idnits 2.17.1 draft-templin-aerolink-23.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- == The 'Obsoletes: ' line in the draft header should list only the _numbers_ of the RFCs which will be obsoleted by this document (if approved); it should not include the word 'RFC' in the list. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (May 30, 2014) is 3618 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Unused Reference: 'RFC0768' is defined on line 1287, but no explicit reference was found in the text == Unused Reference: 'RFC0792' is defined on line 1293, but no explicit reference was found in the text == Unused Reference: 'RFC2460' is defined on line 1299, but no explicit reference was found in the text == Unused Reference: 'RFC4862' is defined on line 1323, but no explicit reference was found in the text == Unused Reference: 'RFC0879' is defined on line 1334, but no explicit reference was found in the text == Unused Reference: 'RFC6204' is defined on line 1386, but no explicit reference was found in the text == Unused Reference: 'RFC6691' is defined on line 1398, but no explicit reference was found in the text == Unused Reference: 'RFC6706' is defined on line 1401, but no explicit reference was found in the text == Unused Reference: 'RFC6980' is defined on line 1417, but no explicit reference was found in the text ** Obsolete normative reference: RFC 2460 (Obsoleted by RFC 8200) ** Obsolete normative reference: RFC 3315 (Obsoleted by RFC 8415) ** Obsolete normative reference: RFC 3633 (Obsoleted by RFC 8415) ** Obsolete normative reference: RFC 6434 (Obsoleted by RFC 8504) -- Obsolete informational reference (is this intentional?): RFC 879 (Obsoleted by RFC 7805, RFC 9293) -- Obsolete informational reference (is this intentional?): RFC 5246 (Obsoleted by RFC 8446) -- Obsolete informational reference (is this intentional?): RFC 5996 (Obsoleted by RFC 7296) -- Obsolete informational reference (is this intentional?): RFC 6204 (Obsoleted by RFC 7084) -- Obsolete informational reference (is this intentional?): RFC 6691 (Obsoleted by RFC 9293) Summary: 4 errors (**), 0 flaws (~~), 11 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group F. Templin, Ed. 3 Internet-Draft Boeing Research & Technology 4 Obsoletes: rfc6706 (if approved) May 30, 2014 5 Intended status: Standards Track 6 Expires: December 1, 2014 8 Transmission of IPv6 Packets over AERO Links 9 draft-templin-aerolink-23.txt 11 Abstract 13 This document specifies the operation of IPv6 over tunnel virtual 14 Non-Broadcast, Multiple Access (NBMA) links using Asymmetric Extended 15 Route Optimization (AERO). Nodes attached to AERO links can exchange 16 packets via trusted intermediate routers on the link that provide 17 forwarding services to reach off-link destinations and/or redirection 18 services to inform the node of an on-link neighbor that is closer to 19 the final destination. Operation of the IPv6 Neighbor Discovery (ND) 20 protocol over AERO links is based on an IPv6 link local address 21 format known as the AERO address. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at http://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on December 1, 2014. 40 Copyright Notice 42 Copyright (c) 2014 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (http://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 58 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 59 3. Asymmetric Extended Route Optimization (AERO) . . . . . . . . 5 60 3.1. AERO Node Types . . . . . . . . . . . . . . . . . . . . . 5 61 3.2. AERO Addresses . . . . . . . . . . . . . . . . . . . . . 6 62 3.3. AERO Interface Characteristics . . . . . . . . . . . . . 6 63 3.3.1. Coordination of Multiple Underlying Interfaces . . . 8 64 3.4. AERO Interface Neighbor Cache Maintenace . . . . . . . . 9 65 3.5. AERO Interface Data Origin Authentication . . . . . . . . 10 66 3.6. AERO Interface MTU Considerations . . . . . . . . . . . . 10 67 3.7. AERO Interface Encapsulation, Re-encapsulation and 68 Decapsulation . . . . . . . . . . . . . . . . . . . . . . 12 69 3.8. AERO Router Discovery, Prefix Delegation and Address 70 Configuration . . . . . . . . . . . . . . . . . . . . . . 13 71 3.8.1. AERO Client Behavior . . . . . . . . . . . . . . . . 13 72 3.8.2. AERO Server Behavior . . . . . . . . . . . . . . . . 15 73 3.9. AERO Redirection . . . . . . . . . . . . . . . . . . . . 16 74 3.9.1. Reference Operational Scenario . . . . . . . . . . . 16 75 3.9.2. Classical Redirection Approaches . . . . . . . . . . 17 76 3.9.3. Concept of Operations . . . . . . . . . . . . . . . . 18 77 3.9.4. Message Format . . . . . . . . . . . . . . . . . . . 19 78 3.9.5. Sending Predirects . . . . . . . . . . . . . . . . . 20 79 3.9.6. Processing Predirects and Sending Redirects . . . . . 21 80 3.9.7. Re-encapsulating and Relaying Redirects . . . . . . . 22 81 3.9.8. Processing Redirects . . . . . . . . . . . . . . . . 22 82 3.10. Neighbor Reachability Maintenance . . . . . . . . . . . . 23 83 3.11. Mobility and Link-Layer Address Change Considerations . . 24 84 3.12. Encapsulation Protocol Version Considerations . . . . . . 25 85 3.13. Multicast Considerations . . . . . . . . . . . . . . . . 25 86 3.14. Operation on AERO Links Without DHCPv6 Services . . . . . 25 87 3.15. Operation on Server-less AERO Links . . . . . . . . . . . 26 88 3.16. Other Considerations . . . . . . . . . . . . . . . . . . 26 89 4. Implementation Status . . . . . . . . . . . . . . . . . . . . 26 90 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 91 6. Security Considerations . . . . . . . . . . . . . . . . . . . 26 92 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27 93 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 27 94 8.1. Normative References . . . . . . . . . . . . . . . . . . 28 95 8.2. Informative References . . . . . . . . . . . . . . . . . 29 96 Appendix A. AERO Server and Relay Interworking . . . . . . . . . 31 97 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 32 99 1. Introduction 101 This document specifies the operation of IPv6 over tunnel virtual 102 Non-Broadcast, Multiple Access (NBMA) links using Asymmetric Extended 103 Route Optimization (AERO). Nodes attached to AERO links can exchange 104 packets via trusted intermediate routers on the link that provide 105 forwarding services to reach off-link destinations and/or redirection 106 services to inform the node of an on-link neighbor that is closer to 107 the final destination. This redirection provides a route 108 optimization capability that addresses the requirements outlined in 109 [RFC5522]. 111 Nodes on AERO links use an IPv6 link-local address format known as 112 the AERO Address. This address type has properties that avoid 113 duplication and statelessly link IPv6 Neighbor Discovery (ND) to IPv6 114 routing. The AERO link can be used for tunneling to neighboring 115 nodes on either IPv6 or IPv4 networks, i.e., AERO views the IPv6 and 116 IPv4 networks as equivalent links for tunneling. The remainder of 117 this document presents the AERO specification. 119 2. Terminology 121 The terminology in the normative references applies; the following 122 terms are defined within the scope of this document: 124 AERO link 125 a Non-Broadcast, Multiple Access (NBMA) tunnel virtual overlay 126 configured over a node's attached IPv6 and/or IPv4 networks. All 127 nodes on the AERO link appear as single-hop neighbors from the 128 perspective of IPv6. 130 AERO interface 131 a node's attachment to an AERO link. 133 AERO address 134 an IPv6 link-local address assigned to an AERO interface and 135 constructed as specified in Section 3.2. 137 AERO node 138 a node that is connected to an AERO link and that participates in 139 IPv6 Neighbor Discovery over the link. 141 AERO Client ("client") 142 a node that configures either a host interface or a router 143 interface on an AERO link. 145 AERO Server ("server") 146 a node that configures a router interface on an AERO link over 147 which it can provide default forwarding and redirection services 148 for other AERO nodes. 150 AERO Relay ("relay") 151 a node that relays IPv6 packets between Servers on the same AERO 152 link, and/or that forwards IPv6 packets between the AERO link and 153 the IPv6 Internet. An AERO Relay may or may not also be 154 configured as an AERO Server. 156 ingress tunnel endpoint (ITE) 157 an AERO interface endpoint that injects tunneled packets into an 158 AERO link. 160 egress tunnel endpoint (ETE) 161 an AERO interface endpoint that receives tunneled packets from an 162 AERO link. 164 underlying network 165 a connected IPv6 or IPv4 network routing region over which AERO 166 nodes tunnel IPv6 packets. 168 underlying interface 169 an AERO node's interface point of attachment to an underlying 170 network. 172 link-layer address 173 an IP address assigned to an AERO node's underlying interface. 174 When UDP encapsulation is used, the UDP port number is also 175 considered as part of the link-layer address. Link-layer 176 addresses are used as the encapsulation header source and 177 destination addresses. 179 network layer address 180 the source or destination address of the encapsulated IPv6 packet. 