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Holland 3 Internet-Draft Akamai Technologies, Inc. 4 Updates: 7450 (if approved) February 14, 2019 5 Intended status: Standards Track 6 Expires: August 18, 2019 8 DNS Reverse IP AMT Discovery 9 draft-ietf-mboned-driad-amt-discovery-01 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 August 18, 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 . . . . . . . . . . . . 12 70 2.5. Guidelines for Restarting Discovery . . . . . . . . . . . 13 71 2.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . 13 72 2.5.2. Updates to Restarting Events . . . . . . . . . . . . 13 73 2.5.3. Tunnel Stability . . . . . . . . . . . . . . . . . . 14 74 2.5.4. Traffic Health . . . . . . . . . . . . . . . . . . . 15 75 2.5.5. Relay Loaded or Shutting Down . . . . . . . . . . . . 16 76 2.5.6. Relay Discovery Messages vs. Restarting Discovery . . 17 77 2.5.7. Independent Discovery Per Traffic Source . . . . . . 17 78 2.6. DNS Configuration . . . . . . . . . . . . . . . . . . . . 18 79 2.7. Waiting for DNS resolution . . . . . . . . . . . . . . . 18 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 . . . . . . . . . . . . . . . . 21 85 3.2.1. Sender-controlled Relays . . . . . . . . . . . . . . 21 86 3.2.2. Provider-controlled Relays . . . . . . . . . . . . . 22 87 4. AMTRELAY Resource Record Definition . . . . . . . . . . . . . 23 88 4.1. AMTRELAY RRType . . . . . . . . . . . . . . . . . . . . . 23 89 4.2. AMTRELAY RData Format . . . . . . . . . . . . . . . . . . 23 90 4.2.1. RData Format - Precedence . . . . . . . . . . . . . . 24 91 4.2.2. RData Format - Discovery Optional (D-bit) . . . . . . 24 92 4.2.3. RData Format - Type . . . . . . . . . . . . . . . . . 24 93 4.2.4. RData Format - Relay . . . . . . . . . . . . . . . . 25 94 4.3. AMTRELAY Record Presentation Format . . . . . . . . . . . 25 95 4.3.1. Representation of AMTRELAY RRs . . . . . . . . . . . 25 96 4.3.2. Examples . . . . . . . . . . . . . . . . . . . . . . 26 98 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 99 6. Security Considerations . . . . . . . . . . . . . . . . . . . 27 100 6.1. Record-spoofing . . . . . . . . . . . . . . . . . . . . . 27 101 6.2. Local Override . . . . . . . . . . . . . . . . . . . . . 27 102 6.3. Congestion . . . . . . . . . . . . . . . . . . . . . . . 27 103 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28 104 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 28 105 8.1. Normative References . . . . . . . . . . . . . . . . . . 28 106 8.2. Informative References . . . . . . . . . . . . . . . . . 29 107 Appendix A. Unknown RRType construction . . . . . . . . . . . . 30 108 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 31 110 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). 430 When the above conditions are met, the gateway has no path within its 431 local network that can receive multicast traffic from the source IP 432 of the (S,G). 434 In this situation, the best way to find a relay that can forward the 435 required traffic is to use information that comes from the operator 436 of the sender. When the sender has configured an AMTRELAY RR, 437 gateways can use the DRIAD mechanism defined in this document to 438 discover the relay information provided by the sender. 440 2.4.2. Preference Ordering 442 This section defines a preference ordering for relay addresses during 443 the relay discovery process. Gateways are encouraged to implement a 444 Happy Eyeballs algorithm, but even gateways that do not implement a 445 Happy Eyeballs algorithm SHOULD use this ordering, except as noted. 447 When establishing an AMT tunnel to forward multicast data, it's very 448 important for the discovery process to prioritize the network 449 topology considerations ahead of address selection considerations, in 450 order to gain the packet replication benefits from using multicast 451 instead of unicast tunneling in the multicast-capable portions of the 452 network path. 454 The intent of the advice and requirements in this section is to 455 describe how a gateway should make use of the concurrency provided by 456 a Happy Eyeballs algorithm to reduce the join latency, while still 457 prioritizing network efficiency considerations over Address Selection 458 considerations. 460 Section 4 of [RFC8305] requires a Happy Eyeballs algorithm to sort 461 the addresses with the Destination Address Selection defined in 462 Section 6 of [RFC6724], but for the above reasons, that requirement 463 is superseded in the AMT discovery use case by the following 464 considerations: 466 1. Prefer Local Relays 468 Figure 5 and Section 3.1.2 provide a motivating example to prefer 469 DNS-SD [RFC6763] for discovery strictly ahead of using the 470 AMTRELAY RR controlled by the sender for AMT discovery. 472 For this reason, it's RECOMMENDED that AMT gateways by default 473 perform service discovery using DNS Service Discovery (DNS-SD) 474 [RFC6763] for _amt._udp. (with chosen as 475 described in Section 11 of [RFC6763]) and use the AMT relays 476 discovered that way in preference to AMT relays discoverable via 477 the mechanism defined in this document (DRIAD). 479 2. Let Sender Manage Relay Provisioning 481 A related motivating example in the sending-side network is 482 provided by considering a sender which needs to instruct the 483 gateways on how to select between connecting to Figure 6 or 484 Figure 7 (from Section 3.2), in order to manage load and failover 485 scenarios in a manner that operates well with the sender's 486 provisioning strategy for horizontal scaling of AMT relays. 