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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 Intended status: Informational June 15, 2021 5 Expires: December 17, 2021 7 Asymmetric Extended Route Optimization (AERO) 8 draft-templin-6man-aero-18 10 Abstract 12 This document specifies an Asymmetric Extended Route Optimization 13 (AERO) service for IP internetworking over Overlay Multilink Network 14 (OMNI) interfaces. AERO/OMNI use an IPv6 link-local address format 15 that supports operation of the IPv6 Neighbor Discovery (ND) protocol 16 and links ND to IP forwarding. Prefix delegation/registration 17 services are employed for network admission and to manage the routing 18 system. Secure multilink operation, mobility management, multicast, 19 traffic selector signaling and route optimization are naturally 20 supported through dynamic neighbor cache updates. AERO is a widely- 21 applicable mobile internetworking service especially well-suited to 22 aviation services, intelligent transportation systems, mobile end 23 user devices and many other applications. 25 Status of This Memo 27 This Internet-Draft is submitted in full conformance with the 28 provisions of BCP 78 and BCP 79. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF). Note that other groups may also distribute 32 working documents as Internet-Drafts. The list of current Internet- 33 Drafts is at https://datatracker.ietf.org/drafts/current/. 35 Internet-Drafts are draft documents valid for a maximum of six months 36 and may be updated, replaced, or obsoleted by other documents at any 37 time. It is inappropriate to use Internet-Drafts as reference 38 material or to cite them other than as "work in progress." 40 This Internet-Draft will expire on December 17, 2021. 42 Copyright Notice 44 Copyright (c) 2021 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (https://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 Table of Contents 59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 60 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 61 3. Asymmetric Extended Route Optimization (AERO) . . . . . . . . 13 62 3.1. AERO Node Types . . . . . . . . . . . . . . . . . . . . . 13 63 3.2. The AERO Service over OMNI Links . . . . . . . . . . . . 14 64 3.2.1. AERO/OMNI Reference Model . . . . . . . . . . . . . . 14 65 3.2.2. Addressing and Node Identification . . . . . . . . . 17 66 3.2.3. AERO Routing System . . . . . . . . . . . . . . . . . 18 67 3.2.4. OMNI Link Segment Routing . . . . . . . . . . . . . . 20 68 3.2.5. Segment Routing Topologies (SRTs) . . . . . . . . . . 26 69 3.2.6. Segment Routing For OMNI Link Selection . . . . . . . 26 70 3.2.7. Segment Routing Within the OMNI Link . . . . . . . . 27 71 3.3. OMNI Interface Characteristics . . . . . . . . . . . . . 33 72 3.4. OMNI Interface Initialization . . . . . . . . . . . . . . 35 73 3.4.1. AERO Proxy/Server and Relay Behavior . . . . . . . . 35 74 3.4.2. AERO Client Behavior . . . . . . . . . . . . . . . . 36 75 3.4.3. AERO Bridge Behavior . . . . . . . . . . . . . . . . 36 76 3.5. OMNI Interface Neighbor Cache Maintenance . . . . . . . . 36 77 3.5.1. OMNI ND Messages . . . . . . . . . . . . . . . . . . 38 78 3.5.2. OMNI Neighbor Advertisement Message Flags . . . . . . 40 79 3.5.3. OMNI Neighbor Window Synchronization . . . . . . . . 41 80 3.6. OMNI Interface Encapsulation and Re-encapsulation . . . . 41 81 3.7. OMNI Interface Decapsulation . . . . . . . . . . . . . . 42 82 3.8. OMNI Interface Data Origin Authentication . . . . . . . . 42 83 3.9. OMNI Interface MTU . . . . . . . . . . . . . . . . . . . 43 84 3.10. OMNI Interface Forwarding Algorithm . . . . . . . . . . . 43 85 3.10.1. Client Forwarding Algorithm . . . . . . . . . . . . 45 86 3.10.2. Proxy/Server and Relay Forwarding Algorithm . . . . 46 87 3.10.3. Bridge Forwarding Algorithm . . . . . . . . . . . . 49 88 3.11. OMNI Interface Error Handling . . . . . . . . . . . . . . 51 89 3.12. AERO Router Discovery, Prefix Delegation and 90 Autoconfiguration . . . . . . . . . . . . . . . . . . . . 53 91 3.12.1. AERO Service Model . . . . . . . . . . . . . . . . . 54 92 3.12.2. AERO Client Behavior . . . . . . . . . . . . . . . . 54 93 3.12.3. AERO Proxy/Server Behavior . . . . . . . . . . . . . 56 94 3.13. The AERO Proxy Function . . . . . . . . . . . . . . . . . 59 95 3.13.1. Detecting and Responding to Proxy/Server Failures . 62 96 3.13.2. Point-to-Multipoint Proxy/Server Coordination . . . 63 98 3.14. AERO Route Optimization . . . . . . . . . . . . . . . . . 64 99 3.14.1. Route Optimization Initiation . . . . . . . . . . . 65 100 3.14.2. Relaying the NS(AR) *NET Packet(s) . . . . . . . . . 66 101 3.14.3. Processing the NS(AR) and Sending the NA(AR) . . . . 66 102 3.14.4. Relaying the NA(AR) . . . . . . . . . . . . . . . . 67 103 3.14.5. Processing the NA(AR) . . . . . . . . . . . . . . . 67 104 3.14.6. Forwarding Packets to Route Optimized Targets . . . 68 105 3.15. Neighbor Unreachability Detection (NUD) . . . . . . . . . 72 106 3.16. Mobility Management and Quality of Service (QoS) . . . . 73 107 3.16.1. Mobility Update Messaging . . . . . . . . . . . . . 74 108 3.16.2. Announcing Link-Layer Address and/or QoS Preference 109 Changes . . . . . . . . . . . . . . . . . . . . . . 75 110 3.16.3. Bringing New Links Into Service . . . . . . . . . . 76 111 3.16.4. Deactivating Existing Links . . . . . . . . . . . . 76 112 3.16.5. Moving Between Proxy/Servers . . . . . . . . . . . . 76 113 3.17. Multicast . . . . . . . . . . . . . . . . . . . . . . . . 77 114 3.17.1. Source-Specific Multicast (SSM) . . . . . . . . . . 78 115 3.17.2. Any-Source Multicast (ASM) . . . . . . . . . . . . . 79 116 3.17.3. Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . . 80 117 3.18. Operation over Multiple OMNI Links . . . . . . . . . . . 80 118 3.19. DNS Considerations . . . . . . . . . . . . . . . . . . . 81 119 3.20. Transition/Coexistence Considerations . . . . . . . . . . 81 120 3.21. Detecting and Reacting to Proxy/Server and Bridge 121 Failures . . . . . . . . . . . . . . . . . . . . . . . . 82 122 3.22. AERO Clients on the Open Internet . . . . . . . . . . . . 82 123 3.23. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . . 84 124 4. Implementation Status . . . . . . . . . . . . . . . . . . . . 85 125 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 85 126 6. Security Considerations . . . . . . . . . . . . . . . . . . . 86 127 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 88 128 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 90 129 8.1. Normative References . . . . . . . . . . . . . . . . . . 90 130 8.2. Informative References . . . . . . . . . . . . . . . . . 91 131 Appendix A. Non-Normative Considerations . . . . . . . . . . . . 98 132 A.1. Implementation Strategies for Route Optimization . . . . 98 133 A.2. Implicit Mobility Management . . . . . . . . . . . . . . 98 134 A.3. Direct Underlying Interfaces . . . . . . . . . . . . . . 99 135 A.4. AERO Critical Infrastructure Considerations . . . . . . . 99 136 A.5. AERO Server Failure Implications . . . . . . . . . . . . 100 137 A.6. AERO Client / Server Architecture . . . . . . . . . . . . 100 138 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 102 139 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 105 141 1. Introduction 143 Asymmetric Extended Route Optimization (AERO) fulfills the 144 requirements of Distributed Mobility Management (DMM) [RFC7333] and 145 route optimization [RFC5522] for aeronautical networking and other 146 network mobility use cases including intelligent transportation 147 systems and enterprise mobile device users. AERO is a secure 148 internetworking and mobility management service that employs the 149 Overlay Multilink Network Interface (OMNI) [I-D.templin-6man-omni] 150 Non-Broadcast, Multiple Access (NBMA) virtual link model. The OMNI 151 link is a virtual overlay configured over one or more underlying 152 Internetworks, and nodes on the link can exchange original IP packets 153 as single-hop neighbors. The OMNI Adaptation Layer (OAL) supports 154 multilink operation for increased reliability and path optimization 155 while providing fragmentation and reassembly services to support 156 Maximum Transmission Unit (MTU) diversity. In terms of precedence, 157 readers may appreciate reading this specification first to gain an 158 understanding of the overall architecture and mobility services then 159 return to the OMNI specification for a deeper analysis of the NBMA 160 link model. 162 The AERO service comprises Clients, Proxy/Servers and Relays that are 163 seen as OMNI link neighbors as well as Bridges that interconnect 164 diverse Internetworks as OMNI link segments through OAL forwarding at 165 a layer below IP. Each node's OMNI interface uses an IPv6 link-local 166 address format that supports operation of the IPv6 Neighbor Discovery 167 (ND) protocol [RFC4861] and links ND to IP forwarding. A node's OMNI 168 interface can be configured over multiple underlying interfaces, and 169 therefore appears as a single interface with multiple link-layer 170 addresses. Each link-layer address is subject to change due to 171 mobility and/or multilink fluctuations, and link-layer address 172 changes are signaled by ND messaging the same as for any IPv6 link. 174 AERO provides a secure cloud-based service where mobile node Clients 175 may use any Proxy/Server acting as a Mobility Anchor Point (MAP) and 176 fixed nodes may use any Relay on the link for efficient 177 communications. Fixed nodes forward original IP packets destined to 178 other AERO nodes via the nearest Relay, which forwards them through 179 the cloud. Mobile node Clients discover shortest paths to OMNI link 180 neighbors through AERO route optimization. Both unicast and 181 multicast communications are supported, and Clients may efficiently 182 move between locations while maintaining continuous communications 183 with correspondents and without changing their IP Address. 185 AERO Bridges peer with Proxy/Servers in a secured private BGP overlay 186 routing instance to provide a Segment Routing Topology (SRT) that 187 allows the OAL to span the underlying Internetworks of multiple 188 disjoint administrative domains as a single unified OMNI link at a 189 layer below IP. Each OMNI link instance is characterized by the set 190 of Mobility Service Prefixes (MSPs) common to all mobile nodes. 191 Relays provide an optimal route from (fixed) correspondent nodes on 192 the underlying Internetwork to (mobile or fixed) nodes on the OMNI 193 link. To the underlying Internetwork, the Relay is the source of a 194 route to the MSP; hence uplink traffic to the mobile node is 195 naturally routed to the nearest Relay. A Relay can be considered as 196 a simple case of a Proxy/Server that provides only forwarding and not 197 proxying services. 199 AERO can be used with OMNI links that span private-use Internetworks 200 and/or public Internetworks such as the global Internet. In the 201 latter case, some end systems may be located behind global Internet 202 Network Address Translators (NATs). A means for robust traversal of 203 NATs while avoiding "triangle routing" and Proxy/Server traffic 204 concentration is therefore provided. 206 AERO assumes the use of PIM Sparse Mode in support of multicast 207 communication. In support of Source Specific Multicast (SSM) when a 208 Mobile Node is the source, AERO route optimization ensures that a 209 shortest-path multicast tree is established with provisions for 210 mobility and multilink operation. In all other multicast scenarios 211 there are no AERO dependencies. 213 AERO provides a secure aeronautical internetworking service for both 214 manned and unmanned aircraft, where the aircraft is treated as a 215 mobile node that can connect an Internet of Things (IoT). AERO is 216 also applicable to a wide variety of other use cases. For example, 217 it can be used to coordinate the links of mobile nodes (e.g., 218 cellphones, tablets, laptop computers, etc.) that connect into a home 219 enterprise network via public access networks with VPN or non-VPN 220 services enabled according to the appropriate security model. AERO 221 can also be used to facilitate terrestrial vehicular and urban air 222 mobility (as well as pedestrian communication services) for future 223 intelligent transportation systems 224 [I-D.ietf-ipwave-vehicular-networking][I-D.templin-ipwave-uam-its]. 225 Other applicable use cases are also in scope. 227 Along with OMNI, AERO provides secured optimal routing support for 228 the "6M's" of modern Internetworking, including: 230 1. Multilink - a mobile node's ability to coordinate multiple 231 diverse underlying data links as a single logical unit (i.e., the 232 OMNI interface) to achieve the required communications 233 performance and reliability objectives. 235 2. Multinet - the ability to span the OMNI link over a segment 236 routing topology with multiple diverse network administrative 237 domains while maintaining seamless end-to-end communications 238 between mobile Clients and correspondents such as air traffic 239 controllers, fleet administrators, etc. 241 3. Mobility - a mobile node's ability to change network points of 242 attachment (e.g., moving between wireless base stations) which 243 may result in an underlying interface address change, but without 244 disruptions to ongoing communication sessions with peers over the 245 OMNI link. 247 4. Multicast - the ability to send a single network transmission 248 that reaches multiple nodes belonging to the same interest group, 249 but without disturbing other nodes not subscribed to the interest 250 group. 252 5. Multihop - a mobile node vehicle-to-vehicle relaying capability 253 useful when multiple forwarding hops between vehicles may be 254 necessary to "reach back" to an infrastructure access point 255 connection to the OMNI link. 257 6. MTU assurance - the ability to deliver packets of various robust 258 sizes between peers without loss due to a link size restriction, 259 and to dynamically adjust packets sizes to achieve the optimal 260 performance for each independent traffic flow. 262 The following numbered sections present the AERO specification. The 263 appendices at the end of the document are non-normative. 265 2. Terminology 267 The terminology in the normative references applies; especially, the 268 terminology in the OMNI specification [I-D.templin-6man-omni] is used 269 extensively throughout. The following terms are defined within the 270 scope of this document: 272 IPv6 Neighbor Discovery (ND) 273 a control message service for coordinating neighbor relationships 274 between nodes connected to a common link. AERO uses the IPv6 ND 275 messaging service specified in [RFC4861]. 277 IPv6 Prefix Delegation 278 a networking service for delegating IPv6 prefixes to nodes on the 279 link. The nominal service is DHCPv6 [RFC8415], however alternate 280 services (e.g., based on ND messaging) are also in scope. A 281 minimal form of prefix delegation known as "prefix registration" 282 can be used if the Client knows its prefix in advance and can 283 represent it in the IPv6 source address of an ND message. 285 Access Network (ANET) 286 a node's first-hop data link service network (e.g., a radio access 287 network, cellular service provider network, corporate enterprise 288 network, etc.) that often provides link-layer security services 289 such as IEEE 802.1X and physical-layer security (e.g., "protected 290 spectrum") to prevent unauthorized access internally and with 291 border network-layer security services such as firewalls and 292 proxys that prevent unauthorized outside access. 294 ANET interface 295 a node's attachment to a link in an ANET. 297 Internetwork (INET) 298 a network topology with a coherent IP routing and addressing plan 299 and that provides a transit backbone service for its connected end 300 systems. INETs also provide an underlay service over which the 301 AERO virtual link is configured. Example INETs include corporate 302 enterprise networks, aviation networks, and the public Internet 303 itself. When there is no administrative boundary between an ANET 304 and the INET, the ANET and INET are one and the same. 306 INET interface 307 a node's attachment to a link in an INET. 309 *NET 310 a "wildcard" term referring to either ANET or INET when it is not 311 necessary to draw a distinction between the two. 313 *NET interface 314 a node's attachment to a link in a *NET. 316 *NET Partition 317 frequently, *NETs such as large corporate enterprise networks are 318 sub-divided internally into separate isolated partitions (a 319 technique also known as "network segmentation"). Each partition 320 is fully connected internally but disconnected from other 321 partitions, and there is no requirement that separate partitions 322 maintain consistent Internet Protocol and/or addressing plans. 323 (Each *NET partition is seen as a separate OMNI link segment as 324 discussed below.) 326 *NET address 327 an IP address assigned to a node's interface connection to a *NET. 329 *NET encapsulation 330 the encapsulation of a packet in an outer header or headers that 331 can be routed within the scope of the local *NET partition. 333 OMNI link 334 the same as defined in [I-D.templin-6man-omni], and manifested by 335 IPv6 encapsulation [RFC2473]. The OMNI link spans underlying *NET 336 segments joined by intermediate nodes in a spanning tree the same 337 as a bridged campus LAN. AERO nodes on the OMNI link appear as 338 single-hop neighbors at the network layer even though they may be 339 separated by multiple underlying *NET hops; AERO nodes can use 340 Segment Routing [RFC8402] to cause packets to visit selected 341 waypoints on the link. 343 OMNI Interface 344 a node's attachment to an OMNI link. Since OMNI interface 345 addresses are managed for uniqueness, OMNI interfaces do not 346 require Duplicate Address Detection (DAD) and therefore set the 347 administrative variable 'DupAddrDetectTransmits' to zero 348 [RFC4862]. 350 OMNI Adaptation Layer (OAL) 351 an OMNI interface service that subjects original IP packets 352 admitted into the interface to mid-layer IPv6 header encapsulation 353 followed by fragmentation and reassembly. The OAL is also 354 responsible for generating MTU-related control messages as 355 necessary, and for providing addressing context for spanning 356 multiple segments of a bridged OMNI link. 358 original IP packet 359 a whole IP packet or fragment admitted into the OMNI interface by 360 the network layer prior to OAL encapsulation and fragmentation, or 361 an IP packet delivered to the network layer by the OMNI interface 362 following OAL decapsulation and reassembly. 364 OAL packet 365 an original IP packet encapsulated in OAL headers and trailers 366 before OAL fragmentation, or following OAL reassembly. 368 OAL fragment 369 a portion of an OAL packet following fragmentation but prior to 370 *NET encapsulation, or following *NET encapsulation but prior to 371 OAL reassembly. 373 (OAL) atomic fragment 374 an OAL packet that does not require fragmentation is always 375 encapsulated as an "atomic fragment" and includes a Fragment 376 Header with Fragment Offset and More Fragments both set to 0, but 377 with a valid Identification value. 379 (OAL) carrier packet 380 an encapsulated OAL fragment following *NET encapsulation or prior 381 to *NET decapsulation. OAL sources and destinations exchange 382 carrier packets over underlying interfaces, and may be separated 383 by one or more OAL intermediate nodes. OAL intermediate nodes re- 384 encapsulate carrier packets during forwarding by removing the *NET 385 headers of the previous hop underlying network and replacing them 386 with new *NET headers for the next hop underlying network. 388 OAL source 389 an OMNI interface acts as an OAL source when it encapsulates 390 original IP packets to form OAL packets, then performs OAL 391 fragmentation and *NET encapsulation to create carrier packets. 393 OAL destination 394 an OMNI interface acts as an OAL destination when it decapsulates 395 carrier packets, then performs OAL reassembly and decapsulation to 396 derive the original IP packet. 398 OAL intermediate node 399 an OMNI interface acts as an OAL intermediate node when it removes 400 the *NET headers of carrier packets received on a first segment, 401 then re-encapsulates the carrier packets in new *NET headers and 402 forwards them into the next segment. OAL intermediate nodes 403 decrement the Hop Limit of the OAL IPv6 header during re- 404 encapsulation, and discard the packet if the Hop Limit reaches 0. 405 OAL intermediate nodes do not decrement the Hop Limit/TTL of the 406 original IP packet. 408 underlying interface 409 a *NET interface over which an OMNI interface is configured. 411 Mobility Service Prefix (MSP) 412 an aggregated IP Global Unicast Address (GUA) prefix (e.g., 413 2001:db8::/32, 192.0.2.0/24, etc.) assigned to the OMNI link and 414 from which more-specific Mobile Network Prefixes (MNPs) are 415 delegated. OMNI link administrators typically obtain MSPs from an 416 Internet address registry, however private-use prefixes can 417 alternatively be used subject to certain limitations (see: 418 [I-D.templin-6man-omni]). OMNI links that connect to the global 419 Internet advertise their MSPs to their interdomain routing peers. 421 Mobile Network Prefix (MNP) 422 a longer IP prefix delegated from an MSP (e.g., 423 2001:db8:1000:2000::/56, 192.0.2.8/30, etc.) and delegated to an 424 AERO Client or Relay. 426 Mobile Network Prefix Link Local Address (MNP-LLA) 427 an IPv6 Link Local Address that embeds the most significant 64 428 bits of an MNP in the lower 64 bits of fe80::/64, as specified in 429 [I-D.templin-6man-omni]. 431 Mobile Network Prefix Unique Local Address (MNP-ULA) 432 an IPv6 Unique-Local Address derived from an MNP-LLA. 434 Administrative Link Local Address (ADM-LLA) 435 an IPv6 Link Local Address that embeds a 32-bit administratively- 436 assigned identification value in the lower 32 bits of fe80::/96, 437 as specified in [I-D.templin-6man-omni]. 439 Administrative Unique Local Address (ADM-ULA) 440 an IPv6 Unique-Local Address derived from an ADM-LLA. 442 AERO node 443 a node that is connected to an OMNI link and participates in the 444 AERO internetworking and mobility service. 446 AERO Client ("Client") 447 an AERO node that connects over one or more underlying interfaces 448 and requests MNP delegation/registration service from AERO Proxy/ 449 Servers. The Client assigns an MNP-LLA to the OMNI interface for 450 use in ND exchanges with other AERO nodes and forwards original IP 451 packets to correspondents according to OMNI interface neighbor 452 cache state. 454 AERO Proxy/Server ("Proxy/Server") 455 a node that provides a proxying service between AERO Clients and 456 external peers on its Client-facing ANET interfaces (i.e., in the 457 same fashion as for an enterprise network proxy) as well as 458 default forwarding and Mobility Anchor Point (MAP) services for 459 coordination with correspondents on its INET-facing interfaces. 460 (Proxy/Servers in the open INET instead configure only an INET 461 interface and no ANET interfaces.) The Proxy/Server configures an 462 OMNI interface and assigns an ADM-LLA to support the operation of 463 IPv6 ND services, while advertising all of its associated MNPs via 464 BGP peerings with Bridges. Note that the Proxy and Server 465 functions can be considered logically separable, but since each 466 Proxy/Server must be informed of all of the Client's other 467 multilink Proxy/Server affiliations the AERO service is best 468 supported when the two functions are coresident on the same 469 physical or logical platform. 471 AERO Relay ("Relay") 472 a Proxy/Server that provides forwarding services between nodes 473 reached via the OMNI link and correspondents on connected 474 downstream networks and links. AERO Relays configure an OMNI 475 interface and assign an ADM-LLA the same as Proxy/Servers, and 476 also run a dynamic routing protocol to discover any non-MNP IP GUA 477 routes in service on its connected downstream network links. The 478 Relay advertises the MSP(s) to its downstream networks, and 479 redistributes downstream network routes into the OMNI link routing 480 system the same as for Proxy/Servers. 482 AERO Bridge ("Bridge") 483 a BGP hub autonomous system node that also provides OAL forwarding 484 services (as well as a security trust anchor) for nodes on an OMNI 485 link. The Bridge forwards carrier packets between OMNI link 486 segments as an OAL intermediate node while decrementing the OAL 487 IPv6 header Hop Limit but without decrementing the network layer 488 IP TTL/Hop Limit. Bridges peer with Proxy/Servers and other 489 Bridges over an OMNI link secured spanning tree to discover the 490 full set of MNPs for the link as well as any non-MNP IP GUA routes 491 reached via Relays. 493 First-Hop Segment (FHS) Proxy/Server 494 a Proxy/Server for an underlying interface of the source Client 495 that forwards packets sent by the source Client over that 496 interface into the segment routing topology. 498 Last-Hop Segment (LHS) Proxy/Server 499 a Proxy/Server for an underlying interface of the target Client 500 that forwards packets received from the segment routing topology 501 to the target Client over that interface. 503 Segment Routing Topology (SRT) 504 a multinet forwarding region between the FHS Proxy/Server and LHS 505 Proxy/Server. FHS/LHS Proxy/Servers and SRT Bridges span the OMNI 506 link on behalf of source/target Client pairs. The SRT maintains a 507 spanning tree established through BGP peerings between Bridges and 508 Proxy/Servers. Each SRT segment includes Bridges in a "hub" and 509 Proxy/Servers in "spokes", while adjacent segments are 510 interconnected by Bridge-Bridge peerings. The BGP peerings are 511 configured over both secured and unsecured underlying network 512 paths such that a secured spanning tree is available for critical 513 control messages while other messages can use the unsecured 514 spanning tree. 516 link-layer address 517 an IP address used as an encapsulation header source or 518 destination address from the perspective of the OMNI interface. 519 When an upper layer protocol (e.g., UDP) is used as part of the 520 encapsulation, the port number is also considered as part of the 521 link-layer address. 523 network layer address 524 the source or destination address of an original IP packet 525 presented to the OMNI interface. 527 end user network (EUN) 528 an internal virtual or external edge IP network that an AERO 529 Client or Relay connects to the rest of the network via the OMNI 530 interface. The Client/Relay sees each EUN as a "downstream" 531 network, and sees the OMNI interface as the point of attachment to 532 the "upstream" network. 534 Mobile Node (MN) 535 an AERO Client and all of its downstream-attached networks that 536 move together as a single unit, i.e., an end system that connects 537 an Internet of Things. 539 Mobile Router (MR) 540 a MN's on-board router that forwards original IP packets between 541 any downstream-attached networks and the OMNI link. The MR is the 542 MN entity that hosts the AERO Client. 544 Route Optimization Source (ROS) 545 the AERO node nearest the source that initiates route 546 optimization. The ROS may be a FHS Proxy/Server or Relay for the 547 source, or may be the source Client itself. 549 Route Optimization responder (ROR) 550 the AERO node that responds to route optimization requests on 551 behalf of the target. The ROR may be an LHS Proxy/Server for a 552 target MNP Client or an LHS Relay for a non-MNP target. 554 MAP List 555 a geographically and/or topologically referenced list of addresses 556 of all Proxy/Servers within the same OMNI link. Each OMNI link 557 has its own MAP list. 559 Distributed Mobility Management (DMM) 560 a BGP-based overlay routing service coordinated by Proxy/Servers 561 and Bridges that tracks all Proxy/Server-to-Client associations. 563 Mobility Service (MS) 564 the collective set of all Proxy/Servers, Bridges and Relays that 565 provide the AERO Service to Clients. 567 Mobility Service Endpoint MSE) 568 an individual Proxy/Server, Bridge or Relay in the Mobility 569 Service. 571 Throughout the document, the simple terms "Client", "Proxy/Server", 572 "Bridge" and "Relay" refer to "AERO Client", "AERO Proxy/Server", 573 "AERO Bridge" and "AERO Relay", respectively. Capitalization is used 574 to distinguish these terms from other common Internetworking uses in 575 which they appear without capitalization. 577 The terminology of IPv6 ND [RFC4861] and DHCPv6 [RFC8415] (including 578 the names of node variables, messages and protocol constants) is used 579 throughout this document. The terms "All-Routers multicast", "All- 580 Nodes multicast", "Solicited-Node multicast" and "Subnet-Router 581 anycast" are defined in [RFC4291]. Also, the term "IP" is used to 582 generically refer to either Internet Protocol version, i.e., IPv4 583 [RFC0791] or IPv6 [RFC8200]. 585 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 586 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 587 "OPTIONAL" in this document are to be interpreted as described in BCP 588 14 [RFC2119][RFC8174] when, and only when, they appear in all 589 capitals, as shown here. 591 3. Asymmetric Extended Route Optimization (AERO) 593 The following sections specify the operation of IP over OMNI links 594 using the AERO service: 596 3.1. AERO Node Types 598 AERO Clients are Mobile Nodes (MNs) that configure OMNI interfaces 599 over underlying interfaces with addresses that may change when the 600 Client moves to a new network connection point. AERO Clients 601 register their Mobile Network Prefixes (MNPs) with the AERO service, 602 and distribute the MNPs to nodes on EUNs. AERO Bridges, Proxy/ 603 Servers and Relays are critical infrastructure elements in fixed 604 (i.e., non-mobile) INET deployments and hence have permanent and 605 unchanging INET addresses. Together, they constitute the AERO 606 service which provides an OMNI link virtual overlay for connecting 607 AERO Clients. 609 AERO Bridges (together with Proxy/Servers) provide the secured 610 backbone supporting infrastructure for a Segment Routing Topology 611 (SRT) that spans the OMNI link. Bridges forward carrier packets both 612 within the same SRT segment and between disjoint SRT segments based 613 on an IPv6 encapsulation mid-layer known as the OMNI Adaptation Layer 614 (OAL) [I-D.templin-6man-omni]. The OMNI interface and OAL provide a 615 virtual bridging service, since the inner IP TTL/Hop Limit is not 616 decremented. Each Bridge also peers with Proxy/Servers and other 617 Bridges in a dynamic routing protocol instance to provide a 618 Distributed Mobility Management (DMM) service for the list of active 619 MNPs (see Section 3.2.3). Bridges present the OMNI link as a set of 620 one or more Mobility Service Prefixes (MSPs) and configure secured 621 tunnels with Proxy/Servers, Relays and other Bridges; they further 622 maintain IP forwarding table entries for each MNP and any other 623 reachable non-MNP prefixes. 625 AERO Proxy/Servers in distributed SRT segments provide default 626 forwarding and mobility/multilink services for AERO Client mobile 627 nodes. Each Proxy/Server also peers with Bridges in a dynamic 628 routing protocol instance to advertise its list of associated MNPs 629 (see Section 3.2.3). Proxy/Servers facilitate prefix delegation/ 630 registration exchanges with Clients, where each delegated prefix 631 becomes an MNP taken from an MSP. Proxy/Servers forward carrier 632 packets between OMNI interface neighbors and track each Client's 633 mobility profiles. Proxy/Servers at ANET/INET boundaries provide a 634 conduit for ANET Clients to associate with peers reached through 635 external INETs. Proxy/Servers in the open INET support INET Clients 636 through authenticated IPv6 ND message exchanges. Source Clients 637 employ First-Hop Segment (FHS) Proxy/Servers to forward packets over 638 the SRT to Last-Hop Segment (LHS) Proxy/Servers which finally forward 639 to target Clients. 641 AERO Relays are Proxy/Servers that provide forwarding services to 642 exchange original IP packets between the OMNI interface and INET/EUN 643 interfaces. Relays are provisioned with MNPs the same as for an AERO 644 Client, and also run a dynamic routing protocol to discover any non- 645 MNP IP routes. The Relay advertises the MSP(s) to its connected 646 networks, and distributes all of its associated MNP and non-MNP 647 routes via BGP peerings with Bridges 649 3.2. The AERO Service over OMNI Links 651 3.2.1. AERO/OMNI Reference Model 653 Figure 1 presents the basic OMNI link reference model: 655 +----------------+ 656 | AERO Bridge B1 | 657 | Nbr: S1, S2, P1| 658 |(X1->S1; X2->S2)| 659 | MSP M1 | 660 +-------+--------+ 661 +--------------+ | +--------------+ 662 | AERO P/S S1 | | | AERO P/S S2 | 663 | Nbr: C1, B1 | | | Nbr: C2, B1 | 664 | default->B1 | | | default->B1 | 665 | X1->C1 | | | X2->C2 | 666 +-------+------+ | +------+-------+ 667 | OMNI link | | 668 X===+===+==================+===================+===+===X 669 | | 670 +-----+--------+ +--------+-----+ 671 |AERO Client C1| |AERO Client C2| 672 | Nbr: S1 | | Nbr: S2 | 673 | default->S1 | | default->S2 | 674 | MNP X1 | | MNP X2 | 675 +------+-------+ +-----+--------+ 676 | | 677 .-. .-. 678 ,-( _)-. ,-( _)-. 679 .-(_ IP )-. +-------+ +-------+ .-(_ IP )-. 