Server. 2707 Clients SHOULD NOT move rapidly between Servers in order to avoid 2708 causing excessive oscillations in the AERO routing system. Examples 2709 of when a Client might wish to change to a different Server include a 2710 Server that has gone unreachable, topological movements of 2711 significant distance, movement to a new geographic region, movement 2712 to a new AERO link segment, etc. 2714 When a Client moves to a new Server, some of the fragments of a 2715 multiple fragment packet may have already arrived at the old Server 2716 while others are en route to the new Server, however no special 2717 attention in the reassembly algorithm is necessary when re-routed 2718 fragments are simply treated as loss. 2720 3.17. Multicast 2722 The AERO Client provides an IGMP (IPv4) [RFC2236] or MLD (IPv6) 2723 [RFC3810] proxy service for its EUNs and/or hosted applications 2724 [RFC4605]. The Client forwards IGMP/MLD messages over any of its 2725 underlying interfaces for which group membership is required. The 2726 IGMP/MLD messages may be further forwarded by a first-hop ANET access 2727 router acting as an IGMP/MLD-snooping switch [RFC4541], then 2728 ultimately delivered to an AERO Proxy/Server acting as a Protocol 2729 Independent Multicast - Sparse-Mode (PIM-SM, or simply "PIM") 2730 Designated Router (DR) [RFC7761]. AERO Relays also act as PIM 2731 routers (i.e., the same as AERO Proxys/Servers) on behalf of nodes on 2732 INET/EUN networks. The behaviors identified in the following 2733 sections correspond to Source-Specific Multicast (SSM) and Any-Source 2734 Multicast (ASM) operational modes. 2736 3.17.1. Source-Specific Multicast (SSM) 2738 When an ROS (i.e., an AERO Proxy/Server/Relay) "X" acting as PIM 2739 router receives a Join/Prune message from a node on its downstream 2740 interfaces containing one or more ((S)ource, (G)roup) pairs, it 2741 updates its Multicast Routing Information Base (MRIB) accordingly. 2742 For each S belonging to a prefix reachable via X's non-AERO 2743 interfaces, X then forwards the (S, G) Join/Prune to any PIM routers 2744 on those interfaces per [RFC7761]. 2746 For each S belonging to a prefix reachable via X's AERO interface, X 2747 originates a separate copy of the Join/Prune for each (S,G) in the 2748 message using its own LLA as the source address and ALL-PIM-ROUTERS 2749 as the destination address. X then encapsulates each message in a 2750 SPAN header with source address set to the ULA of X and destination 2751 address set to S then forwards the message into the spanning tree, 2752 which delivers it to AERO Server/Relay "Y" that services S. At the 2753 same time, if the message was a Join, X sends a route-optimization NS 2754 message toward each S the same as discussed in Section 3.14. The 2755 resulting NAs will return the LLA for the prefix that matches S as 2756 the network-layer source address and TLLAOs with the ULA 2757 corresponding to any ifIndex-tuples that are currently servicing S. 2759 When Y processes the Join/Prune message, if S located behind any 2760 INET, Direct, or VPNed interfaces Y acts as a PIM router and updates 2761 its MRIB to list X as the next hop in the reverse path. If S is 2762 located behind any Proxys "Z"*, Y also forwards the message to each 2763 Z* over the spanning tree while continuing to use the LLA of X as the 2764 source address. Each Z* then updates its MRIB accordingly and 2765 maintains the LLA of X as the next hop in the reverse path. Since 2766 the Bridges do not examine network layer control messages, this means 2767 that the (reverse) multicast tree path is simply from each Z* (and/or 2768 Y) to X with no other multicast-aware routers in the path. If any Z* 2769 (and/or Y) is located on the same AERO link segment as X, the 2770 multicast data traffic sent to X directly using SPAN/INET 2771 encapsulation instead of via a Bridge. 2773 Following the initial Join/Prune and NS/NA messaging, X maintains an 2774 asymmetric neighbor cache entry for each S the same as if X was 2775 sending unicast data traffic to S. In particular, X performs 2776 additional NS/NA exchanges to keep the neighbor cache entry alive for 2777 up to t_periodic seconds [RFC7761]. If no new Joins are received 2778 within t_periodic seconds, X allows the neighbor cache entry to 2779 expire. Finally, if X receives any additional Join/Prune messages 2780 for (S,G) it forwards the messages to each Y and Z* in the neighbor 2781 cache entry over the spanning tree. 2783 At some later time, Client C that holds an MNP for source S may 2784 depart from a first Proxy Z1 and/or connect via a new Proxy Z2. In 2785 that case, Y sends an unsolicited NA message to X the same as 2786 specified for unicast mobility in Section 3.16. When X receives the 2787 unsolicited NA message, it updates its asymmetric neighbor cache 2788 entry for the LLA for source S and sends new Join messages to any new 2789 Proxys Z2. There is no requirement to send any Prune messages to old 2790 Proxys Z1 since source S will no longer source any multicast data 2791 traffic via Z1. Instead, the multicast state for (S,G) in Proxy Z1 2792 will soon time out since no new Joins will arrive. 2794 After some later time, C may move to a new Server Y2 and depart from 2795 old Sever Y1. In that case, Y1 sends Join messages for any of C's 2796 active (S,G) groups to Y2 while including its own LLA as the source 2797 address. This causes Y2 to include Y1 in the multicast forwarding 2798 tree during the interim time that Y1's symmetric neighbor cache entry 2799 for C is in the DEPARTED state. At the same time, Y1 sends an 2800 unsolicited NA message to X with an OMNI option and TLLAO with 2801 ifIndex-tuple set to 0 and a release indication to cause X to release 2802 its asymmetric neighbor cache entry. X then sends a new Join message 2803 to S via the spanning tree and re-initiates route optimization the 2804 same as if it were receiving a fresh Join message from a node on a 2805 downstream link. 2807 3.17.2. Any-Source Multicast (ASM) 2809 When an ROS X acting as a PIM router receives a Join/Prune from a 2810 node on its downstream interfaces containing one or more (*,G) pairs, 2811 it updates its Multicast Routing Information Base (MRIB) accordingly. 2812 X then forwards a copy of the message to the Rendezvous Point (RP) R 2813 for each G over the spanning tree. X uses its own LLA as the source 2814 address and ALL-PIM-ROUTERS as the destination address, then 2815 encapsulates each message in a SPAN header with source address set to 2816 the ULA of X and destination address set to R, then sends the message 2817 into the spanning tree. At the same time, if the message was a Join 2818 X initiates NS/NA route optimization the same as for the SSM case 2819 discussed in Section 3.17.1. 2821 For each source S that sends multicast traffic to group G via R, the 2822 Proxy/Server Z* for the Client that aggregates S encapsulates the 2823 packets in PIM Register messages and forwards them to R via the 2824 spanning tree, which may then elect to send a PIM Join to Z*. This 2825 will result in an (S,G) tree rooted at Z* with R as the next hop so 2826 that R will begin to receive two copies of the packet; one native 2827 copy from the (S, G) tree and a second copy from the pre-existing (*, 2828 G) tree that still uses PIM Register encapsulation. R can then issue 2829 a PIM Register-stop message to suppress the Register-encapsulated 2830 stream. At some later time, if C moves to a new Proxy/Server Z*, it 2831 resumes sending packets via PIM Register encapsulation via the new 2832 Z*. 2834 At the same time, as multicast listeners discover individual S's for 2835 a given G, they can initiate an (S,G) Join for each S under the same 2836 procedures discussed in Section 3.17.1. Once the (S,G) tree is 2837 established, the listeners can send (S, G) Prune messages to R so 2838 that multicast packets for group G sourced by S will only be 2839 delivered via the (S, G) tree and not from the (*, G) tree rooted at 2840 R. All mobility considerations discussed for SSM apply. 2842 3.17.3. Bi-Directional PIM (BIDIR-PIM) 2844 Bi-Directional PIM (BIDIR-PIM) [RFC5015] provides an alternate 2845 approach to ASM that treats the Rendezvous Point (RP) as a Designated 2846 Forwarder (DF). Further considerations for BIDIR-PIM are out of 2847 scope. 2849 3.18. Operation over Multiple AERO Links (VLANs) 2851 An AERO Client can connect to multiple AERO links the same as for any 2852 data link service. In that case, the Client maintains a distinct 2853 AERO interface for each link, e.g., 'aero0' for the first link, 2854 'aero1' for the second, 'aero2' for the third, etc. Each AERO link 2855 would include its own distinct set of Bridges, Servers and Proxys, 2856 thereby providing redundancy in case of failures. 2858 The Bridges, Servers and Proxys on each AERO link can assign AERO and 2859 ULAs that use the same or different numberings from those on other 2860 links. Since the links are mutually independent there is no 2861 requirement for avoiding inter-link address duplication, e.g., the 2862 same LLA such as fe80::1000 could be used to number distinct nodes 2863 that connect to different AERO links. 2865 Each AERO link could utilize the same or different ANET connections. 2866 The links can be distinguished at the link-layer via the SSP in a 2867 similar fashion as for Virtual Local Area Network (VLAN) tagging 2868 (e.g., IEEE 802.1Q) and/or through assignment of distinct sets of 2869 MSPs on each link. This gives rise to the opportunity for supporting 2870 multiple redundant networked paths, where each VLAN is distinguished 2871 by a different SRT color (see: Section 3.2.6). In particular, the 2872 Client can tag its RS messages with the appropriate label to cause 2873 the network to select the desired VLAN. 2875 The Client's IP layer can select the outgoing AERO interface 2876 appropriate for a given traffic profile while (in the reverse 2877 direction) correspondent nodes must have some way of steering their 2878 packets destined to a target via the correct AERO link. 2880 In a first alternative, if each AERO link services different MSPs, 2881 then the Client can receive a distinct MNP from each of the links. 2882 IP routing will therefore assure that the correct Red/Green/Blue/etc. 2883 network is used for both outbound and inbound traffic. This can be 2884 accomplished using existing technologies and approaches, and without 2885 requiring any special supporting code in correspondent nodes or 2886 Bridges. 