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Holland 3 Internet-Draft Akamai Technologies, Inc. 4 Updates: 7450 (if approved) April 04, 2019 5 Intended status: Standards Track 6 Expires: October 6, 2019 8 DNS Reverse IP AMT Discovery 9 draft-ietf-mboned-driad-amt-discovery-03 11 Abstract 13 This document updates RFC 7450 (Automatic Multicast Tunneling, or 14 AMT) by extending the relay discovery process to use a new DNS 15 resource record named AMTRELAY when discovering AMT relays for 16 source-specific multicast channels. The reverse IP DNS zone for a 17 multicast sender's IP address is configured to use AMTRELAY resource 18 records to advertise a set of AMT relays that can receive and forward 19 multicast traffic from that sender over an AMT tunnel. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at https://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on October 6, 2019. 38 Copyright Notice 40 Copyright (c) 2019 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (https://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 56 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 3 57 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 58 1.2.1. Relays and Gateways . . . . . . . . . . . . . . . . . 4 59 1.2.2. Definitions . . . . . . . . . . . . . . . . . . . . . 4 60 2. Relay Discovery Operation . . . . . . . . . . . . . . . . . . 5 61 2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 6 62 2.2. Signaling and Discovery . . . . . . . . . . . . . . . . . 6 63 2.3. Happy Eyeballs . . . . . . . . . . . . . . . . . . . . . 8 64 2.3.1. Overview . . . . . . . . . . . . . . . . . . . . . . 8 65 2.3.2. Connection Definition . . . . . . . . . . . . . . . . 9 66 2.4. Optimal Relay Selection . . . . . . . . . . . . . . . . . 9 67 2.4.1. Overview . . . . . . . . . . . . . . . . . . . . . . 9 68 2.4.2. Preference Ordering . . . . . . . . . . . . . . . . . 10 69 2.4.3. Connecting to Multiple Relays . . . . . . . . . . . . 13 70 2.5. Guidelines for Restarting Discovery . . . . . . . . . . . 13 71 2.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . 13 72 2.5.2. Updates to Restarting Events . . . . . . . . . . . . 14 73 2.5.3. Tunnel Stability . . . . . . . . . . . . . . . . . . 15 74 2.5.4. Traffic Health . . . . . . . . . . . . . . . . . . . 15 75 2.5.5. Relay Loaded or Shutting Down . . . . . . . . . . . . 17 76 2.5.6. Relay Discovery Messages vs. Restarting Discovery . . 17 77 2.5.7. Independent Discovery Per Traffic Source . . . . . . 18 78 2.6. DNS Configuration . . . . . . . . . . . . . . . . . . . . 18 79 2.7. Waiting for DNS resolution . . . . . . . . . . . . . . . 19 80 3. Example Deployments . . . . . . . . . . . . . . . . . . . . . 19 81 3.1. Example Receiving Networks . . . . . . . . . . . . . . . 19 82 3.1.1. Tier 3 ISP . . . . . . . . . . . . . . . . . . . . . 19 83 3.1.2. Small Office . . . . . . . . . . . . . . . . . . . . 20 84 3.2. Example Sending Networks . . . . . . . . . . . . . . . . 22 85 3.2.1. Sender-controlled Relays . . . . . . . . . . . . . . 22 86 3.2.2. Provider-controlled Relays . . . . . . . . . . . . . 23 87 4. AMTRELAY Resource Record Definition . . . . . . . . . . . . . 24 88 4.1. AMTRELAY RRType . . . . . . . . . . . . . . . . . . . . . 24 89 4.2. AMTRELAY RData Format . . . . . . . . . . . . . . . . . . 24 90 4.2.1. RData Format - Precedence . . . . . . . . . . . . . . 25 91 4.2.2. RData Format - Discovery Optional (D-bit) . . . . . . 25 92 4.2.3. RData Format - Type . . . . . . . . . . . . . . . . . 25 93 4.2.4. RData Format - Relay . . . . . . . . . . . . . . . . 26 94 4.3. AMTRELAY Record Presentation Format . . . . . . . . . . . 26 95 4.3.1. Representation of AMTRELAY RRs . . . . . . . . . . . 26 96 4.3.2. Examples . . . . . . . . . . . . . . . . . . . . . . 27 98 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 99 6. Security Considerations . . . . . . . . . . . . . . . . . . . 28 100 6.1. Use of AMT . . . . . . . . . . . . . . . . . . . . . . . 28 101 6.2. Record-spoofing . . . . . . . . . . . . . . . . . . . . . 28 102 6.3. Congestion . . . . . . . . . . . . . . . . . . . . . . . 29 103 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29 104 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 105 8.1. Normative References . . . . . . . . . . . . . . . . . . 29 106 8.2. Informative References . . . . . . . . . . . . . . . . . 31 107 Appendix A. Unknown RRType construction . . . . . . . . . . . . 32 108 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 33 110 1. Introduction 112 This document defines DNS Reverse IP AMT Discovery (DRIAD), a 113 mechanism for AMT gateways to discover AMT relays that are capable of 114 forwarding multicast traffic from a known source IP address. 116 AMT (Automatic Multicast Tunneling) is defined in [RFC7450], and 117 provides a method to transport multicast traffic over a unicast 118 tunnel, in order to traverse non-multicast-capable network segments. 120 Section 4.1.5 of [RFC7450] explains that the relay selection process 121 for AMT is intended to be more flexible than the particular discovery 122 method described in that document, and further explains that the 123 selection process might need to depend on the source of the multicast 124 traffic in some deployments, since a relay must be able to receive 125 multicast traffic from the desired source in order to forward it. 127 That section goes on to suggest DNS-based queries as a possible 128 solution. DRIAD is a DNS-based solution, as suggested there. This 129 solution also addresses the relay discovery issues in the 130 "Disadvantages" lists in Section 3.3 of [RFC8313] and Section 3.4 of 131 [RFC8313]. 133 The goal for DRIAD is to enable multicast connectivity between 134 separate multicast-enabled networks when neither the sending nor the 135 receiving network is connected to a multicast-enabled backbone, 136 without pre-configuring any peering arrangement between the networks. 138 This document updates Section 5.2.3.4 of [RFC7450] by adding a new 139 extension to the relay discovery procedure. 141 1.1. Background 143 The reader is assumed to be familiar with the basic DNS concepts 144 described in [RFC1034], [RFC1035], and the subsequent documents that 145 update them, particularly [RFC2181]. 147 The reader is also assumed to be familiar with the concepts and 148 terminology regarding source-specific multicast as described in 149 [RFC4607] and the use of IGMPv3 [RFC3376] and MLDv2 [RFC3810] for 150 group management of source-specific multicast channels, as described 151 in [RFC4604]. 153 The reader should also be familiar with AMT, particularly the 154 terminology listed in Section 3.2 of [RFC7450] and Section 3.3 of 155 [RFC7450]. 157 1.2. Terminology 159 1.2.1. Relays and Gateways 161 When reading this document, it's especially helpful to recall that 162 once an AMT tunnel is established, the relay receives native 163 multicast traffic and sends unicast tunnel-encapsulated traffic to 164 the gateway, and the gateway receives the tunnel-encapsulated 165 packets, decapsulates them, and forwards them as native multicast 166 packets, as illustrated in Figure 1. 