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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force T. Pusateri 3 Internet-Draft Unaffiliated 4 Intended status: Standards Track S. Cheshire 5 Expires: February 8, 2020 Apple Inc. 6 August 7, 2019 8 DNS Push Notifications 9 draft-ietf-dnssd-push-24 11 Abstract 13 The Domain Name System (DNS) was designed to return matching records 14 efficiently for queries for data that are relatively static. When 15 those records change frequently, DNS is still efficient at returning 16 the updated results when polled, as long as the polling rate is not 17 too high. But there exists no mechanism for a client to be 18 asynchronously notified when these changes occur. This document 19 defines a mechanism for a client to be notified of such changes to 20 DNS records, called DNS Push Notifications. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at https://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on February 8, 2020. 39 Copyright Notice 41 Copyright (c) 2019 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (https://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 57 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 58 1.2. Fatal Errors . . . . . . . . . . . . . . . . . . . . . . 3 59 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4 60 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5 61 4. State Considerations . . . . . . . . . . . . . . . . . . . . 6 62 5. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 7 63 6. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 8 64 6.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 9 65 6.2. DNS Push Notification SUBSCRIBE . . . . . . . . . . . . . 13 66 6.2.1. SUBSCRIBE Request . . . . . . . . . . . . . . . . . . 13 67 6.2.2. SUBSCRIBE Response . . . . . . . . . . . . . . . . . 16 68 6.3. DNS Push Notification Updates . . . . . . . . . . . . . . 20 69 6.3.1. PUSH Message . . . . . . . . . . . . . . . . . . . . 20 70 6.4. DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . . 25 71 6.4.1. UNSUBSCRIBE Message . . . . . . . . . . . . . . . . . 25 72 6.5. DNS Push Notification RECONFIRM . . . . . . . . . . . . . 27 73 6.5.1. RECONFIRM Message . . . . . . . . . . . . . . . . . . 28 74 6.6. DNS Stateful Operations TLV Context Summary . . . . . . . 30 75 6.7. Client-Initiated Termination . . . . . . . . . . . . . . 31 76 6.8. Client Fallback to Polling . . . . . . . . . . . . . . . 32 77 7. Security Considerations . . . . . . . . . . . . . . . . . . . 33 78 7.1. Security Services . . . . . . . . . . . . . . . . . . . . 33 79 7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 34 80 7.3. TLS Early Data . . . . . . . . . . . . . . . . . . . . . 34 81 7.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 35 82 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36 83 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36 84 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 37 85 10.1. Normative References . . . . . . . . . . . . . . . . . . 37 86 10.2. Informative References . . . . . . . . . . . . . . . . . 38 87 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41 89 1. Introduction 91 Domain Name System (DNS) records may be updated using DNS Update 92 [RFC2136]. Other mechanisms such as a Discovery Proxy [DisProx] can 93 also generate changes to a DNS zone. This document specifies a 94 protocol for DNS clients to subscribe to receive asynchronous 95 notifications of changes to RRsets of interest. It is immediately 96 relevant in the case of DNS Service Discovery [RFC6763] but is not 97 limited to that use case, and provides a general DNS mechanism for 98 DNS record change notifications. Familiarity with the DNS protocol 99 and DNS packet formats is assumed [RFC1034] [RFC1035] [RFC6895]. 101 1.1. Requirements Language 103 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 104 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 105 "OPTIONAL" in this document are to be interpreted as described in 106 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 107 capitals, as shown here. These words may also appear in this 108 document in lower case as plain English words, absent their normative 109 meanings. 111 1.2. Fatal Errors 113 Certain invalid situations are described in this specification, like 114 a server sending a Push Notification subscription request to a 115 client, or a client sending a Push Notification response to a server. 116 These should never occur with a correctly implemented client and 117 server, and if they do occur then they indicate a serious 118 implementation error. In these extreme cases there is no reasonable 119 expectation of a graceful recovery, and the recipient detecting the 120 error should respond by unilaterally aborting the session without 121 regard for data loss. Such cases are addressed by having an engineer 122 investigate the cause of the failure and fixing the problem in the 123 software. 125 Where this specification says "forcibly abort", it means sending a 126 TCP RST to terminate the TCP connection, and the TLS session running 127 over that TCP connection. In the BSD Sockets API, this is achieved 128 by setting the SO_LINGER option to zero before closing the socket. 130 2. Motivation 132 As the domain name system continues to adapt to new uses and changes 133 in deployment, polling has the potential to burden DNS servers at 134 many levels throughout the network. Other network protocols have 135 successfully deployed a publish/subscribe model following the 136 Observer design pattern [obs]. XMPP Publish-Subscribe [XEP0060] and 137 Atom [RFC4287] are examples. While DNS servers are generally highly 138 tuned and capable of a high rate of query/response traffic, adding a 139 publish/subscribe model for tracking changes to DNS records can 140 deliver more timely notification of changes with reduced CPU usage 141 and lower network traffic. 143 Multicast DNS [RFC6762] implementations always listen on a well known 144 link-local IP multicast group address, and record changes are sent to 145 that multicast group address for all group members to receive. 146 Therefore, Multicast DNS already has asynchronous change notification 147 capability. However, when DNS Service Discovery [RFC6763] is used 148 across a wide area network using Unicast DNS (possibly facilitated 149 via a Discovery Proxy [DisProx]) it would be beneficial to have an 150 equivalent capability for Unicast DNS, to allow clients to learn 151 about DNS record changes in a timely manner without polling. 153 The DNS Long-Lived Queries (LLQ) mechanism [LLQ] is an existing 154 deployed solution to provide asynchronous change notifications, used 155 by Apple's Back to My Mac [RFC6281] service introduced in Mac OS X 156 10.5 Leopard in 2007. Back to My Mac was designed in an era when the 157 data center operations staff asserted that it was impossible for a 158 server to handle large numbers of mostly-idle TCP connections, so LLQ 159 was defined as a UDP-based protocol, effectively replicating much of 160 TCP's connection state management logic in user space, and creating 161 its own poor imitations of existing TCP features like the three-way 162 handshake, flow control, and reliability. 164 This document builds on experience gained with the LLQ protocol, with 165 an improved design. Instead of using UDP, this specification uses 166 DNS Stateful Operations (DSO) [RFC8490] running over TLS over TCP, 167 and therefore doesn't need to reinvent existing TCP functionality. 168 Using TCP also gives long-lived low-traffic connections better 169 longevity through NAT gateways without depending on the gateway to 170 support NAT Port Mapping Protocol (NAT-PMP) [RFC6886] or Port Control 171 Protocol (PCP) [RFC6887], or resorting to excessive keepalive 172 traffic. 174 3. Overview 176 A DNS Push Notification client subscribes for Push Notifications for 177 a particular RRset by connecting to the appropriate Push Notification 178 server for that RRset, and sending DSO message(s) indicating the 179 RRset(s) of interest. When the client loses interest in receiving 180 further updates to these records, it unsubscribes. 182 The DNS Push Notification server for a DNS zone is any server capable 183 of generating the correct change notifications for a name. It may be 184 a primary, secondary, or stealth name server [RFC7719]. 186 The "_dns-push-tls._tcp." SRV record for a zone MAY reference 187 the same target host and port as that zone's 188 "_dns-update-tls._tcp." SRV record. When the same target host 189 and port is offered for both DNS Updates and DNS Push Notifications, 190 a client MAY use a single DSO session to that server for both DNS 191 Updates and DNS Push Notification Subscriptions. DNS Updates and DNS 192 Push Notifications may be handled on different ports on the same 193 target host, in which case they are not considered to be the "same 194 server" for the purposes of this specification, and communications 195 with these two ports are handled independently. Supporting DNS 196 Updates and DNS Push Notifications on the same server is OPTIONAL. A 197 DNS Push Notification server is not required to support DNS Update. 199 Standard DNS Queries MAY be sent over a DNS Push Notification (i.e., 200 DSO) session. For any zone for which the server is authoritative, it 201 MUST respond authoritatively for queries for names falling within 202 that zone (e.g., the "_dns-push-tls._tcp." SRV record) both for 203 normal DNS queries and for DNS Push Notification subscriptions. For 204 names for which the server is acting as a recursive resolver, e.g., 205 when the server is the local recursive resolver, for any query for 206 which it supports DNS Push Notification subscriptions, it MUST also 207 support standard queries. 209 DNS Push Notifications impose less load on the responding server than 210 rapid polling would, but Push Notifications do still have a cost, so 211 DNS Push Notification clients MUST NOT recklessly create an excessive 212 number of Push Notification subscriptions. Specifically: 214 (a) A subscription should only be active when there is a valid reason 215 to need live data (for example, an on-screen display is currently 216 showing the results to the user) and the subscription SHOULD be 217 cancelled as soon as the need for that data ends (for example, when 218 the user dismisses that display). In the case of a device like a 219 smartphone which, after some period of inactivity, goes to sleep or 220 otherwise darkens its screen, it should cancel its subscriptions when 221 darkening the screen (since the user cannot see any changes on the 222 display anyway) and reinstate its subscriptions when re-awakening 223 from display sleep. 