488 In this example about the sending-side network, the precedence 489 field described in Section 4.2.1 is a critical method of control 490 so that senders can provide the appropriate guidance to gateways 491 during the discovery process. 493 Therefore, after DNS-SD, the precedence from the RR MUST be used 494 for sorting preference ahead of the Destination Address Selection 495 ordering from Section 6 of [RFC6724], so that only relay IPs with 496 the same precedence are directly compared according to the 497 Destination Address Selection ordering. 499 3. Let Sender Manage Non-DRIAD discovery 501 It's also RECOMMENDED that if the well-known anycast IP addresses 502 defined in Section 7 of [RFC7450] are suitable for discovering an 503 AMT relay that can forward traffic from the source, that a DNS 504 record with the AMTRELAY RRType be published by the sender for 505 those IP addresses along with any other appropriate AMTRELAY RRs 506 to indicate the best relative precedences for receiving the 507 source traffic. 509 Accordingly, AMT gateways SHOULD by default prefer relays first by 510 DNS-SD if available, then by DRIAD as described in this document (in 511 precedence order, as described in Section 4.2.1), then with the 512 anycast addresses defined in Section 7 of [RFC7450] (namely: 513 192.52.193.1 and 2001:3::1) if those IPs weren't listed in the 514 AMTRELAY RRs. 516 This default behavior MAY be overridden by administrative 517 configuration where other behavior is more appropriate for the 518 gateway within its network. 520 Among relay addresses that have an equivalent preference as described 521 above, a Happy Eyeballs algorithm for AMT MUST use the Destination 522 Address Selection defined in Section 6 of [RFC6724], as required by 523 [RFC8305]. 525 Among relay addresses that still have an equivalent preference after 526 the above orderings, a gateway MUST make a non-deterministic choice 527 for relay preference ordering, in order to support load balancing by 528 DNS configurations that provide many relay options. (Note that 529 gateways not implementing a Happy Eyeballs algorithm are not required 530 to use the Destination Address Selection ordering, but are still 531 required to use non-deterministic ordering among equally preferred 532 relays.) 534 Note also that certain relay addresses may be excluded from 535 consideration by the hold-down timers described in Section 2.5.4.1 or 536 Section 2.5.5. These relays constitute "unusable destinations" under 537 Rule 1 of the Destination Address Selection, and are also not part of 538 the superseding considerations described above. 540 The discovery and connection process for the relay addresses in the 541 above described ordering MAY operate in parallel, subject to delays 542 prescribed by the Happy Eyeballs requirements described in Section 5 543 of [RFC8305] for successively launched concurrent connection 544 attempts. 546 2.4.3. Connecting to Multiple Relays 548 In some deployments, it may be useful for a gateway to connect to 549 multiple upstream relays and subscribe to the same traffic, in order 550 to support an active/active failover model. A gateway SHOULD NOT be 551 configured to do so without guaranteeing that adequate bandwidth is 552 available. 554 A gateway configured to do this SHOULD still use the same preference 555 ordering logic from Section 2.4.2 for both connections. (Note that 556 this ordering allows for overriding by explicit administrative 557 configuration where required.) 559 2.5. Guidelines for Restarting Discovery 561 2.5.1. Overview 563 It's expected that gateways deployed in different environments will 564 use a variety of heuristics to decide when it's appropriate to 565 restart the relay discovery process, in order to meet different 566 performance goals (for example, to fulfill different kinds of service 567 level agreements). 569 In general, restarting the discovery process is always safe for the 570 gateway and relay during any of the events listed in this section, 571 but may cause a disruption in the forwarded traffic if the discovery 572 process results in choosing a different relay, because this changes 573 the RPF forwarding tree for the multicast traffic upstream of the 574 gateway. This is likely to result in some dropped or duplicated 575 packets from channels actively being tunneled from the old relay to 576 the gateway. 578 The degree of impact on the traffic from choosing a different relay 579 may depend on network conditions between the gateway and the new 580 relay, as well as the network conditions and topology between the 581 sender and the new relay, as this may cause the relay to propagate a 582 new RPF join toward the sender. 584 Balancing the expected impact on the tunneled traffic against likely 585 or observed problems with an existing connection to the relay is the 586 goal of the heuristics that gateways use to determine when to restart 587 the discovery process. 589 The non-normative advice in this section should be treated as 590 guidelines to operators and implementors working with AMT systems 591 that can use DRIAD as part of the relay discovery process. 593 2.5.2. Updates to Restarting Events 595 Section 5.2.3.4.1 of [RFC7450] lists several events that may cause a 596 gateway to start or restart the discovery procedure. 598 This document provides some updates and recommendations regarding the 599 handling of these and similar events. The first 5 events are copied 600 here and numbered for easier reference, and the following events are 601 newly added for consideration in this document: 603 1. When a gateway pseudo-interface is started (enabled). 