168 Multicast +-----------+ Unicast +-------------+ Multicast 169 >---------> | AMT relay | >=======> | AMT gateway | >---------> 170 +-----------+ +-------------+ 172 Figure 1: AMT Tunnel Illustration 174 1.2.2. Definitions 175 +------------+------------------------------------------------------+ 176 | Term | Definition | 177 +------------+------------------------------------------------------+ 178 | (S,G) | A source-specific multicast channel, as described in | 179 | | [RFC4607]. A pair of IP addresses with a source host | 180 | | IP and destination group IP. | 181 | | | 182 | discovery | A broker or load balancer for AMT relay discovery, | 183 | broker | as mentioned in section 4.2.1.1 of [RFC7450]. | 184 | | | 185 | downstream | Further from the source of traffic, as described in | 186 | | [RFC7450]. | 187 | | | 188 | FQDN | Fully Qualified Domain Name, as described in | 189 | | [RFC8499] | 190 | | | 191 | gateway | An AMT gateway, as described in [RFC7450] | 192 | | | 193 | L flag | The "Limit" flag described in Section 5.1.1.4 of | 194 | | [RFC7450] | 195 | | | 196 | relay | An AMT relay, as described in [RFC7450] | 197 | | | 198 | RPF | Reverse Path Forwarding, as described in [RFC5110] | 199 | | | 200 | RR | A DNS Resource Record, as described in [RFC1034] | 201 | | | 202 | RRType | A DNS Resource Record Type, as described in | 203 | | [RFC1034] | 204 | | | 205 | SSM | Source-specific multicast, as described in [RFC4607] | 206 | | | 207 | upstream | Closer to the source of traffic, as described in | 208 | | [RFC7450]. | 209 +------------+------------------------------------------------------+ 211 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 212 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 213 "OPTIONAL" in this document are to be interpreted as described in 214 [RFC2119] and [RFC8174] when, and only when, they appear in all 215 capitals, as shown here. 217 2. Relay Discovery Operation 218 2.1. Overview 220 The AMTRELAY resource record (RR) defined in this document is used to 221 publish the IP address or domain name of a set of AMT relays or 222 discovery brokers that can receive, encapsulate, and forward 223 multicast traffic from a particular sender. 225 The sender is the owner of the RR, and configures the zone so that it 226 contains a set of RRs that provide the addresses or domain names of 227 AMT relays (or discovery brokers that advertise relays) that can 228 receive multicast IP traffic from that sender. 230 This enables AMT gateways in remote networks to discover an AMT relay 231 that is capable of forwarding traffic from the sender. This in turn 232 enables those AMT gateways to receive the multicast traffic tunneled 233 over a unicast AMT tunnel from those relays, and then to pass the 234 multicast packets into networks or applications that are using the 235 gateway to subscribe to traffic from that sender. 237 This mechanism only works for source-specific multicast (SSM) 238 channels. The source address of the (S,G) is reversed and used as an 239 index into one of the reverse mapping trees (in-addr.arpa for IPv4, 240 as described in Section 3.5 of [RFC1035], or ip6.arpa for IPv6, as 241 described in Section 2.5 of [RFC3596]). 243 This mechanism should be treated as an extension of the AMT relay 244 discovery procedure described in Section 5.2.3.4 of [RFC7450]. A 245 gateway that supports this method of AMT relay discovery SHOULD use 246 this method whenever it's performing the relay discovery procedure, 247 and the source IP addresses for desired (S,G)s are known to the 248 gateway, and conditions match the requirements outlined in 249 Section 2.4. 251 Some detailed example use cases are provided in Section 3, and other 252 applicable example topologies appear in Section 3.3 of [RFC8313], 253 Section 3.4 of [RFC8313], and Section 3.5 of [RFC8313]. 255 2.2. Signaling and Discovery 257 This section describes a typical example of the end-to-end process 258 for signaling a receiver's join of a SSM channel that relies on an 259 AMTRELAY RR. 261 The example in Figure 2 contains 2 multicast-enabled networks that 262 are both connected to the internet with non-multicast-capable links, 263 and which have no direct association with each other. 265 A content provider operates a sender, which is a source of multicast 266 traffic inside a multicast-capable network. 268 An end user who is a customer of the content provider has a 269 multicast-capable internet service provider, which operates a 270 receiving network that uses an AMT gateway. The AMT gateway is 271 DRIAD-capable. 273 The content provider provides the user with a receiving application 274 that tries to subscribe to at least one (S,G). This receiving 275 application could for example be a file transfer system using FLUTE 276 [RFC6726] or a live video stream using RTP [RFC3550], or any other 277 application that might subscribe to a SSM channel. 279 +---------------+ 280 | Sender | 281 | | | 198.51.100.15 | 282 | | +---------------+ 283 |Data| | 284 |Flow| Multicast | 285 \| |/ Network | 286 \ / | 5: Propagate RPF for Join(S,G) 287 \ / +---------------+ 288 \/ | AMT Relay | 289 | 203.0.113.15 | 290 +---------------+ 291 | 4: Gateway connects to Relay, 292 sends Join(S,G) over tunnel 293 | 294 Unicast 295 Tunnel | 297 ^ | 3: --> DNS Query: type=AMTRELAY, 298 | / 15.100.51.198.in-addr.arpa. 299 | | / <-- Response: 300 Join/Leave +-------------+ AMTRELAY=203.0.113.15 301 Signals | AMT gateway | 302 | +-------------+ 303 | | 2: Propagate RPF for Join(S,G) 304 | Multicast | 305 Network | 306 | 1: Join(S=198.51.100.15, G) 307 +-------------+ 308 | Receiver | 309 | (end user) | 310 +-------------+ 312 Figure 2: DRIAD Messaging 314 In this simple example, the sender IP is 198.51.100.15, and the relay 315 IP is 203.0.113.15. 317 The content provider has previously configured the DNS zone that 318 contains the domain name "15.100.51.198.in-addr.arpa.", which is the 319 reverse lookup domain name for his sender. The zone file contains an 320 AMTRELAY RR with the Relay's IP address. (See Section 4.3 for 321 details about the AMTRELAY RR format and semantics.) 323 The sequence of events depicted in Figure 2 is as follows: 325 1. The end user starts the app, which issues a join to the (S,G): 326 (198.51.100.15, 232.252.0.2). 328 2. The join propagates with RPF through the multicast-enabled 329 network with PIM [RFC7761] or another multicast routing 330 mechanism, until the AMT gateway receives a signal to join the 331 (S,G). 333 3. The AMT gateway performs a reverse DNS lookup for the AMTRELAY 334 RRType, by sending an AMTRELAY RRType query for the FQDN 335 "15.100.51.198.in-addr.arpa.", using the reverse IP domain name 336 for the sender's source IP address (the S from the (S,G)), as 337 described in Section 3.5 of [RFC1035]. 339 The DNS resolver for the AMT gateway uses ordinary DNS recursive 340 resolution until it has the authoritative result that the content 341 provider configured, which informs the AMT gateway that the relay 342 address is 203.0.113.15. 344 4. The AMT gateway performs AMT handshakes with the AMT relay as 345 described in Section 4 of [RFC7450], then forwards a Membership 346 report to the relay indicating subscription to the (S,G). 348 5. The relay propagates the join through its network toward the 349 sender, then forwards the appropriate AMT-encapsulated traffic to 350 the gateway, which decapsulates and forwards it as native 351 multicast through its downstream network to the end user. 353 2.3. Happy Eyeballs 355 2.3.1. Overview 357 Often, multiple choices of relay will exist for a gateway using DRIAD 358 for relay discovery. It is RECOMMENDED that DRIAD-capable gateways 359 implement a Happy Eyeballs [RFC8305] algorithm to support connecting 360 to multiple relays in parallel. 362 The parallel discovery logic of a Happy Eyeballs algorithm serves to 363 reduce join latency for the initial join of a SSM channel. This 364 section and Section 2.4.2 taken together provide guidance on use of a 365 Happy Eyeballs algorithm for the case of establishing AMT 366 connections. 368 2.3.2. Connection Definition 370 Section 5 of [RFC8305] non-normatively describes success at a 371 connection attempt as "generally when the TCP handshake completes". 