680 (__ EUN )--|Host H1| |Host H2|--(__ EUN ) 681 `-(______)-' +-------+ +-------+ `-(______)-' 683 Figure 1: AERO/OMNI Reference Model 685 In this model: 687 o the OMNI link is an overlay network service configured over one or 688 more underlying SRT segments which may be managed by different 689 administrative authorities and have incompatible protocols and/or 690 addressing plans. 692 o AERO Bridge B1 aggregates Mobility Service Prefix (MSP) M1, 693 discovers Mobile Network Prefixes (MNPs) X* and advertises the MSP 694 via BGP peerings over secured tunnels to Proxy/Servers (S1, S2). 695 Bridges provide the backbone for an SRT that spans the OMNI link. 697 o AERO Proxy/Servers S1 and S2 configure secured tunnels with Bridge 698 B1 and also provide mobility, multilink, multicast and default 699 router services for the MNPs of their associated Clients C1 and 700 C2. (Proxy/Servers that act as Relays can also advertise non-MNP 701 routes for non-mobile correspondent nodes the same as for MNP 702 Clients.) 704 o AERO Clients C1 and C2 associate with Proxy/Servers S1 and S2, 705 respectively. They receive MNP delegations X1 and X2, and also 706 act as default routers for their associated physical or internal 707 virtual EUNs. Simple hosts H1 and H2 attach to the EUNs served by 708 Clients C1 and C2, respectively. 710 An OMNI link configured over a single *NET appears as a single 711 unified link with a consistent underlying network addressing plan; 712 all nodes on the link can exchange carrier packets via simple *NET 713 encapsulation (i.e., following any necessary NAT traversal) since the 714 underlying *NET is connected. In common practice, however, OMNI 715 links often traverse an SRT spanning tree that connects multiple 716 distinct *NET segments that may be managed under different 717 administrative authorities (e.g., as for worldwide aviation service 718 providers such as ARINC, SITA, Inmarsat, etc.). Individual *NETs may 719 also be partitioned internally, in which case each internal partition 720 is seen as a separate segment. 722 The addressing plan of each SRT segment is consistent internally but 723 will often bear no relation to the addressing plans of other 724 segments. Each segment is also likely to be separated from others by 725 network security devices (e.g., firewalls, proxys, packet filtering 726 gateways, etc.), and disjoint segments often have no common physical 727 link connections. Therefore, nodes can only be assured of exchanging 728 carrier packets directly with correspondents in the same segment, and 729 not with those in other segments. The only means for joining the 730 segments therefore is through inter-domain peerings between AERO 731 Bridges. 733 The same as for traditional campus LANs, the OMNI link spans multiple 734 SRT segments that can be joined into a single unified link using the 735 OMNI Adaptation Layer (OAL) [I-D.templin-6man-omni]. The OAL is an 736 OMNI interface sublayer that inserts a mid-layer IPv6 encapsulation 737 header for inter-segment forwarding (i.e., bridging) without 738 decrementing the network-layer TTL/Hop Limit of the original IP 739 packet. An example OMNI link SRT is shown in Figure 2: 741 . . . . . . . . . . . . . . . . . . . . . . . 742 . . 743 . .-(::::::::) . 744 . .-(::::::::::::)-. +-+ . 745 . (:::: Segment A :::)--|B|---+ . 746 . `-(::::::::::::)-' +-+ | . 747 . `-(::::::)-' | . 748 . | . 749 . .-(::::::::) | . 750 . .-(::::::::::::)-. +-+ | . 751 . (:::: Segment B :::)--|B|---+ . 752 . `-(::::::::::::)-' +-+ | . 753 . `-(::::::)-' | . 754 . | . 755 . .-(::::::::) | . 756 . .-(::::::::::::)-. +-+ | . 757 . (:::: Segment C :::)--|B|---+ . 758 . `-(::::::::::::)-' +-+ | . 759 . `-(::::::)-' | . 760 . | . 761 . ..(etc).. x . 762 . . 763 . . 764 . <- Segment Routing Topology (SRT) -> . 765 . . . . . . . . . . . . . .. . . . . . . . . 767 Figure 2: OMNI Link Segment Routing Topology (SRT) 769 Bridge, Proxy/Server and Relay OMNI interfaces are configured over 770 both secured tunnels and open INET underlying interfaces within their 771 respective SRT segments. Within each segment, Bridges configure 772 "hub-and-spokes" BGP peerings with Proxy/Server/Relays as "spokes". 773 Adjacent SRT segments are joined by Bridge-to-Bridge peerings to 774 collectively form a spanning tree over the entire SRT. The "secured" 775 spanning tree supports authentication and integrity for critical 776 control plane messages. The "unsecured" spanning tree conveys 777 ordinary carrier packets without security codes and that must be 778 treated by destinations according to data origin authentication 779 procedures. AERO nodes can employ route optimization to cause 780 carrier packets to take more direct paths between OMNI link neighbors 781 without having to follow strict SRT spanning tree paths. 783 3.2.2. Addressing and Node Identification 785 AERO nodes on OMNI links use the Link-Local Address (LLA) prefix 786 fe80::/64 [RFC4291] to assign LLAs used for network-layer addresses 787 in link-scoped IPv6 ND and data messages. AERO Clients use LLAs 788 constructed from MNPs (i.e., "MNP-LLAs") while other AERO nodes use 789 LLAs constructed from administrative identification values ("ADM- 790 LLAs") as specified in [I-D.templin-6man-omni]. Non-MNP routes are 791 also represented the same as for MNP-LLAs, but may include a prefix 792 that is not properly covered by the MSP. 794 AERO nodes also use the Unique Local Address (ULA) prefix fd00::/8 795 followed by a pseudo-random 40-bit OMNI domain identifier to form the 796 prefix [ULA]::/48, then include a 16-bit OMNI link identifier '*' to 797 form the prefix [ULA*]::/64 [RFC4291]. The AERO node then uses the 798 prefix [ULA*]::/64 to form "MNP-ULAs" or "ADM-ULA"s as specified in 799 [I-D.templin-6man-omni] to support OAL addressing. (The prefix 800 [ULA*]::/64 appearing alone and with no suffix represents "default".) 801 AERO Clients also use Temporary ULAs constructed per 802 [I-D.templin-6man-omni], where the addresses are typically used only 803 in initial control message exchanges until a stable MNP-LLA/ULA is 804 assigned. 806 AERO MSPs, MNPs and non-MNP routes are typically based on Global 807 Unicast Addresses (GUAs), but in some cases may be based on private- 808 use addresses. See [I-D.templin-6man-omni] for a full specification 809 of LLAs, ULAs and GUAs used by AERO nodes on OMNI links. 811 Finally, AERO Clients and Proxy/Servers configure node identification 812 values as specified in [I-D.templin-6man-omni]. 814 3.2.3. AERO Routing System 816 The AERO routing system comprises a private Border Gateway Protocol 817 (BGP) [RFC4271] service coordinated between Bridges and Proxy/ 818 Servers. The service supports carrier packet forwarding at a layer 819 below IP, and does not interact with either the public Internet BGP 820 routing system or any underlying INET routing systems. 822 In a reference deployment, each Proxy/Server is configured as an 823 Autonomous System Border Router (ASBR) for a stub Autonomous System 824 (AS) using a 32-bit AS Number (ASN) [RFC4271] that is unique within 825 the BGP instance, and each Proxy/Server further uses eBGP to peer 826 with one or more Bridges but does not peer with other Proxy/Servers. 827 Each SRT segment in the OMNI link must include one or more Bridges in 828 a "hub" AS, which peer with the Proxy/Servers within that segment as 829 "spoke" ASes. All Bridges within the same segment are members of the 830 same hub AS, and use iBGP to maintain a consistent view of all active 831 routes currently in service. The Bridges of different segments peer 832 with one another using eBGP. 834 Bridges maintain forwarding table entries only for the MNP-ULAs 835 corresponding to MNP and non-MNP routes that are currently active, 836 while carrier packets destined to all other MNP-ULAs are dropped with 837 a Destination Unreachable message returned due to the black-hole 838 route. In this way, Proxy/Servers and Relays have only partial 839 topology knowledge (i.e., they only maintain routing information for 840 their directly associated Clients and non-AERO links) and they 841 forward all other carrier packets to Bridges which have full topology 842 knowledge. 844 Each OMNI link SRT segment assigns a unique ADM-ULA sub-prefix of 845 [ULA*]::/96 known as the "SRT prefix". For example, a first segment 846 could assign [ULA*]::1000/116, a second could assign 847 [ULA*]::2000/116, a third could assign [ULA*]::3000/116, etc. Within 848 each segment, each Proxy/Server configures an ADM-ULA within the 849 segment's SRT prefix, e.g., the Proxy/Servers within [ULA*]::2000/116 850 could assign the ADM-ULAs [ULA*]::2011/116, [ULA*]::2026/116, 851 [ULA*]::2003/116, etc. 853 The administrative authorities for each segment must therefore 854 coordinate to assure mutually-exclusive ADM-ULA prefix assignments, 855 but internal provisioning of ADM-ULAs an independent local 856 consideration for each administrative authority. For each ADM-ULA 857 prefix, the Bridge(s) that connect that segment assign the all-zero's 858 address of the prefix as a Subnet Router Anycast address. For 859 example, the Subnet Router Anycast address for [ULA*]::1023/116 is 860 simply [ULA*]::1000. 862 ADM-ULA prefixes are statically represented in Bridge forwarding 863 tables. Bridges join multiple SRT segments into a unified OMNI link 864 over multiple diverse network administrative domains. They support a 865 bridging function by first establishing forwarding table entries for 866 their ADM-ULA prefixes either via standard BGP routing or static 867 routes. For example, if three Bridges ('A', 'B' and 'C') from 868 different segments serviced [ULA*]::1000/116, [ULA*]::2000/116 and 869 [ULA*]::3000/116 respectively, then the forwarding tables in each 870 Bridge are as follows: 872 A: [ULA*]::1000/116->local, [ULA*]::2000/116->B, [ULA*]::3000/116->C 874 B: [ULA*]::1000/116->A, [ULA*]::2000/116->local, [ULA*]::3000/116->C 876 C: [ULA*]::1000/116->A, [ULA*]::2000/116->B, [ULA*]::3000/116->local 878 These forwarding table entries are permanent and never change, since 879 they correspond to fixed infrastructure elements in their respective 880 segments. 882 MNP ULAs are instead dynamically advertised in the AERO routing 883 system by Proxy/Servers and Relays that provide service for their 884 corresponding MNPs. For example, if three Proxy/Servers ('D', 'E' 885 and 'F') service the MNPs 2001:db8:1000:2000::/56, 886 2001:db8:3000:4000::/56 and 2001:db8:5000:6000::/56 then the routing 887 system would include: 889 D: [ULA*]:2001:db8:1000:2000/120 891 E: [ULA*]:2001:db8:3000:4000/120 893 F: [ULA*]:2001:db8:5000:6000/120 895 A full discussion of the BGP-based routing system used by AERO is 896 found in [I-D.ietf-rtgwg-atn-bgp]. 898 3.2.4. OMNI Link Segment Routing 900 With the Client and SRT segment prefixes in place in Bridge 901 forwarding tables, the OMNI interface sends control and data carrier 902 packets toward AERO destination nodes located in different OMNI link 903 segments over the SRT spanning tree. The OMNI interface uses the 904 OMNI Adaptation Layer (OAL) encapsulation service 905 [I-D.templin-6man-omni], and includes an OMNI Routing Header (ORH) as 906 an OAL header extension. Each carrier packet includes at most one 907 ORH in compressed or uncompressed form (with the uncompressed form 908 shown in Figure 3): 910 0 1 2 3 911 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 1 912 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 913 | Next Header | Hdr Ext Len | Routing Type | Segments Left | 914 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 915 | omIndex | FMT | SRT | LHS (bits 0 - 15) | 916 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 917 | LHS (bits 16 - 31) | ~ 918 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ 919 ~ Link Layer Address (L2ADDR) ~ 920 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 921 | Null Padding (if necessary) | 922 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 923 ~ Destination Trailer ~ 924 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 926 Figure 3: OMNI Routing Header (ORH) Format 928 The ORH includes the following fields, in consecutive order: 930 o Next Header identifies the type of header immediately following 931 the ORH. 933 o Hdr Ext Len is the length of the Routing header in 8-octet units 934 (not including the first 8 octets). The field must encode a value 935 between 0 and 4 (all other values indicate a parameter problem). 937 o Routing Type is set to TBD1 (see IANA Considerations). 939 o Segments Left encodes the value 0 or 1 (all other values indicate 940 a parameter problem). 942 o omIndex - a 1-octet field that informs the LHS Proxy/Server of a 943 specific Client underlying interface when there are multiple 944 alternatives. When FMT-Forward is clear, omIndex determines the 945 interface for forwarding the ORH packet following reassembly; when 946 FMT-Forward is set, omIndex determines the interface for 947 forwarding the raw carrier packets without first reassembling. 948 When omIndex is 0 (or when no ORH is present), the LHS Proxy/ 949 Server selects among any of the Client's available underlying 950 interfaces that it services locally (i.e., and not those serviced 951 by another Proxy/Server). 953 o FMT - a 3-bit "Forward/Mode/Trailer" code corresponding to the 954 included Link Layer Address as follows: 956 * When the most significant bit (i.e., "FMT-Forward") is clear, 957 the LHS Proxy/Server must reassemble. When FMT-Forward is set, 958 the LHS Proxy/Server must forward the fragments to the Client 959 (while changing the OAL destination address to the MNP-ULA of 960 the Client if necessary) without reassembling. 962 * When the next most significant bit (i.e., "FMT-Mode") is clear, 963 L2ADDR is the INET address of the LHS Proxy/Server and the 964 Client must be reached through the LHS Proxy/Server. When FMT- 965 Mode is set, the Client is eligible for route optimization over 966 the open INET where it may be located behind one or more NATs, 967 and L2ADDR is either the INET address of the LHS Proxy/Server 968 (when FMT-Forward is set) or the native INET address of the 969 Client itself (when FMT-Forward is clear). 971 * The least significant bit (i.e., "FMT-Type") is consulted only 972 when Hdr Ext Len is 1 and ignored otherwise. If FMT-Type is 973 clear, the remainder includes an LHS and L2ADDR for IPv4. If 974 FMT-Type is set, the remainder instead includes two null 975 padding octets followed by a Destination Trailer. 977 o SRT - a 5-bit Segment Routing Topology prefix length consulted 978 only when Segments Left is 1, and encodes a value that (when added 979 to 96) determines the prefix length to apply to the ADM-ULA formed 980 from concatenating [ULA*]::/96 with the 32 bit LHS value (for 981 example, the value 16 corresponds to the prefix length 112). 983 o LHS - a 4-octet field consulted only when Segments Left is 1. The 984 field encodes the 32-bit ADM-ULA suffix of an LHS Proxy/Server for 985 the target. When SRT is non-zero, SRT and LHS together determine 986 the ADM-ULA of the LHS Proxy/Server over the spanning tree. When 987 SRT is 0, LHS instead encodes a Peer Index value meaningful to the 988 LHS Bridge. 990 o Link Layer Address (L2ADDR) - an IP encapsulation address 991 consulted only when Segments Left is 1. The L2ADDR IP version is 992 determined by the ORH length, since L2ADDR will always contain 993 exactly 6 octets for UDP/IPv4 or 18 octets for UDP/IPv6. When 994 present, provides the link-layer address (i.e., the encapsulation 995 address) of the LHS Proxy/Server or the target Client itself. The 996 UDP Port Number appears in the first two octets and the IP address 997 appears in the remaining octets. The Port Number and IP address 998 are recorded in network byte order, and in ones-compliment 999 "obfuscated" form per [RFC4380]. The OMNI interface forwarding 1000 algorithm uses L2ADDR as the INET encapsulation address for 1001 forwarding when SRT/LHS is located in the local OMNI link segment. 1002 If direct INET encapsulation is not indicated, L2ADDR is instead 1003 set to all-zeros and the SRT/LHS fields are consulted for LHS 1004 forwarding. 1006 o Null Padding - zero-valued octets added as necessary to pad the 1007 portion of the ORH included up to this point to an even 8-octet 1008 boundary. 1010 o Destination Trailer - a trailing 8-octet field present only when 1011 indicated by the ORH length and FMT-Type (see below) and encodes 1012 the 64-bit ULA suffix for the target. 1014 The ORH Hdr Ext Len field value also serves as an implicit ORH 1015 "Type", with 5 distinct Types possible (i.e., ORH-0 through ORH-4). 1016 All ORH-* Types include the same 6-octet preamble beginning with Next 1017 Header up to and including FMT/SRT, followed by a Type-specific 1018 remainder as follows: 1020 o ORH-0 - The preamble Hdr Ext Len and Segments Left must both be 0. 1021 The remainder includes two null padding octets, and all other 1022 fields are omitted. 1024 o ORH-1 - The preamble Hdr Ext Len is set to 1. If FMT Type is 1025 clear, the remainder includes an LHS and L2ADDR for IPv4. If FMT 1026 Type is set, the remainder includes two null padding octets 1027 followed by Destination Trailer, and all other fields are omitted. 1029 o ORH-2 - The preamble Hdr Ext Len is set to 2. LHS, L2ADDR for 1030 IPv4 and Destination Trailer are included. 1032 o ORH-3 - The preamble Hdr Ext Len is set to 3. LHS and L2ADDR for 1033 IPv6 are included followed by four null padding octets, and 1034 Destination Trailer is omitted. 1036 o ORH-4 - The preamble Hdr Ext Len is set to 4. LHS and L2ADDR for 1037 IPv6 are included followed by four null padding octets followed by 1038 Destination Trailer. 1040 AERO neighbors use OAL encapsulation and fragmentation to exchange 1041 OAL packets as specified in [I-D.templin-6man-omni]. When an AERO 1042 node's OMNI interface (acting as an OAL source) uses OAL 1043 encapsulation for an original IP packet with source address 1044 2001:db8:1:2::1 and destination address 2001:db8:1234:5678::1, it 1045 sets the OAL header source address to its own ULA (e.g., 1046 [ULA*]::2001:db8:1:2), sets the destination address to the MNP-ULA 1047 corresponding to the IP destination address (e.g., 1048 [ULA*]::2001:db8:1234:5678), sets the Traffic Class, Flow Label, Hop 1049 Limit and Payload Length as discussed in [I-D.templin-6man-omni], 1050 then finally selects an Identification and appends an OAL checksum. 1052 If the neighbor cache information indicates that the target is in a 1053 different segment, the OAL source next inserts an ORH immediately 1054 following the OAL header while including Destination Trailer with a 1055 ULA suffix only when necessary (in this case, the ULA suffix is 1056 2001:db8:1234:5678). Next, to direct the packet to an LHS Bridge, 1057 the source prepares an ORH with Segments Left set to 1 and with LHS/ 1058 L2ADDR included, then overwrites the OAL header destination address 1059 with the LHS Subnet Router Anycast address (for example, for LHS 1060 3000:4567 with SRT 16, the Subnet Router Anycast address is 1061 [ULA*]::3000:0000). To direct the packet to the LHS Proxy/Server 1062 (either directly in the local segment or in a remote segment via the 1063 spanning tree), the OAL source instead includes an ORH (Type 0 or 1) 1064 with Segments Left set to 0, and overwrites the OAL header 1065 destination address with either the LHS Proxy/Server ADM-ULA or the 1066 MNP-ULA of the Client itself. 1068 The OAL source then fragments the OAL packet, with each resulting OAL 1069 fragment including the OAL/ORH headers while only the first fragment 1070 includes the original IPv6 header. If FMT-Forward is set, the 1071 Identification used for fragmentation must be within the window for 1072 the Client and a Destination Trailer with the target ULA suffix must 1073 be included when necessary; otherwise the Identification must be 1074 within the window for the LHS Proxy/Server and no Destination Trailer 1075 is needed. (Note that if no actual fragmentation is required the OAL 1076 source still prepares the packet as an "atomic" fragment that 1077 includes a Fragment Header with Offset and More Fragments both set to 1078 0.) The OAL source finally encapsulates each resulting OAL fragment 1079 in an *NET header to form an OAL carrier packet, with source address 1080 set to its own *NET address (e.g., 192.0.2.100) and destination set 1081 to the *NET address of the last hop itself or the next hop in the 1082 spanning tree (e.g., 192.0.2.1). 1084 The carrier packet encapsulation format in the above example is shown 1085 in Figure 4: 1087 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1088 | *NET Header | 1089 | src = 192.0.2.100 | 1090 | dst = 192.0.2.1 | 1091 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1092 | OAL IPv6 Header | 1093 | src = [ULA*]::2001:db8:1:2 | 1094 | dst= [ULA*]::3000:0000 | 1095 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1096 | ORH (if necessary) | 1097 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1098 | OAL Fragment Header | 1099 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1100 | Original IP Header | 1101 | (first-fragment only) | 1102 | src = 2001:db8:1:2::1 | 1103 | dst = 2001:db8:1234:5678::1 | 1104 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1105 | | 1106 ~ ~ 1107 ~ Original Packet Body/Fragment ~ 1108 ~ ~ 1109 | | 1110 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1112 Figure 4: Carrier Packet Format 1114 In this format, the original IP header and packet body/fragment are 1115 encapsulated in an OAL IPv6 header prepared according to [RFC2473], 1116 the ORH is a Routing Header extension of the OAL header, the Fragment 1117 Header identifies each fragment, and the *NET header is prepared as 1118 discussed in Section 3.6. The OAL source transmits each such carrier 1119 packet into the SRT spanning tree, where they are forwarded over 1120 possibly multiple OAL intermediate nodes until they arrive at the OAL 1121 destination. 1123 The SRT forwarding system therefore contains both Client MNP-ULA 1124 prefix information that may change dynamically due to regional node 1125 mobility and per-segment ADM-ULA prefix information that rarely if 1126 ever changes. The SRT forwarding system therefore supports link- 1127 layer bridging through carrier packet forwarding over the spanning 1128 tree according to link-layer information instead of network-layer 1129 routing according to MNP routes. As a result, opportunities for loss 1130 due to node mobility between different segments are mitigated. 1132 Note: The document recommends that AERO nodes transform ORHs with 1133 Segments Left set to 1 into ORH-0/1 during forwarding. While this 1134 may yield encapsulation overhead savings in some cases, the AERO node 1135 can instead simply decrement Segments Left and leave the original ORH 1136 in place. The LHS Proxy/Server or destination Client that processes 1137 the ORH will receive the same information in both cases. 1139 Note: When the OAL source sets a carrier packet destination address 1140 to a target's MNP-ULA but does not assert a specific target 1141 underlying interface, it may omit the ORH whether forwarding to the 1142 LHS Proxy/Server or directly to the target itself. When the LHS 1143 Proxy/Server receives a carrier packet with OAL destination set to 1144 the target MNP-ULA but with no ORH, it forwards over any available 1145 underlying interface for the target that it services locally. 1147 Note: When the OAL source and destination are on the same INET 1148 segment, OAL header compression can be used to significantly reduce 1149 encapsulation overhead as discussed in [I-D.templin-6man-omni]. 1151 Note: Use of an IPv6 "minimal encapsulation" format (i.e., an IPv6 1152 variant of [RFC2004]) based on extensions to the ORH was considered 1153 and abandoned. In the approach, the ORH would be inserted as an 1154 extension header to the original IPv6 packet header. The IPv6 1155 destination address would then be written into the ORH, and the ULA 1156 corresponding to the destination would be overwritten in the IPv6 1157 destination address. This would seemingly convey enough forwarding 1158 information so that OAL encapsulation could be avoided. However, 1159 this "minimal encapsulation" IPv6 packet would then have a non-ULA 1160 source address and ULA destination address, an incorrect value in 1161 upper layer protocol checksums, and a Hop Limit that is decremented 1162 within the spanning tree when it should not be. The insertion and 1163 removal of the ORH would also entail rewriting the Payload Length and 1164 Next Header fields - again, invalidating upper layer checksums. 1165 These irregularities would result in implementation challenges and 1166 the potential for operational issues, e.g., since actionable ICMPv6 1167 error reports could not be delivered to the original source. In 1168 order to address the issues, still more information such as the 1169 original IPv6 source address could be written into the ORH. However, 1170 with the additional information the benefit of the "minimal 1171 encapsulation" savings quickly diminishes, and becomes overshadowed 1172 by the implementation and operational irregularities. 1174 3.2.5. Segment Routing Topologies (SRTs) 1176 The 64-bit sub-prefixes of [ULA]::/48 identify up to 2^16 distinct 1177 Segment Routing Topologies (SRTs). Each SRT is a mutually-exclusive 1178 OMNI link overlay instance using a distinct set of ULAs, and emulates 1179 a Virtual LAN (VLAN) service for the OMNI link. In some cases (e.g., 1180 when redundant topologies are needed for fault tolerance and 1181 reliability) it may be beneficial to deploy multiple SRTs that act as 1182 independent overlay instances. A communication failure in one 1183 instance therefore will not affect communications in other instances. 1185 Each SRT is identified by a distinct value in bits 48-63 of 1186 [ULA]::/48, i.e., as [ULA0]::/64, [ULA1]::/64, [ULA2]::/64, etc. 1187 Each OMNI interface is identified by a unique interface name (e.g., 1188 omni0, omni1, omni2, etc.) and assigns an anycast ADM-ULA 1189 corresponding to its SRT prefix length. The anycast ADM-ULA is used 1190 for OMNI interface determination in Safety-Based Multilink (SBM) as 1191 discussed in [I-D.templin-6man-omni]. Each OMNI interface further 1192 applies Performance-Based Multilink (PBM) internally. 1194 The Bridges and Proxy/Servers of each independent SRT engage in BGP 1195 peerings to form a spanning tree with the Bridges in non-leaf nodes 1196 and the Proxy/Servers in leaf nodes. The spanning tree is configured 1197 over both secured and unsecured underlying network paths. The 1198 secured spanning tree is used to convey secured control messages 1199 between FHS and LHS Proxy/Servers, while the unsecured spanning tree 1200 forwards data messages and/or unsecured control messages. 1202 Each SRT segment is identified by a unique ADM-ULA prefix used by all 1203 Proxy/Servers and Bridges in the segment. Each AERO node must 1204 therefore discover an SRT prefix that correspondents can use to 1205 determine the correct segment, and must publish the SRT prefix in 1206 IPv6 ND messages and carrier packet ORHs. 1208 3.2.6. Segment Routing For OMNI Link Selection 1210 Original IPv6 source can direct IPv6 packets to an AERO node by 1211 including a standard IPv6 Segment Routing Header (SRH) [RFC8754] with 1212 the anycast ADM-ULA for the selected OMNI link as either the IPv6 1213 destination or as an intermediate hop within the SRH. This allows 1214 the original source to determine the specific OMNI link SRT an 1215 original IPv6 packet will traverse when there may be multiple 1216 alternatives. 1218 When an AERO node processes the SRH and forwards the original IPv6 1219 packet to the correct OMNI interface, the OMNI interface writes the 1220 next IPv6 address from the SRH into the IPv6 destination address and 1221 decrements Segments Left. If decrementing would cause Segments Left 1222 to become 0, the OMNI interface deletes the SRH before forwarding. 1223 This form of Segment Routing supports Safety-Based Multilink (SBM). 1225 3.2.7. Segment Routing Within the OMNI Link 1227 OAL sources can insert an ORH for Segment Routing within the same 1228 OMNI link to influence the paths of carrier packets sent to OAL 1229 destinations in remote SRT segments without requiring all carrier 1230 packets to traverse strict SRT spanning tree paths. (OAL sources can 1231 also insert an ORH in carrier packets sent to OAL destinations in the 1232 local segment if additional last-hop forwarding information is 1233 required.) After segment routing has established spanning tree soft 1234 state, OAL nodes can employ header compression for most carrier 1235 packets sent to peers in the same SRT segment to greatly reduce 1236 encapsulation overhead. 1238 When a Route Optimization Source (ROS) has an original IP packet to 1239 send to a target located in the same SRT segment, it places the 1240 packet on a short queue and initiates window synchronization. The 1241 ROS prepares an NS(WIN) (see: Section 3.5.1 and Section 3.14.6 with 1242 its own LLA as the source. If FMT-Forward is clear, the ROS sets the 1243 destination to the ADM-LLA of the LHS Proxy/Server; otherwise, it 1244 sets the destination to the MNP-LLA of the target Client. The ROS 1245 then include an OMNI Interface Attributes option for the underlying 1246 interface of the source Client. (The ROS must ensure that the 1247 NS(WIN) is no larger than the maximum payload size for OAL fragments, 1248 since multi-fragment control message authentication and integrity 1249 cannot be assured for all targets.) 1251 If the ROS is also the source Client, it then performs OAL 1252 encapsulation with an ORH with Segments Left set to 1 with 1253 FMT/SRT/LHS/L2ADDR information for the NS(WIN) target while setting 1254 the OAL source to its own MNP-ULA. The Client then sets the OAL 1255 destination to the ADM-ULA of an FHS Proxy/Server and includes an 1256 authentication signature if necessary then forwards the "atomic 1257 fragment" carrier packet to the FHS Proxy/Server. The FHS Proxy/ 1258 Server verifies the signature if necessary, then rewrites the 1259 Interface Attributes with FMT/SRT/LHS/L2ADDR information for its own 1260 INET interface, transforms the ORH into an ORH-0/1, sets the OAL 1261 destination address to the ADM-ULA of the LHS Proxy/Server then 1262 forwards the carrier packet into the secured spanning tree. (If the 1263 ROS is also the FHS Proxy/Server, it instead prepares the NS(WIN) 1264 itself using its own ADM-LLA as the source and forwards the same as 1265 above.) 1267 When the LHS Proxy/Server receives the NS(WIN) in a carrier packet 1268 with its own ADM-LLA as the destination, it caches the Interface 1269 Attributes and window syncrhonization information and returns an 1270 NA(WIN) (with the LLAs from the NS(WIN) reversed) to the ADM-ULA of 1271 the FHS Proxy/Server via the secured spanning tree (where the NA(WIN) 1272 again must fit within a single atomic fragment carrier packet). When 1273 the LHS Proxy/Server receives an NS(WIN) with an MNP-LLA destination, 1274 it instead locates the NCE corresponding to the MNP-LLA and creates a 1275 Peer List entry for the NS(WIN) source LLA NCE while also caching the 1276 OMNI Interface Attributes information and assigning a Peer Index for 1277 this (NCE, Peer List entry)-tuple. The LHS Proxy/Server then 1278 includes an ORH with SRT set to 0, with LHS set to this Peer Index, 1279 and with L2ADDR set to its INET address. The LHS Proxy/Server then 1280 includes an authentication signature if necessary, changes the OAL 1281 destination to the target MNP-ULA, performs INET encapsulation and 1282 forwards the carrier packet to the target Client. 1284 When the target Client receives the (carrier packet / NS(WIN)), it 1285 verifies the signature then creates or updates an NCE for the NS(WIN) 1286 source LLA while caching window synchronization information and the 1287 (Peer Index, L2ADDR) for the LHS Proxy/Server (and possibly also the 1288 LHS Bridge). The ORH includes the (Peer Index, L2ADDR) information 1289 for the LHS Proxy/Server, while the OMNI Interface Attributes 1290 includes the (Peer Index, L2ADDR) for the LHS Bridge when SRT is 0. 1291 When the target Client forwards future carrier packets with 1292 compressed headers to the ROS, it must use the correct (Peer Index, 1293 L2ADDR) information for the next hop (i..e, the Bridge vs the Proxy/ 1294 Server). The target Client then prepares an NA(WIN) (with the LLAs 1295 from the NS(WIN) reversed), with Interface Attributes for its 1296 underlying interface and with an authentication signature. The 1297 Client then sets the OAL source to its own MNP-ULA and OAL 1298 destination to the ADM-ULA of the LHS Proxy/Server, includes an OMNI 1299 Interface Attributes option with FMT/SRT/LHS/L2ADDR information for 1300 its own underlying interface as well as an ORH with FMT/SRT/LHS/ 1301 L2ADDR information copied from the NS(WIN) OMNI Interface Attributes 1302 then forwards the message to the LHS Proxy/Server. The LHS Proxy/ 1303 Server rewrites the OMNI Interface Attributes FMT/SRT/LHS/L2ADDR 1304 information, changes the ORH to an ORH-0/1, sets the OAL destination 1305 to the ADM-ULA of the FHS Proxy/Server, then forwards the carrier 1306 packet into the secured spanning tree. 1308 When the FHS Proxy/Server receives an NA(WIN) with its own ADM-LLA as 1309 the destination, it processes the message locally. If the NA(WIN) 1310 destination is an MNP-LLA, the FHS Proxy/Server instead forwards the 1311 message to the ROS Client. In the process, the FHS Proxy/Server 1312 locates the NCE for the MNP-LLA and creates a Peer List entry for the 1313 NA(WIN) source LLA while also caching the OMNI Interface Attributes 1314 and assigning a Peer Index for this (NCE, Peer List entry)-tuple the 1315 same as the LHS Proxy/Server. The FHS Proxy/Server then includes an 1316 ORH with SRT set to 0, LHS set to the new Peer Index, and L2ADDR set 1317 to its own INET address, then incudes an authentication signature if 1318 necessary, changes the OAL destination to the MNP-ULA and forwards 1319 the message to the ROS. When either the ROS Client or FHS Proxy/ 1320 Server processes the NA(WIN), it creates or updates an NCE for the 1321 NA(WIN) source LLA while caching window synchronization information, 1322 (Peer Index, L2ADDR) for the Proxy Server (and possibly also the 1323 Bridge) and OMNI Interface Attributes. The ROS then prepares an 1324 NA(WIN) acknowledgement (with the LLAs from the original NA(WIN) 1325 reversed) and forwards it back to the target to complete the third 1326 leg of the window syncrhonization handshake. (Procedures for 1327 preparing and forwarding the NA(WIN) acknowledgement are identical to 1328 those for the initial NA(WIN) above except with the LHS Proxy/Server 1329 as the destination.) 