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Holland 3 Internet-Draft Akamai Technologies, Inc. 4 Updates: 7450 (if approved) May 06, 2019 5 Intended status: Standards Track 6 Expires: November 7, 2019 8 DNS Reverse IP AMT Discovery 9 draft-ietf-mboned-driad-amt-discovery-06 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 November 7, 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 of this document for further information about the 387 relevance of the L flag to the establishment of a Happy Eyeballs 388 connection. See Section 2.5.4 for an overview of how to respond if 389 the connection does not provide multicast connectivity to the source. 391 2.4. Optimal Relay Selection 393 2.4.1. Overview 395 The reverse source IP DNS query of an AMTRELAY RR is a good way for a 396 gateway to discover a relay that is known to the sender. 398 However, it is NOT necessarily a good way to discover the best relay 399 for that gateway to use, because the RR will only provide information 400 about relays known to the source. 402 If there is an upstream relay in a network that is topologically 403 closer to the gateway and able to receive and forward multicast 404 traffic from the sender, that relay is better for the gateway to use, 405 since more of the network path uses native multicast, allowing more 406 chances for packet replication. But since that relay is not known to 407 the sender, it won't be advertised in the sender's reverse IP DNS 408 record. An example network that illustrates this scenario is 409 outlined in Section 3.1.2. 411 It's only appropriate for an AMT gateway to discover an AMT relay by 412 querying an AMTRELAY RR owned by a sender when all of these 413 conditions are met: 415 1. The gateway needs to propagate a join of an (S,G) over AMT, 416 because in the gateway's network, no RPF next hop toward the 417 source can propagate a native multicast join of the (S,G); and 419 2. The gateway is not already connected to a relay that forwards 420 multicast traffic from the source of the (S,G); and 422 3. The gateway is not configured to use a particular IP address for 423 AMT discovery, or a relay discovered with that IP is not able to 424 forward traffic from the source of the (S,G); and 426 4. The gateway is not able to find an upstream AMT relay with DNS-SD 427 [RFC6763], using "_amt._udp" as the Service section of the 428 queries, or a relay discovered this way is not able to forward 429 traffic from the source of the (S,G) (as described in 430 Section 2.5.4.1 or Section 2.5.5); and 432 5. The gateway is not able to find an upstream AMT relay with the 433 well-known anycast addresses from Section 7 of [RFC7450]. 435 When the above conditions are met, the gateway has no path within its 436 local network that can receive multicast traffic from the source IP 437 of the (S,G). 439 In this situation, the best way to find a relay that can forward the 440 required traffic is to use information that comes from the operator 441 of the sender. When the sender has configured an AMTRELAY RR, 442 gateways can use the DRIAD mechanism defined in this document to 443 discover the relay information provided by the sender. 445 2.4.2. Preference Ordering 447 This section defines a preference ordering for relay addresses during 448 the relay discovery process. Gateways are encouraged to implement a 449 Happy Eyeballs algorithm, but even gateways that do not implement a 450 Happy Eyeballs algorithm SHOULD use this ordering, except as noted. 452 When establishing an AMT tunnel to forward multicast data, it's very 453 important for the discovery process to prioritize the network 454 topology considerations ahead of address selection considerations, in 455 order to gain the packet replication benefits from using multicast 456 instead of unicast tunneling in the multicast-capable portions of the 457 network path. 459 The intent of the advice and requirements in this section is to 460 describe how a gateway should make use of the concurrency provided by 461 a Happy Eyeballs algorithm to reduce the join latency, while still 462 prioritizing network efficiency considerations over Address Selection 463 considerations. 465 Section 4 of [RFC8305] requires a Happy Eyeballs algorithm to sort 466 the addresses with the Destination Address Selection defined in 467 Section 6 of [RFC6724], but for the above reasons, that requirement 468 is superseded in the AMT discovery use case by the following 469 considerations: 471 o Prefer Local Relays 473 Figure 5 and Section 3.1.2 provide a motivating example to prefer 474 DNS-SD [RFC6763] for discovery strictly ahead of using the 475 AMTRELAY RR controlled by the sender for AMT discovery. 477 For this reason, it's RECOMMENDED that AMT gateways by default 478 perform service discovery using DNS Service Discovery (DNS-SD) 479 [RFC6763] for _amt._udp. (with chosen as 480 described in Section 11 of [RFC6763]) and use the AMT relays 481 discovered that way in preference to AMT relays discoverable via 482 the mechanism defined in this document (DRIAD). 484 o Prefer Relays Managed by the Containing Network 486 When no local relay is discoverable with DNS-SD, it still may be 487 the case that a relay local to the receiver is operated by the 488 network providing transit services to the receiver. 490 In this case, when the network cannot make the relay discoverable 491 via DNS-SD, the network SHOULD use the well-known anycast 492 addresses from Section 7 of [RFC7450] to route discovery traffic 493 to the relay most appropriate to the receiver's gateway. 495 Accordingly, the gateway SHOULD by default discover a relay with 496 the well-known AMT anycast addresses as the second preference 497 after DNS-SD when searching for a local relay. 