373 There is no normative definition of a connection in the AMT 374 specification [RFC7450], and there is no TCP connection involved in 375 an AMT tunnel. 377 However, the concept of an AMT connection in the context of a Happy 378 Eyeballs algorithm is a useful one, and so this section provides the 379 following normative definition: 381 o An AMT connection is completed successfully when the gateway 382 receives from a newly discovered relay a valid Membership Query 383 message (Section 5.1.4 of [RFC7450]) that does not have the L flag 384 set. 386 See Section 2.5.5 for further information about the relevance of the 387 L flag to the establishment of a Happy Eyeballs connection. 389 2.4. Optimal Relay Selection 391 2.4.1. Overview 393 The reverse source IP DNS query of an AMTRELAY RR is a good way for a 394 gateway to discover a relay that is known to the sender. 396 However, it is NOT necessarily a good way to discover the best relay 397 for that gateway to use, because the RR will only provide information 398 about relays known to the source. 400 If there is an upstream relay in a network that is topologically 401 closer to the gateway and able to receive and forward multicast 402 traffic from the sender, that relay is better for the gateway to use, 403 since more of the network path uses native multicast, allowing more 404 chances for packet replication. But since that relay is not known to 405 the sender, it won't be advertised in the sender's reverse IP DNS 406 record. An example network that illustrates this scenario is 407 outlined in Section 3.1.2. 409 It's only appropriate for an AMT gateway to discover an AMT relay by 410 querying an AMTRELAY RR owned by a sender when all of these 411 conditions are met: 413 1. The gateway needs to propagate a join of an (S,G) over AMT, 414 because in the gateway's network, no RPF next hop toward the 415 source can propagate a native multicast join of the (S,G); and 417 2. The gateway is not already connected to a relay that forwards 418 multicast traffic from the source of the (S,G); and 420 3. The gateway is not configured to use a particular IP address for 421 AMT discovery, or a relay discovered with that IP is not able to 422 forward traffic from the source of the (S,G); and 424 4. The gateway is not able to find an upstream AMT relay with DNS-SD 425 [RFC6763], using "_amt._udp" as the Service section of the 426 queries, or a relay discovered this way is not able to forward 427 traffic from the source of the (S,G) (as described in 428 Section 2.5.4.1 or Section 2.5.5); and 430 5. The gateway is not able to find an upstream AMT relay with the 431 well-known anycast addresses from Section 7 of [RFC7450]. 433 When the above conditions are met, the gateway has no path within its 434 local network that can receive multicast traffic from the source IP 435 of the (S,G). 437 In this situation, the best way to find a relay that can forward the 438 required traffic is to use information that comes from the operator 439 of the sender. When the sender has configured an AMTRELAY RR, 440 gateways can use the DRIAD mechanism defined in this document to 441 discover the relay information provided by the sender. 443 2.4.2. Preference Ordering 445 This section defines a preference ordering for relay addresses during 446 the relay discovery process. Gateways are encouraged to implement a 447 Happy Eyeballs algorithm, but even gateways that do not implement a 448 Happy Eyeballs algorithm SHOULD use this ordering, except as noted. 450 When establishing an AMT tunnel to forward multicast data, it's very 451 important for the discovery process to prioritize the network 452 topology considerations ahead of address selection considerations, in 453 order to gain the packet replication benefits from using multicast 454 instead of unicast tunneling in the multicast-capable portions of the 455 network path. 457 The intent of the advice and requirements in this section is to 458 describe how a gateway should make use of the concurrency provided by 459 a Happy Eyeballs algorithm to reduce the join latency, while still 460 prioritizing network efficiency considerations over Address Selection 461 considerations. 463 Section 4 of [RFC8305] requires a Happy Eyeballs algorithm to sort 464 the addresses with the Destination Address Selection defined in 465 Section 6 of [RFC6724], but for the above reasons, that requirement 466 is superseded in the AMT discovery use case by the following 467 considerations: 469 1. Prefer Local Relays 471 Figure 5 and Section 3.1.2 provide a motivating example to prefer 472 DNS-SD [RFC6763] for discovery strictly ahead of using the 473 AMTRELAY RR controlled by the sender for AMT discovery. 475 For this reason, it's RECOMMENDED that AMT gateways by default 476 perform service discovery using DNS Service Discovery (DNS-SD) 477 [RFC6763] for _amt._udp. (with chosen as 478 described in Section 11 of [RFC6763]) and use the AMT relays 479 discovered that way in preference to AMT relays discoverable via 480 the mechanism defined in this document (DRIAD). 482 2. Prefer Relays Managed by the Containing Network 484 When no local relay is discoverable with DNS-SD, it still may be 485 the case that a relay local to the receiver is operated by the 486 network providing transit services to the receiver. 488 In this case, when the network cannot make the relay discoverable 489 via DNS-SD, the network SHOULD use the well-known anycast 490 addresses from Section 7 of [RFC7450] to route discovery traffic 491 to the relay most appropriate to the receiver's gateway. 493 Accordingly, the gateway SHOULD by default discover a relay with 494 the well-known AMT anycast addresses as the second preference 495 after DNS-SD when searching for a local relay. 497 3. Let Sender Manage Relay Provisioning 499 A related motivating example in the sending-side network is 500 provided by considering a sender which needs to instruct the 501 gateways on how to select between connecting to Figure 6 or 502 Figure 7 (from Section 3.2), in order to manage load and failover 503 scenarios in a manner that operates well with the sender's 504 provisioning strategy for horizontal scaling of AMT relays. 506 In this example about the sending-side network, the precedence 507 field described in Section 4.2.1 is a critical method of control 508 so that senders can provide the appropriate guidance to gateways 509 during the discovery process. 511 Therefore, after DNS-SD, the precedence from the RR MUST be used 512 for sorting preference ahead of the Destination Address Selection 513 ordering from Section 6 of [RFC6724], so that only relay IPs with 514 the same precedence are directly compared according to the 515 Destination Address Selection ordering. 517 Accordingly, AMT gateways SHOULD by default prefer relays first by 518 DNS-SD if available, then with the anycast addresses defined in 519 Section 7 of [RFC7450] (namely: 192.52.193.1 and 2001:3::1), then by 520 DRIAD as described in this document (in precedence order, as 521 described in Section 4.2.1). 523 This default behavior MAY be overridden by administrative 524 configuration where other behavior is more appropriate for the 525 gateway within its network. 527 Among relay addresses that have an equivalent preference as described 528 above, a Happy Eyeballs algorithm for AMT MUST use the Destination 529 Address Selection defined in Section 6 of [RFC6724], as required by 530 [RFC8305]. 532 Among relay addresses that still have an equivalent preference after 533 the above orderings, a gateway MUST make a non-deterministic choice 534 for relay preference ordering, in order to support load balancing by 535 DNS configurations that provide many relay options. (Note that 536 gateways not implementing a Happy Eyeballs algorithm are not required 537 to use the Destination Address Selection ordering, but are still 538 required to use non-deterministic ordering among equally preferred 539 relays.) 