1331 Following window synchronzation, either peer can refresh window 1332 synchronization parameters and/or send other carrier packets 1333 requiring security at any time using the same secured procedures 1334 described above. The peers can also begin exchanging ordinary 1335 carrier packets with Identification values within their respective 1336 send/receive windows using INET encapsulation and OAL header 1337 compression when possible (see below). The source peer sets the OAL 1338 destination to the ADM-ULA of the target Proxy/Server if FMT-Forward 1339 is clear; otherwise, it sets the OAL destination to the MNP-ULA of 1340 the target Client to cause the target Proxy/Server to forward without 1341 reassembling. The source peer also includes an ORH-0/1 with omIndex 1342 set to select a specific target Client underlying interface; 1343 otherwise, it sets omIndex to 0 (or omits the ORH entirely) when no 1344 preference is selected. The source peer finally encapsulates the 1345 carrier packets in INET headers with destination addresses set 1346 according to the target Proxy/Server L2ADDR information. (If the 1347 target peer is the Client itself and FMT-Mode is set, the source peer 1348 can instead forward initial packets via the Proxy/Server then begin 1349 forwarding directly according to the target Client L2ADDR information 1350 discovered through NAT traversal procedures.) 1352 When an ROS has an original IP packet to send to a route optimization 1353 target located in a remote SRT segment, it places the packet on a 1354 short queue and initiates wndow synchronization the same as above. 1355 The ROS then prepares an NS(WIN) and performs OAL encapsulation with 1356 an ORH with Segments Left set to 1 while setting the OAL source to 1357 its own ULA and including an ORH with Segments Left set to 1 and with 1358 FMT/SRT/LHS/L2ADDR information appropriate for the target. If the 1359 ROS is a Client, it sets the OAL destination to the ADM-ULA of a FHS 1360 Proxy/Server and includes an authentication signature if necessary 1361 then forwards the carrier packet to the FHS Proxy/Server. The FHS 1362 Proxy/Server verifies the signature if necessary, then rewrites the 1363 Interface Attributes with FMT/SRT/LHS/L2ADDR information for its own 1364 INET interface and sets the OAL destination address to the Subnet 1365 Router Anycast address for the local SRT segment. The FHS Proxy/ 1366 Server then forwards the carrier packet into the secured spanning 1367 tree which conveys it to the nearest FHS Bridge. 1369 Bridges forward carrier packets not explicitly addressed to 1370 themselves via the spanning tree. For carrier packets that arrived 1371 via the secured spanning tree, the Bridge forwards to the next hop 1372 also via the secured spanning tree. For other carrier packets, the 1373 Bridge forwards to the next hop via the unsecured spanning tree. 1374 When an FHS Bridge receives a carrier packet over the secured 1375 spanning tree destined to its ADM-ULA or the FHS Subnet Router 1376 Anycast address, it processes the packet locally. If the packet 1377 contains an NS(WIN), the FHS Bridge creates or updates a NCE for the 1378 NS(WIN) source LLA and caches the NS(WIN) destination LLA and window 1379 synchronization information in the Peer List for this NCE while also 1380 selecting a unique Peer Index value for this new (NCE, Peer List 1381 entry)-tuple. The FHS Bridge next caches the omIndex and 1382 FMT/SRT/LHS/L2ADDR information found in the OMNI Interface Attributes 1383 as FHS forwarding infomation for the NCE, and caches the omIndex and 1384 FMT/SRT/LHS/L2ADDR information found in the ORH as LHS forwarding 1385 information for the Peer List entry. The FHS Bridge then rewrites 1386 the Interface Attributes SRT/LHS to its own ADM-ULA then re- 1387 encapsulates the NS(WIN) with OAL destination address set to the LHS 1388 Subnet Router Anycast address (found in the ORH SRT/LHS) and forwards 1389 the carrier packet into the secured spanning tree. 1391 When an LHS Bridge receives a carrier packet over the secured 1392 spanning tree destined to its ADM-ULA or the LHS Subnet Router 1393 Anycast address, it processes the packet locally. If the packet 1394 contains an NS(WIN), the LHS Bridge creates or updates a NCE for the 1395 NS(WIN) destination LLA and caches the NS(WIN) source LLA and window 1396 synchronization information in the Peer List for this NCE while also 1397 selecting a unique Peer Index value for this new (NCE, Peer List 1398 entry)-tuple. The LHS Bridge also caches the OMNI option Inteface 1399 Attributes parameters as FHS forwarding information for this Peer 1400 List entry, and caches the ORH parameters as LHS forwarding 1401 information for this NCE. THe LHS Bridge then rewrites the Interface 1402 Attributes SRT/LHS to its own ADM-ULA then re-encapsulates the 1403 NS(WIN) with OAL destination address set to the ADM-ULA of the LHS 1404 Proxy/Server and forwards the carrrier packet into the secured 1405 spanning tree. 1407 When the LHS Proxy/Server receives the carrier packet via the secured 1408 spanning tree, it proceeds the same as for the local segment case 1409 above. Specifically, if the NS(WIN) destination is its own ADM-LLA, 1410 the LHS Proxy/Server prepares an NA(WIN)) and forwards the resulting 1411 carrier packet to the ADM-ULA of the LHS Bridge (found in the NS(WIN) 1412 Interface Attributes) via the secured spanning tree. If the NS(WIN) 1413 destination is the MNP-LLA of a local Client, the LHS Proxy/Server 1414 instead re-encapsulates and forwards the resulting carrier packet to 1415 the local Client. The local Client processes the NS(WIN) and returns 1416 an NA(WIN) the same as describe above, including the necessary OMNI 1417 Interface Attributes and ORH. The LHS Proxy/Server in turn proxies 1418 the NA(WIN) Interface Attributes, re-encapsulates, and forwards the 1419 resulting carrier packet to the ADM-ULA of the LHS Bridge. 1421 Following the initial NS(WIN), both Bridges have established state 1422 and are prepared to process subsequent NA(WIN) messages in a 1423 symmetric fashion. When either Bridge receives an NA(WIN) over the 1424 secured spanning tree destined to its ADM-ULA, it locates the NCE for 1425 the NA(WIN) source LLA then locates the Peer List entry for the 1426 NA(WIN) destination LLA. The Bridge then forwards the NA(WIN) over 1427 the secured spanning tree to the ADM-ULA of the peer Bridge found in 1428 the forwarding information cached in the Peer List entry. The peer 1429 Bridge finally forwards the NA(WIN) to the ADM-ULA of the local 1430 segment Proxy/Server over the secured spanning tree while rewriting 1431 OMNI Interface Attributes with the value 0 in SRT, the Peer Index in 1432 LHS and its own INET information in L2ADDR. In the process, both 1433 Bridges also update their window synchronization state based on the 1434 NA(WIN) information. When the NA(WIN) message finally arrives at the 1435 ROS, the ROS caches the Peer Index and L2ADDR information for this 1436 Bridge in the corresponding (NCE, Peer List entry)-tuple and responds 1437 by returning an NA(WIN) acknolwedgement the same as described above 1438 to complete the three way window synchronization handshake. 1440 Following window synchronization, each peer forwards ordinary carrier 1441 packets directly via their respective FHS Bridges instead of via an 1442 FHS Proxy/Server. The source peer employs OAL header compression as 1443 specified in [I-D.templin-6man-omni] by including a minimal header 1444 that encodes the Peer Index published in the FHS Bridge NA(WIN) 1445 Interface Attributes. When the FHS Bridge receives the carrier 1446 packets, it locates the NCE corresponding to the Peer Index. The FHS 1447 Bridge next examines the Identification and processes the carrier 1448 packet further only if the Identification is within the receive 1449 window for this target peer (otherwise, it drops the carrier packet). 1450 The FHS Bridge next reconstructs the full OAL header based on the 1451 (NCE, Peer List Entry) information, sets the OAL source to the ULA of 1452 the source peer, sets the OAL destination to the ADM-ULA of the LHS 1453 Bridge and includes an ORH with Destination Trailer set to the target 1454 peer ULA suffix. The FHS Bridge then forwards the carrier packet via 1455 the unsecured spanning tree. 1457 When the LHS Bridge receives the carrier packet addressed to its ADM- 1458 ULA, it locates the (NCE, Peer List entry)-tuple corresponding to the 1459 ORH Destination Trailer ULA suffix and OAL source, respectively. The 1460 LHS Bridge next examines FMT-Mode to determine whether the target 1461 peer can accept packets directly (i.e., following any NAT traversal 1462 procedures necessary) while bypassing the LHS Proxy/Server. If the 1463 taget peer can be reached directly and NAT traversal has converged, 1464 the LHS Bridge then employs header compression while including Peer 1465 Index for the (NCE, Peer List entry)-tuple, encapsulates the carrier 1466 packet according to the NATed L2ADDR information then forwards the 1467 carrier packet directly to the target peer. If the target peer 1468 cannot be reached directly (or if NAT traversal has not yet 1469 converged), the LHS Bridge instead performs header compression while 1470 including Peer Index, encapsulates the packet according to the cached 1471 L2ADDR information for the LHS Proxy/Server then forwards the carrier 1472 packet directly to the LHS Proxy/Server. 1474 When an LHS Proxy/Server receives carrier packets with any ORH type 1475 with Segments Left set to 0 and with OAL destination set to its own 1476 ADM-ULA, the LHS Proxy/Server verifies that the Identification is 1477 within its receive window for this source peer then proceeds 1478 according to FMT-Forward and omIndex, while including an 1479 authentication signature if necessary. If FMT-Forward is set, the 1480 LHS Proxy/Server changes the OAL destination to the MNP-ULA of the 1481 target Client found in the ORH Destination Trailer, removes the ORH 1482 and forwards to the target Client interface identified by omIndex. 1483 If FMT-Forward is clear, the LHS Proxy/Server instead reassembles 1484 then re-encapsulates while refragmenting if necessary, removes the 1485 ORH and forwards to the target Client according to omIndex. 1487 When an LHS Proxy/Server receives carrier packets with any ORH type 1488 with Segments Left set to 0 and with OAL destination set to the MNP- 1489 ULA of the target Client, it verifies that the Identification is 1490 within the receive window for this source peer then removes the ORH 1491 and forwards to the target Client according to omIndex. During 1492 forwarding, the LHS Proxy/Server must first verify that the omIndex 1493 corresponds to a target underlying interface that it services locally 1494 and must not forward to other target underlying interfaces. If 1495 omIndex is 0 (or if no ORH is included) the LHS Proxy/Server instead 1496 selects among any of the target underlying interfaces it services. 1498 When an LHS Proxy/Server receives carrier packets with compressed 1499 headers, it first locates the (NCE, Peer List entry)-tuple 1500 corresponding to the Peer Index and LHS Bridge INET address and 1501 verifies that the Identification is acceptable. If the (NCE, Peer 1502 List entry)-tuple indicates that the target peer is the LHS Proxy/ 1503 Server, it reconstitutes the OAL headers, reassembles, decapsulates 1504 then forwards the original IP packet (while re-encapsulating and re- 1505 fragmenting if necessary). If the (NCE Peer List entry)-tuple 1506 indicates that the target peer is a local Client, the LHS Proxy/ 1507 Server instead forwards the packet to the INET destination address of 1508 the local Client while writing its own INET address as the source and 1509 its own Peer Index in the compressed header. 1511 When a target Client receives carrier packets with OAL destination 1512 set to is MNP-ULA, it first verifies that the Identification is 1513 acceptable then reassembles to obtain the OAL packet then 1514 decapsulates and delivers the original IP packet to upper layers. 1515 When a target Client receives carrier packets with a compressed 1516 header, it examines the INET source address. If the source was the 1517 Bridge, the Client locates the (NCE, Peer List entry)-tuple based on 1518 the Bridge INET address and Peer Index; if the source was a Proxy/ 1519 Server, the Client instead locates the (NCE, Peer List entry)-tuple 1520 based on the Proxy/Server INET address and Peer Index. The Client 1521 next verifies that the Identification is acceptable, then 1522 reconstructs the OAL header, reassembles to obtain the OAL packet, 1523 then decapsulates and delivers the IP packet to upper layers. 1525 3.3. OMNI Interface Characteristics 1527 OMNI interfaces are virtual interfaces configured over one or more 1528 underlying interfaces classified as follows: 1530 o INET interfaces connect to an INET either natively or through one 1531 or more NATs. Native INET interfaces have global IP addresses 1532 that are reachable from any INET correspondent. The INET-facing 1533 interfaces of Proxy/Servers are native interfaces, as are Relay 1534 and Bridge interfaces. NATed INET interfaces connect to a private 1535 network behind one or more NATs that provide INET access. Clients 1536 that are behind a NAT are required to send periodic keepalive 1537 messages to keep NAT state alive when there are no carrier packets 1538 flowing. 1540 o ANET interfaces connect to an ANET that is separated from the open 1541 INET by an FHS Proxy/Server. Clients can issue control messages 1542 over the ANET without including an authentication signature since 1543 the ANET is secured at the network layer or below. Proxy/Servers 1544 can actively issue control messages over the INET on behalf of 1545 ANET Clients to reduce ANET congestion. 1547 o VPNed interfaces use security encapsulation over the INET to a 1548 Virtual Private Network (VPN) server that also acts as an FHS 1549 Proxy/Server. Other than the link-layer encapsulation format, 1550 VPNed interfaces behave the same as Direct interfaces. 1552 o Direct (i.e., single-hop point-to-point) interfaces connect a 1553 Client directly to an FHS Proxy/Server without crossing any ANET/ 1554 INET paths. An example is a line-of-sight link between a remote 1555 pilot and an unmanned aircraft. The same Client considerations 1556 apply as for VPNed interfaces. 1558 OMNI interfaces use OAL encapsulation and fragmentation as discussed 1559 in Section 3.2.4. OMNI interfaces use *NET encapsulation (see: 1560 Section 3.6) to exchange carrier packets with OMNI link neighbors 1561 over INET or VPNed interfaces as well as over ANET interfaces for 1562 which the Client and FHS Proxy/Server may be multiple IP hops away. 1563 OMNI interfaces do not use link-layer encapsulation over Direct 1564 underlying interfaces or ANET interfaces when the Client and FHS 1565 Proxy/Server are known to be on the same underlying link. 1567 OMNI interfaces maintain a neighbor cache for tracking per-neighbor 1568 state the same as for any interface. OMNI interfaces use ND messages 1569 including Router Solicitation (RS), Router Advertisement (RA), 1570 Neighbor Solicitation (NS) and Neighbor Advertisement (NA) for 1571 neighbor cache management. In environments where spoofing may be a 1572 threat, OMNI neighbors should employ OAL Identification window 1573 synchronization in their ND message exchanges. 1575 OMNI interfaces send ND messages with an OMNI option formatted as 1576 specified in [I-D.templin-6man-omni]. The OMNI option includes 1577 prefix registration information, Interface Attributes containing link 1578 information parameters for the OMNI interface's underlying interfaces 1579 and any other per-neighbor information. Each OMNI option may include 1580 multiple Interface Attributes sub-options identified by omIndex 1581 values. 1583 A Client's OMNI interface may be configured over multiple underlying 1584 interfaces. For example, common mobile handheld devices have both 1585 wireless local area network ("WLAN") and cellular wireless links. 1586 These links are often used "one at a time" with low-cost WLAN 1587 preferred and highly-available cellular wireless as a standby, but a 1588 simultaneous-use capability could provide benefits. In a more 1589 complex example, aircraft frequently have many wireless data link 1590 types (e.g. satellite-based, cellular, terrestrial, air-to-air 1591 directional, etc.) with diverse performance and cost properties. 1593 If a Client's multiple underlying interfaces are used "one at a time" 1594 (i.e., all other interfaces are in standby mode while one interface 1595 is active), then successive ND messages all include OMNI option 1596 Interface Attributes sub-options with the same underlying interface 1597 index. In that case, the Client would appear to have a single 1598 underlying interface but with a dynamically changing link-layer 1599 address. 1601 If the Client has multiple active underlying interfaces, then from 1602 the perspective of ND it would appear to have multiple link-layer 1603 addresses. In that case, ND message OMNI options MAY include 1604 Interface Attributes sub-options with different underlying interface 1605 indexes. Every ND message need not include Interface Attributes for 1606 all underlying interfaces; for any attributes not included, the 1607 neighbor considers the status as unchanged. 1609 Bridge and Proxy/Server OMNI interfaces are configured over 1610 underlying interfaces that provide both secured tunnels for carrying 1611 IPv6 ND and BGP protocol control plane messages and open INET access 1612 for carrying unsecured messages. The OMNI interface configures both 1613 an ADM-LLA and its corresponding ADM-ULA, and acts as an OAL source 1614 to encapsulate and fragment original IP packets while presenting the 1615 resulting carrier packets over the secured or unsecured underlying 1616 paths. Note that Bridge and Proxy/Server BGP protocol TCP sessions 1617 are run directly over the OMNI interface and use ADM-ULA source and 1618 destination addresses. The OMNI interface employs the OAL to 1619 encapsulate the original IP packets for these sessions as carrier 1620 packets (i.e., even though the OAL header may use the same ADM-ULAs 1621 as the original IP header) and forwards them over the secured 1622 underlying path. 1624 3.4. OMNI Interface Initialization 1626 AERO Proxy/Servers and Clients configure OMNI interfaces as their 1627 point of attachment to the OMNI link. AERO nodes assign the MSPs for 1628 the link to their OMNI interfaces (i.e., as a "route-to-interface") 1629 to ensure that original IP packets with destination addresses covered 1630 by an MNP not explicitly associated with another interface are 1631 directed to an OMNI interface. 1633 OMNI interface initialization procedures for Proxy/Servers, Clients 1634 and Bridges are discussed in the following sections. 1636 3.4.1. AERO Proxy/Server and Relay Behavior 1638 When a Proxy/Server enables an OMNI interface, it assigns an 1639 ADM-{LLA,ULA} appropriate for the given OMNI link SRT segment. The 1640 Proxy/Server also configures secured tunnels with one or more 1641 neighboring Bridges and engages in a BGP routing protocol session 1642 with each Bridge. 1644 The OMNI interface provides a single interface abstraction to the IP 1645 layer, but internally includes an NBMA nexus for sending carrier 1646 packets to OMNI interface neighbors over underlying INET interfaces 1647 and secured tunnels. The Proxy/Server further configures a service 1648 to facilitate ND exchanges with AERO Clients and manages per-Client 1649 neighbor cache entries and IP forwarding table entries based on 1650 control message exchanges. 1652 Relays are simply Proxy/Servers that run a dynamic routing protocol 1653 to redistribute routes between the OMNI interface and INET/EUN 1654 interfaces (see: Section 3.2.3). The Relay provisions MNPs to 1655 networks on the INET/EUN interfaces (i.e., the same as a Client would 1656 do) and advertises the MSP(s) for the OMNI link over the INET/EUN 1657 interfaces. The Relay further provides an attachment point of the 1658 OMNI link to a non-MNP-based global topology. 1660 3.4.2. AERO Client Behavior 1662 When a Client enables an OMNI interface, it assigns either an 1663 MNP-{LLA, ULA} or a Temporary ULA and sends RS messages with ND 1664 parameters over its underlying interfaces to an FHS Proxy/Server, 1665 which returns an RA message with corresponding parameters. The RS/RA 1666 messages may pass through one or more NATs in the case of a Client's 1667 INET interface. (Note: if the Client used a Temporary ULA in its 1668 initial RS message, it will discover an MNP-{LLA, ULA} in the 1669 corresponding RA that it receives from the FHS Proxy/Server and begin 1670 using these new addresses. If the Client is operating outside the 1671 context of AERO infrastructure such as in a Mobile Ad-hoc Network 1672 (MANET), however, it may continue using Temporary ULAs for Client-to- 1673 Client communications until it encounters an infrastructure element 1674 that can provide an MNP.) 1676 3.4.3. AERO Bridge Behavior 1678 AERO Bridges configure an OMNI interface and assign the ADM-ULA 1679 Subnet Router Anycast address for each OMNI link SRT segment they 1680 connect to. Bridges configure secured tunnels with Proxy/Servers in 1681 the same SRT segment and other Bridges in the same (or an adjacent) 1682 SRT segment. Bridges then engage in a BGP routing protocol session 1683 with neighbors over the secured spanning tree (see: Section 3.2.3). 1685 3.5. OMNI Interface Neighbor Cache Maintenance 1687 Each OMNI interface maintains a conceptual neighbor cache that 1688 includes a Neighbor Cache Entry (NCE) for each of its active 1689 neighbors on the OMNI link per [RFC4861]. Each NCE is indexed by the 1690 LLA of the neighbor, while the OAL encapsulation ULA determines the 1691 context for Identification verification. Clients and Proxy/Servers 1692 maintain NCEs through RS/RA exchanges, and also maintain NCEs for any 1693 active correspondent peers through NS/NA exchanges. 1695 Bridges also maintain NCEs for Clients within their local segments 1696 based on NS/NA(WIN) route optimization. When a Bridge creates/ 1697 updates a NCE for a local segment Client based on NS/NA(WIN) route 1698 optimization, it also creates/updates a "Peer List" entry for the NCE 1699 containing the ULA, receive window information and FMT/SRT/LHS/L2ADDR 1700 information for the source of each NS/NA(WIN) message destined to 1701 this local segment Client. The Bridge also assigns a unique "Peer 1702 Index" value for each (NCE, Peer List entry)-tuple. The Bridge 1703 maintains a separate ReachableTime timer for each NCE Peer List entry 1704 individually. When ReachableTimer for an NCE Peer List entry 1705 expires, the Bridge deletes the Peer List entry and frees the 1706 associated Peer Index. When ReachableTime for all of an NCE's Peer 1707 List entries expire, the Bridge deletes the NCE. Proxy/Servers also 1708 create/update a Peer List entry for the NCE of a local Client when 1709 they forward an NA(WIN) that was not processed by a Bridge. Proxy/ 1710 Servers maintain the Peer List and Peer Index values the same as for 1711 Bridges, except that they do not delete the NCE when all Peer List 1712 entries expire. 1714 To the list of NCE states in Section 7.3.2 of [RFC4861], Proxy/Server 1715 OMNI interfaces add an additional state DEPARTED that applies to 1716 Clients that have recently departed. The interface sets a 1717 "DepartTime" variable for the NCE to "DEPART_TIME" seconds. 1718 DepartTime is decremented unless a new ND message causes the state to 1719 return to REACHABLE. While a NCE is in the DEPARTED state, the 1720 Proxy/Server forwards carrier packets destined to the target Client 1721 to the Client's new location instead. When DepartTime decrements to 1722 0, the NCE is deleted. It is RECOMMENDED that DEPART_TIME be set to 1723 the default constant value REACHABLE_TIME plus 10 seconds (40 seconds 1724 by default) to allow a window for carrier packets in flight to be 1725 delivered while stale route optimization state may be present. 1727 Proxy/Servers can act as RORs on behalf of their associated Clients 1728 according to the Proxy Neighbor Advertisement specification in 1729 Section 7.2.8 of [RFC4861]. When a Proxy/Server ROR receives an 1730 authentic NS message used for route optimization, it first searches 1731 for a NCE for the target Client and accepts the message only if there 1732 is an entry. The Proxy/Server then returns a solicited NA message 1733 while creating or updating a "Report List" entry in the target 1734 Client's NCE that caches both the LLA and ULA of ROS with a 1735 "ReportTime" variable set to REPORT_TIME seconds. The ROR resets 1736 ReportTime when it receives a new authentic NS message, and otherwise 1737 decrements ReportTime while no authentic NS messages have been 1738 received. It is RECOMMENDED that REPORT_TIME be set to the default 1739 constant value REACHABLE_TIME plus 10 seconds (40 seconds by default) 1740 to allow a window for route optimization to converge before 1741 ReportTime decrements below REACHABLE_TIME. 1743 When the ROS receives a solicited NA message response to its NS 1744 message used for route optimization, it creates or updates a NCE for 1745 the target network-layer and link-layer addresses. The ROS then 1746 (re)sets ReachableTime for the NCE to REACHABLE_TIME seconds and 1747 performs reachability tests over specific underlying interface pairs 1748 to determine paths for forwarding carrier packets directly to the 1749 target. The ROS otherwise decrements ReachableTime while no further 1750 solicited NA messages arrive. It is RECOMMENDED that REACHABLE_TIME 1751 be set to the default constant value 30 seconds as specified in 1752 [RFC4861]. 1754 AERO nodes also use the value MAX_UNICAST_SOLICIT to limit the number 1755 of NS messages sent when a correspondent may have gone unreachable, 1756 the value MAX_RTR_SOLICITATIONS to limit the number of RS messages 1757 sent without receiving an RA and the value MAX_NEIGHBOR_ADVERTISEMENT 1758 to limit the number of unsolicited NAs that can be sent based on a 1759 single event. It is RECOMMENDED that MAX_UNICAST_SOLICIT, 1760 MAX_RTR_SOLICITATIONS and MAX_NEIGHBOR_ADVERTISEMENT be set to 3 the 1761 same as specified in [RFC4861]. 1763 Different values for DEPART_TIME, REPORT_TIME, REACHABLE_TIME, 1764 MAX_UNICAST_SOLICIT, MAX_RTR_SOLCITATIONS and 1765 MAX_NEIGHBOR_ADVERTISEMENT MAY be administratively set; however, if 1766 different values are chosen, all nodes on the link MUST consistently 1767 configure the same values. Most importantly, DEPART_TIME and 1768 REPORT_TIME SHOULD be set to a value that is sufficiently longer than 1769 REACHABLE_TIME to avoid packet loss due to stale route optimization 1770 state. 1772 3.5.1. OMNI ND Messages 1774 OMNI interfaces prepare IPv6 ND messages the same as for standard 1775 IPv6 ND, but also include a new option type termed the OMNI option 1776 [I-D.templin-6man-omni]. OMNI interfaces prepare IPv6 ND messages 1777 the same as for standard IPv6 ND, and include one or more OMNI 1778 options and any other options then completely populate all option 1779 information. If the OMNI interface includes an authentication 1780 signature, it sets the IPv6 ND message Checksum field to 0 and 1781 calculates the authentication signature over the entire length of the 1782 message (beginning with a pseudo-header of the IPv6 header) but does 1783 not then proceed to calculate the IPv6 ND message checksum itself. 1784 If the OMNI interface forwards the message to a next hop over the 1785 secured spanning tree path, it omits both the authentication 1786 signature an checksum since lower layers already ensure 1787 authentication and integrity. In all other cases, the OMNI interface 1788 calculates the standard IPv6 ND message checksum and writes the value 1789 in the Checksum field. OMNI interfaces verify authentication and 1790 integrity of each IPv6 ND message received according to the specific 1791 check(s) included, and process the message further only following 1792 verification. 1794 OMNI options include per-neighbor information such as Interface 1795 Attributes that provide segment routing, link-layer address and 1796 traffic selector information for the neighbor's underlying 1797 interfaces. This information is stored in the neighbor cache and 1798 provides the basis for the forwarding algorithm specified in 1799 Section 3.10. The information is cumulative and reflects the union 1800 of the OMNI information from the most recent ND messages received 1801 from the neighbor; it is therefore not required that each ND message 1802 contain all neighbor information. 1804 The OMNI option Interface Attributes for each underlying interface 1805 includes a two-part "Link-Layer Address" consisting of an INET 1806 encapsulation address determined by the FMT and L2ADDR fields and an 1807 ADM-ULA determined by the SRT and LHS fields. Underlying interfaces 1808 are further selected based on their associated traffic selectors. 1809 When the SRT is 0, LHS instead includes a Peer Index value meaningful 1810 to the node identified by FMT and L2ADDR. 1812 The OMNI option is distinct from any Source/Target Link-Layer Address 1813 Options (S/TLLAOs) that may appear in an ND message according to the 1814 appropriate IPv6 over specific link layer specification (e.g., 1815 [RFC2464]). If both an OMNI option and S/TLLAO appear, the former 1816 pertains to encapsulation addresses while the latter pertains to the 1817 native L2 address format of the underlying media 1819 OMNI interface IPv6 ND messages may also include other IPv6 ND 1820 options. In particular, solicitation messages may include Nonce and/ 1821 or Timestamp options if required for verification of advertisement 1822 replies. If an OMNI ND solicitation message includes a Nonce option, 1823 the advertisement reply must echo the same Nonce. If an OMNI ND 1824 solicitation message includes a Timestamp option, the advertisement 1825 reply should also include a Timestamp option. 1827 AERO Clients send RS messages to the All-Routers multicast address 1828 while using unicast link-layer addresses. AERO Proxy/Servers respond 1829 by returning unicast RA messages. During the RS/RA exchange, AERO 1830 Clients and Servers include state synchronization parameters to 1831 establish Identification windows and other state. 1833 AERO nodes use NS/NA messages for the following purposes: 1835 o NS/NA(AR) messages are used for address resolution only. The ROS 1836 sends an NS(AR) to the solicited-node multicast address of the 1837 target, and an ROR in the network with addressing information for 1838 the target returns a unicast NA(AR). The NA(AR) contains current, 1839 consistent and authentic target address resolution information, 1840 but only an implicit third-party assertion of target reachability. 1841 NS/NA(AR) messages must be secured. 1843 o NS/NA(WIN) messages are used for establishing and maintaining 1844 window synchronization state (and/or any other state such as 1845 Interface Attributes). The source sends an NS(WIN) to the unicast 1846 address of the target, and the target returns a unicast NA(WIN). 1847 The NS/NA(WIN) exchange synchronizes the sequence number windows 1848 for Identification values the neighbors will include in subsequent 1849 carrier packets, and asserts reachability for the target without 1850 necessarily testing a specific underlying interface pair. NS/ 1851 NA(WIN) messages must be secured. 1853 o NS/NA(NUD) messages are used for determining target reachability. 1854 The source sends an NS(NUD) to the unicast address of the target 1855 while naming a specific underlying interface pair, and the target 1856 returns a unicast NA(NUD). NS/NA(NUD) messages that use an in- 1857 window sequence number and do not update any other state need not 1858 be secured but should include an IPv6 ND message checksum. NS/ 1859 NA(NUD) messages may also be used in combination with window 1860 synchronization (i.e., NUD+WIN), in which case the messages must 1861 be secured. 1863 o Unsolicited NA (uNA) messages are used to signal addressing and/or 1864 other neighbor state changes (e.g., address changes due to 1865 mobility, signal degradation, traffic selector updates, etc.). uNA 1866 messages that include state update information must be secured. 