225 (b) A DNS Push Notification client SHOULD NOT routinely keep a DNS 226 Push Notification subscription active 24 hours a day, 7 days a week, 227 just to keep a list in memory up to date so that if the user does 228 choose to bring up an on-screen display of that data, it can be 229 displayed really fast. DNS Push Notifications are designed to be 230 fast enough that there is no need to pre-load a "warm" list in memory 231 just in case it might be needed later. 233 Generally, as described in the DNS Stateful Operations specification 234 [RFC8490], a client must not keep a DSO session to a server open 235 indefinitely if it has no subscriptions (or other operations) active 236 on that session. A client MAY close a DSO session immediately it 237 becomes idle, and then if needed in the future, open a new session 238 when required. Alternatively, a client MAY speculatively keep an 239 idle DSO session open for some time, subject to the constraint that 240 it must not keep a session open that has been idle for more than the 241 session's idle timeout (15 seconds by default) [RFC8490]. 243 Note that a DSO session which has an active DNS Push Notification 244 subscription is not considered idle, even if there is no traffic 245 flowing for an extended period of time. In this case the DSO 246 inactivity timeout does not apply, because the session is not 247 inactive, but the keepalive interval does still apply, to ensure 248 generation of sufficient messages to maintain state in middleboxes 249 (such at NAT gateways or firewalls) and for the client and server to 250 periodically verify that they still have connectivity to each other. 251 This is described in Section 6.2 of the DSO specification [RFC8490]. 253 4. State Considerations 255 Each DNS Push Notification server is capable of handling some finite 256 number of Push Notification subscriptions. This number will vary 257 from server to server and is based on physical machine 258 characteristics, network bandwidth, and operating system resource 259 allocation. After a client establishes a session to a DNS server, 260 each subscription is individually accepted or rejected. Servers may 261 employ various techniques to limit subscriptions to a manageable 262 level. Correspondingly, the client is free to establish simultaneous 263 sessions to alternate DNS servers that support DNS Push Notifications 264 for the zone and distribute subscriptions at the client's discretion. 265 In this way, both clients and servers can react to resource 266 constraints. 268 5. Transport 270 Other DNS operations like DNS Update [RFC2136] MAY use either User 271 Datagram Protocol (UDP) [RFC0768] or Transmission Control Protocol 272 (TCP) [RFC0793] as the transport protocol, in keeping with the 273 historical precedent that DNS queries must first be sent over UDP 274 [RFC1123]. This requirement to use UDP has subsequently been relaxed 275 [RFC7766]. 277 In keeping with the more recent precedent, DNS Push Notification is 278 defined only for TCP. DNS Push Notification clients MUST use DNS 279 Stateful Operations [RFC8490] running over TLS over TCP [RFC7858]. 281 Connection setup over TCP ensures return reachability and alleviates 282 concerns of state overload at the server which is a potential problem 283 with connectionless protocols using spoofed source addresses. All 284 subscribers are guaranteed to be reachable by the server by virtue of 285 the TCP three-way handshake. Flooding attacks are possible with any 286 protocol, and a benefit of TCP is that there are already established 287 industry best practices to guard against SYN flooding and similar 288 attacks [SYN] [RFC4953]. 290 Use of TCP also allows DNS Push Notifications to take advantage of 291 current and future developments in TCP, such as Multipath TCP (MPTCP) 292 [RFC6824], TCP Fast Open (TFO) [RFC7413], RACK: a time-based fast 293 loss detection algorithm for TCP [I-D.ietf-tcpm-rack], and so on. 295 Transport Layer Security (TLS) [RFC8446] is well understood, and used 296 by many application-layer protocols running over TCP. TLS is 297 designed to prevent eavesdropping, tampering, and message forgery. 298 TLS is REQUIRED for every connection between a client subscriber and 299 server in this protocol specification. Additional security measures 300 such as client authentication during TLS negotiation MAY also be 301 employed to increase the trust relationship between client and 302 server. 304 6. Protocol Operation 306 The DNS Push Notification protocol is a session-oriented protocol, 307 and makes use of DNS Stateful Operations (DSO) [RFC8490]. 309 For details of the DSO message format refer to the DNS Stateful Oper- 310 ations specification [RFC8490]. Those details are not repeated here. 312 DNS Push Notification clients and servers MUST support DSO. A single 313 server can support DNS Queries, DNS Updates, and DNS Push 314 Notifications (using DSO) on the same TCP port. 316 A DNS Push Notification exchange begins with the client discovering 317 the appropriate server, using the procedure described in Section 6.1, 318 and then making a TLS/TCP connection to it. 320 A typical DNS Push Notification client will immediately issue a DSO 321 Keepalive operation to request a session timeout and/or keepalive 322 interval longer than the the 15-second default values, but this is 323 not required. A DNS Push Notification client MAY issue other 324 requests on the session first, and only issue a DSO Keepalive 325 operation later if it determines that to be necessary. Sending 326 either a DSO Keepalive operation or a Push Notification subscription 327 request over the TLS/TCP connection to the server signals the 328 client's support of DSO and serves to establish a DSO session. 330 In accordance with the current set of active subscriptions, the 331 server sends relevant asynchronous Push Notifications to the client. 332 Note that a client MUST be prepared to receive (and silently ignore) 333 Push Notifications for subscriptions it has previously removed, since 334 there is no way to prevent the situation where a Push Notification is 335 in flight from server to client while the client's UNSUBSCRIBE 336 message cancelling that subscription is simultaneously in flight from 337 client to server. 339 6.1. Discovery 341 The first step in establishing a DNS Push Notification subscription 342 is to discover an appropriate DNS server that supports DNS Push 343 Notifications for the desired zone. 345 The client begins by opening a DSO Session to its normal configured 346 DNS recursive resolver and requesting a Push Notification 347 subscription. This connection is made to TCP port 853, the default 348 port for DNS-over-TLS [RFC7858]. If the request for a Push 349 Notification subscription is successful, and the recursive resolver 350 doesn't already have an active subscription for that name, type, and 351 class, then the recursive resolver will make a corresponding Push 352 Notification subscription on the client's behalf. Results received 353 are relayed to the client. This is closely analogous to how a client 354 sends a normal DNS query to its configured DNS recursive resolver 355 which, if it doesn't already have appropriate answer(s) in its cache, 356 issues an upstream query to satisfy the request. 358 In many contexts, the recursive resolver will be able to handle Push 359 Notifications for all names that the client may need to follow. Use 360 of VPN tunnels and split-view DNS can create some additional 361 complexity in the client software here; the techniques to handle VPN 362 tunnels and split-view DNS for DNS Push Notifications are the same as 363 those already used to handle this for normal DNS queries. 365 If the recursive resolver does not support DNS over TLS, or supports 366 DNS over TLS but is not listening on TCP port 853, or supports DNS 367 over TLS on TCP port 853 but does not support DSO on that port, then 368 the DSO Session session establishment will fail [RFC8490]. 370 If the recursive resolver does support DSO but not Push Notification 371 subscriptions, then it will return the DSO error code, DSOTYPENI 372 (11). 374 In some cases, the recursive resolver may support DSO and Push 375 Notification subscriptions, but may not be able to subscribe for Push 376 Notifications for a particular name. In this case, the recursive 377 resolver should return SERVFAIL to the client. This includes being 378 unable to establish a connection to the zone's DNS Push Notification 379 server or establishing a connection but receiving a non success 380 response code. In some cases, where the client has a pre-established 381 trust relationship with the owner of the zone (that is not handled 382 via the usual mechanisms for VPN software) the client may handle 383 these failures by contacting the zone's DNS Push server directly. 385 In any of the cases described above where the client fails to 386 establish a DNS Push Notification subscription via its configured 387 recursive resolver, the client should proceed to discover the 388 appropriate server for direct communication. The client MUST also 389 determine which TCP port on the server is listening for connections, 390 which need not be (and often is not) the typical TCP port 53 used for 391 conventional DNS, or TCP port 853 used for DNS over TLS. 393 The discovery algorithm described here is an iterative algorithm, 394 which starts with the full name of the record to which the client 395 wishes to subscribe. Successive SOA queries are then issued, 396 trimming one label each time, until the closest enclosing 397 authoritative server is discovered. There is also an optimization to 398 enable the client to take a "short cut" directly to the SOA record of 399 the closest enclosing authoritative server in many cases. 