2888 In a second alternative, if each AERO link services the same MSP(s) 2889 then each link could assign a distinct "AERO Link Anycast" address 2890 that is configured by all Bridges on the link. Correspondent nodes 2891 can then perform Segment Routing to select the correct SRT, which 2892 will then direct the packet over multiple hops to the target. 2894 3.19. DNS Considerations 2896 AERO Client MNs and INET correspondent nodes consult the Domain Name 2897 System (DNS) the same as for any Internetworking node. When 2898 correspondent nodes and Client MNs use different IP protocol versions 2899 (e.g., IPv4 correspondents and IPv6 MNs), the INET DNS must maintain 2900 A records for IPv4 address mappings to MNs which must then be 2901 populated in Relay NAT64 mapping caches. In that way, an IPv4 2902 correspondent node can send packets to the IPv4 address mapping of 2903 the target MN, and the Relay will translate the IPv4 header and 2904 destination address into an IPv6 header and IPv6 destination address 2905 of the MN. 2907 When an AERO Client registers with an AERO Server, the Server can 2908 return the address(es) of DNS servers in RDNSS options [RFC6106]. 2909 The DNS server provides the IP addresses of other MNs and 2910 correspondent nodes in AAAA records for IPv6 or A records for IPv4. 2912 3.20. Transition Considerations 2914 SPAN encapsulation ensures that dissimilar INET partitions can be 2915 joined into a single unified AERO link, even though the partitions 2916 themselves may have differing protocol versions and/or incompatible 2917 addressing plans. However, a commonality can be achieved by 2918 incrementally distributing globally routable (i.e., native) IP 2919 prefixes to eventually reach all nodes (both mobile and fixed) in all 2920 AERO link segments. This can be accomplished by incrementally 2921 deploying AERO Relays on each INET partition, with each Relay 2922 distributing its MNPs and/or discovering non-MNP prefixes on its INET 2923 links. 2925 This gives rise to the opportunity to eventually distribute native IP 2926 addresses to all nodes, and to present a unified AERO link view even 2927 if the INET partitions remain in their current protocol and 2928 addressing plans. In that way, the AERO link can serve the dual 2929 purpose of providing a mobility/multilink service and a transition 2930 service. Or, if an INET partition is transitioned to a native IP 2931 protocol version and addressing scheme that is compatible with the 2932 AERO link MNP-based addressing scheme, the partition and AERO link 2933 can be joined by Relays. 2935 Relays that connect INETs/EUNs with dissimilar IP protocol versions 2936 may need to employ a network address and protocol translation 2937 function such as NAT64[RFC6146]. 2939 3.21. Detecting and Reacting to Server and Bridge Failures 2941 In environments where rapid failure recovery is required, Servers and 2942 Bridges SHOULD use Bidirectional Forwarding Detection (BFD) 2943 [RFC5880]. Nodes that use BFD can quickly detect and react to 2944 failures so that cached information is re-established through 2945 alternate nodes. BFD control messaging is carried only over well- 2946 connected ground domain networks (i.e., and not low-end radio links) 2947 and can therefore be tuned for rapid response. 2949 Servers and Bridges maintain BFD sessions in parallel with their BGP 2950 peerings. If a Server or Bridge fails, BGP peers will quickly re- 2951 establish routes through alternate paths the same as for common BGP 2952 deployments. Similarly, Proxys maintain BFD sessions with their 2953 associated Bridges even though they do not establish BGP peerings 2954 with them. 2956 Proxys SHOULD use proactive NUD for Servers for which there are 2957 currently active ANET Clients in a manner that parallels BFD, i.e., 2958 by sending unicast NS messages in rapid succession to receive 2959 solicited NA messages. When the Proxy is also sending RS messages on 2960 behalf of ANET Clients, the RS/RA messaging can be considered as 2961 equivalent hints of forward progress. This means that the Proxy need 2962 not also send a periodic NS if it has already sent an RS within the 2963 same period. If a Server fails, the Proxy will cease to receive 2964 advertisements and can quickly inform Clients of the outage by 2965 sending multicast RA messages on the ANET interface. 2967 The Proxy sends multicast RA messages with source address set to the 2968 Server's address, destination address set to All-Nodes multicast, and 2969 Router Lifetime set to 0. The Proxy SHOULD send 2970 MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated by small delays 2971 [RFC4861]. Any Clients on the ANET interface that have been using 2972 the (now defunct) Server will receive the RA messages and associate 2973 with a new Server. 2975 3.22. AERO Clients on the Open Internet 2977 AERO Clients that connect to the open Internet via INET interfaces 2978 can establish a VPN or direct link to securely connect to a Server in 2979 a "tethered" arrangement with all of the Client's traffic transiting 2980 the Server. Alternatively, the Client can associate with an INET 2981 Server using UDP/IP encapsulation and asymmetric securing services as 2982 discussed in the following sections. 2984 When a Client's AERO interface enables an INET underlying interface, 2985 it first determines whether the interface is likely to be behind a 2986 NAT. For IPv4, the Client might assume it is on the open Internet if 2987 the INET address is not a special-use IPv4 address per [RFC3330]. 2988 Similarly for IPv6, the Client might assume it is on the open 2989 Internet if the INET address is not a link-local [RFC4291] or unique- 2990 local [RFC4193] IPv6 address. 2992 The Client then prepares a UDP/IP-encapsulated RS message with IPv6 2993 source address set to its LLA, with IPv6 destination set to All- 2994 Routers multicast and with an OMNI option with underlying interface 2995 parameters. If the Client believes that it is on the open Internet, 2996 it SHOULD also include an SLLAO set according to the address used for 2997 INET encapsulation (otherwise, it MAY omit the SLLAO). If the 2998 underlying address is IPv4, the Client includes a Teredo address 2999 [RFC4380] using the prefix fe80::/32 with the Server's IPv4 address, 3000 and with the IP address and Port Number used for INET encapsulation 3001 written in obfuscated form and with FMT set to '01' (INET) as 3002 discussed in Section 3.3. If the underlying interface address is 3003 IPv6, the Client instead includes the IPv6 address and Port number in 3004 obfuscated form and sets FMT to '11' (INET). The Client finally 3005 includes a Teredo Authentication option per [RFC4380] to provide 3006 message authentication, sets the UDP/IP source to its INET address 3007 and UDP port, sets the UDP/IP destination to the Server's INET 3008 address and the AERO service port number (8060), then sends the 3009 message to the Server. 3011 When the Server receives the RS, it authenticates the message and 3012 registers the Client's MNP and INET interface information according 3013 to the OMNI option parameters. If the RS message includes an SLLAO, 3014 the Server compares the encapsulation IP address and UDP port number 3015 with the (unobfuscated) SLLAO values. If the values are the same, 3016 the Server caches the Client's information as "INET" addresses 3017 meaning that the Client is likely to accept direct messages without 3018 requiring NAT traversal exchanges. If the values are different (or, 3019 if there was no SLLAO) the Server instead caches the Client's 3020 information as "NAT" addresses meaning that NAT traversal exchanges 3021 may be necessary. 3023 The Server then returns an RA message with IPv6 source and 3024 destination set corresponding to the addresses in the RS, and with a 3025 Teredo Authentication option. For IPv4, the Server also includes an 3026 IPv4 Teredo Origin option per [RFC4380] with the mapped and 3027 obfuscated IPv4 address and port number observed in the encapsulation 3028 headers. For IPv6, the Server instead includes an IPv6 Teredo Origin 3029 option per Figure 7 with the mapped and obfuscated observed IPv6 3030 address and port number (note that the value 0x02 in the second octet 3031 differentiates from other Teredo option types). 3033 +--------+--------+-----------------+ 3034 | 0x00 | 0x02 | Origin port # | 3035 +--------+--------+-----------------+ 3036 ~ Origin IPv6 address ~ 3037 +-----------------------------------+ 3039 Figure 7: IPv6 Teredo Origin Option 3041 When the Client receives the RA message, it compares the mapped IP 3042 address and port from the Teredo Origin option with its own address. 3043 If the addresses are the same, the Client assumes the open Internet / 3044 Cone NAT principle; if the addresses are different, the Client 3045 instead assumes that further Server qualification procedures are 3046 necessary to detect the type of NAT and proceeds according to 3047 standard Teredo procedures. 3049 After the Client has registered its INET interfaces in such RS/RA 3050 exchanges it sends periodic RS messages to receive fresh RA messages 3051 before the Router Lifetime received on each INET interface expires. 3052 The Client also maintains default routes via its Servers, i.e., the 3053 same as described in earlier sections. 3055 When the Client sends messages to target IP addresses, it also 3056 invokes route optimization per Section 3.14 using IPv6 ND address 3057 resolution messaging. The Client sends the NS(AR) message to the 3058 Server wrapped in a UDP/IP header with a Teredo Authentication option 3059 with the NS source address set to the Client's LLA and destination 3060 address set to the target solicited node multicast address. The 3061 Server authenticates the message and sends a corresponding NS(AR) 3062 message over the spanning tree the same as if it were the ROS, but 3063 with the SPAN source address set to the Server's ULA and destination 3064 set to the ULA of the target. When the ROR receives the NS(AR), it 3065 adds the Server's ULA and Client's LLA to the target's Report List, 3066 and returns an NA with OMNI and TLLAO information for the target. 3067 The Server then returns a UDP/IP encapsulated NA message with a 3068 Teredo Authentication option to the Client. 