605 2. When the gateway wishes to report a group subscription when none 606 currently exist. 608 3. Before sending the next Request message in a membership update 609 cycle. 611 4. After the gateway fails to receive a response to a Request 612 message. 614 5. After the gateway receives a Membership Query message with the L 615 flag set to 1. 617 6. When the gateway wishes to report a (S,G) subscription with a 618 source address that does not currently have other group 619 subscriptions. 621 7. When there is a network change detected, for example when a 622 gateway is operating inside an end user device or application, 623 and the device joins a different network, or when the domain 624 portion of a DNS-SD domain name changes in response to a DHCP 625 message or administrative configuration. 627 8. When congestion or substantial loss is detected in the stream of 628 AMT packets from a relay. 630 9. When the gateway has reported one or more (S,G) subscriptions, 631 but no traffic is received from the source for some timeout. 632 (See Section 2.5.4.1). 634 This list is not exhaustive, nor are any of the listed events 635 strictly required to always force a restart of the discovery process. 637 Note that during event #1, a gateway may use DNS-SD, but does not 638 have sufficient information to use DRIAD, since no source is known. 640 2.5.3. Tunnel Stability 642 In general, subscribers to active traffic flows that are being 643 forwarded by an AMT gateway are less likely to experience a 644 degradation in service (for example, from missing or duplicated 645 packets) when the gateway continues using the same relay, as long the 646 relay is not overloaded and the network conditions remain stable. 648 Therefore, gateways SHOULD avoid performing a full restart of the 649 discovery process during routine cases of event #3 (sending a new 650 Request message), since it occurs frequently in normal operation. 652 However, see Section 2.5.4, Section 2.5.6, and Section 2.5.4.3 for 653 more information about exceptional cases when it may be appropriate 654 to use event #3. 656 2.5.4. Traffic Health 658 2.5.4.1. Absence of Traffic 660 If a gateway indicates one or more (S,G) subscriptions in a 661 Membership Update message, but no traffic for any of the (S,G)s is 662 received in a reasonable time, it's appropriate for the gateway to 663 restart the discovery process. 665 If the gateway restarts the discovery process multiple times 666 consecutively for this reason, the timeout period SHOULD be adjusted 667 to provide a random exponential back-off. 669 The RECOMMENDED timeout is a random value in the range 670 [initial_timeout, MIN(initial_timeout * 2^retry_count, 671 maximum_timeout)], with a RECOMMENDED initial_timeout of 4 seconds 672 and a RECOMMENDED maximum_timeout of 120 seconds. 674 Note that the recommended initial_timeout is larger than the initial 675 timout recommended in the similar algorithm from Section 5.2.3.4.3 of 676 [RFC7450]. This is to provide time for RPF Join propagation in the 677 sending network. Although the timeout values may be administratively 678 adjusted to support performance requirements, operators are advised 679 to consider the possibility of join propagation delays between the 680 sender and the relay when choosing an appropriate timeout value. 682 Gateways restarting the discovery process because of an absence of 683 traffic MUST use a hold-down timer that removes this relay from 684 consideration during subsequent rounds of discovery while active. 685 The hold-down SHOULD last for no less than 3 minutes and no more than 686 10 minutes. 688 2.5.4.2. Loss and Congestion 690 In some gateway deployments, it is also feasible to monitor the 691 health of traffic flows through the gateway, for example by detecting 692 the rate of packet loss by communicating out of band with receivers, 693 or monitoring the packets of known protocols with sequence numbers. 694 Where feasible, it's encouraged for gateways to use such traffic 695 health information to trigger a restart of the discovery process 696 during event #3 (before sending a new Request message). 698 However, to avoid synchronized rediscovery by many gateways 699 simultaneously after a transient network event upstream of a relay 700 results in many receivers detecting poor flow health at the same 701 time, it's recommended to add a random delay before restarting the 702 discovery process in this case. 704 The span of the random portion of the delay should be no less than 10 705 seconds by default, but may be administratively configured to support 706 different performance requirements. 708 2.5.4.3. Ancient Discovery Information 710 In most cases, a gateway actively receiving healthy traffic from a 711 relay that has not indicated load with the L flag should prefer to 712 remain connected to the same relay, as described in Section 2.5.3. 714 However, a relay that appears healthy but has been forwarding traffic 715 for days or weeks may have an increased chance of becoming unstable. 716 Gateways may benefit from restarting the discovery process during 717 event #3 (before sending a Request message) after the expiration of a 718 long-term timeout, on the order of multiple hours, or even days in 719 some deployments. 