401 1. The client begins the discovery by sending a DNS query to its 402 local resolver, with record type SOA [RFC1035] for the record 403 name to which it wishes to subscribe. As an example, suppose the 404 client wishes to subscribe to PTR records with the name 405 _ipp._tcp.headoffice.example.com (to discover Internet Printing 406 Protocol (IPP) printers [RFC8010] [RFC8011] being advertised in 407 the head office of Example Company.). The client begins by 408 sending an SOA query for _ipp._tcp.headoffice.example.com to the 409 local recursive resolver. The goal is to determine the server 410 authoritative for the name _ipp._tcp.headoffice.example.com. The 411 closest enclosing DNS zone containing the name 412 _ipp._tcp.headoffice.example.com could be example.com, or 413 headoffice.example.com, or _tcp.headoffice.example.com, or even 414 _ipp._tcp.headoffice.example.com. The client does not know in 415 advance where the closest enclosing zone cut occurs, which is why 416 it uses the iterative procedure described here to discover this 417 information. 419 2. If the requested SOA record exists, it will be returned in the 420 Answer section with a NOERROR response code, and the client has 421 succeeded in discovering the information it needs. 422 (This language is not placing any new requirements on DNS 423 recursive resolvers. This text merely describes the existing 424 operation of the DNS protocol [RFC1034] [RFC1035].) 426 3. If the requested SOA record does not exist, the client will get 427 back a NOERROR/NODATA response or an NXDOMAIN/Name Error 428 response. In either case, the local resolver would normally 429 include the SOA record for the closest enclosing zone of the 430 requested name in the Authority Section. If the SOA record is 431 received in the Authority Section, then the client has succeeded 432 in discovering the information it needs. 433 (This language is not placing any new requirements on DNS 434 recursive resolvers. This text merely describes the existing 435 operation of the DNS protocol regarding negative responses 436 [RFC2308].) 438 4. If the client receives a response containing no SOA record, then 439 it proceeds with the iterative approach. The client strips the 440 leading label from the current query name, and if the resulting 441 name has at least two labels in it, the client sends an SOA query 442 for that new name, and processing continues at step 2 above, 443 repeating the iterative search until either an SOA is received, 444 or the query name consists of a single label, i.e., a Top Level 445 Domain (TLD). In the case of a single-label (TLD), this is a 446 network configuration error, which should not happen, and the 447 client gives up. The client may retry the operation at a later 448 time, of the client's choosing, such after a change in network 449 attachment. 451 5. Once the SOA is known (either by virtue of being seen in the 452 Answer Section, or in the Authority Section), the client sends a 453 DNS query with type SRV [RFC2782] for the record name 454 "_dns-push-tls._tcp.", where is the owner name of 455 the discovered SOA record. 457 6. If the zone in question is set up to offer DNS Push Notifications 458 then this SRV record MUST exist. (If this SRV record does not 459 exist then the zone is not correctly configured for DNS Push 460 Notifications as specified in this document.) The SRV "target" 461 contains the name of the server providing DNS Push Notifications 462 for the zone. The port number on which to contact the server is 463 in the SRV record "port" field. The address(es) of the target 464 host MAY be included in the Additional Section, however, the 465 address records SHOULD be authenticated before use as described 466 below in Section 7.2 and in the specification for using DANE TLSA 467 Records with SRV Records [RFC7673], if applicable. 469 7. More than one SRV record may be returned. In this case, the 470 "priority" and "weight" values in the returned SRV records are 471 used to determine the order in which to contact the servers for 472 subscription requests. As described in the SRV specification 473 [RFC2782], the server with the lowest "priority" is first 474 contacted. If more than one server has the same "priority", the 475 "weight" indicates the weighted probability that the client 476 should contact that server. Higher weights have higher 477 probabilities of being selected. If a server is not willing to 478 accept a subscription request, or is not reachable within a 479 reasonable time, as determined by the client, then a subsequent 480 server is to be contacted. 482 Each time a client makes a new DNS Push Notification subscription 483 session, it SHOULD repeat the discovery process in order to determine 484 the preferred DNS server for subscriptions at that time. However, 485 the client device MUST respect the DNS TTL values on records it 486 receives, and store them in its local cache with this lifetime. This 487 means that, as long as the DNS TTL values on the authoritative 488 records are set to reasonable values, repeated application of this 489 discovery process can be completed nearly instantaneously by the 490 client, using only locally-stored cached data. 492 6.2. DNS Push Notification SUBSCRIBE 494 After connecting, and requesting a longer idle timeout and/or 495 keepalive interval if necessary, a DNS Push Notification client 496 then indicates its desire to receive DNS Push Notifications for 497 a given domain name by sending a SUBSCRIBE request to the server. 498 A SUBSCRIBE request is encoded in a DSO message [RFC8490]. 499 This specification defines a primary DSO TLV for DNS Push 500 Notification SUBSCRIBE Requests (tentatively DSO Type Code 0x40). 502 DSO messages with the SUBSCRIBE TLV as the Primary TLV are permitted 503 in TLS early data, provided that the precautions described in 504 Section 7.3 are followed. 506 The entity that initiates a SUBSCRIBE request is by definition the 507 client. A server MUST NOT send a SUBSCRIBE request over an existing 508 session from a client. If a server does send a SUBSCRIBE request 509 over a DSO session initiated by a client, this is a fatal error and 510 the client MUST forcibly abort the connection immediately. 512 6.2.1. SUBSCRIBE Request 514 A SUBSCRIBE request begins with the standard DSO 12-byte header 515 [RFC8490], followed by the SUBSCRIBE primary TLV. A SUBSCRIBE 516 request message is illustrated in Figure 1. 518 The MESSAGE ID field MUST be set to a unique value, that the client 519 is not using for any other active operation on this DSO session. For 520 the purposes here, a MESSAGE ID is in use on this session if the 521 client has used it in a request for which it has not yet received a 522 response, or if the client has used it for a subscription which it 523 has not yet cancelled using UNSUBSCRIBE. In the SUBSCRIBE response 524 the server MUST echo back the MESSAGE ID value unchanged. 526 The other header fields MUST be set as described in the DSO spec- 527 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 528 for DNS Stateful Operations (6). The four count fields MUST be zero, 529 and the corresponding four sections MUST be empty (i.e., absent). 531 The DSO-TYPE is SUBSCRIBE (tentatively 0x40). 533 The DSO-LENGTH is the length of the DSO-DATA that follows, which 534 specifies the name, type, and class of the record(s) being sought. 536 1 1 1 1 1 1 537 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 538 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 539 | MESSAGE ID | \ 540 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 541 |QR| OPCODE(6) | Z | RCODE | | 542 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 543 | QDCOUNT (MUST BE ZERO) | | 544 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 545 | ANCOUNT (MUST BE ZERO) | | 546 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 547 | NSCOUNT (MUST BE ZERO) | | 548 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 549 | ARCOUNT (MUST BE ZERO) | / 550 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 551 | DSO-TYPE = SUBSCRIBE (tentatively 0x40) | 552 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 553 | DSO-LENGTH (number of octets in DSO-DATA) | 554 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 555 | | \ 556 \ NAME \ | 557 \ \ | 558 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > DSO-DATA 559 | TYPE | | 560 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 561 | CLASS | / 562 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 564 Figure 1: SUBSCRIBE Request 566 The DSO-DATA for a SUBSCRIBE request MUST contain exactly one NAME, 567 TYPE, and CLASS. Since SUBSCRIBE requests are sent over TCP, 568 multiple SUBSCRIBE DSO request messages can be concatenated in a 569 single TCP stream and packed efficiently into TCP segments. 571 If accepted, the subscription will stay in effect until the client 572 cancels the subscription using UNSUBSCRIBE or until the DSO session 573 between the client and the server is closed. 575 SUBSCRIBE requests on a given session MUST be unique. A client MUST 576 NOT send a SUBSCRIBE message that duplicates the NAME, TYPE and CLASS 577 of an existing active subscription on that DSO session. For the 578 purpose of this matching, the established DNS case-insensitivity for 579 US-ASCII letters applies (e.g., "example.com" and "Example.com" are 580 the same). If a server receives such a duplicate SUBSCRIBE message, 581 this is a fatal error and the server MUST forcibly abort the 582 connection immediately. 584 DNS wildcarding is not supported. That is, a wildcard ("*") in a 585 SUBSCRIBE message matches only a literal wildcard character ("*") in 586 the zone, and nothing else. 588 Aliasing is not supported. That is, a CNAME in a SUBSCRIBE message 589 matches only a literal CNAME record in the zone, and no other records 590 with the same owner name. 592 A client may SUBSCRIBE to records that are unknown to the server at 593 the time of the request (providing that the name falls within one of 594 the zone(s) the server is responsible for) and this is not an error. 595 The server MUST NOT return NXDOMAIN in this case. The server MUST 596 accept these requests and send Push Notifications if and when 597 matching records are found in the future. 599 If neither TYPE nor CLASS are ANY (255) then this is a specific 600 subscription to changes for the given NAME, TYPE and CLASS. If one 601 or both of TYPE or CLASS are ANY (255) then this subscription matches 602 any type and/or any class, as appropriate. 