499 o Let Sender Manage Relay Provisioning 501 A related motivating example in the sending-side network is 502 provided by considering a sender which needs to instruct the 503 gateways on how to select between connecting to Figure 6 or 504 Figure 7 (from Section 3.2), in order to manage load and failover 505 scenarios in a manner that operates well with the sender's 506 provisioning strategy for horizontal scaling of AMT relays. 508 In this example about the sending-side network, the precedence 509 field described in Section 4.2.1 is a critical method of control 510 so that senders can provide the appropriate guidance to gateways 511 during the discovery process. 513 Therefore, after DNS-SD, the precedence from the RR MUST be used 514 for sorting preference ahead of the Destination Address Selection 515 ordering from Section 6 of [RFC6724], so that only relay IPs with 516 the same precedence are directly compared according to the 517 Destination Address Selection ordering. 519 Accordingly, AMT gateways SHOULD by default prefer relays in this 520 order: 522 1. DNS-SD 523 2. Anycast addresses from Section 7 of [RFC7450] 524 3. DRIAD 526 This default behavior MAY be overridden by administrative 527 configuration where other behavior is more appropriate for the 528 gateway within its network. 530 Among relay addresses that have an equivalent preference as described 531 above, a Happy Eyeballs algorithm for AMT MUST use the Destination 532 Address Selection defined in Section 6 of [RFC6724], as required by 533 [RFC8305]. 535 Among relay addresses that still have an equivalent preference after 536 the above orderings, a gateway MUST make a non-deterministic choice 537 for relay preference ordering, in order to support load balancing by 538 DNS configurations that provide many relay options. 540 The gateway MAY introduce a bias in the non-deterministic choice 541 according to information obtained out of band or from a historical 542 record about network topology, timing information, or the response to 543 a probing mechanism, that indicates some expected benefits from 544 selecting some relays in preference to others. Details about the 545 structure and collection of this information are out of scope for 546 this document, but a gateway in possession of such information MAY 547 use it to prefer topologically closer relays. 549 Note also that certain relay addresses may be excluded from 550 consideration by the hold-down timers described in Section 2.5.4.1 or 551 Section 2.5.5. These relays constitute "unusable destinations" under 552 Rule 1 of the Destination Address Selection, and are also not part of 553 the superseding considerations described above. 555 The discovery and connection process for the relay addresses in the 556 above described ordering MAY operate in parallel, subject to delays 557 prescribed by the Happy Eyeballs requirements described in Section 5 558 of [RFC8305] for successively launched concurrent connection 559 attempts. 561 2.4.3. Connecting to Multiple Relays 563 In some deployments, it may be useful for a gateway to connect to 564 multiple upstream relays and subscribe to the same traffic, in order 565 to support an active/active failover model. A gateway SHOULD NOT be 566 configured to do so without guaranteeing that adequate bandwidth is 567 available. 569 A gateway configured to do this SHOULD still use the same preference 570 ordering logic from Section 2.4.2 for each connection. (Note that 571 this ordering allows for overriding by explicit administrative 572 configuration where required.) 574 2.5. Guidelines for Restarting Discovery 576 2.5.1. Overview 578 It's expected that gateways deployed in different environments will 579 use a variety of heuristics to decide when it's appropriate to 580 restart the relay discovery process, in order to meet different 581 performance goals (for example, to fulfill different kinds of service 582 level agreements). 584 In general, restarting the discovery process is always safe for the 585 gateway and relay during any of the events listed in this section, 586 but may cause a disruption in the forwarded traffic if the discovery 587 process results in choosing a different relay, because this changes 588 the RPF forwarding tree for the multicast traffic upstream of the 589 gateway. This is likely to result in some dropped or duplicated 590 packets from channels actively being tunneled from the old relay to 591 the gateway. 593 The degree of impact on the traffic from choosing a different relay 594 may depend on network conditions between the gateway and the new 595 relay, as well as the network conditions and topology between the 596 sender and the new relay, as this may cause the relay to propagate a 597 new RPF join toward the sender. 599 Balancing the expected impact on the tunneled traffic against likely 600 or observed problems with an existing connection to the relay is the 601 goal of the heuristics that gateways use to determine when to restart 602 the discovery process. 604 The non-normative advice in this section should be treated as 605 guidelines to operators and implementors working with AMT systems 606 that can use DRIAD as part of the relay discovery process. 608 2.5.2. Updates to Restarting Events 610 Section 5.2.3.4.1 of [RFC7450] lists several events that may cause a 611 gateway to start or restart the discovery procedure. 613 This document provides some updates and recommendations regarding the 614 handling of these and similar events. The first 5 events are copied 615 here and numbered for easier reference, and the following events are 616 newly added for consideration in this document: 618 1. When a gateway pseudo-interface is started (enabled). 620 2. When the gateway wishes to report a group subscription when none 621 currently exist. 623 3. Before sending the next Request message in a membership update 624 cycle. 