3070 Following route optimization, for targets in the same AERO link 3071 segment if the target's TLLAO addresss is on the open INET, the 3072 Client forwards data packets directly to the target INET address. If 3073 the target's TLLAO address is behind a NAT, the Client first 3074 establishes NAT state for the Link Layer Address using the "bubble" 3075 mechanisms specified in [RFC6081][RFC4380]. The Client continues to 3076 send data packets via its Server until NAT state is populated, then 3077 begins forwarding packets via the direct path through the NAT to the 3078 target. For targets in different AERO link segments, the Client 3079 inserts a Segment Routing header and forwards data packets to the 3080 Bridge that returned the NA message. 3082 The ROR may return uNAs via the Server if the target moves, and the 3083 Server will send corresponding Teredo Authentication-protected uNAs 3084 to the Client. The Client can also send "loopback" NS(NUD) messages 3085 to test forward path reachability even though there is no security 3086 association between the Client and the target. 3088 The Client sends Teredo UDP/IP encapsulated IPv6 packets no larger 3089 than 1280 bytes in one piece. In order to accommodate larger IPv6 3090 packets (up to the AERO interface MTU), the Client inserts a SPAN 3091 header with source set to its own ULA and destination set to the ULA 3092 of the target and uses IPv6 fragmentation according to Section 3.9. 3093 The Client then encapsulates each fragment in a UDP/IP header and 3094 sends the fragments to the next hop. 3096 3.22.1. Use of SEND and CGA 3098 In some environments, use of the Teredo Authentication option alone 3099 may be sufficient for assuring IPv6 ND message authentication between 3100 Clients and Servers. When additional protection is necessary, nodes 3101 should employ SEcure Neighbor Discovery (SEND) [RFC3971] with 3102 Cryptographically-Generated Addresses (CGA) [RFC3972]. 3104 When SEND/CGA are used, the Client prepares RS messages with its 3105 link-local CGA as the IPv6 source and All-Routers as the IPv6 3106 Destination, includes any SEND options and wraps the message in a 3107 SPAN header. The Client sets the SPAN source address to its own ULA 3108 and sets the SPAN destination address to the "All-Routers" ULA. The 3109 Client then wraps the RS message in UDP/IP headers according to the 3110 Teredo format and sends the message to the Server. 3112 When the Server receives the message, it first verifies the Teredo 3113 Authentication option (if present) then uses the SPAN source address 3114 to determine the MNP of the Client. The Server then processes the 3115 SEND options to authenticate the RS message and prepares an RA 3116 message response. The Server prepares the RA with its own link-local 3117 CGA and the CGA of the Client as the IPv6 source and destination, 3118 includes any SEND options and wraps the message in a SPAN header. 3119 The Server sets the SPAN source address to its own ULA and sets the 3120 SPAN destination address to the Client's ULA. The Server then wraps 3121 the RA message in UDP/IP headers according to the Teredo format and 3122 sends the message to the Client. Thereafter, the Client/Server send 3123 additional RS/RA messages to maintain their association and any NAT 3124 state. 3126 The Client and Server also may exchange NS/NA messages using their 3127 own CGA as the source and with SPAN encapsulation as above. When a 3128 Client sends an NS(AR), it sets the IPv6 source to its CGA and sets 3129 the IPv6 destination to the Solicited-Node Multicast address of the 3130 target. The Client then wraps the message in a SPAN header with its 3131 own ULA as the source and the ULA of the target as the destination 3132 and sends it to the Server. The Server authenticates the message, 3133 then changes the IPv6 source address to the Client's LLA, removes the 3134 SEND options, and sends a corresponding NS(AR) into the spanning 3135 tree. When the Server receives the corresponding SPAN-encapsulated 3136 NA, it changes the IPv6 destination address to the Client's CGA, 3137 inserts SEND options, then wraps the message in UDP/IP headers and 3138 sends it to the Client. 3140 When a Client sends a uNA, it sets the IPv6 source address to its own 3141 CGA and sets the IPv6 destination address to All-Nodes multicast, 3142 includes SEND options, wraps the message in SPAN and UDP/IP headers 3143 and sends the message to the Server. The Server authenticates the 3144 message, then changes the IPv6 address to the Client's LLA, removes 3145 the SEND options and forwards the message the same as discussed in 3146 Section 3.16.1. In the reverse direction, when the Server forwards a 3147 uNA to the Client, it changes the IPv6 address to its own CGA and 3148 inserts SEND options then forwards the message to the Client. 3150 When a Client sends an NS(NUD), it sets both the IPv6 source and 3151 destination address to its own LLA, wraps the message in a SPAN 3152 header and UDP/IP headers, then sends the message directly to the 3153 peer which will loop the message back. In this case alone, the 3154 Client does not use the Server as a trust broker for forwarding the 3155 ND message. 3157 3.23. Time-Varying MNPs 3159 In some use cases, it is desirable, beneficial and efficient for the 3160 Client to receive a constant MNP that travels with the Client 3161 wherever it moves. For example, this would allow air traffic 3162 controllers to easily track aircraft, etc. In other cases, however 3163 (e.g., intelligent transportation systems), the MN may be willing to 3164 sacrifice a modicum of efficiency in order to have time-varying MNPs 3165 that can be changed every so often to defeat adversarial tracking. 3167 The DHCPv6-PD service offers a way for Clients that desire time- 3168 varying MNPs to obtain short-lived prefixes (e.g., on the order of a 3169 small number of minutes). In that case, the identity of the Client 3170 would not be bound to the MNP but rather the Client's identity would 3171 be bound to the DHCPv6 Device Unique Identifier (DUID) and used as 3172 the seed for Prefix Delegation. The Client would then be obligated 3173 to renumber its internal networks whenever its MNP (and therefore 3174 also its LLA) changes. This should not present a challenge for 3175 Clients with automated network renumbering services, however presents 3176 limits for the durations of ongoing sessions that would prefer to use 3177 a constant address. 3179 4. Implementation Status 3181 An AERO implementation based on OpenVPN (https://openvpn.net/) was 3182 announced on the v6ops mailing list on January 10, 2018 and an 3183 initial public release of the AERO proof-of-concept source code was 3184 announced on the intarea mailing list on August 21, 2015. 3186 As of 4/1/2020, more recent updated implementations are under 3187 internal development and testing with plans to release in the near 3188 future. 3190 5. IANA Considerations 3192 The IANA has assigned a 4-octet Private Enterprise Number "45282" for 3193 AERO in the "enterprise-numbers" registry. 3195 The IANA has assigned the UDP port number "8060" for an earlier 3196 experimental version of AERO [RFC6706]. This document obsoletes 3197 [RFC6706] and claims the UDP port number "8060" for all future use. 3199 The IANA is instructed to assign a new type value TBD in the Segment 3200 Routing Header TLV registry [RFC8754]. 3202 No further IANA actions are required. 3204 6. Security Considerations 3206 AERO Bridges configure secured tunnels with AERO Servers, Realys and 3207 Proxys within their local AERO link segments. Applicable secured 3208 tunnel alternatives include IPsec [RFC4301], TLS/SSL [RFC8446], DTLS 3209 [RFC6347], WireGuard, etc. The AERO Bridges of all AERO link 3210 segments in turn configure secured tunnels for their neighboring AERO 3211 Bridges in a spanning tree topology. Therefore, control messages 3212 exchanged between any pair of AERO link neighbors on the spanning 3213 tree are already secured. 3215 AERO Servers, Relays and Proxys targeted by a route optimization may 3216 also receive data packets directly from arbitrary nodes in INET 3217 partitions instead of via the spanning tree. For INET partitions 3218 that apply effective ingress filtering to defeat source address 3219 spoofing, the simple data origin authentication procedures in 3220 Section 3.8 can be applied. 3222 For INET partitions that require strong security in the data plane, 3223 two options for securing communications include 1) disable route 3224 optimization so that all traffic is conveyed over secured tunnels, or 3225 2) enable on-demand secure tunnel creation between INET partition 3226 neighbors. Option 1) would result in longer routes than necessary 3227 and traffic concentration on critical infrastructure elements. 3228 Option 2) could be coordinated by establishing a secured tunnel on- 3229 demand instead of performing an NS/NA exchange in the route 3230 optimization procedures. Procedures for establishing on-demand 3231 secured tunnels are out of scope. 3233 AERO Clients that connect to secured ANETs need not apply security to 3234 their ND messages, since the messages will be intercepted by a 3235 perimeter Proxy that applies security on its INET-facing interface as 3236 part of the spanning tree (see above). AERO Clients connected to the 3237 open INET can use symmetric network and/or transport layer security 3238 services such as VPNs or can by some other means establish a direct 3239 link. When a VPN or direct link may be impractical, however, an 3240 asymmetric security service such as SEcure Neighbor Discovery (SEND) 3241 [RFC3971] with Cryptographically Generated Addresses (CGAs) [RFC3972] 3242 and/or the Teredo Authentication option [RFC4380] can be applied. 3244 Application endpoints SHOULD use application-layer security services 3245 such as TLS/SSL, DTLS or SSH [RFC4251] to assure the same level of 3246 protection as for critical secured Internet services. AERO Clients 3247 that require host-based VPN services SHOULD use symmetric network 3248 and/or transport layer security services such as IPsec, TLS/SSL, 3249 DTLS, etc. AERO Proxys and Servers can also provide a network-based 3250 VPN service on behalf of the Client, e.g., if the Client is located 3251 within a secured enclave and cannot establish a VPN on its own 3252 behalf. 3254 AERO Servers and Bridges present targets for traffic amplification 3255 Denial of Service (DoS) attacks. This concern is no different than 3256 for widely-deployed VPN security gateways in the Internet, where 3257 attackers could send spoofed packets to the gateways at high data 3258 rates. This can be mitigated by connecting Servers and Bridges over 3259 dedicated links with no connections to the Internet and/or when 3260 connections to the Internet are only permitted through well-managed 3261 firewalls. Traffic amplification DoS attacks can also target an AERO 3262 Client's low data rate links. This is a concern not only for Clients 3263 located on the open Internet but also for Clients in secured 3264 enclaves. AERO Servers and Proxys can institute rate limits that 3265 protect Clients from receiving packet floods that could DoS low data 3266 rate links. 