182 end user network (EUN) 183 an IPv6 network attached to a downstream interface of an AERO 184 Client (where the AERO interface is seen as the upstream 185 interface). 187 Throughout the document, the simple terms "Client", "Server" and 188 "Relay" refer to "AERO Client", "AERO Server" and "AERO Relay", 189 respectively. Capitalization is used to distinguish these terms from 190 DHCPv6 client/server/relay. This is an important distinction, since 191 an AERO Server may be a DHCPv6 relay, and an AERO Relay may be a 192 DHCPv6 server. 194 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 195 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 196 document are to be interpreted as described in [RFC2119]. 198 3. Asymmetric Extended Route Optimization (AERO) 200 The following sections specify the operation of IPv6 over Asymmetric 201 Extended Route Optimization (AERO) links: 203 3.1. AERO Node Types 205 AERO Relays relay packets between nodes connected to the same AERO 206 link and also forward packets between the AERO link and the native 207 IPv6 network. The relaying process entails re-encapsulation of IPv6 208 packets that were received from a first AERO node and are to be 209 forwarded without modification to a second AERO node. 211 AERO Servers configure their AERO interfaces as router interfaces, 212 and provide default routing services to AERO Clients. AERO Servers 213 configure a DHCPv6 relay or server function and facilitate DHCPv6 214 Prefix Delegation (PD) exchanges. An AERO Server may also act as an 215 AERO Relay. 217 AERO Clients act as requesting routers to receive IPv6 prefixes 218 through a DHCPv6 PD exchange via AERO Servers over the AERO link. 219 (Clients typically associate with a single Server at a time; Clients 220 MAY associate with multiple Servers, but associating with many 221 Servers may result in excessive control message overhead.) Each AERO 222 Client receives at least a /64 prefix delegation, and may receive 223 even shorter prefixes. 225 AERO Clients that act as routers configure their AERO interfaces as 226 router interfaces and sub-delegate portions of their received prefix 227 delegations to links on EUNs. End system applications on AERO 228 Clients that act as routers bind to EUN interfaces (i.e., and not the 229 AERO interface). 231 AERO Clients that act as ordinary hosts configure their AERO 232 interfaces as host interfaces and assign one or more IPv6 addresses 233 taken from their received prefix delegations to the AERO interface 234 but DO NOT assign the delegated prefix itself to the AERO interface. 235 Instead, the host assigns the delegated prefix to a "black hole" 236 route so that unused portions of the prefix are nullified. End 237 system applications on AERO Clients that act as hosts bind directly 238 to the AERO interface. 240 3.2. AERO Addresses 242 An AERO address is an IPv6 link-local address assigned to an AERO 243 interface and with an IPv6 prefix embedded within the interface 244 identifier. The AERO address is formatted as: 246 fe80::[IPv6 prefix] 248 Each AERO Server configures the AERO address 'fe80::'; this 249 corresponds to the IPv6 prefix '::/0' (i.e., "default") and provides 250 a handle for Clients to insert into a neighbor cache entry. 252 Each AERO Client configures an AERO address based on the prefix it 253 has received from the AERO link prefix delegation authority (e.g., 254 the DHCPv6 server). The address begins with the prefix fe80::/64 and 255 includes in its interface identifier the base /64 prefix taken from 256 the Client's delegated IPv6 prefix. The base prefix is determined by 257 masking the delegated prefix with the prefix length. For example, if 258 an AERO Client has received the prefix delegation: 260 2001:db8:1000:2000::/56 262 it would construct its AERO address as: 264 fe80::2001:db8:1000:2000 266 The AERO address remains stable as the Client moves between 267 topological locations, i.e., even if its underlying address changes. 269 3.3. AERO Interface Characteristics 271 AERO interfaces use IPv6-in-IPv6 encapsulation [RFC2473] to exchange 272 tunneled packets with AERO neighbors attached to an underlying IPv6 273 network, and use IPv6-in-IPv4 encapsulation [RFC4213] to exchange 274 tunneled packets with AERO neighbors attached to an underlying IPv4 275 network. AERO interfaces can also operate over secured tunnel types 276 such as IPsec [RFC4301] or TLS [RFC5246] in environments where strong 277 authentication and confidentiality are required. When Network 278 Address Translator (NAT) traversal and/or filtering middlebox 279 traversal may be necessary, a UDP header is further inserted 280 immediately above the IP encapsulation header. 282 Servers assign the AERO address fe80:: to their AERO interfaces. 283 Servers and Relays also use (non-AERO) administratively-assigned 284 link-local addresses to support the operation of the inter-Server/ 285 Relay routing system (see: [IRON]). 287 Clients initially use a temporary IPv6 link-local address in the 288 DHCPv6 PD exchanges used to receive an IPv6 prefix and derive an AERO 289 address. If the Client is provisioned with an IPv6 prefix associated 290 with the AERO service, it SHOULD use the AERO address derived from 291 the prefix as the temporary address. Otherwise, the Client uses any 292 randomly-selected link-local address as the temporary address. After 293 the Client receives a prefix delegation, it assigns the corresponding 294 AERO address to the AERO interface. DHCPv6 is therefore used to 295 bootstrap the assignment of unique link-local addresses on the AERO 296 interface for subsequent use in IPv6 ND messaging. 298 AERO interfaces maintain a neighbor cache and use an adaptation of 299 standard unicast IPv6 ND messaging. AERO interfaces use unicast 300 Neighbor Solicitation (NS), Neighbor Advertisement (NA), Router 301 Solicitation (RS) and Router Advertisement (RA) messages the same as 302 for any IPv6 link. AERO interfaces use two redirection message types 303 -- the first being the standard Redirect message and the second known 304 as a Predirect message (see Section 3.9). AERO links further use 305 link-local-only addressing; hence, Clients ignore any Prefix 306 Information Options (PIOs) they may receive in RA messages. 308 AERO interface Redirect/Predirect messages use Target Link Layer 309 Address Options (TLLAOs) formatted as shown in Figure 1: 311 0 1 2 3 312 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 1 313 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 314 | Type = 2 | Length = 3 | Reserved | 315 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 316 | Link ID | Preference | UDP Port Number (or 0) | 317 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 318 | | 319 +-- --+ 320 | | 321 +-- IP Address --+ 322 | | 323 +-- --+ 324 | | 325 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 327 Figure 1: AERO Target Link Layer Address Option (TLLAO) Format 329 In this format, Link ID is an integer value between 0 and 255 330 corresponding to an underlying interface of the source/target node, 331 and Preference is an integer value between 0 and 255 indicating the 332 node's preference for this underlying interface, with 0 being highest 333 preference and 255 being lowest. UDP Port Number and IP Address are 334 set to the addresses used by the target node when it sends 335 encapsulated packets over the underlying interface. When no UDP 336 encapsulation is used, UDP Port Number is set to 0. When the 337 encapsulation IP address family is IPv4, IP Address is formed as an 338 IPv4-compatible IPv6 address [RFC4291]. 340 AERO interface Redirect/Predirect messages can both update and create 341 neighbor cache entries. Redirect/Predirect messages SHOULD include a 342 Timestamp option (see Section 5.3 of [RFC3971]) that other AERO nodes 343 can use to verify the message time of origin. 345 AERO interface NS/NA/RS/RA do not include Source/Target Link Layer 346 Address Options; they may only update existing neighbor cache entires 347 and do not create new neighbor cache entries. NS/RS messages SHOULD 348 include a Nonce option (see Section 5.3 of [RFC3971]). If an NS/RS 349 message contains a Nonce option, the recipient MUST echo the option 350 back in the corresponding NA/RA response. Unsolicited NA/RA messages 351 are not used on AERO interfaces, and SHOULD be ignored on receipt. 353 3.3.1. Coordination of Multiple Underlying Interfaces 355 AERO interfaces may be configured over multiple underlying 356 interfaces. From the perspective of IPv6 Neighbor Discovery, the 357 AERO interface therefore appears as a single logical interface with 358 multiple link-layer addresses the same as described for "Inbound Load 359 Balancing" in Section 3 of [RFC4861]. The load balancing paradigm 360 applies to AERO Servers that are connected to stable backhaul 361 networks, but may not necessarily be appropriate for AERO Clients 362 that connect via multiple diverse media types. 364 For example, common handheld devices of the modern era have both 365 wireless local area network (aka "WiFi") and cellular wireless links. 366 These links are typically used "one at a time" with low-cost WiFi 367 preferred and highly-available cellular wireless as a cold standby. 