626 4. After the gateway fails to receive a response to a Request 627 message. 629 5. After the gateway receives a Membership Query message with the L 630 flag set to 1. 632 6. When the gateway wishes to report a (S,G) subscription with a 633 source address that does not currently have other group 634 subscriptions. 636 7. When there is a network change detected, for example when a 637 gateway is operating inside an end user device or application, 638 and the device joins a different network, or when the domain 639 portion of a DNS-SD domain name changes in response to a DHCP 640 message or administrative configuration. 642 8. When congestion or substantial loss is detected in the stream of 643 AMT packets from a relay. 645 9. When the gateway has reported one or more (S,G) subscriptions, 646 but no traffic is received from the source for some timeout. 647 (See Section 2.5.4.1). 649 This list is not exhaustive, nor are any of the listed events 650 strictly required to always force a restart of the discovery process. 652 Note that during event #1, a gateway may use DNS-SD, but does not 653 have sufficient information to use DRIAD, since no source is known. 655 2.5.3. Tunnel Stability 657 In general, subscribers to active traffic flows that are being 658 forwarded by an AMT gateway are less likely to experience a 659 degradation in service (for example, from missing or duplicated 660 packets) when the gateway continues using the same relay, as long the 661 relay is not overloaded and the network conditions remain stable. 663 Therefore, gateways SHOULD avoid performing a full restart of the 664 discovery process during routine cases of event #3 (sending a new 665 Request message), since it occurs frequently in normal operation. 667 However, see Section 2.5.4, Section 2.5.6, and Section 2.5.4.3 for 668 more information about exceptional cases when it may be appropriate 669 to use event #3. 671 2.5.4. Traffic Health 673 2.5.4.1. Absence of Traffic 675 If a gateway indicates one or more (S,G) subscriptions in a 676 Membership Update message, but no traffic for any of the (S,G)s is 677 received in a reasonable time, it's appropriate for the gateway to 678 restart the discovery process. 680 If the gateway restarts the discovery process multiple times 681 consecutively for this reason, the timeout period SHOULD be adjusted 682 to provide a random exponential back-off. 684 The RECOMMENDED timeout is a random value in the range 685 [initial_timeout, MIN(initial_timeout * 2^retry_count, 686 maximum_timeout)], with a RECOMMENDED initial_timeout of 4 seconds 687 and a RECOMMENDED maximum_timeout of 120 seconds. 689 Note that the recommended initial_timeout is larger than the initial 690 timout recommended in the similar algorithm from Section 5.2.3.4.3 of 691 [RFC7450]. This is to provide time for RPF Join propagation in the 692 sending network. Although the timeout values may be administratively 693 adjusted to support performance requirements, operators are advised 694 to consider the possibility of join propagation delays between the 695 sender and the relay when choosing an appropriate timeout value. 697 Gateways restarting the discovery process because of an absence of 698 traffic MUST use a hold-down timer that removes this relay from 699 consideration during subsequent rounds of discovery while active. 701 The hold-down SHOULD last for no less than 3 minutes and no more than 702 10 minutes. 704 2.5.4.2. Loss and Congestion 706 In some gateway deployments, it is also feasible to monitor the 707 health of traffic flows through the gateway, for example by detecting 708 the rate of packet loss by communicating out of band with receivers, 709 or monitoring the packets of known protocols with sequence numbers. 710 Where feasible, it's encouraged for gateways to use such traffic 711 health information to trigger a restart of the discovery process 712 during event #3 (before sending a new Request message). 714 However, to avoid synchronized rediscovery by many gateways 715 simultaneously after a transient network event upstream of a relay 716 results in many receivers detecting poor flow health at the same 717 time, it's recommended to add a random delay before restarting the 718 discovery process in this case. 720 The span of the random portion of the delay should be no less than 10 721 seconds by default, but may be administratively configured to support 722 different performance requirements. 724 2.5.4.3. Ancient Discovery Information 726 In most cases, a gateway actively receiving healthy traffic from a 727 relay that has not indicated load with the L flag should prefer to 728 remain connected to the same relay, as described in Section 2.5.3. 730 However, a relay that appears healthy but has been forwarding traffic 731 for days or weeks may have an increased chance of becoming unstable. 732 Gateways may benefit from restarting the discovery process during 733 event #3 (before sending a Request message) after the expiration of a 734 long-term timeout, on the order of multiple hours, or even days in 735 some deployments. 737 It may be beneficial for such timers to consider the amount of 738 traffic currently being forwarded, and to give a higher probability 739 of restarting discovery during periods with an unusually low data 740 rate, to reduce the impact on active traffic while still avoiding 741 relying on the results of a very old discovery. 743 Other issues may also be worth considering as part of this heuristic; 744 for example, if the DNS expiry time of the record that was used to 745 discover the current relay has not passed, the long term timer might 746 be restarted without restarting the discovery process. 748 2.5.5. Relay Loaded or Shutting Down 750 The L flag (see Section 5.1.4.4 of [RFC7450]) is the preferred 751 mechanism for a relay to signal overloading or a graceful shutdown to 752 gateways. 