721 It may be beneficial for such timers to consider the amount of 722 traffic currently being forwarded, and to give a higher probability 723 of restarting discovery during periods with an unusually low data 724 rate, to reduce the impact on active traffic while still avoiding 725 relying on the results of a very old discovery. 727 Other issues may also be worth considering as part of this heuristic; 728 for example, if the DNS expiry time of the record that was used to 729 discover the current relay has not passed, the long term timer might 730 be restarted without restarting the discovery process. 732 2.5.5. Relay Loaded or Shutting Down 734 The L flag (see Section 5.1.4.4 of [RFC7450]) is the preferred 735 mechanism for a relay to signal overloading or a graceful shutdown to 736 gateways. 738 A gateway that supports handling of the L flag should generally 739 restart the discovery process when it processes a Membership Query 740 packet with the L flag set. If an L flag is received while a 741 concurrent Happy Eyeballs discovery process is under way for multiple 742 candidate relays (Section 2.3), the relay sending the L flag SHOULD 743 NOT be considered for the relay selection. 745 It is also RECOMMENDED that gateways avoid choosing a relay that has 746 recently sent an L flag, with approximately a 10-minute hold-down. 747 Gateways SHOULD treat this hold-down timer in the same way as the 748 hold-down in Section 2.5.4.1, so that the relay is removed from 749 consideration for short-term subsequent rounds of discovery. 751 2.5.6. Relay Discovery Messages vs. Restarting Discovery 753 A gateway should only send DNS queries with the AMTRELAY RRType or 754 the DNS-SD DNS queries for an AMT service as part of starting or 755 restarting the discovery process. 757 However, all AMT relays are required to support handling of Relay 758 Discovery messages (e.g. in Section 5.3.3.2 of [RFC7450]). 760 So a gateway with an existing connection to a relay can send a Relay 761 Discovery message to the unicast address of that AMT relay. Under 762 stable conditions with an unloaded relay, it's expected that the 763 relay will return its own unicast address in the Relay Advertisement, 764 in response to such a Relay Discovery message. Since this will not 765 result in the gateway changing to another relay unless the relay 766 directs the gateway away, this is a reasonable exception to the 767 advice against handling event #3 described in Section 2.5.3. 769 This behavior is discouraged for gateways that do support the L flag, 770 to avoid sending unnecessary packets over the network. 772 However, gateways that do not support the L flag may be able to avoid 773 a disruption in the forwarded traffic by sending such Relay Discovery 774 messages regularly. When a relay is under load or has started a 775 graceful shutdown, it may respond with a different relay address, 776 which the gateway can use to connect to a different relay. This kind 777 of coordinated handoff will likely result in a smaller disruption to 778 the traffic than if the relay simply stops responding to Request 779 messages, and stops forwarding traffic. 781 This style of Relay Discovery message (one sent to the unicast 782 address of a relay that's already forwarding traffic to this gateway) 783 should not be considered a full restart of the relay discovery 784 process. It is recommended for gateways to support the L flag, but 785 for gateways that do not support the L flag, sending this message 786 during event #3 may help mitigate service degradation when relays 787 become unstable. 789 2.5.7. Independent Discovery Per Traffic Source 791 Relays discovered via the AMTRELAY RR are source-specific relay 792 addresses, and may use different pseudo-interfaces from each other 793 and from relays discovered via DNS-SD or a non-source-specific 794 address, as described in Section 4.1.2.1 of [RFC7450]. 796 Restarting the discovery process for one pseudo-interface does not 797 require restarting the discovery process for other pseudo-interfaces. 798 Gateway heuristics about restarting the discovery process should 799 operate independently for different tunnels to relays, when 800 responding to events that are specific to the different tunnels. 802 2.6. DNS Configuration 804 Often an AMT gateway will only have access to the source and group IP 805 addresses of the desired traffic, and will not know any other name 806 for the source of the traffic. Because of this, typically the best 807 way of looking up AMTRELAY RRs will be by using the source IP address 808 as an index into one of the reverse mapping trees (in-addr.arpa for 809 IPv4, as described in Section 3.5 of [RFC1035], or ip6.arpa for IPv6, 810 as described in Section 2.5 of [RFC3596]). 812 Therefore, it is RECOMMENDED that AMTRELAY RRs be added to reverse IP 813 zones as appropriate. AMTRELAY records MAY also appear in other 814 zones, but the primary intended use case requires a reverse IP 815 mapping for the source from an (S,G) in order to be useful to most 816 AMT gateways. 818 When performing the AMTRELAY RR lookup, any CNAMEs or DNAMEs found 819 MUST be followed. This is necessary to support zone delegation. 820 Some examples outlining this need are described in [RFC2317]. 822 See Section 4 and Section 4.3 for a detailed explanation of the 823 contents for a DNS Zone file. 825 2.7. Waiting for DNS resolution 827 The DNS query functionality is expected to follow ordinary standards 828 and best practices for DNS clients. A gateway MAY use an existing 829 DNS client implementation that does so, and MAY rely on that client's 830 retry logic to determine the timeouts between retries. 