3268 AERO Relays must implement ingress filtering to avoid a spoofing 3269 attack in which spurious messages with ULA addresses are injected 3270 into an AERO link from an outside attacker. AERO Clients MUST ensure 3271 that their connectivity is not used by unauthorized nodes on their 3272 EUNs to gain access to a protected network, i.e., AERO Clients that 3273 act as routers MUST NOT provide routing services for unauthorized 3274 nodes. (This concern is no different than for ordinary hosts that 3275 receive an IP address delegation but then "share" the address with 3276 other nodes via some form of Internet connection sharing such as 3277 tethering.) 3279 The MAP list MUST be well-managed and secured from unauthorized 3280 tampering, even though the list contains only public information. 3281 The MAP list can be conveyed to the Client in a similar fashion as in 3282 [RFC5214] (e.g., through layer 2 data link login messaging, secure 3283 upload of a static file, DNS lookups, etc.). 3285 Although public domain and commercial SEND implementations exist, 3286 concerns regarding the strength of the cryptographic hash algorithm 3287 have been documented [RFC6273] [RFC4982]. 3289 SRH authentication facilities are specified in [RFC8754]. 3291 Security considerations for accepting link-layer ICMP messages and 3292 reflected packets are discussed throughout the document. 3294 Security considerations for IPv6 fragmentation and reassembly are 3295 discussed in Section 3.9.1. 3297 7. Acknowledgements 3299 Discussions in the IETF, aviation standards communities and private 3300 exchanges helped shape some of the concepts in this work. 3301 Individuals who contributed insights include Mikael Abrahamsson, Mark 3302 Andrews, Fred Baker, Bob Braden, Stewart Bryant, Brian Carpenter, 3303 Wojciech Dec, Pavel Drasil, Ralph Droms, Adrian Farrel, Nick Green, 3304 Sri Gundavelli, Brian Haberman, Bernhard Haindl, Joel Halpern, Tom 3305 Herbert, Sascha Hlusiak, Lee Howard, Zdenek Jaron, Andre Kostur, 3306 Hubert Kuenig, Ted Lemon, Andy Malis, Satoru Matsushima, Tomek 3307 Mrugalski, Madhu Niraula, Alexandru Petrescu, Behcet Saikaya, Michal 3308 Skorepa, Joe Touch, Bernie Volz, Ryuji Wakikawa, Tony Whyman, Lloyd 3309 Wood and James Woodyatt. Members of the IESG also provided valuable 3310 input during their review process that greatly improved the document. 3311 Special thanks go to Stewart Bryant, Joel Halpern and Brian Haberman 3312 for their shepherding guidance during the publication of the AERO 3313 first edition. 3315 This work has further been encouraged and supported by Boeing 3316 colleagues including Kyle Bae, M. Wayne Benson, Dave Bernhardt, Cam 3317 Brodie, John Bush, Balaguruna Chidambaram, Irene Chin, Bruce Cornish, 3318 Claudiu Danilov, Don Dillenburg, Joe Dudkowski, Wen Fang, Samad 3319 Farooqui, Anthony Gregory, Jeff Holland, Seth Jahne, Brian Jaury, 3320 Greg Kimberly, Ed King, Madhuri Madhava Badgandi, Laurel Matthew, 3321 Gene MacLean III, Rob Muszkiewicz, Sean O'Sullivan, Vijay 3322 Rajagopalan, Greg Saccone, Rod Santiago, Kent Shuey, Brian Skeen, 3323 Mike Slane, Carrie Spiker, Katie Tran, Brendan Williams, Amelia 3324 Wilson, Julie Wulff, Yueli Yang, Eric Yeh and other members of the 3325 Boeing mobility, networking and autonomy teams. Kyle Bae, Wayne 3326 Benson, Katie Tran and Eric Yeh are especially acknowledged for 3327 implementing the AERO functions as extensions to the public domain 3328 OpenVPN distribution. 3330 Earlier works on NBMA tunneling approaches are found in 3331 [RFC2529][RFC5214][RFC5569]. 3333 Many of the constructs presented in this second edition of AERO are 3334 based on the author's earlier works, including: 3336 o The Internet Routing Overlay Network (IRON) 3337 [RFC6179][I-D.templin-ironbis] 3339 o Virtual Enterprise Traversal (VET) 3340 [RFC5558][I-D.templin-intarea-vet] 3342 o The Subnetwork Encapsulation and Adaptation Layer (SEAL) 3343 [RFC5320][I-D.templin-intarea-seal] 3345 o AERO, First Edition [RFC6706] 3347 Note that these works cite numerous earlier efforts that are not also 3348 cited here due to space limitations. The authors of those earlier 3349 works are acknowledged for their insights. 3351 This work is aligned with the NASA Safe Autonomous Systems Operation 3352 (SASO) program under NASA contract number NNA16BD84C. 3354 This work is aligned with the FAA as per the SE2025 contract number 3355 DTFAWA-15-D-00030. 3357 This work is aligned with the Boeing Commercial Airplanes (BCA) 3358 Internet of Things (IoT) and autonomy programs. 3360 This work is aligned with the Boeing Information Technology (BIT) 3361 MobileNet program. 3363 8. References 3365 8.1. Normative References 3367 [I-D.templin-6man-omni-interface] 3368 Templin, F. and T. Whyman, "Transmission of IPv6 Packets 3369 over Overlay Multilink Network (OMNI) Interfaces", draft- 3370 templin-6man-omni-interface-22 (work in progress), May 3371 2020. 3373 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 3374 DOI 10.17487/RFC0791, September 1981, 3375 . 3377 [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, 3378 RFC 792, DOI 10.17487/RFC0792, September 1981, 3379 . 3381 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 3382 Requirement Levels", BCP 14, RFC 2119, 3383 DOI 10.17487/RFC2119, March 1997, 3384 . 3386 [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in 3387 IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473, 3388 December 1998, . 3390 [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, 3391 "Definition of the Differentiated Services Field (DS 3392 Field) in the IPv4 and IPv6 Headers", RFC 2474, 3393 DOI 10.17487/RFC2474, December 1998, 3394 . 3396 [RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander, 3397 "SEcure Neighbor Discovery (SEND)", RFC 3971, 3398 DOI 10.17487/RFC3971, March 2005, 3399 . 3401 [RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)", 3402 RFC 3972, DOI 10.17487/RFC3972, March 2005, 3403 . 3405 [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and 3406 More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191, 3407 November 2005, . 3409 [RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast 3410 Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005, 3411 . 3413 [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through 3414 Network Address Translations (NATs)", RFC 4380, 3415 DOI 10.17487/RFC4380, February 2006, 3416 . 3418 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 3419 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 3420 DOI 10.17487/RFC4861, September 2007, 3421 . 3423 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 3424 Address Autoconfiguration", RFC 4862, 3425 DOI 10.17487/RFC4862, September 2007, 3426 . 3428 [RFC5175] Haberman, B., Ed. and R. Hinden, "IPv6 Router 3429 Advertisement Flags Option", RFC 5175, 3430 DOI 10.17487/RFC5175, March 2008, 3431 . 3433 [RFC6081] Thaler, D., "Teredo Extensions", RFC 6081, 3434 DOI 10.17487/RFC6081, January 2011, 3435 . 3437 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 3438 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 3439 May 2017, . 3441 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 3442 (IPv6) Specification", STD 86, RFC 8200, 3443 DOI 10.17487/RFC8200, July 2017, 3444 . 3446 [RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., 3447 Richardson, M., Jiang, S., Lemon, T., and T. Winters, 3448 "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", 3449 RFC 8415, DOI 10.17487/RFC8415, November 2018, 3450 . 3452 8.2. Informative References 3454 [BGP] Huston, G., "BGP in 2015, http://potaroo.net", January 3455 2016. 3457 [I-D.bonica-6man-comp-rtg-hdr] 3458 Bonica, R., Kamite, Y., Niwa, T., Alston, A., and L. 3459 Jalil, "The IPv6 Compact Routing Header (CRH)", draft- 3460 bonica-6man-comp-rtg-hdr-22 (work in progress), May 2020. 3462 [I-D.bonica-6man-crh-helper-opt] 3463 Li, X., Bao, C., Ruan, E., and R. Bonica, "Compressed 3464 Routing Header (CRH) Helper Option", draft-bonica-6man- 3465 crh-helper-opt-01 (work in progress), May 2020. 3467 [I-D.ietf-dmm-distributed-mobility-anchoring] 3468 Chan, A., Wei, X., Lee, J., Jeon, S., and C. Bernardos, 3469 "Distributed Mobility Anchoring", draft-ietf-dmm- 3470 distributed-mobility-anchoring-15 (work in progress), 3471 March 2020. 3473 [I-D.ietf-intarea-frag-fragile] 3474 Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O., 3475 and F. Gont, "IP Fragmentation Considered Fragile", draft- 3476 ietf-intarea-frag-fragile-17 (work in progress), September 3477 2019. 3479 [I-D.ietf-intarea-gue] 3480 Herbert, T., Yong, L., and O. Zia, "Generic UDP 3481 Encapsulation", draft-ietf-intarea-gue-09 (work in 3482 progress), October 2019. 3484 [I-D.ietf-intarea-gue-extensions] 3485 Herbert, T., Yong, L., and F. Templin, "Extensions for 3486 Generic UDP Encapsulation", draft-ietf-intarea-gue- 3487 extensions-06 (work in progress), March 2019. 3489 [I-D.ietf-intarea-tunnels] 3490 Touch, J. and M. Townsley, "IP Tunnels in the Internet 3491 Architecture", draft-ietf-intarea-tunnels-10 (work in 3492 progress), September 2019. 3494 [I-D.ietf-rtgwg-atn-bgp] 3495 Templin, F., Saccone, G., Dawra, G., Lindem, A., and V. 3496 Moreno, "A Simple BGP-based Mobile Routing System for the 3497 Aeronautical Telecommunications Network", draft-ietf- 3498 rtgwg-atn-bgp-05 (work in progress), January 2020. 3500 [I-D.templin-6man-dhcpv6-ndopt] 3501 Templin, F., "A Unified Stateful/Stateless Configuration 3502 Service for IPv6", draft-templin-6man-dhcpv6-ndopt-09 3503 (work in progress), January 2020. 3505 [I-D.templin-intarea-grefrag] 3506 Templin, F., "GRE Tunnel Level Fragmentation", draft- 3507 templin-intarea-grefrag-04 (work in progress), July 2016. 3509 [I-D.templin-intarea-seal] 3510 Templin, F., "The Subnetwork Encapsulation and Adaptation 3511 Layer (SEAL)", draft-templin-intarea-seal-68 (work in 3512 progress), January 2014. 3514 [I-D.templin-intarea-vet] 3515 Templin, F., "Virtual Enterprise Traversal (VET)", draft- 3516 templin-intarea-vet-40 (work in progress), May 2013. 3518 [I-D.templin-ironbis] 3519 Templin, F., "The Interior Routing Overlay Network 3520 (IRON)", draft-templin-ironbis-16 (work in progress), 3521 March 2014. 3523 [I-D.templin-v6ops-pdhost] 3524 Templin, F., "IPv6 Prefix Delegation and Multi-Addressing 3525 Models", draft-templin-v6ops-pdhost-25 (work in progress), 3526 January 2020. 