754 A gateway that supports handling of the L flag should generally 755 restart the discovery process when it processes a Membership Query 756 packet with the L flag set. If an L flag is received while a 757 concurrent Happy Eyeballs discovery process is under way for multiple 758 candidate relays (Section 2.3), the relay sending the L flag SHOULD 759 NOT be considered for the relay selection. 761 It is also RECOMMENDED that gateways avoid choosing a relay that has 762 recently sent an L flag, with approximately a 10-minute hold-down. 763 Gateways SHOULD treat this hold-down timer in the same way as the 764 hold-down in Section 2.5.4.1, so that the relay is removed from 765 consideration for short-term subsequent rounds of discovery. 767 2.5.6. Relay Discovery Messages vs. Restarting Discovery 769 A gateway should only send DNS queries with the AMTRELAY RRType or 770 the DNS-SD DNS queries for an AMT service as part of starting or 771 restarting the discovery process. 773 However, all AMT relays are required to support handling of Relay 774 Discovery messages (e.g. in Section 5.3.3.2 of [RFC7450]). 776 So a gateway with an existing connection to a relay can send a Relay 777 Discovery message to the unicast address of that AMT relay. Under 778 stable conditions with an unloaded relay, it's expected that the 779 relay will return its own unicast address in the Relay Advertisement, 780 in response to such a Relay Discovery message. Since this will not 781 result in the gateway changing to another relay unless the relay 782 directs the gateway away, this is a reasonable exception to the 783 advice against handling event #3 described in Section 2.5.3. 785 This behavior is discouraged for gateways that do support the L flag, 786 to avoid sending unnecessary packets over the network. 788 However, gateways that do not support the L flag may be able to avoid 789 a disruption in the forwarded traffic by sending such Relay Discovery 790 messages regularly. When a relay is under load or has started a 791 graceful shutdown, it may respond with a different relay address, 792 which the gateway can use to connect to a different relay. This kind 793 of coordinated handoff will likely result in a smaller disruption to 794 the traffic than if the relay simply stops responding to Request 795 messages, and stops forwarding traffic. 797 This style of Relay Discovery message (one sent to the unicast 798 address of a relay that's already forwarding traffic to this gateway) 799 should not be considered a full restart of the relay discovery 800 process. It is recommended for gateways to support the L flag, but 801 for gateways that do not support the L flag, sending this message 802 during event #3 may help mitigate service degradation when relays 803 become unstable. 805 2.5.7. Independent Discovery Per Traffic Source 807 Relays discovered via the AMTRELAY RR are source-specific relay 808 addresses, and may use different pseudo-interfaces from each other 809 and from relays discovered via DNS-SD or a non-source-specific 810 address, as described in Section 4.1.2.1 of [RFC7450]. 812 Restarting the discovery process for one pseudo-interface does not 813 require restarting the discovery process for other pseudo-interfaces. 814 Gateway heuristics about restarting the discovery process should 815 operate independently for different tunnels to relays, when 816 responding to events that are specific to the different tunnels. 818 2.6. DNS Configuration 820 Often an AMT gateway will only have access to the source and group IP 821 addresses of the desired traffic, and will not know any other name 822 for the source of the traffic. Because of this, typically the best 823 way of looking up AMTRELAY RRs will be by using the source IP address 824 as an index into one of the reverse mapping trees (in-addr.arpa for 825 IPv4, as described in Section 3.5 of [RFC1035], or ip6.arpa for IPv6, 826 as described in Section 2.5 of [RFC3596]). 828 Therefore, it is RECOMMENDED that AMTRELAY RRs be added to reverse IP 829 zones as appropriate. AMTRELAY records MAY also appear in other 830 zones, but the primary intended use case requires a reverse IP 831 mapping for the source from an (S,G) in order to be useful to most 832 AMT gateways. 834 When performing the AMTRELAY RR lookup, any CNAMEs or DNAMEs found 835 MUST be followed. This is necessary to support zone delegation. 836 Some examples outlining this need are described in [RFC2317]. 838 See Section 4 and Section 4.3 for a detailed explanation of the 839 contents for a DNS Zone file. 841 2.7. Waiting for DNS resolution 843 The DNS query functionality is expected to follow ordinary standards 844 and best practices for DNS clients. A gateway MAY use an existing 845 DNS client implementation that does so, and MAY rely on that client's 846 retry logic to determine the timeouts between retries. 848 Otherwise, a gateway MAY re-send a DNS query if it does not receive 849 an appropriate DNS response within some timeout period. If the 850 gateway retries multiple times, the timeout period SHOULD be adjusted 851 to provide a random exponential back-off. 853 As with the waiting process for the Relay Advertisement message from 854 Section 5.2.3.4.3 of [RFC7450], the RECOMMENDED timeout is a random 855 value in the range [initial_timeout, MIN(initial_timeout * 856 2^retry_count, maximum_timeout)], with a RECOMMENDED initial_timeout 857 of 1 second and a RECOMMENDED maximum_timeout of 120 seconds. 859 3. Example Deployments 861 3.1. Example Receiving Networks 863 3.1.1. Tier 3 ISP 865 One example of a receiving network is an ISP that offers multicast 866 ingest services to its subscribers, illustrated in Figure 3. 