541 Note also that certain relay addresses may be excluded from 542 consideration by the hold-down timers described in Section 2.5.4.1 or 543 Section 2.5.5. These relays constitute "unusable destinations" under 544 Rule 1 of the Destination Address Selection, and are also not part of 545 the superseding considerations described above. 547 The discovery and connection process for the relay addresses in the 548 above described ordering MAY operate in parallel, subject to delays 549 prescribed by the Happy Eyeballs requirements described in Section 5 550 of [RFC8305] for successively launched concurrent connection 551 attempts. 553 2.4.3. Connecting to Multiple Relays 555 In some deployments, it may be useful for a gateway to connect to 556 multiple upstream relays and subscribe to the same traffic, in order 557 to support an active/active failover model. A gateway SHOULD NOT be 558 configured to do so without guaranteeing that adequate bandwidth is 559 available. 561 A gateway configured to do this SHOULD still use the same preference 562 ordering logic from Section 2.4.2 for both connections. (Note that 563 this ordering allows for overriding by explicit administrative 564 configuration where required.) 566 2.5. Guidelines for Restarting Discovery 568 2.5.1. Overview 570 It's expected that gateways deployed in different environments will 571 use a variety of heuristics to decide when it's appropriate to 572 restart the relay discovery process, in order to meet different 573 performance goals (for example, to fulfill different kinds of service 574 level agreements). 576 In general, restarting the discovery process is always safe for the 577 gateway and relay during any of the events listed in this section, 578 but may cause a disruption in the forwarded traffic if the discovery 579 process results in choosing a different relay, because this changes 580 the RPF forwarding tree for the multicast traffic upstream of the 581 gateway. This is likely to result in some dropped or duplicated 582 packets from channels actively being tunneled from the old relay to 583 the gateway. 585 The degree of impact on the traffic from choosing a different relay 586 may depend on network conditions between the gateway and the new 587 relay, as well as the network conditions and topology between the 588 sender and the new relay, as this may cause the relay to propagate a 589 new RPF join toward the sender. 591 Balancing the expected impact on the tunneled traffic against likely 592 or observed problems with an existing connection to the relay is the 593 goal of the heuristics that gateways use to determine when to restart 594 the discovery process. 596 The non-normative advice in this section should be treated as 597 guidelines to operators and implementors working with AMT systems 598 that can use DRIAD as part of the relay discovery process. 600 2.5.2. Updates to Restarting Events 602 Section 5.2.3.4.1 of [RFC7450] lists several events that may cause a 603 gateway to start or restart the discovery procedure. 605 This document provides some updates and recommendations regarding the 606 handling of these and similar events. The first 5 events are copied 607 here and numbered for easier reference, and the following events are 608 newly added for consideration in this document: 610 1. When a gateway pseudo-interface is started (enabled). 612 2. When the gateway wishes to report a group subscription when none 613 currently exist. 615 3. Before sending the next Request message in a membership update 616 cycle. 618 4. After the gateway fails to receive a response to a Request 619 message. 621 5. After the gateway receives a Membership Query message with the L 622 flag set to 1. 624 6. When the gateway wishes to report a (S,G) subscription with a 625 source address that does not currently have other group 626 subscriptions. 628 7. When there is a network change detected, for example when a 629 gateway is operating inside an end user device or application, 630 and the device joins a different network, or when the domain 631 portion of a DNS-SD domain name changes in response to a DHCP 632 message or administrative configuration. 634 8. When congestion or substantial loss is detected in the stream of 635 AMT packets from a relay. 637 9. When the gateway has reported one or more (S,G) subscriptions, 638 but no traffic is received from the source for some timeout. 639 (See Section 2.5.4.1). 641 This list is not exhaustive, nor are any of the listed events 642 strictly required to always force a restart of the discovery process. 644 Note that during event #1, a gateway may use DNS-SD, but does not 645 have sufficient information to use DRIAD, since no source is known. 647 2.5.3. Tunnel Stability 649 In general, subscribers to active traffic flows that are being 650 forwarded by an AMT gateway are less likely to experience a 651 degradation in service (for example, from missing or duplicated 652 packets) when the gateway continues using the same relay, as long the 653 relay is not overloaded and the network conditions remain stable. 655 Therefore, gateways SHOULD avoid performing a full restart of the 656 discovery process during routine cases of event #3 (sending a new 657 Request message), since it occurs frequently in normal operation. 659 However, see Section 2.5.4, Section 2.5.6, and Section 2.5.4.3 for 660 more information about exceptional cases when it may be appropriate 661 to use event #3. 663 2.5.4. Traffic Health 665 2.5.4.1. Absence of Traffic 667 If a gateway indicates one or more (S,G) subscriptions in a 668 Membership Update message, but no traffic for any of the (S,G)s is 669 received in a reasonable time, it's appropriate for the gateway to 670 restart the discovery process. 672 If the gateway restarts the discovery process multiple times 673 consecutively for this reason, the timeout period SHOULD be adjusted 674 to provide a random exponential back-off. 676 The RECOMMENDED timeout is a random value in the range 677 [initial_timeout, MIN(initial_timeout * 2^retry_count, 678 maximum_timeout)], with a RECOMMENDED initial_timeout of 4 seconds 679 and a RECOMMENDED maximum_timeout of 120 seconds. 681 Note that the recommended initial_timeout is larger than the initial 682 timout recommended in the similar algorithm from Section 5.2.3.4.3 of 683 [RFC7450]. This is to provide time for RPF Join propagation in the 684 sending network. Although the timeout values may be administratively 685 adjusted to support performance requirements, operators are advised 686 to consider the possibility of join propagation delays between the 687 sender and the relay when choosing an appropriate timeout value. 689 Gateways restarting the discovery process because of an absence of 690 traffic MUST use a hold-down timer that removes this relay from 691 consideration during subsequent rounds of discovery while active. 692 The hold-down SHOULD last for no less than 3 minutes and no more than 693 10 minutes. 695 2.5.4.2. Loss and Congestion 697 In some gateway deployments, it is also feasible to monitor the 698 health of traffic flows through the gateway, for example by detecting 699 the rate of packet loss by communicating out of band with receivers, 700 or monitoring the packets of known protocols with sequence numbers. 701 Where feasible, it's encouraged for gateways to use such traffic 702 health information to trigger a restart of the discovery process 703 during event #3 (before sending a new Request message). 