1868 o NS/NA(DAD) messages are not used in AERO, since Duplicate Address 1869 Detection is not required. 1871 Additionally, nodes may send NA/RA messages with the OMNI option PNG 1872 flag set to receive a solicited NA response from the neighbor. The 1873 solicited NA response MUST set the ACK flag (without also setting the 1874 SYN or PNG flags) and include the Identification used in the PNG 1875 message in the Acknowledgement. 1877 3.5.2. OMNI Neighbor Advertisement Message Flags 1879 As discussed in Section 4.4 of [RFC4861] NA messages include three 1880 flag bits R, S and O. OMNI interface NA messages treat the flags as 1881 follows: 1883 o R: The R ("Router") flag is set to 1 in the NA messages sent by 1884 all AERO/OMNI node types. Simple hosts that would set R to 0 do 1885 not occur on the OMNI link itself, but may occur on the downstream 1886 links of Clients and Relays. 1888 o S: The S ("Solicited") flag is set exactly as specified in 1889 Section 4.4. of [RFC4861], i.e., it is set to 1 for Solicited NAs 1890 and set to 0 for uNAs (both unicast and multicast). 1892 o O: The O ("Override") flag is set to 0 for solicited NAs returned 1893 by a Proxy/Server ROR and set to 1 for all other solicited and 1894 unsolicited NAs. For further study is whether solicited NAs for 1895 anycast targets apply for OMNI links. Since MNP-LLAs must be 1896 uniquely assigned to Clients to support correct ND protocol 1897 operation, however, no role is currently seen for assigning the 1898 same MNP-LLA to multiple Clients. 1900 3.5.3. OMNI Neighbor Window Synchronization 1902 In secured environments (e.g., such as between nodes on the same 1903 secured ANET), OMNI interface neighbors can exchange OAL packets 1904 using randomly-initialized and monotonically-increasing 1905 Identification values (modulo 2*32) without window synchronization. 1906 In environments where spoofing is considered a threat, OMNI interface 1907 neighbors instead invoke window synchronization in NS/NA(WIN) message 1908 exchanges to maintain send/receive window state in their respective 1909 neighbor cache entries as specified in [I-D.templin-6man-omni]. 1911 In the asymmetric window synchronization case, the initial NS/NA(WN) 1912 message exchange establishes only the initiator's send window and the 1913 responder's receive window such that a corresponding exchange would 1914 be needed to establish the reverse direction. In the symmetric case, 1915 the initiator and responder engage in a three-way handshake to 1916 symmetrically establish the send/receive windows of both parties. 1918 When Bridges and LHS Proxy/Servers forward and NS/NA(WIN) exchange 1919 between a ROS and a local segment Client, they also cache receive 1920 window information in a Peer List entry maintained by the local 1921 segment Client NCE. This allows FHS Bridges and Proxy/Servers to 1922 verify that carrier packets produced by the local Client include 1923 Identifications within the ROS' receive window before forwarding. 1925 3.6. OMNI Interface Encapsulation and Re-encapsulation 1927 The OMNI interface admits original IP packets then acts as an OAL 1928 source to perform OAL encapsulation and fragmentation as specified in 1929 [I-D.templin-6man-omni] while including an ORH if necessary as 1930 specified in Section 3.2.4. The OAL encapsulates original IP packets 1931 to form OAL packets subject to fragmentation, then encapsulates the 1932 resulting OAL fragments in *NET headers as carrier packets. 1934 For carrier packets undergoing re-encapsulation at an OAL 1935 intermediate node, the OMNI interface decrements the OAL IPv6 header 1936 Hop Limit and discards the carrier packet if the Hop Limit reaches 0. 1937 The intermediate node next removes the *NET encapsulation headers 1938 from the first segment and re-encapsulates the packet in new *NET 1939 encapsulation headers for the next segment. 1941 When an FHS Bridge or Proxy/Server re-encapsulates a carrier packet 1942 received from a Client with no ORH, it inserts an ORH immediately 1943 following the OAL header and adjusts the OAL payload length and 1944 destination address field. The ORH may be removed by an LHS Bridge 1945 or Proxy/Server, but its insertion and removal will not interfere 1946 with reassembly at the final destination. For this reason, Clients 1947 must reserve 40 bytes for a maximum-length ORH when they perform OAL 1948 encapsulation (see: Section 3.9). 1950 3.7. OMNI Interface Decapsulation 1952 OMNI interfaces (acting as OAL destinations) decapsulate and 1953 reassemble OAL packets into original IP packets destined either to 1954 the AERO node itself or to a destination reached via an interface 1955 other than the OMNI interface the original IP packet was received on. 1956 When carrier packets containing OAL fragments addressed to itself 1957 arrive, the OMNI interface discards the NET encapsulation headers and 1958 reassembles as discussed in Section 3.9. 1960 3.8. OMNI Interface Data Origin Authentication 1962 AERO nodes employ simple data origin authentication procedures. In 1963 particular: 1965 o AERO Bridges and Proxy/Servers accept carrier packets received 1966 from secured underlying interfaces. 1968 o AERO Proxy/Servers and Clients accept carrier packets and original 1969 IP packets that originate from within the same secured ANET. 1971 o AERO Clients and Relays accept original IP packets from downstream 1972 network correspondents based on ingress filtering. 1974 o AERO Clients, Relays and Proxy/Servers verify carrier packet UDP/ 1975 IP encapsulation addresses according to [I-D.templin-6man-omni]. 1977 o AERO nodes accept carrier packets addressed to themselves with 1978 Identification values within the current window for the OAL source 1979 neighbor (when window synchronization is used) and drop any 1980 carrier packets with out-of-window Identification values. (AERO 1981 nodes may forward carrier packets not addressed to themselves 1982 without verifying the Identification value.) 1984 AERO nodes silently drop any packets that do not satisfy the above 1985 data origin authentication procedures. Further security 1986 considerations are discussed in Section 6. 1988 3.9. OMNI Interface MTU 1990 The OMNI interface observes the link nature of tunnels, including the 1991 Maximum Transmission Unit (MTU), Maximum Reassembly Unit (MRU) and 1992 the role of fragmentation and reassembly [I-D.ietf-intarea-tunnels]. 1993 The OMNI interface employs an OMNI Adaptation Layer (OAL) that 1994 accommodates multiple underlying links with diverse MTUs while 1995 observing both a minimum and per-path Maximum Payload Size (MPS). 1996 The functions of the OAL and the OMNI interface MTU/MRU/MPS are 1997 specified in [I-D.templin-6man-omni] with MTU/MRU both set to the 1998 constant value 9180 bytes, with minimum MPS set to 400 bytes, and 1999 with potentially larger per-path MPS values depending on the 2000 underlying path. 2002 When the network layer presents an original IP packet to the OMNI 2003 interface, the OAL source encapsulates and fragments the original IP 2004 packet if necessary. When the network layer presents the OMNI 2005 interface with multiple original IP packets bound to the same OAL 2006 destination, the OAL source can concatenate them together into a 2007 single OAL super-packet as discussed in [I-D.templin-6man-omni]. The 2008 OAL source then fragments the OAL packet if necessary according to 2009 the minimum/path MPS such that the OAL headers appear in each 2010 fragment while the original IP packet header appears only in the 2011 first fragment. The OAL source then encapsulates each OAL fragment 2012 in *NET headers for transmission as carrier packets over an 2013 underlying interface connected to either a physical link (such as 2014 Ethernet, WiFi and the like) or a virtual link such as an Internet or 2015 higher-layer tunnel (see the definition of link in [RFC8200]). 2017 Note: A Client that does not (yet) have neighbor cache state for a 2018 target may omit the ORH in carrier packets with the understanding 2019 that a FHS Bridge or Proxy/Server may insert an ORH on its behalf. 2020 For this reason, Clients reserve 40 bytes for the largest possible 2021 ORH in their OAL fragment size calculations. 2023 Note: Although the ORH may be removed or replaced by an LHS Bridge or 2024 Proxy/Server (see: Section 3.10.3), this does not interfere with the 2025 destination's ability to reassemble since the ORH is not included in 2026 the fragmentable part and its removal/transformation does not 2027 invalidate fragment header information. 2029 3.10. OMNI Interface Forwarding Algorithm 2031 Original IP packets enter a node's OMNI interface either from the 2032 network layer (i.e., from a local application or the IP forwarding 2033 system) while carrier packets enter from the link layer (i.e., from 2034 an OMNI interface neighbor). All original IP packets and carrier 2035 packets entering a node's OMNI interface first undergo data origin 2036 authentication as discussed in Section 3.8. Those that satisfy data 2037 origin authentication are processed further, while all others are 2038 dropped silently. 2040 Original IP packets that enter the OMNI interface from the network 2041 layer are forwarded to an OMNI interface neighbor using OAL 2042 encapsulation and fragmentation to produce carrier packets for 2043 transmission over underlying interfaces. (If routing indicates that 2044 the original IP packet should instead be forwarded back to the 2045 network layer, the packet is dropped to avoid looping). Carrier 2046 packets that enter the OMNI interface from the link layer are either 2047 re-encapsulated and re-admitted into the OMNI link, or reassembled 2048 and forwarded to the network layer where they are subject to either 2049 local delivery or IP forwarding. In all cases, the OAL MUST NOT 2050 decrement the original IP packet TTL/Hop-count since its forwarding 2051 actions occur below the network layer. 2053 OMNI interfaces may have multiple underlying interfaces and/or 2054 neighbor cache entries for neighbors with multiple underlying 2055 interfaces (see Section 3.3). The OAL uses Interface Attributes 2056 traffic selectors (e.g., port number, flow specification, etc.) to 2057 select an outbound underlying interface for each OAL packet based on 2058 the node's own interface attributes, and also to select segment 2059 routing and/or link-layer destination addresses based on the 2060 neighbor's underlying interface attributes. AERO implementations 2061 SHOULD permit network management to dynamically adjust traffic 2062 selector values at runtime. 2064 If an OAL packet matches the traffic selectors of multiple outgoing 2065 interfaces and/or neighbor interfaces, the OMNI interface replicates 2066 the packet and sends one copy via each of the (outgoing / neighbor) 2067 interface pairs; otherwise, it sends a single copy of the OAL packet 2068 via an interface with the best matching traffic selector. (While not 2069 strictly required, the likelihood of successful reassembly may 2070 improve when the OMNI interface sends all fragments of the same 2071 fragmented OAL packet consecutively over the same underlying 2072 interface pair to avoid complicating factors such as delay variance 2073 and reordering.) AERO nodes keep track of which underlying 2074 interfaces are currently "reachable" or "unreachable", and only use 2075 "reachable" interfaces for forwarding purposes. 2077 The following sections discuss the OMNI interface forwarding 2078 algorithms for Clients, Proxy/Servers and Bridges. In the following 2079 discussion, an original IP packet's destination address is said to 2080 "match" if it is the same as a cached address, or if it is covered by 2081 a cached prefix (which may be encoded in an MNP-LLA). 2083 3.10.1. Client Forwarding Algorithm 2085 When an original IP packet enters a Client's OMNI interface from the 2086 network layer the Client searches for a NCE that matches the 2087 destination. If there is a match, the Client selects one or more 2088 "reachable" neighbor interfaces in the entry for forwarding purposes. 2089 If there is no NCE, the Client instead either enqueues the original 2090 IP packet and invokes route optimization or forwards the original IP 2091 packet toward a Proxy/Server. The Client (acting as an OAL source) 2092 performs OAL encapsulation and sets the OAL destination address to 2093 the MNP-ULA of the target if there is a matching NCE; otherwise, it 2094 sets the OAL destination to the ADM-ULA of the Proxy/Server. If the 2095 Client has multiple original IP packets to send to the same neighbor, 2096 it can concatenate them in a single super-packet 2097 [I-D.templin-6man-omni]. The OAL source then performs fragmentation 2098 to create OAL fragments (see: Section 3.9), appends any *NET 2099 encapsulation, and sends the resulting carrier packets over 2100 underlying interfaces to the neighbor acting as an OAL destination. 2102 If the neighbor interface selected for forwarding is located on the 2103 same OMNI link segment and not behind a NAT, the Client forwards the 2104 carrier packets directly according to the L2ADDR information for the 2105 neighbor. If the neighbor interface is behind a NAT on the same OMNI 2106 link segment, the Client instead forwards the initial carrier packets 2107 to the LHS Proxy/Server (while inserting an ORH-0 if necessary) and 2108 initiates NAT traversal procedures. If the Client's intended source 2109 underlying interface is also behind a NAT and located on the same 2110 OMNI link segment, it sends a "direct bubble" over the interface per 2111 [RFC6081][RFC4380] to the L2ADDR found in the neighbor cache in order 2112 to establish state in its own NAT by generating traffic toward the 2113 neighbor (note that no response to the bubble is expected). 2115 The Client next sends an NS(NUD) message toward the MNP-ULA of the 2116 neighbor via the LHS Proxy/Server as discussed in Section 3.15. If 2117 the Client receives an NA(NUD) from the neighbor over the underlying 2118 interface, it marks the neighbor interface as "trusted" and sends 2119 future carrier packets directly to the L2ADDR information for the 2120 neighbor instead of indirectly via the LHS Proxy/Server. The Client 2121 must honor the neighbor cache maintenance procedure by sending 2122 additional direct bubbles and/or NS/NA(NUD) messages as discussed in 2123 [RFC6081][RFC4380] in order to keep NAT state alive as long as 2124 carrier packets are still flowing. 2126 When a carrier packet enters a Client's OMNI interface from the link- 2127 layer, if the OAL destination matches one of the Client's ULAs the 2128 Client (acting as an OAL destination) verifies that the 2129 Identification is in-window for this OAL source, then reassembles and 2130 decapsulates as necessary and delivers the original IP packet to the 2131 network layer. If the OAL destination does not match, the Client 2132 drops the original IP packet and MAY return a network-layer ICMP 2133 Destination Unreachable message subject to rate limiting (see: 2134 Section 3.11). 2136 Note: When an LHS Bridge or Proxy/Server forwards an NA(WIN), it 2137 overwrites its own link-layer address and a Peer Index value in the 2138 Interface Attribute option. When the local Client updates the NCE 2139 for this ROS, it caches the link-layer address and Peer Index 2140 information as the FHS encapsulation values to use when sending 2141 return carrier packets to the ROS. 2143 Note: Clients and their FHS Proxy/Server (and other Client) peers can 2144 exchange original IP packets over ANET underlying interfaces without 2145 invoking the OAL, since the ANET is secured at the link and physical 2146 layers. By forwarding original IP packets without invoking the OAL, 2147 however, the ANET peers can engage only in classical path MTU 2148 discovery since the packets are subject to loss and/or corruption due 2149 to the various per-link MTU limitations that may occur within the 2150 ANET. Moreover, the original IP packets do not include either the 2151 OAL integrity check or per-packet Identification values that can be 2152 used for data origin authentication and link-layer retransmissions. 2153 The tradeoff therefore involves an assessment of the per-packet 2154 encapsulation overhead saved by bypassing the OAL vs. inheritance of 2155 classical network "brittleness". (Note however that ANET peers can 2156 send small original IP packets without invoking the OAL, while 2157 invoking the OAL for larger packets. This presents the beneficial 2158 aspects of both small packet efficiency and large packet robustness, 2159 with delay variance and reordering as possible side effects.) 2161 3.10.2. Proxy/Server and Relay Forwarding Algorithm 2163 When the Proxy/Server receives an original IP packet from the network 2164 layer, it drops the packet if routing indicates that it should be 2165 forwarded back to the network layer to avoid looping. Otherwise, the 2166 Proxy/Server regards the original IP packet the same as if it had 2167 arrived as carrier packets with OAL destination set to its own ADM- 2168 ULA. When the Proxy/Server receives carrier packets on underlying 2169 interfaces with OAL destination set to its own ADM-ULA, it performs 2170 OAL reassembly if necessary to obtain the original IP packet. 2172 The Proxy/Server next searches for a NCE that matches the original IP 2173 destination and proceeds as follows: 2175 o if the packet is an NA(WIN) message for a local Client NCE, the 2176 Proxy/Server examines the Interface Attributes information and 2177 rewrites the fields if the NA(WIN) was not already processed by a 2178 (local segment) Bridge as discussed in Section 3.2.7. 2180 o else, if the original IP packet destination matches a NCE, the 2181 Proxy/Sever uses one or more "reachable" neighbor interfaces in 2182 the entry for packet forwarding using OAL encapsulation and 2183 fragmentation according to the cached link-layer address 2184 information. If the neighbor interface is in a different OMNI 2185 link segment, the Proxy/Server performs OAL encapsulation and 2186 fragmentation, inserts an ORH and forwards the resulting carrier 2187 packets via the spanning tree to a Bridge; otherwise, it forwards 2188 the carrier packets directly to the neighbor via INET 2189 encapsulation. If the neighbor is behind a NAT, this FHS Proxy/ 2190 Server instead forwards initial carrier packets via a Bridge (or 2191 more directly via an LHS Proxy/Server) while sending an NS(NUD) to 2192 the neighbor. When the Proxy/Server receives the NA(NUD), it can 2193 begin forwarding carrier packets directly to the neighbor the same 2194 as discussed in Section 3.10.1 while sending additional NS(NUD) 2195 messages as necessary to maintain NAT state. Note that no direct 2196 bubbles are necessary since the Proxy/Server is by definition not 2197 located behind a NAT. 2199 o else, if the original IP destination matches a non-MNP route in 2200 the IP forwarding table or an ADM-LLA assigned to the Proxy/ 2201 Server's OMNI interface, the Proxy/Server acting as a Relay 2202 presents the original IP packet to the network layer for local 2203 delivery or IP forwarding. 2205 o else, the Proxy/Server initiates address resolution as discussed 2206 in Section 3.14, while retaining initial original IP packets in a 2207 small queue awaiting address resolution completion. 2209 When the Proxy/Server receives a carrier packet with OAL destination 2210 set to an MNP-ULA that does not match the MSP, it accepts the carrier 2211 packet only if data origin authentication succeeds and if there is a 2212 network layer routing table entry for a GUA route that matches the 2213 MNP-ULA. If there is no route, the Proxy/Server drops the carrier 2214 packet; otherwise, it reassembles and decapsulates to obtain the 2215 original IP packet then acts as a Relay to present it to the network 2216 layer where it will be delivered according to standard IP forwarding. 2218 When a Proxy/Server receives a carrier packet from one of its Client 2219 neighbors with OAL destination set to another node, it forwards the 2220 packets via a matching NCE or via the spanning tree if there is no 2221 matching entry. When the Proxy/Server receives a carrier packet with 2222 OAL destination set to the MNP-ULA of one of its Client neighbors 2223 established through RS/RA exchanges, it accepts the carrier packet 2224 only if data origin authentication succeeds. If the NCE state is 2225 DEPARTED, the Proxy/Server inserts an ORH that encodes the MNP-ULA 2226 destination suffix and changes the OAL destination address to the 2227 ADM-ULA of the new Proxy/Server, then re-encapsulates the carrier 2228 packet and forwards it to a Bridge which will eventually deliver it 2229 to the new Proxy/Server. 2231 If the neighbor cache state for the MNP-ULA is REACHABLE, the Proxy/ 2232 Server forwards the carrier packets to the Client which then must 2233 reassemble. (Note that the Proxy/Server does not reassemble carrier 2234 packets not explicitly addressed to its own ADM-ULA, since some of 2235 the carrier packets of the same original IP packet could be forwarded 2236 through a different Proxy/Server.) In that case, the Client may 2237 receive fragments that are smaller than its link MTU but that can 2238 still be reassembled. 2240 Note: Proxy/Servers may receive carrier packets with ORHs that 2241 include additional forwarding information. Proxy/Servers use the 2242 forwarding information to determine the correct NCE and underlying 2243 interface for forwarding to the target Client, then remove the ORH 2244 and forward the carrier packet. If the ORH information instead 2245 indicates that the Proxy/Server is responsible for reassembly, the 2246 Proxy/Server reassembles first before re-encapsulating (and possibly 2247 also re-fragmenting) then forwards to the target Client. For a full 2248 discussion of cases when the Proxy/Server may receive carrier packets 2249 with ORHs, see: Section 3.14.6. 2251 Note: Clients and their FHS Proxy/Server peers can exchange original 2252 IP packets over ANET underlying interfaces without invoking the OAL, 2253 since the ANET is secured at the link and physical layers. By 2254 forwarding original IP packets without invoking the OAL, however, the 2255 Client and Proxy/Server can engage only in classical path MTU 2256 discovery since the packets are subject to loss and/or corruption due 2257 to the various per-link MTU limitations that may occur within the 2258 ANET. Moreover, the original IP packets do not include either the 2259 OAL integrity check or per-packet Identification values that can be 2260 used for data origin authentication and link-layer retransmissions. 2261 The tradeoff therefore involves an assessment of the per-packet 2262 encapsulation overhead saved by bypassing the OAL vs. inheritance of 2263 classical network "brittleness". (Note however that ANET peers can 2264 send small original IP packets without invoking the OAL, while 2265 invoking the OAL for larger packets. This presents the beneficial 2266 aspects of both small packet efficiency and large packet robustness.) 2268 Note: When a Proxy/Server receives a (non-OAL) original IP packet 2269 from an ANET Client, or a carrier packet with OAL destination set to 2270 its own ADM-ULA from any Client, the Proxy/Server reassembles if 2271 necessary then performs ROS functions on behalf of the Client. The 2272 Client may at some later time begin sending carrier packets to the 2273 OAL address of the actual target instead of the Proxy/Server, at 2274 which point it may begin functioning as an ROS on its own behalf and 2275 thereby "override" the Proxy/Server's ROS role. 2277 Note; Proxy/Servers drop any original IP packets (received either 2278 directly from an ANET Client or following reassembly of carrier 2279 packets received from an ANET/INET Client) with a destination that 2280 corresponds to the Client's delegated MNP. Similarly, Proxy/Servers 2281 drop any carrier packet received with both a source and destination 2282 that correspond to the Client's delegated MNP regardless of their 2283 OMNI link point of origin. These checks are necessary to prevent 2284 Clients from either accidentally or intentionally establishing 2285 endless loops that could congest Proxy/Servers and/or ANET/INET 2286 links. 2288 Note: Proxy/Servers forward secure control plane carrier packets via 2289 the SRT secured spanning tree and forward other carrier packets via 2290 the unsecured spanning tree. When a Proxy/Server receives a carrier 2291 packet from the secured spanning tree, it considers the message as 2292 authentic without having to verify upper layer authentication 2293 signatures. When a Proxy/Server receives a carrier packet from the 2294 unsecured spanning tree, it verifies any upper layer authentication 2295 signatures and/or forwards the unsecured message toward the 2296 destination which must apply data origin authentication. 2298 Note: If the Proxy/Server has multiple original IP packets to send to 2299 the same neighbor, it can concatenate them in a single OAL super- 2300 packet [I-D.templin-6man-omni]. 2302 3.10.3. Bridge Forwarding Algorithm 2304 Bridges forward carrier packets while decrementing the OAL header Hop 2305 Count but not the original IP header Hop Count/TTL. Bridges convey 2306 carrier packets that encapsulate critical IPv6 ND control messages or 2307 routing protocol control messages via the secured spanning tree, and 2308 may convey other carrier packets via the unsecured spanning tree. 2309 When the Bridge receives a carrier packet, it removes the outer *NET 2310 header and searches for a forwarding table entry that matches the OAL 2311 destination address. The Bridge then processes the packet as 2312 follows: 2314 o if the carrier packet destination matches its ADM-ULA or the 2315 corresponding ADM-ULA Subnet Router Anycast address and the OAL 2316 header is followed by an ORH, the Bridge processes the carrier 2317 packet locally before forwarding. If the carrier packet contains 2318 an NS/NA(WIN) message, the Bridge creates/updates an NCE for the 2319 destination Client and rewrites the Interface Attributes as 2320 discussed in Section 3.2.7. The Bridge next examines the ORH, and 2321 if FMT-Mode indicates the destination is a Client on the open *NET 2322 (or, a Client behind a NAT for which NAT traversal procedures have 2323 already converged) the Bridge writes the MNP-ULA formed from the 2324 ORH Destination Trailer into the OAL destination. The Bridge then 2325 removes the ORH and forwards the packet using encapsulation based 2326 on the NATed L2ADDR information. If the LHS Proxy/Server will 2327 forward to the Client without reassembly, the Bridge writes the 2328 MNP-ULA into the OAL destination then replaces the ORH with an 2329 ORH-0 and forwards the carrier packet to the LHS Proxy/Server 2330 while also invoking NAT traversal procedures if necessary (noting 2331 that no direct bubbles are necessary since only the target Client 2332 and not the Bridge is behind a NAT). If the LHS Proxy/Server must 2333 perform reassembly before forwarding to the Client, the Bridge 2334 instead writes the ADM-ULA formed from the ORH SRT/LHS into the 2335 OAL destination address, replaces the ORH with an ORH-0 and 2336 forwards the carrier packet to the LHS Proxy/Server. (Note: for 2337 IPv6 ND messages that require security, the Bridge must forward 2338 the message to the LHS Proxy/Server over the secured spanning tree 2339 instead of directly to the Client itself even if NAT traversal has 2340 converged.) 2342 o else, if the carrier packet destination matches its ADM-ULA or the 2343 corresponding ADM-ULA Subnet Router Anycast address and the OAL 2344 header is not followed by an ORH with Segments Left set to 1, the 2345 Bridge submits the packet for reassembly. When reassembly is 2346 complete, the Bridge submits the original IP packet to the network 2347 layer to support local applications such as BGP routing protocol 2348 sessions. 2350 o else, if the carrier packet destination matches a forwarding table 2351 entry the Bridge forwards the carrier packet to the next hop. (If 2352 the destination matches an MSP without matching an MNP, however, 2353 the Bridge instead drops the packet and returns a Destination 2354 Unreachable message subject to rate limiting - see: Section 3.11). 2356 o else, the Bridge drops the packet and returns an Destination 2357 Unreachable as above. 2359 The Bridge decrements the OAL IPv6 header Hop Limit when it forwards 2360 the carrier packet and drops the packet if the Hop Limit reaches 0. 2361 Therefore, only the Hop Limit in the OAL header is decremented and 2362 not the TTL/Hop Limit in the original IP packet header. Bridges do 2363 not insert OAL/ORH headers themselves; instead, they simply forward 2364 carrier packets based on their destination addresses while also 2365 possibly transforming larger ORHs into an ORH-0 (or removing the ORH 2366 altogether). 2368 Bridges forward carrier packets received from a first segment via the 2369 SRT secured spanning tree to the next segment also via the secured 2370 spanning tree. Bridges forward carrier packets received from a first 2371 segment via the unsecured spanning tree to the next segment also via 2372 the unsecured spanning tree. Bridges use a single IPv6 routing table 2373 that always determines the same next hop for a given OAL destination, 2374 where the secured/unsecured spanning tree is determined through the 2375 selection of the underlying interface to be used for transmission 2376 (i.e., a secured tunnel or an open INET interface). 2378 3.11. OMNI Interface Error Handling 2380 When an AERO node admits an original IP packet into the OMNI 2381 interface, it may receive link-layer or network-layer error 2382 indications. The AERO node may also receive OMNI link error 2383 indications in OAL-encapsulated uNA messages that include 2384 authentication signatures. 2386 A link-layer error indication is an ICMP error message generated by a 2387 router in the INET on the path to the neighbor or by the neighbor 2388 itself. The message includes an IP header with the address of the 2389 node that generated the error as the source address and with the 2390 link-layer address of the AERO node as the destination address. 2392 The IP header is followed by an ICMP header that includes an error 2393 Type, Code and Checksum. Valid type values include "Destination 2394 Unreachable", "Time Exceeded" and "Parameter Problem" 2395 [RFC0792][RFC4443]. (OMNI interfaces ignore link-layer IPv4 2396 "Fragmentation Needed" and IPv6 "Packet Too Big" messages for carrier 2397 packets that are no larger than the minimum/path MPS as discussed in 2398 Section 3.9, however these messages may provide useful hints of probe 2399 failures during path MPS probing.) 2401 The ICMP header is followed by the leading portion of the carrier 2402 packet that generated the error, also known as the "packet-in-error". 2403 For ICMPv6, [RFC4443] specifies that the packet-in-error includes: 2404 "As much of invoking packet as possible without the ICMPv6 packet 2405 exceeding the minimum IPv6 MTU" (i.e., no more than 1280 bytes). For 2406 ICMPv4, [RFC0792] specifies that the packet-in-error includes: 2407 "Internet Header + 64 bits of Original Data Datagram", however 2408 [RFC1812] Section 4.3.2.3 updates this specification by stating: "the 2409 ICMP datagram SHOULD contain as much of the original datagram as 2410 possible without the length of the ICMP datagram exceeding 576 2411 bytes". 2413 The link-layer error message format is shown in Figure 5: 2415 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2416 ~ ~ 2417 | IP Header of link layer | 2418 | error message | 2419 ~ ~ 2420 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2421 | ICMP Header | 2422 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --- 2423 ~ ~ P 2424 | carrier packet *NET and OAL | a 2425 | encapsulation headers | c 2426 ~ ~ k 2427 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ e 2428 ~ ~ t 2429 | original IP packet headers | 2430 | (first-fragment only) | i 2431 ~ ~ n 2432 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2433 ~ ~ e 2434 | Portion of the body of | r 2435 | the original IP packet | r 2436 | (all fragments) | o 2437 ~ ~ r 2438 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --- 2440 Figure 5: OMNI Interface Link-Layer Error Message Format 2442 The AERO node rules for processing these link-layer error messages 2443 are as follows: 2445 o When an AERO node receives a link-layer Parameter Problem message, 2446 it processes the message the same as described as for ordinary 2447 ICMP errors in the normative references [RFC0792][RFC4443]. 2449 o When an AERO node receives persistent link-layer Time Exceeded 2450 messages, the IP ID field may be wrapping before earlier fragments 2451 awaiting reassembly have been processed. In that case, the node 2452 should begin including integrity checks and/or institute rate 2453 limits for subsequent packets. 2455 o When an AERO node receives persistent link-layer Destination 2456 Unreachable messages in response to carrier packets that it sends 2457 to one of its neighbor correspondents, the node should process the 2458 message as an indication that a path may be failing, and 2459 optionally initiate NUD over that path. If it receives 2460 Destination Unreachable messages over multiple paths, the node 2461 should allow future carrier packets destined to the correspondent 2462 to flow through a default route and re-initiate route 2463 optimization. 