868 In the example network below, subscribers can join (S,G)s with MLDv2 869 or IGMPv3 as described in [RFC4604], and the AMT gateway in this ISP 870 can receive and forward multicast traffic from one of the example 871 sending networks in Section 3.2 by discovering the appropriate AMT 872 relays with a DNS lookup for the AMTRELAY RR with the reverse IP of 873 the source in the (S,G). 875 Internet 876 ^ ^ Multicast-enabled 877 | | Receiving Network 878 +------|------------|-------------------------+ 879 | | | | 880 | +--------+ +--------+ +=========+ | 881 | | Border |---| Border | | AMT | | 882 | | Router | | Router | | gateway | | 883 | +--------+ +--------+ +=========+ | 884 | | | | | 885 | +-----+------+-----------+--+ | 886 | | | | 887 | +-------------+ +-------------+ | 888 | | Agg Routers | .. | Agg Routers | | 889 | +-------------+ +-------------+ | 890 | / \ \ / \ | 891 | +---------------+ +---------------+ | 892 | |Access Systems | ....... |Access Systems | | 893 | |(CMTS/OLT/etc.)| |(CMTS/OLT/etc.)| | 894 | +---------------+ +---------------+ | 895 | | | | 896 +--------|------------------------|-----------+ 897 | | 898 +---+-+-+---+---+ +---+-+-+---+---+ 899 | | | | | | | | | | 900 /-\ /-\ /-\ /-\ /-\ /-\ /-\ /-\ /-\ /-\ 901 |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| 903 Subscribers 905 Figure 3: Receiving ISP Example 907 3.1.2. Small Office 909 Another example receiving network is a small branch office that 910 regularly accesses some multicast content, illustrated in Figure 4. 912 This office has desktop devices that need to receive some multicast 913 traffic, so an AMT gateway runs on a LAN with these devices, to pull 914 traffic in through a non-multicast next-hop. 916 The office also hosts some mobile devices that have AMT gateway 917 instances embedded inside apps, in order to receive multicast traffic 918 over their non-multicast wireless LAN. (Note that the "Legacy 919 Router" is a simplification that's meant to describe a variety of 920 possible conditions; for example it could be a device providing a 921 split-tunnel VPN as described in [RFC7359], deliberately excluding 922 multicast traffic for a VPN tunnel, rather than a device which is 923 incapable of multicast forwarding.) 925 Internet 926 (non-multicast) 927 ^ 928 | Office Network 929 +----------|----------------------------------+ 930 | | | 931 | +---------------+ (Wifi) Mobile apps | 932 | | Modem+ | Wifi | - - - - w/ embedded | 933 | | Router | AP | AMT gateways | 934 | +---------------+ | 935 | | | 936 | | | 937 | +----------------+ | 938 | | Legacy Router | | 939 | | (unicast) | | 940 | +----------------+ | 941 | / | \ | 942 | / | \ | 943 | +--------+ +--------+ +--------+=========+ | 944 | | Phones | | ConfRm | | Desks | AMT | | 945 | | subnet | | subnet | | subnet | gateway | | 946 | +--------+ +--------+ +--------+=========+ | 947 | | 948 +---------------------------------------------+ 950 Figure 4: Small Office (no multicast up) 952 By adding an AMT relay to this office network as in Figure 5, it's 953 possible to make use of multicast services from the example 954 multicast-capable ISP in Section 3.1.1. 956 Multicast-capable ISP 957 ^ 958 | Office Network 959 +----------|----------------------------------+ 960 | | | 961 | +---------------+ (Wifi) Mobile apps | 962 | | Modem+ | Wifi | - - - - w/ embedded | 963 | | Router | AP | AMT gateways | 964 | +---------------+ | 965 | | +=======+ | 966 | +---Wired LAN---| AMT | | 967 | | | relay | | 968 | +----------------+ +=======+ | 969 | | Legacy Router | | 970 | | (unicast) | | 971 | +----------------+ | 972 | / | \ | 973 | / | \ | 974 | +--------+ +--------+ +--------+=========+ | 975 | | Phones | | ConfRm | | Desks | AMT | | 976 | | subnet | | subnet | | subnet | gateway | | 977 | +--------+ +--------+ +--------+=========+ | 978 | | 979 +---------------------------------------------+ 981 Figure 5: Small Office Example 983 When multicast-capable networks are chained like this, with a network 984 like the one in Figure 5 receiving internet services from a 985 multicast-capable network like the one in Figure 3, it's important 986 for AMT gateways to reach the more local AMT relay, in order to avoid 987 accidentally tunneling multicast traffic from a more distant AMT 988 relay with unicast, and failing to utilize the multicast transport 989 capabilities of the network in Figure 3. 991 3.2. Example Sending Networks 993 3.2.1. Sender-controlled Relays 995 When a sender network is also operating AMT relays to distribute 996 multicast traffic, as in Figure 6, each address could appear as an 997 AMTRELAY RR for the reverse IP of the sender, or one or more domain 998 names could appear in AMTRELAY RRs, and the AMT relay addresses can 999 be discovered by finding A or AAAA records from those domain names. 1001 Sender Network 1002 +-----------------------------------+ 1003 | | 1004 | +--------+ +=======+ +=======+ | 1005 | | Sender | | AMT | | AMT | | 1006 | +--------+ | relay | | relay | | 1007 | | +=======+ +=======+ | 1008 | | | | | 1009 | +-----+------+----------+ | 1010 | | | 1011 +-----------|-----------------------+ 1012 v 1013 Internet 1014 (non-multicast) 1016 Figure 6: Small Office Example 1018 3.2.2. Provider-controlled Relays 1020 When an ISP offers a service to transmit outbound multicast traffic 1021 through a forwarding network, it might also offer AMT relays in order 1022 to reach receivers without multicast connectivity to the forwarding 1023 network, as in Figure 7. In this case it's RECOMMENDED that the ISP 1024 also provide at least one domain name for the AMT relays for use with 1025 the AMTRELAY RR. 1027 When the sender wishes to use the relays provided by the ISP for 1028 forwarding multicast traffic, an AMTRELAY RR should be configured to 1029 use the domain name provided by the ISP, to allow for address 1030 reassignment of the relays