705 However, to avoid synchronized rediscovery by many gateways 706 simultaneously after a transient network event upstream of a relay 707 results in many receivers detecting poor flow health at the same 708 time, it's recommended to add a random delay before restarting the 709 discovery process in this case. 711 The span of the random portion of the delay should be no less than 10 712 seconds by default, but may be administratively configured to support 713 different performance requirements. 715 2.5.4.3. Ancient Discovery Information 717 In most cases, a gateway actively receiving healthy traffic from a 718 relay that has not indicated load with the L flag should prefer to 719 remain connected to the same relay, as described in Section 2.5.3. 721 However, a relay that appears healthy but has been forwarding traffic 722 for days or weeks may have an increased chance of becoming unstable. 723 Gateways may benefit from restarting the discovery process during 724 event #3 (before sending a Request message) after the expiration of a 725 long-term timeout, on the order of multiple hours, or even days in 726 some deployments. 728 It may be beneficial for such timers to consider the amount of 729 traffic currently being forwarded, and to give a higher probability 730 of restarting discovery during periods with an unusually low data 731 rate, to reduce the impact on active traffic while still avoiding 732 relying on the results of a very old discovery. 734 Other issues may also be worth considering as part of this heuristic; 735 for example, if the DNS expiry time of the record that was used to 736 discover the current relay has not passed, the long term timer might 737 be restarted without restarting the discovery process. 739 2.5.5. Relay Loaded or Shutting Down 741 The L flag (see Section 5.1.4.4 of [RFC7450]) is the preferred 742 mechanism for a relay to signal overloading or a graceful shutdown to 743 gateways. 745 A gateway that supports handling of the L flag should generally 746 restart the discovery process when it processes a Membership Query 747 packet with the L flag set. If an L flag is received while a 748 concurrent Happy Eyeballs discovery process is under way for multiple 749 candidate relays (Section 2.3), the relay sending the L flag SHOULD 750 NOT be considered for the relay selection. 752 It is also RECOMMENDED that gateways avoid choosing a relay that has 753 recently sent an L flag, with approximately a 10-minute hold-down. 754 Gateways SHOULD treat this hold-down timer in the same way as the 755 hold-down in Section 2.5.4.1, so that the relay is removed from 756 consideration for short-term subsequent rounds of discovery. 758 2.5.6. Relay Discovery Messages vs. Restarting Discovery 760 A gateway should only send DNS queries with the AMTRELAY RRType or 761 the DNS-SD DNS queries for an AMT service as part of starting or 762 restarting the discovery process. 764 However, all AMT relays are required to support handling of Relay 765 Discovery messages (e.g. in Section 5.3.3.2 of [RFC7450]). 767 So a gateway with an existing connection to a relay can send a Relay 768 Discovery message to the unicast address of that AMT relay. Under 769 stable conditions with an unloaded relay, it's expected that the 770 relay will return its own unicast address in the Relay Advertisement, 771 in response to such a Relay Discovery message. Since this will not 772 result in the gateway changing to another relay unless the relay 773 directs the gateway away, this is a reasonable exception to the 774 advice against handling event #3 described in Section 2.5.3. 776 This behavior is discouraged for gateways that do support the L flag, 777 to avoid sending unnecessary packets over the network. 779 However, gateways that do not support the L flag may be able to avoid 780 a disruption in the forwarded traffic by sending such Relay Discovery 781 messages regularly. When a relay is under load or has started a 782 graceful shutdown, it may respond with a different relay address, 783 which the gateway can use to connect to a different relay. This kind 784 of coordinated handoff will likely result in a smaller disruption to 785 the traffic than if the relay simply stops responding to Request 786 messages, and stops forwarding traffic. 788 This style of Relay Discovery message (one sent to the unicast 789 address of a relay that's already forwarding traffic to this gateway) 790 should not be considered a full restart of the relay discovery 791 process. It is recommended for gateways to support the L flag, but 792 for gateways that do not support the L flag, sending this message 793 during event #3 may help mitigate service degradation when relays 794 become unstable. 796 2.5.7. Independent Discovery Per Traffic Source 798 Relays discovered via the AMTRELAY RR are source-specific relay 799 addresses, and may use different pseudo-interfaces from each other 800 and from relays discovered via DNS-SD or a non-source-specific 801 address, as described in Section 4.1.2.1 of [RFC7450]. 803 Restarting the discovery process for one pseudo-interface does not 804 require restarting the discovery process for other pseudo-interfaces. 805 Gateway heuristics about restarting the discovery process should 806 operate independently for different tunnels to relays, when 807 responding to events that are specific to the different tunnels. 809 2.6. DNS Configuration 811 Often an AMT gateway will only have access to the source and group IP 812 addresses of the desired traffic, and will not know any other name 813 for the source of the traffic. Because of this, typically the best 814 way of looking up AMTRELAY RRs will be by using the source IP address 815 as an index into one of the reverse mapping trees (in-addr.arpa for 816 IPv4, as described in Section 3.5 of [RFC1035], or ip6.arpa for IPv6, 817 as described in Section 2.5 of [RFC3596]). 819 Therefore, it is RECOMMENDED that AMTRELAY RRs be added to reverse IP 820 zones as appropriate. AMTRELAY records MAY also appear in other 821 zones, but the primary intended use case requires a reverse IP 822 mapping for the source from an (S,G) in order to be useful to most 823 AMT gateways. 825 When performing the AMTRELAY RR lookup, any CNAMEs or DNAMEs found 826 MUST be followed. This is necessary to support zone delegation. 827 Some examples outlining this need are described in [RFC2317]. 829 See Section 4 and Section 4.3 for a detailed explanation of the 830 contents for a DNS Zone file. 832 2.7. Waiting for DNS resolution 834 The DNS query functionality is expected to follow ordinary standards 835 and best practices for DNS clients. A gateway MAY use an existing 836 DNS client implementation that does so, and MAY rely on that client's 837 retry logic to determine the timeouts between retries. 839 Otherwise, a gateway MAY re-send a DNS query if it does not receive 840 an appropriate DNS response within some timeout period. If the 841 gateway retries multiple times, the timeout period SHOULD be adjusted 842 to provide a random exponential back-off. 844 As with the waiting process for the Relay Advertisement message from 845 Section 5.2.3.4.3 of [RFC7450], the RECOMMENDED timeout is a random 846 value in the range [initial_timeout, MIN(initial_timeout * 847 2^retry_count, maximum_timeout)], with a RECOMMENDED initial_timeout 848 of 1 second and a RECOMMENDED maximum_timeout of 120 seconds. 850 3. Example Deployments 852 3.1. Example Receiving Networks 854 3.1.1. Tier 3 ISP 856 One example of a receiving network is an ISP that offers multicast 857 ingest services to its subscribers, illustrated in Figure 3. 