368 In a more complex example, aircraft frequently have many wireless 369 data link types (e.g. satellite-based, terrestrial, directional 370 point-to-point, etc.) with diverse performance and cost properties. 372 If a Client's multiple underlying interfaces are used "one at a time" 373 (i.e., all other interfaces are disabled when one interface is 374 active), then Predirect/Redirect messages MUST include only a single 375 TLLAO with the Link ID and Preference values set to 0. If the Client 376 enables multiple underlying interfaces, Predirect/Redirect messages 377 MAY include multiple TLLAOs that each use a different Link ID value. 378 Coordination of multiple active underlying interfaces is outside the 379 scope of this specification and MAY be defined in future 380 specifications. 382 3.4. AERO Interface Neighbor Cache Maintenace 384 Each AERO interface maintains a conceptual neighbor cache that 385 includes an entry for each neighbor it communicates with on the AERO 386 link, the same as for any IPv6 interface (see [RFC4861]). Neighbor 387 cache entries are created and maintained as follows: 389 When an AERO Server relays a DHCPv6 Reply message to an AERO Client, 390 it creates or updates a neighbor cache entry for the Client based on 391 the AERO address corresponding to the prefix in the IA_PD option as 392 the Client's network layer address and with the Client's 393 encapsulation IP address and UDP port number as the link-layer 394 address. 396 When an AERO Client receives a DHCPv6 Reply message from an AERO 397 Server, it creates or updates a neighbor cache entry for the Server 398 based on fe80:: as the network layer address and the Server's 399 encapsulation IP address and UDP port number as the link-layer 400 address. 402 When an AERO Client receives a valid Predirect message it creates or 403 updates a neighbor cache entry for the Predirect target network-layer 404 and link-layer addresses, and also creates an IPv6 forwarding table 405 entry for the predirected (source) prefix. The node then sets an 406 "ACCEPT" timer for the neighbor and uses this timer to determine 407 whether messages received from the predirected neighbor can be 408 accepted. 410 When an AERO Client receives a valid Redirect message it creates or 411 updates a neighbor cache entry for the redirected target network- 412 layer and link-layer addresses, and also creates an IPv6 forwarding 413 table entry for the redirected (destination) prefix. The node then 414 sets a "FORWARD" timer for the neighbor and uses this timer to 415 determine whether packets can be sent directly to the redirected 416 neighbor. The node also maintains a constant value MAX_RETRY to 417 limit the number of keepalives sent when a neighbor may have gone 418 unreachable. 420 When an AERO Client receives a valid NS message it (re)sets the 421 ACCEPT timer for the neighbor to ACCEPT_TIME. 423 When an AERO Client receives a valid NA message, it (re)sets the 424 FORWARD timer for the neighbor to FORWARD_TIME. 426 It is RECOMMENDED that FORWARD_TIME be set to the default constant 427 value 30 seconds to match the default REACHABLE_TIME value specified 428 for IPv6 neighbor discovery [RFC4861]. 430 It is RECOMMENDED that ACCEPT_TIME be set to the default constant 431 value 40 seconds to allow a 10 second window so that the AERO 432 redirection procedure can converge before the ACCEPT timer decrements 433 below FORWARD_TIME. 435 It is RECOMMENDED that MAX_RETRY be set to 3 the same as described 436 for IPv6 neighbor discovery address resolution in Section 7.3.3 of 437 [RFC4861]. 439 Different values for FORWARD_TIME, ACCEPT_TIME, and MAX_RETRY MAY be 440 administratively set, if necessary, to better match the AERO link's 441 performance characteristics; however, if different values are chosen, 442 all nodes on the link MUST consistently configure the same values. 443 In particular, ACCEPT_TIME SHOULD be set to a value that is 444 sufficiently longer than FORWARD_TIME to allow the AERO redirection 445 procedure to converge. 447 3.5. AERO Interface Data Origin Authentication 449 AERO nodes use a simple data origin authentication for encapsulated 450 packets they receive from other nodes. In particular, AERO nodes 451 accept encapsulated packets with a link-layer source address 452 belonging to one of their current AERO Servers and accept 453 encapsulated packets with a link-layer source address that is correct 454 for the network-layer source address. 456 The AERO node considers the link-layer source address correct for the 457 network-layer source address if there is an IPv6 forwarding table 458 entry that matches the network-layer source address as well as a 459 neighbor cache entry corresponding to the next hop that includes the 460 link-layer address and the ACCEPT timer is non-zero. 462 3.6. AERO Interface MTU Considerations 464 The AERO link Maximum Transmission Unit (MTU) is 64KB minus the 465 encapsulation overhead for IPv4 [RFC0791] and 4GB minus the 466 encapsulation overhead for IPv6 [RFC2675]. This is the most that 467 IPv4 and IPv6 (respectively) can convey within the constraints of 468 protocol constants, but actual sizes available for tunneling will 469 frequently be much smaller. 471 The base tunneling specifications for IPv4 and IPv6 typically set a 472 static MTU on the tunnel interface to 1500 bytes minus the 473 encapsulation overhead or smaller still if the tunnel is likely to 474 incur additional encapsulations on the path. This can result in path 475 MTU related black holes when packets that are too large to be 476 accommodated over the AERO link are dropped, but the resulting ICMP 477 Packet Too Big (PTB) messages are lost on the return path. As a 478 result, AERO nodes use the following MTU mitigations to accommodate 479 larger packets. 481 AERO nodes set their AERO interface MTU to the larger of the 482 underlying interface MTU minus the encapsulation overhead, and 1500 483 bytes. (If there are multiple underlying interfaces, the node sets 484 the AERO interface MTU according to the largest underlying interface 485 MTU, or 64KB /4G minus the encapsulation overhead if the largest MTU 486 cannot be determined.) AERO nodes optionally cache other per- 487 neighbor MTU values in the underlying IP path MTU discovery cache 488 initialized to the underlying interface MTU. 490 AERO nodes admit packets that are no larger than 1280 bytes minus the 491 encapsulation overhead (*) as well as packets that are larger than 492 1500 bytes into the tunnel without fragmentation, i.e., as long as 493 they are no larger than the AERO interface MTU before encapsulation 494 and also no larger than the cached per-neighbor MTU following 495 encapsulation. For IPv4, the node sets the "Don't Fragment" (DF) bit 496 to 0 for packets no larger than 1280 bytes minus the encapsulation 497 overhead (*) and sets the DF bit to 1 for packets larger than 1500 498 bytes. If a large packet is lost in the path, the node may 499 optionally cache the MTU reported in the resulting PTB message or may 500 ignore the message, e.g., if there is a possibility that the message 501 is spurious. 503 For packets destined to an AERO node that are larger than 1280 bytes 504 minus the encapsulation overhead (*) but no larger than 1500 bytes, 505 the node uses IP fragmentation to fragment the encapsulated packet 506 into two pieces (where the first fragment contains 1024 bytes of the 507 original IPv6 packet) then admits the fragments into the tunnel. If 508 the encapsulation protocol is IPv4, the node admits each fragment 509 into the tunnel with DF set to 0 and subject to rate limiting to 510 avoid reassembly errors [RFC4963][RFC6864]. For both IPv4 and IPv6, 511 the node also sends a 1500 byte probe message (**) to the neighbor, 512 subject to rate limiting. 514 To construct a probe, the node prepares an NS message with a Nonce 515 option plus trailing padding octets added to a length of 1500 bytes 516 without including the length of the padding in the IPv6 Payload 517 Length field. The node then encapsulates the NS in the encapsulation 518 headers (while including the length of the padding in the 519 encapsulation header length fields), sets DF to 1 (for IPv4) and 520 sends the padded NS message to the neighbor. If the neighbor returns 521 an NA message with a correct Nonce value, the node may then send 522 whole packets within this size range and (for IPv4) relax the rate 523 limiting requirement. (Note that the trailing padding SHOULD NOT be 524 included within the Nonce option itself but rather as padding beyond 525 the last option in the NS message; otherwise, the (large) Nonce 526 option would be echoed back in the solicited NA message and may be 527 lost at a link with a small MTU along the reverse path.) 529 AERO nodes MUST be capable of reassembling packets up to 1500 bytes 530 plus the encapsulation overhead length. It is therefore RECOMMENDED 531 that AERO nodes be capable of reassembling at least 2KB. 533 (*) Note that if it is known without probing that the minimum Path 534 MTU to an AERO node is MINMTU bytes (where 1280 < MINMTU < 1500) then 535 MINMTU can be used instead of 1280 in the fragmentation threshold 536 considerations listed above. 