2465 o When an AERO Client receives persistent link-layer Destination 2466 Unreachable messages in response to carrier packets that it sends 2467 to one of its neighbor Proxy/Servers, the Client should mark the 2468 path as unusable and use another path. If it receives Destination 2469 Unreachable messages on many or all paths, the Client should 2470 associate with a new Proxy/Server and release its association with 2471 the old Proxy/Server as specified in Section 3.16.5. 2473 o When an AERO Proxy/Server receives persistent link-layer 2474 Destination Unreachable messages in response to carrier packets 2475 that it sends to one of its neighbor Clients, the Proxy/Server 2476 should mark the underlying path as unusable and use another 2477 underlying path. 2479 o When an AERO Proxy/Server receives link-layer Destination 2480 Unreachable messages in response to a carrier packet that it sends 2481 to one of its permanent neighbors, it treats the messages as an 2482 indication that the path to the neighbor may be failing. However, 2483 the dynamic routing protocol should soon reconverge and correct 2484 the temporary outage. 2486 When an AERO Bridge receives a carrier packet for which the network- 2487 layer destination address is covered by an MSP, the Bridge drops the 2488 packet if there is no more-specific routing information for the 2489 destination and returns an OMNI interface Destination Unreachable 2490 message subject to rate limiting. 2492 When an AERO node receives a carrier packet for which reassembly is 2493 currently congested, it returns an OMNI interface Packet Too Big 2494 (PTB) message as discussed in [I-D.templin-6man-omni] (note that the 2495 PTB messages could indicate either "hard" or "soft" errors). 2497 AERO nodes include ICMPv6 error messages intended for the OAL source 2498 as sub-options in the OMNI option of secured uNA messages. When the 2499 OAL source receives the uNA message, it can extract the ICMPv6 error 2500 message enclosed in the OMNI option and either process it locally or 2501 translate it into a network-layer error to return to the original 2502 source. 2504 3.12. AERO Router Discovery, Prefix Delegation and Autoconfiguration 2506 AERO Router Discovery, Prefix Delegation and Autoconfiguration are 2507 coordinated as discussed in the following Sections. 2509 3.12.1. AERO Service Model 2511 Each AERO Proxy/Server on the OMNI link is configured to facilitate 2512 Client prefix delegation/registration requests. Each Proxy/Server is 2513 provisioned with a database of MNP-to-Client ID mappings for all 2514 Clients enrolled in the AERO service, as well as any information 2515 necessary to authenticate each Client. The Client database is 2516 maintained by a central administrative authority for the OMNI link 2517 and securely distributed to all Proxy/Servers, e.g., via the 2518 Lightweight Directory Access Protocol (LDAP) [RFC4511], via static 2519 configuration, etc. Clients receive the same service regardless of 2520 the Proxy/Servers they select. 2522 AERO Clients and Proxy/Servers use ND messages to maintain neighbor 2523 cache entries. AERO Proxy/Servers configure their OMNI interfaces as 2524 advertising NBMA interfaces, and therefore send unicast RA messages 2525 with a short Router Lifetime value (e.g., ReachableTime seconds) in 2526 response to a Client's RS message. Thereafter, Clients send 2527 additional RS messages to keep Proxy/Server state alive. 2529 AERO Clients and Proxy/Servers include prefix delegation and/or 2530 registration parameters in RS/RA messages (see 2531 [I-D.templin-6man-omni]). The ND messages are exchanged between 2532 Client and FHS Proxy/Servers according to the prefix management 2533 schedule required by the service. If the Client knows its MNP in 2534 advance, it can employ prefix registration by including its MNP-LLA 2535 as the source address of an RS message and with an OMNI option with 2536 valid prefix registration information for the MNP. If the Proxy/ 2537 Server accepts the Client's MNP assertion, it injects the MNP into 2538 the routing system and establishes the necessary neighbor cache 2539 state. If the Client does not have a pre-assigned MNP, it can 2540 instead employ prefix delegation by including the unspecified address 2541 (::) as the source address of an RS message and with an OMNI option 2542 with prefix delegation parameters to request an MNP. 2544 The following sections specify the Client and Proxy/Server behavior. 2546 3.12.2. AERO Client Behavior 2548 AERO Clients discover the addresses of candidate FHS Proxy/Servers in 2549 a similar manner as described in [RFC5214]. Discovery methods 2550 include static configuration (e.g., from a flat-file map of Proxy/ 2551 Server addresses and locations), or through an automated means such 2552 as Domain Name System (DNS) name resolution [RFC1035]. 2553 Alternatively, the Client can discover Proxy/Server addresses through 2554 a layer 2 data link login exchange, or through a unicast RA response 2555 to a multicast/anycast RS as described below. In the absence of 2556 other information, the Client can resolve the DNS Fully-Qualified 2557 Domain Name (FQDN) "linkupnetworks.[domainname]" where 2558 "linkupnetworks" is a constant text string and "[domainname]" is a 2559 DNS suffix for the OMNI link (e.g., "example.com"). 2561 To associate with a FHS Proxy/Server over an underlying interface, 2562 the Client acts as a requesting router to request MNPs by preparing 2563 an RS message with prefix management parameters. If the Client 2564 already knows the Proxy/Server's ADM-LLA, it includes the LLA as the 2565 network-layer destination address; otherwise, the Client includes the 2566 (link-local) All-Routers multicast as the network-layer destination. 2567 The Client can use its MNP-LLA as the network-layer source address 2568 and include an OMNI option with prefix registration information. If 2569 the Client does not yet have an MNP-LLA, it instead sets the network- 2570 layer source address to unspecified (::) and includes prefix 2571 delegation parameters in the OMNI option (see: 2572 [I-D.templin-6man-omni]). 2574 The Client next includes an authentication sub-option if necessary, 2575 Interface Attributes corresponding to the underlying interface over 2576 which it will send the RS message, and optionally any additional 2577 Interface Attributes corresponding to other underlying interfaces. 2578 Next, the Client submits the RS for OAL encapsulation and 2579 fragmentation if necessary with its own MNP-ULA and the Proxy/ 2580 Server's ADM-ULA or (site-scoped) All-Routers multicast as the OAL 2581 addresses while selecting an Identification value and invoking window 2582 synchronization as specified in [I-D.templin-6man-omni]. 2584 The Client then sends the RS (either directly via Direct interfaces, 2585 via a VPN for VPNed interfaces, via an access router for ANET 2586 interfaces or via INET encapsulation for INET interfaces) then waits 2587 up to RetransTimer milliseconds for an RA message reply (see 2588 Section 3.12.3) (retrying up to MAX_RTR_SOLICITATIONS). If the 2589 Client receives no RAs, or if it receives an RA with Router Lifetime 2590 set to 0, the Client SHOULD abandon attempts through the first 2591 candidate FHS Proxy/Server and try another Proxy/Server. Otherwise, 2592 the Client processes the prefix information found in the RA message. 2594 When the Client processes an RA, it first performs OAL reassembly and 2595 decapsulation if necessary then creates a NCE with the Proxy/Server's 2596 ADM-LLA as the network-layer address and the Proxy/Server's 2597 encapsulation and/or link-layer addresses as the link-layer address. 2598 The Client then caches the FMT/SRT/LHS/L2ADDR information from the 2599 Interface Attributes for omIndex 0 included in the RA as an SRT local 2600 segment reference point for this Proxy/Server. The Client next 2601 records the RA Router Lifetime field value in the NCE as the time for 2602 which the Proxy/Server has committed to maintaining the MNP in the 2603 routing system via this underlying interface, and caches the other RA 2604 configuration information including Cur Hop Limit, M and O flags, 2605 Reachable Time and Retrans Timer. The Client then autoconfigures 2606 MNP-LLAs for any delegated MNPs and assigns them to the OMNI 2607 interface. The Client also caches any MSPs included in Route 2608 Information Options (RIOs) [RFC4191] as MSPs to associate with the 2609 OMNI link, and assigns the MTU value in the MTU option to the 2610 underlying interface. 2612 The Client then registers its additional underlying interfaces with 2613 FHS Proxy/Servers for those interfaces discovered by sending RS 2614 messages via each additional interface as described above. The RS 2615 messages include the same parameters as for the initial RS/RA 2616 exchange, but with destination address set to the Proxy/Server's ADM- 2617 LLA. The Client finally sub-delegates the MNPs to its attached EUNs 2618 and/or the Client's own internal virtual interfaces as described in 2619 [I-D.templin-v6ops-pdhost] to support the Client's downstream 2620 attached "Internet of Things (IoT)". The Client then sends 2621 additional RS messages over each underlying interface before the 2622 Router Lifetime received for that interface expires. 2624 After the Client registers its underlying interfaces, it may wish to 2625 change one or more registrations, e.g., if an interface changes 2626 address or becomes unavailable, if traffic selectors change, etc. To 2627 do so, the Client prepares an RS message to send over any available 2628 underlying interface as above. The RS includes an OMNI option with 2629 prefix registration/delegation information, with Interface Attributes 2630 specific to the selected underlying interface, and with any 2631 additional Interface Attributes specific to other underlying 2632 interfaces. When the Client receives the Proxy/Server's RA response, 2633 it has assurance that the Proxy/Server has been updated with the new 2634 information. 2636 If the Client wishes to discontinue use of a Proxy/Server it issues 2637 an RS message over any underlying interface with an OMNI option with 2638 a prefix release indication. When the Proxy/Server processes the 2639 message, it releases the MNP, sets the NCE state for the Client to 2640 DEPARTED and returns an RA reply with Router Lifetime set to 0. 2641 After a short delay (e.g., 2 seconds), the Proxy/Server withdraws the 2642 MNP from the routing system. 2644 3.12.3. AERO Proxy/Server Behavior 2646 AERO Proxy/Servers act as both IP routers and ND proxies, and support 2647 a prefix delegation/registration service for Clients. Proxy/Servers 2648 arrange to add their ADM-LLAs to a static map of Proxy/Server 2649 addresses for the link and/or the DNS resource records for the FQDN 2650 "linkupnetworks.[domainname]" before entering service. The static 2651 map and/or DNS resource records should be arranged such that Clients 2652 can discover the addresses of Proxy/Servers that are geographically 2653 and/or topologically "close" to their underlying network connections. 2655 When an FHS Proxy/Server receives a prospective Client's RS message, 2656 it SHOULD return an immediate RA reply with Router Lifetime set to 0 2657 if it is currently too busy or otherwise unable to service the 2658 Client. Otherwise, the Proxy/Server performs OAL reassembly if 2659 necessary, then decapsulates and authenticates the RS message before 2660 processing the prefix delegation/registration parameters. The Proxy/ 2661 Server then determines the correct MNPs to provide to the Client by 2662 processing the MNP-LLA prefix parameters and/or the DHCPv6 OMNI sub- 2663 option. When the Proxy/Server returns the MNPs, it also creates a 2664 forwarding table entry for the MNP-ULA corresponding to each MNP 2665 resulting in a BGP update (see: Section 3.2.3). For IPv6, the Proxy/ 2666 Server creates an IPv6 forwarding table entry for each MNP. For 2667 IPv4, the Proxy/Server creates an IPv6 forwarding table entry with 2668 the IPv4-compatibility MNP-ULA prefix corresponding to the IPv4 2669 address. 2671 The Proxy/Server next creates a NCE for the Client using the base 2672 MNP-LLA as the network-layer address. Next, the Proxy/Server updates 2673 the NCE by recording the information in each Interface Attributes 2674 sub-option in the RS OMNI option. The Proxy/Server also records the 2675 actual OAL/*NET addresses and RS message window synchronization 2676 parameters (if any) in the NCE. 2678 Next, the Proxy/Server prepares an RA message using its ADM-LLA as 2679 the network-layer source address and the network-layer source address 2680 of the RS message as the network-layer destination address. The 2681 Proxy/Server sets the Router Lifetime to the time for which it will 2682 maintain both this underlying interface individually and the NCE as a 2683 whole. The Proxy/Server also sets Cur Hop Limit, M and O flags, 2684 Reachable Time and Retrans Timer to values appropriate for the OMNI 2685 link. The Proxy/Server includes the MNPs, any other prefix 2686 management parameters and an OMNI option with an Interface Attributes 2687 sub-option with omIndex 0 and FMT/SRT/LHS/L2ADDR information for its 2688 INET interface and an Origin Indication sub-option with the mapped 2689 and obfuscated Port Number and IP address corresponding to the 2690 Client's RS encapsulation addresses. The Proxy/Server then includes 2691 one or more RIOs that encode the MSPs for the OMNI link, plus an MTU 2692 option (see Section 3.9). The Proxy/Server finally forwards the 2693 message to the Client using OAL encapsulation/fragmentation if 2694 necessary while including an acknowledgement if the RS invoked window 2695 synchronization. 2697 After the initial RS/RA exchange, the Proxy/Server maintains a 2698 ReachableTime timer for each of the Client's underlying interfaces 2699 individually (and for the Client's NCE collectively) set to expire 2700 after ReachableTime seconds. If the Client (or Proxy) issues 2701 additional RS messages, the Proxy/Server sends an RA response and 2702 resets ReachableTime. If the Proxy/Server receives an ND message 2703 with a prefix release indication it sets the Client's NCE to the 2704 DEPARTED state and withdraws the MNP from the routing system after a 2705 short delay (e.g., 2 seconds). If ReachableTime expires before a new 2706 RS is received on an individual underlying interface, the Proxy/ 2707 Server marks the interface as DOWN. If ReachableTime expires before 2708 any new RS is received on any individual underlying interface, the 2709 Proxy/Server sets the NCE state to STALE and sets a 10 second timer. 2710 If the Proxy/Server has not received a new RS or ND message with a 2711 prefix release indication before the 10 second timer expires, it 2712 deletes the NCE and withdraws the MNP from the routing system. 2714 The Proxy/Server processes any ND messages pertaining to the Client 2715 and returns an NA/RA reply in response to solicitations. The Proxy/ 2716 Server may also issue unsolicited RA messages, e.g., with reconfigure 2717 parameters to cause the Client to renegotiate its prefix delegation/ 2718 registrations, with Router Lifetime set to 0 if it can no longer 2719 service this Client, etc. Finally, If the NCE is in the DEPARTED 2720 state, the Proxy/Server deletes the entry after DepartTime expires. 2722 Note: Clients SHOULD notify former Proxy/Servers of their departures, 2723 but Proxy/Servers are responsible for expiring neighbor cache entries 2724 and withdrawing routes even if no departure notification is received 2725 (e.g., if the Client leaves the network unexpectedly). Proxy/Servers 2726 SHOULD therefore set Router Lifetime to ReachableTime seconds in 2727 solicited RA messages to minimize persistent stale cache information 2728 in the absence of Client departure notifications. A short Router 2729 Lifetime also ensures that proactive RS/RA messaging between Clients 2730 and Proxy/Servers will keep any NAT state alive (see above). 2732 Note: All Proxy/Servers on an OMNI link MUST advertise consistent 2733 values in the RA Cur Hop Limit, M and O flags, Reachable Time and 2734 Retrans Timer fields the same as for any link, since unpredictable 2735 behavior could result if different Proxy/Servers on the same link 2736 advertised different values. 2738 3.12.3.1. DHCPv6-Based Prefix Registration 2740 When a Client is not pre-provisioned with an MNP-LLA, it will need 2741 for the FHS Proxy/Server to select one or more MNPs on its behalf and 2742 set up the correct state in the AERO routing service. (A Client with 2743 a pre-provisioned MNP may also request the Proxy/Server to select 2744 additional MNPs.) The DHCPv6 service [RFC8415] is used to support 2745 this requirement. 2747 When a Client needs to have the FHS Proxy/Server select MNPs, it 2748 sends an RS message with source address set to the unspecified 2749 address (::) and with an OMNI option that includes a DHCPv6 message 2750 sub-option with DHCPv6 Prefix Delegation (DHCPv6-PD) parameters. 2751 When the Proxy/Server receives the RS message, it extracts the 2752 DHCPv6-PD message from the OMNI option. 2754 The Proxy/Server then acts as a "Proxy DHCPv6 Client" in a message 2755 exchange with the locally-resident DHCPv6 server, which delegates 2756 MNPs and returns a DHCPv6-PD Reply message. (If the Proxy/Server 2757 wishes to defer creation of MN state until the DHCPv6-PD Reply is 2758 received, it can instead act as a Lightweight DHCPv6 Relay Agent per 2759 [RFC6221] by encapsulating the DHCPv6-PD message in a Relay-forward/ 2760 reply exchange with Relay Message and Interface ID options.) 2762 When the Proxy/Server receives the DHCPv6-PD Reply, it adds a route 2763 to the routing system and creates an MNP-LLA based on the delegated 2764 MNP. The Proxy/Server then sends an RA back to the Client with the 2765 (newly-created) MNP-LLA as the destination address and with the 2766 DHCPv6-PD Reply message coded in the OMNI option. When the Client 2767 receives the RA, it creates a default route, assigns the Subnet 2768 Router Anycast address and sets its MNP-LLA based on the delegated 2769 MNP. 2771 Note: See [I-D.templin-6man-omni] for an MNP delegation alternative 2772 that avoids including a DHCPv6 message sub-option in the RS. Namely, 2773 when the Client requests a single MNP it can set the RS source to the 2774 unspecified address (::) and include a Node Identification sub-option 2775 and Preflen in the OMNI option (but with no DHCPv6 message sub- 2776 option). When the Proxy/Server receives the RS message, it forwards 2777 a self-generated DHCPv6 Solicit message to the DHCPv6 server on 2778 behalf of the Client. When the Proxy/Server receives the DHCPv6 2779 Reply, it prepares an RA message with an OMNI option with Preflen 2780 information (but with no DHCPv6 message sub-option), then places the 2781 (newly-created) MNP-LLA in the RA destination address and returns the 2782 message to the Client. 2784 3.13. The AERO Proxy Function 2786 OMNI link Clients register with one or more FHS Proxy/Servers for 2787 each underlying interface. Each of the Client's FHS Proxy/Servers 2788 must be informed of all of the Client's additional underlying 2789 interfaces. For Clients on Direct and VPNed underlying interfaces, 2790 the FHS Proxy/Server for each interface is directly connected, for 2791 Clients on ANET underlying interfaces the Proxy/Server is located on 2792 the ANET/INET boundary, and for Clients on INET underlying interfaces 2793 the FHS Proxy/Server is located somewhere in the connected 2794 Internetwork. When FHS Proxy/Server "A" processes a Client 2795 registration, it must also forward a proxyed registration to any 2796 Proxy/Servers "B", "C", "D", etc. for the Client's other underlying 2797 interfaces, which it perceives as LHS Proxy/Servers (i.e., and not 2798 FHS) from its own reference point. Proxy/Servers satisfies these 2799 requirements as follows: 2801 o when FHS Proxy/Server "A" receives a Client RS message, it first 2802 verifies that the OAL Identification is within the window for the 2803 NCE that matches the MNP-ULA for this Client neighbor and 2804 authenticates the message. (If no NCE was found, Proxy/Server "A 2805 instead creates one in the STALE state and returns an RA message 2806 with an authentication signature if necessary and any window 2807 synchronization parameters.) Proxy/Server "A" then examines the 2808 network-layer destination address. If the destination address is 2809 the ADM-LLA of a different Proxy/Server "B" (or, if the OMNI 2810 option included MS-Register/Release sub-options with the ADM-LLAs 2811 of one or more different LHS Proxy/Servers "B", "C", "D", etc.), 2812 Proxy/Server "A" prepares a separate proxyed version of the RS 2813 message with an OAL header with source set to its own ADM-ULA and 2814 destination set to the LHS Proxy/Server's ADM-ULA. Proxy/Server 2815 "A" also writes its own FMT/SRT/LHS/L2ADDR information over the 2816 first Interface Attributes supplied by the Client (i.e., by 2817 inserting the FMT/SRT/LHS/L2ADDR fields if not already present, or 2818 increasing/decreasing the L2ADDR field size if the IP version of 2819 the Client underlying interface differs from its own INET 2820 interface). Proxy/Server "A" then sets the S/T-omIndex to the 2821 value for this Client underlying interface, then forwards the 2822 message into the OMNI link secured spanning tree. 2824 o when LHS Proxy/Server "B" receives the RS, it authenticates the 2825 message then creates or updates a NCE for the Client with FHS 2826 Proxy/Server "A"'s Interface Attributes as the link-layer address 2827 information for this S/T-omIndex and caches any window 2828 synchronization parameters supplied by the Client. LHS Proxy/ 2829 Server "B" then prepares an RA message with source set to its own 2830 LLA and destination set to the Client's MNP-LLA, and with any 2831 window synchronization acknowledgements. Proxy/Server "B" then 2832 encapsulates the RA in an OAL header with source set to its own 2833 ADM-ULA and destination set to the ADM-ULA of Proxy/Server "A, 2834 performs fragmentation if necessary, then sends the resulting 2835 carrier packets into the secured spanning tree. 2837 o when Proxy/Server "A" reassembles the RA, it locates the Client 2838 NCE based on the RA destination LLA. Proxy/Server "A" then re- 2839 encapsulates the RA message with OAL source set to its own ADM-ULA 2840 and OAL destination set to the MNP-ULA of the Client, includes an 2841 authentication signature if necessary, and includes an Interface 2842 Attributes sub-option with omIndex 0 and with FMT/SRT/LHS/L2ADDR 2843 information for its INET interface. Proxy/Server "A" then 2844 fragments if necessary and returns the fragments to the Client. 2846 o The Client repeats this process over each of its additional 2847 underlying interfaces while treating each "B", "C", "D" as a FHS 2848 Proxy/Server while providing MS-Register//Release information for 2849 the others as LHS Proxy/Servers. 2851 After the initial RS/RA exchanges each Proxy/Server forwards any of 2852 the Client's carrier packets with OAL destinations for which there is 2853 no matching NCE to a Bridge using OAL encapsulation with its own ADM- 2854 ULA as the source and the destination determined by the ORH supplied 2855 by the Client. The Proxy/Server instead forwards any carrier packets 2856 destined to a neighbor cache target directly to the target according 2857 to the OAL/link-layer information - the process of establishing 2858 neighbor cache entries is specified in Section 3.14. 2860 While the Client is still associated with each Proxy/Server "A", "A" 2861 can send NS, RS and/or unsolicited NA messages to update the neighbor 2862 cache entries of other AERO nodes on behalf of the Client and/or to 2863 convey Interface Attributes updates. This allows for higher- 2864 frequency Proxy-initiated RS/RA messaging over well-connected INET 2865 infrastructure supplemented by lower-frequency Client-initiated RS/RA 2866 messaging over constrained ANET data links. 2868 If any Proxy/Server "B", "C", "D" ceases to send solicited RAs, 2869 Proxy/Server "A" sends unsolicited RAs over the Client's underlying 2870 interface with destination set to (link-local) All-Nodes multicast 2871 and with Router Lifetime set to zero to inform Clients that another 2872 Proxy/Server has failed. Although Proxy/Server "A" can engage in ND 2873 exchanges on behalf of the Client, the Client can also send ND 2874 messages on its own behalf, e.g., if it is in a better position than 2875 "A" to convey Interface Attribute changes, etc. The ND messages sent 2876 by the Client include the Client's MNP-LLA as the source in order to 2877 differentiate them from the ND messages sent by Proxy/Server "A". 2879 If the Client becomes unreachable over all underlying interface it 2880 serves, Proxy/Server "A" sets the NCE state to DEPARTED and retains 2881 the entry for DepartTime seconds. While the state is DEPARTED, 2882 Proxy/Server "A" forwards any carrier packets destined to the Client 2883 to a Bridge via OAL/ORH encapsulation. When DepartTime expires, 2884 Proxy/Server "A" deletes the NCE and discards any further carrier 2885 packets destined to the former Client. 2887 In some ANETs that employ a Proxy/Server, the Client's MNP can be 2888 injected into the ANET routing system. In that case, the Client can 2889 send original IP packets without invoking the OAL so that the ANET 2890 routing system transports the original IP packets to the Proxy. This 2891 can be very beneficial, e.g., if the Client connects to the ANET via 2892 low-end data links such as some aviation wireless links. 2894 If the ANET first-hop access router is on the same underlying link as 2895 the Client and recognizes the AERO/OMNI protocol, the Client can 2896 avoid OAL encapsulation for both its control and data messages. When 2897 the Client connects to the link, it can send an unencapsulated RS 2898 message with source address set to its own MNP-LLA (or to a Temporary 2899 LLA), and with destination address set to the ADM-LLA of the Client's 2900 selected Proxy/Server or to (link-local) All-Routers multicast. The 2901 Client includes an OMNI option formatted as specified in 2902 [I-D.templin-6man-omni]. The Client then sends the unencapsulated RS 2903 message, which will be intercepted by the AERO-Aware access router. 2905 The ANET access router then performs OAL encapsulation on the RS 2906 message and forwards it to a Proxy/Server at the ANET/INET boundary. 2907 When the access router and Proxy/Server are one and the same node, 2908 the Proxy/Server would share and underlying link with the Client but 2909 its message exchanges with outside correspondents would need to pass 2910 through a security gateway at the ANET/INET border. The method for 2911 deploying access routers and Proxys (i.e. as a single node or 2912 multiple nodes) is an ANET-local administrative consideration. 2914 Note: When a Proxy/Server alters the IPv6 ND message contents before 2915 forwarding (e.g., such as altering the OMNI option contents), the 2916 IPv6 ND message checksum and/or authentication signature are 2917 invalidated. If the Proxy/Server forwards the message over the 2918 secured spanning tree, however, it need not re-calculate the 2919 checksum/signature since they will not be examined by the next hop. 2921 Note: When a Proxy/Server receives a secured Client NS message, it 2922 performs the same proxying procedures as for described for RS 2923 messages above. The proxying procedures for NS/NA message exchanges 2924 is specified in Section 3.14. 2926 3.13.1. Detecting and Responding to Proxy/Server Failures 2928 In environments where fast recovery from Proxy/Server failure is 2929 required, Proxy/Server "A" SHOULD use proactive Neighbor 2930 Unreachability Detection (NUD) to track each peer Proxy/Server "B" 2931 reachability in a similar fashion as for Bidirectional Forwarding 2932 Detection (BFD) [RFC5880]. Proxy/Server "A" can then quickly detect 2933 and react to failures so that cached information is re-established 2934 through alternate paths. The NS/NA(NUD) control messaging is carried 2935 only over well-connected ground domain networks (i.e., and not low- 2936 end aeronautical radio links) and can therefore be tuned for rapid 2937 response. 2939 Proxy/Server "A" performs continuous NS/NA(NUD) exchanges with peer 2940 Proxy/Server "B" for which there are currently active Clients in 2941 rapid succession, e.g., one exchange per second. Proxy/Server "A" 2942 sends the NS(NUD) message via the spanning tree with its own ADM-LLA 2943 as the source and the ADM-LLA of the peer Proxy/Server "B" as the 2944 destination, and Proxy/Server "B" responds with an NA(NUD). When 2945 Proxy/Server "A" is also sending RS messages to the peer Proxy/Server 2946 "B" on behalf of Clients, the resulting RA responses can be 2947 considered as equivalent hints of forward progress. This means that 2948 Proxy/Server "B" need not also send a periodic NS(NUD) if it has 2949 already sent an RS within the same period. If the peer Proxy/Server 2950 "B" fails (i.e., if "A" ceases to receive advertisements), Proxy/ 2951 Server "A" can quickly inform Clients by sending unsolicited RA 2952 messages 2954 Proxy/Server "A" sends unsolicited RA messages with source address 2955 set to Proxy/Server "B"'s address, destination address set to (link- 2956 local) All-Nodes multicast, and Router Lifetime set to 0. Proxy/ 2957 Server "A" SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA messages 2958 separated by small delays [RFC4861]. Any Clients that had been using 2959 the failed Proxy/Server "B" will receive the RA messages and 2960 associate with a new Proxy/Server. 2962 3.13.2. Point-to-Multipoint Proxy/Server Coordination 2964 In environments where Client messaging over ANETs is bandwidth- 2965 limited and/or expensive, Clients can enlist the services of FHS 2966 Proxy/Server "A" to coordinate with multiple LHS Proxy/Servers "B", 2967 "C", "D" etc. in a single RS/RA message exchange. The Client can 2968 send a single RS message to (link-local) All-Routers multicast that 2969 includes the ID's of multiple Proxy/Servers in MS-Register/MS-Release 2970 OMNI sub-options. 2972 When FHS Proxy/Server "A" receives the RS and processes the OMNI 2973 option, it sends a separate RS to each MS-Register/MS-Release LHS 2974 Proxy/Server "B", "C", "D", etc. When FHS Proxy/Server "A" receives 2975 an LHS Proxy/Server RA, it can optionally return an immediate 2976 "singleton" RA to the Client or record the LHS Proxy/Server's MSID 2977 for inclusion in a pending "aggregate" RA message. FHS Proxy/Server 2978 "A" can then return aggregate RA messages to the Client including 2979 multiple LHS Proxy/Server MSIDs in order to conserve bandwidth. Each 2980 RA includes a proper subset of the LHS Proxy/Server MSIDs from the 2981 original RS message, and FHS Proxy/Server "A" must ensure that the 2982 message contents of each RA are consistent with the information 2983 received from the (aggregated) LHS Proxy/Servers. 2985 Clients can thereafter employ efficient point-to-multipoint LHS 2986 Proxy/Server coordination under the assistance of FHS Proxy/Server 2987 "A" to reduce the number of messages sent over the *NET. Clients can 2988 further include MS-Release sub-options in IPv6 ND messages to request 2989 FHS Proxy/Server "A" to release from former LHS Proxy/Servers via the 2990 procedures discussed in Section 3.16.5. 2992 The OMNI interface specification [I-D.templin-6man-omni] provides 2993 further discussion of the RS/RA messaging involved in point-to- 2994 multipoint coordination. 2996 3.14. AERO Route Optimization 2998 AERO nodes invoke route optimization when they need to forward 2999 packets to new target destinations. Route optimization is based on 3000 IPv6 ND Address Resolution messaging between a Route Optimization 3001 Source (ROS) and Route Optimization Responder (ROR). Route 3002 optimization is initiated by the first eligible ROS closest to the 3003 source as follows: 3005 o For Clients on VPNed and Direct interfaces, the Client's FHS 3006 Proxy/Server is the ROS. 3008 o For Clients on ANET interfaces, either the Client or the FHS 3009 Proxy/Server may be the ROS. 3011 o For Clients on INET interfaces, the Client itself is the ROS. 3013 o For correspondent nodes on INET/EUN interfaces serviced by a 3014 Relay, the Relay is the ROS. 3016 The route optimization procedure is conducted between the ROS and an 3017 LHS Proxy/Server/Relay for the target selected by routing as the ROR. 3018 In this arrangement, the ROS is always the Client or Proxy/Server/ 3019 Relay nearest the source over the selected source underlying 3020 interface, while the ROR is always an LHS Proxy/Server/Relay for the 3021 target regardless of the target underlying interface. 3023 The AERO routing system directs a route optimization request sent by 3024 the ROS to the nearest available ROR, which returns a route 3025 optimization reply. The exact ROR selected is unimportant as long as 3026 the information returned is current, consistent and authentic. The 3027 ROS is responsible for periodically refreshing the route 3028 optimization, and the ROR is responsible for quickly informing the 3029 ROS of any changes. 3031 The procedures are specified in the following sections. 