604 NOTE: A little-known quirk of DNS is that in DNS QUERY requests, 605 QTYPE and QCLASS 255 mean "ANY" not "ALL". They indicate that the 606 server should respond with ANY matching records of its choosing, not 607 necessarily ALL matching records. This can lead to some surprising 608 and unexpected results, where a query returns some valid answers but 609 not all of them, and makes QTYPE = 255 (ANY) queries less useful than 610 people sometimes imagine. 612 When used in conjunction with SUBSCRIBE, TYPE and CLASS 255 should be 613 interpreted to mean "ALL", not "ANY". After accepting a subscription 614 where one or both of TYPE or CLASS are 255, the server MUST send Push 615 Notification Updates for ALL record changes that match the 616 subscription, not just some of them. 618 6.2.2. SUBSCRIBE Response 620 Each SUBSCRIBE request generates exactly one SUBSCRIBE response from 621 the server. 623 A SUBSCRIBE response begins with the standard DSO 12-byte header 624 [RFC8490]. The QR bit in the header is set indicating it is a 625 response. The header MAY be followed by one or more optional TLVs, 626 such as a Retry Delay TLV. 628 The MESSAGE ID field MUST echo the value given in the MESSAGE ID 629 field of the SUBSCRIBE request. This is how the client knows which 630 request is being responded to. 632 A SUBSCRIBE response message MUST NOT include a SUBSCRIBE TLV. If a 633 client receives a SUBSCRIBE response message containing a SUBSCRIBE 634 TLV then the response message is processed but the SUBSCRIBE TLV MUST 635 be silently ignored. 637 A client MUST NOT send a SUBSCRIBE response. If a client does send a 638 SUBSCRIBE message, with the QR bit set indicating that it is a 639 response, this is a fatal error and the server MUST forcibly abort 640 the connection immediately. 642 1 1 1 1 1 1 643 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 644 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 645 | MESSAGE ID | \ 646 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 647 |QR| OPCODE(6) | Z | RCODE | | 648 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 649 | QDCOUNT (MUST BE ZERO) | | 650 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 651 | ANCOUNT (MUST BE ZERO) | | 652 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 653 | NSCOUNT (MUST BE ZERO) | | 654 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 655 | ARCOUNT (MUST BE ZERO) | / 656 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 658 Figure 2: SUBSCRIBE Response Message 660 In the SUBSCRIBE response the RCODE indicates whether or not the 661 subscription was accepted. Supported RCODEs are as follows: 663 +-----------+-------+-----------------------------------------------+ 664 | Mnemonic | Value | Description | 665 +-----------+-------+-----------------------------------------------+ 666 | NOERROR | 0 | SUBSCRIBE successful. | 667 | FORMERR | 1 | Server failed to process request due to a | 668 | | | malformed request. | 669 | SERVFAIL | 2 | Server failed to process request due to a | 670 | | | problem with the server. | 671 | NOTIMP | 4 | Server does not implement DSO. | 672 | REFUSED | 5 | Server refuses to process request for policy | 673 | | | or security reasons. | 674 | NOTAUTH | 9 | Server is not authoritative for the requested | 675 | | | name. | 676 | DSOTYPENI | 11 | SUBSCRIBE operation not supported. | 677 +-----------+-------+-----------------------------------------------+ 679 Table 1: SUBSCRIBE Response codes 681 This document specifies only these RCODE values for SUBSCRIBE 682 Responses. Servers sending SUBSCRIBE Responses SHOULD use one of 683 these values. Note that NXDOMAIN is not a valid RCODE in response to 684 a SUBSCRIBE Request. However, future circumstances may create 685 situations where other RCODE values are appropriate in SUBSCRIBE 686 Responses, so clients MUST be prepared to accept SUBSCRIBE Responses 687 with any other RCODE value. 689 If the server sends a nonzero RCODE in the SUBSCRIBE response, that 690 means: 692 a. the client is (at least partially) misconfigured, 693 b. the server resources are exhausted, or 694 c. there is some other unknown failure on the server. 696 In any case, the client shouldn't retry the subscription to this 697 server right away. If multiple SRV records were returned as 698 described in Section 6.1, Paragraph 7, a subsequent server can be 699 tried immediately. 701 If the client has other successful subscriptions to this server, 702 these subscriptions remain even though additional subscriptions may 703 be refused. Neither the client nor the server are required to close 704 the connection, although, either end may choose to do so. 706 If the server sends a nonzero RCODE then it SHOULD append a Retry 707 Delay TLV [RFC8490] to the response specifying a delay before the 708 client attempts this operation again. Recommended values for the 709 delay for different RCODE values are given below. These recommended 710 values apply both to the default values a server should place in the 711 Retry Delay TLV, and the default values a client should assume if the 712 server provides no Retry Delay TLV. 714 For RCODE = 1 (FORMERR) the delay may be any value selected by the 715 implementer. A value of five minutes is RECOMMENDED, to reduce 716 the risk of high load from defective clients. 718 For RCODE = 2 (SERVFAIL) the delay should be chosen according to 719 the level of server overload and the anticipated duration of that 720 overload. By default, a value of one minute is RECOMMENDED. If a 721 more serious server failure occurs, the delay may be longer in 722 accordance with the specific problem encountered. 724 For RCODE = 4 (NOTIMP), which occurs on a server that doesn't 725 implement DNS Stateful Operations [RFC8490], it is unlikely that 726 the server will begin supporting DSO in the next few minutes, so 727 the retry delay SHOULD be one hour. Note that in such a case, a 728 server that doesn't implement DSO is unlikely to place a Retry 729 Delay TLV in its response, so this recommended value in particular 730 applies to what a client should assume by default. 732 For RCODE = 5 (REFUSED), which occurs on a server that implements 733 DNS Push Notifications, but is currently configured to disallow 734 DNS Push Notifications, the retry delay may be any value selected 735 by the implementer and/or configured by the operator. 737 If the server being queried is listed in a 738 "_dns-push-tls._tcp." SRV record for the zone, then this is 739 a misconfiguration, since this server is being advertised as 740 supporting DNS Push Notifications for this zone, but the server 741 itself is not currently configured to perform that task. Since it 742 is possible that the misconfiguration may be repaired at any time, 743 the retry delay should not be set too high. By default, a value 744 of 5 minutes is RECOMMENDED. 746 For RCODE = 9 (NOTAUTH), which occurs on a server that implements 747 DNS Push Notifications, but is not configured to be authoritative 748 for the requested name, the retry delay may be any value selected 749 by the implementer and/or configured by the operator. 751 If the server being queried is listed in a 752 "_dns-push-tls._tcp." SRV record for the zone, then this is 753 a misconfiguration, since this server is being advertised as 754 supporting DNS Push Notifications for this zone, but the server 755 itself is not currently configured to perform that task. Since it 756 is possible that the misconfiguration may be repaired at any time, 757 the retry delay should not be set too high. By default, a value 758 of 5 minutes is RECOMMENDED. 760 For RCODE = 11 (DSOTYPENI), which occurs on a server that 761 implements DSO but doesn't implement DNS Push Notifications, it is 762 unlikely that the server will begin supporting DNS Push 763 Notifications in the next few minutes, so the retry delay SHOULD 764 be one hour. 766 For other RCODE values, the retry delay should be set by the 767 server as appropriate for that error condition. By default, a 768 value of 5 minutes is RECOMMENDED. 770 For RCODE = 9 (NOTAUTH), the time delay applies to requests for other 771 names falling within the same zone. Requests for names falling 772 within other zones are not subject to the delay. For all other 773 RCODEs the time delay applies to all subsequent requests to this 774 server. 776 After sending an error response the server MAY allow the session to 777 remain open, or MAY send a DNS Push Notification Retry Delay 778 Operation TLV instructing the client to close the session, as 779 described in the DSO specification [RFC8490]. Clients MUST correctly 780 handle both cases. 782 6.3. DNS Push Notification Updates 784 Once a subscription has been successfully established, the server 785 generates PUSH messages to send to the client as appropriate. In the 786 case that the answer set was already non-empty at the moment the 787 subscription was established, an initial PUSH message will be sent 788 immediately following the SUBSCRIBE Response. Subsequent changes to 789 the answer set are then communicated to the client in subsequent PUSH 790 messages. 792 6.3.1. PUSH Message 794 A PUSH unidirectional message begins with the standard DSO 12-byte 795 header [RFC8490], followed by the PUSH primary TLV. A PUSH message 796 is illustrated in Figure 3. 798 In accordance with the definition of DSO unidirectional messages, the 799 MESSAGE ID field MUST be zero. There is no client response to a PUSH 800 message. 802 The other header fields MUST be set as described in the DSO spec- 803 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 804 for DNS Stateful Operations (6). The four count fields MUST be zero, 805 and the corresponding four sections MUST be empty (i.e., absent). 807 A client MUST NOT send a PUSH message. If a client does send a PUSH 808 message, or a PUSH message is sent with the QR bit set indicating 809 that it is a response, this is a fatal error and the receiver MUST 810 forcibly abort the connection immediately. 812 The DSO-TYPE is PUSH (tentatively 0x41). 814 The DSO-LENGTH is the length of the DSO-DATA that follows, which 815 specifies the changes being communicated. 817 The DSO-DATA contains one or more change notifications. A PUSH 818 Message MUST contain at least one change notification. If a PUSH 819 Message is received that contains no change notifications, this is a 820 fatal error, and the client MUST forcibly abort the connection 821 immediately. 