without forcing the sender to reconfigure 1031 the corresponding AMTRELAY RRs. 1033 +--------+ 1034 | Sender | 1035 +---+----+ Multicast-enabled 1036 | Sending Network 1037 +-----------|-------------------------------+ 1038 | v | 1039 | +------------+ +=======+ +=======+ | 1040 | | Agg Router | | AMT | | AMT | | 1041 | +------------+ | relay | | relay | | 1042 | | +=======+ +=======+ | 1043 | | | | | 1044 | +-----+------+--------+---------+ | 1045 | | | | 1046 | +--------+ +--------+ | 1047 | | Border |---| Border | | 1048 | | Router | | Router | | 1049 | +--------+ +--------+ | 1050 +-----|------------|------------------------+ 1051 | | 1052 v v 1053 Internet 1054 (non-multicast) 1056 Figure 7: Sending ISP Example 1058 4. AMTRELAY Resource Record Definition 1060 4.1. AMTRELAY RRType 1062 The AMTRELAY RRType has the mnemonic AMTRELAY and type code 260 1063 (decimal). 1065 The AMTRELAY RR is class independent. 1067 4.2. AMTRELAY RData Format 1069 The AMTRELAY RData consists of a 8-bit precedence field, a 1-bit 1070 "Discovery Optional" field, a 7-bit type field, and a variable length 1071 relay field. 1073 0 1 2 3 1074 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 1075 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1076 | precedence |D| type | | 1077 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 1078 ~ relay ~ 1079 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1081 4.2.1. RData Format - Precedence 1083 This is an 8-bit precedence for this record. It is interpreted in 1084 the same way as the PREFERENCE field described in Section 3.3.9 of 1085 [RFC1035]. 1087 Relays listed in AMTRELAY records with a lower value for precedence 1088 are to be attempted first. 1090 4.2.2. RData Format - Discovery Optional (D-bit) 1092 The D bit is a "Discovery Optional" flag. 1094 If the D bit is set to 0, a gateway using this RR MUST perform AMT 1095 relay discovery as described in Section 4.2.1.1 of [RFC7450], rather 1096 than directly sending an AMT Request message to the relay. 1098 That is, the gateway MUST receive an AMT Relay Advertisement message 1099 (Section 5.1.2 of [RFC7450]) for an address before sending an AMT 1100 Request message (Section 5.1.3 of [RFC7450]) to that address. Before 1101 receiving the Relay Advertisement message, this record has only 1102 indicated that the address can be used for AMT relay discovery, not 1103 for a Request message. This is necessary for devices that are not 1104 fully functional AMT relays, but rather load balancers or brokers, as 1105 mentioned in Section 4.2.1.1 of [RFC7450]. 1107 If the D bit is set to 1, the gateway MAY send an AMT Request message 1108 directly to the discovered relay address without first sending an AMT 1109 Discovery message. 1111 This bit should be set according to advice from the AMT relay 1112 operator. The D bit MUST be set to zero when no information is 1113 available from the AMT relay operator about its suitability. 1115 4.2.3. RData Format - Type 1117 The type field indicates the format of the information that is stored 1118 in the relay field. 1120 The following values are defined: 1122 o type = 0: The relay field is empty (0 bytes). 1124 o type = 1: The relay field contains a 4-octet IPv4 address. 1126 o type = 2: The relay field contains a 16-octet IPv6 address. 1128 o type = 3: The relay field contains a wire-encoded domain name. 1129 The wire-encoded format is self-describing, so the length is 1130 implicit. The domain name MUST NOT be compressed. (See 1131 Section 3.3 of [RFC1035] and Section 4 of [RFC3597].) 1133 4.2.4. RData Format - Relay 1135 The relay field is the address or domain name of the AMT relay. It 1136 is formatted according to the type field. 1138 When the type field is 0, the length of the relay field is 0, and it 1139 indicates that no AMT relay should be used for multicast traffic from 1140 this source. 1142 When the type field is 1, the length of the relay field is 4 octets, 1143 and a 32-bit IPv4 address is present. This is an IPv4 address as 1144 described in Section 3.4.1 of [RFC1035]. This is a 32-bit number in 1145 network byte order. 1147 When the type field is 2, the length of the relay field is 16 octets, 1148 and a 128-bit IPv6 address is present. This is an IPv6 address as 1149 described in Section 2.2 of [RFC3596]. This is a 128-bit number in 1150 network byte order. 1152 When the type field is 3, the relay field is a normal wire-encoded 1153 domain name, as described in Section 3.3 of [RFC1035]. Compression 1154 MUST NOT be used, for the reasons given in Section 4 of [RFC3597]. 1156 For a type 3 record, the D-bit and preference fields carry over to 1157 all A or AAAA records for the domain name. There is no difference in 1158 the result of the discovery process when it's obtained by type 1 or 1159 type 2 AMTRELAY records with identical D-bit and preference fields, 1160 vs. when the result is obtained by a type 3 AMTRELAY record that 1161 resolves to the same set of IPv4 and IPv6 addresses via A and AAAA 1162 lookups. 1164 4.3. AMTRELAY Record Presentation Format 1166 4.3.1. Representation of AMTRELAY RRs 1168 AMTRELAY RRs may appear in a zone data master file. The precedence, 1169 D-bit, relay type, and relay fields are REQUIRED. 1171 If the relay type field is 0, the relay field MUST be ".". 1173 The presentation for the record is as follows: 1175 IN AMTRELAY precedence D-bit type relay 1177 4.3.2. Examples 1179 In a DNS authoritative nameserver that understands the AMTRELAY type, 1180 the zone might contain a set of entries like this: 1182 $ORIGIN 100.51.198.in-addr.arpa. 1183 10 IN AMTRELAY 10 0 1 203.0.113.15 1184 10 IN AMTRELAY 10 0 2 2001:DB8::15 1185 10 IN AMTRELAY 128 1 3 amtrelays.example.com. 1187 This configuration advertises an IPv4 discovery address, an IPv6 1188 discovery address, and a domain name for AMT relays which can receive 1189 traffic from the source 198.51.100.10. The IPv4 and IPv6 addresses 1190 are configured with a D-bit of 0 (meaning discovery is mandatory, as 1191 described in Section 4.2.2), and a precedence 10 (meaning they're 1192 preferred ahead of the last entry, which has precedence 128). 