3033 3.14.1. Route Optimization Initiation 3035 When an original IP packet from a source node destined to a target 3036 node arrives, the ROS checks for a NCE with an MNP-LLA that matches 3037 the target destination. If there is a NCE in the REACHABLE state, 3038 the ROS invokes the OAL and forwards the resulting carrier packets 3039 according to the cached state then returns from processing. 3040 Otherwise, if there is no NCE the ROS creates one in the INCOMPLETE 3041 state. 3043 The ROS next places the original IP packet on a short queue then 3044 sends an NS message for Address Resolution (NS(AR)) to receive a 3045 solicited NA(AR) message from an ROR. The NS(AR) message must be 3046 sent securely, and includes: 3048 o the LLA of the ROS as the source address. 3050 o the MNP-LLA corresponding to the original IP packet's destination 3051 as the Target Address, e.g., for 2001:db8:1:2::10:2000 the Target 3052 Address is fe80::2001:db8:1:2. 3054 o the Solicited-Node multicast address [RFC4291] formed from the 3055 lower 24 bits of the original IP packet's destination as the 3056 destination address, e.g., for 2001:db8:1:2::10:2000 the NS(AR) 3057 destination address is ff02:0:0:0:0:1:ff10:2000. 3059 The NS(AR) message also includes an OMNI option with an 3060 authentication sub-option if necessary, an Interface Attributes sub- 3061 option for the underlying interface, with S/T-omIndex set to the 3062 underlying interface index and with Preflen set to the prefix length 3063 associated with the NS(AR) source. The ROS then selects an 3064 Identification value submits the NS(AR) message for OAL encapsulation 3065 with OAL source set to its own ULA and OAL destination set to the ULA 3066 corresponding to the target. (The ROS does not include any window 3067 synchronization parameters, since it will not exchange other packet 3068 types with the ROR.) The ROS then sends the resulting carrier 3069 packet(s) into the SRT secured spanning tree without decrementing the 3070 network-layer TTL/Hop Limit field. 3072 When the ROS is an INET Client, it must instead forward the resulting 3073 carrier packets to the ADM-ULA of one of its current Proxy/Servers. 3074 The Proxy/Server reassembles if necessary, then verifies the NS(AR) 3075 authentication signature and writes its FMT/SRT/LHS/L2ADDR 3076 information into the Interface Attributes sub-option the same as 3077 described for RS messages in Section 3.13. The Proxy/Server then re- 3078 encapsulates the NS(AR) with the OAL source set to its own ADM-ULA 3079 and OAL destination set to the ULA corresponding to the target, 3080 fragments if necessary and forwards the resulting carrier packets 3081 into the secured spanning tree on behalf of the Client. 3083 3.14.2. Relaying the NS(AR) *NET Packet(s) 3085 When the Bridge receives the carrier packet(s) containing the RS from 3086 the ROS, it discards the *NET headers and determines the next hop by 3087 consulting its standard IPv6 forwarding table for the OAL header 3088 destination address. The Bridge then decrements the OAL header Hop- 3089 Limit, then re-encapsulates and forwards the carrier packet(s) via 3090 the secured spanning tree the same as for any IPv6 router, where it 3091 may traverse multiple OMNI link segments. The final-hop Bridge will 3092 deliver the carrier packet(s) via the secured spanning tree to a 3093 Proxy/Server or Relay that services the target. 3095 3.14.3. Processing the NS(AR) and Sending the NA(AR) 3097 When an LHS Proxy/Server (or Relay) for the target receives the 3098 secured carrier packet(s), it reassembles if necessary then examines 3099 the NS(AR) target to determine whether it has a matching NCE and/or 3100 non-MNP route. If there is no match, the Proxy/Server drops the 3101 message. Otherwise, the LHS Proxy/Server/Relay continues processing 3102 as follows: 3104 o if the NS(AR) target matches a Client NCE in the DEPARTED state, 3105 the Proxy/Server re-encapsulates while setting the OAL source to 3106 the ULA of the ROS and OAL destination address to the ADM-ULA of 3107 the Client's new Proxy/Server. The (old) Proxy/Server then 3108 fragments if necessary and forwards the resulting carrier 3109 packet(s) over the secured spanning tree then returns from 3110 processing. 3112 o If the NS(AR) target matches the MNP-LLA of a Client NCE in the 3113 REACHABLE state, the Proxy/Server notes whether the NS (AR) 3114 arrived from the secured spanning tree then acts as an ROR to 3115 provide route optimization information on behalf of the Client. 3116 If the message arrived via the secured spanning tree the ROR need 3117 not perform further authentication; otherwise, it must verify the 3118 message authentication signature before accepting. 3120 o If the NS(AR) target matches one of its non-MNP routes, the Relay 3121 acts as both an ROR and a route optimization target, since the 3122 Relay forwards IP packets toward the (fixed network) target at the 3123 network layer. 3125 The ROR next checks the target NCE for a Report List entry that 3126 matches the NS(AR) source LLA/ULA of the ROS. If there is a Report 3127 List entry, the ROR refreshes ReportTime for this ROR; otherwise, the 3128 ROR creates a new entry for the ROS and records both the LLA and ULA. 3130 The ROR then prepares a (solicited) NA(AR) message to return to the 3131 ROS with the source address set to its own ADM-LLA, the destination 3132 address set to the NS(AR) LLA source address and the Target Address 3133 set to the target Client's MNP-LLA. The ROR includes an OMNI option 3134 with Preflen set to the prefix length associated with the NA(AR) 3135 source address, with S/T-omIndex set to the value that appeared in 3136 the NS(AR) and with Interface Attributes sub-options for all of the 3137 target's underlying interfaces with current information for each 3138 interface. 3140 For each Interface Attributes sub-option, the ROR sets the L2ADDR 3141 according to its own INET address for VPNed, Direct, ANET and NATed 3142 Client interfaces, or to the Client's INET address for native Client 3143 interfaces. The ROR then includes the lower 32 bits of the Proxy/ 3144 Server's ADM-ULA as the LHS, encodes the ADM-ULA SRT prefix length in 3145 the SRT field and sets FMT as specified in Section 3.3. 3147 The ROR then sets the NA(AR) message R flag to 1 (as a router) and S 3148 flag to 1 (as a response to a solicitation) and sets the O flag to 0 3149 (as a proxy) and sets the OMNI header S/T-omIndex to 0. The ROR 3150 finally submits the NA(AR) for OAL encapsulation with source set to 3151 its own ULA and destination set to the same ULA that appeared in the 3152 NS(AR) OAL source, then performs OAL encapsulation and fragmentation 3153 using the same Identification value that appeared in the NS(AR) and 3154 finally forwards the resulting (*NET-encapsulated) carrier packets 3155 via the secured spanning tree without decrementing the network-layer 3156 TTL/Hop Limit field. 3158 3.14.4. Relaying the NA(AR) 3160 When the Bridge receives NA(AR) carrier packets from the ROR, it 3161 discards the *NET header and determines the next hop by consulting 3162 its standard IPv6 forwarding table for the OAL header destination 3163 address. The Bridge then decrements the OAL header Hop-Limit, re- 3164 encapsulates the carrier packet and forwards it via the SRT secured 3165 spanning tree the same as for any IPv6 router, where it may traverse 3166 multiple OMNI link segments. The final-hop Bridge will deliver the 3167 carrier packet via the secured spanning tree to a Proxy/Server for 3168 the ROS. 3170 3.14.5. Processing the NA(AR) 3172 When the ROS receives the NA(AR) message from the ROR, it first 3173 searches for a NCE that matches the NA(AR) target address. The ROS 3174 then processes the message the same as for standard IPv6 Address 3175 Resolution [RFC4861]. In the process, it caches all OMNI option 3176 information in the target NCE (including all Interface Attributes), 3177 and caches the NA(AR) ADM-{LLA,ULA} source addresses as the addresses 3178 of the ROR. If the ROS receives additional NA(AR) or uNA messages 3179 for this target Client with the same ADM-LLA source address but a 3180 different ADM-ULA source address, it configues the ADM-LLA 3181 corresponding to the new ADM-ULA, then caches the new ADM-{LLA,ULA} 3182 and deprecates the former ADM-{LLA,ULA}. 3184 When the ROS is a Client, the solicited NA(AR) message will first be 3185 delivered via the SRT secured spanning tree to the Proxy/Server that 3186 forwarded the NS(AR), which reassembles if necessary. The Proxy/ 3187 Server then forwards the message to the Client while re-encapsulating 3188 and re-fragmenting if necessary. If the Client is on a well-managed 3189 ANET, physical security and protected spectrum ensures security for 3190 the unmodified NA(AR); if the Client is on the open INET the Proxy/ 3191 Server must instead include an authentication signature (while 3192 adjusting the OMNI option size, if necessary). The Proxy/Server uses 3193 its own ADM-ULA as the OAL source and the MNP-ULA of the Client as 3194 the OAL destination. 3196 3.14.6. Forwarding Packets to Route Optimized Targets 3198 After the ROS receives the route optimization NA(AR) and updates the 3199 target NCE, it sends additional NS(AR) messages to the ADM-ULA of the 3200 ROR to refresh the NCE ReachableTime before expiration while it still 3201 has sustained interest in this target. While the NCE remains 3202 REACHABLE, the ROS can forward packets along the best underlying 3203 interface paths based on the target's Interface Attributes. The ROS 3204 selects target underlying interfaces according to traffic selectors 3205 and/or any other traffic discriminators, however each underlying 3206 interface selected must first establish window synchronization state 3207 if necessary. 3209 To establish window synchronization state, the ROS performs a secured 3210 unicast NS/NA(WIN) exchange with window synchronization parameters 3211 according to the Interface Attributes FMT code. If FMT-Forward is 3212 set, the ROS prepares an NS(WIN) with its own LLA as the source and 3213 the MNP-LLA of the target Client as the destination; otherwise, it 3214 sets the ADM-LLA of the LHS Proxy/Server as the destination. The ROS 3215 then encapsulates the NS(WIN) in an OAL header with its own ULA as 3216 the source and includes an ORH with Segments Left set to 1 and with 3217 FMT/SRT/LHSL2ADDR addressing information for the LHS Proxy/Server. 3218 If the ROS is the Client, it sets the OAL destination to the ADM-ULA 3219 of its FHS Proxy/Server, includes an authentication signature if 3220 necessary, includes an Interface Attributes sub-option for its 3221 underlying interface and, then forwards the NS(WIN) to its FHS Proxy/ 3222 Server which must reassemble, verify the authentication signature and 3223 rewrite the Interface Attributes FMT/SRT/LHS/L2ADDR information if 3224 necessary. The FHS Proxy/Server then re-encapsulates, re-fragments 3225 and forwards the NS(WIN) carrier packets into the secured spanning 3226 tree with its own ADM-ULA as the OAL source and the Subnet Router 3227 Anycast address for the LHS as the OAL destination. 3229 When an LHS Bridge that configures the Subnet Router Anycast address 3230 receives the NS(WIN) carrier packets over the secured spanning tree, 3231 it creates/updates a NCE for the NS(WIN) destination and creates a 3232 Peer List entry and Peer Index as discussed in Section 3.2.7. The 3233 Bridge then re-encapsulates, re-fragments and forwards the NS(WIN) 3234 carrier packets into the secured spanning tree with the ADM-ULA of 3235 the LHS Proxy/Server as the OAL destination. 3237 When the LHS Proxy/Server receives the NS(WIN) carrier packets it 3238 first reassembles if necessary. If the NS(WIN) destination is its 3239 own ADM-LLA, the LHS Proxy/Server creates an NCE based on the NS(WIN) 3240 source LLA, caches the window synchronization information, and 3241 prepares an NA(WIN) while using its own ADM-LLA as the source and the 3242 ROS LLA as the destination. The LHS Proxy/Server then encapsulates 3243 the NA(WIN) in an OAL header with source set to its own ADM-ULA and 3244 destination set to the Subnet Router Anycast address for the NS(WIN) 3245 OAL source. The LHS Proxy/Server then fragments if necessary, 3246 includes an ORH with FMT/SRT/LHS/L2ADDR found in the NS(WIN) OMNI 3247 option, then forwards the resulting carrier packets into the secured 3248 spanning tree which will deliver them to a FHS Bridge on the ROS 3249 Proxy/Server segment. 3251 If the NS(WIN) destination is the MNP-LLA of the target Client, the 3252 LHS Proxy/Server instead removes the ORH, re-encapsulates using the 3253 same OAL source and the MNP-ULA of the target as the OAL destination 3254 and includes an authentication signature (while adjusting the OMNI 3255 option size) if necessary. The LHS Proxy/Server then forwards the 3256 NS(WIN) to the target Client over the underlying interface identified 3257 by the ORH omIndex (or, over any underlying interface if omIndex is 3258 0). When the target Client receives the NS(WIN), it verifies the 3259 authentication signature if necessary then creates an NCE for the ROS 3260 LLA, caches the window synchronization information and prepares an 3261 NA(WIN) to return to the ROS with its MNP-LLA as the source and the 3262 LLA of the ROS as the destination, and with an Interface Attributes 3263 sub-option and an authentication signature if necessary. The target 3264 Client then encapsulates the NA(WIN) in an OAL header with its own 3265 MNP-ULA as the source, the ADM-ULA of the LHS Proxy/Server as the 3266 destination, and with an ORH with FMT/SRT/LHS/L2ADDR information 3267 copied from the NS(WIN) Interface Attributes. The target Client then 3268 sets the ORH omIndex to the S/T-omIndex value found in the NS(WIN) 3269 OMNI option, then forwards the NA(WIN) message to the LHS Proxy/ 3270 Server. 3272 When the LHS Proxy/Server receives the message, it reassembles if 3273 necessary, verifies the authentication signature and rewrites the 3274 Interface Attributes FMT/SRT/LHS/L2ADDR information if necessary, 3275 then re-encapsulates using its own ADM-ULA as the source and the ADM- 3276 ULA of the LHS Bridge as the destination. The LHS Proxy/Server then 3277 re-fragments and forwards the NA(WIN) carrier packets into the 3278 secured spanning tree which will direct them to the LHS Bridge. 3280 When the LHS Bridge receives the NA(WIN) carrier packets, it 3281 reassembles if necessary then locates the NCE for the local segment 3282 Client along with the Peer List information it had previously cached 3283 for this ROS. The LHS Bridge then re-encapsulates using its own ADM- 3284 ULA as the source and the Subnet Router Anycast for the FHS segment 3285 as the destination. The LHS Bridge then re-fragments and forwards 3286 the NA(WIN) carrier packets into the secured spanning tree which will 3287 direct them to the FHS segment. 3289 When an FHS Bridge that configures the Subnet Router Anycast address 3290 receives the NA(WIN) carrier packets, it creates/updates a NCE for 3291 the NA(WIN) destination, creates a Peer List entry and Peer Index and 3292 rewrites the Interface Attributes FMT/SRT/LHS/L2ADDR information as 3293 discussed in Section 3.2.7. The Bridge then re-encapsulates, re- 3294 fragments and forwards the NA(WIN) carrier packets into the secured 3295 spanning tree with the ADM-ULA of the FHS Proxy/Server as the OAL 3296 destination while leaveing the ADM-ULA of the LHS Bridge as the OAL 3297 source. 3299 When the FHS Proxy/Server receives the NA(WIN), it reassembles if 3300 necessary then updates the NCE for the NA(WIN) target based on the 3301 message contents if the Proxy/Server itself is the ROS. If the 3302 NA(WIN) source was the ADM-LLA of the LHS Proxy/Server, the ROS must 3303 also create and maintain a NCE for the LHS Proxy/Server which it must 3304 regard as a companion to the existing target Client NCE. (The NCE 3305 for the LHS Proxy/Server can also be shared by multiple target Client 3306 NCEs if the ROS communicates with multiple active targets located 3307 behind the same LHS Proxy/Server.) 3309 If the Client is the ROS, the FHS Proxy/Server instead inserts an 3310 authentication signature (while adjusting the OMNI option size) if 3311 necessary, removes the ORH then re-encapsulates and re-fragments if 3312 necessary while changing the OAL destination to the MNP-ULA of the 3313 ROS Client. The FHS Proxy/Server then forwards the NA(WIN) to the 3314 ROS Client over the underlying interface identified by the ORH 3315 omIndex which then updates its own NCE based on the target Client 3316 MNP-LLA or LHS Proxy/Server ADM-LLA. 3318 The ROS (whether the FHS Proxy/Server or the Client itself) finally 3319 arranges to return an acknowledgement if requested by the NA(WIN) by 3320 sending an NA(WIN) acknowledgement in the reverse path over the 3321 secured spanning tree with its own ULA as the source and the ADM-ULA 3322 of the LHS Bridge as the destination. When the NA(WIN) 3323 acknowledgement arrives, the LHS Bridge updates the destination 3324 Client NCE, rewrites the Interface Attributes and securely forwards 3325 the NA(WIN) toward the destination. 3327 After window synchronization state has been established, the ROS and 3328 target Client can begin forwarding carrier packets as specified in 3329 Section 3.2.7 while performing additional NS/NA(WIN) exchanges as 3330 above to update window state and/or test reachability. The ROS sends 3331 carrier packets to the FHS Bridge discovered through the NS/NA(WIN) 3332 exhange which verifies the Identification is in window for the target 3333 Client. The FHS Bridge then forwards the carrier packets over the 3334 unsecured spanning tree to the LHS Bridge, which forwards them via 3335 LHS encapsulation to the LHS Proxy/Server or directly to the target 3336 Client itself. The target Client in turn sends packets to the ROS in 3337 the reverse direction while forwarding through the Bridges to 3338 minimize Proxy/Server load whenever possible. 3340 The above forwarding procedures apply to the case where the selected 3341 target interface SRT/LHS codes indicate that the interface is located 3342 in a remote OMNI link segment. If the SRT/LHS codes indicate that 3343 the interface is in the local OMNI link segment, the ROS can instead 3344 forward carrier packets directly to the LHS Proxy/Server using L2ADDR 3345 encapsulation information following the initial NS/NA(WIN) secured 3346 spanning tree exchange (or even directly to the target Client itself 3347 while invoking NAT traversal if necessary). When the ROS sends 3348 packets directly to the LHS Proxy/Server, it includes an ORH if 3349 necessary to inform the Proxy/Server as to whether it must reassemble 3350 and/or the correct target Client omIndex for (re)forwarding. If the 3351 LHS Proxy/Server is required to reassemble, the ROS sets the OAL 3352 destination to the ADM-ULA of the LHS Proxy/Server; otherwise, it 3353 sets the OAL destination to the MNP-ULA of the target Client itself. 3354 When the ROS sends packets directly to the target Client, it need not 3355 include an ORH. 3357 While the ROS continues to actively forward packets to the target 3358 Client, it is responsible for updating window synchronization state 3359 and per-interface reachability before expiration. Window 3360 synchronization state is shared by all underlying interfaces in the 3361 ROS' NCE that use the same destination LLA so that a single NS/ 3362 NA(WIN) exchange applies for all interfaces regardless of the 3363 (single) interface used to conduct the exchange. However, the window 3364 synchronization exchange only confirms target Client reachability 3365 over the specific interface used to conduct the exchange. 3366 Reachability for other underlying interfaces that share the same 3367 window synchronization state must be determined individually using 3368 NS/NA(NUD) messages which need not be secured as long as they use in- 3369 window Identifications and do not update other state information. 3371 3.15. Neighbor Unreachability Detection (NUD) 3373 AERO nodes perform Neighbor Unreachability Detection (NUD) per 3374 [RFC4861] either reactively in response to persistent link-layer 3375 errors (see Section 3.11) or proactively to confirm reachability. 3376 The NUD algorithm is based on periodic control message exchanges and 3377 may further be seeded by ND hints of forward progress, but care must 3378 be taken to avoid inferring reachability based on spoofed 3379 information. For example, IPv6 ND message exchanges that include 3380 authentication codes and/or in-window Identifications may be 3381 considered as acceptable hints of forward progress, while spurious 3382 random carrier packets should be ignored. 3384 AERO nodes can perform NS/NA(NUD) exchanges over the OMNI link 3385 secured spanning tree (i.e. the same as described above for NS/ 3386 NA(WIN)) to test reachability without risk of DoS attacks from nodes 3387 pretending to be a neighbor. These NS/NA(NUD) messages use the 3388 unicast LLAs and ULAs of the parties involved in the NUD test the 3389 same as for standard IPv6 ND over the secured spanning tree. When 3390 only reachability information is required without updating any other 3391 NCE state, AERO nodes can instead perform NS/NA(NUD) exchanges 3392 directly between neighbors without employing the secured spanning 3393 tree as long as they include in-window Identifications and either an 3394 authentication signature or checksum. 3396 When an ROR directs an ROS to a target neighbor with one or more 3397 link-layer addresses, the ROS probes each unsecured target underlying 3398 interface either proactively or on-demand of carrier packets directed 3399 to the path by multilink forwarding to maintain the interface's state 3400 as reachable. Probing is performed through NS(NUD) messages over the 3401 SRT secured or unsecured spanning tree, or through NS(NUD) messages 3402 sent directly to an underlying interface of the target itself. While 3403 testing a target underlying interface, the ROS can optionally 3404 continue to forward carrier packets via alternate interfaces and/or 3405 maintain a small queue of carrier packets until target reachability 3406 is confirmed. 3408 NS(NUD) messages are encapsulated, fragmented and transmitted as 3409 carrier packets the same as for ordinary original IP data packets, 3410 however the encapsulated destinations are the LLA of the ROS and 3411 either the ADM-LLA of the LHS Proxy/Server or the MNP-LLA of the 3412 target itself. The ROS encapsulates the NS(NUD) message the same as 3413 described in Section 3.2.7, however Destination Trailers (if present) 3414 are set according to the LLA destination suffix (i.e., and not the 3415 ULA/GUA destination). The ROS sets the NS(NUD) OMNI header S/ 3416 T-omIndex to identify the underlying interface used for forwarding 3417 (or to 0 if any underlying interface can be used). The ROS also 3418 includes an ORH with FMT/SRT/LHS/L2ADDR information the same as for 3419 ordinary data packets, but does not include an authentication 3420 signature. The ROS then fragments the OAL packet and forwards the 3421 resulting carrier packets into the unsecured spanning tree or 3422 directly to the target (or LHS Proxy/Server) if it is in the local 3423 segment. 3425 When the target (or LHS Proxy/Server) receives the NS(NUD) carrier 3426 packets, it verifies that it has a NCE for this ROS and that the 3427 Identification is in-window, then submits the carrier packets for 3428 reassembly. The node then verifies the authentication signature or 3429 checksum, then searches for Interface Attributes in its NCE for the 3430 ROS that match the NS(NUD) S/T-omIndex and uses the FMT/SRT/LHS/ 3431 L2ADDR information to prepare an ORH for the NA(NUD) reply. The node 3432 then prepares the NA(NUD) with the source and destination LLAs 3433 reversed, encapsulates and sets the OAL source and destination, sets 3434 the NA(NUD) S/T-omIndex to the index of the underlying interface the 3435 NS(NUD) arrived on and sets the Target Address to the same value 3436 included in the NS(NUD). The target next sets the R flag to 1, the S 3437 flag to 1 and the O flag to 1, then selects an in-window 3438 Identification for the ROS and performs fragmentation. The node then 3439 forwards the carrier packets into the unsecured spanning tree, 3440 directly to the ROS if it is in the local segment or directly to a 3441 Bridge in the local segment. 3443 When the ROS receives the NA(NUD), it marks the target underlying 3444 interface tested as "reachable". Note that underlying interface 3445 states are maintained independently of the overall NCE REACHABLE 3446 state, and that a single NCE may have multiple target underlying 3447 interfaces in various states "reachable" and otherwise while the NCE 3448 state as a whole remains REACHABLE. 3450 Note also that the exchange of NS/NA(NUD) messages has the useful 3451 side-benefit of opening holes in NATs that may be useful for NAT 3452 traversal. 3454 3.16. Mobility Management and Quality of Service (QoS) 3456 AERO is a Distributed Mobility Management (DMM) service. Each Proxy/ 3457 Server is responsible for only a subset of the Clients on the OMNI 3458 link, as opposed to a Centralized Mobility Management (CMM) service 3459 where there is a single network mobility collective entity for all 3460 Clients. Clients coordinate with their associated Proxy/Servers via 3461 RS/RA exchanges to maintain the DMM profile, and the AERO routing 3462 system tracks all current Client/Proxy/Server peering relationships. 3464 Proxy/Servers provide default routing and mobility/multilink services 3465 for their dependent Clients. Clients are responsible for maintaining 3466 neighbor relationships with their Proxy/Servers through periodic RS/ 3467 RA exchanges, which also serves to confirm neighbor reachability. 3468 When a Client's underlying Interface Attributes change, the Client is 3469 responsible for updating the Proxy/Server with this new information. 3470 Note that when there is a Proxy/Server in the path, the Proxy 3471 function can also perform some RS/RA exchanges on the Client's 3472 behalf. 3474 Mobility management messaging is based on the transmission and 3475 reception of unsolicited Neighbor Advertisement (uNA) messages. Each 3476 uNA message sets the IPv6 source address to the LLA of the ROR and 3477 the destination address to the unicast LLA of the ROS. 3479 Mobility management considerations are specified in the following 3480 sections. 3482 3.16.1. Mobility Update Messaging 3484 RORs accommodate Client mobility and/or multilink change events by 3485 sending secured uNA messages to each ROS in the target Client's 3486 Report List. When an ROR sends a uNA message, it sets the IPv6 3487 source address to the its own LLA, sets the destination address to 3488 the ROS LLA (i.e., an MNP-LLA if the ROS is a Client and an ADM-LLA 3489 if the ROS is a Proxy/Server) and sets the Target Address to the 3490 Client's MNP-LLA. The ROR also includes an OMNI option with Preflen 3491 set to the prefix length associated with the Client's MNP-LLA, with 3492 Interface Attributes for the target Client's underlying interfaces 3493 and with the OMNI header S/T-omIndex set to 0. The ROR then sets the 3494 uNA R flag to 1, S flag to 0 and O flag to 1, then encapsulates the 3495 message in an OAL header with source set to its own ADM-ULA and 3496 destination set to the ROS ULA (i.e., the ADM-ULA of the ROS Proxy/ 3497 Server) and sends the message into the secured spanning tree. 3499 As discussed in Section 7.2.6 of [RFC4861], the transmission and 3500 reception of uNA messages is unreliable but provides a useful 3501 optimization. In well-connected Internetworks with robust data links 3502 uNA messages will be delivered with high probability, but in any case 3503 the Proxy/Server can optionally send up to MAX_NEIGHBOR_ADVERTISEMENT 3504 uNAs to each ROS to increase the likelihood that at least one will be 3505 received. Alternatively, the Proxy/Server can set the PNG flag in 3506 the uNA OMNI option header to request a solicited NA acknowledgement 3507 as specified in [I-D.templin-6man-omni]. 3509 When the ROS Proxy/Server receives a uNA message prepared as above, 3510 it ignores the message if the destination is not its own ADM-ULA or 3511 the MNP-ULA of the ROS Client. If the destination was its own ADM- 3512 ULA, the ROS Proxy/Server uses the included OMNI option information 3513 to update its NCE for the target but does not reset ReachableTime 3514 since the receipt of an unsolicited NA message from the ROR does not 3515 provide confirmation that any forward paths to the target Client are 3516 working. If the destination was the MNP-ULA of the ROS Client, the 3517 Proxy/Server instead re-encapsulates with the OAL source set to its 3518 own ADM-ULA, OAL destination set to the MNP-ULA of the ROS Client 3519 with an authentication signature if necessary, and with an in-window 3520 Identification for this Client. Finally, if the uNA message PNG flag 3521 was set, the ROS returns a solicited NA acknowledgement as specified 3522 in [I-D.templin-6man-omni]. 3524 In addition to sending uNA messages to the current set of ROSs for 3525 the target Client, the ROR also sends uNAs to the former Proxy/Server 3526 associated with the underlying interface for which the link-layer 3527 address has changed. These uNA messages update former Proxy/Servers 3528 that cannot easily detect (e.g., without active probing) when a 3529 formerly-active Client has departed. When the ROR sends the uNA, it 3530 sets the IPv6 source address to its LLA, sets the destination address 3531 to the former Proxy/Server's ADM-LLA, and sets the Target Address to 3532 the Client's MNP-LLA. The ROR also includes an OMNI option with 3533 Preflen set to the prefix length associated with the Client's MNP- 3534 LLA, with Interface Attributes for the changed underlying interface, 3535 and with the OMNI header S/T-omIndex set to 0. The ROR then sets the 3536 uNA R flag to 1, S flag to 0 and O flag to 1, then encapsulates the 3537 message in an OAL header with source set to its own ULA and 3538 destination set to the ADM-ULA of the former Proxy/Server and sends 3539 the message into the secured spanning tree. 3541 3.16.2. Announcing Link-Layer Address and/or QoS Preference Changes 3543 When a Client needs to change its underlying Interface Attributes 3544 (e.g., due to a mobility event), the Client requests one of its 3545 Proxy/Servers to send RS messages to all of its other Proxy/Servers 3546 via the secured spanning tree with an OMNI option that includes 3547 Interface Attributes with the new link quality and address 3548 information. 3550 Up to MAX_RTR_SOLICITATIONS RS messages MAY be sent in parallel with 3551 sending carrier packets containing user data in case one or more RAs 3552 are lost. If all RAs are lost, the Client SHOULD re-associate with a 3553 new Proxy/Server. 3555 When the Proxy/Server receives the Client's changes, it sends uNA 3556 messages to all nodes in the Report List the same as described in the 3557 previous section. 3559 3.16.3. Bringing New Links Into Service 3561 When a Client needs to bring new underlying interfaces into service 3562 (e.g., when it activates a new data link), it sends an RS message to 3563 the Proxy/Server via the underlying interface with an OMNI option 3564 that includes Interface Attributes with appropriate link quality 3565 values and with link-layer address information for the new link. 3567 3.16.4. Deactivating Existing Links 3569 When a Client needs to deactivate an existing underlying interface, 3570 it sends an RS message to an FHS Proxy/Server with an OMNI option 3571 with appropriate Interface Attribute values for the deactivated link 3572 - in particular, the link quality value 0 assures that neighbors will 3573 cease to use the link. 3575 If the Client needs to send RS messages over an underlying interface 3576 other than the one being deactivated, it MUST include Interface 3577 Attributes with appropriate link quality values for any underlying 3578 interfaces being deactivated. 3580 Note that when a Client deactivates an underlying interface, 3581 neighbors that have received the RS/uNA messages need not purge all 3582 references for the underlying interface from their neighbor cache 3583 entries. The Client may reactivate or reuse the underlying interface 3584 and/or its omIndex at a later point in time, when it will send new RS 3585 messages to an FHS Proxy/Server with fresh Interface Attributes to 3586 update any neighbors. 3588 3.16.5. Moving Between Proxy/Servers 3590 The Client performs the procedures specified in Section 3.12.2 when 3591 it first associates with a new FHS Proxy/Server or renews its 3592 association with an existing Proxy/Server. The Client also includes 3593 MS-Release identifiers in the RS message OMNI option per 3594 [I-D.templin-6man-omni] if it wants the new Proxy/Server to notify 3595 any old Proxy/Servers from which the Client is departing. 3597 When the new FHS Proxy/Server receives the Client's RS message, it 3598 returns an RA as specified in Section 3.12.3 and sends RS messages to 3599 any old Proxy/Servers listed in OMNI option MS-Release identifiers. 