823 The change notification records are formatted similarly to how DNS 824 Resource Records are conventionally expressed in DNS messages, as 825 illustrated in Figure 3, and are interpreted as described below. 827 The TTL field holds an unsigned 32-bit integer [RFC2181]. If the TTL 828 is in the range 0 to 2,147,483,647 seconds (2^31 - 1, or 0x7FFFFFFF), 829 then a new DNS Resource Record with the given name, type, class and 830 RDATA is added. A TTL of 0 means that this record should be retained 831 for as long as the subscription is active, and should be discarded 832 immediately the moment the subscription is cancelled. 834 If the TTL has the value 0xFFFFFFFF, then the DNS Resource Record 835 with the given name, type, class and RDATA is removed. 837 If the TTL has the value 0xFFFFFFFE, then this is a 'collective' 838 remove notification. For collective remove notifications RDLEN MUST 839 be zero and consequently the RDATA MUST be empty. If a change 840 notification is received where TTL = 0xFFFFFFFE and RDLEN is not 841 zero, this is a fatal error, and the client MUST forcibly abort the 842 connection immediately. 844 There are three types of collective remove notification: 846 For collective remove notifications, if CLASS is not 255 (ANY) and 847 TYPE is not 255 (ANY) then for the given name this deletes all 848 records of the specified type in the specified class. 850 For collective remove notifications, if CLASS is not 255 (ANY) and 851 TYPE is 255 (ANY) then for the given name this deletes all records of 852 all types in the specified class. 854 For collective remove notifications, if CLASS is 255 (ANY), then for 855 the given name this deletes all records of all types in all classes. 856 In this case TYPE MUST be set to zero on transmission, and MUST be 857 silently ignored on reception. 859 Summary of change notification types: 861 Delete all RRsets from a name, in all classes 862 TTL = 0xFFFFFFFE, RDLEN = 0, CLASS = 255 (ANY) 864 Delete all RRsets from a name, in given class: 865 TTL = 0xFFFFFFFE, RDLEN = 0, CLASS gives class, TYPE = 255 (ANY) 867 Delete specified RRset from a name, in given class: 868 TTL = 0xFFFFFFFE, RDLEN = 0 869 CLASS and TYPE specify the RRset being deleted 871 Delete an individual RR from a name: 872 TTL = 0xFFFFFFFF 873 CLASS, TYPE, RDLEN and RDATA specify the RR being deleted. 875 Add individual RR to a name 876 TTL >= 0 and TTL <= 0x7FFFFFFF 877 CLASS, TYPE, RDLEN, RDATA and TTL specify the RR being added. 879 Note that it is valid for the RDATA of an added or removed DNS 880 Resource Record to be empty (zero length). For example, an Address 881 Prefix List Resource Record [RFC3123] may have empty RDATA. 882 Therefore, a change notification with RDLEN = 0 does not 883 automatically indicate a remove notification. If RDLEN = 0 and TTL 884 is the in the range 0 - 0x7FFFFFFF, this change notification signals 885 the addition of a record with the given name, type, class, and empty 886 RDATA. If RDLEN = 0 and TTL = 0xFFFFFFFF, this change notification 887 signals the removal specifically of that single record with the given 888 name, type, class, and empty RDATA. 890 If the TTL is any value other than 0xFFFFFFFF, 0xFFFFFFFE, or a value 891 in the range 0 - 0x7FFFFFFF, then the receiver SHOULD silently ignore 892 this particular change notification record. The connection is not 893 terminated and other valid change notification records within this 894 PUSH message are processed as usual. 896 For efficiency, when generating a PUSH message, a server SHOULD 897 include as many change notifications as it has immediately available 898 to send, rather than sending each change notification as a separate 899 DSO message. Once it has exhausted the list of change notifications 900 immediately available to send, a server SHOULD then send the PUSH 901 message immediately, rather than waiting to see if additional change 902 notifications become available. 904 For efficiency, when generating a PUSH message, a server SHOULD use 905 standard DNS name compression, with offsets relative to the beginning 906 of the DNS message [RFC1035]. When multiple change notifications in 907 a single PUSH message have the same owner name, this name compression 908 can yield significant savings. Name compression should be performed 909 as specified in Section 18.14 of the Multicast DNS specification 910 [RFC6762], namely, owner names should always be compressed, and names 911 appearing within RDATA should be compressed for only the RR types 912 listed below: 914 NS, CNAME, PTR, DNAME, SOA, MX, AFSDB, RT, KX, RP, PX, SRV, NSEC 916 Servers may generate PUSH messages up to a maximum DNS message length 917 of 16,382 bytes, counting from the start of the DSO 12-byte header. 918 Including the two-byte length prefix that is used to frame DNS over a 919 byte stream like TLS, this makes a total of 16,384 bytes. Servers 920 MUST NOT generate PUSH messages larger than this. Where the 921 immediately available change notifications are sufficient to exceed a 922 DNS message length of 16,382 bytes, the change notifications MUST be 923 communicated in separate PUSH messages of up to 16,382 bytes each. 924 DNS name compression becomes less effective for messages larger than 925 16,384 bytes, so little efficiency benefit is gained by sending 926 messages larger than this. 928 If a client receives a PUSH message with a DNS message length larger 929 than 16,382 bytes, this is a fatal error, and the client MUST 930 forcibly abort the connection immediately. 932 1 1 1 1 1 1 933 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 934 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 935 | MESSAGE ID (MUST BE ZERO) | \ 936 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 937 |QR| OPCODE(6) | Z | RCODE | | 938 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 939 | QDCOUNT (MUST BE ZERO) | | 940 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 941 | ANCOUNT (MUST BE ZERO) | | 942 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 943 | NSCOUNT (MUST BE ZERO) | | 944 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 945 | ARCOUNT (MUST BE ZERO) | / 946 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 947 | DSO-TYPE = PUSH (tentatively 0x41) | 948 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 949 | DSO-LENGTH (number of octets in DSO-DATA) | 950 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 951 \ NAME \ \ 952 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 953 | TYPE | | 954 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 955 | CLASS | | 956 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 957 | TTL | | 958 | (32-bit unsigned big-endian integer) | > DSO-DATA 959 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 960 | RDLEN (16-bit unsigned big-endian integer) | | 961 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 962 \ RDATA (sized as necessary) \ | 963 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 964 : NAME, TYPE, CLASS, TTL, RDLEN, RDATA : | 965 : Repeated As Necessary : / 966 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 968 Figure 3: PUSH Message 970 When processing the records received in a PUSH Message, the receiving 971 client MUST validate that the records being added or deleted 972 correspond with at least one currently active subscription on that 973 session. Specifically, the record name MUST match the name given in 974 a SUBSCRIBE request, subject to the usual established DNS case- 975 insensitivity for US-ASCII letters. If the TYPE in the SUBSCRIBE 976 request was not ANY (255) then the TYPE of the record must match the 977 TYPE given in the SUBSCRIBE request. If the CLASS in the SUBSCRIBE 978 request was not ANY (255) then the CLASS of the record must match the 979 CLASS given in the SUBSCRIBE request. If a matching active 980 subscription on that session is not found, then that individual 981 record addition/deletion is silently ignored. Processing of other 982 additions and deletions in this message is not affected. The DSO 983 session is not closed. This is to allow for the unavoidable race 984 condition where a client sends an outbound UNSUBSCRIBE while inbound 985 PUSH messages for that subscription from the server are still in 986 flight. 988 In the case where a single change affects more than one active 989 subscription, only one PUSH message is sent. For example, a PUSH 990 message adding a given record may match both a SUBSCRIBE request with 991 the same TYPE and a different SUBSCRIBE request with TYPE = 255 992 (ANY). It is not the case that two PUSH messages are sent because 993 the new record matches two active subscriptions. 995 The server SHOULD encode change notifications in the most efficient 996 manner possible. For example, when three AAAA records are deleted 997 from a given name, and no other AAAA records exist for that name, the 998 server SHOULD send a "delete an RRset from a name" PUSH message, not 999 three separate "delete an individual RR from a name" PUSH messages. 1000 Similarly, when both an SRV and a TXT record are deleted from a given 1001 name, and no other records of any kind exist for that name, the 1002 server SHOULD send a "delete all RRsets from a name" PUSH message, 1003 not two separate "delete an RRset from a name" PUSH messages. 1005 A server SHOULD combine multiple change notifications in a single 1006 PUSH message when possible, even if those change notifications apply 1007 to different subscriptions. Conceptually, a PUSH message is a 1008 session-level mechanism, not a subscription-level mechanism. 1010 The TTL of an added record is stored by the client. While the 1011 subscription is active, the TTL is not decremented, because a change 1012 to the TTL would produce a new update. For as long as a relevant 1013 subscription remains active, the client SHOULD assume that when a 1014 record goes away the server will notify it of that fact. 1015 Consequently, a client does not have to poll to verify that the 1016 record is still there. Once a subscription is cancelled 1017 (individually, or as a result of the DSO session being closed) record 1018 aging for records covered by the subscription resumes and records are 1019 removed from the local cache when their TTL reaches zero. 1021 6.4. DNS Push Notification UNSUBSCRIBE 1023 To cancel an individual subscription without closing the entire DSO 1024 session, the client sends an UNSUBSCRIBE message over the established 1025 DSO session to the server. The UNSUBSCRIBE message is encoded as a 1026 DSO unidirectional message [RFC8490]. This specification defines a 1027 primary unidirectional DSO TLV for DNS Push Notification UNSUBSCRIBE 1028 Messages (tentatively DSO Type Code 0x42). 