1194 For zone files in name servers that don't support the AMTRELAY RRType 1195 natively, it's possible to use the format for unknown RR types, as 1196 described in [RFC3597]. This approach would replace the AMTRELAY 1197 entries in the example above with the entries below: 1199 10 IN TYPE260 \# ( 1200 6 ; length 1201 0a ; precedence=10 1202 01 ; D=0, relay type=1, an IPv4 address 1203 cb00710f ) ; 203.0.113.15 1204 10 IN TYPE260 \# ( 1205 18 ; length 1206 0a ; precedence=10 1207 02 ; D=0, relay type=2, an IPv6 address 1208 20010db800000000000000000000000f ) ; 2001:db8::15 1209 10 IN TYPE260 \# ( 1210 24 ; length 1211 80 ; precedence=128 1212 83 ; D=1, relay type=3, a wire-encoded domain name 1213 09616d7472656c617973076578616d706c6503636f6d ) ; domain name 1215 See Appendix A for more details. 1217 5. IANA Considerations 1219 This document updates the IANA Registry for DNS Resource Record Types 1220 by assigning type 260 to the AMTRELAY record. 1222 This document creates a new registry named "AMTRELAY Resource Record 1223 Parameters", with a sub-registry for the "Relay Type Field". The 1224 initial values in the sub-registry are: 1226 +-------+---------------------------------------+ 1227 | Value | Description | 1228 +-------+---------------------------------------+ 1229 | 0 | No relay is present. | 1230 | 1 | A 4-byte IPv4 address is present | 1231 | 2 | A 16-byte IPv6 address is present | 1232 | 3 | A wire-encoded domain name is present | 1233 | 4-255 | Unassigned | 1234 +-------+---------------------------------------+ 1236 Values 0, 1, 2, and 3 are further explained in Section 4.2.3 and 1237 Section 4.2.4. Relay type numbers 4 through 255 can be assigned with 1238 a policy of Specification Required (as described in [RFC8126]). 1240 6. Security Considerations 1242 6.1. Use of AMT 1244 This document defines a mechanism that enables a more widespread and 1245 automated use of AMT, even without access to a multicast backbone. 1246 Operators of networks and applications that include a DRIAD-capable 1247 AMT gateway are advised to carefully consider the security 1248 considerations in Section 6 of [RFC7450]. 1250 AMT gateway operators also are encouraged to take appropriate steps 1251 to ensure the integrity of the data received via AMT, for example by 1252 the opportunistic use of IPSec [RFC4301] to secure traffic received 1253 from AMT relays, when IPSECKEY records [RFC4025] are available or 1254 when a trust relationship with the AMT relays can be otherwise 1255 established and secured. 1257 6.2. Record-spoofing 1259 The AMTRELAY resource record contains information that SHOULD be 1260 communicated to the DNS client without being modified. The method 1261 used to ensure the result was unmodified is up to the client. 1263 There must be a trust relationship between the end consumer of this 1264 resource record and the DNS server. This relationship may be end-to- 1265 end DNSSEC validation, a TSIG [RFC2845] or SIG(0) [RFC2931] channel 1266 to another secure source, a secure local channel on the host, DNS 1267 over TLS [RFC7858] or HTTPS [RFC8484], or some other secure 1268 mechanism. 1270 If an AMT gateway accepts a maliciously crafted AMTRELAY record, the 1271 result could be a Denial of Service, or receivers processing 1272 multicast traffic from a source under the attacker's control. 1274 6.3. Congestion 1276 Multicast traffic, particularly interdomain multicast traffic, 1277 carries some congestion risks, as described in Section 4 of 1278 [RFC8085]. 1280 Application implementors and network operators that use DRIAD-capable 1281 AMT gateways are advised to take precautions including monitoring of 1282 application traffic behavior, traffic authentication at ingest, rate- 1283 limiting of multicast traffic, and the use of circuit-breaker 1284 techniques such as those described in Section 3.1.10 of [RFC8085] and 1285 similar protections at the network level, in order to ensure network 1286 health in the event of misconfiguration, poorly written applications 1287 that don't follow UDP congestion control principles, or deliberate 1288 attack. 1290 7. Acknowledgements 1292 This specification was inspired by the previous work of Doug Nortz, 1293 Robert Sayko, David Segelstein, and Percy Tarapore, presented in the 1294 MBONED working group at IETF 93. 1296 Thanks to Jeff Goldsmith, Toerless Eckert, Mikael Abrahamsson, Lenny 1297 Giuliano, Mark Andrews, Sandy Zheng, Kyle Rose, and Ben Kaduk for 1298 their very helpful comments. 1300 8. References 1302 8.1. Normative References 1304 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1305 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 1306 . 1308 [RFC1035] Mockapetris, P., "Domain names - implementation and 1309 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1310 November 1987, . 1312 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1313 Requirement Levels", BCP 14, RFC 2119, 1314 DOI 10.17487/RFC2119, March 1997, 1315 . 1317 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 1318 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 1319 . 1321 [RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A. 1322 Thyagarajan, "Internet Group Management Protocol, Version 1323 3", RFC 3376, DOI 10.17487/RFC3376, October 2002, 1324 . 1326 [RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, 1327 "DNS Extensions to Support IP Version 6", STD 88, 1328 RFC 3596, DOI 10.17487/RFC3596, October 2003, 1329 . 