538 (**) It is RECOMMENDED that no probes smaller than 1500 bytes be used 539 for MTU probing purposes, since smaller probes may be fragmented if 540 there is a nested tunnel somewhere on the path to the neighbor. 541 Probe sizes larger than 1500 bytes MAY be used, but may be 542 unnecessary since original sources are expected to implement 543 [RFC4821] when sending large packets. 545 3.7. AERO Interface Encapsulation, Re-encapsulation and Decapsulation 547 AERO interfaces encapsulate IPv6 packets according to whether they 548 are entering the AERO interface for the first time or if they are 549 being forwarded out the same AERO interface that they arrived on. 550 This latter form of encapsulation is known as "re-encapsulation". 552 AERO interfaces encapsulate packets per the specifications in 553 [RFC2473][RFC4213][RFC4301][RFC5246] except that the interface copies 554 the "Hop Limit", "Traffic Class" and "Congestion Experienced" values 555 in the packet's IPv6 header into the corresponding fields in the 556 encapsulation header. For packets undergoing re-encapsulation, the 557 AERO interface instead copies the "TTL/Hop Limit", "Type of Service/ 558 Traffic Class" and "Congestion Experienced" values in the original 559 encapsulation header into the corresponding fields in the new 560 encapsulation header (i.e., the values are transferred between 561 encapsulation headers and *not* copied from the encapsulated packet's 562 network-layer header). 564 When AERO UDP encapsulation is used, the AERO interface encapsulates 565 the packet per the specifications in [RFC2473][RFC4213] except that 566 it inserts a UDP header between the encapsulation header and IPv6 567 packet header. The AERO interface sets the UDP source port to a 568 constant value that it will use in each successive packet it sends, 569 sets the UDP checksum field to zero (see: [RFC6935][RFC6936]) and 570 sets the UDP length field to the length of the IPv6 packet plus 8 571 bytes for the UDP header itself. For packets sent via a Server, the 572 AERO interface sets the UDP destination port to 8060 (i.e., the IANA- 573 registered port number for AERO) when AERO-only encapsulation is 574 used. For packets sent to a neighboring Client, the AERO interface 575 sets the UDP destination port to the port value stored in the 576 neighbor cache entry for this neighbor. 578 The AERO interface next sets the IP protocol number in the 579 encapsulation header to the appropriate value for the first protocol 580 layer within the encapsulation (e.g., IPv6, UDP, IPsec, etc.). When 581 IPv6 is used as the encapsulation protocol, the interface then sets 582 the flow label value in the encapsulation header the same as 583 described in [RFC6438]. When IPv4 is used as the encapsulation 584 protocol, the AERO interface sets the DF bit as discussed in 585 Section 3.6. 587 AERO interfaces decapsulate packets destined either to the node 588 itself or to a destination reached via an interface other than the 589 receiving AERO interface. When AERO UDP encapsulation is used (i.e., 590 when a UDP header with destination port 8060 is present) the 591 interface examines the first octet of the encapsulated packet. If 592 the most significant four bits of the first octet encode the value 593 '0110' (i.e., the version number value for IPv6), the packet is 594 accepted and the encapsulating UDP header is discarded; otherwise, 595 the packet is discarded. 597 Further decapsulation then proceeds according to the appropriate 598 tunnel type [RFC2473][RFC4213][RFC4301][RFC5246]. 600 3.8. AERO Router Discovery, Prefix Delegation and Address Configuration 602 3.8.1. AERO Client Behavior 604 AERO Clients observe the IPv6 node requirements defined in [RFC6434]. 605 AERO Clients first discover the link-layer addresses of AERO Servers 606 via static configuration, or through an automated means such as DNS 607 name resolution. In the absence of other information, the Client 608 resolves the Fully-Qualified Domain Name (FQDN) 609 "linkupnetworks.domainname", where "domainname" is the DNS domain 610 appropriate for the Client's attached underlying network. The Client 611 then creates a neighbor cache entry with fe80:: as the link-local 612 address and the discovered addresses of one or more Servers as the 613 link-layer addresses. 615 Next, the Client acts as a requesting router to request an IPv6 616 prefix through DHCPv6 PD [RFC3633] using a temporary link-local 617 address (see Section 3.3) as the IPv6 source address and fe80:: as 618 the IPv6 destination address. The Client includes a DHCPv6 Unique 619 Identifier (DUID) in the Client Identifier option of its DHCPv6 620 messages [RFC3315][RFC6355], where the DUID uniquely identifies the 621 Client to the Server. The Client also includes any additional 622 authenticating information necessary to authenticate itself to the 623 DHCPv6 server. If the Client is pre-provisioned with an IPv6 prefix 624 associated with the AERO service, it MAY also include the prefix in 625 an IA_PD option in its DHCPv6 Request to indicate its preferred 626 prefix to the DHCPv6 server. The Client then sends the encapsulated 627 DHCPv6 request via an underlying interface. 629 After the Client receives its prefix delegation, it assigns the link- 630 local AERO address taken from the prefix to the AERO interface and 631 sub-delegates the prefix to nodes and links within its attached EUNs 632 (the AERO link-local address thereafter remains stable as the Client 633 moves). The Client also sets both the ACCEPT and FORWARD timers for 634 each Server to infinity, since the Client will remain with this 635 Server unless it explicitly terminates the association. The Client 636 further renews its prefix delegation by performing DHCPv6 Renew/Reply 637 exchanges with its AERO address as the IPv6 source address, fe80:: as 638 the IPv6 destination address and the same DUID value in the Client 639 Identifier option. If the Client wishes to associate with multiple 640 Servers, it can perform DHCPv6 Renew/Reply exchanges via each of the 641 Servers, which will result in the creation of neighbor cache entries. 643 The Client then sends an RS message to each of its associated Servers 644 to receive an RA message with a default router lifetime and any other 645 link-specific parameters. When the Client receives an RA message, it 646 configures a default route according to the default router lifetime 647 but ignores any Prefix Information Options (PIOs) included in the RA 648 message since the AERO link is link-local-only. The Client further 649 ignores any RS messages it might receive, since only Servers may 650 process RS messages. 652 The Client then sends periodic RS messages to each Server (subject to 653 rate limiting) to obtain new RA messages for Neighbor Unreachability 654 Detection (NUD), to refresh any network state, and to update the 655 default router lifetime and any other link-specific parameters. The 656 Client can also forward IPv6 packets destined to networks beyond its 657 local EUNs via a Server as an IPv6 default router. The Server may in 658 turn return a redirection message informing the Client of a neighbor 659 on the AERO link that is topologically closer to the final 660 destination as specified in Section 3.9. 662 Note that, since the Client's AERO address is configured from the 663 unique DHCPv6 prefix delegation it receives, there is no need for 664 Duplicate Address Detection (DAD) on AERO links. Other nodes 665 maliciously attempting to hijack an authorized Client's AERO address 666 will be denied due to an unacceptable link-layer address and/or 667 security parameters (see: Security Considerations). 669 3.8.2. AERO Server Behavior 671 AERO Servers observe the IPv6 router requirements defined in 672 [RFC6434] and further configure a DHCPv6 relay function on their AERO 673 links. When the AERO Server relays a Client's DHCPv6 PD messages to 674 the DHCPv6 server, it wraps each message in a "Relay-forward" message 675 per [RFC3315] and includes a DHCPv6 Interface Identifier option that 676 encodes a value that identifies the AERO link to the DHCPv6 server. 678 The Server then includes the Client's link-layer address in a DHCPv6 679 Client Link Layer Address Option (CLLAO) [RFC6939] with the link- 680 layer address format shown in Figure 1 (i.e., Link ID followed by 681 Preference followed by UDP Port Number followed by IP Address). The 682 Server sets the CLLAO 'option-length' field to 22 (2 plus the length 683 of the link-layer address) and sets the 'link-layer type' field to 684 TBD (see: IANA Considerations). The Server finally includes a DHCPv6 685 Echo Request Option (ERO) [RFC4994] that encodes the option code for 686 the CLLAO in a 'requested-option-code-n' field. The CLLAO 687 information will therefore subsequently be echoed back in the DHCPv6 688 Server's "Relay-reply" message. 690 When the DHCPv6 server issues the IPv6 prefix delegation in a "Relay- 691 reply" message via the AERO Server (acting as a DHCPv6 relay), the 692 AERO Server obtains the Client's link-layer address from the echoed 693 CLLAO option and obtains the Client's delegated prefix from the 694 included IA_PD option. The Server then creates a neighbor cache 695 entry for the Client's AERO address with the Client's link-layer 696 address as the link-layer address for the neighbor cache entry. The 697 neighbor cache entry is created with both ACCEPT and FORWARD timers 698 set to infinity, since the Client will remain with this Server unless 699 it explicitly terminates the association. 