1030 A server MUST NOT send an UNSUBSCRIBE message. If a server does send 1031 an UNSUBSCRIBE message over a DSO session initiated by a client, or 1032 an UNSUBSCRIBE message is sent with the QR bit set indicating that it 1033 is a response, this is a fatal error and the receiver MUST forcibly 1034 abort the connection immediately. 1036 6.4.1. UNSUBSCRIBE Message 1038 An UNSUBSCRIBE unidirectional message begins with the standard DSO 1039 12-byte header [RFC8490], followed by the UNSUBSCRIBE primary TLV. 1040 An UNSUBSCRIBE message is illustrated in Figure 4. 1042 In accordance with the definition of DSO unidirectional messages, the 1043 MESSAGE ID field MUST be zero. There is no server response to an 1044 UNSUBSCRIBE message. 1046 The other header fields MUST be set as described in the DSO spec- 1047 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 1048 for DNS Stateful Operations (6). The four count fields MUST be zero, 1049 and the corresponding four sections MUST be empty (i.e., absent). 1051 The DSO-TYPE is UNSUBSCRIBE (tentatively 0x42). 1053 The DSO-LENGTH field contains the value 2, the length of the 2-octet 1054 MESSAGE ID contained in the DSO-DATA. 1056 The DSO-DATA contains the value given in the MESSAGE ID field of an 1057 active SUBSCRIBE request. This is how the server knows which 1058 SUBSCRIBE request is being cancelled. After receipt of the 1059 UNSUBSCRIBE message, the SUBSCRIBE request is no longer active. 1061 It is allowable for the client to issue an UNSUBSCRIBE message for a 1062 previous SUBSCRIBE request for which the client has not yet received 1063 a SUBSCRIBE response. This is to allow for the case where a client 1064 starts and stops a subscription in less than the round-trip time to 1065 the server. The client is NOT required to wait for the SUBSCRIBE 1066 response before issuing the UNSUBSCRIBE message. 1068 Consequently, it is possible for a server to receive an UNSUBSCRIBE 1069 message that does not match any currently active subscription. This 1070 can occur when a client sends a SUBSCRIBE request, which subsequently 1071 fails and returns an error code, but the client sent an UNSUBSCRIBE 1072 message before it became aware that the SUBSCRIBE request had failed. 1073 Because of this, servers MUST silently ignore UNSUBSCRIBE messages 1074 that do not match any currently active subscription. 1076 1 1 1 1 1 1 1077 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 1078 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1079 | MESSAGE ID (MUST BE ZERO) | \ 1080 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1081 |QR| OPCODE(6) | Z | RCODE | | 1082 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1083 | QDCOUNT (MUST BE ZERO) | | 1084 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 1085 | ANCOUNT (MUST BE ZERO) | | 1086 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1087 | NSCOUNT (MUST BE ZERO) | | 1088 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1089 | ARCOUNT (MUST BE ZERO) | / 1090 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1091 | DSO-TYPE = UNSUBSCRIBE (tentatively 0x42) | 1092 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 1093 | DSO-LENGTH (2) | 1094 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1095 | SUBSCRIBE MESSAGE ID | > DSO-DATA 1096 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1098 Figure 4: UNSUBSCRIBE Message 1100 6.5. DNS Push Notification RECONFIRM 1102 Sometimes, particularly when used with a Discovery Proxy [DisProx], a 1103 DNS Zone may contain stale data. When a client encounters data that 1104 it believes may be stale (e.g., an SRV record referencing a target 1105 host+port that is not responding to connection requests) the client 1106 can send a RECONFIRM message to ask the server to re-verify that the 1107 data is still valid. For a Discovery Proxy, this causes it to issue 1108 new Multicast DNS queries to ascertain whether the target device is 1109 still present. How the Discovery Proxy causes these new Multicast 1110 DNS queries to be issued depends on the details of the underlying 1111 Multicast DNS implementation being used. For example, a Discovery 1112 Proxy built on Apple's dns_sd.h API responds to a DNS Push 1113 Notification RECONFIRM message by calling the underlying API's 1114 DNSServiceReconfirmRecord() routine. 1116 For other types of DNS server, the RECONFIRM operation is currently 1117 undefined, and SHOULD result in a NOERROR response, but otherwise 1118 need not cause any action to occur. 1120 Frequent use of RECONFIRM operations may be a sign of network 1121 unreliability, or some kind of misconfiguration, so RECONFIRM 1122 operations MAY be logged or otherwise communicated to a human 1123 administrator to assist in detecting and remedying such network 1124 problems. 1126 If, after receiving a valid RECONFIRM message, the server determines 1127 that the disputed records are in fact no longer valid, then 1128 subsequent DNS PUSH Messages will be generated to inform interested 1129 clients. Thus, one client discovering that a previously-advertised 1130 device (like a network printer) is no longer present has the side 1131 effect of informing all other interested clients that the device in 1132 question is now gone. 1134 A server MUST NOT send a RECONFIRM message. If a server does send a 1135 RECONFIRM message over a DSO session initiated by a client, or a 1136 RECONFIRM message is sent with the QR bit set indicating that it is a 1137 response, this is a fatal error and the receiver MUST forcibly abort 1138 the connection immediately. 1140 6.5.1. RECONFIRM Message 1142 A RECONFIRM unidirectional message begins with the standard DSO 1143 12-byte header [RFC8490], followed by the RECONFIRM primary TLV. 1144 A RECONFIRM message is illustrated in Figure 5. 1146 In accordance with the definition of DSO unidirectional messages, the 1147 MESSAGE ID field MUST be zero. There is no server response to a 1148 RECONFIRM message. 1150 The other header fields MUST be set as described in the DSO spec- 1151 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 1152 for DNS Stateful Operations (6). The four count fields MUST be zero, 1153 and the corresponding four sections MUST be empty (i.e., absent). 1155 The DSO-TYPE is RECONFIRM (tentatively 0x43). 1157 The DSO-LENGTH is the length of the data that follows, which 1158 specifies the name, type, class, and content of the record being 1159 disputed. 1161 The DSO-DATA for a RECONFIRM message MUST contain exactly one record. 1162 The DSO-DATA for a RECONFIRM message has no count field to specify 1163 more than one record. Since RECONFIRM messages are sent over TCP, 1164 multiple RECONFIRM messages can be concatenated in a single TCP 1165 stream and packed efficiently into TCP segments. 1167 TYPE MUST NOT be the value ANY (255) and CLASS MUST NOT be the value 1168 ANY (255). 1170 DNS wildcarding is not supported. That is, a wildcard ("*") in a 1171 RECONFIRM message matches only a literal wildcard character ("*") in 1172 the zone, and nothing else. 1174 Aliasing is not supported. That is, a CNAME in a RECONFIRM message 1175 matches only a literal CNAME record in the zone, and no other records 1176 with the same owner name. 1178 Note that there is no RDLEN field, since the length of the RDATA can 1179 be inferred from DSO-LENGTH, so an additional RDLEN field would be 1180 redundant. 1182 1 1 1 1 1 1 1183 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 1184 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1185 | MESSAGE ID | \ 1186 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1187 |QR| OPCODE(6) | Z | RCODE | | 1188 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1189 | QDCOUNT (MUST BE ZERO) | | 1190 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 1191 | ANCOUNT (MUST BE ZERO) | | 1192 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1193 | NSCOUNT (MUST BE ZERO) | | 1194 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1195 | ARCOUNT (MUST BE ZERO) | / 1196 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1197 | DSO-TYPE = RECONFIRM (tentatively 0x43) | 1198 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 1199 | DSO-LENGTH (number of octets in DSO-DATA) | 1200 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1201 \ NAME \ \ 1202 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1203 | TYPE | | 1204 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > DSO-DATA 1205 | CLASS | | 1206 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1207 \ RDATA \ / 1208 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1210 Figure 5: RECONFIRM Message 1212 6.6. DNS Stateful Operations TLV Context Summary 1214 This document defines four new DSO TLVs. As suggested in Section 8.2 1215 of the DNS Stateful Operations specification [RFC8490], the valid 1216 contexts of these new TLV types are summarized below. 1218 The client TLV contexts are: 1220 C-P: Client request message, primary TLV 1221 C-U: Client unidirectional message, primary TLV 1222 C-A: Client request or unidirectional message, additional TLV 1223 CRP: Response back to client, primary TLV 1224 CRA: Response back to client, additional TLV 1226 +-------------+-----+-----+-----+-----+-----+ 1227 | TLV Type | C-P | C-U | C-A | CRP | CRA | 1228 +-------------+-----+-----+-----+-----+-----+ 1229 | SUBSCRIBE | X | | | | | 1230 | PUSH | | | | | | 1231 | UNSUBSCRIBE | | X | | | | 1232 | RECONFIRM | | X | | | | 1233 +-------------+-----+-----+-----+-----+-----+ 1235 Table 2: DSO TLV Client Context Summary 1237 The server TLV contexts are: 1239 S-P: Server request message, primary TLV 1240 S-U: Server unidirectional message, primary TLV 1241 S-A: Server request or unidirectional message, additional TLV 1242 SRP: Response back to server, primary TLV 1243 SRA: Response back to server, additional TLV 1245 +-------------+-----+-----+-----+-----+-----+ 1246 | TLV Type | S-P | S-U | S-A | SRP | SRA | 1247 +-------------+-----+-----+-----+-----+-----+ 1248 | SUBSCRIBE | | | | | | 1249 | PUSH | | X | | | | 1250 | UNSUBSCRIBE | | | | | | 1251 | RECONFIRM | | | | | | 1252 +-------------+-----+-----+-----+-----+-----+ 1254 Table 3: DSO TLV Server Context Summary 1256 6.7. Client-Initiated Termination 1258 An individual subscription is terminated by sending an UNSUBSCRIBE 1259 TLV for that specific subscription, or all subscriptions can be 1260 cancelled at once by the client closing the DSO session. When a 1261 client terminates an individual subscription (via UNSUBSCRIBE) or all 1262 subscriptions on that DSO session (by ending the session) it is 1263 signaling to the server that it is longer interested in receiving 1264 those particular updates. It is informing the server that the server 1265 may release any state information it has been keeping with regards to 1266 these particular subscriptions. 