859 In the example network below, subscribers can join (S,G)s with MLDv2 860 or IGMPv3 as described in [RFC4604], and the AMT gateway in this ISP 861 can receive and forward multicast traffic from one of the example 862 sending networks in Section 3.2 by discovering the appropriate AMT 863 relays with a DNS lookup for the AMTRELAY RR with the reverse IP of 864 the source in the (S,G). 866 Internet 867 ^ ^ Multicast-enabled 868 | | Receiving Network 869 +------|------------|-------------------------+ 870 | | | | 871 | +--------+ +--------+ +=========+ | 872 | | Border |---| Border | | AMT | | 873 | | Router | | Router | | gateway | | 874 | +--------+ +--------+ +=========+ | 875 | | | | | 876 | +-----+------+-----------+--+ | 877 | | | | 878 | +-------------+ +-------------+ | 879 | | Agg Routers | .. | Agg Routers | | 880 | +-------------+ +-------------+ | 881 | / \ \ / \ | 882 | +---------------+ +---------------+ | 883 | |Access Systems | ....... |Access Systems | | 884 | |(CMTS/OLT/etc.)| |(CMTS/OLT/etc.)| | 885 | +---------------+ +---------------+ | 886 | | | | 887 +--------|------------------------|-----------+ 888 | | 889 +---+-+-+---+---+ +---+-+-+---+---+ 890 | | | | | | | | | | 891 /-\ /-\ /-\ /-\ /-\ /-\ /-\ /-\ /-\ /-\ 892 |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| 894 Subscribers 896 Figure 3: Receiving ISP Example 898 3.1.2. Small Office 900 Another example receiving network is a small branch office that 901 regularly accesses some multicast content, illustrated in Figure 4. 903 This office has desktop devices that need to receive some multicast 904 traffic, so an AMT gateway runs on a LAN with these devices, to pull 905 traffic in through a non-multicast next-hop. 907 The office also hosts some mobile devices that have AMT gateway 908 instances embedded inside apps, in order to receive multicast traffic 909 over their non-multicast wireless LAN. (Note that the "Legacy 910 Router" is a simplification that's meant to describe a variety of 911 possible conditions; for example it could be a device providing a 912 split-tunnel VPN as described in [RFC7359], deliberately excluding 913 multicast traffic for a VPN tunnel, rather than a device which is 914 incapable of multicast forwarding.) 916 Internet 917 (non-multicast) 918 ^ 919 | Office Network 920 +----------|----------------------------------+ 921 | | | 922 | +---------------+ (Wifi) Mobile apps | 923 | | Modem+ | Wifi | - - - - w/ embedded | 924 | | Router | AP | AMT gateways | 925 | +---------------+ | 926 | | | 927 | | | 928 | +----------------+ | 929 | | Legacy Router | | 930 | | (unicast) | | 931 | +----------------+ | 932 | / | \ | 933 | / | \ | 934 | +--------+ +--------+ +--------+=========+ | 935 | | Phones | | ConfRm | | Desks | AMT | | 936 | | subnet | | subnet | | subnet | gateway | | 937 | +--------+ +--------+ +--------+=========+ | 938 | | 939 +---------------------------------------------+ 941 Figure 4: Small Office (no multicast up) 943 By adding an AMT relay to this office network as in Figure 5, it's 944 possible to make use of multicast services from the example 945 multicast-capable ISP in Section 3.1.1. 947 Multicast-capable ISP 948 ^ 949 | Office Network 950 +----------|----------------------------------+ 951 | | | 952 | +---------------+ (Wifi) Mobile apps | 953 | | Modem+ | Wifi | - - - - w/ embedded | 954 | | Router | AP | AMT gateways | 955 | +---------------+ | 956 | | +=======+ | 957 | +---Wired LAN---| AMT | | 958 | | | relay | | 959 | +----------------+ +=======+ | 960 | | Legacy Router | | 961 | | (unicast) | | 962 | +----------------+ | 963 | / | \ | 964 | / | \ | 965 | +--------+ +--------+ +--------+=========+ | 966 | | Phones | | ConfRm | | Desks | AMT | | 967 | | subnet | | subnet | | subnet | gateway | | 968 | +--------+ +--------+ +--------+=========+ | 969 | | 970 +---------------------------------------------+ 972 Figure 5: Small Office Example 974 When multicast-capable networks are chained like this, with a network 975 like the one in Figure 5 receiving internet services from a 976 multicast-capable network like the one in Figure 3, it's important 977 for AMT gateways to reach the more local AMT relay, in order to avoid 978 accidentally tunneling multicast traffic from a more distant AMT 979 relay with unicast, and failing to utilize the multicast transport 980 capabilities of the network in Figure 3. 982 3.2. Example Sending Networks 984 3.2.1. Sender-controlled Relays 986 When a sender network is also operating AMT relays to distribute 987 multicast traffic, as in Figure 6, each address could appear as an 988 AMTRELAY RR for the reverse IP of the sender, or one or more domain 989 names could appear in AMTRELAY RRs, and the AMT relay addresses can 990 be discovered by finding a A or AAAA records from those domain names. 992 Sender Network 993 +-----------------------------------+ 994 | | 995 | +--------+ +=======+ +=======+ | 996 | | Sender | | AMT | | AMT | | 997 | +--------+ | relay | | relay | | 998 | | +=======+ +=======+ | 999 | | | | | 1000 | +-----+------+----------+ | 1001 | | | 1002 +-----------|-----------------------+ 1003 v 1004 Internet 1005 (non-multicast) 1007 Figure 6: Small Office Example 1009 3.2.2. Provider-controlled Relays 1011 When an ISP offers a service to transmit outbound multicast traffic 1012 through a forwarding network, it might also offer AMT relays in order 1013 to reach receivers without multicast connectivity to the forwarding 1014 network, as in Figure 7. In this case it's RECOMMENDED that the ISP 1015 also provide at least one domain name for the AMT relays for use with 1016 the AMTRELAY RR. 1018 When the sender wishes to use the relays provided by the ISP for 1019 forwarding multicast traffic, an AMTRELAY RR should be configured to 1020 use the domain name provided by the ISP, to allow for address 1021 reassignment of the relays without forcing the sender to reconfigure 1022 the corresponding AMTRELAY RRs. 1024 +--------+ 1025 | Sender | 1026 +---+----+ Multicast-enabled 1027 | Sending Network 1028 +-----------|-------------------------------+ 1029 | v | 1030 | +------------+ +=======+ +=======+ | 1031 | | Agg Router | | AMT | | AMT | | 1032 | +------------+ | relay | | relay | | 1033 | | +=======+ +=======+ | 1034 | | | | | 1035 | +-----+------+--------+---------+ | 1036 | | | | 1037 | +--------+ +--------+ | 1038 | | Border |---| Border | | 1039 | | Router | | Router | | 1040 | +--------+ +--------+ | 1041 +-----|------------|------------------------+ 1042 | | 1043 v v 1044 Internet 1045 (non-multicast) 1047 Figure 7: Sending ISP Example 1049 4. AMTRELAY Resource Record Definition 1051 4.1. AMTRELAY RRType 1053 The AMTRELAY RRType has the mnemonic AMTRELAY and type code 260 1054 (decimal). 1056 The AMTRELAY RR is class independent. 1058 4.2. AMTRELAY RData Format 1060 The AMTRELAY RData consists of a 8-bit precedence field, a 1-bit 1061 "Discovery Optional" field, a 7-bit type field, and a variable length 1062 relay field. 1064 0 1 2 3 1065 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 1066 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1067 | precedence |D| type | | 1068 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 1069 ~ relay ~ 1070 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1072 4.2.1. RData Format - Precedence 1074 This is an 8-bit precedence for this record. It is interpreted in 1075 the same way as the PREFERENCE field described in Section 3.3.9 of 1076 [RFC1035]. 1078 Relays listed in AMTRELAY records with a lower value for precedence 1079 are to be attempted first. 1081 4.2.2. RData Format - Discovery Optional (D-bit) 1083 The D bit is a "Discovery Optional" flag. 1085 If the D bit is set to 0, a gateway using this RR MUST perform AMT 1086 relay discovery as described in Section 4.2.1.1 of [RFC7450], rather 1087 than directly sending an AMT Request message to the relay. 