3528 [OVPN] OpenVPN, O., "http://openvpn.net", October 2016. 3530 [RFC1035] Mockapetris, P., "Domain names - implementation and 3531 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 3532 November 1987, . 3534 [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - 3535 Communication Layers", STD 3, RFC 1122, 3536 DOI 10.17487/RFC1122, October 1989, 3537 . 3539 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 3540 DOI 10.17487/RFC1191, November 1990, 3541 . 3543 [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers", 3544 RFC 1812, DOI 10.17487/RFC1812, June 1995, 3545 . 3547 [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, 3548 DOI 10.17487/RFC2003, October 1996, 3549 . 3551 [RFC2236] Fenner, W., "Internet Group Management Protocol, Version 3552 2", RFC 2236, DOI 10.17487/RFC2236, November 1997, 3553 . 3555 [RFC2492] Armitage, G., Schulter, P., and M. Jork, "IPv6 over ATM 3556 Networks", RFC 2492, DOI 10.17487/RFC2492, January 1999, 3557 . 3559 [RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4 3560 Domains without Explicit Tunnels", RFC 2529, 3561 DOI 10.17487/RFC2529, March 1999, 3562 . 3564 [RFC2764] Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and A. 3565 Malis, "A Framework for IP Based Virtual Private 3566 Networks", RFC 2764, DOI 10.17487/RFC2764, February 2000, 3567 . 3569 [RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. 3570 Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, 3571 DOI 10.17487/RFC2784, March 2000, 3572 . 3574 [RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE", 3575 RFC 2890, DOI 10.17487/RFC2890, September 2000, 3576 . 3578 [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", 3579 RFC 2923, DOI 10.17487/RFC2923, September 2000, 3580 . 3582 [RFC2983] Black, D., "Differentiated Services and Tunnels", 3583 RFC 2983, DOI 10.17487/RFC2983, October 2000, 3584 . 3586 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition 3587 of Explicit Congestion Notification (ECN) to IP", 3588 RFC 3168, DOI 10.17487/RFC3168, September 2001, 3589 . 3591 [RFC3330] IANA, "Special-Use IPv4 Addresses", RFC 3330, 3592 DOI 10.17487/RFC3330, September 2002, 3593 . 3595 [RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener 3596 Discovery Version 2 (MLDv2) for IPv6", RFC 3810, 3597 DOI 10.17487/RFC3810, June 2004, 3598 . 3600 [RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D., 3601 Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. 3602 Wood, "Advice for Internet Subnetwork Designers", BCP 89, 3603 RFC 3819, DOI 10.17487/RFC3819, July 2004, 3604 . 3606 [RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms 3607 for IPv6 Hosts and Routers", RFC 4213, 3608 DOI 10.17487/RFC4213, October 2005, 3609 . 3611 [RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) 3612 Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251, 3613 January 2006, . 3615 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 3616 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 3617 DOI 10.17487/RFC4271, January 2006, 3618 . 3620 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 3621 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 3622 2006, . 3624 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 3625 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 3626 December 2005, . 3628 [RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery 3629 Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April 3630 2006, . 3632 [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet 3633 Control Message Protocol (ICMPv6) for the Internet 3634 Protocol Version 6 (IPv6) Specification", STD 89, 3635 RFC 4443, DOI 10.17487/RFC4443, March 2006, 3636 . 3638 [RFC4511] Sermersheim, J., Ed., "Lightweight Directory Access 3639 Protocol (LDAP): The Protocol", RFC 4511, 3640 DOI 10.17487/RFC4511, June 2006, 3641 . 3643 [RFC4541] Christensen, M., Kimball, K., and F. Solensky, 3644 "Considerations for Internet Group Management Protocol 3645 (IGMP) and Multicast Listener Discovery (MLD) Snooping 3646 Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006, 3647 . 3649 [RFC4605] Fenner, B., He, H., Haberman, B., and H. Sandick, 3650 "Internet Group Management Protocol (IGMP) / Multicast 3651 Listener Discovery (MLD)-Based Multicast Forwarding 3652 ("IGMP/MLD Proxying")", RFC 4605, DOI 10.17487/RFC4605, 3653 August 2006, . 3655 [RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for 3656 IP", RFC 4607, DOI 10.17487/RFC4607, August 2006, 3657 . 3659 [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly 3660 Errors at High Data Rates", RFC 4963, 3661 DOI 10.17487/RFC4963, July 2007, 3662 . 3664 [RFC4982] Bagnulo, M. and J. Arkko, "Support for Multiple Hash 3665 Algorithms in Cryptographically Generated Addresses 3666 (CGAs)", RFC 4982, DOI 10.17487/RFC4982, July 2007, 3667 . 3669 [RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano, 3670 "Bidirectional Protocol Independent Multicast (BIDIR- 3671 PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007, 3672 . 3674 [RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site 3675 Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, 3676 DOI 10.17487/RFC5214, March 2008, 3677 . 3679 [RFC5320] Templin, F., Ed., "The Subnetwork Encapsulation and 3680 Adaptation Layer (SEAL)", RFC 5320, DOI 10.17487/RFC5320, 3681 February 2010, . 3683 [RFC5522] Eddy, W., Ivancic, W., and T. Davis, "Network Mobility 3684 Route Optimization Requirements for Operational Use in 3685 Aeronautics and Space Exploration Mobile Networks", 3686 RFC 5522, DOI 10.17487/RFC5522, October 2009, 3687 . 3689 [RFC5558] Templin, F., Ed., "Virtual Enterprise Traversal (VET)", 3690 RFC 5558, DOI 10.17487/RFC5558, February 2010, 3691 . 3693 [RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4 3694 Infrastructures (6rd)", RFC 5569, DOI 10.17487/RFC5569, 3695 January 2010, . 3697 [RFC5871] Arkko, J. and S. Bradner, "IANA Allocation Guidelines for 3698 the IPv6 Routing Header", RFC 5871, DOI 10.17487/RFC5871, 3699 May 2010, . 3701 [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 3702 (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, 3703 . 3705 [RFC5991] Thaler, D., Krishnan, S., and J. Hoagland, "Teredo 3706 Security Updates", RFC 5991, DOI 10.17487/RFC5991, 3707 September 2010, . 3709 [RFC6106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, 3710 "IPv6 Router Advertisement Options for DNS Configuration", 3711 RFC 6106, DOI 10.17487/RFC6106, November 2010, 3712 . 3714 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful 3715 NAT64: Network Address and Protocol Translation from IPv6 3716 Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146, 3717 April 2011, . 3719 [RFC6179] Templin, F., Ed., "The Internet Routing Overlay Network 3720 (IRON)", RFC 6179, DOI 10.17487/RFC6179, March 2011, 3721 . 3723 [RFC6221] Miles, D., Ed., Ooghe, S., Dec, W., Krishnan, S., and A. 3724 Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221, 3725 DOI 10.17487/RFC6221, May 2011, 3726 . 3728 [RFC6273] Kukec, A., Krishnan, S., and S. Jiang, "The Secure 3729 Neighbor Discovery (SEND) Hash Threat Analysis", RFC 6273, 3730 DOI 10.17487/RFC6273, June 2011, 3731 . 3733 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 3734 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 3735 January 2012, . 3737 [RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label 3738 for Equal Cost Multipath Routing and Link Aggregation in 3739 Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011, 3740 . 3742 [RFC6706] Templin, F., Ed., "Asymmetric Extended Route Optimization 3743 (AERO)", RFC 6706, DOI 10.17487/RFC6706, August 2012, 3744 . 3746 [RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field", 3747 RFC 6864, DOI 10.17487/RFC6864, February 2013, 3748 . 3750 [RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and 3751 UDP Checksums for Tunneled Packets", RFC 6935, 3752 DOI 10.17487/RFC6935, April 2013, 3753 . 3755 [RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement 3756 for the Use of IPv6 UDP Datagrams with Zero Checksums", 3757 RFC 6936, DOI 10.17487/RFC6936, April 2013, 3758 . 3760 [RFC7269] Chen, G., Cao, Z., Xie, C., and D. Binet, "NAT64 3761 Deployment Options and Experience", RFC 7269, 3762 DOI 10.17487/RFC7269, June 2014, 3763 . 3765 [RFC7333] Chan, H., Ed., Liu, D., Seite, P., Yokota, H., and J. 3766 Korhonen, "Requirements for Distributed Mobility 3767 Management", RFC 7333, DOI 10.17487/RFC7333, August 2014, 3768 . 3770 [RFC7421] Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S., 3771 Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit 3772 Boundary in IPv6 Addressing", RFC 7421, 3773 DOI 10.17487/RFC7421, January 2015, 3774 . 3776 [RFC7739] Gont, F., "Security Implications of Predictable Fragment 3777 Identification Values", RFC 7739, DOI 10.17487/RFC7739, 3778 February 2016, . 3780 [RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I., 3781 Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent 3782 Multicast - Sparse Mode (PIM-SM): Protocol Specification 3783 (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March 3784 2016, . 3786 [RFC8086] Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE- 3787 in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086, 3788 March 2017, . 3790 [RFC8138] Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie, 3791 "IPv6 over Low-Power Wireless Personal Area Network 3792 (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138, 3793 April 2017, . 3795 [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., 3796 "Path MTU Discovery for IP version 6", STD 87, RFC 8201, 3797 DOI 10.17487/RFC8201, July 2017, 3798 . 3800 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 3801 Decraene, B., Litkowski, S., and R. Shakir, "Segment 3802 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 3803 July 2018, . 3805 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 3806 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 3807 . 3809 [RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J., 3810 Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header 3811 (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020, 3812 . 3814 Appendix A. AERO Alternate Encapsulations 3816 When GUE encapsulation is not needed, AERO can use common 3817 encapsulations such as IP-in-IP [RFC2003][RFC2473][RFC4213], Generic 3818 Routing Encapsulation (GRE) [RFC2784][RFC2890] and others. The 3819 encapsulation is therefore only differentiated from non-AERO tunnels 3820 through the application of AERO control messaging and not through, 3821 e.g., a well-known UDP port number. 