1268 After terminating its last subscription on a session via UNSUBSCRIBE, 1269 a client MAY close the session immediately, or it may keep it open if 1270 it anticipates performing further operations on that session in the 1271 future. If a client wishes to keep an idle session open, it MUST 1272 respect the maximum idle time required by the server [RFC8490]. 1274 If a client plans to terminate one or more subscriptions on a session 1275 and doesn't intend to keep that session open, then as an efficiency 1276 optimization it MAY instead choose to simply close the session, which 1277 implicitly terminates all subscriptions on that session. This may 1278 occur because the client computer is being shut down, is going to 1279 sleep, the application requiring the subscriptions has terminated, or 1280 simply because the last active subscription on that session has been 1281 cancelled. 1283 When closing a session, a client should perform an orderly close of 1284 the TLS session. Typical APIs will provide a session close method 1285 that will send a TLS close_notify alert (see Section 6.1 of the TLS 1286 1.3 specification [RFC8446]). This instructs the recipient that the 1287 sender will not send any more data over the session. After sending 1288 the TLS close_notify alert the client MUST gracefully close the 1289 underlying connection using a TCP FIN, so that the TLS close_notify 1290 is reliably delivered. The mechanisms for gracefully closing a TCP 1291 connection with a TCP FIN vary depending on the networking API. For 1292 example, in the BSD Sockets API, sending a TCP FIN is achieved by 1293 calling "shutdown(s,SHUT_WR)" and keeping the socket open until all 1294 remaining data has been read from it. 1296 If the session is forcibly closed at the TCP level by sending a RST 1297 from either end of the connection, data may be lost. 1299 6.8. Client Fallback to Polling 1301 There are cases where a client may exhaust all avenues for 1302 establishing a DNS Push Notification subscription without success. 1303 This can happen if the client's configured recursive resolver does 1304 not support DNS over TLS, or supports DNS over TLS but is not 1305 listening on TCP port 853, or supports DNS over TLS on TCP port 853 1306 but does not support DSO on that port, or for some other reason is 1307 unable to provide a DNS Push Notification subscription. In this case 1308 the client will attempt to communicate directly with an appropriate 1309 server, and it may be that the zone apex discovery fails, or there is 1310 no "_dns-push-tls._tcp." SRV record, or server indicated in the 1311 SRV record is misconfigured, or is unresponsive for some other 1312 reason. 1314 Regardless of the reason for the failure, after being unable to 1315 establish the desired DNS Push Notification subscription, it is 1316 likely that the client will still wish to know the answer it seeks, 1317 even if that answer cannot be obtained with the timely change 1318 notifications provided by DNS Push Notifications. In such cases it 1319 is likely that the client will obtain the answer it seeks via a 1320 conventional DNS query instead, repeated at some interval to detect 1321 when the answer RRset changes. 1323 In the case where a client responds to its failure to establish a DNS 1324 Push Notification subscription by falling back to polling with 1325 conventional DNS queries instead, the polling rate should be 1326 controlled to avoid placing excessive burden on the server. The 1327 interval between successive DNS queries for the same name, type and 1328 class SHOULD be at least the minimum of: 900 seconds (15 minutes), or 1329 two seconds more than the TTL of the answer RRset. 1331 The reason that for TTLs shorter than 898 seconds the query should 1332 not be reissued until two seconds *after* the answer RRset has 1333 expired is to ensure that the answer RRset has also expired from the 1334 cache on the client's configured recursive resolver. Otherwise 1335 (particularly if the clocks on the client and the recursive resolver 1336 do not run at precisely the same rate) there's a risk of a race 1337 condition where the client queries its configured recursive resolver 1338 just as the answer RRset has one second remaining in the recursive 1339 resolver's cache. The client would then receive a reply telling it 1340 that the answer RRset has one second remaining, and then the client 1341 would then re-query the recursive resolver again one second later 1342 when the answer RRset actually expires, and only then would the 1343 recursive resolver issue a new query to fetch new fresh data from the 1344 authoritative server. Waiting until the answer RRset has definitely 1345 expired from the the cache on the client's configured recursive 1346 resolver avoids this race condition and unnecessary additional 1347 queries it causes. 1349 Each time a client is about to reissue its query to discover changes 1350 to the answer RRset, it should first make a new attempt to establish 1351 a DNS Push Notification subscription, using previously cached DNS 1352 answers as appropriate. After a temporary misconfiguration has been 1353 remedied, this allows a client that is polling to return to using DNS 1354 Push Notifications for asynchronous notification of changes. 1356 7. Security Considerations 1358 The Strict Privacy Usage Profile for DNS over TLS is REQUIRED for DNS 1359 Push Notifications [RFC8310]. Cleartext connections for DNS Push 1360 Notifications are not permissible. Since this is a new protocol, 1361 transition mechanisms from the Opportunistic Privacy profile are 1362 unnecessary. 1364 Also, see Section 9 of the DNS over (D)TLS Usage Profiles document 1365 [RFC8310] for additional recommendations for various versions of TLS 1366 usage. 1368 As a consequence of requiring TLS, client certificate authentication 1369 and verification may also be enforced by the server for stronger 1370 client-server security or end-to-end security. However, 1371 recommendations for security in particular deployment scenarios are 1372 outside the scope of this document. 1374 DNSSEC is RECOMMENDED for the authentication of DNS Push Notification 1375 servers. TLS alone does not provide complete security. TLS 1376 certificate verification can provide reasonable assurance that the 1377 client is really talking to the server associated with the desired 1378 host name, but since the desired host name is learned via a DNS SRV 1379 query, if the SRV query is subverted then the client may have a 1380 secure connection to a rogue server. DNSSEC can provided added 1381 confidence that the SRV query has not been subverted. 1383 7.1. Security Services 1385 It is the goal of using TLS to provide the following security 1386 services: 1388 Confidentiality: All application-layer communication is encrypted 1389 with the goal that no party should be able to decrypt it except 1390 the intended receiver. 1392 Data integrity protection: Any changes made to the communication in 1393 transit are detectable by the receiver. 1395 Authentication: An end-point of the TLS communication is 1396 authenticated as the intended entity to communicate with. 1398 Anti-replay protection: TLS provides for the detection of and 1399 prevention against messages sent previously over a TLS connection 1400 (such as DNS Push Notifications). Prior messages cannot be re- 1401 sent at a later time as a form of a man-in-the-middle attack. 1403 Deployment recommendations on the appropriate key lengths and cypher 1404 suites are beyond the scope of this document. Please refer to TLS 1405 Recommendations [RFC7525] for the best current practices. Keep in 1406 mind that best practices only exist for a snapshot in time and 1407 recommendations will continue to change. Updated versions or errata 1408 may exist for these recommendations. 1410 7.2. TLS Name Authentication 1412 As described in Section 6.1, the client discovers the DNS Push 1413 Notification server using an SRV lookup for the record name 1414 "_dns-push-tls._tcp.". The server connection endpoint SHOULD 1415 then be authenticated using DANE TLSA records for the associated SRV 1416 record. This associates the target's name and port number with a 1417 trusted TLS certificate [RFC7673]. This procedure uses the TLS 1418 Server Name Indication (SNI) extension [RFC6066] to inform the server 1419 of the name the client has authenticated through the use of TLSA 1420 records. Therefore, if the SRV record passes DNSSEC validation and a 1421 TLSA record matching the target name is useable, an SNI extension 1422 must be used for the target name to ensure the client is connecting 1423 to the server it has authenticated. If the target name does not have 1424 a usable TLSA record, then the use of the SNI extension is optional. 1425 See Usage Profiles for DNS over TLS and DNS over DTLS [RFC8310] for 1426 more information on authenticating domain names. 1428 7.3. TLS Early Data 1430 DSO messages with the SUBSCRIBE TLV as the Primary TLV are permitted 1431 in TLS early data. Using TLS early data can save one network round 1432 trip, and can result in the client obtaining results faster. 