1089 That is, the gateway MUST receive an AMT Relay Advertisement message 1090 (Section 5.1.2 of [RFC7450]) for an address before sending an AMT 1091 Request message (Section 5.1.3 of [RFC7450]) to that address. Before 1092 receiving the Relay Advertisement message, this record has only 1093 indicated that the address can be used for AMT relay discovery, not 1094 for a Request message. This is necessary for devices that are not 1095 fully functional AMT relays, but rather load balancers or brokers, as 1096 mentioned in Section 4.2.1.1 of [RFC7450]. 1098 If the D bit is set to 1, the gateway MAY send an AMT Request message 1099 directly to the discovered relay address without first sending an AMT 1100 Discovery message. 1102 This bit should be set according to advice from the AMT relay 1103 operator. The D bit MUST be set to zero when no information is 1104 available from the AMT relay operator about its suitability. 1106 4.2.3. RData Format - Type 1108 The type field indicates the format of the information that is stored 1109 in the relay field. 1111 The following values are defined: 1113 o type = 0: The relay field is empty (0 bytes). 1115 o type = 1: The relay field contains a 4-octet IPv4 address. 1117 o type = 2: The relay field contains a 16-octet IPv6 address. 1119 o type = 3: The relay field contains a wire-encoded domain name. 1120 The wire-encoded format is self-describing, so the length is 1121 implicit. The domain name MUST NOT be compressed. (See 1122 Section 3.3 of [RFC1035] and Section 4 of [RFC3597].) 1124 4.2.4. RData Format - Relay 1126 The relay field is the address or domain name of the AMT relay. It 1127 is formatted according to the type field. 1129 When the type field is 0, the length of the relay field is 0, and it 1130 indicates that no AMT relay should be used for multicast traffic from 1131 this source. 1133 When the type field is 1, the length of the relay field is 4 octets, 1134 and a 32-bit IPv4 address is present. This is an IPv4 address as 1135 described in Section 3.4.1 of [RFC1035]. This is a 32-bit number in 1136 network byte order. 1138 When the type field is 2, the length of the relay field is 16 octets, 1139 and a 128-bit IPv6 address is present. This is an IPv6 address as 1140 described in Section 2.2 of [RFC3596]. This is a 128-bit number in 1141 network byte order. 1143 When the type field is 3, the relay field is a normal wire-encoded 1144 domain name, as described in Section 3.3 of [RFC1035]. Compression 1145 MUST NOT be used, for the reasons given in Section 4 of [RFC3597]. 1147 For a type 3 record, the D-bit and preference fields carry over to 1148 all A or AAAA records for the domain name. There is no difference in 1149 the result of the discovery process when it's obtained by type 1 or 1150 type 2 AMTRELAY records with identical D-bit and preference fields, 1151 vs. when the result is obtained by a type 3 AMTRELAY record that 1152 resolves to the same set of IPv4 and IPv6 addresses via A and AAAA 1153 lookups. 1155 4.3. AMTRELAY Record Presentation Format 1157 4.3.1. Representation of AMTRELAY RRs 1159 AMTRELAY RRs may appear in a zone data master file. The precedence, 1160 D-bit, relay type, and relay fields are REQUIRED. 1162 If the relay type field is 0, the relay field MUST be ".". 1164 The presentation for the record is as follows: 1166 IN AMTRELAY precedence D-bit type relay 1168 4.3.2. Examples 1170 In a DNS authoritative nameserver that understands the AMTRELAY type, 1171 the zone might contain a set of entries like this: 1173 $ORIGIN 100.51.198.in-addr.arpa. 1174 10 IN AMTRELAY 10 0 1 203.0.113.15 1175 10 IN AMTRELAY 10 0 2 2001:DB8::15 1176 10 IN AMTRELAY 128 1 3 amtrelays.example.com. 1178 This configuration advertises an IPv4 discovery address, an IPv6 1179 discovery address, and a domain name for AMT relays which can receive 1180 traffic from the source 198.51.100.10. The IPv4 and IPv6 addresses 1181 are configured with a D-bit of 0 (meaning discovery is mandatory, as 1182 described in Section 4.2.2), and a precedence 10 (meaning they're 1183 preferred ahead of the last entry, which has precedence 128). 1185 For zone files in name servers that don't support the AMTRELAY RRType 1186 natively, it's possible to use the format for unknown RR types, as 1187 described in [RFC3597]. This approach would replace the AMTRELAY 1188 entries in the example above with the entries below: 1190 10 IN TYPE260 \# ( 1191 6 ; length 1192 0a ; precedence=10 1193 01 ; D=0, relay type=1, an IPv4 address 1194 cb00710f ) ; 203.0.113.15 1195 10 IN TYPE260 \# ( 1196 18 ; length 1197 0a ; precedence=10 1198 02 ; D=0, relay type=2, an IPv6 address 1199 20010db800000000000000000000000f ) ; 2001:db8::15 1200 10 IN TYPE260 \# ( 1201 24 ; length 1202 80 ; precedence=128 1203 83 ; D=1, relay type=3, a wire-encoded domain name 1204 09616d7472656c617973076578616d706c6503636f6d ) ; domain name 1206 See Appendix A for more details. 1208 5. IANA Considerations 1210 This document updates the IANA Registry for DNS Resource Record Types 1211 by assigning type 260 to the AMTRELAY record. 1213 This document creates a new registry named "AMTRELAY Resource Record 1214 Parameters", with a sub-registry for the "Relay Type Field". The 1215 initial values in the sub-registry are: 1217 +-------+---------------------------------------+ 1218 | Value | Description | 1219 +-------+---------------------------------------+ 1220 | 0 | No relay is present. | 1221 | 1 | A 4-byte IPv4 address is present | 1222 | 2 | A 16-byte IPv6 address is present | 1223 | 3 | A wire-encoded domain name is present | 1224 | 4-255 | Unassigned | 1225 +-------+---------------------------------------+ 1227 Values 0, 1, 2, and 3 are further explained in Section 4.2.3 and 1228 Section 4.2.4. Relay type numbers 4 through 255 can be assigned with 1229 a policy of Specification Required (as described in [RFC8126]). 1231 6. Security Considerations 1233 6.1. Use of AMT 1235 This document defines a mechanism that enables a more widespread and 1236 automated use of AMT, even without access to a multicast backbone. 1237 Operators of networks and applications that include a DRIAD-capable 1238 AMT gateway are advised to carefully consider the security 1239 considerations in Section 6 of [RFC7450]. 1241 AMT gateway operators also are encouraged to implement the 1242 opportunistic use of IPSec [RFC4301] when IPSECKEY records [RFC4025] 1243 are available to secure traffic from AMT relays, or when a trust 1244 relationship with the AMT relays can be otherwise secured. 1246 6.2. Record-spoofing 1248 The AMTRELAY resource record contains information that SHOULD be 1249 communicated to the DNS client without being modified. The method 1250 used to ensure the result was unmodified is up to the client. 1252 There must be a trust relationship between the end consumer of this 1253 resource record and the DNS server. This relationship may be end-to- 1254 end DNSSEC validation, a TSIG [RFC2845] or SIG(0) [RFC2931] channel 1255 to another secure source, a secure local channel on the host, DNS 1256 over TLS [RFC7858] or HTTPS [RFC8484], or some other secure 1257 mechanism. 1259 If an AMT gateway accepts a maliciously crafted AMTRELAY record, the 1260 result could be a Denial of Service, or receivers processing 1261 multicast traffic from a source under the attacker's control. 1263 6.3. Congestion 1265 Multicast traffic, particularly interdomain multicast traffic, 1266 carries some congestion risks, as described in Section 4 of 1267 [RFC8085]. 