3823 As for GUE encapsulation, alternate AERO encapsulation formats may 3824 require encapsulation layer fragmentation. For simple IP-in-IP 3825 encapsulation, an IPv6 fragment header is inserted directly between 3826 the inner and outer IP headers when needed, i.e., even if the outer 3827 header is IPv4. The IPv6 Fragment Header is identified to the outer 3828 IP layer by its IP protocol number, and the Next Header field in the 3829 IPv6 Fragment Header identifies the inner IP header version. For GRE 3830 encapsulation, a GRE fragment header is inserted within the GRE 3831 header [I-D.templin-intarea-grefrag]. 3833 Figure 8 shows the AERO IP-in-IP encapsulation format before any 3834 fragmentation is applied: 3836 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3837 | Outer IPv4 Header | | Outer IPv6 Header | 3838 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3839 |IPv6 Frag Header (optional)| |IPv6 Frag Header (optional)| 3840 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3841 | Inner IP Header | | Inner IP Header | 3842 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3843 | | | | 3844 ~ ~ ~ ~ 3845 ~ Inner Packet Body ~ ~ Inner Packet Body ~ 3846 ~ ~ ~ ~ 3847 | | | | 3848 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3850 Minimal Encapsulation in IPv4 Minimal Encapsulation in IPv6 3852 Figure 8: Minimal Encapsulation Format using IP-in-IP 3854 Figure 9 shows the AERO GRE encapsulation format before any 3855 fragmentation is applied: 3857 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3858 | Outer IP Header | 3859 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3860 | GRE Header | 3861 | (with checksum, key, etc..) | 3862 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3863 | GRE Fragment Header (optional)| 3864 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3865 | Inner IP Header | 3866 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3867 | | 3868 ~ ~ 3869 ~ Inner Packet Body ~ 3870 ~ ~ 3871 | | 3872 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3874 Figure 9: Minimal Encapsulation Using GRE 3876 Alternate encapsulation may be preferred in environments where GUE 3877 encapsulation would add unnecessary overhead. For example, certain 3878 low-bandwidth wireless data links may benefit from a reduced 3879 encapsulation overhead. 3881 GUE encapsulation can traverse network paths that are inaccessible to 3882 non-UDP encapsulations, e.g., for crossing Network Address 3883 Translators (NATs). More and more, network middleboxes are also 3884 being configured to discard packets that include anything other than 3885 a well-known IP protocol such as UDP and TCP. It may therefore be 3886 necessary to determine the potential for middlebox filtering before 3887 enabling alternate encapsulation in a given environment. 3889 In addition to IP-in-IP, GRE and GUE, AERO can also use security 3890 encapsulations such as IPsec, TLS/SSL, DTLS, etc. In that case, AERO 3891 control messaging and route determination occur before security 3892 encapsulation is applied for outgoing packets and after security 3893 decapsulation is applied for incoming packets. 3895 AERO is especially well suited for use with VPN system encapsulations 3896 such as OpenVPN [OVPN]. 3898 Appendix B. Non-Normative Considerations 3900 AERO can be applied to a multitude of Internetworking scenarios, with 3901 each having its own adaptations. The following considerations are 3902 provided as non-normative guidance: 3904 B.1. Implementation Strategies for Route Optimization 3906 Route optimization as discussed in Section 3.14 results in the route 3907 optimization source (ROS) creating an asymmetric neighbor cache entry 3908 for the target neighbor. The neighbor cache entry is maintained for 3909 at most ReachableTime seconds and then deleted unless updated. In 3910 order to refresh the neighbor cache entry lifetime before the 3911 ReachableTime timer expires, the specification requires 3912 implementations to issue a new NS/NA exchange to reset ReachableTime 3913 while data packets are still flowing. However, the decision of when 3914 to initiate a new NS/NA exchange and to perpetuate the process is 3915 left as an implementation detail. 3917 One possible strategy may be to monitor the neighbor cache entry 3918 watching for data packets for (ReachableTime - 5) seconds. If any 3919 data packets have been sent to the neighbor within this timeframe, 3920 then send an NS to receive a new NA. If no data packets have been 3921 sent, wait for 5 additional seconds and send an immediate NS if any 3922 data packets are sent within this "expiration pending" 5 second 3923 window. If no additional data packets are sent within the 5 second 3924 window, delete the neighbor cache entry. 3926 The monitoring of the neighbor data packet traffic therefore becomes 3927 an asymmetric ongoing process during the neighbor cache entry 3928 lifetime. If the neighbor cache entry expires, future data packets 3929 will trigger a new NS/NA exchange while the packets themselves are 3930 delivered over a longer path until route optimization state is re- 3931 established. 3933 B.2. Implicit Mobility Management 3935 AERO interface neighbors MAY provide a configuration option that 3936 allows them to perform implicit mobility management in which no ND 3937 messaging is used. In that case, the Client only transmits packets 3938 over a single interface at a time, and the neighbor always observes 3939 packets arriving from the Client from the same link-layer source 3940 address. 3942 If the Client's underlying interface address changes (either due to a 3943 readdressing of the original interface or switching to a new 3944 interface) the neighbor immediately updates the neighbor cache entry 3945 for the Client and begins accepting and sending packets according to 3946 the Client's new address. This implicit mobility method applies to 3947 use cases such as cellphones with both WiFi and Cellular interfaces 3948 where only one of the interfaces is active at a given time, and the 3949 Client automatically switches over to the backup interface if the 3950 primary interface fails. 3952 B.3. Direct Underlying Interfaces 3954 When a Client's AERO interface is configured over a Direct interface, 3955 the neighbor at the other end of the Direct link can receive packets 3956 without any encapsulation. In that case, the Client sends packets 3957 over the Direct link according to QoS preferences. If the Direct 3958 interface has the highest QoS preference, then the Client's IP 3959 packets are transmitted directly to the peer without going through an 3960 ANET/INET. If other interfaces have higher QoS preferences, then the 3961 Client's IP packets are transmitted via a different interface, which 3962 may result in the inclusion of Proxys, Servers and Bridges in the 3963 communications path. Direct interfaces must be tested periodically 3964 for reachability, e.g., via NUD. 3966 B.4. Operation on AERO Links with /64 ASPs 3968 IPv6 AERO links typically have MSPs that aggregate many candidate 3969 MNPs of length /64 or shorter. However, in some cases it may be 3970 desirable to use AERO over links that have only a /64 MSP. This can 3971 be accommodated by treating all Clients on the AERO link as simple 3972 hosts that receive /128 prefix delegations. 3974 In that case, the Client sends an RS message to the Server the same 3975 as for ordinary AERO links. The Server responds with an RA message 3976 that includes one or more /128 prefixes (i.e., singleton addresses) 3977 that include the /64 MSP prefix along with an interface identifier 3978 portion to be assigned to the Client. The Client and Server then 3979 configure their LLAs based on the interface identifier portions of 3980 the /128s (i.e., the lower 64 bits) and not based on the /64 prefix 3981 (i.e., the upper 64 bits). 3983 For example, if the MSP for the host-only IPv6 AERO link is 3984 2001:db8:1000:2000::/64, each Client will receive one or more /128 3985 IPv6 prefix delegations such as 2001:db8:1000:2000::1/128, 3986 2001:db8:1000:2000::2/128, etc. When the Client receives the prefix 3987 delegations, it assigns the LLAs fe80::1, fe80::2, etc. to the AERO 3988 interface, and assigns the global IPv6 addresses (i.e., the /128s) to 3989 either the AERO interface or an internal virtual interface such as a 3990 loopback. In this arrangement, the Client conducts route 3991 optimization in the same sense as discussed in Section 3.14. 3993 This specification has applicability for nodes that act as a Client 3994 on an "upstream" AERO link, but also act as a Server on "downstream" 3995 AERO links. More specifically, if the node acts as a Client to 3996 receive a /64 prefix from the upstream AERO link it can then act as a 3997 Server to provision /128s to Clients on downstream AERO links. 3999 B.5. AERO Critical Infrastructure Considerations 4001 AERO Bridges can be either Commercial off-the Shelf (COTS) standard 4002 IP routers or virtual machines in the cloud. Bridges must be 4003 provisioned, supported and managed by the INET administrative 4004 authority, and connected to the Bridges of other INETs via inter- 4005 domain peerings. Cost for purchasing, configuring and managing 4006 Bridges is nominal even for very large AERO links. 4008 AERO Servers can be standard dedicated server platforms, but most 4009 often will be deployed as virtual machines in the cloud. The only 4010 requirements for Servers are that they can run the AERO user-level 4011 code and have at least one network interface connection to the INET. 4012 As with Bridges, Servers must be provisioned, supported and managed 4013 by the INET administrative authority. Cost for purchasing, 4014 configuring and managing Servers is nominal especially for virtual 4015 Servers hosted in the cloud. 4017 AERO Proxys are most often standard dedicated server platforms with 4018 one network interface connected to the ANET and a second interface 4019 connected to an INET. As with Servers, the only requirements are 4020 that they can run the AERO user-level code and have at least one 4021 interface connection to the INET. Proxys must be provisioned, 4022 supported and managed by the ANET administrative authority. Cost for 4023 purchasing, configuring and managing Proxys is nominal, and borne by 4024 the ANET administrative authority. 