701 The Server also configures an IPv6 forwarding table entry that lists 702 the Client's AERO address as the next hop toward the delegated IPv6 703 prefix with a lifetime derived from the DHCPv6 lease lifetime. The 704 Server finally injects the Client's prefix as an IPv6 route into the 705 inter-Server/Relay routing system (see: [IRON]) then relays the 706 DHCPv6 message to the Client while using fe80:: as the IPv6 source 707 address, the link-local address found in the "peer address" field of 708 the Relay-reply message as the IPv6 destination address, and the 709 Client's link-layer address as the destination link-layer address. 711 Servers respond to RS/NS messages from Clients on their AERO 712 interfaces by returning an RA/NA message. The Server SHOULD NOT 713 include PIOs in the RA messages it sends to Clients, since the Client 714 will ignore any such options. 716 Servers ignore any RA messages they may receive from a Client. 717 Servers MAY examine RA messages received from other Servers for 718 consistency verification purposes. 720 When the Server forwards a packet via the same AERO interface on 721 which it arrived, it initiates an AERO route optimization procedure 722 as specified in Section 3.9. 724 3.9. AERO Redirection 726 3.9.1. Reference Operational Scenario 728 Figure 2 depicts the AERO redirection reference operational scenario. 729 The figure shows an AERO Server('A'), two AERO Clients ('B', 'D') and 730 three ordinary IPv6 hosts ('C', 'E', 'F'): 732 .-(::::::::) 733 .-(::: IPv6 :::)-. +-------------+ 734 (:::: Internet ::::)--| Host F | 735 `-(::::::::::::)-' +-------------+ 736 `-(::::::)-' 2001:db8:2::1 737 | 738 +--------------+ 739 | AERO Server A| 740 | (C->B; E->D) | 741 +--------------+ 742 fe80:: 743 L2(A) 744 | 745 X-----+-----------+-----------+--------X 746 | AERO Link | 747 L2(B) L2(D) 748 fe80::2001:db8:0:0 fe80::2001:db8:1:0 .-. 749 +--------------+ +--------------+ ,-( _)-. 750 | AERO Client B| | AERO Client D| .-(_ IPv6 )-. 751 | (default->A) | | (default->A) |--(__ EUN ) 752 +--------------+ +--------------+ `-(______)-' 753 2001:DB8:0::/48 2001:DB8:1::/48 | 754 | 2001:db8:1::1 755 .-. +-------------+ 756 ,-( _)-. 2001:db8:0::1 | Host E | 757 .-(_ IPv6 )-. +-------------+ +-------------+ 758 (__ EUN )--| Host C | 759 `-(______)-' +-------------+ 761 Figure 2: AERO Reference Operational Scenario 763 In Figure 2, AERO Server ('A') connects to the AERO link and connects 764 to the IPv6 Internet, either directly or via an AERO Relay (not 765 shown). Server ('A') assigns the address fe80:: to its AERO 766 interface with link-layer address L2(A). Server ('A') next arranges 767 to add L2(A) to a published list of valid Servers for the AERO link. 769 AERO Client ('B') receives the IPv6 prefix 2001:db8:0::/48 in a 770 DHCPv6 PD exchange via AERO Server ('A') then assigns the address 771 fe80::2001:db8:0:0 to its AERO interface with link-layer address 772 L2(B). Client ('B') configures a default route and neighbor cache 773 entry via the AERO interface with next-hop address fe80:: and link- 774 layer address L2(A), then sub-delegates the prefix 2001:db8:0::/48 to 775 its attached EUNs. IPv6 host ('C') connects to the EUN, and 776 configures the address 2001:db8:0::1. 778 AERO Client ('D') receives the IPv6 prefix 2001:db8:1::/48 in a 779 DHCPv6 PD exchange via AERO Server ('A') then assigns the address 780 fe80::2001:db8:1:0 to its AERO interface with link-layer address 781 L2(D). Client ('D') configures a default route and neighbor cache 782 entry via the AERO interface with next-hop address fe80:: and link- 783 layer address L2(A), then sub-delegates the prefix 2001:db8:1::/48 to 784 its attached EUNs. IPv6 host ('E') connects to the EUN, and 785 configures the address 2001:db8:1::1. 787 Finally, IPv6 host ('F') connects to an IPv6 network outside of the 788 AERO link domain. Host ('F') configures its IPv6 interface in a 789 manner specific to its attached IPv6 link, and assigns the address 790 2001:db8:2::1 to its IPv6 link interface. 792 3.9.2. Classical Redirection Approaches 794 With reference to Figure 2, when the IPv6 source host ('C') sends a 795 packet to an IPv6 destination host ('E'), the packet is first 796 forwarded via the EUN to AERO Client ('B'). Client ('B') then 797 forwards the packet over its AERO interface to AERO Server ('A'), 798 which then re-encapsulates and forwards the packet to AERO Client 799 ('D'), where the packet is finally forwarded to the IPv6 destination 800 host ('E'). When Server ('A') re-encapsulates and forwards the 801 packet back out on its advertising AERO interface, it must arrange to 802 redirect Client ('B') toward Client ('D') as a better next-hop node 803 on the AERO link that is closer to the final destination. However, 804 this redirection process applied to AERO interfaces must be more 805 carefully orchestrated than on ordinary links since the parties may 806 be separated by potentially many underlying network routing hops. 808 Consider a first alternative in which Server ('A') informs Client 809 ('B') only and does not inform Client ('D') (i.e., "classical 810 redirection"). In that case, Client ('D') has no way of knowing that 811 Client ('B') is authorized to forward packets from the claimed source 812 address, and it may simply elect to drop the packets. Also, Client 813 ('B') has no way of knowing whether Client ('D') is performing some 814 form of source address filtering that would reject packets arriving 815 from a node other than a trusted default router, nor whether Client 816 ('D') is even reachable via a direct path that does not involve 817 Server ('A'). 819 Consider a second alternative in which Server ('A') informs both 820 Client ('B') and Client ('D') separately, via independent redirection 821 control messages (i.e., "augmented redirection"). In that case, if 822 Client ('B') receives the redirection control message but Client 823 ('D') does not, subsequent packets sent by Client ('B') could be 824 dropped due to filtering since Client ('D') would not have a route to 825 verify the claimed source address. Also, if Client ('D') receives 826 the redirection control message but Client ('B') does not, subsequent 827 packets sent in the reverse direction by Client ('D') would be lost. 829 Since both of these alternatives have shortcomings, a new redirection 830 technique (i.e., "AERO redirection") is needed. 832 3.9.3. Concept of Operations 834 Again, with reference to Figure 2, when source host ('C') sends a 835 packet to destination host ('E'), the packet is first forwarded over 836 the source host's attached EUN to Client ('B'), which then forwards 837 the packet via its AERO interface to Server ('A'). 839 Server ('A') then re-encapsulates and forwards the packet out the 840 same AERO interface toward Client ('D') and also sends an AERO 841 "Predirect" message forward to Client ('D') as specified in 842 Section 3.9.5. The Predirect message includes Client ('B')'s 843 network- and link-layer addresses as well as information that Client 844 ('D') can use to determine the IPv6 prefix used by Client ('B') . 845 After Client ('D') receives the Predirect message, it process the 846 message and returns an AERO Redirect message destined for Client 847 ('B') via Server ('A') as specified in Section 3.9.6. During the 848 process, Client ('D') also creates or updates a neighbor cache entry 849 for Client ('B') and creates an IPv6 forwarding table entry for 850 Client ('B')'s IPv6 prefix. 852 When Server ('A') receives the Redirect message, it re-encapsulates 853 the message and forwards it on to Client ('B') as specified in 854 Section 3.9.7. The message includes Client ('D')'s network- and 855 link-layer addresses as well as information that Client ('B') can use 856 to determine the IPv6 prefix used by Client ('D'). After Client 857 ('B') receives the Redirect message, it processes the message as 858 specified in Section 3.9.8. During the process, Client ('B') also 859 creates or updates a neighbor cache entry for Client ('D') and 860 creates an IPv6 forwarding table entry for Client ('D')'s IPv6 861 prefix. 863 Following the above Predirect/Redirect message exchange, forwarding 864 of packets from Client ('B') to Client ('D') without involving Server 865 ('A) as an intermediary is enabled. The mechanisms that support this 866 exchange are specified in the following sections. 868 3.9.4. Message Format 870 AERO Redirect/Predirect messages use the same format as for ICMPv6 871 Redirect messages depicted in Section 4.5 of [RFC4861], but also 872 include a new "Prefix Length" field taken from the low-order 8 bits 873 of the Redirect message Reserved field (valid values for the Prefix 874 Length field are 0 through 64). The Redirect/Predirect messages are 875 formatted as shown in Figure 3: 877 0 1 2 3 878 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 1 879 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 880 | Type (=137) | Code (=0/1) | Checksum | 881 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 882 | Reserved | Prefix Length | 883 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 884 | | 885 + + 886 | | 887 + Target Address + 888 | | 889 + + 890 | | 891 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 892 | | 893 + + 894 | | 895 + Destination Address + 896 | | 897 + + 898 | | 899 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 900 | Options ... 