1434 However, there are some factors to consider before using TLS early 1435 data. 1437 TLS Early Data is not forward secret. In cases where forward secrecy 1438 of DNS Push Notification subscriptions is required, the client should 1439 not use TLS Early Data. 1441 With TLS early data there are no guarantees of non-replay between 1442 connections. If packets are duplicated and delayed in the network, 1443 the later arrivals could be mistaken for new subscription requests. 1444 Generally this is not a major concern, since the amount of state 1445 generated on the server for these spurious subscriptions is small and 1446 short-lived, since the TCP connection will not complete the three-way 1447 handshake. Servers MAY choose to implement rate-limiting measures 1448 that are activated when the server detects an excessive number of 1449 spurious subscription requests. 1451 For further guidance please see Section 2.3, Section 8, and 1452 Appendix E.5 of the TLS 1.3 specification [RFC8446]. 1454 7.4. TLS Session Resumption 1456 TLS Session Resumption is permissible on DNS Push Notification 1457 servers. The server may keep TLS state with Session IDs [RFC8446] or 1458 operate in stateless mode by sending a Session Ticket [RFC5077] to 1459 the client for it to store. However, closing the TLS connection 1460 terminates the DSO session. When the TLS session is resumed, the DNS 1461 Push Notification server will not have any subscription state and 1462 will proceed as with any other new DSO session. Use of TLS Session 1463 Resumption may allow a TLS connection to be set up more quickly, but 1464 the client will still have to recreate any desired subscriptions. 1466 8. IANA Considerations 1468 This document defines a new service name to be published in the IANA 1469 Registry Service Types [RFC6335][ST] that is only applicable for the 1470 TCP protocol. 1472 +-----------------------+------+----------------------+-------------+ 1473 | Name | Port | Value | Definition | 1474 +-----------------------+------+----------------------+-------------+ 1475 | DNS Push Notification | None | "_dns-push-tls._tcp" | Section 6.1 | 1476 | Service Type | | | | 1477 +-----------------------+------+----------------------+-------------+ 1479 Table 4: IANA Service Type Assignments 1481 This document also defines four new DNS Stateful Operation TLV types 1482 to be recorded in the IANA DSO Type Code Registry. 1484 +-------------+------------+---------+-----------------+------------+ 1485 | Name | Value | Early | Status | Definition | 1486 | | | Data | | | 1487 +-------------+------------+---------+-----------------+------------+ 1488 | SUBSCRIBE | TBA (0x40) | OK | Standards Track | Section | 1489 | | | | | 6.2 | 1490 | PUSH | TBA (0x41) | NO | Standards Track | Section | 1491 | | | | | 6.3 | 1492 | UNSUBSCRIBE | TBA (0x42) | NO | Standards Track | Section | 1493 | | | | | 6.4 | 1494 | RECONFIRM | TBA (0x43) | NO | Standards Track | Section | 1495 | | | | | 6.5 | 1496 +-------------+------------+---------+-----------------+------------+ 1498 Table 5: IANA DSO TLV Type Code Assignments 1500 This document defines no new DNS OPCODEs or RCODEs. 1502 9. Acknowledgements 1504 The authors would like to thank Kiren Sekar and Marc Krochmal for 1505 previous work completed in this field. 1507 This draft has been improved due to comments from Ran Atkinson, Tim 1508 Chown, Sara Dickinson, Mark Delany, Ralph Droms, Jan Komissar, Eric 1509 Rescorla, Michael Richardson, David Schinazi, Manju Shankar Rao, 1510 Robert Sparks, Markus Stenberg, Andrew Sullivan, Michael Sweet, Dave 1511 Thaler, Brian Trammell, Bernie Volz, Eric Vyncke, Christopher Wood, 1512 Liang Xia, and Soraia Zlatkovic. Ted Lemon provided clarifying text 1513 that was greatly appreciated. 1515 10. References 1517 10.1. Normative References 1519 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 1520 DOI 10.17487/RFC0768, August 1980, 1521 . 1523 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 1524 RFC 793, DOI 10.17487/RFC0793, September 1981, 1525 . 1527 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1528 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 1529 . 1531 [RFC1035] Mockapetris, P., "Domain names - implementation and 1532 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1533 November 1987, . 1535 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - 1536 Application and Support", STD 3, RFC 1123, 1537 DOI 10.17487/RFC1123, October 1989, 1538 . 1540 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1541 Requirement Levels", BCP 14, RFC 2119, 1542 DOI 10.17487/RFC2119, March 1997, 1543 . 1545 [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound, 1546 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 1547 RFC 2136, DOI 10.17487/RFC2136, April 1997, 1548 . 1550 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 1551 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 1552 . 1554 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 1555 specifying the location of services (DNS SRV)", RFC 2782, 1556 DOI 10.17487/RFC2782, February 2000, 1557 . 1559 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 1560 Extensions: Extension Definitions", RFC 6066, 1561 DOI 10.17487/RFC6066, January 2011, 1562 . 1564 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. 1565 Cheshire, "Internet Assigned Numbers Authority (IANA) 1566 Procedures for the Management of the Service Name and 1567 Transport Protocol Port Number Registry", BCP 165, 1568 RFC 6335, DOI 10.17487/RFC6335, August 2011, 1569 . 1571 [RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA 1572 Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895, 1573 April 2013, . 1575 [RFC7673] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS- 1576 Based Authentication of Named Entities (DANE) TLSA Records 1577 with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October 1578 2015, . 1580 [RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and 1581 D. Wessels, "DNS Transport over TCP - Implementation 1582 Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016, 1583 . 1585 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1586 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1587 May 2017, . 1589 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 1590 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 1591 . 1593 [RFC8490] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S., 1594 Lemon, T., and T. Pusateri, "DNS Stateful Operations", 1595 RFC 8490, DOI 10.17487/RFC8490, March 2019, 1596 . 1598 [ST] "Service Name and Transport Protocol Port Number 1599 Registry", . 1602 10.2. Informative References 1604 [DisProx] Cheshire, S., "Discovery Proxy for Multicast DNS-Based 1605 Service Discovery", draft-ietf-dnssd-hybrid-10 (work in 1606 progress), March 2019. 1608 [I-D.ietf-tcpm-rack] 1609 Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK: 1610 a time-based fast loss detection algorithm for TCP", 1611 draft-ietf-tcpm-rack-05 (work in progress), April 2019. 1613 [LLQ] Cheshire, S. and M. Krochmal, "DNS Long-Lived Queries", 1614 draft-sekar-dns-llq-03 (work in progress), March 2019. 1616 [obs] "Observer Pattern", 1617 . 1619 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS 1620 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, 1621 . 1623 [RFC3123] Koch, P., "A DNS RR Type for Lists of Address Prefixes 1624 (APL RR)", RFC 3123, DOI 10.17487/RFC3123, June 2001, 1625 . 1627 [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom 1628 Syndication Format", RFC 4287, DOI 10.17487/RFC4287, 1629 December 2005, . 1631 [RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks", 1632 RFC 4953, DOI 10.17487/RFC4953, July 2007, 1633 . 1635 [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, 1636 "Transport Layer Security (TLS) Session Resumption without 1637 Server-Side State", RFC 5077, DOI 10.17487/RFC5077, 1638 January 2008, . 1640 [RFC6281] Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang, 1641 "Understanding Apple's Back to My Mac (BTMM) Service", 1642 RFC 6281, DOI 10.17487/RFC6281, June 2011, 1643 . 1645 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 1646 DOI 10.17487/RFC6762, February 2013, 1647 . 1649 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service 1650 Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, 1651 . 1653 [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, 1654 "TCP Extensions for Multipath Operation with Multiple 1655 Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013, 1656 . 1658 [RFC6886] Cheshire, S. and M. Krochmal, "NAT Port Mapping Protocol 1659 (NAT-PMP)", RFC 6886, DOI 10.17487/RFC6886, April 2013, 1660 . 1662 [RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and 1663 P. Selkirk, "Port Control Protocol (PCP)", RFC 6887, 1664 DOI 10.17487/RFC6887, April 2013, 1665 . 1667 [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP 1668 Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, 1669 . 1671 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, 1672 "Recommendations for Secure Use of Transport Layer 1673 Security (TLS) and Datagram Transport Layer Security 1674 (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 1675 2015, . 1677 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 1678 Terminology", RFC 7719, DOI 10.17487/RFC7719, December 1679 2015, . 1681 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 1682 and P. Hoffman, "Specification for DNS over Transport 1683 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 1684 2016, . 1686 [RFC8010] Sweet, M. and I. McDonald, "Internet Printing 1687 Protocol/1.1: Encoding and Transport", STD 92, RFC 8010, 1688 DOI 10.17487/RFC8010, January 2017, 1689 . 1691 [RFC8011] Sweet, M. and I. McDonald, "Internet Printing 1692 Protocol/1.1: Model and Semantics", STD 92, RFC 8011, 1693 DOI 10.17487/RFC8011, January 2017, 1694 . 1696 [RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles 1697 for DNS over TLS and DNS over DTLS", RFC 8310, 1698 DOI 10.17487/RFC8310, March 2018, 1699 . 1701 [SYN] Eddy, W., "Defenses Against TCP SYN Flooding Attacks", The 1702 Internet Protocol Journal, Cisco Systems, Volume 9, 1703 Number 4, December 2006. 1705 [XEP0060] Millard, P., Saint-Andre, P., and R. Meijer, "Publish- 1706 Subscribe", XSF XEP 0060, July 2010. 1708 Authors' Addresses 1710 Tom Pusateri 1711 Unaffiliated 1712 Raleigh, NC 27608 1713 USA 1715 Phone: +1 919 867 1330 1716 Email: pusateri@bangj.com 1718 Stuart Cheshire 1719 Apple Inc. 1720 One Apple Park Way 1721 Cupertino, CA 95014 1722 USA 1724 Phone: +1 (408) 996-1010 1725 Email: cheshire@apple.com