4026 AERO Relays can be any dedicated server or COTS router platform 4027 connected to INETs and/or EUNs. The Relay connects to the AERO link 4028 and engages in eBGP peering with one or more Bridges as a stub AS. 4029 The Relay then injects its MNPs and/or non-MNP prefixes into the BGP 4030 routing system, and provisions the prefixes to its downstream- 4031 attached networks. The Relay can perform ROS/ROR services the same 4032 as for any Server, and can route between the MNP and non-MNP address 4033 spaces. 4035 B.6. AERO Server Failure Implications 4037 AERO Servers may appear as a single point of failure in the 4038 architecture, but such is not the case since all Servers on the link 4039 provide identical services and loss of a Server does not imply 4040 immediate and/or comprehensive communication failures. Although 4041 Clients typically associate with a single Server at a time, Server 4042 failure is quickly detected and conveyed by Bidirectional Forward 4043 Detection (BFD) and/or proactive NUD allowing Clients to migrate to 4044 new Servers. 4046 If a Server fails, ongoing packet forwarding to Clients will continue 4047 by virtue of the asymmetric neighbor cache entries that have already 4048 been established in route optimization sources (ROSs). If a Client 4049 also experiences mobility events at roughly the same time the Server 4050 fails, unsolicited NA messages may be lost but proxy neighbor cache 4051 entries in the DEPARTED state will ensure that packet forwarding to 4052 the Client's new locations will continue for up to DepartTime 4053 seconds. 4055 If a Client is left without a Server for an extended timeframe (e.g., 4056 greater than ReachableTime seconds) then existing asymmetric neighbor 4057 cache entries will eventually expire and both ongoing and new 4058 communications will fail. The original source will continue to 4059 retransmit until the Client has established a new Server 4060 relationship, after which time continuous communications will resume. 4062 Therefore, providing many Servers on the link with high availability 4063 profiles provides resilience against loss of individual Servers and 4064 assurance that Clients can establish new Server relationships quickly 4065 in event of a Server failure. 4067 B.7. AERO Client / Server Architecture 4069 The AERO architectural model is client / server in the control plane, 4070 with route optimization in the data plane. The same as for common 4071 Internet services, the AERO Client discovers the addresses of AERO 4072 Servers and selects one Server to connect to. The AERO service is 4073 analogous to common Internet services such as google.com, yahoo.com, 4074 cnn.com, etc. However, there is only one AERO service for the link 4075 and all Servers provide identical services. 4077 Common Internet services provide differing strategies for advertising 4078 server addresses to clients. The strategy is conveyed through the 4079 DNS resource records returned in response to name resolution queries. 4080 As of January 2020 Internet-based 'nslookup' services were used to 4081 determine the following: 4083 o When a client resolves the domainname "google.com", the DNS always 4084 returns one A record (i.e., an IPv4 address) and one AAAA record 4085 (i.e., an IPv6 address). The client receives the same addresses 4086 each time it resolves the domainname via the same DNS resolver, 4087 but may receive different addresses when it resolves the 4088 domainname via different DNS resolvers. But, in each case, 4089 exactly one A and one AAAA record are returned. 4091 o When a client resolves the domainname "ietf.org", the DNS always 4092 returns one A record and one AAAA record with the same addresses 4093 regardless of which DNS resolver is used. 4095 o When a client resolves the domainname "yahoo.com", the DNS always 4096 returns a list of 4 A records and 4 AAAA records. Each time the 4097 client resolves the domainname via the same DNS resolver, the same 4098 list of addresses are returned but in randomized order (i.e., 4099 consistent with a DNS round-robin strategy). But, interestingly, 4100 the same addresses are returned (albeit in randomized order) when 4101 the domainname is resolved via different DNS resolvers. 4103 o When a client resolves the domainname "amazon.com", the DNS always 4104 returns a list of 3 A records and no AAAA records. As with 4105 "yahoo.com", the same three A records are returned from any 4106 worldwide Internet connection point in randomized order. 4108 The above example strategies show differing approaches to Internet 4109 resilience and service distribution offered by major Internet 4110 services. The Google approach exposes only a single IPv4 and a 4111 single IPv6 address to clients. Clients can then select whichever IP 4112 protocol version offers the best response, but will always use the 4113 same IP address according to the current Internet connection point. 4114 This means that the IP address offered by the network must lead to a 4115 highly-available server and/or service distribution point. In other 4116 words, resilience is predicated on high availability within the 4117 network and with no client-initiated failovers expected (i.e., it is 4118 all-or-nothing from the client's perspective). However, Google does 4119 provide for worldwide distributed service distribution by virtue of 4120 the fact that each Internet connection point responds with a 4121 different IPv6 and IPv4 address. The IETF approach is like google 4122 (all-or-nothing from the client's perspective), but provides only a 4123 single IPv4 or IPv6 address on a worldwide basis. This means that 4124 the addresses must be made highly-available at the network level with 4125 no client failover possibility, and if there is any worldwide service 4126 distribution it would need to be conducted by a network element that 4127 is reached via the IP address acting as a service distribution point. 4129 In contrast to the Google and IETF philosophies, Yahoo and Amazon 4130 both provide clients with a (short) list of IP addresses with Yahoo 4131 providing both IP protocol versions and Amazon as IPv4-only. The 4132 order of the list is randomized with each name service query 4133 response, with the effect of round-robin load balancing for service 4134 distribution. With a short list of addresses, there is still 4135 expectation that the network will implement high availability for 4136 each address but in case any single address fails the client can 4137 switch over to using a different address. The balance then becomes 4138 one of function in the network vs function in the end system. 4140 The same implications observed for common highly-available services 4141 in the Internet apply also to the AERO client/server architecture. 4142 When an AERO Client connects to one or more ANETs, it discovers one 4143 or more AERO Server addresses through the mechanisms discussed in 4144 earlier sections. Each Server address presumably leads to a fault- 4145 tolerant clustering arrangement such as supported by Linux-HA, 4146 Extended Virtual Synchrony or Paxos. Such an arrangement has 4147 precedence in common Internet service deployments in lightweight 4148 virtual machines without requiring expensive hardware deployment. 4149 Similarly, common Internet service deployments set service IP 4150 addresses on service distribution points that may relay requests to 4151 many different servers. 4153 For AERO, the expectation is that a combination of the Google/IETF 4154 and Yahoo/Amazon philosophies would be employed. The AERO Client 4155 connects to different ANET access points and can receive 1-2 Server 4156 LLAs at each point. It then selects one AERO Server address, and 4157 engages in RS/RA exchanges with the same Server from all ANET 4158 connections. The Client remains with this Server unless or until the 4159 Server fails, in which case it can switch over to an alternate 4160 Server. The Client can likewise switch over to a different Server at 4161 any time if there is some reason for it to do so. So, the AERO 4162 expectation is for a balance of function in the network and end 4163 system, with fault tolerance and resilience at both levels. 4165 Appendix C. Change Log 4167 << RFC Editor - remove prior to publication >> 4169 Changes from draft-templin-intarea-6706bis-48 to draft-templin- 4170 intrea-6706bis-49: 4172 o SPAN Anycast address and SBM/PBM concepts introduced. 4174 Changes from draft-templin-intarea-6706bis-47 to draft-templin- 4175 intrea-6706bis-48: 4177 o SEND/CGA. 4179 Changes from draft-templin-intarea-6706bis-46 to draft-templin- 4180 intrea-6706bis-47: 4182 o Major changes to align with Teredo, including changed AERO "Relay" 4183 to "Bridge", and changed AERO "Gateway" to "Relay". The term 4184 "[Rr]elay" now refers to exactly the same thing in both AERO and 4185 Teredo. 4187 o Changed to use Teredo message authentication instead of SEND. 4189 Changes from draft-templin-intarea-6706bis-42 to draft-templin- 4190 intrea-6706bis-43: 4192 o Segment Routing. 4194 Changes from draft-templin-intarea-6706bis-39 to draft-templin- 4195 intrea-6706bis-40: 4197 o Teredo. 4199 Changes from draft-templin-intarea-6706bis-38 to draft-templin- 4200 intrea-6706bis-39: 4202 o Major clarifications and simplifications of SPAN fragmentation/ 4203 reassembly. 4205 o Revised AERO address format to support prefix lengths up to 112. 4207 o New method for forming SPAN Client Prefixes and population in the 4208 routing system. 4210 o Updates RFC4443 to set a new value in the ICMP PTB Code field. 4212 Changes from draft-templin-intarea-6706bis-35 to draft-templin- 4213 intrea-6706bis-36: 4215 o Clients in the open Internet secured using SEND/CGA. 4217 Changes from draft-templin-intarea-6706bis-32 to draft-templin- 4218 intrea-6706bis-33: 4220 o Updated Proxy discussion with "point-to-multipoint" server 4221 coordination 4223 o Significant updates to Address Resolution and NUD to include 4224 correct addresses in messages 4226 o Differentiate between NS(AR) and NS(NUD) as their addresses and 4227 use cases differ. 4229 Changes from draft-templin-intarea-6706bis-30 to draft-templin- 4230 intrea-6706bis-31: 4232 o Added "advisory PTB messages" under FAA SE2025 contract number 4233 DTFAWA-15-D-00030. 4235 Changes from draft-templin-intarea-6706bis-29 to draft-templin- 4236 intrea-6706bis-30: 4238 o Deprecate "primary" concept. Now, RS/RA keepalives are maintained 4239 over *all* underlying interfaces (i.e., and not just one primary). 4241 Changes from draft-templin-intarea-6706bis-28 to draft-templin- 4242 intrea-6706bis-29: 4244 o Changed OMNI interface citation to "draft-templin-6man-omni- 4245 interface" 4247 o Changed SPAN Service Prefix to fd80::/10. 