901 +-+-+-+-+-+-+-+-+-+-+-+- 903 Figure 3: AERO Redirect/Predirect Message Format 905 3.9.5. Sending Predirects 907 When a Server forwards a packet out the same AERO interface that it 908 arrived on, the Server sends a Predirect message forward toward the 909 AERO Client nearest the destination instead of sending a Redirect 910 message back to the Client nearest the source. 912 In the reference operational scenario, when Server ('A') forwards a 913 packet sent by Client ('B') toward Client ('D'), it also sends a 914 Predirect message forward toward Client ('D'), subject to rate 915 limiting (see Section 8.2 of [RFC4861]). Server ('A') prepares the 916 Predirect message as follows: 918 o the link-layer source address is set to 'L2(A)' (i.e., the 919 underlying address of Server ('A')). 921 o the link-layer destination address is set to 'L2(D)' (i.e., the 922 underlying address of Client ('D')). 924 o the network-layer source address is set to fe80::2001:db8:0:0 925 (i.e., the AERO address of Client ('B')). 927 o the network-layer destination address is set to fe80::2001:db8:1:0 928 (i.e., the AERO address of Client ('D')). 930 o the Type is set to 137. 932 o the Code is set to 1 to indicate "Predirect". 934 o the Prefix Length is set to the length of the prefix to be applied 935 to Target address. 937 o the Target Address is set to fe80::2001:db8:0::0 (i.e., the AERO 938 address of Client ('B')). 940 o the Destination Address is set to the IPv6 source address of the 941 packet that triggered the Predirection event. 943 o the message includes a TLLAO with UDP Port Number and IP Address 944 set to 'L2(B)'. 946 o the message includes a Timestamp option. 948 o the message includes a Redirected Header Option (RHO) that 949 contains the originating packet truncated to ensure that at least 950 the network-layer header is included but the size of the message 951 does not exceed 1280 bytes. 953 Server ('A') then sends the message forward to Client ('D'). 955 3.9.6. Processing Predirects and Sending Redirects 957 When Client ('D') receives a Predirect message, it accepts the 958 message only if the message has a link-layer source address of the 959 Server, i.e. 'L2(A)'. Client ('D') further accepts the message only 960 if it is willing to serve as a redirection target. Next, Client 961 ('D') validates the message according to the ICMPv6 Redirect message 962 validation rules in Section 8.1 of [RFC4861]. 964 In the reference operational scenario, when Client ('D') receives a 965 valid Predirect message, it either creates or updates a neighbor 966 cache entry that stores the Target Address of the message as the 967 network-layer address of Client ('B') and stores the link-layer 968 address found in the TLLAO as the link-layer address(es) of Client 969 ('B'). Client ('D') then sets the neighbor cache entry ACCEPT timer 970 with timeout value ACCEPT_TIME. Next, Client ('D') applies the 971 Prefix Length to the Interface Identifier portion of the Target 972 Address and records the resulting IPv6 prefix in its IPv6 forwarding 973 table. 975 After processing the message, Client ('D') prepares a Redirect 976 message response as follows: 978 o the link-layer source address is set to 'L2(D)' (i.e., the link- 979 layer address of Client ('D')). 981 o the link-layer destination address is set to 'L2(A)' (i.e., the 982 link-layer address of Server ('A')). 984 o the network-layer source address is set to fe80::2001:db8:1:0 985 (i.e., the AERO address of Client ('D')). 987 o the network-layer destination address is set to fe80::2001:db8:0:0 988 (i.e., the AERO address of Client ('B')). 990 o the Type is set to 137. 992 o the Code is set to 0 to indicate "Redirect". 994 o the Prefix Length is set to the length of the prefix to be applied 995 to the Target and Destination address. 997 o the Target Address is set to fe80::2001:db8:1:0 (i.e., the AERO 998 address of Client ('D')). 1000 o the Destination Address is set to the IPv6 destination address of 1001 the packet that triggered the Redirection event. 1003 o the message includes a TLLAO with UDP port number and IP address 1004 set to '0'. 1006 o the message includes a Timestamp option. 1008 o the message includes as much of the RHO copied from the 1009 corresponding AERO Predirect message as possible such that at 1010 least the network-layer header is included but the size of the 1011 message does not exceed 1280 bytes. 1013 After Client ('D') prepares the Redirect message, it sends the 1014 message to Server ('A'). 1016 3.9.7. Re-encapsulating and Relaying Redirects 1018 When Server ('A') receives a Redirect message from Client ('D'), it 1019 accepts the message only if it has a neighbor cache entry that 1020 associates the message's link-layer source address with the network- 1021 layer source address. Next, Server ('A') validates the message 1022 according to the ICMPv6 Redirect message validation rules in 1023 Section 8.1 of [RFC4861] and also verifies that Client ('D') is 1024 authorized to use the Prefix Length in the Redirect message when 1025 applied to the AERO address in the network-layer source of the 1026 Redirect message. If validation fails, Server ('A') discards the 1027 message; otherwise, it copies the correct UDP port numbers and IP 1028 addresses into the TLLAO supplied by Client ('D'). 1030 Server ('A') then re-encapsulates the Redirect and relays it on to 1031 Client ('B') by changing the link-layer source address of the message 1032 to 'L2(A)' and changing the link-layer destination address to 'L2(B)' 1033 . Server ('A') finally forwards the re-encapsulated message to the 1034 ingress node ('B') without decrementing the network-layer IPv6 header 1035 Hop Limit field. 1037 While not shown in Figure 2, AERO Relays relay Redirect and Predirect 1038 messages in exactly this same fashion described above. See Figure 4 1039 in Appendix A for an extension of the reference operational scenario 1040 that includes Relays. 1042 3.9.8. Processing Redirects 1044 When Client ('B') receives the Redirect message, it accepts the 1045 message only if it has a link-layer source address of the Server, 1046 i.e. 'L2(A)'. Next, Client ('B') validates the message according to 1047 the ICMPv6 Redirect message validation rules in Section 8.1 of 1049 [RFC4861]. Following validation, Client ('B') then processes the 1050 message as follows. 1052 In the reference operational scenario, when Client ('B') receives the 1053 Redirect message, it either creates or updates a neighbor cache entry 1054 that stores the Target Address of the message as the network-layer 1055 address of Client ('D') and stores the link-layer address found in 1056 the TLLAO as the link-layer address of Client ('D'). Client ('D') 1057 then sets the neighbor cache entry FORWARD timer with timeout value 1058 FORWARD_TIME. Next, Client ('B') applies the Prefix Length to the 1059 Interface Identifier portion of the Target Address and records the 1060 resulting IPv6 prefix in its IPv6 forwarding table. 1062 Now, Client ('B') has an IPv6 forwarding table entry for 1063 Client('D')'s prefix and a neighbor cache entry with a valid FORWARD 1064 time, while Client ('D') has an IPv6 forwarding table entry for 1065 Client ('B')'s prefix with a valid ACCEPT time. Thereafter, Client 1066 ('B') may forward ordinary network-layer data packets directly to 1067 Client ("D") without involving Server ('A') and Client ('D') can 1068 verify that the packets came from an acceptable source. (In order 1069 for Client ('D') to forward packets to Client ('B') a corresponding 1070 Predirect/Redirect message exchange is required in the reverse 1071 direction.) 1073 3.10. Neighbor Reachability Maintenance 1075 AERO nodes send unicast NS messages to elicit NA messages from 1076 neighbors the same as described for Neighbor Unreachability Detection 1077 (NUD) in [RFC4861]. When an AERO node sends an NS/NA message, it 1078 MUST use its AERO address as the IPv6 source address and the AERO 1079 address of the neighbor as the IPv6 destination address. When an 1080 AERO node receives an NS/NA message, it accepts the message if it has 1081 a neighbor cache entry for the neighbor; otherwise, it ignores the 1082 message. 1084 When a source Client is redirected to a target Client it SHOULD test 1085 the direct path to the target by sending an initial NS message to 1086 elicit a solicited NA response. While testing the path, the source 1087 Client SHOULD continue sending packets via the Server until target 1088 Client reachability has been confirmed. The source Client SHOULD 1089 thereafter continue to test the direct path to the target Client (see 1090 Section 7.3 of [RFC4861]) in order to keep neighbor cache entries 1091 alive. In particular, the source Client sends NS messages to the 1092 target Client subject to rate limiting in order to receive solicited 1093 NA messages. If at any time the direct path appears to be failing, 1094 the source Client can resume sending packets via the Server which may 1095 or may not result in a new redirection event. 