3600 When the new Proxy/Server sends an RS message, it sets the source to 3601 the MNP-LLA of the Client and sets the destination to the ADM-LLA of 3602 the old Proxy/Server. The new Proxy/Server also includes an OMNI 3603 option with Preflen set to the prefix length associated with the 3604 Client's MNP-LLA, with Interface Attributes for its INET underlying 3605 interface, and with the OMNI header S/T-omIndex set to 0. The new 3606 Proxy/Server then encapsulates the message in an OAL header with 3607 source set to its own ADM-ULA and destination set to the ADM-ULA of 3608 the old Proxy/Server and sends the message into the secured spanning 3609 tree. 3611 When an old Proxy/Server receives the RS, it notices that the message 3612 appears to have originated from the Client's MNP-LLA but that it 3613 includes an Interface Attributes sub-option for the new Proxy/Server 3614 and with S/T-omIndex set to 0. The old Proxy/Server then changes the 3615 Client's NCE state to DEPARTED, sets the FMT/SRT/LHS/L2ADDR 3616 information for the Client to point to the new Proxy/Server, and 3617 resets DepartTime. The old Proxy/Server then returns an RA message 3618 with zero Router Lifetime via the secured spanning tree by reversing 3619 the LLA and ULA addresses found in the RS message. After a short 3620 delay (e.g., 2 seconds) the old Proxy/Server withdraws the Client's 3621 MNP from the routing system. After DepartTime expires, the old 3622 Proxy/Server deletes the Client's NCE. 3624 The old Proxy/Server also iteratively sends uNA messages to each ROS 3625 in the Client's Report List with OAL source address set to the ADM- 3626 ULA of the new Proxy/Server and OAL destination address set to the 3627 ULA of the ROS. When the ROS receives the uNA, it examines the uNA 3628 Target Address to locate the target Client's NCE and the LLA source 3629 address to identify the old Proxy/Server. The ROS then caches the 3630 ULA source address as the ADM-{LLA/ULA} for the new Proxy/Server for 3631 this target NCE and marks the entry as STALE. While in the STALE 3632 state, the ROS sends new NS(AR) messages using its own ULA as the OAL 3633 source and the ADM-ULA of the new Proxy/Server as the OAL destination 3634 address. The new Proxy/Server will then process the NS(AR) and 3635 return an NA(AR) response. 3637 Clients SHOULD NOT move rapidly between Proxy/Servers in order to 3638 avoid causing excessive oscillations in the AERO routing system. 3639 Examples of when a Client might wish to change to a different Proxy/ 3640 Server include a Proxy/Server that has gone unreachable, topological 3641 movements of significant distance, movement to a new geographic 3642 region, movement to a new OMNI link segment, etc. 3644 3.17. Multicast 3646 The AERO Client provides an IGMP (IPv4) [RFC2236] or MLD (IPv6) 3647 [RFC3810] proxy service for its EUNs and/or hosted applications 3648 [RFC4605]. Proxy/Servers act as a Protocol Independent Multicast - 3649 Sparse-Mode (PIM-SM, or simply "PIM") Designated Router (DR) 3650 [RFC7761]. AERO Relays also act as PIM routers (i.e., the same as 3651 AERO Proxys/Servers) on behalf of nodes on INET/EUN networks. 3653 Clients on ANET underlying interfaces for which the ANET has deployed 3654 native multicast services forward IGMP/MLD messages into the ANET. 3656 The IGMP/MLD messages may be further forwarded by a first-hop ANET 3657 access router acting as an IGMP/MLD-snooping switch [RFC4541], then 3658 ultimately delivered to an ANET FHS Proxy/Server. 3660 Clients on ANET underlying interfaces without native multicast 3661 services instead send NS(AR) messages to cause their FHS Proxy/Server 3662 to act as an ROS and forward the message to an LHS Proxy/Server ROR. 3663 Clients on INET interfaces act as an ROS on their own behalf and 3664 forward NS(AR) messages directly to the LHS Proxy/Server ROR (i.e., 3665 via the FHS Proxy/Server as a proxy). When the Client receives an 3666 NA(AR) response, it initiates PIM protocol messaging according to the 3667 Source-Specific Multicast (SSM) and Any-Source Multicast (ASM) 3668 operational modes as discussed in the following sections. 3670 3.17.1. Source-Specific Multicast (SSM) 3672 When an ROS "X" (i.e., either a ROS Client or its FHS Proxy Server) 3673 acting as PIM router receives a Join/Prune message from a node on its 3674 downstream interfaces containing one or more ((S)ource, (G)roup) 3675 pairs, it updates its Multicast Routing Information Base (MRIB) 3676 accordingly. For each S belonging to a prefix reachable via X's non- 3677 OMNI interfaces, X then forwards the (S, G) Join/Prune to any PIM 3678 routers on those interfaces per [RFC7761]. 3680 For each S belonging to a prefix reachable via X's OMNI interface, X 3681 sends an NS(AR) message (see: Section 3.14) using its own LLA as the 3682 source address and the LLA of S as the destination address. X then 3683 encapsulates the NS(AR) in an OAL header with source address set to 3684 the ULA of X and destination address set to the solicited node 3685 multicast address for S, then forwards the message into the secured 3686 spanning tree, which delivers it to ROR "Y" that services S. The 3687 resulting NA(AR) will return the LLA for the prefix that matches S as 3688 the network-layer source address and with an OMNI option with 3689 interface attributes for any underlying interfaces that are currently 3690 servicing S. 3692 When X processes the NA(AR) it selects one or more underlying 3693 interfaces for S and performs an NS/NA(WIN) exchange while including 3694 a PIM Join/Prune message for each multicast group of interest in the 3695 OMNI option. If S is located behind any Proxys "Z"*, each Z* then 3696 updates its MRIB accordingly and maintains the LLA of X as the next 3697 hop in the reverse path. Since the Bridges do not examine network 3698 layer control messages, this means that the (reverse) multicast tree 3699 path is simply from each Z* (and/or S) to X with no other multicast- 3700 aware routers in the path. 3702 Following the initial combined Join/Prune and NS/NA messaging, X 3703 maintains a NCE for each S the same as if X was sending unicast data 3704 traffic to S. In particular, X performs additional NS/NA exchanges 3705 to keep the NCE alive for up to t_periodic seconds [RFC7761]. If no 3706 new Joins are received within t_periodic seconds, X allows the NCE to 3707 expire. Finally, if X receives any additional Join/Prune messages 3708 for (S,G) it forwards the messages over the secured spanning tree. 3710 At some later time, Client C that holds an MNP for source S may 3711 depart from a first Proxy/Server Z1 and/or connect via a new Proxy/ 3712 Server Z2. In that case, Y sends a uNA message to X the same as 3713 specified for unicast mobility in Section 3.16. When X receives the 3714 uNA message, it updates its NCE for the LLA for source S and sends 3715 new Join messages to any new Proxys Z2. There is no requirement to 3716 send any Prune messages to old Proxy/Server Z1 since source S will no 3717 longer source any multicast data traffic via Z1. Instead, the 3718 multicast state for (S,G) in Proxy/Server Z1 will soon time out since 3719 no new Joins will arrive. 3721 After some later time, C may move to a new Proxy/Server Y2 and depart 3722 from old Sever Y1. In that case, Y1 sends Join messages for any of 3723 C's active (S,G) groups to Y2 while including its own LLA as the 3724 source address. This causes Y2 to include Y1 in the multicast 3725 forwarding tree during the interim time that Y1's NCE for C is in the 3726 DEPARTED state. At the same time, Y1 sends a uNA message to X with 3727 an OMNI option with S/T-omIndex set to 0 and a release indication to 3728 cause X to release its NCE for S. X then sends a new Join message to 3729 S via the secured spanning tree and re-initiates route optimization 3730 the same as if it were receiving a fresh Join message from a node on 3731 a downstream link. 3733 3.17.2. Any-Source Multicast (ASM) 3735 When an ROS X acting as a PIM router receives a Join/Prune from a 3736 node on its downstream interfaces containing one or more (*,G) pairs, 3737 it updates its Multicast Routing Information Base (MRIB) accordingly. 3738 X then forwards a copy of the message within the OMNI option of an 3739 NS(WIN) message to the Rendezvous Point (RP) R for each G over the 3740 secured spanning tree. X uses its own LLA as the source address and 3741 the LLA for R as the destination address, then encapsulates the 3742 NS(WIN) message in an OAL header with source address set to the ULA 3743 of X and destination address set to the ULA of R's Proxy/Server then 3744 sends the message into the secured spanning tree. 3746 For each source S that sends multicast traffic to group G via R, the 3747 Proxy/Server Z* for the Client that aggregates S encapsulates the 3748 original IP packets in PIM Register messages and forwards them to R 3749 via the secured spanning tree, which may then elect to send a PIM 3750 Join to Z*. This will result in an (S,G) tree rooted at Z* with R as 3751 the next hop so that R will begin to receive two copies of the 3752 original IP packet; one native copy from the (S, G) tree and a second 3753 copy from the pre-existing (*, G) tree that still uses PIM Register 3754 encapsulation. R can then issue a PIM Register-stop message to 3755 suppress the Register-encapsulated stream. At some later time, if C 3756 moves to a new Proxy/Server Z*, it resumes sending original IP 3757 packets via PIM Register encapsulation via the new Z*. 3759 At the same time, as multicast listeners discover individual S's for 3760 a given G, they can initiate an (S,G) Join for each S under the same 3761 procedures discussed in Section 3.17.1. Once the (S,G) tree is 3762 established, the listeners can send (S, G) Prune messages to R so 3763 that multicast original IP packets for group G sourced by S will only 3764 be delivered via the (S, G) tree and not from the (*, G) tree rooted 3765 at R. All mobility considerations discussed for SSM apply. 3767 3.17.3. Bi-Directional PIM (BIDIR-PIM) 3769 Bi-Directional PIM (BIDIR-PIM) [RFC5015] provides an alternate 3770 approach to ASM that treats the Rendezvous Point (RP) as a Designated 3771 Forwarder (DF). Further considerations for BIDIR-PIM are out of 3772 scope. 3774 3.18. Operation over Multiple OMNI Links 3776 An AERO Client can connect to multiple OMNI links the same as for any 3777 data link service. In that case, the Client maintains a distinct 3778 OMNI interface for each link, e.g., 'omni0' for the first link, 3779 'omni1' for the second, 'omni2' for the third, etc. Each OMNI link 3780 would include its own distinct set of Bridges and Proxy/Servers, 3781 thereby providing redundancy in case of failures. 3783 Each OMNI link could utilize the same or different ANET connections. 3784 The links can be distinguished at the link-layer via the SRT prefix 3785 in a similar fashion as for Virtual Local Area Network (VLAN) tagging 3786 (e.g., IEEE 802.1Q) and/or through assignment of distinct sets of 3787 MSPs on each link. This gives rise to the opportunity for supporting 3788 multiple redundant networked paths (see: Section 3.2.5). 3790 The Client's IP layer can select the outgoing OMNI interface 3791 appropriate for a given traffic profile while (in the reverse 3792 direction) correspondent nodes must have some way of steering their 3793 original IP packets destined to a target via the correct OMNI link. 3795 In a first alternative, if each OMNI link services different MSPs the 3796 Client can receive a distinct MNP from each of the links. IP routing 3797 will therefore assure that the correct OMNI link is used for both 3798 outbound and inbound traffic. This can be accomplished using 3799 existing technologies and approaches, and without requiring any 3800 special supporting code in correspondent nodes or Bridges. 3802 In a second alternative, if each OMNI link services the same MSP(s) 3803 then each link could assign a distinct "OMNI link Anycast" address 3804 that is configured by all Bridges on the link. Correspondent nodes 3805 can then perform Segment Routing to select the correct SRT, which 3806 will then direct the original IP packet over multiple hops to the 3807 target. 3809 3.19. DNS Considerations 3811 AERO Client MNs and INET correspondent nodes consult the Domain Name 3812 System (DNS) the same as for any Internetworking node. When 3813 correspondent nodes and Client MNs use different IP protocol versions 3814 (e.g., IPv4 correspondents and IPv6 MNs), the INET DNS must maintain 3815 A records for IPv4 address mappings to MNs which must then be 3816 populated in Relay NAT64 mapping caches. In that way, an IPv4 3817 correspondent node can send original IPv4 packets to the IPv4 address 3818 mapping of the target MN, and the Relay will translate the IPv4 3819 header and destination address into an IPv6 header and IPv6 3820 destination address of the MN. 3822 When an AERO Client registers with an AERO Proxy/Server, the Proxy/ 3823 Server can return the address(es) of DNS servers in RDNSS options 3824 [RFC6106]. The DNS server provides the IP addresses of other MNs and 3825 correspondent nodes in AAAA records for IPv6 or A records for IPv4. 3827 3.20. Transition/Coexistence Considerations 3829 OAL encapsulation ensures that dissimilar INET partitions can be 3830 joined into a single unified OMNI link, even though the partitions 3831 themselves may have differing protocol versions and/or incompatible 3832 addressing plans. However, a commonality can be achieved by 3833 incrementally distributing globally routable (i.e., native) IP 3834 prefixes to eventually reach all nodes (both mobile and fixed) in all 3835 OMNI link segments. This can be accomplished by incrementally 3836 deploying AERO Bridges on each INET partition, with each Bridge 3837 distributing its MNPs and/or discovering non-MNP IP GUA prefixes on 3838 its INET links. 3840 This gives rise to the opportunity to eventually distribute native IP 3841 addresses to all nodes, and to present a unified OMNI link view even 3842 if the INET partitions remain in their current protocol and 3843 addressing plans. In that way, the OMNI link can serve the dual 3844 purpose of providing a mobility/multilink service and a transition/ 3845 coexistence service. Or, if an INET partition is transitioned to a 3846 native IP protocol version and addressing scheme that is compatible 3847 with the OMNI link MNP-based addressing scheme, the partition and 3848 OMNI link can be joined by Bridges. 3850 Relays that connect INETs/EUNs with dissimilar IP protocol versions 3851 may need to employ a network address and protocol translation 3852 function such as NAT64 [RFC6146]. 3854 3.21. Detecting and Reacting to Proxy/Server and Bridge Failures 3856 In environments where rapid failure recovery is required, Proxy/ 3857 Servers and Bridges SHOULD use Bidirectional Forwarding Detection 3858 (BFD) [RFC5880]. Nodes that use BFD can quickly detect and react to 3859 failures so that cached information is re-established through 3860 alternate nodes. BFD control messaging is carried only over well- 3861 connected ground domain networks (i.e., and not low-end radio links) 3862 and can therefore be tuned for rapid response. 3864 Proxy/Servers and Bridges maintain BFD sessions in parallel with 3865 their BGP peerings. If a Proxy/Server or Bridge fails, BGP peers 3866 will quickly re-establish routes through alternate paths the same as 3867 for common BGP deployments. Similarly, Proxys maintain BFD sessions 3868 with their associated Bridges even though they do not establish BGP 3869 peerings with them. 3871 3.22. AERO Clients on the Open Internet 3873 AERO Clients that connect to the open Internet via INET interfaces 3874 can establish a VPN or direct link to securely connect to a FHS 3875 Proxy/Server in a "tethered" arrangement with all of the Client's 3876 traffic transiting the Proxy/Server which acts as a router. 3877 Alternatively, the Client can associate with an INET FHS Proxy/Server 3878 using UDP/IP encapsulation and control message securing services as 3879 discussed in the following sections. 3881 When a Client's OMNI interface enables an INET underlying interface, 3882 it first examines the INET address. For IPv4, the Client assumes it 3883 is on the open Internet if the INET address is not a special-use IPv4 3884 address per [RFC3330]. Similarly for IPv6, the Client assumes it is 3885 on the open Internet if the INET address is a Global Unicast Address 3886 (GUA) [RFC4291]. Otherwise, the Client should assume it is behind 3887 one or several NATs. 3889 The Client then prepares an RS message with IPv6 source address set 3890 to its MNP-LLA, with IPv6 destination set to (link-local) All-Routers 3891 multicast and with an OMNI option with underlying interface 3892 attributes. If the Client believes that it is on the open Internet, 3893 it SHOULD include its IP address and UDP port number in the Interface 3894 Attributes sub-option corresponding to the underlying interface 3895 (otherwise it may omit the FMT/SRT/LHS/L2ADDR fields). If the 3896 underlying address is IPv4, the Client includes the Port Number and 3897 IPv4 address written in obfuscated form [RFC4380] as discussed in 3898 Section 3.3. If the underlying interface address is IPv6, the Client 3899 instead includes the Port Number and IPv6 address in obfuscated form. 3900 The Client finally includes an authentication signature per 3901 [I-D.templin-6man-omni] to provide message authentication, selects an 3902 Identification value and window synchronization parameters, and 3903 submits the RS for OAL encapsulation. The Client then encapsulates 3904 the OAL fragment in UDP/IP headers to form a carrier packet, sets the 3905 UDP/IP source to its INET address and UDP port, sets the UDP/IP 3906 destination to the FHS Proxy/Server's INET address and the AERO 3907 service port number (8060), then sends the carrier packet to the 3908 Proxy/Server. 3910 When the FHS Proxy/Server receives the RS, it discards the OAL 3911 encapsulation, authenticates the RS message, creates a NCE and 3912 registers the Client's MNP, window synchronization state and INET 3913 interface information according to the OMNI option parameters. If 3914 the Interface Attributes sub-option includes an L2ADDR, the Proxy/ 3915 Server compares the encapsulation IP address and UDP port number with 3916 the (unobfuscated) values. If the values are the same, the Proxy/ 3917 Server caches the Client's information as an "INET" address meaning 3918 that the Client is likely to accept direct messages without requiring 3919 NAT traversal exchanges. If the values are different (or, if the 3920 OMNI option did not include an L2ADDR) the Proxy/Server instead 3921 caches the Client's information as a "mapped" address meaning that 3922 NAT traversal exchanges may be necessary. 3924 The FHS Proxy/Server then prepares an RA message with IPv6 source and 3925 destination set corresponding to the addresses in the RS, and with an 3926 OMNI option with an Origin Indication sub-option per 3927 [I-D.templin-6man-omni] with the mapped and obfuscated Port Number 3928 and IP address observed in the encapsulation headers. The Proxy/ 3929 Server also includes an Interface Attributes sub-option with omIndex 3930 0 and FMT/SRT/LHS/L2ADDR information appropriate for its INET 3931 interface, an authentication signature sub-option per 3932 [I-D.templin-6man-omni] and/or a symmetric window synchronization/ 3933 acknowledgement if necessary. The Proxy/Server then performs OAL 3934 encapsulation and fragmentation if necessary and encapsulates each 3935 fragment in UDP/IP headers with addresses set per the L2ADDR 3936 information in the NCE for the Client. 3938 When the Client receives the RA, it authenticates the message then 3939 process the window synchronization/acknowledgement and compares the 3940 mapped Port Number and IP address from the Origin Indication sub- 3941 option with its own address. If the addresses are the same, the 3942 Client assumes the open Internet / Cone NAT principle; if the 3943 addresses are different, the Client instead assumes that further 3944 qualification procedures are necessary to detect the type of NAT and 3945 proceeds according to standard procedures [RFC6081][RFC4380]. The 3946 Client also caches the RA Interface Attributes FMT/SRT/LHS/L2ADDR 3947 information to discover the Proxy/Server's spanning tree orientation. 3948 The Client finally arranges to return an explicit/implicit 3949 acknowledgement, and sends periodic RS messages to receive fresh RA 3950 messages before the Router Lifetime received on each INET interface 3951 expires. 3953 When the Client sends messages to target IP addresses, it also 3954 invokes route optimization per Section 3.14. For route optimized 3955 targets in the same OMNI link segment, if the target's L2ADDR is on 3956 the open INET, the Client forwards carrier packets directly to the 3957 target INET address. If the target is behind a NAT, the Client first 3958 establishes NAT state for the L2ADDR using the "direct bubble" and 3959 NUD mechanisms discussed in Section 3.10.1. The Client continues to 3960 send carrier packets via its Proxy/Server until NAT state is 3961 populated, then begins forwarding carrier packets via the direct path 3962 through the NAT to the target. For targets in different OMNI link 3963 segments, the Client uses OAL/ORH encapsulation and forwards carrier 3964 packets to the Bridge that returned the NA(AR) message. 3966 The Client can send original IP packets to route-optimized neighbors 3967 in the same OMNI link segment no larger than the minimum/path MPS in 3968 one piece and with OAL encapsulation as atomic fragments. For larger 3969 original IP packets, the Client applies OAL encapsulation then 3970 fragments if necessary according to Section 3.9, with OAL header with 3971 source set to its own MNP-ULA and destination set to the MNP-ULA of 3972 the target, and with an in-window Identification value. The Client 3973 then encapsulates each resulting carrier packet in UDP/IP *NET 3974 headers and sends them to the next hop. 3976 Note: The NAT traversal procedures specified in this document are 3977 applicable for Cone, Address-Restricted and Port-Restricted NATs 3978 only. While future updates to this document may specify procedures 3979 for other NAT variations (e.g., hairpinning and various forms of 3980 Symmetric NATs), it should be noted that continuous communications 3981 are always possible through Proxy/Server forwarding even for these 3982 other NAT variations. 3984 3.23. Time-Varying MNPs 3986 In some use cases, it is desirable, beneficial and efficient for the 3987 Client to receive a constant MNP that travels with the Client 3988 wherever it moves. For example, this would allow air traffic 3989 controllers to easily track aircraft, etc. In other cases, however 3990 (e.g., intelligent transportation systems), the MN may be willing to 3991 sacrifice a modicum of efficiency in order to have time-varying MNPs 3992 that can be changed every so often to defeat adversarial tracking. 3994 The DHCPv6 service offers a way for Clients that desire time-varying 3995 MNPs to obtain short-lived prefixes (e.g., on the order of a small 3996 number of minutes). In that case, the identity of the Client would 3997 not be bound to the MNP but rather to a Node Identification value 3998 (see: [I-D.templin-6man-omni]) to be used as the Client ID seed for 3999 MNP prefix delegation. The Client would then be obligated to 4000 renumber its internal networks whenever its MNP (and therefore also 4001 its MNP-LLA) changes. This should not present a challenge for 4002 Clients with automated network renumbering services, however presents 4003 limits for the durations of ongoing sessions that would prefer to use 4004 a constant address. 4006 4. Implementation Status 4008 An early AERO implementation based on OpenVPN (https://openvpn.net/) 4009 was announced on the v6ops mailing list on January 10, 2018 and an 4010 initial public release of the AERO proof-of-concept source code was 4011 announced on the intarea mailing list on August 21, 2015. 4013 AERO Release-3.2 was tagged on March 30, 2021, and is undergoing 4014 internal testing. Additional internal releases expected within the 4015 coming months, with first public release expected end of 1H2021. 4017 Many AERO/OMNI functions are implemented and undergoing final 4018 integration. OAL fragmentation/reassembly buffer management code has 4019 been cleared for public release and will be presented at the June 4020 2021 ICAO mobility subgroup meeting. 4022 5. IANA Considerations 4024 The IANA is instructed to assign a new type value TBD1 in the IPv6 4025 Routing Types registry (IANA registration procedure is IETF Review or 4026 IESG Approval). 4028 The IANA has assigned the UDP port number "8060" for an earlier 4029 experimental first version of AERO [RFC6706]. This document together 4030 with [I-D.templin-6man-omni] reclaims UDP port number "8060" for 4031 'aero' as the service port for UDP/IP encapsulation. This document 4032 makes no request of IANA, since [I-D.templin-6man-omni] already 4033 provides instructions. (Note: although [RFC6706] was not widely 4034 implemented or deployed, it need not be obsoleted since its messages 4035 use the invalid ICMPv6 message type number '0' which implementations 4036 of this specification can easily distinguish and ignore.) 4038 No further IANA actions are required. 4040 6. Security Considerations 4042 AERO Bridges configure secured tunnels with AERO Proxy/Servers and 4043 Relays within their local OMNI link segments. Applicable secured 4044 tunnel alternatives include IPsec [RFC4301], TLS/SSL [RFC8446], DTLS 4045 [RFC6347], WireGuard [WG], etc. The AERO Bridges of all OMNI link 4046 segments in turn configure secured tunnels for their neighboring AERO 4047 Bridges in a secured spanning tree topology. Therefore, control 4048 messages exchanged between any pair of OMNI link neighbors over the 4049 secured spanning tree are already protected. 4051 To prevent spoofing vectors, Proxy/Servers MUST discard without 4052 responding to any unsecured NS(AR) messages. Also, Proxy/Servers 4053 MUST discard without forwarding any original IP packets received from 4054 one of their own Clients (whether directly or following OAL 4055 reassembly) with a source address that does not match the Client's 4056 MNP and/or a destination address that does match the Client's MNP. 4057 Finally, Proxy/Servers MUST discard without forwarding any carrier 4058 packets with an OAL source and destination that both match the same 4059 MNP (i.e., after consulting the ORH if present). 4061 For INET partitions that require strong security in the data plane, 4062 two options for securing communications include 1) disable route 4063 optimization so that all traffic is conveyed over secured tunnels, or 4064 2) enable on-demand secure tunnel creation between Client neighbors. 4065 Option 1) would result in longer routes than necessary and impose 4066 traffic concentration on critical infrastructure elements. Option 2) 4067 could be coordinated between Clients using NS/NA messages with OMNI 4068 Host Identity Protocol (HIP) "Initiator/Responder" message sub- 4069 options [RFC7401][I-D.templin-6man-omni] to create a secured tunnel 4070 on-demand. 4072 AERO Clients that connect to secured ANETs need not apply security to 4073 their ND messages, since the messages will be authenticated and 4074 forwarded by a perimeter Proxy/Server that applies security on its 4075 INET-facing interface as part of the spanning tree (see above). AERO 4076 Clients connected to the open INET can use network and/or transport 4077 layer security services such as VPNs or can by some other means 4078 establish a direct link to a Proxy/Server. When a VPN or direct link 4079 may be impractical, however, INET Clients and Proxy/Servers SHOULD 4080 include and verify authentication signatures for their IPv6 ND 4081 messages as specified in [I-D.templin-6man-omni]. 4083 Application endpoints SHOULD use transport-layer (or higher-layer) 4084 security services such as TLS/SSL, DTLS or SSH [RFC4251] to assure 4085 the same level of protection as for critical secured Internet 4086 services. AERO Clients that require host-based VPN services SHOULD 4087 use network and/or transport layer security services such as IPsec, 4088 TLS/SSL, DTLS, etc. AERO Proxys and Proxy/Servers can also provide a 4089 network-based VPN service on behalf of the Client, e.g., if the 4090 Client is located within a secured enclave and cannot establish a VPN 4091 on its own behalf. 4093 AERO Proxy/Servers and Bridges present targets for traffic 4094 amplification Denial of Service (DoS) attacks. This concern is no 4095 different than for widely-deployed VPN security gateways in the 4096 Internet, where attackers could send spoofed packets to the gateways 4097 at high data rates. This can be mitigated through the AERO/OMNI data 4098 origin authentication procedures, as well as connecting Proxy/Servers 4099 and Bridges over dedicated links with no connections to the Internet 4100 and/or when connections to the Internet are only permitted through 4101 well-managed firewalls. Traffic amplification DoS attacks can also 4102 target an AERO Client's low data rate links. This is a concern not 4103 only for Clients located on the open Internet but also for Clients in 4104 secured enclaves. AERO Proxy/Servers and Proxys can institute rate 4105 limits that protect Clients from receiving packet floods that could 4106 DoS low data rate links. 4108 AERO Relays must implement ingress filtering to avoid a spoofing 4109 attack in which spurious messages with ULA addresses are injected 4110 into an OMNI link from an outside attacker. AERO Clients MUST ensure 4111 that their connectivity is not used by unauthorized nodes on their 4112 EUNs to gain access to a protected network, i.e., AERO Clients that 4113 act as routers MUST NOT provide routing services for unauthorized 4114 nodes. (This concern is no different than for ordinary hosts that 4115 receive an IP address delegation but then "share" the address with 4116 other nodes via some form of Internet connection sharing such as 4117 tethering.) 4119 The MAP list MUST be well-managed and secured from unauthorized 4120 tampering, even though the list contains only public information. 4121 The MAP list can be conveyed to the Client in a similar fashion as in 4122 [RFC5214] (e.g., through layer 2 data link login messaging, secure 4123 upload of a static file, DNS lookups, etc.). 4125 The AERO service for open INET Clients depends on a public key 4126 distribution service in which Client public keys and identities are 4127 maintained in a shared database accessible to all open INET Proxy/ 4128 Servers. Similarly, each Client must be able to determine the public 4129 key of each Proxy/Server, e.g. by consulting an online database. 4130 When AERO nodes register their public keys indexed by a unique Host 4131 Identity Tag (HIT) [RFC7401] in a distributed database such as the 4132 DNS, and use the HIT as an identity for applying IPv6 ND message 4133 authentication signatures, a means for determining public key 4134 attestation is available. 4136 Security considerations for IPv6 fragmentation and reassembly are 4137 discussed in [I-D.templin-6man-omni]. In environments where spoofing 4138 is considered a threat, OMNI nodes SHOULD employ Identification 4139 window synchronization and OAL destinations SHOULD configure an (end- 4140 system-based) firewall. 4142 SRH authentication facilities are specified in [RFC8754]. Security 4143 considerations for accepting link-layer ICMP messages and reflected 4144 packets are discussed throughout the document. 4146 7. Acknowledgements 4148 Discussions in the IETF, aviation standards communities and private 4149 exchanges helped shape some of the concepts in this work. 4150 Individuals who contributed insights include Mikael Abrahamsson, Mark 4151 Andrews, Fred Baker, Bob Braden, Stewart Bryant, Scott Burleigh, 4152 Brian Carpenter, Wojciech Dec, Pavel Drasil, Ralph Droms, Adrian 4153 Farrel, Nick Green, Sri Gundavelli, Brian Haberman, Bernhard Haindl, 4154 Joel Halpern, Tom Herbert, Bob Hinden, Sascha Hlusiak, Lee Howard, 4155 Christian Huitema, Zdenek Jaron, Andre Kostur, Hubert Kuenig, Ted 4156 Lemon, Andy Malis, Satoru Matsushima, Tomek Mrugalski, Thomas Narten, 4157 Madhu Niraula, Alexandru Petrescu, Behcet Saikaya, Michal Skorepa, 4158 Dave Thaler, Joe Touch, Bernie Volz, Ryuji Wakikawa, Tony Whyman, 4159 Lloyd Wood and James Woodyatt. Members of the IESG also provided 4160 valuable input during their review process that greatly improved the 4161 document. Special thanks go to Stewart Bryant, Joel Halpern and 4162 Brian Haberman for their shepherding guidance during the publication 4163 of the AERO first edition. 