4249 o Changed S/TLLAO format to include 'S' bit for ifIndex 4250 corresponding to the underlying interface that is Source of ND 4251 message. 4253 o Updated Path MTU 4255 Changes from draft-templin-intarea-6706bis-27 to draft-templin- 4256 intrea-6706bis-28: 4258 o MTU and fragmentation. 4260 Changes from draft-templin-intarea-6706bis-26 to draft-templin- 4261 intrea-6706bis-27: 4263 o MTU and fragmentation. 4265 o SPAN Service Prefix set to fd00::/10 4267 o Client SPAN addresses defined. 4269 Changes from draft-templin-intarea-6706bis-25 to draft-templin- 4270 intrea-6706bis-26: 4272 o MTU and RA configuration information updated. 4274 Changes from draft-templin-intarea-6706bis-24 to draft-templin- 4275 intrea-6706bis-25: 4277 o Added concept of "primary" to allow for proxyed RS/RA over only 4278 selected underlying interfaces. 4280 o General Cleanup. 4282 Changes from draft-templin-intarea-6706bis-23 to draft-templin- 4283 intrea-6706bis-24: 4285 o OMNI interface spec now a normative reference. 4287 o Use REACHABLE_TIME as the nominal Router Lifetime to return in 4288 RAs. 4290 o General cleanup. 4292 Changes from draft-templin-intarea-6706bis-22 to draft-templin- 4293 intrea-6706bis-23: 4295 o Choice of using either RS/RA or unsolicited NA for old Server 4296 notification. 4298 o General cleanup. 4300 Changes from draft-templin-intarea-6706bis-21 to draft-templin- 4301 intrea-6706bis-22: 4303 o Tightened up text on Proxy. 4305 o Removed unnecessarily restrictive texts. 4307 o General cleanup. 4309 Changes from draft-templin-intarea-6706bis-20 to draft-templin- 4310 intrea-6706bis-21: 4312 o Clarified relationship between OMNI and S/TLLAO ifIndex-tuples. 4314 o Important text in Section 13.15.3 on Servers timing out Clients 4315 that have gone silent without sending a departure notification. 4317 o New text on RS/RA as "hints of forward progress" for proactive 4318 NUD. 4320 Changes from draft-templin-intarea-6706bis-19 to draft-templin- 4321 intrea-6706bis-20: 4323 o Included new route optimization source and destination addressing 4324 strategy. Now, route optimization maintenance uses the address of 4325 the existing Server instead of the data packet destination address 4326 so that less pressure is placed on the BGP routing system 4327 convergence time and Server constancy is supported. 4329 o Included new method for releasing from old MSE without requiring 4330 Client messaging. 4332 o Included references to new OMNI interface spec (including the OMNI 4333 option). 4335 o New appendix on AERO Client/Server architecture. 4337 Changes from draft-templin-intarea-6706bis-18 to draft-templin- 4338 intrea-6706bis-19: 4340 o Changed Proxy/Server keepalives to use "proactive NUD" in a manner 4341 that parallels BFD 4343 Changes from draft-templin-intarea-6706bis-17 to draft-templin- 4344 intrea-6706bis-18: 4346 o Discuss how AERO option is used in relation to S/TLLAOs 4348 o New text on Bidirectional Forwarding Detection (BFD) 4350 o Cleaned up usage (and non-usage) of unsolicited NAs 4352 o New appendix on Server failures 4354 Changes from draft-templin-intarea-6706bis-15 to draft-templin- 4355 intrea-6706bis-17: 4357 o S/TLLAO now includes multiple link-layer addresses within a single 4358 option instead of requiring multiple options 4360 o New unsolicited NA message to inform the old link that a Client 4361 has moved to a new link 4363 Changes from draft-templin-intarea-6706bis-14 to draft-templin- 4364 intrea-6706bis-15: 4366 o MTU and fragmentation 4368 o New details in movement to new Server 4370 Changes from draft-templin-intarea-6706bis-13 to draft-templin- 4371 intrea-6706bis-14: 4373 o Security based on secured tunnels, ingress filtering, MAP list and 4374 ROS list 4376 Changes from draft-templin-intarea-6706bis-12 to draft-templin- 4377 intrea-6706bis-13: 4379 o New paragraph in Section 3.6 on AERO interface layering over 4380 secured tunnels 4382 o Removed extraneous text in Section 3.7 4384 o Added new detail to the forwarding algorithm in Section 3.9 4385 o Clarified use of fragmentation 4387 o Route optimization now supported for both MNP and non-MNP-based 4388 prefixes 4390 o Relays are now seen as link-layer elements in the architecture. 4392 o Built out multicast section in detail. 4394 o New Appendix on implementation considerations for route 4395 optimization. 4397 Changes from draft-templin-intarea-6706bis-11 to draft-templin- 4398 intrea-6706bis-12: 4400 o Introduced Gateways as a new AERO element for connecting 4401 Correspondent Nodes on INET links 4403 o Introduced terms "Access Network (ANET)" and "Internetwork (INET)" 4405 o Changed "ASP" to "MSP", and "ACP" to "MNP" 4407 o New figure on the relation of Segments to the SPAN and AERO link 4409 o New "S" bit in S/TLLAO to indicate the "Source" S/TLLAO as opposed 4410 to additional S/TLLAOs 4412 o Changed Interface ID for Servers from 255 to 0xffff 4414 o Significant updates to Route Optimization, NUD, and Mobility 4415 Management 4417 o New Section on Multicast 4419 o New Section on AERO Clients in the open Internetwork 4421 o New Section on Operation over multiple AERO links (VLANs over the 4422 SPAN) 4424 o New Sections on DNS considerations and Transition considerations 4426 o 4428 Changes from draft-templin-intarea-6706bis-10 to draft-templin- 4429 intrea-6706bis-11: 4431 o Added The SPAN 4432 Changes from draft-templin-intarea-6706bis-09 to draft-templin- 4433 intrea-6706bis-10: 4435 o Orphaned packets in flight (e.g., when a neighbor cache entry is 4436 in the DEPARTED state) are now forwarded at the link layer instead 4437 of at the network layer. Forwarding at the network layer can 4438 result in routing loops and/or excessive delays of forwarded 4439 packets while the routing system is still reconverging. 4441 o Update route optimization to clarify the unsecured nature of the 4442 first NS used for route discovery 4444 o Many cleanups and clarifications on ND messaging parameters 4446 Changes from draft-templin-intarea-6706bis-08 to draft-templin- 4447 intrea-6706bis-09: 4449 o Changed PRL to "MAP list" 4451 o For neighbor cache entries, changed "static" to "symmetric", and 4452 "dynamic" to "asymmetric" 4454 o Specified Proxy RS/RA exchanges with Servers on behalf of Clients 4456 o Added discussion of unsolicited NAs in Section 3.16, and included 4457 forward reference to Section 3.18 4459 o Added discussion of AERO Clients used as critical infrastructure 4460 elements to connect fixed networks. 4462 o Added network-based VPN under security considerations 4464 Changes from draft-templin-intarea-6706bis-07 to draft-templin- 4465 intrea-6706bis-08: 4467 o New section on AERO-Aware Access Router 4469 Changes from draft-templin-intarea-6706bis-06 to draft-templin- 4470 intrea-6706bis-07: 4472 o Added "R" bit for release of PDs. Now have a full RS/RA service 4473 that can do PD without requiring DHCPv6 messaging over-the-air 4475 o Clarifications on solicited vs unsolicited NAs 4477 o Clarified use of MAX_NEIGHBOR_ADVERTISEMENT for the purpose of 4478 increase reliability 4480 Changes from draft-templin-intarea-6706bis-05 to draft-templin- 4481 intrea-6706bis-06: 4483 o Major re-work and simplification of Route Optimization function 4485 o Added Distributed Mobility Management (DMM) and Mobility Anchor 4486 Point (MAP) terminology 4488 o New section on "AERO Critical Infrastructure Element 4489 Considerations" demonstrating low overall cost for the service 4491 o minor text revisions and deletions 4493 o removed extraneous appendices 4495 Changes from draft-templin-intarea-6706bis-04 to draft-templin- 4496 intrea-6706bis-05: 4498 o New Appendix E on S/TLLAO Extensions for special-purpose links. 4499 Discussed ATN/IPS as example. 4501 o New sentence in introduction to declare appendices as non- 4502 normative. 4504 Changes from draft-templin-intarea-6706bis-03 to draft-templin- 4505 intrea-6706bis-04: 4507 o Added definitions for Potential Router List (PRL) and secure 4508 enclave 4510 o Included text on mapping transport layer port numbers to network 4511 layer DSCP values 4513 o Added reference to DTLS and DMM Distributed Mobility Anchoring 4514 working group document 4516 o Reworked Security Considerations 4518 o Updated references. 4520 Changes from draft-templin-intarea-6706bis-02 to draft-templin- 4521 intrea-6706bis-03: 4523 o Added new section on SEND. 4525 o Clarifications on "AERO Address" section. 4527 o Updated references and added new reference for RFC8086. 4529 o Security considerations updates. 4531 o General text clarifications and cleanup. 4533 Changes from draft-templin-intarea-6706bis-01 to draft-templin- 4534 intrea-6706bis-02: 4536 o Note on encapsulation avoidance in Section 4. 4538 Changes from draft-templin-intarea-6706bis-00 to draft-templin- 4539 intrea-6706bis-01: 4541 o Remove DHCPv6 Server Release procedures that leveraged the old way 4542 Relays used to "route" between Server link-local addresses 4544 o Remove all text relating to Relays needing to do any AERO-specific 4545 operations 4547 o Proxy sends RS and receives RA from Server using SEND. Use CGAs 4548 as source addresses, and destination address of RA reply is to the 4549 AERO address corresponding to the Client's ACP. 4551 o Proxy uses SEND to protect RS and authenticate RA (Client does not 4552 use SEND, but rather relies on subnetwork security. When the 4553 Proxy receives an RS from the Client, it creates a new RS using 4554 its own addresses as the source and uses SEND with CGAs to send a 4555 new RS to the Server. 4557 o Emphasize distributed mobility management 4559 o AERO address-based RS injection of ACP into underlying routing 4560 system. 4562 Changes from draft-templin-aerolink-82 to draft-templin-intarea- 4563 6706bis-00: 4565 o Document use of NUD (NS/NA) for reliable link-layer address 4566 updates as an alternative to unreliable unsolicited NA. 4567 Consistent with Section 7.2.6 of RFC4861. 4569 o Server adds additional layer of encapsulation between outer and 4570 inner headers of NS/NA messages for transmission through Relays 4571 that act as vanilla IPv6 routers. The messages include the AERO 4572 Server Subnet Router Anycast address as the source and the Subnet 4573 Router Anycast address corresponding to the Client's ACP as the 4574 destination. 4576 o Clients use Subnet Router Anycast address as the encapsulation 4577 source address when the access network does not provide a 4578 topologically-fixed address. 4580 Author's Address 4582 Fred L. Templin (editor) 4583 Boeing Research & Technology 4584 P.O. Box 3707 4585 Seattle, WA 98124 4586 USA 4588 Email: fltemplin@acm.org