1097 When a target Client receives an NS message from a source Client, it 1098 resets the ACCEPT timer to ACCEPT_TIME if a neighbor cache entry 1099 exists; otherwise, it discards the NS message. 1101 When a source Client receives a solicited NA message from a target 1102 Client, it resets the FORWARD timer to FORWARD_TIME if a neighbor 1103 cache entry exists; otherwise, it discards the NA message. 1105 When the FORWARD timer on a neighbor cache entry expires, the source 1106 Client resumes sending any subsequent packets via the Server and may 1107 (eventually) receive a new Redirect message. When the ACCEPT timer 1108 on a neighbor cache entry expires, the target Client discards any 1109 subsequent packets received directly from the source Client. When 1110 both the FORWARD and ACCEPT timers on a neighbor cache entry expire, 1111 the Client deletes both the neighbor cache entry and the 1112 corresponding IPv6 forwarding table entry. 1114 If the source Client is unable to elicit an NA response from the 1115 target Client after MAX_RETRY attempts, it SHOULD consider the direct 1116 path unusable for forwarding purposes. Otherwise, the source Client 1117 considers the path usable and SHOULD thereafter process any link- 1118 layer errors as a hint that the direct path to the target Client has 1119 either failed or has become intermittent. 1121 3.11. Mobility and Link-Layer Address Change Considerations 1123 When a Client needs to change its link-layer address (e.g., due to a 1124 mobility event), it performs an immediate DHCPv6 Renew/Reply via each 1125 of its Servers using the new link-layer address as the source. The 1126 DHCPv6 Renew/Reply exchange will update each Server's neighbor cache. 1128 Next, the Client sends a Predirect message to each of its active 1129 neighbors via a Server using the new link-layer address as the 1130 encapsulation source address. The Predirect message includes a TLLAO 1131 with UDP Port Number and IP Address set to 0. The Server then copies 1132 the correct UDP port number and IP address into the TLLAO supplied by 1133 the Client and forwards the Predirect message towards the target as 1134 specified in Section 3.9. When the target receives the Predirect 1135 message, it returns a Redirect message which the Client processes as 1136 an indication that the target has received the update and is ready to 1137 accept encapsulated packets with the new link-layer address. 1139 When a Client needs to associate with a new Server, it issues a new 1140 DHCPv6 Renew message via the new Server as the DHCPv6 relay. The new 1141 Server then relays the message to the DHCPv6 server and processes the 1142 resulting exchange. After the Client receives the resulting DHCPv6 1143 Reply message, it sends an RS message to the new Server to receive a 1144 new RA message. 1146 When a Client disassociates with an existing Server, it sends a 1147 "terminating RS" message to the old Server. The terminating RS 1148 message is prepared exactly the same as for an ordinary RS message, 1149 except that the Code field contains the value '1'. When the old 1150 Server receives the terminating RS message, it withdraws the IPv6 1151 route from the routing system and deletes the neighbor cache entry 1152 and IPv6 forwarding table entry for the Client. The old Server then 1153 returns an RA message with default router lifetime set to 0 which the 1154 Client can use to verify that the termination signal has been 1155 processed. The client then deletes both the default route and the 1156 neighbor cache entry for the old Server. (Note that the Client and 1157 the old Server MAY impose a small delay before deleting the neighbor 1158 cache and IPv6 forwarding table entries so that any packets already 1159 in the system can still be delivered to the Client.) 1161 3.12. Encapsulation Protocol Version Considerations 1163 A source Client may connect only to an IPvX underlying network, while 1164 the target Client connects only to an IPvY underlying network. In 1165 that case, the target and source Clients have no means for reaching 1166 each other directly (since they connect to underlying networks of 1167 different IP protocol versions) and so must ignore any redirection 1168 messages and continue to send packets via the Server. 1170 3.13. Multicast Considerations 1172 When the underlying network does not support multicast, AERO nodes 1173 map IPv6 link-scoped multicast addresses (including 1174 "All_DHCP_Relay_Agents_and_Servers") to the underlying IP address of 1175 a Server. 1177 When the underlying network supports multicast, AERO nodes use the 1178 multicast address mapping specification found in [RFC2529] for IPv4 1179 underlying networks and use a direct multicast mapping for IPv6 1180 underlying networks. (In the latter case, "direct multicast mapping" 1181 means that if the IPv6 multicast destination address of the 1182 encapsulated packet is "M", then the IPv6 multicast destination 1183 address of the encapsulating header is also "M".) 1185 3.14. Operation on AERO Links Without DHCPv6 Services 1187 When the AERO link does not provide DHCPv6 services, operation can 1188 still be accommodated through administrative configuration of 1189 prefixes on AERO Clients. In that case, administrative 1190 configurations of IPv6 routes and AERO interface neighbor cache 1191 entries on both the Server and Client are also necessary. However, 1192 this may preclude the ability for Clients to dynamically change to 1193 new Servers, and can expose the AERO link to misconfigurations unless 1194 the administrative configurations are carefully coordinated. 1196 3.15. Operation on Server-less AERO Links 1198 In some AERO link scenarios, there may be no Servers on the link and/ 1199 or no need for Clients to use a Server as an intermediary trust 1200 anchor. In that case, each Client can then act as its own Server to 1201 establish neighbor cache entries and IPv6 forwarding table entries by 1202 performing direct Client-to-Client Predirect/Redirect exchanges, and 1203 some other form of trust basis must be applied so that each Client 1204 can verify that the prospective neighbor is authorized to use its 1205 claimed prefix. 1207 When there is no Server on the link, Clients must arrange to receive 1208 prefix delegations and publish the delegations via a secure alternate 1209 prefix delegation authority through some means outside the scope of 1210 this document. 1212 3.16. Other Considerations 1214 IPv6 hosts serviced by an AERO Client can reach IPv4-only services 1215 via a NAT64 gateway [RFC6146] within the IPv6 network. 1217 AERO nodes can use the Default Address Selection Policy with DHCPv6 1218 option [RFC7078] the same as on any IPv6 link. 1220 All other (non-multicast) functions that operate over ordinary IPv6 1221 links operate in the same fashion over AERO links. 1223 4. Implementation Status 1225 An application-layer implementation is in progress. 1227 5. IANA Considerations 1229 The IANA is instructed to assign a new 2-octet Hardware Type number 1230 for AERO in the "arp-parameters" registry per Section 2 of [RFC5494]. 1231 The number is assigned from the 2-octet Unassigned range with 1232 Hardware Type "AERO" and with this document as the reference. 1234 6. Security Considerations 1236 AERO link security considerations are the same as for standard IPv6 1237 Neighbor Discovery [RFC4861] except that AERO improves on some 1238 aspects. In particular, AERO is dependent on a trust basis between 1239 Clients and Servers, where the Clients only engage in the AERO 1240 mechanism when it is facilitated by a trust anchor. 1242 AERO links must be protected against link-layer address spoofing 1243 attacks in which an attacker on the link pretends to be a trusted 1244 neighbor. Links that provide link-layer securing mechanisms (e.g., 1245 WiFi networks) and links that provide physical security (e.g., 1246 enterprise network wired LANs) provide a first line of defense that 1247 is often sufficient. In other instances, additional securing 1248 mechanisms such as Secure Neighbor Discovery (SeND) [RFC3971], IPsec 1249 [RFC4301] or TLS [RFC5246] may be necessary. 1251 AERO Clients MUST ensure that their connectivity is not used by 1252 unauthorized nodes on EUNs to gain access to a protected network, 1253 i.e., AERO Clients that act as IPv6 routers MUST NOT provide routing 1254 services for unauthorized nodes. (This concern is no different than 1255 for ordinary hosts that receive an IP address delegation but then 1256 "share" the address with unauthorized nodes via an IPv6/IPv6 NAT 1257 function.) 1259 On some AERO links, establishment and maintenance of a direct path 1260 between neighbors requires secured coordination such as through the 1261 Internet Key Exchange (IKEv2) protocol [RFC5996] to establish a 1262 security association. 