832 Otherwise, a gateway MAY re-send a DNS query if it does not receive 833 an appropriate DNS response within some timeout period. If the 834 gateway retries multiple times, the timeout period SHOULD be adjusted 835 to provide a random exponential back-off. 837 As with the waiting process for the Relay Advertisement message from 838 Section 5.2.3.4.3 of [RFC7450], the RECOMMENDED timeout is a random 839 value in the range [initial_timeout, MIN(initial_timeout * 840 2^retry_count, maximum_timeout)], with a RECOMMENDED initial_timeout 841 of 1 second and a RECOMMENDED maximum_timeout of 120 seconds. 843 3. Example Deployments 845 3.1. Example Receiving Networks 847 3.1.1. Tier 3 ISP 849 One example of a receiving network is an ISP that offers multicast 850 ingest services to its subscribers, illustrated in Figure 3. 852 In the example network below, subscribers can join (S,G)s with MLDv2 853 or IGMPv3 as described in [RFC4604], and the AMT gateway in this ISP 854 can receive and forward multicast traffic from one of the example 855 sending networks in Section 3.2 by discovering the appropriate AMT 856 relays with a DNS lookup for the AMTRELAY RR with the reverse IP of 857 the source in the (S,G). 859 Internet 860 ^ ^ Multicast-enabled 861 | | Receiving Network 862 +------|------------|-------------------------+ 863 | | | | 864 | +--------+ +--------+ +=========+ | 865 | | Border |---| Border | | AMT | | 866 | | Router | | Router | | gateway | | 867 | +--------+ +--------+ +=========+ | 868 | | | | | 869 | +-----+------+-----------+--+ | 870 | | | | 871 | +-------------+ +-------------+ | 872 | | Agg Routers | .. | Agg Routers | | 873 | +-------------+ +-------------+ | 874 | / \ \ / \ | 875 | +---------------+ +---------------+ | 876 | |Access Systems | ....... |Access Systems | | 877 | |(CMTS/OLT/etc.)| |(CMTS/OLT/etc.)| | 878 | +---------------+ +---------------+ | 879 | | | | 880 +--------|------------------------|-----------+ 881 | | 882 +---+-+-+---+---+ +---+-+-+---+---+ 883 | | | | | | | | | | 884 /-\ /-\ /-\ /-\ /-\ /-\ /-\ /-\ /-\ /-\ 885 |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| 887 Subscribers 889 Figure 3: Receiving ISP Example 891 3.1.2. Small Office 893 Another example receiving network is a small branch office that 894 regularly accesses some multicast content, illustrated in Figure 4. 896 This office has desktop devices that need to receive some multicast 897 traffic, so an AMT gateway runs on a LAN with these devices, to pull 898 traffic in through a non-multicast next-hop. 900 The office also hosts some mobile devices that have AMT gateway 901 instances embedded inside apps, in order to receive multicast traffic 902 over their non-multicast wireless LAN. (Note that the "Legacy 903 Router" is a simplification that's meant to describe a variety of 904 possible conditions- for example it could be a device providing a 905 split-tunnel VPN as described in [RFC7359], deliberately excluding 906 multicast traffic for a VPN tunnel, rather than a device which is 907 incapable of multicast forwarding.) 909 Internet 910 (non-multicast) 911 ^ 912 | Office Network 913 +----------|----------------------------------+ 914 | | | 915 | +---------------+ (Wifi) Mobile apps | 916 | | Modem+ | Wifi | - - - - w/ embedded | 917 | | Router | AP | AMT gateways | 918 | +---------------+ | 919 | | | 920 | | | 921 | +----------------+ | 922 | | Legacy Router | | 923 | | (unicast) | | 924 | +----------------+ | 925 | / | \ | 926 | / | \ | 927 | +--------+ +--------+ +--------+=========+ | 928 | | Phones | | ConfRm | | Desks | AMT | | 929 | | subnet | | subnet | | subnet | gateway | | 930 | +--------+ +--------+ +--------+=========+ | 931 | | 932 +---------------------------------------------+ 934 Figure 4: Small Office (no multicast up) 936 By adding an AMT relay to this office network as in Figure 5, it's 937 possible to make use of multicast services from the example 938 multicast-capable ISP in Section 3.1.1. 940 Multicast-capable ISP 941 ^ 942 | Office Network 943 +----------|----------------------------------+ 944 | | | 945 | +---------------+ (Wifi) Mobile apps | 946 | | Modem+ | Wifi | - - - - w/ embedded | 947 | | Router | AP | AMT gateways | 948 | +---------------+ | 949 | | +=======+ | 950 | +---Wired LAN---| AMT | | 951 | | | relay | | 952 | +----------------+ +=======+ | 953 | | Legacy Router | | 954 | | (unicast) | | 955 | +----------------+ | 956 | / | \ | 957 | / | \ | 958 | +--------+ +--------+ +--------+=========+ | 959 | | Phones | | ConfRm | | Desks | AMT | | 960 | | subnet | | subnet | | subnet | gateway | | 961 | +--------+ +--------+ +--------+=========+ | 962 | | 963 +---------------------------------------------+ 965 Figure 5: Small Office Example 967 When multicast-capable networks are chained like this, with a network 968 like the one in Figure 5 receiving internet services from a 969 multicast-capable network like the one in Figure 3, it's important 970 for AMT gateways to reach the more local AMT relay, in order to avoid 971 accidentally tunneling multicast traffic from a more distant AMT 972 relay with unicast, and failing to utilize the multicast transport 973 capabilities of the network in Figure 3. 975 3.2. Example Sending Networks 977 3.2.1. Sender-controlled Relays 979 When a sender network is also operating AMT relays to distribute 980 multicast traffic, as in Figure 6, each address could appear as an 981 AMTRELAY RR for the reverse IP of the sender, or one or more domain 982 names could appear in AMTRELAY RRs, and the AMT relay addresses can 983 be discovered by finding a A or AAAA records from those domain names. 