4165 This work has further been encouraged and supported by Boeing 4166 colleagues including Kyle Bae, M. Wayne Benson, Dave Bernhardt, Cam 4167 Brodie, John Bush, Balaguruna Chidambaram, Irene Chin, Bruce Cornish, 4168 Claudiu Danilov, Don Dillenburg, Joe Dudkowski, Wen Fang, Samad 4169 Farooqui, Anthony Gregory, Jeff Holland, Seth Jahne, Brian Jaury, 4170 Greg Kimberly, Ed King, Madhuri Madhava Badgandi, Laurel Matthew, 4171 Gene MacLean III, Kyle Mikos, Rob Muszkiewicz, Sean O'Sullivan, Vijay 4172 Rajagopalan, Greg Saccone, Rod Santiago, Kent Shuey, Brian Skeen, 4173 Mike Slane, Carrie Spiker, Katie Tran, Brendan Williams, Amelia 4174 Wilson, Julie Wulff, Yueli Yang, Eric Yeh and other members of the 4175 Boeing mobility, networking and autonomy teams. Kyle Bae, Wayne 4176 Benson, Madhuri Madhava Badgandi, Vijayasarathy Rajagopalan, Katie 4177 Tran and Eric Yeh are especially acknowledged for their work on the 4178 AERO implementation. Chuck Klabunde is honored and remembered for 4179 his early leadership, and we mourn his untimely loss. 4181 This work was inspired by the support and encouragement of countless 4182 outstanding colleagues, managers and program directors over the span 4183 of many decades. Beginning in the late 1980s,' the Digital Equipment 4184 Corporation (DEC) Ultrix Engineering and DECnet Architects groups 4185 identified early issues with fragmentation and bridging links with 4186 diverse MTUs. In the early 1990s, engagements at DEC Project Sequoia 4187 at UC Berkeley and the DEC Western Research Lab in Palo Alto included 4188 investigations into large-scale networked filesystems, ATM vs 4189 Internet and network security proxies. In the mid-1990s to early 4190 2000s employment at the NASA Ames Research Center (Sterling Software) 4191 and SRI International supported early investigations of IPv6, ONR UAV 4192 Communications and the IETF. An employment at Nokia where important 4193 IETF documents were published gave way to a present-day engagement 4194 with The Boeing Company. The work matured at Boeing through major 4195 programs including Future Combat Systems, Advanced Airplane Program, 4196 DTN for the International Space Station, Mobility Vision Lab, CAST, 4197 Caravan, the NASA UAS/CNS program, the FAA/ICAO ATN/IPS program and 4198 many others. An attempt to name all who gave support and 4199 encouragement would double the current document size and result in 4200 many unintentional omissions - but to all a humble thanks. 4202 Earlier works on NBMA tunneling approaches are found in 4203 [RFC2529][RFC5214][RFC5569]. 4205 Many of the constructs presented in this second edition of AERO are 4206 based on the author's earlier works, including: 4208 o The Internet Routing Overlay Network (IRON) 4209 [RFC6179][I-D.templin-ironbis] 4211 o Virtual Enterprise Traversal (VET) 4212 [RFC5558][I-D.templin-intarea-vet] 4214 o The Subnetwork Encapsulation and Adaptation Layer (SEAL) 4215 [RFC5320][I-D.templin-intarea-seal] 4217 o AERO, First Edition [RFC6706] 4219 Note that these works cite numerous earlier efforts that are not also 4220 cited here due to space limitations. The authors of those earlier 4221 works are acknowledged for their insights. 4223 This work is aligned with the NASA Safe Autonomous Systems Operation 4224 (SASO) program under NASA contract number NNA16BD84C. 4226 This work is aligned with the FAA as per the SE2025 contract number 4227 DTFAWA-15-D-00030. 4229 This work is aligned with the Boeing Commercial Airplanes (BCA) 4230 Internet of Things (IoT) and autonomy programs. 4232 This work is aligned with the Boeing Information Technology (BIT) 4233 MobileNet program. 4235 8. References 4237 8.1. Normative References 4239 [I-D.templin-6man-omni] 4240 Templin, F. L. and T. Whyman, "Transmission of IP Packets 4241 over Overlay Multilink Network (OMNI) Interfaces", draft- 4242 templin-6man-omni-03 (work in progress), April 2021. 4244 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 4245 DOI 10.17487/RFC0791, September 1981, 4246 . 4248 [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, 4249 RFC 792, DOI 10.17487/RFC0792, September 1981, 4250 . 4252 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 4253 Requirement Levels", BCP 14, RFC 2119, 4254 DOI 10.17487/RFC2119, March 1997, 4255 . 4257 [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in 4258 IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473, 4259 December 1998, . 4261 [RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander, 4262 "SEcure Neighbor Discovery (SEND)", RFC 3971, 4263 DOI 10.17487/RFC3971, March 2005, 4264 . 4266 [RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)", 4267 RFC 3972, DOI 10.17487/RFC3972, March 2005, 4268 . 4270 [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and 4271 More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191, 4272 November 2005, . 4274 [RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast 4275 Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005, 4276 . 4278 [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through 4279 Network Address Translations (NATs)", RFC 4380, 4280 DOI 10.17487/RFC4380, February 2006, 4281 . 4283 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 4284 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 4285 DOI 10.17487/RFC4861, September 2007, 4286 . 4288 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 4289 Address Autoconfiguration", RFC 4862, 4290 DOI 10.17487/RFC4862, September 2007, 4291 . 4293 [RFC6081] Thaler, D., "Teredo Extensions", RFC 6081, 4294 DOI 10.17487/RFC6081, January 2011, 4295 . 4297 [RFC7401] Moskowitz, R., Ed., Heer, T., Jokela, P., and T. 4298 Henderson, "Host Identity Protocol Version 2 (HIPv2)", 4299 RFC 7401, DOI 10.17487/RFC7401, April 2015, 4300 . 4302 [RFC7739] Gont, F., "Security Implications of Predictable Fragment 4303 Identification Values", RFC 7739, DOI 10.17487/RFC7739, 4304 February 2016, . 4306 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 4307 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 4308 May 2017, . 4310 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 4311 (IPv6) Specification", STD 86, RFC 8200, 4312 DOI 10.17487/RFC8200, July 2017, 4313 . 4315 [RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., 4316 Richardson, M., Jiang, S., Lemon, T., and T. Winters, 4317 "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", 4318 RFC 8415, DOI 10.17487/RFC8415, November 2018, 4319 . 4321 8.2. Informative References 4323 [BGP] Huston, G., "BGP in 2015, http://potaroo.net", January 4324 2016. 4326 [I-D.bonica-6man-comp-rtg-hdr] 4327 Bonica, R., Kamite, Y., Alston, A., Henriques, D., and L. 4328 Jalil, "The IPv6 Compact Routing Header (CRH)", draft- 4329 bonica-6man-comp-rtg-hdr-24 (work in progress), January 4330 2021. 4332 [I-D.bonica-6man-crh-helper-opt] 4333 Li, X., Bao, C., Ruan, E., and R. Bonica, "Compressed 4334 Routing Header (CRH) Helper Option", draft-bonica-6man- 4335 crh-helper-opt-03 (work in progress), April 2021. 4337 [I-D.ietf-intarea-frag-fragile] 4338 Bonica, R., Baker, F., Huston, G., Hinden, R. M., Troan, 4339 O., and F. Gont, "IP Fragmentation Considered Fragile", 4340 draft-ietf-intarea-frag-fragile-17 (work in progress), 4341 September 2019. 4343 [I-D.ietf-intarea-tunnels] 4344 Touch, J. and M. Townsley, "IP Tunnels in the Internet 4345 Architecture", draft-ietf-intarea-tunnels-10 (work in 4346 progress), September 2019. 4348 [I-D.ietf-ipwave-vehicular-networking] 4349 (editor), J. (. J., "IPv6 Wireless Access in Vehicular 4350 Environments (IPWAVE): Problem Statement and Use Cases", 4351 draft-ietf-ipwave-vehicular-networking-20 (work in 4352 progress), March 2021. 4354 [I-D.ietf-rtgwg-atn-bgp] 4355 Templin, F. L., Saccone, G., Dawra, G., Lindem, A., and V. 4356 Moreno, "A Simple BGP-based Mobile Routing System for the 4357 Aeronautical Telecommunications Network", draft-ietf- 4358 rtgwg-atn-bgp-10 (work in progress), January 2021. 4360 [I-D.templin-6man-dhcpv6-ndopt] 4361 Templin, F. L., "A Unified Stateful/Stateless 4362 Configuration Service for IPv6", draft-templin-6man- 4363 dhcpv6-ndopt-11 (work in progress), January 2021. 4365 [I-D.templin-intarea-seal] 4366 Templin, F. L., "The Subnetwork Encapsulation and 4367 Adaptation Layer (SEAL)", draft-templin-intarea-seal-68 4368 (work in progress), January 2014. 4370 [I-D.templin-intarea-vet] 4371 Templin, F. L., "Virtual Enterprise Traversal (VET)", 4372 draft-templin-intarea-vet-40 (work in progress), May 2013. 4374 [I-D.templin-ipwave-uam-its] 4375 Templin, F. L., "Urban Air Mobility Implications for 4376 Intelligent Transportation Systems", draft-templin-ipwave- 4377 uam-its-04 (work in progress), January 2021. 4379 [I-D.templin-ironbis] 4380 Templin, F. L., "The Interior Routing Overlay Network 4381 (IRON)", draft-templin-ironbis-16 (work in progress), 4382 March 2014. 4384 [I-D.templin-v6ops-pdhost] 4385 Templin, F. L., "IPv6 Prefix Delegation and Multi- 4386 Addressing Models", draft-templin-v6ops-pdhost-27 (work in 4387 progress), January 2021. 4389 [OVPN] OpenVPN, O., "http://openvpn.net", October 2016. 4391 [RFC1035] Mockapetris, P., "Domain names - implementation and 4392 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 4393 November 1987, . 4395 [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers", 4396 RFC 1812, DOI 10.17487/RFC1812, June 1995, 4397 . 4399 [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, 4400 DOI 10.17487/RFC2003, October 1996, 4401 . 4403 [RFC2004] Perkins, C., "Minimal Encapsulation within IP", RFC 2004, 4404 DOI 10.17487/RFC2004, October 1996, 4405 . 4407 [RFC2236] Fenner, W., "Internet Group Management Protocol, Version 4408 2", RFC 2236, DOI 10.17487/RFC2236, November 1997, 4409 . 4411 [RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet 4412 Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998, 4413 . 4415 [RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4 4416 Domains without Explicit Tunnels", RFC 2529, 4417 DOI 10.17487/RFC2529, March 1999, 4418 . 4420 [RFC2983] Black, D., "Differentiated Services and Tunnels", 4421 RFC 2983, DOI 10.17487/RFC2983, October 2000, 4422 . 4424 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition 4425 of Explicit Congestion Notification (ECN) to IP", 4426 RFC 3168, DOI 10.17487/RFC3168, September 2001, 4427 . 4429 [RFC3330] IANA, "Special-Use IPv4 Addresses", RFC 3330, 4430 DOI 10.17487/RFC3330, September 2002, 4431 . 4433 [RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener 4434 Discovery Version 2 (MLDv2) for IPv6", RFC 3810, 4435 DOI 10.17487/RFC3810, June 2004, 4436 . 4438 [RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally 4439 Unique IDentifier (UUID) URN Namespace", RFC 4122, 4440 DOI 10.17487/RFC4122, July 2005, 4441 . 4443 [RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) 4444 Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251, 4445 January 2006, . 4447 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 4448 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 4449 DOI 10.17487/RFC4271, January 2006, 4450 . 4452 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 4453 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 4454 2006, . 4456 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 4457 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 4458 December 2005, . 4460 [RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery 4461 Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April 4462 2006, . 4464 [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet 4465 Control Message Protocol (ICMPv6) for the Internet 4466 Protocol Version 6 (IPv6) Specification", STD 89, 4467 RFC 4443, DOI 10.17487/RFC4443, March 2006, 4468 . 4470 [RFC4511] Sermersheim, J., Ed., "Lightweight Directory Access 4471 Protocol (LDAP): The Protocol", RFC 4511, 4472 DOI 10.17487/RFC4511, June 2006, 4473 . 4475 [RFC4541] Christensen, M., Kimball, K., and F. Solensky, 4476 "Considerations for Internet Group Management Protocol 4477 (IGMP) and Multicast Listener Discovery (MLD) Snooping 4478 Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006, 4479 . 4481 [RFC4605] Fenner, B., He, H., Haberman, B., and H. Sandick, 4482 "Internet Group Management Protocol (IGMP) / Multicast 4483 Listener Discovery (MLD)-Based Multicast Forwarding 4484 ("IGMP/MLD Proxying")", RFC 4605, DOI 10.17487/RFC4605, 4485 August 2006, . 4487 [RFC4982] Bagnulo, M. and J. Arkko, "Support for Multiple Hash 4488 Algorithms in Cryptographically Generated Addresses 4489 (CGAs)", RFC 4982, DOI 10.17487/RFC4982, July 2007, 4490 . 4492 [RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano, 4493 "Bidirectional Protocol Independent Multicast (BIDIR- 4494 PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007, 4495 . 4497 [RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site 4498 Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, 4499 DOI 10.17487/RFC5214, March 2008, 4500 . 4502 [RFC5320] Templin, F., Ed., "The Subnetwork Encapsulation and 4503 Adaptation Layer (SEAL)", RFC 5320, DOI 10.17487/RFC5320, 4504 February 2010, . 4506 [RFC5522] Eddy, W., Ivancic, W., and T. Davis, "Network Mobility 4507 Route Optimization Requirements for Operational Use in 4508 Aeronautics and Space Exploration Mobile Networks", 4509 RFC 5522, DOI 10.17487/RFC5522, October 2009, 4510 . 4512 [RFC5558] Templin, F., Ed., "Virtual Enterprise Traversal (VET)", 4513 RFC 5558, DOI 10.17487/RFC5558, February 2010, 4514 . 4516 [RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4 4517 Infrastructures (6rd)", RFC 5569, DOI 10.17487/RFC5569, 4518 January 2010, . 4520 [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 4521 (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, 4522 . 4524 [RFC6106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, 4525 "IPv6 Router Advertisement Options for DNS Configuration", 4526 RFC 6106, DOI 10.17487/RFC6106, November 2010, 4527 . 4529 [RFC6139] Russert, S., Ed., Fleischman, E., Ed., and F. Templin, 4530 Ed., "Routing and Addressing in Networks with Global 4531 Enterprise Recursion (RANGER) Scenarios", RFC 6139, 4532 DOI 10.17487/RFC6139, February 2011, 4533 . 4535 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful 4536 NAT64: Network Address and Protocol Translation from IPv6 4537 Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146, 4538 April 2011, . 4540 [RFC6179] Templin, F., Ed., "The Internet Routing Overlay Network 4541 (IRON)", RFC 6179, DOI 10.17487/RFC6179, March 2011, 4542 . 4544 [RFC6221] Miles, D., Ed., Ooghe, S., Dec, W., Krishnan, S., and A. 4545 Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221, 4546 DOI 10.17487/RFC6221, May 2011, 4547 . 4549 [RFC6273] Kukec, A., Krishnan, S., and S. Jiang, "The Secure 4550 Neighbor Discovery (SEND) Hash Threat Analysis", RFC 6273, 4551 DOI 10.17487/RFC6273, June 2011, 4552 . 4554 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 4555 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 4556 January 2012, . 4558 [RFC6355] Narten, T. and J. Johnson, "Definition of the UUID-Based 4559 DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355, 4560 DOI 10.17487/RFC6355, August 2011, 4561 . 4563 [RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label 4564 for Equal Cost Multipath Routing and Link Aggregation in 4565 Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011, 4566 . 4568 [RFC6706] Templin, F., Ed., "Asymmetric Extended Route Optimization 4569 (AERO)", RFC 6706, DOI 10.17487/RFC6706, August 2012, 4570 . 4572 [RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and 4573 UDP Checksums for Tunneled Packets", RFC 6935, 4574 DOI 10.17487/RFC6935, April 2013, 4575 . 4577 [RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement 4578 for the Use of IPv6 UDP Datagrams with Zero Checksums", 4579 RFC 6936, DOI 10.17487/RFC6936, April 2013, 4580 . 4582 [RFC7333] Chan, H., Ed., Liu, D., Seite, P., Yokota, H., and J. 4583 Korhonen, "Requirements for Distributed Mobility 4584 Management", RFC 7333, DOI 10.17487/RFC7333, August 2014, 4585 . 4587 [RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I., 4588 Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent 4589 Multicast - Sparse Mode (PIM-SM): Protocol Specification 4590 (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March 4591 2016, . 4593 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 4594 Decraene, B., Litkowski, S., and R. Shakir, "Segment 4595 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 4596 July 2018, . 4598 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 4599 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 4600 . 4602 [RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J., 4603 Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header 4604 (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020, 4605 . 4607 [WG] Wireguard, "WireGuard, https://www.wireguard.com", August 4608 2020. 4610 Appendix A. Non-Normative Considerations 4612 AERO can be applied to a multitude of Internetworking scenarios, with 4613 each having its own adaptations. The following considerations are 4614 provided as non-normative guidance: 4616 A.1. Implementation Strategies for Route Optimization 4618 Route optimization as discussed in Section 3.14 results in the route 4619 optimization source (ROS) creating a NCE for the target neighbor. 4620 The NCE state is set to REACHABLE for at most ReachableTime seconds. 4621 In order to refresh the NCE lifetime before the ReachableTime timer 4622 expires, the specification requires implementations to issue a new 4623 NS/NA(AR) exchange to reset ReachableTime while data packets are 4624 still flowing. However, the decision of when to initiate a new NS/ 4625 NA(AR) exchange and to perpetuate the process is left as an 4626 implementation detail. 4628 One possible strategy may be to monitor the NCE watching for data 4629 packets for (ReachableTime - 5) seconds. If any data packets have 4630 been sent to the neighbor within this timeframe, then send an NS(AR) 4631 to receive a new NA(AR). If no data packets have been sent, wait for 4632 5 additional seconds and send an immediate NS(AR) if any data packets 4633 are sent within this "expiration pending" 5 second window. If no 4634 additional data packets are sent within the 5 second window, reset 4635 the NCE state to STALE. 4637 The monitoring of the neighbor data packet traffic therefore becomes 4638 an ongoing process during the NCE lifetime. If the NCE expires, 4639 future data packets will trigger a new NS/NA(AR) exchange while the 4640 packets themselves are delivered over a longer path until route 4641 optimization state is re-established. 4643 A.2. Implicit Mobility Management 4645 OMNI interface neighbors MAY provide a configuration option that 4646 allows them to perform implicit mobility management in which no ND 4647 messaging is used. In that case, the Client only transmits packets 4648 over a single interface at a time, and the neighbor always observes 4649 packets arriving from the Client from the same link-layer source 4650 address. 4652 If the Client's underlying interface address changes (either due to a 4653 readdressing of the original interface or switching to a new 4654 interface) the neighbor immediately updates the NCE for the Client 4655 and begins accepting and sending packets according to the Client's 4656 new address. This implicit mobility method applies to use cases such 4657 as cellphones with both WiFi and Cellular interfaces where only one 4658 of the interfaces is active at a given time, and the Client 4659 automatically switches over to the backup interface if the primary 4660 interface fails. 4662 A.3. Direct Underlying Interfaces 4664 When a Client's OMNI interface is configured over a Direct interface, 4665 the neighbor at the other end of the Direct link can receive packets 4666 without any encapsulation. In that case, the Client sends packets 4667 over the Direct link according to traffic selectors. If the Direct 4668 interface is selected, then the Client's IP packets are transmitted 4669 directly to the peer without going through an ANET/INET. If other 4670 interfaces are selected, then the Client's IP packets are transmitted 4671 via a different interface, which may result in the inclusion of 4672 Proxy/Servers and Bridges in the communications path. Direct 4673 interfaces must be tested periodically for reachability, e.g., via 4674 NUD. 4676 A.4. AERO Critical Infrastructure Considerations 4678 AERO Bridges can be either Commercial off-the Shelf (COTS) standard 4679 IP routers or virtual machines in the cloud. Bridges must be 4680 provisioned, supported and managed by the INET administrative 4681 authority, and connected to the Bridges of other INETs via inter- 4682 domain peerings. Cost for purchasing, configuring and managing 4683 Bridges is nominal even for very large OMNI links. 4685 AERO INET Proxy/Servers can be standard dedicated server platforms, 4686 but most often will be deployed as virtual machines in the cloud. 4687 The only requirements for INET Proxy/Servers are that they can run 4688 the AERO/OMNI code and have at least one network interface connection 4689 to the INET. INET Proxy/Servers must be provisioned, supported and 4690 managed by the INET administrative authority. Cost for purchasing, 4691 configuring and managing cloud Proxy/Servers is nominal especially 4692 for virtual machines. 4694 AERO ANET Proxy/Servers are most often standard dedicated server 4695 platforms with one underlying interface connected to the ANET and a 4696 second interface connected to an INET. As with INET Proxy/Servers, 4697 the only requirements are that they can run the AERO/OMNI code and 4698 have at least one interface connection to the INET. ANET Proxy/ 4699 Servers must be provisioned, supported and managed by the ANET 4700 administrative authority. Cost for purchasing, configuring and 4701 managing Proxys is nominal, and borne by the ANET administrative 4702 authority. 4704 AERO Relays are simply Proxy/Servers connected to INETs and/or EUNs 4705 that provide forwarding services for non-MNP destinations. The Relay 4706 connects to the OMNI link and engages in eBGP peering with one or 4707 more Bridges as a stub AS. The Relay then injects its MNPs and/or 4708 non-MNP prefixes into the BGP routing system, and provisions the 4709 prefixes to its downstream-attached networks. The Relay can perform 4710 ROS/ROR services the same as for any Proxy/Server, and can route 4711 between the MNP and non-MNP address spaces. 4713 A.5. AERO Server Failure Implications 4715 AERO Proxy/Servers may appear as a single point of failure in the 4716 architecture, but such is not the case since all Proxy/Servers on the 4717 link provide identical services and loss of a Proxy/Server does not 4718 imply immediate and/or comprehensive communication failures. Proxy/ 4719 Server failure is quickly detected and conveyed by Bidirectional 4720 Forward Detection (BFD) and/or proactive NUD allowing Clients to 4721 migrate to new Proxy/Servers. 4723 If a Proxy/Server fails, ongoing packet forwarding to Clients will 4724 continue by virtue of the neighbor cache entries that have already 4725 been established in route optimization sources (ROSs). If a Client 4726 also experiences mobility events at roughly the same time the Proxy/ 4727 Server fails, uNA messages may be lost but neighbor cache entries in 4728 the DEPARTED state will ensure that packet forwarding to the Client's 4729 new locations will continue for up to DepartTime seconds. 4731 If a Client is left without a Proxy/Server for a considerable length 4732 of time (e.g., greater than ReachableTime seconds) then existing 4733 neighbor cache entries will eventually expire and both ongoing and 4734 new communications will fail. The original source will continue to 4735 retransmit until the Client has established a new Proxy/Server 4736 relationship, after which time continuous communications will resume. 4738 Therefore, providing many Proxy/Servers on the link with high 4739 availability profiles provides resilience against loss of individual 4740 Proxy/Servers and assurance that Clients can establish new Proxy/ 4741 Server relationships quickly in event of a Proxy/Server failure. 4743 A.6. AERO Client / Server Architecture 4745 The AERO architectural model is client / server in the control plane, 4746 with route optimization in the data plane. The same as for common 4747 Internet services, the AERO Client discovers the addresses of AERO 4748 Proxy/Servers and connects to one or more of them. The AERO service 4749 is analogous to common Internet services such as google.com, 4750 yahoo.com, cnn.com, etc. However, there is only one AERO service for 4751 the link and all Proxy/Servers provide identical services. 4753 Common Internet services provide differing strategies for advertising 4754 server addresses to clients. The strategy is conveyed through the 4755 DNS resource records returned in response to name resolution queries. 4756 As of January 2020 Internet-based 'nslookup' services were used to 4757 determine the following: 4759 o When a client resolves the domainname "google.com", the DNS always 4760 returns one A record (i.e., an IPv4 address) and one AAAA record 4761 (i.e., an IPv6 address). The client receives the same addresses 4762 each time it resolves the domainname via the same DNS resolver, 4763 but may receive different addresses when it resolves the 4764 domainname via different DNS resolvers. But, in each case, 4765 exactly one A and one AAAA record are returned. 4767 o When a client resolves the domainname "ietf.org", the DNS always 4768 returns one A record and one AAAA record with the same addresses 4769 regardless of which DNS resolver is used. 4771 o When a client resolves the domainname "yahoo.com", the DNS always 4772 returns a list of 4 A records and 4 AAAA records. Each time the 4773 client resolves the domainname via the same DNS resolver, the same 4774 list of addresses are returned but in randomized order (i.e., 4775 consistent with a DNS round-robin strategy). But, interestingly, 4776 the same addresses are returned (albeit in randomized order) when 4777 the domainname is resolved via different DNS resolvers. 4779 o When a client resolves the domainname "amazon.com", the DNS always 4780 returns a list of 3 A records and no AAAA records. As with 4781 "yahoo.com", the same three A records are returned from any 4782 worldwide Internet connection point in randomized order. 4784 The above example strategies show differing approaches to Internet 4785 resilience and service distribution offered by major Internet 4786 services. The Google approach exposes only a single IPv4 and a 4787 single IPv6 address to clients. Clients can then select whichever IP 4788 protocol version offers the best response, but will always use the 4789 same IP address according to the current Internet connection point. 4790 This means that the IP address offered by the network must lead to a 4791 highly-available server and/or service distribution point. In other 4792 words, resilience is predicated on high availability within the 4793 network and with no client-initiated failovers expected (i.e., it is 4794 all-or-nothing from the client's perspective). However, Google does 4795 provide for worldwide distributed service distribution by virtue of 4796 the fact that each Internet connection point responds with a 4797 different IPv6 and IPv4 address. The IETF approach is like google 4798 (all-or-nothing from the client's perspective), but provides only a 4799 single IPv4 or IPv6 address on a worldwide basis. This means that 4800 the addresses must be made highly-available at the network level with 4801 no client failover possibility, and if there is any worldwide service 4802 distribution it would need to be conducted by a network element that 4803 is reached via the IP address acting as a service distribution point. 4805 In contrast to the Google and IETF philosophies, Yahoo and Amazon 4806 both provide clients with a (short) list of IP addresses with Yahoo 4807 providing both IP protocol versions and Amazon as IPv4-only. The 4808 order of the list is randomized with each name service query 4809 response, with the effect of round-robin load balancing for service 4810 distribution. With a short list of addresses, there is still 4811 expectation that the network will implement high availability for 4812 each address but in case any single address fails the client can 4813 switch over to using a different address. The balance then becomes 4814 one of function in the network vs function in the end system. 4816 The same implications observed for common highly-available services 4817 in the Internet apply also to the AERO client/server architecture. 4818 When an AERO Client connects to one or more ANETs, it discovers one 4819 or more AERO Proxy/Server addresses through the mechanisms discussed 4820 in earlier sections. Each Proxy/Server address presumably leads to a 4821 fault-tolerant clustering arrangement such as supported by Linux-HA, 4822 Extended Virtual Synchrony or Paxos. Such an arrangement has 4823 precedence in common Internet service deployments in lightweight 4824 virtual machines without requiring expensive hardware deployment. 4825 Similarly, common Internet service deployments set service IP 4826 addresses on service distribution points that may relay requests to 4827 many different servers. 4829 For AERO, the expectation is that a combination of the Google/IETF 4830 and Yahoo/Amazon philosophies would be employed. The AERO Client 4831 connects to different ANET access points and can receive 1-2 Proxy/ 4832 Server ADM-LLAs at each point. It then selects one AERO Proxy/Server 4833 address, and engages in RS/RA exchanges with the same Proxy/Server 4834 from all ANET connections. The Client remains with this Proxy/Server 4835 unless or until the Proxy/Server fails, in which case it can switch 4836 over to an alternate Proxy/Server. The Client can likewise switch 4837 over to a different Proxy/Server at any time if there is some reason 4838 for it to do so. So, the AERO expectation is for a balance of 4839 function in the network and end system, with fault tolerance and 4840 resilience at both levels. 4842 Appendix B. Change Log 4844 << RFC Editor - remove prior to publication >> 4846 Changes from draft-templin-6man-aero-17 to draft-templin-6man-aero- 4847 18: 4849 o Interim version with extensive new text - cleanup planned for next 4850 release. 4852 Changes from draft-templin-6man-aero-16 to draft-templin-6man-aero- 4853 17: 4855 o Final editorial review pass resulting in multiple changes. 4856 Document now submit for final approval (with reference to rfcdiff 4857 from previous version). 4859 Changes from draft-templin-6man-aero-15 to draft-templin-6man-aero- 4860 16: 4862 o Final editorial review pass resulting in multiple changes. 4863 Document now submit for final approval (with reference to rfcdiff 4864 from previous version). 4866 Changes from draft-templin-6man-aero-14 to draft-templin-6man-aero- 4867 15: 4869 o Final editorial review pass resulting in multiple changes. 4870 Document now submit for final approval (with reference to rfcdiff 4871 from previous version). 4873 Changes from draft-templin-6man-aero-13 to draft-templin-6man-aero- 4874 14: 4876 o Final editorial review pass resulting in multiple changes. 4877 Document now submit for final approval (with reference to rfcdiff 4878 from previous version). 4880 Changes from draft-templin-6man-aero-12 to draft-templin-6man-aero- 4881 13: 4883 o Final editorial review pass resulting in multiple changes. 4884 Document now submit for final approval (with reference to rfcdiff 4885 from previous version). 4887 Changes from draft-templin-6man-aero-11 to draft-templin-6man-aero- 4888 12: 4890 o Final editorial review pass resulting in multiple changes. 4891 Document now submit for final approval (with reference to rfcdiff 4892 from previous version). 4894 Changes from draft-templin-6man-aero-10 to draft-templin-6man-aero- 4895 11: 4897 o Final editorial review pass resulting in multiple changes. 4898 Document now submit for final approval (with reference to rfcdiff 4899 from previous version). 4901 Changes from draft-templin-6man-aero-09 to draft-templin-6man-aero- 4902 10: 4904 o Final editorial review pass resulting in multiple changes. 4905 Document now submit for final approval (with reference to rfcdiff 4906 from previous version). 4908 Changes from draft-templin-6man-aero-08 to draft-templin-6man-aero- 4909 09: 4911 o Final editorial review pass resulting in multiple changes. 4912 Document now submit for final approval (with reference to rfcdiff 4913 from previous version). 4915 Changes from draft-templin-6man-aero-07 to draft-templin-6man-aero- 4916 08: 4918 o Final editorial review pass resulting in multiple changes. 4919 Document now submit for final approval (with reference to rfcdiff 4920 from previous version). 4922 Changes from draft-templin-6man-aero-06 to draft-templin-6man-aero- 4923 07: 4925 o Final editorial review pass resulting in multiple changes. 4926 Document now submit for final approval (with reference to rfcdiff 4927 from previous version). 4929 Changes from draft-templin-6man-aero-05 to draft-templin-6man-aero- 4930 06: 4932 o Final editorial review pass resulting in multiple changes. 4933 Document now submit for final approval. 4935 Changes from draft-templin-6man-aero-04 to draft-templin-6man-aero- 4936 05: 4938 o Changed to use traffic selectors instead of the former multilink 4939 selection strategy. 4941 Changes from draft-templin-6man-aero-03 to draft-templin-6man-aero- 4942 04: 4944 o Removed documents from "Obsoletes" list. 4946 o Introduced the concept of "secured" and "unsecured" spanning tree. 4948 o Additional security considerations. 4950 o Additional route optimization considerations. 4952 Changes from draft-templin-6man-aero-02 to draft-templin-6man-aero- 4953 03: 4955 o Support for extended route optimization from ROR to target over 4956 target's underlying interfaces. 4958 Changes from draft-templin-6man-aero-01 to draft-templin-6man-aero- 4959 02: 4961 o Changed reference citations to "draft-templin-6man-omni". 4963 o Several important updates to IPv6 ND cache states and route 4964 optimization message addressing. 4966 o Included introductory description of the "6M's". 4968 o Updated Multicast specification. 4970 Changes from draft-templin-6man-aero-00 to draft-templin-6man-aero- 4971 01: 4973 o Changed category to "Informational". 4975 o Updated implementation status. 4977 Changes from earlier versions to draft-templin-6man-aero-00: 4979 o Established working baseline reference. 4981 Author's Address 4983 Fred L. Templin (editor) 4984 Boeing Research & Technology 4985 P.O. Box 3707 4986 Seattle, WA 98124 4987 USA 4989 Email: fltemplin@acm.org