1264 7. Acknowledgements 1266 Discussions both on IETF lists and in private exchanges helped shape 1267 some of the concepts in this work. Individuals who contributed 1268 insights include Mikael Abrahamsson, Fred Baker, Stewart Bryant, 1269 Brian Carpenter, Wojciech Dec, Brian Haberman, Joel Halpern, Sascha 1270 Hlusiak, Lee Howard, Joe Touch and Bernie Volz. Members of the IESG 1271 also provided valuable input during their review process that greatly 1272 improved the document. Special thanks go to Stewart Bryant, Joel 1273 Halpern and Brian Haberman for their shepherding guidance. 1275 This work has further been encouraged and supported by Boeing 1276 colleagues including Keith Bartley, Dave Bernhardt, Cam Brodie, 1277 Balaguruna Chidambaram, Wen Fang, Anthony Gregory, Jeff Holland, Ed 1278 King, Gen MacLean, Kent Shuey, Mike Slane, Julie Wulff, Yueli Yang, 1279 and other members of the BR&T and BIT mobile networking teams. 1281 Earlier works on NBMA tunneling approaches are found in 1282 [RFC2529][RFC5214][RFC5569]. 1284 8. References 1285 8.1. Normative References 1287 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 1288 August 1980. 1290 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1291 1981. 1293 [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, 1294 RFC 792, September 1981. 1296 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1297 Requirement Levels", BCP 14, RFC 2119, March 1997. 1299 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 1300 (IPv6) Specification", RFC 2460, December 1998. 1302 [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in 1303 IPv6 Specification", RFC 2473, December 1998. 1305 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., 1306 and M. Carney, "Dynamic Host Configuration Protocol for 1307 IPv6 (DHCPv6)", RFC 3315, July 2003. 1309 [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic 1310 Host Configuration Protocol (DHCP) version 6", RFC 3633, 1311 December 2003. 1313 [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure 1314 Neighbor Discovery (SEND)", RFC 3971, March 2005. 1316 [RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms 1317 for IPv6 Hosts and Routers", RFC 4213, October 2005. 1319 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 1320 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 1321 September 2007. 1323 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 1324 Address Autoconfiguration", RFC 4862, September 2007. 1326 [RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node 1327 Requirements", RFC 6434, December 2011. 1329 8.2. Informative References 1331 [IRON] Templin, F., "The Internet Routing Overlay Network 1332 (IRON)", Work in Progress, June 2012. 1334 [RFC0879] Postel, J., "TCP maximum segment size and related topics", 1335 RFC 879, November 1983. 1337 [RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4 1338 Domains without Explicit Tunnels", RFC 2529, March 1999. 1340 [RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms", 1341 RFC 2675, August 1999. 1343 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 1344 Architecture", RFC 4291, February 2006. 1346 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 1347 Internet Protocol", RFC 4301, December 2005. 1349 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 1350 Discovery", RFC 4821, March 2007. 1352 [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly 1353 Errors at High Data Rates", RFC 4963, July 2007. 1355 [RFC4994] Zeng, S., Volz, B., Kinnear, K., and J. Brzozowski, 1356 "DHCPv6 Relay Agent Echo Request Option", RFC 4994, 1357 September 2007. 1359 [RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site 1360 Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, 1361 March 2008. 1363 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1364 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 1366 [RFC5494] Arkko, J. and C. Pignataro, "IANA Allocation Guidelines 1367 for the Address Resolution Protocol (ARP)", RFC 5494, 1368 April 2009. 1370 [RFC5522] Eddy, W., Ivancic, W., and T. Davis, "Network Mobility 1371 Route Optimization Requirements for Operational Use in 1372 Aeronautics and Space Exploration Mobile Networks", RFC 1373 5522, October 2009. 1375 [RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4 1376 Infrastructures (6rd)", RFC 5569, January 2010. 1378 [RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen, 1379 "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC 1380 5996, September 2010. 1382 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful 1383 NAT64: Network Address and Protocol Translation from IPv6 1384 Clients to IPv4 Servers", RFC 6146, April 2011. 1386 [RFC6204] Singh, H., Beebee, W., Donley, C., Stark, B., and O. 1387 Troan, "Basic Requirements for IPv6 Customer Edge 1388 Routers", RFC 6204, April 2011. 1390 [RFC6355] Narten, T. and J. Johnson, "Definition of the UUID-Based 1391 DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355, August 1392 2011. 1394 [RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label 1395 for Equal Cost Multipath Routing and Link Aggregation in 1396 Tunnels", RFC 6438, November 2011. 1398 [RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)", 1399 RFC 6691, July 2012. 1401 [RFC6706] Templin, F., "Asymmetric Extended Route Optimization 1402 (AERO)", RFC 6706, August 2012. 1404 [RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field", 1405 RFC 6864, February 2013. 1407 [RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and 1408 UDP Checksums for Tunneled Packets", RFC 6935, April 2013. 1410 [RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement 1411 for the Use of IPv6 UDP Datagrams with Zero Checksums", 1412 RFC 6936, April 2013. 1414 [RFC6939] Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer 1415 Address Option in DHCPv6", RFC 6939, May 2013. 1417 [RFC6980] Gont, F., "Security Implications of IPv6 Fragmentation 1418 with IPv6 Neighbor Discovery", RFC 6980, August 2013. 1420 [RFC7078] Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing 1421 Address Selection Policy Using DHCPv6", RFC 7078, January 1422 2014. 1424 Appendix A. AERO Server and Relay Interworking 1426 Figure 2 depicts a reference AERO operational scenario with a single 1427 Server on the AERO link. In order to support scaling to larger 1428 numbers of nodes, the AERO link can deploy multiple Servers and 1429 Relays, e.g., as shown in Figure 4. 1431 .-(::::::::) 1432 .-(::: IPv6 :::)-. 1433 (:: Internetwork ::) 1434 `-(::::::::::::)-' 1435 `-(::::::)-' 1436 | 1437 +--------------+ +------+-------+ +--------------+ 1438 |AERO Server C | | AERO Relay D | |AERO Server E | 1439 | (default->D) | | (A->C; G->E) | | (default->D) | 1440 | (A->B) | +-------+------+ | (G->F) | 1441 +-------+------+ | +------+-------+ 1442 | | | 1443 X---+---+-------------------+------------------+---+---X 1444 | AERO Link | 1445 +-----+--------+ +--------+-----+ 1446 |AERO Client B | |AERO Client F | 1447 | (default->C) | | (default->E) | 1448 +--------------+ +--------------+ 1449 .-. .-. 1450 ,-( _)-. ,-( _)-. 1451 .-(_ IPv6 )-. .-(_ IPv6 )-. 1452 (__ EUN ) (__ EUN ) 1453 `-(______)-' `-(______)-' 1454 | | 1455 +--------+ +--------+ 1456 | Host A | | Host G | 1457 +--------+ +--------+ 1459 Figure 4: AERO Server/Relay Interworking 1461 In this example, Client ('B') associates with Server ('C'), while 1462 Client ('F') associates with Server ('E'). Furthermore, Servers 1463 ('C') and ('E') do not associate with each other directly, but rather 1464 have an association with Relay ('D') (i.e., a router that has full 1465 topology information concerning its associated Servers and their 1466 Clients). Relay ('D') connects to the AERO link, and also connects 1467 to the native IPv6 Internetwork. 1469 When host ('A') sends a packet toward destination host ('G'), IPv6 1470 forwarding directs the packet through the EUN to Client ('B'), which 1471 forwards the packet to Server ('C') in absence of more-specific 1472 forwarding information. Server ('C') forwards the packet, and it 1473 also generates an AERO Predirect message that is then forwarded 1474 through Relay ('D') to Server ('E'). When Server ('E') receives the 1475 message, it forwards the message to Client ('F'). 1477 After processing the AERO Predirect message, Client ('F') sends an 1478 AERO Redirect message to Server ('E'). Server ('E'), in turn, 1479 forwards the message through Relay ('D') to Server ('C'). When 1480 Server ('C') receives the message, it forwards the message to Client 1481 ('B') informing it that host 'G's EUN can be reached via Client 1482 ('F'), thus completing the AERO redirection. 1484 The network layer routing information shared between Servers and 1485 Relays must be carefully coordinated in a manner outside the scope of 1486 this document. In particular, Relays require full topology 1487 information, while individual Servers only require partial topology 1488 information (i.e., they only need to know the EUN prefixes associated 1489 with their current set of Clients). See [IRON] for an architectural 1490 discussion of routing coordination between Relays and Servers. 1492 Author's Address 1494 Fred L. Templin (editor) 1495 Boeing Research & Technology 1496 P.O. Box 3707 1497 Seattle, WA 98124 1498 USA 1500 Email: fltemplin@acm.org