985 Sender Network 986 +-----------------------------------+ 987 | | 988 | +--------+ +=======+ +=======+ | 989 | | Sender | | AMT | | AMT | | 990 | +--------+ | relay | | relay | | 991 | | +=======+ +=======+ | 992 | | | | | 993 | +-----+------+----------+ | 994 | | | 995 +-----------|-----------------------+ 996 v 997 Internet 998 (non-multicast) 1000 Figure 6: Small Office Example 1002 3.2.2. Provider-controlled Relays 1004 When an ISP offers a service to transmit outbound multicast traffic 1005 through a forwarding network, it might also offer AMT relays in order 1006 to reach receivers without multicast connectivity to the forwarding 1007 network, as in Figure 7. In this case it's RECOMMENDED that the ISP 1008 also provide at least one domain name for the AMT relays for use with 1009 the AMTRELAY RR. 1011 When the sender wishes to use the relays provided by the ISP for 1012 forwarding multicast traffic, an AMTRELAY RR should be configured to 1013 use the domain name provided by the ISP, to allow for address 1014 reassignment of the relays without forcing the sender to reconfigure 1015 the corresponding AMTRELAY RRs. 1017 +--------+ 1018 | Sender | 1019 +---+----+ Multicast-enabled 1020 | Sending Network 1021 +-----------|-------------------------------+ 1022 | v | 1023 | +------------+ +=======+ +=======+ | 1024 | | Agg Router | | AMT | | AMT | | 1025 | +------------+ | relay | | relay | | 1026 | | +=======+ +=======+ | 1027 | | | | | 1028 | +-----+------+--------+---------+ | 1029 | | | | 1030 | +--------+ +--------+ | 1031 | | Border |---| Border | | 1032 | | Router | | Router | | 1033 | +--------+ +--------+ | 1034 +-----|------------|------------------------+ 1035 | | 1036 v v 1037 Internet 1038 (non-multicast) 1040 Figure 7: Sending ISP Example 1042 4. AMTRELAY Resource Record Definition 1044 4.1. AMTRELAY RRType 1046 The AMTRELAY RRType has the mnemonic AMTRELAY and type code 260 1047 (decimal). 1049 The AMTRELAY RR is class independent. 1051 4.2. AMTRELAY RData Format 1053 The AMTRELAY RData consists of a 8-bit precedence field, a 1-bit 1054 "Discovery Optional" field, a 7-bit type field, and a variable length 1055 relay field. 1057 0 1 2 3 1058 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 1059 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1060 | precedence |D| type | | 1061 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 1062 ~ relay ~ 1063 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1065 4.2.1. RData Format - Precedence 1067 This is an 8-bit precedence for this record. It is interpreted in 1068 the same way as the PREFERENCE field described in Section 3.3.9 of 1069 [RFC1035]. 1071 Relays listed in AMTRELAY records with a lower value for precedence 1072 are to be attempted first. 1074 4.2.2. RData Format - Discovery Optional (D-bit) 1076 The D bit is a "Discovery Optional" flag. 1078 If the D bit is set to 0, a gateway using this RR MUST perform AMT 1079 relay discovery as described in Section 4.2.1.1 of [RFC7450], rather 1080 than directly sending an AMT Request message to the relay. 1082 That is, the gateway MUST receive an AMT Relay Advertisement message 1083 (Section 5.1.2 of [RFC7450]) for an address before sending an AMT 1084 Request message (Section 5.1.3 of [RFC7450]) to that address. Before 1085 receiving the Relay Advertisement message, this record has only 1086 indicated that the address can be used for AMT relay discovery, not 1087 for a Request message. This is necessary for devices that are not 1088 fully functional AMT relays, but rather load balancers or brokers, as 1089 mentioned in Section 4.2.1.1 of [RFC7450]. 1091 If the D bit is set to 1, the gateway MAY send an AMT Request message 1092 directly to the discovered relay address without first sending an AMT 1093 Discovery message. 1095 This bit should be set according to advice from the AMT relay 1096 operator. The D bit MUST be set to zero when no information is 1097 available from the AMT relay operator about its suitability. 1099 4.2.3. RData Format - Type 1101 The type field indicates the format of the information that is stored 1102 in the relay field. 1104 The following values are defined: 1106 o type = 0: The relay field is empty (0 bytes). 1108 o type = 1: The relay field contains a 4-octet IPv4 address. 1110 o type = 2: The relay field contains a 16-octet IPv6 address. 1112 o type = 3: The relay field contains a wire-encoded domain name. 1113 The wire-encoded format is self-describing, so the length is 1114 implicit. The domain name MUST NOT be compressed. (See 1115 Section 3.3 of [RFC1035] and Section 4 of [RFC3597].) 1117 4.2.4. RData Format - Relay 1119 The relay field is the address or domain name of the AMT relay. It 1120 is formatted according to the type field. 1122 When the type field is 0, the length of the relay field is 0, and it 1123 indicates that no AMT relay should be used for multicast traffic from 1124 this source. 1126 When the type field is 1, the length of the relay field is 4 octets, 1127 and a 32-bit IPv4 address is present. This is an IPv4 address as 1128 described in Section 3.4.1 of [RFC1035]. This is a 32-bit number in 1129 network byte order. 1131 When the type field is 2, the length of the relay field is 16 octets, 1132 and a 128-bit IPv6 address is present. This is an IPv6 address as 1133 described in Section 2.2 of [RFC3596]. This is a 128-bit number in 1134 network byte order. 