1269 Application implementors and network operators that use DRIAD-capable 1270 AMT gateways are advised to take precautions including monitoring of 1271 application traffic behavior, traffic authentication at ingest, rate- 1272 limiting of multicast traffic, and the use of circuit-breaker 1273 techniques such as those described in Section 3.1.10 of [RFC8085] and 1274 similar protections at the network level, in order to ensure network 1275 health in the event of misconfiguration, poorly written applications 1276 that don't follow UDP congestion control principles, or deliberate 1277 attack. 1279 7. Acknowledgements 1281 This specification was inspired by the previous work of Doug Nortz, 1282 Robert Sayko, David Segelstein, and Percy Tarapore, presented in the 1283 MBONED working group at IETF 93. 1285 Thanks to Jeff Goldsmith, Toerless Eckert, Mikael Abrahamsson, Lenny 1286 Giuliano, Mark Andrews, Sandy Zheng, Kyle Rose, and Ben Kaduk for 1287 their very helpful comments. 1289 8. References 1291 8.1. Normative References 1293 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1294 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 1295 . 1297 [RFC1035] Mockapetris, P., "Domain names - implementation and 1298 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1299 November 1987, . 1301 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1302 Requirement Levels", BCP 14, RFC 2119, 1303 DOI 10.17487/RFC2119, March 1997, 1304 . 1306 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 1307 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 1308 . 1310 [RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A. 1311 Thyagarajan, "Internet Group Management Protocol, Version 1312 3", RFC 3376, DOI 10.17487/RFC3376, October 2002, 1313 . 1315 [RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, 1316 "DNS Extensions to Support IP Version 6", STD 88, 1317 RFC 3596, DOI 10.17487/RFC3596, October 2003, 1318 . 1320 [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record 1321 (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September 1322 2003, . 1324 [RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener 1325 Discovery Version 2 (MLDv2) for IPv6", RFC 3810, 1326 DOI 10.17487/RFC3810, June 2004, 1327 . 1329 [RFC4604] Holbrook, H., Cain, B., and B. Haberman, "Using Internet 1330 Group Management Protocol Version 3 (IGMPv3) and Multicast 1331 Listener Discovery Protocol Version 2 (MLDv2) for Source- 1332 Specific Multicast", RFC 4604, DOI 10.17487/RFC4604, 1333 August 2006, . 1335 [RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for 1336 IP", RFC 4607, DOI 10.17487/RFC4607, August 2006, 1337 . 1339 [RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, 1340 "Default Address Selection for Internet Protocol Version 6 1341 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012, 1342 . 1344 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service 1345 Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, 1346 . 1348 [RFC7450] Bumgardner, G., "Automatic Multicast Tunneling", RFC 7450, 1349 DOI 10.17487/RFC7450, February 2015, 1350 . 1352 [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage 1353 Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, 1354 March 2017, . 1356 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1357 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1358 May 2017, . 1360 [RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: 1361 Better Connectivity Using Concurrency", RFC 8305, 1362 DOI 10.17487/RFC8305, December 2017, 1363 . 1365 8.2. Informative References 1367 [RFC2317] Eidnes, H., de Groot, G., and P. Vixie, "Classless IN- 1368 ADDR.ARPA delegation", BCP 20, RFC 2317, 1369 DOI 10.17487/RFC2317, March 1998, 1370 . 1372 [RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B. 1373 Wellington, "Secret Key Transaction Authentication for DNS 1374 (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000, 1375 . 1377 [RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures 1378 ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September 1379 2000, . 1381 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1382 Jacobson, "RTP: A Transport Protocol for Real-Time 1383 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 1384 July 2003, . 1386 [RFC4025] Richardson, M., "A Method for Storing IPsec Keying 1387 Material in DNS", RFC 4025, DOI 10.17487/RFC4025, March 1388 2005, . 1390 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 1391 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 1392 December 2005, . 1394 [RFC5110] Savola, P., "Overview of the Internet Multicast Routing 1395 Architecture", RFC 5110, DOI 10.17487/RFC5110, January 1396 2008, . 1398 [RFC6726] Paila, T., Walsh, R., Luby, M., Roca, V., and R. Lehtonen, 1399 "FLUTE - File Delivery over Unidirectional Transport", 1400 RFC 6726, DOI 10.17487/RFC6726, November 2012, 1401 . 1403 [RFC7359] Gont, F., "Layer 3 Virtual Private Network (VPN) Tunnel 1404 Traffic Leakages in Dual-Stack Hosts/Networks", RFC 7359, 1405 DOI 10.17487/RFC7359, August 2014, 1406 . 1408 [RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I., 1409 Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent 1410 Multicast - Sparse Mode (PIM-SM): Protocol Specification 1411 (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March 1412 2016, . 1414 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 1415 and P. Hoffman, "Specification for DNS over Transport 1416 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 1417 2016, . 1419 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 1420 Writing an IANA Considerations Section in RFCs", BCP 26, 1421 RFC 8126, DOI 10.17487/RFC8126, June 2017, 1422 . 1424 [RFC8313] Tarapore, P., Ed., Sayko, R., Shepherd, G., Eckert, T., 1425 Ed., and R. Krishnan, "Use of Multicast across Inter- 1426 domain Peering Points", BCP 213, RFC 8313, 1427 DOI 10.17487/RFC8313, January 2018, 1428 . 1430 [RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS 1431 (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, 1432 . 1434 [RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 1435 Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, 1436 January 2019, . 1438 Appendix A. Unknown RRType construction 1440 In a DNS resolver that understands the AMTRELAY type, the zone file 1441 might contain this line: 1443 IN AMTRELAY 128 0 3 amtrelays.example.com. 1445 In order to translate this example to appear as an unknown RRType as 1446 defined in [RFC3597], one could run the following program: 1448 1449 $ cat translate.py 1450 #!/usr/bin/env python3 1451 import sys 1452 name=sys.argv[1] 1453 wire='' 1454 for dn in name.split('.'): 1455 if len(dn) > 0: 1456 wire += ('%02x' % len(dn)) 1457 wire += (''.join('%02x'%ord(x) for x in dn)) 1458 print(len(wire)//2) + 2 1459 print(wire) 1461 $ ./translate.py amtrelays.example.com 1462 24 1463 09616d7472656c617973076578616d706c6503636f6d 1464 1466 The length and the hex string for the domain name 1467 "amtrelays.example.com" are the outputs of this program, yielding a 1468 length of 22 and the above hex string. 1470 22 is the length of the wire-encoded domain name, so to this we add 2 1471 (1 for the precedence field and 1 for the combined D-bit and relay 1472 type fields) to get the full length of the RData, and encode the 1473 precedence, D-bit, and relay type fields as octets, as described in 1474 Section 4. 1476 This results in a zone file entry like this: 1478 IN TYPE260 \# ( 24 ; length 1479 80 ; precedence = 128 1480 03 ; D-bit=0, relay type=3 (wire-encoded domain name) 1481 09616d7472656c617973076578616d706c6503636f6d ) ; domain name 1483 Author's Address 1485 Jake Holland 1486 Akamai Technologies, Inc. 1487 150 Broadway 1488 Cambridge, MA 02144 1489 United States of America 1491 Email: jakeholland.net@gmail.com