1331 [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record 1332 (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September 1333 2003, . 1335 [RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener 1336 Discovery Version 2 (MLDv2) for IPv6", RFC 3810, 1337 DOI 10.17487/RFC3810, June 2004, 1338 . 1340 [RFC4604] Holbrook, H., Cain, B., and B. Haberman, "Using Internet 1341 Group Management Protocol Version 3 (IGMPv3) and Multicast 1342 Listener Discovery Protocol Version 2 (MLDv2) for Source- 1343 Specific Multicast", RFC 4604, DOI 10.17487/RFC4604, 1344 August 2006, . 1346 [RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for 1347 IP", RFC 4607, DOI 10.17487/RFC4607, August 2006, 1348 . 1350 [RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, 1351 "Default Address Selection for Internet Protocol Version 6 1352 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012, 1353 . 1355 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service 1356 Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, 1357 . 1359 [RFC7450] Bumgardner, G., "Automatic Multicast Tunneling", RFC 7450, 1360 DOI 10.17487/RFC7450, February 2015, 1361 . 1363 [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage 1364 Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, 1365 March 2017, . 1367 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1368 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1369 May 2017, . 1371 [RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: 1372 Better Connectivity Using Concurrency", RFC 8305, 1373 DOI 10.17487/RFC8305, December 2017, 1374 . 1376 8.2. Informative References 1378 [RFC2317] Eidnes, H., de Groot, G., and P. Vixie, "Classless IN- 1379 ADDR.ARPA delegation", BCP 20, RFC 2317, 1380 DOI 10.17487/RFC2317, March 1998, 1381 . 1383 [RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B. 1384 Wellington, "Secret Key Transaction Authentication for DNS 1385 (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000, 1386 . 1388 [RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures 1389 ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September 1390 2000, . 1392 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1393 Jacobson, "RTP: A Transport Protocol for Real-Time 1394 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 1395 July 2003, . 1397 [RFC4025] Richardson, M., "A Method for Storing IPsec Keying 1398 Material in DNS", RFC 4025, DOI 10.17487/RFC4025, March 1399 2005, . 1401 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 1402 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 1403 December 2005, . 1405 [RFC5110] Savola, P., "Overview of the Internet Multicast Routing 1406 Architecture", RFC 5110, DOI 10.17487/RFC5110, January 1407 2008, . 1409 [RFC6726] Paila, T., Walsh, R., Luby, M., Roca, V., and R. Lehtonen, 1410 "FLUTE - File Delivery over Unidirectional Transport", 1411 RFC 6726, DOI 10.17487/RFC6726, November 2012, 1412 . 1414 [RFC7359] Gont, F., "Layer 3 Virtual Private Network (VPN) Tunnel 1415 Traffic Leakages in Dual-Stack Hosts/Networks", RFC 7359, 1416 DOI 10.17487/RFC7359, August 2014, 1417 . 1419 [RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I., 1420 Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent 1421 Multicast - Sparse Mode (PIM-SM): Protocol Specification 1422 (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March 1423 2016, . 1425 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 1426 and P. Hoffman, "Specification for DNS over Transport 1427 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 1428 2016, . 1430 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 1431 Writing an IANA Considerations Section in RFCs", BCP 26, 1432 RFC 8126, DOI 10.17487/RFC8126, June 2017, 1433 . 1435 [RFC8313] Tarapore, P., Ed., Sayko, R., Shepherd, G., Eckert, T., 1436 Ed., and R. Krishnan, "Use of Multicast across Inter- 1437 domain Peering Points", BCP 213, RFC 8313, 1438 DOI 10.17487/RFC8313, January 2018, 1439 . 1441 [RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS 1442 (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, 1443 . 1445 [RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 1446 Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, 1447 January 2019, . 1449 Appendix A. Unknown RRType construction 1451 In a DNS resolver that understands the AMTRELAY type, the zone file 1452 might contain this line: 1454 IN AMTRELAY 128 0 3 amtrelays.example.com. 1456 In order to translate this example to appear as an unknown RRType as 1457 defined in [RFC3597], one could run the following program: 1459 1460 $ cat translate.py 1461 #!/usr/bin/env python3 1462 import sys 1463 name=sys.argv[1] 1464 wire='' 1465 for dn in name.split('.'): 1466 if len(dn) > 0: 1467 wire += ('%02x' % len(dn)) 1468 wire += (''.join('%02x'%ord(x) for x in dn)) 1469 print(len(wire)//2) + 2 1470 print(wire) 1472 $ ./translate.py amtrelays.example.com 1473 24 1474 09616d7472656c617973076578616d706c6503636f6d 1475 1477 The length and the hex string for the domain name 1478 "amtrelays.example.com" are the outputs of this program, yielding a 1479 length of 22 and the above hex string. 1481 22 is the length of the wire-encoded domain name, so to this we add 2 1482 (1 for the precedence field and 1 for the combined D-bit and relay 1483 type fields) to get the full length of the RData, and encode the 1484 precedence, D-bit, and relay type fields as octets, as described in 1485 Section 4. 1487 This results in a zone file entry like this: 1489 IN TYPE260 \# ( 24 ; length 1490 80 ; precedence = 128 1491 03 ; D-bit=0, relay type=3 (wire-encoded domain name) 1492 09616d7472656c617973076578616d706c6503636f6d ) ; domain name 1494 Author's Address 1496 Jake Holland 1497 Akamai Technologies, Inc. 1498 150 Broadway 1499 Cambridge, MA 02144 1500 United States of America 1502 Email: jakeholland.net@gmail.com