1136 When the type field is 3, the relay field is a normal wire-encoded 1137 domain name, as described in Section 3.3 of [RFC1035]. Compression 1138 MUST NOT be used, for the reasons given in Section 4 of [RFC3597]. 1140 For a type 3 record, the D-bit and preference fields carry over to 1141 all A or AAAA records for the domain name. There is no difference in 1142 the result of the discovery process when it's obtained by type 1 or 1143 type 2 AMTRELAY records with identical D-bit and preference fields, 1144 vs. when the result is obtained by a type 3 AMTRELAY record that 1145 resolves to the same set of IPv4 and IPv6 addresses via A and AAAA 1146 lookups. 1148 4.3. AMTRELAY Record Presentation Format 1150 4.3.1. Representation of AMTRELAY RRs 1152 AMTRELAY RRs may appear in a zone data master file. The precedence, 1153 D-bit, relay type, and relay fields are REQUIRED. 1155 If the relay type field is 0, the relay field MUST be ".". 1157 The presentation for the record is as follows: 1159 IN AMTRELAY precedence D-bit type relay 1161 4.3.2. Examples 1163 In a DNS authoritative nameserver that understands the AMTRELAY type, 1164 the zone might contain a set of entries like this: 1166 $ORIGIN 100.51.198.in-addr.arpa. 1167 10 IN AMTRELAY 10 0 1 203.0.113.15 1168 10 IN AMTRELAY 10 0 2 2001:DB8::15 1169 10 IN AMTRELAY 128 1 3 amtrelays.example.com. 1171 This configuration advertises an IPv4 discovery address, an IPv6 1172 discovery address, and a domain name for AMT relays which can receive 1173 traffic from the source 198.51.100.10. The IPv4 and IPv6 addresses 1174 are configured with a D-bit of 0 (meaning discovery is mandatory, as 1175 described in Section 4.2.2), and a precedence 10 (meaning they're 1176 preferred ahead of the last entry, which has precedence 128). 1178 For zone files in name servers that don't support the AMTRELAY RRType 1179 natively, it's possible to use the format for unknown RR types, as 1180 described in [RFC3597]. This approach would replace the AMTRELAY 1181 entries in the example above with the entries below: 1183 10 IN TYPE260 \# ( 1184 6 ; length 1185 0a ; precedence=10 1186 01 ; D=0, relay type=1, an IPv4 address 1187 cb00710f ) ; 203.0.113.15 1188 10 IN TYPE260 \# ( 1189 18 ; length 1190 0a ; precedence=10 1191 02 ; D=0, relay type=2, an IPv6 address 1192 20010db800000000000000000000000f ) ; 2001:db8::15 1193 10 IN TYPE260 \# ( 1194 24 ; length 1195 80 ; precedence=128 1196 83 ; D=1, relay type=3, a wire-encoded domain name 1197 09616d7472656c617973076578616d706c6503636f6d ) ; domain name 1199 See Appendix A for more details. 1201 5. IANA Considerations 1203 This document updates the IANA Registry for DNS Resource Record Types 1204 by assigning type 260 to the AMTRELAY record. 1206 This document creates a new registry named "AMTRELAY Resource Record 1207 Parameters", with a sub-registry for the "Relay Type Field". The 1208 initial values in the sub-registry are: 1210 +-------+---------------------------------------+ 1211 | Value | Description | 1212 +-------+---------------------------------------+ 1213 | 0 | No relay is present. | 1214 | 1 | A 4-byte IPv4 address is present | 1215 | 2 | A 16-byte IPv6 address is present | 1216 | 3 | A wire-encoded domain name is present | 1217 | 4-255 | Unassigned | 1218 +-------+---------------------------------------+ 1220 Values 0, 1, 2, and 3 are further explained in Section 4.2.3 and 1221 Section 4.2.4. Relay type numbers 4 through 255 can be assigned with 1222 a policy of Specification Required (as described in [RFC8126]). 1224 6. Security Considerations 1226 [ TBD: these 3 are just the first few most obvious issues, with just 1227 sketches of the problem. Explain better, and look for trickier 1228 issues. ] 1230 6.1. Record-spoofing 1232 If AMT is used to ingest multicast traffic, providing a false 1233 AMTRELAY record to a gateway using it for discovery can result in 1234 Denial of Service, or artificial multicast traffic from a source 1235 under an attacker's control. 1237 Therefore, it is important to ensure that the AMTRELAY record is 1238 authentic, with DNSSEC [RFC4033] or other operational safeguards that 1239 can provide assurance of the authenticity of the record contents. 1241 6.2. Local Override 1243 The local relays, while important for overall network performance, 1244 can't be secured by DNSSEC. 1246 6.3. Congestion 1248 Multicast traffic, particularly interdomain multicast traffic, 1249 carries some congestion risks, as described in Section 4 of 1250 [RFC8085]. Network operators are advised to take precautions 1251 including monitoring of application traffic behavior, traffic 1252 authentication, and rate-limiting of multicast traffic, in order to 1253 ensure network health. 1255 7. Acknowledgements 1257 This specification was inspired by the previous work of Doug Nortz, 1258 Robert Sayko, David Segelstein, and Percy Tarapore, presented in the 1259 MBONED working group at IETF 93. 1261 Thanks to Jeff Goldsmith, Toerless Eckert, Mikael Abrahamsson, Lenny 1262 Giuliano, and Mark Andrews for their very helpful comments. 1264 8. References 1266 8.1. Normative References 1268 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1269 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 1270 . 1272 [RFC1035] Mockapetris, P., "Domain names - implementation and 1273 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1274 November 1987, . 1276 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1277 Requirement Levels", BCP 14, RFC 2119, 1278 DOI 10.17487/RFC2119, March 1997, 1279 . 1281 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 1282 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 1283 . 1285 [RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A. 1286 Thyagarajan, "Internet Group Management Protocol, Version 1287 3", RFC 3376, DOI 10.17487/RFC3376, October 2002, 1288 . 1290 [RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, 1291 "DNS Extensions to Support IP Version 6", STD 88, 1292 RFC 3596, DOI 10.17487/RFC3596, October 2003, 1293 . 1295 [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record 1296 (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September 1297 2003, . 1299 [RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener 1300 Discovery Version 2 (MLDv2) for IPv6", RFC 3810, 1301 DOI 10.17487/RFC3810, June 2004, 1302 . 1304 [RFC4604] Holbrook, H., Cain, B., and B. Haberman, "Using Internet 1305 Group Management Protocol Version 3 (IGMPv3) and Multicast 1306 Listener Discovery Protocol Version 2 (MLDv2) for Source- 1307 Specific Multicast", RFC 4604, DOI 10.17487/RFC4604, 1308 August 2006, . 1310 [RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for 1311 IP", RFC 4607, DOI 10.17487/RFC4607, August 2006, 1312 . 1314 [RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, 1315 "Default Address Selection for Internet Protocol Version 6 1316 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012, 1317 . 1319 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service 1320 Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, 1321 . 1323 [RFC7450] Bumgardner, G., "Automatic Multicast Tunneling", RFC 7450, 1324 DOI 10.17487/RFC7450, February 2015, 1325 . 1327 [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage 1328 Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, 1329 March 2017, . 1331 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1332 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1333 May 2017, . 1335 [RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: 1336 Better Connectivity Using Concurrency", RFC 8305, 1337 DOI 10.17487/RFC8305, December 2017, 1338 . 1340 8.2. Informative References 1342 [RFC2317] Eidnes, H., de Groot, G., and P. Vixie, "Classless IN- 1343 ADDR.ARPA delegation", BCP 20, RFC 2317, 1344 DOI 10.17487/RFC2317, March 1998, 1345 . 1347 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1348 Jacobson, "RTP: A Transport Protocol for Real-Time 1349 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 1350 July 2003, . 1352 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1353 Rose, "DNS Security Introduction and Requirements", 1354 RFC 4033, DOI 10.17487/RFC4033, March 2005, 1355 . 1357 [RFC5110] Savola, P., "Overview of the Internet Multicast Routing 1358 Architecture", RFC 5110, DOI 10.17487/RFC5110, January 1359 2008, . 1361 [RFC6726] Paila, T., Walsh, R., Luby, M., Roca, V., and R. Lehtonen, 1362 "FLUTE - File Delivery over Unidirectional Transport", 1363 RFC 6726, DOI 10.17487/RFC6726, November 2012, 1364 . 1366 [RFC7359] Gont, F., "Layer 3 Virtual Private Network (VPN) Tunnel 1367 Traffic Leakages in Dual-Stack Hosts/Networks", RFC 7359, 1368 DOI 10.17487/RFC7359, August 2014, 1369 . 1371 [RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I., 1372 Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent 1373 Multicast - Sparse Mode (PIM-SM): Protocol Specification 1374 (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March 1375 2016, . 1377 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 1378 Writing an IANA Considerations Section in RFCs", BCP 26, 1379 RFC 8126, DOI 10.17487/RFC8126, June 2017, 1380 . 1382 [RFC8313] Tarapore, P., Ed., Sayko, R., Shepherd, G., Eckert, T., 1383 Ed., and R. Krishnan, "Use of Multicast across Inter- 1384 domain Peering Points", BCP 213, RFC 8313, 1385 DOI 10.17487/RFC8313, January 2018, 1386 . 1388 [RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 1389 Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, 1390 January 2019, . 1392 Appendix A. Unknown RRType construction 1394 In a DNS resolver that understands the AMTRELAY type, the zone file 1395 might contain this line: 1397 IN AMTRELAY 128 0 3 amtrelays.example.com. 1399 In order to translate this example to appear as an unknown RRType as 1400 defined in [RFC3597], one could run the following program: 1402 1403 $ cat translate.py 1404 #!/usr/bin/env python3 1405 import sys 1406 name=sys.argv[1] 1407 wire='' 1408 for dn in name.split('.'): 1409 if len(dn) > 0: 1410 wire += ('%02x' % len(dn)) 1411 wire += (''.join('%02x'%ord(x) for x in dn)) 1412 print(len(wire)//2) + 2 1413 print(wire) 1415 $ ./translate.py amtrelays.example.com 1416 24 1417 09616d7472656c617973076578616d706c6503636f6d 1418 1420 The length and the hex string for the domain name 1421 "amtrelays.example.com" are the outputs of this program, yielding a 1422 length of 22 and the above hex string. 1424 22 is the length of the wire-encoded domain name, so to this we add 2 1425 (1 for the precedence field and 1 for the combined D-bit and relay 1426 type fields) to get the full length of the RData, and encode the 1427 precedence, D-bit, and relay type fields as octets, as described in 1428 Section 4. 1430 This results in a zone file entry like this: 1432 IN TYPE260 \# ( 24 ; length 1433 80 ; precedence = 128 1434 03 ; D-bit=0, relay type=3 (wire-encoded domain name) 1435 09616d7472656c617973076578616d706c6503636f6d ) ; domain name 1437 Author's Address 1439 Jake Holland 1440 Akamai Technologies, Inc. 1441 150 Broadway 1442 Cambridge, MA 02144 1443 United States of America 1445 Email: jakeholland.net@gmail.com